Development of High-Efficiency Si Tandem Solar Cell Modules for Solar-Powered Vehicle Applications

Solar-powered vehicles are very attractive for reducing CO₂ emission and creation of new market [1]. Development of high-efficiency (&gt; 30%) and low-cost solar cell modules is very important. Previously, the authors have reported driving test data for Toyota Prius and Nissan eNV 200 demonstration cars installed with Sharp’s high-efficiency III-V 3-junction solar cell modules with a module efficiency of more than 30% and have demonstrated longer driving range of 26 km/day average (at solar irradiation of 4kWh/m²/day) compared to 16 km/day average for Sono Motors Sion installed with Si back contact solar cell modules with a module efficiency of 21%. However, cost reduction of multi-junction (MJ) solar cells is necessary for solar-powered vehicle applications. Development of Si tandem solar cells [2] such as III-V/Si and perovskite/Si tandem solar cells is very promising for cost reduction in addition to high-efficiency.<br/>This paper presents chronological efficiency improvements of the Si tandem solar cells and modules including our new world record efficiency (33.7%) InGaP/GaAs/Si 3-junction tandem solar cell module with an area of 775cm² [3]. The Si 3-junction tandem solar cell modules with an efficiency of more than 35% are shown to have driving distance potential of more than 30 km/day average and more than 50 km/day on a clear day [1]. Toward 35% module efficiency Si tandem solar cell modules, efficiency potential of various Si tandem solar cells was analyzed by using measure of external radiative efficiency (ERE) [4]. The authors have defined bandgap energy Eg dependence of △Voc,rad ( = Eg/q – Voc,rad) for radiative open-circuit voltage Voc,rad relative to Eg/q as △Voc,rad [V] = 0.1836 + 0.0671 Eg [eV] for Si and III-V compounds and △Voc,rad [V] = 0.1708 + 0.0671 Eg [eV] for perovskite [5].<br/>Practically feasible efficiencies of Si tandem solar cells estimated by assuming ERE of 20%, optical loss of 5% and resistance loss of 2% are 37.6% and 43.4% for 2-junction and 3-junction Si tandem solar cells, respectively. Because 3-junction tandem solar cells have higher potential efficiency by about 6% compared to 2-junction tandem solar cells, development of high-efficiency perovskite based 3-junction solar cells is suggested to be very attractive. Because optimum bandgap energy combination [6] of Si tandem solar cells is 1.73 eV/Si and 2.01 eV/1.50 eV/Si, optimization of bandgap energy for perovskite/Si tandem solar cells is necessary. Differences of non-radiative recombination loss in III-V compounds and perovskite materials are also discussed in this paper. Potential of driving distance of solar-powered vehicles installed with current various solar cell modules including our world record efficiency III-V/Si 3-junction solar cell module was estimated based on test driving data [1]. The III-V/Si 3-junction tandem solar cell modules show great potential of solar-powered vehicle applications.<br/><br/><b>References</b><br/>[1] M. Yamaguchi et al., <i>Prog. Photovolt. </i><b>29,</b> 684 (2021).<br/>[2] M. Yamaguchi et al., <i>J. Phys. D: Appl. Phys</i>. <b>51</b>, 133002 (2018).<br/>[3] M. Yamaguchi et al., <i>Prog. Photovolt. </i><b>32,</b> (2024).<br/>[4] U. Rau, <i>Phys. Rev.</i> <b>B76</b>, 085303 (2007).<br/>[5] M. Yamaguchi et al., <i>Solar RRL </i> <b>7</b>, 2300308 (2023).<br/>[6] K-H. Lee et al., <i>Prog. Photovolt. </i><b>24,</b> 1310 (2016).