Efficiency Improvement of Solution‐Processed Dithienopyrrole‐Based A‐D‐A Oligothiophene Bulk‐Heterojunction Solar Cells by Solvent Vapor Annealing

Abstract

provide good solubility in organic solvents and processability, we attached branched alkyl substituents at the DTP-nitrogen (ethylhexyl (EH) 1 , 4 , octylnonyl (ON) 2 , 5 , hexyldecyl (HD) 3 , 6 ) and hexyl side chains to each thiophene unit in a regioregular fashion as a mimic of the well-known and frequently used regioregular poly(3-hexylthiophene). [ 26 ] Thereby, the hexyl chains were attached either at the outer ( 1 -3 ) or inner ( 4 -6 ) positions thus yielding three isomeric pairs, which only differ in the positioning of the side chains. As a result, excellent solubilities of ≥120 mg mL −1 in chloroform for molecules 5 and 6 have been obtained. First solar cell results of oligomer pair 1 and 4 have recently been published and we discovered a striking difference between the photovoltaic behavior of the two isomers. [ 16 ] Whereas the so far highest PCE of 4.8% for any DTP-based oligomer has been obtained for oligothiophene 1 and PC 61 BM in BHJSCs, isomer 4 showed a lower performance (0.8%), which we attributed to the different blend morphologies and phase separation probed by atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. [ 16 ] The detailed photovoltaic behavior, further optimization, and characterization of the photoactive layers of the whole series of soluble DTP-oligothiophenes 1 -6 is now described. Signifi cant improvement of the oligomer BHJSC performance was obtained by SVA, which was investigated by absorption spectroscopy, grazing incidence XRD (GI-XRD), and AFM. After extensive optimization, power conversion effi ciencies up to 6.1% and fi ll factors greater than 70% were achieved.

Efficiency Improvement of Organic Solar Cells Using Two-step Annealing Technique

“…On the other hand, mobility of electron of approximately 3.6 × 10 -4 cm 2 / Vs does not change. In summary, the overall performance of device was dramatically improved whereby a slight decrease in V oc of 0.77 V was observed, as previously reported [11,[22][23][24]. The cause of decrease in V oc is linked with the decrease in Quasi-Fermi levels for the transport of hole and electron [25,26].…”

Dithienopyrrole Based Small Molecule with Low Band Gap for Organic Solar Cells

“…In this work, using the above strategy for designing low band small molecule, herein, we have chosen dithienopyrrole, an efficient electron donating unit widely used in organic optoelectronic materialsP [27,[43][44][45] P as the central building unit to design and synthesize a new A-D-A molecule, named DR3TDTN (Scheme 1) with dithienopyrrole as the core, octyl-rhodanine as the terminal acceptor, and terthiophene as the π-bridge. The new molecule, with strong electron-donating dithieno- …”

Thiophene dendrimer-based low donor content solar cells

“…22,23 We therefore decided to study the effect of solvent annealing on the solution-processed low donor content devices. The 10 mol.…”

“…This trend is in contrast to that previously reported (for dithienopyrrole-oligothiophene non-polymeric materials with PC60BM) where the Jsc and FF were significantly enhanced while the Voc remained relatively unchanged. 23 Given that tetrahydrofuran is a good solvent for the organic semiconductors, we used XRR to see whether solvent annealing induced a significant change in the film structure, leading to the improved Voc. The results show that no change in thickness or out of plane material distribution had taken place.…”