Solution Deposition and Photonic Curing of Hybrid Transparent Conducting Electrodes

Development is still needed to improve flexible transparent conductive electrodes (TCE), the starting point for many optoelectronic devices, such as LEDs, photovoltaics, and touch-sensitive screens, which are the backbones of many modern technologies. PET/ITO is a common TCE but is made with slow vacuum deposition and thermal annealing processes, which are not compatible with high-speed roll-to-roll (R2R) manufacturing. Because of the limited thermal budget, ITO films must be thicker on PET than on glass substrates to maintain similar sheet resistance (R_sh), resulting in lower ITO transmittance. Many solution deposition techniques can already achieve high R2R web speeds &gt; 10 m/min. However, post-deposition thermal annealing of sol-gel TCE precursors requires high temperature (&gt; 300°C) for a long time (&gt; 30 min), leading to either a reduction in R2R web speed or implementation of long, high-energy-consuming ovens. In addition, plastics have higher coefficients of thermal expansion than oxides, resulting in mechanical failures when thermally annealed at high temperatures. A promising alternative to thermal annealing that is R2R compatible is photonic curing. Photonic curing is a photoirradiation process during which a xenon flash lamp emits high-intensity, broadband (200 – 1000 nm) light upon a sample in discreet pulses lasting micro or milliseconds. Light impinging on the sample is absorbed by the film and converted into heat depending on its optical absorption, which drives material transformations: crystallization, phase change, or sol-gel conversion. Because photonic curing uses high-intensity, low-energy light to anneal materials, high-temperature processing can be performed on plastic substrates, which do not absorb a significant amount of light, without damaging them. Photonic curing requires simultaneous optimization of multiple parameters, which will be explored in this work: pulse voltage, pulse length, number of micro-pulses, duty cycle, number of pulses, repetition rate, and radiant exposure.<br/><br/>In this work, a hybrid TCE material is solution deposited and photonic cured on PET substrates. The hybrid TCE consists of a transparent conducting oxide made from sol-gel precursors and a metal network—silver nanowires (AgNW), metal grid traces (MG), or a combination of both. The metal network enhances electrical conductivity without sacrificing optical transmittance. AgNWs and MGs significantly affect the photonic curing process because they absorb light which leads to heating within the material stack. This additional light absorption may enhance the sol-gel oxide conversion during photonic curing. The conversion of the sol-gel metal oxide was tested using a weak acid etch. To evaluate the hybrid TCE performance, we measure sheet resistance (R_sh) and average optical transmittance from 400 to 700 nm (T_avg) to calculate the figure of merit (FOM) [1]. The preliminary results on spin-coated films show the best-performing hybrid TCE made by photonic curing AgNW and IZO composite films achieves R_sh = 16 ± 4 Ω/sq, T_avg = 78 ± 2%, with a FOM = 90 ± 14. This TCE already outperforms some commercially available PET/ITO products, which have FOM ranging from 24 to 110. To transition to R2R processing, we employ blade-coating to deposit the hybrid TCE materials. The blade-coating and photonic curing processes will be optimized together to produce high FOM hybrid TCE films. Finally, perovskite solar cell devices will be fabricated on these hybrid TCEs and compared to devices made on commercially available PET/ITO.<br/><br/>[1] A. Anand, M. M. Islam, R. Meitzner, U. S. Schubert, and H. Hoppe, <i>Adv. Energy Mater.</i>, vol. 11, no. 26, 2021, doi: 10.1002/aenm.202100875.