Correlation of Nanomorphology with Structural and Spectroscopic Studies in Organic Solar Cells

Abstract

The nanomorphology of bulk heterojunction (BHJ) blends based on poly­[[4,8-bis­[(2-ethylhexyl)­oxy]­benzo­[1,2-b:4,5-b′]­dithiophene-2,6-diyl]­[3-fluoro-2-[(2-ethylhexyl)­carbonyl]­thieno­[3,4-b]­thiophenediyl]] (PTB7) blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) is systematically varied by using the volume fraction of the solvent additive 1,8-diiodooctane (DIO) from 0 to 20 vol % in chlorobenzene to prepare organic solar cells (OSCs). Blends prepared without DIO forms large phase-separated domains of PC71BM which are suppressed on addition of 3 vol % DIO, resulting in improved nanoscale features. Addition of 20 vol % DIO results in a finer interconnected morphology along with increased roughness due to polymer aggregation, which contrasts with previous reports. With increase in addition of DIO the photoluminescence (PL) from the blend is reduced; however, a relative increase in PL from 750 nm onward is observed for blends with 20 vol % DIO. As quenching of the blend PL is related to the donor/acceptor interface, structural characterizations in real-space (microscopy) and k-space (scattering) are performed to unravel the nanomorphology and correlate it with photophysical and charge transport processes. Electron-transport length scales measured by scanning photocurrent microscopy are found to increase with the addition of up to 3 vol % DIO associated with the breakup of the large PC71BM agglomerates, while the hole-transport length is found to increase on adding DIO up to 20 vol % due to aggregation of polymer chains. Hence, this work represents a unique set of results systematically examining the effect of nanomorphology on structural and solar cells properties of BHJ blends, which can have a direct implication on better understanding of the emerging high-efficiency OSC systems.

Visualization of sub-nanometer scale multi-orientational ordering in thin films of polymer/non-fullerene acceptor blends

“…This contrasts with polymer/fullerene or polymer/polymer systems in which the current density and the FF reduce drastically, resulting in a reduction of PCE by more than 50% relative to optimized cells for devices with coarse phase-separated morphology. 33,52,53 These results show that although the NFAs are sensitive to processing conditions due to their anisotropic chemical structures, the effect on their device performance is comparatively less; therefore, these systems can be a good candidate for printable and stable OSCs.…”

“…20,[24][25][26][27][28][29][30][31][32] Large-scale phaseseparated morphologies have been observed for many polymer/polymer and polymer/fullerene systems. [33][34][35][36] Due to the crystallisation of both donor polymer and NFA, phase separations into amorphous polymer/crystalline NFA and semicrystalline polymer/crystalline NFA can take place. A correlated orientation of donor and acceptor molecules at the BHJ can improve the free charge carrier separation.…”

Broadband Photon Harvesting in Organic Photovoltaic Devices Induced by Large-Area Nanogrooved Templates

“…However, anti-reflective layers act over a limited wavelength range and suffer from delamination issues, which hinder mechanical stability in the long term. , The use of metallic nanostructures supporting localized plasmon resonances has been extensively studied to exploit the resonant light scattering amplification and/or the near-field enhancement; − however, the ohmic losses limit the efficiency of these strategies. In a complementary approach, anti-reflection and light trapping functionalities can be achieved by introducing subwavelength dielectric nanostructures on the transparent window, which supports the active OPV layer. − A substantial increase of the photon absorption in thin-film photovoltaics has been demonstrated by functionalizing transparent or semiconductor substrates with anisotropic high-aspect aspect ratio nanostructures fabricated via several means, e.g., lithographic techniques, , etching processes through nanofabricated masks, self-organized methods expoliting ion-beams − bottom-up approach using colloidal spheres, or via engineering more complex reconfigurable nanopatterns as recently demonstrated. − Alternatively, flat-optics configurations have been recently devised in the case of ultrathin few-layer transition metal dichalcogenide semiconductors to promote strong in-plane light confinement via resonant excitation of guided photonic anomalies. , In the case of thin-film devices (thickness in the range of 100–300 nm) supported on periodically textured surfaces endowed with subwavelength structures (SWSs) light coupling into the active thin film can be improved by significantly reducing reflection losses, mimicking the moth-eye effect: high-aspect--ratio SWSs fetch gradual changes in refractive index from the value of air to that of the substrate, leading anti-reflective functionality in the broadband spectral range. − In a complementary way, dielectric nanostructures with a lateral size comparable to or larger than the incident wavelength behave as Mie resonators, strongly promote photon scattering and absorption in the active layer, and thus enhance the short-circuit current ( J SC ) in the thin-film PV device.…”

“…In a complementary approach, anti-reflection and light trapping functionalities can be achieved by introducing subwavelength dielectric nanostructures on the transparent window, which supports the active OPV layer. 19 − 21 A substantial increase of the photon absorption in thin-film photovoltaics has been demonstrated by functionalizing transparent or semiconductor substrates with anisotropic high-aspect aspect ratio nanostructures fabricated via several means, e.g., lithographic techniques, 22 , 23 etching processes through nanofabricated masks, 24 self-organized methods expoliting ion-beams 25 − 27 bottom-up approach using colloidal spheres 28 , or via engineering more complex reconfigurable nanopatterns as recently demonstrated. 28 − 30 Alternatively, flat-optics configurations have been recently devised in the case of ultrathin few-layer transition metal dichalcogenide semiconductors to promote strong in-plane light confinement via resonant excitation of guided photonic anomalies.…”

Resonant Energy Transfer-Mediated Efficient Hole Transfer in the Ternary Blend Organic Solar Cells

“…To understand these photophysical pathways, we measured the photoluminescence (PL) on a thin film of neat and ternary blend . As shown in Figures d and S4, the photoluminescence (PL) spectra of TPDI were quenched after excitation at 545 nm. This suggests the possibility of a charge transfer.…”