Search

Perovskite/crystalline-silicon (c-Si) tandem solar cells offer a viable roadmap for reaching power conversion efficiencies beyond 30%. In this configuration, however, the silicon cell now mainly receives an infrared rich illumination spectrum where the absorption coefficient of silicon is poor. To boost the light absorption in this wavelength interval, transmitted through the top cell, a solution is to introduce a dedicated nanoscale texture on the front side of the silicon bottom cell and tweaking the optical elements in its design. These optical elements are manifold, with tunable geometrical dimensions, layer thicknesses, and refractive indices. For optically optimizing nanostructured silicon solar cells, electromagnetic wave solving methods, such as the rigorous coupled-wave analysis (RCWA), are normally used but they become computationally infeasible when several design variables are involved in an optimization problem. In this work, a natural selection algorithm, known as the genetic algorithm, is coupled with RCWA for extracting the optimal values of various design parameters in four-terminal perovskite/c-Si tandem devices in parallel. Both two-side-contacted and interdigitated back-contacted silicon heterojunction cell structures, featuring an inverse nanopyramid grating texture on the front, are optimized for five interdependent variables using a genetic algorithm for the bottom cell application and benchmarked against their random pyramid textured analogs. Our study shows that an optimized inverse nanopyramid grating texture can outperform the standard random pyramid texture when considering incidence angle variations. More importantly, the study illustrates how a genetic algorithm can support modeling complex solar cell structures with numerous degrees of freedom.

Integrating and operating PV in a system-efficient way is the key to increasing penetration levels to evolve toward a more sustainable and clean energy system. Reliable generation forecasting is the solution to tackle fluctuations in PV power generation and its negative effects on power systems and energy markets. Clouds play a key role in solar power, so cloud detection is important in PV power generation forecast. A color-based cloud detection algorithm is proposed in this paper. It considers 12 different color channels together with the K-means algorithm to detect clouds in sky images from an All-Sky Camera. The result of the proposed method has been compared to both classic and state-of-the-art methods. The experimental results show that the proposed approach has better performance in comparison to state-of-the-art methods.

The heterojunction interdigitated back-contact (HJ IBC) cell technology enables remarkably high cell efficiencies, but requirescomplex processing for the rear-side patterning of the interdigitated a-Si:H electron and hole hetero-contacts. Therefore, the HJ IBC process flow must be simplified into a sequence that is cost-effective and industrially-compatible. Towards this goal, a litho-free, all-dry process sequence is being developed. As part of this simplified process flow, a novel in situ dry clean process called “nano-envelope” clean is developed to replace the wet cleaning after dry etching of a-Si:H. Surface contaminationanalysis and passivation studies prove that the developed dry clean is as effective as the standard wet clean. Since the “nano-envelope” clean can bedone in the same PECVD tool, the sequence from dry etching to repassivation can be done fully in situ.

We present a novel approach to simplify the rearside a-Si:H patterning of silicon heterojunction interdigitated back-contact solar cells. In our current process, laser ablation and lift-off are used. Since lift-off is not industrially-viable, we propose to replace it with a selective deposition process which ensures a-Si: H is realised only on c-Si surface and not on the SiOx mask, at the end of the process, based on cycles of a-Si: H deposition and etching. While neither the deposition nor the etching is truly selective, this method relies on the difference of a-Si: H etch rates on c-Si and SiOx surfaces to achieve selectivity as the net end-result. The main challenge is addressing the trade-off between selectivity and c-Si surface passivation. The authors gratefully acknowledge the financial support of imec’s industrial affiliation program for Si-PV. This project has also received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 727523 (NextBase).

Texturing of crystalline silicon at the light wavelength scale is a promising approach for light management in ever thinner cells. However, this approach requires adapting the device design and process, which has so far revealed to be a challenging task. Indeed, successful integration requires compromising the excellent optical properties for preserving the electrical ones. We briefly summarize what has been learned so far in the community and illustrate some of these learnings with the fabrication and improvement of 1-μ m-thin monocrystalline nanotextured silicon solar cells.

We present a novel process scheme for a-Si:H patterning using damage-free laser ablation, resulting in a simple, fast, and photolithography-free emitter patterning of silicon heterojunction interdigitated back-contact (SHJ-IBC) solar cells. An a-Si:H laser-absorbing layer and a stack of sacrificial dielectric layers are deposited on top of a-Si:H/c-Si heterocontact to prevent laser damage. Laser ablation only removes the top aSi: H layer, which limits laser damage to the surface of dielectric layers. These dielectric layers form a distributed Bragg reflector with a high reflectance of 80% at the laser wavelength which results in additional protection of the bottom a-Si:H/c-Si heterocontact. The significant reduction of laser damage is confirmed by atomic-force microscopy and photo-luminescence measurements. Such damage-free laser ablation process was successfully incorporated in a SHJ-IBC process flow and a best efficiency of 21.8% was achieved. The authors gratefully acknowledge the financial support of imec’s industrial affiliation program for Si-PV. The research leading to these results has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 657270. This project has also received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 727523.

Module-level processing of silicon heterojunction interdigitated back-contacted (SHJ-IBC) solar cells while bonded to glass, in the so-called i2-module concept, is discussed. In this approach, a key challenge is the interdigitated patterning of a-Si:H without compromising on the rear-surface passivation. Process adaptations involving more resistant bonding agents and a milder wet etchant enabled bonded cells with the best efficiency of 21.7% and Voc of 734 mV, which are the highest reported for bonded cells processed partially at module level, and which undoubtedly proves the potential of this i2-module concept to reach high Voc and efficiency. Yet another challenge is to make the process flow cost-effective and industrially-relevant, i.e. a litho-free, all-dry process flow, analogous to thin-film PV module fabrication. As a first step, dry etching of a-Si:H was developed to replace wet etching, and was successfully incorporated in a SHJ-IBC process flow to fabricate freestanding cells with the best efficiency of 22.9% and above 20% on thick (190 μm) and thin (56 μm) EVA-bonded silicon.

In this study nanodisks made from aluminum are incorporated in a back-reflector scheme in order to enhance the maximum achievable current in one-micron thick crystalline silicon solar cells. We perform three-dimensional numerical investigations of the backward scattering properties of aluminum nanodisks located at the back side, and optimize them for enhancing the absorption in the silicon layer. We also compare our results with previously optimized silver nanodisks and show that Al nanodisks are nearly as efficient as Ag counterpart. We show that if the absorption in the metallic back reflector (flat metal layer) can be avoided; this results in a further enhancement in absorption in the ultra-thin silicon layer. The proposed configuration results in a broadband (500nm to 1200nm) enhancement of absorption (~ 58 %) for ultra-thin solar cells and has a great potential in thin film photovoltaic.

Foil creation by lifting off a thin layer of a high quality silicon substrate is one of the promising substitutes for wafer sawing to create substrates thinner than 100 µm. The porous silicon-based layer transfer process is a well known method to obtain high quality foils. Despite a number of convincing lab-based solar cell show-cases, there is no breakthrough of this technology at (semi)-industrial level, because of the poor yield of processing free standing foils. This paper presents a method to fabricate back contacted solar cells based on epitaxial foils avoiding processes on free-standing foils. First, a porous silicon layer is electrochemically etched, acting as a weak sacrificial layer to detach the foil that is epitaxially grown on top of the porous silicon layer. Characterization of the epitaxial foils shows a good crystalline quality and an effective lifetime around 100 µs. Those results give indications that the obtained foils are well suited for solar cell fabrication. Front-side processing is done while the epitaxial foil is still attached to its parent substrate. A good yield is obtained for epitaxial foils that underwent the front-side processing sequence consisting of wet chemical texturing, FSF formation, passivation and ARC deposition. Afterwards, the front-side of the foil is bonded to a glass carrier and the foil is detached from its parent substrate. Silicone adhesives are used for this permanent bond. The rear-side of the solar cell is processed while bonded to glass. Therefore, only low temperature processes (

Thin layers of intrinsic and doped amorphous silicon have been used as the emitter in a processing scheme to form heterojunction interdigitated back contact (HJ-IBC) solar cells. Such processing involved depositing the emitter across the wafer, and subsequent patterning to define the base and emitter regions. Average cell efficiencies of 14.6% have been achieved (with a best cell efficiency of 15.2%), with average short circuit current densities as high as 37.8 mA/cm2 (maximum 39.1 mA/cm2), demonstrating the potential of such a solar cell structure. However, the open circuit voltage and fill factor values are lower than could be expected. The reasons behind this are discussed, as are proposed solutions to further enhance the cell performance.

The case for thin-film polycrystalline silicon (pc-Si) solar cells is strong as it combines the cost benefit of thin-films and the quality potential of crystalline Si technology. The challenge is in making high-quality pc-Si layers on non-Si substrates. By studying layers based on aluminum-induced crystallization (AIC) we previously showed that electrically active intragrain defects are a major limiting factor for thin-film polycrystalline silicon solar cells. This paper investigates the use of a recently proposed novel scanning-laser based mixed-phase solidification (MPS) process which results in large grains with a low intragrain defect density, as well as a narrow grain size distribution and strong surface crystallographic texture [6]. Through subsequent epitaxial growth, absorber layers with the desired doping and thickness can be obtained. Defect etching and TEM measurements demonstrate the drastically decreased intragrain defect density compared to the AIC-based samples. The most efficient solar cell so far has an energy conversion efficiency of 5.4% and open circuit voltage (V oc ) of around 500mV. From the preliminary results obtained, we conclude that mixed phase solidification is an attractive technique to crystallize seed layers for thin-film silicon solar cells.

N-type thin-film polycrystalline-silicon solar cells were fabricated on alumina substrates. The polycrystalline silicon material was made by overdoping of AIC seed layers with phosphorus atoms followed by epitaxial thickening with low-pressure chemical vapor deposition (LPCVD) at 1000°C. The cells have i/p+ amorphous Si heterojunction emitters and show an average V OC of 455 mV, an average J SC of 16.5 mA cm−2 and a record efficiency of 5.0 %. The excess out-diffusion of phosphorus from the seed layers during the epitaxial growth resulted in a non-ideal doping profile, which is believed to be the main limitation of the current density besides the material quality. In addition, the non-optimized heterojunction emitter deposition for n-type polycrystalline silicon and the lack of superior light trapping also limited the present cell performance.

Solar modules made from thin-film crystalline-silicon layers of high quality on glass substrates could lower the price of photovoltaic electricity substantially. One possibility is to use polycrystalline-silicon thin films, but the difficulty of this approach is to obtain a sufficiently high material quality and to achieve an effective passivation of the numerous grain boundaries. However, the utilization of a monocrystalline-silicon thin film instead of a polycrystalline-silicon layer should enable simpler device processes, higher process yield and a single-crystal-like efficiency due to the intrinsic higher quality of the material. In this paper, 10-micron thick monocrystalline-silicon layers on transparent glass-ceramic substrates were made by epitaxial thickening of thin monocrystalline-silicon seed layers. By using a very simple cell structure that does not incorporate any light trapping features, silicon single-crystal solar cells on glass-ceramics presented efficiencies of up to 7.5%. However, efficiencies above 9% should be realizable by simply improving the contacting scheme. In addition, by integrating an efficient light trapping scheme and by optimizing the minority carrier diffusion length, the efficiency potential could be as high as 15% for films with a thickness between five to ten microns on glass-ceramics.

We present a new wafering method for the production of ultra-thin crystalline silicon. This new lift-off process, named SLIM-cut (for Stress-induced LIft-off Method), requires only the use of a screen-printer and a belt furnace; no ion-implanted, porous layer or additional thickening by epitaxy is needed to obtain high quality wafers in the thickness range of 50 μm without kerf loss. We deposit on a thick substrate a layer with mismatched coefficient of thermal expansion with respect to the substrate (for instance a metal layer). Upon cooling, the differential contraction induces a large stress field, which is released by the initiation and the propagation of a crack parallel to the surface. The concept has already been demonstrated successfully on both single and multi-crystalline silicon. Very clean self-standing crystalline films with an area of 25 cm2 have been obtained from a high quality wafer. Some Si layers were further processed into solar cells. The first unoptimized solar cell device with a very simple process (no back-surface field, no intentional texturing, heterojunction emitter) showed an energy conversion efficiency of 10.0% (1 cm2). This represents a silicon consumption already as low as 3.1 g/Wp (taking into account a polishing loss of 50 μm), and proves the conservation of the material quality during the process. The potential of the method is estimated by the development of a custom-made numerical model. The model uses a 2D finite element method and is able to propagate a crack in a multi-layered structure. The model proposes a concentric re-meshing in the region of the crack tip at every increment and a crack initiation and propagation based on maximum stress criteria.

Thin-film polysilicon solar cells on foreign substrates are often considered as a promising low cost alternative to bulk silicon solar cells. Until now however, the obtained efficiencies and open-circuit voltages are far below those of other technologies. In this paper, we show how the open-circuit voltage can be enhanced significantly using an amorphous silicon ? crystalline silicon heterojunction emitter instead of a diffused homojunction emitter. Open-circuit voltages up to 536 mV were obtained for polysilicon layers with a heterojunction emitter. This is the highest open-circuit voltage obtained for polysilicon solar cells with a p-n structure on foreign substrates.