Study of Small-Molecule Thin Organic Films
Deposited on Porous Silicon Substrates
Zbigniew Łukasiak*, Andrzej Korcala, Przemysław Płóciennik, Anna Zawadzka
Institute of Physics, Faculty of Physics,Astronomy and Informatics, Nicolaus Copernicus University,
Grudziądzka 5, 87-100 Toruń, Poland
Tel: (4856) 611 3243, e-mail: lukasiak@fizyka.umk.pl
ABSTRACT
In this work time-resolved photoluminescence spectra (PL-TRS) under pulsed excitation of fresh porous silicon
(PSi) and with thin copper phthalocyanine (CuPc) deposited on the top of such structure are presented. The PSi
substrate have been by electrochemical etching method produced. The CuPc coating have been using physical
vapor deposition (PVD) in high vacuum prepared. The PL-TRS of such structures were at room temperature at
vacuum in microsecond delay range registered. We observed that PSi/CuPc samples exhibit strong
photoluminescence in visible region. The registered PL-TRS spectra exhibit multiband structure. Comparison of
PL-TRS for fresh PSi and PSi/CuPc structure shows that organic material can penetrate into PSi skeleton which
leads to interaction between carriers in PSi and organic material deposited on the top of structure.
Keywords: porous silicon, PSi, copper phthalocyanine, CuPc, small-molecule organic thin films, time-resolved
photoluminescence spectra, PL-TRS.
1. INTRODUCTION
The porous silicon (PSi) has focused attention of researchers because of its very efficient photoluminescence
(PL) in visible region of spectrum at room temperature. First observations has promoted PSi to be a candidate for
base of silicon light emitting diodes and silicon colour displays. Today no fast PSi LED has been produced but
photo- and electro- luminescent properties of PSi make it attractive as a highly sensitive and rapid gas sensor or
safe biomedical tag entirely neutral for human body.
Today the organic technology seems to be future of cheap LEDs, displays of computers, cell phones and other
widely used devices. The new materials for opto-electronic organic-based devices: organic light-emitting diodes
(OLEDs), organic photovoltaic devices (OPVs) and organic field effect transistors (OFETs) are intensively
studied. The metallophthalocyanines and other small-molecule organic compounds are very important here
beacuse they usually exhibit better thermal stabilty and durability than polymers. Moreover small-molecule
based thin films prepared by physical vapor deposition (PVD) may form regular quasi-cryslalline structures.
The small-molecule films deposited on PSi substrate can penetrate into PSi skeleton and change of its electrooptlical properties. That phenomena open door for two ways: tuning of PSi based devices and integration of
organic and silicon technology.
2. EXPERIMENTAL
The porous silicon films were prepared by electrochemical anodisation of p type epitaxial layer deposited on low
resistivity p type substrate (0.01 cm). This method in comparison with other ones like spark erosion or laser
ablation connected with an epitaxial structure as a starting material assures good uniformity of prepared layers
and reproducibility of its parameters. The anodisation was carried out in the electrolytic cell (designed for the
large wafer) with backside compartment filled with NaCl solution, to get a good electrical contact with the
wafer. The wafer was at the wall of the cell mounted and sealed by means of o'rings. A coiled Pt wire was as the
cathode used. Such a construction assures almost uniform current density distribution over whole large wafer. As
an electrolyte the HF acid (40% in water) diluted in alcohol (ethanol mainly) was used. The HF concentration
was 10 wt. Anodic current was by a constant current source to maintain accurate anodic charge supplied. Anodic
current density was 25 mA/cm2. Organic layers was by physical vapor deposition in high vacuum on the top of
fresh PSi deposited and at the 475K durig 2 hours annealed. The source material used for high vacuum
deposition was supplied by Sigma Aldrich Co. and used as received. Preparation methods of PSi [1] and organic
films [2] were discussed elsewhere.
Photoluminescence experiments were carried out under pulse excitation. As a source home-made pulse
nitrogen laser was used ( = 337.1 nm, FWHM = 5 ns, power in pulse 20 kW, 10 Hz repetition rate). The proper
band of emission was by a monochromator (SPM-2 ZEISS) selected. Signal was registered using photomultiplier
(HAMAMATSU R928) and analog boxcar (162/164 PAR). Our measurements were limited by the time constant
of the sampling head of boxcar (τSH= 0.5μs). Samples were in vacuum chamber enclosed and room temperature
was maintained. The signals were by personal computer processed and analysed. Parameters of presented
multigaussian decompositions were recovered by an iterative fitting procedure using a nonlinear least squares
search based on the Marquardt routine. Details of PL set-up and its units were discussed elsewhere [3].
3. RESULTS
Figure 1. shows example of PL-TRS spectra for PSi layer (left part) and thin film of CuPc deposited on PSi
substrate (right part) in 0-150 microsecond range for selected delays after laser pulse registered in vacuum at
room temperature. The results have been corrected using aparatus sensitivity curve. All measured PL-TRS
exhibit multiband shape. For all our samples PL observed at room temperature completly decays after 200
microsecond after pulsed excitation. That behaviour is typical for PSi based structures [3].
Figure 1. PL-TRS spectra of as prepared PSi (left part) and thin film CuPc deposited on PSi (right part).
PL-TRS was registered for different delays 0, 5μs, 10μs, 20μs, 40μs, 60μs, 80μs, 100μs and 150μs from top to
down, respectively.
The explanation of multiband shape of PL-TRS spectra results from structure of PSi and nature of radiative
charge recombination inside PSi. For PSi samples prepared by electrochemical anodisation is impossible to
avoid inhomogeneity of Si skeleton. There always exist distribution of pores, nanostructure sizes and
orientations, affecting registered PL emission spectrum, but most of researchers agree that similarly to other Sibased nanoparticles, luminescent PSi samples are usually composed of few “groups” of nanostructures with
given characteristic sizes, dependent on properties of initial material and etching conditions. This become more
clear when we consider that decreasing of diameter of Si nanowire or nanoparticle during electro-anodisation
may be realised only step by step, by removing one monolayer of Si atoms and nearly discrete change of
dimension is equal to lattice constant of crystalline silicon. Based of these considerations, the multiband
structure of presented PL-TRS spectra, which is representative for our PSi samples, can be as light assisted
transitions inside PSi skeleton consist of few groups of nanoparticles and nanowires easy explained.
Both collections of PL-TRS are very similar and it is clear, that PL comes from PSi structure, but CuPc coating
modifies shape of spectra. To investigate of difference all PL-TRS have been converted to energy scale and
decomposed as a sum of few Gaussians (1):
 1  E  E 2 
0i
 
I(E)    i exp  
2

 
i 1
i
 
N
(1)
where I(E) - fit curve, E - energy, i - number of component, N - total number of components which depends on
PSi formation conditions, E0i - centre, αi - amplitude, σi - width. We found that four peaks give best
multigaussian fit for all spectra presented here. Moreover positions of individual peaks on energy scale is
constant in time. That behaviour is obvious because emission represented by given peak originates from
radiative recombination inside specific part of PSi structure. More interesting observation is that positions of
peaks on energy scale are exactly the same for PSi substrate coated with thin CuPc layer. Only relative
amplitudes and widths of peaks are different. The observation leads to conclusion that origin of PL is the same
in both cases (radiative recombination in PSi structure), and recombination processes are affected by organic
material (CuPc) which penetrates pores. The exaple of multigaussian fit for fresh PSi and the same PSi used as
substrate for CuPc for are shown on figure 2.
Figure 2. The exaple of results of multigaussian fit for PL-TRS spectra of as prepared PSi (left part) and thin
film CuPc deposited on PSi (fight part). As example the PL-TRS for zero delay from fig. 1 converted to energy
scale and normalysed have been used.
For PSi and PSi/CuPc samples studied here 4 gaussian peaks have been found marked A, B, C, D (Fig. 2),
respectively. Parameters of fit for zero delay (peak position on energy scale, amplitude and width) are in Table 1.
Table 1. Parameters of gaussians for PSi and PSi/CuPc PL spectrum.
Peak A
E01
[eV]
PSi
Peak B
α1
[a.u.]
σ1
[eV]
2.83
0.12
1.55
PSi/CuPc
E02
[eV]
Peak C
α2
[a.u.]
σ2
[eV]
3.61
0.12
1.68
2.1
0.12
E03
[eV]
Peak D
α3
[a.u.]
σ3
[eV]
5.82
0.16
1.81
3.83
0.16
E04
[eV]
α4
[a.u.]
σ4
[eV]
19.5
0,30
21.8
0.37
1.96
6.88
0.26
The results of fit shows that spectrum of PSi coated with CuPc is shifted to red part in comparison to clean PSi.
Bands of emission represented by peaks B and C are higher and wider and mainly influences on shape of PL
spectrum. It can be explained by influence of CuPc on generation and recombination od charges inside PSi
skeleton. Based on widely accepted quantum confinement model [1, 3, 4] of PL of PSi, radiative recombination
occurs inside nanostructures in PSi skeleton. The constant posiotions of peaks for both types of samples studied
and observation of changes of its amplitudes and widths leads to conclusion that CuPc molecules penetrate deep
to PSi skeleton and coated it not only on the top, but inside the PSi layer, too. This is important because of using
PSi as a material for production of tuneable gas or environmental detectors [4, 5].
For more detailed study of PL bands temporal behaviour of peaks have been analysed. The aplitudes were
plotted as a function of time (Fig. 3.). For peaks A and D (“left” and “roght” on energy scale) no changes have
been found. For peak C change was very slight. The strongest change is for peak B. The linear dependence on
Fig. 3. means that real decay is exponential because of log vertical scale. For PSi sample peak B decays
nonexponentially and more than one exponential component is observed (fast at the begining of decay and slow
after 50 microseconds after excitation). The situation for respective peak B for PSi/CuPc sample looks different
because the fast decay component disappears. The decay shape for that peak looks nonexponential too, but mean
decay time is clearly longer.
Figure 3. The evolution in the time of amplitudes of selected (B and C) peaks from Fig. 2 for PSi and PSi/CuPc.
The plots of temporal behaviour of PL peaks of PSi coated with CuPc shows that coating influences mean decay
tumes, but to study of deacay shapes further measurements of PL decay curves in microsecond range as a
function of wavelength of observation is needed. The PL measurements at low (down to 10K) temperatures is
good idea for further study, too.
4. CONCLUSIONS
The photoluminescence spectra under pulsed excitation at room temperature of electrochemically etched porous
silicon with thin CuPc organic films on the top of structure was studied. The time-resolved PL spectra
in 0-150 ms delay range was registered. PL spectra of PSi/CuPc exhibit the same multiband structure like for
fresh prepared PSi. The recovered gaussians have the same positions on energy scale but different relative
amplitudes and widths. The temporal behaviour of peaks shows that not all peaks are affected by existence of
organic coating. The results shows that origin of PL is the same for fresh and organic coated samples. The
organic material penetrates deep into PSi skeleton and influences generation and recombination processes of
carriers in PSi structure. The broadening of the spectrum in red range due to coating is obsrved. The results are
interesting for aplication of PSi as a part of hybrid PSi-organic photovoltaic devices or selective gas detectors
(“electrical noses”) [5].
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