Development of Hydrogenated Microcrystalline Silicon-Germanium Alloys for Improving Long-Wavelength Absorption in Si-Based Thin-Film Solar Cells

(1)

Research Article

Development of Hydrogenated Microcrystalline

Silicon-Germanium Alloys for Improving Long-Wavelength

Absorption in Si-Based Thin-Film Solar Cells

Yen-Tang Huang, Hung-Jung Hsu, Shin-Wei Liang,

Cheng-Hang Hsu, and Chuang-Chuang Tsai

Department of Photonics, National Chiao Tung University, 1001 University Road, Hsinchu 300, Taiwan Correspondence should be addressed to Yen-Tang Huang; [email protected]

Received 25 April 2014; Accepted 9 July 2014; Published 22 July 2014 Academic Editor: Serap Gunes

Copyright © 2014 Yen-Tang Huang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Hydrogenated microcrystalline silicon-germanium (𝜇c-Si1−𝑥Ge𝑥:H) alloys were developed for application in Si-based thin-film solar cells. The effects of the germane concentration(𝑅GeH₄) and the hydrogen ratio (𝑅H₂) on the 𝜇c-Si1−𝑥Ge𝑥:H alloys and the corresponding single-junction thin-film solar cells were studied. The behaviors of Ge incorporation in a-Si_1−𝑥Ge_𝑥:H and 𝜇c-Si_1−𝑥Ge_𝑥:H were also compared. Similar to a-Si_1−𝑥Ge_𝑥:H, the preferential Ge incorporation was observed in 𝜇c-Si_1−𝑥Ge_𝑥:H. Moreover, a higher𝑅H₂significantly promoted Ge incorporation for a-Si1−𝑥Ge𝑥:H, while the Ge content was not affected by𝑅H₂ in𝜇c-Si_1−𝑥Ge_𝑥:H growth. Furthermore, to eliminate the crystallization effect, the 0.9𝜇m thick absorbers with a similar crystalline volume fraction were applied. With the increasing𝑅_GeH₄, the accompanied increase in Ge content of𝜇c-Si_1−𝑥Ge_𝑥:H narrowed the bandgap and markedly enhanced the long-wavelength absorption. However, the bias-dependent EQE measurement revealed that too much Ge incorporation in absorber deteriorated carrier collection and cell performance. With the optimization of𝑅_H₂and 𝑅GeH4, the single-junction𝜇c-Si1−𝑥Ge𝑥:H cell achieved an efficiency of 5.48%, corresponding to the crystalline volume fraction of

50.5% and Ge content of 13.2 at.%. Compared to𝜇c-Si:H cell, the external quantum efficiency at 800 nm had a relative increase by 33.1%.

1. Introduction

Hydrogenated amorphous silicon (a-Si:H) has been widely

studied [1,2] and employed as an absorber in silicon

thin-film solar cells [3] because of its high absorption coefficient in the visible range of the solar spectrum and the feasibility of large area deposition. However, the solar spectrum is distributed from ultraviolet to near-infrared (IR) region. The bandgap of approximately 1.75 eV [4] for a-Si:H limits the absorption in IR region. On the concept of light absorption, only the photons having the energies larger than the bandgap of absorbers can contribute to photoexcited carriers [5]. For effective use of the low-energy photon in the solar spectrum, the development of a lower-bandgap material is important. Accordingly, the integration of lower-bandgap material and the concept of spectrum splitting have been applied as multi-junction thin-film solar cells for allowing more efficient use

of solar spectrum. Compared to single-junction solar cell, the multijunction cell generally has a broadened and effective spectral response. The more efficient light absorption is attri-buted to the component cells with different bandgap absorbers, which leads to a higher cell efficiency. Yunaz et al. have demonstrated a potential efficiency over 20% by using AMPS-1D simulation for the Si-based multijunction thin-film solar cell [6]. Other groups have integrated a-Si:H and hydrogenated microcrystalline silicon (𝜇c-Si:H) absorbers into tandem structure cells with a stabilized efficiency over 10% [7–9]. Moreover, Yan et al. have reported an a-Si:H/a-SiGe:H/𝜇c-Si:H triple-junction cell reached a recorded effi-ciency of 16.3% [10].

Due to a lower bandgap of 1.1 eV [5],𝜇c-Si:H has been

utilized as an absorber for IR absorption [11–14]. In addition, 𝜇c-Si:H has a minor Staebler-Wronski effect (SWE) [14], which has less impact on the long term film quality and

Volume 2014, Article ID 579176, 7 pages http://dx.doi.org/10.1155/2014/579176

(2)

cell performance than amorphous material. Nevertheless, the

indirect bandgap nature of𝜇c-Si:H leads to a low absorption

coefficient. Therefore, a thick 𝜇c-Si:H absorber is usually

needed to obtain adequate IR absorption. Matsui et al. have reported that the Ge incorporation in microcrystalline silicon network led to a bandgap narrowing and an increase in IR

absorption, with the consequence of a thinner𝜇c-Si_1−𝑥Ge_𝑥:H

absorber in the cells [15–17]. The𝜇c-Si_1−𝑥Ge_𝑥:H consists of

an amorphous-crystalline mixed phase of binary SiGe alloys, which are affected by the deposition parameters including

the hydrogen ratio (𝑅H₂) and the germane concentration

(𝑅GeH₄). The addition of Ge to Si network not only lowers

the bandgap, but could also reduce the crystallization of the films. The crystalline volume fraction can not only influence the electrical properties including bandgap and carrier collection, but also change the optical absorption. The

trade-off between crystallization and Ge incorporation of

𝜇c-Si_1−𝑥Ge_𝑥:H alloys should be carefully manipulated for the

requirement of IR absorption.

Previous works on 𝜇c-Si_1−𝑥Ge_𝑥:H alloy [18, 19] have

reported the effect of Ge incorporation by varying𝑅GeH4but

have not yet considered the accompanied variation of crystal-lization. In this work, to eliminate the effect of different degree

of crystallization, the𝜇c-Si_1−𝑥Ge_𝑥:H absorber with a similar

crystalline volume fraction was applied to indeed discuss the effect of Ge content on cell performance. Furthermore, we compared the behaviors of the Ge incorporation in

a-Si_1−𝑥Ge_𝑥:H and𝜇c-Si_1−𝑥Ge_𝑥:H alloys. The effects of𝑅H₂and

𝑅GeH₄on Ge incorporation were discussed.

2. Experimental Detail

Silicon thin films including 𝜇c-Si_1−𝑥Ge_𝑥:H were deposited

by a single-chamber process in a multichamber plasma-enhanced chemical vapor deposition (PECVD) system

equipped with 27.12 MHz rf power, NF₃in situ plasma

clean-ing, and a load-lock chamber. The films were prepared on

Corning EAGLE XG glass substrate at approximately 200∘C.

A gas mixture of highly H₂-diluted SiH₄ and GeH₄ was

introduced to deposit 𝜇c-Si_1−𝑥Ge_𝑥:H thin films. The 𝑅H₂,

defined as [H₂]/[SiH₄], was varied from 71.4 to 123.0. The

𝑅GeH₄, defined as [GeH4]/[GeH4+ SiH4], was changed from

0 to 6.8%. In contrast, the lower𝑅H₂ varied from 0 to 6 and

the𝑅GeH₄ varied from 8.3% to 16.7% were employed for

a-Si_1−𝑥Ge_𝑥:H deposition. The film Ge content was calculated by

the integrated intensities of Ge3d and Si2p core lines using the quantitative X-ray photoelectron spectroscopy (XPS) analysis [20–22]. A presputtering was conducted to eliminate con-taminations and native oxides on the film surface. We have found in our previous work that the Ge content would have variation in the incubation layer. This incubation region

(approximately 0.1𝜇m) occupied only small part of the

absorbing layer (∼0.9 𝜇m). The measured Ge content shown in the paper should be representative for the absorbing layer. The crystalline volume fraction was estimated from Raman spectra, which were obtained from a high-resolution confocal Raman microscope with an excitation laser at a wavelength of 488 nm. The dark and photocoplanar conductivities of

0 2 4 6 80 100 120 0 10 20 30 40 5.0% 7.1% Amorphous 16.7% 11.1% 8.3% Microcrystalline Film G e co n ten t (a t.% ) RGeH4 RH2 (a) 0 2 4 6 80 100 120 0 1 2 3 4 7.1% 5.0% Amorphous Microcrystalline 16.7% 11.1% 8.3% G e inco rp o ra tio n RGeH4 RH₂ efficienc y ([G e]/ RGe H4 ) (b)

Figure 1: The variations of (a) Ge content and (b) incorporation efficiency versus𝑅H₂in amorphous [23] and microcrystalline SiGe alloys with different𝑅_GeH₄.

the prepared films were obtained by an 𝐼-𝑉 measurement

system equipped with an AM1.5G illumination. A spec-trophotometer was used to determine the transmittance and

the reflectance of the films. The optical bandgap(𝐸₀₄) was

obtained when the absorption coefficient is 104cm−1.

The commercial textured SnO₂:F-coated substrates were

utilized for preparing superstrate p-i-n𝜇c-Si_1−𝑥Ge_𝑥:H cells. A

0.9𝜇m thick 𝜇c-Si_1−𝑥Ge_𝑥:H absorber was employed in

single-junction solar cells with a p-type 𝜇c-Si:H layer and an

n-type hydrogenated microcrystalline silicon oxide

(𝜇c-SiO_𝑦:H) layer. The cell was characterized by an AM1.5G solar

simulator. The area of the device for measurement was

0.25 cm2which was defined by the silver electrode. A

measur-ing system havmeasur-ing monochromator, chopper, lock-in

ampli-fier, and 𝐼-𝑉 meter was applied to measure the external

quantum efficiency (EQE).

3. Results and Discussion

3.1. Ge-Incorporation in Amorphous and Crystalline Silicon-Germanium Alloys. The dependence of Ge content ([Ge]) on

𝑅H2with different𝑅GeH4in amorphous and microcrystalline

SiGe alloys is shown in Figure 1(a). As can be seen, the

Ge content in a-Si_1−𝑥Ge_𝑥:H alloys rapidly increased as𝑅H₂

(3)

saturate as𝑅H₂was larger than 2. The phenomenon suggested

that the hydrogen atoms promoted Ge incorporation in the amorphous network [23]. One possible reason may relate

to the sticky nature of GeH₃ species more than the SiH₃

species. The diffusion length of GeH₃ species is less than

SiH₃ species during the growth of SiGe alloy [24], which

makes it more difficult to reach the energetically favorable sites on the film surface. As a result, Ge is easier to form weak bonds than Si in SiGe binary network. When the atomic hydrogen is sufficient in plasma, a high H-coverage growth surface and local heating lead to well-relaxed network [25–27]. Thus, rigid Ge-related bonds increase as increasing hydrogen. Accordingly, more Ge atoms can be left in the films.

In high hydrogen-containing gas mixture with𝑅H₂over 2,

the saturation of Ge content was observed for a-Si_1−𝑥Ge_𝑥:H

alloys. Presumably, the sufficient hydrogen atoms promote

rigid Ge bonding in the films. Compared to a-Si_1−𝑥Ge_𝑥:H

alloys, a much higher hydrogen diluted gas mixture is needed

for the crystallization of the𝜇c-Si_1−𝑥Ge_𝑥:H. When the𝑅H₂

was over 85 at a fixed𝑅GeH₄, Ge content was not significantly

changed, suggesting that the effect of hydrogen for Ge

incorporation in the 𝜇c-Si_1−𝑥Ge_𝑥:H films has less impact.

The resulting Ge content in the 𝜇c-Si_1−𝑥Ge_𝑥:H film with

increasing𝑅H₂ was kept at approximately 13 and 16.7 at.%,

with𝑅GeH₄of 5.0% and 7.1%, respectively.

In addition to the Ge content, the incorporation efficiency of Ge was also discussed. The incorporation efficiency

repre-sents the ratio of the transformation from GeH₄ to film Ge

content, defined as [Ge]/𝑅GeH₄. As shown inFigure 1(b), the

tendency of incorporation efficiency of a-Si_1−𝑥Ge_𝑥:H and

𝜇c-Si_1−𝑥Ge_𝑥:H films was similar to that of the film Ge content

with the increasing𝑅H₂. The Ge incorporation efficiency was

larger than one in both amorphous and microcrystalline SiGe alloys. This suggests that Ge was preferentially incorporated into films more than Si. The incorporation efficiency over 1

also indicates that the change of𝑅GeH₄alters the Ge content

significantly, as well as the film characteristics. One of the

reasons was the less dissociation energy of GeH₄compared to

SiH₄. The more efficient decomposition of GeH₄was known

from SiH₄-GeH₄-H₂ discharge plasma field [28]. However,

adding more GeH₄decreased the Ge incorporation efficiency.

More produced sticky GeH₃ precursors led to an increase

in the weak Ge-related bonds [29,30]. Consequently, under

the hydrogen-containing atmospheres, the probability of

the SiH₃ replacement on a weak Ge-bonded site may be

enhanced, which reduced the effective Ge incorporation. In short, the preferential incorporation of Ge in SiGe

alloys was observed. Compared to high𝑅H₂ environment,

the Ge content in SiGe alloys was affected by the hydrogen

significantly in low𝑅H₂environment. More Ge content can be

achieved by adding more GeH₄in the gas mixture.

Neverthe-less, with increasing Ge content, the incorporation efficiency

of Ge into solid phase decreased with increasing𝑅GeH₄.

3.2. Effect of the Hydrogen Ratio on Film Properties and Cell Performance. The microstructure of 𝜇c-Si_1−𝑥Ge_𝑥:H films

deposited with different𝑅H₂ at𝑅GeH₄ of 5% was studied by

the Raman spectroscopy.Figure 2shows the resulting Raman

400 450 500 550 480 520 510 In te rm ed ia te 120.3 a-S i 107.6 94.9 88.6 83.5 c-S i R ama n in te n si ty (a.u .) RGeH4= 5% RH₂ Raman shift (cm−1)

Figure 2: The Raman spectra of𝜇c-Si_1−𝑥Ge_𝑥:H films with different 𝑅H2.

spectra, where the transverse optical (TO) modes mainly con-sisted of amorphous, intermediate phase and crystalline Si-Si networks [31]. The TO mode of amorphous Si-Si network is

distributed as a Gaussian function at 480 cm−1. This is

attributed to the Si-Si network in short-range order. The full width of half maximum and the Raman shift of a-Si phase are related to the variation of bonding angle of a-Si network

[32,33]. For the narrow c-Si Lorenzian peak, the TO mode

is at 520 cm−1. When the c-Si grain becomes as small as few

nanometers in a crystalline-to-amorphous transition region, the Raman shift of c-Si peak decreases because of momentum

conservation [34,35]. The peak of intermediate phase is in a

Raman shift ranging approximately from 490 to 510 cm−1.

This is ascribed to the defective part of the Si-Si crystallines, which include small size crystallite, bond dilation at grain boundaries, or a silicon wurtzite phase consisting of twins

[36, 37]. When the 𝑅H₂ increased from 83.5 to 120.3, more

crystalline phase is accompanied with less amorphous phase. However, the resulting c-Si peak constantly appeared near

512 cm−1 as increasing 𝑅H₂. In previous work [17, 38, 39],

when Ge presents nearby the crystallites, the c-Si peak has a red-shift. In addition, the increased Ge content was in a linear correlation with decreasing c-Si peak. As mentioned in

Section 3.1, Ge content was unchanged in the𝜇c-Si_1−𝑥Ge_𝑥:H

films at a fixed𝑅GeH₄. The higher degree of crystallization at a

higher𝑅H₂is contributed to more crystallites in the films. In

addition, there was no significant difference in Raman

spec-tra at approximately 300 cm−1 for 𝜇c-SiGe:H samples. This

may be due to a low Ge content used in this study, which contributed to negligible Ge-Ge TO mode signal from the crystal phase [40].

Effect of𝑅H2on𝑋𝐶and optical bandgap(𝐸04) is shown

inFigure 3. The crystalline volume fraction(𝑋_𝐶) is defined by

(𝐼₅₂₀+ 𝐼₅₁₀)/(𝐼₅₂₀+ 𝐼₅₁₀+ 𝐼₄₈₀), where 𝐼₅₂₀,𝐼₅₁₀, and𝐼₄₈₀were

the integrated intensities of crystalline, intermediate, and

(4)

Table 1: Properties of𝜇c-Si_1−𝑥Ge_𝑥:H absorber and the corresponding performance of single-junction cells with different𝑅H₂of 88.6, 94.9, 101.3, 124.1. The𝑅GeH4of these cells was kept at 5.0%.

𝑅H₂ 𝑋𝐶(%) 𝐸04(eV) 𝑉OC(mV) 𝐽SC(mA/cm2) FF (%) Eff. (%)

88.6 44.0 1.91 485 17.17 58.9 4.90 94.9 52.8 1.90 475 18.61 62.0 5.48 101.3 59.1 1.89 460 18.80 62.4 5.40 124.1 70.6 1.87 430 19.25 59.4 4.91 70 90 110 130 0 20 40 60 80 100 1.85 1.90 1.95 2.00 0% 5.0% 5.0% XC E04 E04 (eV) XC (% ) RGeH4 RH₂

Figure 3: Effect of𝑅H₂ on the properties of𝜇c-Si1−𝑥Ge𝑥:H films prepared with𝑅_GeH₄of 0 and 5.0%. The circle and the square symbols represent the crystalline volume fraction(𝑋_𝐶) and the bandgap (𝐸04), respectively.

the𝑋_𝐶 increased with increasing 𝑅H₂. More H2 in the gas

mixture promoted the crystallization of 𝜇c-Si_1−𝑥Ge_𝑥:H

growth. Moreover, given the same𝑋_𝐶, the𝑅H₂required for

𝜇c-Si_1−𝑥Ge_𝑥:H was much larger than that for𝜇c-Si:H. This

suggests that adding GeH₄ significantly suppressed

crys-talline growth. This should be due to the distorted Si network by incorporating Ge, and more Ge-induced defects in the

film, which needs more H-atom to be eliminated. When𝑅H₂

was varied from 83.5 to 124.1 and𝑅GeH₄ was kept at 5%, the

𝑋_𝐶 increased from 25.2% to 70.6%, corresponding to the

decreased𝐸₀₄from 1.93 to 1.87 eV. The more crystalline phase

led to a narrower bandgap, which shifted light absorption to

IR. To investigate the effect of𝑋_𝐶of𝜇c-Si_1−𝑥Ge_𝑥:H absorbers

on cell performance, we further employed different

𝜇c-Si_1−𝑥Ge_𝑥:H alloys as absorbers by changing the𝑅H₂.

Figure 4 shows the cell structure and the 𝐽-𝑉

charac-teristics of 𝜇c-Si_1−𝑥Ge_𝑥:H p-i-n single-junction cells using

absorbers prepared with different𝑅H₂. This cell performance

is shown inTable 1. Accompanied with the increasing 𝑅H₂

from 88.6 to 124.1, the resulting bandgap narrowing of the absorber influenced the internal electric field and decreased

the𝑉OCfrom 485 to 430 mV. On the contrary, the𝐽SCwas

sig-nificantly enhanced from 17.17 to 19.25 mA/cm2. More

crys-talline phase in the film contributed to more photocurrent in

the cells due to the lower bandgap. When the𝑅H₂was 94.9,

the corresponding𝑋_𝐶of the absorber was 50.5% which led to

an optimal cell efficiency of 5.48%.

0.0 0.1 0.2 0.3 0.4 0.5 0 5 10 15 20 124.1 88.6 94.9 101.3 Voltage (V) C u rr en t den si ty (mA/cm 2) Glass Ag h RGeH4= 5.0% RH2 Textured SnO₂:F p-_𝜇c-Si:H i-𝜇c-SiGe:H 0.9 𝜇m n-𝜇c-SiO_y:H

Figure 4: Schematic diagram of the cell structure and the 𝐽-𝑉 characteristics of 𝜇c-Si_1−𝑥Ge_𝑥:H solar cells with different 𝑅_H₂ as 𝑅GeH₄was 5.0%.𝑅H₂ = 88.6 (dot dash line), 94.9 (black line), 101.3 (gray line), and 124.1 (dash line).

3.3. Effect of the Germane Concentration on Film Properties and Cell Performance. InSection 3.1, we have shown that the

𝑅GeH4 significantly changed the Ge content in the film. To

reveal the effect of 𝑅GeH4 on cell performance is therefore

important for improving long-wavelength absorption. The

𝜇c-Si_1−𝑥Ge_𝑥:H absorbers in single-junction solar cells were

prepared with different𝑅GeH₄of 0, 3.7%, 5.0%, and 6.8%. In

addition, the𝜇c-Si_1−𝑥Ge_𝑥:H absorber with a similar𝑋_𝐶 of

approximately 55% was applied to eliminate the effect of the crystallization of absorber on the cell performance. When the

𝑅GeH₄increased from 0 to 5.0%, the film Ge content increased

from 0 to 13.2 at.%, as shown in Table 2. As a result, the

bandgap decreased from 1.96 to 1.85 eV, corresponding to a

reduction in𝑉OCof 90 mV. The worsened FF from 71.0% to

59.3% may be due to the more Ge-related defects created in the absorber with increasing Ge incorporation. With more Ge incorporation which reduced the bandgap of the absorber,

the𝐽SCsignificantly increased from 17.38 to 18.50 mA/cm2due

to more optical absorption. When the𝑅GeH₄ was 6.8%, the

film Ge content further went up to 18.0 at.%, which resulted

in the degraded cell performance. The 𝑉OC, FF, and 𝐽SC

decreased to 370 mV, 53.0%, and 17.27 mA/cm2, respectively.

The improvement of 𝐽SC according to the change of

Ge content can be revealed by the EQE measurement. As

shown inFigure 5, no significant drop in spectral response in

short-wavelength region was observed as the𝑅GeH₄increased

(5)

Table 2: Properties of𝜇c-Si_1−𝑥Ge_𝑥:H absorber and the corresponding performance of single-junction cells with different𝑅GeH4of 0, 3.7%,

5%, and 6.8%. The𝑋_𝐶of these cells was kept at approximately 55%.

𝑅GeH4 𝑅H2 [Ge] (at.%) 𝐸04(eV) QE800 nm(%) 𝑉OC(mV) 𝐽SC(mA/cm

2₎ _{FF (%)} _{Eff. (%)} 0 81.0 0 1.96 26.6 540 17.38 71.0 6.67 3.7 104.8 8.8 1.89 28.3 490 17.16 62.2 5.23 5.0 109.5 13.2 1.85 35.4 460 18.50 59.3 5.04 6.8 166.1 18.0 1.83 31.0 370 17.27 53.0 3.83 300 500 700 900 1100 0 20 40 60 80 100 0% 6.8% EQ E (%) Wavelength (nm) 5.0% 3.7% 0.9 𝜇m thick absorber XC= 55% RGeH4

Figure 5: The spectral response of𝜇c-Si_1−𝑥Ge_𝑥:H p-i-n solar cells. The𝜇c-Si_1−𝑥Ge_𝑥:H absorbers were prepared with the𝑅_GeH₄of 0% (black fine line), 3.7% (gray bold line), 5% (black bold line), and 6.8% (dash line).

600–1100 nm was enhanced. The external quantum efficiency at 800 nm increased from 26.6% to 35.4%. This relative increase of 33.1% in spectral response suggested that Ge incorporation effectively enhances the optical absorption in the infrared region. However, the red-to-IR response

reduced as the absorber was prepared with𝑅GeH₄ of 6.8%.

Too much Ge incorporation could degrade the transport of carriers generated in the long-wavelength region, which will

be discussed in the next section. Besides, when the 𝑅GeH₄

was 6.8%, the𝜇c-Si_1−𝑥Ge_𝑥:H absorber near p/i interface may

preferentially grow in amorphous phase. Compared to micro-crystalline phase, amorphous phase generally has higher short-wavelength absorption. As a result, the increase in the spectral response range of 300–500 nm was observed.

The results of EQE measurement for the𝜇c-Si_1−𝑥Ge_𝑥:H

cells having absorber prepared with 𝑅GeH4 of 5.0% and

6.8% were presented inFigure 6. The spectral response was

measured under 0 and−2 bias voltages to reveal the difference

in carrier transport. If a reverse voltage bias of −2 V was

applied to the device, the electric built-in field can be enlarged and the photogenerated carriers trapped by the defects can be driven out. If the cell having defects was measured with the reverse bias, the spectral response would be enlarged. For

the𝜇c-Si_1−𝑥Ge_𝑥:H cell employing the absorber prepared by

𝑅GeH₄of 6.8%, the difference of𝐽SCas measured by EQE with

0 and−2 bias voltages was 1.05 mA/cm2. In comparison, the

300 500 700 900 1100 0 20 40 60 80 100 0 20 40 60 80 100 EQ E (%) EQ E (%) Wavelength (nm) 0 V −2 V 6.8% RGeH4= 5.0%

Figure 6: Spectral response of𝜇c-Si_1−𝑥Ge_𝑥:H cell measured with (dash line) and without (solid line) bias voltage. The absorbers were prepared with𝑅_GeH₄of 5.0% and 6.8%.

difference of𝐽SCfor𝜇c-Si1−𝑥Ge𝑥:H cell employing absorber

prepared with 𝑅GeH₄ of 5.0% under the same bias voltages

was less than 0.25 mA/cm2. The result indicates that too

much Ge incorporation would lead to the degraded carrier collection and worsen cell performance. Moreover, in con-trast to the photogenerated electrons, the holes generated by long-wavelength photons near back contact would drift toward longer distance. The change in spectral response was presumably due to the degraded hole collection [43].

4. Conclusion

The effects of𝑅GeH₄ and 𝑅H₂ on𝜇c-Si1−𝑥Ge𝑥:H alloys and

the corresponding single-junction cells were studied.

Simi-lar to a-Si_1−𝑥Ge_𝑥:H, the preferential Ge incorporation was

observed in𝜇c-Si_1−𝑥Ge_𝑥:H. Moreover, a higher𝑅H₂

signif-icantly promoted Ge incorporation for a-Si_1−𝑥Ge_𝑥:H, while

(6)

growth. To eliminate the crystallization effect, the 0.9𝜇m thick absorbers with a similar crystalline volume fraction

were applied. With the increasing𝑅GeH4, the accompanied

increase in Ge content of 𝜇c-Si_1−𝑥Ge_𝑥:H narrowed the

bandgap and edly enhanced the long-wavelength absorption.

When the𝑅GeH₄increased from 0 to 5%, the spectral response

at 800 nm was significantly improved from 26.6% to 35.4%, which was a relative increase by 33.1%. However, the bias-dependent EQE measurement revealed that too much Ge incorporation in absorber deteriorated carrier collection and

cell performance. With the optimization of𝑅H₂ and𝑅GeH₄,

the single-junction𝜇c-Si_1−𝑥Ge_𝑥:H cell achieved an efficiency

of 5.48%, corresponding to the crystalline volume fraction of 50.5% and Ge content of 13.2 at.%. Future work will include

the application of𝜇c-Si_1−𝑥Ge_𝑥:H absorbers in the tandem cell

structure.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

This work was sponsored by National Science Council in Taiwan under Contract no. NSC-102-3113-P-008-001 and no. NSC-2221-E-009-122.

References

[1] W. E. Spear and P. G. Le Comber, “Substitutional doping of amorphous silicon,” Solid State Communications, vol. 17, no. 9, pp. 1193–1196, 1975.

[2] C. C. Tsai, J. C. Knights, G. Chang, and B. Wacker, “Film for-mation mechanisms in the plasma deposition of hydrogenated amorphous silicon,” Journal of Applied Physics, vol. 59, no. 8, pp. 2998–3001, 1986.

[3] D. E. Carlson and C. R. Wronski, “Amorphous silicon solar cell,” Applied Physics Letters, vol. 28, no. 11, article 671, 1976. [4] M. A. Green, Solar Cells: Operating Principles, Technology and

System Applications, University of New South Wales, Sydney, Australia, 1998.

[5] M. A. Green, “Third generation photovoltaics: solar cells for 2020 and beyond,” Physica E: Low-dimensional Systems and Nanostructures, vol. 14, pp. 65–70, 2002.

[6] I. A. Yunaz, A. Yamada, and M. Konagai, “Theoretical analysis of amorphous silicon alloy based triple junction solar cells,” Japanese Journal of Applied Physics, vol. 46, p. L1152, 2007. [7] J. Meier, J. Spitznagel, S. Fay et al., “Enhanced light-trapping

for micromorph tandem solar cells by LP-CVD ZnO,” in Proceedings of the 29th IEEE Photovoltaic Specialists Conference (PVSC ’02), pp. 1118–1121, New Orleans, La, USA, May 2002. [8] K. Yamamoto, A. Nakajima, M. Yoshimi et al., “A high efficiency

thin film silicon solar cell and module,” Solar Energy, vol. 77, no. 6, pp. 939–949, 2004.

[9] J. Meier, S. Dubail, R. Fl¨uckiger, D. Fischer, H. Keppner, and A. Shah, in Proceedings of the 1st World Conference on Photovoltaic Energy Conversion (WCPEC '94), p. 409, Waikoloa, Hawaii, USA, 1994.

[10] B. Yan, G. Yue, L. Sivec, J. Yang, S. Guha, and C.-S. Jiang, “Inno-vative dual function nc-SiO_𝑥:H layer leading to a>16% efficient multi-junction thin-film silicon solar cell,” Applied Physics Letters, vol. 9, Article ID 113512, 2011.

[11] J. Meier, R. Fl¨uckiger, H. Keppner, and A. Shah, “Complete microcrystalline p−i−n solar cell—crystalline or amorphous cell behavior?” Applied Physics Letters, vol. 65, p. 860, 1994. [12] A. V. Shah, J. Meier, E. Vallat-Sauvain et al., “Material and solar

cell research in microcrystalline silicon,” Solar Energy Materials and Solar Cells, vol. 78, no. 1–4, pp. 469–491, 2003.

[13] K. Saito, M. Sano, S. Okabe, S. Sugiyama, and K. Ogawa, “Microcrystalline silicon solar cells fabricated by VHF plasma CVD method,” Solar Energy Materials and Solar Cells, vol. 86, no. 4, pp. 565–575, 2005.

[14] B. Yan, G. Yue, X. Xu, J. Yang, and S. Guha, “High efficiency amorphous and nanocrystalline silicon solar cells,” Physica Sta-tus Solidi A: Applications and Materials Science, vol. 207, no. 3, pp. 671–677, 2010.

[15] T. Matsui, M. Kondo, K. Ogata, T. Ozawa, and M. Isomura, “Influence of alloy composition on carrier transport and solar cell properties of hydrogenated microcrystalline silicon-germ-anium thin films,” Applied Physics Letters, vol. 89, Article ID 142115, 2006.

[16] T. Matsui, K. Ogata, M. Isomura, and M. Kondo, “Micro-crystalline silicon–germanium alloys for solar cell application: growth and material properties,” Journal of Non-Crystalline Solids, vol. 352, no. 9–20, pp. 1255–1258, 2006.

[17] G. Ganguly, T. Ikeda, T. Nishimiya, K. Saitoh, M. Kondo, and A. Matsuda, “Hydrogenated microcrystalline silicon germanium: a bottom cell material for amorphous silicon-based tandem solar cells,” Applied Physics Letters, vol. 69, no. 27, pp. 4224– 4226, 1996.

[18] T. Matsui, C. W. Chang, T. Takada, M. Isomura, H. Fujiwara, and M. Kondo, “Microcrystalline Si_1−𝑥Ge_𝑥solar cells exhibiting enhanced infrared response with reduced absorber thickness,” Applied Physics Express, vol. 1, no. 3, Article ID 031501, 2008. [19] T. Matsui, K. Ogata, C. W. Chang, M. Isomura, and M.

Kondo, “Carrier collection characteristics of microcrystalline silicon-germanium p-i-n junction solar cells,” Journal of Non-Crystalline Solids, vol. 354, no. 19–25, pp. 2468–2471, 2008. [20] G. Lucovsky, S. S. Chao, J. E. Tyler, and G. De Maggio, “An XPS

study of sputtered a−Si,Ge alloys,” Journal of Vacuum Science & Technology A, vol. 21, p. 838, 1982.

[21] M. P. Seah, “The quantitative analysis of surfaces by XPS: a review,” Surface and Interface Analysis, vol. 2, p. 222, 1980. [22] M. P. Seah, “XPS reference procedure for the accurate intensity

calibration of electron spectrometers—results of a BCR inter-comparison co-sponsored by the VAMAS SCA TWA,” Surface and Interface Analysis, vol. 20, no. 3, pp. 243–266, 1993. [23] C. M. Wang, Y. T. Huang, K. H. Yen et al., “Influence of hydrogen

on the germanium incorporation in a-Si_1−𝑥Ge_𝑥:H for thin-film solar cell application,” MRS Proceedings, vol. 1245, 2010. [24] A. R. Middya, S. C. de, and S. Ray, “Improvement in the

prop-erties of a-SiGe:H films: roles of deposition rate and hydrogen dilution,” Journal of Applied Physics, vol. 73, no. 9, pp. 4622– 4630, 1993.

[25] A. Matsuda, “Growth mechanism of microcrystalline silicon obtained from reactive plasmas,” Thin Solid Films, vol. 337, no. 1-2, pp. 1–6, 1999.

[26] A. Matsuda, “Formation kinetics and control of microcrystallite in 𝜇c-Si:H from glow discharge plasma,” Journal of Non-Crystalline Solids, vol. 59-60, no. 2, pp. 767–774, 1983.

(7)

[27] S. Veprek, Z. Iqbal, and F.-A. Sarott, “A thermodynamic cri-terion of the crystalline-to-amorphous transition in silicon,” Philosophical Magazine B, vol. 45, no. 1, pp. 137–145, 1982. [28] M. Stutzmann, R. A. Street, C. C. Tsai, J. B. Boyce, and S. E.

Ready, “Structural, optical, and spin properties of hydrogenated amorphous silicon-germanium alloys,” Journal of Applied Physics, vol. 66, no. 2, pp. 569–592, 1989.

[29] A. Morimoto, T. Miura, M. Kumeda, and T. Shimizu, “ESR and IR studies on a-Si_1−𝑥Ge_𝑥:H prepared by glow discharge decom-position,” Japanese Journal of Applied Physics, vol. 20, no. 11, article L833, 1981.

[30] W. Paul, D. K. Paul, B. von Roedern, J. Blake, and S. Oguz, “Pref-erential attachment of H in amorphous hydrogenated binary semiconductors and consequent inferior reduction of pseudo-gap state density,” Physical Review Letters, vol. 46, no. 15, pp. 1016–1020, 1981.

[31] D. Han, J. D. Lorentzen, J. Weinberg-Wolf, L. E. McNeil, and Q. Wang, “Raman study of thin films of amorphous-to-microcrystalline silicon prepared by hot-wire chemical vapor deposition,” Journal of Applied Physics, vol. 94, no. 5, pp. 2930– 2936, 2003.

[32] L. Houben, M. Luysberg, P. Hapke, R. Carius, F. Finger, and H. Wagner, “Structural properties of microcrystalline silicon in the transition from highly crystalline to amorphous growth,” Philo-sophical Magazine A, vol. 77, pp. 1447–1460, 1998.

[33] R. L. C. Vink, G. T. Barkema, and W. F. Van Der Weg, “Raman spectra and structure of amorphous Si,” Physical Review B, vol. 63, no. 11, Article ID 115210, pp. 1152101–1152106, 2001.

[34] S. Veprek, F.-A. Sarott, and Z. Iqbal, “Effect of grain boundaries on the Raman spectra, optical absorption, and elastic light scat-tering in nanometer-sized crystalline silicon,” Physical Review B, vol. 36, no. 6, pp. 3344–3350, 1987.

[35] Y.-L. He, C.-Z. Yin, G.-X. Cheng, L.-C. Wang, X. Liu, and G. Y. Hu, “The structure and properties of nanosize crystalline silicon films,” Journal of Applied Physics, vol. 75, no. 2, p. 797, 1994. [36] M. Islam and S. Kumar, “Influence of crystallite size distribution

on the micro-Raman analysis of porous Si,” Applied Physics Letters, vol. 78, p. 715, 2001.

[37] M. Luysberg, P. Hapke, R. Carius, and F. Finger, “Structure and growth of hydrogenated microcrystalline silicon: Investigation by transmission electron microscopy and Raman spectroscopy of films grown at different plasma excitation frequencies,” Philosophical Magazine A, vol. 75, no. 1, pp. 31–47, 1997. [38] M. I. Alonso and K. Winer, “Raman spectra of c-Si_1−𝑥Ge_𝑥

alloys,” Physical Review B, vol. 39, Article ID 10056, 1989. [39] T. S. Perova, J. Wasyluk, K. Lyutovich et al., “Composition and

strain in thin Si_1−𝑥Ge_𝑥virtual substrates measured by micro-Raman spectroscopy and x-ray diffraction,” Journal of Applied Physics, vol. 109, no. 3, Article ID 033502, 2011.

[40] M. Krause, H. Stiebig, R. Carius, U. Zastrow, H. Bay, and H. Wagner, “Structural and optoelectronic properties of micro-crystalline silicon germanium,” Journal of Non-Crystalline Solids, vol. 299–302, no. 1, pp. 158–162, 2002.

[41] J. K. Rath, M. Brinza, Y. Liu, A. Borreman, and R. E. I. Schropp, “Fabrication of thin film silicon solar cells on plastic substrate by very high frequency PECVD,” Solar Energy Materials and Solar Cells, vol. 94, no. 9, pp. 1534–1541, 2010.

[42] T. Kaneko, M. Wakagi, K. Onisawa, and T. Minemura, “Change in crystalline morphologies of polycrystalline silicon films pre-pared by radio-frequency plasma-enhanced chemical vapor deposition using SiF₄+H₂gas mixture at 350∘C,” Applied Physics Letters, vol. 64, p. 1865, 1994.

[43] S. Guha, J. Yang, A. Pawlikiewicz, T. Glatfelter, R. Ross, and S. R. Ovshinsky, “Band-gap profiling for improving the efficiency of amorphous silicon alloy solar cells,” Applied Physics Letters, vol. 54, no. 23, pp. 2330–2332, 1989.

(8)

Submit your manuscripts at

http://www.hindawi.com

Hindawi Publishing Corporation

http://www.hindawi.com Volume 2014

Inorganic Chemistry International Journal of

Photoenergy

Carbohydrate

Chemistry

International Journal of

Journal of

Chemistry

Physical Chemistry

Hindawi Publishing Corporation http://www.hindawi.com Analytical Methods in Chemistry Journal of Volume 2014 Bioinorganic Chemistry and Applications

Spectroscopy

The Scientific

World Journal

Medicinal Chemistry

Chromatography Research International Hindawi Publishing Corporation

Applied ChemistryJournal of

Theoretical Chemistry Journal of

http://www.hindawi.com Volume 2014 Journal of

Spectroscopy

Analytical Chemistry

Journal of

Quantum Chemistry Hindawi Publishing Corporation

International

Development of Hydrogenated Microcrystalline Silicon-Germanium Alloys for Improving Long-Wavelength Absorption in Si-Based Thin-Film Solar Cells

Research Article