Molecular photovoltaic devices

The successful design of both natural and artificial molecular photovoltaic devices rests on meeting three fundamental requirements, namely 121 (1) The quantum yield for the charge separation process should be as high as possible. That is, kcs > kd (Figure 1). (2) The lifetime, tcr (= 1 lkcr), of the CS state must be sufficiently long to enable it to carry out... 

There are, however, several fields of current research in which a corresponding level of understanding would be of interest also for large molecular adsorbates. For example, adsorbate-substrate interactions are relevant in the general areas of biocompatibility [51] and chemical sensors [52]. The requirement of dye-sensitization of metal oxide semiconductors also makes this an important aspect of many molecular photovoltaic devices. In fact, a good interfacial contact between dye and substrate, characterized by long-term stability and intimate electric contact, is vital for the efficiency of e.g. the dye-sensitized solar cells which have been at the center of our attention for the last five years. 

The successful design of both natural and artificial molecular photovoltaic devices rests on meeting three fundamental requirements, namely [1] ... 

In summary, the comparatively small internal reorganization energy of C o, together with its excellent redox properties, should make the Cgo chromophore a valuable electron acceptor component in molecular photovoltaic devices. 

Nelson J., Kirkpatrick J. and Ravirajan P. (2004), Factors limiting the effieieney of molecular photovoltaic devices , Phys. Rev. B Condensed Matter 69, 35337-35348. 

Nierengarten, J.-F., Hadziioannou, G., and Armaroli, N. (2001) Molecular photovoltaic devices. Mater. Today, 4,16. 

Nelson J, Kirkpatrick J, Ravirajan P (2004) Factors limiting the efficiency of molecular photovoltaic devices. Phys Rev B 69 035337. doi 10.1103/PhysRevB.69.035337... 

These significant findings form the basis of a set of design principles for the construction of molecular photovoltaic cells and other nanoscale electronic devices in which the control of both the rate and directionality of ET processes is an essential requirement. The successful construction of an artificial light-driven proton pump, based on principles of long-range ET processes illustrates the promise of this approach.1501... 

The conversion of light to electric current in photovoltaic devices is the direct inversion of the electroluminescent process in OLEDs, thus it is not surprising that the same molecular glasses as described above have also been used for the realization of solar cells. There are at least two different types of approaches, however, that shall be described now. 

Esterification constitutes a valuable alternative to the amidation strategy. As with amidation, the formation of the ester bond is performed following a first reaction step with acyl chloride. The ester bond has been extensively utilized to attach many organic and inorganic moieties. Porphyrins are a classic example of substrates covalently bound via esterification strategies their photoinduced electron transfer to the nanotube has been studied for applications in molecular electronics and photovoltaic devices (Fig. 3.6) [21]. 

The preceding sections described molecular interactions important in organic solar cells. This section discusses the impact of those interactions on the overall device behavior. Simulated electrical behavior for a typical solar cell is illustrated in Fig. 10. Under forward bias voltages 0 < V < Vqo typical photovoltaic device under illumination supplies power (P = / x V) to the external circuit (cf. lower panel of Fig. 10, dashed trace in first quadrant). The formalism used here implies that, under reverse bias, the organic material is reduced at the anode and oxidized at the cathode, while, under forward bias, the organic material is oxidized at the anode and reduced at the cathode. The short circuit current, J c, is approximately equal to... 

Perez MD, Borek C, Forrest SR, Thompson ME (2009) Molecular and morphological influences on the open circuit voltages of organic photovoltaic devices. J Am Chem Soc 131 9281... 

Dye-sensitized solar cells (DSSCs) are photoelectrochemical solar devices, currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. DSSCs are based upon the sensitization of mesoporous nanocrystalline metal oxide films to visible light by the adsorption of molecular dyes.5"7 Photoinduced electron injection from the sensitizer dye (D) into the metal oxide conduction band initiates charge separation. Subsequently, the injected electrons are transported through the metal oxide film to a transparent electrode, while a redox-active electrolyte, such as I /I , is employed to reduce the dye cation and transport the resulting positive charge to a counter electrode (Fig. 17.4). 

One of the most promising bottom-up approaches in nanoelectronics is to assemble 7i-conjugated molecules to build nano-sized electronic and opto-electronic devices in the 5-100 nm length scale. This field of research, called supramolecular electronics, bridges the gap between molecular electronics and bulk plastic electronics. In this contest, the design and preparation of nanowires are of considerable interest for the development of nano-electronic devices such as nanosized transistors, sensors, logic gates, LEDs, and photovoltaic devices. 

Specifically, energy- and charge-transfer properties of several different molecular-wire systems have been studied within the framework of photoinduced charge separation and solar-energy conversion. Up front, the conductance behavior of wire-like molecules was of particular interest. Such features have been carefully examined in view of possible applications in the fields of molecular electronics and/or photovoltaic devices. Among the tested systems, 7t-conjugation played a crucial role. 

Dye-sensitized semiconductors continue to be the focus of considerable research as a consequence of their importance in photovoltaic technologies, with the theories and mechanisms behind their operation also emerging. Considerably less is known about photoinduced interfacial processes in large photochemically active adsorbates on metals. ISAs on metals are becoming increasingly prevalent and are possible precursors for suitable molecular electronic devices. It would seem likely that over the coming years fundamental studies on the photophysics and chemistry of these materials will become more widespread. 

Photovoltaic Devices with OPV4—Ceo- The increased lifetime of the charge-separated state, which extends into the millisecond time domain, opens the possibility of using the OPVrt-Coo dyads as the active material in a photovoltaic device. As an important difference with previous bulk heterojunction cells, the covalent linkage between donor and acceptor in these molecular dyads restricts the dimensions of the phase separation between the oligomer and the fullerene that could freely occur in blends of the individual components. This can be considered as a primitive attempt to obtain more ordered and better-defined phase-separated D-A networks. 

K. Yoshino, K. Tada, A. Fujii, E.M. Conwell, A. Zakhidov, Novel photovoltaic devices based on donor-acceptor molecular and conducting polymer systems, IEEE Trans. Electron Devices 44 (1997) 1315-1324. 

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