The improved understanding of defects and related processes is quintessential for fabrication of highly efficient optoelectronic devices such as solar cells, LED, photodetectors, etc. The presence of defects in crystals is very common and spectacular concept in material science which becomes more common on nanoscale. For quite some time, the defect has been thought of as an unwanted feature disturbing the crystal purity and their impact on various applications was really misinterpreted. Defects impact the transport properties (i.e., mobility and diffusion length) of the materials negatively, but they are extremely crucial to tune the carrier concentration and fermi level to accomplish n- and p-type materials, which enables semiconducting p–n junctions. The presence of deep trap states can influence the device performance adversely through carrier trapping and non-radiative recombination (such as Shockley-Read-Hall (SRH), which reduces the voltage in operating solar cells. The non- radiative recombination undesirably affects the collection of photo-generated carriers and carrier lifetimes or suppresses the luminescence in optoelectronic device like LED, solar cells, etc and thus reduces their efficiencies. The quality of interfaces and materialization of defects such as point defects, stacking faults, twins, dislocations, and grain boundaries decreases the solar cell efficiency. Our group employs experimental and computational methods to engineer the energy materials and the effective control of defects. Our defect engineering strategy will help in understanding of the defects and related processes, which will assist us in achieving ultra-high efficiency in perovskite and chalcogenide based solar cells.