Boundary Recombination Research Articles

Three dimensional simulated modelling of diffusion capacitance of polycrystalline bifacial silicon solar cell

A three dimensional (3-D) simulated modelling was developed to analyse the excess minority carrier density in the base of a polycrystalline bifacial silicon solar cell. The concept of junction recombination velocity was ado-pted to quantify carrier flow through the junction, and to examine the solar cell diffusion capacitance for three illumination modes (front side, back side and both front and back sides). Plots of diffusion capacitance against grain size, grain boundary recombination velocity, junction recombination velocity and illumination wavelength were used to study the influence of cell parameters on the capacitance. The results indicated that junction and grain boundary recombination velocities played determinant roles, especially, for small grain size and long wav-elength. Hence, high diffusion capacitance was obtained for high junction recombination velocity, large grain size and long wavelength; while small grain size led to increased recombination centers and corresponding decrease in the diffusion capacitance

New approach of both junction and back surface recombination velocities in a 3D modelling study of a polycrystalline silicon solar cell

A 3D modelling is used to obtain the expression of excess minority carrier density in base region of a polycrystalline silicon solar cell. The concept of the junction recombination velocity S f is used to quantify how excess carrier flow through the junction in actual operating conditions. The plot of the photocurrent density allowed study the influence of the grain boundary recombination velocity and grain size on both, the junction recombination velocity and on the back surface recombination velocity of an n + - p - p + solar cell. This study pointed out the importance of the losses at the back side of solar cell. It's also show that the junction recombination velocity is more important for small grain size with large values of grain boundary recombination velocity.

Numerical simulation of carrier collection and recombination at grain boundaries in Cu(In,Ga)Se2 solar cells

Two-dimensional numerical device simulations investigate the influence of grain boundaries (GBs) on the performance of Cu(In,Ga)Se2 solar cells. We find that the electronic activity of grain boundaries can reduce the efficiency of Cu(In,Ga)Se2 solar cells from 20% to below 12% making proper passivation of GBs a primary requirement for high efficiency. Cell efficiencies larger than 19% require GB defect densities below 1011 cm−2. Also, an internal band offset in the valence band due to a Cu-poor region adjacent to the GBs could effectively passivate grain boundaries that are otherwise very recombination active. It is shown that such a barrier must be more than 300 meV high and at least 3 nm wide to virtually suppress all grain boundary recombination. Contrariwise, such a barrier represents an obstacle for hole transport reducing carrier collection across grain boundaries that are not perpendicular to the cell surface. We further find that inverted grain boundaries lead to an accumulation of the short circuit current along the grain boundary, which in certain situations enhances the total short circuit current. However, we do not find any beneficial effect of any type of grain boundaries on the overall cell efficiency.

Nanocrystalline silicon solar cells from excimer laser crystallization of amorphous silicon

Excimer laser crystallized nanocrystalline silicon layers were used to fabricate solar cells. The laser crystallized layers are characterized with Raman spectroscopy for structural investigations and with atomic force microscopy to study surface modifications upon crystallization. The current–voltage characteristics of the devices completed with aluminium back contacts were investigated with air mass 1.5 G solar simulations. The resulting nanocrystalline solar cells show inferior performance compared with amorphous silicon devices. The degradation of open circuit voltages and short circuit currents with the increase of crystallization energy density is explained to be due to thermal modification of the p-type/intrinsic layer interface and band gap enhancement of fine-grained nanocrystalline layers of the devices along with increased surface and grain boundary recombination of carriers.

A Novel Method for the Estimation of Surface Recombination Velocity of Polycrystalline CIGS Solar Cells

The surface recombination velocity of minority carriers at the grain boundaries severely affects the performance of the polycrystalline solar cells. In the present work, the effect of grain size and grain boundary recombination velocity on the short circuit photocurrent of polycrystalline copper indium gallium di‐selenide (CIGS) solar cell is estimated theoretically. The model has been used to determine the surface recombination velocity for CIGS using our own experimental data on carrier generation rate. A good agreement is found between theory and available experimental results.

Enhancement of Photoelectrochemical Hydrogen Production from Hematite Thin Films by the Introduction of Ti and Si

Doped and undoped hematite (α-Fe2O3) thin film electrodes for photoelectrochemical hydrogen production have been prepared using reactive magnetron sputtering. We present an extensive characterization of the photoelectrochemical, structural, electrical, and optical properties of the α-Fe2O3 films to assess the mechanisms by which the dopants influence photoelectrode performance. The Si-doped and Ti-doped α-Fe2O3 exhibited much higher photoelectrochemical activity than the undoped material. The Ti-doped α-Fe2O3 showed the largest photocurrent as well as the lowest onset potential, the highest electrical conductivity, the lowest activation energy, and the highest charge carrier density. The crystallographic orientation of the films does not appear to be a dominant factor affecting the photocurrent. The changes in conductivity with doping are insufficient to explain the observed changes in the photoelectrochemical activity. The proposed mechanism for the enhanced photocurrent is reduction of recombination due to an improvement of the charge-transfer rate coefficient at the surface and also possibly passivation of the grain boundaries by the dopants. A highly disordered surface and higher grain boundary recombination due to small crystals may explain the lower photocurrent of the Si-doped α-Fe2O3 compared to the Ti-doped material.

Application of genetic algorithms for the extraction of electrical parameters of multicrystalline silicon

The application of the genetic algorithms (GAs) concept in measurements science deals with a fitting procedure used for the numerical prediction of physical parameters from an experimental data curve. In this work, GAs were applied for the extraction of electrical parameters of multicrystalline silicon solar cells. The experimental technique used is the light-beam-induced-current (LBIC). From LBIC measurements, we deduced the values of the diffusion length L and the grain boundary recombination velocity Vr of the minority carriers of the multicrystalline silicon wafers using the fitting procedure. The nonlinear fitting procedure is based on the minimization of the standard deviation of the theoretical LBIC profile from the experimental one. However, this criterion is not convex, and using traditional deterministic optimization algorithms leads to local minima solutions. To overcome this problem, the nonlinear least-square minimization technique was computed with the GAs strategy, increasing the probability of obtaining the best minimum value of the cost function in very reasonable time. The results of the proposed algorithm are presented and compared with the Levenberg–Marquardt algorithm and the Gauss–Newton method. The GAs-based numerical technique was found to be a promising and a powerful technique for numerical evaluation of the electrical parameters of silicon solar cells.

Combination of gettering and etching in multicrystalline silicon used in solar cells processing

Undesired impurities can be removed away from multicrystalline silicon (mc-Si) wafers by combining porous silicon (PS) formation and heat treatments. The gettering procedure used in this work is based on the formation of a PS film at both back and front sides of the mc-Si wafers, followed by a heat treatment. The latter was achieved in an infrared furnace at different temperatures and during various periods. We show that when the based material undergoes such a gettering, the electrical properties (short-circuit current, open-circuit voltage, serial and shunt resistances) and the electronic parameters (diffusion length and grain boundary recombination velocity) of the corresponding solar cells can be improved only if some regions of the wafers are etched. Compared to reference cells based on untreated wafers, the diffusion length and grain boundary recombination velocity of solar cells fabricated from gettered and etched samples was improved by about 30% and reduced by a factor of 10, respectively.

High open-circuit voltage values on fine-grained thin-film polysilicon solar cells

Grain boundaries are known to be the main limiting factor for a high performance of polysilicon solar cells. Defects at these grain boundaries serve as recombination centers for minority and majority carriers. Grain boundaries are also known to be paths for enhanced hydrogen diffusion, which results in passivation of part of the defects. In this paper, we show that grain boundaries are also paths for an enhanced phosphorus diffusion that limits the effect of hydrogen passivation. Phosphorus spikes along the grain boundaries enhance the junction area and determine the collection and the recombination volumes. Avoiding this preferential diffusion of phosphorus atoms during emitter formation, we obtained open-circuit voltages (Voc) up to 536mV on polysilicon material with a grain size of only 0.2μm. These high Voc values can only be accounted for by theory if a much smaller grain boundary recombination velocity is assumed than what was previously accepted for p-n junctions on fine-grained polysilicon solar cells.

Structure optimization according to a 3D model applied on epitaxial Si solar cells: A comparative study☆

Abstract A three-dimensional (3D) analytical model based on the Green's Function method, is applied on n + pp + type epitaxial solar illuminated on the front side. This model is implemented through a simulation program in order to determine the device parameters, which optimize the cell's efficiency. A number of cells fabricated on low-cost UMG substrate have been studied and their spectral response is evaluated. The basic design parameter selected for optimization of these cells is the epitaxial layer thickness, through variation of grain size and grain boundary recombination velocity. In addition, 3D spectral response results are compared with experimental data and the 1D model.

QE and Suns-Voc study on the epitaxial CSiTF solar cells

In order to clarify the major factors having confined the efficiencies of as-prepared crystalline silicon thin film (CSiTF) solar cells on the SSP (silicon sheets from powder) ribbons, QE (quantum efficiency) and Suns-V-oc study were performed on the epitaxial CSiTF solar cells fabricated on the SSP ribbons, the SSP ribbons after surface being zone melting recrystallized (ZMR) and single crystalline silicon (sc-Si) substrates. The results show that the epi-layers deposited on the SSP ribbons have rough surfaces, which not only increases the diffusion reflectance on the surfaces but also makes the anti-reflection coatings become structure-loosened, both of which would deteriorate the light trapping effect; in addition, the epi-layers deposited on the SSP ribbons possess poor crystallographic quality, so the heavy grain boundary (GB) recombination limits the diffusion length of the minority carriers in the epi-layers, which makes the as-prepared CSiTF solar cells suffer the worse spectra response at long-wavelength range. Nearly all the dark characteristic parameters of the CSiTF solar cells are far away from the ideal values. The performances of the CSiTF solar cells are especially affected by too high I-02 (the dark saturation current of space charge region) values and too low R-sh (parallel resistance) values. The higher 102 values are mainly caused by the heavy GB recombination resulting from the poor crystallographic qualities of the silicon active layers in the space charge regions, while the lower R-sh values are attributed to the electrical leakage at the un-passivated PN junction or solar cell edges after the solar cells are cut by the laser scriber.

Study on epitaxial silicon thin film solar cells on low cost silicon ribbon substrates

Heavily B-doped silicon ribbons were prepared by modified silicon sheets from powder (SSP) technique using 9 N purity electronic-grade silicon powder. Utilizing the SSP ribbons as substrates, crystalline silicon thin film (CSiTF) solar cells have been fabricated by direct epitaxy method. As-prepared CSiTF solar cells were characterized by illuminated and dark I– V characteristic measurement, Suns- V oc measurement, quantum efficiency (QE) and reflectance measurement. The results show that the CSiTF solar cells have poor performance with typical efficiency η only about 4.25%, and nearly all the characteristic parameters are far away from the ideal values. By detailed analysis and discussion on the measured results, it was found that grain boundary (GB) recombination in the active layers and electrical leakage at the edge of the PN junctions play a major role in deteriorating the performance of the CSiTF solar cells, which lead to higher dark saturation current I s and lower parallel resistance R sh, respectively. By improving the preparation process, such as optimization of the epitaxy process, use of remote plasma hydrogen passivation (RPHP), optimization of the metallization and use of double layer anti-reflection (DLAR) coatings, etc., all the characteristic parameters of the CSiTF solar cells have been greatly improved, and the best efficiency value increased to 8.02%.

Effects of defects and impurities on minority carrier lifetime in cast-grown polycrystalline silicon

The grain size distribution of B-doped cast-grown polycrystalline Si could not explain the minority carrier lifetime map. The lifetime was not mainly determined by the grain boundary recombination, since the grain size of the present cast-grown polycrystalline Si exceeded 1 cm and was large enough as compared with the minority carrier diffusion length (0.25 μm). In the region where lifetime was relatively short, there were many defects, which appeared as etch-pit by the Secco etching. The relationship between the etch-pit density and the minority carrier lifetime suggested that these defects acted as a recombination center and that they mainly determined the lifetime. There were many C atoms (>10 17 cm 3), which exist as substitutional impurities, in the as-grown wafer. The thermal annealing induced the C segregation, which might deteriorate the solar cell performance through the cell fabrication processes.

Numerical simulation of grain boundary effects in Cu(In,Ga)Se 2 thin-film solar cells

Two-dimensional numerical simulations of polycrystalline Cu(In,Ga)Se 2 thin-film solar cells show that grain boundary (GB) recombination can deteriorate the photovoltaic power conversion efficiency of these devices by about 9% absolute with respect to a starting value of 21.7% that would hold for a material without GBs. The achieved record efficiencies of 19% are only possible if the recombination velocity S at GBs is kept below S=10 3 cm s −1. Comparing devices that have all defects homogeneously distributed in the bulk to devices where the same number of defects is concentrated at GBs only unveils that the latter situation is more favorable because of kinetic restrictions. The efficiency difference between the homogeneous and the concentrated cases is, however, only 1% (absolute). We further model the possible effect of an additional hole barrier at the GB by assuming asymmetric capture cross-sections for electrons and holes. We find that the positive consequence of this feature is rather limited and much dependent on the specific properties of the GB defects. For example, the efficiency improves by 2% when introducing a hole barrier of 120 meV at a GB with midgap defects. The same improvement would result from a reduction of the GB defects by a factor of 2.5.

Thin-film polycrystalline Si solar cells on foreign substrates: film formation at intermediate temperatures (700–1300 °C)

We give an overview and analysis of research on thin-film polycrystalline Si solar cells on foreign substrates, with layers formed at intermediate temperatures (700–1300 °C), covering substrates, deposition techniques and solar cell processing. The main deposition techniques that have been investigated are solution growth (SG) and chemical vapour deposition (CVD). Insufficient nucleation on foreign substrates is an important problem with SG, which could be solved with appropriate surface preparation techniques and growth conditions. With CVD, continuous layers are achieved routinely, but the electronic quality of the material is usually very low. Solar cell performance appears to be limited by a very large recombination activity of grain boundaries. Improvement can be achieved reducing the grain boundary density and recombination activity, and experimental examples are given. Devices have been demonstrated with efficiencies up to 5.5%.

Impacts of spatially reduced grain boundary recombination regions on multicrystalline silicon solar cell performance

In this work, we demonstrate that grain boundaries (GBs) in multicrystalline silicon solar cells may be easily grooved using porous silicon (PS). We then use this interesting property to try reducing the area of these highly recombining regions. To reach this objective a thin PS layer was formed on both front and back sides of the multicrystalline material. After removing the PS films, conventional phosphorus diffusion was achieved. Grooved GBs enable deep penetration of phosphorus and metallic contacts. As a result, we found an improvement of the I–V characteristics under the dark and at AM1.5 illumination. The light-beam-induced current was found to be improved of approximately 16% resulting from an enhancement of the bulk minority carrier collection probability. The internal quantum efficiency was also found to be enhanced by approximately 15% in a rather wide spectral range, approximately between 600 and 1100 nm. These results are discussed and compared to solar cells based on untreated multicrystalline silicon wafers.

Impacts of spatially reduced grain boundary recombination regions on multicrystalline silicon solar cell performance

In this work, we demonstrate that grain boundaries (GBs) in multicrystalline silicon solar cells may be easily grooved using porous silicon (PS). We then use this interesting property to try reducing the area of these highly recombining regions. To reach this objective a thin PS layer was formed on both front and back sides of the multicrystalline material. After removing the PS films, conventional phosphorus diffusion was achieved. Grooved GBs enable deep penetration of phosphorus and metallic contacts. As a result, we found an improvement of the I– V characteristics under the dark and at AM1.5 illumination. The light-beam-induced current was found to be improved of approximately 16% resulting from an enhancement of the bulk minority carrier collection probability. The internal quantum efficiency was also found to be enhanced by approximately 15% in a rather wide spectral range, approximately between 600 and 1100 nm. These results are discussed and compared to solar cells based on untreated multicrystalline silicon wafers.

Effects of Grain Boundaries in Amorphous/Multicrystalline Silicon Heterojunction Photovoltaic Cells

ABSTRACTSpectral response and dark current-voltage characteristics of heterojunctions are used to investigate grain boundary degradation in photovoltaic properties of a-Si/mc-Si heterojunction solar cells. Measured spectral response inside the grain and on the grain boundary shows small but consistent QE degradation due to minority carrier recombination at the grain boundaries. No consistent difference is observed in dark current-voltage characteristics because of large diode area and periphery leakage current in the employed heterojunction diodes. Comparing measurement results and results from device modeling using the simulation software Medici, a recombination velocity of 4900 cm/sec is found at the grain boundaries of employed multicrystalline silicon wafer. The modeling and experimental results can also be used to define an effective grain area that serves as a measure of grain boundary recombination and the influence of grain size.

Modelling recombination current in polysilicon solar cell grain boundaries

Abstract The recombination current in grain boundaries is theoretically investigated for a polycrystalline silicon solar cell n–p junction operating either in short-circuit or in open-circuit configurations. The analysis is carried out using results performed in earlier works, by means of numerical simulations including the presence of grain boundaries. The variation of grain boundary recombination current density with exciting light wavelength is reported for different cell parameters like grain boundary recombination velocity, base dopant density and base thickness. The objective of the present paper is to understand to what extend the light wavelength and the junction configuration allow an optimal measure of the grain boundary recombination parameter.

Diamond-based alpha particle detectors with coplanar geometry

In this paper, we report the results concerning significant improvement of charge collection efficiency for chemical vapor deposition (CVD) diamond-based alpha particle detectors with coplanar geometry by reducing the inter-electrode spacing from 100 to 25 μm. CVD diamond films used were prepared employing hot-filament CVD technique. Electrical contacts of the detectors were only patterned on the growth surface of the polished free-standing 300 μm thick diamond film using lift-off technology. The performance of the fabricated diamond-based detectors was tested using a 241Am alpha particle source with a characteristic energy of 5.5 MeV. Average charge collection efficiency of 52% for detector with narrow inter-electrode spacing of 25 μm and of 32% for detector with wide inter-electrode spacing of 100 μm was obtained, indicating that the detector with narrow inter-electrode has a higher charge collection efficiency than the detector with wide inter-electrode spacing. This should be attributed to fewer boundary trap and recombination centers between electrodes in detector with narrow inter-electrode spacing than that with wide inter-electrode spacing.