The hybrid halide perovskite CH3NH3PbI3 has enabled solar cells to reach an efficiency of about 20%, demonstrating a pace for improvements with no precedents in the solar energy arena. Despite such explosive progress, the microscopic origin behind the success of such material is still debated, with the role played by the organic cations in the light-harvesting process remaining unclear. Here van der Waals-corrected density functional theory calculations reveal that the orientation of the organic molecules plays a fundamental role in determining the material electronic properties. For instance, if CH3NH3 orients along a (011)-like direction, the PbI6 octahedral cage will distort and the bandgap will become indirect. Our results suggest that molecular rotations, with the consequent dynamical change of the band structure, might be at the origin of the slow carrier recombination and the superior conversion efficiency of CH3NH3PbI3.

Organic-inorganic lead-halide perovskites have received a revival of interest in the past few years as a promising class of materials for photovoltaic applications. Despite recent extensive research, the role of cations in defining the high photovoltaic performance of these materials is not fully understood. Here, we conduct nonadiabatic molecular dynamics simulations to study and compare nonradiative hot carrier relaxation in three lead-halide perovskite materials: CHNHPbI, HC(NH)PbI, and CsPbI. It is found that the relaxation of hot carriers to the band edges occurs on the ultrafast time scale and displays a strong quantitative dependence on the nature of the cations. The obtained results are explained in terms of electron-phonon couplings, which are strongly affected by the atomic displacements in the Pb-I framework triggered by the cation dynamics.

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In this review, we present and discussed the main trends in photovoltaics with emphasize on the conversion efficiency limits. The theoretical limits of various photovoltaics device concepts are presented and analyzed using a flexible detailed balance model where more discussion emphasize is toward the losses. Also, few lessons from nature and other fields to improve the conversion efficiency in photovoltaics are presented and discussed as well. From photosynthesis, the perfect exciton transport in photosynthetic complexes can be utilized for PVs. Also, we present some lessons learned from other fields like recombination suppression by quantum coherence. For example, the coupling in photosynthetic reaction centers is used to suppress recombination in photocells.

In the past few years, the efficiency of solar cells based on hybrid organic-inorganic perovskites has exceeded the level needed for commercialization. However, existing perovskites solar cells (PSCs) suffer from several intrinsic instabilities, which prevent them from reaching industrial maturity, and stabilizing PSCs has become a critically important problem. Here we propose to stabilize PSCs chemically by strengthening the interactions between the organic cation and inorganic anion of the perovskite framework. In particular, we show that replacing the methylammonium cation with alternative protonated cations allows an increase in the stability of the perovskite by forming strong hydrogen bonds with the halide anions. This interaction also provides opportunities for tuning the electronic states near the bandgap. These mechanisms should have a universal character in different hybrid organic-inorganic framework materials that are widely used.

The power conversion efficiency of perovskite solar cells is drastically affected by photocarrier dynamics at the interfaces. Experimental measurements show quenching of the photoluminescence (PL) signal from the perovskite layer when it is capped with a hole transport medium (HTM). Furthermore, time-resolved PL (TRPL) data show a faster decay of the PL signal in the presence of the perovskite/HTM interface. The experimental decay is usually fitted using one or two exponential functions with an incomplete physical picture. In this work, an extensive model is used to extract the key physical parameters characterizing carrier dynamics in the bulk and at the interfaces. The decay of the TRPL signal is calculated in the presence of both defect-assisted recombination (Shockley Read Hall) and band-to-band radiative recombination where carrier extraction/ recombination at the interfaces is described by interface recombination velocities. By proper curve fitting of the modeling results and the measured TRPL signal, meaningful optoelectronic parameters governing photophysical processes in mixed halide perovskite thin films and single crystals are extracted. Furthermore, a sensitivity analysis to assess the contribution of these parameters on TRPL kinetics is also performed. Notably, the inclusion of the diffusion and surface recombination velocity at the interfaces allows to obtain the important physical parameters that govern the TRPL kinetics and improve the conformity of fits to experiments.

The high quantum efficiency of photosynthetic complexes has inspired researchers to explore new routes to utilize this process for photovoltaic devices. Quantum coherence has been demonstrated to play a crucial role within this process. Herein, we propose a three-dipole system as a model of a new photocell type which exploits the coherence among its three dipoles. We have proved that the efficiency of such a photocell is greatly enhanced by quantum coherence. We have also predicted that the photocurrents can be enhanced by about 49.5% in such a coherent coupled dipole system compared with the uncoupled dipoles. These results suggest a promising novel design aspect of photosynthesis-mimicking photovoltaic devices.

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