The traditional development of photovoltaics has been based on crystalline-silicon
wafer technology. In the early 1970’s, however, a new approach arises based
on the possibility to grow silicon in the form of a thin film onto a given
substrate. Several techniques are used for such a deposition, among which
plasma-enhanced chemical vapour deposition (PECVD) is clearly outstanding
given its widespread use and success. More recently, very-high-frequency
(VHP PECVD) and hot-wire CVD have appeared as very promising and fast developing
alternatives with important potential and actual advantages. Thin-film
technology introduces completely novel concepts and challenges in silicon
photovoltaics. Low-temperature processes particularly adequate for large-area
devices open up not only very important cost-reduction potential, but also
new possibilities such as making semi-transparent or flexible modules.
Additional important features are a highly automated production system,
an enormous potential for building integration, a good performance at realistic
working temperatures (around 40oC) and an excellent durability
in outdoor conditions among others. Photovoltaics are facing important
challenges for the near future. Silicon-wafer technology is evolving towards
making thinner, cheaper, multicrystalline silicon. Thin-film-silicon researchers
are in turn striving to make thicker, better, more crystalline films. Both
ways seem to converge to new-generation photovoltaics in which wafer and
thin-film technologies may be
used in a synergistic rather than competing manner. Silicon heterojunction
cells (made up of a crystalline silicon absorber onto which one or more
thin-film silicon layers are deposited), such as the well-known HIT cell,
are in the forefront of photovoltaics and may represent a breakthrough
in the next few years.
Keywords: photovoltaics, silicon solar cell, thin-film technology.
The paper describes the status of photovoltaics (PV) in the World. The recent development of PV market and factors influencing its rapid growth are described. The prospects for further growth are analysed. The amount of installed power, production, and current efficiencies in various technologies are presented. The current status and prospects of bulk and thin-film technologies are described. High efficiency cells and new concepts of solar cells are presented.
Keywords: solar energy, photovoltaics, solar cells.
Solar energy can be used by man in a planned direct way. There are two
fundamental methods of solar energy conversion:
photothermal and photoelectric. The paper is dealing with the analysis
of photothermal conversion in the high latitude countries.
Different types of solar active and passive systems are considered.
The more reliable the solar system and the higher the
temperature requirements, the required technology is more modern and
sophisticated, and the construction of the system, together with the automatic
control, is more complicated. The degree of complexity depends on the function
of the system and
required mode of its operation. In the case of solar space heating
in high latitude countries, it is evident that thermal performance of solar
source is not coherent with the heat demand. Therefore the use of solar
energy requires a special upgrading, e.g. in a form of a heat pump or seasonal
storage. However, the best idea for efficient use of solar energy is to
design a building in a proper way, including the passive systems. The architecture
is crucial for the energy balance of a building and it should assure proper
use of environment, improving solar energy gains in winter and protecting
inside of the building against solar overheating in summer.
Keywords: photothermal conversion of solar radiation, solar active heating
systems, heat pumps, solar passive systems,
solar architecture, availability of solar radiation, isorads.
The review gives the introductory information on metastable light-induced
degradation of amorphous silicon called the
Staebler-Wronski effect (SWE). The paper highlights some recent developments
in our understanding of the effect as well as
some successes in reducing the SWE. The latest results of the author’s
experiments on the metastable effects and the degradation in thin film
transistors and solar cells are also presented.
Keywords: hydrogenated amorphous and microcrystalline silicon (a-Si:H),
Staebler-Wronski effect (SWE), degradation
of solar cells.
Measurements of recombination parameters of both photoconductive materials
and structures (solar cells) have been described. For materials, the methods
are based on both steady-state photoconductivity (or quasi steady-state
photoconductivity) and photocurrent decay PCD experiments. Examples of
PCD measurements taken from literature for c-Si
wafers and our own experiments for amorphous a-Si:H and a-SiC:H samples
have been discussed. Investigation of solar
cells based on the most popular photovoltage decay technique is widely
described. Measurement and interpretation details
have been discussed. Theoretical description and experimental evidence
is, however, focused on combined photovoltage and
photocurrent decays technique, developed in the authors’ laboratory.
This technique enables us determination of both minority
carrier lifetime and surface recombination velocity of photocarriers.
The measurement setup enabling determination of both open circuit voltage
and short circuit current decay times has been described.
Keywords: photoconductivity decay, photovoltage decay time, photocurrent
decay time, minority carrier lifetime, surface
recombination velocity.
Amorphous hydrogenated silicon nitride (a-SiNx:H) films were prepared
by a plasma–enhanced chemical vapour deposition
in a conventional direct plasma reactor operating at 13.56 MHz using
a mixture of the silane (SiH4) and ammonia (NH3).
The reflectance of SiNx films deposited onto Cz-Si polished wafers
substrates was measured in the range of 300–1200 nm.
The wavelength dependence of the refractive index n and the extinction
coefficient k was determined by fitting a Cauchy
model to the experimental reflectance. The influence of the flow NH3/SiH4
ratio on the optical constant n and k of SiNx films
is presented. An optimisation of the antireflection coating on the
flat and texturised substrate for encapsulated and
non-encapsulated solar cells was performed using the SUNRAYS program.
Keywords: silicon nitride, silicon solar cell, antireflection coating.
The feasibility of improvement in multicrystalline silicon (mc-Si) solar
cells is considered by applying porous silicon (PSi)
layers obtained and modified with chemical double-step method. PSi
layers have a different diameter of pores which determines electrical and
optical characteristics of solar cells. Structural properties of these
layers were investigated by means of scanning electron microscopy. The
reflectance, internal quantum efficiency, and current-voltage characteristics
of the silicon
solar cells with porous layers are reported. The PC1D computer program
was applied to calculate the correlation between
texture slope angle and electrical parameters of the solar cell. The
diffusion results are also demonstrated using POCl3 as a
donors source in an emitter with a sheet resistance in the range 30–80
W) which depends on a structure of the porous
layer
prepared before a diffusion process. Some technological problems using
PSi in a solar cell structure and correlation between
porous layer morphology and opto-electrical parameters are discussed.
As a result, the mc-Si solar cell with 12.74% efficiency
was obtained. We think that macroporous silicon formation process can
be applied in industry technology that allows
for exceeding efficiency limit of the mc-Si solar cells with TiOx,
at the level of 13% at present.
Keywords: silicon solar cell, porous silicon.
Undoped and Al-doped ZnO films were prepared by a high pressure dc magnetron
sputtering technique on glass substrates at
a temperature of 470 K. Plasma-emission monitoring was used to stabilize
oxygen flow to the deposition chamber. The effect
of the total pressure during deposition and doping level on the structural,
electrical and optical properties of the films was investigated. No preferred
orientation was observed in the case of films prepared at low pressures.
ZnO films deposited at a total pressures above 5 Pa and with a doping level
up to 3 at.% Al had highly oriented crystallites with a c-axis normal to
the
substrate. These films with the electrical resistivity of 2×10–3
cm and the rms roughness of about 20 nm exhibited an average
transmission of 81% over the visible range. At higher doping levels
the preferred orientation gradually disappeared, the
carrier concentration increased by a small amount, but the Hall mobility
decreased drastically because of the ionised impurity
scattering and the lack of oriented growth.
Keywords: Al-doped zinc oxide, reactive magnetron sputtering, transparent electrodes.
TiO2:Au nanocermets and TiO2 thin films covered
with gold precipitates were obtained from RF reactive sputtering of mosaic
Au:Ti or metallic Au targets. The influence of gold content, deposition
temperature and post-deposition annealing on
photoanodic behaviour and the structure of the obtained materials was
examined by optical, structural and electrochemical
measurements.
Keywords: nanocermets, titanium dioxide, water photolysis.
This paper presents high performance setup developed at the SolarLab
to measure current-voltage (I–V) curves of solar cells.
The core of a setup is a steady light solar simulator of class A, according
to specifications of IEC 60904-9 and ASTM E927
standards. Available range of measurements enables us to characterise
not only all kinds of silicon wafer-based solar cells
but also thin film cells and minimodules. A lot of effort has been
done to make a setup as a powerful tool for advanced research
work. For that purpose, such options have been implemented as various
algorithms for I–V curve translation to external
conditions other than those recorded during measurement or several
techniques to determine lumped series resistance of
solar cells (in both cases procedures recommended by IEC 60891 standard
have been included). Advanced numerical fitting
algorithms allow to extract from I–V curves the parameters corresponding
to either of three commonly used equivalent diode
models of a solar cell. Using an independent microprocessor unit, the
temperature of the measuring table may be controlled in the range 0–60
oC due to a system of four Peltier cells attached to its rear
side. This allows for routine determination of thermal coefficients of
basic cell parameters. The paper discusses also some of elemental random
and nonrandom error sources that can be encountered during the standard
I–V measurements of a solar cell. The test results of repeatability of
measurements, problems related to probe configuration, and heating up of
the cells during “light” measurements are presented showing that the developed
system can be successfully used both for laboratory work and as a tester
on a production line. The system meets all requirements of the IEC 60904-1,
IEC 60904-3, and IEC 60904-9 standards.
Keywords: I–V curve, solar cell calibration, solar cell characterisation.
The paper presents the results of the outdoor performance tests made
on several types of commercial PV modules. These were
both single crystal and multicrystalline Si modules as well as thin-film
CIS and two amorphous Si devices – single junction,
marked as SJ, and triple junction, marked as TJ, respectively. Special
emphasis has been put on the effects related to actual
solar spectrum and meteorological parameters like air humidity and
ambient temperature that may influence it. Besides short
term changes that could be observed during a single day, also the results
collected for more than one year of monitoring are
presented. Additionally, the results of some theoretical calculations
supplementing the experimental data are given. Performance
of thin-film CIS module and both a-Si modules after well visible period
of degradation seems to be more affected by
weather and climatic conditions than the modules made of crystalline
Si cells.
Keywords: photovoltaics, PV modules, solar cell characterisation.
The paper presents the advanced measurement system being developed for
the 1 kWp photovoltaic generator installed at the
Technical University of Lodz, at the Department of Microelectronics
and Computer Science, in the city centre. The main aim
of research is verification of feasibility of photovoltaic generator
in polluted environment and influence of I–V characteristics
mismatch on long-term performance. The photovoltaic system is equipped
with comprehensive monitoring system which collects climatic data and I–V
characteristics of all PV modules under operation of PV generator. The
monitoring system is based on the custom-made microprocessor board, designed
with view of reliability of the data collection. The key module of the
system is advanced I–V characteristics scanner with a switching matrix.
Keywords: PV measurements, I–V characteristics, performance evaluation.
Starting from the general equivalent model of a solar cell including
junction capacitance and shunt resistance connected with surface recombination
of photocarriers, the phenomenon of open circuit voltage decay was analysed.
The time decay of a photovoltage is influenced by the junction capacitance
time constant to a great extent. The time constants of both depletion
layer and diffusion capacitances varying with applied bias voltage
were examined. The detailed analysis of these time constants allowed for
evaluation of the forward bias voltage that should be applied to c-Si solar
cell to minimize this detrimental effect. As it was shown, that voltage
bias can be easily generated by the constant illumination, the so-called
bias light. Impedance measurements of solar cells in the frequency range
10 Hz–100 kHz and with varying voltage bias allowed for additional characterisation
of the junction capacitance and resistance. The measured parameters agree
well with these used in model calculation of a photovoltage decay.
Keywords: junction capacitance, open circuit voltage decay, solar cell.
The paper deals with a photo-commutation effect that can be described
most easily by means of elementary photoelectric
motor construction. In the paper, characteristics of the elementary
power photocell and characteristics of a short-circuited
pair of photo-commutator photocells are presented. The elementary photo-commutation
cycle has been analysed as well as
theoretical quantitative relations are given. Analysis of properties
of solar energy powered motor has been performed basing
on relations between light intensity and functional parameters of the
motor. The aim of the research was to examine an effect
of illumination intensity on photoelectric commutator parameters, motor
start-up conditions, and rotor rotational speed.
Measurements of static and dynamic characteristics of photoelectric
motor were performed using specially designed test
setup. In the measurement system, a PC has been employed using LabView
software, playing a role of the measurement process-controlling device
and the means of data acquisition and processing.
Keywords: photoelectric motor, photoelectric commutator, photovoltaic cells, contactless commutator.
The paper describes performance of the grid-connected 1-kW PV system
installed on a grammar school in Warsaw after two
years of operation. The system consists of twenty Millennia MST-50
MV modules and inverter Sunny Boy GCI 1200. The performance of the system
is continuously monitored according to guidelines in IEC 61724. Energy
production in the first year was about 740 kWh and it was slightly higher
than expected with respect to the simulation done before installation.
In the second year, the energy production was about 680 kWh. The measured
efficiency of the PV modules is about 5%, efficiency of the inverter is
about 92% and efficiency of the entire PV system exceeds 4%. The performance
ratio is in the range from 0.5 to 0.8.
Keywords: photovoltaics, PV system, grid-connected, monitoring.
A family of silicon avalanche photodiodes with an n+-p-p-p+
epiplanar structure was developed at the Institute of Electron
Technology (ITE). The diameters of photosensitive area range from 0.3
mm to 5 mm. These photodiodes are optimised for detection of 800–850 nm
radiation and in that range achieve excellent parameters – high gain, low
noise, high detectivity. The
detailed research on their spectral dependencies of the gain and noise
parameters has revealed that their range of operating
is considerable wider and stretches from 550 to 1000 nm. The principles
of operation and design considerations concerning
avalanche photodiodes are outlined in this paper. Next, the design,
technology and properties of silicon avalanche
photodiodes developed at the ITE are discussed. Avalanche photodiodes
are widely used in detection of very weak and very
fast optical signals. Presently in the world, the studies are carried
out on applying the avalanche photodiodes in detection of
X radiation and in the scintillation detection of nuclear radiation.
Keywords: silicon avalanche photodiode, silicon photodiode, photodetector.
The emergence of uncooled detectors has opened new opportunities for IR detection for both military and commercial applications. Development of such devices involves a lot of trade-offs between the different parameters that define the technological stack. These trade-offs explain the number of different architectures that are under worldwide development. The key factor is to find a high sensitivity and low noise thermometer materia³ compatible with silicon technology in order to achieve high thermal isolation in the smallest area as possible. Ferroelectric thermometer based on hybrid technology and electrical resistive thermometer based (microbolometer) technology are under development. However, ferroelectric material suffers from the difficulty to achieve a high figure of merit from thin film that is needed for monolithic structure development. Besides, the microbolometer technology, well adapted for thin film process, leads to higher performance at the expense of more complex readout integrated circuit design. LETI and ULIS have been chosen from the very beginning to develop first, a monolithic microbolometer technology fully compatible with commercially available CMOS technology and secondly, amorphous silicon based thermometer. This silicon approach has the greatest potential for reducing infrared detector manufacturing cost. After the development of the technology, the transfer to industrial facilities has been performed in a short period of time and the production is now ramping up with ULIS team in new facilities. LETI and ULIS are now working to facilitate the IRFPA integration into equipment in order to address a very large market. Achievement of this goal needs the development of smart sensors with on-chip advanced functions and the decrease in manufacturing cost of IRFPA by decreasing the pixel pitch and simplifying the vacuum package. We present in this paper the new designs for readout circuit and packages that will be used for 384×288 and 160×120 arrays with a pitch of 35 µm and advanced results on 35 µm pixel pitch arrays. Thermographic application needs high stable infrared detector with a precise determination of the amount of absorbed infrared flux. Hence, infrared detector with internal temperature stabilized shield has been developed and characterised. The results will be presented.
Keywords: amorphous silicon, microbolometr, focal plane arrays, NETD.
The ultimate performance of long wavelength infrared photodetectors
operating at high temperatures is likely to be limited
by the noise due to the statistical nature of thermal generation of
charge carriers in narrow band gap semiconductors. Additional
obstacles to achieve theoretical performance in practical devices arise
from weak absorption of infrared radiation, short diffusion length of charge
carriers in narrow gap semiconductors and other reasons. Various ways to
improve performance of uncooled photodetector such as the reduction of
thermal generation rate by proper selection of the semiconductor material,
its doping and suppression of thermal generation by the non-equilibrium
mode of operation are considered. Another possibility is the reduction
of physical volume of a detector. This can be done by reducing a detector
physical area and its thickness with appropriate means to preserve the
device field of view and a quantum efficiency. The advanced architectures
of uncooled Hg1-xCdxTe IR photoconductors, photoelectromagnetic and photovoltaic
detector are described. The devices require heterostructures with complex
band gap and doping profiles and can be grown by the low temperature epitaxial
techniques. The most promising device for uncooled detection is heterojunction
photodiode integrated with optical concentrator. The progress in technology
of photodetectors will eventually lead to perfect and fast detection of
long wavelength radiation without cooling.
Keywords: infrared photodetectors, uncooled IR photodetectors, MOCVD growth.
This paper presents an overview of fundamental techniques for planar
junction formation in HgCdTe infrared detectors. At
the beginning, the evolution of HgCdTe photodiode performance is presented.
Further considerations are restricted to modern
methods of p-n junction formation, so the current state of the art
of different types of HgCdTe photodiodes is presented.
The comparison of theoretical and experimental results for planar HgCdTe
photodiodes is finally described.
Keywords: planar photodiodes, heterojunction, HgCdTe.
A review of readouts electronics for optical detectors is presented. General requirements for scientific infrared focal plane arrays readout are discussed. Specific approaches to the unit cell electronics are described with respect to operation, complexity, noise, and other operating parameters.
Keywords: low noise electronics, photoreceivers, focal plane arrays.
InGaAs is a variable band gap semiconductor with excellent transport
and optical properties. This makes it attractive for
electronic and optoelectronic devices. One of the most important applications
is short wavelengh (1–3.6 µm) infrared
photodetectors. Such devices are based on multilayer heterostructures
with complex band gap and doping profiles. Significant
progress in technology of the InGaAs heterostructures has been achieved
with MBE and MOCVD growth. We discuss here the status and perspectives
of infrared photodetectors based on advanced InGaAs heterostructures.
Keywords: InGaAs photodiodes, optical immersion, resonant cavity, Schottky photodiodes, avalanche photodiodes.
The results of the work aiming at development of a RCE photodiode operating
at 1.9 µm are described. Detection is based on
interband absorption in a thin pseudomorphic InGaAs layer placed inside
a resonant cavity which enhances an optical field.
The technology of heterostructures grown by MOCVD has been developed.
The photodiode structure comprises, between two parallel Bragg mirrors,
an InP p-i-n junction with a thin strained InxGa1–xAs (0.65 x 0.8)
layer placed inside an undoped region. The bottom Bragg mirror is composed
of an In0.53Ga0.47As/InP quarter-wave layer stack, the top mirror is made
of Si/SiO2 layers deposited on epitaxial layers by a sputtering method.
Good properties of InxGa1–xAs strained layers and good reflectivity spectra
of the Bragg mirrors enable us to obtain RCE photodetectors with photoresponse
characteristics at wavelengths near 1.9 µm. Photodetectors exhibit
very low dark current densities (of the order of 10–6 A/cm2).
Keywords: RCE photodetector, InGaAs photodiode.
We present an analysis and optimisation of optical resonant cavity enhanced
(ORC) long wavelength Hg1-xCdxTe photovoltaic detectors operating at near
room temperatures. The resonant cavity is based on Hg1-xCdxTe Bragg reflector
and detector heterostructure grown on a GaAs substrate, supplied with a
metal back reflector. Such devices are compatible with
low-temperature epitaxial (MBE and MOCVD) techniques. The optical gain
due to ORC enhancement was calculated for
the ambient temperature and the temperatures achievable with two-stage
Peltier cooled 10.6 µm detectors as a function of
the device design and an incidence angle.
Keywords: infrared detectors, optical resonant cavity, HgCdTe.
The paper presents investigations of optoelectric and thermal properties
of ternary and quaternary II–VI mixed crystals
Zn0.7Mg0.3Se, Zn0.88Be0.12Se,
Zn0.83Mg0.13Be0.04Se, Zn0.86Mn0.08Be0.06Se
grown from the melt. The photoluminescence, photoconductivity and photoacoustic
amplitude and phase spectra were measured. The energy gap and thermal diffusivity
were estimated from photoacoustic spectra. Ionisation energies of defects
responsible for observed photoconductivity were determined.
Keywords: photoluminescence, photoconductivity, photoacoustic.
A design of the structure of an integrated GaN photoreceiver for the
UV range is presented. The circuit includes an MSM
photodetector and an HFET based amplifier. Preliminary data for the
design were obtained from the measurements performed
on the test structures – CTLM, van der Pauw, Schottky contacts and
also performed on discrete devices – MSM detectors,
HFET transistors – all fabricated on AlGaN/GaN/sapphire substrates.
Frequency response characteristics of the HFET
based amplifier and also of the complete MSM-HFET photoreceiver were
computer-simulated. One of the most important
steps in device monolithic integration is isolation of the elements.
In this project were considered two types of possible elements
isolation: plasma etch (mesa) and implant isolation. A set of photolithography
masks for chip fabrication on AlGaN/GaN layers was created. Anticipated
fabrication of the designed MSM-HFET integrated photoreceiver in 1-µm
geometry should allow its RF operation.
Keywords: integrated photoreceiver, MSM, HFET, GaN.