Morris Dahlinger

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.

Surface passivation of heavily boron or phosphorus doped crystalline silicon utilizing amorphous silicon

Excellent surface passivation of heavily boron or phosphorus doped crystalline silicon is presented utilizing undoped hydrogenated amorphous silicon (a-Si:H). For passivating boron doped crystalline silicon surfaces, amorphous silicon needs to be deposited at low temperatures 150 °C≤Tdep≤200 °C, leading to a high bandgap. In contrast, low bandgap amorphous silicon causes an inferior surface passivation of highly boron doped crystalline silicon. Boron doping in crystalline silicon leads to a shift of the Fermi energy towards the valence band maximum in the undoped a-Si:H. A simulation, implementing dangling bond defects according to the defect pool model, shows this shift in the undoped a-Si:H passivation to be more pronounced if the a-Si:H has a lower bandgap. Hence, the inferior passivation of boron doped surfaces with low bandgap amorphous silicon stems from a lower silicon-hydrogen bond energy due to this shift of the Fermi energy. Hydrogen effusion and ellipsometry measurements support our interpretation.

Full area laser doped boron emitter silicon solar cells

We present full area laser doped boron emitter n-type silicon solar cells using sputtered boron as dopant source. Quasi-steady-state photo conductance decay measurements show a low emitter saturation current density J0e and an open circuit voltage limit of Voc,lim 702 mV for a 128 Ω/sq emitter proving the high quality of the laser doped boron emitters. The first 4 cm2 cell yield efficiencies up to η= 16.7% without optimization of the cell structure.

Boron Partitioning Coefficient above Unity in Laser Crystallized Silicon

Boron pile-up at the maximum melt depth for laser melt annealing of implanted silicon has been reported in numerous papers. The present contribution examines the boron accumulation in a laser doping setting, without dopants initially incorporated in the silicon wafer. Our numerical simulation models laser-induced melting as well as dopant diffusion, and excellently reproduces the secondary ion mass spectroscopy-measured boron profiles. We determine a partitioning coefficient kp above unity with kp=1.25±0.05 and thermally-activated diffusivity DB, with a value DB(1687K)=(3.53±0.44)×10−4 cm2·s−1 of boron in liquid silicon. For similar laser parameters and process conditions, our model predicts the anticipated boron profile of a laser doping experiment.

Pulsed laser porosification of silicon thin films

We present a new and simple laser-based process to porosify thin film silicon using a pulsed laser. During deposition, we incorporate gas atoms or molecules into the Si thin film. Pulsed laser radiation of wavelength λ=532nm heats up thin film Si beyond its melting point. Upon heating, gas atoms or molecules form nm-sized thermally expanding gas bubbles in the silicon melt, until they explosively exit the film, leaving pores behind. Rapid heating and fast cooling during pulsed laser processing enable re-solidification of the liquid Si before the created pores contract and pore closure occurs within the liquid phase. Optimized plasma-enhanced chemical vapor deposition or sputtering of amorphous Si thin films on stainless steel substrate incorporates the necessary concentration of gas atoms or molecules. We are able to tailor the pore size between 50 and 550 nm by changing laser pulse energy density and film deposition parameters. Evaporated silicon containing no gas atoms forms only a few very large μm-sized gas bubbles due to laser-induced vapor formation of evaporated solid material at the substrate–silicon interface.

Band gap narrowing models tested on low recombination phosphorus laser doped silicon

This manuscript discusses bandgap narrowing models for highly phosphorus doped silicon. We simulate the recombination current pre-factor J0,phos in PC1Dmod 6.2 of measured doping profiles and apply the theoretical band gap narrowing model of Schenk [J. Appl. Phys. 84, 3684 (1998)] and an empirical band gap narrowing model of Yan and Cuevas [J. Appl. Phys. 114, 044508 (2013)]. The recombination current pre-factor of unpassivated and passivated samples measured by the photo conductance measurement and simulated J0,phos agrees well, when the band gap narrowing model of Yan and Cuevas is applied. With the band gap narrowing model of Schenk, the simulation cannot reproduce the measured J0,phos. Furthermore, the recombination current pre-factor of our phosphorus laser doped silicon samples are comparable with furnace diffused samples. There is no indication of recombination active defects, thus no laser induced defects in the diffused volume.