# The Outdoor Field Test and Energy Yield Model of the Four-Terminal on Si Tandem PV Module  
[PERSON]  
1 Semiconductor Laboratory, Toyota Technological Institute, Nagoya 468-8511, Japan;  
[EMAIL_ADDRESS]  
2 Faculty of Engineering, University of Miyazaki, Miyazaki 889-2192, Japan;  
[EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (H.S. );  
[EMAIL_ADDRESS] (K.N.) 3 Institute for Tenure Track Promotion, University of Miyazaki, Miyazaki 889-2192, Japan;  
[EMAIL_ADDRESS]  
3 March 20  
On-Si tandem solar cells use the widely-used Si solar cell as the bottom junction of the tandem solar cell. Because the technology of Si solar cells is well-established, the production cost of the solar cell is expected to reduce; at least, the one to the substrate or the bottom junction [21]. The III-V/Si (III-V on Si) 3-junction and 2-junction tandem solar cells right now exhibit excellent efficiency with 35.9% [22] and 32.8% [22]. The perovskite on Si 2-junction tandem solar cells reached 28.0% [23]. The efficiency of CdZnTe on Si tandem solar cells was 16.8% [24], and that of GaAs nano-wire on Si tandem solar cells was 11.4% [25]. The III-V/Si tandem solar cell has the best efficiency among them. Related to this III-V/Si tandem solar cell technology, there are several module technologies, including a partial concentrator module [26]. The tested power conversion efficiency of a pair of InGaP/GaAs partial concentrator cell on Si, including optical loss of the concentrator optics, was 27.1% [27]. The partial concentrator module, also using InGaP/GaAs partial concentrator cell on Si and designed for automobile applications, showed 21.5% module efficiency [28]. The non-concentrating module using InGaP/GaAs cell on InGaAs by 4T configuration showed 31.17% [1; 29]. Another big group of the candidates of the on Si tandem cells is Perovskite on Si tandem cells, including a review of the 4T Perovskite on Si cell [30] and challenging the growth on the textured silicon [31; 32; 33]. Related to the topic of this paper, consideration of the performance of the Perovskite on Si tandem cells was done, extending the standard testing condition (AM1.5G spectrum and consideration of diffused sunlight). Until now, it had not been validated by annual outdoor operation [34]. Regardless of the material type combined with Si bottom junction, mismatching loss by the spectrum change is a big issue [35; 36; 37; 38; 39]. The intensive studies of the spectral sensitivity of the tandem cell were firstly done in the analysis of the concentrator photovoltaic (CPV), since tandem solar cells were expensive. Nevertheless, the high performance and the concentration operation were commercially advantageous to the application [40; 41; 42; 43; 44; 45; 46; 47; 48; 49]. The spectrum shift from the standard AM1.5G spectrum destroys the balance of the output electrical current from the sub-cell. Then, the sub-cell with the least output current constrains the total electrical current by the conservation of the carrier or the Kirchhoff's law. It is called \"spectrum-mismatching loss\". It is an inherent loss for every type of tandem solar cell, regardless of CPV or standard flat-plate application, irrespective of on-Si or on-CIGS tandem, and regardless of III-V or Perovskite tandem solar cells, except for more than three terminal configurations where the output of the sub-cells is individually connected to the load. It is essential to note that a variation of the solar spectrum by sun height and fluctuation of scattering and absorption of the air by the seasonal effect will affect the annual performance. However, a lot of research and development was done to minimize the influence of spectrum change by the improvement of the solar cell design [50; 51; 52; 53; 54; 55]. Research and development seeking for the device design with the robustness to spectrum change have been made in the past 20 years, including a computer model named Syracuse by Imperial College London [2; 56; 57].