fabrication and characterization of al2o3 si composite nanodome structures for high efficiency crystalline si thin film solar cells

Fabrication and characterization of Al2O3 /Si composite nanodome structures for high efficiency crystalline Si thin film solar cells , Ruiying Zhang , Jian Zhu, Zhen Zhang, Yanyan Wang, Bocang Qiu, Xuehua Liu, Jinping Zhang, Yi Zhang, Qi Fang, Zhong Ren, and Yu Bai Citation: AIP Advances 5, 127209 (2015); doi: 10.1063/1.4937744 View online: http://dx.doi.org/10.1063/1.4937744 View Table of Contents: http://aip.scitation.org/toc/adv/5/12 Published by the American Institute of Physics AIP ADVANCES 5, 127209 (2015) Fabrication and characterization of Al2O3/Si composite nanodome structures for high efficiency crystalline Si thin film solar cells Ruiying Zhang,1,2,a Jian Zhu,1 Zhen Zhang,1 Yanyan Wang,1 Bocang Qiu,1 Xuehua Liu,3 Jinping Zhang,3 Yi Zhang,3 Qi Fang,4 Zhong Ren,4 and Yu Bai5 Key lab of nanodevices and applications, Chinese Academy of Sciences, Division of nano-devices and related materials, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou, 215123, China State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050 China Platform for Characterization & Test, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou, 215123, China Oxford Instruments Plasma Technology, Yatton, Bristol, BS49 4AP, UK School of Nano-Science and Nano-Engineering, Xi’an Jiaotong University, Suzhou, 215123, China (Received 18 September 2015; accepted 30 November 2015; published online December 2015) We report on our fabrication and characterization of Al2O3/Si composite nanodome (CND) structures, which is composed of Si nanodome structures with a conformal cladding Al2O3 layer to evaluate its optical and electrical performance when it is applied to thin film solar cells It has been observed that by application of Al2O3 thin film coating using atomic layer deposition (ALD) to the Si nanodome structures, both optical and electrical performances are greatly improved The reflectivity of less than 3% over the wavelength range of from 200 nm to 2000 nm at an incident angle from 0◦ to 45◦ is achieved when the Al2O3 film is 90 nm thick The ultimate efficiency of around 27% is obtained on the CND textured µm-thick Si solar cells, which is compared to the efficiency of around 25.75% and 15% for the µm-thick Si nanodome surface-decorated and planar samples respectively Electrical characterization was made by using CND-decorated MOS devices to measure device’s leakage current and capacitance dispersion It is found the electrical performance is sensitive to the thickness of the Al2O3 film, and the performance is remarkably improved when the dielectric layer thickness is 90 nm thick The leakage current, which is less than 4x10−9 A/cm2 over voltage range of from -3 V to V, is reduced by several orders of magnitude C-V measurements also shows as small as 0.3% of variation in the capacitance over the frequency range from 10 kHz to 500 kHz, which is a strong indication of surface states being fully passivated TEM examination of CND-decorated samples also reveals the occurrence of SiOx layer formed between the interface of Si and the Al2O3 film, which is thin enough that ensures the presence of field-effect passivation, From our theoretical and experimental study, we believe Al2O3 coated CND structures is a truly viable approach to achieving higher device efficiency C 2015 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4937744] I INTRODUCTION Although solar cells based on bulk crystalline silicon have been well developed and dominated current photovoltaic markets, much effort has to be made to improve their cost-effeteness in order for a Corresponding author: ryzhang2008@sinano.ac.cn 2158-3226/2015/5(12)/127209/10 5, 127209-1 © Author(s) 2015 127209-2 Zhang et al AIP Advances 5, 127209 (2015) silicon-based solar cells have much wider adoption Thin film Si solar cells are increasingly becoming attractive because of their low manufacturing cost, abundance in raw material supply, and being environment friendly.1–3 However, there are two issues which impact their energy conversion efficiency The first one is the insufficient absorption to the incident light due to the thickness of the active layer is only several microns and conventional pyramid structures are not longer suitable as an antireflectional surface The second one is the strong loss of photon-generated-carriers owing to the fact that the surface non-radiative recombination becomes more severe because of surface-to-volume ratio increases with the decrease in the layer thickness Therefore, it is of critical importance to exercise improvement on both optical and electrical performance by maximizing light absorption and in the mean time minimizing the surface recombination in order for one to achieve high efficiency in thin film solar cells Recently, absorption improvement in c-Si thin film solar cells by means of photonic management (PM) engineering, such as photonic crystals,4,5 nano-wires (nano-rods),6,7 nanopores (nanoholes),8,9 nanocones (nanodomes),10–15 and inverted nano-pyramids16 has been theoretically studied However, all these light-trapping structures inevitably lead to significant increase in the device surface area and introduce surface defects, resulting in elevated carrier loss Consequently, reduction of photo-current loss becomes a critical issue for nano-textured thin film solar cells On the other hand, it has been demonstrated that Al2O3 layer deposited by atomic layer deposition (ALD) technique17–19 is able to passivate many nanostructure surfaces, because of its three inherent advantages: (1) a conformal passivation layer formed by its self-limiting growth process,20 (2) an excellent chemical passivation due to its strong coordination of Si and oxygen (O) atoms at the interface21,22 and selective hydrogenation,23 (3) an excellent field passivation effect due to its capacity to achieve extremely high concentration of fixed negative charges at the interface.24,25 Thus far, this technique has been applied to black silicon solar cells,26 radial junction sub-micron pillar array solar cells27 and µ-Si:H /Si radiation junction nanowire array solar cells28 with promising passivation effect However, the effect of Al2O3 coating on the optical performance has not yet been fully evaluated In this paper, encouraged by the promising passivation effect, we fabricated Al2O3-coated SiCND structures to evaluate both electrical and optical effects on the thin film solar cells To the best of our knowledge, this is the first time demonstration of the application of Al2O3 coating using ALD technique to the Si-CND structures Reflection suppression, which is only weakly dependent on the thickness of the Al2O3 film, was experimentally observed in Al2O3/Si CND structure-decorated Si substrates, and absorption enhancement was also predicted by our theoretical study on Al2O3/Si CND decorated µm thick Si solar cells Meanwhile, it is evident that the surface electrical performance of these CND decorated samples is highly sensitive to Al2O3 film thickness, as it was confirmed by our C-V (capacity-voltage) and J-V (current density-voltage) analysis performed on Al2O3/SiCND-incorporated metal-oxide-semiconductor (MOS) devices The best result in terms of electrical performance was obtained for devices with thickness of 90 nm for the Al2O3 film Our theoretical analysis and experimental work indicate that application of thin film Al2O3 layer to Si-CND solar cells can lead to strong improvement in the device’s electrical performance, and in the meantime, the optical performance is also improved II EXPERIMENT AND SIMULATIONS A Device Fabrication Figure schematically shows the fabrication process of Al2O3/Si CND structures First, monodisperse polystyrene (PS) nanospheres with a diameter of 600 nm were hexagonally close-packed on a cleaned p-type single crystalline Si substrate by spin-coating as shown in figure 1(a) Then, the PS nanospheres were shrunk by O2 plasma to achieve desired mask pattern (figure 1(b)) Subsequently, Si nanodome structures (figure 1(c)) were transferred onto the Si substrate using inductively coupled plasma (ICP) etching Furthermore, PS residue was removed using THF solution, and clean Si nanodome structures were finally obtained as shown in figure 1(d) To avoid surface oxidation, the nanodome structures were further treated using 1% HF solution and DI water immediately before the deposition of the Al2O3 film The deposition of the Al2O3 film was performed in a thermal ALD facility 127209-3 Zhang et al AIP Advances 5, 127209 (2015) FIG Fabrication procedure of Al2O3/Si composite nanodome structures through batch process with the precursors of trimethyalumnum (TMAl) and water at 200 ◦C The pulse time and N2 purge time were chosen to be 15 ms and s for both TMAl and H2O respectively To further improve their electrical performance, post-annealing was performed in N2 ambient environment for 30 at 400 ◦C Finally, the Al2O3/Si CND structures were formed on the Si (figure 1(e)) During deposition process, Al2O3 layer with the identical thickness was also deposited on a separate planar Si surface The planar Si sample was used to measure the optical parameters of Al2O3 for simulations and to compare the electrical performance in the experiments To evaluate the electrical properties of Al2O3/Si CND structures, MOS devices with incorporation of the identical nanostructures were also fabricated The configuration of the MOS devices is AlAl2O3/Si CND -Si-Al, with the size being 200 àmì200 àm B Characterization The etched profile and surface morphology of the Al2O3/Si CND structures were characterized using scanning electron microscopy before and after Al2O3 deposition Interface morphology of the Al2O3/Si CND structures was characterized using high-resolution transmission electron microscopy (HR-TEM) The TEM examinations were carried out using a transmission electron microscope operated at 120 kV, at which electron beam damage induced by TEM operation was minimized to an undetectable level The refractive indices and extinction coefficients of Si and Al2O3 materials were measured using a spectroscopic ellipsometer and the reflectivity of all samples was measured using a spectrophotometer The J-V curves were tested at room temperature using a Keithley 4200-SCS parameters analyzer and the C-V curves were tested at room temperature using an Agilent E4980A Precision LCR Meter C Simulation and calculation To investigate the effect of the CND structures on the optical performance, Si nanodomes and Al2O3/Si CNDs incorporated 2-µm-thick solar cells were simulated by rigorous-coupled-wave-analysis (RCWA) method, which is an exact solution of Maxwell’s equations for the electromagnetic diffraction by the period structures and is widely used for analysis and design of diffractive structures.29 The simulation models are shown in figure The parameters of Si nanodome decorated solar cells, including its diameter, height, and duty cycle were extracted from the SEM images of processed structures The total height of Si material in all samples was µm The nanodomes with a period of 600 nm were hexagonally distributed on a planar Si substrate In addition, Al2O3/Si CND structures as shown in figure 2(a)-2(c) were used in this simulation The optical parameters of Si and Al2O3, as shown in figure 2(d), were obtained from the above measurement In order to simplify the simulation procedure, only specular reflectance was considered Moreover, a perfect-matched layer was selected as the back interface in the reflection simulation, which helps us to compare the measured and 127209-4 Zhang et al AIP Advances 5, 127209 (2015) FIG Simulation models of (a) X-Y plane view, (b) Cross-section view, (c) overlook view of Si and Al2O3/Si CND structure (d) The optical parameters of Si and Al2O3 simulated results The air was selected as the back interface in the absorption simulation, which results in back reflection was included in the absorption simulation Such boundary conditions enable us to correctly evaluate the effect of Al2O3/Si CND structures on their reflection and absorption Based on the above model, parameters and boundary conditions, the reflection and absorption spectra of Al2O3/Si CND structures with Al2O3 thickness of nm, 30 nm, 60 nm, and 90 nm were simulated Then, the weighted reflectance and the ultimate efficiency of the solar cells described above were calculated according to following equations  λg R(λ) f (λ)dλ RA =  ∞ (1) f (λ)dλ λg η= f (λ)A(λ)(λ/λg)dλ ∞ (2) f (λ)dλ Where, RA is the weighted reflectance, λ is the incident wavelength, R(λ) is the reflectivity at the incident wavelength λ, f(λ) is the incident photon-flux spectral density of AM1.5D η is the ultimate efficiency, λg is the wavelength corresponding to the bandgap of Si material, A(λ) is the normalized absorption intensity for the solar cells mentioned above III THEORETICAL AND EXPERIMENTAL RESULTS A Antireflection SEM images of the fabricated Si nanodome and Al2O3/Si CND structures are shown in figure In figure 3(a), Si nanodome structures with an aspect ratio of less than 1.1 and a duty cycle of 75% 127209-5 Zhang et al AIP Advances 5, 127209 (2015) FIG SEM cross-section images of (a) Si nanodome, (b) Al2O3 (90 nm) /Si CND structure at 40K magnification, (c) Al2O3 (30 nm) /Si, (d) Al2O3 (60 nm) /Si and (e) Al2O3 (90 nm) /Si CND structure at 130K magnification are distributed on Si surface The advantage of such morphology with low aspect ratio is that it can avoid the significant increase of non-radiative recombination centers arisen from the increase in the surface area ALD deposition ensures that Al2O3 film covers every Si nanodome completely and forms composite structures as shown in figure 3(b)-3(e) In addition, the gradually increased duty cycle of the composite structures with Al2O3 thickness was observed in these images as shown in figure 3(a)-3(e) Such morphology evolution could influence the optical and electrical performance of these CND textured devices Figure 4(a) and figure 4(b) show the reflection spectra of the Al2O3/Si CND decorated samples measured at ◦ and 45 ◦ respectively, and figure 4(c) shows the weighted reflectance over 300 nm-1000 nm calculated from the measured and simulated reflection spectra Compared with Si nanodome decorated Si substrate, further antireflection is observed in all Al2O3/Si CND decorated Si substrates over broadband wavelength range and wide view as shown in Fig 4(a) and Fig 4(b), which is due to the fact that the refractive index of Al2O3 is in-between that of Si nanodome and the air, and then further graded index is formed in such Al2O3/Si CND structures Moreover, Fig 4(a) and Fig 4(b) also show that the reflectance of Al2O3/Si CND decorated Si substrate varies with the thickness of Al2O3 film, which is the combination effects arisen from both the contributions of high order diffraction induced by Si nanodome and the interference of Si nanodome and Al2O3 film In addition, Fig 4(c) shows that the weighted reflectance for Al2O3/Si CNDs gradually decreases with the increase in the Al2O3 film thickness This is quite different from that mentioned in Ref 25, but the results benefit the performance of Al2O3/Si CNDs decorated Si thin film solar cells Fig 4(c) also shows that the reflectance reduced to around 1% for samples with 90 nm thick Al2O3 coating from around 5.5% for samples without Al2O3 coating Low reflection is highly desirable as it can lead to the absorption enhancement in thin film solar cells Also it is clear when the coating thickness varies from 30 nm to 90 nm, the change in reflectance is not significant, which is attributed to the fact that the antireflection of all these Al2O3/Si CNDs decorated Si substrates is mainly dominated by Si nanodome structure In addition, the measured reflectivity is always lower than that of the simulated one, which is attributed to some disorder induced in the fabricated sample as discussed in Ref 28 B Absorption and ultimate efficiency Figure 5(a) shows the absorption mapping of all samples, including a 2-µm-thick planar Si solar cell as a reference Vertical axis values of nm, 30 nm, 60 nm, and 90 nm correspond to the solar cells 127209-6 Zhang et al AIP Advances 5, 127209 (2015) FIG Measured reflection spectra of Si nanodomes and Al2O3/ Si CNDs over 200-2000 nm at (a) 8◦ and (b) 45◦ (c) The measured and simulated weighted reflectance with Al2O3 thickness with different thicknesses of the Al2O3 coating layer, and Si-planar corresponds to the 2-µm-thick planar Si solar cells Compared with the planar Si solar cells, the absorption is clearly enhanced in nanodome decorated Si solar cells The absorption is further enhanced when the nanodomes were cladded with Al2O3 film when λ < 600 nm Moreover, the absorption for Al2O3/Si CND incorporated solar cells is less dependent on Al2O3 thickness over the whole wavelength range, which is consistent with the antireflection performance As a result, Al2O3 thickness can be designed such that the cells have the best optical as well as electrical performances FIG (a) Simulated absorption spectra; (b) calculated ultimate efficiency of 2-µm-thick planar Si, Si nanodome and Al2O3 (30 nm, 60 nm, 90 nm)/ Si CND textured solar cells 127209-7 Zhang et al AIP Advances 5, 127209 (2015) Figure 5(b) shows the ultimate efficiency of all the above 2-µm-thick solar cells As expected, the ultimate efficiency of all nanodome incorporated Si solar cells is around 27%, which is compared to 15% for the planar devices Furthermore, it is evident that the ultimate efficiency for solar cells with Al2O3 coating is also improved compared to the cells without this coating, albeit the improvement is not very remarkable Moreover, as we reported in Ref 14, the contribution of antireflection to the ultimate efficiency increases with the increase in the thickness of the active layer Therefore, we believe that the ultimate efficiency can be further improved in Al2O3/Si CND decorated Si solar cells by using thicker active layer C Interface morphology A thin layer of SiOx at the Si/Al2O3 interface is reported to affect the field-effect passivation of c-Si surfaces by the Al2O3 layer.30 In order to verify the presence of such an interfacial SiOx layer in our samples, we examined the Al2O3/Si nanodome interface by cross-sectional TEM, where the Al2O3 layer under investigation was 15 nm thick and was deposited in the same batch with other Al2O3/Si CND samples A representative TEM result is shown in figure 6, where figure 6(a) is a low magnification image of two CNDs When the dome was tilted in [011] zone, the lattice structure of fcc of Si crystal as well as the amorphous feature of encapsulated Al2O3 was observable at near Scherzer-condition, where two enlarged images (labeled by I and II) are inserted in figure 6(b) to present a similar feature between the crystalline and the amorphous From Fig 6, one can see that at Si/Al2O3 interface, there exists a thin layer of SiOx as expected, which is only few atomic layers thick Such SiOx layer is too thin to affect the optical performance of the whole structure, but play a dominant role in their electrical properties, because field-effect passivation is strongly dependent on the thickness of the SiOx layer, and independent on the thickness of Al2O3 layer.30,31 The thinner the SiOx layer, the stronger the passivation, and as such, very thin layer of SiOx interface between Si and Al2O3 coating ensures the presence of field-effect passivation as reported in Ref 30 Moreover, as the passivation effect is dominated by the thickness of SiOx located at the interface of Al2O3/Si, but independent of the thickness of Al2O3,30,31 the above TEM examination verifies that a good field-effect passivation exists in our Al2O3/Si CND structures D Electrical performance and analysis To investigate the electrical performance of Al2O3/Si CND structures, Al2O3/Si CND decorated MOS devices were fabricated To make a full comparison, Si nanodome decorated samples as well FIG (a) Low-magnification TEM image of two silicon nanodomes covered with 15-nm-thick Al2O3 film, viewed along [011] as indicated by the inset electron diffraction pattern An enlarged area (marked with a box) is shown in (b), where the amorphous feature of the Al2O3 layer with a sharp transition in few atomic layers at the interface to Si is observed 127209-8 Zhang et al AIP Advances 5, 127209 (2015) FIG Capacitance-Voltage curves at frequency of 500 kHz, 100 kHz, 50 kHz, and 10 kHz for the MOS devices textured by (a) Al2O3 (30 nm)/Si CND, (b) Al2O3 (60 nm)/Si CND and (c) Al2O3 (90 nm)/Si CND as planar Al2O3/Si samples were fabricated with identical fabrication process C-V and J-V curves of all these samples were measured as described in 2.2 Figure shows the C-V curves of all Al2O3/Si CND decorated MOS devices measured at different frequencies Negative capacitance was clearly observed in Al2O3 (30 nm)/Si CND structures at all frequencies and applied voltages as shown in figure 7(a), which is an indication of the transient current lagging behind the applied voltage and is attributed to the carrier capture and emission at the interface states.32 Such phenomenon means that there are a large number of interface states in this CND structure as discussed in Ref 32 However, the capacitance is only partially negative and entirely positive for MOS devices with 60 nm and 90 nm Al2O3 coating respectively It implies that the interface state density of Al2O3/Si CND MOS devices decreases with Al2O3 thickness Figure also shows the dependence of the frequency dispersion of the accumulation region in all these samples The minimum frequency dispersion of 0.3% over 10 kHz to 500 kHz was observed for Al2O3 CND decorated MOS devices with 90 nm thick Al2O3 Small frequency dispersion is a strong indication of low interface trap density at the Al2O3/Si interface Therefore, the above C-V characteristics show that the interface trap density of the Al2O3/Si CND decorated MOS devises decreases with the increase in the Al2O3 thickness Room temperature J-V curves for the samples are shown in figure Except Al/Si nanodome/Al device as shown in the inset of figure 8, other devices exhibit the typical J-V curves of MOS devices For all Al2O3/Si CND decorated MOS devices, the leakage current density decreases with Al2O3 thickness Especially when Al2O3 thickness increases from 60 nm to 90 nm, the leakage current density decreases by several orders of magnitude as shown in figure The value of less than × 10−9 A/cm2 over the voltage range of from -3 V to V was achieved when Al2O3 layer thickness is 90 nm, which is FIG Measured leakage current density-voltage curves of Al2O3 (0 nm, 30 nm, 60 nm, 90 nm) /Si CND decorated MOS devices and planar Si MOS devices 127209-9 Zhang et al AIP Advances 5, 127209 (2015) very close to that of the planar Si MOS structure with 90 nm thick Al2O3 layer coating Furthermore, voltage-shift is also observed in J-V curves of Al2O3 (90 nm)/Si CND decorated and planar Si MOS devices In spite of the fact that four main factors, including shunted system channel, resistance of Al2O3, field-effect passivation and chemical passivation induced by the interface of Al2O3/Si, could contribute to the variation of the leakage current density, it is believed that the decrease in the leakage current density with increase in the Al2O3 thickness in our MOS samples is primarily attributed to the chemical passivation of Al2O3 film First, as is shown in figure 6, all Si nanodome structures are conformally covered by Al2O3 film, therefore, there are no shunted system channels to induce the higher leakage current density in any of our Al2O3/Si CND decorated MOS devices Second, figure shows that the evolution of the leakage current density is far from inversely proportional to the Al2O3 thickness, which indicates that the bulk resistance of Al2O3 film is not the main contribution factor to the reduction of the leakage current density In addition, as shown in figure and reported in Refs 30 and 31, field-effect passivation exist in these Al2O3/Si CND decorated MOS devices, which is independent of Al2O3 thickness Therefore, we conclude that the reduction of the leakage current density with the increase in Al2O3 thickness should be mainly attributed to the decrease of the interface trap density Such analysis is consistent with the above discussion of C-V curves Furthermore, the voltage-shift shown in J-V curves of Al2O3 (90 nm)/Si CND decorated and planar Si MOS devices could be related to photon-generated-voltage This is a good evidence that Si nanodome structures were well passivated by 90-nm-thick Al2O3 and the surface recombination loss in Al2O3 (90 nm)/Si CND structure was greatly suppressed Meanwhile, No voltage-shift phenomenon was observed in the samples decorated by Al2O3 (30 nm, 60 nm)/Si CND structures suggesting that their interface trap density is too high to release the photon-generated-carriers Therefore, we believe that (1) the decrease of the leakage current density with the increase in Al2O3 thickness should be mainly attributed to the chemical passivation induced by the decrease of interface defect state density in Al2O3/Si interface (2) 90 nm thick Al2O3 is thick enough to passivate all the surface states, (3) compared with planar Si, thicker Al2O3 layer is required for Si nanodome structures to passivate, because the surface area and surface defects in Si nanodomes decorated samples is higher than that in planar Si samples IV DISCUSSIONS AND CONCLUSIONS As demonstrated before, thin film solar cells with incorporation Si-nanodome structure has much higher ultimate efficiency than their planar counterparts However extremely high surface recombination renders Si-nanodome structure cells uselessness, unless a solution to the issue of surface recombination is in place Our work demonstrated that application of thin film Al2O3 coating to the nanodome structures using ALD is a perfect approach, as it can fully passivate the surface states, which is confirmed by our MOS device characterization The leakage current is reduced to the very similar level to that for the planar devices, and C-V measurement also shows very low level of capacitance dispersion In addition, Al2O3 coating also has an extra benefit in terms of optical performance improvement, albeit the improvement is less remarkable However, it is possible to further enhance the optical performance by employing thicker active layer and using multi-layer coating to maximize the absorption efficiency In summary, the fabrication and characterization of Al2O3/Si CND structures with Al2O3 thin film thickness of 30 nm, 60 nm and 90 nm have been carried out in order to evaluate its optical and electrical performance in CND-decorated thin film solar cells Compared with the Si nanodome decorated samples, it was observed that by application of Al2O3 thin film through atomic layer deposition (ALD) to the Si nanodome structures, both optical and electrical performance is greatly improved The reflectivity of less than 3% over broadband and omni-direction was achieved when 90 nm thick Al2O3 film was coated on Si nanocone surface with aspect ratio of 1.1 The ultimate efficiency of around 27% was obtained on the CND textured µm-thick Si solar cells, which is compared to efficiency of around 15% for the µm-thick Si planar sample Electrical characterization was made by using CND-decorated MOS devices to measure device’s leakage current and capacitance It has been found that the leakage current, which is less than 4x10−9 A/cm2 over voltage range of from -3 V to V, is reduced by several orders of magnitude when the Al2O3 film is 90 nm thick, compared to uncoated 127209-10 Zhang et al AIP Advances 5, 127209 (2015) MOS devices C-V measurements also shows as small as 0.3% of variation in the capacitance over the frequency range from 10 kHz to 500 kHz if the Al2O3 film is 90 nm thick, which is a strong indication of surface state being fully passivated TEM examination of CND-decorated samples also reveal the occurrence of SiOx layer formed between the interface of Si and the Al2O3 film, which is thin enough that ensures the presence of field-effect passivation, From our theoretical and experimental study, we believe the application of Al2O3 thin film coating to the naodomed thin-film-solar cells is a truly viable approach to achieving higher device efficiency ACKNOWLEDGMENTS This work is funded by the National Natural Science Foundation (No.51202284), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education 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