Stability and Photovoltaic Performance Studies of Bulk Heterojunction Polymer Solar Cells

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Addis Ababa Universty


This dissertation reports a research done on the stability and photovoltaic performance studies of bulk heterojunction polymer solar cells. Different techniques were used, in order to study the stability and performance of polymer solar cells fabricated using different polymers and fullerenes. Two fullerene derivatives with different electron affinity were used to fabricate polymer solar cell based on P3HT as donor. Results obtained from UV-visible, FTIR and J-V measurement showed that the P3HT:PCBM film and solar cells fabricated from P3HT:PCBM as active layer have much better air stability than those made from P3HT:ICBA as active layer. Due to lower electron affinity of ICBA, the blend of P3HT:ICBA photobleached at a faster rate compared to PCBM. Effect of sol-gel synthesized TiOx layer on the stability and performance of P3HT:ICBA based solar cells were investigated by spectroscopic and electrical measurement methods. Results from J-V measurements indicates that P3HT:ICBA/TiOx solar cell has better performance and stability compared to the reference device. The higher value of shortcircuit current density and open-circuit voltage for P3HT:ICBA/TiOx solar cell indicate the role of TiOx layer as the optical spacer and hole blocking layer, respectively. The results from UV-visible absorbance, J-V and C-V measurement on light irradiated P3HT:ICBA/TiOx solar cell demonstrates that TiOx layer can be used to protect P3HT:ICBA active layer from oxygen and water attack under ambient air and to improve the working lifetime of P3HT:ICBA based device. Investigation on the effect of two fullerene derivatives with similar electron affinity on the photovoltaic performance and stability of P3HT based solar cells is presented. From UVvisible measurements it is found out that P3HT mixed with both PC61BM and PC71BM has good ambient air photochemical stability. However, device based on P3HT:PC61BM has better stability compared to P3HT:PC71BM based device. The impact of two different fullerenes with similar electron affinity on the photovoltaic performance and stability of PCDTBT based solar cells were studied. Results from UVvisible absorbance indicate that unlike that of pure film of PCDTBT, blend of PCDTBT with PC61BM, PC71BM and 1:1 mixture of PC61BM and PC71BM has relatively constant absorbance under light exposure. The results show the stabilizing power of acceptor fullerenes when mixed with PCDTBT. The results from J-V measurement indicates that the solar cells made from PCDTBT:PC61BM and PCDTBT:PC61+71BM have better stability under light exposure in ambient air compared to the device based on PCDTBT:PC71BM. When only PC71BM is used as acceptor the device has higher rate of decay of PCE. The effect of DIM on the PCE and stability of PSCs made of PCDTBT:PC71BM is investigated. PCE of the device based on PCDTBT:PC71BM processed with DIM is higher than the reference device. In terms of device stability, the PSCs processed with DIM showed poor stability at longer light exposure time. For the device without DIM especially as the light exposure time was increased, the device stability was better. The result from IS measurement shows that for pristine PCDTBT:PC71BM devices with DIM, the active layer resistance is lower compared to device without DIM. However, after irradiating the device for 5 hr the resistance of the device processed with DIM is higher and it is consistent with decreased PCE of aged device. The investigation presented on the application of NIL for performance improvement of organic solar cells shows that, NIL is really promising technique for improving efficiency of polymer solar cells. Nanopatterning of the PEDOT:PSS electrode can be used as a way to improve the cell efficiency. In our case relative increase of the 60%, from 0.8% to 1.3% of efficiency is obtained. The increased interfacial area and higher hole collection contributed for the improved photovoltaic performance of nanopatterned device. Keywords: polymer solar cells, degradation, photochemical stability, lifetime, charge carrier mobility, P3HT, PCDTBT, ICBA, PC61BM, PC71BM, TiOx, DIM, nanopatterning



polymer solar cells; degradation; photochemical stability; lifetime; charge carrier mobility; P3HT; PCDTBT; ICBA, PC61BM; PC71BM; TiOx; DIM; nanopatterning