Performance Enhancement of Solar Cell by Incorporating Bilayer RGO‐ITO Smart Conducting Antireflection Coating

Abstract Multilayered graphene deposited on a flat resistive surface has twofold benefits. Less electronic scattering reduces the sheet resistance of the combined bilayer and high photon scattering through the unavoidable wrinkles on the chemically synthesized graphene layer leads to decreased effective reflection. In this paper, wet‐chemically‐synthesized reduced graphene oxide (RGO) has been employed on the top of the indium‐doped tin‐oxide (ITO) layer. The ITO layer of optimized thickness has been deposited as an alternative antireflection coating (ARC) on a p/n junction based crystalline silicon solar cell with standard textured surface. Variation in spectral response has been studied experimentally for different thickness and surface coverage of RGO on ITO. The combined effect of reduced sheet resistance due to high surface conductivity and increased photon injection efficiency due to scattering from the wrinkles of RGO results in significant improvement in the performance of the solar cell. By employing optimum thickness of RGO, percentage enhancements of about 18% and 10%, respectively, in efficiency and short‐circuit current density have been achieved over the baseline cell structure. RGO also exhibits an additional benefit as a moisture repelling layer.

introduction of in-plane and inter-planner crystal defect in the deposited ITO layer, which supports the formation of desirable 222 plane, potential as TCO material. All X-Ray diffraction data suggest that the 222 oriented phases is the dominating phases than 400 oriented planes. The higher time oxygen exposure during the growth of 80nm and 100nm ITO layer is a possible reason for the presence of the two peaks suggesting planes 211, 024 and 136. However, we have no clear understanding about the preferential growth of these peaks at present [1,2].

Optimization of ITO layer
A part of the light trapping structure includes lower reflection and absorption loss at the front layer of the device. The reflection takes place because of difference in refractive index between two layers. Refractive index (RI) of c-Si is 3.88 with that of air is 1 [3]. Thus, TCO film having refractive index somewhere around ~2 may show better optical transmission in accordance with equation 1 [4]. The quarter of the wavelength thickness of the ARC layer produces destructive interference causing least reflection overall according to the equation 2 [4].
Selection of ARC is so important, that effect of interference on the coating causes the wave reflected from the top surface of anti-reflection coating to be out of phase with the wave reflected from the semiconductor surfaces. These out-of-phase reflected waves destructively interfere with one another, resulting in zero net reflected energy.
From the equation 1, n 1 is the RI of the intermediate ARC layer i.e. ITO (in our case) between air with refractive index 1 (n 0 ) and silicon layer with refractive index 3.88 (n 2 ). The calculated RI from equation 1 is 1.975, which is almost matched with the RI of ITO [5,6] material as ARC in between air and silicon to harvest maximum nos. of photons. Thus parasitic reflection losses due to mismatch in RI can be minimized by incorporating ARC following the condition stated in equation 1. Lowest reflection from the surface of the ITO layer can be achieved if the thickness of the ARC should be quarter of the mid visible wavelength, required to be placed in between [4]. Solving equation 2, the lowest reflection will be obtained using ~80nm thick ITO.
In this work, ITO films with various thicknesses have been employed. Numerous studies have been carried out on the highly transmissive layer of ITO of different thickness. ITO has been sputtered on polished silicon, glass and textured wafer for 10mins, 14mins, 18mins and 22 mins to grow ITO layer of thicknesses 40nm, 60nm, 80nm and 100nm hence named as ITO_40, It is found that with the increment of ITO thickness on the textured silicon wafer, the reflection minimizes in an almost linear fashion. Bare textured silicon wafer shows a reflection higher than any of ITO coated textured wafer. The integrated total number of photon is found to be 2.1034x10 17 cm -2 s -1 in AM1.5G from 300nm to 1100nm wavelength, which has been calculated by 'Simpsons 1/3 rd ' rule.    In urge to establish the ITO as good TCO, sheet resistance measurement was performed. If transparent oxide (85% transmittance in the visible range from 400nm to 700nm [7,8]) supports electrical resistivity lower than 5.0 x 10 -4 Ω -cm (1.44 to 2.08) can be employed as solar potent TCO material in solar cell structure [8]. Resistivity of ITO strongly associated with a few factors, which are as following i) the resistance of ITO films decreases with increase in oxygen vacancy in the lattice network. The oxygen vacancies create free electrons in the films because one oxygen vacancies creates two extra electrons [11]. The increase in the number of oxygen vacancies leads to an increase in carrier density and a consequent decrease in resistance ii) If many free electrons exist in ITO films, the mobility is rapidly decreased by scattering with carriers or with crystal defects [12] iii) Resistivity can increase due to the change in chemical composition or micro-crystal structure of an ITO film [13] iv) Mobility is said to be increased due to enhanced crystallinity of films deposited at higher substrate temperatures. The results discussed in these studies vary significantly from one another and suggest that the optical and electrical attributes significantly depend on thickness manipulation of ITO layer and demand a careful experimental and mathematical data investigation to select optically perfect thickness of ITO to be used in the solar cell as TCO for reproducible results. In terms of optical and electrical perceptions, 80nm thick ITO (ITO_80) deposited on substrate in our case, shows optimized performance with 8 % transmittance and 50 Ω/□ sheet resistance.

Figure 5: Intensity variation of D and G peaks in Raman spectroscopy of GO and RGO
Raman spectroscopic study (RanishawinVia.) with laser excitation wavelengths of 514 nm ( . 1eV) was used to investigate the G'-band as a function of thickness and is presented in figure   5. For the measurement, graphene thin film was coated on glass substrate by spin coating. Each of the de-convoluted Raman spectra shows two peaks in the figure; disorder-induced (D) band and tangential (G) band. G band is usually assigned to the E 2g phonon of sp 2 carbon atom, while D band is a breathing mode of -point phonons of A 1g symmetry. Two distinct peaks at ca.
1366cm -1 and 1613 cm -1 correspond with D and G bands in GO. The shifting and intensity gain in D band (1358 cm -1 ) indicates that the graphene sheets (RGO) has structural disorder. Intensity ratio of D and G band (I D /I G ) in RGO is 1.04 and in GO is 0.91. The comparison shows a decrease in the average size of sp 2 domain due to the reduction process [14].