A polyradical consisting of alternating triarylamine and perchlorotriphenylmethyl radical moieties was synthesized by Horner-Emmons reaction. This compound is the first polymeric neutral mixedvalence compound that shows an intervalence charge transfer (IV-CT) band in the NIR. Comparison of the absorption spectra of the polymer with those of a reference monomer shows that the IV-CT transition is confined to one repeating unit. HOMO and LUMO levels are at -5.5 and -4.5 eV vs vacuum, respectively, as estimated by cyclic voltammetry. A very low exciton binding energy is indicated by comparison with the optical band gap (1.2 eV). The electron transfer properties of the polymer were investigated in solution by fs-pump-probe transient absorption spectroscopy. After optical excitation, the polymer shows a biexponential decay in the ps time regime. The short-living, solvent-dependent component refers to the direct decay from the IV-CT state to the ground state and the long-living, solvent-independent component is tentatively attributed to an equilibrium formation of the IV-CT state and a completely charge separated state. The charge-transport properties were investigated in films in organic field-effect transistor (OFET) devices. Electron and hole mobilities are both about 3 Â 10 -5 cm 2 V -1 s -1 , demonstrating ambipolar transport behavior of the polymer.

Lead−Tin perovskite solar cells (Pb/Sn PSCs) are limited by the intrinsic instability of Sn(II), which tends to oxidize forming Sn vacancies in perovskite films. Herein, a Lewis base β-guanidinopropionic acid (GUA) and hydrazinium iodide (HAI) are introduced to effectively passivate the perovskite bulk and surface, respectively. The synergistic approach leads to Pb/Sn PSCs with a promising power conversion efficiency of 20.5% owing to the significantly reduced nonradiative recombination and voltage losses. As a result, the V OC × FF product of PSCs is significantly improved, which is among the highest values documented in the literature, being favorable for perovskite-based tandem applications. Additionally, the strategy demonstrated in this work could also improve the stability of PSCs by enhancing the chemical robustness of the perovskite layer. These results emphasize the significance of bulk and surface passivation in the development of efficient and stable PSCs based on Pb/Sn perovskites.

In order to improve the performance of chalcopyrite Cu(In 1 À x ,Ga x )(Se 1 À y S y ) 2 solar cells, the implementation of a bandgap widening at the absorber/buffer interface via an increase of the [S]/[S þ Se] ratio is investigated. In this work we examine industrially processed samples, which were fabricated via the deposition-reaction process with varied H 2 S pressure during rapid thermal processing (RTP). Precursors which were exposed to a crucial amount of sulfur during the RTP step resulted in samples with significantly improved device performance. The increase of the open circuit voltage by about 150 mV cannot sufficiently be explained by bandgap widening.Observation of a strongly reduced saturation current density and ideality factor in intensively sulfurized samples suggest subdued recombination via mid-gap defect states located in the space charge region. This hypothesis is supported by results of deep-level transient spectroscopy measurements, which show that in both samples a mid-gap minority defect is present albeit its concentration is about one magnitude larger in sulfur-poor samples. These results confirm that sulfur passivates recombination centers in the depletion layer and hence significantly increases the open circuit voltage and the overall device performance of the photovoltaic devices.

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The charge transport properties of thin films of sol-gel processed undoped and Al-doped zinc oxide nanoparticles with variable doping level between 0.8 and 10 at.% were investigated. The x-ray diffraction studies revealed a decrease of the average crystallite sizes in highly doped samples. We provide estimates of the conductivity and the resulting charge carrier densities with respect to the doping level. The increase of charge carrier density due to extrinsic doping was compared to the accumulation of charge carriers in field effect transistor structures. This allowed us to assess the scattering effects due to extrinsic doping on the electron mobility. The latter decreases from 4.6 × 10(-3) to 4.5 × 10(-4) cm(2) V(-1) s(-1) with increasing doping density. In contrast, the accumulation leads to an increasing mobility up to 1.5 × 10(-2) cm(2) V(-1) s(-1). The potential barrier heights related to grain boundaries between the crystallites were derived from temperature dependent mobility measurements. The extrinsic doping initially leads to a grain boundary barrier height lowering, followed by an increase due to doping-induced structural defects. We conclude that the conductivity of sol-gel processed nanocrystalline ZnO:Al is governed by an interplay of the enhanced charge carrier density and the doping-induced charge carrier scattering effects, achieving a maximum at 0.8 at.% in our case.

We demonstrate an approach to improve poly-3-hexylthiophene field effect transistors by modifying the gold contacts with monolayer thick pentacenequinone (PQ) or naphthalene (NL). The effective contact resistance is reduced by a factor of two and sixteen for interlayers of PQ and NL, respectively. The observation is attributed to different injection barriers at the metal-organic interface caused by the functionalization and to an additional tunneling barrier enhancing the on/off ratios. This barrier yields to activation energies of 37meV (NL) and 104meV (PQ) below 190K, which are smaller than without functionalization, 117meV.

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