Colloidal quantum dots (CQDs) are an attractive third-generation material for photovoltaics due to their solution-processability, lightweight and flexible nature, and bandgap tunability, allowing them to be used as infrared materials for multi-junction solar cells. Here, we describe several methods for building new lead sulfide-based CQD materials and thin films for improving efficiencies in both single-junction and multi-junction solar cells.

First, we demonstrate that the power conversion efficiency in single-junction PbS CQD solar cells is limited in part by the performance of the hole transport layer (HTL), traditionally made from ethanedithiol-passivated lead sulfide CQDs, due to the sub-optimal carrier mobility and doping density in this material. We use sulfur doping of the HTL, as well as incorporation of 2D transition metal dichalcogenide nanoflakes to address these issues and demonstrate absolute power conversion efficiency improvements of greater than 1% in single-junction devices.

Next, we demonstrate a micrometer-resolution 2D characterization method with millimeter-scale field of view for assessing CQD solar cell film quality and uniformity. Our instrument simultaneously collects photoluminescence spectra, photocurrent transients, and photovoltage transients. We use this high-resolution morphology mapping to quantify the distribution and strength of the local optoelectronic property variations in CQD solar cells due to film defects, physical damage, and contaminants across nearly the entire test device area, and the extent to which these variations account for overall performance losses. We also use the massive data sets produced by this method to train machine learning models that take as input simple illuminated current-voltage measurements and output complex underlying materials parameters, greatly simplifying the characterization process for optoelectronic devices.

Finally, we use artificial photonic band engineering as a method for achieving spectral selectivity in absorbing PbS CQD thin films for applications in multi-junction photovoltaics. We show that a structured periodic CQD thin film is able to maintain a photonic band structure, including the existence of a reduced photonic density of states, in the presence of weak material absorption, enabling modification of the absorption, transmission, and reflection spectra. We use a machine learning-based inverse design process to generate CQD thin film photonic structures with targeted absorption, transmission, and reflection spectra for multi-junction photovoltaics and narrow bandwidth photodetectors.