Tailoring Mixed-Halide, Wide-Gap Perovskites via Multistep Conversion Process

Wide-band-gap mixed-halide CH3NH3PbI3–XBrX-based solar cells have been prepared by means of a sequential spin-coating process. The spin-rate for PbI2 as well as its repetitive deposition are important in determining the cross-sectional shape and surface morphology of perovskite, and, consequently, J–V performance. A perovskite solar cell converted from PbI2 with a dense bottom layer and porous top layer achieved higher device performance than those of analogue cells with a dense PbI2 top layer. This work demonstrates a facile way to control PbI2 film configuration and morphology simply by modification of spin-coating parameters without any additional chemical or thermal post-treatment

Molecular Ligands Control Superlattice Structure and Crystallite Orientation in Colloidal Quantum Dot Solids

Colloidal quantum dot solids represent a new materials platform that has garnered interest for a variety of electronic, optoelectronic, and photovoltaic applications. In such solids, individual quantum dots must be coupled with each other to facilitate charge transport through the solid. Past improvements on charge transport of colloidal quantum dot solids have been achieved primarily through the control of the interparticle spacing. However, the role of morphological ordering of the crystalline facets of individual quantum dots on the charge transport of the quantum dot solid is unknown. Here, we show for the first time that small passivating ligand molecules around the quantum dots can control the arrangement of different facets of quantum dots within the quantum dot solid. The insights from this study provide important directions for future enhancement in orientation of quantum dots in colloidal quantum dot solids

Improving Performance in Colloidal Quantum Dot Solar Cells by Tuning Band Alignment through Surface Dipole Moments

Colloidal quantum dots (CQDs) have received recent attention for low cost, solution processable, high efficiency solid-state photovoltaic devices due to the possibility of tailoring their optoelectronic properties by tuning size, composition, and surface chemistry. However, the device performance is limited by the diffusion length of charge carriers due to recombination. In this work, we show that band engineering of PbS QDs is achievable by changing the dipole moment of the passivating ligand molecules surrounding the QD. The valence band maximum and conduction band minimum of PbS QDs passivated with three different thiophenol ligands (4-nitrothiophenol, 4-fluorothiophenol, and 4-methylthiophenol) are determined by UV–visible absorption spectroscopy and photoelectron spectroscopy in air (PESA), and the experimental results are compared with DFT calculations. These band-engineered QDs have been used to fabricate heterojunction solar cells in both <i>unidirectional</i> and <i>bidirectional</i> configurations. The results show that proper band alignment can improve the directionality of charge carrier collection to benefit the photovoltaic performance

Developing a Robust Recombination Contact to Realize Monolithic Perovskite Tandems With Industrially Common p-Type Silicon Solar Cells

10.1109/JPHOTOV.2018.2820509IEEE JOURNAL OF PHOTOVOLTAICS841023-102

23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability

As the record single-junction efficiencies of perovskite solar cells now rival those of copper indium gallium selenide, cadmium telluride and multicrystalline silicon, they are becoming increasingly attractive for use in tandem solar cells due to their wide, tunable bandgap and solution processability. Previously, perovskite/silicon tandems were limited by significant parasitic absorption and poor environmental stability. Here, we improve the efficiency of monolithic, two-terminal, 1-cm2perovskite/silicon tandems to 23.6% by combining an infrared-tuned silicon heterojunction bottom cell with the recently developed caesium formamidinium lead halide perovskite. This more-stable perovskite tolerates deposition of a tin oxide buffer layer via atomic layer deposition that prevents shunts, has negligible parasitic absorption, and allows for the sputter deposition of a transparent top electrode. Furthermore, the window layer doubles as a diffusion barrier, increasing the thermal and environmental stability to enable perovskite devices that withstand a 1,000-hour damp heat test at 85 °C and 85% relative humidity