Quantum Efficiency in Complex Systems, Part IIFrom Molecular Aggregates to Organic Solar Cells
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A scattering matrix approach is presented which accurately reproduces the observed effects and thus delivers the radiative recombination spectra corrected for the wavelength-dependent out-coupling. This approach is proven to enable the correct determination of charge transfer state energies. Organic photovoltaics OPV has the potential of low-cost and large-area production of flexible and light-weight solar cells.
However, the power conversion efficiencies PCE achieved thus far are not yet competitive to commercially available technologies like, e. In the commonly used organic bulk heterojunction solar cell OSC , a charge transfer CT complex is formed at the donor—acceptor D—A interface due to interaction of the D and A molecules, resulting in an absorption and emission band at lower energies compared to the optical gaps of the individual D and A materials. Although the CT complex does not contribute significantly to the overall absorption in OSC it nevertheless is the intermediate state over which charge carrier generation and recombination occur forming a CT exciton 1 , 2 , 3 , 4.
The term CT state will be used here to refer to the excited state of the CT complex where the electron is located at the acceptor and the hole at the donor where the CT energy E CT is the excitation energy with respect to the CT complex ground state. The energy E CT and occupation of the CT states are crucial as they determine the open-circuit voltage V OC and are thus directly correlated to the PCE 1 , 5 , 6 , 7 , which will be discussed in more detail below.
Therefore, this work will focus on EL, while the results are valid for any kind of radiative emission, such as photoluminescence. For organic light-emitting diodes OLED , microcavity effects have been reported to influence emission 16 , 17 , 18 and optical out-coupling is a central part of the conducted research 19 , Optical simulations in the field of OPV are commonly used to improve wavelength-dependent optical field distribution incoupling of sunlight and therefore generation 21 , 22 , Microcavity effects have been used to enhance quantum efficiency close to the absorption band edge and thus improve device performance 24 , 25 , 26 , 27 , 28 , 29 and they were furthermore utilized to alter transmission and produce color-tunable semitransparent OSCs in the visible range As a matter of fact, these OPV-related optical studies focus on the wavelength regions of the donor and acceptor absorption and thus ultimately on the optimization of the external quantum efficiency EQE.
In contrast, to the knowledge of the authors microcavity effects have not been considered yet when using luminescence spectroscopy to characterize radiative recombination occurring via CT states within OSCs, i. For this reason, this work focuses on the wavelength region of CT state absorption and emission in OSC and takes optical out-coupling effects explicitly into account.
The Role of Molecular Structure and Conformation in Polymer Electronics
It will be shown that it is essential to account for the optical properties of the investigated system when CT state emission is detected and analyzed by luminescence spectroscopy. Absorption and emission spectra of CT states are stokes shifted due to the reorganization of the CT complex E CT can be determined by fitting Eq. Under open-circuit conditions, generation and recombination are equally large which determines the electron and hole concentrations. The open-circuit voltage of a solar cell is given by:.
For conventional semiconductors, E G resembles the band gap. Accounting for energetic disorder and under the assumption of thermodynamic equilibrium between free charge carriers and the occupation of CT states, Burke et al. From Eq. Note that the energy of a particular CT complex depends on the local environment at the D—A interface, which means that not all charge carriers can thermalize down to the low energetic tail of the distribution. From Eqs.
Here we show that interference effects have a significant impact on emission spectra in the relevant range and have to be taken into account when CT state energies are determined by luminescence spectroscopy. Emitted electromagnetic waves are partially reflected at material interfaces in multilayer structures due to differences in their optical coefficients, i.
The direct and reflected waves superimpose leading to constructive and destructive interference. For simulations of the radiance enhancement within the active layer of the OSCs, an implementation 33 of the scattering matrix S matrix method 34 , 35 based on custom code was applied. Within this method, the exact configuration of the investigated OSC layer stacks can be simulated. Experimentally determined n and k values are used as input parameters. All used parameters can be found in the Supplementary Fig.
When the luminescence is observed through the 1. Thereby, no reflection at the glass—air interface is included in the simulation, i. The used layer stacks are one-dimensional photonic structures, therefore a one-dimensional simulation along the direction perpendicular to the electrodes referred to as x -axis fully describes these systems. Luminescence within the photoactive layer PAL was implemented by a dipole-like emitter and its position x was varied throughout the whole PAL. For this, the layer stack was split at position x and two S matrices, one for the lower and one for the upper layers, were calculated.
To obtain a spherically symmetrical emission the dipole was oriented in x -, y -, and z -direction successively and the resulting radiances were averaged. As reference, the free space radiance I hom of a homogeneous system with infinite elongation was calculated. To investigate CT state emission, EL spectroscopy was performed for a variation of photoactive D—A systems and layer stacks. Schematic illustration of device architectures of the used organic solar cells.
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Layer stacks of an optically inverted OSC a , b with additional semitransparent Ag layer. EL spectra and out-coupling factor of an OSC with and without semitransparent layer. The more distinct peak with the additional semitransparent layer originates from the formation of an optical cavity what can be seen from the out-coupling factor. When measuring EL again, drastic changes are observed in the spectrum which can be seen from the solid red line in Fig. It should be pointed out that the PAL and contact materials were not changed and morphologic changes are not expected by mere deposition of such a thin Ag layer on top of the PEDOT:PSS layer and thus the recombination properties are also not expected to change.
Accordingly, the different spectrum has to originate from the change of the optical properties of the system. Therefore, the data hardly allows for any statement about the spectral fingerprint of the emitting photoactive material, i. In this case, the path difference between direct and reflected light depends on the distance of the photon emission from the reflective electrode, thus, the averaging over all emission positions within the PAL leads to a quite homogeneous out-coupling. Still, aspects of the spectral shape like the asymmetry of the spectrum can be partially explained by optical out-coupling.
The previously discussed case has a more exemplary character, since a reflective surface at the illumination side reduces photon absorption and hence is avoided in the design of an OSC. The following part therefore focuses on regularly used architectures of OSC. The corrected intensity I EL,corr represents the expected free space radiance without the optical environment, i. In Fig. In addition, peak amplitudes range from 3. In great contrast, all-corrected intensities I EL,corr are quite similar regarding the shape and amplitude for the whole range of different values of d PAL and for the two different device architectures as could be expected for the same photoactive material used in these devices.
This is a clear indication that the measured spectra are strongly influenced by the optical properties of the layer stacks. These peaks originate from the high reflectance of ITO at the relevant wavelengths as shown in Fig. Materials were measured on glass solid lines and the fits dashed lines to the ITO data for the determination of n and k values. The deviations present in the corrected spectra are due to slight offsets in peak position and peak width between measurement and simulation.
The results shown in Fig. This is shown in Fig. This can clearly be ascribed as to originate from changes in the optical out-coupling properties upon variation of absorber thickness. Thicker photoactive layers could not be realized due to a poor resulting film quality. OSCs usually feature a reflective back contact.
In case of no interference in the device, i. Hence, the total emitted power which is proportional to the square of the electric field of the wave E 2 remains unchanged. In contrast, for coherent light, E is doubled when the criterion for constructive interference is fulfilled. For this reason, the radiance is enhanced by a factor of 4 compared to the free space emission thus resulting in a factor of 2 in total emitted power. As could be shown experimentally by Drexhage, this doubling of the total emitted power is accompanied by a decreased radiative lifetime for the emission This decreased radiative lifetime for emission does also apply for the devices investigated in this study.
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For this reason, interference effects do not have any measurable impact on the overall charge carrier lifetimes in these materials. Gaussian functions are fitted by means of Eq. Gaussian fits dashed lines given by Eq.
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From Fig. This results in a strong dependence of the fit results on the chosen data range making a fit of a Gaussian function questionable in general. These strongly altered spectral shapes make a meaningful analysis of the uncorrected data difficult and prone to misinterpretation. Hence, even in this case the correction procedure is indispensable to achieve a deeper understanding of the underlying processes and material properties. In contrast, the raw E CT energies deviate up to more than 0.
Applications 4. The Modeling Approach 3. Molecular Dynamics Simulations 4. Site Energies 5. Electronic coupling 6. Spatial Correlations of Site Energy Fluctuations 7. Temporal Correlations of Site Energies 8. Spectral Densities 9. Exciton Dynamics and Optical Properties Open-System Hamiltonians and Chain Mappings 3. Numerical Results and Applications 4. Many-Site Polaron Master Equation 3. Non-Markovian Dynamics 4. Born-Markov Approximation in the Polaron Frame 5. Charge Transport in Conjugated Polymers 3. Polymer-Based Photovoltaics 4.
Conjugated Polymers for Biomolecular Recognition 5. Primary Photoexcitation 3. Charge Generation 4. Charge Extraction 5. Summary Acknowledgments. Written and edited by internationally renowned experts Relevant to a wide readership: physicists, chemists, materials scientists, and device engineers in academia, scientific laboratories and modern industry. It can be seen that the cell material can be found all over the sample, even on the border between the two materials, indicating a high tolerance of the cells for oPPy.
From these results, it is clear that certain conjugated polymers have great potential as functional surfaces or as device components in biology and medicine. The high degree of flexibility, versatility, and biocom- patibility are promising for devices that are in direct and extended contact with biological systems. Insights to the correlation between properties at the molecular scale, mesoscopic scale, and macroscopic device scale are given. It is shown that by systematically tuning the molec- ular structure, processes at different length scales can be probed. Three device applications are discussed to shed light on the unique electrical, photovoltaic, and mechanical properties of conducting polymers.
The rela- tionship between molecular ordering and charge transport is discussed in the first part of the chapter. The OFET, a model two-dimensional system, is introduced, and the influence the nature of the side group grafted to the molecule on the macroscopic device performance is demonstrated.
It is revealed that by tuning the molecular stacking properties, the transistor parameters can be improved significantly. In the second part of this chap- ter, the BHJ solar cell is discussed. As the power conversion efficiencies of these cells continue to increase, it is becoming important to gain control over the three-dimensional bulk morphology.
In-plane phase segregation between the donor and the acceptor phases in the bulk is important to facilitate charge transfer as well as charge transport. Molecular ordering at the donor—acceptor interface and at the mesoscopic scale is shown to be directly related to the photocurrent. In addition, the vertical structuring of the morphology is an important factor influencing charge collection and therefore, the solar cell fill factor. In the last part of this chapter, the unique structure—function relationships of polymer layers is used to selectively uptake and release target biomolecules.
The potential-regulated trafficking of L-glutamate is demonstrated with molecularly selective oPPy layers. The oPPy layers are additionally demonstrated to be stable and well tolerated by biological systems. Bao, Z. Baranovski, S. Baranovski, pp. B 62, Barreto, J. Status Solidi. B , Baughman, R. Berggren, M. Benson-Smith, J. Blom, P. Bohnenbuck, B. Bolto, B. Brabec, C. Mater 11, MRS Bull. MRS Bulletin. Bredas, J. Brock, E. Csavinsky, P. Physik Journal.
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