Ternary Blend Bulk Hetero Junction Organic Solar Cells and Isoindigo Organic Solar Cells

This thesis contains two parts to investigate two new types of organic solar cell. The first one is about adding third polymer in the binary blend type of organic solar cells, thus it can enhance the light harvesting or generate more bi-continues phase that can achieve better phase separation in the active layer, finally accomplish the purpose of cell performance improvement; In the second part, we aim to investigate the structure effect of Isoindigo based conducting polymer on the performance of solar cell. This polymer exhibits low bandgap and is available from plant. We use it to make photovoltaic device and tune process conditions to optimize the performance of solar cell.
In the first part, the experimental systems are poly(3-hexylthiophene) [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) and Poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b'']dithiophene-2,6-diyl]]-C61-butyric acid methyl ester (PCPDTBT: PC61BM), P3HT has good crystalline phase after annealing while PCPDTBT has better absorption in the NIR spectra and high Voc. In our experimental design we add low weight ratio of PCPDTBT and P3HT respectively in the P3HT:PC61BM and PCPDTBT:PC61BM binary devices and they thus become ternary blend devices.
We analyze the device by measuring the absorption spectra and external quantum efficiency (EQE), and find that both absorption intensity and external quantum efficiency of P3HT:PC61BM:PCPDTBT are obviously enhanced in the near infrared-visible spectrum compare to the P3HT:PC61BM binary system. In the case of PCPDTBT:PC61BM:P3HT, though the absorption intensity rises in the region related to the added P3HT, the enhancement of external quantum efficiency increases in almost from the UV-NIR region. For going into the details of this different causes of performance improvement, we utilize the atomic force microscopy (AFM) to observe those two systems, and we discover that the surface of P3HT:PC61BM:PCPDTBT smooth and better dispersed than that of P3HT:PC61BM, and the aggregations decreases; In contrast, PCPDTBT:PC61BM:P3HT ternary dissolved in CB+3% DiO has rougher surface, larger aggregation, thus contribute a decrease rate of charge recombination rate. Then we aim at PCPDTBT:PC61BM:P3HT system and further analyze it by transmission electron microscopy (TEM) and grazing incidence X-ray diffraction (GIXRD), thus acquire the PCPDTBT domain and PCBM clusters and detect the signal of P3HT (100) planes.
For enhancing the light harvesting in visible spectrum and tuning the nano-morphology of thin film and binary system, we include third polymer in the original binary blend system, after optimizing the process, the best power conversion efficiency of P3HT:PC61BM:PCPDTBT system is slightly above 4.0%. It is about 10% higher than the highest one of P3HT:PC61BM: binary blend device which is due to the additional absorption in near infrared-visible region. The best PCEs value of PCPDTBT:PC61BM:P3HT dissolved in CB+ 3% DiO is 3.3%, which is also 10~20% higher than that of PCPDTBT:PC61BM. We can attribute the improvement to the morphology changes which result in moderate phase separation, so the above-mentioned phenomena confirm our prediction from the experimental design of
this study.
The second topic in the thesis is concentrated on the effect of Isoindigo device, a widely-used polymer in dye industry from one century ago, also a low band gap material, which is suitable for light harvesting donors for organic solar cells when blended with PCBM. We have evaluated four isoindigo polymers with different side-chain:PC8Ie, PCeI8, PC8I8,and PCeIe which are synthesized in our laboratory. In the attempt to optimize their device performance, we change the process by adding additive, different mixing ratio, and processing spin rate, because the structure of side chain (linear or branch), the length of side chain and the concentration of PCBM all can influence the charge mobility. Eventually, we obtain the best power conversion efficiency on different polymers: 4.0% in PCeIe:PCBM, 3.6% in PCeI8:PCBM, 3.3% in PC8I8:PCBM. Then, we measure and analyze the absorption spectra, external quantum efficiency, atomic force microscopy, and grazing incidence x-ray diffraction series of the active layer of these devices, thus we can provide useful information for manufacturing Isoindigo devices.