This work presents a new type of transparent conductive adhesive and its electrical and optical properties with regards to an application in III-V on Silicon tandem solar cells. The developed adhesive is based on a doped ZnO and deposited by spray coating. The optical interconnection of the two sub-cells is identified as one of the major challenges due to the reflectance at the bond and possible approaches to reduce the reflectance are investigated by optical simulations. Regarding the electrical interconnection, an ITO-based contact layer reduced the lowest measured connecting resistivity from previously 17 Ω•cm² [1] down to 0.12 Ω•cm². It is shown that the homogeneity of the bond correlates with its conductivity and can be improved by adjusting the glueing process according to the calcination process of the adhesive. Taking both the electrical and optical parameters of the transparent conductive adhesive into account, the efficiency of a triple junction solar cell was estimated.

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The currently used options for the monolithic interconnection of sub-cells in a Si and III-V tandem solar cell device are direct wafer-bonding or hetero-epitaxy. Both methods are costly and difficult to transfer into an industrial process. This work presents a novel, scalable and cost-efficient process for the interconnection of semiconductor substrates by using a transparent conductive oxide (TCO) interlayer. The TCO material is sprayed from a solution onto both sub-cells which are subsequently connected by using a hot press. The resulting bond shows optical absorption below 2% and a connecting resistivity of 2 Ωcm². Bonded samples withstood all further processing steps and thereby demonstrated mechanical stability of the bond.

By adjusting the process flow of a newly developed transparent conductive adhesive (TCA), the first mechanically stable interconnection of a glued III-V on Si tandem solar cell was established utilizing a ZnO-based adhesive. The measured VOC of 1691 mV is a first proof of concept for the developed TCA. The functionality of the two sub-cells in the tandem device was proven by external quantum efficiency (EQE) measurements. The reflectance at the bond interface of such a glued tandem solar cell remains the limiting factor. In order to reduce the optical losses, the effect of a textured silicon surface was investigated in test structures, which indeed showed reduced reflectance. Furthermore, this textured test structure showed an improved connecting resistivity as low as 83 mΩcm².

Sprayed transparent conductive oxides (TCOs) are an interesting alternative to sputtered TCOs for many applications due to the possible high throughput and a simple, atmospheric pressure process of spray deposition. In this work, the growth mechanism of sprayed ZnO:In was analyzed by transmission Kikuchi diffraction (TKD) analysis of the thin film’s crystal orientation, which shows a preferred orientation of the growing grains and thus proves that the deposition occurs from the gas phase. It was observed that with increasing thickness of the layer, the average grain size increases and the measured resistivity significantly reduces to ≈5–6 × 10–3 Ω cm for layers of >500 nm thickness. Since many applications also require good electrical contact formation, the contact resistivity and the interface between sprayed IZO and n-type poly-Si and p-type GaAs, two materials that are commonly used in III–V/silicon tandem solar cells, were investigated by electrical measurements and high-resolution transmission electron microscopy (TEM) analyses. The interlayers observed in TEM were investigated by energy-dispersive X-ray spectroscopy (EDS) line scans. The results suggest that oxidic interlayers at the substrate/IZO interface are responsible for the observed higher contact resistivity compared to the contact resistivity of sputtered indium tin oxide (ITO) references. The results presented in this work lead to a better understanding of the deposition process occurring in spray pyrolysis and thus allow a more targeted optimization of process parameters depending on the future requirements of the application.