Light Beam Induced Current Research Articles

Printed metal back electrodes for R2R fabricated polymer solar cells studied using the LBIC technique

The performance of printable metal back electrodes for polymer solar cells were investigated using light beam induced current (LBIC) mapping of the final solar cell device after preparation to identify the causes of poor performance. Three different types of silver based printable metal inks were employed. Organic solvent based, UV-curable and water based silver inks were tested. Both grid electrodes and full electrodes were employed and it was shown via the grid electrode that the organic solvent based ink adversely affects the device performance under the printed metal whereas both the UV-curable and the water based inks were neutral to improving device performance. Complete roll-to-roll (R2R) processed modules were also tested and some limitations of the LBIC technique was identified for serially connected modules.

Efficiency improved by acid texturization for multi-crystalline silicon solar cells

In this paper, we will show that efficiency of multi-crystalline silicon (mc-Si) solar cells may be improved by acid texturization. In order to enhance overall efficiency of mc-Si for solar-cell applications, the surface treatment of texturization with wet etching using appropriate solutions can improve incident light into the cell. Alkali etchant cannot produce uniformly textured surface to generate enough open circuit voltage ( V OC) and high efficiency of the mc-Si due to the unavoidable grain randomly oriented with higher steps formed during etching process. Optimized acid etching conditions can be obtained by decreasing the reflectance ( R) for mc-Si substrate below levels generated by alkali etching. Short-circuit current ( I SC) measurements on acid textured cells reveal that current gain can be significantly enhanced by reducing reflection. The optimal acid etching ratio HF:HNO 3:H 2O = 15:1:2.5 with etching time of 60 s and lowering 42.7% of the R value can improve 112.4% of the conversion efficiency ( η) of the developed solar cell. In order to obtain more detailed information of different defect region, high-resolution light beam induced current (LBIC) is applied to measure the internal quantum efficiency (IQE) and the lifetime of minority carriers. Thus, the acid texturing approach is instrumental to achieve high efficiency in mass production using relatively low-cost mc-Si as starting material with proper optimization of the fabrication steps.

Organoboron Polymers for Photovoltaic Bulk Heterojunctions

We report on the application of three-coordinate organoboron polymers, inherently strong electron acceptors, in flexible photovoltaic (PV) cells. Poly[(1,4-divinylenephenylene)(2,4,6-triisopropylphenylborane)] (PDB) has been blended with poly(3-hexylthiophene-2,5-diyl) (P3HT) to form a thin film bulk heterojunction (BHJ) on PET/ITO substrates. Morphology may be modulated to give a high percentage of domains (10-20 nm in size) allowing exciton separation. The photoelectric properties of the BHJs in devices with aluminium back electrodes were imaged by light beam induced current (LBIC) and light beam induced voltage (LBIV) techniques. Open circuit voltages, short circuit currents and overall external quantum efficiencies obtained are among the highest reported for all-polymer PV cells.

Spatially Resolved Defect Analysis in Cz-Silicon after Copper-Nickel Co-Precipitation by Virtue of Light-Beam-Induced Current Measurements

We report on a light-beam-induced current (LBIC)-analysis of metal silicide defects arising from co-precipitation of copper and nickel in Cz-silicon-bicrystals produced by wafer direct bonding. Large colonies of silicide precipitates in the one wafer emerging from undisturbed growth from few nucleation sites were observed in different orientations with respect to the surface which correspond to Si {110} planes. From this, the colonies formed during copper-nickel co-precipitation reveal the same attributes as those colonies typical for copper precipitation in the absence of nickel. Oxygen related defects associated with a higher defect distribution in the other wafer were characterized by means of high resolution Transmission Electron Microscopy (TEM) and their temperature dependent LBIC signal.

Determination of diffusion length in photovoltaic crystalline silicon by modelisation of light beam induced current

The diffusion length of minority carriers is one of the most important electrical parameters to qualify silicon for photovoltaic applications. One way to evaluate this parameter is to analyse the decay of the current induced when a focused beam is scanned away from the collector using Light Beam Induced Current (LBIC) technique. The LBIC signal was numerically calculated with 2D-DESSIS software under different boundary conditions, as a function of material thickness and surface recombination velocity in order to verify the limitations of analytical models and to fit the LBIC signal measured in thin silicon samples. Samples with thickness ranging from 55 μm to 2500 μm were evaluated with diffusion length values ranging from 70 μm to 2.5 mm. Analytical expressions of the Internal Quantum Efficiency (IQE) were also used to extract the minority carrier bulk and effective diffusion lengths from surface averaged spectral response and reflectivity data in thick solar cells.

Interaction between process technology and material quality during the processing of multicrystalline silicon solar cells

Multicrystalline silicon is the most used material for the production of silicon solar cells. The quality of the as grown material depends on the quality of the feedstock and the crystallization process. Bulk impurities, crystal defects like dislocations and of course the grain boundaries determine the material quality and thus the solar cell conversion efficiency. Therefore minority carrier lifetime measurements are often done to characterize the material quality. But the measured values are from limited use because it is known that the solar cell process itself can dramatically change the minority carrier lifetime and the solar cell efficiency. In order to obtain more detailed information of the behaviour of different defect types additionally high-resolution LBIC (light beam induced current)-measurements have been done. Since LBIC needs a pn-junction for photocurrent generation the LBIC technique has been combined with the a-Si/c-Si heterojunction cell process, which makes it possible to manufacture solar cells even from as cut wafers without changing the material quality. With this combination of measurement and preparation techniques it was possible to analyze the influence of the diffusion process and the firing process on the behaviour of the three different defect types: grain boundaries, dislocation networks and bulk impurities.

Local photo‐response measurements of photovoltaic devices

AbstractLocal photo‐response measurements based on the Light Beam Induced Current (LBIC) were performed on photovoltaic (PV) cells. LBIC is a technique that is used routinely for detecting and mapping local defects in photovoltaic devices [1–4]. A 5.0 mW, 655 nm laser diode focussed to less than 100 µm on a sample surface was used. In this paper we report measurements of photo‐response and relative reflection at various cells biases, looking amongst other aspects on the vicinity of grain boundaries in multi‐crystalline silicon solar cells. LBIC investigation on single crystalline silicon technology was also conducted. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experimental Limits of Light Capture in Thin Film Silicon Devices

AbstractNew approach for the determination of the angular distribution of the scattered light at nano-rough surfaces/interfaces from AFM (Atomic Force Microscopy) data is presented. Calculation comes from modeling the electromagnetic field in the tight vicinity of the nano-rough surface by complex solution of Maxwell's equations and subsequent near field to far field transform. This method is demonstrated for four types of transparent conductive oxides (with rough free surfaces) deposited on glass substrates. As a result we have the amount and angular distribution of the scattered light „observed” in both transmission and reflection. Moreover calculation can be done for real sample dimensions (to compare the results with the measurement of the angular distribution function using LED laser) or for a semi-infinite sample which suppresses the interference effects and thus such distribution functions can be used as an input parameter for our 3-dimensional optical model CELL for thin film silicon solar cell modeling.In the second part of this contribution we describe our experiment of thin film silicon solar cell characterization by Light Beam Induced Current (LBIC). This measurement done for laboratory solar cell structures reveals the light scattering and light trapping properties of the multilayer stack on a glass substrate. We suggest the test structure for the direct back reflector quality comparison and thus also for its optimization.

Regular Dislocation Networks in Si. Part II: Luminescence

Regular dislocation networks formed as a result of the direct bonding of Cz-Si wafers with oxide remnants on the pre-bonding surfaces were investigated. Besides the dislocation network, oxide precipitates were detected at the bonding interface. The precipitate density across the network was ~5×1010 cm-2, except small irregularly distributed circular areas, several mm in diameter, where the density was remarkably lower (<5×108 cm-2). The dislocation network structure was not affected by the change in the precipitate density. Photoluminescence spectroscopy (PL) and light beam induced current (LBIC) mapping were applied for characterization of such dislocation networks. For the locations with high precipitate density, PL signal from dislocations and that from the band-to-band transitions were enhanced. On the other hand, the LBIC results indicated that oxide precipitates are active recombination centers and thus should suppress the observed radiative transitions. The controversy can be explained in the assumption that the D-band PL signal increases due to scattering of excitation light by the precipitates and due to related expansion of the excitation area of the dislocation network. The light reflection from the precipitate layer also enhances the detected band-to-band PL signal. The shape of PL spectra from the samples in the range of photon energies 0.75 – 1.15 eV was not influenced by the oxide precipitates.

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.

Electrical properties of purified solar grade silicon substrates using a combination of porous silicon and SiCl 4

This work investigates the photo-thermal treatment of solar grade (SG) silicon to reduce impurities to a low level suitable for high efficiency low-cost solar cells application. It describes experiment carried out by using a tungsten lamps furnace (rapid thermal processing, RTP) to purify solar grade silicon wafers using a combination of porous silicon (PS) and silicon tetrachloride. This process enables to attract the impurities towards the porous layer where they react with SiCl 4 to form metallic chlorides. The gettering effect was studied using the Hall Effect and the Van Der Pauw methods to measure the resistivity, the majority carrier concentration and mobility. We have obtained a significant improvement of the majority carrier mobility after such thermo-chemical treatment. The gettering efficiency is also evaluated by the relative increase of the minority carrier diffusion length L, measured by the light beam induced current (LBIC) technique.

High saturation solar light beam induced current scanning of solar cells

The response of the electrical parameters of photovoltaic cells under concentrated solar irradiance has been the subject of many studies performed in recent times. The high saturation conditions typically found in solar cells that are subjected to highly concentrated solar radiation may cause electrically active cell features to behave differently than under monochromatic laser illumination, normally used in light beam induced current (LBIC) investigations. A high concentration solar LBIC (S-LBIC) measurement system has been developed to perform localized cell characterization. The responses of silicon solar cells that were measured qualitatively include externally biased induced cell current at specific cell voltages, I(V), open circuit voltage, V(oc), and the average rate of change of the cell bias with the induced current, DeltaV/DeltaI(V), close to the zero bias region. These images show the relative scale of the parameters of a cell up to the penetration depth of the solar beam and can be obtained with relative ease, qualifying important electrical response features of the solar cell. The S-LBIC maps were also compared with maps that were similarly obtained using a high intensity He-Ne laser beam probe. This article reports on the techniques employed and initial results obtained.

Grooving of grain boundaries in multicrystalline silicon: Effect on solar cell performance

In this work, we investigate the effect of grooving of grain boundaries (GB) in multicrystalline silicon using chemical etching in HF/HNO3 solutions. The grain boundaries were grooved in order to reduce the area of these highly recombining regions. Using optimized conditions, grooved GBs enable deep phosphorus diffusion and deep metallic contacts. As a result, the internal quantum efficiency (IQE), and the I–V characteristics under the dark and AM1.5 illumination were improved. It was also observed a reduction of the GB recombination velocity, which was deduced from light-beam-induced-current (LBIC) measurements. Such grooving in multicrystalline silicon enables passivation of GB-related defects. These results are discussed and compared to solar cells based on untreated multicrystalline silicon wafers.

Characterisation of AlGaN MSM by Light Beam Induced Current technique

The influence of the grain structure on the performance of AlGaN MSM detector structures was examined by Light Beam Induced Current (LBIC) technique. LBIC is a mapping technique in which the generated photocurrent is measured for each position of the scanning light beam. Estimated values for the effective carrier collection regions between contact electrodes in AlGaN MSM structures are hundreds of nanometers. A non-uniformity of the carrier distribution was observed. Regions, where the measured signal was one order of magnitude larger than the average signal level, were also observed. LBIC method was applied for investigation of layer quality and for optimisation of MSM detectors. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of Material Inhomogeneity on the Open-Circuit Voltage of String Ribbon Si Solar Cells

The effect of material inhomogeneity on open-circuit voltage (V/sub OC/) of string ribbon Si solar cells is investigated by a combination of experimental results and a simple analytical model. Light beam-induced current (LBIC) measurements showed that a cell with no detectable defective region gave an efficiency of 15.9% with a V/sub OC/ of 616 mV. However, a neighboring cell with highly defective regions covering 38% of its area, as determined by LBIC measurements, gave an efficiency of 14.1% with V/sub OC/ of 578 mV. Another cell with 19% highly defective regions gave an efficiency of 15.0% with V/sub OC/ of 592 mV. A simple and approximate analytical model was developed to quantify the loss in V/sub OC/ on the basis of recombination intensity and the area fraction of defective regions. This model showed that the majority of the loss in V/sub OC/ is associated with the most defective region, even if its area fraction is relatively small.

Evaluation of the diffusion length of gettered multicrystalline silicon using solar cells–cross‐sectional LBIC scan

AbstractThe effective bulk diffusion length (L) in multicrystalline silicon was evaluated using the light‐beam‐induced‐current (LBIC) technique using a geometrical configuration in which the front and back metallic contacts cover the entire front surfaces. The latter technique was performed on solar cells in cross section. We employed this particular configuration in order to evaluate the diffusion length independently of the recombination velocity at the front surface. For this purpose, we propose a new theoretical expression of the LBIC current. The diffusion length was found to be enhanced from 33 µm to 65 µm after gettering of undesired impurities from the bulk of the multicrystalline silicon. The gettering procedure used in this work is based on the formation of sacrificial porous silicon (PS) layers on both sides of the samples and heat treatments at different temperatures. This gettering method seems to concentrate some of the unwanted impurities close to the PS regions and to remove them by dissolving the PS layers. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Gettering impurities from crystalline silicon by aluminum diffusion using a porous silicon layer

In this paper, we report a study on the possibility of gettering transition metal impurities from solar grade crystalline silicon (Si). Porous silicon layers were formed by the stain-etching method on both sides of the Si wafer. Aluminum diffusion was done throughout the PS layer in an infrared furnace under a (N2/O2) controlled atmosphere. This enables to getter eventual metal impurities towards the PS layer. The gettering effect was evaluated by measuring the majority carrier density and mobility and the minority carrier diffusion length (Ld) of the Si substrate. For this purpose, Wander Pauw and Hall Effect measurements together with the Light Beam Induced Current (LBIC) technique were used. We noticed that the best gettering corresponds to a heat treatment at 850 °C for 30 min; in that case an evident decrease of the majority carrier density and an enhancement of the mobility were observed. After gettering, we found an apparent improvement of the minority carrier diffusion length. These results give evidence of the effectiveness of external gettering treatments by combining (Al-PS) layer for an efficient gettering effect in solar grade monocrystalline Si. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Silicon gettering: Some novel strategies for performance improvements of silicon solar cells

This paper reports the recent performance improvements in crystalline silicon solar cells. These have been achieved by a combination of two mechanisms. One is related to the solar cell design which consists of grooving silicon substrates to obtain a structure suitable to perform an efficient gettering process. The proposed structure consists of buried emitter contacts rear locally diffused. Chemical-vapour etching has been used in the process sequence both to realize buried contacts and opening periodic arrangement of small deep grooving holes, for local aluminum diffusion. The second consists to perform a gettering sequence by Rapid Thermal (RT) heat treatments of p-type silicon in an infrared furnace, in controlled silicon tetrachloride (SiCl4) and N2 gas atmosphere. The resulting silicon shows an increase of minority drift mobility determined by Hall Effect to reach 1417 cm2 V− 1 s− 1, and a decrease in resistivity over 40 μ m on both sides of silicon substrates. Moreover, Light Beam Induced Current (LBIC) investigations show an improvement of diffusion bulk lengths (Ln) to ward 210 μ m as compared to silicon starting substrates.

Gettering impurities from crystalline silicon by phosphorus diffusion using a porous silicon layer

The possible benefits of phosphorus-based gettering applied to crystalline silicon wafers have been evaluated. The gettering process is achieved by forming porous silicon (PS) layers on both sides of the Si wafers. The PS layers were formed by the stain-etching technique, and phosphorus diffusion using liquid POCl3-based source was done on both sides of the Si wafer. The realized phosphorus/PS/Si/PS/phosphorus structure undergoes a heat treatment in an infrared furnace under an O2/N2 controlled atmosphere. This heat treatment allows phosphorus to diffuse throughout the PS layer and to getter eventual metal impurities towards the phosphorus doped PS layer. The gettering effect was evaluated using four probe points, Hall effect measurements and the light beam induced current (LBIC) technique. These techniques enable to measure the density and the mobility of the majority carrier and the minority carrier diffusion length (Ld) of the Si substrate. We noticed that the best gettering is achieved at 900 °C for 90 min of heat treatment. After gettering impurities, we found an apparent enhancement of the mobility and the minority carrier diffusion length as compared to the reference substrate.

Physical characterization of thin‐film solar cells

AbstractThe principal techniques used in the physical characterization of thin‐film solar cells and materials are reviewed, these being scanning probe microscopy (SPM), X‐ray diffraction (XRD), spectroscopic ellipsometry, transmission electron microscopy (TEM), Auger electron spectroscopy (AES), secondary‐ion mass spectrometry (SIMS), X‐ray photoelectron spectroscopy (XPS), photoluminescence and time‐resolved photoluminescence (TRPL), electron‐beam‐induced current (EBIC) and light‐beam‐induced current (LBIC). For each method the particular applicability to thin‐film solar cells is highlighted. Examples of the use of each are given, these being drawn from the chalcopyrite, CdTe, Si and III–V materials systems. Copyright © 2004 John Wiley & Sons, Ltd.