From smart building sensors to asset trackers, many indoor IoT devices still rely on disposable batteries for power due to their simple design. However, this dependency brings some challenges, including limited lifespan, maintenance costs, operational downtime, and environmental issues. These factors combined directly affect the reliability of IoT devices.
In addition, frequent battery replacement is both time-consuming and inefficient. This runs counter to the vision of the Internet of Things being "autonomous and devices always online". Therefore, it is necessary to adopt new methods to power indoor IoT nodes to improve reliability, minimize maintenance costs, and promote large-scale deployment.
According to a report by Transforma Insights, it is expected that the growth of IoT devices will increase energy demand by 34 terawatt hours by 2030. Therefore, the key to addressing this challenge is to utilize indoor solar cells for continuous power supply, reduce electronic waste by using sustainable materials and avoiding the use of batteries, and minimize energy consumption costs for computing and transmitting data as much as possible.
In recent years, photovoltaic technology tailored for indoor environments has made significant progress in materials and structures. Crystalline silicon is the standard active material for outdoor solar panels, with a bandgap of 1.12 eV. However, since typical indoor light sources only emit light in the visible range, the optimal bandgap becomes 1.9-2.0 eV.
Therefore, crystalline silicon has poor performance under indoor lighting conditions. To address this issue, the industry has developed indoor alternatives using light harvesting technology, including amorphous silicon, dye-sensitized solar cells (DSSCs), peroxide solar cells, and organic photovoltaic cells.
Figure 1: Panasonic Energy's AM-1456CA-DGK-E amorphous solar cell uses a glass substrate. (Image source: Panasonic Energy)
Key indoor photovoltaic technologies for the Internet of Things
1. Amorphous silicon (a-Si) battery
Amorphous silicon (a-Si) is a mature thin-film solar technology with an optical bandgap of approximately 1.6 eV, which is closer to the optimal value for indoor lighting applications. This is the first technology to be incorporated into low-power indoor IoT devices.
Due to the spectral matching characteristics of amorphous silicon and its relatively high open circuit voltage at low light levels, a-Si performs better than crystalline silicon under typical indoor lighting conditions. Tests have shown that the efficiency of hydrogenated a-Si solar cells under LED indoor lighting can reach 21%.
The main advantage of a-Si solar cells is the use of gaseous plasma sources to manufacture thin films, which is cost-effective. This enables the manufacturing of solar cells on low-cost flexible substrates.
However, this technology has a major limitation - it requires a larger battery area to generate the same power as the new technology. In addition, the voltage generated by each a-Si battery individually is relatively low, so it is usually necessary to connect each battery in series to achieve the voltage required by IoT devices.
Figure 2: BCS4430B6 amorphous thin flexible solar cell from TDK Corporation, with an open circuit voltage of 4.2 V. (Image source: TDK Corporation)
2. Dye sensitized solar cells (DSSCs)
As a new generation photovoltaic device, the working principle of DSSC is similar to photosynthesis. The dye on the working electrode generates electrons through photosensitivity, which are then replenished by the electrolyte through redox reactions. This dye can be optimized based on the emission spectrum of indoor light sources, making it highly suitable for indoor IoT applications.
A different design approach is to use multidimensional nanostructures, such as composite photoanodes. This structure combines scattering functions to enhance light capture and charge collection capabilities. A research paper claims that a new type of nanostructure has achieved a power conversion efficiency of 24% under extremely weak artificial lighting conditions of 0.014 mW/cm2.
3. Peroxide solar cells (PSC)
Another promising alternative for indoor applications is PSC, and research on this material began in 2015. In this study, researchers achieved control over trap states and carrier dynamics in the perovskite active layer by designing an electron transport layer. The resulting PSC achieved a power conversion efficiency of 27.4% in indoor environments.
Perovskite is a type of semiconductor material that can be processed in solution. This material can be adjusted to an ideal bandgap value of 1.8 eV and has high photovoltaic characteristics, thus exhibiting excellent photoelectric conversion efficiency under both LED light sources and fluorescent lighting conditions. The efficiency of perovskite indoor photovoltaic (IPV) devices has reached a historic high. A research report in 2025 showed that the power conversion efficiency at 1000 lux was 42%, the highest record ever.
4. Organic photovoltaic cells (OPVs)
Organic photovoltaic technology (OPV) utilizes carbon based molecules as semiconductors to absorb light and generate electricity. Through molecular design, organic semiconductors can be customized to have strong visible spectrum specificity. The optimized indoor OPV exhibits a power conversion efficiency of nearly 30% under low light conditions, comparable to the best DSSC or peroxide cells.
These characteristics make OPV particularly suitable for irregular shaped discrete IoT deployments, as it can be printed into thin flexible films on substrates such as PET plastic. Some companies even produce flexible indoor solar foils that can bend or adapt to various shapes. For IoT designers, this means that solar cells can be easily integrated into devices, such as as as thin films on sensor surfaces or sticker style power films.