Hydrogenated Nanocrystalline Silicon Research Articles

Development of high conducting phosphorous doped nanocrystalline thin silicon films for silicon heterojunction solar cells application

We have investigated the plasma-enhanced chemical vapor deposition growth of the phosphorus-doped hydrogenated nanocrystalline silicon (n-nc-Si:H) film as an electron-selective layer in silicon heterojunction (SHJ) solar cells. The effect of power densities on the precursor gas dissociation are investigated using optical emission spectra and the crystalline fraction in n-nc-Si:H films are correlated with the dark conductivity. With the P d of 122 mW cm−2 and ∼2% phosphorus doping, we observed Raman crystallinity of 53%, high dark conductivity of 43 S cm−1, and activation energy of ∼23 meV from the ∼30 nm n-nc-Si:H film. The n-nc-Si:H layer improves the textured c-Si surface passivation by two-fold to ∼2 ms compared to the phosphorus-doped hydrogenated amorphous silicon (n-a-Si:H) layers. An enhancement in the open-circuit voltage and external quantum efficiency (from >650 nm) due to the better passivation at the rear side of the cell after integrating the n-nc-Si:H layer compared to its n-a-Si:H counterpart. An improvement in the charge carrier transport is also observed with an increase in fill factor from ∼71% to ∼75%, mainly due to a reduction in electron-selective contact resistivity from ∼271 to ∼61 mΩ-cm2. Finally, with the relatively better c-Si surface passivation and carrier selectivity, a power conversion efficiency of ∼19.90% and pseudo-efficiency of ∼21.90% have been realized from the SHJ cells.

Synergetic Effect of Gas Compositions on Material Properties of n‐Type Nanocrystalline Silicon Oxide Prepared by Plasma‐Enhanced Chemical Vapor Deposition

Hydrogenated nanocrystalline silicon oxide (nc‐SiOx:H) has drawn extensive attention during the past years for the utilization of silicon heterojunction (SHJ) solar cells. This material is demonstrated to be the key to the performance improvement of SHJ solar cells. Herein, the influence of the process gas compositions on the optical, electronic, chemical, and structural properties of n‐type nc‐SiOx:H films at the thickness of 38 nm is systematically investigated. The synergetic effect of CO2 and PH3 gas flow fraction ( and ) and the synergetic effect of CO2 and SiH4 gas flow fraction ( and ) on the growth of nc‐SiOx:H (n) films are demonstrated. Creatively combining the Fourier‐transform infrared, UV‐Raman scattering spectroscopy, spectroscopy ellipsometry, and photo‐thermal deflection spectroscopy measurement, the mechanism behind the synergetic effect of the gas compositions on the material properties is analyzed.

Fabrication of highly conductive phosphorous-doped nc-SiCx:H thin film on PET

Plasma enhanced chemical vapor deposition (PECVD) method has been utilized to fabricate phosphorous-doped hydrogenated nanocrystalline silicon carbide (P-doped nc-SiCx:H) thin films on polyethylene terephthalate (PET) substrate. With the aim at obtaining highly conductive thin films, H2/Ar mixed dilution has been applied for creating plasma during deposition, and the variation of structural, electrical and optical properties with H2/Ar flow ratio RH have been systemically investigated through a series of characterizations. Results show that the highly crystallized P-doped nc-SiCx:H thin film can be prepared while the properties are controllable through adjusting RH. In the case of RH = 0.75, the maximum dark conductivity (6.42 S cm−1) and a wide optical bandgap (1.93 eV) are attained. Finally, detail discussion has been made to illustrate the growth mechanism of the flexible P-doped nc-SiCx:H thin films.

Highly conductive p-type nc-SiOX:H thin films deposited at 130°C via efficient incorporation of plasma synthesized silicon nanocrystals and their application in SHJ solar cells

We present highly conductive and transparent p-type hydrogenated nanocrystalline silicon oxide (p-type nc-SiOX:H) layers produced by Plasma Enhanced Chemical Vapor Deposition (PECVD) at 130°C and 150°C. We report on the crystalline volume fraction (XC), spectral broadening parameter C, and dark conductivity (σ) as functions of the growth temperature and RF power, and how these properties evolve with post-deposition annealing at 250°C and 300°C. Interestingly, we observe that the best layers in terms of crystalline volume fraction and conductivity are obtained at the lowest temperature and RF power, which we attribute to the soft landing of silicon nanocrystals synthesized in the plasma. The p-type nc-SiOX:H layers with the best properties on glass substrates are implemented as carrier-selective contacts in silicon heterojunction (SHJ) solar cells with the structure: (n) a-Si:H / (i) a-Si:H / n-type c-Si / (i) a-Si:H / (i) a-SiOX:H / (p) nc-SiOX:H / (p) nc-Si:H where all films are deposited by PECVD. The cells were completed with sputtered ITO on the front and rear sides plus Ag on the rear side, and Ag grid on the front, with the best devices showing conversion efficiencies of 21.8%, which, contrary to a-Si:H contact layers, are preserved or even slightly improved upon annealing at 240°C.

Improved interface microstructure between crystalline silicon and nanocrystalline silicon oxide window layer of silicon heterojunction solar cells

N-type hydrogenated nanocrystalline silicon oxide (nc-SiOx:H) is potential to enhance the performance of silicon heterojunction solar cells, but the raised plasma damage on underlying layer during the nc-SiOx:H deposition with a high-volume fraction of hydrogen is a burning issue. The underlying intrinsic hydrogenated amorphous silicon (i-a-Si:H) bilayer between n-type crystalline silicon (c-Si) and n-type nc-SiOx:H has been investigated by modulating silane (SiH4) gas flow rate (GFR) of interface porous layer. It has been found that the initial H-rich i-a-Si:H bilayer deposited by interfacial 1600 sccm GFR with relatively stable larger voids diameter and less voids number density can not only withstand hydrogen ions bombardment but also passivate c-Si surface well. Meanwhile, it also has been verified that the optimal n-seed deposition can further enhance both total hydrogen content and hydrogen content in compact structure to promote c-Si surface passivation and carrier transportation. The optimized SiH4 GFR for the interfacial i-a-Si:H growth and the appropriate deposition time of n-seed layer have been applied into the front passivation layer of silicon heterojunction solar cells, thus a high efficiency of approximate 25 % with high VOC and FF has been achieved.

Growth, structural and vibrational properties of hydrogenated nanocrystalline silicon thin films prepared by radiofrequency magnetron sputtering technique at room temperature

The main objective of this paper is to produce the hydrogen-free and hydrogenated nanocrystalline silicon (nc-Si:H(x)) thin films deposited onto the single-crystalline Si(100) wafer by radio-frequency magnetron sputtering (RF-MS) at a substrate temperature of ∼ 300 K. Sputtering was effectuated by a microwave plasma in a mixture of argon and hydrogen (Ar + (H2(x)) gasses taken at different proportions of hydrogen dilution: x = 0, 2.5, and 5 sccm (standard cubic centimeter per minute). The deposition rates of the nc-Si:H were fully controlled, and the thickness of these films was ∼ 200 and 300 Å. Pt-capping layer was deposited in order to protect the nc-Si:H layer from oxidation. The thickness of the capping layer was fixed at ∼ 5 Å.The effects of the hydrogen flow rate and post-growth thermal treatment on the crystalline structure and morphology of as-grown nanocrystalline silicon thin films were systematically investigated by ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS), atomic force microscopy (AFM), grazing incidence x-ray diffraction (GI-XRD) and micro-Raman spectroscopy methods. From the measured data, the average nanocrystallite sizes, surface morphology and other important characteristics of these thin films were analyzed. The grown nc-Si:H thin films may be used as a semiconducting electrode material for electrochemical proton production, which is of great technological importance, especially for solid-state electrochromic device applications.

Achievement of 25.54% power conversion efficiency by optimization of current losses at the front side of silicon heterojunction solar cells

AbstractParasitic absorption in the front window layers of transparent conductive oxide (TCO) films and carrier selective collection layers and the optical shading losses from the metallic finger grid mainly limit the current generation in silicon heterojunction (SHJ) solar cells. In this work, we demonstrate an improved short‐circuit current density (Jsc) of 40.24 mA/cm2 through a combination of novel window layers composed of transition metal doped indium oxide (IMO) and hydrogenated nanocrystalline silicon oxide (nc‐SiOx:H) films and Cu plating for SHJ solar cells. By introducing water vapor during direct current (DC) magnetron sputtering deposition process, IMO films show a large optical band gap (Eg) of about 3.88 eV and high mobility up to 83.2 cm2/V·s, while maintaining a low carrier concentration, which leads to high transparency and low near‐infrared (NIR) free carrier absorption (FCA). In addition to its high deposition rate and crystalline volume fraction, we found that nc‐SiOx:H films deposited by very high frequency (VHF) excited plasma‐enhanced chemical vapor deposition (PECVD) show an excellent surface passivation quality, which not only improves the open circuit voltage (Voc) of SHJ cells but also increases the Jsc through improved carrier selective collection. The quantified Jsc breakdown analysis was performed to identify the room for improvement, and it showed that the front shading loss (about 1.32 mA/cm2) is the largest portion. By combining the benefits of these window layer enhancements with the further use of fine line width and conductivity of Cu plating, SHJ solar cells, with a Jsc improvement of 0.57 mA/cm2 and a certified efficiency of 25.54%, were achieved on a total area of 274.5 cm2 using in‐house pilot production line equipment.

(Invited, Digital Presentation) Advanced Passivating Contacts for Silicon Photovoltaics

Carrier selective passivating contacts, which provide dual functions of surface defect passivation and extracting one type of charge carriers, is a key enabler for high efficiency Si solar cells. Here we introduce two passivating contact materials developed in our lab: one is the hydrogenated nanocrystalline silicon (nc-Si:H) and the other is the metal-oxide (non-silicon) material. For the nc-Si:H, it is demonstrated that the use of the (p)nc-Si:H/(i)a-Si:H stack instead of the conventional (p)a-Si:H/(i)a-Si:H in silicon heterojunction solar cells provides not only a mitigation of the parasitic absorption loss but also improvements in passivation and electrical contact properties. This results in an efficiency increase by 0.3-0.6% absolute compared to our reference cell, and a best cell efficiency of 23.5%. For metal-oxide material, we have developed an atomic-layer-deposited titanium oxide (TiOx) that functions as an efficient and transparent hole-selective passivating contact without using buffer layer, leading to the proof-of-concept solar cells with efficiencies above 20%. An example of applying the developed contact material as a recombination junction layer of the perovskite/Si tandem solar cell will be presented.

Transparent Conductive Oxide Sputtering Damage on Contact Passivation in Silicon Heterojunction Solar Cells with Hydrogenated Nanocrystalline Silicon

Mitigating the adverse effect of transparent conductive oxide (TCO) sputtering on the passivation quality of the heterojunction contact is very critical for achieving high‐efficiency silicon heterojunction (SHJ) solar cells. Herein, n‐type ultra‐thin (5 nm) nanocrystalline silicon (nc‐Si:H) is utilized as a contact layer in rear‐junction SHJ solar cells. It is revealed that the TCO sputtering damage on the contact layer is caused by the ion bombardment during the sputtering process and cannot be fully recovered by low‐temperature annealing. A more severe passivation deterioration is observed for the devices with nc‐Si:H(n) having a higher microstructure factor. This result is explained by an increased ion penetration depth in the porous nc‐Si:H film. By applying denser nc‐Si:H(n) in SHJ solar cells, the front silicon film stack shows more resilience to sputter damage, yielding a remarkable cell performance on the M2‐size wafer with certificated power conversion efficiency (η) of 23.87%. The sputter damage on nc‐Si:H by TCO in SHJ solar cells is explored in depth for the first time from the perspective of silicon thin films.

In Situ Grown Nanocrystalline Si Recombination Junction Layers for Efficient Perovskite-Si Monolithic Tandem Solar Cells: Toward a Simpler Multijunction Architecture.

The perovskite-Si tandem is an attractive avenue to attain greater power conversion efficiency (PCE) than their respective single-junction solar cells. However, such devices generally employ complex stacks with numerous deposition steps, which are rather unattractive from an industrial perspective. Here, we develop a simplified tandem architecture consisting of a perovskite n-i-p stack on a silicon heterojunction structure without applying the typically used indium-tin-oxide (ITO) recombination junction (RJ) layer between the top and bottom cells. It is demonstrated that an n-type hydrogenated nanocrystalline silicon (nc-Si:H) grown in situ on an amorphous silicon hole contact layer of the bottom cell acts as an efficient RJ layer, leading to a high open-circuit voltage (VOC) of >1.8 V and a PCE of 21.4% without optimizing the optical design. Compared to the tandem cell with an ITO RJ layer, the nc-Si:H RJ layer not only improves light management but also achieves a higher VOC due to superior contact properties with an overlying SnO2 electron transport layer of the perovskite top cell. Omitting the costly material and its deposition step offers the opportunity toward realizing industrially feasible high-efficiency tandem solar cells.

Hydrogen effect on structural and optical properties of in-situ fabricated nc-Si:H prepared by facing targets sputtering

We report high-quality hydrogenated nanocrystalline silicon (nc-Si:H) thin films were prepared by facing targets sputtering (FTS) in the environment of Ar and H2 mixture gases. By increasing hydrogen dilution (RH) of FTS deposition, the structural evolution of as-fabricated Si-based films was observed. Fourier infrared spectroscopy (FTIR) shows that decrease in bonded hydrogen (CH) concentration, especially that of (Si-H2)n with increasing RH, indicating that the H etching effect increased atomic density of these films, thus improving film quality. With the RH increased, the Raman spectra show that the morphological transition from amorphous to crystalline states. We further reveal correlations between the degree of structural order in the amorphous network and the RH by calculating experimental data from the Raman measurements. Moreover, as-fabricated nc-Si:H films show large bandgap and narrow band tail states with increasing RH, which allows potential applications in the window passivation layer for HIT solar cells and Si-based charge-coupled devices (CCD). This work offers a novel route to fabricate high-quality nc-Si:H films for optoelectronic applications.

Effect of boron doping on structural, optical, and electrical properties of hydrogenated nanocrystalline silicon thin films grown by Cat-CVD method

Effect of boron doping on structural, optical, and electrical properties of hydrogenated nanocrystalline silicon thin films grown by Cat-CVD method

N-type nc-SiOx:H film enables efficient and stable silicon heterojunction solar cells in sodium environment

Hydrogenated nanocrystalline silicon oxide (nc-SiOx:H) is promising to replace hydrogenated amorphous silicon (a-Si:H) as the window layer of silicon heterojunction (SHJ) solar cells due to its lower parasitic absorption at short wavelength. In this work, by varying the growth conditions of nc-SiOx:H, we prepared highly conductive, highly transparent phosphorus-doped nc-SiOx:H film, and successfully applied as the window layer of SHJ cells. The short-circuit current density gains 0.5 mA/cm2, together with an improvement of power conversion efficiency by 0.3%, whose average value reaches 24.09% on 6-inch total-area wafers. More importantly, we find these devices show better stability in sodium environment at 85 °C and 85% relative humidity.

Ultra‐thin electron collectors based on nc‐Si:H for high‐efficiency silicon heterojunction solar cells

AbstractLow parasitic absorption and high conductivity enable (n)‐type hydrogenated nanocrystalline silicon [(n)nc‐Si:H], eventually alloyed with oxygen [(n)nc‐SiOx:H], to be deployed as window layer in high‐efficiency silicon heterojunction (SHJ) solar cells. Besides the appropriate opto‐electrical properties of these nanocrystalline films, reduction of their thickness is sought for minimizing parasitic absorption losses. Many strategies proposed so far reveal practical limits of the minimum (n)‐layer thickness that we address and overcome in this manuscript. We demonstrated the successful application of an ultra‐thin layer of only 3‐nm‐thick based on (n)nc‐Si:H PECVD plasma growth conditions without the use of additional contact or buffer layers. For simplicity, we still name (n)nc‐Si:H this ultra‐thin layer and the solar cell endowed with it delivers a certified efficiency η of 22.20%. This cell shows a 0.61 mA/cm2 overall JSC gain over the (n)a‐Si:H counterpart mainly owing to the higher transparency of (n)nc‐Si:H, while maintaining comparable VOC > 714 mV and FF > 80%. Our optimized (n)nc‐Si:H layer yields low absorption losses that are commonly measured for (n)nc‐SiOx:H films. In this way, we are able to avoid the detrimental effect that oxygen incorporation has on the electrical parameters of these functional layers. Further, by applying a MgF2/ITO double‐layer anti‐reflection coating, a cell with 3‐nm‐thick (n)nc‐Si:H exhibits a JSC,EQE up to 40.0 mA/cm2. By means of EDX elemental mapping, we additionally identified the (n)nc‐Si:H/ITO interface as critical for electron transport due to unexpected oxidation. To avoid this interfacial oxidation, insertion of a 2‐nm‐thick (n)a‐Si:H on the 3‐nm‐thick (n)nc‐Si:H contributes to FF gains of 1.4%abs. (FF increased from 78.6% to 80.0%), and showing further room for improvements.

Very Thin (56 μm) Silicon Heterojunction Solar Cells with an Efficiency of 23.3% and an Open‐Circuit Voltage of 754 mV

The silicon heterojunction (SHJ) is a crystalline silicon (c‐Si) solar cell device architecture capable of achieving high efficiencies due to excellent surface passivation realized by hydrogenated amorphous silicon (a‐Si:H) thin layers. The SHJ architecture also allows to process thinner Si wafers due to the structural symmetry and the low processing temperature. Herein, an attempt to increase the performance of very thin c‐Si solar cells is made, using an advanced SHJ architecture. By replacing the conventional a‐Si:H by hydrogenated nanocrystalline silicon (nc‐Si:H) in the hole contact layer, an increase in all solar cell parameters over a wide range of wafer thicknesses from 50 to 400 μm is observed. This also provides an open‐circuit voltage (V OC) of 754 mV with the very thin absorber, which is the highest V OC independently confirmed under standard test conditions (STCs) among any c‐Si solar cells ever reported. As a result, a notable efficiency of 23.27% is realized in a SHJ cell with an average thickness of only 56.2 μm. These results demonstrate the high potential of very thin c‐Si solar cells that may open various opportunities for flexible and lightweight c‐Si modules, as well as reducing the carbon footprint of solar cell manufacture.

Effects of Chamber Pressures on the Passivation Layer of Hydrogenated Nano-Crystalline Silicon Mixed-Phase Thin Film by Using Microwave Annealing

This paper proposes the effects of chamber pressures on the passivation layer of hydrogenated nano-crystalline silicon (nc-Si:H) mixed-phase thin film using microwave annealing (MWA) to achieve a high-quality thin film. The use of 40.68 MHz very-high-frequency plasma-enhanced chemical vapor deposition (VHFPECVD) deposited the nc-Si:H mixed-phase thin film on the top and bottom of the n-type crystalline silicon substrate. The chamber pressures (0.2, 0.4, 0.6, and 0.8 Torr) of the VHFPECVD were critical factors in controlling the carrier lifetime of the symmetric structure. By using the VHFPECVD to deposit the nc-Si:H and using the MWA to enhance the quality of the symmetric structure, the deposited nc-Si:H’s properties of a crystalline volume fraction of 29.6%, an optical bandgap of 1.744 eV, and a carrier lifetime of 2942.36 μs were well achieved, and could be valuable in thin-film solar-cell applications.

Three Generations of Solar Cells

In photovoltaic system the major challenge is the cost reduction of the solar cell module to compete with those of conventional energy sources. Evolution of solar photovoltaic comprises of several generations through the last sixty years. The first generation solar cells were based on single crystal silicon and bulk polycrystalline Si wafers. The single crystal silicon solar cell has high material cost and the fabrication also requires very high energy. The second generation solar cells were based on thin film fabrication technology. Due to low temperature manufacturing process and less material requirement, remarkable cost reduction was achieved in these solar cells. Among all the thin film technologies amorphous silicon thin film solar cell is in most advanced stage of development and is commercially available. However, an inherent problem of light induced degradation in amorphous silicon hinders the higher efficiency in this kind of cell. The third generation silicon solar cells are based on nano-crystalline and nano-porous materials. Hydrogenated nanocrystalline silicon (nc-Si:H) is becoming a promising material as an absorber layer of solar cell due to its high stability with high Voc. It is also suggested that the cause of high stability and less degradation of certain nc-Si:H films may be due to the improvement of medium range order (MRO) of the films. During the last ten years, organic, polymer, dye sensitized and perovskites materials are also attract much attention of the photovoltaic researchers as the low budget next generation PV material worldwide. Although most important challenge for those organic solar cells in practical applications is the stability issue. In this work nc-Si:H films are successfully deposited at a high deposition rate using a high pressure and a high power by Radio Frequency Plasma Enhanced Chemical Vapor Deposition (RF PECVD) technique. The transmission electron microscopy (TEM) studies show the formations of distinct nano-sized grains in the amorphous tissue with sharp crystalline orientations. Light induced degradation of photoconductivity of nc-Si:H materials have been studied. Single junction solar cells and solar module were successfully fabricated using nanocrystalline silicon as absorber layer. The optimum cell is 7.1 % efficient initially. Improvement in efficiency can be achieved by optimizing the doped layer/interface and using Ag back contact.

Improvement in a-Si:H silicon solar cells with using double p-type window layers based on nanocrystalline silicon oxide

In this study, our objective was to analyze by simulation the effect of using two different double p-type layers, one p-type based on hydrogenated nanocrystalline silicon (nc-Si:H) and the other p-type based on hydrogenated nanocrystalline silicon oxide (nc-SiOx:H) on the performances of n-i-p a-Si:H based solar cell. The AMPS-1D computer code is used to realize the simulation. Two devices were simulated and compared, the first structure device is:metal/n-a-Si:H/i-a-Si:H/p-nc-SiOx:H/p+nc-SiOx:H/ITO and the second structure device is:metal/n-a-Si:H/i-a-Si:H/p-nc-Si:H/p+nc-Si:H/ITO. An improvements in the JSC in the case of structure with p nc-SiOx:H layers is mainly due to the good absorption in the short wavelength range which is causes by the presence of oxygen in the high band gap p-nc-SiOx:H layer and which makes it more transparent. This good light absorption leads to a high photocarriers generation which combined with a high doping concentration to extract holes from the active layer. The presence of the p-nc-SiOx:H buffer layer at the i/p-window interface reduces the recombination at this interface and improves the VOC. For the optimized window layer parameters values: NDG = NAG = 1017 cm−3, L = 15 nm, NA = 5 × 1019 cm−3 and with NA = 1018 cm−3 for the buffer layer, the best performances were obtained in the case of solar cell with double p-nc-SiOx:H type window layers (JSC = 13.80 mA/cm2, VOC = 934 mV, FF = 79.1% and Eff = 10.21%).

Influence of the Dopant Gas Precursor in P-Type Nanocrystalline Silicon Layers on the Performance of Front Junction Heterojunction Solar Cells

Silicon heterojunction solar cells can employ p-type hydrogenated nanocrystalline silicon nc-Si:H(p) on their front side, since these can provide better transparency and contact resistance compared to hydrogenated p-type amorphous silicon layers. We investigate here the influence of trimethyl boron (TMB) and BF3 as dopant source on the layer properties and its performance in solar cells. Both gases enable high efficiencies but yield a different crystallinity and effective doping. A high BF3 flow lowers the series resistance through a low activation energy of dark lateral conductivity and maintains a high crystallinity. This allows fill factors up to 83%, however with the apparition of a parasitic absorption in the UV. A low TMB flow enables simultaneously a high crystallinity and a low activation energy. As an illustration of this layer potential, a 23.9%-certified efficiency is achieved with a 2 × 2 cm2 screen-printed device. We finally suggest that similar transport versus transparency trade-offs can be reached for both dopant types for front junction application, while high BF3 flow allowing lower series resistance might be of interest when placed on the rear side.

Performances analysis of heterojunction solar cells through integration of hydrogenated nanocrystalline silicon bilayer by using numerical study

This study was conducted to simulate a pin-type thin film solar cell by integrating nc-Si:H as p-window and buffer layers. The structures proposed to investigated are ITO/(p)nc-Si:H/((i)a-Si:H/(n)a-Si:H/Ag and ITO/(p)nc-Si:H/(p')nc-Si:H(buff)/(i)a-Si:H/(n)a-Si:H/Ag simulated with the AFORS-HET simulator. In an effort to improve the electrical and optical properties of the heterojunction solar cell, the dopant concentration for the p-window and n-layers, the absorber bandgap, and the absorber thickness were optimized. The result showed that the Eff of p-p’-i-n is 9.60% (VOC = 936.6 mV, JSC = 13.86 mA/cm2, FF = 0.738) were obtained when values of Na, Nd, absorber bandgap, and absorber thickness parameters are 1.0 x 1017 particles/cm3, 1.0 x 1019 particles/cm3, 1.80 eV, and 600 nm, respectively.