Electronics 2014, 3(2), 351-380; doi:10.3390/electronics3020351
Review

Bandgap Science for Organic Solar Cells

1,2,* email, 1,2email, 1,2email, 1,2email, 1,2email, 3email, 4email and 2,5email
Received: 18 February 2014; in revised form: 28 April 2014 / Accepted: 26 May 2014 / Published: 11 June 2014
(This article belongs to the Special Issue Organic Semiconductors)
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Abstract: 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.
Keywords: 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
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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MDPI and ACS Style

Hiramoto, M.; Kubo, M.; Shinmura, Y.; Ishiyama, N.; Kaji, T.; Sakai, K.; Ohno, T.; Izaki, M. Bandgap Science for Organic Solar Cells. Electronics 2014, 3, 351-380.

AMA Style

Hiramoto M, Kubo M, Shinmura Y, Ishiyama N, Kaji T, Sakai K, Ohno T, Izaki M. Bandgap Science for Organic Solar Cells. Electronics. 2014; 3(2):351-380.

Chicago/Turabian Style

Hiramoto, Masahiro; Kubo, Masayuki; Shinmura, Yusuke; Ishiyama, Norihiro; Kaji, Toshihiko; Sakai, Kazuya; Ohno, Toshinobu; Izaki, Masanobu. 2014. "Bandgap Science for Organic Solar Cells." Electronics 3, no. 2: 351-380.

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