Solar Energy Harvesting with Photosynthetic Pigment-Protein Complexes by Sai Kishore Ravi & Swee Ching Tan

Author:Sai Kishore Ravi & Swee Ching Tan
Language: eng
Format: epub
ISBN: 9789811563331
Publisher: Springer Singapore

Several recent studies focus on overcoming this limitation in OPVs by understanding the interfacial electronic structure and by engineering the energy-level alignment in the device interfaces [56, 84, 85]. With the recent developments in biophotovoltaics focussing more on solid-state devices, understanding the interfacial energetics becomes crucial. With differences only in the level of sophistication, the interfaces formed by a simple organic molecule or by a photosynthetic complex with an electrode can fairly be understood by the same of set of governing principles established by the band theory of solids.

For an interface formed by an organic layer in an OPV, the possibility of an energy barrier at the interface is often investigated [89–91]. When a thick organic layer is interfaced with an electrode, there is a chance of interfacial band-bending which stems from the non-equilibrium state created when materials of two different work functions are brought into contact [58, 92, 93]. In order to reach an electrical equilibrium where the Fermi levels of the two materials are to be at the same level, charge redistribution occurs around the metal/organic interface [58, 94–96]. The flow/distribution of charges into either side of the interface continues until the Fermi energies in the bulk of the two materials are aligned, resulting in a diffusion layer with band bending [58, 92, 93]. This results in a built-in potential in the device. While the same effect is possible in photoprotein/electrode interface, the interfacial electronic structure can no more be treated as the junction of two solids of different work functions. As a photoprotein complex is an assemblage of polypeptides and pigments and other cofactors each having a distinct Fermi level, it isn’t possible to assign it a single value of work-function. Though when a photoprotein is interfaced with an electrode ideally it is only one part of the protein with a specific organic molecule that is intended to be in contact with the electrode, in most practical cases it is hard to ensure this. An exception to this would be those devices employing ordered monolayers of proteins with genetic tags and functionalized electrodes, however, these architectures are far from practical high-output biophotovoltaics where high loading/concentration of light absorbing units is necessary [6]. Hence simplification of the protein/electrode interface to an organic/electrode interface is unsound in any biophotovoltaic device with high photoprotein loading. The interfacial electronic properties (especially the energy barrier at the interface) are often studied by different kinds of in-device photoelectron spectroscopies under ultra-high vacuum (UHV) [58, 91]. These studies haven’t been so far attempted in biophotovoltaic devices due to the challenges in preserving the structural integrity of the photoproteins under the test conditions.

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