Abstract
The boron-tailing effect in hydrogenated amorphous silicon (a-Si:H) solar cells describes the reduced charge collection specifically in the blue part of the spectrum for absorber layers deposited above a critical temperature. This effect limits the device performance of state-of-the art solar cells: For enhanced current density (reduced bandgap), the deposition temperature should be as high as possible, but boron tailing gets detrimental above 200 degrees C. To investigate this limitation and to show potential paths to overcome it, we deposited high-efficiency a-Si: H solar cells, varying the deposition temperatures of the p-type and the intrinsic absorber (i) layers between 150 and 250 degrees C. Using secondary ion mass spectroscopy, we study dedicated stacks of i-p-i layers deposited at different temperatures. This allows us to track boron diffusion at the p-i and i-p interfaces as they occur in the p-i-n and n-i-p configurations of a-Si: H solar cells for different deposition conditions. Finally, we prove step-by-step that the common explanation for boron tailing-boron diffusion from the p layer into the i layer leading to enhanced recombination-is not generally true and propose an alternative explanation for the experimentally observed drop in the external quantum efficiency at short wavelengths. (C) 2015 AIP Publishing LLC.