Laser Beam Induced Current Technique Research Articles

Interior and Surface Degradation Analysis of Multicrystalline Si Solar Cell Module Using Laser-Beam-Induced Current Technique

The interior and surface degradation of a multicrystalline silicon (mc-Si) solar cell module was investigated using the laser beam-induced current (LBIC) technique. Reverse voltage was applied to avoid the influence of the solar cells not irradiated with light and to evaluate the LBIC intensity only of the illuminated solar cell. In addition, we found that the mc-Si, but not the solar cell surface, is comprised of homogeneous crystals and the degraded layer is estimated to be $\sim 0.6~\mu\text{m}$ . The LBIC technique was applied at several wavelengths under reverse bias conditions in order to analyze degraded solar cells.

Defect detection in solar cells via electroluminescence, LBIC, and EBIC methods

Structural defects affecting the efficiency of multicrystalline silicon solar cells are investigated by the electron-beam-induced current (EBIC) mode of a scanning electron microscope and the laser-beam-induced current (LBIC) method. It is experimentally demonstrated that the LBIC technique is more sensitive to electrically active 2D defects than the EBIC approach at large values of the diffusion length and excitation-beam penetration depth. The comparison of LBIC (or EBIC) and electroluminescence images enables us to reveal the correlation between solar-cell short circuits and electrically active structural defects.

Laser beam induced current measurements of Cd1−xZnxS/CdTe solar cells

The role of the window layer in Cd1−xZnxS/CdTe photovoltaic (PV) device structures was investigated using a triple wavelength laser beam induced current (LBIC) technique. Full pn structures were deposited using metal organic chemical vapour deposition (MOCVD). The triple wavelength LBIC technique allowed for the diagnostic analysis of carrier collection uniformity at 3 wavelength dependent photon penetration depths, δp. An assessment of the Cd1−xZnxS window layer film thickness and surface coverage showed a correlation between lateral device performance and window layer thickness distribution. Islands of thick Cd1−xZnxS were shown to increase localised blue absorption, correlating to isolated regions of increased red and IR photoresponse. Overall device open-circuit voltage, Voc is shown to demonstrate a linear dependence on thick (∼300nm) Cd1−xZnxS surface coverage. Voc and shunt resistance Rsh losses attributed to the window layer were found to jointly contribute to the overall cell efficiency losses. A non-uniform pin-hole distribution, leading to Rsh loss, was found to be independent of the overall Cd1−xZnxS thickness distribution; thus the triple wavelength LBIC measurement was able to identify causes for the observed losses in both Voc and Rsh.

Evaluation of decay photocurrent measurements in dye-sensitized solar cells: Application to laser beam-induced current technique

SUMMARY The use of the laser beam-induced current (LBIC) technique in photovoltaic devices is widespread, but its use in photoelectrochemical cells, such as dye-sensitized solar cells (DSSCs), is limited due to the configuration of these devices. The main reason is the very slow response time of DSSCs in the decay process, and therefore LBIC scans take too long to perform. We have designed a procedure published in the literature to correct the photocurrent values obtained by the LBIC technique, based on an algorithm that uses a decreasing mono-exponential model. This work presents a study of the decay measurements in DSSCs using several functions, in order to improve the algorithm designed. It concludes that functions such as a decreasing bi-exponential or Becquerel function generate better fits to experimental data. Copyright © 2010 John Wiley & Sons, Ltd.

Improving photoresponse characterization of dye-sensitized solar cells: application to the laser beam-induced current technique

The photocurrent response of dye-sensitized solar cells (DSSCs) to light excitation from focused and non-focused laser beams is investigated. We observe that part of the photocurrent is produced by the activation of the irradiated area, whereas another part is generated by the previously photoexcited area. A mathematical algorithm has been devised to describe the rise and decay processes. The application of this algorithm leads to a significant improvement in the surface photoresponse and quantum yield measurements in DSSCs by means of the laser beam-induced current (LBIC) technique. This algorithm enhances the quality and definition of the LBIC images and opens the way to use this technique to cope with the biphasic features of these photovoltaic devices and extracting key properties for device performance such as internal quantum efficiencies and electron diffusion lengths.

Suppression of extension of the photo-sensitive area for a planar-type front-illuminated InGaAs detector by the LBIC technique

To suppress the extension of the photo-sensitive area of a planar-type InGaAs detector, the structure of the detector was modified, and the small-diffusion-area diffusion method, circle-type covering contact and guard-ring were introduced. The laser-beam-induced-current (LBIC) technique was used to study the photo responsive characteristics of the photo-sensitive area of different detector structures. It was indicated that, by modifying the size of the diffusion area, the width of the circle-type covering contact, the distance between the guard-ring and the photo-sensitive area and the working status of the guard-ring, extension of the photo-sensitive area could be effectively suppressed, and the detector photo-sensitive area could be exactly defined.

Opto-electronic analysis of silicon solar cells by LBIC investigations and current–voltage characterization

A different laser beam induced current (LBIC) mapping technique has been used for the measurements of spatial variation of light generated current of a solar cell. These variations are caused by parasitic resistances and defects at grain boundaries (GBs) in multicrystalline silicon solar cells (mc-Si). This study investigates and identifies the regions within mc-Si solar cells where dominating recombination and lifetime limiting processes occur. A description of the LBIC technique is presented and the results show how multicrystalline GBs and other defects affect the light generated current of a spot illuminated mc-Si solar cell. The results of the internal quantum efficiency (IQE) at wavelength of 660 nm revealed that some regions in mc-Si solar cell give rise to paths that lead current away from the intended load.

In-depth resolution for LBIC technique by two-photon absorption

A detailed study of the in-depth dependence of the laser-beam-induced current (LBIC) technique by sub-bandgap two-photon absorption (TPA) has been carried out in this paper. The strong focal dependence mechanism for TPA has been demonstrated by our studies through comparing the TPA technique with traditional single-photon-absorption-based ones. Dependence of the TPA-induced single-event transient response in linear integrated circuits on depth and position is investigated. Our results illustrate an interesting in-depth resolution for the TPA technique, which enables three-dimensional imaging of charge-collecting volumes through the backside of integrated circuits.

Screening of high-power diode laser bars by optical scanning

High-power diode laser bar arrays (808 nm) with very uniform emission properties are inspected by the laser-beam-induced current technique (LBIC). Setting the excitation energy to 50 meV below the lasing energy, we observe distinctive signatures within the LBIC scans at certain locations on the devices. After 1000 h of high-power operation, we observe degradation at exactly those positions that previously showed a characteristic LBIC signature. A cathodoluminescence analysis reveals that the quantum wells of these anomalous device sections suffer from the existence of spots with reduced luminescence efficiency. Additionally, a concomitant 2.3 meV redshift of the quantum-well cathodoluminescence spectrum is also observed. Our measurements demonstrate the efficiency of the LBIC approach as a screening tool as well as its capacity for predicting device failure well before any degradation of the emission properties is observed.

Laser beam induced current imaging of reactive ion etching induced n-type doping in HgCdTe

The nondestructive optical characterization technique of laser beam induced current (LBIC) has been used to illustrate the effects of reactive ion etching (RIE) of mid-wavelength infrared n-type HgCdTe. RIE may be used as a method of n-p junction formation, as a means of forming n+ ohmic contacts to wider bandgap HgCdTe, or for mesa isolation etching of epilayers for HgCdTe detectors and emitters. Along with experimental measurements of the LBIC phenomena, this paper introduces the simulation of LBIC signals using a commercial semiconductor device modeling package. A number of LBIC maps are presented for different wafer processing conditions, with the results being explained using the simulation software. The experimental and calculated results bring to light a number of previously unreported characteristics associated with the LBIC phenomena, including the effect of junction depth, temperature, and grading of the junction region. In addition to the LBIC technique confirming the presence of an n+ region after RIE processing, it also provides information regarding the depth of the n+ region and lateral extent of the doping.

A discrete element model of laser beam induced current (LBIC) due to the lateral photovoltaic effect in open-circuit HgCdTe photodiodes

The non-destructive optical characterization technique of Laser-Beam-Induced-Current (LBIC) imaging has proven useful in qualitatively assessing electrically active defects and localized non-uniformities in HgCdTe materials and devices used for infrared photovoltaic arrays. To further the development of a quantitative working model for LBIC, this paper focuses on the application of the technique to photovoltaic structures that are represented by a discrete element equivalent circuit. For this particular case the LBIC signal arises due to the lateral photovoltaic effect in non-uniformly illuminated open-circuit photodiodes. The outcomes of the model predict all of the experimentally observed geometrical features of the LBIC image and signal. Furthermore, the model indicates that the LBIC signal has an extremely weak dependence on the p-n junction reverse saturation current, and shows a linear dependence with laser power. This latter feature map be useful for non-contact measurement of the quantum efficiency of individual photodiodes within a large two-dimensional focal plane array. The decay of the LBIC signal outside the physical boundary of the p-n junction is of the same form as the roll-off in the short circuit photoresponse and, therefore, can be used to extract the diffusion length of minority carriers. Experimental data is obtained from an arsenic implanted p-on-n junction fabricated on MBE grown Hg/sub 1-x/Cd/sub x/Te material with an x-value of 0.3. The p-on-n diode is shown to be uniform and of high quality with an R/sub 0/A product of 1/spl times/10/sup 8/ /spl Omega//spl middot/cm/sup 2/ at 77 K. The validity of the simple model developed in this paper, is confirmed by the excellent agreement with experimental results. Consequently, the LBIC technique is shown to be an appropriate diagnostic tool for non-contact quantitative analysis of semiconductor materials and devices.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Spatially resolved characterization of HgCdTe materials and devices by scanning laser microscopy

Spatially resolved characterization of HgCdTe materials and p-n junction diodes using scanning laser microscopy is reviewed. Several techniques that yield spatial maps of electrical inhomogeneities in HgCdTe material and non-uniformities in various performance parameters of p-n junctions fabricated using these materials have been developed. Many of the techniques are non-destructive, or can be made such with minor changes in sample preparation, and are scalable to large full wafers. A high-resolution and non-destructive technique called 'laser beam induced current (LBIC)' has been developed for spatial mapping of electrically active regions in HgCdTe. When applied to unprocessed HgCdTe material, LBIC images represent spatial distributions of electrically active defects including inclusions, strain, damage, precipitates, stacking faults, twin boundaries, dislocation clusters, bandgap and doping variations. Device structures such as p-n junctions are special cases of electrically active regions, therefore the LBIC technique lends itself to a non-destructive study of p-n junction arrays without requiring any direct electrical contact to the individual elements of the array. The remote contacting, especially using pressure contacts, makes the application of the LBIC technique non-destructive, allowing testing at various stages of device processing to identify particular processing procedures that need optimization. LBIC has also been used to spatially map electrical non-uniformities at the HgCdTe surface near its interface with an insulating passivation layer. Besides LBIC imaging, scanning laser microscopy has been used for several other applications. For HgCdTe p-n junctions, applications include photoresponse mapping (uniformity, active area and diffusion length determination, contact bonding effects), spatially resolved light-induced degradation, and avalanche photodiode properties (ionization coefficients, localized breakdown). A key technique currently in development is the non-contact diode R0A determination based on trapping mode photoconductivity. Examples of scanning laser microscopy on HgCdTe materials are infrared transmission mapping (thickness and bandgap variations), photoluminescence mapping, and minority carrier lifetime mapping (distribution of recombination centres).

Identifiability of Semiconductor Defects from LBIC Images

The identifiability of defects in a semiconductor from its laser-beam-induced current (LBIC) image is studied. It is shown that the LBIC technique is reliable for detecting any spatial defects in the semiconductor material. Continuous dependence of the current measurements on the spatial defects is proved, and sensitivity is also discussed in certain cases.

Spatial mapping of electrically active defects in HgCdTe using laser beam-induced current

A high-resolution and nondestructive optical characterization technique called laser beam-induced current (LBIC) has been developed and utilized to obtain maps of electrically active defects in liquid phase epitaxy HgCdTe. The LBIC technique is also suitable for studying the p–n junction detector elements in an array nondestructively, without requiring any electrical contacts to individual detector elements.