Category SOLAR CELLS – NEW ASPECTS AND SOLUTIONS
Acetylene-substituted PPV derivatives can be synthesized via the Wittig – Horner Reaction (Figure 15). The coplanar and rigid nature of the acetylene moiety in the polymer chain may have the potential to obtain a higher degree of packing and thus improve the photovoltaic performance of such devices. Having coplanar electron-rich anthracene units and triple bond bridges, P18 exhibits broader absorption, a lower HOMO level, and a smaller optical band gap of 1.9 eV, compared to MDMO-PPV. A device with the configuration of ITO/PEDOT/P18:PCBM (1:2, w/w)/LiF/Al, achieved a PCE value of up to 2% with a high Voc of 0.81 V. Figure 16 shows the chemical structures of a series of acetylene-substituted PPV derivatives synthesized by similar procedures. For polymers P21 and P22 (Egbe et al...Read More
Cyano-substituted poly(p-phenylene-vinylene)s (CN-PPV) with electron deficient cyano groups on the vinyl units are synthesized by Knoevenagel polycondensation polymerization of terephthaldehyde and 1,4-bis(cyanomethyl)benzene in the presence of the base f-BuOK (Figure 12). Hence, the LUMO and HOMO levels of PPV derivatives can also be tuned by incorporating electronic substituent into the vinylene bridges (Cheng et al., 2009).
CN-PPVs show high electron affinity to reduce the barrier to electron injection and good electron-transport properties as a result of the electron-withdrawing effect of the cyano side group and suitable electron acceptors in organic photovoltaic devices (Granstrom et al., 1998; Halls et al., 1995; Gupta et al., 2007)...Read More
Poly(p-phenylene-vinylene)s (PPVs) and its derivatives are one of the most promising classes of conjugated polymers for organic solar cells due to their ease of processability as
well as tunability of their optical and electronic properties through chemical modifications. Since the first report of electroluminescence from PPV, a great research attention has been focused on these types conjugated polymers (Burroughes et al., 1990). This focus was moreover up heaved after the discovery of an ultrafast photoinduced charge transfer from alkoxy-substituted PPV to the buckminsterfullerene (Sariciftci et al., 1992)...Read More
Conjugated polymer thin films sandwiched between two metal electrode are usually described using a metal-insulator-metal (MIM) picture (Parker, 1994). The different operating regimes the MIM device due to externally applied voltages is shown in Figure 3. As illustrated in Figure 3(a), the vacuum levels (Evac) of the stacked materials shall align themselves (Shottky-Mott model).
Figure 3(a) indicates the energy diagram of a bulk heterojunction solar cell in open circuit condition. The Evac of the different materials are aligned as explained above, and no electrical field is present within the device. Figure 3 (b) represents the short circuit condition...Read More
The sequential process involved in the light into electricity conversion can be summarized by the following steps: First, incident light is absorbed within the photoactive layer leading to the created of a bound electron-hole pairs (singlet excitons); the created excitons start to diffuse within the donor phase leading to charge separation; the separated charge carriers are transported to the corresponding electrodes.
Fig. 1. (a) Schematic device structure and (b) energy diagram for an organic bilayer solar cell
Figure 1 (a) shows the simplest structure of an organic bilayer solar cell appears to be the superposition of donor and acceptor materials on top of each other, providing the interface needed to ensure the charge transfer...Read More
The first organic solar cells consisted of a single layer of photoactive material sandwiched between two electrodes of different work functions (Chamberlain, 1983; Wohrle & Meissner, 1991). However, due to the high binding energy of the primary photoexcitations, the separation of the photogenerated charge carriers was so inefficient that far below 1% power conversion efficiency could be achieved.
The next breakthrough was achieved in 1986 by introducing the bilayer heterojunction concept, in which two organic layers with specific electron or hole transporting properties were sandwiched between the electrodes (Tang, 1986)...Read More
Cigdem Yumusak12 and Daniel A. M. Egbe2 1Department of Physics, Faculty of Arts and Sciences, Yildiz Technical University,
Davutpasa Campus, Esenler, Istanbul, 2Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University of Linz, Linz,
Since the discovery of electrical conductivity in chemically doped polyacetylene (Shirakawa et al., 1977; Chiang et al., 1977; Chiang et al., 1978), enormous progress has been made in the design, synthesis and detailed studies of the properties and applications of ^-conjugated polymers (Yu et al., 1998; Skotheim et al., 1998; Hadziioannou et al., 1998). The award of the Nobel prize in Chemistry three decades later in the year 2000 to Alan J. Heeger, Alan G...Read More
Although conventional SCs based on inorganic materials specially Si exhibit high efficiency, very expensive materials and energy intensive processing techniques are required. In comparison with the conventional scheme, the hybrid Si-based SC system has advantages such as; (1) Higher charging current and longer timescale, which make the hybrid system have improved performances and be able to full-charge a storage battery with larger capacity during a daytime so as to power the load for a longer time; (2) much more cost effective, which makes the cost for the hybrid PV system reduced by at least 15% (Wu et al.,
2005) . Therefore, hybrid SCs can be suitable alternative for conventional SCs...Read More