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

Competition between charge collection and non-geminate recombination in bulk heterojunction solar cells

This thesis is concerned with quantification of non-geminate recombination losses in organic bulk heterojunction solar cells. After description of the context of this work, the theoretical background and the methodology employed are presented. In this thesis, many different polymer:fullerene systems are investigated. In the next chapter, we show that the study of non-geminate losses using charge extraction/transient photovoltage analysis can be applied to many different systems away from P3HT/P3HS blends. We see to what extent ideality factors can give a more precise description of the exact recombination mechanism. Then, the change of optoelectronic properties of a high performance polymer:fullerene blend upon a blend ratio perturbation is investigated. The resulting shifts in energetics and dynamics of the blends are quantified. A quantitative agreement between two methods (charge extraction and electroluminescence) probing the shifts in the energetics at the heterojunction is presented. In the next two chapters, two limits of the common vision of polymer: fullerene systems are explored by combining experiment and 1D drift-diffusion modelling. First, the impact of the variations of the spatial distribution of carriers on the apparent reaction order is experimentally investigated. The study reconciles the apparent contradictions currently in the literature regarding the meaning of high reaction orders. In the following chapter, the often underestimated effects of unintentional doping in polymer blends are addressed experimentally. In particular, its effect on device optimisation, understanding of carrier collection and recombination dynamics are explored. This study suggests that many donor/acceptor blends are not, contrary to common belief, intrinsic semiconductors. Finally, an analysis of the Langevin and non-Langevin behaviour of some efficient systems is presented. The different interpretations of regular observations disproving non-geminate recombination following the Langevin type mechanism are reviewed. We suggest the ratio ’recombination over collision’ is often overlooked and question the underlying assumption that it should be unity.Open Acces

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

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

Electron Collection as a Limit to Polymer:PCBM Solar Cell Efficiency: Effect of Blend Microstructure on Carrier Mobility and Device Performance in PTB7:PCBM

The poor photovoltaic performance of state-of-the-art blends of poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl] (PTB7) and [6,6]-phenyl-C61-butyric acid (PCBM) at large active layer thicknesses is studied using space-charge-limited current mobility and photovoltaic device measurements. The poor performance is found to result from relatively low electron mobility. This is attributed to the low tendency of PTB7 to aggregate, which reduces the ability of the fullerene to form a connected network. Increasing the PCBM content 60–80 wt% increases electron mobility and accordingly improves performance for thicker devices, resulting in a fill factor (FF) close to 0.6 at 300 nm. The result confirms that by improving only the connectivity of the fullerene phase, efficient electron and hole collection is possible for 300 nm-thick PTB7:PCBM devices. Furthermore, it is shown that solvent additive 1,8-diiodooctane (DIO), used in the highest efficiency PTB7:PCBM devices, does not improve the thickness dependence and, accordingly, does not lead to an increase in either hole or electron mobility or in the carrier lifetime. A key challenge for researchers is therefore to develop new methods to ensure connectivity in the fullerene phase in blends without relying on either a large excess of fullerene or strong aggregation of the polymer