Finding universal rules for efficiency enhancing layers in thin film PV

点击翻译按钮获取中文摘要

1 PV market and the challenge for new technologiesIt 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 PVIt 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.