Charge Carrier Extraction in Organic Solar Cells Governed by Steady-State Mobilities

Charge transport in organic photovoltaic (OPV) devices is often characterized by steady-state mobilities. However, the suitability of steady-state mobilities to describe charge transport has recently been called into question, and it has been argued that dispersion plays a significant role. In this paper, the importance of the dispersion of charge carrier motion on the performance of organic photovoltaic devices is investigated. An experiment to measure the charge extraction time under realistic operating conditions is set up. This experiment is applied to different blends and shows that extraction time is directly related to the geometrical average of the steady-state mobilities. This demonstrates that under realistic operating conditions the steady-state mobilities govern the charge extraction of OPV and gives a valuable insight in device performance

Relating polymer chemical structure to the stability of polymer::fullerene solar cells

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

1,8-diiodooctane acts as a photo-acid in organic solar cells

The last decade saw myriad new donor polymers, among which benzodithiophene-co-thienothiophene polymers are attractive due to their relatively high power conversion efficiency in bulk heterojunction solar cells. We examine the effect of UV-light on the stability of these polymers. The relationship between the polymer chemical structure and the UV-stability of the cells is explored on the one hand, and on the other hand, the effect of additives on their UV-stability: 1,8-diiodooctane against 1-chloronaphthalene in the cells and 1,8-octanedithiol in solution. For example, PBDTTT-E with 18% efficiency loss is more stable than PBDTTT-ET with 36% loss throughout the exposure. While 1,8-diiodooctane acts as photo-acid and leads to accelerated degradation of the solar cells, 1-chloronaphthalene does not. Acidity is known to be detrimental to the efficiency and stability of organic solar cells. The degradation is initiated upon UV-irradiation by the cleavage of the side chains, resulting in more electron traps and by the formation of iodine, dissolved HI and carbon-centered radicals from 1,8-diiodooctane as revealed by 1H NMR spectrum. The 1,8-octanedithiol spectra do not show such species. Finally, the mechanisms behind the effect of 1,8-diiodooctane are explained, paving the way for the design of new, efficient as well as stable materials and additives

Can Ferroelectricity Improve Organic Solar Cells?

Blends of semiconducting (SC) and ferroelectric (FE) polymers have been proposed for applications in resistive memories and organic photovoltaics (OPV). For OPV, the rationale is that the local electric field associated with the dipoles in a blend could aid exciton dissociation, thus improving power conversion efficiency. However, FE polymers either require solvents or processing steps that are incompatible with those required for SC polymers. To overcome this limitation, SC (poly(3-hexylthiophene)) and FE (poly(vinylidene fluoride-trifluoroethylene)) components are incorporated into a block copolymer and thus a path to a facile fabrication of smooth thin films from suitably chosen solvents is achieved. In this work, the photophysical properties and device performance of organic solar cells containing the aforementioned block copolymer consisting of poly(vinylidene fluoride-trifluoroethylene): P(VDF-TrFE), poly(3-hexylthiophene): P3HT and the electron acceptor phenyl-C-61-butyric acid methyl ester: [60]PCBM are explored. A decrease in photovoltaic performance is observed in blends of the copolymer with P3HT:[60]PCBM, which is attributed to a less favorable nanomorphology upon addition of the copolymer. The role of lithium fluoride (the cathode modification layer) is also clarified in devices containing the copolymer, and it is demonstrated that ferroelectric compensation prevents the ferroelectricity of the copolymer from improving photovoltaic performance in SC-FE blends

Efficient polymer light-emitting diode with air-stable aluminum cathode

The fast degradation of polymer light-emitting diodes (PLEDs) in ambient conditions is primarily due to the oxidation of highly reactive metals, such as barium or calcium, which are used as cathode materials. Here, we report the fabrication of PLEDs using an air-stable partially oxidized aluminum (AlOx) cathode. Usually, the high work function of aluminum (4.2 eV) imposes a high barrier for injecting electrons into the lowest unoccupied molecular orbital (LUMO) of the emissive polymer (2.9 eV below the vacuum level). By partially oxidizing aluminum, its work function is decreased, but not sufficiently low for efficient electron injection. Efficient injection is obtained by inserting an electron transport layer of poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT), which has its LUMO at 3.3 eV below vacuum, between the AlOx cathode and the emissive polymer. The intermediate F8BT layer not only serves as a hole-blocking layer but also provides an energetic staircase for electron injection from AlOx into the emissive layer. PLEDs with an AlOx cathode and F8BT interlayer exhibit a doubling of the efficiency as compared to conventional Ba/Al PLEDs, and still operate even after being kept in ambient atmosphere for one month without encapsulation

Improved photostability in ternary blend organic solar cells:The role of [70]PCBM

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

Reaching a Double-Digit Dielectric Constant with Fullerene Derivatives

The dielectric constant (ϵr) of organic semiconductors is a key material parameter for improving device performance in the field of organic electronics. However, the effect of the dielectric constant on the electronic and optoelectronic properties of materials remains unclear due to the scarcity of known organic semiconductors with an ϵr value higher than 6. Herein, the optical and electronic properties of a homologous series of fullerene derivatives with high ϵr are studied. The low frequency (<106 Hz) ϵr is extracted from the capacitance measured using impedance spectroscopy, and the effect of length (n) and geometrical arrangement of the polar ethylene glycol (EG) side chains is investigated. The ϵr is found to correlate with length for the symmetrical Bingel adducts, whereas for the unsymmetrical branched-EG chain adducts there is no significant difference between the two EG chain lengths. For BTrEG-2, the ϵr reaches 10, which is an unprecedented value in monoadduct fullerene derivatives. These materials open up new possibilities of studying the effect of ϵr in organic electronic devices such as organic photovoltaics, organic thermoelectrics, and organic field-effect transistors

Conjugated Polyions Enable Organic Photovoltaics Processed from Green Solvents

This paper describes the design, synthesis, and optical and electronic properties of two conjugated polymers CPIZ-B and CPIZ-T that incorporate closed-shell cations into their conjugated backbones, balanced by anionic pendant groups. The zwitterionic nature of the polymers renders them soluble in and processable from polar, protic solvents to form semiconducting films that are not doped. These unique properties are confirmed by absorption and electron paramagnetic resonance spectroscopy. The energies of the unoccupied states respond to the tritylium moieties in the conjugated backbone, while the occupied states respond to the electron-donating ability of the uncharged, aromatic units in the backbone. Films cast from 80:20 HCOOH/H2O by volume show good electron mobilities, enabling a photovoltaic effect in proof-of-concept, bilayer solar cells

Effects of the Reduction and/or Fluorination of the TT‐Units in BDT‐TT Polymers on the Photostability of Polymer:Fullerene Solar Cells

Polymer solar cells have a promising future for applications in our day to day usage of energy in small appliances and portable devices. However, their performance in terms of efficiency is limited by a number of factors, among which is their low open circuit voltage (Voc). It is, therefore, understandable that much effort is channeled by the scientific community in improving the Voc. One way to achieve this goal is the development of novel materials, for example, polymers, through chemical structure modification. Typical examples are addition (chlorination, fluorination, or sulfonylation) and/or reduction (from alkyl‐ester to ketone substituents) mechanisms. This paper reports on the study of the effect of these structural changes for Voc enhancement on the performance of the polymers in polymer:fullerene solar cells. In particular, it looks at seven polymers of the polybenzodithiophene‐thienothiophene family, identifying the structural changes in the thienothiophene units or their moieties as a function of Voc behavior in relation to their UV‐stability. The findings reveal that the fluorination of the TT‐units or having alkyl‐ester groups as substituents on the TT‐units is bad for photostability. However, when these alkyl‐ester groups are reduced into ketone substituents, the photostability behavior improves

Effect of the layer thickness on the efficiency enhancement in bilayer polymer light-emitting diodes

To eliminate quenching of excitons at the metallic cathode of a polymer light-emitting diode (PLED) the emitting layer is separated from the cathode by a hole-blocking layer (HBL). We investigate a wide range of single-layer and bilayer PLEDs with different thicknesses consisting of a poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) emitting layer and a 20 nm poly(9,9'-dioctylfluorene) (PFO) HBL. The highest efficiency for both single-layer and bilayer devices is achieved when the total polymer layer thickness is similar to 90 nm. As a result, addition of an HBL to reduce cathode quenching is only effective when the luminescence enhancement due to microcavity effects in PLEDs is restored. The relative efficiency enhancement in bilayer devices as compared to single-layer devices varies from 283% for a 30 nm active layer to 20% for a 250 nm device. (C) 2016 Elsevier B.V. All rights reserved