Light Beam-induced Current Mapping Research Articles

Processing and characterization of large area InP nanowire photovoltaic devices

III−V nanowire (NW) photovoltaic devices promise high efficiencies at reduced materials usage. However, research has so far focused on small devices, mostly ≤1 mm2. In this study, the upscaling potential of axial junction InP NW photovoltaic devices is investigated. Device processing was carried out on a full 2″ wafer, with device sizes up to 1 cm2, which is a significant increase from the mm-scale III−V NW photovoltaic devices published previously. The short-circuit current density of the largest 1 cm2 devices, in which 460 million NWs are contacted in parallel, is on par with smaller devices. This enables a record power generation of 6.0 mW under AM1.5 G illumination, more than one order of magnitude higher than previous III−V NW photovoltaic devices. On the other hand, the fill factor of the larger devices is lower in comparison with smaller devices, which affects the device efficiency. By use of electroluminescence mapping, resistive losses in the indium tin oxide (ITO) front contact are found to limit the fill factor of the large devices. We use combined light-beam induced current (LBIC) and photoluminescence (PL) mapping as a powerful characterization tool for NW photovoltaic devices. From the LBIC and PL maps, local defects can be identified on the fully processed devices.

Optical Characterization and Photovoltaic Performance Evaluation of GaAs p-i-n Solar Cells with Various Metal Grid Spacings

GaAs p-i-n solar cells are studied using electroreflectance (ER) spectroscopy, light beam induced current (LBIC) mapping and photovoltaic characterization. Using ER measurements, the electric field across the pn junction of a wafer can be evaluated, showing 167 kV/cm and 275 kV/cm in the built-in condition and at −3 V reverse bias, respectively. In order to understand the effect of the interval between metal grids on the device’s solar performance, we performed LBIC mapping and solar illumination on samples of different grid spacings. We found that the integrated photocurrent intensity of LBIC mapping shows a consistent trend with the solar performance of the devices with various metal grid spacings. For the wafer used in this study, the optimal grid spacing was found to be around 300 μm. Our results clearly show the importance of the metal grid pattern in achieving high-efficiency solar cells.

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.

Low-cost system for micrometer-resolution solar cell characterization by light beam-induced current mapping

Light-beam induced current (LBIC) mapping is an increasingly utilized characterization technique for laboratory-scale as well as industrial-scale solar cells, which measures the local solar cell photocurrent by point illumination. This contribution demonstrates the design and testing of an LBIC mapping device capable of measuring LBIC maps of solar cells using inexpensive materials. With a spatial resolution of 4 µm and an auto-focused beam spot size of about 2 µm, obtained from a standard CD/DVD pickup, high-resolution LBIC maps of thin-film solar cells are obtained. The system was demonstrated by measuring LBIC maps on thin-film solar cells, revealing significant, micrometer-sized photocurrent heterogeneities that are otherwise unseen when using typical commercial LBIC systems with lower resolution.

An applied light-beam induced current study of dye-sensitised solar cells: Photocurrent uniformity mapping and true photoactive area evaluation

The conditions for light-beam induced current (LBIC) measurement were experimentally optimised for dye-sensitised solar cells. The impacts of too fast a laser diode modulation frequency (f) and too short a dwell time (t0) were investigated for their distortions, artefacts, and noise on the overall photocurrent map image. Optimised mapping conditions for fastest measurement were obtained at a f = 15 Hz and t0 = 900 ms. Whole device maps (nominal area 4 × 4 mm2) were obtained on devices in which fabrication defects were intentionally induced. The defects were readily resolved with the LBIC setup and conditions. The inclusion of defects had the effect of broadening the photocurrent distribution and producing a sub-optimal tail to photocurrent histograms. Photoactive areas were derived from LBIC maps and were larger than those predicted by the projected screen printing pattern by up to 25%, which has obvious implications for efficiency measurements made on nominal projected active area.

High-Resolution Lock-in Thermography Investigation on Industrial Multicrystalline Silicon Solar Cells

Industrial multicrystalline silicon (mc-Si) solar cells with different types of shunts have been analyzed in detail by dark lock-in thermography (DLIT). Several types of nonlinear shunts were found in our samples and most of them could only be detected in low forward-bias images of DLIT. However, we also observed nonlinear shunts that are only visible or have much stronger signal under the low reverse-bias than equivalent forward-bias condition, which is a new finding compared with the common observations. The edge of an mc-Si solar cell was found vulnerable for shunting. A weak leakage current around edges was frequently observed under 0.5 V forward bias on both shunted and normal cells. It reveals that the edge is one of the major recombination paths under the cell operation condition, which is due to the imperfect edge passivation and material quality limitation of mc-Si. Light-beam-induced current (LBIC) was also applied on one material-induced shunt. LBIC mapping with long wavelength revealed the degraded current response due to poor wafer quality. However, an LBIC image of short wavelength did not show the defect structure because the current was dominated by the Auger recombination, while not influenced by the bulk lifetime. Some pre-breakdown sites were found in the material-induced shunt sample and were only visible under the reverse-bias condition of DLIT.

Influence of different surface treatments on multicrystalline silicon wafers for defect characterization by LBIC

In this paper, we investigate the influence of different chemical treatments on the surface of multicrystalline Silicon (mc-Si) wafers, for both revealing grain boundaries and intra-grain defects. Electrical characterization by light beam-induced current (LBIC) was also carried out after the treatments. Several pieces of ~2 × 2 cm2, from mc-Si wafers, were mechanically polished and chemically etched and subsequently metallized with gold (on both surfaces) by sputtering, using optimized deposition times for doing transparent electrodes suitable for LBIC mapping. The surface treatments are discussed in terms of their capabilities to reveal the crystal defects and to provide the best conditions for efficient LBIC signals directly on silicon substrates, in the absence of a p–n junction. The best surface treatments allowing both revealing crystallographic defects and permitting the measurement of highly contrasted LBIC maps is the KOH etching. A large number of intra-grain defects are clearly revealed, being their electrical activity sensitively higher than that of the grain boundaries.

Recombination Strength at Intra and Intergrain Defects in Crystalline Silicon Investigated by Low Temperature Lbic Scan Maps

AbstractAnnealing of multicrystalline silicon wafers in argon at temperatures higher than 800°C increases the recombination strength of extended defects irreversibly, while external gettering techniques applied at the same temperatures reduce their recombination strength. It is observed that in the light beam induced current (LBIC) scan maps at 80 K the contrast of some extended defects in raw samples is enhanced and additional defects are detected. The same features of defects are revealed by the LBIC maps of raw samples at 80 K as well as at 300 K after annealing. In other words, the LBIC scan maps at low temperature detect “sleeping” defects, which do not recombine at 300 K, but which are activated by thermal treatments during processing steps. The results can be explained by the Shockley-Read-Hall statistic involving the presence of deep and shallow energy levels in the band gap associated with the impurity-defect interaction.

Silicon Films Deposited by LPE on Low Cost Multicrystalline Silicon Substrates

AbstractP type silicon layers for photovoltaic applications have been deposited on low cost multicrystalline silicon substrates prepared by directional solidification or by electromagnetic casting with solar grade charges. The thickness is between 4 to 20 μm. The layers were characterized by Light Beam Induced Current (LBIC) mappings at different wavelengthes. Collecting structures are p-n junctions or Al-Si semitransparent diodes. The layer and the substrate can be investigated separately by changing the wavelength of the LBIC maps, or/and by realizing a collecting structure on the free surface of the substrate.It is found that the same extended crystallographic defects are detected by LBIC in the layer and in the substrate. However their recombination strength in the layer is less marked probably due to a higher purity of the layer. The computed values of minority carrier diffusion length are close or larger than the film thickness and the investigated layers could be used to make solar cells.

Mapping of defects and their recombination strenght by a light-beam-induced current in silicon wafers

Knowledge of the correlation between the nature of recombining defects (extended crystallographic defects, impurities etc.) and the local electrical properties of wafers are necessary for the control of semiconductor materials. The control of the evolution of the electrical properties during the processing steps used to manufacture a device is of a great interest provided that the investigation technique is non-destructive. The light-beam-induced current (LBIC) mapping technique is able (1) to recognize and reveal the existence of extended crystallographic defects, such as grain boundaries, dislocations and stacking faults, when they recombine minority carriers; (2) to evaluate the recombination strength of the preceding defects and (3) to suggest the existence of impurity clouds or precipitates in the homogeneous regions of the wafers. In silicon wafers and devices, LBIC mapping can reveal bulk and surface defects, by changing the wavelength of the light only. The LBIC map can be transformed into a quantitative minority-carrier diffusion length L map in silicon, because the light beam excitation is compatible with the determination of L from the spectral variations in the photocurrent and in the light absorption coefficient in the near IR. Finally, LBIC maps can also be used to detect and evaluate any improvements associated with treatments such as hydrogenation, internal and external getterings.