Thomas Buck

Large Area Screen Printed N-Type Silicon Solar Cells with Rear Aluminium Emitter: Efficiencies Exceeding 16%

We present low cost, screen printed industrial solar cells with a rear Al-emitter for n-type mono- and mc-Si large area thin wafers showing efficiencies of 16.4% and 14.4% respectively. The gap of 2% absolute between the cell efficiencies for mono- and mc-Si is not material related but is largely due to the non-textured surface of the mc-Si substrate. Applying a surface texture by acidic wet chemical etching could lead to efficiencies above 15% on the mc-Si material. The process is based on the standard industrial p-type firing through solar cell concept with slight modifications in individual processing steps similar to the Phostop cell presented by Ebara Solar. There is still need for further optimisation before industrialisation for instance in the fields of wafer bowing reduction and the implementation of AgAl pads for wafer interconnection. We propose two different ways of pad printing-one directly to the Al-emitter, the other on top of the Al rear contact

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

Large Area Screen Printed N-Type MC-SI Solar cells With B-Emitter: Efficiencies Close to 15% and Innovative Module Interconnection

In this paper we present n-type Si solar cells on large area mc-Si wafers with a boron diffused emitter at the front side. The focus of our studies is mainly related to the front surface of the solar cell. We have optimised BBr3-diffusion and in-situ oxidation with respect to the homogeneity of the sheet resistance and substrate degradation. After diffusion even a slight improvement of the minority charge carrier lifetime was measured, which can be related to B-gettering. The emitter is contacted by AgAl-paste and passivated by thermal SiO2. The development and optimisation of all processes led to solar cells with efficiencies of 14.7% on mc-Si and 17.1% on Cz-Si substrates. In addition to this we present an innovative interconnection of modules using our developed cell (patent pending). We show an alternate serial interconnection of p- and n-type solar cells resulting in easier module processing

N-type multicrystalline silicon: material for solar cell processes with high efficiency potential

We present the characterisation of directionally solidified n-type Si ingots. Three ingots with a range of bulk resistivities and different n-type doping elements (Sb, P and As) were studied. We show from Hall measurements that the mc-Si material has excellent electrical transport properties. The mobilities are close to the theoretical limit, which is given mainly by scattering at acoustical phonons. Mobilities so close to the theoretical value have, to our knowledge, not been demonstrated for comparable p-type mc-Si wafers. Additional measurements on high quality p-type mc-Si material support this statement. This means that other scattering mechanisms reduce the mobility in p-type mc-Si material, but are not present in n-type silicon. Lifetime measurements were conducted by /spl mu/W-PCD using an iodine-ethanol surface passivation. This passivation was used preferably to SiN/sub x/, as in some experiments the hydrogen from the PECVD SiN seemed to passivate the bulk at deposition temperatures. Average values in excess of 120 /spl mu/s over large areas were measured. In order to exploit the good material properties of n-type mc-Si, solar cell concepts must be developed and the processes optimised. B-diffusion is the most problematic step as it is considered to be both destructive to material quality and energy consuming. In this paper, we show that a BBr/sub 3/-emitter diffusion is possible at moderate temperatures without degrading the carrier lifetime of the mc-Si material. An additional contribution from Libal et al. on solar cell processing is included in this conference.

Realization of Thin MC-Silicon Pert-Type Bifacial Solar Cells in Industrial Environments

We present bifacial solar cells processed with a sequence suitable for industrial production. This method uses the LPCVD silicon nitride deposition based on DCS (dichlorosilane) or BTBAS (bis-(tertiary butyl amino)-silane). The bifacial solar cell process on wafers of 200 mum thickness has the following steps: (1) boron doped BSF of Rsheet =60 ohm/sq; (2) POCl3 emitter (on front side) of Rsheet= 50-55 ohm/sq; (3) thermal oxidation of the wafer surfaces; (4) the deposition of DCS or BTBAS based LPCVD silicon nitride on either sides of the wafer (5); finger grid printing on both sides and firing; (6) edge isolation. The solar cells produced with the DCS based silicon nitride process exhibit fill factor (FF) values of 76% on p-type and 75% on n-type solar cells with a rear to front efficiency ratio etarear/etafront of 67% for the p-type solar cells and 43% for the n-type solar cells. The solar cells with the BTBAS silicon nitride show FF values close to 72% and etarear/etafront 68% for p-type solar cells