Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV16)
Publication date: 28th March 2016
The rapid degradation of organic photovoltaic (OPV) devices compared to conventional inorganic solar cells is one of the critical issues that have to be solved in order to make OPV a competitive commercial technology. The understanding of the fundamental mechanisms that reduce the power conversion efficiency (PCE) over time will be beneficial for the design of new materials with enhanced stability. We focus on bulk heterojunction organic solar cells based on thieno[3,4-b]thiophene-alt-benzodithiophene (PTB7) mixed with [6,6]-phenyl-C71-butyric acid methyl esther ([70]PCBM). In spite of being promising in terms of PCE, devices based on this blend are unstable and have a short lifetime. The existing literature about the stability of PTB7-based solar cells mainly regards photochemical degradation pathways that require the combined exposure to both light and oxygen (e. g. Soon et al. Chem. Commun. 2013, 49,1291). The UV-induced degradation of the devices in inert atmosphere has received little attention. In particular, the impact of UV light on the charge transport properties of PTB7:[70]PCBM has not been previously investigated, in spite of the crucial importance that the transport of charge has in determining the efficiency of a solar cell. Here, we analyze what happens when PTB7:[70]PCBM blend is exposed to UV light in inert atmosphere: for the first time, we characterize transport of charges in the pristine materials and in the blend for different exposure times. The hole transport in pristine PTB7 is deteriorated by UV exposure; on the other hand, the electron transport properties of pristine [70]PCBM are stable. Surprisingly, the effect of UV light on the blend is the opposite: the electron transport is deteriorated, while the transport of holes appears unaffected. Additionally, we find that the PCE drops by 15% in less than one hour and by 35% in eight hours. We show evidence for the increase of electron trapping in the blend upon UV exposure. By selectively exposing the two materials, we prove that the loss of power conversion efficiency in PTB7:[70]PCBM solar cells is mostly due to a photochemical reaction which involves both donor and acceptor, while the intrinsic instability of the polymer plays a marginal role. These findings highlight the issue of the compatibility of donor and acceptor materials under UV light, which has to be taken into consideration for the design of a new class of donor materials aiming at improving the stability of the devices.
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International Conference on Hybrid and Organic Photovoltaics
International Conference on Hybrid and Organic Photovoltaics (HOPV) is celebrated yearly in May. The main topics are the development, function and modeling of materials and devices for hybrid and organic solar cells. The field is now dominated by perovskite solar cells but also other hybrid technologies, as organic solar cells, quantum dot solar cells, and dye-sensitized solar cells and their integration into devices for photoelectrochemical solar fuel production.
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