Potassium post deposition treatment (K-PDT) has been a major breakthrough for CIGSSe (Cu(In,Ga)(S,Se)2) solar cells yielding world record efficiencies. K-PDT yields significant improvements in open circuit voltage, fill factor and allows thinning CdS thus leading to efficiencies above 20% for CIGSSe material. Here we present a similar K-PDT approach for solution processed kesterite solar cells. K-PDT improves the open circuit voltage of kesterite solar cells, however a severe blocking of the short circuit current and reduction of fill factor reduces the overall device performance. Furthermore, quantum efficiency measurements indicate that K-PDT alters the properties of the CdS layer whereas time-resolved photoluminescence measurements exhibit an additional third exponential component, which can be attributed to minority carrier trapping at the interface.