Properties of boron doped ZnO films prepared by reactive sputtering method: Application to amorphous silicon thin film solar cells

doi:10.1016/j.jmst.2019.12.004

Abstract

Abstract:

Reactive sputtered boron-doped zinc oxide (BZO) film was deposited from argon, hydrogen and boron gas mixture. The reactive sputtering technique provides us the flexibility of changing the boron concentration in the produced films by using the same intrinsic zinc oxide target. Textured surface was obtained in the as-deposited films. The surface morphology and the opto-electronic properties of the films can be controlled by simply varying the gas concentration ratio. By varying the gas concentration ratio, the best obtained resistivity ~ 6.51 × 10^-4 Ω-cm, mobility ~ 19.05 cm² V^-1 s^-1 and sheet resistance ~ 7.23 Ω/□ were obtained. At lower wavelength of light, the response of the deposited films improves with the increase of boron in the gas mixture and the overall transmission in the wavelength region 350-1100 nm of all the films are >85 %. We also fabricated amorphous silicon (a-Si) thin film solar cell on the best obtained BZO layers. The overall efficiency of the a-Si solar cell is 8.14 %, found on optimized BZO layer.

Key words: Magnetron reactive sputtering, BZO, Amorphous silicon, Solar cells, Zinc oxide (ZnO)

Sukanta Bose, Sourav Mandal, Asok K. Barua, Sumita Mukhopadhyay. Properties of boron doped ZnO films prepared by reactive sputtering method: Application to amorphous silicon thin film solar cells[J]. J. Mater. Sci. Technol., 2020, 55: 136-143.

Figures/Tables 15

Table 1 Process parameters for the development of BZO layers.

Parameters	Value
Working pressure (mbar)	0.01
Substrate temperature (°C)	275
Target-substrate distance (cm)	7
Sputtering power density(W/cm²)	2.25
Film thickness (nm) approx	900
Ar: 1 % B₂H₆ in H₂ (flow in sccm)	5:0.75, 5:1.00, 5:1.25, 5:1.50, 5:1.75, 5:2.00

Fig. 1. X-ray diffraction pattern of BZO thin films deposited at different PB2H6.

Fig. 2. Variation of 2θ peak position, FWHM of (002) reflections and Grain size with different PB2H6.

Table 2 Strain, stress, lattice spacing & lattice parameter of BZO layers.

Sample ID	P_B2H6	Lattice spacing, d (?)	Lattice parameter, c (?)	Strain (× 10^-3)	Stress (GPa)
BZO-1	13.04	2.614	5.228	4.41	-1.02
BZO-2	16.67	2.612	5.224	3.65	-0.85
BZO-3	20.00	2.606	5.212	1.34	-0.31
BZO-4	23.08	2.617	5.234	5.57	-1.30
BZO-5	25.93	2.618	5.236	5.95	-1.39
BZO-6	28.57	2.619	5.238	6.34	-1.48

Fig. 3. Shifting of XRD (002) peak with changing of PB2H6.

Fig. 4. FESEM images of (a) BZO-1 (b) BZO-2 (c) BZO-3 (d) BZO-4 (e) BZO-5 (f) BZO-6.

Fig. 5. 3D AFM images of (a) BZO-1 (b) BZO-2 (c) BZO-3 (d) BZO-4 (e) BZO-5 (f) BZO-6.

Fig. 6. Electrical properties of BZO films as a function of different PB2H6: (a) resistivity, (b) mobility, (c) carrier concentration.

Fig. 7. Transmission and reflection spectra of BZO films with different PB2H6. The inset shows the effect of B2H6 gas concentration ratio on the lower wavelength transmission.

Fig. 8. Plot of (αhυ)2 vs. photon energy (hυ) of BZO thin films prepared at different PB2H6. The inset shows the optical bandgap variation with PB2H6.

Fig. 9. Plot of refractive index (n) w.r.t wavelength of BZO thin films prepared at different PB2H6.

Fig. 10. Haze ratio and direct transmittance spectra of BZO thin films prepared at different PB2H6.

Fig. 11. Average figure of merit of BZO samples with different PB2H6.

Table 3 Comparison table of J-V parameters of the devices fabricated on different BZO substrates.

Device ID	Substrate type	V_oc (V)	J_sc (mA/cm²)	FF (%)	η (%)
Cell-A	BZO-1	0.880	12.97	69.0	7.87
Cell -B	BZO-2	0.879	13.11	68.8	7.92
Cell -C	BZO-3	0.877	13.56	68.5	8.14
Cell -D	BZO-4	0.878	12.89	68.9	7.79
Cell -E	BZO-5	0.881	12.77	68.9	7.75
Cell -F	BZO-6	0.882	12.63	69.0	7.68

Fig. 12. (a) J-V characteristics, (b) normalized EQE and reflection of the a-Si solar cell fabricated on BZO-3 substrates, (c) the enlarge cross-sectional image of the BZO and a-Si solar cell interface, (d) the cross-sectional image of the full a-Si solar cell with BZO as front contact.

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