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Highly-efficient charge separation and polaron delocalization in polymer-fullerene bulk-heterojunctions: a comparative multi-frequency EPR and DFT study.

ABSTRACT: The ongoing depletion of fossil fuels has led to an intensive search for additional renewable energy sources. Solar-based technologies could provide sufficient energy to satisfy the global economic demands in the near future. Photovoltaic (PV) cells are the most promising man-made devices for direct solar energy utilization. Understanding the charge separation and charge transport in PV materials at a molecular level is crucial for improving the efficiency of the solar cells. Here, we use light-induced EPR spectroscopy combined with DFT calculations to study the electronic structure of charge separated states in blends of polymers (P3HT, PCDTBT, and PTB7) and fullerene derivatives (C60-PCBM and C70-PCBM). Solar cells made with the same composites as active layers show power conversion efficiencies of 3.3% (P3HT), 6.1% (PCDTBT), and 7.3% (PTB7), respectively. Upon illumination of these composites, two paramagnetic species are formed due to photo-induced electron transfer between the conjugated polymer and the fullerene. They are the positive, P(+), and negative, P(-), polarons on the polymer backbone and fullerene cage, respectively, and correspond to radical cations and radical anions. Using the high spectral resolution of high-frequency EPR (130 GHz), the EPR spectra of these species were resolved and principal components of the g-tensors were assigned. Light-induced pulsed ENDOR spectroscopy allowed the determination of (1)H hyperfine coupling constants of photogenerated positive and negative polarons. The experimental results obtained for the different polymer-fullerene composites have been compared with DFT calculations, revealing that in all three systems the positive polaron is distributed over distances of 40-60 Å on the polymer chain. This corresponds to about 15 thiophene units for P3HT, approximately three units for PCDTBT, and about three to four units for PTB7. No spin density delocalization between neighboring fullerene molecules was detected by EPR. Strong delocalization of the positive polaron on the polymer donor is an important reason for the efficient charge separation in bulk heterojunction systems as it minimizes the wasteful process of charge recombination. The combination of advanced EPR spectroscopy and DFT is a powerful approach for investigation of light-induced charge dynamics in organic photovoltaic materials.

SUBMITTER: Niklas J

PROVIDER: S-EPMC4985177 | biostudies-literature | 2013 Jun

REPOSITORIES: biostudies-literature

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Highly-efficient charge separation and polaron delocalization in polymer-fullerene bulk-heterojunctions: a comparative multi-frequency EPR and DFT study.

Niklas Jens J Mardis Kristy L KL Banks Brian P BP Grooms Gregory M GM Sperlich Andreas A Dyakonov Vladimir V Beaupré Serge S Leclerc Mario M Xu Tao T Yu Luping L Poluektov Oleg G OG

Physical chemistry chemical physics : PCCP 20130514 24

The ongoing depletion of fossil fuels has led to an intensive search for additional renewable energy sources. Solar-based technologies could provide sufficient energy to satisfy the global economic demands in the near future. Photovoltaic (PV) cells are the most promising man-made devices for direct solar energy utilization. Understanding the charge separation and charge transport in PV materials at a molecular level is crucial for improving the efficiency of the solar cells. Here, we use light- ...[more]

PMID: 23670645

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Project description:Recent efforts and progress in polymer solar cell research have boosted the photovoltaic efficiency of the technology. This efficiency depends not only on the device architecture but also on the material properties. Thus, insight into the design of novel semiconductor materials is vital for the advancement of the field. This paper looks from a theoretical viewpoint into two of the factors for the design of semiconductor materials with applications to bulk heterojunction solar cells: the charge transfer exciton binding energy and the nanoscale arrangement of donor and acceptor molecules in blend systems. Being aware that the exciton dissociation of local excitons in charge transfer states initiates the charge generation process, the excited state properties of four oligomers (one donor-type: PEO-PPV; and three donor-acceptor-types: PTFB, PTB7, and PTB7-Th) and two fullerene derivatives ([60]PCBM and [70]PCBM), previously reported in the literature as having high electrical conductance, are studied. With such a study, the donor molecules, either of donor-type or donor-acceptor type, are screened as candidates for [60]PCBM- and/or [70]PCBM-based bulk heterojunctions. The charge transfer energy and charge transfer exciton binding energy of suitable donor:acceptor bulk heterojunctions, some of them not yet fabricated, are studied. Further, the charge transfer exciton binding energies of [60]PCBM- and [70]PCBM-based blends are compared. A combination of molecular dynamics simulations with calculations based on Kohn-Sham density functional theory (KS-DFT) and its time-dependent extension (KS-TDDFT) is used. An important feature of this work is that it incorporates the effect of the environment of the quantum chemical system in KS-DFT or KS-TDDFT calculations through a polarizable discrete reaction field (DRF). Our predictions in terms of the influence of the nanoscale arrangement of donor and acceptor molecules on the performance of organic solar cells indicate that bulk heterojunction morphologies for donor-acceptor-type oligomers lead to their lowest excited states having charge transfer character. Further, we find that in terms of favorable charge transfer exciton binding energy, the PTB7-Th:[70]PCBM blends outperform the other blends.

Dataset Information

Highly-efficient charge separation and polaron delocalization in polymer-fullerene bulk-heterojunctions: a comparative multi-frequency EPR and DFT study.

Publications

Highly-efficient charge separation and polaron delocalization in polymer-fullerene bulk-heterojunctions: a comparative multi-frequency EPR and DFT study.

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