Evaluation of Cost-Effective Technologies for Highly Efficient Silicon-Based Solar Cells

The work underscores the feasibility of highly efficient silicon solar cell structures manufactured with high throughput machines. The main challenge consists in the implementation of more performant structures of intrinsic higher complexity. These structures are meant to be fabricated within similar constraints on time and on cost as for conventional devices of industrial manufacture. Achievements in this direction foster larger economical deployment of this kind of renewable energy. The structure chosen for the implementation of "high efficiency" concepts requires an advanced machining of the silicon substrate. Etching techniques, dielectric coating preparations, and metal to semiconductor contact formation were evaluated for their integration in a complete silicon solar cell fabrication sequence. These sequences were later tested to gain knowledge regarding the effects of the aforementioned advanced processing. Prototypes were created using the fabrication sequence. They were analysed to acquire a further understanding of the advanced processing influences. Statistical interpretation of the data obtained was used to support physical interpretation of the observed phenomena. For one particular implementation of a surface passivation (floating junction passivation) an attempt of modelling aiming to unveil its specific dynamics is reported. A resulting solar cell concept compatible with high throughput equipment achieved a certified efficiency of 18% (VOC = 635 mV, JSC = 37.3 mA/cm2, FF = 76 %) on substrate with a reduced thickness (120 µm). The resulting sequence prepares the back surface with a thermal oxidation for passivation purposes and with a laser technique to locally contact the bulk. The thickness is compatible with the intent of cost reduction through the decrease in material consumption. Other approaches performed on the same substrate achieved 17.1% efficiency (VOC = 617 mV, JSC = 36.1 mA/cm2, FF = 75 %). In this case the passivation was achieved with deposited dielectric layers on the back surface. Furthermore, the investigated dynamics of the floating junction passivation allows for better insight into its traits. The proposed solution has an interesting ratio of efficiency versus costs. The general consequence is a higher market appeal of this particular renewable source on the energy market. The study on the floating junction passivation allows a better exploitation of this particular implementation towards more performant silicon solar cells.

A Material Approach to Dye-Sensitized Solar Cells

Philippe Pierre Labouchère

With growing concerns on future energy supplies, solar energy appears as an energy source whose potential remains to be tapped at a large scale. In the last two decades, dye-sensitized solar cells (DSCs) have been considered as a competitive means to convert solar energy into electricity thanks to their low manufacturing and environmental costs, simple fabrication and high efficiency. Indeed, record efficiencies as high as 13% have been obtained for liquid DSCs and recently, solid-state DSCs using solid hole transport materials as well as perovskite materials as sensitizers have showed efficiencies above 17%. From a physical point of view, charge carrier generation in a DSC takes place in a photoactive light harvester chemisorbed on a mesoscopic semiconductor oxide. Due to the excellent electronic overlap, the exciton separation occurs by the injection of the excited electrons and holes from the dye into the conduction band of the semiconductor and redox electrolyte, respectively. A high surface area mesoscopic photoanode is necessary to ensure a high-density bulk heterojunction and efficient light harvesting in the visible part of the solar spectrum due to effective dye loading. This implies that the DSC has a large, heterogeneous interface through which photogenerated electrons may be intercepted as a result of the slow electron transport. The latter is governed by an ambipolar diffusion mechanism controlled by trap-limited hopping through a long path to the transparent conductive electrode. It is however hindered by the low electron mobility in anatase TiO2 nanoparticles and the multiple grain boundaries in the mesoporous film. The work conducted during this thesis aimed at developing new architectures for the electrodes of DSCs using notably atomic layer deposition (ALD) and inverse opal host-guest structures with different semiconducting metal oxides. First, using a self-assembled polymeric template and ALD, we fabricated a porous ZnO inverse opal backbone. Advantages are numerous: a high electronic mobility of ZnO, a precise control of the metal oxide deposition using ALD and in situ growth of ZnO nanowires at low temperatures. Besides increasing the surface area available for photoactive dye molecules, single-crystal nanowires offer a defect-free path for photoinjected electrons. However, because of the relative instability of ZnO in acidic environments, a TiO2 overlayer was deposited by ALD. This proved to increase the recombination resistance of photogenerated electrons and increased their lifetime in the porous film. This host-guest approach was further transposed to three related fields in photo-electrochemistry using different ALD precursors. For water splitting, a niobium-doped tin oxide inverse opal backbone was constructed, which served as a host for hematite deposited by atmospheric pressure chemical vapour deposition or ALD. In the field of plasmonics, a TiO2 inverse opal host was made to act as a scaffold for silver nanoparticles. For solid-state DSCs, we strived to achieve an ultra-thin ZnO inverse opal, which acted as a host for perovskite crystals. Finally, we studied the effect of very thin TiO2 and ZnO overlayers deposited by ALD on top of mesoporous ZnO and TiO2, respectively. We were able to accurately monitor the thickness of overlayers and study the effects on photovoltaic parameters such as transport and recombination rates of photogenerated electrons, photocurrent, photovoltage, fill factor and efficiency.

EPFL2014

Computational Modeling of the Oxygen Evolution Reaction at Semiconductor-Water Interfaces: A Path Towards Breaking Linear Scaling Relationships

Patrick Gono

Photoelectrochemical water splitting is a promising source of clean, renewable fuel in the form of hydrogen. Despite extensive research endeavors, the widespread adoption of this technology is impeded due to suboptimal catalysts for the oxygen evolution reaction (OER) taking place on the anode. Studies have shown that linear scaling relationships exist between the binding energies of the OER intermediates. These relationships are responsible for the emergence of an overpotential which limits the catalytic efficiency of electrochemical cells. This work is devoted to the study of the OER, the emergence of linear scaling relationships, and possible avenues for overcoming them. We employ a selection of ab-initio computational free energy methods based on density functional theory in order to investigate the OER at atomistic model semiconductor interfaces. An approach based on thermodynamic integration and \textit{ab-initio} molecular dynamics simulations is used to isolate the effect of the solvent on the OER free energy steps. The observed effect is twofold. First, the presence of explicit water affects the equilibrium configurations of the reaction intermediates. Second, explicit water modifies the OER free energy steps by up to 0.5 eV. This effect is rationalized in terms of electrostatic interactions at the interface. Next, we verify the existence of the linear scaling relationships across a set of semiconductor and metal interfaces. We establish the robustness of the scaling relationships with respect to the adopted energy functional. Two potential approaches of breaking the linear scaling are considered. First, a bifunctional mechanism that involves two close but functionally different active sites is investigated. The performance of the proposed mechanism is studied on a set of model interfaces, and the results indicate that specific combinations of catalysts may overcome the limitations imposed by the linear scaling relationship. We were able to establish general trends governing the efficiency of the bifunctional mechanism. In particular, a correlation between the valence band maximum and the hydrogen binding energy may be used as a descriptor in future searches for suitable hydrogen acceptors within the bifunctional scheme. Next, a detailed study of NiOOH/FeOOH catalysts identifies features most favorable to the bifunctional mechanism. Overpotentials as low as 0.13 eV are found for specific configurations. These results support the bifunctional mechanism as a means to break the linear scaling relationships. Second, hole polarons at the TiO

_2

surface induced by electronegative adsorbates in the OER intermediates are studied. The computational hydrogen electrode method in conjunction with a hybrid density functional is employed to quantify the effect of the surface polarons on the OER free energy steps. We find that the hole bipolarons reduce the overpotential of the reaction-determining step leading to good agreement with experiment. The stability of the polarons is further confirmed at the hydrated surface through a free energy study involving the explicit treatment of the solvent. Finally, the polaron energy level is aligned with respect to the redox levels in water to better understand the role of the polarons in the OER. Since the occurrence of surface hole polarons is unrelated to the scaling relationships, it offers an additional handle in the search for improved catalysts.

EPFL2020

Investigation on the impact of passivation in perovskite solar cells

Erfan Shirzadi

Conversion of sunlight to electricity is the most plausible approach towards the generation of renewable energies. During recent decade lead halide perovskites (LHPs) have proven to be promising candidates for photovoltaic applications such as solar cells. LHPs can be made easily in laboratories around the world via solution process methods and many universities can afford to participate in their development. How-ever, LHPs suffer from instability issues and their efficiencies can be further improved. In this thesis, both instability and efficiency challenges are addressed. We showed by doping the precursor of methylammonium lead iodide (MAPbI3) perovskite with a relatively large cation of 2-Choloroethylammonium (CLEA), perovskites films with CLEA incorporated into their structures are formed. The novel perovskite is able to withstand high temperatures as high as 80°C while achieving efficiencies > 19% in MAPbI3-based perovskite solar cells (PSCs). Various approaches have been employed for the passivation of the defects in perovskite solar cells Vari-ous approaches have been employed to passivate the defects in perovskite films used in perovskite solar cells (PSCs). However, the passivation mechanism is often unclear at a molecular level and the reason for enhanced PSC performance remains elusive. Here, we describe passivation based on the deposition of two dimensional (2D) perovskites (obtained from para-halophenylethylammonium iodide) on the surface of perovskite films. Moreover, the mechanism of passivation was elucidated and was traced to the high work function of the 2D perovskites, which induce band-bending at the surface of perovskite film and therefore reduces the charge carrier recombination at the interface of perovskite and the hole transporting material (HTM). Efficiencies >22% with average voltages >1.15 V were achieved using a typical mesoporous-TiO2 and Spiro-OMeTAD device configuration. This study paves the way for PSCs efficiency improvement by rational design. The employment of 2D perovskites is a promising approach to tackle the stability and voltage issues inherent in perovskite solar cells. It remains unclear however whether other perovskites with different dimensionalities have the same effect on efficiency and stability. Here, we report the use of quasi-3D azetidinium lead iodide (AzPbI3) as a secondary layer on top of the primary 3D perovskite film that results in significant improvements in the photovoltaic parameters. Remarkably, utilization of AzPbI3 leads to the passivation resulting in a power conversion efficiency (PCE) of 22.4 %. The open-circuit voltage obtained is as high as 1.18 V, which is among the highest reported to date for single-junction perovskite solar cells, corresponding to a voltage deficit of 0.37 V for a bandgap of 1.55 eV. Studying the compositional instability of mixed ion perovskites under light illumination is important to understand the mechanisms underlying their efficiency and stability. However, current techniques are limited in resolution and are unable to deconvolute minor ion migration phenomena. Here, we describe a method that enables ion migration to be studied allowing different segregation mechanisms to be elucidated. We applied statistical analysis to cathodoluminescence data to generate compositional distribution histograms. Using these histograms, we identified two different ion migration phenomena, horizontal ion migration (HIM) and vertical ion migration (

EPFL2021