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

doi:10.1016/j.jmst.2019.03.041

Abstract

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-SiO_x: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-SiO_x: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-SiO_x: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.

Key words: Microcrystalline silicon oxide, Building-integrated photovoltaics, Semitransparent, Thin film, Solar cells

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]. J. Mater. Sci. Technol., 2019, 35(8): 1563-1569.

Figures/Tables 13

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.

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.

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. 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)	V_oc (V)	J_sc (mA/cm²)	FF (%)	η (%)	R_s (Ω?cm²)
0.2	0.787	9.30	64.4	4.71	49.5
0.4	0.799	9.59	66.7	5.11	48.5
0.6	0.804	9.99	67.2	5.40	40.8
0.8	0.818	9.87	63.8	5.15	45.3
1.0	0.823	9.78	58.6	4.67	51.6

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.

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.

Film	H₂/SiH₄	CO₂/SiH₄	B₂H₆/SiH₄	Dark conductivity (S/cm)	E_a (eV)	E_g (eV)
p-a-SiO_x:H	5	0.9	0.3	8.89 × 10^-8	0.54	2.08
i-a-Si:H	10			2.11 × 10^-9	0.64	1.88

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

Structure	V_oc (V)	J_sc (mA/cm²)	FF (%)	η (%)	R_s (Ω?cm²)	R_sh (Ω?cm²)	J₀ (A/cm²)
A	0.804	9.99	67.2	5.40	40.8	5460	3.12 × 10^-5
B	0.873	10.03	68.5	6.00	39.7	6535	7.01 × 10^-8
C	0.883	10.12	71.7	6.41	35.6	7125	1.91 × 10^-10

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.

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).

Structure	η (%)	AT (%)	FOM
A	5.40	28.2	152.3
B	6.00	29.0	173.6
C	6.41	29.3	187.7

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