Evaluating Carrier Accumulation in Degraded Bulk Heterojunction Organic Solar Cells by a Thermally Stimulated Current Technique

• Published in: Advanced functional materials Vol. 19; no. 24; pp. 3934 - 3940
• Main Authors: Kawano, Kenji, Adachi, Chihaya
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 23.12.2009; WILEY‐VCH Verlag
• Subjects: carrier accumulation; organic solar cells; thermally stimulated current technique
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.200901573

Here, the initial photo‐degradation of encapsulated P3HT:PCBM bulk heterojunction organic solar cells is investigated. The degraded device is recovered by thermal annealing treatment. Thermally stimulated current measurements reveal that the cause of photo‐degradation is carrier accumulation and that the degraded organic solar cell has two broad trap levels, of 0.71 and 0.81 eV. These traps are independent of the thickness of the photoactive layers, the mixing ratio of the photoactive materials and the cathode materials. In addition, it is confirmed that there is a close relationship between the degree of degradation and the amount of accumulated charge carriers. Degraded organic solar cells can be recovered by thermal annealing treatment. Thermally stimulated current measurement reveals that the cause of photo‐degradation is carrier accumulation. When the degraded organic solar cell is annealed, the accumulated carriers escape from the trap sites, and oppositely charged carriers then recombine in the photoactive layer.