Introduction
Introduction
Many features of dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs) are unique and advantageous over the solar cells based on crystalline or amorphous silicon. Dye-sensitized and perovskite solar cells have gained intense interest due to their potential advantages such as low production cost, easy fabrication, transparency, multi-color options, flexibility, light weight, and short energy payback time.
Dye Sensitized and Perovskite Solar Cells
Examples of ETA are CdTe and CdSe.24 The motivation to find more advanced light absorbers compared to dyes led Miyasaka and co-workers to report perovskite-sensitized solar cells for the first time between 2006 and 2008. They used CH3NH3PbI3 and CH3NH3PbBr3 absorbers with either an iodide triiodide redox couple or a polypyrrole carbon black composite solid hole conductor and measured full solar energy conversion efficiencies ranging between 0.4 and 2% for solid-state and liquid electrolyte cells, respectively.25-26 The first peer- peer-reviewed journal publication of a perovskite-sensitized solar cell came in 2009, where the CH3NH3PbI3 absorber led to a 3.5% efficient sensitized solar cell using the iodide/triiodide redox couple.22 While the charge-transport- investigated properties of the perovskite-sensitized solar cells and co-worker observed that the charge-extraction rates were significantly faster for the perovskite-sensitized solar cell compared to conventional DSSCs.
Device Architecture and Working Principle
- Dye Sensitized Solar Cells
- Perovskite Solar Cells
The architecture of commonly used single-junction perovskite solar cells can be classified into mesoscopic and planar heterojunction structures (Figure 1.2). Electron and hole injection is efficient due to the long diffusion lengths of the charge carriers in perovskite solar cells.
Components of Typical Dye Sensitized and Perovskite Solar Cells
- Working Electrode
- Electron Transporting Material
- Active Layer
- Hole Transporting Materials/Electrolyte
- Counter Electrode/Back Contact
To further reduce the cost and improve the performance and stability of the PSC, nickel oxide (NiOx) was used as the HTL for the final work (Chapter 6). Some additional layers can also be used to further improve the performance and/or stability of a device.
Device Measurements
- J-V Characterization
- Impedance Spectroscopy Measurements
IPCE is the ratio of charge carriers collected by a solar cell to the number of photons shining on the cell and is calculated using equation 1.3. The expected short-circuit photocurrent density to be generated by the solar cell is calculated by integrating the product of IPCE and Jsc (λ) over the wavelength of the input light.
Engineering Dye-sensitizer and Perovskite Layer
These defects present in the perovskite film induce carrier recombination centers, resulting in non-radiative recombination and thus loss of open circuit voltage and fill factor. This could also significantly reduce the rate of non-radiative recombination in the perovskite films80.
Thesis Synopsis
To achieve this, the Lewis acid base approach was combined with the hot casting technique to control the grain growth of perovskite films by varying the amount of DMSO added to the perovskite precursor solution of lead iodide (PbI2) and methylammonium chloride (MACl) in dimethylformamide. (DMF) carefully. A brief summary of research work carried out and possibilities of investigations necessary for the commercialization of perovskite solar cells in the near future is presented.
To further understand the carrier dynamics across the PSCs, transient photocurrent (TPC) and transient photovoltage (TPV) experiments were performed for the pristine and the OA-modified devices.43 As shown in the TPC curves (Figure 5.8(b)), the OA-based device showed a faster decay compared to the pristine device. When the TA concentration increased to 0.9%, the grain size increased with the presence of some small TA particles on the surface.
Effect of Fluorine Substitution
Introduction
The properties of these dyes were compared with those of the reference fluorine-free dye MA0F. It was observed that substitution of monofluorine in the ortho position significantly improves the photovoltaic properties of the dye.
Results and Discussion
The results show that the power conversion efficiency (PCE) of DSSCs clearly changed with the fluorine substitution position in the phenyl ring of the dyes. The energy of the titanium electrode depends on the dipole moment of the sensitizers in the DSSC.
Conclusions
This result indicated that the equilibrium between the electron injection and the electron recombination at the TCO surface was reached.
Experimental Section
- Materials
- Synthesis
- Fabrication of solar cell
- Characterization
TiO2 photoelectrodes were prepared according to the previously described procedure.31 Briefly, a transparent TiO2 layer with a thickness of 12 μm was used as the photoanode. The IPCE measurement was performed on an Optosolar SR 300, Germany, using a 250 W xenon lamp as the light source.
In Figure 3.5, the PCE as a function of time is plotted for a period of 25 days, and it is found that the ss-DSSCs exhibit excellent stability. As shown in Figure 5.5(b), the MAPbI3 perovskite film prepared with 5 mg mL-1 OA has a higher emission intensity compared to the pristine perovskite film, demonstrating that the non-radiative carrier recombination was effectively suppressed due to the lower defect density and high quality of the perovskite film with OA, as evidenced by XRD and SEM measurements.40-41 This suppression of nonradiative recombination indicates a lower.
Effect of Mono- and Di-anchoring
Introduction
The advantage of structural diversity in metal-free organic dyes allows easy tuning of optical and electrochemical properties. Although a decent PCE of ~14% has been achieved with metal-free dyes,7 research into new organic dyes is still challenging and desirable to overcome the energy crisis.
Results and Discussion
Each equivalent circuit consisted of several components, as shown in the inset of Figure 3.6(a): interfacial resistance (Rpt) of electron transfer at the Pt/electrolyte interface, recombination resistance (Rk) in TiO2 matrix, diffusion resistance (RD) in electrolyte and constant phase element (CPE-P), the value of CPE-P is associated with the surface roughness and porosity of the photoelectrode.18,27-28 Nyquist spectra of the DSSCs showed three semicircles: the first high-frequency region represents Rpt; The second center frequency range represents Rk and the third lower frequency range represents RD. A lower αc slope value was found in the liquid electrolyte-based DSSCs, as shown in Figure 3.6(b) and Table 3.3. a) (b).
Conclusions
The decreased slope αc indicates an enhanced reaction of triiodide at the sensitized TiO2/I-/I3- electrolyte photoanode interface.
Experimental Section
- Materials
- Synthesis
- Preparation of electrolytes
- Fabrication of DSSCs
- Characterization
The solution of acetonitrile:chloroform (2:1) was added to the mixture of Cz-PhF2-CHO (0.2 mmol) and cyanoacetic acid (0.3 mmol) and thoroughly degassed. The solution of acetonitrile:chloroform (2:1) was added to the mixture of Cz-2PhF2-CHO (0.2 mmol) and cyanoacetic acid (0.5 mmol) and thoroughly degassed.
During the past decade, the rapid growth of perovskite solar cells (PSCs) has become feasible due to the cheap perovskite materials, ease of fabrication and high power conversion efficiency (PCE).1-7 The organic-inorganic mixed perovskites of methylammonium lead halide (MAPbX3 , MA = CH3NH3+, X = I−, Br− or Cl−) possess outstanding properties of an active material for photovoltaic application, such as strong absorption with high extinction coefficient, large diffusion length and lifetime of charge carriers, and low recombination rate together with the ability to tune its band gap and crystallinity.8-11 Optimization of the methods for perovskite film formation,4,12-14 development of new materials for improved performance,3,5-9 and flexibility of device architectures15 -16 have facilitated the PSCs to be recently progressing an initial efficiency of 3.8 % to a certified 23.3 %17, making them the fastest growing photovoltaic technology to date. Finally, chapter six elucidated the effect of benzenecarboxylic acid derivatives in the passivation of perovskite film.
Crystallization and Grain Growth Regulation
Introduction
The quality of the perovskite film depends on the intermediate formed and can be controlled by using strong Lewis bases in combination with DMF. This method resulted in an excellent quality perovskite film with good crystallinity and large grains.40.
Results and Discussion
There was an improvement in the absorption intensity of perovskite films with increasing amount of DMSO up to 1.5 equiv, but further addition led to reduced absorption. However, further addition of DMSO to the precursor resulted in the decrease of its PCE.
Conclusions
The degradation of perovskite films at grain boundaries takes place at a much faster rate compared to the grain surface,50-51 therefore the improved stability observed in the case of DMSO-modified perovskite compared to the control cell with only DMF can be attributed to the larger grain size. The air stability test was also performed (Figure 4.9), which confirmed the improved stability of the device made from 1.5 equiv. DMSO solution.
Experimental Section
- Materials
- Fabrication of Perovskite solar cell
- Characterization
A thin film of PEDOT:PSS was first deposited by spin-coating on the cleaned FTO at 5000 rpm for 30 s, followed by heat treatment for 20 min at 150 ˚C. The perovskite films were coated in a one-step hot casting method which is described in detail elsewhere.40 In short; the substrates were preheated to 190 ˚C and transferred to the spin-coater chuck followed by spin coating for 30 s at a speed of 4000 rpm.
However, further increase in concentration resulted in non-uniform grains with many pinholes observed at the surface of the perovskite layer. The effect of the Lewis base concentration in the perovskite precursor solution on the solar cell performance had been investigated.
Oxalic Acid Induced Perovskite Formation
Introduction
Remarkable improvements have been observed in the crystallinity, surface coverage, and grain size of perovskite films via Ostwald ripening,24 solvent annealing,25 solvent engineering,26-. This bidirectional OA-assisted growth enables a significant improvement in the quality of perovskite films, leading to an improvement in PCE to 17.12% compared to the control unit by 14.06%.
Results and Discussion
The J−V curves of PSCs without and with different amount of OA are illustrated in Figure 5.6 and the corresponding photovoltaic performance parameters are summarized in Table 5.1. A minimum hysteresis behavior was obtained in both the PSCs without and with 5 mg mL-1 OA as shown in Figure 5.7(a) (Photovoltaic data presented in Table 5.2).
Conclusions
The OA-free PSC retained only 14% of its initial PCE after heat treating the device at 100. However, under the same condition, the OA-based device retained 90% of its initial PCE after 9 h and 70% after 19 h.
Experimental Section
- Materials
- Device Fabrication
- Characterization
The reduction in PCE loss is mainly attributed to the reduced degradation of the perovskite layer.45-46 These results show that the perovskite layer prepared with OA results in improved thermal stability, which is related to the superior morphology obtained. Sixty microliters of these precursor solutions were spin-coated on top of the PEDOT:PSS films for 20 s at 750 rpm, followed by 60 s at 4000 rpm.
Benzoic acid (BA), isophthalic acid (ITA) and trimesic acid (TA) (with one, two and three carboxylic acid groups, respectively), were used as additives in the precursor solution leading to high quality perovskite films with large grains and reduced trapping conditions. The efficiency of the trap state passivation can also be correlated from a significant improvement in the Voc after passivation.
Benzene Carboxylic Acid Derivative Assisted Passivation
Introduction
On the other hand, some additives such as IT-4F,34 F4TCNQ,35 PVP (poly(4-vinylpyridine)),36 polyamino acid and polyimide,37 fullerene derivatives and ITIC,21,27 organic dye (AQ310), 38 p-type conjugated polymers PBDB-T,39 P3HT26 were only effective in defect passivation of perovskite films, while no improvement was observed in crystallization. One possible way to simultaneously obtain an improved perovskite film with good morphology and reduced defects is to use several additives with different functions in the perovskite precursor.
Results and discussion
When FESEM analysis was performed on the ITA-modified perovskite film (Figure 6.2), an improvement in the quality of the perovskite film ensued, although it was not as proficient as in the case of BA. The increase in PL intensity for perovskite films with BA, ITA and TA additives provides strong evidence that non-radiative recombination is suppressed due to reduction.
Conclusions
The PCE of the pristine MAPbI3 device retained only 13% of its initial PCE after 20 h, while the PCE of the doped PSC can retain ~80% under the same test conditions at 20 h and ~40% at 60 h. The maximum PCE of 18.08% is the highest value obtained for PSC with optimal TA concentration (with three –COOH groups).
Experimental Section
- Materials
- Device Fabrication
- Characterization
The reduction of trap density has been successfully investigated by PL and EIS measurements. In the second step, 160 µL of anhydrous toluene was dropped after 20 seconds as an anti-solvent and the substrates were baked at 80 °C for 10 min.
In the second chapter, the effect of fluorine substitution and position on the phenylene spacer in carbazole-based organic sensitizers for dye-sensitized solar cells was studied. Effect of fluorine substitution and position on the phenylene spacer in carbazole-based organic sensitizers for dye-sensitized solar cells.
Conclusions and Prospects
Conclusions
Dye-sensitized and perovskite solar cells have become one of the most promising photovoltaic solar cells that have led the field of third-generation photovoltaic technologies. This thesis aimed to overcome the limitations of dye-sensitive solar cells and discuss the relationship between structural properties of metal-free organic photosensitizing dyes in the first part.
Prospects
Crystallization and grain growth regulation by formation of Lewis acid-base adducts in hot cast perovskite-based solar cells. Benzenecarboxylic acid derivative assisted passivation of perovskites for stable and high-performance inverted perovskite solar cells (submitted).
