Heterojunction devices

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-... 

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...

Ng AMC, Djurisic AB, Chan WK, Nunzi JM (2009) Near infrared emission in rubrene fullerene heterojunction devices. Chem Phys Lett 474 141... 

Dupuis RD (2000) III-V semiconductor heterojunction devices grown by metalorganic chemical vapor deposition. leee Journal of Selected Topics in Quantum Electronics 6(6), 1040-1050... 

These data prove that thin layer multi-heterojunction devices are enabled by OVPD with accurate thickness reproducibility and layer performance, which are essential manufacturing requirements in industrial mass production. The OVPD technology also enables controlled material transport and a high material utilization efficiency of 50-70%, based on condensation on to the substrate only. For state-of-the art VTE manufacturing technology material utilization efficiencies are in the range of 1 to 6% [23],... 

The valence band offset between two dissimilar semiconductors is an important parameter in predicting the behaviour of heterojunction devices. The theoretical and experimental band offset values for several different III-N/III-N and III-N/substrate interfaces have been summarised. 

Fig. 5.6. Schematic drawing of a bulk heterojunction device. Charge generation occurs throughout the bulk, but the quality of the two transport networks (p-and n-type channels) is essential for the functionality of the blend as an intrinsic, ambipolar semiconductor. Light emission occurs at the semi-transparent ITO electrode. Electron transport on the fullerenes is marked by full arrows and hole transport along the polymer by dotted arrows...

Fig. 5.12. Temperature dependent I/V characteristics of a bulk heterojunction device (ITO/PEDOT/MDMO-PPV PCBM/LiF-Al) in the dark (top) and under illumination (bottom)...

The previous section gave an overview of the transport and junction properties of conjugated materials regarding their importance for photovoltaic devices. In this chapter, the bulk heterojunction device itself will be in the spotlight. Device properties will be discussed and evaluated as for classical inorganic solar cells, concentrating on the short-circuit current /sc, the open-circuit voltage Foc, the fill factor FF, and the spectral sensitivity. 

Table 5.3. Solar cell characteristics (PF and Voc) of MDMO-PPV/PCBM bulk heterojunction devices for various interfacial layers (LiF, SiO ) with different thicknesses compared to a solar cell with a pristine A1 electrode, and also calculated diode characteristics Rs and Rp found using (5.39) for the various interfacial layers...

Table 5.4. PV performance parameters of various bulk heterojunction devices comprising a low bandagap p-type polymer PTPTB under AM 1.5 conditions...

The closest parallel to the a-Si devices described in this paper seems to be the observation of memory switching in heterojunctions of n-type ZnSe grown epitaxially on p-type (single-crystal) Ge substrates, reported by Hovel (1970) and by Hovel and Urgell (1971). The ZnSe-Ge heterojunction devices are polar and the transition times for the OFF-ON and ON-OFF operation are both in the region of 100 nsec or less. Similar, but not so... 

The heterojunction devices were fabricated by masking the surface, then using a 10 % HNO3 aqueous solution to etch down through 2x2 mm windows to remove ZnO and leave regions of the AlGaN layer exposed. Ohmic contacts to the n-ZnO and / -Alo i2Gao 88N were made by thermal vacuum evaporation of Al and Ni, respectively. ... 

We have for a long time considered the Ti02/CuSCN heterojunction device to be a frame structure for the ETA solar cell, since only the absorber layer is missing. Much of our work therefore focused on finding a suitable absorber, but chemical compatibility and processing characteristics have precluded a successful strategy so far. This issue will be discussed in more detail in Section 6.5. 

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... 

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