ETIP Photovoltaics

Roadmap 4

Tandem PV modules

Rationale for support

Modules made from tandem PV cells achieve higher ef- ficiency than single-junction cells. As explained in the in- troduction of this objective efficiency is one of the main drivers to reduce cost, area requirement and resource demands.

Novel material systems, like perovskites, have made cost-efficient multijunction PV modules possible. They are an area where Europe has a high level of expertise. Sup- porting EU manufacturers will help them create skilled jobs and export earnings from a technology vital for the world for ecological and competitiveness-related reasons. Since questions remain about the reliability and scalability of perovskite technology, silicon-based tandems with III-V top material (the dominant technology) should also be fur- ther developed and explored. A threshold of 20 % efficien- cy for the tandem PV modules is anticipated.


Multijunction III-V solar cells (based on III–V compound semiconductors) currently exhibit the highest conversion efficiencies and are applied in concentrating photovol- taics (CPV) and in space. While space is a stable field for commercial activities, only little commercial or industrial activity is visible for terrestrial CPV. The reason are the very high costs associated with the epitaxial deposition of the III-V materials. Since 2009 the record efficiency for a three-junction cell is 41.5 % from a GaInP/GaInAs/Ge up- right-grown solar cell under concentrated sunlight. Adding p-n junctions increases efficiency, e.g. to 47.1 % at 143 suns in an inverted metamorphic six-junction cell structure based on III-V semiconductor material.

On the other hand, silicon based tandem solar cells, espe- cially perovskite-silicon tandem devices promise consider- ably lower costs and with 29.5 % efficiency. (30) They are be- ing tested in the field. In the laboratory scale, Perovskite/ CIGS, Perovskite/Perovskite, and Organic/Organic tandem devices are being investigated.

Researchers have not yet settled on the right number of terminals to put on tandem cells. Two terminal devices (2T) require series interconnection of the tandem sub-cells, but the resulting solar cell has similar features as current single-junction devices regarding module interconnection and system aspects. In four terminal devices (4T), the so- lar cells are operated independently, making the need for current matching obsolete but requiring adapted module concepts. Three terminal structures (3T) seek to combine the advantages of 2T and 4T concepts.

Targets, Type of Activity and TRL

Our vision for 2030 is that tandem technologies will reach a market share of more than 5 % and will be successful- ly transitioning from niche to mass market applications by 2030. The technologies will successfully demonstrate long-term performance comparable to the single-junction technologies, clear advantages in terms of LCOE and in the environmental footprint to be achieved. In consequence, technology acceptance will be high, and no risk premium must be paid to finance projects that use them.

R&D specific to tandem cell material

Low-cost Silicon bottom cells with good red re- sponse and high voltage should be developed. Works include bottom cell adaptation for IR spec- trum, different tandem compatible texture technol- ogies, and bifocality.

Pilot-line production of low-cost Silicon bottom cells derived from hetero-junction technologies on the one hand, and alternatively derived from PERC and TOPCon technologies.

Perovskite/silicon tandem solar cells

Long-term stable perovskite absorbers with the optimal bandgap for maximum energy yield under operation conditions. To realise these, formation of the absorber during the different possible dep- osition processes (wet-chemically, co-evaporation, hybrid-processing) needs to be understood more deeply.

Deepen the understanding of the interface process- es to optimise recombination layers and the charge selective layers, which might also enable reducing the number of layers through combined functional- ities, e.g. by tunnel junctions.

Develops bifacial multi-junction perovskite PV de- vices with suitable materials, e.g. for future vertical PV elements in noise barriers or on agriculture land areas.

Demonstrate industrially feasible processes and equipment for reliable, low cost and high through- put production of silicon-perovskite tandem solar cells with high efficiency, low material usage, low energy and media consumption.

Pilot lines showing the potential of different pro- duction pathways for perovskite silicon tandem solar cells and modules should be established in Europe.

Perovskite/Perovskite tandem technology

Improve stability of perovskite absorber layers with low band gap (typically containing Sn) to be used as bottom cell

Develop stable high-bandgap absorbers for the top-cell. Work on the absorber and charge-selective contact layers to reduce Voc deficit for top cell ab- sorbers with Eg > 1.65 eV.

Anti-reflection measures and photon management to mitigate weak surface texturing

The NIR (near-infrared) transparency of the top cell needs to be improved, in particular by reducing the parasitic absorption in TCO layers.

The technology for monolithic 2T tandem modules needs to be developed. This requires non-damaging (laser) scribing technologies and novel concepts for series interconnections.

R&D priorities independent of tandem cell materials:

Absorber material chemical compositions need to be finetuned for highest stability and maximum ef- ficiency. This includes bandgap tuning and the de- velopment of absorber material combinations for application in triple-junction devices. The question of bandgap tuning is not trivial, as bandgaps change due to temperature effects and in 2T devices, cur- rent matching has to be achieved at elevated oper- ational temperatures.

For the best optical performance, parasitic absorp- tion in adjacent layers, TCOs etc needs to be min- imized and light management ensuring photons are absorbed in the best suited junction within the multilayer stacks needs to be developed. To this end electro-optical modelling frameworks have to be developed for 2T, 3T and 4T multijunction solar cells.

Furthermore, perspectives for the replacement of certain elements that are critical either due to tox- icity issues (e.g. Pb) or due to limited availability not compatible with multi-TW scale deployment (e.g. In) need to be developed. Concepts need to be developed and the critical issues need to be under- stood for maximizing circularity, sustainability, and recyclability.

A task for many 2T tandem technologies is the de- velopment of stable high-quality recombination layers that interconnect the different sub-cells and ideally combine functionalities i.e. that of charge selective contacts, to reduce the number of overall layers.

Concepts and module technology need to be de- veloped for 3T and 4T architectures with regard to voltage- or current-matching, and the consideration of system aspects, e.g. module integrated inverters/ mpp tracking etc. For all technologies module tech- nology needs to be further developed to enable customization, achieve light weight/flexibility (in- cluding packaging) and enable bifocality.

Characterisation methods and equipment (inline, offline) need to be developed to enable loss analysis from material and sub cell/cell level up to module level, for guidance for target-oriented optimisation to realise the full efficiency potential. Furthermore, failure mode analysis and mitigation strategies are needed to maximize stability.

Reliable energy yield modelling needs to be devel- oped which considers all relevant effects, such as changes of the spectral conditions and temperature effects. Accordingly, input data should be generat- ed to enable energy yield modelling for distinct cli- matic conditions.

High-throughput processing up to module level, i.e. including high-throughput interconnection and lamination technology needs to be demonstrated with advanced process (inline) monitoring.

Outdoor tests and reliability testing must be under- taken. Solar cell and module development should aim at achieving the same stability as established single-junction technologies while transferring the tandem efficiency advantage into a substantially increased energy yield. To this end power man- agement systems and electronics integration that maximize energy yield in rapidly varying illumina- tion conditions (weather variation but also mobile applications) need to be applied.

Market opportunities need to be evaluated in tech- no-economic analyses for more-than-two junction solar cells (multijunction solar cells), new applica- tions (electric vehicles, drones, HAPS, space). Ac- cordingly, business models for re-use and recycling strategies must be calculated.

The successful integration of tandem technologies needs to be validated across all potential applica- tions (vehicle, building, infrastructure, power plant ...).

Pilot-lines for solar cell processing and module pro- duction must be put into operation with high ver- satility to combine different PV technologies in tan- dem modules with different configurations.

Other Thin Film Tandem Technologies

Improve stability of perovskite absorber layers with low band gap (typically containing Sn) to be used as bottom cell

Develop stable high-bandgap absorbers for the top-cell. Work on the absorber and charge-selective contact layers to reduce Voc deficit for top cell ab- sorbers with Eg > 1.65 eV.

Anti-reflection measures and photon management to mitigate weak surface texturing

To enable CIGS as bottom cell in combination with a Pk top cell it is important to adjust the CIGS mate- rial composition to reduce the bandgap in the bot- tom cell while maintaining high efficiency. The CIGS bottom cell needs improved infrared absorption with improved performance under top cell filtered light, while preferably reducing the surface rough- ness to enable good homogeneous coverage for the top cell. Optimise post-deposition treatments for lower bandgap compositions of CIGS bottom cell to achieve smooth surface.

Similarly, contact and (transparent) electrode mate- rials, identified in low-TRL work from high-through- put screening) and processes need to be adapted to create effective recombination junctions at the front side of the CIGS bottom cell. This includes a.o. improvement of the thermal stability of the CIGS bottom cell to tolerate thermal stress from top cell deposition and for deposition of highly transparent TCOs and top-device deposition in monolithic con- figuration.

The variety of materials in the full tandem stack requires specific development of technology for such monolithic 2T tandem modules. Laser scribing technology for cell-to-cell interconnection which is versatile but also selectively non-damaging for the complicated layer stacks requires dedicated tool and process development. Novel concepts for series connections can come into play to address these challenges in an innovative way.

A clear differentiator for thin-film tandems is its po- tential to yield fully flexible devices and modules. To gain market interest for this unique selling point, it will be crucial to demonstrate scalable processes for the full tandem stack and its interconnection for modules. While it’s obviously best to show this for high performing and stable tandem materials and stacks as to be developed in above actions, the cru- cial points here should focus more on the process flow development. Large area homogeneity, con- formality, accuracy etc are quality parameters that come into play. Processes with higher CAPEX should yield high-throughput thin film deposition with high quality to balance the overall cost/performance ra- tio for final modules at competitive level. Process conditions (like thermal budget) are additionally constrained when flexible carriers are introduced.

III-V/silicon tandem solar cells

To reduce the amount of epitaxy required maximiz- ing absorption through increasing optical thickness photonic structures or plasmonic elements need to be developed.

The concept of terrestrial CPV should be re-evalu- ated considering new module concepts (especially hybrid modules which also capture the diffuse part of sunlight, micro CPV combined with self-assembly technologies) and improved tracking.

The overwhelming priority is to reduce costs, as ef- ficiency are already the highest of any PV technology for III-V multijunction devices. The most important leverages that need to be demonstrated and for which equipment at an industrial scale needs to be developed are

  • Low-cost high-throughput epitaxy on large are- as of III-V materials either via heteroepitaxy and subsequent lift-off or directly on Si substrates.
  • Low-cost high-throughput large area epitaxial lift-off (porous subsurface layers, spalling). In this context the reuse of the wafer/substrates needs to be maximised.
  • High-throughput transfer and long-lasting con- nection with silicon bottom solar cells or other substrates for epitaxial films
  • Low-cost processing of and especially the re- placement of lithographic steps.


KPIs that can be utilised for capturing progress in the above identified R&I fields are:

KPITarget Value (2030)

Efficiency of at least 5 % absolute above respective single junction technology/p>


Lifetime of the tandem device is the same as the lifetime of the bot- tom single junction device.


Production of additional junction for less than 8 €/m2