Improved Film Morphology Reduces Charge Carrier Recombination into the Triplet Excited State in a Small Bandgap Polymer‐Fullerene Photovoltaic Cell

Nuzzo, Daniele Di; Aguirre, Aránzazu; Shahid, Munazza; Gevaerts, Veronique S.; Meskers, Stefan C. J.; Janssen, Raj René

doi:10.1002/adma.201001452

Cited by 158 publications

(205 citation statements)

References 26 publications

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“…The results confi rm that CT dissociation is favored by donor-acceptor nanomorphologies with separated and ordered phases. [ 24,25 ] Large and ordered domains can be expected to result in enhanced delocalization of the electron and hole wave functions, which would in turn reduce the effective binding energy of the CT pairs. [ 48 ] Moreover, Si-PCPDTBT:PCBM blends exhibit less geminate recombination to the ground state and to the triplet state of the polymer compared to C-PCPDTBT:PCBM, [ 49 ] supporting the idea of enhanced CT dissociation effi ciency.…”

Section: Resultsmentioning

confidence: 99%

“…It is well established that the nanoscale morphology of the blends plays a role in the kinetics of charge separation and recombination. [24][25][26] In intimately, almost molecularly mixed, donor-acceptor blends charges have higher chance of recombining. In contrast, the presence of nanocrystalline fullerene domains has been found to enhance the dissociation of charge transfer states into free charge carriers.…”

Section: Wileyonlinelibrarycommentioning

confidence: 99%

“…In contrast, the presence of nanocrystalline fullerene domains has been found to enhance the dissociation of charge transfer states into free charge carriers. [ 24,25 ] Several recent studies also suggest that the delocalization of electron and hole wave functions is benefi cial for the effi ciency of CT dissociation, because delocalized charges result in a lower net binding energy at the interface. [ 14,15,[27][28][29] Molecular structures favoring such delocalization are therefore the best suited to obtain effi cient CT dissociation.…”

Section: Wileyonlinelibrarycommentioning

confidence: 99%

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The Role of Photon Energy in Free Charge Generation in Bulk Heterojunction Solar Cells

Nuzzo

Koster

Gevaerts

et al. 2014

Advanced Energy Materials

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To determine the role of photon energy on charge generation in bulk heterojunction solar cells, the bias voltage dependence of photocurrent for excitation with photon energies below and above the optical band gap is investigated in two structurally related polymer solar cells. Charges generated in (poly[2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′′]dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)] (C‐PCPDTBT):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) solar cells via excitation of the low‐energy charge transfer (CT) state, situated below the optical band gap, need more voltage to be extracted than charges generated with excitation above the optical band gap. This indicates a lower effective binding energy of the photogenerated electrons and holes when the excitation is above the optical band gap than when excitation is to the bottom of the CT state. In blends of PCBM with the silicon‐analogue, poly[(4,4‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl] (Si‐PCPDTBT), there is no effect of the photon energy on the electric field dependence of the dissociation efficiency of the CT state. C‐PCPDTBT and Si‐PCPDTBT have very similar electronic properties, but their blends with PCBM differ in the nanoscale phase separation. The morphology is coarser and more crystalline in Si‐PCPDTBT:PCBM blends. The results demonstrate that the nanomorphological properties of the bulk heterojunction are important for determining the effective binding energy in the generation of free charges at the heterojunction.

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Section: Resultsmentioning

confidence: 99%

Section: Wileyonlinelibrarycommentioning

confidence: 99%

Section: Wileyonlinelibrarycommentioning

confidence: 99%

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The Role of Photon Energy in Free Charge Generation in Bulk Heterojunction Solar Cells

Nuzzo

Koster

Gevaerts

et al. 2014

Advanced Energy Materials

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“…20,24 However, when charge transfer and the subsequent dissociation of CT states into free charge carriers is faster than intersystem crossing, this does not constitute a major loss mechanism. 23 The intensity borrowing mechanism proposed in this paper should be considered as another possible loss mechanism limiting the photocurrent for small "driving force" polymer:fullerene systems. This loss mechanism, however, can be reduced by reducing the electronic coupling matrix element V * by, for example, increasing the distance between donor and acceptor molecule.…”

Section: Implications For Polymer:fullerene Photovoltaics and Concmentioning

confidence: 88%

“…14,22 For example, for such low driving force material systems, triplet energies of polymer and/or fullerene can be below the CT state, sparking debate as to whether population of the triplet state in such systems constitutes a fundamental loss. [23][24][25] Another topic of intense debate relates to the excess energy from excited donor states and if it is exploited as kinetic energy to increase the intercharge pair separation distance, improving the free charge carrier generation rate. [25][26][27][28][29][30][31] Therefore, theoretical and experimental investigations of the electronic structure of the CT state, focusing on the influence of the E D * − E CT difference, are of interest.…”

Section: Introductionmentioning

confidence: 99%

Polarization anisotropy of charge transfer absorption and emission of aligned polymer:fullerene blend films

2012

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An improved understanding of the electronic structure of interfacial charge transfer (CT) states is of importance due to their crucial role in charge carrier generation and recombination in organic donor-acceptor (DA) solar cells. DA combinations with a small difference between the energy of the CT state (E CT ) and energy of the donor exciton (E D * ) are of special interest since energy losses due to electron transfer are minimized, resulting in an optimized open-circuit voltage. In that case, the CT state can be considered as a resonance mixture, containing character of a fully ionic state (D + A − ) and of the local polymer excited state (D * A). We show that the D * A contribution to the overall CT state wave function can be determined by measurements of the polarization anisotropy of CT absorption and emission of polymer:fullerene blends with aligned polymer chains. We study two donor polymers, P3HT and TQ1, blended with fullerene acceptors with different ionization potentials, allowing variation of the E D * − E CT difference. We find that, upon decreasing E D * − E CT , the local excitonic D * A character of the CT state increases, resulting in a decreased fraction of charge transferred and an increased transition dipole moment. For typical polymer:fullerene systems, this effect is expected to become detrimental for device performance if E D * − E CT < 0.1 eV. This however, depends on the electronic coupling between D * A and D + A − , which we experimentally estimate to be ∼ 6 meV for the TQ1:PCBM system.

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Quantification of Quantum Efficiency and Energy Losses in Low Bandgap Polymer:Fullerene Solar Cells with High Open‐Circuit Voltage

Vandewal

Bergqvist

et al. 2012

Adv Funct Materials

202

200

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FTPS-EQE measurement : FTPS-EQE was carried out using a Vertex 70 from Bruker optics, equipped with a QTH lamp, quartz beamsplitter and external detector option. A low noise current amplifi er (SR570) is used to amplify the photocurrent produced upon illumination of the photovoltaic devices with light modulated by the FTIR. The output voltage of the current amplifi er is fed back into the external detector port of the FTIR, in order to be able to use the FTIR's software to collect the photocurrent spectrum.

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Improved Film Morphology Reduces Charge Carrier Recombination into the Triplet Excited State in a Small Bandgap Polymer‐Fullerene Photovoltaic Cell

Cited by 158 publications

References 26 publications

The Role of Photon Energy in Free Charge Generation in Bulk Heterojunction Solar Cells

The Role of Photon Energy in Free Charge Generation in Bulk Heterojunction Solar Cells

Polarization anisotropy of charge transfer absorption and emission of aligned polymer:fullerene blend films

Quantification of Quantum Efficiency and Energy Losses in Low Bandgap Polymer:Fullerene Solar Cells with High Open‐Circuit Voltage

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