Polymer bulk heterojunction solar cells employing Fo ¨rster resonance energy transfer Jing-Shun Huang 1 , Tenghooi Goh 1 , Xiaokai Li 1 , Matthew Y. Sfeir 2 , Elizabeth A. Bielinski 3 , Stephanie Tomasulo 4 , Minjoo L. Lee 4 , Nilay Hazari 3 and Andre ´ D. Taylor 1 * There are two crucial tasks for realizing high-efficiency polymer solar cells (PSCs): increasing the range of the spectral absorption of light and efficiently harvesting photogenerated excitons. Here, we describe Fo ¨rster resonance energy transfer-based heterojunction polymer solar cells that incorporate squaraine dye. The high absorbance of squaraine in the near-infrared region broadens the spectral absorption of the solar cells and assists in developing an ordered nanomorphology for enhanced charge transport. Femtosecond spectroscopic studies reveal highly efficient (up to 96%) excitation energy transfer from poly(3-hexylthiophene) to squaraine occurring on a picosecond timescale. We demonstrate a 38% increase in power conversion efficiency to reach 4.5%, and suggest that this system has improved exciton migration over long distances. This architecture transcends traditional multiblend systems, allowing multiple donor materials with separate spectral responses to work synergistically, thereby enabling an improvement in light absorption and conversion. This opens up a new avenue for the development of high-efficiency polymer solar cells. P olymer solar cells (PSCs) are promising candidates for provid- ing low-cost, lightweight, large-area and mechanically flexible energy conversion devices 1–4 . Cells incorporating a binary bulk heterojunction (BHJ) blend based on regioregular poly(3- hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) have been studied widely, providing power conversion efficiencies (PCEs) of 4–5% (refs 5–8). A critical step towards improving the PCE is to increase the quantum yields of incident photons. P3HT, with its bandgap of ≏2 eV, is only able to harvest 22% of the total photons in solar light 9,10 . A common route towards realizing broadband light harvesting makes use of low- bandgap polymers, which often require processing additives 11–14 , making it difficult to precisely control both the crystallinity and phase separation of the photoactive layer. Additionally, the photogenerated excitons in the photoactive layer must reach the donor–acceptor interface to dissociate before recombination 4 . Unfortunately, an ideally bicontinuous interpenetration network is difficult to control 15 , and results in ≏50% energy loss to recombina- tion 16 . Recent studies in dye-sensitized solar cells have demonstrated that Fo ¨rster resonance energy transfer (FRET) is a promising strat- egy to improve exciton migration over long distances 17–21 . Although it was thought that FRET may occur in P3HT–TiO 2 nanostructured solar cells featuring squaraine dye 22 , to the best of our knowledge, experimental evidence that FRET can enhance exciton harvesting in polymer BHJ solar cells has never been presented. Furthermore, several groups have studied ternary-blend PSCs that utilize a third material 23 to either extend the solar spectrum 24–29 or control the phase separation 30 . In many cases, however, the third material in PSCs, such as porphyrin-based dye, has even reduced device performance through the formation of recombina- tion centres and/or detrimental effects on blend morphology 24,25 . In this Article, we demonstrate, for the first time, efficient FRET in BHJ PSCs by incorporating 2,4-bis[4-(N,N-diisobutylamino)-2,6- dihydroxyphenyl] squaraine (SQ) in P3HT:PCBM blends to improve both the photon absorption range and exciton harvesting. Unlike common multiblend systems, FRET-based systems enable the effective use of multiple donors, thereby bringing significant improvements in light absorption and conversion. Femtosecond flu- orescence and transient absorption (TA) spectroscopy on the ternary blends show energy transfer efficiencies of up to 96% (within the first few picoseconds) due to the large spectral overlap between the P3HT emission and SQ absorption. In addition to FRET, the SQ functions as a long-wavelength absorber due to its high absorbance (≏3 × 10 5 M 21 cm 21 ) in the red and near-infrared (NIR) spectral regions 31,32 . Both processes enhance the overall photocurrent in the resulting solar cells and we show that a nano- morphology with an interpenetrating network is well developed by SQ. As a result, the overall PCE is dramatically improved from 3.27% to 4.51%. Optical characterization of P3HT and SQ The molecular structures and optical properties of the materials used in this study are shown in Fig. 1. The P3HT absorption shows a broad spectrum from 400 to 650 nm with a peak at 515 nm and two shoulders at 550 and 600 nm. The vibronic feature at 600 nm indicates a high degree of ordered crystalline lamellae in the P3HT film due to strong interchain interactions 33 . The SQ dye has a high extinction coefficient of ≏3 × 10 5 M 21 cm 21 at 647 nm corresponding to p–p* charge transfer transitions. The cooperative absorption of P3HT and SQ covers a significant portion of the solar spectrum, with only limited overlap. This ensures that the addition of SQ will not inhibit the light absorption of the P3HT. Furthermore, the absorption of SQ strongly overlaps with the photoluminescence of P3HT, making these two materials a good FRET pair. FRET is a non-radiative energy transfer process that acts through long-range dipole–dipole interactions between donor and acceptor molecules 34 . The strength of this interaction is strongly dependent on the overlap integral of the donor emission (P3HT) and the accep- tor absorption (SQ) and can be summarized by the Fo ¨rster 1 Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, USA, 2 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA, 3 Department of Chemistry, Yale University, New Haven, Connecticut 06511, USA, 4 Department of Electrical Engineering, Yale University, New Haven, Connecticut 06511, USA. *e-mail: andre.taylor@yale.edu ARTICLES PUBLISHED ONLINE: 5 MAY 2013 | DOI: 10.1038/NPHOTON.2013.82 NATURE PHOTONICS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturephotonics 1 © 2013 Macmillan Publishers Limited. All rights reserved.