The ultrafast nonradiative bimolecular recombination of charge carriers was investigated in amorphous and microcrystalline silicon. The photoelectric method based on measurement of extraction time of the charge carrier reservoir in the space-charge-limited photocurrent mode was used. A series of models (Auger, diffusion-controlled, charge-carrier-fluctuation dominated) were experimentally tested but were not able to explain all features of bimolecular recombination in a-Si:H. The model of diffusion limited recombination taking into account the intrinsic random potential seems most suitable for a description of the observed suppressed bimolecular recombination.
Recently we presented a detailed theory of the space-charge-limited current (SCLC) transients generated by an instantaneous pulse of light, in which the realistic absorption profile has been taken into account. In this paper we present a theory that, in addition, takes into account the bimolecular recombination and the value of the detection resistor. This allowed us to propose a convenient method to study the fast recombination by electrical instead of the usual optical techniques. The method allowed us to present experimental results related to subnanosecond nonradiative bimolecular recombination in amorphous hydrogenated silicon (a-Si:H) and to discuss the microscopic origin of bimolecular recombination. The importance of understanding bimolecular recombination in a-Si:H is underscored by the fact that bimolecular recombination is the driving force of a-Si:H degradation.
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.
The transport properties of microcrystalline silicon, namely, mobility and conductivity, are investigated by a new method, for which the simple theory as well as numerical modeling is presented. The basic idea of the new method is verified on amorphous hydrogenated silicon by comparison with the widely used time-of-flight method. Contrary to time of flight, the new method can be used even for relatively conductive materials. Preliminary results on microcrystalline silicon clearly indicate the critical role of amorphouslike tissue in transport in microcrystalline silicon.
A simple theoretical model explaining the increase of X-ray sensitivity caused by adding tungsten nanoparticles into thin layers of organic materials is proposed. The mentioned increase of sensitivity is caused by quenched electron multiplication due to secondary electron emission from tungsten particles. After some simplifying assumptions, an expression of the electron multiplication factor K is derived for the case when tungsten atoms are uniformly mixed with the matrix material. The main assumption of the model is the existence of a threshold energy E min of the order of 0.1 eV, below which the recombination of charge carriers prevents them from being accelerated by the electric field to energies sufficient for impact ionization. It is shown that this assumption makes the increase of K and photocurrent with increasing electric field much slower than the exponential increase commonly associated with an electron avalanche, and K may even start to decrease when the electric field strength exceeds a certain value. Another factor, which has an adverse effect on the X-ray sensitivity, is the ionization energy loss of photoelectrons inside metallic nanoparticles. The results of Monte Carlo simulations show that in the case of spherical tungsten particles with 0.8 μm diameter, the latter phenomenon may cause an additional decrease of the sensitivity by as much as 75%. In order to reduce this effect, the size of nanoparticles should be reduced, or, alternatively, most of the photoelectrons should be generated in the organic matrix rather than inside the nanoparticles.
For the first time, a blend of carbazolyl‐containing small molecules and tungsten particles for X‐ray‐sensitive layers is developed using the solvent‐free melt spin‐coating (MSC) method. The composite films fabricated on a glass substrate, using aluminum as electrodes, are applied for direct current conversion of X‐rays with high signal‐to‐noise ratio, reliable on–off switching characteristics, and short‐ and long‐term stability. To elucidate the processes of charge carrier generation by X‐rays, Monte Carlo simulations of X‐ray‐induced current are performed. The response and charge transfer processes investigated by an X‐ray pulse experiment indicate possibilities for real‐time detection.
We investigated an open ionization cell based on the Geiger-Müller counter principle in a gas mixture at atmospheric pressure and demonstrated that the photoemission signals as weak as 1 electron per second are detectable. This finding allowed us to investigate more accurately the photoemission spectrums, especially in the vicinity of the photoemission threshold. Using such a cell, we investigated a number of organic semiconductor materials, tested various ways to analyze the results of the measurements of photoemission spectrums, and demonstrated an efficient way to determine ionization potential by using the square root of the derivative of the yield dependence on the light quanta energy (dY1/d(hν))1/2. This method leads to more evident graphical representation of the measurement results and better Ip estimation in comparison to the results estimated by using the traditional method of plotting Y1/n dependence on the quanta energy hν.
In μc-Si:H/a-Si:H superlattices, using time-of-flight (TOF), photo and equilibrium charge carrier extraction by linearly increasing voltage (CELIV) methods the mobility, bulk conductivity, equilibrium carrier density and recombination transients have been investigated. It was obtained that the interfaces between μc-Si:H and a-Si:H layers have no influence on charge carrier transport, and the conductivity, equilibrium charge carrier density and lifetime of such superlattice are substantially higher than those of a single a-Si:H layer. Obtained experimental results are promising for application of μc-Si:H/a-Si:H superlattices in solar energetics. The superlattice consisting of approximately 80 % of microcrystalline substance is optimal for these purposes.
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