Organic solar cells: An overview
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Organic solar cells: An overview Harald Hoppea) and Niyazi Serdar Sariciftci Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University, 4040 Linz, Austria (Received 8 March 2004; accepted 12 March 2004)
Organic solar cell research has developed during the past 30 years, but especially in the last decade it has attracted scientific and economic interest triggered by a rapid increase in power conversion efficiencies. This was achieved by the introduction of new materials, improved materials engineering, and more sophisticated device structures. Today, solar power conversion efficiencies in excess of 3% have been accomplished with several device concepts. Though efficiencies of these thin-film organic devices have not yet reached those of their inorganic counterparts ( ≈ 10–20%); the perspective of cheap production (employing, e.g., roll-to-roll processes) drives the development of organic photovoltaic devices further in a dynamic way. The two competitive production techniques used today are either wet solution processing or dry thermal evaporation of the organic constituents. The field of organic solar cells profited well from the development of light-emitting diodes based on similar technologies, which have entered the market recently. We review here the current status of the field of organic solar cells and discuss different production technologies as well as study the important parameters to improve their performance.
I. INTRODUCTION
Though common materials used for photovoltaics (i.e., the conversion of sunlight into electrical energy) are inorganic,1 there has been a tremendous effort to develop organic solar cells within the last three decades.2–9 The field started by the application of small organic molecules (pigments),2,3,9 and since the development of semiconducting polymers,10–13 these materials were incorporated into organic solar cells resulting in remarkable improvements within the past years.4,5,8,14 The potential of semiconducting organic materials to transport electric current and to absorb light in the ultraviolet (UV)-visible part of the solar spectrum is due to the sp2-hybridization of carbon atoms. For example, in conducting polymers the electron in the pZ-orbital of each sp2-hybridized carbon atom will form -bonds with neighboring pZ electrons in a linear chain of sp2hybridized carbon atoms, which leads then to dimerization (an alternating single and double bond structure, i.e., Peierls distortion). Due to the isomeric effect, these -electrons are of a delocalized nature, resulting in high electronic polarizability. An important difference to inorganic solid-state semiconductors lies in the generally poor (orders of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0252 1924
J. Mater. Res., Vol. 19, No. 7, Jul 2004
magnitudes lower) charge-carrier mobility in these materials,15 which has a large effect on the design and efficiency of organic semiconductor devices. However, organic semiconductors have relat
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