posted on 2018-03-23, 00:00authored byJin Fang, Dan Deng, Zaiyu Wang, Muhammad Abdullah Adil, Tong Xiao, Yuheng Wang, Guanghao Lu, Yajie Zhang, Jianqi Zhang, Wei Ma, Zhixiang Wei
An inverted device
structure is a more stable configuration than
a regular device structure for solution-processed organic solar cells
(OSCs). However, most of the solution-processed small-molecule OSCs
(SM-OSCs) reported in the literature used the regular device structure,
and a regular device normally exhibits a higher efficiency than an
inverted device. Herein, a representative small-molecule DR3TBDTT
was selected to figure out the reason for photovoltaic performance
differences between regular and inverted devices. The mechanisms for
a reduced open-circuit voltage (Voc) and
fill factor (FF) in the inverted device were studied. The reduced Voc and FF is due to the vertical phase separation
with excess [6,6]-phenyl-C71-butyric acid methyl ester
near the air/blend surface, which leads to a reduction in build-in
voltage and unbalanced charge transport in the inverted device. Another
reason for the reduced FF is the unfavorable DR3TBDTT crystallite
orientation distribution along the film thickness, which is preferential
edge-on crystallites in the top layer of the blend film and the increased
population of face-on crystallites in the bottom layer of the blend
film. This study illustrates that the morphology plays a key role
in photovoltaic performance difference between regular and inverted
devices and provides useful guidelines for further optimization of
the morphology of solution-processed SM-OSCs.