Heterojunction Silicon Wafer Solar Cells Research Articles

Investigation of Field‐Effect Passivation Created by Hydrogen Plasma Etching of Radio Corporation of America Formed Chemical Oxides on Crystalline Silicon Wafers

This work investigates the interface properties of intrinsic hydrogenated amorphous silicon film passivated wafers that underwent hydrogen plasma cleaning. A high level of interface band bending of nearly −0.6 eV, which corresponds to a fixed charge of −2.2 × 1012 cm−2, is found to be responsible for an effective minority carrier lifetime of over 6 ms on the 4.5 Ω cm n‐type wafer, while such field‐effect passivation is missing in hydrofluoric acid (HF) cleaned wafers. Further study indicates a positive correlation between the extent of surface band bending and doping concentration, together with an inverted U‐shape with respect to the increased annealing condition. The fixed charge on p‐type wafer is found to have a higher “formation energy” compared with the n‐type case, which renders its field‐effect passivation much less effective due to H effusion at high annealing temperatures. With reference to the theory on donor/acceptor‐H complex upon H plasma treatment, the origin and observed properties of the surface band bending on both dopant types are discussed. The unique presence of field effect on hydrogen plasma cleaned n‐type wafers can provide new insights into passivation material selection and structural design of heterojunction silicon wafer solar cells.

Effects of indium tin oxide on the performance of heterojunction silicon wafer solar cells

The effects of indium tin oxide (ITO) films on the performance of heterojunction silicon wafer solar cells is investigated, using heterojunction (HET) solar cell precursors. Different ITO deposition conditions are used, which result in significant differences in the performance of HET solar cells. It is found that HET solar cells with ITO films deposited at room temperature exhibit severer sputter damage, while those with substrate heating show less damage. Besides the ITO deposition temperature, the sputtering gas ambient is also investigated. The hydrogen gas used in the ITO deposition can greatly affect the interface properties between the ITO film and the amorphous silicon layers. The champion solar cell fabricated under the optimum ITO deposition conditions (a deposition temperature of 150 °C with optimal gas concentration) shows a conversion efficiency of 19.7%.

Three-dimensional numerical analysis of hybrid heterojunction silicon wafer solar cells with heterojunction rear point contacts

This paper presents a three-dimensional numerical analysis of homojunction/heterojunction hybrid silicon wafer solar cells, featuring front-side full-area diffused homojunction contacts and rear-side heterojunction point contacts. Their device performance is compared with conventional full-area heterojunction solar cells as well as conventional diffused solar cells featuring locally diffused rear point contacts, for both front-emitter and rear-emitter configurations. A consistent set of simulation input parameters is obtained by calibrating the simulation program with intensity dependent lifetime measurements of the passivated regions and the contact regions of the various types of solar cells. We show that the best efficiency is obtained when a-Si:H is used for rear-side heterojunction point-contact formation. An optimization of the rear contact area fraction is required to balance between the gains in current and voltage and the loss in fill factor with shrinking rear contact area fraction. However, the corresponding optimal range for the rear-contact area fraction is found to be quite large (e.g. 20-60 % for hybrid front-emitter cells). Hybrid rear-emitter cells show a faster drop in the fill factor with decreasing rear contact area fraction compared to front-emitter cells, stemming from a higher series resistance contribution of the rear-side a-Si:H(p+) emitter compared to the rear-side a-Si:H(n+) back surface field layer. Overall, we show that hybrid silicon solar cells in a front-emitter configuration can outperform conventional heterojunction silicon solar cells as well as diffused solar cells with rear-side locally diffused point contacts.

Study of hydrogen influence and conduction mechanism of amorphous indium tin oxide for heterojunction silicon wafer solar cells

The optoelectrical properties of the indium tin oxide (ITO) films are investigated at low‐power and low‐temperature conditions for heterojunction silicon wafer solar cell applications. ITO films deposited at 100 W in hydrogen‐diluted argon ambient have the best optoelectrical quality, giving transparent and conductive (carrier mobilities in the 50–60 cm2/Vs range) films. X‐ray diffraction analysis shows that the ITO films deposited at 100 W are primarily of amorphous (or quasi‐amorphous) nature, regardless of the deposition conditions. Based on several other characterisation methods, it is found that the high electron mobility achieved by the hydrogenated ITO films is mainly due to the reduction of scattering centres. A few In2O3 samples are prepared at similar deposition conditions to investigate the electrical conduction mechanism of ITO films. Due to the low doping efficiency of tin atoms in amorphous ITO, the main source of the free electron charge carriers are the oxygen vacancies.

Comparative study of amorphous indium tin oxide prepared by pulsed-DC and unbalanced RF magnetron sputtering at low power and low temperature conditions for heterojunction silicon wafer solar cell applications

Comparative study of indium tin oxide (ITO) was performed between films prepared by pulsed direct current (PDC) and radio frequency (RF) magnetron sputtering methods at low power and low temperature conditions. The best achieved film resistivity is in the range of 3–6 × 10−4 Ω cm with the highest electron carrier mobility of ∼60 cm2/V. The properties of the surface layer and the bulk material were analysed in details by means of X-ray photoelectron spectroscopy (XPS). Although there is a little difference in elemental compositions, it is revealed that the ITO's properties are directly related to the chemical bonding conditions. Owing to the fast film growth rate of PDC sputtering method, second phase Sn3O4 is formed which impedes the optical performance of the ITO films. Our observations suggest that the darkening of ITO is most likely due to the existence of Sn3O4 second phases that trapped inside of the film bulk. Besides material properties, we have demonstrated that the PDC sputtering method introduces more particle bombardment damage, while for RF sputtering the damage is mainly contributed by ultraviolet radiation emission. The major sputtering induced damage can be recovered by 15 min thermal annealing at 200 °C in air.

Hybrid silver nanoparticle and transparent conductive oxide structure for silicon solar cell applications

AbstractTransparent conductive oxides (TCOs) have been widely used as electrodes for various solar cell structures. For heterojunction silicon wafer solar cells, the front TCO layer not only serves as a top electrode (by enhancing the lateral conductance of the underlying amorphous silicon film), but also as an antireflection coating. These requirements make it difficult to simultaneously achieve excellent conductance and transparency, and thus, only high‐quality indium tin oxide (ITO) has as yet found its way into industrial heterojunction silicon wafer solar cells. In this Letter, we present a cost‐effective hybrid structure consisting of a TCO layer and a silver nano‐particle mesh. This structure enables the separate optimization of the electrical and optical requirements. The silver nanoparticle mesh provides high electrical conductance, while the TCO material is optimized as an antireflection coating. Therefore, this structure allows the use of cost‐effective (and less conductive) TCO materials, such as aluminium‐doped zinc oxide. The performance of the hybrid structure is demonstrated to achieve a similar visible transmission (∼86% in the 380–780 nm range) as an 80 nm thick ITO layer, but with 10 times better lateral conductance. The presented hybrid structure thus seems well suited for a variety of photovoltaic devices. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Novel Hybrid Electrode Using Transparent Conductive Oxide and Silver Nanoparticle Mesh for Silicon Solar Cell Applications

Transparent conductive oxides (TCOs) have been widely used as the front electrodes for various solar cell structures, including heterojunction silicon wafer solar cells and the vast majority of thin-film solar cells. For heterojunction silicon wafer solar cells, the front TCO layer not only serves as a top electrode (by enhancing the lateral conductance of the underlying amorphous silicon film), but also as an antireflection coating. These requirements make it difficult to simultaneously achieve excellent conductivity and transparency, and thus only high-quality indium tin oxide (ITO) has as yet found its way into industrial heterojunction silicon wafer solar cells. For thin-film solar cells, in order to provide efficient lateral conductance of the charge carriers, normally a TCO layer of a few hundred nanometers thickness is used which impedes the optical transparency due to the enhanced free carrier absorption. To reduce the conflict between conductivity and transparency, and to separately engineer the electrical and optical properties, a hybrid electrode is proposed and fabricated by us which consists of a TCO layer (optical layer) and a silver nanoparticle mesh (electrical layer). This hybrid electrode is demonstrated to have a 10 times higher lateral conductance compared to a single TCO layer, while maintaining high light transmission in a wide wavelength range. Due to the excellent performance of the hybrid electrode, it is demonstrated that such an electrode is suitable for various solar cell structures.

Excellent Silicon Surface Passivation Achieved by Industrial Inductively Coupled Plasma Deposited Hydrogenated Intrinsic Amorphous Silicon Suboxide

We present an alternative method of depositing a high-quality passivation film for heterojunction silicon wafer solar cells, in this paper. The deposition of hydrogenated intrinsic amorphous silicon suboxide is accomplished by decomposing hydrogen, silane, and carbon dioxide in an industrial remote inductively coupled plasma platform. Through the investigation on CO2partial pressure and process temperature, excellent surface passivation quality and optical properties are achieved. It is found that the hydrogen content in the film is much higher than what is commonly reported in intrinsic amorphous silicon due to oxygen incorporation. The observed slow depletion of hydrogen with increasing temperature greatly enhances its process window as well. The effective lifetime of symmetrically passivated samples under the optimal condition exceeds 4.7 ms on planarn-type Czochralski silicon wafers with a resistivity of 1 Ωcm, which is equivalent to an effective surface recombination velocity of less than 1.7 cms−1and an implied open-circuit voltage (Voc) of 741 mV. A comparison with several high quality passivation schemes for solar cells reveals that the developed inductively coupled plasma deposited films show excellent passivation quality. The excellent optical property and resistance to degradation make it an excellent substitute for industrial heterojunction silicon solar cell production.

Micro Raman Spectroscopy Analysis of Doped Amorphous and Microcrystalline Silicon Thin Film Layers and its Application in Heterojunction Silicon Wafer Solar Cells

Micro Raman Spectroscopy Analysis of Doped Amorphous and Microcrystalline Silicon Thin Film Layers and its Application in Heterojunction Silicon Wafer Solar Cells

Analysis of intrinsic hydrogenated amorphous silicon passivation layer growth for use in heterojunction silicon wafer solar cells by optical emission spectroscopy

The structure and quality of intrinsic hydrogenated amorphous silicon thin films are studied with intended use as passivation layer in heterojunction silicon wafer solar cells. The thin film layers are formed by radio-frequency parallel-plate plasma-enhanced chemical vapor deposition. While the passivation quality of the films is found to improve steadily with increasing deposition temperature, a very narrow process window in terms of pressure variation is observed. The best effective lifetime is obtained at a hydrogen to silane dilution ratio of 1 and a pressure of 66.7 Pa for the used tool configuration. Raman crystallinity and Urbach energy obtained from fitting ellipsometry data confirm that the degradation of the passivation quality outside the process window is due to a phase change into microcrystalline silicon with different growth mechanisms and an increase in bonding related defects. Film growth mechanisms are proposed to account for the observed narrow process window, which are verified by optical emission spectroscopy measurements.

Characterisation and Optimisation of Indium Tin Oxide Films Deposited by Pulsed DC Magnetron Sputtering for Heterojunction Silicon Wafer Solar Cell Applications

Amorphous/crystalline heterojunction silicon wafer solar cells are attracting attention in recent years due to their potential to achieve high conversion efficiencies at low fabrication temperatures. However, to date, only SANYO (Japan) has brought this technology to commercial mass production, mainly due to the high sensitivity of the solar cell parameters to the growth conditions. One significant difference between a heterojunction and a standard screen- printed silicon wafer solar cell is the current collection scheme. As a heterojunction silicon wafer solar cell is limited by its low emitter lateral conductivity, a transparent conductive oxide (TCO) is employed to improve the carrier transport, whilst also acting as an antireflective coating (ARC) for the front side. From the variety of TCOs, indium tin oxide (ITO) is one of the most promising candidates due to its high electrical conductivity and excellent optical transmittance. For deposition of very thin ITO films (in the 70-90nm range, in order to function as ARC), the restrictions and requirements for both high electrical and optical properties are rather challenging. In this study we focus on TCO deposition using an industrial pulsed direct current (DC) magnetron sputtering system (FHR, Germany). The effects of the applied DC power, the oxygen partial pressure, the deposition temperature, and the annealing conditions are investigated. Encouraging results demonstrate a resistivity in the range of (4.6-5)x10-4Ωcm and a transmission of above 90% in the visible range.

Detailed Micro Raman Spectroscopy Analysis of Doped Silicon Thin Film Layers and Its Feasibility for Heterojunction Silicon Wafer Solar Cells

Hydrogenated doped silicon thin films deposited using RF (13.56 MHz) PECVD were studied in detail using micro Raman spectroscopy to investigate the impact of doping gas flow, film thickness, and substrate type on the film characteristics. In particular, by deconvoluting the micro Raman spectra into amorphous and crystalline components, qualitative and quantitative information such as bond angle disorder, bond length, film stress, and film crystallinity can be determined. By selecting the optimum doped silicon thin film deposition conditions, and combining our p-doped and n-doped silicon thin films in different heterojunction structures, we demonstrate both (i) an efficient field effect passivation and (ii) further improvement to c-Si/a-Si:H(i) interface defect density with observed improvement in implied open-circuit voltage VOC and minority carrier lifetimes across all injections levels of interest. In particular, the heterojunction structure (a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(p)) demonstrates a minority carrier lifetime of 2.4 ms at an injection level of 1015 cm-3, and a high implied open-circuit voltage of 725 mV. Simulation studies reveal a strong dependence of the interface defect density Dit on the heterojunction silicon wafer solar cell performance, affected by the deposition conditions of the overlying doped silicon thin film layers. Using our films, and a fitted Dit of 5 × 1010 cm-2·eV-1, we demonstrate that a solar cell efficiency of ~22.5% can be potentially achievable.

Heterojunction Silicon Wafer Solar Cells using Amorphous Silicon Suboxides for Interface Passivation

Current industrial monocrystalline silicon wafer solar cells based on screen-printing technology for contact formation and selective emitter have an efficiency potential of around 20%. Heterojunction silicon wafer solar cells combine plasma-deposited amorphous silicon thin films and n-type crystalline silicon wafers to industrially viable high-efficiency solar cell devices. The excellent surface passivation provided by an intrinsic amorphous silicon interlayer leads to a very high open-circuit voltage, enabling industrial heterojunction cells with efficiencies of over 23%, as demonstrated by the Japanese company Sanyo. The key point of these structures is the removal of the highly recombination-active solar cell contacts from the crystalline surface, by insertion of a thin film with a wide bandgap. To reach the full device potential, the defect density at the hetero-interface must be minimised. Commonly used hydrogenated amorphous silicon films of only a few nanometer thickness are appealing candidates for this. Their bandgap (∼1.7eV) is much larger than that of c-Si and, when intrinsic, such films can reduce the c-Si surface state density by hydrogenation. Additional oxygen built into the amorphous network results in amorphous silicon suboxides, which in turn leads to an improved surface passivation. With this approach we are able to achieve implied voltages of 740mV and an optical bandgap of about 2eV, which improves the cell's blue response. In addition, microcrystalline silicon emitter and back-surface-field layers provide enhanced doping efficiency, enabling the fabrication of contacts with low saturation current density values. This new approach has already demonstrated promising cell efficiencies of about 21%. This paper summarises the recent progress in heterojunction silicon wafer solar cell research using amorphous silicon suboxides.

Optimisation of p-doped μc-Si:H Emitter Layers in Crystalline-amorphous Silicon Heterojunction Solar Cells

Heterojunction silicon wafer solar cells, using a microcrystalline silicon (μc-Si:H) thin-film emitter and a very thin intrinsic amorphous silicon (a-Si:H) passivation layer between the crystalline silicon wafer and the emitter layer, have been reported to exhibit stable performance and high efficiency. Desired properties for the emitter layer include wide bandgap, low surface and interface recombination, and good doping efficiency. In this study, we report on the thin-film properties of p-doped μc-Si:H emitter layers deposited using RF (13.56MHz) PECVD, at different SiH4/H2 gas flow ratios, pressures, and temperatures at the same RF power. Trends relating deposition conditions to relevant film characteristics such as thickness, crystalline fraction and conductivity are discussed. Finally, device relevant symmetrical p+/i/c-Si/i/p+ heterojunction lifetime test structures are investigated, using the optimised parameters for p-doped μc-Si:H layers (discussed in this paper) and for intrinsic a-Si:H layers (discussed in a companion paper [1]). These exhibit promising effective lifetimes of up to 2.4ms at an injection level of 1015cm-3.

Optimisation of Intrinsic a-Si:H Passivation Layers in Crystalline-amorphous Silicon Heterojunction Solar Cells

Heterojunction silicon wafer solar cells using an intrinsic amorphous silicon (a-Si:H) thin-film passivation layer between the crystalline c-Si substrate and the thin-film emitter layer have proven to be a viable device structure for high efficiency. While microcrystalline μc-Si:H is a good candidate for the emitter layer due to its high doping efficiency, intrinsic a-Si:H with its low interfacial defect density, high optical bandgap and good passivation ability makes it the ideal buffer layer to passivate the crystalline silicon interface. In this study, we report the film properties of intrinsic a-Si:H passivation layers deposited using RF (13.56MHz) PECVD, at different SiH4/H2 gas flow ratios, pressures and temperatures. Trends relating deposition conditions to relevant film characteristics, such as thickness, hydrogen bonding, optical bandgap, and effective carrier lifetime of the samples are discussed. Finally, symmetrical p+/i/c-Si wafer/i/p+ heterojunction lifetime test structures, using 20nm thick p-doped μc-Si:H emitter layers and 10nm thick intrinsic a-Si:H layers, were made using the optimised parameters for intrinsic a-Si:H layers (discussed in this paper) and for p-doped μc-Si:H layers (discussed in a companion paper [1]). These yield an effective lifetime of 2.4ms at an injection level of 1015cm-3, and an implied Voc of 730mV.