Polyoxometalate-Based Sensitizers for p-Type Dye-Sensitized Solar Cells

Lead Research Organisation: University of East Anglia
Department Name: Chemistry

Abstract

Photonic materials interact with light in interesting and useful ways. They are vital to many current and emerging technologies, such as biological imaging, optical data processing, telecommunications and solar energy. This project will investigate the properties of a promising, but little explored class of photonic materials and test them as a means to improve the performance of an emerging type of solar cell - the p-type dye sensitized photocathode (p-DSSC). In this way, our long-term goal is to develop low cost, high-efficiency solar energy devices which will help reduce carbon emissions and dependence on imported fossil fuels.

Dye-sensitized solar cells (DSSCs), based on a dye-sensitized n-type titanium dioxide photoanode, promise a low-cost alternative to conventional semiconductor photovoltaic (PV) materials like silicon. They function well in northern-European, low-light conditions but their peak power conversion lags far behind that of the best semiconductor designs, which combine several different semiconductors optimized to absorb different portions of the solar spectrum. DSSC performance may be improved through an analogous approach - tandem DSSCs which pair the usual dye-sensitized photoanode (n-DSSC) with a dye-sensitized photocathode (p-DSSC). With complementary absorption profiles, the n- and p-DSSCs absorb more sunlight together in the tandem DSSC than either can alone. Currently, though, the efficiency of the p-DSSC (record 1.3%) is far from matching that of n-DSSCs (10 to 15%). This means that tandem DSSCs perform worse than n-DSSCs by themselves, and p-DSSCs must improve dramatically for the tandem DSSC to become a viable device.

Both n- and p-DSSCs depend on efficient charge separation at the interface between a dye and a metal oxide support to generate electricity. n-DSSCs achieve useful efficiencies because light causes the dyes to rapidly inject electrons into an n-type (electron transporting) metal oxide. Transport of electrons through the metal oxide, and filling of "holes" formed in the dyes by electrons from a redox electrolyte, is much faster than recombination (return of electrons) to the dye from the metal oxide. p-DSSCs work in the opposite sense, injecting holes into a p-type (hole transporting) metal oxide, with the redox electrolyte taking electrons from the dyes. The problem for p-DSSCs is that transport of holes through oxides is slow, and recombination from the dye and electrolyte is fast. This leads to low efficiency.

In this project, we will synthesize a novel class of dye for the p-DSSC, based on connection of electron accepting multi-metallic clusters (polyoxometalates, POMs) to organic groups. By holding electrons away from the metal oxide surface, the POM electron acceptor groups will slow recombination and improve performance. The proposed POM-based sensitizers have an electronic structure that will favour charge separation, and are expected to have important advantages - in stability and ability to rapidly transfer electrons to the redox electrolyte - over the current purely organic materials.

Planned Impact

The multidisciplinary nature of this project gives rise to potential for impact in three main areas. These are:

1. Economic and Social Impact: This project will develop new photonic materials, with the goal of improving the performance of an emerging type of solar cell. The economic impact of a low cost, high efficiency means to generate solar electricity cannot be overstated. It would reduce our dependence on costly fossil fuel imports, reduce the need for expensive and inefficient electricity transmission networks by enabling localized power generation, and provide the first step towards converting sunlight to chemical fuels. The new materials proposed are also potentially relevant to a whole range of technologies requiring the manipulation of light - for example optical switches, optical computing/telecommunications, optical power limiters, and imaging agents - all of which can provide economic impact.

In addition, the project contributes to supporting the UK economy by training highly skilled workers - as it involves synthesis, advanced photophysical measurement and device assembly/testing, the PDRA will gain skills which are widely marketable in academia and beyond. Such training can have a direct economic impact by forming individuals with high level problem solving and other transferrable skills, as well as specific scientific knowledge.

2. Environmental Impact: All photovoltaics have the potential to produce large quantities of low-to-zero carbon electricity, reducing our CO2 emissions and helping minimize climate change. Dye-sensitized solar cells (DSSCs) in general offer lower processing costs (both environmental and monetary) and embedded energy than traditional silicon-based PV, and often give better performance in northern-European light conditions. The efficient p-type DSSCs targeted here, through pairing with conventional n-type DSSCs in "tandem" devices, could lead to a step change in the applicability of DSSCs because once optimized, tandem DSSCs are expected to offer better performance than any other affordable PV technology in UK conditions. These factors give this project potential for a very positive environmental impact.

3. Social and Cultural Impact - through fundamental science: As the project involves synthesis of new materials and evaluating their properties, it clearly impacts on fundamental science. The photonic properties of the class of material I am interested in are almost completely unexplored, and the p-DSSC is at an embryonic stage. Therefore the discoveries we make in this project could influence science significantly, for some time. Topics like energy are an excellent vehicle for interesting students in science, and for this reason the original n-DSSCs have become an undergraduate experiment and textbook example of photochemistry. My hope and belief is that p-DSSCs will follow their example.

Publications

10 25 50
 
Description We have discovered that using polyoxometalates (POMs) as co-absorbents in p-type dye-sensitized solar cells (p-DSSCs) dramatically increases photovoltages and slows recombination (the undesirable tendency for electrons to return to the p-type substrate, which causes inefficiency). However, overall efficiency gains are small at best, because currents are reduced as forward electron transfer from the POMs is also slow.

POM derivatives by themselves also have a weak sensitizing effect on the p-type substrates. However, attempts to make efficient photosensitizers based on POMs met with unexpected problems in synthesis and stability.

We have however found a solution to the stability problems through synthesis of some unprecedented mixed-metal POM derivatives. The PDRA working on this award also contributed to a closely related project investigating imido-Lindqvist POMs as a new class of non-linear optical chromophore. This has demonstrated that the materials combine a high activity for laser frequency doubling, with relatively high optical transparency in the visible region - a potentially valuable combination for use in optical telecommunications or computing.
Exploitation Route Our use of POM coabsorbents is new and could provide a general method to improve p-DSSCs. It appears to address two of the most important problems of the device. However, we need to find ways to improve the onward electron transfer and possibly integrate the POM with the sensitizer.

Our new mixed-metal systems could find wide application as they connect a POM to an organic fragment in a very stable way, opening use of hybrid POM-organic systems in many applications (molecular electronics, energy, catalysis) where they are currently impractical. The results on NLO chromophores are the most promising and are currently the subject of an EPSRC grant application by us.

In the short term, these findings will be taken forward by us and other academic groups. In the long term, industrial concerns interested in energy, molecule-based electronics, and catalysis could exploit them.
Sectors Electronics,Energy

 
Description RSC Researcher Mobility Fellowship
Amount £6,500 (GBP)
Organisation Royal Society of Chemistry 
Sector Learned Society
Country United Kingdom
Start 04/2015 
End 05/2017
 
Description Redox Switchable Photonic Materials Based on Organoimido-Polyoxometalate/Cyclodextrin Host-Guest Complexes
Amount £349,185 (GBP)
Funding ID EP/R042675/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 09/2018 
End 09/2021
 
Description Royal Society Research Grant
Amount £15,000 (GBP)
Organisation The Royal Society 
Sector Academic/University
Country United Kingdom
Start 04/2015 
End 04/2017
 
Description UEA Faculty of Science Studentship
Amount £58,251 (GBP)
Organisation University of East Anglia 
Sector Academic/University
Country United Kingdom
Start 10/2018 
End 09/2021
 
Description Dr Bruce Brunschwig, Caltech, Stark Spectroscopy 
Organisation California Institute of Technology
Department Beckman Institute
Country United States 
Sector Academic/University 
PI Contribution Funding, manpower, materials for study.
Collaborator Contribution Equipment, expertise, consumables for spectroscopy.
Impact Collaboration has enhanced understanding of photophysical properties of the POM derivatives we work on in the grant. A manuscript will shortly be submitted to inorganic chemistry.
Start Year 2015
 
Description Elizabeth Gibson - p-DSSCs 
Organisation Newcastle University
Department School of Chemistry
Country United Kingdom 
Sector Academic/University 
PI Contribution Synthesis and characterization of samples Manpower for measurements (through secondments) and help with interpretation
Collaborator Contribution Assembly of p-type dye-sensitized solar cells and performance measurements. Data analysis and interpretation
Impact Outcome - we are able to increase p-DSSC photovoltage and decrease recombination. Results are still being analysed however
Start Year 2015
 
Description Invited talk at Newcastle University 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact This was a research talk (ca. 50 min) to members of the Newcastle University School of Chemistry. There were several questions and discussion afterwards. The visit reinforced the existing collaboration with Dr. Elizabeth Gibson that forms part of the award, and will potentially initiate new ones with Dr. R. John Errington and Dr. Fabio Cucinotta.
Year(s) Of Engagement Activity 2016
URL http://www.ncl.ac.uk/chemistry/about/seminars/item/to-be-confirmed15
 
Description Invited talk at University of Southampton 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact Visited the National Crystallography Service with a sample related to the grant. Met other members of Southampton academic staff and gave a talk featuring work on polyoxometalate derivatives as charge transfer chromphores for non-linear optical and solar energy applications.

I also discussed other research projects with members of staff.
Year(s) Of Engagement Activity 2016
 
Description Invited talk at the FMOCS-PoCheMON 2016 meeting in Newcastle 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact I gave a talk featuring results from the grant at the FMOCS-PoCheMON polyoxometalate meeting in Newcastle, July 2016.
Year(s) Of Engagement Activity 2016
URL https://conferences.ncl.ac.uk/pochemon2016fmocs/
 
Description Invited talk at the University of Sussex 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact Visited University of Sussex and gave a talk containing research from this award.
The audience were a mix of postgraduates, post-docs and academic staff. I received some engaged questions afterwards.
The main outcome is potential future collaboration and use of Sussex mass spectroscopy facility by me and other UEA faculty.
Year(s) Of Engagement Activity 2018
 
Description Poster presentation at the Inorganic Chemistry Gordon Research Conference, 2016 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact One of two posters I took to the 2016 Inorganic Chemistry GRC featured results from the grant.
Year(s) Of Engagement Activity 2016
URL https://www.grc.org/programs.aspx?id=15122
 
Description Talk at European Materials Research Society Spring Meeting 2016 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact I gave a talk featuring results from this grant at the European Materials Research Society conference in Lille, in May 2016.
Year(s) Of Engagement Activity 2016
URL http://www.european-mrs.com/meetings/2016-spring-meeting
 
Description Talk at Polychar conference, Poznan, Poland 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact I gave a talk featuring results from the grant at the Polychar polymer/materials conference (Polychar24).
Year(s) Of Engagement Activity 2016
URL http://www.polychar24.divisia.pl/teksty.php?plik=index.php