High Thickness Tolerance in All-Polymer-Based Organic Photovoltaics Enables Efficient and Stable In-Door Operation.

Organic photovoltaics (OPVs) have great potential to drive low-power consumption electronic devices under indoor light due to their highly tunable optoelectronic properties. Thick devices (>300 nm photo-active junctions) are desirable to maximize photocurrent and to manufacture large-scale modules via solution-processing. However, thick devices usually suffer from severe charge recombination, deteriorating device performances. Herein, the study demonstrates excellent thickness tolerance of all-polymer-based PVs for efficient and stable indoor applications. Under indoor light, device performance is less dependent on photoactive layer thickness, exhibiting the best maximum power output in thick devices (34.7 µW cm-2 in 320-475 nm devices). Thick devices also exhibit much better photostability compared with thin devices. Such high thickness tolerance of all-polymer-based PV devices under indoor operation is attributed to strongly suppressed space-charge effects, leading to reduced bimolecular recombination losses in thick devices. The unbalanced charge carrier mobilities are identified as the main cause for significant space-charge effects, which is confirmed by drift-diffusion simulations. This work suggests that all-polymer-based PVs, even with unbalanced mobilities, are highly desirable for thick, efficient, and stable devices for indoor applications.

Nurse understaffing associated with adverse outcomes for surgical admissions.

BACKGROUND: Nurses play a crucial role in maintaining the safety of surgical patients. Few nurse staffing studies have looked specifically at surgical patients to examine the impact of exposure to low staffing on patient outcomes. METHODS: A longitudinal patient analysis was conducted in four organizations in England using routine data from 213 910 admissions to all surgical specialties. Patients' staffing exposures were modelled as counts of understaffed registered nurse and nurse assistant days in the first 5 inpatient days. Understaffing was identified when staffing per patient-day was below the mean for the ward. Cox models were used to examine mortality within 30 days of admission and readmission within 30 days of discharge. Generalized linear models were used to investigate duration of hospital stay and occurrence of hospital-acquired conditions. RESULTS: Increased exposure to registered nurse understaffing was associated with longer hospital stay and increased risk of deep vein thrombosis, pneumonia, and pressure ulcers. This was also true for nurse assistant understaffing, but the effect sizes tended to be smaller. In the Cox models, there were similarly increased hazards of death for registered nurse understaffing (HR 1.09, 95% c.i. 1.07 to 1.12) and nurse assistant understaffing (HR 1.10, 1.08 to 1.13), whereas the effect size of registered nurse understaffing for readmission (HR 1.02, 1.02 to 1.03) was greater than that seen with nurse assistants (HR 1.01, 1.01 to 1.02). CONCLUSION: Understaffing by both registered nurses and nursing assistants is associated with increased risks of a range of adverse events, and generally larger effects are seen with registered nurse understaffing.

Nursing Team Composition and Mortality Following Acute Hospital Admission.

Importance: Many studies show the adverse consequences of insufficient nurse staffing in hospitals, but safe and effective staffing is unlikely to be just about staff numbers. There are considerable areas of uncertainty, including whether temporary staff can safely make up shortfalls in permanent staff and whether using experienced staff can mitigate the effect of staff shortages. Objective: To explore the association of the composition of the nursing team with the risk of patient deaths. Design, Setting, and Participants: This patient-level longitudinal observational study was conducted in 185 wards in 4 acute hospital trusts in England between April 2015 and March 2020. Eligible participants were patients with an overnight stay and nursing staff on adult inpatient wards. Data analysis was conducted from month April 2022 to June 2023. Exposure: Naturally occurring variation during the first 5 days of hospital admission in exposure to days of low staffing from registered nurses (RNs) and nursing support (NS) staff, the proportion of RNs, proportion of senior staff, and proportion of hospital-employed (bank) and agency temporary staff. Main Outcomes and Measures: The primary outcome was death within 30 days of admission. Mixed-effect Cox proportional hazards survival models were used. Results: Data from 626 313 admissions (319 518 aged ≥65 years [51.0%]; 348 464 female [55.6%]) were included. Risk of death was increased when patients were exposed to low staffing from RNs (adjusted hazard ratio [aHR], 1.08; 95% CI 1.07-1.09) and NS staff (aHR, 1.07; 95% CI, 1.06-1.08). A 10% increase in the proportion of temporary RNs was associated with a 2.3% increase in the risk of death, with no difference between agency (aHR, 1.023; 95% CI, 1.01-1.04) and bank staff (aHR, 1.02; 95% CI, 1.01-1.04). A 10% increase in the proportion of agency NS was associated with a 4% increase in risk of death (aHR, 1.04; 95% CI, 1.02-1.06). Evidence on the seniority of staff was mixed. Model coefficients were used to estimate the association of using temporary staff to avoid low staffing and found that risk was reduced but remained elevated compared with baseline. Conclusions and Relevance: This cohort study found that having senior nurses in the nursing team did not mitigate the adverse outcomes associated with low nurse staffing. These findings indicate that while the benefits of avoiding low staffing may be greater than the harms associated with using temporary staff, particularly for RNs, risk remains elevated if temporary staff are used to fill staffing shortages, which challenges the assumption that temporary staff are a cost-effective long-term solution to maintaining patient safety.

Exciton diffusion in organic semiconductors: precision and pitfalls.

Nanometer exciton diffusion is a fundamental process important in virtually all applications of organic semiconductors. Many measurement techniques have been developed to measure exciton diffusion length (LD) at the nanometer scale; however, these techniques have common challenges that the community has worked for decades to overcome. In this perspective, we lay out the principal challenges researchers need to overcome to obtain an accurate measurement of LD. We then examine the most common techniques used to measure LD with respect to these challenges and describe solutions developed to overcome them. This analysis leads to the suggestion that static quenching techniques underestimate LD due to uncertainties in the quenching behavior, while time-resolved exciton-exciton annihilation (EEA) techniques overestimate LD based on experimental conditions, we advance steady-state EEA techniques as an alternative that overcome many of the challenges of these other techniques while preserving accuracy. We support this hypothesis with a meta-analysis of LD measured across various organic semiconductors and measurement techniques. We intend this investigation to provide a framework for researchers to interpret and compare findings across measurement techniques and to guide researchers on how to obtain the most accurate results for each technique in question.

Enhanced SWIR Light Detection in Organic Semiconductor Photodetectors through Up-Conversion of Mid-Gap Trap States.

Shortwave-infrared (SWIR) photodetectors are vital for many scientific and industrial applications including surveillance, quality control and inspection. In recent decades, photodetectors based on organic semiconductors have emerged, demonstrating potential to add real value to broadband and narrowband imaging and sensing scenarios, where factors such as thermal budget sensitivity, large area aperture necessity, cost considerations, and lightweight and conformal flexibility demands are prioritized. It is now recognized that the performance of organic photodetectors (OPDs), notably their specific detectivity, is ultimately limited by trap states, universally present in disordered semiconductors. This work adopts an approach of utilizing these mid-gap states to specifically create a SWIR photo-response. To this end, this work introduces a somewhat counter-intuitive approach of "trap-doping" in bulk heterojunction (BHJs) photodiodes, where small quantities of a guest organic molecule are intentionally incorporated into a semiconducting donor:acceptor host system. Following this approach, this work demonstrates a proof-of-concept for a visible-to-SWIR broadband OPD, approaching (and, to some extent, even exceeding) state-of-the-art performance across critical photodetector metrics. The trap-doping approach is, even though only a proof-of-concept currently, broadly applicable to various spectral windows. It represents a new modality for engineering photodetection using the unconventional strategy of turning a limitation into a feature.

A New Framework for Understanding Recombination-Limited Charge Extraction in Disordered Semiconductors.

Recombination of free charges is a key loss mechanism limiting the performance of organic semiconductor-based photovoltaics such as solar cells and photodetectors. The carrier density-dependence of the rate of recombination and the associated rate coefficients are often estimated using transient charge extraction (CE) experiments. These experiments, however, often neglect the effect of recombination during the transient extraction process. In this work, the validity of the CE experiment for low-mobility devices, such as organic semiconductor-based photovoltaics, is investigated using transient drift-diffusion simulations. We find that recombination leads to incomplete CE, resulting in carrier density-dependent recombination rate constants and overestimated recombination orders; an effect that depends on both the charge carrier mobilities and the resistance-capacitance time constant. To overcome this intrinsic limitation of the CE experiment, we present an analytical model that accounts for charge carrier recombination, validate it using numerical simulations, and employ it to correct the carrier density-dependence observed in experimentally determined bimolecular recombination rate constants.

Inpatient midwifery staffing levels and postpartum readmissions: a retrospective multicentre longitudinal study.

BACKGROUND: Preventing readmission to hospital after giving birth is a key priority, as rates have been rising along with associated costs. There are many contributing factors to readmission, and some are thought to be preventable. Nurse and midwife understaffing has been linked to deficits in care quality. This study explores the relationship between staffing levels and readmission rates in maternity settings. METHODS: We conducted a retrospective longitudinal study using routinely collected individual patient data in three maternity services in England from 2015 to 2020. Data on admissions, discharges and case-mix were extracted from hospital administration systems. Staffing and workload were calculated in Hours Per Patient day per shift in the first two 12-hour shifts of the index (birth) admission. Postpartum readmissions and staffing exposures for all birthing admissions were entered into a hierarchical multivariable logistic regression model to estimate the odds of readmission when staffing was below the mean level for the maternity service. RESULTS: 64 250 maternal admissions resulted in birth and 2903 mothers were readmitted within 30 days of discharge (4.5%). Absolute levels of staffing ranged between 2.3 and 4.1 individuals per midwife in the three services. Below average midwifery staffing was associated with higher rates of postpartum readmissions within 7 days of discharge (adjusted OR (aOR) 1.108, 95% CI 1.003 to 1.223). The effect was smaller and not statistically significant for readmissions within 30 days of discharge (aOR 1.080, 95% CI 0.994 to 1.174). Below average maternity assistant staffing was associated with lower rates of postpartum readmissions (7 days, aOR 0.957, 95% CI 0.867 to 1.057; 30 days aOR 0.965, 95% CI 0.887 to 1.049, both not statistically significant). CONCLUSION: We found evidence that lower than expected midwifery staffing levels is associated with more postpartum readmissions. The nature of the relationship requires further investigation including examining potential mediating factors and reasons for readmission in maternity populations.

Safer and more efficient vital signs monitoring protocols to identify the deteriorating patients in the general hospital ward: an observational study.

Background: The frequency at which patients should have their vital signs (e.g. blood pressure, pulse, oxygen saturation) measured on hospital wards is currently unknown. Current National Health Service monitoring protocols are based on expert opinion but supported by little empirical evidence. The challenge is finding the balance between insufficient monitoring (risking missing early signs of deterioration and delays in treatment) and over-observation of stable patients (wasting resources needed in other aspects of care). Objective: Provide an evidence-based approach to creating monitoring protocols based on a patient's risk of deterioration and link these to nursing workload and economic impact. Design: Our study consisted of two parts: (1) an observational study of nursing staff to ascertain the time to perform vital sign observations; and (2) a retrospective study of historic data on patient admissions exploring the relationships between National Early Warning Score and risk of outcome over time. These were underpinned by opinions and experiences from stakeholders. Setting and participants: Observational study: observed nursing staff on 16 randomly selected adult general wards at four acute National Health Service hospitals. Retrospective study: extracted, linked and analysed routinely collected data from two large National Health Service acute trusts; data from over 400,000 patient admissions and 9,000,000 vital sign observations. Results: Observational study found a variety of practices, with two hospitals having registered nurses take the majority of vital sign observations and two favouring healthcare assistants or student nurses. However, whoever took the observations spent roughly the same length of time. The average was 5:01 minutes per observation over a 'round', including time to locate and prepare the equipment and travel to the patient area. Retrospective study created survival models predicting the risk of outcomes over time since the patient was last observed. For low-risk patients, there was little difference in risk between 4 hours and 24 hours post observation. Conclusions: We explored several different scenarios with our stakeholders (clinicians and patients), based on how 'risk' could be managed in different ways. Vital sign observations are often done more frequently than necessary from a bald assessment of the patient's risk, and we show that a maximum threshold of risk could theoretically be achieved with less resource. Existing resources could therefore be redeployed within a changed protocol to achieve better outcomes for some patients without compromising the safety of the rest. Our work supports the approach of the current monitoring protocol, whereby patients' National Early Warning Score 2 guides observation frequency. Existing practice is to observe higher-risk patients more frequently and our findings have shown that this is objectively justified. It is worth noting that important nurse-patient interactions take place during vital sign monitoring and should not be eliminated under new monitoring processes. Our study contributes to the existing evidence on how vital sign observations should be scheduled. However, ultimately, it is for the relevant professionals to decide how our work should be used. Study registration: This study is registered as ISRCTN10863045. Funding: This award was funded by the National Institute for Health and Care Research (NIHR) Health and Social Care Delivery Research programme (NIHR award ref: 17/05/03) and is published in full in Health and Social Care Delivery Research; Vol. 12, No. 6. See the NIHR Funding and Awards website for further award information.

Patient recovery in hospital is tracked by measuring heart rate, blood pressure and other 'vital signs' and converting them into a score. These are 'observed' regularly by nursing staff so that deterioration can be spotted early. However, taking observations can disturb patients, and taking them too often causes extra work for staff. More frequent monitoring is recommended for higher scores, but evidence is lacking. To work out how often patients should be monitored, we needed to know how likely it is for patients to become more unwell between observations. We analysed over 400,000 patient records from two hospitals to understand how scores change with time. We looked at three of the most serious risks for patients in hospital. These risks are dying, needing intensive care or having a cardiac arrest. We also looked at the risk that a patient's condition would deteriorate significantly before their measurements were taken again. We identified early signs of deterioration and how changes in vital signs affected the risk of a patient's condition becoming worse. From this we calculated a maximum risk of deterioration. We then calculated different monitoring schedules that keep individual patients below this risk level. Some of those would consume less staff time than current National Health Service guidelines suggest. We also watched staff record patients' vital signs. We learnt it takes about 5 minutes to take these measurements from each patient. This information helped us calculate how costs would change if patients' vital signs were taken more or less often. We found that patients with a low overall score could have their vital signs monitored less often without being in danger of serious harm. This frees up nursing time so that patients with a higher score can be monitored more often. Importantly, this can be achieved without employing more staff.

The association between midwifery staffing and reported harmful incidents: a cross-sectional analysis of routinely collected data.

BACKGROUND: Independent inquiries have identified that appropriate staffing in maternity units is key to enabling quality care and minimising harm, but optimal staffing levels can be difficult to achieve when there is a shortage of midwives. The services provided and how they are staffed (total staffing, skill-mix and deployment) have been changing, and the effects of workforce changes on care quality and outcomes have not been assessed. This study aims to explore the association between daily midwifery staffing levels and the rate of reported harmful incidents affecting mothers and babies. METHODS: We conducted a cross-sectional analysis of daily reports of clinical incidents in maternity inpatient areas matched with inpatient staffing levels for three maternity services in England, using data from April 2015 to February 2020. Incidents resulting in harm to mothers or babies was the primary outcome measure. Staffing levels were calculated from daily staffing rosters, quantified in Hours Per Patient Day (HPPD) for midwives and maternity assistants. Understaffing was defined as staffing below the mean for the service. A negative binomial hierarchical model was used to assess the relationship between exposure to low staffing and reported incidents involving harm. RESULTS: The sample covered 106,904 maternal admissions over 46 months. The rate of harmful incidents in each of the three services ranged from 2.1 to 3.0 per 100 admissions across the study period. Understaffing by registered midwives was associated with an 11% increase in harmful incidents (adjusted IRR 1.110, 95% CI 1.002,1.229). Understaffing by maternity assistants was not associated with an increase in harmful incidents (adjusted IRR 0.919, 95% 0.813,1.039). Analysis of specific types of incidents showed no statistically significant associations, but most of the point estimates were in the direction of increased incidents when services were understaffed. CONCLUSION: When there is understaffing by registered midwives, more harmful incidents are reported but understaffing by maternity assistants is not associated with higher risk of harms. Adequate registered midwife staffing levels are crucial for maintaining safety. Changes in the profile of maternity service workforces need to be carefully scrutinised to prevent mothers and babies being put at risk of avoidable harm.

Outcomes of Mohs micrographic surgery for periocular squamous cell carcinoma.

INTRODUCTION: Periocular cutaneous squamous cell carcinoma (cSCC) accounts for 5-10% of all eyelid malignancies. Periocular cSCC carries a low mortality rate but can be destructive to local tissues. Due to the unique function and anatomy of the eyelids, Mohs micrographic surgery (MMS) is gold standard for treating cSCC to preserve healthy tissue and reduce rates of local recurrence. In this study, we describe the success and outcomes of MMS and subsequent oculoplastic reconstruction for periocular cSCC in the North East of England. METHODS: Retrospective analysis of 34 patients who underwent MMS for periocular cSCC in the North of England between 2013 and 2020. Primary outcome measure of success is defined as no recurrence of cSCC after minimum 24 months' time elapsed post-MMS. Secondary outcome measures included analysis of disease characteristics, describing the surgical techniques utilised for oculoplastic reconstruction following MMS and surgical complications should they occur. RESULTS: Two patients (5.9%) had local recurrence of periocular cSCC. Median time elapsed since MMS was 60 months. A variety of oculoplastic surgical techniques were utilised in the repair of the Mohs defect. One patient (2.9%) developed a significant post-operative reconstruction complication. CONCLUSION: Periocular cSCC recurrence following MMS in the North of England is 5.9%, which is comparable to the literature. Significant post-operative complications following oculoplastic reconstruction of periocular MMS are very low, occurring in 2.9% of cases in this study.

Costs and cost-effectiveness of improved nurse staffing levels and skill mix in acute hospitals: A systematic review.

BACKGROUND: Extensive research shows associations between increased nurse staffing levels, skill mix and patient outcomes. However, showing that improved staffing levels are linked to improved outcomes is not sufficient to provide a case for increasing them. This review of economic studies in acute hospitals aims to identify costs and consequences associated with different nurse staffing configurations in hospitals. METHODS: We included economic studies exploring the effect of variation in nurse staffing. We searched PubMed, CINAHL, Embase Econlit, Cochrane library, DARE, NHS EED and the INAHTA website. Risk of bias was assessed using a framework based on the NICE guidance for public health reviews and Henrikson's framework for economic evaluations. Inclusion, data extraction and critical appraisal were undertaken by pairs of reviewers with disagreements resolved by the entire review team. Results were synthesised using a hierarchical matrix to summarise findings of economic evaluations. RESULTS: We found 23 observational studies conducted in the United States of America (16), Australia, Belgium, China, South Korea, and the United Kingdom (3). Fourteen had high risk of bias and nine moderate. Most studies addressed levels of staffing by RNs and/or licensed practical nurses. Six studies found that increased nurse staffing levels were associated with improved outcomes and reduced or unchanged net costs, but most showed increased costs and outcomes. Studies undertaken outside the USA showed that increased nurse staffing was likely to be cost-effective at a per capita gross domestic product (GDP) threshold or lower. Four studies found that increased skill mix was associated with improved outcomes but increased staff costs. Three studies considering net costs found increased registered nurse skill mix associated with net savings and similar or improved outcomes. CONCLUSION: Although more evidence on cost-effectiveness is still needed, increases in absolute or relative numbers of registered nurses in general medical and surgical wards have the potential to be highly cost-effective. The preponderance of the evidence suggests that increasing the proportion of registered nurses is associated with improved outcomes and, potentially, reduced net cost. Conversely, policies that lead to a reduction in the proportion of registered nurses in nursing teams could give worse outcomes at increased costs and there is no evidence that such approaches are cost-effective. In an era of registered nurse scarcity, these results favour investment in registered nurse supply as opposed to using lesser qualified staff as substitutes, especially where baseline nurse staffing and skill mix are low. REGISTRATION: PROSPERO (CRD42021281202). TWEETABLE ABSTRACT: Increasing registered nurse staffing and skill mix can be a net cost-saving solution to nurse shortages. Contrary to the strong policy push towards a dilution of nursing skill mix, investment in supply of RNs should become the priority.

Subgap Absorption in Organic Semiconductors.

Organic semiconductors have found a broad range of application in areas such as light emission, photovoltaics, and optoelectronics. The active components in such devices are based on molecular and polymeric organic semiconductors, where the density of states is generally determined by the disordered nature of the molecular solid rather than energy bands. Inevitably, there exist states within the energy gap which may include tail states, deep traps caused by unavoidable impurities and defects, as well as intermolecular states due to (radiative) charge transfer states. In this Perspective, we first summarize methods to determine the absorption features due to the subgap states. We then explain how subgap states can be parametrized based upon the subgap spectral line shapes. We finally describe the role of subgap states in the performance metrics of organic semiconductor devices from a thermodynamic viewpoint.

Efficient Nanoscale Exciton Transport in Non-Fullerene Organic Solar Cells Enables Reduced Bimolecular Recombination of Free Charges.

The highest-efficiency organic photovoltaic (OPV)-based solar cells, made from blends of electron-donating and electron-accepting organic semiconductors, are often characterized by strongly reduced (non-Langevin) bimolecular recombination. Although the origins of the reduced recombination are debated, mechanisms related to the charge-transfer (CT) state and free-carrier encounter dynamics controlled by the size of donor and acceptor domains are proposed as underlying factors. Here, a novel photoluminescence-based probe is reported to accurately quantify the donor-acceptor domain size in OPV blends. Specifically, the domain size is measured in high-efficiency non-fullerene acceptor (NFA) systems and a comparative conventional fullerene system. It is found that the NFA-based blends form larger domains but that the expected reductions in bimolecular recombination attributed to the enhanced domain sizes are too small to account for the observed reduction factors. Further, it is shown that the reduction of bimolecular recombination is correlated to enhanced exciton dynamics within the NFA domains. This indicates that the processes responsible for efficient exciton transport also enable strongly non-Langevin recombination in high-efficiency NFA-based solar cells with low-energy offsets.

Combining viral genomics and clinical data to assess risk factors for severe COVID-19 (mortality, ICU admission, or intubation) amongst hospital patients in a large acute UK NHS hospital Trust.

Throughout the COVID-19 pandemic, valuable datasets have been collected on the effects of the virus SARS-CoV-2. In this study, we combined whole genome sequencing data with clinical data (including clinical outcomes, demographics, comorbidity, treatment information) for 929 patient cases seen at a large UK hospital Trust between March 2020 and May 2021. We identified associations between acute physiological status and three measures of disease severity; admission to the intensive care unit (ICU), requirement for intubation, and mortality. Whilst the maximum National Early Warning Score (NEWS2) was moderately associated with severe COVID-19 (A = 0.48), the admission NEWS2 was only weakly associated (A = 0.17), suggesting it is ineffective as an early predictor of severity. Patient outcome was weakly associated with myriad factors linked to acute physiological status and human genetics, including age, sex and pre-existing conditions. Overall, we found no significant links between viral genomics and severe outcomes, but saw evidence that variant subtype may impact relative risk for certain sub-populations. Specific mutations of SARS-CoV-2 appear to have little impact on overall severity risk in these data, suggesting that emerging SARS-CoV-2 variants do not result in more severe patient outcomes. However, our results show that determining a causal relationship between mutations and severe COVID-19 in the viral genome is challenging. Whilst improved understanding of the evolution of SARS-CoV-2 has been achieved through genomics, few studies on how these evolutionary changes impact on clinical outcomes have been seen due to complexities associated with data linkage. By combining viral genomics with patient records in a large acute UK hospital, this study represents a significant resource for understanding risk factors associated with COVID-19 severity. However, further understanding will likely arise from studies of the role of host genetics on disease progression.

Systematic in situ hydration neutron reflectometry study on Nafion thin films.

Reported herein is a neutron reflectometry (NR) study on hydrated Nafion thin films (∼30 nm) on a silicon substrate with native oxide. The Nafion morphology is investigated systematically across the whole relative humidity range using both H2O and D2O vapours to enable a comparative study. By utilising this systematic approach two key results have been obtained. The first is that by leveraging the strong positive scattering signal from the D2O vapour, a complete and systematic water adsorption isotherm (Type II) for a Nafion thin film is produced. Utilising the slight negative scattering signal of the H2O enabled the quantification of the hydration dependent evolution of the formation of Nafion/water lamellae near the substrate surface. The number of lamellae layers increases continuously with hydration, and does not form abruptly. We also report the effects of swelling on the thin films across the relative humidity ranges. The work reported should prove useful in quantifying other hydration dependent properties of Nafion thin films such as conductivity and understanding Nafion/semiconductor based devices, as well as showcasing a NR methodology for other hydrophilic polymers.

Static Disorder in Lead Halide Perovskites.

In crystalline and amorphous semiconductors, the temperature-dependent Urbach energy can be determined from the inverse slope of the logarithm of the absorption spectrum and reflects the static and dynamic energetic disorder. Using recent advances in the sensitivity of photocurrent spectroscopy methods, we elucidate the temperature-dependent Urbach energy in lead halide perovskites containing different numbers of cation components. We find Urbach energies at room temperature to be 13.0 ± 1.0, 13.2 ± 1.0, and 13.5 ± 1.0 meV for single, double, and triple cation perovskite. Static, temperature-independent contributions to the Urbach energy are found to be as low as 5.1 ± 0.5, 4.7 ± 0.3, and 3.3 ± 0.9 meV for the same systems. Our results suggest that, at a low temperature, the dominant static disorder in perovskites is derived from zero-point phonon energy rather than structural disorder. This is unusual for solution-processed semiconductors but broadens the potential application of perovskites further to quantum electronics and devices.

Probing Charge Generation Efficiency in Thin-Film Solar Cells by Integral-Mode Transient Charge Extraction.

The photogeneration of free charges in light-harvesting devices is a multistep process, which can be challenging to probe due to the complexity of contributing energetic states and the competitive character of different driving mechanisms. In this contribution, we advance a technique, integral-mode transient charge extraction (ITCE), to probe these processes in thin-film solar cells. ITCE combines capacitance measurements with the integral-mode time-of-flight method in the low intensity regime of sandwich-type thin-film devices and allows for the sensitive determination of photogenerated charge-carrier densities. We verify the theoretical framework of our method by drift-diffusion simulations and demonstrate the applicability of ITCE to organic and perovskite semiconductor-based thin-film solar cells. Furthermore, we examine the field dependence of charge generation efficiency and find our ITCE results to be in excellent agreement with those obtained via time-delayed collection field measurements conducted on the same devices.

Role of Exciton Diffusion and Lifetime in Organic Solar Cells with a Low Energy Offset.

Despite general agreement that the generation of free charges in organic solar cells is driven by an energetic offset, power conversion efficiencies have been improved using low-offset blends. In this work, we explore the interconnected roles that exciton diffusion and lifetime play in the charge generation process under various energetic offsets. A detailed balance approach is used to develop an analytic framework for exciton dissociation and free-charge generation accounting for exciton diffusion to and dissociation at the donor-acceptor interface. For low-offset systems, we find the exciton lifetime to be a pivotal component in the charge generation process, as it influences both the exciton and CT state dissociation. These findings suggest that any novel low-offset material combination must have long diffusion lengths with long exciton lifetimes to achieve optimum charge generation yields.

Direct observation of trap-assisted recombination in organic photovoltaic devices.

Trap-assisted recombination caused by localised sub-gap states is one of the most important first-order loss mechanism limiting the power-conversion efficiency of all solar cells. The presence and relevance of trap-assisted recombination in organic photovoltaic devices is still a matter of some considerable ambiguity and debate, hindering the field as it seeks to deliver ever higher efficiencies and ultimately a viable new solar photovoltaic technology. In this work, we show that trap-assisted recombination loss of photocurrent is universally present under operational conditions in a wide variety of organic solar cell materials including the new non-fullerene electron acceptor systems currently breaking all efficiency records. The trap-assisted recombination is found to be induced by states lying 0.35-0.6 eV below the transport edge, acting as deep trap states at light intensities equivalent to 1 sun. Apart from limiting the photocurrent, we show that the associated trap-assisted recombination via these comparatively deep traps is also responsible for ideality factors between 1 and 2, shedding further light on another open and important question as to the fundamental working principles of organic solar cells. Our results also provide insights for avoiding trap-induced losses in related indoor photovoltaic and photodetector applications.