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In contrast to the gradual development of mature wafer-scale silicon photovoltaic (PV) technologies, more recent development of nanostructured PV devices has experienced an explosive growth. These materials are solution-processable and as such can be deposited on flexible substrates, in a form of semitransparent devices, and potentially at low cost. In this chapter, we review our present understanding of the basic mechanisms that govern operation of nanowire-based bulk heterojunction PV devices by focusing on current limitations and future opportunities. In particular, colloidal quantum dots (QDs) have been studied as promising candidates for single-junction and tandem solar cell applications due to their direct and tunable band gap in the visible and near-IR spectral regions. However, a mismatch between the optical absorption length and the carrier collection length prevents these devices from achieving optimal photocurrent generation. To enhance charge collection, it is possible to combine (1) hydrothermally grown ZnO nanowire arrays to form an ordered bulk heterojunction and (2) a band alignment engineered PbS QD film that utilizes inorganic and organic ligands to generate cascaded energy-level offsets. Furthermore, the hydrothermal method enables the growth of high-quality ZnO nanowires on graphene electrodes resulting in flexible semitransparent devices. |
