By molecular design - low-cost solar cell systems

Materials World magazine
,
1 Dec 2010

The search for materials to develop low-cost solar cell systems continues. Professor Guosheng Shao from the University of Bolton, UK, explores titanium dioxide at the molecular level.

There has been an intense drive in the exploration of titanium dioxide phases as multifunctional materials. This is because they are promising candidates for a wide range of applications, including low-cost photovoltaic cells, water-splitting and environmental cleaning. However, this potential is seriously limited or invalidated by the intrinsic wide energy gaps of titanium dioxide phases – 3.2eV for anatase and 3.0eV for rutile – which confines their desirable functionalities to be viable only under ultraviolet (UV) irradiation, accounting for less than five per cent of the whole solar spectrum.

Alloying, or doping, of the titanium dioxide phases for improved functionalities has been extensively explored worldwide, but achievement has been limited, largely due to the lack of in-depth and systematic theoretical understanding to guide the formulation of effective doping/alloying schemes.

The team at the University of Bolton, UK, has attempted to chart a road map for doping formulation, using a molecular design approach within the framework of the density functional theory (DFT).

Metal selection

A case study has been undertaken to examine the effects of incorporating 3d transition metal incorporation in titanium dioxide. The team has substituted one-sixteenth of the titanium atoms for both anatase and rutile with a 3d transition metal atom. This corresponds to replacing a titanium atom by a dopant in either a 2x2x2 super cell of rutile or a 2x2x1 super cell of anatase, with each super cell containing 48 atoms. The band structures of 3d metal-doped rutile are displayed in the graphs (bottom charts, top) – black lines for up-spin and red lines for a down-spin state. The 3d substitution of titanium leads to intermediate states, (localised impurity or defect state) or bands of adequate width and curvature, within the forbidden band of titanium dioxide.  

 

Density functional theory calculated band structures of 3d doped rutile titanium dioxide, with one of the titanium atoms replaced in each 2x2x2 super cell. All but zinc doping induces intermediate bands or states within the forbidden band. The calculated band gap for the virgin phase is shown by the vertical bars


The DFT calculated band gap for rutile is in the graphs (above) as a reference marker. There are two ways to promote the optical absorption of light of a longer wavelength, or lower photon energy, by 3d doping – (a) the overall narrowing of the band gap, and (b) the introduction of intermediate states or bands within the forbidden band.

The intermediate states offer stepping stones to relay electrons from the valence band up to the conduction band via the absorption of lower energy photons, so that photons of both longer and shorter wavelengths can be absorbed to generate electron-hole pairs. Curved intermediate bands with noticeable bandwidths are preferred because of the larger accumulated density of states (DOS) and some carrier mobility necessary to enhance carrier separation.

With these in mind, manganese stands out among the 3d dopants, due to the considerable widths of the intermediate bands and the lack of localised intermediate state. In the case of zinc, which contains completed 3d and 4s orbitals, no intermediate states/bands are induced due to its substitution of titanium atoms. Zinc doping is shown to induce a shallow acceptor state over the top of the valence band of titanium dioxide, while V (see graph, above, top left) is shown to offer a shallow donor band.

Band gap

It is known that DFT alone is inadequate for calculating band structures of transition metal oxides. Such methodological inadequacy can be largely corrected by considering the onsite Coulomb interaction, using the DFT+U approach. On the basis of initial DFT modelling, refined modelling was carried out with the DFT+U approach. The DFT+U calculated band structures of manganese doped anatase and rutile are compared in the graph (below, top). Comparing the DFT+U band structures of both virgin and manganese-doped titanium dioxide, it is evident that manganese doping also induces significant reduction of the overall forbidden gap, showing a reduction of gap value by 0.36 for anatase and 0.21eV for rutile. With reference to the DFT results, one notices that the Hubbard correction enhances spin splitting in the dopant-induced intermediate states/bands, when the occupancy of the majority up- and minority down-spin states can be differentiated. The up-spin intermediate bands, owing to manganese 3d and O 2p hybridisation, are shifted downwards into deeper levels, and the down-spin manganese 3d states are raised into the conduction band.

Overall, theoretical modelling indicates that manganese has the biggest potential among 3d transition metals for effective red shift in optical absorption. In addition to narrowing the band gaps of titanium dioxide phases, manganese in the titanium sub-lattices of both anatase and rutile induces effective intermediate bands to allow multiband optical absorption, which are similar to those induced by quantum wells/dots.

The fairly curved intermediate bands due to manganese doping indicate good charge carrier mobilities, which are essential for using the doped materials as catalysts or for photonic/photovoltaic devices. The capacity in inducing such promising electronic characteristics through bulk-alloying, and hence a low-cost approach, is of great technological significance, so that an economical material system could find numerous ways for both photonic (including photovoltaic) and photocatalytic applications.

Optical absorption

Using theoretical modelling, the team has synthesised nanocrystalline titanium dioxide powders with varied manganese doping levels. X-ray diffraction shows that the powders, annealed at 400ºC for an hour, were of the anatase structure (see graph, below, (a)). Powders annealed at 800ºC for an hour were turned into the rutile structure, with manganese (III) oxide precipitated out of those containing over five per cent manganese (see charts, below, (c)).

The corresponding optical absorption spectra are shown in the graphs (see charts below, (b) and (d) respectively). For both anatase and rutile, manganese doping induces significant optical absorption for both visible and infrared light. The band gap of manganese (III) oxide was reported to be 2.5-2.6eV, so its presence in the rutile powders does not have an evident effect on the long wavelength part of the absorbance spectrum. The shift in the main optical absorption edge is attributed to the narrowed band gap and the secondary absorption shoulders in the optical spectra of both doped titanium dioxide phases are consistent with the presence of intermediate band/state in the forbidden gap (see graphs, below).

The use of fundamental modelling as guidance has allowed us to bring about substantial redshift in titanium dioxide within a rather short period of time.

 

Band structures of manganese-doped (a) 2x2x1 super cell of anatase, and (b) 2x2x2 super cell of rutile phases, both of the same level of titanium substitution of one-sixteenth. The corresponding virgin gaps calculated using the same of 7eV for both virgin phases are indicated as vertical bars


(a) X-ray diffraction patterns of pure and manganese-doped titanium dioxide powder annealed at 400ºC for an hour, all made of the anatase phase. (b) Optical absorption spectra for the powders corresponding to (a). (c) X-ray diffraction patterns of pure and manganese-doped titanium dioxide powder annealed at 700ºC for an hour. (d) Optical absorption spectra for the powders corresponding to (c)

 

Further information

Guosheng Shao, Institute for Materials Research and Innovation, University of Bolton, Bolton, BL3 5AB, UK. Tel: +44 (0)1204 903592. Email: g.shao@bolton.ac.uk

The work has been supported in part by the Joule Centre – a partnership of UK Northwest Universities – and the UK Technology Strategy Board