| Solar cells are a very promising alternative to fossil fuels and other renewable energy sources to supply some or all of the increasing demand for electricity since they are renewable, relatively maintenance free, can be installed on preexisting structures, and don't pollute. The most common solar cell technology uses crystalline silicon, but it is a relatively poor light absorber requiring a thick active region, which is expensive to fabricate. Dye sensitized solar cells (DSSCs) use low cost materials and fabrication techniques and can be alternatives to crystalline silicon solar cells. The efficiency of the DSSC is limited by the loss of photo-excited electrons at the TiO2/dye/electrolyte and indium tin oxide (ITO)/electrolyte interfaces. Atomic layer deposited (ALD) hafnium oxide (HfO2) treatment on porous titanium oxide (TiO2) has been used to improve DSSC efficiency from 4.2% to 7.1%, but the reason for the improvement was uncertain. Solution deposited TiO2 nanoparticle compact layer at the ITO/electrolyte interface has been used to improve DSSC efficiency from 5.7% to 7.6%, but this compact layer was relatively thick, and can disrupt the injection of electrons from sensitized TiO2 to ITO. The objective of this research was to determine interfacial recombination mechanisms for DSSCs with ALD HfO2 treated TiO2 by characterizing the structural, electronic, and photovoltaic properties, and to improve the DSSC efficiency by treating the ITO/electrolyte interface. DSSCs were fabricated with ALD HfO2 treated TiO2/dye/electrolyte and ITO/electrolyte interfaces. Five ALD cycles of HfO2 at the TiO2/dye/electrolyte interface provided optimal treatment thickness, and improved the JSC, V OC, and efficiency of the DSSC from 10.5 mA/cm2 to 17.5 mA/cm2, 730 mV to 745 mV, and 4.2% to 7.1%, respectively, compared to an untreated interface. The dominant recombination mechanism at TiO 2/dye/electrolyte interface of a DSSC was by a trapassisted process, since a reduced density and activity of TiO2 surface states with HfO2 treatment correlated with a decrease in recombination indicated by the improved JSC, VOC, and efficiency. The ALD method was capable of growing a thin film with uniform thickness onto porous TiO 2 film without filling the pores. An ALD HfO2 compact layer at the ITO/electrolyte interface improved JSC, VOC, and efficiency from 10.0 mA/cm2 to 14.3 mA/cm2, 600 mV to 680 mV, and 3.6% to 6.0%, respectively, compared to the conventional sol-gel TiO2 compact layer. ALD grown films are dense and pin-hole free that can prevent contact between electrolyte and ITO with a thin layer. Future work should address the additional improvement of DSSC efficiency by the incorporation of both ALD HfO2 interface treatments in a single device, and the further optimization of the treatment thickness. |
