Novel Concepts for Silicon Based Photovoltaics and Photoelectrochemistry Lihao HAN 韩李豪

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Novel Concepts for Silicon

Based Photovoltaics and

Photoelectrochemistry

Lihao HAN

韩李豪

Photovoltaic Materials and Devices (PVMD) Laboratory Electrical Sustainable Energy (ESE) Department Electrical Engineering, Mathematics and Computer Science (EEMCS) Faculty Delft University of Technology, the Netherlands

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Novel Concepts for Silicon

Based Photovoltaics and

Photoelectrochemistry

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College van Promoties,

in het openbaar te verdedigen op donderdag 15 januari 2015 om 10:00 uur door

Lihao HAN

Master of Microelectronic Engineering, Tsinghua University

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Copromotor: Dr. A.H.M. Smets

Samenstelling promotiecommissie:

Rector Magnificus, voorzitter Prof. Dr. M. Zeman, Technische Universiteit Delft, promotor Dr. A.H.M. Smets, Technische Universiteit Delft, copromotor Prof. Dr. B. Dam, Technische Universiteit Delft Prof. Dr. J.A. Ferreira, Technische Universiteit Delft Dr. F. Finger, Forschungszentrum Jülich GmbH, Duitsland Prof. Dr. T. Gregorkiewicz, Universiteit van Amsterdam Prof. Dr. M.C.M. van de Sanden, Dutch Institute for Fundamental Energy Research This project was financially supported by the VIDI projected granted to Associate Prof. Dr. A.H.M. Smets by NWO-STW (the Netherlands Organization for Scientific Research - Dutch Foundation for Applied Sciences).

L. Han

Novel Concepts for Silicon Based Photovoltaics and Photoelectrochemistry Ph.D. thesis, Delft University of Technology, with summary in Dutch Published and distributed by Lihao Han

No part of this thesis may be reproduced, stored in a retrieval system, or transmitted in any form or by any means without the prior written permission of the copyright owner.

Cover design by Lihao Han

Printed and bound by CPI Wöhrmann Print Service B.V., Zutphen, the Netherlands A digital copy is available at http://repository.tudelft.nl

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三十功名尘与土，八千里路云和月。

——岳飞

We should consider every day lost on which we have not danced at least once. And we should call every truth false which was not accompanied by at least one laugh.

—Friedrich Wilhelm Nietzsche

Veni. Vidi. Vici.

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1. Introduction

From the sun I learned this: when he goes down, over-rich; he pours gold into the sea out of inexhaustible riches so that even the poorest ﬁsherman still rows with golden oars. For this I once saw and I did not tire of my tears as I watched it.

—Friedrich Wilhelm Nietzsche: Thus Spoke Zarathustra

1.1 Solar energy

The Sun is indeed as “rich” as what Nietzsche thought. The Sun is a bright star that locates in the center of our Solar System, and has an average diameter of approximately 1,392,684 km (~109 times that of Earth).1, 2_{The Sun has a mass of 1.989×10}30_{kilograms, ~330,000 times the mass of} the Earth,3_{consisting of hot plasma interwoven with strong magnetic fields.}4-6_{The energy source} of the plasma is the nuclear fusion of hydrogen atoms, and helium and other elements. According to the Theory of Relativity by Albert Einstein, the Sun is continuously generating a huge amount of energy from these reactions using this mass-energy equivalence:

2

E



mc

(1.1)

where c is the light speed in vacuum (c ≈ 3×108_{m s}-1_{), and m is the mass converted into energy.} The total amount of power that the Sun irradiates is about 3.6×1018_{MW, which is close to the} blackbody radiation spectrum of an object having a hot surface with a temperature of ~6000 K (following Planck’s Law, see Figure 1.1). Considering the potential amount of mass the Sun is able to convert into energy, it can be considered as the most sustainable energy source for humankind. The tiny amount of the total amount of power that reaches the Earth has a power density as large as 1353 W m-2_{. This value refers to the solar intensity at the outside of the Earth’s} atmosphere, and is noted as Air Mass (AM) 0. AM 1 is referred as the solar vertical irradiation spectrum at the sea surface on a sunny summer noon. When the Earth is illuminated with a zenith angle of θ, the Air Mass coefficient is defined as7





1

cos

AM   (1.2)

For example, AM 1.5 is the solar spectrum when illuminated with a zenith angle of 48.2°. The solar cells used for space power applications are generally characterized according to AM 0 spectrum.8_{The terrestrial solar cells are usually optimized in reference to AM 1.5 spectrum,}9 which is therefore the most important reference irradiance spectrum in this thesis. The spectra are illustrated in Figure 1.1.

There are a few major energy sources available to human beings today: fossil fuels, hydro-electricity, nuclear energy, wind energy, biomass and solar energy. Fossil fuels were formed by the long-term natural processes such as anaerobic decomposition and reformation of the buried

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dead organisms, which can also be considered as a stored form of underground solar energy for millions of years.10_{Hydropower is the conversion of potential or kinetic energy of running water} into electricity by mechanical devices, and it can be referred as another form of solar energy. Nuclear energy can be used to generate heat and even electricity via the exothermic nuclear processes, such as atomic fission or fusion.11_{Solar energy, is generally basic to man's continued} survival on the Earth.12_{It is a renewable energy source being utilized by human beings in various} ways.13-15 0 1000 2000 3000 4000 0.0 0.5 1.0 1.5 2.0 2.5 AM 1.5 radiation

Spectral irra

diance (W m

-2

nm

-1

)



nm



6000 K Blackbody radiation AM 0 radiation

Figure 1.1 Blackbody radiation spectrum of an object at the temperature of 6000 K (black curve) and the solar radiation spectra (blue curve: AM 0 radiation, red curve: AM 1.5 radiation).

Figure 1.2 Total energy consumption by human beings towards sustainable energy systems. Data adapted from the German Advisory Council on Global Change.16

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The development of the human society is accompanied with the exploration of all available forms of energies. The total energy consumption by human beings is increasing almost linearly every year in the recent decades, as illustrated in Figure 1.2.

Among all the energy sources mentioned in Figure 1.2, wind and geothermal are among the cheapest renewables. There is potential for significant growth but they cannot ultimately solve our energy problem due to the restrictions in natural geography and climate conditions. Biomass has the potential to provide part of transportation energy needs. Solar energy is possible to provide all our energy needs, but it is intermittent as the wind energy and currently relatively expensive (levelized cost of electricity, LCOE: 0.08-0.11 €/kWh for utility PV, compared with 0.04-0.06 €/kWh for lignite coal, according to the data analyzed by Fraunhofer ISE, Germany in November 2013). In addition, effective storage is another big challenge for most of the renewables.

Fossil fuels have excellent energy characteristics and have been intensively explored by humankind. However, the fossil fuels cannot meet the needs of the present without compromising the ability of future generations to meet their needs. The reproduction or regeneration of oil, coal and gas takes thousands of times longer than the life expectation of human beings. This means the total amount of these forms of energy stored in the Earth is limited and can therefore be considered not sustainable. Thus, seeking alternative sustainable energy sources has become super urgent in recent decades. Figure 1.2 shows that with the rapid development of the technology, the amount of fossil fuels consumption is expected to stop increasing after the year of 2030, and a rapid booming of sustainable energy utilization will begin from 2040. Among all the renewable sources, the solar power conversion, including photovoltaics (PV), solar thermal generation and solar fuels, is the most promising option, aiming to reach ~64% of the total annual energy consumption at the end of this century.

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Figure 1.3 (a) Annual CO2 emissions for various energy sources. Coal, oil and gas share 43%, 33% and 18% of the total global emission in 2012 respectively. Data adapted from the Global

Carbon Project 2013.17_{(b) Potential of CO}

2-free energy sources. The red bar shows the total amount of the CO2-free energy consumed in 2005. Data adapted from a DOE report by N.S. Lewis et al.18

Long term concerns about climate change and fossil fuel depletion will require a transition towards energy systems powered by solar radiation or other renewable sources. Carbon dioxide (CO2) excessive emissions from the burning of fossil fuel are the culprit for the climate change among various factors. Figure 1.3 (a) shows the enhancement of the annual CO2 emission in recent five decades monitored by the Carbon Dioxide Information Analysis Center (CDIAC) at U.S. Department of Energy. Among the global emissions in 2012, the energy utilization in the form of coal, oil and gas is responsible for 43%, 33% and 18% of the total amount of CO2 emission, respectively. The numbers on the right side indicate the growth rate of each energy source from 2011 to 2012.

The dramatic enhancement of CO2 concentration in the atmosphere has various negative effects to the Earth’s environment, such as the global warming, sea level rise, species extinction, etc. Seeking alternative methods for energy utilization in a CO2-free procedure has been an urgent issue for our sustainable society. Figure 1.3 (b) illustrates the potential for various CO2-free energy sources and the corresponding potentials. The huge potential of solar energy implies that large scale applications of photovoltaics, solar thermal and solar fuel devices are the most promising solutions for the concerns about both climate change and fossil fuel depletion in long term.

1.2 Photovoltaic effect and characteristics of solar

cells

Photovoltaic effect, first discovered by the French physicist Edmond Becquerel in 1839 at age 19, is the basic operation principle for solar cells. It is the generation of a voltage and/or current in a

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semiconductor device upon the absorption of light. This principle can be described by three steps: generation, separation and collection of photo-generated charge carriers in the junction.19

When a doped semiconductor that contains mostly free holes (p-type) is combined with another doped semiconductor that contains mostly free electrons (n-type), a p/n junction is formed. The electrons in the valence band absorb the energy from the incident light and get excited to the conduction band in the case that the energy of the photon is larger than the bandgap of the semiconductor (1st_{step: generation).}

These excited electrons in the conduction band can freely diffuse and can reach the other side of the junction. In the depletion zone around the interface between the p- and n-layer, a build-in electric filed is formed. The light-excited minority carriers drift across this depletion zone (2nd step: separation).

Finally, the photo-generated holes and electrons that extracted from the solar cell are collected at the terminals of the junction consisting of two metal contacts (3rd_{step: collection). These carriers} flow through the external circuit, and the solar energy is converted into electricity.

The solar-to-electricity energy conversion efficiency η can be defined by the ratio of the maximum power (PMPP) and the average power density of the incident AM 1.5 (1000 W cm-2, Pin) spectrum under standard test conditions (STC, 25 °C), i.e.

MPP in

P P

 (1.3)

The most important external parameters used to describe the performance of the solar cell are open-circuit voltage (VOC), short-circuit current density (JSC) and the power at the maximum power point (PMPP) as illustrated by a typical j-V curve of a single-junction a-Si:H solar cell shown in Figure 1.4 (a). From physical viewpoint, the current density should be negative since the current flows from the cathode to the anode in the solar cell. However, it is generally expressed in its absolute value for the sake of brevity.

In Figure 1.4 (a), the area ratio of the two squares (the area of blue rectangular divided by the area of the dashed purple rectangular) is defined as the fill factor (FF), which is another key external parameter for a solar cell:

MPP OC SC P FF V J   (1.4)

The FF of a commercialized solar cell is typically above 0.7. A device with a high FF value means a low internal loss of the produced photocurrent and voltage, which can be evaluated by a high equivalent shunt resistance and a low equivalent series resistance.

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The external quantum efficiency (EQE) describes the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy incident on the cell. By integrating the EQE of a solar cell at V = 0V over the whole incident AM 1.5 spectrum, one can calculate the total JSC of a solar cell illuminated by that spectrum. The JSC value of lab-scale devices determined from the j-V measurement is very likely to be slightly overestimated due to the extra current collected from the area without the metal contact or slightly underestimated /overestimated due to the spectral mismatching between the light source and the AM 1.5 spectrum. However, the JSC integrated from the EQE measurement is both contact-size-independent and light-source-contact-size-independent. Therefore, the JSC should be normalized by the value from the EQE integration to determine a reliable energy-conversion efficiency.

(a) 0.0 0.2 0.4 0.6 0.8 1.0 0 5 10 15 20

V

_OC

= 0.86 V

J

_SC

= 17.9 mA cm

-2

FF = 69.5%



= 10.7%

J

_MPP

V

_MPP

V

_OC

J

_SC

MPP

j (mA cm

-2

)

V (V)

j (b) 300 400 500 600 700 800 0.0 0.2 0.4 0.6 0.8 1.0

J

_SC

= 17.9 mA cm

-2

EQE



(nm)

Figure 1.4 The j-V curve and external characteristics of a typical single-junction a-Si:H solar cell (a) and its EQE spectrum (b).

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A typical EQE spectrum of a single-junction Si:H solar cell is shown in Figure 1.4 (b). The Si:H layers can absorb up to ~85% of light in the visible range (500-600 nm). The bandgap of a-Si:H is ~1.8 eV, which means that photons with a wavelength longer than 800 nm cannot be absorbed any more. The penetration depth of the light is comparable with the absorber layer thickness in the range of range of 600-800 nm. If a significant fraction of the light coupled into the solar cell is coherent, the interference in the absorber cause the interference in the spectrum. The JSC value integrated from this EQE spectrum is 17.9 mA cm-2.

There are two dominant loss mechanisms in a single-junction solar cell. If the incident photon energy is lower than the bandgap of the absorber, it cannot be absorbed and is transmitted through the absorber layer. If the photon energy is higher than the bandgap of the absorber, the energy exceeding the bandgap is lost in the thermalization processes. The theoretical maximum solar-to-electricity conversion efficiency is determined by these spectrum losses, blackbody radiation and radiative combination. This single bandgap limit of a solar cell was first calculated by W. Shockley and H. J. Queisser in 1961,20_{referred as Shockley-Queisser limit from then on.}

1.3 Three generations of photovoltaics

The development of photovoltaic technology can be traditionally categorized in three generations according to the cost and efficiency shown in Figure 1.5.21

Figure 1.5 Illustration of the (expected) cost and (potential) efficiency of the three generations of photovoltaics. Data adapted from G. Conibeer.21

The first generation of photovoltaic technology (1GPV) generally refers to Si wafer based solar cells, specifically noted as single-crystalline Si (mono-c-Si) and multi-crystalline (or polycrystalline) Si (poly-c-Si). This technology was first developed at Bell Labs in USA in 1954, when Daryl Chapin, Calvin Fuller and Gerald Pearson fabricated a prototype silicon-based device to convert solar energy into electricity.22_{Bell Telephone Laboratories managed to produce a c-Si}

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PV device with ~4% efficiency and quickly improved the efficiency to 11%.23_{Nowadays, the c-Si} based solar device is the most popular PV technology due to the earth-abundance of Si (27% of the Earth crust mass, ranked the second just after oxygen),24_{relatively high efficiency and good} stability. The Si wafer based panels count for 85-90% in the PV market worldwide.25_{After the} rapid development in recent decades (especially in the 1990s as shown in the NREL record efficiency chart),26_{the state-of-the-art silicon heterojunction with intrinsic thin (HIT) layer solar} cells based on the interdigitated back contact (IBC) technology have achieved the highest efficiency of 25.6% (PANASONIC Co.)14_{approaching the theoretical maximum conversion} efficiency of 29.4% (for the c-Si material with a bandgap of 1.1 eV under AM 1.5 spectrum illumination), known as the Shockley–Queisser limit (including Auger recombination).20, 27_Only little room is left for the further optimization on the loss mechanisms such as preventing contact losses and parasitic absorption losses, minimizing the reflection from the surface and introducing light trapping technique in the absorber layers.23, 28, 29_{The majority of the PV products nowadays} are based on poly-c-Si and mono-c-Si materials. In October 2014 in South and Southeast Asia, the net price of c-Si modules has become as low as 0.48 €/Wp (euro per Watt peak).30

The second generation of photovoltaic technology (2GPV) is based on the thin-film (TF) technology using less amount of raw material and aiming for a lower cost compared with 1GPV. Cadmium telluride (CdTe),31-33_{copper indium gallium (di-)selenide (CIGS)}34-36_{and amorphous} silicon (a-Si:H)37-39_{are materials that are most frequently used in 2GPV applications. Other} options can be the gallium arsenide (GaAs)40_{usually for concentrated photovoltaics (CPV) or} organic solar cells such as dye sensitized solar cells (DSSC, or known as Grätzel cells).41_The 2GPV used to have a 10-15% PV market occupancy, but its share in the market reduced to ~9% in 2013.25

The solar cells implemented and studied in Chapter 4-6 in this thesis are based on TF-Si technology, in which hydrogenated amorphous silicon (a-Si:H) and/or hydrogenated nano-crystalline silicon (nc-Si:H, a transition material from a-Si:H to c-Si) are the main absorber layers. Sometimes silicon carbide/nitride/oxide layers are also deposited as the supporting layers. TF-Si technologies are quite cost-effective in 2GPV, but devices based on a-Si:H materials are suffering from the light-induced degradation, referred to as the Staebler–Wronski effect (SWE) discovered in 1977.42_{The prolonged illumination (generally the first a few hundreds of hours) leads to the} creation of metastable defects in the a-Si:H absorber layer,43_{therefore decreasing the initial} efficiency value to typically 10-20% relatively.

Two types of TF-Si solar cell configurations are utilized in this thesis. The first type is a superstrate configuration. The illumination light enters through the glass superstrate, a front TCO layer, and then enters the “p/i/n” layers. The second type is a substrate configuration in which the light enters through a front TCO layer and the “n/i/p” layers before reaching the substrate. The advantage of the “n/i/p” structures is that the substrates do not have to be transparent, allowing more freedom for the introduction of light-trapping techniques in the device. TF-Si PV technology is flexible and mature, and therefore it can be easily integrated in various hybrid devices.

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The concept of third generation photovoltaics (3GPV) was proposed to overcome the Shockley-Queisser limit of a single-junction cell aiming to achieve a higher efficiency than the 1GPV and 2GPV, but still using earth-abundant cost-effective materials. 3GPV is the main focus of this thesis.21, 44, 45_{Many novel concepts can be applied in the 3GPV, such as multi-junctions,}46 nanocrystals (NCs, or called quantum dots, QDs),47_{intermediate band solar cell,}48, 49_{solar thermal} technology,15, 50_{hot carrier solar cells,}51-53_{perovskite (CaTiO}

3)54, 55 or even non-semiconductor technologies including biomimetics and polymers.56, 57

Among all these 3GPV techniques, multi-junction is the only type that has been commercialized for large-scale applications. The thin-film technology allows the single “p/i/n” or “n/i/p” structures stack to flexibly form a multi-junction solar cell (therefore this technology may sometimes be considered as 2GPV as well). The VOC value of a multi-junction device can be close to the sum of the VOC of each single-junction as they are connected in series. The absorption of the solar spectrum is distributed over the various junctions. The JSC value of the multi-junction device is limited by the junction generating the lowest current. Therefore, for an optimal performance of such device, the current generation should be the equally distributed over all the junctions.

NCs are materials whose excitons are confined in all three spatial dimensions.58_{Therefore NCs} can have promising 3GPV applications because of these quantum confinement effects. Solar cells based on NCs have the potential to break through the Shockley-Queisser limit, using mechanisms like multiple exciton generation (MEG)59_{and spectral conversion like up or down conversion by} space separated quantum cutting (SSQC).60, 61_{In a conventional solar cell, the absorbed photon is} only capable to excite one electron from the valence band to the conduction band, where the excess energy is lost in the form of heat. MEG is a carrier multiplication procedure in which more than one electron can be excited in a NC by a photon of high energy.59_{This MEG phenomenon} has the potential to enhance the efficiency of the solar cell. In 2011, above 100% external quantum efficiency (EQE) value in a certain spectral range has been achieved in a cell based on PbSe NCs.62

The concept of spectral conversion is aimed to modify the incident spectrum, so that a better match can be achieved between the spectral response of the PV absorber and the converted light spectrum offered. In the so-called “up-conversion” mechanism in a NC, two photons of lower energy can be combined to generate one photon of higher energy.63_{This procedure can convert} the red spectrum transmitted through a solar cell to the blue range. The second mechanism is noted as “down-shifting”, in which a photon of higher energy is transformed into a photon of lower energy in the form of luminescence.64_{The blue response of a solar cell based on NCs can} be improved by shifting the incident photons to a spectral range where the device has a higher response. The third mechanism is called “down-conversion” or “SSQC”,60, 61_{in which one photon} of higher energy can be transferred into two photons of lower energy.65_{Up- and down-conversion} mechanism has the potential to break through the Shockley-Queisser limit. For instance, by using down- and up-conversion mechanism, extra 32% and 35% higher intensity can be absorbed by c-Si material illuminated by the AM 1.5 spectrum respectively.66

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1.4 Photoelectrochemistry

The Sun continuously provides 1.2 × 1017_{W of power to the Earth,}17_{and in one hour it delivers} approximately the entire amount of energy humans use in a year (4.6× 1020_J).67_{Figure 1.6 (a)} shows the rapid development of the effective installed sustainable energy worldwide. It is predicated that within a decade the installation of solar and wind energy can be almost the same level of nuclear and hydro energy, if the growth of the last years continues at the same rate. Figure 1.6 (b) shows an example of the solar panels on the roof of Prof. M. Zeman, and it is obvious that this PV system generated 5-6 times more energy in summer time than in the winter time in Delft. However, the Dutch population generally needs more energy in the winter than they do in the summer. Due to these seasonally or even daily fluctuations and intermittency of sustainable energy, effective collection, storage and distribution are critical in the utilization of this largest carbon-neutral energy source for our society.

Solar fuels are fuels produced from sunlight via artificial photosynthesis or thermochemical reaction.68, 69_{It is a process that partly mimics what plants do by splitting water into hydrogen fuel} and oxygen, or reducing carbon dioxide (CO2) to methane (CH4) or organic components.70 This fuel forming process is typically performed in a photoelectrochemical (PEC) cell, and generally uses photoelectrodes that are constructed from semiconductor(s) and electrocatalysts.

(a) 1980 1990 2000 2010 2020 0.1 1 10 100 1000

Hydro C

_F

=0,40

Nuclear C

_F

=0,90

Wind C

_F

=0,30

Solar C

_F

=0,15

Effective installe

d Power (GW)

Year

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(b)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 0 10 20 30 40 50 60

Genera

ted en

ergy (kW h)

Month

Figure 1.6 The (expected) installation of several renewable energy sources with the corresponding capacity factor (a). An example of the seasonal fluctuation of solar energy in the year of 2003 in Delft (b). Data adapted from Solar Energy, a Massive Open Online Course given by A.H.M. Smets.71

Figure 1.7 illustrates the prototype structure of a PEC device proposed by JCAP at Caltech. The sunlight incidents on the photoanode material which can be a semiconductor having relatively high bandgap such as BiVO4 (Chapter 4 and 5),72 Fe2O3,73 WO374 and is generally coated with O2 evolution reaction (OER) catalysts.75_{A photocathode material, such as GaAs,}76_Si77_{(Chapter 6)} that has a relatively low bandgap in reference to that of the photoanode material, is located at the bottom part of the cell and absorbs the photons with longer wavelengths. Hydrogen evolution reaction (HER) catalysts can be metal oxide or metal in the form of thin coatings or nano-particles (NPs) made of materials like Ni,78_Pt79_{(Chapter 7). A H}+_{permeable membrane is inserted} between the two photoelectrodes and it can separate the generated O2 and H2 gases.80

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The electrolyte of the redox reaction can either be in acid or in base solutions. In the acidic solution, the reactions on the cathode and anode happen as:82

0 2 ox 4H4e2H ; E 0.0 V . NHEvs (1.5) 0 2 2 red 2H O4h4H +O ; E  1.229 V . NHEvs (1.6) Herein e-_{and h}+_{represent the electrons and holes respectively.}

In the alkaline solution, the reactions on the cathode and anode happen as:82

0

2 2 red

4H O 4e 2H +4OH ; E  0.828 V . NHEvs (1.7)

4OH





4h





2H O+O ; E

₂ ₂ _ox0



0.401 V . NHE

vs

(1.8) Therefore, the overall water-splitting reaction can be described as:

2H O

₂



Sunlight

2H +O ;

₂ ₂

 

G

237 kg mol

1 (1.9) A potential higher than 1.23 V between the cathode and anode is required for water-splitting reactions to occur in principle, which corresponds to a Gibbs free energy change of 237 kJ mol -1_.82_{In practice, more energy than thermodynamically is expected to drive the redox reaction, and} this extra energy is defined as overpotential. This overpotential originates from various reasons such as defects in the semiconductor, surface recombination between the semiconductor and electrolyte, low charge carrier collection (sometimes called as charge carrier separation). Novel techniques are proposed and applied in this thesis to reduce this overpotential and consequently to improve the solar-to-hydrogen efficiency. Particularly, we focus on the integration of the photoelectrode with a solar cell, which provides the potential for water-splitting devices with high efficiencies. Various configurations for PEC/PV devices can be explored. Figs. 1.8 (a) and (b) illustrate a photoanode and a photocathode configuration developed in this thesis. The rear PV junction should work efficiently under the illumination of the spectrum transmitted through the front PEC junction. The spectrum utilization and the j-V curve matching between the PEC and PV junctions will be studied in Chapter 4-6.

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(a)

(b)

Figure 1.8 PEC/PV monolithic water-splitting photoanode (a) and photocathode (b) configurations investigated in this thesis. These two figures have been highlighted as the cover image of ChemSusChem (issue of October 2014) and Journal of Materials Chemistry A (issue of February 2015) respectively.

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1.5 Outline of this thesis

As mentioned in the previous sections, this doctoral thesis introduces some novel concepts for silicon based photovoltaics and photoelectrochemistry. Chapters 2 and 3 focus on the processing and characterization of Si NCs as a potential material for 3GPV applications. Chapter 4-6 focus on the development of PEC/PV devices where the PV part is based on TF-Si technology. Chapter 7 focuses on a high rate deposition approach for water reduction catalysts.

The main scientific questions tackled in this thesis are:

 What temperature can the free-standing Si NCs be heated when they are illuminated by a low intensity but sharply focused laser beam? (Chapter 3)

 What are the design rules of a double-junction TF-Si solar cell for a hybrid water-splitting device in reference to the transmitted spectrum through the front photoanode? (Chapter 4)

 Where is room for the further optimization of an efficient hybrid BiVO4 /double-junction TF-Si photoanode device? (Chapter 5)

 What is the optimal device structure for a photocathode based on TF-Si alloys? (Chapter 6)

 What is the optimal high-rate deposition conditions for the nano-structured Pt as the water reduction catalyst by ALD technique? (Chapter 7)

Chapter 2 focuses on the deposition and characterization methodology of Si NCs, TF-Si solar cells, metal-oxide photoanodes, TF-Si photocathodes and nano-structured metal catalysts on photocathodes. A high rate fabrication of Si NCs by a state-of-the-art technique called expanding thermal plasma chemical vapor deposition (ETP-CVD) is introduced. The advantages and challenges for the Si NCs fabrication are discussed. The absorber layers of the TF-Si solar cells in this thesis are deposited by PECVD. The BiVO4 photoanodes are synthesized by spay pyrolysis, and the nano-structured Pt are synthesized by atomic layer deposition (ALD) technique for hydrogen evolution reaction (HER) catalysts. The morphology and size distribution of the Si NCs is characterized by SEM and TEM. Analytical methods are employed to investigate material and device properties, such as Fourier transform infrared spectroscopy (FTIR) to study the oxidation process of this material with large surface-area-to-volume-ratio, as well as the carbon ratio in the a-SiC:H films. X-ray diffraction (XRD) measurement is carried out to analyze the crystallinity of the NCs and the various orientation of the crystals. Raman spectroscopy (RS) is another important tool to observe the crystallinity of the NCs, allowing the identification of species presented in the material. XPS is utilized to study the chemical components of the ALD-grown Pt. The performance of the photoelectrodes are verified by PEC measurements.

In Chapter 3, a fundamental study of heating effects of Si NCs induced by Raman laser beam is presented. The free-standing Si NCs can be easily heated using the low intensity but sharply focused Raman probe beam, because the heat loss mechanism is not dominated by the thermal conduction like Si NCs embedded in a host matrix. This thermal expansion results in a significant

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red-shift and broadening of peak width from the initial Si crystalline peak. By analyzing the ratio of Anti-Stokes-to-Stokes peak intensities of the first order Si-Si transverse optical (TO) phonon mode, the temperatures of Si NCs under laser exposure can be determined. It is found that the laser heating effects are reversible to a large extent, however the nature of the free-standing Si NCs is slightly modified after intensive illumination.

In Chapter 4, a PEC water-splitting photoanode based on bismuth vanadate (BiVO4) and an amorphous silicon tandem cell (a-Si:H/a-Si:H) is demonstrated. The front BiVO4 photoanode is optimized first for effective oxygen evolution reactions (OER). An a-Si:H/a-Si:H cell is chosen as the rear power source. The tandem solar cells are then optimized in reference to the spectrum transmitted through the photoanode. Finally, the stability of the photoanode is addressed. In Chapter 5, the further optimization of the hybrid PEC/PV water-splitting device previously demonstrated in Chapter 4 is presented. The improvement with respect to the cell in Chapter 4 that also used gradient tungsten (W) doped BiVO4 has been realized by simultaneously introducing a textured substrate to enhance light trapping in the BiVO4 photoanode and further optimization of the W gradient doping profile in the photoanode. Various TF-Si PV cells have been studied in combination with this BiVO4 photoanode, such as Si:H single-junction, a-Si:H/a-Si:H double-junction and a-Si:H/nc-Si:H double-junction. The highest conversion efficiency - which is also the record efficiency for metal-oxide-based water-splitting devices - is reached for a tandem consisting of the optimized W:BiVO4 photoanode and the micromorph Si (a-Si:H/nc-Si:H) cell.

In Chapter 6, a cost-effective, earth-abundant photocathode based on a-SiC:H for hydrogen evolution reaction (HER) is studied and developed. This monolithic a-SiC:H PEC cathode integrated with an a-Si:H/nc-Si:H double PV junction to achieve a high unbiased photocurrent density. The SiC:H photocathode used no HER catalyst and the boron doping profile in the a-SiC:H layers is studied. The growth of high quality nc-Si:H PV junctions in combination with high spectral utilization is achieved using glass substrates with integrated micro-textured photonic structures. Advanced Semiconductor Analysis (ASA) software is employed as well to simulate the performance of the PEC/PV cathode.

In Chapter 7, the ALD technique to fabricate nano-structured Pt onto etched p-type Si(111) wafers and glassy carbon discs as hydrogen-evolution reaction (HER) catalysts is introduced. New types of precursors are utilized to increase the Pt growth rate. X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), ellipsometry and scanning-electron microscopy (SEM) are used to analyze the composition, structure, morphology and thickness of the ALD-grown Pt nanoparticle films. The catalytic activity of the ALD-grown Pt for the HER is compared with electron-beam evaporated Pt on glassy carbon electrodes.

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2. Processing and characterization of silicon

nanocrystals, solar cells and photoelectrodes

2.1 Chemical vapor deposition techniques

2.1.1 Expanding thermal plasma chemical vapor deposition

Up to date, a few fabrication methods of Si NCs have been proposed. Conibeer et al. sputtered Si rich oxide (SRO) and quartz (SiO2) layer by layer on a wafer. The thickness of each layer ranges from 2-6 nm. After annealing at 900-1100 °C for 1 hour, dense Si NCs were formed in the SiOx matrix.83_{The size of Si NCs can be controlled by the layer thickness of SRO and SiO}

2. Han et al. employed pulsed laser deposition (PLD) tools with Si rich SiOx target, followed by similar annealing procedure as well.84_{Molecular beam epitaxy (MBE),}85_{non-thermal plasma,}86_chemical wet etching87_{can also be used for Si, SiGe or Ge NCs fabrications. Most of these techniques} suffer from the low deposition rate that limits large scale applications of Si NCs. In addition, the high temperature procedure is not cost-effective, as well as the maintenance cost for some setup such as MBE.

In this chapter, a novel fabrication processing of Si NCs by expanding thermal plasma (ETP-CVD) is proposed. A photo of ETP-CVD setup is shown in Figure 2.1. The plasma creation, plasma transport and film deposition occur in separate regions of the reactor. Therefore, ETP-CVD can be considered as a remote plasma technique.88, 89

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At the top of the ETP-CVD reactor, six electrically insulated copper plates are stacked, acting as the cascaded arc. The bottom plate serves as the anode, while the three cathode tips are located at the top of the arc and each tip stands in the centers of the copper plate. Each plate has a circular open space in the center to form the plasma channel (ф =2.5 mm), from which the argon (Ar) gas is injected and is operated by a mass flow controller. The plasma is ignited and sustains due to the high voltage/current (typically 70~100 V / 20~60 A) between the cathode tips and the anode plate at pressures commonly in the 0.5 bar range.

With the help of two stacked root blowers, the deposition chamber is kept at a much lower pressure than the arc, typically around 0.5-2 mbar. The distance from the arc to the substrate holder is as long as 38 cm. The pressure difference between the arc and the vessel results in a supersonic expansion of the plasma and a subsonic expansion after a stationary shock. At ~4.5 cm below the nozzle a stainless steel injection ring is located, which serves as an injection point for silane (SiH4) gas.

The SiH4 gas decomposes and reacts with the plasma to form predominantly silyl radicals. During their transportation to the substrate, these silyl radicals form Si NCs due to the lower temperature of the plasma and larger pressure. These Si NCs “freely stand” in the form of nano-powder on top of the substrate. This substrate is pre-cleaned first in acetone and then in IPA in a heated water bath chest. The substrate is kept at room temperature during the plasma synthesis, and the substrate material does not play an important role in this Si NCs fabrication process, which means either a piece of Corning glass or c-Si wafer is suitable as substrate.

The ETP-CVD is highly efficient and several micrometers of the Si NCs porous layer can be deposited on the substrate in 1 second. Small Si NCs with a narrow size distribution of a few nanometers can be collected on the substrate spots which are exactly below the holes in the SiH4 injection ring. We believe that these small NCs were formed in the plasma beam and directly deposited on the substrate. However, free-standing larger NCs (diameter of dozens of nanometers) are also observed, which were collected from the other areas on the substrate. They have a larger average diameter due to the recycling from the back ground volume into the plasma beam.90, 91_{By reducing the diameter of the deposition chamber, the amount of the larger NCs can} be reduced.

2.1.2 Plasma-enhanced chemical vapor deposition

The thin-film silicon (TF-Si) for the solar cells (Chapter 4-6) and photocathodes (Chapter 6) were deposited by plasma-enhanced chemical vapor deposition (PECVD). PECVD is a frequently used technique for PV and semiconductor industry due to its relatively large areas (1.5 m2_for example), low substrate temperature (180-250 °C) during processing and device-grade deposition quality with a good coverage of textured morphologies, uniformity and adhesion.

A sketch of the PECVD setup is illustrated in Figure 2.2 (a). The plasma is initiated by an oscillating voltage with a radio frequency (RF) of 13.56 MHz applied between the electrode plates. Free electrons start accelerating in the oscillating electric field. Ionized gas molecules are

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created due to the inelastic collisions between the free electrons and the neutral gas molecules. This results in the production of secondary electrons that in turn cause gas molecules to react or dissociate and maintain the plasma. Thin films (TFs) of various materials can be deposited according to the precursor gas that is used. The deposition rate and the physical properties of the films, such as the refractive index (n), extinction coefficient (k), can be turned by varying the RF power, chamber pressure, substrate temperature, electrode distance, amount of gas flows, etc.

(a) (b)

(c)

Figure 2.2 A sketch illustration of the PECVD setup (a),92_{together with a sketch (b) and a photo} (c) of the AMIGO multi-chamber PECVD setup located in Cleanroom Class 10000 of the PVMD group. LLC means the load lock chamber, DPC 1-5 are the depositions process chambers used to synthesis intrinsic and doped TF-Si layers, and DPC 6 is a sputtering chamber for TCO layers.

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Figures 2.2 (b) and (c) show a sketch and a photo of the so-called AMIGO multi-chamber PECVD utilized in this work. The intrinsic (non-intentionally doped, more scientifically) a-Si:H films are deposited by the silane (SiH4) gas (sometimes together with H2 for a better passivation) using RF power. The p-doped films were deposited in another chamber with SiH4 diluted by diborane (B2H6), and the n-doped films were deposited with SiH4 diluted by phosphine (PH3). CO2 is introduced for the fabrication of oxide materials such as nc-SiOx:H, and CH4 is injected for the a-SiC:H depositions.

To guarantee high quality nc-Si:H films, the deposition requires a higher power, pressure and lower deposition rate compared with the parameters of the a-Si:H. The material of nc-Si:H was deposited in a chamber powered by very high frequency (VHF), which guarantees a larger processing window for high quality nc-Si:H.

2.1.3 Atomic layer deposition

Atomic Layer Deposition (ALD) is a CVD thin film (TF) deposition technique that is based on self-limiting or sequentially self-terminating gas phase chemical process to form the TFs or nano-particles (NPs). Chemical precursors are carried by nitrogen (N2) gas from an injection hole into the chamber. After reacting with the surface of a substrate, the extra precursors are pumped away through the taphole. Through the repeated exposure to the precursors, a thin film is deposited in a slow rate and the thickness can be controlled up to angstrom scale. Both amorphous and crystalline materials are possible depending on substrate and temperature.

Advantages of a self-limiting film coated by ALD include uniform surfaces with low defect density, high conformity to surface features, high control and accuracy of atomic level thickness, wide process windows (no sensitivity to temperature or precursor dose variations) and excellent reproducibility. ALD does have some limitations, including incomplete reactions and slow reaction rates. In addition, some ALD process mechanism has not yet been completed understood. The Cambridge Nanotech S200 ALD system based in the Noyes Laboratory at Caltech is employed for the deposition of nano-structured platinum (Pt) for the water reduction catalysts in Chapter 7. Figure 2.3 (a) is the photo of the ALD system and (b) is the sketch illustration its deposition chamber and precursor system.

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(a) (b)

Figure 2.3 A photo of Cambridge Nanotech S200 ALD (a) and the sketch of its deposition chamber and precursor system (b).

2.1.4 Spray pyrolysis

The bismuth vanadate (BiVO4) photoanodes discussed in Chapter 4 and 5 were deposited by spray pyrolysis in the Chemistry and Chemical Engineering department in TU Delft, and the recipe was developed by Abdi et al.93-96_{A sketch of the spray pyrolysis setup is shown in Figure} 2.4.

Before the deposition, 4 mM Bi(NO3)3·5H2O (98%, Alfa Aesar) was dissolved in acetic acid (98%, Sigma Aldrich), and an equimolar amount of vanadium in the form of VO(AcAc)2 (99%, Alfa Aesar) was dissolved in absolute ethanol (Sigma Aldrich). The precursor solution in the reservoir was made by mixing these two solutions. Two types of fluorine-doped tin dioxide (FTO) coated glass substrates are used as the substrate. One is TEC-15 (15 Ω/□, Hartford Glass Co.) that is initially used in Chapter 4, and the other is ASAHI VU-type (8 Ω/□, Asahi Co.) that is optimized in Chapter 5. The substrates were cleaned by ultrasonic rinsing treatments for successive 15 min for three times, in a Triton® solution, acetone and ethanol respectively. The substrate temperature during spraying was kept at 450 °C on a heating element, as measured by a thermocouple pressed to the substrate surface. The Quick-Mist Air Atomizing Spray nozzle was located 20 cm above the substrate surface, and the nozzle was connected by a programmed controller.

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Figure 2.4 A sketch illustration of spray pyrolysis setup.

The precursor solution was placed 20 cm below the nozzle and fed to the nozzle via the siphoning effect induced by the nitrogen gas flow with an overpressure of 0.06 MPa. Each spray cycle was consisted of 5 seconds of spray time and 55 seconds of delay time to allow sufficient solvent evaporation. 100 cycles were needed to deposit the film with a deposition rate of ~1 nm per cycle. The doping profile was investigated in this thesis as well.95, 97_{The 1% W-doped BiVO}

4 sample was prepared by spraying 200 cycles of the BiVO4 precursor solution containing 1 at% of W. The W:BiVO4 homo-junction was prepared by spraying 100 cycles of the BiVO4 precursor solution containing 1 at% of W, followed by 100 cycles of the BiVO4 precursor solution. This sequence was reversed for the deposition of the W:BiVO4 reverse homo-junction. To deposit the gradient-doped W:BiVO4, the concentration of W in the BiVO4 precursor solution was changed in step every 20 cycles, starting from 1 to 0 at%.

Prior to praying the BiVO4, a SnO2 interfacial layer (~80 nm) was coated onto the FTO substrate to prevent possible recombination at the FTO/BiVO4 interface.98, 99 A 0.1 M SnCl4 (99%, Acros Organics) solution in ethyl acetate (99.5%, J. T. Baker) was used as the precursor solution. The SnO2 layer was deposited at 425 °C using 5 spray cycles (5 s on, 55 s off) in a gravity-assisted siphoning mode, where the precursor solution was placed 30 cm above the nozzle. After deposition, the SnO2/BiVO4 samples were subjected to an additional 2-hour heat treatment in a tube furnace at 450 °C in air atmosphere to further improve the crystallinity.

A 30-nm CoPi catalyst was electrodeposited according to the recipe developed by Kanan and Nocera.100_{The electro-deposition was performed at a constant voltage of 1.7 V versus RHE for} 15 min. Care was taken to always keep the electrodeposited CoPi layer wet, as intermediate drying of the CoPi was found to adversely affect the stability.

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2.2 Physical vapor deposition techniques

Physical vapor deposition (PVD) describes a variety of vacuum deposition methods used to deposit TFs by the condensation of a vaporized form of the desired film material onto various substrates. No chemical reaction takes place during the vacuum deposition, but the PVD is normally realized by the material phase change, such as from solid to gas phase during the deposition, and back to solid with a different morphology during the transportation onto the substrate. These PVD techniques include pulsed laser deposition (PVD), sputtering, evaporation, cathodic arc deposition (CAD), etc. In this section, the sputtering and evaporation are introduced for the deposition of transparent conductive oxide (TCO) and metal contacts of the PV and/or PEC devices respectively.

2.2.1 Sputtering

Physical sputtering is a popular PVD tool for TF depositions that involves eroding material from a target onto a substrate. The deposition is driven by momentum exchange between the ions and atoms in the materials due to collisions.

Argon (Ar) is introduced as the processing gas in the vacuum chamber with a sputter target, which is powered by radio frequency (RF) source. Electrons from the target surface ionize the Ar atoms, creating a plasma. The target attracts positive Ar ions, which in turn ejects neutrally charged atoms from the sputter target towards the substrate’s surface. Subsequently, a uniform film is deposited on the substrate’s surface.

Figure 2.5 RF magnetron sputtering system (Kurt J. Lesker) for the deposition of ITO and AZO layers in the PVMD lab.

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The RF magnetron sputter employed in this thesis is made by RF magnetron sputtering system Kurt J. Lesker, as shown by Figure 2.5. The samples are placed in the load lock and transferred to the process chamber through a gate valve for depositions. The targets in the sputter are indium tin oxide (ITO, 10% SnO2 and 90% In2O3) or aluminum doped zinc oxide (AZO, 2% Al2O3 and 98% ZnO) deposition. By controlling the RF power intensity, electrode distance, substrate temperature and chamber pressure, ITO or AZO layer with good transparency and conductivity can be sputtered, which are both important transparent conductive oxides (TCO) in TF-Si PV technology.

2.2.2 Evaporation

The metal contact of the solar cells and PEC devices in this thesis were deposited using an evaporation PVD technique. In an evaporator, a target anode is bombarded with an electron-beam given off by a charged tungsten (W) filament in a high vacuum chamber. The atoms in the target material is transformed into a gaseous phase when exposed to the intensive electron-beam. These atoms precipitate into solid form after they are evaporated out of the boat and cooled down in the vacuum chamber. A thin layer of the anode material is coated on the superstrate placed above the target. A scheme of the evaporator is depicted in Figure 2.6 (a).

A PROVAC PRO500S evaporator is utilized in this thesis to deposit the metal contacts of the solar cells and PEC devices, as shown in the photo in Figure 2.6 (b). Ag can be deposited by thermal evaporation (or known as resistance heating evaporation). Other materials such as Al, Cr, Ti and ceramics with higher melting points should be deposited by electron-beam evaporation. This system is equipped with a 4 pocket electron-beam gun for evaporation. A few substrates up to 10 cm ×10 cm can be loaded into the evaporator, and large area patterns can be created by using shadow masks. Pumping cycles and deposition recipes are fully programmable.

The front contacts of the PV and PEC devices in this thesis consist of a 300 nm thick aluminum (Al) bar on the edge of the TCO-coated substrate (such as ASAHI VU-type). The back contact of the solar cell is a stack of 100 nm Ag, 30 nm Cr and 300-500 nm Al. Ag can form an alloy with TF-Si after annealing and reduce the Ohmic resistance of the contact. Besides, Ag is “shiny” metal that can reflect the light back to the absorption layers. The Al layer prevents the oxidation of the Ag layer, whereas the Cr interlayer avoids mixing of Ag and Al in post-deposition annealing treatments.

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(a)

(b)

Figure 2.6 (a) Schematic overview of Resistance heating evaporation (left) and electron-beam evaporation (right) physical vapor deposition; illustrations adapted from Ref.,101_{and (b) a photo} of PROVAC PRO500S evaporator located in PVMD group.

2.3 Characterization tools

2.3.1 Scanning electron microscope

Scanning electron microscope (SEM) is an electron microscope that frequently utilized for the nano-technology research. Electron-beam generated by the electron gun is focused by more than one condenser lens in a magnetic field, and scans fast over the surface of the specimen in vacuum. Various signals can be generated when the electronic beam interacts with atoms of the sample. These detected signals reflect the topography, morphology and composition of the sample’s

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surface. A real-time image can be produced when the beam's position is combined with the detected signal. This technique can achieve a resolution of a few nanometers minimum.

The SEM setup used in this thesis is FEI Quanta 200 FEG, located in the Charged Particle Optics (CPO) group, Department of Imaging Physics at TU Delft. This technique was employed to observe the morphology of the Si NCs, photoanodes, photocathodes and metal catalysts in Chapter 3-7 at various operating voltages from 3 to 15 eV. A photo is shown in Figure 2.7.

Figure 2.7 The SEM setup located in CPO group at Delft utilized in this thesis.

2.3.2 Transmission electron microscopy

Transmission electron microscopy (TEM) is a microscopy technique that operates on similar basic principles with that of optical microscopes but using electron-beams instead of visible light beams. This electron-beam transmitted though the ultra-thin specimen in vacuum, and meanwhile interact with the materials of the specimen. The traveling route of the electrons is changed due to the collisions with the atoms when transmitting though the specimen, and these electrons are scattered in various directions. The scattering angles indicate the density and thickness of the specimen, and can be detected with different brightness and contrast. A real-time image is formed on a CCD camera when these signals are focused and magnified.

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Figure 2.8 A photo of the TEM system utilized in this thesis. Photo adapted from Ref.102

The advantage of this technique is that the de Broglie wavelength of the electrons can be thousands of times smaller than the visible light source. As a result, this technique can observe nanostructures in the scale of 0.1-0.2 nm, and it can operated as high resolution transmission electron microscopy (HR-TEM) when observing specimen with a big magnification.

The HR-TEM setup used in this thesis is TECNAI G2 S-Twin F20, which is located in State Key Laboratory of Inorganic Synthesis & Preparative Chemistry at Jilin University in China. When observing the lattice of the Si NCs, the operating voltage is 200 kV. A photo of that system is shown in Figure 2.8.

2.3.3 Fourier transform infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy is an indirect technique to simultaneously collect a transmission or absorption specimen over a wide infrared spectrum and then convert the measured data into the actual spectrum using a mathematical process known as Fourier transform. In an FTIR setup, the light from the polychromatic infrared source is equally spitted into two beams. One beam is refracted towards a fixed mirror, and the other beam is transmitted through the specimen and arrives at a dynamic mirror that is linearly moving at a constant speed. These two beams are reflected by the mirrors and return to the splitter. An interference beam is formed due to the optical path difference of the two beams, and this beam is transmitted through the specimen and collected by the detector. The measured interferogram is processed by Fourier transformation, and a transmittance or absorbance spectrum is obtained.

The most frequently studied spectrum range is the mid-infrared (400-4000 cm-1_{) and the} near-infrared (4000-13300 cm-1_{) region. The advantage of this technique is that an FTIR spectrometer}

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measures much faster than the other traditional spectral measurement methods using a dispersive monochromatic light beam at a sample.

The FTIR technique was utilized to analyze the Si NCs oxidation procedure in this thesis, as well as the a-SiC:H films for photocathode applications. A photo of the FTIR setup located in the PVMD cleanroom is shown in Figure 2.9.

Figure 2.9 The FTIR setup employed in this thesis.

2.3.4 Raman spectroscopy

Raman spectroscopy is named after Sir C.V. Raman for his great contribution in physics. It is a spectroscopic technique that is frequently used in solid state chemistry and bio-pharmaceutical industry to analyze the vibrational and rotational modes of the molecules by studying the scattering spectrum that have a different frequency from the incident light. Raman scattering occurs when the incident laser light illuminates the molecule and interacts with the bonds in the molecule. The most intensive (about a few thousandths of the intensity of the incident light) signals can be observed are from Rayleigh scattering, which has the same frequency as the incoming light (υ0). The scattering light with higher or lower frequency than υ0 can also be observed and they are designated as the Raman spectrum. The smaller frequency than the incident light is known as the Stokes shift (υ0 - υ1), and the larger frequency than the incident light is defined as the anti-Stokes shift (υ0 + υ1). Stokes shift can be observed when the molecules absorb photons with a frequency of υ0 during the inelastic scattering and then emit photons with a frequency of (υ0 - υ1). The absorbed energy is higher than the emitted energy and meanwhile the molecule transits from the ground electronic state to an excited electronic state. Anti-Stokes is the opposite. The emission energy is higher than the absorption energy and meanwhile the molecule transits from an excited electronic state to the ground electronic state.

The Raman measurements in Chapter 3 in this thesis were performed by Renishaw inVia Micro-Raman microscope. Two same Notch filters (Model NF03-514E-25, Laser 2000 Benelux CV Co.)

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were equipped in our Micro-Raman spectra setup to fully filter out the excitation wavelength, so that both the Stokes and Anti-Stokes shift can be observed at the same time.

The Si NCs were illuminated by the visible light (λ = 514 nm) from an argon (Ar) ion laser, and the intensity of the laser power can be varied by switching lens of various magnifications (e.g., 5×, 10×, 20×, 50×, 100×). A photo of this setup located in the PVMD lab is shown in Figure 2.10.

Figure 2.10 Renishaw inVia Micro-Raman microscope utilized in Chapter 3 of this thesis.

2.3.5 X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS), sometimes called as Electron Spectroscopy for Chemical Analysis (ESCA), is a spectroscopic technique to characterize the elemental composition at the surface of a material. The basic principle of XPS is that the valence electrons or the inner-shell electrons of an atom or molecular can be emitted due to the excitation by the intensive focused beam of X-ray in high vacuum (10-8_{mbar). The emitted electrons are excited by} the photons, therefore can be called as photoelectrons. These photoelectrons are focused by electron collection lens and then detected by an electron energy analyzer. A spectrum figure can be plotted using the number of detected photoelectrons per second as the y-axis, and binding energy of the photoelectrons as the x-axis. Binding energy means the energy required to disassemble a whole system into separate parts, and can be determined as

binding photon

(

kinetic

)

E



E



E





(2.1)

Here Ephoton is the energy of photons, i.e., hυ. Ekinetic is the kinetic energy and Φ is the work function.

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XPS was employed to characterize the carbon concentration in the a-SiC:H films in Chapter 5 and the platinum elements in Chapter 7 of this thesis. A photo of the XPS setup based in the Molecular Materials Research Center (MMRC) at Caltech is shown in Figure 2.11.

Figure 2.11 A photo of the XPS setup located in MMRC at Caltech.

2.3.6 Photoelectrochemical measurement

Since last decade, there is intensive debate in the solar fuel society about how to define the solar-to-hydrogen efficiency and measure the actual performance of a photoelectrode. The main reason is that the photoelectrochemical (PEC) measurement is sensitive to many relevant parameters that are not fully understood.82

(a) (b)

Figure 2.12 Different methods to measure the j-V curves of a photoelectrode. A two-electrode setup with a working electrode (WE) and counter electrode (CE) (a); and a three-electrode setup with a WE, CE and a reference electrode (RE) (b).

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From Chapter 4 to 7, the PEC j-V curve measurements were done in either a two-electrode or three-electrode setup and the sketches are depicted in Figure 2.12.

In the two-electrode setup, as depicted by Figure 2.12 (a), the current flowing through the electrolyte (or between WE and CE) is monitored as a function of the voltage in the EG&G potentiostat. A solar-to-hydrogen conversion efficiency can therefore be determined by measuring the j-V curve in the two-electrode setup, when the WE is immersed in the electrolyte and illuminated by the AM 1.5 simulated solar spectrum (Newport Sol3A Class AAA solar simulator).

The difference between the above two configurations is that a reference electrode (RE) is introduced in the circuit in the three-electrode setup, as shown by Figure 2.12 (b). In our case we utilize the most commonly used RE, which is a silver electrode in silver chloride solution (Ag/AgCl). The working electrode (WE) means the photoelectrode, and the counter electrode (CE) is usually a coiled platinum (Pt) coil for the other half of redox reaction. The influence of the CE can be excluded by the introduction of RE, and only the performance of the WE can be evaluated, which can be designated as the photoelectrode efficiency. The measured potential is V

vs. RE (i.e., Ag/AgCl), and can be converted into the V vs. reversible hydrogen electrode (RHE)

using Equation (2.2) to exclude the influence of the pH value in the electrolyte:82 0

RHE Ag/AgCl Ag/AgCl . SHEvs

0.059 pH

V



V



V





(2.2)

Where VAg/AgCl is the measured voltage, V0Ag/AgCl vs. SHE is the potential of the Ag/AgCl RE with respect to the standard hydrogen electrode, and the value is 0.199 V at 25 °C. The pH value factor of 0.059 comes from the Nernst equations.82_{The potential of the WE is plotted against the RHE} as the x-axis, and the current density is the y-axis, which represents the amount of hydrogen generated by the illuminated photoelectrode under a certain voltage.

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Novel Concepts for Silicon Based Photovoltaics and Photoelectrochemistry Lihao HAN 韩 李 豪

Novel Concepts for Silicon

Based Photovoltaics and

Photoelectrochemistry

Lihao HAN

韩 李 豪

Novel Concepts for Silicon

Based Photovoltaics and

Photoelectrochemistry

Proefschrift

Lihao HAN

Contents

1. Introduction

1.1 Solar energy

E



mc





Spectral irra

diance (W m

nm

)



nm



1.2 Photovoltaic effect and characteristics of solar

cells

V

= 0.86 V

J

= 17.9 mA cm

FF = 69.5%



= 10.7%

J

V

V

J

MPP

j (mA cm

)

V (V)

J

= 17.9 mA cm

EQE



(nm)

1.3 Three generations of photovoltaics

1.4 Photoelectrochemistry

Hydro C

=0,40

Nuclear C

=0,90

Wind C

=0,30

Solar C

=0,15

Effective installe

d Power (GW)

Year

Genera

ted en

ergy (kW h)

Month

4OH



4h



2H O+O ; E



0.401 V . NHE

vs

2H O



2H +O ;

 

G

237 kg mol

1.5 Outline of this thesis

Novel Concepts for Silicon Based Photovoltaics and Photoelectrochemistry Lihao HAN 韩李豪

韩李豪