Unveiling Excitonic Dynamics in High-Efficiency Nonfullerene Organic Solar Cells to Direct Morphological Optimization for Suppressing Charge Recombination

Abstract

Nonfullerene acceptors (NFAs)-based organic solar cells (OSCs) have recently drawn considerable research interests; however, their excitonic dynamics seems quite different than that of fullerene acceptors-based devices and remains to be largely explored. A random terpolymer of PBBF11 to pair with a paradigm NFA of 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC) such that both complementary optical absorption and very small offsets of both highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels are acquired is designed and synthesized. Despite the small energy offsets, efficient electron/hole transfer between PBBF11 and ITIC is both clearly observed from steady-state photoluminescence and transient absorption spectra and also supported by the measured low exciton binding energy in ITIC. Consequently, the PBBF11:ITIC-based OSCs afford an encouraging power conversion efficiency (PCE) of 10.02%. Although the good miscibility of PBBF11 and ITIC induces a homogenous blend film morphology, it causes severe charge recombination. The fullerene acceptor of PC₇₁BM with varying loading ratios is therefore added to modulate film morphology to effectively reduce the charge recombination. As a result, the optimal OSCs based on PBBF11:ITIC:PC₇₁BM yield a better PCE of 11.4% without any additive or annealing treatment.

Keywords: charge recombination; film morphology; hole and electron transfer; nonfullerene acceptors; organic solar cells.

Figure 1

a) Molecular structures and b) UV–vis absorption spectra of PBBF11 and ITIC neat, and PBBF11:ITIC blend films. c) Determination of E _B of ITIC by temperature‐dependent PL measurements with a linear fitting curve. Inset: A simulation of the free charge fraction over the total excitation density at 300 K under thermal equilibrium. The shaded area represents PV operating conditions (PV regime). d) TRPL decay of ITIC neat film.

Figure 2

GIWAXS images of a) PBBF11 and b) ITIC neat film, and c) PBBF11:ITIC (1:1, wt%) binary blend film. d) TEM and e) AFM images of PBBF11:ITIC (1:1, wt%) binary blend film.

Figure 3

a) J–V curve and b) EQE spectrum of the PBBF11:ITIC‐based optimal OSC.

Figure 4

TA spectrograms of a) PBBF11 and b) ITIC both excited at 400 nm, PBBF11:ITIC blend excited at c) 700 nm and d) 400 nm, respectively. TA kinetics of the samples illustrating e) hole transfer from ITIC to PBBF11 and f) electron transfer from PBBF11 to ITIC.

Figure 5

a) Energy level diagrams of PBBF11, ITIC, and PC₇₁BM, respectively. b) UV–vis absorption spectra of the blend with different PC₇₁BM ratios. c) J–V curves and d) EQE spectra of the optimal ternary OSCs.

Figure 6

GIWAXS, TEM, and corresponding AFM images of ternary PBBF11:ITIC:PC₇₁BM blends at a weight ratio of a,e,i) 1:1:0.1, b,f,j) 1:1:0.3, c,g,k) 1:1:0.5, and d,h,l) binary PBBF11:PC₇₁BM (1:1, wt%) blend.

Figure 7

a) TA kinetics (open dots) of the blends with various PC₇₁BM weight ratios along with their global fitting (solid lines) of tri‐exponential functions, b) SVD fitting of the TA spectrogram of optimal blend, c) schematics of the charge recombination pathways, and d) evolution of the amplitudes among three decay components in the blend samples.

Figure 8

Schematics of charge transportation pathways in a) PBBF11:ITIC (1:1, wt%) binary, b) PBBF11:ITIC:PC₇₁BM (1:1:0.1, wt%) ternary, c) PBBF11:ITIC:PC₇₁BM (1:1:0.3, wt%) ternary, and d) PBBF11:ITIC:PC₇₁BM (1:1:0.5, wt%) ternary blends.