Radiation Chemistry Department, IRI, Delft University of Technology
Mekelweg 15, 2629 JB, Delft, The Netherlands
For the realisation of integral solid state photovoltaic cells based on organic materials, there is considerable interest in the use of bilayers of a wide band-gap inorganic semiconductor and a p-type conjugated polymer, such as dialkoxypoly (phenylene-vinylene) (PPV). On excitation of such a bilayer with visible light an exciton is created in the polymer layer which can dissociate at the interface, resulting in an electron in the semiconductor and a hole in the polymer as shown in Figure 1. Using such a bilayer in a sandwich cell configuration, the polymer should thus function as both an antenna and as a conducting layer for hole transport to an electrode. Many processes and parameters contribute to the overall efficiency of such a device, including the exciton lifetime and diffusion coefficient, the rates of interfacial charge separation and recombination, the mobilties of charge carriers, and the wavelength dependence of photocarrier generation.
Figure 1
Optoelectronic
processes following excitation of the polymer :
1: photo excitation
2: (non)radiative decay
3: exciton diffusion
4: interfacial charge separation
5: charge recombination
For studying these factors the application of metal electrodes often introduces additional complications, such as non-Ohmic contacts or space charge limited currents which mask the photophysical processes of interest within the bilayer itself. Recently we have developed a technique by which the charge separation within semiconductor/polymer bilayers can be monitored without the necessity of electrodes thus considerable simplifying sample preparation and data interpretation [1,2]. The method depends on the use of microwaves to monitor the change in conductivity on illumination. Figure 2 shows the details of the measurement cell.
Figure 2
Microwave cavity used
for monitoring the photo-induced charge separation within bilayers of an organic and
an
inorganic semiconductor. The sample is mounted at such a position that there is
maximum
overlap between the sample and electric field vector of the microwaves.
An important parameter which can be extracted from the photoconductivity transients is the efficiency of the charge separation between a spin-coated thin polymer layer and a smooth thin anatase TiO2 layer. This efficiency can be calculated by comparing the conductivity transients observed at irradiation at 544 nm with that from direct band-gap excitation of the TiO2 at 308 nm. Maximum values for the IPCE of 6% are found for 30-50 nm thick spin coated layers of dialkoxy PPV’s on an 80 nm thick film of anatase TiO2.
Very recently we have extended
the capabilities
of the technique to wavelength coverage from the UV (240 nm) to the infra-red (2000
nm) using
either a continuous light source or a continuously-wavelength-variable,
nanosecond-pulsed
laser. In this way both steady-state and dynamic photoconductivity action spectra can
be
obtained. For similar samples to those described above steady state action spectra
ave been
recorded in the visible part of the spectrum using front- and backside excitation and
are shown
in Figure 3. By simulation of the action spectra an exciton diffusion length in PPV of
ca 20
—40 nm is obtained.
Figure 3 Absorption (¾) and steady state microwave conductivity spectra (+) using illumination through the 100 nm PPV layer (front side, left) and through the TiO2 layer (backside, right). Conductivity spectra are fitted (--) by assuming that all absorbed photons within 20 nm of the TiO2 contribute to the conductivity.
References
[1] T.J.Savenije, M.P. de Haas, J.M. Warman The Yield and Mobility of Charge Carriers in Smooth and Nanoporous TiO2 Films, Zeitschrift für Physikalische Chemie, 212 (1999) 201-206.
[2] T.J.Savenije, M.J.W. Vermeulen, M.P. de Haas and J.M. Warman Contactless determination of interfacial charge separation in a TiO2/phenylene vinylene polymer junction, Solar Energy Materials and Solar Cells, 61 (2000) 9-18.
