Ultrathin Solar Cells Research Articles

Light absorption enhancement in ultrathin film solar cell with embedded dielectric nanowires

A novel design of thin-film crystalline silicon solar cell (TF C-Si-SC) is proposed and numerically analyzed. The reported SC has 1.0 µm thickness of C-Si with embedded dielectric silicon dioxide nanowires (NWs). The introduced NWs increase the light scattering in the active layer which improves the optical path length and hence the light absorption. The SC geometry has been optimized using particle swarm optimization (PSO) technique to improve the optical and electrical characteristics. The suggested TF C-Si-SC with two embedded NWs offers photocurrent density ({J}_{ph}) of 32.8 mA cm−2 which is higher than 18 mA cm−2 of the conventional thin film SC with an enhancement of 82.2%. Further, a power conversion efficiency of 15.9% is achieved using the reported SC.

Vacuum-Free Fabrication Strategies for Nanostructure-Embedded Ultrathin Substrate in Flexible Polymer Solar Cells

In this paper, we discuss a method for fabricating an ultrathin polymer substrate with one-dimensional nanograting patterns to improve the power conversion efficiency (PCE) of ultrathin polymer solar cells (PSCs) and suppress the dependence on the incident angle of light. Because the fabricating process of the ultrathin polymer substrate was carried out using a solution process, it can be manufactured in a large area, and the PCE of the patterned ultrathin substrate-based PSC is improved by 8.9% compared to the non-patterned device. In addition, triple-patterned ultrathin PSCs incorporating the same nanograting pattern as the substrate were fabricated in the electron transport (ZnO) layer and the photoactive layer (PBDTTT-OFT and PC71BM mixture (ratio-1: 1.5)) to achieve PCE of 10.26%. Thanks to the nanograting pattern introduced in the substrate, ZnO layer, and photoactive layer, it was possible to minimize the PCE change according to the incident angle of light. Moreover, we performed 1000 cycles of compression/relaxation tests to evaluate the mechanical properties of the triple-patterned ultrathin PSCs, after which the PCE remained at 71% of the initial PCE.

Light-trapping structure based on ultra-thin GaAs solar cell

It is well-known that the light-trapping effect is very important for improving cell efficiency and reducing material cost. Certain kinds of light-trapping schemes have been explored and applied to thin-film cells, especially to Si-based thin-film solar cells. This is considered less attractive in GaAs thin film cells, due to the fact that GaAs has a high absorption coefficient, a direct bandgap and suffers from strong surface recombination. In this paper, we describe the development of a highly efficient light-trapping structure utilizing periodically patterned front and back dielectric nanopyramid arrays keeping a completely flat GaAs active layer. It was found that our proposed structure was superior for ultra-thin active layers. The optimized structure yielded a photocurrent density of 20.94 mA cm−2 with an active layer thickness of 0.1 μm, which by far exceeded the reference cell photocurrent of 15.31 mA cm−2 with an equivalent thickness. These results are very significant for directing research into the light trapping and cost reduction of thin-film GaAs solar cells.

Double grating high efficiency nanostructured silicon-based ultra-thin solar cells

In this paper, the cylindrical, conical and parabolic nanostructures inherited from self-organized anodic aluminum oxide (AAO) are applied to silicon-based ultra-thin solar cells aiming for a new design concept for low-cost, high-efficiency double-grating solar cells. Numerical results reveal that the optimal bottom metal grating can enhance the absorption capacity of ultra-thin solar cell at 0.5 µm–1.1 µm waveband with a very small parasitic absorption. Meanwhile the surface grating with a period of 0.1 µm can strongly enhance the absorption of solar cell in 0.3 µm–0.5 µm waveband. Consequently, the silicon-based ultra-thin solar cell inherited their characteristics very well when both the optimal silicon surface grating and bottom metal grating are applied at the same time, thereby increasing the absorption rate of the solar cell in the entire waveband. Compared to the solar cell with single surface grating and the one with single bottom metal grating, the photocurrent density increments of the double-grating nanostructured solar cells can reach as high as 75.47% and 40.69%, respectively. Further results show that the conical and parabolic nanostructured gratings have higher light absorption efficiency and lower structure morphology-dependent sensitivity than the cylindrical nanostructured one, which is more conducive to the application of high efficiency ultra-thin solar cells.

High open-circuit voltage in transition metal dichalcogenide solar cells

The conversion efficiency of ultra-thin solar cells based on layered materials has been limited by their open-circuit voltage, which is typically pinned to a value under 0.6 V. Here we report an open-circuit voltage of 1.02 V in a 120 nm-thick vertically stacked homojunction fabricated with substitutionally doped MoS2. This high open-circuit voltage is consistent with the band alignment in the MoS2 homojunction, which is more favourable than in widely-used TMDC heterostructures. It is also attributed to the high performance of the substitutionally doped MoS2, in particular the p-type material doped with Nb, which is demonstrated by the observation of electroluminescence from tunnelling graphene/BN/MoS2 structures in spite of the indirect nature of bulk MoS2. We find that illuminating the TMDC/metal contacts decreases the measured open-circuit voltage in MoS2 van der Waals homojunctions because they are photoactive, which points to the need of developing low-resistance, ohmic contacts to doped MoS2 in order to achieve high efficiency in practical devices. The high open-circuit voltage demonstrated here confirms the potential of layered transition-metal dichalcogenides for the development of highly efficient, ultra-thin solar cells.

Reducing toxicity and enhancing broadband solar energy harvesting of ultra-thin perovskite solar cell via SiO2 nanophotonic structure

Perovskite solar cell had the advantages of low cost, high efficiency and flexibility, which had significant development potential. Reducing the thickness of the perovskite absorber layer was a practicable method to reduce the content of toxic lead elements, however, an insufficient light absorption and low photocurrent were usually observed due to the shorten optical path length. In this study, SiO2 nanophotonic structure was introduced and designed by electromagnetic theory to allow the perovskite absorber layer to become thinner, while did not decrease its light absorption and photocurrent. The geometrical parameters of SiO2 nanophotonic structure was optimized by finite-difference time-domain (FDTD) method combined with particle swarm optimization algorithm to obtain the largest light absorption enhancement. When SiO2 nanophotonic structure was used, the perovskite solar cell with a 200 nm thickness perovskite absorber layer can generate the same photocurrent as the planar perovskite solar cell with a 300 nm thickness perovskite absorber layer, which can reduce toxic lead elements of perovskite material as high as 33%. The feasibility of applying SiO2 nanophotonic structure to different perovskite solar cells was further investigated.

Fabrication of quasi-hexagonal Si nanostructures and its application for flexible crystalline ultrathin Si solar cells

Optimally designed Si nanostructures can serve as effective light trapping structures for flexible crystalline ultrathin Si solar cells. In this study, we develop a unique quasi-hexagonal inverted nano-pyramid in a hexagonal array, fabricated by a combined process of nanosphere lithography and wet etching. Self-assembled silica nanoparticles were deposited on Si wafers by spin coating, followed by deposition of triangular metal nanodisks in a hexagonal array. The metal nanodisks were used as a wet etch mask in an alkaline solution to create the quasi-hexagonal Si nanostructures. We investigated a temporal evolution of the Si nanostructures with increasing the etch time to optimize optical performances. The newly developed nanostructures take a quasi-hexagonal inverted pyramid, which provides geometrically high compatibility with the self-assembled monolayers in a hexagonal array. We incorporated these quasi-hexagonal nanostructures to the flexible crystalline ultrathin Si solar cells, and the novel nanostructures exhibited optical performances comparable to conventional micro-pyramid textures while showing the enhanced mechanical flexibility of the ultrathin Si-based solar cells.

Efficiency enhancement of an ultra-thin film silicon solar cell using conical-shaped nanoparticles: similar to superposition (top, middle, and bottom)

The use of conical-shaped plasmonic nanostructures for light management in an ultra-thin silicon solar cell has been investigated. The optical absorption and hence photocurrent are obtained for several cases of structures using finite difference time domain simulations. In this paper, we demonstrate that the use of superposition theorem causes significant photocurrent enhancement due to the surface plasmonic effects of nanoparticles. For this, at first, we used one conical-shaped nanoparticle at the top side, then in the rear side, and finally, three nanoparticles are used in the top, bottom, and middle sides. Depending on the incident light wavelength, each nanoparticle manipulates the part of the spectrum. The photocurrents of 9.165, 10.463, 16.402, 17.761, and 18.072 mA/cm2, are obtained for a cell without nanoparticles, with one conical-shaped NP at the top, with one conical-shaped NP at the bottom, with two NPs at top and bottom and with three NPs at the top, bottom, and middle, respectively. Finally, the electrical field distribution and generation rate are calculated for proposed structures.

Design of suppressing optical and recombination losses in ultrathin CuInGaSe2 solar cells by Voronoi nanocavity arrays

Cu(In, Ga)Se 2 (CIGS) solar cells have reached efficiencies over 23%. However, the scarcity of In and Se makes reducing the thickness of CIGS thin-film absorber desirable for mass production with low-cost and time-saving processes. Meanwhile, CIGS solar cells based on different architectures and strategies have drawn much attention for significant performance over conventional technologies, but rational design guidelines for optical and electrical performance optimization are yet to be completed. In this work, the reduced thickness of the CIGS thin film with the optical and electrical loss-passivated molybdenum Voronoi nanocavity arrays (VNCAs) electrodes were demonstrated. A structural-model under conformally coating criteria was investigated and optimized by a finite-difference time-domain method. The reduced parasitic absorption and rear surface recombination by Al 2 O 3 passivation layers on VNCAs substrates give rise to enhanced V OC and J SC up to 9.5 and 8.5%, respectively. These results confirm that the novel VNCAs electrodes can provide a cost-effective way for ultrathin CIGS solar cells with high efficiency. Design of suppressing optical and recombination losses in ultrathin CuInGaSe 2 solar cells by voronoi nanocavity arrays was demonstrated. The reduced parasitic absorption and rear surface recombination by Al 2 O 3 passivation layers on VNCAs substrates give rise to enhanced V OC and J SC up to 9.5 and 8.5 %, respectively. • The passivation of electrical and optical losses was achieved by a glancing angle deposition system. • The criteria of conformal coating is applied in the detailed optical modeling. • The effect of rear surface passivation was studied before and after post-selenization process. • The Al 2 O 3 layer shows superior passivation effects in electrical and optical losses.

Ultrathin CIGSe Solar Cells with Integrated Structured Back Reflector

To reduce the thickness of CuIn1‐xGaxSe2 (CIGSe) solar cells, light management concepts must be used. One concept is the combination of an Al/indium1‐tin1‐oxide (ITO) reflector/contact and an optical scattering element at the back side of the device. Herein, nanostructured substrates, obtained by laser interference lithography and lift‐off process, are covered with an Al/ITO back contact. Structure dimensions are a pitch of 1.96 μm and heights of 100–700 nm. Solar cells with 600 nm CIGSe absorber having this integrated structured back reflector exhibit an increase in JSC of up to 3.2 mA cm−2 compared with the Mo reference sample of identical thickness. To understand this optical gain, 3D finite‐difference time‐domain (FDTD) simulations are performed. To this end, a model is built to simulate structure formation. It is shown that locally enhanced optical absorption can be achieved by the integrated structured back reflector. It is also shown that parasitic absorption in the back ITO diffusion barrier needs to be minimized.

Persistent Double-Layer Formation in Kesterite Solar Cells: A Critical Review

In kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell research, an asymmetric crystallization profile is often obtained after annealing, resulting in a bilayered - or double-layered - CZTSSe absorber. So far, only segregated pieces of research exist to characterize the appearance of this double layer, its formation dynamics and its effect on the performance of devices. In this work, we review the existing research on double-layered kesterites and evaluate the different mechanisms proposed. Using a cosputtering-based approach, we show that the two layers can differ significantly in morphology, composition and optoelectronic properties, and complement the results with a large statistical dataset of over 850 individual CZTS solar cells. By reducing the absorber thickness from above 1000 nm to 300 nm, we show that the double layer segregation is alleviated. In turn, we see a progressive improvement in the device performance for lower thickness, which alone would be inconsistent with the well-known case of ultrathin CIGS solar cells. We therefore attribute the improvements to the reduced double-layer occurrence, and find that the double-layer limits the efficiency of our devices to below 7%. By comparing the results with CZTS grown on monocrystalline Si substrates, without a native Na supply, we show that the alkali metal supply does not determine the double layer formation, but merely reduces the threshold for its occurrence. Instead, we propose that the main formation mechanism is the early migration of Cu to the surface during annealing and formation of Cu2-xS phases, in a self-regulating process akin to the Kirkendall effect. Finally, we comment on the generality of the mechanism proposed, by comparing our results to other synthesis routes, including our own in-house results from solution processing and pulsed laser deposition of sulfide- and oxide-based targets. We find that although the double layer occurrence largely depends on the kesterite synthesis route, the common factors determining the double-layer occurrence appear to be the presence of metallic Cu and/or a chalcogen deficiency in the precursor matrix. We suggest that understanding the limitations imposed by the double layer dynamics could prove useful to pave the way for breaking the 13% efficiency barrier for this technology.

Innovative and industrially viable approach to fabricate AlOx rear passivated ultra-thin Cu(In, Ga)Se2 (CIGS) solar cells

In this work, an industrially viable and novel rear surface passivation approach for Copper Indium Gallium di-Selenide, Cu(In,Ga)Se2, CIGS, ultra-thin (500 nm) solar cells is developed. The passivation layer was deposited by atomic layer deposition (ALD), and an alkali treatment was applied via spin coating. It was observed that selenization of the samples is required to create contact openings. The openings were visualized by SEM, and these results were supported by EDS. The impact of the oxide layer’s thickness, as well as the alkali solution’s molarity, was studied. Solar cells were produced for the optimal combination of these two parameters. As a result, with a relative 13% increase, the highest Voc of 623 mV was achieved. Hence, the efficiency of the passivated solar cell was relatively increased by one-third, by using an industrially feasible, fast, and repeatable technique.

Envisaging radio frequency magnetron sputtering as an efficient method for large scale deposition of homogeneous two dimensional MoS2

2D MoS2 has the potential to be used in demanding products such as transistors with capability of overcoming short-channel effects of silicon, ultra-thin solar cells because of huge optical absorption in 2D form, memristors, capacitors, sensors etc. However, the commercial use of 2D MoS2 is hindered by the lack of synthesis methods which provide with high quality large scale deposition. Here we demonstrate the deposition of large area 2D 2H-MoS2 from bulk to bilayers on quartz substrates by industry compatible RF sputtering technique in a sulphur sufficient environment provided by evaporation of sulphur flakes by an in-situ effusion cell. Absorbance spectra of sputtered samples showed peaks corresponding to A, B, convoluted C and D and E excitons. Raman signature peaks of MoS2, A1g near 410 cm−1 and E12g around 380 cm−1 was found in all samples. A1g-E12g value decreased with decreasing deposition time, indicating the formation of thin MoS2 layers up to bilayer which was further supported by the evolution of photoluminescence spectra. AFM measurements exhibited a uniform surface morphology. XPS ascertained the purity of the samples. Thus we could prove that radio frequency magnetron sputtering could be used as an efficient method for large area deposition of 2D MoS2.

Design of high performance and low resistive loss graphene solar cells

Despite metallic plasmonic excitations can enhance the performance of ultra-thin solar cells however these so-called plasmonic solar cells suffer from a large resistive (Ohmic) loss caused by metallic elements. In this work, we report on a new design that uses graphene nanoribbons (GNRs) in a two-dimensional (2D) grating form at the top of the semiconductor-on-insulator (SOI) solar cells aimed to reduce the resistive loss. The results showed that GNRs can remarkably reduce the resistive loss compared to the SOI cell with Ag nanograting, while keeping all other cell’s parameters, comparable with those of Ag SOI cell. Optical absorption and short-circuit current density of the graphene cells showed, respectively, enhancements of 18 and 1.7 times when optimizations were done with respect to width and the grating period. Our calculations showed that the graphene solar cells dissipate at most 5% of incident sunlight power as narrow and tiny peaks around 508 nm, which is noticeably lower than those of Ag solar cells with high and broad band peaks with the maximum values of 29% at 480 nm and 24% at 637 nm.

Fully Inkjet‐Printed, Ultrathin and Conformable Organic Photovoltaics as Power Source Based on Cross‐Linked PEDOT:PSS Electrodes

AbstractUltra‐lightweight solar cells have attracted enormous attention due to their ultra‐conformability, flexibility, and compatibility with applications including electronic skin or miniaturized electronics for biological applications. With the latest advancements in printing technologies, printing ultrathin electronics is becoming now a reality. This work offers an easy path to fabricate indium tin oxide (ITO)‐free ultra‐lightweight organic solar cells through inkjet‐printing while preserving high efficiencies. A method consisting of the modification of a poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) ink with a methoxysilane‐based cross‐linker (3‐glycidyloxypropyl)trimethoxysilane (GOPS)) is presented to chemically modify the structure of the electrode layer. Combined with plasma and solvent post‐treatments, this approach prevents shunts and ensures precise patterning of solar cells. By using poly(3‐hexylthiophene) along rhodanine‐benzothiadiazole‐coupled indacenodithiophene (P3HT:O‐IDTBR), the power conversion efficiency (PCE) of the fully printed solar cells is boosted up to 4.73% and fill factors approaching 65%. All inkjet‐printed ultrathin solar cells on a 1.7 µm thick biocompatible parylene substrate are fabricated with PCE reaching up to 3.6% and high power‐per‐weight values of 6.3 W g−1. After encapsulation, the cells retain their performance after being exposed for 6 h to aqueous environments such as water, seawater, or phosphate buffered saline, paving the way for their integration in more complex circuits for biological systems.

Design of wave-optical structured substrates for ultra-thin perovskite solar cells

Photonic micro/nano-structures in the wave-optics regime have shown to be a promising strategy for effective broadband light capture in ultra-thin devices, opening a window of opportunity for cheap, efficient, lightweight and flexible photovoltaics (PV). Here we design, from an optical standpoint, a novel industrially-attractive concept where light trapping is obtained by conformably depositing the solar cell materials onto previously-patterned photonic substrates. This solution is applied and optimized for perovskite solar cells (PSCs) with distinct thicknesses of the perovskite absorber - the conventional (500 nm) and ultra-thin (300 nm) in view of enhanced flexibility - yielding photocurrent improvements up to 22.8% in superstrate cell configuration and 24.4% in substrate-type configuration; thereby coming relatively close to the fundamental Lambertian limits. Furthermore, these structures also show an omni-direction optical response for incidence angles up to 70° for all cases, therefore demonstrating the viability of this light trapping method for implementation in flexible PV devices operating under bending. The photonic-enhanced ultra-thin solar cells designed here ultimately support the reduction of material usage in PSC technology, which is especially beneficial to mitigate lead usage, without impacting the device's performance.

Absorption-Enhanced Ultra-Thin Solar Cells Based on Horizontally Aligned p-i-n Nanowire Arrays.

A horizontally aligned GaAs p–i–n nanowire array solar cell is proposed and studied via coupled three-dimensional optoelectronic simulations. Benefiting from light-concentrating and light-trapping properties, the horizontal nanowire array yields a remarkable efficiency of 10.8% with a radius of 90 nm and a period of 5 radius, more than twice that of its thin-film counterpart with the same thickness. To further enhance the absorption, the nanowire array is placed on a low-refractive-index MgF2 substrate and capsulated in SiO2, which enables multiple reflection and reabsorption of light due to the refractive index difference between air/SiO2 and SiO2/MgF2. The absorption-enhancement structure increases the absorption over a broad wavelength range, resulting in a maximum conversion efficiency of 18%, 3.7 times higher than that of the thin-film counterpart, which is 3 times larger in GaAs material volume. This work may pave the way for the development of ultra-thin high-efficiency solar cells with very low material cost.

Strategically Manipulated Polymer Solar Cells to Incorporate Plasmonically Enhanced Spectral Upconversion Backplane

AbstractSpectral upconversion systems placed underneath solar cells have the considerable potential for enhancement of the photovoltaic performance as they allow additional absorption for the solar photons with energy below the bandgap of active materials. However, their application to a type of ultrathin solar cells for achieving the meaningful benefit of the efficiency improvement is challenging, because a pre‐existing rear‐side reflector that substantially increases the photon absorption needs to be eliminated for photonic interaction between photovoltaic active layer and upconversion medium, and hence a level of cell efficiency becomes limited. Herein a facile strategy is presented that can circumvent the issue of performance deterioration arising from the expelled reflector for integrating plasmonically enhanced upconversion systems with ultrathin nonfullerene‐based polymer solar cells. By employing a wavelength‐selectively reflective rear electrode of metal/dielectric multilayer that enables the photon penetration only at excitation and emission wavelengths of the upconversion process, the effect of photocurrent improvement with uncompromising efficiency levels can be expected from the plasmonic upconversion backplane comprising NaYF4:Yb3+,Er3+ core‐shell nanoparticles and metallic nanostructure. Systematic studies of optical process and resulting device performance in both experiments and numerical modeling provide the optimal design scheme for high‐performance polymer solar cells assisted with upconversion systems.

Numerical investigation of CIGS thin-film solar cells

The high efficiency of thin Cu(In1−xGax)Se2 solar cells is investigated using the Atlas SILVACO-TCAD tools. The CIGS model is based on an experimental study and takes into account the physical properties, dimensions, and thicknesses of the different layers. The main features model used consists of the thermionic emission concept that was used to describe the carrier transport model across the buffer/absorber heterojunction. Thermionic emission was found to be an important mechanism for carrier transport across the hetero-interface since it produced a matching behaviour for the cell under study with experimental behaviour, thus validating the considered CIGS model for further investigation. To improve efficiency, magnesium fluoride (MgF2) is used as the preferred anti-reflective coating to maximize the photocurrent from the incident light intensity. The effect of varying the single-layer antireflective coating thickness variation, surface recombination velocity at the hetero-interface between CdS and CIGS layers, absorber thickness variation, and gallium content in the Cu(In1−xGax)Se2 on cell performance are studied under AM 1.5G, 1 sun illumination. The novelty of this research includes the configuration of the p-CIGS/n-CdS cells with thin CIS material used for the back surface field layer to improve efficiency. The result has shown that a cell performance of 25% efficiency can be achieved using a 1 µm thick absorber and an MgF2 anti-reflective layer. This result can further aid the development of ultrathin, flexible, and tandem solar cell devices.

The Future of Flexible Organic Solar Cells

AbstractExtensive efforts have been devoted during the last decade to organic solar cell research that has led to remarkable progress and achieved power conversion efficiencies (PCEs) in excess of 10%. Among the existing flexible organic solar cells, ultrathin organic solar cells with a total thickness <10 µm have important advantages, including good mechanical bending stabilities and good conformability. These advantages have led to power generation solutions for wearable electronics. In this essay, the progress of flexible and ultrathin organic solar cells, and the future research directions pertaining to these cells are discussed based on the potential applications of textile‐compatible solar cells. Both process engineering and development of the material of ultrathin substrate films have improved the PCE of ultrathin organic solar cells, which in turn have led to the small PCE difference between flexible organic solar cells with substrate thickness >10 µm and ultrathin organic solar cells with substrate thickness ≤10 µm. Key technologies for the further improvement of PCE of flexible/ultrathin organic solar cells are discussed. Strategies to improve the stability and some important aspects, which determine the mechanical robustness of flexible organic solar cells, are also presented and discussed.