Recently, organic-inorganic perovskites were identified as promising absorbers for solar cells.¹ In the three years since, the performance of perovskite-based solar cells has improved rapidly to reach efficiencies as high as 15 %.^2-4 We developed metal oxide free methylammonium lead iodide perovskite cells with high power-conversion efficiencies.⁵ The effect of the organic charge transporting layers on the performance of these solar cells will be presented as well as the effect of different layer thicknesses. The power conversion efficiency increases from 4.7 % for a device with only an organic hole transporting/electron blocking layer to 12 % when the perovskite layer is sandwiched in between suitable organic electron and hole blocking layers. 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.
1 Kojima, A., Teshima, K., Shirai, Y. & Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. JACS131, 6050-6051 (2009).
2 Liu, D. & Kelly, T. L. Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques. Nat Photon8, 133-138 (2014).
3 Liu, M., Johnston, M. B. & Snaith, H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature501, 395-398 (2013).
4 Burschka, J. et al. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature499, 316-319 (2013).
5 Malinkiewicz, O. et al. Perovskite solar cells employing organic charge transport layers. Nature Photonics8, 128 (2014).

An intense effort is boosting the development of third generation photovoltaic (PV) cells, to obtain cheap, high efficiency and environmentally friendly devices. One of the most promising solar cell architectures is based on quantum dots (QDs). The photoconversion efficiency (PCE) have reached to above 7%, by using near infrared (NIR) PbS QDs.[1] Electrophoretic deposition (EPD) has been demonstrated for preparation of high efficiency photo-anodes for QD solar cells, in which QDs are grafted to a mesoporous TiO₂ NP thin film. As the performance of QD solar cell is highly dependent on not only the loading amounts, but also the QDs dispersion in TiO₂ film, it is very important to control the QDs loading process.
Here, for the first time, we report a systematic investigation and modeling of the dynamics of NIR QDs loaded into TiO₂ mesoporous film via EPD. We used PbS@CdS core@shell QDs and investigated the influence of EPD time, QD&’s concentration and voltage on the QD uptake process via Rutherford backscattering for Pb depth profiling. The optical density of the obtained film is strongly dependent on the applied voltage, the deposition time and the concentration of solution containing the QDs. We modeled the deposition process using Fick&’s diffusion law and explained the observed trends as a fast (and depth-independent) QD uptake induced by the presence of the electric field, followed by a diffusion-induced QD migration from outside the film, due to the fast creation of a QD concentration gradient. In addition, we demonstrated the increased stability of the core@shell structure compared to PbS QDs in terms of structure and optical property, based on X-ray photoelectron spectrometry and photoluminescence measurements. Thanks to the much higher stability of the core@shell QDs as compared to pure PbS QDs, our findings suggest that the PbS@CdS QDs loaded with EPD can be profitably used for the development of highly efficient and stable light absorbers in PV devices.
[[1]] Salant, A.; Shalom, M.; Hod, I.; Faust, A.; Zaban, A.; Banin. “U. Quantum Dot Sensitized Solar Cells with Improved Efficiency Prepared Using Electrophoretic Deposition”, ACS Nano, 4, 2010, pp. 5962minus;5968

Recently, the low-cost organolead halide perovskites have emerged as the most promising absorber materials for the development of next generation high efficiency and cost-effective photovoltaic devices. Owing to its impressive properties such as high absorption coefficient over a broad region of visible light spectrum and extremely long carrier diffusion lengths, a device power conversion efficiency (PCE) as high as 25 % could well be within reach in the future. This is comparable to the best commercial single-crystalline silicon solar cells which are substantially more expensive than the perovskite materials. Although there have been a number of reports on the development of high efficiency perovskite-based solar cells demonstrate significant potential in achieving high device efficiency, the basic understanding of the materials, device properties, working mechanisms as well as the manufacturing processes are still at the early stages of development. In this work, we report on the fabrication and systematic investigations of high efficiency planar CH₃NH₃PbI₃-based solar cells (FTO/TiO₂ compact layer/CH₃NH₃PbI₃/spiro-MeOTAD/metal electrode). A two-step spin coating technique was used to fabricate the devices. Through careful optimization of the fabrication and film formation processes we have achieved a high PCE of 15.4% measured under the calibrated ABET Technologies SUN 2000 solar simulator equipped with AM 1.5 filter at 100mW/cm², which is a record efficiency, at the time of the composition of this abstract, for all-solution processed CH₃NH₃PbI₃-based devices with a planar structure. Detailed investigations, including I-V characteristics, external quantum efficiencies, carrier lifetimes, impedance spectroscopy and low-frequency noise measurements, were performed on the devices to examine the underlying mechanisms responsible for the observed improvements in the PCEs of the devices. In particular, systematic studies on the impact of the optimized fabrication process on the density of the localized states and their effects on the performance of the devices were performed. From the experimental results, it is observed that performance of perovskite solar cells is strongly affected by concentration of the material defects which could be highly sensitive not only to the processing parameters but also the post-deposition treatments of the films. The results of our investigations point to a direction for future improvements of perovskite-based solar cells.

Mixed-halide hybrid perovskites such as CH₃NH₃Pb(Br_xI_1-x)₃ are a promising family of photovoltaic absorber materials that have achieved power conversion efficiencies of over 17%. By varying the halide composition, the optical bandgap can be tuned over the range 1.6-2.3 eV, making this family of materials a suitable candidate for both single-junction solar cells as well as the large bandgap absorber of a tandem solar cell. However, reports of mixed CH₃NH₃Pb(Br_xI_1-x)₃ devices with higher bromine content have so far not been able to achieve the increase in open circuit voltage that may be expected from the larger bandgap of these materials. We observe photo-induced halide segregation in bromine-rich CH₃NH₃Pb(Br_xI_1-x)₃ and other mixed-halide perovskites as evidenced by the appearance of intense photoluminescence and absorption features from a new iodide-rich phase upon continuous illumination and the disappearance of these features with time in the dark. We suggest that photoexcitation may induce halide migration, resulting in iodide-rich domains that act as traps and pin the open circuit voltage at a lower energy. The kinetics of this process have a similar temperature dependence to the hysteretic behavior in planar CH₃NH₃PbI_3-xCl_x solar cells which is suggestive of a prominent role of halide migration in perovskite photovoltaic hysteresis. These observations are reminiscent of photo-initiated halide migration in lead halides and other metal halides, which has been proposed to occur via a halide-vacancy diffusion mechanism from surface sites. This suggests that improved control of the perovskite stoichiometry, crystallinity and surface passivation are potential strategies towards reducing halide migratory effects and improving the stability of halide perovskite optoelectronic devices.

High voltage output is a central feature of the high efficiency of perovskite photovoltaic cells. Open-circuit voltage can reach 1.1V or more by constructing multilayer structures of uniform thickness and minimized interfacial resistance with use of suitable compact layers and hole transport materials. For the narrow band gap tri-iodide perovskite (Eg=1.55 eV ), relatively high voltage (V_oc >1.05V) is obtained with use of Al₂O₃ as a mesoporous and insulating scaffold when compared to semiconductive TiO₂. Planar structured perovskite cells without using mesoporous scaffold is also able to generate high voltage. Here a common point of their structures is that electron transfer interface is formed at the peroskite/compact layer junction rather than perovskite/mosoporous TiO₂ interface. In the latter case, number of hetero interfaces for electron transfer becomes larger (perovskite/mesoTiO₂ and meso-TiO₂/compact TiO₂); this increases internal resistance to reduce V_oc.
Apart from high voltage, I-V characteristics of perovskite cell are often accompanied by serious hysteresis, which impairs the reliability of cell performance. We found that the cell constructed of a thin compact layer (<20 nm), mesoporous TiO₂, mixed halide (Cl-doped) perovskite, and spiroMeOTAD has I-V performance that is perfectly reversible without hysteresis for a wide range of voltage scanning rate and incident intensity. We measure the extent of hysteresis as a function of compact layer and meso-TiO2 thicknesses and as the influence of different cell structures. These experiments and impedance analysis led us to locate the origin of hysteresis based on the physical properties of the hybrid perovskite crystal.
For high voltage generation, one of the best cell structure we found is use of a well-oriented crystalline hole transport material in junction with an oriented high quality perovskite layer. Crystalline thin film of perylene is found to be an efficient hole conductor for this purpose. Self-organised formation of perylene on the surface of perovskite was influenced its orientation by the orientation of underlying perovskite. The fully crystalline perovskite-perylene hybrid cell is capable of V_oc exceeding 1.2V sustaining high conversion efficiency. The voltage loss of this cell, <0.35 eV, is one of smallest value ever achieved by solid state thin photovoltaic cells, and can be compared to GaAs solar cells capable of high voltage (1.12V).

We examine the voltage and energy loss in methyl-ammonium lead-halide perovskite solar cells in comparison to crystalline silicon (c-Si) and polymer:fullerene systems. The contributions to voltage and energy loss can be interpreted either in terms of a balance of generation and recombination events or in terms of energy levels. Using the principles of detailed balance, combined with electroluminescence spectroscopy (EL) and sub-bandgap quantum efficiency measurements, we derive the theoretical upper limit of the open-circuit voltage (V_oc,rad), when only radiative recombination occurs. The voltage difference ΔV_oc,nr between the actual V_oc and V_oc,rad is attributed to non-radiative recombination.
We show that pervoskite solar cells have a slightly larger non-radiative voltage loss (0.28 V) than c-Si (0.22 V) but smaller than the best organic system (0.35 V). The voltage drop between optical bandgap (E_opt/q) and V_oc,rad for perovskite and c-Si are similar, at about 0.26 V for both, whilst the organic devices investigated exhibit a much larger voltage drop due to the need for a heterojunction to separate excitons. The voltage loss can, alternatively, be divided into components representing geminate and non-geminate recombination. The results show that recombination losses in perovskite devices are dominated by non-geminate processes in contrast to c-Si and organic solar cells which show a higher proportion of geminate loss.
We also show that radiative recombination in perovskite devices is independent of the hysteresis behavior that is often observed in these devices. The hysteresis behavior can strongly affect the non-radiative recombination process. Also, a correlation is seen between the photocurrent, photovoltage, photoluminescence and electroluminescence during the time dependent relaxation due to the hysteresis effects. The magnitudes and relaxation times of these measurements vary with temperature from 90K to 330K in perovskite devices, with less hysteresis observed at lower temperatures.
The understanding of different factors contributing to recombination and limiting V_oc in the different material systems can be used in future research to minimize energetic losses and increase stability in perovskite solar cells.

Perovskite compounds of the general formula ABX₃ have played a central role in the evolution of condensed matter physics and materials chemistry over the last 70 years. Together with the growing interested in dense metal organic frameworks (MOFs), the research field of organic-inorganic hybrid perovskites combines two major fields of materials science.[1]
In particular, the family of lead and tin based hybrid perovskites with the general formula [AmH]MX₃ (AmH⁺=protonated amine, M²⁺=Sn²⁺,Pb²⁺ and X=Cl^-,Br^-,I^-) was recently discovered to exhibit impressive performances in solar cell applications and additionally takes advantage of straightforward processing methods. The electrical power conversion efficiencies of lead iodides have increased from 3.8% in 2009 to over 16% by the end of 2013, leading to a surge of interest in the study of hybrid perovskites.[2]
In general the complex interplay between different types of bonding interactions in hybrid perovskites makes crystal engineering a challenging task, even though some important studies have addressed this issue.[3,4] In this work we apply a more fundamental approach towards designing hybrid perovskite materials focusing on their relationship to their inorganic analogues using Goldschmidt&’s concept of Tolerance Factors.[5] Goldschmidt&’s concept is a semi-empirical approach that combines the idea of dense ionic packing with ionic radii and purely inorganic perovskites usually show Tolerance Factors between ~0.8 and 1. Based on reported crystallographic data we estimated a consistent set of effective ionic radii for different organic ions which then allows for the calculation of Tolerance Factors of hybrid perovskites. To our knowledge, all organic-inorganic hybrid perovskites currently show Tolerance Factors between 0.81 and 1, thereby emphasizing the close relationship to their inorganic analogues. Our approach is applicable to all families of hybrid perovskites including lead and tin iodides, transition metal formates and azide frameworks with a perovskite-like architecture. We further show how Tolerance Factors can be used to predict the existence of hitherto unknown hybrid perovsite-like compounds.
References
1. A. K. Cheetham, C.N.R. Rao, Science 318, 2007, 58.
2. M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501, 395-398.
3. D. B. Mitzi, J. Chem. Soc., Dalton Trans., 2001, 1-12
4. Z. Wang, K. Hu, S. Gao, H. Kobayashi, Adv. Mater., 2010, 22, 1526-1533.
5. V. M. Goldschmidt, Naturwissenschaften, 1926, 14, 477-485.

Hybrid organic / inorganic perovskites such as methylammonium lead tri-halides (MAPbX_3-nY_n: X, Y = halogen, n = 0-3) are materials of substantial interest as the light-harvester in photovoltaic devices. There is currently some debate about the effect of processing upon the structure and composition of the resulting material, with suggestion, in the case of the mixed halide MAPbI_3-nCl_n, that there is no chlorine present in the resulting material. We have used thermal analysis, spectroscopy and ‘hyphenated&’ techniques, which facilitate evolved gas analysis, to understand changes occurring during material processing.
Samples of MAPbI_3-nCl_n were prepared by processing aliquots of 40 % precursor solution (in DMF) at different cure temperature / time combinations. TGA and DSC were used to monitor mass loss and heat flow isothermally (during the cure process) and on a temperature ramp (after the material had formed). The hyphenated techniques TGA-GCMS and DTA-FTIR were used to analyse any volatiles released during each process.
TGA-FTIR and TGA-GCMS showed only solvent evolution during the cure step. Post-cure TGA-FTIR analysis of material prepared at 100 °C (15 minutes) and 30 °C (240 minutes) shows that the resulting material still contains a significant amount of residual solvent, which is released at increasing temperature; it is possible that a small amount of solvent becomes incorporated within the perovskite matrix, requiring increased energy for its release.
Post-cure Differential Scanning Calorimetry (DSC) shows differences in thermodynamic properties. A sample cured at 100 °C for 15 minutes shows no features over the temperature range 20 - 120 °C but a sample cured at 30 °C shows overlapping features around 80 °C; these are not present on subsequent scans. As solvent is demonstrably trapped within the sample matrix this is not surprising; however, the number and nature of the features is interesting. They could be attributed to simple release of solvent or something more complex: if the incorporated solvent acted as a plasticizer, the material may have formed in an amorphous state; the thermodynamic features could therefore include a phase change.
DSC also demonstrates that the material resulting from the MAPbI_3-nCl_n precursor solution is not simply MAPbI₃: it is well documented that MAPbI₃ undergoes a defined, reversible, tetragonal - cubic phase change around 55 °C; this is easy to replicate using the MAPbI₃ / DMF precursor solution; however, this phase change not present in materials produced from MAPbI_3-nCl_n / DMF; this shows that the material resulting from the mixed halide cannot be MAPbI₃.

The rapid rise of research efforts on photovoltaic devices utilizing organic-inorganic hybrid perovskites has led to power conversion efficiencies (PCEs) exceeding 15 % over the past year¹. Initially, these devices employed nanostructured charge transport layers until planar device structures were realized to have comparable or greater performance². Inverted planar structures, which extract holes through the transparent electrode, have made recent progress on the hole transport layer with materials such as PEDOT:PSS and NiOx^3,4,5. In this work, we have developed a p-type copper (II) oxide (CuO) as an alternative to current hole transport materials to construct planar CH_shy;3NH₃PbI₃ solar cells using a sequential deposition method. To the best of our knowledge, this is the first report to demonstrate efficient perovskite solar cells with p-type CuO as the hole transport layer. By depositing the CuO layer from a low-temperature sol-gel solution process, we are able to fabricate cells with PCEs reaching 14.3%, higher than that of PEDOT:PSS based devices. The key aspects to form the perovskite layer will also be discussed. Our results indicate that the implementation of CuO as a hole transport material broadens the scope of potential architectures for perovskite solar cells.
References
1. J. Mater. Chem. A, 2014, 2, 5994
2. Nat. Phot. , 2014, 8, 133
3. Nat. Commun. , 2013, 4, 2761
4. Sci. Rep. , 2014, 4, 4756
5. Phys. Chem. Lett. , 2014, 5, 1748

Hybrid halide perovskite solar cells have demonstrated efficiencies approaching those of crystalline silicon. Although these perovskite absorbers were initially used in a mesostructured architecture, high efficiencies have been reported in simple planar heterojunction devices. These thin films of methylammonium lead mixed halide perovskite (CH₃NH₃PbI_3-xCl_x) are often solution-processed, and the role of chlorine in the development of the perovskite phase is of particular interest to the community, but poorly understood. Deposition methods involving chlorine have been shown to increase carrier lifetime, improve absorption at the band edge, and decrease series resistance [1]; however, the presence of chlorine in these films has not been confirmed. We have investigated the presence and chemical state of chlorine in thin films spin-cast from a solution of PbCl₂ and CH₃NH₃I using X-ray absorption spectroscopy at the chlorine K-edge. Films were thermally annealed for different durations in order to correlate chlorine composition with annealing procedure. This has enabled us to track the evolution of chlorine from the film and determine how much remained after a standard annealing time of 2 hours at 95°C. We have found that chlorine remains present in all films including those annealed up to 6 hours, dramatically longer than annealing times reported in the fabrication of high efficiency devices.
[1] P. Docampo, F. Hanusch, S. Stranks, M. Döblinger, J. Feckl, M. Ehrensperger, N. Minar, M. Johnston, H. Snaith and T. Bein, 'Solution Deposition-Conversion for Planar Heterojunction Mixed Halide Perovskite Solar Cells', Advanced Energy Materials, 2014.

Organometal halide perovskites have become a highly interesting class of photovoltaic materials with an efficiency of almost 20%, as reported very recently. At the same time, a quantum efficiency for light emission on the order of 70%, and optically pumped lasing has been achieved very recently.^[1] The electro-optical properties of this class of materials are strongly linked to the preparation technique, the resulting composition and micro-/nanostructure of the materials. Two step formation processes, initially explored by Mitzi and coworkers,^[2] recently turned out to yield excellent solar cells with an apparently low defect density.^[3] However, factors governing the material formation process and the resulting light emitting characteristics remain to be understood. In this work, perovskite layers were formed by the thermal evaporation of 100 nm of PbCl2 or PbI2, followed by dipping into a CH3NH3I solution. In the case of PbCl2, a significantly faster conversion to CH3NH3PbClxI3-x is evidenced by x-ray diffraction than in the case of PbI2. Concomitantly, a substantially increased luminescence intensity of about an order of magnitude for CH3NH3PbClxI3-x compared to CH3NH3PbI3 is evidenced, which hints to a lower defect density in CH3NH3PbClxI3-x. The influence of pre-wetting the metal halide layer with isopropanol on the kinetics of perovskite formation and on the resulting morphology will be discussed in detail. In the case of pre-wetting the lead-halide layer, we observe the formation of dense, pinhole-free perovskite layers with small sized crystallites after only few minutes without any further change with time especially for PbCl2, and no traces of non-converted PbCl2 or PbI2 are found in the respective layers. Without pre-wetting, the conversion is significantly slower. For PbCl2 the conversion shows a strong XRD signal of a pure CH3NH3PbCl3 phase in the first minutes before further conversion to CH3NH3PbClxI3-x. We monitor the I and Cl elemental concentration in the layers by Rutherford backscattering (RBS). In contrast to the pre-wetted case, the formation of large micron sized cuboids, plates, and wires on a time scale of tens of minutes is found in non-pre-wetted samples. Very interestingly, in optical emission microscopy a particularly strong light emission is seen for the cuboids while the plates appear largely non-emissive. Large diffusion lengths reported for these perovskites are expected to render recombination via defect states at the surface of the crystallites to become of critical importance. ^[4] Our findings are of general relevance to gather insight of organometal halide perovskite formation and their resulting photo-physical properties.
[1] F. Deschler, et al., The Journal of Physical Chemistry Letters 2014, 5, 1421.
[2] K. Liang, et al., Chem Mater 1998, 10, 403.
[3] J. Burschka, et al., Nature 2013, 499, 316.
[4] S. D. Stranks, et al., Science 2013, 342, 341.

The “hot off the press” perovskite-based solar cell technology is revolutionizing the photovoltaic field with an enormous increase in the power conversion efficiency to more than 16% since their first introduction in 2012. Hybrid perovskites consists of a 3D crystal with ABX₃ structure (A are organic cations, i.e. methylammonium CH₃NH₃; B a Pb²⁺metal ion; X a halide anion, i.e. Cl-, Br-, I-). Within this family, methyl ammonium lead tri-iodide perovskite (CH₃NH₃PbI₃) and its Chlorine-doped counterpart (Cl-doped CH₃NH₃PbI₃) have been the pioneer materials for solar application. They can be incorporated both as light antenna and electron/hole transporting layers. The perovskite absorbers thus appear capable of operating similar to excitonic absorbers, but also in a comparable configuration and with comparable performance to the best inorganic thin-film semiconductors. Whilst we have borne witness to unbelievable progress and evolution of photovoltaic technology based on these materials, the huge gap in understanding when moving from the pristine molecules to their embodiment in a device has severely hampered their widespread adoption in optoelectronic applications.
Here we present a comprehensive picture of the main photophysical properties of the material with a particular focus on the structure-optical properties relationship, emphasizing the role of the interfaces from a molecular to mesoscopic level. Firstly we address the nature of the primary photo-excitation. By temperature dependent linear absorption measurements we observe an excitonic transition at the on-set of the semiconductor optical absorption and we estimate the exciton binding energy to be about 50 meV. However, this holds true only for large perovskite crystals. In fact we show that tuning the crystallization process and the crystal size we can control the interplay between the organic and inorganic moieties and tune the optical band-gap and the nature of the electronic state at the on-set of the semiconductor optical absorption [1-2]. Then, by exploiting a large set of time-resolved optical spectroscopy tools we investigate time scale and dynamics of carrier thermalization, state filling effect, band gap renormalization, exciton formation, and ionization. We show how structural properties and the total photo-excitation - critical parameter for the definition of the operational condition of different optoelectronic devices, i.e. solar cells, light-emitting diodes and lasers - can influence the non-equilibrium dynamics in CH₃NH₃PbI₃ and Cl-doped CH₃NH₃PbI₃ films. This allows us to map the potential of these functional materials for their application in a wide host of technological application.
[1] C. Quarti et al., J. Phys. Chem. Lett., 2014, 5 (2), 279-284;
[2] D'Innocenzo V., Nature Communications, 2014, 5, doi:10.1038/ncomms4586.

Recently, organometal halide perovskite solar cells have been attracted intense research interest due to the extraordinary energy conversion efficiency. The light generated carrier dynamics in the perovskites as well as their corresponding spectral features remain questions. Here we investigated charge carrier dynamics in planar CH₃NH₃PbI_3xCl_x films by transient absorption (TA) and time resolved terahertz (THz) spectroscopy. The comparison of the THz and TA measurement showed that the free carriers were generated instantaneously by absorbing the photon with energy lager than bandgap, and the exciton formation was not observed. Based on the dynamic Burstein-Moss shift model, we attributed the TA bleach close bandgap to the band filling effect and also correlated the bleach signals with the carrier density so as to determine the carrier recombination dynamics from the bleach recovery. The trap related monomolecular recombination (first order), electron-hole bi-molecular recombination (second order) and Auger recombination (third order) rate were all resolved from the excitation intensity dependent measurement.

The efficiency of single junction solar cells based on semiconductor materials is mainly determined by transmission and thermalization losses. In strongly correlated oxides the excitation of the quasiparticles depends on the interaction between spin, charge, orbital and lattice degrees of freedomshy;shy;. These interactions may offer new mechanisms to overcome the above limitation, i.e. by slowing down the charge carrier thermalization time. Also for perovskite materials, e.g., the existence of long living states of small polarons is reported [1].
This represents the general motivation for our studies of photovoltaic properties of pn-junctions prepared by epitaxial thin films of p-doped Pr_0.67Ca_0.33MnO₃ (PCMO) on n-doped SrTi_0.998Nb_0.002O₃ (STNO) single crystal substrates via ion beam sputtering. Because of the high doping levels, the expected width of the depletion layer is only of the order of a few nanometers. Therefore, it is necessary to ensure a high quality of the interface with respect to structural and chemical properties.
TEM-analysis shows an atomically sharp interface with a very small dislocation density and a B-site interdiffusion only on a length scale of a few unit cells [2]. In order to characterize the pn-junction, IV-characteristics are measured as a function of temperature and wavelength. The data are first fitted to the one diode model as derived by Shockley. Its applicability to a correlated pn-junction is critically evaluated.
Preliminary EBIC-studies indicate long living excited states in the PCMO absorber. Using standard semiconductor theory, a diffusion length of about 10 nm is estimated, which is considerably larger than the depletion layer width.
At the present state the main photovoltaic effect is only caused by the UV part of illumination with a Xe lamp, which can be attributed to polaron interband transitions in the PCMO. Probably the small electronic overlap of Mn eg and Ti t2g orbitals at the interface limits the separation of IR induced excited carriers. Therefore, conclusions for a pn-junction with improved orbital overlap at the interface will be presented. The results will be discussed in terms of polaron dynamics and the opportunities for conversion of optical polaronic excitations into a photovoltaic energy.
We thank the German Research Society for funding through CRC 1073.
[1] P. Grossmann, I. Rajkovic, R. Moré, J. Norpoth, S.Techert, C. Jooss, K. Mann, Rev. Sci. Instrum. 83, 053110 (2012).
[2] G. Saucke, J. Norpoth, D. Su, Y. Zhu, Ch. Jooss, Phys. Rev. B 85 (2012) 165315.

Perovskite-based solar cells have recently demonstrated highly efficient energy conversion up to 15% [1]. Early studies of the characteristics of mixed halide (CH₃PH₃PbI_3-xCl_x) and triiodide (CH₃PH₃PbI₃) perovskites showed that these promising performances result from the combined high charge-carrier mobility [2] and long-lived charge-carriers with long diffusion lengths [3]. Nevertheless, understanding of the optical and electronic properties of Perovskites materials embedded in solar cell structures is crucial both from a fundamental point of view and for future improvement of device performances.
We present a first approach to the characterization of the optical and photo-electronic properties of triiodide perovskite with original domain-based micro-structures. Both photoluminescence response and dynamics are investigated with respect to different microscopic designs of the perovskite layer, providing notably the structure-dependent decay lifetime and diffusion length of the charge-carriers. Furthermore, scanning photocurrent technique is applied to perovskite-based field-effect transistor devices, providing new insights into the comprehension of the photo-electric response of these materials.
[1] Hodes, G. Perovskite-Based Solar Cells, Science 342, 317-318 (2013).
[2] C. C. Stoumpos, C. D. Malliakas, M. G. Kanatzidis, Inorg. Chem. 52, 9019 (2013).
[3] G. Xing et al., Science 342, 344 (2013), S. D. Stranks et al, Science 342, 341 (2013).

Understanding the crystal growth and improving material quality is of critical importance for improving semiconductors for electronic, photovoltaic, and optoelectronic applications. Amidst the surging interest in photovoltaic cells based on the hybrid organic-inorganic lead halides perovskite, despite the exciting recent progress in device performance achieved, improved understanding and better control of the crystal growth of these perovskites could further boost their solar performance. Here, we provide new insights on the crystal growth of the perovskite materials, especially nanostructures. Single-crystal methylammonium lead triiodide (CH₃NH₃PbI₃) nanowires, nanorods, and nanoplates were grown via a facile solution conversion technique. The as-grown CH₃NH₃PbI₃ nanostructures are confirmed to be single crystals of tetragonal phase by powder X-ray diffraction and transmission electron microscopy. These CH₃NH₃PbI₃ nanostructures are found to be n-type semiconductors using surface photovoltage measurement. More importantly, the as-grown CH₃NH₃PbI₃ nanorods and nanoplates display strong room-temperature photoluminescence, which is indicative of good photophysical properties and has not been observed in bulk single crystals of CH₃NH₃PbI₃. The better understanding of the crystal growth of CH₃NH₃PbX₃ can enable improved material synthesis to achieve better solar performance.

Due to their low cost, ease of fabrication, and unprecedented rate of increase in solar-to-electrical power conversion efficiency, organo-lead halide perovskite-based solar cells have attracted significant interest in recent years. In an effort to gain deeper insight into fundamental properties behind the high efficiency, as well as the role of structural phase transformations, temperature-dependent optical and electronic properties of perovskite materials were studied. Mixed halide CH₃NH₃PbI_3-xCl_x and CH₃NH₃PbI_3-xBr_x perovskite thin films deposited by spin coating for optical and electrical characterization. Temperature-dependent photoluminescence, absorption, transient absorption, time resolved fluorescence, and Raman spectra were measured in a wide temperature range from ~10 to 380 K to establish the temperature-dependence of optical bandgap and the role of carrier phonon coupling. XANES and EXFAS were utilized to obtain local element specific structural information on the effects of ligand orientation disorder as a function of composition. These results are correlated with charge transport measurements and provide important information about thermally activated transport and trapping processes in this important class of materials.

Pb halide perovskite solar cells have attracted interest because of the high efficiency reaching 19%. The process consists of 450-500 degree C process during which porous titania and conpact titania layers are fabricated. Low temperature fabrication employing porous alumina has been reported, however, they need compact layers fabricated at high temperatures. We now report Pb halide perovskite solar cells which can be prepared under 150 degree process including compact layer. The solar cells are composed of FTO layered glass/compact ZnO/porous ZnO/Pb halide perovskite/spiro-OMeTAD/Ag/Au. The compact and porous ZnO was prepared by spin-coating diethyl zinc solution at 150 degree C. The ZnO layers worked as electron transport layers even when the preparation temperature was low. Both of the porous and compact ZnO can be prepared from diethyl zinc solution, by changing the preparation conditions. The relationship between solar cell performances and preparation conditions are discussed from the view point of crystallinity and morphology. 8 % efficiency after optimization of ZnO thin film preparation conditions were recorded for Pb halide perovskite solar cells where all process including compact layers were prepared under 150 degree C. This will open the way to all plastic perovskite solar cells.

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) was used to identify the energetically favorable energy offsets at the interfaces of electron transport layer (ETL) (e.g. TiO2minus; perovskite and ZnO - perovskite) and hole transport layer(HTL) (e.g. perovskite - PDPP3T, Provskite-Spiro-Meotad). The energetically favorable offsets of 0.2 eV and 0.15eV were found at the TiO2minus; perovskite and ZnO - perovskite interfaces. Hole transport from perovskite to polymer was found to be energetically favorable with a offsets of 0.13 eV in case of PDPP3T and 0.17 eV for Spiro-Meotad HTL. Spatial maps of surface potential of perovskite film showed higher positive potential (130 meV) at grain boundary compared to the surface of the grains which decreases (110 meV) upon illumination by 20 meV. Transient photocurrent (TPC) and transient photovoltage (TPV) analysis shows that charge carrier transport time is faster than the charge carrier recombination time for high performance device and agrees with the increase in short current density for best performing cell.

There have been rapid advances in photovoltaic devices containing hybrid organic-inorganic ABX₃ (A = CH₃NH₃; B = Pb, Sn; X = Cl, Br, I) perovskite absorbers. Most devices utilise a 3D electron selective layer but research efforts have so far focussed on a limited number of transport materials. TiO₂ is by far the most widely used material followed by Al₂O₃, ZnO and ZrO₂. In this work, an alternative heterojunction based on both planar and nanostructured CdS is presented. CdS nanoparticle synthesis is well understood and therefore it offers a simple route to nanoparticle morphology, whilst planar CdS is widely used in inorganic thin-film solar cells. Here sputtering is used to deposit the planar layers, while nanorods are synthesised in solution befor being dropcast and annealed.
Complete cell structures were fabricated on coated glass substrates with an FTO/CdS/CH₃NH₃PbI_(3-x)Cl_x/PTAA/Au heterostructure and characterized using J-V and EQE analysis. Planar and nanostructured cells have maximum efficiencies of 3.3% and 7.3% respectively. Although devices are limited by a low FF (35-40%) the latter device has a particularly high V_oc, 0.95V, and J_sc, 18.5 mA cm^-2, suggesting that there is potential this junction could match the very high efficiencies achieved for TiO₂ transport layers.

Organolead trihalide perovskites are at the heart of the new high-efficiency perovskite-based solar cells. However, reproducible synthesis/processing of high-quality perovskite thin films still remains a challenge. Typically fabricated perovskite thin films can be phase-impure and fine-grained, and they can have pinholes and incomplete coverage. In this study, we present various strategies, both vapor-based or solution-based, to fabricate high-quality, pinhole-free perovskite (CH3NH3PbI3 or CH(NH2)2PbI3) films that are compact, smooth, and coarse-grained. The mechanisms by which these films form are elucidated, and guidelines for the fabrication of future perovskite-based thin films for solar cells are provided. The near-ideal film morphology of these perovskite films has enabled the investigation of the extraordinary properties (optical absorption, ferroelectricity, etc.) of the organolead trihalide perovskites and the unique characteristics (transport, recombination, carriers-diffusion lengths, etc.) of devices based on these materials. The basic understanding gained from these investigations is presented. Finally, properties of perovskite-based solar cells with high efficiencies (>10%) and/or uniquely simplified architecture (hole/electron-conductor-free planar structures), enabled by these compact films, are presented.

To advance the performance of the polycrystalline thin film devices, it requires a delicate control of its grain structures. As one of the most competitive candidates among current thin film photovoltaic techniques, the organic/inorganic hybrid perovskites inherit polycrystalline nature, and exhibit compositional/structural dependent optoelectronic properties. Here, we demonstrate a controllable passivation technique for perovskite films, which enables a compositional change, and allows substantial enhancement in corresponding device performance. By releasing the organic species during annealing, the inorganic PbI₂ phase is presented in perovskite grain boundaries and at the relevant interfaces. The consequent passivation effects and underlying mechanisms are examined with complementary characterizations, including SEM, XRD, TRPL, SKPM and UPS. This controllable self-induced passivation technique represents an important step to understand the polycrystalline nature of hybrid perovskites thin films, and contributes to the development of perovskite solar cells judiciously.

Very recently, significant progress has been realized in solid-state inorganic-organic hybrid perovskite solar cells, with high efficiency over 15%, attracting tremendous attention in the field of photovoltaics [1-6]. Devices fabricated from alkylammonium metal trihalide perovskite absorbers achieve very high power conversion efficiencies, already superior to amorphous Si. One key aspect for the highest device performances reported to date is film uniformity and coverage of the perovskite film. Recently, Snaith et. al, reported that under-coordinated iodine ions within the perovskite structure are responsible and establish a supramolecular strategy to successfully passivate these sites [7]. Our research on surface passivation of inorganic-organic hybrid perovskite also supports this finding. This work highlights the criticality of controlling the thin film crystallization mechanism of hybrid perovskite materials and the chemical treatments of this surface can offers a simple pathway for further enhancements in perovskite solar cells. These results will lead to more efficient and cost-effective inorganic-organic hybrid heterojunction solar cells in the future.
[1] Kojima, A., Teshima, K., Shirai, Y. & Miyasaka, T. Organometal Halide Perovskites as Visible-Light
Sensitizers for Photovoltaic Cells. JACS 131, 6050-6051 (2009).
[2] Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M.,
Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 2013, 499,
316-319.
[3] Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Science 2012, 338, 643minus;647.
[4] M. Liu, M. B. Johnston, H. J. Snaith, Nature, 2013, 501, 395.
[5] Y. Wu, X. Yang, Han C., Kun Z., C. Qin, J. Liu, W. Peng, A. Islam, E. Bi, F. Ye, M. Yin, P. Zhang and L. Han. Highly compact TiO₂ layer for efficient hole-blocking in perovskite solar cells. 2014 Appl. Phys. Express7 052301
[6] D. Liu, T. L. Kelly, Nat. Photonics, 2014, 8, 133.
[7] A. Abate, M. Saliba, D. J. Hollman, S. D. Stranks, K. Wojciechowski, R. Avolio, G. Grancini, A. Petrozza, and H. J. Snaith. Supramolecular Halogen Bond Passivation of Organicminus;Inorganic Halide Perovskite Solar Cells. dx.doi.org/10.1021/nl500627x | Nano Lett. 2014.

Carrier diffusion length is paramount in determining the thickness and performance of the photovoltaic devices. Here we will report that the solvent annealing can be applied to organic-inorganic hybrid materials which significantly increase the charge carrier diffusion length of CH₃NH₃PbI₃ to over 1 µm due to the increased crystallinity and grain size.^[1]
One issue with solution-processed trihalide perovskite thin films is that the polycrystalline films have a relatively small grain size of a couple of hundred nanometers (nm) due to the quick reaction of PbI₂ and methylamonium iodine (MAI) and the quick crystallization of these perovskite materials.^[2] Most of the best performing devices have a perovskite thickness of around 300 nm. A thicker perovskite film is desired so that more sunlight can be absorbed, which, however, is limited by the low carrier diffusion length of around 100-300 nm.^[3-4] Another merit of having a thicker perovskite film is that the device&’s manufacturing yield can be increased, which is especially important in larger scale manufacturing.
After solvent annealing of the perovksite film, the average grain sizes were increased to be comparable to film thickness so that most photo-generated charges can be extracted within single grain without crossing grain boundaries. The long charge diffusion length, over 1 µm, enables high efficiency devices based on thick perovskite films. A high power conversion efficiency of 15.6% was achieved using the 630 nm thick MAPbI₃ perovskite film under one sun illumination, and the efficiency kept above 14.5% when the thickness changes from 250 nm to over 1 µm. The high tolerance of the efficiency on the film thickness after solvent annealing would enable it one of the most promising treatments for perovksite toward its commercialization.
[1] Z. Xiao; Q. Dong; C. Bi; Y. Shao; Y. Yuan; J. Huang, Adv. Mater. 2014, Under review,
[2] E. Edri; S. Kirmayer; A. Henning; S. Mukhopadhyay; K. Gartsman; Y. Rosenwaks; G. Hodes; D. Cahen, Nano Letters 2014,
[3] Z. Xiao; C. Bi; Y. Shao; Q. Dong; Q. Wang; Y. Yuan; C. Wang; Y. Gao; J. Huang, Energy Environ. Sci. 2014,
[4] G. Xing; N. Mathews; S. Sun; S. S. Lim; Y. M. Lam; M. Grätzel; S. Mhaisalkar; T. C. Sum, Science 2013, 342, 344

Power conversion efficiencies of over 17% have been achieved using methylammonium lead iodide (CH₃NH₃PbI₃) perovskites, however their bandgap (~1.6eV) is significantly higher than the ideal for a single-junction solar cell (~1.3-1.4eV). With this in mind, formamidinium lead iodide (CH(NH₂)₂PbI₃) has been identified as an alternative absorber with a more ideal bandgap of 1.45eV. Despite this improved bandgap, the efficiencies of formamidinium containing devices still lag behind their methylammonium containing counterparts. One reason for this lower efficiency is the relatively weak near-bandgap absorption of CH(NH₂)₂PbI₃ - its reported light-harvesting efficiency is less than half that of CH₃NH₃PbI₃ near the band edge (~30% versus ~65%). Here we present a simple strategy to overcome this problem by increasing the light path-length through solar cells made with organometal halide perovskites, such as CH(NH₂)₂PbI₃, by texturing the back electrode for improved scattering. Through a simple PDMS stamping method we are able to replicate a pattern of nanodomes from a silicon master into substrates coated with mesoporous titania. This patterning is then preserved through subsequent perovskite and electrode deposition processes, resulting in a final device with a textured and therefore scattering back electrode. The integration of this patterned electrode is sufficient to increase the light path and therefore absorption in our perovskite devices, and represents a facile way of tuning the device architecture to increase the photocurrent in these already impressively efficient perovskite solar cells.

High performance (>17%) organometaltrihalide based solar cells have been demonstrated in recent years using mesostructured composites and has attracted significant attention in the photovoltaic community. Planar thin film perovskite solar cells, which are more easily fabricated, however, is still under development to achieve similar high performances. In addition, the planar devices provide a great platform to investigate the perovskite film properties. Here, it is shown that the film transformation of perovskite materials is a critical factor to reach high performance in planar heterojunction CH₃NH₃PbI_3-xCl_x solar cells. Secondary phases can be observed and carefully controlled by tuning processing conditions during film formation. The physical properties of CH₃NH₃PbI_3-xCl_x films are investigated and a possible formation pathway is proposed. It is demonstrated that the high performance devices are attainable with a small portion of secondary phases in CH₃NH₃PbI₃ film and power conversion efficiencies of up to 14% are achieved. The correlations between the phases, device performance and physical properties are discussed to identify the role of the secondary phases in CH₃NH₃PbI_3-xCl_x materials.

Present day high efficiency solar to electrical energy technologies are enabled by the use of high purity single crystalline semiconductors. This was a direct consequence of the discovery of the crystal growth that over years was refined to obtain inch scale wafers of semiconductors(1) from a seed crystal. Several new solar cell technologies based on nanomaterials that promise to deliver even lower cost solar power have emerged but despite tremendous advances in these new technologies over the past 15 years, their commercial viability (necessitating about 15% PCE) is still crippled by their relatively low PCE currently capped by about 10%(2). Recent discovery of incorporating organic-inorganic hybrid perovskites such as CH₃NH₃PbX₃ (X = Cl, Br, I) as donor materials in a simple planar bilayer photovoltaic devices has revealed very high power conversion efficiency exceeding 12%(2-5)without the need for nanostructturing or creating complex device architectures. The excellent absorption coefficient and long-range charge transfer length leads to remarkable solar cell performance. Here we show, for the first time, large-scale single crystal formation by low temperature solution process. We report the synthesis of large area (>200 microns) growth of single crystal domains in organometallic halide perovskites using three different approaches. The large area single crystal domains form a continuous laterally across the device. We correlate the power conversion efficiency to the domain size and show that as anticipated, the largest domain size gives us the highest PCE of ~12.5% for a single junction bilayer device. We also perform detailed crystal structure, spectroscopy and transport measurements to understand the atomistic origins of the domain formation and high PCE.
1. J. Czochralski, A new method for the measurement of crystallization rate of metals. Zeitschrift für physikalische Chemie92, 219 (1918).
2. Z. He et al., Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nat Photon6, 591 (09//print, 2012).
3. P. Docampo, J. M. Ball, M. Darwich, G. E. Eperon, H. J. Snaith, Efficient organometal trihalide perovskite planar-heterojunction solar cells on flexible polymer substrates. Nat Commun4, (11/12/online, 2013).
4. D. Liu, T. L. Kelly, Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques. Nat Photon8, 133 (02//print, 2014).
5. O. Malinkiewicz et al., Perovskite solar cells employing organic charge-transport layers. Nat Photon8, 128 (02//print, 2014).

Perovskites are the wonder compounds of materials science, with examples of ferromagnets, ferroelectrics, multiferroics, superconductors, semiconductors, ion conductors, insulators and, most recently, highly efficient photovoltaics. This talk will address the chemical and physical factors that make these materials, and in particular hybrid organic-inorganic halide perovskites, unique.
Recently, we have been addressing the success of methylammonium lead iodide in solar cells from atomistic and electronic structure modelling [1-3]. The hybrid material satisfies the basic optoelectronic criteria essential for an active photovoltaic layer (spectral response with light electron and hole effective masses). Relativistic and many-body corrections are shown to be essential to describing the electronic band structure. In addition, the system is structurally and compositionally flexible with large dielectric constants, and the ability to alloy on each of the three perovskite lattice sites.
One anomalous behaviour of hybrid perovskite solar cells is hysteresis in the photovoltaic current-voltage response, which we demonstrate has a contribution from the orientational disorder of the methylammonium cations. The rotation-libration of the molecular dipoles results in a rich domain structure that is sensitive to both temperature and the external electric field.
1. F. Brivio, A. B. Walker and A. Walsh, APL Materials 1, 042111 (2013)
2. J. M. Frost, K. T. Butler, F. Brivio, C. H. Hendon, M. van Schilfgaarde and A. Walsh, Nano Letters, 14, 2584 (2014)
3. F. Brivio, K. T. Butler, A. Walsh and M. van Schilfgaarde, Physical Review B 89, 115204 (2014)

Rapid improvement in photovoltaic efficiency in hybrid lead halide perovskite materials has provided the impetus for understanding the correlations between crystal chemistry, electronic structure, and optoelectronic properties of these materials. It is interesting to ask whether related compounds can show promising optoelectronic properties. We have developed simple routes to complex metal halides using organic solvents, thereby improving the more time-consuming solid state syntheses applied in the past. By using solvothermal conditions, samples suitable for single crystal X-ray diffraction were obtained. The band gaps and band positions have been obtained using combinations of diffuse reflectance UV/Vis spectroscopy and ultraviolet photoelectron spectroscopy and are compared to band offset calculations carried out using carried out using density functional theory (DFT).

A new generation of thin-film photovoltaic cells based on organic-inorganic metal halide perovskites has recently emerged with extraordinary power conversion efficiencies. We recently found that charge carriers can travel over distances of up to a micron in solution-processed CH₃NH₃PbI_3minus;xCl_x perovskite absorbers [1], thus making these materials suitable for planar-heterojunction solar cells. We further showed [2] that this effect arises from a combination of high charge-carrier mobility (11.6#8239;cm²#8239;V^{minus;1#8239;}s^minus;1) and abnormally low bi-molecular charge recombination rates that defy the Langevin limit by at least four orders of magnitude.
We demonstrate here [3] that CH₃NH₃PbI_3minus;xCl_x films deposited through thermal evaporation exhibit even higher charge-carrier mobility of 33 cm²#8239;V^{minus;1#8239;}s^minus;1 and similarly low bimolecular recombination rates to those observed for solution processed CH₃NH₃PbI_3minus;xCl_x. At charge carrier densities below 10¹⁷ cm^-3, intrinsic diffusion lengths approach 3 microns, highlighting the capability of dual-source evaporation as a fabrication route for highly efficient planar-heterojunction solar cells. We further show that the emission spectrum of such films is predominantly homogenously broadened through strong coupling of charge-carriers to phonons [4]. With the observed spectral width of 103 meV, such gain media can sustain amplification of light pulses as short as 6.4 fs, opening a perspective for electrically-pumped femtosecond-pulsed lasers.
In addition, we examine the charge-carrier dynamics in the lead-free perovskite CH₃NH₃SnI₃ for which we have recently demonstrated solar cells processed on a mesoporous TiO₂ scaffold that reach efficiencies of over 6% [5]. Using transient THz spectroscopy, we find that the ultrafast charge-dynamics are dominated by a first-order decay that may originate from electron recombination with an unintentional hole doping density of ~10¹⁸ cm³. We establish an effective charge-carrier mobility of 1.6#8239;cm²#8239;V^{minus;1#8239;}s^minus;1, yielding a charge-carrier diffusion length of 30nm [5]. We show that the effective bi-molecular recombination constant in solution-processed CH₃NH₃SnI₃ is similarly low to that observed for CH₃NH₃PbI_3minus;xCl_x. Hence if mono-molecular processes arising from unintentional doping or charge trapping are resolved, charge diffusion lengths in excess of one micron can be reached for CH₃NH₃SnI₃, opening the way for lead-free perovskite photovoltaics with planar-heterojunction architecture.
[1] Stranks, Eperon, Grancini, Menelaou, Alcocer, Leijtens, Herz, Petrozza, Snaith, Science 342 (2013) 341.
[2] Wehrenfennig, Eperon, Johnston, Snaith, Herz, Adv. Mater. 26 (2014), 1584.
[3] Wehrenfennig, Liu, Snaith, Johnston, Herz, Energy Environ. Sci. doi: 10.1039/C4EE01358A
[4] Wehrenfennig, Liu, Snaith, Johnston, Herz, #8232;J. Phys. Chem. Lett., 5 (2014), 1300.
[5] Noel, Stranks, Abate, Wehrenfennig, Guarnera, Haghighirad, Sadhanal, Eperon, Johnston, Petrozza, Herz, Snaith, #8232;Energy Environ. Sci. doi: 10.1039/C4EE01076K

Rapid improvements in the power conversion efficiency have been shown in organometallic mixed halide perovskite-based solar cells. Solution-processed devices with power conversion efficiencies of 10-12%^1-3 were reported in 2012, and have more recently exceeded 17% in devices processed by evaporation^4,5 and sequential deposition^6,7. New perovskite types have been developed by modifying the chemical composition of the materials^8,9. Recently, high photoluminescence quantum efficiencies, of over 70%, have been reported¹⁰.
We investigate the photoluminescence properties of mixed-halide lead perovskite films with sub-micrometer spatial resolution. Thin films are prepared by spin coating from solution varying the ratio of bromine and iodine from pure bromine to pure iodine. Using a near-field scanning optical microscopy (NSOM) system we excite the sample through a sub-wavelength aperture and measure localized photoluminescence spectra with spatial resolution below the diffraction limit. By concomitantly probing the topography, we can correlate emission properties with film structure. We find that our samples show spatial inhomogeneity in the photoluminescence intensity as well as in the emission energy for different regions of the film. We show how these inhomogeneities correlate with structural features observed in the film morphology.
1 Ball, J. M., et al. Low-temperature processed meso-superstructured to thin-film perovskite solar cells. Energy & Environmental Science (2013).
2 Kim, H.-S., et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Scientific Reports2 (2012).
3 Lee, M. M., et al. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science338, 643-647 (2012).
4 Liu, M., et al. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature501, 395-398 (2013).
5 Chen, Q., et al. Planar Heterojunction Perovskite Solar Cells via Vapor-Assisted Solution Process. J. Am. Chem. Soc. 136 (2), pp 622-625 (2014)
6 Burschka, J., et al. Sequential Deposition as a Route to High-performance Perovskite-sensitized Solar Cells. Nature499, 316-319 (2013).
7 CNRE, Best Research Cell Efficiency, http://www.nrel.gov/ncpv/images/efficiency_chart.jpg (2014.05)
8 Noh, J. H., et al. Chemical Management for Colorful, Efficient, and Stable Inorganic-Organic Hybrid Nanostructured Solar Cells. Nano Letters13, 1764-1769 (2013).
9 Stoumpos, C. C., et al. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties. Inorg. Chem.52, 9019-9038 (2013).
10 Deschler, F., et al. High Photoluminescence Efficiency and Optically Pumped Lasing in Solution-Processed Mixed Halide Perovskite Semiconductors. J.Phys.Chem.Lett. 5, 1421minus;1426 (2014)

Halide perovskites are known to undergo a series of phase transitions as function of temperature. Here we study their relation to soft-phonons by carrying out first-principles calculations using the linear response method as implemented in ABINIT for CsSnX₃, X=I, Br, Cl. The cubic α-phase is found to have a soft phonon branch between the M and R points of the Brillouin zone corresponding to octahedron rotations. Using group-theoretical analysis, the transition to the tetragonal β-phase is shown to be induced by the condensation of the M₂⁺ soft mode. The transition from the β-phase to the orthorhombic γ-phase is induced by the Z₅^minus; soft mode which involves rotation not only of the octahedra but also of the Cs atoms around two different axes perpendicular to the tetragonal symmetry axis. No soft phonons are found in the γ-phase, indicating its mechanical stability. However, another orthorhombic phase in CsSnI₃ the so-called yellow phase, and a monoclinic phase exist in CsSnCl₃. They contain edge-sharing octahedra and thus require more significant distortions of the bonding not describable in terms of soft-phonons. We have shown[1] using quasiparticle self-consistent GW calculations that these have much higher band gaps, unsuitable for photovoltaic applications. The driving force behind these phase transitions is to make the structure more dense because the cages between the octahedral in which the Cs ions reside is too large for the Cs. Thus it seems desirable to reduce the lattice constants to avoid these phase transitions. We compare the effects on the electronic band structure of replacing I by Br and Sn by Ge. Both preserve the inverted band structure with group IV element s-like valence band maximum and p-like conduction band maximum which underlies the strong optical absorption in the visible, and the low hole mass which play a significant role in the success of these compounds for photovoltaics.
[1] Ling-yi Huang and W. R. L. Lambrecht, Phys. Rev. B 88, 165203 (2013)

During the last few years, 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 leed 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.

Perovskite solar cell based on lead iodide light harvester is an emerging photovoltaic technology due to extremely low cost and superb photovoltaic performance. The first report on a long-term durable perovskite solar cell with a PCE of 9.7% in 2012 has triggered researches on perovskite solar cells. As a result PCE approaches 20%. In this talk, Role of perovskite crystal size and mesoporous TiO₂ layer on photovoltaic performance of perovskite solar cell will be discussed. CH₃NH₃PbI₃ (MALI) and HC(NH₂)₂PbI₃ (FALI) are typical materials for high efficiency perovskite solar cells. TiO₂ layer was found to useful in extension of electron diffusion by compensating rather shorter diffusion length for electrons than for holes. However, difference in electron mobility between TiO₂ and perovskite may lead to poor charge collection. ZnO nanorod was found to be better than TiO₂ in terms of electron collection. Photovoltaic parameters depend significantly on perovskite crystal size. Large size was beneficial to charge extraction and dipole relaxation. Morphology and size are critical in achieving high performance. PCEs of 17% and 16% were achieved from MALI and FALI, respectively, by controlling morphology and crystal size together with engineering of TiO₂ nanostructure.

Recently, ammonium metal halide perovskites have emerged as an exciting new area of research in the fields of thin film and nanostructured solar cells. Solar cells making use of ammonium lead iodide chloride (CH3NH3PbI3-xClx) perovskite absorbing layers have been reported with power conversion efficiencies of >19%, a massive increase in efficiency since 2009, with further improvements likely in the future¹. Although several key studies of perovskite conduction and carrier lifetimes have been reported, there are still many questions as to the spectral features observed in CH3NH3PbI3-xClx and related materials and how these relate to device performance². Here we report several key observations into the excited state and charge dynamics present in ammonium metal halide thin films. Perovskites investigated include mixed and pure lead halide perovskites recently reported as the active component in solar cells and lasing thin films. Perovskite thin films are produced on alumina nanoparticle scaffolding via solvent deposition methods to emulate device performance. We then use fluence dependent ultrafast transient absorption spectroscopy to resolve the evolution of electroabsorption, ground state bleach and excited state absorption features. Using an ultrafast broadband Kerr gate photoluminescence (PL) spectrometer, we can verify our transient absorption assignment with the complementary observation of ultrafast PL spectral relaxation and depolarization, and we can temporally resolve the onset of amplified stimulated emission (ASE). With these tools we observe charge dynamics on sub picosecond timescales giving us insight into features indicative of charge separation, thermal state filling, and hot carrier relaxation to the band edge. We note that charge separation appears to be very fast in CH3NH3PbI3-xClx and that relatively long thermalization times coincide with the onset of amplified stimulated emission. These results have several implications, and indicate the possibility of hot carrier extraction to increase the effective V_oc of perovskite solar cells.
1. Robert Service, Science May 2014: Vol. 344 no. 6183 p. 458 DOI: 10.1126/science.344.6183.458
2. T.C. Sum and N. Mathews, Energy Environ. Sci., May 2014, ASAP, DOI: 10.1039/C4EE00673A

Organometal trihalide perovskite materials, such as CH₃NH₃PbI₃, are most interesting materials for solar cells with their remarkable power conversion efficiencies and cost-efficient, solution-based processing. Solar cells based on these materials are fabricated similar to dye-sensitized solar cells: A mesoporous TiO₂ scaffold is infiltrated by the perovskite absorber. The materials distribution within this mesoporous layer can be visualized using e.g. electron energy-loss spectroscopic methods [1, 2]. Although the applied spectroscopic method does provide information about excitable electronic states and thus materials distribution, it cannot derive detailed information on the structure of the materials. In our present study we use (Scanning) Transmission Electron Microscopy ((S)TEM) to correlate STEM Electron Energy-Loss Spectroscopy (EELS) and Nanobeam Diffraction (NBD). We found that the perovskite crystals are present in the devices but also an amorphous phase of presumably organometallic halogenoplumbate. NBD and STEM EELS measurements show that crystalline areas correlate with the appearance of a signal in the low-loss region of the EEL spectrum at about 7-8 eV loss, which is not present in amorphous regions. For these STEM studies a thin lamella has to be prepared from the device using focused ion beam (FIB) milling. This is a time consuming process, which yields a low statistical output. To correlate the now found crystalline and amorphous phases further with device performance we use a novel Energy Selective Backscattered electron detector (ESB) in a Scanning Electron Microscope (SEM). This novel imaging modality provides contrast between the crystalline and amorphous phases and allows a large-scale sampling of device areas, which is currently under way.
Acknowledgements:
Financial support by the German Federal Ministry of Education and Research (FKZ 03EK3505J/L/K, FKZ 13GW0044, FKZ 13N10794) is gratefully acknowledged.
References:
[1] M. Pfannmöller et al. Nano Lett. 11, 3099-3107 (2011).
[2] D. Nanova et al. Nano Lett. 14, 2735-2740 (2014).

The perovskite lead halides possess many important properties for solar cells: Ease of preparation as well-formed crystals, earth-abundant elements, long charge lifetimes and effective diffusion lengths and high values of open circuit voltage (V_OC) relative to the bandgap. The last of these is of particular interest to us in our search for a cheap and effective high V_OC solar cell that can be used, e.g., as a top cell in a tandem photovoltaic device. For this reason, we have concentrated our effort on CH₃NH₃PbBr₃ (MeAPbBr)-based cells. MeAPbBr has a bandgap of 2.3 eV. While this is a bit high, particularly for a 2-cell tandem using Si cells as the low Eg part (a likely initial application for perovskite cells), it is a good starting material to test for high V_OC cells.
In initial work on these cells, we focused mainly on the HOMO level of the hole conductor to reach a V_OC of 1.3 V, albeit with a low efficiency (0.56%). Using the electron beam induced current (EBIC) technique with the more common iodide perovskite, we demonstrated experimentally that these cells worked as p-i-n cells with contributions to the photovoltage from both interfaces with the perovskite. Subsequent improvements to the bromide cells (adding Cl to the perovskite, changing the hole conductor and controlling its doping) led to increase in V_OCshy; to >1.5 V and a considerably improved efficiency of 2.7%. In this talk, we describe extension of this research including morphological modification of the perovskites, the role of added Cl, and solid solutions between the Br and I perovskites to reduce the bandgap to the more useful (for tandem cells) 1.8-2.0 eV region.

All photovoltaic device efficiencies are limited by the ‘threshold&’ process inherent in how photovoltaic devices work: a photon above a certain energy level is required to excite an electron that will later be extracted as electrical current. This sets a limit to the efficiency of solar power conversion to electricity of a “single threshold” system to about 30%. Differentiating the threshold to two ‘steps&’ increases the theoretical limit to 45%. One of the proposed ways to achieve this is by splitting the spectrum to several devices, each with a different threshold energy matching a different portion of the solar spectrum. Consequently, a photovoltaic device that uses the high-energy portion of the solar spectrum is required. Current available options are extremely costly and are not feasible on a large-scale application, or are very inefficient.
Using the high-energy portion of the spectrum means fewer photons will be used but those that are, will have higher energies, meaning the excited electrons will have high potential energy, which translates in a photovoltaic device to a high open circuit voltage (V_OC). In this research we compare the design in two PV systems with potential to have high V_OC, namely certain extremely thin absorber (ETA) solar cells and hybrid organic-inorganic perovskite (OIP) materials-based devices.
Starting from an ETA system based on a CdS absorber (2.4 eV bandgap) with initial V_OC ~ 200 mV, the reasons for this very low V_OC were investigated. Using chemical treatments and process modifications, a uniform surface coverage of the mesoscopic ZnO scaffold and an improved front contact is achieved. Charge trapping at certain interfaces and ways to minimize them were also investigated. This led to an overall increase of the V_OC to 860 mV.
With OIP based devices, we were able to achieve initially a V_OC of 1300 mV and then improve it to 1570 mV by improving the nucleation and crystal growth process and better match the energy levels in the device.
While both cells are modelled as p-i-n, the electronic quality of the absorber determines the design that best suits the materials&’ properties. General guidelines for further advancement are suggested.

Perovskite photovoltaic is emerging as one of the most competitive solar technology due to its optimum band gap, high absorption coefficient, and balanced ambipolar carrier transportation property. Advancing perovskite solar cell technologies toward higher power conversion efficiency (PCE) requires delicate control over the perovskite film and the relevant interfaces, and deep understanding of defects. The properties of the perovskite can be manipulated (or optimized) during film growth by careful control of the intercalation reaction between the organic and inorganic species. We developed two approaches for perovskite film growth: 1) vapor assisted solution process, and 2) one-step solution process with enhanced film reconstruction under controlled humidity. The underlying film growth mechanism, defect property and the corresponding device performance have been carefully examined. We further control the carrier dynamics throughout the entire device, based on careful choices of the carrier transport materials, and the modification of electrodes. These efforts lead to suppressed carrier recombination in the absorber, efficient carrier injection into the carrier transport layers, and good carrier extraction at the electrodes. The fabrication of our perovskite solar cells was conducted from solution at low temperatures, which should simplify future manufacturing of high performance, low-cost, and large-area perovskite solar cells.

Organometallic mixed halide perovskite-based solar cells have shown a breakthrough in power conversion efficiency^1,2 with latest power conversion efficiencies exceeding 17%. Recently, novel mixed-halide bromine-iodine perovskites have been demonstrated which allow a tuning of the bandgap across the visible wavelength range by changing the bromine-iodine ratio ^3,4. An open question is how these changes in halide composition affect the underlying photo-physical properties of the excited states.
We investigate bromine-iodine lead perovskite films prepared by spincoating from solution for different iodine contents from 0% to 100%. We measure the absorption onset with high sensitivity using photo-thermal deflection spectroscopy and find materials with surprisingly clean band-edges and low levels of sub-bandgap absorption. We study the recombination of the photo-excited states by photoluminescence (PL) spectroscopy and correlate our spectroscopic findings with the crystal structure determined from X-ray diffraction measurements. Additionally, we investigate the properties of the photo-excited species in these mixed-halide films with transient absorption (TA) and transient photoluminescence spectroscopy. In order to gain insights on the photo-physical nature of the excited species, we compare the extracted PL kinetics and intensities for different fluences. We find that varying the bromine-iodine ratio leads to significant changes in the lifetime and properties of the excited species.
1 Kim, H.-S. et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Scientific Reports2, doi:10.1038/srep00591 (2012).
2 Lee, M. M., Teuscher, J., Miyasaka, T., Murakami, T. N. & Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science338, 643-647, doi:10.1126/science.1228604 (2012).
3 Noh, J. H., Im, S. H., Heo, J. H., Mandal, T. N. & Seok, S. I. Chemical Management for Colorful, Efficient, and Stable Inorganic-Organic Hybrid Nanostructured Solar Cells. Nano Letters13, 1764-1769, doi:10.1021/nl400349b (2013).
4 Stoumpos, C. C., Malliakas, C. D. & Kanatzidis, M. G. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties. Inorg. Chem.52, 9019-9038, doi:10.1021/ic401215x (2013).

Hybrid perovskite materials CH₃NH₃PbI₃ (MAPI) and CH₃NH₃PbI_3-xCl_x (MAPIC) are used as optically active components in high efficiency solution processed solar cells. Within the perovskite crystal structure the methyl ammonium (MA) ions are caged between lead halide octahedra. The MA ions are electrical dipoles which have the potential to contribute to ferroelectric properties of the material. These ions are also speculated to play a critical role in the stability and hysteresis of MAPI and MAPIC photovoltaic devices.
We present neutron diffraction and quasi-inelastic neutron scattering (QENS) data to examine the structure and behaviour of the MA ions in MAPI and MAPIC for the temperature range 7 - 380 K. Neutron diffraction allows the crystal positions of hydrogen nuclei to be determined which cannot be easily achieved using X-ray diffraction. Two crystalline phase transitions are observed at 150-170 K and ~ 330-350 K. QENS focusses on the dynamic motion of hydrogen nuclei within the structure.
The data is consistent with the rotation of the hydrogen ions around the C-N axis. A second rotational mode is observed above 140 K which can be attributed to reorientation of the C-N axis with respect to the crystal. Activation energies for these rotational movements are estimated, and the residence times in the possible orientations are obtained.
The inferred active fraction of rotating MA is analysed. The proportion of CH3-rotors undergoing reorientation around the C-N axis increases linearly with temperature, which could be consistent with the reported H-bonds between MA and the halides of the inorganic moiety. The fraction experiencing reorientations of the C-N axis itself is independent of temperature, thus pointing at steric hindrance due to the extreme softness of the material at atomic level.
Different geometries of reorientation are compared corresponding to hops between different possible lowest energy configurations. Molecular dynamics simulations are used to determine the most likely geometry.
Finally we speculate on the possible consequences of these reorientations for the material properties. In particular, we use Ising-type simulations to show how the different possible MA arrangements could contribute to ferro- or antiferro-electric properties¹, and the possible consequences for the charge transport characteristics of the material. We examine whether realignment of the MA ion domains under varying electric fields could contribute to the hysteresis observed in the current-voltage curves of MAPI and MAPIC solar cells.
1- J. M. Frost, K. T. Butler, A. Walsh, "Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells", submitted.

A key requirement for obtaining the maximum performance of perovskite based photovoltaic devices is the perfect alignment of the energy levels in the perovskite film to the adjacent transport layers. This becomes even more important as we envision the implementation of perovskite absorbers into tandem solar cells, where a precise control over the electrical current into the recombination layer is needed. On the other end the ongoing research efforts of the community introduce an increasing number of prospective cell designs many of them relying on new materials as transport layers. Whether the electronic alignment at the respective interfaces to the perovskite film is conforming to the ideal case is often only considered in approximated energy diagrams.
In my talk I will show how we experimentally access the position of the electronic energy levels and electronic band gap of various organometal lead halides by employing direct and inverse photoemission spectroscopy. Our results are in good agreement with estimates derived from optical experiments and recently obtained values from numerical computations. Subsequently we trace the position of the frontier molecular orbitals of adjacent organic transport layers. Our observation reveals that tuning the energetics at this interface for an improved match of the electronic levels significantly enhances the solar cell characteristics. But rather than just tailoring this transport layer we now find experimental evidence that the energetics in the perovskite layer itself can be controlled. Oxide interlayers such as MoO₃ or NiO can be used to induce band bending in the perovskite layer or force it to become p-type. This route enabled us to map out the energy diagram of a prototypical inverted perovskite solar cell and will be a foundation for the successful and flexible integration of perovskite films in novel cell architectures.

A high performance planar heterojunction photovoltaic device needs a uniform and compact perovskite layer with full surface coverage, which provides low leakage, small roughness, longer charge diffusion length and reduced charge recombination at surface. While thermal evaporation technique can be used to form a uniform perovskite film, it is not cost-competitive as compared to solution process. However,compact and pinhole-free films are difficult to be fabricated through solution processes: the film&’s surface coverage was usually lower than 90% with large roughness, yielding a low device performance^[1]. In this talk, we will report a simple solution approach to prepare uniform and compact perovskite films. The devices based on it show over 15% power conversion efficiency with a high device yield of above 85% ^[2].
Uniform perovskite filmswere fabricated through a thermal dynamic interdiffusion process of two stacking precursor layers where a layer of methylamine iodide was on the top of a compact PbI₂ layer. Thermal annealing was found to be essential for interdiffusion-grown perovskite film for two aspects:(1) It drove the interdiffusion process to form the perovskite films with right composition, removing possible defects and charge traps caused by impurity phases; (2) It led to the recrystallization and grain growth in the formed perovskite films. In addition, the enlarged grains and improved crystallinity resulted in varied electronic properties,including hole mobility, extrinsic hole concentration and Fermi level. We will discuss the influence of these parameters, especially the doping in perovskite film, on the open circuit voltage of the devices. ^[3]
[1] Giles E. Eperon, Victor M. Burlakov,Pablo Docampo, Alain Goriely andHenry J. Snaith. Morphological Control for High Performance, Solution-Processed Planar Heterojunction Perovskite Solar Cells, Advanced Functional Materials. 24, 151-157 (2014).
[2] Zhengguo Xiao, Cheng Bi, Yuchuan Shao, Qingfeng Dong, Qi Wang, Yongbo Yuan, Chenggong Wang, Yongli Gao and Jinsong Huang. Efficient, High yield Perovskite Photovoltaic Devices Grown By Interdiffusion Of Solution-processed Precursor Stacking Layers, Energy and Environmental Science, DOI: 10.1039/C4EE01138D (2014).
[3] Cheng Bi, Yuchuan Shao, Yongbo Yuan, Zhengguo Xiao, Chenggong Wang, Yongli Gao and Jinsong Huang. Understanding the Formation and Evolution of Interdiffusion Grown Organolead Halide Perovskite Thin Films by Thermal Annealing, Advanced Functional Materials. Submitted (2014)

Fundamental understanding of photoinduced charge separation and recombination processes of organometal halide perovskites is imperative for the optimization of optoelectronic and photovoltaic devices. Femtosecond transient absorption spectroscopy is a valuable tool to characterize the photogenerated charge carriers and establish the excited state dynamics. A two-body charge carrier recombination mechanism is established based on the second order kinetics of the bleach recovery at 760 nm, with a rate constant of 2.3 ± 0.6 10^-9 cm³ s^-1. A key finding of the present study is the absence of higher order Auger processes even at the largest photogenerated carrier densities. In addition an increase in the band edge transition with increasing charge carrier density suggests a band filling mechanism or dynamic Burstein-Moss effect is operative when subjected these films to bandgap excitation. The abrupt onset of this band filling effect provides an initial measure of the balance between excitons and free carriers in perovskite films. We have also probed the spectral changes associated with the formation of perovskite structure as we anneal the films at 150#730;C. The dependence of spectral shifts seen with films PbI₂/CH3NH₃I complex at different annealing times and different ratio of Pb:I offers new insights into the evolution of perovskite structure (CH₃NH₃PbI₃) from the initial lead halide complex PbI_x formed in precursor solution. Analysis of time-resolved transient absorption of different spectral bands to obtain further insight into the photoinduced charge separation will be discussed.

According to the low band-gap property of mixed lead halide perovskite material (CH₃NH₃PbI_3-xCl_x) that a large amount of separated electron-hole pairs with low binding energy, namely Wannier exciton, can be generated within perovskite crystalline domains under photoexcitation. Therefore, the low binding energy electron-hole pairs and internal spontaneous polarizations provide un-doubtable advantage to develop high-efficiency mixed lead halide perovskite based solar cells. For fully investigating the insight of photovoltaic response of mixed lead halide perovskite material, we introduce magnetic field effects of photocurrent (MFE_PC) and photoluminescence (MFE_PL) to study the properties of photoexcited excitons and the charge dissociation processes based on the planar-heterojuction ITO/PEDOT/CH₃NH₃PbI_3-xCl_x/PCBM/TiOx/Al device. In general, magnetic field effects of photoluminescence can be conveniently developed when a magnetic field changes the density of light-emitting states by re-orientating the singlet-triplet ratio through spin mixing. It has been found that magnetic field effects of photoluminescence can be conveniently generated based on the following three processes. First, a photoexcitation is allowed to generate singlet excitons due to spin-selection rule. Second, the singlet excitons can partially convert into triplet excitons through singlet-triplet transitions governed mainly by spin flipping mechanisms through internal magnetic interactions from hyperfine or spin-orbital coupling which can be induced by heavy lead atoms. Third, a magnetic field can perturb the singlet-triplet transitions by contributing to the spin flipping and consequently changes the singlet/triplet density, leading to magnetic field effects of photoluminescence, namely magneto-photoluminescence. Therefore, magneto-photoluminescence can elucidate the properties of photoexcited excitons in lead halide perovskite materials. On one hand, the magnetic field effects of photocurrent can be observed when a magnetic field changes the singlet/triplet population by perturbing the singlet-triplet transitions in mixed lead halide perovskite solar cells. This is because the singlets and triplets excitons can exhibit different dissociation rates driven by either different energies or different polarizations towards the generation of free charge carriers. Clearly, magnetic field effects of respective photoluminescence and photocurrent can elucidate more insight of the properties of photoexcited excitons and the charge dissociation processes in the perovskite solar cells based on ITO/PEDOT/CH₃NH₃PbI_3-xCl_x/PCBM/TiOx/Al device structure, and toward to higher photovoltaic efficiency.

Organic-inorganic perovskites are attracting increasing attention for their use in high-performance solar cells. Nevertheless, detailed understanding of charge generation, interplay of excitons and free charge carriers, and recombination pathways, crucial for further device improvement, are still incomplete. In this work we present a very generic yet analytically solvable model describing both equilibrium properties of free charge carriers and excitons in the presence of electronic sub-gap trap states, and their kinetics after photo-excitation, in the perovskite CH₃NH₃PbI_3-xCl_x. At low fluences the charge trapping pathways limit the photoluminescence quantum efficiency whereas at high fluences the traps are predominantly filled and recombination of the photo-generated species is dominated by efficient radiative bimolecular processes. The model is able to reproduce the time-resolved photoluminescence decays and photoluminescence quantum efficiencies, which we show approach 100% at low temperatures and at high fluences. The results from the model strongly indicate that the trap concentration increases with increasing temperature, suggesting an intrinsic origin of trap states. Our work provides an understanding of how to further enhance the material performance for high-efficiency perovskite solar cells and light-emitting diodes.

Thin film photovoltaic cells based on hybrid organic/inorganic perovskite absorbers, such as methylammonium lead halide CH₃NH₃PbX₃ (X is Cl, Br and I), have received great attentions recently because of their extraordinary power conversion efficiencies (PCEs). Initial PCEs of around 10% have been rapidly replaced by higher values ranging from 12% to over 15% as materials processing and device architectures are improved, making perovskite solar cells one of the most exciting new photovoltaics technologies available today. In contrast to the fast progress in device efficiency, the fundamental understanding concerning the defect properties, which play major roles in controlling the overall device performance of crystalline semiconductors, remains unknown. This is a particularly sensitive issue when a semiconductor such as a perovskite is doped by its own crystalline defects, since their energy levels and spatial distributions are extremely important factors that determine perovskite&’s electrical properties and success as a photovoltaic absorber. In this study, we conducted the admittance spectroscopy measurements on different device structures to identify the defect response. Furthermore, the results of temperature dependent admittance spectra reveal the defect energy distribution within the bandgap in the perovskite absorber. A deep defect state is found to be ~0.16 eV above the valence band. According to the theoretical calculation, the defect state is attributed to iodide interstitials (I_i), which can become the non-radiative recombination centers in the absorber.

Solar cells based on perovskite light absorbing materials reached power conversion efficiencies >15%. Today, the knowledge about the local charge generation processes inside these solar cells is limited. The aim of our study is to measure the electrical potentials under working conditions inside the device via scanning force microscopy methods [1]. In particular, we prepared smooth cross sections by means of focused ion beam milling such that the full structure and functionality of the devices were preserved. This way, the internal interfaces between the different materials in the cell are accessible for frequency modulation Kelvin Probe Force Microscopy (FM-KPFM). Our measurements indicated that mesoscopic lead methylammonium tri-iodide solar cells exhibit a homogeneous electric field throughout the device representing a p-i-n type junction. Upon illumination under short-circuit conditions, holes accumulate in front of the hole transport layer, which is proof of an unbalanced charge transport. This potential barrier reduces the charge transfer towards the electrode. Furthermore after light illumination, we measured remaining charges inside the active device area. These charges were attributed to traps in the material. In conclusion, the FM-KPFM method allows us not only to map the local contact potential variation but also to correlate it with the local structure of the functional layers.
References
[1] Q. Peng et al. Nanoscale 6, 1508 (2014).

The presence of the anomalous photocurrent hysteresis in many organometal trihalide perovskite solar cells was recorded by many groups. Sometimes an unreasonable fill factor (FF) larger than unit was observed, and in many cases the efficiency measured in devices with photocurrent hysteresis can be seriously overestimated. The mystery photocurrent hysteresis has become one major hindrance for the accurate power conversion efficiency (PCE) measurements. Thus understanding and elimination of the photocurrent hysteresis is an essential challenge for the whole perovskite photovoltaic research society now. Three possible mechanisms for the photocurrent hysteresis were proposed previously¹. 1) Ferroelectric photovoltaic effect. 2) The migration of ions. 3) The charge trapping and detrapping effect caused by a large defect density in perovskite bulk or at the surfaces.
In this talk, we will report our carefully analysis on the three mechanisms by test the ferroelectricity and ion drift effect. We identified a large density of trap states near the top surface and grain boundaries of the perovskite thin films as the origin of photocurrent hysteresis. We also introduced a simple and effective method to eliminate the notorious photocurrent hysteresis by depositing a phenyl-C61-butyric acid methylester (PCBM)/C₆₀ double fullerene layer on perovskites². The photocurrent hysteresis completely disappeared with optimum passivation, meanwhile the PCE doubles from 7~8% to 15~17%.
To verify our scenario, a series of characterizations like photoluminescence, thermal admittance spectroscopy, transient photocurrents and impedance spectroscopy were carried out for devices with and without the double fullerene layer. The results demonstrated the excellent passivation effect of PCBM/C₆₀ by reducing the density of surface traps in a complementary way and explain the huge improvement of PCE as the enhancement of the electronic transport properties and the decrease of the surface charge recombination.
1 Snaith, H. J. et al. Anomalous Hysteresis in Perovskite Solar Cells. The Journal of Physical Chemistry Letters5, 1511-1515 (2014).
2 Yuchuan Shao, Zhengguo Xiao, Bi Cheng, Yongbo Yuan & Jinsong Huang. Elucidating the Origin and Elimination of Photocurrent Hysteresis by Fullerene Passivation in CH3NH3PbI3 Planar Heterojunction Solar Cells. under review (2014).

The recent emergence of organometal halide perovskite-based materials for highly-efficient (ge;15%) and low-cost photovoltaics have focused interest on understanding and controlling the already low-cost and simple processing approaches toward even higher efficiencies, which may exceed 25% by improved material processing control and advanced optical management.^1-2 In addition, understanding how to extend the long term stability of these materials is another key challenge.¹ Both device efficiency and stability in perovskite absorbers (e.g., CH₃NH₃PbI_3-xCl_x, CH₃NH₃PbI₃) are believed to be significantly affected by the evolution of phase transitions and morphology during synthesis and environmental exposure, where fundamental systematic studies are needed. Here, we apply in-situ X-ray diffraction to identify the structural phase transitions of spin-coated organometal halide perovskite films as temperature is increased from room temperature to 250 °C in vacuum. As-grown perovskites (e.g., CH₃NH₃PbI₃) were shown to experience a tetragonal-cubic phase transition, and start decomposing to lead iodide (PbI₂) at temperatures above 135 °C, eventually completing the decomposition at 180 °C. Time-resolved in-situ x-ray diffraction measurements were also applied to understand the kinetics of crystal growth and evolution of crystalline order during growth. In situ neutron reflectivity techniques were applied to understand the structure and vertical phase morphology of the perovskite films. The electrical and photovoltaic performance of the perovskite solar cells was found to be closely related to the phase transition, crystal growth, and phase morphology. The best device performance was obtained by annealing the perovskite films at 100 °C for tetragonal phase. Process improvements to address control over phase transitions and film morphology as a result of these studies will be discussed.
This research was conducted at the Center for Nanophase Materials Sciences (CNMS) and the Spallation Neutron Source (SNS) which are sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.
References:
1. Robert F. Service, Science, 2014, 344 , 458
2. M. D. McGehee, Nature, 2013, 501, 323

Recently, organometal trihalide perovskites have revolutionized the field of thin film solar cells due to their remarkably fast increase in performance. However, the understanding of the working principles of these devices is lagging behind this rapid success. Further improvement can be expected from this material class by optimizing the energetic alignment and minimizing energetic barriers present in these devices.
We use UV and x-ray photoelectron spectroscopy (UPS/XPS) to probe films of perovskites that are either prepared by thermal evaporation in vacuum or by common solution processing approaches. We investigate the differences in the occupied density of states using the two preparation methods and look at the influence of preparation conditions, e.g. solvent effects, depositions rates or annealing procedures. Using high resolution UPS measurements we have the unique opportunity to compare the density of trap states present in these layers.
Furthermore, the vacuum evaporation approach allows for the preparation of films ranging from the sub-nanometer scale up to several tenths or hundreds of nanometers. So for the first time we are able to investigate interactions of perovskite monolayers with various commonly used contact materials and look at the energetic alignment and band bending effects when increasing the sample thickness.

This paper reports the interface effects on spin-dependent photoexcited excitons in an organo-metal halide perovskite by using both magnetic and electric field-dependent photoluminescence. We observe, for the first time, that a magnetic field can decrease the photoluminescence intensity in perovskite and consequently generates a magneto-photoluminescence. This experimental result leads to a hypothesis that the photoexcited excitons are populated in different spin states, forming singlet and triplet Wannier excitons, with the inter-conversions from high-energy singlets to low-energy triplets via exothermic process and spin flipping. Furthermore, we find that introducing the donor/acceptor (D/A) interface of perovskite/PCBM can affect both amplitude and line-shape in magneto-photoluminescence. This suggests that the D/A interface can dissociate the photoexcited excitons in bulk perovskite film and consequently changes the force-constant of singlet-triplet transitions. On the other hand, we show, by using electric field-dependent photoluminescence, that the D/A interface is formed with a local electric field under photoexcitation. This local electric field provides a driving force to dissociate the photoexcited excitons inside the perovskite film. Clearly, our magneto- and electro-photoluminescence studies provide a new understanding on the interfacial effects on spin-dependent photoexcited excitons in perovskite materials.

Advanced characterization of perovskite solar cells is already in progress yielding highly significant data. However, studies of isolated perovskite thin films under the working conditions of solar cells are still scarce. Presented here are photoconductivity measurements of CH₃NH₃PbI₃ deposited between two dielectric-protected Au electrodes spaced ~2000 nm apart. The photo response of the CH₃NH₃PbI₃ which is subjected to a dc bias, involves two time constants one of which is extremely long, lasting for several seconds. Similar time scale is observed upon transformation back to dark. Our findings seem to clarify the origin of the well-known hysteresis in perovskite solar cells.

Recent discoveries of highly efficient solar cells based on organometal trihalide perovskites have led to a surge in research activity on these materials. Much experimental effort has been devoted to understanding photophysics in organometal trihalide perovskites with excitations at or above the bandgaps, but little is known about below gap states that may be detrimental to solar cell performance. Here we show excitonic states below the optical gaps of three-dimensional (3D) CH₃NH₃PbI_3minus;xCl_x, CH₃NH₃PbI₃ and two dimensional (2D) (C₄H₉NH₃I)₂(CH₃NH₃I)_n-1(PbI₂)_n (n = 1, 2, 3) perovskites. These below-gap states possess weak optical transition strengths, exhibit dipole moments, can be populated from the trapping of above gap optical excitations, and become more significant as dimensionality decreases (from 3D to 2D and, for the 2D family, as n decreases from 3 to 1). The below-gap excitonic states are likely stabilized at surfaces/interfaces and must be minimized for optoelectronic applications.

Organometal halide perovskite are emerging as a new generation of solution processable, low cost photovoltaic materials. Many fabrication protocols for perovskite solution coating on flat substrates are exploited to control perovskite film morphology for achieving high efficiency planar heterojunction perovskite solar cells. Here, we report that perovskite film with thickness approximately 600nm, which is suitable for roll-to-roll manufacturing process, is greatly sensitive to the timescale of solution process. By tuning the timescale during the solution process, we are able to form uniform and high quality perovskite thin films with controllable morphology. The perovskite thin film exhibits superior optical and electrical properties and ordered crystal structures, resulting in a promising efficiency of over 14% in fully solution processed planar heterojunction perovskite solar cells.

Hybrid organic-inorganic halide perovskites have created a new excitement in the photovoltaic community. CH₃NH₃PbI₃, the primary semiconductor of interest, form nearly defect free, crystalline films at low temperatures with attractive properties including efficient charge transport and high optical absorption coefficients which make them amenable to a wide variety of device configurations. Perovskite based solar cells are currently the most efficient solution processed/3rd generation photovoltaic technology, where PCE approaching, or even exceeding 20% is a challenging but reasonable objective. These excellent electrical properties have been exploited in photovoltaic devices based on different electron transporting materials such as electrospun nanofibres and ZnO nanorods, which have allowed the fabrication of efficient flexible solar cells. In addition to balanced charge transport and long diffusion lengths, these hybrid materials have demonstrated a huge potential for a wide variety of electronics applications, such as light emission.
Research is rapidly switching from the archetypical methyl ammonium lead halide to other analogues which include substitution of the organic cation with formamidinium or caesium, substitution of lead with tin, and inclusion of various halides in the system. The opportunity of these substitutions include better control on the bandgap and thus light harvesting capabilities as well as improved stability and processability. Needless to say that the push for lead-free perovskite solar cells would be the top priority to enable potential scalability and commercialization potential. This presentation will address the various challenges and opportunities in perovskite solar cells beyond methyl ammonium lead iodide.

Organo-metal halide perovskite materials, CH₃NH₃PbI₃ or CH₃NH₃PbI_3-xCl_x, have been attractive in the 3^rd generation solar cells due to the excellent photovoltaic properties such as strong light absorption, long exciton diffusion, competitive power conversion efficiency (PCE) and potentiality of low production-cost. State-of-the-art device performance of perovskite solar cells is approaching that of commercialized silicon solar cells and expected to be further increased as high as 25%. Therefore, it can be competitive in the market as it is now if it can be produced via low cost manufacturing process such as roll-to-roll printing. However, the current perovskite solar cells have been made by vapor deposition process or spin coating-based process, and no scalable method has been used to fabricate perovskite solar cells. For those reasons, we demonstrated the fabrication of fully-printed perovskite solar cells by a scalable process, slot-die coator. Formation of perovskite layer and growth of perovskite crystals were analyzed by XRD, UV-visible spectroscopy, SEM and optical microscope. Finally, the effects of printing conditions on the film morphology and device performance will be discussed.

Low cost and high efficient photovoltaics have been persistently pursued during the past decade for renewable solar energy conversion. Recently, organometal halide perovskites have arisen as outstanding earth abundant photovoltaic materials, most commonly CH₃NH₃PbIshy;₃ or close variants, due to their small bandgap, large absorption coefficient, excellent crystallinity, and long charge diffusion length. These perovskite absorber layers could be fabricated via solution process, allowing for relative simple and economical production of solar cells. However, the electrode deposition of both sides still requires high vacuum process. The counter electrodes include noble metals such as Au or Ag prepared by thermal evaporation, and vacuum sputtered fluorine doped tin oxide(FTO) or indium tin oxide(ITO) films are commonly used as a transparent electrode. Since the vacuum evaporation and sputtering process are energy consuming and raise the manufacturing cost, alternative solution method to replace the vacuum processed both electrodes is highly demanding to achieve an ultimate goal of the perovskite solar cells; inexpensive, high efficient photovoltaic system.
Here, we suggest the silver nanowire (AgNW) based composite electrode as an effective, solution-processable electrode for the perovskite solar cells. The AgNW composite electrode has a low sheet resistance and high transparency, and also it can be easily fabricated through solution process in air. For preventing the silver from diffusing into perovskite film and acting as a hole blocking layer, a low temperature TiO₂ layer was used to construct AgNW composite electrode. All layers were deposited at low temperature ~100 ^oC including the electrode, TiO₂ blocking layer, and perovskite film. The resulting cells showed high I-V performances comparable to the conventional cell fabricated on FTO substrate. In addition, the counter electrode also could be achieved by solution-processed AgNW film. This leads to the semi-transparent solar cells due to the light transmission through AgNW electrode, which is different from an opaque metal contact. By adopting solution-processed electrodes into the perovskite solar cell successfully, the possibility of the fully printable and low cost perovskite solar cell will be proven.

This talk reviews recent ultra-violet, X-ray and inverse photoemission spectroscopy (UPS, XPS IPES) measurements of the electronic structure of three organo-metal halide perovskites, i.e., methylammonium lead tri-halides (MAPbX, X=Br₃, I₃, I_3-xCl_x), and of the energetics and chemistry of some of their interfaces with molecular hole-transport layers (HTL) and oxides. The measurements first provide the necessary quantitative information on the position of the valence and conduction band edges of the MAPbX films, and associated ionization energy and electron affinity (IE, EA) [1]. The measurements show the fairly good alignment of the MAPbX conduction band minimum with that of the electron-collecting TiO₂, a result that suggests minimal energy loss during electron extraction at this interface. The hole-extraction interface of the device is investigated with spiro-MeOTAD vacuum evaporated on the three perovskites, as well as with PTCDI-C₁ films on MAPbBr₃. The positions of the perovskite valence and conduction band edges relative to the HTM highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO), respectively, as determined via UPS and IPES, clearly puts into evidence the role of interface energetics in terms of quasi energy loss-less hole extraction from, and electron confinement in, the perovskite layer [2]. Finally, the high work function conducting oxide MoO₃ is introduced on the hole-extraction side to provide a large built-in potential across the perovskite-based device. XPS demonstrates that vacuum-evaporated MoO₃ undergoes considerable interface chemical reaction with the perovskite, leading to the reduction of Mo (Mo⁶⁺ → Mo⁴⁺) and formation of interface states, which likely lead to carrier recombination and poor device efficiency [3]. The reaction is essentially eliminated by introducing a thin (2-3 nm) protective organic HTL interlayer, i.e. spiro-MeOTAD. MoO₃-induced core and valence level shifts suggest the formation of a beneficial 0.4-0.5 V built-in potential across, with eventual band bending in, the peroskite layer.
[1] P. Schulz, E. Edri, S. Kirmayer, G. Hodes, D. Cahen and A. Kahn, Energ. & Envir. Sci. 7, 1377 (2014)
[2] E. Edri, S. Kirmayer, M. Kulbak, G. Hodes and D. Cahen, J. Phys. Chem. Lett., 5, 429 (2014)
[3] Y. Zhao, A. M. Nardes, and K. Zhu, Appl. Phys. Lett. 104, 213906 (2014)

CH₃NH₃PbI_3-xCl_x is a commonly used chemical formula to represent the methylammonium lead halide perovskite fabricated from mixed chlorine- and iodine-containing salt precursors. Despite the rapid progress in improving its photovoltaic efficiency, fundamental questions remain regarding the atomic ratio of Cl in the perovskite as well as the reaction mechanism that leads to its formation and crystallization. In this work we investigated these questions through a combination of chemical, morphological, structural and thermal characterizations. The elemental analyses reveal unambiguously the negligible amount of Cl atoms in the CH₃NH₃PbI_3-xCl_x perovskite. By studying the thermal characteristics of methylammonium halides as well as the annealing process in a polymer/perovskite/FTO glass structure, we show that the formation of the CH₃NH₃PbI_3-xCl_x perovskite is likely driven by release of gaseous CH₃NH₃Cl (or other organic chlorides) through an intermediate organometal mixed halide phase. Furthermore, the comparative study on CH₃NH₃I/PbCl₂ and CH₃NH₃I/PbI₂ precursor combinations with different molar ratios suggest that the initial introduction of a CH₃NH₃⁺ rich environment is critical to slow down the perovskite formation process and thus improve the growth of the crystal domains during annealing; accordingly, the function of Cl^- is to facilitate the release of excess CH₃NH₃⁺ at a relatively low annealing temperatures.

Recent progress in using hybrid organic-inorganic perovskites as thin-film solar cell absorber materials has demonstrated their potential as cheap, high-efficiency alternatives to silicon-wafer-based photovoltaics. In the few years since their introduction into solar cell devices, power conversion efficiencies up to 17.9% have been demonstrated.¹ For solar cell applications, mixed methyl ammonium organometallic halide perovskites, CH₃NH₃XY₃ (X = Pb or Sn and Y = I, Br, and/or Cl), are predominantly used, in particular CH₃NH₃PbI_3-xCl_x. To date, the fundamental properties of these materials remain poorly understood.
We have used a variety of x-ray and electron spectroscopies with varying information depths to study the chemical and electronic structure of CH₃NH₃PbI_3-xCl_x with a particular focus on identifying the chemical environment of chlorine. Lab-based x-ray and ultra-violet photoelectron spectroscopies (XPS and UPS) have been used to investigate the core and valence levels at the surface of CH₃NH₃PbI_3-xCl_x layers of increasing thickness on compact TiO₂. We find that the perovskite layer does not completely cover the TiO₂ surface, in agreement with the island formation observed in scanning electron micrographs,² and that no chlorine is present at the perovskite surface. Synchrotron-based hard x-ray photoelectron spectroscopy (HAXPES), which has a larger information depth than lab-based XPS and UPS, has been used to investigate the near-surface region of the perovskite layers and their buried interface with the TiO₂. We observe “metallic” Pb in the bulk of the studied CH₃NH₃PbI_3-xCl_x perovskite samples, the absence of chlorine, and that the valence band offset between CH₃NH₃PbI_3-xCl_x and TiO₂ significantly depends on the perovskite “thickness” and/or the type of TiO₂ used. Furthermore, bulk-sensitive fluorescence yield x-ray absorption spectroscopy (XAS) measurements at the Cl K edge reveal that chlorine is present in the proximity of the deeply buried CH₃NH₃PbI_3-xCl_x/TiO₂ interface. The chemical identity of the chlorine species is currently ongoing.
Based on these results, we will present a detailed picture of the chemical and electronic structure of CH₃NH₃PbI_3-xCl_x thin films and their interfaces and correlate the findings with the performance of related solar cell devices.
¹http://www.nrel.gov/ncpv/images/efficiency-chart.jpg, last accessed 2014-06-17.
²G. E. Eperon et al., Adv. Funct. Mater. 24, 151 (2014)

Organic-Inorganic mixed halide perovskites MAPbX₃, (MA=CH₃NH₃⁺, X=Cl^-, Br^-, I^-), both in their single halide [1] and in their mixed halide [2] crystals, have been shown to quantitatively enhance the PCE of photovoltaic devices up to values one and a half higher than those of traditional DSSC [3]. Their extremely recent appearance in PV in 2009 [4] suggests the high potentiality in solar-to-energy conversion and similarly that there is plenty of room to further improve the performances of such perovskite based devices. Very interestingly these perovskites are not simple light harvester; they additionally are indeed both good electron [1] and hole [2] transport materials, in other words they show ambipolar behaviour [5, 6]. In the present contribution we report our results obtained via a first-principle plane wave based analysis [7].
We focus on the impact that relativistic effects (Spin-Orbit Coupling, SOC) exert on the electronic and optical properties of MAPbI₃ [6] and similarly on the role played by the two cations (MA [8], Pb) on the same properties of these extremely appealing materials. By means of both a DFT and a DFT+SOC approach, we calculated the photocarrier effective masses of MAPbI₃ confirming the experimentally reported ambipolar nature of these organic-inorganic perovskites, obtaining values that are comparable with those of commercially available silicon solar cells [6]. We similarly give clear evidence that the ambipolar behaviour is attributed by the presence of the organic cation. Its removal indeed induces a flattening with subsequent energetic reordering of the valence band maximum that causes a marked electron transport nature to the PbI₃^- inorganic skeleton. Finally, we compare the electronic properties of MAPbI₃ with those of the most studied fully inorganic counterpart, i.e. CsPbI₃, finding the origin for the reduced bandgap of the latter species.
[1] Lee et al., Science, 338, 634, (2012).
[2] Heo et al., Nature Phot., 7, 486 (2013).
[3] KRICT researchers have reported a PCE of 16.2%.
[4] Kojima et al., JACS, 131, 6050 (2009).
[5] Etgar et al., JACS, 134, 17396 (2012).
[6] Giorgi et al., J. Phys. Chem Lett. 4, 4213 (2013).
[7] (a) Kresse et al. Comput. Mater. Sci. 6, 15 (1996); (b) Phys. Rev. B, 54, 11169 (1996).
[8] Giorgi et al., J. Phys. Chem. C 118, 12176 (2014).

A microscopic picture of the crystal structure and relevant binding processes in organic-inorganic perovskites is imperative for understanding the remarkable semiconducting and photovoltaic properties of these materials. Due to the distinctly different building blocks present in the material, a range of interactions could, in principle, contribute to cohesion. In particular, weak interactions may play an important role and significantly influence the cohesive properties of organic-inorganic pervoskites. On the basis of density functional theory, we provide detailed insights into the crystal binding of lead-halide perovskites and quantify the effect of different types of interactions on the structure. Furthermore, in light of the recently elucidated importance of spin-orbit coupling for the electronic structure of lead-halide perovskites, we examine the extent to which it affects structural properties.

Halides are not usually considered as electronic or optoelectronic materials because their transport properties are usually inferior compared to more covalent semiconductors. However, several halide perovskite materials have recently been found to exhibit exceptionally good transport properties (e.g., high mobility, long carrier lifetime and diffusion length) for both electrons and holes. Methylammonium lead iodide chloride (CH₃NH₃PbI_1-xCl_x) is reported to have electron and hole diffusion lengths both exceeding 1 mu;m. Density functional calculations are performed to study the electronic structure, dielectric properties, and defect properties of β-CH₃NH₃PbI₃. The results show that Pb chemistry plays an important role in a wide range of material properties, i.e., small effective masses, enhanced Born effective charges and lattice polarization, and the suppression of the formation of deep defect levels, all of which contribute to the exceptionally good carrier transport properties observed in CH₃NH₃PbI₃. Defect calculations show that the iodine interstitial is the only low-energy native point defect that acts as a deep trap and non-radiative recombination center. Interestingly, the iodine vacancy is a shallow electron donor. This is in contrast to the majority of the anion vacancies in halides, which act as deep electron trap (F center). The relation between the crystal structure and the nature of the anion vacancy in halides (shallow vs. deep) is discussed.

Solar cells based on organo-metal halide perovskites represent the fastest developing photovoltaic technology so far, and are thus major contestants in the quest for inexpensive and high-efficiency photovoltaics.^[1-3] Two of the three principal requirements for the breakthrough of this technology are already fulfilled: (i) the starting products and cell production process are very inexpensive and (ii) with the current record cell efficiency approaching 18%,^[4] perovskites exhibit high performance that is not far away from challenging polycrystalline and monocrystalline silicon. To meet the third requirement, however, is likely the most challenging: the guarantee of a decent device lifetime that is at least comparable with existing established technologies. Despite the obvious urgency of this aspect, the vast majority of reports on perovskite solar cells have been focusing on efficiency enhancement, whereas the degradation mechanisms are far from clarified. Yet, these best performing perovskites are highly sensitive to humidity due to the presence of an alkylammonium cation in the crystal matrix. In addition, there is no clear picture of the perovskite's ability to withstand typical outdoor operating temperatures (which are very similar to the formation temperature of the perovskite). To bring more clarity into these issues, this contribution focuses on the degradation mechanisms of organo-metal halide perovskites under the influence of different stress factors. The chemical composition of degraded perovskites resulting from mono-stress tests is investigated with X-ray photoelectron spectroscopy (XPS), allowing to pinpoint the main culprits for the decay of the perovskite. In parallel, the perovskite's crystal structure and its optical and morphological properties are investigated, and the correlation is made with full devices.
[1] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science2012, 338, 643.
[2] B. Conings, L. Baeten, C. De Dobbelaere, J. D'Haen, J. Manca and H.-G. Boyen, Adv. Mater.2014, 26 (13), 2041.
[3] D. Liu and T. L. Kelly, Nat. Photonics 2014, 8 (2), 133.
[4] http://www.nrel.gov/ncpv/images/efficiency_chart.jpg

Perovskite CH₃NH₃PbX₃ (X=Cl, Br, and I) semiconductors recently attract growing interests because of their advanced photovoltaic properties. The power conversion efficiencies of perovskite solar cells attain nearly 20 % to date. It is significant to reveal the key mechanism that provides higher conversion efficiencies of perovskite solar cells. For this purpose, we need to understand the fundamental optical properties of perovskite semiconductor itseilf. We have reported the static optical properties and determined the bandgap energy [1]. It was also demonstrated that the near-band-edge optical absorption is determined by the Urbach tail. To gain a further understanding of photoconversion mechanism in perovskite solar cells, dynamical behaviors of photoexcited carriers should be clarified.
In this study, we investigated the photocarrier recombination dynamics and their time variation after sample fabrication by means of time-resolved photoluminescence (PL) spectroscopy. We fabricated CH₃NH₃PbI₃ films on glass substrates by a spin-coating method. Samples were kept in argon atmosphere before and during experiments to avoid degradation due to air exposure and left for a few days after fabrication to stabilize the material properties. All measurements were conducted at room temperature.
We observed excitation-intensity dependent PL intensity and dynamics. PL intensity shows quadratic dependence on the excitation intensity, meaning that the two-carrier radiative recombination process determines the PL intensity. This is the direct evidence that the photoexcited electrons and holes do not form excitons but behave as free carriers at room temperature. PL lifetime is also sensitive to the excitation intensity. PL lifetime keeps almost constant value (~200 ns) under weak excitation and is reduced with an increase in intensity above 1 nJ/cm². We found that the PL dynamics are described by a simple rate equation based on nonradiative single-carrier trapping and two-carrier radiative recombination process, and determined the radiative recombination coefficient.
In addition, we will report the time variation of PL lifetime. Just after fabrication, PL dynamics has a fast decay component (tau;~ ns). We consider that this fast component originates from the defects in the samples. However, the fast component gradually disappears within two days and only a slow decay component (~200 ns) is observed. This result implies that the samples just after fabrication contain large amount of defects that are gradually compensated by thermal annealing at room-temperature. We believe that this finding provides an important insight that helps to fabricate high-efficiency perovskite solar cells. The correlation between the time-dependent PL lifetime and the solar cell efficiency 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. Express 7, 032302 (2014).

Since the first report in 2009, perovskite solar cells (PSCs) have been attracting great attention for the high energy conversion efficiency and the low cost of materials and fabrication process. However, there is still a big challenge to maintain high stability and reproducibility of high performance PSCs.
In the first part of this presentation, I will demonstrate the effect of nanoscale pinholes in hole-blocking layer on the device performance. Surface morphology and electrical studies show that the film resistance of hole-blocking layer is largely enhanced by reducing the density of nanoscale pinholes. A large shunt resistance was obtained by the forming of a nanoscale pinhole-free hole-blocking layer in cells, which shielded the device against unwanted charge recombination and enabled a high power conversion efficiency of 14.9%.
Secondly, I will present an efficient method to shield the device against the humid-induced degradation. A dopant-free hole-transporting material (HTM) was introduced in PSCs without using of p-type dopants like Lithium salts. The dopant-free HTM based PSCs performed comparable efficiency to cells based on well-known p-type doping spiro-OMeTAD; moreover, the stability was greatly improved by 2 fold in air at a relative humidity of ~40%, owing to the shield by dopant-free HTM. This work paves the way for the design of novel dopant-free HTMs and will promote the advancement of cost-effective and practical PSCs.
References
[1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc., 2009, 131, 6050.
[2] Yongzhen Wu1, Xudong Yang, Han Chen, Kun Zhang, Chuanjiang Qin, Jian Liu, Wenqin Peng, Ashraful Islam, Enbing Bi, Fei Ye, Maoshu Yin, Peng Zhang, and Liyuan Han. Applied Physics Express 2014, 7, 052301.
[3] Jian Liu, Yongzhen Wu, Chuanjiang Qin, Xudong Yang, Takeshi Yasuda, Ashraful Islam, Wei Chen, Kun Zhang, Wenqin Peng and Liyuan Han. Energy Environ. Sci., in press.
[4] Yongzhen Wu, Ashraful Islam, Xudong Yang, Chuanjiang Qin, Jian Liu, Kun Zhang, Wenqin Peng and Liyuan Han. Energy Environ. Sci., in press.

Organo-halide perovskites have attracted huge amounts of attention for their use in solution processable solar cells due to their reported efficiencies surpassing 19%. We report on the effect of preparing these lead-based perovskites under highly controlled air and moisture free conditions. When prepared under completely inert conditions, only amorphous powders could be obtained. It was found, however, that on exposure to moisture these amorphous powders very rapidly crystallized in to the expected cubic or tetragonal cells. We compare the photophyiscal properties of both states and discuss the role moisture plays in causing the differences. We further characterize the structure of the crystalline and amorphous phases using high resolution synchrotron X-ray scattering and Pair Distribution Function analysis.

Solar cell research based on (CH₃NH₃)[PbX₃] (X = Cl, Br, I) and related perovskite materials has been ubiquitous in the photovoltaic community. With their high efficiencies and facile processing, perovskite devices show promise for commercial viability. However, there remain a few concerns with the inherent properties of these materials: the toxicity of lead and the moisture sensitivity of the perovskite. Recently, lead was replaced by the significantly less toxic tin analog in solar cells with efficiencies of up to ca. 6%. However, the moisture-sensitivity of the material remains a problem. Devices fabricated in humid environments do not achieve high solar conversion efficiencies and research on long-term device performance in the presence of moisture is still lacking. Device-centric fixes such as encapsulation can help remedy these problems but inherent stability to moisture will likely be necessary for large-scale, cost-efficient manufacture and for achieving the long lifetimes required for broad commercialization. I will present our recent results on new structural modifications that improve the moisture stability of these perovskites and provide additional routes for tuning the properties of these versatile and well-defined materials.

A summary of results of our characterization and stability studies on polymer, DSSC, and MAPI perovskite solar cells will be presented. Concerning perovskite solar cells, we have examined the JV hysteresis of mp-TiO2/MAPI/HC and bilayer MAPI/HC cells as a function of temperature using a simultaneous stepped voltage and stepped light experiment. We have extracted the activation energy of the process controlling the hysteresis. Formamadinium ( FAPI cells) have a similar hysteresis signature, indicating the hysteresis is not due to diffusion of the B site ion. We have characterized the stability of the MAPI and FAPI material under various conditions. For example, we find bare MAPI films are completely unstable to 48 hours at 85 oC in a glove box. We find full MAPI/Spiro cells are similarly unstable. We have heated sealed bare MAPI films to 85 oC, in the glove box, followed by unsealing and cell completion with Spiro/Gold. We find mp-TiO2/MAPI films are stable if the headspace for sublimation is sufficiently small (~20 µm). However, sealed bare MAPI films without mesoporous substrates are unstable at 85 oC; they do not make efficient cells after unsealing. We propose this is due to a different crystal growth occurring in the two cases. Results of on accelerated aging of MAPI and FAPI cells under 40 suns illumination will be discussed. In the realm of DSSCs, we will summarize our recent work on the under-appreciated importance of the dye/electrolyte "interface". We have shown that both iodine and guanidinium bind to the dye, the former having a negative impact and the latter positive. Literature reports also show that TBP has specific interaction with some dyes, possibly competing with iodine binding, but also slowing injection. The implication is that the electrolyte composition may control the success or failure of a new dye class. However, up till now, other explanations (frequently the catch-all "aggregation") have been favored, perhaps helping to explain the very slow progress in replacing ruthenium dyes in DSSCs. Finally, in the realm of polymer cells we have measured a new effect, Voc saturation, at light levels from 10 to 100 suns. The Voc saturation maximum varies by >300 mV between different donor /acceptor pairs. We are developing a model to explain the observation. We expect this observation to help distinguish between Voc that is limited by the "surface bound charge pair " (as measured by sub-band gap luminescence), the HOMO-LUMO gap, or the electrode materials. Some cell types show a unique Voc decrease with increasing illumination above the saturation value (not do to heating). We think this is a signature of a second charge generating interface running in reverse to the main donor/acceptor pair. This could be due to the contacts, or possibly to a pure polymer or pure PCBM layer at one side or the other. W hopee to present polymer cell stabilities well beyond the 20 hours at 40 suns that we have recently published.

Thin-film solar cells based on organometallic halide perovskite absorber layers are emerging as a high-performance photovoltaic technology. Since the first report on perovskite-based solar cells by Kojima et al. [1] the efficiency of these cells has increased rapidly up to 17.9% [2]. Fast progress has at least partially been enabled by excellent photovoltaic properties of CH₃NH₃PbI₃ perovskite films, featuring very high absorption in the whole visible part of the solar spectrum and well-ordered microstructure of the deposited films, as evidenced by their sharp absorption edge [3].
However, perovskite solar cells are known to degrade at moderate temperatures and upon moisture ingress. Here, we present micro-Raman spectra of CH₃NH₃PbI₃ perovskites films that reveal this phenomenon locally on the micrometer scale. Raman spectra of freshly prepared non-degraded CH₃NH₃PbI₃ perovskites films were measured and the degradation process was observed by repeatedly measuring the Raman signal on the same spot of the sample. Great care was taken to reduce the impact of the laser light on the perovskite sample during the measurements as much as possible to ensure a well-controlled degradation. During the degradation process, Raman spectra are changing from pure CH₃NH₃PbI₃ perovskites spectra to spectra that closely resemble those of PbI₂ thin films. At the same time, the absorbance of the film locally changes and the film becomes more transparent at the excitation wavelength of 514 nm, which is in good agreement with results obtained in Ref. [3]. Interestingly, the Raman spectra of the degraded perovskite layer is very similar to a recently published Raman study of CH₃NH₃PbI₃ perovskite films [4], indicating that the results reported there most probably originate from degraded rather than pristine perovskite layers. To gain more insight into the degradation mechanisms at work during Raman measurements, we also performed measurements in vacuum.
Based on our results, the role of humidity, temperature and illumination on the CH₃NH₃PbI₃ degradation will be discussed.
[1] A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, “Organometal halide perovskites as visible-light sensitizers for photovoltaic cells” J. Am. Chem. Soc. 131 (2009) 6050.
[2] NREL chart - Best Research-Cells Efficiencies.
[3] S. De Wolf, J. Holovsky, S.-J. Moon, P. Löper, B. Niesen, M. Ledinsky, et al., "Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance," J. Phys. Chem. Lett. 5 (2014) 1035.
[4] C. Quarti, G. Grancini, E. Mosconi, P. Bruno, J. M. Ball, M. M. Lee, H. J. Snaith, A. Petrozza, and F. De Angelis, “The Raman Spectrum of the CH(3)NH(3)Pbl(3) Hybrid Perovskite: Interplay of Theory and Experiment” J. Phys. Chem. Lett. 5 (2014) 279.

Although one of the most widely-used hole transport materials (HTMs) is 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9'-spirobifluorene (spiro-MeOTAD), it has some disadvantages such as high cost, hydrophilic condition and low hole mobility (~10^minus;4 cm² V^minus;1s^minus;1) and low conductivity (~10^-5 Scm^-2) in its pristine form.^1,2 Further enhancement in the field of perovskite solar cells could be realized by developing new hole transporting materials with high electrical properties and proper oxidation potential with respect to the energy level of perovskite. Diketopyrrolopyrrole-containing polymers are known to be an alternative HTM due to a high hole mobility (0.32 cm²V^-1s^-1).³ We recently reported the development of DPP-based highly p-extended polymers, namely, poly[2,5-bis(2-decyldodecyl)-pyrrolo[3,4-c] pyrrole-1,4(2H,5H)-dione-(E)-1,2-di(2,2'-bithio-phen-5-yl)-ethene] (PDPPDBTE), which provided high-performance field effect transistors that exhibited a m_FET = 0.32 cm²V^-1s^-1.²⁴ The DPP moiety exhibited excellent hole mobilities in OFETs and successfully functioned in organic photovoltaics (OPVs).^4,5 With an appropriate oxidation potential of 5.4 eV vs. the vacuum level, the PDPPDBTE conjugated polymer is expected to function efficiently as an HTMs when incorporated into perovskite-based hybrid solar cells. Herein, we present the preparation of efficient solid-state hybrid solar cells using PDPPDBTE as a HTM. A PCE of 9.2% was achieved, along with excellent long-term stability over 1000 hrs. This is the first successful demonstration of a conjugated polymer-based solar cell PCE exceeding the maximum value obtained from a spiro-MeOTAD-based solar cell (7.6%). Considering that the averaged PCE of a perovskite cell prepared using spiro-MeOTAD lies between 7.5 and 8.5%, further performance enhancements are expected of PDPPDBTE-based cells with careful optimization of the device architecture.
References
1 J. Burschka, A. Dualeh, F. Kessler, E. Baranoff, N. L. Cevey-Ha, C. Y. Yi, M. K. Nazeeruddin, and M. Grätzel, J. Am. Chem. Soc., 2011,133, 18042.
2 T. Leijtens, J. Lim, J. Teuscher, T. Park, and H. J. Snaith, Adv. Mater., 2013,25, 3227.
3 I. Kang, T. K. An, J.-A. Hong, H.-J. Yun, R. Kim, D. S. Chung, C. E. Park, Y.-H. Kim, and S.-K. Kwon, Adv. Mater., 2013,25, 524.
4 H. Bronstein, Z. Y. Chen, R. S. Ashraf, W. M. Zhang, J. P. Du, J. R. Durrant, P. S. Tuladhar, K. Song, S. E. Watkins, Y. Geerts, M. M. Wienk, R. A. J. Janssen, T. Anthopoulos, H. Sirringhaus, M. Heeney, and I. McCulloch, J. Am. Chem. Soc., 2011,133, 3272.
5 B. Walker, J. H. Liu, C. Kim, G. C. Welch, J. K. Park, J. Lin, P. Zalar, C. M. Proctor, J. H. Seo, G. C. Bazan, and T.-Q. Nguyen, Energy Environ. Sci., 2013,6, 952.

Perovskite solar cells have advantages of high energy conversion efficiency and low cost. Transparent electrode is one important part of perovskite solar cells. Although both ITO and FTO have been used as the transparent electrode of perovskite solar cells, they are too expensive. Here, I will present high-performance perovskite solar cells without ITO and FTO. A solution processable transparent and conductive material is used as the transparent electrode. This new transparent electrode material can further lower the cost of perovskite solar cells. The devices are fabricated through a low-temperature process.

The organic-inorganic hybrid perovskite materials such as CH₃NH₃PbI₃ have recently attracted great attention as light harvesters in solid-state sensitized solar cells since they has led to the remarkable improvement in the power conversion efficiency up to 18 %. This high performance is attributed to the superior electrical transport properties including the long diffusion length of photogenerated electron and holes and fast charge transport at the interfaces with semiconductors. [1] Recently, it was argued that the excellent electrical charge transport in organic lead halide perovskites can be related to the highly flexible electrical properties, which depends on point defects. [2] In this context, the comprehension of defect chemistry is a critical requirement to design functional materials with even higher photovoltaic performance. However, the defect chemistry and related charge carriers chemistry have not been investigated in organic-inorganic hybrid perovskite materials. Herein, through dc polarization and ac impedance measurements under systematically controlled atmosphere, we identified the major ionic and electronic charge carriers and showed the influences of defect generation on their conduction for the first time.
The electrical measurements were conducted in cold-pressed CH₃NH₃PbI₃ pellets with ion blocking graphite electrodes under Ar-atmosphere with controlling iodine partial pressure. In dc polarization test, the typical behavior of stoichiometric polarization was observed. Under these conditions, the majority mobile charge carrier was identified to be iodine ions. The ionic conductivity was even higher than the electronic conductivity. At high iodine partial pressure, the ionic conductivity was reduced, while the electronic conductivity increased by more than an order of magnitude. These results indicate that the electronic conduction is p-type, and the conductivity strongly depends on the concentration of iodine defects. Our findings suggest that the point defects and the ionic space charge region in the perovskite materials can significantly affect the energy conversion efficiency of a solar cell.
[1] G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S. Mhaisalkar, T. C. Sum, Science 2013, 342, 344-347
[2] W.-J. Yin, T. Shi, Y. Yan, Appl. Phys. Lett. 2014, 104, 063903

Modification by magnesium oxide(MgO) was successfully introduced into perovskite solar cells to protect perovskite from corrosion by the photo-catalysis of TiO₂ under ultraviolet(UV) light. MgO modification was fabricated on mesoporous TiO₂ film using a simple method by the oxidation of magnesium acetate and therefore could block the contact between perovskite and TiO₂. After radiation under UV light, UV-vis absorption spectra of perovskite with MgO modification revealed an enhanced absorption compared to the un-modificated one, especially in long wavelength range. Besides the photo current, photo-voltage and fill factor also showed an enhancement after modification, resulting in the overall efficiency increasing from 9.9% to 13.2%. With further study, we used found the MgO could increase the bandgap of TiO₂ to raise the photo-voltage via UV-vis DRS spectra and the EIS results gave the prove that MgO could act as a insulating layer to reduced charge recombination and therefore increase the fill factor. Thus, the application of MgO as a functional layer between TiO₂ and perovskite is a useful strategy to preparing highly efficient hybrid perovskite solar cells with anti corrosion by UV-light, increased photo-voltage and retarded charge recombination. At last, we raised a new mechanism of UV-vis corrosion cooperated with H₂O absorbed on TiO₂ and further investigation is necessary to understand the inside of the UV light degradation mechanism.

Hybrid organic-inorganic perovskite photovoltaics (PVs) have achieved tremendous progress over the last few years and promise to advance the thin film PV technology with considerably high efficiency and low cost. The p-i-n planar heterojunction soalr cell (PHSC) device structure allows simple and low temperature processing which is potentially compatible with flexibly roll-to-roll production. It has been reported that the organic-inorganic hybrid perovskite materials possess ambipolar transport properties with high electron-transport lengths, indicating the capability of free carriers (electrons and holes) transporting toward electrodes with little recombination. For further simplifying the procedure of device fabrication and realizing room-temperature (RT) processing, in this presentation we&’ll for the first time demonstrate an electron transporting layer free (ETL-free) device. Polymers containing simple aliphatic amine groups (i.e. polyethylenimine ethoxylated (PEIE)) was used to mediate the electron collection between absorbers and transparent electrode fluorine-doped tin oxide (FTO). The RT processed ETL-free perovskite solar cell shows significant PCEs of 10.5%. The work function control and charge collection efficiency were systematically investigated by Kelvin probe formce microscope (KPFM) and Impedance modeling. Our perovskite solar cell omits high temperature sintering for TiO₂ compact layer or the synthesis step of metal oxide nanoparticles (e.g. ZnO or TiO₂) and thus simplifies the device manufactory. The developed ETL-free PHSC based on RT solution processing promises extremely low-cost solar energy and potentially allows monolithic multi-junction device fabrication for breaking the PCE record.

Organometal halide perovskite-based solar cells have recently been reported to be highly efficien. However, much of the fundamental physical and chemical properties underlying this performance have remained unknown.
In this work, we report on our recent studies on the effects of film processing conditions (e.g., solvent and temperature), film thickness and alternative top contacts on charge transport, recombination, and device characteristics of perovskite-based solar cells.
Particularly, we will present our newly developed one-step solution approach to prepare perovskite CH3NH3PbI3 on a mesoporous TiO2 film or on a planar, compact TiO2 layer on FTO. In this new synthetic approach, CH3NH3Cl is added to the standard CH3NH3PbI3 precursor (equimolar mixture of CH3NH3I and PbI2) solution to adjust the crystallization process for CH3NH3PbI3. Depending on the amount of MACl used in the precursor solution and the annealing temperature, the optimum crystallization time for forming pure CH3NH3PbI3 with the strongest absorption varies from a few minutes to several tens of minutes. The use of MACl not only leads to enhanced absorption of CH3NH3PbI3 but also improves significantly coverage of CH3NH3PbI3 on a planar substrate.
Moreover, pushing our device optimization and better understanding of its operation, we examined charge transport, recombination, and device characteristics of solid-state mesostructured perovskite CH3NH3PbI3 solar cells based on 0.24 to 1.65 mu;m thick TiO2 films and spiro-MeOTAD hole conductor. Charge transport and recombination in the solid-state mesostructured perovskite cells are similar to those in the solid-state DSSC and exhibit little dependence on the TiO2 film thickness. The performance of perovskite cells increases with TiO2 film thickness up to 650minus;850 nm, resulting primarily from the enhanced light harvesting. Further increasing film thickness results in lower cell efficiencies, mainly caused by the reduced FF or Jsc. The electron diffusion length in mesostructured perovskite cells is found to be longer than 1 mu;m under normal cell operation conditions.
Finally, we will demonstrate the effectiveness of using a combination of a thin layer of molybdenum oxide and aluminum as the top-contact structure for extracting photogenerated holes from perovskite solar cells, instead of nobel metals such as Au or Ag. The device performance of perovskite solar cells using a MoOx/Al top contact is comparable to that of cells using the standard Ag top contact. Impedance measurements suggest that the extraction of photogenerated holes is not affected by the MoOx/metal interface when proper MoOx thickness is used. Using a thicker (20-nm) MoOx layer leads to decreased cell performance resulting primarily from a reduced fill factor.
We will discuss our result in terms of device performance in order to serve as guide toward highly efficiency cells. These results have direct implications for device manufacturing and upscaling issues.

The solution processable perovskite solar cells exhibit outstanding power conversion efficiency (PCE) up to 17.9% due to low exciton binding energy and high charge mobility of pervoskite. However, the pervoskite is fragile and weak absorption beyond 600nm. On the other hand, low band gap polymer has merits of mechanical flexibility, strong absorption beyond 600nm to near IR, semi-transparency, low weight. Thus, we would like to fabricate solar cell from both pervoskite and low band gap polymer to cover the whole solar spectrum that can increase J_sc and PCE of solar cell. But in the device fabrication, the low band gap polymer cannot endure a thermal annealing process which is required in the formation of pervoskite from its precursor. We have solved the problem by fabricate a tandem solar cell using a step-wise process. First, we fabricate the perovskite solar cell on substrate with the thermal annealing process. Next, we stack the polymer solar cell on top of the perovskite cell as a rear cell to make a tandem. The device performance is currently under investigation and expected to be increased as compared with a plain perovskite cell.

ZnO / II-VI core shell nanowire (NWs) array-based solar cells have recently received increasing interest to decrease material consumption while maintaining high power conversion efficiency. The core can be composed of ZnO NWs as electron transporting layer and the shell can comprise CdTe as absorbing and hole transporting layer in order to form type II radial heterostructures [1]. These NW-based heterostructures are expected to favour light trapping as well as efficient charge carrier separation and collection. The combination of ZnO with CdTe is further potentially attractive since the conversion efficiency of 12.3% has been achieved in ZnO/CdTe planar layers [2]. In this work, the low-cost fabrication of ZnO / CdTe core shell NW arrays is shown by combining chemical bath deposition with close space sublimation. Annealing effects in chlorine compound atmosphere are revealed on their physical properties [3]. It is found that recrystallization phenomena are induced by annealing in the CdTe shell composed of nanograins: its crystallinity is improved while grain growth and texture randomization occur [3]. The chlorine doping of the CdTe shell with the formation of chorine A-centers is also shown by photoluminescence measurements. A special emphasis is made on the absorption mechanisms, which are investigated by systematic optical computations of the ideal short-circuit current density and careful optical mode analysis [4]. It is shown that the high absorptance of these NW-based heterostructures is driven by two different regimes originating from the combination of individual NW effects (i.e., optically guided modes) and NW arrangement effects (i.e., diffraction) [4]. The geometrical dimensions of ZnO / CdTe core shell NW based solar cells are further optimized [4]. Eventually, the transport mechanisms and recombinations at the heterojunction interface are studied by electrical characterizations versus temperature.
[1] V. Consonni et al., Appl. Phys. Lett. 98, 111906 (2011).
[2] M.G. Panthani et al., Nano Lett. 14, 670 (2014).
[3] V. Consonni et al., Nanscale Res. Lett. 9, 222 (2014).
[4] J. Michallon et al., Optics Express, in press (2014).

Ferroelectric (FE) perovskites are an important class of materials for potential absorber in next-generation FE solar cells. With spontaneous polarization properties, FE materials facilitate electron-hole separation and drive charge carriers at their opposite ends allowing voltages higher than their bandgap [1]. To this end, Pb-free LiNbO₃ (LN)-type ZnSnO₃, a derivative of the perovskite structure (ABO₃), is lately in focus because of its theoretically predicted high spontaneous polarization, [2] and experimentally reported high remanent polarizations both in epitaxial and nanowire-arrayed thin films. [3,4] Composed of earth abundant non-toxic zinc and tin elements, and being essentially a direct band-gap semiconductor, ZnSnO₃ holds promise to be a high performance solar absorber material. However, the wide band-gap energy (~ 3.5 eV in bulk) of ZnSnO₃ is not ideal for absorbing broad range of the solar spectrum and hence band-gap engineering is a subject of interest in this material. In this work, we discuss the effects of cation doping (Sb, Cu, Ca) on the bandgap tunability in ultra-thin LN-type ZnSnO₃ nanorods. The undoped and doped ZnSnO₃ nanorods with an average size of 8 nm were synthesized by a surfactant and template free low temperature solvothermal process. The bandgap in ZnSnO₃ nanorods (~ 4 eV when undoped) is found to be an extremely strong function of the lattice strain, which is also investigated by surface modification of these nanorods through a core-shell approach choosing ZnSnO₃ as the core and ZnO, SnO₂ and TiO₂ with moderate lattice mismatches as shell materials. We provide a detailed structural and optical analyses to understand the effects of substitution and lattice strain in modifying the energy-band diagrams in this important FE photovoltaic material. Photoconductivity studies were performed on the prototype devices and the results are discussed in terms of understanding the possibility of using ZnSnO₃ as advanced solar cell materials.
[1] I. Grinberg, D. V. West, M. Torres, G. Gou, D. M. Stein, L. Wu, G. Chen, E. M. Gallo, A. R. Akbashev, P. K. Davies, J. E. Spanier, A. M. Rappe, Nature 2013, 503, 509.
[2] Y. Inaguma, M. Yoshida, T. Katsumata, J. Am. Chem. Soc. 2008, 130, 6704.
[3] J. Y. Son, G. Lee, M. -H. Jo, H. Kim, H. M. Jang, Y. -H. Shin, J. Am. Chem. Soc. 2009, 131, 8386.
[4] A. Datta, D. Mukherjee, C. Kons, S. Witanachchi, P. Mukherjee, Small, 2014 (accepted).

Ferroelectric materials are gaining importance for photovoltaic device applications. The inherent spontaneous polarization present in these materials facilitates the required separation of the photo generated carriers. Due to this the photovoltaic effect is not limited to the band gap of the materials and thus leads to the possibility of having efficiencies beyond that of conventional p-n junction solar cells. In this paper we report a considerable photovoltaic (PV) effect in aurivillius phase Bi₅FeTi₃O₁₅ (BFTO) thin films deposited on platinum coated silicon substrates by pulsed laser deposition with SrRuO₃ (SRO) as top electrode. The structural and micro structural properties of these films were analysed by X-ray diffraction and atomic force microscopy techniques. The films showed a photo sensitive ferroelectric behaviour with a notable remnant polarization in the range of 10-15 mu;C/cm². Domain switching and hysteresis loops of BFTO films were investigated via piezoresponce force microscopy (PFM). From the amplitude and phase hysteresis curves obtained from the piezoresponce spectroscopy it was found that these films can be switched below 5 V. The films also exhibited a switchable type photo-response and this parameter was observed to be sensitive to polarization field and the polarization direction. The device shows a large ON OFF photo current ratio with an open circuit voltage of 0.14 V. The photo response at zero bias of this BFTO based heterostructures showed rapid increase to a saturation value of 6 mu;A at zero bias.

We report on the first principles calculation of the electronic, structural and optical properties of BaTaO₂N, using density functional theory (DFT) and Finite Difference Time Domain (FDTD) methods. Band structure calculations were performed to calculate the direct and indirect bandgaps of the material. Density of states and Mulliken charge analysis as well as the electronic contour maps were established to determine the type of bonding and hybridization between the various electronic states. The dielectric constant, reflectivity, absorption, optical conductivity and energy-loss function were also calculated. Moreover, FDTD was used to investigate the optical properties of a larger and more reliable structure for BaTaO₂N powder with a good agreement with the reported experimental parameters. The calculated electronic, structural and optical properties showed the potential of BaTaO₂N for solar energy conversion and optoelectronic applications.

Quantum-dot (QD) photovoltaic cells are promising candidates for low-cost, terawatt-scale solar power, which is critical to meet the energy requirements of the 21st century. Unfortunately, current QD cells suffer from incomplete light absorption and charge-transport issues that limit their overall photo-conversion efficiency. Advanced photon management techniques, such as periodic nanostructuring, could improve in-coupling and light-trapping in the active layer. This would enhance absorption while reducing the thickness requirements of the absorbing material. Additional benefits include the mitigation of bulk charge recombination and reduction of production costs. Herein we investigate a periodic nanostructured CdS/PbS-QD bilayer heterojunction solar cell in substrate configuration. In general, this configuration involves the sequential deposition of a back contact, the active materials, and a transparent conductor (TC) on a supporting substrate. Because light enters the cell through the TC, the transparency of the support is unimportant, and flexible, low-cost metal foils or nontransparent plastics such as polyethylene naphthalate can be used. Substrate configuration also allows nanostructuring to extend to the air interface, thereby maximizing the in-coupling of light. To exploit these benefits, we fabricated our devices by first using template stripping to prepare high-quality, periodic nanostructured gold electrodes. The remaining layers of the solar cell were then deposited on these electrodes via all-solution-processing at temperatures below 100°C. Full three-dimensional topology and electromagnetic simulations were used to optimize the geometry of the structuring. For cells with a chemical-bath-deposited film of CdS on top of a layer of PbS QDs, we measured a photo-conversion efficiency of 4.2%. For comparison, we measured optimized planar cells (i.e. without nanostructuring) with the same volume of CdS and PbS, obtaining a photo-conversion efficiency of 3.7%. Thus, a significant improvement due to the nanostructuring was observed. More generally, our approach to periodic nanostructured QD cells in substrate configuration can be adapted to different materials and is amenable to roll-to-roll processing on low-cost, flexible substrates.

Over the years, semiconductor material options have slowly shifted to smaller bandgaps towards semi-metals and have been incorporated into a variety of new devices with a focus on mid- and far-infrared applications [1,2] . In addition to their small negative bandgaps, semi-metals can also be grown epitaxially on a semiconductor to form a new semiconductor with properties between those of a semi-metal and the original semiconductor. It is because of their versatility and tunability that studying them in more detail becomes of increasing importance. In this study, we focus our attention to an understudied tertiary compound, Ga_1-xTl_xP, with a decreasing bandgap as the Thallium (Tl) content is increased. TlP is a semi-metal with a bandgap less than zero [1]: This implies that the material bandgap can change from 2.26 eV (x=0: GaP, semiconductor) to -0.27 eV (x=1: TlP, semi-metal). Presented in this study is the band structure change and associated properties; such as, the optical bandgap and lattice constant of Ga_1-xTl_xP with varying Tl content. The properties of the compound are simulated using WIEN2k, a commercial software used to calculate the electronic structure of solids using density functional theory (DFT) [3,4]. It is based on the full-potential augmented plane-wave (APW) and local orbitals methods. Given said results, we then model the semi-metal/semiconductor for various applications; including, as a photovoltaic cell in a multijunction and compare it with a multijunction solar cell operating under the same conditions to demonstrate its functional advantage.
[1] M. Van Schilfgaarde, A-B. Chen, S. Krishnamurthy, and A. Sher “InTlPthinsp;—thinsp;a proposed infrared detector material”, Appl. Phys. Lett. 65 2714 (1994).
[2] M. Van Schilfgaarde, A. Sher, and A-B Chen, “InTlSb: An infrared detector material?” Appl. Phys. Lett., 62, 1857-1859 (1993)
[3] N. Saidi-Houat, A. Zaoui, and M. Ferhat, “Structural stability of thallium-V compounds,” J. Phys.: Condens. Matter 19 (2007) 106221
[4] M. Ferhat and A. Zaoui, “Structural and electronic properties of III-V bismuth compounds,” Phys Rev B 73, 115107 (2006)

Atomic Layer Deposition (ALD) is nowadays a well-known technique adopted in crystalline silicon and thin films photovoltaic technologies to fabricate highly dense, uniform and conformal nanoscale structures. Recently, thermal ALD has been successfully applied also to organometal halide perovskite solar cells which have attracted enormous attention during the last 3 years due the exponential growth in device performance. Particularly, conformal TiO₂ blocking layer on mesoscopic TiO₂ photoelectrode and hole-blocking layer over glass/TCO substrates have been recently reported [1,2]. Plasma-assisted ALD (PA-ALD) enables the deposition of higher quality films in the range of temperature compatible with conductive plastic substrates, compared to conventional thermal process [3]. Here, we explore the potential of PA-ALD to deposit ultra-thin, highly compact TiO₂ blocking layers on ITO-polymer substrates for a hybrid organolead halide perovskite solar cell configuration. The layers were prepared in a remote plasma reactor (FlexAL^TM) at 150 °C using an heteroleptic alkylamido precursor Ti(CpMe)(NMe₂)₃ step alternated with an O₂ plasma exposure.
For perovskite solar cells, the presence of a blocking layer (BL) on the TCO surface becomes crucial to reduce the recombination of the electrons in ITO with the holes in the perovskite (CH₃NH₃PbI₂Cl- based). The Tafel plot analysis has shown that current exchange (J₀) at the interface ITO/perovskite is almost 3 order of magnitude higher compared for instance to conventional Dye Sensitized Solar Cells (DSCs) where electrons reduce I₃^- in the liquid electrolyte. In particular for our mesostructured perovskite solar cells on ITO/polymer we observe that the absence of the TiO₂ blocking layer is extremely detrimental for the performance of the device, due to the very low open circuit voltage generated (50mV). The deposition of 11 nm (190 ALD cycles) of TiO₂ leads to a significant decrease of the current exchange (4 order of magnitude compared to the device without the BL), and consequently it allows to fabricate a flexible device (0.12 cm²) with an efficiency of 7.4%. This behaviour differs enormously from the case of DSCs where the addition of the TiO₂ blocking layer only slightly improves the performance of the device (+5%). Finally, we report also the first flexible perovskite module (8 cm²) with an overall performance of 3.1%.
[1] A.K. Chandiran, A. Yella, M.T. Mayer, P. Gao, M.K. Nazeeruddin, M. Grätzel , Adv. Mater. 2014, DOI: 10.1002/adma.201306271
[2] Y.Wu, X. Yang, H. Chen, K. Zhang, C. Qin, J. Liu, W. Peng, A. Islam, E. Bi, F. Ye, M. Yin, P. Zhang, L. Han, Applied Physics Express 7, 052301 (2014).
[3] D. Garcia-Alonso, V. Zardetto, A.J.M. Mackus, F. De Rossi, M.A. Verheijen, T.M. Brown, W.M.M. Kessels, M. Creatore, Adv. En. Mater. 4, 1300831 (2014).

B.V. Korzun¹, V.R. Sobol¹, M. Rusu², R.M. Savizky³, A.A. Fadzeyeva⁴, A.N. Gavrilenko⁵, V.L. Matukhin^5
1Belarusian State Pedagogical University, Sovetskaya 18, Minsk, 220030, Belarus
²Helmholtz Zentrum Berlin für Materialen und Energie, Hahn-Meitner Platz 1, Berlin, 14109, Germany
³The Cooper Union for the Advancement of Science and Art, New York, NY 10003, USA
⁴State Scientific and Production Association “Scientific-Practical Materials Research Centre of the National Academy of Sciences of Belarus,” P. Brovki 19, Minsk, 220072, Belarus
⁵Kazan State Power University, Krasnoselskaya 51, Kazan, 420066, Russia
Copper indium diselenide (CuInSe₂) belongs to the I-III-VI₂ group compounds and is extensively studied and used as an absorbing material in solar cells. To optimize its physical properties, it is necessary to develop the technology of doping of this material. One of the doping elements can be antimony. The goal of the present paper is to study phase relations in the CuSbSe₂ - CuInSe₂ system and determine the limits of solubility.
The initial elements for the preparation of CuInSe₂ and CuSbSe₂ ternary compounds were 99.9998% copper, 99.9997% indium, 99.999% antimony, and 99.9999% selenium. The synthesis of the initial ternary compounds CuInSe₂ and CuSbSe₂ was performed in quartz ampoules by melting from the elements. Nine alloys of the (CuSbSe₂)_1-x (CuInSe₂)_x system with molar part of CuInSe₂ (x) equaling to 0.05, 0.15, 0.25, 0.375, 0.50, 0.625, 0.75, 0.85, and 0.95 were prepared. The required amounts of powder of the corresponding ternary compounds CuInSe₂ and CuSbSe₂ were weighted, mixtured and homogenized. Samples of the alloys were prepared by melting, where the mixtures of ternary compounds were heated up to the 1280 K temperature, which exceeds by 20 K the melting point of the compound with the highest melting point (CuInSe₂).
The phase relations in the CuSbSe₂-CuInSe₂ system were investigated by means of X-ray powder diffraction, optical microscopy, scanning electron microscopy, and differential thermal analysis. Microstructure was studied on the freshly-polished samples without additional etching.
The XRPD and microstructure studies showed that all of the prepared alloys of the CuSbSe₂-CuInSe₂ system were two-phased. The primary crystals with the chalcopyrite structure based on the CuInSe₂ ternary compound are crystallized in regular form.
In addition to the thermal effect of melting, the thermograms of the alloys based on CuInSe₂ have the heat peaks corresponding to the polymorphous transformation from chalcopyrite to zinc blend structure. The T-x phase diagram of the CuSbSe₂-CuInSe₂ system has a peritectics character with the peritectics temperature of 748 K. It was established that the solubility does not exceed 0.05 molar part of the corresponding ternary compound.

High efficiency hybrid halide perovskite solar cells have developed as a technology faster than the understanding of the underlying device physics. Here we model these unique features based on electronic structure methods.
We look at the interaction[1,2] and dynamics[3] of the organic cation, the role of its polarisation in creating molecular ferroelectric domains within the active device[3], and the interaction of these domains with both excitons and polarons. Ab-initio molecular dynamics is used to understand the microscopic motion of the cations, providing inspiration and time constants for a physical model of the interaction, and statistics of motion which can be fitted to quasi-inelastic neutron scattering data[4]. Electronic structure calculations provide parameters (elastic strain, and dipole strength) for an on-lattice Monte Carlo simulation of ferroelectric domains, capable of accessing far larger length scales and longer time scales than ab-initio molecular dynamics.
We find that the built in field of the solar cell at short circuit is capable of modifying the local electric potential structure of the solar cell[3], as the domain boundary between twinned molecular dynamics responds slowly to applied electric field. This we connect to the observed hysteresis in these materials, as the built in field at short circuit generates electrostatic traps within the perovskite.
The inhomogeneous electric potential & other values derived from electronic structure calculations are used as inputs into a Monte Carlo model of polaron transport and recombination, to understand how the small scale structure relates to device operation.
1. F Brivio, KT Butler, A Walsh, M van Schilfgaarde, Physical Review B 89 (15), 155204
2. JM Frost, K Butler, F Brivio, CH Hendon, M van Schilfgaarde, A Walsh, Nano Lett., 2014, 14 (5), pp 2584-2590
3. JM Frost, K Butler, A Walsh, Under Review (2014), arXiv preprint http://arxiv.org/abs/1405.5810
4. A Leguy, JM Frost, A Walsh, P Barnes, in preparation (2014).

Hybrid perovskite materials CH₃NH₃PbI₃ (MAPI) and CH₃NH₃PbI_3-xCl_x (MAPIC) are used as optically active components in high efficiency solution processed solar cells. Despite the tremendous interest focusing on these semiconductors, a range of significantly different possible energy band diagrams have been suggested and the optical constants of the material have not yet been reported.
In this work, we solve these issues with a complementary study ellipsometry modelling and the highest level of ab initio quantum chemical calculations available for crystal structures: relativistic quasi-particle self-consistent GW simulations with no adjustable parameters¹.
Ellipsometry looks at the change of polarization state of a reflected light beam which can be modelled to yield information about the optical properties of a material. An ensemble of critical points of the joint density of states (four in this case) can be fitted to precisely derive the index of refraction and extinction coefficient of both MAPI and MAPIC. The nature of the critical points can also be used infer detailed information about the energy band structure of MAPI and MAPIC.
We show excellent agreement between the optical constants of MAPI calculated from ellipsometry and the ab initio band structure calculations, the clear first experimental validation of ab-initio calculations on this material. The very close match emphasises the superiority of the GW approximation over density-functional based approaches and the importance of accounting for spin-orbit coupling which contributes 1 eV to the band gap.
The combined results of ellipsometry modelling and quantum chemical calculations enable us to highlight the following interesting features which could have implications for device preparation:
(i) The bandgap edge is found to be anisotropic (2D). This means that absorption occurs only in two directions <100> and <010> but not in the third <001> around the first critical point of the joint density of states. If oriented crystals of MAPI can be grown, the active layer used in solar cells can become significantly thinner!
(ii) The valence band maximum and conduction band minimum are non-parabolic. Their relative flatness is due to spin-orbit degeneration and suggests a high effective mass of the charge carriers. This implies that mobilities could be further enhanced by doping the material.
1- F. Brivio, K.T. Butler, A. Walsh, M.v. Schilfgaarde, "Relativistic quasiparticle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers", Condensed Matter and Materials Physics, 2014

Perovskite based inorganic/organic hybrid solar cells have gained much attention due to their superior performance with easy fabrication procedure. Following the progress since 2009, now conversion efficiency has reached up to ~19%. Perovskite is a light absorbing semiconducting material with generalized formula CH₃NH₃MX₃, where M= Sn, Pb and X is a halide atom (I, Cl, Br). It has been shown that the perovskite structure itself is conducting and can also work as electron and hole conductor. Various schemes have been employed to achieve the highest possible efficiency, structural and interface characteristics of perovskite layer remains very crucial to determine the cell performance. Therefore, further development in this field is required to understand the structural properties of perovskite layer in combination with electro-optical characteristics at interface to get the optimal performance. Here, we present our investigations on planer structure solar cell device with different morphologies of perovskite layer. A two-step fabrication method was employed to achieve the pin-hole free and uniform coverage of the perovskite layer over the hole conducting layer. In brief, first PbI₂ layer was spin coated on already prepared substrate (ITO/PEDOT:PSS), followed by the spin coating of CH₃NH₃I (MAI) layer. This has been achieved by varying the annealing temperature and duration to form the perovskite structure. In current-voltage (I-V) measurements, the fill factor (FF) of solar devices prepared at lower annealing temperature of perovskite layer became smaller in comparison of one at high annealing temperature. We analysed the lower FF by using impedance spectroscopy. At high temperature, the two typical semi-circles were observed in Cole-Cole plot. The two semi-circles are due to the PCBM or perovskite layers, and can be explained by ideal RC parallel circuit model. On the other hands, at lower annealing temperature, anomalous arc were observed at high frequency region and is similar to the shape of the transmission line model of meso-porous TiO₂ films in dye-sensitized solar cells. The resistance of anomalous semi-circle was much larger than that at high annealing temperature and is due to lower FF value. These samples were further characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD pattern shows the presence of (110) diffraction peak assigned to the mixed halide perovskite with an orthorhombic crystal structure onset of annealing. SEM micrographs of PbI₂ and PbI₂/MAI layer revealed the growth of CH₃NH₃PbI₃ crystal (size~200 nm) on the PbI₂ surface in the as-prepared sample. Further annealing of PbI₂/MAI samples leads to the dissolution of bigger size CH₃NH₃PbI₃ crystals, forming the uniform and pin-hole free perovskite film. As the results of XRD and SEM, the CH₃NH₃PbI₃ crystals are formed on PbI₂ layer at lower annealing temperature, which induces the anomalous arc at high frequency region in impedance spectroscopy.

Inkjet printing of organometal halide perovskites (e.g., CH₃NH₃PbI₃) is a promising approach for low-cost and scalable manufacturing of future thin-film solar cells. Hybrid organic-inorganic perovskites exhibit efficient carrier transport and low internal losses, leading to high open-circuit voltages and solar power conversion efficiencies. To achieve large-scale deployment, however, emerging thin-film photovoltaic technologies must achieve high efficiencies using high-throughput manufacturing techniques. Inkjet printing offers several advantages over conventional lab-scale solution deposition methods (e.g., spin-coating or dip-coating), such as compatibility with roll-to-roll processing, ease of patterning, substrate temperature control, and increased material utilization.
In this work we demonstrate inkjet printing of thin CH₃NH₃PbX₃ perovskite films with controlled morphology and microstructure on a variety of substrates. We further report the effect of key printing parameters, such as substrate temperature, pulse frequency, and pulse amplitude, on film thickness, morphology, and homogeneity

Organic-inorganic hybrid perovskite materials, such as CH₃NH₃PbI₃, have gained much attention in the past years because of their high efficiency, low cost, and the ease to make these materials solution processable.¹ Their ambipolar semiconducting nature and good charge transporting ability enabled either sensitized mesoporous device structure or planar multi layer device structures. Although a quick increase in the power conversion efficiency has been witnessed in a quite short period, there are still a lot of problems that need to be in-depth studied for its practical applications.
Currently, perovskite solar cells show low reproducibility that large deviation exists in the result of photovoltaic performance. One possible reason is the poly-crystalline structure of the active perovskite layer, which varied significantly in crystal size, crystallinity as well as film morphology from batch to batch. In this presentation, I will demonstrate some universally useful ideas for the crystallization and morphology controlling of perovskite thin films with the purpose of highly reproducible photovoltaic devices. Firstly, we introduce a new strategy of controlling the crystallization of hybrid perovskite through polar-nonpolar solvents interactions and study the mechanism of nonpolar solvents induced crystal growth. By using such strategy, highly uniform and compact perovskite films could be fabricated on mesoporous substrates, resulting in large increase of device reproducibility with respect to those using heat induced perovskite crystallization. Secondly, we demonstrate that using strongly coordinative solvents to retard the crystallization of PbI₂, and fabricate planar-structured perovskite solar cells via sequential deposition.² This strategy overcomes the problem of incomplete conversion and uncontrolled particle sizes of perovskite in absence of mesoporous scaffolds, greatly increasing the film reproducibility. Highly efficient and reproducible planar-structured perovskite solar cells were obtained with high efficiency of 13.5% and a small standard deviation in a batch of cells. The present finding paves the way for reproducible perovskite solar cells, which promote the advancement of cost-effective and practical perovskite solar cells.
[1] T. Miyasaka, et al., Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc., 2009, 131, 6050-6051.
[2] Y. Z. Wu, L. Y. Han, et al., Retarding the crystallization of PbI₂ for highly reproducible planar-structured perovskite solar cells via sequential deposition. Energy Environ. Sci., in press.

Recently, a stir of excitement has been created around lead-based organic-inorganic halide perovskite materials used as solar absorber materials. We have used grazing incidence wide angle x-ray scattering (GIWAXS) to investigate how the fabrication methodology of planar perovskite films determines the crystal structure, lattice parameters, absolute crystallinity and crystallite texture. We have looked at three of the fabrication routes which lead to high efficiency planar films: the two step conversion (first reported by Burschka), the single step equimolar spin casting (first reported by the Graetzel ), and vapor conversion (recently reported by Yang). We find that although these methodologies have been reported to all produce the tetragonal perovskite phase there are in fact significant differences within the lattice parameters between these synthetic routes. Furthermore, we find that the crystallite texture vary considerably depending on the synthetic method with the two-step conversion route resulting in the most highly textured films. We speculate on how the interplay between relative crystallinity, crystallite texture and crystal structure contribute to the differences in observed power conversion efficiencies between these synthetic methods.

Solar cells based on hybrid perovskites have garnered increasing attention with their simple solution processing, high open circuit voltages (V_OC>1.0 eV - 86 % of their Shockley-Queisser limit), and hence the lower manufacturing costs compared to other thin film solar cells such as CZT(S_xSe_1-x)₄ and CIGS. The power conversion efficiencies (PCE) are developed very rapidly particularly methyl ammonium lead halides (MAPbX₃, X = Cl^-, Br^-, I^-), to over 17.9% in less than 5 years. Between the two hybrid perovskite solar cell architectures, the mesoporous scaffold configuration shows the highest performance. The scaffold configuration requires a high temperature (~500 0C) calcination step for the TiO₂ nanoparticles; thus the scaffold configuration inhibits the advantages of low-temperature fabrication and tandem solar cell applications of MAPbX₃ as top cells. The elimination of mesoporous layer by using only a planar heterojunction removes the high-temperature process steps. Moreover, the planar heterojunction configurations are estimated to be ultimately the most efficient architectures according to their 100 % internal quantum efficiency calculations, which further simplify the solar cell architecture. However, non-continuous coverage of the perovskite films, especially with MAPbI₃, is a common problem with planar heterojunctions. The poor coverage is believed to be the reason for the lower PCEs in these architectures due to the contact paths between the electron and hole accepting layers. Therefore, improving the substrate coverage with the MAPbX₃ layer is crucial. Herein, we show full coverage with pure MAPbI₃ in a planar architecture. We deposit perovskite films via hybrid solution process that consists of a thermal evaporation step of lead iodide (PbI₂) and a spin coating step of methyl-ammonium iodide (MAI) in isopropanol that results in MAPbI₃. Complete substrate coverage is obtained with pinholes (~100 nm) between the formed MAPbIshy;₃ crystals. Simple grain growth of MAPbI₃ is challenging due to the high volatility and decomposition of MAI. We control the morphology of the MAPbIshy;₃ crystals via low-temperature (110 0C) post-annealing at atmospheric pressures in an isolated environment. The post-annealing treatment resulted in highly compact (e.g., pinhole-free with ~100 % substrate coverage), micrometer wide (~1 mm), and film-thickness high (~350 nm) grains without decomposing the MAPbI₃ structure. The smaller grain sizes before the post-annealing process is attributed to the incomplete MAPbI₃ formation (e.g., residual PbI₂ phases) as confirmed by X-Ray diffraction. This hybrid solution method resulted in longer carrier lifetimes (~40 ns), twice of what has been reported in the literature, even before annealing; and V_OC&’s above 1.0 V (with E_g<1.60 eV) are obtained. In this talk, the improvement in optoelectronic properties and solar cell performance as a function of microstructure, phase purity, and film thickness will be discussed in detail.

Hybrid organic/inorganic perovskite based solar cells are showing promising performance with an increase in power conversion efficiency (PCE) to over 15% from 4% in last three years. The reasons for this rapid increase include suitable bandgaps, long exciton diffusion lengths, and high absorption coefficients of the perovskite materials. To exploit these excellent electrical and optical properties, it is extremely important to have good morphology and higher absorption. Here, we use a nano-structured porous glass substrate with high roughness to increase the light absorption in the solar cell. The transparent conductive oxide (TCO) (e. g. indium tin oxide (ITO), fluorine doped tin oxide (FTO), etc.) layer is integrated on this glass with enhanced haze, yet maintaining high transparency and low sheet resistance. When light enters the TCO, the surface roughness increases light scattering and hence the effective light travelling length through the cell, which results in higher absorption and short circuit current density. We also apply this nano-structured glass to a tin-based perovskite, which is lead-free and a preferred choice for the future perovskite based solar cell technology but has a low absorption coefficient, intrinsic to this type of perovskite absorber. By using the nano-structured substrate, we can increase the absorption, and hence the device efficiency towards the commercial threshold, i.e. 10%.
This research was conducted at the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Flexible and lightweight thin film solar cells have attracted considerable attention for their potential applications such as portable electronic chargers, electronic textiles, and large-scale industrial roofing on account of their convenience of integration and versatile functionality. To realize flexible thin film solar cells, many different kinds of photovoltaic technologies have been employed. However, achieving high power conversion efficiency (PCE) and low-temperature process are the most important issues in this area. The most recently developed state-of-the-art organic-inorganic perovskite (CH3NH3PbX3, X=I, Br, Cl) based solid state solar cells are highly promising candidates for high efficient flexible solar devices due to their low temperature process, as well as a high PCE of above 15 %, which was achieved within only a few years from the first report in 2009.
Recently, a few groups have attempted to fabricate flexible perovskite solar cells with an all-low-temperature (< 130 ^oC) process and have shown the possibility of higher efficiency than that of organic flexible solar cells. So far, the best PCE of the flexible perovskite solar cells has been 10.2 %, obtained by using a 300 nm-thick CH₃NH₃PbI₃ layer; however, this is still inferior to the rigid-type perovskite solar cells due to lower short-circuit current density (J_sc). The main reasons for the relatively poorer J_sc are the inferior charge collection layer in the flexible substrate. Also, there is insufficient understanding of mechanical properties of organic-inorganic perovskite light absorber, such as the matching of Poisson&’s ratio and elastic modulus between perovskite layers and their underlying substrate or substrate/charge collection layer, for flexible devices. Therefore, it is highly desirable to invest the mechanical properties of perovskite layer and conceive a appropriate low-temperature-processed charge collection layer for flexible perovskite solar cells.
Here in, we present mechanical properties of organic-inorganic perovskite (CH₃NH₃PbI_3-xCl_x) and realized high efficient and bending durable flexible thin film solar cells with low-temperature-processed TiO_x charge collection layer. Moreover, we investigate the systematical bending stability of the devices, with three values of bending radii, 400 mm (R₄₀₀), 10 mm (R₁₀), and 4 mm (R₄), respectively. Our devices performance is withstood up to 1000 cycles of the bending test for R₄₀₀ and R₁₀. In the case of R₄, the 90 % of initial PCE remained until 25 cycles, but after 1000 cycles, the PCE was significantly deteriorated to 50 % of the initial efficiency. This study demonstrates that the organic-inorganic perovskite materials themselves possess excellent mechanical properties, which can contribute to the realization of foldable solar cells, well beyond the capabilities of current flexible thin film solar cells.

Heterojunction solar cells based on organometal trihalide perovskite absorbers have emerged as a promising low cost alternative to inorganic bulk and thin films solar cells. The efficiencies exceeding 15% have been demonstrated in these cells within a short period. Nevertheless, various studies conducted on these materials indicated existence of enough room for improving the cell parameters further and that the technology could compete with single crystal silicon technology in terms of performance. With this in mind we investigated effect of post-fabrication treatments on these cells and noticed that the cell performance could be improved dramatically by treating the cells in different ambient conditions. The open circuit voltages close to 1.1 V were obtained in our high efficiency TiO₂/CH₃NH₃PbX₃ (X= halogen) hybrid heterojunction cells. We will present the details of the post-fabrication treatments, their influence on cell parameters as well as the impedance spectroscopic analysis of the observed correlation.

A low band gap poly(diketopyrrolopyrrole-terthiophene) PDPP3T, was studied and is shown to act as hole transport material in organic-inorganic perovskite based solar cells. CH3NH3PbI3 was synthesized using already reported literature. SEM and XRD was done 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 here showed that PDPP3T acts as efficient hole transport material in conjuction with perovskite layer giving initial device performance with decent Jsc of 15.28 mA/cm2, Voc of 0.67 V and a fill factor of 46%, resulting in an overall efficiency of 4.71%. Transient measurement shows a lifetime and charge transport time in the order of microseconds. Further optimization of the device in terms of layer thickness and perovskite morphology will lead to efficient device performance.

Hybrid perovskites (HPs) are rapidly developing into high performing photovoltaic materials. Solution processed solar cells with 17.9% power conversion efficiency have been certified.[1] Furthermore, HP devices have achieved open circuit-voltages of 1.07 V,[2] which is 86.1% of their Shockley-Queisser limit based on a 1.55 eV band gap. Only GaAs outperforms HPs in this respect. The HP halide composition has been observed by others to effect band gap, 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. Coupling composition spread libraries with local measurements is a powerful technique[3] to rapidly explore the vast composition space of HPs due to their alloying and substituting possibilities. We develop inks, spray coating methods, and optoelectronic measurement techniques to create and analyze HP compositional gradients. In this study, we have created composition spread libraries in the system CH₃NH₃Pb(Cl,Br,I)₃. High resolution profiles of absolute intensity photoluminescence and UV-Vis spectroscopy are measured along the compositional gradient. Both the PL peak position and the quasi-Fermi level splitting plateau after the HP band gap exceeds 1.7 eV. We hypothesize this limit to be due to the precipitation of a bromide rich phase. Below this critical bromide content, HP films reach quasi-Fermi level splitting values over 90% of their theoretical maximum. With these initial results, the stage is set to rapidly discover other superior HP ink formulations. Expanding our compositional spread library to the remainder of the HP halide composition space is a trivial yet important extension to this data set. Future ink formulations that incorporate other metal and organic components are also proposed.
[1] NREL Research Cell Efficiency Records. http://www.nrel.gov/ncpv/. (accessed June 6).
[2] Liu, M. Z.; Johnston, M. B.; Snaith, H. J., Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 2013, 501, 395-397.
[3] Collord, A.; Hugh, H. In Mapping the Composition Dependence of Cu₂ZnSn(S,Se)₄ Absorber Quality using Composition-Spread Libraries, Photoluminescence, and Raman, 39th IEEE Photovoltaic Specialists Conference, Tampa, FL, 2013; pp 0368 - 0370.

Organic-inorganic perovskites containing organic amines and metal halides have recently attracted much attention due to their unique electrical and optical properties, along with the high conversion efficiency of the perovskite solar cells Organic-inorganic perovskites have wide range of structural variation by changing organic amine species and metal species. We have already reported many kinds novel perovskites containing various amines such as cyclic amines and aminopolydiacetylene.^1,2 The fabrication of high quality thin films of perovskites is very important from the viewpoint of optical applications. Spin-coating, vacuum evaporation, and Langmuir-Blodgett (LB) techniques are employed to prepare well-ordered thin films of these organic-inorganic perovskites. However, it is difficult for the spin-coating method to control uniformity and thickness over large areas. The fabrication of various perovskites using different organic molecules is also limited because of the poor solubility of lead halides. In addition, it is difficult for the vacuum evaporation and LB techniques to find empirical conditions of the equipment and processes.
In this work, we tried to prepare the nanostructured films of organic-inorganic perovskites using a self-assembly method and self-intercalation methods. The self-assembly method is based on alternate dipping in solutions of alkyldication halides and lead halides with washing steps to remove excess reactants.³ The self-intercalation method is two-step procedures involving conventional spin-coating and intercalation of lead halide into organic ammonium framework films.⁴ The benefits of these methods are that it is possible to control the thickness in nano order and to construct heterostructures using different organic salts and metal halides. We have already succeeded in fabrication two-dimensional perovskites (H₃NRNH₃)PbX₄ by these methods. Here, we tried to fabricate three-dimensional perovskites by these methods and investigated the effect of crystal size or purity of films on the photovoltaic properties.
Reference
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Y. Takeoka, K. Asai, M. Rikukawa, and K. Sanui, Chem. Commun., 24, 2592-2593 (2001).
T. Matsui, A. Yamaguchi, Y. Takeoka, M. Rikukawa, and K. Sanui, Chem. Commun., 10, 1094-1095 (2002).
Y. Takeoka, M. Fukasawa, T. Matsui, K. Kikuchi, M. Rikukawa, and K. Sanui, Chem. Commun., 3, 378-380 (2005).
Acknowledgrments
This work is supported by The Science Research Promotion Fund, Japan.

Organic-inorganic hybrid solar cells based on methylammonium-lead-halide perovskite absorbers are attracting increasing interest due to high efficiencies coupled with low cost and ease of fabrication. In this work, a two-step deposition process is utilised to create thick films and their optical properties were assessed through variable angle ellipsometery. Precursor solutions containing various lead iodide and lead chloride ratios were spin-coated, followed by a vapour-phase reaction with methylammonium iodide to form the perovskite. Ellipsometry measurements were then used to create a multi-oscillator optical model for the material and dispersion relations extracted.
Absorption profiles gave an absorption coefficient above 10⁵ cm^-1 for much of the visible spectrum and a bandgap of ~1.5 eV extracted from an (αhnu;)^½ vs. hnu; Tauc plot. The n and k values extracted from ellipsometery data are used in a multi-layer model, employing a transfer matrix method, to predict the maximum J_sc possible for several systems. A planar structure of perovskite on a compact TiO₂ blocking layer as a function of perovskite thickness is shown to achieve 95% of the maximum attainable J_sc (22.57 mA cm^-2) for a thickness of 575 nm.

In the past two years, organometal halide perovskites such as CH₃NH₃PbX₃(X = Cl, Br, I) have shown great potential as a photovoltaic material that achieves over 15% solar to electrical power conversion efficiency (PCE) in nanostructured devices or in planar heterojunction structures. The generation of high efficiency perovskite solar cells, by vapour deposition, is likely to greatly shorten the timeline to large-scale manufacturing of the first generation of perovskite solar cells. After pushing the PCE of vapour-deposited perovskite solar cells to over 15%, we further performed a series of fundamental studies to compare the photophysics and charge mechanism of perovskite film produced between by solution-process and vapour deposition. Our results suggest that the charge mechanism is heavily influenced by the fabrication process, which alters the crystallization and film morphology. Next, we sought to further probe the superiority of vapour deposition as a technique to develop perovskite films by establishing control over the crystallization and morphology of the perovskite film by vapour deposition and studying the outcome. In the previous pioneering work, perovskite solar cells synthesized by dual-source evaporation have shown optimal efficiency and uniformity in film formation. However, the uncontrolled precipitation from the interactions between the organic and inorganic sources and the non-linear relationship between device performance and evaporation parameters (e.g., deposition rate, deposition time) has led to inconsistencies in the use of the dual-source evaporation method. An alternative sequential two-step vapour deposition method results in more predictable perovskite film morphology, where device performance is more closely correlated to fabrication parameters. This outcome confers greater reproducibility to device performance and more importantly, establishes the principle that the properties of the perovskite film are sensitive to changes in the fabrication process, which is closely correlated to the resulting film thickness and morphology.

Perovskite materials are the hottest issues in the filed of recent photovoltaic researches. They already broken through the best efficiency of photochemical solar cells and higher performance is still expected. Its surprise appearance attracted considerable attention but left insufficient studies in parallel. This work focused on optical properties of perovskite (lead methylammonium tri-iodide: CH₃NH₃PbI₃) films. The adsorption of perovskite materials is very fast as some minutes. Therefore, the concentration of lead iodide (PbI₂) and methylammonium iodide (CH₃NH₃I) is one of importanta parameters in perovskite absorption and overall performance.
The concentration of PbI₂ and CH₃NH₃I was varied from 10 to 200 mM. The solutions were spin-coated on the sintered TiO₂/glass substrate and perovskite/TiO₂/glass electrodes were dried. Completed films were optically analyzed from 250 to 1100 nm by an ultraviolet-visible (UV-vis) spectrophotometer.
Under fixed concentration of PbI₂, the absorption of perovskite films was increased with the concentration of CH₃NH₃I. Especially, the color of perovskite film was changed from yellow to brown with CH₃NH₃I over 100mM regardless of PbI₂ concentration. That is, high concentration of CH₃NH₃I was necessary for better absorption. Under fixed concentration of CH₃NH₃I, the optical properties of perovskite films were also analyzed according to the concentration of PbI₂. Contrary to above case, all films based on 20mM CH₃NH₃I were yellow and the color was not changed. They were just deepened with PbI₂ concentration. In other words, the dependence on PbI₂ concentration was not observed. This tendency was maintained under high concentration of CH₃NH₃I (200 mM). The color of perovskite films was changed to brown but it did not depend on the concentration of PbI₂. They were also deepened with PbI₂ concentration. That is, under fixed concentration of CH₃NH₃I, the absorption ranges of perovskite films were not changed but the absolute values were increased with PbI₂ concentration.
We conducted optical analysis with detailed concentration of PbI₂ and CH₃NH₃I and they were applied to solar cells. Further results and analysis will be presented on conference site.

All-solid state solar cells consisting of Pb halide perovskite have recently attracted interest because of the high efficiency reaching 12-19%. Light absorption spectrum edge for the Pb halide solar cells is limited to 800 nm. It has been reported that Sn halide perovskites have the light absorption up to 1200nm. However, CH₃NH₃SnI₃ is not stable for handling in air. We found that the Sn halide perovskite became stable in air when Pb halide perovskite was added. In this report, CH₃NH₃Sn_xPb_(1-x)I₃ is abbreviated as MASnPb(x/(1-x)).The spectrum edge of MASnPb(x/(1-x)) shifted from 800 nm to 1200 nm when x was changed from 0 to 1. 2theta;in XRD changed from 14.45 to 15.56 gradually when x increased from 0 to 1.The relationship between x and lattice constant become linear and obeyed Vegard's law, suggesting that these cystals are mixed crystal. This reflects that Pb ion (ionic radius: 1.3-1.5 Å) is replaced partially with Sn ion (ionic radius: 0.93 Å). XPS peaks for Sn 3d shifted from 492.33 eV to 492.91 eV (higher binding energy) when x was changed from 0.3 to 1.0, suggesting that Sn⁴⁺ content may increase with higher x value. Dark current of the cocktail perovskite solar cells with higher x (1,0.9, and 0.7) showed ohmic properties and did not show any diode properties. This is consistent to the speculation that Sn⁴⁺ content of cocktail perovskite with high x value is higher and conductivity increased with an increase in the Sn⁴⁺ content. It has been reported that Sn⁴⁺ is associated with the conductivity. Diode performance was improved when x is lower than X:0.5. IPCE of MASnPb(0.5/0.5) covers from 300 nm to 1060 nm.The MASnPb(0.5/0.5) with 4.2 % efficiency was compared with MASnPb(0/1) with 14.2% efficiency. Series resistances (Rs) of both solar cells were almost the same, however, large difference was observed in shunt resistance (Rsh). This suggests that charge recombination frequently occur at some interfaces, which have to be suppressed for further improvements. HOMO of P3HT is at -4.67 from vacuum level which is much lower than that of SPIRO (-5.22). The difference between TiO₂ conduction band (-4.0) and P3HT HOMO (-4.67) is 0.67 V which corresponds to the expected maximum Voc for MASnPb(0.5/0.5). The difference between the expected 0.67 V and the experimentally obtained Voc (0.42 V) is 0.25 V. The Voc loss is not large, compared with other solar cells. For improving the Voc further, to employ hole transporting materials with deep HOMO and to suppress charge recombination are needed. In order to supress charge recombination between electrons in porous titania and holes in perovskite, titania surface was passivated with metal oxide thin films or three amino acid HI salts (alanine HI salt (AHI salt), β-glycine HI salt (GHI salt), γ-amino butyric acid (GABAHI salt)). GABA passivation and Y2O3 thin layer passivation gave the best results. These results were well explained, using transientn spectra, electron life time, and trap density changes.

The advantage of thin film solar cells based on new materials such as polymers has just begun to be explored in industries for applications such as BIPV (Building Integrated Photovoltaic). The light weight and flexibility of the thin film solar cells has attracted many players in industries so far, and the further improvement in power conversion efficiency is inevitable for continued development of such new solar cell devices. In fact, perovskite based solar cells have been developed very recently and its power conversion efficiency reached almost 20% and further improvement is well expected. The rapid growth of the efficiency as well as the solution processability have made the perovskite solar cells as one of the most promising materials for the new PV applications such as the BIPV where light weight and flexibility are strongly required, although the high-temperature treatment often required for high quality TiO₂ compact layer for those with high efficiency perovskite-based devices hampers such applications. It is therefore critical to develop new materials and/or device configurations for high performance low-temperature solution-processed perovskite solar cells. Here, we describe the planar structure perovskite solar cells processed below 140 #730;C on ITO coated substrates, using the device structure of ITO/PEDOT:PSS/Perovskite/C₆₀ derivative/electron selective layer/metal electrodes. The C₆₀ derivative includes PCBM and other triple-bond attached fullerene derivatives, and very thin BCP or PFN layers were spin-coated as electron selective layer before evaporating the metals such as Al, Ca/Al, or Ca/Ag as cathode electrodes. In our hands, we found that the devices without the electron selective layer and calcium metals often gave S-shaped I-V curves and low fill factors, resulted in low power conversion efficiencies. Insertion of thin calcium layer between the fullerene derivatives and the cathode electrode was often resulted in high fill factors over 0.75 and also higher power conversion efficiencies. In fact, our preliminary study using PCBM as electron transport layer and Ca/Ag as cathode metal, with the configuration of ITO/PEDOT:PSS/CH₃NH₃PbI₃/PCBM/Ca/Ag, gave power conversion efficiencies over 10% with FF= 0.75, Jsc= 15.4 mA/cm², and Voc= 0.9 V, and further improvement is expected after the optimization of the electron selective layers and fullerene derivatives. The details on the choice of fullerene derivatives, electron selective materials, and cathode metals will be discussed with the emphasis on the device performances as well as the electro-optical characteristics at interfaces derived from the impedance spectroscopy measurements.

In the last two years perovskite based solar cells have been characterized by a fast development, achieving power conversion efficiencies up to 16.1%. Impressively, most of the latest advancements have been the result of an improved design of the device architecture. On the other hand, our poor understanding of the relationship between structure and optoelectronic properties of these hybrid semiconductors is still a limiting factor for further improvement in devices efficiency and to frame them in a defined technology. In this work we report vibrational, optical and structural investigations of both methylammonium lead tri-iodide (CH3NH3PbI₃), and Cl-doped CH3NH3PbI₃ perovskite polycrystalline films that demonstrate how the crystallization procedure, i.e. nature of the substrate and of the precursors, can intrinsically influence the hybrid perovskite thin film properties. For CH3NH3PbI₃ crystals, we identify a rearrangement of the organic cation in the inorganic cage - random orientation versus head to tail orientation - moving from smaller crystals grown in a mesoporous scaffold to larger crystals grown on a flat substrate. This induces a different degree of strain in the Pb-I bond and a different degree of screening in the crystal which eventually affects the semiconductor optical band-gap and the nature of the electronic transition at the onset of the absorption spectrum. We demonstrate that when the PbI₂ precursor, generally used for the synthesis of CH3NH3PbI₃, is replaced by PbCl₂, the Cl^- ions play an important role in driving the formation of crystals with a preferential order, from a molecular (organic-inorganic moieties interplay) to a mesoscopic level (larger crystals with anisotropic shape), independently of the growing substrate Finally, we show that, when the crystallization is carried out on a flat substrate, most Cl 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 interface.

To understand the thickness effects of ZnO film which fabricated by ALD for triiodide perovskite absorber (TPA) based photovoltaics, comprehensive studies including absorption spectrum, photoluminescence, nanosecond time-resolved photoluminescence (NTRPL), and photo-induced absorption (PIA) of TPA/ZnO/FTO/glass were carried out to explore the Urbach energy, exciton binding energy, and exciton dissociation in TPA film. The results show that the thickness of ZnO film significantly influences the photovoltaic performances in terms of open-circuit voltage (V_oc), fill factor (FF), and short-circuit current density (J_sc). In the case of the thicker ZnO film, the photovoltaics have the better FF and V_oc, which is the result of the smaller electron recombination. It means that the thicker ZnO film can block the electron transition from the Fermi level of ITO to the valance band of TPA. On the other hand, the thicker ZnO film results in the higher J_sc due to the better exciton dissociation at the interface between TPA and ZnO, which indicates that the electron mobility of the thicker ZnO is higher. Consequently, the photovoltaic performances are expected to be improved by using a transparent cathode electrode with high conductivity and electron mobility.

The superhydrophobic self-assembled monolayer (SAM) is introduced to increase the power conversion efficiency (PCE) on the surface of the TiO₂ photoanode in dye-sensitized solar cells (DSCs). 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FSAM) and triethoxyoctylsilane (HSAM) were deposited without loss of dyes via the sequential adsorption after dye (MK-2 organic dye) dipping process. The absorption spectra revealed that the aggregation peak of MK-2 dye at 370 nm decreased upon deposition of SAM, whereas the maximum peak at 480 nm increased in the order of reference < HSAM < FSAM. According to the chemical capacitance results collected by cyclic voltammetry, the SAM materials seem not to shift the TiO₂ conduction band edge due to the small dipole moment of both FSAM and HSAM. Meanwhile, the impedance results imply that the SAM materials induced the reduction of recombination reaction rate in the order of FSAM > HSAM > reference attributable to their effects for TiO₂ surface passivation. As a result, the FSAM-modified superhydrophobic photoanode exhibited a high PCE of over 9.5%, compared to 8.5 % for the reference without the SAM deposition, evaluated under 1 sun illumination, based on MK-2 and iodide redox couples in DSCs.

In recent years there has been increased attention to perovskite solar cells due to their attractive features such as high energy conversion efficiency and low energy production cost. Due to its ambipolar and high light absorbing properties, perovskite can be used as efficient light absorber, e^- or h⁺ transport layer. During the past several years, the energy conversion efficiencies of the perovskite solar cells have been dramatically increased. The enhancement in the perovskite solar cell performances is mostly attributed to the development of efficient perovskite materials and structural configuration. However, many aspects of the photovoltaic properties are needed to investigate for further improvement of the solar cell efficiency. In this work, we have studied the effects of photoelectrode and hole transport material (HTM) additives (e.g. t-BP and Li salt) on the photovoltaic characteristics. The perovskite solar cells were prepared by using conventional organometal halides (e.g. CH₃NH₃PbI₃) and HTMs (e.g. Spiro-MeOTAD) as the base materials. A uniform and pinhole-free hole-blocking layer is necessary for high-performance perovskite-based solar cells. Several metal oxides were investigated to form thin blocking layers. We utilized the spin coating or screen printing method to coat the thin metal oxide layer on transparent conductive oxide substrate. Highly ordered nanoporous TiO₂ layer was also prepared via a surfactant-templating method for efficient charge transport. The effects of various additives on the photovoltaic properties were systematically studied in terms of the electron-hole transports. The charge transport characteristics in the photoelectrode were investigated through IMVS/IMPS and SLIM-PCV measurements.
Acknowledgements : This work was supported in part by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korea government (MSIP) (No. 2012013811) and by the New & Renewable Energy R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry & Energy (MOITE) (No. 20123010010070).

Recently, the perovskite solar cells draw lots of attentions because of low cost, solution-processable, flexibility, broad absorption range and promising high efficiency which is expected to approach the silicon solar cell in near future. The perovskite solar cell was first developed from the dye-sensitized solar cell (DSSC) and the power conversion efficiency (PCE) can achieve 15%. In the DSSC system, the perovskite active layer was deposited on the TiO_x mesoporous and compact layer which helps to transport the electrons from active layer to electrode. Due to the TiO_x mesoporous and compact layer both have to be sintered over 500^oC, it is not eco-friendly and energy-efficient. As a result, we fabricate the perovskite solar cell which has a structure of FTO/TiO₂ nanoparticles/CH₃NH₃PbI_3-xCl_x/spiro-OMeTAD/Au instead of conventional TiO_x mesoporous and compact layer. We use the TiO₂ nanoparticles as the electron transporting material which doesn&’t need high temperature sintering and a PCE of 12% can be achieved. By controlling the reaction time and temperature, we synthesize a series of TiO₂ nanoparticles with different particle sizes. We study the size effect of TiO₂ nanoparticles on the performance of perovskite solar cells. The size of TiO₂ nanoparticles affects the morphology of perovskite active layer. By tuning the morphology this way, the charge transport and PCE of the solar cell can be improved.

Organometal halide perovskites have attracted interest as the next generation photovoltaic materials. The mainly used perovskite photovoltaic materials are methylammonium lead halide, expressed as CH₃NH₃MX₃, where M = Pb, Sn and X = I, Cl, Br or combination of these elements. Recently, a planar heterojunction perovskite solar cell, without mesoporous TiO₂ scaffold, was reported with excellent performances due to its long charge diffusion lengths (100-1000 nm). In this planar type structure, controlling the crystallinity and coverage of perovskite film play an important role to enhance the device performances. The thermal evaporated CH₃NH₃PbI_3-XCl_X film with perfect surface coverage shows high energy conversion efficiency over 15%. However, although solution process is more cost-efficient method, spin coated perovksite film exhibits relatively lower performance than evaporation system because of pin-hole formation and deficient coverage which can be a shunting path and has the reduced absorbing area. To resolve this problem, researchers tried to optimize solvents, annealing temperature, and solution concentration, or using some additives in perovskite solution for enhancing the crystallization. It is recognized that the crystallization rate is a key factor to control the morphology and coverage of perovksite film.
In this study, we apply mixed solvent system to enhance crystallization and coverage of spin coated perovskite film. Through the investigation of various solvents, it was found that the crystallization rate and morphology are greatly influenced by the physical and chemical properties of solvent such as evaporation speed and interaction with precursors, etc. Certain solvent aids the grain growth effectively by assisting the transport of solute during crystallization and other kinds of solvents promote the uniform coverage by interacting with Pb ions showing the uniform distributed film. In addition, the solvent evaporation rate is important to induce the uniform crystallization throughout the film. By selecting the solvents and their mixing ratio, the well crystallized perovskite film with good coverage was obtained. The resulting cells showed ~10% higher efficiencies as compared to the cell prepared by dimethylformamide single solvent. This mixed solvent system is a quite simple and effective method to fabricate perovskite film via solution process without vacuum condition or two-step solution deposition.

Perovskite (organometal halide) solar cells have attracted much attention recently as the low cost and high performance solar cells. Several structures to fabricate perovskite solar cells have been studied to enhance performances; planar heterojunction, soaking into mesoporous scaffolds or 1-D TiO₂ or ZnO nanorod array. Extensive investigations on photovoltaic properties and carrier transport properties indicated that the structure and shape of perovskite film significantly affect photoelectrical characteristics. Interestingly, the perovskite cell based on TiO₂/ZnO nanorod arrays shows higher electron mobility than the mesoporous structured device due to direct pathway for electron transport. However, the reason for enhanced performance was not fully understood whether either the shape of perovskite or the structure of TiO₂/ZnO nanorod scaffold play a dominant role. For a clear understanding of the shape/structure of perovskite layer, the inactive scaffold that does not influence the charge transport is necessary like aluminum oxide.
We fabricated for the first time vertical one dimensional (1-D) nanostructured perovskite solar cell using anodized aluminum oxide (AAO). Anodized aluminum oxide is one of the popular nanostructured materials which have one dimensional pore with controlled pore diameters. AAO template was successfully fabricated on the compact TiO₂ layer by anodizing and widening of the deposited Al film. Using AAO as a scaffold for perovskite, we obtained 1-D shaped perovskite absorber inside of AAO template whose pore diameter is 80 nm and thickness is 400-500 nm. Over 10% efficiency was obtained by coating methylammonium lead mixed halide perovskite (CH₃NH₃PbI_3-xCl_x) on the FTO/TiO₂/AAO substrate. I-V measurement, photoluminescence, impedance, and time-limited current collection are performed to determine vertically arrayed 1-D perovskite solar cells shaped in comparison with planar heterojunction and mesoporous scaffold solar cells. Our findings lead to reveal the influence of the shape of perovskite layer on photoelectrical properties.

Organic/inorganic perovskite solar cells (PeSCs) have been considered one of the most competitive next generation power source due to their light weight, low cost of manufacture, and low-temperature solution processing. To date, light-to-electric conversion efficiencies have rapidly increased to over 10%, and further improvements are expected. However, the poor device reproducibility of PeSCs ascribed to their inhomogeneous and partially covered film morphology has hindered their practical application. Here, we demonstrate high-performance PeSCs with superior reproducibility by introducing a morphology controllable solvent with a high boiling point and a low vapor pressure into N,N-dimethylformamide (DMF). As a result, highly homogeneous film morphology, similar to that achieved by vacuum-deposition methods, as well as a high PCE of 10% and an extremely small performance deviation within 0.2% were achieved. This study represents a method for realizing efficient and reproducible PeSCs through morphology control, taking a major step forward in the low-cost and rapid production of PeSCs by solving one of the biggest problems of perovskite photovoltaic technology through a facile method.

Perovskite solar cells are very attractive for multijunction applications because the bandgap of perovskite semiconductors can be easily tuned in the range of 1.6 to 2.3 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.

Methylammonium lead halide perovskites have been intensively studied as promising photo absorption and carrier transporter materials in solar cells due to excellent semiconducting properties, a broad range of light absorption, and a high extinction coefficient. Although PCE of perovskite solar cells has been increased remarkably, a few reports have considered solution-processed planar heterojunction (SP-PHJ) structure solar cells without using a mesoporous or compact semiconducting metal oxide (e.g, TiO₂) layer processed by high-temperature sintering, and the SP-PJH solar cells to date have shown lower PCE than those with a mesoporous or compact TiO₂ layer. Commercialization of perovskite solar cells requires easy, scalable and low-temperature methods to fabricate them efficiently by a solution process without sintering. PEDOT:PSS can be considered as a good HEL because of simple solution processibility, planarization effect on the underlying ITO layer, and a low-temperature annealing process. . However the work function WF of PEDOT:PSS (4.9 to 5.2 eV depending on the ratio of PEDOT to PSS) is lower than the ionization potential IP of perovskite (e.g. 5.4 eV for methylammonium lead iodide (CH₃NH₃PbI₃)), so the potential energy loss at PEDOT:PSS/Perovskite interface decreased built-in potential in perovskite solar cells.
The WF in HELs can be tuned by using molecular surface engineering to control the surface composition in HEL films, which depends on the surface-enriched molecules and their concentration relative to the conducting polymer. Thus we used a self-organized HEL (SOHEL) which is composed of a conducting polymer composition (e.g., PEDOT:PSS) and a perfluorinated ionomer (PFI), i.e., tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid copolymer. Here, we present solution-processed methylammonium lead iodide CH₃NH₃PbI₃-based perovskite solar cells with a high-WF SOHEL for good energy level alignment with the IP level of CH₃NH₃PbI₃. The SOHEL at the hole extraction interface can increase the built-in potential, the photocurrent, and thus the PCE of perovskite solar cells. We obtained high PCE of 11.7% in SP-PHJ perovskite solar cells under 100-mW/cm² illumination. We also demonstrated flexible perovskite solar cells on a poly(ethylene terephthalate) (PET) substrate; they had PCE as high as 8.0%.

An effective approach to further improve the performance beyond achievable efficiencies of state-of-the-art single junction solar cells is to combine them to form a tandem structure, in which the solar spectrum is optimally utilized to the largest extent and the thermal loss of high photon energy is minimized. Although the Cu(In, Ga)Se₂ (CIGS) solar cells have been widely studied as bottom cell in tandem structure combined with dye-sensitized solar cells (DSSC) [1] and organic solar cells [2], the overall tandem cell efficiency was still limited to about 15% [3] due to the lower efficiency and parasitic absorption losses in the top cell. Moreover, the liquid electrolyte needed in the DSSC top cell led to process incompatibilities. The emerging photovoltaic technology based on perovskite absorber developed rapidly during the past 5 years with certified efficiency as high as 17.9% [4], offering the possibility to overcome these limitations. In this work we systematically investigate the design, fabrication, and performance of the perovskite/CIGS tandem solar cells from both simulation and experimental perspectives. The perovskite top cell and CIGS bottom cell are individually grown and afterwards mechanically stacked. Various kinds of transparent contacts are explored and optimized to realize high transmittance through the top cell and front contact of CIGS. In addition, the limiting factors for improving the performance of the tandem structure are thoroughly discussed and a future outlook of this tandem is proposed. This work provides preliminary results on the perovskite/CIGS-based tandem structure and sheds light on further optimization of this architecture.
[1] S. Seyrling et al., Thin Solid Film 517, 2411 (2009)
[2] M. Reinhard et al., J. Appl. Lett. 103, 143904(2013)
[3] P. Liska et al., Appl. Phys. Lett. 88, 203103 (2006)
[4] NREL efficiency chart, May 2014.

Metal chalcogenides, such as Sb₂S(e)₃, SnS, CdS, and PbS, have been successfully applied as light sensitizers to the inorganic-organic hybrid solar cells. Among them, Sb chalcogenides (Sb-Chs), Sb₂S(e)₃, are of particular interest because of their excellent optical properties including easily tunable band-gaps by adjustable composition, high molar extinction coefficients, and large intrinsic dipole moments. Very recently, we achieved the high efficiency of ~ 7.5 % in the Sb₂S₃-sensitized devices via a post-surface-treatment with thioacetamide [1]. However, the photovoltaic performance of the solar cells sensitized with Sb-Chs is still restricted by strong charge recombination (low open circuit voltage V_OC) and insufficient light-harvesting (low short circuit current density J_SC). Here, we introduce one approach for enhancing both V_OC and J_SC by applying Sb₂(S/Se)₃ light sensitizers in the inorganic-organic heterojunction solar cells. The Sb₂(S/Se)₃ light sensitizers were synthesized either by mixing two single source precursors of Sb₂S₃/Sb₂Se₃ or sequential deposition of Sb₂Se₃/Sb₂S₃. We observed that the V_OC was gradually decreased with increase of J_SC as increasing ratio of Se/S in Sb₂(S/Se)₃. The best cell exhibited high efficiency of ~ 8.1 % with high J_SC of 25.8 mA cm^-2 and V_OC of 530.6 mV. Our efforts towards further improvement in efficiency will pave a new way for highly efficient inorganic-organic hybrid solar cells.
[1] Y. C. Choi, D. U. Lee, J. H. Noh, E. K. Kim, and S. I. Seok, Highly Improved Sb₂S₃ Sensitized-Inorganic-Organic Heterojunction Solar Cells and Quantification of Traps by Deep-Level Transient Spectroscopy, Adv. Funct. Mater., 2014, 24, 3587-3592.

Indium gallium nitride nanocubes were syntheized via a low temperature chemical route. Energy-dispersive X-ray spectroscopy and X-ray diffraction analyses confirmed the successful fabrication of InGaN with various indium mole fractions. The bandgap of the material was tunded as a function of the indium content. The fabricated nanocubes showed a deep level photoluminescence emission at 734 nm as well as in the visible region at 435-520 nm. The Hall effect measurements showed the hole concentration to constantly increase from 6.2×1016 to 2.3×1018 cm^minus;3, while the hole mobility to decrease from 0.92 to 0.1 cm² /(V s) as the doping ratio increases from 0.005 to 0.025 cm^-3. The solar cell device made of nanocubes film containing 0.4 indium on flexible substrates showed a short-circuit current density of 12.47 mA/cm² and an open-circuit voltage (Voc) of 0.48V with 54% fill factor. The relationship between Voc and indium content in the fabricated films was also investigated.

Dual junction solar cells can harvest the solar spectrum more efficiently compared to conventional single junction solar cells. In this paper, we present a computer based simulation of a high efficiency dual junction solar cell. This device has a bandgap combination close to the ideal one for high efficiency device. AlGaAs with 1.82 eV energy bandgap and CIGS with 1.15 eV energy bandgap were used as the top and bottom cells on the dual junction device, respectively. The AlGaAs/CIGS dual junction solar cell was able to achieve high conversion efficiency of 32.3% under 1 sun, 25 °C and AM1.5G with 6.11 mu;m thickness. The device could achieve 36.4% efficiency under 1000 suns.

Recent emergence of inorganic-organic halide perovskite-based materials as efficient solar cell absorbers has sparked an interest in the development of multijunction solar cells based on a perovskite top cell and a silicon bottom cell, the latter of which leverages nearly 40 GW of installed industrial capacity. We recently reported a 4-terminal perovskite-silicon multijunction solar cell reaching 17% efficiency, even when low-quality silicon (11% single-junction device) is used for the bottom silicon cell in combination with a 12.7% semi-transparent perovskite top cell. In this work, we present a roadmap for the development of the bottom silicon solar cell suitable for a 2-terminal multijunction configuration with a monolithically integrated perovskite top cell.
The requirements for the bottom silicon solar cell (both performance and structure) will be assessed using a combined optical and electrical PV device model. Specifically, we compare 2- and 4-terminal tandem designs, and theoretically assess performance and energy-yield trade-offs. For each design approach, we will present a performance sensitivity analysis defining key technical parameters of merit.

I will present a comparative analysis of physical mechanisms of operation of perovskite and colloidal quantum dot solar cells. This will include a discussion of mobilities, trap states, and their combined role in determining diffusion length; as well as our current understanding of the role of contacts in each case. I will also discuss ways in which rapid advances in field can be usefully combined.