Journal of Materials Science & Technology  2019 , 35 (8): 1563-1569 https://doi.org/10.1016/j.jmst.2019.03.041

Orginal Article

Adoption of wide-bandgap microcrystalline silicon oxide and dual buffers for semitransparent solar cells in building-integrated photovoltaic window system

Johwa Yanga, Hyunjin Joa, Soo-Won Choia, Dong-Won Kangb*, Jung-Dae Kwona*

a Surface Technology Division, Korea Institute of Materials Science, Changwon, 641-831, Republic of Korea
b School of Energy Systems Engineering, Chung-Ang University, Seoul, 06974, Republic of Korea

Corresponding authors:   *Corresponding authors.E-mail addresses: kangdwn@cau.ac.kr (D.-W. Kang), jdkwon@kims.re.kr(J.-D. Kwon).*Corresponding authors.E-mail addresses: kangdwn@cau.ac.kr (D.-W. Kang), jdkwon@kims.re.kr(J.-D. Kwon).

Received: 2018-10-28

Revised:  2018-11-30

Accepted:  2018-12-4

Online:  2019-08-05

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

We focused on developing penetration-type semitransparent thin-film solar cells (STSCs) using hydrogenated amorphous Si (a-Si:H) for a building-integrated photovoltaic (BIPV) window system. Instead of conventional p-type a-Si:H, p-type hydrogenated microcrystalline Si oxide (p-μc-SiOx:H) was introduced for a wide-bandgap and conductive window layer. For these purposes, we tuned the CO2/SiH4 flow ratio (R) during p-μc-SiOx:H deposition. The film crystallinity decreased from 50% to 13% as R increased from 0.2 to 1.2. At the optimized R of 0.6, the quantum efficiency was improved under short wavelengths by the suppression of p-type layer parasitic absorption. The series resistance was well controlled to avoid fill factor loss at R = 0.6. Furthermore, we introduced dual buffers comprising p-a-SiOx:H/i-a-Si:H at the p/i interface to alleviate interfacial energy-band mismatch. The a-Si:H STSCs with the suggested window and dual buffers showed improvements in transmittance and efficiency from 22.9% to 29.3% and from 4.62% to 6.41%, respectively, compared to the STSC using a pristine p-a-Si:H window.

Keywords: Microcrystalline silicon oxide ; Building-integrated photovoltaics ; Semitransparent ; Thin film ; Solar cells

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Johwa Yang, Hyunjin Jo, Soo-Won Choi, Dong-Won Kang, Jung-Dae Kwon. Adoption of wide-bandgap microcrystalline silicon oxide and dual buffers for semitransparent solar cells in building-integrated photovoltaic window system[J]. Journal of Materials Science & Technology, 2019, 35(8): 1563-1569 https://doi.org/10.1016/j.jmst.2019.03.041

1. Introduction

With dramatic global increases in both building-energy consumption and greenhouse gas emissions, solar cells have been widely studied as environmentally benign power sources. One application of solar cells is building-integrated photovoltaic (BIPV) window systems [1]. In realizing BIPV window systems, solar cell transparency is critical. To reduce transparency losses in BIPV window systems, semitransparent solar cells (STSCs) employing metal mesh and transparent conductive oxides (TCO) as electrodes have been investigated, instead of opaque metals (i.e., Al and Ag). Improving the conversion efficiency and transparency of STSCs is necessary but difficult because of their trade-off relationship.

STSCs have been realized using dye-sensitized solar cells (DSSCs), as well as organic and thin-film Si solar cells. DSSCs have various benefits including low production cost, relatively high efficiency and transparency [2,3]. However, they show poor stability caused by electrolyte leakage and dye degradation at high temperatures [4]. Organic solar cells are more stable than DSSCs; however, thin-film Si solar cells show superior environmental stability because they use inorganic Si-based materials. Semitransparent hydrogenated amorphous Si (a-Si:H) is suitable for use in BIPV window systems for the following reasons. First, large-area a-Si:H can be produced easily using plasma-enhanced chemical vapor deposition (PE-CVD). Second, the thickness of the a-Si:H absorber layer can be very low, between 100 and 250 nm, because of its high absorption coefficient. Third, the transparency can be controlled by changing the thickness of the absorber layer. Finally, semitransparent a-Si:H solar cells are resistant to high temperatures because of their low temperature coefficients [5,6].

In STSCs, both the power conversion efficiency (PCE) and transmittance should be considered. Hence, we introduced a figure of merit (FOM) as multiplication of average optical transmittance and PCE to evaluate these factors. For a-Si:H p-i-n-type solar cells, a p-type window layer significantly affects the optoelectronic properties of the device. Until now, p-a-Si:H windows have been widely used, with p-a-Si:H-based STSCs recently achieving the high FOM of 164.7 [7]. In the report of these devices, the optimized PCE and average transmittance under 500-800-nm irradiation were 5.38% and 30.7%, respectively. Because they used a p-a-Si:H window, further transmittance enhancements could potentially be realized by reducing parasitic absorption in the p-type layer. To reduce transparency losses, a p-nanocrystalline (nc)-SiC:H window was reported in an nc-STSC. This work showed the PCE of 4.27% and average transmittance of 17.3%. However, the FOM for the device was moderate at 73.8 [8].

In this work, we focus on Si oxide materials to suppress parasitic absorption and improve the transmittance of devices. We recently reported on Si oxide materials for STSCs [9], investigating p-type amorphous Si oxide (p-a-SiOx:H). Hence, further improvements in the optoelectronic properties may be attained by using crystalline Si oxide. In this study, p-type microcrystalline (μc) Si oxide (p-μc-SiOx:H) films were fabricated and introduced as window layers; this is the first report of p-μc-SiOx:H-based STSCs. The enhanced transmittance and electronic conductivity of the crystalline p-type layer are expected to improve the STSC photovoltaic characteristics [[10], [11], [12], [13]]. In addition, carrier recombination problems were found at the p-μc-SiOx:H/i-a-Si:H interface in the p-i-n-type architecture. Therefore, we developed p-a-SiOx:H/i-a-Si:H bilayer buffers to alleviate energy-band mismatches and thus enhance carrier transport at the p/i interface. We have achieved the highest FOM of 187.7 among the STSCs using the proposed window and buffer layers.

2. Experimental

To examine the crystallinity and optoelectronic properties of the p-type window layer, 60-nm-thick p-μc-SiOx:H films were deposited on 1.8-mm-thick soda-lime glass (2 cm × 2 cm) substrates by using radio-frequency (RF, 13.56 MHz) PE-CVD at a substrate temperature of 200 °C. The crystalline volume fraction (Xc) of the deposited films was analyzed by Raman spectroscopy (Horibo, Ltd., Kyoto, Japan) with a 514-nm laser. The dark conductivities of the p- and n-type Si layers were measured using the transfer length method (TLM) after evaporating Ag electrodes onto them. The refractive indices and thicknesses of the films were measured using spectroscopic ellipsometry (SE MG-1000, nano-view). To evaluate the bandgap (Eg) by the Tauc plot method of the Si films and the transmittance of the semitransparent a-Si:H solar cells, UV-vis spectrophotometry (Varian, Cary 5000) was utilized. For the current density-voltage (J-V) properties of the a-Si:H-based STSCs under dark conditions, a probe station (MS-TECH) was used at room temperature (25 °C). In the previous work [14], the activation energy (Ea) was estimated from the dark conductivity at room temperature, presuming the pre-factor value σ0 of 150 S/cm, following Eq. (1):

σd0exp[$\frac{-E_{a}(RT)}{KT}$] (1)

where σ0 is the conductivity pre-factor, k is the Boltzmann constant, and T is the absolute temperature.

For STSC fabrication, the cell structure was < glass substrate/textured F-doped Sn oxide (FTO; 600 nm)/Al-doped Zn oxide (AZO; 30 nm)/p-μc-SiOx:H ($\widetilde{2}$0 nm)/p-a-SiOx:H (5 nm) and/or i-a-Si:H ($\widetilde{2}$0 nm)/i-a-Si:H ($\widetilde{1}$50 nm)/n-μc-SiOx:H (30 nm)/AZO (700 nm)>, as depicted in Fig. 1. The AZO buffer layer was used to protect the underlying FTO from H plasma during the deposition of p-μc-SiOx:H. For the deposition of Si layers, p- and n-type layers were deposited under the RF power of 50 W while the intrinsic Si layer was prepared under the very high frequency (VHF, 40.68 MHz) power of 20 W from a p-i-n cluster system. The gases used in PE-CVD were silane (SiH4), hydrogen (H2), carbon dioxide (CO2), phosphine (PH3, 1% diluted in H2), and diborane (B2H6, 1% diluted in H2). The CO2/SiH4 flow ratio R was controlled to manipulate the crystallinity of the films. For the n-doped layer, n-μc-SiOx:H was suggested to replace conventional n-a-Si:H layer as a better back reflector [15]. The AZO rear transparent electrode was formed using a direct-current (DC) magnetron sputtering system in high vacuum of $\widetilde{2}$× 10-6 Torr ($\widetilde{2}$.67 × 10-4 Pa) and a substrate temperature of 150 °C using a shadow mask that defined the active cell area of 0.25 cm2. The J-V characteristics of the solar cells were measured using a solar simulator (Oriel 300, Newport Co.) under 100 mW/cm2 (AM 1.5 G) irradiation. A photo-mask was carefully aligned to avoid potential overestimation of the photocurrent. The external quantum efficiencies (EQEs) of the solar cells were obtained by quantum efficiency measurements (IQE-200, Newport Co.) using a 240-W tungsten halogen lamp and grating monochromator.

Fig. 1.   Schematics of semitransparent a-Si:H solar cells with p-μc-SiOx:H window layers. Structures B and C include additional single and dual buffers, respectively, at p/i interface.

3. Results and discussion

As shown in Fig. 2, the crystallinities of the p-μc-SiOx:H films are investigated from Raman spectroscopy by changing R from 0.2 to 1.2. The Si transverse optical (TO) peaks are fitted and divided into the integrated crystalline, amorphous, and intermediate Gaussian peaks, denoted I520 cm-1, I480 cm-1, and I510 cm-1, respectively. As R is increased from 0.2 to 1.2, the crystalline and intermediate Gaussian peaks are decreased in intensity, while that of the amorphous Gaussian peak is gradually increased. The crystalline volume fraction Xc was calculated from the widely known relation, Xc = ((I510 + I520)/(I480 + I510 + I520)). Xc declined from 50% to 11% as R increased from 0.2 to 1.0, indicating that the microcrystalline phase of the p-μc-SiOx:H was deteriorated by the increase of the a-SiOx:H matrix in the mixed phases [10,16].

Fig. 2.   Raman spectra of p-μc-SiOx:H films at changing R of (a) 0.2, (b) 0.6, and (c) 1.0. (d) Crystalline fraction of p-μc-SiOx:H films is decreased from 50 to 13% with increasing R from 0.2 to 1.2.

In order to optimize the electrical conductivities and parasitic absorption properties, the influence of R on the p-type layer characteristics was investigated. The dark conductivities and Eg of the p-μc-SiOx:H films were measured by varying R; the results are depicted in Fig. 3(a). By increasing R from 0.2 to 1.2, Eg is increased from 2.10 to 2.38 eV, while the dark conductivity is decreased from 1.2 × 10-1 to 8.9 × 10-6 S/cm. It is generally agreed that the oxygen concentration and Si-O bonding in SiOx films are increased with an increase in R. Si-O bonds have higher energies than Si-Si and/or Si-H, thus causing increases in Eg and decreases in dark conductivity because of the stronger electronegativity of O atoms [[17], [18], [19]]. The increased electronegativity can allow for blue-shifting of optical bandgap [20]. To confirm the reduction of parasitic absorption losses in p-μc-SiOx:H films with enhanced R, the transmittance spectra at 300-800 nm are shown in Fig. 3(b). The transmittance increases throughout this range without variation in the thickness of the p-type layers. As shown in Fig. 3(c), the refractive indices of the p-μc-SiOx:H films are also investigated as a function of R at 550 nm to estimate the light reflection at the front AZO/p-type layer interface. Our experimental results show a decrease in the refractive index from 4.19 to 2.97, which benefits decreases in reflective losses at the AZO/p interface because of the refractive index matching between the front TCO (n@550 nm = 1.93) and the doped p-type layer. Thus, enhanced optical properties are expected to reduce parasitic absorption in the p-type layer and enhance light incoupling, which can yield improvements in light absorption in the Si photoactive layer.

Fig. 3.   (a) Dark conductivity and optical bandgap (Eopt), (b) transmittance in the range 300-800 nm, and (c) refractive index at a wavelength of 550 nm (n@550 nm) of p-μc-SiOx:H films deposited at various R.

Fig. 4(a) exhibits the J-V characteristics of the fabricated STSCs depending on R under 1-sun illumination. Photovoltaic parameters including the open-circuit voltage Voc, short-circuit current Jsc, fill factor FF, and series resistance Rs are summarized in Fig. 4(b) and Table 1. The Voc of the STSCs is gradually increased from 787 mV to 823 mV as R increases. This Voc enhancement is related to the increase in Eg of the p-type layer, as depicted in Fig. 3(a). In Fig. 5, the EQE spectra of the STSCs are shown to understand Jsc in combination with the J-V curves. The Jsc increases, approaching 10.0 mA/cm2, with R until R = 0.6. This increase is matched well with the EQE in that the EQE significantly increases in the range 300-550 nm for R increasing from 0.2 to 0.6. This can be attributed to the reduced reflection at the TCO/p-type layer interface as well as to the reduced parasitic absorption in the p-type layer, as confirmed above. However, Jsc decreases for R > 0.8, despite the reduced parasitic absorption in the p-μc-SiOx:H films. This is related to the deteriorated conductivity of the p-type layer and increased Rs of the device. The FF shows behavior similar to that of the Jsc. The FF drop for R > 0.8 can be understood by the increased Rs of the cells. This is attributable to void-rich and high-defect-density films with increased internal void surfaces in the Si network, caused by the enriched Si-H2 bonding and back-bonding to O atoms as the O content increases in Si films [9,21,22]. Furthermore, dangling-bond defects in films are increased and optoelectronic properties such as mobility and carrier lifetime are decreased as the O concentration of Si films is increased [23,24]. Based on these experiments regarding p-type layer tuning, the best-performing STSC is achieved at R = 0.6 with the PCE of 5.40%, Voc of 804 mV, Jsc of 9.99 mA/cm2, and FF of 0.672. We compared this device to one employing conventional p-a-Si:H to evaluate our suggested p-type window material. The fabricated cell with p-a-Si:H exhibited the PCE of 4.62%, Voc of 770 mV, Jsc of 9.01 mA/cm2, and FF of 0.666, which were much lower than those of the suggested device. In addition, the suggested device with p-μc-SiOx:H exhibits improved transmission throughout the spectral wavelength range by the suppression of parasitic absorption, as found in Fig. 6. The average transmittance of the cell with p-μc-SiOx:H is 28.2%, which is much higher than the 22.9% transmittance achieved using p-a-Si:H.

Fig. 4.   Performance characteristics of semitransparent a-Si:H solar cells as function of R under 1-sun illumination: (a) J-V characteristics of devices and (b) variation of cell parameters. The best efficiency (5.40%) of p-μc-SiOx:H is achieved at R = 0.6.

Table 1   PV parameters of semitransparent a-Si:H solar cells as function of R.

Sample R (CO2/SiH4)Voc (V)Jsc (mA/cm2)FF (%)η (%)Rs (Ω∙cm2)
0.20.7879.3064.44.7149.5
0.40.7999.5966.75.1148.5
0.60.8049.9967.25.4040.8
0.80.8189.8763.85.1545.3
1.00.8239.7858.64.6751.6

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Fig. 5.   EQE spectra of semitransparent a-Si:H solar cells at R of 0.2-1.0.

Fig. 6.   Optical transmittances of STSCs using various window layers, such as conventional p-a-Si:H and p-μc-SiOx:H.

We believe that further improvement of PV performance can be attained by optimizing the p/i interface, because the wide-bandgap p-μc-SiOx:H has been introduced as the p-type layer instead of conventional p-a-Si:H. Thus, employing the proper buffer layer at the p/i interface may improve carrier collection in the device. As a first approach, we introduced a thin $\widetilde{5}$-nm p-a-SiOx:H buffer at the p/i interface to reduce Eg and the hole-transfer barrier height, as shown in Fig. 7(b). Here, the total p-type layer thickness (window + buffer) is fixed at 20 nm, as in Fig. 7(a) and structure B in Fig. 1. Enhanced PV characteristics such as increased Voc (804 → 873 mV) and FF (67.2% → 68.5%) are observed. In order to further improve hole collection and suppress charge-carrier recombination, a 20-nm-thick i-a-Si:H buffer with strong hydrogen dilution (H2/SiH4 = 10) is introduced before the main i-layer deposition, as displayed in Fig. 7(c) and structure C of Fig. 1. To realize this approach in devices, we optimized the dual buffers; their deposition parameters and optoelectronic properties are summarized in Table 2. The PV performances of the fabricated STSCs with various buffers at the p/i interface are exhibited in Table 3 and Fig. 8(a). The suggested STSC using the dual buffers delivers the much improved PCE of 6.41% because of the reinforced Voc (883 mV) and FF (71.7%).

Fig. 7.   Energy band diagrams of different p/i interfaces: (a) no buffer layer, (b) single buffer layer with p-a-SiOx:H layer, and (c) dual buffers comprising p-a-SiOx:H and i-a-Si:H.

Table 2   Process condition and resulted optoelectronic properties (dark conductivity, activation energy, and bandgap) for the suggested buffer layers.

FilmH2/SiH4CO2/SiH4B2H6/SiH4Dark conductivity (S/cm)Ea (eV)Eg (eV)
p-a-SiOx:H50.90.38.89 × 10-80.542.08
i-a-Si:H102.11 × 10-90.641.88

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Table 3   PV parameters under illumination and dark condition for semitransparent a-Si:H solar cells with various structures shown in Fig. 1.

StructureVoc (V)Jsc (mA/cm2)FF (%)η (%)Rs (Ω∙cm2)Rsh (Ω∙cm2)J0
(A/cm2)
A0.8049.9967.25.4040.854603.12 × 10-5
B0.87310.0368.56.0039.765357.01 × 10-8
C0.88310.1271.76.4135.671251.91 × 10-10

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Fig. 8.   PV cell performances of structure A, device using p-μc-SiOx:H film at R = 0.6; structure-B, single buffer layer at p/I interface; and structure C, dual buffer layers at p/i interface: (a) J-V under 1-sun illumination, (b) J-V under dark condition.

The increase in Voc can be understood in terms of the suppressed carrier recombination at the p/i interface and enhanced built-in potential. The diode saturation current J0, related with carrier recombination, affects Voc based on the Shockley diode equation; we previously explained a J0 extraction method from the dark J-V curve [9,25]. The Shockley diode equation relating Voc and J0 is expressed in Eq. (2):

Voc=$\frac{nkT}{q}(ln\frac{Jph}{J0}+1)$ (2)

where Jph is the photocurrent, n is the diode ideality factor, k is the Boltzmann constant, and T is the absolute temperature. As shown in Table 3, the J0 values of the devices are decreased as buffer layers are added, which could correspondingly increase the Voc. Accordingly, structure C in Fig. 1 employing dual buffers provides the highest Voc (883 mV) among all cells tested herein. This can be attributed to the reduced carrier recombination rate by the introduced buffer layers at p/i interface. The bandgap mismatch at the p/i interface is alleviated by the stepwise formation of band profiles using the buffers. Therefore, buffers decrease cell Rs by increasing the effectiveness of photo-generated hole collection, which in turn increases FF, as indicated in Table 3.

In this work, the optical transmittance in the visible region and the STSC PCE are critical for STSC application in a BIPV window system. Hence, we present the transmission characteristics of the advanced device structures in Fig. 9. To evaluate the principle properties of the STSCs, the average transmittance (AT) in the visible range 500-800 nm and the FOM are extracted as summarized in Table 4. By adopting the suggested dual buffers, we demonstrate remarkable improvement in not only the AT but also the PCE of STSCs. Furthermore, the semitransparent a-Si:H solar cells with double buffer layers achieved the FOM of 187.7, which is the highest value among the a-Si:H-based STSCs reported to date.

Fig. 9.   Optical transmittance properties of STSCs of structures A, B, and C in the visible region.

Table 4   Performances of STSCs with various buffer structures, as shown in Fig. 1: PCE (η), average transmittance at wavelengths 500-800 nm, and Figure of Merit (FOM).

Structureη (%)AT (%)FOM
A5.4028.2152.3
B6.0029.0173.6
C6.4129.3187.7

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4. Conclusion

We have improved the optoelectronic performances of semitransparent p-i-n single-junction thin-film solar cells by introducing p-μc-SiOx:H as a window layer with combined dual buffers at the p/i interface. The employed p-μc-SiOx:H window layer successfully reduced parasitic absorption losses and increased the Voc because of the enhanced Eg. As R increased, transmittance was increased and the refractive index of the p-type layer was decreased. This phenomenon induced low parasitic absorption loss and lower reflection of light at the TCO/p interface. To relieve the energy-band mismatch between the inserted p-μc-SiOx:H and i-a-Si:H photoactive layers, dual buffers comprising p-a-SiOx:H (5 nm) and i-a-Si:H (20 nm) using strong hydrogenation were introduced to increase hole collection from the lower valence band offsets at the p/i interface. By developing these buffers, simultaneous improvements in the PCE from 5.40% to 6.41% and transmittance from 28.2% to 29.3% were demonstrated. Also, the significant FOM of 187.7, a new record for a-Si:H-based STSCs, was achieved in this work. These promising results support the rapid industrialization of STSCs based on a-Si:H that use the suggested p-μc-SiOx:H window and efficient control of the p/i interface.

Acknowledgments

This research was supported by the Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) under grant Nos. 20163010012560 and 20172010104940.

Abbreviations: a-Si:H, amorphous hydrogenated Si; a-SiOx:H, amorphous hydrogenated SiOx; AZO, aluminum-doped zinc oxide; BIPV, building-integrated photovoltaics; DSSC, dye-sensitized solar cell; Eg, bandgap energy; EQE, external quantum efficiency; FF, fill factor; FOM, figure of merit; FTO, fluorine-doped tin oxide; Jsc, short-circuit current density; PCE, photoconversion efficiency; PE-CVD, plasma-enhanced chemical vapor deposition; p-μc-SiOx:H, p-type microcrystalline hydrogenated SiOx; R, flow rate ratio of CO2/SiH4; RF, radio-frequency; Rs, series resistance; Rsh, sheet resistance; STSC, semitransparent solar cell; TCO, transparent conducting oxide; UV, ultraviolet; Voc, open-circuit voltage; Xc, crystalline volume fraction.

The authors have declared that no competing interests exist.


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