The Raman scattering spectra are de-convoluted into two Gaussian-shaped peaks: (i) the transverse optical (TO) mode of the a-SiO_x:H phase (centered around 480 cm⁻¹) and (ii) the TO vibrational mode of the nc-Si:H phase (centered around 520 cm⁻¹).^[19] The crystalline volume fraction of the n-nc-SiO_x:H film is defined as the Raman intensity ratio I_c which is equal to the ratio of the integrated area of the nc-Si:H peak to the total peak area (a-SiO_x:H + nc-Si:H) as given in the following equation:

Figure 1 shows the variations of I_c (calculated from Eq. (1)) with deposition power for the n-nc-SiO_x:H films: the I_c increases from 22% to 31% with the power increasing up to 40 W because of the etching effect of H. However, when the power is higher than 40 W, the I_c decreases to 28% because of the inhibitory action of forming Si–Si network in nanocrystalline. The increasing power enhances the dissociation of CO₂ to assist the O to form Si–O amorphous phase rather than Si–Si. And when the gradient power is introduced to grow the n-nc-SiO_x:H, the I_c is improved from 24% to 33% significantly. So the microstructure of the n-nc-SiO_x:H film with gradient power should be further studied.

Fig. 1.

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	Fig. 1. Plots of Ic versus deposition power for n-nc-SiOx:H films.

Figure 2 shows the corresponding peaks and the I_c profiles of samples B, F, and G. Double-side incident method is used to investigate the mixing uniformity of n-nc-SiO_x:H films in the vertical direction.

Fig. 2.

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	Fig. 2. Raman spectra of the n-nc-SiOx:H films with the laser incidence from ((a), (b), and (c)) the glass side (samples B, F, G) and ((d), (e), and (f)) from film side (samples B, F, G), respectively.

From the Raman spectra shown in Figs. 2(a) and 2(d), the n-nc-SiO_x:H material grown under constant power is not uniform, and the crystalline volume fraction I_c = 15% (from glass side) increases up to 24% (from film side) in the growth direction. Figures 2(b) and 2(e) shows that by increasing power in the deposition process, the crystallinity volume fraction increases (approximately 30%) with power increasing, and the n-nc-SiO_x:H material becomes more uniform. On the one hand, high power produces more H atoms and it would achieve higher energy in the plasma to break the weak Si–Si bond to rebuild ordered structure. On the other hand, high power also enhances the dissociation efficiency of CO₂,^[12] and the O content changes the a-SiO_x:H phase proportion in n-nc-SiO_x:H film. These two processes are both existent during the deposition, and the H-etch effect dominantly affects the results. In addition, the 20-s interval during power change is favorable because of the highest crystallinity of 33% (Figs. 2(c) and 2(f)). A proposed explanation for this observation involves the changing deposition power between two different n-nc-SiO_x:H structures, higher power leading to higher crystallinity, and an interval that can make the pre-growth film relaxed sufficiently and microstructures get a steady-state condition. The new-forming layer can be beneficial to the later crystallization. The residual stress is calculated by Wolf’s empirical formula ^[20] and shown in Fig. 3.

Fig. 3.

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	Fig. 3. Variations of residual stress of the deposited n-nc-SiOx:H films with power from film side and glass side, respectively.

It is evident from Fig. 3 that the low I_c has a lowest residual stress of 308 Mpa from glass-side at 30 W and with I_c increasing during the film growth, the residual stress increases up to 1380 Mpa. The Si–Si network in a-SiO_x phase can increase the residual stress in the film, and the 20-s interval helps relax strained Si–Si network in a-SiO_x phase and the residual stress drops from 1250 Mpa to 965 Mpa from glass-side.

Figure 4 shows the surface morphologies of the n-nc-SiO_x:H films deposited on the a-Si:H/a-SiGe:H tandem solar cells. Compared with the solar cell without SiO_x BR in Fig. 4(a), the solar cells fabricated with the n-nc-SiO_x:H BR keep the column-growth mode from the cell substrate. Distinctly, in the n-nc-SiO_x:H column there are two separate phases — small grains scattered throughout in the uniform matrix and indicated separately by the circle and arrow in Fig. 4. In Figs. 4(b) and 4(c) corresponding to the cells with different n-nc-SiO_x:H BRs (sample B and sample F). The diminished grains are distributed more uniformly, meaning that more O atoms are incorporated into the material since the gradient deposition power is introduced. Moreover, 20-s interval during power change which improves the crystallinity, can lead to the smaller grains and higher density (Fig. 4(d), sample G). Generally, the crystallinity volume fractions and mixing uniformities of the n-nc-SiO_x:H films have all been improved by increasing deposition power in the growth process.

Fig. 4.

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	Fig. 4. SEM images of a-Si:H/a-SiGe:H solar cell with different n-nc-SiOx:H layers as BR. Panel (a) shows the case without SiOx back reflector, and panels (b)–(d) show the cases corresponding to samples B, F, and G.

The oxygen-rich region in n-nc-SiO_x:H film, totally amorphous, supplies low refractive index and high band gap because of incorporated oxygen;^[21] the n-doped nanocrystalline silicon region provides sufficient electrical conductivity.^[22,23] The influences of deposition parameters on lateral conductivity and growth rate are presented in Fig. 5(a). The average deposition rates are little enhanced from 11 Å/min to 13 Å/min with the increase of deposition power. But the lateral conductivities of all n-nc-SiO_x:H layers are greatly different. Increasing power will assist the dissociation of SiH₄ to enhance the deposition rate, and the H-etching effect can also promote the formation of nanocrystalline silicon which has high doping efficiency to achieve a high lateral conductivity up to 11.3 S/cm when the power is at 40 W. But with power increasing further, the increase of CO₂ dissociation reduces the nanocrystalline phase in n-nc-SiO_x:H film, which leads a decrease of lateral conductivity. Furthermore, due to the non-uniformity of the crystallinity in the growth direction, amorphous incubation layer is formed during the early stage of the n-nc-SiO_x:H layer deposition and lower H-etching effect leads to the low crystallinity volume fraction with poor lateral conductivity (0.4 S/cm) under the condition of constant deposition power of 30 W. Increasing power in the deposition process can improve the crystallinity volume fraction and increase the lateral conductivity up to 9.1 S/cm. However, when the 20-s interval is introduced in the deposition process before increasing power, the lateral conductivity drops from 9.1 S/cm to 4.6 S/cm. Figure 4(d) shows that the crystalline grains which have smaller sizes and higher density distributed in uniformly amorphous phase (red circled) and more grain boundaries become potential energy barriers to restrict the thermally activated transport of carriers.^[24]

Fig. 5.

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	Fig. 5. Average deposition rate (Rd) and lateral conductivity (a), absorption coefficient and n, k (b) of the different n-nc-SiOx:H films.

The optical properties of the n-nc-SiO_x:H films are also studied. Figure 5(b) presents the absorption coefficient α, the index of refraction n, and the extinction coefficient k (inset graph), which are obtained by calculating transmission spectra.^[25] It is obvious that absorption coefficient α of the film fabricated by increasing the power during deposition process is reduced and the optical gap E₀₄ is increased from 2.4 eV to 2.6 eV. Increasing power causes more CO₂ to decompose and higher O content of the n-nc-SiO_x:H film to increase its optical gap.^[21,26] Besides, it is also found in Fig. 5(b) that 20-s interval during power change increases the E₀₄ slightly (2.7 eV). We have mentioned that the sample G have more grain boundaries, and this highly disordered grain boundary region becomes poly-hydride bonding configuration which mades it likely exhibit higher optical gap.^[27–,30] Furthermore, higher O content could also reduce the n from 3.2 to 2.9 (at 1000 nm), and it would introduce the refractive gradient between the above layers, thereby increasing the light-trapping effect of solar cells.

Figure 6(a) shows the J–V curves of n-nc-SiO_x:H films serving as BRs in a-Si:H/a-SiGe:H solar cells. It is found in Fig. 6(b) that the short circuit current density (J_sc) of the cells increases significantly when using n-nc-SiO_x:H materials as BR (reference cell without SiO_x BR, sample RC). The open circuit voltage (V_oc) decreases rapidly from 1.56 V to 1.43 V when the deposition power reaches as high as 60 W because of electrically-shielded effect of carriers at i/n interface with high O content n-nc-SiO_x:H film. The comparison of the optimized SiO_x BR (gradient deposition power with 20-s interval) with the standard SiO_x BR (constant deposition power) shows that the J_sc is enhanced largely (6.85 mA/cm² to 7.42 mA/cm²), maximally reaching up to 8.3%. As a consequence, the cell efficiency increases from 6.82% to 7.88% with a relative increase of 15.5%. By observing the change trend of fill factor (FF), the results show that the n-nc-SiO_x:H film which has the highest lateral conductivity is not suitable for the a-Si:H/a-SiGe:H solar cell (FF = 65%). Because the highest lateral conductivity can collect more local current drains, which relate to the weak diodes and shunts, FF of solar cell decreases. The 20-s interval is beneficial to the solar cells because it has high FF (FF = 68%). The small grains with highest density form high potential energy barrier to prevent the collection of local current drains, thus increasing FF of solar cell. However, because of the electrically-shielded effect of carriers, the increase of series resistance (R_s) causes the FF to reduce significantly when the deposition power of n-nc-SiO_x:H film is as high as 60 W.

Fig. 6.

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	Fig. 6. (a) J–V curves of solar cells, the inset showing the top cell and bottom cell’s Jsc tendency, and (b) the parameters of the solar cell.

The variations of external quantum efficiency (EQE) of a tandem cell with wavelength for different n-nc-SiO_x:H serving as BRs are shown in Fig. 7. The optimized n-nc-SiO_x:H layer used as BR affects both bottom and top cell: the spectrum response in the whole wavelength region of bottom cells is improved, especially in the long wave-length region from 650 nm to 850 nm; the top cell spectrum response slightly reduces in the range from 500 nm to 750 nm because this part of light is utilized by the bottom cell fabricated with additional n-nc-SiO_x:H layer. This corresponds to the top cell J_sc dropping from 7.69 mA/cm² to 7.26 mA/cm² and the bottom cell J_sc rising from 6.85 mA/cm² to 8.09 mA/cm² (see the inset in Fig. 6(a)). The refractive index step between the absorbing layer and n-nc-SiO_x:H layer leading to the reflection of light at interface increases the optical light path in the absorbing layer,^[22] and this mainly affects the long wavelength part corresponding to the bottom cell. Moreover, the 20-s interval during changing deposition power in sample SC7 shows its better performance than sample SC6 because of higher refractive index step. But with the deposition power of nc-SiO_x:H film further increasing from 40 W to 60 W, the bottom cell J_sc begins to increase and becomes higher than top cell J_sc (see the inset in Fig. 6(a)), which causes the J_sc of tandem solar cells to decrease. So by using the optimized n-nc-SiO_x:H as BR, a better current matching between top and bottom cell is obtained.

Fig. 7.

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	Fig. 7. Plots of external quantum efficiency versus wavelength of a-Si:H/a-SiGe:H solar cells with different SiOx back reflector.