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 3 rd molecules allowed us to fabricate 5.3% efficient cells based on 1 µm-thick fullerene:phthalocyanine (C 60 :H 2 Pc) co-deposited films.
Control of the energetic structure of photovoltaic co-deposited films consisting of fullerene and α-sexithiophene was demonstrated by ppm-level doping with molybdenum oxide (MoO3). The transition from an n-type Schottky junction via a metal/insulator/metal junction to a p-type Schottky junction by increasing the MoO3 doping concentration was verified by observing the photovoltaic properties. Direct ppm-level doping into photoactive co-deposited films could become a powerful tool for designing the appropriate built-in potential for efficient organic photovoltaic cells.
Tandem photovoltaic cells were formed in single fullerene films by doping with molybdenum oxide and cesium carbonate. A heavily doped n+p+-homojunction acted as an ohmic interlayer between the two pn-homojunction cells. The observed photovoltaic properties of the tandem cell were shown to be consistent with the energy band diagram mapped using a Kelvin probe.
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