Application of phosphorus diffusion gettering process on upgraded metallurgical grade Si wafers and solar cells

Although phosphorus (P) diffusion gettering process has been wildly used to improve the performance of Si solar cells in photovoltaic technology, it is a new attempt to apply P diffusion gettering process to upgraded metallurgical grade silicon (UMG-Si) wafers with the purity of 99.999%. In this paper, improvements on the electrical properties of UMG-Si wafers and solar cells were investigated with the application of P diffusion gettering process. To enhance the improvements, the gettering parameters were optimized on the aspects of gettering temperature, gettering duration and POCl₃ flow rate, respectively. As we expected, the electrical properties of both multicrystalline Si (multi-Si) and monocrystalline Si (mono-Si) wafers were significantly improved. The average minority carrier lifetime increased from 0.35 μs to nearly about 2.7 μs for multi-Si wafers and from 4.21 μs to 5.75 μs for mono-Si wafers, respectively. Accordingly, the average conversion efficiency of the UMG-Si solar cells increased from 5.69% to 7.03% for multi-Si solar cells (without surface texturization) and from 13.55% to 14.55% for mono-Si solar cells, respectively. The impurity concentrations of as-grown and P-gettered UMG-Si wafers were determined quantitively so that the mechanism of P diffusion gettering process on UMG-Si wafers and solar cells could be further understood. The results show that application of P diffusion gettering process has a great potential to improve the electrical properties of UMG-Si wafers and thus the conversion efficiencies of UMG-Si solar cells.     

Preparation of size-controlled fine Al particles for application to rear electrode of Si solar cells

Micron-sized aluminum (Al) particles are widely used in the fabrication of rear electrodes of Si solar cells. Moreover, the rear electrode of Si solar cells can be fabricated at relatively low electrode firing temperature using submicron Al particles whose sizes can be easily controlled, but there have not been any clear methods to obtain submicron Al particles yet. In this study, we first successfully prepared size-controlled Al submicron particles via a wet chemical process using dibutyl ether solvent and then to fabricate rear electrodes of Si solar cells; to our knowledge, this is the first such application of submicron Al particles. The geometric mean diameter (D_p) of the Al particles could be controlled from 139 to 614 nm by adjusting the reaction temperature, and the prepared Al particles showed geometric standard deviations (σ_g) of 1.25-1.30. The Al particle size was reduced to ∼35 nm by adding an organic surfactant to the precursor solution for Al particle preparation. Rear electrodes were fabricated by firing the screen printed Al paste films comprising Al particles with geometric mean diameters of ∼379 nm from 600 to 900° C. The electrode fired at 750 °C showed the minimum electrical sheet resistivity of 77.9 mΩ/□ (specific resistivity: 97.3 μΩ cm) and contained a BSF (back surface field) with a thickness of ∼3 μm. Our results indicate that the reported method can be used to minimize the thermal defects in the rear electrode of Si solar cells by lowering the electrode firing temperature.     

Development of an emitter for industrial silicon solar cells using the doped oxide solid source diffusion technique

The aim of this paper is to demonstrate for the first time the feasibility of fabricating large-area screen-printed monocrystalline silicon solar cells using the Doped Oxide Solid Source (DOSS) diffusion technique. This process was applied to form the n⁺p emitter junction from highly doped sources prepared in a POCl₃ ambient. The diffusions were performed under a pure nitrogen flow in the temperature range 900–1050°C. In this investigation attention was devoted to the influence of the source doping level on the fill factor. The solar cells were fabricated on industrial quality 4-inch Cz wafers using a simple processing sequence incorporating screen-printed contacts and a TiO₂ antireflection coating deposited by spin-on. Fill factors as high as 79% were obtained. The potential benefit of retaining for passivation purposes the thin residual oxide grown during phosphorus diffusion was evaluated. These first experiments led to a cell efficiency close to 10%.     

Optimized resistivity of p-type Si substrate for HIT solar cell with Al back surface field by computer simulation

For HIT (heterojunction with intrinsic thin-layer) solar cell with Al back surface field on p-type Si substrate, the impacts of substrate resistivity on the solar cell performance were investigated by utilizing AFORS-HET software as a numerical computer simulation tool. The results show that the optimized substrate resistivity (R_op) to obtain the maximal solar cell efficiency is relative to the bulk defect density, such as oxygen defect density (D_od), in the substrate and the interface defect density (D_it) on the interface of amorphous/crystalline Si heterojunction. The larger D_od or D_it is, the higher R_op is. The effect of D_it is more obvious. R_op is about 0.5 Ω cm for D_it = 1.0 × 10¹¹/cm², but is higher than 1.0 Ω cm for D_it = 1.0 × 10¹²/cm². In order to obtain very excellent solar cell performance, Si substrate, with the resistivity of 0.5 Ω cm, D_od lower than 1.0 × 10¹⁰/cm³, and D_it lower than 1.0 × 10¹¹/cm², is preferred, which is different to the traditional opinion that 1.0 Ω cm resistivity is the best.     

The influence of porous silicon on junction formation in silicon solar cells

In this work the results of a structural investigation by SEM of porous silicon (PS) before and after diffusion processes are reported. The formation of PS n⁺/p structures were carried out on PS p/p silicon wafers with two methods: from POCl₃ in a conventional furnace and from a phosphorous doped paste in an infrared furnace. Sheet resistance was found to be a strong function of PS structure. Further details on sheet resistance distribution are reported. The electrical contacts in prepared solar cells were obtained by screen printing process, with a Du Ponte photovoltaic silver paste for front contacts and home-prepared silver with 3% aluminium paste for the back ones. Metallization was done in the infrared furnace. Solar cell current–voltage characteristics were measured under an AM 1.5 global spectrum sun simulator. The average results for multi-crystalline silicon solar cells without antireflection coating are: I_sc=720 (mA), V_oc=560 (mV), FF=69%, Eff=10.6% (area 25 cm²).     

Effect of electrode geometry on the photovoltaic performance of dye-sensitized solar cells

Effect of electrode geometry on the photovoltaic performance of dye-sensitized solar cell (DSSC) has been investigated to optimize the device geometry for reliable energy conversion efficiency assessment. Mesoporous TiO₂ layers with an identical active area (0.40 cm²) and different dimension are prepared on FTO glass substrate by the screen printing method and used as photoanodes for DSSCs. Under 1 sun illumination (AM 1.5G, 100 mW cm⁻²), both the open-circuit voltage and the short-circuit current density are independent of electrode geometry whereas the fill factor and hence energy conversion efficiency show strong dependency. Electrochemical impedance spectroscopy analysis indicates that the distance between active layer and ohmic contact directly contributes to internal series resistance and influence photovoltaic performance.     

Electrochemically deposited p-n homojunction cuprous oxide solar cells

The electrical properties of both p- and n-type cuprous oxide (Cu₂O) films electrochemically deposited from two electrolyte solutions were examined by current-voltage measurements. The resistivity of p-type Cu₂O varied from 3.2×10⁵ to 2.0×10⁸ Ω cm, while that of n-type Cu₂O from 2.5×10⁷ to 8.0×10⁸ Ω cm, depending on deposition conditions such as solution pH, deposition potential and temperature. With optimized deposition conditions for minimum resistivity, p-n homojunction Cu₂O solar cells were fabricated by a two-step deposition process. The p-n homojunction Cu₂O solar cells showed a conversion efficiency of 0.1% under AM1 illumination. The low efficiency is attributed to the high resistivity of p- and n-type Cu₂O, which require doping to reduce.     

Effect of series resistance on metal-wrap-through multi-crystalline silicon solar cells

Metal-wrap-through (MWT) is a promising technique to improve the solar cell performance cost effectively because it can be easily integrated into the current production line with only two additional processing steps. Metal filling through the via-holes is a key to obtain low series resistances and good FFs. In this study, several screen printing process conditions were examined to find out an optimal filling state of the metal contacts. Various shapes of the filled metals in the via-holes were formed with different printing conditions, and the shape of the filled metal results in different series resistance values. Optimization of the printing conditions dramatically reduced the series resistance of the MWT cells. The maximum and minimum series resistance values of the cells obtained are 8.56 and 0.114 Ω cm², respectively. As a result, we achieved an efficiency of 16.3% using the optimal printing condition on 156 mm×156 mm solar-grade multi-crystalline silicon wafer, which was 0.8% absolute higher than the baseline cell efficiency.     

Gravure printed flexible organic photovoltaic modules

In this letter, organic solar cell modules based on poly-3-hexylthiophene (P3HT) and [6.6]-phenyl-C61-butyric acid methyl ester (PCBM) blend films with a module active area of 15.45 cm² prepared by roll-to-roll (R2R) compatible gravure printing method are demonstrated. The gravure printed organic photovoltaic modules consist of eight serially connected solar cells in same substrate. Indium-tin-oxide (ITO) is patterned by screen printable etching paste. Hole injection layer and active layer are prepared by gravure printing method. All processing steps excluding cathode evaporation are performed in air. Electrical measurements are done to modules consisting of 5-8 serially connected solar cells. The photovoltaic modules comprising 5, 7 and 8 serially connected cells exhibit an active area power conversion efficiency of 1.92%, 1.79% and 1.68%, respectively (Oriel Sol3A Class AAA, AM1.5G, 100 mW cm⁻²).     

Influence of deep level defects on the performance of crystalline silicon solar cells: Experimental and simulation study

Introduction of deep level defects during thermal diffusion of phosphorous (P) in silicon (Si) using spin-on-doping (SOD) from phosphosilicate glass (PSG) was studied using deep level transient spectroscopy (DLTS). The structure was utilized as a solar cell and defect-induced-degradation of the cell efficiency was studied and modeled. The light current-voltage (LIV) measurements performed on as-fabricated solar cell yielded open circuit voltage, short-circuit current density, fill factor (FF) and efficiency to be 540 mV, 24 mA/cm², 40% and 5%, respectively. Whilst the simulation of the similar solar cell using AFORS-HET software revealed significantly higher data than the experimental ones. However, by including three deep level defects H₁-H₃ (holes) having activation energies (eV) 0.23, 0.33 and 0.41 in the modeled solar cell, the simulated results were observed in remarkably good agreement with experimental data. Our DLTS measurements practically witnessed H₁-H₃ defect levels in p-layer of the cell.     

Preparation of transparent and conducting boron-doped ZnO electrode for its application in dye-sensitized solar cells

A simple and economic chemical spray pyrolysis method is used to prepare transparent and conducting boron-doped zinc oxide (B_nZnO) electrode having potential applications in dye-sensitized solar cells (DSSCs). The B_nZnO electrodes were critically characterized for their structural, morphological and electrical properties. The B_nZnO electrode with 2 at% boron doping showed average grain size of 20(±1) nm, surface roughness of 9 nm, ?95% transparency and resistivity of 4.5×10⁻³ Ω cm⁻¹. Furthermore, doping concentration of boron could also be easily controlled for achieving desired properties. Using this electrode as a substrate in DSSCs, the solar-to-electrical conversion efficiency with N3 dye as a sensitizer was noted to be 1.53%. This work suggests that the B_nZnO electrodes could be used as promising alternative to presently used indium- or fluorine-doped tin oxide as substrates.     

Impact of sheet resistance on 2-D modeling of thin-film solar cells

A rigorous mathematical approach was used to find a relation between the transparent-conductive-oxide (TCO) sheet resistance ρ_S (Ω/□) of a thin-film solar cell and the parameter R (Ω) that describes the TCO resistance in a two-dimensional circuit model. Additionally, the mathematical relationship that connects experimentally derived series resistance R_S (Ω cm²) of the solar cell to the TCO sheet resistance ρ_S (Ω/□) and the bulk semiconductor resistivity ρ (Ω cm) was derived. It was found that the fill factor of the solar cell is governed by a reduced dimensionless TCO sheet resistance that depends only weakly on the type and quality of the solar cell. Parameters corresponding to thin-film Cu(In,Ga)Se₂, two-junction a-Si, and an ideal solar cell were used as concrete examples.     

Optimization of lead- and cadmium-free front contact silver paste formulation to achieve high fill factors for industrial screen-printed Si solar cells

Screen-printed n⁺–p–p⁺ solar cells were fabricated on Cz single crystalline Si material, with a 45 Ω/sq emitter and PECVD SiNx antireflective coating with a thickness of 700 Å, using different Ag pastes and commercial leaded reference paste (CN33-462, Ferro Corp.). Ag and Al contacts were co-fired using a mass-production line equipped with mesh belt conveyer furnace systems (Centrotherm thermal solution GmbH & Co. KG). The average results for single crystalline Si solar cells (156 cm²) are: I_sc=5.043 A, V_oc=0.621 V, R_s=0.0087 Ω, R_sh=15.3 Ω, FF=0.773, and E_ff=16.45%. R_sh and fill factor values of fabricated cells were slightly higher when compared with the commercial leaded Ag paste, although cells were fabricated by metallizing the lead-free silver pastes. For the lead-free Ag paste used in this study, the line pattern continuity is retained with improved edge definition in sharp contrast to that of reference Ag paste. Average value of R_s was also equivalent approximately to that of the leaded Ag paste.     

Organic solar cells with 2-Thenylmercaptan/AU self-assembly film as buffer layer

The interface between an electrode and the organic active layer is an important factor in organic solar cells (OSCs) that influences the power conversion efficiency (PCE). In this report, a buffer layer of 2-thenylmercaptan/Au self-assembly film is introduced into OSCs as a substitute for the poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT: PSS) layer. The electrode/active layer interface is meliorated by Au-S coordinate bond of self assembly after applying this buffer layer. The series resistance reduces from 20 Ω cm² in a device based on PEDOT:PSS to 10.2 Ω cm². Correspondingly, the fill factor (FF) increases from 0.50 to 0.64. Moreover, due to the dipole of this self-assembled layer, the open circuit voltage (V_oc) also increases slightly from 0.54 V to 0.56 V and the PCE reaches 2.5%.     

Reactive Spray Deposition Technology - An one-step deposition technique for Solid Oxide Fuel Cell barrier layers

In order to reduce the cost of the manufacturing of Solid Oxide Fuel Cells (SOFC), and to enable metal supported cell fabrication, a new fabrication method called Reactive Spray Deposition Technology (RSDT) for direct deposition of the material onto ceramic or metal support for low temperature SOFC is currently being developed. The present work describes the effect on the performance of a SOFC when a Gd_0.2Ce_0.8O_1.9 (GDC) layer has been introduced as diffusion barrier layer between the yttria stabilized zirconia (YSZ) electrolyte and the La_0.6Sr_0.4CoO_3−δ (LSC) cathode. The dense, thin and fully crystalline GDC films were directly applied by RSDT, without any post-deposition heating or sintering step. The quality of the film and performance of the cell prepared by RSDT was compared to a GDC blocking layer deposited by screen printing (SP) and then sintered. The observed ohmic resistance of the ASC with a GDC layer deposited by RSDT is 0.24 Ω cm², which is close to the expected theoretical value of 0.17 Ω cm² for a 5-μm thick 8 mol% yttria YSZ (8YSZ) electrolyte at 873 K.     

Inkjet printed chalcopyrite CuInxGa1−xSe2 thin film solar cells

In this paper, we report a novel approach for the fabrication of chalcopyrite CuIn_xGa_1−xSe₂ thin film solar cells by inkjet printing. The short circuit current (J_sc), open circuit voltage (V_oc), fill factor (FF), and total area power conversion efficiency (η) of the device are 29.78 mA/cm², 386 mV, 0.44%, and 5.04%, respectively. Inkjet printing at atmospheric environment offers an opportunity for the direct patterning of absorber materials at large scale. This provides a potential cost advantage over conventional fabrication process that involves sequential deposition, patterning, and etching of selected materials. In addition, inkjet printing increases the raw material utilization ratio compared to more wasteful vacuum-based deposition techniques.     

A novel UV-mediated low-temperature sintering of TiO2 for dye-sensitized solar cells

Titania pastes were fired at 450 °C in oxygen to give white titania that was used to prepare dye-sensitized solar cells (DSSC). Titania fired at lower temperature and/or under inert atmosphere have brown stripes and cells made from these stripes had no measurable efficiency. When the titania paste was screen printed and then heated and simultaneously irradiated with UV light, white stripes were obtained. Improved efficiency was noted for PV cells made from pastes heated at lower temperature under irradiation vs. cells made from low-temperature heated paste but without irradiation. UV irradiation appears to facilitate clean oxidation of residual organic materials in the titania precursor pastes. The best cells in our study made with our titania paste treated at 450 °C in oxygen had the following characteristics: efficiency=3.45%; V_oc=630 mV; J_sc=8.5 mA/cm²; and a fill factor=0.64.     

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.     

High efficiency flexible dye-sensitized solar cells by multiple electrophoretic depositions

A multiple electrophoretic deposition (EPD) of binder-free TiO₂ photoanode has been developed to successfully fill the crack occurring after air-drying on the first EPD-TiO₂ film surface. With the slow 2nd EPD, high quality TiO₂ thin films are acquired on flexible ITO/PEN substrates at room temperature and the device efficiency of the dye-sensitized solar cell achieved 5.54% with a high fill factor of 0.721. Electrochemical impedance spectroscopy measurements analyze the great enhancement of the photovoltaic performance through multiple EPD. The electron diffusion coefficient improved by about 1 order of magnitude in crack-less multiple-EPD TiO₂ films. With the scattering layer, the device reveals a high conversion efficiency of up to 6.63% under AM 1.5 G one sun irradiation, having a short circuit current density, open circuit voltage, and filling factor of 12.06 mA cm⁻², 0.763 V and 0.72, respectively.     

Significant impact of the current collection material and method on the performance of Ba0.5Sr0.5Co0.8Fe0.2O3−δ electrodes in solid oxide fuel cells

The effects of the current collection material and method on the performance of SOFCs with Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) cathodes are investigated. Ag paste and LaCoO₃ (LC) oxide are studied as current collection materials, and five different current collecting techniques are attempted. Cell performances are evaluated using a current-voltage test and electrochemical impedance spectra (EIS) based on two types of anode-supported fuel cells, i.e., NiO + SDC|SDC|BSCF and NiO + YSZ|YSZ|SDC|BSCF. The cell with diluted Ag paste as the current collector exhibits the highest peak power density, nearly 16 times that of a similar cell without current collector. The electrochemical characteristics of the BSCF cathode with different current collectors are further determined by EIS at 600 °C using symmetrical cells. The cell with diluted Ag paste as the current collector displays the lowest ohmic resistance (1.4 Ω cm²) and polarization resistance (0.1 Ω cm²). Meanwhile, the surface conductivities of various current collectors are measured by a four-probe DC conductivity technique. The surface conductivity of diluted Ag paste is 2-3 orders of magnitude higher than that of LC or BSCF. The outstanding surface conductivity of silver may reduce the contact resistance at the current collector/electrode interface and, thus, contributes to better electrode performance.