Bandgap Science for Organic Solar Cells

• Main Authors: Masahiro Hiramoto, Masayuki Kubo, Yusuke Shinmura, Norihiro Ishiyama, Toshihiko Kaji, Kazuya Sakai, Toshinobu Ohno, Masanobu Izaki
• Format: Article
• Language: English
• Published: MDPI AG 2014-06-01
• Series: Electronics
• Subjects: organic solar cell; doping; bandgap science; seven-nines purification; phase-separation; pn-control; co-deposited film; Kelvin band mapping; carrier concentration; ionization efficiency; built-in potential design; pn-homojunction; metal/organic ohmic junction; organic/organic ohmic homojunction; tandem cell
• Online Access: http://www.mdpi.com/2079-9292/3/2/351

The concept of bandgap science of organic semiconductor films for use in photovoltaic cells, namely, high-purification, pn-control by doping, and design of the built-in potential based on precisely-evaluated doping parameters, is summarized. The principle characteristics of organic solar cells, namely, the exciton, donor (D)/acceptor (A) sensitization, and p-i-n cells containing co-deposited and D/A molecular blended i-interlayers, are explained. ‘Seven-nines’ (7N) purification, together with phase-separation/cystallization induced by co-evaporant 3rd molecules allowed us to fabricate 5.3% efficient cells based on 1 µm-thick fullerene:phthalocyanine (C60:H2Pc) co-deposited films. pn-control techniques enabled by impurity doping for both single and co-deposited films were established. The carrier concentrations created by doping were determined by the Kelvin band mapping technique. The relatively high ionization efficiency of 10% for doped organic semiconductors can be explained by the formation of charge transfer (CT)-complexes between the dopants and the organic semiconductor molecules. A series of fundamental junctions, such as Schottky junctions, pn-homojunctions, p+, n+-organic/metal ohmic junctions, and n+-organic/ p+-organic ohmic homojunctions, were fabricated in both single and co-deposited organic semiconductor films by impurity doping alone. A tandem cell showing 2.4% efficiency was fabricated in which the built-in electric field was designed by manipulating the doping.