Thermal degradation related to the PEDOT:PSS hole transport layer and back electrode of the flexible inverted organic photovoltaic module

The hole transport layer (HTL) and back electrode play a significant role in the stability of the flexible organic photovoltaic (OPV) module. In particular, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), a widely used hole transport material, is known to be associated with many degrading factors in the OPV field. This study highlights the impact of the PEDOT:PSS layer on thermal stability using a thermal acceleration test of flexible OPV modules with the inverted structure of indium tin oxide/ZnO/photoactive layer/PEDOT:PSS/Ag. The results confirm that thermal degradation of the OPV devices depends on heat temperature, in which the OPV performance degrades by a notable decrease in the open-circuit voltage (V_oc) as the temperature increases from 65 °C to 85 °C. Moreover, the stability of V_oc is enhanced when the PEDOT:PSS layer is thicker and contains polar solvent DMSO as an additive, suggesting that the thermal degradation can correlate with the properties of the PEDOT:PSS layer. In addition, microscopy images of the active layers show that the surface damage is attributed to a residual solvent of the printed Ag electrode, thereby resulting in a thermally induced drop in the short circuit current density (J_sc). More detailed descriptions are presented in this paper, and the results are expected to offer a comprehensive understanding of the thermal degradation mechanism of OPV modules.