Photovoltaic diodes

Marsh RA, Hodgkiss JM, Friend RH (2010) Direct measmement of electric field-assisted charge separation in polymer fullerene photovoltaic diodes. Adv Mater 22 3672... 

Penmans P., Bulovic V. and Forrest S. R. (2000), Efficient photon harvesting at high photon intensities in ultra-thin organic double-heterostructure photovoltaic diodes , Appl. Phys. Lett. 76, 2650-2653. 

GranstrOm M, Petritsch K, Arias AC, Lux A, Andersson MR, Friend RH (1998) Laminated fabrication of polymeric photovoltaic diodes. Nature 395 257... 

McNeill CR, AbrusciA, ZaumseilJ, Wilson R, McKiernanMJ, Burroughes JH, Halls JJM, Greenham NC, Friend RH (2007) Dual electron donor/electron acceptor character of a conjugated polymer in efficient photovoltaic diodes. Appl Phys Lett 90 193506... 

Russell DM, Arias AC, Friend RH, SUva C, Ego C, Grimsdale AC, Mullen K (2002) Efficient light harvesting in a photovoltaic diode composed of a semiconductor conjugated copolymer blend. Appl Phys Lett 80 2204... 

Snaith HJ, Whiting GL, Sun B, Greenham NC, Huck WTS, Friend RH (2005) Self-organization of nanocrystals in polymer brushes application in heterojunction photovoltaic diodes. Nano Lett 5 1653... 

The polymers used in this chapter are all polyfluorene derivatives. They are ABAB copolymers with A being the fluorene group shown in Fig. 2.4(a). Due to their high chemical stability, flexible chemistry and good charge transport and luminescence properties, polyfluorenes are very common materials for polymer optoelectronics and have been used to make highly efficient LEDs [26, 13] as well as good photovoltaic diodes [27, 28] and transistors [29]. Table 2.1 lists the abbreviated and full names of all the polymers that appear in this chapter. The chemical structures of each of the polymers are displayed in Fig. 2.4. 

This is sometimes used to alter the color of LEDs or to improve light harvesting in photovoltaic diodes. 

We investigate the properties of two different type-II heterojunction systems that sit on either side of the exciton destabilization threshold blends of the hole-accepting TFB with the electron-accepting F8BT that show excellent LED performance (see below) and blends of the hole-accepting PFB with F8BT that make poor LEDs but show promise for use as photovoltaic diodes [27]. These and related polymers have been used extensively for LEDs [10, 26, 36]. 

The operation and physics of optoelectronic devices based on conjugated molecules such as light-emitting diodes (LEDs) and photovoltaic diodes, where organic thin films are sandwiched between different electrodes, is critically influenced by the barriers at electrode/semiconductor interfaces and by the electric fields present within the different functional layers of the devices. 

Gonzalez-Rabade, A., Morteani, A.C., and Friend, R.H. (2009) Correlation of heterojunction luminescence quenching and photocurrent in polymer-blend photovoltaic diodes. Adv. Mater., 21, 3924-3927. 

While photoconductors are passive elements that need an external power supply, photovoltaic diodes are active elements that generate their own photovoltage upon illumination, although they are often used with an external bias voltage. The principle of the photogenerated voltage is shown in Fig. 4.84. 

Fig. 4.84a-c. Photovoltaic diode (a) schematic structure and (b) generation of an electron-hole pair by photon absorption within the p-n junction, (c) Reduction of the diffusion voltage Vb for an open circuit... 

This limitation can be lifted, however, as shown in the case of a polymer (4). The singlet excited state of this material is photosensitized in the presence of a dye (5) that absorbs in the visible with great efficiency (ultrafast charge injection, lifetime of the latter compound, 10 fs to 10 ps when interfaced with ZnO as a prototypal electron-acceptor compound) (see Scheme 6). This principle can be exploited and leads to a panchromatic photoresponse in prototype polymer/oxide bilayer photovoltaic diodes. 

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