Intrinsic Amorphous Si Research Articles

Influence of Intrinsic Silicon Layer and Intermediate Silicon Oxide Layer on the Performance of Inline PECVD Deposited Boron-Doped TOPCon

In this article, the development and optimization of carrier-selective and passivating contacts by industry-scale inline plasma-enhanced chemical vapor deposition and their successful integration into solar cells are reported. Amorphous Si thin films with varying carbon content (SiC <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) were deposited on a thermally grown ultrathin tunnel oxide (TOPCon) and electrically characterized. Furthermore, the impact of a vacuum break (VB) during the deposition of a layer stack consisting of intrinsic amorphous Si [a-Si:H(i)] and boron-doped SiC <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> was investigated. That is, samples that were processed with VB were exposed to ambient air, and hence, a thin native oxide was formed on the a-Si:H(i) layer, which affected the boron diffusion into the absorber resulting in a distinct anneal behavior of the contacts. Upon optimization, these layers provided an excellent surface passivation quality, which was reflected in an implied open-circuit voltage of 733 mV for n-type and 716 mV for p-type TOPCon structures, respectively. In addition, very low contact resistivities of 0.3 mΩ-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for n-type and 0.5 mΩ-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for p-type TOPCon were measured, respectively. These optimized TOPCon structures were implemented into both sides contacted p-type laboratory solar cells. After a two-step furnace anneal, these cells achieved a maximum energy conversion efficiency of 22.7% with evaporated contacts.

Phosphorus-doped polycrystalline silicon passivating contacts via spin-on doping

Polycrystalline silicon (poly-Si) passivating contacts are promising technologies to promote the efficiency of silicon solar cells, due to their low carrier recombination and low contact resistivity. In this work, we present phosphorus spin-on doping as an alternative doping method to fabricate high performance poly-Si passivating contacts. The influences of thermal treatments and intrinsic amorphous Si thickness on poly-Si passivating contact quality were investigated. A high implied open-circuit voltage of above 730 mV together with a low contact resistivity below 4 mΩ⋅cm2 were obtained for 100 – 230 nm thick poly-Si layers after a thermal treatment at 975 °C for 60 min followed by a forming gas annealing. The promising results presented in this work imply that phosphorus spin-on doping can be an effective doping method alternative to conventional POCl3 diffusion.

Lithography-free and dopant-free back-contact silicon heterojunction solar cells with solution-processed TiO2 as the efficient electron selective layer

Lithography-free interdigitated back-contact silicon heterojunction (IBC-SHJ) solar cells with dopant-free metal oxides (TiO2 and MoOx) as the carriers selective transport layers were investigated. Spin-coating and hot-wire reactive-sublimation deposition together with low cost mask technology were used to fabricate the solar cells. Insertion of a SiOx layer with the thickness of about 2.4 nm between the intrinsic amorphous Si (a-Si:H(i)) passivation layer and the spin-coated TiO2 layer greatly improves the solar cell performance due to the enhanced field-effect passivation of the a-Si:H(i)/SiOx/TiO2 layer stack. Efficiency up to 20.24% was achieved on the lithography-free and dopant-free IBC-SHJ devices with a-Si:H(i)/SiOx/TiO2 layer stack as the electron selective transport layer, a-Si:H(i)/MoOx as the hole selective transport layer, and WOx as the antireflection layer. The novel IBC-SHJ solar cells show significant advantages in simplification of the technology and process compared with the IBC-SHJ devices whose back surface pattering and carrier selective layers relied on photolithography and plasma enhanced chemical vapor deposition (PECVD).

Catalytic phosphorus and boron doping of amorphous silicon films for application to silicon heterojunction solar cells

We investigate a novel doping method, catalytic impurity doping (Cat-doping), for application to the fabrication of silicon heterojunction (SHJ) solar cells. Thin n- or p-type doped layers can be formed on intrinsic amorphous Si (a-Si) films by exposing P- or B-related radicals generated by the catalytic cracking of phosphine (PH3) or diborane (B2H6) gas molecules. The passivation quality of underlying a-Si films can be maintained both for phosphorus (P) and boron (B) Cat-doping if we carefully choose the appropriate substrate temperature during Cat-doping. We confirm the rectifying and photovoltaic properties of an SHJ solar cell containing a B Cat-doped layer as a p-type a-Si emitter. These findings suggest the applicability of Cat-doping to SHJ solar cells.

On the recombination behavior of p+-type polysilicon on oxide junctions deposited by different methods on textured and planar surfaces

We investigate the passivation quality of hole-collecting junctions consisting of thermally or wet-chemically grown interfacial oxides, sandwiched between a monocrystalline-Si substrate and a p-type polycrystalline-silicon (Si) layer. The three different approaches for polycrystalline-Si preparation are compared: the plasma-enhanced chemical vapor deposition (PECVD) of in situ p+-type boron-doped amorphous Si layers, the low pressure chemical vapor deposition (LPCVD) of in situ p+-type B-doped polycrystalline Si layers, and the LPCVD of intrinsic amorphous Si, subsequently ion-implanted with boron. We observe the lowest J0e values of 3.8 fA cm−2 on thermally grown interfacial oxide on planar surfaces for the case of intrinsic amorphous Si deposited by LPCVD and subsequently implanted with boron. Also, we obtain a similar high passivation of p+-type poly-Si junctions on wet-chemically grown oxides as well as for all the investigated polycrystalline-Si deposition approaches. Conversely, on alkaline-textured surfaces, J0e is at least 4 times higher compared to planar surfaces. This finding holds for all the junction preparation methods investigated. We show that the higher J0e on textured surfaces can be attributed to a poorer passivation of the p+ poly/c-Si stacks on (111) when compared to (100) surfaces.

Alternative device structures for CIGS-based solar cells with semi-transparent absorbers

Abstract In this work, we discuss two alternative structures of CIGS-based solar cells, a bi-facial configuration and a back-wall configuration. One of the critical issues in these structures is fabrication of a transparent back contact which facilitates not only ohmic contact for holes in the valance but also an effective reflector for electrons in the conduction band. In order to meet these requirements of the transparent back contact, a p-type amorphous Si (p-a-Si)/intrinsic amorphous Si (i-a-Si) bi-layer was deposited onto the ITO back contact. Absorber films were formed by a metal precursor reaction in H2Se and H2S, and their thicknesses were adjusted to about 0.25 µm to enhance current collection from back-side illumination. The resulting devices with a bi-facial structure of SLG/ITO/p-Si/i-Si/Cu(In, Ga)(Se,S)2/CdS/i-ZnO/ITO/Ni-Al exhibited a comparable open-circuit voltage to a Cu(In, Ga)(Se,S)2 cell on a Mo substrate in tests with illumination of both sides. Back-wall cells with a structure of SLG/ITO/p-Si/i-Si/Cu(In, Ga)(Se,S)2/CdS/i-ZnO/Ag-reflector were also fabricated and improved current collection by greater light trapping was achieved compared to the bi-facial cell with back-side illumination.

Surface Optimization of Random Pyramid Textured Silicon Substrates for Improving Heterojunction Solar Cells

Key steps in the fabrication of high-efficiency a-Si:H/c-Si heterojunction solar cells are the controlled pyramid texturing of the c-Si substrates to minimize reflection losses and the subsequent passivation by deposition of a high-quality a-Si:H layer to reduce recombination losses. This contribution reviews our recent results on the optimization of the wet-chemical texturing of crystalline Si wafers for the preparation of heterojunction solar cells with respect to low reflection losses, low recombination losses and long minority carrier lifetimes. It is demonstrated, that by joint optimization of both saw damage etch and texture etch the optical and electronic properties of the resulting pyramid morphology can be controlled. Effective surface passivation and thus long minority charge carrier lifetimes are achieved by deposition of intrinsic amorphous Si ((i) a-Si:H) layers. It is shown, that optimized (i) a-Si:H deposition parameters for planar Si (111) wafers can be transferred to a-Si:H layer deposition on random pyramid textured Si (100) wafers. Statistical analysis of the pyramid size distribution revealed that a low fraction of small pyramids leads to longer minority charge carrier lifetimes and, thus, a higher Voc potential for solar cells.

Suppression of the epitaxial growth of Si films in Si heterojunction solar cells by the formation of ultra-thin oxide layers

The epitaxial growth of silicon (Si) films during the catalytic chemical vapor deposition (Cat-CVD) of intrinsic amorphous Si (i-a-Si) passivation films on crystalline Si (c-Si) wafers is suppressed by the oxidation of c-Si surfaces simply by dipping the c-Si wafers in hydrogen peroxide (H2O2). This oxidation treatment is also effective for (111)-oriented c-Si surfaces particularly at high a-Si deposition temperatures. The suppression of the epitaxial growth leads to the better effective minority carrier lifetime (τeff) of c-Si wafers passivated with Cat-CVD i-a-Si films. SHJ solar cells show remarkably high open-circuit voltage (Voc) exceeding 0.7 V. These results clearly show the effectiveness of the insertion of SiOx layers on the improvement in Cat-CVD a-Si/c-Si interfaces.

Interaction of KCN solutions with Si-based thin films

AbstractThis contribution deals with the interaction of cyanide solutions with two types of double-layer structures. A 0.1 M KCN methanol solution, one of the very strong alkaline solutions, was used for treatment as an alternative to 0.1 M KCN aqueous solution. The Fourier transfer infrared (FTIR) spectra of samples deposited on glass contain the interference behaviors from which corresponding changes of optical thicknesses were calculated. After interaction with all types of KCN solutions there was an observed decrease in the values of optical thickness on the double-layer structure intrinsic amorphous Si/p-type amorphous SiC/glass. The optical thickness of an n-type microcrystalline Si/intrinsic amorphous Si/glass structure increased after interaction with KCN methanol solutions. This effect is partly related with the increase of the complex refractive index of the intrinsic amorphous Si sublayer. The values of the changes of optical thickness of the structures were obtained with corresponding confidence intervals determining the range of error in the measurement and evaluation procedures used. The results indicate that this approach can be successfully used also for research into the interaction of other thin film structures with various types of solution with real-time measurements, as well as for the successful monitoring of passivation of defect states inside pin amorphous solar cells.Keywords: KCN solutionamorphous siliconoptical thicknessPACS: 78.20. ± e42.70. ± a71.20.Nr81.16. ± c AcknowledgmentWe thank to Dr. M. Jergel of the IoP SAS Bratislava, for help with XRD measurements and in discussions.Additional informationFundingThis work was financially supported by the Japan Society for Promotion of Science (JSPS) and by the following Slovakian agencies: APVV [project number 0888/11]; and VEGA [project numbers 2/0076/15, 1/0818/13, and 1/0853/13].

Rear-emitter Si heterojunction solar cells with over 23% efficiency

We have developed highly crystallized n-type microcrystalline Si layers as window layers for rear emitter Si heterojunction solar cells. We introduce a seed layer between an n-type microcrystalline Si layer and an intrinsic amorphous Si layer to improve the crystallinity of the n-type microcrystalline Si layer. By using this stacked layer instead of an n-type amorphous Si layer, the contact resistance between the n-type thin layer and In2O3:H is reduced without Al-doped ZnO. As a result, we obtain a high short-circuit current and a high fill factor simultaneously, and achieve a solar cell efficiency of 23.43%.

Preparation of nano-patterned Si structures for hetero-junction solar cells

The 220nm and 300nm periodically nano-patterned Si structures with low aspect ratio were fabricated by nano-sphere lithography technique. A good anti-reflection properties in a broadband spectral range (300–1200nm) was exhibited due to the gradually changing refractive index of the formed Si nanostructures. After deposition of the intrinsic and phosphorous-doped (P-doped) amorphous Si (a-Si) film, the weighted mean reflection of the 220nm and 300nm periodic nanostructures was further reduced to 3.30% and 2.96%, respectively. Due to the enhanced light absorption, both the IQE and EQE of the nano-patterned cells were improved in a wide spectral range. For the 300nm periodically nano-patterned prototype hetero-junction solar cell, the short circuit current density was increased to 34.5mA/cm2, which was obviously improved compared with 26.8mA/cm2 for the flat cell.

Passivation of Textured Silicon Wafers:Influence of Pyramid Size Distribution, a-Si:H Deposition Temperature, and Post-treatment

In this study we focus on optimizing the passivation of pyramid textured n-type crystalline silicon (c-Si) wafers by deposition of intrinsic amorphous Si (a-Si:H(i)) layers. By statistical analysis of the pyramid size distribution it is revealed that a decreased fraction of small pyramids leads to longer minority charge carrier lifetimes and, thus, a higher Voc potential for solar cells. Further, we demonstrate that optimized parameters for the deposition of a-Si:H(i) layers on planar Si(111) wafers can be transferred to the a-Si:H deposition on random pyramid textured Si(100) wafers exhibiting facets with {111} orientation. In particular the influence of the deposition temperature on the optical layer properties is elucidated. Furthermore, the favorable impact of post-deposition plasma-hydrogenation and annealing on the charge carrier lifetime (> 5ms) and the implied open-circuit voltage (up to 738mV) are demonstrated.

Thin macroporous silicon heterojunction solar cells

AbstractWe demonstrate the processing of a heterojunction solar cell from a purely macroporous silicon (MacPSi) absorber that is generated and separated from a monocrystalline n‐type Cz silicon wafer by means of electrochemical etching. The etching procedure results in straight pores with a diameter of (4.7 ± 0.2) µm and a distance of 8.3 µm. An intrinsic amorphous Si (a‐Si)/p+‐type a‐Si/indium tin oxide (ITO) layer stack is on the front side and an intrinsic a‐Si/n+‐type a‐Si/ITO layer stack is on the rear side. The pores are open when depositing the layers onto the 3.92 cm2‐sized cell. The conductive layers do not cause shunting through the pores. A silicon oxide layer passivates the pore walls. The energy‐conversion efficiency of the (33 ± 2) µm thick cell is 7.2%. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Low-temperature Phosphorus Doping To Silicon Using Phosphorus-related Radicals

ABSTRACTPhosphorus (P) doped ultra thin n+-layer is formed on crystalline silicon (c-Si) at low substrate temperatures of 80 – 350 °C using radicals generated by the catalytic reaction of phosphine (PH3) with a tungsten catalyzer heated at 1300 °C. The sheet carrier concentration obtained by Hall effect is in the range between 3×1012cm-2 and 8×1012cm-2. The distribution of P atoms obtained by secondary ion mass spectrometry (SIMS) indicates that P atoms locate within the depth of 4 nm from surface and the profile has almost the same distribution independent of any doping conditions such as substrate temperature or radical exposure time. The sheet carrier concentration is 1.15 – 2.12% of the amount of P atoms incorporated through the radical doping. The ratio of activated donors increases with substrate temperature during the radical doping, suggesting that P-related species bonded on the c-Si surface require thermal energy for their activation. Using the n+-layer formed by radical doping, the reduction of surface recombination velocity for n-type c-Si wafer is attempted. The effective minority carrier lifetime of the n-type c-Si sample covered with 6-nm-thick intrinsic amorphous Si (i-a-Si) layers on both side increases from 32 μs to1576 μs by the radical doping of P atoms to n-type c-Si surface, suggesting that the radical doping can be utilized for the formation of passivation layers on a-Si/ n-c-Si hetero-interface.

Applications of the Heterojunction with Intrinsic Thin Layer Solar-Cell Structure on Photodetectors

We have utilized the high-efficiency heterojunction with intrinsic thin-layer (HIT) solar-cell structure for applications in photodetectors. In this research, we have investigated the relationship between responsivities and thicknesses of active layers, including top intrinsic amorphous Si and bulk crystalline Si. The thickness of a HIT photodetector should depend on the detection wavelength desired. For a typical HIT structure with a 5-nm-thick top intrinsic amorphous Si layer, the responsivities at 450, 650, and 850 nm wavelengths could be 0.511, 0.529, and 0.641 A/W, respectively. The responsivity at the 450 nm wavelength can reach 0.591 A/W for a 20-nm-thick top intrinsic amorphous Si layer even when the bulk Si is only 1.5 µm thick.

Fabrication of selective-emitter silicon heterojunction solar cells using hot-wire chemical vapor deposition and laser doping

The Si heterojunction (HJ) solar cells were fabricated on the textured p-type mono-crystalline Si (c-Si) substrates using hot-wire chemical vapor deposition (HWCVD). In view of the potential for the bottom cell in a hybrid junction structure, the microcrystalline Si (μc-Si) film was used as the emitter with various PH 3 dilution ratios. Prior to the n-μc-Si emitter deposition, a 5 nm-thick intrinsic amorphous Si layer (i-a-Si) was grown to passivate the c-Si surface. In order to improve the indium-tin oxide (ITO)/emitter front contact without using the higher PH 3 doping concentration, a laser doping technique was employed to improve the ITO/n-μc-Si contact via the formation of the selective emitter structure. For a cell structure of Ag grid/ITO/n-μc-Si emitter/i-a-Si/textured p-c-Si/Al-electrode, the conversion efficiency (AM1.5) can be improved from 13.25% to 14.31% (cell area: 2 cm × 2 cm) via a suitable selective laser doping process.

Fabrication and characteristics of n-Si/ c-Si/ p-Si heterojunction solar cells using hot-wire CVD

A double-side (bifacial) heterojunction (HJ) Si solar cell was fabricated using hot-wire chemical vapor deposition. The properties of n-type, intrinsic and p-type Si films were investigated. In these devices, the doped microcrystalline Si layers (n-type Si for emitter and p-type Si for back contact) are combined with and without a thin intrinsic amorphous Si buffer layer. The maximum temperature during the whole fabrication process was kept below 150 °C. The influence of hydrogen pre-treatment and n-Si emitter thickness on performance of solar cells have been studied. The best bifacial Si HJ solar cell (1 cm 2 sample) with an intrinsic layer yielded an active area conversion efficiency of 16.4% with an open circuit voltage of 0.645 V, short circuit current of 34.8 mA/cm 2 and fill factor of 0.73.

On-chip inductors incorporating porous-Si and intrinsic-amorphous-Si films for rf integrated circuits

The quality factor (Q) of inductors on silicon (Si) is limited by the series resistance of the metal at low frequency and by the substrate resistivity at high frequency. Oxide is generally used to isolate the useful signal of the inductor from the lossy Si substrate. However, stoichiometric silica (SiO2) is processed at high temperature which restricts the possibility of post-complementary metal-oxide semiconductor integration, and due to the large thermal mismatch with Si, thick oxide films introduce considerable stress. In this work, we apply electrochemically formed porous Si (PS) and intrinsic amorphous Si (i-a-Si:H) deposited at low temperature (250°C) in an approach asan isolation bilayer for planar inductors on Si for rf integrated circuits. An improvement of morethan 45% in Q was measured at a frequency of 6.4GHz for 1.6nH inductor incorporating PS andi-a-Si:H films. The experimental results presented indicate that these low-temperature materials are promising for the isolation of rf devices on low-resistivity Si.

Passivation of Defect States in Amorphous and Crystalline Si by use of Cyanide Treatment and Improvement of Solar Cell Characteristics

AbstractWe have developed a new method of eliminating defect states in Si. This method called cyanide treatment simply includes immersion of Si in KCN solutions followed by the rinse. The contamination by potassium ions can be completely prevented by the inclusion of 18-crown-6 in the KCN solutions (crown-ether cyanide treatment). When the crown-ether cyanide treatment was performed on intrinsic amorphous Si (a-Si) films, decreases in the photo-and dark current densities with the irradiation time were completely prevented. When cyanide treatment using aqueous KCN solutions was applied to pin-junction a-Si solar cells, the conversion efficiencies measured before and after light-induced degradation became higher than those with no treatment. These improvements are attributed to the elimination of defect and defect precursor states by the reaction with cyanide ions, resulting in the formation of Si-CN bonds. From density functional calculations, Si-CN bonds are found to possess a high bond energy of 4.5 eV. Due to the high bond energy, the bonds are not ruptured by heat treatment at 800°C and upon irradiation, resulting in the thermal and irradiation stability of the cyanide treatment.

Photoemission measurements on aluminium and amorphous silicon by pulsed laser illumination in presence of a plasma

In this paper the effect of the illumination of a surface by a pulsed laser beam was studied. Al layers and intrinsic and doped amorphous silicon films were illuminated in vacuum, in the presence of different gases and under a DC plasma. The wavelength of the pulsed beam ranged from 308 to 348 nm. Applied bias influenced strongly the measured signal when performing experiments in a gas atmosphere. While no photoemission was detected on both intrinsic and p-type amorphous Si:H, a signal was obtained on n-type a-Si:H if a gas atmosphere is present.