Improved light trapping in thin-film silicon solar cells via alternated n-type silicon oxide reflectors

Solar Energy Materials & Solar Cells 119 (2013) 77–83

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Improved light trapping in thin-film silicon solar cells via alternated n-type silicon oxide reflectors Seung Yeop Myong n, La Sun Jeon Energy R&D Center, KISCO, National Nanofab Center, 335 Gwahangno, Yuseong-gu, Daejeon 305-806, Republic of Korea

art ic l e i nf o

a b s t r a c t

Available online 15 June 2013

We have investigated light trapping of p–i–n type hydrogenated amorphous silicon (a-Si:H) singlejunction and p–i–n–p–i–n type a-Si:H/hydrogenated microcrystalline silicon (μc-Si:H) double-junction solar cells by adopting the oxygen-content graded or the oxygen-content alternated hydrogenated n-type silicon oxide (SiOx:H) reflectors. The graded n-type SiOx:H back reflector effectively increases the optical path length of the p–i–n type a-Si:H single-junction solar cells due to the refractive index grading. Moreover, the alternated n-type SiOx:H back reflector comprising of highly hydrogen-diluted n-type a-Si: H (n-a-Si:H) sublayers having a high refractive index and highly hydrogen-diluted n-type a-SiOx:H sublayers having a low refractive index provides the further improvement for the optical path length of the p–i–n type a-Si:H single-junction solar cells due to the multiple reflection. Furthermore, the developed alternated n-type SiOx:H reflector is suitable for the back reflector as well as the intermediate reflector of the p–i–n–p–i–n type a-Si:H/μc-Si:H double-junction solar cells. As a result, the high initial efficiency (η) of 13.1% and stabilized η of 11.5% are achieved. The considerably thin (30–45 nm) alternated n-type SiOx:H reflectors can be easily prepared using the in situ plasma enhanced chemical vapor deposition (PECVD) technique. Since the alternated n-type SiOx:H reflector can avoid the lateral shunting problem during the monolithic series integration of segments, it is a promising option for cost effective mass production of large-area thin-film silicon solar modules. & 2013 Elsevier B.V. All rights reserved.

Keywords: Thin-film silicon solar cell Light trapping Internal reflection Graded n-type silicon oxide reflector Alternated n-type silicon oxide reflector

1. Introduction It is well known that the recent global warming caused by excess emission of CO2 warns about the climate change, and nuclear power plants are no longer stable and cheap energy source after the Fukushima nuclear meltdowns. Sufficient supplies of clean energy are intimately linked with global stability, economic prosperity, and quality of life. Accordingly, clean renewable energy including solar, wind, and hydrogen energy becomes a prime issue. A solar module using solar light is a promising candidate among the renewable energy sources because the sun is our primary source of clean, and abundant energy. Actually, there has been an explosive, worldwide increase in solar module market during the last decade with an annual increase of ∼40%. However, the oversupply of bulk crystalline silicon (c-Si) solar modules that currently shares 80% of products and the decrease in government subsidies due to the recent global economic crisis threat the photovoltaic business by causing the rapid drop in the module price. The thin-film silicon (Si) solar modules using hydrogenated amorphous Si (a-Si:H) and/or hydrogenated microcrystalline Si (mc-Si:H) absorbers have been

n

Corresponding author. Tel.: +82 42 861 1556; fax: +82 42 861 1554. E-mail address: [email protected] (S.Y. Myong).

0927-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2013.05.033

considered as promising alternatives to the bulk c-Si solar modules due to the various advantages of remarkably low consumption of raw Si material (o1% of consumption of bulk c-Si solar modules), large-area deposition, low-temperature production, and low temperature coefficient. The thin-film Si photovoltaic technology also profits from the wide experience base of the display industries [1]. However, the recent sharp drop in the module price gives rise to a need for a new breakthrough in the conversion efficiency (η) as well as the cost of the thin-film Si solar modules. However, the so-called “Staebler–Wronski effect (SWE)” in a-Si:Hbased films remains as a major technical challenge for the commercialization of the thin-film Si solar modules. SWE is the light-induced degradation arising from the photocreation of dangling bonds accomplished by the nonradiative recombination of photogenerated electron-hole pairs [2]. To reduce SWE in a-Si:H absorbers that leads to the degradation of thin-film Si solar cells, there have been extensive investigations during the past 30 years. Double-junction solar cells composed of a-Si:H top and the mc-Si:H bottom cells stacked in series have been developed to achieve a high value of stabilized η by guiding incident light to the appropriate absorbers [3– 7]. Because the mc-Si:H bottom cell is very stable against red light irradiation, the reduced thickness of the a-Si:H absorber in the top cell compared to that of a-Si:H absorbers in conventional singlejunction solar cells provides a good stability against light soaking. The

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stability of the a-Si:H/μc-Si:H double-junction solar cells mainly depends on the light-induced degradation as well as the thickness of the absorber in the top cell. Thus, improved light trapping in the a-Si:H top cell using an intermediate reflector becomes of paramount interest. Lower refractive index (n) materials for the intermediate reflector compared to n of Si layers around 4.0 are essential in order to effectively enhance the internal reflection [8]. It was reported that highly conductive and transparent zinc oxide (ZnO) intermediate reflectors having n of ∼2.0 significantly increased stabilized η of superstrate type a-Si:H/μc-Si:H double-junction solar cells [9–10]. Despite the improve the cell performances, such a metal oxide intermediate reflector including ZnO, unfortunately, is not suitable for mass production of superstrate type thin-film Si solar modules due to the lateral shunting [11] caused by the leakage current path generation between the intermediate reflector and the metal back contact. To prevent a poor fill factor (FF) as a result of the monolithic series integration, at least an additional step is required for the isolation of the exposed intermediate reflector after the laser scribe of Si layers from the subsequently coated metal back contact. This additional step gives rise to a high production cost. In the case of substrate type a-Si:H/μc-Si:H double-junction solar cells, improved light trapping by employing a textured ZnO intermediate reflector was also reported [12]. However, the substrate type a-Si:H/μc-Si:H double-junction solar modules employing the textured ZnO intermediate reflector also suffer from a similar lateral shunting. A leakage current path occurs between the highly conductive ZnO intermediate reflector and the subsequently prepared transparent front electrode like the indium tin oxide (ITO) and ZnO. Alternatively, hydrogenated n-type amorphous silicon-oxide (n-a-SiOx:H) and hydrogenated n-type nanocrystalline siliconoxide (n-nc-SiOx:H) intermediate reflectors with a constant n were developed [13,14]. The SiOx:H intermediate reflectors can considerably reduce the lateral shunting due to the lower lateral conductivity than the ZnO intermediate reflectors. Also, the SiOx: H intermediate reflectors can be removed simultaneously with adjacent Si layers via the laser scribe of Si layers. Hence, no additional step for the monolithic integration of a-Si:H/μc-Si:H double-junction solar modules is necessary. Moreover, in situ plasma enhanced chemical vapor deposition (PECVD) for the preparation of SiOx:H intermediate reflectors are possible. However, a thick (∼100 nm) SiOx:H intermediate reflector designed to enhance the internal reflection can lead to the increased optical absorption in the intermediate reflector and the reduced throughput of mass production. Therefore, we need to find the optimum oxygen addition strategy during the deposition in order to achieve considerably thin SiOx intermediate reflectors. In this work, we investigate thin alternated n-type SiOx:H intermediate reflectors for the a-Si:H/μc-Si:H double-junction solar cells. In addition, the feasibility of alternated n-type SiOx:H back reflectors is investigated to replace the conventional ZnO back reflector or to reduce the thickness (80–100 nm) of the ZnO back reflector.

2. Experimental details A series of p–i–n type a-Si:H single-junction solar cells with an area of 1.02 cm2 were fabricated on Asahi VU-type (textured SnO2: F) glass substrates with a size of 20 cm
20 cm by the 13.56 MHz radio-frequency (RF) PECVD technique. Each type of layers was deposited in the respective separated chamber to minimize the residual impurity cross-contamination. The fabricated solar cells has the structure of glass/SnO2:F/low hydrogen-diluted p-type amorphous silicon-carbide (p-a-SiC:H) window (12 nm)/highly hydrogendiluted p-a-SiC:H buffer (4 nm)/intrinsic a-Si:H (i-a-Si:H) absorber

(160 nm)/low hydrogen-diluted n-type a-Si:H (n-a-Si:H) (5 nm)/ntype back reflector (30 nm)/Al (300 nm). The p-a-SiC:H double layer structure comprising of the low hydrogen-diluted heavily-doped p-a-SiC:H window layer and the highly hydrogen-diluted lightlydoped p-a-SiC:H buffer layer was employed to reduce the carrier recombination loss at the p/i interface and the experimental details are described in our previous works [15–18]. For the deposition of the considerably thin i-a-Si:H absorbers, the twostep deposition method was used at the substrate temperature of 190 1C under the low pressure of 1 Torr by varying silane concentration (SC), i.e., SC ¼[SiH4]/([SiH4]+[H2]) from 10.2% to 11.4% without any turn-off of the plasma power. The average deposition rate of the i-a-Si:H absorbers is 0.25 nm/s. All the n-type layers were deposited at the substrate temperature of 180 1C. For the deposition of low hydrogen-diluted n-a-Si:H layers, the process parameters of SC, doping ratio (PH3/SiH4), process pressure, and plasma density were maintained at 20%, 3333 ppm, 0.5 Torr, and 60.3 mW/cm2, respectively. The deposition rate of the low hydrogen-diluted n-a-Si:H layers is 0.25 nm/s. In the case of the deposition of n-type back reflectors including highly hydrogendiluted n-a-Si:H, the process parameters of SC, PH3/SiH4, process pressure, and plasma density were maintained at 0.3%, 4000 ppm, 5 Torr, and 603 mW/cm2, respectively. With the increase in the oxygen addition rate (CO2/SiH4) from 0 to 1.2, the deposition rate of the n-type back reflectors gradually decrease from 0.28 to 0.13 nm/s. Two-kinds of 30-nm-thick n-type a-SiOx:H back reflectors having a nonuniform oxygen content in the thickness direction were prepared. One is the graded n-type a-SiOx:H reflector deposited via the stepwise increase in CO2/SiH4 from 0 to 1.2 with the interval of 0.4. Thus, the first sublayer contacting to the low hydrogen-diluted n-a-Si:H layer is the highly hydrogen-diluted n-a-Si:H sublayer, and all the sublayers have the identical thickness of 7.5 nm. The other is the alternated n-type SiOx:H reflector formed by modulating CO2/SiH4 between 0 and 1.2. Actually, two cycles of the deposition for the highly hydrogen-diluted n-a-Si:H (7.5 nm)/n-a-SiOx:H (7.5 nm) multilayer were executed. The Al back contact was formed via the thermal evaporation. On the other hand, the p–i–n–p–i–n type a-Si:H/μc-Si:H doublejunction solar cells with an area of 1.02 cm2 were fabricated with the reference structure of glass/SnO2:F/low hydrogen-diluted p-a-SiC:H window (12 nm)/highly hydrogen-diluted p-a-SiC:H buffer (4 nm)/ia-Si:H (200 nm)/low hydrogen-diluted n-a-Si:H (5 nm)/highly hydrogen-diluted n-a-Si:H (30 nm)/hydrogenated p-type mc-Si:H (p-mc-Si:H) (15 nm)/intrinsic mc-Si:H (i-mc-Si:H) (2300 nm)/low hydrogen-diluted n-a-Si:H (5 nm)/highly hydrogen-diluted n-a-Si:H (30 nm)/Ag (100 nm)/Al (200 nm). For the deposition of the i-mc-Si:H absorbers, the 40 MHz very-high-frequency (VHF) PECVD technique was used. The i-mc-Si:H absorbers were deposited at the low temperature of 150 1C under the high pressure of 8 Torr. Multistep hydrogen dilution profiling in the SC range from 0.5% to 1.5% was used to remove an incubation layer formation at the p/i interface and to maintain the crystal volume fraction (Xc) in i-mc-Si:H at a uniform value. The average deposition rate of i-mc-Si:H is 0.35 nm/s. The Ag back contact was formed via the thermal evaporation in order to prevent the Al interband absorption near the wavelength of 800 nm [19]. The highly hydrogen-diluted n-a-Si:H layers in the tunnel recombination junction and the bottom cell can be replaced by the alternated n-type SiOx:H intermediate reflector and the alternated n-type SiOx:H back reflector, respectively. The Raman measurements were performed using the Horiba Jobin Yvon, LabRAM HR UV/Vis/NIR system with a He–Ne laser having a wavelength of 633 nm. The value of n at the wavelength of 632 nm and the thickness of the deposited films were evaluated via an ex-situ variable-angle spectroscopic ellipsometer (J. A. Wollam Co., Inc.) [18]. The photo current density–voltage (J–V) characteristics for the fabricated cells were measured at 25 1C under 1 sun (AM 1.5G,

S.Y. Myong, L.S. Jeon / Solar Energy Materials & Solar Cells 119 (2013) 77–83

100 mW/cm2) solar simulator irradiation. The 1 sun light soaking test was performed at 50 1C for 192 h based on the International Electrochemical Commission (IEC) 61646 standard for thin-film terrestrial photovoltaic modules. The biased external quantum efficiency (EQE) was measured using the JASCO, CEP-25 ML spectral response system. In order to reduce the side collection effect, the peripheral Si outside the metal back contact was removed via plasma etching and the short-circuit current (Jsc) was evaluated by integration over the measured EQE curve, weighted with the AM 1.5G solar spectrum [20].

3. Results and discussion To study the electrical, structural, and optical properties of ntype SiOx:H layers, a series of films with the thickness of 30–50 nm were deposited on glass substrates by varying CO2/SiH4. Fig. 1 (a) shows the dark conductivity (sD) and Xc for the n-type SiOx:H layers as a function of CO2/SiH4. sD was measured using an Al coplanar electrode configuration formed on the layers. Xc was evaluated from Raman spectra of the layers. These Raman spectra can be deconvoluted to three Gaussian peaks centered near 480, 510, and 520 cm−1, corresponding to the transverse optical (TO) mode of a-Si:H, intermediate fraction, and TO mode of c-Si, respectively. The intermediate fraction originates from the defective crystalline phase [21]. The degree of crystallinity is indicated 60 10

1

50 10-1

30

10-5

D

10-7

Xc

Xc (%)

D (Scm-1)

40 10-3

20

10

10-9 10-11

0 0.0

0.5 CO2 /SiH4

1.0

79

by Xc given by X c ¼ ðI 510 þ I 520 Þ=ðI 480 þ I 510 þ I 520 Þ;

ð1Þ

where Ii denotes an integrated intensity at i cm−1. As shown in the Fig. 1(a), sD is rapidly diminished from 5.2 to 2.5
10−10 S cm−1 with the increase in CO2/SiH4 from 0 to 1.2, which is mainly due to the decrease in Xc from 47% to 0%. From the Xc point of view, n-mcSi:H is obtained with CO2/SiH4 of 0, while n-nc-SiOx:H layers are obtained in the CO2/SiH4 range of 0.4–0.8. When the CO2/SiH4 is 1.2, the deposited layer is a fully n-a-SiOx:H layer. In Fig. 1(b), n at the wavelength of 632 nm for the deposited layers is gradually reduced from 3.6 to 2.2 with the increase in CO2/SiH4 from 0 to 1.2. Thus, the refractive index engineering is easy for the n-type SiOx:H reflecting layers by controlling the process parameter of CO2/SiH4. However, the compromise between the electrical conductivity and the optical transparency is requested for use in thin-film Si solar cells. Fig. 2 exhibits the Raman spectrum of the fabricated p–i–n type a-Si:H single-junction solar cell employing the 30-nm-thick alternated n-type SiOx:H reflector as the back reflector. This spectrum is measured by irradiating the fabricated solar cell from the n-type reflector side opposite to the transparent substrate. Different from the n-mc-Si:H and n-nc-SiOx:H layers in Fig. 1(a), the fabricated solar cell does not show any crystalline phase. Accordingly, the first sublayer contacting to the low hydrogen-diluted n-a-Si:H layer is the highly hydrogen-diluted n-a-Si:H layer because the underlying low hydrogen-diluted n-a-Si:H layer hinders the crystal growth and the alternated reflector is composed of the highly hydrogen-diluted n-a-Si:H sublayers and highly hydrogen-diluted n-a-SiOx:H sublayers. Table 1 provides the initial performances of the p–i–n type a-Si:H single-junction solar cells employing four different structures of 30-nm-thick n-type back reflectors. Here, we kept the thickness values of the graded n-type SiOx:H and alternated n-type SiOx:H back reflectors at 30 nm in order to be the alternatives to the highly hydrogen-diluted n-a-Si:H layer. The a-Si:H solar cell with the highly hydrogen-diluted n-a-Si:H layer shows good FF of 0.709, however low Jsc of 12.1 mA/cm2 limits η to 7.5%. We tested a constant refractive index (n ¼3.0) n-type SiOx:H back reflector deposited with CO2/SiH4 ¼ 0.4 by replacing the highly hydrogendiluted n-a-Si:H layer. Despite the small increase in Jsc, the fabricated solar cell shows no enhancement of η mainly due to reduced FF. On the contrary, the a-Si:H solar cell with the graded n-type SiOx:H back reflector shows the improved η value of 8.0% mainly due to considerably improved Jsc of 12.9 mA/cm2. The comparable open-circuit voltage (Voc) and FF values to those of

4.0

highly hydrogen-diluted n-a-Si:H reflector on n-a-Si:H

Intensity (a. u.)

3.5

n

3.0 2.5 2.0 1.5

a-Si:H solar cell

0.0

0.5

1.0

1.5

CO2 /SiH4 Fig. 1. Variation of the electrical, structural, and optical properties of n-type SiOx:H films deposited on glass substrates as a function of CO2/SiH4. (a) sD and Xc; (b) n at the wavelength of 632 nm. The solid and dashed lines are guides to eye for the films.

300

400

500

600

Raman shift (cm-1) Fig. 2. Raman spectra of the fabricated p–i–n type a-Si:H single-junction solar cell employing the alternated n-type SiOx:H reflector.

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Table 1 Initial performances of the fabricated a-Si:H single-junction solar cells having different n-type reflectors. n-type reflector Highly hydrogen-diluted n-a-Si:H Highly hydrogen-diluted n-a-Si:H/ZnO:Al (50 nm) Graded n-type SiOx:H Graded n-type SiOx:H/ZnO:Al (50 nm) Alternated n-type SiOx:H

Voc (V) 0.876 0.873 0.874 0.881 0.883

Jsc (mA/cm2) 12.1 13.9 12.9 14.0 13.9

FF 0.709 0.697 0.711 0.695 0.698

< Structure of back reflectors > n-a-Si:H/ ZnO:Al

η (%) 7.5 8.5 8.0 8.6 8.6

n-a-Si:H (300 nm) CO2/SiH4 = 0

Graded SiOx:H

Alternated SiOx:H

CO2 /SiH4 = 0 (75 nm)

CO2 /SiH4 = 0 (75 nm)

CO2 /SiH4 = 0.4 (75 nm) CO2 /SiH4 = 1.2 (75 nm) CO2 /SiH4 = 0.8 (75 nm)

CO2 /SiH4 = 0 (75 nm)

CO2 /SiH4 = 1.2 (75 nm) CO2 /SiH4 = 1.2 (75 nm) Al

ZnO:Al (50 nm)

Al

Al

16

Current density (mA/cm2)

14 12 10 8 6

n-a-Si:H n-a-Si:H/ZnO:Al Graded SiOx:H

4

Alternated SiOx:H

2 0 0.0

0.2

0.4 0.6 Voltage (V)

0.8

1.0

1.0

0.8

0.6 EQE

the a-Si:H solar cell with the highly hydrogen-diluted n-a-Si:H layer can be ascribed for the highly conductive part of the graded n-type SiOx:H back reflector contacting to the low hydrogendiluted n-a-Si:H layer. Thereby, the highly hydrogen-diluted n-aSi:H sublayer was selected for the first sublayer of the alternated n-type SiOx:H back reflector. Nonetheless the tiny decrease in FF (¼ 0.698), the fabricated a-Si:H single-junction solar cell with the alternated n-type SiOx:H back reflector shows the further improvement of Jsc up to 13.9 mA/cm2, which leads to improved η of 8.6%. The initial photo J–V curves for the fabricated a-Si:H single-junction solar cells under consideration are displayed in Fig. 3(a), together with the detailed structure of the reflectors. As shown in the figure, the graded n-type SiOx:H and alternated n-type SiOx:H back reflectors have the same mean value of CO2/ SiH4 (¼0.6) during the depositions. Under these limited conditions of the fixed thickness and mean value of CO2/SiH4, the alternated n-type SiOx:H back reflector can be superior to the graded n-type SiOx:H back reflector. However, it should be noted that this result is probably because the thickness and the n value of the graded n-type SiOx:H back reflector are not optimized. Here, we selected the high pressure deposition at 5 Torr to obtain the high deposition rate. Actually, higher sD with lower n has been reported by reducing the deposition pressure [22]. Thus, the thicker graded n-type SiOx:H back reflector with an optimized oxygen grading scheme may provide a further improved internal reflection. For comparison, the performances of the a-Si:H singlejunction solar cell employing a conventional 50-nm-thick Al-doped ZnO (ZnO:Al) back reflector prepared by RF magnetron sputtering between the 30-nm-thick highly hydrogen-diluted n-aSi:H layer and the Al back contact are included. The initial η value of 8.5% is achieved with the Jsc value of 13.9 mA/cm2. This solar cell has slightly low Voc (¼0.873 V) and FF (¼ 0.697) compared to the solar cell without the ZnO:Al back reflector. The initial η value of the fabricated a-Si:H solar cell employing the alternated n-type SiOx:H back reflector is comparable to that employing the more complicated back reflector structure of graded n-type SiOx:H (30 nm)/ZnO:Al (50 nm). Therefore, the alternated n-type SiOx:H back reflector can successfully replace the conventional ZnO back reflector or reduce its thickness, which is very favorable for the low-cost fabrication. It has been reported that the resistive n-type SiOx:H interlayer inserted between the active layer and back contact improves the electrical properties of the a-Si:H singlejunction solar cells fabricated on the highly textured substrates [23,24]. It is suggested that the resistive interlayer can effectively prevent the undesired local current paths of solar cells and resulting in the reduction of the overall leakage current and the improvement of FF. In our study, however, the improvement of the electrical properties for the a-Si:H single-junction solar cells by employing the graded n-type SiOx:H back reflector or the alternated n-type SiOx:H back reflector is not clear because FF tends to decrease slightly despite the small increase in Voc. It may be ascribed for the increased device series resistance due to low sD of

0.4

0.2

n-a-Si:H n-a-Si:H/ZnO:Al Graded SiOx:H Alternated SiOx:H

0.0 400

500

600 700 Wavelength (nm)

800

Fig. 3. Initial characteristics for the fabricated p–i–n type a-Si:H single-junction solar cells having different n-type reflectors. (a) photo J–V curves; (b) EQE spectra.

the graded n-type SiOx:H and alternated n-type SiOx:H back reflectors deteriorates FF. Fig. 3(b) depicts EQE spectra for the fabricated a-Si:H singlejunction solar cells under consideration. It is found that the spectral response in the visible range of 500–700 nm is considerably elevated by employing the graded n-type SiOx:H back reflector. Thus, the refractive index grading effectively increases the optical path length in the solar cell by increasing the reflectance (R) at the back side. The reduced optical loss in the graded n-type SiOx:H back reflector also contributes to the optical enhancement. In addition, the diminished plasmonic absorption losses in the wavelength range is suggested because the insertion of the n-type SiOx:H back reflectors can shift the localized plasmon

S.Y. Myong, L.S. Jeon / Solar Energy Materials & Solar Cells 119 (2013) 77–83

resonances to higher energy [24,25] like the ZnO:Al back reflector [25,26]. The alternated n-type SiOx:H back reflector leads to the further improvement of the spectral response in the wavelength range compared to the graded n-type SiOx:H back reflector. Thereby, the optical improvement of the alternated n-type SiOx: H back reflector is comparable to that of the 50-nm-thick ZnO:Al back reflector. It can be speculated that the multiple reflection caused by the alternately stacking structure comprising of the highly hydrogen-diluted n-a-Si:H sublayers with high n of 3.6 and the highly hydrogen-diluted n-a-SiOx:H sublayers with low n of 2.2 effectively enhances the optical path length of the a-Si:H single-junction solar cells. It should be noted that the enhancement of the spectral response between the alternated n-type SiOx: H and ZnO:Al back reflectors are different. In the wavelength range of 500–600 nm, ZnO:Al back reflector displays the higher EQE values. On the contrary, the alternated n-type SiOx:H back reflector permits the higher EQE values in the wavelength range of 600– 660 nm. However, the EQE spectra become identical at the longer wavelengths. The similar trends of EQE spectra are observed in the literature employing alternative n-type SiOx:H back reflector with the constant n value of 2.5 [24]. Recently, thick (4100 nm) n-μc-SiOx:H back reflector with the constant n value of 1.8 has been reported for use in the μc-Si:H single-junction solar cells [27]. The thick n-μc-SiOx:H back reflector improves η over the ZnO:Al back reflector by enhancing all the parameters (Voc, Jsc, and FF). In the meantime, the application of the 35-nm-thick alternative n-type SiOx:H layer with the constant n value of 1.8 for the bottom cell of the a-Si:H/μc-Si:H doublejunction solar cells has been developed [22]. The n-type SiOx:H layer shows a considerably enhanced spectral response for near-

81

infrared (IR) light compared to an n-μc-Si:H layer. In our case, the developed alternated n-type SiOx:H reflectors were employed as the alternative n-type layers for the a-Si:H/μc-Si:H doublejunction solar cells. Fig. 4 shows the biased EQE spectra of the fabricated p–i–n–p–i–n type a-Si:H/μc-Si:H double-junction solar cells. One cell adopted a 30-nm-thick alternated n-type SiOx:H intermediate reflector as an alternative to the 30-nm-thick highly hydrogen-diluted n-a-Si:H layer in the top cell from the aforementioned reference double-junction solar cell structure. Similarly, the other cell adopted 45-nm-thick alternated n-type SiOx:H back reflector in the bottom cell without any intermediate reflector. All the sublayers of the alternated n-type SiOx:H intermediate reflector and alternated n-type SiOx:H back reflector have the identical thickness of 7.5 nm. The thicker back reflector was chosen in order to enhance R effectively in the long wavelength region by increasing the number of sublayers. R of the alternated n-type SiOx:H reflectors can be simply estimated by [28] 2 2N R ¼ ½ðn2N R −1Þ=ðnR þ 1Þ

ð2Þ

where nR is the refractive index ratio of the highly hydrogendiluted n-a-Si:H sublayer over the highly hydrogen-diluted n-aSiOx:H sublayer and N is the number of total stacking cycles. With the nR value of 1.64, the alternated n-type SiOx:H intermediate reflector has R of 0.57, whereas the alternated n-type SiOx:H back reflector has higher R of 0.81. By comparing the short-circuit current for the top cell (Jsc,top), it is found that the alternated n-type SiOx:H intermediate reflector can afford to increase Jsc,top from 11.0 to 12.1 mA/cm2 due to the reinforced multiple internal reflection. From the big difference of the short-circuit current for the bottom cell (Jsc,bottom) between 12.0 and 13.8 mA/cm2, it is

< Structure of alternated n -type SiOx:H reflectors > Intermediate reflector

Back reflector

a-Si:H top cell

CO2 /SiH4 = 0 (75 nm)

CO2 /SiH4 = 0 (75 nm)

CO2 /SiH4 = 1.2 (75 nm)

CO2 /SiH4 = 12 (75 nm)

CO2 /SiH4 = 0 (75 nm)

CO2 /SiH4 = 0 (75 nm)

CO2 /SiH4 = 1.2 (75 nm)

CO2 /SiH4 = 1.2 (75 nm)

CO2 /SiH4 = 0 (75 nm)

μc-Si:H bottom cell

CO2 /SiH4 = 1.2 (75 nm) Al

1.0 Application of an alternated n-type SiOx:H reflector

as an intermediate reflector as a back reflector

0.8

EQE

0.6

Application

Jsc, top

Jsc, bottom

intermediate reflector

12.1 mA/cm2

12.0 mA/cm2

back reflector

11.0 mA/cm2

13.8 mA/cm2

0.4

0.2

0.0 400

600 800 Wavelength (nm)

1000

1200

Fig. 4. Comparison of the spectral response between the fabricated a-Si:H/mc-Si:H double-junction solar cell with the alternated n-type SiOx:H intermediate reflector and that with the alternated n-type SiOx:H back reflector.

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obvious that the alternated n-type SiOx:H back reflector can elevate Jsc,bottom. Since the total n-type layer thickness of the top cell is identical, the gap of EQE between the bottom cells in the wavelength range of 500–720 nm is not considered due to the optical loss in the intermediate reflector. However, this gap can be attributed to the enhanced reflection of shorter wavelength light back into the top cell by the intermediate reflector [14,22,29,30] and the increased reflection in the long wavelength range by the solar cell adopting the effective back reflector [22,27]. The difference of EQE between the bottom cells in the near-IR range is mainly due to the improved reflection caused by the solar cell adopting the alternated n-type SiOx:H back reflector. Therefore, both the alternated n-type SiOx:H intermediate and back reflectors can effectively improve the optical path length of the a-Si:H/μc-Si: H double-junction solar cells. Based on the aforementioned results, we optimized the p–i–n– p–i–n type a-Si:H/μc-Si:H double-junction solar cell by changing the parameters of absorbers. Table 2 describes the initial and stabilized performances of the optimized the p–i–n–p–i–n type a-Si:H/μc-Si:H double-junction solar cell simultaneously employing the alternated n-type SiOx:H intermediate reflector and the alternated n-type SiOx:H back reflector. 50-nm-thick ZnO:Al back reflector is added between the alternated n-type SiOx:H back reflector and the Ag/Al back contact for the further improvement [25]. Fig. 5 exhibits the corresponding photo J–V curves of the optimized solar cell. As a result, the high initial η value of 13.1% is achieved (Voc ¼ 1.41 V, Jsc ¼12.8 mA/cm2, and FF¼0.724). After light soaking, η is deteriorated to 11.5% with the degradation rate (¼ Δη/initial η) of 12.2% (Voc ¼1.39 V, Jsc ¼12.3 mA/cm2, and FF¼0.670). The corresponding light-induced behaviors for the solar cell against the light soaking test are plotted in Fig. 6. Thus, it is demonstrated that the developed alternated n-type SiOx:H intermediate and back reflectors are good for light trapping in the p–i–n–p–i–n type a-Si:H/μc-Si:H double-junction solar cells. Furthermore, the considerably thin alternated n-type SiOx:H intermediate and back reflectors are very promising for mass Table 2 Effect of light soaking on the fabricated a-Si:H/mc-Si:H double-junction solar cell. Status

Voc (V)

Jsc (mA/cm2)

FF

η (%)

Initial Stabilized

1.41 1.39

12.8 12.3

0.724 0.670

13.1 11.5

Current density (mA/cm2)

15

10 initial stabilized

100

η / initial η (%)

82

95

90

85

80

0

50 100 150 Light soaking time (h)

200

Fig. 6. Light-induced degraded behaviors of the optimized a-Si:H/mc-Si:H doublejunction solar cell against the light soaking test.

production of large-area p–i–n–p–i–n type a-Si:H/μc-Si:H doublejunction solar modules due to the cost-effective in situ deposition and monolithic series integration processes. On the other hand, the proposed n-type SiOx:H intermediate and back reflectors may also be adopted for good light trapping of the thin-film Si triplejunction solar cells because the high efficient (initial η of 16.3%) thin-film Si triple-junction solar cell is achieved by employing the n-type SiOx:H intermediate reflector with the constant n value of ∼2.0 between the middle and bottom cells [31].

4. Conclusion We have investigated the improvement of p–i–n type a-Si:H single-junction and p–i–n–p–i–n type a-Si:H/μc-Si:H doublejunction solar cells by applying the n-type SiOx:H intermediate and back reflectors with the nonuniform oxygen content in the thickness direction. It is found that the graded n-type SiOx:H back reflector elevates the optical path length of the p–i–n type a-Si:H single-junction solar cells due to the relevant refractive index grading. Moreover, the alternated n-type SiOx:H back reflector composed of the highly hydrogen-diluted n-a-Si:H sublayers and the highly hydrogen-diluted n-a-SiOx:H sublayers effectively improves the optical path length of the a-Si:H single-junction solar cell due to the multiple reflection. Furthermore, it is verified that the developed alternated n-type SiOx:H reflector is suitable for the back reflector as well as the intermediate reflector of the p–i–n–p–i–n type a-Si:H/μc-Si:H double-junction solar cells. The considerably thin (30–45 nm) alternated n-type SiOx:H reflectors can be easily prepared using the in situ PECVD technique. Furthermore, since the alternated n-type SiOx:H reflector can avoid the lateral shunting problem during the monolithic series integration of segments, it is a promising option for cost-effective mass production of large-area thin-film Si solar modules. References

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0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Voltage (V) Fig. 5. Initial and stabilized photo J–V curves for the optimized a-Si:H/mc-Si:H double-junction solar cell.

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