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Nano-Copper Iodide: A New Material for High Performance P-Type Dye-Sensitized Solar Cells

Periodic Reporting for period 1 - NanoCuI (Nano-Copper Iodide: A New Material for High Performance P-Type Dye-Sensitized Solar Cells)

Reporting period: 2015-09-03 to 2017-09-02

The problem we chose to address in NanoCuI was the very weak performance of p-type dye sensitized cells (p-DSSCs), a new type of solar energy device. p-DSSCs promise to increase the efficiency of dye-sensitized solar cells (DSSCs) by enabling tandem cells that absorb light at both electrodes, and lead to a new generation of efficient low-cost photovoltaics. However, no component of the p-DSSC is fully optimised, to the extent that tandem DSSCs are currently less efficient than conventional DSSCs based on a photoanode alone. The major bottleneck in the p-DSSC appears to be nickel oxide (NiO) substrates used as the semiconductor, due their inefficient charge transport. In NanoCuI we investigated the synthesis of new copper(I) iodide electrodes which in theory, should be capable of transporting charge much better, and could form a basis for new, efficient p-DSSCs. This work is important for society because it could contribute to development of clean, low-cost energy sources. Moreover, CuI has other potential uses in electronics and photonics that make development of nanoporous CuI electrodes, our goal, important. Nanoporosity is essential to provide the high surface area for dye binding, that gives high light absorption and efficient photosensitisation.

The overall objectives were:

1. Develop routes to nanoporous CuI (np-CuI) electrodes supported on conductive glasses
2. Characterize the np-CuI materials
3. Synthesize sensitizers for np-CuI
4. Assemble and test np-CuI based p-DSSCs

The overall conclusions are:

1) Nanoporous CuI can be produced by a very simple, chemical method but our characterisation has indicated that it is not the required, gamma form of the material. Instead, we appear to form an unusual, metastable high temperature form of the material known as CuI-IV. Previously, this has only been observed at ca. 670 K during the transition between the gamma and beta forms. Formation at room temperature is thus remarkable, though not yet understood.

2) Electrodeposition produced nano-sized gamma-CuI particles, however the resulting materials are not porous.

3) Pyridines seem to be the best binding groups for dyes on CuI.

4) Films of CuI-IV are not conductive enough to work well in p-DSSCs, while our gamma-CuI films are not porous enough. Thus, so far, we have been unable to produce CuI-based p-DSSCs that perform as well as existing devices based on NiO.
We have worked in three areas: synthesis of sensitiser dyes, synthesis of nanoporous CuI substrates, and testing devices based on dye-sensitized nanoporous CuI (and also NiO).

Synthesis of Sensitiser Dyes: We synthesised and fully characterised a total of 8 new organic sensitizer dyes, in addition to several known organic and ruthenium based sensitizers. The new dyes were all based on triarylamine donors, with dicyano, pyridinium or indolium acceptors. The indoliums showed the most promising properties, with strong extinction coefficients and lamda max shifted as far to the red as 580 nm. Comparison to known materials with carboxylate and thiocyanate anchoring groups suggested that pyridine was the best for binding CuI, and multiple pyridine binding groups increase the binding of dye.

Nanoporous-CuI: Initially, we pursued electrochemical deposition methods - first by deposition of Cu or Cu2O and conversion to CuI, and subsequently by direct deposition of CuI from stabilised solutions of Cu salts and iodide. However, we discovered that prior reports describing direct nanoparticle-to-nanoparticle conversion of Cu2O to CuI by displacement of oxide are likely erroneous. We were only able to effect this conversion in the presence of oxygen or electrochemical oxidation, indicating that the route must be by oxidation to Cu2+ and reaction with iodide, and found that nanostructures were substantially changed after this occurred (particle sizes became far too large). Direct deposition of CuI was ultimately more successful, with control of potential and concentration enabling formation of densely packed films of CuI nanoparticles with sizes of the order of several hundred nanometres. However, the ideal size is still around an order of magnitude smaller and the films lacked poroosity. Introduction of templates (polystyrene nanospheres) made the films less flat and dense but removal of template led to disruption of the film structure.

A chemical deposition approach (using copper sulfate, plus KI and thiosulfate) generated structures of long (up to one micron), thin (sub 100 nm) particles, interpenetrating with large void space. These structures appear very promising, but our characterisation (powder X-ray) indicates that they are not the required gamma form of CuI. Instead, they are a very unusual, metastable form of CuI known as CuI-IV. This is normally observed as a high temperature intermediate (at around 670 K). We are continuing to study this material, to understand its properties and how it could have formed, as isolating it this way is quite remarkable. It may be possible to transform to gamma-CuI by heat treatment.

Solar devices: Conductivity measurements on the CuI-IV suggest it does not have the desired high conductivity. This is consistent with measurements on solar cells assembled with this material, which have efficiencies around an order of magnitude lower than systems based on NiO. The electrodeposited gamma-CuI also failed to perform well, probably due to its lack of porosity. However, we observed some trends in the performance of the different dyes on both substrates that could be informative for future work.

In addition, the fellow contributed to measurements completing another project running on p-DSSCs, resulting in a publication in Phys. Chem. Chem. Phys.

Exploitation/Dissemination: This project has not produced results suitable for commercial exploitation. Apart from the paper in PCCP mentioned above, results from the project were presented at the Southern Dalton meeting in London (28th June) and the 22nd International Symposium on Photochemistry and Photophysics of Coordination Compounds (ISPPCC 2017, Oxford, 9th-14th July). We are currently preparing manuscripts describing the performance of the new sensitizers on NiO, and synthesis, characterisation and photosensitisation of nanoporous CuI-IV.
The key progress beyond the state of the art is that we have managed to produce a nanostructured, porous form of CuI and sensitizer dyes that bind it effectively. It is also remarkable that the CuI we isolate is usually considered a high temperature metastable form. The properties of this material are not well known and our procedure will facilitate its study.

It is possible that heat treatment will convert our CuIV to the desired gamma-CuI. This is currently under investigation with a new collaborator we acquired during the course of this project - Dr Geoff Hyett at the University of Southampton. If this can be done, we will have discovered a route to a potentially very useful material for solar energy. Thus, in the long term, our project could yet have major scientific and economic impact.

NanoCuI has also had positive impact through training of the fellow, Dr Anil Reddy Marri, who has learned new skills in synthesis and characterisation of nanomaterials and will be able to deploy them in his home country, India.
Chemically and electrochemically deposited CuI films