Exciton generation layer

The second type of solar cell is based on a /m-heterojunction in analogy to semiconductor devices [274]. Excitons generated by light, diffuse and dissociate at the interface between a hole and an electron-conducting material. The optimum layer thickness was calculated to be 1.5 times the exciton diffusion length [275]. 

Experiments using a two-layer heterostructure in which the photocurrent action spectra are observed both for front and rear (symbatic and antibatic) illumination of the interfaee between a photosensitizer and a hole transport layer have shown that the surface enhaneement of bound pair generation is due to a layer typieally 300-500 nm thiek [13]. Within this distance of the interface, excitons generated by the optieal absorption may diffuse toward the interface and initiate bound pair generation. The importanee of these excitons for a specific photoreceptor can be iden-... 

With sufficiendy thin absorber layers, particnlarly with quantum-dot absorbers, it also appears possible to harvest hot carriers and reduce the thermalisation loss in solar cells. This idea was pursned at an early time by Cooper et al. (1983). Hot electron transfer has been demonstrated in an electrochemical arrangement, and the possibility of hot-carrier transfer across solid interfaces is starting to be realised. Furthermore, the associated phenomenon of mnltiple exciton generation from a single hot electron-hole pair created by a single photon within a semicondnctor qnantnm dot has now been realised, as Art Nozik discnsses in Chapter 3. 

This is given by g = gexdex, where gex is the exciton generation rate and /ex the exciton dissociation probability. The position dependence of gex can be found by solving Maxwell s equations for light absorption in the thin, layered device structure. Because of the low thicknesses and low dielectric permittivity, absorption is strongly influenced by optical interference. In an effective medium, tj x can be treated as a... 

Fig. 9 In single layer single material devices, charge carriers can only be dissociated at the Schottky junction. Therefore only excitons generated close to the depletion region W can contribute to the photocurrent (denoted as the active zone )...

Bulk heterojunction active layer in organic thin film solar cells is a typical example of mixture for functional blends. The mixture of p- and n-types of semiconductors have to form an appropriate geometry of two compounds to make effective charge transport paths for hole and electron as well as effective interfaces at which charges are generated with less efficient recombination. In addition, excitons generated by photon abosorption have to migrate for a certain distance to reach to the interface and it also needs an appropriate path. 

The author with co-workers [39] has investigated systems with the structures Al/Cso/PANI+CdS/ITO and Al/Cgo/PPV-l-CdInS/ITO. Most worthy of note are the dependencies of the short circuit current and the open circuit voltage on the nanoparticles concentration (Fig. 21). Since the fullerene molecule acts as a strong electron acceptor, excitons generated both in the polymer matrix and in the CdS particles are decomposed, electrons are accepted by the Ceo layer and the holes are transported to the anode through the polymer. Once the concentration of the nanoparticles exceeds the percolation threshold value, the system becomes short contacted, since the electrons can pass from the anode to the cathode through the barrier-free connected network of CdS clusters this latter fact leads to the disappearance of the photovoltaic effect, as illustrated in Fig. 21b. The increase in the photocurrent and the open circuit... 

In an all-polymer system, the excitons generate in the donor phase after the absorption of photons in active layer and then transform to the charge-transfer state, which will dissociate into free charge or recombine geminately. Those two processes compete with each other. A promoted efficiency of OSCs requires the inhibition of geminated recombination and the transformation of more excitons into free charges [134,135]. 

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