Effects of bathocuproine/Ag composite anode on the performances of stability polymer photovoltaic devices

Abstract

In this work, the composite anode of BCP/Ag replaces the composite anode of Ca/Al, and the PTB7:PC71BM acts an as active layer for polymer solar cells. Calcium (Ca) is not a desirable candidate as electron extraction layer (EEL) for long-term stability polymer solar cells (PSCs) on account of its nature of active metal. And then, due to the poor stability of Al, which is not a desirable candidate as electrode, the bathocuproine (BCP) layer acts as an exciton blocking layer in organic device such OLEDs and small molecule solar cells, which has a k value that is close to zero for a broad range of wavelengths. The Ag has the nature of better chemical stability and conductivity than Al. In the device architecture described below, we replace the typical back metal electrode composed of a thin Ca layer and a thicker Al electrode by a few nanometer thick bathocuproine (BCP) layer and a thick 150 nm Ag layer. We investigate the effects of BCP thickness on the power conversion efficiency (PCE) and stability. The results reveal that the photovoltaic performances are improved, and a PCE of 6.82% at the 5 nm of BCP thickness, higher than the PCE of Ca/Al acted composite anode, is achieved. The substitution of BCP for Ca, can largely enhance light harvesting and exhibits an optimal light absorption by the active layer. This enhanced reflectivity of the buffer layer/electrode back contact results in an increase of the short circuit current. Compared with the devices of Ca/Al composite anode, it increases Jsc and external quantum efficiency with BCP/Ag composite anode. At the same time, it has the better stability of BCP/Ag composite anode of device, and almost the same PCE decrease ratio as free BCP devices and significantly improves the stability compared with Ca/Al composite anode. The stability test shows the better stability of BCP/Ag as composite anode than that of Ca/Al composite anode. The PCE of the device with Ca/Al as composite anode rapidly decreases by about 70% after 50 hour servicing due to the poor stabilities of Ca and Al. The device with BCP/Ag as composite anode shows favorable stability, owing to the PCE moderate decrease by less than 30% after the same story time. Our results indicate that substitution of BCP/Ag for Ca/Al composite anode is an alternative candidate for high performance and longterm photo stability PSCs.