Air-stable inverted structure of hybrid solar cells using a cesium-doped ZnO electron transport layer prepared by a sol–gel process

We have developed an air-stable inverted structure of poly(3-hexylthiophene) (P3HT) : cadmium selenide (CdSe) hybrid solar cells using a cesium-doped ZnO (ZnO:Cs) electron transport layer. The ZnO:Cs layer was simply prepared at low temperature by the sol–gel process using a ZnO solution containing cesium carbonate (Cs2CO3). With increasing Cs-doping concentration, the conduction band edge of ZnO is decreased, as confirmed by scanning Kelvin probe microscopy. The energy level of ZnO:Cs is effective for electron transport from CdSe. Consequently, the power conversion efficiency (PCE) of the inverted P3HT : CdSe hybrid solar cells using the ZnO:Cs electron transport layer is 1.14%, which is significantly improved over that (0.43%) of another device without Cs. X-ray photoelectron spectroscopy analysis revealed that the amount of CdSe on the substrate (or the bottom surface) is larger compared with the air (or top) surface regardless of the P3HT : CdSe weight ratio. The vertically inhomogeneous distribution of CdSe in the hybrid solar cells gives better charge transport from CdSe to ZnO:Cs in the inverted structure of the device compared with that in the normal structure. As a result, the inverted hybrid solar cell consisting of 1 : 4 (wt/wt) P3HT : CdSe shows the best efficiency, while the best efficiency of a normal hybrid solar cell is achieved at 1 : 9 (wt/wt) P3HT : CdSe.