Elucidating the role of chlorine in perovskite solar cells

It has been proposed that introducing the chlorine anion into a CH3NH3PbI3 perovskite material can substantially improve the materials properties as well as the solar cell performance. To elucidate the role of chlorine in perovskite solar cells (PSCs), here we introduced PbCl2 into the precursor, and studied the chlorine configuration evolution during perovskite film formation and the associated influence on PSC performance in detail. We found that chlorine could be successfully incorporated into the precursor film in the form of PbICl or PbCl2 through a properly designed preparation, and it was conserved in the final perovskite film with a configuration of MAPbCl(3), PbICl or PbCl2 depending on the fabrication process. However, no evidence of a MAPbI(3-x)Cl(x) phase was observed, and it is considered that MAPbI(3-x)Cl(x) might be metastable or possesses a higher formation energy. In addition, we demonstrate that the formation of a porous PbICl scaffold in the precursor film plays a key role in high quality perovskite film realization, benefiting from an effective stress release during structure expansion after methylammonium iodide dripping. Moreover, we propose that residual amorphous PbCl2 can effectively passivate defects in perovskite film, and dramatically improve the film's electrical properties. Finally, n-i-p type planar PSCs with efficiencies up to 19.45% were achieved. It should be mentioned that the whole process for the formation of the PSCs is performed at less than 100 degrees C, which is beneficial for a wide range of applications, such as flexible and tandem solar cells.

Self-tracking Solar Concentration

Volker Günter Zagolla

Solar concentration is intended to decrease the cost of expensive solar cell material by replacing the solar cell with a focusing optical system as the initial receiving aperture. This reduces the amount of solar cell material needed but adds the need of tracking to the entire system in order to keep the smaller solar cell at the focal spot. In state of the art concentrators, precise tracking (less than 1°) is needed in two directions to cover diurnal and seasonal variations. This adds to the cost of the system, diminishing the savings from the smaller solar cell, and consumes energy which in turn decreases the overall system efficiency. A self-tracking concentrator has the potential to increases the tolerances of the system (less precise tracking needed) and cover seasonal variations (only tracking in one dimension remains). Self-tracking makes use of the energy of the spectrum that is typically not utilized in PV, thus self-tracking does not consume an outside surplus of energy. During the thesis such a self-tracking system was developed. This thesis describes two concepts for the realization of a self-tracking concentrator, one of which is then developed into a working demonstration device with an angular acceptance of 32° (±16°). The work covers both the development of these two different mechanisms as well as an analysis of their performance as a single element and a discussion of their performance as part of a much larger device. The first concept, the micro-fluidic concentrator, uses vapor bubbles to efficiently couple light into a liquid waveguide (chapter 3), while the second concept, based on a phase change actuation uses the thermal expansion of paraffin wax to create a coupling feature at a waveguide (chapter 4). Based on the findings, the phase-change concentrator was then expanded into a self-tracking concentrator with a possible effective concentration of 10x (chapter 5). As a result this thesis shows the feasibility of a self-tracking concentrator and describes in detail the processes and techniques for its realization. Future work based on the designs presented in this thesis can optimize the concepts. Simulations show that a system based on this approach can achieve 150x effective concentration by scaling the system collecting area to reasonable dimensions (30 x 1 cm²). An optimized optical system can then also increase the acceptance angle of the self-tracking concentrator to the ±23° needed in order to cover seasonal variations. In addition, the presented concept is based on earth abundant, inexpensive materials, making it technologically and economically viable to extend it to much larger formats.

EPFL2015

Window Layers for Silicon Heterojunction Solar Cells

Johannes Peter Seif

Currently, crystalline silicon (c-Si) wafer-based solar cells dominate the photovoltaic market (80-90%). In this thesis we concentrate on silicon heterojunction (SHJ) solar cells that--in contrast to diffused homojunction cells--rely on the application of amorphous silicon (a-Si:H) thin films. Unlike standard homojunction devices, which are typically limited by their highly recombination-active semiconductor-to-metal contacts, SHJ devices exhibit excellent surface passivation enabled by intrinsic and doped a-Si:H films. These a-Si:H layers, however, entail drawbacks for optical performance and carrier transport, two topics that will be addressed in this work. To this end we investigate non-traditional materials for SHJ devices, with the goal of replacing the a-Si:H or the transparent electrodes. These materials include microcrystalline silicon (uc-Si:H) and organic semiconductors for contact formation; amorphous silicon suboxides (a-SiOx:H) for surface passivation; and transparent electrodes applied by atomic layer deposition (ALD) as protective layers against subsequent processing steps. Along with the optical and electrical properties of these materials, we study the impact on device performance associated with their deposition. For this we test the devices under standard testing conditions (25 °C) and at elevated temperatures closer to those encountered in the field. For the investigations on uc-Si:H layers, we vary process parameters (including temperature, pressure, power, excitation frequency and hydrogen dilution) as well as pre-treatments, gas variations and nucleation layers. We assess the suitability of these approaches for SHJ solar cells and apply selected measures in devices. Thereby we demonstrate a gain in short-circuit current density in the range of 0.5-1 mA.cm-2 and good fill factor values of up to 79.2% using either n- and p-type uc-Si:H layers. Furthermore, with the goal of reducing optical losses, we test wide-bandgap a-SiOx:H layers for passivation. In terms of current gain without negative side effects on transport, we argue that these layers are best applied to the electron-collecting contact--put at the front of the device--as their application to the hole-collecting contact introduces a transport barrier for holes. This barrier deteriorates the device performance at 25 °C, but shows a beneficial effect on the temperature coefficient of the device, yielding coefficients as low as -0.1%/°C. In some cases--compared to standard devices--devices with a-SiOx:H layers exhibit superior performance at elevated temperatures, which can be of interest in warmer climates. In parallel to these main topic we also study aluminium-doped zinc oxide (ZnO:Al) layers deposited by ALD as a protective layer against sputter-induced damage and organic semiconductors as transparent electrodes for the hole-collecting contact. In both cases we observe a gain in terms of surface passivation, which indicates that these materials may be beneficial for the contact formation in future device structures. In addition to these material-related investigations, we unravel the temperature-dependence of each individual cell parameter and present a brief comparison of state-of-the-art technologies and their respective temperature dependence.

EPFL2015

Material Development for Perovskite/Silicon Tandem Photovoltaics

Peter Joseph Fiala

The developed world is built on the fact that energy is readily available and functionally infinite. The electricity from the wall, the gas at the station, and the heat in our homes are reliable and low-cost. But this comfort is so far only possible through the combustion of fossilized carbon sources. This is not a sustainable system. Not only are these sources finite, but the long-term effects of this much combustion are catastrophic for our planet and species. Transitioning from current energy production to sustainable sources will require the development and deployment of a diverse group of technologies. Chief among these is the generation of electricity from sunlight, or photovoltaics (PV).Most modern PV is based on crystalline silicon. Silicon PV alone could be scaled to provide the necessary energy production, but this could be realized more quickly and efficiently if more advanced technologies are used. The defining metric is the cost per generated electricity. In the case of silicon, the current cost breakdown of materials, manufacturing, installation, and system costs leaves little room for further price reduction; and light-to-electricity efficiencies are near their practical limit. Of the proposed alternatives, the pairing of silicon with a thin film of lead-halide perovskite in a multijunction or "tandem" architecture is one of the most promising. This splits the solar spectrum in two, and raises the theoretical maximum efficiency from 33.3% to 45.1% based on each material better using its segment than any one material could use the full spectrum. The central goal of this thesis was therefore to advance the capability of this type of solar cell while remaining within the constraints of industrially-plausible production methods.Towards this goal, we first isolated perovskite layers to study what limits their performance. This led to improvements in the bulk material quality, the interfaces with adjacent materials, and the layer morphology. We reduced bulk disorder via balancing the annealing temperature, time, and environment. The interfaces were improved by selecting passivating materials which were compatible with our deposition methods. We further observed a complex interplay between the materials used and the environment of the annealing, which illuminated critical dependencies of device performance and gave us a more complete understanding of our material. Finally the morphology was improved via high-temperature annealing. This was enabled by a surface-binding additive which reduced the detrimental effects of such an anneal while preserving the benefits, resulting in a perovskite with less internal resistance. From there, we moved outside of the perovskite layer and developed a light scattering ZnO layer which increased the optical performance and absorption of our cells. Returning to the main goal of the thesis, the lessons learned from work on isolated perovskites were applied in tandems on textured silicon. We additionally increased the perovskite layer thickness and made the bottom-cell more robust to processing damages. Together, the result was a stabilized efficiency of 27.3%. This is better than any silicon cell to date, but work remains to achieve 30% and beyond, extend material stability, scale up the deposition area, and transfer these capabilities to industry. Together, the work done as a part of this thesis represents a step forward in the journey towards carbon-neutral energy production.

EPFL2022

Window Layers for Silicon Heterojunction Solar Cells

Johannes Peter Seif

EPFL2015

Material Development for Perovskite/Silicon Tandem Photovoltaics

Peter Joseph Fiala

EPFL2022

Self-tracking Solar Concentration

Volker Günter Zagolla

EPFL2015