Structural, Optical and Electrical properties of ZnSe thin films for solar cell applications
INTRODUCTION
In the last two decades considerable work has been done in developing thin film solar cells to replace the costly
single crystal and wafer based solar cells and the progress achieved is encouraging. Research in CdTe dates
back to the 1950s[1], because it was quickly identified as having a band gap (about 1.5 eV) almost perfectly
matched to the distribution of photons in the solar spectrum in terms of optimal conversion to electricity. A
simple heterojunction design evolved in which p-type CdTe was matched with n-type cadmium sulfide (CdS).
Small-area CdTe cells with efficiencies of greater than 15% [2] and CdTe modules with efficiencies of greater
than 9% [3] have been reported. The field stability of CdTe modules has also been confirmed. Its ability to be
doped both n and p-type and the possibility of a variety of preparation techniques such as vacuum deposition
[4], electrodeposition [5], molecular beam epitaxy [6], metal- organic chemical vapour deposition [7], close
space sublimation [8] and the screen printing [9].
The use of a buffer layer has been found to improve solar cell performance in particular when the CdS thickness
is relatively small. The primary function of a buffer layer in a hetero-junction is to form a junction with the
absorber layer while admitting a maximum amount of light to the junction region and absorber layer [10], no
photo current generation occurs in the window layer. The efficiency gain of the thin film solar cells greatly
depends upon the quality and thickness of the buffer layer. The addition of a high- resistivity buffer layer,
placed between the TCO and the CdS layers, could improve the performance and reproducibility of the
CdS/CdTe devices. Several resistive oxides have been found to be effective buffer layers. These include SnO2,
In2O3, and Zn2SnO4 [11]. Recently, ZnSe has been found that an important material for buffer layer [12] with
large direct band gap (2.7eV) which allows transmission of higher energy photons than the band gap of CdS
(2.4eV). Also it has a better lattice match with CuInGaSe (CIGS), thin films absorbers. In view of these, the
performance of ZnSe thin films will be studied for various parameters such as thickness and temperature
dependencies on glass and transparent oxides by cost effective sol-gel spin coating technique.
AIMS AND OBJECTIVES
The main objectives of the present proposal are;

Optimization of the ZnSe thin film with respect to the thickness, concentration.

Optimisation for the uniformity of ZnSe multiple layers on different substrates.

Characterization of the each layer for their electrical, optical, structural and morphological properties
METHODOLOGY
In the proposed work, ZnSe thin film will be developed by spin coating by sol-gel technique. The flow process
regarding the solution preparation, coating and characterization is as shown below.
Zinc Acetate (0.1-0.5M) +
2-Methoxyethanol
Selenic Acid (0.1M) +
Ammonium Hydroxide
10min stirring
Acetic Acid (Catalyst)
12hrs stirring
ZnSe Solution
Aging (12hrs)
Spin Coating (3krpm
for 60sec)
Characterizatio
ns
XRD, SEM, UV-VIS
SPECTROPHOTOMETER
Samples will be characterized for structural studies using X-Ray Diffractometer (XRD). From the XRD results,
studies will be made on their nature and formation of the film such as amorphous and crystalline. Morphological
studies will be carried out using SEM (Scanning Electron Microscopy) or AFM (Atomic Force Microscopy)
images. Structural and morphological studies will be carried to understand the films for their uniformity,
continuity, grain size, etc. The electrical properties will be studied using 4 probe technique. Optical
characteristics of the prepared films will be studied using double source UV-VIS-NIR spectrophotometer. The
optical properties such as absorption, transmittance, reflectance, band gap energy will be studied which is useful
for device applications. The thickness and refractive index can be determined by both envelope technique using
reflection spectrum and by using ellipsometer.
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