Ultrafast dynamics of charge carrier photogeneration and geminate recombination in conjugated polymer: fullerene solar cells -: art. no. 195208

被引:123
作者
Müller, JG
Lupton, JM
Feldmann, J
Lemmer, U
Scharber, MC
Sariciftci, NS
Brabec, CJ
Scherf, U
机构
[1] Univ Munich, Lehrstuhl Photo & Optoelektron, Dept Phys & CeNS, D-80799 Munich, Germany
[2] Univ Karlsruhe, Light Technol Inst, D-76131 Karlsruhe, Germany
[3] CFN, D-76131 Karlsruhe, Germany
[4] Johannes Kepler Univ Linz, Linz Inst Organ Solar Cells, A-4040 Linz, Austria
[5] Konarka Technol, A-4040 Linz, Austria
[6] Univ Wuppertal, Fachbereich C, D-42097 Wuppertal, Germany
关键词
D O I
10.1103/PhysRevB.72.195208
中图分类号
T [工业技术];
学科分类号
08 ;
摘要
We investigate the nature of ultrafast exciton dissociation and carrier generation in acceptor-doped conjugated polymers. Using a combination of two-pulse femtosecond spectroscopy with photocurrent detection, we compare the exciton dissociation and geminate charge recombination dynamics in blends of two conjugated polymers, MeLPPP [methyl-substituted ladder-type poly(p-phenylene)] and MDMO-PPV [poly(2-methoxy,5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene], with the electron accepting fullerene derivative PCBM [1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C-61]. This technique allows us to distinguish between free charge carriers and Coulombically bound polaron pairs. Our results highlight the importance of geminate pair recombination in photovoltaic devices, which limits the device performance. The comparison of different materials allows us to address the dependence of geminate recombination on the film morphology directly at the polymer:fullerene interface. We find that in the MeLPPP:PCBM blend exciton dissociation generates Coulombically bound geminate polaron pairs with a high probability for recombination, which explains the low photocurrent yield found in these samples. In contrast, in the highly efficient MDMO-PPV:PCBM blend the electron transfer leads to the formation of free carriers. The anisotropy dynamics of electronic transitions from neutral and charged states indicate that polarons in MDMO-PPV relax to delocalized states in ordered domains within 500 fs. The results suggest that this relaxation enlarges the distance of carrier separation within the geminate pair, lowering its binding energy and favoring full dissociation. The difference in geminate pair recombination concurs with distinct dissociation dynamics. The electron transfer is preceded by exciton migration towards the PCBM sites. In MeLPPP:PCBM the exciton migration time decays smoothly with increasing PCBM concentration, indicating a trap-free exciton hopping. In MDMO-PPV:PCBM, however, the exciton migration time is found to show a threshold-like dependence on the PCBM concentration. This observation is explained by efficient interchain energy transfer in ordered MDMO-PPV domains in conjunction with exciton trapping.
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页数:10
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