R. Ferre

Effect of amorphous silicon carbide layer thickness on the passivation quality of crystalline silicon surface

Surface passivation of p-type crystalline silicon wafers by means of phosphorus-doped hydrogenated amorphous silicon carbide films [a-SiCx(n):H] has been investigated. Particularly, we focused on the effects of layer thickness on the c-Si surface passivation quality resulting in the determination of the fixed charge density, Qf, within the a-SiCx(n):H film and the fundamental recombination of holes, Sp0. The main result is that surface recombination velocity decreases with film thickness up to 40nm and then saturates. The evolution of the interface parameters indicates that Qf could be located in a layer less than 10nm thick. In addition, Sp0 increases with thinner films probably due to different hydrogenation and saturation of interface dangling bonds during forming gas annealing.

n-type emitter surface passivation in c-Si solar cells by means of antireflective amorphous silicon carbide layers

Emitter saturation current densities (JOe) of phosphorus-diffused planar c-Si solar cell emitters passivated by silicon carbide (SiCx) layers have been determined in a wide sheet resistance range (20–500Ω∕sq). Phosphorus diffusions were performed using solid planar diffusion sources without employing any drive-in step. Stacks of two SiCx layers were deposited by plasma enhanced chemical vapor deposition: first a thin silicon rich layer with excellent passivating properties and then an antireflective carbon rich layer. The thickness of the passivating layer was optimized, reaching a trade-off between the better passivation achieved for thicker layers and the increased light absorption within the layer, which reduced the photocurrent. The surface recombination velocity and the optical losses were determined for each configuration and used to calculate photovoltaic conversion efficiency limits for 50 and 90Ω∕sq emitters. In both cases, optimum configuration is for the stacks with passivating layers that are ...

Analysis of Series Resistance Losses in a-Si:H/c-Si Heterojunction Solar Cells

We present an experimental method to quantify the series resistance R_a-Si/ITO through the a-Si:H layers and the a-Si:H/ITO interface on test structures. In order to optimize R_a-Si/ITO, we apply different a-Si:H and ITO deposition parameters. We find the best value for R_(p)-a-Si/ITO of 0.42 Ω·cm² for an ITO double layer with a 10-nm-thin starting layer that provides good contact resistance and an additional 90-nm top layer that provides good conductivity. For R_(n)-a-Si/ITO, we reach values below 0.1 Ω·cm². We present an analysis of the series resistance and shading losses of our 100-cm² bifacial screen-printed a-Si:H/cSi heterojunction solar cells, which show an open-circuit voltage of V_oc = 733 mV, demonstrating the excellent level of interface passivation. The efficiency of 20.2% is limited by a low short-circuit current density of 37.1 mA/cm² and fill factor of 76%.

Improvements in the Passivation of P+-Si Surfaces by PECVD Silicon Carbide Films

We present further results of a surface passivation study of p⁺-Si emitters by both intrinsic and boron-doped amorphous SiC_x films, deposited in two different standard PECVD reactors. For comparison, thermally grown SiO₂ and PECVD-SiN _x layers with refractive indices of n=2.0 and n=2.4 were examined on the same test structures. While thermal SiO₂ exhibits passivating properties comparable to those on n⁺-Si emitters, PECVD-SiN_x is found to even deteriorate the surface passivation, especially after firing (without metal contacts). On the other hand, PECVD-SiC_x yields, to our knowledge, the best p ⁺-Si passivation so far obtained by an industrially relevant low temperature process. It is expressed by an implied V_oc of 635 mV for a symmetrically 60 Omega/sq BBr₃-diffused n-type CZ-wafer with a base resistivity of 4.6 Omegacm

c-Si surface passivation for photovoltaic applications by means of antireflective amorphous silicon carbide layers

Surface passivation of p-type silicon wafers was performed by amorphous silicon carbide films (SiCx) deposited by plasma enhanced chemical vapor deposition (PECVD). Stacks of two different SiCx layers were applied. The inner layer was rich in silicon and offered good passivation properties. The outer layer was a carbon rich, antireflective coating. Anneals in forming gas were performed to improve surface passivation. Simulation of lifetime measurements indicated the nature of the passivating mechanism. Finally, optical constants were determined by ellipsometry measurements.

Macroporous silicon as an absorber for thin heterojunction solar cells

Meso- and macroporous silicon is widely studied for several applications in photovoltaic devices. We investigate macroporous silicon as an absorber for thin-film silicon solar cells. Here we review the progress of our work. We demonstrate the separation of 85 × 85 mm2-sized macroporous silicon layer from a monocrystalline n-type Cz silicon wafer. The measured optical absorption of a 26 μm thick macroporous silicon layer allows for a short-circuit current density of 37.6 mA cm-2. We measure an effective carrier lifetime of up to (38.8 ± 3.9) μs for (33 ± 2) μm thick surface passivated macroporous silicon layer. We use an analytical model to determine average surface recombination velocity S= (10 ± 2) cm s-1 for the MacPSi layer. We prepare macroporous silicon heterojunction solar cells with an energy-conversion efficiency of 7.2 %.

Simultaneous gettering and emitter formation in multicrystalline-Si wafers by annealing phosphorus doped amorphous silicon compounds

We demonstrate gettering of metal impurities in p-type multicrystalline silicon (mc-Si) wafers by annealing wafers that are surface passivated by double layers of amorphous silicon-based compounds acting as gettering sites. As inner layer we use a phosphorus-doped amorphous silicon-carbon-nitrogen alloy, providing surface passivation and acting as dopant source for the emitter formation during subsequent anneal. The outer layer is silicon nitride with antireflective properties. Anneals are done at 750, 800, and 850 °C for 30 and 60 min. The gettering effect is as good as for a conventional POCl3 diffusion followed by extended gettering at low temperature, and it is weakly influenced by the temperature step chosen. In the range explored, the sheet resistances of the emitters and the junction depths lay between 3000 to 60 Ω/sq. and 100–300 nm, respectively.

Investigation of the Surface Passivation of P+-Type Si Emitters by PECVD Silicon Carbide Films

The surface passivation of an industrial applicable (e.g. screen printing) 60 Omegasq/ p+ boron emitter on Cz n-type c-Si wafers by amorphous SiCx films is investigated. A partial optimization of the deposition conditions of the SiCx films was performed resulting in an improved passivation quality of the SiCx films which also serves as anti-reflection coating. Passivation quality is determined by measuring the injection level dependency of the effective minority carrier lifetime using the quasi-steady state photoconductance method. Combining the information form the boron diffusion profile measured by SIMS and injection level dependent lifetime curves it can be concluded that a boron depletion layer formed during in-situ drive-in and oxidation has a detrimental effect on the passivation quality. Changing the diffusion conditions to prepare an improved boron profile with a similar sheet resistance should result in a further improvement of the passivation quality. This could include a reduction or a removal of the boron depletion layer at the emitter surface

Influence of RF power on c-Si surface passivation by amorphous a-SiC/sub x/:H layers deposited by PECVD

Surface passivation of p-type crystalline silicon (c-Si) wafers has been achieved by depositing phosphorus-doped amorphous silicon carbide films (a-SiC/sub x/:H) in a PECVD reactor. We explored the dependence of effective surface recombination velocity (S/sub eff/) on the RF power supplied to the reactor during film deposition. The effective lifetime (/spl tau//sub eff/) as a function of excess carrier concentration (/spl Delta/n) was measured by quasi steady state-photoconductance (QSS-PC) technique in order to extract the surface recombination rate. We fitted the experimental /spl tau/eff(/spl Delta/n) curve by an insulator/semiconductor model resulting in the determination of the fixed charge density at the interface (Q/sub f/) and the fundamental recombination velocity of holes (S/sub po/). Additional experiments to determine the uniformity of the films along the PECVD reactor plate were carried out.

Characterization of industrial p-type CZ silicon wafers passivated with a-SiC/sub x/:H films

In this work we investigate the impact of surface passivation on the efficiency of solar cells produced on industrial p-type Czochralsky(CZ)-grown crystalline silicon wafers as those used in the standard fabrication process of ISOFOTON. The investigation has two objectives. The first objective is to estimate the bulk minority carrier lifetime of the CZ silicon material. For this purpose the effective minority carrier lifetime of silicon wafers with different thicknesses is determined using the quasi steady-state photoconductance (QSS-PC) method. To eliminate the effect of surface recombination, amorphous silicon carbide films are deposited by plasma enhanced chemical vapour deposition (PECVD) on both wafer surfaces. The second objective is to find out how solar cell parameters like open circuit voltage and efficiency are modified when simultaneously to a wafer thickness reduction an effective back surface passivation is applied. This is done through the analysis of injection level dependent lifetime data together with PC-ID simulation.