Polymer Fullerene Bilayer Diffusion

PCBM BCBG OFET measured before annealing. This device had a channel length and width of 7.5 and 10000pm, respectively, and P3HT and [fiOjPCBM thicknesses of 8 and 31 nm, respectively. (Adapted from Ref [101] with permission of American Physical Society.) 

---

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]. 

It is seen from Table 5.1 that the values of the conversion efficiency in bilayer solar cells also is quite low. As mentioned in the introduction it is difficult to dissociate excitons in the conducting polymers. The Donor/Acceptor (D/A) junction between the polymer and the fullerene is rectifying and can be used for designing photovoltaic cells or photodetectors. In this bilayer cell also the conversion efficiency is low. The cause of the low efficiency is that the charge separation occurs only at the D/A interface that results low collection efficiency. The diffusion length of the exciton is a factor 10, lower than the typical penetration depth of the photon. 

---