Hysteresis in Hole-free Perovskite Solar Cells with Carbon Counter Electrode Doped Reduced Graphene Oxide ( RGO )

In recent few years, perovskite solar cells based on CH3NH3PbI3 and CH3NH3PbI3-xClx have attracted researchers due to their unique properties such as high light absorbance and high power conversion efficiency (PCE) [1]. The first use of CH3NH3PbI3 as a sensitizer in solar cells was in 2009 [2], PCE of perovskite solar cells has risen from 3.8% [3,4] to more than 22.3% [5] within eight years. Although the high PCE, anomalous hysteresis appeared in (J-V) curve which made a problem on the cell stability. Hysteresis in perovskite solar cells depends on a lot of factors such as: cell structure, methods of fabrication, measurements methods, capacitive effect [6], and the effect of illumination, temperature and humidity [7]. The reverse scan (from the open circuit to the short circuit current) always shows higher PCE compared to that of forward scan (from short circuit to the open circuit) which introduces the undesirable hysteresis.[8]. The origin of hysteresis has been discussed on the intrinsic properties such as ferroelectric polarization [9], ionic migration [10] ,the charge trapping/detrapping caused by the lattice mismatch or voids at this interface which act as capacitors. In this paper to discuss the relation between the cell area and the hysteresis we used (RGO) [11] as a dopant of carbon counter electrode to enhance its conductivity, moisture impermeability and photoelectric properties. This leads to favorable physical properties, more charge separation and extraction. Hysteresis is usually small for C-PSCs because the thick TiO2 scaffold may eliminate charge accumulation at the TiO2/perovskite interface.[12] .


Introduction
In recent few years, perovskite solar cells based on CH 3 NH 3 PbI 3 and CH 3 NH 3 PbI 3-x Cl x have attracted researchers due to their unique properties such as high light absorbance and high power conversion efficiency (PCE) [1].The first use of CH 3 NH 3 PbI 3 as a sensitizer in solar cells was in 2009 [2], PCE of perovskite solar cells has risen from 3.8% [3,4] to more than 22.3% [5] within eight years.Although the high PCE, anomalous hysteresis appeared in (J-V) curve which made a problem on the cell stability.Hysteresis in perovskite solar cells depends on a lot of factors such as: cell structure, methods of fabrication, measurements methods, capacitive effect [6], and the effect of illumination, temperature and humidity [7].The reverse scan (from the open circuit to the short circuit current) always shows higher PCE compared to that of forward scan (from short circuit to the open circuit) which introduces the undesirable hysteresis.[8].The origin of hysteresis has been discussed on the intrinsic properties such as ferroelectric polarization [9], ionic migration [10] ,the charge trapping/detrapping caused by the lattice mismatch or voids at this interface which act as capacitors.
In this paper to discuss the relation between the cell area and the hysteresis we used (RGO) [11] as a dopant of carbon counter electrode to enhance its conductivity, moisture impermeability and photoelectric properties.This leads to favorable physical properties, more charge separation and extraction.Hysteresis is usually small for C-PSCs because the thick TiO 2 scaffold may eliminate charge accumulation at the TiO 2 /perovskite interface.[12] .

Experimental Works
The perovskite solar cell (PSC) was fabricated by multiple layers; each layer was prepared individually as follows:

preparation and deposition of blocking layer.
Each pre-cleaned FTO substrate was coated with a 60nm TiO 2 by spinning a sol-gel solution (0.25 M Titanium Isopropoxide in Ethanol) at 2000 rpm.The layer was dried at 100 °C for 10 minutes and then annealed at 500 °C for 30 min to allow sufficient crystallization in ambient air.

Preparation and deposition of mesoporous TiO 2 paste.
This layer fabricated as reported in [13] but it was deposited by spin-coating method at 5000 rpm ,the layer was dried at 100 °C for 10 minutes and annealed at 450 °C .The morphology of the mesoporous TiO 2 is shown in Figure .1.
Abstract.Perovskite solar cells achieved high efficiencies but they suffer from the hysteresis of (J-V) curve that depends on the scan direction.The architecture introduced in this paper is holefree cell with Reduced Graphene Oxide (RGO) as a dopant.The good electrical and optical properties of (RGO) make it acts as a hole transfer layer (HTL) beside its basic role as a dopant of carbon counter electrode.For different areas, the hysteresis decreases by decreasing the cell area.The TiO2/CH3NH3PbI3-xClx interface is truly responsible for the hysteresis in the Perovskite solar cells due to the dependence of the porosity from area to other.Finally, the mixture was stirred vigorously using ball milling for 5 h to get the carbon paste.The normal carbon paste was prepared according to the same procedures as this paste without the addition of RGO.

Hysteresis in
The prepared carbon paste as shown in figure 2 was deposited on the top of the TiO 2 film by the doctorblading technique, followed by heat-treatment at 500 °C for 30 min.   1 for different cell areas and different scanning directions(forward-reverse) .The hysteresis index is calculated by [14]: Where: J RS is the current density in reverse direction, J FS is the current density in forward direction.
To examine the surface morphology of the perovskite alone we have used scanning electron microscope (JSM-6360).The structure and phases of the materials are examined byX-ray Diffraction (schimadzu-7000) [11].theabsorption and estimated band gap of the perovskite (Ch 3 NH 3 PbI 3-x Cl x ) was measured using UV-Visible spectroscopy (EVO300 PC) as shown in next figures(4-6), the absorption peak at the wavelength of around 750 nm of CH3NH3PbI 3-x Cl x film was observed as shown in figure (6) .Perovskite film exhibits very strong absorption intensity [15].https://doi.org/10.1051/matecconf/201821002046CSCC 2018 Fig. 6.UV-Visible absorption spectrum of Ch3NH3PbI3-xClx

Results
After measuring the (J-V) curve, the results shown in table 1: T Table 1 Reduction in photovoltaic parameters due to Hysteresis.
From the table we found that as the cell area increases, the hysteresis index (HI) increases which means that the main reason for the hysteresis occurrence comes from the perovskite layer interfaces .The hysteresis less than for cells containing hole transporting layers (HTL) with Perovskite layer as reported with planar structure [16] .The measurement protocol used is voltage sweeping with scan rate 0.3mv/sec which is very slow if the scan rate increased to be 0.3v /sec, it also may reduce the hysteresis index.

Conclusion
For the hole -free Perovskite solar cell structure, the hysteresis decreases with the absence of the interface perovskite/HTL .Additionally ,when the area decreases, the hysteresis decreases.This ensures that the area of the interface between perovskite /TiO 2 should be small to reduce the trapping /detrapping of charges.This research was supported by Arab Academy for Science ,Technology and Maritime Transport(AASTMT).We thank Gehan (Technical Engineer , Institute of Graduate studies and research(IGSR) laboratory) for helping in cells fabrication and characterization .We thank Sherif Abdullah(teaching assistance, college of Engineering and Technology ,Electronics and Communications department,AASTMT) for his comments that helped in improving this paper research.

Hole-free Perovskite Solar Cells with Carbon Counter Electrode Doped Reduced Graphene Oxide (RGO)
, College of Engineering and Technology, Electronics and Communications Department , Alexandria , Egypt. 2 IGSR, ALEXANDRIA UNIVERSITY, Materials Science Department, Alexandria, Egypt. 1AASTMT