To estimate the effect on solar cell efficiency, we chose the emitters with 50 Ω/sq and 90 Ω/sq from the whole series of wafers. Using PC1D software, we calculate the limits for the open circuit voltage (Voc), the short circuit current (Jsc) and the cell efficiency (η) operating at 1 sun.
Bulk resistivity and thickness were the same as employed in the experimental procedure (0.95 Ω cm, 300 µm). Also, the emitter profiles were taken from the SUPREM simulations. To account for the optical losses calculated in the previous section, four different input files (for the four different passivation schemes) were built containing the effective reflectivity, i.e. considering the absorption in the PAS layer. For the electrical losses we consider Auger recombination as the only recombination process within the bulk, and perfect surface passivation in the rear side (SRV = 0). For the front side we employed SRV = 106, 105, 104 and 104 cm s-1 for 0, 30, 60 and 90 s of PAS layer respectively, which are values derived from Fig 4.13. We assumed values of 10-4 Ω for
0 4 8 12 610 620 630 640 650 660 670 17 18 19 32 33 34 Vo c ( m V )
passivating layer (PAS) thickness (nm)
η limit with SRV = 0 Jsc η ( % ) Jsc ( m A c m -2 ) η Voc 50 Ω/sq emitter
Figure 4.16. Theoretical limits of Voc, Jsc and η for 50 Ω/sq emitters under
standard test conditions (AM 1.5G, 300K), calculated from the J0e and SRV
values. The maximum efficiency of 18.4% is achieved for the PAS layer 8 nm thick. The limit for the efficiency with no surface recombination is also plotted.
0 4 8 12 610 620 630 640 650 660 670 17 18 19 32 33 34 Vo c ( m V )
passivating layer (PAS) thickness (nm)
η limit with SRV = 0 Jsc η ( % ) Jsc ( m A c m -2 ) η Voc 90 Ω/sq emitter
Figure 4.17. Theoretical limits of Voc, Jsc and η for 90 Ω/sq emitters under
standard test conditions (AM 1.5G, 300K), calculated from the J0e and SRV
values. The maximum efficiency of 18.9% is achieved for the PAS layer 8 nm thick. The limit for the efficiency with no surface recombination is also plotted.
both emitter and base contacts, and the shunt resistance was considered to be infinite. Finally, the I-V curves of the structures were simulated under standard test conditions (AM 1.5G, 300 K).
Figure 4.16 and 4.17 show the calculated results for Voc, Jsc and η as a function of the nominal PAS layer thickness, showing that both emitter structures behave in the same way. As the PAS layer thickness increases the Voc increases and reaches saturation. The trend for the Jsc is, as expected, the opposite. An optimum point is found for 8 nm of PAS layer thickness, whose corresponding efficiencies are 18.4 and 18.9% for the emitters
with 50 Ω/sq and 90 Ω/sq respectively. To make clear the good emitter passivation achievable we also plot the limits for the efficiency considering a perfect surface passivation in the front side (SRV = 0) independently of the structure used to passivate. In the ideal case (no absorption in the passivating layer and no surface recombination) the maximum efficiencies achieved would be 19.2 and 19.8% for the emitters with 50 Ω/sq and 90 Ω/sq respectively. Clearly, the passivation obtained by the SiCx stacks is excellent and the efficiency obtained in our case study is mostly limited by low Jsc due to the nonexistence of a texturing treatment.
4.6 Chapter conclusions
Surface passivation by silicon carbide stacks has been analyzed by varying the thickness of the inner layer (PAS). The study has been performed on low resistivity
p-type wafers, suitable for solar cells performance, and pre-diffused n+-type emitters,
without requiring a further drive-in step.
The use of a carbon rich (ARC) led to several benefits. Firstly, the transparency and antireflective properties provided by a suitable refractive index (n ≈ 2) are pre-requisites when applied at the front side of solar cells. For the rear side passivation this also helps to enhance internal reflection towards the bulk silicon. Secondly, the high content of hydrogen available in these films allows further improvement in surface passivation after forming gas anneal. Thirdly, it presents better stability than thin Si rich samples in front of long forming gas anneals as it may act as a hydrogen barrier. Lastly, together with the Si rich film it provides good electronic conditions when applied in PERC structures, as the stack prevents from shunting effects and is well behaved with the metal work function. This helped to achieve solar cells with a-SiC rear side passivation above 20% in efficiency.
However, the use of the passivating film (PAS), with undesired absorption of light, is still required to provide good surface passivation, either in p-type bases or in n+-type emitters. Si rich layers thicker than 8 nm prevent from degradation of the surface passivation quality due to de-hydrogenation of the a-SiC/c-Si interface. This thickness is precisely the optimum for the passivation of n+-type emitters at the front side, since it reaches a trade-off between surface passivation and optical losses.
CHAPTER 5
Phosphorus doped silicon carbon
nitrogen alloys, SiCN(n)
5.1 Introduction
In Chapters 3 and 4 we have shown the applicability of phosphorus doped silicon carbide films to perform surface passivation of crystalline silicon. It was already shown in a previous work that the introduction of phosphorus represents a significant improvement respect to intrinsic films [32]. We have been able to resolve partially the absorptive behaviour of the passivating films by reducing their thickness and complete the passivation with antireflective layers. However, to provide enough amount of hydrogen this antireflective film had to be grown at a lower temperature than the passivating film. A process like this, involving two different temperature processes, is clearly far from the objectives of simplicity and robustness claimed for mass production of silicon solar cells. Furthermore, it has been found that when the passivating films are 4 nm or thinner the
surface passivation degrades after a period of time at room temperature. Therefore, it would be preferable to find new growing conditions that ensure good surface passivation, antireflective properties and stability.
In the present Chapter, these new growing conditions are focused on the introduction of nitrogen to the silicon carbide material. It has already been demonstrated that intrinsic carbon nitrogen alloys a-SiCxNy(i) represent a further improvement compared to a-SiC(i) for the passivation of n-type wafers [148], reaching values of Seff as low as 16 cm s-1. With the aim combining the benefits of nitrogen and phosphorus, we study phosphorus- doped hydrogenated-amorphous silicon carbon nitrogen alloys, a-SiCxNy:H(n) as a new option for surface passivation. Hence, it represents the mixing between silicon nitride and silicon carbide.
The introduction of a film with five elements (silicon, carbon, nitrogen, phosphorus and hydrogen) could be regarded as complicated and therefore useless for the photovoltaic industry, which is in the search of low-cost and simplified processes for mass production, especially when the silicon nitride has demonstrated excellent capabilities. However, some different properties between silicon nitride and silicon carbide (or silicon carbon nitrogen alloys) can be of interest in certain fabrication processes. Firstly, silicon nitride provides a strong field effect passivation mainly due to a high positive charge that is calculated to be around 2.5 × 1012 cm-2 [99], while in silicon carbide it appears to be one order of magnitude lower (see Chapter 3 or reference [149]). A second property is that silicon nitride can be etched in HF, while silicon carbide cannot. A third issue concerns the introduction of phosphorus in the film. It is interesting since it could serve as an emitter layer in a HIT structure, with the benefit of more transparency achieved thanks to the carbon/nitrogen incorporation, thus avoiding loss in photocurrent. Finally, a recovery of surface passivation after long time anneals at high temperature has been recently observed after initial strong degradation at short annealing times [150] (see Chapter 6). Another interesting property of nitrogen in SiC systems is that it acts as a shallow donor impurity of microcrystalline silicon carbide phases [151]. This concept has been used to fabricate n-i-p amorphous silicon solar cells with a µc-SiC
n-type region acting as open window and hence allowing a higher short circuit currents
than in typically amorphous silicon phases [152] . All this issues make interesting the study of such ternary alloys.
With the aim to investigate the applicability of a-SiCxNy:H(n) films to all kind of concepts of solar cells, the surface passivation is tested on low resistivity p-type and n- type silicon wafers, as well as on n-type emitters. It would be very interesting to study also their possibilities in p-type emitters. However, the technical opportunities of our laboratory kept this option unavailable at the moment of performing this work. Variation of the composition from silicon rich (highly absorptive) to carbon or nitrogen rich (highly transparent) films is performed to find optimum surface passivation. As in Chapter 4, stacks of different compositions are applied to combine excellent passivation and good antireflective properties.