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Conjugated polymers incorporating glycolated side chains have been widely used for those organic electronic devices that use the properties of the doped state, such as organic thermoelectrics (OTEs) and organic electrochemical transistors (OECTs). This work pioneers the study of adjusting the molecular weight of such conjugated polymers for optimizing the electronic transport of both OTE and OECT devices. The example conjugated polymer P-3O has a naphthalene-alt-bithiazole-based backbone bearing polar side chains. The molecular weight of P-3O was tuned from Mw = 37 kDa to Mw = 81 kDa by controlling the polymerization conditions. The molecular weight strongly influenced the crystalline domain size and π-stacking paracrystallinity, while the interaction between polar side chains and the dopant molecules largely dictates the doping. This finding seems simple but is significant as it enables us to individually control free charge density and mobility in the doped state. As such, an appropriate molecular weight (Mw = 68 kDa) promotes the formation of broad charge transport pathways, leading to the best thermoelectric performance (σ ≈ 7 S cm−1 and PF ≈ 25 μW m−1 K−2) and the highest μC* ≈ 27 F cm−1 V−1 s−1 for OECT devices. This study proves the strength of regulating molecular weight to enhance electronic transport in extrinsically-doped conjugated polymers. [ABSTRACT FROM AUTHOR]

Doping to enhance the electrical conductivity of organic semiconductors is not without its challenges: The efficacy of this process depends on many factors and it is not always clear how to remedy poor doping. In the case of doping with salts, one of the possible causes of poor doping is a limited yield of integer charge transfer resulting in the presence of both cations and anions in the film. The charge of such ions can severely limit the electrical conductivity, but their presence is not easily determined. Here we introduce a set of simple conductivity measurements to determine whether poor doping in the case where the dopant is a salt is due to limited integer charge transfer. By tracking how the conductivity changes over time when applying a bias voltage for an extended amount of time we can pinpoint whether unwanted ions are present in the film. Firstly, we introduce the principle of this approach by performing numerical simulations that include the movement of ions. We show that the conductivity can increase or decrease depending on the type of ions present in the film. Next, we show that the movement of these dopant ions causes a build-up of space-charge, which makes the current–voltage characteristic non-linear. Next, we illustrate how this approach may be used in practice by doping a fullerene derivative with a series of organic salts. We thus provide a tool to make the optimization of doping more rational. [ABSTRACT FROM AUTHOR]

Polymer solar cells are potentially key contributors to the next-generation organic photovoltaics for sustainable green sources of energy. In the past few years, ternary organic solar cells have emerged with promising characteristics. They have proven to yield high efficiency at about 15% for single junction donor:acceptor (D:A) solar cells. However, the low stability of organic solar cells is a hindrance to the commercialisation of this technology, and thus, needs more attention. Here, we show that with the right ratio of D : A1 : A2, ternary blend solar cells can be more efficient and more photostable than their D:A binary blend solar cells. We add [70]PCBM to PBDB-T:ITIC and PTB7-Th:ITIC binary blend solar cells in various ratios to fabricate ternary solar cells. The ternary solar cells outperform all binary cells in terms of efficiency and photostability with only a 10% average loss in efficiency under continuous illumination irrespective of the device structure. We identify changes in the molecular structure of the active layer blends as the main reason behind the observed degradation behaviour of the solar cells. The ternary blends are the most resilient to photo-induced molecular structural changes. This finding suggests that ternary organic solar cells could be a way to achieve photostable devices. [ABSTRACT FROM AUTHOR]

This paper describes a promising n-type doping system with high performance for thermoelectric applications. By introducing the polar triethylene glycol (TEG) side chain onto both fullerene host (PTEG-1) and dopant (TEG-DMBI) materials, the TEG-DMBI doped PTEG-1 films obtained through solution processing provide a better miscibility compared with films doped with commercially available N-DMBI (bearing a dimethylamino group instead of TEG), as determined by phase imaging AFM (atomic force microscopy) measurements and coarse-grain molecular dynamics simulations, leading to high doping efficiency up to 18% at 20 mol% doping concentration and thus high carrier density and mobility, which are critical to the electrical conductivity. Therefore a record power factor of 19.1 μW m−1 K−2 is obtained with an electrical conductivity of 1.81 S cm−1, one of the highest values reported for solution processable fullerene derivatives as n-type organic materials for thermoelectric applications to date. [ABSTRACT FROM AUTHOR]

The design of novel polymers has brought more attention to bulk heterojunction polymer:fullerene solar cells in the past decade. A typical example is the synthesis, through chemical structure engineering, of the benzodithiophene-co-thieno[3,4-b]thiophene (BDT-TT) polymers leading to power conversion efficiency of over 10%. In this work, we study the stability for a set of PBDT-TT polymers. We conduct a systematic UV-degradation study on the solar cells. Most importantly, the paper shows clearly the effect of polymer chemical structure on the UV-degradation pathway of the solar cells. We find that based on the polymer chemical structure, solar cells of polymers with alkoxy side chains are more stable (＜20% loss in PCE) than those with alkylthienyl side chains (∼48% loss in PCE) over the period of study. These findings pave the way for new materials that yield efficient as well as stable organic solar cells. [ABSTRACT FROM AUTHOR]

We studied three n-type polymers of the naphthalenediimide-bithiophene family as electron extraction layers (EELs) in hybrid perovskite solar cells. The recombination mechanism in these devices is found to be heavily influenced by the EEL transport properties. The maximum efficiency of the devices using the n-type polymers EELs did not exceed substantially that of the devices using PC60BM (about 11%), while a substantial improvement in their ambient stability (87% of the initial value after 270 minutes) compared to that using PC60BM (3.5% of the initial value after 270 minutes) was detected. [ABSTRACT FROM AUTHOR]

The power conversion efficiency of photovoltaic cells based on the use of hybrid halide perovskites, CH3NH3PbX3 (X = Cl, Br, I), now exceeds 20%. Recently, it was suggested that this high performance originates from the presence of ferroelectricity in the perovskite, which is hypothesized to lower charge recombination in the device. Here, we investigate and quantify the influence of mesoscale ferroelectric polarization on the device performance of perovskite solar cells. We implement a 3D drift diffusion model to describe the solar cell operation. To account for the mesoscale ferroelectricity, we incorporate domains defined by polarization strength, P, in 3D space, forming different polarization landscapes or microstructures. Study of microstructures with highly-ordered polarized domains shows that charge transport and recombination in the solar cell depends significantly on the polarization landscape viz. the orientation of domain boundaries and the size of domains. In the case of the microstructure with random correlated polarization, a realistic scenario, we find indication of the existence of channels for efficient charge transport in the device which leads to lowering of charge recombination, as evidenced by the high fill factor (FF). However, the high open-circuit voltage (VOC), which is typical of high performance perovskite solar cells, is unlikely to be explained by the presence of ferroelectric polarization in the perovskite. [ABSTRACT FROM AUTHOR]

Solar cells are generally optimised for operation under AM1.5 100 mW cm−2 conditions. This is also typically done for polymer solar cells. However, one of the entry markets for this emerging technology is portable electronics. For this market, the spectral shape and intensity of typical illumination conditions deviate considerably from the standard test conditions (AM1.5, 100 mW cm−2, at 25 °C). The performance of polymer solar cells is strongly dependent on the intensity and spectral shape of the light source. For this reason the cells should be optimised for the specific application. Here a theoretical model is presented that describes the light intensity dependence of P3HT:[C60]PCBM solar cells. It is based on the Shockley diode equation, combined with a metal–insulator–metal model. In this way the observed light intensity dependence of P3HT:[C60]PCBM solar cells can be described using a 1-diode model, allowing fast optimization of polymer solar cells and module design. [ABSTRACT FROM AUTHOR]

The invention of new organic materials with high dielectric constants is of extreme importance for the development of organic-based devices such as organic solar cells. We report on a synthetic way to increase the dielectric constant of fullerene derivatives. It is demonstrated that introducing triethylene glycol monoethyl ether (teg) side chains into fulleropyrrolidines increases the dielectric constant by ~46 percent without devaluation of optical properties, electron mobility and the energy level of the compound. [ABSTRACT FROM AUTHOR]