Thickness-dependent stress relaxation and its unreported effect on optical band gap of Al-doped ZnO thin films have been investigated. The thinnest film $\left(\sim 84\text{nm}\right)$ had a stress of $-8.39\times {10}^9{\text{Nm}}^{-2}$ , carrier concentration of $1.73\times {10}^{19}{\text{cm}}^{-3}$ and optical band gap of 3.69 eV, a value significantly higher than the reported ones. With increase in thickness, magnitude of the stress decreased, and correspondingly a redshift of fundamental absorption band edge was observed. A linear dependence of optical band gap on stress in the films with a coefficient of 54.6 meV/GPa has been observed.

Ambient energy harvesting has great potential to contribute to sustainable development and address growing environmental challenges. Converting waste energy from energy-intensive processes and systems (e.g. combustion engines and furnaces) is crucial to reducing their environmental impact and achieving net-zero emissions. Compact energy harvesters will also be key to powering the exponentially growing smart devices ecosystem that is part of the Internet of Things, thus enabling futuristic applications that can improve our quality of life (e.g. smart homes, smart cities, smart manufacturing, and smart healthcare). To achieve these goals, innovative materials are needed to efficiently convert ambient energy into electricity through various physical mechanisms, such as the photovoltaic effect, thermoelectricity, piezoelectricity, triboelectricity, and radiofrequency wireless power transfer. By bringing together the perspectives of experts in various types of energy harvesting materials, this Roadmap provides extensive insights into recent advances and present challenges in the field. Additionally, the Roadmap analyses the key performance metrics of these technologies in relation to their ultimate energy conversion limits. Building on these insights, the Roadmap outlines promising directions for future research to fully harness the potential of energy harvesting materials for green energy anytime, anywhere.

An enhanced conversion efficiency of ∼3.10% for PbS/CdS thin film solar cells without the involvement of quantum dots is demonstrated by focusing on the origin of the enhancement. The optical band gap of the PbS absorber is optimized toward a higher value by utilizing near room temperature deposition in chemical baths. Only the highest band gap of ∼1.61 eV for p-type PbS, which was obtained at 40 °C, results in a promising band alignment with an n-type CdS layer for effective light absorption and charge transfer. Both open circuit voltage and current density increase substantially to 280 mV and 20.93 mA cm⁻², respectively, with the ideal adjustment of the relative band gaps. The variations in crystallite size, surface roughness, the stoichiometry ratio of S/Pb and carrier concentration are discussed as key parameters in relation to the improved band alignment and photovoltaic properties.

Halide perovskite materials have been recently recognized to possess potentially strong ferroelectricity and piezoelectricity even though the experimental evidence is very limited. Herein, we report high piezoelectricity of representative inorganic halide CsPbBr₃ thin films and their energy harvesting and current-sensing characteristics with an optimization process. This is the very first report of introducing an effective piezoelectric coefficient and energy-harvesting performance for inorganic halide perovskites. A record high piezoelectric coefficient of ∼40.3 pm V⁻¹ was obtained for the optimized CsPbBr₃, which was attributed to the extended bonding angles of Pb–Br–Pb and Br–Pb–Pb arising from the applied electric field according to structural simulations. As expected from the high piezoelectricity, the electromechanical energy-harvesting performance was very competitive with excellent values of a ∼16.4 V output voltage and ∼604 nA output current achieved for a large-scale harvester of 5 cm². Highly strain-sensitive devices were also successfully exemplified to identify specific body-motion from the detected current values in the sensitive inorganic halide patches.

Wearable pressure sensors having versatile device structures have been extensively investigated to achieve high sensitivity under mechanical stimuli. Here, we introduce piezoelectric pressure sensors based on fabrics woven using polyvinylidene fluoride (PVDF) weft and polyethylene terephthalate (PET) warp yarns with different weave structures: 1/1 (plain), 2/2, and 3/3 weft rib patterns. The dependence of the pressure-sensing performance on the weave pattern is demonstrated with an actual large-scale fabric up to the ~2 m scale. An optimized pressure sensor with a 2/2 weft rib pattern produced a high sensitivity of 83 mV N⁻¹, which was 245% higher than that of the 1/1 pattern. The detection performance of the optimal fabric was extensively evaluated with a variety of ambient input sources, such as pressing, bending, twisting, and crumpling, as well as various human motions. Further, a large all-fabric pressure sensor with arrayed touch pixel units demonstrated highly sensitive and stable sensing performance.