Flexible Electronics Materials and Applications
Flexible Electronics: Materials and Applications surveys the materials systems and processes that are used to fabricate devices that can be employed in a wide variety of applications, including flexible flat-panel displays, medical image sensors, pho
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Physics and Materials Issues of Organic Photovoltaics Shawn R. Scully and Michael D. McGehee
11.1 Introduction Organic materials hold promise for use in photovoltaic (PV) devices because of their potential to reduce the cost of electricity per kWh ultimately to levels below that of electricity produced by coal-fired power plants. Deposition of organics by techniques such as screen printing, doctor blading, inkjet printing, spray deposition, and thermal evaporation lends itself to incorporation in high-throughput low-cost roll-to-roll coating systems. These are low-temperature deposition techniques which allow the organics to be deposited on plastic substrates such that flexible devices can easily be made. In addition to the inherent economics of high-throughput manufacturing, lightweight and flexibility are qualities claimed to offer a simple way to reduce the price of PV panels by reducing installation costs. Flexible PVs also open niche markets like portable power generation and aesthetic-PV in building design. This chapter reviews the current state-of-the-art in making efficient organic and hybrid inorganic–organic PV devices. We discuss the basic physics of operation in a systematic way and also discuss current material limitations and identify areas that need improvement. Special emphasis is given to materials design and materials and device characterization. This chapter should serve as a guide to researchers in the field who plan to develop better material systems and optimize devices to push organic PV power conversion efficiencies above 10%.
11.2 Basic Operation The efficient conversion of a photon absorbed by an organic chromophore to an electron–hole pair extracted and driven through an external circuit involves many steps. Upon photon absorption a strongly bound electron–hole pair, known as an S.R. Scully (B) Department of Materials Science and Engineering, Stanford University, Stanford, CA, USA e-mail: [email protected]
W.S. Wong, A. Salleo (eds.), Flexible Electronics: Materials and Applications, Electronic Materials: Science & Technology, DOI 10.1007/978-0-387-74363-9 11, C Springer Science+Business Media, LLC 2009
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S.R. Scully and M.D. McGehee
exciton, is formed [1]. The exciton must migrate to an interface where there is a sufficient electrochemical potential drop to drive exciton dissociation into an electron– hole pair that spans the interface across the donor (material with low electron affinity) and acceptor (material with high electron affinity). This geminate pair is still coulombically bound [2–6] and consequently must be dissociated. After dissociation, each charge must be transported through the device, avoiding trapping or bimolecular recombination, to the appropriate contact where the excess potential energy left over after all the above processes can be used to drive a load in an external circuit. One of the primary ways to characterize a solar cell is by measuring current as a function of applied voltage in the dark and under illumination. The figures-of-merit are
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