Inductively coupled plasma grown semiconductor films for low cost solar cells with improved light-soaking stability

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Inductively coupled plasma grown semiconductor films for low cost solar cells with

improved light-soaking stability

Chang-Hong Shen, Jia-Min Shieh, Jung Y. Huang, Hao-Chung Kuo, Chih-Wei Hsu, Bau-Tong Dai, Ching-Ting Lee, Ci-Ling Pan, and Fu-Liang Yang

Citation: Applied Physics Letters 99, 033510 (2011); doi: 10.1063/1.3615650 View online: http://dx.doi.org/10.1063/1.3615650

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Inductively coupled plasma grown semiconductor films for low cost solar

cells with improved light-soaking stability

Chang-Hong Shen,1Jia-Min Shieh,1,2,a)Jung Y. Huang,2Hao-Chung Kuo,2,b) Chih-Wei Hsu,2Bau-Tong Dai,1Ching-Ting Lee,3Ci-Ling Pan,2,4and Fu-Liang Yang1

1

National Nano Device Laboratories, No. 26, Prosperity Road 1, Hsinchu 30078, Taiwan

2

Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan

3

Institute of Microelectronics and Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan

4

Department of Physics and Institute of Photonics Technologies, National Tsing Hua University, Hsinchu 30013, Taiwan

(Received 31 December 2010; accepted 3 July 2011; published online 22 July 2011)

We investigate the performance of a single-junction amorphous Si (a-Si) solar cell fabricated with inductively coupled plasma (ICP) deposition technique. The high-density plasma resulting from high dissociation capacity of ICP enables good-quality hydrogenated Si films to be synthesized at low temperatures. High-density ICP also promotes the diffusion of reactive radicals on substrates and forms a-Si:H films with low defect density (3 1015_cm3_{). We demonstrate single-junction}

a-Si solar cells with a conversion efficiency of 9.6% and improved light-soaking stability. This low thermal-budget thin-film technique could open up the feasibility of efficient thin film solar cells on flexible substrates.VC _{2011 American Institute of Physics. [doi:}_{10.1063/1.3615650}_]

Energy security has become the major concern of human being. Photovoltaic (PV) technology is attractive for its potential as the major carbon-free renewable energy source. Due to the long-term efforts in microelectronic industry, silicon-based PV technology has reached a mature status. However, currently single-crystal solar cells are still much more expensive than thin-film PV devices, which signifi-cantly limits the wide spread use of large-area single-crystal solar-cell panel. Hydrogenated amorphous silicon (a-Si:H) is well suited for PV applications.1,2 To further expand its applications on compliant substrates, thin-film solar cells must be fabricated at low temperature. Unfortunately, a-Si solar cells fabricated by radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) below 150C revealed fairly poor PV conversion efficiencies.3 This is because low thermal energy of reactive radicals in RF-PECVD at low deposition temperature leads to a film with low mass density and poor electrical properties.4,5 Further-more, the resulting solar cell is quickly degraded by the photo-generated defects.6–8 Although by using thinner absorber layer in an a-Si solar cell, researchers can reduce the photo-induced degradation to 15% after 1000-h exposure with one-sun irradiance at 50C, they need to employ a more sophisticated light trapping scheme in the device to maintain the PV efficiency.7,8

Typically in an a-Si:H film, recombination of photo-gen-erated charge carriers creates extra dangling bonds.8,9 It is highly desired to develop a low-thermal budget thin-film deposition technique for fabricating low cost PV panel with high light-soaking stability. Capacitor-type low-density plasma such as RF-PECVD generates abundant short-life sil-ane-related radicals SiHx (0 5 x 5 2), which eventually

form weak bonding configuration.4,8,9 Inductively coupled plasma chemical vapor deposition (ICP-CVD) system,10,11 because of its high fractional ionization capacity, can pro-duce high-density plasma to improve diffusion of the reac-tive radicals on substrates even at fairly low deposition temperatures. The configuration of ICP system with high density plasma separated from the thin film growing region effectively reduces the possibility of ion bombardment on growing surface, resulting in a film with low density of defects. In this letter, by using ICP-CVD, we demonstrated single-junction a-Si solar cell with a conversion efficiency of 9.6% and improved light-soaking stability.

For a side-by-side comparison, we employed ICP-CVD and very high frequency PECVD (VHF-PECVD) to deposit a-Si:H thin films at 140C. Our ICP-CVD system is equipped with a 13.56-MHz RF electrical source operating at a power density of 135 mW/cm2. During the thin film dep-osition, we maintained the pressure and hydrogen dilution ra-tio (H2/SiH4) at 700 mTorr and 10, respectively. This results

in a deposition rate of a-Si:H film about 0.3 nm/s. VHF-PECVD system was operated at a frequency of 40 MHz with a power density of 83 mW/cm2. Similar dilution ratio and working gas pressure were used.

The concentration of hydrogen inclusion CH(%) and the

microstructure parameter R of the resulting a-Si:H films were deduced from the integrated peak area of the 630-cm1 peak and ratio of the spectral peaks I2070cm1=

ðI2070cm1þ I_2000cm1Þ of Fourier-transform infrared (FTIR)

absorption spectra. Here I2070cm1 and I_2000cm1 denote the

integrated infrared absorption peak areas of stretching mode of Si-H bonds locating at internal interfaces and of the iso-lated Si-H bonds, respectively.8,12We determined the defect densities of intrinsic a-Si:H layers with drive-level capaci-tance profiling (DLCP) technique.13,14

For the fabrication of a-Si solar cell, ap-layer/intrinsic i-layer/n-layer (p-i-n) stack was deposited on Asahi

a)_{Electronic addresses: [email protected] and [email protected].}

edu.tw.

b)

Electronic mail: [email protected].

0003-6951/2011/99(3)/033510/3/$30.00 99, 033510-1 VC2011 American Institute of Physics

APPLIED PHYSICS LETTERS 99, 033510 (2011)

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substrates (SnO2:F/glass) by ICP-CVD or VHF-PECVD. The

thickness of the intrinsic layer (i-layer) was kept at 400 nm. Doped layers with thickness of 12 nm (p-layer) or 20 nm (n-layer) were synthesized with an admixture of either 13% B2H6(p-layer) or 5% PH3(n-layer). A reflective contact of

in-dium-tin-oxide (ITO)/Al was then deposited by direct current-sputtering on the back side to yield efficient light trapping. The ITO layers have high electrical conductivity (<1 103 X cm) and optical transmittance (>90%, 400–900 nm), which significantly improve the light collection and reduce the series resistance of the devices. We characterized the performance of the PV devices with an AM1.5G Global sun simulator (Oriel Sol3A). The light-soaking measurements were con-ducted with a white light source with an irradiance of 600 mW/cm2(6 Sun). During the light-soaking measurements, the device under test was not temperature-controlled. The device can rapidly reach a steady state temperature at 60C. There-fore, the condition of our light soaking can be considered to be irradiated with 6 Sun at 60C.

In Fig.1(a), the FTIR absorption spectra of a-Si:H films prepared with ICP-CVD and VHF-PECVD were presented. From the 630-cm1peak, we estimated the hydrogen content of the a-Si:H films to be about 9.3% for the ICP a-Si:H and 11.7% for the VHF-PECVD film. Note that the hydrogen content in an a-Si:H film deposited by PECVD typically lies in the range of 11%–12%.15 The growth mechanism with combined effect from the formation of dense film by higher surface mobility of reactive species and the lower deposition rate due to hydrogen out-gassing predicts the hydrogen con-tent in a film to be decreased as the deposition temperature is increased.16–18However, we synthesized our ICP a-Si:H at a low deposition temperature (140C) with a fairly high deposition rate (0.3 nm/s) by using a high power density (135 mW/cm2). The resulting film still reveals lower hydrogen content than that prepared by PECVD and VHF-PECVD. The estimated microstructure parameter R in our ICP a-Si:H from I2070cm1=ðI_2070cm1þ I_2000cm1Þ is nearly

zero, implying a dense lattice network with a very low level of voids.8,12 The cross-sectional transmission electron mi-croscopy (TEM) image of a-Si solar cell and the spatially

resolved diffraction pattern at the p-i-n stack presented in Fig.1(b)clearly indicates the p-i-n stack to be an amorphous phase.

The current-voltage (I-V) curves of the solar cells pre-pared by ICP-CVD and VHF-PECVD are shown in Fig.2(a). The sizes andi-layer thickness of the devices are 1 cm2and 400 nm, respectively. For the ICP a-Si solar cell, the PV con-version efficiency (g) was measured to be 9.6% under AM1.5 illumination. The device also has a fairly broad quan-tum-efficiency (QE) spectral profile (300–750 nm) (Fig.

2(b)), which can be attributed to a low density of defects in both thei-layer and at the interfaces. The broad QE spectrum also improves the absorption of broadband solar radiation by the i-layer, resulting in a high short-circuit current density (Jsc) of 15.7 mA/cm

2

. We had succeeded in the preparation of good-quality heavily doped p- and n-layers, ensuring an open-circuit voltage Voc as high as 0.91 V to be achieved.

The low dark saturation current of 1.3 109 (A/cm2), extracted from the dark I-V characteristics of the device, leads to an observed fill factor (FF) of 67.2%. For compari-son, the VHF-PECVD a-Si solar cell reveals a conversion ef-ficiency of 8.8% (Fig.2(a)) with similar Voc(0.92 V) and FF

(67.6%). The short-circuit current density (14.2 mA/cm2) is lower than that of the ICP device due to a lower quantum ef-ficiency in the long wavelength (550 nm–750 nm), suggest-ing a higher defect density in the absorber layer. We measured the quantum efficiency loss QEloss, which is

defined as the ratio of integration values of QE spectra meas-ured at1V and 0 V, QE(1 V)/QE(0 V), and found QEloss

of ICP-CVD and VHF-PECVD a-Si solar cells to be 1.015 and 1.027, respectively (the inset table of Fig. 3). The slightly higher QElossof the VHF PECVD a-Si solar cell

sup-ports the notion that its i-layer has a higher defect density than the one grown with ICP-CVD.19

The major obstacle of amorphous Si PV technology is the low stability to light exposure. This Staebler-Wronski effect (SWE) originates from generation of extra defects in a-Si:H after recombination of charge carriers.8,9In Fig.2(a), the current-voltage (I-V) curves of the ICP a-Si solar cell shows a reduced conversion efficiency from 9.6% to 8.5% after 104-s exposure with 6 Sun irradiance at 60C. The PV efficiency drops by 11% (Fig.3). While for the VHF PECVD a-Si solar cell, the I-V curves indicate a decrease in PV effi-ciency from 8.8% to 7.2%. The PV effieffi-ciency was degraded by 18% after 104-s exposure under the same light-soaking condition (Fig. 3). The light-induced changes in I-V curves FIG. 1. (Color online) (a) FTIR absorption spectra of a-Si:H films deposited

by ICP-CVD and VHF-PECVD. (b) The cross-sectional TEM image show-ing the Si pin stack in a solar cell fabricated by high-density ICP-CVD on Asahi U substrate. The thickness of the i-layer was 400 nm. Spatially resolved diffraction pattern of the a-Si:H p-i-n stack showing to verify the amorphous phase of the stack.

FIG. 2. (Color online) (a) I-V characteristics and (b): QE spectra of p-i-n solar cells fabricated by ICP-CVD (red-colored triangles) and VHF-PECVD (blue-colored circles) at 140C. For comparison, the device characteristics (open symbols) after 104-s exposure with 6-Sun irradiance at 60C are also presented.

033510-2 Shen et al. Appl. Phys. Lett. 99, 033510 (2011)

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mainly occur at a decrease in the fill factor and short-circuit current.

For thin-film solar cells, the major limitation on the PV efficiency originates from the trap-controlled transport. The defects in a-Si:H can act as recombination centers of carriers, which reduce the carrier lifetime and form charged defects that can further screen the electric field in the absorber layer of a solar cell. To more clearly reveal the underlying light-induced degradation process, we measured the defect density of intrinsic a-Si:H films as a function of exposure time with the DLCP method13,14(Fig.4). For the measurement, a-Si:H film with thickness of 400 nm was deposited on heavily doped p-type crystalline Si substrate at a temperature of 140C. A defect density of 3 1015cm3in the ICP a-Si:H film was found, comparing to 6 1015 cm3 in the VHF-PECVD a-Si:H. The lower defect density in the ICP a-Si:H may be due to more effective hydrogen passivation of dan-gling bond during layer deposition.8The defect density of ICP a-Si:H film increases to 1.5 1016 cm3 after 104-s light-soaking with 6 Sun irradiance, comparing to 3.5 1016cm3 in the VHF-PECVD a-Si:H film under the same condition. Note that due to slower surface diffusion of SiH3on substrate

at lower temperature, a defect density of 1015 cm3 in a PECVD grown a-Si:H can only be achieved at a substrate temperature >250C. ICP system can maintain the mobility of the reactive radicals on substrate even at fairly low temper-atures, combing with low ion bombardment on growing sur-face, leads to the deposition of a-Si:H layer at 140C with low defect density.

Light-induced degradation in a-Si:H was found to strongly depend on the amount of nano-sized voids and hydrogen content.12Recent experiments have shown that the SWE is accompanied by metastable structural changes of the underlying amorphous network.20The results of lower defect density in our ICP a-Si:H layers from the DLCP measure-ments and fewer defects being created by light soaking from our light soaking test support that the Si network in ICP deposited films is fairly stable and is therefore more resistant

to prolonged light exposure. The stability of ICP a-Si solar cell with low hydrogen content and nano-sized voids is attributed to the unique feature of high fractional ionization capacity of ICP-CVD.12

In summary, we developed high-density ICP-CVD sys-tem for solar cell applications. High quality a-Si:H films were deposited at a fairly low deposition temperature of 140C. Single-junction pin solar cells with 400-nm thick i-layer achieved a conversion efficiency of 9.6% with an improved light-soaking stability.

The authors would like to thank the National Science Council of the Republic of China, Taiwan, for partially sup-porting this research.

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grown by ICP-CVD (open triangles) and VHF-PECVD (open circles) as a function of exposure time with a light irradiance of 6 Sun at 60C. The inset table shows the key parameters of above solar cells, including Jsc, Voc, FF,

g, and QEloss.

FIG. 4. (Color online) The defect densities retrieved from DLCP measure-ments of a-Si:H films deposited with ICP-CVD (open triangles) and VHF-PECVD (open circles) as a function of exposure time with a light irradiance of 6 Sun. The a-Si:H films with thickness of 400 nm were deposited on heav-ily dopedp-type crystalline Si substrate at substrate temperature 140_C.

033510-3 Shen et al. Appl. Phys. Lett. 99, 033510 (2011)