In this Letter, we investigate the temperature dependence of the optical properties of methylammonium lead iodide (MAPbI3 = CH3NH3PbI3) from room temperature to 6 K. In both the tetragonal (T > 163 K) and the orthorhombic (T < 163 K) phases of MAPbI3, the band gap (from both absorption and photoluminescence (PL) measurements) decreases with decrease in temperature, in contrast to what is normally seen for many inorganic semiconductors, such as Si, GaAs, GaN, etc. We show that in the perovskites reported here, the temperature coefficient of thermal expansion is large and accounts for the positive temperature coefficient of the band gap. A detailed analysis of the exciton line width allows us to distinguish between static and dynamic disorder. The low-energy tail of the exciton absorption is reminiscent of Urbach absorption. The Urbach energy is a measure of the disorder, which is modeled using thermal and static disorder for both the phases separately. The static disorder component, manifested in the exciton line width at low temperature, is small. Above 60 K, thermal disorder increases the line width. Both these features are a measure of the high crystal quality and low disorder of the perovskite films even though they are produced from solution.

We investigate the role of n-type organic molecule, bathocuproine (BCP), additive in perovskite semiconductor film based solar cells using first principle calculations and optoelectronic studies. A state-of-art high fill factor of 0.82 and improved open circuit voltage of 0.95 V for CH3NH3PbI3 (MAPI) based perovskite solar cells are achieved in addition with highly improved (more than one order) electroluminescence efficiency. Delayed emission spectroscopy does not show any blue shift peak or double peak emission suggests no structural changes in 3D perovskite. Contact angle studies using water droplet supports BCP is forming a capping layer to boost moisture barrier. Our experimental findings regarding no 2D structure formation has also been supported by first-principles electronic structure calculations based on DFT in MAPI after insertion of BCP. Furthermore, calculations suggest that a physisorption type of interaction has been found in between MAPI and BCP with an average distance of 2.9 Å. These results were found to be in agreement with steady-state and transient photoluminescence studies where an improved PL is observed by using perovskite solution with BCP additive. Chemical analysis and electroluminescence study confirmed the reduction in defect sites by BCP addition. The study suggests that BCP being a small molecule plays an important role in passivating and improving the MAPI film quality and points to an approach toward utilizing other such additives. Furthermore, this approach can be useful for other optoelectronic devices too based on wider bandgap perovskite semiconductors.

We provide in-depth insight into the role of 1,8-diiodooctane (DIO) as a solvent additive in wide bandgap CH3NH3PbBr3 (MAPbBr3) films which results in altering the kinetics of crystal growth.

The charge carrier diffusion length and dielectric relaxations are important parameters which decide the performance of various optoelectronic devices, in particular for photovoltaic devices. A comparative study is carried out on the charge transport length scale (L) for passivated and pristine CH3NH3PbI3 (MAPI) thin-film-based perovskite solar cells (PSCs) through scanning photocurrent microscopy (SPM). The SPM study suggested an improved L and degree of ambipolarity of photogenerated charge carriers (electron and hole) in passivated as compared to pristine MAPI-based PSCs. These results were found to be correlated with frequency-dependent photocurrent measurement, which shows that the relaxation time of the charge carrier is relatively lower in passivated MAPI-based PSCs. This mechanism could be explained by trap-assisted recombination, where trap states are induced by ion migration in halide perovskite films. Furthermore, passivation of traps showed an increased degree of ambipolarity in the perovskite semiconductor thin film.

Imperfections such as heterogeneity at different length scales, static versus dynamic disorders, defects in the bulk, surface imperfections, grain boundaries, and interface imperfections of solution-processed hybrid metal—halide perovskite semiconductors are known to be detrimental to the solar cell performance. These imperfections influence voltage losses and charge transport by the formation of undesirable non-radiative channels. Photo-generated charge carriers recombine via these non-radiative channels and hamper the performance of perovskite solar cells (PSCs). Scientists are aiming to decode the nature of these defects by a better understanding of their origins and by developing novel engineering techniques for the passivation of defect states. In this review article, we explain the different kinds of imperfection and discuss their impact on charge carrier transport in PSCs through optical studies. Furthermore, we summarize the efforts made in the community to passivate these defect states by various kinds of additive engineering such as solvent additives, small-organic-molecule additives, potassium-ion additives, graded 3D—2D perovskite materials, etc. Finally, this review provides an insight into defect dynamics and passivation strategies that allows us to understand the nature of defects and helps in the development of future trends in passivation methods.