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Über dieses Buch

The book describes emerging strategies to circumvent transmission and thermalization losses in solar cells, and thereby redefine the limits of solar power conversion efficiency. These strategies include the use of organic molecules and rare-earth metal materials. Approaches to augment the efficiency of these processes via near-field enhancement are described as well. This book includes a discussion of state-of-the-art implementations of these emerging strategies in solar cells, both internally, as in molecular intermediate band and charge carrier multiplication, and externally, such as photon up- and down-conversion. Tools for characterization are also provided. Written by leading researchers in the field, this book can be useful to both beginners and experienced researchers in solar energy.



Chapter 1. Introduction: Solar Cell Efficiency and Routes Beyond Current Limits

The energy carried by sunlight has an enormous potential to provide humanity with all the energy it needs while limiting the catastrophic consequences of climate change caused to a large extent by current combustion of fossil fuels. A convenient way to harvest solar energy is by making use of the photovoltaic effect of certain semiconducting materials. When a solar photon interacts with such a material, its energy can be converted into electronic energy, which can be extracted to an external electrical circuit, providing both a photovoltage and a photocurrent. The material absorbing the sunlight imposes certain boundary conditions on its electrons which allow electronic states to have only specific distinct energies. Importantly, there will always be a certain minimum energy needed to promote a “resting” electron in a material to a higher electronic state. Therefore, all materials used for the absorption of sunlight will have a requirement on the minimal energy that must be carried by a photon in order for that photon to be absorbed by the material. Photons with energies lower than this absorption threshold will be transmitted by the material. In a solar cell, this portion of solar energy will be lost. This loss of solar photons, known as a transmission loss, restricts the photocurrent achievable from sunlight (see Fig. 1.1a). In a hypothetical solar cell with an optimal absorption threshold (also known as band gap) of 1.3 eV, the fraction of incident solar energy lost by transmission is ∼25%.
Jonas Sandby Lissau, Morten Madsen

Addressing Transmission Losses: Sequential Absorption via Triplet Fusion in Organic Materials


Chapter 2. Photophysics

Sensitized triplet-triplet annihilation-based photochemical upconversion (TTA-UC) or sensitized triplet fusion (TF) is a process by which low-energy photons are converted into high-energy light through a sequence of bimolecular triplet-triplet energy-transfer (TTET) reactions. TTA-UC attracts interest due to its potential applications in solar energy conversion and storage, biological imaging, photochemical synthesis, photocatalysis, photopolymerization, and photoresponsive devices. In this chapter, we present the principles and photophysics governing the observation, mechanism, and chemical kinetics of TTA-UC to establish general energetic and kinetic rules which dictate the conditions under which the phenomenon can be observed.
Nancy Awwad, Mo Yang, Felix N. Castellano

Chapter 3. Near-Infrared-to-Visible Photon Upconversion

One of the promising methods to overcome the Shockley-Queisser limit in solar energy conversion is triplet-triplet annihilation-based photon upconversion (TTA-UC) from near-infrared (NIR,  > 700 nm) light to visible (Vis,  < 700 nm) light. However, it had been difficult to achieve efficient NIR-to-Vis TTA-UC mainly due to the absence of appropriate triplet sensitizers with less or no energy loss associated with intersystem crossing (ISC). In this chapter, we overview recent successful examples of NIR-to-Vis TTA-UC based on the developments of new NIR-absorbing triplet sensitizers, such as semiconductor nanocrystals with small singlet-triplet exchange splitting and Os complexes with direct singlet-to-triplet (S-T) absorption.
Yoichi Sasaki, Nobuhiro Yanai, Nobuo Kimizuka

Chapter 4. Photon Upconversion Based on Sensitized Triplet-Triplet Annihilation (sTTA) in Solids

The conversion of low-energy photons into radiation of higher energy is useful for bioimaging, 3D displays and other applications. In particular, upconversion of the infrared portion of the solar spectrum, which is typically not absorbed by the light-harvesting materials used in solar technologies, allows additional photons to be harnessed and boosts the efficiency of photovoltaic and photocatalytic devices. Therefore, low power photon upconversion of non-coherent light based on sensitized triplet-triplet annihilation (sTTA) has been recently recognized as a potential viable approach towards enhancing the efficiency of sunlight-powered devices through sub-bandgap photon harvesting.
The sTTA permits the conversion of light into radiation of higher energy involving a sequence of photophysical processes between two moieties, respectively a light harvester/triplet sensitizer and an annihilator/emitter. High up-conversion yields under solar irradiance can be observed in low viscosity solutions of dyes, but in solid materials, which are better suited for integration in devices, the process is usually less efficient. The ability to control triplet excitons in the solid state is therefore crucial to obtain high performance solid upconverters. In this chapter, we will discuss the results obtained in several systems, such as dye-doped polymers/nanoparticles, amorphous/crystalline supramolecular structures and many others, highlighting the materials design guidelines to obtain efficient upconverters at the solid state that can match the strict requirements of solar technologies.
Angelo Monguzzi

Chapter 5. Organic Triplet Photosensitizers for Triplet-Triplet Annihilation Upconversion

Triplet-triplet annihilation (TTA) upconversion, a photophysical phenomenon of conversion of low-energy photon into high-energy photon, is a promising approach due to its low excitation power requirement and high upconversion quantum yields. Most of the triplet photosensitizers (PSs) used for TTA upconversion are based on transition metal complexes and organic chromophores containing heavy atom. Recently TTA upconversion with heavy atom-free PSs has gained much attention due to its minimal detrimental effects. In this chapter we summarize organic triplet PSs with different intersystem crossing (ISC) mechanism used in TTA upconversion. We briefly describe different types of triplet PSs for TTA upconversion such as heavy atom-containing organic chromophores, BODIPY dimers involving exciton coupling, and electron spin convertor-bearing dyes. Recently new triplet PSs involving charge recombination (CR)-induced ISC and thermally activated delayed fluorescence (TADF) have been used in the TTA upconversion. We discuss the benefits of these PSs for TTA upconversion. This book chapter provides current progress in the field of TTA upconversion and designing new efficient triplet PSs.
Zafar Mahmood, Shaomin Ji, Jianzhang Zhao, Mushraf Hussain, Farhan Sadiq, Noreen Rehmat, Muhammad Imran

Chapter 6. Plasmon-Enhanced Homogeneous and Heterogeneous Triplet-Triplet Annihilation

Triplet-triplet annihilation (TTA) process includes two categories, a homogeneous TTA occurring between two triplet excited molecules of the same type such as the homogeneous TTA upconversion (TTA-UC) and a heterogeneous TTA occurring between two triplet excited molecules of different types such as the heterogeneous TTA-UC, or between a triplet excited state and a triplet ground state such as the sensitized singlet oxygen generation. To the other front, noble metal nanostructures are known to exhibit an extraordinary capability to manipulate light through the collective oscillations of their conduction-band electrons, the so-called localized surface plasmon resonances (LSPR). Plasmonic nanostructures have been shown to be able to dramatically enhance the performances of many optical systems. In this book chapter, we will use a few examples to demonstrate that LSPR of noble metal nanoparticles can enhance the efficiency of both categories of TTA, and to discuss the conditions where such plasmonic enhancement would occur. The results shed light onto ways to improve the overall TTA efficiency, which would be relevant to the broad applications involving TTA-UC or sensitized singlet oxygen generation.
Emily Westbrook, Xian Cao, Peng Zhang

Molecular Oxygen and Triplets: Photophysics and Protective Strategies


Chapter 7. Molecular Oxygen in Photoresponsive Organic Materials

The presence of molecular oxygen in organic materials designed for use in photoresponsive devices (e.g., solar cells, photon up-converters) can adversely influence the performance of the device in several ways. The most important of these is arguably through reactions that oxygenate and/or oxidize the organic components and thereby change properties relevant for a functioning device. The ground electronic state of molecular oxygen is a spin triplet, O2(X3Σg ). As such, it behaves as a biradical in its chemical reactions, trapping adventitious organic free radicals to yield reactive peroxyl radicals. The lowest excited electronic state of molecular oxygen is a spin singlet, O2(a1Δg), and can be formed in appreciable yield by energy transfer from a photoexcited organic molecule to O2(X3Σg ). Functional groups common to molecules used in photoresponsive materials (e.g., double bonds, sulfides) can react with O2(a1Δg) to form peroxides, which likewise are reactive and propagate disruption. The quenching of an excited-state organic component by O2(X3Σg ) also points to other ways in which oxygen can influence device performance. For example, the oxygen-mediated deactivation of comparatively long-lived triplet states can adversely influence the energy fusion process essential for some up-conversion devices. Likewise, electron transfer from an excited-state organic molecule to O2(X3Σg ) and/or O2(a1Δg) can not only interfere with desired charge movement (e.g., in a photovoltaic device), but it will also produce the superoxide radical ion that, in turn, can contribute to oxygenation reactions. Thus, to exploit fully the functional capabilities of photoresponsive organic materials and to prolong device longevity, and in lieu of completely excluding oxygen, it is necessary to monitor, understand, and ultimately control the behavior of oxygen in such systems.
Mikkel Bregnhøj, Peter R. Ogilby

Chapter 8. Protective Strategies Toward Long-Term Operation of Annihilation Photon Energy Upconversion

The process of triplet-triplet annihilation photon energy upconversion (TTA-UC) performed in soft-matter environment relays on optically created densely populated organic triplet ensembles. In soft-matter matrix, it is a diffusion-controlled process, simultaneously demonstrating essential dependences on the environmental parameters such as matrix temperature, matrix viscosity, and presence of molecular oxygen, dissolved into the solvent or adsorbed on the polymer film. It is important to notice that all these environmental parameters are strongly interrelated and their impact on the temporal evolution of the densely populated triplet ensembles is not a linear combination of its partial impacts. If the TTA-UC process is applied toward solar energy storage and/or conversion technologies, the influence of the singlet oxygen generation generally leads to lower quantum yield (QY) and simultaneously to accelerated aging of the upconversion device. The generation of singlet oxygen is much more harmful for the studied object, if the process of TTA-UC is used as a sensing mechanism for probing of the local temperature/local oxygen content in cell cultures. In this chapter, the development of an effective protection strategy against quenching by molecular oxygen and protection against the subsequent photooxidation caused by singlet oxygen is discussed.
Stanislav Baluschev

Chapter 9. Additive-Assisted Stabilization Against Photooxidation of Organic and Hybrid Solar Cells

Thin-film organic and perovskite solar cells have attracted the interest of research and industry. The main reason behind this is the low price, high tunability, and great potential of these technologies. Lightweight, color tunability, translucence, flexibility, and ease and low cost of processing are a few of the excellent characteristics of these technologies. However, they have a high sensitivity to light, oxygen, temperature, and humidity, which are common environmental conditions. Upon exposure to these environmental stresses, the chemical structure and morphological arrangement undergo complex changes that result in dramatic reduction of device functionalities over time. This chapter collects the approaches currently reported in the field of additive-assisted stabilization, a method in which solar cells’ active layers are blended with additives that promote their stability (e.g., hydrogen donors, radical scavengers, hydroperoxide decomposers, quenchers, UV absorbers).
Michela Prete, Um Kanta Aryal, Jonas Sandby Lissau, Horst-Günter Rubahn, Morten Madsen, Vida Turkovic

Implementation of Photochemical Upconversion in Solar Cells


Chapter 10. Optically Coupled Upconversion Solar Cells

Conventional solar cells do not use photons with energy less than the band gap. Upconversion uses sensitizers to enable capture of the remaining light. Since upconversion is a nonlinear process, efficient device design requires detailed knowledge of the coupling of sunlight to the sensitizer. This chapter describes the properties of the principal components of an upconversion-augmented solar cell.
Laszlo Frazer, Timothy W. Schmidt

Chapter 11. Electronically Coupled TTA-UC Solar Cells

Optical and electronic coupling architectures are two distinct strategies for harnessing photon upconversion via triplet-triplet annihilation (TTA-UC) in a solar cell. The former combines a standard solar cell with an UC film (Chap. 10), while the latter integrates TTA-UC directly into the solar cell. In this chapter we will review the various strategies for integrating TTA-UC into dye-sensitized and layered heterojunction solar cells. These strategies include heterogeneous sensitization, multilayers, metal-organic frameworks, co-deposition, and more. We describe these architectures, note advantages and disadvantages of each, and summarize progress in the field to date. We also discuss the efficiency-limiting factors of current devices and the prospects for integrated TTA-UC solar cells.
Yan Zhou, Kenneth Hanson

Addressing Transmission Losses: Sequential Absorption in Rare Earth Ions


Chapter 12. Rare-Earth Ion-Based Photon Up-Conversion for Transmission-Loss Reduction in Solar Cells

Photon up-conversion (UC) describes an anti-Stokes emission process, in which a luminophor emits one higher energy photon after being excited by multiple low-energy photons, among which rare-earth (RE) ion-doped materials present promising UC properties due to unique electron configuration. RE UC materials have been widely studied in solar cells with the purpose to reduce transmission losses, i.e., achieve wide/full solar spectral harvesting and high-power conversion efficiency, by converting unutilized sub-bandgap photons into sensitive resonant photons. This chapter exclusively focuses on RE-doped UC materials and their applications in solar cells. The RE-based UC photophysics, UC enhancement, and applications in solar cells will be reviewed and briefly discussed.
Hai-Qiao Wang, Andres Osvet, Miroslaw Batentschuk, Christoph J. Brabec

Chapter 13. Nanophotonics for Photon Upconversion Enhancement

Lanthanide-based upconversion materials convert low-energy infrared photons into high-energy visible photons with much higher efficiency than two-photon absorption or second harmonic generation. Naturally, they have attracted much attention for potential applications in solar energy harvesting, photocatalysis, security, and biological imaging. Despite the high promise, the intrinsic conversion efficiency of upconversion materials remains low for most applications. The emergent nanophotonic technologies could provide a powerful tool to boost the upconversion efficiency and enable novel applications. In this chapter, we provide an in-depth review of the theoretical foundation for light-matter interaction, dynamics of luminescence upconversion process, and nanophotonic enhancement mechanisms. We then provide a comprehensive survey of recent progress in nanophotonic enhancement of upconversion. The use of plasmonic and dielectric nanostructures has led to 2–3 orders of magnitude improvements in upconversion efficiency. The accelerating pace of progress makes nanophotonically enhanced upconversion a highly promising platform for novel photonic applications.
Wounjhang Park, Ananda Das, Kyuyoung Bae

Addressing Thermalisation Losses: Singlet Fission and Quantum Cutting


Chapter 14. Singlet Fission: Mechanisms and Molecular Design

Exciton multiplication processes provide a means of overcoming thermalization losses in photovoltaic devices. In this chapter we will introduce one promising exciton multiplication process, termed singlet fission, which occurs in carbon-based semiconductors. After introducing the photophysics of organic semiconductors, we discuss the mechanism of singlet fission and the role of spin and electronic structure in the singlet fission process. Based on this mechanistic discussion we will introduce design rules for singlet fission materials related to their energy level alignment, molecular structure and crystal packing.
Victor Gray, Leah Weiss, Akshay Rao

Chapter 15. Singlet Fission Solar Cells

The high-energy (blue) part of the solar spectrum is inefficiently converted in conventional solar cells, mainly because the high-energy excitations thermalize to the bandgap before they are extracted. Several strategies have been devised to tackle these thermalization losses, most prominently tandem solar cells. However, these tandem cells require an intricate device design and current matching in case of a series connection. Downconversion via singlet fission and quantum cutting promise a better use of the high-energy photons, avoiding a large fraction of the thermalization losses, but without the intricate fabrication and design constraints of tandem cells. In this chapter we review the progress made towards efficient singlet fission and quantum cutting downconversion. We start with the potential for solar cell integration, reviewing the different integration schemes and their efficiency potential. In the second part we review the progress towards solar cells that utilize singlet fission and quantum cutting from all-organic devices to hybrid two-bandgap devices and fully optical integration. Finally, we lay out the challenges for using these downconversion schemes in commercial solar cells.
Bruno Ehrler


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