Abstract
Charge-transfer state (CTS) – coulombically bound electron-hole pair at donor/acceptor interface – is the principal intermediate of photoelectric conversion in organic solar cells and photodetectors. It is generated upon illumination of donor/acceptor bulk heterojunction (BHJ) – the common active layer material for these devices. There are two different ways of CTS photogeneration in organic donor/acceptor BHJ. The photons with sufficiently high energy (above-gap photons) are absorbed by donor or acceptor molecule, creating molecular excitons. The excitons are spitted donor/acceptor interface, producing the exciton-mediated CTSs. An alternative way of CTS generation is operative for below-gap photons. It still can be captured by BHJ blend to produce CTS directly, bypassing molecular excites states. This process is responsible for charge transfer band (CT-band) in absorption spectrum of organic BHJs, and can be utilized in organic IR-photodetectors. The properties of CTS, in particular, the electron-hole distance within the CTS, significantly influence the performance of organic solar cells and photodetectors. However, the data on the CTS structure are scarce at present, and the geometry of CTS produced by direct and exciton-mediated path was not compared before. In the present work we used electron spin echo (ESE) spectroscopy to perform such comparison. For BHJ blend of donor polymer P3HT and non-fullerene acceptor IT-4F we detected ESE signal of CTS under above gap (λ = 527 nm) and below gap (λ = 1064 nm) pulse laser excitation. Analysis of out-of-phase ESE traces allowed to reconstruct the electron-hole distribution function. Surprisingly, we found that the electron-hole distance within CTS is larger for its direct generation than for exciton-mediated generation, both for thermalized CTS and for CTS during thermalization. Presumably, this is explained by more efficient direct CTS generation in more ordered crystalline-like regions of D/A interface, while the efficiency exciton-mediated CTS generation is homogeneous over the BHJ. Therefore, low photon energy does not necessary mean short charge thermalization length and fast geminate recombination for directly generated CTS. Instead, direct CTS generation may facilitate further CTS dissociation and photocurrent production, which can be used for design of sensitive organic IR photodetectors.