Polymer tandem cells

Following the demonstration of solution processed tandem polymer solar cells in 2007, the performance of these devices has developed significantly. In this section, we highlight some of the advanees made since then. 

In 2010, Yang et al published the first inverted configuration tandem polymer solar cell using P3HT PC6iBM and PSBTBT PC7iBM subcells. As a recombination layer, Yang et al used thermally evaporated MoOj and Al, covered with a sol-gel ZnO layer to reach a PCE of 5.1%. Almost at the same 

6% PCE achieved by Yang et al showed that designing materials tailored to operate in tandem cells can lead to considerable improvements. 

Several groups have reported new and speeifieally designed materials for tandem cells. In this respect it is not only of interest to design small bandgap 

By the end of 2014, a number of polymer tandem cells reached PCEs very close to or even exceeding 10%. Among these are a series of tandem cells developed by Yang et al as reference cells for a record high triple junction cell of 11.6% thatwill be discussed in detail in Section 11.5.3. The reference tandem cells use wide (1.90 eV), medium (1.58 eV), and small (1.40 eV) band-gap photoactive layers in each of the three possible mixed configurations to give 9.6%, 10.7%, and 9.8% inverted tandem cells (see Table 11.1). The novelty is the use of WO3 nanoparticles as an interlayer between the photoactive 

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The use of low bandgap polymers (ER < 1.8 eV) to extend the spectral sensitivity of bulk heterojunction solar cells is a real solution to this problem. These polymers can either substitute one of the two components in the bulk hetero junction (if their transport properties match) or they can be mixed into the blend. Such a three-component layer, comprising semiconductors with different bandgaps in a single layer, can be visualized as a variation of a tandem cell in which only the current and not the voltage can be added up. 

Transparent polymer solar cells (i.e., polymer solar cells with transparent electrodes) can be easily fabricated based on inverted architecture and have important application in tandem architectures as well. We can form transparent solar cells by replacing the Al top electrode with 12 nm Au in the inverted structure. The J-V curves for this transparent polymer solar cell, with light incident from ITO and Au side, are shown in Figure 11.17. The difference between the two J-V curves is due to the partial loss by the reflection and absorption at the semitransparent Au electrode. To provide sufficient electrical conductance, Au layer thickness has to be sufficient and the optical loss at Au electrode becomes significant. However, the inverted solar cell structure has the V2O5 layer which is not only transparent but also provides effective protection to the polymer layer. A transparent conductive oxides electrode, such as ITO, can therefore be deposited without compromising device performance. 

At the early development of polymer solar cells, a planar p-n junction structure represented the mainstream in mimicking conventional silicon-based solar cells. However, the obtained devices demonstrated poor photovoltaic performances due to the long distance between the exciton and junction interface and insufficient light absorption due to the thin light absorber. It was not until 1995 that the dilemma was overcome with the discovery of a novel bulk heterojunction in which donor and acceptor form interpenetrated phases. Poly[2-methoxy-5-(2 -ethylhexyloxy)-p-phenylene vinylene] was blended with Ceo or its derivatives to form the bulk heterojunction. A much improved power conversion efficiency of 2.9% was thus achieved under the illumination of 20 mW/cm. (Yu et al., 1995). The emergence of the donor/acceptor bulk-heterojunction structure had boosted the photovoltaic performances of polymer solar cells. Currently, a maximal power conversion efficiency of 10.6% had been reported on the basis of synthesizing appropriate polymer materials and designing a tandem structure (You et al., 2013). The detailed discussions are provided in Chapter 5. 

A polymer tandem solar cell with 10.6% power conversion efhciency. Nat. Commun. 4,1446. 

J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T.-Q. Nguyen, M. Dante, A. J. Heeger, Efficient Tandem Polymer Solar Cells Fabricated by All-Solution Processing. Science 2007,317,222-225. 

A.F. Mitul, et al. Low temperature efficient interconnecting layer for tandem polymer solar cells. Nano Energy, 2015.11 p. 56-63. 

V.S. Gevaerts, et al. Solution processed polymer tandem solar cell using efficient small and wide bandgap polymer fullerene blends. Advanced Materials, 2012 p. 1-5. 

J. Yang, et al, A robust inter-connecting layer for achieving high performance tandem polymer solar cell. Advanced Materials, 2011. 23 p. 3465-3470. 

L. Don, et al.. Tandem polymer solar cells featuring a spectrally matched low-bandgap polymer. Nature Photonics, 2012. 

L. Dou, et al.. Systematic investigation of benzodithiophene- and diketopyr-rolopyrrole-based low-bandgap polymers designed for single junction and tandem polymer solar cells. Journal of the American Chemical Society, 2012. 134(24) p. 10071-10079. 

J.Y. Kim, et al.. Efficient tandem polymer solar cells fabricated by all-solution processing. Science, 2007. 317 p. 222-225. 

Recent progress of naphthalimide-based dendrimers 12CJO304. Recent trends in polymer tandem solar cells research 13PPS1909. Rod—coil and all-conjugated block copolymers for photovoltaic applications 13PPS791. 

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