EFFECTS OF ACID TREATMENTS ON HYBRID SOLAR CELL PERFORMANCE

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Characterization of materials and solar cells

Scanning electron microscope (SEM) is an imaging technique used to characterize the surface of samples. Under the vacuum less than 10‐5 Torr, the focused high energy electron beam scans the surface of samples. As the beam is negatively charged, it sensitively interacts with the specimen. Secondary electron, X‐ray, Auger electron and back scattered electrons and cathodoluminescence (electron‐induced luminescence from sample) are detected as signals by the interaction between sample and electron beam. In this study, secondary electron, which is the most common imaging mode for surface characterization, is used as the signal to image SiNWs and hybrid solar cells. By the inelastic scattering of electron beam with the sample, secondary electron is ejected from the k‐shell of sample atoms. It could be generated within only a few nanometer depths from the sample’s surface due to its low energy (under 50 eV).
The magnification of SEM ranges about 6 orders of magnitude, for examples, 10 to 500,000 times which is easily controlled by the modification of current of x, y scanning coils or voltage of x, y detector plate. In addition, SEM is able to produce high resolution images of sample’s surface (1~5 nm). Therefore, SEM is useful for imaging numerous nanostructures. For example, Figure 1.9 (a) and (b) show the SEM image of tin (Sn) and bismuth (Bi)‐ catalyzed SiNWs, respectively. Sn‐catalyzed SiNWs exhibits straight and tapered shape. In contrast, Bi‐catalyzed SiNWs shows winding shape. The difference in the appearance of SiNWs catalyzed by different metals is observable by taking SEM images.

Raman spectroscopy

Raman spectroscopy is a spectroscopic method that is involved in vibrational spectroscopy domain, to reveal the molecular structure and characteristic by exploiting vibrational modes of the molecule. Raman spectroscopy is known as a simple nondestructive method suitable for nano‐ materials [45]. Monochromatic light, which is usually laser in visible region, is used as a probe to observe the vibrational modes. The incident monochromatic light into samples undergoes the inelastic scattering, namely, Raman scattering. On the pathway of the incident light through the sample, it can gain or lose the energy corresponding to the inherent vibrational energy of the samples. Also, it can be just passed the samples without energy change. The scattered light could possibly have energies; 1) higher (anti‐stokes shift) 2) lower (stoke shift) than the incident light or 3) same (Rayleigh scattering) energy with it. By measuring the shifts, the molecule is characterized. In general, stoke shift is mainly used due to higher intensity than others. Raman shifts are expressed using the equation given by ∆ν (1) where ∆ν is Raman shift which has unit of inverse length, λ0 is the excitation wavelength and λ is Raman spectrum wavelength. In this study, Raman spectroscopy is employed to verify crystallinity of PECVD grown SiNWs and to characterize SiNWs array. Figure 1.10 shows Raman shift of Sn‐catalyzed SiNWs as an example. It exhibits the peak of crystalline silicon at 518 cm‐1.

X‐ray photoemission spectroscopy

X‐ray photoemission spectroscopy called XPS is a spectroscopic approach to measure the elemental composition, the empirical formula and the electronic states of samples. XPS measures the kinetic energy and the number of electrons which are ejected (photoelectrons) from the samples by the X‐ray irradiation. From 1 to 10 nm depth of samples could be analyzed under the ultra‐high vacuum condition. The binding energy of the electron is extracted by the following equation. where Ephoton is the energy of X‐ray with particular wavelength, which is already knows, Ekinetic is the measured energy of electron and ф is work function of the spectrometer. The unique characteristic of electronic states is analyzed based on the electron binding energy. Here, XPS is employed to reveal the catalyst removal by hydrochloric acid treatment. The binding energy identifies metallic and oxidized catalysts. Besides, the count gives information for the catalyst removal in Chapter 4.

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UV‐Vis spectroscopy

The optical characteristics such as absorbance, transmittance and reflectance are closely related with the solar cell performance. UV‐Vis spectroscopy measures the absorption or transmittance in UV‐Vis region. Thin‐films for solar cells such as P3HT and SiNWs can be characterized by measuring the absorbance of materials as a function of wavelength. The molecule undergoes an electrical transition from ground states to the one of discrete energy levels by absorbing the incident light. As a result, the received light at the detector differs from the incident light in terms of intensity after passing the sample. Absorbance is generally given by log (3) where Aλ is the absorbance at a specific wavelength λ, I is the light intensity after passing a sample and I0 is the intensity of incident light. From the absorbance equation, transmittance (I/I0) could be extracted. Figure 1.11 shows the absorbance of SiNWs, P3HT:PCBM blend and SiNWs with P3HT:PCBM.

Table of contents :

CHAPTER 1 ‐ BACKGROUND KNOWLEDGE
1.1 INTRODUCTION
1.2 SILICON MATERIALS
1.2.1 SILICON NANOWIRES
1.2.2 HYDROGENATED AMORPHOUS SILICON
1.3 ORGANIC SEMICONDUCTOR
1.3.1 Π‐CONJUGATED SYSTEM
1.3.2 P3HT AND PCBM
1.3.3 PEDOT:PSS
1.4 CHARACTERIZATION OF MATERIALS AND SOLAR CELLS
1.4.1 SCANNING ELECTRON MICROSCOPE
1.4.2 RAMAN SPECTROSCOPY
1.4.3 X‐RAY PHOTOEMISSION SPECTROSCOPY
1.4.4 UV‐VIS SPECTROSCOPY
1.4.5 CURRENT‐VOLTAGE CHARACTERISTICS OF SOLAR CELLS
1.5 HYBRID SOLAR CELLS BASED ON SILICON AND ORGANIC MATERIALS
1.6 APPROACHES OF THESIS
CHAPTER 2 ‐ PECVD GROWTH OF SINWS AND NANO‐MORPHOLOGY
2.1 INTRODUCTION
2.2 PLASFIL SYSTEM (PECVD)
2.3 VLS GROWTH OF SINWS AND CATALYST CHOICE
2.4 SN‐CATALYZED SINWS
2.4.1 GROWTH TIME DEPENDENCE
2.4.2 DENSITY CONTROL BY H2 PLASMA
2.5 BI‐CATALYZED SINWS
2.5.1 N‐TYPE CHARACTERISTIC OF BI‐CATALYZED SINWS
2.5.2 TEMPERATURE DEPENDENCE
2.5.3 RF POWER DEPENDENCE
2.5.4 GAS FLOW RATIO DEPENDENCE
2.6 CONCLUSION: PROPER USE OF EACH SINWS
2.6.1 SN‐CATALYZED SINWS
2.6.2 BI‐CATALYZED SINWS
CHAPTER 3 ‐ NANO‐STRUCTURED HYBRID SOLAR CELLS
3.1 INTRODUCTION
3.2 DEVICE DESIGN, FABRICATION AND WORKING PRINCIPLE
3.3 PHOTOVOLTAIC EFFECT ESTABLISHMENT BY PEDOT:PSS
3.4 MORPHOLOGY OF SINWS & PHOTOVOLTAIC EFFECTS
3.5 CONCLUSION: REQUIREMENTS FOR PHOTOVOLTAIC ACTION
CHAPTER 4 ‐ OPTIMIZATION OF SINWS FOR EFFICIENT LIGHT TRAPPING
4.1 INTRODUCTION
4.2 ACID TREATMENTS FOR RESIDUAL CATALYST REMOVAL
4.2.1 HCL TREATMENT OF SINWS AND ITS EFFECT ON SINWS ARRAY
4.2.2 EFFECTS OF ACID TREATMENTS ON HYBRID SOLAR CELL PERFORMANCE
4.3 A‐SI:H ETCHING BY HYDROGEN PLASMA
4.3.3 SELECTIVE ETCHING OF A‐SI:H FROM SINWS ARRAY
4.3.4 ENHANCED SOLAR CELL PERFORMANCE BY A‐SI:H ETCHING
4.4 CONCLUSION: BENEFITS FROM SN‐CATALYZED SINWS
CHAPTER 5 ‐ HYBRID HETEROJUNCTION BASED SOLAR CELL
5.1 INTRODUCTION
5.2 HYBRID INTERFACE: P3HT AND SILICON MATERIALS
5.3 REALIZATION OF HYBRID SOLAR CELLS
5.4 DEVICE PERFORMANCES
5.4.1 DARK CURRENT‐VOLTAGE CHARACTERISTIC
5.4.2 SOLAR CELL PERFORMANCE
5.5 CONCLUSION: CHALLENGES AND PERSPECTIVES
Reference

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