氫化非晶矽薄膜太陽能電池之最佳化研究

國立交通大學

顯示科技研究所

碩士論文

Optimization of Hydrogenated Amorphous

Silicon Single-Junction Solar Cells

氫化非晶矽薄膜太陽能電池之最佳化研究

研究生: 陳達欣 Da-Shin Chen

指導教授: 蔡娟娟 教授 Prof. C.C. Tsai

氫化非晶矽薄膜太陽能電池之最佳化研究

Optimization of Hydrogenated Amorphous Silicon Single-Junction

Solar Cells

研究生 : 陳達欣 Student: Da-Shin Chen

指導教授 : 蔡娟娟 教授 Advisor: Prof. C.C. Tsai

國立交通大學

顯示科技研究所

碩士論文

A thesis

Submitted to Department of Photonics Display Institute

College of Electrical Engineering and Computer Science National Chiao Tung University

In partial Fulfillment of the Requirements For the Degree of

Master In

Electro-Optical Engineering August 2009

Hsinchu, Taiwan, Republic of China

中文摘要

中文摘要

中文摘要

中文摘要

Abstract

In this study, hydrogenated amorphous silicon (s-Si:H) solar cell was fabricated by

plasma enhanced chemical vapor deposition (PECVD). First, we optimized condition of the

deposited single layer for p-layer, i-layer and n-layer, respectively. In order to investigate film

property, the optoelectronic and optical properties was measured by Fourier Transform

Infrared Spectroscopy (FTIR), UV/VIS/NIR spectrometers. The property of hydrogenated

amorphous silicon carbide (a-SiC:H) p-layer was measured and discussed. Comparing the

photovoltaic performances of the as grown solar cell with p-layer for a-Si:H and

a-SiC:H ,respectively. By using wide bandgap p-layer, the open-circuit voltage (Voc) increased

from 0.75V to 0.78V with corresponding short-circuit current (Jsc) increased from

10.23mA/cm2 to 12.76mA/cm2. Post-treatment of the cell was also carried out and significant

increase in the fill factor (FF), efficiency, and Voc were observed. The experiment result

showed an improvement between the Ag back electrode and amorphous n-layer. Different cell

area of 2×2 cm2 and 1×1 cm2 were also fabricated. A cell conversion efficiency of 8.67% was

誌謝

誌謝

誌謝

誌謝

本論文得以順利完成，真的要感謝許多曾幫過我的人，首先必須要感

謝我的指導教授蔡娟娟老師，他教導我許多做人處事方法以及研究學問的

態度，在這兩年研究生涯的敦敦教誨之下使我受益匪淺。尤其是實驗研究

部份，老師的全心投入與栽培，更是支持學生繼續走下去的動力泉源。光

電所冉曉雯老師，在學生感到困惑時，給學生許多的支持與鼓勵，以及在

實驗上給我的各種幫忙。交通大學綠色能源科技中心的支持，內心亦不勝

感激。

還要感謝光電所博士班顏國錫、

黃 彥 棠

學長、博後徐振航學長、光電

所王建敏同學、顯示所曾威豪、姚芳弘同學及

許翼鵬、許宏榮、李建亞、

鄭柏翔學弟除了協助實驗之外並

在我最艱苦的時候陪我渡過最後的關頭，

謝謝大家。此外，交通大學奈米中心的崔秉鉞主任、林聖欽先生、倪月珍

小姐、黃國華先生、何惟梅小姐、優貝克的陳江耀、張文心及張智浩 ，核

能研究所的蔡文發，陳泳智跟余沛慈老師實驗室的黃正宇對於實驗上的幫

助跟協助，在此獻上我最誠摰的祝福與謝意

還要感謝口試委員林明璋、冉曉雯、李柏璁於百忙之中撥冗前來，提

供我許多寶貴意見，使得本論文更臻於完善。最後，要深深感謝我最愛的

家人，陪我度過許多挫折及分享我的喜悅，並在精神上永遠支持我，有你

們在背後的支持真好，在此，願將這份榮耀與你們一同分享。

Contents

1.2 Amorphous Silicon and Crystalline Silicon 3

1.3 The Structure of Thin Film Solar Cell 4

1.4 AM1.5 Light Source 6

1.5 Staebler–Wronski Effect 8

1.6 PECVD 9

1.7 An Overview of Amorphous Silicon Solar Cell 9

1.8 Motivation 11

1.9 Thesis Outline 12

Chapter 2 Experimental Details 14

2.1 Radio-Frequency Plasma-Enhanced Chemical Vapor Deposition 14

2.2 Introduction of Experiment 17

2.3 Determination of Thin Film Thickness 18

2.4 Conductivity Analysis 19

2.5 Determination of Optical Properties 20

Chapter 3 Result and Discussion 26 3.1 Optimization of Intrinsic Hydrogenated Amorphous Silicon 26 3.1.1 Effect of the Silane Flow Rate on the Film Property 26 3.1.2 Effect of the Electrode Spacing on the Film Property 29 3.2 Doping of Hydrogenated Amorphous Silicon 32

3.2.1 Phosphorus Doping of n-type a-Si:H 32

3.2.2 Boron Doping of p-type a-Si:H 34

3.3 Optimization of Hydrogenated Amorphous Silicon Carbide 35 3.4 Hydrogenated Amorphous Silicon Solar Cell 38 3.4.1 Solar Cell Fabrication on TCO-Coated Glass 38

3.4.2 Effect of Annealing on cell Performance 40

Chapter 4 Conclusion and Future Work 46

4.1 Conclusion 46

Figure Captions

Figure 1.1 _{Transforming the global energy mix: The exemplary path until 2050/2100 [1] 1}

Figure 1.2 Efficiency and cost projections for first-, second- and third-generation PV technologies (wafer-based, thin films, and advanced materials and structures, respectively) [2]

2

Figure 1.3 Silicon network in (a) crystalline silicon (b) hydrogenated amorphous silicon. 3

Figure 1.4 (a) The superstrate cell configuration and (b) the substrate cell configuration. 5

Figure 1.5 Schematic of air mass 7

Figure 2.1 Schematic diagram of capacitively-coupled rf PECVD system 14

Figure 2.2 Surface reactions during the a-Si:H film growth [23] 16

Figure 2.3 The dependence of etching rate of the [100] silicon in 30 wt.% KOH on

temperature [25] 18

Figure 2.4 The dependence of etching rate of the silicon dioxide in 30 wt.% KOH on

temperature[25] 19

Figure 2.5 Sample configuration for the conductivity measurement of a-Si:H, where t is the

film thickness 20

Figure 2.6

(

α

ω

)

( )

ω

Figure 2.7 (a) The I-V characteristics and (b) power density versus voltage of solar cell, Vm

and Im are the voltage and current corresponding to the maximum power density of the cell 24

Figure 3.1 The dark-conductivity (σdark) and the photo-conductivity (σphoto) as a function of

the silane flow rate 26

Figure 3.3 The microstructure parameters and the hydrogen content as a function of the

silane flow rate 28

Figure 3.4 The integrated absorption of the SiH and SiH2 bonding configurations as a

function of the silane flow rate 28

Figure 3.5 Dependence of photo- (σphoto) and dark-conductivity (σdark) of intrinsic a-Si:H on

the electrode spacing 29

Figure 3.6 The deposition rate as a function of the electrode spacing 30

Figure 3.7 The hydrogen content and microstructure parameters versus the electrode

distance, respectively 31

Figure 3.8 The integrated absorption of the SiH and SiH2 bonding configurations versus the

electrode spacing 31

Figure 3.9 Dark-conductivity as function of the phosphine-to-silane flow ratio 33

Figure 3.10 The bandgap and deposition rate as function of the silane-to-phosphine flow

ratio 33

Figure 3.11 The dark-conductivity of the p-type a-Si:H as a function of the diborane–to-

silane flow ratio 34

Figure 3.12 Bandgap and deposition rate versus the diborane-to-silane flow ratio 35

Figure 3.13 The conductivity of a-SiC:H versus the methane-to-silane flow ratio 36

Figure 3.14 The bandgap and deposition rate as a function of the methane-to-silane flow

ratio 37

Figure 3.15 Photovoltaic performance of the as-deposited solar cell with a-Si:H and

a-SiC:H as window layers 39

Figure 3.16 The I-V curves for the as-deposited, and the annealed solar cell after annealing

Figure 3.17 Voc, Jsc, fill factor and efficiency of the solar cell as a function of the annealing

temperature 42

Figure 3.18 The I-V curves of the solar cells which were annealed before and after the Ag electrode was deposition, as compared to the as-deposited cell 43

Figure 3.19 Diagrams of Voc, Jsc, fill factor and efficiency with different solar cell area of

Table Captions

Table 3.1 The deposition condition of each layer in the fabricated a-Si:H solar cell 39

Table 3.2 The solar cell parameters with different p-layer material 40

Table 3.3 The performance of the as-deposited solar cell and the annealed solar cell where the annealing was done before and after the silver deposition, respectively. 44

Table 3.4 The solar-cell performance with cell area of 0.5×0.5 cm2, 1.0×1.0 cm2 and

Chapter 1 Introduction

1.1

The world is currently dependent on fossil fuels and other sources of

non-renewable energy. The constant consumption and world-wide demography expansion add

to the potential risks of ecological and human disaster associated with global warming. These

reserves are finite resources and as such the need to reduce our dependency on them is widely

agreed on, to the point where ideally energy generation could be self sustaining over the

whole planet. This makes it a necessity to develop renewable energy technologies. Renewable

energy can be harvested from many natural sources including wind, geothermal heat, sunlight,

hydroelectricity, etc. From the Figure1.1, the solar power has more potential to compensate

energy gap.

Solar Cells are classified into three generations which indicates the order of which

each became important as shown in Figure 1.2. First generation cells consist of wafer base,

high quality and single-junction devices. But the cost is too high. Second generation cells is

thin film solar cell such as a-Si:H solar cell ,CIGS, CdTe, etc. These materials are applied in a

thin film to a supporting substrate such as glass reducing material mass and therefore costs.

Third generation technologies aim to enhance poor electrical performance of second

generation (thin-film technologies) while maintaining very low production costs. It involves

new material and new solar cell structure. In this work, we will focus on second generation,

amorphous silicon solar cell.

Figure 1.2 Efficiency and cost projections for first-, second- and third-generation PV

1.2 Amorphous Silicon and Crystalline Silicon

Figure 1.3 illustrates the different silicon network in crystalline silicon and hydrogenated

amorphous silicon. Crystalline silicon is a four-fold coordinated atom that is normally

tetrahedrally bonded to four neighboring silicon atoms. The tetrahedral structure is continued

over a large range, forming a well-ordered lattice (crystal), as shown in Figure 1.3 (a).

Hydrogenated amorphous silicon (a-Si:H) is the non-crystalline allotropic form of silicon.

It can be deposited in thin films at low temperatures onto a variety of substrates. In

amorphous silicon this long range order is not present and the atoms form a continuous

random network, as shown in Figure 1.3 (b). Although amorphous silicon lacks the long range

order, it has the same short range as single crystalline silicon. Due to the disordered nature of

the amorphous silicon, some silicon atoms have a dangling bond. There are covalently bonded

to only three silicon atoms and have on unpaired electron, a so-called dangling bond. These

dangling bonds are defects in the silicon random network, which cause anomalous electrical

behavior.

When amorphous silicon was deposited in such a way that hydrogen can be incorporated

in the atomic network, the material can be passivated by hydrogen. Thus, the dangling bond

density can reduce by several orders of magnitude. In general, there exist about 10 at.%

hydrogen concentrations in hydrogenated amorphous silicon thin film. Because hydrogen

radical would passivate dangling bond, hydrogenated amorphous silicon has a sufficiently low

amount of defects to be used within devices. Due to Si-H bonding (1.7eV) was formed,

amorphous silicon bandgap (1.75eV) is large than crystalline silicon bandgap (1.12eV).

However, the hydrogen is unfortunately associated with light induced degradation of the

material, termed the Staebler-Wronski effect.

One advantage is that a-Si or its alloys can be deposited at very low temperatures. This

allows for deposition on not only glass, but plastic as well, making it a candidate for a

roll-to-roll processing technique [4-6]. Another advantage is that amorphous silicon can be

deposited over large areas by PECVD.

1.3 The Structure of Thin Film Solar Cell

There are two basic configurations of thin film solar cell, as shown in Figure 1.4. From

the Figure 1.4(a), it is termed superstrate configuration and the substrate usually is used

transparent material. The p-layer is deposited first, then the intrinsic layer and the n-layer

deposited last. From the Figure 1.4(b), it is substrate configuration and occurs when a cell is

configuration. In the case of the superstrate configuration, the glass usually was coated with

transparent conductive oxide (TCO) film. In addition, the TCO layer has to be surface

textured in order to enhance light absorption inside the solar cell due to the scattering at

internal rough interface.

Figure 1.4 (a) The superstrate cell configuration and (b) the substrate cell configuration.

Thin film a-Si:H must be grown with an intrinsic region as opposed to pn junction to

compensate for low carrier drift mobility. The electric field across the intrinsic region is

required to achieve effective separation of generated carriers and a rectifying junction. (b)

i-layer

n-type

Metal Electrode

p-type

TCO

Glass

i-layer

n-type

Metal Electrode

p-type

TCO

Substrate

L

L

i

i

g

g

h

h

t

t

L

L

i

i

g

g

h

h

t

t

Both configurations are usually illuminated through a TCO/p-layer interface. This

improves cell performance by generating most carriers close to this interface, lowing hole

recombination probability. Due to the low hole mobility of a-Si:H, efficient collection is a

priority for ensuring good cell performance. So the p-layer also call window layer in the solar

cell structure.

1.4 AM1.5 Light Source

Solar radiation closely matches a black body radiator at 5,800 K. As the sunlight travels

though the atmosphere, chemicals react with the sunlight and absorb certain wavelengths.

Perhaps the best known example is the stripping of ultraviolet light by ozone in the upper

atmosphere, which dramatically reduces the amount of short wavelength light reaching the

Earth's surface. A more active component of this process is water vapor, which results in a

wide variety of absorption bands at many wavelengths, while molecular nitrogen, oxygen and

carbon dioxide add to this process.

Atmospheric scattering also plays a role, removing higher frequencies from direct

sunlight and scattering it about the sky. The greater the distance of atmosphere the sunlight

travels through, the greater this effect, which is why the sky looks pink at sundown when the

For a thickness lo of the atmosphere, the path length l through the atmosphere for solar

radiation incident at angle θ relative to the normal to the Earth's surface is

l = lo / cos θ (1) The ratio l / lo is the air mass coefficient.

As shown in Figure 1.5, the spectrum outside the atmosphere, the 5,800 K black body, is

referred to as "AM0", meaning "zero atmospheres". Cells used for space power applications,

like those on communications satellites are generally characterized using AM0.

Figure 1.5 Schematic of air mass

The spectrum after traveling through the atmosphere to sea level with the sun directly

overhead is referred to as "AM1". This means "one atmosphere". Conveniently, silicon solar

cell development is concentrated in the United States, Europe and Japan, an AM number

representing the spectrum at mid-latitudes is much more common. "AM1.5", which is

1.5 times the atmosphere thicknesses, corresponds to a solar zenith angle of 48°, and is almost

universally used to characterize solar panels. "AM1.5G", where G stands for ‘global’ and

includes both direct and diffuse radiation.

1.5 Staebler–Wronski Effect

The Staebler-Wronski Effect (SWE) refers to light-induced metastable changes in the

properties of hydrogenated amorphous silicon. The defect density of hydrogenated amorphous

silicon (a-Si:H) increases with light exposure, to cause an increase in the recombination

current and lead to the reduction in the sunlight to electricity conversion efficiency. It was

discovered by Staebler and Wronski in 1977 [3]. They showed that the photo-conductivity of

hydrogenated amorphous silicon can be reduced significantly by prolonged illumination with

intense light. However, it could reverse the effect by heating the samples to above 150℃.

The efficiency of an amorphous silicon solar cell typically drops during the first six

months of operation. This drop may be in the range from 10% up to 30% depending on the

material quality and device design. After this initial drop, the effect reaches equilibrium and

causes little further degradation. Most commercially available a-Si modules have SWE

degradation in the 10 to 15% range and suppliers typically specify efficiency based on

cell the efficiency is reduced by up to 30% in the first 6 months as a result of the

Staebler–Wronski effect, and the fill factor falls from over 0.7 to about 0.6. This light induced

degradation is the major disadvantage of amorphous silicon as a photovoltaic material.

1.6 PECVD

Plasma enhanced chemical vapor deposition (PECVD) system is widely used to fabricate

silicon thin film solar cell. Using PECVD system to deposit silicon thin film is a low

temperature process, usually less than 300℃. So silicon thin film can deposit onto variety of

substrates such as glass, stainless steel, or plastic substrate. Comparing the silicon wafer

substrate, employing the above substrates is the good point of view to industry for large scale

productions and cost drop. Besides, doping and alloy layers are made easily by using PECVD

method. For example, introducing source gases SiH4 and B2H6 or SiH4 and PH3 mixture,

doping layers can be easily deposited onto different substrates.

1.7 An Overview of Amorphous Silicon Solar Cell

First commercially available thin-film solar cells based on a-Si:H were produced for

consumer applications such as pocket calculators and solar watches. Today, a new generation

of a-Si:H-based solar modules enters the market focusing on power applications. Several

manufacturing facilities with multi-megawatt capacity have started production, are currently

being installed, or are announced [4, 7, 8].

in 1976 [9]. This single junction p-i-n a-Si:H solar cell deposited on a glass substrate coated

with transparent conductive oxide (TCO) and aluminium back contact exhibited a 2.4%

conversion efficiency. Only one year later Staebler and Wronski discovered the light-induced

degradation of a-Si:H (Staebler–Wronski effect, SWE) [3]– a severe drawback in a-Si:H solar

cell technology. During exposure of solar cells to light the SWE results in a decrease of

efficiency until a saturation value is reached. Since the recognition of the SWE, development

strategies have focused on improvements of the stabilized efficiency reached after long-term

operation. The stacked-cell concept has emerged as a powerful tool to enhance the stabilized

efficiency [10]. Moreover, the use of component cells with different optical bandgaps

provides a better utilisation of the solar spectrum [11, 12].

Much research in the field of a-Si:H solar cell was devoted to developing and

optimizing a-Si:H base alloys in the 1980s. A p type hydrogenated amorphous silicon carbide

(a-SiC:H) was incorporated in solar cells as a low absorbing layer, usually denoted as a

window layer [13]. Hydrogenated amorphous silicon germianium (a-SiGe:H) became an

attractive low bandgap material for stacked solar cells [14]. Surface textured substrates were

introduced to enhance optical absorption [15]. Optimized transparent conductive oxide (TCO)

films and TCO/metal back reflectors minimize reflection losses and provide an effective light

trapping.

a-SiGe:H (hydrogenated amorphous silicon germanium) films and corresponding alloys

improves the electronic properties of these materials in both the initial and the light-soaked

state [16–18]. All these techniques are combined in a triple-junction a-Si/a-SiGe:H/a-SiGe:H

cell which yields a stable active area efficiency of 13% [12]. This value represents the current

world record for solar cells based on a-Si:H and its alloys. Hydrogenated microcrystalline

silicon deposited by the low temperature PECVD technique emerged in this period as a new

candidate for the low bandgap material in multijunction a-Si:H based solar cells [19].

Research has concentrated on understanding and improving light trapping

techniques, where surface textures as well as new TCO material play a crucial role. This

activity has resulted in the commercialization of novel deposition techniques for ZnO as an

alternative TCO material for SnO2 [20]. Several deposition machine manufacturers have

started developing commercial production machines for the fabrication of thin film silicon

solar cell [21]. Most improvements in stabilized solar cell efficiency are based on adapted cell

designs and advanced light-trapping concepts. Many fundamental questions remained

regarding the growth and the material properties of a-Si:H and its alloys as well as the optical

and electrical function of complete solar cell devices.

1.8 Motivation

Thin film silicon in general, are expected to be promising material for applications to

device-grade a-Si:H, reactive sputtering, mercury-sensitized photo-chemical-vapor deposition

(CVD), direct-photo CVD, hot-wire CVD, and plasma-enhanced CVD (PECVD). Among the

variety of growth methods, PECVD is widely used due to its high potential for yielding a

uniform high-quality silicon thin film on a large-area substrate.

In this study, hydrogenated amorphous silicon (s-Si:H) solar cell was fabricated by

Radio-Frequency Plasma Enhanced Chemical Vapor Deposition (PECVD). The absorption in

the a-Si:H intrinsic layer contributes to the current generation, the optimal optical and

optoelectronic property was crucial role for solar cell performance. Due to lower mobility of

holes in comparison to electrons in a-Si:H , the p-layer a-Si:H of quality is the other effect for

solar cell performance. Because light enters the solar cell through the p-layer, there is

substantial absorption in this layer. The photogenerated carriers in the p-layer do not

contribute to the photocurrent because the electron quickly recombined. Therefore, the

absorption of the p-layer has to minimize, which is done by minimizing the thickness and

alloying the p-layer with carbon.

1.9 Thesis Outline

This thesis is organized into the following chapters:

In the chapter 1, a brief overview of thin film solar cell technology is introduced to

describe its advantages and bottlenecks. The motivation of the studies is also described in

In the chapter 2, the process flow of the samples is introduced. The measurement method

also shows here in detail.

In the chapter 3, here shows the dependence of material properties and device

performance on various conditions with different hydrogen dilution ratios.

In the chapter 4, this chapter will discuss the results from chapter 3. The effect and the

dependence of the material properties on different conditions will be explain here.

In the chapter 5, the results of our experiments in chapter 3 and discussions in chapter 4

Chapter 2 Experimental Details

Plasma-enhanced chemical vapor deposition (PECVD) is common technique used

during the manufacturing of most microeletronic device. The role of the plasma is to provide

a source of energy to dissociate silicon bearing gas, which is usually silane (SiH4), hydrogen

(H2), etc. This is done by collisions with electrons, which originate as secondary electrons in

the plasma an build up their energy by acceleration in an electric field. The growth of an

a-Si:H film is accomplished by attaching reactive particles of dissociated silane molecules,

call radicals, to the surface of the growing film. It consists in the excitation of a gas, of a mix

gases, by an electric field between two electrodes as sketched in Figure 2.1.

Figure 2.1 Schematic diagram of capacitively-coupled rf PECVD system

RF

Pumping

Plasma

Substrate

Grounded Electrode Gas phase reactions

Showerhead

The initial event in the growth process of a-Si:H is electron-impact dissociation of

the source gas materials in silane (SiH4) and silane-to-hydrogen (SiH4/H2) glow-discharge

plasma. SiH4 and H2 molecules were through electronic excited states of these molecules by

inelastic collisions with high-energy electros in the plasma. Electronic excited states of

complicated molecules such as SiH4 are usually dissociating states at which dissociation

occurs spontaneously to SiH3, SiH2, SiH, Si, H2 and H.

Reactive neutral and ionic species produced in the plasma undergo secondary

reaction mostly with SiH4 and H2 molecules, forming a steady state. Rate constants for each

reaction are summarized in the literature [22]. Therefore, highly reactive species such as SiH2,

SiH, and Si (short-lifetime species) have much smaller values of densities than SiH3 in the

steady-state plasma, although the generation rates of those species are not very different from

that of SiH3, which indicates low reactivity with SiH4 and H2( long-lifetime species).

In the growing amorphous silicon, the SiH3 radicals reaching the film-growing

surface start to diffuse on the surface. During surface diffusion, SiH3 abstracts

surface-covering bonded hydrogen, forming SiH4 and leaving a dangling bond on the surface

(growth-site formation). Another SiH3 radical toward the dangling-bond site on the surface

and forms the Si-Si bond (film growth), as schematically shown in Figure 2.2. This surface

reaction scheme for film growth has been proposed on the basis of two experimental results

Figure 2.2 Surface reactions during the a-Si:H film growth [23]

Through other radicals contribute much less to the growth, they do play an important

role in determining the properties of the film. The SiH2 and higher silane radicals have higher

sticking coefficients than SiH3 and be incorporated directly into the hydrogen terminated

surface [24]. However, the contribution of these radical to the growth results in poor quality

film, and therefore the presence of these radicals in the plasma should to be avoided.

An important advantage of plasma enhanced CVD deposition is that the deposition is

2.2 Introduction of Experiment

Hydrogenated amorphous silicon films were deposited using pure SiH4 by radio

frequency (27.12 MHz) plasma enhanced chemical vapor deposition (PECVD) system.

Doping layers were prepared by using B2H6 or PH3 with pure SiH4 and H2 dilution.

Amorphous silicon carbide was deposited by mixing SiH4 and CH4. Prior to deposit all the

samples, the chamber was introduced gas NF3 and Ar to clean the chamber. Corning

Eagle2000 glass and crystalline silicon wafer substrate were clean by KG Cleaner and RCA

standard cleaning process, respectively. All the samples were started to prepare after the

background pressure of the reactor chamber reach to 10-7 torr. In this study, samples were

prepared simultaneously on 5.0 cm * 5.0 cm Corning Eagle 2000 glass and 1.5cm * 1.5cm

c-si wafer. The samples gown on glass substrate was used to analysis optoelectrial properties

and the films on c-si wafer were used to determine the hydrogen bonding configuration. The

devices were deposited on the glass coated TCO.

The complete range of deposition conditions include: substrate temperature (TS) :

190~210°C, pressure: 0.3~1 torr, background pressure: 10-7 torr, power: 20~40 W, electrode

2.3 Determination of Thin Film Thickness

The measurement of thin film thickness has a lot of methods such aAlpha stepper and

ellipsometery. In the measurement of the alpha stepper, it has to be step profile. So chemical

etching is necessary to make step high. Chemical etching may occur by any of several

different processes. The simplest modes of etching involve dissolution of the material in a

liquid solvent without any change in the chemical nature of the dissolved species. So

choosing solvent is important for chemical etching. And we find the material, 30 wt.% KOH,

can etch silicon. Although it still etches silicon dioxide, it etches slowly at room temperature.

Figure 2.3 The dependence of etching rate of the [100] silicon in 30 wt.% KOH on temperature [25]

Figure 2.4 The dependence of etching rate of the silicon dioxide in 30 wt.% KOH on temperature[25]

The Figure 2.3 and Figure 2.4 show the correction between the etching rate and

temperature[25]. The 30 wt.% KOH can etch amorphous silicon. We use the tape attach a part

of sample area. It immerses in the 30 wt.% KOH, until the amorphous silicon was removed.

This method leaves a well defined step profile which was measured at intervals of 25mm

along the length of the slide using a mechanical alpha step.

2.4 Conductivity Analysis

In order to investigate the conductivity of the a-Si:H, metal contacts were deposited by

thermal evaporation method onto the film. Conductivity (σ) is calculated by equation (2),

to conductivity is determined by the width (W), length (L), and thickness (t) of a cuboid

located between the conducting contacts used to probe the sample in the Figure 2.5.

(2)

A prepared a-Si:H film with silver contacts is show in Figure 2.5. Photo-conductivity

was measured by AM1.5G light spectrum with power at 100mw/cm2. For the amorphous

silicon germanium film, the thickness is about 600 nm. For the intrinsic amorphous silicon,

the thickness is about 600 nm. The thickness of doping layer is about 200 nm .

Figure 2.5 Sample configuration for the conductivity measurement of a-Si:H, where t is the film thickness

2.5 Determination of Optical Properties

2.5.1 Tauc Bandgap

The optical bandgap of thin film use UV/VIS/NIR spectrometers to measure

substrate

a-Si:H

Ag

W

t

Ag

L





































=

t

1

W

L

V

I

σ

transmission (T) spectra were carried out between 200nm and 1.3 µm in the step of 2 nm.

Calculating optical bandgap (Eg) has to use transmission spectra and thickness of thin film in

equation (3).

(

α

h

ω

)

=

c

(

h

ω

−

E

)

1

(3)

Where α is the absorption coefficient of a-Si:H,

h

ω

optical bandgap. From Figure 2.6,

(

α

ω

)

( )

h

ω

bandgap value can be determined from the expanding linear region line intercepts a-axis.

1.0 1.5 2.0 2.5 3.0 3.5 0.00 0.02 0.04 0.06 0.08 0.10 0.12 (αααα hωωωω ) 1 /2 hωωωω (eV) bandgap

Figure 2.6

(

α

ω

)

( )

ω

2.5.2 Fourier Transform Infrared Spectroscopy

Since hydrogen is an important element in passivation, the incorporation and stability of

hydrogen in a-Si:H has the topic of intensive research. Infrared absorption spectroscopy is

Three characteristic infrared absorption bands are observed in a-Si:H: a peak at 640 cm-1, and

absorption peaks in the rage of 2000-2200cm-1. The peak at 640 cm-1 reflects the rocking

mode of hydrogen covalently bonded in all possible bonding configurations, such as silicon

monohydride (SiH), dihydride (SiH2), and trihydride (SiH3) and polymeric (Si-H2)n bonding

configurations. Thus, this peak is used to determine the hydrogen content in a-Si:H [26]. And

peak around 2000cm-1 isassigned to the stretching mode of the isolated Si-H bonds and a peak

in the range of 2060-2160cm-1 includes contributions from the stretching mode of Si-H bonds

at internal surface, such as voids, Si-H2 and Si-H3.

Infrared absorption measurements were made with Fourier Transform Infrared

Spectroscopy on films deposited on crystalline silicon. The data were analyzed following

Brodksy et al. [27]. The integrated absorption, I, for each bond was determined from the

relationship.

( )

d

I

ω

ω

ω

α

∫

=

Where α(ω) is the absorption coefficient of the film at the wavenumber ω. A ‘microstructure

parameter’, denoted as R, is determined from the equation (5).

SiH2 SiH SiH2

I

I

I

R

+

=

Where ISiH and ISiH2 are the integrated absorption strength of the peak at wavenumber

integration of the Si-H rocking-wagging mode [28] at 640 cm-1. The complete procedure can

be expressed by the following equation (6):

(

)

=

_∫

( )

≈

∑

( )

∆

N

A

d

N

A

at.%

C

ω

ω

ω

α

ω

ω

ω

α

Where Aw=1.6x1019 cm-2 is the proportionality constant, and Nsi=5x1022 cm-3 is the

atomic density of pure silicon.

2.6 Measurement of Thin Film Solar Cell

An I-V characteristic is measured by performing a voltage sweep from a small reverse

of forward bias whilst measuring current flow at each bias point. An illuminated I-V

characteristic provides a figure for cell efficiency and incident light source is AM 1.5G with

power density 100mW/cm2. Isc and Voc can be determined directly from the vertical and

horizontal intercepts of the sweep.

From Figure 2.7, it shows a typical illuminated I-V characteristic using real measured

data. The current density is that measured current divided by cell area. The corresponding

power function is shown alongside it and this is created by multiplying every voltage bias

point of the I-V characteristic by the current measured at that bias. Important labeled features

of the graphs include Voc, Jsc, the bottom of the graph is maximum power density

Figure 2.7 (a) The I-V characteristics and (b) power density versus voltage of solar cell, Vm and Im are the voltage and current corresponding to the maximum power density of the cell

The solar cell efficiency (η) is the ratio between incident light power density (Pin) and

the device convert the maximum power density. Equation (7) can calculate the cell efficiency.

P

I

V

P

P

=

=

η

V

J

P

(V

, J

)

P

(V

,J

)

Where the ratio of the maximum power density (PMax = Vm Jm) and Pin is the light source

power density. And the other parameter is the fill factor (FF) can describe PV solar cell quality.

It is the ratio of the maximum power density (PMax = Vm Jm) and a cell should supply if it was

ideal (P = Voc Jsc) like equation (8). And the FF can be connect with the efficiency, it like the

equation (9). sc oc Max

J

V

P

FF

=

P

FF

J

V

P

I

V

=

=

η

Chapter 3 Result and Discussion

3.1.1

The optoelectronic properties of the a-Si:H deposited by the PECVD are dominated by

deposition parameters , such as pressure of the gas, flow rate , substrate temperature , power

density, the electrode spacing, etc. In this work, we discuss the effects of the silane flow rate

and electrode distance on the qualities of amorphous silicon thin films.

Here, the substrate temperature was held at 190℃. As shown in Figure 3.1, the dark-

conductivity (σdark) and photo-conductivity (σphoto) as a function of the silane flow rate are

demonstrated. 20 40 60 80 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5

σσσσ

σσσσ

Figure 3.1 The dark-conductivity (σdark) and the photo-conductivity (σphoto) as a function of

From Figure 3.2, the dependence of deposition rate with on silane flow is also illustrated.

The deposition rate significantly increased with raising silane flow rate from 15 to 30 sccm.

When silane flow rate was raised from 30sccm to 80 sccm, the deposition rate didn’t change

too much. The surface reaction caused the restriction of deposition rate. Both the σphoto and

σdark decreased with increasing of the silane flow rate.

20 40 60 80 2.5 3.0 3.5 4.0 D e p o s it io n r a te ( A /s ) SiH 4 Flow (sccm) Figure 3.2 The dependence of deposition rate on silane flow rate

The dependence of hydrogen bonding configuration on silane flow rate is illustrated in

the Figure 3.3. The integrated absorption of SiH and SiH2 species depended on the silane flow

rate, as shown in Figure 3.4. The hydrogen content almost was constant. When the silane flow

rate varied from 30sccm to 50sccm, the R value was minimized. The silane was depleted

20 40 60 80 0.0 0.2

R

SiH

Flow (sccm)

H

c

o

n

te

n

t

(a

t.

%

)

Figure 3.3 The microstructure parameters and the hydrogen content as a function of the silane flow rate

0 20 40 60 80 10 20 30

SiH

Flow (sccm)

In

te

g

ra

te

d

A

b

s

o

rp

ti

o

n

(

a

rb

.u

n

it

)

SiH (2000cm

)

SiH

(2090cm

)

Figure 3.4 The integrated absorption of the SiH and SiH2 bonding configurations as a

On the contrary, the silane flow rate was so much that induced gas phase reaction

drastically. The SiH2 bonding increased slowly as silane flow rate from 50sccm to 80 sccm.

The good qualities of amorphous silicon thin films were attained while the silane flow rate

varied from 30 sccm to 50 sccm. The films contained a majority of SiH2 bonding while the

silane entirely decomposed, which was in agreement with the study of Knights and Lujan [29]

and Street et al. [30].

3.1.2 Effect of the Electrode Spacing on the Film Property

The photo- and dark-conductivity of intrinsic a-Si:H was depended on electron spacing,

as shown in the Figure 3.5. The dark-conductivity decreased with increasing electrode

spacing. 16 20 24 10-11 10-10 10-9 10-8 10-7 10-6 10-5

σσσσ

Electrode Spacing (mm)

C

o

n

d

u

c

ti

v

iy

c

m

σσσσ

Figure 3.5 Dependence of photo- (σphoto) and dark-conductivity (σdark) of intrinsic a-Si:H on

Due to the property of defect would cause the film to be more conductive or resistive,

the quality of films didn’t determine by the dark-conductivity. From the Figure 3.6, deposition

rate significantly increased as raising electrode spacing. Increasing electrode spacing induced

more gas-phase reaction. That is reason the deposition rate increase strongly when the

electron spacing varied from 20mm to 25mm.

16

20

24

1

2

3

4

5

6

Figure 3.6 The deposition rate as a function of the electrode spacing

The results of IR spectra were shown in the Figure 3.7, including the hydrogen content

and microstructure parameters with different electrode spacing, respectively. From

the Figure 3.8, the hydrogen bond configuration of SiH and SiH2 bonding was as a function of

16 20 24 0.0 0.2 0.4 R Electrode Spacing (mm) 6 8 10 12 H C o n te n t (a t. % )

Figure 3.7 The hydrogen content and microstructure parameters versus the electrode distance, respectively 12 16 20 24 0 10 20 30 Electrode Spacing (mm) In te g ra te d A b s o rp ti o n ( a rb .u n it )

SiH

(2090cm

)

SiH (2000cm

)

Figure 3.8 The integrated absorption of the SiH and SiH2 bonding configurations versus the

It figured out that the increases of microstructure and hydrogen content were due to

increase electrode spacing. The electrode spacing significantly affected on SiH and SiH2

species contained in the film. When the electrode distance increased, it caused more gas-phase

reaction. Therefore, it induced the increase of SiH2 bonding. It concluded the optimal

electrode spacing of film quality was in the range of from 14mm to 17mm.

The different hydrogen bonding configuration within the film can be understood from

Paschen’s law. Increasing pressure or electrode spacing, the electrons are more likely to

collide with the plasma constituents than the electrodes. Thus, it promotes polymerization and

can lead to the inclusion of SiH2 chain within the film. On the other hand, as pressure is

lowered or as electrode spacing is decreased, the electron energy is limit by collision with the

electrodes. In this study, it concluded that increasing electrode spacing induces more

gas-phase reaction.

3.2 Doping of Hydrogenated Amorphous Silicon

3.2.1 Phosphorus Doping of n-type a-Si:H

The purpose of doping is to change electrical conductivity and its magnitude by adding

a controlled amount of impurity atoms. The principal doping elements used in a-Si:H are the

same as in crystalline silicon, boron for p-type and phosphorus for n-type material. It change

conductivity of a-Si:H by mixing the silicon source gas, silane (SiH4), with phosphine (PH3)

2 4

10-3

10-2

10-1

PH₃/SiH₄ Flow Ratio (x10-3)

D a rk C o n d u c ti v it y ( ΩΩΩΩ −1−1−1−1 c m -1 )

Figure 3.9 Dark-conductivity as function of the phosphine-to-silane flow ratio

2 4

2 4

PH₃/SiH₄ Flow Ratio (x10-3)

D e p o s it io n r a te (A /s ) 1.0 1.5 2.0 2.5 B a n d g a p (e v )

Figure 3.10 The bandgap and deposition rate as function of the silane-to-phosphine flow ratio

From the Figure 3.9, the dark-conductivity as a function of the diborane-to-silane flow

ratio. As a result of heavy doping, the dark-conductivity didn’t change too much. The

dark-conductivity was almost 10-2 (Ω-1Cm-1). As shown in the Figure 3.10, the bandgap and

deposition rate as a function of phosphine-to-silane flow ratio. The doping material was too

small concentration to change the bandgap.

3.2.2 Boron Doping of p-type a-Si:H

The dark-conductivity (σd) of p-type a-Si:H as a function of the diborane-to-silane flow

ratio, as shown in the Figure 3.11. The conductivity increased as increasing flow ratio.

But the diborane-to-silane flow ratio was large than 1, the conductivity didn’t increase.

Because the doping of a-Si:H inevitably caused the creation of dangling bond [32], it reduced

the conductivity. 0 1 2 10-6 10-5 10-4 D a rk C o n d u c ti v it y ( ΩΩΩΩ -1 c m -1 ) B

2H6/SiH4 Flow Ratio (x10

-2 )

An additional important different between a-Si:H and single crystal silicon is that when

the concentration of boron and phosphorous atoms in a-Si:H increase, then Fermi level

doesn’t move close to the valence and conduction band mobility edges than 0.3eV and to

0.15eV, respectively. 0 1 2 2 4 D e p o s it io n R a te (A /s ) 1 2 B

2H6/SiH4 Flow Ratio (x10 -2 ) B a n d G a p (e v )

Figure 3.12 Bandgap and deposition rate versus the diborane-to-silane flow ratio

The presence of tail and defect states in the bandgap does not allow a full shift of the

Fermi level towards the band edges. This is the reason the conductivity of p-type amorphous

silicon was lower than n-type.

3.3 Optimization of Hydrogenated Amorphous Silicon Carbide

The purpose of alloying a-Si:H for photovoltaic application is to shift the optical

absorption spectrum to higher of lower photon energies and in this way to adjust the optical

slightly changed by varying the hydrogen content in a film [33]. This is done by changing the

deposition conditions, such as the substrate temperature of dilution of silane with hydrogen. A

substantial shift of the absorption coefficient can be achieved by alloying a-Si:H with carbon,

oxygen, nitrogen or germanium.

1.0 1.5 2.0

10-6 10-5 10-4

CH₄/SiH₄ Flow Ratio

C o n d u c ti v it y ( ΩΩΩΩ −1−1−1−1 c m -1 )

Figure 3.13 The conductivity of a-SiC:H versus the methane-to-silane flow ratio

Alloying can easily be accomplished by adding the appropriate gases to the silane

source gas in the CVD process. Amorphous silicon carbide is mixed silane with methane in

the process. It can easily change the amorphous silicon bandgap by varying the methane-to-

silane flow rate ratio[34, 35]. The conductivity and bandgap depended on methane-to-silane

flow ratio, as shown in the Figure 3.13. The conductivity of doped a-SiC:H decreased strongly,

more by alloying carbon. 1.0 1.5 2.0 2 4 D e p o s it io n R a te (A /s ) CH

4/SiH4 Flow Ratio

1.8 2.0 B a n d G a p (e v )

Figure 3.14 The bandgap and deposition rate as a function of the methane-to-silane flow ratio

The bandgap verses methane-to-silane flow ratio as shown in the Figure 3.14.

The effective of alloying carbide can improve bandgap for amorphous silicon.

The bandgap of the p-type amorphous silicon typical is 1.7 eV, but the amorphous silicon

carbide was large than 2 eV. Therefore, amorphous silicon carbide can improve the device

performance by inducing more photo energy to the device. On the other hand, alloying carbon

would reduce the conductivity. Thus, The p-layer a-SiC:H is a trade-off between conductivity

3.4 Hydrogenated Amorphous Silicon Solar Cell

3.4.1 Solar Cell Fabrication on TCO-Coated Glass

In a-Si:H, the diffusion length of the charge carriers is shorter than in crystalline silicon.

In intrinsic a-Si:H, the ambipolar diffusion length is about 0.1 to 0.3 µm. In doped a-Si:H layers,

the defect density of doping a-Si:H is two or three orders of magnitude higher than intrinsic

a-Si:H. Due to the short diffusion length the photogenerated carriers would virtually all

recombine in the doped a-Si:H layers before reaching the depletion region of the p-n junction.

Therefore, an a-Si:H solar cell is designed differently compared to the standard p-n junction of

a crystalline silicon solar cell.

The doped layer are usually very thin at a-Si:H thin film solar cell. The thickness of

p-type a-SiC:H layer is less than 30nm thick, the thickness of the a-Si:H p-layer is less than

30nm thick, and an n-type a-Si:H is less than 30nm thick. The doping layer has two function of

the amorphous silicon solar cell. First, a sufficiently high electrical conductivity is required for

both p- and n- layers in order to form a high built-in voltage across the p-i-n junction. Second,

the doping layers establish low loss ohmic electrical contact with the electrode. The

steabler-Wronski effect deteriorate the collection of carriers, the thickness of intrinsic

amorphous silicon can’t be too thick. So the optimal thickness of the thickness of the intrinsic

layer is in the range of 250nm to 320 nm [36]. The deposition conditions of each layer in the

p-layer (a-Si:H) p-layer (p-a-SiC:H) i-layer (i-a-Si:H) n-layer (n-a-Si:H) Substrate temperature (℃℃℃) ℃ 190 190 190 190 Deposited

material SiH4+B2H6 SiH4+CH4+B2H6 SiH4 SiH4+PH3+H2

Table 3.1 The deposition condition of each layer in the fabricated a-Si:H solar cell

-0.5 0.0 0.5 1.0 1.5 -10 0 10 20

a-SiC:H

p-layer

a-Si:H

p-layer

As-deposited

Figure 3.15 Photovoltaic performance of the as-deposited solar cell with a-Si:H and a-SiC:H as window layers

From the Figure 3.15, photovoltaic performances of the as-deposited solar cell with

a-Si:H p-layer and a-SiC:H p-layer, respectively. The characteristic of solar cell with different

shows increase in the open-circuit voltage (Voc) from 0.75V to 0.78V and in the short–circuit

current (Jsc) from 10.23 mA/cm2 to 12.76 mA/cm2 at the same time. Because the wide bandgap

p-layer also increases the open-circuit voltage by reducing recombination at the p-i interface

[37]. p-layer

_V

J

FF

η

Table 3.2 The solar cell parameters with different p-layer material

But in the I-V curve shows an S-shaped character, which the current starts falling off

already at low forward bias.The curve bending in the Figure 3.15 has shown an imperfection of

the cell. Therefore we performed a thermal annealing process try to eliminate the current

instability and enhance the performance of the solar cell.

3.4.2 Effect of Annealing on cell Performance

In the Figure 3.15 shows an S-shaped character, which the current starts to fall off already

at low forward bias. In order to find the origin for this S-shaped I-V characteristic, the

photovoltaic property of a pin type solar cell is characterized by the series resistance Rs and the

performance. The bias-annealing increased the open-circuit voltage, fill factor, and the cell

efficiency [38]. Their conclusion is the series resistance and the shunt resistance of the

hydrogenated amorphous silicon solar cell can be observed to decrease and increase

respectively after bias-annealing [39].

-0.5 0.0 0.5 1.0 1.5 -10 0 10 As-deposited 75oC anneal 100oC anneal 150oC anneal 210oC anneal C u rr e n t D e n s it y (m A /c m 2 ) Voltage (V)

AM1.5 Cell Area=0.25cm2

150oC

Figure 3.16 The I-V curves for the as-deposited, and the annealed solar cell after annealing at 75℃, 100℃, 150℃ 210℃, respectively ,

The important parameter is annealing temperature. Y. Arai et al. eliminated the process of

bias-annealing. They also can improve solar cell performance by annealing without bias [40].

Figure 3.16 shows the I-V curve for the as-deposited, and the annealed solar cell after annealing

temperature at 75℃, 100℃, 150℃, 210℃, respectively. The result shows that with adequate

Figure 3.17. The results in the Figure 3.17 indicate significant increase of Voc and FF, while Jsc

remains the same.

0 50 100 150 200 250 0.7 0.8 0.9 Annealing Temperature(oC) V o c (v ) 12 14 J sc (m A /c m 2 ) 60 75 F F ( % ) 6 8 10 ηηηη ( % )

Figure 3.17 Voc, Jsc, fill factor and efficiency of the solar cell as a function of the annealing

temperature

In the thin film silicon solar cell, there are three main factors can affect the fill factor.

First, the recombination loss in the i-layer or at the p/i and n/i interface. Second, partial

micro-shunts through the i-layer. Third, the contact and doped layers are too high resistance

which may due to the degradation of interface quality.

In order to investigate the thermal annealing effect for the solar cell, experiment of the

sequence between Ag deposition and annealing was carried out Ag deposition and annealing

was carried out. The IV curve of the solar cell which were annealed before and the Ag

electrode was deposited, as compared to the as-deposited cell, as shown in the Figure 3.18.

The result indicates that the n/Ag interface and contact quality is main issue for the thermal

annealing. The as-deposited and the annealing without Ag electrode didn’t change observably.

But the annealing after Ag electrode deposited improved performance with annealing 150 ℃.

-0.5 0.0 0.5 1.0 1.5 -10 -5 0 5 10 15 20

C

u

rr

e

n

t

D

e

n

s

it

y

(m

A

/c

m

)

Voltage (V)

Anneal before Ag deposited Anneal after Ag deposited

Annealing Temprature = 150

C

Figure 3.18 The I-V curves of the solar cells which were annealed before and after the Ag electrode was deposition, as compared to the as-deposited cell

Voc(V) Jsc (mA/Cm2) FF (%)

Table 3.3 The performance of the as-deposited solar cell and the annealed solar cell where the annealing was done before and after the silver deposition, respectively.

0 1 2 3 4 0.8 1.0 Cell Area (cm2) V o c (V ) 12 14 J s c ( m A /c m 2 ) 60 75 F F ( % ) 8.0 8.5 9.0 ηηηη (% )

Figure 3.19 Diagrams of Voc, Jsc, fill factor and efficiency with different solar cell area of

Cell area (cm2) V_oc (V) J_sc (mA/cm2) FF (%) η ηη η (%) 0.5×0.5 0.85 13.75 74.11 8.68 1.0×1.0 0.85 13.57 74.66 8.69 2.0×2.0 0.85 14.4 70.75 8.67

Table 3.4 The solar-cell performance with cell area of 0.5×0.5 cm2, 1.0×1.0 cm2 and 2.0×2.0

cm2

It can conclude the thermal annealing can get better between silver and n-type layer

interface. Because the defect in the interface introduce states in the semiconductor bandgap

which can trap charges and influence the potential distribution at the junction. A high density

of interface states reduces the open-circuit voltage and degrades the photovoltaic performance

of the devices. The cells with size of 0.5×0.5 cm2, 1.0×1.0 cm2 and 2.0×2.0 cm2 were

fabricated to verify the cell performance related to uniformity. The results can be seen in the

Figure 3.19 and Table 3.9. The result shows cell performance did not degraded on the cell

area was enlarged to 2.0×2.0 cm2. The efficiency of about 8.5 % and high performance were

Chapter 4 Conclusion and Future Work

In this work, we have studied the property of a-Si:H as function of the silane flow rate,

electrode spacing and dopant concentration. Increasing the electrode spacing promoted

polymerization and can lead to the inclusion of SiH2 chain within the film. The high quality

intrinsic a-Si:H was obtain at which contains 8~9 atomic % of hydrogen predominated

bonded as SiH configuration. The p-layer and n-layer have been doped with appropriate

dopant concentration. Alloying a-Si:H with carbon causes the bandgap to widen, but it also

reduce the conductivity. Usage of the wide bandgap p-layer would increases the open-circuit

voltage and the short-circuit current of the solar cell by increasing incident light and reducing

electron-hole recombination at the p-i interface. In this study, the optimal a-SiC:H window

layer has bandgap 1.94ev and carbon composition 20 %.The annealing would affect solar cell

performance significantly with optimal annealing temperature of 150℃ The n/Ag interface is . the main improvement of the solar cell when underwent thermal annealing. The structure of

a-Si:H solar cell with area sizes of 2×2cm2 and 1×1cm2 was also fabricated, respectively. The

cell performance did not degrade on the cell area was enlarge to 2×2cm2. The best conversion

4.2 Future Work

The performances of single junction a-Si:H solar cells decrease during the initial

stage of due to light induced degradation. The solar cell degradation due to illumination is the

manifestation of the Staebler-Wronski effect. After the initial degradation, the performance of

solar cells stabilizes. Therefore, the a-Si:H solar cell structure and the properties of the

individual a-Si:H based layers must be optimized for the light soaked state. In addition, in this

study we considered only single-junction solar cell. This work can be extended for tandem

solar cells, with a-Si:H cell as the bottom cell and the a-SiGe or µc-Si thin film solar cell as