Investigation of various surface passivation schemes for silicon solar cells

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Abstract

In this work, we have investigated three different surface passivation technologies: classical thermal oxidation (CTO), rapid thermal oxidation (RTO) and silicon nitride by plasma enhanced chemical vapor deposition (PECVD). Eight different passivation properties including SiO2/SiNx stacks on phosphorus diffused (100 and 40 Ω/Sq) and non-diffused 1 Ω cm FZ silicon were compared. Both types of SiO2 layers, CTO and RTO, yield a higher effective lifetime on the emitter surface than on the non-diffused surface. For the SiNx layers the situation is reverted. On the other hand, with SiO2/SiNx stacks high lifetimes are obtained not only non-diffused surface but also on the diffused surface. Thus, we have chosen the RTO/SiNx stack layers as front and rear surface passivation in solar cells, which passivate relatively good on the surface and has very low-weighted reflection. On planar cells passivated with RTO/SiNx a very high Voc of 675.6 mV and a Jsc of 35.1 mA/cm2 was achieved. Compared to a planar cell using CTO the efficiency of RTO/SiNx cell is 0.8% higher (4.5% relative). It can be concluded that the RTO/SiNx layers are the optimal passivation for the front and rear surface. On the other hand, for textured cells, the Jsc and FF of RTO/SiNx cells are lower than those of CTO cells. The main reasons of these Jsc and FF losses were also discussed systematically.

Introduction

Currently, to fabricate high efficiency and low-cost solar cells is one of main issue in the photovoltaic field. Low surface recombination is obviously one of prerequisites to reach high efficiencies. Silicon dioxide (SiO2) is formed by classical thermal oxidation (CTO) and by rapid thermal oxidation (RTO). CTO is used for laboratory high-efficiency solar cells as passivation of the rear and front surface [1], [2] in spite of high process temperatures and long duration. Rapid thermal oxidation (RTO) is very fast (<5 min) and an efficiency of 18.6% [3] can be reached for solar cells passivated by 12 nm thick RTO-layer.

An alternative passivation layer is silicon nitride (SiNx). SiNx can be grown using fast and cost-effective methods. SiNx by plasma enhanced chemical vapor deposition (PECVD) is deposited at low temperatures (⩾400°C). Also, the refractive index is adjustable easily and the passivation quality is very high [4].

The aim of this work is to study systematically and comprehensively the different surface passivation properties on non-diffused and diffused wafer. Furthermore, the passivation properties of the combined oxide/silicon nitride stacks layers are investigated. Finally, the chosen optimal passivation layers are applied to fabricate solar cells.

Section snippets

Passivation on phosphorus non-diffused and diffused surface

In this work 1 Ω cm p-type FZ silicon wafers of 250 μm thickness were used in order to investigate the passivation quality of the various layers. After RCA cleaning, emitters were diffused on both sides in a classical furnace. The study was performed on emitters with 100 and 40 Ω/Sq to be applicable to high-efficiency and industrial solar cells, respectively.

CTO at temperatures of 1050 °C for 38 min results in an oxide thickness of approximately 105 nm, which is used in high-efficiency solar cells as

Passivation on phosphorus non-diffused and on diffused surface

Silicon surface passivation by SiO2(l), SiNx(2), SiO2/SiN1(3), and SiO2/SiN2(4) stacks was investigated on phosphorous diffused and non-diffused surface as shown in Fig. 2.

Oxides (1): The effective lifetime (τeff) of sample with SiO2 layers (RTO and CTO) on the 100 Ω/Sq emitter is 2–3 times higher than on the non-diffused surface. On the 100 Ω/Sq the τeff of the samples passivated with CTO and RTO is 306 and 166 μs, respectively. It is well known that SiO2 layers passivate better on n-type surface

Higher reflection losses

To investigate the optical properties of the four different type of solar cells, the reflection, R(λ) was measured as a function of the wavelength λ between 390 and 1200 nm. Fig. 3 shows the R(λ) for the four different type of solar cells. In the case of planar cells the weighted reflection (Rw) of RTO/SiNx/MgF cells is 16%, while cell with CTO shows a Rw of 25%. On the textured cells the Rw of RTO/SiNx/MgF cells and CTO cells is reduced to 5.4% and to 11%, respectively. Note that the reflection

Conclusion

In this study surface passivation and optical properties of three different types were investigated: SiO2 layers by CTO and RTO, two different SiNx layers by PECVD, and combined SiO2/SiNx stack layers. On the phosphorus diffused emitter, both CTO and RTO layers are achieved a better effective passivation compared to the non-diffused surface. In contrast, SiNx layers passivate on the non-diffused surface than on the diffused emitter surface. SiO2/SiNx stack layers passivate good not only on

Acknowledgments

The authors would like to thank Professor W. Wettling for fruitful discussions, T. Leimenstoll for technical assistance and for the processing of the solar cells, E. Schäffer for measurements, and J. Dicker for simulation.

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