Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
DOI: https://doi.org/10.29363/nanoge.hopv.2018.034
Publication date: 21st February 2018
In organic photovoltaics (OPV) devices, the optimal photoactive layer is typically less than 100nm, with the efficiency often dropping significantly when photoactive junction thickness increases, due to losses of both Jsc and FF. This thickness dependence is one of the main challenges for the commercialization of OPV devices which require active layer thicknesses larger than 300nm.
When OPV active layer thicknesses are increased from 100nm to 300nm, the collected photocurrents typically drops 30% to 40% relative to the generated photocurrent. In this study, we demonstrate and address the importance of getting high Jsc in order to maintain the efficiency of thick devices. Studies have efficiency losses for thick OPV devices have previously primarily focused on FF losses assigned to increased bimolecular recombination losses during collection. Herein we experimentally quantified the non-geminate recombination flux in four device series as a function of active layer thickness, and find that this recombination loss is too small at short circuit to explain the loss of JSC for thicker devices. Our studies rather identify that the main reason for JSC loss in thick devices is that the device thickness becomes large than the space charge layer width, which limits the effective thickness for charge generation. We assign the origin of this space charge layer to the accumulation, under irradiation, of charge carriers in sub-bandgaptail states,. This limitation on photocurrent generation in thick devices is shown to correlate with the density of these tail states, and is shown to be distinct from space charge layer thickness limitations resulting from dopants or impurities ionization. It is also important to note that tail states are widely observed in organic semiconductors due to the disordered nature of organic material and bulk heterojunction architecture. As such the selection of organic photoactive layers with low tail state densities is shown to be a key requirement for the fabrication of thick, efficient organic solar cells.
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