Tailoring band alignment for improved charge extraction of colloidal quantum dot photovoltaics

Abstract

Due to limitation of resources and environmental issue of conventional energy sources, demand for renewable energy is increasing. Though the silicon photovoltaics are highly developed device, next generation photovoltaics are still in need in terms of utility and low processing cost. Next generation photovoltaics are solution processible, and should be fabricated in thin film, applicable for flexible device. Among them, colloidal quantum dot photovoltaics (CQDPV) have great potential in terms of efficiency and stability. The development of CQDPV started in 2010, and reached the best efficiency of 10.7% in 2015, and all inorganic passivated colloidal quantum dots (CQD) are very stable in air.1 But still further efficiency improvement is required to compete silicon photovoltaics. To prevent schottky junction formation, hole transport materials are inserted between active layer and top metal. Many materials have been studied and so far, n-type transition metal oxide, such as molybdenum oxide and vanadium oxides are found to provide the best efficiency to the device. However, molybdenum oxide is poor hole conductor and unstable in air. Due to its poor hole conductivity, the thickness of molybdenum oxide has to be delicately controlled

otherwise, the device loses its performance, and current density characteristic becomes poor. Also, oxidation of vacancy oxygen site of molybdenum oxide is very critical for cell performance. In 10 days, the photovoltaic loses its performance under 25% of its own. In this research, we used PEDOT:PSS layer as hole transport material. Diluting PEDOT:PSS solution in methanol could successfully reserve active layer quality, and current density was increased around 20%. Additional alpha-sexithiophene $(\alpha -6T)$ layer could boost open circuit voltage by blocking leakage current. Α-6T has huge band gap of 2.2eV which forms huge potential barrier. Also $\alpha -6T$ could improve the stability. Due to hygroscopic PEDOT:PSS, colloidal quantum dot layer is degraded and results in poor device performance. But $\alpha -6T$ could block direct contact and improve stability and efficiency. Compared to Molybdenum oxide device, $\alpha -6T$ and PEDOT:PSS layer could improve the efficiency from 5.8% to 8.06%.