Thermally stable MoOx hole selective contact with Al2O3 interlayer for industrial size silicon solar cells

So far, intrinsic hydrogenated amorphous silicon (a-Si:H(i)) has been commonly used below molybdenum oxide (MoOx) to form a good contact. An a-Si:H(i)/MoOx stack gives good surface passivation, but often results in poor carrier selectivity after exposure to slightly elevated temperatures >130 °C (Geissbühler et al., 2015) [1]. For this reason, we have investigated an alternative interface layer, a very thin Al2O3 tunneling layer (<2 nm), deposited by atomic layer deposition (ALD), that can provide surface passivation, higher transparency and thermal stability without affecting the hole transport across the contact. To demonstrate this new passivating contact a 6" moly-poly cell, with an Al2O3/MoOx stack at the front side and n-type doped polysilicon at the rear side, was made using a high- throughput spatial ALD tool, and E-beam PVD, for the Al2O3 and MoOx layers, respectively. This resulted in an efficiency of 18.2% with a Voc of 651 mV, a FF of 75.6% and a Jsc of 36.9 mA/cm2. A post-deposition anneal (PDA) of the thin Al2O3 interlayer has significant effect on the Al2O3 thickness, layer stoichiometry, contact selectivity, and sputtering-induced damage. Annealing at higher TPDA (350–600 °C) results in ineffective hole carrier transport and makes the stack more sensitive to ITO damage. The best performing device was, therefore, made using an Al2O3 layer without a PDA treatment. Moreover, we have found that this solar cell structure is thermally stable up to at least 210 °C, and even slightly improves under annealing which makes this device industrially appealing.