低温沉积p型氢化纳米硅与非晶硅pin顶衬太阳电池

Low temperature deposition of p-type nc-Si:H thin films for

superstrate a-Si:H based p-i-n solar cells

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Hu Zhihua1,2, Shi Qingnan1, Cai Yi1, Elvira Fortunato2, Rodrigo Martins2, Diao

Hongwei3, Xu Ying3, Liao Xianbo3

Instrumental Center, Kunming University of Science & Technology, Kunming, China (650031) Department of Material Sciences and CEMOP/UNINOVA, New University of Lisbon, Portugal

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Institute of semiconductors, Chinese Academy of Science, Beijing, China (100083)

E-mail：zhuahu1963@yahoo.com

Abstract

Boron doped nc-Si:H p-layers were deposited by PECVD technique at a low substrate temperature (~60 0

C) with various hydrogen dilution ratio of 150, 100 and 50 respectively. Transmission studies were carried out on these nc-Si:H films to understand the systematic variation of the optical band gap. A detailed and well organized investigation was employed on nc-Si:H films using Raman scattering measurements to identify the exact origin of optical and acoustic vibration modes from Si nanocrystallites as well as from a-Si. Using peak fit software analysis the crystalline volume fraction of nc-Si:H p-layers was estimated as ~73%, 52% and ~16% respectively for 150, 100 and 50 hydrogen dilution. The presently developed nc-Si:H p-layers with different crystalline fraction have been incorporated in single junction a-Si:H solar cells with a superstrate configuration of TCO/p-nc-Si:H (20nm) /i-a-Si:H (350nm) /n-nc-Si:H (30nm) /Al (200nm). It was found that nc-Si:H p-layer with a slightly crystalline fraction is more beneficial for solar cell performances. Applying this slightly crystallized p-layer, together with a H2-plasma treatment of the p-layer prior to deposition of i-layer, an efficiency of 9.0 % with an open circuit voltage of 0.90 V, a fill factor of 0.65 and a short circuit current density of 15.2 mA/cm2 has been achieved.

Keywords: Solar cells; nacocrystalline silicon films.

1. Introduction

The p-layer is critical to hydrogenated amorphous silicon (a-Si:H) alloy solar cell performance [1] since the p-i junction is the dominant junction in the p-i-n structure. One of the most significant developments in a-Si:H solar cell technology was the incorporation of C in the a-Si:H p-layer by the group at Osaka University in the early 1980’s [2]. Alternatively, there has been considerable effort focused on changing the structure of the p-layer from amorphous a-Si:H to “nanocrystalline” (nc-Si:H) [,4,5,6]. However, incorporation of nc-Si:H p-layers without carbon addition to a solar cell in a superstrate sequence having an amorphous silicon i-layer has not been much successful[7], although significant progresses have been achieved by Hamma and Roca [8].

The challenge regarding p-layers is to develop a process that results in homogeneous films of high conductivity and wide optical band gap which could be deposited on transparent conductive oxides (TCO) for pin solar cells. One possible way to achieve a wide band gap film is to keep low substrate temperature for increasing hydrogen content and/or to decrease the Si crystallites size that could be present in a-Si network.

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2. Experimental

Ultra thin boron doped nc-Si:H thin films were prepared with a conventional capacitance-coupled RF-PECVD multi-chamber system using a gas mixture of SiH4, H2 and B2H6 at a low substrate temperature (~600C) under the condition of high RF power density (~1W/cm2), high pressure (~200 Pa) and varied hydrogen dilution (H2/SiH4) of 150, 100 and 50. Films were deposited on ZGO coated glasses. Since the subsequent i-layer and n-layer in solar cells were deposited at a higher temperature of

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170 0C, all the samples have been heated from 60 0C to 170 0C and kept in vacuum for 2 hours before cooling down and taking out. Thicknesses were measured by using a stylus type Dektak profilometer. An UV–VIS–NIR double beam spectrophotometer was used to estimate the bandgap in the visible spectral range. Samples were also examined by using Raman scattering to detect the crystallinity. Raman spectra of the films were taken in backscattering geometry with the 514.5 nm line of an Ar laser. The power of the laser was kept small to prevent crystallization of these films during Raman spectroscopy measurements.

Table I PECVD conditions for each layer in pin solar cells

Substrate temperature

(0C) rf power density (mW/cm2)Chamber pressure (Pa) H Dilution Deposition rate (H2/ time

(min) SiH4)

Doping gas

concentration

p-nc-Si:H 60 1000 200 150,100,502 1-1.5%B2H6 i-a-Si:H 170 70 70 10 30 n-nc-Si:H 170 70 70 60 5 1-1.5% PH3

Single junction solar cells were fabricated on Gallium doped Zinc Oxide (GZO) coated glasses under conditions presented in Table 1. The p-layers were deposited at 60 0C and then heated to 170 0C. For deposition of intrinsic active layers, standard conditions were used with a pressure of 70 Pa, a power density of 70 mW/cm2, a hydrogen dilution ratio of 10 and a substrate temperature of 170 0C. For deposition of n-layer, a higher hydrogen dilution ratio of 60 was employed. The area of solar cells was defined by a mask-evaporation of Aluminum rear spots with an area of 0.1256 cm2. Solar cells were made in configuration of GZO/p-nc-Si:H (20nm) /i-a-Si:H (350nm) /n-nc-Si:H (30nm) /Al (200nm). Illuminated I-V characteristics of solar cells were measured under a sun simulator (AM1.5, 100 mW/cm2) at room temperature.

3. Results and discussion

Transmission spectra of p-type nc-Si:H thin films (~50nm) prepared on ZGO coated glasses under different hydrogen dilution ratio (H2/SiH4) from 150 to 50 are shown in Figure 1 a to c. It is noted from Fig 1 that the absorption spectra of the nc-Si:H films is monotonically shifted towards longer wavelength with increasing hydrogen dilution ratio. However, the absorption behavior of Fig 1a and b in the shorter wavelength region are markedly different from Fig 1c, a typical transmission spectrum of quasi-direct band gap thin film. This could be originated from the dual band gap nature of thin films prepared under higher hydrogen dilution [150 for (a) and 100 for (b)] which facilitates the formation of Si crystallites. The effective optical bandgaps calculated from the Tauc’s plot are 1.68 eV [Fig 1(a)], 1.78 eV [Fig 1(b)] and 1.90 eV [Fig 1(c)] respectively. The systematic decreases in optical band gap for the a-Si film are brought out evidently with increasing hydrogen dilution.

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100

(a)

(b)

80

(c)

Transmission60

40

20

0

400600800

Wavelength/nm

Fig.1. Transmission spectra of nc-Si:H p-layers prepared on ZnO:Ga with hydrogen dilution ratios of 150 (a), 100

(b) and 50 (c)

Figure 2a, b and c presents the Raman spectra recorded in the range of 100 to 650 cm-1 taken from annealed nc-Si:H p-layers deposited on GZO coated glass substrates with different hydrogen dilution ratio of 150, 100 and 50 respectively. They were well fitted with to a sum of four Gaussian and two Lorentzian line-shapes with a constant baseline to obtain the parameters like peak position, FWHM and area. It is to be noted in Figure 2 that the TO mode of Si nanocrystallites broadens, becomes asymmetric and the peak position shifts to lower wavenumber in comparison with that of crystalline silicon (521 cm-1 not shown in Figure 2). The finite size of Si nanocrystallites relaxes the wavevector (q) selection rule, which results in the broadening and downshift of the optical Raman spectrum. This Red shift of the optical phonon Raman spectra for the decrease in hydrogen dilution indicates the quantum confinement effect.

Fig 2 also shows five broad bands around 135, 330, 440, 480 and 500 cm-1 which originated from a-Si TA, LA, LO, TO modes and one from grain boundary (GB) phase, a silicon Wurzite phase [9], or even smaller crystallites [10] that could result from twinning defects. These assignations are clearly labeled in Fig 2b. The increase of a-Si TA, LA, LO and TO modes intensities also suggests the reduction of Si nanocrystallites volume fraction in a-Si:H matrix. These changes in Si nanocrystallites in a-Si might be the reason for the notable difference in transmission spectra in the short wavelength region (see Fig.1).

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10nano Si:H p layer150 H2 dilution 8100 H2 dilution 50 H2 dilution (a)Intensity (arb. units)6TOTO4TA(b)LAGBLO2(c)0100200Raman shift ( cm -1 )

300400500600

Fig.2. Fitted Raman spectra of nc-Si:H p-layers prepared on ZnO:Ga with hydrogen dilution ratios of 150 (a), 100

(b) and 50 (c)

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http://www.paper.edu.cn The observed Raman spectra in TO region in Fig 2 show features of nc-Si:H films, which could be deconvoluted into three parts corresponding to an amorphous phase mode around 480 cm-1, a GB phase mode around 500 cm-1 and a crystalline phase mode around 515 cm-1. The crystalline volume fraction XC = (IC+IGB)/(IC+IGB+IA) were estimated to be ~72%, 52% and ~16%, respectively for samples (a), (b) and (c), where IC, IGB, IA are integrated intensities of the crystalline, GB and amorphous Si TO modes [11,12]. 5Photocurrent density (mA/cm)02-5 Jsc Voc FF Eff(a)16,9 0,50 0,56 4,7%(b)15,4 0,69 0,60 6,4%(c)15,0 0,82 0,63 7,7%(a)(b)(c)-10-15-20-0.2

0.00.20.40.60.81.0

Voltage (V)

Fig.3. Light J-V curves of solar cells with nc-Si:H p-layer under hydrogen dilution ratios of 150 (a), 100 (b) and 50

(c)

The thin p-type nc-Si:H films developed in this work have been incorporated to single junction solar cell in a superstrate structure. The intrinsic layers were directly deposited on top of p-layer at 170 0C under a standard condition with a hydrogen dilution ratio of 10 (see Table 1). No buffer layer has been intentionally inserted into p/i interface. Fig. 3 displays the light J-V characteristics of solar cells with p-layers prepared under different hydrogen dilution ratios from 150 to 50 as (a) to (c). It is observed that the overall performances are improved while hydrogen dilution was decreased from 150 to 50. An efficiency (Eff) of 7.7 % with an open circuit voltage (Voc) of 0.82 V and a fill factor (FF) of 0.63 (see curve c in Fig 3) has been achieved for a slightly crystallized p-layer (Xc ~16%). However, the Voc and FF are 0.50, 0.69 and 0.56, 0.60 respectively for (a), (b). This drop in the solar cell parameters should be accounted for defects and the band edge discontinuities at p/i interface, which might cause recombination of photocarriers at the junction while highly crystallized p-layers were used [13]. This initial result is in agreement with Hamma and Roca [8] who intended that the optimum crystalline volume fraction of p-layer should be around 30 % when a-Si:H is used as i-layer of p-i-n solar cells. To ensure formation of a high quality p/i interface and therefore decrease the possible recombination of photocarriers at the junction, a hydrogen plasma treatment for the underlying p-layer surface was used

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http://www.paper.edu.cn prior to deposition of i-layer. This treatment could improve the material quality of i-layer [14], because there are possible weak bonds on the p-layer surface. Fig 4 gives the light J-V characteristics of a-Si:H p-i-n solar cell with a slightly crystallized nc-Si:H p-layer and a H2-plasma treatment incorporated, 5Photocurrent density (mA/cm)02-5Jsc Voc FF Eff15,2 0,90 0,65 9,0% -10-15-20-0.2

0.00.20.40.60.81.0

Voltage (V)

Fig.4. Light J-V curve of solar cell with nc-Si:H p-layer under hydrogen dilution ratios of 50 combined with a

hydrogen plasma treatment prior to deposition of intrinsic a-Si:H layer

which shows an Eff of 9.0 % with a Voc of 0.90 V, FF of 0.65 and a short circuit current density (Jsc) of 15.2 mA/cm2, under AM1.5, 100 mW/cm2 and 25 0C. As can be seen, although the Voc and Jsc are acceptable, the FF value is relatively low, indicating that the material quality and the optimization of process need further improvements, which are still going on in our lab.

4. Conclusions

Boron doped nc-Si:H p-layers were deposited by PECVD technique using a gas mixture of SiH4, H2 and B2H6 at a low substrate temperature (~600C). Visible transmission and Raman scattering spectra were employed for optical and microstructural characterization. Nc-Si:H p-layers with different crystalline fraction have been incorporated in single junction a-Si:H solar cells with a superstrate configuration of ZGO/p-nc-Si:H (20nm) /i-a-Si:H (350nm) /n-nc-Si:H (30nm) /Al (200nm). It was found that solar cell with a slightly crystallized nc-Si:H p-layer has the best performance. Applying these slightly crystallized nc-Si:H p-layer, together with an initial H2-plasma treatment of the p-layer prior to deposition of i-layer, an efficiency of 9.0 % with an open circuit voltage of 0.90 V, a fill factor of 0.65 and a short circuit current density of 15.2 mA/cm2 has been achieved.

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Acknowledgments

The first author would like to thank the Foundation of Science and Technology of Portugal for scholarship support. Apart from that, I also would like to thank Kunming Institute of Physics for financial support.

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Author Brief Introduction： Zhihua Hu, 1963-, PhD, currently works in Solar Research Institute, Yunnan Normal University.

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