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.
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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.
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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 relati