Toward Current Matching in Tandem Dye-Sensitized Solar Cells

The tandem pn-type dye-sensitized solar cells (pn-DSCs) have received much attention in the field of photovoltaic technologies because of their great potential to overcome the Shockley-Queisser efficiency limitation that applies to single junction photovoltaic devices. However, factors governing the short-circuit current densities (J(sc)) of pn-DSC remain unclear. It is typically believed thatJ(sc)of the pn-DSC is limited to the highest one that the two independent photoelectrodes can achieve. In this paper, however, we found that the availableJ(sc)of pn-DSC is always determined by the largerJ(sc)that the photoanode can achieve but not by the smaller one in the photocathode. Such experimental findings were verified by a simplified series circuit model, which shows that a breakdown will occur on the photocathode when the photocurrent goes considerably beyond its threshold voltage, thus leading to an abrupt increase inJ(sc)of the circuit. The simulation results also suggest that a higher photoconversion efficiency of the pn-DSCs can be only achieved when an almost equivalent photocurrent is achieved for the two photoelectrodes.