High open circuit voltage stacked bulkheterojunction organic solar cells fabricated by the solution processing

Abstract

We investigate the stacked structure of bulkheterojunction (BHJ) organic solar cells based on conjugated polymer and fullerene derivative by the solution processing, especially spin coating method. Poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene], (MDMO-PPV) blended with [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) are used as a photo-active layer. The stacked-cell structure is as follows: ITO/(lower BHJ cell) PEDOT:PSS/MDMO-PPV:PCBM/inter-layer/(upper BHJ cell) PEDOT:PSS/MDMO-PPV:PCBM/Al. We evaluate Ag and ITO as materials of inter-layer to connect the upper BHJ cell with the lower BHJ cell. High open circuit voltage (V_OC) BHJ organic solar cell is obtained by using ITO inter-layer. V_OC of stacked-cell with ITO inter-layer is 1.31 ± 0.04 V under 100 mW/cm² illumination intensity, the AM 1.5 solar simulator. This is about 1.6 times as large as that of single BHJ organic solar cell. By virtue of inserting ITO inter-layer, the upper BHJ cell can be formed without damaging the lower BHJ cell and consequently stacked-cell in which two BHJ cells are connected in series can be fabricated. Moreover, the origin of V_OC of stacked-cell is likely the sum of each V_OC of the upper and the lower BHJ cells, and is different from that of stacked small molecule heterojunction organic solar cells. In the case of Ag interlayer, stacked-cell does not work at all. From the elemental analyses of device cross section, Ag layer seems to be damaged by PEDOT:PSS solution.