Characterization of the $SnO_2$/p contact resistance and $SnO_2$ properties in operating a-Si:H p-i-n solar cells

Özet

A method is presented to characterize the TCO/p contact and the TCO sheet resistance in a-Si TCO/p-i-n superstrate devices. It is extremely useful for understanding resistance losses in modulus and diagnosing how plasma processing influences the TCO layers. Analysis of 4-terminal dark J-V measurements as a function of temperature on devices with varying TCO geometry yields the TCO/p contact resistance $R_{TCO}/p$, its activation energy $E_a$ or barrier height, and the TCO sheet resistance $R_{SH}$ in an integrated device structure. The method is applied to devices fabricated on different brands of commercial $SnO_2$ substrates with different p-layers. Important new results are found. $E_a$ for the $SnO_2/p$ contact resistance are about 40-50 meV which is < 2kT, and therefore not a rectifying barrier. $R_{SH}$ in one brand of $SnO_2$ has a benefical decrease of 60% after a-Si deposition while another brand is unaffected. The impact of $R_{SH}$ or $R_{TCO}$ losses on the FF (fill factor) are determined.