Nanostructured photovoltaics : improving device efficiency and measuring carrier transport

Abstract

Photovoltaics (PV) offer a promising route to combat climate change. However, the growth rate of the dominant commercial photovoltaic (PV) technology is limited by large capital expenditure requirements. This motivates fundamental research into thin-film materials, such as lead sulfide (PbS) quantum dots (QDs), that are composed of earth-abundant elements, can be produced through low-cost deposition techniques, and are stable under operating conditions. In this thesis, a device architecture that combines a zinc oxide (ZnO) nanowire ordered bulk heterojunction (OBHJ) architecture with band alignment engineering of the PbS QD film to enhance charge extraction is demonstrated. This approach results in PV devices with photocurrent density greater than 30 mA/cm2, which represents a 15% improvement compared to planar devices and enables solar cells with power conversion efficiency up to 9.6%. This photocurrent density is the highest achieved for QDs with a 1.3 eV band gap, which is the optimal band gap in the detailed balance limit. The enhanced photocurrent in the nanowire devices is shown to be a result of both improved light harvesting due to improved in-coupling of light after the addition of the ZnO nanowire array and improved carrier collection due to the bulk heterojunction effect. Furthermore, electron beam-induced current (EBIC) was used to study charge transport in PbS QD films. It is shown that holes are the minority carrier in PbS QD films treated with tetrabutylammonium iodide (TBAI). This finding indicates that the thickness of OBHJ devices composed of a PbS-TBAI film paired with an n-type nanowire array are constrained by minority carrier transport. Moreover, quantitative EBIC was applied for the first time on PbS QD diodes to measure the bulk minority carrier diffusion length (Lbulk). Lbulk was extrapolated by comparing the effective diffusion length measured at different beam energies. EBIC injection leads to high-level injection conditions, therefore a lower bound for the hole diffusion length in PbS-TBAI QD films is established, with Lbulk e 110 nm. This provides a critical design parameter for OBHJ solar cells. This thesis motivates further work on optimization of ZnO nanowire arrays for PbS QD OBHJ solar cells through array patterning, acceptor-doping, and passivation of the nanowire surface. Furthermore, the EBIC technique developed in this work can be applied to quantitatively measure nanoscale carrier diffusion lengths in other thin-film PV materials.