Dependence of recombination mechanisms and strength on processing conditions in polymer solar cells

Abstract

Charge carrier recombination due to carrier trapping is not often considered in polymer based solar cells, except in those using non-fullerene acceptors or new donor polymers with limited short-range order. However, we show that even for the canonical poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester (P3HT:PCBM) system, relative strengths of bimolecular and trap-assisted recombination are strongly dependent on processing conditions. For slow-grown active-layers, bimolecular recombination is indeed the major loss mechanism under one sun illumination. However, for fast-grown active-layers, trap-assisted recombination dominates over bimolecular recombination by an order of magnitude, and recombination strength at short-circuit condition is 3-4 times higher, leading to loss of photocurrent and lowering of fill factor.

Reducing Trap‐Assisted Recombination in Small Organic Molecule‐Based Photovoltaics by the Addition of a Conjugated Block Copolymer

“…The average PCE of the B‐DTS was 4.17% ± 0.67%. The PCE distribution of the B‐DTS was comparable to the previously reported PCE values of DTS based OPVs utilizing the same device structure, additives, and electrode . Another control device of the B‐D130 exhibited maximum PCE of 5.03% with V OC , J SC , FF values of 0.97 V, 8.99 mA cm −2 , 0.57, respectively.…”

“…The slope of V OC versus ln( I ) is known to approach k b T / q when bimolecular recombination (Langevin recombination) is dominant. A stronger dependence on light intensity is expected for trap‐assisted recombination, i.e., a slope greater than k b T / q . As shown in Figure c, the value for B‐DTS deviated significantly from that expected in ideal devices and was determined to be 5.0 k b T / q .…”

“…As shown in Figure c, the value for B‐DTS deviated significantly from that expected in ideal devices and was determined to be 5.0 k b T / q . This finding suggests that recombination in open‐circuit conditions is dominated by charge carrier trapping and through traps, rather than by bimolecular recombination . Mandoc et al showed that the introduction of 7,7,8,8‐tetracyanoquinodimethane electron traps in the blends of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylenevinylene] (MDMO‐PPV):PC 60 BM strongly modifies the open circuit voltage and its dependence on light intensity (e.g., the slope of S changes from 1.05 kT / q to 3.04 kT / q ) …”

High fill factor and thermal stability of bilayer organic photovoltaic cells with an inverted structure

“…Increasing power intensity, the bimolecular recombination becomes dominant. 34 Fig. 5 also showed the dependence of FF upon power intensity, where FF of 0.74 under 100 mW/cm 2 was observed.…”

The effect of processing additives for charge generation, recombination, and extraction in bulk heterojunction layers of all-polymer photovoltaics

“…3(c). In case of bimolecular recombination of free carriers being the only loss mechanism, it has been shown that the Voc is given by 27 Voc…”