Low-temperature solution-processed photovoltaics suffer from low efficiencies because of poor exciton or electron-hole diffusion lengths (typically about 10 nanometers). Recent reports of highly efficient CH3NH3PbI3-based solar cells in a broad range of configurations raise a compelling case for understanding the fundamental photophysical mechanisms in these materials. By applying femtosecond transient optical spectroscopy to bilayers that interface this perovskite with either selective-electron or selective-hole extraction materials, we have uncovered concrete evidence of balanced long-range electron-hole diffusion lengths of at least 100 nanometers in solution-processed CH3NH3PbI3. The high photoconversion efficiencies of these systems stem from the comparable optical absorption length and charge-carrier diffusion lengths, transcending the traditional constraints of solution-processed semiconductors.

This work reports a study into the origin of the high efficiency in solution-processable bilayer solar cells based on methylammonium lead iodide (CH 3 NH 3 PbI 3) and [6,6]-phenyl-C 61-butyric acid methyl ester (PC 61 BM). Our cell has a power conversion efficiency (PCE) of 5.2% under simulated AM 1.5G irradiation (100 mW cm À2) and an internal quantum efficiency of close to 100%, which means that nearly all the absorbed photons are converted to electrons and are efficiently collected at the electrodes. This implies that the exciton diffusion, charge transfer and charge collection are highly efficient. The high exciton diffusion efficiency is enabled by the long diffusion length of CH 3 NH 3 PbI 3 relative to its thickness. Furthermore, the low exciton binding energy of CH 3 NH 3 PbI 3 implies that exciton splitting at the CH 3 NH 3 PbI 3 /PC 61 BM interface is very efficient. With further increase in CH 3 NH 3 PbI 3 thickness, a higher PCE of 7.4% could be obtained. This is the highest efficiency attained for low temperature solutionprocessable bilayer solar cells to date. Broader context Low-temperature solution-processable bilayer solar cells with their simple architecture provide an inexpensive and straightforward platform for device fabrication without the necessity for extensive morphological optimization. For efficient solar cells, good light absorption accompanied by efficient conversion of photons to electrons is critical. This work shows that solar cells based on hybrid organic-inorganic lead halide as the donor and [6,6]-phenyl-C 61-butyric acid methyl ester (PC 61 BM) as the acceptor are able to resolve the conicting lm thickness requirements of high absorption together with efficient exciton diffusion. As a result, practically all the photons absorbed by the active layer can be converted to electrons.

This review provides an overview of factors affecting film morphology and how together with device architecture they impact perovskite cell performance.

The chemically reduced graphene oxide (rGO) was transferred onto polyethylene terephthalate (PET) substrates and then used as transparent and conductive electrodes for flexible organic photovoltaic (OPV) devices. The performance of the OPV devices mainly depends on the charge transport efficiency through rGO electrodes when the optical transmittance of rGO is above 65%. However, if the transmittance of rGO is less than 65%, the performance of the OPV device is dominated by the light transmission efficiency, that is, the transparency of rGO films. After the tensile strain (∼2.9%) was applied on the fabricated OPV device, it can sustain a thousand cycles of bending. Our work demonstrates the highly flexible property of rGO films, which provide the potential applications in flexible optoelectronics.

cell (i.e., mesoscopic, meso-superstructured and planar heterojunctions), the power conversion effi ciencies (PCEs) of these cells have improved tremendously from 3.8% to 20% within a few years. [1][2][3][4][5][6][7] The high effi ciencies obtained with the hybrid perovskites are attributed to the high absorbance and long-range balanced charge transport lengths within the hybrid perovskites. [ 8,9 ] While most studies focus on improving the device performance, equal emphasis should also be given to the fundamental device physics. Among the several open questions on perovskite solar cells, the most challenging issue to date is the hysteresis effect (or dynamic lag) in current -voltage ( I-V ) measurements. [ 10,11 ] It was found that the PCEs measured is highly dependent on scan rate, scan direction, scan history, and light exposure. This could lead to the inaccurate reporting of PCEs, which would undermine the credibility and progress of this nascent photovoltaic technology. Consensus on the origin(s) of the hysteresis has proven elusive. Proposed origins include slow trapping and detrapping of charges due to subgap traps of solution-processed perovskites; changes to the ferroelectric structure and ion migration, etc. [10][11][12][13] Detailed investigations are therefore urgently needed to unravel their complicated mechanisms and elucidate their physical origins. Such fi ndings would be highly essential for establishing clear design rules needed for further performance improvements in halide organic-inorganic perovskite solar cells.The electrical properties and optical properties of an optoelectronic material are intimately coupled; both are the macroscopic refl ection of the intrinsic electronic physics. Studying both the electrical and optical behavior in a photovoltaic device is an ideal approach to uncover the physics shared by the two. Till now, very few reports have concurrently studied the optical and electrical phenomena that occur in perovskite solar cells with hysteresis. Herein, through versatile combined electrical and optical measurements, we uncover that the hysteresis effect in CH 3 NH 3 PbI 3 (MAPbI 3 ):TiO 2 -based perovskite solar cells is dominated by distinct slow processes persisting from hundreds of milliseconds to tens of seconds. These processes originate from the dynamic rearrangement of the perovskite structure that is mediated by applied electric fi elds and accumulated