Comparison of MWA/MWT solar cells with conventional BCSCs for different substrate sizes 95

The experimental results in the previous section indicated that MWA/MWT solar cells can be considered as conventional BCSCs with deviations in resistive as well as shadowing losses. In this section, calculations are carried out by the definition of a “normalised”

efficiency which considers only these two deviations. The normalised efficiency ηn is given by

ηshad is due to the shadowing losses of the front metallisation and M is the metallised fraction of the front surface. ηRs determines the reduction in fill factor due to the series resistance. The influence of Rs on the fill factor is given by [Gre82]:

R )

Rchar is given by Voc/Jsc, which does not significantly depend on the actual solar cell parameter and a value of 17.6 Ωcm² was taken. The normalised efficiency will be equal to one for negligible shading (M=0) as well as resistive (Rs=0) losses.

As parameters for the calculations, a line resistance of the finger metallisation of 500 mΩ/cm and a groove width of 30 µm were taken which are standard values of conventional BCSCs. For the calculations of the series resistance contributions of the emitter, busbar and the finger metallisation were considered. For the busbars of the conventional cells, a standard thickness for the Cu ribbons of 125 µm was taken [Rob00b], whereas the width was 1.5 mm (2.0mm, 2.5 mm) for 10x10 cm² (12.5x12.5 cm², 15x15 cm²) solar cells. Thicker ribbons are difficult to use, since the stress on the wafer edges causes fracture problems during module fabrication. For the back contact solar cells it was assumed that the interconnection on the rear side leads to negligible resistive losses since thicker ribbons or adapted interconnection schemes can be used. For the conventional and MWT cells two busbars were considered independent of the wafer size. For each device design and cell area, the optimum finger spacing was determined and Remitter, Rfinger and the shadowing losses M were calculated. For MWA solar cells a design with wrap around contacts at all four wafer edges was assumed leading to no area losses due to edge isolation.

These cells have to be cut into two halves for cell interconnection (see section 5). For MWT and conventional solar cells 1 mm is removed during edge isolation and cleaving [Brut94]

which is also included in the calculations. The results of the calculations are illustrated in Figure 4.8.

10.0 12.5 15.0 0.90

0.91 0.92 0.93 0.94 0.95

0.96 three busbars

two busbars conventional

MWA MWT

normalised efficiency η n

cell size [cm]

Figure 4.8: Calculated normalised efficiency for different cell sizes of conventional, MWA and MWT solar cells.

The calculations demonstrate that the back contact devices are superior to conventional solar cells independent of the investigated substrate sizes. For a cell area of 10x10 cm², the MWA and MWT solar cell design lead to a comparable performance. For larger cell areas the MWT cells reaches the highest normalised efficiency. For conventional and MWA solar cells, the efficiency decreases substantially with increasing substrate sizes, whereas the influence of an increase in solar cell area is only minor for MWT solar cells. For MWT cells with a cell area of 15x15 cm², an increase in efficiency can be achieved by an additional busbar. The calculations demonstrate that the gain in efficiency for the best scenario will be between 4 and 5% compared to conventional BCSCs.

In this section it was demonstrated, that MWA and MWT solar cells lead to higher efficiencies as conventional solar cells. The choice for industrial production will mainly depend on the technique applied for contact groove formation. If contact grooves are formed by mechanical abrasion, the MWA solar cell design is favourable, since it can be fabricated without laser processing. Applying laser ablation for contact groove formation, the holes for MWT solar cells can be formed during groove formation by stopping the laser beam. Also the interconnection of MWA cells seems easier (see section 5), which might favour this design for industrial manufacturing.

4.4 Emitter Wrap Through Solar Cells

In the first section the solar cell design of Buried Contact EWT solar cells is described as it was investigated in this work. The second section deals with the manufacturing of EWT solar cells and two types of conventional cells which were processed for comparison. A detailed analysis of the three device designs investigates the differences and leads to a further understanding of EWT cells. The work on BC-EWT solar cells was motivated by the development of a device design without metallisation on the front surface which is important for some applications of photovoltaic requiring a very high optical appearance.

Additionally this design leads to high currents for crystalline silicon materials with a low ratio of bulk diffusion length to cell thickness.

4.4.1 Solar cell description

The principle device design of EWT solar cells was illustrated in Figure 4.2. The current collected in the front side emitter is conducted through laser drilled, diffused and metallised vias to the rear side emitter contact. The rear surface consists of an interdigitated pattern of p- and n-contacts. The specific device design of this work (see Figure 4.9) includes a selective emitter structure. The front surface is coated with LPCVD SiNx for front surface passivation and ARC. Also the rear side is covered by LPCVD SiNx which is opened locally for the n- and p-type contacts. An Al-BSF is formed within the p-type grooves for surface passivation. An emitter is diffused within the n-type contact grooves with a coverage of about 20%. This additional p/n-junction on the rear also collects minority charge carriers leading to an increase in Jsc.

plated Ni/Cu n⁺⁺

n⁺ p⁺

LPCVD SiN_x

p

Laser drilled vias

LPCVD SiN_x

Figure 4.9: Schematic drawing of an Emitter Wrap Through (EWT) solar cell with buried contact metallisation. The current collected in the front side emitter is conducted through diffused and metallised holes to the n-type contact finger on the rear. Front and rear surface are coated with LPCVD SiN_x, which is locally opened by laser ablation for the p- and n-contacts at the rear surface.