Si-based Tandem Solar Cells Research Articles

Two-step synthesis of antimony sulfide thin films: enhancement in physical properties through sulfurization

Recently, there has been an increase in the use of antimony sulfide (Sb2S3) in Si-based tandem solar cells as a potential absorber material for top sub-cells. The choice of the material stems from the favoured properties such as appropriate bandgap, simple binary composition, nontoxic elements, and long-term stability. However, the physical properties and practical applicability of these materials depend largely on their synthesis conditions. In this work, we investigate the role of sulfurization on the structural, morphological, compositional, and optical properties of Sb2S3 thin films deposited on soda-lime glass via a thermal evaporation technique. Sulfurization was performed on the as-prepared thin films in a customized Chemical Vapor Deposition (CVD) chamber at five different temperatures. Analysis of the crystallinity of the film using the x-ray diffraction technique illustrates the transformation of the film from impure, poor crystalline phase to phase-pure, and highly crystalline orthorhombic structure due to sulfurization. Scanning electron microscopic investigations of the samples revealed better grains with nanorods on the surface at a temperature of 400 °C. For the samples investigated here, the energy values estimated via density functional theory (DFT) calculations agreed well with the experimental data obtained from UV-visible absorption spectral studies. Additionally, it was observed that the desired near-stoichiometric Sb2S3 thin films could be achieved via sulfurization, and the presence of Sb2S3 in all samples was confirmed via Raman spectroscopic studies. Additionally, the defects and trap states of the prepared films were investigated using photoluminescence studies, and donor and acceptor defects were identified. Our study revealed that sulfur rich Sb2S3 films prepared at a sulfurization temperature of 400 °C produced the desired structure, morphology, and optical qualities for future photovoltaic applications.

The Intermediate Connection of Subcells in Si-based Tandem Solar Cells.

Tandem solar cells are rationally designed and fabricated by stacking multiple subcells to achieve power conversion efficiency well above the Shockley-Queisser (SQ) limit. There is a large selection pool for the subcell candidates, such as Si, III-V, Kesterite, Perovskite, and organic solar cells. A series of different combinations of these subcells have been successfully demonstrated in practical tandem solar cell devices. However, there has not been a systematic summary of how to connect subcells in a tandem solar cell using a practical, cost-effective, and efficiency-beneficial fashion. In this work, the connection manners of subcells within a tandem cell are classified into three main categories, performing sequential growth, using the physical connection, and applying an intermediate layer, focusing on systematical description of intermediate layers using different materials. The advantages and disadvantages of these connection methods and their applicability to tandem cell types are further elaborated using two typical example models, III-V/Si and Perovskite inclusive tandem cell devices. Eventually, this work can provide useful guidance on how to carry out a suitable intermediate connection in the design of tandem solar cells depending on the selected subcells and device structure.

GaAs//Si Multijunction Solar Cells Fabricated via Mechanical Stack Technology Using Pd Nanoparticles and Metal-Assisted Chemical Etching

Multijunction solar cells (MJSCs) have attracted attention as next-generation solar cells. In particular, GaAs//Si-based MJSCs are highly efficient with low cost and are expected to gain new applications, such as on-vehicle integrations. In this article, we examined a highly efficient In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.49</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.51</sub> P/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.06</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.94</sub> As//Si three-junction solar cell. The bottom Si cell has a tunnel oxide passivated contact structure. The key technology used to fabricate this solar cell is a stacking method that uses Pd nanoparticles (Pd-NPs) and metal-assisted chemical etching (MacEtch) for the bonding interface, which is improved from our previous “smart stack” technology. The MacEtch method has a feature of selective etching for Si around a metal body. Pd-NPs selectively invade the Si cell through the surface of the Si oxide layer, thereby improving the bonding resistivity between the GaAs-based cell and Si cell. Further, this technology aids the management of the bonding gap width by controlling the Pd-NP invasion depth. As a result, an efficiency of 27.6% for the aperture area was attained. The proposed technology is useful for the connection of Si-based cells, enhancing the development of GaAs//Si-based tandem solar cells.

Hyperdoping of silicon films with titanium via nanosecond-laser melting: Structure evolution, impurity distribution, sub-bandgap formation, and doping mechanism

Hyperdoping of Si films with Ti was achieved via nanosecond (ns)-laser melting of Si/Ti composite films under different film deposition and laser treatment conditions. The structural evolution (from amorphous to nanocrystalline structure), Ti impurity distribution and concentration variation, and sub-bandgap (Ti impurity band) formation in the films, were carefully studied to optimize the hyperdoping process. The results show that the hyperdoping concentration and depth can be manipulated by controlling the Si and Ti co-evaporation speed ratio, Si/Ti composite film thickness together with a matched ns-laser fluence. Furthermore, Ti impurity bands were determined with two sub-bandgaps of energy 385–448 and 759 meV. In addition, the mechanism of ns-laser hyperdoping of Si films was clarified into the heat transfer, material ejection, partial ablation and redeposition, melting, and nanocrystallization (simultaneous hyperdoping) processes. Thus, hyperdoped nanocrystalline films up to the micron-level thickness with a crystallinity of ∼ 70%, an optical absorptance of ∼ 90% from the visible to the long-wavelength near-infrared spectrum, and good electronic transport properties were obtained. These hyperdoped Si films exhibit a high potential in the development of Si-based broad-spectrum tandem or thin film solar cells and room-temperature infrared photodetectors.

Si tunnel junctions obtained by proximity rapid thermal diffusion for tandem photovoltaic cells

Low-resistance c-Si Esaki tunnel junctions (TJ) can be applied in two-terminal Si-based tandem solar cells to electrically connect two sub-cells. Proximity rapid thermal diffusion (PRTD) is an economical and facile method to fabricate the Si tunnel junctions with a damage-free surface. The p++/n++ Si TJ on (111)-oriented c-Si wafer produced by combining PRTD and photovoltaic industrial techniques is reported in this work. The adjustment of the n++ emitter by a two-step rapid thermal annealing effectively facilitates the realization of the p++/n++ TJ. The peak current density of a tunnel diode based on this TJ is within the range 140–192 A cm−2 with a peak to valley current ratio of 1.9–3.2. Such a p++/n++ TJ is implemented in III–V nanowires (NWs) on Si tandem solar cells. Despite the defectuosity of the NWs array, we demonstrate that an increase of the open-circuit voltage is observed compared with the sole single-junction Si solar cell. This kind of TJ can also be integrated with other top cell materials such as perovskites and copper indium gallium selenide. Low-cost and high-efficiency c-Si based tandem solar cells might be produced with the application of Si TJs obtained by PRTD.

Possible top cells for next-generation Si-based tandem solar cells.

Si-based solar cells, which have the advantages of high efficiency, low manufacturing costs, and outstanding stability, are dominant in the photovoltaic market. Currently, state-of-the-art Si-based solar cells are approaching the practical limit of efficiency. Constructing Si-based tandem solar cells is one available pathway to break the theoretical efficiency limit of single-junction silicon solar cells. Various top cells have been explored recently in the construction of Si-based tandem devices. Nevertheless, many challenges still stand in the way of extensive commercial application of Si-based tandem solar cells. Herein, we summarize the recent progress of representative Si-based tandem solar cells with different top cells, such as III-V solar cells, wide-bandgap perovskite solar cells, cadmium telluride (CdTe)-related solar cells, Cu(In,Ga)(Se,S)2 (CIGS)-related solar cells, and amorphous silicon (a-Si) solar cells, and we analyze the main bottlenecks for their next steps of development. Subsequently, we suggest several potential candidate top cells for Si-based tandem devices, such as Sb2S3, Se, CdSe, and Cu2O. These materials have great potential for the development of high-performance and low-cost Si-based tandem solar cells in the future.

Characterization of ZnSiP2 Films Grown on Si Substrate by Liquid Phase Epitaxy: Morphology, Composition, and Interface Microstructure

The exploration and growth of novel cost-effective top cell materials is one of the great hot topics for the development of Si-based tandem solar cells. ZnSiP2 was thought to be a promising candidate of the top cell material for the Si tandem solar cell. In this work, ZnSiP2 single crystal film with the stoichiometric ratio was successfully grown on a Si substrate by the liquid phase epitaxy method for the first time. The surface morphology, thickness, composition, and the orientation of the film were characterized by SEM, EDS, EPMA, and the high-resolution XRD. The results show that a continuous epitaxial ZnSiP2 film with the thickness of 3 μm was grown on the Si substrate. In addition to this continuous film, many island single crystals were also observed on the surface of the film. The high-resolution XRD φ scan results indicated that the film as well as these isolated islands crystallized with the same orientation, c-orientation with respect to the (100) Si substrate, without any misoriented domains. The microstructure of the ZnSiP2/Si interface characterized by the high-resolution transmission electron microscopy image, as well as the selected area electron diffraction patterns of the ZnSiP2 thin films and Si substrates, also confirmed the epitaxial growth of ZnSiP2 on the Si substrate. The electrical conductivity σ, Hall mobility μH, and carrier concentrations n of the ZnSiP2 film were 1.38 × 10–5 Ω–1 cm–1, 94.5 cm–2 V–1 s–1, and 9.11 × 109 cm–3, respectively. Our work paves the way for the development of ZnSiP2 silicon-based tandem solar cell devices.

Structural optimization and quantum size effect of Si-nanocrystals in SiC interlayer fabricated with bio-template

Amorphous SiC/Si multilayers were fabricated using alternately magnetron sputtering of SiC and electron beam evaporation of Si targets. The as-deposited films were annealed at different temperatures from 800 till 1000 °C to form Si-nanocrystals (Si-NCs) and study the layered structural stability by different analytical techniques, including x-ray reflectivity (XRR), x-ray diffraction (XRD), secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM), and surface morphology by atomic force microscopy (AFM). XRR demonstrated that SiC/Si multilayers annealed at temperatures up to 1000 °C retain their layered structure. XRD and TEM confirmed the formation of Si-NCs after annealing at ≥800 °C. The Si-NCs size estimated from the TEM images is ∼4 nm for the multilayered sample with alternating 2 nm SiC and 4 nm Si layers. SIMS revealed that SiC/Si multilayers annealed at 800 °C possess the best periodic structure possibly due to the lowest carbon interdiffusion as compared to the samples annealed at higher temperatures. AFM micrograph confirmed that SiC/Si multilayers annealed at 800 °C characterized by the lowest surface roughness having RMS value of 0.123 nm as compared to the samples annealed at 900 °C and 1000 °C having RMS values of 0.207 and 0.530 nm, respectively. Size-controlled 3D array of Si-NCs separated by SiC barriers was fabricated by neutral beam etching of annealed SiC/Si multilayers with different Si layers thickness from 2 to 6 nm using a bio-template consisting of ferritin molecules. The size-dependent band gap energy of Si-NCs was estimated by optical spectroscopy to vary from 1.61 till 1.92 eV owing to the quantum confinement effect as the Si-NCs size decreases, which is appropriate for application in Si-based tandem solar cells.

Fabrication of Sb2S3 thin films by sputtering and post-annealing for solar cells

The semiconductor antimony sulfide (Sb2S3) is a potential absorber materials for the top sub-cell of Si-based tandem solar cells because of its appropriate band-gap, simple binary composition, nontoxic elements, and long-term stability. In this study, polycrystalline Sb2S3 films were fabricated by post-annealing of radio frequency (RF) magnetron sputtered precursors using an Sb2S3 target. The effects of the post-annealing temperature and atmosphere on Sb2S3 film properties and device performances were investigated. A high-performance device having a 2.41% power conversion efficiency was obtained by making use of a uniform Sb2S3 absorber layer. This preliminary experimental study shows that Sb2S3 thin films could be used as top sub-cell absorber materials for third-generation high efficiency, stable, and environmentally friendly Sb2S3/Si tandem solar cells.

In situ growth of Sb2S3 thin films by reactive sputtering on n-Si(100) substrates for top sub-cell of silicon based tandem solar cells

Si-based tandem solar cells have emerged as a promising alternative to replace the current high efficiency silicon single junction solar cells. Sb2S3 is a promising candidates for top cell absorbers due to its suitable band gap, non-toxic and earth-abundant constituent, simple composition, and long term stability. Here, in situ growth of Sb2S3 thin films by radio frequency (RF) reactive sputtering on n-Si(100) substrates is reported. The effects of the growth temperature on morphology, structure, optical and electrical properties of the films were evaluated. The top sub-cell device shows an open circuit voltage (Voc) of 367mV, indicating the potential to be the top cell of the next generation Si-based tandem solar cells.

Realization of GaInP/Si Dual-Junction Solar Cells With 29.8% 1-Sun Efficiency

Combining a Si solar cell with a high-bandgap top cell reduces the thermalization losses in the short wavelength and enables theoretical 1-sun efficiencies far over 30%. We have investigated the fabrication and optimization of Si-based tandem solar cells with 1.8-eV rear-heterojunction GaInP top cells. The III–V and Si heterojunction subcells were fabricated separately and joined by mechanical stacking using electrically insulating optically transparent interlayers. Our GaInP/Si dual-junction solar cells have achieved a certified cumulative 1-sun efficiency of 29.8% ± 0.6% (AM1.5g) in four-terminal operation conditions, which exceeds the record 1-sun efficiencies achieved with both III–V and Si single-junction solar cells. The effect of luminescent coupling between the subcells has been investigated, and optical losses in the solar cell structure have been addressed.

III-V/Si wafer bonding using transparent, conductive oxide interlayers

We present a method for low temperature plasma-activated direct wafer bonding of III-V materials to Si using a transparent, conductive indium zinc oxide interlayer. The transparent, conductive oxide (TCO) layer provides excellent optical transmission as well as electrical conduction, suggesting suitability for Si/III-V hybrid devices including Si-based tandem solar cells. For bonding temperatures ranging from 100 °C to 350 °C, Ohmic behavior is observed in the sample stacks, with specific contact resistivity below 1 Ω cm2 for samples bonded at 200 °C. Optical absorption measurements show minimal parasitic light absorption, which is limited by the III-V interlayers necessary for Ohmic contact formation to TCOs. These results are promising for Ga0.5In0.5P/Si tandem solar cells operating at 1 sun or low concentration conditions.

Annealing of nm-thin Si1−xCx/SiC multilayers

Amorphous hydrogenated silicon carbide (a-SiC:H) multilayers consisting of nm-thin, alternating Si-rich Si1−xCx and stoichiometric SiC layers were prepared using plasma enhanced chemical vapour deposition. Such multilayers are possible precursor layers for forming Si nanocrystals (NCs) for Si-based tandem solar cells. The multilayers were annealed at temperatures up to 1000°C for 30min and the crystallization of Si NCs in a SiC matrix was investigated. The annealing temperature affected the nanostructure of the multilayers and was studied using various techniques: by scanning electron microscopy we observed that the total thickness of the multilayers was reduced during annealing by 30%. X-ray diffraction revealed the crystallization of Si and SiC NCs when annealing temperatures exceeded 900°C. The crystal structure and the nanostructure were investigated by Transmission Electron Microscopy (TEM) bright-field imaging, high resolution TEM and electron diffraction. By electron diffraction it was proved that SiC crystallized in the cubic polytype 3C-SiC during the annealing process. Energy-filtered TEM showed a roughening of the multilayer structure when crystalline phases were formed. Aberration-corrected high-resolution images yielded lattice fringe images of the SiC and Si NCs. The size of the NCs was 5nm for Si and 2nm for SiC. Raman spectroscopy and electron diffraction showed that a large fraction of Si was still amorphous after annealing at 900°C. Annealing at 1000°C reduced the fraction of the amorphous phase by ∼35% for Si and ∼25% for SiC and strongly affected the crystalline perfection of the NCs and also the integrity of the layers. In contrast to Si NCs in a SiO2 matrix, Si NCs in a SiC matrix did not yield significant photoluminescence possible reasons will be discussed.

Direct observation of quantum confinement of Si nanocrystals in Si-rich nitrides

The band gap of the active layers in stacked Si-based tandem solar cells can be fine tuned by changing the size of embedded silicon nanocrystals (Si NCs). Although SiO${}_{2}$ matrices have been predominantly used for such applications, nitride phases have recently emerged as a promising alternative. In this paper, we use high-resolution scanning transmission electron microscopy and energy-loss spectroscopy to report on the electronic structure of individual Si NCs embedded in silicon nitride films. Si NCs were produced by rf sputtering and exhibited controllable crystallite size and quality via different thermal annealing conditions. Quantum confinement effects were observed through a blue shift in both conduction band edges and volume plasmon energies as a function of particle size and structure. We show that, in good agreement with theoretical models, the volume plasmon energy ${E}_{p}$ (eV) is related to the size $d$ (nm) of Si NCs by ${E}_{p}$ $=$ 16.89$+$ 23.90/${d}^{2}$. Lattice distortion in twinned Si NCs and dangling bonds at defect centers are shown to be the cause of a weakening in quantum size effects and a reduction in the light emission efficiency of the films. Both electron spectroscopy and optical results are consistent in explaining the correlation between structure and optoelectronic properties of Si NCs.

Luminescence mechanisms in Si quantum dots-SiN[sub x] nanocomposite structures

Silicon quantum dots have been grown in situ in amorphous silicon nitride (SiNx) films on silicon substrate by plasma-enhanced chemical vapor deposition using ammonia (NH3) and silane (SiH4) as reactant gases. After deposition, films were annealed in N2 atmosphere at temperatures ranging from 700 to 1000 °C for 1 min using rapid thermal annealing system. The evolution of the Si quantum dots (QDs) and the chemical composition of the films have been investigated by Raman scattering and infrared absorption spectroscopy, and the correlation with the optical properties is presented. From the photoluminescence measurements, the position and the intensity of the photoluminescence peak were studied as a function of the annealing temperature (Ta), and the luminescence mechanism in Si QDs-SiNx composite structures is attributed to the quantum confinement in silicon clusters. These results lead to further understanding the physical and optical properties of Si QDs embedded in silicon nitride films and thus open up the possibility of realizing efficient Si-based tandem solar cells.

Optimization of Silicon-Based 2-Terminal Tandem Solar Cells with GaAs1-xPx and InGa1-xPx as Top Cell Material

A new structure of Si-based 2-terminal tandem solar cells was numerically analyzed. The structure was optimized to obtain high efficiency considering realistic material parameters. Gallium arsenide phosphide (GaAs_1-xP_x) and indium gallium phosphide (In_1-xGa_xP) were proposed as top cell materials. The conversion efficiency of the cell was calculated by simulating its voltage–current performances under the operating conditions. An efficiency of 33.1% can be expected for a GaAs_0.73P_0.27/Si cell and 34.4% for an In_0.57Ga_0.43P/Si cell. In_1-xGa_xP was shown to be more promising as a top cell material because it has a larger absorption coefficient than aAs_1-xP_x.