Understanding the effect of unintentional doping on transport optimization and analysis in efficient organic bulk-heterojunction solar cells

In this paper, we provide experimental evidence of the effects of unintentional p-type doping on the performance and the apparent recombination dynamics of bulk-heterojunction solar cells. By supporting these experimental observations with drift-diffusion simulations on two batches of the same efficient polymer-fullerene solar cells with substantially different doping levels and at different thicknesses, we investigate the way the presence of doping affects the interpretation of optoelectronic measurements of recombination and charge transport in organic solar cells. We also present experimental evidence on how unintentional doping can lead to excessively high apparent reaction orders. Our work suggests first that the knowledge of the level of dopants is essential in the studies of recombination dynamics and carrier transport and that unintentional doping levels need to be reduced below approximately 7 × 1015 cm-3 for full optimization around the second interference maximum of highly efficient polymer-fullerene solar cells.F. D. and J. R. D. are thankful of the support from the EPSRC APEX Grant No. EP/H040218/2 and SPECIFIC Grant No. EP/1019278. T. K. acknowledges funding by an Imperial College Junior Research Fellowship. We are grateful to the Ministerio de Economa y Competitividad for funding through the project PHOTOCOMB, Reference No. MAT2012-37776.Peer Reviewe

Understanding the Effect of Unintentional Doping on Transport Optimization and Analysis in Efficient Organic Bulk-Heterojunction Solar Cells

In this paper, we provide experimental evidence of the effects of unintentional p-type doping on the performance and the apparent recombination dynamics of bulk-heterojunction solar cells. By supporting these experimental observations with drift-diffusion simulations on two batches of the same efficient polymer-fullerene solar cells with substantially different doping levels and at different thicknesses, we investigate the way the presence of doping affects the interpretation of optoelectronic measurements of recombination and charge transport in organic solar cells. We also present experimental evidence on how unintentional doping can lead to excessively high apparent reaction orders. Our work suggests first that the knowledge of the level of dopants is essential in the studies of recombination dynamics and carrier transport and that unintentional doping levels need to be reduced below approximately 7×1015  cm−3 for full optimization around the second interference maximum of highly efficient polymer-fullerene solar cells

The effect of thiadiazole out-backbone displacement in indacenodithiophene semiconductor polymers †

We describe the synthesis and characterisation of two new polymers consisting of an electron-rich backbone containing indacenodithiophene (IDT) and dithiophene (DT) with the electron-poor units benzothiadiazole (BT) and benzopyrazolothiadiazole (BPT) fused on top of DT. The effect of this substitution has been studied and discussed by optical, electrochemical and computational means. Despite having very similar molecular distribution as well as thermal and electrochemical properties, the addition of the stronger electron-withdrawing BPT unit leads to a substantial change on the absorption properties by promoting the intramolecular charge transfer (ICT) band alongside the p-p*. Furthermore, we also report organic field effect transistor and solar cells device results, giving hole mobilities of 0.07 cm 2 V À1 s À1 with low threshold voltage (<10 V) and power conversion efficiencies of up to 2.2%

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

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

Isostructural, Deeper Highest Occupied Molecular Orbital Analogues of Poly(3-hexylthiophene) for High-Open Circuit Voltage Organic Solar Cells

We present the synthesis and characterization of two novel thiazole-containing conjugated polymers (<b>PTTTz</b> and <b>PTTz</b>) that are isostructural to poly­(3-hexylthiophene) (P3HT). The novel materials demonstrate optical and morphological properties almost identical to those of P3HT but with HOMO and LUMO levels that are up to 0.45 eV deeper. An intramolecular planarizing nitrogen–sulfur nonbonding interaction is observed, and its magnitude and origin are discussed. Both materials demonstrate significantly greater open circuit voltages than P3HT in bulk heterojunction solar cells. <b>PTTTz</b> is shown to be an extremely versatile donor polymer that can be used with a wide variety of fullerene acceptors with device efficiencies of up to 4.5%. It is anticipated that this material could be used as a high-open circuit voltage alternative to P3HT in organic solar cells

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