Titanium oxide: A re-emerging optical and passivating material for silicon solar cells

https://doi.org/10.1016/j.solmat.2016.05.006Get rights and content

Highlights

  • The recombination decreases drastically with increasing TiO2 thickness on p, n and p+(c-Si) surfaces, and saturates at 15 nm.

  • Effective passivation were achieved on both undiffused n-type (J0=6.4 fA/cm2) and p-type (J0=7.3 fA/cm2) (c-Si) wafers, as well as boron-diffused p+ surfaces (J0=19 fA/cm2), without any post deposition treatment.

  • Planar {111} Si exhibits a 1.39-fold higher recombination than planar {100} Si; and recombination at a textured surface is similar about 1.37 times to that at a planar {111} after surface area correction.

  • Application of a single layer TiO2 ARC on rear locally-diffused p+nn+solar cells resulted in average cell efficiency over 20% and the best efficiency of 20.45%.

Abstract

We demonstrate effective passivation of a variety of crystalline silicon (c-Si) surfaces by a thin layer of thermal atomic layer deposited (ALD) titanium oxide (TiO2). Surface recombination velocities of 0.8 cm/s, 2.5 cm/s and 9.8 cm/s have been obtained on n-type 10 cm, 1 Ω cm and p-type 1 Ω cm undiffused wafers, respectively. Recombination current densities of 19 fA/cm2 and 780 fA/cm2 have been measured on 120 Ω/□ boron diffused p+ and 100 Ω/□ phosphorus diffused n+ regions. In addition to providing a superior passivation on p+ over n+ (c-Si) surfaces, the ALD TiO2 layers produces a strong injection-dependent effective lifetime on the n-type substrates, , both of which are consistent with the possible presence of negative charge in the passivating layer. The recombination at the interface between TiO2 and planar <100>, planar <111> and alkaline textured (c-Si) surfaces with upright pyramids is compared. We find that (i) a planar <111> surface exhibits a 1.39 times higher recombination than a planar <100> surface, and (ii) after area correction, the ratio of recombination on the textured and planar <111> surfaces is 1.37. A thin film of TiO2 deposited by ALD has been applied to the front surface of a rear locally diffused p+nn+ front junction solar cell, performing the dual role of surface passivation and single-layer antireflection coating on the textured p+ diffusion. The best solar cell achieved Voc=655 mV, FF=79.9% and efficiency=20.45%. The results presented in this work demonstrate that TiO2 re-emerging as a suitable optical and passivating material to produce high performance solar cells.

Introduction

In recent years, double-side passivated silicon solar cells featuring localized contacts on the rear surface have received broad acceptance in the solar photovoltaic (PV) industry, due to higher open circuit voltage, improved electrical and optical performance parameters and improved efficiency. The performance gain is largely attributed to the reduction of the surface recombination loss, which is obtained by a reduced density of interface states (Dit) and/or depletion of minority carriers from the surface. Dielectrics with negative charge are especially favoured in the p-type rear passivated cell and n-type cell with a boron-diffused p+ front surface. Aluminium oxide (Al2O3) is so far the best candidate, because of its low Dit and strong negative charge (elementary charge concentration in the range ~1012–1013 cm−2) [1], [2], [3], [4]. Large efforts have been spent in the development of alternative materials to Al2O3, such as titanium oxide (TiO2) [5], [6], [7], hafnium oxide [8], [9], aluminium nitride [10], gallium oxide [11], and tantalum oxide [12], with various levels of surface passivation reported.

Titanium oxide was used extensively from the 1970s to 1990s in the PV industry as an antireflection coating (ARC) for screen-printed solar cells [13]. Its high refractive index (typically n=1.9–2.45) and low extinction coefficient (i.e., k<0.1 for λ>350 nm) make it well-suited for reducing reflection losses of glass-encapsulated silicon solar cells [14]. Many technologies for depositing TiO2 have been explored, including evaporation [15], [16], sputtering [17], sol-gel method [5], spray pyrolysis deposition [13], [18], atmospheric pressure chemical vapour deposition (APCVD) [6], [19] and, more recently, ALD [7]. With respect to the application of TiO2 to surface passivation, however, only modest results have been reported, either using APCVD [6] or the sol-gel method [5]. Very recently, effective passivation by ALD was reported, but only after complex post deposition annealing and extended light soaking [7].

In this work, we investigate the surface recombination on n, p, p+ and n+ (c-Si) surfaces passivated with ALD TiO2, as well as its dependence on crystal orientation. We demonstrate excellent ARC and surface passivation of (c-Si) by a thin layer of thermal ALD TiO2 deposited at 75 °C without any post deposition treatment. An average conversion efficiency over 20% is achieved, when such a layer is deposited on the front boron diffused p+ surface of a rear locally diffused p+nn+ solar cell.

Section snippets

Experimental

Samples for surface recombination investigation were fabricated on (i) undiffused 1 Ω cm p-type and n-type as well as 10 Ω cm n-type, <100> float-zone (FZ) (c-Si) wafers, with a thickness of 180 µm, and (ii) phosphorus diffused (n+) 100 Ω cm p-type and boron diffused (p+) 100 Ω cm n-type FZ (c-Si) wafers with a thickness of 390 and 280 µm, respectively. In Section 3.2, surface recombination was also studied on (i) n-type FZ 1 Ω cm <111> oriented planar wafer, with a thickness of 290 µm, and (ii) alkaline

Surface passivation of (c-Si): p, n, p+ and n+

Fig. 2 shows the measured surface passivation quality presented as a recombination current density, J0 (extracted at Δn=1016 cm−3), provided by thermal ALD TiO2 on p, n, and p+ (c-Si) substrates. Fig. 2(a) depicts the effect of TiO2 film thickness on J0 on FZ n-type 10 Ω cm undiffused wafers. As shown in the figure, an ultra-thin TiO2 with thickness less than 2.2 nm provides poor surface passivation, with J0 close to 1000 fA cm−2. The level of surface passivation is dramatically improved with a

Conclusion

We have demonstrated thermal ALD TiO2 as an effective dual ARC and surface passivation layer on the front surface of p+nn+ cells, with average efficiency over 20% and a highest efficiency of 20.45%. The utilisation of a single TiO2 film on the boron-diffused p+ surface simplifies the cell fabrication process, which might enable potential cost savings when ALD is replaced by an industrial-feasible process, such as spatial ALD [52] or APCVD. The change in its crystalline phase at higher

Acknowledgement

This work has been supported by the Australian government through the Australian Renewable Energy Agency (ARENA) and by Trina Solar (No: 2014/RND003). Ellipsometer facility at the Australian National Fabrication Facility (ANFF) ACT node was used in this work. The author would also like to thank Dr. Kean Chern Fong for discussion on the solar cell performance analysis.

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