Visible to Near-Infrared Light Harvesting in TiO₂ Nanotube Array−P3HT Based Heterojunction Solar Cells

Abstract

The development of high-efficiency solid-state excitonic photovoltaic solar cells compatible with solution processing techniques is a research area of intense interest, with the poor optical harvesting in the red and near-IR (NIR) portion of the solar spectrum a significant limitation to device performance. Herein we present a solid-state solar cell design, consisting of TiO 2 nanotube arrays vertically oriented from the FTOcoated glass substrate, sensitized with unsymmetrical squaraine dye (SQ-1) that absorbs in the red and NIR portion of solar spectrum, and which are uniformly infiltrated with p-type regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) that absorbs higher energy photons. Our solidstate solar cells exhibit broad, near-UV to NIR, spectral response with external quantum yields of up to 65%. Under UV filtered AM 1.5G of 90 mW/cm 2 intensity we achieve typical device photoconversion efficiencies of 3.2%, with champion device efficiencies of 3.8%.The best performing bulk heterojunction solar cells (η ∼ 6%) employ an optimized blend of a polymeric donor and a fullerene acceptor. 1,2 The fullerene acceptor absorbs very little light and is primarily used in blends to provide an efficient interface for exciton dissociation. Various efforts toward efficiency improvement in these devices are directed toward the development of low band gap polymers to absorb a broad swathe of the solar spectrum, 3-6 lowering the molecular energy levels of the semiconducting polymer to enhance the open circuit voltage of the organic solar cells, 1,7 and the control of the blended film morphology for enhanced exciton harvesting. [8][9][10][11][12] In the best performing solid-state dyesensitized solar cells (SS-DSSCs), η ∼ 5%, the only photon absorber is a dye, while the electron and hole transport functions are performed, respectively, by a disordered nanoparticulate TiO 2 network and a transparent small molecule spiro-OMeTAD. [13][14][15][16] To achieve higher efficiency devices, research efforts have focused on improved porefilling by the hole transporter, the use of higher mobility hole transporters and the synthesis of dyes with a broader and more-intense absorption spectrum.Red/NIR radiation (650-1000 nm) accounts for approximately 33% of the solar energy arriving at the surface of the Earth, while UV-visible radiation (350-650 nm) accounts for about 40%. Hence a key issue toward achieving higher efficiency organic solar cells is in the development of red and NIR absorbing molecules to utilize more of the solar spectrum. As an approach for efficiently utilizing visible to NIR solar radiation, herein we describe an inorganic-organic hybrid solar cell, see Figure 1a, where electron transporting TiO 2 nanotube arrays, sensitized with red and NIR light absorbing organic dye, are used in combination with hole transporting and visible light absorbing regioregular P3HT. [17][18][19] It should be noted that this low band gap organic dye should not block transmission of the high-energy photons of near-UV-visible range pa...

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