钛白粉用于染料敏化太阳能电池的数码印刷

Summary

This paper investigates the suitability of inkjet printing for the manufacturing of dye-sensitized solar cells. A binder-free TiO₂ nanoparticle ink was formulated and printed onto a FTO glass substrate. The printed layer was fabricated into a cell with an active area of 0.25 cm² and an efficiency of 3.5%.

Abstract

Silicon solar cell manufacturing is an expensive and high energy consuming process. In contrast, dye sensitized solar cell production is less environmentally damaging with lower processing temperatures presenting a viable and low cost alternative to conventional production. This paper further enhances these environmental credentials by evaluating the digital printing and therefore additive production route for these cells. This is achieved here by investigating the formation and performance of a metal oxide photoelectrode using nanoparticle sized titanium dioxide. An ink-jettable material was formulated, characterized and printed with a piezoelectric inkjet head to produce a 2.6 µm thick layer. The resultant printed layer was fabricated into a functioning cell with an active area of 0.25 cm² and a power conversion efficiency of 3.5%. The binder-free formulation resulted in a reduced processing temperature of 250 °C, compatible with flexible polyamide substrates which are stable up to temperatures of 350 ˚C. The authors are continuing to develop this process route by investigating inkjet printing of other layers within dye sensitized solar cells.

Introduction

Conventional silicon solar cells are made from highly pure materials that require expensive and high-energy consuming specialist equipment. These conventional silicon cells incorporate a p-n junction that requires highly pure materials at the interface to generate electron-hole pairs. Dye-sensitized solar cells (DSSCs) have a fundamentally different working principle, where charge generation takes place at the materials interface. This means that processing under vacuum, ultrahigh temperatures or the use of clean room facilities are not required¹. Therefore they are seen as a potentially low cost alternative; however up-scaling from small laboratory test cells into large prototypes for industrial manufacturing involves overcoming several issues including the rapid patterning of substrates.

Electronics manufacturing generally requires a degree of patterning, which is either achieved by masking or selective removal of the material after deposition. These steps can be removed through the use of “additive” digital printing techniques such as inkjet printing or spray coating. Digital printing is a promising method for direct deposition of functional materials for electronic devices. The technique can be described as printing from a digital-based pattern directly to a variety of substrates². They are non-contact methods, which will not damage or contaminate the substrate surface and deposit material only where it is required, resulting in little or no wastage³. These techniques have been highlighted as being ideally suited to being scaled up to high-volume production³. Since digital printing methods use liquid forms of materials dispersed in a solvent, it is critical to understand the deposition of ink to determine the applications of the technique.

DSSCs have three main components: a porous layer of wide bandgap metal oxide material, a dye that covers the particles, and a “charge transporter” that infiltrates the pores within the porous layer of semiconductor. These are sandwiched in between a transparent conductive electrode and a counter electrode⁴. The counter electrode is coated with a catalytic material for electron transfer, which in most cases is platinum. Under illumination, the dye molecules will absorb energy in the form of photons. The dye molecules then become excited and charge separation occurs at the interface of the titanium dioxide and the dye. Electrons are ejected into the adjacent metal oxide particles and ‘holes’ are left behind on the dye molecule. The injected electrons travel through the metal oxide particles and reach the transparent conductive electrode. When a load is connected, the electrons move to the counter electrode through the external circuit and are finally reunited with their counter charges through the redox couple present in the electrolyte¹. The nano-structured metal oxide layer within DSSCs plays a critical role in the overall performance of the cell, with material choice, processing methods and nature of the structure all having influencing factors^5-10. One of the most important requirements for the photoanode is that it needs to have an extremely large surface area. This is achieved through the deposition of nanoparticle materials, commonly TiO₂^1,11. This has been fabricated by countless different processes, however wet coating techniques such as screen-printing and doctor-blading, are still the most popular approach^9,12,13.

Inkjet technology is a potential manufacturing route for dye-sensitized solar cells. It uses the movement of a piezoelectric crystal to expel a fixed quantity of liquid through a nozzle onto the desired substrate. This deposition method allows material to be jetted very accurately but also at high frequency with a potentially high print speed or deposition rate. Inkjet technology is sensitive to the viscosity of the ink used and this was previously a barrier to the development of functional inks. Recent work in the development of solvents suitable for ink formulation has helped to alleviate this problem, and printing of electronic components using 2D layered materials such as graphene has been demonstrated¹⁴. The viscosity of nanoparticle suspensions such as these has been found to depend on the nanoparticle size and concentration¹⁵. High concentrations of nanoparticles result in higher viscosities, therefore particle loadings are usually around 10 wt% to avoid nozzle blockages¹⁶, however higher concentrations have been achieved¹⁷.

The key advantages of inkjet technology include it being non-contact, additive patterning and maskless¹⁸. The latter two attributes are due to the ability to position many nozzles together on one or more printheads, with each nozzle separately addressable by the control software. This allows highly complex, multi-layered patterns to be created very rapidly as the printheads move across the substrate. No masking between materials or layers is required as the position of each ink drop is accurately controlled, in some systems to an accuracy of ~1.5 µm¹⁹. One of the key benefits is that inkjet technology is mature, with significant development carried out in the latter half of the twentieth century. The result is that the inkjet is a very scalable technology, with roll-to-roll systems capable of printing accurately onto flexible substrates at rates of many meters per second. Traditionally this was used for high volume production, e.g., newspapers. However, developments in technology have allowed the inkjet to be used in roll-to-roll production of electronic circuits using nanoparticulate silver inks²⁰. The inkjet is therefore an attractive process for the potential production of dye-sensitized solar cells by digital printing.

Protocol

1.墨水配方注：油墨配方经常被制造商一个高度保守的秘密。成功的配方平衡喷射，形成滴，湿润和干燥性能旁边的功能表现。一般的功能性材料分散在溶剂中，至少一种其它组分以使它们可喷射。本节详细介绍了TiO 2的墨水喷墨印刷中使用的发展。墨的一小批料通过以下方法制备。 注意：油墨制剂应以适当通风面积， 例如来进行，在通风橱下，同时穿眼护目镜和乳胶手套。 制备盐酸（HCl）的0.1 mM溶液，以产生pH为约4。 32克酸溶液添加至8g相容溶剂的具有较高沸点和较低的表面张力比水（如二甲基甲酰胺（DMF））。加入共SOLV的耳鼻喉科充当干燥剂以诱导墨滴的墨水蒸发内循环流，导致纳米颗粒的均匀放置液滴21的表面上。 添加1.5克分散添加剂（丙二醇和水四甲基-5-癸炔-4,7-二醇的45％活性溶液）。 加入10克乙二醇，作为保湿剂以防止干燥的喷嘴。 0.5克消泡剂（在甲氧基炔二醇的20％活性溶液）添加到墨，以防止气泡的产生。 通过取墨水的等分试样放入密闭容器中执行简单的抖动测试，并用手摇动60秒。如果观察到任何泡沫再加入另一0.5克消泡剂的墨水。 混合使用磁力搅拌棒以确保均匀在RT 8小时的溶液中。 添加1.5克二氧化钛（TiO 2的）纳米粒子具有21纳米的初级颗粒尺寸和表面积的35 – 65米2 /克。 使用15分钟的超声波探头，在60赫兹的频率超声处理该混合物。 测量粒径，根据制造商的协议使用适当的测量技术，例如动态光散射（DLS），以确保它们将通过喷嘴开口容易流动。使在相同的条件下进行测量（ 例如 ，相同的溶剂，pH值，分散剂的浓度），以用于油墨作为各组分可在油墨中影响附聚物的形成。对于成功的喷射，在流体中的颗粒应比喷嘴开口小100倍。 根据制造商的协议测量油墨的粘度，用适当的测量技术，如旋转粘度计，以确保从打印头可靠喷射喷墨印刷需要2至20厘泊（cP）的粘度低的油墨。通过additi增加粘度上的聚合物材料或基于纤维素的材料;然而这些需要在沉积之后被移除，以释放用于印刷膜22内的染色位。 根据制造商的协议测量油墨的表面张力，使用合适的测量技术，如张力，以确保可靠的喷射。喷墨打印机的可喷射液配方指南建议28和33达因/厘米之间的表面张力，以实现可靠的打印。 2.喷墨打印在打印之前，浸泡在玻璃基板在清洁洗涤剂（阴离子和非离子表面活性剂的混合物，稳定剂，碱，非磷酸盐洗涤剂助洗剂和多价螯合剂，在含水碱）的2重量％的溶液中去离子水。一旦冲洗用去离子水彻底玻璃，因为它们是从清洗液中除去以除去污染和清洗洗涤剂的痕迹。 </l我> 测量基底的表面能，使用合适的测量技术，例如根据制造商的协议的张力。对良好的粘合性，基材的表面能应不超过超过10的流体的表面张力 – 15达因/厘米。修改使用方法在衬底的表面能，如电晕处理23，等离子体处理24和 ​​化学蚀刻25，如果它是不适合的。 根据制造商的协议衬底装入打印机。 通过位于头部一侧的端口冲洗与墨打印头以取代贮存器和喷嘴中的任何空气或清洁溶液。 将打印头插入打印机。与连接头专用板的打印头。 刚装入墨盒去除大颗粒聚集，可堵塞喷嘴前过滤通过正确的大小过滤器的墨水。该在这项工作中使用的打印头具有直径为40μm的喷嘴（ 例如 ，柯尼卡KM512）;因此墨不应该包含具有直径大于400nm的粒子。通过5微米，接着通过1.2微米聚偏二氟乙烯（PVDF）过滤器通过混悬液以除去任何大颗粒。 加载墨入150毫升注射器位于打印头以上，供应墨水至打印头。附着在针筒顶部的气密盖和开启真空泵。 通过按压位于真空泵'清洗'按钮清除通过喷嘴的油墨。 通过地理信息系统（GIS）的打印服务器，设置波形和打印参数。注意，打印机可以打印多达每秒1.5米的速度，但是对于这种油墨已经发现每秒0.3米打印速度，以提供最佳涂层开放的GIS用户界面软件和装入所需的图案。 镨根据制造商的协议从装载盒诠释。 除去从压纸基材并加热，在150ºC30分钟的印刷薄膜，随后250ºC另外30分钟或在热板上或在烘箱中。 3.印刷薄膜的分析使用光学显微镜或扫描电子显微镜（SEM），以看在低放大倍数（100X）印刷膜的表面来分析表面形貌和在高放大倍率（35,000X）来分析在印刷膜的孔隙率。检查的图像显示了均匀覆盖，没有裂纹和良好的孔隙率。在SEM操作的更多详细信息可在下列参考26,27找到。 测量印刷层的厚度，使用合适的测量技术，例如根据制造商的协议的表面轮廓。在TiO 2层机智的厚度和孔隙率欣DSSC中影响可被吸收到纳米颗粒，其中以此来影响细胞18的整体电转换效率的表面上的染料的量。因此，它是评价的一个重要参数。用表面轮廓（1纳米精度）来测量在印刷膜的厚度。 测量该膜的透光率，利用适当的测量技术，如紫外可见（UV-VIS）光谱，以确定多的可见光将如何通过印刷膜发送。使用制造商的协议。 4.上述细胞通过用磁力搅拌器8小时混合20ml乙醇和2mg钌染料在玻璃烧杯中让染料溶液。 淹没在TiO 2涂覆的玻璃在室温（20至25℃）溶液24小时，以使染料吸收到TiO 2颗粒的表面上。 除去TiO 2的</从溶液和地点到薄纸子>涂覆的玻璃，以吸收任何过量染料溶液（用TiO 2的朝上以避免污染）。 放置在导电玻璃顶部的预切为60μm厚的热塑性密封垫片，周边的 TiO 2涂层。 放置在预切为60μm厚的热塑性密封垫片的顶部的铂涂覆的反电极，以使阳极和阴极的有源侧面彼此面对。允许将两块玻璃，使得电接触可与导电玻璃制成之间有足够的重叠。这应该具有在中心的预钻孔，以允许电解质以后填充。 热的热板上，以110℃的温度和施加轻的压力用镊子在密封垫片的面积。 30秒后，将电极应一起密封。 填充在两个电极之间的间隙与碘化物/三碘化物的电莱特在乙腈中的50mM的浓度，通过在使用注射器的铂涂覆的玻璃的预钻孔注入。

Representative Results

一个二氧化钛油墨根据列出的程序制定。悬挂在油墨中的颗粒的大小，使用动态光散射（DLS）和80纳米（nm）中观察到的平均粒径进行测定。在这项工作中的墨的粘度被发现是3厘泊，使用具有小样品适配器旋转粘度计，并用18毫米的心轴直径测量。使用张力计测定表面张力和经计算为26达因/厘米的平均值。 在FTO玻璃的表面能是根据欧洲标准EN 828用于通过测量接触角和表面自由能确定的固体表面的润湿性来计算。三种不同的液体（水，乙二醇和二碘甲烷）十滴分配到一个平面试验片表面。对于每个液滴，左和右的接触角分别为测量电编辑。从各液体在其表面张力相结合的平均接触角，试片的表面自由能的计算方法。在授权给Fowkes方法计算来自从色散相互作用（的γd）和γnon色散相互作用（γP）的贡献的总和的总的表面能（γ）。这种方法导致了26.45达因/厘米为在FTO涂覆玻璃的表面自由能。 印刷根据上述步骤，以生产500平方毫米进行。玻璃上的印刷层的厚度使用表面轮廓仪测定的。在印刷层的中心的最大厚度经测定为2.6微米。涂层玻璃的透射率使用UV-VIS分光计测量。在700纳米的波长，60％的透射率对在TiO 2印刷膜测得的FTO玻璃78％进行比较。 <p class="jove_content" fo:keep-together.within-page ="“1”">光伏器件是根据上述过程的轮廓产生，并直接特征在于制造之后以最小化降解的空气中的引起的水和氧的影响。但是也有一些用于表征和比较太阳能电池28 5个电性能参数。短路电流（I SC）和开路电压（V OC）的值可从电流-电压（IV）曲线导出。然后，这些可被用于确定填充因子（FF）和功率转换效率（η）。将FF给出了细胞的实际最大功率输出的比率的开路电压和短路电流29的产物。这是在评价太阳能电池的性能的关键参数。高FF是指低电亏损，而低FF显示有改进的余地。几个因素是已知的以影响FF包括在细胞内的各层的质量和接口。 DS旺纳入碘化物/三碘化物的氧化还原夫妇用11.9％的报告记录效率填补0.71 30个因素。所有这些参数的需要的标准测试条件，其中设备温度为25℃时，光的光谱辐照分布具有1.5的气团下确定，总辐照测量（E m）上的太阳能电池单元100毫瓦/平方厘米2。用于单个pn结电池的转换效率的理论最大值已被广泛报道为37.7％31，但是对于DSSC中已报道的最大的效率是接近15.1％与吸收起始在920处32。 输出电流和电压分别利用源表，同时将细胞与100毫瓦/厘米2的光装有一个过滤器，以配合1.5的气团的光谱辐照分布源照射进行测定。结果进行比较，以小区使用刮刃的 TiO 2层使用市售的糊具有锐钛矿颗粒20纳米和450纳米的共混物制备。印刷层具有0.25 平方厘米的面积，这是使用表面轮廓仪测得的18微米的平均厚度。在两个设备之间的光电性能的对比示于图1和表1。 几项研究已经调查了在TiO 2层的厚度和DSSC中内转换效率之间的关系。结果显著变化，具有从任何地方9.5微米和20微米之间33-39。 表1列出了TiO 2的印刷层和效率的厚度报告最佳膜厚度。印刷的 TiO 2的喷墨的厚度大于刀刃的 TiO 2医生显著少，导致在一个较低的效率。今后的工作将调查墨水制剂中使用的有机粘合剂，以增加在喷墨印刷层的厚度。 图1.喷墨印刷和医生Bladed中二氧化钛 层 DSSC中的性能曲线 。电流密度/电压曲线结合印刷的 TiO 2层和医生刃二氧化钛层的喷墨DSSC中。在设备的短路电流密度印制二氧化钛喷墨2层比与医生刃二氧化钛层导致较低的整体转换效率的设备显著低。 请点击此处查看该图的放大版本。 <td行跨度=“3”> 短路电流开路电压填充因子效率厚度 （毫安/平方厘米2） （毫伏） （％） （微米） 喷墨印刷 9.42 760 0.49 3.5 2.6 医生刀片 11 756 0.58 4.8 18 的细胞在图1该表的太阳能电池的关键参数包括开路电压（V OC），短路电流（I SC），其确定所述效率（η）指定的光下进行比较表1中的关键性能特性情况介绍。该参数o使用医生刃二氧化钛层产生的发细胞也被列入了比较。这两种设备的填充因子（FF）是其通常归因于小区内的高内电阻相当低。

Discussion

A particular challenge when formulating inks is the natural tendency for nanoparticles to cluster together. These are known as either aggregates or agglomerates, depending on the nature and strength of the bonds between the particles. The energy of simply stirring particles into water or binder is not great enough to overcome the particle attractive forces preventing the breakup of agglomerates. Ball milling, high shear mixing or ultrasonication are commonly used to break up agglomerated nanoparticles. Various anionic, nonionic, and cationic surfactants and polymers can also be used to provide long-term stabilization. By minimizing the number of these agglomerates, a good quality suspension can be achieved. The fluids should be filtered through the correct size filter just before loading into the cartridge to remove large particle aggregates which can clog the nozzles.

The particle size within the TiO₂ layer also has been shown to influence the overall efficiency of DSSCs. The photocatalytic activity of titanium dioxide increases as particle sizes decrease due to an increase in the specific surface area⁴⁰. A study comparing the efficiency of DSSCs incorporating TiO₂ nanoparticles with 5 different sizes ranging from 400 nm to 14 nm and found that those with smaller particle sizes resulted in better electrical conversion efficiencies³³.

Inkjet printing is a non-contact deposition technique capable of multi-pass printing. This presents the unique opportunity to rapidly fabricate multilayer devices in one operation on a wide range of substrates with minimal material waste. It also potentially provides a way to integrate other components (such as batteries) into the system through the printing of functional materials⁴¹. Although the representative results shown for the inkjet printed devices do not perform as well as the doctor-bladed devices, it demonstrates the potential for the deposition technique. With further ink optimization, it could perform on a comparable level to currently used methods and may provide further scope for cost-effective, environmentally friendly integration of photovoltaic cells onto a wide range of substrates. We hope to improve the efficiency of the inkjet printed devices by increasing the thickness of the printed layer closer to that of the doctor-bladed TiO₂ and will continue to look at the printing of other materials and layers within DSSCs.

Materials

Titanium dioxide	Sigma Aldrich	718467
Deionized water			Supplied from a filter in the laboratory
Hydrochloric acid, 2M(2N)	Fisher Scientific	J/4250/17
Dimethylformamide (DMF)	Fisher Scientific	D/3840/08
Ethanol	VWR Chemicals	20721.33
Dispersing additive	Air Products
Defoaming agent	Air Products
Ethylene glycol	Fluka	107-21-1
Polyvinylidene fluoride (PVDF) syringe filter	VWR International
Cleaning detergent	Fisher Scientific	10335650
Fluorine doped tin oxide (FTO) glass, 8 Ω/sq	Pilkington
Ruthenizer dye	Solaronix	21613
Pre-cut 60 µm thick thermoplastic sealing film	Solaronix	74301
50 mM iodide/tri-iodide electrolyte  in acetonitrile	Solaronix	31111
Platinum coated FTO glass	Solaronix	74201
Vac'n'Fill Syringe	Solaronix	65209
Polyimide tape (6.35 mm)	Onecall Farnell	1676087