Mechanically stacked bifacial III-V/HIT multijunction silicon solar cells optimized with spectral albedo for high efficiency

Abstract

This study explores mechanically stacked tandem solar modules’ design, fabrication, and performance. These modules have an III-V top cell and silicon heterojunction with intrinsic thin-layer (HIT) bottom cells. The top cell includes three layers: GaInP (∼0.5 µm), GaAs (∼3.0 µm), and Ge (∼150 µm). Under standard sunlight (1-sun AM1.5G), the III-V top cell, measuring 1 cm × 1 cm, showed a V_oc of 2.74 V, a J_sc of 12.17 mA/cm², and an efficiency of 29.14 %. Four HIT bottom cells measuring 1.5 cm × 1.5 cm were connected in series to match this voltage. Each HIT cell had a crystalline silicon layer (∼120 µm thick) and very thin amorphous silicon layers (∼5–10 nm) with a transparent conductive oxide (ITO, ∼75 nm). These bottom cells reached a voltage of about 2.70 V. To match currents, backside illumination (albedo) ranged from 0.1-sun to 0.4-sun, while the front side remained at 1-sun. With increased rear illumination, current improved significantly from 8.96 mA/cm² (0.1-sun) to 17.97 mA/cm² (0.4-sun), increasing efficiency from 21.27 % to 39.74 %. However, the fill factor slightly decreased from 87.94 % to 81.91 % at higher illuminations due to resistive and recombination losses. Under standard test conditions, the tandem solar module achieved an efficiency of 28.52 %, a current of 11.97 mA/cm², and a fill factor of 88.26 %. This study clearly shows how mechanical stacking and rear-side albedo illumination effectively enhance the performance of tandem solar cells.