RUI: Printable Metal Contacts to Realize Fully Printable All-Inorganic CdTe Photovoltaics

光伏（PV）能源转换可以提供比我们全球使用的更多的电力；然而，这种可再生能源仍然只占我们能源组合的3%，这主要是由于通过高温或真空沉积的材料加工成本过高。为了应对这种情况，各种低成本和快速的卷对卷（R2R）印刷技术被发明出来用于印刷介质，现在正在复兴，并适应于沉积纳米级电子墨水，以快速生产速度制造轻质柔韧性的电子设备。固态光伏器件是一层一层沉积的，在两个金属触点之间夹有半导体吸光材料。每一层的化学成分和加工条件可以调节器件的性能，这些性能受到层间界面上电子能级排列的显著影响。当电荷流过器件电路时，选择具有低电子能量的贵金属触点作为阳极，而选择活性金属在阴极进行合适的触点。光收集半导体和金属触点之间的能量不匹配可能导致产生的电荷的重大损失并限制器件性能。尽管如此，可印刷的金属目前仅限于金、银和碳的纳米材料，而可印刷的阴极金属由于其前驱体油墨的空气敏感性而没有出现在文献中。本研究提出了对低成本可打印阳极材料替代品和新型可打印阴极电极的需求，以实现更高效率的器件，成本仅为传统光伏的一小部分。此外，本研究将展示第一个完全打印的全无机光伏器件，包括通过可打印材料及其界面的组成和界面工程进行优化。该项目将在一个主要的本科院校进行，并将为代表性不足的群体提供纳米科学研究经验的机会，同时为学生准备材料科学的职业道路。本科生还将参加与当地学校和STEM营地的公共宣传化学演示。该项目通过研究可印刷金属触点，特别是低成本太阳能光伏（PV）的高通量卷对卷（R2R）生产，直接解决了可印刷电子领域的一个主要限制。金属触点的电子特性对带弯曲有很大的影响，这将极大地影响器件的效率。特别是对于溶液处理的CdTe/ZnO纳米晶PV，需要高功函数金属和低功函数金属分别与p型CdTe和n型ZnO进行欧姆接触。然而，目前已证实的可打印的高功功能金属仅限于银、金和碳的纳米颗粒，而可打印的低功功能金属由于其前驱体油墨的空气敏感性而在文献中缺失。出于这个原因，研究可打印PV材料的研究人员通常从底部接触的真空沉积ITO开始，尽管在CdTe的情况下使用费米水平钉住p型材料，然后在顶部接触的n型层上真空沉积铝。为了解决低成本兼容R2R的油墨材料与尺寸/速度受限的真空触点之间的配对差异，我们将利用透明和不透明可打印的低功功能触点和高功功能触点的组成和界面工程来实现和优化完全印刷的坚固耐用的全无机CdTe光伏器件。用开尔文探针测量功函数的变化，用紫外/可见和x射线荧光光谱、霍尔效应测量、原子力显微镜和x射线衍射对印刷薄膜进行表征。完成的设备将用研究级喷墨和R2R兼容的凹版打印机打印，随后进行1次太阳光照测量。将这些工程材料应用于印刷的CdTe PV将揭示它们对能带能量排列的影响及其产生的器件性能，从而推动可印刷电子领域的发展。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Photovoltaic (PV) energy conversion can provide more power than we use globally; however, this renewable energy source still only contributes to 3% of our energy portfolio largely due to the prohibitive cost of material processing via high temperature or vacuum-based deposition. In response to this, a variety of low cost and rapid Roll-to-Roll (R2R) printing techniques that were invented for printing media are now being revived and adapted to deposit nanoscale electronic inks to build lightweight and flexible electronic devices at rapid production speeds. Solid state PV devices are deposited layer-by-layer and contain semiconducting light absorbing materials sandwiched between two metal contacts. The chemical composition and processing conditions of each layer can be tuned to adjust the device properties, which are dramatically affected by the alignment of the electron energy levels at the interfaces between layers. As charge flows through the device circuit, noble metal contacts with low electron energies are chosen for the anode whereas reactive metals make suitable contact at the cathode.¬¬ Energy mismatch between the light harvesting semiconductor and the metal contact can lead to significant loss of generated charge and restrict device performance. Despite this, printable metals are currently limited to nanomaterials of gold, silver and carbon, and printable cathode metals are absent from the literature because of the air-sensitivity of their precursor inks. This proposed study addresses the need for low-cost printable anode me¬¬tal alternatives and novel printable cathode electrodes to achieve higher efficiency devices at a fraction of the cost of conventional PV. Additionally, this research will demonstrate the first fully printed all-inorganic PV devices including optimization through composition and interface engineering of the printable materials and their interfaces. This project will be conducted at a primarily undergraduate institution and will provide nanoscience research experience opportunities for underrepresented groups while preparing students for materials science career pathways. Undergraduate research students will also participate in public outreach chemistry demonstrations with local schools and STEM camps. This project directly addresses a major limitation in the field of printable electronics by investigating printable metal contacts with a specific focus on high throughput Roll-to-Roll (R2R) production of low-cost solar photovoltaics (PV). The electronic properties of metal contacts have a major influence on band bending, and this dramatically affects device efficiency. Specifically for solution processed CdTe/ZnO nanocrystal PV, high work function and low work function metals are required to make ohmic contact with p-type CdTe and n-type ZnO, respectively. Currently, however, demonstrated printable high work function metals are limited to nanoparticles of silver, gold and carbon, and printable low work function metals are absent from the literature due to the air-sensitivity of their precursor inks. For this reason, researchers investigating printable PV materials typically start with vacuum-deposited ITO for the bottom contact, despite Fermi level pinning with the p-type material in the case of CdTe, followed by vacuum-deposited aluminum on the n-type layer for the top contact. To address the discrepancy between pairing low cost R2R compatible ink-based materials with size/speed restricted vacuum-based contacts, we will use composition and interface engineering of transparent and opaque printable low and high work function contacts to realize and optimize fully printed robust all-inorganic CdTe PV devices. Changes in work function will be measured with a Kelvin Probe, and printed films will be characterized with UV/Vis and X-ray fluorescence spectroscopy, Hall effect measurements, atomic force microscopy and X-ray diffraction. Completed devices will be printed with research grade InkJet and R2R compatible rotogravure printers followed by 1 sun illumination measurements. Applying these engineered materials to printed CdTe PV will uncover their effects on band energy alignments and their resulting device properties to advance the field of printable electronics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.