Optical Performance Analysis Of Organic Solar Cells

With the increasing demand for energy in human society and the growing concern about climate issues, solar energy as a clean, renewable resource, is more widely used. Because of their low cost, flexibility and other advantages, the organic solar cells have attracted much research attention and made many achievements in recent years. The optical analysis of active layer can provide a direction for the further research of organic solar cells.In this thesis, the bulk heterojunction solar cell with the P3HT: PCBM material system as the active layer is used as the research object. And the UV- visible absorption spectroscopy, photoluminescence spectroscopy and Raman spectra are used to investigate the active layer. Through analysis, the following results are obtained:Firstly, in the P3HT: PCBM active layer, the principal light absorption material is P3 HT.Its absorption is due to the π-π* electronic transition in the conjugated structure in the main absorption range of 300-650nm; its main photoluminescence peaks are at 640 nm, 710 nm and 780 nm. Compared with P3 HT material, the P3HT: PCBM layer has lower absorption intensity in absorption spectroscopy, and there is the quench of the intensity in photoluminescence spectroscopy. The peaks in both kinds of spectroscopy appear blue shift. The Raman spectra test results shows that the addition of PCBM can destroy the order of conjugated chains in polymer. However, the energy levels of P3 HT and PCBM match well, that can provide convenience for the process of exciton dissociation. By increasing the ratio of PCBM in the active layer, the absorption of the active layer decreases. When the ratio is 1:0.8, the photoluminescence intensity is the lowest, and combined with the performance results of the corresponding device, the cell efficiency and short-circuit current density reach the highest value. The explanation for this phenomenon is properly that the interactive network in the active layer is formed best. Meanwhile, the open circuit voltage of the devices is gradually rising with the increasing proportion of PCBM. The congestion of PCBM near the electrodes may make a contribution to the change of energy which is inferred from Raman spectroscopy results.Secondly, annealing treatment will improve the performance of P3HT: PCBM solar cells. Through the optical analysis, after annealing, the P3HT: PCBM active layer has enhanced absorption intensity with the peaks movement to the long wavelength direction, while the photoluminescence intensity quenches and the peak intensity in X-ray diffraction pattern increases. Anneal can increase the crystallization degree of P3 HT in the active layer, which not only make phase separation of two materials, but also increase the carriers mobility. When the annealing time and temperature rise, the phase separation distance is large that may be a negative impact on the exciton dissociation. The increase on the mobility will improve the carriers transport efficiency. So there will be a balance point in this process. After spectra analysis and the cell efficiency measures, a temperature of 150 ℃ for 15 minutes will be the optimal setting.