T. C. Röder

18.9% efficient full area laser doped silicon solar cell

A record in full area laser doped emitter solar cells with an efficiency η=18.9% is reported. Our patented, scanned laser doping process allows for the fabrication of defect free pn junctions via liquid state diffusion of predeposited dopant layers in ambient atmosphere without the need of clean room conditions. Our cells display an open circuit voltage Voc=677 mV, demonstrating laser doping to be comparable to furnace diffusion. Combining laser diffused pn junctions with a textured front side has the potential to boost the short circuit current density Jsc and thus the solar cell efficiency η to η>21%.

0.4% absolute efficiency gain of industrial solar cells by laser doped selective emitter

The formation of selective emitter with lower doped areas between the contact fingers and higher doped areas beneath the front side metallization increases the efficiency ¿ of crystalline silicon solar cells. The efficiency increases as a result of higher open circuit voltage and enhanced short circuit current density. The ipe laser doping process offers an elegant and reliable approach for the local formation of selective emitters. We use only one additional laser processing step after furnace diffusion. Thus, this patented process is easily integrated into an industrial production line without changing previous and subsequent processing steps. The phosphor silicate glass (PSG) layer, already grown on top of the emitter during furnace diffusion, serves as doping precursor. A laser locally melts the emitter surface and additional phosphorus atoms diffuse from the PSG layer into the emitter, increasing the doping concentration. This single additional processing step increases the efficiency ¿ of monocrystalline industrial solar cells by ¿¿ = 0.4% absolute.

Physical model for the laser induced forward transfer process

This paper presents a numerical model which describes the underlying physical processes during laser induced forward transfer. The laser induced forward transfer uses a pulsed laser to transfer thin layers from a transparent support to a substrate. The model predicts the threshold energies Eth as well as the blow-off time tblow, thus allowing a profound physical understanding of the transfer process. The good agreement of simulated with measured Eth and tblow of thin nickel layers demonstrates the accuracy of the model. The model shows that gasification of the soda-lime glass support is the main driving force of the transfer process.This paper presents a numerical model which describes the underlying physical processes during laser induced forward transfer. The laser induced forward transfer uses a pulsed laser to transfer thin layers from a transparent support to a substrate. The model predicts the threshold energies Eth as well as the blow-off time tblow, thus allowing a profound physical understanding of the transfer process. The good agreement of simulated with measured Eth and tblow of thin nickel layers demonstrates the accuracy of the model. The model shows that gasification of the soda-lime glass support is the main driving force of the transfer process.

30 µm wide contacts on silicon cells by laser transfer

We present a new fine line metallization process, called laser transferred contacts (LTC), with finger width w < 30 µm. This method uses a laser transfer of metals to contact the emitter directly through the anti-reflection coating without the need of high temperature steps. The laser transfer process is not limited to a special material; hence a variety of metals can be used for contact formation. After the laser transfer, electroplating thickens the contacts to increase the conductivity of the fingers. First solar cells with efficiency η = 17.4% and fill factor FF = 77.7% demonstrate the great potential of LTC.

Laser-Doped Back-Contact Solar Cells

We present laser-doped interdigitated back-contacted solar cells with a record efficiency η = 22.0%. The high versatility and spatial resolution of our laser doping process enable local n-type and p-type doping with a precision below 30 μm and avoid any masking for doping. Nevertheless, the presented solar cells use photolithography (masking) to define the contact area and metallization layout. Quasi-steady-state photoconductance measurements prove the low-saturation current density of the laser doping. We process solar cells with varied pitch and emitter fraction and compare their measured current density-voltage characteristics with a 3-D solar cell simulation. The good agreement of the simulation and experimental data allows a reliable efficiency forecast when optimizations are applied. Furthermore, the influence of the base-busbar region on solar cell performance is discussed.

Self-doping laser transferred contacts for c-Si solar cells

Laser transferred antimony contacts provide a self-aligned n-type selective emitter and simultaneously form the contacts to the front side of the solar cell. A pulsed laser beam transfers antimony from a transparent support through the passivation layer of a crystalline silicon solar cell and melts the underlying silicon. Antimony diffuses into the melt creating a locally highly doped n-type emitter. Between the transferred antimony contacts, the emitter remains shallowly doped, reducing Auger recombination. The antimony doped contacts act as seed layer for subsequent nickel and copper plating, resulting in a fine line front metallization with finger width w_F= 20 μm, contact resistivity as low as ρ_C= 30 μΩcm², and metallization resistivity ρ_M= 2.0×10^-6 Ωcm. First solar cells show an efficiency η= 17.5%.

Laser transferred contacts: Modeling of the transfer process

This work presents a numerical model for the physical processes during the transfer process of our nickel/copper laser fine line metallization. Here, a pulsed laser transfers a thin nickel layer from a glass support and contacts the solar cell directly through the anti-reflection coating. The model simulates the temperature distribution in the nickel as well as the glass support and calculates the arising gas pressures at the nickel/glass interface. The gasification of the soda-lime glass support is the main driving force of the transfer process.

18.9% efficient silicon solar cell with laser doped emitter

A new record in laser doped solar cells with an efficiency ¿ = 18.9% is here reported. The ipe laser doping process allows for the fabrication of emitters of silicon solar cells via liquid state diffusion of predeposited dopant layers. Laser doping of sputtered phosphorus precursors results in an open circuit voltage Voc = 677 mV, thus, ipe laser doping is comparable to furnace diffusion.