We have characterized lateral imperfections of photovoltaic modules based on solution processed polymer-fullerene semiconductor blends by means of lock-in thermography (LIT). The active layer of the solar cell modules is based on the heterogeneous organic semiconductor system poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester and the power conversion efficiency of the modules reached nearly 2% under irradiation of an AM 1.5 solar simulator. Applying highly sensitive LIT allowed us to detect several kinds of laterally distributed defects originating from imperfections in the respective functional layers as well as in the quality of encapsulation. We show that LIT is a powerful method for the quality control of large area polymer solar cells and modules, enabling fast feedback for optimization of production parameters.
In order to study the influence of the organic solar cell device layout on the photovoltaic parameters, we systematically varied its geometry. By knowledge of all sheet resistances in the device, we were able to correlate the series resistance with the geometry of the device using a simple model for its calculation. Deviations between experiment and calculation could be related with the solar cell geometry and understood by postulating curved transport ways of the current within the largely resistive ITO‐layer. Thus, a further refinement of the calculation is required in order to minimize the deviation between calculation and experiment. Short solar cell lengths and ITO‐bridges yield minimal series resistance and best conversion efficiency.
A schematic cross‐section of a polymer solar cell displays the current flow through the device. The corresponding contributions to the series resistance are depicted as well.
P3HT:PCBM (poly(3-hexylthiophene-2,5-diyl): ([6,6]-phenyl-C61-butyric acid methyl ester)-based bulk heterojunctions (BHJs) were doped by using 4-toluenesulfonic acid (TSA) as dopant. This approach was inspired by the well-known interfacial doping of the active layer via the electron-blocking layer PEDOT:PSS (poly(3,4-ethylenedioxy-thiophene):poly-(styrenesulfonate)) at its interface. TSA is amphiphilic, acidic, and structurally very similar to the monomeric building block of PSS. Upon TSA doping, a notable increase in the light absorption in the sub-bandgap region of pristine P3HT was observed. These features are assigned to polaron transitions within P3HT; however, the TSA impact on polaron absorption features in the BHJ is rather small. Although, for small TSA concentrations and thick active layers (∼220 nm) the fill factor of the solar cells improved dramatically with increasing TSA content in the active layer, which is discussed in terms of contact resistances at interfaces in the present paper. For 0.5% TSA concentration in the active layer solution the maximum of the power conversion efficiency was obtained. At the same time, the reproducibility of solar cell performance parameters was considerably improved.
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