Moving away from the high-performance achievements in organometal halide perovskite (OHP)-based optoelectronic and photovoltaic devices, intriguing features have been reported in that photocarriers and mobile ionic species within OHPs interact with light, electric fields, or a combination of both, which induces both spatial and temporal changes of optoelectronic properties in OHPs. Since it is revealed that the transport of photocarriers and the migration of ionic species are affected not only by each other but also by the inhomogeneous character, which is a consequence of the route selected to deposit OHPs, understanding the nanostructural evolution during OHP deposition, in terms of the resultant structural defects, electronic traps, and nanoscopic charge behaviors, will be valuable. Investigation of the film-growth mechanisms and strategies adopted to realize OHP films with less-defective large grains is of central importance, considering that single-crystalline OHPs have exhibited the most beneficial properties, including carrier lifetimes. Critical factors governing the behavior of photocarriers, mobile ionic species, and nanoscale optoelectronic properties resulting from either or all of them are further summarized, which may potentially limit or broaden the optoelectronic and photovoltaic applications of OHPs. Through inspection of the recent advances, a comprehensive picture and future perspective of OHPs are provided.
Semiconductor sensitized solar cells, a promising candidate for next-generation photovoltaics, have seen notable progress using 0-D quantum dots as light harvesting materials. Integration of higher-dimensional nanostructures and their multi-composition variants into sensitized solar cells is, however, still not fully investigated despite their unique features potentially beneficial for improving performance. Herein, CdSe/CdSexTe1−x type-II heterojunction nanorods are utilized as novel light harvesters for sensitized solar cells for the first time. The CdSe/CdSexTe1−x heterojunction-nanorod sensitized solar cell exhibits ~33% improvement in the power conversion efficiency compared to its single-component counterpart, resulting from superior optoelectronic properties of the type-II heterostructure and 1-octanethiol ligands aiding facile electron extraction at the heterojunction nanorod-TiO2 interface. Additional ~32% enhancement in power conversion efficiency is achieved by introducing percolation channels of large pores in the mesoporous TiO2 electrode, which allow 1-D sensitizers to infiltrate the entire depth of electrode. These strategies combined together lead to 3.02% power conversion efficiency, which is one of the highest values among sensitized solar cells utilizing 1-D nanostructures as sensitizer materials.
With growing demands on the stability of perovskite photovoltaics against various degradation factors, understanding and controlling the defect characteristics of devices have become the most essential issues to be resolved. In this work, the organometal halide perovskite is modified with a lithium−fluoride ionic passivator that enables highly stable and efficient solar cells with a power-conversion efficiency of over 21%, retaining up to ∼90% after 1000 h at 85 °C. The thermal degradation regressions of the films and devices have been temporally investigated, and the trap density of states has been scrutinized as a function of time. Surprisingly, the electronic traps of the solar cells exhibit exponential relaxations in both the trap densities and energy levels as thermally stressed, and the incorporation of LiF has greatly enhanced this relaxation with the mitigation of the following degradation. It is suggested that LiF not only passivates the initial formation of the traps but also controls their roles and behaviors under the thermal degradation of devices.
The migration of ionic defects in grain boundaries plays a critical role in the stability and efficiency of organic−inorganic perovskite solar cells. Furthermore, the ionic defects of perovskites also contribute to the generation of hysteresis. Herein, we alloy KF with perovskites to passivate ionic defects, leading to the improved stability and performance. We obtain a power conversion efficiency of 21.2% by alloying KF with triple cation perovskites [Cs 0.05 (FA 0.83 MA 0.17 ) 0.95 Pb(I 0.83 Br 0.17 ) 3 ]. With KF, the current−voltage hysteresis of solar cells becomes negligible, and the trap density of the device is reduced. Photostability and thermal stability of the devices were also improved. Approximately 90% of the initial efficiency was maintained after 1200 h under 1 sun illumination, and ∼80% for 2000 h at 85 °C/85% relative humidity. This study confirmed that K + ions bond to halide ions in the alloyed perovskite and suppress the ionic defects, thereby reducing the trap density and metallic lead impurities resulting in the improved stabilities.
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