Mikroskopische Charakterisierung von LFCs für dünne multikristalline Si-Solarzellen

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Surface Texturing for Silicon Solar Cell Application Using ICP-PECVD Plasma Technique

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Spatially resolved analysis of selectively doped regions via confocal Raman microscopy

More or less all proposed highly efficient solar cell concepts use quite successfully laterally selective boron doping. However, quality control regarding achieved doping level, lateral extension, etc. on real devices or precursors gets more and more complicated with these small structures as the typical characterisation techniques simply do not work well on small scale. High resolution mapping confocal Raman spectroscopy is considered a possible technique to tackle this challenge. In this contribution it is demonstrated on IBC precursor structures that the contrast in local doping level can be finely resolved via mapping Raman spectroscopy, even though the determined absolute value is found to be too small. It is discussed on the basis of a few fundamental calculations that the depth sensitivity is accountable for this drawbac

p⁺-doping analysis of laser fired contacts for silicon solar cells by Kelvin probe force microscopy

Local rear contacts for silicon passivated emitter and rear contact solar cells can be established by point-wise treating an Al layer with laser radiation and thereby establishing an electrical contact between Al and Si bulk through the dielectric passivation layer. In this laser fired contacts (LFC) process, Al can establish a few μm thick p+-doped Si region below the metal/Si interface and forms in this way a local back surface field which reduces carrier recombination at the contacts. In this work, the applicability of Kelvin probe force microscopy (KPFM) to the investigation of LFCs considering the p+-doping distribution is demonstrated. The method is based on atomic force microscopy and enables the evaluation of the lateral 2D Fermi-level characteristics at sub-micrometer resolution. The distribution of the electrical potential and therefore the local hole concentration in and around the laser fired region can be measured. KPFM is performed on mechanically polished cross-sections of p+-doped Si regions formed by the LFC process. The sample preparation is of great importance because the KPFM signal is very surface sensitive. Furthermore, the measurement is responsive to sample illumination and the height of the applied voltage between tip and sample. With other measurement techniques like micro-Raman spectroscopy, electrochemical capacitance-voltage, and energy dispersive X-ray analysis, a high local hole concentration in the range of 1019 cm−3 is demonstrated in the laser fired region. This provides, in combination with the high spatial resolution of the doping distribution measured by KPFM, a promising approach for microscopic understanding and further optimization of the LFC process.publishe

Al₂O₃ rear surface passivation for silicon ribbon solar cells

In this work the application of an Al2O3 surface passivation layer to low-cost multicrystalline silicon ribbon material is investigated. Symmetrical lifetime samples are prepared from adjacent p-doped EFG (Edge-defined Film-fed Growth) wafers and spatially resolved μPCD (Microwave detected PhotoConductance Decay) measurements are carried out to determine the minority charge carrier lifetime. It is shown that even very thin layers (5 nm) of Al2O3 can provide an excellent surface passivation of the mentioned silicon ribbon material. Instead of annealing in N2 ambience a MIRHP (Microwave Induced Remote Hydrogen Plasma) treatment is applied. This process step allows hydrogen atoms to pass the thin Al2O3 layer and to increase lifetime by passivating recombinative defects in the bulk material. In addition to the Al2O3 layer, hydrogen-rich silicon nitride (SiNx:H) is deposited by PECVD (Plasma Enhanced Chemical Vapor Deposition) on top of selected Al2O3 samples to increase the mechanical and chemical stability during a subsequent solar cell process. This deposition can replace the necessary Al2O3 annealing step leading to a comparable passivation quality. Finally, 2 x 2 cm2 solar cells are processed including the Al2O3 rear side passivation in a photolithography based highefficiency process reaching efficiencies above 18% on multicrystalline EFG material [1]

P⁺ Doping Analysis of Laser Fired Contacts by Raman Spectroscopy

Laser firing of contacts is a simple method to establish local rear contacts of silicon PERC (passivated emitter and rear contact) solar cells. The silicon bulk is contacted point-wise by Al driven by the laser through the rear dielectric layer. The Al is deposited on the full area by physical vapor deposition or screen-printing. Al and B, if the Al paste contains B additives, can thereby establish a p+-doped Si region below the Laser Fired Contact (LFC), which helps to lower carrier recombination because of the local back surface field and also to reduce contact resistance. In this work Raman spectroscopy is used to detect and investigate the p+-layer established by laser firing through screen-printed Al. Scanning Raman measurements allow spatially resolved determination of the free hole concentration in the contact area. In a line scan through a LFC, the step in doping concentration between the lowly doped bulk Si and the highly doped LFC area is clearly seen by a high local hole concentrations in the range of 1019 cm-3 in the LFC region. This shows that scanning Raman spectroscopy is a useful method for the microscopic understanding of LFCs and optimization of the process parameters

Manufacturing 100-µm-thick silicon solar cells with efficiencies greater than 20% in a pilot production line

Reducing wafer thickness while increasing power conversion efficiency is the most effective way to reduce cost per Watt of a silicon photovoltaic module. Within the European project 20 percent efficiency on less than 100-µm-thick, industrially feasible crystalline silicon solar cells (“20plµs”), we study the whole process chain for thin wafers, from wafering to module integration and life-cycle analysis. We investigate three different solar cell fabrication routes, categorized according to the temperature of the junction formation process and the wafer doping type: p-type silicon high temperature, n-type silicon high temperature and n-type silicon low temperature. For each route, an efficiency of 19.5% or greater is achieved on wafers less than 100 µm thick, with a maximum efficiency of 21.1% on an 80-µm-thick wafer. The n-type high temperature route is then transferred to a pilot production line, and a median solar cell efficiency of 20.0% is demonstrated on 100-µm-thick wafers

Evaluating the efficiency limits of low cost mc Si materials using advanced solar cell processes

The evaluation of the efficiency potential of Si materials for solar cell production is one key aspect for strategic decisions in today’s photovoltaic business. In this work a flexible photolithography-based cell process is presented which is in particular well-suited for defect-rich multicrystalline Si material. One decisive feature is the low overall thermal budget of the process since it is based on only one longer high-temperature step (the P diffusion) and a short firing step to obtain a decent hydrogen passivation from a hydrogen-rich PECVD (Plasma-Enhanced Chemical Vapor Deposition) SiNx:H layer. A further MIRHP (Microwave Induced Remote Hydrogen Plasma) step at a temperature below 400°C completes the hydrogen passivation of bulk defects. The process is derived from the standard photolithography based process at the University of Konstanz (UKN) and can easily be adapted to all kinds of dielectric rear side passivation patterns like a-Si, SiO2, SiCx and Al2O3 or stack systems. The rear side contact in this approach is established by Laser Fired Contacts (LFCs). Results presented in this work originate from a process based on an Al2O3 rear side passivation which is deposited at less than 200°C and subsequently annealed at about 400°C. Efficiencies above 18% on EFG and Calisolar polysilicon material, above 14% on RGS and above 20% on FZ reference material are demonstrated on 2 x 2 cm2 solar cells. For all mc-Si materials these efficiencies are very close to the highest efficiencies ever obtained by applying other already established high efficiency processes