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

Recombination within the bulk and at the surfaces of crystalline silicon has been investigated in this thesis. Special attention has been paid to the surface passivation achievable with plasma enhanced chemical vapour deposited (PECVD) silicon nitride (SiN) films due to their potential for widespread use in silicon solar cells. The passivation obtained with thermally grown silicon oxide (SiO2) layers has also been extensively investigated for comparison. Injection-level dependent lifetime measurements have been used throughout this thesis to quantify the different recombination rates in silicon. New techniques for interpreting the effective lifetime in terms of device characteristics have been introduced, based on the physical concept of a net photogeneration rate. The converse relationships for determining the effective lifetime from measurements of the open-circuit voltage (Voc) under arbitrary illumination have also been introduced, thus establishing the equivalency of the photoconductance and voltage techniques, both quasi-static and transient, by allowing similar possibilities for all of them. The rate of intrinsic recombination in silicon is of fundamental importance. It has been investigated as a function of injection level for both n-type and p-type silicon, for dopant densities up to ~5x10cm. Record high effective lifetimes, up to 32ms for high resistivity silicon, have been measured. Importantly, the wafers where commercially sourced and had undergone significant high temperature processing. A new, general parameterisation has been proposed for the rate of band-to-band Auger recombination in crystalline silicon, which accurately fits the experimental lifetime data for arbitrary injection level and arbitrary dopant density. The limiting efficiency of crystalline silicon solar cells has been re-evaluated using this new parameterisation, with the effects of photon recycling included. Surface recombination processes in silicon solar cells are becoming progressively more important as industry drives towards thinner substrates and higher cell efficiencies. The surface recombination properties of well-passivating SiN films on p-type and n-type silicon have been comprehensively studied, with Seff values as low as 1cm/s being unambiguously determined. The well-passivating SiN films optimised in this thesis are unique in that they are stoichiometric in composition, rather than being silicon rich, a property which is attributed to the use of dilute silane as a process gas. A simple physical model, based on recombination at the Si/SiN interface being determined by a high fixed charge density within the SiN film (even under illumination), has been proposed to explain the injection-level dependent Seff for a variety of differently doped wafers. The passivation obtained with the optimised SiN films has been compared to that obtained with high temperature thermal oxides (FGA and alnealed) and the limits imposed by surface recombination on the efficiency of SiN passivated solar cells investigated. It is shown that the optimised SiN films show little absorption of UV photons from the solar spectrum and can be easily patterned by photolithography and wet chemical etching. The recombination properties of n and p emitters passivated with optimised SiN films and thermal SiO2 have been extensively studied over a large range of emitter sheet resistances. Both planar and random pyramid textured surfaces were studied for n emitters, where the optimised SiN films were again found to be stoichiometric in composition. The optimised SiN films provided good passivation of the heavily doped n-Si/SiN interface, with the surface recombination velocity increasing from 1400cm/s to 25000cm/s as the surface concentration of electrically active phosphorus atoms increased from 7.5x10cm to 1.8x10cm. The optimised SiN films also provided reasonable passivation of industrial n emitters formed in a belt-line furnace. It was found that the surface recombination properties of SiN passivated p emitters was poor and was worst for sheet resistances of ~150Ω/ . The hypothesis that recombination at the Si/SiN interface is determined by a high fixed charge density within the SiN films was extended to explain this dependence on sheet resistance. The efficiency potential of SiN passivated np cells has been investigated, with a sheet resistance of 80-100Ω/ and a base resistivity of 1-2Ωcm found to be optimal. Open-circuit voltages of 670-680mV and efficiencies up to ~20% and ~23% appear possible for SiN passivated planar and textured cells respectively. The recombination properties measured for emitters passivated with SiO2, both n and p, were consistent with other studies and found to be superior to those obtained with SiN passivation. Stoichiometric SiN films were used to passivate the front and rear surfaces of various solar cell structures. Simplified PERC cells fabricated on 0.3Ωcm p-type silicon, with either a planar or random pyramid textured front surface, produced high Voc’s of 665-670mV and conversion efficiencies up to 19.7%, which are amongst the highest obtained for SiN passivated solar cells. Bifacial solar cells fabricated on planar, high resistivity n-type substrates (20Ωcm) demonstrated Voc’s up to 675mV, the highest ever reported for an all-SiN passivated cell, and excellent bifaciality factors. Planar PERC cells fabricated on gettered 0.2Ωcm multicrystalline silicon have also demonstrated very high Voc’s of 655-659mV and conversion efficiencies up to 17.3% using a single layer anti-reflection coating. Short-wavelength internal quantum efficiency measurements confirmed the excellent passivation achieved with the optimised stoichiometric SiN films on n emitters, while long-wavelength measurements show that there is a loss of short-circuit current at the rear surface of SiN passivated p-type cells. The latter loss is attributed to parasitic shunting, which arises from an inversion layer at the rear surface due to the high fixed charge (positive) density in the SiN layers. It has been demonstrated that that a simple way to reduce the impact of the parasitic shunt is to etch away some of the silicon from the rear contact dots. An alternative is to have locally diffused p regions under the rear contacts, and a novel method to form a rear structure consisting of a local Al-BSF with SiN passivation elsewhere, without using photolithography, has been demonstrated.

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Surface, Emitter and Bulk Recombination in Silicon and Development of Silicon Nitride Passivated Solar Cells

A solar cell comprises a crystalline substrate having projections and depressions formed on either side or both sides of the substrate, wherein a projection-depression depth is 25 μm or more.

Solar cell and production process therefor

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.

High-efficiency silicon solar cells for low-illumination applications

At ISE, a research project has been carried out which aims at the development of single crystalline solar cells of high efficiency. For this task a clean room laboratory for solar cell processing has been established, various measuring stations for the characterization of processes and cells have been set up, and a processing sequence for the manufacturing of such cells has been established. All systems and all processes are working well. The highest efficiencies (AM1.5) obtained to date are 21.1% for FZ-, 19.3% for CZ-, and 16.2% for MC (multicrystalline)-silicon. >

F.J. Kamerewerd

Papers

High-efficiency silicon solar cells for low-illumination applications

High-efficiency solar cells from FZ, CZ and MC silicon material