Book Description

Increasing global energy demand requires the development of clean energy sources that will help reduce the consumption of fossil fuels. Solar energy, the most abundant renewable source, is converted to electricity using photovoltaic devices. The photovoltaics market has witnessed rapid growth in the past decade, and today many photovoltaic strategies aim at low-cost, solution-processed manufacture. Colloidal quantum dots (CQDs), an emerging semiconductor material, have attracted attention in view of their spectral tunability. The bandgap of CQDs is readily tuned to harvest infrared solar energy. This could enable both full-spectrum devices and also tandem solar cells that can be integrated with wider-bandgap semiconductors. Unfortunately, a high density of surface-associated trap states, low carrier mobilities, and an inhomogeneous energy landscape have previously limited CQD photovoltaic performance. I introduce three strategies to address these problems. Through new materials processing approaches, I succeeded in increasing charge extraction, reducing bandtail states, and lowering the barrier to carrier hopping in CQD solids. The benefits from enhanced charge extraction were demonstrated in a double-sided junction architecture enabled by the engineering of an electron-accepting layer. This architecture resulted in an increase in the width of the carrier depletion region and a resultant decrease in recombination. I elucidated the effect of bandtail states on carrier transport and designed a solution-phase ligand-exchange method to create CQD inks that can be deposited as an active layer in a single step. The resulting CQD films exhibited a flattened energy landscape that increased the carrier diffusion length and contributed to solar cells having certified solar power conversion efficiencies of 11.3%. I then explored the translation of this strategy to small-bandgap infrared CQDs. Through management of surface ligands, I improved CQD passivation while reducing CQD fusion. I conclude with a new strategy for the design of a hybrid material system that combined CQDs with epitaxially-grown inorganic metal halide perovskites. The matrix-passivated CQD films showcased a two-fold increase in carrier mobility and superior thermal stability compared to pristine CQDs. This work provides promising pathways to achieve more fully the potential of CQD solids, and to showcase these advances in improved performance for CQD solar cells.