Excited state organic solar cells

Through exothermic dissociation of a neutral excited state in molecule by electron transfer to an adjacent molecule. This process leads to the generation of geminately bound electron-hole pairs as precursors of free positive and negative charges in an organic solar cell. 

Fig. 8 Schematic illustration of donor/acceptor energies relevant for charge-transfer in organic solar cells. Straight lines represent ground state binding energies, while wavy lines represent excited state binding energies...

A bulk heterojunction (BHJ) organic solar cell (Figure 3) has essentially the same stmcture as a single-layer LED, except that the active layer is a mixmre of an electron acceptor and an electron donor. By contrast in a bilayer device, the electron donor and acceptor are deposited as separate layers. In both cases, the devices produce electricity by separation of an excited state (exciton) at the interface between the donor and acceptor, followed by transport of the charges so formed to the electrodes. 

A 2-level system is the most general electronic model of a solar cell. It allows a simple but nevertheless rigorous treatment of optical transitions and formulation of the excited state. For organic materials, a 2-level system might even be more appropriate than a band system. 

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]. 

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