Improving the Stability of Bulk Heterojunction Solar Cells by Incorporating pH-Neutral PEDOT:PSS as the Hole Transport Layer

Abstract

In the application of traditional bulk heterojunction polymer solar cells, to prevent the etching of ITO by the acidic poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and thereby improve the device stability, pH-neutral PEDOT:PSS is introduced as the hole transport layer (HTL). After treating the neutral PEDOT:PSS with UV-ozone and with an oxygen plasma, the average power conversion efficiency (PCE) of the device increases from 3.44% to 6.60%. Such surface treatments reduce the energy level offset between the HTL and the active layer, which increases the open circuit voltage and enhances hole transportation, leading to the PCE improvement. Moreover, the devices with the neutral PEDOT:PSS HTL are more stable in air than those with the acidic PEDOT:PSS HTL. The PCE of the devices with the acidic PEDOT:PSS HTL decreases by 20% after 7 days and 45% after 50 days under ambient conditions, whereas the PCE of the devices with the pH-neutral PEDOT:PSS HTL decreases by only 9 and 20% after 7 and 50 days, respectively. X-ray photoelectron spectroscopy shows that the acidic PEDOT:PSS etches the indium from the indium-tin-oxide (ITO) electrode, which is responsible for the degradation of the device. In comparison, the diffusion of the indium is much slower in the devices with the pH-neutral PEDOT:PSS HTL.