Conjugated polymer-based photovoltaic cells

Fig. 12 Examples of device architectures of conjugated polymer-based photovoltaic cells a single layer b bilayer c disordered bulk heterojunction d ordered bulk heterojunction. (Reproduced with permission from [71], 2005, American Chemical Society)...

Figure 6.1 Four device architectures of conjugated polymer-based photovoltaic cells (a) single-layer polymer PV cell (b) bilayer polymer PV cell (c) disordered bulk heterojunction (d) ordered...

There is a close relationship between the morphology of active layers and performance (physical processes) of conjugated-polymer-based photovoltaic cells, which is quite important for the further performance improvement. The morphological characteristics include ordered packing of donor/acceptor materials, domain size, phase-separation stracture and interfacial diffusion structure, while physical processes include exciton diffusion, charge-transfer state separation, lifetime of charge carrier, and carrier mobility. 

An alternative inexpensive organic polymer-based photovoltaic solar cell has been invented. In this device, p-type and n-type semiconductors are sequentially stacked on top of each other. In such devices, absorption of a photon by a ji-conjugated polymer results in the formation of an excited state, where coulom-bicaUy bound electron-hole pair (exciton) is created. This exciton diffuses to a region of interface of n-type semiconductor where exciton dissociation takes place and transport of charge to the respective electrodes occurs. For example, the photo-induced electron transfer from a donor layer (p-type) to acceptor layer (n-type) takes place in a polymer/fullerene-based organic bilayer solar cell, MDMO-PPV PCBM, with power conversion efiiciency of 2.5 % (Fig. 11.8) [13]. 

Besides ruthenium complexes, rhenium complexes were also used as the photosensitizers in photovoltaic cells. Bulk heterojunction photovoltaic cells fabricated from sublimable rhenium complexes exhibited a power conversion efficiency of 1.7%.75,76 The same rhenium complex moiety was incorporated into conjugated polymer chains such as polymer 16a c (Scheme 9). Fabrication of devices based on conjugated rhenium containing polymers 17a c and SPAN by the LbL deposition method was reported.77 The efficiencies of the devices are on the order of 10 4%. 

Finally, conjugated materials 40 based on poly(phenylene thiophene) and poly (fluorene thiophene) main chain polymers functionalized with pendant trithiocyanato ruthenium terpyridine complexes were synthesized by the Suzuki coupling reaction. Heterojunction photovoltaic cells with the simple structure ITO/polymer/C-60/Al were fabricated. Under simulated AM1.5 solar light illumination, the short circuit currents, open circuit voltages, and power conversion efficiencies of the photovoltaic cells were measured to be 1.53-2.58 mAcm 2, 0.12-0.24 V, and 0.084-0.12%, respectively [77]. 

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