Free charge photogeneration in a single component high photovoltaic efficiency organic semiconductor

Organic photovoltaics (OPVs) promise cheap and flexible solar energy. Whereas light generates free charges in silicon photovoltaics, excitons are normally formed in organic semiconductors due to their low dielectric constants, and require molecular heterojunctions to split into charges. Recent record efficiency OPVs utilise the small molecule, Y6, and its analogues, which – unlike previous organic semiconductors – have low band-gaps and high dielectric constants. We show that, in Y6 films, these factors lead to intrinsic free charge generation without a heterojunction. Intensity-dependent spectroscopy reveals that 60–90% of excitons form free charges at AM1.5 light intensity. Bimolecular recombination, and hole traps constrain single component Y6 photovoltaics to low efficiencies, but recombination is reduced by small quantities of donor. Quantum-chemical calculations reveal strong coupling between exciton and CT states, and an intermolecular polarisation pattern that drives exciton dissociation. Our results challenge how current OPVs operate, and renew the possibility of efficient single-component OPVs.

When light hits organic semiconductors, bound charge pairs, called excitons, are usually produced. Here, the authors show that in the best performing organic solar material to date, free charges, rather than excitons, are directly created by light.

Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors

Engineering a low singlet-triplet energy gap (ΔEST) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient = 3.8 × 105 cm-1) and a relatively large ΔEST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (delayed lifetime = 260 μs), but in aggregated films, BF2 generates intermolecular charge transfer (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a ~24 ns timescale and have an average electron-hole separation of ≥1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states, which is possible even at low BF2 doping concentrations of 4 wt%, resolves the conflicting requirements of fast radiative emission and low ΔEST in organic DF emitters.

Carbene metal amide photoemitters: tailoring conformationally flexible amides for full color range emissions including white-emitting OLED

Conformationally flexible "Carbene-Metal-Amide" (CMA) complexes of copper and gold have been developed based on a combination of sterically hindered cyclic (alkyl)(amino)carbene (CAAC) and 6- and 7-ring heterocyclic amide ligands. These complexes show photoemissions across the visible spectrum with PL quantum yields of up to 89% in solution and 83% in host-guest films. Single crystal X-ray diffraction and photoluminescence (PL) studies combined with DFT calculations indicate the important role of ring structure and conformational flexibility of the amide ligands. Time-resolved PL shows efficient delayed emission with sub-microsecond to microsecond excited state lifetimes at room temperature, with radiative rates exceeding 10⁶ s^-1. Yellow organic light-emitting diodes (OLEDs) based on a 7-ring gold amide were fabricated by thermal vapor deposition, while the sky-blue to warm-white mechanochromic behavior of the gold phenothiazine-5,5-dioxide complex enabled fabrication of the first CMA-based white light-emitting OLED.

A Simple Molecular Design Strategy for Delayed Fluorescence toward 1000 nm

Harnessing the near-infrared (NIR) region of the electromagnetic spectrum is exceedingly important for photovoltaics, telecommunications, and the biomedical sciences. While thermally activated delayed fluorescent (TADF) materials have attracted much interest due to their intense luminescence and narrow exchange energies (ΔE_ST), they are still greatly inferior to conventional fluorescent dyes in the NIR, which precludes their application. This is because securing a sufficiently strong donor–acceptor (D–A) interaction for NIR emission alongside the narrow ΔE_ST required for TADF is highly challenging. Here, we demonstrate that by abandoning the common polydonor model in favor of a D–A dyad structure, a sufficiently strong D–A interaction can be obtained to realize a TADF emitter capable of photoluminescence (PL) close to 1000 nm. Electroluminescence (EL) at a peak wavelength of 904 nm is also reported. This strategy is both conceptually and synthetically simple and offers a new approach to the development of future NIR TADF materials.

Efficient Vacuum-Processed Light-Emitting Diodes Based on Carbene-Metal-Amides

Efficient vacuum-processed organic light-emitting diodes are fabricated using a carbene-metal-amide material, CMA1. An electroluminescence (EL) external quantum efficiency of 23% is achieved in a host-free emissive layer comprising pure CMA1. Furthermore external quantum efficiencies of up to 26.9% are achieved in host-guest emissive layers. EL spectra are found to depend on both the emissive-layer doping concentration and the choice of host material, enabling tuning of emission color from mid-green (Commission Internationale de l'Éclairage co-ordinates [0.24, 0.46]) to sky blue ([0.22 0.35]) without changing dopant. This tuning is achieved without compromising luminescence efficiency (>80%) while maintaining a short radiative lifetime of triplets (<1 μs).

Leaving a mark: the frequency and accuracy of tattooing prior to laparoscopic colorectal surgery

AIM

Intra-operative localization of small cancers and polyps during laparoscopic colorectal surgery is difficult due to reduced tactile feedback. The consequences of failing to identify the lesion for resection can result in open conversion or removal of the wrong segment of bowel.

METHOD

Data were collected from a prospectively-kept database over a 12-month period from April 2008 to March 2009 and analysed retrospectively. Details concerning the documentation, visibility and accuracy of tattoos were recorded.

RESULTS

Eighty-five patients (88 lesions) underwent laparoscopic resection for a benign or malignant colorectal tumour during 1 year from April 2008. Eighty-one patients underwent endoscopic visualization of the tumour as a first or second procedure. Of these 81 patients, 83 lesions were visualized endoscopically and 54 (65.1%) were tattooed in 52 patients. In the 52 patients, 36 (69%) of the tattoos were carried out on the first endoscopy. At operation the tattoo was judged to be visible and accurate in 70%, visible but inaccurate in 7% and not visible in 15%. It was significantly easier to see the tattoo in women (19/21 women vs 21/29 men; P=0.03) but there was no relationship between tattoo visibility and BMI. An accurate tattoo did not reduce the conversion rate (P=0.71). No tattoo-related complications were encountered.

CONCLUSION

The practice of tattooing colorectal cancers is variable in frequency, technique and accuracy. We advocate that all colonic lesions suspicious for cancer should be tattooed during endoscopy at a defined distance below the tumour, adhering to a departmental protocol in case surgery is required.

Randomized clinical trial of the effectiveness of emergency day surgery against standard inpatient treatment

BACKGROUND

The aim of this study was to assess the feasibility of treating patients with minor and intermediate general surgical emergency conditions as day cases.

METHODS

Emergency referrals for minor and intermediate general surgical conditions were assessed by a surgeon. Those fitting day-case criteria and requiring operation under general anaesthesia were randomized to receive standard inpatient care or day surgery. Patients in the latter group were booked on to day-case lists or gaps on inpatient elective lists for surgery within 48 h. The process was coordinated by an experienced theatre sister.

RESULTS

One hundred patients were randomized. There was a reduction in the number of nights spent in hospital in the day-case group (median 0 versus 2 nights; P < 0.001). The median time from diagnosis to treatment was 1 day in both groups, although there was a small but significant delay in the day-case group (P = 0.018). There was no significant difference in postoperative outcome or patient and general practitioner satisfaction. The day-case option had no increased impact on primary care services but was associated with a significant saving of about pound sterlings 150 per patient (P < 0.001).

CONCLUSION

Certain general surgical emergencies may be managed as day cases with cost saving but without detriment to patient care.

Education provision for surgical senior house officers

The provision of formal education in surgical SHO training is a requirement for posts to be accredited by postgraduate deans. We carried out a questionnaire survey of surgical SHOs attending a Royal College of Surgeons of England study day to quantify the amount of time given to formal education and supervised operating. We found that 79% of SHOs at district general hospitals and 87.5% at teaching hospitals were provided with a formal education programme. Of these SHOs, 77% were able to attend the teaching on a regular basis. In an average week, 8% of SHOs received no supervised operating experience and 37% had no operations supervised by a consultant. These results suggest that many hospitals fall short of the accepted guidelines on basic surgical training. This issue must be addressed by the Royal Colleges.