Thermoelectric nanoantennas have been extensively investigated due to their ability to directly convert infrared (IR) radiation into direct current without an additional rectification device. In this study, we introduce a thermoelectric nanoantenna geometry for maximum output voltage (V_oc) and propose an optimal series array configuration with a finite number of antennas to enhance the V_oc. A finite and open-ended SiO₂ substrate, with a thickness of a quarter-effective wavelength at a frequency of 28.3 THz, is used to generate standing waves within the substrate. An array of antennas is then positioned optimally on the substrate to maximize the temperature difference (∆T) between hot and cold areas, thereby increasing the average V_oc per antenna element. In numerical simulations, a linearly polarized incident wave with a power density of 1.42 W/cm² is applied to the structure. The results show that a single antenna with the optimum geometry on a substrate measuring 35 µm × 35 µm generates a ∆T of 64.89 mK, corresponding to a V_oc of 1.75 µV. Finally, a series array of 5 × 6 thermoelectric nanoantennas on a 150 µm × 75 µm substrate including measurement pads achieves an average ∆T of 49.60 mK with a total V_oc of 40.18 µV, resulting in an average V_oc of 1.34 µV per antenna element and a voltage responsivity (β_v) of 0.77 V/W. This value, achieved solely by optimizing the antenna geometry and open-ended substrate, matches or exceeds the V_oc and β_v of approximately 1 µV and 0.66 V/W, respectively, from suspended thermoelectric antenna arrays over air cavities. Therefore, the proposed thermoelectric nanoantenna array device, characterized by high stability and ease of fabrication, is suitable for manufacturing massive nanoantenna arrays for high-output IR-DC energy harvesters.