All the reviewed tandem cells so far were based on two-terminal devices, comprising two or more cells connected in series. However, several groups have suggested other approaches to realise multijunction devices. This part of the study will discuss some of these novel concepts.
Shrotriya and coworkers have reported a stacking method to overcome all processing difficulties. They superimposed two independent devices, and connected them either in series or in parallel [207]. To do so, the bottom cell required a semitransparent cathode that was realised with a 1-nm LiF/2-nm Al/12-nm Au layer sequence showing a maximum transparency of almost 75%. Both active layers were made of a blend of poly(2-methoxy-5- (2/-ethyl)-hexyloxy)-1,4-phenylene-vinylene (MEH-PPV) and PCBM.
In 2007 Hadipour et al. introduced an additional solution-processable, transparent, and insulating layer between the bottom and the top subcell that serves as an optical spacer and also allows the fabrication of a monolithic four-terminal device [208]. The cathode of the bottom cell (P3HT:PCBM) was evaporated with 3 nm of samarium (Sm) and 12 nm of Au. Then, 250 nm of polytrifluoroethylene (PTrFE), dissolved in methyl ethyl ketone (MEK), was spin coated onto the bottom cell to separate the two subcells, as suggested earlier by Perrson et al. [209]. The top cell was made of PTBEHT:PCBM on top of a 20-nm Au/50- nm PEDOT:PSS anode. They concluded that the most efficient connection for the active materials employed was the parallel one; with a Voc of 0.59 V, a Jsc of 9.2 mA/cm2 and a FF of 0.54.
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Table 8.2 Nonexhaustive survey of reports dealing with solution-processed tandem organic solar cells. (Reproduced with permission from De Vos, 1980. Copyright © IOP Publishing, 1980.)
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Figure 8.27 Sketch of the folded tandem cell realised by Tvingstedt et al. [210]. The chemical structures of the exploited alternating polyfluorenes, namely APFO3, APFO Green-9, and the acceptor molecule PCBM, are also shown.
Tvingstedt and colleagues proposed a novel geometrical modification of multijunction cells called a “folded reflective tandem device” [210]. The device scheme is depicted in Figure 8.27. This geometry enables the construction of tandem or multiple-bandgap solar cells with arbitrary electrical parallel or series connection. In addition, this geometry allowed the authors to benefit from three major advantages: First, the reflected light of one cell is directed toward the second one, which ideally has a complementary absorption spectrum. Secondly, the folded structures cause light trapping at high angles and absorb more photons from incoming solar light. Finally, the tilting of the cells enhances the light path within the active layer [211]. The used polymers were based on alternating fluorine copolymers (APFs) combined with PCBM, APFO3:PCBM for one cell and APFO-Green9:PCBM for the other cell. The conversion efficiency for a series connection increased from 2% up to 3.7% upon folding the V-shaped device from 0° to 70°. The advantage of this approach is that all problems related to multijunction stacking, extra transparent electrodes, and solvent incompatibility are simply avoided.
Zhang et al. demonstrated a simple alternative to a parallel interconnection in 2008 [212]. In this device structure, a PCBM layer is employed to simultaneously form a bilayer heterojunction PV subcell with the underlying CuPc layer and a BHJ photovoltaic subcell blended with P3HT. In comparison with the conventional tandem structure, the omission of the semitransparent intercellular connection layer reduces the complexity of the device processing and the light losses. According to the working principle of the bilayer PV cells, only the excitons created in a 5-10-nm thickness of CuPc can diffuse to the interface of the CuPc/PCBM where they are separated into free carriers. The enhanced Jsc = 8.63 mA/cm2 and n = 2.79% of the tandem structure were nearly the sum of those of the standalone cells of CuPc/PCBM and P3HT:PCBM.
