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Applying silicon solar cell technology to revolutionize the design of thin-film solar cells and enhance their efficiency, cost and stability

Periodic Reporting for period 4 - Uniting PV (Applying silicon solar cell technology to revolutionize the design of thin-film solar cells and enhance their efficiency, cost and stability)

Reporting period: 2021-09-01 to 2022-08-31

Thin film solar cells have the possibility to be made flexible, semi-transparent and/or may be applied for tandem structures or building integrated. Having multiple usages, it is of interest to make these thin film solar cells as fast and cheap as possible. From the available thin films, CIGS has one of the best solar cell performance. There are concerns about the usage of indium though when CIGS will be widely applied. Therefore, making the CIGS layer thinner and thereby reducing the amount of In, is an interesting option to explore. When the absorber material becomes thin, interfaces are generally limiting the performance and the path length to absorb all the incoming light may be too short to absorb the longer wavelengths. There are various approaches to tackle these problems. Passivation of the back contact by applying a dielectric is the main route investigated. Not only reduces it the interface recombination at the CIGS back contact but it also increases the reflection. We have investigated various approaches which are aimed to be industrial viable and easy to make to reduce the losses in ultrathin CIGS solar cells. This also holds for the absorber layers which are made by a single stage co-evaporation process. In this case, the Ga gradient at the back is replaced by a passivation layer and no copper rich stage is applied. The effect of the simplified growth is mostly visible in the grain boundaries. As passivation layer, AlOx is often used. As this layer blocks the current flow, it has to be either sufficiently thin to allow tunneling or requires holes allowing for the current to flow to the Mo back contact. We did show that the latter approach can be applied for layers up to 6 nm without lithography steps by simply adding NaF before CIGS growth. When applying a thin Ag layer under the AlOx the cells also show optical improvement. This is likely due to formation of scattering particles, making the CIGS rougher and thereby increasing the optical path length. To improve the absorber layer itself alkali treatments are applied. At the front the CIGS/buffer interface may also need to be adapted. Application of a passivation layer at the front, either with holes or thin enough for tunneling, has been investigated. Before applying a passivation layer on the CIGS surface, cleaning treatments need to be applied. To analyze the effect of the various treatments a combinatorial approach based on bias dependent admittance spectroscopy and (time resolved (TR)) photoluminescence (PL) is developed. Generally, with (TR)-PL a quick analysis whether the treatments are improving the absorber/interface quality before finishing into a device is possible. Features like interference and blue shifts of the spectra may be observed as well and can be related to the emission profile and scale of the potential fluctuations in the absorber layer. However, improvement due the various treatments observed in the (TR)-PL does not necessarily translates into better solar cell performance. This can indicate that the treatments are sensitive to the processing conditions of the window layers for instance and/or barriers are formed. With bias-dependent admittance spectroscopy losses at interfaces and bulk can be found by introducing a so-called CVf loss map. These maps visualize the losses in the solar cell, which makes it possible to distinguish defects in the bulk from the interface and barriers at front or back interfaces.
Scientific outcomes: New insights into ultra-thin CIGS materials, its surface passivation and optical confinement have been successfully delivered. Besides the novel scientific approaches for passivation and light management and simplified and advanced solar cell models, innovative characterization devices and approaches have been sought, found, and applied.

Technological outcomes: The successfully achieved scientific outcomes like novel passivation approaches and/or characterization techniques can be applied to other prominent thin-film materials that have similar characteristics to CIGS, i.e. cadmium telluride (CdTe) and perovskite. Furthermore, the techniques proposed and achieved at the end of this project are scalable due to one of the major focuses of this project, i.e. industrial viability.

Social outcomes: Uniting PV aimed to merge the characterization and production approaches used in the PV market lead, i.e. silicon solar cells, with thin film PV. As a result of this merger, one of the main objectives of Uniting PV was to lead collaborations between Si and thin film PV. Considering the efficiency limit of the single junction PV technologies, the trend for a while is to 'merge' two different technologies to get more efficient solar cells, the so-called tandem PV technology. Uniting PV had anticipated this and showed the competitive Si and TF technologies experts that it is possible to integrate process and characterization techniques into different solar cell architectures, which is leading to more fruitful collaborations for tandem PV applications.
Additionally, also an interdisciplinary link with the field of photo- and/or electrocatalysis has been made. Chalcogenide materials (e.g. CIGS) are very promising for the conversion of CO2 by photo- and/or electrocatalysis as they can enable high activity and selectivity, but also and especially high-value products (i.e. methanol or dimethoxy ethane). Industry releases relatively large amounts of CO2, which by use of such processes could be captured, fed back into the process as a resource, or temporarily stored through so-called Carbon Capture and Storage (CCS). Hence, this field of research is currently attracting a lot of interest