Charge photogeneration

Clarke TM, Durrant JR (2010) Charge photogeneration in organic solar cells. Chem Rev... 

This reaction has been studied in some detail [2,4,31,32] and will be considered only briefly here. It is a remarkably slow process (microseconds to milliseconds) at short circuit and, thus, does not limit the short-circuit photocurrent density, Jsc. However, the rate of reaction (3) [33] and of the other recombination reactions increases as the potential of the substrate electrode becomes more negative [e.g., as the cell voltage charges from short-circuit (0 V) to its open-circuit photovoltage, Voc, (usually between —0.6 V and —0.8 V versus the counterelectrode)]. At open circuit, no current flows and the rate of charge photogeneration equals the total rate of charge recombination. 

There are numerous mechanisms of charge photogeneration. The most common are the direct production of electron-hole pairs, exciton dissociation, and photoinjection from electrodes. 

Polyaromatics such as perylene and bis-azo pigments have been employed in low-volume copiers that normally utilize low intensity and relatively long exposures. For high-volume copiers, instead, it is necessary to employ photoreceptors with high speed and high charge photogeneration quantum... 

Table 1. Rate of response of selected photorefractive polymer composites a comparison of the experimental fast time constant of growth of contrast in refractive index in response to a nonuniform intensity pattern and the theoretical rate limit based on charge photogeneration rate.

AODCST, 2-[4-bis(2-methoxyethyl)amino]benzylidene malononitrile PTPDac-BA2, copolymer, 65% wt N-(4-acryloyloxymethylphenyl)-N -phenyl-N,N -bis(4-methylphenyl)-[ 1,1 -biphenyl]-4,4 -diamine, 35% wt A-butylacetate DOP, dioctyl phthalate DRl-DCTA, 4,4 -di(carbazol-cl-yl)-4"-(2- N-ethyl-N-[4-(4-nitrophenyldzo)phenyl]amine ethoxy)-triphenylamine other abbreviations are defined in the text and Figures, quantum effieieney of mobile charge photogeneration has been estimated where necessary, °a relative static dielectric constant of 3 and a linear electro-optic response have been assumed. 

In low-dimensional systems, such as quantum-confined semiconductors and conjugated polymers, the first step of optical absorption is the creation of bound electron-hole pairs, known as excitons [34]. Charge photogeneration (CPG) occurs when excitons break into positive and negative carriers. This process is of essential importance both for the imderstanding of the fundamental physics of these materials and for applications in photovoltaic devices and photodetectors. Since exciton dissociation can be affected by an external electric field, field-induced spectroscopy is a powerful tool for studying CPG. 

T.M. Clarke, A.M. Ballantyne, J. Nelson, D.D.C. Bradley, and J.R. Durrant, Free energy control of charge photogeneration in polythiophene/fullerene solar cells The influence of thermal annealing on P3HT/PCBM blends, Adv. Fund Mater., 18, 4029 035 (2008). 

The photorefractive effect in polymeric systems is different from that in inorganic crystals because the charge photogeneration quantum yield, the carrier mobility and (in case of low-Tg materials) the electro-optic activity are clearly field-dependent parameters. Polymeric materials optimized for photorefractive response have to fulfill a number of important requirements in addition to those known for conventional applications. These include spectrally optimized charge... 

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