Study On The Charge Carrier Recombination, Transport Mechanism And Device Performance In Polymer Solar Cells

ABSTRACT:The physical processes in polymer solar cells provide significant guidance for the improvement of device power conversion efficiency. Charge carrier recombination, transport and collection are very important for enhancing the device photovoltaic characteristics in polymer:fullerene solar cells. The charge transfer state (CTS) recombiantion at the donor:acceptor interface has a direct influence on device open circuit voltage (voc) and short circuit current (Jsc). The interface modification between electrode and active layer can improve charge carrier transport and collection, therefore improve device performances.In this thesis, the absorptions and radiative recombianations of charge transfer state in polymer solar cells were firstly investigated by external quantum efficiency (EQE) response and electroluminescence (EL) measurements respectively. We found that except the characteristic responses of donor and acceptor materials, additional red-shifted CTS responses appeared. This CTS response energy position is normally consistent with the energy difference between the highest occupied molecular orbital (HOMO) level of the donor and the lowest unoccupied molecular orbital (LUMO) level of the acceptor. Besides, the relation between EQE and EL responses of CTS were studied by detailed balance theory, the calculated EQE curves from EL were consistent with the experimental EQE results. The device theoretical value of Voc was calculated using the measured EQE and EL of CTS, the offsets between calculated Foc and measured Voc were about0.3-0.45V. This offset was further reduced to0.15V by taking the EL external quantum efficiency (QLED) and characteristic energy (Ech) into account. The behaviour of the EL of CTS was studied as a function of applied voltage (or injection current). We found that the measured EL had a blueshift of the peak with increasing injection current, but this shift was much smaller that expected from simulation. The possible explanation is that the CTS recombination is unlikely without a preceding detrapping event that lifts one of the two carriers to a higher energy level, where it is free to move and find a partner for radiative recombination.The influence of buffer layer on charge carrier transport and recombination in polymer solar cell was investigated by capacitance-voltage (C-V) measurement. The C-V characteristic in polymer solar cell was firstly introduced, and the parameters that influence C-V curve were studied from the aspects of charge carrier transport and interface injection barrier. The effects of MoO3and NPB anode buffer on device performances were investigated, we found that proper thickness of buffer layer can improve device charge transport and reduce interface injection barrier, therefore enhance Voc and Jsc. Too thick of these buffer layers can induce large series resistance, therefore block charge injection and collection, further led to significant charge recombination. C-Vmeasurement was performed to investigate the effects of MoO3and NPB buffer layer on charge collection and recombination. The relation between Voc and the peak position of maximum capacitance (VCmax) was studied. We found that the Voc represents the balance of charge generation and recombination; the Vcmax. represents that the charge accumulation in acceptor LUMO states is suppressed by charge recombination. The values of Voc and VCmax are related but not the same.The influence of interface recombination on device performance was investigated. Firstly, the different of J-V and QLED characteristic between bilayer and bulkheterojunction was studied. Because of the unefficient exciton dissociation in bilayer devices, the Jsc are much smaller than the Jsc of bulkheterojunction structure devices. The influence of charge carrier mobility on QLED was studied, we found the larger charge carrier mobility is, it is more likely that the exciton recombination at the electrode will happen. The relation between Jsc and QLED was also investigated, the QLED represents the extent of CTS recombination, the less the recombination extent is, the more charge will be collected, and led to large value of EQE and Jsc. It is noticed, large proportion of the radiative recombination (large QLED) is better for Voc, but small proportion of the radiative recombination (small QLED) is expected for Jsc, so there is optimal QLED value.