Bulk heterojunction mechanism

FIGURE 10.8 The energy diagram of the photoinduced electron transfer and the main energy loss mechanisms in donor-acceptor bulk heterojunction solar cells. 

Photogenerated electron transfer reactions in chemistry typically occur on a timescale significantly greater than the decay timescale of coherent processes. Electron transfer reactions fail this criterion in poly(phenylen-evinylene) based polymers, for which the photogenerated coherent wave-function persists for 25 fs (in solution), and electron transfer occurs in =45 fs in the solid BHJ material. In P3HT, the coherent wavefunction persists for 100 fs in solution, while the electron transfer timescale is <100 fs in the BHJ material. It is therefore necessary to develop testable hypotheses regarding the mechanism of electron transfer in situations where a coherent photoexcited state is involved in the ultrafast electron transfer in bulk heterojunction solar cells. 

Fig. 2.1 Illustration of the two mechanisms for electron-hole capture discussed in the text. Electrons and holes are transported through their respective transport materials and accumulate at the heterojunction, a) Injection of one of the charges into the opposite polymer makes possible charge capture within the polymer bulk and formation of intramolecular excitons. b) Barrier-free electron-hole capture...

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