Paper is a flexible material, commonly used for information storage, writing, packaging, or specialized purposes. It also has strong appeal as a substrate in the field of flexible printed electronics. Many applications, including safety, merchandising, smart labels/packing, and chemical/biomedical sensors, require an energy source to power operation. Here, progress regarding development of photovoltaic and energy storage devices on cellulosic substrates, where one or more of the main material layers are deposited via solution processing or printing, is reviewed. Paper can be used simply as the flexible substrate or, exploiting its porous fiber‐like nature, as an active film by infiltration or copreparation with electronic materials. Solar cells with efficiencies of up to 9% on opaque substrates and 13% on transparent substrates are demonstrated. Recent developments in paper‐based supercapacitors and batteries are also reviewed with maximum achieved capacity of 1350 mF cm⁻² and 2000 mAh g⁻¹, respectively. Analyzing the literature, it becomes apparent that more work needs to be carried out in continuing to improve peak performance, but especially stability and the application of printing techniques, even roll‐to‐roll processing, over large areas. Paper is not only environmentally friendly and recyclable, but also thin, flexible, lightweight, biocompatible, and inexpensive.

In response to the current trend of miniaturization of electronic devices and sensors, the complementary coupling of high-efficiency energy conversion and low-loss energy storage technologies has given rise to the development of photocapacitors (PCs), which combine energy conversion and storage in a single device. Photovoltaic systems integrated with supercapacitors offer unique light conversion and storage capabilities, resulting in improved overall efficiency over the past decade. Consequently, researchers have explored a wide range of device combinations, materials, and characterization techniques. This review provides a comprehensive overview of photocapacitors, including their configurations, operating mechanisms, manufacturing techniques, and materials, with a focus on emerging applications in small wireless devices, Internet of Things (IoT), and Internet of Everything (IoE). Furthermore, we highlight the importance of cutting-edge materials such as metal-organic frameworks (MOFs) and organic materials for supercapacitors, as well as novel materials in photovoltaics, in advancing PCs for a carbon-free, sustainable society. We also evaluate the potential development, prospects, and application scenarios of this emerging area of research.

Indoor photovoltaics (IPVs) harvest ambient light to produce electricity and can cleanly power the rapidly growing number of Internet-of-Things (IoT) sensors. The surge in IPV development, with new proposed materials, devices and products, creates the need to critically evaluate how IPV devices have advanced and to assess their prospects. In this Review, we analyse the status, challenges and opportunities of established and emerging IPV technologies, including metal-halide perovskite, organic photovoltaics, dye-sensitized solar cell and perovskite-inspired materials. Many emerging low-toxicity semiconductor materials could reach IPV efficiencies of up to 50%, but carrier localization and defect trapping hinder their performance. Wide adoption of standardized performance assessment methods is essential, and further harmonization is needed for stress tests, qualification standards and energy rating assessments. For seamless IPV integration in IoT devices, series-connected cell modules and appropriate power management hardware are crucial to maximize energy extraction. IPV device stability, technology upscaling and cost-effective integration in IoT sensors must be further developed but balanced with sustainability across the entire value chain.

Electron transport layers (ETLs) play a fundamental role in perovskite solar cells (PSCs) through charge extraction. Here, we developed flexible PSCs on 12 different kinds of ETLs based on SnO₂. We show that ETLs need to be specifically developed for plastic substrates in order to attain 15% efficient flexible cells. Recipes developed for glass substrates do not typically transfer directly. Among all the ETLs, ZnO/SnO₂ double layers delivered the highest average power conversion efficiency of 14.6% (best cell 14.8%), 39% higher than that of flexible cells of the same batch based on SnO₂-only ETLs. However, the cells with a single ETL made of SnO₂ nanoparticles were found to be more stable as well as more efficient and reproducible than SnO₂ formed from a liquid precursor (SnO₂-LP). We aimed at increasing the understanding of what makes a good ETL on polyethylene terephthalate (PET) substrates. More so than ensuring electron transport (as seen from on-current and series resistance analysis), delivering high shunt resistances (R_SH) and lower recombination currents (I_off) is key to obtain high efficiency. In fact, R_SH of PSCs fabricated on glass was twice as large, and I_off was 76% lower in relative terms, on average, than those on PET, indicating considerably better blocking behavior of ETLs on glass, which to a large extent explains the differences in average PCE (+29% in relative terms for glass vs PET) between these two types of devices. Importantly, we also found a clear trend for all ETLs and for different substrates between the wetting behavior of each surface and the final performance of the device, with efficiencies increasing with lower contact angles (ranging between ∼50 and 80°). Better wetting, with average contact angles being lower by 25% on glass versus PET, was conducive to delivering higher-quality layers and interfaces. This cognizance can help further optimize flexible devices and close the efficiency gap that still exists with their glass counterparts.

Long-term stability of perovskite solar cells (PSCs) is one of the main issues to be solved for forthcoming commercialization of this technology. In this work, thermosetting polyurethane (PU)-based resins are proposed as effective encapsulants for perovskite solar cells to prevent degradation caused by both moisture and oxygen. Application consists of drop-casting the precursor mixture directly over the devices followed by in situ polymerization, avoiding the use of other adhesives. PUs are cost-effective, lightweight, thermal, and light-stable materials whose mechanical, chemical, and physical properties can be easily tuned by thoughtful choice of their precursor. Encapsulated PSCs show extremely good stability when stored under ambient light (maximum, 1000 lux), controlled humidity (28–65%), and temperature (18–30 °C) by retaining 94% of the initial power conversion efficiency after 2500 h (4 months), whereas control devices lose 90% of their performance after 500 h (T₈₀ = 37 h); once stored according to ISOS-D-1, PU-protected devices showed T₈₀ > 1200 h. Encapsulated devices are stable even when immersed in pure water. The demonstration of PUs as promising solution-processed encapsulant materials for PSCs can pave the way for these to become a cost-effective encapsulation route alternative for future industrialization of this technology.

Flexible perovskite solar cells (FPSCs) are prime candidates for applications requiring a highly efficient, low-cost, lightweight, thin, and even foldable power source. Despite record efficiencies of lab-scale flexible devices (19.5% on a 0.1 cm² area), scalability represents a critical factor toward commercialization of FPSCs. Large-area automized deposition techniques and efficient laser scribing procedures are required to enable a high-throughput production of flexible perovskite modules (FPSMs), with the latter being much more challenging compared to glass substrates. In this work, we introduce the combined concept of laser scribing optimization and automatized spray-coating of SnO₂ layers. Based on a systematic variation of the incident laser power and a comprehensive morphological and electrical analysis of laser-based cell interconnections, optimal scribing parameters are identified. Furthermore, spray-coating is used to deposit uniform compact SnO₂ films on large-area (>120 cm²) plastic substrates. FPSCs with spray-coated SnO₂ show comparable performance as spin-coated cells, delivering up to 15.3% efficiency on small areas under 1 sun illumination. When upscaling to large areas, FPSMs deliver 12% power conversion efficiency (PCE) and negligible hysteresis on 16.8 cm² and 11.7% PCE on a 21.8 cm² active area. Our perovskite devices preserved 78% efficiency when the active area increased from 0.1 to 16.8 cm², demonstrating that our combined approach is an effective strategy for large-area manufacturing of perovskite devices on flexible substrates.