Minority Carrier Transport in Lead Sulfide Quantum Dot Photovoltaics

Lead sulfide quantum dots (PbS QDs) are an attractive material system for the development of low-cost photovoltaics (PV) due to their ease of processing and stability in air, with certified power conversion efficiencies exceeding 11%. However, even the best PbS QD PV devices are limited by diffusive transport, as the optical absorption length exceeds the minority carrier diffusion length. Understanding minority carrier transport in these devices will therefore be critical for future efficiency improvement. We utilize cross-sectional electron beam-induced current (EBIC) microscopy and develop methodology to quantify minority carrier diffusion length in PbS QD PV devices. We show that holes are the minority carriers in tetrabutylammonium iodide (TBAI)-treated PbS QD films due to the formation of a p–n junction with an ethanedithiol (EDT)-treated QD layer, whereas a heterojunction with n-type ZnO forms a weaker n⁺–n junction. This indicates that modifying the standard device architecture to include a p-type window layer would further boost the performance of PbS QD PV devices. Furthermore, quantitative EBIC measurements yield a lower bound of 110 nm for the hole diffusion length in TBAI-treated PbS QD films, which informs design rules for planar and ordered bulk heterojunction PV devices. Finally, the low-energy EBIC approach developed in our work is generally applicable to other emerging thin-film PV absorber materials with nanoscale diffusion lengths

Dimension- and Surface-Tailored ZnO Nanowires Enhance Charge Collection in Quantum Dot Photovoltaic Devices

The use of zinc oxide (ZnO) nanowires improves charge collection, and consequently power conversion efficiency, in quantum dot (QD) based photovoltaic devices. However, the role of the nanowire geometry (e.g., density, length, and morphology, etc.) relative to the QD properties remains unexplored, in part due to challenges with controlled nanowire synthesis. Here, we independently tailor nanowire length and the active device layer thickness to study charge collection in lead sulfide (PbS) QD photovoltaic devices. We then demonstrate consistently high internal quantum efficiency in these devices by applying quantum efficiency and total reflectance measurements. Our results show that significant losses originate from ZnO nanowire–QD interfacial recombination, which we then successfully overcome by using nanowire surface passivation. This geometry-tailored approach is generally applicable to other nanowire–QD systems, and the surface passivation schemes will play a significant role in future development of nanostructured photovoltaics