Flat heterojunction

Fig. 6.18 Schematic depiction of a flat-heterojunction organic solar cell.

Scheme 2 A simplified view of a the solar cell energy diagram as well as the solar cell with b flat heterojunction (FHJ) and c bulk heterojunction (BHJ).

Scheme 3 a A simplified energy diagram and b principle of operation of a flat heterojunction solar cell. The numbered cell operation steps are explained in text. Adapted from Ref. [5] with permission from The Royal Society of Chemistry... 

Australia, and scaled up by BP Solar in Spain, the heterojunction with intrinsic thin layer (HIT) cells developed by Sanyo by replacing the diffused P-doped emitter with an amorphous silicon layer and the back contact cells developed by Stanford University for use in concentrator technology and now converted to a large area for flat plate use. All three use single-crystalline silicon, while the majority of screen-printed cells use multicrystalline silicon wafers. 

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. 3.35. Experimental J—V characteristics of an ITO/PEDOT PSS/PCBM/Au injection limited electron current (triangles) and calculated space charge limited hole current in OC4C10-PPV (circles) for a thickness of L = 170 nm and temperature T = 290 K. The inserted figure represents die device band diagram under the flat band condition of a bulk heterojunction solar cell using Au as a top electrode [65]. 

Furthermore phase separation of Ceo and CuPe is deseribed at elevated temperatures [55] whieh affeets the moleeular arrangement, but ean not be de-teeted by the teehniques used here. Altogether, film morphology and strueture of blends of flat CuPe and spherieal Ceo moleeules are still not very well understood and need further investigation. This will beeome partieularly important in photovoltaic cells, where this material combination is a potentially promising candidate for so-called buUc-heterojunction cells [21, 56],... 

Phase boundaries can be like GBs where the adjoining two grains may not only be rotated relative to one another but will also (or instead) be structurally and/or chemically different of course, one or both phases may be a glass, which means the interface is not structured. As with heterojunctions, the word hetero is implied when we say phase boundaries. Semiconductor heterojunctions are examples of PBs heterojunction usually indicates that we are talking about flat interfaces and a thin-film geometry. As is the case with GBs, almost all the detailed studies have been concerned with special PBs. We can summarize this idea and compare some PBs to GBs. 

Another method that was successfully applied for the formation of a nano-structured bulk heterojunction organic solar cell is nanoimprint lithography (NIL). In direct comparison with a flat bilayer organic solar cell design, the nanostructured version exhibited nearly a double increase in solar PCE [119]. 

The first smaU-molecule SC was devised as early as three decades ago [4]. Depending on the nature of the interface between the electron-donor and electron-acceptor films in this SC, one can distinguish flat (or planar) heterojunction (FHJ) and bulk heterojunction (BHJ) SCs (Scheme 2). 

Typical experimental data of capacitance measurements carried out at 100 kHz "with solar cells in darkness are shown in figure 19c The parameter indicated is the duration of heat treatment at 180 which controls diffusion of Cu atoms in the CdS layer. The shift of the 1/c lines is attributed to an increasing compensation layer both in compensation degree and in extension of the i-CdS layer. This characteristic behaviour is observed only when the heterojunction is flat i.e. by avoiding the etching process with a plane geometrical structure. 

Structural, optical, and electronic properties of n-Si/n-BP and p-Si/n-BP heterojunctions have been investigated by Goossens et al. (77,78). Impedance spectroscopy has been used to obtain Mott-Schottky (MS) plots of (Csc) versus DC bias, V (Fig. 24). The slope of the MS plot was positive and concordant with an effective donor density No - Na of about 5 X 10 cm for all studied samples. For crystalline CVD layers of BP (100), the flat-band potential was -0.55 V versus SCE at pH 4.6 and was observed to show a Nernstian -60 mV/pH dependence. 

Figure 3.20 Shows schematically the band edge offsets in the flat band condition for the types of semiconductor heterojunctions.

The major question is where to put the Fermi level with respect to the flat-band band edges (step 3). Marking this position will determine on which side of the heterojunction most of the band bending occurs. When one semiconductor is very heavily doped the choice is easy. Mark the Fermi level with respect to the flat band edge of the heavily-doped material. There will be very httle band bending in that material (see Figure 3.22). 

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