The intermediate band scheme: mimicking the Z scheme of photosynthesis with one single-phase material

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Oral, Perla Wahnón, presentation 103
Publication date: 5th February 2015

Intermediate band materials, having a partially filled band in the bandgap of a semiconductor, have been proposed to boost photovoltaic efficiency by absorbing two low energy photons to excite an electron to higher energy (like in the photosynthesis Z-scheme). The optimal bandgap is then ~2.0 eV. We have proposed, based on DFT calculations, several such materials based on III-V or chalcopyrite hosts [2], and have realized later experimentally the concept [3]with transition metal-substituted sulphides. We show here that, as evidenced experimentally in that latter work, inserting V in In₂S₃ (having gap=2.0 eV) allows exciting efficiently electrons across the gap using sub-bandgap photons. First, photoluminescence giving 2.0 eV photons is excited, unlike in In₂S₃, with photons having lower energies (>1.59 eV). The process behaves linearly, therefore is not a frequency doubling effect. To our knowledge this effect is observed for the first time in such materials.  GW-type quantum calculations confirmed that a partially filled band formed by V(3d) orbitals appears inside the In₂S₃ gap in this type of material. We showed also that the same sub-bandgap photons can drive on this material (but not on In₂S₃) a simple photocatalytic reaction (photo-oxidation of aqueous HCOOH), evidencing that CB and VB carriers thus generated can migrate to the surface and be transferred to external acceptors. Similar effect should be achievable in photovoltaics. Furthermore,holes generated by sub-bandgap photons in V:In₂S₃ (but not in In₂S₃) are oxidizing enough to produce OH radicals from water. Similar spectral response extension in photocatalysis occurs also for V:SnS₂ [4]. We extend now this concept in different directions. Thus, preliminary work shows that a hydrogenase enzyme anchored on In₂S₃ catalyzes the photo-generation of H₂ from water using a sacrificial agent (acetate); we expect that the same can happen on the said V:In₂S₃ using sub-bandgap photons, facilitating the production of solar fuels. Also, we explore now how to obtain intermediate band materials based on organohalide perovskites of the kind giving recently outstanding results in photovoltaics, for which we showed recently that hybrid DFT theory describes accurately the electronic structure [5]. Some promising preliminary results of this work will be discussed at the Conference.
Figure- Upper left: The intermediate band scheme. Lower left: density of states computed at GW level for V:MgIn2S4 analog of V:In2S3. Centre: spectral response of photocatalytic activity (measured with reaction rate constant k) and optical spectra (from the diffuse reflectance R), of In2S3 and V:In2S3. Right: excitation spectra (in sub-bandgap range) of 600 nm photoluminiscence (see spectrum of the latter in inset) for the same samples (same colour coding)
[1] Luque, A.; Martí, A. Increasing the efficiency of ideal solar cells by photon induced transitions at intermediate levels. Phys. Rev. Lett. 1997, 78, 5014. [2] Palacios, P.; Sánchez, K.; Conesa, J.C.; Fernández, J.J.; Wahnón, P. Theoretical modelling of intermediate band solar cell materials based on metal-doped chalcopyrite compounds . Thin Solid Films 2007, 515, 6280; and references therein [3] Lucena, R.; Conesa, J.C.; Aguilera, I.; Palacios, P.; Wahnón, P. V-substituted In2S3: an intermediate band material with photocatalytic activity in the whole visible light range. J. Mater. Chem. A 2014, 2, 8236. [4] Wahnón , P.; Conesa, J.C.; Palacios, P.; Lucena, R.; Aguilera, I.; Seminovski, Y.; Fresno, F. V-doped SnS2: a new intermediate band material for a better use of the solar spectrum. PhysChemChemPhys 2011, 13, 20401 [5] E. Menéndez-Proupin, E.; Palacios, P.; Wahnón, P.; Conesa, J.C. Self-consistent relativistic band structure of the CH3NH3PbI3 perovskite. Phys. Rev. B 2014, 90, 045207.

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