| Solution-processed semiconductors, including both conjugated polymers and colloidal quantum dots, have the potential to significantly decrease the manufacturing costs of many optoelectronic devices. An increased understanding of the fundamental physics underlying device operation is critical to realizing this goal. In this dissertation, we present our work using a combination of spectroscopy, microscopy and device studies toward understanding photoinduced, interfacial charge transfer, which is one of the essential process in operating mechanism of these devices. In particular, we study blend films of semiconducting polymers and infrared absorbing colloidal quantum dots, such as PbS and PbSe, with applications to both photovoltaics and photodetectors.;While IR-absorbing quantum dots have long been recognized for their potential to harvest large fractions of the solar spectrum, devices made from bulk heterojunction blends of these materials with commonly studied conjugated polymers failed to operate as efficiently as most other material combinations. Using photoinduced absorption spectroscopy (PIA) to probe for long-lived polymer polarons, we demonstrated that blends of common polymers with PbSe do not undergo photoinduced charge transfer, and that this was the likely explanation of the low efficiencies measured for devices made from these blends. We hypothesized that the reason for our observation is that the energy levels of PbSe quantum dots do not form a type-II heterojunction with common polymers, meaning that photoinduced charge transfer is not energetically favorable for these particular material combinations.;Building on the results from these initial studies, we selected three new polymers, synthesized by collaborators, that we believed were likely to form type-II heterojunctions with PbS quantum dots. Using PIA, we screened blends of PbS quantum dots with each of the new polymers, finding one in particular, PDTPQx, that outperformed the others. Photovoltaics devices made from PDTPQx/PbS blends exhibited solar power conversion efficiencies 100x greater than any previously reported hybrid organic-inorganic blend devices incorporating IR bandgap quantum dots, demonstrating the efficacy of PIA as a screening technique.;Finally, we observed via PIA that the dynamics of photoinduced charge carriers in blends containing inorganic acceptors, such as quantum dots, are very different than all-organic blends with the former exhibited much longer recombination lifetimes. We report on some initial experiments that we've conducted to try to explain these dynamics, with a likely explanation being trapping by surface states on the nano-structured inorganic semiconductors. |
