Development of an inorganic cesium carbonate-based electron transport material for a 17% power conversion efficiency perovskite solar cell device

Brahim Aissa, Mohammad Khaja Nazeeruddin, Iwan Zimmermann
2020
Article

Résumé

A low-temperature solution process technique is employed to develop an inorganic cesium carbonate (Cs2CO3) as an electron transport material for inorganic-organic hybrid double cation (FAPbI(3))(0.85)(MAPbBr(3))(0.15) perovskite solar cells, as an alternative to the conventional thick and meso-TiO2. A device structure of compact-TiO2/Cs2CO3 (0.2 wt. %)/perovskite/spiro-OMETAD leads to enhanced performance of the photovoltaic device, achieving a short-circuit current density (J(sc)) of 22.26 mA/cm(2), an open-circuit voltage (V-oc) of 1054 mV, a fill factor (FF) of 71.6%, and a power conversion efficiency (PCE) of about 17% under one sun illumination, whereas the controlled device structure shows an efficiency of 16.58% without such surface modification layer. Additionally, a device structure of Cs2CO3 (6 wt. %)/perovskite/spiro-OMETAD without any TiO2 ETM has shown a J(sc) of 15.40 mA/cm(2), V-oc of 1023 mV, FF of 51.7%, and a PCE of 8.14%. On the other hand, external quantum efficiency (EQE) data yields around 85% of incident photon to electron conversion for c-TiO2/Cs2CO3 (0.2 wt. %)/perovskite/spiro-OMETAD structure and integrated J(sc) extracted from EQE data confirms that J(sc) obtained from the current-voltage test is within a close agreement. The obtained results indicate that there is a possibility to further increase the performance of perovskite-based cells and reduce their processing cost by replacing the thick mesoporous TiO2 by Cs2CO3. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)

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Brahim Aissa, Mohammad Khaja Nazeeruddin, Iwan Zimmermann
2020
Article

Résumé

Official source

https://infoscience.epfl.ch/record/286225?ln=fr

À propos de ce résultat

Concepts associés (14)

Concepts associés

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Publications associées (117)

Publications associées

Chargement

Strategies to Optimizing Dye-Sensitized Solar Cells

Sophie Wenger

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.

EPFL2010

Chargement

Charge collection and pore filling in solid-state dye-sensitized solar cells

Ilkay Cesar, Michael Graetzel, Robin Humphry-Baker, Henry Snaith, Shaik Mohammed Zakeeruddin

The solar to electrical power conversion efficiency for dye-sensitized solar cells (DSCs) incorporating a solid-state organic hole-transporter can be over 5%. However, this is for devices significantly thinner than the optical depth of the active composites and by comparison to the liquid electrolyte based DSCs, which exhibit efficiencies in excess of 10%, more than doubling of this efficiency is clearly attainable if all the steps in the photovoltaic process can be optimized. Two issues are currently being addressed by the field. The first aims at enhancing the electron diffusion length by either reducing the charge recombination or enhancing the charge transport rates. This should enable a larger fraction of photogenerated charges to be collected. The second, though less actively investigated, aims to improve the physical composite formation, which in this instance is the infiltration of mesoporous TiO2 with the organic hole-transporter 2,2',7,7'-tetrakis(N,N-di-p-methoxypheny-amine)-9,9'-spirobifluorene (spiro-MeOTAD). Here, we perform a broad experimental study to elucidate the limiting factors to the solar cell performance. We first investigate the charge transport and recombination in the solid-state dye-sensitized solar cell under realistic working conditions via small perturbation photovoltage and photocurrent decay measurements. From these measurements we deduce that the electron diffusion length near short-circuit is as long as 20 µm. However, at applied biases approaching open-circuit potential under realistic solar conditions, the diffusion length becomes comparable with the film thickness, ~2 µm, illustrating that real losses to open-circuit voltage, fill factor and hence efficiency are occurring due to ineffective charge collection. The long diffusion length near short-circuit, on the other hand, illustrates that another process, separate from ineffective charge collection, is rendering the solar cell less than ideal. We investigate the process of TiO2 mesopore infiltration with spiro-MeOTAD by examining the cross-sectional images of and performing photo-induced absorption spectroscopy on devices with a range of thickness, infiltrated with spiro-MeOTAD with a range of concentrations. We present our interpretation of the mechanism for material infiltration, and by improving the casting conditions demonstrate efficient charge collection through devices of over 7 µm in thickness. This investigation represents an improvement in our understanding of the limiting factors to the dye-sensitized solar cell. However, much work, focused on composite formation and improved kinetic competition, is required to realize the true potential of this concept.

2008

Chargement

Key elements involved in the fabrication of Micromorph thin-film silicon solar cells, a tandem device including an amorphous silicon top cell and a microcrystalline silicon bottom cell, are studied in this manuscript. Due to the very short diffusion lengths of photogenerated carriers in both materials, the photoactive layers of both sub-cells of Micromorph devices must be kept thin enough to ensure carrier collection. Due to this limited thickness, not all valuable light (i.e. light of higher energy than the band-gap) can be fully absorbed after one pass through the absorber layers. Advanced light-harvesting schemes are thus mandatory to achieve high conversion efficiencies. Random rough interfaces are typically used to induce light scattering in the photoactive layers, thus elongating the light path through these layers, enhancing their absorption. A simple and analytical way of modeling light harvesting in thin-film solar cells is developed. Its validity is demonstrated by comparing with experimental measurements involving different types of rough interfaces. It is shown that present light-scattering schemes come close to the best theoretically achievable scattering from random rough interfaces. With the morphology of a state-of-the-art rough ZnO layer, 32mA/cm2 can be obtained for a 1-μm-thick μc-Si:H layer, compared to 33.2 mA/cm2 for the Yablonovitch limit. Most of the gains in terms of light management are therefore to be made by making non-active layers more transparent (since these layers are presently responsible of ∼ 7 mA/cm2 of losses for a 1-μm-thick μc -Si:H layer). Parasitic absorption in non-active layers is also shown to be equally detrimental on both sides of the cell in the infrared part of the spectrum, corresponding to the wavelength range for which light trapping is most important. To improve significantly light trapping, complementary strategies to random rough interfaces must therefore be applied. For such a strategy, part of the light has to be prevented to escape from the cell. This can be obtained for example by using an angular filter, transmitting all light up to a certain incidence angle, and reflecting all light of higher incidence angle. An experimental optical setup based on spatial filtering is presented. It is shown to prevent 80% of light from escaping the cell, additionally to other light trapping strategies. A strong absorption enhancement of the complete device is demonstrated, at the cost of a reduction of the acceptance angle. However, most of the spared light is shown to be absorbed in non-active layers. A drastic reduction of parasitic absorption from these layers is therefore identified as a prerequisite to benefit from a better light trapping. Turning then to complete device analysis, the requirements of the front electrode for high- efficiency Micromorph devices are discussed point-by-point, focusing both on optical and electrical requirements. The need for sharp and relatively small features for an efficient coupling of light in the top cell is notably pointed out, as well as the need for features large enough to scatter light of wavelength up to 1100 nm for a high bottom cell current. The impact of sharp and large features on the electrical performances of the cells is also underlined. An optimal morphology is proposed, exhibiting features that are the smallest enabling scattering of light up to 1100 nm, and the sharpest that do not harm the electrical quality of the bottom cell. A device with a stable efficiency of 11.8% could be obtained on such a substrate. To avoid this trade-off, the combination of features with different sizes in a multi-scale elec- trode architecture is studied. It is shown that even though no gain in terms of light scattering is seen compared to a state-of-the-art single-scale morphology, a better electrical quality can be obtained, making the multi-scale approach of interest. A noteworthy 14.1% initial efficiency is demonstrated with such an architecture, with a short-circuit current density of 14 mA/cm2 for a total thickness of the silicon layers of 3 μm. Another way to add a degree of freedom in the design of a Micromorph device is also presented, employing a smoothening intermediate layer between both sub-cells. A separate tuning of the morphology inducing light scattering in each sub-cell can be made. Several routes are explored, out of which promising results are obtained by using a spin-coated lacquer on the top cell. A slight etching uncaps the tips of the top cell surface and allow for electrical conduction, whereas the smoothening effect is preserved. A large top cell current boost (up to+2.5mA/cm2)andopen-circuitvoltageimprovement(+50mV)areobtained,withmany adjustable processing parameters to obtain various morphologies. The newly developed concepts enable a better understanding of the present limitations of Micromorph devices. We believe that, by implementing these concepts in Micromorph tandem devices employing state-of-the-art amorphous and microcrystalline single-junction solar cells, stable 13.5% to 14% efficiency are within reach. Yet, increasing further the efficiency of Micromorph devices will likely require material improvements.

EPFL2012

Chargement

Strategies to Optimizing Dye-Sensitized Solar Cells

Sophie Wenger

EPFL2010

Charge collection and pore filling in solid-state dye-sensitized solar cells

Ilkay Cesar, Michael Graetzel, Robin Humphry-Baker, Henry Snaith, Shaik Mohammed Zakeeruddin

2008

EPFL2012