Correlation of structural organisation and electronic properties of P3HT: Ni-complexes thin films for organic bulk heterojunction solar cells
Résumé
The performance of the so-called "bulk-heterojunction" (BHJ) solar cells is strongly influenced by the interpenetrating electron donor/acceptor (D/A) material network composition. Ideal blend structure is a bicontinuous-interpenetrating network of D and A domains with optimized interfaces and energy levels matched in a suitable way for efficient exciton dissociation. BHJs deposited by spin coating techniques lead to either highly crystalline or completely amorphous materials, or intermediate. So, material characterization at different scales could provide key results to understand physical mechanisms in charges photo-generation and to improve BHJ composition. Film as cast need to be optimized according to thickness, solvent deposition and other parameters. In this context, we investigated electrical and optical properties of BHJ poly(3-hexylthiophene) (P3HT) as electron donor and nickel complexes ([Ni(4dodpedt) 2 ]) as electron acceptor. [Ni(4dodpedt) 2 ] is constituted of a planar nickel dithiolene core surrounded by linear alkyl peripheral chains. 1 Owing to its structure, this complexes may be amorphous or crystalline in solid state and exhibit hexagonal columnar liquid crystal (LC) properties between 83°C and 100°C. Previous investigations have demonstrated that blending P3HT with [Ni(4dopedt) 2 ] improved polymer chain ordering without thermal annealing as soon as the 1:1 wt. ratio is reached. 2 However the nanostructure of the blend is not regular as fibrils formed of [Ni(4dopedt) 2 ] aggregates are identified by AFM technics. 3 This work aims to improve conduction pathways in BHJ of (i) P3HT and (ii) [Ni(4dodpedt) 2 ] (iii) to obtain the largest D/A interface area for enhanced exciton dissociation. Hence, we use thermal annealing to remove those fibrils by modifying [Ni(4dodpedt) 2 ] phase between solid, LC, and liquid states. These states determine P3HT polymer chains mobilities. We show that P3HT crystallinity and [Ni(4dodpedt) 2 ] aggregates sizes strongly depend on the films thicknesses. The influence of thermal and solvent annealing on thin film morphology is studied at nano/micro scales in order to understand surface and bulk contributions in structural mechanisms. In particular, we examine annealing impact on blends nanomorphology by using complementary techniques. Analysis of the symmetrical C=C stretch mode (1450 cm-1) and CC intraring stretch mode (1380 cm-1) by RAMAN spectroscopy on pristine or annealed P3HT:[Ni(4dopedt) 2 ] blends shows that thin film annealing improves P3HT π-stacking in these BHJs. Photoluminescence in conjunction with electronic absorption is also used for evidencing P3HT changes due to post deposition treatment. Then, the correlation between electrical properties and film morphology is investigated using AFM and Kelvin Probe AFM (KPFM). KPFM helped in discriminating D and A domains with highest resolution. This set of characterizations provides complete information on structural organization of blends that are helpful to understand the BHJmorphology and consequently lead to optimized OPV devices.