Mechanism investigation and structure design of organic photovoltaic cells for improved energy conversion efficiency

Abstract

The performance of organic solar cells (OSCs) is severely limited by poor light absorption, exciton dissociation and charge transport. This challenge can be partially overcome through the use of the bulk heterojunction (HJ) solar cell structure because it has the potential to guarantee the effective exciton dissociation and carrier transport by forming the interpenetrating network. The purpose of this thesis is to investigate the mechanisms of bulk HJ solar cells and then design the novel solar cell structure to improve the power conversion efficiency. In this thesis, the microscopic mechanisms of the short circuit current density (JSC) and the open circuit voltage (VOC) in bulk HJ solar cells are investigated. JSC suffers from the serious optical interference effect, non-ideal exciton dissociation probability, low mobility and short carrier lifetime. All these factors are investigated and considered to predict JSC. Another parameter, VOC has a direct relationship with the offset energy between the donor (D) and the acceptor (A) materials both in layered and bulk HJ solar cells. However, in the two types of devices, VOC shows different dependences on the electrodes. VOC of layered HJ OSCs shows a very weak dependence on the electrodes, while VOC of bulk HJ solar cells shows a strong dependence on the electrodes. It is suggested that their distinct structures lead to the different dependences of VOC on the electrodes, although VOC of the two types of solar cells follow the same mechanism and are mainly determined by the light injected carriers at the D/A interface and the electrodes. Based on the above understandings, experimental studies are carried out to increase JSC and enhance VOC of the poly(3-hexylthiophene-2,5-diyl): [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells. It is found that the sequence of the thermal annealing is critical for the performance of the polymer-fullerene bulk HJ solar cells. The post-annealed device shows a higher JSC. This is attributed to the improved contact at polymer/aluminum interface, the improved phase-structured morphology due to the prohibition of the overgrowth of PCBM and the enhanced P3HT crystallinity. It is also found that a significant increase of VOC is obtained in polymer-fullerene bulk HJ solar cells by using e-beam evaporated Al cathodes. This is because that the energetic particles of Al in the e-beam deposition damage the surface of P3HT and induce deep hole traps at the P3HT/Al interface while leave fullerene unaffected. These deep hole traps will induce the negative image charges in the cathode and form ¿dipoles¿. The ¿dipoles¿ lower down the Al effective work function and induce a very strong increase of VOC. Based on these findings, the post-annealed devices with the e-beam Al cathodes are optimized around the first and second optical interference peaks. At last, a simple tandem structure design for efficient light harvesting is proposed. In this device structure, PCBM is employed simultaneously to form a bilayer HJ photovoltaic (PV) subcell with the underlying CuPc and a bulk HJ PV subcell with the blended P3HT. In comparison with the conventional tandem structure, the omission of the semitransparent intercellular connection layer reduces the complexity of the device and the light loss. This structure effectively improves JSC and the overall power conversion efficiency (PCE).