基于氧化锌的纳米管染料敏化太阳能电池翻译.doc

基于氧化锌的纳米管染料敏化太阳能电池 纳米快报 2007 Vol.7，No.8 21832187 Alex B. F. Martinson, Jeffrey W. Elam, Joseph T. Hupp,*, and Michael J. Pellin 伊利诺斯州60208，谢里丹路2145号，西北大学，阿贡国家实验室，,和南方卡斯大道9700号 ，阿贡国家实验室，伊利诺斯60439 文章于2007年一月收到， 于2007年五月收到修改后的底稿 文章摘要我们介绍了被使用在染料敏化太阳能电池(DSSCs)中以氧化铝膜为模板的高表面积氧化锌纳米管的光阳极。原子沉积层是被用于涂层孔隙共形 ，为了收集超过数十微米厚的电荷而所提供的直接路径的方法。相比起同类以ZnO为基础的设备，氧化锌的纳米管除了能源利率高达1.6%之外，还表现出出色的光电压和填充因子。这种新颖的制造技术提供了一种简便、金属氧化物综合的途径去清晰明确地定义敏化太阳能电池的光阳极。具有良好捕光效率的高标准染料敏化太阳能电池的是一种染上适度消光物质并拥有高表面积光电阳极（1200倍的扁平电极的面积）的产品。这种联合体能充分吸收大部分的可见光谱和红色波长所提升的空间。在最有效的电池中，光子被染料分子产生几十飞秒的时间尺度上的激子所拦截，因而产生电荷分离的效率接近于统一。最后能效的级别和电荷收集量是由电荷迁移率和电荷重组率之间的不同所支配。 为了能与相对缓慢（毫秒级）的纳米颗粒传输相竞争，一个例如碘化/三碘化物这样极缓慢的氧化还原穿梭必须采用阻碍其重组的方式。因此，典型的染料敏化太阳能电池，基于在TiO2和碘化物/三碘化物材料上，尽管出现了小的电子扩散系数，但是表现出非常良好的电荷收集性能。这些显著的反应对这些电池为了最高能效记录（11%）的记述是非常重要的。为了推进装置的性能超越其目前的限制，更快的氧化还原穿梭（这需要一个较小的过电压以减少染料氧化）可以用于光伏的增加（前提是暗电流不增强）。另外，有较高的表面积结构将增加光捕获效率，因而增加光电流。任何一个方案的成功实施，都需要放缓电荷的重组。降低伴随着氧化还原穿梭的寄生反应的反应速率的方式包括了对无机屏障层上的金属氧化物的结构和染料的修改。 尽管金属氧化物的结构可允许在电池工作状态下完整的电荷收集，但是更快的氧化还原穿梭或高表面积的框架可能还需要更快的电荷运输条件。事实上，仅仅简单快速地替换碘化/三碘化物，而没有采取其他的变化，这已经被证实会由于电子收集问题而降低功率转换效率。为此，几种新型光阳极体系结构已经被制备，其中包括但不局限于水热法产生氧化锌纳米棒阵列、电沉积的氧化锌小板，和TiO2 通过钛阳极氧化形成气孔。由于其较低的陷阱密度和更直接路径的集电极，表面上一维（1-D）的纳米结构阵列被预期加快电荷迁移的速度而对重组产生不利的影响。然而，纳米棒设备的效率是受限于较低的光捕获效率。增加纳米棒阵列的表面面积依赖于通过水热法增长较高宽高比杆方法，这仍然是一个重大的技术挑战。在这里，我们介绍这一种新的光阳极设计具有非常高的宽高比子结构的特性和超过1000粗糙度系数（虽然我们最初的研究仅限于射频 E 450）。本设计实现策略结合阳极氧化铝（AAO）的模板和原子层沉积（ALD）产生定向电互连阵列的半导体纳米管。因为它是一个逐步和保形涂层技术，原子层沉积提供出色地控制纳米设备组成和结构。大量的金属氧化物通过ALD16接近（包括但不限于，二氧化钛，氧化锌，二氧化锡，氧化锆，和NiO）使得该技术可能是非常广泛适用于新光电极的开发。在这封信中我们说明这些结构的ZnO版本的可行性，例如描述染料敏化特征和形态，捕光效率和光伏性能。一个具有25-50为可穿透的200nm细孔的60微米厚薄膜是使用氮原子层沉积法涂有氧化锌，通过交替暴露于二乙基锌和水温度在200 C使用反应物接触6次数和在接触期间用氮气扫吹5次。再将膜在空气中焙烧400至30分钟，以增加结晶度。1 m厚的电极组成的透明、 导电掺铝（AZO）氧化锌通过ALD一侧沉积。本研究选择的商业AAO膜具有缩小至20nm内最后一个微米厚度的一侧。在偶氮沉积期间，除了薄膜的小缝隙表面其他都被金属所覆盖固定。为了提高电接触偶氮染料涂覆面积，100 nm的Au被蒸发到涂层的多孔氧化铝膜的边缘。经过加热到 200 C 和随后冷却至 80 C，微热的膜被引入到0.5毫米 (Bu 4 N) 2 - Ru(4,4-(COOH)-2,2-bipyridine) 2 (NCS) 2 (“N719”, Dyesol, B2 dye) 乙醇30 min后，用乙腈快速冲洗。一个50米厚的沙林结构是类似三文治一样介于膜的开孔一侧与镀铂掺氟氧化锡（FTO）电极之间。而光压力施加在130以密封电池。0.5M的的LiI的溶液，0.05mM I2和0.5M叔丁基吡啶在3-甲氧基丙腈作用下通过填充真空在FTO电极上的一个孔而被引入电池中。从而额外的沙林和显微镜的盖玻片密封电解液进入电池中。单色照明是通过一个Jobin Yvon公司的荧光激发单色器实现。入射光子-电流效率（IPCE）可由 CH1202 恒电位仪测定仪器测定。有源区由沙林框架限制为0.28 cm2 和被另外遮住照明了的黑色绝缘胶带为相同的大小。一类新技术从光谱与906瓦/米2电力的太阳能电池分析仪测定AM1.5效率。为了更好的了解电荷收集，电池也通过通过PtFTO电极被启动，可见其衰减20%的可见光波长的光吸收主要是由于铂。对同一批光阳极，染料负载量由10毫KOH测定从膜N719解吸的吸光度定量在Varian卡里5000。 图1。商业AAO 模板的横断面扫描电镜图像。 膜孔涂覆有通过ALD20纳米的氧化锌 作为一种惰性、强大且呈无色结构，阳极氧化铝是用于太阳能电池的理想基础电极。有大量的文献记载用于制造独立六角形从10到300 nm，孔密度超过100000000000孔/厘米2 排列毛孔大小不等的阳极氧化铝膜的描述和理论方法。在这项研究中，商业膜被选定为他们的准备情况。商业膜的限制包括定义孔隙排序，可见光的强散射，并限制到只有几个孔隙的大小和长度。 六角形排列的孔的几何考察使粗糙度因子分别是从R，L的估计，孔隙半径D，膜的厚度和中心孔的间距。虽然缺乏有序性，一个210纳米，329纳米的间距，和64微米的长度的平均孔径通过扫描电子显微镜（SEM）进行估计可得出射频450。这个比拟方便测量BET比表面积为487cm2/cm2的膜。正如预期ALD序列需要足够的曝光时间，由此产生的多晶ZnO薄膜是连续的保形，如图1所示。通过对64米厚的膜的电阻的测量（48，8nm厚的ZnO），这提供了额外的证据表明涂表明涂料涵盖细孔的长度。由于沉积，ALD氧化锌有许多氧空位，使薄膜为半导电性并导致较低的电阻。ZnO 薄膜的晶性质可能直接观察扫描电镜和 x-射线衍射数据的印证。这两种方法都表明了晶粒约20纳米的粒径。图2。横断面扫描电镜图像的商业AAO膜 涂有透明导电氧化物偶氮染料如图2，AZO的薄膜涂层通过ALD选择性地施加到所述膜的一侧。窄孔末端和短曝光时间的组合（0.15s）防止一般保形沉积技术在涂层孔隙内部出现。图3。在500毫微米（蓝色、打开符号）和计算的粗糙度因子（橙色、封闭符号）吸染料的吸光度。线是最佳线性表示数据的。如图3中，通过用表面面积计算协议来减少涂孔r的染料装载，峰值吸收率是 0.71 时 在500 nm处为2nm氧化锌涂层随着厚度的增加减少。图4.短路光电流（蓝色，空心符号）和开路光电压（橙，关闭符号）使用ZnO管壁厚度的函数。 正如预期的那样，适度的射频（1 /3这些最佳的染料敏化太阳能电池）会正如我们希望的极限电流密度那样导致整体吸光效率相对较低。图4显示了了短路光电流密度（J SC）随着一系列装置的纳米管管壁厚度增加而增加。在该控制装置中，当缺少的ZnO时，J SC是非常低的。这并不奇怪,因为有用的表面积是类似于一个平面电极的。随着管壁的变薄，偶氮电极收集的电荷也相对减少了。最有可能是由于通过在ZnO缓慢电荷输送并由于染料分子电子的高稳态浓度加速电荷的组合。当管壁增厚，电子更自由地流动，J SC急剧上升。随后的锥形壁较厚，随后的锥形壁较厚，与随着减少的染料装载为相一致的。 在仅为1纳米氧化锌沉积之后，开路光电压（V OC）急剧上升。在缺少ZnO的情况下非常低的电压可能是由与重掺杂AZO层的氧化还原穿梭直接接触而引起的，其导电带（相对于氧化锌的）不能固定电池光电压是由于直接接触的而引起的。光电压的随着ZnO的增加而增加直到ZnO厚度为7纳米。这个V OC（739 MV）超过了以前报道的ZnO的染料敏化太阳能电池的开路电压得最高值（约670-710 MV）。但由于低区透明导电氧化物(TCO)暴露在新设备几何形状，优良的光电压是最有可能不是改善电荷传输的结果。虽然许多基于TiO2的太阳能电池使用致密的阻挡层或四氯化钛处理，以抑制在所述TCO氧化还原梭的寄生反应，但很少有优先对这种类型的钝化氧化锌的系统。增加ZnO膜厚度提供第二功能，即缩小标称20nm的孔直接相邻的TCO。光电压的增长与这些逐渐变尖的细孔直到电压达到峰值周围的毛孔的厚度将完全限制电解质进入TCO。抑制重组通过另外一个更小的TCO（1 / 3）重组ZnO的表面面积，相对于纳米薄膜，也许可以解释上的V OC。由V oc值观察到虽然比较高,仍然是大约200 - 260 mV小于最大理论上获得基于氧化还原穿梭和染料激发态的潜力的差异。原则上，它应该有可能通过抑制暗电流（减慢电荷重组）来捕获的剩余200-260毫伏一些部分。阻挡层的沉积是可以实现这样的效果的一种方式。 了解电池性能的改善与提高ZnO层的厚度通过调查电荷载流子寿命（电子/三碘化物重组次）而获得。光电压衰减的分析提供相关电子寿命(n)的光电压与时间的图标,k B是波尔兹曼常数的表达式,绝对温度T,q是正电荷。电子寿命作为光电压的函数显示了两个氧化锌薄膜厚度,如图5。相对于其他氧化锌设备,最好的氧化锌纳米管设备的光电压衰减是相似的。在相同电位下(0.6 V)、电子寿命提高是因为氧化锌作用于管壁厚度高达9纳米的孔隙，插图图5。图5。电荷寿命与光的装置与4 nm孔壁（蓝色）和9 nm的孔壁（橙色）。插图显示充电寿命等电位的函数的孔壁厚度（600MV）。图7。的I-V曲线是最有效的细胞，根据7 nm的ZnO。模拟AM1.5光照。图6。入射光子-电流效率（蓝色，打开符号）和光捕获效率（橙色，封闭的符号）ZnO细胞与5 nm的孔壁。图8.比光电流，背面VS前端照明。的比例并没有得到纠正对于光损失（约20）通过在背面照度的镀铂电极。 这一趋势是惊人的相似，正如图4所示的变化，（即，J SC随ZnO层的厚度增加而增加）。这是否反映了随着厚度的增加氧化锌反应活性发生内在变化，或简单地更好阻断暴露的TCO电子重组，这个结论还有待证实。图6显示了在LET 在500m的峰值时N719的特性。正如预期那样，合适的表面面积的电极，所述的AM1.5光谱的一小部分是由电池吸收。峰值入射光子的 - 电流效率（IPCE）出现20nm的红移从表观LHE峰与解吸染料评估。ZnO Nanotube Based Dye-SensitizedSolar CellsAlex B. F. Martinson, Jeffrey W. Elam, Joseph T. Hupp,*, andMichael J. PellinNorthwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208, andArgonne National Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439Received January 20, 2007; Revised Manuscript Received May 25, 2007ABSTRACTWe introduce high surface area ZnO nanotube photoanodes templated by anodic aluminum oxide for use in dye-sensitized solar cells (DSSCs).Atomic layer deposition is utilized to coat pores conformally, providing a direct path for charge collection over tens of micrometers thickness.Compared to similar ZnO-based devices, ZnO nanotube cells show exceptional photovoltage and fill factors, in addition to power efficienciesup to 1.6%. The novel fabrication technique provides a facile, metal-oxide general route to well-defined DSSC photoanodes.The good light-harvesting efficiency of the best dyesensitizedsolar cells (DSSCs) is the product of a dye withmoderate extinction and a photoanode of high surface area(1200 times the area of a flat electrode). This combinationallows for ample absorbance over the majority of the visiblespectrum with room for improvement in the red wavelengths.1 In the most efficient cells, the photons interceptedby these molecular dyes create excitons that split on the tensof femtoseconds time scale resulting in charge separationefficiencies approaching unity.2,3 The efficiency of the finalstep, charge collection, is governed by the difference betweenthe rate of charge transport and the rate of charge recombination.In order to compete with relatively slow (millisecond)transport through the nanoparticle network, an exceedinglyslow redox shuttle such as iodide/triiodide must be employedto hinder recombination.4 Thus, prototypical DSSCs, basedon TiO2 and iodide/triiodide, exhibit very good electroncollection, despite small apparent electron diffusion coefficients.This remarkable behavior is important in accountingfor the record high efficiency (11%) of these cells.1In order to push device performance beyond its currentlimits, a faster redox shuttle (that requires a smaller overpotentialto reduce the oxidized dye) may be employed toincrease the photovoltage (provided that dark current in notenhanced). Alternatively, a higher surface area frameworkwould increase light-harvesting efficiency and, therefore,photocurrent. Successful implementation of either scenariowill require slowing charge recombination. Methods todecrease the rate of parasitic reactions with the redox shuttleinclude inorganic barrier layers on the metal oxide frameworkand modification of the dyes.5-7 A faster redox shuttle orhigher surface area framework may also require faster chargetransport though the metal oxide framework to allow forcomplete charge collection under cell operating conditions.Indeed, simply replacing iodide/triiodide with faster shuttles,but without implementing other changes, has been shownto decrease power conversion efficiencies due to theemergence of electron collection problems.8,9 To this end,several novel photoanode architectures have been fabricated,including but not limited to hydrothermally grown ZnOnanorod arrays, electrodeposited ZnO platelets, and TiO2pores formed via titanium anodization.10-12 Due to their lowertrap density and more direct path to the current collectingelectrode, arrays of nominally one-dimensional (1-D) nanostructuresare expected to speed charge migration withoutadversely affecting recombination. In the most successfulapplication of this idea to date, a 1.5% efficient ZnO nanorodarray has been shown to exhibit much faster transport thannanoparticle networks.10,13,14 The efficiency of the nanoroddevices, however, is limited by low light-harvesting efficiency.Increasing the surface area of the nanorod arraydepends on growing higher aspect ratio rods via hydrothermalmethods, which remains a significant technological challenge.15Here we introduce a new photoanode design featuring veryhigh aspect ratio substructures and having the potential forroughness factors (RFs) greater than 1000 (although ourinitial studies are limited to RF e 450). The designimplementation strategy combines anodic aluminum oxide(AAO) templating and atomic layer deposition (ALD) toyield oriented arrays of electrically interconnected semiconductornanotubes. Because it is both a stepwise and conformalcoating technique, ALD provides exceptional control* Corresponding author: e-mail, J-; telephone,1-(847)-491-3504; fax, 1-(847)-467-1425. Northwestern University. Argonne National Laboratory.NANOLETTERS2007Vol. 7, No. 82183-218710.1021/nl070160+ CCC: $37.00 2007 American Chemical SocietyPublished on Web 06/29/2007over nanoscale device composition and architecture. Thelarge number of metal oxides accessible by ALD16 (including,but not limited to, TiO2, ZnO, SnO2, ZrO2, and NiO) makesthe technique potentially very widely applicable for thedevelopment of new photoelectrodes. In this Letter wedemonstrate the viability of ZnO versions of these structuresas dye-sensitized electrodes by characterizing their morphology,light-harvesting efficiency, and photovoltaic performance.A nominally 60 m thick membrane with 200 nm poresthat is 25-50% porous (Anodisc, Whatman) was coated withZnO by atomic layer deposition via alternate exposure todiethyl zinc and water at a temperature of 200 C usingreactant exposure times of 6 s and nitrogen purge periods of5 s between exposures.17 The membranes were fired at400 C in air for 30 min to increase crystallinity. A 1 mthick electrode composed of transparent, conductive aluminumdopedzinc oxide (AZO) was deposited on one side by ALD.The commercial AAO membranes chosen for this study havepores that narrow to 20 nm within the last micrometer ofthickness of one side. During AZO deposition, a steel fixturemasked all but the small-pore face of the membrane. Toimprove the electrical contact to the AZO coating, 100 nmof Au was evaporated onto the coating along the edges ofthe AAO membrane.After heating to 200 C and subsequent cooling to 80 C,the warm membranes were introduced to 0.5 mM (Bu4N)2-Ru(4,4-(COOH)-2,2-bipyridine)2(NCS)2 (“N719”, Dyesol,B2 dye) in ethanol for 30 min followed by a quick rinsewith acetonitrile. A 50 m thick Surlyn frame was sandwichedbetween the open-pore side of the membrane and aplatinized fluorine-doped tin oxide (FTO) electrode. Lightpressure was applied at 130 C to seal the cell. A solutionof 0.5 M LiI, 0.05 mM I2, and 0.5 M tert-butylpyridine in3-methoxypropionitrile was introduced into the cell viavacuum backfilling through a hole in the FTO electrode.Additional Surlyn and a microscope cover slip sealed theelectrolyte into the cell. Monochromatic illumination wasachieved through the excitation monochromator of a Jobin-Yvon fluorescence spectrometer. Incident photon-to-currentefficiencies (IPCE) were measured with a CH Instruments1202 potentiostat. Active areas were limited to 0.28 cm2 bythe Surlyn frame and were additionally masked fromillumination by black electrical tape to the same size. AM1.5efficiencies were measured on a class A solar cell analyzerfrom Spectra-Nova Technologies with a power of 906 W/m2.To better understand charge collection, cells were alsoilluminated through the Pt/FTO electrode, which attenuated20% of the light at visible wavelengths due mostly to Ptabsorption. For an identical batch of photoanodes, dyeloading was quantified on a Varian Cary 5000 by measuringthe absorbance of N719 desorbed from the membranes by10 mM KOH.As an inert, robust, and colorless framework, anodicaluminum oxide is an ideal foundation for solar cellelectrodes. A large amount of literature exists describing thetheory and methodology for fabricating freestanding AAOmembranes with hexagonally ordered pores ranging in sizefrom 10 to 300 nm and pore densities in excess of 100 billionpores/cm2.18-20 For this study, commercial membranes wereselected for their ready availability. Limitations of commercialmembranes include ill-defined pore ordering, strongscattering of visible light, and restriction to only a few poresizes and lengths. Geometric consideration of hexagonallyarranged pores allows roughness factors to be estimated fromwhere r, l, and d are the pore radius, membrane thickness,and center-to-center pore spacing, respectively. Althoughpoorly ordered, an average pore diameter of 210 nm, spacingof 329 nm, and length of 64 m were estimated by scanningelectron microscopy (SEM), giving RF 450. This compareswell to the measured BET surface area of 487 cm2/cm2 ofthe membrane.As expected for an ALD sequence entailing sufficientexposure times, the resulting polycrystalline ZnO film iscontinuous and conformal, Figure 1. Measurements of theresistance through the 64 m thickness of the membrane(48 , 8 nm thick ZnO) provide additional evidence thatcoatings span the length of the pores. As-deposited, ALDZnO has numerous oxygen vacancies that make filmsmoderately conductive and account for the relatively lowresistance. The polycrystalline nature of the ZnO films maybe directly observed by SEM and is corroborated by X-raydiffraction data. Both methods suggest a grain size of20 nm.As shown in Figure 2, a thick coating of AZO was appliedselectively to one side of the membrane by ALD. Thecombination of narrow pore termini and short exposure times(0.15 s) prevented the typically conformal deposition techniquefrom significantly coating the pore interiors.As shown in Figure 3, reducing r by coating the poresreduces the dye loading, in agreement with surface areacalculations. The peak absorbance is 0.71 at 500 nm for a2 nm coating of ZnO and decreases with increasing thickness.Figure 1. Cross-sectional SEM image of commercial AAOmembrane pores coated with 20 nm of ZnO by ALD.RF ) 4x3rld2 (1)2184 Nano Lett., Vol. 7, No. 8, 2007As expected, the modest RFs (1/3 those of the best DSSCs)result in relatively low overall light-harvesting efficienciesthat we expect to limit photocurrent densities.Figure 4 shows the short-circuit photocurrent densities (Jsc)for a series of devices with increasing nanotube wallthickness. In the control device, lacking ZnO, Jsc is extremelylow. This is not surprising given that the useful surface areais similar to that of a flat electrode. With the thinnest tubewalls, relatively small amounts of charge are collected bythe AZO electrode, most likely due to a combination of slowcharge transport through the ZnO and accelerated recombinationowing to high steady-state concentrations of dyeinjectedelectrons. As the nanotube walls thicken, electronsflow more freely and Jsc rises sharply. Subsequent taperingwith thicker walls is consistent with decreasing dye loading(Figure 3).The photovoltage at open circuit (Voc) rises sharply afterdeposition of only 1 nm of ZnO. The exceptionally lowvoltage in the absence of ZnO is likely caused by the directcontact of the redox shuttle with the heavily doped AZOlayer, whose conduction band (in contrast to ZnOs) is unableto fix the cell photovoltage. The photovoltage increases withincreasing ZnO wall thickness until it plateaus after 7 nm.This Voc (739 mV) exceeds that of the highest previouslyreported ZnO DSSC photovoltages (ca. 670-710 mV).10,21The excellent photovoltage is most likely not a result ofimproved charge transport but a consequence of the low areaof transparent conducting oxide (TCO) exposed in the newdevice geometry. While many TiO2-based DSSCs employ adense blocking layer or TiCl4 treatment to inhibit the parasiticreaction of the redox shuttle at the TCO, there is littleprecedence for this type of passivation in ZnO systems. Forthe photoanodes of interest here, an increasing ZnO filmthickness serves a second function, namely to narrow thenominally 20 nm pores directly adjacent to the TCO. Anincrease in photovoltage coincides with tapering these smallpores until the photovoltage peaks around the thicknessexpected to completely restrict electrolyte access to the TCO.Inhibited recombination via the TCO in addition to asignificantly smaller (1/3) ZnO surface area for recombination,relative to nanoparticle films, may explain the superiorVoc. The Voc value observed here, while high, is still about200-260 mV less than the maximum theoretically obtainablebased on the difference of redox-shuttle and dye-excitedstatepotentials. In principle, it should be possible to capturesome fraction of the remaining 200-260 mV by suppressingd