Tuning the active materials for the engineering of tandem solar cells is an arising field of research among the perovskite community. In this post, I present the main targets of tandem solar cells engineering: band-gap tuning. Then, I showcase two solutions that have been reported by students I am currently working with.
Tandem Solar Cells or the Necessity of Wide Band-gaps
Metal halide perovskites, of the form ABX₃ (where A can be methyl ammonium, formamidinium, cesium or a mixed of both, B lead or tin and X an halide, typically iodide, bromide, or mix of both), are gaining much attention for thin film photovoltaics, thanks to their long carrier diffusion lengths, their defect tolerance enabling versatile solution (or vapour) processing and a very strong optical absorption. The efficiencies of so-called perovskite solar cells are comparable to the one achieved with the incumbent silicon technologies.
Two market solutions can be proposed for perovskites: either challenging the incubent silicon, or use perovskite as a layer that can be stacked on top of the silicon cell, boosting its efficiency. For this later solution, a wide band-gap needs to be achieved with perovskites, in order to maximise the energy collection. In fact, the silicon solar cells have a very low band-gap. It means that every incident photon having an energy higher than the silicon band-gap will be collected, but the energy difference (the energy higher than the bang-gap), will be dissipated as heat loss. Thus the need of a layer that would extract a portion of higher voltage photo-generated carriers will be valuable in order to overpass the fundamental efficiency limit for single junctions solar cells, commonly known as the Shockley-Queisser limit.
Because the perovskite have a tunable band-gap, that can be controlled with halide substitution (for example), they tend to be considered as the best solution for more efficient solar cells. Their ability to be solution-processed or vapour-deposited would allow to benefit high quality scalable inexpensive integration on existing devices. State of the art perovskite-silicon tandem solar cells achieved record efficiencies of 23.6% for monolithically integrated cells, and 26.4% for mechanically stacked configuration. Attempts to make perovskite-perovskite tandem solar cells have also been tried with record efficiencies of 18.5%, but these rely largely on the development of more efficient and more stable low-end-gap perovskite absorbers (1.1 to 1.3eV). The PCE figures are expected to grow significantly with deeper understanding of the materials.
All these aspects make the development of perovskites for tandem devices an active field of research.
Band-gap Tuning Induced via Compositional Engineering
In order to achieve higher band-gap solar devices, compositional tuning of the solar cells have been tried by the materials Science & Engineering group led by Mike McGehee. His students, Bush et al. describe their result in their paper: “Compositional Engineering for Efficient Wide Band-Gap Perovskites with Improved Stability to Photo-induced Phase Segregation”.
They explore various compositions of hybrid perovskites of the form FA1-xCsxPb(I1-yBry)3. They investigate the performance of solar cells as well as the photo-stability by varying the composition between formamidinium (FA) and cesium (Cs) in the perovskite A-site and between iodide and bromide on the X-site. With that, they show that increasing the concentration in cesium into the solid-solution thin film results in higher VOC and greater photo-stability, as it raises the bandgap. This can be compared with the previously known raise of bromide, that yields higher band-gap, with a fewer photo-stability and VOC.
That article might be particularly important in the development of high efficiency perovskite tandem solar cells, as the authors identify stable compositions with high band-gap (1.68 to 1.75eV), that demonstrate high device efficiencies.
Band-gap Tuning Induced with Change of the Lattice Constants
Variation of those A-site cation changes the band-gap. Prasanna et al. proposed an interpretation in their JACS paper: “Band-gap Tuning via Lattice Contraction and Octahedral Tilting in Perovskite Materials for Photovoltaics”. By exploring both lead and tin on the B-site, they allow different octahedral sizes, as the length of the metal-halide bond changes. In this paper, they show that the use of a smaller A-site cation can distort the perovskite lattice in two different ways: either by tilting the BX₆ octahedra (pictured on the right), or by contracting the lattice isotropically (depicted on the left). The first effect yields to a change of the metal-halide orbital overlap, that varies the band-gap. The second results in a relative contraction that increases the orbital overlap, thus cutting down the band-gap.
Those two strategies can be achieved with partial substitution of the large FA cation with the smaller Cs. With lead halide perovskites, this results in octahedral tilting but with tin based materials to lattice contraction, due to the smaller size of the tin.
The authors therefore provide a framework to tune the band-gap as well as the valence and conduction band positions by controlling the A-site cation composition.
In order to achieve high efficiency perovskite-based tandem solar cells, it is necessary to adjust the band-gap of the top cells. As shown on this post, this can be improved through control of the cation composition, resulting in higher band-gaps. These results will, for sure, be needed for future high efficiency solar panels.
The cover picture is from Michigan Engineering on Flickr. Scheme from Prasanna et al.