The conjugated polymer poly-[3-hexylthiophene] (P3HT) has been the benchmark donor in bulk-heterojunction organic solar cells (OSCs) and opened a very fruitful research route for sustainable photovoltaics [1]. New derivatives of poly-thiophenes have driven a large interest in new approaches for polymeric solar cells with non-fullerene acceptors, for example P5TCN-Fx with cyano-group substitutions and varied fluorination degrees delivered high efficient OSCs [2], and more recently a 20% power conversion efficiency was achieved in polymeric OSCs in which a dual fibrillar molecular structure with a thiophene terminated non-fullerene acceptor, L8-ThCl, was used to induce the fibrillization of both polymer donor and host acceptor polymers [3]. Additionally, poly-[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)], (PCPDT-R), copolymers with low band-gap are widely used for high-performance OPV devices with excellent charge-carrier properties in which the side-chains are varied with the aim to control its morphology [4,5]. Here, we report small angle neutron scattering (SANS) measurements on solutions of P3HT in deuterated toluene carried out at LOQ SANS instrument at ISIS spallation neutron source, and of PCPDT-R with -R= -P, -F8, -FEH, -BT and -T2 carried out at D11 instrument at ILL neutron source. For P3HT, the Gaussian coil globular shape of the macromolecule in solution changes to an elongated semi-rigid cylindrical shape when the temperature is reduced below a critical temperature triggering a gelation process in which the semi flexible rods are entangled and coexist with free dissolved polymer. On the PCPDT-R samples, effects of fibrillation are also observed, with a rigid rod behaviour like P3HT behaviour but showing an onset of aggregation at two length scales depending on -R and with a clear influence of temperature, which upon a slight heating clearly reinforces the aggregation effect. Our results hence show that relatively small chemical changes can have noticeable effects on the aggregation behaviour in solution with implications with respect to the selection of processing conditions from solution especially when considering a future development of the process towards controlling the fibrillar structure of the bulk heterojunction which is crucial to achieve higher efficiencies in organic solar cells.