Abstract
ZnO and TiO2 are two of the most commonly used n-type metal oxide semiconductors in new generation solar cells due to their abundance, low-cost, and stability. ZnO and TiO2 can be used as active layers, photoanodes, buffer layers, transparent conducting oxides, hole-blocking layers, and intermediate layers. Doping is essential to tailor the materials properties for each application. The dopants used and their impact in solar cells are reviewed. In addition, the advantages, disadvantages, and commercial potential of the various fabrication methods of these oxides are presented.
Highlights
We focus on the recent developments on tuning the properties of these materials through doping and we showcase the most promising methods for fabricating high performance doped materials in a commercially viable way
The devices covered in this research update are: inorganic solar cells, hybrid solar cells, bulk heterojunction (BHJ) solar cells, dye-sensitized solar cells (DSSCs), and quantum dot solar cells
Having the quantum dots (QDs) conduction band exactly aligned with the TiO2 conduction band (as in the case of the Zr:TiO2 in Fig. 2(b)) was found to produce the most efficient devices because when the TiO2 conduction band was too low, the VOC was reduced by a lower quasi-Fermi level offset between the TiO2 and QDs (lower VBI, see Figs. 1(a) and 1(b)), and possibly in addition to electron thermalization down the metal oxide
Summary
Doping TiO2 with Mg has been found to raise the conduction band This was attributed to the formation of Mg-Ti mixed oxides, since MgTiO3 and MgTi2O5 have larger bandgaps than undoped TiO2.16,25 The raised conduction band resulted in an increased VBI and VOC for DSSCs (Fig. 1(d)). These were further increased by changing the commonly used I3−,I− redox couple with Br3−,Br− which has a lower redox couple potential, along with surface treatment of the doped TiO2 electrodes using MgO and ethanoic acid to reduce back-electron transfer. Having the QD conduction band exactly aligned with the TiO2 conduction band (as in the case of the Zr:TiO2 in Fig. 2(b)) was found to produce the most efficient devices because when the TiO2 conduction band was too low (as with Sb:TiO2), the VOC was reduced by a lower quasi-Fermi level offset between the TiO2 and QDs (lower VBI, see Figs. 1(a) and 1(b)), and possibly in addition to electron thermalization down the metal oxide
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