Lead halide perovskite is an excellent candidate for use as a light harvester in solar cells. Our work presents a depleted hole conductor free CH₃NH₃PbI₃/TiO₂ heterojunction solar cell using a thick CH₃NH₃PbI₃ film. The CH₃NH₃PbI₃ formed large crystals which function simultaneously as light harvesters and as hole transport materials. We performed capacitance voltage measurements, which show a depletion region which extends to both n and p sides. The built-in field of the depletion region assists in the charge separation and suppresses the back reaction of electrons from the TiO₂ film to the CH₃NH₃PbI₃ film. This depleted hole conductor free CH₃NH₃PbI₃/TiO₂ heterojunction solar cell provides a power conversion efficiency of 8% with a current density of 18.8 mA cm⁻², the highest efficiency achieved to date for perovskite based solar cells without a hole conductor.

The inorganic–organic perovskite is currently attracting a lot of attention due to its use as a light harvester in solar cells. The large absorption coefficients, high carrier mobility and good stability of organo-lead halide perovskites present good potential for their use as light harvesters in mesoscopic heterojunction solar cells. This work concentrated on a unique property of the lead halide perovskite, its function simultaneously as a light harvester and a hole conductor in the solar cell. A two-step deposition technique was used to optimize the perovskite deposition and to enhance the solar cell efficiency. It was revealed that the photovoltaic performance of the hole conductor free perovskite solar cell is strongly dependent on the depletion layer width which was created at the TiO₂–CH₃NH₃PbI₃ junction. X-ray diffraction measurements indicate that there were no changes in the crystallographic structure of the CH₃NH₃PbI₃ perovskite over time, which supports the high stability of these hole conductor free perovskite solar cells. Furthermore, the power conversion efficiency of the best cells reached 10.85% with a fill factor of 68%, a V_oc of 0.84 V, and a J_sc of 19 mA cm⁻², the highest efficiency to date of a hole conductor free perovskite solar cell.

This perspective paper focuses on the dimensionality of organic–inorganic halide perovskites and their relevant advantages over 3D perovskites. The charges in two-dimensional (2D) materials are restricted in their movement to the two-dimensional plane; however, their confined structure allows one to tune the optical and electronic properties by varying their thickness. Here we focus on the main advantages of 2D halide perovskites including their enhanced stability; several recent reports discussing this point are summarized. It was observed that 2D perovskites exhibit enhanced moisture resistivity, starting from films to complete solar cells. The power conversion efficiency of photovoltaic solar cells based on 2D- and quasi-2D perovskites (2D + 3D perovskites) as light harvesters has been increasing but still not as much as 3D perovskites. However, their other advantages (e.g., stability, tuning of optical and electronic properties, and the large amount of possible organic cations that can be integrated into their structure) make them a stable alternative for efficient and large-scale module solar cells, which may constitute the next step for commercializing perovskite-based solar cells. To sum up, several strategies for improving the photovoltaic performance of low dimensional perovskites are discussed.

Organometal halide perovskite is a promising material in photovoltaic (PV) cells. Within a short time, its performance has increased dramatically to become a real competitor to silicon solar cells. Here we report on the temperature dependence (annealing temperature and the dependence of the photovoltaic parameters on temperature) of formamidinium (FA) lead iodide (FAPbI₃), methylammonium (MA) lead iodide (MAPbI₃) and their mixture (MAPbI₃ : FAPbI₃) in hole conductor free perovskite solar cells. These three types of perovskites function both as light harvesters and as hole conductors. Surface photovoltage and optical characterization reveal the p-type behavior and the band gap of the different perovskites. We observed that the ratio between the MA and FA cations might change during the annealing process, affecting the band gap and the stability of the layers. The PV parameters at different temperatures show better stability for the pure MAPbI₃ and FAPbI₃ solar cells compared to their mixture. Using intensity modulated photovoltage/photocurrent spectroscopy, we found that the diffusion length is weakly dependent on the light intensity, while the charge collection efficiency drops with light intensity for the FAPbI₃-based cells. However, for MAPbI₃ and the mixture, the charge collection efficiency remains constant for a wide range of light intensities.