Role of aluminum distribution on the growth of CuIn0.75Al0.25Se2 films and numerical simulation of p-CuIn0.75Al0.25Se2/n-CuAlSe2 heterojunction solar cell

Chalcopyrite thin films of CuIn 0.75 Al 0.25 Se 2 have been grown by a two-stage process containing e-beam evaporation of a threefold (In/Cu/Al/Se) precursor deposition onto glass substrates in a high vacuum followed by post selenization at various temperatures (300–550 °C) using a horizontal tubular furnace. The X-ray diffraction pattern of precursor layers selenized at ≤ 525 °C shows the formation of co-existence of CuInSe 2 and Cu(In,Al)Se 2 phases, as well as a mixture of two Cu(In,Al)Se 2 phases with different Al content. The precursor layers selenized at 550 °C results in the formation of single-phase Cu(In,Al)Se 2 thin films. The presence of an intense A 1 mode at 174.7 cm −1 in the Raman spectra of selenized films at 550 °C confirms the growth of Cu(In,Al)Se 2 phase. The energy-dispersive spectra of stacked layers selenized at 550 °C shows a Cu-poor and (In + Al)-rich composition with atomic ratios of Cu/(In + Al) = 0.79, Al/(In + Al) = 0.25, and Se/(Cu + In + Al) = 0.98. The secondary ion mass spectra depth study reveals a shift in Al distribution from graded to uniform with an increase in selenization temperature to 550 °C. The stacked layers selenized at 550 °C reveal a uniform distribution of void-free dense grains (~ 0.7 μm). Optical and electrical studies of selenized Cu(In,Al)Se 2 films at 550 °C show a direct band gap of 1.22 eV with a higher hole mobility of 14.0 cm 2 /V-s. The heterojunction solar cell of p -Cu(In,Al)Se 2 / n -CuAlSe 2 was numerically simulated using SCAPS-1D software, yielding a high power conversion efficiency ( η ) of 21.01%.