Formation of porous silicon for large-area silicon solar cells: A new method

Luminescent porous silicon (PS) was prepared for the first time using a spraying set-up, which can diffuse in a homogeneous manner HF solutions, on textured or untextured (1 0 0) oriented monocrystalline silicon substrate. This new method allows us to apply PS onto the front-side surface of silicon solar cells, by supplying very fine HF drops. The front side of N⁺/P monocrystalline silicon solar cells may be treated for long periods without altering the front grid metallic contact. The monocrystalline silicon solar cells (N⁺/P, 78.5 cm²) which has undergone the HF-spraying were made with a very simple and low-cost method, allowing front-side Al contamination. A poor but expected 7.5% conversion efficiency was obtained under AM1 illumination. It was shown that under optimised HF concentration, HF-spraying time and flow HF-spraying rate, Al contamination favours the formation of a thin and homogeneous hydrogen-rich PS layer. It was found that under optimised HF-spraying conditions, the hydrogen-rich PS layer decreases the surface reflectivity up to 3% (i.e., increase light absorption), improves the short circuit current (I_sc), and the fill factor (FF) (i.e., decreases the series resistance), allowing to reach a 12.5% conversion efficiency. The dramatic improvement of the latter is discussed throughout the influence of HF concentration and spraying time on the I–V characteristics and on solar cells parameters. Despite the fact that the thin surfae PS layer acts as a good anti-reflection coating (ARC), it improves the spectral response of the cells, especially in the blue-side of the solar spectrum, where absorption becomes greater, owing to surface band gap widening and conversion of a part of UV and blue light into longer wavelengths (that are more suitable for conversion in a Si cell) throughout quantum confinement into the PS layer.     

Polycrystalline silicon solar cells with V-grooved surface

Mechanical grooving techniques are effective to uniform reduction of surface reflectance over all polycrystalline silicon solar cells. Furthermore, to reduce the surface reflectance, a V-shaped grooving technique was newly examined. To improve the short-circuit current (I_sc) and the open-circuit voltage (V_oc), a shallow n⁺/p junction was also examined for the grooved surface. By forming the shallower junction, both I_sc and V_oc remarkably increased. Consequently, a record high conversion efficiency of 17.2% has been confirmed at Japan Quality Assurance Organization (JQA) for a 10 × 10 cm² area polycrystalline silicon solar cell.     

Performance enhancement of dye‐sensitized solar cells via cosensitization of ruthenizer Z907 and organic sensitizer SQ2

Cosensitization is a highly effective technique to enhance the photovoltaic performance of a dye‐sensitized solar cell. The main objective of this work is to improve the performance of dye‐sensitized solar cell using cosensitization approach and investigation of the effect of the organic cosensitizer concentration on the power conversion efficiency of the fabricated solar cell devices. In this work, Z907, a ruthenium dye, has been cosensitized with SQ2, an organic sensitizer, and an overall efficiency of 7.83% has been achieved. The fabricated solar cells were evaluated using UV‐Vis spectroscopy, current‐voltage (I‐V) characteristics, and electrochemical impedance spectroscopy analysis. Our results clearly indicate that the concentration of organic cosensitizer strongly affects the photovoltaic performance of fabricated solar cells. Upon optimization, the cell fabricated with 0.3 mM Z907 + 0.2 mM SQ2 dye solution demonstrated J_sc (mA/cm²) = 21.38, V_oc (mV) = 698.37, FF (%) = 52.46, and power conversion efficiency of η (%)  = 7.83 under standard AM1.5G 1 sun illumination (100 mW/cm²). It was observed that the efficiency of cosensitized solar cells is significantly superior than that of individual sensitized solar cells (Z907 [η  = 5.08%] and SQ2 [η  = 1.39%]). This enhancement in efficiency could be attributed to the lower electron‐hole recombination rate, decrease in competitive absorption of I^?/I^?₃, and less dye aggregation because of the synergistic effect in cosensitized solar cells.     

Performance of p-type silicon-oxide windows in amorphous silicon solar cell

a-SiO_x films have been prepared using silane and pure oxygen as reactive gases in plasma CVD system. Diborane was introduced as a doping gas to obtain p-type conduction silicon oxide. Infrared absorption spectra show the incorporation of Si–O stretch mode around 1000 cm⁻¹. The optical bandgap increases with the oxygen to silane gas ratio, while the electrical conductivity decreases. Hydrogenated amorphous silicon solar cells have been fabricated using p-type a-SiO_x with around 1.85 eV optical bandgap and conductivity greater than 10⁻⁷ S/cm. The measured current–voltage characteristics of the solar cells under 100 mW/cm² artificial light are V_oc=0.84 V, J_sc=14.7 mA/cm², FF=0.635 with a conversion efficiency of 7.84%.     

Large-area multicrystalline silicon solar cell fabrication using reactive ion etching (RIE)

Surface texturing of crystalline silicon wafer improves the conversion efficiency of solar cells by the enhancement in antireflection property and light trapping. Compared to antireflection coating, it is a more permanent and effective scheme. Wet texturing with the chemicals such as alkali (NaOH, KOH) or acid (HF, HNO₃, CH₃COOH) is too difficult for thinner wafer to apply due to a large amount of silicon loss. However, Plasma surface texturing using Reactive Ion Etching (RIE) can be effective in reducing the surface reflectance with low silicon loss. In this study, we have fabricated a large-area (156×156 mm) multicrystalline silicon (mc-Si) solar cell by mask less surface texturing using a SF₆/O₂ reactive ion etching. We have accomplished texturing with RIE by reducing silicon loss by almost half of that in wet texturing process. By optimizing the processing steps, we achieved conversion efficiency, open circuit voltage, short circuit current density, and fill factor as high as 16.1%, 619 mV, 33.5 mA/cm², and 77.7%, respectively. This study establishes that it is possible to fabricate the thin multicrystalline silicon solar cells of low cost and high efficiency using surface texturing by RIE.     

Large-area high efficiency single crystalline silicon solar cells

This paper reports on a 100 cm² single crystalline silicon solar cell with a conversion efficiency of 19.44% (J_sc = 37.65 mA/cm², V_oc = 638 mV, FF = 0.809). The cell structure is as simple as only applying the textured surface, oxide passivation, and back surface field by the screen printing method. The comparison between cell performances of the CZ (Czochralski) and FZ (Floating zone) silicon substrates was investigated. The higher efficiency cells were obtained for the FZ substrate rather than the CZ substrate. The influence of the phosphorus concentration of the emitter on the cell efficiency has also been investigated. A good result was obtained when the surface concentration of phosphorus was 3 × 10²⁰ cm⁻³ and the junction depth was about 0.6 μm.     

High-efficiency μc-Si/c-Si heterojunction solar cells

P-type microcrystalline silicon (μc-Si (p)) on n-type crystalline silicon (c-Si(n)) heterojunction solar cells is investigated. Thin boron-doped μc-Si layers are deposited by plasma-enhanced chemical vapor deposition on CZ-Si and the V_oc of μc-Si/c-Si heterojunction solar cells is higher than that produced by a conventional thermal diffusion process. Under the appropriate conditions, the structure of thin μc-Si films on (1 0 0), (1 1 0), and (1 1 1) CZ-Si is ordered, so high V_oc of 0.579 V is achieved for 2×2 cm² μc-Si/multi-crystalline silicon (mc-Si) solar cells. The epitaxial-like growth is important in the fabrication of high-efficiency μc-Si/mc-Si heterojunction solar cells.     

Solar cells using iodine-doped polythiophene–porphyrin polymer films

Wet-type organic solar cells containing 5,10,15,20-3-tetrathienylporphyrin (TThP) and polythiophene (PTh) films were fabricated. The TThP/PTh film was prepared on indium-tin-oxide (ITO) glass using an electrochemical polymerization method in an n-Bu₄NPF₆/CH₂Cl₂ solution. It was found that a small amount of iodine doping of the film improved the incident photon-to-electron conversion efficiency (IPCE) of a solar cell consisting of a TThP/PTh film and an aqueous electrolyte. A HOMO level measurement suggested that a modified HOMO level of the low iodine-doped TThP/PTh film allowed a fast electron transfer from PTh to a porphyrin moiety. To obtain further improvement, a sandwich-type solar cell using a 5% (w/w) aqueous solution of acetonitrile containing 0.05 M iodine and 0.5 M lithium iodide as an electrolyte was then fabricated. The solar cell absorbed light in the 300–800 nm wavelength range, converting this to a cathodic photocurrent with a maximum IPCE of 32% at 430 nm under irradiation of 5.0×10¹⁴ photon cm⁻² s⁻¹. This value is about 10 times higher than that of the solar cells using an aqueous electrolyte. The total energy conversion efficiency (η) of the solar cell under simulated sunlight reached 0.12% for 2.59 mW cm⁻² at AM1.5 and 0.05% for 100 mW cm⁻² at air mass 1.5.     

Experimental evidence of very high open-circuit voltages of inversion–layer silicon solar cells

In this paper the first experimental evidence of the high V_oc-potential of inversion-layer silicon solar cells is given. Minority-carrier lifetime measurements on inversion-layer emitters have been performed and the diffused p–n contact of PN-IL silicon solar cells has been optimized for high open-circuit voltages. PN-IL silicon solar cells with open-circuit voltages of 693 mV have been fabricated on 0.2 and 0.5-Ω cm FZ p-Silicon wafers. These values are the highest ever reported V_oc's for inversion-layer silicon solar cells on p-Silicon. This demonstrates that inversion-layer silicon solar cells exhibit a similar potential for achieving high open-circuit voltages as silicon solar cells with a diffused p–n junction.     

A new route for fabricating CdO/c-Si heterojunction solar cells

CdO/c-Si solar cells have been made by depositing CdO thin films on p-type monocrystalline silicon substrate by means of the rapid thermal oxidation (RTO) technique using a halogen lamp at 350 °C/45 s in static air. Results on structural, optical, and electrical properties of grown CdO films are reported. The electrical and photovoltaic properties of CdO/Si solar cells are examined. Under AM1 illumination condition, the cell shows an open circuit voltage (V_OC) of 500 mV, a short circuit current density (J_SC) of 27.5 mA/cm², a fill factor (FF) of 60%, and a conversion efficiency (η) of 8.84% without using frontal grid contacts and/or post-deposition annealing. Furthermore, the stability of solar cells characteristics is tested.     

Simulation of an efficient silicon heterostructure solar cell concept featuring molybdenum oxide carrier‐selective contact

Transition metal oxides/silicon heterocontact solar cells are the subject of intense research efforts owing to their simpler processing steps and reduced parasitic absorption as compared with the traditional silicon heterostructure counterparts. Recently, molybdenum oxide (MoO_x, x < 3) has emerged as an integral transition metal oxide for crystalline silicon (cSi)‐based solar cell based on carrier‐selective contacts (CSCs). In this paper, we physically modelled the CSC‐based cSi solar cell featuring MoO_x/intrinsic a‐Si:H/n‐type cSi/intrinsic a‐Si:H/n⁺‐type a‐Si:H for the first time using Silvaco technology computer‐aided design simulator. To analyse the optical and electrical properties of the proposed solar cell, several technological parameters such as work function and thickness of MoO_x contact layer, intrinsic a‐Si:H band gap, interface recombination, series resistance, and temperature coefficient have been evaluated. It has been shown that higher work function of MoO _x induces the formation of a favourable Schottky barrier height as well as an inversion at the front interface, stimulating least resistive path for holes. Utilising thinner MoO_x layer implies reduced tunnelling of minority charge carriers, thus enabling the device to numerically attain 25.33% efficiency. With an optimised interface recombination velocity and reduced parasitic absorption, the proposed device exhibited higher V_oc of 752 mV, J_sc of 38.8 mA/cm², fill‐factor of 79.0%, and an efficiency of 25.6%, which can be termed as the harbinger for industrial production of next‐generation efficient solar cell technology.     

High efficiency AlxGa1−xAs/GaAs solar cells: Fabrication, irradiation and annealing effect

High efficiency Al_xGa_1−xAs/GaAs heteroface solar cells have been fabricated by an improved multi-wafer squeezing graphite boat liquid phase epitaxy (LPE) technique, which enables simultaneous growth of twenty 2.3 × 2.3cm² epilayers in one run. A total area conversion efficiency of 17.33% is exhibited (1sun, AMO, 2.0 × 2.0cm²). The shallow junction cell shows more resistance to 1 MeV electron radiation than the deep one. After isochronal or isothermal annealing the density and the number of deep level traps induced by irradiation are reduced effectively for the solar cells with deep junction and bombardment under high electron fluences.     

The study on high efficient AlxGa1−xAs/GaAs solar cells

The investigation of Al_xGa_1−xAs/GaAs solar cells is carried out by means of both metalorganic chemical vapor deposition (MOCVD) and liquid-phase epitaxial (LPE) technique. The measurements of illuminated I–V characteristics, dark I–V characteristics and quantum efficiencies were performed for the GaAs solar cells made in author's laboratory. The measuring results revealed that the quality of materials in GaAs solar cell's structures is the key factor for getting high-efficient GaAs solar cells, but the effect of post-growth technology on the performances of GaAs solar cells is also very strong. The 21.95% (AM0, 2×27 cm², 25°C) high conversion efficiency in a typical GaAs solar cell has been achieved owing to improving the quality of materials as well as optimizing the post-growth technology of devices.     

p-doped μc-Si:H window layers prepared by hot-wire CVD for amorphous solar cell application

We report on boron-doped μc-Si:H films prepared by hot-wire chemical vapor deposition (HWCVD) using silane as a source gas and trimethylboron (TMB) as a dopant gas and their incorporation into all-HW amorphous silicon solar cells. The dark conductivity of these films was in the range of 1–10 (Ω cm)⁻¹. The open circuit voltage V_oc of the solar cells was found to decrease from 840 mV at low hydrogen dilution H-dil=91% to 770 mV at high H-dil =97% during p-layer deposition which can be attributed to the increased crystallinity at higher H-dil and to subsequent band edge discontinuity between μc-Si:H p- and amorphous i-layer. The short circuit current density J_sc and the fill factor FF show an optimum at an intermediate H-dil and decrease for the highest H-dil. To improve the conversion efficiency and the reproducibility of the solar cells, an amorphous-like seed layer was incorporated between TCO and the bulk p-layer. The results obtained until now for amorphous solar cells with and without the seed layer are presented. The I–V parameters for the best p–i–n solar cell obtained so far are J_sc=13.95 mA/cm², V_oc=834 mV, FF=65% and η=7.6%, where the p-layers were prepared with 2% TMB. High open circuit voltages up to 847 mV could be achieved at higher TMB concentrations.     

SOLAR CELLS WITH 15.6% EFFICIENCY ON MULTICRYSTALLINE SILICON,USING IMPURITY GETTERING,BACK SURFACE FIELD AND EMITTER PASSIVATION

Three features have been combined to raise the efficiency of solar cells made on industrial multicrystalline silicon wafers: 1) reduction of bulk recombination by a special gettering process, 2) reduction of back recombination by using a p/p ⁺ junction, 3) reduction of front recombination by emitter back-etching and passivation.

A conversion efficiency of 15.6% has been achieved on 2 × 2 cm² solar cells. Spectral response measurements are used to identify the role of each processing parameter.     

Polycrystalline silicon solar cells on low cost foreign substrates

The use of polycrystalline silicon layers on low-cost substrates is a promising approach for the fabrication of low-cost solar cells. Using low-carbon steel and graphite as substrates, solar cell structures have been deposited by the thermal decomposition of silane and appropriate dopants.Steel was selected as a substrate on the sole basis of its low cost. However, steel and silicon are not compatible in their properties, and an interlayer of a diffusion barrier, such as borosilicate, must be used to minimize the diffusion of iron from the substrate into the deposit. The deposited silicon on borosilicate/steel substrates is polycrystalline with a grain size of 1–5 μm, depending on deposition conditions. P-n junction solar cells were found to have low open-circuit voltages and poor current-voltage characteristics, and Schottky-barrier solar cells were found to show negligible photovoltages.Graphite is more compatible with silicon in properties than steel, and silicon deposited on graphite substrates shows considerably better microstructures. A number of solar cells, 2·5×2·5 cm in area, have been fabricated from n⁺-silicon/p-silicon/p⁺-silicon/graphite structures. The best cell to date had a V_oc of 0·35 V and an AMO efficiency of 1·5% (no antireflection coating). This type of solar cell is very promising because of the simplicity in fabrication.     

Calibration of solar simulator for evaluation of dye-sensitized solar cells

The photo-to-electricity energy conversion efficiencies of ruthenium-dye-sensitized solar cells (DSC) are measured under a solar simulator. The error in conversion efficiencies was compared under a variety of spectral conditions. Measurements of the conversion efficiencies of DSC between a solar simulator and outdoor sunlight result in about 10% error. This error was seen when the spectral intensity of a xenon-lamp solar simulator (imitating an air mass (AM) 1.5 spectrum) was adjusted by the short-circuit photocurrent I_Si of a crystalline silicon (c-Si) standard cell. In order to adjust the energetic intensity of AM 1.5 for DSC that has a spectrum response only in the visible region light, the c-Si reference cell is modified with a glass UV filter (KG-5, Schott) and the solar simulator was adjusted by I_{IR-cut Si}. The energetic spectrum of the solar simulator has a good accuracy over the wavelength range 300–750 nm, giving the conversion efficiency of DSC an accuracy of about 2%. The dependency of the ratio of I_Si to I_{IR-cut Si} on natural sun power is discussed in view of scattering of the visible light under changing natural sun light.     

Single-junction a-Si solar cells with over 13% efficiency

Single junction hydrogenated amorphous silicon solar cells having a high conversion efficiency of 13.2% were developed by combining three approaches. First, a new type of p-layer, such as (a-Si/a-C)_n multilayers, was investigated. The high open-circuit voltage was obtained without lowering the short-circuit current and the fill factor. Second, alternately repeating deposition and hydrogen plasma treatment method was applied to the fabrication of an a-SiC or wide gap a-Si : H films for p/i interface layer. High photoconductive and wide bandgap materials were obtained applicable to the p/i buffer layers. Third, the relationships between defect density of films or fill factors of solar cells and hydrogen radical in plasma were investigated. It was suggested that the H^*/SiH^* ratio was an effective parameter to improve the defect and fill factor, and the excess hydrogen radical deteriorated quality of films and cells.     

Thin crystalline silicon solar cell bonded to sintered substrate with aluminum paste

Making thinner wafers is a simple way to reduce the production cost of silicon solar cells. However, thin wafers need to be supported mechanically in order to avoid the problem of breakage. Among the several possible supporting materials, silicon substrate made from the sintering of silicon powder, which is produced during the slicing process is the most favorable one because of its abundance and its similar thermal expansion coefficient with silicon wafers. For the bonding of the substrate and thin silicon wafers, aluminum paste is selected because of its compatibility with silicon and the possible BSF effect. Silicon solar cells of 150 μm with the sintered substrate on the back show 5.42% in solar cell conversion efficiency. Compared to commercial silicon cells, lower J_sc is obtained. This might be due to the poor conduction in the back layer of aluminum, which is absorbed into the supporting substrate during the annealing process.     

Dependence of the I–V characteristics of amorphous silicon solar cells on illumination intensity and temperature

Current-voltage characteristics of amorphous silicon (a-Si) solar cells are systematically investigated as functions of the illumination intensity and ambient temperature. The principle of superposition of the short-circuit current and the dark current, which is usually assumed for crystalline silicon solar cells, is not applicable to a-Si solar cells. It is shown, that the output current of a-Si solar cells at a given illumination intensity E₂mW/cm²I_E2(V) is expressed by a relatively simple equation, I_E2(V) = I_d(V) + (_{E2/100) × (I100(V) — Id(V))}, when the series resistance of the solar cells is negligible. Here, I_d(V) is the dark current, I₁₀₀(V) is the output current at an illumination of 100 mW/cm², and V is the applied voltage. Empirical formula to describe the dependence of the current-voltage characteristics on the illumination intensity and the temperature are presented and discussed.