G. Sliaužys

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.

Double injection as a technique to study charge carrier transport and recombination in bulk-heterojunction solar cells

Ambipolar charge carrier mobility and recombination in bulk-heterojunction solar cells based on the mixture of regioregular poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM) has been studied using injection current transients. The experimental results demonstrate double injection with bimolecular recombination limiting the injection current. We found that charge carrier bimolecular recombination is significantly reduced compared to Langevin recombination. We have measured the temperature and electric field dependence of the reduced bimolecular recombination coefficient and the results suggest that the electron and hole pathways are different and the recombination is controlled by the probability of the carriers to meet at the polymer/PCBM interface.

Double-injection current transients as a way of measuring transport in insulating organic films

We propose a double-injection current transient technique for the study of charge-carrier transport in thin insulating films of low-mobility materials with reduced carrier bimolecular recombination compared to the Langevin type. This experimentally simple technique, allows us to estimate the sum of the faster carrier and the slower carrier mobility’s (μf+μs) and the slower carrier mobility (μs). Furthermore, in thin films when the RC current overlaps the injection current transients we propose to estimate these transport parameters using the extracted charge as a function of injection pulse duration. The method is applied on bulk-heterojunction solar cells made from blends of regioregular poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene. We have experimentally verified the technique by measuring the charge carrier mobility’s and compared them with results obtained using standard time-of-flight and carrier extraction using linearly increasing voltage techniques.

TiO2-PHT interface influence to charge carrier photo generation and recombination

In this paper the influence of TiO2/polymer (RR-P3HT) interface to photo generated charge carriers separation and recombination have been studied. The quantum efficiency and charge carrier recombination were studied in heterojunctions of TiO2 (prepared with porous or flat interface) and spin coated RR-P3HT polymer film using time of flight (TOF) and double injection (DoI) transient methods. From the shape of the TOF transients it was determined that the charge carrier generation is taking place in the vicinity of the TiO2/P3HT interface. Samples with porous TiO2 films show better charge carrier separation, but the deep trapping of charge carriers at the interface appear. The charge carrier recombination is trimolecular (Auger type recombination) with the similar values of recombination coefficients in both types of samples.

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.