**3. Optical characterization of AlSb thin film**

346 Solar Cells – New Aspects and Solutions

produce 1 micron AlSb film in different deposition ratio for Al:Sb. The film was annealed at 200 C in vacuum for 2 hrs and cooled down naturally. Table 1 summarizes the deposition

(W) Ar Gas Pressure

(kÅ) Al Sb Al Sb

(mTorr)

Film Thickenss

Sputtering Power

1:3 2 6 104 37 20.1 10 2:5 2 5 104 33 20.1 10 3:7 3 7 150 42 20.1 10 1:1 1 1 150 24 20.1 10 7:3 7 3 261 24 20.1 10

The film was deposited on glass slides for electrical and optical characterization. The microscopic glass substrate (1 cm x 1 cm) was cleaned using standard substrate cleaning procedure as follows: soaked in a solution of 90% boiling DI and 10% dishwashing liquid for five minutes, followed by soaking in hot DI (nearly boiled) water for five minutes. The substrate was then ultra sonicated, first in Acetone (Fisher Scientific) and then isopropyl alcohol (Fisher Scientific) for 10 minutes each. The substrate was then blown dry with

The morphology of the AlSb film was checked by SEM and was used to validate the grain

(a) (b)

The AlSb grains were found to have been developed after annealing of the film due to proper diffusion and bonding of Al and Sb. Only low magnified image could be produced before annealing the film and holes were seen on the surface. The AlSb microcrystal is formed with an average grain size of 200 nm. Also seen are holes in the film which are primarily the defect area, which could act as the recombination centers. Better quality AlSb film could be produced

Fig. 3. SEM images of the AlSb thin film (a) before annealing, and (b) after annealing.

if proper heating of the substrate is employed during deposition process.

parameters of different AlSb films.

Deposition Rate (Å/s)

Table 1. Deposition Parameters of Different AlSb films.

size and crystalline nature of AlSb particles and shown in Fig. 3.

Al: Sb Ratio

nitrogen.

The transmittance in the thin film can be expressed as (Baban et al. 2006):

$$T = \frac{I\_T}{I\_0} = \left(1 - R1\right)\left(1 - R2\right)\left(1 - R3\right)\left(1 - S\right)e^{-\alpha d} \tag{1}$$

Where, *α* is the absorption coefficient antd *d* is the thickness of the semiconductor film. *R1*, *R2* and *R3* are the Fresnel power reflection coefficient and the Fresnel reflection coefficient at semiconductor - substrate and substrate – air interface. *S* measures the scattering coefficient of the surface.

UV Visible Spectrophotometer (Lambda 850) was used to measure the absorption and transmission data. This system covered the ultraviolet-visible range in 200 – 800 nm. The procedures in the Lambda 850 manual were followed. Figure 4 shows the transmittance spectra of the AlSb thin films. The films have an strong absorption in the visible spectral range up to 550 nm for film with Al:Sb ratio 2:5. Similarly for films with Al:Sb in the ratio of 1:3, 1:1 and 3:7 have strong absorption up to 700 nm. The films were transparent beyond these levels.

Fig. 4. Transmittance Spectra of AlSb films with different Al:Sb growth ratios.

The film with Al:Sb ratio of 7:3 didn't have a clear transmittance spectra and thus not shown in the figure. This was because the increasing the content of aluminum would make the film metallic thus absorbing most of the light in visible spectrum.

Absorption coefficient of a film can be determined by solving equation 1 for absorption and normalizing the Transmittance in the transparent region as (Baban et al. 2006):

$$\alpha = -\frac{1}{d} \ln \left( T\_{normalized} \right) \tag{2}$$

Optical band gap of the film was calculated with the help of transmission spectra and reflectance spectra by famous using Tauc relation (Tauc, 1974)

$$
abla \nu = d \left( \hbar \nu - E\_g \right)^n \tag{3}$$

AlSb Compound Semiconductor as Absorber Layer in Thin Film Solar Cells 349

*measured measured*

(4)

(5)

The sheet resistance,

*RCF*

*<sup>V</sup> RCF I*

*R*s, could be thus be calculated as *R*s = *ρ*/*d* and measured in ohms per square. Conductivity (*σ*) is measured as reciprocal of resistivity and could be related to the activation energy as

0

Where, ∆E is the activation energy. This describes the temperature dependence of carrier mobility. The dark conductivity of AlSb film measured as a function of temperature and is

Fig. 7. Temperature dependence of annealed AlSb (3:7) film when heated from 26 - 240 C

 *e* 

*E k Tb*

Where, RCF is the resistivity correction factor and given by ln 2

Fig. 6. Schematic diagram of Four point probe configuration. The sheet resistivity of a thin sheet is given by (Chu et al. 2001):

(Chu et al., 2001):

shown in Fig. 7.

(dot line for guiding the eyes).

Where *Eg* is optical band gap and the constant *n* is 1/2 for direct band gap material and n is 2 for indirect band gap. The value of the optical band gap, *Eg*, can be determined form the intercept of 1 2 *h* Vs Photon energy, *hν*, at 1 2 *h* = 0.

Fig. 5. Bandgap estimation of AlSb semiconductor with Al:Sb growth ratios (a) 1:3, (b) 2:5, (c) 1:1 and (d) 3:7.

The optical absorption coefficient of all the films was calculated from the transmittance spectra and was found in the range of 105 cm-1 for photon energy range greater than 1.2 eV. Fig. 5 shows the square root of the product of the absorption coefficient and photon energy (*hν*) as a function of the photon energy. The band gap of the film was then estimated by extrapolating the straight line part of the (αhv)1/2 vs *hν* curve to the intercept of horizontal axis.

This band gap for Al:Sb growth ratio 1:3, 2:5, 1:1 and 3:7 was found out to be 1.35 eV, 1.4 eV, 1.25 eV and 1.44 eV respectively. Since the ideal band gap of AlSb semiconductor is 1.6 eV we have taken the Al:Sb growth ratio to be 3:7 to characterize the film and fabricate the solar cells.

### **4. Electrical characterization**

Material's sheet resistivity, *ρ*, can be measured using the four point probe method as show in Fig. 6. A high impedance current source is used to supply current (*I*) through the outer two probes and a voltmeter measures the voltage (*V*) across the inner two probes.

Where *Eg* is optical band gap and the constant *n* is 1/2 for direct band gap material and n is 2 for indirect band gap. The value of the optical band gap, *Eg*, can be determined form the

Fig. 5. Bandgap estimation of AlSb semiconductor with Al:Sb growth ratios (a) 1:3, (b) 2:5,

the straight line part of the (αhv)1/2 vs *hν* curve to the intercept of horizontal axis.

two probes and a voltmeter measures the voltage (*V*) across the inner two probes.

The optical absorption coefficient of all the films was calculated from the transmittance spectra and was found in the range of 105 cm-1 for photon energy range greater than 1.2 eV. Fig. 5 shows the square root of the product of the absorption coefficient and photon energy (*hν*) as a function of the photon energy. The band gap of the film was then estimated by extrapolating

This band gap for Al:Sb growth ratio 1:3, 2:5, 1:1 and 3:7 was found out to be 1.35 eV, 1.4 eV, 1.25 eV and 1.44 eV respectively. Since the ideal band gap of AlSb semiconductor is 1.6 eV we have taken the Al:Sb growth ratio to be 3:7 to characterize the film and fabricate the solar cells.

Material's sheet resistivity, *ρ*, can be measured using the four point probe method as show in Fig. 6. A high impedance current source is used to supply current (*I*) through the outer

 *h* = 0.

*h* Vs Photon energy, *hν*, at 1 2

intercept of 1 2 

(c) 1:1 and (d) 3:7.

**4. Electrical characterization** 

Fig. 6. Schematic diagram of Four point probe configuration.

The sheet resistivity of a thin sheet is given by (Chu et al. 2001):

$$\rho = \text{RCF} \frac{V\_{measured}}{I\_{measured}} \tag{4}$$

Where, RCF is the resistivity correction factor and given by ln 2 *RCF* The sheet resistance,

*R*s, could be thus be calculated as *R*s = *ρ*/*d* and measured in ohms per square. Conductivity (*σ*) is measured as reciprocal of resistivity and could be related to the activation energy as (Chu et al., 2001):

$$
\sigma = \sigma\_o e^{\frac{-\Lambda E}{k\_b T}} \tag{5}
$$

Where, ∆E is the activation energy. This describes the temperature dependence of carrier mobility. The dark conductivity of AlSb film measured as a function of temperature and is shown in Fig. 7.

Fig. 7. Temperature dependence of annealed AlSb (3:7) film when heated from 26 - 240 C (dot line for guiding the eyes).

AlSb Compound Semiconductor as Absorber Layer in Thin Film Solar Cells 351

Fig. 8. Current-voltage simulation of AlSb p-i-n junction structure in AMPS 1D software.

illuminated under one sun at standard AM 1.5 spectrum.

19% by doubling the thickness of AlSb layer to 2 micron.

Fig. 9. Solar Cell Design (a) p-n and (b) p-*i*-n structure.

**6. Solar cell fabrication** 

CuSCN as a p-type layer.

Fig. 8 shows the current voltage simulation curve of pin junction solar cell - CuSCN/AlSb/ZnO with AlSb as an intrinsic layer. CuSCN was used as a p layer and ZnO as a n layer. The cell was

The simulation result shows that the solar cell has the FF of 55.5% and efficiency of 14.41%. The short circuit current for the cell was observed to be 21.7 mA/cm2 and the open circuit voltage was observed to be 1.19 V. AlSb is thus the promising solar cell material for thin film solar cells. The efficiency of the same cell structure could be seen increased up to

Both p-n and p-*i*-n junction solar cells were designed and fabricated in 1cm x 2 cm substrate with AlSb as a p type and an absorber material respectively. Variety of n type materials including TiO2 and ZnO were used to check the photovoltaic response of AlSb thin film. Fig. 9 shows the p-n and p-*i*-n based solar cell design with ZnO and TiO2 are an n-type layer and

The annealed film shows a linear lnσ vs 1/*T* relationship. The activation energy of the dark conductivity was estimated to be 0.68 eV from the temperature dependence of the conductivity curve for AlSb film. This value is in good agreement with work done by Chen et al. (Chen et al., 2008). This curve also confirms the semiconducting property of the AlSb (3:7) film because the conductivity of the film was seen to be increasing with increasing the excitation.
