Time carrier transport

Another important problem of the photoconductivity of poly-N-vinylcarbazole is the carrier transport. With the so-called time of flight method this problem is well investigated. 

Figure 3.4 Typical transient response characteristic for hole carrier transport in a-As2Sc3 at room temperature. The behavior is displayed with both linear (a) and logarithmic (b) axes of current and time.

In this section, we propose that the transit time of transport photocarriers can be obtained by an analysis of transient current in a double layer that consists of a thin chalcogenide nnder test and another material with higher mobility snch as a-Se. Figure 4.19(a) shows the type of carrier transit pulse that is frequently enconntered in the study of amorphous semiconductors. 

One of principal causes of increase of P-conductivity in MF can be magnetosensitive non-equilibrium processes connected with charges carriers transport or change of intensity of capture (or release) by traps of electrons and holes. High times of increase and decrease of P-conductivity confirm the given assumption, indicating the contribution of defect structure to P-conductivity of C6o single crystal in MF. 

For reasonable functioning of these low cost, low mobility semiconductor solar cells, a considerable amount of the photogenerated chemical potential epc — fv of the electron hole ensemble must be used for carrier transport. An acceptable charge collection may be achieved if the extraction times for electrons and/or holes are smaller than their recombination lifetimes, i.e.,... 

Figure 92 Typical transient current pulses for holes in amorphous selenium (see Ref. 422a), illustrating the effect of temperature on the degree of the dispersion in carrier transport. Left linear current (i) and time (t) axes right normalized values in logarithmic axes log(i/io) vs. log(t/t0). The arrows indicate the position of the knee dividing the two regimes of logarithmic dependence. Similar behavior can be observed in organic solids (see e.g. Ref. 422b).

Mort and Lakatos (1970), Mort (1972), and Mort et al. (1972) reported hole mobilities in PVK of 10-7 cm2/Vs at 1.0 x 105 V/cm. The mobilities were field dependent and described by trap-free lifetimes of the order of 10-1 s The activation energy was 0.50 eV. It was reported that the width of the transport band was less than 0.10 eV. Pai (1970) described hole transport in PVK via a transit time with a logit)-1 pE1/2 field dependence. The temperature dependence was described by a field-independent activation energy of 0.36 eV. It was suggested that the field dependence of the transit time was due to the field dependence associated with the time carriers spend in localized states. Bulk trapping was reported for low fields. Mort and Lakatos (1970) and Pai (1970) are the first literature reference to dispersive transport in polymers. 

Kinetically, the overall dissolution process consists of carrier transport in the semiconductor, electrochemical reactions at the interface, and mass transport of the reactants and reaction products in the electrolyte. Also, toe are a number of reactions involved at the interface and these reactions consist of several steps and subreactions. At any given time the dissolution kinetics can be controlled by any one or several of these steps. The distribution of reactions along a pore bottom under a steady-state condition during pore propagation must be such that pore walls are relatively less active than the pore tip. Then, the dissolution reactions are concentrated at the pore tip resulting in the preferential dissolution and formation of pores. The formation of pores is the consequence of spatially and temporally distributed reactions. 

Diffusion length (or lifetime) is a key parameter for the performance of solar cells. It is usually admitted that diffusion length of minority carriers has to be four times the thickness of the film to assure good photovoltaic efficiencies. At 1,050°C, appropriated values of 136 and 120 pm were obtained with In and Sn, respectively. The lower performance of epilayer grown from Sn melt can be explained by its high solid solubility (5 x 1019 cm 3 at 1,050°C). Incorporation of Sn atoms within the Si crystal could create a large stress and affect carrier transport. It is clearly related to the defects density of epitaxial film (measured by SECCO etching). 

Peak photocurrents excited In a polymer of bis ( -toluene-sulfonate) of 2,4-hexadlyne-l,6-dlol (PTS) by N2-laser pulses vary superquadratically with electric field. The ratio ip(E)/((i(E), where ()i denotes the carrier generation efficiency, increases linearly with field. This indicates that on a 10 ns scale the carrier drift velocity is a linear function of E. Information on carrier transport kinetics in the time domain of barrier controlled motion is inferred from the rise time of photocurrents excited by rectangular pulses of A88 nm light. The intensity dependence of the rate constant for carrier relaxation indicates efficient interaction between barrier-localized carriers and chain excitons promoting barrier crossing. 

The challenging information of this contribution is that in the short time limit a PTS crystal behaves like a conventional semiconductor as far as carrier transport is concerned. Clearly, more work is required, notably, quantitative assessments. However, for reasons of conclstency, the drift velocities measured at highest fields employed in this study cannot exceed those measured earlier (4). This yields a mobility of order 10 cm (Vs)"l, A similar estimates results from the schubweg travelled by a carrier before immobilized by a large barrier. It has to be shorter than the distance travelled between localization events (-10 pm). The conclusion is that carrier motion is polaronic as described by Cade and Mova-ghar (27). 

From the above, it can be appreciated that the details of the initial carrier distributions subsequent to field acceleration are important for fully optimising the energy conversion process. The most detailed study of the field acceleration process was conducted through a series of real-time measurements of the carrier transport directly in the space-charge region (Min and Miller, 1989 and 1990). From Poisson s equation for the space-charge field 6... 

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