鐘志有,顧錦華,何 翔,孫奉?yuàn)?陳首部

(1中南民族大學(xué)電子信息工程學(xué)院,武漢430074;2中南民族大學(xué)計(jì)算與實(shí)驗(yàn)中心,武漢430074)

有機(jī)太陽(yáng)能電池?zé)o銦透明電極的光電性能研究

鐘志有1,顧錦華2,何 翔1,孫奉?yuàn)?,陳首部1

(1中南民族大學(xué)電子信息工程學(xué)院,武漢430074;2中南民族大學(xué)計(jì)算與實(shí)驗(yàn)中心,武漢430074)

以氧化鋅鋁陶瓷靶為濺射源,采用射頻磁控濺射方法制備了鋁摻雜氧化鋅(A ZO)無(wú)銦透明導(dǎo)電薄膜,通過(guò)X射線衍射儀、分光光度計(jì)和四探針儀等測(cè)試分析,研究了基板溫度對(duì)薄膜晶體結(jié)構(gòu)、力學(xué)和光電性能的影響.結(jié)果表明:所制備的A ZO薄膜均為六角纖鋅礦結(jié)構(gòu),并具有(002)擇優(yōu)取向,其晶體結(jié)構(gòu)、殘余應(yīng)力、方塊電阻、光學(xué)帶隙以及優(yōu)良指數(shù)等都與基板溫度相關(guān),當(dāng)溫度為400℃時(shí),A ZO薄膜的優(yōu)良指數(shù)最大(0.40Ω-1),具有最好的光電綜合性能.

有機(jī)太陽(yáng)能電池;電極;光電性能

有機(jī)太陽(yáng)能電池(OSC)具有成本低、輕薄、能卷曲、環(huán)境友好、可實(shí)現(xiàn)大面積制造等潛在優(yōu)勢(shì)[1-7],有望在便攜式計(jì)算器、半透光式充電器、柔性可卷曲系統(tǒng)等體系中發(fā)揮供電作用,是未來(lái)最為廉價(jià)和有吸引力的能源模式之一.自從1986年美國(guó)柯達(dá)公司Tang[1]首次報(bào)道電子給體/受體雙層結(jié)構(gòu)的OSC以來(lái),人們從器件結(jié)構(gòu)、光伏作用機(jī)理、新材料合成和制備工藝等各個(gè)方面開展了許多富有成效的研究,OSC器件的光伏性能得到了明顯改善[2-7].眾所周知,OSC的典型結(jié)構(gòu)為透明導(dǎo)電陽(yáng)極/有機(jī)光敏層/金屬陰極,通常情況下人們普遍使用摻錫氧化銦(ITO)透明導(dǎo)電玻璃作為OSC器件陽(yáng)極[2-7],但由于銦、錫等材料自然儲(chǔ)量少、制備工藝復(fù)雜、成本高、有毒、穩(wěn)定性差等缺點(diǎn),從而將大大影響未來(lái)OSC的推廣應(yīng)用,為此,研制ITO的替代產(chǎn)品已經(jīng)成為當(dāng)前透明導(dǎo)電薄膜領(lǐng)域的一個(gè)重要課題.氧化鋅(ZnO)是Ⅱ-Ⅵ族n型半導(dǎo)體材料,室溫下的直接光學(xué)帶隙為3.37 eV,由于其良好的導(dǎo)電性和透光性,在太陽(yáng)能電池、發(fā)光器件、液晶顯示器、透明電磁屏蔽以及觸敏覆蓋層等領(lǐng)域得到了廣泛的應(yīng)用[8-10].與目前常用的ITO相比,ZnO薄膜具有價(jià)格便宜,在活性氫和氫等離子體環(huán)境下穩(wěn)定性高等優(yōu)點(diǎn)而備受青睞.在ZnO 薄膜中摻入B、A l、In、Ga、Zr等都可以有效改善其光電性能[11,12],摻入A l時(shí),由于鋁離子的半徑(RAl=0.057 nm)比鋅離子的半徑(RZn=0.083 nm)小,鋁原子容易變成替位原子占據(jù)鋅原子的位置,也容易成為間隙原子而存在.A l原子趨向于以A l3+的方式發(fā)生固溶,A l3+離子占據(jù)晶格中Zn2+離子的位置,與周圍元素形成共價(jià)鍵.A l的價(jià)電子數(shù)比Zn多一個(gè),出現(xiàn)了弱束縛的電子,這個(gè)多余的電子只需要很小的能量,就可以擺脫束縛,成為ZnO薄膜中共有化運(yùn)動(dòng)的自由電子,即導(dǎo)帶中的電子.鋁摻雜ZnO(A ZO)薄膜中,導(dǎo)電非平衡載流子主要是電子,它是n型半導(dǎo)體,A l原子為施主雜質(zhì),摻A l的結(jié)果是增加了凈電子,使晶粒電導(dǎo)率增加,因此具有優(yōu)異的導(dǎo)電性能.A ZO薄膜是20世紀(jì)80年代發(fā)展起來(lái)的一類新型透明導(dǎo)電材料,它不僅具有與ITO可比擬的光電性能,而且還具有原材料儲(chǔ)量豐富、價(jià)格便宜、無(wú)毒性等優(yōu)點(diǎn),有望替代ITO薄膜在平板顯示器件和太陽(yáng)能電池等領(lǐng)域得到廣泛應(yīng)用[13-16],特別是由于它在氫等離子體環(huán)境中化學(xué)穩(wěn)定性高,可以用作制備太空中太陽(yáng)能電池的電極材料.

幾乎所有的薄膜制備方法都可以用來(lái)制備A ZO透明導(dǎo)電薄膜,如反應(yīng)蒸發(fā)法、電子束蒸發(fā)法、分子束外延法、脈沖激光沉積法、化學(xué)氣相沉積法、噴霧熱分解法、濺射法、溶膠-凝膠法等[17-27],相對(duì)于其他制備技術(shù),由于磁控濺射法具有設(shè)備簡(jiǎn)單、價(jià)格便宜、易于摻雜等特點(diǎn),所制備的A ZO薄膜不僅均勻致密、表面平整、附著性好,而且c軸取向性好、可見光透過(guò)率高,因此它是目前應(yīng)用最為廣泛一種ZnO摻雜薄膜制備技術(shù).本文以ZnA l2O4陶瓷靶為靶材,采用射頻磁控濺射技術(shù)制備了A ZO無(wú)銦透明導(dǎo)電薄膜,通過(guò)XRD、分光光度計(jì)和四探針儀等測(cè)試技術(shù),研究了基板溫度對(duì)薄膜晶體結(jié)構(gòu)、力學(xué)性質(zhì)和光電性能的影響.

1 實(shí)驗(yàn)

1.1 基板處理

實(shí)驗(yàn)選用普通玻璃作為基板材料,首先采用丙酮擦拭玻璃基板表面,然后用清水沖洗干凈,再依次使用丙酮、無(wú)水乙醇和去離子水各超聲清洗約20 m in,最后在無(wú)水乙醇中煮沸,吹干待用.

1.2 薄膜制備

無(wú)銦透明電極A ZO薄膜利用射頻磁控濺射技術(shù)制備,實(shí)驗(yàn)設(shè)備為國(guó)產(chǎn)KDJ567型高真空復(fù)合鍍膜系統(tǒng),濺射靶材為高密度氧化鋅鋁陶瓷靶(ZnO和A l2O3的純度為99.99%,其中A l2O3的含量為2.0 w t%).濺射所用氣體為純度99.99%的高純氬氣.薄膜沉積之前,先將玻璃基板放置于鍍膜系統(tǒng)的真空室中,待氣壓抽至低于～5×10-4Pa后通入氬氣,并先采用氬離子體對(duì)基板表面清洗約3 m in,然后再對(duì)靶材表面預(yù)濺射約5 m in以去除其表面的雜質(zhì)和污染物.制備A ZO薄膜的具體工藝參數(shù)如下:濺射功率為100W,靶基距為65 mm,工作壓強(qiáng)為0.3 Pa,氬氣流量為25 sccm,濺射時(shí)間為30 m in,基板溫度分別為250℃、300℃、400℃和500℃.

1.3 性能表征

A ZO薄膜的晶體結(jié)構(gòu)采用R igaku D/M ax-A型X射線衍射儀(Cu Kα,射線源波長(zhǎng)(λ=0.154 nm)分析,光學(xué)性能利用W FZ-2102PC型紫外/可見/近紅外分光光度計(jì)表征,薄膜的方塊電阻使用SZ-82型四探針儀測(cè)試.

2 結(jié)果與討論

2.1 力學(xué)性能

標(biāo)準(zhǔn)ZnO晶體(002)晶面所對(duì)應(yīng)的衍射角(2θ)為34.45°,晶格常數(shù)c0為0.52 nm,(002)晶面所對(duì)應(yīng)的晶面間距d0為0.2603 nm.A ZO薄膜的XRD分析結(jié)果如表1所示,可以看出,所有薄膜的衍射角(2θ)均偏離34.45°,但偏離較小,計(jì)算獲得的晶面間距d與標(biāo)準(zhǔn)值d0之間的最大偏差約為0.73%,這表明實(shí)驗(yàn)制備的A ZO薄膜為(002)擇優(yōu)取向的六角纖鋅礦ZnO結(jié)構(gòu),其衍射角的變化是由于A l3+對(duì)Zn2+的摻雜替代造成的晶格畸變所形成的.由于A l3+比Zn2+的原子尺寸小,在結(jié)晶過(guò)程中將產(chǎn)生殘余應(yīng)力,即A l3+對(duì)Zn2+的摻雜替代必然將造成晶格畸變,而隨著應(yīng)力逐漸減小,晶格畸變現(xiàn)象將減弱.

表1 AZO樣品的XRD分析結(jié)果Tab.1 XRD data of the AZO samples

根據(jù)雙軸應(yīng)力模型[28],薄膜的殘余應(yīng)力σ可以表示為:

(1)式中,cij為標(biāo)準(zhǔn)ZnO薄膜的彈性模量,其中c11=208.8 GPa,c12=119.7 GPa,c13=104.2 GPa,c33=213.8 GPa,計(jì)算得到σ和∈之間的關(guān)系為:

公式(1)和(2)中,∈為A ZO 薄膜(002)方向的相對(duì)應(yīng)變,可用公式(3)表示:

(3)式中,d和d0分別為A ZO薄膜和標(biāo)準(zhǔn)ZnO薄膜(002)方向的晶面間距.圖1給出了A ZO薄膜殘余應(yīng)力隨基板溫度的變化曲線,從圖中看出,基板溫度為 250～500℃時(shí),σ均為負(fù)值,這說(shuō)明所制備的A ZO薄膜沿c軸方向受到了壓縮作用或處于壓應(yīng)力狀態(tài).另外,從圖中還看到,當(dāng)溫度從250℃升高到300℃時(shí),σ的值從1.65 GPa變?yōu)?.96 GPa;而當(dāng)溫度從400℃升高到500℃時(shí),σ值從0.32 GPa變?yōu)?.23 GPa,可見隨著基板溫度升高,σ的數(shù)值逐漸減小,并且其下降速率呈愈來(lái)愈小的趨勢(shì).

圖1 AZO樣品殘余應(yīng)力隨基板溫度的變化Fig.1 Variation in the film stress as a function of substrate temperature

2.2 光學(xué)性能

圖2為不同基板溫度下所沉積A ZO樣品的光學(xué)透過(guò)率曲線,它們的透過(guò)率曲線顯示了清晰、光滑的干涉條紋,這說(shuō)明所制備的A ZO薄膜表面具有很高的平整度.另外,溫度為400℃時(shí)薄膜在可見光波段的平均透過(guò)率最高,而500℃時(shí)最低,這可能是因?yàn)闇囟葹?00℃時(shí)所制備的A ZO薄膜的晶化程度較高,晶粒尺寸較大,晶界缺陷密度較小,對(duì)可見光能量的散射也相對(duì)較少,因此其透光性能較好;而溫度過(guò)高時(shí),薄膜表面的粗糙度增加,使光線在薄膜表面不斷形成漫反射和散射,因此反射率和吸收率增加,進(jìn)而導(dǎo)致A ZO薄膜的光學(xué)透過(guò)率降低.從圖2中還可以看出,隨著基板溫度的升高,薄膜的紫外吸收邊向短波方向移動(dòng),即出現(xiàn)藍(lán)移現(xiàn)象,這說(shuō)明A ZO薄膜的光學(xué)帶隙隨基板溫度升高而增大.

圖2 AZO樣品透過(guò)率隨基板溫度的變化Fig.2 Transm ission spectra of AZO at different substrate temperature

對(duì)于直接帶隙半導(dǎo)體材料,在基本吸收區(qū)域,薄膜的光學(xué)透過(guò)率T可表示為:

(4)式中,C、α、d分別為常數(shù)、吸收系數(shù)和薄膜厚度[30,31].對(duì)于吸收邊緣有C≈1,因此,利用(4)式由薄膜厚度d以及對(duì)應(yīng)的吸收邊附近的光學(xué)透過(guò)率T就可以獲得薄膜的光學(xué)吸收系數(shù)α.根據(jù)Tauc定律[31,32],在薄膜的吸收邊附近,光學(xué)吸收系數(shù)α與入射光子能量hv之間滿足如下關(guān)系:

(5)式中,B為常數(shù),Eg為薄膜的光學(xué)帶隙,指數(shù)n取決于躍遷的類型.n=1/2時(shí),對(duì)應(yīng)于直接躍遷,而n=2時(shí)對(duì)應(yīng)于間接躍遷.A ZO薄膜屬于直接躍遷,取n=1/2作(α·hv)2與hv之間的關(guān)系曲線圖,根據(jù)外推法得到橫軸(hv)上的交點(diǎn)(α·hv=0)后,即可計(jì)算出薄膜的光學(xué)帶隙Eg.

圖3為300℃時(shí)所沉積A ZO樣品的(α·hv)2-hv關(guān)系曲線,利用外推法容易得到它的光學(xué)帶隙Eg為3.41 eV,采用同樣的方法可以計(jì)算出所有樣品的Eg,其結(jié)果如圖4所示,所制備A ZO薄膜的Eg值為3.38～3.61 eV,均大于標(biāo)準(zhǔn)ZnO薄膜的光學(xué)帶隙(3.37 eV)[8],同時(shí)Eg隨著基板溫度的升高而增大,這主要是由于Burstein-M oss效應(yīng)[33,34]所引起的,即由于導(dǎo)帶底部附近量子態(tài)基本上已被電子占據(jù),故價(jià)帶中的電子欲想直接躍遷到導(dǎo)帶中時(shí),則必須吸收更多的能量才能躍遷到導(dǎo)帶中較高的空位上,就如同禁帶寬度增加了.A ZO薄膜在紫外光區(qū)的吸收是由薄膜中的載流子濃度n所決定的,由Burstein-M oss效應(yīng)而引起的寬化帶隙可以表示為[33,34]:

式(6)中,?為普朗克常量,m＊為價(jià)帶中導(dǎo)帶中電子的有效質(zhì)量,n為載流子的濃度.式(6)表明薄膜中載流子濃度的增大將導(dǎo)致薄膜帶隙的增加.

圖3 AZO 樣品的(α·hν)2-hν關(guān)系曲線Fig.3 (α·hν)2as a function ofhνof AZO at temperature of 300℃

圖4 AZO樣品的光學(xué)帶隙隨基板溫度的變化Fig.4 Optical energy gap of AZO at different substrate temperature

2.3 電學(xué)性能

圖5為A ZO樣品方塊電阻Rs隨基板溫度變化的關(guān)系曲線,當(dāng)溫度由250℃升高至400℃時(shí),Rs的值急劇下降,而溫度大于400℃時(shí),Rs隨溫度的變化趨于平穩(wěn),其下降幅度很小(由16.1Ω/sq變?yōu)?5.2 Ω/sq).A ZO電學(xué)性能的變化是與薄膜質(zhì)量密切相關(guān)的,基板溫度較低時(shí),A ZO薄膜的結(jié)晶質(zhì)量較差,晶粒尺寸較小,過(guò)多的薄膜晶界和缺陷,導(dǎo)致A ZO的導(dǎo)電能力較差.但當(dāng)基板溫度升高時(shí),A ZO薄膜的晶粒尺寸增大、缺陷減少,削弱了晶界的散射作用,增加了載流子的遷移率,從而使A ZO的方塊電阻降低.另外,基板溫度的升高還能增加靶材粒子在成膜過(guò)程中的活性,遷移更加充分,提高了成膜質(zhì)量,因此改善了A ZO薄膜的電學(xué)性能[35].

圖5 AZO樣品方塊電阻隨基板溫度的變化Fig.5 Sheet resistance of AZO at different substrate temperature

2.4 光電綜合性能

優(yōu)良指數(shù)(Φ)是評(píng)價(jià)透明導(dǎo)電薄膜綜合光電性能的重要參數(shù),根據(jù)Φ的計(jì)算公式[36]:(7)式中,Rs為A ZO樣品的方塊電阻,T為可見光波段的平均透過(guò)率,所得Φ值如圖6所示,可以看出,溫度從250℃升高到400℃時(shí),Φ增大,而當(dāng)溫度繼續(xù)升高到500℃時(shí),Φ反而減小,400℃時(shí)A ZO薄膜的Φ值最大(0.40Ω-1),這表明基板溫度為400℃時(shí)所制備的A ZO薄膜具有最好的光電綜合性能.

3 結(jié)語(yǔ)

采用高密度氧化鋅鋁陶瓷靶作為濺射源,利用射頻磁控濺射技術(shù)制備了A ZO透明導(dǎo)電薄膜,研究了基板溫度對(duì)薄膜的力學(xué)、光學(xué)和電學(xué)性能的影響.結(jié)果表明:基板溫度的升高有利于減小A ZO薄膜的殘余應(yīng)力和方塊電阻,但溫度過(guò)高時(shí),它將明顯降低薄膜的透過(guò)率和優(yōu)良指數(shù),當(dāng)溫度為400℃時(shí),A ZO薄膜的優(yōu)良指數(shù)接近0.40Ω-1,具有最好的光電綜合性能.由于受Burstein-M oss效應(yīng)的影響,A ZO薄膜的光學(xué)帶隙也與基板溫度密切相關(guān),溫度升高,則光學(xué)帶隙增加.

圖6A ZO樣品優(yōu)良指數(shù)隨基板溫度的變化Fig.6 Figure of merit of AZO at different substrate temperature

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Optical and Electrical Properties of Indium-Free Transparent Electrodes for Organ ic Solar Cells

Zhong Zhiyou1,Gu J inhua2,H e X iang1,S un Feng lou1,Chen S houbu1
(1 College of Electronics and Information Engineering;South-CentralU niversity for N ationalities,W uhan 430074,China;2 Center of Computing&Exper imenting,South-CentralU niversity for N ationalities,W uhan 430074,China)

A lum inum-doped zinc oxide(A ZO)thin film s were deposited by radio-frequency magnetron sputtering method using a sintered ceram ic target.The influence of substrate temperature on crystalline,mechanical,optical and electrical properties of A ZO film s was investigated by X-ray diffraction,four-point probe and optical transm ission spectroscopy.The results show that all the obtained film s are polycrystalline w ith a hexagonal wurtzite structure and grow preferentially in the(002)direction.The mechanical and optoelectrical properties of A ZO film s are closely related to the substrate temperature.The A ZO film deposited at substrate temperature of 400℃possesses the best synthetic optoelectrical properties,w ith the highest transm ittance and the highest figure of merit(0.4Ω-1).

organic solar cells;electrode;optical and electrical properties

TM 914

A

1672-4321(2011)01-0064-06

2010-12-01

鐘志有(1965-),男,博士,副研究員,研究方向:光電材料及其器件,E-mail:zhongzhiyou@163.com

湖北省自然科學(xué)基金資助項(xiàng)目(2009CDB166);中南民族大學(xué)學(xué)術(shù)團(tuán)隊(duì)基金資助項(xiàng)目(XTZ09003)