AZO透明导电薄膜的工艺优化及其在叠层硅薄膜太阳能电池上的应用.pdf

Vol. 34, No. 6 Journal of Semiconductors June 2013Development of aluminum-doped ZnO films for a-Si:H/ c-Si:H solar cellapplicationsLei Zhifang( 雷志芳 ), Chen Guangyu( 陈光羽 ), Gu Shibin( 谷士斌 ), Dai Lingling( 代玲玲 ),Yang Rong( 杨荣 ), Meng Yuan(孟原 ), Guo Ted(郭铁 ), and Li Liwei( 李立伟 )ENN Solar Energy Co., Ltd., Langfang 065001, ChinaAbstract: This study deals with the optimization of direct current (DC) sputtered aluminum-doped zinc oxide(AZO) thin films and their incorporation into a-Si:H/ c-Si:H tandem junction thin film solar cells aiming forhigh conversion efficiency. Electrical and optical properties of AZO films, i.e. mobility, carrier density, resistiv-ity, and transmittance, were comprehensively characterized and analyzed by varying sputtering deposition condi-tions, including chamber pressure, substratetemperature, and sputtering power. The correlations between sputteringprocesses and AZO thin film properties were first investigated. Then, the AZO films were textured by diluted hy-drochloric acid wet etching. Through optimization of deposition and texturing processes,AZO films yield excellentelectrical and optical properties with a high transmittance above 81% over the 380– 1100nm wavelength range, lowsheet resistance of 11 / and high hazeratio of 41.3%. In preliminary experiments, the AZO films were applied toa-Si:H/ c-Si:H tandem thin film solar cells as front contact electrodes, resulting in an initial conversion efficiencyof 12.5% with good current matching between subcells.Key words: aluminum-doped zinc oxide; magnetron sputtering; tandem silicon thin film solar cellDOI: 10.1088/1674-4926/34/6/063004 PACC: 6855; 7360; 8115C1. IntroductionTransparent conductive oxide (TCO) thin films have awide range of applications in the photovoltaic (PV) industry.As a transparent electrode material, TCO has to provide highlight transparency, low electrical resistance, and strong capa-bility of light scattering. Currently fluorine-doped tin oxide(SnO2 :F or FTO) films are widely used as TCO films in thePV industry. However, as an alternative approach, aluminumdoped zinc oxide (ZnO:Al or AZO) film, which can be pro-duced on large areas in a controllable manner by magnetronsputtering, is considered as a substitute for use in amorphoussilicon solar cells. Compared to well established FTO films bychemical vapor deposition (CVD), AZO films maintain goodstability in ambient hydrogen plasma?1; 2 , lower resistivity,less absorption, and improved light scattering thanks to thefact that a precise control of the morphology can be achievedfrom the wet chemical etching process?3 5 . In particular, ina-Si:H/ c-Si:H tandem thin film solar cells, AZO has a betterperformance in increasing the short circuit current, reducingsilicon thickness, and lowering down the production cost ofsolar modules ?6; 7 .So far, among various deposition processes, magnetronsputtering is one of the most promising methods for ZnO depo-sition owing to the inherent easewith which the deposition con-ditions can be controlled well ?8 . In this study, AZO thin filmswere prepared by a DC sputtering system with ZnO:Al 2O3 ce-ramic targets, and the correlations between the sputtering pro-cessesand properties of AZO thin films were established. AZOthin films with high transmittance, high light scattering, andlow electrical resistivity were obtained through the optimiza-tion of sputtering processes. As a result, the AZO thin filmswere incorporated into a-Si:H/ c-Si:H thin film solar cells, andan initial conversion efficiency up to 12.5% has been achievedin preliminary experiments.2. ExperimentsAZO thin films were deposited by a high vacuum mag-netron sputtering system on 3.2 mm thick, 140 140 mm2sized low iron glass substrates using ceramic targets with anAl 2O3 -concentration of 1 wt%. The deposition pressure wasadjusted through Ar gas flow; the substrate temperature waspre-calibrated on 3.2 mm thick glass with a thermal sensorand controlled by heater temperature during film deposition,and the film thickness was controlled by sputtering powerand deposition time. Then, the as-deposited AZO thin filmswere textured by wet-chemical etching with diluted hydrochlo-ric acid (HCl). The surface morphology was controlled byetchant concentration, etching time, and temperature. Imme-diately after the chemical etching process the samples werecleaned by D.I. water. Finally, the glass substrates with tex-tured AZO were usedto fabricate Si thin film solar cells. The a-Si:H/ c-Si:H tandem junctions were prepared using a plasma-enhanced chemical vapor deposition (PECVD) system, fol-lowed by AZO/Ag back electrodes that were formed by sput-tering with a shadow mask to define a small cell area around0.5 cm2.Electrical resistivity of the AZO films was tested by afour-point probe meter (RTS-9). Hall mobility and carrier con-centration, were measured by a Hall meter (Ecopia, HMS-2000). Transmittance and haze of the films were measuredus-? Corresponding author. Email: liliwei@enn.cnReceived 22 October 2012, revised manuscript received 18 January 2013 ? 2013 Chinese Institute of Electronics063004-1J. Semicond. 2013, 34(6) Lei Zhifang et al.Fig. 1. X-ray diffraction pattern for the AZO film.ing a UV/VIS spectrometer (Perkin Elmer, Lambda 750) inthe wavelength range of 300– 1100nm. Film thickness wasscanned by a surface profiler (KLA, P-16+). Surface mor-phology was inspected by an atomic force microscopy (AFM)(Agilent, 5400 SPM). The crystal structure of the thin filmswas investigated using X-ray diffraction (XRD, Bruker ad-vance D8) with Cu K ? radiation ( D 0.1540 nm). Current –voltage (IV) and quantum – efficiency(QE) characteristics ofsolar cells were tested at standard testing conditions (AM1.5,100 mW/cm 2, 25 ?C).3. Results and discussion3.1. Structure of AZO thin filmsFigure 1 shows the typical XRD pattern of the AZO thinfilm with a thickness of 860 nm. It can be found that thefilm has a hexagonal wurtzite crystal structure (P63mc). In thiswork, all the deposited films have prominent h002i orientationas is observed from the XRD spectra, indicating a preferentialc -axis orientation perpendicular to the substrate?9 .3.2. Effects of the deposition processesAZO thin films were sputtered with a similar thicknessaround 860 nm with the deposition temperature varied as othersputtering conditions are fixed. Then the electrical and opti-cal properties were analyzed. The dependence of carrier con-centration (n/ , Hall mobility ( ), and electrical resistivity ( )on deposition temperature is shown in Fig. 2(a). It is observedthat carrier concentration lies in a narrow range of (4 – 5)1020 cm 3 , showing a weak dependenceon deposition temper-ature. The Hall mobility shows an initial sharp rise and thena gradual decrease as the deposition temperature further in-creases. Co-determined by carrier concentration and mobility,electrical resistivity shows a minimum value at an intermediatetemperature. The above data can be interpreted as follows: (1)a higher deposition temperature can enhance film crystalliza-tion, increase grain sizes,and reduce defects, resulting in betterHall mobility due to less carrier scattering and trapping; (2) Atoo high deposition temperature leads to excessincrease of Alprecipitation at grain boundaries in the form of Al 2 O3 , whichcausesvariation of grain orientation, increases scattering, andFig. 2. (a) Carrier concentration (n), Hall mobility ( ), and electri-cal resistivity ( ) versus deposition temperature. (b) Spectral trans-mittance versus deposition temperature.thus reduces carrier mobility ?10; 11 . Both factors work togetherand co-determine that minimal electrical resistivity occurs atthe intermediate temperature.Figure 2(b) shows the optical transmittance in the UV-VISrange of AZO films deposited at different deposition temper-atures, with sputtering pressure and deposition power fixed.By increasing the deposition temperature, the transmittance in-creasesin the short wavelength range (340 – 450nm), and thenreaches saturation at an intermediate temperature. As men-tioned above, temperature increase leads to higher carrier con-centration and wider optical bandgap, which shortens the ‘ cut-off ’ wavelength of light absorption, leading to a higher trans-mittance in the short wavelength range?10; 11 .Effects of sputtering pressure on AZO properties wereplotted in Fig. 3. As shown in Fig. 3(a), sputtering pressureplays a negligible role in carrier concentration in the wholepressure range examined in this study. However, Hall mobil-ity has a decreasing trend with increase of sputtering pressure,especially in the higher pressure range. Accordingly, electricalresistivity basically remains at a low level of 4 10 4 cm atthelower pressurerange, but evidently increases after that. Thismay be explained by less collision among sputtered atoms andargon ions, and thus less energy loss at a low sputtering pres-sure. Higher energy of sputtering species helps improve thecrystallization of the AZO film, resulting in less carrier scat-tering and thus higher mobility. At a higher sputtering pres-sure, a higher probability of collision between argon ions andsputtered atoms results in more energy loss when the atoms063004-2J. Semicond. 2013, 34(6) Lei Zhifang et al.Fig. 3. (a) Carrier concentration (n), Hall mobility ( ) and electri-cal resistivity ( ) versus sputtering pressure. (b) Spectral transmit-tance versus sputtering pressure.(Deposition temperature and sput-tering power were fixed, and AZO thickness is 860 nm).reach the substrate surface, which degrades the crystallizationof the AZO film, leading to lower mobility and higher resistiv-ity ?12; 13 .Sputtering pressure shows very minor effects on transmit-tance of AZO films as illustrated in Fig. 3(b). As mentionedearlier, AZO optical properties are considerably dominated byAl concentration, which is not sensitive to sputtering pressure.Thus, sputtering pressure influences electrical resistivity in-stead of film transmittance, unlike the casein which sputteringtemperature influences both properties.Dependence of carrier concentration, Hall mobility, andelectrical resistivity on sputtering power is shown in Fig. 4(a).It is observed that as sputtering power increases, carrier con-centration and Hall mobility increase initially and then satu-rate. Accordingly, electrical resistivity decreasesfirst and thenincreases, with a minimum value at a higher sputtering powervalue. At lower sputtering power, the sputtering atoms mightnot have sufficient energy to break the Al –O bonds in Al 2 O3 .Thus, zinc atoms and oxygen vacancies play a dominant rolein electrical conductivity. At higher power, more Al –O bondscan be broken up, and Al plays a key role in electrical conduc-tivity. The high energy of sputtering atoms may also improvethe crystallization of AZO films, making grains bigger and re-ducing defect numbers. All these contribute to the electricalconductivity as illustrated in Fig. 4. A slight increase of elec-trical resistivity at high power may be explained as that sput-Fig. 4. (a) Carrier concentration (n), Hall mobility ( ) and electricalresistivity ( ) versussputtering power. (b) Spectral transmittance ver-sussputtering power (Deposition temperature andsputtering pressurewere fixed, andAZO thickness is 860 nm).tering species with very high energy can damage the growingsurface, which leads to deterioration of crystal structures andAZO properties ?14; 15 .Figure 4(b) shows spectral transmittance of AZO films atdifferent sputtering power. AZO transmittance increases in theshort wavelength range (340 – 450nm) as sputtering power in-creases. This may be attributed to a higher carrier concentra-tion andwider optical bandgap at a higher sputtering power ac-cording to the Burstein – Mosseffect ?14 16 . When carrier con-centration increases, the lower levels in the conduction bandareoccupied by electrons, resulting in an increase in the Fermilevel and then the optical band gap widens. The optical trans-mittance in the short wavelength range (340 – 450nm) of thefilm deposited at higher power is larger than that of AZO filmsdeposited at lower power, suggesting that higher power is morehelpful for the effective substitution of Zn2C ions by Al 3 C ionsreleasing extra electrons into the conduction band.It should be noted that, although film thickness is targetedat 860 nm, the compactness of the AZO films varies among dif-ferent processes; and actual deposition thickness also has mi-nor variation, therefore the optical thickness of the AZO filmsseemsto be slightly different, as shown in Figs. 2(b), 3(b) and4(b).Considering the electrical and optical performance of AZOthin films for tandem thin film Si solar cell applications, thepreferred ranges of sputtering process parameters from this063004-3J. Semicond. 2013, 34(6) Lei Zhifang et al.Fig. 5. Haze ratio (averageover 380– 760nm) variation of post etchedAZO films in 0.3% HCl with different etching time at room tempera-ture.study are advised as follows: temperature 250– 350?C, pres-sure 0.25 – 0.68Pa, and power 400– 500W.3.3. Effects of texturing processesSuitable surface morphology and haze ratio of texturedAZO films can enhance light scattering and absorption insidethe cell. The rough surface structure can reduce directional re-flection, increase internal reflection effects, enhance the effec-tive absorption of solar energy, and consequently improve thepower efficiency of the solar cells. To create the desired tex-tures, the etching processes were varied as follows: the as-deposited AZO films were etched for 10, 20, 30, 40, 60 and80 s, in 0.3% hydrochloric acid solution (8.1 mL HCl/1 L D.I.water). Then, the etchant concentration was varied to find anappropriate etching rate. Additionally, the etching temperaturerange of 25– 48?C was also examined.Figure 5 shows the haze ratio obviously increases alongwith the etching time varying from 10 to 80 s. A longer etch-ing time leads to more craters, a larger etching depth, and sig-nificantly higher haze ratio. At a constant average etching rate,however, long etch time causescraters to reach the substrateand thus increases resistance significantly (not shown here).The AFM micrographs of the textured AZO films for differentetching times also show a similar tendency. The etch depth andlateral diameter increase linearly with the etching time. Also,the surface of films receiving a long etching time (more than40 s) is full of deep craters approaching the substrate.With the increase of the concentration of etchant (HCl %),a dominant rise in film haze ratio and etching rate is observedin Fig. 6. AFM images show that a higher concentration has alarger etching depth. When the concentration further increases,excessive corrosion occurs, more craters touch the substrateand result in resistance increase (not shown here).Meanwhile, the etching temperature was also investigated.Figure 7 illustrates the effects of various etching temperatureson the AZO film haze ratio and etchin