We studied the thickness variation of equally doped ZnO:Al films used as conductive window layer in Cu(In,Ga)(Se,S)2 (CIGSSe) thin film solar cells. The IV-characteristics of solar cells with window layer thickness of d1=200nm exhibit a strong enhancement of the short-circuit current density JSC (ΔJSC = 3mA/cm2) as compared to samples with module-like ZnO:Al-film thickness (d2=1200nm). Accordingly, the quantum efficiency reveals the spectral regimes where the JSC-gain occurs. Moreover, current-voltage measurements reveal that the cells with thicker ZnO:Al exhibit slightly decreased open circuit voltage VOC. This finding can be assigned to a decreased net-doping density NA, which appears to be introduced by additional heat flux during the longer process time required for deposition of thicker ZnO:Al films. However, the improved efficiency of solar cells with thinner window layer comes along with an increase of the series resistance (RS) by almost a factor of 2, which will have consequences for the series connection of elements in a module. XRD-diffractograms and SEM cross-section imaging suggest that the enhanced RS in cells with thin ZnO:Al is not exclusively related to the thickness but is also due to a reduced (002)-texture and an elongated lateral charge carrier pathway.