Proceedings of the European Summer School of Photovoltaics 4 – 7 ...

Multifunctional coatings for solar cells application
Karolina Sieradzka, Michał Mazur, Danuta Kaczmarek, Jarosław Domaradzki,
Damian Wojcieszak
Wroclaw University of Technology, Faculty of Microsystem Electronics and Photonics
In recent years, an increased interest of nanocrystalline oxide materials
was been observed [1, 2]. Such oxides join a few selected,
well-defined properties such as high transparency, good electrical
conductivity, hydrophilic or hydrophobic properties, antireflective
properties, etc. [3, 4]. In contrary to conventional semiconductors,
such thin oxide films, prepared for transparent electronics
or solar cells application, combine mainly two specific features
[5]: high transparency in visible light and the ability of electrical
conduction at room temperature. These oxides, depending on the
level of electrical resistivity are divided into two groups of materials:
Transparent Conducting Oxide (TCO) or Transparent Oxide
Semiconductor. However, in the literature, there is increasing
number of reports about trying to get the additional properties,
but most showed examples of multilayers. For example, it might
be a thin oxide films with additional antireflective properties, prepared
as TiO 2
/SiO 2
multilayers [4, 6] in order to obtain the largest
possible reduction of light reflectance. Pemble et al. described
dual functionality self-cleaning thermochromic films prepared by
APCVD method. The multilayers based on VO 2
and TiO 2
revealed
good degradation of stearic acid under UV radiation and thermochromic
properties with the switching temperature of 55 o C.
In the first part of this paper, the possibilities of characterization
of nanostructures used in the Laboratory of Optoelectrical
Diagnostics of Nanomaterials located at Wroclaw Univeristy of
Technology have been showed. Then antireflective, photoactive
nanocrystalline Ti-V oxide as a multifunctional thin film has been
presented.
Directions of current research
The scope of current work carried out in the Laboratory involves
the research for new, nanocrystalline oxide materials with unique
electrical and optical properties, which might be applied as multifunctional
coatings for example solar cells, glasses for special
application or optoelectronic elements.
The subject of interest of staff from our Laboratory are closely
related to preparation and analysis of mainly the optical, electrical,
hydrophilic and hydrophobic and antistatic properties of thin oxide
films. A modified magnetron sputtering method [7], developed
and patented by the staff from our Division enabled to achieve
transparent and semiconducting oxide based on TiO 2
doped with
different metal ions with n- or p-type of electrical conduction.
Review of measurements apparatus for thin
oxide films characterization used in Laboratory
Optical properties are analysed based on spectrophotometry
measurements. Transmittance and reflectance spectra are studied
in the spectral range from 200 nm to 2600 nm at normal
incident. The spectra are recorded with the use of Ocean Optics
spectrophotometers (QE65000 and NIR-256.2.1) and coupled
DH-2000-BAL deuterium-halogen or XE-450 xenon lamps as an
excitation light source. Additionally, the optical measurements
performed with the aid of Janis ST-100H cryostat in a varied temperature
conditions (from ~2K up to 700K) allow for the analysis
of gasochromic and thermochromic properties.
Electrical properties are examined in the wide temperature range
in order to determine the basic electrical parameters, e.g. resistivity,
type of electrical conductivity. The electrical temperature
dependent measurements are performed in the range from 25 o C
to 300 o C using air-cooled thermal chuck operated by ERS SP72
controller. In case of heterojunction structure the fundamental property
is their current-to-voltage performance. The measurements
102
are performed using Keithley 4200-SCS semiconductor characterization
system equipped with Cascade MicroTech probe station
with thermal chuck heater. The system allows the measurements
from 100 aA up to 100 mA within a voltage source-measure range
from 1 mV up to 210 V. Additonally, measurements performed at
different temperature with and without light illumination provide to
study of their influence on electrical performance of solar cells. A
special software Keithley Interactive Test Environment enables for
the rapid determination of solar cells parameters such as: shortcircuit
current, open circuit voltage, maximum power, maximum
current, maximum voltage, fill factor and cell efficiency.
The analysis of electrical conduction mechanisms are done
using the impedance spectroscopy (IS) method. Capacitance and
conductivity data measured in a wide frequency range from 40 Hz
to 110 MHz are recorded with a help of an Agilent Technologies
4294A precision impedance analyser.
Electrical investigations are completed with stationary and
transient photoelectrical measurements. The measurements performed
with a monochromatic light on the range of 200 nm up to
1400 nm allow to investigate photoelectrical effect at the prepared
heterostructures and to study theirs spectral sensitivity. Investigation
of photoelectrical properties performed by a scanned
light beam allows building two-dimensional maps of photocurrent
distribution at the active area of the junction. These optical beam
induce current (OBIC) maps usually allow to study generation-recombination
centres in junctions, inter-level shorts and a latch-up
mechanism.
The surface wettability of the thin films are evaluated by contact
angle measurement of deionized water under room temperature
and daylight illumination. The measurement are carried out
with a computer controlled Theta Lite goniometer system manufactured
by Attension.
The antistatic properties of thin films are investigated using a
modern measurement station – JCI 155v5 Charge Decay Test
Unit operated in the chamber with controlled humidity and temperature.
The ability for static charge dissipation from samples
surface vs. time were measured.
Results of the measurments
Thin oxide films were prepared mainly based on TiO 2
, which is
in particular non-toxic, transparent to light in wide spectral range
and thermally, chemically, mechanically stable material. The thin
films were deposited in pure oxygen atmosphere by developed,
modified high energy reactive magnetron sputtering (HE RMS)
method. As a result the nanocrystalline oxides with thermodynamically
stable rutile form with average crystallites size under
10 nm (Fig. 1) has been achieved.
Typically, in a conventional magnetron sputtering process, directly
after deposition of TiO 2
thin films, have the anatase phase,
which is less thermodynamically stable. Then, the crystalline rutile
form of TiO 2
is received only when additional thermal heating
at 750 o C has been used. These achievements have contributed
to a number of unique and desirable properties. These films,
in addition to high transparency for light in the visible range, revealed
among others increased hardness. The results of hardness
measurements using nanoindenter (Fig. 2) showed that the nanocrystalline
TiO 2
thin films deposited in HE RMS process has
a high hardness of 14.3 GPa, what is comparable to DLC materials.
While in case of TiO 2
films sputtered in the conventional
process the hardness was 3.5 GPa and 7.9 GPa respectively for
thin films after deposition and after annealing at 800 o C.
Elektronika 6/2012