Direct measurements of exciton diffusion length limitations on organic solar cell performance

Kozub, Derek; Vakhshouri, Kiarash; Kesava, Sameer Vajjala; Wang, Cheng; Hexemer, Alexander; Gomez, Enrique D.

doi:10.1039/c2cc31925j

Cited by 42 publications

(51 citation statements)

References 21 publications

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“…[ 12 ] Domain spacings of 20-50 nm are consistent with measurements of the microstructure of other polymer/ fullerene blends. [ 10,12,22 ] The similar qualitative shape of the data between samples of different compositions implies that the microstructure is not changing signifi cantly. Nevertheless, the scattering intensities shown in Figure 3 vary between samples.…”

Section: Resultsmentioning

confidence: 95%

“…[ 12,[29][30][31][32][33] Figure 3 shows scattering profi les as a function of scattering vector, q , which are typical for small angle scattering of polymer/fullerene mixtures. [ 10,12,22,27,[34][35][36][37][38][39][40] Assuming the scattering is dominated by structure factors, we estimate a domain spacing from the infl ection point in the data near q = 0.015 Å −1 that corresponds to an average distance between domains of approximately 40 nm. Previous work on poly(3-hexylthiophene)/PCBM fi lms suggests that at 284.4 eV contrast between pure polymer phases and mixed domains is dominant (even when fullerene aggregates are present), such that the domain spacing corresponds to the distance between polymer fi brils.…”

Section: Resultsmentioning

confidence: 99%

“…EFTEM images taken at energies near core excitations can generate elemental maps which provide quantitative information about domain compositions with nanometer resolution. [ 10,12,15,22 ] We focus on a specifi c system, poly[(4,4′-bis(2-ethylhexyl)dithieno [3,2-…”

Section: Introductionmentioning

confidence: 99%

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Domain Compositions and Fullerene Aggregation Govern Charge Photogeneration in Polymer/Fullerene Solar Cells

Kesava

Fei

Rimshaw

et al. 2014

Advanced Energy Materials

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The complex microstructure of organic semiconductor mixtures continues to obscure the connection between the active layer morphology and photovoltaic device performance. For example, the ubiquitous presence of mixed phases in the active layer of polymer/fullerene solar cells creates multiple morphologically distinct interfaces which are capable of exciton dissociation or charge recombination. Here, it is shown that domain compositions and fullerene aggregation can strongly modulate charge photogeneration at ultrafast timescales through studies of a model system, mixtures of a low band‐gap polymer, poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]germole)‐2,6‐diyl‐alt‐(2,1,3‐benzothia‐diazole)‐4,7‐diyl], and [6,6]‐phenyl‐C71‐butyric acid methyl ester. Structural characterization using energy‐filtered transmission electron microscopy (EFTEM) and resonant soft X‐ray scattering shows similar microstructures even with changes in the overall film composition. Composition maps generated from EFTEM, however, demonstrate that compositions of mixed domains vary significantly with overall film composition. Furthermore, the amount of polymer in the mixed domains is inversely correlated with device performance. Photoinduced absorption studies using ultrafast infrared spectroscopy demonstrate that polaron concentrations are highest when mixed domains contain the least polymer. Grazing‐incidence X‐ray scattering results show that larger fullerene coherence lengths are correlated to higher polaron yields. Thus, the purity of the mixed domains is critical for efficient charge photogeneration because purity modulates fullerene aggregation and electron delocalization.

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Section: Resultsmentioning

confidence: 95%

Section: Resultsmentioning

confidence: 99%

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Domain Compositions and Fullerene Aggregation Govern Charge Photogeneration in Polymer/Fullerene Solar Cells

Kesava

Fei

Rimshaw

et al. 2014

Advanced Energy Materials

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“…[19][20][21][22][23] This has strong implications on the photophysics of the materials. [24][25][26][27][28][29][30] In the present contribution, we demonstrate that the dynamics of charge generation in polymer:fullerene blends is largely determined by the microstructure. To that aim, we have used femtosecond transient absorption (TA) spectroscopy to investigate CS in several systems with well-characterized BHJ structure.…”

Section: Introductionmentioning

confidence: 85%

“…It is likely that not all excitons reach pBTTT during their lifetime (as previously suggested by incomplete quenching of the fullerene emission), especially if the PCBM domains are larger than the 5 nm exciton diffusion length. 26,27 Although we have discussed HT as splitting of a PCBM exciton in the presence of pBTTT, we cannot exclude the alternative scenario of hole injection into the polymer following spontaneous charge dissociation within the PCBM clusters and charge migration (this would lead to similar evolution of the TA spectra). The process has been demonstrated, 51,52 but is probably slower than what we observe here and therefore has a minor inuence on our results.…”

Section: Resultsmentioning

confidence: 99%

The influence of microstructure on charge separation dynamics in organic bulk heterojunction materials for solar cell applications

et al. 2014

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Light-induced charge formation is essential for the generation of photocurrent in organic solar cells. In order to gain a better understanding of this complex process, we have investigated the femtosecond dynamics of charge separation upon selective excitation of either the fullerene or the polymer in different bulk heterojunction blends with well-characterized microstructure. Blends of the pBTTT and PBDTTPD polymers with PCBM gave us access to three different scenarios: either a single intermixed phase, an intermixed phase with additional pure PCBM clusters, or a three-phase microstructure of pure polymer aggregates, pure fullerene clusters and intermixed regions. We found that ultrafast charge separation (by electron or hole transfer) occurs predominantly in intermixed regions, while charges are generated more slowly from excitons in pure domains that require diffusion to a charge generation site.The pure domains are helpful to prevent geminate charge recombination, but they must be sufficiently small not to become exciton traps. By varying the polymer packing, backbone planarity and chain length, we have shown that exciton diffusion out of small polymer aggregates in the highly efficient PBDTTPD:PCBM blend occurs within the same chain and is helped by delocalization.

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Controlling Polymorphism in Poly(3‐Hexylthiophene) through Addition of Ferrocene for Enhanced Charge Mobilities in Thin‐Film Transistors

Smith

Clark

Kuang

et al. 2014

Adv Funct Materials

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Crystalline organic molecules often exhibit the ability to assemble into multiple crystal structures depending on the processing conditions. Exploiting this polymorphism to optimize molecular orbital overlap between adjacent molecules in the unit lattice is an effective method for improving charge transport within the material. In this study, grazing incident X-ray diffraction was employed to demonstrate the formation of tighter π-π stacking poly(3-hexylthiophene-2,5-diyl) polymorphs in fi lms spin coated from ferrocenecontaining solutions. As a result, the addition of ferrocene to casting solutions yields thin-fi lm transistors which exhibit approximately three times higher source-drain currents and charge mobilities than neat polymer devices. Nevertheless, XPS depth profi ling and NMR analyses of the active layer reveal that all ferrocene is removed during the spin coating process, which may be an essential factor to achieve high mobilities. Such insights gleaned from ferrocene/poly(3-hexylthiophene-2,5-diyl) mixtures can serve as a template for selection and optimization of other small molecule/polymer systems with greater baseline charge mobilities.

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Direct measurements of exciton diffusion length limitations on organic solar cell performance

Cited by 42 publications

References 21 publications

Domain Compositions and Fullerene Aggregation Govern Charge Photogeneration in Polymer/Fullerene Solar Cells

Domain Compositions and Fullerene Aggregation Govern Charge Photogeneration in Polymer/Fullerene Solar Cells

The influence of microstructure on charge separation dynamics in organic bulk heterojunction materials for solar cell applications

Controlling Polymorphism in Poly(3‐Hexylthiophene) through Addition of Ferrocene for Enhanced Charge Mobilities in Thin‐Film Transistors

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