High and Low: CIGS all thin film tandem solar cells

Research question

The new generation solar cells include not only one, but several semiconductor materials and patterning processes. In this project we enable two semiconductors to interact in a tandem solar cell, with the goal of increasing the efficiency and yearly yield of the solar cell. The light absorpbing material is Cu(In,Ga)Se₂, or CIGS for short, that has a direct bandgap and therefore can be made very thin and still retain full absorption of sunlight. The solar cell materials are made by scalable vacuum processes in the clean-room at the Ångström laboratory and the solar cell absorber is made by co-evaporation of the elements. By varying the Ga/(Ga+In) ratio in the CIGS, a high bandgap material, suitable for the top cell in a tandem, and a low bandgap, suitable for the bottom cell. By using a transparent back contact, further absorption of light from albedo can contribute to the yearly energy yield. Research challenges relate to obtaining the best semiconductor materials for both top and bottom cell and to get recombination-free interfaces.

Sustainability aspects

Solar cell is today growing in all parts of the world at an increasing pace and the prognosis for 2023 predicts that 10% of the world electricity generation will be from solar cells, including also self-consumption in e.g. residential systems. Solar cells are emission-free during their long active production life-span (>25 years), and have an energy payback time of less than 2 years, today.A large part of the reduction in system prices and thereby the price per kWh have originated from the un-precedented increase of solar cell efficiencies at production scale. A high efficiency is necessary to reduce the area-related costs and thus the need for materials can be minimized, both module materials but also balance-of-cost, such as mounting and cabling. Thin films have the advantage of needing fewer active materials and lower energy needs during production (only about 1/100 of the active material needs as compared to silicon), but the thin film solar cells need to compete with the silicon-based technologies also regarding efficiency, which is the motivation for this project. A combination of sustainable production and low prices are important characteristics for a further penetration of solar energy to the electricity system