Nils Brinkmann

>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.

Effect of Oxygen during Thermal Annealing on the Electrical and Optical Properties of Sputter Deposited Al-Doped ZnO Films for Heterojunction Solar Cell Application

The influence of oxygen during short post deposition annealing on Al-doped zinc oxide (AZO) films for heterojunction solar cells grown by sputter deposition is investigated in terms of their electrical, optical and structural properties. AZO films were annealed for 4 min at 300°C and 450°C in ambient air and N2 atmosphere. It has been shown, that at 450°C the elimination of oxygen during annealing allows an increase in carrier concentration and reduction of the sheet resistance of AZO films four times higher than annealing in ambient air. This effect has not been observed for annealing temperatures below 300°C, where annealing in ambient air proved to have superior influence on the electrical properties. The optical bandgap of all annealed samples was enlarged and for an annealing temperature of 450°C the optical transmission weighted by ASTM G173-03 solar spectrum from 300 to 1100 nm was increased to over 80%. The changes observed in all samples were primarily explained by means of oxygen chemisorption and desorption processes. It is concluded, that oxygen plays an important role in the improvement of electrical and optical properties of AZO thin films during thermal annealing.