Controlling the structural properties of conjugated polymers for efficient all-polymer solar cells

Abstract

In this thesis, 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 $\pi - \pi$ intermolecular interactions, and (4) the controllable physicochemical properties through facile functionalization on the nitrogen-position 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 all-PSCs. Interestingly, the third component of $PC_{71}BM$ 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 $PC_{71}BM$ 30% while retaining their mechanical properties (crack onset strain: 11.6% for all-PSCs binary blend and 10.7% for $PC_{71}BM$ 30% ternary blend). Finally, we also investigated the new class of non-conjugated polymer additive of poly(2-vinylpyridine) (P2VP) for modifying the interfaces between active layer and electrodes. We found out that the P2VP was vertically self-assembled on ZnO layer (electron transporting layer) via a single coating process in the deposition of active materials. Consequently, we successfully improved PCEs by adding a small amount of P2VP in both the highly efficient fullerene-PSC and all-PSC systems: $PTB7:PC_{71}BM$ (PCE: 7.37→8.67%), $PTB7-Th:PC_{71}BM$ (10.53→11.14%), and PTB7-Th:P(NDI2HD-T) (5.52→6.14%).