Researchers at the Hebrew University of Jerusalem have developed flexible, semi-transparent perovskite solar cells whose transparency and apparent color can be tuned without altering the light-absorbing material.
Semi-transparent solar cells are often discussed in the context of windows and building-integrated photovoltaics, but in practice they tend to force a compromise. Most existing designs achieve transparency by thinning the absorber layer or altering its chemical composition, and both approaches usually come with costs in stability or performance.
In this work, the researchers chose a different path. They kept the same mixed-cation, mixed-halide perovskite composition, Cs₀.₁₅FA₀.₈₅Pb(I₀.₈₅Br₀.₁₅)₃, and instead created microscopic areas that simply do not contain the active material.
Printing transparency into perovskite cells
The devices are built on flexible plastic substrates coated with indium tin oxide (ITO), which makes low-temperature processing essential. Because conventional tin oxide (SnO₂) electron transport layers normally require heat that would damage plastic, the team deposited SnO₂ from a commercial colloidal solution and tested annealing at 60 °C, 100 °C, and 180 °C, followed by oxygen plasma treatment.
Although higher temperatures improved conductivity, layers annealed at 100 °C offered the best compromise: they produced higher fill factors and open-circuit voltages than the low-temperature films, while delivering efficiencies comparable to the high-temperature ones. This 100 °C process was therefore used for all subsequent devices.
Instead of thinning the perovskite, the researchers printed the transparency into the device. Before depositing the active layer, they inkjet-printed arrays of polymer pillars using a UV-curable, solvent-free ink designed to form neat, circular features and crosslink into a solvent-resistant polymer. When the perovskite is deposited, it fills the spaces between the pillars but leaves the pillar regions clear, producing a regular array of microscopic, inactive windows through the cell.
By adjusting the spacing between the pillars, the team could precisely control how much light passes through the cell. Closer spacing increases transparency and reduces absorption by shrinking the active area: at the tightest spacing, average visible transmittance reached about 34%, compared with about 25% for an otherwise identical device without pillars. Photoluminescence measurements showed only minor changes in recombination behavior, indicating that the patterning does not significantly disrupt charge transport.
The semi-transparent devices used Spiro-OMeTAD as the hole transport layer and a transparent MoOx/Au/MoOx top contact. They achieve open-circuit voltages of about 1.0 V, current densities of 13-15 mA cm⁻², and fill factors of roughly 62–67%. Although adding more pillars slightly reduces current and voltage, the best flexible cells still reach about 9–10% efficiency while maintaining around 35% visible transparency, corresponding to a light utilization efficiency of 3.23, above a commonly cited threshold for building-integrated applications.
The pillars did more than shape the optics and also improved durability. In bending tests, devices without pillars lost about 50% of their efficiency at a 10 mm radius, while pillar-based cells retained roughly 85-90%. After 1000 bending cycles, they still kept 80-98% of their initial performance. In 1200-hour stability tests under ambient conditions and illumination, the same trend held, with pillar-based devices retaining about 80% of their performance, compared with about 40% for unstructured cells.
With transparency established, the researchers turned to color. Without changing the perovskite or adding layers, they tuned the thickness of the top molybdenum oxide in the transparent electrode. This oxide-metal-oxide stack acts as an optical cavity, and varying the oxide thickness from 15 to 45 nm shifts the reflected color from purple toward yellow while keeping transparency above 30%. The resulting colored devices reached efficiencies of about 6 to 8%, depending on the thickness.
The work shows how a solar cell’s appearance, transparency, and mechanical robustness can be tuned without changing the absorber’s chemistry, and the authors note that future improvements, especially better encapsulation and barrier layers, should further enhance durability and performance.
3D printing and solar energy
3D printing allows the structure and the energy-generating function of a solar cell or panel to be designed together rather than as separate parts, making it possible to build lighter, more efficient, and more application-specific solar systems.
In an interview, British construction giant Foster + Partners explained the development of a self-deploying Tall Lunar Tower meant to combine communications and power generation for future lunar bases. Working with Branch Technology under NASA’s SBIR Phase I program, the spiral, diagrid structure is designed to be built in situ by robotic systems using Branch’s C-Fab 3D printing process, a concept already demonstrated in a 5 m full-scale prototype.
Its geometry has been optimized using finite element analysis for lunar gravity, and climber robots are envisioned to print the structure along integrated spiral rails. The roughly 50 m tower would support deployable solar arrays and is being evaluated through vacuum, thermal, and structural tests to assess its survival in lunar conditions.
Elsewhere, T3DP applied its patented volumetric 3D printing method to fabricate scaffold-reinforced, perovskite-based solar panels inspired by a fly’s-eye geometry. The process cured structures in a single step and produced a copper-plated hexagonal scaffold that routed electricity in parallel through decentralized sub-cells.
This approach addressed perovskite fragility by mechanically supporting the material while allowing higher packing density. Using 20% efficient CIGS solar triangles arranged in this 3D architecture, the company demonstrated about 36% panel efficiency, which was validated by Infinity Energy, and outlined a path toward even higher efficiencies through tandem designs.
The researchers’ findings are detailed in their paper titled “Semitransparent color tunable perovskite solar cells with 3D pillar structure,” by Vikas Sharma, Ouriel Bliah, Tal Binyamin, Shlomo Magdassi and Lioz Etgar.
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Featured image shows a schematic presentation of the main steps involved in the fabrication of a colourful, semi-transparent, flexible perovskite solar cell. Image via EES Solar.
With a background in journalism, Ada has a keen interest in frontier technology and its application in the wider world. Ada reports on aspects of 3D printing ranging from aerospace and automotive to medical and dental.
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