Polymer multi-junction cells

A further increase in the efficiency of polymer solar cells can be achieved by extending the multi-junction configuration from two to three or more photoactive layers. Multi-junction cells with more than two absorber layers are readily obtained using thermal evaporation of photoactive and 

Polymer multi-junction cells have also achieved noticeable attention. In Table 11.2 we provide an overview of the multi-junction cells that had been published up to the end of 2014. In 2007, Gilot et aL showed that it is possible fully to solution process three photoactive layers and two recombination layers based on ZnO/PEDO PSS and obtain a working triple junction solar cell with koc = 2. 19 By the same procedure, six-fold junction cells with 

Cell configuration (eV) Foe (V) lype PCE (%) Recombination layer front/back Ref. 

These advanees in triple-junction polymer solar cells show the potential for this type of device. Considering that in these triple junctions the photon energy loss ( ioss) in each subcell is at least 0.7 eV, and sometimes even above 1.0 eV (Table 11.2), it can be expected that by developing new materials in which E oss is more tailored to 0.6 eV it will be possible to increase the efficiency of triple-junction solar cells considerably. In our view efficiencies in the 15-20% range are within reach. 

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Several excellent reviews exist on polymer tandem solar cells and we refer to these publications for a comprehensive overview. In this chapter we focus on the materials requirements for creating efficient polymer solar cells and intermediate contact layers. In the following paragraphs, we will outline the operational principles in more detail. We will then review the most important photoactive materials used in polymer multi-junction cells and outline the material combinations that can used for the recombination layer. We conclude with an overview of recent achievements that have pushed the efficiency to well over 10% and address the progress in processing of large area tandem polymer solar cells. 

The improvements described above have been obtained via the design and development of new semiconducting polymers, with wide, medium, and small bandgaps, and by the improvement of the interconnecting layers in regular or inverted series-connected multi-junction cells. In addition to the... 

One of the problems facing manufacturers of polymer-based photovoltaic cells is their low light-to-energy conversion efficiency of 5% maximum, whereas multi-junction crystalline silicon-cells have achieved a figure of 37%. One of the flexible plastic solar cells exhibiting the 5% efficiency contains zinc oxide. 

Principles of Multi-Junction Polymer Solar Cells... 

Figure 11.7 Structures of fullerene acceptors used in multi-junction polymer solar cells. References to the publications using these acceptors can be found in Table 11.1.

Table 11.2 Solution processed multi-junction polymer solar cells.

Although multi-junction polymer solar cells are a valid strategy for reaching high PCEs, the increased number of layers that have to be processed on top of eaeh other increases the complexity of the fabrieation process. The first important factor is solvent orthogonality. Each of the layers has to be proeessed from a solution that will not damage or wash away any of the previously deposited layers. For processing on top of a bulk heterojunction polymenfullerene, only a few solvents can be used in praetiee water, aeetone, and aleohols. Typically, the photoactive layers are hydrophobie and this represents an obstacle to the deposition of materials from aqueous solutions beeause of poor wettability. Therefore, extra measures have to be taken such... 

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