High-performance Thin-film Solar Cells Research Articles

Novel Mg‐ and Ga‐doped ZnO/Li‐Doped Graphene Oxide Transparent Electrode/Electron‐Transporting Layer Combinations for High‐Performance Thin‐Film Solar Cells (Small 22/2023)

High-Performance Thin-Film Solar Cells In article number 2207966, Jung-Je Woo, Jin Hyeok Kim, Hyeong-Jin Kim, and co-workers demonstrate the Mg- and Ga-doped ZnO/Li-doped graphene oxide transparent electrode (TE)/electron-transporting layer (ETL) combination in thin-film solar cells. This newly designed TE/ETL combination not only increases the electron collection rate but also suppresses the open-circuit voltage deficit, thereby improving the conversion efficiency of the solar cells.

Novel Mg- and Ga-doped ZnO/Li-Doped Graphene Oxide Transparent Electrode/Electron-Transporting Layer Combinations for High-Performance Thin-Film Solar Cells.

Herein, a novel combination of Mg- and Ga-co-doped ZnO (MGZO)/Li-doped graphene oxide (LGO) transparent electrode (TE)/electron-transporting layer (ETL) has been applied for the first time in Cu2 ZnSn(S,Se)4 (CZTSSe) thin-film solar cells (TFSCs). MGZO has a wide optical spectrum with high transmittance compared to that with conventional Al-doped ZnO (AZO), enabling additional photon harvesting, and has a low electrical resistance that increases electron collection rate. These excellent optoelectronic properties significantly improved the short-circuit current density and fill factor of the TFSCs. Additionally, the solution-processable alternative LGO ETL prevented plasma-induced damage to chemical bath deposited cadmium sulfide (CdS) buffer, thereby enabling the maintenance of high-quality junctions using a thin CdS buffer layer (≈30nm). Interfacial engineering with LGO improved the Voc of the CZTSSe TFSCs from 466 to 502mV. Furthermore, the tunable work function obtained through Li doping generated a more favorable band offset in CdS/LGO/MGZO interfaces, thereby, improving the electron collection. The MGZO/LGO TE/ETL combination achieved a power conversion efficiency of 10.67%, which is considerably higher than that of conventional AZO/intrinsic ZnO (8.33%).

Self-assembled monolayers as emerging hole-selective layers enable high-performance thin-film solar cells

Self-assembled monolayers as emerging hole-selective layers enable high-performance thin-film solar cells

Origin of Broadband Emission and Large Stokes Shift in Antimony Trisulfide.

The antimony trisulfide (Sb2S3) has been theoretically predicted to have various merits in exploiting high-performance thin-film solar cells and attracted intense attention. However, the power conversion efficiency of Sb2S3-based solar cells is yet to be satisfactory in experiments and the origin of large open circuit voltage (VOC) loss is still a controversial question. Based on first-principles calculations, we have systematically analyzed the excited state behavior and dynamics images of carriers in Sb2S3 materials. Our calculations showed that intrinsic defects like vacancy (VSb and VS) and antisites (SbS and SSb) are energetically accessible. More importantly, we found that the sulfide vacancy-bound excitons can produce a large Stokes shift of ∼0.66 eV, which could well rationalize the experimental observations like the reduction of VOC. These new findings suggest that the performance of Sb2S3-based solar cells might be largely enhanced by avoiding sulfide vacancy defects.

A general and facile solution approach for deposition of high-quality metal oxide charge transport layers

Metal oxide charge transport layers (CTLs) are significant components for fabricating high-performance thin film solar cells. Generally, sol-gel methods have been widely used to deposit high-quality metal oxide CTLs, but they usually suffer from the choices of solvents, ligands and metal precursors. Here we report a general and facile ionic liquid (n-butylammonium butyrate) approach to prepare V2O5, MoO3, WO3, NiO, ZnO and SnO2 thin films, which exhibits strong dissolving ability and good film-forming property. The thermal volatilization of ionic liquid is mild, which provides a slow and stable decomposition process of metal compounds and favors the formation of smooth and compact CTLs. All thin films are highly transparent in the visual range from the optical transmittance spectra and suitable to be CTLs in photovoltaic devices. Encouragingly, we have received the efficiencies of 15.06% and 13.38% based on the area of 0.09 and 1.21 cm2 (NiO as the hole transport layer), and 19.13% and 16.71% based on the area of 0.09 and 1.21 cm2 (SnO2 as the electron transport layer), which are obviously higher than those of devices prepared by traditional sol-gel methods.

Design of a Highly Efficient Subwavelength Antireflective Structure for Solar Cells

An efficient optical antireflective (AR) structure plays a vital role in high-performance thin-film solar cells. Here, we design a surface relief AR structure consisting of a two-dimensional (2D) array of a subwavelength ring and pillar-shaped feature, capable of suppressing optical reflection over a wide spectral window of the solar spectrum. Our simulations show that the weighted average reflectance of the subwavelength AR structure is as low as 4.2% in the 400–1100 nm spectral range in the normal incidence condition and almost 10-fold reduction compared with a bare silicon surface. When placed on the front side of a simple Si thin-film photovoltaic solar cell, this subwavelength AR structure leads to an improved light absorption with simulated results showing an increase of 53% short-circuit current compared to a flat solar cell. Besides, our simulations show that this AR structure could, in principle, perform well against reasonable fabrication errors.

Theoretical modeling and optimization: Cd-free CTS/Zn(O,S)/ZnO thin film solar cell

Latterly Zn(O, S) has gained utmost attention as an eco-friendly buffer layer with controllable energy levels for high-performance thin-film solar cell (TFSCs) devices. The article discusses that the conventional detrimental Cd-containing buffer is replaced by the non-toxic Zn(O,S) buffer and yielded a comparable high efficiency. Using SCAPS software, the modeling and numerical simulations of the CTS based solar cell are systematically analyzed. Detail investigations of the band alignment between the CTS absorber and Zn(O, S) buffer layer has been studied as a function of the S/(S+O) ratio along with the effects of thickness, carrier concentration, defect density of buffer layer and absorber layer with interface defect density. The final optimized results of p-CTS/Zn(O,S)/n-ZnO/Al heterojunction revealed that the obtained figure of merit value is comparably high (ɳ=13.65%, VOC = 0.3802 V, JSC = 48.33 mA/cm2, and FF = 74.45%) than the previously reported values. All in all, the present study suggests that Zn(O, S) can be a suitable alternative candidate for the CdS-buffer layer in a CTS-based solar cell.

Metasurface-assisted broadband optical absorption in ultrathin perovskite films.

Ultrathin hybrid organic-inorganic perovskite (HOIP) films have significant potential for use in integrated high-performance photoelectric devices. However, the relatively low optical absorption capabilities of thinner films, particularly in the long-wavelength region, pose a significant challenge to the further improvement of photoelectrical conversion in ultrathin HOIP films. To address this problem, we propose a combining of ultrathin HOIP film with plasmonic metasurface to enhance the absorption of the film effectively. The metasurface excites localized surface plasmon resonances and deflects the reflected light within the HOIP film, resulting in an obvious enhancement of film absorption. Finite-difference time-domain simulation results reveal that the far-field intensities, deflection angles, and electric field distributions can be effectively varied by using metasurfaces with different arrangements. Examination of the reflection and absorption spectra reveals that embedding a specifically designed metasurface into the HOIP film produces an obvious enhancement in broadband optical absorption compared with pure HOIP films. We further demonstrate that this broadband absorption promotion mechanism can be effective at a wide range of HOIP film thicknesses. Comparison of the absorption spectra at various incidence angles of ultrathin HOIP films with and without underlying metasurfaces indicates that the addition of a metasurface can effectively promote absorption under wide-angle incident light illumination. Moreover, by extending the metasurface structure to a two-dimensional case, absorption enhancements insensitive to the incident polarization states have also been demonstrated. This proposed metasurface-assisted absorption enhancement method could be applied in designing novel high-performance thin-film solar cells and photodetectors.

Study the Properties of Solution Processable CZTS Thin Films Induced by Annealing Treatment: Study of Annealing Time

Cu2ZnSnS4 is suitable for high-performance thin-film solar cell because of its high absorbance coefficient, presence of non-toxic elements, excellent optoelectronic properties, and a near-perfect direct band gap. The effect of thermal annealing time (1–4 h) on the optical, morphological, and structural properties of Cu2ZnSnS4 coated through a simple solution processable method has been studied in the present work. All the CZTS films are crystalline in nature with kesterite structure as shown by X-ray diffraction studies. Crystallite size, strain, and dislocation density were calculated. However, no notable changes in these parameters were obtained by varying the annealing time in the above range. Field emission scanning electron microscopy images show good quality compact films with particle size in the order of 10–15 nm. Absorption spectroscopy results show an optical band gap of 1.46 eV. Raman spectroscopy was used to check binary or ternary phases present. It shows that the impurity phase decreases and the pure Cu2ZnSnS4 phase was obtained by increasing the annealing time to 3 and 4 h.

Light Propagation in Flexible Thin-Film Amorphous Silicon Solar Cells with Nanotextured Metal Back Reflectors.

Nanostructured metal back reflectors (BRs) are playing an important role in thin-film solar cells, which facilitates an increased optical path length within a relatively thin absorbing layer. In this study, three nanotextured plasmonic metal (copper, gold, and silver) BRs underneath flexible thin-film amorphous silicon solar cells are systematically investigated. The solar cells with BRs demonstrate an excellent light harvesting capability in the long-wavelength region. With the combination of hybrid cavity resonances, horizontal modes, and surface plasmonic resonances, more incident light is coupled into the photoactive layer. Compared to the reference cells, the three devices with plasmonic BRs show lower parasitic absorptions with different individual absorption distributions. Both experimental and simulated results indicate that the silver BR cells delivered the best performance with a promising power conversion efficiency of 7.26%. These rational designs of light harvesting nanostructures provide guidelines for high-performance thin-film solar cells and other optoelectronic devices.

Investigation of physical properties of F-and-Ga co-doped ZnO thin films grown by RF magnetron sputtering for perovskite solar cells applications

F-and-Ga co-doped ZnO films were sputter-deposited on glass substrates by RF magnetron sputtering method. The fabricated films were characterized by different techniques. It was found that all the films were polycrystalline with a hexagonal wurtzite structure with a c-axis preferred orientation of growth. The effect of the substrate temperature on the surface morphology, electrical and optical properties of the films was also investigated. The optimal growth temperature was found to be 440 oC which led to the optimal film with the resistivity of 6.81 × 10−4 Ωcm, carrier concentration of 2.61 × 1020 cm−3, mobility of 35.1 cm2/V, over-90% transmittance in the region of 400–1200 nm and a wide optical bandgap of 3.49 eV. This optimal film was employed in as the front contact in perovskite solar cells and resulted in a high power-conversion efficiency of 15.32%. This indicates that such a film can be promisingly useful for high-performance thin-film solar cells.

Two-dimensional GeSe for high performance thin-film solar cells

Multilayer GeSe can be a promising candidate for flexible photovoltaic devices because of the low Schottky barrier at the back electrode and high PCE of ∼18%.

A novel MoS2-based hybrid film as the back electrode for high-performance thin film solar cells

A MoS2-based complex film was developed with a high work function and thickness-dependent conductivity, which greatly improved the CdTe cell efficiency.

Broadband Antireflection and Light Trapping in Thin Film Silicon Solar Cells With Hemisphere Arrays

An efficient photon management scheme for high-performance thin film solar cells requires both antireflection and light trapping. In this letter, we propose a double-sided grating design, by patterning hemispherical pit arrays (HPAs) on the front and back surfaces of glass substrates, to satisfy such a dual requirements. The HPAs are designed with different period sizes on the two sides: For one thing, the large-sized hemisphere arrays placed at the top surface allow multiple rebounds of incident light, thereby suppressing reflection. For another, wavelength-sized arrays on the bottom side provide strong diffraction scattering and, then, trap photons inside the cell devices. Electromagnetic simulations show that a-Si:H solar cells incorporated with the double-sided configuration gain a significant enhancement in spectral response from 600 to 800 nm, as well as the photo-generated current is improved by 21.76% compared with the random textured reference solar cells.

Novel p-Type Conductive Semiconductor Nanocrystalline Film as the Back Electrode for High-Performance Thin Film Solar Cells.

Thin film solar cells, due to the low cost, high efficiency, long-term stability, and consumer applications, have been widely applied for harvesting green energy. All of these thin film solar cells generally adopt various metal thin films as the back electrode, like Mo, Au, Ni, Ag, Al, graphite, and so forth. When they contact with p-type layer, it always produces a Schottky contact with a high contact potential barrier, which greatly affects the cell performance. In this work, we report for the first time to find an appropriate p-type conductive semiconductor film, digenite Cu9S5 nanocrystalline film, as the back electrode for CdTe solar cells as the model device. Its low sheet resistance (16.6 Ω/sq) could compare to that of the commercial TCO films (6-30 Ω/sq), like FTO, ITO, and AZO. Different from the traditonal metal back electrode, it produces a successive gradient-doping region by the controllable Cu diffusion, which greatly reduces the contact potential barrier. Remarkably, it achieved a comparable power conversion efficiency (PCE, 11.3%) with the traditional metal back electrode (Cu/Au thin films, 11.4%) in CdTe cells and a higher PCE (13.8%) with the help of the Au assistant film. We believe it could also act as the back electrode for other thin film solar cells (α-Si, CuInS2, CIGSe, CZTS, etc.), for their performance improvement.

Robust AZO/i-ZnO bilayer front contact for high-performance thin film solar cells

Resistivity of AZO/ZnO is lower than that of an AZO film after annealing at 400 °C for 60 minutes.

A modeling method to enhance the conversion efficiency by optimizing light trapping structure in thin-film solar cells

Nano-scaled dielectric and metallic structures are popular light tapping structures in thin-film solar cells. However, a large parasitic absorption in those structures is unavoidable. Most schemes based on such structures also involve the textured active layers that may bring undesirable degradation of the material quality. Here we propose a novel light trapping structure based on the prism structured SiO2 for thin-film solar cells, and a flat active layer is introduced purposefully. Such a light trapping structure is imposed by the geometrical shape optimization to gain the best optical benefit. A self-consistent simulation method including the solutions of Maxwell, drift–diffusion and Poisson equations is adopted for modeling the optical and electrical performance of the solar cell. By examining our scheme, it is disclosed that the conversion efficiency of the flat a-Si:H thin-film solar cell (8.1%) can be promoted to an impressive value (10.74%) that is higher than previous reports. As the method proposed in this work is also applicable to design the thin-film solar cell systems with other materials, this proposal would provide useful guides for developing high-performance thin-film solar cells.

New strategy to promote conversion efficiency using high-index nanostructures in thin-film solar cells.

Nano-scaled metallic or dielectric structures may provide various ways to trap light into thin-film solar cells for improving the conversion efficiency. In most schemes, the textured active layers are involved into light trapping structures that can provide perfect optical benefits but also bring undesirable degradation of electrical performance. Here we propose a novel approach to design high-performance thin-film solar cells. In our strategy, a flat active layer is adopted for avoiding electrical degradation, and an optimization algorithm is applied to seek for an optimized light trapping structure for the best optical benefit. As an example, we show that the efficiency of a flat a-Si:H thin-film solar cell can be promoted close to the certified highest value. It is also pointed out that, by choosing appropriate dielectric materials with high refractive index (>3) and high transmissivity in wavelength region of 350 nm–800 nm, the conversion efficiency of solar cells can be further enhanced.

Development of thin-film solar cells using solar spectrum splitting technique

To increase the performance of thin-film solar cells, the solar spectrum splitting technique has been considered and studied. It was found from simulations that the total efficiency of nearly 25% can be obtained at the splitting wavelength of 600 nm with the top cell using higher band gap material. Experiments have been carried out to verify the simulation results. Up to now the total efficiency of about 22.0% has been obtained using a-Si:H and Cu(In1−x,Gax)Se2 as the top and bottom cells, respectively, at the splitting wavelength of 614 nm which is similar to the simulation results. This result shows that our developed splitting technique is promising for high-performance thin-film solar cells.

Surface plasmon excitation via Au nanoparticles in n-CdSe∕p-Si heterojunction diodes

We report on the significant enhancement of photocurrent in pn heterojunction diode, consisting of n-CdSe∕p-Si substrates, in situ deposited with Au nanoparticles on the surface by the pulsed-laser deposition technique. This is attributed due to the large enhancement in electromagnetic field that occurs in the vicinity of the metal surface, causing surface plasmons. The large enhancement in Raman and photoluminescence intensity was observed due to surface plasmon resonance. Our results suggest that the photodetectors, optoelectronic, such as high-performance thin-film solar cells, optical communication, and sensing devices, including bio- and molecular sensors, can be fabricated with improved functionality.