Improved device performance stability of bulk-heterojunction hybrid solar cells with low molecular weight PCPDPTB

The molecular weight effect of low band-gap conjugated polymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)], PCPDTBT, with an optical energy gap of Eg similar to 1.4 eV, on the device stability of quantum dot polymer hybrid solar cells has been studied. The PCPDTBT is integrated into hybrid polymer/QDs solar cells, where PCPDTBT acts as electron donor and CdSe QDs as electron acceptor. Morphology investigation and results on device performance over time indicate that low molecular weight PCPDTBT is preferable to fabricate high efficiency hybrid solar cells with long-term stability. An average power conversion efficiency of 2% was measured for PCPDTBT-CdSe QD devices with a molecular weight of < 12 kDa. The long-term stability of the photovoltaic performance of the devices was investigated and found superior compared to devices with PCPDTBT of higher molecular weight. About 96% of the PCE remained after a week of storage of solar cell devices without any further encapsulation in a glove-box. It is assumed that the improvement is mainly due to the better distribution of CdSe QDs within the polymer matrix of low molecular weight PCPDTBT in hybrid solar cells and an improved preservation of the nanomorphology of the hybrid film over time.