The mechanism of burn-in loss in a high efficiency polymer solar cell.

A new class of push-pull polymers combined with a fullerene derivative in a bulk heterojunction (BHJ) architecture have enabled organic photovoltaic (OPV) efficiencies approaching 10% with the highest NREL certified efficiency now at 8.3%.[1–8] This is a significant improvement over the 5% efficiency achieved by the well-studied polymer poly(3-hexylthiophene) (P3HT) in 2005.[9] These efficiency gains have brought organic solar cells closer to commercial viability, highlighting the importance of studying the lifetime and reliability of OPV devices. The lifetime and degradation mechanisms of BHJ solar cells that employ P3HT have been well studied,[10–15] however, lifetime studies on the recently developed high performance polymers have been limited. Many of these high performance polymers generally show less structural order than the more crystalline polymer P3HT[6,16,17] and as such, may exhibit lifetimes and degradation pathways that are quite different from P3HT. Another reason to expect different behavior is that fullerenes can intercalate between the polymer sidechains of some polymers to create a molecular mixture that enables rapid exciton dissociation.[18,19] One such polymer, poly[N-9”-hepta-decanyl2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT), achieved an efficiency of 6.1% in 2009[4] and more recently 7.2%.[20] Previous work by our group[21] on PCDTBT in BHJ composites with the fullerene derivative [6,6]-phenyl C70-butyric acid methyl ester (PC70BM) showed extrapolated lifetimes approaching 7 years for encapsulated devices, which is the longest reported lifetime for polymer solar cells and double the lifetime of P3HT:PCBM devices fabricated in a similar manner. The efficiency as a function of aging time curve (Figure 1) showed a severe loss in the first few hundred hours of operation followed by a remarkably stable period that lasted for the duration of the 4400 hour study. The initial efficiency loss, termed “burn-in,” was shown to be primarily the result of a drop in both fill factor (FF) and open-circuit voltage (Voc)