Donor-acceptor bilayer devices

Organic bilayer solar cells have improved steadily since the report by Tang in 1986. Developments have been assisted by detailed modelling and advances in the understanding of molecnlar materials, as well as by innovations in device design and fabrication techniques. 

Despite attempts to increase efficiency using lower optical gap materials, the highest power conversion efficiencies are still achieved using CuPc as the donor material and Ceo as acceptor. The most important improvements have come about through improved device design. Key developments include the following  

In addition to vacuum-deposited small-molecule structures, bilayer devices have been fabricated from conjugated polymers and other solution-processible materials. One study reports a 1.9% efficient device fabricated by lamination of two spin-coated polymer layers (Granstrom et al, 1998). In principle, polymer bilayers can also be fabricated by spin-coating of successive layers using incompatible solvents. A more practical route to planar bilayers is the deposition of successive layers that have previously been spin-coated and removed from the substrate by a float-off technique (Ramsdale et al, 2002). 

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Originally, a donor-acceptor bilayer device of two films was used as an n-p junction in solar cells. Thus, they were fabricated as sandwich structures. An example would be one where a transparent substrate is first coated with a conductor, like indium-tin oxide. A conducting polymer like, poly (ethylene dioxythiphene), doped with polystyrene-sulfonic acid, would then be applied from and aqueous solution. The indium-tin oxide acts as an electrode for hole injection or extraction. The polymer is then covered with a conductor, an aluminum foil. The doped polymer can be illustrated as follows ... 

Comparison of the spectral response and of the power efficiency of these first conjugated polymer/fullerene bilayer devices with single layer pure conjugated polymer devices showed that the large potential of the photoinduced charge transfer of a donor-acceptor system was not fully exploited in the bilayers. The devices still suffer from antibatic behavior as well as from a low power conversion efficiency. However, the diode behavior, i.e. the rectification of these devices, was excellent. 

Bilayer devices [27,65] apply the donor-acceptor concept introduced above here the exciton is dissociated at their interface, leading to holes on the donor and electrons on the acceptor. Thus, the different types of charge carriers may travel independently within separate materials and bimolecu-lar recombination is largely suppressed. Therefore fight intensity-dependent... 

As shown in Figure 5.9, randomly mixed polymer-inorganic BHJ and ordered heterojunction (OHJ) are two common structures to prepare hybrid solar cells [44]. Similar to polymer-fullerene BHJ solar cells, polymer-inorganic hybrid BHJ solar cells (Figure 5.9a) can overcome the limitations of bilayer devices having small donor-acceptor interfacial area with inefficient exciton dissociation. NCRs can be surface modified to render them soluble in organic solvents (such as chloroform, toluene and chlorobenzene) to facilitate solution processability. Various surface... 

Hayashi, Y, et al. 2005. Influence of structure and C o composition on blends and bilayers of organic donor-acceptor polymer/Ceo photovoltaic devices, fpn J Appl Phys 44 1296. 

Yan, H Swaraj, S Wang, C., Hwang, I., Greenham, N.C., Groves, C Ade, H and McNeill, C.R. (2010) Influence of armealing and interfacial roughness on the performance of bilayer donor/acceptor polymer photovoltaic devices. Adv. Funct. Mater, 20, 4329-2337. 

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