All indoor radiation is optimized for its visibility by the recipient. For humans, this is the band between 380 nm and 780 nm. Any radiation beyond this narrow band is generally unused and thus lost. The maximum indoor efficiencies therefore are significantly higher than outdoor photovoltaic efficiencies. The radiation filtered in windows outside the visibility spectrum could also be used for photovoltaic conversion.
Resulting from the peak of the human visibility function, the ideal indoor photovoltaic material has a higher band gap than for outdoor applications. Indoor cells optimized to indoor solar radiation have ideal band gaps between 1.4 to 1.6 eV, whilst cells for current artificial light sources have ideal band gaps between 1.8 to 2 eV.
Detailed model and technology discussions for the most interesting indoor materials are provided in the following chapters of this book.
Figure 3.2 Efficiencies for indoor spectra calculated following the Shockley-Queisser limit (adapted from [10]). As a reference, efficiency for outdoor AM 1.5 is given. Due to the adaption of functional window coatings to the human eye, the shift of the ideal band gap to higher values also holds true for indoor solar radiation.
3.3 State of IPV Design, Issues, Approaches
As the irradiance level in indoor surroundings generally is very low, the ratio of active conversion to loss area is very small. Thus, efficiency losses need to be avoided where possible. Therefore, a good indoor design avoids shading caused by module integration, ensures good electric contacts, and aims to avoid recombination losses at surfaces, including edges.
Another issue is a reliable characterization of the device. As standards and equipment are missing, reproducibility and comparability pose a challenge both to designers and customers. This is enforced by the fact that today’s most common light sources vary in their spectral distribution and that the available measurement instruments vary in their response to these light sources, too. Furthermore, photometric measurement devices optimized to measure human visibility exclude spectral radiation that is usable for IPV devices. These issues and approaches to solutions are outlined and discussed in Chapters 4 and 5. Müller (Freunek) demonstrated the use of a radiometric reference cell and pyranometers for indoor applications [12, 14]. Today, Enlitech offers an indoor solar cell light simulator, which is capable of measuring both photometric illuminance and radiometric irradiance for low irradiance levels and narrow-band light sources such as LED [28].
Contrary to outdoor applications, where the geographic location and the azimuth might be sufficient for the first stage of planning, in IPV spectra, intensity and contribution of direct irradiance vary from application to application. As these measurements can become too elaborate for the first stage of product development, simulations can be alternatively used. Different research groups have evaluated this for indoor conditions [13, 14, 29, 30]. Chapter 4 introduces the simulation of indoor irradiance conditions.
3.4 Fields of Application
Everywhere humans live and work, some kind of lighting optimized for the human eye is installed. Indoor photovoltaic devices can be operated with it, and often the available light is defined in its spectral range and irradiance following obligatory standards. This eases the design process. Although the classical and well-known IPV products include niche products such as kitchen scales, LED powered lamps, and solar calculators, the potential applications reliably realized with a proper IPV design are as varied as human life. The recent industrial takeoff of the Internet of Things (IoT) with billions of required sensor nodes in the field might drive a wide use of IPV. This section outlines the most common or important application fields for IPV today and in the near future.
3.4.1 Customer and Office Applications
Charging stations for mobile phones are common consumer IPV applications. Further examples include solar calculators, LED powered lamps such as the IKEA Sunnan lamp, USB chargers such as IXYS Green Energy products, and wireless keyboards such as the Logitech K750 [31, 32]. Figure 3.3 shows the Soehnle Easy Solar White kitchen scale.
However, regarding the size of the consumer market, the distribution of products powered by optical radiation is surprisingly low. Reasons for this might be found in spectrally mismatched design, resulting reliability issues, and standard mass production lines that are not optimized for low power consumption.
3.4.2 Ambient Assisted Living and Building Automatization
The concept of ambient assisted living is strongly related to building automation, smart homes, and the IoT. The idea is to support humans in their homes with automated and often customized electronic assistance, often with a focus on the elderly or handicapped individuals. In most applications, reliability is critical. That said, with ambient assisted living still in its early stage of development, this application with its many required sensors has huge potential for IPV; however, so far, the market coverage is very low.
Figure 3.3 Solar powered kitchen scale. (Photo reproduced by permission of Soehnle-Leifheit, 2019).
The aim of building automation is to provide comfort and efficiency and to automate energy-saving measures, such as switching lights on and off or regulating room temperatures. Besides financial losses or discomfort, a malfunction of a sensor poses no significant risks. On the other hand, installing automated energy-saving technologies will pay off for most applications very quickly. EnOcean provides different batteryless IPV powered sensors in this application field [33].
3.4.3 Industry, Agriculture, Horticulture, Retail, and Logistics
These applications show some of the highest potential for a significant market growth of IPV, as they are classical applications for IoT and cloud-assisted AI solutions. Typically, the IoT edge node needs to transmit or receive a sensor or actor signal in a regular period or even only under certain circumstances. Therefore, the required average power consumption of such an IoT edge node can range below a few milli- or even micro-watts, which can be provided from photovoltaic generators in most cases. There are different optimized products available on the market, including electronics in the case of the TI Bluetooth Low Energy Beacon [34]. IXYS specializes in manufacturing high-efficiency crystalline silicon modules that have been proven to be reliable indoors [10, 35]. The challenge and in some cases the advantage for some of these applications is the frequency of incident radiation. Applications with a typical schedule, such as production hours or even automated and spectrally defined radiation, such as in horticultural application, are an ideal case for IPV design due to their known operating conditions.
Logistics and retail have often been discussed as possible fields of application for micro energy harvesting. However, these applications often suffer from sufficient radiation and/or recycling issues in single-use applications, such as in tracking logistics. As proposed in the introduction, a dedicated radiation power beam might be an approach to some of those applications.
3.4.4 Relation of IPV to Outdoor Applications – Hiking, Emergency Kits
Many small-scale products available on the market were designed for outdoor use or outdoor charging. Among these are emergency kits for hikers, and phone chargers, torches and lamps such as the Little Sun products or the SOLVINDEN table lamp by IKEA [36, 37]. Generally, the module size of these products is a few square centimeters and the material is crystalline or amorphous Si. Although some products use high efficiency cells, they are not optimized for