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

St Mary'S College Of Maryland, Saint Marys City MD

Investigators

Abstract

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.