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

Abstract

Light-induced charge formation is essential for the generation of photocurrent in organic solar cells. In order to gain a better understanding of this complex process, we have investigated the femtosecond dynamics of charge separation upon selective excitation of either the fullerene or the polymer in different bulk heterojunction blends with well-characterized microstructure. Blends of the pBTTT and PBDTTPD polymers with PCBM gave us access to three different scenarios: either a single intermixed phase, an intermixed phase with additional pure PCBM clusters, or a three-phase microstructure of pure polymer aggregates, pure fullerene clusters and intermixed regions. We found that ultrafast charge separation (by electron or hole transfer) occurs predominantly in intermixed regions, while charges are generated more slowly from excitons in pure domains that require diffusion to a charge generation site.The pure domains are helpful to prevent geminate charge recombination, but they must be sufficiently small not to become exciton traps. By varying the polymer packing, backbone planarity and chain length, we have shown that exciton diffusion out of small polymer aggregates in the highly efficient PBDTTPD:PCBM blend occurs within the same chain and is helped by delocalization.

A Close Look at Charge Generation in Polymer:Fullerene Blends with Microstructure Control

“…In the 1:1 pBTTT:PCBM blend (by weight), processed by wire bar coating at 35°C (see the Supporting Information for details), there is quasi-complete intercalation of fullerenes between the polymer side chains, yielding an intimately intermixed cocrystal phase, as schematically shown in Figure 2A. 20,22,24,48 Both the PCBM and the polymer signatures are present in the absorption spectrum of this blend ( Figure 1B). While the fullerene displays a similar signature as in the neat system, the absorption of the polymer shows a pronounced structure due to a vibronic progression and is more red-shifted (maximum at 558 nm) compared to the band in neat pBTTT.…”

“…50 For the 1:4 pBTTT:PCBM blend (by weight), there is excess PCBM present, which forms relatively pure clusters around the intermixed regions, yielding a two-phase scenario ( Figure 2B). 20,22,48 As a consequence, the PCBM The different processing conditions yielding each microstructure are also summarized, together with the abbreviated sample names used in the main text (color coded according to the absorption spectra in Figure 1). absorption is strongly enhanced, while the structured shape of the pBTTT band confirms that the polymer is still predominantly present in the co-crystalline domains ( Figure 1B).…”

“…We have discussed the detailed TA analysis for the two samples elsewhere. 48 In brief, all pBTTT excitons in the 1:1 and 1:4 blends dissociate into charges on the ultrafast ∼100 fs time scale, given the proximity of the polymer and fullerene in the co-crystal phase (no exciton diffusion is necessary).…”

“…Here, we directly observe the geminate charge recombination (gCR) as a significant decay of all TA features with a relatively short 211 ps time constant ( Figure 5A, dynamics of the 605 nm GSB and 850 nm charge absorption in Figure 5D, amplitude spectra in Figure S6, Supporting Information). 48 Only a small fraction of free charges is formed, leading to a weakly pronounced longlived TA spectrum at our longest time delay of 1 ns. In the 1:4 blend, the presence of PCBM clusters largely suppresses gCR, 20,22,28,62 so that the decay of the TA features on the 1 ns time scale is notably reduced (Figure 5C,D).…”

“…Our previous TA study has revealed this process to be prompt in the 1:1 blend (∼100 fs), but to have a distribution of ultrafast to slow components (ranging from sub-ps to ∼500 ps) in the 1:4 blend due to diffusion of PCBM excitons to a quenching site through the fullerene clusters (see amplitude spectra in Figure S6, Supporting Information). 48 This causes the weak offset at long time delays in the emission dynamics of the 1:4 blend excited at 400 nm ( Figure 4C). …”