Performance, Stability and Environmental Impact of Organic Solar Cells: Towards Technology Scale-up

The effects of global climate changes are progressively observable on the environment. Their direct correlation with the CO2 emission coming from the fossil fuel combustion and industrial processes elucidates the urgent need of renewable energies. In this context, organic photovoltaics (OPVs) attracts much attention because of the cost-competitiveness and the new device functionalities over existing solar cells. Indeed, the interesting properties of organic materials open the road to the economically sustainable production of flexible and light-weight devices, which also meet aesthetical requirements as semi-transparency and color-tunability. This Ph.D. thesis focuses on the performance, stability and environmental impact of solution-processed polymer solar cells (PSCs). In a preliminary part, the development of novel materials, the optimization of the processing conditions and a deeper understanding of the device physics elucidate the pathways towards the enhancement of the PSCs efficiency. However, PSCs still present some issues of stability under operation conditions, which slow down the widespread commercialization of this technology. To this end, part of the thesis discusses some specific aspects related to the stability of critical materials/layers of organic solar cells. In particular, the light stability of different active materials and ZnO layers is investigated in order to provide guidelines for the development of advanced materials for PSCs. Then, the impact of the processing conditions on the thermal stability of the resulting photovoltaic devices is studied. In particular, the effect of the replacement of common chlorinated solvents with a “greener” analogous is investigated both in terms of device efficiency and thermal stability. Finally, a contribution to the understanding of state-of-the-art tandem architectures is reported as a perspective for the large scale deployment of highly performing solar cells. The analysis of these crucial aspects of OPVs provides the basis for the development of improved devices heading to the widespread deployment of this technology

2,3-Thienoimide-ended oligothiophenes as ambipolar semiconductors for multifunctional single-layer light-emitting transistors

In view of developing multifunctional OLETs, 2,3-thienoimide-ended oligothiophenes are proposed as ideal candidates to effectively ensure good ambipolar field-effect mobility, self-assembly capability and high luminescence in solid state

Organic Light-Emitting Transistors in a Smart-Integrated System for Plasmonic-Based Sensing

AbstractThe smart integration of multiple devices in a single functional unit is boosting the advent of compact optical sensors for on‐site analysis. Nevertheless, the development of miniaturized and cost‐effective plasmonic sensors is hampered by the strict angular constraints of the detection scheme, which are fulfilled through bulky optical components. Here, an ultracompact system for plasmonic‐sensing is demonstrated by the smart integration of an organic light‐emitting transistor (OLET), an organic photodiode (OPD), and a nanostructured plasmonic grating (NPG). The potential of OLETs, as planar multielectrode devices with inherent micrometer‐wide emission areas, offers the pioneer incorporation of an OPD onto the source electrode to obtain a monolithic photonic module endowed with light‐emitting and light‐detection characteristics at unprecedented lateral proximity of them. This approach enables the exploitation of the angle‐dependent sensing of the NPG in a miniaturized system based on low‐cost components, in which a reflective detection is enabled by the elegant fabrication of the NPG onto the encapsulation glass of the photonic module. The most effective layout of integration is unraveled by an advanced simulation tool, which allows obtaining an optics‐less plasmonic system able to perform a quantitative detection up to 10−2 RIU at a sensor size as low as 0.1 cm3

Improved Performance of Organic Light-Emitting Transistors Enabled by Polyurethane Gate Dielectric

Organic light-emitting transistors (OLETs) are multifunctional optoelectronic devices that combine in a single structure the advantages of organic light emitting diodes (OLEDs) and organic field-effect transistors (OFETs). However, low charge mobility and high threshold voltage are critical hurdles to practical OLETs implementation. This work reports on the improvements obtained by using polyurethane films as dielectric layer material in place of the standard poly(methylmethacrylate) (PMMA) in OLET devices. It was found that polyurethane drastically reduces the number of traps in the device thereby improving electrical and optoelectronic device parameters. In addition, a model was developed to rationalize an anomalous behavior at the pinch-off voltage. Our findings represent a step forward to overcome the limiting factors of OLETs that prevent their use in commercial electronics by providing a simple route for low-bias device operation.Comment: 25 pages, 5 figures, 1 tabl

A new quinoxaline and isoindigo based polymer as donor material for solar cells: Role of ecofriendly processing solvents on the device efficiency and stability

A new semiconducting polymer based on two different electron deficient (quinoxaline and isoindigo) and electron rich (benzodithiophene) moieties is synthesized, characterized and used as donor material for photovoltaic devices. Blade‐coated bulk heterojunction solar cells are fabricated in air by using chlorinated (o‐dichlorobenzene) and nonchlorinated (o‐xylene) solvents for the deposition of the active layer. The use of o‐xylene allows a ∼10% improvement of the device efficiency in comparison to the analogous system processed from o‐dichlorobenzene. In addition, the evolution of the photovoltaic parameters of the resulting devices during thermal stress is monitored and compared, demonstrating a nearly identical resistance against temperature. The reported results not only highlight the promising properties of the new polymer in terms of environmental stability and compatibility with nonhalogenated solvents, but also show an easy and ecofriendly way to further improve the device performance without altering the corresponding thermal stabilit

Impact of environmentally friendly processing on polymer solar cells: Performance, thermal stability and morphological study by imaging techniques

The combination of mass-production compatible coating techniques and environmentally friendly solvents to process bulk heterojunction solar cells represents a key issue to scale up this technology. In this work we demonstrate that using a benchmark polymer HBG-1 blended with PC61BM, the replacement of a common chlorinated processing solvent (orthodichlorobenzene) with a non-chlorinated analogous (o-xylene) not only allows the fabrication of blade-coated bulk heterojunction devices with identical photovoltaic performance, but also determines a great enhancement of the resulting thermal stability. Thermal degradation tests were carried out in inert atmosphere, by keeping the solar cells onto a hot plate at 85\ua0\ub0C and monitoring their OPV performance. In parallel, the morphological changes of the active layers induced by thermal stress are investigated by combining two complementary light-based imaging techniques, laser scanning confocal and photocurrent microscopy, which offer the great advantage to simultaneously study on complete devices the blend morphology and the electrical properties, point-by-point, of the active layer even in regions unlikely accessible (e.g. the active area under the top electrode) using other techniques. As a result, we found that solar cells processed from a non-chlorinated based solvent, in comparison to an analogous reference system, exhibit a different evolution of the resulting BHJ morphology during thermal ageing, in perfect agreement with the corresponding photovoltaic responses

Interplay between Charge Injection, Electron Transport, and Quantum Efficiency in Ambipolar Trilayer Organic Light-Emitting Transistors

The fascinating characteristic of organic light-emitting transistors (OLETs) of being electrical switches with an intrinsic light-emitting capability makes them attractive candidates for a wide variety of applications, ranging from sensors to displays. To date, the OLET ambipolar trilayer heterostructure is the most developed architecture for maximizing device performance. However, a major challenge of trilayer OLETs remains the inverse correlation between external quantum efficiency and brightness under ambipolar conditions. The complex interconnection between electroluminescent and ambipolar charge transport properties, in conjunction with the limited availability of electron transport semiconducting materials, has indeed hampered the disruptive evolution of the OLET technology. Here, an in-depth study of the interplay of the key fundamental features that determine the device performance is reported by exploring electron transport semiconductors with different properties in ambipolar trilayer OLETs. Through the selection of compounds with tailored chemical structures, the relation between intrinsic optoelectronic characteristics of the electron transport semiconductor, energy level alignment within the structure, and morphological features is unraveled. Furthermore, the introduction of a suitable electron injector at the emissive/semiconducting layers interface sheds light into the bidimensional nature of OLETs that is a distinguishing factor of this class of devices with respect to prototypical organic light-emitting diodes.Funding Agencies|European Unions Horizon 2020 Research and Innovation Programme [101016706]</p

Organic Light-Emitting Transistors with Simultaneous Enhancement of Optical Power and External Quantum Efficiency via Conjugated Polar Polymer Interlayers.

Organic light-emitting transistors (OLETs) show the fascinating combination of electrical switching characteristics and light generation capability. However, to ensure an effective device operation, an efficient injection of charges into the emissive layer is required. The introduction of solution-processed conjugated polyelectrolyte (CPE) films at the emissive layer/electrode interface represents a promising strategy to improve the electron injection process by dipole formation. However, their use in optoelectronic devices also involves some limitations because of the ionic nature of CPEs. In this context, neutral conjugated polar polymers (CPPs) represent a valid alternative to CPEs because the conjugated backbones of CPPs are functionalized with polar nonionic side groups, thus avoiding ion-dependent drawbacks. By introducing a layer of polyfluorene-containing phosphonate groups underneath the metal electrodes, we here demonstrate a substantial improvement of the electron injection properties into the OLET-emissive layer and, accordingly, a more than 2-fold increased light power and a 5 times higher external quantum efficiency of p-type OLETs in comparison with reference devices without any interlayer. The great benefit of using a transparent glass substrate allowed to selectively investigate the morphological and photoluminescent characteristics of both CPE- and CPP-buried interlayers within complete OLETs by means of an optical scanning probe technique. This, together with a thorough optoelectronic characterization of the figures of merit of working light-emitting devices, allowed to disclose the origin of the improved optical performance of CPP-based devices as well as the operation mechanisms of the investigated interlayer in the corresponding OLETs

Induced photodegradation of quinoxaline based copolymers for photovoltaic applications

We report here the synthesis and characterization of a series of p-type copolymers, which combine a fluorinated quinoxaline (FQ) acceptor unit either with a differently substituted benzodithiophene (BDT) or an unsubstituted thieno[3,2-b]thiophene (TT). The effect of the structural modifications on the photochemical stability of the resulting films is investigated and then correlated with the photovoltaic performance and lifetime measurements of corresponding photovolatic devices. To this end, we firstly studied the intrinsic stability of each polymer film by monitoring the UV–vis absorption decay, under simulated sunlight, as a function of ageing time. Bulk heterojunction solar cells, based on these polymers as donor materials, were fabricated and tested. Beside the initial values, we monitored the photovoltaic performance during prolonged light soaking in order to evaluate and compare the photostability of more complex systems such as working solar cells

Tuning the Electron-Acceptor Properties of [60]Fullerene by Tailored Functionalization for Application in Bulk Heterojunction Solar Cells

The synthesis of a series of fullerene derivatives designed to act as photoactive acceptor materials in polymer:fullerene photovoltaic blends has been reported. DFT calculations, which provide the LUMO energy values as a function of the fullerene substitution pattern, were employed in the design of these compounds. The functionalizations focused on the type of addition (1,2- or 1,4-addition) and electronic properties of the addends. Cyclic voltammetry determined the LUMO energies to be in good agreement with the calculated values. The photovoltaic efficiencies of the solar cells, composed of poly(3-hexylthiophene) (P3HT):fullerene blends, were measured. EPR spectroscopy was used to characterize the magnetic interactions and symmetry properties of radical anions and the excited triplet states of the derivatives. An explanation of the observed LUMO energy shifts based on the symmetry change in the frontier orbitals of the derivatives has been proposed