Session Th-B1

Organic Semiconductors I

Chair: Zheng-Hong Lu, University of Toronto

Th-B1.1 8:20–8:40

Influence of Annealing to Mobility of Holes in Layers of Derivatives of Diphenylethenyl Substituted Triphenylamines

K. Arlauskas (1), G. Juska Jr. (1), S. Tumenas (2), R. Juskenas (3), and V. Getautis (4)

1. Vilnius University, Department of Solid State Electronics, Saulėtekio al. 9, k. 3, LT-10222 Vilnius, Lithuania

2. Center for Physical Sciences and Technology, Department of Optoelectronics, A. Goštauto 11, LT-01108 Vilnius, Lithuania

3. Center for Physical Sciences and Technology, Department of Characterization of Materials Structure, A. Goštauto 9, LT-01108 Vilnius, Lithuania

4. Kaunas University of Technology, Department of Organic Chemistry, Radvilėnų Rd. 19, LT-50254 Kaunas, Lithuania

Successful application of organic materials not only for xerography, organic light emitting devices, but also for organic solar cells stimulated search and synthesis of new chemical structures. One of targets of this search is to synthesize organic materials with as high as possible mobility of charge carriers. However, the mobility of charge carriers is influenced not only by chemical formulae of organic material but also by microstructure of deposited layer.

In this research we investigate hole mobility dependencies on electric field, temperature and annealing of vacuum deposited layers of derivatives of diphenylethenyl substituted triphenylamines. The structures of as deposited and of annealed layers have been investigated using small angle X-ray (SAXS) spectroscopy and variable angle spectroscopic ellipsometry (VASE) methods.

The hole mobility of layers was measured using time-of-flight (TOF) technique in 200K–300K temperature range. In as deposited tris[4-(2,2-diphenylethenyl)phenyl]amine layers the highest mobility of the order of 10–2 cm2/Vs has been estimated. An annealing of investigated layers increased zero electric field mobility, dependently on chemical formulae, up to four times and changed character of mobility dependencies on electric field and temperature. The investigations using SAXS and VASE methods demonstrated that annealing influenced structural anisotropy of layers. However, hole mobility dependencies on temperature and electric field after annealing demonstrated, that the parameters of energy and space disorder, estimated according Bässler's formalism, changed insignificantly. An obtained experimental results are discussed taking into account structural changes of layers and mobility description expressions of other authors.

Keywords: organics, electronic transport

Th-B1.2 8:40–9:00

ReRAM Based on Switching in Metal Particles Doped Polymer Films

Mikhail Dronov (1), Maria Kotova (2), and Ivan Belogorohov (3)

1. A. M. Prokhorov General Physics Institute, Moscow, Russian Federation

2. M. V. Lomonosov Moscow State University, Moscow, Russian Federation

3. Federal State Research and Design Institute of Rare Metal Industry ("Giredmet"), Moscow, Russian Federation

Resistive switching, the reversible modulation of electronic conductivity, is of interest for prospective memory devices that could be perfect electronic memory. Despite different types of ReRAM (Resistive Random Access Memory), organic based ReRAM is an object of special interest due to ability to produce high performance devices using relatively simple technology.

We report on demonstration of a rewritable memory device structure based on metal particles doped polymer films with the nonvolatile states, on/off resistance ratio up to 106 and the switching time on the order of several ns.

We present two device types: sandwich structure metal/polymer blend/metal, which could be used for crossbar arrays, and coplanar type with two parallel contacts, usable for cheap and simple memory devices. A number of different metals: Cu or Al sheets, ITO on glass, silver paste were used as electrodes. Organic films were spin coated from a solution or prepared by lamination. Two different polymers polystyrene (PS) and polyvinyl chloride (PVC) were used as a base component of the polymer blend both undoped and with different concentration of metal particles. As metal particles we used zinc, aluminum and silver particles with sizes from 1 to 5 μm. The thickness of polymer blend layer was varied from 1 μm to 50 μm. The minimal electrode area was about 100 μm2.

Resistive switching is observed even for undoped polymer layers (with thickness below 20 μm) but we demonstrate the ability to control switching voltages and maximum device dimensions by changing metal particle concentrations with optimal (for RRAM) device behavior near the percolation limit.

A bistable operating mode is achievable with nonvolatile "ON" (conducting) and "OFF" (isolating) states, for which the resistance ratio varies from 2 to 106 for devices of different thickness and prepared using different materials. Minimal obtainable switching voltages are of order of few volts, with the device with switching voltages below 3 V at the best. For some of the devices, a good stability of states for more than 106 switching cycles was observed.

The most possible explanation of the effect observed is a filament formation. The filaments could be possibly originating from metal particles used as dopant and metal particles injected into an organic film from metallic contacts during the initial device electroforming. However, states not only with different resistances, but also with different capacities were observed. This effect cannot be explained only in terms of the filament formation. It is necessary to consider additional charge trapping mechanisms that may affect the device properties.

Keywords: ReRAM, polymer memory, resistive switching

Th-B1.3 9:00–9:20

Extraordinary Broad Band Light Enhancement Near the Lambertian Limit in Organic Solar Cells Using a Photonic Crystal Architecture

Rana Biswas (1), Erik Timmons (1), and Stephen Bergeson (2)

1. Ames Laboratory and Microelectronics Center; Dept. of Physics and Astronomy and Dept. of Electrical and Electrical and Computer Engineering, Iowa State University, Ames, IA 50011

2. Dept. of Physics and Astronomy, Iowa State University, Ames, IA 50011

A critical step to achieving high efficiency thin film solar cells is the broad band harvesting of solar photons. Spectacular progress has recently been achieved in improving the power conversion efficiency of organic solar cells to near 8% for single junction, and 10.6% for tandem junction cells [1]. In spite of rapid efficiency gains, these cells still do not absorb up to ~50% of the incident solar spectrum.

We have designed and developed a solar cell architecture that can boost the absorption of photons by 40% and the photo-current by 50% for organic absorber layers of typical device thicknesses. Our solar cell architecture is based on all layers of the solar cell being patterned in a conformal two-dimensional periodic photonic crystal architecture. This results in very strong diffraction of photons that increases the photon path length in the absorber layer, and plasmonic light concentration near the patterned organic-metal cathode interface. The absorption and photo-current approach the Lambertian limit. Conformal patterning is far more effective with much stronger diffraction than texturing the organic-metal cathode interface. The simulations utilize an established rigorous scattering matrix approach [2] and provide important fundamental limits of light absorption in periodically textured organic solar cells. We describe results for both P3HT-PCBM and PTB7 absorber layers. This solar cell architecture has the potential to increase the power conversion efficiency of single band gap organic solar cells utilizing long wavelength absorbers such as PTB7 to exceed 10%.

We will discuss how the plasmonic enhancement of fields has the potential to increase exciton dissociation at donor-acceptor interfaces. We will survey and compare with ongoing experimental approaches that have achieved patterned organic cells. We will discuss and compare these results with alternative light enhancement mechanisms using microlens arrays to focus incoming light. We will compare these results for organic cells with our previous results for photonic and plasmonic crystal enhanced thin film silicon cells [2]. There are significant differences in the plasmonic enhancement in organic vs. silicon based systems, which will be discussed. Sources of losses within solar architecture will be identified. Although the Lambertian limit in organic semiconductors is much lower than in Si-based semiconductors, it may still represent a fundamental limit for broad-band light absorption.

[1] J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C-C. Chen, J. Gao, G. Li, and Y. Yang, Nature Comm. 4, 1446 (2013)

[2] R. Biswas and C. Xu, Journal of Non-Crystalline Solids 358, 2289-2294 (2012) (Invited ICANS-24 presentation); Optics Express, 19, A664–A672 (2011)

Keywords: light trapping, organics, optical simulation, organic solar cell

Th-B1.4 9:20–9:40

Fundamental Studies of Degradation of Organic Solar Cells

Vikram Dalal, Joydeep Bhattacharya, Meharan Samiee, and Pranav Joshi

Iowa State University, Dept. of Electrical and Computer Engr., Ames, Iowa 50011, USA

Organic solar cells are an important new technology for inexpensive thin film solar cells. While a number of investigators have recently achieved a solar conversion efficiency of over 10%, the organic cells still suffer from significant instability upon exposure to light or the environment. It is generally believed that most of the instability is due to degradation of external layers such as contacts and hole and electron transmitting layers. In this paper, we show that such is not the case, and that the instability is intrinsic to the material and heterojunction device interface. The polymer solar cells and materials were fabricated from P3HT as the donor, and PCBM or ICBA as the acceptor, and had efficiencies ranging from 4.5% to 6.3%. and were subjected to different light intensities from a full spectrum xenon solar simulator inside an environment that was free of oxygen and moisture, and properties such as light I-V curves, dark I-V curves and defect densities measured in-situ during exposure. It is shown that light exposure causes increases in defect densities in the donor material, both at the midgap and in shallow traps near the valence band, and also causes increases in defect densities at the interface between the donor and the acceptor. The increases in defect density then causes increases recombination at the interface, and a reduction in external carrier collection and fill factor, as well as increases in dark current. The dark current increase is directly related to the reduction in open circuit voltage. We also find that a full spectrum illumination does far more damage to the material and to the interface than a spectrum that excludes blue and uv photons. Thus, it is important that full spectrum lamps be used for any reliable degradation studies.

We also find that the increase in defect densities tends to saturate over time, implying that there is a reverse annealing mechanism present in the device. Upon thermal annealing subsequent to degradation, we find that the devices recover some, but not all, of their loss due to illumination. A measurement of defect densities also reveals a recovery in defects to a lower value after thermal annealing compared to before. The recovery is quite rapid, occurring within 30 minutes at 100°C. We also discovered that the extent of the recovery depended on the molecular weight of the donor polymer material, with higher molecular weight materials recovering more than materials with lower molecular weights. The annealing and saturation results indicate that perhaps, defect states are being created by breaking of C-H bonds, and that H motion plays a role in thermal annealing.

We will also discuss similar measurements on PTB7 materials and devices, where the device efficiency was ~8.5%. Finally, we will contrast this intrinsic degradation with degradation due to environment, which has a very different signature. We will show how to separate the external (e.g. contact) degradation from the intrinsic degradation.

Th-B1.5 (invited) 9:40–10:10

Photocarrier Recombination Kinetics in a Bulk Heterojunction Solar Cell Studied by a Frequency-domain Measurement

Takashi Kobayashi (1,2), Takashi Nagase (1,2), and Hiroyoshi Naito (1,2,3)

1. Department of Physics and Electronics, Osaka Prefecture University, Osaka, Japan

2. The Research Institute for Molecular Electronic Devices, Osaka Prefecture University, Osaka, Japan

3. Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology (JST), Tokyo, Japan

Bulk-heterojunction solar cells based on electron-donating polymers and electron-accepting fullerenes are extensively studied because of their strong potential to cut the cost of solar energy. In this type of solar cell, the photocarrier generation process is known to be very efficient; however, a large portion of generated photocarriers are lost in the subsequent transport process by an unfavorable recombination with oppositely charged carriers. It is, therefore, important to understand photocarrier recombination kinetics in actual solar cells to further enhance device performance. For this purpose, time-domain optical measurements with a pulse laser, e.g. pump-probe and transient absorption measurements, have been used widely. Although these time-domain techniques have excellent time resolution, they are rather unsuitable for investigation of recombination kinetics in a microseconds–milliseconds time range, the time scale on which photocarriers recombine or sweep out in actual solar cells. Thus, we have alternatively applied a frequency-domain optical measurement, continuous-wave photoinduced absorption (cw-PIA) spectroscopy, which has a high sensitivity for detecting signals due to excitations with a lifetime in the microseconds–milliseconds time range [1].

In this presentation, we show that cw-PIA spectroscopy allows us to investigate hole transport process in actual solar cells at various operating points, including open- and short-circuit conditions [2]. From modulation frequency dependence of a PIA signal, it is also possible to estimate hole lifetime, hole drift mobility, and collection efficiency. For solar cells based on P3HT and PCBM, holes created in crystalline regions of P3HT recombine in an ideal bimolecular recombination manner under open-circuit conditions and exclusively contribute to short-circuit current, whereas the carrier transit cannot be observed for holes created in amorphous regions of P3HT. It is known that power conversion efficiency of P3HT:PCBM solar cells largely depends on fabrication conditions, in particular on thermal annealing. cw-PIA spectroscopy also helps in understanding the effects of thermal annealing on hole transport process. Thus, cw-PIA spectroscopy is a powerful tool to investigate photocarrier recombination kinetics in actual solar cells.

[1] T. Kobayashi, K. Kinoshita, T. Nagase, and H. Naito, Phys. Rev. B 83, 035305 (2011)

[2] T. Kobayashi, Y. Terada, T. Nagase, and H. Naito, Appl. Phys. Express 4, 126602 (2011)

Keywords: bulk-heterojunction solar cell, photoinduced absorption spectroscopy, photocarrier recombination, bimolecular recombination, hole drift mobility