Antireflection and surface passivation behaviour of SiO2/Si/SiO2 quantum wells on silicon

Quantum wells (QWs) consisting of Si and Si-related materials (such as SiO₂) are of interest for solar cell work because they can possibly be used as a surface passivating antireflection (AR) coating or as the top cell in an all-silicon tandem solar cell. In this study, we fabricate SiO₂/Si/SiO₂ QW layers by RF magnetron sputtering and thermal oxidation. On high-resistivity (300 Ω cm) n-type silicon wafer substrates, the effective surface recombination velocity provided by our SiO₂/Si/SiO₂ QWs is around 4 cm/s for 13 Å Si thickness and 480 cm/s for 150 Å Si thickness. The parasitic optical absorption in the well-passivating QWs is negligible for terrestrial photovoltaic applications. However, they have very poor AR properties on Si wafers and hence would have to be covered by an additional reflection reducing dielectric film.     

Low temperature passivation of silicon surfaces by polymer films

A novel surface passivation method for silicon carrier lifetime measurements and solar cells using a polymer film is introduced. It is easy to apply, no special pre-treatment, e.g. no hydrofluoric acid (HF)-treatment, is necessary. The surfaces to be passivated are covered with the polymer solution, dried at 90°C and encapsulated. Surface recombination velocities (S) as low as S=30 cm/s for various doping concentrations have been observed, nearly independent of the bulk injection level. The passivation is stable for at least 6 h. For a polymer-passivated rear contact solar cell the same open circuit voltage is achieved as for a cell with thermally grown oxide.     

Surface passivation in high efficiency silicon solar cells

Surface passivation for crystalline silicon solar cells is particularly important for devices with open-circuit voltages in excess of 650 mV. Thick passivating thermal oxides, originally developed for use with buried contact solar cells, are shown to produce the most effective and stable surface passivation particularly in conjunction with lightly phosphorus diffused surfaces. However, for improved optical performance, antireflection coatings are only effective with surface oxide thicknesses reduced to 100–200 Å. Thinner passivating oxides cause significant voltage loss, most of which can be recovered through hydrogen passivation. Throughout this study, variation in surface passivation approaches has produced open-circuit voltages ranging from 620 mV to record voltages of 720 mV.     

Analysis of the effect of hydrogen-radical annealing for SiO2 passivation

The effect of hydrogen-radical annealing for SiO₂ passivation was examined. The annealing effect was analyzed by measuring effective lifetime and C-V characteristics and was compared with the effects of forming gas annealing (FGA) and hydrogen RF plasma annealing. The effect of hydrogen-radical annealing is much higher than those of FGA and hydrogen RF plasma annealing. It was also confirmed that both changes of surface recombination velocity and interface state density showed the same tendency. Furthermore, the investigations of hydrogen-radical density showed that by using microwave afterglow method, hydrogen-radicals could be generated much more than that by RF plasma. Accordingly, more interface trap density could be terminated and surface recombination velocity was effectively decreased by using microwave afterglow method.     

Effect of high temperature steam annealing for SiO2 passivation

We introduced high-temperature steam annealing (HSA) as a low-cost and effective post-annealing method for c-Si solar cell processing. The annealing effects were analyzed by measuring effective lifetime and C–V characteristics and were compared with the effects of forming gas annealing (FGA) and hydrogen-radical annealing (HRA). By using this method, effective lifetime of a SiO₂-coated wafer was increased in a very short annealing time compared to the conventional FGA. It was determined that the improvement of lifetime by HSA can be attributed to the decrease of interface state density.     

Overview on SiN surface passivation of crystalline silicon solar cells

Silicon nitride (SiN) fabricated by plasma-enhanced chemical vapour deposition (PECVD) is increasingly used within the crystalline silicon (c-Si) photovoltaic industry as it offers the possibility to fabricate a surface and bulk passivating antireflection coating at low temperature (450°C). This article presents an overview on the present status of SiN for industrial as well as laboratory-type c-Si solar cells. Topics covered include the fundamentals of the PECVD technology, the present status of high-throughput PECVD machines for the deposition of SiN onto c-Si wafers, and a review of the fundamental properties of Si–SiN interfaces fabricated by PECVD.     

Low temperature surface passivation for silicon solar cells

Surface passivation at low processing temperatures becomes an important topic for cheap solar cell processing. In this study, we first give a broad overview of the state of the art in this field. Subsequently, the results of a series of mutually related experiments are given about surface passivation with direct Plasma Enhanced Chemical Vapour Deposition (PECVD) of silicon oxide (Si-oxide) and silicon nitride (Si-nitride). Results of harmonically modulated microwave reflection experiments are combined with Capacitance-Voltage measurements on Metal-Insulator-Silicon structures (CV-MIS), accelerated degradation tests and with Secondary Ion Mass Spectrometry (SIMS) and Elastic Recoil Detection (ERD) measurements of hydrogen and deuterium concentrations in the passivating layers. A large positive fixed charge density at the interface is very important for the achieved low surface recombination velocities S. The density of interface states D_it is strongly reduced by post deposition anneals. The lowest values of S are obtained with PECVD of Si-nitride. The surface passivation obtained with Si-nitride is stable under typical operating conditions for solar cells. By using deuterium as a tracer it is shown that hydrogen in the ambient of the post deposition anneal does not play a role in the passivation by Si-nitride. Finally, the results of CV-MIS measurements (Capacitance-Voltage measurements on Metal-Insulator-Silicon structures) on deposited Si-nitride layers are used to calculate effective recombination velocities as a function of the injection level at the surface, using a model that is able to predict the surface recombination velocity S at thermally oxidized silicon surfaces. These results are not in agreement with the measured increase of S at low injection levels.     

Electrodeposition of TiO2/SiO2 nanocomposite for dye-sensitized solar cell

For the working electrode of dye-sensitized solar cell (DSC), TiO₂/SiO₂ nanocomposite materials were electrodeposited on transparent fluorine doped tin oxide-coated glass by cathodic electrodeposition at room temperature. The electrode and DSC fabricated with TiO₂/SiO₂ nanocomposite were characterized with photocurrent density, X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM) and a photovoltaic performance test. On the electrodeposition, the addition of an appropriate amount of SiO₂ in the bath containing TiO₂ slurry was essential to achieve the superior crystallinity, photocurrent density and photovoltaic performance of the resulting TiO₂/SiO₂ electrode, which was significantly superior to a bare TiO₂ electrode. This enhanced performance of optimized TiO₂/SiO₂ electrode was ascribed to the role of SiO₂ as an energy barrier, increasing the physical separation of injected electrons and oxidized dyes/redox couple, and thereby retarding the recombination reactions in the resulting DSC.     

Tri-layer antireflection coatings (SiO2/SiO2–TiO2/TiO2) for silicon solar cells using a sol–gel technique

Antireflection coatings (ARCs) have become one of the key issues for mass production of Si solar cells. They are generally performed by vacuum processes such as thermal evaporation, reactive sputtering, and plasma-enhanced chemical vapor deposition. In this work, a sol–gel method has been demonstrated to prepare the ARCs for the non-textured monocrystalline Si solar cells. The spin-coated TiO₂ single-layer, SiO₂/TiO₂ double-layer and SiO₂/SiO₂–TiO₂/TiO₂ triple-layer ARCs were deposited on the Si solar cells and they showed good uniformity in thickness. The measured average optical reflectance (400–1000 nm) was about 9.3, 6.2 and 3.2% for the single-layer, double-layer and triple-layer ARCs, respectively. Good correlation between theoretical and experimental data was obtained. Under a triple-layer ARC condition, a 39% improvement in the efficiency of the monocrystalline Si solar cell was achieved. These indicate that the sol–gel ARC process has high potential for low-cost solar cell fabrication.     

Highest-quality surface passivation of low-resistivity p-type silicon using stoichiometric PECVD silicon nitride

The surface passivation properties of silicon nitride (SiN) films fabricated by high-frequency direct plasma-enhanced chemical vapour deposition (PECVD) on low-resistivity (1 Ω cm) p-type silicon solar cell substrates have been investigated. The process gases used were ammonia and a mixture of silane and nitrogen. In order to find the optimum set of SiN deposition parameters, a large number of carrier lifetime test structures were prepared under different deposition conditions. The optimised deposition parameters resulted in outstandingly low surface recombination velocities (SRVs) below 10 cm/s. Interestingly, we find the lowest SRVs for stoichiometric SiN films, as indicated by a refractive index of 1.9. In former studies similarly low SRVs had only been obtained for silicon-rich SiN films. The fundamentally different passivation behaviour of our SiN films is attributed to the addition of nitrogen to the process gases.     

Carrier transport in high-efficiency ZnO/SiO2/Si solar cells

Carrier transport in ZnO/SiO₂/n-Si solar cell has been theoretically analyzed with a consideration that the photo-carrier transport from silicon to ZnO layer through the barrier is dominated by quantum mechanical tunneling process of minority carrier. It was found that the highest efficiency of the cell could be achieved at SiO₂ layer thickness of around 20 Å. The efficiency of the cells decreases as the surface states density Q_ss becomes higher. Moreover, the efficiency increases as the electron concentration of ZnO layer is increased due to the decrease of work function of ZnO. It was also found that the lower transmittance of the high carrier concentration ZnO due to the free-carrier absorption at infrared wavelength region does not give any significant effect to the cell performance. The efficiency of higher than 25% is achievable by optimizing the involved device parameters.     

Ruthenium dye-sensitized SnO2/TiO2 coupled solar cells

The photoelectrochemical behaviors of RuL₂(NCS)₂ dye-sensitized SnO₂/TiO₂ coupled solar cell was studied and compared with TiO₂ single system. The coupled system shows higher incident photon-to-current conversion efficiency (IPCE) value than the single system. A maximum IPCE value in the coupled system with 3.5 μm-thick SnO₂ and 7 μm-thick TiO₂ attained 82.4% at 530 nm wavelength. The higher IPCE value in the coupled system is attributed to the charge separation by fast electron transfer process from the excited RuL₂(NCS)₂ dye to TiO₂ to SnO₂ in the system with different energy level.     

Effect of hydrogen plasma passivation on performance of HIT solar cells

We studied the performance improvement of HIT solar cells by optimizing H₂ plasma exposure and deposition of thin a-S:H layer on c-Si. With increasing H₂ treatment time, the V_OC increases until 80 s and then decreases, indicating the optimum time is 80 s. It is found that the cell performance is almost the same with and without a thin a-Si:H layer when 80 s plasma is treated on the c-Si before i-layer deposition. The conversion efficiency of 14.04% was achieved at the substrate temperature of 160 °C.     

Surface passivation for germanium photovoltaic cells

Passivation of a germanium surface has proved to be challenging. Various materials have been examined for this purpose, like for example silicon nitride and amorphous silicon. In this work the optimisation of PECVD amorphous silicon and the influence of the preliminary surface treatment for passivation purposes are described. Furthermore, experiments done to extract the surface recombination velocity and the bulk lifetime of a germanium substrate will be presented. Optimisation of the deposition parameters, in combination with a pre-deposition in situ hydrogen plasma, results in a surface recombination velocity of 17 cm/s on a lowly doped germanium substrate.     

Control of μc-Si/c-Si interface layer structure for surface passivation of Si solar cells

Outstanding passivation properties for p-type crystalline silicon surfaces were obtained by using very thin n-type microcrystalline silicon (μc-Si) layers with a controlled interface structure. The n-type μc-Si layers were deposited by the RF PE-CVD method with an insertion of an ultra-thin oxide (UTO) layer or an n-type amorphous silicon (a-Si : H) interface layer. The effective surface recombination velocity (SRV) obtained was very small and comparable to that obtained using thermal oxides prepared at 1000°C. The structural studies by HRTEM and Raman measurements suggest that the presence of UTO produces a very thin a-Si : H layer under the μc-Si. A crystal lattice discontinuity caused by these interface layers is the key to a small SRV.     

A detailed study of H2 plasma passivation effects on GaAs/Si solar cell

The hydrogen plasma passivation effects of MOCVD-grown GaAs solar cell on Si substrate have been studied in detail. To get a more reproducible increase of conversion efficiency and test the thermal stability of the plasma-exposed GaAs/Si solar cell, both the plasma exposure and post-passivation annealing conditions were optimized. Annealing the H₂ plasma passivated GaAs/Si solar cell at 450°C in AsH₃/H₂ ambient seems a very essential parameter to restore the carrier concentration, especially, without losing the beneficial effects of H incorporation into GaAs on Si. For the H₂ plasma passivated GaAs/Si solar cell, a highest conversion efficiency of 18.3% was obtained compared with that of the as-grown cell (16.6%) due to the H passivation effects on nonradiative recombination centers, which increased the minority carrier lifetime.     

Surface ligand effects in MEH-PPV/TiO2 hybrid solar cells

TiO₂ nanorods (NRs) were synthesized by hydrolysis of titanium tetraisopropoxide (TTIP) using oleic acid (99%) as surfactant at low temperatures (80-100 °C) and are modified with different ligands: oleic acid (OLA), n-octyl-phosphonic acid (OPA) and thiophenol (TP) in order to investigate the effect of surface ligand on the excition dissociation and the charge transport in hybrid MEH-PPV/TiO₂ photovoltaic (PV) cells. The morphology and crystalline form of as-prepared TiO₂ NRs are examined by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD) and Raman spectrometer (RS). The FTIR analysis confirms all the ligands coordinated with the Ti center of TiO₂ NRs. The optical properties of the modified TiO₂ NRs are characterized by UV-vis absorption spectra and photoluminescence (PL) spectra. Thiophenol modified TiO₂ NRs quench the PL of MEH-PPV more effectively than OLA-TiO₂ NRs and OPA-TiO₂ NRs. The power conversion efficiency of hybrid PV cells from thiophenol modified TiO₂ NRs and MEH-PPV is the highest among the investigated TiO₂ NRs.     

A new back surface passivation stack for thin crystalline silicon solar cells with screen-printed back contacts

In order to manufacture high-efficiency Si solar cells with a passivated rear surface and local contacts, it is necessary to develop both an excellent rear-passivation scheme compatible with screen-printing technology and a robust patterning technique for local contact formation. In this work, we have fabricated Si solar cells on ∼130 μm thick substrates using manufacturable processing, where rear side was passivated with a plasma-enhanced chemical vapor deposited SiO_x/SiN_x/SiO_xN_y stack and local back contacts using laser. As a result of both the rear surface passivation stack and the laser-fired local contacts, cell efficiencies of up to 17.6% on a 148.6 cm² Float-zone Si wafer and 17.2% for a 156.8 cm² multicrystalline Si wafer were achieved. PC-1D calculations revealed that the cells had a back surface recombination velocity (BSRV) of ∼400 cm/s and a back surface reflectance (BSR) of over 90%, as opposed to standard full Al-BSF cells having a BSRV of ∼800 cm/s and a 70% BSR. This result clearly indicates that the new technique of the passivation scheme and the patterning using laser developed in this study are promising for manufacturing high-efficiency PERC-type thin Si solar cells.     

Enhanced CO2 stability of oxyanion doped Ba2In2O5 systems co-doped with La, Zr

In the solid oxide fuel cell (SOFC) field, proton conducting perovskite electrolytes offer many potential benefits. However, an issue with these electrolytes is their stability at elevated temperatures in the presence of CO₂. Recently we have reported enhanced oxide ion/proton conductivity in oxyanion (silicate, phosphate) doped Ba₂In₂O₅, and in this paper we extend this work to examine the stability at elevated temperatures towards CO₂. The results show improved CO₂ stability compared to the undoped system, and moreover this can be further improved by co-doping on either the Ba site with La, or the In site with Zr. While this co-doping strategy does reduce the conductivity slightly, the greatly improved CO₂ stability would suggest there is technological potential for these co-doped samples.     

Surface and interface behaviors of (La0.8Sr0.2)xMnO3 air electrode for solid oxide cells

In solid oxide cell operation, the stoichiometry of the air electrode is an important factor for its interaction with electrolyte and interconnect and long-term cell performance. In this study, tri-layer samples of yttria stabilized zirconia (YSZ)/(La_0.8Sr_0.2)_xMnO₃ (LSM)/AISI 441 stainless steel are made and thermally treated in dry air atmosphere at 800 °C for 500 h. The air electrode composition is varied by changing the x value in (La_0.8Sr_0.2)_xMnO₃ from 0.95 to 1.05 (LSM95, LSM100, and LSM105). The LSM composition segregation, YSZ/LSM/AISI 441 interfacial interaction, and the reaction of volatile chromium species with the LSM surface are characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Surface segregation of Sr and La are detected for all the LSM samples. Cr deposition is found across the LSM surface. For the LSM95 sample, Sr-containing compound leads to a high Cr content at the YSZ/LSM interface. For the LSM105 sample, on the other hand, the enrichment of La at the YSZ/LSM interface hinders the Cr deposition, leading to a very low Cr content. The mechanisms of LSM elemental surface segregation and Cr deposition are discussed.