Prof Henry Snaith FRS

Abstract:

The ability to process amorphous or polycrystalline solar cells at low temperature (<150 °C) opens many possibilities for substrate choice and monolithic multijunction solar cell fabrication. Organometal trihalide perovskite solar cells have evolved rapidly over the last two years, and the CHNHPbX (X = Cl, I or Br) material is processed at low temperature. However the first embodiments of the solar cell were composed of high temperature processed (500 °C) compact and mesoporous layers of TiO. The sintering of the mesoporous TiO has been negated by replacing this with a mesoporous insulating scaffold in the meso-superstructured solar cell (MSSC), yet the high temperature processed compact TiO layer still persists in the most efficient devices. Here we have realised a low temperature route for compact TiO, tailored for perovskite MSSC operation. With our optimized formulation we demonstrate full sun solar power conversion efficiencies of up to 15.9% in an all low temperature processed solar cell. © The Royal Society of Chemistry 2014.

Abstract:

The highest efficiencies in solution-processable perovskite-based solar cells have been achieved using an electron collection layer that requires sintering at 500 °C. This is unfavorable for low-cost production, applications on plastic substrates, and multijunction device architectures. Here we report a low-cost, solution-based deposition procedure utilizing nanocomposites of graphene and TiO2 nanoparticles as the electron collection layers in meso-superstructured perovskite solar cells. The graphene nanoflakes provide superior charge-collection in the nanocomposites, enabling the entire device to be fabricated at temperatures no higher than 150 °C. These solar cells show remarkable photovoltaic performance with a power conversion efficiency up to 15.6%. This work demonstrates that graphene/metal oxide nanocomposites have the potential to contribute significantly toward the development of low-cost solar cells.

Abstract:

Solid-state dye-sensitized solar cells rely on effective infiltration of a solid-state hole-transporting material into the pores of a nanoporous TiO 2 network to allow for dye regeneration and hole extraction. Using microsecond transient absorption spectroscopy and femtosecond photoluminescence upconversion spectroscopy, the hole-transfer yield from the dye to the hole-transporting material 2,2′,7,7′-tetrakis(N,N-di-p- methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) is shown to rise rapidly with higher pore-filling fractions as the dye-coated pore surface is increasingly covered with hole-transporting material. Once a pore-filling fraction of ≈30% is reached, further increases do not significantly change the hole-transfer yield. Using simple models of infiltration of spiro-OMeTAD into the TiO2 porous network, it is shown that this pore-filling fraction is less than the amount required to cover the dye surface with at least a single layer of hole-transporting material, suggesting that charge diffusion through the dye monolayer network precedes transfer to the hole-transporting material. Comparison of these results with device parameters shows that improvements of the power-conversion efficiency beyond ≈30% pore filling are not caused by a higher hole-transfer yield, but by a higher charge-collection efficiency, which is found to occur in steps. The observed sharp onsets in photocurrent and power-conversion efficiencies with increasing pore-filling fraction correlate well with percolation theory, predicting the points of cohesive pathway formation in successive spiro-OMeTAD layers adhered to the pore walls. From percolation theory it is predicted that, for standard mesoporous TiO2 with 20 nm pore size, the photocurrent should show no further improvement beyond an ≈83% pore-filling fraction. Solid-state dye-sensitized solar cells capable of complete hole transfer with pore-filling fractions as low as ∼30% are demonstrated. Improvements of device efficiencies beyond ∼30% are explained by a stepwise increase in charge-collection efficiency in agreement with percolation theory. Furthermore, it is predicted that, for a 20 nm pore size, the photocurrent reaches a maximum at ∼83% pore-filling fraction. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

Mesoporous single crystals (MSCs) of TiO2 are promising materials for more efficient dye sensitized solar cells and other energy conversion or storage devices, since they combine high surface area with large crystalline domain size. In this work, we investigate the charge transport properties of TiO2 MSCs after annealing them within a confining template at temperatures from 500 to 850 C. We observe that higher anneal temperatures do not change the crystal phase, as in nanocrystalline TiO 2, but do influence the MSC absorption spectrum in a manner consistent with the signature of increased oxygen-vacancy defects. By comparing MSC film conductivity in vacuum and in air, we infer that these anneal-induced defects increase the background charge density in TiO2. Subsequently, we measure higher effective mobility in annealed MSCs using transient mobility spectroscopy (TMS), consistent with higher anneal temperatures filling sub-bandgap trap states by n-doping TiO2. Finally, we measure faster charge transport rates in solid-state dye sensitized solar cells as well as increased open-circuit voltages at low light intensity with increasing MSC anneal temperature. This study leverages the fixed geometry and crystal phase of MSCs under thermal treatment to identify and isolate the doping effect of annealing at high temperature, previously inaccessible for mesoporous anatase TiO2. The results offer insight into the influence of doping on charge transport in TiO2-based solar cells and the tunability of MSCs for use in enhancing device performance. © 2013 American Chemical Society.

Abstract:

Neutral-colored semitransparent solar cells are commercially desired to integrate solar cells into the windows and cladding of buildings and automotive applications. Here, we report the use of morphological control of perovskite thin films to form semitransparent planar heterojunction solar cells with neutral color and comparatively high efficiencies. We take advantage of spontaneous dewetting to create microstructured arrays of perovskite "islands", on a length-scale small enough to appear continuous to the eye yet large enough to enable unattenuated transmission of light between the islands. The islands are thick enough to absorb most visible light, and the combination of completely absorbing and completely transparent regions results in neutral transmission of light. Using these films, we fabricate thin-film solar cells with respectable power conversion efficiencies. Remarkably, we find that such discontinuous films still have good rectification behavior and relatively high open-circuit voltages due to the inherent rectification between the n- and p-type charge collection layers. Furthermore, we demonstrate the ease of "color-tinting" such microstructured perovksite solar cells with no reduction in performance, by incorporation of a dye within the hole transport medium.