Pb halide based perovskite materials are important for photovoltaic devices. In this paper, we systematically discuss the influence of different processing conditions of perovskite devices for both p-i-n and n-i-p configurations. In a p-i-n device, light enters from the p side whereas in the n-i-p device, light enters from the n side. We discuss properties of materials and devices produced using three processing conditions : complete solution growth, partial solution growth, and complete vapor growth where no liquids are used. The properties measured include grain size, structure using x-ray spectrum, deep defects using capacitance-frequency-temperature spectroscopy, drift mobility of holes, type of doping ( p or n), which carrier controls transport and minority carrier diffusion length measured directly using electronic measurements. We show that the complete vapor grown device has the lowest deep defect density, the largest grains size, and a high photovoltaic efficiency of ~15%. We also discuss the differences in properties of materials devices fabricated using either methyl-ammonium iodide (MAI) or formamidinium iodide (FAI). We show that the materials and devices prepared using the vapor process for FAI are far more stable physically and allow a much larger processing range, including processing in vacuum at higher temperatures. In contrast, the devices produced with MAI decompose in vacuum at elevated temperatures. The differences in physical properties between these two classes of materials lead to much better stability for the devices prepared using FAI compared to devices prepared using MAI. We also investigate systematically the influence of Chlorine (eg use of PbCl₂ instead of PbI₂)during the vapor deposition process for devices. We do not find that Cl has any influence on the electronic properties of the material when the grain size is significant. We find that the material is n type independently of the substrate on which it is deposited, and that the transport is unambiguously controlled by holes and not electrons. Therefore it is inaccurate to say that the device is controlled by ambipolar transport. We also find that the drift mobility of holes, measured using time of flight techniques, is ~10^-1 cm2/V-s. The Urbach energy of valence band tails is~16 meV.

Organometal halide perovskites have recently attracted enormous attention since CH₃NH₃PbX₃ can be successfully applied as photoactive material in photovoltaic devices, yielding solar cells with an efficiency exceeding 15%. Surprisingly, the exact mechanism how charges are generated and extracted so well is unclear. In this paper, we report the dynamics of electron injection from CH₃NH₃PbI₃ into PCBM and hole injection into Spiro-OMeTAD, using time resolved microwave conductivity measurements. For intrinsic CH₃NH₃PbI₃ deposited on an inert substrate we observed fast formation of microsecond lived charge carriers. At low laser fluences a maximum charge carrier mobility of about 5 cm²/Vs, yielding charge carrier diffusion lengths well above 5 mu;m is found. In a CH₃NH₃PbI₃/PCBM bilayer electron injection into PCBM occurs on a sub-ns timescale, similar as has been found for TiO₂. In contrast in a CH₃NH₃PbI₃/Spiro-OMeTAD bilayer, hole injection into Spiro-OMeTAD is much slower, extending up to a few hundreds of ns. This large difference in dynamics is related to the type of junction formed at both interfaces. Furthermore, the low conductivity of Spiro-OMeTAD causes the hole to be essentially immobile at the interface enabling fast recombination with electrons residing in the perovskite. These results highlight the need to optimize the conductivity of hole transporting materials in order to enhance the hole extraction to push the overall power conversion efficiency further.

Since the first report on high efficiency perovskite solar cells based on CH₃NH₃PbI₃, organo-lead halide perovskites have recently received significant attention as a promising photovoltaic absorber and near 20 % of power conversion efficiency is achieved over the past years. To improve the photon-to-electron conversion efficiency, the band gap of a photovoltaic absorber should be optimized. Although the band gap of organo-lead halide perovskites as an absorber material is one of fundamental material properties, our understanding of their electronic band structures is lacking. Although there have been a lot of computational studies on the electronic band structures of organo-lead halide perovskites, most studies have interest on the calculation of a specific materials so that a universal model remains undeveloped.
Here, we present a theoretical model to understand band gap variations of organo-lead halide perovskites through density functional theory including spin-orbit coupling and band correction by hybrid density functional. Based on the molecular orbital analysis and electronic band features of lead halide perovskites, we reveal that the factors governing their band gap engineering are the lattice constant, and the positional distortion of iodine from cubic symmetry sites. The antibonding characters of both conduction band and valence band edges and the p-orbital hybridization in Pb atoms imposed by spin orbit coupling are responsible for the unique band gap dependence on lattice deformation. Our results suggest that reducing lattice constants while keeping the cubic symmetry of the PbI₃ inorganic framework is essential for the development of lead halide perovskites having a lower band gap.
This model can be applied to understand uncovered physical origin of band gap variation of organo-lead halide perovskites. For example, the prior study presented that the less Pb-I tilting of formamindinium lead iodide caused the reduced band gap from methylammonium lead iodide but the origin of conduction band lifting induced by Pb-I tilting is not clearly described. In this study, it is found that the occurrence of the antibonding between Pb 6p_y and I₁ 5p_x raises the conduction band energy minimum with the help of spin-orbit coupling. In addition, various puzzling band gap behaviors predicted by theoretical methods can be successfully interpreted with the developed model.
The intense competition to develop more efficient photovoltaic cells using organo-metal halide perovskites today drives the synthesis of various kinds of alloys, and trials involving substitutions of elements. We expect that this elucidation of the factors governing band gap modulation can contribute to interpreting the band gap changes of newly synthesized materials and the improvement of material design.

Methylammonium lead halide perovskites are promising candidates as active materials for photovoltaics owing to their solution processability, tunable bandgap and strong optical absorption. Understanding the nature of the photoconductivity in these materials is mandatory in order to promote further this technology. Here, we use Terahertz (THz) spectroscopy to unravel the nature of photoconductivity in chlorine based methylammonium lead iodide perovskite films as a function of temperature in the ranges 77K-150K (orthorhombic phase) and 150-300K (tetragonal phase). Femtosecond laser-based THz spectroscopy provides a non-contact way to determine the conductivity of the material following optical injection of charge carriers, with sub-picosecond time resolution. The measurements reveal long-lived trap-free free carrier dynamics on a nanosecond timescale for the tetragonal phase and the activation of carrier recombination for temperatures below ~150 K (orthorhombic phase). The latter could be correlated with the activation of below bandgap emission as resolved by photoluminescence.
The THz frequency-resolved complex photoconductivity measurements reveal that: (i) free charge carriers (in contrast to excitons) are the primary photoproduct after light excitation and (ii) free carriers do not experience confinement, as concluded from to the absence of backscattering at grain boundaries, independently of temperature and crystal structure. The Drude model of carrier mobility describes the data well, providing scattering rates as a function of temperature. These indicate that phonon-electron scattering is the main physical mechanism impeding larger charge mobilities in these materials.

Organometal halide perovskite-based solar cells have recently realized large conversion efficiency over 19% showing great promise for a new large scale cost-competitive photovoltaic technology. The proficient operation of the CPbX3 perovskite solar cell, where C is an organic cation, has been accomplished by many different approaches, and it points to a robust photovoltaic operation mechanism that so far has not yet been fully understood. We report the behaviour of lead halogenate perovskite solar cell probed by a number of dynamic techniques including impedance spectroscopy and time transient dynamics. We compare a number of compositions and morphologies and we show the general characteristics of the observed processes. New and interesting phenomena govern the solar cell behaviour in the long time scale, that influence the solar cell performance in phenomena as hysteresis or time dependent luminescence. These results indicate the need for detailed studies to relate structural and electronic behaviours in the perovskite solar cells.

Plumbate perovskite solar cells based on CH₃NH₃PbI₃ have demonstrated efficiencies comparable to commercial PV technology. The simple fabrication techniques and lower carbon footprint makes the new system attractive candidate for designing next generation solar cells. These cells have already delivered solar cell efficiencies in the range of 17-19%. So far, the major thrust is directed towards processing of solar cells with reproducible and stable performance. By using transient absorption spectroscopy we have succeeded in elucidating the excited state behavior of CH₃NH₃PbI₃ as well as the nature of the charge separation responsible photocurrent generation. A charge separated state and charge transfer complex state of CH₃NH₃PbI₃ have been identified using the nature of different relaxation pathways. The dominant relaxation course however, remains charge recombination between free electrons and holes. Accumulation of charges following laser pulse excitation of the perovskite films leads to an increase in the intrinsic bandgap as expected from the Burstein-Moss band filling model. The carrier density dependent recombination process and the band-edge shift provide insight into the behavior of photogenerated charge carriers.

Organometal halide perovskite materials have been studied extensively for photovoltaic applications due to their high power conversion efficiency and compatibility with simple fabrication processes. Despite the theoretical studies and macroscopic electrical characterizations on the electrical and electro-optical properties of the perovskites, a microscopic picture that correlates the chemical composition with electric dipoles in the perovskite solids is still lacking.
Herein, we investigate the compositional dependence of electric dipoles in AMX₃ (A: organic; M: metal; X: halogen) perovskite structures using modulation electroabsorption (EA) spectroscopy, which measures the change in the reflection of light through a material upon application of a modulated electric field. By sampling various device structures we show that the second harmonic EA spectra reflect the intrinsic, rather than interfacial, properties of the perovskite films. A quantitative analysis of the EA spectra of CH₃NH₃PbI₃, NH₂CHNH₂PbI₃ and CH₃NH₃Sn_0.4Pb_0.6I₃ is provided to compare the impact of the organic and metal cations on the photoinduced response of dipole moment. Based on the EA results, we propose that the A and M cations could both largely affect the dielectric and dipolar properties of the perovskite materials, but through different mechanisms, such as ionic polarization, rotation of molecular dipoles and charge migration. These processes occur at different time scales and thus result in a frequency-dependent dipole response. We further correlate the dielectric property with charge transport and charge trapping processes in the perovskites using charge modulation spectroscopy, electrochemical impedance spectroscopy and time-resolved THz experiments. Our fundamental spectroscopic measurement together with theoretical calculations can help to improve the understanding on the remarkable electronic properties of the AMX₃ perovskites and provide guidelines on the design of perovskite materials with new functionalities.

Organohalide perovskites have recently emerged as a leading next-generation photovoltaic technology [1]. However, there is considerable uncertainty as to how such devices operate and major knowledge gaps exist in the fundamental perovskite material properties such as the exciton binding energy, dielectric constants, and free carrier dynamics. Much of the electro-optical property knowledge for these systems is derived from solid-state physics of the 1990s [2], and has to be revisited with better materials and updated methods and theories - specifically for high quality organohalide perovskite thin films.
Motivated by these factors we have undertaken a detailed study of CH₃NH₃PbI₃ perovskite thin films deposited by vacuum co-evaporation [3]. We have measured the optical constants from the UV to NIR using a combination of ellipsometry and near-normal incidence reflectometry. Using this information we used transfer matrix analysis to model and optimize simple linear homojunction cell structures composed of a p-type polymer modified anode, thin perovskite active layer (100 - 300 nm), and fullerene modified cathode. Such devices delivered a maximum Power Conversion Efficiciency (PCE) at AM1.5G of 16.5% (hysteresis free) with Internal Quantum Efficiencies (IQEs) of ~100% that were spectrally flat ruling out any hot species effects. Both experiment and simulations confirmed that these perovskite solar cells operate in two distinct cavity regimes: within the Beer-Lambert limit for lambda;<500 nm; and as low finesse interference cavities for c >500 nm. Furthermore, we also observed lossless charge extraction even at light intensities higher than 1 sun.
In addition, we also measured the dielectric constants from static to optical frequencies. Specifically we obseverd the static value to be ~70 from which we calculated an exciton binding energy of ~2 meV - much lower than previous estimations [2,4] and closer to recent bare-band DFT calculations by Frost et al. [5]. In our paper we will describe these findings in detail, and furthermore discuss how they establish systematic design rules for simple homojunction organohalide perovskite solar cells, which are generic and relevant to other architectures such as tandems.
References:
[1] Green et al. Nature Photon. 8, 506 (2014).
[2] Hirasawa et al. Physica B201, 427 (1994).
[3] Lin and Armin et al. Nature Photon., Accepted mansucript.
[4] D'Innocenzo et al. Nature Commun. 5, 3586 (2014).
[5] Frost et al. Nano Lett. 14, 2584 (2014).

Organometal halide perovskites are promising photovolatic materials that have been recently reported of high power conversion efficiency. Extraordinary charge transport ability is associated with a long free charge carrier diffusion length in their lattice structures. In this work, we report a direct measurement of the charge carrier diffusion coefficient in methylammonium lead iodine perovskite (CH₃NH₃PbI₃) film using transient absorption imaging (TAM) technique. The time dependent charge carrier diffusion profiles were recorded soon after a point source excitation with a point spread function of less than 340 nm FWHM. The charge carrier diffusion coefficient was determined as 0.063±0.013 cm²/s in 3D perovskite through fitting the gaussian profile variance change. Time dependent 2D diffusion profiles can be satisfactorily described by a diffusion equation including both spatial diffusion and high order recombination terms at different pump fluences. We have also investigated the photophysics of 2D perovskite structures composed of few layers of perovskite lattices in one dimension. Unlike in the bulk, electrons and holes are bound in to excitons in 2D CH₃NH₃PbI₃ perovskite. The exciton diffusion coefficient in a tri-layer 2D perovskite was found to be below 0.003 cm²/s. Ground state and time resolved absorption spectra as well as time resolved photoluminescence of mono-, bi-, and tri-layer perovskite films have also been obtained to identify spectral feature corresponding to 2D perovskite band structures.

Solar cells based on hybrid metal-organic halide perovskites have been polarizing the attention of the photovoltaics community in the past 3 years due to their ever increasing efficiencies, currently over 19%. This success is largely due to methylammonium lead iodide and mixed halide having band gaps approaching the Shockley-Queisser limit (1.55 eV) and electron and hole diffusion lenghts exceeding 1 micron. Additionally, these electronic properties can be tuned by mixing different halides, metal atoms or cations.
In this context computational design can assist the search for novel and even more efficient perovskites by exploring a vast materials library in silico.
In this work we elucidate the interplay between the electronic properties and the structure of the lead-iodide perovskite network. We model the perovskite unit cell by four ideal corner sharing octahedra, which can rigidly rotate within the constraints of an orthorhombic unit cell. We show that the structure can be uniquely identified two geometrical parameters, the apical and equatorial Pb-I-Pb bond angles. Furthermore we show a strong correlation between the calculated band gap and these angles. In addition, we show that the magnitude of the Pb-I-Pb angles can be directly controlled by changing the size of the cation component at the center of the cuboctahedral cavity.
Based on this finding we propose 17 lead-iodide perovskite absorbers containing different cations at the center of the lead-iodide network,
14 of which have not been reported thus far. The band gaps for these potential novel absorbers are theoretically tunable over 1 eV. Moreover, the band gap trends exhibited are consistent accross different computational approaches (density functional theory and GW - with and without spin-orbit coupling) [1,2] Experiments motivated by this study not only confirmed our predicted trends, but also lead to the synthesis of the novel mixed Rb{1-x}Cs{x}PbI3 perovskite.
[1] Filip, M. R., Eperon, G., Snaith, H. J. & Giustino, F., http://arxiv.org/abs/1409.6478 (2014)
[2] Filip, M. R. & Giustino, F., http://arxiv.org/abs/1410.2029 (2014)
This work was supported by the European Research Council (EU FP7 / ERC grant no. 239578), the UK Engineering and Physical Sciences Research Council (Grant NO. EP/J009857/1) and the Leverhulme Trust (Grant RL-2012-001). G.E.E. is supported by the UK EPSRC
and Oxford Photovoltaics Ltd. through a Nanotechnology KTN CASE award. Calculations were performed at the Oxford Supercomputing Centre and the Oxford Materials Modelling Laboratory.

Recently, solution-processible organic-inorganic metal halide perovskites have come to the fore as a result of their high power-conversion efficiencies (PCE) in photovoltaics, exceeding 17%. To attain reproducibility in the performance one of the critical factors is the processing conditions of the perovskite film, which directly influences the photo-physical properties and hence the device performance. Here we study the effect of annealing parameters on the change in crystal structure of the perovskite films and correlate these changes with its photo-physical properties. We find that the crystal formation is kinetically driven by the annealing atmosphere, time and temperature. Annealing in air produces a more defined tetragonal symmetry as compared to nitrogen. In addition, annealing the perovskite films at 100 °C for more than an hour in nitrogen results in deterioration of crystallinity, unlike those annealed in air, which retain their crystallinity even after 2.5 hours of annealing. Lower photoluminescence quantum efficiency (PLQE) and shorter photoluminescence (PL) lifetimes are observed for nitrogen annealed perovskite films as compared to the air-annealed counterparts. This indicates a critical impact of the atmosphere upon crystallisation and material quality.

Recently, organometal halide perovskite materials were identified as promising light harvesters to achieve rapidly boosted performance, providing great potential for developing low-cost next-generation photovoltaic devices. The highly crystalline perovskite is required either to absorb most of the sunlight or deliver efficient charge transport pathways for photogenerated carriers. Here, we use a sequential deposition technique for prepared perovskite crystals under various conversion ratios to demonstrate the mechanisms of an extended three-dimensional network of corner-sharing [PbI₆]^4- octahedral and then filled the methylammonium (MA) to 12-fold iodide coordinated interstitial sites among the octahedral by X-ray diffraction (XRD) spectrum and X-ray photoelectron spectroscopy (XPS), respectively, during crystal growth. Furthermore, the vertical distributions of electronic structure and chemical composition have important implication for analyzed depth profile of the perovskite structures using the Pb 4f core level XPS spectra measurement. These results indicate that through clearly realized material engineering, and the most significant differences in efficiency are attributed to whether enhances transformation of perovskite by the orderly built the inorganic frameworks and completely inserted the organic molecules.
Keywords: Perovskite, Photovoltaics, Depth profile, X-ray photoelectron spectroscopy and Crystal growth

Perovskite-sensitized solar cells have been attracted attentions as a potential alternative to the silicon solar cells due to their low cost and promising photovoltaic efficiency. Many study is focusing on fabricating a planar device architecture because of their simple structure and flexible application, where the perovksite layer is sandwiched between the hole conductor and electron conductor. Perovskite layer made by conventional spin-coating method tends to form pin-holes and uncovered area, which will cause shunting to the device. In this work, a microreactor-assisted continuous flow method is employed to synthesize and fabricate the perovskite solar cells in order to improve the perovskite layer quality and achieve a relatively high efficiency.

Recently, CH₃NH₃PbI₃ perovskite has been proved to be a superb light absorber together with balanced charge transporter. It was recently shown that the grain size of perovskite played important role in determining photovoltaic performance[ref]. In this study, we report a study on the crystal growth mechnaism of perovskite in a two-step deposition procedure. A theoretical model is developed by considering the variation of Gibbs free energy of the chemical reaction at a two-step method and the probability of nucleation and growth. The change in crystal size with the CH₃NH₃I concentration fits well with the developed theorectical equation lnY = B/(lnX - A)² + C (Y: grain size, X: CH₃NH₃I concentraion, A, B, and C: constants at same condition except for concentration). Our model suggests that the temperature of the reaction could also play an important role in controlling the perovskite crystal size. This theoretical model could be utilized to find the optimum conditions of perovskite photovoltaics.
Ref: J. Im, I. Jang, N. Pellet, M. Gratzel, and N. Park, “Growth of CH3NH3PbI3 cuboids with controlled size for high-efficiency perovskite solar cells,”Nat. Nanotechnol. (published online 2014).

Perovskite solar cells (PeSCs) have attracted great attention as next power sources because the methylammonium lead halide perovskite materials (e.g., CH₃NH₃PbI₃), as a light absorber, possess decent optoelectronic properties such as direct bandgap, excellent charge transport, high absorption coefficient, and defect tolerability. Although PeSCs with high power conversion efficiency (PCE) up to 19 % were recently reported, the understanding of mechanism for formation of perovskite crystalline structure associated with device performance variations has seldom been reported. To solve these low reproducibility issues, we introduced only small amount of N-cyclohexyl-2-pyrrolidone (CHP) solvent as an additive, which has similar amide functional groups with main solvent N,N-dimethylformamide (DMF). In doing so, homogeneous and uniform perovskite thin film morphology with small device-to-device PCE variations (0.14 %) were successfully obtained by such a facile method. However, the obvious mechanism and origin of these decent film morphologies were not clearly understood yet. Therefore, we implemented investigations to figure out the relationship between the film morphology and characteristics of additives such as vapor pressure, boiling point, functional group, and miscibility. Finally, we will also discuss the perovskite crystalline behavior contingent upon some selected solvents in precursor solution such as the N-methyl-2-pyrrolidone, N-octyl-2-pyrrolidone, Benzyl benzoate, Benzyl ether, etc.

During the past years, the swift surge of perovskite solar cells has been witnessed. Power conversion efficiency (PCE) has been rapidly boosted, with the understanding of film formation and crystal growth of perovskite materials. In this work, we report the tuning of PbI₂ film morphology through the control of solvent evaporation, leading to the mesoporous PbI₂ structure under optimized conditions. Perovskite film was then prepared based on the mesoporous PbI₂ film via sequential deposition process^[1]. Results revealed that the particle sizes and porosity of the mesoporous PbI₂ played critical roles for the growth of CH₃NH₃PbI₃ crystal, providing an efficient approach to optimize the film morphology. Based on the optimized morphology of perovskite film, the perovskite solar cell with conventional planar structure^[2] could achieve a PCE of 15.6% with a J_sc of 22.1 mAmiddot;cm^-2, V_oc of 1.01 V and fill factor of 0.70. For the inverted planar heterojunction structure (ITO/PEDOT:PSS/CH₃NH₃PbI₃/PC₆₀BM/Al^[3,4]), a PCE of 10.9% was obtained under standard AM 1.5 1 Sun condition. The J_sc, V_oc, and fill factor (FF) values were 14.5 mAmiddot;cm^-2, 0.96 V, and 0.78. Hysterisis effect was also compared in the different device architecture.
Reference:
[1] J. Burschka; N. Pellet; S. J. Moon; R. Humphry-Baker; P. Gao; M. K. Nazeeruddin; M. Gratzel; Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 2013, 499, 316-319.
[2] D. Y. Liu; T. L. Kelly; Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques. Nat. Photonics 2014, 8, 133-138.
[3] Chien-Hung Chiang; Zong-Liang Tseng; Chun-Guey Wu; Planar heterojunction perovskite/PC₇₁BM solar cells with enhanced open-circuit voltage via (2/1)-step spin-coating process. J. Mater. Chem. A,2014, 2, 15897-15903.
[4] Zhengguo Xiao; Cheng Bi; Yuchuan Shao; Qingfeng Dong; Qi Wang; Yongbo Yuan; Chenggong Wang; Yongli Gao; Jinsong Huang; Efficient, high yield perovskite photovoltaic devices grown by interdiffusion of solution-processed precursor stacking layers. Energy Environ. Sci., 2014, 7, 2619-2623.

Organometal halide perovskite solar cells have recently emerged as promising cost-effective and high-efficiency nanostructured solar cells. Unfortunately, perovskite solar cells have a problem with the reduction of power conversion efficiencies (PCE) due to degradation in the air. It is discussed that the degradation of perovskite solar cell is caused by moisture and oxygen intercalations into perovskite layer¹⁾. To avoid this problem of degradation in air, we introduced the system without air exposure from fabrication to measurement by connecting with globe box and evaporation chamber. In this work, we try to detect the influence of air exposure among fabrication steps.
Device structure of our perovskite solar cells is ITO / compact-TiOx / perovskite (CH₃NH₃PbI₃) / spiro-OMeTAD (HTL) / Au electrode. The CH₃NH₃PbI₃ layer was deposited by sequential vacuum deposition of PbI₂ and CH₃NH₃I (MAI) layers, not co-evaporation method. To remove MAI remains on CH₃NH₃PbI₃ surface, the film deposited by sequential vacuum deposition of PbI₂ and MAI were rinsed with 2-propanol solvent by dipping for 10 seconds. Then, HTL and Au electrode were deposited in glove box and evaporation chamber without air expose, respectively. Measurements of solar cell parameters were done in encapsulated container filled in nitrogen gas. To clarify the influence of air exposure on perovskite solar cells, we exposed deposited films in the air for 30 minutes among various fabrication steps. Three perovskite solar cells and reference without exposure were fabricated. We exposed the PbI₂/MAI film deposited by sequential vacuum deposition, the rinsed film by 2-propanol solvent, and the deposited film after HTL layer spin-cast among fabrication process for 30 minutes in the air. The PCE of reference without air exposure shows 4.1%. The PCEs of perovskite solar cells exposed after 2-propanol rinse and after HTL spin cast are 3.6% and 2.5%, respectively. The PCE of perovskite solar cell exposed after PbI₂/MAI sequential vacuum deposition is dramatically decreased to 0.45%, which is 90% reduction compared with reference without exposure. These results indicated whatever air exposure among fabrication process reduces the solar cell properties. Especially, perovskite solar cell exposed after PbI₂/MAI sequential vacuum deposition is damaged deeply. This result suggests that the MAI remains on perovskite surface are trigger of degradation.
[1]Severin N. Habisreutinger, Tomas Leijtens, Giles E. Eperon, Samuel D. Stranks, Robin J. Nicholas and Henry J. Snaith, Nano Lett., 2014, 14 (10), pp 5561-5568

In this study, we report the utilization of solution-processed high electrical conductive [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) combined with solution-processed TiO₂ as the electron transport layer (ETL) to overcome extremely low electrical conductivity of solution-processed TiO₂ ETL in planar heterojunction (PHJ) perovskite hybrid solar cells (pero-HSCs). Due to the much more preferable electron extraction and transportation of PC₆₁BM at the cathode side, tremendously boosted short-circuit current density (J_SC), fill factor (FF) with enhanced power conversion efficiency (PCE) are observed. In order to further address the wettability issues of perovskite materials on the top of PC₆₁BM, water soluble fullerene derivative is applied to modulate the surface of PC₆₁BM. Consequently, further advanced FF with slightly enlarged J_SC and open-circuit voltage (V_OC) are observed. The resulted PCE is comparable with the meso-superstructured solar cells in which high PCEs can be produced. Our studies certainly provide a simple approach to boost the efficiency of PHJ pero-HSCs.

We demonstrate that charge carrier diffusion lengths of two classes of perovskites, CH3NH3PbI3-shy;xClx and CH3NH3PbI3, are both highly sensitive to film processing conditions and optimal processing procedures are critical to preserving the long carrier diffusion lengths of the perovskite films. This understanding, together with the improved cathode interface using bilayer-shy;#8208;#8209;structured electron transporting interlayers of [6,6]-shy;phenyl-shy;C61-shy;butyric acid methyl ester (PCBM)/ZnO lead to the successful fabrication of highly efficient, stable and reproducible planar heterojunction CH3NH3PbI3-shy;#8208;#8209;xClx solar cells with impressive power-shy;conversion efficiencies (PCEs) up to 15.9%. A 1-shy;square-shy;centimeter device yielding a PCE of 12.3% has been realized, demonstrating that this simple planar structure is promising for large-shy;area devices.

Methyl ammonium lead iodine (CH₃NH₃)PbI₃ crystalline control is critical for developing high efficient solar cells by enhancing charge carrier collection and light harvesting. In a mesostructured perovskite solar cell, the TiO₂ porous layer facilitates the electron transport and more importantly act as the scaffold to promote (CH₃NH₃)PbI₃ crystallization in a solution process. Therefore, studying the effect of this layer on perovskite crystallization and charge transport and optimizing its structure is useful for further improvement of the perovskite solar cell performance. Here we report the study of the effects of blending TiO₂ nanorods into the commonly used nanoparticles in terms of morphology, crystallization, charge transport, light harvesting, and device characteristics. High quality perovskite film was deposited on top of mesoporous structure by using one-step solution processing. By adjusting the nanorod-to-nanoparticle ratio, the power conversion efficiency was improved by about 25% in comparison to the nanoparticle-based devices. The underlying charge transport and recombination properties were investigated by intensity-modulated photocurrent spectroscopy (IMPS) and electrochemical impedance spectroscopy (EIS). A favorable charge transport/recombination property was observed in the TiO₂ nanorods/nanoparticle blending structure. The impact of the adding nanorods on the light absorption of perovskite solar cells will also be discussed. Our findings contribute to the rational design of mesoporous electron transport layer for high-performance perovskite solar cells.

Perovskite hybrid solar cells (pero-HSCs) have attracted significant attention in the past four years due to their promising photovoltaic efficiencies. However, the correlations between device performance of pero-HSCs and the film morphology and film thickness of CH₃NH₃PbI_3-xCl_x perovskite layers are rarely addressed. In this study, we systematically study the film morphology and film thickness of CH₃NH₃PbI_3-xCl_x layers on device performance of planner heterojunction (PHJ) pero-HSCs. It was found that PHJ pero-HSCs performance is significantly dependent on the film morphology and film thickness of CH₃NH₃PbI_3-xCl_x layers. As the thickness of CH₃NH₃PbI_3-xCl_x layer is gradually increased, the device performance of PHJ pero-HSCs is increased and then decreased. A short-circuit current of 19.79 mA/cm², an open-circuit voltage of 0.95, a fill factor of 63.20% and an corresponding efficiency of 11.88% were observed from the pero-HSCs fabricated by solution-processed CH₃NH₃PbI_3-xCl_x film with a thickness of 575 nm. The correlations between PHJ pero-HSCs performance and the film morphology and film thickness of CH₃NH₃PbI_3-xCl_x layers were systematically investigated through optical properties, x-ray diffraction patterns, atomic force microscopy and scanning electron microscopy images of CH₃NH₃PbI_3-xCl_x layers. Our studies provide a promising pathway for fabricating high performance PHJ pero-HSCs.

Organometal halide based perovskite solar cells are rapidly developing with current devices reaching ~ 19% efficiency.¹ Due to perovskite&’s low temperature processing, and low cost of materials it is a promising material for low cost solar cells. In this work we introduce a new method of perovskite synthesis by hybrid chemical vapor deposition (HCVD), and demonstrate efficiencies as high as 11.8%.² These cells were found to retain almost the same efficiency after approximately 1100 h storage in dry N₂ gas. This method is particularly attractive because of its ability to scale up to industrial levels and the ability to precisely control gas flow rate, temperature, and pressure with high reproducibility. To achieve easier fabrication and better reproducibility we used a two-step method of perovskite synthesis where a layer of metal halide (e.g. PbCl₂, PbI₂) is deposited, followed by the formation of perovskite by exposing the pre-deposited metal halide film to the flow of ammonium halide vapor (e.g. methyl ammonium iodide, formamidinium bromide). In this method, a solid powder of ammonium halide is converted to gas phase in a dedicated zone of the tube furnace and then deposited onto the substrates pre-deposited with metal halide downstream in another zone of the tube furnace. Because of the use of solid precursors this method can be viewed as a form of hybrid chemical vapor deposition (HCVD).
1. H. Zhou, Q. Chen, G. Li, S. Luo, T.-b. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu and Y. Yang, "Interface engineering of highly efficient perovskite solar cells". Science345, 542-546 (2014).
2. Matthew R. Leyden, Luis K. Ono, Sonia R. Raga, Yuichi Kato, Shenghao Wang and Yabing Qi *, "High performance perovskite solar cells by hybrid chemical vapor deposition". J. Mater. Chem. A (2014). doi:10.1039/C4TA04385E

Perovskite solar cells that utilize organometal halides as light absorber have been fabricated with either “mesoporous” or “planar” device structure. The “planar” structure clearly has the advantage in terms of low temperature and simple fabrication over the “mesoporous” structure, although high power conversion efficiencies have been achieved in both structures. A distinctive difference of the two device structure is the interfaces between organometal halide and titanium oxide, with planar and macro scale interfaces in “planar” device, and round and namometer scale interfaces in “mesoporous” devices. In order to clarify whether the device structure makes any difference in device performance, we studied perovskite solar cells with three different interfaces. And we found that organometal halide/titanium oxide interfaces have significant effects on film morphology, crystal structure, optical properties, device stability and power conversion efficiency. Especially, time-resolved confocal microscopy study shows that the film morphologies induced by organometal halide/titanium oxide interfaces may influence the crystal structure and energy band gap of Methylammonium Iodide.

we developed a new spiro type 4,4',4'',4'''-(2H,2'H,4H,4'H-3,3'-spiro-bi[thieno[3,4-b][1,4]dioxepine]-6,6',8,8'-tetrayl)tetrakis(N,N-bis(4-methoxyphenyl)aniline) (Spiro-OMeTABPDOT, PST1) as efficient hole transporting material for perovskite solar cells (PSCs) that works efficiently even without cobalt dopant. The compound is obtained by employing facile synthetic routes. A preliminary X-ray diffraction study of PST1 revealed a unique quasi-spiro molecular configuration and found multiple CH/π and π-π intermolecular short contacts. For the first time, the crystal structure of 2,2&’,7,7&’-tetrakis(N,N&’-di-p-methoxyphenylamine)-9,9&’-spirobifluorene (Spiro-OMeTAD) is parallel studied and compared with that of PST1. The device based on doped PST1 exhibited PCE of 13.44% and a comparable 12.74% PCE was achieved with its undoped form, which paves the way for developing new low cost hole transporting materials as promising alternative to the widely employed Spiro-OMeTAD.

Perovskite solar cell is the most promising emerging thin film solar cell technology. It has recently hit a photo conversion efficiency of 19% after being invented and developed only few years ago. The efficiency is expected to further increase within few years. On the other hand, spray coating is a viable technique for the fabrication of large area solar cells with an affordable cost and in a vacuum-free process. This makes the spray-on perovskite solar cells a very attractive candidate for large scale electric energy generation. In this work, we have fine-tuned and optimized processing parameters of spray coating technique to make it suitable for manufacturing high efficiency solution-processed perovskite solar cells. All solution-processed layers of Glass-FTO-coated/TiO₂/Al₂O₃/Methylammonium lead halide perovskite/HTM/Au solar cell are fabricated by an automated ultrasonic spray coating machine, where FTO stands for fluorine-doped tin oxide and HTM stands for hole-transforming-material, which may be a polymer. The solution-processed layers include a dense TiO₂ layer, a supporting Al₂O₃ scaffold layer, the perovskite-based active layer, and an HTM. The Au electrode is deposited by thermal evaporation. Currently, we are conducting experiments, improving the device stability and life time and performing characterization of solar cell films as well as the complete device. Optimized spraying strategies, such as the optimum number of spraying passes for the best performance already tested on organic solar cells are being applied to perovskite solar cells. Results on topography, roughness, and morphology of spray-on solar cell layers, as well the photo-current conversion efficiency of prepared solar cell devices will be reported and discussed.

Perovskite solar cells have been under active research cell because of high efficiency and low cost materials and process. Perovskite solar cells are composed of mesoporous-TiO₂ nanopariticles(mp-TiO₂ NPs), perovksite (CH₃NH₃PbI₃) light absorber and hole transport materials. Full surface coverage of perovskite layer is of great importance to achieve a high efficiency. So far, two-step method, evaporation, and solvent engineering method have been exploited to improve surface coverage of peorvskite [1,2,3].
In the present study, we fabricated fully covered perovskite layer which was also completely filled into mp-TiO₂ NPs. This layer was formed by penetration of dense PbO layer into mp-TiO₂ NPs using sol-gel method. After substitution reaction in I₂ atmosphere, the dense PbO was transformed into dense PbI₂, consequently forming dense perovskite layer. This process will be potentially used to realize conformal perovskite layer coating, hence improving photovoltaic properties of perovskite solar cell.
[1] Julian Burschka, Norman Pellet, Soo-Jin Moon, Robin Humphry-Baker, Peng Gao, Mohammad K. Nazeeruddin& Michael Gratzel, Nature, 499, 316-319 (2013)
[2] Qi Chen, Huanping Zhou, Ziruo Hong, Song Luo, Hsin-Seng Duan, Hsin-Hua Wang, Yongsheng Liu, Gang Li & Yang Yang, Journal of the American Chemical Society, 136, 622-625 (2014)
[3] Nam Joong Jeon, Jun Hong Noh, Young Chan Kim, Woon Seok Yang, Seungchan Ryu & Sang Il Seok, Nature materials, 13, 897-903 (2014)

Organo-lead tri-halide perovskite-based solar cells have the potential for widespread usage due to low manufacturing cost and high efficiency. Within five years, efficiencies of ~19% have been achieved. Understanding how the thin film morphology affects the performance will enable further rapid improvements in the performance of these solar cells.
In this work, we develop a morphology aware computational framework to model how the morphology affects device physics. We utilize an excitonic drift-diffusion model to represent the device physics of perovskite solar cells. We include all relevant phenomena, namely 1) electrostatic potential distribution with the presence of charge carriers flow; 2) exciton generation, dissociation, recombination, decay and diffusion; 3) free charge carrier generation, recombination and drift/diffusion; and 4) charge carrier extraction. We explore how morphology affects the device characteristics using cross-sectional data of TiO₂|CH₃NH₃PbI₃|Spiro-OMeTAD BHJ.

Three-dimensional (3D) nanowire (NW) architectures are considered as superior electrode design for photovoltaic devices compared to NWs or nanoparticle systems in terms of improved large surface area and charge transport properties. In this paper, we report a development of lead iodide perovskite solar cells based on a novel 3D TiO₂ NW architectures. The 3D TiO₂ nanostructure was synthesized via surface-reaction-limited pulsed chemical vapor deposition (SPCVD) technique that also implemented the Kirkendall effect for complete ZnO NW template conversion. It was found that the film thickness of 3D TiO₂ can significantly influence the photovoltaic performance. Short-circuit current increased with the TiO₂ length; while open-circuit voltage and fill factor decreased with the length. Highest power conversion efficiency (PCE) of 9.0% was achieved with ~600 nm-long 3D TiO₂ NW structure. Compared to other 1D nanostructure arrays (TiO₂ nanotubes and ZnO NWs), 3D TiO₂ NW architecture was able to achieve larger amount of perovskite loading, enhanced light harvesting efficiency and increased electron transport property. Therefore, its PCE is 1.5 and 2.8 times higher than those of TiO₂ nanotubes and ZnO NWs, respectively. The unique morphological advantages, together with the largely suppressed hysteresis effect, make 3D hierarchical TiO₂ a promise electrode selection in designing high-performance perovskite solar cells.

Recently, organic-inorganic hybrid perovskites, CH₃NH₃PbX₃ (X is iodine or a mixture of iodine and chlorine), have been intensively pursued as solid-state sensitizers. In a typical perovskite solar cell, several hundred nanometer thick absorber layers is sandwiched between mesoporous or planner structure of TiO₂ electron transporting layer (ETL) and the hole transport layers. Only ZnO has been investigated as an alternative electron transporting layer so far, and there is an urgent need to explore wide-band-gap oxides with suitable band alignments.
In this work, we explored perovskite oxide SrTiO₃ (STO) for the first time as the electron-transporting layer in organolead perovskite solar cells. A suitable band alignment with very low band conduction band offset of the perovskite on the mesoporous STO was observed by the photo-electron emission spectroscopy. The transient absorption experiments and steady-state photoluminescence (PL) quenching revealed efficient photoelectron transfer from CH₃NH₃PbI_3-xCl_x to STO. Perovskite solar cells with meso-STO electron transporting layers exhibit an average open circuit voltage (V_OC) of 0.93 V which higher than the value of 0.81 V achieved in the control device with the conventional meso-TiO₂. With increasing the meso-STO thickness the average V_OC can reach up to 1.01 V and with 17% increase in the fill factor but the short circuit current density decreases. We found that the application of STO leads to uniform perovskite layers with large grains and complete surface coverage, leading to a high shunt resistance and improved performance. These findings suggest STO as a competitive candidate as electron transport material in organometal perovskite solar cells.

Perovskite based solar cells has recently shown high power conversion efficiency >15%. Many of these cells utilize air-processed spiro-OMeTAD as hole transport layer, thus degrading perovskite layer by exposing it in in atmospheric moisture. We report a low band gap poly(diketopyrrolopyrrole-terthiophene) PDPP3T polymer, to act as efficient hole transport material in organic-inorganic perovskite based solar cells, which can be processed in inert atmosphere, thus preventing the degradation of perovskite layer. Perovskite (CH₃NH₃PbI₃) was deposited on mesoporous titanium dioxide layer using two-step sequential dip coating technique. SEM and XRD was performed to study the morphology and structure of CH₃NH₃PbI₃ film. UV-vis absorption spectrum showed broad range absorption of CH₃NH₃PbI₃ film, with optical band gap of 1.4 eV. Device structure consisted of FTO/TiO₂/ CH₃NH₃PbI₃/PDPP3T/Ag. We showed that PDPP3T acts as efficient hole transport material in conjunction with perovskite layer giving initial device performance with Jsc of 17.53 mA/cm², Voc of 0.97 V and a fill factor of 49.8%, resulting in an overall efficiency of 8.46%. Further optimization of the device in terms of TiO₂ and polymer layer thickness will lead to efficient device performance.

Organometallic hybrid perovskite solar cells have received a great deal of attention for next generation renewable energy sources because of their high efficiencies and low material costs compared to other solar cells. However, cost-effectiveness is still necessary to facilitate commercialization of perovskite solar cells. In terms of material cost, the light absorber layer composed of organic-inorganic perovskite material is not expensive. The n-type substrate which contains relatively expensive TiO₂ thin film and transparent conductive oxide (TCO) coated glass is one of bottle neck for commercial viability. Herein, we present a simple and effective method to reuse n-type substrate. We found that the perovskite light absorber was perfectly soluble in some solvents (Acetone and Dimethylformamide, etc.) with proper polarity, consequently yielding fairly clean n-type substrate without any residual perovskite/Spiro-MeOTAD/Ag components which was evidenced by X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS) analysis. The recycled perovskite solar cell exhibited photo-conversion efficiency (PCE) of 11.52 % which is identical to the PCE of initial perovskite solar cell. We strongly believe that this technique will significantly impact on paradigm change of photovoltaic applications.

Hybrid organic/inorganic perovskite solar cells are among the most competitive emerging photovoltaic technologies. Here, we report NiO-based inverted structure perovskite solar cells with a high power conversion efficiency of 10.68%, which is achieved by adding a small percentage (1.5 wt%) of high molecular weight polystyrene (PS) into the PCBM electron transport layer (ETL). The addition of PS facilitates the formation of a highly smooth and uniform PCBM ETL that is more effective in preventing undesirable electron-hole recombination between the perovskite layer and the top electrode. As a result, the V_OC of the PCBM:PS-based cells is increased from 0.97 V to 1.07 V, which leads to significantly enhanced power conversion efficiencies of the solar cells. Our study provides a simple and low-cost approach to improving the ETL film quality and the performance of inverted perovskite solar cells.

Organolead trihalide perovskites (OTPs) are emerging as a new generation of solution processable photovoltaic materials which are low-cost and nature-abundant. It was later found that the diffusion length in mixed trihalide perovskite films in the order of 1 micrometer is ten times longer than that in triiodide perovskite films (~100 nm), which is a significant discovery because a longer carrier diffusion length enables the application of thicker mixed trihalide perovskite films for stronger absorption. However, there is barely any study on the origin for the much longer diffusion length in mixed halide OTPs from the material microscopic structures. It was recently reported by many groups that Cl could not be traced in final OTP films formed from the coating of mixed halide precursor solution, accompanied by the absence of lattice constant change in mixed halide OTPs with respect to MAPbI3.
In this work, we report effective incorporation of chlorine into the mixed halide OTPs for the formation of MAPbI_3-xCl_x thin films using a low temperature solution process. A multiple-circle coating method was used to incorporate Cl which yielded a large tunable composition range in the formed MAPbI_3-xCl_x films. A two-step process for the formation of MAPbI_3-xCl_x was revealed. We studied the growth mode of grains with and grain orientation in the mixed halide perovskite films. A correlation between the film microstructure and electronic property and device performance was identified. An excellent yield of high efficiency MAPbI_3-xCl_x devices was achieved with an average fill factor of 79% and the best power conversion efficiency of 16.01% without photocurrent hysteresis. This method can be applied to other types of perovskite materials for incorporating Br or other ions and the low temperature process was compatible with plastic flexible substrates.

The effect of hole transport layer (HTL) conductivity on the performance of perovskite solar cells was systematically investigated. We fabricated methylammonium lead iodide (CH₃NH₃PbI₃) based planar structure perovskite solar cells using conductivity-controlled poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as the HTL. The conductivity was systematically modulated by 7 orders of magnitude by doping and composition variation. The resultant photovoltaic devices showed significant increment in photocurrent generation with increasing PEDOT:PSS conductivity, and consequently the 30% increased power conversion efficiency. These enhancement were attributed to the elongated charge carrier lifetime by facilitated hole extraction, which was confirmed by transient photovoltage and photocurrent analysis. Due to its simplicity and excellent characteristics, our approach of using conductivity-controlled HTL would be suitable for high performance printed pervskite solar cells.

Flexibility, compliance and weight will turn out to be key metrics for future electronic appliances and power supplies. Imperceptible plastic electronic wraps integrate nanometer thin film active components on sub-2-mu;m polymer foils and create devices unmatched in mechanical flexibility, stretchability and weight.
Organometallic halide perovskites are capable of delivering very high power per weight when fabricated on ultrathin substrates, an important metric for wearable and ultraportable electronics, for remote sensing or for space applications.
Here we demonstrate methods to fabricate perovskite solar cells on 1.4mu;m thick PET substrates with 12% power conversion efficiency and a record high solar cell specific weight of 30W/g. The solar cells are less than 2mu;m in total thickness and can be bent into radii smaller than 50mu;m. Our devices are fabricated from solution in ambient air at temperatures below 120°C to ensure process compatibility with ultrathin polymer foil substrates. Their unique mechanical properties are achieved with an all ITO/FTO free device architecture that does not require titanium oxide interlayers and avoids high sintering temperatures typically employed for rigid devices on glass substrates. These potentially low cost power sources conform to arbitrary shapes and are expected to provide electrical energy wherever high specific power is critical, as in next generation ultra light portables, wearables, small-scale autonomous robots or space technologies.
The authors acknowledge funding from the Wittgenstein award and the ERC advanced investigators grant “Soft Map”.

Hybrid organometal trihalide perovskite (CH₃NH₃PbX₃, X = I, Br, or Cl) solar cells have garnered significant attention in the last five years, with unprecedented efficiency improvements that have rapidly escalated from about 4% to over 19%. Two different device architectures, one based on mesoporous and the other on planar heterojunctions with TiO₂, have been used to fabricate perovskite solar cells. In both cases, a uniform and pinhole free perovskite layer is needed to achieve high efficiency. Here, we present a two-step synthetic methodology for the fabrication of high performance, uniform halide perovskite films with tunable composition and without the need for sophisticated deposition systems. Planar heterojunction solar cell consisting of a dense, e-beam deposited TiO₂ electron transport layer, a perovskite active layer, a spiro-OMeTAD hole transport layer, and an Au top contact exhibits a power conversion efficiency of 16.8%, which is the highest reported efficiency for planar heterojunction solar cells. This versatile synthetic approach provides high degree of control over morphology and composition of the perovskite layer through a solution assisted low-pressure deposition. In addition, this low pressure process allows for reduced processing temperatures compared to other two-step synthesis methods, which is critical for achieving significantly improved efficiency. The influence of the composition of the perovskite films on the efficiency and hysteresis of the devices are also investigated.

Organolead mixed halide perovskites have been extensively investigated in the last few years. As a result, power conversion efficiency (PCE) of perovskite solar cells has risen steeply from 3 % in 2009 to 19.3 % confirmed efficiency in August 2014. Some of the reported growth techniques include solvothermal processing of CH₃NH₃I and PbCl₂, two-step sequential deposition, dual-source vapor deposition, and vapor assisted solution process. Reacting excess CH₃NH₃I with PbCl₂ is the conventional method of fabricating mixed halide perovskite CH₃NH₃PbI_3-xCl_x, but the synthesis of CH₃NH₃I is quite expensive and laborious. In this work we present a single-step solution approach to prepare perovskite CH₃NH₃PbI_3-xCl_x #64257;lms by the direct reaction of the commercially available CH₃NH₃Cl (or MACl) and PbI₂. The growth process includes the nebulization of a stoichiometric mixture of MACl and PbI₂ dissolved in DMF, and injection of the aerosol into a low-pressure chamber and deposition on to a substrate. Our results show that the film crystallinity and the formation of multiple phases depend strongly on the precursor concentration and the deposition temperature. I-V characteristics show orders of magnitude increase in charge transport properties in nebulizer assisted vacuum spray deposited perovskite films in comparison to solution casted films. Structural, optical and electrical properties of films fabricated under various growth conditions will be presented.

Perovskite based absorbers in solid state photovoltaics have emerged as a promising class of materials for high efficiency solar cells. An appropriate electronic band alignment between electron transport layer(ETL) and perovskite absorber layer, and hole transport layer(HTL) is required to improve the device performance. Kelvin probe force microscopy (KPFM) measurement shows that charge transport in perovskite solar cell increases with increasing annealing temperature. The electron barrier increases to 0.378 eV from 0.09 eV between TiO₂ and perovskite upon annealing suppressing the back recombination of electrons from TiO₂ and holes from perovskite. XRD spectra shows formation of PbI₂ phase upon annealing which reduces the recombination. Device with 9% efficiency have been achieved with noble highly stable polymers upon annealing. Spatial maps of surface potential of perovskite film shows higher positive potential (130 meV)at grain boundary compared to the surface of the grains which decreases (110 meV) upon illumination. Transient analysis shows that charge carrier transport time is faster than the carrier recombination time for all the devices.

Perovskite based absorbers in solid state photovoltaics have emerged as a promising class of materials for high efficiency solar cells. An appropriate electronic band alignment between electron transport layer(ETL) and perovskite absorber layer, and hole transport layer(HTL) is required to improve the device performance. Kelvin probe force microscopy (KPFM) measurement shows that charge transport in perovskite solar cell increases with increasing annealing temperature. The electron barrier increases to 0.378 eV from 0.09 eV between TiO₂ and perovskite upon annealing suppressing the back recombination of electrons from TiO₂ and holes from perovskite. XRD spectra shows formation of PbI₂ phase upon annealing which reduces the recombination. Device with 9% efficiency have been achieved with noble highly stable polymers upon annealing. Spatial maps of surface potential of perovskite film shows higher positive potential (130 meV)at grain boundary compared to the surface of the grains which decreases (110 meV) upon illumination. Transient analysis shows that charge carrier transport time is faster than the carrier recombination time for all the devices.

We here address the upper limit to open circuit voltage for the novel type of photovoltaic cells based on lead iodide perovskites. By accurate determination of the solar cells radiative efficiency, that is their ability to emit light, we conclude how far these new type of solar cells are from their own thermodynamic limit.[1] Perovskite PVs have gained significant power conversion efficiency in a very short time period and apart from a very high current, they also retain a comparatively high open circuit voltage. We herein explain the reason for the high voltage and also put it in relation to those of earlier generation photovoltaic technologies.
We also highlight that, as the perovskite steady state photoluminescence is rather strong at open circuit conditions, and substantially quenched only at short circuit, it performs just as a good solar cell should do, and also very different from most OPVs or DSSC cells studied so far.
[1] Scientific Reports 4 6071 (2014) doi:10.1038/srep06071

Photovoltaics based on organic-inorganic perovskites offer new promise to address the contemporary energy and environmental issues. These solar cells have so far largely relied on small-molecule hole transport materials (HTMs) such as spiro-OMeTAD, which commonly suffer from high cost and low mobility. In principle, polyfluorene (PF) copolymers can be an ideal alternative to spiro-OMeTAD, given their low price, high hole mobility and good processability, but this potential has not been explored. Herein, we demonstrate polyfluorene derived polymers-TFB and PFB, which contains fluorine and arylamine groups, can indeed rival or even outperform spiro-OMeTAD as efficient hole-conducting materials for perovskite solar cells. In particular, under the one-step perovskite deposition condition, TFB has achieved a 10.92 % power conversion efficiency that is considerably higher than that with spiro-OMeTAD (9.78%), while with the two-step perovskite deposition method, we were able to achieve about 13% efficient solar cells for TFB (12.80%) and spiro-OMeTAD (13.58%). Photoluminescence (PL) revealed the efficient hole extraction and diffusion at the interface between CH₃NH₃PbI₃ and the hole conducting polymer (HCP). By using the impedance spectroscopy technique, we have uncovered the higher electrical conductivity and lower series resistance than spiro-OMeTAD, which naturally explain the significantly higher fill factor, photocurrent and open-circuit voltage of the TFB-derived cells than with spiro-MeOTAD.

Performance of a perovskite based solar cell is highly determined by the crystalline qualities of the perovskite thin film sandwiched between an electron and a hole transfer layer in a typical perovskite solar cell structure, such as the size of perovskite grains and the uniformity of thin films, Here, we demonstrated a novel hybrid technique to grow high quality perovskite films combining evaporation and chemical vapor deposition (CVD) methods. First a PbI₂ film was spun coated on a mesoporous TiO₂ (m-TiO₂)/compact TiO₂ (c-TiO₂)/FTO substrate in ambient environment. Then, purified CH₃NH₃I crystal material was evaporated and the vapor reacted with the PbI₂ film in a vacuum pressure/temperature accurately controlled quartz tube furnace. In this technique, high vacuum (1x10^-2 mTorr) and low temperature (100#8451;) were applied to decrease perovskite film growth rate and reduce perovskite film defects. After vapor reaction, the perovskite film was annealed at 100#8451; for 10min in 1x10^-2 mTorr vacuum to recrystallize and remove CH₃NH₃I residue in order to further improve crystal quality of the thin film. The crystal quality of this perovskite thin film was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD). The SEM and AFM results illustrate perovskite thin films growing by this technique have larger grain sizes (from 200nm to 1um) and more uniformity (root-mean-square value/RMS 18nm) superior to most current methods, such as one-step spin coating and sequential two-steps spin coating method. Strong peaks show in XRD results at 14.1°, 28.4°, 43.2°, assigned to (110), (220), (330) miller indices of CH₃NH₃PbI₃ perovskite crystal respectively, indicate the complete reaction between vapor CH₃NH₃I and PbI₂ layer. Perovskite based solar cells were fabricated with the perovskite thin film sandwiched between an electron transfer layer of FTO/c-TiO₂/m-TiO₂, a hole transfer layer of Spiro-OMeTAD and a sputtered or evaporated gold contact layer. High power conversion efficiency (PCE) up to 9.2% was achieved by these solar cells and remained 7% after two month stored in dark and dry air, which demonstrates the effectiveness and stability of this novel hybrid growth technique for perovskite thin films. This vacuum/vapor based technique is compatible with conventional semiconductor fabrication methods and high quality perovskite film would be achieved through delicate process control. Eventually, perovskite based solar cells could be mass produced in low cost for large scale applications by this novel technique.

Organic-inorganic metal trihalide perovskites are promising materials for photovoltaic applications and significant progress has been made in a short period of time with a high efficiency close to 19%. In this work we introduce a new and facile one step method to fabricate perovskite solar cell by chemical vapor deposition (CVD) with the achieved energy conversion efficiency of 11%. CVD method has a special advantage because of its ability to fabricate large scale solar cell and scale up to industrial levels. Also, it is possible to precisely control gas flow rate, pressure and temperature with high reproducibility using CVD method. The results of Scanning Electron Microscopy (SEM) and Time Resolved Photoluminescence (TRPL) of Methylammonium Lead Iodide perovskite (CH₃NH₃PbI₃) synthesized with CVD showed that the perovskite has a large grain size more than 1 micron and carriers life time more than 10ns. Furthermore, the fabricated solar cell devices with CVD were found to be reasonably stable with time. This is the first demonstration of an efficient perovskite solar cell using one step chemical vapor deposition and there is likely still room for significant optimization to improve the efficiency.

In the last 5 years, a new class of inorganic-organic PV devices based on perovskite absorbers (CH₃NH₃PbX₃, X = Br, Cl, I) has attracted considerable attention mainly due to three significant advantages: inexpensive precursors, a variety of available fabrication methods, and consistently high power conversion efficiency values.[1,2] Among these reports, thin-film vapor deposition has proved to be a successful method to fabricate uniform, flat perovskite films and yield high-efficiency devices.[1,3,4] In addition to continual optimization of perovskite active-layer deposition methods and post-deposition treatments, a large recent effort has focused on the selection of appropriate transport layers that allow for improved fabrication and enhanced performance. In particular, the maximum open-circuit photovoltage of the device, approximated by the perovskite energy gap, can be limited by the relative energy difference between the ionization potential/electron affinity of the hole-/electron-transporting material causing energy losses in the system. Minimizing these energy losses while favoring high charge-extraction rates is fundamental to take greater advantage of the intrinsic properties of the perovskites and achieve efficiencies beyond the current status. Herein, we demonstrate the use of optimized perovskite active layers in combination with industry standard vapor-deposited hole- and electron-transport materials that are carefully selected to addresses the effect of energy level alignment between the hole- and electron- transporting material and the active layer in vacuum deposited, planar-heterojunction perovskite solar cells. This study provides a direct comparison between perovskite and PV device architectures, while showcasing a well-established, highly reproducible, industrially applicable device fabrication technique.
[1] Liu, M.; Johnston, M. B.; Snaith, H. J. Nature2013, 501, 395.
[2] Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Nature2013, 499, 316.
[3] Malinkiewicz, O.; Yella, A.; Lee, Y. H.; Espallargas, G. M.; Gräetzel, M.; Nazeeruddin, M. K.; Bolink, H. J. Nature Photon.2014, 8, 128-132.
[4] Polander, L. E.; Pahner, P.; Schwarze, M.; Saalfrank, M.; Körner, C.; Leo, K. APL Materials2014, 2, 081503.

Graphene is atom-thick two-dimensional (2D) material, which has remarkably high charge mobility and electronic conductivity.^[1] Therefore, it has aroused interest in applications for graphene-like materials on photovoltaics, photocatalysis, nanoelectronics, and so forth. Meanwhile, organometal halide based perovskites are promising materials for solar cell applications and are rapidly developing with current devices reaching ~ 19% efficiency.^[2] Although the PCE of perovskite solar cells has been increased remarkably, few reports have considered solution-processed planar heterojunction structure solar cells without using a mesoporous semiconducting metal oxide layer processed by high-temperature sintering, and the solution-processed planar heterojunction structure solar cells to date have shown lower PCE than those with a mesoporous or compact TiO₂ layer.^[3] Basically, in addition to the perovskite CH₃NH₃PbI₃ photoactive layer, hole transport material and noble metal counter electrode, a successful planar perovskite solar cell requires a compact layer of TiO₂ or ZnO to block holes and to ensure a mechanically robust perovskite layer. ^[4] Therefore, to improve the device efficiency of solution-processed planar heterojunction structure solar cells, interfacial engineering at electrodes is of prime importance because the interface between a photoactive layer and electrodes significantly influences both built-in potential and charge carrier extraction. Recently, we reported a TiO₂ compact film prepared by atomic layer deposition, which offered a large shunt resistance and enabled a high power conversion efficiency of 12.56%. ^[5] In this work we introduced graphene-like TiO₂ (G-TiO₂) ultrathin nanosheets synthesized by solvothermal assembly method and designed to act as electron transport layers in efficient TiO₂/perovskite CH₃NH₃PbI₃/spiro-OMeTAD/Au solid-state solar cells. The graphene-like TiO₂ layer with a sufficient roughness exhibits an effective retard of interfacial charge recombination inside the devices and greatly increases the shunt resistance, resulting to a high-energy conversion efficiency of 14.78 % for perovskite solar cells.
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[3] Liu M, Johnston M B, Snaith H J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature, 2013, 501, 395-398.
[4] Wu Y, Islam A, Yang X, et al. Retarding the crystallization of PbI 2 for highly reproducible planar-structured perovskite solar cells via sequential deposition. Energy & Environmental Science, 2014, 7, 2934-2938.
[5] Wu Y, Yang X, Chen H, et al. Highly compact TiO₂ layer for efficient hole-blocking in perovskite solar cells. Applied Physics Express, 2014, 7, 052301.

Significant progress in high-performance photovoltaics based on solution-processable organic-inorganic hybrid perovskites, such as CH₃NH₃PbX₃ (X=Cl, Br, I) as light-harvesting materials, has been realized with achieving high power conversion efficiencies up to 19 %. One prominent feature of the perovskite material is that the electron or hole has long diffusion lengths (>100 nm) as compared to those in conventional organic photovoltaic materials (~10 nm). Most state-of-the-art perovskite solar cells typically consist of a device structure based on a mesoporous metal oxide based scaffold, on which the CH₃NH₃PbI₃ light-absorption layer can be grown, followed by the deposition of a hole transport layer. Mesoporous TiO₂ films, which are typically used in DSSCs, are the most popular scaffold material in perovskite-based solar cells, responsible for accepting electrons from the absorber and transporting them to the electrode. However, the synthesis of mesoporous titania films usually requires sintering at high temperature of 450-550 °C prior to use, which may cause the limitation of perovskite solar cells to be deposited on flexible substrates or to be compatible with fabrication processes in multi-junction solar cells. Many efforts have been done to search for new scaffold or interlayer materials or to develop new deposition processes for perovskite-based solar cells to be fabricated at low-temperature. In this work, we would like to propose a new heterojunction-typed perovskite solar cell, inspired by the device concept of “bulk heterojunctions” in polymer solar cells, by intermixing dispersed TiO₂ nanorods with the perovskite precursor to form perovskite/TiO₂ nanorods hybrids. The bulk heterojunction-typed solar cells fabricated using spin-coating the perovskite/TiO₂ nanorod hybrids achieve a promising PCE corresponding to ~80% of that of the conventional standard perovskite solar cells using the sintered mesoporous TiO₂ scaffold. The bulk heterojunction-typed perovskite/TiO₂ nanorod hybrid solar cells have the advantages not only to be deposited on various substrates at low temperature by spin-coating but also to enable the layer-by-layer deposition for future development in perovskite-based multi-junction solar cells.

Simplifying the process of fine-tuning the electronic and optical properties of graphene oxide (GO) is of important in order to fully utilize it as a hole interfacial layer (HIL). We introduced silver trifluoromethanesulfonate (AgOTf) inorganic chemical dopant that tunes and controls these properties of single-layer GO films synthesized by chemical vapor deposition. The morphology, work function, mobility, sheet resistance, and transmittance of GO film were systematically tuned by various doping concentrations. We further developed solution-processable low-temperature hole interfacial layer (HIL) poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS):AgOTf-doped GO HIL in highly efficient perovskite solar cells. The PEDOT:PSS:AgOTf-doped GO HIL grants desirable charge-collection in the HIL allowing the entire device to be prepared at temperatures less than 120 #730;C. The fabricated perovskite solar cells utilizing rigid substrate demonstrate compelling photovoltaic performance with power conversion efficiency (PCE) of 11.90%. Moreover, flexible devices prepared using a polyethylene terephthalate (PET)/ITO demonstrate a PCE of 9.67%, while ITO-free flexible devices adopting PET/aluminum doped zinc oxide (AZO)/silver (Ag)/AZO demonstrate a PCE of 7.97%. This study shows that PEDOT:PSS:AgOTf-doped GO HIL has a significant potential to contribute the development of low-cost solar cells.

Organometallic halide perovskite-based solar cells have exhibited rapidly increasing efficiencies through the use of mesoporous composites. The addition of materials used in organic solar cells to perovskite-based solar cells (PSCs) enables the fabrication of low-cost, flexible, low-temperature, solution-processed PSCs. However, obtaining sufficient coverage of the organic layer, usually poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), with CH₃NH₃PbI_3-xCl_x films remains difficult in spite of the advances. In this study, we investigated the influence of controlling the solvent evaporation rate on the degree of PEDOT:PSS surface coverage by CH₃NH₃PbI_3-xCl_x. we controlled spinning speeds (1500 and 3000 rpm) and carried out different annealing conditions (room temperature drying, stepwise ramp and rapid annealing type) to gain insight into the influence of the solvent evaporation rate on the surface coverage of PEDOT:PSS by CH₃NH₃PbI_3-xCl_x for the optimization of photovoltaic performance in ph-PSCs. CH₃NH₃PbI_3-xCl_x films spin coated at 1500 rpm were strongly dependent on different annealing type, whereas we found that CH₃NH₃PbI_3-xCl_x films spin coated at 3000 rpm were less affected by different annealing type. We determined that an adequately fast spinning speed, drying at room temperature, and stepwise ramp annealing are critical for obtaining optimized planar hybrid perovskite solar cells with an ITO/PEDOT:PSS/CH₃NH₃PbI_3-xCl_x/PCBM/Al structure and efficiencies of up to 11.8%.

Organic-inorganic perovskites employing methylammonium lead halide have resulted in remarkable progress in power conversion efficiency (PCE) in a very short timescale. General perovskite solar cells (PeSCs) of inverted planar structures consist of an ITO anode, PEDOT:PSS as a hole-transporting material (HTM), the perovskite as a light absorber, PCBM as an electron-transporting material, and a low work-function metal as a cathode. Among these layers, the charge-transporting layers play a key role in determining the device performance because the interfaces between perovskite absorbers and adjacent layers could manipulate the p-i-n characteristics and charge-transporting performances in the PeSCs. However, very little interest has been paid to develop hole-transporting materials in PeSCs, particularly in the inverted planar structures. Moreover, general PEDOT:PSS HTM should be replaced because of the detrimental effects of its hygroscopicity and acidity on the device stability. Herein, we demonstrate a simple solution and room-temperature processed reduced graphene oxide (RGO) as a new class of a hole-transporting material (HTM) for highly efficient and highly stable CH₃NH₃PbI₃ perovskite solar cells. The effects of RGO HTM are systemically investigated in terms of PeSC-efficiency, PeSC-stability, perovskite film morphology, the recombination dynamics, and the charge-transport across the interface of CH₃NH₃PbI₃/HTM. The resultant PeSC with configuration of glass/ITO/RGO/CH₃NH₃PbI₃/PC₆₁BM/bathocuproine (BCP)/Ag exhibits superior device efficiency with high reproducibility than those of the conventional devices using PEDOT:PSS HTMs. Also, the RGO-based PeSCs show highly desirable long-term device stability in comparison to the PEDOT:PSS PeSCs.

Organometal halide perovskite solar cell has attracted tremendous attention owing to its boosted development in the efficiency of converting solar energy into electricity. The large absorption coefficient covering the visible spectrum, high carrier mobility and extremely long electron-hole diffusion lengths of perovskite assures the fabrication of high-performing solar cell. The shiny over 15% efficiency has been achieved in four years, since the 3.8% efficiency was achieved in its original attempt as absorber in dye sensitized solar cell. The conductor plays a critical role in the device working mechanism. An efficient conductor assures the charge transfer at cathode or anode interface, thereby guiding the charge transport to corresponding electrode selectively. Herein, graphene oxide (GO) is employed as a hole conductor in inverted planar heterojunction perovskite solar cells, and the devices with CH₃NH₃PbI_3-xCl_x as absorber achieve an efficiency of over 12%. The perovskite film grown on GO exhibits enhanced crystallization, high surface coverage ratio as well as preferred in-plane orientation of (110) plane. Efficient hole extraction form the perovskite to GO is also demonstrated.

Organic-inorganic hybrid solar cells based on organometal halide perovskite have recently emerged as one of the transformative photovoltaic technologies. With various interface engineering technologies, the power conversion efficiency has now exceeded 19% within just four years. In this work, we have investigated the thickness dependence of active layer and electron transporting/hole blocking layer in inverted organometal halide perovskite solar cells based on CH₃NH₃PbI_3-xCl_x as an active material and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) layer as a hole blocking layer fabricated by low temperature solution process. The performance of the inverted perovskite solar cells is strongly depends on the thickness of CH₃NH₃PbI_3-xCl_x active layer and PCBM electron transporting/hole blocking layer. By optimizing the thickness of CH₃NH₃PbI_3-xCl_x layer and PCBM layer, we achieved a 10.4% power conversion efficiency from the inverted perovskite solar cells.

Hybrid organic-inorganic perovskite solar cells have attracted tremendous attention in the past two years, recently producing a record power conversion efficiency of ~19.2%. However, “all” fabrications of perovskite solar cells require thermal annealing of spin-coated thin films that is a critical step to remove resident solvents and form necessary crystalline structures. Such annealing step is difficult to control and more importantly unsuitable for low-temperature, solution processing of perovskite solar cells on flexible substrates. Here we report a novel one-step solution method of preparing perovskite CH₃NH₃PbI₃ films without the need of thermal annealing. The key to this method is adding 1 wt% of small-molecule additives to the standard precursors consisting of an equimolar mixture of CH₃NH₃I and PbI₂ solution. The additives were shown to strongly accelerate the crystallization process of CH₃NH₃PbI₃, leading to flat and uniform perovskite films with dark brown color immediately after spin-coating. Built upon these unannealed CH₃NH₃PbI₃ films, regular solar cells were constructed based on planar bilayer heterojunctions using PCBM as electron transporting layer, yielding an average power conversion efficiency of ~10%. Importantly, the device performance exhibited good reproducibility and no hysteresis behavior was observed. In addition, transport studies confirmed ambipolar characteristics of perovskite materials by showing balanced hole and electron transport in solar cells. In short, our studies further demonstrated the commercialization potential of perovskite thin film solar cells.

Recently, the scientific community has witnessed the amazing rise of metal halide perovskite as new and powerful photovoltaic light harvesters. Both mesosopic and planar embodiments are presently being investigated. The simple structure of the planar embodiment for a PSC suggests that ultimately all devices may adopt this configuration. However, presently mesoscopic systems with a certified power conversion efficiency of 17.9% have a lead over planar embodiments, whose efficiencies remain uncertified till now [1]. The main reason for this is that planar films are prone to show an unwanted hysteresis in the photocurrent - voltage curve, which has rendered difficult the correct assessment of their performance. Our recent studies suggest that the hysteresis arises from the slow migration of ions across the perovskite films screening the electric field. The use of a mesoporous TiO₂ scaffold to host the perovskite material strongly attenuates the hysteresis effects and facilitates greatly the collection of photo-induce charge carriers. Another advantage of the mesoscopic PSC embodiment over the planar configuration is that it enables carrier collection efficiencies and therefore external quantum efficiencies for photocurrent generation close to 100 %, even with systems where the exciton or charge carrier diffusion length is much smaller than the photon absorption length. Contrary to a widespread belief, employing a mesoporous TiO₂ scaffold does not necessarily entail a loss in V_OC. A rational and data to support this contention will be presented.
1. M. Grätzel. Nature Mat. 2014, 13, 838.

The efficiency of perovskite solar cells has soared from a few percent to over 17% in the last 2 years. They are very attractive for multijunction solar cell applications because the bandgap of perovskite semiconductors can be easily tuned in the range of 1.55 to 2.2 eV and the open circuit voltage of the cells is large. We have made highly efficient semitransparent perovskite solar cells using silver nanowire meshes as the top electrode. These cells can be used in combination with either silicon or copper indium gallium diselenide solar cells to make four-terminal tandems. We will also present detailed characterization of perovskite semiconductors made with different processing conditions to show what needs to be done to minimize recombination and make the solar cells stable.

Organolead trihalide perovskite (OTP) materials are emerging as naturally abundant materials for low-cost, solution processed and highly efficient solar cells. Here we are excited to show that, in OTP-based photovoltaic devices with vertical and lateral cell configuration, the photocurrent direction can be switched repeatedly by applying a small electric field of <1 V/mu;m. ^[1] The switchable photocurrent, generally observed in devices based on ferroelectric materials, reached 20.1 mA/cm² under one sun illumination in OTP devices with vertical architecture, which is four orders of magnitude larger than that measured in other ferroelectric photovoltaic devices. The lateral photovoltaic devices with 125 devices connected in series have an open circuit voltage of 47 V under a quarter sun illumination. We will present the study of the origin of the switchable photovoltaic effect by examining the three possible mechanisms which have been reported for switchable photovoltaic or memristor behavior, which was also speculated to result in photocurrent hysteresis with a changed photocurrent scanning direction and scanning rate in some perovskite photovoltaic devices: 1) ferroelectricity of the photoactive layer; 2) charge traps in the active layer&’s surface; and 3) motion and accumulation of ions induced doping effect. The demonstration of switchable OTP photovoltaics and electric-field manipulated doping paves the way for new perovskite photovoltaic device designs such as homojunction solar cells, and opens up new applications for perovskite devices, such as both electrically- and optically- readable memristors and circuits.
[1] Giant Field Switchable Photovoltaic Effect in Organometal Trihalide Perovskite Devices
Z. Xiao, et al. Nature materials, Accepted, 2014.

ABSTRACT. Hybrid organic/inorganic perovskite solar cells have emerged as among the most competitive photovoltaic technologies thanks to their superb and rapidly improving power conversion efficiencies (PCEs), advancing to nearly 20%. For realistic deployment of the perovskite photovoltaic technology in large scale, however, device stability, cost and environmental impact become more important issues than ever. Toward this end, we have developed inexpensive hole extraction nanostructures and hole transport polymers for perovskite solar cells with high efficiency, some of which can even be processed with no need of a standard glovebox. In the present contribution, I will present our most recent work on (1) using NiO and carbon nanostructures for efficient hole extraction and (2) demonstrating a series of polyfluorene derivatives as high-mobility hole transport alternatives to the standard small molecule version for perovskite solar cells. These studies have set the stage for developing high-performance perovskite solar cells, which embrace not only high efficiency but also high stability and high environmental friendliness.
References:
1. “Cost-efficient Clamping Solar Cells Using Candle Soot for Hole Extraction from Ambipolar Perovskite”, Wei, Yang et al., Energy Environ. Sci. 7, 3326 (2014).
2. “High-performance Hole Extraction Layer of Sol-Gel Processed NiO Nanocrystals for Inverted Planar Perovskite Solar Cells”, Zhu, Yang et al., Angew. Chem., Int. Ed. 49, xx (2014).
3. “Inkjet Printing and Instant Chemical Transformation of CH₃NH₃PbI₃/Nanocarbon Electrode and Interface for Planar Perovskite Solar Cell”, Wei, Yang et al., Angew. Chem., Int. Ed. 49, xx (2014).
4. “Polyfluorene Derivatives are High-performance Organic Hole-Transporting Materials for Inorganicminus;Organic Hybrid Perovskite Solar Cells”, Zhu, Yang et al., Adv. Funct. Mater.xx, xx (2014).
Acknowledgement. Collaborators and supports (HK-RGC-GRF No. HKUST 606511 and 605710) are acknowledged.

Among many methodologies aimed at producing low cost, efficient photovoltaic cells, semiconductor nanocrystals and inorganic-organic hybrid perovskites offer promise of a breakthrough for next-generation solar devices. This is mainly due to the combination of superior optical properties with the opportunities for inexpensive, solution-based device fabrication. In this presentation, I will talk about several steps toward efficient inorganic-organic hybrid solar cells employing Sb₂S₃ (Sb₂Se₃) and chemically managed perovskite materials with hole conducting polymers. Our results showed the power conversion efficiency (PCE) of 7.9 % and 8.1 %, respectively, with a metal mask. In inorganic-organic hybrid perovskite materials, a solvent-engineering technology enabled the extremely uniform, dense perovskite layers, and remarkably improved the performance of the cells. Very recently, the certified PCE exceeding 20 % under air-mass 1.5 global (AM 1.5G) illumination of 100 mW cm^-2 intensity, based on the perovskite materials, was achieved. These results will lead to more efficient and cost-effective inorganic-organic hybrid solar cells in the future.

Perovskite is a promising light harvester for use in photovoltaic solar cells. In recent years, the power conversion efficiency of perovskite solar cells has been dramatically increased, making them a competitive source of renewable energy.
This work will discuss several topics related to perovskite based solar cells:
1. An in-depth study on two-step deposition, separating the perovskite deposition into two precursors.The effects of spin velocity, annealing temperature, dipping time and methylammonium iodide concentration on the photovoltaic performance are studied.
2. High Voltage hole conductor free perovksite solar cells- first demonstration to gain high voltage in perovksite solar cells without hole conductor.
3. Kelvin probe force microscopy is used to measure cross-sections of hole conductor free CH₃NH₃PbI₃ perovskite solar cells. The work function change are measured at the interfaces between the CH₃NH₃PbI₃ perovskites and a metal oxide, nanocrystalline TiO₂ and Al₂O₃, respectively. The findings from this research are critical for the understanding and further improvement of perovskite based solar cells, and are valid for cells with a hole transport material.

Current state-of-the-art photovoltaics utilize high purity single-crystal semiconductors grown by sophisticated, high temperature crystal-growth processes. Alternative emerging solar cell technologies based on solution-processed materials promise to deliver power at lower cost, but so far have had limited success due to the presence of defects in the bulk and at the grain boundaries. Improving the crystal size and quality of solution-processed films could lead to unprecedented improvements in photovoltaic performance. However, large-scale crystal growth is challenging in solution based deposition where molecular kinetics are limited by the low processing temperature. Here, for the first time we demonstrate organometallic perovskites thin-films composed of extra-ordinarily large crystalline grains (1-2 mm), fabricated using a new processing technique. The large-area mm-scale grains form a continuous pinhole free film laterally across the substrate. The increase in the grain-size directly correlates to a dramatic increase in the efficiency, as a direct consequence of increased charge carrier mobility and reduction in defect densities. Our measured efficiency values approach ~18%, which is among the highest reported. These devices are extremely robust, exhibiting no degradation with voltage sweep direction or the rate at which the voltage was scanned. We anticipate that this technique for growing high quality large-area crystals at low temperature with low defect concentration will be universally applicable to a wide variety of other material systems thus providing a solution to a long-standing scientific challenge of overcoming polydispersity, defects and grain boundary recombination in solution-processed thin-films. From the perspective of the global photovoltaics community, these results are expected to lead the field towards the synthesis of wafer-scale crystalline perovskites necessary for the fabrication of high-efficiency single-junction and hybrid (semiconductor and perovskite) tandem planar cells.

A key step in enabling the large scale manufacture of inorganic-organic perovskite solar cells [1] is the development of a low cost flexible transparent electrode that can be easily applied in a continuous process to replace the vacuum evaporated gold contact.
Here we describe a major breakthrough in electrode engineering that entails the combination of a polymer embedded nickel grid with a transparent conducting contact adhesive that can be applied to perovskite devices providing conductivity, charge extraction, mechanical adhesion and environmental protection. This has enabled the development of an indium-tin oxide (ITO), Au and Ag free entirely non-vacuum processed perovskite device to be manufactured.
In laboratory devices charge collection is usually achieved through the evaporation of an opaque gold metallic contact onto the active material. This limits their potential for scale-up as the materials used are inherently expensive to use and deposit. Furthermore the standard architecture generally dictates that the working electrode of the cell must be transparent. Current solution processable electrodes employ silver nanowires [2,3] that, although capable of making good electrical contact, can give rise to silver halide formation and degradation of performance.
The new system is a blend of an acrylic emulsion pressure sensitive adhesive (PSA) with a very low loading (0.018 volume fraction) PEDOT:PSS. This causes a phase segregation to occur between the PEDOT:PSS and the polymer adhesive domains which is advantageous for achieving conductivity at an order of magnitude lower volume fraction below conventional percolation threshold. The PEDOT:PSS assembles into a honeycomb arrangement around the larger domains typical of and acrylic emulsion. The spanning network is more ordered and allows conductivity to be evolved. The low loading gives excellent transparency of over 85%. To complete the electrode design the conducting adhesive is applied onto a corrosion proof Ni mesh electrode embedded into conventional PET film. The whole electrode can be fabricated separately to the perovskite material and then laminated together at room temperature
Device performance is comparable to that of a gold vacuum deposited electrode with champion efficiencies achieving 16.7% for the gold and 15.5% for the laminate on a glass substrate [4]. This paper also presents for the first time a fully flexible indium and silver free perovskite solar cell using metal foil as a substrate incorporating the new laminate with a PCE exceeding 10%.
[1] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science.2012,338, 643.
[2] B. E. Hardin, W. Gaynor, I-K. Ding, S-B. Rim, P. Peumans, M. D. McGhee, Org. Electron.2011, 12, 875.
[3] W. Gaynor, J-Y. Lee, P. Peumans, ACS Nano.2010, 4, 30.
[4] D. Bryant, P. Greenwood, J. Troughton, M. Wijdekop, M. Carnie, M. Davies, K. Wojciechowski, H. Snaith, T. Watson, D. Worsley, Adv. Mater. 2014 doi: 10.1002/adma.201403939

Perovskite solar cells have come in the spotlight as a merging research topic under intensive investigation. It has been reported that the latest efficiency record was achieved in excess of 19%. My group has recently developed a novel hybrid deposition method to prepare perovskite films for solar cell applications [1]. In our method, the growth of perovskite films relies on the control of the CH₃NH₃I flow and vapor pressure inside a vacuum chamber, which is designed on the basis of the chemical reaction nature of perovskite formation. Using the hybrid deposition method, it is possible to deposit semi-transparent perovskite films with centimeter-scale uniformity, good crystallinity and high reproducibility. Solar cell devices based on the prepared semi-transparent perovskite films as thin as ~135 nm achieved efficiencies of 9.9% and high open circuit voltages of 1.09 V. Also, the deposited perovskite films exhibit superior uniformity with an intrinsic roughness as low as 4.6 nm and the film thickness variation of ~2% for the film area of 5cm x 5cm. Superior film uniformity achieved by our hybrid deposition method is expected to be an important step for the scaling up of perovskite solar cell fabrication.
[1] Luis K. Ono +, Shenghao Wang +, Yuichi Kato, Sonia R. Raga and Yabing Qi *, "Semi-Transparent Perovskite Films with Centimeter-scale Superior Uniformity by the Hybrid Deposition Method", Energy Environ. Sci., in press (2014). (+ equal contribution) (http://dx.doi.org/10.1039/C4EE02539C)

With solution-processed, organic-inorganic hybrid lead halide perovskite-based solar cell developments being meteoric over the last few years, how these solar cells work is of fundamental interest. Here we will describe the insights that we have gained from several measurement approaches that are less common among those mostly employed, viz. Electron Beam Induced Current (EBIC), a scanning electron microscopy-based technique, applied to cell cross-sections, Surface photovoltage spectroscopy (SPS) on partial and complete cells, aided by several photoelectron spectroscopic measurements (in collaboration with A. Kahn and group, Princeton) and scanning probe measurements. For thin film, non-mesoporous cells the best model remains that of a solar cell with a low-doped, high electronic quality photovoltaic semiconductor layer between a p- and an n-layer, a less common type of “p-i-n” cell. Under certain conditions, if the cell is very thin, it will become hard to distinguish this readily from a p-n cell, except by way of scanning probe methods. This model alongside the high effective diffusion lengths we measured in working cells, rather than indirectly by optical spectroscopy, explains the remarkably high open circuit voltage (V_OC) to E_gap obtained with these cells.
Work done with Gary Hodes, Boris Rybtchinsky and former group members Eran Edri, Saar Kirmayer, Yaron Tidhar, Lee Barnea and others at the Weizmann Institute.

Organohalide lead-perovskites have revolutionized the hybrid/organic photovoltaics landscape. Despite the fast efficiency increase, some of the materials properties related to their extraordinary photovoltaic performance remain largely not understood.
Further advances in the perovskite solar cells (PSCs) field may be boosted by computational design and screening of new materials, with researchers examining material characteristics that can improve device performance and/or stability. Suitable modeling strategies may allow researchers to observe the otherwise inaccessible but crucial hetero-interfaces that control the operation of PSCs, allowing researchers the opportunity to develop new and more efficient materials and optimize processes.
We illustrate here the performance of an integrated simulation toolbox, rooted into Density Functional Theory and many body GW methods including spin-orbit coupling, that can provide atomistic electronic structure information on the materials properties and on the crucial perovskite absorbers/metal-oxide/hole transporter material heterointerfaces. We critically assess the accuracy of various computational approaches against the related experimental data and analyze the representative interfaces that control the device operational mechanism.
In particular, we describe the structural and electronic features of the methylammonium lead iodide perovskite with various metal oxide substrates, i.e. TiO₂, Al₂O₃ and ZnO, with emphasis on the influence of the substrate in determining the perovskite growth, the preference for specific surfaces and the underlying electronic interfacial properties.
Despite there is in principle no need of an electron transporter in perovksite solar cells, yet we demonstrate how the surface deeply influences the ensuing perovskite material properties.
References:
1) De Angelis, F. Acc. Chem. Res2014, DOI: 10.1021/ar500089n.
2) Colella, S.; Mosconi, E.; Pellegrino, G.; Alberti, A.; Guerra, V. L. P.; Masi, S.; Listorti, A.; Rizzo, A.; Guido Condorelli, G.; De Angelis, F.; Gigli J. Phys. Chem. Lett.2014, 5, 3532
3) Mosconi, E.; Ronca, E.; De Angelis, F. J. Phys. Chem. Lett.2014, 5, 2619.
4) Amat, A.; Mosconi, E.; Ronca, E.; Quarti, C.; Umari, P.; Nazeeruddin, M. K.; Graetzel, M.; De Angelis, F. Nano Lett.2014, 14, 3608.
5) Umari, P.; Mosconi, E.; De Angelis, F. Sci. Rep.2014, 4, 4467.
6) Roiati, V.; Mosconi, E.; Listorti, A.; Colella, S.; Gigli, G.; De Angelis F. Nano Lett. 2014, 14, 2168.

The field of thin-film photovoltaics is witnessing a revolution of sorts with the advent of perovskite solar cells where organometallic halide perovskites are used as the light absorber. While great progress is being made in the solution processing of the perovskite thin films, reproducible deposition of high quality material remains a challenge. Here we demonstrate the feasibility of new solution-processing methods for the deposition of CH₃NH₃PbI₃-based perovskite thin films, and elucidate their formation mechanisms. The characterization of these thin films reveals high crystallinity, uniform coverage/density (pinhole free), high smoothness, large grain size, uniform tunable thickness from <20 nm to a micron, and vivid colors. Some of these characteristics allow detailed measurement of dielectric and ferroelectric properties of the CH₃NH₃PbI₃-based perovskite thin films. These results, together with a discussion of the role of structure and defects in determining these properties, are presented. The effects of the dielectric and ferroelectric properties on the photovoltaic performance of solar cells made from these CH₃NH₃PbI₃-based perovskite thin films are also studied and presented. The connections between processing - structure/defects - properties - performance of organometallic-halide-based perovskites are discussed comprehensively, together with guidelines for future directions for research in this blossoming field.

Methylammonium lead halides have demonstrated great promise as photovoltaic absorbing materials, recently achieving power conversion efficiencies over 17%. However, there are many light-induced phenomena that have yet to be explained. For example, it has been shown that perovskite solar cells have slow photoconductivity responses, which cause I-V hysteresis only under illumination. In addition, we have previously demonstrated that mixed halide perovskites such as CH₃NH₃Pb(Br_xI_1-x)₃ undergo a reversible transformation under constant 1-sun illumination over the course of minutes, resulting in the formation of defect states that pin the photoluminescence and open circuit voltage of devices made with this material at a lower energy, which reduces the power conversion efficiency.^[1] We also have observed that illumination increases the corrosion of aluminum electrodes on perovskite devices, which suggests that light allows reactive ions to move towards the electrode. In this work, we provide the first direct evidence that methylammonium lead iodide exhibits lattice distortion under illumination and voltage biases relevant for photovoltaic operation. We use in-situ high resolution synchrotron x-ray powder diffraction to resolve the changes in lattice parameters, and investigate the coupled influence that photoexcitation and electric field have on producing this structural change. This finding has important implications for device reliability and can help explain the origin of hysteresis.
[1] E.T. Hoke et al. Reversible photo-induced halide segregation in mixed-halide hybrid perovskites for photovoltaics, Materials Research Society, Boston, December 2014.

Intentional doping, through which carrier concentration of semiconductor could be deliberately controlled, ignited the era of semiconductor development in history. Nowadays, intentional doping techniques have been widely used in both inorganic and organic semiconductor based photovoltaic (PV) devices. Many key parameters of PV devices, such as short circuit current, fill factor and open circuit voltage (V_OC), are all strongly related to doping level because it largely influences carrier transport, recombination and quasi-Fermi energy splitting. Understanding the doping mechanism of a semiconductor is crucial in predicting electronic properties for rational design of efficient PV devices. Recently, methylammonium lead halides (MAPbI₃) have arisen as one of the most attractive candidates for the next generation of solar cell materials. Since the formation of perovskite involves a reaction of organic and inorganic precursors, the electronic properties of perovskite films are supposed to be different with varied film composition. More interestingly, a unique property, namely self doping, was first experimentally demonstrated by us, in which conductivity of CH₃NH₃PbI₃ perovskite can be either p-type or n-type by tuning the composition of the perovskite.
In this talk, we will report the self-doping effect in MAPbI₃ perovskite and simple approaches to tune the carrier concentration of the films by as large as six orders of magnitude. The electronic properties of the perovskite films, such as carrier concentration, mobility, conductivity type and energy level, were found closely related to the film compositions. Many experimental procedures that change the perovskite film composition, such as precursor ratio in solution, thermal annealing time, thermal annealing temperature, and film formation method, were all found played crucial roles in determining the electronic properties of the formed perovskite films. Perovskite film was found to be either n- or p- doped by changing the precursor ratio in solution. Thermal annealing was another effective way to tune film composition because of the low dissociation energy of MAPbI₃. The possible point defects that contributed to the self-doping effect in perovskite films will also be discussed.
Finally, the influence of the doping level on device performance will also be reported. The changed doping profile was found have strong impact on device performance, especially V_OC as it&’s directly related to electron-hole quasi-Fermi energy splitting. A clear correlation between doping level and open circuit voltage was observed in perovskite devices made by two different formation methods. The influence of thermal annealing on device performance was also studied. It was found that the work function change in the perovskite films, caused by annealing-induced composition change, is linearly correlated with the device V_OC.

Organic-inorganic halide perovskites such as CH₃NH₃PbX₃ (X= Cl, Br, I), have attracted great attention as absorbers for solar cells. Recent reports have shown that these perovskites have very long electron-hole diffusion lengths. Thin-film solar cells based on CH₃NH₃PbI_3-xCl_x have demonstrated very high conversion efficiencies and high open circuit voltages. Here we show by first principles calculation that halide perovskites exhibit a number of unique properties such as extremely high optical absorption, small effective masses for electrons and holes, and intrinsic low non-radiative recombination rate: explaining their long carrier diffusion length and high efficiency. We reveal that their unique properties are attributed to the combination of perovskite symmetry and the existence of the lone-pair s orbitals, which enable halide perovskites semiconductors to have direct bandgap p-p transition. Our results demonstrate the halide perovskite, with superior intrinsic photovoltaic properties, is likely to be among the highest efficiency solar cell materials. We will further discuss the strategies for n-type and p-type doping of halide perovskite materials.

Lead halide perovskite semiconductors CH₃NH₃PbX₃(X=Cl, Br, and I) recently attract much attention as a new class of solar-cell materials. The power conversion efficiencies of perovskite solar cells are being drastically improved and attain nearly 20 % to date. However, the physical mechanism that determines the conversion efficiency of perovskite solar cells has not been clarified yet. Therefore, it is necessary to understand the fundamental optical properties of perovskite semiconductors. Thus far, the optical spectra and photocarrier recombination dynamics of perovskite CH₃NH₃PbI₃ thin films have been intensively investigated using various optical measurements [1-3]. The optical properties of thin films are usually dependent on the sample fabrication condition and are sensitive to extrinsic defects and impurities.
In this study, we investigated the optical spectra and photocarrier recombination dynamics of perovskite CH₃NH₃PbI₃ single crystals to reveal intrinsic properties of perovskite semiconductors in comparison with thin film samples.
We fabricated single crystal samples by a solution growth method and their optical properties were studied by time-resolved photoluminescence (PL) and femtosecond transient reflectance (TR) measurements. All measurements were conducted at room temperature.
We determined the direct band gap of perovskite CH₃NH₃PbI₃ single crystals as 1.63 eV from reflection spectrum. The band-to-band PL was observed at 1.60 eV. The band-to-band PL dynamics are well described by a simple rate equation including single-carrier trapping and electron-hole radiative recombination. Under high-density excitation, the PL lifetime becomes shorter with an increase in excitation intensity because of electron-hole radiative recombination. Based on these results, we successfully determined the two-carrier radiative recombination coefficient of perovskite CH₃NH₃PbI₃ single crystals. Furthermore, we investigated the femtosecond TR of perovskite CH₃NH₃PbI₃ single crystals using a white-light pump-probe technique. In the TR spectra, we observed the photobleaching signal that has a derivative shape of Lorentzian function centered at the band-gap energy. The excitation-intensity dependence of TR dynamics agrees well with that of PL dynamics. The global features of photocarrier recombination dynamics in perovskite semiconductors will be discussed.
This work was supported by the Sumitomo Electric Industries group CSR foundation, JST-CREST, and JST-PRESTO.
[1]Y. Yamada, et al., Appl. Phys. Exp., 7, 032302 (2014).
[2]Y. Yamada, et al., J. Am. Chem. Soc., 136, 11610 (2014).
[3]Y. Yamada, et al., IEEE J. Photovolt. (in press).

Hybrid perovskites represent a new, disruptive, technology in the field of optoelectronics. They have the potential to overcome the performance limits of current technologies and achieving low cost and high integrability. Hybrid halide perovskite, e.g. CH₃NH₃PbX₃ [X = Cl, Br, or I], are usually deposited as polycrystalline thin-films with variable mesoscale morphology depending on the growth conditions. The obtained grain size ranges from tens to thousands of nm. Over the last two years the impressive improvement of photovoltaic performance has been driven by radical empirical evolution of the device architecture and processing methodologies. However, there is a considerable lack of understanding of material properties, both as pristine films and their embodiment in a device.
Here we demonstrate, through a combination of femtosecond transient absorption spectroscopy and multiscale modeling as a function of crystal size and temperature, that the electron-hole interaction is sensitive to the microstructure of the material. We find that by control of the material processing during fabrication both free carrier and Wannier excitonic regimes are accessible, with strong implications for optoelectronic devices. The long-range order of the organic cation dipole field is disrupted by polycrystalline disorder introducing domain walls where dipole twinning breaks down. The variations in electrostatic potential found for smaller crystallites suppress exciton formation, while larger crystals of the same composition demonstrate an unambiguous excitonic state. In addition, within the free carrier regime, we also observe a reduction in the dielectric permittivity of the material after the photo-excitation. We attribute this effect to the saturation of the molecular dipole response induced by the polaronic effects (a result of Frohlich electron-phonon coupling) as the carrier concentration is increased. We show that such a reduction in the dielectric constant results in the quenching of the excitonic screening and thus favouring the formation of a photo-induced meta-stable excitonic transition.

In the past few years, CH₃NH₃PbX₃ (X = Cl, Br, and/or I) perovskite materials have shown great promise for PV solar cells and are close to being commercialized. However, the devices need to be more efficient and stable. To optimize the solar cells, it is critical to know the band energies of the perovskite material and how these energies depend on the hole and electron transport layers. Therefore, we investigate the influence of various substrates on the Fermi level of CH₃NH₃PbI₃ thin films with photoelectron spectroscopy. The substrate conductivity type (n- or p-type) tracks with the measured conductivity type of the CH₃NH₃PbI₃ thin film. This result highlights that the electron and hole transport layer can impact the CH₃NH₃PbI₃ type and band alignment in solar cells. In addition to optimizing the band alignment within a solar cell, perovskite materials are limited by the formation and degradation with time, temperature, and humidity. We use photoelectron spectroscopy to follow the change in atomic core level positions and intensities to formulate formation and degradation mechanisms for perovskites. This could result in making the perovskite material more stable and commercially viable.

Amplification of photo-induced current output is mandatory for high-resolution image sensing and photon counting for environmental monitoring, bioimaging, and medical diagnosis. High gain of such amplified photo-induced current in large excess of the theoretical limit of quantum efficiency (100%) is achieved by commercial avalanche photodiode made of inorganic semiconductors, such as GaAs and Si. The mechanism here depends on ionization of the semiconductor crystal by high voltage (50-150V) applied in reverse-biased conditions. However, these conventional avalanche photodiodes require expensive crystal growth technology (vapor-phase epitaxy) and high drive voltage for the device.
In this report, we show that avalanche-like giant diode current can be obtaied by low-cost solution-processed films of an organo-lead halide perovskite material that strongly absorbs visible light. We found that the avalanche-like photo-induced current in the perovskite occurs with external quantum conversion efficiency (EQE) as high as ~1700% by the visible light absorption and considerably low applied voltage of <1 V. With a flat spectral response up to wavelength of 800 nm, current magnitude showed linear dependence on light intensity with the high signal/noise ratio (>700) that the current is perfectly switched off under dark. This photodiode function shows usefulness for photon detection with high sensitivity. Regarding the mechanism, the hybrid perovskite is characterized as a ferroelectric material due to the special shift of organic cation in the crystal lattice of hybrid structure. Our results demonstrate how this ferroelectric property of the hybrid perovskite enables generation of giant photodiode current under low bias voltage. The perovskite-based low cost device can be quickly fabricated by solution processes under ambient air conditions, in which optical band gap of the perovskite is tunable by material synthesis. The perovskite-based avalanche-like photodiode opens many applications not only as photon detector and image sensor but also optical switch for information and communication.

Recently, agreat deal of effort has been devoted to explore the unique properties of perovskite materials and the development of theoptoelectronic devices. In UCLA, we have made substantialprogress on highly efficientpervoskite solar cells, where the findings are beneficialto a deep understanding of this unique materials/devices system, and continuous advanceof the associated researchfrom academic/industry perspectives. Here, we summarizeourrecent progressin the perovskitefilm formation, defect passivation, transport materials design, interface engineering toward high performance solar cell, as well as the exploration of its applications beyond photovoltaics. These achievements include:1)two approaches for perovskite film growth: vapor assisted solution process (VASP)and moisture assisted solution process, which provide advanced alternatives to process high performance perovskite filmandphotovoltaic devices; 2)self-induced passivation of the perovskitematerials to reduce the carrier recombination and improve the device properties based on VASP method; 3)interface engineering based on careful choices of the carrier transport materials, and the modification of electrodes, in combination withmoisture assisted solution process, which enable the resulting15~20% PCEs; 4) a novel design of hole transport materials (HTM) without any dopants/additives to achievePCEup to 15% PCEs; 5)fabrication of ITO/PEDOT:PSS/CH₃NH₃PbI_3-xCl_x/PCBM/Al deviceswith high efficiency and flexibility and 6) exploration of the pervoksite materials intophotodetector application. Moreover, we will report theunderlying film growth mechanism, film properties,interfaceproperties and device physics based on various characterizations.

Next-generation solar cells based on new concepts and/or novel materials are currently attracting wide interests. In this study, several types of tandem solar cells based on organic and hybrid photovoltaics are investigated. Among the emerging solar cells, dye-sensitized solar cells (DSSCs) have received much attention as the low-cost solar cells. However, the energy conversion efficiency should be improved for the practical use. In order to improve the energy conversion efficiency, the extension of absorption range of the sensitizers to near-infrared regions is an important issue. In our study, panchromatic photoelectric conversion up to around 1000 nm has been accomplished by the use of new sensitizers DX1, 2, and 3. The panchromatic DSSC are useful for a series-connected tandem solar cell. We prepared the various tandem solar cells, including the stable perovskite solar cell, showing a high overall power conversion efficiency (eta;) of about 17.7%. In this study, we used planar or meso-porous heterojuction perovskite solar cells with high quality perovskite (CH₃NH₃PbI₃-xCl_y) films through a simple solution process. Analysis of content of chloride and iodide in the final annealed perovskite films revealed that the low chloride content of the perovskite formed through chloride-assisted growth. We observed that the quality of the film depends significantly on the level of humidity in the atmosphere. The effects of dry and humid condition in air on the perovskite film quality and eventually on cell performance have been investigated.

The dependence of the current-voltage curve on voltage sweep direction, sweep rate, and the situation (pre-bias, illumination) a CH₃NH₃PbI₃ perovskite solar cell has faced before a scan, makes a simple determination of the efficiency more difficult and results in what is called a rate-dependent hysteresis in the current-voltage relation.
In this work we show that the rate-dependent hysteresis is related to a slow field-induced process that tends to cancel the electric field in the device at each applied bias voltage. It is attributed to the built-up of space charge close to the contacts, independent of illumination and most likely due to ionic movement, which is enhanced when the device undergoes aging. This process can also lead to a reduction of the steady-state photocurrent and does not directly correlate with the development of the hysteresis if it is measured at a fixed voltage sweep rate. Consequently, investigating the hysteresis itself at a given voltage sweep rate is not sufficient to understand the effects causing the hysteresis. We show that the difference between the photocurrent when scanning from positive to negative bias and the other way around is not related to a displacement current, but to a modified charge-carrier collection efficiency. Our experimental approach allows to discriminate between slow and fast processes, where we compare planar architectures with devices based on a mesoscopic scaffold. We apply a device model to assist the analysis of the experimental data.

The recent and fast evolving interest in Perovskite-based solar cells has led to a development of several device architectures and new designs. The standard architecture of the devices employing a flat junction of perovskite between a compact layer of TiO₂ as electron transporting layer and Spiro-OMeTAD as hole transporting material has reported surprising high efficiencies. Anyway this structure shows several unsolved issues with a change in the overall global efficiencies of the devices depending on the measurement conditions (in particular the scan voltage direction and the dwell time) resulting in a strong hysteresis effect. This is limited, but still present when a mesoporous TiO₂ scaffold and smaller perovskite crystallites are embodied in the solar cell. On the other hand, perovskite solar cells based on an inverted structure and employing a flat junction between a bottom layer of PEDOT:PSS as a selective hole extraction layer, perovskite and a top layer of fullerene derivatives in order to extract the electrons, shows extremely reproducible results with high conversion efficiencies and negligible hysteresis effects.
Starting from these observations in this work we present a deep investigation on several device configurations, moving from flat to meso and from standard to inverted structures and we shed light the origin of the hysteresis in solar cells. By combining Photoluminescence and Raman spectroscopy, XRD analysis and an investigation of the diodes electrical characteristic in dark and under illumination we have pinned down the effect of electric field and light polarization on the structural and electrical properties of the hybrid semiconductor and on the complete device structure. We suggest that the hysteresis phenomenon observed in the operation of the solar cell is caused by an intrinsic property of the active material which gets polarized under application of an electric field and under photo-excitation. Such polarization strongly depends on the level of poly-crystallinity and disorder of the thin film. We find that it affects the energy alignment of the interface between the semiconductor and the electron/hole extracting layer and according to the nature of such layer (i.e. organic, oxide) a dipole will be formed which will improve or be detrimental for the carrier extraction.

Within the last few years organic-inorganic halide perovskites have risen to become a very promising PV material, captivating he research community. In the most efficient devices, which now exceed 18% solar to electrical power conversion efficiency, the perovskite is present as a solid absorber layer sandwiched between n and p-type charge collection contacts. Increasing importance of improving solar cell operation is reliant upon understanding and controlling thin-film crystallisation and controlling the nature of the p and n-type contacts. Here I will present our recent progress with understanding the thin-film formation and crystallisation of the perovskite absorber layers, and the critical role of the p and n-type contacts on device efficiency. I will furthermore highlight the areas of the perovskite solar cell susceptible to instabilities under operation, and how the correct choice of materials and device architecture can lead top exceptionally stable so cell operation.

The most straightforward method to surpass the crystalline silicon (c-Si) single-junction solar cell efficiency limit is to stack a high-band gap top cell on a c#8209;Si bottom cell in a tandem solar cell. For many years, no appropriate low-cost high-efficiency top cell for Si-based tandems was available. This situation changed drastically with the emergence of perovskite solar cells. Interestingly, the bandgap energy of CH₃NH₃PbI₃, the perovskite with the highest reported solar cell efficiency so far, is around 1.55 eV [1]. As a consequence, the spectral sensitivity as well as the high open-circuit voltages of CH₃NH₃PbI₃ make it also attractive as top cell material for c#8209;Si-based tandems [2].
In this paper, we outline our steps taken towards the integration of a perovskite cell on top of a c#8209;Si bottom cell. Firstly, in a tandem device, all involved layers of the perovskite top cell should exhibit excellent infrared (IR) transparency in order to utilize the potential of the c-Si bottom cell. For this purpose, we measured the absorption spectra of CH₃NH₃PbI₃ using ultra-sensitive photo-thermal deflection spectroscopy, and observed that the sub-bandgap absorption of this material is remarkably low. Secondly, based on the experimentally determined absorption coefficient, we have built an optical model of perovskite/c-Si tandems, enabling us to assess the limiting efficiency of CH₃NH₃PbI₃/c-Si tandems. Thirdly, we replaced the metal back contact in our perovskite devices by an IR transparent contacting material and realized first tandem test devices. Our IR transparent perovskite top cell utilizes the common device architecture described in [3]: Fluorinated tin oxide (FTO) together with a compact and a mesoporous TiO₂ layer as electron-selective, and Spiro-OMeTAD as hole-selective contact. To achieve IR transparency, we replaced the metal back electrode by a transparent conductive oxide (TCO) with excellent transparency from 350-1200 nm. With this, we constructed a first experimental four-terminal tandem test device. Up to 65% of the incident photons below the top cell bandgap are transmitted to the c-Si bottom cell. However, the tandem is strongly affected by parasitic absorption in the top cell. We identify this loss as being caused by free-carrier absorption in the highly doped TCO layers. As doping of the TCOs is required in four-terminal tandems to provide lateral conductance, but at the same time induces absorption of IR light, the efficiency is a delicate trade off depending on the TCOs figure of merit.
Based on these results, we address current technological bottlenecks for tandem integration and large-scale fabrication of perovskite/c#8209;Si tandem modules targeting ultra-high conversion efficiencies.
[1] T. Baikie, et al., J. Mater. Chem. A, vol. 1, p. 5628, 2013.
[2] S. De Wolf, et al., J. Phys. Chem. Lett., vol. 5, p. 1035, 2014.
[3] J. Burschka, et al., Nature, vol. 499, p. 316, 2013.

With rapid increase of efficiency from 3.8% to 17.9%, hybrid organic-inorganic perovskite solar cells have attracted much attention because of the excellent properties of organo-lead halide perovskite materials, which makes perovskite solar cells to be a promising candidate of next generation photovoltaic technology. However, the employment of organic hole transport materials and noble metal counter electrode increased its material cost and added the complexity of manufacture process, which has been regard as a potential hurdle for large-scale production and practical application. It is desirable to develop perovskite solar cells using low-cost materials with simple manufacturing process. Here, we reported perovskite heterojunction solar cells, fabricated with fully screen printing method in atmosphere environment using mesoporous TiO₂ as electron transport layer, mesoporous ZrO₂ as spacer layer and mesoporous NiO as active interfacial layer, respectively, whose pore was penetrated perovskite (CH₃NH₃PbI₃) as light absorber, achieving a promising power conversion efficiency of 12.4% when measured under AM1.5G illumination. This fully printable device with all-inorganic materials offers a viable pathway to develop efficient low-cost solar cells with attractive properties for scale up and practical applications.

Perovskite solar cell is like the jewel in the crown of solar cell research because of its stunning surge in the past few years. Various approaches have been carried out to produce perovskite films either in mesoporous or planar structures. Common perovskite deposition approaches include one-step solution process (stoichiometric or non-stoichiometric, various additives, fast crystallization), two-step process, vacuum deposition, just to name a few. Two-step sequential solution process generally requires the deposition of a uniform, compact PbI₂ film, which is subsequently converted to perovskite film through a second-step conversion with methylammonium iodide. It can be divided into three different categories based on media used in the second step: liquid solution based dipping, vapor assisted conversion, and solid state inter-diffusion. All solution based two-step process possesses the advantage of low-temperature process and scalability readiness compared to other two approaches. However, a main challenge with the conventional two-step solution process is the incomplete conversion of PbI₂ to perovskite especially in the planar structure. Here we demonstrate a modified two-step solution process to convert PbI₂ much more efficiently and completely, especially for the planar perovskite structure. In addition, the morphology, optical property, and charge transport of perovskite from this modified method are improved significantly. Devices based on the modified two-step solution process both in mesoporous and planar structures show a higher performance compared to the conventional approach. These results and others will be discussed. This modified two-step solution approach could open up a new avenue for preparing low-temperature processed high-quality perovskites.

Very recently a generation of mixed organic-inorganic lead-halide perovskites materials has emerged as a promising solution for solar cells with power conversion efficiencies exceeding 19%. Moreover, after revolutionising photovoltaics, organo-metal halide perovskites are now emerging as viable materials for other optoelectronic applications such as light emitting devices and lasers. Thus, the understanding of the physics underlying perovskite&’s photophysics and in particular the origin and the dynamics of photoluminescence (PL) has become of critical importance.
It has already been shown that the light emission in these materials is dominated by free charge carriers recombination as in conventional 3D semiconductors with PL dynamics characterized by a recombination rate showing a bi-molecular character. However, the optoelectronic properties of perovskite have shown to be strongly dependent on the thin film morphology, at molecular and mesoscopic level, mainly determined by different processing conditions.
Here we address the complex relationship between the morphological and optoelectronic properties. In particular we show that when MAPbI3 crystallites grows in a nanometre size porous scaffold present a shorter-living and blue-shifted emission with respect to the perovskite crystals which are free to grow without any constraints. While, at first glance, this effect could be assigned to an increased non-radiative decay channels created by larger grain boundaries, we show that it is also associated to a change in the intrinsic radiative recombination rate. By varying average crystal dimension we observe that there is a consistent red shift of the optical band-edge as the crystal dimension is increased, and through Raman spectroscopy we assign this effect to a relaxation of the I-Pb-I bending angle. In addition, we observe a correlated increase in the PL lifetime with the crystal size. Intensity dependent and temperature dependent time resolved photoluminescence measurements demonstrate that the increase in the lifetime is indeed related to the change in the intrinsic radiative rate that is driven by the change in the bandgap within different crystal morphologies. Finally we have proven that this also directly affects the thin film optical gain, with consequences in the optimization of lasing devices.

Until now, organohalide lead perovskites of AMX₃ structure have been mostly focused on fabricating high-performance hybrid lead based solar cells have efficiencies close to 20%.^[1] With this rapid development, a photovoltaic technology capable of large-scale production for commercialization is needed. At the same times, environmentally benign materials during manufacturing steps need to take into consideration.
In this work, we first introduce a new class of lead-free cubic crystals taking the form A₂MX₆ (A= Cs, Rb, CH₃NH₃; M= Sn, Ge; X=F, I, Cl, Br) as a hole transporter and then a sensitizer. While in AMX₃ structure, Sn and Ge have been used in their +2 oxidation state, the A₂MX₆ compounds have a high symmetry cubic structure with good air and moisture stability in the +4 oxidation state.^[2,3]
We demonstrate here that Cs₂SnI₆ can serve as a hole transporting material (HTM) in a dye sensitized solar cell. In this way, a Z907(dye)/Cs₂SnI₆/DSSC delivers about 4.7% of energy conversion efficiency. By substituting a more efficient mixture of N719 with YD2-oC8 and YDD6 dyes, we further increase the efficiency of our cell to nearly 8% with photon confinement.
At the same time, we are developing a simpler architectural solar cell using electrospray technique for solar light harvesting. In this system, costly gold and organic hole transporting materials (HTMs) were replaced by a novel large-effective-surface-area polyaromatic hydrocarbon.^[4] The resulting device based Cs₂SnI₄Br₂ has an initial V_oc = 0.461 V, J_sc = 7.34 mA/cm², and FF = 69.8%, and an efficiency = 2.36%. Continuing efforts are under way aimed at optimizing the coating conditions to further improve the cell conversation efficiency.
1. Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y., Science 2014,345 (6196), 542-546.
2. Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G.,Inorganic Chemistry 2013,52 (15), 9019-9038.
3. Lee, B.; Stoumpos, C. C.; Zhou, N.; Hao, F.; Malliakas, C. D.; Yeh, C.-Y.; Marks, T. J.; Kanatzidis, M. G.; Chang, R. P. H., Journal of the American Chemical Society 2014.
4. Lee, B.; Buchholz, D. B.; Chang, R. P. H., Energy & Environmental Science 2012,5 (5), 6941-6952.

The certified efficiency of perovskite solar cells is now 17.9% and is reaching that of inorganic solar cells such as 20.4% of poly Si solar cells. According to our simulation, 20.4 % efficiency is obtained (IPCE:0.8 and FF:0.7) by using photo-absorber with 900nm absorption spectrum edge, when 0.3 V was taken as the voltage loss from the ideal voltage. We focused on infrared-photoconversion, because the infrared-responsible solar cells should be useful for the bottom cells of the tandem cells and for enhancing the efficiency of single cells by harvesting light up to 900 nm after the band gap tuning.
It is well-known that CH₃NH₃SnI₃ Perov Sn) has absorption in the area of infrared. However, it was difficult to handle the Perov Sn in open air condition. We found that the mixture of Sn perovskite and Pb perovskite (CH₃NH₃Sn_(0.5)Pb_(0.5)I₃, Perov Pb/Sn) has improved stability under air with maintaining the photoconversion in the infrared area. The IPCE of the 950nm was 0.2. However, it was found that the absolute photoconversion efficiency (IQE: Internal quantum efficiency) was 0.75 (75%). The low IPCE in the infrared area was explained by the loss of light uptake. Therefore, it was concluded that the photon confinement in the layer of Perov Pb/Sn is needed for enhancing the conversion. The charge recombination between electrons in porous titania and P3HT for Perov Pb and that in Perov Pb/Sn(1:1) was 600mu;sec and 30mu;sec respectively. In order to retard the charge recombination, porous titania was passivated by wide gap oxide thin layers such as Al₂O₃, ZrO₂ and Y₂O₃. Among then, Y₂O₃ passivation gave the best results. Urbach tail measured by Photo Acoustic Spectroscopy was compared between Perov Pb/Sn and Perov Pb and it was found that the Urbach tail for the Perov Pb/Sn was lower than that of Perov Pb. The tail and band gaps of Perov Pb/Sn varied, depending on the substrates from which Perov Pb/Sn crystals grow, suggesting that Perov Pb/Sn has gradient structures affected by the surface of the substrates and Sn:Pb ratio changed from the bottom layer to the top layer. The relationship between the structure of Perov Sn/Pb and the substrate surface is reported in detail.

Lead-based halide perovskites with the chemical formula of APb(II)X₃ (A = Cs, CH₃NH₃, or CH₂NH=CH₂; X = I, Br or Cl) have attracted enormous interest for solar cell applications, primarily due to the high photoelectric conversion efficiency (PCE) up to 19.3%. However, the stability and toxicity are the major issues restricting the commercialization of these lead perovskites based PVs. ASn(II)X₃ perovskites are lead-free, but they are extremely sensitive to the ambient air. To resolve the issues, an air-stable class of perovskite materials with the chemical formula of A₂Sn(IV)X₆, a defect variant of the ASn(II)X₃, has been very recently introduced to PV applications. Particularly, Cs₂SnI₆ exhibits an ideal bandgap of 1.26 eV, high carrier mobility, and have demonstrated high PCE up to 7.8%. On the other hand, the electronic structure and the chemical bonding nature of Cs₂SnI₆ have not been understood properly; i.e., Sn in Cs₂SnI₆ is expected to be the +4 charge state upon assumption of Cs⁺ and I^-, which would lead to expectation that the conduction band minimum is formed by Sn 5s similar to SnO₂.
Here, we performed hybrid density functional theory calculations with the HSE06 functionals for Cs₂SnI₆. We found that the band gap is formed mainly by anti-bonding and bonding states of I 5p - I 5p slightly hybridized with Sn 5s because the Cs₂SnI₆ structure is composed of isolated Cs ions and [SnI₆] clusters. Unexpectedly, the Sn 5s state is very deep, ~7 eV deeper than the I 5p valence band (VB), and has little contribution to the VB; while, Sn 5s has more contribution to the conduction band. As a consequence, the charge state of Sn is not +4, but much closer to +2 like in CsSnI₃ and SnO. Then, the apparent charge state of I is a bit smaller than -1 and to be of -4/6, which difference comes from the direct covalent bonds between the adjacent I atoms in the [SnI₆] cluster. We also studied the defect physics and revealed the origin of ambipolar conduction in Cs₂SnI₆.
These results break the conventional common sense about chemical bonding nature in ionic semiconductors and provide a guiding principle to design new pervoskite-based PV materials.

Organolead-halide-perovskite solar cells containing absorber materials, with general formula (CH₃NH₃)PbX₃, are a recent breakthrough in the photovoltaic research field [1]. Thanks to a close to ideal band-gap (1.55 eV) of the lead based perovskite, its high absorption coefficient, low exciton binding energy and high electron and hole diffusion lengths, power conversion efficiencies greater than 19% have already been reached [2]. Perovskite thin films are low-temperature and solution processable from low cost precursurs, which makes this technology viable for commercialisation in large scale production. The main drawback of perovskite sensitizers is the high lead content, a material which is toxic, polluting and bio-accumulates in the eco-system. Thus, it is highly desirable to find suitable replacements to lead. Bi-dimensional perovskites with general formula (CH₃NH₃)₂MX₄, where M is a transition metal, are possible green alternatives to lead based perovskites [3].
In this work we present the synthesis and characterization of novel bi-dimensional (2D) Cu-based hybrid perovskite for photovoltaic applications. The series (CH₃NH₃)₂CuCl_4-xBr_x was synthetized, and the optoelectronic properties and thin film processing conditions were deeply investigated in view of integration into photovoltaic devices. We observed a strong effect of chlorine in the stabilization of the material against Cu²⁺ reduction. The optical properties are strongly dependent on the Br/Cl ratio, allowing tuning of the band-gap associated to metal-to-ligand charge transfer (CT) transitions from 2.48 eV to 1.8 eV by increasing Br content. Additionally, d-d transitions between 700-900 nm are found to further contribute to the absorption, in agreement with ab-initio calculations. Preliminary solar cell results highlighted the strong interdependence of morphological and electronic properties of (CH₃NH₃)₂CuCl_4-xBr_x films. Charge carrier generation, dynamic and recombination in this class of new lead-free materials will be investigated by steady-state and ultrafast optical and photocurrent spectroscopy, to clarify the factors affecting photovoltaic performance.
[1] M. A. Green, A. Ho-Baillie, and H. J. Snaith, "The Emergence of Perovskite Solar Cells," Nat Photon, 8[7] 506-14 (2014)
[2] Zhou, Q. Chen, G. Li, S. Luo, T.-b. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu, and Y. Yang, "Interface Engineering of Highly Efficient Perovskite Solar Cells," Science, 345[6196] 542-46 (2014).
[3] Ziyong Cheng, Jun Lin, “Layered organic-inorganic hybrid perovskites_structure, optical properties, film preparation, patterning and templating engineering CrystEngComm” , 12, 2646-2662, (2010)

The development of all solid-state thin-film solar cells has reached a new milestone when the devices made of organometallic lead halide perovskite materials were reported with power conversion efficiency (PCE) exceeding 19 %. The key issue to make a device with a great photovoltaic performance for perovskite solar cells is to control the film morphology of perovskite under different experimental conditions. Diverse processing techniques were reported according to either a one-step or a sequential method to synthesize the required perovskite layer on top of the contact electrode with either a mesoscopic or a planar interface. In this lecture, I will demonstrate how the film morphology of perovskite can be controlled via varied synthetic approaches. For example, the perovskite layer can be produced under the condition of fast crystallization deposition using either toluene or chlorobenzene as an anti-solvent to induce a fast crystallization. The other approach is to use a proper additive such as hydroiodic acid (HI) to produce homogeneous precursor solutions prior to the following spin-coating step. Without adding the HI additive, one-dimensional dendroid microcrystals were produced with a poor surface coverage. When the HI additive was added in the perovskite stock solution, uniform and pinhole-free perovskite nanocrystals with full surface coverage were observed. For a p-type planar device ITO/PEDOT:PSS/CH₃NH₃PbI₃/PCBM/Ag fabricated using such a synthetic approach to generate the CH₃NH₃PbI₃ layer, the power conversion efficiency attained 8 % with the short-circuit current density over 18 mA cm^-2. Photo-induced absorption (PIA) spectra and nanosecond transient absorption (ns-TAS) kinetics were also performed to understand the electron-hole recombination rates responsible for the corresponding device performances.

The photovoltaic community has witnessed a unprecedented and stunning emergence of the new type of lead halide perovskite based solar cells, whose efficiency has reached to beyond 17% up to date and has become promising for commercialization. Meanwhile, research has shown great properties for high absorbance coefficiencts, weak exciton binding energy and high carrier mobilities for perovskite and long lifetime and diffusion length of charge carriers in the device. However, despite of the great increase of device performance, understanding of the working mechanisms of the device in terms of charge generation, transfer and recombination process has still be limited.
In this work, transient photovoltage/current behavior was investigated in details on mesoscopic and thin film perovskite devices, in order to reveal the charge transfer properties of the devices. Fast phtovoltage/current decay and shorter lifetime of charge carriers in mesoscopic perovskite with thicker capping layer suggest the effectiveness of using mesoscopic oxide semiconductor as the selective contact for charge injection.
In addition, charge extraction measurement was conducted to reveal the charge accumulation of devices under illumination and to study the
kinetics of electron decay at open circuit by varying the delay between interrupting the illumination and short-circuiting the cell. Dependence of the decay process on the total electron concentration will be discussed.
Lastly, the anomalous hysteresis effect observed in perovskite based devices has not been fully understood yet. Attempts to elucidate this hysteresis effect from the ferroelectricity and ion diffusion pespective have been carried out. With the results obtained, better understanding of the charge transfer and recombination process and the hysteresis effect, we expect it's benefitial for future further optimization in order to realizing the full potential of the perovskite material in photovoltaic devices.

Recently, perovskite solar cells have attracted intense research interest due to the rapid rise in the experimentally measured solar energy power conversion efficiency. The light generated carrier dynamics as well as their corresponding spectral features remain relatively unstudied. Here we investigated charge carrier dynamics in planar CH₃NH₃PbI_3-xCl_x films by transient absorption (TA) spectroscopy. We find that the hot carrier cooling process exhibits strong dependence on the carrier density. The hot carrier cooling time can vary from <1ps to >100 ps depending on the carrier density. Based on the dynamic Burstein-Moss shift model, we attribute the TA bleach near the bandgap to the band filling effect and also correlate the bleach signals with the carrier density so as to determine the carrier recombination dynamics from the bleach recovery. The monomolecular recombination (first order), electron-hole bi-molecular recombination (second order) and Auger recombination (third order) rate are all resolved from the excitation intensity dependent measurement. Thus, we estimate the time constant of each recombination channel under solar photon flux, suggesting that the monomolecular recombination rate is at least 3 orders of magnitude faster than the other two recombination channels.

Perovskite solar cells are driven a paradigm shift in photovoltaics (PV) since they are breaking with the existing tradeoff between efficiency and fabrication cost.^1,2 A little over two years this solar cell technology is at the top of the emerging PV efficiency race, reaching certified values that surpass all the ones attained in cells based on solution-processed materials.
Herein we provide a full description of the optical behavior of perovskite solar cell devices. We performed an in-depth experimental and theoretical analysis of the optical effects occurring in state-of-the-art solution-processed perovskite cells, i.e. organic-inorganic metal halide perovskite, prepared using a one-step deposition method, sandwiched between an n-type and p-type charge collection layer. We developed a theoretical model using a method based on the transfer matrix formalism,³ which allows us to calculate the electric field intensity within the layered structure and thus to visualize the effect of each layer comprising the cell on the spatial distribution of the radiation in the PV device. To check the suitability of the proposed theoretical model, we fitted the experimental reflectance, transmittance and absorptance spectra of devices measured at normal incidence. The layout of the device determines the field intensity distribution along the cell and hence the spatial and spectral distribution of optical absorption in the device. This allows us to estimate the amount of light that is captured by each absorbing layer in the perovskite cell, and hence to discriminate between productive (absorbed in the perovskite material) and parasitic (absorbed in other components such as FTO, or the metallic contact) absorption. Our work also provides a theoretical framework in which the optical response of solar cells integrating photonic enhancing components can be rationalized. Also, as demonstrated in other PV technologies, photonic designs may lead to resonant photocurrent generation⁴ to improve the efficiency of the device.
References:
1. Lee, M.M., Teuscher, J., Miyasaka, T., Murakami, T. N., & Snaith, H. J. Science338, 643-647 (2012).
2. Liu, M., Johnston, M. B. & Snaith, H. J. Nature 501, 395-398 (2013).
3. Lozano, G., Colodrero, S., Caulier, O., Calvo, M. E., & Míguez, H. J. Phys. Chem. C114, 3681-3687 (2010).
4. Anaya, M., Calvo, M. E., Luque-Raigoacute;n, J. M. & Míguez. J. Am. Chem. Soc. 135, 7803-7806 (2013).

Organometal trihalide perovskites have recently emerged as one of the most promising class of absorber materials for high efficiency solar cells, with power conversion efficiency (PCE) above 15% being consistently achieved even in a simple planar thin film configuration. These outstanding performances enable for the fabrication of devices incorporating very thin absorber layers, which are semitransparent in the visible spectrum and still provide enough power conversion. Such devices are particularly appealing for building integrated photovoltaics and for multi-junction (tandem) structures.
In this talk, we will focus on the fabrication of high efficiency, semi-transparent perovskite solar cells. The two key areas of discussion will be (i) optimization of the deposition method to enable a reduction of the perovskite layer thickness down to ~50 nm without major structural defects (such as pinholes), and (ii) the use of a multilayer transparent top electrode to replace the commonly adopted thick metal cathode of non-transparent devices. We will discuss in details how the structure of the transparent top electrode and the thickness of its different layers affect its transparency and the device performances, with a particular focus on the role of the interface between the hole transport material (Spiro-OMeTAD) and the composite electrode.
Through this effective combination of processing and device architecture developments, we have fabricated semitransparent perovskite solar cells with 13.6% PCE and an average visible transmittance (AVT) of 7% through the whole device. By controlling the absorber layer thickness we have obtained devices with 10.1% PCE (at 16% AVT), 8.8% PCE (at 19% AVT) and 5.3% PCE (at 31% AVT), respectively. These devices exhibit the highest reported PCEs at every AVT level assessed to date for semitransparent perovskite cells.

Perovskite solar cells (PSCs) have been intensively studied by using methylammonium lead tri-iodide (MAPbI₃) after the report of high efficiency cell fabrication [1]. MAPbI₃ has superior light absorption property, high carrier mobility, and appropriate band structures for TiO₂ as its PSC has reached efficiency close to 20% [2]. Because photocurrent output of MAPbI₃-based PSC almost reaches the theoretical limit (~25 mA/cm²) on sunlight absorption up to 800 nm, development of new absorbers are required for further improvement of efficiency. In this respect, we have investigated formamidinium-based perovskite (FAPbI₃) that absorbs in longer wavelengths compared to MA based perovskite [3,4]. Difficulty of optimizing the solution processing FAPbI₃ and cell fabrication has so far encouraged limited number of research in spite of the potential for higher conversion efficiency. Recently, we conducted research by systematic observation on the effects of precursor materials of FAPbI₃ and meso-porous layer in PSC device in order to improve the photovoltaic characteristics of FAPbI₃-based PSCs.
We used PbCl₂ as a precursor of FAPbI₃ perovskite based on our experience that chloride ion improves the quality of perovskite films of MAPbI₃, leading to higher conversion efficiency and durablity [1]. In this report, we show how the impact of ratio of precursor materials, film deposition, and annealing conditions can change the perovskite film morphology and photovoltaic properties. Further, we will demonstrate the influence of meso-porous scaffolds on perovskite film morphology and photovoltaic performance including the hysteresis behavior in current-voltage characteristics.
References
[1] M. M. Lee, et al., Science, 2012, 338, 643; [2] H. Zhou, et al., Science, 2014, 345, 542; [3] G. E. Eperon, et al., Energy Environ. Sci., 2014, 7, 982; [4] J.-W. Lee, et al., Adv. Mater., 2014, DOI: 10.1002/adma.201401137

Over the past two years, organic/inorganic hybrid halide perovskite (e.g., CH₃NH₃PbX, X=Clminus;, Brminus;, Iminus;), have made significant impact on photovoltaic research field because of their dramatic rise in the power conversion efficiency (PCE) to over 17%. Many studies have been carried out on the performance and optimization of the perovskite solar cells. Zhou et al. reported that planar cell PCE was boosted to 16.6% on average by interface engineering,¹ Nam Joong Jeon et al. demonstrated that solvent engineering enabled the fabrication of remarkably improved solar cells with a certified PCE of 16.2% and no hysteresis,² Jeong-Hyeok Im et al. suggested that average efficiencies of 16.4% (s.d. ± 0.35) can be achieved by a two-step size-controlled growth method of the CH₃NH₃PbI₃ cuboid species³ and so on. In this work, an oxygen treatment method was developed for p-i-n two-step method planar perovskite solar cell. In order to understand the O₂ diffusion and interaction in planar perovskite solar cell, the impact of tuning the O₂ post-annealing temperature on the performance of planar heterojunction devices is investigated. Interestingly, we find that the performances of O₂-treated devices are very sensitive to the change of the O₂ post-annealing temperature. Through systematic control of the treatment process, high PCEs of above 15% is achieved under the optimal post-treating temperature (65#8451;) with good reproducibility. To specifically identify and elucidate the mechanism underlying the observed phenomenon, a series of optical and electronic characterizations were performed, e.g. impedence spectroscopy (IS), the time-resolved photoluminescence decay (TRPL), photothermal deflection spectroscopy (PDS), low frequency noise (LFN). Our experimental results indicated significant reduction in the defect density of the perovskite film due to the O₂ annealing process. This phenomenon accounts for the observed improvement in the PCE of the device.
References
1. Huanping Zhou, Qi Chen, Gang Li, Song Luo, Tze-bing Song, Hsin-Sheng Duan, Ziruo Hong, Jingbi You, Yongsheng Liu, Yang Yang, Science2014, 345, 542
2. Nam Joong Jeon, Jun Hong Noh, Young Chan Kim, Woon Seok Yang, Seungchan Ryu and Sang II Seok, Nature Mater.2014, 897-903
3. Jeong Hyeok Im, In Hyuk Jang, Norman Pellet, Michael Gratzel and Nam Gyu Park, Nature Nano.2014, 181

Thin-film photovoltaics based on organo-metal halide perovskite absorbers have recently emerged as a new generation of solution process with low-cost and high power conversion efficiency of around 20% in just a few years. Though progress in device efficiencies has been remarkable, the lead-halide perovskite fabricated by chemical solution deposition have as main drawback the instability of the material to atmospheric moisture. In addition, films produced by the conventional spin-coating methods were found to have many uncovered pin-hole areas. To date, the CH₃NH₃PbI₃ layer in the most efficient planar solar cells has been fabricated by either vapor deposition, a two-step sequential solution method, or a vapor-assisted solution process. In this work, we studied on the coverage improvement and humidity stability of films. We have deposited CH₃NH₃I and PbI₂ layer by using RF magnetron sputtering (physical deposition method) which forms a compact perovskite film that is significantly more uniform than that of the film produced by spin-coating. The grain size and roughness of CH₃NH₃PbI₃ layer on ITO substrate were 500 nm and 20 nm, respectively. Also, the CH₃NH₃PbI₃ film without shielding had almost not changed over time.

Using density functional theory (DFT)-based calculations, we explore the extent to which achievable modes of structural modification can tune the near-gap electronic structure of tin and lead halide perovskites with applications in dye-sensitized solar cells. We show that regardless of how atomic layering is achieved - whether by the growth of layered inorganic phases such as the Ruddlesden-Popper series, hybrid perovskites connected by organic linker molecules, or layered perovskite heterostructures - their band gaps can similarly be widened by several tenths of an eV or more. Furthermore, subjecting perovskites to moderate amounts of epitaxial strain can controllably tune band gaps to either larger or smaller values. When accompanied by the types of structural distortions known to exist in these materials at varying temperatures, the landscape in which to tune band gaps and band edges becomes even richer. Because tin and lead halide perovskites are known to have band gaps spanning much of the visible region of the solar spectrum, the ability to controllably tune their near-gap electronic structure could further optimize their performance in solar energy conversion applications. Throughout our work, trends in band gap are explained based on the effects of atomic layering, quantum confinement, strain, and structural distortions on the character and energy of band-edge crystal orbitals.

Since the first report on the long-term durable perovskite solar cell in 2012, perovskite solar cells have been studied intensively with worldwide attention for potential energy device. As a result, power conversion efficiency (PCE) of perovskite solar cells greater than 17% were reported in 2014 with potential for further improvement in device performance. However, one of critical challenges of organometallic halide perovskite materials is durability and environmental concern in water and humidity condition. Therefore, protection of perovskite solar cells under outdoor operating conditions is important for long term stability. We demonstrate a simple and versatile method for the long term stable self-cleaning perovskite solar cells with structural light trapping effect using lotus leaf-inspired hierarchical pyramidal arrays. The method consists of replica molding of micro pyramid structures and Ar ion surface treatment. Fabricated hierarchical pyramidal PDMS film by micromolding and Ar ion surface treatment was easily attached to/detached from a bare glass, which is substrate of perovskite solar cell device, without any additional adhesives. Such robust and flexible PDMS films were turned out to have anti-reflection property with structural advantages of the pyramid shapes, leading to an improvement of PCE from 13.12% to 14.01%. Furthermore, the film demonstrated excellent self-cleaning behavior due to superhydrophobicity. It enables perovskite solar cells to protect from environmental contamination. This bifunctional property has feasibility potential for a practical application of photovoltaic technology including perovskite solar cells.

Titanium di oxide (TiO₂), a wide band gap semiconducting material, has attracted wide attention due to its chemical stability, high resistance to photo corrosion, non-toxicity and low cost. TiO₂ acts as a light absorbing material and an electron transporting material for the third generation solar cell and TiO₂ based solar cell has shown efficiency of ~17.1 % and ~13% for pervoskite and DSSC solar cell, respectively [1,2].
The band gap of TiO₂ is large ~3.2 eV which limits, it ability to absorb only 4-5% part of solar spectrum. The band gap tuning of TiO₂ can be expected to enhance its photovoltaic performance. Recently it is demonstrated that rutile phase TiO₂ based solar cell shows better performance as compared to anatase phase TiO₂ solar cell [3]. The band gap of TiO₂ can also be tuned by doping of elements.
In the present work, we have prepared nanostructured TiO₂ and co-doped (N (nitrogen), Nd (neodymum)) TiO₂ by hydrothermal method and compared the photovoltaic performance of TiO₂/CH₃NH₃PbI₃ and co-doped TiO₂/CH₃NH₃PbI₃ based solar cell. Structural, optical, and compositional characterization of nanoparticles confirms the single phase formation of anatase TiO₂. The doping of N and Nd has been observed to reduce the band gap of TiO₂. Optical band gap for the nanostructured TiO₂ has been obtained as ~3.2eV whereas for the nanostructured co-doped TiO₂, the optical band gap ~2.5eV. The band gap reduction of TiO₂ nanoparticles by the doping of N and Nd is due to formation intra-gap states between the valance band and conduction band. The performance of co-doped TiO₂/CH₃NH₃PbI₃ solar cell is better as compared to the TiO₂/CH₃NH₃PbI₃ solar cell. The boosts in the performance has been attributed to the lower band gap of nanostructured co-doped TiO₂ and lowering of Fermi level, which decreases the open circuit voltage, but increases the short circuit current, fill Factor and leads to improvement in the overall solar cell efficiency.
References
[1] J-H Im, In-H Jang, N. Pellet, M Grätzel and N G Park, Nature Nanotechnology (2014) (doi:10.1038/nnano.2014.181)
[2] S Mathew, A Yella, P Gao, R H Baker, B F Curchod, N A Astani, I Tavernelli, U Rothlisberger, M K Nazeeruddin and M Grätzel Nature Chemistry 6, 242 (2014)
[3] J-W Lee, T-Y Lee, P J Yoo, M Gr#168;atzel, S Mhaisalkar and N G Park, Journal of Materials Chemistry A 2, 9251 (2014)

Because titanium dioxide has various crystal phases which affect activity of photo-functional materials, such as photocatalyst, photovoltaic cell, research of controllable synthesis of crystal is not only aimed at crystal phase but also crystal face that occupies the surface of TiO₂. The latter became particularly important for preparation of functional nanoparticle. Recently, a characteristic decahedral shape TiO₂ nanoparticle with high rate of anatase (001) face on the surface was synthesized and has been attracting considerable attention. It was known that the (001) plane of decahedral TiO₂ promotes photocatalytic decomposition of organic pollutants since surface energy of (001) face is higher than that of (101) face. Additionally, photoinduced electrons transfer is known to occur quickly between (001) plane and surface modified metal ions or absorbed dyes.
In this report, we show preparation of thin films composed of 2D oriented array of decahedral TiO₂ nanocrystal, which was synthesized on a fluorine-doped tin oxide (FTO) glass substrate by simple hydrothermal reaction without using hydrofluoric acid. Decahedral TiO₂ particle was grown in direction perpendicular to the surface of FTO substrate to give a tabular shape, in which (001) plane occupies the side of tabular grain facing the inside of internal pore. These oriented decahedral TiO₂ films were applied to methylammonium lead halide perovskite solar cells as a scaffold of perovskite crystal formation as well as semiconductor hole blocking layer for investigation of the influence of TiO₂ crystalline orientation on photovoltaic properties.

Organo-lead-halide perovskite (OHP) based solar cells were reported to achieve energy-to-electricity power conversion efficiency as high as ~19.3%,¹ which combined with reported methods for low-cost, flexible, and large-area solar cell fabrication technology makes OHP cell technology amenable to scaling up to production levels. Methylammonium (MA) lead iodide (CH₃NH₃PbI₃) and mixed lead iodide chloride (CH₃NH₃PbI_3-xCl_x), the most commonly employed material in halide perovskite solar cells, were reported to have both high absorption coefficient (direct bandgap of ~1.55 eV) and high mobilities for electrons and holes resulting in long carrier diffusion lengths (100 nm - 3 mu;m).² Despite all the superb properties, perovskite solar cells suffer from strong hysteresis in the current-voltage (I-V) measurements typically conducted under AM1.5G illumination. In this work,³ we performed staircase voltage sweep measurements at three different temperatures of 250 K, 300 K, and 360 K to quantify the photocurrent transient behavior on the complete perovskite cell composed of FTO/TiO₂/MAPbI_3-xCl_x/spiro-MeOTAD/Au. Our photocurrent data suggest multiple charging-discharging processes take place within the perovskite cell. Semi-logarithmic plots of the photocurrent responses reveal a linear regime showing the slowest transient process (well-defined mono-exponential trend) with a time constant (tau;_slow) in the order of seconds. This process was attributed to the polarization response of the perovskite layer. I-V curves under steady-state conditions were extracted from the transient photocurrent data. The hysteresis effects were smaller at 360 K and higher at 300 K and 250 K. On the basis of our study, to compare the results from different laboratories, it is essential to establish a protocol for extracting hysteresis-free steady sate I-V curves on perovskite solar cells. The extrapolation method used in this work to extract the steady-state photocurrents is suggested as a possible method.
(1) H. Zhou, Q. Chen, G. Li, S. Luo, T.-b. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu and Y. Yang, "Interface engineering of highly efficient perovskite solar cells", Science345, 542-546 (2014).
(2) M. Gratzel, "The light and shade of perovskite solar cells", Nature Mat.13, 838-842 (2014).
(3) L.K. Ono, S.R. Raga, S. Wang, Y. Kato, Y. Qi*, "Temperature-dependent hysteresis effects on perovskite-based solar cells", submitted to J. Mater. Chem. A (2014).

Metal oxide porous layer is the one of the most important components in the energy conversion devices. Generally, it played many important roles of a charge transport channel, scaffold for loading of light absorbing materials, and electron-hole separator. Among the various metal oxide materials, TiO₂ is the one of the most worldwide researched metal oxide materials for energy applications, i.e. dye-sensitized solar cells (DSSCs), quantum dot solar cells (QDSCs), photoelectrochemical water-splitting devices, photocatalysis, and lithium ion batteries due to its superior physical and chemical properties.
Recently, perovskite solar cells, based on state-of-the-art light harvesting materials, are one of the most promising photo-energy conversion devices, which show the rapid efficiency increment up to 19.3% during last two years. Perovskite solar cells have two different common types which are based on mesoscopic heterostructure with metal oxide porous layer and planar heterojunction (PHJ) structure. In the former structure, mesoporous-TiO₂ (mp-TiO₂) is used as efficient electron transfer layer as well as scaffold layer for the formation of perovskite film. Moreover, compared to the PHJ structure, the mp-TiO₂ based solar cell exhibited the relief of I-V hysteresis problem. So far, the effect of TiO₂ particle size on the relief of hysteresis has not been clearly understood. In this presentation, we controlled the particle size of TiO₂ using two-step sol-gel method and investigated the hysteresis behavior of solar cell. This study will give a clue to solve the hysteresis problem of perovskite solar cell.

In the past two years organometal halide perovskites have emerged as a revolutionary photovoltaic material due to its high power conversion efficiency (certificated record ~17.9%) and solution processability. Despite the rapid improvement in device performance, many fundamental questions regarding the structure-performance correlation of the perovskite solar cells remain unclear. In particular, the ubiquitous hysteretic phenomenon that the forward scanning curves (from Jsc to Voc) are superior to the reverse scanning curves (from Voc to Jsc) in the I-V measurements of the solar cells have received much attention. Three possible origins have been proposed for such device behavior: ferroelectric properties of the perovskite films, migration of interstitial defects (excess CH₃NH₃⁺ or I^- ) in the perovksite films, and poor charge extraction at the interfaces between the perovskite films and transport layers. In this work we developed a protocol to systematically control the morphology (e.g. grain size) and composition (e.g. dopants) of the perovskite films from the initial precursor solutions. We then correlated these material parameters with the hysteresis characteristics in two planar structure device architectures, TiO₂/perovskite/sprio-OMeTAD and PEDOT:PSS/perovskite/n-type organic semiconductors. The temperature-dependent I-V measurements and electrochemical impedance analysis reveal that the hysteresis variation manifests an interplay between the interface traps and bulk impurities in the perovskite solar cells.

Perovskite solar cells are composed of organic-inorganic hybrid structure (CH₃NH₃PbI₃) due to the high extinction coefficient, the wide range of absorption, and the excellent electrical properties.^[1] Mesoscopic perovskite solar cells contain mesoporous (mp) -TiO₂ nanoparticle layer for electron transport and collection, CH₃NH₃PbI₃ for light harvesting, and hole transport materials. Power conversion efficiency of perovskite has reached 17.9% ^[2] and it is expected to touch down the efficiency of over 20%. ^[3] To commercialize this solar cell, long-term stability is fairly important as well as high efficiency. However, studies on long-term stability have been sparse. It is well known that perovskite materials are easily decomposed under moisture condition. Recently, S. Ito et al. reported the different mechanism, i.e. perovskite is decomposed by generated electron in mp-TiO₂ NPs under UV irradiation.^[4] In that study, Sb₂S₃ layer was found to retard the photodegradation of perovskite material.
In this study, mp-TiO₂ NPs was uniformly coated with MgO thin layer possessing high band gap. We observed the photovoltaic (PV) characteristics of pristine TiO₂ NPs-based and MgO-coated TiO₂ NPs-based perovksite solar cells as a function of light soaking time. As a result, MgO-coated TiO₂ NPs-based perovksite solar cells exhibited excellent stability. The decreasing rate of short circuit current for the MgO-coated TiO₂ NPs based cells (<5%/hour) was slower than TiO₂ NPs based perovskite solar cells (~15%/hour) under AM 1.5 condition. Degradation of perovskite was analyzed by using X-ray diffraction patterns (XRD) as a function of light soaking time. In addition, the mp-TiO₂ NPs based cells show the similar photovoltaic property in comparison with MgO-coated TiO₂ NPs based cell under illumination of UV cutoff light, which indicates that degradation of perovskite was significantly retarded by blocking back electron transfer of highly energetic electrons. This study demonstrates that controlling the interface between the electron transport materials and perovskite is of great importance for improving long-term stability of the perovskite solar cell.
[1] H. Snaith, J. Phys. Chem. Lett. 2013, 4, 3623
[2] N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu, S. I. Seok, Nat. Mater. 13, 897
[3] N-G. Park, J. Phys. Chem. Lett.2013, 4, 2423
[4] S. Ito, S. Tanaka, K. Manabe, H. Nishino, J. Phys. Chem. C2014, 118, 16995

The unprecedented rapid rate improvement in the performance of thin-film organometal halide perovskite photovoltaic devices based on [CH₃NH₃][PbI₃] and related materials over the past several years has led to near-commercial levels of solar conversion efficiency in these types of devices. While historically the ability of new materials to deliver technologically useful conversion efficiency has been the key gating metric for development of commercially interesting photovoltaic technologies, this requirement is in itself necessary but not sufficient to insure commercial relevance. High efficiency is insufficient without acceptable levels of reliability and long-term durability. To date these types issues have received much less attention for perovskite based PV devices. In this presentation we will focus on our initial investigations of the chemical stability of [RNH₃][PbX₃] thin films with an aim to better understanding key chemical degradation pathways of these materials under conditions relevant both to device fabrication and to normal PV device operation. We consider this type of understanding to be key to developing strategies to produce perovskite devices with both high efficiency and long lifetime.

Long term stability of mixed perovskite compounds is one of the important concerns for prolonged viability and economical use of perovskite based solar cells. Degradation in perovskite films mainly occurs due to exposure to moisture. Hence, a controlled atmospheric condition and lower humidity is preferred for device fabrication. Many different strategies such as use of thin a wide band gap semiconductor layer, improvement in pour filling of metal oxide film, and utilization of AgTFSI have been attempted to improve device stability. Most of these degradation mitigation strategies are effective only for a few hundred hours of air and light exposure. Hence, a more detailed study is needed to improve long-term film stability without compromising optical performance. In this study we examined several new water resistant additives, non-volatile additives, structural modifications, barrier films with heat and vapor resistant properties, and stoichiometric modification for enhanced film durability. These strategies and preliminary results are discussed in this report.

In recent years, hybrid organo-metal halide perovskites have been intensely studied as potential absorbers for large-scale cost-effective thin-film based photovoltaic energy generation. The progress in device fabrication, leveraging prior development in organic optoelectronic materials and solution processing know-how, has resulted in efficiencies in excess of 17% in both meso-structured and planar device architectures. The unprecedented advances, while promising the advent of a new material with tremendous technological potential, have also raised the awareness on challenges in reliability and long-term durability, metrics that are key to commercial viability. While the reported device efficiencies increase continuously, inquiries on the control of solar cell performance under optical and electrical bias are being raised. Observations of slow transient electrical behavior, hysteresis and catastrophic failures of devices under photovoltaic operation, point towards the possible role of transient polarization and metal and halide ion migration on relevant length scales, and the effect of photo-excitation on these processes. In this presentation, we report our initial investigations on the optoelectronic stability of organometal halide perovskite thin film solar cells. Chemical information derived from time-of-flight mass spectrometry data is used to track the mobility of metal and halide ions through the bulk and through the different interfaces in variously stressed and aged devices. We further seek to correlate the information from these measurements to understand the generation and evolution of mobile ions in the different layers of the solar cell during operation, and their effect on charge extraction, carrier recombination, optoelectronic characteristics and device stability.

Perovskites with ABX₃-structure show a huge possibility on element substitutions on A-,B- and C-site which leads to a broad variety of physical properties. In recent years the interest become focused on hybrid perovskites as a future photovoltaic material. Our field of interest lays in lead methylammonium triodide in which A is the organic unit [CH₃NH₃]⁺ , B=Pb²⁺ and X=I₃^-.
The different perovskite crystal structures can be classified by their octahedral coordinated B-cation. The aristotype-structure (P m-3m) is symmetry lowered due to tilting, distortion of [BX₆] - octahedra or displacement of B-cation from center of octahedron. These different types of displacements are shown in the ‘Bärnighausen-Stammbaum&’ [1] a tree diagram which was extended in 2002 by Bock & Müller [2] who presented a tree diagram where all group-subgroup relations of octahedral tilted space groups are taken into account, based on the work of Bärnighausen and ‘Glazer&’s tilt systems&’ [3].
First structure analysis and Rietveld refinements confirmed the MAPbI₃-perovskite belongs to the tetragonal space group I 4/mcm.
Further investigations showed that MAPbI₃ shows a highly disordered structure with large atomic displacement factors of the anions meaning, that the [PbI₆]-octahedra show a tilting. The analyses furthermore revealed a distortion and statistical distributed tilting of the octahedra. The synthesis was done by two different routes: first, single crystals as well as polycrystalline material was prepared by precipitation from hydroiodic solution, described by Poglitsch and Weber [4]. Second, polycrystalline perovskite samples were synthesized from a equimolar mixture of methylammonium iodide, described by Im et al. [5].
For structural investigations Synchrotron and Neutron diffraction experiments were performed at the Helmholtz-Centre Berlin for Materials and Energy. The data treatment was performed by F.O.X. (Free Objects for Xtallography) and FullProf Suite software.
The presentation will give an overview of the results of this structural study with a certain focus on the statistic disorder of the methylammonium molecule, and anion displacement. Moreover, structural trends will be discussed.
1. Bärnighausen, H., Group-subgroup relations between space groups: A useful tool in crystal chemistry. MATCH Communications in Mathematical and in Computer Chemistry, 1980. 9: p. 139-175.
2. Bock, O. and U. Müller, Symmetrieverwandtschaften bei Varianten des ReO₃-Typs. Zeitschrift für anorganische und allgemeine Chemie, 2002. 628(5): p. 987-992.
3. Glazer, A.M., The classification of tilted octahedra in perovskites Acta Crystallographica B, 1972. 28: p. 3384-3392, .
4. Poglitsch, A. and D. Weber, Dynamic disorder in methylammoniumtrihalogenoplumbates (II) observed by millimeter#8208;wave spectroscopy. The Journal of Chemical Physics, 1987. 87(11): p. 6373-6378.
5. Im, J.H., et al., 6.5% efficient perovskite quantum-dot-sensitized solar cell. Nanoscale, 2011. 3(10): p. 4088-93.

In recent years, the application of perovskites structured materials in solar cells has attracted much attention because of their high power conversion efficiencies. Organic-lead halide perovskites are one of the most widely studied materials due to their ability to harvest light efficiency, ease of manufacture and low price. However, the toxicity of lead is a problem which needs to be resolved before this class of solar cell can be widely adopted. In order to try to resolve this issue, we have investigated bismuth and antimony as an alternative to lead in the perovskite materials. The use of bismuth and its alloys in various fields is increasingly common as a replacement for lead due to its low toxicity. The structure of the bismuth and antimony perovskite layer is compared to that found for lead perovskites using SEM for samples prepared at the various spin speed. Their suitability for solar cell applications is also determined by absorbance spectroscopy. Finally, the performance data for planar bismuth and antimony perovskite solar cells which have been fabricated using a PEDOT:PSS hole transfer layer and PCBM as electron transfer layer (ITO/PEDO:PSS/Perovksite/PCBM/Ca/Al) through by spin coating will be presented.

We report a new ternary Zn₂SnO₄ (ZSO) electron-transporting electrode of a CH₃NH₃PbI₃ perovskite solar cell as an alternative to the conventional TiO₂ electrode. The ZSO-based perovskite solar cells have been prepared following a conventional procedure known as a sequential (or two-step) process with ZSO compact/mesoscopic layers instead of the conventional TiO₂ counterparts, and their solar cell properties have been investigated as a function of the thickness of either the ZSO compact layer or the ZSO mesoscopic layer. The presence of the ZSO compact layer has a negligible influence on the transmittance of the incident light regardless of its thickness, whereas the thickest compact layer blocks the back-electron transfer most efficiently. The open-circuit voltage and fill factor increase with the increasing thickness of the mesoscopic ZSO layer, whereas the short-circuit current density decreases with the increasing thickness except for the thinnest one (~ 100 nm). As a result, the device with a 300 nm-thick mesoscopic ZSO layer shows the highest conversion efficiency of 7%. In addition, time-resolved and frequency-resolved measurements reveal that the ZSO-based perovskite solar cell exhibits faster electron transport (~ 10 times) and superior charge-collection capability compared to the TiO₂-based counterpart with similar thickness and conversion efficiency.

We show that Ge-halides octahedron in perovskite solar materials are structurally and chemically stable via density-functional theory (DFT) calculations. When the A-site cation is replaced by an organic molecule, the crystal structure becomes less cubic due to distortion along the <111> axis. Both DFT and DFT plus dispersion (DFT-D) schemes are employed in the geometrical optimization of our crystal models. The norm-conserving pseudopotentials are used with cutoff energy of 830 eV. The lattice vectors and atomic positions are optimized. Electronic band structure calculations show that all Ge-halide based perovskite solar materials remain direct with minimum gap at k=(0.5, 0.5, 0.5) 2π/a. According to the projected density of states (PDOS) analysis, the band-gap values are slightly wider when Cs cation is replaced by an organic molecule in perovskite solar materials containing Ge-halides octahedron, which implies that CH₃NH₃GeI₃ could be promising photovoltaic solar cell material.

The high efficiency of perovskite solar cells and the wide energy band gap of the absorber (ge; 1.57 eV) which is tunable over a large range make them well suitable for the application as top cells in a tandem structure with Cu(In,Ga)Se₂ (CIGS) bottom cells. A prerequisite for such tandem cells is the development of perovskite solar cells with high near infrared (NIR) transparency. In this work, we present results on the development of semi-transparent perovskite solar cells and discuss the design and efficiency potential of perovskite/CIGS tandem solar cells.
In this contribution, we present perovskite solar cells that are grown in planar structure. The CH3NH3PbI₃:Cl absorber layer is deposited by vacuum evaporation which allowed the growth of single phase and pin-hole free perovskite layers. The use of FTO/TiO₂ and spiroOMeTAD/Au as electron and hole collecting contacts, respectively, enabled up to 15% conversion efficiency. For the growth of semi-transparent perovskite solar cells the spiroOMeTAD/Au contact layer was replaced by different transparent hole collecting contact layers and the device properties and transmission was investigated. The highly transparent inorganic hole conductor CuSCN combined with ZnO:Al transparent conductive oxide (TCO) allowed a total device transmittance of ~80% for wavelengths between 800 and 1000 nm while enabling efficiencies up to 7.8%. Possibilities to further improve efficiency and transparency of the device will be discussed. This includes the replacement of the FTO layer by more transparent layers like the high mobility TCO In₂O₃:H. The use of such semi-transparent perovskite solar cells in tandem structures with CIGS is discussed and the design, fabrication, and performance of the perovskite/CIGS tandem are investigated using both experimental and numerical approaches.

Organic-inorganic metal halide perovskites, with tuneable bandgap and high open-circuit voltage in solar cells, make them ideal to use in hybrid tandem structures with existing low bandgap photovoltaics. For a tandem solar cell to operate ideally, there needs to be the same current generated in the top cell as that generated in the bottom cell after the light has passed through the top cell. CH₃NH₃PbI₃ is closely matched for c-Si, and will a small substitution of Br for I in the perovskite close to perfect current matching should be achievable. Here we present results on 4-terminal tandem solar cells consisting of perovskite based solar cells and (a-Si:H)/(c-Si) solar cells. In order to make this structure it is necessary to obtain as high as possible transparency in the near IR region of the perovskite solar cell. We have developed sputter coated ITO as transparent electrode of perovskite cell, and tuned both the p-type hole conductor and the ITO deposition to achieve high efficiency perovskite solar cell operation. Integrating this semi-transparent perovskite cells with c-Si HIT cells delivers expected enhancement in open-circuit voltage. We will present detailed characterization of these semi-transparent perovskite cells and tandem solar cells.

Recently, lead perovskite-based solar cells have gained intense research because they have reached astounding efficiencies in a very short time. Many works, describing their applicability in different photovoltaic structures, synthesis, and growth mechanisms have been developed, still, there is a plenty of work to be done on this material. In this work, we report the synthesis and characterization of lead perovskites (CH3NH3PbI3) thin films by spin coating. Precursor solutions containing lead iodide, methylammonium iodide and ammonium iodide were used for the coating. The deposition parameters such as spin speed and time were varied to form uniform thin films with good adhesion. Crystalline Structure and composition of the perovskite thin films were characterized using X-Ray diffraction and X-ray Photoelectron spectrometer. The morphology of the films was analyzed by Atomic force microscopy. The optical properties of the films such as absorption coefficient and band gap energy were determined using Uv-Vis spectroscopy. The electrical conductivity under dark and illumination were studied. The results of a systematic investigation to elucidate the role of the halide concentration and the deposition parameters of the spin coating process on the properties of the perovskite thin films will be presented.

Organometallic halide perovskites are a class of spectrally tunable semiconductors possessing remarkable optoelectronic and photophysical properties despite being fabricated under non-expitaxial growth conditions. Thin films of this material have enabled great strides in third-generation solar cell technology, as well as in light-emissive devices. We've developed a new facile method for perovskite thin film fabrication utilizing atomic layer deposition. This method—perovskite ALD—enables conformal perovskite thin films to be grown onto structured substrates. Conformal integration of semiconductor active media is important to materials science and optoelectronic devices broadly. We demonstrate that films grown via perovskite ALD have exceptional properties as an optical gain media. Leveraging the quality of the medium, as well as the conformality of the method, we report perovskite lasers on spherical optical cavities.

We demonstrate the fabrication of organometal halide perovskite light-emitting diodes based on nano-structured TiO₂ electrodes. The nano-patterns were created by the soft-nanoimprinting of resist using perfluoropolyether (PFPE) moulds, followed by reactive ion etching. TiO₂ electrodes with nano-hole arrays with a diameter as small as 25 nm and a period as small as 50 nm can be fabricated. LEDs based on this structure have the benefits of enhanced charge carrier confinement and bimolecular recombination. In another example, TiO₂ electrodes with 280 nm period nano-gratings were fabricated. Due to their periodic refractive index, they provide a promising structure for distributed feedback perovskite lasers.

We present a new family of UV-NIR wavelength-tunable planar nanolasers based on organic-inorganic perovskite CH₃NH₃PbX₃ (X= I, Br, Cl) nanoplatelets. The perovskite nanoplatelets were grown on muscovite mica utilizing van der Waals epitaxy by Chemical Vapor Deposition method. By tuning the composition of halides in CH₃NH₃PbX₃ (X= Cl, Br, I), we are able to tune the spontaneous emission peak from ~ 400 nm up to 800 nm. The grown perovskite platelets exhibit excellent Whisper Gallery Mode (WGM) lasing action with the highest quality factor (Q) of 1100 at room temperature for CH₃NH₃PbBr_xI_3-x platelets (with emission peak at ~800 nm). The result is comparable with a recent report on the lasing of the material based on a commercial Distributed Bragg Reflector (DBR) cavity. The lasing can be obtained in platelets with thickness of as thin as 40 nm which is the thinnest WGM cavity ever reported to date. The good naturally formed WGM cavity in this material may result from high crystallinity in the platelet forms prepared by our special synthesis method. Moreover, we demonstrate the integration of these platelets onto some common conductive substrates such as Si, Au and indium-tin oxide (ITO) without losing their performance on lasing (i.e, Q factor, lasing threshold, etc.). Our findings open up a new class of wavelength tunable planar nanomaterials potentially suitable for on-chip integration.

One major advantage of solar cells based on CH₃NH₃PbI₃ perovskite is the remarkably high open-circuit voltage (V_oc) of approximately 1.1 V, which demands for low non-radiative recombination losses.
In this contribution we show that the V_oc of CH₃NH₃PbI₃ perovskite solar cells can be theoretically calculated from the reciprocity relation between photovoltaic external quantum efficiency and electroluminescence spectra. Investigating different device architectures (TiO₂ or Al₂O₃ mesoscopic scaffold, planar devices, varied hole-transport layer thickness), we find that charge is photogenerated in the perovskite itself, not requiring any interface with titanium dioxide (TiO₂) or with the hole transport layer. A share of electrons and holes recombines in the perovskite independently of whether the charges are photogenerated (photoluminescence) or electrically injected (electroluminescence). Measuring the external electroluminescence yield we explain measured values for V_oc and give estimations on the maximum V_oc obtainable with CH₃NH₃PbI₃ perovskite-based solar cells. We demonstrate the two major sources of non-radiative recombination that decrease V_oc in this type of solar cell: surface recombination due to non-selective contacts if the hole-transport layer is not sufficiently thick and a Shockley-Read-Hall type of recombination in the perovskite layer itself giving rise to a strong dependence of the electroluminescence emission yield on injection current.

The degradation of methylammonium lead iodide (MAPbI₃) upon air exposure is a potentially limiting effect for large scale MAPbI₃ photovoltaic production. Curiously, it is also known that a small amount of air exposure actually improves solar cell device performance. In this paper, we report systematic studies of MAPbI₃ exposed to air as compared to encapsulated samples. The photoluminescence yield and carrier lifetime, measured with time-resolved photoluminescence, both increase with exposure. X-ray diffraction studies also reveal a shrinking volume of MAPbI₃ as the material reverts back to its precursors, methyl ammonia and lead iodide (PbI₂). Further analysis of our data reveals that these phenomena can be explained by PbI₂ forming a passivating layer around MAPbI₃ grains that reduce the number of non-radiative recombination centers. Extended studies show that this process is not self-limiting such that the process eventually leads to a film that has completely reverted to its precursor state. This work helps to understand the finding that air exposure helps to improve device performance.

Highly efficient energy conversion above 15% has been reached in perovskite-based solar cells in less than two years. Early studies performed on small-grain (<10 microns) structured perovskite showed that these promising performances seem to be resulting from the combined high charge-carrier mobility and long-lived charge-carriers with long diffusion lengths. However, dynamics of carriers and processes of absorption/recombination are still under debate and inconsistent from one perovskite material type structure to the other as due to the large spectrum of synthesis techniques. Recently, we have developed a new synthesis method yielding up to millimeter-scale crystal grains, i.e. about hundred times larger than grain sizes reported in the literature. The large size and good crystallinity of grains in our material improve the performance stability of perovskite-based solar cells, suggesting a reduction in defect state concentration and paving the way for investigating the closer-to-intrinsic properties of perovskite.
In this study, we examine the charge-carrier dynamics in the large grain perovskite structure by means of time-correlated single-photon counting, and attempt to relate our results to the overall performances of our solar cell devices. In complement, we analyze emission and absorption processes occurring at the micrometer level providing key parameters for the interpretation of the dynamics of carriers. By tuning of both the excitation power and the temperature, we propose a more complete picture of the recombination kinetics of carriers intrinsic to perovskite.

The outstanding photovoltaic performance of hybrid organic-inorganic perovskites have brought these materials to the forefront of solar cell research. Perovskite solar cells were first unveiled in 2009 with efficiency of 3.8% and it have been reported to reach of 19.3 % only in five years.^[1-2] Moreover, these materials can be made from solution at the low temperature, suggesting a great potential for future deployment of this technology at industrial level. Despite the extremely fast progress, fundamental aspects of the photophysics underlying device operation which are key to the photovoltaic and optoelectronic applications are relatively little understood.^[3-5] Here we use steady-state and time-resolved photoluminescence spectroscopy to investigate photogeneration and recombination of charge carriers in CH3NH3PbI3. We provide evidence that free carriers are photoexcited at room temperature whereas Wannier-Mott excitons are dominant at low temperature. Furthermore, the excitons screening effect plays important role on photoexcitation dynamics in organometal halide perovskites. The results presented here provide important fundamental insights into the intrinsic photophysics of semiconducting organometal halide perovskites, and such insights are essential for the further development of highly efficient solar cells and other optoelectronic devices based on this unique class of hybrid materials.
[1] Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. J. Am. Chem. Soc. 2009, 131, 6050minus;6051.
[2] Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Science 2014, 345, 542.
[3] Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Science 2013, 342, 341
[4] Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Science 2013, 342, 344
[5] Wehrenfennig, C.; Eperon, G. E.; Johnston, M. B.; Snaith, H. J.; Herz, L. M. Adv. Mater. 2014, 26, 1584- 1589

Organo-metal halide perovskites like methylammonium lead iodide show extraordinary performance when used as photovoltaic cells. Open circuit voltages close to and even exceeding 1.0V can be observed, which is closer to their maximum possible value defined by the Shockley-Queisser limit than many other PV technologies.[1] However, a fundamental understanding of the physical processes in perovskite solar cells is still lacking but is essential for further development in this quickly emerging research field. Here, we present our recent studies on the charge carrier recombination in methylammonium lead iodide perovskite solar cells in a planar configuration without porous transport layers. Thereby, the perovskite layer was made by vapor deposition. These results are compared with the recombination behavior in reference state-of-the-art polymer-fullerene bulk heterojunction solar cells.[2] In open circuit voltage decay measurements, we observed two very different time domains in the voltage transients. On short time scales, we observed very similar voltage drops to reference organic solar cells. In addition, a second much slower decay on longer time scales was observed only in case of the methylammonium lead iodide solar cell. More interesting, in the perovskite solar cell, the recombination dynamics at all timescales were found to be dependent on the starting illumination intensity. We discuss the potential origins of these unique behaviors. Furthermore, we compare the recombination dynamics of the vapor deposited perovskite solar cells to solution processed methylammonium lead iodide perovskite solar cells, which are made also in a planar device configuration.
[1] K. Tvingstedt et al., Sci. Rep. 4, 6071 (2014), [2] A. Baumann et al., APL Mater. 2, 081501 (2014)

Renewable energy generation and storage are indisputably the greatest challenges of the 21^st century. The ever increasing need for low-cost ultrathin photovoltaic (PV) devices that can efficiently harness solar energy has invigorated the search for alternate materials that can overcome the limitations of the conventional technology. In this regard, perovskite materials have attracted considerable attention owing to their intrinsic ambipolar nature, broad absorption spectra, weak exciton binding energy, high carrier mobilities, long carrier diffusion lengths, and ease of fabrication and processing. Despite the plethora of research pertaining to the PV performance of the perovskites, understanding of their fundamental photophysics still leaves much to be explored. To this end, we have studied carrier dynamics in trihalide perovskites (CH₃NH₃PbI₃) using ultrafast pump-probe (PP) spectroscopy, as a function of temperature. The salient features of the photobleach and photo-induced absorption (PIA) peaks show a systematic dependence on the temperature. The origins of these transient absorption features are further discussed in the context of phase transition and change in the nature of photogenerated carriers. It is observed that the time evolution of bleaching and PIA signatures are highly correlated on the picosecond time scale suggesting a common origin. Furthermore, we demonstrate energy transfer from perovskite thin film to silicon substrates advocating their potential use as the top layer in a two-level tandem solar cell configuration.

Femtosecond fluorescence up-conversion system was used to investigate the carrier and excitonic relaxation dynamic of perovskite on mesoporous Al₂O₃, TiO₂ or NiO thin films with excitation at 450 nm and monitored at 670 nm and 770 nm. The excitation density dependence of the intensity of photoluminescence was followed by the power law which is described by the function: . For monitored at 670 nm, it shows the number of 1.5 for b coefficient as we monitored the intensity of photoluminescence at peak of spike after photo-excitation, but it does 2.2 at 5 ps. This means that the free carriers were generated upon excitation. Otherwise, it is observed that a broadening and shift to longer time delay for the spike as the excitation density increased. This may attributed to the Auger-like energy transfer [1] and band filling effect [2] to retard the fluorescence formation and decreasing. As a result, we obtained that the hole relaxation rate constant is 2 10¹² s^-1 and auger type energy transfer rate constant is around 1.5 10^-4 cm³ s^-1 photon^-1. On the contrary, at low photon flux density (< 4 mJ cm^-2), the slope of power law (b coefficient) exhibit 2, but it does 1 at high photon flux density (> 5 mJ cm^-2). This means that the carriers were generated at low photon flux density but excitons did at high photon flux density after photon excitation. For time-resolved fluorescence decay process, we observed a fast decay process around 50 ps and a slow decay process exceed than 1 ns for all systems. In our previous report, we assigned that the fast and slow decay process are electron relaxation from conduction band to surface state and to valance band, respectively [3]. In power dependent time-resolve fluorescence decay measurement, the lifetime of slow decay process at low photon flux density was longer than that at high photon flux density, but the fast decay process was independent with the photon flux density. The acceleration of slow decay may be attributed to the Auger recombination resulted by high concentration of excitons or carriers. As a result, we introduced the Auger recombination model to analyse the time-resolved fluorescence spectra: for carrier relaxation process and for excitonic relaxation process. We obtained that the Auger recombination rate constant of carriers and excitons are 8.87 10^-10 cm s^-1 counts^-1 and 3.68 10^-9 cm s^-1 counts^-1, respectively. When we considered that the carrier transfer dynamic form perovskite to Al₂O₃ is negligible, the electron and hole transfer rate constant are 12 ns^-1 and 3.6 ns^-1.
[1]A. L. Efrosa, V. A. Kharchenkob, and M. Rosena,Solid State Commun., 1995, 93, 281.
[2]J. S. Manser and P. V. Kamat, Nature Photon., 2014, 8, 737.
[3]H.-Y. Hsu, C.-Y. Wang, A. Fathi, J.-W. Shiu, C.-C. Chung, P.-S. Shen, T.-F. Guo, P. Chen, Y.-P. Lee, and E. W.-G. Diau, Angew. Chem. Int. Ed., 2014, 53, 9339.

The investigations and developments of photo-energy conversion by various types of photovoltaic devices are very rapid in the late 20^th century because of the upcoming energy crisis resulting from the depletion of fossil fuels in the near future. The silicon-based photovoltaic device is the most notable type of developed photovoltaic devices in the last decade, but the high cost of the processing and the inflexible substrate limit its application. Finding alternative architectures is an important issue currently.
The dye-sensitized solar cell (DSSC) is a notable alternative and has grabbed wide attention. Recently, a novel kind of sensitized solar cells based on the organic-inorganic hybrid halide perovskite material methylammonium lead iodide (CH₃NH₃PbI₃ or MAPbI₃) stimulates scientists&’ interests because this kind of solar cells can reach high power conversion efficiency (PCE) and it is relatively low-cost and easy in processing. As the material which is used in the active layer of solar cell devices, the absorption coefficient of MAPbI₃ is large near the visible-light region, which was reported in previous results. Although the strong absorption is identified, the optical transition mechanism, which is the key factor for characterizing and designing devices, is still inconclusive now. The theoretical derivation would be a promising tool to solve this problem.
In this research, first principles calculations were employed to investigate the electronic structures and properties of perovskite MAPbI₃. Optical absorption spectra were evaluated and further be analyzed by applying the “band-resolved absorption density” technique [1]. The theoretical results provide detail information of optical absorptions and also suggest the mechanism of optical transitions.
[1] Lee, M.H., C.H. Yang, and J.H. Jan, Band-resolved analysis of nonlinear optical properties of crystalline and molecular materials. Physical Review B, 2004. 70(23).

The high efficiency of lead halide organic perovskite solar cells has raised many questions on the mechanisms at work here. An accurate description of the macroscopic dielectric properties is essential for understanding the origin of the materials ability to convert light to electricity. In this talk, we present an accurate account of the electronic, optical and excitonic properties of twelve halide organometal perovskites ABX₃ (A = CH₃NH₃⁺, HC(NH₂)₂⁺; B = Pb, Sn; X = Cl, Br, I) by means of many-body first principles methods. We use optimized structures obtained at room temperature using parallel tempering molecular dynamics. Quasi particle band structures and absorption spectra are calculated at the GW₀ level. Electron-hole interactions have been included in the dielectric function by solving a Bethe-Salpeter equation for the polarizability. We demonstrate that a description beyond independent particles is necessary to describe the onset of the optical absorption. The calculated degree of localization of the excitons in k-space indicates Wannier-Mott type excitons with binding energies ranging from 30 meV (ABI₃) and 100-200 meV (ABCl₃). To validate our predictions, we compare the results with available experimental data (band gap and optical absorption).

We have performed an in-depth characterization of local opto-electronic properties of three hybrid organic-inorganic perovskite materials for solar cells using scanning probe techniques. Perovskite solar cells have the potential to be highly efficient and low-cost compared to other third-generation solar technologies. Since 2009, power conversion efficiency of perovskite solar cells has rapidly increased from 3.8% to almost 20%. Currently, one of the main challenges is understanding the exact operating mechanism in perovskite solar cells to make strategic adjustments to fabrication techniques. The effect of morphology and optoelectronic properties on performance in these materials is hardly understood. We have studied three promising solution processed hybrid organic-inorganic perovskite materials, CH₃NH₃PbI₃, CH₃NH₃PbI_3-xCl_x, and CH₃NH₃PbI_3-xBr_x using conductive AFM. In these materials, we observed differences in local morphology and photocurrent generation. Specifically, we have measured significant differences in local photocurrent generation that do not correlate with topography but rather with local electronic structure.

Halide perovskite semiconductors CH₃NH₃PbX₃ (X=Cl, Br, and I) have attracted great attention in recent years as promising high performance solar-cell materials. The power conversion efficiency of perovskite-based solar cells dramatically increases in a few years and recently reaches to 19.3 %. For the further increase of power conversion efficiency toward the theoretical limit (~30 %), it is needed to clarify the efficiency loss mechanism in the photoconversion processes. We have so far studied the near-band-edge optical properties of CH₃NH₃PbI₃ thin films by static optical measurements such as photoabsorption, photoluminescence (PL), and photocurrent [1,2]. Furthermore, we recently revealed the photocarrier recombination dynamics of CH₃NH₃PbI₃ thin films by means of time-resolved PL and femtosecond transient absorption (TA) spectroscopy [3]. The carrier recombination process is dominated by nonradiative single-carrier trapping under weak excitation and electron-hole radiative recombination under high-density excitation. According to this result, the PL and TA lifetimes under weak excitation density reflects nonradiative single-carrier trapping due to impurities and defects in the samples, which reduces the power conversion efficiency of perovskite solar cells.
In this study, we estimated the nonradiative recombination rate in CH₃NH₃PbI₃ bulk crystals and thin films fabricated by different methods and under different conditions. Thin film samples were formed on SiO₂ glass substrate by one-step (simple spin-coating) or two-step (sequential deposition) method. We confirmed that the CH₃NH₃PbI₃-based solar cells we fabricated by two-step method display conversion efficiency of more than 13 % with high reproducibility [4]. Time-resolved PL measurements were conducted using a streak camera and a wavelength-tunable femtosecond laser system. Our thin film samples fabricated by two-step method display a very long PL lifetime of ~300 ns at a maximum, which is much longer than that reported previously. We will present the effect of sample fabrication method and condition such as annealing time and temperature on the nonradiative recombination process. In addition, the correlation between the carrier lifetime and solar cell efficiency will be discussed.
Part of this work was supported by The Sumitomo Electric Industries group CSR foundation, JST-CREST, and JST-PRESTO.
References
[1] Y. Yamada et al., Appl. Phys. Express 7, 032302 (2014).
[2] Y. Yamada et al., IEEE J. Photovoltaics, (in press).
[3] Y. Yamada et al., J. Am. Chem. Soc. 136, 11610 (2014).
[4] A. Wakamiya, et al., Chem. Lett. 43, 711 (2014).

Lead-based hybrid organicminus;inorganic halide perovskites have emerged as one class of promising light-harvesting materials for next-generation solar cells because of their exceptional properties. To develop environmentally friendly solar cells, it is necessary to find an alternative light-harvesting material that does not contain toxic lead. Here we studied the structural and electronic properties of the hybrid perovskites CH₃NH₃SnI₃ and CH₃NH₃SnBr₃ as well as their band alignments with respect to the electron-conducting TiO₂ electrode using first-principles hybrid functional calculations. Our hybrid functional calculations yielded band gaps of 1.3 and 1.84 eV for CH₃NH₃SnI₃ and CH₃NH₃SnI_3, respectively, which are consistent with experimental values. In addition, our calculations show that the organic cation [CH3NH3]⁺ does not take part in the formation of the valence band nor the conduction band and only plays a role in donating one electron in each material. Our band alignment calculations show that introducing substitutional Br dopants for I anions in CH₃NH₃SnI₃could facilitate charge transfer from the hybrid perovskite to the TiO₂ electrode, enabling the development of more efficient solar cell architectures.

Solution processable hybrid perovskite semiconductor materials have been demonstrated to give excellent photovoltaic (PV) power conversion efficiencies, now exceeding 19%. ^[1-3] The family of the mixed halide perovskites, CH₃NH₃Pb(I_1-xBr_x)₃ (0 le; x le; 1), have attracted special attention due to their easy bandgap tunability and good performance in PVs and light emitting diodes (LED).^[4-6] Although decent power conversion efficiencies have been achieved from these perovskite PVs, the fundamental issue of open circuit voltage losses compared to the pure tri-halide perovskite based PVs, is not resolved. Also, the effect of halide ratio on the recombination properties in this family of perovskites is still largely unexplored.
We study the photo-induced changes in these iodide-bromide perovskites CH₃NH₃Pb(Br_1-xI_x)₃ (0 le; x le; 1) using photothermal deflection spectroscopy, transient absorption and transient photoluminescence measurements. We find changes in the absorption and recombination behavior of the photo-excited states in these materials under background white light illumination. We compare the photo-physical properties and diffusion lengths of the photo-excited states at different excitation densities and temperatures, ranging from 293K to 4K.
[1] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science2012, 6107,338, 643.
[2] M. A. Green, A. Ho-Baillie, H. J. Snaith, Nat. Photonics2014, 8, 506.
[3] H. Zhou, Q. Chen, G. Li, S. Luo, T. -b. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Science 2014, 6196, 345, 542.
[4] Z.-K. Tan, R. S. Moghaddam, M. L. Lai, P. Docampo, R. Higler, F. Deschler, M. Price, A. Sadhanala, L. M. Pazos, D. Credgington, F. Hanusch, T. Bein, H. J. Snaith, R. H. Friend, Nat. Nanotechnol.2014, ADVANCE ON, DOI 10.1038/nnano.2014.149.

Solution-processable organic-inorganic lead iodide perovskite (CH₃NH₃PbI₃) based solar cells exhibit remarkably high power conversion efficiency (PCE).¹ The key issues for increasing PCE are to control crystallization and morphology of perovskite² and to match the electronic energy levels of the perovskite to the adjacent organic hole and electron transporting layers.³ Here we used ab initio calculations and photoelectron spectroscopy to unravel the electronic structure and charge redistribution at the interfaces between CH₃NH₃PbI₃ with different surface orientations and organic charge transporting materials (Spiro-OMeTAD and PCBM). The estimated energy levels of organic/CH₃NH₃PbI₃ systems from DFT calculations match well with the experimental values interpreted from ultraviolet photoelectron spectroscopy measurements. The presence of large interfacial dipole induced by hole/electron accumulation layer appear at the CH₃NH₃PbI₃ surfaces prove that the significant interfacial charge transfer happens at the heterojunctions. The calculated results suggest that CH₃NH₃PbI₃(001) surface tends to favor hole injection into Spiro-OMeTAD and CH₃NH₃PbI₃(100) surface inclines to help electron transfer to PCBM due to large surface delocalized states and orbital coupling. Molecular dynamic simulations confirm that the stronger electronic coupling at the hybrid organic/perovskite interfaces, especially in CH₃NH₃PbI₃(001)/Spiro-OMeTAD, is due to thermal fluctuations at room temperature. These findings could apply to a wider range of organic/perovskite interfaces, where control of perovskite crystallization and surface orientation are key factors to optimize photovoltaic device performance.
References
1. Zhou, H. P.; Chen, Q.; Li, G.; Luo, S.; Song, T. B.; Duan, H. S.; Hong, Z. R.; You, J. B.; Liu, Y. S.; Yang, Y. Science, 2014, 345, 542.
2. Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Il Seol, S. Nature Materials, 2014, 13, 897.
3. Schulz, P.; Edri, E.; Kirmayer, S.; Hodes, G.; Cahen, D.; Kahn, Energ Environ Sci, 2014, 7, 1377.

Lead-based hybrid organic-inorganic perovskite (CH₃NH₃PbI₃) has recently attracted a lot of attention in photovoltaic community, with power conversion efficiency already approaching 20%. The optical properties of this class of materials have been extensively characterized, and used to obtain indirect information about charge carrier dynamics and other transport properties. For instance, a long exciton diffusion length of ~1 mm was inferred by ultrafast pump-probe spectroscopy, and used to correlate the good photovoltaic performance to the efficient charge carrier extraction^1,2.
Surprisingly, direct electrical characterization of the transport properties of this class of materials is still lacking. In this work, we fabricated bottom contact, bottom gate field-effect transistor (FET) devices incorporating CH₃NH₃PbI₃ in the active region to derive intrinsic transport parameters such as carrier mobility. FET transfer and output characteristics were measured both in dark and under illumination, showing large hysteresis and sweep-rate dependence similar to reported current-voltage characteristics of photovoltaic devices³.
Hysteresis, most likely originating from ferroelectricity of CH₃NH₃⁺ cation, has a strong influence on the extraction of transport parameters and power conversion efficiency from photovoltaic current-voltage measurements. Effective field-effect mobility derived from both linear and saturation regimes is of the order of 10^-3 to 10^-2 cm²V^-1s^-1. Comprehensive device modeling, including ferroelectric effects of lead-based perovskite, is developed to account for the time-dependence of transport parameters.
This work highlights the importance of ferroelectric CH₃NH₃⁺ cation on the charge transport properties of lead-based perovskite CH₃NH₃PbI₃, suggesting that a deeper understanding of this phenomenon may be useful to further optimize photovoltaic device performance.
1 Xing et al Science 2013 Vol. 342 no. 6156 pp. 344-347
2 Stranks et al Science 2013 Vol. 342 no. 6156 pp. 341-344
3 Kim etl al J. Phys. Chem. Lett., 2014, 5 (17), pp 2927-2934

Perovskite-based solar cells have achieved high solar-energy conversion efficiencies and attracted wide attentions nowadays. Despite the rapid progress in solar-cell devices, many fundamental issues of the hybrid perovskites have not been fully understood. Experimentally, it is well known that in CH₃NH₃PbI₃, the organic molecules CH₃NH₃ are randomly orientated at the room temperature, but the impact of the random molecular orientation has not been investigated. Using linear-scaling ab-initio methods, we have calculated the electronic structures of the tetragonal phase of CH₃NH₃PbI₃ with randomly orientated organic molecules in large supercells up to ~20,000 atoms. Due to the dipole moment of the organic molecule, the random orientation creates a novel system with long-range potential fluctuations unlike alloys or other conventional disordered systems. We find that the charge densities of the conduction-band minimum and the valence-band maximum are localized separately in nanoscales due to the potential fluctuations. The charge localization causes electron-hole separation and reduces carrier recombination rates, which may contribute to the long carrier lifetime observed in experiments. We have also proposed a model to explain the charge localization.

It has recently been realized that upon variations in synthesis parameters different film qualities of perovskite can be obtained. Up to now, two main different procedures have been employed to deposit perovskite films: one-step and two-step procedures. For the one-step procedure, different precursors have been used. For instance, lead chloride, lead iodide have been widely used as lead source for synthesis of mixed methylammonium lead trihalide perovskite, CH₃NH₃PbI_3-xCl, while methylammonium chloride (MACl) and methylammonium iodide (MAI) have been used as methylammonium (MA) source. The most common solution precursors for synthesis of such mixed perovskites are mixtures of PbCl₂, MAI, and DMF or mixtures of PbI₂, MAI, MACl, and DMF. Both procedures use an excess of MA and Cl during the growth of perovskite. However, the amount of chloride remaining in the perovskite films has been demonstrated to be very low, which is attributed to sublimation of MACl during the annealing process. Current research has investigated the role of Cl on crystal growth. Excessive MA has been considered responsible for the orientation of perovskite films along the [110] direction of the tetragonal phase. Since the mechanism of crystal growth of such mixed perovskites is not fully understood at this point, we synthesized mixed perovskites by using the two precursor solutions via the one-step deposition process. In addition, we also studied the effect of substrates on the crystal growth of perovskite films.

Since the first report on a long-term durable solid-state perovskite solar cell with a PCE of 9.7% in 2012, perovskite solar cell has been intensively researched. As a result PCE approaches almost 20% in about 2 years. In this talk, opto-electronic properties of perovskite materials and high efficiency device engineering will be discussed. Photoferroic property of methylammonium lead iodide is observed, which is significantly dependent on perovskite crystal size. Size-dependent photoferroic effect is found to influence on I-V hysteresis. Interfaces play important roles in photovoltaic performance, where the blocking layer/perovskite interface affects photocurrent but voltage is influenced by the perovskite/HTM interface. Imperfect blocking layer is however found to deteriorate voltage significantly according to ZnO nanorod study. Photo-generated electrons are transported both in perovskite and TiO₂ as observed in TiO₂ nanohelix based perovskite solar cell, where injection into TiO₂ depends on surface area of TiO₂ helicies. By interfacial and morphological optimization, power conversion efficiency (PCE) over 17% is routinely attained. Furthermore, feasibility study shows that hybridization of perovskite solar cell with a conventional semiconductor solar cell can lead to PCE approaching 30%.

Perovskite metal halides, represented by CH₃NH₃PbI₃, have yielded unprecedented advances in photovoltaics with power conversion efficiencies close to 20%, rivaling the performance of industry standard silicon. The excellent electronic properties, especially the balanced electron and hole diffusion lengths (1), has positioned this material at the forefront of solution processed photovoltaics, as well as an excellent candidate for light emission and lasing applications (2).
Most of the recent improvements of devices based on perovskites have pursued the pathway of new device architectures and on incremental improvements of the fabrication processes. Although these strategies have yielded success, it is essential to expand the family with new materials that can match the advantages of CH₃NH₃PbI₃ and address some of its drawbacks, such as moisture stability and toxicity.
Although alternatives to methyl ammonium lead halide with substitutions of the organic cation with formamidinium or caesium (3), lead with tin, and halide substitutions are beginning to be investigated; there are clear indications that these represent only the beginning of the perovskite era. It is possible to prepare a large variety of 0, 1, 2, 3-D hybrid organic-inorganic structures with potentially important technological properties. The layered 2-D systems (A₂MX₄) can accommodate much larger and more complex organic cations and thus offers a greater control over their structural and physical properties. Whereas, the 0 & 1-D systems synthesised via manipulation of the organic and inorganic components would yield critical control of the photovoltaics and light emission properties. This presentation will address the various challenges and opportunities in perovskite solar cells beyond methyl ammonium lead iodide with particular emphasis on their optoelectronic properties.
1. G Xing, N Mathews, S Sun, S S Lim, YM Lam, M Gratzel, S Mhaisalkar, TC Sum, Science, 2013, 342, 344-347
2. G Xing, N Mathews, S. S. Lim, N. Yantara, X. Liu, D. Sabba, M Grtzel, S Mhaisalkar and TC Sum, Nature materials, 2014, 13, 476-480
3. HK Mulmudi, D Sabba, WL Leong, PP Boix, R Ramanujam, T Baikie, C Shi, H Ding, R Ramesh, M Asta, M Graetzel, SG Mhaisalkar, N Mathews, Advanced Materials, DOI: 10.1002/adma.201401991

Planar heterojunction organic-perovskite hybrid solar cell is one of the most promising photovoltaic architecture to achieve high efficiency with low temperature process. A new two-step solution process to synthesis high quality perovskite film at room temperature was disclosed. Combining with smooth PEDOTLPSS hole transport layer and high mobility electron transport (PC₇₁BM) layer, the planar heterojunction perovskite-PC₇₁BM solar cell achieves the power conversion efficiency above 16%. This champion cell shows no current hysteresis both in voltage scan directions and rates.. The crystalline perovskite film was made by first spin-coating PbI₂ film then adding CH₃NH₃I via spin-coating. The growth and purity of perovskite film can be manipulated individually, providing a simple and reproducible way to fabricate perovskite based solar cells via low temperature solution process in an ambient atmosphere.

In the recent past, organometalhalide perovskite-based solar cells (1-2) have gained enormous attention because of rapid progress in cell efficiency, reaching up to almost 20% (3), and its promisingly high potential to improve the cell performance further. However, unlike other types of solar cells, perovskite cells exhibit hysteresis in their current-voltage (I-V) characteristics, resulting in difference in the power conversion efficiencies estimated from forward scan (short circuit to open circuit) and backward scan (open circuit to short scan). Such hysteretic behavior of these solid-state perovskite cells has created ambiguity about working mechanism of the device and has been a serious concern to all. In fact, such characteristics of these cells have created doubts about the cell performance, which is being suspected to have been overestimated (4,5). Although it has been found that hysteresis in perovskite cells depends strongly on the cell structure and the process/method by which they are fabricated (6), the origin of such behavior is not yet completely understood. A lot of speculations and hypotheses (6-8) are being made that hysteresis can be caused by ferroelectric polarization, ion migration, deeper trap states, etc. However, there is no direct evidence that can support the proposed mechanism of origin of hysteresis.
The present study investigates the role of different interfaces (e.g. FTO/TiO₂ compact layer, TiO₂ compact layer/Perovskite, FTO/perovskite interfcae) and bulk of perovskite in a planar heterojunction CH₃NH₃PbI_3-xCl_x cell in causing hystresis. Although there is no direct evidence to ferroelectric polarization causing hysteresis, the same hysterestic I-V charatceristics observed in a cell made with a nonferroelectric PbI₂ used in place of ferroelectric perovskite indicates that ferroelectric polarization may not be only origin to hysteresis. Investigation of different interfaces reveals that FTO/TiO₂ interface has a definite contribution to hysteresis. I-V characterization across FTO (substrate)/TiO₂ compact layer interface shows hysteresis in a same way as observed in a planar cell, confirming that carrier tarnsfer at the interface certainly play a role in causing hysteresis. It was found that FTO/TiO₂ interface hysteresis changed with bias voltage, amount of current allowed to flow across it and with modification of the interfacial structure.
References
(1) Lee, et.al. Science, 2012, 338, 643-647, (2) Burschka, et. al., Nature, 2013, 499, 316-319, (3) Huanping Zhou, et al. Science, 2014, 345, 6196, (4) Michael D. McGehee, Nature Materials, 2014, 13, 845-846, (5) Editorial, Nature Materials, 2014, 13, 837-837, (6) Snaith, et. al., J. Phys. Chem. Lett., 2014, 5, 1511minus;1515, (7) Rafael S. Sanchez et al, J. Phys. Chem. Lett., 2014, 5, 2357minus;2363, (8) Emilio J. Juarez-Perez et al, J. Phys. Chem. Lett., 2014, 5, 2390minus;2394

Methylammonium lead iodide perovskite has shown very promising optoelectronic properties and produced solar cells with extraordinary efficiency improvement in just a few years. In a planar structure of perovskite solar cells (PSCs), the perovskite film, usually a few hundred nanometer thick, serves the dual roles for light absorption and charge transport, and thus is a critical functional component of the device. It is desirable that the perovskite film has a uniform microstructure and is pinhole free. But normal one-solution spinning coating produced films consisting of large dendritic perovskite grains and many pores, leading to devices with poor power conversion efficiency. A low level of supersaturation in the perovskite solution is found to be a major issue, as it results in slow nucleation coupled with a rapid grain growth during precipitation. A critical measure to achieve monodisperse perovskite grains in spinning coating is to decouple the nucleation from growth. Instant nucleation would cause large amounts of nuclei to form in the solution, upon which uniform and fine grains would subsequently grow. In this talk, the nucleation and grain growth in the perovskite solution will be discussed and some techniques that can control the nucleation and growth and, thus the microstructure of perovskite films introduced.

Controlling the morphology and composition of lead halide perovskites (e.g., CH₃NH₃PbI₃) has been found critical to developing high-performance perovskite solar cells. In this presentation, we demonstrate that CH₃NH₃Cl can work as a sacrificial additive to fabricating high-quality CH₃NH₃PbX₃ (x=Br, I) in both one-step and two-step sequential solution deposition approaches. When added to the standard one-step precursor solution, CH₃NH₃Cl functions as a glue or soft template to control the initial formation of a solid solution. Interestingly, CH₃NH₃Cl is found to release from the precursor solid solution in a temperature range when perovskite CH₃NH₃PbX₃ is still thermally stable. This CH₃NH₃Cl-assisted thermal decomposition process facilitates the formation of a series of phase-pure perovskite films, including CH₃NH₃PbI₃, CH₃NH₃PbI₂Br, CH₃NH₃PbIBr₂ and CH₃NH₃PbBr₃, at relatively mild temperatures.^1,2 For two-step sequential solution deposition, it is critical to effectively convert PbI₂ to perovskite. To facilely convert PbI₂ into CH₃NH₃PbI₃ without PbI₂ residues, we developed a CH₃NH₃Cl-assisted three-step sequential solution method to fabricate CH₃NH₃PbI₃. In this method, the PbI₂middot;CH₃NH₃Cl precursor film was first deposited on the mesoporous TiO₂ substrate, followed by thermal decomposition to form PbI₂. This PbI₂ film prepared from CH₃NH₃Cl-assisted thermal decomposition can further be converted quickly and completely into CH₃NH₃PbI₃ by dipping it in a regular isopropanol solution of CH₃NH₃I for a short time.³ Thus, we found that CH₃NH₃Cl is an effective additive for facile preparation of high quality CH₃NH₃PbX₃ films in both one-step and two-step deposition method. Impact of various processing conditions on the structural, optical, and electronic properties of the perovskite films prepared via CH₃NH₃Cl-assisted thermal decomposition in both one-step and two-step approaches will be discussed. Device characteristics will also be presented.
Reference:
1. Zhao, Y. and K. Zhu, CH₃NH₃Cl-Assisted One-Step Solution Growth of CH₃NH₃PbI₃: Structure, Charge-Carrier Dynamics, and Photovoltaic Properties of Perovskite Solar Cells. J. Phys. Chem. C, 2014. 118(18): p. 9412-9418.
2. Zhao, Y. and K. Zhu, Efficient Planar Perovskite Solar Cells Based on 1.8 eV Band Gap CH₃NH₃PbI₂Br Nanosheets via Thermal Decomposition. J. Am. Chem. Soc., 2014. 136(35): p. 12241-12244.
3. Zhao, Y. and K. Zhu, Three-Step Sequential Solution Deposition of PbI₂-Free CH₃NH₃PbI₃ Perovskite. J. Mater. Chem. A, submitted.

The role of chloride in the MAPbI_3-xCl_x perovskite is still limitedly understood, albeit subjected of much debate. Here, we present a combined angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and first-principles DFT modeling to investigate the MAPbI_3-xCl_x/TiO₂ interface. AR-XPS analyses carried out on ad hoc designed bilayers of MAPbI_3-xCl_x perovskite deposited onto a flat TiO₂ substrate reveal that the chloride is preferentially located in close proximity to the perovskite/TiO₂ interface. DFT calculations indicate the preferential location of chloride at the TiO₂ interface compared to the bulk perovskite due to an increased chloride-TiO₂ surface affinity. Furthermore, our calculations clearly demonstrate an interfacial chloride-induced band bending, creating a directional “electron funnel” that may improve the charge collection efficiency of the device and possibly affecting also recombination pathways. Our findings represent a step forward to the rationalization of the peculiar properties of mixed halide perovskite, allowing one to further address material and device design issues.

In the last two years methylammonium lead halide perovskite based solar cells have been characterized by a fast development, achieving power conversion efficiencies approaching 20% ^{1, 2}. Impressively, most of the latest advancements have been the result of a higher control on the material processing and on the crystallization steps. On the other hand, the poor understanding of the relationship between structure and optoelectronic properties is still a limiting factor for a further improvement in the efficiency.
Here, we demonstrate how the crystallization procedure as well as the nature of the precursors, i.e. the presence of chlorine, can affect the macroscopic structural and optical properties of CH₃NH₃PbI₃ and the chlorine-doped CH₃NH₃PbI₃ perovskite films. We investigate the optical and micro-structural properties of hybrid perovskite polycrystalline films deposited either on flat glass substrate or infiltrated into a mesoporous scaffold combining Raman spectroscopy ³ with X-Ray Diffraction measurements, High Resolution Scanning Electron Microscopy and Energy Dispersive X-ray spectroscopy ⁴. The specific organic-inorganic interactions are revealed by monitoring the Raman signal related to the Pb-I stretching modes and the CH₃NH₃⁺ rocking modes, as important markers of local distortion of the inorganic cage³. The experimental findings point to a clear trend, that is, an ordered arrangement of the organic cation in CH₃NH₃PbI₃ crystallites grown on “flat” substrates with respect to a fully randomly orientation in the mesoporous scaffold. Interestingly, we observe that the displacement of the organic cations and their interaction with the inorganic cage result in a strain felt by the lattice that manifest as a band gap shrinking moving from meso to flat CH₃NH₃PbI₃ film. On the other hand the presence of chlorine in the precursor to obtain the Chlorine-doped CH₃NH₃PbI₃ also affects the crystal formation both at a molecular scale and at a mesoscopic level during crystal growth. In particular, the chlorine assists the crystallization favouring a preferential ordered arrangement of the organic cation in the unit cell even in the mesoporous scaffold, as revealed by XRD.
We demonstrate that, although we do not find any chloride signature in the crystal unit cell, chlorine ions play an important role at a larger scale in driving the formation of crystals with a preferential order. We show that, when the crystallization is carried out on a flat substrate, most chlorine ions are segregated from the film in the forms of large perovskite crystals. When instead the perovskite film is crystallized in the presence of an oxide scaffold, these are retained at the mesoporous interface.
References
[1] Zhou, H. et al.Science.345, 542-546 (2014).
[2] Im, G. H. et al.Nat. Nanotechnol (2014). doi:10.1038/nnano.2014.181
[3] Grancini, G. et al.J. Phys. Chem. Lett. (2014). doi:10.1021/jz501877h
[4] Quarti, C. et al. J. Phys. Chem. Lett. 5, 279minus;284 (2014)

A new-generation of thin-film photovoltaics have emerged in the past two year with the introduction of solution-processable organometallic halide perovskites (e.g. CH₃NH₃PbI₃) as light absorbers. Owing to the extraordinary crystallization kinetics of such organic-inorganic hybrid perovskites from precursor solutions at elevated temperatures of about 70 to 150 ^oC, the reproducible deposition of high quality perovskite thin films has been a challenge. To address this issue, we describe here a universal low-temperature one-step solution processing method which enables the deposition of organometallic halide perovskite thin films with superior uniformity over square-centimeter area and smoothness at the nanometer scale. Using this method, CH₃NH₃PbI₃-based photovoltaics can be reproducibly fabricated with high efficiencies up to 15%. It is also demonstrated that ultrathin perovskite films with thickness below 100 nm can form in complete coverage on substrates and their based full devices deliver surprisingly high efficiency, which are promising for building-integrated photovoltaic applications with low lead levels. Furthermore, partial or full substitution of iodine with bromine in the precursor solution allows band-gap tuning of the perovskite thin films, resulting in tunable vivid colors in the solar cells for potential application as decorative or tandem photovoltaics. The simple and mild deposition conditions used here also expand the selection of materials used in electron/hole selective layers and substrates. This makes the resulting solar cells more versatile in terms of multifunctionality and the types of fundamental studies that can be carried out. This new solution-processing method for the deposition of organometallic halide perovskite thin films is potentially low-cost, considering the low temperature, ambient pressure and one-step nature involved. Also, the rapid perovskite conversion from precursor solutions makes this process amenable to roll-to-roll continuous fabrication and automation for high throughput manufacturing.

Perovskite based solar cells, mostly employ solution processed perovskite layers. Evaporated methylammonium lead iodide perovskite layers have also been reported and been employed in solar cells.^1-3 Recently, we developed metal oxide free perovskite cells with high power-conversion efficiencies by sandwiching these evaporated perovskite films in between organic charge transporting layers .^4-9 We will present recent developments in these metal oxide free perovskite solar cells, such as semi-transparent, flexible and large area cells as well as insight in to their operational mechanism.
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(5) Tvingstedt, K.; Malinkiewicz, O.; Baumann, A.; Deibel, C.; Snaith, H. J.; Dyakonov, V.; Bolink, H. J. Sci. Rep. 2014, 4, 6071.
(6) Roldan-Carmona, C.; Malinkiewicz, O.; Soriano, A.; Minguez Espallargas, G.; Garcia, A.; Reinecke, P.; Kroyer, T.; Dar, M. I.; Nazeeruddin, M. K.; Bolink, H. J. Energy & Environ. Sci. 2014, 7, 994.
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Organo-lead halide perovskite solar cells have gained enormous significance and approached the power conversion efficiencies of commercialized c-Si solar cells and some other thin film photovoltaic solar cells recently. However, one major concern comes from the potential toxicology of the soluble lead. Efficiency boosts for the lead-free perovskite solar cells are thus highly desirable in this regard. Here we show that the crystallization of the methylammonium tin triiodide (CH₃NH₃SnI₃) perovskite can be controlled through the choice of solvents used in the spin coating process, thus enabling a faster crystallization rate in comparison with the CH₃NH₃PbI₃ analog. Careful evaporation of the solvents and the convective self-assembly process during spinning effectively assist the formation of well-crystallized perovskite films due to the favorable interactions between the solvent molecules and the solvated CH₃NH₃⁺ and [SnI₃]^-. The crystallization processes in the selected polar solvents (N,N-dimethylmethanamide (DMF), γ-butyrolactone(GBL) and dimethylsulfoxide (DMSO)) have been investigated and compared. Intermmediate compounds of the perovskite withcrystallized solvent molecules were recognized to be important for the resultant film morphology. Efficient lead-free pervoskite solar cells have been realized with a conductive poly(triarylamine) hole-transporting material. Our results provide important progress towards the understanding of the role of crystallization engineering in the realization of low-cost and highly efficient lead-free perovskite solar cells.

Chalcogenide (CZTSe, CIGS) and organolead halide perovskite solar cells have optoelectronic characteristics and processing requirements well suited for monolithic tandem integration. Highest efficiency CZTSe and CIGS solar cells have lower band gaps (<1.2 eV) and require high temperature processing on a typically non-transparent molybdenum back contact. This makes them a natural choice for bottom devices but until recently the low stability of the p-n junction at temperatures above 200 C was a severe limitation for growing efficient high band gap partners on top. Perovskite solar cells have reached remarkable efficiencies of over 17% with absorber fabrication requiring less than 150°C. The already high 1.55eV band gap of pure CH₃NH₃PbI₃ can be further increased by Br and Cl addition allowing straightforward band gap tuning for current matching with the bottom chalcogenide device.
We report monolithic tandem photovoltaic devices consisting of perovskite solar cells grown directly on chalcogenide solar cells. A proof of concept was first carried out with CH₃NH₃PbI₃ and CZTSSe absorbers, achieving cumulative Voc of 1.4 V, close to the sum of individual cell voltages. These initial devices were limited by the highly reflective and light-absorbing aluminum layer used as a top contact as well as the non-optimal band gap matching of the two absorbers. Recent progress with improved transparent top contacts and band gap optimization of perovskite on CZTSe and CIGS absorbers will be presented.

The past five years have witnessed a tremendous surge in the popularity of hybrid organic-inorganic halide perovskites among several research groups around the globe. The impulse for this trend can be largely attributed to CH₃NH₃PbI₃, a compound semiconductor adopting a distorted perovskite crystal structure that was originally discovered in the late 70&’s. CH₃NH₃PbI₃ has been successfully used for the fabrication of inexpensive, high-efficiency solar cells when used as a light absorber and it has been shown to operate under various device architectures. CH₃NH₃PbI₃ is a member of a wider class of halide perovskites, AMX₃, where A is a univalent cation able to stabilize the perovskite structure, M is typically a group 14 bivalent metal ion and X is a halide anion. Other than CH₃NH₃PbI₃, various line compounds and solid solutions within the AMX₃ system have been demonstrated as efficient photosensitizers, thus highlighting the universally good semiconducting properties for this class of compounds.
Despite the fact that the perovskite compounds are efficient photosensitizers and promise further improvements in the photovoltaic efficiency in the near future, the fundamental optical and electronic properties of the compounds themselves are not yet properly understood. In particular, we observe a significant variation in the photoluminescence (PL) properties of the compounds depending on whether the bulk material was isolated by means of a solution- or solid-state-based process. In the present work we study the properties of selected compounds from the APbI₃ and ASnI₃ systems as a function of the preparation method and we evaluate the resulting materials in terms of vacancy formation by employing a combination of single-crystal X-ray diffraction and theoretical DFT calculations. We further attempt to rationalize on the PL properties (PL emission vs. PL quenching) and electrical properties (resistivity, Hall effect) of these materials and correlate the estimated carrier density with the observed effects.

Perovskite solar cells (PSCs) have attracted much attention in recent years. Their metal halide composition holds the promise of achieving highly efficient and cost effective devices due to high quality crystalline perovskite thin films with tunable absorption edge and high extinction coefficient. Fully solution processed devices are advantageous for future large-scale industrial applications. Methylammonium lead iodide (MAPbI₃) with a bandgap of 1.55 eV and an absorption onset in the near infra-red (800 nm) is mostly investigated for PSCs since it can absorb photons in both visible and near-infrared solar spectrum. Furthermore, its capability of acting simultaneously as a hole conductor and electron transporter makes it suitable for planar heterojuntion PSCs. Recently, it was observed that replacing the methylammonium cation (CH₃NH₃⁺) by a formamidinium cation (CH(NH₂)₂⁺) in the lead iodide perovskite lead to a decreased band gap value (1.47 eV) and a shift in the absorption edge to 850 nm of the perovskite thin film. In this research, we report the fabrication of a high efficiency planar heterojuction formamidinium PSCs. The device architecture is defined by a fluorine-doped tin oxide glass substrate coated with a titanium dioxide (TiO₂) thin film, a FAPbI₃ absorber layer, a 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD) hole transporting layer and silver electrodes. The purity and quality of the perovskite thin film were determined by UV-vis spectroscopy, X-rays diffraction (XRD) and energy diffraction spectroscopy (EDS). Field emission scanning electron microscopy (FE-SEM) was used to characterize the morphology of the absorber layer. Furthermore, the device performances were studied as a function of the relative humidity (RH) (%).The impact of RH content on the device characteristics and charge carrier dynamics will be discussed. The stability of FAPbI₃ based films and devices will be compared to those based on MAPbI₃.

The ability to solution-process at a low temperature makes organometal halide perovskite a particularly attractive semiconductor for optoelectronic applications. In this work, we demonstrate electroluminescence in these hybrid perovskites using two unique device architectures. In one design, a double heterostructure was implemented, where a thin perovskite layer was sandwiched between large-bandgap titania and polyfluorene for effective electroluminescence. The larger bandgap semiconductors confine injected charges within the perovskite layer to enhance electron-hole capture and radiative recombination. In this structure, a near-infrared radiance of 13.2 W sr^-1 m^-2 was achieved at a current density of 363 mA cm^-2, with external and internal quantum efficiencies of 0.76% and 3.4% respectively. We also demonstrate electroluminescence in the green and the red, using a ITO/PEDOT:PSS/perovskite/F8/Ca/Ag structure. The emission wavelength is tuned by varying the halide content in the perovskite thin films. This demonstration of electroluminescence in perovskite semiconductor offers exciting opportunities for future applications in low-cost and large-area displays and lighting.

We report a strategy of using vapor transport chemical vapor deposition (CVD) to prepare well-faceted, high crystal quality organic-based lead halide perovskite platelets on muscovite mica substrates. The well-defined single crystal nanoplatelets of lead halides were first grown on mica substrate utilizing van der Waals epitaxial growth following by thermally intercalating of methyl amino halide (gas phase) into pre-grown lead halide platelets. The resulted CH₃NH₃PbI₃ platelets showed an electron diffusion length of more than 200 nm which is approximately two times higher than that of the solution processed film. This synthesis approach will create a new platform to exploit the physical properties of the organic-based lead halide perovskites. The as-grown platelet crystals exhibit excellent optical properties, as studied by optical absorption and photoluminescence spectroscopy. Notably, we demonstrate an optically pumped room-temperature near infrared laser based on the CH₃NH₃PbI_aX_3-a (X= I, Br, Cl) nanoplatelets. Their large exciton binding energies, long diffusion lengths and naturally formed high-quality planar whispering-gallery mode cavities ensure adequate gain and efficient optical feedback for low-threshold in-plane lasing. The whispering-gallery type planar perovskite nanolasers have pronounced optical gain and tunable optical modes, which can be potentially expanded with controllable emission from UV to NIR. Our research opens alternative routes beyond III-V nanostructures in achieving near infrared solid state nanolasing and will inspire more designs of low-threshold near-infrared nanolasers pumped optically and electrically.

Third generation lead-halide perovskite photovoltaic solar devices are exciting not only in terms of potential progress in power converging efficiency, but also in terms of fundamental science. At present, perovskite light absorber based solar devices have already overcome the efficiency of many emerging and commercial photovoltaics such as dye-sensitized solar cell (DSSC), organic, and amorphous silicon solar cell¹. However for defining the “design rules” structural and morphological properties of perovskite layer and energy level alignment between the interfaces have been very crucial. Irregularities in device structure either within perovskite layer or interface layer often results in the form of unusual hysteresis in current density (J_sc)-voltage (V) curve and also affects the long term stability. Therefore, investigation of material response to light induced process, recombination processes, and role of junction properties are need to be paid more attention.
Here, we present our in-depth investigation on charge carrier recombination mechanisms and estimation of the device parameters (shunt resistance, series resistance and ideality factor) by simulating the I-V characteristics under dark and light, using standard current-voltage model for single heterojunction solar cell. For this study the planer device structure ITO/PEDOT:PSS/Perovskite/PC₆₁BM/Ca/Ag was used. Devices with efficiency over 11% show a high shunt resistance of the order of 10¹⁵#8486;, low series resistance ~3 #8486; and ideality factor of 1.37, which are in good agreement with the well behaved heterojunction solar devices. Furthermore, a generalized impedance model was applied to a series of perovskite solar devices with variations in structure and perovskite constituents, to understand the recombination properties. We demonstrated that our devices exhibit diode saturation current, capacitance, recombination resistance parameters comparable to those extracted from high efficiency thin film solar cell, despite the great differences between materials and design methodology. Further progress in device performance are anticipated through rational interface engineering.
Ref: (1.) B. Wang, X. Xiao and T. Chen, Nanoscale, 6, 12287, 2014.

The hybrid organic-inorganic methylammonium lead halide perovskites (CH₃NH₃PbX₃, X = Cl^-, Br^-, I^-) pioneered for use in thin film transistors by Mitzi, et al., and introduced as a light harvester in dye sensitized solar cell configurations by Miyasaka et al., have attracted intense attention for thin-film photovoltaics, due to their large absorption coefficient, high charge carrier mobility and diffusion length. Power conversion efficiencies (PCEs) of over 15% were obtained with both mesoporous metal oxide scaffold and in planar heterojunction architectures.
Despite the rapid increase in efficiency associated with the evolution of different types of perovskites and device fabrication techniques, the hole transporting material (HTM) used were mainly limited to organic compounds, the state-of-the-art 2,2',7,7'-tetrakis (N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD) and other small molecules, such as pyrene arylamine derivatives, and conducting polymers. Their relatively high cost when compared to the n-type semiconductors (TiO2, ZnO) and to the perovskite itself is one of the main limitation of these materials for further large scale application. Compared to organic HTMs, inorganic p-type semiconductors appear to be an ideal choice given their high mobility, stability, ease of synthesis and low cost. Copper iodide (CuI) was just reported as a hole conductor in lead halide perovskite based devices, showing a PCE of 6% [by Kamat et al.]. Another interesting inorganic p-type semiconductor is copper thiocyanate (CuSCN), which shows good transparency throughout the visible and near infrared spectrum, high hole mobility of 0.01 - 0.1 cm² V^-1 s^-1 (as compared to 4 × 10^-5 cm² V^-1 S^-1 for spiro-MeOTAD), and good chemical stability. Furthermore, it can be deposited through a solution-processed processing at low temperature, making it compatible with also flexible substrates.
Herein, we demonstrate a device structure consisting of titanium dioxide as a scaffold as well as electron collector, lead-halide perovskite as the light harvester, and copper thiocyanate as the hole transporting material. The combination of CH₃NH₃PbI₃ perovskite with CuSCN shows high power conversion efficiency of 12.4% under full sun illumination, indicating their further application in cost-effective photovoltaic devices.

All solid-state solar cells based on organometal trihalide perovskite absorbers have already achieved distinguished power conversion efficiency (PCE) to over 15% and further improvements are expected up to 20%. These novel organometal halide perovskite absorbers which possess exceptionally strong and broad light absorption enable to approach the performances of the best thin film technologies. Especially, the cost-effective solution process for perovskite solar cells at a low temperature makes them viable to realize flexible thin film solar cells.
Efficient charge collection is one of critical issues in perovskite solar cells [1-3]. The charge collection efficiency for perovskite solar cells can be enhanced by controlling nanostructure or exploiting new materials. In this presentation, we demonstrate that nanostructured materials such as SnO2@TiO2 core-shell nanowires and 3D-ITO nanowire/TiO2 nanoparticle materials can facilitate charge transport in perovskite solar cell. Also, MgO nanolayer coated on TiO2 nanoparticles can efficiently retard charge recombination.
Finally, we introduce highly bendable 12 % perovskite solar cells based on ITO/PEN substrates [4]. The energy conversion efficiency did not change after 1000 cycle of bending test with 10mm bending radius which demonstrates a feasibility of highly bendable perovskite solar cells without efficiency degradation.
References
(1) K. Mahmood, B. S. Swain and H. S. Jung, Nanoscale, 2014, 6, 9127 (2014).
(2) G. S. Han, S. Lee, J. H. Noh, H. S. Chung, J. H. Park, B. S. Swain, J.-H. Im, N.-G. Park and H. S. Jung, Nanoscale, 6, 6127 (2014).
(3) G. S. Han, H. S. Chung, B. J. Kim, D. H. Kim, J. W. Lee, B. S. Swain, K. Mahmood, J. S. Yu, N.-G. Park, J. H. Lee and H. S. Jung,, J. Mater. Chem. A DOI: 10.1039/C4TA03684K (2014).
(4) B.-J. Kim, D. H. Kim, Y.-Y. Lee, H.-W. Shin, G. S. Han, J. S. Hong, K. Mahmood, T. Ahn, Y.-C. Joo, K. S. Hong, N.-G. Park, S. Lee and H. S. Jung, Energy & Environmental Science, DOI: 10.1039/C4EE02441A (2014).

In the past few years there have been extensive research efforts on developing high performance organolead halide based perovskite solar cells. While most studies focused on optimizing the deposition processes of the perovskite films, the selection of the precursors is rather limited to the PbI₂ (or PbCl₂) and methylammonium (or formamidium) halide combination. In this work, we developed a new precursor compound, HPbI₃, to replace PbI₂. The compound shows excellent solubility (up to ca. 2.5 M) in DMF at room temperature, and can react with formamidium iodide (FAI) to produce formamidium lead iodide (FAPbI₃) perovskite films with high uniformity and excellent thermal stability. As compared to the previously reported FAPbI₃ films produced from the PbI₂ based precursor combinations, the HPbI₃ based FAPbI₃ films exhibit much purer crystalline phase with strong (110) preferred orientation. Such high crystallinity benefits from a slow crystallization process involving exchange of H⁺ and FA⁺ ions in the PbI₆ framework. The high quality FAPbI₃ films are integrated in a simple planar solar cell structure and yield an average efficiency of 15.4% and champion efficiency of 17.5% under AM 1.5 illumination. The simple preparation of HPbI₃, together with the facial solution deposition technique, are expect to improve the reproducibility and scalability of perovskite solar cells. The versatility of the new precursor compound are further demonstrated by the success of using (1) FAI/ HPbI3 to fabricate FAPbI₃ perovskite on mesoporous TiO₂ substrates, and (2) MAI/HPbI₃ to synthesize MAPbI₃. The film formation mechnism for each material system will be discussed in details in this talk. We believe that the engineering of the HPbI₃ precursor systems may open new possibilities to fabricate perovskites with new functionalities.

Hybrid perovskites (HPs) are rapidly evolving into high performing photovoltaic materials. Solution processed solar cells with 17.9% power conversion efficiency have been certified. Furthermore, HP devices have achieved open circuit-voltages of 1.07 V [1], which is 86.1% of their theoretical limit (based a 1.55 eV bandgap). Only GaAs outperforms HPs in this respect. In addition, if the bandgap of HPs could be increased slightly (over 1.7 eV) with similar optoelectronic quality, a new generation of efficient tandem solar cells could be possible using silicon, CIGSe or CZTSe as the bottom cell.
The halide composition of HPs has been observed by others to effect bandgap, carrier lifetime, and material stability. However, the ternary alloy space of Cl-Br-I has barely been explored, and it is likely that the best HP formulations have not yet been discovered. Previously, we have developed a combinatorial spray coating method to deposit thousands of compositions of the pentenary semiconductor CZTSSe on a single substrate [2]. In order to screen the materials for optoelectronic quality, we have also developed a theory and model of absolute intensity photoluminescence (AIPL) [3] to extract the quasi-Fermi level splitting. Here, we use ultrasonic spray coating of HP precursor inks to deposit composition spread libraries in the CH3NH3Pb(Cl,Br,I)3 system. We use AIPL to extract and map the quasi-Fermi splitting, bandgap, and presence of sub-bandgap states. We couple the AIPL with EDS, UV-Vis, and lifetime measurements. We show the ability to reach 90% of the theoretical quasi-Fermi level splitting over certain compositions.
In addition to bandgap and optoelectronic quality, controlling HP film growth to yield continuous films is crucial for high performance single junction or tandem devices. As a result, we have also developed a new growth method that yields dramatically improved morphology and higher minority carrier lifetimes. For the pure iodide, typical single solution or two-step approaches yield MAPbI3 with weighted average lifetimes of less than 10 ns (longer lifetimes typically require sacrificial chloride). Here, we show that our growth method yields pin-hole free smooth films of pure iodide HPs with lifetimes greater than 200 ns (the weighted average lifetime from a biexponential decay). We will also report results of larger bandgap HPs.
[1]. Liu, M. Z.; Johnston, M. B.; Snaith, H. J., “Efficient planar heterojunction perovskite solar cells by
vapour deposition,” Nature 2013, 501, 395-397.
[2]. Collord, A.D., Xin, H., and Hillhouse, H.W., “Combinatorial Exploration of the Effects of Intrinsic and Extrinsic Defects in Cu2ZnSn(S,Se)4,” IEEE Journal of Photovoltaics 2014, DOI: 10.1109/jphotov.2014.23610536.
[3]. Katahara, J.K. and Hillhouse, H.W., “Quasi-Fermi level splitting and Sub-bandgap Absorptivity from Semiconductor Photoluminescence” Journal of Applied Physics 2014, DOI: 10.1063/1.48948346

As newly arrived family of materials, organic-inorganic hybrid perovskites (A = alkyl-NH₃, B = Pb or Sn, X = I, Cl or Br) have much attired attention in the photovoltaic community, mainly because of the rapid evolution of cell efficiencies that is now comparable with thin film technologies such as CdTe and CIGS. Nevertheless, despites this fast increase in reported performance there are many point on the physics of these materials that still unknown or in which our knowledge is limited.
Among the interesting properties of hybrid perovskites, the possibility of bandgap tuning with the composition, via modification of Br/I ratio, is particularly attractive for developing advanced devices such as window integrated solar cells or tandem solar cells. In any case, a comprehensive study of the optical properties of these materials is needed for the design of this kind of devices.
In this work, the optical characterization of MAPb(Br_xI_1minus;x)_3minus;yCl_y perovskites, with different Br/I ratio, has been performed. Spectroscopic ellipsometry in the absorbing range of the materials (250-850nm) has enabled to obtain the optical indexes for the different compositions. For validating these indexes, transmission and reflection spectra were calculated and compared with the measured ones. Also low and room temperature photoluminescence measurements have been done in an effort to understand the excitonic behavior of the materials.
Finally, optical indexes are used for a 1D optical model of a tandem solar cell, determining the optimal bandgaps for top and bottom subcells and their respective thicknesses. The theoretical study of a tandem cell of perovskite and CIGS is also reported. This case is extremely interesting from the point of view of industrial applications as they take advantage of the already existing technology for production of Si and CIGS photovoltaic devices.

The field of thin film photovoltaics has been recently reinvigorated by the development of solid state organic-inorganic perovskite solar cells [1,2]. Attention is now starting to turn to the manufacturing processes required to scale these lab devices into modules, for high volume output roll to roll processing on low cost substrates such as metal foils and plastic sheeting.
Here we present three technical process breakthroughs which can support scale up. Firstly a reduction in manufacturing time to enable continuous processing of both the TiO₂ blocking layer and the active perovskite layer (from 90 minutes to 3 seconds), secondly the translation of conventional laboratory spin coating to continuous slot die coating and thirdly the removal of the expensive SPIRO OMeTad layer for a simplified manufacturing process.
The alternative method of heat treating perovskite films was achieved by exposing the freshly deposited perovskite film to a short burst of near-infrared (NIR) radiation using commercially available lamps. The NIR region of the electromagnetic spectrum is situated between the visible and the infrared with wavelengths ranging from 700 nm to 2500 nm, peaking 1000 nm. The perovskite precursor solution has a relatively low absorbance in these wavelengths, as does the c- TiO₂ layer beneath it. FTO or metal absorbs NIR radiation far more readily, causing it to rapidly heat under irradiation. This fast heating of the FTO layer, combined with the relative transparency of the perovskite film to NIR radiation allows for rapid (<3 second), indirect annealing of the device&’s active material producing devices with PCEs similar (10%) to that of a 90 minute heat treatment.
Slot die coating of the perovskite active layer can replace laboratory spin coating as a continuous deposition process. This method pumps the perovskite precursor through a die and perpendicularly onto the moving substrate below, allowing for variables such as flow rate, coating speed and die substrate gap to be tuned to give a thinly deposited wet film. Particular challenges which have been overcome include maintaining heterogeneous nucleation of the perovskite and tightly controlling film thickness.
The third technological development is the ability to remove the SPIRO-OMeTAD layer and replace it with an ultra-dry spray of conducting polymer material which can be applied directly to the perovskite in a continuous manufacturing process, although a slight performance loss is incurred (with 7% efficiency) this represent an intriguing route to large scale manufacture since it replaces a component that currently costs $500 per gram.
[1] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science.2012,338, 643.
[2] H-S. Kim, C-R. Lee, J-H. Im, K-B. Lee, T. Moehl, A. Marchioro, S-J Moon, R. Humphry-Baker, J-H. Yum, J. Moser, M. Graetzel, N-G. Park, Sci. Rep.2012, 2, 1.

Organometal trihalide perovskites are emerging as a new generation of photovoltaic materials. The power conversion efficiencies (PCE) of solution-processed photovoltaics have rapidly increased to beyond 17%. Here, we developed a simple and effective low-temperature layer-by-layer solution processing approach to fabricate CH₃NH₃PbI₃ perovskite photovoltaics with high efficiencies exceeding 17%. Cross-sectional transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) mapping was used to characterize the crystallinity, structure, interfaces, and degradation characteristics of perovskite materials in PV devices. We find that low-temperature thermal annealing not only drives the interdiffusion of the precursor layers to form the perovskite but also promotes grain growth, yielding better crystallinity and larger grain size. However, high-temperature or lengthy annealing drives decomposition of the perovskite and reduces the device efficiency. Electron beam induced current (EBIC) microscopy and time-resolved photoluminescence spectroscopy were used to understand how material crystallinity and grain boundaries affect charge carrier dynamics and device performance in high-efficiency CH₃NH₃PbI₃-based perovskite photovoltaics.
This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility

Organic-inorganic hybrid perovskite solar cells have recently become strong candidates for photovoltaic applications, already achieving power conversion efficiencies similar to other established thin film technologies. Their facile low temperature solution processing also makes them potential candidates for combinations with conventional solar cells in tandem device architectures. However, the bandgap of the usual perovskite structure based on methylammonium lead iodide is too narrow to be optimally used in tandem solar cells. The bandgap can be widened by exchanging some or all of the iodide ions by bromide. On the other hand, the resulting methylammonium lead bromide is limited by short diffusion lengths and therefore requires a high temperature sintered mesoporous titania scaffold to perform efficiently.^[1] This eliminates these types of solar cells as potential candidates for a tandem configuration with conventional inorganic solar cells that are not stable at the required temperatures. In this context, we present, for the first time, highly efficient wide bandgap solar cells with a planar heterojunction layout based on formamidinium lead bromide.^[2] It exhibits a similar crystal structure as methylammonium lead bromide and comparable bandgap and absorption properties. We show that this material can be easily prepared by solution processing below 100°C, exhibits very slow photoluminescence decay dynamics, and therefore charge diffusion lengths approaching the micron scale. It performs efficiently even without a mesoporous charge extraction layer, exhibiting power conversion efficiencies approaching 7 %. This makes it a suitable wide bandgap absorber for tandem solar cell applications or building-integrated photovoltaics.
[1] S. Ryu, J. H. Noh, N. J. Jeon, Y. C. Kim, W. S. Yang, J. W. Seo, S. I. Seok, Energ. Environ. Sci. 2014, 7, 2614-2618
[2] Hanusch, F. C.; Wiesenmayer, E.; Mankel, E.; Binek, A.; Angloher, P.; Fraunhofer, C.; Giesbrecht, N.; Feckl, J. M.; Jaegermann, W.; Johrendt, D, Bein, T., Docampo, P., J Phys Chem Lett 2014, 5, 2791-2795.

Solar cells that convert solar energy into electrical energy have been widely recognized as one of the promising cost-effective, safe, and renewable alternatives to fossil fuels. The discovery of photovoltaic activities in organolead halide perovskites leads to a breakthrough in exploring high-efficiency, low-cost, and large-area thin film solar cells. The power conversion efficiency (PCE) of organolead halide perovskite solar cells have been skyrocketing from 3.8 % in a pioneering work to recent values approaching 20% over the past 5 years. On the other hand, the absorption onset of prototypical CH₃NH₃PbI₃ perovskite is limited to ~1.5 eV, making it not able to absorb the incident light over 800 nm which allows for further improvement of device efficiency. In this context, we report a strategy of improving the photovoltaic performances of CH₃NH₃PbI₃ perovskite solar cells by incorporating upconversion nanoparticles (i.e., NaYF₄:Yb/Er). These hydrophilic monodisperse upconversion nanoparticles were synthesized by the multiarm star-like block copolymer nanoreactor technique recently developed in our group, resulting in monodisperse, stable, spherical upconversion nanoparticles covered by hydrophilic polymer ligands. The as-prepared upconversion nanoparticles can be employed to effectively improve the optoelectronic efficiency of CH₃NH₃PbI₃ perovskite active layer by converting near-infrared (NIR) light to visible high-energy light (i.e., green).

Hybrid organic/inorganic perovskite solar cells are experiencing an explosive development with efficiency rising from the initial 3 % to a certified 17.9 %. A typical perovskite solar cell consists of TiO₂ (mesoporous or compact), CH₃NH₃PbI₃, hole transport material (HTM) and noble metal counter electrode. However, the conventional organic HTMs (such as spiro-OMeTAD) are not only expensive but also air-unstable. The noble metal electrode is also costly and requires demanding vacuum thermal evaporation deposition. Carbon materials are expected to be excellent hole extractor for HTM-free perovskite solar cells due to their suitable Fermi level (- 5.0 eV), earth abundance, low cost and superior environmental stability. In this work, we propose and develop the concept of clamping solar cells by joining together separately optimized perovskite photoanode and candle soot hole extraction electrode.
We established a cost-efficient, environmentally stable and abundant candle soot as an efficient hole extractor and developed a concept of clamping solar cells by judiciously interfacing the candle soot with CH₃NH₃PbI₃ films. In this effort, three generations of clamping solar cells were evolved from direct clamping to rolling-transfer clamping and to chemically promoted clamping accelerated by the mechanistic understanding of inner workings. Up to this stage, the third generation clamping solar cells have already achieved a remarkable efficiency of 11.02 % and good long term stability. Femtosecond time resolved PL and distance dependent PL measurements have confirmed the conclusion that the improved PCE is largely due to the enhanced directional hole extraction at the candle soot/perovskite interface through the mechanical and chemical promotion strategies. Thus we have shown that the clamping solar cells represent a new type of easily processable but high performing photovoltaic devices. The present work has not only promoted the fundamental understanding on the clamping cell working mechanisms but also made great headway towards roll-to-roll production and realistic commercialization of perovskite solar cells.