Date of Award
Doctor of Philosophy
Electrical and Computer Engineering
Vikram L. Dalal
Polymer solar cell has been an active field of research in recent years. Thin film inorganic and organic solar cells are gaining significant momentum to outperform the already commercialized high efficiency crystalline counterpart. However the fundamental limitation of inorganic solar cell is limited number of active layer material. Organic conjugated polymer presents huge prospect in providing flexibility in designing photo active organic molecules which allows us to have tunable optical as well as electrical properties. Recent use of lower band gap polymer such as PCDTBT(6-7)%, PDTSTPD(6.7%), p-DTS(FBTTh2)2(7%), PTB7(9.2%-9.6%) has shown promising future for reaching highly efficient single junction solar cell. By using two different polymer or small molecule in two different tandem structure namely parallel tandem and series tandem were shown to enhance the efficiency in excess of 10.5% - 12%.
However degradation is the fundamental drawback that has limited the widespread commercialization of organic photovoltaic. Both in ambient atmosphere as well as under light, organic photovoltaic tend to degrade rapidly with time. In this work we report on the measurement of fundamental properties such as deep defects and hole mobility in poly-3-hexyl-thiophene (P3HT)/[6,6]-phenyl-C60-butyric acid methyl ester(PCBM) solar cells when exposed to solar radiation without any atmospheric exposure. It was observed that light itself can contribute significantly to the degradation of organic photovoltaic. It was found that the mid-gap defect densities in P3HT and the interface density between P3HT and PCBM increase upon light soaking. Such increase in defect density leads to an increase in reverse saturation current of the diode which can be correlated with the decrease in basic parameters of OPV that determine the final power conversion efficiency of solar cells. It was also found that thermal treatment after photo degradation can recover the solar cell performance completely. Some simultaneous processes consisting of metal cathode diffusion into active layers were found to influence the post production annealing of organic solar cells with regular architecture. Photo degradation study on inverted organic solar cells were performed and found that similar degradation behavior observed irrespective of the cell architecture which corroborated the fact that photo induced degradation in organic solar cells are an active layer phenomena which is intrinsic to the blend properties and does not depend on any interface layers.
Bhattacharya, Joydeep, "Understanding the physics of degradation of polymer solar cells" (2013). Graduate Theses and Dissertations. 13475.