Void formation on PERC solar cells and their impact on the electrical cell parameters verified by luminescence and scanning acoustic microscope measurements

Ideally formed local Al-contacts of passivated emitter and rear contact solar (PERC) cells feature an eutectic and an uniform local back surface field (LBSF). Under certain conditions the eutectic is missing after the co-firing process, referring to the well-known voids. So far light beam induced current (LBIC) measurements are used to obtain information concerning the passivation quality of the LBSF in local contacts in general. In addition, the destructive technique of scanning electron microscopy (SEM) is established for distinguishing whether a void features a sufficiently thick BSF-layer or a very thin/no BSF-layer. However, both methods are very time consuming.This paper shows a non-destructive and fast characterization of solar cells by applying electroluminescence (EL) and photoluminescence (PL) measurements to investigate the effect on the electrical parameters after locating the voids by scanning acoustic microscopy (SAM). For filled contacts EL and PL measurements correlate well with the resulting values for series resistance (RS) and dark saturation current density (j0): the formed LBSF leads to a good surface passivation (high PL signal intensity, low value for j0) and the eutectic layer ensures a good electrical contact (high EL signal intensity, low value for RS). Voids with a sufficiently thick LBSF show a high PL signal intensity whereas the intensity is significantly reduced for a very thin or completely missing LBSF. Increased values for RS can be explained by the missing eutectic layer. In addition, the electrical connection of the LBSF to the paste can be derived from the value of RS.publishe

Temperature dependence of void formation in PERC cells and their spatially resolved detection by combining scanning acoustic microscopy and electroluminescence measurements

Key aspect of this work is the investigation of local Al contacts with regard to void formation under a non-uniform temperature of the solar cell during the firing process in the belt furnace. The impact on electrical properties is determined by IV measurements and electroluminescence imaging (EL). Scanning acoustic microscope (SAM) measurements on full cell area are used for a spatially resolved localization of voids. Combining all these characterization techniques, the impact of thermal non-uniformity on contact formation can be determined spatially resolved. Furthermore, a detailed investigation of BSF thickness of two different Al-pastes shows a clear correlation with temperature distribution on the wafer during the co-firing process. All results demonstrate the importance of a well optimized Al-paste and an absolutely uniform temperature on the Si wafer for an excellent formation of local rear contacts

Comparison of BO Regeneration Dynamics in PERC and Al-BSF Solar Cells

Boron-oxygen related lifetime degradation is a severe problem for high-efficiency solar cells as the applied concepts like PERC suffer strongly from the observed lifetime degradation. However, these efficiency losses can be avoided via the Regeneration process eliminating the harmful defects. Within this contribution the effect of device structure on Regeneration kinetics is investigated. It is found that PERC-type cells regenerate faster than full area Al-BSF-type cells. It is pointed out why PERC-type cells benefit from an injection level effect. But this effect is not the only reason for the accelerated Regeneration. By means of especially adapted PERC cells, which do not benefit from the injection level advantage, it is shown that a second hydrogen containing layer on the rear has a positive influence, too, probably due to an increased hydrogenation of the silicon bulk.publishe

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.publishe

Layer Transfer from Chemically Etched 150 mm Porous Si Substrates

We demonstrate for the first time the successful layer transfer of an epitaxially grown monocrystalline Si film from a purely chemically etched porous Si substrate of 150 mm diameter to a glass carrier. The surface conditioning for all Si layer transfer processes based on porous Si has been, up to now without exception, carried out by electrochemical etching. In contrast, our chemical stain etching process uses an aqueous HF-rich HF/HNO3 solution. The porosity increases with increasing doping concentration of the Si substrate wafer and with increasing porous layer thickness. In contrast to the electrochemically etched double layers, the porosity profile of the stain etched substrates is highest at the original wafer surface and lowest at the interface between the porous layer and the Si bulk. The epitaxy process is adapted to the high porosity at the surface with regard to the reorganization of the porous layer

Surface recombination velocity of local Al-contacts of PERC 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.publishe

Al-density variation as one driving force for void formation in PERC solar cells

The reduction of void formation in local Al contact structures is of high interest in studies dealing with passivated emitter and rear contact (PERC) solar cells. So far, several processing parameters and their impact on local contact formation were investigated in detail. However, up to now density variation of Al in dependence on temperature and Si content in the melt were not taken into account as a principal reason for void formation. In this context the current assumption of a constant volume of the Al paste particles is discussed in more detail. Based on the results of energy dispersive X-ray spectroscopy, void formation implies either an expansion of paste particles or their burst during contact Formation.publishe