Armin G. Aberle

Potential-induced degradation in photovoltaic modules: a critical review

Potential-induced degradation (PID) has received considerable attention in recent years due to its detrimental impact on photovoltaic (PV) module performance under field conditions. Both crystalline silicon (c-Si) and thin-film PV modules are susceptible to PID. While extensive studies have already been conducted in this area, the understanding of the PID phenomena is still incomplete and it remains a major problem in the PV industry. Herein, a critical review of the available literature is given to serve as a one-stop source for understanding the current status of PID research. This paper also aims to provide an overview of future research paths to address PID-related issues. This paper consists of three parts. In the first part, the modelling of leakage current paths in the module package is discussed. The PID mechanisms in both c-Si and thin-film PV modules are also comprehensively reviewed. The second part summarizes various test methods to evaluate PV modules for PID. The last part focuses on studies related to PID in the omnipresent p-type c-Si PV modules. The dependence of temperature, humidity and voltage on the progression of PID is examined. Preventive measures against PID at the cell, module and system levels are illustrated. Moreover, PID recovery in standard p-type c-Si PV modules is also studied. Most of the findings from p-type c-Si PV modules are also applicable to other PV module technologies.

The realistic energy yield potential of GaAs-on-Si tandem solar cells: a theoretical case study.

Si based tandem solar cells represent an alternative to traditional compound III-V multijunction cells as a promising way to achieve high efficiencies. A theoretical study on the energy yield of GaAs on Si (GaAs/Si) tandem solar cells is performed to assess their energy yield potential under realistic illumination conditions with varying spectrum. We find that the yield of a 4-terminal contact scheme with thick top cell is more than 15% higher than for a 2-terminal scheme. Furthermore, we quantify the main losses that occur for this type of solar cell under varying spectra. Apart from current mismatch, we find that a significant power loss can be attributed to low irradiance seen by the sub-cells. The study shows that despite non-optimal bandgap combination, GaAs/Si tandem solar cells have the potential to surpass 30% energy conversion efficiency.

Periodic Upright Nanopyramids for Light Management Applications in Ultrathin Crystalline Silicon Solar Cells

Motivated by the primary benefit of reduced material cost, the thickness of crystalline silicon solar cells has been continuously reduced. Laboratory and industrial studies have explored ultrathin crystalline silicon solar cells below 50 μm with ambitious endeavors toward thicknesses of only a few micrometers. Ultrathin crystalline silicon solar cells require compatible small-scale surface textures to enhance the optical absorption. For this purpose, a novel submicron periodic nanostructure—periodic upright nanopyramids (PuNPs)—is fabricated by an integrated process of laser interference lithography and anisotropic etching of silicon in an alkaline solution. By simulation and measurements, we demonstrate that PuNPs are able to reduce front surface reflectance more effectively than conventional micron-scale pyramid textures and previously investigated periodic inverted nanopyramids (PiNPs). With a silicon nitride antireflection coating, we predict that PuNPs reduce the front surface reflectance to below 1% at an angle of incidence of 8°, which is comparable to black silicon. The superior antireflective property of PuNPs contributes to an absorbed photocurrent density of 40.8 mA/cm2 for a 40 μ m silicon absorber layer, which is 0.7 mA/cm2 higher than PiNPs, 0.8 mA/cm2 higher than inverted pyramids and 1 mA/cm2 higher than upright pyramids.

Numerical Simulation of Doping Process by BBr3 Tube Diffusion for Industrial n -Type Silicon Wafer Solar Cells

Efficient optimization of the boron-doped region of silicon solar cells requires reliable process simulation of boron tube diffusion. Established simulation models and parameters are mostly calibrated for complementary metal oxide semiconductor device fabrication, where the doping processes are significantly different from those used in solar cell fabrication. In this paper, we present models and a set of corresponding parameters that are suitable for process simulation of BBr3 tube diffusion for solar cell applications with Sentaurus TCAD. Experimental doping profiles obtained with a wide range of diffusion recipes are compared with simulation results. Additionally, with the process parameter sensitivity analysis, we demonstrate the dominant process parameters that alter the boron distribution profile and its effect on the electrical performance.

Surface passivation investigation on ultra-thin atomic layer deposited aluminum oxide layers for their potential application to form tunnel layer passivated contacts

The surface passivation performance of atomic layer deposited ultra-thin aluminium oxide layers with different thickness in the tunnel layer regime, i.e., ranging from one atomic cycle (~0.13 nm) to 11 atomic cycles (~1.5 nm) on n-type silicon wafers is studied. The effect of thickness and thermal activation on passivation performance is investigated with corona-voltage metrology to measure the interface defect density D it(E) and the total interface charge Q tot. Furthermore, the bonding configuration variation of the AlO x films under various post-deposition thermal activation conditions is analyzed by Fourier transform infrared spectroscopy. Additionally, poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) is used as capping layer on ultra-thin AlO x tunneling layers to further reduce the surface recombination current density to values as low as 42 fA/cm2. This work is a useful reference for using ultra-thin ALD AlO x layers as tunnel layers in order to form hole selective passivated contacts for silicon solar cells.

Effects of indium tin oxide on the performance of heterojunction silicon wafer solar cells

The effects of indium tin oxide (ITO) films on the performance of heterojunction silicon wafer solar cells is investigated, using heterojunction (HET) solar cell precursors. Different ITO deposition conditions are used, which result in significant differences in the performance of HET solar cells. It is found that HET solar cells with ITO films deposited at room temperature exhibit severer sputter damage, while those with substrate heating show less damage. Besides the ITO deposition temperature, the sputtering gas ambient is also investigated. The hydrogen gas used in the ITO deposition can greatly affect the interface properties between the ITO film and the amorphous silicon layers. The champion solar cell fabricated under the optimum ITO deposition conditions (a deposition temperature of 150 °C with optimal gas concentration) shows a conversion efficiency of 19.7%.

Sensitivity analysis for III–V/Si tandem solar cells: A theoretical study

Material and structural parameters may affect the efficiency of a tandem solar cell differently from the way they do in a single-junction solar cell. We fabricated a III–V/Si four-terminal tandem solar cell and developed an opto-electronic model simulating this device. The optical properties were simulated with the transfer matrix method, while the electrical properties were simulated using the numerical device simulator PC1D. For this simulated tandem structure, we determined the parameters which have the largest potential impact on the device efficiency. A sensitivity analysis of the impact of these parameters on the device efficiency was also performed. In addition, to reduce the cost of the tandem solar cells, we identified the parameters that do not require tight control during the manufacturing process. The Si cell was simulated both as a single-junction cell and as the bottom cell of a tandem device. Finally, we determined those device parameters that are more critical in a tandem configuration than in a single-junction configuration.

Flash spectral imaging for optical metrology of solar cells

Flash spectral imaging of full area (156 mm by 156 mm) silicon solar wafers and cells is realized in a setup integrating pseudo-monochromatic LEDs over the wavelength range of 370 to 1050 nm and a high-resolution monochrome camera. The captured information allows the computation of sample reflectance as a function of wavelength and coordinates, thereby constituting a spectral reflectance map. The derived values match that obtained from monochromator-based measurements. Optical inspection is then based on the characteristic reflectance of surface features at optimally contrasting wavelengths. The technique reveals otherwise hidden stains and anti-reflection coating (ARC) non-uniformities, and enable more selective visualization of grains in multicrystalline Si wafers. Optical contrast enhancement of metallization significantly improves accuracy of metal detection. The high effective resolution of the monochrome camera also allows fine metallization patterns to be measured. The rapid succession of flash-and-image-capture at each wavelength makes the reported optical metrology technique amenable in photovoltaic manufacturing for solar wafers/cells sorting, monitoring and optimization of processes.

Impact of Nonuniform Illumination and Probe Bar Shading on Solar Cell I–V Measurement

This paper examines the effect of nonuniform and probe-bar-shaded solar illumination in the I–V characterization of large area Si wafer solar cells. The illumination conditions were experimentally implemented by masking the solar cell during measurement. The results are examined in detail with simulations in Griddler, a finite element analysis software developed at the Solar Energy Research Institute of Singapore. The light I–V characteristics, particularly the fill factor, depend on the specific irradiance profiles relative to the current-collecting busbars due to effects on the effective series resistance Rs. The effect is slight for small light nonuniformities (IEC 60904 Class A nonuniformity classification or <2%) but for light nonuniformities up to 20%, the standard deviation in the fill factor can reach about 0.19% absolute. Probe bar shading directly leads to an underestimation of the measured fill factor due to longer current paths and larger voltage drops.

Modulated Photoluminescence Lifetime Measurement of Bifacial Solar Cells

The frequency response of photoluminescence (PL) emission in a semiconductor sample is influenced by the effective carrier lifetime. As it is easy to induce and capture PL in solar cells at different stages of processing, modulated PL poses as a versatile technique to track the progression of effective lifetime in silicon wafer solar cells in the production line. This paper demonstrates the application of modulated PL to extract the effective carrier lifetime in n-type silicon bifacial solar cell samples before and after metalization. For nonmetalized samples, good agreement is found in the lifetime curves generated by modulated PL and the more established microwave photoconductance decay technique. For single and double-side metalized samples, the deduced lifetime is also consistent with the implied open-circuit voltage obtained by steady-state PL imaging.