fluorinedoped tin oxide ( fto ) glass substrates ( pilkington , tec15 ) were cleaned ( in the order of detergent / water solution , and ethanol ) using an ultrasonic bath .
then , a solution of 10  mm titanium(iv ) isopropoxide ( ti[och(ch3)2]4 ) in ethanol was spincoated on the glass at 2000  rpm for 20  s in air .
the substrate was put in a water vapor environment for 30  min for a hydrolysis process to form tio2 .
the substrate was treated again , with 40  mm aqueous ticl4 solution at 70  c for 30  min to generate a good blocking layer on the substrate . a layer mesoporous tio2 ( dsl 18nrt , dyesol ) that was 5  m thick and 18  nm in size and a layer of tio2 ( wer2o , dyesol ) that was 5  m thick and 150250  nm in size were screenprinted on to the pretreated fto glass surface .
the doublelayer tio2 electrodes ( area : 55  mm ) were heated in air at 480  c for 30  min .
the sintered films were treated with 40  mm aqueous ticl4 solution again at 70  c for 30  min , and heated to 450  c for 15  min .
the films were then immersed into a 210 
 m dye 3(6(4(bis(2,4dibutoxy[1,1biphenyl]4yl)amino)phenyl)4,4dihexyl4hcyclopenta[1,2b:5,4b]dithiophen2yl)2cyanoacrylic acid ( leg4 ) solution in ethanol and maintained in the dark for 18  h. the resulting sensitized tio2 electrodes were then rinsed with ethanol and dried . hermetically sealed cells were fabricated by assembling the dyeloaded film as the working electrode and a platinized counter electrode ( tec7 ) separated with a hotmelt surlyn 1702 film ( 25  m , dupont ) .
 abstracta new tempo  co tandem redox system with tempo and co(bpy)3 
 2+/3 
 + has been investigated for the use in dyesensitized solar cells ( dsscs ) . 
 a large opencircuit voltage ( v 
 oc ) increase , from 862  mv to 965  mv , was observed in the tandem redox system , while the shortcircuit current density ( j 
 sc ) was maintained . the conversion efficiency was observed to increase from 7.1  % for cells containing the single co(bpy)3 
 2+/3 + redox couple , to 8.4  % for cells containing the tempo  
 co tandem redox system . 
 the reason for the increase in v 
 oc and overall efficiency is ascribed to the involvement of partial regeneration of the sensitizing dye molecules by tempo . 
 this assumption can be verified through the observed much faster regeneration dynamics exhibited in the presence of the tandem system . using the tandem redox system , 
 the faster recombination problem of the single tempo redox couple is resolved and the masstransport of the metalcomplexbased electrolyte is also improved . this tempo  
 co tandem system is so far the most effienct tandem redox electrolyte reported not involving iodine . 
 the current results show a promising future for tandem system as replacements for single redox systems in electrolytes for dsscs . 
Experimental Section