Interface Investigation On High Efficiency And Stable Organic Solar Cells

Organic solar cells(OSCs) have attracted more and more attention because of their easy manufacturing process, flexibility, light weight and low cost. The studies of OSCs present great significance and potential value for the development and application of renewable energy. The power conversion efficiency(PCE) of polymer solar cells based on blends of conjugated polymers and fullerene derivatives has reached more than 12.0%, yet there are still lots of problems need to be solved, such as complex synthesis of polymer materials, stability of polymer solar cells and environmental issues since most of spin-coating interfacial materials requires organic solvents. Also despite the PCE of organic-inorganic hybrid perovskite solar cells have reached 20%, there are still two main challenges in the fabrication process of solution-processed perovskite films involve controlling the crystallization process and ensuring high film quality. In this thesis, our goal is to optimize the fabrication process of the OSCs in order to increase device PCE and stability, develop environmentally friendly technology of fabricating interfacial layer and understand the physical processes of device. Our research mainly includes the following four parts:1. We demonstrate an environmentally friendly way for the fabrication of metaloxide materials. Various metal oxide material films are obtained by directly dissolving their powder into deionized water, such as MoO3, GeO2, V2O5, CrO3. Device stability has been largely enhanced by using these aqueous solution-processed materials as hole transport layer. The aqueous solution-processed MoO3-based OSCs exhibit comparable PCE with PEDOT:PSS based devices. However, even more importantly, the stability of aqueous solution-processed based devices has been greatly improved in air. The Interface Investigation on High Efficiency and Stable Organic Solar Cells Abstract aqueous solution-processed GeO2 and V2O5 film have been used successfully with improved PCE and device stability. The calculations of optical intensity in a whole cell demonstrate that a thin layer of GeO2 and V2O5 film could function as an optical spacer in the based OSCs for enhancing the light harvesting in the active layer. Interfacial evaluation by impedance spectroscopy shows that the GeO2 and V2O5 film based cell exists less charge carrier recombination and lower contact resistance. Moreover, the aqueous solution-processed film fabrication is very simple and environmentally friendly, which has potential applications in green and low-cost organic electronics in the future.2. Solution-processed Cs2CO3 thin film obtains work function adjustability viaMoO3 and/or Na2WO4 doping. The doped Cs2CO3 is successfully used as a cathode interfacial layer in P3HT:IC60BA based solar cells with improved open-circuit voltage and unaffected short-circuit current density. X-ray photoelectron spectroscopy evaluation is conducted to verify the formation of the new composites of W–O–Cs and Mo–O–Cs after doping of MoO3 and/or Na2WO4 into Cs2CO3. The change of the work function of MoO3- and/or Na2WO4-doped Cs2CO3 is further confirmed by ultraviolet photoelectron spectroscopy measurements.3. Light absorption in OSCs creates strongly bound electron-hole pairs(excitons)which need to diffuse to a donor/acceptor interface to be dissociated into free charges. Whereas, there exists a trade-off between light absorption and exciton harvesting efficiency because of complicated design and optimization of OSC devices. The effects of localized surface plasmon resonance LSPR in an inverted polymer/fullerene solar cell by incorporating Au and/or Ag nanoparticles(NPs) into the TiO2 buffer layer has been investigated. Enhanced light harvesting via plasmonic resonance of metal NPs has been observed. It results in improved short-circuit current density while the corresponding open-circuit voltage is maintained. A maximum PCE of 7.52% is obtained in the case of introducing 30% Ag NPs into the TiO2, corresponding to a 20.7% enhancement compared with the reference device without the metal NPs. The device photovoltaic characteristics, photocurrent properties, steady-state and dynamic photoluminescences of active layer on metal NP-doped TiO2, and electric field profile in metal NP-doped TiO2 layers are systematically investigated to explore how the plasmonic effects of Au and/or Ag NPs influence the OSCs performance.4. CH3NH3PbI3 is commonly used in perovskite solar cells due to its long diffusionlength and good crystallinity. Commonly, these perovskite films are annealed at 100°C or higher and the annealing time is long. In order to improve the film quality for the CH3NH3PbI3 in the perovskite cells, we present two steps annealing and a novel low temperature annealing approach of gradually increasing the temperature to fabricate perovskite films. With the two steps annealing, device performance has been improved. Various temperatures and temperature ranges for the formation of perovskite films have been studied. Using the gradual annealing process, the quality of perovksite film and importantly the device performance have been influenced. The results show that through the optimized process, the film quality is improved with high surface coverage and good photoluminescence and reproducibility. The device using gradual annealing at 60°C-80°C has achieved a PCE with a value of 13.3%(average) and 14.9%(best) with no obvious hysteresis.