Enhanced Thermoelectric Efficiency of ZnO-Based Transparent Tandem Thermoelectric Generators

Transparent thin film thermoelectric materials are essential to utilize waste heat from optoelectrical devices such as hybrid solar cells, smart augmented reality (AR) glasses, and smart windows. Zinc oxide (ZnO) is a widely used transparent material for thin film thermoelectric devices because of its wide bandgap (3.37 eV), nontoxicity, chemical and thermal stability.[1, 2] However, a low thermoelectric figure of merit (ZT) of ZnO-based thermoelectric materials (ZT &lt; 0.5), [3] resulting in the low output power density (power generation capacity per unit area), limits their application in conventional thermoelectric generator(TEG). An introduction of a Tandem structure TEG consisting of vertically stacked thin films can be a strategy to enhance the heat-collecting efficiency and output power density since the devices connected in series provide the integration of the thermoelectric voltage of each TE legs.[4]<br/>Recently, a thin-film TEG with a metal–insulator–semiconductor (m-i-s) structure has been demonstrated with enhanced thermoelectric performance.[5] This m-i-s structure, however, has an intrinsic drawback for transparent applications due to the high light absorption by the metal layers. Additionally, the low Seebeck coefficient and high thermal conductivity of the metal layer can result in a decrease in the average performance of Tandem structure TEG. Here, we aim to enhance the performance of the transparent Tandem structure TEG by stacking transparent n and p-type ZnO based thermoelectric materials vertically.<br/>We successfully fabricated a transparent Tandem structure TEG using Al-doped ZnO for the n-type thermoelectric layer, Cu-doped ZnO for the p-type thermoelectric layer, and aluminum oxide for the insulating layer on the quartz substrate. Atomic layer deposition (ALD) was employed to deposit thin films systematically. The stacked structures and thicknesss of each layers were observed in transmission electron microscope images, and XPS depth profile. 2% Al-doped ZnO thin film and 1% Cu doped ZnO thin film showed -150μV/K, and 90μV/K of Seebeck coefficient at single layer, respectively. After the n-i-p structure is assembled, TEG showed -220μV of voltage difference for 1 K of temperature difference. The voltage difference increased as the TEG layers increased, and reached the maximum value at 10 layers. The output power of the TTEGs also enhanced with the number of stacked layers, and achieved the highest output power of 100 nW.<br/>[1] A.T.T. Pham, O.K.T. Le, T.T.T. Phan, D. Van Hoang, T.H. Nguyen, N.D. Le, T.B. Phan, V.C. Tran, Enhancing transparent thermoelectric properties of Sb-doped ZnO thin films via controlled deposition temperature, Vacuum 202 (2022) 111137.<br/>[2] B. Kucukgok, B. Hussain, C. Zhou, I.T. Ferguson, N. Lu, Thermoelectric properties of ZnO thin films grown by metal-organic chemical vapor deposition, MRS Online Proceedings Library (OPL) 1805 (2015) mrss15-2136936.<br/>[3] S. Biswas, S. Singh, S. Singh, S. Chattopadhyay, K.K.H. De Silva, M. Yoshimura, J. Mitra, V.B. Kamble, Selective enhancement in phonon scattering leads to a high thermoelectric figure-of-merit in graphene oxide-encapsulated ZnO nanocomposites, ACS Applied Materials & Interfaces 13(20) (2021) 23771-23786.<br/>[4] C.A. Hewitt, A.B. Kaiser, S. Roth, M. Craps, R. Czerw, D.L. Carroll, Multilayered carbon nanotube/polymer composite based thermoelectric fabrics, Nano Lett. 12(3) (2012) 1307-1310.<br/>[5] X. Tao, B. Hao, H.E. Assender, Novel Stacking Design of a Flexible Thin-Film Thermoelectric Generator with a Metal–Insulator–Semiconductor Architecture, Advanced Electronic Materials 7(12) (2021) 2100201.