Organic–inorganic lead halide perovskites have rapidly emerged over the last few decades as photovoltaic materials due to their high power conversion efficiency (PCE). The high efficiency of these kinds of semiconducting materials is attributed to some of their useful and interesting properties, viz., high absorption coefficients, long carrier lifetimes, long diffusion lengths and high charge carrier mobilities. However, the toxic nature and the instability of lead halide perovskites limit their commercialization. In this regard, to provide a solution to these problems, we have designed five different lead (Pb) free Ga and In based halide perovskites ((AM)₃(Ga_0.50In_0.50)₂I₉, (MA)₃(Ga_0.50In_0.50)₂I₉, (DMA)₃(Ga_0.50In_0.50)₂I₉, (TMA)₃(Ga_0.50In_0.50)₂I₉, and (TTMA)₃(Ga_0.50In_0.50)₂I₉) having the general formula A₃B₂X₉. The trivalent metal cation based A₃B₂X₉ type perovskite materials exhibit non-toxicity and more ambient stability compared to the conventional ABX₃ type perovskite materials. Structural parameter analysis shows that the designed perovskite materials are capable of forming structurally and thermodynamically stable materials. The study of optical properties reveals that all the designed compounds possess absorption maxima within the visible range of the electromagnetic spectrum. In addition, among the studied compounds, (TTMA)₃(Ga_0.50In_0.50)₂I₉ shows the highest theoretical PCE of 16.43%.