Cellules TANDEMS

Research topic : Tandem Solar Cells among FedPV laboratories.

Contacts:
Olivier.durand@insa-rennes.fr
Alain.fave@insa-lyon.fr

- Introduction and context

The photovoltaic (PV) sector is experiencing a fast growth and becoming one of the main pillars for the energy transition. In this context a challenging roadmap was issued during COP2 ^₍₁₎, initiated by IPVF and supported by ten other institutes worldwide, named “30-30-30”. The so-called thirty cube objective aims to reach 30% conversion efficiency for modules at 30 cents per Watt in 2030. This target cannot be achieved with standard solar cells based on single junction. It is due to the absolute Shockley-Queisser (SQ) limit of about 33% at maximum, for an optimized 1.4 eV bandgap of the absorber material and the theoretical maximum efficiency is limited to about 29.1% for silicon with a band gap of 1.12 eV.^₍₁₎ The present cell records are very close to the limit, at 29.1 % for GaAs (30.5% under 258 suns) and 26.7 % for Si ^₍₂₎ (27.6% under 92 suns). It will become more and more difficult to increase further the efficiency. Searching for alternative routes to overpass the SQ limitation becomes now a major objective for PV R&D worldwide along the 30 cube objective. A straightforward strategy is to shift from single to tandem junctions which can reach 43% theoretically by combining 2 subcells: a bottom and a top cell collecting respectively the visible/near Infrared and the blue/UV range of the solar spectrum^₍₃₎. Reaching 30% in production would be feasible with tandem devices with about 20% efficiency for each junction⁽⁴⁾.This high efficiency technology is currently based on III-V materials grown on expensive GaAs substrates. Indeed, the optimal experimental efficiency has been obtained with the well-known GaInP/GaAs bandgap combination with a state-of-the-art efficiency of 32.9% (35.5% under 38.1 suns). However, the multi-junction approach is considered to be limited by high production costs for large-scale applications ⁽⁵⁾. It is possible to reduce the cost of such device by using the existing silicon technology for the bottom cell and/or CIGS, Perovskites for the top cell.

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- The FedPV position

The targeted tandem cell architectures rely on a gap stacking of 1.7eV/1.1 eV, which provides the maximum theoretical power conversion efficiency with Si as a bottom cell.
Candidates are either to combine silicon bottom cells with perovskite (PRX) top cells, with an efficiency of 29.8% (HZB), or with wide bandgap CIGS, or with III-V (planar or nanowires). Additionally, tandem cells combining thin film devices are also investigated.
There are several approaches pursued: monolithic growth, monolithic integration (with smart stacking), and 4T tandems. All these are covered by the projects in FedPV.

Tandem cells made of III-V nanowires on Si substrates
Most direct band-gap III-V compounds are lattice-mismatched with silicon, leading to detrimental dislocation densities. Therefore, the approach of growing nanowire (NW) arrays on top of the silicon avoids the lattice-mismatch issues. Moreover, this approach benefits from a large light trapping effect, a reduction of the reflection losses and a production cost reduction as compared with the planar III-V layers approach.
It has been proposed to develop two-terminal tandem cells made of a top III-V nanowires cell onto a bottom silicon cell, (111)-oriented, electrically connected through an all-Si tunnel junction (C2N).

Towards this aim, the C2N has developed a top NW core-shell solar cell proof-of-concept, either using a GaAs homojunction⁽⁶⁾ , a GaAsP homojunction⁽⁷⁾, or a GaAs/GaInP heterojunction, passivated with a AlInP window layer, and epitaxially grown onto a patterned silicon substrate. The top electrical contact is made with n-GaAs/ITO. The NWs core/shell are grown by the VLS method for the core and a VS one for the shell.
Moreover, the ANR-HETONAN consortium (INL, C2N, IMEP-LAHC and a start-up-SILSEF) has developed tandem cells composed of a top cell of GaAs/AlGaAs core/shell nanowires on a silicon bottom junction. Again, the nanowires are grown by the VLS method, using molecular beam epitaxy, and the top electrical contact is made with ITO. BCB is used as an encapsulation layer.^(8,9)

Tandem cells made of III-V planar layers on Si substrates
Within FedPV (and IPVF), the LPICM lab, in collaboration with the 3-5 lab, proposes also to develop two-terminals tandem cells made of III-V materials onto cheap silicon substrates. They avoid the well-known issue of the growth of a III-V polar material onto a Silicon non-polar one through an inverted metamorphic approach where the Si is deposited by low-temperature PECVD onto the GaAlAs material, itself grown by MOVPE on a GaAs(001) substrate (removed thereafter and which can be recycled for other processes). Then, the final targeted structure is a AlGaAs top-cell onto an Epi Si(Ge) bottom-cell, electrically connected by a tunnel junction (TJ), and reported on a low-cost host substrate. A more recent approach developed by LPICM in collaboration with C2N, III-V Labs and IES (Madrid) consists on the use of ultrathin Ge layers deposited at low temperature by PECVD, which allow to grow the III-V materials on a c-Si substrate used as a bottom cell. Last but not least, the low cost deposition of III-V materials at low temperature using PVD and PECVD techniques is studied at LPICM in the framework of the IPVF common research program.
MBE-grown III-V ternary compounds with a bandgap around 1.8-1.9 eV are also studied to develop the top-junction subcell: after unsuccessful development of a n-GaInP/p-GaInP cell, due to a poor carriers collection generated in the p-GaInP absorber ⁽¹¹⁾, a consortium composed of IPVF, Total new energies, C2N and EDF R&D have studied heterojunctions composed of n-GaInP/p-GaAlAs structure, the GaAlAs being the absorbing layer with a gap of 1.73eV. The efficiency of 18.7% at the 1.73 eV bandgap appears suited for Si-based tandem solar cells⁽¹²⁾. Moreover, the AlGaAs compound displays the advantages of a tunable bandgap, and no consumption of In.

(6) Romaric de Lépinau, Capucine Tong, Andrea Scaccabarozzi, Fabrice Oehler, Hung-Ling Chen, et al, "Direct growth of III-V nanowire-based top cell for tandem on Silicon", 2020 47th IEEE Photovoltaic Specialists Conference (PVSC), Aug 2020, Calgary (virtual), Canada. ‌10.1109/PVSC45281.2020.9300864. hal-03328688
(7) Romaric de Lépinau, Andrea Scaccabarozzi, Gilles Patriarche, Laurent Travers, Stéphane Collin, et al, "Evidence and control of unintentional As-rich shells in GaAs 1– x P x nanowires" Nanotechnology, Institute of Physics, 2019, 30 (29), pp.294003. 10.1088/1361-6528/ab14c1. hal-02351891
(8) M Vettori et al, "Growth optimization and characterization of regular arrays of GaAs/AlGaAs core/shell nanowires for tandem solar cells on silicon", Nanotechnology 30 (2019) 084005
(9) X. Li, A. Fave, M. Lemiti, "Si tunnel junctions obtained by proximity rapid thermal diffusion for tandem photovoltaic cells", 2021, Semicond. Sci. Technol. 36 125004
(10)  Monalisa Ghosh, Pavel Bulkin, François Silva, Erik Johnson, Ileana Florea, Daniel Funes-Hernando, Alexandre Tanguy, Charles Renard, Nicolas Vaissiere, Jean Decobert, Iván García, Ignacio Rey-Stolle, and Pere Roca i Cabarrocas, "Ultrathin Ge epilayers on Si produced by low-temperature PECVD acting as virtual substrates for III-V / c-Si tandem solar cells", Solar Energy Materials and Solar Cells 236 (2022) 111535. https://doi.org/10.1016/j.solmat.2021.111535
(11) A. Michaud et al, "Elaboration of III-V top cell for tandem cell with Silicon", 2019 IEEE 46th Photovoltaic Specialists Conference (PVSC), Jun 2019, Chicago, United States. pp.1034-1038, 10.1109/PVSC40753.2019.8980524. hal-03084896
(12) Ahmed Ben Slimane, Amadeo Michaud, Olivia Mauguin, Xavier Lafosse, Adrien Bercegol, et al, "1.73 eV AlGaAs/InGaP heterojunction solar cell grown by MBE with 18.7% efficiency", Progress in Photovoltaics, Wiley, 2020, 28 (5), pp.393-402. 10.1002/pip.3249. hal-03084744

Tandem cells made of antimonides on GaSb substrates
IES proposes to explore the III-Sb antimonide-based material pathway offering the possibility of direct gaps tunable over a wide range, very useful for photovoltaic applications. The 1.45 eV/0.73 eV combination seems to be the most promising to reach an efficiency of 38% under 1 sun, using the quaternary material AlInAsSb having a good lattice matching with GaSb, which presents the interest of a strong variation of the gap with the composition, and with a direct gap. For the tunnel junction separating the quaternary cell from the GaSb one, the choice was made for the InAs/GaSb type III heterojunction without the need for high doping. A cell with Al0. 75In0.25As0.29Sb0.71/GaSb heterojunction was fabricated.⁽¹³⁾ In this case, the electrical contact is a front-back type. The solution proposed to overcome the problem of lattice mismatch with silicon is to perform a metamorphic growth of a III-V semiconductor buffer layer whose composition varies gradually with its thickness to compensate the lattice mismatch.
IES is also developing another tandem cell to be the bottom part of a 4-junction cell. This tandem cell is composed of a Top junction made with Al0.263Ga0.737As0.02Sb0.98 (1.06eV) associated with a Bottom junction in GaSb (0.72eV). Both cells were grown lattice-matched on GaSb substrate by Molecular Beam Epitaxy.

Tandem cells made of CIGS planar layers on Si substrates
Another option (ANR-EPCIS) is to take benefit of the already industrial CIGS thin film technology to combine it with the silicon technology for the development of a high efficiency, stable and low cost CIGS/Si tandem solar cells⁽¹⁴⁾. To this end, the ANR-EPCIS consortium (IMN, INL, Institut FOTON, IPVF, Riber) proposes to develop epitaxial wide bandgap pure sulfide CIGSu top cell on c-silicon bottom cell, through an epitaxial GaP selective contact.

Perovskite/Si Tandem Solar Cells
Hybrid deposition process for Monolithic Two-terminal Halide Perovskite/Si tandem solar cells (PVK/Si) have raised much attention due to the compatibility of the PVK with silicon bottom cells, in particular their compatibility to the textured silicon bottom cell. A record efficiency of a PVK /Si tandem cell of 29.5% was reported by OxfordPV in 2020. A collaboration between IPVF, EDF R&D, and CEA-INES evaluates the scalability of tandem devices fabrication towards industrial mass production, using a mixed halide MA0_.3FA_0.7Pb(I_0.84Br_0.16)₃. In particular, they use slot-die coating as an upscalable technique for the PbI2 conversion into perovskite, these perovskite layers showed conformity on the pyramidal texture present on silicon substrates surface.
Large size four-terminal PVK/Si tandem solar cells are also studied by IPVF, EDF R&D, C2N and LRCS, using first a spin-coating deposition process, then a slot die coating process. As a first step, full-stack devices have been made, based both on functional semitransparent perovskite solar cells and filters that optically replicate the stacks. These final objects allow reaching 23.3% efficiency (16.9% for the perovskite cell and 6.4% for the c-Si cell) with stable encapsulation. the fabrication of a functional 4T tandem device with a potential efficiency beyond 25% is currently in progress.⁽¹⁵⁾

(13) Kret J, Tournet J, Parola S, Martinez F, Chemisana D, Morin R, de la Mata M, Fernandez-Delgado N, Khan AA, Molina SI, Rouillard Y, Tournié E, Cuminal Y, "Investigation of AlInAsSb/GaSb tandem cells - A first step towards GaSb-based multi-junction solar cells", Solar Energy Materials and Solar Cells 219 (2021) 110795.
(14) Nicolas Barreau, Olivier Durand, Eugène Bertin, Antoine Létoublon, Charles Cornet, Polyxeni Tsoulka, Eric Gautron, Daniel Lincot, "Epitaxial growth of CIGSe layers on GaP/Si(001) pseudo-substrate for tandem CIGSe/Si solar cells", Sol. Energ. Mat. Sol. Cells 233 (2021) 111385. DOI : 10.1016/j.solmat.2021.111385
(15) E. Raoult et al., "Toward a highly efficient large surface Perovskite Silicon 4-Terminal tandem module," 2021 IEEE 48th Photovoltaic Specialists Conference (PVSC), 2021, pp. 0290-0293, doi: 10.1109/PVSC43889.2021.9518929.

PVK/CIGS tandem cells
The PERCISTAND project is a European collaboration whose aim is to develop monolithic integration of two-terminal perovskite/CI(G)S tandem cells in order to reach 30% power conversion efficiency within three years, in both two and four-terminal architecture, with the low band-gap CIGS subcell as the bottom cell and the perovskite solar cell (PSC) as the top cell.
In the two-terminal (2T) configuration, a junction will be formed between n-type IZO on top of the CI(G)S subcell and p-type NiO at the bottom of the perovskite subcell.
PERCISTAND gathers IPVF, GeePs, UPMC, ILV as French partners, and TNO (NDL), ZSW and KIT (G), IMEC (B).

All Si Tandem Solar Cells
Tandem c‐Si/c‐Si solar cells have the capacity to overcome Shockley Queisser limit and reach conversion efficiencies up to 30.7% based on the modeling performed by F. J. Haug & Ch. Ballif. According to them high excess carrier density in the top‐cell allows for a voltage in the tandem device that is more than twice the voltage of an equally thick single junction solar cell. Using this V_OC engineering, the LPICM used plasma enhanced epitaxial growth to experimentally investigate this type of tandem device which could benefit from the current development of c-Si heterojunction solar cells and offer to this technology an easy to implement solution to increase its efficiency while lowering the requirements on transparent conductive materials. This approach offers an elegant alternative to the current singling approach developed by c-Si industry and targeting at reduction of the cells short circuit current.

3-terminal tandem solar cells
Geeps is developing an architecture of three-terminal (3T) photovoltaic tandem solar cells which combines an interdigitated back contacted (IBC) bottom lateral subcell with a heterojunction vertical top cell⁽¹⁶⁾. In this 3T architecture the two subcells work independently and there is no need for any current matching between the 2 subcells, so that no tunnel junction is required. It is particularly well suited to silicon back contact subcells and to various types of top cell materials from III-V compounds or perovskites. The 3T concept cell can be realized with less technological steps and at a lower cost compared to the conventional 4T process.⁽¹⁷⁾
In particular, The ANR THESIS project aims at realizing demonstrators of tandem cells consisting of a top cell based on perovskite and a bottom cell made of crystalline silicon with interdigitated contacts on the back side (IBC). The consortium is composed of GeePS, INL, CEA-INES and EDF R&D – IPVF. A European project, BOBTANDEM, is also dedicated to the perovskite/silicon 3T architectures⁽¹⁸⁾. Note that this 3T architecture on Si IBC is open to other materials like organic semiconductors.

Bonded 2-terminal tandem solar cells
C2N is developing a 2T architecture made of a thin-film top solar cells bonded on Si. The transparent and conductive bonding layer made of low refractive index materials enable current matching, bonding of rough or even textured cells, with overall maximum efficiencies even higher than enabled by direct bonding thanks to photon recycling effects. It releases many constrains commonly found in the fabrication of 2T tandem solar cells (requirement of flat surfaces, tunnel junction, temperature constrains…). First proof-of-concepts are made of III-V//Si (collaboration with Fraunhofer ISE, IPVF common research program)⁽¹⁹⁾. This architecture is being extended to other solar cell technologies with UMR IPVF (CIGS//Si, PVK//Si,…).

(16) Brevet WO/2017/093695, “Photovoltaic cell”, by Zakaria Djebbour, Anne Migan, Jean-Paul Kleider and Walid El-Huni
(17) Zakaria Djebbour, Walid El-Huni, Anne Migan, Jean-Paul Kleider, “Bandgap engineered smart three-terminal solar cell: New perspectives towards very high efficiencies in the silicon world", Prog Photovolt Res Appl. 28 (2019) 306, https://doi.org/10.1002/pip.3096
(18) James P. Connolly, Koffi Ahanogbe, Jean-Paul Kleider, José Alvarez, Hiroyuki Kanda, Mohammad K. Nazeeruddin, Valentin Mihailetchi, Philippe Baranek, Malte Vogt, Rudi Santbergen, Olindo Isabella, "Recent results on carrier selective three terminal perovskite on silicon-IBC tandem solar cells", 37th European Photovoltaic Solar Energy Conference and Exhibition (EU PVSEC 2020), Sep 2020, Lisbon, Portugal
(19) Phuong-Linh Nguyen et al., “New Architecture and Bonding Process for III-V//Si Tandem Solar Cells”, OSA Advanced Photonics Congress 2021. https://doi.org/10.1364/PVLED.2021.PvW1E.4