K. Arlauskas

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.

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.

Effect of 2-D Delocalization on Charge Transport and Recombination in Bulk-Heterojunction Solar Cells

Charge-carrier transport and recombination in thermally treated and untreated films of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)propyl-l-phenyl-[6,6]-methanofullerene (PCBM) bulk-heterojunction solar cells (BHSCs) have been measured using various electrooptical techniques. The formation of lamellar structure in P3HT has a large effect on the efficiency, carrier transport, and recombination of photogenerated charge carriers. Treated P3HT/PCBM solar cells show greatly reduced carrier recombination compared to what is typically expected in low-mobility materials and electric-field-independent carrier generation. In untreated films, the recombination is close to Langevin-type with electric-field-dependent quantum efficiency, consistent with the typically observed Onsager-type generation. Furthermore, we observe an increased effective capacitance in treated films, consistent with increased charge screening. The importance of the interface between the lamellar structured P3HT and PCBM is evident from optical spectroscopies showing that 2-D polarons are directly generated using sub-gap excitation. We conclude that the formation of lamellar structures in the polymer donor, and subsequent, derealization of the charges is favorable for making efficient BHSCs.

Features of carriers at very high electric fields in a-Se and a-Si:H

Abstract Transient photocurrent, carrier mobility and quantum efficiency have been studied by time-o of flight technique. The dependence of hot carrier energy on electric field was evaluated from impact ionization phenomena. It was concluded that in a-Se disactivation of the hole drift mobility was due to the sharp field dependence of capture time into shallow traps.

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.

Electric field heated electrons in a-Si:H — new features

Abstract The subnanosecond time-of-flight measurement at very high electric field (≤ 0.55 MV/cm) and at high temperatures (300 K

Separation of fast and slow transport in regiorandom poly(3-hexylthiophene)

Transport properties of regiorandom poly(3-hexyl thiophene) (RRaPHT) were investigated using integral mode time-of-flight (TOF), and novel extraction photo current transient (photo- CELIV) methods. Time dependence of the charge carrier mobility was observed. We measured a mobility μ ∼ 10 -5 - 10 -6 cm 2 /Vs in long time scales, with the photogenerated carrier lifetime τ = 1.5ms. From the inital tranport distauce we measured the fast mobility-lifeting product μ f τ f = 2 × 10 -11 cm 2 /V, from which we estimated μ f ∼ 0.2cm 2 /Vs by using the previously reported lifetime of the fast transport as τ f ≃ 100ps [G, Yu, et al., Phys. Rev. B42. 3004(1990)]. The time dependence of the charge carrier mobility is obtained over several orders of magnitude.

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.