The carrier dynamics in organic photovoltaic (OPV) cells were investigated by impedance spectroscopy. We introduced a novel impedance spectrum representation called dynamic modulus plot (DMP), which allowed us to observe the layer-to-layer carrier injection behavior graphically. In this work, the impedance responses were characterized in the N, N’-diphenyl- N, N’-di-m-tolyl- 4,4’-diaminobiphenyl (TPD)/C 60 p-n heterostructured OPV cells against applied voltages. The dependence of impedance responses on the layer thickness revealed a constant internal electric field that disturbed the carrier transport within the OPV cells . We applied this technique to new donor materials, in which thiophene units were inserted to the center of TPD. By increasing the number of thiophene units in TPD the fill-factor (FF) improved from 33% to 59%, which increased the power conversion efficiency (PCE). Based on the DMP analysis, we assigned the improvement in device performance to the reduction of the internal electric field.
In 1986, Tang fabricated a bilayer heterojunction solar cell with an efficiency approaching 1%, which was a milestone in the development of organic photovoltaic (OPV) cells [
In this study, the basic carrier dynamics in p-n heterostructured OPV cells were investigated using impedance spectroscopy. We introduced a novel representation of the impedance spectrum dynamic modulus plot (DMP), which allowed us to graphically observe the layer-to-layer carrier injection and carrier accumulation behavior. The diode characteristics of the OPV cells were discussed based on the analysis of the charge carrier behavior by means of DMP analysis in the dark. We revealed the existence of an internal electric field at the donor- acceptor interface, which governed the OPV device performance under the illumination.
The compounds TPD, C60 and bathocuproine (BCP) were purchased from Sigma-Aldrich Co. Ltd. Bis-diphe- nylaminophenyloligothiophene (BDA-Tn [n = 1 - 3]) derivatives were synthesized using the Suzuki-Miyaura coupling reaction in our laboratory. These materials were purified by vacuum sublimation at 10−3 Pa. OPV cells with the layered structure ITO/p-type/C60/BCP/Al were fabricated by vacuum deposition. ITO substrates, purchased from Asahi Glass Co., Ltd, with a sheet resistance of 25 Ω/sq were ultrasonicated first in 2-propanol and then in ultra-pure water. They were later exposed to an ultraviolet-ozone surface treatment and transferred to a vacuum chamber. The organic materials were evaporated onto the substrates under a vacuum pressure of less than 10−4 Pa, with deposition rates between 1 and 5 Å/s. The active area of the diodes was 5.8 mm2. The devices were measured in ambient atmosphere without encapsulation.
The highest occupied molecular orbital (HOMO) energy levels of the organic materials were determined by photoelectron yield spectroscopy in air using an AC-3 spectrometer (Riken Keiki Co. Ltd., Japan). The film absorption spectra were recorded by a UV-3600 UV-Vis-NIR spectrophotometer (Shimadzu Co. Ltd., Japan). The optical band gaps were estimated from the absorption edge of the thin films. The lowest unoccupied molecular orbital (LUMO) energy levels were estimated by the subtraction of the optical band gap from the HOMO levels.
The J-V characteristics of the devices were measured at room temperature with a source measurement unit (Keithley Instruments Inc., 2400) in the dark and under illumination of 100-mW/cm2 white light provided by a 1.5 AM solar simulator.
The impedance measurements were performed with an impedance analyzer (Agilent Technologies Inc., 4294A) in the frequency range from 100 Hz to 1 MHz in the dark. The AC modulation amplitude was kept as low as 50 mV, where the response to the AC amplitude was linear. The complex impedance data were transferred to a PC and processed mathematically to appropriate representations.
In general, electronic devices are represented by a combination of capacitance and resistance components in response to an external field. Carrier density and electron levels are evaluated based on C-V and f-C measurements in semiconductor engineering. In the electrochemical field, Cole-Cole plots, which are a representation of the impedance spectrum, have been used frequently [
When an arc appears in the impedance Cole-Cole plot, a corresponding arc appears in the modulus Cole-Cole plot. Its diameter is the reciprocal value of capacitance (C) (
Here we propose a new graphical representation for the voltage-current response function, the dynamic modulus plot (DMP). The DMP is a compilation of the modulus plots obtained by frequency sweep, which are measured under various bias voltage conditions. The frequency sweep of the current response to the small AC modulation voltage is recorded for various DC bias voltages. The data obtained from the frequency sweep are converted to complex modulus data. By plotting the complex modulus data on a complex plain, we obtain a modulus Cole-Cole plot. The modulus Cole-Cole plots for various bias voltages are re-plotted so their imaginary parts are offset by the bias voltage, and the imaginary parts are rescaled by arbitrary values. Usually, the semicircles of organic electronic devices in the Cole-Cole plots are expressed by precise circles such that the lack of the scale of the imaginary part does not cause any confusion. We call this compilation of the modulus plots the DMP. Here, we note that the envelope at the lowest frequency points is the Mott-Schottky plot.
a power conversion efficiency (η) of 0.67% under AM 1.5, 100 mW・cm−2 simulated solar light. Impedance and modulus Cole-Cole plots of the OPV cell are shown in
In
From DMP, changes to the circuits of OPV devices can be seen. At sufficiently low bias voltage, there is only a small semicircle and a vertical line at the high frequency side. The vertical line means the device is an insulator and the electric response is purely dielectric. As the bias voltage increases, several semicircles appear and merge. The small semicircles correspond to p-type and n-type organic layers in which carriers were injected and were conductive. The equivalent circuits of the devices changed to a parallel connection of resistance and capacitance from pure capacitance. Finally, only one semicircle remained at sufficiently high voltage. There was a small semicircle at the high frequency side below a bias voltage of −1.0 V in all three devices. Compared with the designed thickness, the semicircle was assigned to a BCP layer. It is surprising that electrons accumulate in the BCP layer at such low bias voltages. The carrier accumulation in a BCP layer adjacent to a C60 layer was confirmed by photoelectron yield spectroscopy and displacement current measurements [
The radius of the semicircle at higher frequency becomes smaller by decreasing the layer thickness of the C60 layer. The separation of these two semicircles comes from the Maxwell-Wagner effect, i.e. the difference in the relaxation time in each layer [
suppress the charge dispersion along the donor-acceptor interface in the dark condition of the OPV cells, the reduction of the internal electric field is quite important to improve of the diode characteristics of OPV cells.
The charge accumulation properties of bilayer devices composed of new p-type materials, bis-diphenylamino- phenyloligothiophenes (BDA-Tn), were examined by impedance spectroscopy. Oligothiophenes are one of the largest families of organic semiconductors, and have been widely used in OPVs because of their high charge- carrier mobility and facile synthesis for tuning energy levels [
The new donor materials BDA-Tn were expected to show high PCEs, because their conjugation lengths are longer than mTPD, i.e. they can absorb more of the AM 1.5 spectrum. To investigate the effect of conjugation length, we inserted different numbers of thiophene units into the TPD skeleton.
As the number of thiophene units increases, Voc of the BDA-Tn/C60 cells was reduced and the Jsc and FF improved significantly. The highest power conversion efficiency in this series of materials was achieved with BDA-T3, which reached 1.33%.
The reduction of the Voc was attributed to the relatively low ionization potential of the BDA-Tn materials. The improvement of the Jsc was caused by the stronger absorption of the solar light. BDA-T3, which has three thiophene units, exhibited an absorption peak around 440 nm which is a bathochromic shift of ~30 nm compared with mTPD (λmax = 410 nm).
The layer resistivity is closely related to the carrier mobility of the material and the FF in OPV cells. We measured the mobilities of the BDA-Tn derivatives by TOF methods. The measured mobilities of the BDA-Tn derivatives were 1.0 × 10−3 cm2∙V−1∙s−1 (BDA-T0 or mTDP), 4.8 × 10−4 cm2∙V−1∙s−1 (BDA-T1), 1.6 × 10−4 cm2∙V−1∙s−1 (BDA-T2), and 4.9 × 10−5 cm2∙V−1∙s−1 (BDA-T3). It is puzzling that the mobilities decrease as the number of thiophene units increases, whereas the FF improves with the number of thiophene units. Generally, TOF measurements are carried out with a high electric field, but OPV cells are driven under very low electric fields (so called built-in potentials). These electric field differences may make it difficult to interpret the TOF results.
To understand why the FF improves, we have carried out an impedance analysis of the OPV cells composed of BDA-Tn derivatives. In
Voc (V) | Jsc (mA∙cm−2) | FF | η (%) | Ip (eV) | Af (eV) | |
---|---|---|---|---|---|---|
BDA-T0 | 0.86 | 2.37 | 0.33 | 0.67 | 5.5 | 2.5 |
BDA-T1 | 0.82 | 2.42 | 0.53 | 1.05 | 5.4 | 2.7 |
BDA-T2 | 0.79 | 2.62 | 0.53 | 1.09 | 5.4 | 3.0 |
BDA-T3 | 0.76 | 2.94 | 0.59 | 1.33 | 5.3 | 3.1 |
In this work, we have investigated the impedance response of TPD/C60 p-n heterostructured OPV cells against applied voltages. The equivalent circuit of p-n heterostructured OPV cells can be explained by three connected CR parallel circuits, each of which corresponds to a layer of the device.
As a result of the modulus analysis, the threshold voltage for charge injection and accumulation within the C60 layer varied in proportion to the film thickness. We have found that a constant internal electric field exists which disturbs the carrier transport to the electrodes in OPV cells because of interactions at the donor-acceptor interface. It is thought that this internal electric field is the reason for the reduction of the FF of OPV cells.
Next, new donor materials BDA-Tn were investigated. By increasing the number of thiophene units in BDA- Tn the power conversion efficiency and FF increased significantly, from 33% to 59%. Based on the DMP analysis, we found that the internal electric field decreased by introducing more thiophene units and concluded that the reduction of the internal electric field was the reason for the improved device performance. Further investigation of this phenomenon to reduce the internal electric field will potentially lead to higher-efficiency organic heterostructured solar cells.