Local Optoelectronic Visualisation for Enhancing Tandem Perovskite/Silicon Solar Cells

Project Information

LOVETandemSolar

Grant agreement ID: 841265

DOI

10.3030/841265

Project closed

EC signature date 11 April 2019

Start date 1 October 2019

End date 30 May 2022

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 212 933,76

EU contribution

€ 212 933,76

Coordinated by

THE CHANCELLOR MASTERS AND SCHOLARS OF THE UNIVERSITY OF CAMBRIDGE
United Kingdom

Periodic Reporting for period 1 - LOVETandemSolar (Local Optoelectronic Visualisation for Enhancing Tandem Perovskite/Silicon Solar Cells)

Reporting period: 2019-10-01 to 2021-09-30

Summary of the context and overall objectives of the project

Standard Silicon (Si) solar cells achieve stable and high efficiencies, however, this photovoltaic (PV) technology faces challenges ahead, as the demand for renewable energy solutions continues to rise amidst the global energy transition. The world requires a large roll out of PV to meet development and energy goals, and a boost in power-conversion efficiency (η) of these devices enables more power output per area. Importantly, this means less materials will be needed to meet the equivalent energy demand, which leads to an ease of pressure on land use, industry manufacturing, and supply chains.

To achieve higher efficiency PV technologies that are still based on low-cost Si, cutting-edge researchers are adding an additional, microscopic layer of a semiconductor material, perovskite, on top of the Si device. The perovskite layer conformally coats the Si and absorbs portions of the sun’s light (specifically, bluer light or higher energy light) that enables an increase in η. The perovskite absorber on top of the Si is what researchers call a monolithic perovskite/Si tandem device, or a multi-junction solar cell.

However, the coating process of the microscopic perovskite layer is difficult, particularly because the surface of Si-based PV devices is often not flat. Rougher surfaces are used in PV technology because this supports the scattering of light and enables reabsorption, yet it makes it difficult to coat on. State-of-the-art PV technology shows that coating perovskite on rough Si surfaces is possible and can be done in a conformal manner – but the optimal texturing for balancing both device performance and ease-of-fabrication remained undetermined.

As such, the goal of this project was to interrogate the textured nature of perovskite/Si tandem solar cells and its influence on the device performance at a microscopic level. The primary research questions were: “How do the distinct micro- and nanoscale constructs effect device performance locally?”, “What happens to the optical properties of both the perovskite and Si materials when they are fabricated as a stack, rather than independently?”, and “Will fabricators need to re-tune the texturing pattern on this device stack to achieve enhanced solar harvesting efficiency?”

To answer these questions, and subsequently build upon their learnings, the follow objectives were designed for this project:
• Objective 1: Develop and implement advanced metrology and microscopy techniques for textured perovskite/Si tandem solar cells
• Objective 2: Macroscopic characterisation and modelling of the optical properties of multi-junction solar cells
• Objective 3: Use data analytics to correlate complex and/or difficult-to-find scientific findings
• Objective 4: Boost the power-conversion efficiency of textured perovskite/Si tandems

In conclusion, we have successfully implemented the first three objectives during the project, all of which were presented in a paper published in ACS Energy Letters (DOI: 10.1021acsenergylett.1c00568). The contributions of this research project then informed our collaborators of the potential texturing pattern that could enable a boost in efficiency (Objective 4). I am delighted to remark that earlier this year (2022) our collaborators indeed were able to obtain the new record η for perovskite/Si tandem textured (and non-textured) devices, and fabricate the first of such kind that broke through the 30% efficiency barrier: https://www.csem.ch/page.aspx?pid=172296.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The research was conducted in order of the objectives outlined above.

The first task was to set up the hyperspectral microscope, used for the primary and novel results obtained in the project. The hyperspectral microscope was newly purchased in the lab and required calibration and optimization. The results from this microscope yield the perovskite’s performance characteristics, specifically its photoluminescence (PL). The PL of a material dictates how well a semiconductor material absorbs light. When the PL is high, the perovskite absorbs well, and vice versa. Once the calibration of the hyperspectral microscope was accomplished, the evaluation of the perovskite/Si samples at the microscale was possible. The methodology for acquiring consistent data on the investigation then began, setting the course for Objective 2.

We determined that to fully understand the dimensions of the texture, it was imperative to measure hyperspectral maps in the same exact location as atomic force microscopy (AFM) maps. AFM results reveal the precise dimensions (i.e. length, width, and height) of the Si texturing that lies below the perovskite layer (in our case the Si texturing design was in pyramids). By obtaining the AFM map in the same location as the hyperspectral maps, we were then able to correlate high levels of light absorption (i.e. higher performance) with the height and location of the underlying texturing. Note: this was the first time such a correlation was ever performed, let alone on these materials, indicating the novelty of such measurements.

Obtaining excellent, reliable, and publishable results took a few months to accomplish (results that were published in the ACS Energy Letters paper were achieved about 6 months into the project, c. February 2020). Once achieved, the next phase of the project, Objective 3, was initiated. The timing was fortunate in this case, as lab work was not possible for nearly four months due to the COVID-19 pandemic, thus, data analytics on the microscopy data were performed and the write up of the publication and its results began.

Once the lab was reopened in the Summer of 2020, finalization of results was made possible, and the paper was first submitted for publication. The primary paper was successfully published in May of 2021, accomplishing objectives 1-3.

Throughout this period, I was also supporting many other projects, particularly ones that made use of the novel microscopy techniques that were developed at the start of this project. The DOI’s of each are listed in the ‘Publications’ section, but I have co-authored >10 peer-reviewed papers (one in Nature another in Science) throughout this project, with approximately six more are still under development.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

As mentioned earlier, the colleagues that we worked with on the project (also co-authors on the primary paper for this project) contributed to the collaboration by providing the state-of-the-art perovskite/Si tandem devices, with variable textures of the underlying Si (different sizes and adjusted shaped pyramids).

In 2022, they have announced via the press release: https://www.csem.ch/page.aspx?pid=172296 that they have achieved a perovskite on textured-Si tandem solar cell that achieved 31.3% power-conversion efficiency. This is remarkable, as prior to that, the highest efficiency for this type of technology was just under 30%. Although the underlying scientific solutions for the higher efficiency must remain confidential for intellectual property reasons, it is our expectation that our collaboration played a small role to the ultimate achievement of this new benchmark.

From a broader perspective, these results could mark the scale up and commercialization of this technology, which would be revolutionary for the PV industry. It would signal a high-performance solar cell market which would mean consumers would need less solar panels to output equivalent power.

Perovskite's optoelectronic performance on top of a textured Silicon solar cell