The Stability of PbS Quantum Dot Solar Cells

Solution-based lead sulfide (PbS) quantum dots (QDs) may enable lightweight solar cells that facilitate high-throughput module manufacturing, simplify module installation, and bring light-harvesting functionality to unconventional or weight-restricted surfaces. Proven long-term stability is critical for QD solar cells to reach commercial viability, since solar cells deployed in the field must typically operate for years with minimal loss in efficiency. However, the operational stability and associated aging processes of QD solar cells are not fully understood. Existing measurements of QD solar cell lifetimes are often conducted under non-standardized stress conditions and primarily highlight relative improvements over baseline devices rather than underlying causes of performance degradation. In this work, we systematically investigate aging processes that lead to QD solar cell performance evolution during short-term shelf storage and long-term continuous operation. First, we analyze the role of short-term air exposure in improving the initial efficiency of PbS QD solar cells. We show that brief air exposure treatments elicit multiple oxidation processes that benefit PbS QD solar cell performance in the near-term. In particular, post-fabrication air exposure is necessary to heal the QD-top electrode interface following thermal evaporation of electrodes onto QDs. Next, we characterize the operational stability of PbS QD solar cells under systematically tuned stress conditions such as continuous illumination, heating, environment, and electrical bias. We demonstrate conventional QD solar cells are capable of operating lifetimes approaching 1000 hours or longer if actively cooled, but photothermal instabilities likely limit their current utility for applications requiring sustained operation above room temperature. We also provide evidence of electrode migration-induced degradation in hole transport layer-free QD solar cells, highlighting the need to consider diffusion barrier properties when evaluating next-generation hole transport materials.Ph.D

An interface stabilized perovskite solar cell with high stabilized efficiency and low voltage loss

Stabilization of the crystal phase of inorganic/organic lead halide perovskites is critical for their high performance optoelectronic devices. However, due to the highly ionic nature of perovskite crystals, even phase stabilized polycrystalline perovskites can undergo undesirable phase transitions when exposed to a destabilizing environment. While various surface passivating agents have been developed to improve the device performance of perovskite solar cells, conventional deposition methods using a protic polar solvent, mainly isopropyl alcohol (IPA), results in a destabilization of the underlying perovskite layer and an undesirable degradation of device properties. We demonstrate the hidden role of IPA in surface treatments and develop a strategy in which the passivating agent is deposited without destabilizing the high quality perovskite underlayer. This strategy maximizes and stabilizes device performance by suppressing the formation of the perovskite δ-phase and amorphous phase during surface treatment, which is observed using conventional methods. Our strategy also effectively passivates surface and grain boundary defects, minimizing non-radiative recombination sites, and preventing carrier quenching at the perovskite interface. This results in an open-circuit-voltage loss of only ∼340 mV, a champion device with a power conversion efficiency of 23.4% from a reverse current–voltage scan, a device with a record certified stabilized PCE of 22.6%, and enhanced operational stability. In addition, our perovskite solar cell exhibits an electroluminescence external quantum efficiency up to 8.9%. ©2019Institute for Soldier Nanotechnology (Grant W911NF-13-D-0001)NASA (Grant NNX16AM70H)DOE Division of Materials Sciences and Engineering (Award DE-FG02-07ER46454)NSF (Grant CBET-1605495

Strongly Enhanced Photovoltaic Performance and Defect Physics of Air-Stable Bismuth Oxyiodide (BiOI)

Bismuth-based compounds have recently gained increasing attention as potentially nontoxic and defect-tolerant solar absorbers. However, many of the new materials recently investigated show limited photovoltaic performance. Herein, one such compound is explored in detail through theory and experiment: bismuth oxyiodide (BiOI). BiOI thin films are grown by chemical vapor transport and found to maintain the same tetragonal phase in ambient air for at least 197 d. The computations suggest BiOI to be tolerant to antisite and vacancy defects. All-inorganic solar cells (ITO|NiO x |BiOI|ZnO|Al) with negligible hysteresis and up to 80% external quantum efficiency under select monochromatic excitation are demonstrated. The short-circuit current densities and power conversion efficiencies under AM 1.5G illumination are nearly double those of previously reported BiOI solar cells, as well as other bismuth halide and chalcohalide photovoltaics recently explored by many groups. Through a detailed loss analysis using optical characterization, photoemission spectroscopy, and device modeling, direction for future improvements in efficiency is provided. This work demonstrates that BiOI, previously considered to be a poor photocatalyst, is promising for photovoltaics.National Science Foundation (U.S.) (Grant CBET-1605495)United States. Department of Energy. Office of Basic Energy Sciences (Grant DE-SC0001088)National Science Foundation (U.S.) (Grant DMF-08019762