Ultrathin Solar Cells Research Articles

Modeling free-carrier absorption in ultrathin III-V solar cells with light management.

Ultrathin III-V solar cells with proper light management have become more attractive than their optically thick counterparts as they are less expensive and lightweight, can maintain photon absorption, and have high radiation tolerance for space-related applications. Comprehensive optical modeling efforts have provided pathways to improve device efficiency in ultrathin GaAs solar cells with light trapping structures. Usually, the absorption mechanism known as free-carrier absorption (FCA) is ignored in these models due to the ultrathin layers and the direct bandgap of GaAs. This manuscript reports the significance of considering FCA as a parasitic loss caused by the optical enhancement in highly doped non-active layers between the ultrathin solar cell and backside light trapping structures. We model FCA based on Drude theory in a p-type AlGaAs layer behind ultrathin GaAs solar cells with a planar mirror and cylindrical gratings. Our results show that, depending on the AlGaAs thickness and doping concentration, free carriers will absorb transmitted photons and reduce the backside reflectance, degrading the current and voltage output from ideal conditions. One example shows that for a 300 nm-thick GaAs solar cell, the Ag mirror's peak reflectance decreases nearly 12% when the AlGaAs back layer is 800 nm-thick at a doping concentration of 4x1019 cm-3. Notably, the cylindrical grating designs with 38.5%, 46.5%, and 64.9% AlGaAs coverage resulted in an absolute efficiency reduction of 0.6%, 1.8%, and 2.9% at a doping concentration of 4x1019 cm-3, respectively. This novel study demonstrates that FCA in non-active layers must be properly addressed in the device design to progress the efficiency of ultrathin III-V solar cells with light trapping structures.

Cation disorder engineering yields AgBiS2 nanocrystals with enhanced optical absorption for efficient ultrathin solar cells

Strong optical absorption by a semiconductor is a highly desirable property for many optoelectronic and photovoltaic applications. The optimal thickness of a semiconductor absorber is primarily determined by its absorption coefficient. To date, this parameter has been considered as a fundamental material property, and efforts to realize thinner photovoltaics have relied on light-trapping structures that add complexity and cost. Here we demonstrate that engineering cation disorder in a ternary chalcogenide semiconductor leads to considerable absorption increase due to enhancement of the optical transition matrix elements. We show that cation-disorder-engineered AgBiS2 colloidal nanocrystals offer an absorption coefficient that is higher than other photovoltaic materials, enabling highly efficient extremely thin absorber photovoltaic devices. We report solution-processed, environmentally friendly, 30-nm-thick solar cells with short-circuit current density of 27 mA cm−2, a power conversion efficiency of 9.17% (8.85% certified) and high stability under ambient conditions.

Designing transparent nanophotonic gratings for ultra-thin solar cells.

Integration of a rear surface nanophotonic grating can increase photocurrent in ultra-thin solar cells. Transparent gratings formed of dielectric materials and high bandgap semiconductors can offer efficient diffraction with lower parasitic absorption than more widely studied metal/dielectric equivalents. In these systems, the maximum photocurrent which can be obtained for a grating made of a given combination of materials is shown to follow a simple empirical model based on the optical constants of these materials and independent of grating dimensions. The grating dimensions still require optimization in order to maximize the photocurrent for a given active layer thickness by balancing the effects of diffraction outside the front surface escape cone and the tuning of waveguide modes in long wavelength regions which are poorly absorbed in an ultra-thin film. The optimal grating pitch is shown to be of particular relevance for both effects, changing nonmonotonically as the absorber gets thicker in order to track favourable waveguide mode resonances at wavelengths near the absorber bandgap. These trends together with the empirical model for material selection drastically reduce the design space for highly efficient light trapping with transparent gratings.

Simulations of ultrathin monolayer/multilayer molybdenum disulfide heterojunction solar cell

Ultrathin solar cell has good potential in photovoltaics, monolayer and multilayer transition metal dichalcogenide materials could be used to make ultrathin solar cell because of their high absorption coefficients. The photovoltaic characteristics of monolayer MoS2/multilayer MoS2 vertical heterojunction ultrathin solar cell was calculated in this work based on SCAPS simulation software, monolayer MoS2 was used to make window layers and transparent conducting layers at the same time, multilayer MoS2 was used to make absorber layer. The highest efficiency of the cell with the thickness of 200 nm could reach 16.8% when the lifetime of multilayer MoS2 was larger than 250 ns, the electron and hole mobility of multilayer MoS2 was larger than 14 cm2/V, the recombination velocity of monolayer/multilayer MoS2 interface was smaller than 1 × 106 cm/s. The transmittance of the cell with the thickness of 200 nm approached 1 when wavelength was larger than 740 nm. Thus, monolayer/multilayer MoS2 vertical heterojunction solar cell has good potential in semitransparent solar cell, and the cell could be used as the top cell of silicon based tandem cell.

Ultrathin GaAs solar cells with a high surface roughness GaP layer for light‐trapping application

AbstractBy reducing the thickness of the absorber layers, ultrathin GaAs solar cells can be fabricated in a more cost‐effective manner using less source material and shorter deposition times. In this work, ultrathin GaAs solar cells are presented with a diffuse scattering layer based on wide bandgap GaP grown directly on the device layers of the cells with MOCVD. The roughness and surface morphology are quantified using atomic force microscopy and the resulting diffuse scattering capability is assessed using wavelength‐dependent reflectance measurements. Ohmic rear contacts are made using contact points etched through the GaP layer, for which an etching procedure using I2:KI was developed and optimized. The performance of the GaP textured ultrathin GaAs cells are compared with equivalent planar cells using current density‐voltage measurements and external quantum efficiency measurements, where the GaP textured cells demonstrate an increase of 6.7% in the short‐circuit current density (JSC), which was found to be as high as 21.9 mA·cm−2 as a result of increased photon absorption by light‐trapping.

Picosecond laser patterning for ultrathin spectrally selective solar mini-modules with transparent metal-oxide multilayer electrodes

Semitransparent solar cells featuring spectrally selective light transmission have been developed for agrivoltaic applications. One novel approach for this technology is the combination of a metal-oxide multilayer electrode with an ultrathin germanium solar cell. However, the combination of this electrode possessing multiple thin silver layers with the ultrathin absorber imposes additional challenges for the laser scribing processes that are used for the module integration. In this work, we report on a picosecond laser patterning approach for the monolithic interconnection of a spectrally selective solar cell on mini-module level. We found, that a green (532 nm) and an infrared laser (1064 nm) are suitable for the scribing of the metal-oxide multilayer electrode and the ultrathin absorber. Our approach is an important step towards large scale module fabrication for versatile agrivoltaic applications.

The role of photonic and plasmonic modes in ultrathin amorphous silicon solar cells using finite difference time domain method

In photovoltaic market, the enhancement of optical performance in an ultra-thin film amorphous silicon (a-Si) solar cell is significant. Our main objective is to design and simulate a highest current density by using various nanostructures integrated with ultrathin a-Si solar cells. In this proposed work, the various photonic and plasmonic nanostructures were used in front side of the absorber layer and enhanced the collection of charge carriers due to that an improved the current density in ultrathin a-Si solar cells. Using finite-difference time-domain (FDTD) method, the obtained results indicate the optimized SiO2 nanogratings and Si3N4 thin films acting as an anti-reflection coating (ARC) layer with air medium (AM1.5) which showed significant current density of 16.25 mA/cm2 in ultra-thin a-Si solar cells. Finally, the light trapping mechanism evidenced the importance of nanostructures using transverse electric (TE) and magnetic (TM) field distributions by changing the various incident wavelength.

Bifacial semi-transparent ultra-thin Cu(In,Ga)Se2 solar cells on ITO substrate: How ITO thickness and Na doping influence the performance

Ultra-thin Cu(In,Ga)Se2 (CIGSe) is a promising absorber for thin-film solar cells, as it combines the advantages of low raw material consumption and high conversion efficiency. In addition, ultra-thin absorbers on transparent back contacts bring the advantage of semitransparency, which is essential for e.g. tandem or bifacial solar cells. This work optimizes ultra-thin CIGSe on In2O3:Sn (ITO) for application in bifacial semi-transparent ultra-thin (BSTUT) CIGSe solar cells. Firstly, 100–400 nm ITO were coated onto glass substrates, and it was revealed that the thickness of ITO influences its optical bandgap Eg due to the Burstein-Moss (B-M) shift. The band gap of 400 nm ITO increased by 0.14 eV compared to the 100 nm thick ITO, and the Voc of the related BSTUT CIGSe solar cells raised by 0.043 V as a result of the diminished Schottky barrier Φb at the ITO/CIGSe interface. Secondly, 0–8 mg of NaF used for post deposition treatment (PDT) of the CIGSe were applied to the BSTUT solar cells. Compared to the reference without NaF, 8 mg NaF PDT enhanced the carrier density NA from 2 × 1015cm−3 to 1.2 × 1016cm−3 and diminished the ITO/CIGSe Schottky barrier Φb by 0.21 eV. In conclusion, we found that NaF PDT can tune the carrier density of ultra-thin CIGSe on ITO, and both thicker ITO and higher NaF PDT dose can reduce the ITO/CIGSe Schottky barrier. These discoveries enable future optimization of BSTUT CIGSe solar cells.

Effects of frequency upconversion layer and two-dimensional arrays of inverted nanocone hole on the performance of ultrathin crystalline silicon solar cells

We report the design and optimization of photonic crystal (PhC) structures within the crystalline silicon absorption layer in ultrathin solar cells with a frequency upconversion layer. In the PhC structure, inverted nanocone hole (ICH) arrays with different diameters and inclination angles are used, and the light-trapping performances of air and Si3N4 filled ICH arrays are systematically studied by COMSOL Multiphysics, respectively. The simulation results indicate that the average optical absorption of the solar cells and the local electric field intensity in the upconversion layer can be greatly improved by using PhC structures within the absorption layer. For ICH in height of 1.6 μm, the maximum absorptivity of the solar cells with air filled ICH arrays is 90.75%, while the maximum absorptivity of solar cells with Si3N4 filled ICH arrays is 85.62%. In addition, the upconversion effects of β-NaYF4:Er3+/Yb3+ nanomaterials on the power conversion efficiency of the solar cells were also investigated. The presented model will be the basis for further preparations of high-efficiency ultrathin crystalline silicon solar cells.

Ultrathin Bifacial Passivated Emitter and Rear Cell with Inverted Pyramid Textures

Wafer thinning is a crucial technique for high‐efficiency solar cells. Herein, an inverted pyramid (IP) texture is prepared on a 35 μm‐thick flexible silicon (Si) wafer with a standard area of 156 × 156 mm2. Based on the experimental results, an ultrathin bifacial passivated emitter and rear cell (PERC) with IP textures using PC1D simulation is designed. The influence of wafer thickness, IP texture, bifacial structure, thickness of the antireflection coating, and doping concentration on device performance is investigated. The results show that the ultrathin IP‐based bifacial PERC possesses better output performance than the traditional cell. Finally, a simulated maximum efficiency of 23.44% is obtained using PC1D software, with an open‐circuit voltage of 0.7127 V and a short‐circuit current of 9.272 A. This ultrathin PERC with IP textures provides an effective way to improve the efficiency of ultrathin silicon solar cells.

High-Efficiency Ultrathin Si-Based Solar Cells by Cascading Dilute-Nitride GaNAsP.

Thin-film solar cells are currently an important research subject. In this study, a lattice-matched GaNAsP/Si tandem cell was designed. We adopted the drift-diffusion model to analyze the power conversion efficiency (PCE) of the solar cell. To find the maximum solar cell PCE, the recombination terms and the interlayer between subcells was omitted. For an optimal tandem cell PCE, this study analyzed the mole fraction combinations of GaNAsP and the thickness combinations between the GaNAsP and the Si subcells of the tandem cell. Our results showed the superiority of the tandem cell over the Si cell. The 4.5 μm tandem cell had a 12.5% PCE, the same as that of the 10.7 μm Si cell. The 11.5 μm tandem cell had 20.2% PCE, while the 11.5 μm Si cell processed 12.7% PCE. We also analyzed the Si subcell thickness ratio of sub-12 μm tandem cells for maximum PCE. The tandem cell with a thickness between 40% to 70% of a Si cell would have a max PCE. The ratio depended on the tandem cell thickness. We conclude that the lattice-matched GaNAsP/Si tandem cell has potential for ultrathin thin Si-based solar cell applications.

Cation disorder engineering yields AgBiS2 nanocrystals with enhanced optical absorption for efficient ultrathin solar cells

Cation disorder engineering yields AgBiS2 nanocrystals with enhanced optical absorption for efficient ultrathin solar cells

Enhanced absorption in thin and ultrathin silicon films by 3D photonic band gap back reflectors

Since thin and ultrathin silicon films have limited optical absorption, we explore the effect of a nanostructured back reflector to recycle the unabsorbed light. As a back reflector, we investigate a three-dimensional (3D) photonic band gap crystal made from silicon that is readily integrated with the thin silicon films. We numerically obtain the optical properties by solving the 3D time-harmonic Maxwell equations using the finite-element method, and model silicon with experimentally determined optical constants. The absorption enhancement spectra and the photonic band gap generated current density are obtained by weighting the absorption spectra with the AM 1.5 standard solar spectrum. We study thin films in two different regimes, much thicker (L Si = 2400 nm) or much thinner (L Si = 80 nm) than the wavelength of light. For L Si = 2400 nm thin film, the 3D photonic band gap crystal enhances the spectrally averaged (λ = 680 nm to 880 nm) silicon absorption by 2.22 × (s −pol.) to 2.45 × (p −pol.), which exceeds the enhancement of a perfect metal back reflector (1.47 to 1.56 ×). The absorption is considerably enhanced by the (i) broadband angle and polarization-independent reflectivity in the 3D photonic band gap, and (ii) the excitation of many guided modes in the film by the crystal’s surface diffraction leading to greatly enhanced path lengths. For L Si = 80 nm ultrathin film, the photonic crystal back reflector yields a striking average absorption enhancement of 9.15 ×, much more than 0.83 × for a perfect metal. This enhancement is due to a remarkable guided mode that is confined within the combined thickness of the ultrathin film and the photonic crystal’s Bragg attenuation length. An important feature of the 3D photonic band gap is to have a broad bandwidth, which leads to the back reflector’s Bragg attenuation length being much shorter than the silicon absorption length. Consequently, light is confined inside the thin film and the remarkable absorption enhancements are not due to the additional thickness of the photonic crystal back reflector. We briefly discuss a number of high-tech devices that could profit from our results, including ultrathin film solar cells.

Performance improvement of semi-transparent ultra-thin CIGSe solar cell by transferring exfoliated WTe2 multilayered-2D flakes to ITO substrate

Cu(In1-x,Gax)Se2 (CIGSe)-based solar cells have a high absorption coefficient (∼105), by which they can be used as thin-film solar cells that have the potential to be applied in various fields. To employ such a CIGSe-based solar cell as a transparent type, the conventional Mo substrate should be replaced with a transparent conductive oxide (TCO) substrate as the back electrode, and indium tin oxide (ITO) is one of the candidate TCO materials. However, various problems need to be addressed because the utilization of ITO may hinder solar cell performance, one of which is the contact occurring at the interface between CIGSe and ITO. This can be solved by adding an appropriate material at the interface while preserving the inherent purpose fulfilled by using the ITO electrode. In this study, we attempt to improve the contact properties using semi-metallic WTe2 flakes dispersed on the surface of the ITO substrate to form an electrical path between the CIGSe and the ITO substrate, which exhibit outstanding electrical conductivity among two-dimensional transition metal dichalcogenide materials. Accordingly, the effect of applying WTe2 flakes at the ITO and CIGSe interfaces on the performance of semi-transparent ultra-thin (STUT) CIGSe solar cells is discussed.

Fourteen percent efficiency ultrathin silicon solar cells with improved infrared light management enabled by hole‐selective transition metal oxide full‐area rear passivating contacts

AbstractThe present study investigates the application of hole‐selective transition metal oxide (TMO) layers (MoOx, V2Ox, and WOx) with silver (Ag) as full‐area rear contact to 22.5 μm‐thick low‐quality Cz p‐type c‐Si solar cells. Thin films of metal oxides are deposited directly on p‐type c‐Si by thermal evaporation at room temperature. The large work function of these TMOs creates strong accumulation at the interface with p‐type c‐Si, which allows only holes to transport and simultaneously suppress the interfacial recombination current density (J0) and contact resistivity (ρc). The current generation and losses of 22.5 μm‐thick solar cells with different hole‐selective TMO/Ag at the rear are simulated. The presence of TMO/Ag at the rear is found to significantly reduce parasitic light absorption at longer wavelengths which becomes more pronounced for ultrathin wafers, providing significant advantages over conventional Al contact. The best device performance was attained by the MoOx/p‐type c‐Si solar cells, demonstrating a considerably high efficiency (η) of 14% with Voc of 555 mV, FF of 76.0%, and Jsc of 33.2 mA/cm2. Furthermore, the present work is the first to employ MoOx, V2Ox, and WOx as rear contact in ultrathin p‐type c‐Si solar cells.

Ultrathin Solar Cell With Magnesium-Based Optical Switching for Window Applications

Photovoltaic windows that can be switched between transparent and energy harvesting mode can be realized by using ultrathin solar absorbers embedded in an optical nanocavity. In the present work, we use a 5 nm thick amorphous germanium absorber integrated in a magnesium-based thin film optical cavity, which switches from an absorptive to a transparent state due to hydrogen absorption. We analyze the influence of the mirror layer thickness on the light absorption, photocurrent generation, and transmission as well as color neutrality of the device. The optical properties are studied by 1-D transfer-matrix method by changing Mg thickness between 0 and 100 nm, then compared to the experimental results of fabricated devices. When the thickness of Mg increases, the switchable average transparency varies between 25% and 0%, while the power conversion efficiency rises up to 2.3%. The applicability of the device is tested by modeling the annual power generation in realistic scenarios. The influence of the cardinal orientation and the seasons on the switchable photovoltaic window implemented in a building facade with the abovementioned parameters is analyzed for different switching scenarios.

3‐D Modeling of Ultrathin Solar Cells with Nanostructured Dielectric Passivation: Case Study of Chalcogenide Solar Cells (Adv. Theory Simul. 11/2021)

3-D Modeling of Ultrathin Solar Cells In article number 2100191, Waseem Raja, Erkan Aydin, Thomas G. Allen, and Stefaan De Wolf develop a novel 3-D optoelectrical finite-element model to analyze the performance of ultrathin solar cells. In particular, the 3-D model investigates the light trapping and field passivation in which the negative charges induced on the nanostructured passivation layer repel the minority charge carriers, increasing the solar cell's open-circuit voltage and power conversion efficiency.

Is a passivated back contact always beneficial for Cu (In,Ga)Se2solar cells?

AbstractUltrathin CIGSe solar cells on TCOs (transparent conductive oxides) have many promising potential applications due to their transparency. However, their performance is strongly constrained due to the back contact, often referred to acting as a Schottky junction. In this work, we show that the current limitation due to a back Schottky junction can be responsible for the apparent photocurrent loss and suppression of the forward current, which deteriorates cell performance and leads to the S‐shapedJ‐Vcharacteristics, respectively. However, due to the non‐negligible photocurrent in the back Schottky diode and the current amplification effect of a transistor, cells with a reduced absorber thickness overall exhibit a relieved current suppression, resulting in a lightJ‐Vcurve closer to the one with an Ohmic back contact. Notably, a higher back interface recombination velocity is capable of increasing the forward current of the Schottky diode by recombination, which mitigates the limitation of current flow through the back Schottky junction and thus improves the cell performance. The results show strong implications that introducing high density defects at the back interface is favorable for CIGSe solar cells on TCOs, rather than employing point‐contact structures for interface passivation. This suggests a different path to improve ultrathin CIGSe solar cells on TCOs compared to the devices on Mo with an Ohmic back contact.

High-Performance Laterally Oriented Nanowire Solar Cells with Ag Gratings

A laterally oriented GaAs p-i-n nanowire solar cell with Ag gratings is proposed and studied via coupled three-dimensional optoelectronic simulations. The results show that the gratings significantly enhance the absorption of nanowire for both TM and TE polarized light due to the combined effect of grating diffraction, excitation of plasmon polaritons, and suppression of carrier recombination. At an optimal grating period, the absorption at 650–800 nm, which is an absorption trough for pure nanowire, is substantially enhanced, raising the conversion efficiency from 8.7% to 14.7%. Moreover, the gratings enhance the weak absorption at long wavelengths and extend the absorption cutoff wavelength for ultrathin nanowires, yielding a remarkable efficiency of 13.3% for the NW with a small diameter of 90 nm, 2.6 times that without gratings. This work may pave the way toward the development of ultrathin high-efficiency nanoscale solar cells.

Probing the significance of RF magnetron sputtering conditions on the physical properties of CdS thin films for ultra-thin CdTe photovoltaic applications

We comprehensively explored the impact of RF magnetron sputtering conditions on the physical properties of CdS thin films for the first time. Structural, morphological, and compositional properties were altered to a great extent by the sputtering conditions. Very high average transmittances of 94%, 92.5%, 91% and 89% were achieved in CdS thin films with thicknesses of 40, 60, 80, and 100 nm, respectively. A 100-nm thick CdS thin film showed an excessively high mobility of 92 cm2/V·s with an optimum carrier concentration of 1017 cm−3 and resistivity of 103 Ω·cm. An ultra-thin CdTe film (480 nm) with a smooth and crack-free surface was achieved with RF magnetron sputtering. As proof of concept, ultra-thin CdTe solar cells were fabricated by incorporating CdS thin films with thicknesses varying from 40 to 100 nm, and the power conversion efficiency was enhanced from 1.18% to 4.13%, respectively. This work highlights the importance of investigating sputtering conditions to achieve CdS with superior physical properties and enhance ultra-thin CdTe solar cell performance.