Towards multifunctional device concepts utilizing light absorption and charge storage in carbon nitrides

Gouder, Andreas (2023): Towards multifunctional device concepts utilizing light absorption and charge storage in carbon nitrides. Dissertation, LMU München: Fakultät für Chemie und Pharmazie

Abstract

This thesis comprises 5 parts. In Part 1, we start in Chapter 1 by giving a general introduction which includes a motivation (Section 1.1), an introduction into carbon nitrides as the main material class we utilize (Section 1.2), and an overview of important concepts from the field of energy conversion, energy storage, and new device concepts beyond energy storage (respective Section 1.3, Section 1.4, and Section 1.5). Since solar batteries are a multidisciplinary research endeavor that require input from very different research field directions, it is important to have a broad understanding of key concepts. We underline important messages in grey boxes and then proceed in Chapter 2 to underline the fundamental physical and electrochemical background ( Section 2.1 and Section 2.2) as well as electrochemical measurement techniques to characterize the devices (Section 2.3). Part 2 consists of two perspective papers, which propose fundamental considerations and design guidelines for two emerging research fields: optoionic devices and solar batteries. In our perspective in Chapter 4, we explain the emerging concept of light-assisted ionic effects, which are generally termed optoionic. We start by giving a historical overview over the field and also over related light-ion interaction effects termed photoionic – a term which is used far more often, but only partially relates to optoionics as we understand it today. We then proceed to explain our current understanding of optoionic effects in layered compounds and propose an extension to the picture, that is, the impact of short- or long-range field effects via a case study in carbon nitrides. Our perspective on solar batteries in Chapter 5 starts by explaining the fundamental Solar Battery Experiment and with this underline how light energy affects the energy and power density as well as operation modes of this new class of devices. We classify two main design routes for the devices: (1) Solar cell and battery can operate in parallel to the consumer and as such the light energy produces a photocurrent that increases the overall current output (IEC). (2) Solar cell and battery can operate in series to the consumer and the photopotential reduces the overall required charging voltage (VEC). We then continue to give an overview and classify all current solar battery designs in the respective category (two or one device designs, the latter with bifunctional electrodes or bifunctional materials) and explain how the respective electrochemical signature of the devices can be understood. We proceed in Part 3 to discuss the main three research projects associated to this thesis, namely optical design and proof-of-concept device of a solar battery and a photomemristive sensing concept. We start in Chapter 6 with a theoretical study of how to design an integrated solar battery with KPHI acting as photoanode (i.e., light absorber and electron storage electrode) and all-organic polymer hole transport and hole storage materials, with the hole transporter acting as battery electrolyte as well as performing photogenerated hole transfer via a rectified redox ladder-type charge transfer mechanism. We first design an optical model of the device and calculate optimized respective layer thicknesses and illumination geometries (front or rear illumination; light absorption with a high internal quantum efficiency occurring only in a small collection layer at the junction of KPHI and hole transporter) by using charging time as a figure of merit. We conclude that rear illumination significantly reduces parasitic absorption of parts of KPHI not participating in light absorption and thus increases the photocurrent. We then propose several optimization strategies to enhance light absorption in the collection layer: via scattering of a random textured surface, diffraction by quasi-random binary gratings, diffraction of arrays of dielectric nanoparticles, or excitation of localized surface plasmons in metal nanoparticles. We then simulate how light absorption improves energy and power density in a Ragone plot – up to 60 % increase in energy output. The latter is based on a study of photochromic effects as well as the effect of a charging state dependent photocurrent (i.e., the more the battery is charged, the smaller the photocurrent gets) – the latter required an electrochemical study of the photoanode and cathode half cells. We proceed in Chapter 7 to design a proof-of-concept device by using the knowledge gained from the previous chapter. We start by designing the multilayer configuration of the device, which required a thorough study of thick KPHI film preparation via dip coating and hole transport / hole storage film fabrication via spin coating, as well as an electrochemical study to underline the material's suitability for the desired charge transfer and charge storage dynamics at respective junctions and in the bulk of the layers. We have then performed a study of charging solely via illumination, either when operated as a planar heterojunction solar cell (OCP of 0.45 V, maximum power of 0.326 µW/cm, FF of \num{0.73), or when charged in open circuit conditions and subsequently discharged in the dark with an applied discharging current, i.e., solar battery operation. We analyzed the latter in regard to charge, energy, and power output as a function of illumination time (after illumination of 10000 s: extracted charge of 1.5 mAh/g and energy of 0.60 Wh/kg) and electric discharging current (most efficient operation at smallest current of 5.25 mA/g). We then proceeded to investigate further solar battery modes with an applied current during charging: (1) Both charging and discharging in the dark, (2) charging under illumination, or (3) charging and discharging under illumination. We concluded with looking at how light modifies charge output, electric coulombic efficiency, and Ragone plots. Illumination can yield an increase in extracted energy and charge by 94.1 % and 243 %, respectively. In Chapter 8, we use knowledge gained throughout this thesis on light charging dynamics of KPHI (i.e., optoionic and optoelectronic properties) to modify the photogenerated hole extraction mechanism. Herein, we sacrificially oxidize organic electron donor molecules, which serve as the analyte in an electrolyte, and quantitatively relate the change in photophysical properties accompanying KPHI charging to the amount of analyte. Thus, this device can be understood as a sensor, which senses via charge storage, and thereby imparting a memory function to this electrochemical sensor. Different operations occur: (1) Charging of the sensor "writes" the concentration information onto the device. (2) Reading is performed via different electrochemical and optical techniques. (3) Resetting occurs by quenching the charging state with the sacrificial electron acceptor oxygen. We term this sensor a photomemristive sensor, since electrochemical properties such as resistance depend on the charging history. Note that KPHI acts simultaneously as receptor, transducer, and memristive amplifier. We start this work by underlining KPHI's photoelectrochemical amperometric sensing ability with manifold analytes (sugars, alcohols, ascorbic acid, dopamine), with glucose showing a LOD of 11.4 µM. We then proceed to investigate charging as a function of illumination time and use glucose as a case study analyte. Readout is performed by evaluating the change in OCP (Potentiometric sensing), resistance (Impedimetric sensing), charge (Coulometric sensing), radiative emission (Fluorometric sensing), and change in color (Colorimetric sensing) – all readout methods showing different levels of invasiveness, readout times (instant to <300 s), and sensing over a wide range of concentrations (up to 50 µM to 50 mM). In Part 4 and Part 5, we conclude with a conclusion and outlook towards new research/application directions as well as appendices consisting of the supporting information of Chapter 6, Chapter 7, and Chapter 8.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: carbon nitride, PHI, solar battery, photobattery, energy storage, energy conversion, sensor, memristor
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 540 Chemie
• Fakultäten: Fakultät für Chemie und Pharmazie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 22. Mai 2023
• 1. Bericht­erstatter:in: Lotsch, Bettina