Acceptor materials, properties

In a penalty test, a property cf the system is modified to reduce the probability of the desired result. For example, to predict safety, a particular expl train interface may be tested with a standard donor and a more sensitive acceptor conversely, to predict reliability, a less sensitive acceptor material is used. If this probability is reduced sufficiently, it is possible to obtain mixed responses (that is, some fires and some no-fires) with samples of reasonable size, and to develop data from which the mean value of the penalty and its standard deviation (as well as confidence limits) can be established. These estimates can be used iri statistical extrapolation to estimate safety or reliability under the original design conditions. The term VARICOMP (VARIation of explosive COMPosition) was coined by J.N. Ayres for a method developed at the Naval Ordnance Lab, White Oak, in the 1950 s and early 1960 s (Ref 1)... 

If the electrodes match the lowest unoccupied molecular orbital (LUMO) of the acceptor and the highest occupied (HOMO) level of the donor, respectively, the contacts can be regarded as ohmic. The maximum Voc for this case is schematically indicated by yoci in Fig. 5.13 and is thus controlled by the bulk active layer material properties. Non-ohmic contacts, as shown in Fig. 5.13, a Voc with magnitude V0c2 should be observed, according to the MIM model. However if the Fermi level of the contact metal is pinned at the LUMO and of the anode with the HOMO, the observed F0c by the properties of the acceptor and the donor and will become insensitive to the work function difference of the electrodes. 

The diode quality factor n of type A devices decreases as a function of substrate temperature in Figure 2a from 2.4-2.6 at Tsubstrate = 131°C or Tsubstrate = 187°C to 1.5 at Tsubstrate = 151°C. The behavior of the series resistance (Rs) with temperature is shown in Fig. 2b. The minimum Rs = 0.27 Q x cm2 is achieved for type B OSCs at Tsubstrate = 148°C. The enhancement of the devices PV and diode parameters with the temperature up to 150°C can be explained by (i) an improved separation of the CuPc and Cm donor and acceptor materials in an interpenetrated absorber network, (ii) enhanced crystalline perfection of the CuPc domains [4] and therefore improved transport properties, i.e., better collection efficiency of photogenerated carriers at the respective electrode. Alteration of the photoelectrical parameters at higher temperatures can be attributed to the potential degradation of the PEDOT buffer layer. 

If instead, indium is the impurity in the silicon crystal structure, the opposite effect is produced. Such material contains a number of energy levels only 0.06 eV above the valence band the result is holes in the val ce bands. Such material is referred to as acceptor material. Silicon with acceptor material is called p-type silicon, since the holes are considered to be positively charged. Conduction is in this case by movement of holes. The addition of controlled amounts of impurity atoms thus provide charge carriers (as the electrons and holes collectively are called, c.f. 8.2) and produces the desired properties in semiconductor materials. 

Our calculations also suggest that the redox properties of both donor and acceptor materials undergo considerable modifications. For example, both occupied and unoccupied MOs of TTF experience an increase in their energy in a TTF/TCNQ dimer with respect to the MOs of the isolated TTF molecule (Fig. 2). On the contrary, the MOs of TCNQ experience an increase in their energy in the TTF/TCNQ dimer. 

Rand, B. R, Xue, J., Uchida, S., and Forrest, S. R. 2005. Mixed donor-acceptor molecular heterojunctions for photovoltaic applications. I. Material properties. Journal of Applied Physics 98 (12) 124902. 

It has been demonstrated. In the case of TCNQ-based materials that vibrational spectroscopy Is a useful tool In studying the state of the acceptor species, the knowledge of which is essential In understanding the materials properties (15). In this paper we wish to report on the Raman, infrared and uv-vis spectra of (2,5-DM-DCNQOgM (M=Cu, Ag, Na) materials. Thin film formability of the Cu- and Ag-com-pounds has been also investigated,and preliminary results are reported here. 

The current-voltage characteristics under white illumination exhibit a dramatic increase in short-circuit current density and open-circuit voltage for a 30% I1O2 content. There the best dispersion of the Ti02 particles takes place. It has been concluded that the photovoltaic properties of these nanocomposites are controlled by the interfacial area between the donor and the acceptor material and further are Umited by the dispersion of the nanoparticles in the polymer [210]. 

In addition to performance evaluations, many photoelectrochemical experiments are aimed to identify performance-Umiting steps or to determine certain materials properties. Examples of the latter are donor or acceptor densities and the flatband potential of a material, which can be determined by electrochemical impedance measurements. The challenge with these measurements is that they always yield data, but that it can be difficult - and sometimes even impossible - to translate the measured data to the desired materials parameters. Carefully performed control experiments and a good basic understanding of the measurement equipment -in particular, the potentiostat and the frequency response analyzer (FRA) - are essential for obtaining meaningful results. 

We shall consider next the theory of condition 2 for an extrinsic photoconductor, then the theory of G RA for condition 3, and finally the dependence of t on material parameters. We shall analyze the geometrical model of Fig. 4.8 and assume a simple energy level model of an n-type extrinsic semiconductor consisting of a photoionizable donor level and a compensating acceptor level properties of a corresponding p-type model would be analogous. This material is extrinsic as both a photoconductor and semiconductor. 

Due to the successful use of PCBM as acceptor material in solar cells, various attempts have been made to link Ceo covalently to an oligothiophene backbone in order to prepare D-A-based materials for photovoltaic devices by tuning their electronic properties [45, 46, 200, 201]. The attachment of fullerene units prevented large-scale phase separation in thin films and, despite the inclusion of bulky fullerene units, the soluble polymers retained their high order in thin films. However, the efficiency of such devices has been limited by competition between photoinduced electron transfer and energy transfer which occurs from the donor component to the fullerene. 

Colloidal approaches also frequently accoimt for van der Waals interactions, ie interactions due to fluctuating dipoles. For atomic species, these interactions vary as distance to the minus sixth power. For protein/surface systems modeled via a colloidal description, this 1/r dependence is integrated over the volumes of the interacting bodies. The result is the product of a Hamaker constant, which depends upon material properties, and a term dependent on the system s geometry. In addition, forces related to solvation (114) and donor/acceptor (115) affects may also be included. 

The use of quantum-chemical (electronic-structure) methods has proved to be a powerful complement for designing new molecular/polymer-based materials and for understanding their performance in OP Vs [100]. Our goal in this contribution wOl be to discuss the application of these techniques to study the intrinsic electronic and optical materials properties and the broader processes involved in OPV operation. We will first outline the current understanding of the complex processes involved in OPV operation. We will then consider how these processes dictate the materials design process and why the donor-acceptor (DA) architecture, with a particular emphasis on DA copolymers used as HTMs, is an appealing way to meet the materials needs. This will be followed by an overview of how electronic-stmcture methods can be employed in the study of these materials. We will conclude with an outlook for future investigations. 

Neuteboom EE, Meskers SCJ, Van Hal PA, Van Duien JKJ, Meijer EW, Janssen RAJ, Dupin H, Pourtois G, Comil J, Lazzaroni R, Bredas J-L, Beljonne D (2003) Alternating oligo(p-phenylene vinylene)-perylene bisimide copolymers synthesis, photophysics, and photovoltaic properties of a new class of donor-acceptor materials. J Am Chem Soc 125 8625... 

Springborg and Kirtman have investigated the impact of donor/acceptor substitutions on the linear and nonlinear response properties of long polyenes and have demonstrated that, for sufficiently long chains, the responses per unit of a push-pull system becomes independent of the donor and acceptor groups. This conclusion, which concerns both the electronic and structural/geometrical relaxations, implies that, in that case, material properties cannot be improved upon substitution. Their predictions have been further illustrated and analyzed by adopting a Huckel like model. 

In our further considerations we start from the assumption that the geometrical and physical parameters of the photodetector are already defined. Geometrical parameters include the area and the thickness of the active region, i.e., its total volume. Physical parameters are the detector material properties. These include optical properties (complex refractive index, determined by the material type and composition), and electrical ones (donor and acceptor concentration, i.e., the concentrations of majority and minority carriers at a given temperature). 

---