{"id":5360,"date":"2012-07-18T22:28:43","date_gmt":"2012-07-18T22:28:43","guid":{"rendered":"https:\/\/mtlsites.mit.edu\/annual_reports\/2012\/?p=5360"},"modified":"2012-07-18T22:28:43","modified_gmt":"2012-07-18T22:28:43","slug":"solution-processed-nanowire-based-quantum-dot-photovoltaics","status":"publish","type":"post","link":"https:\/\/mtlsites.mit.edu\/annual_reports\/2012\/solution-processed-nanowire-based-quantum-dot-photovoltaics\/","title":{"rendered":"Solution-Processed Nanowire-based Quantum Dot Photovoltaics"},"content":{"rendered":"

Thin-film solar cells incorporating colloidal quantum dot active layers have recently emerged as a notable third-generation photovoltaic (PV) technology, largely due to the strong absorption, tunable infrared bandgap, and ambient-atmosphere stability of lead sulfide quantum dots (PbS QDs). Photoactive PbS QDs can be solution-deposited on a transparent zinc oxide (ZnO) film to form a depleted np-heterojunction device (Figure 1a,b). However, this standard planar architecture incurs a fundamental trade-off between light absorption and carrier collection: to absorb most incident light, we need a ~1-\u00b5m-thick QD film [1<\/a>] <\/sup>, but to collect most photocarriers, we need absorption to occur within a minority carrier diffusion length (~100 nm) of the ~150-nm-thick depletion region [2<\/a>] <\/sup>. By introducing 1-D nanostructures (Figure 1c), we can decouple these parallel requirements and optimize for each independently. A vertical, QD-infiltrated array of ZnO nanowires orthogonalizes the mechanistic length scales of absorption and collection. Absorption is maximized as light traverses a thick QD film in the axial direction, while field-driven carrier collection is retained throughout the film as photogenerated electrons drift to nearby PbS\/ZnO interfaces in the radial direction.<\/p>\n

Our research demonstrates that moving from a planar ZnO film to a nanowire array can significantly improve QDPV performance, increasing short-circuit current density (JSC<\/sub><\/em>) by ~40% and overall power conversion efficiency by ~15% [3<\/a>] <\/sup>. We confirm the near-complete infiltration of PbS QDs into the ZnO nanowire array via cross-sectional scanning electron microscopy (Figure 1d) and elemental mapping with energy-dispersive x-ray spectroscopy. We further demonstrate a fast solution treatment to assist interfacial charge transfer using a bifunctional linker molecule, 3-mercaptopropionic acid (MPA). A simple MPA treatment increases both JSC<\/sub><\/em> and open-circuit voltage (VOC<\/sub><\/em>) of nanowire-QD devices (see Figure 2). Our work on ZnO nanowire-based QD solar cells\u2014along with the recent demonstration of a 5.6%-efficient TiO2<\/sub>nanopillar-based QDPV [4<\/a>] <\/sup>\u2014suggests that 1-D nanostructures may be the key to enhancing the efficiency and hence the economic viability of quantum dot photovoltaics.<\/p>\n\n\t\t