Single-junction InGaNAs solar cells were grown by MBE with active layers based on a GaAsN/InAs superlattice. Dependence of defect formation on thickness of InGaNAs was explored in the study. Thickness increase from 900 nm to 1200 nm leads to defect formation with two activation energies of 0.20 eV and 0.50 eV but the value of quantum efficiency stays almost the same. Further thickness increase up to 1600 nm leads to the increase of defect concentration in the InGaNAs active layer. These defects are non-radiative recombination centres because a significant decrease of solar cell quantum efficiency was observed. The existence of a critical thickness for defect-free growth of InGaNAs based on GaAsN/InAs superlattice is proposed. 1. Introduction In 2015 multi-junction solar cells (MJ SC) based on III-V compounds with a record efficiency of 46% were fabricated [1]. Triple-junction SC based on the GaInP (1.85 eV) / GaAs (1.42 eV) / Ge (0.7 eV) system are used in industry for space applications where the key factors are efficiency and resistance to radiation. But its concentrator efficiency has almost reached the theoretical limit (46%) and now its value is more than 40% [2]. Fortunately, according to theoretical estimations an additional subcell with energy bandgap of 1 eV could increase the SC efficiency up to 52% [3]. However, such subcell should be lattice-matched to Ge and GaAs wafers. Ga1-xInxNyAs1-y with a small content of nitrogen is one of the most promising alloys to reach this goal. Such III-V-N alloys (GaPNAs, InGaNAs etc.) with nitrogen content less than 5% are called dilute nitrides. It has been shown that the small addition of nitrogen leads to large bowing parameters for the bandgap in quaternary Ga1-xInxNyAs1-y alloys: already few percent of nitrogen reduces the bandgap by hundreds of meV [4] and a value of 1 eV can be achieved. Furthermore, these layers can be epitaxially grown on Ge and GaAs wafers when y=0.35x. Various groups [3, 5-8] studied InGaNAs layers grown by epitaxy, but the quality is not sufficient for fabrication of high-efficiency solar cells. The low lifetime of charge carriers is the most crucial issue in dilute nitrides. The main reason is the high concentration of non-radiative recombination centers in active layers. In MOCVD (metal-organic chemical-vapor deposition) it comes from the background doping of carbon and hydrogen during the process. On the other hand, MBE (molecular-beam epitaxy) allows one to avoid these problems and enables to control the flow of nitrogen more precisely. However, a lower process