• Energy Research

AbstractIn this work, we present our progress in the industrial p-type PERC technology [1, 2]. Based on device simulations we continuously develop an efficiency roadmap for a steady improvement of our PERC (passivated emitter and rear cell) process. Following this simulation based approach, we effectively improve the front side metallization and the emitter characteristics. Currently, our best prototype process has reached a conversion efficiency well over 21% which enables the manufacturing of a 60-cell based module with a power of 310W. Our best cell so far has a conversion efficiency of 21.5% which has been confirmed by the calibration laboratory of Fraunhofer ISE. This is to our knowledge the highest efficiency reported for industrial-size silicon solar cells with screen-printed metal front and rear contacts.

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Abstract With efficiencies of industrial type PERC solar cells exceeding 22% and reduced electrical and optical losses in these high efficiency cells, it becomes more and more important for manufacturers to rely on high quality silicon wafers. In this contribution, we present data of the bulk excess carrier lifetime τ bulk and a doping density independent material quality parameter at maximum power point, the material saturation current density j 0,mat , of different p-type monocrystalline silicon materials. These values are obtained by measuring the effective carrier lifetime of neighbouring wafers of different thickness and thus eliminating the surface recombination contribution. We show that j 0,mat is an adequate parameter to evaluate the bulk quality of a given material. To further validate these findings, we compare the results to PERC cell efficiencies obtained with the same material.

AbstractPrecise quantitative assessment of c-Si wafer quality is of crucial importance for the development and manufacturing of high efficiency solar cells. For this purpose, lifetime samples are typically fabricated with very well cleaned and passivated surfaces. Under those conditions the measured effective lifetime τeff is almost equal to the silicon bulk wafer lifetime τwafer, i.e. a material related quality parameter. Those lifetime measurements are typically carried out with a photo-conductance decay method (PCM) e.g. with a Sinton-WCT tool. The measurement result is an effective excess carrier lifetime τeff which typically exhibits a strong dependence on the excess carrier injection density Δn within the wafer. Stating τeff –values thus necessitates to specifiy Δn. The PV community typically reports at a fixed Δn in the range of 1×1014 cm-3 to 1×1016 cm-3 or for varying wafer doping density Ndop at Δn = Ndop/10. The latter allows for a comparison from the point of view of the Shockley-Read-Hall (SRH) formalism. Unfortunately, the impact of a certain lifetime for device performance changes with Ndop, due to the law of mass action. In this paper a wafer doping density dependent Δn which is relevant for the injection density at maximum power point (MPP) is derived. This Δn@MPP shows a contrary behaviour compared to the often used and accepted reporting method to set Δn = Ndop/10. Additionally, a wafer doping density independent material quality parameter, called material saturation current density j0,mat at MPP, is proposed to improve the comparability of measured effective lifetimes of differently doped wafers.

Simulation is essential for a comprehensive analysis of the performance of solar cells. The rear contact pitch of passivated emitter and rear cells (PERC cells) is a crucial device parameter that influences not only the electrical performance but also the optical performance of these cells. This article investigates the applicability of the analytical light trapping model by Basore to account for the optical influence of the rear contact pitch as a simpler alternative to ray tracing. First, we manufacture three different groups of cells with different rear contact pitches, where the metallization fraction f met varies between 0 and 54%. Second, the reflectance of the cells is measured. Subsequently, we fit the model parameters R f and R b (internal reflection on the front and back surface, respectively) to the measured reflectance. While we confirm a nonlinear relation between f met and the measurable spectra found in previous works, our results reveal linear relations between f met and R b with the adjusted coefficients of determination of R adj² > 0.97, as well as between f met and the charge carrier generation rates with R adj² > 0.94. These relations allow a simple and rapid optical simulation of f met variation for PERC cells. The same approach is likely applicable to any local contacts also in other cell concepts.

This article investigates the impact of the back-surface-field (BSF) thickness variation within a local aluminum contact on the performance of passivated emitter and rear contact solar cells. A significant difference of BSF thickness between contact endings and the center of dash-shaped contacts is verified experimentally by a comprehensive statistical analysis using scanning electron microscopy. The impact of local BSF thickness differences on the cell performance is studied with 3-D technology computer-aided design (TCAD) device simulations. Several device parameters such as BSF thicknesses, the doping concentration in the BSF profile at rear contacts, or the metallized area fraction at the cell rear side are varied. Our simulation study shows that the open-circuit voltage is mainly affected by locally reduced BSF thicknesses, resulting in an efficiency loss up to 0.14%abs or 0.84%abs, respectively, if an area fraction of 1% or 20% within a local contact has reduced BSF thicknesses. This effect can be minimized either by reducing the metallized area fraction at the cell rear side or by increasing the doping concentration in the BSF profile at aluminum rear contacts. In addition, we demonstrate that the 3-D simulations can be approximated with 2-D simulations by applying a single doping profile with an average BSF thickness, calculated with the harmonic mean.

AbstractIn this work we compare experimental results of an industrial passivated emitter and rear cell (PERC) high volume pilot line production in the SolarWorld Innovations technology center with a simulation model based on 2D Sentaurus Device Simulations The PERC solar cell design shows in a well-controlled experiment a 1.1% absolute higher median efficiency compared to the aluminium back surface field (Al-BSF) solar cell reference group. We derive a calibrated simulation model by the characterization of cells and test structures. We show that the simulation model reproduces well the measured I-V data. In consequence this enables us to estimate the solar cell performance when an applied process or silicon wafer parameters will be changed for further optimization. In the second part of this study we investigate the impact of front and rear side recombination on the solar cell parameters by simulation and sensitivity analysis. We show that the quality of the local BSF underneath the local rear contacts has an important impact on the electrical solar cell performance.

Sufficiently deep local back-surface-fields (LBSF) are crucial for achieving low contact recombination and thus high solar cell efficiencies in passivated emitter and rear contact (PERC) solar cells. In this article, we investigate spatial variations of LBSF thickness 1) across dashed contacts, 2) between lateral positions within a cell, and 3) from cell to cell. The objective of this experimental study is to proof the impact of LBSF thickness on open-circuit voltage in PERC solar cells. Our measurements and analysis of variance (ANOVA) reveal that variations in LBSF thickness across dashed contacts exceed observed variations in LBSF thickness between lateral positions within a cell as well as cell-to-cell variations. Thus, we statistically treat all LBSF measurements of a single experimental group of PERC solar cells as one population of measurements. The two investigated experimental sets of PERC cells fired using different fast-firing profiles show a difference in LBSF thickness of about 1 μm. The resulting difference in open-circuit voltages arises to 1.3 mV. ANOVA-based analysis shows for our experimental samples that at least 19 random measurements are necessary in order to resolve LBSF thickness difference of 0.5 μm with sufficient statistical significance.