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Flexibility and delamination

Dechan Angmo

Mechanical flexibility has been a major selling point of OPV. This has been drawn from the premise of fast processing of OPVs which is envisioned best accomplishable on flexible substrates. Traditionally, the use of ITO as a transparent conductor has been a major limitation to flexibility. ITO is an inorganic brittle material which can be can be sputter coated on a flexible substrate such as PET. However, ITO films tends to crack upon flexing (bending, twisting, strectching, etc). Figure 1 shows an example of the change in resistance observed in ITO/PET upon bending and corresponding optical image of the ITO surface before and after the bending test. Within few bending cycles, the sheet resistance of ITO dramatically increases due the formation of cracks in the ITO film. For an in-depth study on flexing of ITO and the various bending parameters that that influence the sheet resistance, refer to other studies, for example ref.DOI:10.1109/JDT.2011.2151830. In solar cells, the increase in sheet resistance would mean that the photovoltaic performance will degrade rapidly as charges generated will not be transported by the ITO film to the external circuit. Depending on the conditions of bending and resulting increase in the sheet resistance of ITO, the solar cells will observe some loss in performance (particularly in FF) and in severe cases, the solar cell may become non-functional. Several ITO-free alternatives including thin-metal films, metal grids, PEDOT:PSS, nanowires, nanotubes, graphene etc. have been shown to have superior flexibility than ITO.

Figure 1: The evolution of sheet resistance of ITO on PET in inward (compression) and outward (compression) bending with a bending radius of 5 mm (A). Optical microscope image [B] of ITO surface. The arrow shows before bending and after 4000 bending cycles. High density of cracks is observed in the latter. The Scale bar is 2 µm.Angmo, et. al. ITO-free roll-coated tandem polymer solar cells based on solution-processed metal film: an all solution ambient route. Energy technology (submitted)

Another degradation mechanism is a result of delamination which in turn is caused by poor adhesion at one or several interfaces. For example, PEDOT:PSS/P3HT:PCBM interface in an inverted solar cell has very poor adhesion because P3HT:PCBM is hydrophobic and PEDOT:PSS is hydrophilic (Figure 2). The actual fracture energy of this interface can depend on the ratio of P3HT:PCBM, the formulation of PEDOT:PSS, and other processing conditionsDOI:10.1109/JDT.2011.2151830; nonetheless the fracture energy of this interface has been found to be an order of magnitude lower than the interface of P3HT:PCBM with other HTL such as V2O5DOI:10.1016/j.solmat.2011.10.012. In encapsulated P3HT:PCBM based solar cell modules that were investigated over 1 year in outdoor and storage conditions, the delamination at this interface has been found to be the primary source of degradationDOI:10.1002/adem.201400002.

Figure 2 P3HT:PCBM/PEDOT:PSS interface has the poor adhesion in an inverted device stack (left). This can be readily evidenced upon ripping apart an encapsulated OPV module (right).

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