The performance of non-fullerene acceptor (NFA)- based organic solar cells (OSC) has shown continuous increase in recent years, reaching power-conversion efficiencies up to 17% through the design and synthesis of efficient acceptor materials. Recent research is directed towards achieving higher efficiency of OSC, which is limited by the open-circuit voltage (Voc) which is lower than the Voc values achieved in inorganic or perovskites solar cells with comparable bandgaps. In this work, voltage losses in NFA based OSC were calculated by investigating charge-transfer state energy (ECT) using electroluminescence spectroscopy and sensitive external quantum efficiency in three polymer:non-fullerene bulk heterojunction solar cells. PCE10:ITIC device acquired the highest ECT with a Voc of 0.82V, and a a power conversion efficiency (PCE) of 7.91%. While PCE10:O-IDTBR obtained the highest Voc of 1.03V, a PCE of 8.02% compared to PCE10:O-IDTBCN solar cell that has a lower Voc of 0.73V with a PCE of 7.98%. Both radiative and non-radiative voltage losses were calculated. In this thesis, the high open circuit voltage of PCE10:O-IDTBR is explained by the low non-radiative voltage losses compared to PCE10:O-IDTBCN and PCE10:ITIC devices.

In this dissertation, we investigate two emerging strategies for enhancing the performance of organic photovoltaics. The first takes advantage of a process called singlet exciton fission, and the second embodies an exodus from the fullerene electron acceptors prominent in organic solar cells. Indeed, this versatile class of tunable small molecules are aptly termed nonfullerene acceptors. However, both strategies would benefit from a greater understanding of underlying principles. Singlet exciton fission is a photon-multiplying process in which a singlet exciton from a high-energy absorbed photon splits into two triplet excitons. The process could significantly reduce energy lost to heat in photovoltaic devices, but its mechanisms are still misunderstood. One model involves direct coupling between the singlet and triplet states, and another model involves an intermediate charge transfer state. Transient absorption spectroscopy allowed us to examine singlet fission in films of pentacene, fluorinated pentacene, and coevaporated blends of various mixing ratios. We directly observe an intermolecular charge transfer state during singlet fission in solid films of coevaporated pentacene and peruoropentacene, which supports the model of charge transfer state-mediated singlet fission. Furthermore, we successfully induced singlet fission in one blend by directly exciting the charge transfer state below the bandgap. We use various types of steady state and time-resolved spectroscopy to characterize two types of nonfullerene electron acceptors. The first type is a group of tetraazabenzodiuoranthene diimide (BFI) dimers and a BFI monomer. The BFI dimers were designed to have twisted, nonplanar 3-dimensional structures and have helped achieve power conversion efficiencies of over 8% in organic solar cells. The other type of nonfullerene acceptor is a calamitic small molecule, and we consider the BAF-4CN electron acceptor, which has also been used in a solar cell whose efficiency exceeded 8%. Spectroscopic studies give insight into the performances of these nonfullerene devices in relation to fullerene-derivative counterparts. We find that the nonfullerene blends suffer from more geminate charge recombination. However, despite this drawback, in some cases, slower rates of nongeminate recombination may lead to successful power conversion efficiencies in nonfullerene solar cells.

<p>Processes where a molecule absorbs visible light and then disposes of the excess energy via electron/energy transfer reactions have an important role both in nature (e.g. in photosynthesis) and in many technical applications (e.g. in photography and photovoltaics). This thesis uses different spectroscopical techniques, mainly ultrafast transient absorption, to study such processes. The thesis can roughly be divided into three parts.</p><p>In the first part, donor-acceptor systems linked by different conjugated bridges are studied. The objective was to see to what extent the conjugated link could enhance excited state energy or electron transfer, via so-called superexchange processes. The studied links do enhance the electron/energy transfer but in the electron transfer study the resulting charge separated state was very short lived.</p><p>The second part explores the possibility of constructing acceptor-donor-acceptor triads where the direction of electron transfer is determined by the electronic state of the donor. Direct evidence of electron transfer in the form of radical absorption was found from both the first and the second excited states of the donor.</p><p>In the last part, two common chromophores were investigated by transient absorption anisotropy. In the case of Ru(bpy)₃²⁺, it was found that the complex lost all memory of the polarization of the exciting light much faster than what was previously thought. This means that electron transfer between ligands is normally not the rate limiting step in electron transfer reactions involving this complex. In the case of zinc porphyrin, it was seen that the measured anisotropy differed depending on which electronic state was excited suggesting differences in the degree of coherence.</p>

Processes where a molecule absorbs visible light and then disposes of the excess energy via electron/energy transfer reactions have an important role both in nature (e.g. in photosynthesis) and in many technical applications (e.g. in photography and photovoltaics). This thesis uses different spectroscopical techniques, mainly ultrafast transient absorption, to study such processes. The thesis can roughly be divided into three parts. In the first part, donor-acceptor systems linked by different conjugated bridges are studied. The objective was to see to what extent the conjugated link could enhance excited state energy or electron transfer, via so-called superexchange processes. The studied links do enhance the electron/energy transfer but in the electron transfer study the resulting charge separated state was very short lived. The second part explores the possibility of constructing acceptor-donor-acceptor triads where the direction of electron transfer is determined by the electronic state of the donor. Direct evidence of electron transfer in the form of radical absorption was found from both the first and the second excited states of the donor. In the last part, two common chromophores were investigated by transient absorption anisotropy. In the case of Ru(bpy)32+, it was found that the complex lost all memory of the polarization of the exciting light much faster than what was previously thought. This means that electron transfer between ligands is normally not the rate limiting step in electron transfer reactions involving this complex. In the case of zinc porphyrin, it was seen that the measured anisotropy differed depending on which electronic state was excited suggesting differences in the degree of coherence.

Organic photovoltaic (OPV) is a promising technology for renewable energy at low cost. Over the last five years, the power conversion efficiency (PCE) has doubled to 12%, which still is clearly lower than commercially available inorganic solar cells with a PCE around 20%. One approach to improve the PCE is to complement the conversion of light into electrical energy with the infrared (IR) part of the solar spectrum. However, the fundamental difference between organic and inorganic semiconductors is the exciton binding energy. Compared to inorganic semiconductors, in organic materials the exciton binding energy is more than 20 times higher, leading to strongly bound electron hole pairs, which are primarily generated upon photo-excitation. To dissociate these charges, in OPV, a donor-acceptor system is used. However, the energetics of this donor-acceptor system lowers the obtained open circuit voltage, representing one major loss of OPV as compared to inorganic solar cells. In the first part of this work, three benzannulated 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (aza-BODIPY) infrared absorbing donor dyes with methyl, methoxy, or without side-group attached are investigated. The solar cells with the highest PCE, i.e. the devices using the donor molecule without a side-group, exhibit a difference between the optical and the effective gap of 0.17 eV. It reflects the "driving force" for the electron to transfer from the donor to the acceptor, and is lowest for the best performing device, indicating that in the devices used efficient charge dissociation does not require large electron transfer energy loss. A - for OPV - relatively high open circuit voltage of 0.81 V is measured and when compared to the optical gap of the donor, a voltage loss of 0.74 V is obtained, reflecting the high voltage losses in OPV. In inorganic devices these voltage loss is around 0.4 V. One approach to lower this difference in the voltage loss is to lower the exciton binding energy of neat organic materials, leading to a larger exciton size. A saturation of the exciton size at about 1.2 nm is calculated by time dependent density functional method (TD-DFT) for one dimensional conjugated organic molecules with a size larger than 4nm. For the largest size of the exciton, provided by the poly(benzimidazobenzophenanthroline) (BBL), a Coulomb interaction of 0.4 eV between the electron and hole wave function is calculated, leading to an estimated exciton binding energy of about 0.2 eV, serving as a lower limit for the organic molecules investigated. The exciton binding energy can further be lowered by increasing the dielectric constant or by introducing a charge transfer (CT) state between two adjacent molecules. It is shown for the ladder polymer BBL that the dielectric function, including ionic and electronic contributions, can be calculated by a new method within the DFT and TD-DFT framework. In agreement with ellipsometry measurements, a highly anisotropic dielectric constant is obtained, which is 8.3 along the backbone of the polymer and around 3 perpendicular to the polymer. The high dielectric constant along the backbone originates from the strong delocalization of the electrons along the π-system. The ionic contribution increases the mean value of the dielectric constant from 3.6 to 4.2. In order to further increase the dielectric constant, different polar side-chains are attached to the ladder polymer BBL and their dielectric constant is calculated. A strong increase of the dielectric constant to about 17 is obtained by attaching a zwitterionic side-chain to the BBL monomer. In order to lower the exciton binding energy by a CT state, a charge transfer from a donor to an acceptor molecule must be introduced. The Coulomb binding energy of intermolecular CT states are calculated. It is shown that an intermolecular CT state of two π-stacked BBL oligomers does not exhibit a lower Coulomb binding energy as compared to the intramolecular binding energy. However, by a spatial separation of the donor and the acceptor molecule, in-line of the polymer backbone, the Coulomb binding energy is reduced from 0.40 eV to 0.24 eV. Combining such CT states with the high dielectric constant obtained by zwitterionic side-chains would lead to an exciton binding energy close to the thermal energy, resulting in spontaneous free carrier generation on neat materials. This could potentially reduce the voltage losses and increase the PCE in OPV devices significantly. / Die organische Photovoltaik stellt eine kostengünstige, erneuerbare und daher zukunftsgerichtete Energieversorgung dar. Die Umwandlungseffizienz organischer Solarzellen von Sonnenenergie in elektrische Energie konnte über die letzten fünf Jahre auf 12% verdoppelt werden. Kommerziell erhältliche anorganische Solarzellen weisen im Vergleich dazu eine Effizienz von ca. 20% auf. Eine Möglichkeit, die Effizienz organischer Solarzellen zu erhöhen, ist die Umwandlung von Licht in Elektrizität nicht nur im sichtbaren Bereich, sondern zusätzlich auch im infraroten Bereich des Sonnenspektrums. Der größte Unterschied zwischen den organischen und anorganischen Solarzellen liegt allerdings in der Exzitonbindungsenergie, welche in organischen Materialien ca. 20 Mal größer ist. Um das Exziton in freie Ladungsträger zu trennen, wird in organischen Solarzellen deshalb ein Donator-Akzeptor-Übergang benutzt, welcher unter anderem auch für den Spannungs- und damit für den Effizienzverlust von organischen Solarzellen verantwortlich ist. Im ersten Teil der Dissertation werden verschiedene funktionalisierte Donator-Moleküle, die infrarotes Licht absorbieren, untersucht. Die Donator-Moleküle ohne zusätzliche Funktionalisierungsgruppe weisen dabei die höchste Umwandlungseffizienz auf. In den besten Zellen kann ein Unterschied zwischen der optischen und effektiven \"Bandlücke\" von 0,17 eV gemessen werden. Dieser Unterschied stellt die treibende Kraft für den Übergang des Elektrons vom Donator zum Akzeptor dar. Da jedoch dieser Unterschied in der besten Solarzelle am geringsten ist, scheint die Dissoziation der Ladungsträger in den untersuchten Donator-Akzeptor-Systemem nicht vom ihm abzuhängen. Die gemessene relative hohe Leerlaufspannung von 0,81 V ist 0,74 V kleiner als die effektive Bandlücke und zeigt die hohen Spannungsverluste organischer Solarzellen. Die Spannungsverluste anorganischer Solarzellen liegen im Bereich von 0,4 V. Ein Ansatz, um die Spannungsverluste zu verkleinern, liegt in der Reduzierung der Exzitonbindungsenergie, woraus ein größeres Exziton erfolgt. Mit der zeitabhängigen Dichtefunktionaltheorie wird an einer Reihe organischer Moleküle gezeigt, dass die Exzitongröße bei einer Moleküllänge (oder Konjugationslänge) größer als 4nm bei 1,2nm sättigt. Für das größte Exziton, welches im Leiterpolymer Poly(benzimidazobenzophenanthroline) (BBL) vorhanden ist, wird eine Coulomb-Bindungsenergie von 0,4 eV berechnet und eine Exzitonbindungsenergie von 0,2 eV abgeschätzt. Die Exzitonbindungsenergie kann entweder durch Erhöhung der Dielektrizitätskonstante oder durch Erzeugung eines Ladungstransfer-Zustandes weiter verringert werden. Es wird gezeigt, dass mit einer neu entwickelten Methode auf Basis der Dichtefunktionaltheorie die ionischen und elektronischen Beiträge zur dielektrischen Funktion von BBL berechnet werden können. Die berechneten anisotropen Werte stimmen gut mit Werten aus Ellipsometriemessungen überein. Entlang der Polymerkette erhalten wir eine hohe Dielektrizitätskonstante von 8,3 und senkrecht dazu von ca. 3. Die hohe Dielektrizitätskonstante entlang der Polymerkette kann auf die starke Delokalisation der π-Elektronen zurückgeführt werden. Der Mittelwert der Dielektrizitätskonstante wird durch die ionischen Beiträge von 3,6 auf 4,2 erhöht. Um die Dielektrizitätskonstante weiter zu erhöhen, werden verschiedene polare Seitenketten am BBL-Polymer angebracht und die Dielektrizitätskonstante berechnet. Es wird gezeigt, dass die Anbringung einer zwitterionischen Seitenkette am BBL-Monomer die Dielektrizitätskonstante auf 17 erhöht. Um die Exzitonbindungsenergie durch einen Ladungstransfer-Zustand zu verringern, werden ein Donator- und ein Akzeptor-Molekül benötigt. Die Coulomb-Bindungsenergien der intermolekularen Ladungstransfer-Zustände werden berechnet. Es wird gezeigt, dass intermolekulare Ladungstransfer-Zustände zwischen zwei π-gestapelten BBL-Oligomeren keine Verringerung der Coulomb-Bindungsenergie bewirken. Bei einer räumlichen Trennung des Donator- und Akzeptor-Moleküls entlang der Polymerkette kann die Coulomb-Bindungsenergie von 0,40 eV auf 0,24 eV gesenkt werden. Eine Kombination aus diesem Ladungstransfer und der Erhöhung der Dielektirizitätskonstante durch zwitterionische Seitenketten kann zu einer niedrigen Exzitonbindungsenergie, nahe der thermischen Energie, und damit zu freien Ladungsträgern führen. Der damit verringerte Spannungsverlust kann die Umwandlungseffizienz organischer Solarzellen signifikant erhöhen.

Organic photovoltaic (OPV) is a promising technology for renewable energy at low cost. Over the last five years, the power conversion efficiency (PCE) has doubled to 12%, which still is clearly lower than commercially available inorganic solar cells with a PCE around 20%. One approach to improve the PCE is to complement the conversion of light into electrical energy with the infrared (IR) part of the solar spectrum. However, the fundamental difference between organic and inorganic semiconductors is the exciton binding energy. Compared to inorganic semiconductors, in organic materials the exciton binding energy is more than 20 times higher, leading to strongly bound electron hole pairs, which are primarily generated upon photo-excitation. To dissociate these charges, in OPV, a donor-acceptor system is used. However, the energetics of this donor-acceptor system lowers the obtained open circuit voltage, representing one major loss of OPV as compared to inorganic solar cells. In the first part of this work, three benzannulated 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (aza-BODIPY) infrared absorbing donor dyes with methyl, methoxy, or without side-group attached are investigated. The solar cells with the highest PCE, i.e. the devices using the donor molecule without a side-group, exhibit a difference between the optical and the effective gap of 0.17 eV. It reflects the "driving force" for the electron to transfer from the donor to the acceptor, and is lowest for the best performing device, indicating that in the devices used efficient charge dissociation does not require large electron transfer energy loss. A - for OPV - relatively high open circuit voltage of 0.81 V is measured and when compared to the optical gap of the donor, a voltage loss of 0.74 V is obtained, reflecting the high voltage losses in OPV. In inorganic devices these voltage loss is around 0.4 V. One approach to lower this difference in the voltage loss is to lower the exciton binding energy of neat organic materials, leading to a larger exciton size. A saturation of the exciton size at about 1.2 nm is calculated by time dependent density functional method (TD-DFT) for one dimensional conjugated organic molecules with a size larger than 4nm. For the largest size of the exciton, provided by the poly(benzimidazobenzophenanthroline) (BBL), a Coulomb interaction of 0.4 eV between the electron and hole wave function is calculated, leading to an estimated exciton binding energy of about 0.2 eV, serving as a lower limit for the organic molecules investigated. The exciton binding energy can further be lowered by increasing the dielectric constant or by introducing a charge transfer (CT) state between two adjacent molecules. It is shown for the ladder polymer BBL that the dielectric function, including ionic and electronic contributions, can be calculated by a new method within the DFT and TD-DFT framework. In agreement with ellipsometry measurements, a highly anisotropic dielectric constant is obtained, which is 8.3 along the backbone of the polymer and around 3 perpendicular to the polymer. The high dielectric constant along the backbone originates from the strong delocalization of the electrons along the π-system. The ionic contribution increases the mean value of the dielectric constant from 3.6 to 4.2. In order to further increase the dielectric constant, different polar side-chains are attached to the ladder polymer BBL and their dielectric constant is calculated. A strong increase of the dielectric constant to about 17 is obtained by attaching a zwitterionic side-chain to the BBL monomer. In order to lower the exciton binding energy by a CT state, a charge transfer from a donor to an acceptor molecule must be introduced. The Coulomb binding energy of intermolecular CT states are calculated. It is shown that an intermolecular CT state of two π-stacked BBL oligomers does not exhibit a lower Coulomb binding energy as compared to the intramolecular binding energy. However, by a spatial separation of the donor and the acceptor molecule, in-line of the polymer backbone, the Coulomb binding energy is reduced from 0.40 eV to 0.24 eV. Combining such CT states with the high dielectric constant obtained by zwitterionic side-chains would lead to an exciton binding energy close to the thermal energy, resulting in spontaneous free carrier generation on neat materials. This could potentially reduce the voltage losses and increase the PCE in OPV devices significantly. / Die organische Photovoltaik stellt eine kostengünstige, erneuerbare und daher zukunftsgerichtete Energieversorgung dar. Die Umwandlungseffizienz organischer Solarzellen von Sonnenenergie in elektrische Energie konnte über die letzten fünf Jahre auf 12% verdoppelt werden. Kommerziell erhältliche anorganische Solarzellen weisen im Vergleich dazu eine Effizienz von ca. 20% auf. Eine Möglichkeit, die Effizienz organischer Solarzellen zu erhöhen, ist die Umwandlung von Licht in Elektrizität nicht nur im sichtbaren Bereich, sondern zusätzlich auch im infraroten Bereich des Sonnenspektrums. Der größte Unterschied zwischen den organischen und anorganischen Solarzellen liegt allerdings in der Exzitonbindungsenergie, welche in organischen Materialien ca. 20 Mal größer ist. Um das Exziton in freie Ladungsträger zu trennen, wird in organischen Solarzellen deshalb ein Donator-Akzeptor-Übergang benutzt, welcher unter anderem auch für den Spannungs- und damit für den Effizienzverlust von organischen Solarzellen verantwortlich ist. Im ersten Teil der Dissertation werden verschiedene funktionalisierte Donator-Moleküle, die infrarotes Licht absorbieren, untersucht. Die Donator-Moleküle ohne zusätzliche Funktionalisierungsgruppe weisen dabei die höchste Umwandlungseffizienz auf. In den besten Zellen kann ein Unterschied zwischen der optischen und effektiven \"Bandlücke\" von 0,17 eV gemessen werden. Dieser Unterschied stellt die treibende Kraft für den Übergang des Elektrons vom Donator zum Akzeptor dar. Da jedoch dieser Unterschied in der besten Solarzelle am geringsten ist, scheint die Dissoziation der Ladungsträger in den untersuchten Donator-Akzeptor-Systemem nicht vom ihm abzuhängen. Die gemessene relative hohe Leerlaufspannung von 0,81 V ist 0,74 V kleiner als die effektive Bandlücke und zeigt die hohen Spannungsverluste organischer Solarzellen. Die Spannungsverluste anorganischer Solarzellen liegen im Bereich von 0,4 V. Ein Ansatz, um die Spannungsverluste zu verkleinern, liegt in der Reduzierung der Exzitonbindungsenergie, woraus ein größeres Exziton erfolgt. Mit der zeitabhängigen Dichtefunktionaltheorie wird an einer Reihe organischer Moleküle gezeigt, dass die Exzitongröße bei einer Moleküllänge (oder Konjugationslänge) größer als 4nm bei 1,2nm sättigt. Für das größte Exziton, welches im Leiterpolymer Poly(benzimidazobenzophenanthroline) (BBL) vorhanden ist, wird eine Coulomb-Bindungsenergie von 0,4 eV berechnet und eine Exzitonbindungsenergie von 0,2 eV abgeschätzt. Die Exzitonbindungsenergie kann entweder durch Erhöhung der Dielektrizitätskonstante oder durch Erzeugung eines Ladungstransfer-Zustandes weiter verringert werden. Es wird gezeigt, dass mit einer neu entwickelten Methode auf Basis der Dichtefunktionaltheorie die ionischen und elektronischen Beiträge zur dielektrischen Funktion von BBL berechnet werden können. Die berechneten anisotropen Werte stimmen gut mit Werten aus Ellipsometriemessungen überein. Entlang der Polymerkette erhalten wir eine hohe Dielektrizitätskonstante von 8,3 und senkrecht dazu von ca. 3. Die hohe Dielektrizitätskonstante entlang der Polymerkette kann auf die starke Delokalisation der π-Elektronen zurückgeführt werden. Der Mittelwert der Dielektrizitätskonstante wird durch die ionischen Beiträge von 3,6 auf 4,2 erhöht. Um die Dielektrizitätskonstante weiter zu erhöhen, werden verschiedene polare Seitenketten am BBL-Polymer angebracht und die Dielektrizitätskonstante berechnet. Es wird gezeigt, dass die Anbringung einer zwitterionischen Seitenkette am BBL-Monomer die Dielektrizitätskonstante auf 17 erhöht. Um die Exzitonbindungsenergie durch einen Ladungstransfer-Zustand zu verringern, werden ein Donator- und ein Akzeptor-Molekül benötigt. Die Coulomb-Bindungsenergien der intermolekularen Ladungstransfer-Zustände werden berechnet. Es wird gezeigt, dass intermolekulare Ladungstransfer-Zustände zwischen zwei π-gestapelten BBL-Oligomeren keine Verringerung der Coulomb-Bindungsenergie bewirken. Bei einer räumlichen Trennung des Donator- und Akzeptor-Moleküls entlang der Polymerkette kann die Coulomb-Bindungsenergie von 0,40 eV auf 0,24 eV gesenkt werden. Eine Kombination aus diesem Ladungstransfer und der Erhöhung der Dielektirizitätskonstante durch zwitterionische Seitenketten kann zu einer niedrigen Exzitonbindungsenergie, nahe der thermischen Energie, und damit zu freien Ladungsträgern führen. Der damit verringerte Spannungsverlust kann die Umwandlungseffizienz organischer Solarzellen signifikant erhöhen.

No description available.

Time resolved spectroscopy is one of the crucial methods used to study processes on molecular level in biological systems. It is useful especially for monitoring fast processes that take a place in photosynthetic apparatus of photosynthetic organisms, such as electron and energy transfer. The integral parts of photosynthetic apparatus are carotenoids, whose role in the photosynthetic apparatus is not as well explored as it is for chlorophylls. It was proved that carotenoids actively participate in energy transfer processes in photosynthetic antennas. They have a crucial role in protection against excess energy damage. They are also electron donors in both antennas and reaction centers. The fact that photo-physical properties of carotenoids are much different from properties of others organic pigments, complicates studies of their functions in photosynthesis as well as in other biological systems. This thesis employs advanced methods of femtosecond spectroscopy to obtain more information about carotenoid functions in some biological systems and in solution with special focus on carotenoids containing carbonyl group.