The highest confirmed efficiency of single-junction solar cells under standard conditions (1-sun AM1.5G spectrum at 25 oC) has remained at 28.8 % for the past 5 years [1, 2]. Although improvements towards reaching the Shockley-Queisser limit (33.5 %) [3, 4] are possible with a single-junction structure, tandem/multi-junction solar cells have demonstrated a pathway to achieving higher efficiencies, e. g., the current 1-sun solar conversion efficiency record, 38.8 % was made by Spectrolab with a 5-junction bonded cell , and under concentrated illumination, 46.0 % was demonstrated by a 4-junction bonded cell . However, the cell cost is a key factor for deployment to impact real-world energy production. Si-based tandems are good candidates for low-cost high-efficiency solar cells. They have achieved 31–36 % by mechanical stacking or wafer bonding techniques . A monolithic approach is preferred in terms of cost and manufacturability, since it only requires a single substrate, and fewer growth and processing steps.
Dilute nitride GaNP can be grown lattice-matched to Si, and is a candidate absorber for making monolithic high-efficiency solar cells. We are theoretically and experimentally investigating the properties of GaNP relevant to photovoltaics. Here we report on the theoretical aspects of GaNP/Si tandems using a multi-scale approach starting with the electronic structure and phonon dispersion of GaNP using semi-empirical tight binding calculations. They are then fed into a Monte Carlo simulator to calculate the transport properties. The optical absorption is calculated from the electronic structure, which is poorly understood for the dilute nitrides. These calculated properties will later be input into a commercial device package (Silvaco ATLAS) to simulate the performance of the tandem structures.
 BM Kayes, H Nie, R Twist, SG Spruytte, F Reinhardt, IC Kizilyalli, GS Higashi, "27.6% Conversion efficiency, a new record for single-junction solar cells under 1 sun illumination", Proc. 37th IEEE Photovolt. Spec. Conf., Jun. 2011.
 MA Green, Y Hishikawa, W Warta, ED Dunlop, DH Levi, J Hohl-Ebinger, AWY Ho-Baillie, "Solar cell efficiency tables (version 50)", Prog. Photovolt. Res. Appl. 25, 2017.
 W Shockley, HJ Queisser, "Detailed balance limit of efficiency of p-n junction solar cells", J. Appl. Phys., Vol. 32, Mar. 1961.
 OD Miller, E Yablonovitch, SR Kurtz, "Strong Internal and External Luminescence as Solar Cells Approach the Shockley–Queisser Limit", IEEE J. Photovolt., Vol. 2, No. 3, Jul. 2012.
 PT Chiu, DL Law, RL Woo, S Singer, D Bhusari, WD Hong, A Zakaria, JC Boisvert, S Mesropian, RR King, NH Karam., “35.8% space and 38.8% terrestrial 5J direct bonded cells”, Proc. 40th IEEE Photovolt. Spec. Conf., Jun. 2014.
 Press Release, Fraunhofer ISE, Dec. 2014 (https://www.ise.fraunhofer.de/en/press-media/press-releases/2014/new-world-record-for-solar-cell-efficiency-at-46-percent.html)