Diketopyrrolopyrrole-based oligomer modified TiO2 nanorods for air-stable and all solution processed poly(3-hexylthiophene):TiO2 bulk heterojunction inverted solar cell

Diketopyrrolopyrrole-based oligomer was synthesized and used to modified TiO2 nanorods. The surface modified TiO2 was employed in the fabrication of air-stable and all solution processed poly(3-hexylthiophene):titanium dioxide nanorods (P3HT:TiO2 nanorods) bulk heterojunction (BHJ) inverted solar cells. The oligomer (copolymerized 4,5-diaza-9,9′-spirobifluorene with diketopyrrolopyrrole (PZFDPP)) was synthesized by Stille coupling reaction. The PZFDPP was coated on TiO2 nanorods by refluxing the TiO2 nanorods in oligomer containing solution at low temperature (70 °C). A concentration gradient profile of polymer/nanocrystals (P3HT/TiO2 nanorods) BHJ was observed for the first time by X-ray photoelectron spectroscopy (XPS) technique together with in situ ion sputtering, showing that the TiO2-rich region and P3HT rich region are aggregated adjacent to electron transport layer (ETL) and hole transport layer (HTL) respectively. The obtained depth profile indicates the inverted device structure is more suitable for polymer/inorganic nanocrystals BHJ solar cells. Furthermore, instead of using an energy consuming process for ETL layer deposition, the PZFDPP modified TiO2 nanorods were used to deposit the ETL layer by spin coating. The surface features and properties of deposited TiO2 ETL that was coated by PZFDPP were systematically investigated. The developed photovoltaic device shows a promising power conversion efficiency (PCE) of 1.2% benefited from improved electron mobility in P3HT:TiO2 BHJ film and across the ETL/active layer interfaces. Moreover, the device is extremely stable stored in ambient condition without encapsulation (less than 10% loss over 1000 h test). The results of this work demonstrate the successful development of highly efficient and air-stable polymer/inorganic nanocrystal hybrid BHJ inverted solar cells based on chemically modified nanocrystals which significantly extend the current knowledge of device fabrication.