ETA cells

Woerdenbag et al also evaluated the influence of chiral center configurations present in artemisinin (1) structure on the proliferation of Ehrlich ascites tumor (ETA) cells. Compounds 11-hydroxyartemisinin (47) and 11-hydroxy-11-epi-artemisinin (48) (Fig. 4) were synthesized and the... 

EFFICIENCY ENHANCEMENT OF ETA-CELLS FABRICATED BY SILAR DEPOSITION... 

Nanosized TiOi powders are of outstanding importance in this context. Aqueous suspensions of 30 nm particulate TiOi (mostly in the rutile form) are the active agent in many of the photocatalytic systems described by Serpone and Emetine in Chapter 5. Agglomerations of Ti02 nanoparticles into mesoporous films of pore size 2-50 nm which allow the penetration of liquid are the basis of the important dye-sensitised solar cell (DSSC) discussed by Grateel and Durrant in Chapter 8, as well as most of the hybrid devices described by Nelson and Benson-Smith in Chapter 7, and some of the ETA (Extremely Thin Absorber) cells described by Konenkamp in Chapter 6. The term eta-solar cell was actually introduced by Konenkamp and co-workers (Siebentritt et al., 1997), who had earlier used the term sensitisation cell for the same type of device (Wahi and Konenkamp, 1992). Precursors to ETA cells with liquid electrolytes as hole conductors were developed by Vogel et al. (1990), Ennaoui et al. (1992) and Weller (1993). Similar electrolytic cells with RuSa (Ashokkumar et al, 1994) and InP (Zaban et al, 1998) nanoparticle absorbers have also been demonstrated. 

Figure 1.3 Typical ETA cell geometries. The interfaces between the absorber and electron and hole transport layers are structured, usually in porous (Fig. 1.3a) or columnar (Fig. 1.3b) form. The interfaces between the transport layers and the contact layers are planar. If the substrate morphology is porous, both transport layers should be transparent to avoid shadowing effects. The contact layer on the light entry side must be transparent and the back contact should be reflective, to minimise optical losses outside the absorber layer. If the interfacial structuring is not very deep, it is possible to omit the hole transport layer, and deposit the back contact straight onto the ETA layer, which greatly simplifies device fabrication.

At their present state of development, ETA cells typically exhibit open-circuit voltages of 0.6-0.7 V and photocurrents of 5-15 mA cm . While the fill factors in the earlier ETA cells were often poor, typically -20%, more recent cells have improved fill factors, typically -60%. Overall, the solar conversion efficiencies are modest, as shown in Fig. 1.2 to date ri p values of approximately 5% have been obtained (Nanu et al., 2005). Insertion of an ultrathin mnnel barrier layer of an insulator such as AI2O3 or MgO can improve the open-circuit voltage (Wienke et al., 2003). Incorporation of quantum... 

Figure 1.4 Band structure in an ETA cell, showing the majority-carrier quasi-Fermi levels i p andi p p in the electron and hole conductor, respectively, and the photovoltage V. The conduction-band edge of the absorber must be above that of the electron conductor, while the valence-band edge of the absorber must be below that of the hole conductor. The conduction-band and valence-band offsets and AE between the electron or hole...

The technology of ETA cells is not yet mature, and future improvements in their efficiency can reasonably be expected. Modelling calculations (Taretto and Rau, 2005) indicate that 15% efficient CdTe ETA cells are possible even at electron diffusion lengths as low as 10 nm, provided that the built-in voltage is optimised to restrict recombination over the working bias range of the cell. If efficiency improvements of this order can be achieved and fabrication methods can be satisfactorily scaled up, ETA cells could offer a low-cost, stable alternative to traditional photovoltaic cells and dye-sensitised solar cells. 

Figure 6.1 Absorber thickness in today s solar cells as compared with the thickness of extremely-thin-absorber cells (ETA cells) using CdTe or quantum dot PbS absorbers.

In the next section we will discuss the basic design rules for a non-planar, solid, inorganic solar cell with an extremely thin absorber layer. This type of cell has often been called the ETA cell (for extremely thin absorber Ernst et al, 2000) or ETA cell, and we will use the latter acronym here. Materials aspects, structural, electronic and optical considerations will be introduced. 

Ernst (2001) has examined the minority-carrier transport in the CdTe/TiOa/Au ETA cell in some detail. Based on the accurate thickness determination that is possible in the CdTe electrodeposition process, it was possible to estimate the minority-carrier diffusion length in the nanocrystalline CdTe material. 

If diffusion or drift lengths are low, the only way out is to change the layout of the cell in such a way that the local distance to the collecting contact falls within the possible transport path length. This guideline was followed in the CdTe-based ETA cell, where the local absorber thickness corresponded quite accurately to the estimated diffusion length. 

Early work attempted to draw a close analogy between the dye-sensitised cell and the ETA cell. Much of the experimental work therefore focused on nanoporous TiOa as the substrate material. For the deposition of a CulnSi absorber layer into the nanoporous network, several methods such as electrodeposition, spray pyrolysis and atomic layer deposition have been explored. Our own efforts have concentrated on combining the ILGAR technique and electrodeposition to prepare the structure glass/SnOi/nanoporous TiOa/CulnSi/ CuSCN/Au. Krunks et al. (1999, 2001) and Wienke et al. (2003) successfully used solution chemistry and spray pyrolysis to deposit CuInSi absorbers on TiOz, and Nanu et al. (2003, 2004) have applied atomic layer deposition for FeSz and CulnSz deposition in nanoporous TiOz. 

Wienke et al. (2003) and Bayon et al. (2004) showed that substantial improvement in conversion efficiency is obtained when a thin indium hydroxysulphide (ln c(OH)j,S0 layer with a bandgap of approximately 2.5 eV is deposited prior to the CuInSz deposition. The observed improvement is attributed to a reduced hole density in the vicinity of the ln ,(OH)j,S2 contact. The CuInSz was prepared in a spray pyrolysis process. Nanu et al. (2003, 2004) employed atomic-layer chemical vapour deposition to obtain ultra-thin CuInSz layers inside the pores of nanostructured TiOz. CuCl, InCh and HzS are used as precursors in this process. A growth rate of 1-3 A s is observed, and conversion efficiencies shghtly above 4% have been obtained. While the process may not be suited for large-area applications, the results can clearly be taken as another confirmation that the ETA cell concept constitutes a feasible route towards efficient solar cells. 

Figure 6.25 (a) Ti02/CdTe interface in the CdTe ETA cell (b) Band energy diagram under zero bias and... 

Concepts for tandem arrangements have been suggested for dye-sensitised solar cells using dye sensitisation at both electrodes and an electrolyte that provides the coupling between the two dyes (He et al, 2000). A similar concept appears feasible also for solid-state devices, althongh it has not been explored yet in the context of ETA cells. 

Dye-sensitization is an example of a broader class of photophysical processes that involve charge transfer from an excited state. Other examples include sensitization of wide-band-gap semiconductors by quantum-confined nanoparticles (quantum dots) and by thin layers of semiconductors (extremely thin absorber layer - ETA cells). Charge transfer at the interface between donor... 

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