Photovoltaic Materials and Devices 2016View this Special Issue
Wide-Range Enhancement of Spectral Response by Highly Conductive and Transparent μc-SiOx:H Doped Layers in μc-Si:H and a-Si:H/μc-Si:H Thin-Film Solar Cells
The enhancement of optical absorption of silicon thin-film solar cells by the p- and n-type μc-SiOx:H as doped and functional layers was presented. The effects of deposition conditions and oxygen content on optical, electrical, and structural properties of μc-SiOx:H films were also discussed. Regarding the doped μc-SiOx:H films, the wide optical band gap () of 2.33 eV while maintaining a high conductivity of 0.2 S/cm could be obtained with oxygen incorporation of 20 at.%. Compared to the conventional μc-Si:H(p) as window layer in μc-Si:H single-junction solar cells, the application of μc-SiOx:H(p) increased the and led to a significant enhancement in the short-wavelength spectral response. Meanwhile, the employment of μc-SiOx:H(n) instead of conventional ITO as back reflecting layer (BRL) enhanced the external quantum efficiency (EQE) of μc-Si:H single-junction cell in the long-wavelength region, leading to a relative efficiency gain of 10%. Compared to the reference cell, the optimized a-Si:H/μc-Si:H tandem cell by applying p- and n-type μc-SiOx:H films achieved a of 1.37 V, of 10.55 mA/cm2, FF of 73.67%, and efficiency of 10.51%, which was a relative enhancement of 16%.
Over the past decade, silicon-based thin-film solar cell technology has become a viable approach for mass production due to its characteristics such as large-scale production, low temperature process, and abundant storage of silicon in the earth crust [1, 2]. However, compared with the mainstream of photovoltaics, the conversion efficiency of silicon-based thin-film solar cells still needs to be improved. One of the approaches is to fabricate multijunction solar cells comprising subcells with absorbers having different band gaps to convert broad-band solar energy into electricity. Among various silicon-based materials, the hydrogenated microcrystalline silicon (μc-Si:H) has been reported as a promising candidate for the absorber of middle or bottom cells in multijunction cells. The μc-Si:H has the advantages of higher optical absorption coefficients at the infrared region and less light-induced degradation than the hydrogenated amorphous silicon (a-Si:H) . Intensive researches have focused on the development of a-Si:H/μc-Si:H tandem solar cells [4–6]. The band gap combination of a-Si:H (~1.8 eV) and μc-Si:H (~1.1 eV) provides a great benefit of utilizing broad sunlight spectrum . However, the absorber thickness of μc-Si:H is relatively thick (~1–3 μm) compared with a-Si:H owing to its nature of indirect band gap. To further improve the efficiency of μc-Si:H solar cells without increasing the thickness of the absorbers, the light management of the solar cells is a critical factor.
Studies have reported that the p- and n-type μc-Si:H are commonly used as doped layers in μc-Si:H solar cells due to its properties of low activation energy and high conductivity, which increased the build-in field and reduced the series resistance of solar cells [7, 8]. However, the optical band gap (, the photon energy at which the absorption coefficient is 104 cm−1) could only be adjusted in a small range of 1.9~2.1 eV for the doped μc-Si:H layers [9, 10]. Besides, the parasitic absorption of the doped μc-Si:H layers contributes to the loss in photocurrent. To further improve the optical properties of the doped layers, hydrogenated microcrystalline silicon oxide (μc-SiOx:H) has been developed for thin-film solar cells. The μc-SiOx:H has been proposed to be a mixed-phase material comprising a-SiOx:H phase and μc-Si:H phase [11, 12]. The oxygen-rich amorphous phase supplied low refractive index and high , while the conductive doped μc-Si:H phase provided the low-resistance pathway for carrier transport . The incorporation of oxygen in the silicon network effectively widened the and reduced the refractive index. As a result, the doped μc-SiOx:H can be used as doped layers as well as intermediate reflecting layer (IRL), tunneling recombination junction (TRJ) layer, and back reflecting layer (BRL), owing to its tunable optical and electrical properties [13–17].
Nevertheless, excessive oxygen content in μc-SiOx:H film significantly suppressed the crystallization, which adversely affected the electrical property. Reports have shown that compared to μc-Si:H having conductivity of 10~102 S/cm and of 1.9~2.1 eV, the conductivity of the doped μc-SiOx:H films degraded to 10−3~10−4 S/cm as the was as high as 2.3 eV [18, 19]. Therefore, to further improve the electrical and optical property by optimizing the deposition conditions were needed for enabling the extensive applicability of the μc-SiOx:H films.
This work aims to efficiently enhance the light absorption in solar cells. The optoelectrical properties of the doped μc-SiOx:H materials as the window and the back reflector layers employed in μc-Si:H single-junction and a-Si:H/μc-Si:H tandem solar cells were investigated.
2. Experimental Details
Silicon-based thin-films were prepared with a single chamber process in multichamber plasma-enhanced vapor deposition (PECVD) system equipped with 27.12 MHz RF power and NF3 in situ plasma cleaning. Gas mixture of SiH4, CO2, B2H6, PH3, and H2 was used as source gases. The μc-SiOx:H(p) and μc-SiOx:H(n) films were deposited onto Corning EAGLE XG glass substrate at approximately 190°C. The incorporation of oxygen was achieved by introducing CO2 in highly H2-diluted SiH4. The oxygen content, [O], of μc-SiOx:H films was examined by an X-ray photoelectron spectroscopy (XPS). The crystalline volume fraction () of μc-SiOx:H was calculated from the ratio of the integrated intensities of deconvoluted peaks centered at 480, 510, and 520 cm−1 from a Raman spectrum with a probe laser of 488 nm excitation. The optical band gap () was obtained by an ultraviolet-visible-near-infrared spectrophotometer. The refractive indices were estimated from an ellipsometry measurement by applying Fresnel equation. The dark conductivity () was measured with coplanar Ag electrodes at room temperature.
The μc-Si:H solar cells were deposited in a superstrate configuration on textured SnO2:F glass substrate. The structure of μc-Si:H single-junction solar cells was glass/SnO2:F/μc-SiOx:H(p)/μc-Si:H(i)/μc-SiOx:H(n)/Ag with 1.4 μm thick μc-Si:H absorber. The thicknesses of the absorbers of a-Si:H and μc-Si:H component cells were 240 nm and 1.4 μm, respectively. The Ag electrode was prepared by thermal evaporator with the area of 0.25 cm2 defined by the shadow mask. The cells were characterized by an AM1.5G illuminated J-V measurement system and an external quantum efficiency (EQE) instrument.
3. Results and Discussion
3.1. Electrical and Optical Properties of Doped μc-SiOx:H Thin-Films
The absorption coefficients of the p-type μc-SiOx:H as a function of [O] are shown in Figure 1(a). Compared to the a-SiOx:H(p), the μc-SiOx:H(p) exhibited lower absorption coefficient owing to the indirect band gap of μc-Si:H phase while the oxygen content was 20 at.% in both cases. In addition, the optical absorption coefficient of μc-SiOx:H(p) shifted toward higher photon energy as the oxygen content was increased from 0 (μc-Si:H(p)) to 40 at.%, which was attributed to the increased a-SiOx:H phase in the films . The n-type μc-SiOx:H layers showed similar trends .
The refractive index (, at the wavelength of 800 nm) and the versus the oxygen content are demonstrated in Figure 1(b). As the [O] was increased from 0 (μc-Si:H(p)) to 40 at.%, the was increased from 1.97 to 2.56 eV. This can be ascribed to the increased Si-O bond in a-SiOx:H phase since the bonding energy of Si-O bond was higher than that of Si-Si and Si-H bonds in a-SiOx:H phase. On the other hand, the refractive index was decreased from 2.90 to 2.15 accordingly. The tunable refractive index was beneficial when employing μc-SiOx:H doped layers as the index-matching layer, IRL, and BRL.
The electrical properties of μc-SiOx:H doped layers mainly depended on and doping, which was significantly affected by the deposition conditions in the PECVD system. The dependence of of μc-SiOx:H(p) and μc-SiOx:H(n) films on [O] is shown in Figure 2(a). In the case of μc-SiOx:H(p) with H2-to-SiH4 flow ratio () of 300, was decreased from 65 to 32% when the [O] was increased from 0 to 33 at.%. Similarly, in the case of μc-SiOx:H(n) with of 214, as the [O] was increased from 0 to 37 at.%, was decreased from 64% to 16%. This suggested that the incorporation of oxygen into μc-Si:H network suppressed the formation of crystalline phase due to the oxygen-induced defects . It should be noted that of μc-SiOx:H(p) under similar was higher than that of μc-SiOx:H(n), which can be ascribed to the boron that had more impact on disrupting the crystalline structure than that of phosphorus . It has been reported that the energy-favorable configuration of phosphorus atoms in the Si matrix was tetrahedral, which made phosphorus act as the crystallization centers . In contrast, the boron may adopt other energy-favorable configurations that were not tetrahedral. The inactive boron atoms could segregate to the grain boundaries, which hindered the crystalline formation . Therefore, of μc-SiOx:H(p) was higher than that of μc-SiOx:H(n) in order to assist the crystallization in our case.
The dependence of on the [O] for μc-SiOx:H(p) and μc-SiOx:H(n) films was shown in Figure 2(b). In the case of μc-SiOx:H(p), the decreasing trend on from 16.7 to 2.1 × 10−7 S/cm with the increase [O] from 0 to 36 at.% was observed. In comparison, in the case of μc-SiOx:H(n), was decreased from 72.4 to 3.4 × 10−9 S/cm as [O] was increased from 0 to 39 at.%. The reduced can be attributed to the decreased arising from the increased defect density by the oxygen incorporation into the films. Moreover, the doping efficiency was reduced with the decrease in , which further degraded at high [O]. In Figure 2(b), the p-type and n-type a-SiOx:H were also shown for comparison. Regarding p-type layers, it can be seen that of μc-SiOx:H with [O] of 20 at.% was higher than that of a-SiOx:H with [O] of 18 at.%. Similarly, μc-SiOx:H(n) with [O] of 8 at.% exhibited higher than a-SiOx:H(n) with [O] of 7 at.%. The significantly enhanced was due to the crystalline structure of μc-SiOx:H layer which facilitated the carrier transport and the higher doping efficiency in μc-Si:H phase of μc-SiOx:H films compared with the a-SiOx:H doped layers. As a result, μc-SiOx:H(p) with [O] of 20 at.%, of 2.33 eV, and the high of 0.2 S/cm were obtained. Regarding the μc-SiOx:H(n), high of 0.08 S/cm was obtained as [O] was 27 at.% with lower refractive index of 2.55 (n at 800 nm). μc-SiOx:H(p) and μc-SiOx:H(n) with tunable optical and electrical properties were applied as window layer and BRL in silicon thin-film solar cells, respectively.
3.2. Application of μc-SiOx:H(p) as Window Layer in μc-Si:H Solar Cells
The μc-SiOx:H(p) with varied [O] were utilized in μc-Si:H single-junction solar cells. The EQE and cell reflection are illustrated in Figure 3. With the [O] increased from 0 (μc-Si:H(p)) to 31 at.%, the short-wavelength spectral response was enhanced significantly. This was due to the reduced parasitic absorption loss by using wide-band gap μc-SiOx:H(p) and the increased light-incoupling owing to graded refractive index . The decreased cell reflection in the short-wavelength range with the increasing oxygen content corresponded to the increased EQE in the short-wavelength range. This indicated that the refractive index of μc-SiOx:H(p) ( at 800 nm) in between the front TCO and the μc-Si:H absorber may act as an antireflection layer by the graded refractive index. The corresponding cell performance of μc-Si:H single-junction solar cells is summarized in Table 1. With the [O] increase from 0 to 31 at.%, the short-circuit current density () was increased from 19.73 to 21.62 mA/cm2. Compared with the cell using μc-Si:H(p) ([O] = 0%), the cell employing μc-SiOx:H(p) with [O] of 12 at.% and 20 at.% exhibited enhancement in fill factor (FF). This can be attributed to the shunt quenching effect by μc-SiOx:H(p) layer which suppressed the carrier recombination at the p/i interface [12, 27]. With [O] increased from 0 to 20 at.%, the open-circuit voltage () was increased from 0.49 to 0.51 V, which were due to the increase in band gap of μc-SiOx:H(p) window layer. In addition, the utilization of wide-band gap p-layer presented a potential barrier on the conduction band, which suppressed the diffusion of electron across the p/i interface and reduced the carrier recombination in bulk region or at p/i interface, leading to an improved . Further increase in the [O] to 31 at.% resulted in the decrease in and FF, which can be attributed to the increase in oxygen-induced defect states existing near the p/i interface and the decrease in of μc-SiOx:H(p) layer. In our case, the cell efficiency of 7.33% was obtained by employing the μc-SiOx:H(p) window layer with [O] of 20 at.% in μc-Si:H single-junction solar cell.
3.3. Application of Different Back Reflecting Layers in μc-Si:H Solar Cells
The back reflector comprising the BRL and metal contact is critical to reflect unabsorbed photons back into the absorber, resulting in the improved long-wavelength absorption and cell performance. The role of BRL was to reduce the parasitic absorption loss of the back reflector. We employed three BRLs in μc-Si:H single-junction solar cells, whose schematic structures are shown in Figure 4.
Figure 5(a) shows the EQE and cell absorptance of μc-Si:H solar cells. Compared to the cell using μc-Si:H(n) as BRL, the cell employing the sputtered ITO as BRL exhibited the lower cell absorptance but slightly higher EQE at wavelengths from 600 to 700 nm. This can be ascribed to the less plasmonic absorption loss by employing ITO than that employing μc-Si:H(n) as BRL [28, 29]. The ITO layer has lower refractive index () than μc-Si:H(n) ( at 800 nm), which shifted the plasmonic absorption to short-wavelength region, resulting in the enhanced optical reflection of back reflector. This led to the increase in from 18.95 to 20.01 mA/cm2 as shown in Table 2. Similar effect was found by using μc-SiOx:H(n) ( at 800 nm) as BRL. Compared with the cell employing μc-Si:H(n) as BRL, the cell with μc-SiOx:H(n) exhibited significantly improved EQE from 500 to 1000 nm, which was due to the reduction in plasmonic absorption loss. was significantly increased from 18.95 to 20.83 mA/cm2. On the other hand, in comparison to the cell employing the ITO layer as BRL, the cell using μc-SiOx:H(n) as BRL ([O] of 27 at.%) exhibited higher absorptance and EQE. This enhancement in EQE was likely due to the improved interface quality by the in situ fabrication process, which avoided the sputtering damage and reduced the formation of native oxide at the interface [11, 30].
In order to have a clear picture on the carrier collection of μc-Si:H solar cells by employing different BRLs, the internal quantum efficiency (IQE) of μc-Si:H solar cells is demonstrated in Figure 5(b). The IQE denoted the ratio of the collected charge carriers to the absorbed photons by the solar cells. Compared to the case using μc-Si:H(n) as BRL, the enhancement of IQE from 600 to 750 nm was due to the reduction in the parasitic absorption loss near the BRL/Ag interface, which led to the more photons being reflected back to the absorber.
Similar to the case using ITO layer, the cell with μc-Si:H(n) as BRL exhibited significantly enhanced IQE compared with the cell using μc-Si:H(n). This was due to the reduction in parasitic absorption loss by shifting the plasmonic absorption to the less-critical spectral range. In addition, by replacing ITO with μc-SiOx:H(n) as BRL, the IQE was enhanced at the wavelength ranging from 450 to 1100 nm. This manifested that the carrier collection was improved owing to the removal of sputtering damage and the contamination during the air exposure. In our case, the optimized cell efficiency of 7.2% with of 0.5 V, of 20.83 mA/cm2 and FF of 70.0% was obtained by employing the μc-SiOx:H(n) as BRL in μc-Si:H single-junction solar cell.
3.4. Optimization of a-Si:H/μc-Si:H Solar Cells by Employing Different p-Layer and Back Reflecting Layer in Bottom Cell
The schematic structures of a-Si:H/μc-Si:H tandem cells with different p-layers and BRLs in μc-Si:H bottom cells are shown in Figure 6. The EQE of a-Si:H/μc-Si:H tandem cell employing different p-layers and BRLs in the bottom cell are illustrated in Figure 7. As can be seen, compared to the tandem cell using μc-Si:H(n) as BRL, the cell employing the sputtered ITO as BRL exhibited higher spectral response in wavelengths ranging from 600 to 1000 nm. This was coincided with the results shown in Figure 5, which was due to the decreased plasmonic absorption loss by employing the ITO layer. Therefore, of bottom cell was increased from 9.78 to 10.62 mA/cm2 as shown in Table 3. The employment of μc-SiOx:H(n) ([O] of 27 at.%) as BRL exhibited similar effect as the case of ITO. Compared to μc-Si:H(n) as BRL, the tandem cell employing μc-SiOx:H(n) as BRL had a significant enhancement in the spectral response of long-wavelength range, which led to the increase in of bottom cell from 9.78 to 11.58 mA/cm2. In addition, compared to the sputtered ITO as BRL, the replacement of μc-SiOx:H(n) as BRL showed higher spectral response from 600 to 1000 nm. The increase of in the bottom cell from 10.62 to 11.58 mA/cm2 was more likely because of the decreased plasmonic losses by the μc-SiOx:H(n) as BRL. Another factor could ascribe to the all in situ PECVD process which reduced the interface defects .
In addition, the cell with μc-SiOx:H(p) as the replacement of μc-Si:H(p) of the bottom cell is shown for comparison. The significant enhancement in spectral response of long-wavelength range can be ascribed to two factors. First, the absorption coefficient of μc-SiOx:H(p) is lower than that of μc-Si:H(p), which resulted in the reduced parasitic absorption loss by the p-layer. Second, the replacement of μc-Si:H(p) by μc-SiOx:H(p) could increase the build-in field due to the higher optical band gap of μc-SiOx:H(p), resulting in the increased and the enhanced carrier collection in long-wavelength region. These two factors led to the advanced optical absorption in long-wavelength range, contributing in the significantly enhanced of bottom cell from 11.58 to 13.08 mA/cm2.
The performance of tandem cell with μc-SiOx:H(p) replacing μc-Si:H(p) of the bottom cell is shown in Table 3. The increased FF from 71.9 to 73.7% can be ascribed to the shunt quenching effect by the μc-SiOx:H(p) layer, which led to reduced leakage current of the cell . In addition, the carrier recombination in tunneling recombination junction (TRJ) may be enhanced by replacing μc-Si:H(p) with μc-SiOx:H(p), resulting in the increase in and FF . Combining the effects discussed above, the a-Si:H/μc-Si:H tandem solar cell employing the μc-SiOx:H(p) as p-layer of bottom cell and μc-SiOx:H(n) as BRL with efficiency of 10.51%, of 1.37 V, of 10.55 mA/cm2, and FF of 73.67% was obtained.
The p-type and the n-type μc-SiOx:H with varied [O] have been prepared and employed as window layer and back reflecting layer, respectively, in μc-Si:H single-junction solar cells. Regarding the μc-SiOx:H films, the decreased refractive index accompanied with the wider with increasing [O] can be ascribed to the increased Si-O bond in amorphous silicon oxide phase, while structural defects could disrupt crystalline structure, resulting in the degraded and .
By utilizing μc-SiOx:H(p) with increasing [O] in μc-Si:H single-junction solar cell, the short-wavelength spectral response enhanced. However, too much oxygen incorporation induced defects in μc-SiOx:H(p) which could decrease the and FF. Meanwhile, ITO in μc-Si:H single-junction cell was replaced by the highly conductive μc-SiOx:H(n) with [O] of 27 at.%. The employment of μc-SiOx:H(n) enhanced spectral response and increased FF, which were due to the elimination of ex-situ ITO sputtering step and the unavoidable sputtering damage at the μc-Si:H(n)/ITO interface. Finally, further replacing μc-Si:H(p) by μc-SiOx:H(p) as p-layer of bottom cell enhanced FF and , which might result from the increased carrier recombination between TRJ layers. In addition, the higher of bottom cell can be ascribed to the less parasitic absorption loss of μc-SiOx:H(p) than that of μc-Si:H(p). Compared to the reference cell, the optimized cell efficiency by employing μc-SiOx:H(p) and μc-SiOx:H(n) achieved cell efficiency from 9.06 to 10.51%, with of 1.37 V, of 10.55 mA/cm2, and FF of 73.67%, which had a relative efficiency enhancement of 16%.
The authors do not have any competing interests concerning the content of the paper.
This work was sponsored by Ministry of Science and Technology in Taiwan under Grant no. 103-3113-P-008-001. Besides, the authors gratefully thank Hung-Jung Hsu for continuous support and encouragement.
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