Improved Charge Generation via Ultrafast Effective Hole‐Transfer in All‐Polymer Photovoltaic Blends with Large Highest Occupied Molecular Orbital (HOMO) Energy Offset and Proper Crystal Orientation

Abstract

Improved charge generation via fast and effective hole transfer in all-polymer solar cells (all-PSCs) with large highest occupied molecular orbital (HOMO) energy offset (ΔE H ) is revealed utilizing ultrafast transient absorption (TA)spectroscopy. Blending the same nonfullerene acceptor poly{[N,N′-bis(2octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′bithiophene) (N2200) with three different donor polymers produces all-polymer blends with different ΔE H . The selective excitation of N2200 component in blends enables to uncover the hole transfer process from hole polaron-induced bleaching and absorption signals probed at different wavelength. As the ΔE H is enhanced from 0.14 to 0.37 eV, the hole transfer rate rises more than one order and the hole transfer efficiency increases from 12.9% to 86.8%, in agreement with the trend of internal quantum efficiency in the infrared region where only N2200 has absorption. Additionally, Grazing-incidence wide-angle X-ray scattering measurements indicate that face-on crystal orientation in both polymer donor and acceptor also plays an important role in facilitating the charge generation via hole transfer in all-PSCs. Hence, large ΔE H and proper crystal orientation should be considered in material design for efficient hole transfer in N2200-based heterostructures. These results can provide valuable guidance for fabrication of all-PSCs to further improve power conversion efficiency.

Ultrafast Exciton Dissociation in Block Copolymer toward Efficient Single Material Organic Solar Cells

“…films. [45][46][47] It is noteworthy that the PB and PA peaks of the PBDB-Tb-PTY6+PS film display ≈10-15 nm blueshift compared with those of the PBDB-T-b-PTY6 film (Figure 2d,g), conformed to the steady-state absorption peaks because of the PBDB-T-b-PTY6 block copolymer spreading throughout the insulating PS to form the isolated block copolymer chain. Thus, for the PBDB-T-b-PTY6+PS film, the PB peaks ≈625 and 815 nm are the ground state bleaching features of the PBDB-T and PTY6 component, respectively, and the PA peak ≈900 nm corresponds to the absorption of singlet excitons in PTY6.…”

“…The corresponding PB signals of PBDB‐T‐ b ‐PTY6+PS, PBDB‐T‐ b ‐PTY6, and PBDB‐T:PTY6 films exhibit a rising stage with a time scale of 3, 50, and 100 ps, respectively, but in contrast, the signals in neat PBDB‐T and PTY6 films probed at 640 nm manifest as an exponential decaying dynamics, followed with the initially rapid rising process within 0.5 ps (Figure S3a,b, Supporting Information). Because the rising PB signal isresulted from the HT from PTY6 to PBDB‐T, [ 46,47 ] these results point to the faster HT in the PBDB‐T‐ b ‐PTY6+PS film.…”

“…In the neat PTY6 film excited by 800 nm pluses, the PB and PA signals of singlet excitons decay rapidly within 100 ps, and beyond that only a small residual PB signal persists (Figures S2 and S3, Supporting Information), owing to the formation of long‐lived triplet excitons by the intersystem crossing. [ 46,47 ] In contrast, in the PBDB‐T‐ b ‐PTY6 and PBDB‐T:PTY6 films under selective excitation of PTY6, the PB signal ≈640 nm, characteristic ground state bleaching band of PBDB‐T, exhibits a pronounced rising process (Figure 2d–f), indicating efficient HT from the PTY6 to PBDB‐T component. We also note the rising PB signal ≈625 nm in the PBDB‐T‐ b ‐PTY6+PS sample, which directly prove that the intramolecular HT is also effective.…”

Resolving the Mechanisms of Photocurrent Improvement in Ternary Organic Solar Cells

“…Figures a,b,d,e and S7 show the spectra recorded at greater than 900 nm in which only the photoinduced absorption (PIA) features of the excitons and charges are observed. The PIA peak was observed at ∼1150 nm for the PTB7-Th:PC 71 BM system, corresponding to the hole polaron absorption of PTB7-Th . The hole polaron absorption band in the ternary film was red-shifted (∼30 nm) compared to the binary blend films, possibly suggesting that the charge in the ternary blend film is not localized to single chains but delocalized over the crystalline phase. , Figure c shows the polaron generation and decay kinetics of PTB7-Th for the binary and ternary systems.…”

Controllable Exciton Diffusion Length and Ultrafast Charge Generation in Ternary Organic Solar Cells

“…The 800 nm excitation is used to excite the NFA molecules selectively in BHJs because the main absorption peaks are well‐ separated in UV‐vis region performed in Figure 1b. [ 44‐45 ] Many‐ body annihilation process, including exciton‐exciton and exciton‐ charge annihilation is avoided by using low excitation fluence of ca . 4 μJ/cm 2 .…”