Design and Fabrication of TiO2\G Nanocomposite as Electron Transport Layer for Perovskite QD Solar Cells

INTRODUCTION
Research in the field of photovoltaic technology has become a priority for researchers in the field of energy [1]. Advanced solar cell research is being carried out in the fields of colloidal quantum dots (CQDs) [2], ink layer[3], dye[4], up conversion[5], and perovskites solar cells[6]. Among these types of solar cells. The crystalline silicon-based solar cells dominate the market share since the 1950s [7]. But the perovskite solar cells have received great interest from scientists due to their cost-effective and high-throughput[8], whereas hybrid perovskite materials have shown great possibilities for solar energy conversion by virtue of their high conversion efficiency, low cost, and The abundance of their preparation materials[9,10]. The perovskite material is an intrinsic (p-type nor n-type) semiconductor [11]. The important obstacle facing perovskite materials for employ in photovoltaic applications is the large and indirect energy gap [12]. Therefore, the general trend has been become focused in many studies on engineering the energy gap to fold this type of solar cell [13]. The general structure of the perovskite is described by the AMX3 structure, where A is an organic cation, M is a divalent group (cations also) such as (Sn2+, Pb2+), and X is a halide anion (I-, Br-, Cl-) that provides a charge equilibrium for the cations [14], Fig. 1 represents a perovskite crystal structure. There is a high probability of exciton generation in perovskite cells within the ETL/perovskite/HTL layered structure by optical injection of charge carriers into both the ETL and HTL layers. Several oxides have been investigated for these layers, including molybdenum trioxide (MoO3) [11], ZnO [15], Cu2O NiO [16], SnO2 [17], and TiO2 [18]. The interface between the HTL and the perovskite layer is the most important layer for its role in electron collection. TiO2 got attention due to its appropriate conduction band, easy deposition methods, and chemical stability [19].

MATERIALS AND METHOD
The Spherical TiO2 nanoparticles (Anatase) (TiO2, 99.5%, 10-30 nm) and ethanol (99.9%), zirconium (IV) oxide (ZrO2) (powder, 5 μm, 99% ), Methylammonium iodide (CH3NH3I), and PbI2 were obtained from Sigma Aldrich. While, the nanosheets graphene (G, 15 micros) with a platelet morphology were obtained from (skyspring Nanomaterials).
Characterization
The structural properties of the deposited layers were achieved by employment the x-ray diffraction (XRD) spectrum, with Cu Kα1 radiation (λ=1.54060 Å), 30 kV, and 10 mA). Field emission scanning electron microscope (FE-SEM) and transmission electron microscopy TEM (TEM) were used to determine the morphological and topological properties. Moreover, the optical properties were measured based on Uv-Vis. absorbance spectrum (300-800) nm of the wavelength. Perovskite solar cell (PSC) performance was determined based on measurements of the current-density-voltage (I-V) curves.  

Fabrication of (FTO \TiO2-G\CH3NH3PbI3\ZrO2-G\Pt)
To obtain the sample of (FTO \c-TiO2-G\ TiO2-G (ETL)\CH3NH3PbI3\ZrO2-G) and (FTO \c-TiO2-G\ TiO2-G (ETL)\CH3NH3PbI3 QD\ZrO2-G) the same steps must be followed. The compact layer TiO2\G was deposited by spin coating on pre-transparent conductive glass FTO, and then sintering substrates at 450℃, sintering time is 2h. TiO2\G film ETL is prepared using the doctor’s blade method; by shedding drops of the solution on the FTO\ c-TiO2\G at room temperature, then left to dry. The fabricated TiO2\G film (ETL) has been annealed at 450℃ for better adhesion between the film and FTO\c-TiO2-G Multi-layer. Afterwards, the perovskite CH3NH3PbI3 layer was deposited by spin coating on FTO\c-TiO2-G\TiO2-G ETL. Next, the structural layers were annealed at 100℃. Later, ZrO2\G layer by spin coating method was deposited and annealed at 100℃. After that, the method of thermal evaporation is used to prepare a grid .Platinum (Pt) wire to be vaporized is placed at a pressure of (10-5 tor), and with a surface area of the counter electrode (CE) film (4 cm2), to fabricated (FTO \TiO2-G\CH3NH3PbI3\ZrO2-G\Pt). In (FTO \c-TiO2-G\ TiO2-G (ETL)\CH3NH3PbI3 QD\ZrO2-G) case, Perovskite layer was prepared according to Zhang et al [20], and replaced by perovskite QD. Fig. 2 presents a structure of the fabricated solar cell.

RESULT AND DISSECTION 
Structural properties (XRD analysis)
The XRD analysis of TiO2/Graphene (ETL) with (Cellulose Ether 0.25) (ETL) showed dominance anatase (nanocrystalline). The dominant phase emerged at 2θ = 25.33o (011), addition to other positions of 2θ= (37.79, 48.04, 62.62, 68.83, 70.22, and 75.154) o respectively. The possibility of dissociation of titanium dioxide resulting from the presence of both pressure and temperature may lead to hydrogen-bonded to form H0.5Ti0.374O and oxygen to form Ti3O5. Meanwhile, no peak appeared related to graphene due to its low percentage of added, except for the shifting in some positions of peaks related to anatase Fig. 3 represents XRD of the prepared deposited ETL film on FTO.
To confirm the structural properties of the raw materials used in the preparation., the X-ray diffraction spectra of both compounds have been compared with (ICDS 00-007-0235 code) for PbI2 and for CH3NH3I according to Stamplecoskie et. al.[14], the dominate phase of PbI2 was (101) with 86 nm of crystallite size while CH3NH3I showed a tetragonal crystalline structure. However, the XRD pattern of CH3NH3PbI3 showed a polycrystalline structure with (211) as a dominate phase and 78.28 nm as crystallite size as shown in Fig. 4. That agreement most likely with previous studies [21-23].

TEM and FESEM images
TEM image of CH3NH3PbI3 shows a spherical as shape and size distribution between 77 nm to 0.846 μm with about 232 nm of average size as shown in Fig. 5. FESEM image of TiO2/G film consisted of semi-spherical clusters, were average of 132 nm in size. These clusters consist of an aggregate of spherical particles, its size ranges from (7.5 to 46.4) nm with an average of 17.5 nm in size. At the same time, the porosity of this film was about 55% according to Abdullah and Khairurrijal[24] as shown in Fig. 6a. Fig. 6b represent the FESEM image of the used PbI2 powder. While, the prepared perovskite layer shown as multi-plane prismatic structures with 1.38071±0.78847 μm of the length average and 0.34694±0.18069 μm of the diameter average, and about 72.3 % as a porosity.
Fig. 6 FESEM image and size distribution histogram (a) of deposited TiO2/G (ETL) film, (b) PbI2, (c) CH3NH3PbI3 film on FTO.

Fourier transform infrared (FTIR)
The dynamic evolution of IR transmission property (Fig. 7) of the prepared perovskite layer was studied by Fourier transform infrared (FTIR) spectra that were recorded between 400 cm-1 to 4000 cm-1 of wavenumber as shown in Fig.7. However, the change in the IR spectrum of CH3NC3PbI3 and CH3NC3PbI3 QD were compared with that of CH3NC3I. Generally, It is noticeable that there are changes in both the width and the intensity of the recorded contrast broad peak with all bonds when incorporation the PbI2. Both peaks (3467 and 3370) cm-1 are associated with O-H stretch vibrations in isolated water molecules with hydrogen bonding [25]. The proposed contribution to peaks (3007, 2941, and 1289) cm-1 is for C-H bonds. While the peaks of CHO bond were detected at both (2774.3 and 2734) cm-1. All peaks in region 1780 cm-1 to 940 cm-1 attribute to binding with NH3+ and CH3 bend [26]. It is possible that there is a significant effect of the humidity on the absorbent properties of the material [27]. For the CH3NC3PbI3 QD, the peaks suffered a decrease in intensity and shifting in positions with the disappearance of some of them.

Optical Properties
Fig. 8 (a and b) illustrates transmittance of ETL film and ZrO2-G film on FTO glass for (300-800) nm of wavelength, where it be increase from 69.7% at 350 nm to 90% at 800 nm for ETL. The addition of the thermal treatment (annealing to 450oC), which made to supply complete crystalline which enhanced the electronic transition property, this provides an abundance of transient electrons. Moreover, ZrO2-G film had about 95% along of the range 300 nm to 800 nm. Energy gap (Eg) was estimated by Tauc equation as follows [28]; 

where, A, α, h, v, and Eg  are a constant, the absorption coefficient, Plank’s and the frequency of incident photon energy respectively, while n is (2) with direct energy gab. Based on that, Eg of ETL was 2.95 eV, this result is roughly consistent with what has been reported by Shi et al [29].     
Fig. 9(a, b) illustrate absorption behavior of PbI2, CH3NH3I, CH3NH3PbI3 and CH3NH3PbI3 QD.  There are a noted deceasing in the transmittance of CH3NH3PbI3 in the region (346-622) nm compared both PbI2 and CH3NH3I. On the other side, due to the greater absorption coefficients of all PbI2, CH3NH3I, and CH3NH3PbI3 (>104cm-1), indicates the presence of a direct energy gap transition. Eg decreased from 2.03 eV and 2.08 eV for PbI2 and CH3NH3I, respectively to 1.95 eV for CH3NH3PbI3 as illustrated in Fig. 10. The formation of a tetragonal perovskite phase with the reaction of lead halides with CH3NH3I may be a lead change in the optical bandgap. The low difference in the bandgap value can be indicative of the partial retention of iodine ions without bonding [30].While the absorbance of CH3NH3PbI3 was 88.7% at 550 nm of the wave length. Moreover, CH3NH3PbI3 QD film had a linear increasing of absorbance along (300-800) nm of wavelength from 82.5 to 94.5 respectively as shown in Fig. 9(c and d). 

Characterization of solar cell
To estimate the quality of a solar cell and its electrical behavior; current-voltage (I-V) measurements were achieved. I-V characteristics of the prepared (FTO\TiO2-G\CH3NH3PbI3\ZrO2-G\Pt) and (FTO \TiO2-G\CH3NH3PbI3 QD\ZrO2-G\Pt) solar cells under 100 mW.cm-2 of the illumination powers with the forward applied voltage is illustrated in Fig. 10. The dark voltage (Voc), short circuit current (Isc), and fill factor (FF) parameters were determined from I-V curves of the solar cell. Fill factor can be calculate by the following [31].

Where Vmp is the voltage at the maximum power point; Imp is the current at the maximum power, P; Vos is the no-load voltage; Isc is the short-circuit current. While the efficiency, PCE can be calculate by the following [21],

Where Pin is the incident light power. If the conduction band edge of the quantum dots is located between the conduction edge of the perovskite and the top occupied molecular orbital of the carriers transition layer, the transition of charge carriers is improved, thus improving the value of the fill factor, which leads to the improvement of the conversion efficiency of the solar cell [32]. 
The prepared solar cells have 15.48% and 16.96% as an efficiency for both FTO \TiO2-\CH3NH3PbI3\ZrO2-G\Pt and FTO \TiO2-G\CH3NH3PbI3 QD\ZrO2-G\Pt respectively as shown in Fig. 11 and Table 1.

CONCLUSION
This study succeeded in presenting a perovskite solar cell based on perovskite QD material. Because ZrO2 and TiO2/G possess energies gap in the deep UV and Visible regions respectively, they generate an abundance of electron-hole pairs that improves the transition of holes to graphene and electron traps in ZrO2. Moreover, the presence of the interface between HTL and the perovskite layer and the interface between ETL and the perovskite layer provides a suitable position for the conduction band edge of the quantum dots in relation to the perovskite conduction edge and the occupied molecular orbital of the HTL and ETL. That improved the factor of the fill factor.  Therefore، leads to the improvement of the conversion efficiency of the solar cell. So was the case with the carriers in ETL.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.