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.
Highlights
Organic solar cells consisting of vacuum-deposited small-molecular thin films have been intensively studied [1,2,3], following the two-layer cell reported by Tang [4]
When an electron-donating molecule (D) and an electron-accepting molecule (A), for which the energetic relationship of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), are shifted in parallel with each other and are contacted or mixed, a charge transfer (CT) exciton is formed in which the positive and negative charges are separated on the neighboring D and A molecules due to photoinduced electron transfer (Figure 2b)
The present results clearly show that pn-homojunctions were fabricated in the single C60 films by doping alone
Summary
Organic solar cells consisting of vacuum-deposited small-molecular thin films have been intensively studied [1,2,3], following the two-layer cell reported by Tang [4]. Pn-control by doping are indispensable for the solid-state physics of inorganic semiconductors The authors strongly expect that the unknown physical phenomena, particular to organic semiconductors will be discovered during the course of research to establish the solid-state physics for organic semiconductors. From these standpoints of view, the authors chose the term “bandgap science”. Factors influencing bandgap science for organic solar cells, such as ‘seven-nines’ purification, pn-control by ppm-level doping for both single and for co-deposited organic semiconductor films, and built-in potential design based on precise evaluation of doping parameters, are summarized
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