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

Hydrogenated microcrystalline silicon-germanium (μc-Si 1−x Ge x :H) alloys were developed for application in Si-based thin-film solar cells. The effects of the germane concentration GeH4 ) and the hydrogen ratio H2 ) 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−x Ge x :H and μcSi 1−x Ge x :H were also compared. Similar to a-Si 1−x Ge x :H, the preferential Ge incorporation was observed in μc-Si 1−x Ge x :H. Moreover, a higher H2 significantly promoted Ge incorporation for a-Si1−xGex:H, while the Ge content was not affected by H2 in μc-Si 1−x Ge x :H growth. Furthermore, to eliminate the crystallization effect, the 0.9 μm thick absorbers with a similar crystalline volume fraction were applied. With the increasing GeH4 , the accompanied increase in Ge content of μc-Si1−xGex: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 of H2 and GeH4 , the single-junction μc-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 to μc-Si:H cell, the external quantum efficiency at 800 nm had a relative increase by 33.1%.


Introduction
Hydrogenated amorphous silicon (a-Si:H) has been widely studied [1,2] and employed as an absorber in silicon thinfilm 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 multijunction 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 attributed 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 thinfilm solar cell [6].Other groups have integrated a-Si:H and hydrogenated microcrystalline silicon (c-Si:H) absorbers into tandem structure cells with a stabilized efficiency over 10% [7][8][9].Moreover, Yan et al. have reported an a-Si:H/a-SiGe:H/c-Si:H triple-junction cell reached a recorded efficiency of 16.3% [10].
Due to a lower bandgap of 1.1 eV [5], c-Si:H has been utilized as an absorber for IR absorption [11][12][13][14].In addition, c-Si:H has a minor Staebler-Wronski effect (SWE) [14], which has less impact on the long term film quality and cell performance than amorphous material.Nevertheless, the indirect bandgap nature of c-Si:H leads to a low absorption coefficient.Therefore, a thick c-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 thinner c-Si 1− Ge  :H absorber in the cells [15][16][17].The c-Si 1− Ge  :H consists of an amorphous-crystalline mixed phase of binary SiGe alloys, which are affected by the deposition parameters including the hydrogen ratio ( H 2 ) and the germane concentration ( GeH 4 ).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 c-Si 1− Ge  :H alloys should be carefully manipulated for the requirement of IR absorption.
Previous works on c-Si 1− Ge  :H alloy [18,19] have reported the effect of Ge incorporation by varying  GeH 4 but have not yet considered the accompanied variation of crystallization.In this work, to eliminate the effect of different degree of crystallization, the c-Si 1− Ge  :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− Ge  :H and c-Si 1− Ge  :H alloys.The effects of  H 2 and  GeH 4 on Ge incorporation were discussed.

Experimental Detail
Silicon thin films including c-Si 1− Ge  :H were deposited by a single-chamber process in a multichamber plasmaenhanced chemical vapor deposition (PECVD) system equipped with 27.12 MHz rf power, NF ], was changed from 0 to 6.8%.In contrast, the lower  H 2 varied from 0 to 6 and the  GeH 4 varied from 8.3% to 16.7% were employed for a-Si 1− Ge  :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][21][22].A presputtering was conducted to eliminate contaminations 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 0.1 m) occupied only small part of the absorbing layer (∼0.9 m).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  the prepared films were obtained by an - measurement system equipped with an AM1.5G illumination.A spectrophotometer was used to determine the transmittance and the reflectance of the films.The optical bandgap ( 04 ) was obtained when the absorption coefficient is 10 4 cm −1 .
The commercial textured SnO 2 :F-coated substrates were utilized for preparing superstrate p-i-n c-Si 1− Ge  :H cells.A 0.9 m thick c-Si 1− Ge  :H absorber was employed in singlejunction solar cells with a p-type c-Si:H layer and an n-type hydrogenated microcrystalline silicon oxide (c-SiO  :H) layer.The cell was characterized by an AM1.5G solar simulator.The area of the device for measurement was 0.25 cm 2 which was defined by the silver electrode.A measuring system having monochromator, chopper, lock-in amplifier, and - meter was applied to measure the external quantum efficiency (EQE).

Ge-Incorporation in Amorphous and Crystalline Silicon-Germanium Alloys. The dependence of Ge content ([Ge]
) on  H 2 with different  GeH 4 in amorphous and microcrystalline SiGe alloys is shown in Figure 1(a).As can be seen, the Ge content in a-Si 1− Ge  :H alloys rapidly increased as  H 2 increased from 0 to 2 at a fixed  GeH 4 and tended to saturate as  H 2 was 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 GeH 3 species more than the SiH 3 species.The diffusion length of GeH 3 species is less than SiH 3 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][26][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 with  H 2 over 2, the saturation of Ge content was observed for a-Si 1− Ge  :H alloys.Presumably, the sufficient hydrogen atoms promote rigid Ge bonding in the films.Compared to a-Si 1− Ge  :H alloys, a much higher hydrogen diluted gas mixture is needed for the crystallization of the c-Si 1− Ge  :H.When the  H 2 was over 85 at a fixed  GeH 4 , Ge content was not significantly changed, suggesting that the effect of hydrogen for Ge incorporation in the c-Si 1− Ge  :H films has less impact.The resulting Ge content in the c-Si 1− Ge  :H film with increasing  H 2 was kept at approximately 13 and 16.7 at.%, with  GeH 4 of 5.0% and 7.1%, respectively.
In addition to the Ge content, the incorporation efficiency of Ge was also discussed.The incorporation efficiency represents the ratio of the transformation from GeH 4 to film Ge content, defined as [Ge]/ GeH 4 .As shown in Figure 1(b), the tendency of incorporation efficiency of a-Si 1− Ge  :H and c-Si 1− Ge  :H films was similar to that of the film Ge content with the increasing  H 2 .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 of  GeH 4 alters the Ge content significantly, as well as the film characteristics.One of the reasons was the less dissociation energy of GeH 4 compared to SiH 4 .The more efficient decomposition of GeH 4 was known from SiH 4 -GeH 4 -H 2 discharge plasma field [28].However, adding more GeH 4 decreased the Ge incorporation efficiency.More produced sticky GeH 3 precursors led to an increase in the weak Ge-related bonds [29,30].Consequently, under the hydrogen-containing atmospheres, the probability of the SiH 3 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 high  H 2 environment, the Ge content in SiGe alloys was affected by the hydrogen significantly in low  H 2 environment.More Ge content can be achieved by adding more GeH 4 in the gas mixture.Nevertheless, with increasing Ge content, the incorporation efficiency of Ge into solid phase decreased with increasing  GeH 4 .

Effect of the Hydrogen Ratio on Film Properties and Cell
Performance.The microstructure of c-Si 1− Ge  :H films deposited with different  H 2 at  GeH 4 of 5% was studied by the Raman spectroscopy.Figure 2 shows the resulting Raman spectra, where the transverse optical (TO) modes mainly consisted 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  H 2 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  H 2 .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 the c-Si 1− Ge  :H films at a fixed  GeH 4 .The higher degree of crystallization at a higher  H 2 is contributed to more crystallites in the films.In addition, there was no significant difference in Raman spectra at approximately 300 cm −1 for c-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 of  H 2 on   and optical bandgap ( 04 ) is shown in Figure 3.The crystalline volume fraction (  ) is defined by ( 520 +  510 )/( 520 +  510 +  480 ), where  520 ,  510 , and  480 were the integrated intensities of crystalline, intermediate, and amorphous phase, respectively [41,42].With a kept  GeH 4 ,  the   increased with increasing  H 2 .More H 2 in the gas mixture promoted the crystallization of c-Si 1− Ge  :H growth.Moreover, given the same   , the  H 2 required for c-Si 1− Ge  :H was much larger than that for c-Si:H.This suggests that adding GeH 4 significantly suppressed crystalline 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.When  H 2 was varied from 83.5 to 124.1 and  GeH 4 was kept at 5%, the   increased from 25.2% to 70.6%, corresponding to the decreased  04 from 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 of   of c-Si 1− Ge  :H absorbers on cell performance, we further employed different c-Si 1− Ge  :H alloys as absorbers by changing the  H 2 .
Figure 4 shows the cell structure and the - characteristics of c-Si 1− Ge  :H p-i-n single-junction cells using absorbers prepared with different  H 2 .This cell performance is shown in Table 1.Accompanied with the increasing  H 2 from 88.6 to 124.1, the resulting bandgap narrowing of the absorber influenced the internal electric field and decreased the  OC from 485 to 430 mV.On the contrary, the  SC was significantly enhanced from 17.17 to 19.25 mA/cm 2 .More crystalline phase in the film contributed to more photocurrent in the cells due to the lower bandgap.When the  H 2 was 94.9, the corresponding   of the absorber was 50.5% which led to an optimal cell efficiency of 5.48%.

Effect of the Germane Concentration on Film Properties
and Cell Performance.In Section 3.1, we have shown that the  GeH 4 significantly changed the Ge content in the film.To reveal the effect of  GeH 4 on cell performance is therefore important for improving long-wavelength absorption.The c-Si 1− Ge  :H absorbers in single-junction solar cells were prepared with different  GeH 4 of 0, 3.7%, 5.0%, and 6.8%.In addition, the c-Si 1− Ge  :H absorber with a similar   of approximately 55% was applied to eliminate the effect of the crystallization of absorber on the cell performance.When the  GeH 4 increased 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 in  OC of 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, the  SC significantly increased from 17.38 to 18.50 mA/cm 2 due to more optical absorption.When the  GeH 4 was 6.8%, the film Ge content further went up to 18.0 at.%, which resulted in the degraded cell performance.The  OC , FF, and  SC decreased to 370 mV, 53.0%, and 17.27 mA/cm 2 , respectively.The improvement of  SC according to the change of Ge content can be revealed by the EQE measurement.As shown in Figure 5, no significant drop in spectral response in short-wavelength region was observed as the  GeH 4 increased from 0 to 5%, while the spectral response in the range of  The c-Si 1− Ge  :H absorbers were prepared with the  GeH 4 of 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 with  GeH 4 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  GeH 4 was 6.8%, the c-Si 1− Ge  :H absorber near p/i interface may preferentially grow in amorphous phase.Compared to microcrystalline 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 the c-Si 1− Ge  :H cells having absorber prepared with  GeH 4 of 5.0% and 6.8% were presented in Figure 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 the c-Si 1− Ge  :H cell employing the absorber prepared by  GeH 4 of 6.8%, the difference of  SC as measured by EQE with 0 and −2 bias voltages was 1.05 mA/cm 2 .In comparison, the  difference of  SC for c-Si 1− Ge  :H cell employing absorber prepared with  GeH 4 of 5.0% under the same bias voltages was less than 0.25 mA/cm 2 .The result indicates that too much Ge incorporation would lead to the degraded carrier collection and worsen cell performance.Moreover, in contrast 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].

Conclusion
The effects of  GeH 4 and  H 2 on c-Si 1− Ge  :H alloys and the corresponding single-junction cells were studied.Similar to a-Si 1− Ge  :H, the preferential Ge incorporation was observed in c-Si 1− Ge  :H.Moreover, a higher  H 2 significantly promoted Ge incorporation for a-Si 1− Ge  :H, while the Ge content was not affected by  H 2 in c-Si 1− Ge  :H

Figure 1 :
Figure 1: The variations of (a) Ge content and (b) incorporation efficiency versus  H 2 in amorphous [23] and microcrystalline SiGe alloys with different  GeH 4 .

2 Figure 2 :
Figure 2: The Raman spectra of c-Si 1− Ge  :H films with different  H 2 .

Figure 3 :
Figure 3: Effect of  H 2 on the properties of c-Si 1− Ge  :H films prepared with  GeH 4 of 0 and 5.0%.The circle and the square symbols represent the crystalline volume fraction (  ) and the bandgap ( 04 ), respectively.

Figure 6 :
Figure 6: Spectral response of c-Si 1− Ge  :H cell measured with (dash line) and without (solid line) bias voltage.The absorbers were prepared with  GeH 4 of 5.0% and 6.8%.

Table 1 :
Properties of c-Si 1− Ge  :H absorber and the corresponding performance of single-junction cells with different  H 2 of 88.6, 94.9, 101.3, 124.1.The  GeH 4 of these cells was kept at 5.0%.

Table 2 :
Properties of c-Si 1− Ge  :H absorber and the corresponding performance of single-junction cells with different  GeH 4 of 0, 3.7%, 5%, and 6.8%.The   of these cells was kept at approximately 55%.