A fast and low temperature preparation method of ZnO films for PSC applications

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Standard solar spectral irradiance

Classical photovoltaic solar cells are electronic devices that use p-n junctions to directly convert sunlight into electrical power. The purpose of the solar cell research is to develop photovoltaic devices that can efficiently convert the energy coming from sun into a usable electrical energy. Because the actual solar spectrum received by a device will vary due to the weather, season, time of day, and location, a standard spectrum has been defined for the determination of the solar cell characteristics and to permit the comparison of devices prepared in various laboratories.
The spectra are standardized by the American Society for Testing and Materials (ASTM). In the Figure 1.8, the solar spectrum used for extraterrestrial application is denoted as Air Mass zero (AM0) based on ASTM standard E 490, and the integrated power of AM0 is 1366.1W m-2. Due to the absorption and scattering when passing through the atmosphere, the solar spectral irradiance is decreased. In the case a solar zenith angle of 48.2°, this solar spectrum is denoted as the AM1.5G (global) standard by ASTM G173, and used for terrestrial applications including direct and diffuse light. The integrated spectral power of the AM1.5G is taken at 1000 W m-2. The AM1.5D spectrum, also based on ASTM G173, is for terrestrial applications but only includes direct light. It integrates to 888 W m-2.[19]

Current-voltage characteristics

The current-voltage characteristics (J-V curves) are measured in the dark and under illumination, for evaluating the performance of solar cells. This process is done by applying an external potential bias when measuring the current-voltage characteristics. They are measured in the dark and under AM1.5G calibrated illumination. Basically, the forward current, for which the applied potential is referred to as forward bias, involves electrons injection into the cell from the photoanode. The reverse current, for which the applied potential is referred to as reverse bias, involves electrons injection from the counter electrode side.
An ideal dye sensitized solar cells shows a typical diode behavior, the corresponding dark current is shown in the Figure 1.9 (curve 1, red dash line). We found that at low applied potential, very little current (even no current) flow through the device due to the low charge density in the dark. When increasing the applied potential, the charge density increases and the quasi Fermi level is also increased in the metal oxide. When the quasi Fermi level reaches the conduction band of the semiconductor, the electron flow unhindered to the HTM (or liquid electrolyte). So the electrons are injected into the metal oxide and it results in the dark current increase. We can conclude that the dark current is governed by the same electron-hole recombination process as described in the Figure 1.7.

Recent milestones in the halide perovskite solar cell

In 2009, Miyasaka first reported the use of organo-lead halide perovskite compounds to efficiently sensitize TiO2 for visible-light conversion in solar cell. The resulting PCEs were 3.81% for CH3NH3PbI3 and 3.31% for the CH3NH3PbBr3.[26] A schematic of the perovskite sensitized TiO2 and the IPCE action spectra are shown in Figure 1.17. In 2011, N.G. Park and co-authors fabricated solar cells using TiO2 layer sensitized by about 2-3 nm sized CH3NH3PbI3 pigment and bumped up the conversion efficiency to 6.54%.[28] However, the stability of these cells was poor and they were degraded in a few minutes due to the perovskite dissolution in the liquid electrolyte.
In 2012, M. Grätzel and N. G. Park reported the heterojunction solar cell using CH3NH3PbI3 nanoparticles as the light harvesters, and introducing the 2,2’,7,7’-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9’-spirobifluorene (spiro-OMeTAD) as a hole transport material to replace the liquid electrolyte. This breakthrough step allowed to boost the power conversion efficiency up to 9.7%, the relative cell device scheme and the I-V curve are shown in Figure 1.18.[29].
In the same year, Henry J. Snaith’s group constructed solar cells where the mesoporous TiO2 was replaced by insulating mesoporous Al2O3 to study their performance. They showed that the charge transport was faster and the photocurrent unaffected when TiO2 was replaced by Al2O3. The open-circuit voltage increased to more than 1.1 volts, the power conversion efficiency increased to the 10.9%. The interesting aspect of this discovery is that the fundamental loss in energy is very low, which gives great promise for significant future increases in efficiency. A schematic illustration of the operating principles of a TiO2-based perovskite-sensitized and alumina-based meso superstructured solar cell (MSSC) is shown in Figure 1.19.[24], [30].

The compact metal oxide blocking layer

In order to prevent the direct contact between the TCO and perovskite, a dense metal oxide blocking layer, which fully covers the TCO surface is first deposited on the TCO substrate. Usually, the blocking layer and mesoporous film (electron transport layer) are made of the same material. Several techniques have been developed to fabricate the metal oxide blocking layer. Aerosol spray pyrolysis and spin-coating are most commonly techniques used.[29], [32] The typical thickness of a blocking layer ranges between 20 and 100 nm, which can be controlled by the concentration of the precursor solution and the spin-coating time.

The electron transport layer

Basically, the metal oxide layer works as a scaffold for the sensitizer and as an electron transport layer for the transfer of the electrons from the sensitized surface to the conductive front substrate. So far, the TiO2 mesoporous layer, which pores are filled with the perovskite material, yield to a higher PCE, and is the most popular structure for the perovsktie solar cell.[29] TiO2 nanorods[46] and TiO2 nanowire[47], which have the higher electron mobility in the one dimensional structure compared the TiO2 nanoparticle, have been used to fabricate a perovskite sensitized solar cell with efficiency of 9.4% and 4.29%. For the other morphology, a TiO2 nanotube,[48] TiO2 nanofiber[49] and TiO2 nanoplatelets[50] have also been used for the PSSC device.
Compared to TiO2 nanostucture, the ZnO nanostructure is second in popularity. A close inspection of the perovskite sensitized ZnO show that they can achieve a higher short circuit current, but a lower recombination resistance and a higher recombination rate induces a lower fill factor, which resulted a lower PCE for the ZnO.[51] D. Liu fabricated the CH3NH3PbI3-based solar cell on ZnO planar nanostructure, and got a conversion efficiency as high as 15.7% on ITO glass and 10.2% on a flexible substrate.[52] Except the TiO2 and ZnO, m-Al2O3 or ZrO2 and also phenyl-C61-butyric acid methyl ester (PCBM), an organic ETL could work as scaffold in the hybrid perovskite solar cell.[30], [45]


The halide perovskite layer

Material for the solar light harvesting must have a broad and strong absorbance over the visible to near infrared region of the solar spectrum, this implies a bandgap optimum of 1.4-1.5 eV. This bandgap determines the strength of electrical field which is the voltage. When it is too low, the cell with this semiconductor will collect extra current by absorbing more photons, but having a small voltage. In this case, to balancing these two effects, the optimal band gap for a single junction solar cell is between about 1.4-1.5 eV.[53].
Halide perovskite (AMX3) optical bandgap can be varied by changing the A cation, the metal cation and the halide cation. The bandgap of perovskite (CH3NH3PbI3 and CH3NH3PbI3-xClx), usually used in the literature, is 1.58 eV[29] and 1.55 eV[30], respectively. For the small A cation, such as Cs+, methylammonium (MA+) or formanidinium (FA+), APbI3 prefer to form a three dimensional framework with PbI6 network, and an increase in the cation size (RCs+ < RMA+ < RFA+) results in a reduction in the band gap, since the values are 1.73 eV, 1.58 eV and 1.48 eV for the CsPbI3, MAPbI3 and FAPbI3, respectively. Therefore, a higher efficiency is expected for the FAPbI3 compound compared to MAPbI3.[53] For the influence of the metal cation, taking the example of AMI3 (M=Sn, Pb), the band gap of AMSnI3 (1.20 eV) is lower than AMPbI3 (1.58 eV).[54] People have investigated the influence of the halide anion, the band gap of this group follows the trend AMPbI3 (1.5 eV) < AMPbBr3 (2.2 eV), but the efficiency depends on the ratio of I and Br in AMPbI3-xBrx.[55].
Compared to the traditional dye solution, the fabrication techniques of the perovskite layers are diverse. The common and simple film deposition methods are one-step precursor solution depositions[56] and the two-step sequential deposition,[32] which two methods have reported the highest efficiency of 19.3% [27] and 15% [32]. The dual-source vapor deposition method permits to prepare an extremely uniform perovskite layer without pin-holes. An efficiency of 15.4% was reported by Liu et al.[44] Y. Yang group reported a low-temperature vapor-assisted solution process to get perovskite film with full surface coverage, small surface roughness and grain size up to the micron scale. The device in a planar architecture with excellent CH3NH3PbI3 film achieved a PCE of 12.1%.[57].

The characteristics of CH3NH3PbI3 layer

¬ SEM: By this two-step method, CH3NH3PbI3 was infiltrated into the space between the ZnO NWs, which means a good sensitization performance (Figure 3.2a), but we did not obtain a capping layer. Therefore, on FE-SEM top view images we could observe the presence of white spots of uncovered ZnO structures (Figure 3.2b).
¬ XRD: A typical XRD pattern of perovskite deposited on the ZnO nanowires is displayed in the Figure 3.3. It shows a well crystalline CH3NH3PbI3 perovskite by the two-step method which is suited for the deposition of perovskite layer on the ZnO structures. In this pattern, we also observe the presence of (001) and (110) PbI2 diffraction peaks at 12.8° and 39.7°, respectively, which indicates traces of PbI2 that were not be fully converted into the organometal halide perovskite.[8]–[10].
¬ Optical characteristics: The absorbance spectrum of CH3NH3PbI3/ZnO heterstructure is presented in Figure 3.4 a, which shows a panchromatic absorption from near-UV to the near infra-red wavelength region. CH3NH3PbI3 has a direct bandgap of 1.58 eV (Figure 3.4 b), corresponding to an absorption onset at 770 nm.

Table of contents :

Chapter 1: Context
1.1 Introduction
1.1.1 Solar energy
1.1.2 Photovoltaic solar cells
1.2 Dye-Sensitized solar cells
1.3 Electron transport process in the ssDSSCs
1.4 Solar cell photovoltaic characteristics
1.4.1 Standard solar spectral irradiance
1.4.2 Current-voltage characteristics
1.4.3 Quantum efficiency measurement
1.4.4 Impedance Spectroscopy
1.5 Perovskite sensitized solar cells
1.5.1 Introduction of perovskite component
1.5.2 Recent milestones in the halide perovskite solar cell
1.6 The components in the perovskite solar cell
1.6.1 The compact metal oxide blocking layer
1.6.2 The electron transport layer
1.6.3 The halide perovskite layer
1.6.4 The hole transport layer
1.6.5 The back contact
Chapter 2: Synthesis and morphological properties of oxide contact layers
2.1 Introduction
2.2 ZnO layers electrodeposited from nitrate precursor
2.2.1 Experimental
2.2.2 Results
2.3 Electrodeposition of ZnO in the Chloride/Oxygen System
2.3.1 Experimental
2.3.2 Results
2.4 One-dimensional TiO2 nanotube arrays
2.4.1 Experimental
2.4.2 Results and discussions
2.5 Thin mesoporous TiO2 layer
2.5.1 Experimental
2.5.2 Results
2.6 Conclusion
Chapter 3: Electrochemical design of ZnO structured layers for efficient PSC application
3.1 Sensitization of ZnO electrodeposited structures by CH3NH3PbI3 perovskite layers
3.1.1 Experimental
3.1.2 The characteristics of CH3NH3PbI3 layer
3.2 Perovskite sensitized solar cells
3.3 Conclusion
Chapter 4: A fast and low temperature preparation method of ZnO films for PSC applications
4.1 Perovskite sensitized solar cells based on i-ZnO ETL
4.1.1 Effect of the deposition time of i-ZnO layer
4.1.2 Effect of the transparent conductive oxide (TCO) substrate
4.2 Conclusion
Chapter 5: Effect of oxide contact layer on the properties of CH3NH3PbI3 for PSC applications
5.1 The preparation and properties of oxide contact layer
5.1.1 Preparation of oxide contact layer
5.1.2 Oxide layer characterizations
5.2 Effect of oxide substrate on the perovskite preparation
5.2.1 Effect of oxide substrate on the one-step perovskite preparation
5.2.2 Effect of oxide substrate on the two-step perovskite preparation
5.2.3 The properties of the heterostructures
5.3 Effect of ETL and perovskite preparation route on the solar cell performances
5.3 Conclusion
Chapter 6: One-dimensional self-standing TiO2 nanotube arrayed layers designed for PSC applications
6.1 The preparation and properties of TiO2 layers
6.1.1 The preparation of TiO2 nanotube arrays.
6.1.2 The properties of TiO2 nanotube arrays.
6.2 The properties of perosvskite sensitized solar cells
6.3 Conclusions


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