In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination1. In contrast, the photogenerated excitons in organic photovoltaic cells are dissociated at a single donor–acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors2. The nanoscale morphology and high charge densities give a high rate of electron–hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes3. Although organic photovoltaic cells would have poor quantum efficiencies if every encounter led to recombination, state-of-the-art examples nevertheless demonstrate near-unity quantum efficiency4. Here we show that this suppression of recombination arises through the interplay between spin, energetics and delocalization of electronic excitations in organic semiconductors. We use time-resolved spectroscopy to study a series of model high-efficiency polymer–fullerene systems in which the lowest-energy molecular triplet exciton (T1) for the polymer is lower in energy than the intermolecular charge transfer state. We observe the formation of T1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate charge transfer states with both spin-singlet (1CT) and spin-triplet (3CT) characters. We show that the formation of triplet excitons can be the main loss mechanism in organic photovoltaic cells. But we also find that, even when energetically favoured, the relaxation of 3CT states to T1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of 3CT states back to free charges, thereby reducing recombination and enhancing device performance. Our results point towards new design rules both for photoconversion systems, enabling the suppression of electron–hole recombination, and for organic light-emitting diodes, avoiding the formation of triplet excitons and enhancing fluorescence efficiency.
Subscribe to Journal
Get full journal access for 1 year
only $3.90 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
Blankenship, R. E. Molecular Mechanisms of Photosynthesis ch. 7 (Blackwell Science, 2001)
Bakulin, A. A. et al. The role of driving energy and delocalized states for charge separation in organic semiconductors. Science 335, 1340–1344 (2012)
Wallikewitz, B. H., Kabra, D., Gelinas, S. & Friend, R. H. Triplet dynamics in fluorescent polymer light-emitting diodes. Phys. Rev. B 85, 045209 (2012)
Park, S. H. et al. Bulk heterojunction solar cells with internal quantum efficiency approaching 100%. Nature Photon. 3, 297–302 (2009)
Hu, B., Yan, L. & Shao, M. Magnetic-field effects in organic semiconducting materials and devices. Adv. Mater. 21, 1500–1516 (2009)
Morteani, A. C., Sreearunothai, P., Herz, L. M., Friend, R. H. & Silva, C. Exciton regeneration at polymeric semiconductor heterojunctions. Phys. Rev. Lett. 92, 247402 (2004)
Veldman, D., Meskers, S. C. J. & Janssen, R. A. J. The energy of charge-transfer states in electron donor–acceptor blends: insight into the energy losses in organic solar cells. Adv. Funct. Mater. 19, 1939–1948 (2009)
Vandewal, K., Tvingstedt, K., Gadisa, A., Inganas, O. & Manca, J. V. On the origin of the open-circuit voltage of polymer-fullerene solar cells. Nature Mater. 8, 904–909 (2009)
Hammond, M. R. et al. Molecular order in high-efficiency polymer/fullerene bulk heterojunction solar cells. ACS Nano 5, 8248–8257 (2011)
Lou, S. J. et al. Effects of additives on the morphology of solution phase aggregates formed by active layer components of high-efficiency organic solar cells. J. Am. Chem. Soc. 133, 20661–20663 (2011)
Hilczer, M. & Tachiya, M. Unified theory of geminate and bulk electron-hole recombination in organic solar cells. J. Phys. Chem. C 114, 6808–6813 (2010)
Baldo, M. A. et al. Highly efficient phosphorescent emission from organic electroluminescent devices. Nature 395, 151–154 (1998)
Uoyama, H., Goushi, K., Shizu, K., Nomura, H. & Adachi, C. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 492, 234–238 (2012)
Deibel, C., Baumann, A. & Dyakonov, V. Polaron recombination in pristine and annealed bulk heterojunction solar cells. Appl. Phys. Lett. 93, 163303 (2008)
Koster, L. J. A., Mihailetchi, V. D. & Blom, P. W. M. Bimolecular recombination in polymer/fullerene bulk heterojunction solar cells. Appl. Phys. Lett. 88, 052104 (2006)
Guangjin, Z., Youjun, H. & Yongfang, L. 6.5% efficiency of polymer solar cells based on poly(3-hexylthiophene) and indene-C60 bisadduct by device optimization. Adv. Mater. 22, 4355–4358 (2010)
Hoke, E. T. et al. The role of electron affinity in determining whether fullerenes catalyze or inhibit photooxidation of polymers for solar cells. Adv. Energy Mater. 2, 1351–1357 (2012)
Schlenker, C. W. et al. Polymer triplet energy levels need not limit photocurrent collection in organic solar cells. J. Am. Chem. Soc. 134, 19661–19668 (2012)
Zhang, Y. et al. Indacenodithiophene and quinoxaline-based conjugated polymers for highly efficient polymer solar cells. Chem. Mater. 23, 2289–2291 (2011)
Peet, J. et al. Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols. Nature Mater. 6, 497–500 (2007)
Di Nuzzo, D. et al. Improved film morphology reduces charge carrier recombination into the triplet excited state in a small bandgap polymer-fullerene photovoltaic cell. Adv. Mater. 22, 4321–4324 (2010)
Soon, Y. W. et al. Energy versus electron transfer in organic solar cells: a comparison of the photophysics of two indenofluorene:fullerene blend films. Chem. Sci. 2, 1111–1120 (2011)
Scharber, M. C. et al. Charge transfer excitons in low band gap polymer based solar cells and the role of processing additives. Energy Environ. Sci. 4, 5077–5083 (2011)
Gélinas, S. et al. The binding energy of charge-transfer excitons localized at polymeric semiconductor heterojunctions. J. Phys. Chem. C 115, 7114–7119 (2011)
Jamieson, F. C. et al. Fullerene crystallisation as a key driver of charge separation in polymer/fullerene bulk heterojunction solar cells. Chem. Sci. 3, 485–492 (2012)
Credgington, D., Hamilton, R., Atienzar, P., Nelson, J. & Durrant, J. R. Non-geminate recombination as the primary determinant of open-circuit voltage in polythiophene:fullerene blend solar cells: an analysis of the influence of device processing conditions. Adv. Funct. Mater. 21, 2744–2753 (2011)
Shockley, W. & Queisser, H. J. Detailed balance limit of efficiency of p-n junction solar cells. J. Appl. Phys. 32, 510–519 (1961)
Etzold, F. et al. The effect of solvent additives on morphology and excited-state dynamics in PCPDTBT:PCBM photovoltaic blends. J. Am. Chem. Soc. 134, 10569–10583 (2012)
Cirmi, G. et al. Few-optical-cycle pulses in the near-infrared from a noncollinear optical parametric amplifier. Opt. Lett. 32, 2396–2398 (2007)
Hodgkiss, J. M. et al. Exciton-charge annihilation in organic semiconductor films. Adv. Funct. Mater. 22, 1567–1577 (2012)
We thank N. Greenham for discussions. A.R. thanks Corpus Christi College, Cambridge for a Research Fellowship. S.G. thanks Fonds Québécois de Recherche sur la Nature et les Technologies for funding. This work is supported by the EPSRC and the Winton Programme for the Physics of Sustainability. C.W.S. was supported by the National Science Foundation (DMR-1215753). D.S.G., C.-Z.L., H.-L.Y. and A.K.-Y.J. acknowledge support from the Office of Naval Research (N00014-11-1-0300). Some of the work was done at the UW NanoTech User Facility, a member of the NSF National Nanotechnology Infrastructure Network. We thank J. Richards and D. Pozzo for performing grazing-incidence small-angle X-ray scattering measurements, S. Williams for transmission electron microscopy and G. Shao for help with atomic force microscopy measurements.
The authors declare no competing financial interests.
About this article
Cite this article
Rao, A., Chow, P., Gélinas, S. et al. The role of spin in the kinetic control of recombination in organic photovoltaics. Nature 500, 435–439 (2013). https://doi.org/10.1038/nature12339
Enhanced photovoltaic performance of quinoxaline-based donor-acceptor type polymers with monocyano substituent
Journal of Power Sources (2021)
Nature Photonics (2021)
Nature Communications (2021)
Advanced Functional Materials (2021)
Reliable Predictions of Benzophenone Singlet–Triplet Transition Rates: A Second-Order Cumulant Approach
The Journal of Physical Chemistry A (2021)