Understanding the Apparent Charge Density Dependence of Mobility and Lifetime in Organic Bulk Heterojunction Solar Cells

Energetic disorder in organic semiconductors leads to strong dependence of recombination kinetics and mobility on charge density. However, observed mobilities and reaction orders are normally interpreted assuming uniform charge carrier distributions. In this paper, we explore the effect of the spatial distribution of charge on the determination of mobility and recombination rate as a function of average charge density. Since the spatial gradient changes when the thickness of a device is varied, we study thickness series of two different polymer:fullerene systems and measure the charge density dependence of mobility and lifetime. Using simulations, we can show that the high apparent reaction orders frequently observed in the literature result from the spatial gradient of charge density at open circuit. However, the mobilities, measured at short circuit, are less affected by the gradients and therefore may show substantially different apparent charge density dependence than the recombination constants, especially for small device thicknesses

On the Differences between Dark and Light Ideality Factor in Polymer:Fullerene Solar Cells

Ideality factors are derived from either the slope of the dark current/voltage curve or the light intensity dependence of the open-circuit voltage in solar cells and are often a valuable method to characterize the type of recombination. In the case of polymer:fullerene solar cells, the ideality factors derived by the two methods usually differ substantially. Here we investigate the reasons for the discrepancies by determining both ideality factors differentially as a function of voltage and by comparing them with simulations. We find that both the dark and light ideality factors are sensitive to bulk recombination mechanisms at the internal donor:acceptor interface, as is often assumed in the literature. While the interpretation of the dark ideality factor is difficult due to resistive effects, determining the light ideality factor <i>differentially</i> indicates that the open-circuit voltage of many polymer:fullerene solar cells is limited by surface recombination, which leads to light ideality factors decreasing below one at high voltage

Scoperta e valorizzazione del talento. Per la cittadinanza dell'allievo con disabilità. Discovery and appreciation of talent. For the citizenship of the student with disabilities

In Italia, dal 1977, gli allievi con disabilità certificata frequentano le scuole comuni, ufficialmente fanno parte di una classe, ma poiché seguono un piano educativo individualizzato, per lo più privo di collegamenti con i curricoli degli altri studenti, rischiano una nuova e più subdola esclusione. Come promuovere l’inclusione didattica? Bisogna partire da ciò che vale per l’allievo, da ciò che per lui ha senso; non da ciò che è eccezionale rispetto agli altri, ma da ciò che lui fa bene e gli piace fare bene. Si parte dal talento perché è presente in tutti, non dalla genialità che è appannaggio di pochissimi. Con una attività di ricerca-azione “Scoperta e valorizzazione didattica del talento”, oltre 2500 insegnanti della scuola secondaria hanno cercato il potenziale formativo di sviluppo esistenziale nel ragazzo che essi seguivano nel tirocinio. Con questo articolo si avvia un ampio lavoro di analisi dei dati, con due interrogativi: 1) Chi sono, come e dove si collocano gli studenti disabili oggi? 2) In quali ambiti, situazioni e contesti si manifesta il talento

Fused Dithienogermolodithiophene Low Band Gap Polymers for High-Performance Organic Solar Cells without Processing Additives

We report the synthesis of a novel ladder-type fused ring donor, dithienogermolodithiophene, in which two thieno­[3,2-<i>b</i>]­thiophene units are held coplanar by a bridging dialkyl germanium. Polymerization of this extended monomer with <i>N</i>-octylthienopyrrolodione by Stille polycondensation afforded a polymer, <b>pDTTG-TPD</b>, with an optical band gap of 1.75 eV combined with a high ionization potential. Bulk heterojunction solar cells based upon <b>pDTTG-TPD</b>:PC₇₁BM blends afforded efficiencies up to 7.2% without the need for thermal annealing or processing additives