Si Solar Cell Applications Research Articles

Structural, electronic, and optical-absorption properties of 2D Si thin films

AbstractRecent experimental studies highlighted the potential of thin-film crystalline silicon (Si) for high-efficiency solar cells. Using density functional theory, we investigated 2D Si thin films across various orientations, thicknesses, and surface structures to elucidate their structure–property relationships. Through surface-energy calculations and Wulff construction, we determined the crystal habit of Si, which aligns with available experimental observations. Electronic-structure calculations underscored the critical role of valence saturation on surfaces in enabling semiconducting behavior in Si thin films, essential for optical applications. From optical-absorption calculations, we identified the surface index exhibiting the highest absorption coefficients for thin films Si solar cell applications. Graphical abstract

Broadband anti-reflection coating for Si solar cell applications based on periodic Si nanopillar dimer arrays & Si3N4 layer

A structure of periodic Si nanopillar dimer array & Si3N4 layer which sits on Si substrates is presented to obtain a broadband high transmission and low reflection. We show numerically that the average reflection of this structure can reach 1.8%, and the average transmission can reach 93.1% in the 400–1100 nm range, due to the combined effects of the forward scattering effects of Si nanopillar dimers and Si3N4 layer’s anti-reflection effects. Si nanopillars’ diameter and height, Si3N4 layer’s height, the gap of dimers, and the period of the array have significant impacts on the transmittance and reflection. This work supplies a practicable way for decreasing broadband surface reflection and increasing the absorption of light for Si solar cell applications.

A novel multifunctional C3-symmetric triphenylamine discotic liquid crystal: synthesis, columnar self-assembly, organogel behavior, selective detection of PA and application in Si solar cells

A novel C3-symmetric triphenylamine discotic liquid crystal with fluorescent gel properties has important applications in detection of PA and Si hybrid solar cells.

Near infrared fluorescence properties of Tb4+ and Tb3+ inter-conversion states in CaF2:Yb3+nanocrystals

Near infra-red emission was enhanced in calcium fluoride (CaF2) co-doped with terbium ions and ytterbium (Yb3+) under a 325 nm laser excitation wavelength, during investigations performed for possible application in Si solar cells. Tb3+ ions were oxidized to Tb4+ ions and emission enhancement occurred due to a cooperative energy transfer mechanism between the Tb4+-Yb3+ couple. The oxidation of Tb3+ ions in CaF2 was achieved by annealing the nanocrystals at 450 °C in air. X-ray diffraction analysis confirmed the cubic face of the CaF2 crystal without any impurity peaks. UV–VIS luminescence and X-ray photoelectron spectroscopy were conducted to probe the conversion of Tb3+ to Tb4+ ions as a function of the annealing time. The main conclusion was that the NIR emission from the Tb4+-Yb3+ couple revealed that down-conversion occurred through a cooperative energy transfer mechanism.

Efficient down-conversion ZnO codoped (Er, Yb) nanopowders synthesized via sol-gel process for Si solar cell applications

This study presents ZnO nanopowders codoped by (Er, Yb) prepared by the sol-gel process using the supercritical drying of ethanol and then annealed at 500 °C, 800 °C and 1000 °C. The molar percentage of Yb was fixed at 5%, and different molar percentages (5% and 10%) of Er at different temperatures were investigated. The structural and morphological studies show that the nanopowders are crystallized in a wurtzite ZnO structure and that the grain size is around 60 nm. The variation of the annealing temperature and the doping percentage have a significant effect on the morphology and the crystallinity of the nanopowders. Down-conversion emissions were investigated under 378 nm excitation and the (Er, Yb) codoped ZnO nanopowders exhibit interesting emissions in the green, red, and infrared ranges.

Excellent omnidirectional light trapping properties of inverted micro-pyramid structured silicon by copper catalyzed chemical etching

Nano/micro structuring of silicon (Si) attracts a lot of research interests for minimizing the reflective losses (>30%) of Si surface limiting the solar cell performance majorly. Here, arrays of inverted micro-pyramids Si (IP–Si) have been fabricated on large area of the solar-grade, as-cut Si wafers by a simple, one-step copper (Cu) catalyzed chemical etching (CuCCE) in aqueous etch bath of 5 mM-Cu(NO3)2/4.6M-HF/0.55M − H2O2 at 50 °C. Influence of etching time is investigated under constant etch bath parameters and starting surface properties of the Si wafers. The CuCCE mechanism is compared with the standard alkali (aq. KOH) texturing of Si(100) wafers. The formation of inverted structure is attributed to Cu nanoparticles catalyzed localized anisotropic as well as isotropic etching of Si(100) as compared to only anisotropic etching in the KOH texturing. The IP-Si surface offers a superior light trapping ability in broad spectral range compared to the conventional KOH based random upright micro-pyramids (UP–Si). The IP-Si surface could achieve solar weighted reflectance (SWR) as low as <7% in 400–1000 nm spectral range as compared to ∼14% of the UP-Si surfaces. This has been attributed to higher percentage of triple bounce interaction of the incident light with Si surface offered by the IP-Si. Further, the IP-Si offers enhanced omnidirectional light trapping for wide angle of incidence (AOI) of light (−40° to +40°) as compared to that in UP-Si. The enhanced omnidirectional light trapping of the IP-Si and its mechanism has further been explained by finite difference time domain (FDTD) simulation. Further, potential of IP-Si for enhanced solar cell performance is proposed on the basis of theoretical calculation of total photocurrent for planar, UP-Si and IP-Si surfaces. A simple, economic yet effective process for large area fabrication of the inverted micro/nano-pyramid arrays of Si with enhanced and broad band omnidirectional light trapping properties has been shown which has potential for efficient Si solar cell application.

Hole selective WOx and V2Ox contacts using solution process for silicon solar cells application

We have prepared solution processed V2Ox and WOx (s-V2Ox and s-WOx) thin films as hole selective contacts for Si solar cell applications. The material properties of s-V2Ox and s-WOx thin films were investigated to shed light on their performance as selective contacts, and their hole selectivity on Si were also characterized. It is found that there is a larger density of oxygen vacancies in s-V2Ox than s-WOx resulting in a larger amount of gap states, but their energy levels in s-V2Ox might not be favourable for carrier transport. We have measured a high work function of 5.17 eV for s-V2Ox and the Fermi level is located above the conduction band suggesting its n-type characteristic. On the other hand, the work function of s-WOx is only 4.71 eV and it is also a n-type semiconductor with the Fermi level situated below the conduction band. From the carrier lifetime studies, both materials are found to exhibit moderate passivation effect on Si and overall they can serve as good hole selective contacts for Si. The optimized power conversion efficiencies of planar s-V2Ox/Si and s-WOx/Si heterojunction solar cells are 10.8% and 9.3% respectively. To the best of our knowledge, the performance of the planar s-V2Ox/Si HSC is better than other reported result for planar configuration using solution process, and this is the first time that the performance of the planar s-WOx/Si HSC is reported.

Omnidirectional and Broadband Antireflection Effect with Tapered Silicon Nanostructures Fabricated with Low-Cost and Large-Area Capable Nanosphere Lithography.

In this report, we present a process for the fabrication and tapering of a silicon (Si) nanopillar (NP) array on a large Si surface area wafer (2-inch diameter) to provide enhanced light harvesting for Si solar cell application. From our N,N-dimethyl-formamide (DMF) solvent-controlled spin-coating method, silica nanosphere (SNS in 310 nm diameter) coating on the Si surface was demonstrated successfully with improved monolayer coverage (>95%) and uniformity. After combining this method with a reactive ion etching (RIE) technique, a high-density Si NP array was produced, and we revealed that controlled tapering of Si NPs could be achieved after introducing a two-step RIE process using (1) CHF3/Ar gases for SNS selective etching over Si and (2) Cl2 gas for Si vertical etching. From our experimental and computational study, we show that an effectively tapered Si NP (i.e., an Si nanotip (NT)) structure could offer a highly effective omnidirectional and broadband antireflection effect for high-efficiency Si solar cell application.

Enhanced near-infrared downconversion luminescence in ZnxNb(1-x)O composite host co-doped Bi3+ /Yb3+ phosphor for Si solar cell applications

The spectral modification is noticed through near Infrared (IR) downconversion (DC) in Yb3+, Bi3+ co-doped ZnxNb(1-x)O composite host. A high-intensity blue emission centered at 450 nm is observed upon UV excitation at 270 nm for the host ZnxNb(1-x)O at x = 0.5 (ZN). Doping with the optimized concentration at 4 mol% of Yb3+ with ZN (YZN) initiates an energy transfer from the niobate group to two neighboring Yb3+ ions in a co-operative energy transfer (CET) process. The resulting Yb3+ emission spectrum is around 1000 nm, which matches well with the spectral response of Si solar cells. Introducing 1 mol% of trivalent Bi3+ ions as an additional sensitizer to YZN (BZN), the excitation wavelength is further extended from UV at 270 nm to visible at 450 nm region. As an effect of this, UV excitations at 270 nm, 330 nm, and visible excitation at 450 nm in BZN enhance the luminescence in the near IR region. The DC process observed in BZN is due to the CET process from both the niobate group and Bi3+ ions to Yb3+ ions. Solar panel coated with BZN shows an increase in conversion efficiency which in turn indicates that the synthesized phosphor is a promising material for increasing Si solar cell efficiency.

Investigation of Pd/MoOx/n-Si diodes for bipolar transistor and light-emitting device applications

Sub-stoichiometric molybdenum oxide (MoOx) has recently been investigated for application in high efficiency Si solar cells as a “hole selective” contact. In this paper, we investigate the electrical and light-emitting properties of MoOx-based contacts on Si from the viewpoint of realizing functional bipolar devices such as light-emitting diodes (LEDs) and transistors without any impurity doping of the Si surface. We realized diodes on n-type Si substrates using e-beam physical vapor deposition of Pd/MoOx contacts and compared their behavior to implanted p+n-Si diodes as a reference. In contrast to majority-carrier dominated conduction that occurs in conventional Schottky diodes, Pd/MoOx/n-Si diodes show minority-carrier dominated charge transport with I–V, C–V, and light-emitting characteristics comparable to implanted counterparts. Utilizing such MoOx-based contacts, we also demonstrate a lateral bipolar transistor concept without employing any doped junctions. A detailed C–V analysis confirmed the excessive band-bending in Si corresponding to a high potential barrier (&amp;gt;0.90 V) at the MoOx/n-Si interface which, along with the observed amorphous SiOx(Mo) interlayer, plays a role in suppressing the majority-carrier current. An inversion layer at the n-Si surface was also identified comprising a sheet carrier density greater than 8.6×1011 cm−2, and the MoOx layer was found to be conductive though with a very high resistivity in the 104 Ω-cm range. We refer to these diodes as metal/non-insulator/semiconductor diodes and show with our device simulations that they can be mimicked as high-barrier Schottky diodes with an induced inversion layer at the interface.

Surface Modifications for Light Trapping in Silicon Heterojunction Solar Cells: A Brief Review

Reducing crystalline silicon (c-Si) wafer thickness is an effective method to reduce the fabrication cost as it constitutes a major portion of the photovoltaic module cost. However, the open-circuit voltage and fill factor depend on the wafer thickness; further, the short-circuit current density (JSC), affects the device performance negatively. Therefore, light trapping is vital for increasing the JSC of Si solar cells. Consequently, it is essential for improving the conversion efficiency of the solar cell and reduce its production cost by decreasing the wafer thickness. It can be assumed that the thickness of the Si wafer will gradually achieve a minimum value of ~ 100 μm in the future. Therefore, reducing the as-cut wafer thickness will result in a more efficient use of Si. This paper reports the surface modification for light trapping based on the Si solar cell application. Additionally, we introduce methods for surface modification, such as front-side texturing and rear-side polishing.

The effects of post heat treatment on ITO/AgAl/ITO/p-Si multilayer films

For the past few decades, the quest for an improve indium tin oxide (ITO) films have attracted a lot of interest by scientists and industries for application in advanced optoelectronic devices. ITO/AgAl/ITO multilayer films were deposited on p-silicon (Si) substrates at room temperature by radio frequency (RF) and direct current (DC) magnetron sputtering respectively. The effects of post-heat (annealing) treatment on the structural, optical and electrical properties of the ITO/AgAl/ITO multilayer films were investigated at different temperature of 200 °C, 300 °C, 400 °C and 500 °C respectively. X-ray diffraction (XRD) results reveal an amorphous structure for the as-deposited film whereas the post annealed films show a polycrystalline and cubic bixbyite structure with preferential peaks orientation along ITO (222), Ag (111), and ITO (440) crystalline directions. Atomic force microscopy (AFM) analysis indicates a smoother surface morphology and improved grain size after post heat treatment. Ultraviolet-visible spectrophotometer measurements show a significant increase in optical transmittance spectra as the annealing temperature increases. Maximum transmittance peak of 87.2 % was obtained by film annealed at 500 °C. Four-point probe measurements exhibit a substantial decrease in sheet resistance with respect to increasing post-annealing temperature. The sheet resistance of the as-deposited film is about 7.85 Ω/sq and falls down to 3.23 Ω/sq as the post-heat temperature is increased to 500 °C. Compared to as-deposited film, the post annealed multilayer films optical and electrical properties have been successfully enhanced, fine turned and favourable for Si solar cell application.

Prediction of nano etching parameters of silicon wafer for a better energy absorption with the aid of an artificial neural network

To enhance energy absorption of photovoltaics, several etching experiments with various parameters were performed. In addition, an Artificial Neural Network (ANN) simulation was utilized to predict chemical nano etching parameters such as masking and etching durations for Silicon (Si) solar cell applications to reach minimum surface reflectance in an optimum etching duration. Experiments were performed with different masking and etching durations to determine the characteristics of surface reflectance of micro textured n-type single crystalline Si wafers in 25mmx25mm width and 300 µm thickness to provide training data for ANN. For this purpose, solutions with identic properties including Ag nanoparticles were applied with different application durations on the surfaces of n-type single crystalline Si wafers to cover partially the Si surfaces with Ag nano-particles at masking step. After, partially masked Si surfaces were exposed to chemical nano etching to develop nano-sized porous structures under different etching durations in an identic acidic etching solution. For the etching of Si wafers, 32 masking and etching processes were performed. Reflectance measurements and SEM images were evaluated to determine the optimum etching duration resulting the best surface quality with minimum reflectance. In addition, reflectance values were utilized as input data for training, testing and validation steps of developed ANN. In the ANN simulation, 70% of reflectance values were used as training, 15% of reflectance values were used as validation and 15% of reflectance values were used to test data in the ANN. Consequently, surface reflectance values under different masking and etching durations were predicted with the new parameter set by using the trained ANN with a success level above 99%.

Graphene as transparent electrode in Si solar cells: A dry transfer method

This paper present an experimental study on a new dry transfer method of graphene applied as transparent conducting electrode in textured silicon solar cells. Raman spectra of dry exfoliated graphene indicated a monolayer graphene was transferred onto textured Si cells and the dry exfoliated graphene with better crystalline quality is attained. Photovoltaic result shows the short circuit current of the dry transferred graphene on texture Si solar cell has an increase of 28% compared to the wet transferred graphene. It proves the possibility of dry transferred graphene as transparent conducting electrode in textured Si solar cell applications.

Energy transfer study between Ce3+ and Tb3+ ions in a calcium fluoride crystal for solar cell applications

Energy transfer from Ce3+ to Tb3+ ions in a single crystal of CaF2 has been analyzed for down-conversion application in Si solar cells. The nanophosphor samples were successfully synthesized by the co-precipitation method. Results obtained with x-ray diffraction indicated that the crystallite size were in the range of 30–40nm. X-ray photoelectron spectroscopy confirmed the formation of the matrix and the presence of the dopants. The luminescence data obtained clearly indicated that energy transfer between Ce3+ and Tb3+ took place inside the crystalline matrix. The calculated energy transfer efficiency was relatively efficient at the high Tb3+ concentration. The results suggested that Ce3+ may therefore be used as an efficient sensitizer to feed the Tb3+ ions in a CaF2 crystal. UV light may be down shifted to the visible region for solar cell applications.

Textured AZO for Thin-Film Si Solar Cells: Towards Understanding the Effect of AZO Film Thickness on the Surface Texturing Properties

Sputter-deposited Al-doped ZnO (AZO) films have been widely used as the front transparent conductive electrode for superstrate thin-film silicon (Si) solar cells. The AZO layer must be conductive enough to enable the lateral transport of electrons and sufficiently transparent to incoming light. In addition, the front AZO layer requires a rough surface texture morphology for light trapping, which improves the solar cell's current and efficiency. In this work, three types of AZO films with different layer thicknesses are prepared by pulsed DC magnetron sputtering. The post-deposition texturing is performed by using diluted hydrochloric (HCl) acid. After the HCl texturing, we find that not only the electrical performance, but also the optical scattering and morphological properties of the surface-etched AZO films can greatly depend on the initial AZO layer thickness. The AZO layer needs to be thick enough (∼700nm) in order to obtain good surface texturing properties for thin-film Si solar cell applications.

Towards the Growth of SiGeC Epitaxial Layers for the Application in Si Solar Cells

For the improvement of today's well established silicon solar cells a further significant increase in light conversion efficiency needs the consideration of multi junction aspects. In this context 3C-SiC with a bandgap of 2.3eV has already been discussed in the past. However, an ideal bandgap match within a silicon tandem cell (Eg(Si) = 1.1eV) requires a lower bandgap of 1.8eV which may be reached by the incorporation of Ge in the 3C-SiC lattice.In this work we report on the growth of Si-Ge-C hetero-epitaxial layers on silicon in a MOCVD reactor. A major obstacle in realizing a high Ge-content in the cubic 3C-SiC lattice is the strong trend towards segregation and phase separation in the ternary system. In terms of the atomic diameter, Ge should be more likely to substitute Si in the SiC lattice. However, concerning the excess energy the bond between Ge and C is not supposed to be stable.We will present several growth runs at different precursor flow-rates and different temperatures in the range of 1090°C - 1330°C and discuss the appearance of a SiGe phase. In order to detect the different bonds in the layers, especially Ge-C bonds, the samples were investigated by means of Raman-spectroscopy and x-ray diffraction. The Ge concentration was investigated by EDX measurements. No proof for the formation of Ge-C was found so far.

Optical absorption and photoluminescence of Ag interlayer modulated ZnO film in view of their application in Si solar cells

In this paper, the effect on the optical absorption efficiency and photoluminescence properties of monolayer ZnO and multilayer ZnO/Ag/ZnO films as anti-reflection layers of Si substrates were investigated. By adding Ag thin film to form the sandwich structures, the intensity of Raman scattering was enhanced, and the energy band structure of sandwich ZnO/Ag/ZnO films was optimized by changing the band gap and the Fermi level of ZnO. The absorption efficiency had a remarkable increase caused by interlayer Ag. Noticeably, the absorption efficiency rose several times in 1200–1500nm region. This Ag interlayer modulated ZnO film can be one of the promising materials used in solar cells.

Investigation of the thickness effect on material and surface texturing properties of sputtered ZnO:Al films for thin-film Si solar cell applications

Transparent conductive Al-doped ZnO (AZO) layers are widely used as the front electrode for thin-film silicon solar cells. For superstrate configurations, the front AZO layer simultaneously provides both electrical conductance and optical scattering. To improve the device performance, a textured surface is needed to enhance the photogeneration inside the solar cell through better light scattering. One approach is to wet-chemically etch the AZO films using hydrochloric (HCl) acid. In this work, AZO films with different initial layer thicknesses (900, 700 and 500 nm) are deposited onto soda-lime glass via magnetron sputtering and their texturing behaviours are compared. It is found that not only the material properties but also the surface texturing process greatly depends on the as-grown thickness of the AZO layer. The increased resistivity is mainly caused by a deteriorating carrier mobility as the thickness reduces. In terms of morphology, thick AZO films (i.e., 900 and 700 nm) show similar texture features after etching. In contrast, the thin AZO films (i.e., 500 nm) show irregularly shaped textures and over-etching, which leads to limited scattering. Hence the AZO layer thickness must be suitably chosen to achieve both good electrical conductance and optical scattering for high-efficiency thin-film Si solar cell applications.

X-ray photoelectron spectroscopy and luminescent properties of Y2O3:Bi3+ phosphor

X-ray photoelectron spectroscopy (XPS) results provided proof for the blue and green emission of Bi3+ in the Y2O3:Bi3+ phosphor. The Y2O3:Bi3+ phosphor was successfully prepared by the combustion process during the investigation of down-conversion materials for Si solar cell application. The X-ray diffraction (XRD) patterns indicated that a single-phase cubic crystal structure with the Ia3 space group was formed. X-ray photoelectron spectroscopy (XPS) showed that the Bi3+ ion replaces the Y3+ ion in two different coordination sites in the Y2O3 crystal structure. The O 1s peak shows five peaks, two which correlate with the O2− ion in Y2O3 in the two different sites, two which correlate with O2− in Bi2O3 in the two different sites and the remaining peak relates to hydroxide. The Y 3d spectrum shows two peaks for the Y3+ ion in the Y2O3 structure in two different sites and the Bi 4f spectrum shows the Bi3+ ion in the two different sites in Bi2O3. The photoluminescence (PL) results showed three broad emission bands in the blue and green regions under ultraviolet excitation, which were also present for panchromatic cathodoluminescence (CL) results. These three peaks have maxima at ∼3.4, 3.0 and 2.5eV. The PL emission ∼3.0eV (blue emission) showed two excitation bands centered at ∼3.7 and 3.4eV while the PL emission at ∼2.5eV (green emission) showed a broad excitation band from ∼4 to 3.4eV. The panchromatic CL images were obtained for selected wavelengths at (2.99±0.08)eV (for blue emission) and (2.34±0.06)eV (for green emission). These luminescence results correlate with the XPS results that show that there are two different Bi3+ sites in the host lattice.