Intermediate layer contacts for tandem solar cells based on ALD SnO2

Abstract: In this project, samples with a metal/semiconductor/metal structure were fabricated and investigated with the potential application as the interconnecting layer of a tandem solar cell in mind. Degenerately doped p-Si and n-Si were used as bottom (metal like) contacts, as Si represents one of the most common materials for the bottom cell of tandem devices. A transparent, wide bandgap semiconductor in the form of SnO₂ was investigated for the intermediate layer as it is a common choice for the selective back contact of top cells based on perovskites. However, atomic layer deposition (ALD) was used as an alternative to the typical solution based application of the SnO₂ layer. The top layer was simply chosen as a triple layer metal contact stack (Ni-Al-Ni) to provide for good contact with the SnO₂.The goal of the project was to study the electrical properties of the samples through I-V measurements and how the I-V characteristic depends on the oxide’s thickness under the possible influence of the contact areas. Three different thicknesses of the SnO2 layer were used for the p-Si sample: 50, 200 and 400 Å. For the n-Si samplesonly one thickness (400 Å) was studied. Using the diode equation, four parameterswere calculated (Jo, Rsh, Rs and n) for different measurements combing different contact configurations. The latter included measurements between the front and the back of the samples and measurements between contacts on the front with and/orwithout SnO2 layer. From the results, it was concluded that as the thickness of SnO₂ increases, the saturation current (Jo) decreases while both shunt resistance (Rsh) andseries resistance (Rs) increase. The ideality factor (n) neither depends significantly on effective area, nor on SnO2 thickness. The p-Si and n-Si samples show similar behavior in the case of 400 Å SnO2 thickness. The contact areas only appreciatively affect Jo, but it is not clear what lies behind this dependence. In all cases, the top contacts obtained major wear during measurements, reducing the number of trustworthy measurements that could be used on the smaller areas. The resistivity through the oxide layer was calculated to ρSnO₂ = 247±96 MΩ cm, which is higher than for SnO₂ deposited by other techniques, and too high for tandem cell application. Schottky barriers formed at the interfaces will typically limit the charge transport further.