Intercalated vs Nonintercalated Morphologies in Donor–Acceptor Bulk Heterojunction Solar Cells: PBTTT:Fullerene Charge Generation and Recombination Revisited

Abstract

In this Letter, we study the role of the donor:acceptor interface nanostructure upon charge separation and recombination in organic photovoltaic devices and blend films, using mixtures of PBTTT and two different fullerene derivatives (PCBM and ICTA) as models for intercalated and nonintercalated morphologies, respectively. Thermodynamic simulations show that while the completely intercalated system exhibits a large free-energy barrier for charge separation, this barrier is significantly lower in the nonintercalated system and almost vanishes when energetic disorder is included in the model. Despite these differences, both femtosecond-resolved transient absorption spectroscopy (TAS) and time-delayed collection field (TDCF) exhibit extensive first-order losses in both systems, suggesting that geminate pairs are the primary product of photoexcitation. In contrast, the system that comprises a combination of fully intercalated polymer:fullerene areas and fullerene-aggregated domains (1:4 PBTTT:PCBM) is the only one that shows slow, second-order recombination of free charges, resulting in devices with an overall higher short-circuit current and fill factor. This study therefore provides a novel consideration of the role of the interfacial nanostructure and the nature of bound charges and their impact upon charge generation and recombination.

Exceptionally low charge trapping enables highly efficient organic bulk heterojunction solar cells

“…As such, it is likely that Y6 exciton separation in this blend is primarily entropy driven, as we discuss further below. 39,40 The small energy offset is consistent with the low energy loss (E loss B 0.55 eV) for this blend, one of the smallest reported for high EQE organic solar cells.…”

“…The free energy decrease resulting from this increasing entropy has been estimated as 0.1-0.2 eV, dependent upon the magnitude of the spatial separation, the blend nanomorphology and the degree of localisation of excitons, charges and CT states. 39,40,67,68 This entropic free energy increase is large enough to have a substantial role in stabilising charge separation in OSCs, and is also likely to be a key contributor to E loss . Many studies have highlighted the role of electronic energy offsets and energetic disorder, as well as 31G(d,p).…”

Impact of Fullerene Intercalation on Structural and Thermal Properties of Organic Photovoltaic Blends

“…Schematic of the donor:acceptor interaction in intercalated (1:1) and high fullerene loading (1:4) blends of PBTTT: PC70BM.This model of the of the blend film morphology can be verified by considering the charge carrier generation and recombination dynamics. Transient Absorption Spectroscopy (TAS) is an established tool for probing charge generation in organic semiconductor blend films, and the specific application to PBTTT-based devices have been previously addressed in detail 11,12,14,24. Figures 3 & 4show the transient decay signals of the polymer polaron band (1 μm wavelength) for the 1:1 and 1:4 PBTTT:PC71BM blend film ratios, respectively, comparing traces measured at different light intensities.…”

“…Figures 3 & 4show the transient decay signals of the polymer polaron band (1 μm wavelength) for the 1:1 and 1:4 PBTTT:PC71BM blend film ratios, respectively, comparing traces measured at different light intensities. We have explored the transient response of nonintercalated blends elsewhere 24.…”

Impact of Initial Bulk‐Heterojunction Morphology on Operational Stability of Polymer:Fullerene Photovoltaic Cells

“…2D Raman mapping of phase‐segregated areas reveals that as PCBM domains develop from a nucleation site even at a molecular level, PCBM diffuses out from a well‐mixed area to create larger PCBM domains such that distinctive areas of PCBM‐deprived regions form around the large PCBM aggregates (Figure S5, Supporting Information). Morphology variations (well‐mixed to phase‐segregated) determine charge extraction efficiency where both extremes: well‐mixed and phase‐segregated morphologies suffer from immediate geminate recombination and the solution has a balance of both phases . Thus, the macroscopic para‐meters such as current–voltage characteristics of OPV devices must be investigated together to understand the influence of different morphologies on device performance and stability thereafter.…”