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heterojunction may recombine before reaching the heterojunction. Also, the donor
and acceptor layers are limited to tens of nanometers which lead to weak absorption.
To ensure that excitons are generated near the heterojunction, interference effects
have to be considered fully during the design of bilayer solar cells. These mutual
tradeoff factors lead to low EQE and impose challenges in the design of bilayer OSCs.
1.4.2 Bulk Heterojunction Solar Cells
One of the most important breakthroughs in the field of OSCs is arguably the
discovery of the bulk heterojunction (BHJ) in the mid 1990s [ 46 ]. The BHJ
structure is shown in Fig. 1.6 . Although thermal co-deposition methods can be
used to fabricate a BHJ [ 47 ], the junction is commonly formed by intermixing
donor and acceptor materials in a solution, then forming the active layer by spin-
coating of the mixed solution on a substrate. The resulting film is an interpene-
trating nanoscale network of donor and acceptor materials. The phase separation
within the film is commonly 10-20 nm, which is within the exciton diffusion
length of many organic semiconductors. Consequently, nearly unity internal
quantum efficiency have been achieved for BHJ solar cell [ 48 ], which means that
nearly all photogenerated excitons are dissociated. Carriers are then transported
through percolated pathways within the active layer toward the respective contacts
for collection.
Due to the small nanoscale phase separation in BHJs, a thicker active layer can
be fabricated in these cells when compared to bilayer solar cells. However, as the
spin-coating process is inherently less controlled than the vapor deposition process
commonly used in bilayer solar cells, the performance of BHJ solar cells is sus-
ceptible to various parameter changes. The efficiency of solar cells is strongly
dependent on the morphology of the BHJ and various methods such as thermal
annealing [ 49 ], solvent annealing [ 50 ], and modifying polymer functional groups
[ 51 ] have been studied to optimize the performance of OSCs.
Fig. 1.6 Structure of a bulk
heterojunction solar cell
 
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