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The real-time morphological evolution of vacuum-deposited α-sexithiophene (α-6T) on a weakly interacting (glass) substrate at ambient temperature is reported. In situ grazing-incidence small-angle X-ray scattering (GISAXS) enabled the observation of nanoscale aggregates, while in situ grazing-incidence wide-angle scattering (GIWAXS) allowed the study of the molecular-scale morphology. The in situ GISAXS measurements revealed that the α-6T growth proceeds via a Stranski-Krastanov mode, whereby 2-4 complete monolayers are deposited, followed by subsequent layers formed via island growth. In situ GIWAXS also showed the evolution of the polymorph composition during the thin-film growth. Initially, the disordered β-phase and the low-temperature (LT)-phase are deposited in nearly equal proportion until a thickness of 8 nm, whereby the LT-phase begins to dominate until a final α-6T thickness of 50 nm where the scattering intensity of the LT-phase is more than double that of the β-phase. The change in the polymorph composition coincided with an increase in the LT-phase d-spacing, indicating a lattice strain relief as the thin film moves from surface to bulk-mediated growth. The GISAXS findings were confirmed through direct imaging using ex situ atomic force microscopy (AFM) at various thicknesses, revealing the existence of both initial the initial and intermediate monolayers and final island morphologies. The findings reveal the real-time morphological evolution of α-6T across both the molecular scale and the nanoscale and highlight the role of strain in polymorph growth. Due to the importance of the thin-film microstructure in device performance, it is expected that these results will aid in the development of structure-property relationships necessary to realize the full potential of organic electronics.

Colloidal quantum dot (CQD)-based photovoltaics are an emerging low-cost solar cell technology with power conversion efficiencies exceeding 10%, i.e., high enough to be interesting for commercialization. Well-controlled and understood charge carrier transport through the device stack is required to make the next step in efficiency improvements. In this paper, polymer-wrapped single-walled carbon nanotube (SWNT) films embedded in an insulating poly(methyl methacrylate) (PMMA) matrix and capped by a thermally evaporated Au electrode are investigated as a composite hole transport layer and optical spacer. Employing transient absorption spectroscopy we show that the SWNTs enhance the charge transfer rate from CQD to CQD, ZnO, or SWNT. In order to pinpoint the underlying mechanism for the improvement, we investigate the energetics of the junction by measuring the relative alignment of the band edges, using Kelvin probe and cyclic voltammetry. Measuring the external quantum efficiency and absorption we find that the improvement is not mainly from electronic improvements but from enhanced absorption of the CQD absorber. We demonstrate experimentally and theoretically, by employing a transfer-matrix model, that the transparent PMMA matrix acts as an optical spacer, which leads to an enhanced absorption in the absorber layer. With these electronic and optical enhancements, the efficiency of the PbS CQD solar cells improved from 4.0% to 6.0%.

Hybrid perovskites form an extremely attractive class of materials for large scale, low-cost photovoltaic applications. Fullerene-based charge extraction layers have emerged as a viable n-type charge collection layer, and in “inverted” p–i–n device architectures the solar cells are approaching efficiencies of 20%. However, the regular n–i–p devices employing fullerenes still lag behind in performance. Here, we show that partial solubility of fullerene derivatives in the aprotic solvents used for the perovskites makes it challenging to retain integral films in multilayer solution processing. To overcome this issue we introduce cross-linkable fullerene derivatives as charge collection layers in n–i–p planar junction perovskite solar cells. The cross-linked fullerene layers are insolubilized and deliver improved performance in solar cells enabled by a controllable film thickness.

Organic–inorganic perovskites are highly promising solar cell materials with laboratory-based power conversion efficiencies already matching those of established thin film technologies. Their exceptional photovoltaic performance is in part attributed to the presence of efficient radiative recombination pathways, thereby opening up the possibility of efficient light-emitting devices. Here, we demonstrate optically pumped amplified spontaneous emission (ASE) at 780 nm from a 50 nm-thick film of CH3NH3PbI3 perovskite that is sandwiched within a cavity composed of a thin-film (~7 µm) cholesteric liquid crystal (CLC) reflector and a metal back-reflector. The threshold fluence for ASE in the perovskite film is reduced by at least two orders of magnitude in the presence of the CLC reflector, which results in a factor of two reduction in threshold fluence compared to previous reports. We consider this to be due to improved coupling of the oblique and out-of-plane modes that are reflected into the bulk in addition to any contributions from cavity modes. Furthermore, we also demonstrate enhanced ASE on flexible reflectors and discuss how improvements in the quality factor and reflectivity of the CLC layers could lead to single-mode lasing using CLC reflectors. Our work opens up the possibility of fabricating widely wavelength-tunable "mirror-less" single-mode lasers on flexible substrates, which could find use in applications such as flexible displays and friend or foe identification.

We measure photoinduced excitations in a dicyanovinyl end-capped methylated quaterthiophene derivative in blends with the electron acceptor C60, as already employed in organic photovoltaics. By usi...

Losses of charge carriers, due to the interfacial charge recombination processes, in small molecule organic solar cells (SMOSCs) have been investigated under operating conditions. The devices consist of zinc phthalocyanine (ZnPc) as electron donor material and C60 as electron acceptor. The results obtained by using time-resolved techniques such as charge extraction (CE) and photoinduced transient photovoltage (TPV) have been compared to the measurements carried out with impedance spectroscopy (IS) and show good agreement. Significantly, much difference is observed in either the charge density distribution versus the device voltage or the charge carriers lifetime when comparing bulk heterojunction versus bilayer-type ZnPc:C60 devices. The implications of the faster charge carrier recombination with the device fill factor (FF) and the open circuit voltage (VOC) are discussed.

By performing microscopic charge transport simulations for a set of crystalline dicyanovinyl-substituted oligothiophenes, we find that the internal acceptor-donor-acceptor molecular architecture combined with thermal fluctuations of dihedral angles results in large variations of local electric fields, substantial energetic disorder, and pronounced Poole-Frenkel behavior, which is unexpected for crystalline compounds. We show that the presence of static molecular dipoles causes large energetic disorder, which is mostly reduced not by compensation of dipole moments in a unit cell but by molecular polarizabilities. In addition, the presence of a well-defined π-stacking direction with strong electronic couplings and short intermolecular distances turns out to be disadvantageous for efficient charge transport since it inhibits other transport directions and is prone to charge trapping.