One-Step Synthesis of ZnO Nanocrystals in <i>n</i>‑Butanol with Bandgap Control: Applications in Hybrid and Organic Photovoltaic Devices

Solution-grown ZnO nanocrystals (NCs) offer a viable path for large-area, low-temperature processing required for optoelectronics on flexible substrates. They can function as the acceptor in hybrid photovoltaic (HPV) devices or the electron transport layer (ETL) in organic photovoltaic (OPV) devices. We demonstrate a one-step synthesis, using diethanolamine (DEA) and water as additives, to produce ZnO NCs in <i>n</i>-butanol with controlled bandgap. We found that the addition of DEA enhances ZnO NC dispersion, which facilitates the formation of smooth films. For a given DEA concentration, varying the water volume fraction in precursor solution controls the ZnO NC grain size, hence its bandgap. We studied the impact of ZnO bandgap on HPV and OPV device performance. While ZnO NCs with higher bandgap increase the open circuit voltage of bilayer polymer/ZnO HPV devices, ETLs of ZnO NCs with different bandgaps result in identical inverted OPV device performance. The ZnO NC suspensions in <i>n</i>-butanol also allow the fabrication of conventional OPV devices, as they form uniform thin films on top of organics without additional processing. Compared to devices with Ca/Al contacts, these conventional devices exhibit equivalent performance but superior stability in air. These ZnO NC <i>n</i>-butanol suspensions could be readily applied to other optoelectronic and photovoltaic applications, for which type II heterojunction is desired and low-temperature solution processing is required