Charge formation and transport in bulk-heterojunction solar cells based on alternating polyfluorene copolymers blended with fullerenes

Jespersen, Kim G.; Zhang, Fengling; Gadisa, Abay; Sundström, Villy; Yartsev, Arkady; Inganäs, Olle

doi:10.1016/j.orgel.2006.03.001

Cited by 59 publications

(74 citation statements)

References 28 publications

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“…[85,86] Nevertheless, most femtosecond transient absorption studies carried out on polymer:fullerene blends with optimized composition and morphology evidence the appearance of charges faster than the 100-200 fs time resolution of the experiments. [54][55][56][57][58][59]63,87] Admitting that the charge separation indeed occurs on the 100 fs time scale, this implies that exciton diffusion to a fullerene interface also occurs in <100 fs. According to the exciton diffusion coefficients reported in literature for poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and poly-3(hexylthiophene) (P3HT) (3 × 10 −3 cm 2 s −1 and 1.8 × 10 −3 cm 2 s −1 , respectively), excitons can diffuse by only 0.1-0.2 nm in 100 fs.…”

Section: Photoexcitation and Charge Transfermentioning

confidence: 95%

“…Many time-resolved studies have shown that the charge transfer to the fullerene materials is nearly quantitative and ultrafast (<100 fs for many optimized systems). [54][55][56][57][58][59][60][61][62][63] Based on relaxation time scales found during time-resolved fluorescence power conversion efficiency. [71,81] Determining primary recombination mechanisms and their physical origins are therefore essential to the future of organic BHJ solar cells.…”

Section: Introductionmentioning

confidence: 99%

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Charge Formation, Recombination, and Sweep‐Out Dynamics in Organic Solar Cells

Cowan

Banerji

Leong

et al. 2012

Adv Funct Materials

307

252

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This article presents a critical discussion of the various physical processes occurring in organic bulk heterojunction (BHJ) solar cells based on recent experimental results. The investigations span from photoexcitation to charge separation, recombination, and sweep‐out to the electrodes. Exciton formation and relaxation in poly[N‐9″‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and poly‐3(hexylthiophene) (P3HT) are discussed based on a fluorescence up‐conversion study. The commonly accepted paradigm describing the conversion of incident photons into charge carriers in the BHJ material is re‐examined in light of these femtosecond time‐resolved measurements. Transient photoconductivity, time‐delayed collection field, and time‐delayed dual pulse experiments carried out on BHJ solar cells demonstrate the competition between carrier sweep‐out by the internal field and the loss of photogenerated carriers by recombination. Finally, an emerging hypothesis is discussed: that bimolecular recombination accounts for the majority of recombination from short circuit to open circuit in optimized solar cells, and that bimolecular recombination is bias‐ and charge‐density‐dependent. The study of recombination loss processes in organic solar cells leads to insights into what must be accomplished to achieve the “ideal” solar cell.

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Section: Photoexcitation and Charge Transfermentioning

confidence: 95%

Section: Introductionmentioning

confidence: 99%

Charge Formation, Recombination, and Sweep‐Out Dynamics in Organic Solar Cells

Cowan

Banerji

Leong

et al. 2012

Adv Funct Materials

307

252

View full text Add to dashboard Cite

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“…Experimental evidence points to the appearance of charges in efficient BHJ blends within less than 100 fs. [35][36][37][38] However, pointlike excited state diffusion in 100 fs is limited to the 0.1-0.2 nm length scale, according to published exciton diffusion coefficients. 102,103 We have also seen that hopping of the excited state by Förster EET typically occurs on the slower >1 ps time scale.…”

Section: 86-99mentioning

confidence: 99%

“…CS components faster than 100 fs have been reported on numerous occasions. [35][36][37][38] In order to fully exploit the potential of conjugated polymers and to allow their intelligent design for highly efficient devices, it is therefore essential to understand what happens in their excited state on the sub-picosecond time scale. Nevertheless, the nature of the primary photoexcitation, its ultrafast relaxation, as well as its dissociation into charges within pure polymers and at the fullerene interface is still not univocally elucidated.…”

Section: Introductionmentioning

confidence: 99%

Sub-picosecond delocalization in the excited state of conjugated homopolymers and donor–acceptor copolymers

2013

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In this feature article, we review and examine evidence that the primary photoexcited species in conjugated polymers is considerably delocalized. Localization occurs via a series of complex relaxation mechanisms on the <200 femtosecond time scale. We show that short-lived delocalization in the neutral excited state and charge separated state of bulk heterojunction blends might play an essential role in ensuring efficient formation of free charge carriers for photovoltaic applications. Finally, the additional parameter of intramolecular charge transfer character in the excited state of more recently developed donor-acceptor copolymers is discussed. Both delocalization and partial charge transfer reduce the binding of the electron and hole in photoexcited organic semiconductors and can help to overcome the bottleneck to macroscopic current generation in polymer solar cells.

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“…[60]PCBM is sufficient to effectively quench polymer emission, 22 and transient absorption measurements show only minor differences in geminate recombination times between different stoichiometries. 11 The polymer is, by far, the strongest absorber at most of the visible wavelengths even at very high levels of [60]PCBM loading in these binary blends.…”

Section: Mobility Manipulation and Experimental Systemmentioning

confidence: 99%

Mobility and fill factor correlation in geminate recombination limited solar cells

Andersson¹,

Müller²,

Badada³

et al. 2011

Journal of Applied Physics

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Empirical data for the fill factor as a function of charge carrier mobility for two different polymer:fullerene systems is presented and analyzed. The results indicate that charge extraction depth limitations and space charge effects are inconsistent with the observed behavior, and the decrease in the fill factor is, instead, attributed to the field-dependent charge separation and geminate recombination. A solar cell photocurrent limited by the Onsager-Braun charge transfer exciton dissociation is shown to be able to accommodate the experimental observations. Charge dissociation limited solar cells always benefit from increased mobilities, and the negative contribution from the reduced charge separation is shown to be much more important for the fill factor in these material systems than any adverse effects from charge carrier extraction depth limitations or space charge effects due to unbalanced mobilities. The logarithmic dependence of the fill factor on the mobility for such a process is also shown to imply that simply increasing the mobilities is an impractical way to reach very high fill factors under these conditions since unrealistically high mobilities are required. A more controlled morphology is, instead, argued to be necessary for high performance.

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Charge formation and transport in bulk-heterojunction solar cells based on alternating polyfluorene copolymers blended with fullerenes

Cited by 59 publications

References 28 publications

Charge Formation, Recombination, and Sweep‐Out Dynamics in Organic Solar Cells

Charge Formation, Recombination, and Sweep‐Out Dynamics in Organic Solar Cells

Sub-picosecond delocalization in the excited state of conjugated homopolymers and donor–acceptor copolymers

Mobility and fill factor correlation in geminate recombination limited solar cells

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