G. Juška

Charge transport and recombination in bulk heterojunction solar cells studied by the photoinduced charge extraction in linearly increasing voltage technique

Charge carrier mobility and recombination in a bulk heterojunction solar cell based on the mixture of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-phenylene vinylene] (MDMO-PPV) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)-C61 (PCBM) has been studied using the novel technique of photoinduced charge carrier extraction in a linearly increasing voltage (Photo-CELIV). In this technique, charge carriers are photogenerated by a short laser flash, and extracted under a reverse bias voltage ramp after an adjustable delay time (tdel). The Photo-CELIV mobility at room temperature is found to be μ=2×10−4cm2V−1s−1, which is almost independent on charge carrier density, but slightly dependent on tdel. Furthermore, determination of charge carrier lifetime and demonstration of an electric field dependent mobility is presented.

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.

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.

Time-of-flight measurements in thin films of regioregular poly 3-hexyl / thiophene

We have studied the transport of holes in thin films of regioregular poly(3-hexyl thiophene) (RRPHT). In comparison to regiorandom poly(3-hexyl thiophene) (PHT), RRPHT is a promising material for electronic devices, due to the high field effect mobility observed. We have studied the photogenerated charge carrier transport by using the integral time-of-flight (TOF) method. We have also studied the transport properties of the equilibrium charge carriers by a newly developed method of equilibrium charge carrier extraction in the case of a linearly increasing voltage. From TOF measurements in the subnanosecond time scale, it was experimentally obtained that the drift distance of holes is smaller than the inter-electrode distance in both PHT and RRPHT films and the mobility is higher than 10−2 cm2/Vs. In the millisecond time scale we obtained additional low mobility of equilibrium holes in RRPHT.

Transport study of self‐supporting porous silicon

We have measured dark DC conductivity and time‐of‐flight (TOF) of carriers in self‐supporting porous silicon films in the temperature range 298–480 K. The dark I‐V curves show superlinear behavior with activation energies of 0.38–0.67 eV. The TOF measurements allowed us to evaluate the drift‐length of non‐equilibrium carriers and revealed a significant decrease of the collected charge with increasing delay (tdel≥1 ms) of the exciting 3 ns laser pulse after the voltage application, probably due to field redistribution in the Si crystallites.

New features in space-charge-limited-photocurrent transients

Abstract We present a theory of space-charge limited photocurrent transients generated by an instantaneous light pulse, in which the realistic absorption profile of the photogenerated charge is taken into account. This theory allows us to explain the following features, all observed experimentally but not predicted by the standard space-charge-limited-current theory: (a) the appearance of an initial current spike related to the screening of the electric field in the generation region; (b) a stronger light intensity dependence of the transit time and current caused by a decrease in the effective sample thickness; (c) the appearance of another kink in the space-charge-limited photocurrent transient related to the full charge reservoir extraction, which may be useful for investigation of the recombination losses in the generation region of, for example, hydrogenated amorphous silicon solar cells.

Charge transport in π-conjugated polymers from extraction current transients

Abstract For the study of the charge transport mechanism of equilibrium charge carriers in π-conjugated polymers we use a novel, still technically simple method, namely charge extraction in a linearly increasing voltage (CELIV). We briefly outline the method and use it to study the temperature and electric field dependence of the mobility in regioregular poly(3-octylthiophene). We find that mobility can be explained using the well-known Poole–Frenkel type of law μ∝μ 0 ( T )exp[B(T)√E], with an activated zero-field mobility. We also find that the nature of the mobility is determined by the release time from traps in regioregular poly(3-hexylthiophene).

New method of drift mobility evaluation in μc-Si:H, basic idea and comparison with time-of-flight

We illustrate problems of measurement of the drift mobility by time-of- flight method on microcrystalline silicon. A new method, based on charge extraction in linearly increasing voltage, is introduced. Simple theory as well as numerical modelling of this new method are presented together with the first experimental results on microcrystalline silicon.

SCLC transients in a-Si:H — New features and possibilities

The new possibilities and advantages of SCLC limited TOF transients are demonstrated. First, the effects related with the realistic absorption profile, screening current and effective thickness are used to explain the shift of the “cusp” position with the increasing laser pulse intensity. The model of a full charge extraction is used to explain the observed “extraction” time (t*) and exemplified by c-Si diode. The first results on a-Si:H p-i-n junctions together with the corresponding theory are presented.

Subnanosecond bimolecular non-radiative recombination in a-Si:H

Abstract We have introduced a new method of the study of fast recombination in a-Si:H, based on the measurement of the extraction time of the charge carrier reservoir generated by a short intensive light pulse. We demonstrate the advantages of the integral mode of the measurement of these photo-SCLC transients. We have found a clear transition from monomolecular to bimolecular recombination at a generation rate of about 10 18 cm −3 . The deduced coefficient of the bimolecular recombination is smaller than the same coefficient found from optical pump and probe measurements. to explain the experimental results we propose a model based on the Auger recombination of two free and one trapped charge carriers.