Effects of amorphous poly(3‐hexylthiophene) on active‐layer structure and solar cells performance

Abstract

A key challenge to the development of polymerbased organic solar cells is the issue of long-term stability, which is mainly caused by the unstable time-dependent morphology of active layers. In this study, poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C60-butyric acid methyl ester (PCBM) blend is used as a model system to demonstrate that the long-term stability of power conversion efficiency can be significantly improved by the addition of a small amount of amorphous regiorandom P3HT into semicrystalline regioregular one. The optical properties measured by UV-vis absorption and photoluminescence reveal that regiorandom P3HT can intimately mix with PCBM and prevent the segregation of PCBM. In addition, X-ray scattering techniques were adopted to evidence the retardation of phase separation between P3HT and PCBM when regiorandom P3HT is added, which is further confirmed by optical microscopy that shows a reduction of large PCBM crystals after annealing at high temperature in the presence of regiorandom P3HT. The improvement of the long-term stability is attributed to the capability of amorphous P3HT to be thermodynamically miscible with PCBM, which allows the active layer to form a more stable structure that evolves slower and hence decelerates the device decay. V C 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 975-985

Spectroelectrochemistry of poly(3-hexylthiophenes) in solution

“…Conversely, sweeping the potential in the RR-P3HT solution, surface coverage is marginal and the formed layer has the appearance of veined blotches. The difference in film morphology is evidence that the ST-P3HT layer is significantly more amorphous than RR-P3HT, as expected on grounds of the existing literature reports (Liu et al 2016; Tremel and Ludwigs 2014). Conversely, the poor surface coverage observed for RR-P3HT indicates that most of the electro-generated oxidised polymer remains in solution as no solid residue was left in the experimental cell.…”

Photoluminescence of P3HT:PCBM bulk heterojunction thin films and effect of external electric field

“…This effect indicates that no complexes existed between P3HT and PCBM, but the blue shift of the 0–1 line of P3HT was associated with decreased crystalline order in the polymeric chains of P3HT because of the presence of PCBM molecules. [ 16 ]…”

“…The shift of G1 and G2 bands, analogous to absorption, is associated with a decreased conjugation length of P3HT chains in the presence of PCBM. [ 16 ] The PL intensity showed a significant decrease even with a small proportion of PCBM, that is, P3HT:PCBM = 10:1 (PCBM 9.1% mixture), and thereafter a small but monotonic decrease occurred with further increased PCBM content in the mixed films. The PL spectra shown in Figure 1 were normalized to the absorption intensity of the maximum at 523 nm to make corrections as the increase of the PCBM proportion decreased the P3HT absorption for a constant thickness of the sample film.…”

“…[ 7 ] P3HT:PCBM bulk heterojunction samples are generally a poor choice for solar cells, but they are highly preferable for fundamental studies because much photophysical investigation has been performed using both steady‐state and transient optical techniques such as ultraviolet (UV)–vis absorption, photoluminescence (PL), ultrarapid transient absorption (vis‐IR), fluorescence up‐conversion and time‐domain Raman spectra to understand their mode of operation. [ 8–24 ]…”

“…In a mixture of P3HT with PCBM, the absorption spectra of P3HT typically show a blue shift, broadening, and a poor absorption intensity when blend morphologies are ununiform or the conjugation length of crystalline P3HT molecules is disturbed. [ 5,14–16 ] PL quenching occurs similarly with an increased proportion of PCBM molecules. [ 12–14,17,18 ] The mechanism of this quenching was explained with a dynamic and apparently static quenching model.…”