N. Nekrašas

Trimolecular recombination in polythiophene: fullerene bulk heterojunction solar cells

In this work, reduced Langevin recombination in poly(3-hexylthiophene): 1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61 blends has been studied with various methods. Using photogenerated charge carrier extraction by linearly increasing voltage technique, it was observed that charge carrier recombination transient corresponds to trimolecular recombination. By double injection current transient technique, it was shown that the influence of deep charge carrier trapping is insignificant and that trimolecular recombination coefficient γ is independent of the electric field. Finally, the temperature dependence of γ was measured by integral time-of-flight method.

Two dimensional Langevin recombination in regioregular poly(3-hexylthiophene)

In this work, it is shown that recombination in regioregular poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (RRP3HT:PCBM) bulk-heterojunction solar cells is caused by the two dimensional (2D) Langevin recombination in the lamellar structures of RRP3HT, which are formed after annealing process. Due to 2D Langevin process, bimolecular recombination coefficient is reduced in comparison with three dimensional Langevin case, and bimolecular recombination coefficient depends on the density of charge carriers n1/2. Data obtained from the different experimental techniques (charge extraction with linearly increasing voltage, integral time of flight, double injection current transients and transient absorption spectroscopy) confirms 2D Langevin recombination in RR3PHT.

Hole drift mobility in μc-Si:H

In microcrystalline hydrogenated silicon (μc-Si:H), the drift mobility dependencies of holes on electric field and temperature have been measured by using a method of equilibrium charge extraction by linearly increasing voltage. At room temperature the estimated value of the drift mobility of holes is much lower than in crystalline silicon and slightly higher than in amorphous hydrogenated silicon (a-Si:H). In the case of stochastic transport of charge carriers with energetically distributed localized states, the numerical model of this method gives insight into the mobility dependence on electric field. From the numerical modeling and experimental measurement results, it follows that the hole drift mobility dependence on electric field is predetermined by electric field stimulated release from localized states.

Current transients in organic field effect transistors

In this work the detailed charging and extraction current transients in the channel of organic field effect transistors are presented. This allows for precise estimation of the charge carrier mobility along the channel and in the direction perpendicular to the layer. The charge carrier mobility is experimentally obtained by various methods: from the extraction transients (in small charge and space charge limited current modes) as well as from the duration of the gate charging. The results are in good agreement with the results obtained by steady state current, photogenerated carrier extraction by linearly increasing voltage, and time-of-flight.

Features of charge carrier transport determined from carrier extraction current in μc-Si:H

Temperature and electric field dependencies of mobility and concentration transients of electrons and holes using modified charge extraction by the linearly increasing voltage (CELIV) method in slightly doped n-type, p-type and undoped microcrystalline silicon (μc-Si:H) have been investigated. The results indicates that: the mobility of majority carriers causes temperature and electric field dependencies of conductivity; the photoconductivity transient is mainly determined by transient of charge carrier concentration; at room temperature the charge carrier transport is controlled by multiple trapping to energetic distributed localised states; at lower temperature the features characteristic of hopping transport have been obtained.

The determination of charge carrier mobility from the current transients in organic field effect transistor

In this paper, we present features of current transients in organic field effect transistors obtained by the computer simulation and experimentally. We analyse the influence of the charge carrier reservoir at the organic semiconductor-insulator interface and the load resistance on the current transients. We demonstrate how to determine the amount of charge carriers in the reservoir and how to take it into account when calculating the mobility values estimated from current transients. Such correction reduces the estimated mobility values and changes mobility dependence on the electric field.

Plasma injection as techniques to study charge carrier transport and recombination in organic solar cells and LED's

We propose a double injection current transient technique to study the charge carrier mobilities and recombination in thin films of bulk heterojunction structures. This experimentally simple technique allows estimation of bimolecular recombination coefficient and charge carrier mobility, either of a sum of electron and hole mobilities in double injection into insulator mode, or the ambipolar mobility in semiconductor mode. The double injection current transient technique is used to study RRPHT/PCBM bulk heterojunction solar cells, and the obtained results are compared with results obtained using TOF and CELIV techniques.