S. Hardman1, D. Graham1, B. Spencer1, W. R. Flavell1, D. Binks1, F. Sirotti2, M. El Kazzi2, M. Silly2, J. Aktar3, M. A. Malik3 and P O'Brien3
1 School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester, M13 9PL
2 TEMPO beamline, Société civile Synchrotron SOLEIL, L'Orme des Merisiers, Saint-Aubin - BP 48, 91192 Gif-sur-Yvette, France
3 School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL
Hybrid photovoltaic cells combine the light-harvesting and charge transport properties of semiconductor and metal oxide nanoparticles with polymer semiconductors. Cells based purely on semiconducting polymers suffer from poor harvesting of red light, poor long term stability, and fast charge recombination. In contrast, nanoparticles can have good photostablility, long recombination lifetimes and an absorption edge that can be tuned, by control of their size, to optimise energy extraction from the solar spectrum. Lead sulphide, PbS, nanoparticles in particular have been suggested for use in photovoltaic devices because the absorption edge of PbS nanoparticles has been shown to be size-tuneable across much of the visible/near IR region, making them well-suited as photo-absorbers for solar cells. The tuneable band gap of PbS nanoparticles relative to bulk PbS is due to shifts in the valence and conduction bands arising from quantum confinement effects. A good knowledge of the position of these bands is essential to the rational design of efficient photovoltaic cells. Hence, using the TEMPO beamline at the SOLEIL synchrotron, we have investigated the electronic structure of butylamine capped PbS in nanoparticulate form using soft X-ray photoemission spectroscopy.
The figures show valence band and core level spectra of butylamine capped PbS nanoparticles compared with previously recorded data for single-crystal PbS and 4-ethypyridine capped PbS nanoparticles. The binding energy of the leading edge of the valence band corresponds to the energy gap between the valence band and Fermi level of the compound. By combining this value with the size of the band gap we can deduce that the bulk sample and the butylamine capped nanoparticles are both n-type semiconductors, whilst the 4-ethylpyridine capped nanoparticles are p-type semiconductors. This has a significant impact on the function of the nanoparticles in photovoltaic cells. From the core level spectra it is possible to see that the bulk sample contains only PbS. The 4-ethylpyridine capped nanoparticulate sample also contained mainly PbS although it also contained a large proportion of neutral Pb and S. The butylamine capped nanoparticulate sample mostly contained PbSO4 with a small proportion of compounds such as PbS, PbS2O3 and PbSO3. The XPS spectra presented here make it clear that the chemical composition of nanoparticulate PbS is very much more complex than that of the bulk material and the nature of the capping group appears to significantly affect the energy level positions, the chemical composition of the nanoparticle and the stability of the nanoparticles under high energy photons. We conclude that the choice of capping group is a very important factor in photovoltaic cell design.
