Fabrication And Study On High-Performance Organic Solar Cells

Organic solar cells (OSCs) have been considered as one of the most promising technology for economically viable alternative energy source owing to their advantages of being potentially low-cost, flexible, light-weight, large-scale roll-to-roll fabrication process and have the versatility of material design. Over the past two decades, significant progresses have been made in OSCs by virtue of rapid advances in material development, comprehensive investigation of novel device architecture, and in-depth understanding of the photophysical process in the device. Although OSCs have approached commercialization, the high cost, short lifetime, and low efficiency still need to be addressed. Therefore, the synthesis of novel materials, optimization of device architecture and understanding of device mechanism are still playing an important role to further improve the device performance. In this dissertation, an integrated approach is taken to improve the overall performance of OSCs by systematical investigation of new organic semiconductor materials, interface engineering of the contacts between layers, new transparent electrodes, and area-scaling effect.1. The dependence of fill factor (FF) on material properties, e.g. hole mobility and highest occupied molecular orbital (HOMO) energy level has been investigated in planar heterojunction (PHJ) OSCs with various donors based on Poole-Frenkel model and charge trapping effect. A wide bandgap phosphorescent material of bis[2-(4-tertbutylphenyl)benzothi azolato-N, C2,] iridium (acetylacetonate) ((t-bt)2lr(acac)) with low HOMO energy level has also been introduced as a donor to study the effect of layer thickness on the FF. The simulation results of the photocurrent density-voltage characteristics of the devices revealed that the charge collection efficiency was significantly enhanced in high mobility cells, which was the main factor that affected the FF. The FF and efficiency were significantly improved by 26% and 35%, respectively, in (t-bt)2Ir(acac)/C6o device by heating the substrate at 70℃ during the (t-bt)2Ir(acac) deposition. This is due to the increased hole mobility in (t-bt)2Ir(acac) film, which is one-ordered higher than that of the unheated film.2. The effect of 4-(5-hexylthiophene-2-yl)-2,6-bis(5-trifluoromethyl) thiophen-2-yl) pyridine (TFTTP) as a cathode buffer layer on the performance of OSCs based on subphthalocyanine (SubPc)/C60 heterojunction was systematically investigated. The results showed that with the introduction of 10 nm TFTTP layer, the power conversion efficiency of the conventional device was increased from 0.54% to 1.25%, which is 20% higher compared with device using bathocuproine (BCP) buffer layer. It was found that the enhancement of device performance was mainly contributed from the improvement of charge transfer exciton (CTE) dissociation efficiency due to the increased built-in potential (Vbi). In addition, the use of TFTTP layer was beneficial for the formation of good Ohmic contact between C6o and Ag cathode, facilitating more efficient charger collection. Moreover, the simulation of optical field distribution inside devices showed that the optical spacer effect of TFTTP was minimal.3. The influence of cesium carbonate (Cs2CO3) cathode buffer layer and molybdenum oxide (MoO3) anode buffer layer on the electrical and optical characteristics of the device with an inverted structure was studied. The results showed that by inserting Cs2CO3 between indium tin oxide (ITO)/C6o interface, the efficiency of SubPc/C60 based device was remarkably enhanced by 173%. This was attributed to the increase of charge transfer efficiency and charge collection efficiency due to the better contact between ITO and C60 and a lower series resistance. It was also found that due to the better protection of C60 layer from air, the stability of inverted devices is better than that of the conventional devices with the lifetimes (T50) of 1440 and 135 min for inverted and conventional devices, respectively.4. High-performance non-fullerene organic solar cells based on a new acceptor perylene bisimides (di-PBI) have been demonstrated with combined molecular, interfacial, and device engineering. The results showed that by using a Pseudo-2D conjugated polymer donor, the PBDTT-F-TT:di-PBI based devices exhibited much higher performance compared with 1D PTB7:di-PBI system. In addition to the molecular engineering of donor/acceptor pair to obtain complementary absorption and matched frontier energy levels in bulk heterojuntion film, the comprehensive device and interface engineering have resulted in a higher device efficiency of～6%by using an inverted device configuration with a self assemble monolayer (SAM)-modified ZnO interface. Optical modeling using transfer matrix formalism reveals that better optical field distribution and exciton generation achieved in inverted device plays an important role in enhancing device performance. Moreover, the SAM-modification on ZnO was found to prevent trap-assisted recombination at the interface to further enhance device performance.5. To replace conventional ITO electrodes for fabricating large area polymer sola cells, highly conductive ultra-thin metal film (UTMF) has been investigated a transparent electrodes. The effect of transparent electrode conductivity and device are on photovoltaic parameters was also studied. The results showed that a significar increase of series resistance resulting from the poor conductivity of ITO transparer electrode is the main reason for the decrease of efficiency in large area devices. B applying a thin layer of ZnO as a seed layer and a 11-mercaptoundecanioc acid (MUA SAM to improve the quality of the ultra-thin Ag film, the transparency of UTMF wa improved by 30% while the electrode sheet resistance (～5Ω/sq) was decreased b 200% compared to the commonly used ITO electrode (～15Ω/sq). As a result, polyme solar cell with a maximum power conversion efficiency of 3.07% has been achieved i the OSCs with cell area as large as 10.0 cm2 using UTMF/sub-electrode hybrid structur for efficient charge carrier collection, which was approximately 58% efficiency of a analogous 0.1 cm2 device. Also, the flexible large area device using UTMF electrod was showed superior performance (115% higher) compared to the device made fro ITO electrode.