Thin crystalline silicon solar cells

A 50 μm thin layer of high quality crystalline silicon together with efficient light trapping and well passivated surfaces is in principle all that is required to achieve stable solar cell efficiencies in the 20% range. In the present work, we propose to obtain these layers by directly cutting 50 μm thin wafers from an ingot with novel cutting techniques. This development is discussed in the frame of a defect tolerant mass production scenario and aims at obtaining twice the amount of wafers as compared to present wire/slurry technology. The ability to process such mechanically flexible wafers into solar cells with standard laboratory equipment is experimentally verified.     

Radiation degradation of large fluence irradiated space silicon solar cells

In this paper, we present data on the electrical properties of 50 gm thick space silicon BSFR cells irradiated with 10 MeV protons with a fluence exceeding 1 x 10¹³ p/cm² and irradiated with 1 MeV electrons with a fluence exceeding 1 x 10¹⁶ e/cm², and discuss the anomalous degradation which was found in these large-fluence regions. These data show an increase of saturation current density and a decrease of diffusion voltage of the pn junction, and a decrease of majority carrier density and an increase of series resistance of the p-substrate as a result of the formation of a large amount of carrier traps by the large-fluence irradiation.     

Modelling of material properties influence on back junction thin polycrystalline silicon solar cells

The influence of polycrystalline silicon properties on the performances of thin back junction solar cells has been investigated by means of a 3-dimensional model taking into account grain size, grain boundary recombination, volumic recombination, and surface recombination. The drastic influence of front surface recombination has been confirmed. The grain size has been shown to be of minor importance provided the grain size is not too small and the grain boundaries are correctly passivated. An optimal base thickness has been determined which is all the smaller that the material is more imperfect.     

High-efficiency silicon space solar cells

SHARP's activities on Si solar cells developments and features of Si solar cells for space use in comparison with GaAs solar cells are presented. Two types of high-efficiency silicon solar cells and the same kinds of high-efficiency solar cells with integrated bypass function (IBF cells) were developed and qualified for space applications. The NRS/LBSF cells and NRS/BSF cells showed an average of 18% and 17% efficiencies, respectively, at AMO and 28°C conditions. The IBF cells have P⁺N⁺ diodes on the front surface to protect itself from reverse voltage due to shadowing. The designs and features of these solar cells are presented. The radiation tests results of these solar cells are also presented. The NRS/BSF cells showed lower degradation rate compared to conventional BSFR cells with the same thickness (100 μm). But the NRS/LBSF cells showed a higher degradation rate than the BSFR cells. The IBF cells showed almost the same radiation characteristics as the same kinds of cells without IBF. The results of radiation tests on these high-efficiency solar cells and the discussions about the radiation characteristics of them are presented. In the last section, the future silicon solar cell development plan is discussed.     

Base limited carrier transport and performance of double junction rear point contact silicon solar cells

Rear point contact (locally diffused) solar cells with double collecting p/n junctions are investigated through numerical three-dimensional simulation and comparison with corresponding single junction solar cells. Their illuminated I–V characteristics are simulated and the series resistance of both structures is calculated. Results are presented which show the influence of the additional collecting junction to the cell's series resistance and performance. It is also shown that by biasing both junctions under slightly different voltages, the efficiency of the cell slightly increases.     

The influence of porous silicon on junction formation in silicon solar cells

In this work the results of a structural investigation by SEM of porous silicon (PS) before and after diffusion processes are reported. The formation of PS n⁺/p structures were carried out on PS p/p silicon wafers with two methods: from POCl₃ in a conventional furnace and from a phosphorous doped paste in an infrared furnace. Sheet resistance was found to be a strong function of PS structure. Further details on sheet resistance distribution are reported. The electrical contacts in prepared solar cells were obtained by screen printing process, with a Du Ponte photovoltaic silver paste for front contacts and home-prepared silver with 3% aluminium paste for the back ones. Metallization was done in the infrared furnace. Solar cell current–voltage characteristics were measured under an AM 1.5 global spectrum sun simulator. The average results for multi-crystalline silicon solar cells without antireflection coating are: I_sc=720 (mA), V_oc=560 (mV), FF=69%, Eff=10.6% (area 25 cm²).     

Contribution of silicon substrates to the efficiencies of silicon thin layer solar cells

Although silicon solar cells based on layers less than 50 μm thick have become very popular, little attention has been paid to the role of the underlying silicon substrate. This treatment uses the device simulation program PC-1D and the ray tracing program SUNRAYS to examine the role of the substrate in contributing to the current and efficiency of textured and non-textured thin layer solar cells. For the case of a heavily doped silicon substrate, substrate contributions can be significant for cells with sufficiently thin base layers. For example, for the case of a silicon thin layer cell with a base layer thickness of 20 μm and a substrate doping of 6 × 10¹⁸ cm⁻³, the substrate contributes no more than 4% of the total short-circuit current. However, decreasing the base width to 5 μm results in an increase in this substrate contribution to 20%. Light trapping tends to alleviate the substrate contribution by increasing the effective path length in the base. Examination of the current components under forward bias reveals that for a thin layer cell with a high quality base and good front surface passivation, back diffusion of electrons into the substrate limits cell performance.     

Surface passivation in high efficiency silicon solar cells

Surface passivation for crystalline silicon solar cells is particularly important for devices with open-circuit voltages in excess of 650 mV. Thick passivating thermal oxides, originally developed for use with buried contact solar cells, are shown to produce the most effective and stable surface passivation particularly in conjunction with lightly phosphorus diffused surfaces. However, for improved optical performance, antireflection coatings are only effective with surface oxide thicknesses reduced to 100–200 Å. Thinner passivating oxides cause significant voltage loss, most of which can be recovered through hydrogen passivation. Throughout this study, variation in surface passivation approaches has produced open-circuit voltages ranging from 620 mV to record voltages of 720 mV.     

Radiation-resistant solar cells for space use

This paper reviews the present status of radiation-resistant solar cells made with Si, GaAs, InP and InGaP/GaAs for space use. At first, properties of radiation-induced defects in semiconductor materials and solar cells are described based on an anomalous degradation of Si space solar cells under high-energy, high-fluence electron and proton irradiations. Advantages of direct bandgap materials as radiation-resistant space cells are presented. Unique properties of InP as radiation-resistant cells have also been found. A world-record efficiency of 26.9% (AM0) has been obtained for an InGaP/GaAs tandem solar cell. Radiation-resistance of the InGaP/GaAs tandem cells is described.     

Aluminum alloy back p–n junction dendritic web silicon solar cell

A new silicon solar cell structure is presented in which the p–n junction is formed by alloying aluminum with n-type silicon, and where this p–n junction is located at the back (unilluminated) side of the cell. With a phosphorus front diffusion, the resultant n⁺np⁺ structure has been implemented using dendritic web silicon substrates which are 100 μm thick and doped with antimony to 20 Ω cm. Such a structure eliminates shunting of the p–n junction, provides an effective front surface field, enables a high minority carrier lifetime in the base, and is immune to light-induced degradation. Using only production-worthy, high-throughput processes, aluminum alloy back junction dendritic web cells have been fabricated with efficiencies up to 14.2% and with corresponding minority carrier (hole) lifetime in the base of 115 μs.     

Experimental and theoretical radiation damage studies on crystalline silicon solar cells

An experimental facility was developed to asses in situ the degradation of crystalline silicon solar cells, fabricated by the Solar Energy Group of the National Atomic Energy Commission (CNEA), by measuring the current–voltage characteristic curve. The cells were irradiated with 10 MeV protons and fluences between 10⁸ and 10¹³ p/cm², using an external beam of the linear tandem accelerator TANDAR, at CAC-CNEA. Furthermore, theoretical simulations were performed to establish the relation between the variation of the electrical parameters and the degradation of the lifetime of minority carriers in the base. The damage constant for 10 MeV proton irradiated silicon solar cells of n⁺–p–p⁺ structure and 1 Ω cm base resistivity was determined. Finally, a proposal of a new model of radiation damage for silicon solar cells is discussed.     

Characteristics of vapor-liquid-solid grown silicon nanowire solar cells

We report fabrication and characterization of solar cells based on vapor-liquid-solid (VLS) grown silicon nanowires (NWs) that form core-shell radial p-n junction structures. We observe efficiency enhancement due to the presence of the NWs that increase the light trapping within the device, while the use of gold as VLS catalyst results in increased carrier recombination within the wires. From the spectral efficiency data, we identify that the surface recombination effect becomes more significant in the large surface area NW cells. To remedy this issue we demonstrate the efficacy of a highly conformal Al₂O₃ film grown by atomic layer deposition to serve as surface passivation layer. This work highlights the key issues confronted by NW-based solar cells grown by VLS technique.     

Microcrystalline silicon solar cells fabricated by VHF plasma CVD method

A series of systematic investigations on microcrystalline silicon (μc-Si:H) solar cells at high deposition rates has been studied. The effect of high deposition pressure and narrow cathode-substrate (CS) distance on the deposition rate and quality of microcrystalline silicon is discussed. The microcrystalline silicon solar cell is adopted as middle cell and bottom cell in a three-stacked junction solar cell. The characteristics of large area three-stacked junction solar cells, whose area is 801.6 cm² including grid electrode areas, are studied in various deposition rates from 1 to 3 nm/s of microcrystalline silicon. An initial efficiency of 13.1% is demonstrated in the three-stacked junction solar cell with microcrystalline silicon deposited at 3 nm/s.     

Laser structuring of crystalline silicon thin-film solar cells on opaque foreign substrates

Crystalline silicon thin-film solar cells were fabricated on graphite substrates. A laser ablation process was developed for edge isolation of the thin-film cells. The shunt resistance was comparable to otherwise identical cells isolated by plasma etching, while the reproducibility of the laser isolation process was higher. The solar cells were characterized by current-voltage and light beam induced current measurements (LBiC). No interference was detected along the ablated edges. Spatial variations of the minority carrier lifetime are attributed to the grain structure of the seeding layer obtained by the zone melting recrystallization (ZMR).     

a-Si/mc-Si hybrid solar cell using silicon sheet substrate

A hybrid junction solar cell with amorphous silicon (a-Si) and multicrystalline silicon (mc-Si) was fabricated using a mc-Si sheet substrate, which is produced directly from molten silicon using a novel rotational solidification method. The efficiency of 11.6% was obtained for the hybrid junction cell, while 10.2% for the single junction cell made of a mc-Si sheet substrate, which confirmed that the hybrid structure is effective to improve the solar cell property made of a mc-Si substrate. With introducing light trapping structure, the efficiency was improved to be 12.0%. Moreover, the possibility of Jsc improvement was investigated using the advanced light trapping structure. Jsc of 15.6 mA/cm² was obtained and it was confirmed that the hybrid junction is a promising structure.     

Radiation response of InP/Si and InGaP/GaAs space solar cells

An analysis of the radiation response of state-of-the-art InP/Si, InGaP, and dual junction (DJ) InGaP/GaAs space solar cells under both electron and proton irradiated is presented. The degradation data are modeled using the theory of displacement damage dose. For each technology, a characteristic curve which describes the cell degradation in any radiation environment is determined, and the characteristic curves are used to compare the radiation resistance of the different technologies on an absolute scale. The radiation data are used as input to a code which predicts the end-of-life (EOL) performance of a solar panel in earth orbit. The results show that in orbits outside the earth's radiation belts, the high-efficiency DJ InGaP/GaAs solar panels provide the highest EOL specific power. However, in orbits which pass through the belts, the radiation hard InP/Si panels provide the highest specific power by as much as 30%.     

GaInP single junction and GaInP/GaAs two junction thin-film solar cell structures by epitaxial lift-off

The epitaxial lift-off (ELO) technique was used in forming a thin-film GaInP/GaAs two-junction monolithic tandem solar cell structure. First, the GaInP single junction solar cell to be used in the tandem cell structure as a top cell was thinned by the ELO process. Although the ELO process and the transfer to the quartz substrate caused a strain in the thin-film cell after separation from the GaAs substrate, the photoluminescence peak intensity was not decreased. This shows that defects, such as those causing carrier loss, were not introduced on the thin-film cell during the thinning process. The key issue for thin-film cell fabrication is to avoid damaging the AlInP window layer during the selective etching (HF etchant), by which the thin-film cell is released from the GaAs substrate. A GaInP/GaAs monolithic tandem structure was also thinned by the same process with a GaInP single junction cell. Characteristics of the single-junction GaInP cell and individual cells in the GaInP/GaAs tandem structure were examined. It was found that the spectral response remains almost the same as that for cells with a GaAs substrate, thus confirming the feasibility of using the ELO process to fabricate thin-film GaInP/GaAs cells.     

A computer analysis of double junction solar cells with a-Si : H absorber layers

An integrated electrical-optical model has been used to examine the design of double junction solar cells, where the component cells have a-Si : H absorber layers of identical material quality in the initial state. The model takes into account both specular interference effects; and diffused reflectances and transmittances due to interface roughness. The carrier transport at the junction between the two p–i–n subcells is simulated with the help of a thin heavily defective “recombination” layer with a reduced mobility gap.Analysis of the transport properties as a function of position in the device indicates that for the highest double junction cell efficiency the thicknesses of the absorber layers of the component subcells are such that the electric fields over these absorber layers are high simultaneously. Our results also show that whereas in the initial state, the open-circuit voltage and the fill factor of the double junction cell are heavily dependent on the electric field in the thicker bottom subcell; in the light-stabilised state, the more degraded top subcell plays an important role in limiting double junction cell performance. The quantum efficiency under AM 1.5 bias light has been shown to be very sensitive to thickness variations of the component subcells. Using this tool we have arrived at a simplified procedure for designing the double junction structure likely to exhibit the highest efficiency in the stabilised state.     

Effect of edge junction isolation on the performance of laser doped selective emitter solar cells

The effect of laser and chemical edge junction isolation on electrical performance of industrially manufactured laser doped selective emitter solar cells with light induced plated n-type contacts is investigated in this work. Directly after the formation of the aluminium back surface field, photoluminescence images indicates that laser edge junction isolation causes substantial damage around the perimeter of the cell, extending several millimeters from the laser edge isolation groove. On finished devices, regions of high series resistance are evident around the perimeter, caused by parasitic plating nucleating in the damaged laser grooved region which induce shunting and inhibits further plating taking place in the surrounding regions. The use of chemical edge junction isolation eliminates both of these issues and can result in efficiency gains of more than 2% absolute compared to that fabricated using laser edge isolation, suggesting a far superior method of edge junction isolation for the industrial manufacture of laser doped selective emitter solar cells with light induced plated contacts.     

High-efficiency drift-field thin-film silicon solar cells grown on electronically inactive substrates

A drift-field in the base region of a solar cell can enhance the effective minority-carrier diffusion length, thus increasing the long-wavelength spectral response and energy-conversion efficiency. Silicon thin-films of 20–32 μm thickness as a cell base layer were grown by liquid-phase epitaxy (LPE) on electronically inactive heavily doped p⁺⁺-type CZ silicon substrates. Growth was performed from In/Ga solutions, and in a purified Ar/4%H₂ forming gas ambient, rather than pure H₂. The Ga dopant concentration was tailored throughout the p-type film to create a drift-field in the base layer of the solar cell. An independently confirmed efficiency of 16.4% was achieved on such an LPE drift-field thin-film silicon solar cell with a total cell area of 4.11 cm². Substrate thinning, combined with light trapping which is encouraged by the textured front surface and a highly reflective aluminium rear surface, is demonstrated to improve the long-wavelength response and thus, increase cell efficiency by a factor of up to 23.7% when thinned to a total cell thickness of 30 μm.