Microscopic Analysis of Organic Solar Cells by Simultaneous Measurements of ESR and Device Performance
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Microscopic Analysis of Organic Solar Cells by Simultaneous Measurements of ESR and Device Performance Kazuhiro Marumoto1,2,3, Tatsuya Nagamori1, and Masaki Yabusaki1 1 Division of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan 2 Japan Science and Technology Agency (JST), PRESTO, Kawaguchi, Saitama 322-0012, Japan 3 Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan ABSTRACT Light-induced electron spin resonance (LESR) study of polymer solar cells has been performed to investigate accumulated hole carriers in these devices under device operation. We analyzed clear correlation between the number of accumulated holes in regioregular poly(3hexylthiophene) (P3HT) evaluated by LESR and the deterioration of device performance (Voc, Jsc) observed using the same device under simulated solar irradiation. The effects of hole accumulation with deep trapping levels formed in P3HT at the organic interfaces on the performance are examined by considering interfacial electric dipole layers and charge-carrier scattering by accumulated holes. INTRODUCTION Organic thin-film solar cells are a promising alternative source of electrical energy because of their printable and flexible device structure, light weight, and low-cost production [13]. There has been a significant amount of interest in the high power conversion efficiency (PCE) of more than 10% due to their potential practical applications [4]. The durability of solar cells is another important problem for the practical use of solar cells. The reversible initial deterioration of device performance has been reported for an organic thin-film solar cell with blend films of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) [5,6]. The deterioration has been ascribed to an accumulation of photogenerated charge carriers during device operation rather than to the degradation of organic materials and/or device structures; the accumulation sites were found to be located at interfaces between active layers and electrodes with various trapping levels [5,6]. A more detailed study clarifying molecules and these sites where charge carriers are accumulated (trapped) is important for further improvements of device performance and durability. Electron spin resonance (ESR) is one promising method for such a microscopic characterization of charge-accumulation sites because it is a highly sensitive and powerful approach that is capable of investigating organic devices at the molecular level [7,8]. The ESR method has also been applied to organic thin-film solar cells with oligomers [9] and polymers [10]. For polymer solar cells, the accumulation sites with deep trapping levels for photogenerated hole carriers under device operation were identified as being formed in P3HT at the interfaces between P3HT:PCBM and poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)
(PEDOT:PSS) layers [10]. A clear correlation between the increase in the number of a
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