Influence of doping on charge carrier collection in normal and inverted geometry polymer: fullerene solar cells

While organic semiconductors used in polymer:fullerene photovoltaics are generally not intentionally doped, significant levels of unintentional doping have previously been reported in the literature. Here, we explain the differences in photocurrent collection between standard (transparent anode) and inverted (transparent cathode) low band-gap polymer:fullerene solar cells in terms of unintentional p-type doping. Using capacitance/voltage measurements, we find that the devices exhibit doping levels of order 1016 cm−3, resulting in space-charge regions ~100 nm thick at short circuit. As a result, low field regions form in devices thicker than 100 nm. Because more of the light is absorbed in the low field region in standard than in inverted architectures, the losses due to inefficient charge collection are greater in standard architectures. Using optical modelling, we show that the observed trends in photocurrent with device architecture and thickness can be explained if only charge carriers photogenerated in the depletion region contribute to the photocurrent

Grand Challenges in global eye health: a global prioritisation process using Delphi method

Background We undertook a Grand Challenges in Global Eye Health prioritisation exercise to identify the key issues that must be addressed to improve eye health in the context of an ageing population, to eliminate persistent inequities in health-care access, and to mitigate widespread resource limitations. Methods Drawing on methods used in previous Grand Challenges studies, we used a multi-step recruitment strategy to assemble a diverse panel of individuals from a range of disciplines relevant to global eye health from all regions globally to participate in a three-round, online, Delphi-like, prioritisation process to nominate and rank challenges in global eye health. Through this process, we developed both global and regional priority lists. Findings Between Sept 1 and Dec 12, 2019, 470 individuals complete round 1 of the process, of whom 336 completed all three rounds (round 2 between Feb 26 and March 18, 2020, and round 3 between April 2 and April 25, 2020) 156 (46%) of 336 were women, 180 (54%) were men. The proportion of participants who worked in each region ranged from 104 (31%) in sub-Saharan Africa to 21 (6%) in central Europe, eastern Europe, and in central Asia. Of 85 unique challenges identified after round 1, 16 challenges were prioritised at the global level; six focused on detection and treatment of conditions (cataract, refractive error, glaucoma, diabetic retinopathy, services for children and screening for early detection), two focused on addressing shortages in human resource capacity, five on other health service and policy factors (including strengthening policies, integration, health information systems, and budget allocation), and three on improving access to care and promoting equity. Interpretation This list of Grand Challenges serves as a starting point for immediate action by funders to guide investment in research and innovation in eye health. It challenges researchers, clinicians, and policy makers to build collaborations to address specific challenge

Analysis of the Relationship between Linearity of Corrected Photocurrent and the Order of Recombination in Organic Solar Cells

We address the claim that the dependence of the “corrected photocurrent” (defined as the difference between the light and dark currents) upon light intensity can be used to determine the charge recombination mechanism in an organic solar cell. We analyze a poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) device using corrected photocurrent and transient photovoltage experiments and show that whereas the corrected photocurrent is linear in light intensity the charge recombination rate scales superlinearly with charge carrier density. We explain this apparent discrepancy by measuring the charge carrier densities at different applied voltages and light intensities. We show that it is only safe to infer a linear recombination mechanism from a linear dependence of corrected photocurrent on light intensity under the following special conditions: (i) the photogenerated charge carrier density is much larger than the dark carrier density and (ii) the photogenerated carrier density is proportional to the photogeneration rate

Limits on the Fill Factor in Organic Photovoltaics: Distinguishing Nongeminate and Geminate Recombination Mechanisms

In this Letter, we present transient optoelectronic experimental studies of the recombination processes limiting the fill factor (FF) in three conjugated polymer:fullerene systems, poly­(3-hexylthiophene) (P3HT) and two lower-band-gap polymers that exhibit lower FFs poly­[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta­[2,1-<i>b</i>;3,4-<i>b</i>′]­dithiophene)-<i>alt</i>-4,7-(2,1,3-benzothiadiazole) (PCPDTBT) and poly­(2,7-(9,9-dioctylfluorene)-<i>alt</i>-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-3). Using transient absorption spectroscopy, charge extraction, and transient photovoltage experiments, we show that the lower FF observed for the PCPDTBT-based device results from enhanced nongeminate recombination even at short circuit, In contrast, we show that for APFO-3 devices, the FF is primarily limited by a voltage-dependent free charge generation, which we assign to a geminate recombination process

Interlaboratory indoor ageing of roll-to-roll and spin coated organic photovoltaic devices: Testing the ISOS tests

The inter-comparability of the stability measurements of organic photovoltaic (OPV) technologies in dark is addressed. Four primary factors that affect the reproducibility of the ageing rate determination and inter-comparison are discussed: production/encapsulation of the samples, current-voltage (IV) characterization, testing conditions for ageing and lifetime determination from a decay curve. Results of inter-laboratory ageing studies of roll-to-roll and spin coated samples with correspondingly flexible plastic packaging and glass encapsulation tested under dark test conditions among 7 laboratories are presented. ISOS test conditions, proposed recently as guiding protocols for testing OPV stability, are applied in the study. The reproducibility of the performance versus the production and encapsulation techniques is firstly studied. The results reveal a significant improvement in the reproducibility when going from manual spin coating to roll-to-roll production. Furthermore, the reproducibility of current-voltage (IV) measurement and preconditioning (light soaking treatments) are addressed. Additionally, the inter-comparison of the degradation rates of the samples tested under three different dark test conditions (ambient, dry/heat, damp heat) reported by different groups are analyzed revealing a reasonable agreement on the degradation rates of the stability curves for each test condition. Finally, a logarithmic diagram for OPV lifetime associated with common time units is proposed that allows conveniently categorizing and intercomparing the stability performance of different samples tested under different test conditions.JRC.F.7-Renewables and Energy Efficienc