Perovskite/silicon tandem is a new high-efficiency photovoltaic platform that converts sunlight into electricity with excellent performance. In few years, this driving factor have pushed the tandem technology already at the entrance of the photovoltaic market. However, to succeed towards their commercialization, the tandems need to secure the stability of the performances, in line with the warranty certificates of commercial crystalline silicon modules. Enhancing the stability while preserving record performances is a challenge that can be addressed with different approaches. In this direction, bifacial tandem solar cells are a potential solution. In this configuration, light is absorbed both at the sun-side and at the rear-side of the device by exploiting the albedo - the scattered and reflected photons from the ground. Thanks to the additional photons arising from the albedo, the bifacial configuration enhances its current above the conventional configuration, resulting in even better performances. The current enhancement is achieved by tailoring the perovskite bandgap, resulting in bromide-free composition. This composition significantly improve the stability of the performances, by suppressing the mechanism of halide segregation. Overall, the bifacial configuration combine together enhanced efficiency and better stability, promising the technology at the utility scale, towards a carbon-sustainable economy.

The usage of self-assembled monolayers (SAMs) as hole-selective layers was a key advancement for achieving >29%-efficient monolithic perovskite/silicon and >24%-efficient perovskite/CIGSe tandem solar cells. The SAMs also enabled model systems for a systematic study of charge extraction by transient methods.

This talk covers a short introduction into the development of SAMs for perovskite solar cells (PSCs), shows their application in perovskite tandem solar cells and shows which design guidelines could be concluded for a high-performance hole-selective interface. A combination of time-resolved and absolute photoluminescence spectroscopy, together with solar cell characterization identified how non-radiative recombination losses were minimized at the hole-selective interface, leading to high open-circuit voltages. The SAM model system further allowed an analysis of the influence of hole extraction speed on the fill factor of the PSCs. We found that hole extraction is around 100-times slower than electron extraction in p-i-n PSCs, which still leaves room for further fill factor gains. The dissection of charge extraction and non-radiative recombination in a perovskite absorber deposited on hole-selective layers was further detailed by a transient surface photovoltage study, confirming and completing the photoluminescence and solar cell analysis.

Finally, an overview of the wide variety of use cases for the SAMs in perovskite solar cells is shown, underlining their versatility, simplicity and robustness.

Perovskite-based multijunction photovoltaics are about to surpass power conversion efficiencies of 30% on a laboratory scale and thereby have irrevocably developed into one of the most promising next-generation photovoltaic technologies with enormous market potential. Unfolding this potential requires solving scientific and technological challenges related to high-efficiency device architectures, the development of stable perovskite materials, light management, and scalable fabrication processes of the perovskite top solar cell. In this contribution, our recent progress on novel passivation strategies using 2D/3D heterostructures, optimization of light management textures, and scalable fabrication processes (evaporation, lamination, and solution-based coating) for perovskite-based tandem photovoltaics will be presented. Prototype perovskite/Si and perovskite/CIGS tandem solar cells in 4-terminal architecture, as well as 2-terminal architecture, will demonstrate the maturity of these developments.

Tandem solar cells based entirely on thin-film metal halide perovskite absorbers hold the potential of high efficiencies at low cost and large flexibility.^[1] Prototypes of such technology have been fabricated successfully, and efficiencies exceeding 26% have been reached.^[2] However, compared to the more established combination of metal halide perovskite top cells with high-efficiency silicon bottom cells, the performance potential and limitations of all-perovskite tandems are less explored. One major difference with respect to the perovskite-silicon reference is the relevance of wave optical effects and of photon recycling for both, top and bottom absorbers. Hence, the optical coupling of the subcells needs to be considered rigorously in a simulation-based assessment of the performance potential.    

Here, we present an extension of our recently introduced model for photon recycling in thin-film perovskite absorbers^[3] to the situation of all-perovskite tandem solar cells, including a consistent consideration of luminescent coupling due to re-absorption in the bottom perovskite cell of the light emitted by the top perovskite absorber. The impact of photon-recycling and luminescent coupling on the photovoltaic device performance is assessed in the limit of ideal transport – using detailed balance – and under consideration of realistic charge transport based on a drift-diffusion model. The latter includes also losses due to both non-idealities in the subcell connection via the recombination junction and non-radiative recombination in the bulk and at interfaces.

This fully coupled optoelectronic approach enables an in-depth analysis of subcell performance issues and their propagation to the performance of the tandem device.^[4]

Solution-processed all-perovskite tandem solar cells have the potential to surpass the efficiency limits for single-junction devices by using complementary ~1.8 eV and ~1.2 eV absorbers, thereby reducing thermalization and transmission losses. However, both absorber categories, mixed-halide wide-bandgap and lead-tin narrow-bandgap, suffer from significant non-radiative recombination losses that can limit the overall open-circuit voltage of the multijunction device. Additionally, the complex optical stack in a tandem device can introduce optical losses due to parasitic absorption and reflection of incident light. This work presents an integrated all-perovskite tandem where the sub-cells use surface passivation strategies to reduce non-radiative recombination at the perovskite-fullerene charge-selective interfaces, yielding a high open-circuit voltage. Further, by using optically benign transparent electrode and charge-selective layers, the external quantum efficiency in the narrow-bandgap sub-cell is improved leading to reduced current-mismatch between sub-cells. Cumulatively, these strategies allow the development of a monolithic tandem solar cell exhibiting a power-conversion efficiency of over 23%.

Meeting the ambitious challenge of net-zero greenhouse gas emissions by 2050 and holding the average increase of global temperature below 1.5°C necessitates the upscaling of readily available renewable energy sources, the window of time to achieve this goal is closing fast, so it is key to accelerate the decarbonisation of the global energy system by increasing the power output of solar cells. In this presentation, I will show how the integration of perovskites into the well-established silicon production infrastructure can raise the rate of solar deployment. Perovskite/silicon tandems deliver a technological disruption in efficiency while maintaining compatibility with the present photovoltaics industry, making it the fastest route to enhance the silicon market and rapidly address climate change.

Tandem solar cells based on Perovskite with silicon or another perovskite can further reduce the cost of photovoltaics due to their high efficiency, and low cost (including low material and fabrication cost). To realize their full potential, the perovskites need to be deposited by scalable fabrication process, and the efficiency of each subcells need to reach their maximum. In this talk, I will present our recent progress in understanding the issues that limit open circuit voltage in wide bandgap perovskites and solutions to address them.  I will also discuss the progress in stabilize narrow bandgap Sn-based narrow bandgap perovskites based on the understanding of its degradation mechanism.  Finally, I will present the demonstrate of large area perovskite/silicon and perovskite/perovskite minimodules with high efficiency.

Tandem solar cells made of perovskite semiconductors offer unique advantages over existing PV technologies. Specifically exciting are opportunities to integrate high tandem PV into novel form factors and enable integration of PV into new areas. Here, we will give an update on R&D progress at Swift, focusing on developing scalable fabrication methods well suited to all kinds of perovskite tandem solar cell manufacturing. We will discuss the development of flexible perovskite tandems and show some early product prototypes and highlight manufacutirng and stability challenges unique to applications in the consumer electronics space as well as for integration into electric vehicles.

Hybrid halide perovskites are excellent building-blocks for multi-junction architectures, that provide the prospect to overcome fundamental efficiency limits of single-junctions. While perovskite/silicon or all-perovskite tandem cells have shown some remarkable progress, as of yet, perovskite/organic tandem cells show subpar efficiencies of ~20 per cent, limited by the low open circuit voltage of wide-gap perovskite cells, and serious optical/electrical losses introduced by the interconnect between the sub-cells [1].

Organic and perovskite semiconductors share similar processing technologies, which makes them attractive partners in multi-junction architectures. More importantly, the introduction of non-fullerene acceptors has boosted the efficiency of organic solar cells to levels beyond 18 per cent [2].

Here, we demonstrate perovskite/organic tandem cells with an efficiency up to 24 per cent, setting a new milestone for perovskite/organic tandem devices, which now for the first time outperform the most efficient single junction perovskite cells in p-i-n architecture [3].

This achievement draws from progress in all parts of the tandem:

Firstly, our ternary organic sub-cells (PM6:Y6:PC61BM) provide an enhanced efficiency in the near infrared spectral region and complement the perovskite cell. Most strikingly, under the filtered illumination conditions in the tandem, where excitons are solely generated on the Y6 acceptor, they are outstandingly stable even under long-term continuous operation of more than 5000 hours. This result is also in notable contrast to all-perovskite tandems, where stability issues of Sn-based narrow‑gap perovskite cells are a very serious issue.

Secondly, we managed to overcome interfacial losses, that are the predominant factors limiting the performance of wide-gap perovskite cells with high Br content. This allows us to access previously unreached territory of combined high open circuit voltage and fill factor.

Finally, we introduce a novel recombination interconnect for the two sub-cells, that is based on an ultra-thin (~1.5 nanometers) ALD grown indium oxide layer, that shows metallic properties and offers unprecedented low optical and electrical losses.

In an optimistic scenario, we envision that perovskite/organic tandem architectures bear a realistic prospect to reach efficiencies above 31 per cent.

Metal halide perovskite solar cells have gained tremendous attention due to their high power conversion efficiencies and potential for low‐cost manufacturing. Their wide and tunable bandgap makes perovskites ideal candidates for tandem solar cells with narrow bandgap photovoltaic technologies, such as crystalline silicon, Cu(In,Ga)Se₂, and low-bandgap perovskite, to go beyond the efficiency beyond the Shockley–Queisser single-junction limit. The past 5 years have witnessed unprecedented advancements in perovskite-based tandems, with lab-scale devices (up to around 1cm²) approaching or even surpassing efficiency records of their single-junction building blocks. Currently, the majority of the perovskite-based tandems are fabricated on rigid glass substrates. It is highly desirable to develop perovskite tandem solar cells on flexible and lightweight substrates. This would open up the possibilities for high throughput roll-to-roll manufacturing and various attractive applications including the portable and wearable electronics, smart buildings, transportations, etc., where a combination of high efficiency, lightweight, and flexibility are important considerations. In this contribution, we will present our progress on developing highly efficient flexible wide-bandgap perovskite solar cells with high near-infrared transmittance and flexible narrow-bandgap CIGS and Pb-Sn perovskite solar cells. We demonstrate over 22% and 24% in all-perovskite and perovskite-CIGS flexible thin film tandems, respectively.

Organic-inorganic halide perovskites have received widespread attention thanks to their strong light absorption, long carrier diffusion lengths, tunable bandgaps, and low temperature processing. Single-junction perovskite solar cells (PSCs) have achieved a boost of the power conversion efficiency (PCE) from 3.8% to 25.5% in just a decade. With the continuous growth of PCE in single-junction PSCs, exploiting of monolithic all-perovskite tandem solar cells is now an important strategy to go beyond the efficiency available in single-junction PSCs. However, their actual efficiencies today are diminished by the subpar performance of mixed lead-tin narrow-bandgap subcells. We firstly reported a strategy to reduce Sn vacancies in mixed Pb-Sn perovskites that use metallic tin to reduce the Sn to Sn via a comproportionation reaction. We increased thereby the charge carrier diffusion length in narrow-bandgap perovskites from sub-micrometer to 3 micrometers. We further reported simultaneous enhancements in the efficiency, uniformity and stability of narrow-bandgap subcells using strongly-reductive surface-anchoring zwitterionic molecules. The zwitterionic antioxidant inhibits Sn oxidation and passivates defects at the grain surfaces in mixed lead-tin perovskite film, enabling an efficiency of 21.7% (certified 20.7%) for single-junction solar cells. We obtained a certified efficiency of 24.2% in 1-cm -area all-perovskite tandem cells, and in-lab PCEs of 25.6% and 21.4% for 0.049 and 12 cm devices, respectively. We further obtained record-performing, 26.4%-efficient tandem devices by increasing the thickness of narrow-bandgap subcell without sacrificing the electrical performance. We recently also achieved a certified PCE of 21.7% for 20-cm2 all-perovskite tandem solar modules fully using scalable processing techniques. The encapsulated tandem devices retain 88% of their initial performance following 500 hours of operation under one-sun illumination in ambient conditions.

Resource limitations are becoming increasingly important for Terawatt-scale photovoltaics.  Reducing the CO2-emissions from processing, substantially reducing the silver demand and ramping up flat-glass production capacity must become key priorities. Furthermore, high-efficiencies obtainable by perovskite-silicon tandem solar cells are one strong leverage to reduce overall resource demand. Here, we report on record high currents for planar perovskite silicon tandem devices by more transparent TCOs and adapted layer thicknesses.  Using a TCO stack design, soft damage free processing could be combined with low parasitic absorption. We outline how further layer and interface optimization, as well as the application of textured bottom solar cells can lead to over 30% efficiency. We discuss, how usage of critical materials and costs can be reduced. First by the implementation of TOPCon-based bottom solar cells, with a PERC-like rear structure and a silicon tunnel junction front. Second, by the first-time successful demonstration of copper plating on perovskite solar cells.

As the efficiency of 2-terminal PK/Si tandem devices continues to increase towards 30% in the laboratory, the question of manufacturability of these devices must be addressed. This is important as the efficiency gap between what is being currently made in laboratory and what is possible using more scalable methods and materials is a critical issue for the future of this technology. Specifically, the integration of these devices into established PV process flows must be investigated to understand where adaption is possible and where new processes must be made. To this end, we present a proposed process flow for the fabrication of large area 2-terminal PK/Si tandems based on a heterojunction bottom cell. Methods to adapt chemical etch, metallization, and encapsulation are presented. Additionally, a high throughput meniscus coating approach for the perovskite layer will be demonstrated and the integration challenges discussed. Using this process flow, we demonstrate that PK/Si tandem solar cells based on commercially available CZ M2 wafers can be fabrication with efficiencies of over 22% on an active area of 100 cm².