For each solar cell semiconductor material, there is an intrinsic limit to how much light can be converted into electricity. For silicon PV technology, for example, that limit is around 30%, and the industrial solar panels are now reaching results that are plateauing some 10% below this limit. From now on, additional gains will be small and not so easy to attain.
Therefore there is a growing interest in tandem cells, solar cells that are made by stacking two or more cells on top of each other to reach a power conversion efficiency that is higher than what would be possible with the separate technologies. An obvious requirement is that the material of the top cell is transparent to the light that it doesn’t convert. An additional boost is given if the materials in the stack are sensitive to a distinct part of the light spectrum, so that they don’t scavenge on each other’s conversion potential. The material that captures the shortest wavelengths (e.g. blue and green) will then be on top, while the bottom material converts the longer wavelengths (e.g. red and near-infrared).
Tandem cells, or multi-junction cells as they are also called, were originally used for spacecraft, where expensive III-V materials such as gallium-arsenide (GaAs) or germanium (Ge) were first combined to reach higher efficiencies than what was possible with any single-junction cell. These cells, however, are much too expensive for terrestrial, large-scale use. To build cost-effective tandem cells, the most obvious way is to add a new material on top of today’s conventional mono-junction cells, which are either based on silicon or on a thin film material such as CIGS (copper indium gallium selenide).
A new, intriguing material
A low-cost candidate material for the top cells is perovskite, a material that only recently appeared on the PV technology radar. Perovskite is a family of materials that have been known for some time because of their intriguing physical properties. They have been studied e.g. as a piezoelectric material, as superconductors, or to catalyze chemical processes. And only as recently as 2009 also to convert light into electricity, integrated in a dye-sensitized solar cell architecture with a modest 3.8% conversion efficiency and a short lifetime. Since then, labs around the world have steadily improved the architecture, reliability and efficiency of perovskite solar cells, now above 22%.
Perovskite solar cells have many desirable properties. They are potentially cheap to produce: they can be made with simple fabrication techniques such as coating and printing with ink-like materials. And they have a high absorption efficiency for sunlight, so not much of the material is needed, a layer of at most a few hundred nanometers. Moreover, the material can be carefully engineered to result in various optical and electronic properties, even transparent cells are possible.
Just what the experts were looking for as a top cell material: thin, transparent, cost-effective, tunable.
A turbo boost for silicon and CIGS
Imec’s researchers chose silicon and CIGS technologies – the most widely fabricated and installed PV technologies – to add a perovskite boost layer. An obvious choice, because imec has a decade long expertise with improving solar technology in an industrially relevant way and in collaboration with solar cell manufacturers.
A first result was obtained by stacking a 0.13 cm² spin-coated perovskite cell stacked on top of a 4 cm² industrial silicon cell in a 4-terminal configuration (more about that later). The silicon cell used in this case has an IBC architecture (interdigitated back-contact), meaning that it has both contacts on the backside, leaving the front side on which the perovskite cell sits completely free to convert incoming light. With a power conversion efficiency of 27.1 percent, this tandem already beats the most efficient standalone silicon solar cell, with room for further gains.
And a second record, 24.6%, was obtained again with a 0.13 cm² perovskite cell, but this time stacked on top of a 0.5 cm² CIGS module. This work was done in collaboration with one of the world-leaders in CIGS technology, the Zentrum für Solare Energiewirtschaft (ZSW, Stuttgart, Germany). This result beats that of last year’s demonstrator with a whopping 5%, showing the potential for improvement that can be made with tandem combinations like these. Compared to the previous year’s tandem, the translucence of the perovskite cell for near-infrared light was improved, and the perovskite material itself was engineered to have a better tandem efficiency.
A notable difference between imec’s two tandem cells is that unlike silicon technology, CIGS is a thin-film technology. That makes the perovskite-CIGS cell a full thin-film solution that can be fabricated on flexible foils. To be used for example in building-integrated PV applications.