Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions

Yu, Gang; Gao, Jun; Hummelen, Jan C.; Wudl, Fred; Heeger, Alan J.

doi:10.1126/science.270.5243.1789

Cited by 9,924 publications

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“…There, the large energy offset (  0.5 eV) between the lowest unoccupied molecular orbitals (LUMOs) of polymer and fullerene, possibly gated by the formation of hot delocalized states 9 , drives exciton dissociation and charge separation. This is supposed to occur only after diffusion of the excitons in the polymer, which is usually forming an interpenetrating network of domains with the fullerene 12 . However, several experimental and theoretical studies have suggested that the primary photoexcitations in pristine conjugated polymers are not only excitons but also polaron pairs, the latter being more separated electrons and holes with a weaker Coulomb attraction [13][14][15] .…”

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Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics

et al. 2012

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Polymeric semiconductors are materials where unique optical and electronic properties often originate from a tailored chemical structure. This allows for synthesizing conjugated macromolecules with ad hoc functionalities for organic electronics. In photovoltaics, donoracceptor co-polymers, with moieties of different electron affinity alternating on the chain, have attracted considerable interest. The low bandgap offers optimal light-harvesting characteristics and has inspired work towards record power conversion efficiencies. Here we show for the first time how the chemical structure of donor and acceptor moieties controls the photogeneration of polaron pairs. We show that co-polymers with strong acceptors show large yields of polaron pair formation up to 24% of the initial photoexcitations as compared with a homopolymer (η = 8%). π-conjugated spacers, separating the donor and acceptor centre of masses, have the beneficial role of increasing the recombination time. The results provide useful input into the understanding of polaron pair photogeneration in low-bandgap co-polymers for photovoltaics.

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confidence: 99%

Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics

et al. 2012

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“…Exciton diffusion lengths (∼10 nm) are an order of magnitude smaller than absorption lengths. 1,2 This discrepancy is ameliorated by coprecipitating a bicontinuous donor and acceptor phase, a disordered bulk heterojunction (BHJ), 1,3 but inherent disadvantages persist: low exciton dissociation probability, 4 mismatched band gaps, 5 and optical losses. 6 Optimized BHJ devices have active layer thicknesses of ∼100 nm due to the device-limiting trade-off between optical absorption and electrical performance.…”

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Photonic Crystal Geometry for Organic Solar Cells

et al. 2009

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We report organic solar cells with a photonic crystal nanostructure embossed in the photoactive bulk heterojunction layer, a topography that exhibits a 3-fold enhancement of the absorption in specific regions of the solar spectrum in part through multiple excitation resonances. The photonic crystal geometry is fabricated using a materials-agnostic process called PRINT wherein highly ordered arrays of nanoscale features are readily made in a single processing step over wide areas (∼4 cm 2 ) that is scalable. We show efficiency improvements of ∼70% that result not only from greater absorption, but also from electrical enhancements. The methodology is generally applicable to organic solar cells and the experimental findings reported in our manuscript corroborate theoretical expectations.Incommensurate length scales conspire to degrade photovoltaic efficiencies in organic solar cells. Exciton diffusion lengths (∼10 nm) are an order of magnitude smaller than absorption lengths. 1,2 This discrepancy is ameliorated by coprecipitating a bicontinuous donor and acceptor phase, a disordered bulk heterojunction (BHJ), 1,3 but inherent disadvantages persist: low exciton dissociation probability, 4 mismatched band gaps, 5 and optical losses. 6 Optimized BHJ devices have active layer thicknesses of ∼100 nm due to the device-limiting trade-off between optical absorption and electrical performance. 7,8 These thicknesses are adequate to absorb most photons in the visible range of the solar spectrum due to the strong extinction coefficient of contemporary BHJ materials. 2,9 However, the sun's maximum photon flux is located around λ ) 700 nm, which is near the band edge of many BHJ materials where absorption is weak. Energy-level engineering via custom synthesis 5 has produced so-called low-bandgap polymers 10 that exhibit broader absorption tails but are located further toward the near-infrared. The active layer thickness constraints imposed on BHJ solar cells make it imperative to develop innovative ways to enhance absorption in a specific spectral range with weak absorption without increasing photoactive layer thickness.Light trapping schemes based on ray optics have provided enhancement in optical absorption, e.g. collector mirrors, 11 microprism substrates, 12 and V-folded configurations, 13 but in these schemes absorption enhancement was not tailored to a desired spectral range. Methods based on wave optics have shown greater promise, and both diffraction gratings [14][15][16] and photonic crystal (PC) designs have been investigated. In particular, theoretical considerations of PC geometries suggest efficiency improvements would be anticipated [17][18][19] for organic solar cells similar to theoretical work on inorganic devices. 20,21 In spite of this theoretical promise, there has been little progress in experimentally demonstrating PC effects in organic solar cells. While there has been success in nanopatterning the photoactive layer, the primary intention has been to provide an undulating surface for evaporating a ...

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“…Lately, a new polymer, PDTP-DFBT, 40 as well as polythiophene SMPV1, 54 have established new landmarks, by overcoming the 10% efficiency threshold, in tandemmode devices. As regards electron acceptors, the most important development consists of fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM) 22,23 which has been the most successful and widely used analogue.…”

Section: Feature Article Chemcommmentioning

confidence: 99%

New generation solar cells: concepts, trends and perspectives

2015

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Organic, dye-sensitized and perovskite solar cell technologies have triggered widespread interest in recent years due to their very promising potential towards a high solar electricity future. A number of important milestones have marked the roadmap of each sector on the way to today's outstanding performances, but there still remains plenty of scope for further improvement. The most influential landmarks, together with basic concepts and future perspectives are unraveled in this review.

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Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions

Cited by 9,924 publications

References 22 publications

Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics

Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics

Photonic Crystal Geometry for Organic Solar Cells

New generation solar cells: concepts, trends and perspectives

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