Graphene as a transparent conducting and surface field layer in planar Si solar cells
© Kumar et al.; licensee Springer. 2014
Received: 24 May 2014
Accepted: 6 July 2014
Published: 13 July 2014
This work presents an experimental and finite difference time domain (FDTD) simulation-based study on the application of graphene as a transparent conducting layer on a planar and untextured crystalline p-n silicon solar cell. A high-quality monolayer graphene with 97% transparency and 350 Ω/□ sheet resistance grown by atmospheric pressure chemical vapor deposition method was transferred onto planar Si cells. An increase in efficiency from 5.38% to 7.85% was observed upon deposition of graphene onto Si cells, which further increases to 8.94% upon SiO2 deposition onto the graphene/Si structure. A large increase in photon conversion efficiency as a result of graphene deposition shows that the electronic interaction and the presence of an electric field at the graphene/Si interface together play an important role in this improvement and additionally lead to a reduction in series resistance due to the conducting nature of graphene.
Graphene has been considered as one of the promising materials for photovoltaic device applications due to its two-dimensional nature with extraordinary optical (transmittance ~98%), electronic (such as low resistivity, high mobility, and zero bandgap), and mechanical properties (Young's modulus 1.0 TPa) [1–3]. Many attempts have been made to utilize the extraordinary properties of graphene in electronic applications, such as solar cells, light-emitting diodes (LEDs), lithium-ion batteries, and supercapacitors. In particular, graphene can be used as an active (for electron-hole separation) or supporting layer in solar cell applications [4–11]. The superior flexibility and abundance of carbon source at lower costs make graphene a good alternative to indium tin oxide (ITO) as a transparent conducting electrode in numerous applications such as flexible solar cells, touch screens, and liquid crystal displays (LCDs) [12–14]. The advancements in the synthesis of large-area graphene with high crystallinity and transfer techniques make it suitable for its applications in solar cells .
In silicon solar cell, the power conversion efficiency is limited by many fundamental losses such as incomplete absorption of the solar spectrum, recombination of the photo-generated charge carriers, shading losses, and series resistance losses [16, 17]. Antireflection coatings and passivation layers of oxides are used to overcome these losses [18, 19]. Apart from these, front surface field (FSF) is also a very important technique to passivate the front surface by introducing an electric field at the surface to enhance the performance of silicon solar cell . In a number of studies, the formation of a graphene/silicon (G/Si) junction for solar cell application has been studied. Li et al. reported the first demonstration on the G/Si solar cell with about 1.65% power conversion efficiency . After that, many attempts have been made to improve the performance of graphene-based Si solar cells by modifying the work function and reducing the sheet resistance of graphene [22–25]. Although high optical transmittance and good electrical conductivity of graphene layer are well reported, there are limited studies in which the transparent conducting property has been studied by depositing the graphene layers onto fabricated solar cells. Difficulty in transferring a uniform graphene layer onto highly textured surfaces in normally available commercial-grade Si solar cells could be one of the possible reasons for this.
In this paper, we investigate the transparent conducting and surface field properties of graphene layers onto planar and untextured crystalline Si surface by carrying out experimental investigations and finite difference time domain (FDTD) calculations. In addition, the effect of graphene layer on the photovoltaic parameters and spectral responses of planar and untextured Si solar cell has also been investigated.
Synthesis and transfer of graphene
The growth of graphene has been carried out on a 25-μm-thick Cu foil (99.98%, Sigma-Aldrich, St. Louis, MO, USA, item no. 349208) using an atmospheric pressure chemical vapor deposition (APCVD) system at a temperature of 1,030°C. A split-type furnace with a quartz tube reactor was used for graphene growth. Before loading into the reaction tube, the Cu foil was cleaned in acetic acid followed by acetone, deionized water, and isopropyl alcohol to remove the copper oxide present at the surface. A mixture of Ar (500 sccm) and H2 (30 sccm) was then introduced into the reaction tube for degassing the air inside. The flow rate of Ar was kept constant (500 sccm) for all the experiments mentioned in this manuscript. The reactor was heated up to 1,030°C in 30 min, and this temperature was kept constant for the next 30 min to anneal the Cu foil. Then, CH4 (3 sccm) was fed into the reactor. After 30 min, the feeding of CH4 was cut off and the reactor was cooled down to room temperature naturally in an Ar and H2 environment. The flow of all the gases was stopped as the temperature reached close to the room temperature.
On successful growth of graphene on Cu foil, polymethyl methacrylate (PMMA) (Sigma-Aldrich, average MW ~996,000, item no. 182265, 10 mg/ml in anisole) was used for the transfer of graphene onto different substrates like quartz, Si, SiO2-sputtered Si, and solar cells to study graphene quality and its electronic and optical properties. In the first step, the graphene-deposited Cu foil was attached to a glass slide with the help of a scotch tape and then PMMA was spin coated on one side of the Cu foil. The other side of the foil was immersed into 10% HNO3 solution for 2 min to etch out the graphene from that side. Subsequently, the Cu foil was etched using FeCl3 (10% wt./vol.) for 3–4 h. The PMMA coated graphene film was transferred to the desired substrate (quartz, Si or SiO2/Si, and solar cell) on several dips in deionized (DI) water as a cleaning step. In the final step, PMMA was etched out using acetone at 80°C for a duration of 2 h. Some residual PMMA was further removed by annealing in a H2 (500 sccm) and Ar (500 sccm) environment at a temperature of 450°C for 2 h.
Solar cell fabrication
In order to study the effect of graphene on photon absorption and carrier collection, we first fabricated Si solar cells with planar and untextured surfaces. A 156-mm monocrystalline silicon wafer was dipped in high-concentration alkali solution at 80°C for 1 to 2 min to remove the roughness of the wafer. A p-n junction was then formed on the polished wafer through a high-temperature, solid-state diffusion process. Phosphorous oxy-chloride (POCl3) liquid dopant was used, and the wafers were subjected to elevated temperature in a furnace resulting in the formation of a thin layer of n-doped region (~0.5 μm). The wafers were etched using freon-oxygen (CF4) gas mixture in dry plasma etch machine to remove the junction regions created on the edge. These wafers were then chemically etched to remove the oxides and phosphorous glass formed on their surfaces. The entire backside was metallized with Ag-Al paste. Front contacts on the wafer surface were formed by screen printing the required pattern with a suitable metallic paste on them. The metal paste was dried and sintered in an infrared sintering belt furnace where temperature and belt speed were optimized to achieve a sharp temperature profile. The printed cells were then cut into smaller cells of dimension 10 mm × 10 mm for deposition of graphene. A similar printed cell is kept for comparative studies.
A 100-nm-thin film of SiO2 layer was deposited over mc-Si solar cell after the deposition of graphene layer using radio frequency (RF)-magnetron sputtering from a Si target (<111>) with 99.99% purity. The sputtering was carried out for 22 min by introducing Ar (15.8 sccm) and O2 (2.8 sccm) gases at room temperature with an applied RF power of 100 W.
Characterization and measurements
Raman spectroscopic measurements were carried out in backscattering geometry using the 514.5-nm line of Ar+ laser for excitation. The scattered light was analyzed with a Renishaw spectrometer having a charged couple device for detection. All the optical measurements were carried out on a Lambda 35 UV/Vis spectrophotometer (PerkinElmer, Waltham, MA, USA). The photovoltaic characterization of the solar cell was carried out by measuring the I-V behavior using a 2400 SourceMeter (Keithley Instruments, Inc., Cleveland, OH, USA) under simulated AM 1.5 solar illumination at 100 mW/cm2 from a xenon arc lamp in ambient atmosphere.
Results and discussion
In order to optimize the CH4/H2 flow rate for growing good-quality single-layer graphene, five flow rates of CH4/H2 content were chosen, i.e., 01/10, 03/30, 05/50, 10/100, and 20/200 sccm, while keeping the CH4:H2 flow rate ratio (1:10) constant. The growth temperature was set at the optimized value of 1,030°C with a deposition time of 30 min to ensure complete coverage of graphene. Raman spectra of graphene samples grown at different CH4/H2 flow rates are shown in Figure 1c, while the corresponding I2D/IG ratio and FWHM data are shown in Figure 1d. The Raman spectra show very-low-intensity D peak (at ~1,353 cm-1) and large and symmetrical graphene G (~1,580 cm-1) and 2D (~2,700 cm-1) peaks. The D peak is negligible in all the cases, indicating a defect-free graphene growth. Furthermore, the FWHM of the 2D peak increases gradually from 30 to 65 cm-2 (as shown in Figure 1d) and the I2D/IG peak ratio changes from 1.3 to 0.3. The optimal CH4/H2 ratio to produce monolayer graphene, determined experimentally, is 03/30. The decrease in I2D/IG and increase in FWHM with the increase in CH4/H2 flow rate indicate an increase in the number of graphene layers upon increasing the CH4/H2 flow rate. The values of I2D/IG (>5) and FWHM (≈32 cm-1) in graphene grown at 1,030°C and 03/30-sccm CH4/H2 flow rate match well with the previously reported values for monolayer graphene [26, 28–30]. Based on the above study, graphene layer grown for 30 min at a deposition temperature of 1,030°C with 03 sccm of CH4 and 30 sccm of H2 flow rates was used for investigating the effect of graphene and G/SiO2 layers on Si solar cell as a transparent conducting and antireflection layer.
A comparison of transmittance and sheet resistance values of graphene layers used in reported studies on Si solar cells
Method of preparation
Sheet resistance (Ω/□)
CVD using Cu foil
96 to 98
CVD using Cu foil
95 to 97
CVD using Ni foil
54 to 70
Fame synthesis using Ni foil
CVD using Ni foil
CVD using Cu foil
8.94 (in the present study)
Performance parameters of planar (Si), G/Si, and SiO 2 /G/Si cells
IPCE (%) (at 600 nm)
Planar (Si) cell
The I-V behavior of graphene/Si (G/n-Si) structure was obtained to study the nature of G/n-Si junction. Figure 4d shows the I-V characteristics of the G/n-Si in dark and light. The forward bias condition was observed with graphene connected to the negative terminal with respect to n-Si. This shows that the interface between the graphene and n-Si behaves like a n+-n junction. The favorable direction of the electric field formed at the interface helps in the reduction of the effective recombination at the front surface and enhances the collection of light-generated free carriers and thus improves the efficiency of solar cell. The n-type or p-type nature of graphene is very sensitive to the synthesis method, adsorbed molecules, nature of the substrate underneath, etc. [42–45]. It can be conjectured that the graphene deposited onto Si (n-type) in G/Si cells in the present study acts like an n-type layer.
The present study is a clear demonstration of the useful combination of the properties of graphene: (i) as a transparent conducting layer, which provides high transmittance (97%) and reduces the series resistance of planar p-n Si solar cell; (ii) as an antireflection layer, which reduces the reflectance of the planar p-n Si solar cell due to the presence of wrinkles; and (iii) as a surface field layer onto n-type Si due to n+-n nature of the interface, which provides a favorable electric field for reducing the carrier recombination. Due to these effects, an increase in efficiency from 5.38% to 7.85% is observed. Deposition of a layer of SiO2 of an optimized thickness value leads to a further increase in the short circuit current density due to its antireflection properties.
RK and MB are PhD students in the Department of Physics, IIT Delhi, India. BRM is a professor (Schlumberger Chair) in the Department of Physics, IIT Delhi, India. SM, SS, and PJ are photovoltaics engineers at BHEL, India.
The support provided by the Nanomission Programme of the Department of Science and Technology, Department of Electronic and Information Technology, Government of India, and Schlumberger Chair Professorship is acknowledged. One of the authors, RK, is thankful to IIT Delhi for providing senior research fellowship.
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