Researchers from Kanazawa University, University of California, Islamic University of Madinah and Queens University have developed a scalable, lithography-free approach to fabricate quasi-interdigitated back-contact (QIBC) perovskite solar cells (PSCs), addressing a known efficiency bottleneck in conventional device architectures: optical losses in front-contact layers.
Schematic illustration of the fabrication process for QIBC PSCs via thermal evaporation. Image from: Solar Energy
Standard “sandwich” PSC architectures tend to suffer from parasitic optical losses, as incident light must pass through transparent conductive oxides and charge transport layers before reaching the absorber. These losses, arising from reflection, scattering, and absorption in non-active layers, limit photon harvesting across the UV-visible spectrum. Back-contact designs offer a compelling alternative by relocating both electrodes to the rear side of the device, thereby eliminating front-side shading. Inspired by interdigitated back-contact (IBC) silicon solar cells, the QIBC architecture adapts this concept to perovskites by arranging electron and hole transport layers laterally on the back side. Optical simulations in this work confirm that such geometries significantly reduce parasitic absorption and enhance photon collection.
Despite this promise, practical implementation of QIBC PSCs has been constrained by fabrication challenges. Conventional photolithography, while capable of producing micron-scale electrode features, is costly, requires cleanroom processing, and is incompatible with solvent-sensitive perovskite materials. Additionally, perovskites exhibit carrier diffusion lengths of only ~1 µm, far shorter than silicon, imposing strict geometric constraints: if electrode spacing exceeds this diffusion length, photogenerated carriers recombine before collection.
To overcome these limitations, the researchers introduced a solvent-free thermal evaporation technique using high-precision shadow masks. This method enables the fabrication of defect-free interdigitated electrodes with feature sizes down to 20 µm, avoiding the chemical damage and complexity associated with lithography. The approach also supports scalable manufacturing.
Device optimization revealed a strong dependence of performance on electrode geometry. Specifically, reducing the inter-electrode gap from 200 µm down to 20–50 µm significantly improved charge extraction and fill factor (FF). This behavior is explained by a charge-transport model: narrower gaps ensure that carrier transport distances remain within the perovskite diffusion length, thereby suppressing bulk recombination. In contrast, wider gaps (>100 µm) lead to substantial recombination losses and poor FF.
Further improvements were achieved through interface engineering. Incorporating a thermally evaporated CuI hole-transport layer reduced interfacial defect density, enhanced rectification behavior, and improved charge selectivity. This modification lowered recombination velocities and contact resistance, highlighting the importance of optimized interfaces in back-contact architectures.
Overall, this work demonstrates a practical route to bridge the gap between the theoretical optical advantages of QIBC designs and scalable device fabrication. By combining micron-scale electrode control (20–200 µm), diffusion-aware design (optimal 20–50 µm spacing), and improved interface engineering, the study establishes a viable pathway toward high-performance back-contact perovskite photovoltaics.
Beyond solar cells, the presented patterning strategy may be extended to other perovskite-based optoelectronic devices, including photodetectors, LEDs, and field-effect transistors, offering a versatile platform for next-generation technologies.
