The 25th International Conference on Amorphous and Nano-crystalline Semiconductors
August 18–23, 2013 Toronto, Ontario Canada
Organic Semiconductors II
Chair: Eric Schiff, Syracuse University
Characterization Limit on Charge Injection Barriers in Organic Semiconductor Devices
Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, 199 Ren-Ai Road, Suzhou Industrial Park, Suzhou, Jiangsu 215123, China
The charge injection barrier height at the electrode interfaces of organic semiconductor devices has substantial effect on the electronic or optoelectronic properties of the devices, such as the amount of injected charges, the balance of the opposite charges injected from the opposite electrodes, the recombination efficiency of the opposite charges, the efficiency of exiton generation or separation. 0.1 eV of difference in the injection barrier height may result in more than 50 times of difference in injected charge density. Therefore, precise characterization of the energy levels of the organic semiconductor layer at the electrode interfaces are crucial.
There are several methods to characterize the energy levels of organic semiconductors, such as cyclic voltammetry, UV-Vis absorption spectroscopy, ultraviolet photoelectron spectroscopy, inversed photoelectron spectroscopy, photoelectron spectroscopy using a synchrotron radiation source. However, those methods either do not have a characterization precision better than 0.1 eV or are very expensive and inconvenient.
Temperature-dependent J-V characterization combined with a suitable charge injection model, a conventional method for inorganic semiconductor analysis, can be easily and precisely used for the investigation of the charge injection barrier heights in organic semiconductor devices.
In this presentation, the author will compare several of the characterization methods and illustrate the advantages of the temperature-dependent J-V characterization for use in the research of organic light-emitting diodes and organic photovoltaic devices.
Keywords: energy level measurement, charge injection barrier, organic semiconductor devices
Exciton Dissociation in P3HT:PCBM Bulk-Heterojunction Organic Solar Cell
1. Centre for Renewable Energy, Charles Darwin University, Darwin NT 0909, Australia
2. School of Engineering and IT, Charles Darwin University, Darwin NT 0909, Australia
Research interest in organic solar cells (OSCs) has escalated recently due to their cost effectiveness and easy fabrication techniques. However, their conversion efficiency is still low (5%–7%) . One of the reasons for this is that the dissociation process of excitons is not fully understood, neither theoretically nor experimentally, although it is the most critical point to influence the operation of OSCs [2,3]. In this paper, a comprehensive study is carried out on the dissociation of excitons in poly(thiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk-heterojunction OSC. P3HT and PCBM are the most popular and highly studied donor and acceptor organic materials, respectively, used for fabrication of bulk heterojunction OSCs .
In such OSCs, Frenkel excitons are generated upon photon absorption in P3HT. Then they diffuse to the P3HT:PCBM interface, where they may dissociate. The offset in the lowest unoccupied molecular orbital (LUMO) energy between P3HT and PCBM provides the excess energy for this dissociation. The newly derived interaction operator between Frenkel excitons and phonons at the P3HT:PCBM interface is used as the perturbation operator [5,6] to calculate the rate of Frenkel exciton dissociation. The rate thus obtained is given by
where EBP3HT is the Frenkel exciton binding energy in P3HT, (LUMOPCBM – LUMOP3HT) is the LUMO offset between PCBM and P3HT, ħων is the phonon energy, Meff is the effective nuclear mass, ax is the excitonic Bohr radius, and ε is the dielectric constant of P3HT. For an effective dissociation, one requires
The intermediate state of the dissociation process generates a charge transfer exciton, whereby the electron moves to the LUMO of PCBM and the hole remains in the highest occupied molecular orbital (HOMO) of P3HT . The charge-transfer exciton gets dissociated by the excess energy of the offset and the work function difference between the electrodes of an OSC may be expected to assist the transport of the thus dissociated pair of charge carriers to the opposite electrodes.
The effect of tuning the LUMO energy of P3HT with the view of optimizing the dissociation rate is investigated in this study. It is found that the larger the LUMO energy difference between PCBM and P3HT, the larger is the dissociation rate of Frenkel excitons. The time of dissociation of Frenkel excitons is also calculated and is found to be in the femtosecond range. This implies that the conversion efficiency of OSCs should be high and not as low as 5%–7%, as observed experimentally. Issues related with this discrepancy will also be presented in detail in this presentation.
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Keywords: Frenkel exciton, charge-transfer exciton, dissociation rate, P3HT:PCBM bulk-heterojunction organic solar cell
Fundamental Material Physics of PTB7 Solar Cells
1. Iowa State University, Dept. of Electrical and Computer Engr., Ames, Iowa 50011, USA
2. Nazarbayev University, Astana, Kazakhstan
PTB7 is an important organic PV material in which solar cells with high efficiency (~9%) have already been fabricated. In this paper, we report on a systematic investigation of the fundamental material parameters in high efficiency cells, such as trap states, mobility, defect density at the interface between the donor and the acceptor materials, deep defect states within PTB7, collection length, interfacial recombination velocity, and dark current in this material system. Mobility was measured using space charge limited current and is in the range of a few x10–4 cm2/V-s. The interfacial defect density vs. energy was measured using capacitance frequency techniques from1 Hz to 1 MHz at various temperatures, and is in the range of 4x1010–1011 /cm2eV. It peaks at about 0.75 eV above the HOMO level of PTB7. The trap states in PTB7 near the HOMO level were measured using both subgap quantum efficiency techniques and C-f techniques, and they show an exponential Urbach edge of ~25 meV. Detailed analysis of dark I-V shows two distinct regions, one corresponding to interfacial recombination, and one to bulk recombination.
Two types of cells were fabricated, one with PCBM70 as the acceptor, and one with ICBA as the acceptor. The cells with ICBA showed an increase in open circuit voltage of approximately 0.2 V. Detailed analysis of subgap QE in these two types of cells showed a distinct shift in the curves of ~0.2 eV, confirming that the effective hetero-bandgap (between the HOMO level of the donor and the LUMO level of the acceptor) is ~0.2 eV higher when ICBA is used as compared to when PCBM70 is used. This report is the first direct observation of the shift in hetero-bandgap in any organic solar cell when different acceptors are used, thereby conclusively proving that the increase in voltage is related to the increase in hetero-bandgap. Similar measurements for the familiar P3HT/PCBM and P3HT/ICBA system will also be reported to confirm the accuracy of this measurement technique. The subgap QE also shows that the interfacial trap states are very different when ICBA is used as the acceptor when compared to when PCBM70 is used.
In summary, we have done a comprehensive study on the fundamental material parameters in the high efficiency when PTB7/acceptor system.
Investigation of the Degradation of Bulk Heterojunction Polymer Solar Cells by Low-frequency Noise Spectroscopy
1. Faculty of Mathematics and Computer Science, FernUniversität Hagen, 58084 Hagen, Germany
2. Dipartimento di Fisica "E.R. Caianiello" and CNR-SPIN Salerno, Università di Salerno, 84084 Fisciano, Salerno, Italy
3. Institute of Physics, Carl von Ossietzky University of Oldenburg, 26111 Oldenburg, Germany
4. Dipartimento di Ingegneria Industriale, Università di Salerno, 84084 Fisciano, Salerno, Italy
Solar cells based on polymer:fullerene bulk-heterojunctions (BHJ) can be processed from solution at low temperatures and low production costs to offer a promising technology for flexible and large area photovoltaic applications. The investigated organic solar cell is based on a blend between poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C6l-butyric acid methyl ester (PCBM), representing one of the reference structure in the polymer:fullerene devices with a power conversion efficiency of 6.5%. To evaluate the role of the charged defects on the electronic device performances, low-frequency noise spectroscopy can be used in dark conditions at 300K. The solar cell has been modeled at low frequencies as a parallel connection between a fluctuating resistance Rx(t) and a capacitance Cx. Under dc biasing, the carriers injected into the active layer modify the equivalent electrical impedance thus changing the noise spectra. The experimental spectral trace can be interpreted by means of a theoretical model based on the capacitance Cμ, which takes into account the excess of minority carriers in the blend, and the device resistance Rrec. The measured electric noise is of 1/f-type up to a cut-off frequency fx = (2πRrecCμ)–1, after which a 1/f3 dependence has been observed. The analysis of fx gives information regarding the recombination lifetime of the electrons in the active layer. Moreover, from the known chemical capacitance due to the storage of the electrons in the blend, the density of states (DOS) for the lowest unoccupied molecular orbital (LUMO) of the PCBM can be extracted. Subsequently, the solar cells have been subjected to a thermal cycle up to 340K in order to induce a morphological modification of the active layer. After the thermal cycle a shift of the current-voltage characteristics to lower voltages has been observed. The voltage drop is due to the presence of the defects at the interface between the cathode and the blend. The defects induce a band-discontinuity giving the formation of an injection barrier, which acts as a blocking contact for the electrons: the device becomes under these conditions a single carrier device. This is reflected in the low-noise voltage-spectral density as a shift of fx to frequencies higher than 100 kHz, above the experimental bandwidth. The resulting noise spectrum for the deteriorated solar cell shows a 1/f-type behavior over the whole investigated frequency range.
Keywords: organic photovoltaics, noise processes and phenomena
Th-B2.5 (invited) 11:50–12:20
Charge Carriers Transport in Organic Field Effect Transistors
Department of Solid State Electronics, Vilnius University, 10222Vilnius, Lithuania
Usually in the organic field effect transistors (OFET) the values of the mobility of the charge carriers obtained from steady state I-V characteristics differ significantly from the values measured by time-of-flight or other techniques .
In this work we are presenting current transients in OFET, from which the mobility of the charge carriers along and across the layer can be estimated. The current transients in OFET are governed by the dynamics of the charge carrier injection from the source electrode and the charge accumulation at the organic charge transporting layer/insulator interface. The numerically calculated distributions of the potential and the electric field along the channel at different time moments and current transients when the voltage pulse is applied between source and drain are presented . The theoretical transients of the source-drain current allow the estimation of the mobility of the charge carriers along the channel in more accurate way than proposed in Ref.[3–5].
The method of the extraction of the injected charge carriers by the linearly increasing voltage (i-CELIV) allows the determination of the thicknesses of both organic charge transporting and insulating layers, threshold voltage and the mobility of charge carriers across the layer .
The charge carrier mobilities have been obtained experimentally by the various methods: across the layer, from the extraction transients (in small charge and space charge limited current modes), and along the channel, from the duration of the channel charging and steady state current for the different organic materials. The results have been compared with the results obtained by the photogenerated carrier extraction by linearly increasing voltage and time-of-flight methods.
The discrepancy of the mobility values measured by different techniques could be caused by the following reasons: the incorrect interpretation of the experimental results, the mobility dependencies on the electric field or on the densities of the charge carriers, the different morphology of the organic material across and along the sample, the presence of charge carriers reservoir at organic charge transporting layer/insulator interface.
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 G. Juška, N. Nekrašas, and K. Genevičius, J. Non-Cryst. Solids 358, 748 (2012)
Keywords: organics, electronic transport, TFTs