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Poly(diethynylthiophene) (PDET) synthesized via Hay coupling polycondensation retains terminal alkynes for tethering polystyrene (PS) blocks by a simple Cu-catalyzed azide–alkyne click (CuAAC) reaction. The successful synthesis of triblock copolymers was confirmed by 1H NMR, FTIR, and GPC measurements. Optical and electrochemical properties of the PDET block were conserved in the triblock copolymers, as determined from the UV-vis absorption spectra and redox potentials. Surface topography of the polymer films revealed the micrometer-scale features attributable to phase separation, which was supported by thermal analyses. The compatibilizer functions of PDET and triblock copolymer P1 were investigated and compared in all-polymer solar cells (all-PSCs). Addition of 1 wt% P1 was shown to result in an enhanced power conversion efficiency (PCE) from 5.90% to 6.24%, corresponding to a relative increase of ∼6%, whereas the addition of 1 wt% PDET decreased the resultant PCE. Notably, adding a proper compatibilizer helped reduce the device's potential loss, as evidenced by the improved Voc in the 1 wt% P1 device. Our results highlight the critical role of the coil segment in designing block copolymer-based compatibilizers for all-PSCs. Also, this study demonstrates a straightforward synthetic route for coil–rod–coil triblock copolymers that afford a compatibilizer function suitable for all-PSCs. [ABSTRACT FROM AUTHOR]