Revealing the Interfacial Photoreduction of MoO₃ with P3HT from the Molecular Weight-Dependent “Burn-In” Degradation of P3HT:PC₆₁BM Solar Cells

“Burn-in” degradation occurs in many polymer solar cells, which dramatically reduces the overall power output of the cells at the early hundred hours. Understanding the “burn-in” degradation mechanism is therefore highly important to improve the lifetime of the cell. In this article, the decay behaviors of P3HT:PC61BM solar cells depending on the molecular weight of P3HT were systematically investigated. Although all of these P3HTs were highly crystalline with regioregularity of 94–97%, the stability of P3HT:PC61BM cells showed a nonmonotonic dependence on P3HT molecular weight. The cells based on P3HT with a weight average molecular weight (Mw) of 20 K showed much faster decadence in open circuit voltage (VOC) and fill factor (FF) during aging, yielding the lowest stability in comparison with that based on P3HT of 10, 25, and 30 K. UV–vis absorption and external quantum efficiency spectra demonstrated that the performance decay is not attributed to the change in the photoactive layer. The recovery of VOC and FF of the aged cells after renewing the MoO3/Al electrode revealed that the performance decay is mainly because of the interfacial degradation of P3HT:PC61BM/MoO3. Electron spin resonance spectroscopy and X-ray photoelectronic spectroscopy confirmed the photon-induced redox reaction between P3HT and MoO3 under light illumination, where P3HT is oxidized to the polaron and Mo(VI) was partially reduced to Mo(V). The photon chemical reduction (PCR) of MoO3 by P3HT is then ascribed as the essential reason for the fast VOC and FF decays of the cells during aging. The surface morphology of the photoactive layer measured by the atomic force microscope revealed the much rougher surface of the P3HT-20 K/PC61BM film. Such a rough surface increases the contact area between P3HT and MoO3, and consequently enhances the PCR of MoO3 and P3HT, which is considered as the main reason for the molecular weight-dependent degradation behaviors. For the first time, the current work clearly demonstrates that the photoreduction of the metal oxide and photoactive layer would lead to fast VOC and FF decays, which could be a very important degradation pathway for polymer solar cells.