Developing Nanoscale Passivation Layers for Tandem Solar Cell Interfaces: Towards Terawatt-Scale Solar PV
Lead Research Organisation:
University of Oxford
Department Name: Materials
Abstract
To reduce CO2 emissions, it is critical to shift towards renewable electricity generation. The cheapest way to do so is to replace fossil fuel generation with solar photovoltaics. In 2022, the world reached a milestone of 1 terawatt (TW) - 1 million * 1 million watts - of cumulative installed solar PV. Experts predict that as much as 70 TW of solar PV must be installed by 2050 to rapidly electrify the energy economy. Therefore, a tremendous effort is required by solar scientists and industrial manufacturers to avoid climate disaster. Recently, perovskite / silicon tandem solar cells achieved efficiencies exceeding 30%, however, the design of the silicon cell used to achieve this feat is not compatible with mass production. Therefore, it is critical to redesign the silicon cell to enable the industrial mass manufacture of high efficiency perovskite / silicon tandem cells that will further reduce the levelized cost of electricity (LCOE) from solar PV.
The goals of this research project can be concisely described as:
1. The development and optimisation of nanolayer passivation and interconnection layers for the front surface of a silicon PERC to accommodate a tandem structure.
2. The demonstration of a perovskite / silicon tandem solar cell with efficiency >28% using the developed cell design. This will be the highest efficiency large area industrially feasible tandem cell.
The goals of this research project can be concisely described as:
1. The development and optimisation of nanolayer passivation and interconnection layers for the front surface of a silicon PERC to accommodate a tandem structure.
2. The demonstration of a perovskite / silicon tandem solar cell with efficiency >28% using the developed cell design. This will be the highest efficiency large area industrially feasible tandem cell.
Publications
Liu D
(2025)
The impact of surface polarisation on the degradation of tunnel oxide passivating contacts in silicon solar cells
in Solar Energy Materials and Solar Cells
McNab S
(2024)
Hole-Selective SiN x and AlO x Tunnel Nanolayers for Improved Polysilicon Passivating Contacts
in ACS Applied Energy Materials
Shi Y
(2023)
Characterization of solar cell passivating contacts using time-of-flight elastic recoil detection analysis
in Applied Physics Letters
Vicari Stefani B
(2024)
Ring Defects Associated with Boron-Oxygen-Related Degradation in p-Type Silicon Heterojunction Solar Cells
in Advanced Energy and Sustainability Research
Wang Y
(2025)
Impact of precursor dosing on the surface passivation of AZO/AlO x stacks formed using atomic layer deposition
in Energy Advances
Wright M
(2024)
Characterisation of SiOX / SiNX Surface Passivation Using Time-of-Flight Elastic Recoil Detection Analysis
in SiliconPV Conference Proceedings
| Description | - Key to the success of the project is the ability for the AZO material to passivate the surface of a silicon solar cell. A methodological approach for achieving excellent surface passivation with ALD deposited AZO/Al2O3 stacks was developed. The associated milestone was to achieve surface passivation with J0s < 20 fA/cm2. A J0s of < 1 fA/cm2 was achieved on n-type Cz wafers, with corresponding implied open circuit voltage of 740 mV. This represents excellent surface passivating properties, indicating the suitability of AZO to be incorporated into perovskite silicon tandem solar cells. Future work is exploring the fabrication of full solar cell devices using such passivating layers. - Work package 2 has focused on advanced characterisation of AZO films, with a particular focus on the hydrogen distribution through the stack. Since beginning this project, a collaboration with the Surrey ion Beam Centre (University of Surrey) has been established. Time-of-flight elastic recoil detection analysis (ToF-ERD) has been used to produced 1D depth profiles of the atomic distribution within both dielectric films as well as passivating contacts. We showed that ToF-ERD is a powerful tool for characterising the distribution of hydrogen (H) and deuterium (D) in a passivating contact. However, the sensitivity of the hydrogen quantification is not sufficient to provide a direct correlation between hydrogen distribution and passivation quality. This realisation led us to reflect upon existing reports, purporting to correlate hydrogen distribution with passivation properties. - An unexpected finding in this project, has been the usefulness for ToF-ERD measurements in characterising thin film perovskites. These materials are lauded as being the most appropriate top cell material for a two (or more) junction solar cell. The addition of more junctions requires an opening of the top cell band gap to fully optimise the cell efficiency. These wide gap perovskite materials suffer from compositional gradients, which drastically limit the efficiency of such multijunction devices. We found that ToF-ERD, due to its ability to resolve different masses, is an excellent too to study these compositional gradients. In particular, we are able to unambiguously plot a 1D trace of the bromine distribution in FAPbBr3 films. - Once the excellent passivation properties of AZO were demonstrated, it was then necessary to assess the stability of this passivation under conditions that are relevant for the operation of a solar cell. The passivation was tested under dark annealing at 75 C and 1 sun light soaking for 1000 hours. Under dark annealing conditions, no loss in passivation was observed. A slight reduction in iVOC of 4 mV was observed under 1 sun light soaking. A secondment at UNSW Sydney was undertaken to further assess the stability using a high intensity laser. Using accelerated conditions of 300 C and up to 100 sun light intensity, the passivation from the AZO was stable. |
| Exploitation Route | The demonstrated excellent passivation properties of AZO, with J0s < 1 fA/cm² and an iVOC of 740 mV, highlight its strong potential for integration into perovskite-silicon tandem solar cells, providing a key pathway for advancing high-efficiency photovoltaics. Future research can build upon this foundation to develop full solar cell devices utilising AZO-based passivation layers. The collaboration with the Surrey Ion Beam Centre and the use of ToF-ERD for hydrogen and deuterium characterisation provides a valuable methodology for further understanding passivating contacts, though refinement in sensitivity is needed for direct hydrogen-passivation correlation. This insight encourages a critical reassessment of existing studies and can guide future work in hydrogen-based passivation strategies. Additionally, the unexpected discovery of ToF-ERD's effectiveness in characterising thin-film perovskites presents an opportunity for researchers to optimise wide-bandgap perovskites for multi-junction solar cells, particularly in addressing compositional gradients that limit efficiency. The stability testing of AZO passivation, including long-term light soaking and accelerated laser testing, establishes confidence in its durability under real-world solar cell conditions, providing essential data for manufacturers and researchers aiming to commercialise high-performance tandem solar cells. These findings open new avenues for materials optimisation, solar cell fabrication, and advanced characterisation techniques, supporting further collaboration and innovation in the photovoltaics community. |
| Sectors | Electronics Energy Environment Manufacturing including Industrial Biotechology |
| Description | Preliminary results of the excellent passivation achieved with AZO films and stability of passivation have been presented at 4 conferences. One of the biggest barriers to high-efficiency tandem solar cells is the reliance on indium-containing transparent conducting oxides, e.g. ITO. These materials have excellent optoelectronic properties; however, the scarcity of indium means that the full potential of tandem solar cells, fabricated on a terawatt (TW) scale, cannot be realised. The results generated in this work, indicating that aluminium-doped zinc oxide (AZO) can function as a surface passivation layer in silicon solar cells, represent a significant step towards demonstrating indium-free tandem solar cells, which could be mass-produced on a TW scale. |
| First Year Of Impact | 2024 |
| Sector | Digital/Communication/Information Technologies (including Software),Electronics,Energy,Environment,Manufacturing, including Industrial Biotechology |
| Impact Types | Societal Economic |
| Description | Commonwealth Science and Industrial Research Organisation |
| Organisation | Commonwealth Scientific and Industrial Research Organisation |
| Department | CSIRO Energy |
| Country | Australia |
| Sector | Public |
| PI Contribution | We are working in collaboration on several projects, including the stability of AZO passivation as well as an analysis of the specific power of PV modules. |
| Collaborator Contribution | We are working in collaboration on several projects, including the stability of AZO passivation as well as an analysis of the specific power of PV modules. |
| Impact | N/A |
| Start Year | 2023 |
| Description | University of New South Wales |
| Organisation | University of New South Wales |
| Country | Australia |
| Sector | Academic/University |
| PI Contribution | We are working on studying the stability of AZO passivation layers. I fabricated the samples at Oxford and travelled to UNSW to do measurements. |
| Collaborator Contribution | Providing facilities for stability measurements. |
| Impact | N/A |
| Start Year | 2023 |
| Description | University of Surrey |
| Organisation | University of Surrey |
| Department | Ion Beam Centre |
| Country | United Kingdom |
| Sector | Public |
| PI Contribution | We have been making solar cell samples for Ion Beam Analysis measurements. We have published two papers from this work. |
| Collaborator Contribution | The Surrey Ion Beam Centre has been performing time-of-flight elastic recoil analysis (ToF-ERDA) measurements. |
| Impact | We have published two papers and have been invited to submit a paper for the 15th Silicon PV conference. |
| Start Year | 2023 |
| Description | 2024 Oxford Solar Research Symposium |
| Form Of Engagement Activity | Participation in an activity, workshop or similar |
| Part Of Official Scheme? | No |
| Geographic Reach | International |
| Primary Audience | Postgraduate students |
| Results and Impact | I organised a research symposium to boost collaboration between the UK and Australian research institutes. |
| Year(s) Of Engagement Activity | 2024 |
| URL | https://interface.materials.ox.ac.uk/solar-symposium |
| Description | North West Science Residential |
| Form Of Engagement Activity | Participation in an open day or visit at my research institution |
| Part Of Official Scheme? | No |
| Geographic Reach | Local |
| Primary Audience | Schools |
| Results and Impact | High School tutoring at Queen's College Oxford for prospective Oxford students. |
| Year(s) Of Engagement Activity | 2024 |