Luana Mazzarella

p-type microcrystalline silicon oxide emitter for silicon heterojunction solar cells allowing current densities above 40 mA/cm2

We have developed a microcrystalline silicon oxide (μc-SiOx:H) p-type emitter layer that significantly improves the light incoupling at the front side of silicon heterojunction solar cells by minimizing reflection losses. The μc-SiOx:H p-layer with a refractive index of 2.87 at 632 nm wavelength and the transparent conducting oxide form a stack with refractive indexes which consecutively decrease from silicon to the ambient air and thus significantly reduce the reflection. Optical simulations performed for flat wafers reveal that the antireflective effect of the emitter overcompensates the parasitic absorption and suggest an ideal thickness of about 40 nm. On textured wafers, the increase in current density is still more than 1 mA/cm2 for a typical emitter thickness of 10 nm. Thus, we are able to fabricate heterojunction solar cells with current densities significantly over 40 mA/cm2 and power conversion efficiency above 20%, which is yet mainly limited by the cells fill factor.

Silicon Heterojunction Solar Cells With Nanocrystalline Silicon Oxide Emitter: Insights Into Charge Carrier Transport

We recently demonstrated how the short-circuit current density of an a-Si:H/c-Si heterojunction solar cell can be significantly improved to above 40 mA/cm2 by replacing the standard a-Si:H(p) emitter by a silicon oxide emitter containing p-doped silicon nanocrystallites. While we could obtain a conversion efficiency of 20.3%, the cell suffered from a lower fill factor of 72.9%, compared with 77.0% for our standard process. In this paper, we address this issue both theoretically and experimentally. We found that a thin (~3 nm) highly doped nanocrystalline silicon layer on top of the emitter can greatly improve the fill factor. Using 1-D device simulation, we explain the prevalent loss mechanism, which originates mostly from poor tunnel recombination at the transparent conducting oxide/emitter interface rather than in the bulk of the emitter. We suspect that have their origin in the lower effective dopant concentration of the nanocrystalline silicon oxide emitter. From the model, implications for further developments can be derived.

Backside contacted solar cells with heterojunction emitters and laser fired absorber contacts for crystalline silicon on glass

Backside contacted cells were fabricated on p- and n-type, liquid phase crystallized silicon on glass. The heterojunction emitter and transparent contacting layer were opened locally using an inkjet printer and the absorber was contacted with laser fired point contacts. An analytical resistance model was derived to determine the resistance losses for the point contact geometry. Using test structures and the model it was found that for p-type cells the main contribution to the cell resistance came from the absorber contact and the absorber. Both were strongly reduced by increasing the absorber doping. For n-type cells the main contributions originated from the emitter and emitter-TCO contact, which was improved by changing the emitter type, resulting in an 11.6 % solar cell.

Optimization of PECVD process for ultra-thin tunnel SiO x film as passivation layer for silicon heterojunction solar cells

Ultra-thin silicon oxide (a-SiOx:H) films have been grown by means of plasma enhanced chemical vapor deposition (PECVD) to replace the standard hydrogenated amorphous silicon (a-Si:H) passivation layer for silicon heterojunction solar cells to reduce parasitic absorption. Additionally, silicon oxide surfaces are well known as superior substrates for the nucleation enhancement for nanocrystalline silicon doped films. Symmetrical passivation samples were fabricated with variable a-SiOx:H layers with a thickness of 10-1.5 nm and characterized after several annealing steps (25-650 °C). The best value reached so far on <;100> oriented Si wafers is: implied open circuit voltage of 686 mV and minority carrier lifetime of 1.6 ms after annealing at 300 °C. Such values were found to be reproducible even for ultra-thin a-SiOx:H layers (1.5 nm).