Improving the stability of P3HT/PC61BM solar cells by a thermal crosslinker

Abstract

Stabilizing the optimal film morphology of bulk heterojunction polymer solar cells is key for realizing long-term stable devices. A crosslinker, octane-1,8-diyl bis(1,4-dihydrobenzo[d][1,2]oxathiine-6-carboxylate 3-oxide) (OBOCO), was designed to crosslink PC₆₁BM through a heat-triggered Diels–Alder reaction to suppress the aggregation tendency of fullerene molecules, thus helping to stabilize the film morphology and to improve device stability. Devices containing OBOCO (0–25% wt) in the active layer were fabricated and studied. Open-circuit voltage (V_oc) shows an interesting increasing tendency as the OBOCO content increases from 0% to 20%. Short-circuit current (J_sc) continuously decreases as the crosslinker content increases. Fill factor (FF) remains stable (>60%) at low OBOCO content (0–15%) but decreases quickly at high OBOCO content (20–25%). The power conversion efficiency (PCE) shows a decreasing tendency with OBOCO content increasing. Atomic force microscopy (AFM) indicates that the nanoscale polymer/fullerene phase separation is retained with 5% addition of OBOCO but deteriorates when more crosslinker is added. Space Charge Limited Current (SCLC) measurements indicate that the electron mobility of fullerene continuously decreases while the hole mobility of P3HT remains stable as the OBOCO content increases, suggesting that the crosslinking took place at the fullerene domains. After the devices were heated at 150 °C for 4 days, the PCE of the devices without OBOCO decreased significantly from 2.74% to 0.78%, while that of the devices with 5% OBOCO decreased from 2.77% to 1.72%. Morphological studies indicate that the OBOCO crosslinker can effectively impede heat-promoted fullerene aggregation, thus leading to a stable morphology and stable device performance.