Improved Morphology and Efficiency of n–i–p Planar Perovskite Solar Cells by Processing with Glycol Ether Additives

Planar perovskite solar cells can be prepared without high-temperature processing steps typically associated with mesoporous device architectures; however, their efficiency has been lower, and producing high-quality perovskite films in planar devices has been challenging. Here, we report a modified two-step interdiffusion protocol suitable to preparing pinhole-free perovskite films with greatly improved morphology. This is achieved by simple addition of small amounts of glycol ethers to the preparation protocol. We unravel the impact the glycol ethers have on the perovskite film formation using in situ ultraviolet–visible absorbance and grazing incidence wide-angle X-ray scattering experiments. From these experiments we conclude that addition of glycol ethers changes the lead iodide to perovskite conversion dynamics and enhances the conversion efficiency, resulting in more compact polycrystalline films, and it creates micrometer-sized perovskite crystals vertically aligned across the photoactive layer. Consequently, the average photovoltaic performance increases from 13.5% to 15.9%, and reproduciability is enhanced, specifically when 2-methoxyethanol is used as the additive

Impact of Nonfullerene Acceptor Core Structure on the Photophysics and Efficiency of Polymer Solar Cells

Small-molecule “nonfullerene” acceptors are promising alternatives to fullerene (PC61/71BM) derivatives often used in bulk heterojunction (BHJ) organic solar cells; yet, the efficiency-limiting processes and their dependence on the acceptor structure are not clearly understood. Here, we investigate the impact of the acceptor core structure (cyclopenta-[2,1-b:3,4-b′]­dithiophene (CDT) versus indacenodithiophene (IDTT)) of malononitrile (BM)-terminated acceptors, namely CDTBM and IDTTBM, on the photophysical characteristics of BHJ solar cells. Using PCE10 as donor polymer, the IDTT-based acceptor achieves power conversion efficiencies (8.4%) that are higher than those of the CDT-based acceptor (5.6%) because of a concurrent increase in short-circuit current and open-circuit voltage. Using (ultra)­fast transient spectroscopy we demonstrate that reduced geminate recombination in PCE10:IDTTBM blends is the reason for the difference in short-circuit currents. External quantum efficiency measurements indicate that the higher energy of interfacial charge-transfer states observed for the IDTT-based acceptor blends is the origin of the higher open-circuit voltage

Molecular Doping of the Hole-Transporting Layer for Efficient, Single-Step-Deposited Colloidal Quantum Dot Photovoltaics

Employment of thin perovskite shells and metal halides as surface-passivants for colloidal quantum dots (CQDs) has been an important, recent development in CQD optoelectronics. These have opened the route to single-step-deposited high-performing CQD solar cells. These promising architectures employ a CQD hole-transporting layer (HTL) whose intrinsically shallow Fermi level (<i>E</i>_F) restricts band-bending at maximum power-point during solar cell operation limiting charge collection. Here, we demonstrate a generalized approach to effectively balance band-edge energy levels of the main CQD absorber and charge-transport layer for these high-performance solar cells. Briefly soaking the CQD HTL in a solution of the metal–organic p-dopant, molybdenum tris­(1-(trifluoroacetyl)-2-(trifluoromethyl)­ethane-1,2-dithiolene), effectively deepens its Fermi level, resulting in enhanced band bending at the HTL:absorber junction. This blocks the back-flow of photogenerated electrons, leading to enhanced photocurrent and fill factor compared to those of undoped devices. We demonstrate 9.0% perovskite-shelled and 9.5% metal-halide-passivated CQD solar cells, both achieving ca. 10% relative enhancements over undoped baselines