Just to clarify: Are you asking why a more crystalline film *of the same material* has a smaller optical gap than a fairly amorphous film or do you want to know why *different materials* with a tendency to form crystalline films have smaller gaps than materials which tend to form amorphous films?

same material such as polyaniline

Crystallinity means high order of atoms/molecules in solid materials.

In organic semiconductors conjugated systems are responsible for absorption of UV or visible light. The absorption causes a transition dipol due to different electron density in HOMO and LUMO of the molecule. 

These transition dipols can interact with other transition dipols in other  molecules and causes an energetic splitting. This is also known as Davydov splitting. You can find various literature about this topic.

Please look also for

a) A.S. Davydov, „Theory of Molecular Excitons“, McGraw-Hill Book Company: New York, 1962;

(b) M. Kasha in DiBartolo (Ed.): Spectroscopy of the Excited State“ Plenum Verlag, New York, 1976, 337ff

I hope, this short information could help.

This can be related to both inter- and intra-molecular electronic coupling in the polymer.

For example, we can compare samples of the semiconducting polymer poly(3-hexylthiophene) (P3HT) that are either regioregular (RR, where the monomers all linked with head-to-tail orientation of the hexyl groups so the polymer chains can lie flat >> "more crystalline" material) or regiorandom (RRa, where the head-to-head couplings result in twisting of the polymer backbone >> "less crystalline"). Since the RR polymer has a more planar backbone, the wavefunction is more delocalised (extended conjugation length) and so the absorption corresponding to the intrachain pi-pi* transition is red-shifted compared to the RRa sample. In addition, the more planar backbone of the RR sample allows the polymer chains to pack better with each other/aggregate (hence "more crystalline") and this can lead to additional low-energy transitions (shoulders) in the absorption spectrum corresponding to interchain pi-pi interactions. 

There are many great papers out there with more comprehensive physical explanations, see for example:

Yamagata & Spano, J. Chem. Phys., 136, 184901 (2012); Brown et al. Phys. Rev. B,  67, 064203 (2003)

The energy and strength of these different transitions will of course depend on the polymer, and many of the more amorphous semiconducting polymers (D-A polymers) will not show the same effects. But in general more crystalline > more planar pi-system > more red-shifted absorption. At least this is my understanding...