**4.1. Organic Photovoltaic (OPV)**

A typical OPV cell consists of a transparent conductor, a photoactive layer, and an electrode. OPV cells rely on organic small molecules or polymers for light absorption and charge transport. Different from the traditional inorganic semiconductors where free electrons and holes are easily generated under solar illumination, in OPV device, neither bilayer and planar heterojunction structure or an intermixed bulk heterojunction (BHJ) structure, a strongly

**Figure 7.** Organic solar cells prepared by using P3HT/graphene composite as photoactive layer.

bound electron–hole pair is generated, and the electron–hole pair needs to be separated by a acceptor material.[95, 96]

As shown in Figure 7, for P3HT-modified graphene-based solar cells, graphene is expected to be a good electron-acceptor candidate, because large donor/acceptor interfaces for charge generation and a continuous pathway for electron transfer will be formed owing to the large two-dimensional plane structure and the one-atom thickness property of graphene.

Dai *et al.* created a bilayer photovoltaic device with solution-cast P3HT-GO heterostructure as donor and electron acceptor of thermally evaporated C60 layer C60.[38] The power conversion efficiency (PCE) of P3HT-GO solar cells is two times larger than that of the P3HT/C60 device. Liu *et al*. prepared a novel type OPV device with BHJ active layer of a solution-processable functionalized graphene (SPFGraphene) as the acceptor material and electron donor com‐ pound of P3HT.[97] In the ITO/PEDOT:PSS/P3HT:graphene/LiF/Al photovoltaic device, a PCE of 1.1% was achieved. The SPFGraphene acts as charge dissociation and provide percolation paths for electron transfer, which promote the active layer yields better carrier mobility, easy excitons splitting, and suppression of charge recombination, thereby improving photovoltaic action. Liu and coworkers introduced SPFGraphene in P3HT/PCBM photovoltaic devices. By taking advantage of the electron-accepting feature of fullerenes and the high electron transport capability of graphene, a PCE as high as 1.4% was obtained.[98]
