Plating of metal front contacts

Other than the point contacts applied to the rear side of the cell within the PassDop approach, the SE-PassDop process requires line contacts to form a contact grid. This is accomplished by accurately adjusting the scanner system of the laser to obtain over-lapping laser pulses on the sample surface (Fig. 7-10 a and b). The resulting line width therefore depends on the diameter of a single laser point. The latter was found to in-crease linearly with the applied laser power in the low power regime investigated (Fig.

7-9).

The preferable metallization technique in combination with the SE-PassDop ap-proach consists of a two-step process: a seed layer forming a mechanical and an elec-trical contact to the emitter surface (at the laser processed regions) is deposited by chemical nickel plating [195]. A high lateral conductivity is then assured by thickening of the seed layer through light-induced silver or copper plating (LIP) [196]. Experi-mental details regarding the two-step metallization process used within this work can be found e.g. in [197].

SE-PassDop-prepared surfaces prior and after nickel plating on shiny etched and textured (random pyramids) surfaces are depicted in Fig. 7-10. It becomes apparent that the laser power and the pulse overlap have to be adjusted with regard to a continu-ous seed layer/silver finger depending on the surface condition. Finally, the resulting finger width depends on the height of the plated silver since the metal growth happens to be isotropic.

Fig. 7-9: Linear relation between the applied laser power and the diameter of the resulting laser point on shiny etched surfaces (in the given power regime). The hatched bar refers to the threshold of silicon melting and hence to the onset of laser doping.

136 Amorphous silicon carbide for the solar cell front side

Fig. 7-10: Laser opened (molten) lines on shiny etched surfaces (images a and b) and on surfaces featuring random pyramids (images c and d). The samples were coated by an amorphous silicon carbide passivation and anti-reflection scheme. SEM images a) and c) show the laser processed surfaces prior, images b) and d) after nickel plating. Note the increased magnification of image b).

7.5 Solar cell results

First solar cells featuring the PassDop rear (chapter 6.2) and the SE-PassDop front side approach (chapter 7.4) for the passivation and contacting of respective surfaces were fabricated. The sketch of the process sequence is depicted in Fig. 7-11. The first step consists in the front side texture (random pyramids) of the wafers (p-Fz, 0.5 Ωcm). Next the emitter formation of the front side is performed resulting in a phosphorous emitter featuring a doping profile as illustrated in Fig. 7-1 and a sheet resistance of approximately 120 Ω/sq. All subsequent PECVD depositions were then performed in the AK400M reactor. The rear sides were coated by a layer stack consist-ing of an intrinsic, a p-doped Si-rich a-Si1-xCx and a C-rich a-SiyC1-y film. For the front side a layer system consisting of an intrinsic (5 nm), a n-doped Si-rich a-Si_1-xC_x (5 nm) and a C-rich a-SiyC1-y (70 nm) film was applied. The approaches for the front and rear side therefore basically only differ in the doping polarity of the intermediate layer and in the film thicknesses. It has to be mentioned that the optimized anti-reflection layer

Amorphous silicon carbide for the solar cell front side 137

on the basis of C-rich a-SiyC1-y presented in section 7.3 was only developed later on.

The C-rich film used as ARC for the fabricated solar cells therefore still exhibits a strongly elevated absorption for wavelengths up to 600 nm. After PECVD, the contact areas on the rear (point contacts) and on the front (line contacts) are opened allowing for additional laser doping from the doped (SE-)PassDop stacks (formation of local back surface field and selective emitter). The rear metallization is performed by evapo-rated aluminium (e-gun process) and finally the front side is metallized by nickel plating (and sintering for the formation of nickel silicide) and subsequent thickening of the front grid by silver plating (LIP).

The solar cell results are summarized in Table 7-1 and the spectral response meas-urements of the best cell of the respective categories (shiny etched or textured front side) are given in Fig. 7-12. Since all cells feature the same rear side in terms of elec-trical performance (as evidenced by the IQE data at long wavelengths in Fig. 7-12), the measured open-circuit voltages are a direct measure for the level of front surface pas-sivation. Voltages of up to 672 mV point to a very effective suppression of front sur-face recombination by the amorphous silicon carbide stack on shiny etched sursur-faces.

On the other hand, the level of passivation on random pyramid textured surfaces is poor (Voc = 623 mV) which was already anticipated from the results on the lifetime

Fig. 7-11: Sketch of the process sequence for the fabrication of all-SiC passivated solar cells. The opening of the contact area on the rear as well as on the front side is performed during a laser process (PassDop approach).

138 Amorphous silicon carbide for the solar cell front side

level (section 7.2). The short-circuit current density is very low for all cells which is attributed to the strong absorption in the front layer stack (blue area in Fig. 7-12). The little gain in current (approx. 1 mA/cm²) for the textured as compared to the planar cells is addressed to a non-optimized ARC thickness for the pyramid texture and to the strong carrier recombination at the front side. Furthermore, the texture itself revealed to be poor, exhibiting a relatively large area of planar (non-textured) areas. Assuming identical absorption losses at the front side of the planar and the textured cells, the difference in IQE in the wavelength region 300-700 nm can be assigned to the recom-binative loss at the textured surface (yellow area in Fig. 7-12 right). As to the pseudo fill-factors (pFF) measured by the Suns-Voc technique (chapter 3.6) at the finished solar cells, the high values encountered for the planar cells evidence no laser induced damage in the space charge region (SCR) of the front emitter. Slightly reduced pFFs for the textured cells may point to minor damage in the SCR due to the interaction of the laser light with the pyramid edges. The fill factors from the illuminated IV-measurements of up to 79 % strongly support the excellent performance of the front and rear contacting scheme. The measured series resistances between 1-2 Ωcm² (from fitting of the dark IV curve) evidence that the fill factor loss is due to series resistance.

The finger width of the front grid was measured to be 55-60 µm.

The impact of the doped intermediate layer was investigated in another solar cell batch. On cells featuring no additional doping source on the frontside (10 nm of

intrin-Table 7-1: One-sun parameters (aperture area of 4 cm²) of p-type cells featuring amorphous silicon carbide films on the rear as well as on the front side. Passivating and opti-cal properties are provided by layer stacks and rear and front contacting are per-formed on the basis of laser processes (PassDop/SE-PassDop approach).

Voc(mV) Jsc(mA/cm²) ^*)pFF (%) FF (%) η ( %)

670±2 32.4±0.3 83.1±0.1 75.7±2.4 16.4±0.6 random

619±3 33.4±0.6 81.8±0.2 76.5±2.8 15.8±0.6

*) measured by the Suns-Voc technique

Amorphous silicon carbide for the solar cell front side 139

sic a-Si1-xCx), comparably high fill factors can only be reached by increasing the sinter temperature of the front contact. However, this often goes along with a decrease in passivation quality. The cells with intermediate doped layers furthermore exhibit open-circuit voltages constantly exceeding the ones from their intrinsic counterparts by about 10 mV. The latter may be attributed to an increased level of surface passivation of the intrinsic/doped a-Si1-xCx layer system and possibly to a decreased recombination at the contacts due to a locally increased doping level (selective emitter formation by laser doping). Although the results do not allow for a final differentiation of its impact, the incorporation of a thin doped a-Si1-xCx film in the front side passivation scheme certainly turns out to be beneficial for the device performance.