Solar Energy Materials & Solar Cells 218 (2020) 110758 Available online 6 September 2020 0927-0248/© 2020 Elsevier B.V. All rights reserved. Engineering aluminum oxide/polysilicon hole selective passivated contacts for high efficiency solar cells Gurleen Kaur a, b, * , Zheng Xin b , Ranjani Sridharan b , Aaron Danner a, b , Rolf Stangl b a Department of Electrical and Computer Engineering, National University of Singapore, 119077, Singapore b Solar Energy Research Institute of Singapore, National University of Singapore, 117574, Singapore A R T I C L E INFO Keywords: Aluminium oxide Passivated contact Carrier selectivity Annealing Polysilicon Atomic layer deposition Surface passivation High efficiency Tunnel layer ABSTRACT Tunnel layer passivated contact technology is already highly efficient in case of selective electron extraction but not as efficient in case of selective hole extraction. Thus far, SiO x /p + -poly-Si contacts have resulted only in ef- ficiencies above ~20.1% for rear-side deployed hole selective contacts. We investigate if hole extraction selec- tivity can be further improved by substituting the ‘conventionally’ used SiO x tunnel layer exhibiting moderate or even high positive fixed charge density by AlO x tunnel layers, exhibiting high negative fixed charge density. The merits of using atomic layer deposited ultrathin AlO x tunnel layers are investigated and compared with wet chemically formed SiO x tunnel layers to form AlO x /p + -poly-Si and SiO x /p + -poly-Si hole selective passivated contacts respectively. The AlO x thickness (0.13–2 nm) and its thermal budget including annealing time, tem- perature and ambient were varied. The quality of the resulting AlO x /p + -poly-Si passivated contacts was deter- mined by measuring the recombination current density (J c ) and the effective contact resistivity (ρ c ). Finally, using the measured values of J c and ρ c , we predict the efficiency potential and selectivity of the passivated contact using Brendel’s model. We show that for 425 ◦ C annealed AlO x samples prior to poly-Si capping, there is an improvement in passivation quality due to the high negative AlO x interface charge, which forms only for “thick” tunnel layers (≥1.5 nm). However, after high-temperature poly-Si capping, enhanced boron in-diffusion and charge compensation are degrading the overall passivation quality of “thick” AlO x /p + -poly-Si passivated contacts. The best AlO x /p + -poly-Si passivated contacts use ultra-thin AlO x tunnel layers (efficiency potential of 26.9%), which is only marginally better than the SiO x reference samples, but still improves hole selectivity. 1. Introduction It is well established that contact passivation is one of the main en- ablers of high efficiency crystalline silicon solar cells. Sandwiching an ultrathin dielectric tunnel layer and a highly p-/n-doped capping layer between the metal contact and the c-Si wafer results in reduced contact recombination, resulting in improved solar cell efficiencies [1,2]. This ultrathin dielectric layer serves not only as the passivation layer but also as a tuneable diffusion barrier, which is important for keeping most of the dopants within the capping layer. Recently, much attention and effort has been directed at developing polysilicon (poly-Si) carrier selective passivating contacts due to their excellent passivating properties and industrial manufacturability. The application of n + -poly-Si (phosphorous doped) as a rear contact for high efficiency solar cells has resulted in conversion efficiencies of 25.8% and 26.1% for solar cells with full area back contact and passivated inter- digitated back contact (IBC) respectively using an ultrathin silicon oxide (SiO x ) in a rear-side electron selective configuration [3,4]. Many re- searchers have used SiO x /p + -poly-Si (boron doped) contacts and yielded efficiencies up to 20.1% [5–7]. However, the achieved passivation quality of p + -poly-Si is generally lower than that of n + -poly-Si. This is a hindrance in realizing the full potential of polysilicon based hole se- lective contacts especially p + -poly-Si contacts for their application in IBC, bifacial or even in PERX (PERC, PERL and PERT) solar cell archi- tectures. One way of achieving high efficiency hole selective polysilicon based passivated contacts would be by replacing the conventionally used SiO x tunnel layer (moderate positive charge density of ~10 10 cm  2 ), with an aluminium oxide (AlO x ) tunnel layer (high negative charge density of ~10 12 cm  2 ). It has been shown that the just cited high negative charge density of AlO x will form upon annealing at 425 ◦ C [8,9] as well as at 800 ◦ C [10], which is a typical deposition temperature for the poly-Si capping layer. A high negative fixed charge of the tunnel layer would not only improve the passivation quality but also enhance the hole extraction capability [9,11,12] of the formed passivated * Corresponding author. Department of Electrical and Computer Engineering, National University of Singapore, 119077, Singapore. E-mail address: gurleen.kaur@u.nus.edu (G. Kaur). Contents lists available at ScienceDirect Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat https://doi.org/10.1016/j.solmat.2020.110758 Received 29 February 2020; Received in revised form 11 August 2020; Accepted 17 August 2020