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...

A new home for organic photovoltaics.

Photoinduced degradation of individual methylammonium lead triiodide (MAPbI3) perovskite nanocrystals was studied using super-resolution luminescence microspectroscopy under intense light excitation. The photoluminescence (PL) intensity decrease and blue-shift of the PL spectrum up to 60 nm together with spatial shifts in the emission localization position up to a few hundred nanometers were visualized in real time. PL blinking was found to temporarily suspend the degradation process, indicating that the degradation needs a high concentration of mobile photogenerated charges to occur. We propose that the mechanistic process of degradation occurs as the three-dimensional MAPbI3 crystal structure smoothly collapses to the two-dimensional layered PbI2 structure. The degradation starts locally and then spreads over the whole crystal. The structural collapse is primarily due to migration of methylammonium ions (MA+), which distorts the lattice structure causing alterations to the Pb–I–Pb bond angle and in turn changes the effective band gap.

A probable limiting factor for efficiency and fill factors of organic solar cells originates from the cathode-polymer interface. We utilize various forms of cathode layer such as Al, Ca, oxidized Ca, and low melting point alloys in model systems to emphasize this aspect in our studies. The current-voltage ͑JV͒ response in the fourth quadrant indicates a general trend of convex shaped JV characteristics ͑d 2 J / dV 2 Ͼ 0͒ for illuminated devices with good cathode-polymer interfaces and linear or concave JV responses ͑d 2 J / dV 2 Ͻ 0͒ for inefficient cathode-polymer interfaces.

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.

We address here the need for a general strategy to control molecular assembly over multiple length scales. Efficient organic photovoltaics require an active layer comprised of a mesoscale interconnected networks of nanoscale aggregates of semiconductors. We demonstrate a method, using principles of molecular self-assembly and geometric packing, for controlled assembly of semiconductors at the nanoscale and mesoscale. Nanoparticles of poly(3-hexylthiophene) (P3HT) or [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were fabricated with targeted sizes. Nanoparticles containing a blend of both P3HT and PCBM were also fabricated. The active layer morphology was tuned by the changing particle composition, particle radii, and the ratios of P3HT:PCBM particles. Photovoltaic devices were fabricated from these aqueous nanoparticle dispersions with comparable device performance to typical bulk-heterojunction devices. Our strategy opens a revolutionary pathway to study and tune the active layer morphology systematically while exercising control of the component assembly at multiple length scales.

Hybrid perovskites have been widely used in solar cells and light-emitting diode applications due to superior optoelectronic properties. However, ion migration in these materials causes photo- and thermal instability. On the other hand, mixed electronic–ionic conduction could be advantageous in electrochemical energy storage applications. We have fabricated porous electrodes from three-dimensional (3D) bulk and 2D layered perovskite single crystals and demonstrated that the ion migration could play a significant role in determining the overall performance of the electrochemical supercapacitor. The areal capacitance (∼58 mF cm–2), specific capacitance (∼36.82 F g–1), and energy density (∼9 W h kg–1) calculated at a current density of 0.6 mA cm–2 are higher in 3D perovskite-based supercapacitors, while the maximum power density (∼400 W kg–1) is significantly higher in 2D perovskite-based supercapacitors due to faster intercalation/deintercalation of the electrolyte ions into the porous electrode. We have also estimated the amount of diffusion-controlled charge storage to that of electric double-layer capacitance and surface redox reaction (pseudo-) capacitance from the power law relation in both the samples. The major difference is observed at a low-field regime, where ionic conductivity in 3D bulk perovskites is significantly higher than that in 2D-layered perovskites mainly due to strong electron–ion coupling. Therefore, in 3D perovskite-based supercapacitors, only 2% is diffusion-controlled charge storage compared to 40% in 2D samples at a low-field regime. With the increasing applied voltage, both capacitive and diffusion-controlled charge storage become comparable in both the samples. The 3D sample stability is ∼98%, while the 2D sample stability is almost 100% even after 1000 cycles of operation.