High-efficiency polymer tandem solar cells with three-terminal structure.

Figure 1. a) Two diodes connected in parallel with the anodes of the two diodes connected together and the cathodes connected together. b) Schematic of the three-terminal device and the dotted squares on Polymer solar cells are a promising alternative to future photovoltaic applications due to various advantages such as low cost and ease of processing. Ever since the invention of bulk heterojunctions (BHJs) utilizing a blend of donor polymer and acceptor fullerene derivative, there have been increasing efforts to enhance the power-conversion efficiency (PCE) of polymer solar cells. However, narrow absorption range and low carrier mobility of polymer solar cells limit the thickness of the active layer and, hence, the absorption efficiency. One of the strategies to increase its efficiency is to make use of tandem architectures. In a two-terminal tandem solar cell, two subcells with complementary absorption bands are connected in series through an interlayer that acts as a recombination zone for electrons from one subcell and holes from the other. The series connection leads to summation of the open-circuit voltage (VOC); however, the overall current is limited by the subcell that delivers the smaller photocurrent. Therefore, the photocurrentmatching criterion between the two subcells must be satisfied for efficient working of a two-terminal tandem cell. Because of the current-matching criterion, the two subcells might not be readily incorporated into a tandem cell under their optimal conditions. An ideal tandem structure would consist of two subcells that operate separately, so that the efficiency of the tandem cell is simply the sum of the efficiencies of the two subcells. To circumvent the challenge of photocurrent matching in the two-terminal architecture, two solar cells can be stacked together to form a four-terminal tandem cell. In this case, the two cells can be connected either in series, to add up the VOC, or connected in parallel (shown in Fig. 1a) to add up the short-circuit current (JSC). However, in these devices, considerable optical losses are encountered because of the presence of an additional indium tin oxide (ITO)-coated glass substrate and semitransparent metal electrode that compensates for the gain from absorption. In this manuscript, tandem structures with a three-terminal (3T) configuration are demonstrated, as seen in Figure 1b, in which two subcells are connected in parallel through a transparent conducting interlayer that acts as a common electrode to the two subcells. The advantage of this structure is that it is convenient to characterize the two subcells independently as well as when connected in parallel. If two subcells with similarVOC are used, they can be connected in parallel to sum the JSC without