ADVANCED PHOTOVOLTAICS
Tuned Bottom Silicon Devices (PP3.4)
PP3.4: Tuned Bottom Silicon Devices
Aim
The aim of this work package is the demonstration of highly tuned bottom silicon devices and development of enabling technologies, guided by a detailed technology roadmap based on current technology projections and state-of-the-art device simulations.
Background
Tandem PV with a bottom silicon cell is projected to enter the PV market within the next 5 years and occupy a ~5% market share within the next 10 years (ITRPV, 2021). The suitability of silicon devices as the bottom cell is driven by both economic and technical factors. Silicon devices are a highly matured PV product with over 90% of the PV market, owing to its potential for high efficiencies 25-27%, low-cost fabrication processes ($/Wp), and low LCOE.
Silicon as a bottom cell with an energy band gap of ~1.12eV is capable of effectively absorbing and converting long wavelength light up to 1000nm with near 100% internal quantum efficieny (IQE), enabling a highly synergistic combination with the higher bandgap materials in emerging PV materials such as perovskites, GaAs, CZTS and various OPV materials.
Two-terminal tandems with silicon as the bottom cell have been demonstrated to achieve very high efficiencies, achieving 32.9% with GaAs/silicon, and 29.2% Perovskite/silicon tandems and showing rapid progression in recent years. Development of such ultra-high efficiency modules is particularly advantageous to Australia where high balance-of-system costs for residential PV installations represents >50% of the total PV market.
A vast range of research, development, integration, commercialisation and demonstration activities are needed to realise a commercial silicon-tandem product within the next decade. This work package will address the most pressing issues relating to design and development of the optimal bottom silicon cell structure incorporating passivated contacts, optimal optical light trapping schemes, and advanced interface contact technologies.
PP3.4.1 Silicon Bottom Cell Simulation
A crucial tool for the development of commercial silicon tandem modules is the need for advanced simulation capabilities that can provide accurate representation of its opto-electrical characteristics based on latest empirical values of material properties, and in-depth loss analysis to aid development of a technology roadmap.
Activities towards development of accurate tandem structure will be invaluable towards materials screening where electrical and optical properties of newly developed materials can be ‘plugged-in’ to established simulation models to provide a comprehensive evaluation of its optimal performance.
Complex performance trade-offs such as ones between parasitic absorption, conductivity, surface recombination, and carrier collection efficiency can be simultaneously evaluated and based on up-to-date empirical models.
Such ‘virtual experiments’ provides a preliminary insight into the complex relationship of new materials, and greatly accelerates the pace of technology innovation.
Simulations tools such as Sentaurus, Sunsolve and Quokka3, with features developed specifically for PV applications, have proven invaluable to the optimisation, analysis and technology roadmap development of Si devices. This activity will build upon the nurtured expertise within the ACAP groups and the active collaboration with the developers to further extend capabilities toward improved monolithic tandem optical and electrical simulations.
PP3.4.2 Monolithic Silicon Bottom Cell
The evolution of bottom silicon cell architectures in champion monolithic tandem cells have closely shadowed the progression of single-junction silicon devices. Early devices inherited features from conventional homojunction Al-BSF cells and PERC cells with front surface modifications to accommodate electrical contact to top devices. With the recent emergence of silicon heterojunction (SHJ) and polysilicon based passivated contacts, bottom Si devices have rapidly followed suit with highly promising results, demonstrating a rapid acceleration in device performances within the last 5 years.
Activities in this subsection will further accelerate the development and demonstration of state-of-the-art silicon device architectures for tandem application, particularly focusing on silicon passivated contact solar cells, and innovation towards upcoming generation of hetero-contact based silicon devices.
PP3.4.3 Silicon-Passivated Contact Bottom Cell
Opto-electrical properties of silicon based passivated contacts (eg: tunnel-oxide-polysilicon and a-Si:H) provides highly synergistic properties to its application in bottom silicon devices. Strengths include (i) the parasitic absorption in the silicon-based passivation layers is less dominant, and (ii) the lower current density in combination with the large contact surface area balance losses from the inherent higher contact resistivity.
Monolithic bottom silicon devices based on polysilicon and PERC contact structures are actively fabricated within ANU laboratories in multi-institutional collaborations toward demonstration of the completed tandem devices. Advances in both front and rear surface technologies have been identified within this work-package towards improved optical and electrical performances.
Another key area of innovation is further development of polysilicon-based recombination junctions. Novel interlayer free recombination junctions have been demonstrated in a monolithic tandem at ANU, featuring doped polysilicon and mesoporous TiOx layers.
Further development of such technologies is highly attractive as doped polysilicon provides significant advantage of being thermally stable and is also highly compatible with subsequent top cell fabrication processes. In close collaboration with PP1.2 activities, this activity aims to continue innovation towards development of a stable, polysilicon-based recombination junction which is electrically and optically optimised for perovskite and other top cells.
On the rear surface, research will focus towards incorporating rear texturing and a compatible, doped polysilicon contact layer, in combination with enhancement to rear optical properties by inclusion of optimised dielectrics. Patterning technologies for contact opening on doped polysilicon layers shall also be investigated to enable optimised contact opening through the rear dielectric layers.
Additionally, research efforts will be directed towards development of all-polysilicon based tunnel-junction, addressing reliability and stability issues of metal-oxide based carrier-transport inter-layers, towards a more stable and robust bottom silicon device.
PP3.4.4 Low-Cost TCA Interconnected >4” Silicon-Based Tandem Cells
Low-cost intermediate connection technology is a complementary alternative to the direct growth of top cell on silicon, allowing wide range of top cell options to partner with silicon for 2 terminal/4 terminal (2T/4T) tandem cells.
This can be integrated with the semitransparent top cell obtained in PP3.4.3 for two-step tandem cell manufacturing. UNSW researchers’ recent invention of a novel interconnection technology using low-cost TCA bonding layer will be further developed here, which has been proven successful on small-sized silicon-based tandem cells with various semitransparent top cells (e.g., perovskite, III-V, antimony chalcogenide).
In this work package, we will adapt this bonding technology for different silicon bottom cells, develop optical design database for various combinations of top cells and silicon bottom cells, and demonstrate large area (>4”) processing capability. Encapsulation and long-term stability test will be co-investigated through the collaboration with industry partners. Life cycle performance, cost and commercialisation pathways will also be evaluated.
PP3.4.5 Multi-Terminal Silicon Bottom Cell
Multi-terminal tandem devices are composed of a stack of two or more electrically independent PV devices mechanically, stacked with semi-transparent top cells to allow transmission of un-absorbed light to the bottom devices.
Key advantages to the electrical decoupling of the devices are the reduced device fabrication complexity, and the electrical decoupling of the devices. Each device can be fabricated independently, avoiding complex materials engineering to achieve matching energy levels across interface materials, and process compatibility issues.
Furthermore, the electrical decoupling allows independent maximum-power tracking to each layer, avoiding complex optical optimisation towards current or power matched devices as is a
necessity in monolithic tandem devices.
These advantages allow circumventing of numerous technological challenges common to monolithic device technology development, therefore enabling a shift in the focus of activity towards achieving optimal optical and electrical performance within each device layer.
ANU are among leaders in development of four-terminal (4T) tandem solar cells.
Absorption within each perovskite, GaAs and silicon layers and the simulation of loss analysis within the silicon layer only, indicates a shift in the major loss mechanisms within the bottom silicon device. Preliminary simulations indicate a significant advantage in adoption of silicon based passivated contacts for silicon bottom cells. Activities within this area will be carried out in close collaboration with activities within the PP1.3, towards fabrication of highly tuned multi-terminal bottom silicon devices.
The expected outcome of this activity is the development of a technology roadmap as guidance to device technology development, the development of novel device fabrication technologies and silicon bottom device architectures, towards higher efficiency and industrial manufacturability of silicon-tandem devices.
