<p>The role of polymer side chains in dopant diffusion during solution sequential doping is systematically investigated using poly(3–2-methyl-2-hexylcarboxylate)thiophene (P3ET), a polythiophene derivative with thermocleavable side chains. Two types of polymer films, P3ET with intact side chains and side-chain-free P3ET-COOH obtained by thermovleavage, are subjected to sequential treatment with an acetonitrile solution of the benchmark molecular dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and compared. Spectroscopic and microstructural analyses reveal that although comparable total amounts of F4TCNQ are incorporated into both films, their spatial distribution and aggregation behavior differ markedly. In P3ET-COOH, F4TCNQ is strongly concentrated and crystallized on the film surface, whereas in P3ET it is less concentrated and only weakly crystallized. The difference is attributed to restricted F4TCNQ diffusion in the P3ET-COOH film, arising from the absence of side chains and minimized swelling. These findings highlight the critical role of polymer side chains in enabling efficient and uniform doping of conjugated polymers via solution sequential doping.</p> Graphical abstract <p></p>