InGaN nano-pyramid-based Multi-terminal multi-junction Solar cells – INMoSt
InGaN for flexible photovoltaics: a last (successful) try?
The purpose of INMoSt is the realization of low-cost, high-efficiency, multi-junction solar cells using a single material family, namely III-nitride semiconductors. This target becomes possible by combination of a series of innovative technologies. <br />The first milestone will be the demonstration of beyond-state-of-the-art, free-standing, and flexible InGaN-based SCs. The ultimate goal will be the fabrication of a stack of such SCs, using a process fully compatible with conventional IC technology.
Many challenges to take up
In terms of efficiency, as of today, multi-junction SC are the only concept to demonstrate performance overcoming the single-junction Schockley-Queisser limit (33%) and reach industrial applications. They rely on the fact that higher conversion efficiencies can be reached by dividing the solar spectrum into several wavelength ranges, and converting those spectral windows with distinct cells of suited band gap. This strategy, using two terminals and four junctions of III-V semiconductors, has led to the highest reported conversion efficiency to date at 46% . The high cost of those devices prevents their application as flat panels, but they can find commercial applications for concentrated PV, or for space applications. Thus, during the last five years, interest in developing a low-cost multi-junction SC has taken off. INMoSt is part of this framework with an original approach.<br /><br />In INMoSt, to lower the cost while maintaining high efficiency, we propose the development of multi-terminal, multi-junction, InGaN-based SCs. Over other approaches, this material family offers the advantage of high mechanical, thermal and chemical stability, the absence of toxic elements in the SC, and an environmental-friendly production process. It has been theorized that a 2-terminal, triple-junction, InGaN SC could reach an efficiency larger than 40%. However, such a structure is not suitable for our purpose because: <br /><br />(i) It does not allow decreasing drastically the cost because of the substrate on which the structure is grown, generally sapphire.<br /><br />(ii) It requires tunnel junctions in a lattice-mismatched scheme, which complicates the technological processes. <br /><br />(iii) Due to the strain resulting from the absence of a lattice matched substrate and the propensity of InGaN to phase separation , only limited indium composition and thickness of the InGaN layer can be reached with a reasonable material quality. This is unsurprising since the critical layer thickness for a fully strained high quality InGaN with medium In content is only a few nanometers.<br /> <br />(iv) It presents problems of high resistance of the p-type GaN contact layer.<br /><br />(v) InGaN/GaN interface polarization charges, which were not considered in the model, are expected to decrease the collection efficiency.<br />InGaN based devices with different absorber designs including InGaN bulk layer , InGaN/GaN semibulk and superlattice structures, and InGaN/GaN multiple quantum well PIN structures have been demonstrated , . However, due to the above-referred issues, the best reported efficiency in InGaN SCs is only 5.9% and has been obtained for MQW structures, with a total InGaN absorber thickness of 46 nm and an In content of 19%. <br /><br />Our approach to overcome these difficulties is described below.
Our approach includes:
- Growth of the InGaN SC using Van der Walls epitaxy on top of h-BN/Al2O3 templates using MOVPE. This technique allows the simple mechanical release of the epitaxial structure, its transfer to a host substrate, and the multiple re-use of the template.
- Growth of the SC using selective area growth MOVPE, which allows the synthesis of InGaN nano-structures with high crystalline quality, instead of planar InGaN, containing high density of dislocations and stacking faults. The nano-pyramid geometry increases the critical layer thickness, resulting in dislocation-free structures with enhanced indium incorporation. This technique allows the fabrication of junctions with high indium content.
- Growth of the p-(In)GaN layer by low-temperature PAMBE. This technique allows high doping levels while preventing the problems of hydrogen passivation, polarity inversion domains and self-passivation that appear in the case of MOVPE growth. Therefore, layers with high Mg (p-type dopant) concentration can be achieved without significant degradation of the material quality.
- Implementation of n+/p+ tunnel contacts to reduce the resistance of the p-contact. This technology has been recently introduced in III-nitride LEDs, with successful results in terms of contact resistance and current spreading. When reverse bias is applied to a heavily doped p+/n+ junction, electrons can tunnel from the valence band of the p-region to the conduction band of the n-region. Due to the requirement of very high doping levels to favor field emission current, such tunnel contacts are fabricated by PAMBE, since this technique is able to attain record n+ doping levels (up to ˜ 1021 cm-3) without degradation of the structural properties.
- Use of a multi-terminal configuration where the different cells of the stack are connected in parallel. This relaxes the necessity of current matching and avoids the growth of complicated tunnel junctions required in the case of cells connected in series (2 terminals).
The simulation work led to the design of the solar cells to be realized as well as to the realization of photo-lithography and e-beam lithography (software) masks. The design was the subject of a publication, and another is in progress. We show that the optimization performed in this study, which takes into account the real technical possibilities of material growth and component fabrication, leads to a theoretical efficiency close to 20%, which is well beyond the state of the art.
The material growth and fabrication work is a bit behind schedule. A first trial, not quite conclusive, has tested the MBE/MOVPE growth recovery and the tunnel junction realization. A series of substrates with silica mask and h-BN layer could be realized and used for the realization of solar cell based on quantum wells. These structures were used to optimize the process of separation and transfer of cells on thin copper substrate.
The highlight of this early project is the demonstration of peeling and transfer of large active layers (up to several cm2, necessary for solar cells) on different types of substrates (copper for heat dissipation and flexibility, polymer for flexibility, silicon for tandem cells) with no or very few cracks and without alteration of the performances.
The work of the next months will consist in comparing a standard InGaN solar cell made only by MOVPE to a cell made by epitaxy with or without tunnel junction. In a second step, we will evaluate the contribution of selective growth at the nanoscale by comparing a standard cell and a nano-pyramid cell. If the results confirm our theoretical predictions, we will study the realization of a tandem cell by transferring, after detachment, a nano-pyramid cell on a copper inverted nano-pyramid cell. It is the design of this last structure that our simulations indicate as being the most efficient. It requires the growth of active layers on a substrate finished by a thin layer of h-BN and the control of the detachment and transfer, steps that we have validated during this first part of the project.
1. Multi-microscopy nanoscale characterization of the doping profile in a hybrid Mg/Ge-doped tunnel junction.E Di Russo, A Mavel, VF Arcara, B Damilano, I Dimkou, S Vézian, ..., Nanotechnology 31 (46), 465706 (2020)
2. Control of the Mechanical Adhesion of III–V Materials Grown on Layered h-BN, P Vuong, S Sundaram, A Mballo, G Patriarche, S Leone, F Benkhelifa, ..., ACS Applied Materials & Interfaces 12 (49), 55460-55466 (2020)
3. Effectiveness of selective area growth using van der Waals h-BN layer for crack-free transfer of large-size III-N devices onto arbitrary substrates, S Karrakchou, S Sundaram, T Ayari, A Mballo, P Vuong, A Srivastava, ..., Scientific Reports 10 (1), 1-9 (2020)
4. Monolithic Free-Standing Large-Area Vertical III-N Light-Emitting Diode Arrays by One-Step h-BN-Based Thermomechanical Self-Lift-Off and Transfer, S Karrakchou, S Sundaram, R Gujrati, P Vuong, A Mballo, HE Adjmi, ..., ACS Applied Electronic Materials (2021)
5. MOVPE of GaN-based mixed dimensional heterostructures on wafer-scale layered 2D hexagonal boron nitride—A key enabler of III-nitride flexible optoelectronics, S Sundaram, P Vuong, A Mballo, T Ayari, S Karrakchou, PL Voss, ..., APL Materials 9, 061101 (2021)
In order to reduce carbon emissions and mitigate the effects of climate change, the energy sector requires an urgent energy transition at a global scale. In the domain of photovoltaics, despite the great effort devoted for large scale implementation, price reduction is still the main concern to become fully cost-competitive with traditional energy sources. In this frame, two main parameters can lead to photovoltaic cost-per-Watt reduction, namely higher conversion efficiency and lower production cost.
The purpose of INMoSt is the realization of low-cost, high-efficiency, multi-junction solar cells using a single material family, namely III-nitride semiconductors. This target becomes possible by combination of a series of innovative technologies. First, recent developments of the InGaN-nanopyramid growth method have made it possible to enhance the In incorporation in the material which reducing the density of structural defects. Then, the implementation of an h-BN-based simple lift-off and transfer process allow a drastic reduction of the fabrication costs. Finally, the improvement of the conductivity of the p-region and of the p-contact is now possible by depositing Mg-doped layers by molecular-beam epitaxy and using an n+/p+ tunnel contact scheme. The combination of these recent breakthroughs have set the basis for the implementation of low-cost (re-use of the substrate) and high-efficiency InGaN solar cells. The first milestone will be the demonstration of beyond-state-of-the-art, free-standing, and flexible InGaN-based solar cells. This will be realized by the encapsulation into PDMS of the lifted-off solar cells. The ultimate goal will be the fabrication of a stack of such solar cells, each step with a different band gap in order to grant access to a large region of the solar spectrum, and using a process fully compatible with conventional integrated circuit production technology.
The INMoSt consortium brings together two partners with complementary experimental and theoretical expertise and capabilities: GT CNRS and CEA-IRIG-PHELIQS. INMoSt researchers possess backgrounds in science and engineering with expertise in experimental and theoretical aspects of nitride materials and nanostructures, growth kinetics, semiconductor fabrication processes, material characterization, and device physics. The functional strategy of the project is based on three main building blocks of technology optimization: simulation and design, epitaxial growth and device fabrication. Assessment of these building blocks will be assisted by material characterization and device tests and measurements.
Photovoltaics is becoming a major industry, with constant growth in terms of economic and social benefits. Preparing the next steps of development, in particular the 30-30-30 challenge (production of photovoltaic modules with a >30% energy conversion efficiency for a <30 c$/Wp price by 2030), starting from basic research and innovation is extremely important. INMoSt will provide the first low-cost, high-efficiency, multi-junction solar cells (SC) using a single material family, namely III-nitride semiconductors.
Monsieur Jean Paul Salvestrini (Unité Mixte Internationale GT CNRS)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
PHELIQS Photonique Electronique et Ingénierie Quantiques
UMI GT CNRS Unité Mixte Internationale GT CNRS
Help of the ANR 386,883 euros
Beginning and duration of the scientific project: January 2020 - 48 Months