Solar cells interfacial effects

Miyashita M, Sunahara K, Nishikawa K, Uemura Y, Koumura N, Kara K, Mori A, Abe T, Suzuki E, Mori S (2008) Interfacial electron-transfer kinetics in metal-free organic dye-sensitized solar cells combined effects of molecular structure of dyes and electrolytes. J Am Chem Soc 130 17874-17881... 

The promise of photoelectrochemical devices of both the photovoltaic and chemical producing variety has been discussed and reviewed extensively.Cl,, 3,4) The criteria that these cells must meet with respect to stability, band gap and flatband potential have been modeled effectively and in a systematic fashion. However, it is becomirg clear that though such models accurately describe the general features of the device, as in the case of solid state Schottky barrier solar cells, the detailed nature of the interfacial properties can play an overriding role in determining the device properties. Some of these interface properties and processes and their potential deleterious or beneficial effects on electrode performance will be discussed. 

D. R. Lilliongton and W. G. Townsend, Effects of interfacial oxide layers on the performance of sihcon Schottky-barrier solar cells, Appl. Phys. Lett. 28(2), 97, 1976. 

Here we show that the polarity of polymer solar cells can be reversed by changing the position of two interfacial layers vanadium oxide (V2O5) layer as hole injection and cesium carbonate (CS2CO3) layer as electron injection, independent of the top and bottom electrodes. ° Since our first demonstration of inverted solar cells, more and more interests have focused on this new architecture. Waldauf et al. demonstrated inverted solar cells with a solution-processed titanium oxide interfacial layer. White et al. developed a solution-processed zinc oxide interlayer as efficient electron extraction contact and achieved 2.58% PCE with silver as a hole-collecting back contact. It is noteworthy to mention that EQE value for inverted solar cells approaches 85% between 500 and 550 nm, which is higher than that of normal polymer solar cells. This is possibly due to (i) the positive effect of vertical phase separation of active layer to increase the selection of electrode and (ii) lower series resistance without the PEDOT PSS layer. 

Y.-K. Han, Y.-J. Lee, and P.-C. Huang, Regioregularity effects in poly(3-hexylthiophene) PCBM-based solar cells incorporating acid-doped polyaniline nanotubes as an interfacial layer, J. Electrochem. Soc., 156, K37-K43 (2009). 

The metal-oxide interaction also depends upon the rate of the metal deposition. Experimental results show that an increase in the deposition rate affects the structure and composition of the interfacial layer in much the same way as an increase in substrate temperature [6]. The effects of the rate of metal deposition, the substrate temperature, and the layer thickness on the electrical and physical properties of solar cells may be understood in terms of the physical and chemical interactions in Ti-SiO -Si cells [5, 6, 8]. 

In summarizing, one can say that the conversion efficiencies of electrochemical solar cells with semiconductor electrodes are very similar to those for solid state devices. Additional problems arise by the possibility that the electron transfer reactions at the interfaces can be slow. This disadvantage may however be compensated by the larger flexibility in the adjustment of the redox potentials of the electrolytes to the properties of the semiconductors and by the very simple formation of the heterojunction at which the unfavorable effects of interfacial electronic states are less pronounced. The most serious problem of such cells remains the photodecomposition which has to be overcome before such devices can reach practical importance. 

The high contact area of the junction nanocrystaUine solar cells renders mandatory the grasp and control of interfacial effects for future improvement of cell performance. 

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