Optoelectronics applications of electrodeposited p- and n-type Al₂Se₃ thin films

In recent times, compound semiconductor materials, such as cadmium telluride (CdTe), zinc oxide (ZnO), zinc sulfide (ZnS), lead sulfide (PbS), aluminum selenide (Al₂Se₃), and cadmium selenide (CdSe) to mention a few, have experienced renewed scientific attention as they form the bedrock of many scalable technologies [1]. Among the compound semiconductors, aluminum selenide (Al₂Se₃) thin films are the most infrequently described despite their potential in optoelectronic applications [2]. Aluminum (Al) as an elemental semiconductor has been extensively studied because of its ease of growth, promising optical and electrical properties, and abundance in the earth's crust, after oxygen and silicon [2,3,4]. Although compound semiconductor has more functionalities than elemental semiconductor [5,6,7], compound semiconductors also possess more functionalities than their respective components. In addition, compound semiconductors with selenium as a constituent portend viable nanomaterials in energy conversion solar cells and sensor devices [2, 8,9,10]. Al₂Se₃ has also been described as a perfect window layer material that is capable of forming a suitable heterojunction with absorber layer materials such as cadmium telluride and lead sulfide. The suitability of this material in the formation of heterostructure-based optoelectronic devices necessitate the study of junction-based Al₂Se₃. Compound semiconductor materials based thin films have been synthesized by various deposition techniques [11,12,13]. Among all the synthesis routes, the electrodeposition (ED) technique plays a significant role in the synthesis of cost-effective nanomaterials for nanodevice applications [14,15,16,17]. This method allows tunability and control of nanomaterials’ properties by changing the preparative parameters of the solution, such as ionic concentration (electrolyte), pH value, temperature, deposition time, and cathodic voltage [18,19,20].

To our knowledge, researchers over the years have paid little to no attention to the electrical conduction type of Al₂Se₃, especially when the cathodic potential varies. In this work, the focus is directed toward the possibility of fabrication of both p-and n-type Al₂Se₃ without the inclusion of external dopants. It is believed that most nanodevices are p-n junction-based devices; therefore n-type and p-type Al₂Se₃ will be of commercial value in the fabrication of single material-based junction devices such as diodes.

The film of Al₂Se₃ sourced from the solution of high-grade aluminum chloride (AlCl₂, 99%) and selenium dioxide (SeO₂, 99%) without further purification were subject to ED on a thoroughly degreased conducting fluorine-doped tin oxide (FTO)/substrate of 2.3 by 4 cm² dimension. The formation of electrolytic bath of Al₂Se₃ contained 29 g of AlCl₂ and 0.9 g of SeO₂. The admixed solution of 400 mL in a 500 mL beaker was magnetically stirred for 2 h to ensure homogeneous solution and the acidic level was tested and adjusted using both pH probe (pH = 2.5) and ammonium solution, since the synthesis technique used is favored by acidic medium [20]. Different films of Al₂Se₃ at a bath temperature of 90 °C in 15 min were achieved potentiostatically in two-electrode configurations; graphite was used as a counter electrode (cathode) and carbon as a reference electrode (anode) by the variation of cathodic potential and duration of deposition, respectively. The film's optical properties and electrical conductivity type were achieved using ultraviolet-visible spectroscopy at wavelength range 200–900 nm and photoelectrochemical cells (PEC) measurements. Thin-film thickness, optical properties, and electrical conductivity type were obtained using Eqs (1)–(4) [21, 22].

The film thickness as illustrated in Table 1 is denoted as T, J is the current density of the electrodeposited Al₂Se₃, t is the deposition time, ρ is the density of Al₂Se₃, n is the total number of electrons transferred per ion of the deposited material, F is the faraday's constant with a numerical value of 96,485 C · mol⁻¹, and M is the molar weight of the deposited Al₂Se₃. (2) (αhv)=A(hv−Eg)n (\alpha hv) = A{\left( {hv - {E_g}} \right)^n} where α is absorption coefficient, hν is photon energy, A is a constant usually equal to one, E_g is the energy band gap, and n is the transition between the valence band and conduction band, which is 0.5 for direct and 1 for indirect transition. (3) k=αλ4π=αλ12.57 k = {{\alpha \lambda } \over {4\pi }} = {{\alpha \lambda } \over {12.57}} where α is absorption coefficient, k is extinction coefficient, and 4π carries a numeric value of 12.57. (4) PEC Signal=VL−VD PEC\;Signal = {V_L} - {V_D} where V_L is voltage under illumination and V_D is voltage under dark.

The achievement of junction-based electrode-posited aluminum selenide revealed the potential of the films in optoelectronics applications. The result obtained in the optical phenomena and constants suggests cathodic potential and time of deposition as good preparative parameters in the tunability of optical properties of electrodeposited Al₂Se₃ thin films. The film energy bandgap varied from 3.23 eV to 2.95 eV as cathodic voltage increases, and from 3.3 eV to 2.7 eV as the time of deposition increases, respectively. The other optical properties with relatively low absorbance, averagely high reflectance, and high transmittance indicate the viability of junction-based electrodeposited Al₂Se₃ as good buffer/window layers in solar cell device architecture. The absorption and extinction coefficient showed a sharper absorption edge and higher extinction coefficient edge in the visible region of the electromagnetic spectrum. The films possess excellent optical constant, which helps in the selection of good window layer materials capable of providing a receptive surface for absorber layers in the fabrication of junction-based optoelectronics devices. The successful fabrication of p-type and n-type Al₂Se₃ suggests cathodic potential as an optimized preparative procedure in the ED technique.