Approaches for High-Efficiency and Low-Cost Multi-Junction Solar Cells

PV-powered vehicle applications 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. The III-V multi-junction (MJ) soar cells have shown high-efficiency of 37.9% and 39.2% with 3-junction and 6-junction solar cells [2]. Although Toyota Prius and Nissan eNV 200 demonstration cars installed with Sharp’s high-efficiency III-V 3-junction solar cell modules with an efficiency of more than 30% have demonstrated longer driving range of 26km/day average compared to 16km/day average for Sono Motors Sion installed with Si back contact solar cell modules with an efficiency of 21.5%, cost reduction of MJ solar cells is necessary for PV-powered vehicle applications. Development of Si tandem solar cells and high speed deposition such as hydride vapor phase epitaxy (H-VPE) is very promising for cost reduction. However, efficiencies of 35.9% with III-V/Si 3-junction tandem cells and 28.2% with H-VPE grown III-V 2-junction cells are lower compared to 37.9% with III-V 3-junction and 32.9% with MOVPE (metalorganic vapor phase epitaxy)-grown III-V 2-junction tandem solar cells. Therefore, it is necessary to clarify and reduce several losses of MJ solar cells. This paper presents high efficiency potential of III-V/Si 3-junction solar cells and H-VPE grown III-V 2-junction solar cells analyzed by using our analytical procedure [3] and discusses about non-radiative recombination, optical and resistance losses in those MJ tandem cells.<br/>One of problems to attain the higher efficiency MJ solar cells is to reduce non-radiative recombination loss. The open-circuit voltage V_oc drop compared to bandgap energy (E_g/q - V_oc) is dependent upon non-radiative voltage loss (V_{oc, nrad}) that is expressed by external radiative efficiency (ERE). Open-circuit voltage is expressed by<br/>Voc = Voc,rad + (kT/q)LN(ERE), (1)<br/>where the second term shows non-radiative voltage loss, is radiative open-circuit voltage and 0.28V for III-V compound and 0.26V for Si were used as △Voc,rad (= E_g/q - Voc,rad ) in this study.<br/>Correlation between V_oc values for III-V/Si 3-juction and H-VPE grown III-V 2-junction solar cells reported in the references and ERE values estimated by eq. (1) suggests that those MJ solar cells have larger non-radiative loss and further improvements in efficiency are thought to be possible by improving ERE. The ERE values estimated in this study are 0.51% for 35.9% III-V/Si 3-junction tandem solar cells and 0.17% for 28.2% H-VPE grown III-V 2-junction solar cells that are lower compared to 2.2% for 37.9% III-V 3-junction and 5.6% for 32.9% MOVPE-grown III-V 2-junction tandem solar cells. In addition, optical and resistance losses of H-VPE grown III-V 2-junction solar cells are higher compared to those of MOVPE-grown III-V 2-junction solar cells. According to our analysis [3], the 2-junction and 3-junction tandem solar cells have potential efficiencies of more than 36% and 42%, respectively.<br/>Several loses such as non-radiative recombination, optical and resistance in MJ solar cells are discussed in this paper.<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. Appl. Phys</i>. <b>129</b>, 240901 (2021).<br/>[3] M. Yamaguchi et al., <i>J. Mater. Res. </i> <b>32</b>, 3445 (2017).