Best J-V curves for a solar cell with 40 % BTO nanoparticles is shown in Figure 6.12. 40 % BTO nanoparticle concentration was chosen based on structural and optical characterization of BTO: perovskite nanocomposites.
Figure 6.12. J-V characteristic of a representative solar cell with BTO nanoparticles.
The structure was poled by applying 9 V (Cu electrode positive and FTO electrode grounded) for 10 s and then solar cells were short circuited for 5 s after poling to avoid the effect of capacitive charging during the poling. The photo current density is higher by one order of magnitude after poling the structure. The measured photocurrent density is higher than from the solar cells without BTO nanoparticles. The enhancement of photo current density can be explained using energy band diagrams.
The detailed band diagram of the isolated TiO2, CH3NH3PbI3-xClx and P3HT with
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P3HT layer is unknown, it is challenging to quantitatively draw the band diagrams of the contacted structure.
Figure 6.13. Schematic of electronic band structure of isolated TiO2, CH3NH3PbI3-xClx, and P3HT.
The band alignment of the contacted structures with quantitative values on certain important energy levels are shown. It shall be noted that such band alignment is based on an approximate assumption of the position of the P3HT's Fermi level. The schematic of band diagrams under intrinsic condition and positive poling condition are shown in Figure 6.14 and 6.15 respectively. In the absence of a ferroelectric polarization (before poling the structure), the energy band diagram can be modeled as a n–i–p structure shown in Figure 6.14.
Ebi is a general term for the built-in electric field in the perovskite layer, E1 is the built-in
electric field in the perovskite layer close to the ETL side, E2 is the built- in electric field in the
perovskite layer close to the HTL side; similarly, W1 and W2 represent their corresponding widths
of depletion regions in the perovskite layer, respectively. 4.8 eV 4.3 eV TiO2 P3HT MAPbI3-xClx EF -4.2 -7.4 -3.7 -5.4 Vacuum Level -3.0 -5.2
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Figure 6.14. Schematic of band structure of BTO nanoparticles embeded perovskite solar cells without poling.
Ep represents ferroelectric polarization field, W1+ is the width of depletion region after
poling the structure in the perovskite layer close to the ETL side, W2+ is the width of depletion
region after poling the structure in the perovskite layer close to the HTL side, P denotes the ferroelectric polarization of BTO nanoparticles.
Figure 6.15. Schematic of electronic band structure BTO nanoparticles embeded perovskite solar cells with positive
poling. ETL HTL MAPbI3-xClx EF W1 W2
Without Poling
ETL HTL MAPbI3-xClx EF W1+ W2+With Poling
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When Cu electrode is positively poled (ETL side is grounded), BTO nanoparticles polarize with net negative ferroelectric charges towards the HTL layer and positive charges towards the TiO2 side. This ferroelectric polarization induced local electric field, Ep in the same direction as
Ebi. This leads to the increase of the width of depletion regions in the perovskite layer (W1+ and
W2+) and enlarges the band bending at both perovskite-p and n-perovskite interfaces (Figure 6.15).
Accordingly, as a result, under poling, the driving forces either to extract electrons to ETL or to push holes to HTL become higher. As a result, a boosting of JSC would be observed due to
additional band bending and better charge separation.
6.7. Chapter Summary
This chapter presented the fabrication and characterization of perovskite solar cells with and without BaTiO3 nanoparticles. BaTiO3 nanoparticles embeded perovskite solar cells generated
a higher photo current density after poling the cells compared with solar cells without BTO nanoparticles. Ferroelectric polarization of BTO nanoparticles within the perovskite solar absorber plays a dominant role in charge separation and transport. Ferroelectric polarization induced local electric field (EP) is expected to increase the width of the depletion regions inside the perovskite
layer, which is expected to regulate the separation of photo-generated charges. Therefore, ferroelectric polarization induced local electric fields act as another driving force for charge separation. The ferroelectric polarization of BTO nanoparticles within the perovskite solar absorber provides a new perspective for tailoring the working mechanism and photovoltaic performance of perovskite solar cells.
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CHAPTER 7
Conclusions
This work presented a novel growth process for the deposition of CH3NH3PbI3-xClx
perovskite thin films starting with CH3NH3Cl and PbI2 precursors. Adetailed study of structural
and optical properties of AACVD deposited CH3NH3PbI3-xClx perovskite thin films was presented.
AACVD system was custom made in LAMSAT for this work. The coverage of deposited films on planar substrates depended on chamber pressure and nozzle to substrate distance. The structure- property relationship of the films grown at various concentrations and growth temperatures pointed to a specific window of growth parameters. The fabricated films at low precursor concentrations showed excess PbI2 showing incomplete crystallization of the perovskite. CH3NH3PbI3-xClx
perovskite films deposited with 0.33 M MACl and 0.99 M PbI2 resulted narrowest XRD peaks
showing highest crystallinity and optical absorption. According to the study, 120 °C is the optimum deposition temperature. At lower temperatures, needle-like structures formed. At higher temperatures (above 140 °C), the perovskite decomposes into PbI2. Optical analysis of the films
showed that direct bandgap energy of the perovskite is 1.57 eV. Absorption and photoluminescence studies showed that the fabricated films have 30 meV stokes shift indicating superior crystallinity of the films. Post-deposition annealing is not a factor to increase the crystallinity or the absorption. Results were compared with solution casted and spin coated perovskite thin films and AACVD deposited thin films have better optoelectronic properties hence
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AACVD technique is a better approach to fabricate CH3NH3PbI3-xClx perovskite thin films starting
with PbI2 and CH3NH3Cl precursors.
Another aim of this work is to enhance charge carrier separation of these thin films by establishing a local internal electric field that can reduce electron-hole recombination resulting in increased photo current. The intrinsic ferroelectricity in nanoparticles of Barium Titanate(BaTiO3
-BTO) embedded in the solar absorber can generate such an internal field. The AACVD technique was extended to deposit BTO: CH3NH3PbI3-xClx nanocomposites by adding BTO nanoparticles to
the perovskite precursors. Optical absorbance and degree of crystallinity of BTO: CH3NH3PbI3- xClx nanocomposites were systematically studied at different BTO nanoparticle concentrations
starting from 0 wt% to 100 wt% in 10 wt% increments. It was found that, optical absorbance of nanocomposite films decreases as the BTO nanoparticle concentration increases. And the upper limit to retain the crystallinity of the perovskite is 46.6 wt% BTO nanoparticles in the nanocomposites. Perovskite grain size decreases as the BTO nanoparticle concentration in the nanocomposite increases. Based on structural and optical characterization of BTO: perovskite nanocomposite thin films, optimum BTO nanoparticle concentration in the nanocomposite is estimated to be 40 wt%. XRD data and SEM images confirm that AACVD technique is capable of depositing BTO nanoparticles embeded perovskite solar absorber thin films. P-E measurements of nanocomposites showed that nanocomposites are ferroelectric. Producing P-E hysteresis loops at all the BTO nanoparticle concentrations were unsuccessful due to leakage currents, poor electrodes, or instability of nanocomposites.
While high percentage of BTO in the nanocomposite film increases the carrier separation, it also leads to reduced carrier generation. Assuming a uniform distribution of BTO nanoparticles, a simple model was developed to estimate the optimum BTO nanoparticle concentration in the
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nanocomposite thin films quntitatively. Modeling each BTO nanoparticle as an electric dipole, variation of electric field within a cubic unit cell (placing 8 dipoles at the corners of the cube) was calculated. The variation of maximum electric field within the unit cell as a function of distance between 2 nanoparticles was calculated. Also, the volume fraction of the solar absorber within the unit cell as a function of distance between 2 nanoparticles was calculated. Optimum BTO nanoparticle concentration was determined to be 36 vol % or 65 wt % in the nanocomposite. The estimated optimum BTO nanoparticle concentration using this model is quite high from the experimentally determined optimum BTO nanoparticle concentration which is 40 wt%. In the model, a uniform distribution of BTO nanoparticles was considered rather than random positions of BTO nanoparticles. A computational model is required to include the randomness of BTO nanoparticles which will estimate a more accurate optimum BTO nanoparticle concentration.
Finally, to study the effect of ferroelectric polarization of BTO nanoparticles on solar cells, a hybrid structure of CH3NH3PbI3-xClx perovskite and ferroelectric BTO nanoparticles
FTO/TiO2/CH3NH3PbI3-xClx: BTO/P3HT/Cu as a new type of photovoltaic device was fabricated.
Using Au-Pd by hummer sputtering and Ag dots (manually) as back contacts was not that successful. Hence, a thermal evaporator was developed in LAMSAT to deposit electrodes on solar cells. In perovskite solar cells, Au is mostly used as the back electrodes. Unfortunately, thermal evaporation of Au or Al was not recommended by the furnace supplier. Hence Cu contacts were used as back contacts deposited via thermal evaporation. One must keep in mind that contacts play a significant role on performance of solar cells. In early attempts, AZO nanoparticles layer was tried as ETL but obtaining a good coverage on FTO was unsuccessful, Furthermore, resulted J-V measurements were not that great. Also employing PEDOT: PSS and NiO nanoparticles layers as HTL was unsuccessful. Spiro-OMeTAD is the standard HTL in perovskite solar cells but
138
employing it was not practical since it is very expensive. Finally, TiO2 and P3HT with t-BP and
Li-TFSI additives were spin coated on fluorine-doped tin oxide (FTO) as ETL and HTL respectively. These layers were then applied to perovskite solar cells and perovskite solar cells were fabricated with and without BTO nanoparticles. Electrical characterization of perovskite solar cells indicated that ferroelectric polarization of BTO nanoparticles increases the depletion region width in the perovskite hence the photo current was increased by one order of magnitude after poling the devices. One must rememeber that all the measurements were carried out under ambient conditions and humidity, oxygen, and UV light play a significant role on the performance of perovskite solar cells. Unfortunately, unavailability of a solar simulator was another barrier to measure all the device parameters.
The ferroelectric polarization of BTO nanoparticles within the perovskite solar absorber provides a new perspective for tailoring the working mechanism and photovoltaic performance of perovskite solar cells. The results presented are of great interest to the researchers involved in this growing field of perovskite solar cells.
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