Laser-beam-induced current mapping evaluation of porous silicon-based passivation in polycrystalline silicon solar cells

In the present work, we report on the effect of introducing a superficial porous silicon (PS) layer on the performance of polycrystalline silicon (pc-Si) solar cells. Laser-beam-induced current (LBIC) mapping shows that the PS treatment on the emitter of pc-Si solar cells improves their quantum response and reduce the grain boundaries (GBs) activity. After the porous silicon treatment, mapping investigation shows an enhancement of the LBIC and the internal quantum efficiency (IQE), due to an improvement of the minority carrier diffusion length and the passivation of recombination centers at the GBs as compared to the reference substrate. It was quantitatively shown that porous silicon treatment can passivate both the grains and GBs.     

Light trapping in layer-transferred quasi-monocrystalline porous silicon solar cell

Quasi-monocrystalline porous silicon (QMPS) layers have a top surface like crystalline silicon with small voids in the body. Such layers are reported to have significantly higher absorption coefficient compared to crystalline silicon at the wavelength of interest for solar cells. A model has been developed to account for higher absorption coefficient of QMPS layer. The model conforms to the experimental results. The model is then extended to predict absorption coefficient of QMPS layer for different thickness, porosity and void size. Interesting results are obtained, particularly regarding the dependence of absorption coefficient on thickness and void diameter of QMPS layers. Computed values of absorption coefficient and some experimental results relating to electronic properties of QMPS layers are used to investigate the solar cell potential of QMPS layers. Short circuit current density of about 31 mA/cm² is predicted for a QMPS layer of thickness 4 μm having average void radius of about 15 nm assuming effective diffusion length to be 5 μm.     

Solar cells from thin silicon layers on Al2O3

The surface of the applied Al₂O₃ ceramic substrate consists of small crystals with a maximum grain size of 5 μm. A 40 μm thick layer deposited on this surface shows polycrystalline quality and grain sizes in the order of 10 μm. Silicon layers of different thickness and doping have been deposited on Al₂O₃ substrates to make thin film silicon solar cells. These structures have been processed to solar cells in a two mask process. From measurements of the spectral response a diffusion length of 8 μm can be extracted.     

Numerical analysis of bulk diffusion length in thin-film c-Si solar cells

We propose a novel technique of determining relationship between effective and bulk diffusion length of single-crystalline Si (c-Si) thin-film solar cells using two-dimensional device simulator. In addition, bulk diffusion length was obtained using the result of the simulation. Effective diffusion length was measured by LBIC method in order to presume bulk diffusion length of c-Si thin film. We obtained 6.7 μm for effective diffusion length of c-Si thin-film solar cell whose thickness was about 7 μm. We compared the result of measurement and simulation, bulk diffusion length of c-Si thin film prepared by CVD method was estimated more than 30 μm and recombination velocity was presumed <10⁴ cm/s for front surface and 10³ cm/s for rear surface of the cell.     

Monitoring of silicon solar cell technology via the surface photovoltage method

The surface photovoltage (SPV) technique adapted to thin samples was used to monitor solar cell technology. The relatively short minority carrier diffusion length from 70 to 80 μm found in p-bulk of the cells results from the presence of a layer with structural defects near the surface. The measurement of successively etched samples reveals that freshly cut off silicon wafers are already strongly destroyed to a depth of at least 35 μm. A diffusion length of about 300 μm was evaluated in the samples after removing the disturbed layer.     

Optimisation of interdigitated back contacts solar cells by two-dimensional numerical simulation

In this paper we present the results of the simulation of interdigitated back contacts solar cell on thin-film (50 μm) silicon layer. The influence of several parameters (surface recombination rate, substrate thickness and type, diffusion length, device geometry, doping levels) on device characteristics are simulated using the accurate two-dimensional numerical simulator DESSIS that allows to optimise the cell design.     

Analysis of thin silicon solar cells for high efficiency

A comprehensive theoretical analysis taking into account the contribution from both the emitter and base regions having finite surface recombination velocity has been developed for computing short-circuit current, open-circuit voltage, and efficiency of thin AR coated thin silicon solar cells with textured front surface. The dependence of efficiency on the front surface and back surface recombination velocities and on the cell parameters have been investigated in details for varying cell thickness considering the effects of bandgap narrowing and Auger recombination in the material. It is shown that efficiency exceeding 24% can be attained with silicon solar cells having thickness as low as 25 μm provided both front and back surfaces are well passivated (S < 10³cm/s) and the doping concentration in the base and emitter are in the range of 5 × 10¹⁶ to 10¹⁷cm⁻³ and 10¹⁸ to 5 × 10¹⁸cm⁻³, respectively. It is also shown that an efficiency of about 23% can be obtained for thin cells of 25 μm thickness with a much inferior quality materials having diffusion length of about 40 μm.     

Optimum two-dimensional short circuit collection efficiency in thin multicrystalline silicon solar cells with optical confinement

The two-dimensional short-circuit AM1.5 collection efficiency is studied in thin multicrystalline silicon solar cells with optical confinement. The collection efficiency is calculated by linking an optical analytical generation profile with the two-dimensional collection probability in pn junction solar cells. The calculations are carried out for variable grain boundary recombination velocity, cell thickness, grain width, diffusion length, and back surface recombination velocity. The role of optical confinement leading to a strong dependence of the collection efficiency on the cell thickness in very thin cells is confirmed. The optimum cell thickness for maximum collection efficiency increases in cells with low back reflection or poor back surface passivation. Also, the optimum thickness in very thin cells increases significantly with increasing the diffusion length. It is also found that the effect of grain boundary recombination is predominant if the cell thickness is larger than the diffusion length and if the diffusion length is larger than half the grain width, especially, in cells with unpassivated grain boundaries. On the other hand, back surface recombination dominates the response in cells with unpassivated back surface if the thickness is smaller than or comparable to the diffusion length.     

Investigation of the damage as induced by 1.7 MeV protons in an amorphous/crystalline silicon heterojunction solar cell

Current–voltage under illumination and quantum yield characteristics of an amorphous silicon/crystalline silicon hetero solar cell have been measured before and after exposure to high-energy (1.7 MeV) protons. A comparison of the measured wavelength-dependent quantum yield with calculated values enabled to determine the effective electron diffusion length of the crystalline silicon, that dropped from a value of 434 μm before to a value of 4 μm after irradiation with 5×10¹² cm⁻² protons. Good agreement has been obtained between measured and simulated data using DIFFIN,¹ a finite-element simulation program for a-Si:H/c-Si heterojunction solar cells, enabling us to extract the depth profile of the recombination rate and the density of states distribution in the semiconductor layers before and after irradiation.     

Current collecting channels in RGS silicon solar cells—are they useful?

Ribbon growth on substrate (RGS) silicon could be the crystalline silicon material for PV of the future. The extremely fast production technique avoiding any material losses due to sawing drastically reduces the wafering costs. On the other hand, one has to deal with more crystal defects (grain boundaries, dislocations, impurities), which especially limit the diffusion length and normally result in small short-circuit current densities J_sc. The charge carrier collection probability can be increased by a macroscopic V-texture of the surface, but even more effective would be a 3-dimensional emitter structure within the whole bulk cell volume. This was observed in some RGS solar cells showing minority carrier lifetimes of only around 0.4 μs after cell processing, but J_sc of above 34 mA/cm². In these cells, the whole bulk volume collects current despite the small diffusion lengths. This behaviour was investigated using spatially resolved lifetime and internal quantum efficiency mappings, capacitance measurements and a special EBIC technique, where the electron beam hits the backside of the wedge-shaped solar cell. From our results, we conclude that the collecting structures may be caused by inversion in combination with a high O content. Cells with large areas of collecting channels exhibit lower fill factors, but nearly no loss in open-circuit voltage as compared to the standard RGS cells. For both types of cells, confirmed record efficiencies of 12.5% have been obtained.     

Hydrogen passivation of defects in multicrystalline silicon solar cells

The knowledge of how hydrogen interacts with defects and impurities in silicon is crucial for the understanding of device performance, especially for solar cells made from disordered silicon wafers. Hydrogen can be introduced in silicon by several techniques, but this paper will be focused on hydrogenation by means of plasma enhanced chemical vapor deposition of hydrogen-rich silicon nitride layer on the surface of the wafer. Passivation effects are observed after annealing and evaluated using minority carrier diffusion length measurements and light-beam-induced current scan maps.It was found that individual intragrain defects are well passivated, while deep levels are transformed into poorly recombining shallow levels at grain boundaries and dislocation clusters. In solar cells, the stability of the hydrogen passivation is much higher with this technique than with other hydrogenation techniques. This is probably due to an encapsulation of hydrogen by the frontwall silicon nitride coating layers and by the backside aluminum film.     

Effects of grain boundaries in polycrystalline silicon thin-film solar cells based on the two-dimensional model

The two-dimensional calculation for polycrystalline Si thin-film solar cells was performed. Two models, “stripe structure” and “columnar structure”, were applied for the solar cells composed of grains. For the stripe structure of 20 μm active layer, to keep the efficiency distribution within 5% for individual unit cells, the stripe width requires more than 500 μm for a minority-carrier lifetime of 1×10⁻⁵ s and recombination velocity at the grain boundary of 1×10⁴ cm/s. For the columnar structure of 10 μm active layer, to keep the efficiency independent of grain size, the recombination velocity should be kept less than 1×10³ cm/s. If imperfect passivation of a grain boundary is given, the way of decreasing carrier concentration to 10¹⁴ cm⁻³ in an active layer may realize insusceptible output. An appropriate device modeling is needed in the two-dimensional calculation for polycrystalline Si thin films with an electron diffusion length close to or more than grain size and with a poorly passivated grain boundary. The calculated efficiency using bad model will include an error of about 1% as overestimation.     

Mono- and tri-crystalline Si for PV application

Crystalline silicon wafers are by far the dominant absorber materials for today's production of solar cells and modules due to their good price/performance relation and their proven environmental stability. These wafers are mainly produced either by a solar-optimized Czochralski (Cz)-growth method yielding crystalline silicon with low defect density (c-Si) or by a directional solidification or a ribbon growth method yielding large grained multi-crystalline (mc-Si) wafers with higher defect density. To further improve the price/performance relation of Cz solar cells, tri-crystalline silicon (tri-Si) is being developed as a high-quality wafer material that combines both the high diffusion length of minority carriers of up to 1300 μm of c-Si and the productivity of mc-Si. More than 1000 μm LID free diffusion length could be reached with specially doped tri-crystals. Due to an increased mechanical stability tri-Si allows both quasi-continuous pulling and thin slicing with higher mechanical yields. This paper reviews the structural, electronic, and mechanical properties of tri-crystalline silicon wafers with respect to c-Si wafers for solar applications. Actual non-textured solar cells processed with a simple cost effective fabrication process exhibit the same cell efficiencies up to 15.9% for both tri-silicon and mono-silicon wafers. With an improved process, up to 18% cell efficiency can be obtained with textured mono-Si.     

Silicon tubes by a closed molten zone: a characterisation study

We report on the characterisation of silicon tubes recrystallised by closed molten zone, a technique developed as a step to a possible process for thin silicon sheet production. The tube faces are quite flat and have a smooth surface. For the electrical characterisation, samples were cut from the tube faces and simple photovoltaic solar cells were formed. The average diffusion length of minority carriers was found, from spectral response, to be around 100 μm. Low-resolution LBIC measurements showed lower diffusion lengths (around 40 μm) in the regions close to the tube edges. This behaviour was correlated to measurements of residual thermal stresses using infrared photoelasticity. Measurements of the changes of spectral response with increasing bias light intensity reveal an increase in the effective diffusion length, a known effect that is interpreted in terms of a density of trapping states.     

SEM investigation of structural defect hydrogen passivation in silicon

The results of the investigation of the structural defect passivation in solar cells made from both shaped polycrystalline and monocrystalline CZ-silicon by means of scanning electron microscope (SEM) in the electron beam induced current mode (EBIC) are reported. Atomic hydrogen effectively passivates the different types of grain boundaries in the shaped polysilicon as well as the microdefects in the CZ-silicon. The changes of the hydrogenated sample electrophysical properties are observed during the electron beam irradiation in SEM (experiments “in situ”). These changes indicate a hydrogen state variation in silicon crystal. The character of these changes depends on the sample nature. In the shaped polysilicon the irradiation leads to both the increase of the GB recombination activity and the local decrease of EBIC, but in the CZ-silicon samples with microdefects it leads to the formation of fields with an increased EBIC value. The mechanism of the observed effect is discussed.     

Dependence of the recombination in thin-film Si solar cells grown by ion-assisted deposition on the crystallographic orientation of the substrate

One promising strategy for achieving high-quality polycrystalline silicon thin-film solar cells on glass is based on low-temperature ion-assisted deposition for epitaxial thickening of a thin, large-grained seeding layer on glass. The crystal growth on the seeding layer is influenced by various factors, amongst which the crystal orientation of the grains plays a substantial role. In this paper we investigate how the electronic properties of solar cells grown on “ideal” seeding layers (Si wafers) are influenced by the crystallographic orientation of the substrate. The Si wafers are heavily doped p-type, ensuring that their contribution to the photogenerated current is small. The films grown on (1 0 0)-oriented Si substrates have a very low density of structural defects, while the films grown on (1 1 1)-oriented Si substrates display a high density of twin defects. The electronic properties of the thin-film solar cells were investigated by means of open-circuit voltage measurements as a function of the incident light intensity. The (1 0 0)-oriented diodes consistently exhibit a higher V_oc than the (1 1 1)-oriented diodes throughout the entire illumination range from 10⁻³ to 10³ Suns. We determine 7 μm as the bulk minority carrier diffusion length of the as-grown (1 0 0)-oriented Si film. A lower bound of 3 μm was found for the bulk minority carrier diffusion length in the as-grown (1 1 1)-oriented Si film. The performances of both types of solar cells were improved by hydrogenation in an ammonia plasma. At voltages around the 1-Sun maximum power point the improvement is due to a reduction of non-ideal current mechanisms. The diffusion length of the (1 0 0) diode remains unaffected by hydrogenation while the lower bound of the diffusion length of the (1 1 1) diode improves to 10 μm.     

Numerical simulation of electron-beam-induced current near a silicon grain boundary and impact of a p–n junction space charge region

Three-dimensional numerical simulations of electron-beam-induced current (EBIC) near a vertical silicon grain boundary are demonstrated. They are compared with an analytical model which excludes the effect of carrier generation other than in the bulk base region of a solar cell structure. We demonstrate that in a wide range of solar cell structures recombination in the space charge region (SCR) significantly affects the EBIC results and hence needs to be included in the data evaluation. Apart from these findings, simulations of a realistic silicon solar cell structure (thick emitter, field-dependent mobility, etc.) are demonstrated.     

Effect of hydrogen plasma treatment on grain boundaries in polycrystalline silicon solar cell evaluated by laser-beam-induced current

Effects of hydrogen plasma treatment on minority carrier diffusion length and recombination velocity at grain boundaries in polycrystalline silicon solar cells have been evaluated by the scanned laser-beam-induced current technique. We have successfully evaluated the two-dimensional minority carrier diffusion length. On the basis of the evaluated diffusion length, the recombination velocity at the grain boundaries was obtained. The recombination velocity was improved by the hydrogen plasma treatment from 15,000—20,000 to 5000—10,000 cm/s. It was quantitatively confirmed that the hydrogen plasma treatment is very effective in both grains and grain boundaries.     

Utilisation of a micro-tip scanning Kelvin probe for non-invasive surface potential mapping of mc-Si solar cells

We have applied a micro-tip Scanning Kelvin Probe to produce high-resolution surface potential maps of silicon nitride (Si₃N₄) coated multi-crystalline Silicon (mc-Si) solar cells in a non-contact, non-invasive fashion. We show this technique highlights two types of defects: localised surface charge and shunts. In the latter case we contrast the non-contact surface potential maps with contact measurements made by the Shuntscan technique.Using a guarded micro-tip with active shield we show for the first time surface potential changes at the mc-Si grain boundaries which are due to different mc-Si polytypes. The high-resolution scanning Kelvin probe (HR-SKP) has a surface potential resolution of <10 mV at a tip diameter <200 μm.     

High-efficiency μc-Si/c-Si heterojunction solar cells

P-type microcrystalline silicon (μc-Si (p)) on n-type crystalline silicon (c-Si(n)) heterojunction solar cells is investigated. Thin boron-doped μc-Si layers are deposited by plasma-enhanced chemical vapor deposition on CZ-Si and the V_oc of μc-Si/c-Si heterojunction solar cells is higher than that produced by a conventional thermal diffusion process. Under the appropriate conditions, the structure of thin μc-Si films on (1 0 0), (1 1 0), and (1 1 1) CZ-Si is ordered, so high V_oc of 0.579 V is achieved for 2×2 cm² μc-Si/multi-crystalline silicon (mc-Si) solar cells. The epitaxial-like growth is important in the fabrication of high-efficiency μc-Si/mc-Si heterojunction solar cells.