Finding universal rules for efficiency enhancing layers in thin film PV

1 PV market and the challenge for new technologies

It remains the case worldwide that solar photovoltaic electricity (PV) has yet to reach parity with grid connected power generation. Although there have been significant price reductions in recent years, these have only partly been driven by genuine improvements in volume manufacturing methods. The recent price crash is due to overproduction in China - it has lead to the market price being less than the cost price - a fundamentally unstable situation that must soon see a market correction. The market position is further complicated by the fact that most of the international sales are driven by government subsidy regimes that are aimed at stimulating the solar industry. In the UK, the Feed In Tariff has increased the industry from 3000 to 25000 jobs - and indeed has created such market expansion that it has had to be curtailed.

In the longer term, PV must become genuinely cheaper - and it is the aim of our research work to achieve this by making significant improvements to the cost effectiveness of new technologies.

2 Capacity for cost reduction and the drive to more sustainable thin film PV

It should be mentioned that even modest sounding improvements in efficiency are very significant: for a 10% efficient solar cell, an increase of +1% to 11% will result in a 9% reduction in the specific cost ($/Wp). This means that for a company selling 1GWp of modules at $0.5 per watt, with a turnover of $500m, there is a saving of $45m.

In recent years, the intrinsically cheaper 'thin film' solar cells have reached market, but the technology, and market position, are far from secure and mature. While the present day leaders, CdTe and Cu(InGa)Se2 (CIGS) are doing well, holding about 10% of the market, significant challenges remain. The products are changing dynamically - the technology development is far from complete, and there is pressure to increase efficiency and reduce costs.

Moreover, in the medium term future, when the thin film PV industry grows to 50 -100 times its current size, the cost and availability of some of the raw materials used will become limiting. New, low-cost, sustainable PV materials must be found. Since the development of PV devices is historically slow - half a percent improvement per year - the search has started that could influence the future of solar electricity in 20 years time.

3 Step change in understanding of efficiency improving layers in thin film PV.

In this work we are seeking to make a step change in the understanding of a method of improving the efficiency of thin film PV devices. The method is based on including an 'extra' layer into the device structure, that being a transparent but highly resistive layer. Paradoxically this improves the efficiency of the cells by increasing their open circuit voltages and fill factors, while in principle one might have expected a decrease due to the additional series resistance. At present there is no consensus in the literature as to how this benefit comes about, although it has been seen to work in a wide range of disparate technologies, including thin film CdTe, GIGS and silicon.

The aim of this proposal is to make a wide-ranging and thorough investigation of this effect, using CdTe as an exemplar for the study. Our objective is to discover the universal rules of operation that will allow the improvements to solar cell performance to be understood.

The benefit of this will be that, if successful, we will be able to recommend changes to current industrial practice that will slash costs and hence promote the solar PV manufacturing industry. More importantly, the same benefits may be applied to new materials in PV, and this would give a boost to the emerging UK effort in these new and sustainable materials. The work is therefore timely, in that it would bring understanding based advances to a field where the UK lead research has the capacity to make an impact on future technology.

1.Climate change.
Photovoltaic solar electricity is widely appreciated as being a technology that has direct and long lasting benefits in carbon emissions reduction. It is the present aim of the UK government to reduce CO2 emissions by 80% relative to 1990 levels. In all scenarios, it is expected that solar photovoltaic power production will have an increasing role to play in this achieving this aim. It is generally understood that the international effort to deal with climate change has the strongest possible benefits to society.

2. Low-cost driven model of PV expansion.
The research action proposed here is intended to assist with the viability of new and advanced low-cost solar cells. Presently the market for solar electricity is driven by subsidies, while future expansion must move to a cost competitive model in order to be successful and genuinely sustainable.

3. Future UK position in new PV industry.
In the medium to long term, the aim is that the research will contribute to an as yet unborn PV industry founded on new and sustainable materials, that on the timescale of 20 - 50 years will supersede present day market leaders, or at least form a significant fraction of the market. The UK could contribute to this by a) having a home grown cell - making industry and/or b) expanding its materials supply to the worldwide PV industry.
It has been the experience in Germany that the production of chalcopyrite based solar cells has not been dominated by big name companies: it is new start ups - often initiated by former University postdocs have been very significant. It is not therefore possible to name the companies that may benefit at this stage.

4. UK contribution to short - medium term global PV industry.
In the short to medium term, it is the existing PV industries and glass supply industries that may benefit most strongly from this research. In their simplest form, the 'HRT layer' that is the subject of this proposal is part of a multilayer coated glass product that is supplied by specialist manufacturers direct to the PV industry.
a) Glass industry
The UK already has the largest manufacturer of PV glass worldwide (Pilkington), who are supporting the project. This project carries considerable technical risk, and requires know-how and laboratories that Pilkington do not have in-house. It is for those reasons that they have not funded this work themselves. Accordingly, while Pilkington are a potential beneficiary, our collaboration agreement makes it clear that we will not be tied to them exclusively for exploitation.
b) PV industry
It is likely that the mainstream thin-film PV industries will benefit from the research, possibly via licensing of the new technology. As has been mentioned elsewhere, modest gains in efficiency shall have a significant impact on the specific cost $/Wp, and leading to very signifiant improvements in competitiveness.

5. Training.
The project will be staffed by a postdoctoral research associate and assisted by a senior experimental officer and other postdoctoral associates in the Liverpool research team. The PDRA and the existing team will all benefit from learning and the exchange of knowledge in this project. Their talents shall be relevant to not only the PV industry, but to the coatings, glass and electronics industries also.