Aggregates -that is short-ranged ordered moieties in the solid-state of p-conjugated polymers -play an important role in the photophysics and performance of various optoelectronic devices. We have previously shown that many polymers change from a disordered to a more ordered conformation when cooling a solution below a characteristic critical temperature T c . Using in situ time-resolved absorption spectroscopy on the prototypical semiconducting polymers P3HT, PFO, PCPDTBT, and PCE11 (PffBT4T-2OD), we show that spincoating at a temperature below T c can enhance the formation of aggregates with strong intra-chain coupling. An analysis of their time-resolved spectra indicates that the formation of nuclei in the initial stages of film formation for substrates held below T c seems responsible for this. We observe that the growth rate of the aggregates is thermally activated with an energy of 310 meV, which is much more than that of the solvent viscosity (100 meV). From this we conclude that the rate controlling step is the planarization of a chain that is associated with its attachment to a nucleation center. The success of our approach for the rather dynamic deposition method of spin-coating holds promise for other solution-based deposition methods.So far, however, only limited approaches have been reported to induce aggregate formation in a controlled fashion, including slow solidification in marginal solvents, 26-29 control of entanglements and sonication, 30-34 and blending 35 -many using poly(3-hexyl thiophene) (P3HT) as model system and many relying on relatively time-consuming methodologies. Approaches to control the formation of aggregates during the solution deposition should ideally be based on considering thermodynamics of the solution as well as by taking the kinetics of film formation into account. 9 In particular with respect to the latter, several methods have been reported, such as varying the boiling point of the solvent, 9,21,36 varying Additional Supporting Information may be found in the online version of this article.

Additives are frequently used to enhance material properties. The addition of the processing additive 5‐aminovaleric acid iodide (5‐AVAI) into printed mesoscopic perovskite solar cells is shown to have a strong impact on the device performance and stability. Although it is difficult to understand the impact of 5‐AVAI as a processing additive by examining only the final thin films, the evolution of morphology with and without 5‐AVAI reveals that 5‐AVAI influences the crystallization behavior of the perovskite. In situ grazing incidence wide angle X‐ray scattering (GIWAXS) is performed to follow the perovskite formation within the printable all‐porous TiO2/ZrO2/carbon architecture and investigate the influence of 5‐AVAI on the perovskite crystallization within the scaffold. Using such time‐resolved measurements, the suppression of large crystalline perovskite grains is identified early in the fabrication process when 5‐AVAI is present, resulting in improved material backfilling. These observations highlight the importance of 5‐AVAI in the precursor solution for reliable fabrication of printed perovskite solar cells relying on the infiltration of a scaffold structure.

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