1 Modeling of  graded In(x)Ga(1-x)N solar cells: comparison of strained and relaxed features Mirsaeid Sarollahi a , Mohammad Zamani Alavijeh e , Manal A. Aldawsari b , Rohith Allaparthi a , Reem Alhelais b , Malak A. Refaei b , Md Helal Uddin Maruf c , Morgan E. Ware a,b,c,d* a University of Arkansas, Electrical Engineering Department, 3217 Bell Engineering Center, Fayetteville, AR 72701 b University of Arkansas, Microelectronics-Photonics Program, 731 West Dickson Street, Fayetteville, Arkansas 72701, c University of Arkansas, Material Science and Engineering 731 West Dickson Street, Fayetteville, Arkansas 72701, d Institute for Nanoscience and Engineering, Fayetteville, Arkansas 72701 e University of Arkansas, Physics Department, Fayetteville, AR 72701 Abstract. The optical properties of  graded InGaN solar cells are studied. Graded InGaN well structures with the indium composition increasing then decreasing in a  shaped pattern have been designed. Through polarization doping, this naturally creates alternating p-type and n-type regions. Separate structures are designed by varying the indium alloy profile from GaN to maximum indium concentrations ranging from 20% to 80%, while maintaining a constant overall structure thicknesses of 100 nm. The solar cell parameters under fully strained and relaxed conditions are considered. The results show that a maximum efficiency of ≅ 5.5%, under fully strained condition occurs at x=60%. Solar cell efficiency under relaxed conditions increases to a maximum of 8.3% at 90%. While Vegard’s law predicts the bandgap under relaxed conditions, a Vegard-like law is empirically determined from the output of Nextnano for varying In compositions in order to calculate solar cell parameters under strain. Keywords: Optical properties, Polarization doping, graded structure, solar cell, InGaN *Corresponding Author, E-mail: meware@uark.edu 1 Introduction Ternary alloys of Group III-N materials are great candidates to be used in photovoltaic devices. This is due to interesting properties such as high thermal conductivity, high optical absorption, and high radiation resistance 1–4 . In addition, the direct band gap, which is tunable over most of the usable solar spectrum 5,6 makes them an appropriate choice for photovoltaic devices 7–9 . Several simulation studies for InGaN homojunctions have been reported. The solar efficiency, , of a single junction  0.65  0.35  solar cell was reported to be 20.28% 10 , for example. However, a higher efficiency of =24.95% was demonstrated using the same indium composition due to adoption of the density of states (DOS) model 11 . This presented much more information about recombination/generation in the solar cell than the lifetime model by neglecting defects.