New designs for thin film solar cells

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To limit global warming to 2degC by 2050, renewable power generation must increase from 15 to 65% (Int Renewable Energy Association [1]), and all scenarios concur that solar photovoltaic electricity generation will play a large part in this: The International Energy Agency (IEA) World Energy Outlook forecasts that solar electricity production should increase from 430 Tw-h in 2017 to 6,410 Tw-h in 2040 in its 'Sustainable Scenario' [2].Solar PV (photovoltaics) is making a strong contribution, with a 24% year on year increase in sales from 2010 to 2017. This has been driven by the tumbling costs of the leading solar PV technology, wafer silicon, which has seen massive production expansion in the Far East. It accounts for 95% of the market. Costs continue to fall by about 25% for each doubling of cumulative installed capacity ('Moore's law for solar'). Low cost thin film competitors to silicon have the advantage that they use thin coatings of highly absorbing compound semiconductors rather than sliced wafers. They are big business in themselves, with a turnover of an estimated $1,000 million per annum for CdTe alone. Nevertheless, the thin film technologies must continually improve their cost/performance ratio in order to stay ahead of the ever-falling costs of wafer silicon production. For example, the largest thin film PV manufacturer worldwide, First Solar, has just doubled the size of its CdTe modules and changed its junction design in order to maintain its market position as having the lowest cost PV available. Despite the commercial success of thin film PV, there are relatively few large commercial players. One of the reasons for this is the technological risk, and the amount of know-how required to produce the solar cells. In the case of the market leader, CdTe for example, the basic design of the semiconductor p-n junction has remained unchanged for 40 yrs. However, it contains some basic materials weaknesses that cannot be overcome. In particular, the CdTe absorber is p-type so as to make it compatible with the n-type transparent electrode. For fundamental materials reasons this limits the voltage achievable, and it makes true Ohmic contacting impossible. Both of these factors act to limit the energy conversion performance of the solar cells in practice. In this programme we will design and fabricate a new type of thin film solar cell architecture that avoids these issues and has the potential for higher efficiency performance with simplified manufacturing protocols. We will use n-type CdTe absorbers, which do not suffer from either the doping limitations or contacting issues of p-type. The work programme will centre on re-designing the solar cell architecture in order to accommodate the n-type CdTe and to realize its advantages. We expect that the new design will be capable of generating open circuit voltages of 1 V compared to the 0.87 V of present day structures. This will push solar energy conversion efficiencies up to about 25%. Moreover, since the device processing and contacting will not need to rely on 'black box' processes and know how, the new design will be more producible and will present an attractive alternative to manufacturers.[1] Renewable Power Generation Costs in 2017https://www.irena.org/publications/2018/Jan/Renewable-power-generation-costs-in-2017[2] International Energy Agency (IEA) World Energy Outlook 'Sustainable Scenario'https://www.iea.org/weo2018/