NOVELTY - Cleaning a glass substrate comprises providing the glass substrate; and exposing the glass substrate to a cleaning solution, where the cleaning solution includes an organic acid.    USE - The methods and cleaning solution are useful for cleaning a glass substrate, and for removing metal ion contaminates from a glass substrate having a TCO layer (all claimed).    ADVANTAGE - The present invention provides methods and related cleaning solutions that can remove metal ion contaminates from TCO glass used in the manufacture of thin-film solar cells. Such methods and related cleaning solutions can be particularly effective in the removal of metal ion contaminates from TCO glass used in the formation of thin-film single-junction and multi-junction solar cells. The removal of metal ion contaminates from TCO glass may help to avoid possible degradation in solar cell performance over time that may result from the use of lower-cost production line produced TCO glass.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are:    (1) a method of removing metal ion contaminates from a glass substrate having a transparent conductive oxide (TCO) layer by providing the glass substrate having a TCO layer; and exposing the glass substrate to a cleaning solution, where the cleaning solution includes a 0.5-5% organic acid, and where the organic acid includes citric acid, acetic acid, or oxalic acid;    (2) a cleaning solution, for removing metal ion contaminates from a glass substrate having a TCO layer, comprising 0.5-5% organic acid; and    (3) a method of making a solar cell by providing a glass substrate having a TCO layer; cleaning the glass substrate by exposing the glass substrate to a cleaning solution, where the cleaning solution includes a 0.5% to 5% organic acid; forming a first p-i-n junction over the glass substrate; forming a second p-i-n junction over the first p-i-n junction; forming a TCO layer over the second p-i-n junction; and forming a metal back layer over the TCO layer; where the first p-i-n junction comprises a p-type amorphous silicon layer, an intrinsic type amorphous silicon layer over the p-type amorphous silicon layer, and an n-type microcrystalline silicon layer over the intrinsic type amorphous silicon layer; and where the second p-i-n junction comprises a p-type microcrystalline silicon layer, an intrinsic type microcrystalline silicon layer over the p-type microcrystalline silicon layer, and an n-type amorphous silicon layer over the intrinsic type microcrystalline layer.