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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - p-TYPE (Transparent p-type semiconductors for efficient solar energy capture, conversion and storage.)

Teaser

Summary

p-TYPE addresses the scientific and technological challenges required to transform the use of solar energy by reducing payback time, improving manufacturability, innovating the technology for integration into buildings or devices and solving the energy storage issue. We are developing tandem dye-sensitized solar cells, which are a unique way to make a step change in the efficiency of solar cells which function at a molecular level. An efficient tandem DSC has not yet been developed because p-type DSCs are much less efficient than n-type cells. The goal of this project is to increase the efficiency from < 2% to > 20% by replacing the nickel oxide conventionally used with a new material which is optimised for dye-sensitized photocathodes: a wide band gap to allow the dye to absorb the light; better conductivity for higher photocurrent; a higher ionisation potential for higher voltage. By adding catalysts to the device, we can drive chemical reactions such as carbon dioxide reduction or hydrogen production from water to make fuel, so the dual challenges of energy conversion and storage are addressed.
n-Type transparent conducting oxides are present in many devices but their p-type counterparts are not largely commercialized as they exhibit much lower conductivities. The core part of the project focuses on making libraries of mixed metal oxides and selecting those which are promising p-type semiconductors. Our high-throughput synthesis and screening system will enable us to accelerate the discovery and optimisation processes. Promising materials are assembled in tandem DSCs and tested. Our objectives are: A) Improve the efficiency of p-type dye sensitized solar cells and water-splitting cells by incorporating new p-type semiconductors which, for the first time, combine good transparency and high conductivity. B) Drive innovative engineering for the fabrication of high-efficiency low-cost tandem devices incorporating new photocathodes as a means of converting the majority of solar radiation striking the Earth. C) Underpin our research with state-of-the-art techniques for solar cell characterization to connect the fundamental research carried out at the molecular level and the events that take place in the device as a whole.

Work performed

An objective of this work is to determine the structure property relationships which affect electron transfer and hole transport in p-type semiconductors to guide the optimisation of photoelectrochemical devices for sustainable energy generation and storage in the future. To achieve this, we have developed three a rapid and controlled routes to new semiconductor materials and morphologies to obtain libraries of compounds from which those with desired properties can be selected. The first is a bespoke co-precipitation reactor to prepare libraries of potential NiO replacements. The precursor salts are premixed before entering the reactor, where they are hydrolysed to form mixed metal hydroxides, which are collected and processed for applying in solar cells. The level of control allows us to reliably and reproducibly prepare libraries of compounds with different electronic and morphological properties. Secondly, we have installed an ultrasonic spray system for depositing thin, uniform films of metal oxides as an alternative approach. This can also be used for depositing thin insulating layers to prevent shunting in the device or to deposit catalysts on the counter electrodes. Finally, we have built a robot for depositing materials by successive ionic layer adsorption and reaction (SILAR), which can be used to grow or coat thin films of semiconductors. We are also applying this to deposit thin layers of inorganic absorbers such as metal sulphides (Sustainable Energy and Fuels, 2019).

We have optimised a formulation which allows us to print arrays of compounds on conductive glass, for screening according to their transparency, conductivity and sensitization by a dye. We have also developed and validated a method to screen the arrays new materials with combinations of dyes. From our simple diagnostic tests, we are able to identify the materials fulfilling most or all of the optoelectronic criteria for efficient tandem cells. We characterise and classify interesting materials according to their chemical and electronic structure, both at the surface and the bulk, their morphology and optoelectronic properties. Specifically, we have shown that XPS can be used to probe the energy alignment of the electronic orbitals of the dye with the states in the semiconductor, which determines the performance of the device. We then use these results to understand how the structure and properties of the materials affects the dynamics of charge-separation at the dye-semiconductor interface, under operational conditions. The results of new materials, initially based on NixMyOz and CuxMyOz, are compared to our benchmark materials, NiO and CuCrO2. As well as dyes, we have tested inorganic absorbers (quantum dots), which have tuneable band-gaps, large absorption coefficients, can transfer charge rapidly to a second semiconductor, but are faster to synthesise than dyes.

We have presented results at several major national and international conferences. We have also carried out public engagement activities to showcase our research at the Science Museum London and the Great North Museum (Hancock) Newcastle. We are also working on technology transfer and hosted an intern from an international company, who we trained to make flexible solar cells.

Final results

The validation of the co-precipitation reactor has enabled us to begin assembling the library of materials in a controlled way. This has increased our capacity to study these materials beyond that typical of an academic research group. In the remaining time we expect to generate a large number of materials with different compositions, structures, morphology and doping.

To quickly screen the materials, we have taken a technique used widely in medicinal chemistry and pioneered its application towards materials discovery and solar cell device assembly and characterisation for the first time. This provides a rapid and non-destructive method to scan combinations of materials/processing techniques/module configurations to accelerate the development of thin-film photovoltaic (PV) technology.

We have also shown how XPS using hard and soft X-rays from a synchrotron source can be used to probe the energy alignment of dyes with the valence band of semiconductors. Previously this was used for looking at the alignment of the energy of the states in the dye with those in n-type semiconductors, which was a simpler process. This new approach is providing us with more information about the interface between the molecules and the material which cannot be achieved with such accuracy by any other technique.

Our research in tandem quantum-dot sensitized solar cells has advanced the efficiency beyond what has been achieved with dyes and has demonstrated the potential for flexible devices. It has also provided a route to address the mass-transport limitations in our solar cells. We still have some concerns over the sustainability of some of the materials used, and future work will address this. There is much optimisation still left to do, but we anticipate that we can exceed 15% efficiency during the period of the grant.