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Free charge generation in organic solar cells generally proceeds via (1) the formation of charge transfer (CT) excitons after photoexcitation of donor or acceptor molecules, and (2) CT dissociation into the charge separated (CS) state. While the efficiency of CT formation depends on the energetic difference between local excitation (LE) singlet and CT states, the CT dissociation efficiency is determined by the energetic barrier between the CT and CS states. For a long time, research either studied the combined effect of CT formation and dissociation on photocurrents without distinguishing the individual processes, or primarily focused on understanding and improving CT formation to increase photocurrents, neglecting the efficiency of CT dissociation all-together. In this work, we provide evidence that CT dissociation rather than CT formation presents a major bottleneck for free charge generation in fullerene-based blends with low energetic offsets between LE and CT states. We fabricate devices based on dilute donor content blends of ZnPc or its fluorinated derivatives and C60. Fluorination of ZnPc simultaneously shifts the molecular orbitals away from the vacuum level, increasing the CT state energy at the donor-acceptor interface with C60. Through experimental device characterization, density functional theory (DFT) calculations, and time-resolved electron paramagnetic resonance (trEPR) measurements, we draw a comprehensive picture of how LE, CT, and CS state energies and the transitions among states change upon fluorination of ZnPc. We find that, upon fluorination of ZnPc, primarily CT dissociation decreases, resulting in a significant decrease in the photocurrents of the blends.

Electronic doping of organic semiconductors is essential for their usage in highly efficient optoelectronic devices. Although molecular and metal complex-based dopants have already enabled significant progress of devices based on organic semiconductors, there remains a need for clean, efficient and low-cost dopants if a widespread transition towards larger-area organic electronic devices is to occur. Here we report dimethyl sulfoxide adducts as p-dopants that fulfil these conditions for a range of organic semiconductors. These adduct-based dopants are compatible with both solution and vapour-phase processing. We explore the doping mechanism and use the knowledge we gain to 'decouple' the dopants from the choice of counterion. We demonstrate that asymmetric p-doping is possible using solution processing routes, and demonstrate its use in metal halide perovskite solar cells, organic thin-film transistors and organic light-emitting diodes, which showcases the versatility of this doping approach.

We report on the characterization of the growth of vacuum-deposited zinc phthalocyanine (ZnPc) thin films on glass through a combination of in situ grazing incidence x-ray scattering, x-ray reflectivity, and atomic force microscopy. We found that the growth at room temperature proceeds via the formation of two structurally unique substrate-induced interfacial layers, followed by the growth of the γ -ZnPc polymorph thereafter (thickness ≈ 1.0 nm). As the growth of the bulk γ -ZnPc progresses, a substantial out-of-plane lattice strain ( ≈ 15 % relative to γ -ZnPc powder) is continually relaxed during the thin film growth. The rate of strain relaxation was slowed after a thickness of ≈ 13 nm, corresponding to the transition from layer growth to island growth. The findings reveal the real-time microstructural evolution of ZnPc and highlight the importance of substrate-induced strain on thin film growth.

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.

For semiconductors, doping is an efficient approach to tune their energy bandgaps, charge transport, and optical properties which could enable the improvement of the corresponding performances and open up the possibility of multifunction integration. Recently, significant advances have been achieved in molecular doped organic semiconductors, especially doped organic semiconductor single crystals (OSSCs) which have features of well-defined packing structures, long-range molecular orders, and low-density defects for fundamental studies and improved properties. In this review, we will give a summary of the exciting progress of molecular doped OSSCs from the aspects of selection criteria of molecular dopants, general growth methods, and resulting optoelectronic properties as well as their applications in optoelectronic devices. Finally, a brief conclusion is given with challenges and perspectives of molecular doped OSSCs and their related promising research directions in this field.

Vacuum-thermal evaporation (VTE) is a highly relevant fabrication route for organic solar cells (OSCs), especially on an industrial scale as proven by the commercialisation of organic light emitting diode (OLED) based displays. While OSC performance is reported for a range of VTE-deposited molecules, a comprehensive assessment of donor:acceptor blend properties with respect to their photovoltaic performance is scarce. Here, we fabricate organic thin films and solar cells of three select systems and measure ellipsometry, external quantum efficiency with high dynamic range, as well as OTRACE to quantify absorption, voltage losses, and charge carrier mobility. These parameters are key to explain OSC performance and will help to rationalize the performance of other material systems reported in literature as our methodology is applicable beyond VTE systems. Furthermore, it could help to judge the prospects of new molecules in general. We find large differences in the measured values and find that today's VTE OSCs can reach high extinction coefficients, but only moderate mobility and voltage loss compared to their solution-processed counterparts. We outline what needs to improve for VTE organic solar cells to again catch up with their solution-processed counterparts in terms of power conversion efficiency. This article is protected by copyright. All rights reserved.

cThe use of liquid crystalline (LC) compounds in organic photovoltaics has revealed to be an effective strategy to optimise the bulk heterojunction morphology, repairing structural defects through their self-assembling properties. Nonetheless, the design of LC materials has mostly been limited to donor molecules in previous reports. Here we introduce a Non-Fullerene Acceptor (NFA), 4TICO, characterised by an improved structural flexibility, which is imparted by the alkoxy sidechain and favours the formation of LC phases at high temperature. This structural polymorphism also occurs in films where the 4TICO is blended with the PBTZT-stat-BDTT-8 polymer. The high-temperature LC polymorph brings to the formation of a smooth surface morphology with less structural defects, providing solar cells with improved short-circuit current (Jsc) and fill factor (FF), by 14% and 20% respectively. An in-depth investigation of the NFA structural properties in relation to the solar cells performance and charge transport is carried out in comparison to the 4TIC crystalline isomer., P. M. and F. S. acknowledge the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska Curie Grant agreement no. 722651 (SEPOMO) for the support in the realization of this work. This work has been supported by the Spanish Government under the project PID2019-110907GB-I00, the “Severo Ochoa” Program for Centres of Excellence in R&D (CEX2019-000917-S) and by the European Union’s Horizon 2020 Research and Innovation Program under grant agreement no. 730872. E.B. and F.S. also acknowledge the Generalitat de Catalunya grant 2017 SGR668. P.M has carried out his work within the framework of the doctoral program (PhD) in Optics and Photonics of Condensed Matter Physics (Department of Physics) at the Chemnitz University of Technology and F. S. in Material Science (Department of Physics) at the Universitat Autònoma de Barcelona (UAB). GIWAXS experiments were performed at NCD-SWEET beamline at ALBA synchrotron (Beamtime ID: 2019093873) with the collaboration of ALBA staff. The authors thank Dr Pascal Kaienburg of the University of Oxford and Dr Mariano Campoy-Quiles of ICMAB for their valuable suggestions and fruitful discussions.

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%.

Organic solar cells (OSCs), also known as organic photovoltaics (OPVs), are an emerging solar cell technology composed of carbon-based, organic molecules, which convert energy from the sun into electricity. Key for their performance is the microstructure of the light-absorbing organic bulk heterojunction. To study this, organic solar films composed of both fullerene C60 as electron acceptor and different mole percentages of di-[4-(N,N-di-p-tolyl-amino)-phenyl]-cyclohexane (TAPC) as electron donor were evaporated in vacuum in different mixing ratios (5, 50 and 95 mol%) on an ITO-coated glass substrate held at room temperature and at 110 °C. The microstructure of the C60: TAPC heterojunction was studied by grazing incidence wide angle X-ray scattering to understand the effect of substrate heating. By increasing the substrate temperature from ambient to 110 °C, it was found that no significant change was observed in the crystal size for the C60: TAPC concentrations investigated in this study. In addition to the variation done in the substrate temperature, the variation of the mole percent of the donor (TAPC) was studied to conclude the effect of both the substrate temperature and the donor concentration on the microstructure of the OSC films. Bragg peaks were attributed to C60 in the pure C60 sample and in the blend with low donor mole percentage (5%), but the C60 peaks became nondiscernible when the donor mole percentage was increased to 50% and above, showing that TAPC interrupted the formation of C60 crystals.

The β-phase, in which the intermonomer torsion angle of a fraction of chain segments approaches ∼180°, is an intriguing conformational microstructure of the widely studied light-emitting polymer poly(9,9-dioctylfluorene) (PFO). Its generation can in turn be used to significantly improve the performance of PFO emission-layer-based light-emitting diodes (LEDs). Here, we report the generation of β-phase chain segments in a copolymer, 90F8:10BT, containing 90% 9,9-dioctylfluorene (F8) and 10% 2,1,3-benzothiadiazole (BT) units and show that significant improvements in performance also ensue for LEDs with β-phase 90F8:10BT emission layers, generalizing the earlier PFO results. The β-phase was induced by both solvent vapor annealing and dipping copolymer thin films into a solvent/nonsolvent mixture. Subsequent absorption spectra show the characteristic fluorene β-phase peak at ∼435 nm, but luminescence spectra (∼530 nm peak) and quantum yields barely change, with the emission arising following efficient energy transfer to the lowest-lying excited states localized in the vicinity of the BT units. For ∼5% β-phase chain segment fraction relative to 0% β-phase, the LED luminance at 10 V increased by ∼25% to 5940 cd m-2, the maximum external quantum efficiency by ∼61 to 1.91%, and the operational stability from 64% luminance retention after 20 h of operation to 90%. Detailed studies addressing the underlying device physics identify a reduced hole injection barrier, higher hole mobility, correspondingly more balanced electron and hole charge transport, and decreased carrier trapping as the dominant factors. These results confirm the effectiveness of chain conformation control for fluorene-based homo- and copolymer device optimization.

The electronic wavefunctions of an atom or molecule are affected by its interactions with its environment. These interactions dictate electronic and optical processes at interfaces, and is especially relevant in the case of thin film optoelectronic devices such as organic solar cells. In these devices, charge transport and interfaces between multiple layers occur along the thickness or vertical direction, and thus such electronic interactions are crucial in determining the device properties. Here, we introduce a new in-situ spectroscopic ellipsometry data analysis method called DART with the ability to directly probe electronic coupling due to intermolecular interactions along the thickness direction using vacuum-deposited organic semiconductor thin films as a model system. The analysis, which does not require any model fitting, reveals direct observations of electronic coupling between frontier orbitals under optical excitations leading to delocalization of the corresponding electronic wavefunctions with thickness or, equivalently, number of molecules away from the interface in C60 and MeO-TPD deposited on an insulating substrate (SiO2). Applying the same methodology for C60 deposited on phthalocyanine thin films, the analyses shows strong, anomalous features - in comparison to C60 deposited on SiO2 - of the electronic wavefunctions corresponding to specific excitation energies in C60 and phthalocyanines. Translation of such interactions in terms of dielectric constants reveals plasmonic type resonance absorptions resulting from oscillations of the excited state wavefunctions between the two materials across the interface. Finally, reproducibility, angstrom-level sensitivity and simplicity of the method are highlighted showcasing its applicability for studying electronic coupling between any vapor-deposited material systems where real-time measurements during deposition are possible., Comment: 12 pages, 6 figures, Supplementary information

Elucidating the interplay between film morphology, photophysics, and device performance of bulk heterojunction (BHJ) organic photovoltaics remains challenging. Here, we use the well-defined morphology of vapor-deposited di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane (TAPC):C60 blends to address charge generation and recombination by transient ultrafast spectroscopy. We gain relevant new insights to the functioning of dilute-donor (5% TAPC) fullerene-based BHJs compared to molecularly intermixed systems (50% TAPC). First, we show that intermolecular charge-transfer (CT) excitons in the C60 clusters of dilute BHJs rapidly localize to Frenkel excitons prior to dissociating at the donor:acceptor interface. Thus, both Frenkel and CT excitons generate photocurrent over the entire fullerene absorption range. Second, we selectively monitor interfacial and bulk C60 clusters via their electro-absorption, demonstrating an energetic gradient that assists free charge generation. Third, we identify a fast (

Organic solar cells have the potential to become the cheapest form of electricity, beating even silicon photovoltaics. This article summarizes the state of the art in the field, highlighting research challenges, mainly the need for an efficiency increase as well as an improvement in long‐term stability. It discusses possible current and future applications, such as building integrated photovoltaics or portable electronics. Finally, the environmental footprint of this renewable energy technology is evaluated, highlighting the potential to be the energy generation technology with the lowest carbon footprint of all.

The development of Non-Fullerene Acceptors (NFAs) for Organic Photovoltaics (OPV) has pushed the device power conversion efficiency over 15%. However, the commercialisation of OPV modules by the roll-to-roll industries requires the processing of thick active-layer films (150-300 nm) and therefore materials with high charge carrier mobilities. The anisotropy and bidimensionality of the NFAs conjugated structure are critical to their solid-state organisation, which in turn affects their electronic functions (e.g. carrier mobility). With our work, we show first insights into the crystallisation of a series of ten commonly used NFAs and its effect on the charge transport, identifying how the structural design of the NFAs facilitates their organisational motifs in the solid-state and exploring the importance of the crystal packing and topological connectivity on the charge transport.

Elucidating the interplay between film morphology, photophysics, and device performance of bulk heterojunction (BHJ) organic photovoltaics remains challenging. Here, we use the well-defined morphology of vapor-deposited di-[4-(

The use of non-fullerene acceptors (NFAs) in organic solar cells has led to power conversion efficiencies as high as 18%1. However, organic solar cells are still less efficient than inorganic solar cells, which typically have power conversion efficiencies of more than 20%2. A key reason for this difference is that organic solar cells have low open-circuit voltages relative to their optical bandgaps3, owing to non-radiative recombination4. For organic solar cells to compete with inorganic solar cells in terms of efficiency, non-radiative loss pathways must be identified and suppressed. Here we show that in most organic solar cells that use NFAs, the majority of charge recombination under open-circuit conditions proceeds via the formation of non-emissive NFA triplet excitons; in the benchmark PM6:Y6 blend5, this fraction reaches 90%, reducing the open-circuit voltage by 60 mV. We prevent recombination via this non-radiative channel by engineering substantial hybridization between the NFA triplet excitons and the spin-triplet charge-transfer excitons. Modelling suggests that the rate of back charge transfer from spin-triplet charge-transfer excitons to molecular triplet excitons may be reduced by an order of magnitude, enabling re-dissociation of the spin-triplet charge-transfer exciton. We demonstrate NFA systems in which the formation of triplet excitons is suppressed. This work thus provides a design pathway for organic solar cells with power conversion efficiencies of 20% or more. A substantial pathway for energy loss in organic solar cells may be suppressed by engineering hybridization between non-fullerene acceptor triplet excitons and spin-triplet charge transfer excitons.

Confining electric fields to a nanoscale region is challenging yet crucial for applications such as high-resolution probing of electrical properties of materials and electric-field manipulation of nanoparticles. State-of-the-art techniques involving atomic force microscopy typically have a lateral resolution limit of tens of nanometers due to limitations in the probe geometry and stray electric fields that extend over space. Engineering the probes is the most direct approach to improving this resolution limit. However, current methods to fabricate high-resolution probes, which can effectively confine the electric fields laterally, involve expensive and sophisticated probe manipulation, which has limited the use of this approach. Here, we demonstrate that nanoscale phase switching of configurable thin films on probes can result in high-resolution electrical probes. These configurable coatings can be both germanium-antimony-tellurium (GST) as well as amorphous-carbon, materials known to undergo electric field-induced nonvolatile, yet reversible switching. By forming a localized conductive filament through phase transition, we demonstrate a spatial resolution of electrical field beyond the geometrical limitations of commercial platinum probes (i.e., an improvement of ∼48%). We then utilize these confined electric fields to manipulate nanoparticles with single nanoparticle precision via dielectrophoresis. Our results advance the field of nanomanufacturing and metrology with direct applications for pick and place assembly at the nanoscale.

Phase-change materials, such as the well-known ternary alloy Ge2Sb2Te5, are essential to many types of photonic devices, from re-writeable optical disk memories to more recent developments such as phase-change displays, reconfigurable optical metasurfaces, and integrated phase-change photonic devices and systems. The successful design and development of such applications and devices requires accurate knowledge of the complex refractive index of the phase-change material being used. To this end, it is common practice to rely on published experimental refractive index data. However, published values can vary quite significantly for notionally the same composition, no doubt due to variations in fabrication/deposition processes. Rather than rely on published data, a more reliable approach to index determination is to measure the properties of as-fabricated films, and this is usually carried out using specialized and dedicated ellipsometric equipment. In this paper, we propose a simple and effective alternative to ellipsometry, based on spectroscopic reflectance measurements of Fabry–Perot phase-change nanocavities. We describe this alternative approach in detail, apply it to measurement of the complex index of the archetypal phase-change materials Ge2Sb2Te5 and GeTe, and compare the results to those obtained using conventional ellipsometry, where we find good agreement.

The quenching of excitons in organic solar cells can play a significant role in limiting their power conversion efficiency (PCE). In this article, we investigate the effect of a thin layer of hexapropyltruxene inserted at the interface between the electron donor boron subphthalocyanine chloride (SubPc) and its underlying hole contact in planar heterojunction solar cells. We find that a 3.8 nm hexapropyltruxene interlayer between the molybdenum oxide (MoOx) hole contact and SubPc is sufficient to improve PCE in SubPc/C60 fullerene solar cells from 2.6% to 3.0%, a ∼20% performance improvement. While the absorption stays roughly the same, the comparison of external and internal quantum efficiencies reveals a significant increase in SubPc's contribution to the current for light with wavelengths between 520 and 600 nm. Microstructure and surface morphology assessed with in situ Grazing-Incidence Wide-Angle X-Ray Scattering (GIWAXS) and Atomic Force Microscopy (AFM), are evaluated alongside in situ spectroscopic ellipsometry, and photoluminescence measurements. The microstructural investigations demonstrate changes to the surface and bulk of SubPc grown atop a hexapropyltruxene interlayer indicating that the latter acts as a template layer in a similar way as MoOx. However, the improvement in PCE is found to be mainly via reduced exciton quenching at the MoOx contact with the insertion of the hexapropyltruxene layer.

A correct determination of voltage losses is crucial for the development of organic solar cells (OSCs) with improved performance. This requires an in-depth understanding of the properties of interfacial charge transfer (CT) states, which not only set the upper limit for the open-circuit voltage of a system, but also govern radiative and non-radiative recombination processes. Over the last decade, different approaches have emerged to classify voltage losses in OSCs that rely on a generic detailed balance approach or additionally include CT state parameters that are specific to OSCs. In the latter case, a correct determination of CT state properties is paramount. In this work, we summarize the different frameworks used today to calculate voltage losses and provide an in-depth discussion of the currently most important models used to characterize CT state properties from absorption and emission data of organic thin films and solar cells. We also address practical concerns during the data recording, analysis, and fitting process. Departing from the classical two-state Marcus theory approach, we discuss the importance of quantized molecular vibrations and energetic hybridization effects in organic donor-acceptor systems with the goal to providing the reader with a detailed understanding of when each model is most appropriate.

We report on a method for fabricating balanced hole and electron transport in ambipolar organic field-effect transistors (OFETs) based on the co-evaporation of zinc-phthalocyanine (ZnPc) and its fluorinated derivative (F 8 ZnPc). The semiconducting behaviour of the OFET can be tuned continuously from unipolar p-type, with a hole mobility in the range of (1.7 ± 0.1) × 10 -4 cm 2 /Vs, to unipolar n-type, with an electron mobility of (1.0 ± 0.1) × 10 -4 cm 2 /Vs. Devices of the pristine ZnPc and F 8 ZnPc show a current on/off ratio of 10 5 . By co-evaporating the p-type ZnPc with the n-type F 8 ZnPc, we fabricate ambipolar transistors and complementary-like voltage inverters. For the ambipolar devices, the optimum balance between the hole and electron mobilities is found for the blend of 1:1.5 weight ratio with hole and electron mobilities of (8.3 ± 0.2) × 10 -7 cm 2 /Vs and (5.5 ± 0.1) × 10 -7 cm 2 /Vs, respectively. Finally we demonstrate application of the ambipolar devices in a complementary-like voltage inverter circuit with the performance comparable to an inverter based on separate ZnPc and F 8 ZnPc OFETs.

Carbon nanotube (CNT) solubilization via non-covalent wrapping of conjugated semiconducting polymers is a common technique used to produce stable dispersions for depositing CNTs from solution. Here, we report the use of a non-conjugated insulating polymer, ethylene vinyl acetate (EVA), to disperse multi- and single-walled CNTs (MWCNT and SWCNT) in organic solvents. We demonstrate that despite the insulating nature of the EVA, we can produce semitransparent films with conductivities of up to 34 S/cm. We show, using photoluminescence spectroscopy, that the EVA strongly binds to individual CNTs, thus making them soluble, preventing aggregation, and facilitating the deposition of high-quality films. To prove the good electronic properties of this composite, we have fabricated perovskite solar cells using EVA/SWCNTs and EVA/MWCNTs as selective hole contact, obtaining power conversion efficiencies of up to 17.1%, demonstrating that the insulating polymer does not prevent the charge transfer from the active material to the CNTs.

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.

The article has been amended to correct Figure 6 and a typographical error.

Previous theoretical calculations show azetidinium has the right radial size to form a 3D perovskite with lead halides [1], and has been shown to impart, as the A-site cation of ABX3 unit, beneficial properties to ferroelectric perovskites [2]. However, there has been very limited research into its use as the cation in lead halide perovskites to date. In this communication we report the synthesis and characterization of azetidinium-based lead mixed halide perovskite colloidal nanocrystals. The mixed halide system is iodine and chlorine unlike other reported nanocrystals in the literature where the halide systems are either iodine/bromine or bromine/chlorine. UV-visible absorbance data, complemented with photoluminescence spectroscopy, reveals an indirect-bandgap of about 1.96 eV for our nanocrystals. Structural characterization using TEM shows two distinct interatomic distances (2.98 +/- 0.15 Angstroms and 3.43 +/- 0.16 Angstroms) and non-orthogonal lattice angles (approximately 112 degrees) intrinsic to the nanocrystals with a probable triclinic structure revealed by XRD. The presence of chlorine and iodine within the nanocrystals is confirmed by EDS spectroscopy. Finally, light-induced electron paramagnetic resonance (LEPR) spectroscopy with PCBM confirms the photoinduced charge transfer capabilities of the nanocrystals. The formation of such semiconducting lead mixed halide perovskite using azetidinium as the cation suggests a promising subclass of hybrid perovskites holding potential for optoelectronic applications such as in solar cells and photodetectors., Comment: 8 pages, 6 Figures, 1 Supplementary information pdf file, 1 XRD indexing information text file

Organic solar cells with an electron donor diluted in a fullerene matrix have a reduced density of donor-fullerene contacts, resulting in decreased free-carrier recombination and increased open-circuit voltages. However, the low donor concentration prevents the formation of percolation pathways for holes. Notwithstanding, high (>75%) external quantum efficiencies can be reached, suggesting an effective hole-transport mechanism. Here, we perform a systematic study of the hole mobilities of 18 donors, diluted at similar to 6 mol % in C-60, with varying frontier energy level offsets and relaxation energies. We find that hole transport between isolated donor molecules occurs by long-range tunneling through several fullerene molecules, with the hole mobilities being correlated to the relaxation energy of the donor. The transport mechanism presented in this study is of general relevance to bulk heterojunction organic solar cells where mixed phases of fullerene containing a small fraction of a donor material or vice versa are present as well. The work was funded by the German Federal Ministry for Education and Research (BMBF) through the InnoProfile Projekt "Organische p-i-n Bauelemente 2.2" (03IPT602X). This work was supported by the Deutsche Forschungsgemeinschaft (project OR 349/1-1), by the Department of the Navy, Office of Naval Research Award No. N00014-14-1-0580 (CAOP MUM), by the German Ministry of Science and Education (BMBF), project UNVEiL, and through a state sponsored scholarship for graduate students to Y.F. from the China Scholarship Council. M.N. acknowledges funding from "Svenska Tekniska Vetenskapsakademien i Finland". Grants for computing time from the Center for Information Services and High Performance Computing (ZIH) are gratefully acknowledged.

To date, the most efficient hybrid metal halide peroskite solar cells employ TiO2 as electron-transporting material (ETM), making these devices unstable under UV light exposure. Replacing TiO2 with fullerene derivatives has been shown to result in improved electronic contact and increased device lifetime, making it of interest to assess whether similar improvements can be achieved by using other organic semiconductors as ETMs. In this work, we investigate perylene-3,4:9,10-tetracarboxylic bis(benzimidazole) as a vacuum-processable ETM, and we minimize electron-collection losses at the electron-selective contact by depositing pentamethylcyclopentadienyl cyclopentadienyl rhodium dimer, (RhCp*Cp)2, on fluorinated tin oxide. With (RhCp*Cp)2 as an interlayer, ohmic contacts can be formed, there is interfacial doping of the ETM, and stabilized power conversion efficiencies of up to 14.2% are obtained.

The effciency of organic solar cells is not only determined by their absorber system, but also strongly dependent on the performance of numerous interlayers and charge transport layers. In order to establish new custom-made materials, the study of structure-properties relationships is of great importance. This publication examines a series of naphthalenetetracarboxylic diimide molecules (NTCDI) with varying side-chain length intended for the use as n-dopable electron transport materials in organic solar cells. While all compounds basically share very similar absorption spectra and energy level positions in the desired range, the introduction of alkyl chains has a large impact on thin film growth and charge transport properties: both crystallization and the increase of conductivity by molecular doping are suppressed. This has a direct influence on the series resistance of corresponding solar cells comprising an NTCDI derivative as electron transport material (ETM) as it lowers the power conversion efficiency to ≪1%. In contrast, using the side-chain free compound it is possible to achive an efficiency of 6.5%, which is higher than the efficiency of a comparable device comprising n-doped C60 as standard ETM.

2017 saw the publication of several new material systems that challenge the long-held notion that a driving force is necessary for efficient exciton dissociation in organic photovoltaics (OPVs) and that a loss of ≈0.6 eV between the energy of the charge transfer state E ct and the energy corresponding to open circuit is general. In light of these developments, the authors combine insights from device physics and spectroscopy to review the two key tradeoffs limiting OPV performances. These are the tradeoff between the charge carrier generation efficiency and the achievable open circuit voltage (V oc ) and the tradeoff between device thickness (light absorption) and fill factor. The emergence of several competitive nonfullerene acceptors (NFAs) is exciting for both of these. The authors analyze what makes these materials compare favorably to fullerenes, including the potential role of molecular vibrations, and discuss both design criteria for new molecules and the achievable power conversion efficiencies.

The well-known organic semiconductor C60 is attracting renewed attention due to its centimetre-long electron diffusion length and high performance of solar cells containing 95% fullerene. Yet, its photophysical properties remain poorly understood. Here, we elucidate the dynamics of Frenkel and intermolecular (inter- C60) charge transfer (CT) excitons in neat and diluted C60 films from high quality femtosecond transient absorption (TA) measurements, performed at low fluences and free from oxygen or pump-induced photo-dimerization. We find from preferential excitation of either species that the CT excitons give rise to a strong electro-absorption signal but are extremely short-lived. The Frenkel exciton relaxation and triplet yield depend strongly on the C60 aggregation. Finally, TA measurements on full devices with applied electric field allow us to optically monitor the dissociation of CT excitons into free charges for the first time and to demonstrate the influence of cluster size on the spectral signature of the C60 anion.

The well known organic semiconductor C

We report an in-situ study of stacking fault evolution in C 60 thin films using grazing-incidence x-ray scattering (GIXS). A Williamson-Hall analysis of the main scattering features during growth of a 15 nm film on glass indicate lattice strain as high as 6% in the first 5 nm of the film, with a decrease to 2% beyond 8 nm thickness. Deformation stacking faults along the {220} plane are found to occur with 68% probability, and closely linked to the formation of a nanocrystalline powder-like film. Our findings, which capture monolayer-resolution growth, are consistent with previous work on crystalline and powder C60 films and provide a crystallographic context for the realtime study of organic semiconductor thin films.

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.

In this work, we introduce models for deriving lower limits for the key parameters doping efficiency, charge carrier concentration, and charge carrier mobility from conductivity data of doped organic semiconductors. The models are applied to data of thin layers of Fullerene C60 n-doped by four different n-dopants. Combining these findings with thermoelectric Seebeck data, the energetic position of the transport level can be narrowed down and trends for the absolute values are derived.