High efficiency silicon solar cells generally feature heterojunction or carrier-selective contact architectures, for which there is current interest in developing structures using a wide range of materials. The electrical and optical requirements that these layers must fulfill have been investigated previously for standard test conditions. Here, we investigate how the required work functions and layer thickness differ under other illumination conditions. The differences will be important for the optimization of tandem device subcells, and for devices which are intended for use in low-light conditions or under low-level concentration. Heterojunction cells are fabricated and the effect of reduced contact thickness and doping at different illumination levels is experimentally demonstrated. Simulations of a-Si/c-Si heterojunctions and ideal metal-semiconductor junctions reveal a logarithmic variation with illumination level of 0.1-10 suns in the electrode work function, and the heterojunction contact layer work function and thickness required to minimize efficiency losses.