Donor-acceptor “bulk heterojunction

Vandewal K, Tvingstedt K, Gadisa A, Inganas O, Manca JV (2010) Relating the open-circuit voltage to interface molecular properties of donor acceptor bulk heterojunction solar cells. Phys Rev B 81 125204... 

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]. 

Fig. 69 Ideal structure of a donor-acceptor bulk heterojunction polymer solar cell...

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 10.8 The energy diagram of the photoinduced electron transfer and the main energy loss mechanisms in donor-acceptor bulk heterojunction solar cells. 

PLA coil block followed by subsequent backfilling with fullerene derivatives for ordered donor/acceptor bulk heterojunctions can provide efficient pathways for transport in OSCs. 

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

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])...

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...

Fig. 5.32. Voc for solar cells using PCBM, azafulleroid 5 and ketolactam 6 as the acceptor component in bulk heterojunction solar cells comprising MDMO-PPV as electron donor...

The observed experimental result that Voc decreases linearly for bulk heterojunction solar cells allows us to conclude that, at least in the high temperature range (T > 200 K), these solar cells may be described by a diode model with Ip exp(E/kT). Here E is a parameter analogous to Eg for conventional semiconductors. For conjugated polymer/fullerene bulk heterojunction solar cells, E should correspond to the energy difference between the HOMO level of the donor and the LUMO level of the acceptor components of the active layer [as also suggested by the extrapolated value of V oc(0 K)]. 

Interpenetrating Network of Donor-Acceptor Organics. Bulk Heterojunction Solar Cells... 

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... 

Several organics, e.g. pristine poly(3-octylthiophene), polyfluorene, bifunctional spiro compounds and polyphenyleneethynylene derivative, have been used for fabricating photOFETs. Responsivity as high as 0.5-1 A/W has been achieved in some of these transistors. We have already discussed the bulk heterojunction concept in Chapter 5. The bulk heterojunctions are fabricated using acceptor materials with high electron affinity (such as C<5o or soluble derivatives of C6o) mixed with conjugated polymers as electron donors. PhotOFETs based on conjugated polymer/fullerene blends are expected to show... 

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