Amorphous silicon solar cells typically consist of stacked layers deposited on plastic or metallic substrates making sample preparation for transmission electron microscopy (TEM) difficult. The amorphous silicon layer - the active part of the solar cell - is sandwiched between 10-nm-thick n- and p-doped layers. The typical boron concentration in the p-doped layer is ~10^21cm -3 and should not exceed 1017cm-3 in the neighbouring intrinsic (i) layer , where it acts as a charge recombination centre and decreases the internal electric field . The detection of low boron concentrations with high spatial resolution using TEM is highly challenging . Recently, scanning TEM (STEM) combined with electron energy-loss spectroscopy (EELS) and spherical aberration-correction has allowed the direct detection of dopant concentration of 10^20cm-3 in 65-nm-wide silicon devices . Here, we prepare TEM samples by focused ion beam milling in order to map the boron distribution across a 200-nm-thick n-p amorphous silicon junction using energy-filtered TEM and EELS spectrum
acquisition. EELS line scans are used to detect boron concentrations as low as 10^20cm-3. We also use monochromated EELS to measure changes in the energies of
plasmon peaks in the low loss region . We use these approaches to characterize both a thick n-p junction and the 10-nm-thick p-doped layer of a working solar cell.
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