Category Materials Challenges

Materials for Improved Photon Energy Collection

Over the past years a variety of organic dyes have been extensively employed in fluorescent collectors because of their high absorption coefficients combined with high fluorescence quantum yields. However, they are not photostable and the few dyes with high quantum efficiency emit in the green-yellow spectral region which introduces a severe constraint on the absorption efficiency in application with commercial solar cells.35

The BASF laboratories have developed organic fluorophores based on perylene and naphthalimide dyes for application in fluorescent collec­tors.24’36 The Lumogen F series,37 in particular, have been used extensively in almost every fabricated collector since 2007...

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Basic Theory

Excitation energy transfer results from the resonance interaction between the electronic transition dipole moments of a donor molecule (D) in the excited state and an acceptor molecule (A) in the ground state (Figure 9.3). It is a near field interaction which occurs during the excitation lifetime of the donor molecule and the electronic excitation energy transfer is radiationless. The FRET mechanism is effective over distances ranging from <1 nm to about 10 nm. The pioneering theoretical work was carried out half a century ago by Theodor Forster who showed that the excitation energy transfer rate varies with the separation distance r between their transition dipole centres as:32’33

(r> = SD (r)* (9’15>

where sD is the lifetime of the excited state for the donor molecule in the absence of...

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Forster Resonance Energy Transfer

9.3.1 Introduction

Forster Resonance energy transfer (FRET)6’32 is a powerful tool for efficient photon management. An example of such process is photosynthesis which makes use of efficient energy transfer in the antenna system to increase the absorption efficiency of energy conversion. Light is absorbed by chlorophylls and other accessory pigments that surround the reaction centre and the molecular excitation energy created is transported to the reaction centre. Borrowing concepts from light harvesting in photosynthesis suitable antenna pigment structures can be envisaged to improve the capture of solar radiation in artificial structures used in photovoltaic and photochemical conversion.13

Fluorescent collectors and down-shifting structures can benefit from a similar process but for a s...

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Photon Balance in the Collector

The performance of the collector is usually assessed by means of optical effi­ciency hopt, equal to the ratio of photon flux! Vwork emitted from the collector and reaching the solar cell and the photon flux Io incident on the collector:

Here, we also define the absorption efficiency:

and the photon collection efficiency:

In terms of the incident photon flux F(l) and absorbance A(1) = s(X)Cd (where C is the concentration, d is the thickness of the collector and e(l) is the decadic extinction coefficient) the absorption efficiency can be written as:

where min and g are the minimum and maximum (near bandgap) wave­length in solar spectrum that are taken into consideration. In our analysis below we take min = 300 nm, unless otherwise stated.

An expression for Qc has been obtained by different...

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Viewed quite generally, the probability r* that light emitted in a volume V at wavelength l is re-absorbed is given by:

where ‘ denotes the optical path length of a ray inside the volume, a* is the absorption coefficient at wavelength l, and p(r) describes the probability distribution of emission events inside V. Eqn (9.1) assumes that emission occurs isotropically over all elements of solid angle dU. For simplicity we have also assumed that a ray is either completely transmitted or completely reflected at the edges of the volume, for example, by total internal reflection. Specific examples of the re-absorption probability [eqn (9.1)] are considered below.

In terms of r*, the total re-absorption probability for light emitted with spectrum fi(l) is equal to:

The spectrum f1(X)—the first gen...

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9.2.1 Introduction

In contrast to geometric concentrators, the opportunity to change frequency (invariably implemented through absorption and re-emission as lumines­cence) introduces a new degree of freedom which opens up new avenues to enhance the capture of sunlight. The stochastic nature of these processes, however, must be taken fully on board for a satisfactory understanding and description of device operation. Viewed in a more fundamental setting, the ergodic features of re-absorption (also known as photon recycling) introduce a fundamentally new facet to optical devices and move optics into the novel arena of the thermodynamics of light.24

Detailed numerical models of photon transport in fluorescent collectors have emerged in recent years...

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Photon Frequency Management Materials for Efficient Solar Energy Collection




aSolar Energy Laboratory, Engineering Materials, School of Engineering Sciences, University of Southampton SO17 1BJ, UK; bTeknova Renewable Energy Research Group, Teknova, Gimlemoen 19, 4630, Kristiansand, Norway; cDepartment of Physics, Faculty of Science, KingMongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand *E-mail: l. danos@lancaster. ac. uk

9.1 Introduction

Photon management in solar cells usually refers to processes that aim to enhance light capture as the first step in photovoltaic solar energy conversion (Figure 9.1)...

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Integration of Metal Nanoparticles into Silicon Solar Cells

Metal nanoparticles have been integrated into many different types of solar cell and photodetectors. In this section we review the experimental results for metal nanoparticles integrated into crystalline (wafer-based), polycrystalline and amorphous silicon solar cells.

Nanoparticles deposited onto the front surface of thick, wafer-based silicon solar cells primarily increase photocurrent by reducing reflection. Lim et al.143 deposited a sparse array of 100 nm Au spheres onto the front surface of a silicon photodiode, and found that photocurrent was increased at short wavelengths and decreased at long wavelengths. The decrease was attributed to interference between scattered and unscattered photons. Pillai et al...

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