Charge Carrier and Spin Transport Properties in Diketopyrrolopyrrole-Based Polymers

The organic active layer in organic spin-valves (OSVs) plays a vital role in regulating the device's performance, and the variations in molecular structure can significantly modulate their physicochemical properties. Herein, we synthesized two diketopyrrolopyrrole-based donor-acceptor copolymers with distinct alkyl side chains, namely DPP-BTCN-C1 (with side-chain branching points closer to the conjugated skeleton) and DPP-BTCN-C3. To evaluate the charge carrier transport properties, a polymer field-effect transistor and space charge limited current method based on the two molecules were conducted. The experimental results demonstrated that the DPP-BTCN-C1 materials had a higher electron transport mobility. Moreover, the spin transport properties were also revealed by fabricating OSVs devices. In the OSVs with a 50 nm interlayer, DPP-BTCN-C1 exhibited a higher magnetoresistance (RM) value of up to 24.6 %. By fitting the polymer thickness dependence of the MR value at 10 K, a longer spin diffusion length and higher spin polarization injection efficiency were achieved for the DPP-BTCN-C1 structure. We attributed the differences in charge and spin transport performance to the modulation of the film microstructure and energy levels by the alkyl side chains. This work studied the structure-property relationship of polymer OSVs from the perspective of side-chain engineering, providing valuable insights for the design of polymers with enhanced spin properties.