Photovoltaic properties of n-C:P/p-Si cells deposited by XeCl eximer laser using graphite target

This paper reports on the successful deposition of phosphorous (P)-doped n-type (p-C:P) carbon (C) films, and fabrication of n-C:P/p-Si cells by pulsed laser deposition (PLD) using graphite target at room temperature. The cells performances have been given in the dark I–V rectifying curve and I–V working curve under illumination when exposed to AM 1.5 illumination condition (100 mW/cm², 25 °C). The n-C:P/p-Si cell fabricated using target with the amount of P by 7 weight percentages (Pwt%) shows highest energy conversion efficiency, η=1.14% and fill factor, FF=41%. The quantum efficiency (QE) of the n-C:P/p-Si cells are observed to improve with and Pwt%. The dependence of P content on the electrical and optical properties of the deposited films and the photovoltaic characteristic of the n-C:P/p-Si heterojunction solar cell are discussed.     

A novel Al and Y codoped ZnO/n-Si heterojunction solar cells fabricated by pulsed laser deposition

Al and Y codoped ZnO (AZOY) transparent conducting oxide (TCO) thin films were first deposited on n-Si substrates by pulsed laser deposition (PLD) to form AZOY/n-Si heterojunction solar cells. However, the properties of the AZOY emitter layers are critical to the performance of AZOY/n-Si heterojunction solar cells. To estimate the properties of AZOY thin films, films deposited on glass substrates with various substrate temperatures (T_s) were analyzed. Based on the experimental results, optimal electrical properties (resistivity of 2.8 ± 0.14 × 10^?4 Ω cm, carrier mobility of 27.5 ± 0.55 cm²/Vs, and carrier concentration of 8.0 ± 0.24 × 10²⁰ cm^?3) of the AZOY thin films can be achieved at a T_s of 400 °C, and a high optical transmittance of AZOY is estimated to be >80% (with glass substrate) in the visible region under the same T_s. For the AZOY/n-Si heterojunction solar cells, the AZOY thin films acted not only as an emitter layer material, but also as an anti-reflected coating thin film. Thus, a notably high short-circuit current density (J_sc) of 31.51 ± 0.186 mA/cm² was achieved for the AZOY/n-Si heterojunction solar cells. Under an AM1.5 illumination condition, the conversion efficiency of the cells is estimated at only approximately 4% (a very low open-circuit voltage (V_oc) of 0.24 ± 0.001 V and a fill factor (FF) of 0.51 ± 0.011) without any optimization of the device structure.     

Electrical and optical properties of point-contacted a-Si:H/c-Si heterojunction solar cells with patterned SiO2 at the interface

We report on the electrical and optical characteristics of a-Si:H/c-Si heterojunction solar cells with point-contact junction via patterned SiO₂ layer at the interface. The new structure showed improved electrical properties, having a smaller leakage current and a larger shunt resistance. The electrical conduction of the point-contacted samples followed the diffusion dominant process with bulk recombination, but the control samples without SiO₂ showed the space-charge region recombination dominant process. The point-contacted samples showed increased internal quantum efficiency in the bulk region, but decreased internal quantum efficiency in the surface region. As the distance between the holes decreased, the point-contacted solar cells showed an improved efficiency with a larger fill-factor but smaller open-circuit voltage and short-circuit current.     

Investigation of non-aqueous electrodeposited CdS/Cd1−xZnxTe heterojunction solar cells

Polycrystalline Cd_1−xZn_xTe solar cells with efficiency of 8.3% were grown by cathodic electrodeposition on glass/ITO/CdS substrates using non-aqueous ethylene glycol bath. The deposit is characterised versus the process conditions by XRD and found to possess a preferred (1 1 1) orientation on Sb doping in the electroplating bath. The surface morphology of the deposit is studied using atomic force microscope. The average RMS roughness for the ternary film was higher than that for the binary CdTe. Optical properties of the films were carried out to study the band gap and calculation of molar concentration ‘x’. The effects of Sb doping in CdS/Cd_1−xZn_xTe heterojunctions have been studied. The short circuit current density (c) was found to improve and series resistance (R_s) reduced drastically upon Sb doping. This improvement in J_sc is attributed to an increase in quantum efficiency. The evaluation of solar cell parameters was also carried out using the current–voltage characteristics in dark and illumination. The best results were obtained when 2×10⁻³ M ZnCl₂ along with antimony were present in the deposition bath. Under AM 1.5 conditions the open circuit voltage, short circuit current density, and fill factor of our best cell were V_oc=600 mV, J_sc=26.66 mA/cm², FF=0.42 and efficiency, η=8.3%. The carrier concentration and built-in potential of Cd_1−xZn_xTe calculated from Mott–Schottky plot was 2.72×10¹⁷ cm⁻³ and 1.02 eV.     

Influence of laser power on the properties of laser doped solar cells

In recent years, increased attention has been focused on the use of lasers in different fabrication steps of solar cells, in particular laser doping to form emitter and/or selective emitter. In this method the laser energy is used to melt silicon, allowing the diffusion of dopant atoms to occur in the liquid phase. The main advantage of this method is the localised nature of the laser beam, which melts and diffuses a limited area without heating the bulk, therefore reducing the possible degradation associated with high temperature processes. At the University of New South Wales a novel laser doping method was developed, which combines the formation of the selective emitter with a self-aligned metallisation pattern. Despite achieving high efficiencies, concerns arose regarding the adhesion of the metal to the shallow laser doped areas. This issue may be alleviated by increasing the roughness of the surface or even more so by creating holes/grooves in the laser doped areas. One simple way of achieving this is by carrying out the laser doping at higher laser energies to deliberately create some ablation. This paper examines the influence of the laser power on the solar cell electrical parameters to ascertain the relationship and the tradeoff between surface roughness and electrical performance. Efficiencies above 18% on a large area commercial grade p-type CZ substrate were achieved despite some ablation, confirming the potential for using this method to improve adhesion. Efficiency of 18.7% on the same substrate, using lower laser power, demonstrates the capability of the laser doping method.     

Synthesis and photocatalytic performance of MoS2/Polycrystalline black phosphorus heterojunction composite

As a promising catalyst for solar hydrogen production, black phosphorus (BP) has received widespread attention due to variable band gaps, high carrier mobility, and strong light absorption performance. Herein, we use MoS₂ as a cocatalyst to synthesize BP/MoS₂ catalyst with polycrystalline BP to improve photocatalytic performance under visible light irradiation. A small amount of MoS₂ can reduce the recombination of electron-hole pairs in the composite, increase carrier transport efficiency, and then improve photocatalytic performance. As expected, the 10/0.5 ratio of BP/MoS₂ catalyst exhibits the highest photocatalytic hydrogen evolution performance with a hydrogen evolution rate of 575.4 μmol h^?1 g^?1, which is 2.5 times of pure BP. Based on the results above, a simple method is provided to synthesize low-cost black phosphorus-based photocatalysts.     

Influence of defects and band offsets on carrier transport mechanisms in amorphous silicon/crystalline silicon heterojunction solar cells

We have investigated the carrier transport mechanisms in undoped a-Si:H/p-type c-Si heterojunctions with and without a μc-Si buffer layer, as well as their effects on the photovoltaic properties of the junction. The conduction behavior of the junction is strongly affected by the defect state distribution and band offset at the hetero-interface. The recombination process involving the interface states on the thin film silicon (a-Si:H/μc-Si) side dominates at low forward bias (V<0.3 V), whereas multistep tunneling capture emission (MTCE) dominates in the higher bias region (0.3<V<0.55 V) until the conduction becomes space charge limited (V>0.55 V). The MTCE process seems to be more closely related to the bulk defects in the thin film silicon than the interface states. In addition, the position of a trapping level, where the tunneling process occurs, seems to be determined by the hole energy at the edge of the c-Si and the trap distribution in the thin film silicon. Despite the domination of MTCE in the indicated voltage range, the reduced band offset at the interface increases current levels by the enhanced diffusion and/or emission processes. The insertion of a 200 Å thick μc-Si buffer layer between the a-Si:H (700 Å)/c-Si increases the solar cell efficiency to 10%, without an antireflective coating, by improving both the carrier transport and the red response of the cell.     

Boron doped amorphous diamond window layer deposited by filtered arc for amorphous silicon alloy p-i-n solar cells

In order to improve the conversion efficiency of amorphous silicon (a-Si:H) alloy p-i-n solar cells, the original p-a-Si:H window layer is substituted by the boron-doped amorphous diamond (a-D:B) films deposited using filtered cathodic vacuum arc technology. The microstructural, optical and electrical properties as functions of the boron concentrations in the films were, respectively, evaluated by an X-ray photoemission spectroscopy, an ultraviolet-visible spectrometer and a semiconductor parameter analyzer. The photovoltaic parameters of the solar cell modules were also detected as functions of boron concentration. It has been shown that the conductive a-D:B films could be obtained and still remained a wide optical gap. The p-i-n structural amorphous silicon solar cell using the a-D:B window layer increased the conversion efficiency by a roughly 10% relative improvement compared to the conventional amorphous silicon solar cell because of the enhancement of short wavelength response.     

A thin-film solar cell of high-quality -FeSi2/Si heterojunction prepared by sputtering

High-quality (1 1 0)/(1 0 1)-oriented epitaxial β-FeSi₂ films were fabricated on Si (1 1 1) substrate by the sputtering method. The critical feature was the formation of a high-quality thin β-FeSi₂ template buffer layer on Si (1 1 1) substrate at low temperature. It was demonstrated that the template is very important for the epitaxial growth of thick β-FeSi₂ films and for the blocking of Fe diffusion into the Si at the β-FeSi₂/Si interface. Hall effect measurements for β-FeSi₂ films showed n-type conductivity, with residual electron concentration around 2.0 × 10¹⁷ cm⁻³ and mobility of 50–400 cm²/V s. A prototype thin-film solar cell was fabricated by depositing n-β-FeSi₂ on p-Si (1 1 1). Under 100 mW/cm² sunlight, an energy conversion efficiency of 3.7%, with an open-circuit voltage of 0.45 V, a short-circuit current density of 14.8 mA/cm² and a fill factor of 0.55, was obtained.     

Synthesis and characterization of boron-doped Si quantum dots for all-Si quantum dot tandem solar cells

Multiple layers of Si quantum dots (QDs) in SiO₂ with a narrow size distribution were synthesized by a co-sputtering technique. Structural, electrical and optical properties of Si QD/SiO₂ multilayer films with various boron (B) concentrations introduced during the sputtering process were studied. X-ray photoelectron spectroscopy (XPS) revealed B-B/B-Si bonding, which suggests possible boron inclusion in the nanocrystals. The addition of boron was observed to suppress Si crystallization, though the boron concentration was found to have little effect on the QD size. Reductions in film resistivity were observed with the increase in boron concentration, which is believed to be a consequence of an increase in carrier concentration. This is supported by a large decrease in the activation energy accompanying the drop in resistivity, consistent with the Fermi energy moving towards the valence bands. The photoluminescence (PL) intensity was found to decrease with increase in boron concentration.     

Dark I–V–T measurements and characteristics of (n) a-Si/(p) c-Si heterojunction solar cells

Heterojunction solar cells have been manufactured by depositing n-type a-Si:H on p-type 1–2 Ω cm Cz single-crystalline silicon substrates. An efficiency of 14.2% has been obtained for 1 cm² solar cells by using a simple (Al/(p) c-Si/(n) a-Si:H/ITO/metal grid) structure. With an additional surface texturing, we have reached an efficiency of 15.3% for 1 cm² solar cells. We have investigated the dark IV-curves in order to contribute to a better understanding of the basis of solar cells.     

Comparison of multicrystalline silicon surfaces after wet chemical etching and hydrogen plasma treatment: application to heterojunction solar cells

The modifications of the surface and subsurface properties of p-type multicrystalline silicon (mc-Si) after wet chemical etching and hydrogen plasma treatment were investigated. A simple heterojunction (HJ) solar cell structure consisting of front grids/ITO/(n)a-Si:H/(p)mc-Si/Al was used for investigating the conversion efficiency. It is found that the optimized wet chemical etching and cleaning processes as a last technological step before the deposition of the a-Si:H emitter are more favorable to HJ solar cells fabrication than the hydrogenation. Solar cells on p-type mc-Si were prepared without high-efficiency features (point contacts, back surface field). They exhibited efficiencies up to 13% for a cell area of 1 cm² and 12% for a cell area of 39 cm².     

Interface properties determined the performance of thermally grown GaN/Si heterojunction solar cells

We report the fabrication of heterojunction solar cells via the thermal chemical vapor deposition (CVD) of gallium nitride (GaN) nanostructures on clean Si substrates. GaN epitaxial layers were synthesized via the direct reaction of Ga vapor and NH₃ solution at 1050 °C. The structural and optical characteristics of the as-grown GaN layers were investigated. The effects of Si orientation (100 vs 111) and doping type (n- vs p-) on the structural and optical properties of the deposited GaN nanostructures and solar cell performance were explored. The fabricated GaN nanostructures exhibited p-type behavior at the GaN/Si interface as revealed from the Hall-effect measurements. The J–V characteristics showed rectifying behavior for the GaN/n-Si junction and Ohmic behavior for the GaN/p-Si junction. Upon illumination (30 mW/cm²), the as-deposited heterojunction solar cell devices showed conversion efficiencies of 6.18% and 3.69% for GaN/n-Si (1 1 1) and GaN/n-Si (1 0 0) heterojunctions, respectively. The growth of GaN on Si substrates in the presence of NH₃ solution has strong effect on the morphological, optical and electrical properties and consequently on the efficiency of the solar cell devices made of such substrates.     

Investigation of the damage as induced by 1.7 MeV protons in an amorphous/crystalline silicon heterojunction solar cell

Current–voltage under illumination and quantum yield characteristics of an amorphous silicon/crystalline silicon hetero solar cell have been measured before and after exposure to high-energy (1.7 MeV) protons. A comparison of the measured wavelength-dependent quantum yield with calculated values enabled to determine the effective electron diffusion length of the crystalline silicon, that dropped from a value of 434 μm before to a value of 4 μm after irradiation with 5×10¹² cm⁻² protons. Good agreement has been obtained between measured and simulated data using DIFFIN,¹ a finite-element simulation program for a-Si:H/c-Si heterojunction solar cells, enabling us to extract the depth profile of the recombination rate and the density of states distribution in the semiconductor layers before and after irradiation.     

Optical and structural properties of pulsed laser deposited Ti:Al2O3 thin films

Spectrally selective solar absorber coatings using metal–dielectric composites offer a high degree of flexibility since their solar selectivity can be optimized by the proper choices of constituents, coating thickness, metal particle concentration, size, shape, and orientation. In this article, Ti:Al₂O₃ composite films of various Ti contents were prepared by the pulsed laser deposition technique, and their structural and optical properties were investigated by scanning electron microscope, X-ray photoelectron spectroscope, and X-ray diffraction methods. The results demonstrate promising solar spectrally selective behavior indicating Ti:Al₂O₃ composite as an excellent choice for solar thermal applications.     

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.     

Fabrication and characterization of 4-tricyanovinyl-N,N-diethylaniline/p-silicon hybrid organic–inorganic solar cells

Hybrid organic–inorganic solar cell was fabricated by thin film of 4-tricyanovinyl-N,N-diethylaniline deposited on p-Si substrates. The capacitance–voltage characteristics indicated that the junction is of abrupt nature. The dark forward current density-voltage characteristics indicated a tunneling conduction at relatively low voltages followed by a space-charge-limited-conduction mechanism at relatively high voltages. Under illumination, the cell exhibits photovoltaic characteristics with an open-circuit voltage of 0.70 V, a short-circuit current density of 9.15 mA cm⁻², and a power conversion efficiency of 3.10%. The effect of γ-rays irradiation (100 kGy absorbed dose) on the characteristics of the cell was also investigated. The fill factor and the power conversion efficiency decrease by 20.9% and 39% of the original value, respectively.     

Simple and fast fabrication of a-Si:H/c-Si hetero-junction solar cells by dual-chamber hot wire chemical vapor deposition

One of the fabrication issues in hetero-junction crystalline Si solar cells is the overhead time between the deposition steps of the top and bottom surfaces, because flipping of the progressing wafer is necessary to process the both sides of the wafer. To reduce the overall processing time by reducing the overhead time, we propose a dual-chamber deposition system, where thin films on the top and bottom surfaces of the Si wafer are simultaneously deposited. We have evaluated the proposed deposition system by demonstrating fabricated hetero-junction crystalline Si solar cells, which were compared with solar cells fabricated by a conventional plasma-enhanced chemical deposition system. We have obtained the power conversion efficiency of 15.5% from solar cells fabricated by our dual-chamber system; and additional analyses confirmed that the proposed dual-chamber system is, in principle, competitive with conventional systems in terms of the fabricated solar cell performance. This novel concept for the fabrication of a hetero-junction crystalline Si solar cell is expected to lay an important foundation in the future thin film crystalline Si based photovoltaic industry.     

Investigation of pulsed non-melt laser annealing on the film properties and performance of Cu(In,Ga)Se2 solar cells

Pulsed non-melt laser annealing (NLA) has been used for the first time to modify near-surface defects and related junction properties in Cu(In,Ga)Se₂ (CIGS) solar cells. CIGS films deposited on Mo/glass substrates were annealed using a 25 ns pulsed 248 nm laser beam at selected laser energy density in the range 20–60 mJ/cm² and pulse number in the range 5–20 pulses. XRD peak narrowing and SEM surface feature size increase suggest near-surface structure changes. Dual-beam optical modulation (DBOM) and Hall-effect measurements indicate NLA treatment increases the effective carrier lifetime and mobility along with the sheet resistance. In addition, several annealed CdS/CIGS films processed by NLA were fabricated into solar cells and characterized by photo- and dark-J–V and quantum efficiency (QE) measurements. The results show significant improvement in the overall cell performance when compared to unannealed cells. The results suggest that an optimal NLA energy density and pulse number for a 25 ns pulse width are approximately 30 mJ/cm² and 5 pulses, respectively. The NLA results reveal that overall cell efficiency of a cell processed from an unannealed film increased from 7.69% to 13.41% and 12.22% after annealing 2 different samples at the best condition prior to device processing.     

DFT study of boron doped MgH2: Bonding mechanism,hydrogen diffusion and desorption

The impact of boron doping on MgH₂ bonding mechanism, hydrogen diffusion and desorption was calculated using density functional theory (DFT). Atomic interactions in doped and non-doped system and its influence on hydrogen and vacancy diffusion were studied in bulk hydride. Slab calculations were performed to study hydrogen desorption energies from (110) boron doped surface and its dependence on the surface configuration and depth position. To study kinetics of hydrogen diffusion in boron vicinity and hydrogen molecule desorption activation energies from boron doped and non-doped (110) MgH₂ surface Nudged Elastic Band (NEB) method was used. Results showed that boron forms stronger, covalent bonds with hydrogen causing the destabilization in its first and second coordination. This leads to lower hydrogen desorption energies and improved hydrogen diffusion, while the impact on the energy barriers for H₂ desorption from hydride (110) surface is less pronounced.