Silicon-based Thin Film Solar Cells Research Articles

Review of the research on nano-structure used as light harvesting in perovskite solar cells

In recent years, nano-structures used as light harvesting have been widely used in perovskite cells to enhance the photon absorption of cells. The introduction of trapping structures in perovskite cells can change the photon propagation in the cell and the photon energy absorbed by the cell. The nano-structure used in different interfaces of perovskite cells can increase the absorption of light by the device to different degrees, and ultimately improve the efficiency of the solar cell. Therefore, the effective light trapping structure has become trending in the application of perovskite cells. How to effectively apply the such nano-structure is the key to improve the power conversion efficiency(PCE) of perovskite cells. So far, there is three ways including surface antireflection nanostructure, texture structure and plasmon nanostructure to apply to perovskite solar cell. The first one is ordered and disordered antireflection nanostructure that enhance the absorption of light on the surface of perovskite cells and makes visible light scatter at the interface of the nanostructure to reflection probability, the second one is texture structure that can not only improve the light absorption but avoid the formation of short-circuit channel inside the cell, the third one is plasmon nanostructure that can further improve the absorption of the thin film absorption material in the long band, so as to achieve the effect of improving the light utilization and cell efficiency. The trap structure is expected to achieve good photon absorption performance in wide spectral range and wide incidence angle range under the condition of reducing the thickness of active layer. At the same time, it has the advantages of good repeatability, easy to simulate and easy to change the structure. Therefore, using various trap technologies to design efficient trap structure has become a research hotspot in the field of solar cells. So far, most of the reports on the trapping structure have been applied to the silicon-based thin film solar cells, but few of them have been reported on the perovskite cells. This paper starts from the description of the perovskite cell with different nano-structures, comparing and summarizing the different structures, and analyzes the advantages and Disadvantage.

Efficiency improvement of a silicon-based thin-film solar cell using plasmonic silver nanoparticles and an antireflective layer

This paper presents a silicon thin-film solar cell (TFSC) integrated with the silver nanoparticles. It consists of anti-reflection, absorption and reflective layers in which the anti-reflective layer is made of pyramids of TiO2. The purpose of this structure is to allow sunlight to enter the cell at any angle with the minimum reflection and absorption in the wavelength range of 300–1100 nm. The absorbing layer is composed of silicon and when sunlight enters this layer, the molecular bonds break down and release many electrons due to its high absorption coefficient. In this layer, silver spherical nanoparticles are placed to increase the absorption of solar energy by the localized surface plasmon resonances, which will increase the efficiency of the TFSC. The last layer of the structure is a reflective surface of aluminum, which aims to reflect the light into the upper layer to enhance its absorption. We will calculate the key performance metrics for a solar cell such as short-circuit current, open-circuit voltage, fill-factor, and photovoltaic efficiency considering the effects of recombination between silicon substrate and other materials. The numerical results based on the finite-difference time-domain method reveal that the proposed structure has much more absorption due to the anti-reflection layer and the presence of silver nanoparticles that leads to light scattering, light localization, and guided mode excitation compared to conventional TFSC. Our simulations based on the finite-element method show the presented TFSC integrated with silver nanoparticles has a fill-factor of 0.82 and an efficiency of 16.18%.

Using Oxygen Plasma Pretreatment to Enhance the Properties of F-Doped ZnO Films Prepared on Polyimide Substrates

In this study, a radio frequency magnetron sputtering process was used to deposit F-doped ZnO (FZO) films on polyimide (PI) substrates. The thermal expansion effect of PI substrates induces distortion and bending, causing FZO films to peel and their electrical properties and crystallinity to deteriorate. To address these shortcomings, oxygen (O2) plasma was used to pretreat the surface of PI substrates using a plasma-enhanced chemical vapor deposition system before the FZO films were deposited. The effects of O2 plasma pretreatment time on the surface water contact angle, surface morphologies, and optical properties of the PI substrates were investigated. As the pretreatment time increased, so did the roughness of the PI substrates. After the FZO films had been deposited on the PI substrates, variations in the surface morphologies, crystalline structure, composition, electrical properties, and optical properties were investigated as a function of the O2 plasma pretreatment time. When this was 30 s, the FZO films had optimal optical and electrical properties. The resistivity was 3.153 × 10−3 Ω-cm, and the transmittance ratios of all films were greater than 90%. The X-ray photoelectron spectroscopy spectra of the FZO films, particularly the peaks for O1s, Zn 2p1/2, and Zn 2p3/2, were determined for films with O2 plasma pretreatment times of 0 and 30 s. Finally, a HCl solution was used to etch the surfaces of the deposited FZO films, and silicon-based thin-film solar cells were fabricated on the FZO/PI substrates. The effect of O2-plasma pretreatment time on the properties of the fabricated solar cells is thoroughly discussed.

Advanced light trapping scheme in decoupled front and rear textured thin-film silicon solar cells

We present the study of an advanced light trapping scheme applied to thin-film silicon-based solar cells, overcoming the broadband Green absorption limit, that is the generalized case of the 4n2 classical absorption limit for all wavelengths. This result is achieved by the 3-dimensional optical modelling of a fully functional thin-film hydrogenated nano-crystalline silicon (nc-Si:H) solar cell endowed with decoupled front and back textures. Our results stem from rigorously characterized optical properties of state-of-the-art materials, optimized geometric nano-features on the front and rear surfaces of the solar cell, and thickness optimization of the front transparent oxide. The simulated improvements derive from a gain in light absorption, especially in the near-infrared part of the spectrum close to the band gap of nc-Si:H. In this wavelength region, the material is weakly absorbing, whereas we now find significant absorptance peaks that can only be explained by the concurrent excitation of guided resonances by front and rear textures. This insight indicates the need to modify the temporal coupled-mode theory, which fails to predict the absorption enhancement achieved in this work, extending its validity to the case of decoupled front/back texturing. Our approach results in substantially high photocurrent density (>36 mA/cm2), creating a platform suitable for high efficiency single and multi-junction thin-film solar cells based either on typical silicon alloys or on the novel and promising barium (di)silicide (BaSi2) absorber. In the latter case, using the same advanced light trapping employed for nc-Si:H, we demonstrate a very high implied photocurrent density of 41.1 mA/cm2, for a device endowed with 2-μm thick absorber.

Influence of oxygen concentration on the optoelectronic properties of hydrogenated polymorphous silicon thin films

Hydrogenated polymorphous silicon (pm-Si:H) is a suitable candidate for application in silicon-based thin film solar cells because it exhibits improved electronic transport properties and stability with respect to conventional hydrogenated amorphous silicon (a-Si:H). The use of chlorinated precursors to grow pm-Si:H thin films in a plasma enhanced chemical vapor deposition systems increases photoconductivity and photo-stability, which is one of the most desired aspects for modern solar cell applications. However, the effects of chlorine and hydrogen chemistry should be more extensively studied because it may lead to the generation of a secondary SiOx phase, after exposure to the ambiance and/or during the growth processes. In the present work, X-Ray photoelectron spectroscopy results showed the depth profile of oxygen in the polymorphous silicon thin film. Finally, we have analyzed the effect of different concentrations and profile of oxygen on the electronic transport properties by means of dark and photoconductivity analysis. Sample with high crystalline fraction and oxygen concentration showed an increase in photo-stability and conductivity due to the passivation of free terminal bonds with oxygen. Understanding chemical and structural properties of pm-Si: H thin films based on chlorine, with additional oxygen incorporation to the films, is a key towards optimizing their electrical and optical properties for the practical implementation of solar cells.

Development of Improved n-μc-SiO$_x$ :H Films and Its Innovative Application in Silicon-Based Single Junction Thin Film Solar Cells

In this paper, we have discussed about the development of high-quality n-μc-SiO:H films by a seeding technique working as a potential back reflector layer (BRL). Highly crystalline and conducting n-μc-Si:H films are used as a seed layer. Phosphorous-doped SiO $_x$ :H film with suitable optoelectronics properties has been deposited by a radio frequency plasma enhanced chemical vapor deposition technique using parallel plate reactors (capacititively coupled). Optoelectronics properties have been controlled and optimized varying the deposition parameters such as process pressure, power density, partial pressure of CO2, etc. We have also innovatively replaced the n-layer, i.e., n-a-Si:H in case of the a-Si:H cell and n-μc-Si:H for μc-Si:H cells by this optimized seed + n-μc-SiO:H films. The performance of this n-μc-SiO:H film grown on the n-μc-Si:H seed layer as n-layer as well as BRL of single junction a-Si:H and μc-Si:H solar cells has been evaluated and better Photovoltaic (PV) characteristics of the solar cell are realized as compared to those of the films developed without any seed layer. a-Si:H solar cells with initial efficiency of 9.38% and μc-Si:H solar cell of 8.50% have been successfully fabricated by using improved doped SiO $_x$ :H based BRL replacing conventional n-layer of the single junction cells.

Thin-film amorphous silicon germanium solar cells with p- and n-type hydrogenated silicon oxide layers

Mixed-phase hydrogenated silicon oxide (SiOx:H) is applied to thin-film hydrogenated amorphous silicon germanium (a-SiGe:H) solar cells serving as both p-doped and n-doped layers. The bandgap of p-SiOx:H is adjusted to achieve a highly-transparent window layer while also providing a strong electric field. Bandgap grading of n-SiOx:H is designed to obtain a smooth transition of the energy band edge from the intrinsic to n-doped layer, without the need of an amorphous buffer layer. With the optimized optical and electrical structure, a high conversion efficiency of 9.41% has been achieved. Having eliminated other doped materials without sacrificing performance, the sole use of SiOx:H in the doped layers of a-SiGe:H cells opens up great flexibility in the design of high-efficiency multi-junction thin-film silicon-based solar cells.

Efficient amorphous silicon solar cells: characterization, optimization, and optical loss analysis

Hydrogenated amorphous silicon (a-Si:H) has been effectively utilized as photoactive and doped layers for quite a while in thin-film solar applications but its energy conversion efficiency is limited due to thinner absorbing layer and light degradation issue. To overcome such confinements, it is expected to adjust better comprehension of device structure, material properties, and qualities since a little enhancement in the photocurrent significantly impacts on the conversion efficiency. Herein, some numerical simulations were performed to characterize and optimize different configuration of amorphous silicon-based thin-film solar cells. For the optical simulation, two-dimensional finite-difference time-domain (FDTD) technique was used to analyze the superstrate (p-i-n) planar amorphous silicon solar cells. Besides, the front transparent contact layer was also inquired by using SnO2:F and ZnO:Al materials to improve the photon absorption in the photoactive layer. The cell was studied for open-circuit voltage, external quantum efficiency, and short-circuit current density, which are building blocks for solar cell conversion efficiency. The optical simulations permit investigating optical losses at the individual layers. The enhancement in both short-circuit current density and open-circuit voltage prompts accomplishing more prominent power conversion efficiency. A maximum short-circuit current density of 15.32 mA/cm2 and an energy conversion efficiency of 11.3% were obtained for the optically optimized cell which is the best in class amorphous solar cell.

Role of oxygen and nitrogen in n-type microcrystalline silicon carbide grown by hot wire chemical vapor deposition

N-type microcrystalline silicon carbide (μc-SiC:H(n)) deposited by hot wire chemical vapor deposition provides advantageous opto-electronic properties for window layer material in silicon-based thin-film solar cells and silicon heterojunction solar cells. So far, it is known that the dark conductivity (σd) increases with the increase in the crystallinity of μc-SiC:H(n)films. However, due to the fact that no active doping source is used, the mechanism of electrical transport in these films is still under debate. It is suggested that unintentional doping by atmospheric oxygen (O) or nitrogen (N) contamination plays an important role in the electrical transport. To investigate the impact of O and N, we incorporated O and N in μc-SiC:H(n) films and compared the influence on the microstructural, electronic, and optical properties. We discovered that, in addition to increasing the crystallinity, it is also possible to increase the σd by several orders of magnitude by increasing the O-concentration or the N-concentration in the films. Combining a high concentration of O and N, along with a high crystallinity in the film, we optimized the σd to a maximum of 5 S/cm.

Effective Light Trapping in Thin Film Silicon Solar Cells with Nano- and Microscale Structures on Glass Substrate.

For thin film silicon-based solar cells, effective light trapping at a broad range of wavelengths (400-1100 nm) is necessary. Normally, etching is only carried out with TCOs, such as SnO2:F and impurity doped ZnO, to form nano-sized craters in the surface morphology to confer a light trapping effect. However, in this study, prior to ZnO:Al etching, periodic structures on the glass substrates were made by photolithography and wet etching to increase the light scattering and internal reflection. The use of periodic structures on the glass substrate resulted in higher haze ratios in the range from 550 nm to 1100 nm, which is the optical absorption wavelength region for thin film silicon solar cells, than obtained by simple ZnO:Al etching. The periodically textured glass with micro-sized structures compensates for the low haze ratio at the middle and long wavelengths of wet etched ZnO:Al. ZnO:Al was deposited on the periodically textured glass, after which the ZnO:Al surface was also etched randomly using a mixed acid solution to form nano-sized craters. The thin film silicon solar cells with 350-nm-thick amorphous silicon absorber layer deposited on the periodic structured glass and etched ZnO:Al generated up to 10.68% more photocurrent, with 11.2% increase of the conversion efficiency compared to the cell deposited on flat glass and etched ZnO:Al.

Hydrogenated Amorphous Silicon-Based Thin Film Solar Cell: Optical, Electrical and Structural Properties

Hydrogenated amorphous silicon (a-Si:H) has been developed as an important materials in thin film-based photovoltaic technologies because of considerable cost reduction as a result of low material consumption and low-temperature process. Among the materials used for thin film solar cells, amorphous silicon is the most important material in the commercial production. Despite of these benefits, the efficiency limit for a single band gap thin film based solar cell predicted by Shockley and Queisser (i.e. ~31%) has become a matter of challenge for current research community. Considering the thermodynamic behavior of a single threshold absorber in generating electricity from solar irradiance, this limit seems inevitable, and thus a tremendous investigation is now being carried out in different dimensions such as hot carrier generation, rainbow solar cell, multiple exciton generation, multiband absorber etc. Nonetheless, so far reported efficiency (ηlab~12%) provide enough room to improve and take challenge to reach to the highest value for a-Si:H based solar cell design. Further to improve architectural design as well as engineer the materials, it is indispensable to understand the optical, electrical and structural properties of aSi:H as an active layer. Here in this article, an attempt was taken into account to focus on such characteristics that affect the overall cell efficiency.

Highly transparent Zn1−xMgxO/ITO multilayer for window of thin film solar cells

Texture-etched Zn1−xMgxO films were fabricated and applied as front transparent electrodes for superstrate type thin film solar cells. The Zn0.65Mg0.35O film (x = 0.35) showed optical transparency better than commercially available Asahi VU and double-textured ZnO (WT-ZnO) substrates. To provide pertinent conductivity, ITO film was coated on the texture-etched Zn0.65Mg0.35O. By employing the Zn0.65Mg0.35O/ITO substrate instead of the SnO2, we demonstrated an enhancement of quantum efficiency for amorphous silicon thin film solar cell devices, resulted in efficiency improvement from 8.92 to 9.56%. We also examined effectiveness of the Zn0.65Mg0.35O/ITO substrate for the microcrystalline silicon solar cells which delivered an efficiency of 9.73% with proper anti-reflection coating. Our experimental results suggest that the Zn0.65Mg0.35O/ITO multilayer front contact can be beneficial for reinforcing performances of silicon-based thin film solar cell devices.

The application of infrared thermo-camera in testing of silicon-based thin-film solar cells

ABSTRACTThermo-camera is employed here to analyze kinds of quality abnormal and improve production process during manufacture procedure of silicon-based thin-film solar modules. It shows that thermo-camera device can help engineers to solve problem of production line quickly and accurately, and save the manpower and financial resources at the same time.

Investigation of H 2 /CH 4 mixed gas plasma post-etching process for ZnO:B front contacts grown by LP-MOCVD method in silicon-based thin-film solar cells

A new plasma post-etching method, H2/CH4 mixed gas plasma, is introduced to modify ZnO:B films grown by LP-MOCVD technique, successfully relaxing the double trade-offs, i.e., transparency/conductivity trade-off and surface texture/Voc and FF trade-off. To deeply evaluate the post-etching process, optical emission spectroscopy technique is applied to diagnose the plasma condition. Upon different etching power, three distinct possible etching mechanisms are identified by analyzing the evolution of Hα*, Hβ*, CH* emission species in the plasma space. It is demonstrated that Hβ* and CH* species are responsible for the physical etching process and chemical etching process, respectively, from which a new “soft” surface morphology is formed with a combination of micro- and nano-sized texture. Additionally, Hα* species can bond with ZnO and also passivate the grains boundaries, thereby making both the carrier concentration and hall mobility increase. This process is defined as chemical bonding process. Finally, pin-type a-Si:H single-junction solar cells with an optimized device structure is grown on the etched ZnO:B substrate. The corresponding electrical parameters, such as Jsc, Voc and FF, are simultaneously improved compared with the solar cell deposited on as-grown ZnO:B substrate with the same fabrication process. As a consequence, a noteworthy 8.85% conversion-efficiency is achieved with an absorber layer thickness only 160 nm.

Quadruple-junction thin-film silicon-based solar cells with high open-circuit voltage

We have fabricated a-SiOx:H/a-Si:H/nc-Si:H/nc-Si:H quadruple-junction thin-film silicon-based solar cells (4J TFSSCs) to obtain high spectral utilization and high voltages. By processing the solar cells on micro-textured superstrates, extremely high open-circuit voltages for photovoltaic technology based on thin-film silicon alloys up to 2.91 V have been achieved. Optical simulations of quadruple-junction solar cells using an advanced in-house model are a crucial tool to effectively tackle the challenging task of current matching among the individual sub-cells in such devices. After optimizing the optical design of the device and the absorber thicknesses, an energy conversion efficiency of 11.4% has been achieved. The open-circuit voltage, short-circuit current density, and fill factor were 2.82 V, 5.49 mA/cm2, and 73.9%, respectively. Based on this demonstration, strategies for further development of highly efficient 4J TFSSCs are proposed.

Optimization of a-Si1–xGex:H single-junction and a-Si:H/a-Si1–xGex:H tandem solar cells with enhanced optical management

This work aimed at improving the optical management of a-Si:H/a-Si1–xGex:H tandem cells and a-Si1–xGex:H single-junction cells. To improve the optical management, the effects of the a-Si1–xGex:H bandgap, the bandgap graded absorber and the n-type μc-SiOx:H back reflecting layer on the cell performance were investigated. Optical reflection spectra, internal quantum efficiency, external quantum efficiency (EQE), and cell performance were used to evaluate the improvement of the optical properties of solar cells. The tandem cells with a-Si1–xGex:H bandgap of 1.53 eV exhibited sufficient optical absorption from 630 to 900 nm and thus lead to higher JSC. Second, the EQE of a-Si1–xGex:H single-junction cell was significantly enhanced from 630 to 720 nm by employing bandgap graded absorber that relatively improved the JSC by 3.8% despite that the reduction in EQE from 720 to 900 nm compared to the cell without bandgap grading. Moreover, the μc-SiOx:H(n)/Ag back reflector showed higher optical reflection than a-Si:H(n)/Ag did, which relatively improved the JSC by 12.3%. The cell with μc-SiOx:H(n)/Ag back reflector exhibited a comparable JSC and efficiency to the cell with ITO–Ag. The previously mentioned approaches are relevant to enhance the optical management in cells and can be applied to silicon-based thin-film solar cells.

High performance and high stability mechanisms of microcrystalline silicon-based thin-film solar cells deposited by laser-assisted plasma-enhancement chemical vapor deposition system

The laser-assisted plasma-enhanced chemical vapor deposition (LAPECVD) system was proposed to deposit high performance and high stability Si-based thin-film solar cells. In the LAPECVD system, the CO2 laser and plasma were simultaneously utilized to effectively decompose the SiH4 reaction gas. Consequently, the hydrogen concentration in the i-Si absorption film was reduced with an increase of CO2 laser power. Furthermore, the microcrystalline i-Si film could be formed due to the formation of more Si nucleation seeds. Si-nanoclusters were formed on the microcrystalline i-Si films deposited in the LAPECVD system. The associated carrier mobility was increased with increasing the CO2 laser power. The XRD measurements demonstrated that a gradual transformation from amorphous to crystalline as guiding the assisting laser. According to the FTIR measurement, the estimated hydrogen content reduction ratio of the light-soaked i-Si films decreased from 16.5% to 5% as the assisting laser power increased from 0W to 80W. The corresponding conversion efficiency degradation ratio of 20.20% and 5.74% was obtained, the high performance and high stability of the resulting Si-based p–i–n thin film solar cells were obtained.

A new kind of superimposing morphology for enhancing the light scattering in thin film silicon solar cells: Combining random and periodic structure

In this article, a new type of superimposing morphology comprised of a periodic nanostructure and a random structure is proposed for the first time to enhance the light scattering in silicon-based thin film solar cells. According to the framework of the Reyleigh—Sommerfeld diffraction algorithm and the experimental results of random morphologies, we analyze the light-scattering properties of four superimposing morphologies and compare them with the individual morphologies in detail. The results indicate that the superimposing morphology can offer a better light trapping capacity, owing to the coexistence of the random scattering mechanism and the periodic scattering mechanism. Its scattering property will be dominated by the individual nanostructures whose geometrical features play the leading role.

Thin-film silicon-based quadruple junction solar cells approaching 20% conversion efficiency

Thin-film silicon-based solar cells are a well-established photovoltaic (PV) technology. The best reported initial and stabilized conversion efficiency is 16.3% and 13.4%, respectively. Thin-film silicon PV technology needs to achieve initial conversion efficiency approaching 20% in order to stay competitive with other PV technologies. The multi-junction approach is regarded as the main strategy for improving cells efficiency. In this contribution, we study thin-film silicon-based solar cells based on a quadruple junction device and discuss their potential for achieving a high efficiency. We carried out optical modelling of this novel device structure using state-of-the-art materials and light management techniques. We demonstrate a quadruple junction cell with simulated photo-generated current density of 8.7mA/cm2 in current-matching condition and potential initial conversion efficiency of 19.6%. A significant spectral overlap is observed between the component cells that makes the design of the current-matched device complex. We can control the spectral overlap by employing band-gap engineering of absorber layers and design an improved current-matched quadruple junction solar cell with potential initial conversion efficiency equal to 19.8%.

Effects of One-Dimensional Photonic Crystal on Thin Film Silicon Solar Cells

We have designed lateral contact thin film silicon-based solar cells with and without one-dimensional photonic crystals as back surface field layer. The photonic crystal comprises a distributed Bragg reflector (DBR) for trapping the light. Simulations demonstrate that energy conversion efficiency and short circuit current ISCfor c-Si solar cells with the photonic crystal structure are increased to 21.11% and 27.0 mA, respectively, from 18.33% and 22.8mA of the one without photonic crystal. In addition, the effects of DBRs consisting of different materials are investigated in our simulations. When the refractive index difference between sub-layers of the DBR is larger, the forbidden band width is broader, the reflectance of the DBR is higher, and more photons are reflected and trapped into the active region, then the absorption efficiency and the energy conversion efficiency of the solar cell are both increased. The bigger the refractive index difference of the DBRs sub-layers is, the broader the forbidden band width is. In addition, a-Si solar cells with and without DBR are also discussed.