Acceptor material

Figure 15-29. Chemical structures of the conjugated polymers used in the device and the device structure of the laminated solar cell. For the top half of the device, A1 or Ca was evaporated on glass substrates, and the acceptor material MEH-CN-PPV (and a small amount of POPT, usually 5%) was spin coaled. The half with the POPT (and a small amount of MEH-CN-PPV, usually 5%) was spin coaled on 1TO substrates and heated to 200"C under vacuum belore the device was laminated together by applying a light pressure.

A general problem with CaO, limestone and dolomite is the limited lifetime of the C02 acceptor material [32], The capacity for C02 is initially very high, but is depleted to almost zero after several cycles. Although the minerals are relatively cheap, this would imply a very considerable stream of waste material coming out of the hydrogen plant. Novel materials are in development with a higher stability [39]. 

Liu Z, Liu Q, Huang Y et al (2008) Organic photovoltaic devices based on a novel acceptor material graphene. Adv Mater 20 3924—3930... 

Fig. 4 Schematic illustration of the processes leading to photocurrent generation in organic solar cells, (a) Photon absorption in Step 1 leads to excitons that may diffuse in Step 2 to the donor/ acceptor (D/A) interface. Quenching of the exciton at the D/A interface in Step 3 leads to formation of the charge-transfer (CT) state. Note that processes analogous to Steps 1-3 may also occur in the acceptor material, (b) Charge separation in Step 4 leads to free polarons that are transported through the organic layers and collected at the electrodes in Steps 5 and 6, respectively, (c) The equilibria involved in Steps 1-4- strongly influence device efficiency...

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)... 

Several organics, e.g. pristine poly(3-octylthiophene), polyfluorene, bifunctional spiro compounds and polyphenyleneethynylene derivative, have been used for fabricating photOFETs. Responsivity as high as 0.5-1 A/W has been achieved in some of these transistors. We have already discussed the bulk heterojunction concept in Chapter 5. The bulk heterojunctions are fabricated using acceptor materials with high electron affinity (such as C<5o or soluble derivatives of C6o) mixed with conjugated polymers as electron donors. PhotOFETs based on conjugated polymer/fullerene blends are expected to show... 

In addition to the reqnirements on the energy levels of donor and acceptor for interfacial charge separation, it is necessary that the work function of the electroncollecting electrode shonld be well matched to the LUMO of the acceptor material, and that of the hole-collecting material be well matched to the HOMO of the donor. In bnlk heterojunctions, photovoltaic action can only be achieved if electronic contact is made to the photoactive material nsing snitable asymmetric electrodes. For high efficiency the electrodes shonld also be condnctive and well matched in energy to the electron or hole transport levels. The constraints on electrode materials will be treated in more detail in Sections 7.5 and 7.6.5. The development and optimisation of bilayer and BHJ device types are reviewed in Sections 7.3 and 7.4 respectively. 

Bulk heterojunction device performance has improved by strides since the first reports of charge separation in bulk heterojunctions, with power conversion efficiencies that now approach 5% (Li et al., 2005 Reyes-Reyes et al, 2005 Kim et al, 2006a). Smdies have focused on varying the donor and acceptor materials, optimising the... 

Once the charge carriers have been successfully separated, they need to be transported to the respective electrodes to provide an external direct ciu rent. Here the donor material serves to transport the holes whereas the electrons travel within the acceptor material. Thus, percolation paths for each type of charge carrier are required to ensure that the charge carriers will not experience the fate of recombination due to trapping in dead ends of isolated domains [60-62]. As such the bulk heterojunction has to consist of percolated, interpenetrating networks of the donor and acceptor phases. 

This limitation was finally overcome by the concept of the bulk heterojunction, where the donor and acceptor materials are intimately blended throughout the bulk [28-30]. In this way, excitons do not need to travel long distances to reach the donor/acceptor interface, and charge separation can take place throughout the whole depth of the photoactive layer. Thus the active zone extends throughout the volume, as illustrated in Fig. 11. Conse-... 

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. 

Various acceptor materials had been assessed on both these points previously in which the reaction cycle was simulated in batch equipment (2). Various stones were tested, and the following conclusions were tentatively reached. Limestones and dolomitic limestones were unsuitable, owing to a rapid deactivation. Many pure dolomites were not suitable because of poor physical strength. Of the dolomites tested that showed good physical strength, the stones from the Greenfield formation in Ohio showed the best resistance to chemical deactivation. 

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. 

Many new electron acceptor materials have been tested in the polymer BHJ blend system, including polymer polymer blend, polymerrCd, polymer oxide, i etc. However, currently they are not as effective as the polymer PCBM system. To further improve the performance of the promising polymer PF-co-DTB, we work on new fullerene derivative as electron acceptor materials. It is shown up to a large amount of (80wt.%) PCBM is required for optimal performance arising from a strong enhancement of... 

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