Donor-acceptor bulk heterojunction devices

The open-circuit voltage of BFH devices has been found to be correlated to the energy difference between the LUMO energy of the electron acceptor, E lumo.a and the HOMO of the electron donor, E homo.d (Brabec et al, 2001 MihaUetchi et al. 

2003 Mihailetchi et al, 2004), when the electrodes of the device make ohmic contacts with the acceptor and donor materials. Progress in nnderstanding the origin of the open-circuit voltage of organic solar cells and its dependence on device parameters will be discussed in Section 7.6.2 below. 

2 Progress with polymer fullerene bulk heterojunction devices 

Bulk heterojunction device performance has improved by strides since the first reports of charge separation in bulk heterojunctions, with power conversion efficiencies that now approach 5% (Li et al., 2005 Reyes-Reyes et al, 2005 Kim et al, 2006a). Smdies have focused on varying the donor and acceptor materials, optimising the 

3 Progress with polymer polymer bulk heterojunction devices 

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Fig. 15 Donor-acceptor bulk heterojunction device and the relevant elementary states. Right HOMO and LUMO energies of P3HT and PCBM in the 1 1 blend as determined by photoelectron spectroscopy (values taken from [160])...

While in the early 1990s power conversion efficiencies in single layer, single component devices were still limited to less than 0.1% [22-25], improvements over the turn of the millennium are attributed to a great extent to the introduction of the donor-acceptor bulk heterojunction concept, which makes use of two electronic components that exhibit an energy offset in their molecular orbitals [26-34]. 

At the early development of polymer solar cells, a planar p-n junction structure represented the mainstream in mimicking conventional silicon-based solar cells. However, the obtained devices demonstrated poor photovoltaic performances due to the long distance between the exciton and junction interface and insufficient light absorption due to the thin light absorber. It was not until 1995 that the dilemma was overcome with the discovery of a novel bulk heterojunction in which donor and acceptor form interpenetrated phases. Poly[2-methoxy-5-(2 -ethylhexyloxy)-p-phenylene vinylene] was blended with Ceo or its derivatives to form the bulk heterojunction. A much improved power conversion efficiency of 2.9% was thus achieved under the illumination of 20 mW/cm. (Yu et al., 1995). The emergence of the donor/acceptor bulk-heterojunction structure had boosted the photovoltaic performances of polymer solar cells. Currently, a maximal power conversion efficiency of 10.6% had been reported on the basis of synthesizing appropriate polymer materials and designing a tandem structure (You et al., 2013). The detailed discussions are provided in Chapter 5. 

Figure 17 Scheme of working photovoltaic devices, (a) single layer (b) bilayer (c) interpenetrating donor/acceptor (bulk heterojunction) photovoltaic cells. 

In the bilayer heterojunction devices, the donor-acceptor phases are separated from each other and can selectively contact the anode and cathode, whereas in the bulk heterojunction both phases are intimately mixed and there is no preferred direction for the internal fields of separated charges. Fig. 20. The electrons and holes are thus created within the volume having concentration gradient (diffusion) as driving force. The separated charges require percolated pathways and the donor-acceptor phases form bicontinuous interpenetrating network [123]. Bulk heterojunction devices are sensitive to the morphology in the blend [124]. Majority of... 

Fig. 3 Contemporary organic solar cell devices are based on donor/acceptor heterojunction device architectures, (a) Energy level diagram, (b) Planar heterojunction conligmation. (c) Bulk heterojunction configuration...

In a bulk-heterojunction photovoltaic cell with methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as an electron acceptor, alternating copolymer 19 (Fig. 9), derived from 2,7-fluorene and 2,5-dithienylsilole, can show impressive performance as the electron donor.31 In a device configuration of ITO/PEDOT/active layer/Ba/Al, the dark current density—bias curve shows a small leakage current, suggesting a continuous, pinhole-free active layer in the device. Under illumination of an AM 1.5 solar simulator at 100 mW/cm2, a high short-circuit current of 5.4 mA/cm2, an open-circuit voltage of 0.7 V, and a fill factor of 31.5% are achieved. The calculated energy conversion efficiency is 2.01%. 

Photovoltaic Devices with OPV4—Ceo- The increased lifetime of the charge-separated state, which extends into the millisecond time domain, opens the possibility of using the OPVrt-Coo dyads as the active material in a photovoltaic device. As an important difference with previous bulk heterojunction cells, the covalent linkage between donor and acceptor in these molecular dyads restricts the dimensions of the phase separation between the oligomer and the fullerene that could freely occur in blends of the individual components. This can be considered as a primitive attempt to obtain more ordered and better-defined phase-separated D-A networks. 

Fig. 5.3. Formation of a bulk heterojunction and subsequent photoinduced electron transfer inside such a composite formed from the interpenetrating donor/acceptor network, plotted with the device structure for such a junction (a). The diagrams showing energy levels of an MDMO-PPV/PCBM system for flat band conditions (b) and under short-circuit conditions (c) do not take into account possible interfacial layers at the metal/semiconductor interface...

Mihailetchi [134] investigated the open circuit voltage of the bulk heterojunction organic solar cells based on methanol-fullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as electron acceptor and poly[2-methoxy-5(3 ,7 -dimethyloctyloxy)-p-phenylene vinylene] (OC1C10-PPV) as an electron donor. It is known that a single layer device follows the MIM model [166] and the open circuit voltage V0c is equal to the difference in the work functions of the metal electrodes [134], If charges accumulate in the... 

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