Extremely thin absorber

Fig. 3.18 Schematic outline and ideal band diagram of an extremely thin absorber solar cell. The n-Ti02 crystallites are clustered together to form a relatively open, network-like morphology, accommodating a thin layer of CdTe absorber, with p-ZnTe at the back contact. (Reprinted from [270], Copyright 2009, with permission from Elsevier)...

In this chapter, I give an account of the historical development of semiconductor photoelectrochemistry and nanostructured photovoltaic devices in Section 1.2, and then Sections 1.3-1.6 provide a brief introduction to the major cell types discussed in the remainder of the book the ETA (extremely thin absorber) cell, organic and hybrid cells, dye-sensitised solar cells (Gratzel cells) and regenerative solar cells. 

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. 

Ernst K., Belaidi A. and Konenkamp R. (2003), Solar cell with extremely thin absorber on highly structured substrate , Semicond. Sci. Technol. 18, 475 79. 

Moller J., Fischer C.-H., Siebentritt S., Konenkamp R. and Lux-Steiner M. (1998), CuInSi as extremely thin absorber in the eta-solar cell , Proc. 2nd. World Conf. and... 

Taretto K. and Ran U. (2005), Influence of built-in voltage in optimized extremely thin absorber solar cells , Thin Solid Films 480-481, 447-451. 

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. 

Figure 6.2 Schematic diagram of solar cells with extended junctions and an extremely thin absorber (a) Layer structure for a superstrate n-i-p cell in this configuration a highly structured ra-layer is deposited on a transparent conductive oxide (TCO) contact layer, then a conformal absorber layer is deposited, followed by a transparent p-type transport layer and finally a reflective metal contact (b) Band diagram for the n-i-p heterojunction. The valence-band edges Ey) and conduction band edges Ec) for the absorber and transport layers and the electron and hole quasi-Fermi levels are shown (c) Illustration of reduced transport paths in the absorber layer and extended optical paths due to scattering in the heterostructure (d) Extremely thin absorber cell with a comparably shallow structure and a metal back contact in place of a transparent transport layer.

The second case is where both transport layers are transparent metallic condnctors with a work function difference A4>. If the metallic layers are separated by an extremely thin absorber of thickness da, the work function difference will be dropped nearly completely across the absorber layer. This will give rise to an electric field of strength = A lqd. Clearly, if the semiconductor is sufficiently thin this field can also be quite large, and this configuration would therefore also produce a useful driving force for charge separation. 

A detailed optical study of this cell structure indicates that the optical path enhancement in this arrangement is only approximately fivefold. Thus this cell should be considered as a moderate version of an extremely thin absorber cell, in which the transport length has been reduced by a factor 10-15 and the optical path is enhanced by a factor of five. 

Extremely thin absorber layers may open a new way to obtain inexpensive and, at the same time, efficient inorganic solar cells. This expectation is based on the realisation that short transport distances in the absorber and long optical paths can be obtained in heterostructures with extended junction areas. The anticipated effects contribute to better usage of material and permit less stringent quality requirements. The present state-of-the-art indicates that this expectation is well founded. 

Ernst K., Lux-Steiner M.-C. and R. Konenkamp (2000), All-solid-state and inorganic solar cell with extremely thin absorber based on CdTe , Proc. 16th. Eur. Photovoltaic Solar Energy Conf., Glasgow, UK, p. 63. 

Rost C. (1999b), Cul and CnSCN as transparent p-type semicondnctors in a p-i-n solar cell with an extremely thin absorber . Diploma thesis. Physics Department, Freie Universitat Berlin, 1999. 

Extremely Thin Absorber-Host-Scaffold Approach... 

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... 

Majidi H, Baxter JB (2011) Electrodeposition of CdSe coatings on ZnO nanowire arrays for extremely thin absorber solar cells. Electrochim Acta 56 2703... 

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