In thin films of semiconductor polymers, the polymer chains often exhibit distinct orientation with respect to the substrate. The planar π-face of the backbone typically orients either in an edge-on ...or face-on manner. Generally, an edge-on alignment is thought to be favorable for transport in thin film transistors, whereas face-on alignment is considered to improve vertical transport as desired in solar cells. However, molecular orientation is among the very few parameters that usually cannot be controlled when tailoring new semiconducting polymers. Here we show for an important class of semiconducting polymer that both the mode of orientation as well as the degree of alignment can be well-controlled by exploiting diffusive noncovalent interactions along the backbone. Studying polydiketopyrrolopyrroles (PDPPs) as a case study, by strategically varying chemical structure, we demonstrate systematic variation in molecular orientation with degree of chain planarization resulting from different degrees of diffusive noncovalent interactions. This observation opens the possibility of controlling and optimizing the orientation of semiconducting polymer chains in thin films by rational design. We anticipate our findings to open the door to new high-performance organic semiconductors with the additional benefit of tailored orientation that fits the desired application.
In this contribution, the optical losses and gains attributed to periodic nanohole array electrodes in polymer solar cells are systematically studied. For this, thin gold nanomeshes with hexagonally ...ordered holes and periodicities (P) ranging from 202 nm to 2560 nm are prepared by colloidal lithography. In combination with two different active layer materials (P3HT:PC
BM and PTB7:PC
BM), the optical properties are correlated with the power conversion efficiency (PCE) of the solar cells. A cavity mode is identified at the absorption edge of the active layer material. The resonance wavelength of this cavity mode is hardly defined by the nanomesh periodicity but rather by the absorption of the photoactive layer. This constitutes a fundamental dilemma when using nanomeshes as ITO replacement. The highest plasmonic enhancement requires small periodicities. This is accompanied by an overall low transmittance and high parasitic absorption losses. Consequently, larger periodicities with a less efficient cavity mode, yet lower absorptive losses were found to yield the highest PCE. Nevertheless, ITO-free solar cells reaching ~77% PCE compared to ITO reference devices are fabricated. Concomitantly, the benefits and drawbacks of this transparent nanomesh electrode are identified, which is of high relevance for future ITO replacement strategies.
The question of designing high electron mobility polymers by increasing the planarization using diffusive nonbonding heteroatom interactions in diketopyrrolopyrrole polymers is addressed in this. For ...this, three different diketopyrrolo3,4‐cpyrrole (DPP) derivatives with thienyl‐, 2‐pyridinyl‐, and phenyl‐flanked cores are copolymerized with an electron‐rich thiophene unit as well as an electron‐deficient 3,4‐difluorothiophene unit as comonomer to obtain diverse polymeric DPPs which vary systematically in their structures. The crystallinity differs significantly with clear trends on varying both flanking unit and comonomer. The optical gap and energy levels depend more on the nature of the flanking aryl units rather than on fluorination. Additionally, the charge transport properties are compared using different methods to differentiate between interface or orientation effects and bulk charge carrier transport. In organic field effect transistor devices with very high electron as well as hole mobilities (up to 0.6 cm2 V−1 s−1) are obtained and fluorination leads to a more pronounced n‐type nature in all polymers, resulting in ambipolar behavior in otherwise p‐type materials. In contrast, space‐charge limited current measurements show a strong influence of the flanking units only on electron mobilities. Especially, the elegant synthetic strategy of combining pyridyl flanking units with difluorothiophene as the comonomer culminates in a record bulk electron mobility of 4.3 × 10−3 cm2 V−1 s−1 in polymers.
An excellent bulk electron mobility of 4.3 × 10−3 cm2 V−1 s−1 is obtained for diketopyrrolo3,4‐cpyrrole copolymers by systematically tuning the diffusive nonbonding heteroatom interactions and dihedral angles between the aryl flanking units, DPP core and comonomer. Differences in crystalline packing, absorption, energy levels, and charge carrier properties are comparatively studied in a series of copolymers.
We report the morphological and photovoltaic evaluation of a novel fully conjugated donor/acceptor block copolymer system based on the P3HT-b-PFTBT scaffold. The incorporation of hydrophilic ...tetraethylene glycol side-chains into the PFTBT acceptor block generates an amphiphilic species whose properties provide demonstrable benefits over traditional systems. This design strategy facilitates isolation of the block copolymer from homopolymer impurities present in the reaction mixture, and we show that this purification leads to better-defined morphologies. The chemical disparity introduced between donor and acceptor blocks causes spontaneous microphase separation into well-defined domains, which we demonstrate with a combination of spectroscopy, microscopy, and X-ray scattering. The morphological advantages of this system are significant; however, preliminary device characterization indicates a loss of electron mobility in the hydrophilic acceptor block.
Thin hole transport layers are important elements in organic semiconductor-based devices. Metal oxides are an encouraging material class for this purpose, as they may provide sufficient hole ...conduction in combination with excellent electron blocking properties. Both, long-term device stability, which may often be limited by the thermal stability of interfaces, and higher temperature processing steps, benefit strongly from the existence of thermally stable metal oxide interlayers. Provided that thermally stable electrodes can be fashioned, the stability of organic active layers—for example, in organic field effect transistors, light emitting diodes, or photovoltaic (OPV) devices can be investigated. Here, we apply this concept and report about the study of hole mobility (µh) in single-carrier-hole-only devices in dependence of thermal annealing up to the above the actual melting temperature of regio-regular poly(3-hexylthiophene-2,5-diyl) (P3HT).
We studied structure and charge transport properties of thin films of donor–acceptor block copolymers, poly(3-hexylthiophene-block-perylene bisimide acrylate), using a combination of X-ray ...scattering, AFM and vertical charge transport measurements in diode devices. Block copolymer self-assembly and crystallization of the individual components are interrelated and different structural states of the films could be prepared by varying preparation conditions and thermal history. Generally the well-defined microphase structures found previously in bulk could also be prepared in thin films, in addition alignment induced by interfacial interactions was observed. Microphase separated block copolymers sustain ambipolar charge transport, but the exact values of electron and hole mobilities depend strongly on orientation and connectivity of the microdomains as well as the molecular order within the domains.
Nanostructured metal mesh structures demonstrating excellent conductivity and high transparency are one of the promising transparent conducting electrode (TCE) alternatives for indium tin oxide ...(ITO). Often, these metal nanostructures are to be employed as hybrids along with a conducting filler layer to collect charge carriers from the network voids and to minimize current and voltage losses. The influence of filler layers on dictating the extent of such ohmic loss is complex. Here, we used a general numerical model to correlate the sheet resistance of the filler, lateral charge transport distance in network voids, metal mesh line width and ohmic losses in optoelectronic devices. To verify this correlation, we prepared gold or copper network electrodes with different line widths and different filler layers, and applied them as TCEs in perovskite solar cells. We show that the photovoltaic parameters scale with the hybrid metal network TCE properties and an Au-network or Cu-network with aluminum-doped zinc oxide (AZO) filler can replace ITO very well, validating our theoretical predictions. Thus, the proposed model could be employed to select an appropriate filler layer for a specific metal mesh electrode geometry and dimensions to overcome the possible ohmic losses in optoelectronic devices.
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