Introduction to Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 10

Amongst various materials, P3HT:PCBM BHJ is the most commonly used and
well-optimized active layer used in OSCs. Using these blend materials, Schilinsky
et al. demonstrated a short-circuit current of 8.7 mA/cm 2 , which was the highest
current at that time [ 52 ]. Within a year, the efficiency of P3HT:PCBM solar cells
was pushed up to 3.5 % efficiency [ 53 ] and the P3HT:PCBM came under intense
research. Thermal annealing of P3HT:PCBM BHJ was found to improve carrier
mobility and cause the crystallization of P3HT in the BHJ [ 54 ]. The effect of the
regioregularity of P3HT was studied, with improving regioregularity showing
improved performance due to better stacking of P3HT chains [ 55 ]. Studies such as
optimization P3HT:PCBM ratio [ 56 ] and using additives in solution [ 57 ] were also
conducted to optimize the performance. Eventually, over 5 % efficiency was
reported by Yang et al. using a solvent annealing approach which improved the
morphology of the P3HT:PCBM layer [ 58 ]. Due to the consistency and simple
fabrication process of this material, P3HT:PCBM OSC has become a benchmark
solar cell for investigating various device mechanisms in solar cells.
1.4.3 Tandem Solar Cells
To overcome the limitation of weak absorption strength and absorption range of
the active layer of OSCs, fabricating solar cells in tandem has been proposed
(see Fig. 1.7 ). Stacking the solar cells in series (a 2-terminal structure) will pro-
duce a large V oc and active layers with different absorption regions in the tandem
structure can allow the cell to absorb light over a wide wavelength range. For
instance, this was demonstrated in a P3HT:PCBM and ZnPc:C60 tandem solar cell
which showed improved V oc and wide absorption range [ 59 ]. A thin layer of Au or
Ag as the intermediate layer fabricated by thermal evaporation was also suc-
cessfully used in tandem cells [ 60 , 61 ]. It has also been demonstrated that tandem
cells can be entirely solution processed, by using polymer: small molecule active
layers and titanium oxide/PEDOT:PSS as the interlayer between subcells [ 62 ].
Other interlayers such as ZnO/PEDOT:PSS were also demonstrated successfully to
Fig. 1.7 Structure of a
tandem cell