Semi-Transparent Polymer Solar Cells with Excellent Sub-Bandgap Transmission for Third Generation Photovoltaics

Semi-transparent organic photovoltaics are of interest for a variety of photovoltaic applications, including solar windows and hybrid tandem photovoltaics. The figure shows a photograph of our semi-transparent solar cell, which has a power conversion efficiency of 5.0%, with an above bandgap transmission of 34% and a sub-bandgap transmission of 81%

Tuning the Properties of Polymer Bulk Heterojunction Solar Cells by Adjusting Fullerene Size to Control Intercalation

Pour la France contemporaine, René Lévy notait en 1996, l’hétérogénéité des recherches d’histoire du crime produites depuis le milieu des années 1980. Il en soulignait les raisons d’ordres divers : approche peu critique des sources, absence de dialogue entre deux pans de la corporation historienne : historiens de lettres et historiens du droit, faible spécialisation du domaine, difficultés de publication. Près d’une décennie plus tard, l’évolution est nette. Les deux derniers siècles ont fait..

Charge transfer dynamics in conjugated polymer/MoS2 organic/2D heterojunctions

Heterojunctions between organic and two-dimensional (2D) semiconductors show promising applications in ultrathin electronic and optoelectronic devices, including field-effect transistors, light-emitting diodes, and photovoltaics. These organic/2D heterojunctions form ideal interfaces due to the lack of dangling bonds at the surfaces of the neat (i.e., individual) materials and their propensity to interact via van der Waals forces. Despite this, organic/2D heterojunction devices have had relatively low quantum efficiencies, suggesting limitations on the charge transport within these devices. Understanding the charge transfer dynamics across organic/2D semiconductor interfaces at fundamental time scales is an important part of overcoming these limitations. In this work, we investigate the photoexcited charge carrier dynamics in organic/2D heterojunctions comprised of large-area monolayer MoS2 and solution-deposited organic semiconducting conjugated polymer thin-films. Using photoluminescence and femtosecond transient absorption spectroscopy, we compare the efficiencies of charge transfer for three different conjugated polymer/MoS2 heterojunctions: P3HT, PCDTBT, and PTB7. We show that electron transfer occurs from MoS2 to P3HT in under 9 ps, and from MoS2 to PCDTBT or PTB7 in under 120 fs. Despite this, we demonstrate that the P3HT/MoS2 heterojunction is the most efficient because the transferred charges have an order-of-magnitude increase in their lifetimes, giving rise to enhanced photoluminescence. This work will help guide designs of future organic/2D heterojunctions using scalable fabrication technologies

Impact of Blend Morphology on Interface State Recombination in Bulk Heterojunction Organic Solar Cells

International audienceThis work is a re-investigation of the impact of blend morphology and thermal annealing on the electrical performance of regioregular P3HT:PC 60 BM. The blend is first characterized by combining atomic force microscopy, X-rays diffraction and Time-of-Flight experiments. Then, current-voltage characteristics of photodiode devices are measured in the dark and under illumination. Finally, the existence of exponential tails of electronic gap states is experimentally confirmed by measuring the device spectral response in the sub-band gap regime. This method reveals the existence of a large density of gap states, which is partially reduced by successive annealing steps. The comparison between drift and diffusion 2 simulations and charge transport experiments show that, when band gap tails are properly taken into account, simulations can satisfactorily reproduce experimental currents under both dark and illumination conditions as a function of voltage and annealing time. This work further confirms the critical impact of tails states on the performance of solar cells

An Efficient, “Burn in” Free Organic Solar Cell Employing a Nonfullerene Electron Acceptor

A comparison of the efficiency, stability, and photophysics of organic solar cells employing poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′″-di(2-octyldodecyl)-2,2′;5′,2″;5″,2′″-quaterthiophen-5,5′″-diyl)] (PffBT4T-2OD) as a donor polymer blended with either the nonfullerene acceptor EH-IDTBR or the fullerene derivative, [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as electron acceptors is reported. Inverted PffBT4T-2OD:EH-IDTBR blend solar cell fabricated without any processing additive achieves power conversion efficiencies (PCEs) of 9.5 ± 0.2%. The devices exhibit a high open circuit voltage of 1.08 ± 0.01 V, attributed to the high lowest unoccupied molecular orbital (LUMO) level of EH-IDTBR. Photoluminescence quenching and transient absorption data are employed to elucidate the ultrafast kinetics and efficiencies of charge separation in both blends, with PffBT4T-2OD exciton diffusion kinetics within polymer domains, and geminate recombination losses following exciton separation being identified as key factors determining the efficiency of photocurrent generation. Remarkably, while encapsulated PffBT4T-2OD:PC71BM solar cells show significant efficiency loss under simulated solar irradiation (“burn in” degradation) due to the trap-assisted recombination through increased photoinduced trap states, PffBT4T-2OD:EH-IDTBR solar cell shows negligible burn in efficiency loss. Furthermore, PffBT4T-2OD:EH-IDTBR solar cells are found to be substantially more stable under 85 °C thermal stress than PffBT4T-2OD:PC71BM devices

Molecular Dynamics Study of the Local Structure of Photovoltaic Polymer PCDTBT

To meet the huge demand for renewable energy, significant research effort focuses on creating efficient organic photovoltaic (OPV) devices. In comparison to silicon-based semiconductors, OPV materials have many superior properties such as cost effectiveness, being lightweight, and flexibility, which lead to a high potential for the replacement of silicon-based semiconductors. Recently, a large number of new alternating copolymer materials have demonstrated high power conversion efficiency (PCE). These successful polymers typically have low long-range order but a high hole mobility which directly affects the PCE which depends on polymer structure. In this study, a solution molecular model for poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]:[6,6]-phenyl (PCDTBT) is developed and subsequently a molecular dynamics simulation conducted in order to understand the structure of the polymer solution. The simulation results are consistent with a low-solubility polymer that requires long equilibration times to planarize. The structural addition of side chains to inhibit rotation of thiophene rings could improve the conjugation and processability of PDCTBT leading to further improvements in OPV efficiency or hole mobility

Correlating molecular morphology with optoelectronic function in solar cells based on low band-gap copolymer:fullerene blends

We review recent progress in the development of organic bulk heterojunction (BHJ) solar cells employing donor–acceptor copolymers as the electron-donor and fullerene derivatives as the electron-acceptor. We discuss the role of the donor and acceptor moieties, side-chains, bridging units and atomic substitutions of the copolymers on their optoelectronic functionality. The physical properties, e.g. molecular conformation, miscibility, phase-separated lateral and vertical morphology, of various photovoltaic blends prepared via solution casting and post-treatments are reviewed and correlated with photovoltaic device performance. Factors influencing the morphological stability of polymer:fullerene BHJ thin-films are briefly discussed. Finally, we address the use of thin organic interlayers to increase the efficiency of BHJ solar cells

Mental Health And Wellness Chatbot

This project is a mental health and wellness chatbot. Chatbots are computer applications that exchange messages with a human user. There are many ways to access chatbots, including Facebook, Twitter, smartphone apps, and messaging platforms such as Skype and Discord. These chatbots all follow a general formula. The user can send messages to the chatbot, and it responds with an appropriate message or action. This chatbot was built to run in Discord, a popular text and audio communication platform. Discord supports development of custom bots and was a natural choice for this project. This chatbot has several features, including mood responses, mood graphing, and a therapist search. Each of these features are designed to improve the general well-being of the chatbot users. The target demographic is adults who frequently use computers, since they are particularly vulnerable to mental health issues such as depression (Madhav, 2017). A user study was conducted to examine the user experience. Initial results indicate a positive user experience, but further studies are required to draw concrete conclusions about the chatbot and its efficacy