StreP (Stretchable PV foil)

Project Information

StreP

Grant agreement ID: 101123147

DOI

10.3030/101123147

Project closed

EC signature date 31 May 2023

Start date 1 November 2023

End date 30 April 2025

Funded under

• European Research Council (ERC)

Total cost

No data

EU contribution

€ 150 000,00

Coordinated by

UNIVERSITEIT HASSELT
Belgium

Periodic Reporting for period 1 - StreP (StreP (Stretchable PV foil))

Reporting period: 2023-11-01 to 2025-04-30

Summary of the context and overall objectives of the project

Solar energy is essential for climate-neutral energy systems, but land for its deployment is scarce in urban and densely populated areas. One solution is to integrate photovoltaics (PV) into lightweight, flexible structures, such as tents, agricultural covers, and architectural membranes, without requiring additional land. However, existing flexible PV modules cannot stretch, which is a key requirement for textiles that elongate by 3–10%.
The StreP (Stretchable PV Foil) project aimed to develop stretchable PV foil using thin-film CIGS (copper–indium–gallium–selenide) solar cells combined with novel, patent-pending interconnect and encapsulation architectures. The goal was to maintain electrical performance and environmental protection during mechanical stretching.
Two main challenges were addressed:
1) Mechanical mismatch: Standard flexible PV breaks under tensile stress. A stretchable foil must deform without losing function.
2) Environmental durability: Outdoor textiles require protection from moisture, temperature fluctuations, and fatigue.
StreP uses a six-step, roll-to-roll compatible process involving thin, lightweight CIGS cells on metal substrates: punching and pre-stretching a polymer foil, placing interconnects, laminating cells, and releasing to create stretchable slack. Barrier coatings seal against humidity and wear.
The goal was to demonstrate a reliable and scalable stretchable photovoltaic (PV) foil at technology readiness level (TRL) 5–6 for integration into real-world textile applications. The demonstration progressed in three phases:
- Fabrication and validation of the stretchable interconnect;
- Assembly and environmental testing of the semi-finished foil;
- Integration into a demonstrator, such as a tent, agricultural cover, or architectural textile.
Impact Potential:
- Humanitarian and Defense: Tents with integrated PV panels provide clean, off-grid power.
- Agriculture: Dual-use shading covers that generate energy.
- Architecture: Tensile membranes that produce electricity.
- Commercialization: The interconnect process supports low-cost production, and licensing is being explored.
By unlocking PV stretchability, StreP expands solar energy into new fabric-integrated applications where rigid panels are ineffective.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The StreP project addressed the mechanical and environmental durability challenges of CIGS-based stretchable foils by combining materials testing, module design, and process development.
The project's core focus was to understand failure modes in flexible photovoltaics (PV) under textile-like conditions. This was achieved through accelerated damp-heat and cyclic bending tests, which led to new tools and protocols for optimization.
Key technical results:
1. Quantification of moisture ingress: A color-change perovskite indicator visualized humidity ingress under 85 °C/85% RH conditions, enabling the comparison of seal and lamination designs.
2. Adhesive strength testing: T-peel tests based on ISO 11339 before and after 2,000 hours of damp heat revealed a loss of 44–60% in bond strength, providing information for improving encapsulant formulations.
3. Cyclic Bending: A custom roll tester simulated real-world strain at ~20 cm over 20,000 cycles.
4. Electrical stability: After 10,000 bending cycles, the modules retained 94–95% of their performance. Degradation was linked to series resistance and leakage paths.
5. Encapsulation guidelines: Best practices include thicker POE layers, robust edge sealing, and optimized lamination to ensure durability.
6. Stretchable PV Demonstrator: This validated TRL 5–6 demonstrator integrates CIGS cells, stretchable interconnects, and sealing layers into textile substrates.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

StreP introduced a new class of stretchable copper indium gallium (di)selenide (CIGS) foils for textile-based photovoltaics (PV). In addition to the demonstrator, the project provided methodologies, insights, and design rules for integrated photovoltaics (IPV).
Key advances:
1. Environmental durability: A color-based humidity test revealed that edge seals and thicker encapsulants delay water ingress, doubling the time until degradation occurs.
2. Adhesion under humidity: Peel testing showed that moisture exposure leads to mechanical failure, with bond loss of up to 60%. This confirms the need for optimized encapsulants and lamination.
3. Mechanical fatigue: Stretchable foils retained over 94% of their efficiency after 10,000 cycles. Losses were minor and were traced back to microcracks in the contact layer.
4. Design rules: Recommendations include a POE of at least 500 µm, optimized lamination to avoid voids, and robust seal geometries.
5. Proof of Concept: A stretchable foil demonstrator (TRL 5–6) confirmed the feasibility and readiness for integration of the design.
Potential impacts and next Steps:
StreP’s results expand the possibilities of PV integration in dynamic, fabric-based systems. The technology could transform:
- Emergency response: Clean energy for deployable shelters;
- Agrivoltaics: multifunctional shading nets.
- Architecture: Solar-enabled membranes for large venues.
Next steps:
- Field validation of durability.
- Scale up via roll-to-roll adaptation.
- Licensing and partnerships (e.g. with EnFoil, CarPro, and Vermako).
- IP exploitation and regulatory standardization (e.g. adapting IEC 61215 for stretchable PV).
Conclusion:
StreP has created a robust foundation for stretchable PV foils. They did this by solving critical mechanical and environmental barriers and delivering a validated demonstrator. These results support new classes of solar-integrated fabrics and advance the EU’s goals for clean, embedded energy technologies.