All-solution processed regular organic solar cells using a new inkjet-printable cathode

The current developments in polymer organic solar cells are inspired by the idea that they can be processed entirely from solution. In all polymer cells the photoactive layer is processed from solution, most often the electrodes are not. Holst Centre has already developed a solution-processable anode, this project focusses on the development of a solution-processable cathode1 for solar cells with a "regular" configuration - that is, the cathode has to be processed on top of the other layers. To make a solution-processable cathode it is required to use high-workfunction metals which can be processed in the form of ink, like silver. When silver is used a cathode modification layer (Electron Transport Layer or ETL) is required. Out of several of materials for these layers that have been studied two were successful: zinc oxide (ZnO) processed from a nanoparticle dispersion in acetone and the polymer PFN which has been dissolved in ethanol. Solar cells with these ETLs and an evaporated silver cathode perform 12% lower than the reference design (with a LiF aluminum cathode). When comparing the reference design with an evaporated silver electrode without ETL the performance loss is 43%, thus the ETL significantly improves performance. The printing of silver on top of the ZnO layer was problematic because of crack formation. It was however shown that the principle worked, thus several methods were employed to prevent cracks. None gave reliable results. The layers were studied using AFM, conductive AFM, SEM and cross-sectional TEM. The printing of silver on top of PFN was successful: 35 out of 40 produced cells were working with the best performances reaching over 50% of the performance of the reference design. This success warranted the combination of the Holst anode with the new cathode to create all-solution processed devices with photovoltaic power conversion efficiencies reaching 1.95%.. - First prize Shell Bachelor Master 2012 "Organische zonnecellen : van lab tot fab". The current developments in polymer organic solar cells are inspired by the idea that they can be processed entirely from solution. In all polymer cells the photoactive layer is processed from solution, most often the electrodes are not. Holst Centre has already developed a solution-processable anode, this project focusses on the development of a solution-processable cathode1 for solar cells with a "regular" configuration - that is, the cathode has to be processed on top of the other layers. To make a solution-processable cathode it is required to use high-workfunction metals which can be processed in the form of ink, like silver. When silver is used a cathode modification layer (Electron Transport Layer or ETL) is required. Out of several of materials for these layers that have been studied two were successful: zinc oxide (ZnO) processed from a nanoparticle dispersion in acetone and the polymer PFN which has been dissolved in ethanol. Solar cells with these ETLs and an evaporated silver cathode perform 12% lower than the reference design (with a LiF aluminum cathode). When comparing the reference design with an evaporated silver electrode without ETL the performance loss is 43%, thus the ETL significantly improves performance. The printing of silver on top of the ZnO layer was problematic because of crack formation. It was however shown that the principle worked, thus several methods were employed to prevent cracks. None gave reliable results. The layers were studied using AFM, conductive AFM, SEM and cross-sectional TEM. The printing of silver on top of PFN was successful: 35 out of 40 produced cells were working with the best performances reaching over 50% of the performance of the reference design. This success warranted the combination of the Holst anode with the new cathode to create all-solution processed devices with photovoltaic power conversion efficiencies reaching 1.95%.. - First prize Shell Bachelor Master 2012 "Organische zonnecellen : van lab tot fab"

FluorMODleaf: A new leaf fluorescence emission model based on the PROSPECT model

A new model of chlorophyll a fluorescence emission by plant leaves, FluorMODleaf, is presented. It is an extension of PROSPECT, a widely used leaf optical properties model that regards the leaf as a pile of N absorbing and diffusing elementary plates. In FluorMODleaf, fluorescence emission of an infinitesimal layer of thickness dx is integrated over the entire elementary plate. The fluorescence source function is based on the excitation spectrum of diluted isolated thylakoids and on the emission spectra of isolated photosystems, PSI and PSII, which are the main pigment-protein complexes involved in the initial stages of photosynthesis. Scattering within the leaf is produced by multiple reflections within and between elementary plates. The input variables of FluorMODleaf are: the number of elementary plates N, also called leaf structure parameter, the total chlorophyll content Cab, the total carotenoid content Ccx, the equivalent water thickness Cw, and the dry matter content Cm (or leaf mass per area), as in the new PROSPECT-5, plus the σII/σI ratio referring to the relative absorption cross section of PSI and PSII, and the fluorescence quantum efficiency of PSI and PSII, τI and τII, that are introduced here as mean fluorescence lifetimes. The model, which considers the reabsorption of emitted light within the leaf, allows good quantitative estimation of both upward and downward apparent spectral fluorescence yield (ASFY) at different excitation wavelengths from 400 nm to 700 nm. It also emphasizes the role of scattering in fluorescence emission by leaves having high chlorophyll content

Hybrid solar cells

Solarzellen sind als "alternative Energiequelle" mehr denn je im Fokus von Forschung und Entwicklung. Derzeit basieren praktisch alle kommerziell erhältlichen Module auf klassischen Halbleitermaterialien wie Silizium. Dieses ist in sehr hoher Reinheit und einkristallin verfügbar, woraus sehr gute Materialeigenschaften resultieren. Gewinnung und Reinigung sind allerdings sehr kostenintensiv und energieaufwendig. Insbesondere weist Silizium, im Vergleich zu Farbstoffmolekülen, einen sehr geringen Absorptionskoeffizienten auf. Durch Verwendung von organischen Halbleitern kann daher u.a. die Schichtdicke von Solarzellen drastisch reduziert werden. Neben d\"unnen Schichten verspricht man sich von günstiger Prozessierung erheblich niedrigere Herstellungskosten. Aufgrund anderer Transportmechanismen (Hopping-Transport) zeigen organische Halbleiter eine erheblich niedrigere Ladungsträgermobilität als anorganische Halbleiter (Bandtransport). Zudem ist in organischen Solarzellen nach der Photonenabsorption eine Trennung der noch gebundenen Ladungträger (Exzitonen) nötig. Die Kombination aus organischen und anorganischen Halbleitern für die Photovoltaik wird Hybrid-Solarzellen genannt. Hiervon verspricht man sich die Nutzung der hohen Absorbanz des organischen Materials und der guten Transporteigenschaften der verwendeten anorganischen Halbleiter. Bislang kamen hauptsächlich Polymere zum Einsatz. Wenig Erfahrung gibt es hingegen in der Kombination von anorganischen Halbleitern und kleinen Molekülen mit aromatischen Ringen. Diese zeigen gute optische Eigenschaften. Dies wurde in der vorliegenden Arbeit am Beispiel von Zink(II)-Phthalocyanin (ZnPc) nachgewiesen. Optische Spektroskopie wurde verwendet, um die optischen Konstanten, Schichtdicke und Rauigkeit der Schichten simultan zu bestimmen. Eine organisch-anorganische Grenzfläche innerhalb einer Hybrid-Solarzelle wurde aus ZnPc und Zinkoxid hergestellt und charakterisiert. Hierfür wurde mittels Photelektronenspektroskopie der Verlauf der Bandstruktur innerhalb des Bauelemtes nachvollzogen. Mit Hilfe dieser Methode wurden Abschätzungen für die Leerlaufspannung getroffen und anhand von Strom-Spannungs-Kennlinien überprüft. Die Kennlinien weisen einen sehr geringen Photostrom auf. Die Ursache dafür scheint eine schlechte Exzitonendissoziation zu sein. Hierfür wurden zwei Verbesserungsansätze gewählt. Zum einen wurde die Bandstruktur mittels Dotierung modifiziert, um die Energie zu erhöhen, welche für die Exzitonentrennung zur Verfügung steht. Zum anderen sollte durch Nanodrähte die Distanz zum dissoziationsverursachenden p-n-Übergang verringert werden, um so in die Reichweite der Exzitonen zu gelangen. Anhand von spektral aufgelösten Photostrommessungen konnte die Exzitonendiffusionslänge auf 16 nm bestimmt werden. Eine Steigerung der Effizienz wurde leider nicht erzielt

A Theoretical Perspective of the Photochemical Potential in the Spectral Performance of Photovoltaic Cells

We present a novel theoretical approach to the problem of light energy conversion in thermostated semiconductor junctions. Using the classical model of a two-level atom, we deduced formulas for the spectral response and the quantum efficiency in terms of the input photons' non-zero chemical potential. We also calculated the spectral entropy production and the global efficiency parameter in the thermodynamic limit. The heat transferred to the thermostat results in a dissipative loss that appreciably controls the spectral quantities' behavior and, therefore, the cell's performance. The application of the obtained formulas to data extracted from photovoltaic cells enabled us to accurately interpolate experimental data for the spectral response and the quantum efficiency of cells based on Si-, GaAs, and CdTe, among others

Photonics and transport in bulk heterojunction organic solar cells

In this thesis, the groundwork is established for a new type of bulk heterojunction (BHJ) organic solar cell geometry that has photonic crystal (PC) photoactive layers. This design is motivated by the need to improve light absorption without increasing active layer thickness, which for many BHJ systems, degrades electrical performance. It is demonstrated that with the right choice of materials and cell dimensions, quasiguided or resonant modes are excited near the band edge of a variety of BHJ blends to enhance absorption. Resonant modes are predicted by first developing a scattering matrix optical model and then observed in wavelength-, polarization-, and angular-dependent reflection and photocurrent measurements. PC cells are fabricated using a facile nanopatterning technique, where highly ordered arrays of submicron features are constructed over large areas in a single step. Optical and electrical function of this new cell architecture is fully explored in this thesis. Through optical measurements and modeling, PC devices show clear enhancements in light absorption. On the other hand, the impact of the nonplanar geometry on electrical performance is not as easily deduced due to the multitude of electrical processes that lead to photocurrent generation. First, the electrical properties of the electron transporting layer that interfaces with the BHJ nanopattern and provides optical contrast in the PC greatly affect parasitic resistances in the solar cell. By including resistance losses in a drift/diffusion numerical model that describes electrical performance, it is shown that these losses greatly influence fundamental steps leading to photocurrent generation. This is confirmed with experiment by comparing two BHJ material systems that have different affinities for exciton separation. Second, significant levels of free carrier recombination are predicted by the electro-optical model due to the relatively long transport paths in the nanopattern features. To test this prediction, an experimental technique is developed to measure the transport lengths of photogenerated electrons and holes in BHJ solar cells. It is found that transport lengths of positive and negative carriers are mismatched and helps explain both PC electrical performance and recent conflicting results of planar BHJ solar cells in the literature

Strategies to Optimizing Dye-Sensitized Solar Cells:Organic Sensitizers, Tandem Device Structures, and Numerical Device Modeling

Dye-sensitized solar cells (DSCs) constitute a novel class of hybrid organic-inorganic solar cells. At the heart of the device is a mesoporous film of titanium dioxide (TiO2) nanoparticles, which are coated with a monolayer of dye sensitive to the visible region of the solar spectrum. The role of the dye is similar to the role of chlorophyll in plants; it harvests solar light and transfers the energy via electron transfer to a suitable material (here TiO2) to produce electricity — as opposed to chemical energy in plants. DSCs are fabricated of abundant and cheap materials using inexpensive processes (e.g. screen-printing) and are likely to be a significant contributor to the future commercial photovoltaic technology portfolio. The work conducted during this thesis aimed at optimizing the DSC using three different strategies: The use of versatile organic sensitizers for stable and efficient DSCs, the study of tandem device architectures in combination with other solar cells to harvest a larger fraction of the solar spectrum, and the development of a validated optoelectric model of the DSC. Organic donor-π-acceptor dyes are an interesting alternative to the standard metal-organic complexes used in DSCs. Efficient photovoltaic conversion and stable performance could be demonstrated with three classes of donor systems, namely diphenylamine, difluorenylaminophenyl, and π-extended tetrathiafulvalene. The highest conversion efficiencies were obtained with a difluorenylaminophenyl donor system (η = 8.3 % with a volatile electrolyte and η = 7.6 % with a solvent-free ionic liquid, which was a new record for organic dyes at the time of publication). Surprisingly, efficient regeneration of the oxidized dye by the I-/I3- redox mediator was found with the π-extended tetrathiafulvalene system, even though the thermodynamic driving force was as low as 150 mV. So far driving forces of 300-500 mV had been regarded as necessary for efficient regeneration of the dye cation. Also, important structure-property relationships pertaining to the recombination of electrons with the electrolyte and to the stability of the device could be identified (i.e. effect of linear vs. branched structure, linker length, and moieties used). The power conversion efficiency of solar cells can be extended beyond the limit for a single cell (∼ 30 %) by using multiple cells with different optical gaps in a tandem device. DSCs and chalcopyrite Cu(In,Ga)Se2 (CIGS) solar cells have complementary optical gaps and are thus suitable systems for integration in a tandem device. It was shown that a monolithic DSC/CIGS tandem device has the potential for increased efficiency over a mechanically stacked device due to increased light transmission to the bottom cell, and a monolithic DSC/CIGS device with an initial efficiency of η = 12.2 % was demonstrated. The degradation of the devices — induced by the corrosion of the CIGS cell in contact with the I-/I3- redox mediator — could be retarded with a protective thin conformal ZnO/TiO2 oxide layer coated on the CIGS cell by atomic layer deposition. Finally, an experimentally validated optical and electrical model of the DSC has been developed to assist the optimization process, which is predominantly conducted by empirical means in the DSC research community. The optical model allows to accurately calculate the internal quantum efficiency of devices, i.e. the ratio of extracted electrons to absorbed photons by the dye, a crucial and so far difficult to determine characteristic. Intrinsic parameters — like injection efficiency, electron diffusion length, or distribution of trap states in the TiO2 — can be extracted from experimental steady-state and time-dependent data with the electric model. The model allows to make a comprehensive and quantitative loss analysis of the different optical and electric loss channels in the DSC. The model has been implemented with a graphical user interface for straightforward usage. All three optimization strategies — organic dyes, tandem architecture, and device modeling — developed during this thesis make a valuable contribution to the development and commercialization of inexpensive and high efficiency DSCs. They enable a comprehensive view of the system and pave the way for a systematic analysis and reduction of losses, which has been the ultimate route to success for several established photovoltaic technologies