Solar cells fabricated using alkyl ammonium metal halides as light absorbers have the right combination of high power conversion efficiency and ease of fabrication to realize inexpensive but efficient thin film solar cells. However, they degrade under prolonged exposure to sunlight. Herein, we show that this degradation is quasi-reversible, and that it can be greatly lessened by simple modifications of the solar cell operating conditions. We studied perovskite devices using electrochemical impedance spectroscopy (EIS) with methylammonium (MA)-, formamidinium (FA)-, and MA(x)FA(1-x) lead triiodide as active layers. From variable temperature EIS studies, we found that the diffusion coefficient using MA ions was greater than when using FA ions. Structural studies using powder X-ray diffraction (PXRD) show that for MAPbI3 a structural change and lattice expansion occurs at device operating temperatures. On the basis of EIS and PXRD studies, we postulate that in MAPbI3 the predominant mechanism of accelerated device degradation under sunlight involves thermally activated fast ion transport coupled with a lattice-expanding phase transition, both of which are facilitated by absorption of the infrared component of the solar spectrum. Using these findings, we show that the devices show greatly improved operation lifetimes and stability under white-light emitting diodes, or under a solar simulator with an infrared cutoff filter or with cooling.

low exciton binding energy [ 20,21 ] and long carrier diffusion length, [21][22][23] metal halide perovskites with organic counterions have enabled both mesoscopic and planar solar cells to achieve power conversion effi ciencies (PCEs) >18%, [24][25][26][27][28][29] with state-of-theart mesocopic devices reaching a certifi ed PCE of 20.1%. [ 27 ] To date, perovskite solar cells with planar heterojunction structures are slightly less effi cient than their mesoscopic counterparts, but their fabrication is straightforward and compatible with well-established solution-based low temperature fabrication roll-to-roll procedures used for the production of polymer solar cells. [24][25][26][27] The incorporation of charge selective transport layers at the electrode/active layer junctions has often been regarded as a prerequisite to realize effi cient charge extraction in planar perovskite solar cells. [ 30 ] Thus, great effort has been focused on the development and understanding of interfacial engineering between perovskite and electron transport layers (ETLs) or hole transport layers (HTLs) for effective charge carrier separation. [31][32][33][34][35] In perovskite solar cells, the diffusion length of electrons is shorter than holes and it is regarded as a major limitation associated with these devices. [ 36,37 ] To address this limitation, compact semiconducting metal oxide (e.g., ZnO, TiO 2 ) ETLs have been used to facilitate electron transport in planar heterojunction devices. [ 2,14,38,39 ] In addition to the use of metal oxide layers, electrode work function modifi cation by an interlayer can further improve the performance of perovskite solar cells. [ 26,[40][41][42][43][44][45][46][47] For example, Yang et al. incorporated polyethyleneimine ethoxylated (PEIE) between indium tin oxide (ITO) electrode and TiO 2 to signifi cantly increase the PCE of planar heterojunction perovskite solar cells, identifying that reduction of ITO's work function (Φ) by PEIE, due to the presence of a negative interfacial dipole, was a leading contributor to the observed device performance improvement. [ 26 ] Phenyl-C 61 -butyric acid methyl ester (PC 61 BM) has been used as an alternative ETL to metal oxide layers in planar heterojunction devices, providing more effi cient charge injection from perovskite, [ 25 ] while allowing for low-temperature solution processing that precludes ITO's use as an electron-extracting electrode. [ 25,48,49 ] In addition, the deposition of PC 61 BM on perovskite fi lm [ 50 ] or making perovskite-PC 61 BM hybrid active layer [ 51 ] is effective to passivate charge trap states and defects Interface engineering is critical for achieving effi cient solar cells, yet a comprehensive understanding of the interface between a metal electrode and electron transport layer (ETL) is lacking. Here, a signifi cant power conversion effi ciency (PCE) improvement of fullerene/perovskite planar heterojunction solar cells from 7.5% to 15.5% is shown by inserting a fulleropyrrolidine interlayer between the silver electrode an...

Perovskite-containing tandem solar cells are attracting attention for their potential to achieve high efficiencies. We demonstrate a series connection of a ∼ 90 nm thick perovskite front subcell and a ∼ 100 nm thick polymer:fullerene blend back subcell that benefits from an efficient graded recombination layer containing a zwitterionic fullerene, silver (Ag), and molybdenum trioxide (MoO3). This methodology eliminates the adverse effects of thermal annealing or chemical treatment that occurs during perovskite fabrication on polymer-based front subcells. The record tandem perovskite/polymer solar cell efficiency of 16.0%, with low hysteresis, is 75% greater than that of the corresponding ∼ 90 nm thick perovskite single-junction device and 65% greater than that of the polymer single-junction device. The high efficiency of this hybrid tandem device, achieved using only a ∼ 90 nm thick perovskite layer, provides an opportunity to substantially reduce the lead content in the device, while maintaining the high performance derived from perovskites.

A significant challenge in the rational design of organic thermoelectric materials is to realize simultaneously high electrical conductivity and high induced-voltage in response to a thermal gradient, which is represented by the Seebeck coefficient. Conventional wisdom posits that the polymer alone dictates thermoelectric efficiency. Herein, we show that doping — in particular, clustering of dopants within conjugated polymer films — has a profound and predictable influence on their thermoelectric properties. We correlate Seebeck coefficient and electrical conductivity of iodine-doped poly(3-hexylthiophene) and poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-3,6-diyl)-alt-(2,2′;5′,2′′;5′′,2′′′-quaterthiophen-5,5′′′-diyl)] films with Kelvin probe force microscopy to highlight the role of the spatial distribution of dopants in determining overall charge transport. We fit the experimental data to a phonon-assisted hopping model and found that the distribution of dopants alters the distribution of the density of states and the Kang–Snyder transport parameter. These results highlight the importance of controlling dopant distribution within conjugated polymer films for thermoelectric and other electronic applications.