Yvonne Schiele

Contacting and recombination analysis of boron emitters via etch-back for advanced n-Type Si solar cells

In p-type c-Si solar cells, selective emitter structures generated by emitter etch-back (EEB) have been introduced in recent years in order to minimize electrical losses in the phosphorous emitter being one of the dominant factors limiting the performance of standard screen-printed p-type c-Si solar cells. In this work, a homogeneously or selectively etched-back boron emitter is demonstrated to provide additional benefits yielding an enhanced conversion efficiency in n-type Si solar cells. By means of subsequent B-EEB, contacting and recombination properties of B emitters dependent on their sheet resistance, surface concentration, and profile depth are studied indicating the latter to be the crucial parameter. Based on this, the characteristics of the optimal B emitter regarding low saturation current density and low specific contact resistivity are determined for the cases of homogeneous and selective etch-back. By employing the selectively etched-back B emitter in initial solar cells, a VOC gain of 5 mV and a significant shunting reduction compared with homogeneously doped devices is achieved.

Co-diffused APCVD boron rear emitter with selectively etched-back FSF for industrial N-type Si solar cells

The employment of a B-doped atmospheric pressure chemical vapor deposited (inline belt APCVD) borosilicate glass is an elegant technology for industrially realizing a p emitter. By drive-in of B and a subsequent POCl3 co-diffusion, p emitter and n front surface field (FSF) are established in a single process step. APCVD-SiOx is used to prevent the p emitter from being compensated during P diffusion. Its thickness needs to be adapted in order not to affect the p profile during POCl3 diffusion while keeping it removable. For rear junction solar cells, it is crucial to ensure low recombination activity at the front. Therefore, a selectively etched-back FSF is to be established in the solar cell. An adjusted etch-back solution increases n Rsheet successively and well controllably, accompanied by a drastic j0FSF reduction while simultaneously almost completely maintaining p Rsheet. A 43 /sq APCVD-AlOx passivated p emitter achieves j0E of only 52 fA/cm. Total implied VOC of a pseudo solar cell structure attains up to 695 mV. The newly developed APCVD p emitter combined with the co-diffused and selectively etched-back FSF employed in an industrial n-type solar cell achieves 18.8% efficiency in a first experiment being still limited by a poor Ag/Al contact to the B-emitter.

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 regio...

>20% efficient 80 µm thin industrial-type large-area solar cells from 100 µm Sawn c-Si Wafers

Reducing the thickness of crystalline Si wafers to be processed into solar cells yields several significant benefits: PV module manufacturing cost can be reduced and the required diffusion length of minority carriers is smaller. The latter in turn enables a higher efficiency potential and a larger spread of Si materials to be employed for rear junction solar cell concepts which are advantageous for n-type devices. Industrial-type 80 μm thin large-area rear junction solar cells manufactured from 100 μm wire-sawn wafers exhibit an independently certified efficiency of 20.1% with VOC of 672 mV.

Comparison of bifacial and monofacial large-area n-type Si solar cells from 100 μm thin wire-sawn wafers

Reducing wafer thickness provides the most effective potential to lower the production cost of c-Si PV modules. Two thin n-type solar cell concepts are compared in terms of their optical and electrical properties: a monofacial device with a full-area metal surface at the rear which is beneficial in particular to such thin solar cells and a very similar but even better industrially applicable bifacially collecting device. The monofacial solar cell exhibits a 0.7 mA/cm higher jSC and 9 mV greater VOC due to better light trapping and less recombination. Rseries and FF discrepancies of both solar cell concepts nearly compensate themselves which is revealed by an Rseries itemization and FF loss analysis. The independently certified 20.1% efficiency of the monofacial solar cell exceeds that of the bifacial rear junction device by 0.7%abs under one-sided illumination. However, since the bifacial solar cells feature a very high bifaciality of 99.4%, a total power output comparable to a 23.4% efficient monofacial solar cell can be achieved assuming a typical albedo of 20%.

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 10 cm 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.