Fabricating electronic devices on extremely thin substrates enables the realization of highly flexible and lightweight electronics, which significantly decreases discomfort in wearable applications. Our research group focuses on developing electronics on ultra-flexible polymer films, approximately 1 µm in thickness, to enhance mechanical robustness against bending by minimizing the applied strain [1]. In a simplified mechanical model, film thickness is inversely proportional to strain, meaning that thinner films experience less strain for the same bending radius. This presentation will discuss the potentials of ultraflexible electronics, latest advancements in environmental and photo-stability, and the challenges in standardizing mechanical characterization.

Recently, we have made significant progress in improving both the power conversion efficiency (PCE) and the environmental stability of these devices across different material systems. We developed waterproof, high-performance ultrathin organic solar cells (OPVs) and realized rechargeable soft robot systems [2,3]. Furthermore, we achieved ultra-flexible perovskite solar cells with unprecedented stability using dual hole transport layers and Al₂O₃ moisture barriers [4]. To ensure long-term reliability under sunlight, we addressed photo-degradation by developing a 3.6 µm thick polyimide substrate with intrinsic UV-filtering properties (380 nm cutoff) [5]. This substrate effectively protects the device's active and interfacial layers, enabling ultraflexible OPVs to maintain long-term operational stability under 1-sun in ambient air while enduring up to 4,000 bending cycles.

As flexible photovoltaics approach technological maturity, standardized characterization becomes essential. We have proposed a unifying bending test protocol recommending 1% strain over 1,000 bending cycles and introduced the "flexible photovoltaic fatigue factor" F as a new figure of merit that integrates PCE retention, strain, and bending cycles [6]. Reflecting the growing importance of these methodologies, a dedicated section for evaluating the performance and mechanical reliability of flexible solar cells has been newly established within the latest comprehensive "Emerging PV Report" [7]. This integration provides the scientific community with standardized benchmarks to accelerate the development of next-generation solar cells.