Impact of Film Thickness and Thermal Treatment on the Excellent Surface Passivation of c-Si by ALD Al2O3 for Solar Cell Applications

The surface passivation of c-Si by atomic layer deposited (ALD) Al2O3 has recently gained considerable interest after extremely low surface recombination velocities (<10 cm/s) have been reported for low resistivity n- and p-type c-Si wafers [1]. The incorporation of an Al2O3 film for boron doped emitter passivation led to enhanced efficiencies of 23.2% for n-type c-Si solar cells [2]. From the cumulative research, various questions related to the thermal stability and other processing aspects of the Al2O3 films appeared. In this contribution we will show that a decrease of film thickness down to ~5 nm does not compromise the passivation quality, enabling a straightforward reduction of deposition time and providing freedom in the design of optimal front passivation/antireflection schemes. To activate the Al2O3 surface passivation a post-deposition anneal is required, but also the thermal budget during the plasma enhanced chemical vapor deposition of an a-SiNx:H capping layer was found sufficient to activate the passivation. Although an anneal effect can be observed in a large temperature range (~350 - ~600oC) the optimal post-deposition anneal temperature window was found to be much smaller. Within the optimum temperature range, the largest anneal effect was observed to take place during the first minutes of the process. For the successful implementation of Al2O3 in conventional screen printed solar cells, thermal stability is required. In this paper we demonstrate the relative stability of Al2O3 and Al2O3/a-SiNx:H passivation/antireflection stacks against an industrial "firing" process reaching temperatures > 800oC. Although the minority carrier lifetime was found to decrease during the firing process, values in excess of 1 ms were obtained on 2 O cm n-Si wafers after firing. These lifetimes suggest that surface recombination will not likely be the efficiency limiting step for solar cells that combine Al2O3 passivation and screen printed metallization as recombination in the metalized area will be dominant. The findings in this paper demonstrate the suitability of thin ALD-synthesised Al2O3 passivation films for large scale photovoltaic applications. [1] B. Hoex, J. Schmidt, P. Pohl, M. C. M. van de Sanden, and W. M. M. Kessels, J. Appl. Phys. 104, 044903 2008 [2] J. Bennick, B. Hoex, M.C.M. van de Sanden, W.M.M. Kessels, O. Schultz, S. Glunz, Appl. Phys. Lett. 92, 253504 (2008

Surface Passivation Mechanism of Atomic Layer Deposited Al2O3 Films on c-Si Studied by Optical Second-Harmonic Generation

Recently, it was shown that Al2O3 thin films synthesized by (plasmaassisted) atomic layer deposition (ALD) provide excellent surface passivation of n, p and p+ type c-Si as highly relevant for c-Si photovoltaics. It was found that a large negative fixed charge density (up to 1013 cm-2) in the Al2O3 film plays a key role in the passivation mechanism of Al2O3 [1, 2]. The surface passivation quality of Al2O3 strongly increases with film thickness before reaching saturation around 10 nm as determined by carrier lifetime spectroscopy. In this contribution a study into the thickness effect will be presented in order to distinguish between the influence of fieldeffect passivation, i.e. electrostatic shielding of charge carriers by the fixed negative charge, and chemical passivation, i.e. by a reduction of the interface defect density. To this goal the nonlinear optical technique of second-harmonic generation (SHG) has been utilized. SHG is highly surface and interface specific and allows for the contactless determination of internal electric fields (= 105 V/cm-1). Spectroscopic SHG, carried out with a femtosecond Ti:sapphire laser tunable in the 1.33-1.75 eV photon energy range, has revealed a thickness independent electric field for Al2O3 films with thicknesses ranging from 2 to 20 nm. This implies that the fieldeffect passivation is not affected by the film thickness and that the thickness dependence in passivation quality can be attributed to a changing level of chemical passivation. Moreover, this result confirms that the fixed negative charges are located at the Al2O3 interface as also indicated by conventional C-V measurements. In addition, SHG shows clear differences between measurements performed on Al2O3 films grown by thermal and plasmaassisted ALD. These are likely related to the properties of the interfacial SiOx induced by either growth process. The presence of this oxide is suggested to be responsible for the chemical passivation quality. Furthermore, the differences indicate a smaller contribution of field-effect passivation for the Al2O3 grown with thermal ALD compared to the film from the plasma-assisted process. These results have led to a deeper understanding of the c-Si surface passivation by Al2O3 as will be discussed

Comparison between Al2O3 surface passivation films deposited with thermal ALD, plasma ALD and PECVD

Surface passivation schemes based on Al2O3 have enabled increased efficiencies for silicon solar cells. The key distinguishing factor of Al2O3 is the high fixed negative charge density (Qf = 1012-1013 cm-2), which is especially beneficial for p- and p+ type c-Si, as it leads to a high level of field-effect passivation. Here we discuss the properties of Al2O3 surface passivation films synthesized with plasma atomic layer deposition (ALD), thermal ALD (using H2O as oxidant) and PECVD. We will show that with all three methods a high level of surface passivation can be obtained for Al2O3 deposited at substrate temperatures in the range of 150-250oC. Furthermore, the role of chemical and field-effect passivation will be briefly addressed. It is concluded that the passivation performance of Al2O3 is relatively insensitive to variations in structural properties. Al2O3 is therefore a very robust solution for silicon surface passivation

Neurotoxicity of Brominated Flame Retardants: (In)direct Effects of Parent and Hydroxylated Polybrominated Diphenyl Ethers on the (Developing) Nervous System

Background/objective: Polybrominated diphenyl ethers (PBDEs) and their hydroxylated (OH-) or methoxylated forms have been detected in humans. Because this raises concern about adverse effects on the developing brain, we reviewed the scientific literature on these mechanisms

Probing a non-biaxial behavior of infinitely thin hard platelets

We give a criterion to test a non-biaxial behavior of infinitely thin hard platelets of $D_{2h}$ symmetry based upon the components of three order parameter tensors. We investigated the nematic behavior of monodisperse infinitely thin rectangular hard platelet systems by using the criterion. Starting with a square platelet system, and we compared it with rectangular platelet systems of various aspect ratios. For each system, we performed equilibration runs by using isobaric Monte Carlo simulations. Each system did not show a biaxial nematic behavior but a uniaxial nematic one, despite of the shape anisotropy of those platelets. The relationship between effective diameters by simulations and theoretical effective diameters of the above systems was also determined.Comment: Submitted to JPS