A common obstacle of many organic semiconductors is that they show highly unipolar charge transport. This unipolarity is caused by trapping of either electrons or holes by extrinsic impurities, such as water or oxygen. For devices that benefit from balanced transport, such as organic light-emitting diodes, organic solar cells and organic ambipolar transistors, the energy levels of the organic semiconductors are ideally situated within an energetic window with a width of 2.5 eV where charge trapping is strongly suppressed. However, for semiconductors with a band gap larger than this window, as used in blue-emitting organic light-emitting diodes, the removal or disabling of charge traps poses a longstanding challenge. Here we demonstrate a molecular strategy where the highest occupied molecular orbital and lowest unoccupied molecular orbital are spatially separated on different parts of the molecules. By tuning their stacking by modification of the chemical structure, the lowest unoccupied molecular orbitals can be spatially protected from impurities that cause electron trapping, increasing the electron current by orders of magnitude. In this way, the trap-free window can be substantially broadened, opening a path towards large band gap organic semiconductors with balanced and trap-free transport.

facile processability. [1,2] However, the application of perovskites in field-effect transistors (FETs) has received less attention and has remained challenging because of ion migration under operational conditions at room temperature due to the low formation energy of mobile ions or ionic defects in these ionic materials. [3,4] Mobile ions in perovskite FETs screen the applied gate field and reduce the gate modulation of the current yielding low fieldeffect mobility and large hysteresis. [5] In contrast, 2D Sn-based perovskites reveal favorable properties due to the insulating property of bulky organic ligands. The advantages of dielectric confinement in 2D layered structures are expected to significantly suppress ion movement in the device. [6] More importantly, the device performance can be tuned by tailoring the chemical structure of the spacer cations. [7] 2D Sn-based perovskites are promising semiconductors for high-performance FETs. [8,9] The Sn-based perovskites typically show high charge carrier mobility due to the smaller in-plane effective mass and longer carrier lifetime compared with their Pb analogs. [10] Nevertheless, there are several drawbacks to 2D Sn-based perovskite FETs. First, easy oxidation of Sn 2+ to its tetravalent state Sn 4+ , especially during solution processing, gives rise to ionic defects and leads to p-type self-doping. [11] Second, the fast Understanding and controlling the nucleation and crystallization in solutionprocessed perovskite thin films are critical to achieving high in-plane charge carrier transport in field-effect transistors (FETs). This work demonstrates a simple and effective additive engineering strategy using pentanoic acid (PA). Here, PA is introduced to both modulate the crystallization process and improve the charge carrier transport in 2D 2-thiopheneethylammonium tin iodide ((TEA) 2 SnI 4 ) perovskite FETs. It is revealed that the carboxylic group of PA is strongly coordinated to the spacer cation TEAI and [SnI 6 ] 4− framework in the perovskite precursor solution, inducing heterogeneous nucleation and lowering undesired oxidation of Sn 2+ during the film formation. These factors contribute to a reduced defect density and improved film morphology, including lower surface roughness and larger grain size, resulting in overall enhanced transistor performance. The reduced defect density and decreased ion migration lead to a higher p-channel charge carrier mobility of 0.7 cm 2 V −1 s −1 , which is more than a threefold increase compared with the control device. Temperaturedependent charge transport studies demonstrate a mobility of 2.3 cm 2 V −1 s −1 at 100 K due to the diminished ion mobility at low temperatures. This result illustrates that the additive strategy bears great potential to realize high-performance Sn-based perovskite FETs.

Studies of ion-specific effects on oligopeptides have aided our understanding of Hofmeister effects on proteins, yet the use of different model peptides and different experimental sensitivities have led to conflicting conclusions. To resolve these controversies, we study a small model peptide, L-Alanyl-L-alanine (2Ala), carrying all fundamental chemical protein motifs: C-terminus, amide bond, and N-terminus. We elucidate the effect of GdmCl, LiCl, KCl, KI, and KSCN by combining dielectric relaxation, nuclear magnetic resonance (1H-NMR), and (two-dimensional) infrared spectroscopy. Our dielectric results show that all ions reduce the rotational mobility of 2Ala, yet the magnitude of the reduction is larger for denaturing cations than for anions. The NMR chemical shifts of the amide group are particularly sensitive to denaturing anions, indicative of anion-amide interactions. Infrared experiments reveal that LiCl alters the spectral homogeneity and dynamics of the carboxylate, but not the amide group. Interaction of LiCl with the negatively charged pole of 2Ala, the COO− group, can explain the marked cationic effect on dipolar rotation, while interaction of anions between the poles, at the amide, only weakly perturbs dipolar dynamics. As such, our results provide a unifying view on ions’ preferential interaction sites at 2Ala and help rationalize Hofmeister effects on proteins.

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