A Novel Photoconductive PVK/SiO2 Interpenetrated Network Prepared by the Sol-Gel
Process

Gonzalo Ramos,†,‡ Tomás Belenguer,†,§ Eusebio Bernabeu,§ Francisco del Monte,*,‡ and
David Levy†,‡

Instituto Nacional de Tecnologia Aeroespacial-INTA, Laboratorio de Instrumentacion Espacial-LINES,
Torrejón de Ardoz, 28850, Madrid, Spain, Instituto de Ciencia de Materiales de Madrid-ICMM, Consejo
Superior de InVestigaciones Cientı´ficas-CSIC, Cantoblanco, 28049, Madrid, Spain, and Facultad de Ciencias
Fı́sicas, UniVersidad Complutense de Madrid, Ciudad UniVersitaria, 28040, Madrid, Spain

ReceiVed: August 14, 2002

In this work, we describe the preparation of a novel photoconductive sol-gel material based on an organic/
inorganic interpenetrating network (IPN). The composition of the sol-gel photoconductive material mimics
the well-known polymeric one based on poly(N-vinylcarbazole) (PVK) as the charge-transporting matrix and
2,4,7-trinitro-9-fluorenone (TNF) as the sensitizer. The resulting photoconductive material (PVK/SiO2 IPN)
shows a photosensitivity of 10-11 cm/(Ω W) in range to that reported for some analogue polymeric compounds
and the highest ever reported for hybrid sol-gel materials.

Photoconductivity in organic materials consists of two
processes (photogeneration of charge carriers and their subse-
quent transport) and is usually wavelength and electric-field
dependent.1 The organic photoconductive materials have drawn
a great deal of attention during the last three decades, given
their involvement in interesting fields as xerography (both
classical and holographic)2-4 and photorefractivity.5-10 Besides
thermoplastic behavior in holographic xerography applications
or optical nonlinearity in photorefractivity, photoconductivity
is a required property that will determine the optimum work of
the material.

Photoconductivity has been observed in a number of polymers
doped with either organic or inorganic entities which act as
charge-transfer complexes.1,2,6,11,12 Among them, poly(vinyl-
carbazole)/2,4,7-trinitro-9-fluorenone (PVK/TNF) composites
have been widely explored because of the extraordinary proper-
ties of PVK as charge-transporting matrix (hole transporting)
and of TNF as sensitizer.4,6,10,13-17 The photoconductivity in
PVK/TNF composites results from the formation of a charge-
transfer complex between the TNF and the carbazole units of
the PVK.18,19

The sol-gel approach appears as a very promising route for
the preparation of photoconductive materials. The mild synthesis
conditions offered by the sol-gel route (the process begins at
the solution stage and takes place at room temperature) allow
the incorporation of the optically active organic molecules within
the porosity of the resulting matrix. Thus, functional sol-gel
materials can be prepared by mimicking the polymeric ones,
but in the supporting material (silica versus polymer) which
will provide an improvement of the stability besides optimum
optical quality.20,21During the last years, photoconductive sol-
gel materials have been mainly obtained through the incorpora-

tion of disperse carbazole units within the porosity of the silica
matrix, whether or not covalently bonded to the silica network,
e.g., carbazole functionalized silane sol-gel precursor,1,20 or
ethylcarbazole,22 respectively.

This work describes the preparation and characterization of
a novel sol-gel material with photoconductive properties. The
novelty of this approach consists of the formation of a PVK/
SiO2 interpenetrated network (IPN) rather than in the discrete
incorporation of carbazole units dispersed within the silica
matrix. The sol-gel photoconductive materials were prepared
through the acid hydrolysis (pH) 1, 11.0 mmol) of the sol-
gel precursor, glycidoxypropyltrimethoxysilane (GPTMS, 5.5
mmol), under vigorous stirring. On the clear and homogeneous
sol, 0.65 mmol ofN-vinylcarbazole (NVC, from Aldrich), 5×
10-3 mmol of 1,1-dimethoxy-1-phenylacetophenone (IRG-651,
from CYBA), 4 × 10-2 mmol of 2,4,7-trinitro-9-fluorenone
(TNF, from Ultra Scientific) and 0.9 mL of pyridine (from
Aldrich) were added drop by drop under vigorous stirring. The
sol was heated at 75°C for 10 min prior to the addition of 25
µL of 2-(dibutylamino)-ethanol (from Aldrich). The resulting
brownish sol was filtered (0.2µm) prior to the dip coating of
an indium tin oxide glass substrate (ITO). Films with thickness
of 8 µm (Deck Tak III profilometer) were obtained. Samples
were thermal treated at 50°C overnight. The PVK network was
obtained through the photopolymerization (UV illumination
along 2 h with a ULTRA-VITALUX lamp) of the NVC caged
within the porous silica matrix. NVC polymerization was
monitored by the appearance of the peaks at 328 and 342 nm
at the UV-Vis spectrum (Figure 1).23,24 The photoconductive
measurements were carried out using a simple DC photocurrent
technique.5 Counter electrodes were fabricated on the film
surface just by Ag painting (0.02Ω/cm2). Polyimide tape
(Kapton), with thickness of 80µm, was used to avoid the Ag
paint impregnation of the sol-gel matrix. The irradiance of the
excitation source (Ar+ laser) used to measure the photo-
conductivity ranged from 3 to 9 mW/cm2.

Photosensitivity is defined as the photoconductivity (σph,
photocurrent density per unit of applied electric field), normal-

* Corresponding author. Fax:+34 91 3720623. E-mail: delmonte@
icmm.csic.es.

† Instituto Nacional de Tecnologia Aeroespacial-INTA, Laboratorio de
Instrumentacion Espacial-LINES.

‡ Instituto de Ciencia de Materiales de Madrid-ICMM, Consejo Superior
de Investigaciones Cientı´ficas-CSIC.

§ Universidad Complutense de Madrid, Ciudad Universitaria.

110 J. Phys. Chem. B2003,107,110-112

10.1021/jp026759+ CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/10/2002



ized by the illumination irradiance (I), according to eq 1:5,20

The PVK/SiO2 IPN shows a photosensititivity of 1.4× 10-11

cm/(Ω W) for an applied external electric field of around 12
V/µm (Figure 2) which, up to our knowledge, is the highest
ever reported in a hybrid sol-gel material (Table 1). It is
remarkable that theSph value measured for the PVK/SiO2 IPN
is in range to those recently reported for analogue polymers
based on PVK/TNF (see Table 1).

Figure 2 also shows how the use of shorter wavelengths (e.g.,
476 nm) results in an improvement of the photoresponse of the

PVK/SiO2 IPN photoconductive material (up to 30%). This
behavior has been reported as a consequence of the better photon
absorption of the PVK/TNF charge-transfer complex at 476 than
at 514 nm.25 The low level of dark current found in the PVK/
SiO2 IPN photoconductive material (Jdark in the order of 10-13

A/cm2 for no applied field and 10-11 A/cm2 for a moderate
applied field of around 10 V/µm) is also remarkable, since it is
highly desirable for a photoconductor to exhibit no conductive
activity under dark conditions.5,26 In addition, it is noteworthy
the photoresponse observed at non external electric field
(Jph ∼ 10-13 A/cm2, for an irradiance of 3 mW/cm2). This
behavior must be explained in terms of the generation of a
gradient of photoexcited charge-carriers across the sample as a
consequence of the main light absorbance at the incoming
surface of the film.26

The improvement in theSph found for the PVK/SiO2 IPN
photoconductive material as compared to the nonpolymerized
sample (NVC/TNF sol-gel material prior irradiation) was found
to be∼50% (Figure 3). Such an improvement must be related
to the favored transport of the charge-carriers in the continuous
medium formed by the PVK chain. Larger differences inSph

between nonpolymerized and polymerized samples should
therefore be expected for lower NVC concentrations. The
improvement of theSph in the polymerized sample reflects the
IPN character of the PVK/SiO2 material. Besides, the silanol
groups of the porous surface can be considered as impurities
which it is well-known that act as trapping centers for the holes.8

In our case, the existence of the glycidoxypropyl groups provides
a local environment which isolates the PVK network from the
silica backbone and results in the enhancement on the charge
carriers mobility. Furthermore, the PVK/SiO2 IPN photoconduc-
tive material shows higher stability in the photocurrent signal
than the NVC/TNF sol-gel material.

In summary, the PVK/SiO2 IPN photoconductive material
reported in this work has exhibited an excellent low level of
dark current besides a photosensitivity in the range to that
reported for PVK/TNF polymeric samples,25 and better than
photoconductive sol-gel materials based on the incorporation
of disperse carbazole units.20,22 Furthermore, the goodness of
the photoconductivy data measured on the PVK/SiO2 IPN
photoconductive material is promising for the future application
of sol-gel materials in photorefractivity, since it provides an
improvement of the response time.20,22

Acknowledgment. The authors are grateful to the CICYT
for the research grant MAT 2001-5073-E. Gonzalo Ramos is
also grateful to INTA for a Rafael Calvo Rode´s fellowship.

Figure 1. Absorption spectra of the NVC/TNF sol-gel material before
(dash line) and after UV illumination (solid line). UV illumination gives
rise to the formation of the PVK/SiO2 IPN.

Figure 2. Photosensitivity response of the PVK/SiO2 IPN photocon-
ductive material at 514 nm ([), 501 nm (O), 496 nm (2), 488 nm (3),
and 476 nm (9).

TABLE 1: Photosensitivity Values Reported for the PVK/
SiO2 IPN Photoconductive Material and for Some Analogue
Polymeric Compoundsa

Sph
a

cm/(Ω W) type of material reference

1.4× 10-11 PVK/TNF/SiO2 IPN this work
10-12 PVK/CdS-nanocrystal polymer composite 5
10-11 full-functionalized polymer composite 9
10-14 PVK/ECZ/TNF/chromophore polymer composite 14
10-14 carbazole discrete units within a sol-gel matrix 20
10-10 PVK/TNF/chromophore polymer composite 25
10-10 PVK/ECZ/C60/chromophore polymer composite 28

a TheSph value has been calculated through the application of eq 1
in those cases where it was not provided.

Figure 3. Photosensitivity response of the NVC/TNF sol-gel material
(2), and the PVK/ SiO2 IPN photoconductive material (9), at 514 nm.

Sph )
Jph

EI
)

σph

I
(1)

Novel Photoconductive PVK/SiO2 Network J. Phys. Chem. B, Vol. 107, No. 1, 2003111



References and Notes

(1) Zhang, Y.; Burzynski, R.; Ghosal, S.; Casstevens, M. K.AdV. Mater.
1996, 8, 111.

(2) Penwell, R. C.; Ganguly, B. N.; Smith, T. W.J. Polym. Sci.
Macromol. ReV. 1978, 13, 63.

(3) (a) Urbach, J. C.; Meier, R. W.Appl. Opt. 1966, 5, 666. (b)
Hariharan, P.Optical Holography. Principles, Techniques and Applications;
Cambridge University Press: Cambridge, 1984.

(4) Carren˜o, F.; Martı́nez-Antón, J. C.; Bernabeu, E.Thin Solid Films
1995, 263, 206.

(5) (a) Winiarz, J. G.; Zhang, L.; Lal, M.; Friend, C. S.; Prasad, P. N.
Chem. Phys.1999,245, 417. (b) Winiarz, J. G.; Zhang, L.; Lal, M.; Friend,
C. S.; Prasad, P. N.J. Am. Chem. Soc.1999, 121, 5287.

(6) Moerner, W. E.; Silence, S. M.Chem. ReV. 1994, 94, 127.
(7) Bolink, H. J.; Krasnikov, V. V.; Malliaras, G. G.; Hadziioannou,

G. J. Phys. Chem.1996, 100, 16356.
(8) Malliaras, G. G.; Krasnikov, V. V.; Bolink, H. J.; Hadziioannou,

G. Appl. Phys. Lett.1995, 67, 455.
(9) Gubler, U.; He, M.; Wright, D.; Roh, Y.; Twieg, R.; Moerner, W.

E. AdV. Mater. 2002, 14, 313.
(10) West, K. S.; West, D. P.; Rahn, M. D.; Shakos, J. D.; Wade, F. A.;

Khand, K.; King, T. A.J. Appl. Phys.1998, 84, 5893.
(11) Jung, J.; Glowacki, I.; Ulanski, J.J. Chem. Phys. 1999, 110, 7000.
(12) Okamoto, K.; Nomura, T.; Park, S.; Ogino, K.; Sato, H.Chem.

Mater. 1999, 11, 3279.
(13) Winiarz, J. G.; Zhang, L.; Park, J.; Prasad, P. N.J. Phys. Chem. B

2002, 106, 967.
(14) Däubler, T. K.; Bittner, R.; Meerholz, K.; Cimrova´, V.; Neher, D.

Phys. ReV. B 2000, 61, 13515.

(15) Cottin, P.; Lessard, R. A.; Knystautas, EÄ . J.; Roorda, S.Nucl.
Instrum. Methods Phys. Res. B1999, 151, 97.

(16) Bittner, R.; Meerholz, K.; Stepanov, S.Appl. Phys. Lett. 1999, 74,
3723.

(17) Safoula, G.; Bernede, J. C.; Touihri, S.; Alimi, K.Eur. Polym. J.
1998, 34, 1871.

(18) West, K. S.; Rahn, D. P.; Shakos, J. D.; Wade, F. A.; Khand, K.;
King, T. A. J. Appl. Phys.1998, 84, 5893.

(19) Mercher, E.; Bra¨uchle, C.; Ho¨rhold, H. H.; Hummelen, J. C.;
Meerholz, K.Phys. Chem. Chem. Phys. 1999, 1, 1749.

(20) Darracq, B.; Chaput, F.; Lahlil, K.; Boilot, J. P.; Levy, Y.; Alain,
V.; Ventelon, L.; Blanchard-Desce, M.Opt. Mater.1998, 9, 265.

(21) Wang, Y.; Wang, X.; Li, J.; Mo, Z.; Zhao, X.; Jing, X.; Wang, F.
AdV. Mater. 2001, 13, 1582.

(22) Cheben, P.; del Monte, F.; Worsfold, D. J.; Carlsson, D. J.; Grover,
C. P.; Mackenzie, J. D.Nature2000, 408, 64.

(23) Wang, S.; Yang, S.; Yang, C.; Li, Z.; Wang, J.; Ge, W.J. Phys.
Chem. B2000, 104, 11853.

(24) Chen, Y.; Cai, R. F.; Xiao, L. X.; Huang, Z. E.; Pan, D.J. Mater.
Sci. 1998, 33, 4633.

(25) Sandalphon, Kippelen, B.; Peyghambarian, N.; Lyon, S. R.; Padias,
A. B.; Hall, H. K., Jr.Opt. Lett.1994,19, 68.

(26) Saleh, B. E. A.; Teich, M. C.Fundamental of Photonics; Goodman,
J. W., Ed.; John Wiley & Sons: New York, 1991; Chapter 18.

(27) Herlocker, J. A.; Fuentes-Herna´ndez, C.; Ferrio, K. B.; Hendrickx,
E.; Blanche, P. A.; Peyghambarian, N.; Kippelen, B.; Zhang, Y.; Wang, J.
F.; Marder, S. R.Appl. Phys. Lett.2000, 77, 2292.

(28) Hendrickx, E.; Zhang, Y.; Ferrio, K. B.; Herlocker, J. A.; Anderson,
J.; Armstrong, N. R.; Mash, E. A.; Persoons, A. P.; Peyghambarian, N.;
Kippelen, B.J. Mater. Chem.1999, 9, 2251.

112 J. Phys. Chem. B, Vol. 107, No. 1, 2003 Ramos et al.