Effect of solvent volume of CuAlO 2 thin films for solar cell active layer

Preparation of thin film

The CAO thin films are synthesised by dissolving (CuCl₂.2H₂O) and (AlCl₃) in doubly deionised water with [Cu]/[Al] molar ratio of 10:1. Four different volumes (10, 20, 30 and 40 mL) of precursor solutions are prepared, and they are sprayed onto the ultrasonically cleaned glass substrates that are kept at a temperature of 400°C. A fully automated spray pyrolysis system ((HOLMARC, HO-TH-01, India) is used for synthesis of CAO films. Table 1 describes the control parameter used in the present study during the synthesis of CAO films.

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Table 1

Control parameter

Control parameter	Value
Solution flow rate	2 mL min− 1
Nozzle to substrate distance	15 cm
Substrate temperature	400°C
Solvent volume	10, 20, 30 and 40 mL
Spray angle with respect to plane of substrate	90o
Spray interval	20 s

Characterisation

The thickness of the films is measured using a profilometer (Mitutoyo, SJ 301, USA), and the thickness is found to be ∼760 nm. The structural studies are carried out using X'Pert Pro MPD X-ray diffractometer with Cu K_α (λ = 0·15406 nm) radiation. The Hall effect studies are carried out using Ecopia HMS-3000 measurement system, which is used to measure resistivity, mobility, conductivity, Hall coefficient and charge carrier concentration of CAO films. The optical band gap and transparency are investigated using UV–Vis spectrophotometer (UV-3150). The surface morphology and elemental identification were obtained using field emission scanning electron microscopy (JEOL-6701F, Japan).

Structural properties

The structural studies of the CAO are investigated using X-ray diffraction (XRD) measurement, which is used to identify the crystal structure and existing phases of CAO thin films. Figure 1 represents XRD patterns of samples for different solvent volume ratios, deposited at 400°C. All the prepared CAO films exhibit polycrystallinity, which is inferred with different plane orientations from the XRD spectrum.

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1

XRD patterns of CAO film for different thickness

The peaks in the XRD exactly match with International Centre for Diffraction Data card (JCPDS card no. 77-2494). Two prominent peaks are observed in the XRD pattern, which corresponds to (1 0 0) and (1 0 2) planes. This clearly confirms that the prepared films are the ternary compound of the CAO and the other phases are not present in the film. The mean crystallite size is calculated from well known Scherrer's equation, ¹⁸ which is given as where D is the crystallite size (nm), β is the full width at half maximum, λ is the X-ray wavelength (0·154 nm) and θ is the diffraction angle. The calculated grain size is tabulated in Table 2, in which it is observed that the grain sizes gradually increases when solvent volume increases.

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Table 2

Energy gap, Hall coefficient and grain size of CAO films

Solvent volume/mL	Eg/eV	RH, 107/cm3 C− 1	D/nm
10	2·295	63·2	21·6
20	2·227	27·9	22·6
30	2·114	13·7	23·4
40	2·095	09·9	28·4

Morphological properties

Figure 2 illustrates scanning electron micrograph (SEM) image of the CAO thin films deposited on glass substrates at 400°C for disparate solvent volume. From the SEM image, the morphology of the film shows uniformly distributed nanocrystallites. It is also observed that in CAO film, a highly oriented crystallites are formed for different solvent volume ratios. Moreover, the texture of the film is observed to be rough, which results in multiple reflection of light to take place on the surface of the film. This is one of the important characteristics of the film, which improves the p type active layer of solar cells. It is also observed that when the solvent volume increases, the size of polycrystalline grains increases. It can be concluded that CAO films are effective light scatters, which is one of the favourable properties to improve the efficiency of solar cell. ¹⁹ However, the rough surface of CAO is feasible for light trapping effect on amorphous silicon based solar cell, which was reported by Hu and Gordon.^20,21

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2

Surface morphology SEM images of solvent volume: a 10 mL, b 20 mL, c 30 mL and d 40 mL of CAO films

Optical Properties

Figure 3 shows the transmittance of spray deposited CAO films. The film shows a high transmittance in the near IR range, which shows that all the visible wavelengths are absorbed and it can be used for active layer in a solar cell.

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3

Optical transmittance spectrum of CAO thin films

The high transmission of ∼45% is observed in the IR region for a solvent volume of 10 mL. From Fig. 3, it is observed that the transmission of such film is strongly affected by the solvent volume of the films. The optical band gap E_g of CAO films is obtained by the absorption coefficient α and the incident photon energy hν, which is the relation of where hν is the incident photon energy, A is the constant related to the effective mass, and the exponent n depends on the type of transition (n = 1/2 and 2 for direct and indirect transition respectively). The direct band gap values can be calculated by extrapolating the linear portion of these plots to the energy axis, which is used to obtain better linearity in the (αhν)^1/2 versus hν plots (shown in Fig. 4). The calculated E_g values are 2·29, 2·22, 2·11 and 2·09 eV for the film of solvent volumes 10, 20, 30 and 40 mL respectively. Interestingly, the shift in the optical band gap is observed with the increase in solvent volume. ²² In the present work, the Cu/Al ratio is lesser than the earlier reported work, ⁴ which results in lesser transparency.

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4

Graph between (αhν)^1/2 versus hν for a 10 mL, b 20 mL, c 30 mL and d 40 mL solvent volume

Electrical properties

The Hall effect studies of CAO thin film with different solvent volume shows p type behaviour, and Hall coefficient (R_H) decreases with increasing the solvent volume. The mobility, resistivity and carrier concentration are plotted in Fig. 5 as a role of CAO solvent volume. From Fig. 5, it is observed that the resistivity drops from 1·12 × 10^− 6 to 6·2 × 10^− 8 Ω cm for the solvent volume from 10 to 40 mL. In addition, the carrier concentration is found to be >0·98 × 10¹⁰/cm³ and increases with solvent volume. The decrease in resistivity for the CAO films from 10 to 40 mL volumes may be attributed to the increase in hole concentration originating from the increase in volume of precursor solution. Moreover, the charge carrier density substantially increases with increase in volume of precursor solution. From Table 2, it was observed that the grain size increases with the increment of solvent volume due to grain boundary scattering effect. ¹⁷

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5

Plot between sample thickness versus resistivity and charge carrier concentrations

The cation vacancies (such as Cu vacancies) or oxygen interstitials are supposed to induce holes within the materials that can enhance conductivity. ²³ The grain size dependence of carrier mobility is expressed from Fig. 6. The carrier mobility amends linearly with solvent volume, which is consistent with the enhancement preferred growth.

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6

Plot between sample thickness versus mobility and grain size

Figure 7 shows the current–voltage characteristics of CAO thin films. It is observed that all the films exhibit ohmic behaviour. The current increases as the solvent volume increases. This behaviour may be due to more scattering regions present on the surface of the films. The presence of scattering regions of thin film surfaces is due to the increasing grain size, which is confirmed from the XRD and SEM studies. However, for the first two cases, the grain size is almost equal and there is no appreciable variation in the current value.

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7

I–V characteristics