Towards highly efficient solar cells based on merocyanine dyes
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Towards highly efficient solar cells based on merocyanine dyes Vera Steinmann1, Nils M. Kronenberg1, Martin R. Lenze1, Manuela Deppisch2, Hannah Bürckstümmer2, Frank Würthner2, and Klaus Meerholz1 1 Department für Chemie, Universität Köln, Luxemburger Straße 116, 50939 Köln, Germany 2 Institut für Organische Chemie and Röntgen Research Center for Complex Material Systems, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany ABSTRACT We present the development of highly efficient merocyanine solar cells at the example of the dye MD376. Due to their unique processing flexibility, merocyanines have been applied in solution- as well as vacuum-processed organic photovoltaics. In fully vapor-deposited MD376 single cells, efficiencies up to 5.0% were achieved while maintaining a rather simple device setup with only four organic layers. Moreover, MD376 has been successfully introduced to vacuum-processed tandem cell structures, reporting a high open-circuit voltage of 2.1 V. INTRODUCTION In recent years, increased demand for low-cost, renewable energy has spurred the research on organic solar cells (OSC). To date, power conversion efficiencies (PCE) beyond 8% have been reported for polymer OSCs,[1] and up to 6.7% for small molecules (SM) devices.[2-5] In contrast to most polymers, SMs show remarkably high absorption coefficients and can be easily synthesized at large scale and high purity. We have recently introduced merocyanine (MC) dyes as electron donor compounds in SM-based organic photovoltaics (OPV). Due to their large absorption coefficients, polarizabilities as well as strong dipole moments, MC dyes are already well established in the field of nonlinear optics.[6, 7] Throughout the last years, a vast majority of dyes has also been tested in solution-processed (SOL) bulk heterojunction (BHJ) solar cells. Due to their unique chemical configuration, MC dyes exhibit easily tunable, intense absorption bands, making this class of traditional colorants greatly suitable for OPV applications.[6, 8] In early studies, the dye MD376 was identified as a promising candidate in efficient OSCs (see Figure 1a for the chemical structure). Its maximum absorption at rather long wavelengths (λmax ≈ 600 nm) guarantees a good overlap with the solar spectrum. In 2008, PCEs of 1.5% were reported for the first MD376 SOL solar cells, demonstrating – at that time – the great potential of this dye. Due to very intense absorption bands, comparably thin BHJ films (≈ 50 nm) already led to effective light harvesting. A high open-circuit voltage (VOC) of 0.90 V was obtained.[9] Most MC dyes exhibit good solubility as well as thermal stability due to their rather compact chemical structure, offering a unique processing flexibility. Hence in 2010, we introduced the first vacuum-processed (VAC) BHJ solar cells based on the same MC dyes as used in SOL-processing. Consequently, a first direct comparison between the two deposition techniques (SOL and VAC) became possible.[10] In this paper, we will cover the development of VAC MC solar cells within t
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