Characterisation zone

Head: Daniel MATHIOT, Contact

In organic photovoltaic cells the active layer is made of a blend of materials donor and acceptor of electrons sandwiched between the anode and the cathode. Different devices may be distinguished depending on whether the cell is illuminated from the anode or from the cathode: devices using the standard architecture (see the pattern opposite) are illuminated from the anode; the succession of layers from the glass substrate is: anode (transparent conducting layer, generally a bilayer made of ITO and PEDOT:PSS), active layer and cathode (metallic layer, generally a bilayer made of calcium and aluminium); devices using the inverted architecture are illuminated from the cathode; the succession of layers from the glass substrate is: cathode (transparent conducting layer, generally a bilayer made of ITO and PEIE), active layer and anode (metallic layer, generally a bilayer made of molybdene oxide MoO3 and silver).

Evaporation masks used for the different characterisations carried out in the laboratory allow to realise cells with four diodes of 12 mm² or two diodes of 24 mm² on each glass substrate. For particular needs, other device architectures and other electrodes sizes can be considered (requests can simply be sent by e-mail).

The characterisation of organic photovoltaic cells is performed in the dedicated zone of one of the glove boxes systems by means of the methods described below.

Measurement of the electrical characteristics under solar simulator

The electrical characteristics of organic photovoltaic cells are evaluated from the measurement of the variation of the current according to the applied voltage, at first in a mode called "in the dark", which means that the cell is not illuminated, then in the mode "under illumination", that is while the cell is illuminated by a solar simulator (Fig. 1).
A software developed in house (Fig. 2) extracts, from the characteristics I(V) measured in both modes (Fig. 3), the various photovoltaic parameters of the cells: the open circuit voltage V_oc, the short circuit current density J_sc, the fill factor FF and the power conversion efficiency PCE.

The solar simulator is a class AAA AM1.5G simulator that delivers 100 mW/cm². This intensity can be reduced, until 0.9 mW/cm², by interposing neutral filters between the light source and the cell.
The apparatus for the measurement of the electrical characteristics is an arm with 4 tips that allows to measure simultaneously the characteristics of 4 diodes of the cell. All the measurements are automated and the calibration of the equipment is regularly verified by means of a reference silicon solar cell.

Fig. 1: Measurement of the I(V) characteristics under illumination	Fig. 2: Monitoring software for the measurement of the I(V) characteristics	Fig. 3: I(V) characteristics measured in the dark (blue curve) and under illumination (red curve) for a 12 mm2 photovoltaic cell whose donor material of the active layer is a fluorinated polymer. Photovoltaic parameters: Voc = 752 mV; Jsc = 15.1 mA/cm2; FF = 67.5%; PCE = 7.6%

External quantum efficiency measurement

The spectral response of organic photovoltaic cells is obtained from the measurement, as a function of the wavelength, of the intensity of the incident light, the intensity of the reflected light, and the photogenerated current.
In the measurement bench of the spectral response (Fig. 4) installed in the glove box, the incident white light is first separated by a monochromator whose resolution is 5 nm for wavelengths between 350 and 1300 nm. Incident and reflected light intensities, as well as the photogenerated current, are then simultaneously registered at each wavelength. At last, these data are used to calculate the External Quantum Efficiency (EQE, Fig. 5) and to estimate the Internal Quantum Efficiency (IQE).

Fig. 4: Measurement bench of the spectral response	Fig. 5: EQE spectrum of an organic photovoltaic cell whose active layer is a P3HT:PCBM blend

Transient measurements

A dedicated measurement bench provides transient characteristics: the Transient PhotoVoltage (TPV) and the Charge Extraction (CE). For these measurements, two ligth emitting diodes are used to illuminate the organic photovoltaic cells: the one delivers a white ligth whose emitting spectrum, corresponding to the radiation of a black body at 5900 K, is close to the solar spectrum the other delivers a pulsed green ligth at 535 nm

For the measurement of the TPV, the cell is first illuminated with the white ligth, used in continuous mode, and its intensity sets the open circuit voltage V_oc. It is then illuminated with the green ligth to increase V_oc from ΔV (Fig. 6). The return to equilibrium that follows the end of the green pulse is characterised by a time τ_Δn that reflects the recombination of charge carriers. In practice, this measurement is repeated for different values of the white light intensity so as to obtain the variation of τ_Δn according to V_oc (Fig. 7).

Fig. 6: Principle of the TPV that enables the estimation of the variation of the characteristic time of charge carriers recombination according to Voc	Fig. 7: Variation of τΔn according to Voc for a cell whose active layer is a P3HT:PCBM blend

For the measurement of the CE, the cell is illuminated with the white ligth, used at the same intensity as for the TPV but in the pulsed mode. At the end of the pulse the resistance at the connections of the oscilloscope varies from 5 MΩ to 50 Ω (Fig. 8). The charge carriers concentration n depends on the open circuit voltage Voc so that, by repeating this measurement for various values of the ligth intensity, it is possible to obtain the variation of the charge carriers concentration n according to Voc (Fig. 9). The combination of TPV and CE measurements allows to obtain the variation of the characteristic time τΔn of charge carriers recombination according to their concentration n (Fig. 10).

Fig. 8: Principle of CE that enables the estimation of n according to Voc

Fig. 9: Variation of the concentration n of charge carriers according to Voc for a cell whose active layer is a P3HT:PCBM blend	Fig. 10: Variation of τΔn according to the concentration n of charge carriers for a cell whose active layer is a P3HT:PCBM blend