Exploring the High-Temperature Synthesis of Thin-Film Solar Absorbers on Silicon for Monolithic Tandem Solar Energy Conversion Devices

At the time of writing (August 2020), 95% of the global photovoltaic (PV) market is comprised of crystalline silicon (c-Si) solar cells. This dominance is a result of significant cost improvements due to economies of scale and technological innovation. Based on a simple extrapolation model, we forecast that the current pace of development and adoption of solar PV can lead to it providing up to 50% of the world’s energy needs before 2050.

We assess the challenges of the current PV technologies, and show that there are no fundamental upscaling constraints on achieving the volumes forecasted by our model, even without the introduction of new technologies. In this context, c-Si is therefore expected to continue being the main PV technology into the future.

When asserting a roadmap for future improvements, we show that the emphasis of new innovations should be on increasing the efficiency (or power output per unit area), as it has the biggest impact on reducing the overall costs per unit of PV power. One such innovation consists in using more than one absorber material in a tandem, to selectively absorb different sections of the solar spectrum.

This transition from single-junction to multijunction tandem devices has occurred now more than 20 years ago in the space industry, due to their superior efficiencies and reliability. We review these technologies, and estimate the potential of achieving low cost implementations of this technology using c-Si as a bottom cell.

We show that although there are very promising new concepts, there is still no material which in combination with c-Si can simultaneously achieve a high powerto-cost ratio and a long lifetime of operation. One of the challenges in achieving this is that a degradation of the c-Si bottom cell structures can occur during the fabrication of the top cell.

This is particularly the case when the top cell processes involve high temperatures or the potential of introducing contaminants in Si by diffusion. Therefore, in this PhD thesis, we set out to explore how the photovoltaic structures of a Si bottom cell can be tuned to accommodate the fabrication of top cell materials under harsh conditions.

We select the tunneling oxide passivated contact (TOPCon) structure for the Si bottom cell due to its high thermal resilience, and use the kesterite Cu2ZnSnS4 (CZTS) as a model material to test its direct fabrication on Si (i.e. a monolithic integration). Being a high bandgap absorber material, CZTS is a promising partner for a CZTS/Si tandem.

However, the high fabrication temperatures used in CZTS synthesis (>550 °C), together with a reactive sulfur atmosphere, leads to the contamination of Si with the elements from CZTS, in particular Cu and S. This causes a two order of magnitude decline in the effective minority carrier lifetime of the Si bottom cell, from initial values on the order of 1 ms to final values of 10 µs after CZTS fabrication.

This decline in lifetime results in a severe performance degradation of the Si bottom cell. To circumvent this problem, we have developed specific modifications of the basic TOPCon Si structure and processing sequence, namely: 1. we introduced a thin titanium nitride-based layer between CZTS and Si, with the multipurpose function of low resistance recombination layer and diffusion barrier; 2. we increased the thickness of the n+ polysilicon selective contact, from 40 nm up to 400 nm, and find that it has a positive effect in protecting the bottom Si bulk by gettering contaminants from CZTS.

This simultaneously allows a reduction in the thickness of the diffusion barrier to <5 nm; 3. we implemented a sacrificial hydrogenated silicon nitride (SiN:H) layer on the backside of the Si bottom cell during the full processing of the CZTS top cell. This SiN:H simultaneously improves the passivation of the Si interfaces, and protects the Si backside from contamination during CZTS processing.

As a result of these configuration improvements, we achieve an in-house CZTS/Si tandem device with 3.9% efficiency, the highest of its kind. For further proof of concept and extending our findings, we have validated our configuration with other top cell materials made in external institutions, including CuGaSe2 and AgInSe2.

Additionally, we have applied our Si bottom cell developed in a tandem device for photoelectrochemical water splitting applications developed externally, using the metal oxide BiVO4, which also requires high temperatures for its synthesis (>475 °C). In all cases, we were able to confirm the generality of the resilience of the Si structures developed in-house, by demonstrating low degradation in Si lifetime during processing, with endprocess lifetimes close to 1 ms, implying that the top cell fabrication is compatible with state-ofthe-art Si open-circuit voltages close to 700 mV.

On the CZTS development side, we use our cosputtering-based synthesis route to optimize single-junction CZTS cells, allowing the transfer of high-quality CZTS absorbers to Si tandem devices. We find that the performance of our single-junction CZTS devices is limited to 7% due the occurrence of a crystallization asymmetry during annealing, leading to a double-layer in the CZTS absorbers.

We review this phenomenon, and find that the limiting factors appear to be related to the presence of metallic Cu and a sulfur deficiency in the precursor matrix.