Organic-based Solar Cells Research Articles

Engineering promising A-π-D type molecules for efficient organic-based material solar cells

ABSTRACT Within this work, to promote the efficiency of organic-based solar cells, a series of novel A-π-D type small molecules were scrutinised. The acceptors which we designed had a moiety of N, N-dimethylaniline as the donor and catechol moiety as the acceptor linked through various conjugated π-linkers. We performed DFT (B3LYP) as well as TD-DFT (CAM-B3LYP) computations using 6-31G (d,p) for scrutinising the impact of various π-linkers upon optoelectronic characteristics, stability, and rate of charge transport. In comparison with the reference molecule, various π-linkers led to a smaller HOMO–LUMO energy gap. Compared to the reference molecule, there was a considerable red shift in the molecules under study (A1–A4). Therefore, based on the analysis of energy level, A4 and A3 were shown to be promising non-fullerene acceptors with the designed donors for applications in solar cells. It is hoped that the current study would provide theoretical insights into the design and amplification of optoelectronic characteristics of suggested frameworks on a grand scale in comparison with the reference molecules.

Charge dissociation in organic solar cells—from Onsager and Frenkel to modern models

Organic-based solar cells have developed for the last three decades. Moving forward generally requires the assistance of useful models that are adapted to currently used materials and device architectures. The least understood part of the charge generation is the first step of the exciton dissociation, and new or refined models are being suggested. However, many of today's questions have been asked before, going back almost an entire century. We have gone to the 1930s and attempted to critically review significant contributions on equal footing. We find that Onsager's and Frenkel's models have a similar foundation but were developed to suit very different materials (ions in solutions vs electrons in semiconductors). The contribution by Braun or the Onsager–Braun model can be considered wrong, yet it was instrumental for the field's development. The community practically ignores one of the most promising models (Arkhipov–Baranovskii). Hot exciton dissociation has many faces due to “hot” being a relative term and/or the heat being stored in different ways (electronic, vibronic, etc.). Entropy considerations are instrumental in simplifying the picture, yet they add no physics compared to the full-3D models. We hope that by emphasizing the physical picture of the various models and the underlying assumptions, one could use them as a stepping stone to the next generation models.

From simple molecules to nanotubes. Reliable predictions of ionization potentials from the ΔMP2-SCS methods

The vertical ionization potentials for systems of various sizes, ranging from simple molecules, DNA/RNA bases, donor and acceptor organic molecular systems as well as nanotubes are calculated using the ΔMP2-SCS family of methods [Śmiga et al 2018 J. Chem. Theory Comput. 14 4780–90]. We have shown that for all investigated cases, the ΔMP2-SCS methods, being almost cost free single-step post-Hartree–Fock calculation, provide very accurate vertical ionization potentials comparable in quality with state-of-art outer valence Green’s function methods. Moreover, we show that a combination of the ΔMP2-SCS methods with the resolution of identity technique is effective and reliable an alternative to the semi-empirical density functional theory methods and Green’s function-based calculations of ionization potentials for large molecular systems such as silicon-based or organic-based solar cells, for which the IP-EOM-CCSD calculations are too expensive for routine calculations.

Methodology of the first combined in-flight and ex situ stability assessment of organic-based solar cells for space applications

Abstract

On the Growth, Percolation and Wetting of Silver Thin Films Grown by Atmospheric-Plasma Enhanced Spatial Atomic Layer Deposition

Silver (Ag) thin-films have attracted interest for applications in sensing1, light management2, imaging beyond the diffraction limit3, interlayer in tri-layer transparent conductors (TCs)4 and even as possible replacement for Cu interconnects5. For tri-layer TCs, a narrow optimum exists between optical transparency and electrical conductivity6. The former decreases exponentially whereas the latter increases with thickness. The ability to precisely control both thickness and electrical percolation is crucial in tri-layer TCs, for which ultra-thin and uniform, yet conductive Ag layers are required over large areas. Here, Atomic Layer Deposition (ALD) can fulfill the requirements for precise thickness control and conformal step coverage over large area substrates. In this work we studied the deposition of Ag by spatial-ALD on molybdenum (Mo) layers serving as a growth model substrate. Mo-layers were specifically selected for testing the suitability of spatial-ALD to fabricate MoO3/Ag/MoO3 tri-layer TCs for organic-based solar cells. Since Mo-films are known to have high surface energy (3 Jm-2), Ag is expected to wet a Mo surface. However, the presence of native MoO3 on the surface is likely to reduce the surface energy resulting in poor wetting. Therefore, an H2/N2 plasma pre-treatment was introduced before Ag deposition in order to reduce the native MoO3 and promote coalescence. Ultrathin Ag films were deposited at 120 °C using Ag(fod)(PEt3) as the precursor and an H2/N2 plasma generated by a surface dielectric barrier discharge (SDBD) source as the co-reactant. Ag(fod)(PEt3) was evaporated into the deposition chamber using Ar as carrier gas flowing at 100 sccm, at the same temperature as that of the reactor. An H2/N2 co-reactant gas mixture was composed from 800 sccm H2 and 600 sccm N2. Silicon substrates with sputter-deposited Mo were used as a model substrate for Ag growth, and compared with the growth on bare silicon substrates. Top view and cross-sectional SEM images of the Ag layers show that a more compact and closed Ag layer was achieved on Mo upon introduction of the plasma pre-treatment (Fig. 1). Spectroscopic ellipsometry measurements demonstrate that the use of an H2/N2 plasma pre-treatment significantly enhances the wetting, and therefore, the coalescence of Ag owing to the high surface energy of the Mo-layers (Fig. 2). Furthermore, the better wetting and interfacial quality of the Ag deposited on plasma pre-treated Mo, compared to Ag on Si was probed by post-growth rapid thermal annealing experiments and XRD stress measurements. It was found that at 450 °C the Ag films de-wetted from Si, resulting in discontinuous films whereas the films deposited on plasma pre-treated Mo remained unchanged. Since both Ag films show similar stress content, the de-wetting most likely originates from interfacial properties rather than stress phenomena. We can therefore conclude that a plasma pre-treatment of Mo layers promotes the Ag wetting. This leads to earlier film coalescence compared to that of Ag films grown on Si, as demonstrated by both morphological and optical inspections of the Ag thin films. Thus, upon plasma pre-treatment ALD deposited MoO3 layers can act as wetting layers for subsequent Ag spatial-ALD in tri-layer TCs. This can open up alternative pathways for the fabrication of TCs using ALD. Figure 1

A Quasi-Solid State DSSC with 10.1% Efficiency through Molecular Design of the Charge-Separation and -Transport.

Organic-based solar cells potentially offer a photovoltaic module with low production costs and low hazard risk of the components. We report organic dye-sensitized solar cells, fabricated with molecular designed indoline dyes in conjunction with highly reactive but robust nitroxide radical molecules as redox mediator in a quasi-solid gel form of the electrolyte. The cells achieve conversion efficiencies of 10.1% at 1 sun, and maintain the output performance even under interior lighting. The indoline dyes, customized by introducing long alkyl chains, specifically interact with the radical mediator to suppress a charge-recombination process at the dye interface. The radical mediator also facilitates the charge-transport with remarkably high electron self-exchange rate even in the quasi-solid state electrolyte to lead to a high fill factor.

Investigation of the Use of Stereo-Pair Data Sets in Electron Tomography Characterization of Organic-Based Solar Cells

Investigation of the Use of Stereo-Pair Data Sets in Electron Tomography Characterization of Organic-Based Solar Cells

The effect of ITO/Mo/MoO3 anode multilayer film on efficient hole extraction in MEH–PPV/ZnO NP hybrid solar cells

Acidic PEDOT:PSS anode buffer layers (ABLs) are widely used for improving efficiency in organic-based solar cells. Recently stable P-type metal oxides, such as NiO, MoO3, and V2O5, have been applied for replacing the commonly used acidic PEDOT:PSS ABL because of the difficulty in coating PEDOT:PSS solutions onto electrodes or polymer active layers. This highlights the importance of the fabrication and characterization of various ABLs that affect the efficiency of hole extraction and the blocking of electron backflows. We studied MoO3 ABLs formed by the rapid thermal annealing (RTA) of molybdenum (Mo) in oxygen atmosphere. In particular, we propose the use of an indium tin oxide (ITO)/Mo/MoO3 anode multilayer for efficient hole extraction in bulk heterojunction (BHJ)-structure organic-based solar cells. The formation of Mo and MoO3 thin films was controlled by the RTA time with a fixed annealing temperature of 350°C. We obtained a two-fold improvement in the short circuit current (Jsc) of the device with the ITO/Mo/MoO3 anode multilayer film (AMF), compared to a device with a PEDOT:PSS ABL.

Recent Advances in Sensitized Mesoscopic Solar Cells

Perhaps the largest challenge for our global society is to find ways to replace the slowly but inevitably vanishing fossil fuel supplies by renewable resources and, at the same time, avoid negative effects from the current energy system on climate, environment, and health. The quality of human life to a large degree depends upon the availability of clean energy sources. The worldwide power consumption is expected to double in the next 3 decades because of the increase in world population and the rising demand of energy in the developing countries. This implies enhanced depletion of fossil fuel reserves, leading to further aggravation of the environmental pollution. As a consequence of dwindling resources, a huge power supply gap of 14 terawatts is expected to open up by year 2050 equaling today's entire consumption, thus threatening to create a planetary emergency of gigantic dimensions. Solar energy is expected to play a crucial role as a future energy source. The sun provides about 120,000 terawatts to the earth's surface, which amounts to 6000 times the present rate of the world's energy consumption. However, capturing solar energy and converting it to electricity or chemical fuels, such as hydrogen, at low cost and using abundantly available raw materials remains a huge challenge. Chemistry is expected to make pivotal contributions to identify environmentally friendly solutions to this energy problem. One area of great promise is that of solar converters generally referred to as "organic photovoltaic cells" (OPV) that employ organic constituents for light harvesting or charge carrier transport. While this field is still in its infancy, it is receiving enormous research attention, with the number of publications growing exponentially over the past decade. The advantage of this new generation of solar cells is that they can be produced at low cost, i.e., potentially less than 1 U.S. $/peak watt. Some but not all OPV embodiments can avoid the expensive and energy-intensive high vacuum and materials purification steps that are currently employed in the fabrication of all other thin-film solar cells. Organic materials are abundantly available, so that the technology can be scaled up to the terawatt scale without running into feedstock supply problems. This gives organic-based solar cells an advantage over the two major competing thin-film photovoltaic devices, i.e., CdTe and CuIn(As)Se, which use highly toxic materials of low natural abundance. However, a drawback of the current embodiment of OPV cells is that their efficiency is significantly lower than that for single and multicrystalline silicon as well as CdTe and CuIn(As)Se cells. Also, polymer-based OPV cells are very sensitive to water and oxygen and, hence, need to be carefully sealed to avoid rapid degradation. The research discussed within the framework of this Account aims at identifying and providing solutions to the efficiency problems that the OPV field is still facing. The discussion focuses on mesoscopic solar cells, in particular, dye-sensitized solar cells (DSCs), which have been developed in our laboratory and remain the focus of our investigations. The efficiency problem is being tackled using molecular science and nanotechnology. The sensitizer constitutes the heart of the DSC, using sunlight to pump electrons from a lower to a higher energy level, generating in this fashion an electric potential difference, which can exploited to produce electric work. Currently, there is a quest for sensitizers that achieve effective harnessing of the red and near-IR part of sunlight, converting these photons to electricity better than the currently used generation of dyes. Progress in this area has been significant over the past few years, resulting in a boost in the conversion efficiency of the DSC that will be reviewed.

Low vacuum process for polymer solar cells: Effect of TiOx interlayer

Here, the effect of integration of a TiOx interlayer on low vacuum-processible organic-based solar cells is reported. Devices with an Al electrode prepared at a low vacuum condition shows lower efficiency compared to those with an Al electrode prepared at a high vacuum condition because the interface between the active layer and Al electrode is too rough. When a TiOx layer is inserted between the active layer and Al electrode of the device, the cell efficiency is regained and much less dependent on the level of vacuum during Al deposition so that it may be possible to develop low cost process.

Absorbing infrared light in polymer solar cells

New polymers absorb light from the ultraviolet to the infrared, extending the reach of organic-based solar cells.

Interface dipole at organic/metal interfaces and organic solar cells

In organic-based solar cells, the interface dipole present at the organic/metal interface participates to the collection and injection of charges between the electrode and the active organic material. The origins of the interface dipole is illustrated for a model system of the organic/metal interface composed of the electron–donor molecule p-phenylenediamine (PPDA) interacting with a nickel surface. The interface dipole created at the PPDA/Ni interface is characterized in a joint experimental and theoretical study using photoelectron spectroscopy and density functional theory calculations. The formation of strong interface dipoles upon chemisorption of a PPDA mono-layer is accompanied by a significant decrease (1.5 eV) of the metal work function reaching 3.6 eV.

Interface dipole at organic/metal interfaces and organic solar cells

In organic-based solar cells, the interface dipole present at the organic/metal interface participates to the collection and injection of charges between the electrode and the active organic material. The origins of the interface dipole is illustrated for a model system of the organic/metal interface composed of the electron-donor molecule p-phenylenediamine (PPDA) interacting with a nickel surface. The interface dipole created at the PPDA/Ni interface is characterized in a joint experimental and theoretical study using photoelectron spectroscopy and density functional theory calculations. The formation of strong interface dipoles upon chemisorption of a PPDA mono-layer is accompanied by a significant decrease (1.5eV) of the metal work function reaching 3.6eV.

Excitonic Solar Cells

Existing types of solar cells may be divided into two distinct classes: conventional solar cells, such as silicon p−n junctions, and excitonic solar cells, XSCs. Most organic-based solar cells, including dye-sensitized solar cells, DSSCs, fall into the category of XSCs. In these cells, excitons are generated upon light absorption, and if not created directly at the heterointerface as in DSSCs, they must diffuse to it in order to photogenerate charge carriers. The distinguishing characteristic of XSCs is that charge carriers are generated and simultaneously separated across a heterointerface. In contrast, photogeneration of free electron−hole pairs occurs throughout the bulk semiconductor in conventional cells, and carrier separation upon their arrival at the junction is a subsequent process. This apparently minor mechanistic distinction results in fundamental differences in photovoltaic behavior. For example, the open circuit photovoltage Voc in conventional cells is limited to less than the magnitude of the band bending Øbi; however, Voc in XSCs is commonly greater than Øbi. Early work on solid-state excitonic solar cells is described as are excitonic processes in general and the use of carrier-selective (energy-selective) contacts to enhance Voc. Then studies of DSSCs, which provide a particularly simple example of XSCs, are described. A general theoretical description applicable to all solar cells is employed to quantify the differences between conventional and excitonic cells. The key difference is the dominant importance, in XSCs, of the photoinduced chemical potential energy gradient ∇μhν, which was created by the interfacial exciton dissociation process. Numerical simulations demonstrate the difference in photoconversion mechanism caused solely by changing the spatial distribution of the photogenerated carriers. Finally, the similarities and differences are explored between the three major types of XSCs: organic semiconductor cells with planar interfaces, bulk heterojunction cells, and DSSCs.

Fabrication of bulk heterojunction plastic solar cells by screen printing

We demonstrate the use of screen printing in the fabrication of ultrasmooth organic-based solar cells. Organic films on the order of several tens of nanometers in thickness and 2.6 nm surface roughness were made. The first-generation screen-printed plastic solar cells demonstrated 4.3% in power conversion efficiency when using an aluminum electrode and 488 nm illumination.

Organic-Based Photovoltaic Cell Mimicking Photosynthesis.

Solar energy conversion by organic photoelectrochemical cells has been intensively investigated. Dye-sensitized cells such as Gratzel cell which is made from titanium dioxide coated with ruthenium complex seem to be promising as solar cell in a level of practical application. Organic-based photovoltaic cells, which mimic photoinduced multistep electron transfer processes in photosynthesis, have been developed in recent years. Sequential electron relay in primary processes of photosynthesis has been realized in multi-arrays of pigments connected with covalent bonds as photosynthetic model. A variety of techniques, such as Langmuir-Blodgett (LB) films, lipid bilayer membranes, and self-assembled monolayers, have been applied for construction of photovoltaic device where pigments including donor, sensitizer, and acceptor, are arranged unidirectionally at the molecular level on electrode surfaces. In the light of the rapid progress in these fields organic-based solar cells will be much improved in the near future.