| Summary: | 博士 === 國立交通大學 === 光電工程系所 === 96 === The power conversion efficiency of organic photovoltaic devices up to ~5% has been achieved recently by creating organic p-n bulk-heterojunction, such as that of p-type poly(3-hexylthiophene) (P3HT) and n-type [6,6]-phenyl C61-butyric acid methyl ester (PCBM), in the active layer. Because of the advantages, such as low-cost, light-weight, flexible and large area fabrication capability, developing high throughput and efficiency methods are thus essential to enable the photovoltaic system more competitive in price and performance.
Solvent annealing is one of the candidates to improve the efficiency of solar cells. For investigating the effect of solvent annealing on the performance of polymer solar cells, we studied two important parameters such as the composition ratio and solvent evaporation by several measurements and morphology characterizations. The optimized composition ratio of P3HT/PCBM was 1 to 1 from light J-V curves and EQE value. When the concentration of PCBM increases, the light absorption and crystallinity of P3HT were reduced. The confocal laser scanning microscopy (CLSM) shows that the uniform and large amounts of interface area were obtained while ratio is equal to one. As a result, the moderate quantity of PCBM can effectively create exciton dissociation cites, to produce free electrons and holes and improve the performance of polymer solar cells. As for the influence of solvent evaporation, the crystallinity and light absorption was improved in longer drying time. In addition, red-shift of vibration peaks indicates the conjugated polymer chain has closed packing structure. From the optical sitmulation, the higher absorption in the active layer was resulted from optimized index-matching to form grading structure in the multi-layer device architecture.
To increase the fabrication throughput, we demonstrated a microwave annealing method to treat the polymer devices and found that it will increase the performance of the photovoltaic cells. The microwave irradiation will selectively heat the active layer and cathode and reduce energy loss in the ITO substrate. The metal cathode heat by microwave is due to the thickness which is lower than the penetration depth of microwave. Finally, the optimized efficiency of polymer solar cells can be improved to 3.6% in only 1.5 min. The microwave is an efficient way to improve the performance of the photovoltaic device.
To pursue higher power conversion efficiency in polymer PV, highly conductive PEDOT:PSS was used to reduce the series resistance in equivalent circuit of the polymer solar cells When the mannitol doped in the PEDOT:PSS, the conductivity was 40 fold improvement compared to the pristine device. The power conversion efficiency was improved and series resistance was reduced as mannitol concentration increased. Comparing with the pristine PEDOT:PSS,. Thus, the short circuit current increased from 12 to 14.7 mA/cm2. Finally, the optimized power conversion of polymer solar cell was 5.2% with 60mg/mL manntiol doping.
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