Charge-Transfer State Dynamics Following Hole and Electron Transfer in Organic Photovoltaic Devices

Abstract

The formation of bound electron-hole pairs, also called charge-transfer (CT) states, in organic-based photovoltaic devices is one of the dominant loss mechanisms hindering performance. While CT state dynamics following electron transfer from donor to acceptor have been widely studied, there is not much known about the dynamics of bound CT states produced by hole transfer from the acceptor to the donor. In this letter, we compare the dynamics of CT states formed in the different charge-transfer pathways in a range of model systems. We show that the nature and dynamics of the generated CT states are similar in the case of electron and hole transfer. However the yield of bound and free charges is observed to be strongly dependent on the HOMO D -HOMO A and LUMO D -LUMO A energy differences of the material system. We propose a qualitative model in which the effects of static disorder and sampling of states during the relaxation determine the probability of accessing CT states favourable for charge separation.

Charge generation in polymer–fullerene bulk-heterojunction solar cells

“…67 This hot CT state theory was further supported by their subsequent work on other polymer:fullerene blends as well as polymer:perylene diimide blends ( Figure 6). [69][70][71][72][73][74] In addition to the above work, there are some other reports supporting the hot CT state theory. For example, the sub-bandgap CT excitation was reported to generate polarons which are more localized compared with those from the above-gap excitation, implying the importance of the excess thermal energy.…”

Charge Separation in Intermixed Polymer:PC70BM Photovoltaic Blends: Correlating Structural and Photophysical Length Scales as a Function of Blend Composition

“…This ability to generate dissociated charges (albeit with only a 70 % yield) in the absence of fullerene domains is most probably associated with the reasonably large energy offset ECS driving charge generation in this blend. 30,[63][64][65] However the absence of any phase structure to drive spatial separation of electrons and holes, and the absence of pure fullerene domains to facilitate rapid electron transport, these dissociated charges undergo relatively fast non-geminate recombination losses and a poor charge collection efficiency (except under strong reverse bias). The presence of aggregated fullerene domains suppresses the formation of bound charge pairs and the resultant geminate recombination losses.…”

The influence of microstructure on charge separation dynamics in organic bulk heterojunction materials for solar cell applications

“…12 Free charges are therefore generated in quasi quantitative yield by both the ET and HT pathways, even if the driving force for HT and ET is different and populates the CT state with more or less excess energy. 11,12 This is evidence that charges formed in the PBDTTPD:PCBM blend (at domain interfaces or in the intermixed phase) are particularly well spatially separated, possibly thanks to the driving force provided by the pure regions, or to the delocalization in the polymer (see below).…”

“…[8][9][10] Also, CS via hole transfer (HT) from photoexcited fullerene is not accounted for. [11][12][13][14][15][16][17][18] Moreover, the microstructure of the BHJ is much more complex in most polymer:fullerene blends. Amorphous and/or crystalline pure domains oen coexist with a phase where the polymer and fullerene are intimately mixed.…”