Emitter saturation current

The open-circuit voltage of an ideal solar cell (one-diode model) is determined by the saturation current density which is the sum of the saturation current densities stemming from the bulk (including back surface recombination) and the emitter, denoted as and , respectively. is given by [45, 48]

1 . (3.2)

In order to improve it is worth knowing which factor of and actually dominates and hence limits . Furthermore, recalling the high emitter near surface concentrations 10²⁰ cm^-3 (and the resulting high Auger recombination) obtained with the P509 as well as with the P507, it has to be assessed whether the emitters actually benefit from surface passivation, e.g. such as the one provided by growth of a thin RTO oxide.

Experimental

was determined by microwave reflectance photocurrent decay measurements (MW-PCD) of double-sided diffused p-type wafers. For the measurements, the emitters featured no metallic contacts. In this floating n⁺pn⁺-structure the n⁺p-junctions act for the electrons of the base region as a surface with an effective surface recombination velocity . According to the quasi-static emitter approximation [75], the charge carrier decay after a laser pulse is determined by the recombination of bulk electrons whose lifetime is much higher than that of the holes in the emitter. As a prerequisite for the validity of the assumption, the measurements have to be performed under low level injection conditions. In general, this implies the use of p-type Si with a base doping 10¹⁶ cm^-3. For the determination of , the effective lifetime is measured by MW-PCD and is calculated from this value and the values for the bulk lifetime , the electron diffusion constant and the wafer bulk thickness . Under the assumption of equally passivated front and back surfaces it holds [159]

1 1

(3.3) with found from the relationship

2 . (3.4)

Assuming that the quasi-static emitter approximation holds and that all space charge region effects as well as emitter shunting can be neglected, is given by [75]

2 , (3.5)

where is the elementary charge and the intrinsic carrier concentration which depends strongly on temperature. At 25 C, = 8.6 10⁹cm^-3[173].

For the determination of , virgin, bright-etched FZ-Si wafers (5 5 cm², 250 m) were used. Please note that for the investigations with the P509 wafers with a specific resistivity of 1.25 cm ( = 1.2 10¹⁶ cm^-3) were used whereas for the P507, wafer resistivity was 0.77 cm ( = 2.0 10¹⁶cm^-3). This is important for the interpretation of the measured according to Eq. 3.5.

After a standard RCA clean, the respective SOD was deposited double-sided and RTD was carried out. For the highly concentrated P509, RTD was performed at 875 C and 900 C, whereas the medium concentrated P507 was diffused at 975 C. In both cases, the diffusion time was varied in a way to obtain emitters with ranging from 40 to 120 /sq and similar due to the same applied. After removal of the PSG in HF, RTO was carried out at 950 C for 30 s in case of the P509 and for 40 s in case of the P507, yielding an approximately 10 nm thick oxide on the n -emitter. After a forming gas anneal ( 400 C, 30 min) similar to the one applied in the solar cell fabrication sequence, representing the passivated emitter structure was determined by MW-PCD. Afterwards, in order to actually see the influence of the surface passivation, the oxide was etched off in HF and was determined again.

Regarding the MW-PCD measurements it has to be noted that all measurements were per-formed under a constant bias-light intensity of 0.5 sun (50 mWcm^-2). The laser pulse intensity was adjusted as low as possible in order to cause just a small perturbation of the background injection level provided by the bias-light. This means, that actually differential values were measured instead of absolute values [1]. In first approximation, depends linearly on injection. Thus, the absolute at 1 sun illumination, i.e. at the injection condition present in a solar cell under open-circuit condition, corresponds to under the 0.5 sun illumination at which the PCD measurements were performed (see Fig. 3.17). For the determination of an infinite was assumed which means that the calculated values represent an upper limit.

Results and discussion

Fig. 3.9 shows the variation of as a function of for the P509. After RTO and subsequent FGA at 400 C, increases by almost one order in magnitude as

decreases from 120 to 40 /sq. This observation can be explained by the fact that the emitter depth increases alongside the decreasing , causing enhanced doping-related Auger recombination in the emitter bulk. At some point, the overall emitter recombination is dominated by recombination in the bulk and emitter surface recombination becomes negligible, i.e. the emitter becomes insensitive to surface passivation. For the P509 this seems to be the case for below around 50 to 60 /sq. Consequently, after removal of the oxide, the -curves for the passivated and the unpassivated emitter coincide for such low . However, for in the range of 80-90 /sq, as typically used for evaporated front contacts, the emitter is definitely sensitive to surface passivation. Despite the very high carrier surface concentration of 3 to 4 10²⁰ cm^-3(see Fig 3.6 and Fig. 3.8), the oxide passivation improves by almost one order in magnitude. This improvement reflects a decrease of at 25 C from 1 10^-12 down to 2-3 10^-13Acm^-2for a 80 to 90 /sq emitter. Taking these values one can calculate

40 60 80 100 120 10²

10³

SOD P509 RTO + FGA oxide removed

S

[c m /s ]

R

[

/sq]

40 60 80 100 120

10^-13 10^-12

J

[ A /c m

]

R

[

/sq]

Fig. 3.9: as a function of for the SOD P509. RTD was carried out at 875 C or 900 C with varying . The measurements were performed after RTO (950 C, 30 s) followed by FGA and after removal of the oxide in HF. is calculated for 25 C. Solid lines are guides to the eye.

40 60 80 100 120

10² 10³ 10⁴

SOD P507 RTO + FGA oxide removed

S

[c m /s ]

R

[

/sq]

40 60 80 100 120

10^-13 10^-12

J

[ A /c m

]

R

[

/sq]

Fig. 3.10: as a function of for the SOD P507. RTD was carried out at 975 C with varying . The measurements were carried out after RTO (950 C, 40 s) followed by FGA and after removal of the oxide. was calculated for 25 C. Solid lines are guides to the eye.

the maximum achievable under the assumption that = 0 and = 44 mA/cm^-2 [45].

Using Eq. 3.2 one gets a maximum of 625 mV for the unpassivated and approximately 660 mV for the passivated 80 to 90 /sq emitter, respectively.

In Fig. 3.10, is shown as a function of when the P507 SOD is used. In the passivated state, follows the same trend as discussed above for the P509. But, in contrast to the P509, the beneficial effect of surface passivation vanishes only for 40 /sq. This can be attributed to the lower electron surface concentration of about 1-2 10²⁰cm^-3after RTO when the P507 is used compared to 3-4 10²⁰ cm^-3 obtained with the P509 (see Fig. 3.8). In addition, for the unpassivated emitter, increases with increasing . Hence, it is crucial to provide surface passivation if emitters with of 80 to 90 /sq are used. In terms of , a decrease from 1.5 10^-12Acm^-2down to around 1 10^-13Acm^-2is achieved by RTO passivation, restricting the maximum achievable to 609 and 678 mV, respectively.

One has to keep in mind that, when different back surface passivation schemes are to be assessed by the preparation of solar cells, must not be limited by which in turn implies the use of passivated emitters for such investigations.