B. Romero

High-Bandwidth Organic Photodetector Analyzed by Impedance Spectroscopy

An organic bulk heterojunction photodetector (OPD) is fabricated on the basis of a blend of poly(3-hexyl thiophene):1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C61. The OPD responsivity is calculated from current density-voltage characteristic (J-V) under green-LED illumination obtaining 1 A/W. Frequency response at different biases is measured, which shows a high-3-dB cutoff frequency of 343 kHz at -3 V . Impedance measurements are carried out at different reverse biases, and the results are fitted with a small signal equivalent circuit. The device cutoff frequency (fc) can be estimated with the parameters extracted from this circuit obtaining good agreement between measured and calculated (fc).

Bis(pyridylpyrazolate)platinum(II): a mechanochromic complex useful as a dopant for colour-tunable polymer OLEDs

The photoluminescence and mechanochromic behaviour of the bis(3-(3,5-bis(dodecyloxy)phenyl)-(5-pyridin-2-yl)pyrazolate)platinum(II) complex PT12, selected from a series of Pt(II) compounds with N,N′-pyridylpyrazolate ligands, has been investigated. The electroluminescence properties of polymer OLEDs based on PT12-doped polyfluorene (PFO) are also described. Addition of PT12 induces the formation of the PFO β-phase, a much more ordered phase with enhanced colour stability at high bias and high photoluminescence quantum efficiency. Additionally, the characteristic blue emission of PFO is red-shifted for PT12 concentrations equal to or higher than 3%. Depending on the dopant concentration and the applied current, the colour coordinates of these polymer OLEDs undergo a broad shift in the CIE colour space. In addition, through the strategic use of keto defect sites, white-emitting devices can be fabricated with a very small addition of PT12.

Extraction of circuital parameters of organic solar cells using the exact solution based on Lambert W-function

The electrical behavior of organic solar cell (OSC) has been analyzed using a simple circuital model consisting on an ideal diode together with a series and parallel resistances (RS and RP respectively). Applying Kirchhoffs Laws to the circuit leads to a transcendental equation that can be solved numerically without approximations using the Lambert W function. Theoretical expression has been fitted to experimental current-voltage (I-V) curves under forward bias, obtaining fairly accurate values for the electrical parameters. This model has been validated comparing the extracted parameters for dark and illumination conditions of different devices. Results show good agreement for RS, and ideality factor (η). Electrical parameters obtained in this work are also compared to those ones extracted using an approximated method often employed by other authors 1. We conclude that approximated method leads to reasonable good values for RS, RP and η. However, in the case of Rp the voltage range chosen to fit the data with the exact method must be constrained to the fourth quadrant, where the role of parallel resistance is more critical. To validate the model, a bunch of organic solar cells with structure ITO/ poly(3,4-ethylenedioxythiophene)-poly (4-styrene sulfonate (PEDOT:PSS)/ poly(3-hexylthiophene) (P3HT): 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM)/Al has been fabricated in inert atmosphere. Different active layers were deposited varying the P3HT:PCBM ratio (1:0.64, 1:1, 1:1.55) and the active layer thickness (ranging from 100 to 280 nm). Devices are encapsulated inside the glove-box prior its characterization outside the glove-box. Electro optical characterization has been performed with a halogen lamp. Values extracted for RS range from 142 Ω to 273 Ω, values for RP range from 25 kΩ to 331 kΩ. Ideality factor ranges from 5 to 17.

P-172: Determination of Hole Mobilities in New Blue Emitting Organic Diodes by Means of Impedance Spectroscopy

In this work we have fabricated passive diode matrices based on a new blue emitting organic electroluminescent polymer. In order to determine the capacitive response at typical display operating frequencies, impedance vs frequency measurements have been carried out at different bias. In addition, hole mobilities have been extracted by fitting a new theoretical model to the measured admittance, that takes into account the effect of an injection barrier, obtaining values from 10−12 m2/Vs to 10−10 m2/Vs with increasing bias.

Influence of cathode in organic solar cells performance

In this work we have fabricated and characterized organic solar cells (OSC) based on blends of polyhexylthiophene (P3HT) and [6,6]-phenil-C61-butyric acid methyl ester (PCBM) with two different cathodes: Al and LiF/Al. Current- Voltage (I-V) curves under illumination have been measured and fitted with the standard equivalent circuit in order to extract circuital parameters, such as series and parallel resistances (Rs and Rp) ideality factor (n) and inverse saturation current (I0). The introduction of a thin layer (0.5 nm) of LiF between active layer and Al increases the open circuit voltage (Voc) but slightly reduces short current (Isc) and fill factor (FF). The overall result is a slight increase of the efficiency, from 2.53 % to 2.57 %. Circuital parameters have been extracted with two methods (approximated an exact), and results show that structures with LiF layer have lower parasitic effects (lower Rs and higher Rp).