Low-refractive-index and high-transmittance silicon oxide with a mixed phase of n-type microcrystalline silicon as intermediate reflector layers for tandem solar cells

doi:10.1016/j.solmat.2013.10.036

Solar Energy Materials and Solar Cells

Volume 121, February 2014, Pages 1-7

https://doi.org/10.1016/j.solmat.2013.10.036 Get rights and content

Highlights

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Low-refractive-index and high-transmittance SiO_x films are developed.
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SiO_x films are formed by PECVD under high H₂ dilution and high pressure conditions.
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SiO_x films are used as intermediate reflection layers (IRLs) for tandem solar cells.
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Characterization and optimization of the SiO_x IRL are presented.
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Stabilized module efficiency of 11.12% is demonstrated using the SiO_x IRL.

Abstract

Low-refractive-index and high-transmittance silicon oxide (SiO_x) with a mixed phase of n-type microcrystalline silicon was developed for intermediate reflector layers (IRLs) of high-efficiency amorphous Si and microcrystalline-Si tandem solar cells. The refractive index, crystalline fraction, and conductivity of the SiO_x IRLs were characterized as functions of the deposition conditions. The SiO_x films were prepared by plasma-enhanced chemical vapor deposition, at a high pressure of 9 Torr and high hydrogen dilution ratio of 400, and using a narrow electrode gap of 12 mm. The films showed a refractive index of 1.85, crystalline fraction of ~50%, and conductivity of ~10⁻⁷ S/cm. Furthermore, the transmittance of the films was more than 90% at wavelengths between 600 nm and 1100 nm. We present here the procedure to optimize the SiO_x IRLs, which is to compare the changes in the top and bottom cells' current densities of the tandem solar cells fabricated with 10–20% thinner absorption layers. A stabilized efficiency of 11.12% and light-induced degradation of −9.3% could be achieved in a tandem module containing an optimized IRL by enhancing the top cell current. The SiO_x IRLs developed and the optimization procedure discussed in this paper can be very useful for the fabrication of high-efficiency thin-film Si tandem modules.

Introduction

Thin-film silicon (Si) solar cells have gained much interest and attention as a powerful product that offers many advantages to the photovoltaic industry: mass–volume module production on glass substrates measuring several square meters and low material usage are feasible; lower cost-per-watt and lower pay back time in comparison with crystalline Si solar cells are expected [1]. Thin-film Si solar cells with stacked multijunctioned structures have achieved high efficiencies [2], [3], [4], [5]. As tandem modules consisting of a high-bandgap, hydrogenated-amorphous-Si (a-Si:H) top cell and a low-bandgap, microcrystalline-Si (μc-Si) bottom cell are considered to be effective at reasonable production costs, most of the advanced thin-film Si solar modules produced these days have tandem structures [4], [5]. To mitigate light-induced degradation (LID) of the a-Si:H top cell [6] and to obtain a highly stabilized efficiency, intermediate reflector layers (IRLs) are used between the top and bottom cells in the tandem structure. An IRL reflects the shorter wavelength portion of the solar spectrum to the a-Si:H top cell and enhances light absorption of the top cell. Consequently, the IRL allows reduction of the thickness of the top cell, which is favorable for the transport of generated carriers, resulting in the reduction of LID of the a-Si:H solar cells.

It is preferable to use materials with low refractive index and high transmittance as IRLs. When the refractive index of the IRL is much lower than that of the a-Si:H top cell, the IRL can effectively reflect the shorter wavelength portion to the top cell; when the optical loss of the IRL is low, it can transmit the longer wavelength portion to the bottom cell as well. Only junction-layer materials with reasonable conductivities can be used as IRLs, however. For example, transparent conducting oxides (TCOs) such as doped ZnO [7], [8] or materials based on n-type silicon oxide (SiO_x) have been used as IRLs [9], [10], [11], [12], [13]. Doped ZnO, with a refractive index of around 1.8–2.0 and a conductivity of more than ~10² S/cm, is a suitable IRL material for a unit cell. However, when a module is monolithically fabricated on glass on a large scale using a conventional three-scribe laser patterning process, the TCO IRL forms an unintended lateral shunt at the back reflecting metal layer. Therefore, TCO IRLs require the use of four laser scribes at the expense of the effective module area [8], as shown in Fig. 1(a). Another drawback of using TCO-based IRLs is that the deposition and patterning of TCO are discontinuous steps undertaken outside the process chamber during the plasma-enhanced chemical vapor deposition (PECVD) of the top and bottom cell layers. n-Type a-SiO_x or mixed-phase SiO_x with n-type μc-Si can be grown by PECVD [14], [15], [16] and the refractive index (or optical transmission loss) and conductivity of SiO_x can be adjusted with the gas flow ratios of CO₂/SiH₄ and PH₃/SiH₄ [17]. With a mixed phase of μc-Si and appropriate choice of doping concentrations, SiO_x IRLs in tandem solar cells have shown normal light I–V curves at conductivities in the10⁻³–10⁻⁷ S/cm range in the lateral direction [10], [11], [12], [13]. Because the a-Si:H top cell, SiO_x IRL, and μc-Si can be grown continuously in the same CVD system, SiO_x IRLs could be more attractive than TCO IRLs in terms of ease of fabrication and productivity. Recently, SiO_x-based layers were applied not only to low-loss n-type layers [18] but also to advanced IRLs adopting photonic structures [19], [20], [21], [22], [23]. Thus, the deposition and characterization of mixed-phase SiO_x with μc-Si are important for the fabrication of thin-film Si solar cells.

There have yet to be detailed reports on the growth conditions and film characteristics of SiO_x IRLs that are suitable for high-efficiency tandem solar cells. Furthermore, there has not been a complete study that covers topics from characterization to optimization of SiO_x IRLs for tandem modules. Generally, the oxygen content increases with the CO₂/SiH₄ source gas ratio (R_C–S), and the refractive index and optical loss become lower with increasing R_C–S [11], [12], [13], [14], [15], [16], [17], [18]. However, since the volume fraction of crystalline Si and the conductivity of the SiO_x film decrease simultaneously, the refractive indices of SiO_x films used for tandem cells have been restricted to values above 2.0 in the case of SiO_x films grown with R_C–S <3 and at pressures below 3 Torr by the use of a conventional 13.56-MHz PECVD system [11], [12], [16], [18]. In other cases, very-high-frequency PECVD (VHF PECVD) systems have been used [9], [10], although low-frequency PECVD technology is still very valuable since it is commonly used in the solar, semiconductor, and flat panel industries, and low-frequency PECVD systems enable the fabrication of larger-sized photovoltaic modules more inexpensively than VHF PECVD systems. In this paper, we will demonstrate that high-transmittance SiO_x films with low refractive indices (<2.0) can be obtained using a conventional 13.56-MHz PECVD system under high-hydrogen dilution and high-pressure deposition conditions. We will also discuss the procedure for optimizing the SiO_x IRL based on our experimental evaluation of the photogenerated current density under short-circuit conditions (J_SC). Furthermore, we fabricated a tandem module with the SiO_x IRL using batch deposition (growing all the p-, i-, and n-type films of a-Si:H and μc-Si:H in a single chamber of the PECVD system, which is very useful for industrial production) and laser patterning processes to show that the stabilized module efficiency can be enhanced. The developed SiO_x IRL can be very useful in the fabrication of high-efficiency thin-film Si tandem solar cells

Section snippets

Experiments and solar cell structures

Thin films of a-Si:H, SiO_x, and μc-Si were grown in a conventional PECVD system excited at a low radio frequency (RF) of 13.56 MHz. The general conditions used for depositing a-Si:H and μc-Si films are described elsewhere [24], [25]. All layers except IRLs were deposited at 180 °C under a pressure of 5 Torr using a narrow electrode gap of 12 mm. The H₂/SiH₄ ratio was fixed at 15 for a-Si:H and at 64 for μc-Si. For the deposition of the SiO_x IRL, we investigated the effects of varying R_C–S, the

Deposition and characteristics of SiO_x films

We first attempted to use n-type doped a-SiO_x layers as IRLs. At a pressure of 5 Torr, hydrogen dilution ratio (HDR) of 100, and RF power of 300 W, we could control the refractive index of the a-SiO_x films in a wide range (1.5–2.4) by changing the value of R_C–S, as shown in Fig. 2(a). However, under a doping gas flow of 2% PH₃, the measured conductivities of the grown films were below 10⁻¹⁰ S/cm. In addition, owing to the low conductivity, tandem cells using a-SiO_x with a refractive index in the

Conclusions

We developed a low-refractive-index SiO_x IRL with a mixed phase of n-doped μc-Si for application in thin-film Si tandem solar cells. We investigated the effects of deposition at high pressure and high hydrogen dilution of the SiO_x films. At 9 Torr and HDR=400, deposition of a SiO_x film with a refractive index of 1.85 occurred at a R_C–S value of approximately 6 CO₂/SiH₄. The film consisted of a crystalline fraction of ~50% and conductivity of ~10⁻⁷ S/cm at a PH₃/SiH₄doping gas ratio that was fixed

Acknowledgments

This research was supported by the Solar Energy Group of the Samsung Electronics Inc. Myunghun Shin was supported by the Korea Institute of Energy Research (No. GP2012-0024-01) and the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (2013M3C1A3065055) for preparing the paper. The authors acknowledge the technical support and the assistance offered during the recording of measurements by Dr. Czang-Ho Lee,

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The fundamental operation mechanisms of nc-SiO<inf>X≥0</inf>:H based tunnel recombination junctions revealed
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Two terminal multi-junction (MJ) photovoltaic (PV) devices are well established concepts to increase the solar-to-electrical power conversion in reference to single PV junctions. In multi-junction PV devices two consecutive sub-cells are interconnected using a tunnel recombination junction (TRJ) in which the light excited holes of one sub-cell recombine with the light excited electrons of the other sub cell. An ideal TRJ is an ohmic contact with non-rectifying behaviour. TRJ’s based on p- and n-doped silicon-oxides have been successfully applied in a variety of hybrid multi-junction PV devices in which tunnelling and trap-assisted tunnelling over width of 5–20 nm rules the TRJ’s recombination kinetics.
In this contribution the qualitative fundamental working principles of tunnel recombination junctions based on p- and n-doped silicon and silicon-oxide alloys are revealed using both electrical modelling and experiments based on a unique set of tandem lab cells (four types based on four different PV materials) combined with structural variations in TRJ architectures. The study results in design rules for the integration of silicon-oxide based TRJ’s and provides fundamental insights into the sensitivity of the electrical performance of the TRJ’s to doping concentrations, to alignment of the conduction and valence bands of consecutive sub-cells, to the nature of interface defects, to the growth of amorphous and crystalline phases and its dependence on substrate or seed layers and to the nanoscale thicknesses of the TRJ layers.
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2021, Materials Science and Engineering: B
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Addition of P atoms as impurities in this disordered system modifies the nanostructures and leads to changes in the electrical conduction mechanism. During last few decades, several researchers have investigated on the correlation among the crystallinity, bonding characteristics and the optoelectronic properties of the films prepared by changing plasma parameters e.g. precursor gas ratio, diluents ratio etc. [1,2,6]. A two phase structure of nc-SiOX:H involving Si nanocrystals embedded in the amorphous oxide matrices, in its both undoped as well as doped configurations, has been proposed by several researchers to validate the carrier transport characteristics in dc-electric field at low as well as high temperatures [10–13].
Phosphorus-doped nc-SiO_X:H films with elevated dark-conductivity and wide optical band gap are prepared via one-step low-temperature (~ 250 °C) processing, by optimizing H₂-dilution [R_H = H₂/(He + H₂)] of the (SiH₄ + CO₂ + PH₃) precursor gasses in RF plasma-CVD. The band tail defect dynamics in covalently bonded disordered oxide-semiconductor system has been extensively studied via frequency and temperature dependent conductivity-response, impedance-characteristics, and dielectric-properties, in doped-silicon-oxygen and amorphous-nanocrystalline complex system. The temperature dependent ac-conductivity obeys the Universal Jonscher power-law and the behaviour of extracted parameters from nonlinear-fitting identifies non-overlapping small-polaron-tunnelling (NSPT) model to prevail in ac-conduction mechanism. Modelling of the impedance spectra over wide frequency range via equivalent electric circuit consisting of two parallel R||CPE connected serially, confirms individual contribution of separate nc-Si and a-SiO_X:H moieties. The charge carrier density-of-states, N(E_F), participating in ac-conduction at ~ 373 K and ~ 10⁴ Hz increases from ~ 7.6 × 10¹⁵ eV⁻¹cm⁻³ to ~ 1.4 ×10¹⁷ eV⁻¹cm⁻³ with R_H elevated from 0.1 to 0.4, corresponding to increased O-content simultaneous to reduced H-content in the Si-network possessing sequentially higher-crystallinity and wider optical band gap.
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Transparent hydrogenated amorphous silicon thin-film solar modules are fabricated using oxide-metal-oxide (OMO) electrodes as the back electrode for building-integrated photovoltaic applications. The outer aluminum-doped zinc oxide and inner silver layers constitute a thin OMO electrode (~110 nm thick), exhibiting a sheet resistance of 6.8 Ω/□ and an average transmittance of ~88% in the visible range of 400–800 nm. The external quantum efficiency and average transmittance of the cell were investigated for the absorber-layer thickness using the finite-difference time-domain method, and it was found that the optical loss in the cell was mainly due to the absorption of the front electrode in the ultra-violet region and free-carrier absorption of the OMO in the infrared region. Fabrication issues are introduced for a 532 nm short-pulse high-power laser patterning process for transparent modules with thin OMO electrodes. Optimization of the laser power for the P2 and P3 laser processes is demonstrated by observing the profiles and measuring the shunt resistance of the laser-patterned edges. Furthermore, the cell width is optimized based on an equivalent circuit model using PSpice simulation. The highest module efficiency and average transparency achieved in the range of 500–800 nm were 5.6% and 15.2%, respectively. The short-circuit current density, fill factor, and open-circuit voltage per cell of the module were found to be 10.8 mA/cm², 62.7%, and 0.830 V, respectively.
Advanced nanocrystallinity with widened optical gap realized via microstructural control in P-doped silicon oxide thin films used as window layer in nc-Si solar cells
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Its wide optical band gap remains tuneable through controlled incorporation of O-atoms into the network and also via changing size of the nanocrystals, while the electrical conductivity is retained by the proportional volume fraction of crystallinity. When used as window layer for tandem silicon solar cells in n-i-p superstrate configuration, the n-SiOx:H layer needs to possess a relatively high degree of crystallinity in order to provide favourable nucleation of crystallites in the i–nc-Si absorber layer of the top sub-cell sequentially grown on it [6–14]. Doped nc-SiO:H layers are in good use as contact layers in heterojunction solar cells [15,16].
The prime objective of the present investigation was focused on simultaneous improvement in crystallinity of the oxygenated wide band gap Si-network, while increasing its doping efficiency via increased H₂-dilution (R_H) of the primarily He-diluted (SiH₄+CO₂+PH₃)-plasma. The P-doped nc-SiO_X:H films were grown at adequate radio frequency (rf) power (200 W) and low substrate temperature (~250 °C) by plasma-enhanced chemical vapour deposition (PECVD). Along with increasing crystallinity, significant contribution of atomic H from the increased R_H resulted in the formation of enough OH-radicals that facilitated growing Si–OH bonds and produced the Si–O–Si network. The effective P-doping gradually advanced within the crystalline dominated network grown at higher R_H, due to increased density of atomic H feasibly influencing sensible etching of the grain boundary and/or amorphous tissue region. In view of increased crystallinity, elevated grain-size, enhanced O-content and simultaneous continuous increase in P/Si ratio from ~1.07% to ~1.89% during increasing R_H from 0 to 0.5, it was envisaged that the dopants preferably resided within crystalline-prone domain of the network which was consistently improved at elevated H₂-dilution of the plasma. The P-doped nc-SiO_x:H films grown with an advancement of the overall nanocrystallinity from ~7 to 60 vol% via increased H₂-dilution to the SiH₄ plasma attained the dark electrical conductivity increased from 10⁻⁴ to 2.3 S cm⁻¹. Simultaneously, a significant widening of optical band gap from ~1.92–2.14 eV has been obtained via a limited increase in the Si–O bond density along with quantum confinement effect arising from increased volume fraction of Si nanocrystals with dimension <5 nm. Elevated number density of tiny Si-ncs played a significant role in maintaining both the increased electrical conductivity and the widened optical band gap, which had been attained via proper micro-structural control using H₂ with He as a tricky diluent to the SiH₄ plasma. Here lies the novelty of the work which led to the development of n-type nc-SiO_X:H films suitably used as the window-layer of thin-film nc-Si solar cell in n-i-p superstrate configuration with an efficiency of ~5.74%.
Development of highly conducting n-type micro-crystalline silicon oxide thin film and its application in high efficiency amorphous silicon solar cell
2017, Materials Science in Semiconductor Processing
Wide band gap and highly conducting n-type nano-crystalline silicon film can have multiple roles in thin film solar cell. We prepared phosphorus doped micro-crystalline silicon oxide films (n-μc-SiO:H) of varying crystalline volume fraction (X_c) and applied some of the selected films in device fabrication, so that it plays the roles of n-layer and back reflector in p-i-n type solar cells. It is generally understood that a higher hydrogen dilution is needed to prepare micro-crystalline silicon, but in case of the n-μc-SiO:H an optimized hydrogen dilution was found suitable for higher X_c. Observed X_c of these films mostly decreased with increased plasma power (for pressure<2.0 Torr), increased gas pressure, flow rate of oxygen source gas and flow rates of PH₃>0.08 sccm. In order to determine deposition conditions for optimized opto-electronic and structural characteristics of the n-μc-SiO:H film, the gas flow rates, plasma power, deposition pressure and substrate temperature were varied. In these films, the X_c, dark conductivity (σ_d) and activation energy (E_a) remained within the range of 0–50%, 3.5×10⁻¹⁰ S/cm to 9.1 S/cm and 0.71 eV to 0.02 eV, respectively. Low power (30 W) and optimized flow rates of H₂ (500 sccm), CO₂ (5 sccm), PH₃ (0.08 sccm) showed the best properties of the n-μc-SiO:H layers and an improved performance of a solar cell. The photovoltaic parameters of one of the cells were as follows, open circuit voltage (V_oc), short circuit current density (J_sc), fill-factor (FF), and photovoltaic conversion efficiency (η) were 950 mV, 15 mA/cm², 64.5% and 9.2% respectively.
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We fabricated hydrogenated amorphous silicon (a-Si: H) transparent solar cells using ultrathin transparent multi-layered electrodes (TMEs) as rear-side transparent electrodes for building-integrated photovoltaic (BIPV) windows. Each TME included a bottom layer, thin Ag layer, and optoelectronic controlling layer (OCL). The TMEs were experimentally designed to have high transparency and conductance. The fabricated a-Si:H transparent solar cells showed optimal performance with a 6.36% power-conversion efficiency and 23.5% average transmittance (500–800 nm) when TMEs with a thickness less than 160 nm were incorporated. We demonstrated that the reflection color of the cell could be tuned without serious loss of cell efficiency by varying the thickness of the OCL. The backside colors were predicted to be the coordinates on a CIE 1931 chromaticity diagram using the reflection spectrum from the cell and emission spectrum of a 50 W LED light source. The developed a-Si:H transparent solar cells exhibited high efficiency and show feasibility for incorporating various colors in the photovoltaic and aesthetic functionalities of BIPV windows.

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Solar Energy Materials and Solar Cells

Highlights

Abstract

Introduction

Section snippets

Experiments and solar cell structures

Deposition and characteristics of SiO_x films

Conclusions

Acknowledgments

References (25)

Remarkable progress in thin-film silicon solar cells using high-efficiency triple-junction technology

Sol. Energy Mater. Sol. Cells

Silicon filaments in silicon oxide for next-generation photovoltaics

Adv. Mater.

A high efficiency thin film solar cell and module

Sol. Energy

Reversible conductivity changes in discharge-produced amorphous Si

Appl. Phys. Lett.

New progress in the fabrication of n–i–p micromorph solar cells for opaque substrates

Sol. Energy Mater. Sol. Cells

LPCVD ZnO-based intermediate reflector for micromorph tandem solar cells

Sol. Energy Mater. Sol. Cells

Optical management in high-efficiency thin-film silicon micomorph solar cells with a silicon oxide based intermediate reflector

Phys. Status Solidi RRL

In situ silicon oxide based intermediate reflector for thin-film silicon micromorph solar cells

Appl. Phys. Lett.

n-Type hydrogenated amorphous silicon oxide containing a microcrystalline silicon phase as an intermediate reflector in silicon thin film solar cells

Phys. Status Solidi C

Improved micromorph solar cells by means of mixed-phase n-doped silicon oxide layers

Prog. Photovoltaics Res. Appl.

Cited by (24)

The fundamental operation mechanisms of nc-SiO<inf>X≥0</inf>:H based tunnel recombination junctions revealed

Frequency and temperature dependent electrical characteristics of P-doped nc-SiO<inf>X</inf>:H thin films

Optimization of laser-patterning process and module design for transparent amorphous silicon thin-film module using thin OMO back electrode

Advanced nanocrystallinity with widened optical gap realized via microstructural control in P-doped silicon oxide thin films used as window layer in nc-Si solar cells

Development of highly conducting n-type micro-crystalline silicon oxide thin film and its application in high efficiency amorphous silicon solar cell

Colored a-Si:H transparent solar cells employing ultrathin transparent multi-layered electrodes

Low-refractive-index and high-transmittance silicon oxide with a mixed phase of n-type microcrystalline silicon as intermediate reflector layers for tandem solar cells

Highlights

Abstract

Introduction

Section snippets

Experiments and solar cell structures

Deposition and characteristics of SiOx films

Conclusions

Acknowledgments

Remarkable progress in thin-film silicon solar cells using high-efficiency triple-junction technology

Sol. Energy Mater. Sol. Cells

Silicon filaments in silicon oxide for next-generation photovoltaics

Adv. Mater.

A high efficiency thin film solar cell and module

Sol. Energy

Reversible conductivity changes in discharge-produced amorphous Si

Appl. Phys. Lett.

New progress in the fabrication of n–i–p micromorph solar cells for opaque substrates

Sol. Energy Mater. Sol. Cells

LPCVD ZnO-based intermediate reflector for micromorph tandem solar cells

Sol. Energy Mater. Sol. Cells

Optical management in high-efficiency thin-film silicon micomorph solar cells with a silicon oxide based intermediate reflector

Phys. Status Solidi RRL

In situ silicon oxide based intermediate reflector for thin-film silicon micromorph solar cells

Appl. Phys. Lett.

n-Type hydrogenated amorphous silicon oxide containing a microcrystalline silicon phase as an intermediate reflector in silicon thin film solar cells

Phys. Status Solidi C

Improved micromorph solar cells by means of mixed-phase n-doped silicon oxide layers

Prog. Photovoltaics Res. Appl.

Deposition and characteristics of SiO_x films