Impurities and deep levels in CdS/CdTe thin film solar cells with no back-contact layer were studied by deep level transient spectroscopy and photoluminescence. They could lower the device performance notably. Distribution of net carrier concentration was obtained. Two deep levels at Ev+0.365 eV and Ev+0.282 eV were determined with concentration of 1.67×1012 cm-3 and 3.86×1011 cm-3, respectively, and with capture cross section of 1.43×10-14 cm2 and 1.53×10-16 cm2, respectively. They are attributed to chemical impurities like Au and/or a singly charged tellurium vacancy complex, or related to O atoms introduced by the deposition of CdTe in O2 and Ar ambient.