Photovoltaic (PV) converters on the centimeter scale are considered to be the most promising energy supplier for energy-autarkic microsystems in indoor applications, i.e., to power wireless sensor nodes in automated buildings. We provide the first systematic discussion of the optimum material and efficiency limits of indoor photovoltaics. The limiting efficiency η of an ideal photovoltaic converter for various indoor radiation sources and two calculation models ranges between 46% for a fluorescent tube and a bandgap Eg = 1.95 eV and 67% for a sodium discharge lamp and Eg = 2.10 eV. The optimal bandgap for typical narrow-band artificial light sources is 1.90-2.00 eV. For Eg = 2.25 eV, i.e., matched to the photon energy of a monochromatic source emitting at 555 nm, η calculated from the detailed balance model reaches 72.98%. The performance when irradiated by indoor Planckian radiators, such as incandescent bulbs and halogen tubes, is always less than under any solar irradiance. The experimental efficiencies of 11 sample groups with a maximum efficiency of 16%, which were measured with a radiometric indoor characterization setup especially developed for this study, support the theoretically predicted changes in efficiency when compared with efficiencies under the conditions of IEC 60904-3.

Maximum efficiencies of indoor photovoltaic devices

Summary of Chapter 2 The three-dimensional transport in solar cells with periodically arranged rear point contacts wassuccessfully reduced to a one-dimensional calculation. A new analytical formulation attributes aneffective surface recombination velocity S eff to the inhomogeneous rear surface. It is based on arelationship between the effective diffusion length and the series resistance in the base that be-came obvious after rigorously decoupling the transport problem. An approximation for this seriesresistance was derived which, for the first time, remains valid in every asymptote of the involvedparameters. Scaling laws helped to reduce the number of these parameters to only two; the cover-age fraction f and the period to cell thickness ratio l P . The final equation for the effective SRVreads fsf l f l f sls passP P metPeff −++−= − 11 1exp2arctan2 1 ππ,with s i = S i W / D , the SRV at the contacts S met and between the contacts S pass . The results are in goodagreement with three-dimensional simulations using a numerical Fourier-transform technique.A new method was found to discuss the effects of carrier lifetime variations in multicrystal-line solar cells without the need to assume a particular lifetime distribution. It consists in rules,expressed as a number

Loss analysis of crystalline silicon solar cells using photoconductance and quantum efficiency measurements

For most wireless sensor networks, one common and major bottleneck is the limited battery lifetime. The frequent maintenance efforts associated with battery replacement significantly increase the system operational and logistics cost. Unnoticed power failures on nodes will degrade the system reliability and may lead to system failure. In building management applications, to solve this problem, small energy sources such as indoor light energy are promising to provide long-term power to these distributed wireless sensor nodes. This article provides comprehensive design considerations for an indoor light energy harvesting system for building management applications. Photovoltaic cells characteristics, energy storage units, power management circuit design, and power consumption pattern of the target mote are presented. Maximum power point tracking circuits are proposed which significantly increase the power obtained from the solar cells. The novel fast charge circuit reduces the charging time. A prototype was then successfully built and tested in various indoor light conditions to discover the practical issues of the design. The evaluation results show that the proposed prototype increases the power harvested from the PV cells by 30p and also accelerates the charging rate by 34p in a typical indoor lighting condition. By entirely eliminating the rechargeable battery as energy storage, the proposed system would expect an operational lifetime 10--20 years instead of the current less than 6 months battery lifetime.

Design considerations of sub-mW indoor light energy harvesting for wireless sensor systems

The current-voltage (I-V) characteristics of most industrial silicon solar cells deviate rather strongly from the exponential behavior expected from textbook knowledge. Thus, the recombination current may be orders of magnitude larger than expected for the given material quality and often shows an ideality factor larger than 2 in a wide bias-range, which cannot be explained by classical theory either. Sometimes, the cells contain ohmic shunts although the cell’s edges have been perfectly insolated. Even in the absence of such shunts, the characteristics are linear or super-linear under reverse bias, while a saturation would be classically expected. Especially in multicrystalline cells the breakdown does not tend to occur at -50 V reverse bias, as expected, but already at about -15 V or even below. These deviations are typically caused by extended defects in the cells. This paper reviews the present knowledge of the origin of such non-ideal I-V characteristics of silicon solar cells and introduces new results on recombination involving coupled defect levels.

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Influence of Defects on Solar Cell Characteristics

Simultaneously achieving high voltage and high efficiency in thin-film solar cells is of paramount importance for real-world applications. While solar cells fabricated from the Earth-abundant kesterite absorber Cu2ZnSn(S
 x
 Se1−x
 )4 provide an attractive, non-toxic, energy harvesting solution, their utilization has been constrained by relatively low open-circuit voltages that limit efficiency. Increasing the sulfur content to widen the bandgap boosts the voltage, but usually at the expense of efficiency. Here, we report important progress on this fundamental problem by fabricating solar cells with high sulfur content that exhibit efficiencies up to 11.89% with open-circuit voltages as high as 670 mV. In a multistep process, fully functional solar cells are separated from their growth substrate, and a high-work-function back contact is subsequently deposited. With this approach, we fabricated a series-connected device that produces 5.7 V under 1 Sun illumination and ~2 V under low lighting conditions, below 10−3 Suns. Kesterite thin-film solar cells feature abundant non-toxic elements. Here, Antunez et al. present a process to simultaneously optimize the conversion efficiency and voltage over a wide range of light intensities appropriate for small-scale, distributed and indoor applications.

Efficient kesterite solar cells with high open-circuit voltage for applications in powering distributed devices

At Fraunhofer ISE the fabrication of high-efficiency solar cells was extended from a laboratory scale to a small pilot-line production. Primarily, the fabricated cells are used in small high-efficiency modules integrated in prototypes of solar-powered portable electronic devices such as cellular phones, handheld computers etc. Compared to other applications of high-efficiency cells such as solar cars and planes, the illumination densities found in these mainly indoor applications are significantly below 1 sun. Thus, special care was taken to keep the cell efficiency level high even at very low illumination levels. For this reason, particularly the cell border was analyzed and optimized carefully. The excellent cell characteristics achieved at low illumination densities increase the benefit of a solar power supply for such devices by an order of magnitude if compared to standard solar cells.

H. Schmidt

Papers

High-efficiency silicon solar cells for low-illumination applications