Bilayer heterojunction

Although the quantum efficiency for photoinduced chaige separation is close to unity for a D/A pair, the conversion efficiency in a bilayer heterojunction device is limited ... 

The spectral dependence of the photoresponse of these bilayer heterojunction devices, illuminated from the 1TO side, is displayed in Figure 15-22. The onset of photocurrent at hv— 1.7 cV follows the absorption of the fullerene, indicating a symmetric hole transfer from the excited fullerene to the MEH-PPV. The minimum in the photocurrent at /iv=2.5 eV corresponds to the photon energy of maximum absorption of MEH-PPV. The MEH-PPV layer, therefore, acts as a filter, which reduces the number of photons reaching the MEH-PPV/C()0 interlace. Thus, the thickness of the MEH-PPV layer determines the anlibatic spectral be-... 

Figure 7.4 (a) A donor-acceptor bilayer heterojunction. Donor regions are shaded blue and acceptor... 

D.S. Yu, et al. Soluble P3HT-grafted graphene for efficient bilayer-heterojunction photovoltaic devices. ACS Nano, 2010. 4(10) p. 5633-5640. 

Bilayer Heterojunction Devices Bulk Heterojunction Devices... 

Roncali et al. prepared tetrahedral oligothienylsilane derivatives 5.44 and 5.45 by reaction of lithiated terthiophenes with SiCU (Chart 1.72) [486]. In comparison with the linear parent terthiophenes, the tetrahedral structures gave a red shift of 17-19 nm in the absorption spectra. These materials were implemented as the donor component in bilayer heterojunction solar cells, showing a significant increase in performance (jj < 0. 20 %) compared with cells including only the terthiophene branches (t] = 0.04 %) as active material. In bulk-heterojunction solar cells with PCBM as acceptor, 5.45 showed an efficiency of 0.3 %, which is fairly low compared with standard P3HT-PCBM solar cells [195, 196, 487-490]. 

Figure 6.19 Working principle of a bilayer heterojunction organic solar cell.

Efficiency of planar heterojunction devices is limited by low exciton diffusion lengths in organic senuconductors that typically stay below 20 nm." This means that excitons generated more than 10-20 nm away from the interface between the p- and n-type materials do not contribute to the charge generation. As a consequence of this drawback, the power conversion efficiencies of the planar bilayer heterojunction devices typically stay in the range of 1.0-1.5% and approach 2-3% only in rare examples." " ... 

There are two main types of PSCs including bilayer heterojunction and bulk-heterojunction [7]. Bulk-heterojunction PSCs are more attractive due to their high surface area junction that increases conversion efficiency. This type of polymer solar cell consists of Glass, ITO, PEDOT PSS, active layer, calciiun and aluminum in which conjugated polymer are used as active layer [8]. The organic solar cells with maximum conversion efficiency about 6% still are at the begiiming of development and have a long... 

Bilayer heterojunction devices consisting of aromatic diamine hole transporting layers and 8-hydroxyquinoline aluminium (Alq) [8] or dye-doped Alq [9] emissive layers sandwiched between indium tin oxide (ITO) and Mg/Ag electrodes have been found to exhibit high external efficiency, luminous efficiency and brightness. 

The application to the case of the polymers of the bilayer heterojunction structure that has been used successfully with molecular semiconductors [139] has recently been reported. The fullerene ) provides a useful high electron affinity semiconductor, and it is found that blends of this with a range of soluble PPV and polythiophene derivatives show very efficient charge separation following photoexcitation [146-148]. Two-layer diodes of C60 and MEH-PPV were reported initially to show poor efficiencies [149], but photovoltaic efficiencies (short circuit) up to 9% have now been obtained for PPV/ Ceo diodes of the type shown in Fig. 29.20 [150]. Charac-... 

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... 

The open circuit voltage (Vqc) of bilayer heterojunction solar cells is governed by the energetics at the D/A interface rather than that involving the electrodes, as pointed out already by Tang in his seminal paper [26]. Conversely, bulk heterojunc-... 

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