Session V49: Focus Session: Organic Electronics and Photonics - Solar Cells and Light Emitting Devices

Frequency dependence of magnetoresistance in MEH-PPV

The organic magnetoresistance (OMAR) in organic light emitting diodes (OLED) made of MEH-PPV was investigated by means of DC transport and the admittance spectroscopy in the range of 1 Hz to 10 MHz at room temperature. The measurements were carried out on unipolar and bipolar OLEDs made of pristine MEH-PPV as well as MEH-PPV with traps introduced by the UV light irradiation. We found that in bipolar, UV-exposed OLEDs, the magnitude of magnetoresistance effect in real part of admittance increases with DC bias, reaches very high value of 35 {\%} (in the field 30mT) at bias 4.8 V and decreases at higher bias voltages. Also, we observed that the cutoff frequency of OMAR effect monotonically increases with DC bias voltage. The cutoff has extrinsic origin and is likely caused by a dissipative process related to the reorientation of permanent dipoles. At the highest tested bias voltage 6.7 V, we were able to detect the OMAR at the highest frequency of our system, 10 MHz. We have found that imaginary part of the admittance is also affected by magnetic field. The effect of magnetic field on dynamical capacitance of the device at low frequencies is very strong and opens up a possibility of using these devices as magnetic field sensors.   

Graphene-Based Polymer Bulk Heterojunction Solar Cells

The requirement of exciton dissociation in organic photovoltaics necessitates the presence of a large-area interface accessible to the interior of the active layer. Traditionally, such bulk heterojunctions (BHJ) have been spin-coated from a blend of conjugated polymer and functionalized fullerene molecules. We propose and demonstrate BHJs that utilize chemically modified graphene nanoparticles in order to facilitate charge transfer in polymeric solar cells. Devices based on P3HT:PCBM:graphene were fabricated on patterned ITO glass, and the effect of graphene on performance was investigated. Various device parameters including short-circuit current density, open-circuit voltage, fill factor, power conversion efficiency, and external quantum efficiency are compared with traditional BHJs. Results are discussed in the context of the morphology of the active layer, and the distribution and orientation of graphene platelets, as characterized by SEM, AFM and TEM.   

Planar Organic Photovoltaics for more Opportunities of Efficiency Enhancement and Parameters Controlling

Organic photovoltaics have been intensively studied as a cheap, easy processed and reliable source of energy that will eventually substitute the inorganic photovoltaics. Commonly, PV devices are made in vertical geometry where the BHJ active material is sandwiched between two electrodes one of them must be transparent to shin the light through. This vertical geometry created some challenges such as requirement of transparent ITO electrode as well as tying up the active material film thickness and electrodes separation. As an approach to overcome these challenges, we utilize the planar geometry to fabricate PV device where the poly (3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methylester (P3HT/PCBM) blend is deposited between two asymmetric metallic electrodes. We investigated the PV behavior for different metal electrodes which is an advantage provided by planar structure. Also, we discuss the behavior of the power conversion efficiency (PCE) with independently varying the active material film thickness and electrodes separation.   

Low band gap polymer bulk heterojunction solar cell on a co-planar digitated electrodes structure

Usage of additive in a bulk heterojunction organic solar (BHJ) cell to enhance absorption of solar spectrum in the NIR range is attracting significant research interest. At present, the BHJ solar cells are fabricated in a vertical geometry where a transparent electrode is necessary limiting the choice to mostly ITO. This may create a significant challenge as the performance of organic solar cell is dependent on the work function matching of active materials and the electrode. In order to address this challenge, we fabricated solar cell by dropcasting a ternary blend of P3HT:PC$_{60}$BM: PCPDTBT on a simple structure comprising of co-planar digitated electrodes with different work functions. We will discuss the performance of our device and its implication.   

Charge Accumulation and Internal Photovoltaic Processes in Organic Solar Cells

The accumulation of dissociated charge carriers plays an important role in reducing the loss occurring in open-circuit voltage (Voc), short-circuit photocurrent (Isc), fill factor (FF) in organic solar cells. We found from light-assisted capacitance measurements that the charge accumulation inevitably occurs at device interfaces in bulk-heterojunction ITO/PEDOT/P3HT:PCBM/Ca/Al solar cells. Our experimental studies have indicated that the charge accumulation can reduce the Voc through charge injection, Isc through charge collection, and FF through charge transport. Furthermore, our light-assisted capacitance measurements reveal that using a dielectric thin film of TiOx can decrease charge accumulation in the ITO/PEDOT/P3HT:PCBM/TiOx/Ca/Al solar cell. In particular, we find that decreasing the charge accumulation can reduce the loss occurring in Voc, Isc, and FF. Clearly, controlling charge accumulation presents a new mechanism to improve photovoltaic performance in organic solar cells.   

Photocurrent noise in organic bulk heterojunction solar cells

We report the first electrical noise measurements from illuminated bulk heterojunction polymer based solar cells. The dependence of photocurrent fluctuations on temperature, light intensity and device conditions was analyzed. We find flicker noise of the form 1/f$^\alpha$ at low frequencies ($<$1 kHz). An unusual log-normal feature in the noise power spectrum is observed in the frequency regime $>$ 5 kHz. We develop a theoretical description employing kinetic Monte-Carlo simulations that points to the importance of a mobility edge in the understanding of fluctuations in the low frequency regime, while also capturing the temperature dependence of the noise amplitude. We find that a Gaussian disorder model with uncorrelated traps is not sufficient to describe the log-normal feature, thus highlighting the significance of spatial and/or energetic correlations among the trap states.   

Relating charge transport and performance in single-layer graded-composition organic light-emitting devices

Organic light-emitting devices (OLEDs) continue to receive interest for application in displays and as solid-state lighting sources. While OLEDs can exhibit high external quantum and luminous power efficiencies, high performance often requires the use of complex, multilayer architectures. Consequently, there has been interest in the development of OLED architectures containing fewer device active layers or perhaps, a single active layer. Here, we describe an approach to realize efficient electroluminescence from a single active layer through the use of engineered composition gradients. Graded-emissive layer (G-EML) devices contain a single layer consisting of nearly 100{\%} hole-transporting material (HTM) at the anode and nearly 100{\%} electron-transport material (ETM) at the cathode, and having a continuously varying HTM:ETM composition across the active layer. Electroluminescence originates from a phosphorescent guest that is uniformly doped throughout the G-EML. For red-, green-, and blue-emitting phosphors, efficiencies are realized that rival those of more complex, multilayer structures. The G-EML balances electron and hole injection and transport leading to effective charge carrier confinement and exciton formation. This talk will examine how charge confinement in the G-EML is realized through a spatial variation in the carrier mobility across the active layer. In addition, separate measurements of the G-EML exciton recombination zone show that it is substantially broader than that of conventional, abrupt heterojunction OLEDs, a feature which may help to reduce bimolecular exciton quenching in these structures.   

Sources of high photo-current in inverted organic solar cells

Inverted organic solar cells have been proved to render exceptional environmental stability compared to the conventional solar cell architecture. On the other hand, polymer/fullerene based inverted solar cells produce more photo-current compared to conventional cells comprising the same active layer thickness. The origin of this current has never been clearly stated so far. We have investigated the photovoltaic properties of inverted solar cells comprising a bulk heterojunction layer of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM). The blend layer was formed by spin casting the blend solution on ITO substrate, covered with an Al doped zinc-oxide layer (ZnO-Al) deposited through pulsed laser deposition technique. The inverted solar cells show over 15{\%} increase in photo-current yield compared to conventional solar cells. We have discovered that the inverted solar cells produce additional photo-current through dissociation of excited electron-hole pairs near the polymer/ZnO-Al interfaces. Since ZnO-Al is a good electron conductor, the electrons generated at the polymer/ZnO-Al interface are more efficiently collected compared to photo-current produced in the bulk of the active film. External quantum efficiency exceeding 70{\%} was recorded in the ZnO-Al based inverted solar cells. In general, ZnO-Al is not only characterized by its high electron conductivity, and transparency but also serves as electron acceptor.   

Near-Infrared Photodetector Consisting of J-Aggregating Cyanine Dye and Metal Oxide Thin Films

We demonstrate a photodetector structure that employs metal-oxide charge transport layers and that is sensitized at near-infrared wavelengths by a thin film of a J-aggregating cyanine dye. The high absorption coefficient of the J-aggregate film, combined with the use of a reflective anode and optical spacer layer, enables an external quantum efficiency (EQE) of 16.1 $\pm $ 0.1{\%} ($\lambda $ = 756 nm) to be achieved at zero-bias in a device consisting of an 8.1 $\pm $ 0.3 nm-thick dye film. The specific detectivity (D*) and response speed (f$_{3dB})$ of the fully-optimized device are measured to be (4.3 $\pm $ 0.1)$\times $10$^{11}$ cm Hz$^{1/2}$ W$^{-1 }$and 91.5 kHz, respectively. Modeling of our structure reveals that the photocurrent is limited by the diffusion of photo-generated excitons to the metal oxide/J-aggregate hetero-interface and we determine the exciton diffusion length in the J-aggregate film to be L$_{D}$ = 2.0 $\pm $ 0.4 nm. This work provides insights relevant to the use of J-aggregating cyanine dyes in photodetector and photovoltaic applications and highlights the importance of engineering the optical field profile within such structures in order to maximize performance.   

Electrical Stability Tests of Polymer Light Emitting Devices

The degradation of polymer light emitting devices (PLEDs) is a main concern influencing the commercial production of functioning devices. There are various sources of degradation related to the polymer film, the interfaces, and the cathodes. The fundamental understanding of these processes helps to develop strategies for fabrication of devices with longer lifetimes. We are reporting on devices in which delamination of the metal cathode is the dominating degradation mechanism. We have performed stability tests at constant current and constant voltage accompanied by current-voltage characteristics. The results indicate initial improvement of functionality followed by degradation. The change in the current-voltage characteristics indicates modifications of the electron and hole transport through the polymer layer in addition to the delamination. The delamination appears only if the current is above a certain threshold value. We have studied the kinetics of the delamination which gradually increased with time. Several types of semitransparent anodes were used to clarify the origin of the observed delamination, e.g. gold, platinum, and ZnO:Al. The devices were also exposed to thermal stress tests to verify if the evolution of volatile molecules is involved in the observed degradation.   

Rational design of charge transport molecules for blue organic light emitting devices

The efficiency and stability of blue OLEDs continue to be the primary roadblock to developing organic solid-state white lighting as well as power efficient displays. It is generally accepted that such high quantum efficiency can be achieved with the use of organometallic phosphor doped OLEDs. The transport layers can be designed to increase the carrier density as a way to reduce the drive voltage. We have developed a comprehensive library of charge transporting molecules using combination of theoretical modeling and experimental evidence. Our work focuses on using chemical structure design and computational methods to develop host, transport, emitter, and blocking materials for high efficiency blue OLEDs, along with device architectures to take advantage of these new materials. Through chemical modification of materials we are able to influence both the charge balance and emission efficiency of OLEDs, and understand the influence of the location of photon emission in OLEDs as a function of minor chemical modifications of host and electron transport materials. Design rules, structure-property relationships and results from state of the art OLEDs will be presented.   

Transient Capacitance of Light-Emitting Electrochemical Cells

Although the steady-state behavior of light-emitting electrochemical cells (LECs) has been addressed theoretically, the transient properties of LECs have yet to be studied in detail. We present time- and frequency-dependent measurements of the capacitance, current, and optical emission of LECs as a constant voltage bias is applied and removed. We find that the capacitance increases more rapidly than the light or current and, unlike the light and current behavior, can be oscillatory and even negative at lower frequencies. Variable temperature experiments were performed to enable observation of a range of transient phenomena that cannot be fully explored at room temperature. The transient behavior suggests that the capacitance is determined by a combination of ion distribution, free carrier screening, and junction width. We interpret our data by qualitatively extending the ideas of existing steady-state theory.