Cathodic multilayer transparent electrodes for ITO-free inverted organic solar cells

• Cathodic multilayer transparent electrodes based on ZnS/Ag/TiOx (ZAT) were successfully applied to organic solar cells.
• Solution-processed TiOx on Ag enabled a cathodic functionality.
• The effect of optical spacer and cavity resonance was balanced to yield the best result.
• The optimized ZAT-based devices yielded the efficiency of 5.1%, which is comparable to that of ITO-based devices.

We demonstrate cathodic multilayer transparent electrodes based on a ZnS/Ag/TiOx (ZAT) structure for ITO-free inverted organic solar cells. A quality solution-based TiOx layer is adopted as an inner dielectric layer to modify the effective work function of Ag, ensuring the ZAT electrode works as a cathode. The effect of the TiOx layer is seen on the open-circuit voltage of a solar cell incorporating this layer, increasing to 900 mV from 600 mV in the case of a cell with a bare Ag layer for a bulk-heterojunction of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C70-butyric acid methyl ester (PCBM70). The results of a joint theoretical and experimental study indicate that the photocurrent of a ZAT-based solar cell can be significantly enhanced by carefully balancing the optical-spacer and cavity-resonance effects, both of which are modulated by the thickness of the WO3 layer used as a hole-collection layer at the top anode side. ZAT-based inverted solar cells with an optimized structure exhibit a power conversion efficiency as high as 5.1%, which is comparable to that of the ITO-based equivalent.

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