Interface Engineering for Terawatt Scale Deployment of Perovskite-on-Silicon Tandem Solar Cells
Lead Research Organisation:
UNIVERSITY OF OXFORD
Department Name: Materials
Abstract
Terawatt (TW) deployment of renewable energy is critical for the world to achieve net-zero emissions. Solar power is one of the most promising technologies for renewable electricity generation and has the largest available resource for exploitation. To boost solar electricity to TW levels, we must accelerate the development of new technologies enabling ever higher efficiencies. At present, the dominant silicon technology is close to reaching its practical efficiency limit. For higher performance to be unlocked, other semiconductor absorbers must be adopted in what is known as a tandem architecture: where two or more light absorbers are integrated on top of each other to make better use of high energy visible photons, reduce thermalisation losses and convert a higher fraction of the solar energy into electrical energy. Among such new absorbers, mixed organic-inorganic metal halide perovskite semiconductors have recently witnessed unprecedented progress and are the most promising technology to integrate into a tandem device. Significant advances have already been made integrating perovskites with silicon to make high efficiency tandems, but efforts so far have almost ubiquitously employed high-end silicon heterojunction rear cells, which do not represent the main-stream mass-produced Si PV technology. In this project, we will tackle the development of perovskite-on-silicon tandem solar cells based on the lowest cost "PERC" and "TOPCon" silicon cells. Our goal is to deliver a novel tandem technology with the potential to scale up to TW levels, due to moving away from the use of rare materials, and employing fully-scalable manufacturing methodologies, for both the silicon and perovskite cells. Enabling the vast installed capacity for silicon cell production to "upgrade" to perovskite tandem technology will accelerate deployment of perovskite-on-silicon tandems in a way that it is not yet possible with current designs. Most importantly, a shift towards scalable tandems will produce a step change in energy capture per metre square as high as 45%rel (from 24% to 35%abs), at a marginal extra cost. Because half the CO2 emissions of PV manufacturing come from silicon production, tandem higher efficiencies greatly reduce the carbon footprint per unit energy generated, potentially to the lowest level of any electricity generating technology to date.
Organisations
- UNIVERSITY OF OXFORD (Lead Research Organisation)
- UNIVERSITY OF OXFORD (Collaboration)
- Oxford Photovoltaics (Collaboration)
- OXFORD PHOTOVOLTAICS LIMITED (Project Partner)
- University of New South Wales (Project Partner)
- Trina Solar (Project Partner)
- Inst Solar Energy Res Hameln Emmerthal (Project Partner)
Publications
Bonilla R
(2025)
The impact of transparent conducting electrodes on tandem solar cell efficiency
in Joule
Kuang X
(2026)
NiOx hole transport layers enable industrial-scale, large-area heterojunction solar cells with efficiencies approaching 24 %
in Solar Energy Materials and Solar Cells
Matsui T
(2025)
Monolithic Perovskite/Silicon Tandem Solar Cells Enabled by Multifunctional TiO x Interconnects
in Small
Sharpe M
(2026)
Characterisation of solar cell nanolayer interfaces using time-of-flight elastic recoil detection analysis
in Solar Energy Materials and Solar Cells
Tao X
(2026)
Roll-to-Roll Manufacturing of Flexible Thermoelectric Semiconductors Using Low-Level Direct-Current Sputtering
in Advanced Materials Technologies
Vicari Stefani B
(2026)
Increasing specific power and the emergence of new markets for crystalline silicon photovoltaics
in Cell Reports Physical Science
Wang J
(2025)
Mercapto-functionalized scaffold improves perovskite buried interfaces for tandem photovoltaics.
in Nature communications
Wang Y
(2025)
Impact of precursor dosing on the surface passivation of AZO/AlO x stacks formed using atomic layer deposition.
in Energy advances
Xiong Y
(2025)
A Failure Mode Affecting the Reliability of LECO-Treated High-Efficiency TOPCon Solar Cells
in Solar RRL
| Description | 1. We have now developed fully-functional TOPCon-based tandem-compatible (i.e. with a full-area top contact) Si bottom cells, with an efficiency of 20%. Indeed, this efficiency is compatible with >30% PCE tandem devices, demonstrating the viability of TOPCon-based Si bottom cells for tandems. This result was achieved using benchmark ITO as the top full-area contact, but as the principal innovations were in the modification and treatment of the TOPCon, other TCOs (including In-free) can now also be readily adapted to these sub-cells 2. Our modelling of the tandem device stack external-quantum-efficiency (EQE) now closely reproduces the EQE spectrum of the perovskite-TOPCon tandem solar cell (34.2% efficient) used as the benchmark, indicating reliable modelling of the device's optical properties and short-circuit generation. Additionally, the electrical model developed is now approaching the current-voltage profile of the benchmark tandem device 3. Our development of indium-free zinc-oxide based TCOs has yielded samples of gallium-doped zinc-oxide (GZO) with resistivities of 0.38 m?cm which, deployed as a 10 nm recombination layer, would be expected to enable sheet resistance < 400?/? and in fact represents the lowest resistivity ever achieved using our available chemical precursors. This result was achieved by carefully controlling and optimising atomic layer deposition (ALD) process parameters and post-deposition processing 4. We developed a process method to make industrial TOPCon Si solar cells compatible with a tandem architecture. TOPCon solar cells cannot conventionally be integrated into a tandem solar cell, as this requires the removal and patterning of the top SiNx dielectric, followed by deposition of a recombination TCO on an exposed nano-crystalline-Si layer. Both of these processes can potentially induce surface defects in the Si subcell, dramatically reducing the efficiency and limiting the performance possible in a tandem device. By optimising our patterning and etching processes, and employing low-impact remote sputtering of a recombination layer (in this case, the benchmark ITO), we have achieved this process without significant surface damage or associated performance reduction. 5. Following on from using the benchmark ITO as a transparent contact, we have enabled the deposition of atomic layer-deposited AZO as a recombination junction. The continued use of ITO is problematic due to the scarcity of indium, so alternative transparent conductors are required. We have applied these optimised AZO layers to fabricate complete indium-free Si subcells compatible with a tandem architecture. These layers have demonstrated the properties of passivating transparent conductive electrodes. Summary of completed objectives and milestones. Objective 1 fully achieved. Objectives 2 to 5, partially achieved. Milestones 1, 3, 4, 5, 6 and 9 are fully achieved. Milestones 2, 7 and 8 are partially achieved. Project is still ongoing and working to achieve as much of these objectives and milestones as possible. |
| Exploitation Route | The research outcomes demonstrate effective strategies to design tandem solar cells compatible with production at unprecedented scales. Within this project, methods have been developed to adapt TOPCon-based solar cells for tandem applications, while maintaining the necessary high performance to enable efficiencies beyond the single-junction limit. These Si bottom cell-designs in fact enable a range of new tandem designs, being compatible with multiple top-cell architectures, that can be taken forward in both research and industry. Coupled with the work developing high-performance indium-free TCEs, this work in combination leads the way to achieving tandem technology with bottom cells and interconnection layers that do not rely upon scarce or critical minerals. The enabling innovations of low-damage etching treatments and atomic-layer deposition of TCEs could lead to new tandem designs that have a major impact on the solar energy industry, by removing several of the constraints posed by elemental scarcity. Since tandems also remove the constraint of the single-junction efficiency limits, ever-more efficient solar cells can drop the cost of primary energy to levels never seen before. |
| Sectors | Chemicals Digital/Communication/Information Technologies (including Software) Education Electronics Energy Environment Manufacturing including Industrial Biotechology |
| Description | Si-Pvk Tandem solar cells now are entering the commercial market, with a range of designs competing to enable power-conversion efficiencies beyond the 29% limit for single-junction silicon. However, most current tandem designs rely upon silicon heterojunction cells, which currently comprise only <10% of the total Si solar cell production. By demonstrating the viability of TOPCon cells, which form ~80% of the Si PV market, for tandem applications, this project is expected to enable tandem designs incorporating the Si cells produced at the greatest scale, and so enable a more rapid scale-up of tandem solar cell technology. This work has already started to generate impact in the private sector, as we are working in partnership with Oxford PV to integrate the Si bottom cells enabled by our process into tandems compatible with Oxford PV's industrial production methods. Challenges include the adaptation of existing TOPCon solar cell designs and processing methods to enable tandem integration. For instance, tandems require a bottom cell with a full-area transparent conductive electrode, whereas the surface of TOPCon cells are generally covered with an insulating dielectric material with small-area metal contacts, neither of which couples effectively with a tandem architecture. It is not trivial to replace these industrially-optimised protective layers and electrodes with a full-area contact while maintaining high performance in the TOPCon. This project has demonstrated a means to effectively adapt TOPCon to tandem compatibility without compromising performance, enabling significant impact. Within academia, the project has also catalysed the rapid expansion of work into spectroscopic characterisation of interfaces relevant to tandem solar cells from 2025 onwards. This work has deployed the world-class facilities of the UK National Synchrotron Facility, Diamond Light Source, which have largely been utilised by the electrochemical community, and applied its capabilities to understanding the physics and chemistry of next-generation solar cells, where work had been comparatively limited in recent years. This impact is evidenced by our successful beamtime applications, and the manuscripts and conference presentations enabled by these sessions. |
| First Year Of Impact | 2025 |
| Sector | Digital/Communication/Information Technologies (including Software),Education,Electronics,Energy,Environment |
| Impact Types | Economic |
| Description | Nanochemical spectroscopy for interface engineering in advanced tandem photovoltaics |
| Amount | £19,277,500 (GBP) |
| Organisation | Engineering and Physical Sciences Research Council (EPSRC) |
| Sector | Public |
| Country | United Kingdom |
| Start | 02/2026 |
| End | 01/2029 |
| Title | Data in support of Impact of precursor dosing on the surface passivation of AZO/AlOx stacks formed using atomic layer deposition |
| Description | High-efficiency solar cell architectures, including silicon heterojunction (SHJ) and perovskite/silicon tandems, rely heavily on the unique properties of transparent conducting oxides (TCOs). The push towards terawatt-scale PV manufacturing means it is increasingly desirable to develop indium-free TCOs to facilitate the upscaled manufacturing of high-efficiency cell designs. Aluminium-doped ZnO (AZO) deposited by atomic layer deposition (ALD) has emerged as a promising candidate due to its combination of optical transparency and electrical conductivity. In addition, AZO has also been shown to passivate the c-Si surface. The ability for one material to provide all three properties without requiring any indium is advantageous in single junction and tandem solar devices. Herein, we demonstrate exceptional silicon surface passivation using AZO/AlOx stacks deposited with ALD, with a J0 < 1 fA cm-2 and corresponding implied open circuit voltage (iVOC) of 740 mV. We provide a comprehensive analysis of the role of ALD precursor dosing to achieve optimised performance. A broad range of characterisation approaches were used to probe the structural, compositional, and chemical properties of AZO films. These indicated that the passivation properties are governed by a delicate interplay between the Zn and Al concentrations in the film, highlighting the importance of precise process control. Optical modelling in a single junction SHJ architecture indicates these AZO films are close in performance to high-mobility indium-containing TCOs. |
| Type Of Material | Database/Collection of data |
| Year Produced | 2025 |
| Provided To Others? | Yes |
| Impact | The insights provided by this work may help to further the case of indium-free TCOs, which is critical for upscaled production of high-efficiency solar cells. |
| URL | https://ora.ox.ac.uk/objects/uuid:90456e7b-8932-4a1e-88e6-a470a654348e |
| Description | OxMat and OxPhys Tandem Solar Cell |
| Organisation | University of Oxford |
| Department | Department of Physics |
| Country | United Kingdom |
| Sector | Academic/University |
| PI Contribution | This collaboration between the Departments of Materials and Physics at the University of Oxford is at the core of this project. Materials (OxMat) has contributed through the development of silicon solar cells compatible with a tandem architecture, based on the TOPCon design. TOPCon is rapidly becoming the Si cell design deployed on the largest scale worldwide, but it is challenging to integrate into tandems, due to the lack of a transparent conductive electrode. The expertise in Si solar cells in OxMat has allowed us to successfully adapt the TOPCon design to produce cells which can function as the bottom cells in a complete tandem device. As such, we have made major contributions to work package 1 in the project plan. Additionally, we have developed a complete model of the TOPCon cell using the Sentaurus TCAD modelling software, enabling improved understanding of device operation and contributing to work package 3. |
| Collaborator Contribution | The Department of Physics (OxPhys) have contributed through the development of perovskite solar cells which form the top cell within the tandem cells produced by this collaboration. The world-leading expertise in perovskite solar cells at OxPhys has enabled the successful production of working Si-perovskite tandem devices based on our TOPCon bottom cells with an open-circuit voltage of 1.6V. Developments in vapour-phase deposited perovskite solar cells in OxPhys provides a promising route to forming reliable tandem solar cells using textured Si, raising the ceiling on efficiencies these cells are capable of. All of this work has contributed to work packages 2 and 3 in the project plan. |
| Impact | The main outcome of this collaboration to date has been a demonstration that a complete tandem solar cell can be fabricated based on a TOPCon Si bottom cell. These cells are in early stages and have demonstrated efficiencies of 10%. However, they have also demonstrated a promising open-circuit voltage of 1.6 V, indicating that the losses in these cells are primarily resistive, rather than resulting from carrier loss through non-radiative recombination processes. Further device engineering is therefore expected to deliver efficiencies greater than the highest performing Si single junction. This demonstration is a crucial output because TOPCon is rapidly becoming the most widely-produced Si solar cell globally, and it does not rely on the scarce element indium. The results of this collaboration are therefore expected to support high--efficiency tandem cells with much greater scaleability than current designs, providing a route to increase efficiency, reduce the cost per watt, and drive an accelerated expansion of solar photovoltaics. These results have not yet been published, but there is a clear route to impact once we have done so. |
| Start Year | 2024 |
| Description | Oxford PV Collaboration |
| Organisation | Oxford Photovoltaics |
| Country | United Kingdom |
| Sector | Private |
| PI Contribution | We have prepared and provided tandem-compatible TOPCon-based Si bottom cells to the collaborator, so that they can optimise their own development of TOPCon-based tandem solar cells. We have carefully optimised the preparation process to ensure the modification of TOPCon cells does not compromise the performance of the devices. |
| Collaborator Contribution | The collaborator has access to industrially-scalable production techniques for Si-perovskite tandem solar cells. By putting the Si subcells we have prepared into this process, they can identify the key bottlenecks in the production of high-performance devices. This, in turn, will help us to identify which aspects of the device require further fundamental study to allow improvements. For example, by identifying which materials interfaces act as sites of energy loss in the tandems, the collaborator will enable us to highlight and study the processes behind carrier recombination lossesin equivalent samples. |
| Impact | Different methods are required at different scales to ensure solar cells can reach high-performance at low cost and most rapidly reduce the costs of primary energy. Through our partnership, a feedback loop is established whereby the collaborator can carry-out high-throughput optimisation of tandem devices based on our bottom-cell design and identify their functional limitations. We are then well-placed to apply fundamental knowledge and characterisation techniques to identify novel processing routes that can overcome these limitations. By cycling through this process, the performance of globally-scalable tandem solar cells can be continually improved. Improved device efficiency is one of the most effective routes to reduce the cost-per-watt of solar energy generation, enabling these new technologies to provide secure, abundant and clean energy with major societal benefits. |
| Start Year | 2025 |
| Description | Sebastian Bonilla: New solar cells break efficiency record - they could eventually supercharge how we get energy from the Sun (Article for The Conversation) |
| Form Of Engagement Activity | A magazine, newsletter or online publication |
| Part Of Official Scheme? | No |
| Geographic Reach | International |
| Primary Audience | Public/other audiences |
| Results and Impact | The article provided a publicly-accessible discussion of the recent scientific advances in photovoltaic solar devices. This has the purpose of educating the public on what tangible efficiency increases and price reductions can be expected over coming years. These are industry trends with direct relevance to consumer energy bills, and so to the majority of readers of The Conversation. The article attracted comments and questions from readers which the author engaged with, further building upon the informative value of this article. |
| Year(s) Of Engagement Activity | 2024 |
| URL | https://theconversation.com/new-solar-cells-break-efficiency-record-they-could-eventually-supercharg... |