Spray Pyrolysis Deposition of Semiconducting Thin Films for Solar Cells

Spray Pyrolysis Deposition of Semiconducting Thin Films for Solar Cells

著者 Kosugi Tsuyoshi

journal or

publication title

静岡大学大学院電子科学研究科研究報告

volume 20

page range 137‑140

year 1999‑03‑31

出版者 静岡大学大学院電子科学研究科

URL http://hdl.handle.net/10297/1559

氏名 。(本籍

) 

)

学 位 の種 類

 (工

⁾

学 位 記番 号

  165  

学位授与の日付

     ̲

学位授与の要件

学位論文題目

   Spray Pyrolysis Deposition of SeEniCOnducting Thin Films for Solar Cells

(太陽電池 を目指 した半導体薄膜のスプレー熱分解法によ る形成

)

論 文 審 査 委 員   (委員長)

教 授 畑 中 義 式

論 文 内 容 の 要 旨

Recent global environment requires rapid progress and development of high performance solar cells as clean energy source. At present, silicon is the most applicable solar cell material, howevero the semiconducting alter- natives of high capacity via low cost processing have been sought for coming into wide use. [n fact, compound semiconductors have been proposed for the applications to solar cells, almost all of which are composed of toxic atoms such as cadmium and arsenicn or of scanty atoms such as gallium and indium. AIso, indium tin oxide (fTO) is chiefly used as transparent electrode of ^solar cells, which is composed of scanty indium. The weak points will become more serious with the spread of ^solar cells in the near future. Therefore, the study of the semiconducting materials overcoming such problems is essential for the sustainable development of the solar cell technology.

Spray pyrolysis deposition, one of the chemical techniques suitable for a molecular levelUesign of thin films,

has been applied to deposit a wide variety of thin films including SnO2, In2O3, CdS, CdTe and CuInSe2. It has

a feature of a large-scale thin film formation using simple apparatus with good productivity. The spray pyrolysis

technique is essentially suitable for the solar cell prodcution, because large-area deposition of thin films with

low cost is required.

In this study, I have attempted the spray pyrolysis deposition of SnO2 thin films which is fit for front electrode layer of solar cell, and Cu2O and SnS for absorbing layer, and carried out the characterization of them. I

have found a novel compound of tin oxysulfide (Sn2OS) in the course of the deposition of SnS thin films.

Tin oxide (SnO) is an imponant semiconducting material with ^a wide band gap, ahigh donor concentrationn and a large mobility. This material is electrically conductive and transparent in the visible light region. SnO2 thin film can be applied both to front electrode and window layer in solar cell. In application of SnO2 to window layer, high orientation of SnO2 thin film is favorable to form clean heterojunction with absorber. The formation process of (200) and (l 10) oriented SnO2 thin films, which were deposited on glass substrates by spray pyroly-

sis of organotin compounds with butyl and/or acetate group: (C+Hg)zSn(OOCCH3)2 and (CaHs)+ Sn, was stud- ied in tenns of film thickness, solution concentration, and substrate temperature. A simple but more adequate texture coefficient for ^preferred orientation was introduced for the determination of the microstructure of film.

A growth model was proposed for the oriented tin oxide thin film based on the consideration of the geometrical obstruction.

Cuprous oxide (Cu2O) is a nonstoichiometric p-type semiconductor having the bandgap energy of ^2.0 eV.

The spray pyrolysis deposition of Cu2O thin films ^{has not} been attempted yet. The formation of Cu2O thin film on glass substrate by the spray pyrolysis of ^aqueous solution including ^{copper ( II} ) ^{acetate, glucose, and} i- propanol has been investigated in terms of solution concentration and substrate temperature. The deposition temperature of the film ^was varied according to the copper ( II ) ^acetate concentration; i.e., 230T to 27OT at 0.02M and2TOT to 280C ^at 0.04M. The deposition process seemed to include the following steps: (1) the arrival of droplet onto the substrate, (2) thereduction of Cu2* by glucose, (3) the formation of Cu cluster, (4) the oxidation of Cu cluster to Cu2O, (5) the growth of ^Cu2O cluster. The kinetic study on the formation of Cu2O

film gave the apparent activation energies of 42 ^and 166 kJ/mol for Cu to Cu2O and Cu2O to CuO processes, respectively. The Cu2O film obtained at 280C which looked reddish yellow was of smooth surface and dense body, compared with those in a previous paper. The indirect and directbandgap enegies of this film showing the resistivity of 100 Ocm ^were determined to be 1.95 and 2.60 eY from the optical measurement, respectively.

The photovoltaic activity to incident solar light appeared in the heterojunction with zinc oxide.

Tin sulfide (SnS) is a p-type semiconductor having indirect bandgap energy of ^{1.05 eV and} large absorption coefficient over 105 cm-l in the visible light region. These properties make the material fit for ^solar cell ab- sorber. From the aqueous solution containing ^0.09 to 0.2M SnCl2 and thiourea at the substrate temperature of

175 to Z7ST, a SnS thin film was successfully deposited. This temperature was over l00C ^{lower than the}

reported value in the spray pyrolysis deposition of SnS thin films from different compound, which leads to the merit of preventing the reaction between SnS and window layer in the formation of ^solar cell. The film was dense but its ^surface has needle-like structure. The absorption coefficient was over 105 cm-t below the wave- length of 600 nm. The direct and indirect bandgap energies were 1.09 and 1.25 eY, respectively. The resistivity

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was 105 f,)cm almost independent of the deposited conditions.

From the aqueous solution containing 0.01 to 0.07M SnCl2 and thiourea atthe substrate temperature of 225 to 375"C, a novel compound thin film could be obtained. The compound was identified to tin oxysulfide, Sn2OS, which has the simple cubic crystal lattice with the laffice constant of 1.157 nm. This film also was dense and had smooth surface. The absorption coefficient was over 105 cm-l below the wavelength of 600 nm, and the indirect bandgap energy was 1.50 eV, which are suitable for solar cell. The photovoltaic ability to incident solar light

appeared in the heterojunction with indium tin oxide.

In conclusion, the spray pyrolysis depositions of semiconducting thin films of SnO2, Cu2O, SnS and Sn2OS, were successful. These compounds have a merit that the constituent atoms are of nontoxicity and availability. It

is further assured that the compounds had a physical properties fit for solar cells.

論 文 審 査 結 果 の 要 旨

本論文には太陽電池への応用を目指 した、スプレー熱分解法による幾つかの化合物半導体薄膜の形 成に関する研究が記述 されている。第^1章は序論であ り、近年、資源問題 と共に環境保全の観点か ら

も太陽電池への関心が高 まっているが、 しか し、太陽電池を普及 させるためには、コス トが高い、材 料中には有毒あるいは資源に制約のある元素を含む、などの問題点の有ることを指摘 している。これ らへの対応策 として、①太陽電池の省資源化 と製造時の省エネルギー化が可能な多結晶薄膜を使用、

②膜形成に大面積の薄膜 を低 コス トで形成できるスプレー熱分解法を利用、そ して③毒性及び資源の 制約が無い半導体材料の採用を提案 している。この考え方を基本 にして以下の各章でスプレー熱分解 法 により形成 した半導体薄膜の解析 と評価 を行 っている。

第

^2章

(S■

^3章

とから明らかにしている。第

^5章

^6章

^2章

以上の様 に、本論文ではスプレー熱分解法を用いて毒性及び資源の制約が少ない元素から成る薄 膜 を形成 し、それらの諸特性 を明 らかにすると共に、太陽電池への応用を試みて多 くの成果が得 られ ている。化合物半導体薄膜の新規合成法に関する知見のみならず、太陽電池の実用化に与える貢献 も 大 きいと判断される。本論文は博士(工学)の学位 を授与するに十分な内容を持つ ものであると認定す る。

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