Emission Spectroscopy Investigation of the Enhancement of Carrier Collection Efficiency in AgBiS 2 -Nanocrystal/ZnO-Nanowire Heterojunction Solar Cells.

Eco-friendly solar cells were fabricated using interdigitated layers comprising ZnO nanowires (NWs) and infrared absorbing AgBiS 2 nanocrystals (ITO/ZnO NWs/AgBiS 2 /P3HT/Au). The quality of ZnO NWs was studied using photoluminescence and Raman spectroscopy to identify the defects in ZnO NWs influencing solar cell performance. Oxygen vacancies and Zn interstitial sites, among various recombination sites, were observed to be the main sites for carrier recombination, which hinders the carrier collection in the solar cells. Accordingly, the power conversion efficiency of AgBiS 2 solar cells exhibited a good correlation with the number of oxygen vacancies. The structural order and electron-phonon interaction in ZnO NWs were also investigated via Raman scattering spectroscopy. A lower concentration of oxygen vacancies and zinc interstitials (Zn i ) resulted in a higher structural order as well as a weaker electron-phonon interaction in ZnO NWs. When ZnO NWs were treated at 500 °C in oxygen with the lowest oxygen vacancy concentration, the solar cells (500-O 2 solar cell (SC)) demonstrated an external quantum efficiency of approximately 70% in the visible region and a corresponding internal quantum efficiency of more than 80%. The 500-O 2 SC exhibited a power conversion efficiency of 5.41% ( J SC = 22.21 mA/cm 2 , V OC = 0.41 V, and FF = 60%) under quasi one-sun illumination. New methods that can efficiently reduce oxygen vacancies and Zn i without affecting the structural order of ZnO NWs would further enhance the carrier collection efficiency. Moreover, since ZnO is a key electron transport material for constructing not only colloidal quantum dot solar cells but also other emerging solar cells, such as organic thin-film solar cells, the present findings provide significant information for improving their performance.