Controlling the Structural Properties 

of Conjugated Polymers  

for Efficient All- Polymer Solar Cells 

Wonho Lee 

We developed high- performance all- polymer solar cells (all- PSCs) by controlling the structural properties of conjugated polymers. In particular, among the variety of the n- type polymers, the naphthalenediimide (NDI)- based copolymers have been studied, due to (1) their excellent thermal and oxidative stabilities, (2) the high electron affinities of the NDI core, (3) the remarkable electron- transporting properties by strong π–π intermolecular interactions, and (4) the controllable physicochemical properties through facile functionalization on the nitrogenposition in the diimide group. Using this class of n- type conjugated polymers, we synthesized a series of NDI- T2 copolymers P(NDIR- T2), where R represents the different alkyl side chains, i.e., 2- hexyldecyl (2- HD), 2- octyldodecyl (2- OD), and 2- decyltetradecyl (2- DT) groups. The P(NDI2HD- T2) exhibits more noticeable crystalline behaviors than P(NDI2OD- T2) and P(NDI2DT- T2), thereby facilitating superior three- dimensional charge transport. The P(NDI2HD- T2) based all- PSCs produce much higher power conversion efficiency (PCE) irrespective of the electron donors. In particular, the PTB7- Th:P(NDI2HD- T2) forms highly ordered, strong face- on interchain stackings, and has better intermixed bulk- heterojunction morphology, producing the highest PCE of 6.11% that has been obtained by P(NDIR- T2) based all- PSCs to date. In addition, we also demonstrated that ternarby blend solar cells combinig P(NDI2HD- T2) and PC71BM as electron acceptors for efficient and mechcanically stable allPSCs. Interestingly, the third component of PC71BM in all- PSCs of PTB7- Th:P(NDI2HD- T2) was found to be largely well mixed in the amorphous phase of D/A interface, which increased the efficiency of free charge carrier generation. As a result, PCE was improved from 6.32 to 7.33% by incorporating PC71BM 30% while retaining their mechanical properties (crack onset strain: 11.6% for all- PSCs binary blend and 10.7% for PC71BM 30% ternary blend).