Photovoltaic Function and Exciton/Charge Transfer Dynamics in a Highly Efficient Semiconducting Copolymer

Abstract

Exciton dissociation is a key step for the light energy conversion to electricity in organic photovoltaic (OPV) devices. Here, excitonic dissociation pathways in the high-performance, low bandgap "in-chain donor-acceptor" polymer PTB7 by transient optical absorption (TA) spectroscopy in solutions, neat fi lms, and bulk heterojunction (BHJ) PTB7:PC 71 BM (phenyl-C 71 -butyric acid methyl ester) fi lms are investigated. The dynamics and energetics of the exciton and intra-/intermolecular charge separated states are characterized. A distinct, dynamic, spectral red-shift of the polymer cation is observed in the BHJ fi lms in TA spectra following electron transfer from the polymer to PC 71 BM, which can be attributed to the time evolution of the hole-electron spatial separation after exciton splitting. Effects of fi lm morphology are also investigated and compared to those of conjugated homopolymers. The enhanced charge separation along the PTB7 alternating donor-acceptor backbone is understood by intramolecular charge separation through polarized, delocalized excitons that lower the exciton binding energy. Consequently, ultrafast charge separation and transport along these polymer backbones reduce carrier recombination in these largely amorphous fi lms. This charge separation mechanism explains why higher degrees of PCBM intercalation within BHJ matrices enhances exciton splitting and charge transport, and thus increase OPV performance. This study proposes new guidelines for OPV materials development.

Study of Photoelectric Conversion in Benzotrithiophene-Based Conjugated Semiconducting Polymers

“…[1][2][3] Polymers applied in OPVs are mostly made of two alternating monomers with different electron affinities. 4) Hence, the polymer backbone has repeating units of electron donor (D) and electron acceptor (A) molecules, referred to as D-A copolymers. Utilizing D-A copolymers is an effective strategy to narrow optical bandgap via intramolecular charge transfer (CT) band.…”

Homogeneous phase separation in polymer:fullerene bulk heterojunction organic solar cells

“…The fluorescence lifetime of a pristine PCBM film is about 688 ps. After blending with PTB7, the fluorescence lifetime of PC 70 BM decreased to 626 ps because of energy or charge transfer to the polymer [6,29]. It is difficult to distinguish two different processes owing to the similar time scale.…”

“…PTB7 has a strong absorption in the wavelength range from 600 nm to 710 nm, while the absorption of PC 70 BM is in the visible wavelength range from 400 nm to 575 nm. When the two materials were blended with a volume ratio of 1:1.5 [29], the absorption spectrum of the blend layer overlaps well with the solar spectrum allowing for efficient light absorption. A small red-shift occurs in the absorption of the blend film compared with a pristine PTB7 film, which can be attributed to charge transfer states due to the percolation of PC 70 BM in the PTB7 polymer [29,30].…”

“…When the two materials were blended with a volume ratio of 1:1.5 [29], the absorption spectrum of the blend layer overlaps well with the solar spectrum allowing for efficient light absorption. A small red-shift occurs in the absorption of the blend film compared with a pristine PTB7 film, which can be attributed to charge transfer states due to the percolation of PC 70 BM in the PTB7 polymer [29,30]. The additive does not show significant influence on the absorption spectrum of PTB7:PC 70 BM blend film.…”

Theoretical study on the cooperative exciton dissociation process based on dimensional and hot charge-transfer state effects in an organic photocell

“…To obtain these calculation results, we set τ h = 1.0 ps. 17,29,46,47 In Figure 3(a), 2D and 3D model results are presented. The 1D case is also shown for comparison purpose.…”