Generation of Photoexcitations and Trap-Assisted Recombination in TQ1:PC₇₁BM Blends

Abstract

The generation and recombination of long-lived photoexcitations is clarified in TQ1 films and TQ1:PC 71 BM 1:1 and 1:3, by weight, blends using photoinduced absorption measurements. At 80 K triplets are formed in TQ1 films, while both triplets and polarons are formed in the 1:1 and 1:3 blends. We suggest that the triplet state acts as a loss mechanism for generation of free charges in these blends and suggest an energy diagram for the photoexcitations in the blends. We estimate the triplet polaron annihilation (γ TPA ) constant to be 1.12 × 10 −14 and 3.10 × 10 −13 cm 3 s −1 for the TQ1:PC 71 BM 1:1 and 1:3 blends, respectively. At 300 K triplets are mainly formed in the TQ1 films, while only polarons are present in the TQ1:PC 71 BM blends. Using frequency measurements we show that the TQ1:PC 71 BM 1:1 blend shows nondispersive kinetics (ζ = 1), while the 1:3 blend exhibits dispersive kinetics (ζ = 0.87). Using intensity-dependent measurements, we show that trap-assisted recombination is the dominating recombination mechanism for polarons in TQ1:PC 71 BM blends. Assuming an exponential trap-density we show that the characteristic energy E ch , the mean trap-depth, is E ch = 38.7 ± 2 meV in 1:1 blends and slightly deeper in 1:3 blends, E ch = 48.4 ± 2 meV. The trap density is shown to be higher in the 1:1 compared to the 1:3 blends.

The Effect of Dopant‐Free Hole‐Transport Polymers on Charge Generation and Recombination in Cesium–Bismuth–Iodide Solar Cells

“…The t e for the devices with TQ1 and P3TI were very similari nt he same voltage region between0 .45 and 0.55 V. An increaseo f t e at lower light intensity was observed, which is expected when the concentration of photo-induced charges in the film decreases. TQ1 exhibits polarona bsorption at approximately 950 nm, [46,47] whicha greesw itht he new peak observed in the PIA spectrum of the device with TQ1 in Figure 7. Therefore, it is difficult to directly comparet he t e of the device based on P3HT with the other devices,ast he measurements were not performed at the same voltage.…”

Buildup of Triplet-State Population in Operating TQ1:PC71BM Devices Does Not Limit Their Performance

“…Triplet states are long-lived in the blend and can accumulate to densities of around half the free charge carrier density under short circuit conditions, their density only limited by triplet–triplet annihilation. The high triplet densities coupled with very low charge recombination losses indicate that triplet states do not necessarily reduce the performance of organic solar cells, in contrast to previous conclusions. , We stress that triplet formation is a very common feature in OPV blends, − and that given their possibly large equilibrium density, it seems important to consider their impact on performance. The approach presented here is fairly universal and enhances our understanding of the role of triplet states in increasing or reducing carrier recombination losses.…”

“…8 Moreover, studies by Gehrig et al demonstrated that when triplets are formed on the polymer, they are a reservoir of excitations, that, to a certain extent, can replenish the charge carrier population via triplet-triplet annihilation in PBDTTC-C:PC61BM blends. 18 For the present study, we chose blends of the donor polymer TQ1 and fullerene derivative PC71BM, since they exhibit clear triplet formation, 27 yet they also exhibit high internal quantum efficiencies in devices. [28][29][30][31] We observed, quantified, and parametrized both charge and triplet state formation and decay, which in turn enabled us, not only to simulate charge densities in operating devices, but also to calculate the corresponding triplet densities by solving the triplet rate equation under steady-state conditions.…”

“…For the present study, we chose blends of the donor polymer TQ1 and fullerene derivative PC 71 BM, since they exhibit clear triplet formation, yet they also exhibit high internal quantum efficiencies in devices. − We observed, quantified, and parametrized both charge and triplet-state formation and decay, which in turn enabled us, not only to simulate charge densities in operating devices but also to calculate the corresponding triplet densities by solving the triplet rate equation under steady-state conditions. More precisely, the kinetics of charges and triplets were acquired by transient absorption (TA) spectroscopy combined with multivariate curve resolution-alternating least-squares (MCR-ALS) data analysis.…”

Emerging Role of the Band‐Structure Approach in Biohybrid Photovoltaics: A Path Beyond Bioelectrochemistry

“…The defects in the active layer bring rise to new energy states within the band gap called electronic traps . These traps are potential sites for recombination to occur and hence the defect density in the material has to be minimized …”