Extended infrared response of silicon solar cells and the impurity photovoltaic effect

Sub-bandgap spectral response measurements on silicon solar cells are used to characterise the infrared response of present devices, and to investigate the impurity photovoltaic (IPV) effect for improving their infrared response. The former has, aside from establishing a baseline case, led to an improved determination of the subgap absorption coefficient of crystalline silicon. Absorption coefficient values as low as 10-7 cm-1 have been determined, revealing structure due to 3- and 4-phonon assisted absorption. These values are compared with a more recent determination of the absorption edge based on photoluminescence measurements. The influences of free carrier absorption, bandgap narrowing, and the Franz-Keldysh effect on cell infrared response are considered. Investigation of the IPV effect of indium in high efficiency bulk and thin film cells reveals that indium improves their infrared response. The cross section for electron photoemission from the indium level, a crucial parameter for modelling indium's IPV effect, is determined.