Design and Development of ZnO Morphologies for Enhanced Photovoltaic Characteristics: Synthesis, Characterization and Fabrication of Photoanodes for Semiconductor Quantum dot/ Dye Sensitized Solar Cells.

Characterization technique of the devices using basic and specific instrumental techniques/methods such as solar simulator (Newport Sol3A), incident photon-to-current analyzer (IQE 200) and electrochemical impedance spectroscopy (EIS) measurements are also discussed. Using this specific strategy dramatically improves the photovoltaic properties of the material compared to its counterparts such as ZnO nanoparticles (NPs), nanorods (NRs) and porous nanoparticles (PNPs); the conventionally and commonly used ZnO morphologies in solar cells.

Chapter 4 presents a simple, green hydrothermal synthetic protocol for exotic 3D ZnO superstructures (ZnO Cages) assembled with compact 1D ZnO nano-rods which can provide faster

A comparative study of the photovoltaic performance of the solar cell was conducted with the devices fabricated with pristine ZnO HMSPs and ZnO NWs. Morphological characterizations of the synthesized materials and the fabricated photoanode are shown in Figure 3.

Figure 2. FESEM images (A) and (B) represents as synthesized cage like ZnO superstructures at different magnifications

In this work, we have presented the idea to improve the PCE (η) of SnO2-based DSSCs by introducing an electronic barrier layer in the form of 3D hierarchical porous MgO composed of 2D nanosheets by a simple ultrasonic route. followed by the mixing process. Optimized PCE up to 3.71 % is achieved by introducing 15 wt % MgO and using the redox mediator [Co(bpy)3]2/3 Moreover, we observed a significant improvement in Voc.

Introduction and Literature Review

AN OVERVIEW OF PHOTOVOLTAIC TECHNOLOGY DEVELOPMENT

Although second generation solar cells are economical, they are less efficient compared to first generation solar cells. Third-generation solar cells are based on nanostructured photoactive materials.16 The objectives of third-generation solar cells are: (i) fabrication of high-efficiency, low-cost, and light-weight solar cells from inexpensive materials, land-abundant and environmentally friendly, ii) simple manufacturing processes and (iii) large-area fabrication.

Figure 1.1.2. (A) Flexible DSSC device fabricated by G24i Ltd., UK and (B) its integration to backpacks

WORKING PRINCIPLE OF DSSC/QDSSC DEVICES

Excitation of the sensitizer upon irradiation with sunlight is followed by injection of photoexcited electrons into the conduction band (CB) of the MO (reactions 1 and 2). However, the time scale of electron injection in the case of semiconductor QD sensitizers is in the range of picoseconds (10-12 s), which is slower in contrast to the dye molecules.43-45 It has also been shown from experimental observations that intra -band- recombination process (direct band-to-band recombination and trap-mediated recombination) of photoexcited electrons in semiconductor QD sensitizers varies in orders of magnitude (1011 to 106 s) and depends largely on the specific semiconductor and of the nature of the semiconductor surface.43.

Figure 1.2.2 Schematic of the interior and basic working principle of dye-or semiconductor-sensitized solar cells with favorable energy levels of all the cell components

A large variety of ZnO morphologies such as nanoparticles,93 nanowires,94. i) Transport of random charges (ii) High area (iii). In this context, different strategies can be integrated to improve the photovoltaic performance of ZnO-based photoanodes.

Figure 1.3.1 Schematic representation of different photoanodic architectures of ZnO made with (A) zero dimensional (0-D) nanoparticles, (B) 1-D nanorod (NR) networks, (C) 2-D nanosheets (NS) and 3-D microspheres (MS)

MOTIVATION AND OBJECTIVES OF THE PRESENT WORK

Scale-up and Product-Development Studies of Dye-Sensitized Solar Cells in Asia and Europe”, edited by K. This chapter describes fundamental instrumentation techniques and methodologies used for material characterization and fabrication of dye/semiconductor QD-sensitized solar cells, where the emphasis is placed on ZnO-based photoanode preparation.

Material Synthesis

INTRODUCTION

This chapter provides a list of all materials used at different stages of the study and basic characterization techniques for the materials synthesized. Characterization techniques of the fabricated photovoltaic devices and performance parameters of solar cells are discussed in detail.

EXPERIMENTAL METODS

MATERIALS AND CHEMICALS USED
CHARACTERIZATION OF MATERIALS AND DEVICES
GENERAL FABRICATION PROCESSES OF DSSC AND QDSSC (i) Photoanode Preparation
EVALUATION OF SOLAR CELL PERFORMANCE
ELECTROCHEMICAL IMPEDENCE SPECTROSCOPY (EIS) MEASUREMENT To monitor electron transport and recombination kinetics at the solid/liquid heterojunctions

Another fundamental measurement of the performance of a solar cell is the incident photon-to-current conversion efficiency (IPCE). The maximum peak frequency of the second semicircle (fmax) is correlated to the rate of recombination of electrons (keff) at working electrode/electrolyte interfaces.

Figure 2.2.3 Pictorial presentation of step-by-step counter electrode preparation process

Bode phase plots also provide the characteristic peak frequency in the medium frequency range (fmax) which is used in the calculation of photoinduced electron lifetime (τe) in the photovoltaic devices. Higher τe value derives maximum photoinduced electron density availability in the photovoltaic devices for extraction to the external circuit.

Hexagon Shaped ZnO Nanodisks with Exposed ± {0001}

Polar Facets for Enhanced Photovoltaic Performance in Quantum Dot Sensitized Solar Cells

INTRODUCTION

To control the exposure of ZnO crystal facets, it is necessary to minimize surface energy of the polar high-energy crystal facets during crystal growth. Controlled reactions are performed to understand the mechanism of formation of ZnO NDs and evaluate their photovoltaic properties in CdS/CdSe-sensitized solar cells.

EXPERIMENTAL METHODS

SYNTHESIS OF ZnO NANODISKS
SYNTHESIS OF ZnO NANOPARTICLES
FABRICATION OF PHOTOANODES AND DEVICES

In SILAR method, the electrodes are first dipped in an ethanolic solution of Cd(NO3)2.6H2O (0.5 M) for 1 min, rinsed with ethanol and dried on a hot plate at 70 C. ZnO NPs-based photoanodes are fabricated following the same procedure used for ZnO NDs-based photoanodes.

RESULTS AND DISCUSSIONS

POWDER X-RAY DIFFRACTION ANALYSIS

The active area for all fabricated devices is the same and is found to be ~0.25 cm2.

2  (degree)

The PXRD pattern of the ZnO precursor complex agrees well with the diffraction pattern of Zn5(OH)8(NO3)2.2H2O (JCPDS card no. 25-1028). After calcination, the precursor complex is converted into pure ZnO, as confirmed from the PXRD pattern, trace (f).

ZnO Nanostructures

UV-VISIBLE ABSORPTION AND PHOTOLUMINESCENCE ANALYSIS

A moderate blue shift of the absorption onset is observed in the case of ZnO NS (~25 nm) and ZnO NR (~20 nm) compared to ZnO ND. The observed decrease in band gap values for ZnO NRs and ZnO NDs in contrast to ZnO NSs is consistent with the blue shift of the absorption profiles.

Wavelength (nm)

BET SURFACE AREA ANALYSIS

Volume adsorbed (cm3/g)

Relative Pressure (P/P

UV–VISIBLE ABSORPTION AND DIFFUSE REFLECTANCE ANALYSIS OF THE PHOTOANODES

In order to investigate the light scattering properties of all photoanode films of ZnO heterostructures, diffuse reflectance measurements are performed and analyzed. This is an indication of the greater light scattering ability of the ZnO NR photoanode than the photoanode based on ZnO ND, ZnO NS and ZnO NP.4,59.

Figure 3.3.10 (A) UV-Vis diffused reflectance spectra of CdS/CdSe co-sensitized ZnO ND based photoanodes with different Se precursor concentrations during the chemical bath deposition (CBD) methods

PHOTOVOLTAIC CHARACTERISTICS OF THE SOLAR CELLS

From Figure 3.3.12, it is clear that no detectable impurity peaks are observed in the PXRD pattern of ZnO ND-CdS-CdSe2.0. CdSe2.0 photoanode compared to ZnO ND-CdS-CdSe1.5 photoanode is illustrated in Scheme 3.3.3.

Figure 3.3.12 Powder X-ray Diffraction pattern for the photoanodes (a) ZnO ND-CdS-CdSe2.0 and (b) ZnO ND- ND-CdS-CdSe1.5 respectively

Voltage (mV)Jsc (mA/cm2)

The enhanced charge separation in the ZnO ND-based device can be well explained with the help of a model of charge separation between reactive polars populated with Zn (0001) [i.e., Zn-ZnO] and populated with O ( 0001̅) [i.e., O-ZnO ]. This may be caused by the minimal pores available in the ZnO NS-based photoanode for electrolyte diffusion compared to the ZnO ND, ZnO NP, and ZnO NR-based photoanodes.

Figure 3.3.14 UV-visible diffused reflectance spectra of CdS/CdSe co-sensitized ZnO ND, ZnO NP, ZnO NR and ZnO NS based photoanodes with 1.5 mM Se precursor concentration used during the chemical bath deposition (CBD) methods

Z ' (Ohm)-Z" (Ohm)

However, in the case of ZnO NP and ZnO NS devices, the photo-induced electrons repeatedly interact with grain boundary traps as they move randomly throughout the film. As a result, the recombination reactions in these two photoanodes (ZnO NPs and ZnO NSs) become more pronounced than in the ZnO ND-based photoanode, which is reflected in the deprived FF and Voc values for the devices. B) Bode phase plots for individual devices.

Frequency (Hz)

Phase ()

CONCLUSIONS

Superiority of the ZnO ND-based device is proven by comparing the cell performance with the similar device fabricated with conventional ZnO NPs (PCE ~3.14 %), ZnO NRs (PCE ~2.52 %) and ZnO NSs (PCE ~1.64. Higher PCE obtained for the ZnO ND-based device is mainly due to efficient photoinduced charge separation enhanced by the exposed polar ±(0001) facets.

Cage-like ZnO Superstructures with Compacted 1D Building Blocks for Enhanced Photovoltaic

INTRODUCTION

Synthesis of 3-D superstructures assembled with 1-D nanostructures is a major challenge as they are enriched with superior optical and electrical properties for photovoltaic as well as photocatalytic applications. sensitizing molecule adsorption, excellent light scattering ability along with direct and faster transport pathways for photogenerated electrons.6 In addition, porous and hollow characteristics of these materials are conducive to the diffusion of electrolyte for better functioning of the redox couple in the photovoltaic devices. In the synthesis process, we have used a soft biotemplate "sodium alginate" (ALG) as a structure-controlling agent and ammonium hydroxide (NH4OH) as a precipitation base.

EXPERIMENTAL METHODS

SYNTHESIS OF ZnO CAGES

After complete addition of NH4OH to 1.5 mL, the pH of the solution was found to be 10. The thickness of the ZnO layer was measured with a surface profilometer (Veeco Dektak-150) and found to be in the range of 12−15 µm.

RESULTS AND DISCUSSIONS

The electrolyte solution was introduced into the devices after sealing (with a low-temperature thermoplastic sealant, thickness ∼50 μm) through a hole in the counter electrode made by drilling with a glass drill. It is worth noting that the intensity ratio of (101̅0) to (0002) crystal planes varied considerably for all three types of ZnO which is an indication of differential tropism of the products in the three reaction conditions namely reaction 1, 2 and 3.26.

Reaction conditions

MATERIAL MORPHOLOGY

Trace (D) represents inverse fast Fourier transform (IFFT) of atomic planes for the specified part of trace (C) with corresponding FFT in the inset. Increasing the amount of NH4OH to 1.0 mL leads to the formation of cage like ZnO with smaller diameter (500 nm) with uniform size distribution; see trace (E).

Figure 4.3.4 FESEM images (A, B, C, D and E) of as synthesized cage like ZnO superstructures at different magnifications

60 min

The morphology of the product at 40 minutes of reaction time is presented in Figure (A) and inset, Figure 4.3.8. This is an indication of the incompleteness of the reaction and is confirmed by the low crystallinity of the product compared to the other two cases, viz. reactions with 60 min and 90 min.

90 min

Please note that the crystallinity variation is also verified for all products by PXRD patterns and is shown in Figure 4.3.9. We have listed the values of full width half maximum (FWHM), crystallite size and calculated intensity in counts per second (c.p.s) of the most intense peak in the PXRD patterns in Table 4.3.1.

120 min

40 min

MECHANISM OF ZnO SUPERSTRUCTURE FORMATION

In hydrothermal reaction conditions, the growth units initiate the nucleation process and form numerous nuclei of ZnO. Thus, the growth of ZnO NRs in the presence of ALG is inhibited due to the minimized contact between the growth units and (0001) crystal surfaces.

UV-VISIBLE ABSORPTION AND PHOTOLUMINESCENCE ANALYSES

At this stage, the existence of the optimal amount of NH4OH and a gas-liquid equilibrium in the autoclave are the decisive factors to achieve the expected size and shape of the product. As the reaction proceeds, some of the multi-cage superstructures crack at the adjacent points and result in single-cage superstructures.

Wavelength

BET SURFACE AREA ANALYSIS

Nitrogen adsorption and desorption isotherms of surface area analysis for all the as-synthesized ZnO with different morphologies are shown in Figure 4.3.12. The wide pore size distribution observed in the case of ZnO cages is due to the presence of two types of pores in them, viz. (i) pores between the small NRs and (ii) the macropores formed due to breaking of multi cages at the adjacent points as we have discussed in the FESEM analysis section.

Pore Volume (cm3/g)

Pore Diameter (nm)

UV–VISIBLE ABSORPTION AND DIFFUSE REFLECTANCE ANALYSIS OF THE PHOTOANODES

This confirms the efficient light scattering by the ZnO cages in the photoanode to enhance the repetitive excitation process of dye molecules across the ZnO NR and ZnO PNP based photoanodes. The hollow nature of ZnO cages is believed to be the most likely reason for superior light scattering, which is not the case with ZnO PNP and ZnO NR based photoanodes.

PHOTOVOLTAIC CHARACTERISTICS OF THE SOLAR CELLS

The film based on ZnO cages has a markedly higher reflectivity in the wavelength range of 500–. The inset shows the advantages of ZnO cages in terms of (1) light scattering, (2) electrolyte diffusion, and (3) electron transport in solar cell devices.

Voltage (mV)

However, an appreciated value of FF (~50%) provided by device (c) is indicative of facilitated diffusion of electrolyte in the photoanode compared to the ZnO PNP-based photoanode. Electron transport and recombination kinetics in the photovoltaic devices, namely device (a) ZnO cage, device (b) ZnO PNP and device (c) ZnO NR, are monitored using electrochemical impedance spectroscopy (EIS).

Figure 4.3.15 Cross-sectional (A & B) and top view (C & D) FESEM images of as fabricated ZnO cage based Photoanode

Phase ()

Z ' (Ohm)

CONCLUSIONS

A reliable growth mechanism is proposed for assembled 1-D ZnO NR superstructures and their photovoltaic properties are evaluated. The improved photovoltaic performance of ZnO lattice-based devices is mainly achieved due to higher photoinduced electron flux, enhanced light scattering, easy electrolyte diffusion probability, and efficient charge separation through 1-D ZnO channels. Nano-sized NRs.

Fabrication of Bilayer ZnO Based Hybrid Photoanode for Enhanced Photovoltaic Performance in CdS

INTRODUCTION

An optimized hybrid photoanode structure with different ZnO morphologies has the potential to exploit the beneficial effects of the size- and shape-dependent properties of the semiconductor oxide material. In this chapter, we introduced a strategy to integrate 1D structures and 3D microstructures in such a way that their combination provides increased surface area for the adsorption of sufficient sensitizer particles, facilitated electrolyte diffusion, efficient photogenerated charge collection, and light scattering effects.

EXPERIMENTAL METHODS

SYNTHESIS OF HOLLOW MESOPOROUS ZnO MICROSPHERES (ZnO HMSP) 3D ZnO microspheres were synthesized by following a reported protocol and is presented
SYNTHESIS OF CdS NANOPARTICLES
GROWTH OF 1D ZnO NWs ON FTO SUBSTRATES
FABRICATION OF (ZnO NW ZnO HMSPCdS) HYBRID PHOTOANODE
DEVICE FABRICATION AND CHARACTERIZATION

ZnO NWZnO HMSP electrodes were sensitized in-situ with CdS QDs using the sequential ion layer adsorption and reaction (SILAR) process. The FTO substrates having bare ZnO NWs (without ZnO microspheres) and ZnO HMSPs (without ZnO NWs) were sensitized by CdS QDs following the same procedure and were further named as ZnO NWCdS and ZnO HMSPCdS, respectively.

RESULTS AND DISCUSSIONS

MATERIAL MORPHOLOGY
DIFFUSE REFLECTANCE UV–VISIBLE ABSORPTION SPECTRA
PHOTOLUMINESCENCE ANALYSES OF THE MATERIALS
PHOTOVOLTAIC CHARACTERISTICS OF THE SOLAR CELLS

It should be noted that slightly higher absorption in the UV region was observed for ZnO NW–ZnO HMSP–CdS compared to that of ZnO HMSP–. From the figure 5.3.9 (A) we observed a significant quenching in the PL intensity for ZnO NWZnO HMSPCdS [trace (c)] compared to ZnO HMSPCdS [trace (a)] and ZnO NW CdS.

Figure 5.3.1 Powder X-ray diﬀraction patterns for (a) ZnO NW–ZnO HMSP–CdS, (b) CdS NPs, (c) ZnO HMSPs and (d) ZnO NWs grown on glass substrates

CONCLUSIONS

INTRODUCTION

In this chapter, we have carried out a systematic study on the effects of g-C3N4 in photovoltaic performance parameters of ZnO NR-based CdS QD-sensitized solar cell devices by introducing a binary hybrid composition of g-C3N4 and ZnO, i.e. (g-C3N4) ZnO NR) in the photoanodic segment. A comparative study of (g-C3N4ZnO) devices based on various morphologies (i.e., NR and NP) shows that the performance parameters of ZnO NR-based devices are better, suggesting a better/smoother electron track.

EXPERIMENTAL METHODS

SYNTHESIS OF g-C 3 N 4
SYNTHESIS OF ZnO NANORODS
SYNTHESIS OF ZnO NANOPARTICLES
PREPARATION OF (g-C 3 N 4 ZnO NR) AND (g-C 3 N 4 ZnO NP) COMPOSITES The g-C 3 N 4 composites of ZnO NR and ZnO NP are prepared by ultra-sonication treatment

A series of composites (g- C3N4ZnO NR) are prepared following the same methodology with different weight ratios of g-C3N4 to ZnO NR. Photoanodes with composites (g-C3N4ZnO NR) are produced by preparing a homogeneous paste of composite powders.

RESULTS AND DISCUSSIONS

DIFFUSE REFLECTANCE UV-VISIBLE ABSORPTION SPECTRA ANALYSES Diffuse reflectance UV-visible (UV-vis) absorption spectra of as prepared (g-C 3 N 4 ZnO
STEADY STATE AND TIME RESOLVED PHOTOLUMINESCENCE ANALYSES Excited state charge transfers and recombination process in the photoactive electrode
DIFFUSE REFLECTANCE UV–VISIBLE ABSORPTION SPECTRA OF THE PHOTOANODES
PHOTOVOLTAIC CHARACTERISTICS OF THE SOLAR CELLS
MORPHOLOGY DEPENDENT PHOTOVOLTAIC CHARACTERISTICS

From FESEM images of the composites, it is clear that the ZnO NRs and ZnO NP are well embedded in the g-C3N4 sheets. It is observed that the peaks of g-C3N4 are suppressed and broadened in (g-C3N4ZnO NR) hybrid composites.

Figure 6.3.2 (A) & (B) are FESEM Images showing flake like features of g-C 3 N 4 at different magnifications

CONCLUSIONS

Boosting Photovoltaic Performance of SnO 2 Based Dye Sensitized Solar Cells by Utilizing 2D MgO

INTRODUCTION

Meanwhile, a suitable redox couple completes the transport cycle in DSSCs by regenerating the oxidized dye molecules.35 Numerous wide band gap semiconductors, such as ZnO,6 Nb2O5,7 SnO2,810 SrTiO3,11 etc., have been investigated as an alternative to TiO2 in DSSCs. Dye molecules that absorb solar radiation in a wide wavelength range to get excited and efficiently inject electrons into the wide bandgap semiconductor scaffold are considered an ideal candidate for use in DSSC devices.25 Nowadays, metal-free TH.

EXPERIMENTAL SECTION

SYNTHESIS OF SK1 DYE
SYNTHESIS OF 3D HIERARCHICAL SnO 2 MICROSPHERES
SYNTHESIS OF POROUS 3D HIERARCHICAL MgO

Finally, the solution was transferred to a Teflon-lined stainless steel autoclave and kept at 180 C for 14 h. After that, the autoclave was naturally cooled to room temperature, and the obtained product was centrifuged and rinsed thoroughly with distilled water and absolute ethanol several times, dried in hot air oven at 60 C for 24 h and calcined at 500 C for 2 h. get the final product.

Figure 7.2.1 400 MHz 1 H NMR spectrum of metal-free carbazole SK1 dye recorded in CDCl 3 as the solvent

RESULTS AND DISCUSSIONS

DIFFUSE REFLECTANCE UV–VISIBLE ABSORPTION SPECTRA
DENSITY FUNCTIONAL THEORY (DFT) STUDY OF SK1 DYE

From (E) and (F) it is clear that the diameter of the MgO microspheres is in the range of 24 m. From the Tauc's plot (inserted in figure 7.3.5) it is found that the band gap of 3D SnO2 and SnO2–MgO films are ~3.57 eV and ~3.42 eV respectively where (αhν)2 versus the photon energy (hν) and α is the absorption coefficient.

Figure 7.3.2 FESEM images of 3D porous SnO 2 microstructures (A and B) which are composed of randomly arranged 2D nanoplates as we see from high resolution image (C)

ELECTROCHEMICAL ANALYSIS OF SK1 DYE

The highest occupied molecular orbital (HOMO) level of SK1 dye was estimated in the anhydrous acetonitrile using 0.1 M Tetra-n-butylammonium hexafluorophosphate (TBAP) electrolyte. From the optical energy band gap (Eg) value and the HOMO level of SK1 dye, the lowest unoccupied molecular orbital (LUMO) energy level was calculated using the following expression, i.e.

PHOTOVOLTAIC CHARACTERISTICS OF THE SOLAR CELLS

The lower Voc values of pristine SnO2 units (357 mV and 445 mV for I/I3and [Co(bpy)3]2+/3+ redox couple, respectively) may be due to faster back electron transfer from CB of SnO2 to the redox shuttle (about 3 times faster than TiO2), due to the reactive low-energy trap states of SnO2.16,35,36. The Voc values for SnO2-MgO units are observed to be significantly higher (i.e., 550 mV, 619 mV for I/I3and [Co(bpy)3]2+/3+ redox couples, respectively), indicating significant decrease in reverse tunneling probability in the presence of MgO.