11.4% Efficiency non-fullerene polymer solar cells with trialkylsilyl substituted 2D-conjugated polymer as donor

Abstract

Simutaneously high open circuit voltage and high short circuit current density is a big challenge for achieving high efficiency polymer solar cells due to the excitonic nature of organic semdonductors. Herein, we developed a trialkylsilyl substituted 2D-conjugated polymer with the highest occupied molecular orbital level down-shifted by Si-C bond interaction. The polymer solar cells obtained by pairing this polymer with a non-fullerene acceptor demonstrated a high power conversion efficiency of 11.41% with both high open circuit voltage of 0.94 V and high short circuit current density of 17.32 mA cm^-2 benefitted from the complementary absorption of the donor and acceptor, and the high hole transfer efficiency from acceptor to donor although the highest occupied molecular orbital level difference between the donor and acceptor is only 0.11 eV. The results indicate that the alkylsilyl substitution is an effective way in designing high performance conjugated polymer photovoltaic materials.

Figure 1. Synthetic route of J71 and the effect of the alkylsilyl substitution on the physicochemical properties of the monomers.

(a) Synthetic route of J71 with the structure of BDTT-Si obtained by X-ray crystallography. (b) Absorption spectra of the monomers BDTT-Si and BDTT-C chloroform solutions with concentration of 1 × 10⁻⁵ mol l⁻¹. (c) Cyclic voltammograms of BDTT-Si and BDTT-C in 0.1 mol l⁻¹ Bu₄NPF₆ acetonitrile solution at a scan rate of 20 mV s⁻¹, the ferrocene/ferrocenium (Fc/Fc⁺) couple was also provided for an internal reference. (d) Energy level diagrams of BDTT-Si and BDTT-C.

Figure 2. Chemical structure and physicochemical properties of J71.

(a) Chemical structures of J71 polymer donor and ITIC n-OS acceptor. (b) Absorption spectra of J71 and ITIC. (c) Cyclic voltammogram of J71 polymer film on a platinum electrode measured in 0.1 mol l⁻¹ Bu₄NPF₆ acetonitrile solutions at a scan rate of 20 mV s⁻¹, the inser figure (blue line) shows the Cyclic voltammogram of ferrocene/ferrocenium (Fc/Fc⁺) couple used as an internal reference. (d) Energy level diagram of J71 and ITIC.

Figure 3. Photovoltaic performance of the PSCs based on J71:ITIC without (open circles) and with (filled circles) thermal annealing at 150 °C for 10 min.

(a) J–V curves of the champion PSCs, under the illumination of AM 1.5 G, 100 mW cm⁻², (b) IPCE spectra of the corresponding PSCs; (c) Light intensity dependence of J_sc of the PSCs; (d) dark currents of the PSCs, the inset shows the equivalent circuit of the PSCs.

Figure 4. Plots and images of the GIWAXS measurements.

Line cuts of the GIWAXS images of (a) neat ITIC film and (b) neat J71 film, (c) as cast J71: ITIC blend films and (d) thermal annealed J71: ITIC blend films. GIWAXS images of (e) the neat ITIC film, (f) neat J71 film, (g) as acst J71: ITIC film and (h) thermal annealed J71: ITIC film.

Figure 5. Transient absorption measurements for the study of hole transfer dynamics.

transient absorption signal recorded from the films of (a) neat ITIC and (b) J71: ITIC (1:1, w/w) blend excited by 710 nm. (c) Transient absorption spectra from the blend film exicted by 710 nm (orange line) at different time delays. The lower panel shows the transient absorption spectra recorded at 1 ps from ITIC excited by 710 nm and J71 excited by 540 nm (blue line), respectively. (d) Dynamics probed at 710 nm recorded from the films of neat ITIC and blend J71: ITIC (1:1, w/w). (e) Dynamic curves probed at 540 and 590 nm recorded from the film of blend. (f) A schematic digram of the hole transfer in the film of J71: ITIC (1:1, w/w) blend.