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Hybrid organic-inorganic semiconducting perovskite photovoltaic cells are usually coupled with organic hole conductors. Here, we report planar, inverse CH3NH3PbI3-xClx-based cells with inorganic hole conductors. Using electrodeposited NiO as hole conductor, we have achieved a power conversion efficiency of 7.3%. The maximum VOC obtained was 935 mV with an average VOC value being 785 mV. Preliminary results for similar cells using electrodeposited CuSCN as hole conductor resulted in devices up to 3.8% in efficiency. The ability to obtain promising cells using NiO and CuSCN expands the presently rather limited range of available hole conductors for perovskite cells.

Highly reproducible and reversible thermochromic nature of dihydrated methylammonium lead iodide is found. A wide bandgap variation of the material (∼2 eV) is detected between room temperature and 60 °C under ambient condition as a result of phase transition caused by moisture absorption and desorption. In situ X-ray diffraction and Fourier transform infrared spectroscopy studies are performed to understand the mechanistic behavior during the phase transition. This thermochromic property is further explored as absorber material in mesostructured solar cells. Temperature-dependent reversible power conversion efficiency greater than 1% under standard test conditions is demonstrated; revealing its potential applicability in building integrated photovoltaics.

Frequency domain techniques are useful tools to characterize processes occurring on different time scales in solar cells and solar fuel devices. Intensity-modulated photocurrent spectroscopy (IMPS) is one such technique that links the electrical and optical responses of the device. In this review, a summary of the fundamental application of IMPS to semiconductor photoelectrodes and nanostructured solar cells is presented, with a final goal of understanding the IMPS response of the perovskite solar cell (PSC) to shed light on its complex physical mechanisms of operation. The historical application of IMPS that connects its transfer function to the charge transfer efficiency of the semiconductor electrode and subsequently the considerations of diffusive transport for the dye-sensitized solar cell is summarized. These models prioritize the association of spectral features with time constants, which has led to a neglect of other absolute aspects of the spectra by the photovoltaic community. We clarify these aspects by developing the fundamental connection between the absolute value of the IMPS transfer function and the external quantum efficiency (EQEPV) of a photovoltaic cell. Basic models for the solar cell are developed using kinetic equations and equivalent circuits (EC), stressing their equivalence and the advantage of the EC representation to adequately account for different capacitances in the system. A critique of the current interpretations of the PSC IMPS spectra is performed, where time constants and their evolution are associated with characteristic transport processes of either electronic or ionic carriers within the PSC. These are clarified using the EC representation to identify that the generated characteristic processes are only related to coupling between different elements of the EC and are not reflective of transport phenomena in general. Furthermore, a general model is developed that identifies charge accumulation at the interfaces as a general feature for both low- and high-efficiency PSCs, whose charging/discharging resistances are the main factor in controlling the electrical response of the device. This model shows a separation of the photovoltage within the PSC that causes a reduction in its EQEPV at low frequencies. Further development of the PSC will involve gaining control over the low-frequency charge kinetics in the device to overcome these limitations.

Pseudohalide thiocyanate anion (SCN(-)) has been used as a dopant in a methylammonium lead tri-iodide (MAPbI3) framework, aiming for its use as an absorber layer for photovoltaic applications. The substitution of SCN(-) pseudohalide anion, as verified using Fourier transform infrared (FT-IR) spectroscopy, results in a comprehensive effect on the optical properties of the original material. Photoluminescence measurements at room temperature reveal a significant enhancement in the emission quantum yield of MAPbI3-x(SCN)x as compared to MAPbI3, suggestive of suppression of nonradiative channels. This increased intensity is attributed to a highly edge specific emission from MAPbI3-x(SCN)x microcrystals as revealed by photoluminescence microscopy. Fluoresence lifetime imaging measurements further established contrasting carrier recombination dynamics for grain boundaries and the bulk of the doped material. Spatially resolved emission spectroscopy on individual microcrystals of MAPbI3-x(SCN)x reveals that the optical bandgap and density of states at various (local) nanodomains are also nonuniform. Surprisingly, several (local) emissive regions within MAPbI3-x(SCN)x microcrystals are found to be optically unstable under photoirradiation, and display unambiguous temporal intermittency in emission (blinking), which is extremely unusual and intriguing. We find diverse blinking behaviors for the undoped MAPbI3 crystals as well, which leads us to speculate that blinking may be a common phenomenon for most hybrid perovskite materials.