Light Beam Induced Current Measurements Research Articles

Defect characterization of UMG mc-Si solar cells using LBIC and luminescence imaging techniques

ABSTRACTUpgraded metallurgical-grade silicon (UMG Si) solar cells with different ranges of efficiencies were characterized through electroluminescence imaging (ELi) and light-beam induced current (LBIC) measurements. The results showed a good correlation between the EL intensity and the efficiency of the solar cells. ELi images gave a bright contrast at the defects, grain boundaries and intragrain defects, and dark contrast inside the grain bodies. Metallic impurities are much more present in some cells due to the directional solidification of the Si ingot. Local short-circuit current mapping with LBIC measurements revealed a bright zone in the neighborhoods of the defects due to the depletion of impurities. Internal quantum efficiencies (IQE) and effective diffusion lengths (Leff) were calculated using different excitation wavelengths. High resolution LBIC measurements revealed micrometric clusters of impurities around intragrain defects.

Investigation of rough surfaces on Cu2ZnSn(SxSe1-x)4 monograin layers using light beam induced current measurements

Monograin technology has proven to be a successful way of manufacturing low cost photovoltaic applications using the pentanary Cu2ZnSn(SxSe1-x)4 (CZTSSe) as an absorber material in an industrial roll-to-roll process. For high efficient CZTSSe monograin device fabrication a thorough understanding of the impacts of the device characteristics and surface structure is important. A new evaluation method of Light Beam Induced Current (LBIC) images had to be developed to distinguish between different effects resulting from different surface orientations, grain sizes, packing densities and contacting areas. In this work we will show that with LBIC measurements it is possible to evaluate the quality and differences in produced CZTSSe monograin cells in a post-production and non-destructive step. The high spatial resolution evaluation allows investigating the homogeneity of single crystalline grains as well as certain areas of a CZTSSe device. By introducing a statistical method the active area as a major factor for the current density of a device will be calculated and evaluated. The results show that with LBIC measurements the active area can be quantified, which differs for the investigated cells up to 9%. Additionally, the homogeneity of short circuit current densities of the monograins and also of certain areas of a cell can be detected and quantified.

Compressed Sensing Current Mapping Spatial Characterization of Photovoltaic Devices

A new photovoltaic (PV) device current mapping method has been developed, combining the recently introduced compressed sensing (CS) sampling theory with light beam induced current (LBIC) measurements. Instead of a raster scan, compressive sampling is applied using a digital micromirror device. The aim is to significantly reduce the time required to produce a current map, compared to conventional LBIC measurements. This is achieved by acquiring fewer measurements than a full raster scan and by utilizing the fast response of the micromirror device to modulate measurement conditions. The method has been implemented on an optical current mapping setup built at the National Physical Laboratory, U.K. Measurements with two different PV cells are presented in this paper and an analytical description for realization of an optimized CS current mapping system is provided. The experimental results illustrate the feasibility of the method and its potential to significantly reduce measurement time of current mapping of PV devices.

Surface Recombination Velocity of Local Al-contacts of PERC Solar Cells Determined from LBIC Measurements and 2D Simulation

Industrial production volumes of passivated emitter and rear contact (PERC) solar cells increase due to significantly higher cell efficiencies compared to full area back surface field (BSF) solar cells at similar costs. The main features of PERC cells are dielectric surface passivation of the rear and local contact formation with Al leading to a p+p junction beneath the Al/eutectic. For non-optimized process conditions, the eutectic in the local Al contact area does not form and so-called voids result. Since it is known that there are voids causing high or low recombination activity, a determination of the surface recombination velocity (SRV) is necessary for identification of the potential for process optimization. The passivation quality of the BSF, locally formed in the rear side contacts, is studied in detail via local internal quantum efficiency (IQE) measured by high resolution light beam induced current (LBIC). The significant spreading of the IQE values is attributed to a variation in local BSF layer thickness at different areas. The SRV of the local contact is determined by fitting the LBIC measurements of voids by 2D simulations. These simulations are based on a detailed modeling of SRV in local contact areas involving a non-uniform SRV in the void's vicinity. The non-uniform SRV in voids is traced back to laser induced damage nearby the local contact opening in the dielectric layer. Additionally the existence of laser damaged areas close to filled contacts is demonstrated in this work.

Characterization of local Al‐contacts by light beam induced current measurements and their verification by 2D simulation using flexPDE

In recent years the relevance of passivated emitter and rear contact solar cells for industrial application increased due to significantly higher cell efficiencies compared to full area back surface field solar cells. However, the formation of local Al contacts is particularly sensitive to the manufacturing process parameters. Under non‐optimized conditions, the eutectic in local Al contacts is missing and so‐called voids are formed. So far, their impact onto the electrical parameters of the solar cells is not fully understood, mainly because of their difficult spatially resolved detection. Within this work the application of scanning acoustic microscopy circumvents this difficulty and allows a classification of local contacts in terms of “voids” or “filled contacts” on large cell area. The passivation quality of the BSF in the local contacts is investigated in detail through the local internal quantum efficiency (IQE) measured by light beam induced current (LBIC). It is found that there is a large spreading for the IQE values, attributed to a variation in BSF layer thickness. In addition, LBIC measurements of voids are fitted by 2D simulations, according to a detailed modelling of the surface recombination velocities (SRV) in local contacts. The final model is based on a non‐uniform SRV in the void's vicinity, attributed to laser damage close to the interface. It is demonstrated that the electrical parameters of the solar cell are affected by voids only when their surface is not sufficiently passivated by a local BSF.

Probing individal subcells of fully printed and coated polymer tandem solar cells using multichromatic opto-electronic characterization methods

In this study, a method to opto-electronically probe the individual junctions and carrier transport across interfaces in fully printed and coated tandem polymer solar cells is described, enabling the identification of efficiency limiting printing/coating defects. The methods used are light beam induced current (LBIC) mapping, External quantum efficiency (EQE) measurements, and monochromatic current–voltage (I–V) characterization. Using these methods, inherent limitations to the accuracy of EQE and LBIC measurements on non-ideal tandem solar cells are identified and described through the use of a small-signal electrical model. The model is able to predict the EQE spectrum of the non-ideal polymer tandem solar cell, using extracted values of shunt- and series resistance of the individual junction of the tandem cell. This finally enables LBIC mapping of the individual junctions of the tandem polymer solar cells, using a combination of light and voltage-biasing.

Investigation of the short-circuit current increase for PV modules using halved silicon wafer solar cells

It is well established that using halved silicon wafer solar cells in a photovoltaic (PV) module is an efficient way to reduce cell-to-module resistive losses. In this work we have shown that PV modules using halved cells additionally show an improvement in their optical performance, resulting in a higher current generation. We attribute this increase in current to gains in light reflected from the backsheet area. An optical model is presented that quantitatively determines the influence of the backsheet on the short-circuit current of a PV module. We find that, for an accurate prediction, several factors have to be taken into account, including the geometry of the module, the backscattering properties of the backsheet and the illumination spectrum. Particularly the angularly and spectrally resolved scattering properties of the backsheet are shown to have a large impact on the current generation. Furthermore, light beam induced current (LBIC) measurements are used to test the backscattering properties of the backsheet and also the influence of the illumination spectrum. LBIC measurements are also used to verify the simulation results, giving good agreement. Thus the design of a PV module can be optimized by simulation. A standard full-size cell module and a halved-cell module with optimized cell spacing are fabricated. Compared to the standard module, the half-cell module is shown to have 4.60% more power (315.3 vs. 329.8W), 1.46% higher fill factor (75.5 vs. 76.6%), and 3.08% more current (9.08 vs. 9.36A).

Application of lateral photovoltage towards contactless light beam induced current measurements and its dependence on the finite beam size.

The nature of the signal due to light beam induced current (LBIC) at the remote contacts is verified as a lateral photovoltage for non-uniformly illuminated planar p-n junction devices; simulation and experimental results are presented. The limitations imposed by the ohmic contacts are successfully overcome by the introduction of capacitively coupled remote contacts, which yield similar results without any significant loss in the estimated material and device parameters. It is observed that the LBIC measurements introduce artefacts such as shift in peak position with increasing laser power. Simulation of LBIC signal as a function of characteristic length L(c) of photo-generated carriers and for different beam diameters has resulted in the observed peak shifts, thus attributed to the finite size of the beam. Further, the idea of capacitively coupled contacts has been extended to contactless measurements using pressure contacts with an oxidized aluminium electrodes. This technique avoids the contagious sample processing steps, which may introduce unintentional defects and contaminants into the material and devices under observation. Thus, we present here, the remote contact LBIC as a practically non-destructive tool in the evaluation of device parameters and welcome its use during fabrication steps.

Application of LBIC measurements for characterisation of triple junction solar cells

In this study the Light Beam Induced Current (LBIC) imaging technique was used to characterise InGaP/InGaAs/Ge triple junction solar cells. The study focused on the use of monochromatic and solar light as beam probes to obtain photocurrent response maps from which the presence of any current reducing features on the solar cell were identified. Point illuminated current voltage (I–V) curves were obtained simultaneously while LBIC scanning measurements were being made. Curve fitting using an interval division algorithm based on the single diode model was performed to extract basic point device and performance parameters to give a rough indication of the functioning of the triple junction device. Using red and blue lasers as beam probes, reverse voltage breakdown was observed on the I–V curves which could be attributed to the Ge bottom subcell not being fully activated. The extracted parameters obtained when using monochromatic and solar light beam probes showed a large variation, indicating the dependence of I–V parameters on the spectral content of the beam probe.

LBIC measurement optimization to detect laser annealing induced defects in Si

Because of the short penetration depth of ultraviolet (UV) in semiconductor, the realization of UV sensors requires the reduction of the junction thickness. Excimer laser annealing (ELA) is a new annealing technical allowing to achieve a thin layer (<30 nm) heavily boron doped (Rsq < 350 Ω/sq) on n-type silicon substrate together with a good profile abruptness (<3 nm/dec). The small size of the laser beam requires a scanning procedure for processing large surfaces. That could generate non-homogeneities resulting in defect growth. In order to investigate these phenomena, we developed a Light Beam Induced Current (LBIC) measurement set-up. LBIC analyses consist in the measurement of the photocurrent generated by a localized irradiation of the sample. The presence of defects in the irradiated zone leads to a local decrease of the photo-current. Moreover, thanks to different probe beams with wavelengths ranging from 193 nm to 800 nm, we can control the depth of photo-carriers generation. Thus, we are able to perform a 3D-localization of the defects which helps in understanding their origins. We have developed and validated this detection method of defects generated in the ultra-thin junctions, down to thirty nanometers, with a spatial resolution of ten micrometers at the surface of the sample. First experimental results demonstrate that most of the defects are localized at the edge of the irradiated zone within the first ten nanometers from the surface.

Light beam induced current measurements on ZnO Schottky diodes and MESFETs

AbstractThe homogeneity of the Schottky barrier potential of reactively sputtered PdOy/ZnO Schottky contacts has been investigated by light beam-induced current measurements on the micrometer scale. It is found that a metallic capping layer, acting as an equipotential surface, is not necessary for PdOy/ZnO Schottky contacts in contrast to AgxO/ZnO Schottky diodes. Further, we probed the generated photocurrent of a ZnO-based metal-semiconductor field-effect transistor for a closed and open channel, respectively. The photocurrent is, in general, one order of magnitude larger for closed channel conditions. The position of maximum photocurrent generation shifts towards the drain for higher source drain voltages for closed channel conditions, whereas it is nearly independent of the source drain potential for an open channel.

Bias-dependent high saturation solar LBIC scanning of solar cells

A light beam-induced current measurement system that uses concentrated solar radiation as a beam probe to map spatially distributed defects on a solar cell has been developed and tested [F.J. Vorster, E.E. van Dyk, Rev. Sci. Instrum., submitted for review]. The induced current response from a flat plate EFG Si solar cell was mapped as a function of surface position and cell bias by using a solar light beam induced current (S-LBIC) mapping system while at the same time dynamically biasing the whole cell with an external voltage. This paper examines the issues relating to transient capacitive effects as well as the electrical behaviour of typical solar cell defect mechanisms under spot illumination. By examining the bias dependence of the S-LBIC maps, various defect mechanisms of photovoltaic (PV) cells under concentrated solar irradiance may be identified. The techniques employed to interpret the spatially distributed IV curves as well as initial results are discussed.

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.

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.

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.

CELLO: an advanced LBIC measurement technique for solar cell local characterization

A solar cell is a large area device, and thus its global IV-characteristic and efficiency depend strongly upon its local properties. Local defects, such as a locally reduced diffusion length, a strong local shunt resistance or a high local series resistance will adversely influence the cell’s global properties. Experimental techniques suitable for mapping the spatial distribution of such local parameters can provide valuable information, and thus help to improve the technology for the production of efficient and reproducible solar cells. The LBIC (Light Beam Induced Current) technique [1] allows calculation of the local diffusion length of the solar cell material from local photocurrent data obtained under short-circuit conditions. As well as allowing LBIC measurements, the PVScan 5000 mapping analyzer by NREL [2] can be used to map defects and grain boundaries using reflectivity data and special surface etching. Localized shunts can be mapped by CCD cameras [3,4] or nematic liquid crystals [5]. The electron-beam-induced current (EBIC) is an alternative technique for the investigation of defects in solar cells [6]. Destructive techniques, MASC [7] for local IV-characterisation and RAMP [8] using a scanning (by scratching) tungsten electrode, have also recently been developed.

Analysis of Photocurrent in Screen‐Printed CdS / CdTe Solar Cells

Spatial photocurrent distribution in screen‐printed solar cells has been investigated using the light‐beam‐induced current (LBIC) technique. Furthermore, losses in short‐circuit current density have been analyzed through measurements of illuminated current‐voltage characteristics, spectral responses, and reflectance spectra. The LBIC measurements revealed that the p‐n junction area of the cell extended beyond the edge of a carbon (C) electrode on a film, but that the active area corresponded to the junction area covered with the C electrode. The photocurrent distribution in the active area was nonuniform. Its fluctuation increased with the film thickness, without being affected appreciably by voltage and white‐light biases. These results indicate that the fluctuation originated mainly from inhomogeneities of the film. From the optical analysis of the , the improvement in film quality proved essential for the increase in .