Charge carrier mobility of the organic photovoltaic materials PTB7 and PC71BM and its influence on device performance

The mobility is an important parameter for organic solar cell materials as it influences the charge extraction and recombination dynamics. In this study, the time of flight technique is used to investigate the charge mobility of the important organic photovoltaic materials PC71BM, PTB7 and their blend. The electron mobility of PC71BM is in the region of 1 × 10−3 cm2/Vs for the neat fullerene film, and has a positive electric field dependence. The hole mobility of PTB7 is 1 × 10−3 cm2/Vs for the neat film and 2 × 10−4 cm2/Vs for their blend. The hole mobility of the blend reduces by a factor of a thousand when the sample is cooled from room temperature to 77 K. This finding is compared with the device performance of efficient PTB7:PC71BM solar cells for varying temperature. At 77 K the solar cell efficiency halved, due to losses in fill factor and short circuit current. Bimolecular and trap-assisted recombination increase at low mobility (low temperature) conditions, whereas at high mobility conditions the open circuit voltage reduces. The power conversion efficiency as a function of temperature has a maximum between 260 K and 295 K, revealing an optimized mobility at room temperature.PostprintPublisher PDFPeer reviewe

Photocarrier lifetime and recombination losses in photovoltaic systems

[Extract] To the Editor — The photocarrier lifetime is commonly employed to describe the performance of photovoltaic and photodetecting devices fabricated from crystalline or non-crystalline materials. Numerous studies have obtained photocarrier lifetimes from transient decay signals in a multitude of non-crystalline systems, such as dye-sensitized solar cells, organic donor–acceptor blends, perovskites, nanoparticles, quantum dots and nanowires

The effect of polymer solar cell degradation on charge carrier dynamics in benzodithiophene-diketopyrrolopyrrole polymers

Degradation in polymer based organic solar cells still remains poorly understood despite prominent research efforts in recent years. In this work, the impact of photostability and documented morphological evolution of a couple of poly(benzodithiophene-diketopyrrolopyrrole) polymers in their blends with phenyl-C-61-butyric acid methyl ester (PCBM) are investigated by a combination of charge extraction techniques. Photo-induced charge carrier extraction by linearly increasing voltage (photo-CELIV) and integral-time of flight (TOF) methods are employed to determine charge carrier mobility and recombination at various stages of photo-degradation in the absence of air. We report the progressive formation of deeper trap states as crucial outcome of photo-degradation within these blends as deduced from our measurements. (C) 2016 Elsevier B.V. All rights reserved

Effect of the Nature of the Core on the Properties of the Star-Shaped Compounds Containing Bicarbazolyl Moieties

Three star-shaped compounds containing bicarbazolyl side arms and various core moieties are described. Bicarbazolyl moiety proves to be a stronger donor than single carbazolyl group, providing lower ionization potential and superior thermal and electrochemical stability. The influence of the central core, that is, 2,4,6-triphenyl-1,3,5-triazine, 1,3,5-triphenylbenzene, and 9-phenylcarbazole fragments, on the properties of the compounds is investigated and supported by DFT calculations. The dependence of photophysical properties on the rigidity and polarity of the media is discussed. Owing to the differences in molecular architecture, tuning of singlet and triplet energy values can be achieved. The synthesized compounds showed good hole-transporting properties with the mobility values exceeding 10^–3 cm² V^–1 s^–1

Doping-induced screening of the built-in-field in organic solar cells: effect on charge transport and recombination

We report on the effects of screening of the electric field by doping-induced mobile charges on photocurrent collection in operational organic solar cells. Charge transport and recombination were studied using double injection (DI) and charge extraction by linearly increasing voltage (CELIV) transient techniques in bulk-heterojunction solar cells made from acceptor-donor blends of poly(3-n-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PC60BM). It is shown that the screening of the built-in field in operational solar cells can be controlled by an external voltage while the influence on charge transport and recombination is measured. An analytical theory to extract the bimolecular recombination coefficient as a function of electric field from the injection current is also reported. The results demonstrate that the suppressed (non-Langevin) bimolecular recombination rate and charge collection are not strongly affected by native doping levels in this materials combination. Hence, it is not necessary to reduce the level of doping further to improve the device performance of P3HT-based solar cells

Effect of Ethynyl Linkages on the Properties of the Derivatives of Triphenylamine and 1,8-Naphthalimide

Three new derivatives of triphenylamine containing different number of 1,8-naphthalimide moieties linked via ethynyl linkages were obtained by a Sonogashira cross-coupling reaction and their properties were studied. The glass-forming derivatives showed glass transition temperatures ranging from 73 to 96 °C. They showed high thermal stabilities with the temperatures of the onset of thermal degradation ranging from 421 to 462 °C. Fluorescence quantum yields of the dilute solutions in nonpolar solvents of the compounds ranged from 0.063 to 0.94 while those of the solid films were in the range of 0.011–0.25. Small singlet–triplet gap was predicted by DFT calculations, which was confirmed by the similarity between the prompt and the delayed fluorescence energies. Cyclic voltammetry measurements revealed close values of the solid state ionization potentials ranging from 5.48 to 5.61 eV and electron affinities ranging from −3.29 to −3.16 eV. The layer of 4,4′-((di­(<i>N</i>-(2-ethylhexyl)-1,8-naphthalimide-4-yl)­phenyl)­ethynyl)­benzenamine exhibited effective charge-transport with hole drift mobilities exceeding 10^–2 cm²/V·s. Good intrinsic hole transport parameters were predicted by theoretical estimations. Both efficient fluorescence and high hole mobilities are found to be positively influenced by the presence of ethynyl bridges, (i) allowing for molecular planarization and high hole mobility and (ii) inducing large space separation between the HOMO and LUMO localizations, with direct impact in singlet–triplet splitting

Photocarrier lifetime and recombination losses in photovoltaic systems

[Extract] To the Editor — The photocarrier lifetime is commonly employed to describe the performance of photovoltaic and photodetecting devices fabricated from crystalline or non-crystalline materials. Numerous studies have obtained photocarrier lifetimes from transient decay signals in a multitude of non-crystalline systems, such as dye-sensitized solar cells, organic donor–acceptor blends, perovskites, nanoparticles, quantum dots and nanowires

Main-chain alternating fullerene and dye oligomers for organic photovoltaics

This work demonstrates for the first time that it is possible to prepare alternating oligomers, containing both dyes and fullerenes in repeating structures, that act as electron acceptors in bulk heterojunction devices. A sterically controlled azomethine ylide cycloaddition polymerization is employed with either C-60 or phenyl-C-61-butyric acid methyl ester (PCBM) and the dye diketopyrrolopyrrole (DPP). The former results in low molecular weights of around 5600 g mol(-1), whereas the latter, PCBM, enables the formation of more soluble chains with higher molecular weights of ca 11 200 g mol(-1). Remarkably, cyclic voltammetry shows that the incorporation of PCBM into the main-chain raises the lowest unoccupied molecular orbital by ca 380 meV due to the in-chain bis-additions. The observation of the complete quenching of DPP fluorescence by the fullerene moiety, combined with computer modelling studies, indicates both electron and energy transfers between intra-chain moieties. Proof-of-concept devices show low efficiencies most likely due to as-yet-unoptimized preparation and structures, but hint at the possibilities of these novel bi-functionalized, in-chain fullerenes due to their high Voc of 0.89 V with an example low-bandgap polymer, KP115, and reasonable charge mobilities of ca 1x10(-4) cm(2) V-1 s(-1),making this new class of materials of strong interest for applications. Furthermore, their good thermal stability to above 300 degrees C and their stabilization of photovoltaic devices against thermal degradation confirm that this new pathway to a wide range of dye/fullerene structures is extremely promising. (C) 2016 Society of Chemical Industr