Chapter 4 Results and Discussion
4.2 The Inverted Structure of Perovskite Solar Cells
4.2.1 Introduction
The structure of PSCs can be mainly divided into two species: the conventional structure originated from the concept of DSSCs [3, 5, 38, 39], and the inverted structure originated from the concept of organic solar cells (OSCs) [12, 40-42]. The inverted structure is composed of TCO substrate / HTL / perovskite layer / ETL / cathode. The TCO substrate used in the conventional and inverted structure are FTO and ITO, respectively, which is attributed to the fabrication process in temperature treatment. The HTL and ETL materials for the inverted structure are PEDOT: PSS and [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) or Bathocuproine (BCP), respectively [12, 40-42].
4.2.1.1 PEDOT: PSS structure
The poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT: PSS) is a conductive polymer mixture of two polymers (PEDOT and PSS). Part of the sulfonyl groups are deprotonated and carry a negative charge. The other component, PEDOT, is a conjugated polymer and carries positive charge. PEDOT is insoluble in water and based on the 3,4-ethylenedioxythiophene (EDOT) monomer unit. PEDOT can form an aqueous suspension when combined with PSS, which can be processed with other treatment conveniently. The chemical structure of PEDOT: PSS was shown in Figure 4-12 [46-47].
4.2.1.2 The treated PEDOT: PSS of this study
Previous investigations of our laboratory about how to improve and enhance the high transparent conductive thin film have mentioned that PEDOT: PSS have higher transmission and lower sheet resistance with treatments like acid-doped, heated-stirred, etc. [46-47]. Based on this theory, experiment processes were added to remove the extra solvents of PEDOT: PSS by treatment of
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heating and stirring which is referenced the optimal data from our laboratory.
It is mentioned that the thin films fabricated by treated PEDOT: PSS spin coated at specific temperature and removal solvent in regular volume were optimized. Table 4-4 showed the performance parameters of the devices fabricated by PEDOT: PSS without treatment (The device structure was shown in Figure 4-10 and Ag was deposited to the thickness of 100 nm), but returned to room temperature corresponded specific temperature of the treated PEDOT: PSS. Obviously, the performance parameters of treated PEDOT: PSS have higher JSC and VOC because of better charge carrier collection efficiency. In addition, the substrate was cleaned by the oxygen plasma to reduce the carbon contamination and enhance the work function before the treated PEDOT: PSS was spin coated on it [32]. Therefore, VOC is improved and enhanced to stable value by treatment of oxygen plasma and treated PEDOT: PSS no matter the performance of the devices.
4.2.2 Experimental Section
In our experiment, the structure of PSCs was shown in Figure 4-10, and the schematic energy levels of the device was shown in Figure 4-11. The performance of treated PEDOT: PSS with different volumes (1700, 1500 and 1300 μL) was investigated. PEDOT: PSS as the HTL was stirred for 20 min at 90 ◦C on hotplate and spin coated on ITO substrate. We extracted 100 μL of different volumes of PEDOT: PSS solution to accomplish the fabrication of the PSCs. Perovskite as the active layer spin coated on treated PEDOT: PSS is the vital material in device for light absorption. The organic material of C60 and BCP were deposited to thicknesses of 20 and 10 nm used as an electron accepter to enhance the absorption effect of the active layer and an ETL, respectively. Finally, the silver was deposited to thicknesses of 50 and 100 nm by vacuum thermal evaporation as the cathode.
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4.2.3 Results and Discussion
4.2.3.1 Influence of PEDOT: PSS on the Performance of Devices
It was found that all PSCs display stable and perfect VOC according to the careful fabrication of growing perovskite and organic material. Besides, the interface between CH3NH3PbI3 and C60 also affected the transportation of carriers owing to the good quality of thin films. The difference between the highest occupied molecular orbital of the donor (HOMOD) and the lowest unoccupied molecular orbital of the acceptor (LUMOA) was clear to display the maximum value of the VOC from perovskite and C60 [34, 48-49].
The performance parameters of the devices fabricated by treated PEDOT: PSS and Ag deposited to thickness of 50 nm were listed in Table 4-5. Figure 4-13 showed the J-V characteristics of the PSCs fabricated by treated PEDOT: PSS and Ag deposited to thickness of 50 nm. The improved PCE from 5.28 to 9.71 % of the devices fabricated by treated PEDOT: PSS and Ag deposited to thickness of 50 nm, which is attributed to the additional amount of transportation between photos and electronics, and excellent open-circuit voltage (VOC) values of the PSCs in Figure 4-13 and 4-15 (a), leading to the increase of short-circuit density from 12.98 to 17.39 mA/cm2. Although the fill factor was decreased from 0.64 to 0.48, the Rs was decreased from 136.3 to 72.42 Ω*cm2.
The performance parameters of the devices fabricated by treated PEDOT: PSS and Ag deposited to thickness of 100 nm were listed in Table 4-6. Figure 4-14 showed the J-V characteristics of the PSCs fabricated by treated PEDOT: PSS and Ag deposited to thickness of 100 nm. The devices with 1500 μL PEDOT: PSS had the highest PCE of 11.0 %, compared to other devices with volumes of 1700 and 1300 μL. The improved PCE from 8.0 to 11.0 % of the devices fabricated by treated PEDOT:
PSS and Ag deposited to thickness of 100 nm, which is attributed to the additional amount of transportation between photos and electronics in Figure 4-13 and 4-5 (b), leading to the increase of short-circuit density from 13.91 to 18.6 mA/cm2 because of the superb surface morphology of the device with 1500 μL PEDOT: PSS to promote the ability of carrier transportation. Although Voc was decreased from 0.87 to 0.84 and the fill factor is 0.71 which was lower than the device treated PEDOT:
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PSS of 1700
μL
, the Rs was decreased from 72.3 to 61.1 Ω*cm2 and Rsh was increased largely than others.On the experiment of the treated PEDOT: PSS solution, we controlled the experiment parameters, such as, the removal solvent in regular amount of 650 to 700 μL and the temperature as spin coating the treated solution in 18 to 33 ℃. Compare to the Table 4-4 and 4-6, it was found that the treated PEDOT: PSS (Table 4-6) displays better performance of the devices than without treatment one (Table 4-4), especially for the open-circuit voltage (Voc). The Voc enhancement is attributed to the work function of ITO improved by the treatment of oxygen plasma to reduce the carbon contamination and enhance the work function on the substrate (from 4.75 to 5.2eV) [32] and the solution characteristic of treated PEDOT: PSS, increasing the difference of work function between two electrodes, and leading to improvement of the built-in electric field (Ebi) in device. The large Ebi of the devices could increase the collection efficiency of free carriers and decrease the recombination probability.
On the other hand, the reason for the unchanged PCE of device with 1300 and 1700 μL was directly decided to the effect of heating time on PEDOT: PSS solution. At the same time, it was found that the proper moisture will optimize the viscosity of the solution, becoming the standard to judge the convenience of the process and the quality of film growing. There are no apparent differences between the original solution and the treated PEDOT: PSS with 1700 μL, expect the viscosity of the solution. After stirring and heating, the extra solvent was removed from the solution and the viscosity was enhanced. Moreover, we found that PEDOT: PSS with 1300 μL showed worse performance after stirring and heating. The solution with 1300 μL was too less in container leading to solution was evaporated too much, so that there was solution-solid state in container. This treated solution caused that we could not control the quality of the thin film fabricated by spin coating. We obviously found that the PDEOT particles after spin coating on the substrate and the thin film was rough and ragged, leading to the performance, the active area and the reproducibility of the devices.
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4.2.3.2 The External Quantum Efficiency of Devices
Figure 4-15 showed EQE spectra of devices fabricated by treated PEDOT: PSS and Ag deposited to thickness of (a) 50 and (b) 100 nm.
Figure 4-15 (a) showed that the devices have peak in 392, 390 and 386 nm corresponded to treated PEDOT: PSS in volumes of 1700, 1500 and 1300 μL, respectively. The value of blue, red, black line, EQE of device fabricated by treated PEDOT: PSS in volumes of 1700, 1500, 1300 μL was 61.12, 58.60, 54.75 % in average conversion efficiency from 350 to 750 nm. In 552 nm, they presented the highest efficiency of 69.97, 76.59 and 66.94 % corresponded to treated PEDOT: PSS in volumes of 1700, 1500 and 1300 μL.
Figure 4-15 (b) showed that the devices have peak in 388, 388 and 392 nm corresponded to treated PEDOT: PSS in volumes of 1700, 1500 and 1300 μL, respectively. The value of blue, red, black line, EQE of device fabricated by treated PEDOT: PSS in volumes of 1700, 1500, 1300 μL was 56.42, 64.77, 58.19 % in average conversion efficiency from 350 to 750 nm. In 552 nm, they presented the highest efficiency of 69.98, 80.61 and 72.62 % corresponded to treated PEDOT: PSS in volumes of 1700, 1500 and 1300 μL.
The function of BCP layer is to protect the acceptor layer when the metal cathode deposition, and it can decrease the exciton recombination at the interface of acceptor/cathode that reduce the leakage current.
4.2.3.3 The Morphology Analysis of PEDOT: PSS Layer
Figure 4-16 showed the surface morphologies fabricated by treated PEDOT: PSS in volumes of (a) 1700, (b) 1500 and (c) 1300 μL. The average roughness (Rq) of surface fabricated by treated PEDOT: PSS in volumes of 1700, 1500 and 1300 μL were corresponded to 1.190, 1.513 and 1.502 nm, respectively. By contrast, the surface morphology fabricated by treated PEDOT: PSS in volumes of 1500 μL showed the smooth appearance without apparent defect on the substrate, which is attributed that the optimized viscosity of solution with the proper moisture leading to increase the Rsh of the device, improved FF from 0.66 to 0.71, and the PCE from 8.0 to 11.0 %.
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