@misc{oai:ir.soken.ac.jp:00004050,
 author = {石山, 仁大 and イシヤマ, ノリヒロ and ISHIYAMA, Norihiro},
 month = {2016-02-17, 2016-02-26},
 note = {Organic solar cells consisting of vacuum deposited films have been actively studied due to their potential use in the fabrication of low-cost solar cells.  Most recent cells use a mixture of two kinds of organic semiconductors, since cells constructed from single organic semiconductors generate little photocurrent.  Co-deposition from different evaporation sources is a convenient way to mix two different organic semiconductors.  Organic solar cells that include co-deposited films can generate photocurrents of significant magnitude, since the efficient dissociation of excitons (bound states consisting of an electron and a hole) occurs in co-deposited films due to photoinduced electron transfer.

On the other hand, impurity-doping has been developed to precisely control the energy structures of inorganic solar cells.  In the case of organic solar cells, doping techniques have not been reliably established.   In particular, there has been no attempt to control the energy structures of organic co-deposited films by the use of doping.

In this thesis, the author has developed doping techniques for photovoltaic organic co-deposited films.  p+in+-homojunctions and an n+p+-homojunction, which act as photoactive layers and as an ohmic interlayer, respectively, were fabricated in co-deposited films.  A 2.4% efficient tandem organic solar cell was constructed by simply doping into co-deposited films.

This thesis consists of seven chapters.

In Chapter 1, the history and principles of organic photovoltaic cells are described.

In Chapter 2, fundamental equipment and methods are described.  A co-deposited film consisting of fullerene and α-sexithiophene (C60:6T), which exhibits a large open-circuit voltage (reaching 0.8 V) was used.  Molybdenum oxide (MoO3) was used as an acceptor dopant to create p-type films.  The author found out that cesium carbonate (Cs2CO3) acted as a donor dopant to produce n-type C60 and 6T films.  In order to introduce dopants into co-deposited films, the author developed a ‘three component co-evaporation’ technique, in which three different evaporation sources were used.  Precise monitoring of the deposition rates of the dopants using a computer monitoring system enabled us to dope as low as 40 ppm by volume concentration.  Direct energy-band mapping of doped junctions was achieved using a Kelvin probe.

In Chapter 3, pn-homojunctions and an n+p+-homojunction, which act as photoactive layers and as an ohmic interlayer, respectively, were fabricated in single C60 films by simply controlling the doping concentrations of MoO3 and Cs2CO3.  A tandem photovoltaic cell, whose open circuit voltage (Voc) is double that of the unit cells, was incorporated in single C60 films by doping alone.  The energy-band diagram of the overall tandem cell was depicted based on Kelvin probe measurements for the pn- and n+p+homojunctions. The pn-homojunctions, in which an exciton is dissociated into a hole and an electron under photo-irradiation, have 130 nm-wide depletion layers.  The n+p+-homojunction, in which a hole and an electron neutralize each other due to recombination or tunneling, has a 20 nm-wide depletion layer.  A doping technique for controlling the energy structures of single C60 films was established.

In Chapter 4, control of the energy structure of a C60:6T co-deposited film was achieved by ppm-level doping with MoO3.  The conduction types of C60:6T films were intentionally tuned from n-type, via intrinsic, to p-type by controlling the MoO3 doping concentration.  The potential profiles of MoO3-doped C60:6T films mapped using a Kelvin probe enabled us to confirm the transition of the energy structure.  The results confirmed that MoO3 acts as an acceptor dopant in the case of C60:6T co-deposited films.

	In Chapter 5, tuning of the barrier parameters of n-type Schottky junctions formed in C60:6T co-deposited films was achieved by ppm-level control of Cs2CO3 doping.  The carrier concentration of electrons, as evaluated by capacitance measurements, showed a clear proportional relationship to the overall doping concentration of Cs2CO3.  The results confirmed that Cs2CO3 acts as a donor dopant for the C60:6T co-deposited films.  In addition, the doping efficiency was found to be around 0.15.

	In Chapter 6, since the pn-properties of the C60:6T co-deposited films could be completely controlled by doping with MoO3 and Cs2CO3, organic solar cells were designed in the C60:6T films by use of these doping techniques.  The author fabricated a series of fundamental junctions, that is, p- and n-type Schottky junctions, pn, p+in+, and ohmic n+p+ homojunctions, and ohmic junctions between metal electrodes and heavily-doped p+ and n+ layers.  Based on these doping techniques, a tandem organic solar cell was formed in a C60:6T film by connecting two photoactive p+in+-homojunctions via a heavily doped n+p+-ohmic interlayer.  The value of Voc and the conversion efficiency of the tandem cell reached 1.69 V and 2.4%, respectively.

	In Chapter 7, the conclusion of this thesis is described.  The author constructed doping techniques for designing the energy structures of photovoltaic C60:6T co-deposited films.  Complete control of the pn-properties of photovoltaic C60:6T co-deposited films was achieved by doping with MoO3 and Cs2CO3.  A series of fundamental junctions, that is, p- and n-type Schottky junctions, pn, p+in+, and ohmic n+p+ homojunctions, and ohmic junctions between metal electrodes and heavily-doped p+ and n+ layers, were fabricated.  A 2.4% efficient tandem organic solar cell was built in a C60:6T co-deposited film fabricated by doping only.  Energy-band mapping and capacitance measurements strongly assisted in the clarification of the operating mechanisms of the doped junctions.

	The introduction of direct doping into bulk co-deposited films can provide the following improvements in the design of organic solar cells.  (i) A built-in electric field can be constructed directly in the co-deposited region where the generation and transport of photocarriers occurs.  (ii) A reduction of the bulk resistance of co-deposited films by doping can enable the growth of co-deposited films that are sufficiently thick (e.g. 1 m) to absorb the whole of the incident solar light and to convert it to a photocurrent.  Therefore, these doping techniques could significantly help in the development of an efficient organic solar cell., 総研大甲第1576号},
 title = {Design of the Energy Structures of Photovoltaic Organic Co-deposited Films by Impurity Doping},
 year = {}
}