Molecular Engineering Strategies for Morphology Control in Organic Semiconductors for Optoelectronics

It is understood that the optoelectronic performance of organic electronic devices is determinant on the macroscopic solid state self-assembly of organic semiconductors (OSC), which is driven by the supramolecular Ï-Ï stacking interactions between the conjugated segments in its molecular structure. Molecular engineering approaches directly impacts the intrinsic self-assembling properties of the OSCs, but the flexibility of organic chemistry on modulating the backbone architecture of the Ï-conjugated system has led a vast library of OSCs with various functionalities, which fundamentally changes the electronic properties of the Ï-conjugated system. It is highlighted that introducing conjugation break spacers (flexible linkers) between the Ï-conjugated segments in the OSC, showed great promise in controlling supramolecular self-assembly without altering the semiconducting core of the OSC. Previous examples on controlling the morphology of the molecular OSC, DPP(TBFu)2 using horizontal and vertical dimers, and a horizontal flexibly linked polymer analogues, have shown great promise in effecting the self-assembly behaviour and stabilizing the morphology of the parent (non-flexibly linked) DPP(TBFu)2 molecule, when used as an additive. In this thesis, an extension of this work is presented whereby the flexible linking approach is used to design and synthesize two molecular compatibilizers that consists of the donor component, DPP(TBFu)2 that is linked with an aliphatic spacer to an acceptor component, based on a fullerene and perylenediimde small molecule OSC. In chapter 2, a comparison between the compatiblizer (CP) and an in-situ linker approach (ISL) was explored to elucidate the impact of stabilizing a multi-component bulk heterojunction (BHJ) morphology and its device performance for organic photovoltaics (OPVs). It was concluded that the CP approach shows the most promise in stabilizing the BHJ morphology for OPVs, which was then applied onto a highly crystalline BHJ system with a perylenediimde acceptor to demonstrate its versatility as described in chapter 3. Taking leverage from this demonstration, chapter 4 discusses how the CP approach is used to tune the phase-domain size of the BHJ that is processed from a homogeneous single-phase melt, to obtain a photoactive BHJ in an OPV device. This unique demonstration, ultimately opens up vast new possibilities for solvent free âgreenâ processing of OPVs. Lastly in Chapter 5, the approached used to address BHJ morphological stabilization is slightly different from that of previous chapters, where the (kinetic) stability of a binary donor-acceptor BHJ is addressed. In this chapter, a fully-conjugated block copolymer (BCP) consisting of donorâacceptor blocks is used to demonstrate its applicability for a single-component BHJ for OPVs. However, the main challenge for this approach is in the synthetic methodology, and to overcome this, a modular synthetic strategy using Heck coupling between two functionalized donor and accepting marcromonomers showed promise in obtaining a fully conjugated BCP for OPV

Hybrid heterojunctions of solution-processed semiconducting 2d transition metal dichalcogenides

Exfoliated transition metal dichalcogenides (2D-TMDs) are attractive light-harvesting materials for large-area and inexpensive solar energy conversion given their ability to form highly tolerant heterojunctions. However, the preparation of large-area heterojunctions with these materials remains a challenge toward practical devices, and the details of photogenerated charge carrier harvesting are not well established. In this work, we use all solution-based methods to prepare large-area hybrid heterojunction films consisting of exfoliated semiconducting 2H-MoS2 flakes and a perylene-diimide (PDI) derivative. Hybrid photoelectrodes exhibited a 6-fold improvement in photocurrent compared to that of bare MoS2 or PDI films. Kelvin probe force microscopy, X-ray photoelectron spectroscopy, and transient absorption measurements of the hybrid films indicate the formation of an interfacial dipole at the MoS2/organic interface and suggest that the photogenerated holes transfer from MoS2 to the PDI. Moreover, performing the same analysis on MoSe2-based hybrid devices confirms the importance of proper valence band alignment for efficient charge transfer and photogenerated carrier collection in TMD/organic semiconductor hybrid heterojunctions

Light-Responsive Oligothiophenes Incorporating Photochromic Torsional Switches (PTS)

We present a quaterthiophene and sexithiophene that can reversibly change their effective π-conjugation length via photoexcitation. The reported compounds make use of light-responsive molecular actuators consisting of an azobenzene attached to a bithiophene unit by both direct and linker-assisted bonding. Upon exposure to 350 nm light the azobenzene undergoes trans -to- cis isomerization mechanically inducing the oligothiophene to assume a planar conformations (extended π-conjugation). Exposure to 254 nm wavelenght promotes azobenzene cis -to- trans isomerization, forcing the thiophenic backbones to twist out of planarity (confined π-conjugation). Twisted conformations are also reached by cis -to- trans thermal relaxation with rate that increases proportionally with the conjugation length of the oligothiophene moiety. The molecular conformations of quaterthiophene and sexithiophene were characterized using steady-state UV-vis, X-ray crystallography and quantum-chemical modelling. Finally, we tested the proposed light-responsive oligothiophenes into field-effect transistors to probe the photo-induced tuning of their electronic properties

Morphology stabilization strategies for small-molecule bulk heterojunction photovoltaics

The greater crystallinity of solution-processed small-molecule organic semiconductors, compared to their polymer counterparts, renders the bulk heterojunction (BHJ) more susceptible to phase separation under thermal stress, decreasing device performance. Here we demonstrate and compare strategies to stabilize the donor: acceptor BHJ in DPP(TBFu)(2):PC61BM solar cells using molecular additives designed to either afford compatiblization (CP) of the bulk heterojunction, or to in situ link (ISL) the components using a functional azide group. Both additives were found to stop phase segregation of the BHJ under thermal stress. At 5 wt% loading the ISL additive prevents phase segregation, while altering the azide reaction mechanism by using UV-induced linking versus thermal induced linking was found to significantly affect the device performance. Including 5 wt% of the CP additive slowed phase segregation and devices retained 80% of their optimum performance after 3000 min of thermal treatment at 110 degrees C (compared to 50% with the control). The CP additive at 10 wt% changed drastically the kinetics of phase segregation leading to devices with no decrease in performance over 3000 min thermal treatment. Thin film morphology characterization together with photoluminescence and impedance spectroscopy give further insight into the performance differences between the additives. These results reinforce the conclusion that the compatiblization method is the most promising strategy to engineer highly-efficient thermally-stable organic photovoltaics based on solution-processed small molecules

Engineering the self-assembly of diketopyrrolopyrrole-based molecular semiconductors via an aliphatic linker strategy

The solid-state self-assembly of molecular semiconductors is a key aspect for controlling the optoelectronic properties of organic electronic materials. Herein, we investigate the use of a flexible linker strategy to control the self-assembly of a solution-processable diketopyrrolopyrrole semiconductor coded as DPP(TBFu)(2). Two distinct dimers-prepared with varied linker position relative to the orientation of the conjugated core-reveal the effect of connectivity on the solid-state self-assembly and optoelectronic properties-favoring either Hor J-type aggregation. The dimer with a "vertical"linker orientation exhibits a poor crystallinity in neat films, but improves hole mobility in OFETs 10-fold, reaching 3.0 x 10(-3) cm(2) V-1 s(-1) when used as an additive with DPP(TBFu)(2). Distinctively, the dimer with a "horizontal" linking orientation does not enhance charge carrier transport, but is found to affect the thermal stability of donor : acceptor blends in OPVs with PCBM. Devices retain 90% of their initial conversion efficiency after 5 hours of thermal stress, compared to only 45% for control devices. Thermodynamic and kinetic rationales further suggest that this flexible linker strategy represents a powerful tool to control supramolecular assembly in molecular semiconductors without altering the nature of the core conjugated segment

Molecular Strategies for Morphology Control in Semiconducting Polymers for Optoelectronics

Solution-processable semiconducting polymers have been explored over the last decades for their potential applications in inexpensively fabricated transistors, diodes and photovoltaic cells. However, a remaining challenge in the field is to control the solid-state self-assembly of polymer chains in thin films devices, as the aspects of (semi)crystallinity, grain boundaries, and chain entanglement can drastically affect intra-and inter-molecular charge transport/transfer and thus device performance. In this short review we examine how the aspects of molecular weight and chain rigidity affect solid-state self-assembly and highlight molecular engineering strategies to tune thin film morphology. Side chain engineering, flexibly linking conjugation segments, and block co-polymer strategies are specifically discussed with respect to their effect on field effect charge carrier mobility in transistors and power conversion efficiency in solar cells. Example systems are taken from recent literature including work from our laboratories to illustrate the potential of molecular engineering semiconducting polymers

Sustainable stretchable batteries for next-generation wearables

Next-generation wearables will interface intimately with the human body either on-skin, implanted or woven into clothing. This requires electrical components that match the mechanical properties of biological tissues - stretchability (up to 60% strain) and softness (Youngs modulus of similar to 1 kPa to 1 MPa). As wearables become increasingly complex, the energy and mechanical requirements will increase, and an integrated power supply unit such as a soft and stretchable battery is needed to achieve autonomy and wireless operation. However, two key challenges remain for current stretchable battery technology: the mechanical performance (softness and stretchability) and its relation to the size and charge storage capacity (challenge I), and the sustainability and biocompatibility of the battery materials and its components (challenge II). Integrating all these factors into the battery design often leads to a trade-off between the various properties. This perspective will evaluate current strategies for achieving sustainable stretchable batteries and provide a discussion on possible avenues for future research. Stretchable battery technology still faces several challenges to progress the development of next-generation wearables. This perspective will evaluate current strategies and provide a discussion on possible avenues for future research.Funding Agencies|Swedish Governmental Agency for Innovation Systems, VINNOVA [2021-01668]; Knut and Alice Wallenberg Foundation; Linkoeping University; Wallenberg Wood Science Centre; Swedish Research Council [2020-05218]; Wallenberg Initiative Materials Science for Sustainability (WISE) - Knut and Alice Wallenberg Foundation</p