Carrier Selective Contact Solar Cells Research Articles

Effect of annealing on MoOx and interface properties for carrier-selective contact solar cells with different passivation layers

The structural, electrical and optical properties of molybdenum oxide (MoOx) thin films prepared by magnetron sputtering are studied with annealing temperature. The interface properties of silicon with the intrinsic amorphous silicon (a-Si:H(i)) and alumina (AlOx) passivation layers are investigated by the minority carrier lifetime and implied-Voc (iVoc). The AlOx shows the better passivation properties and thermal stability for the carrier-selective contact solar cells. The Ce-doped In2O3(ICO) is deposited on the surface of MoOx thin films. It is found that the optimized annealing temperature is 125 °C for the carrier selective contact solar cells with the a-Si:H(i) passivation layer. It could increase to 175 °C for the AlOx passivation layer.

Synthesis and simulation study of titanium dioxide-based nanomaterial for electron carrier selective contact solar cell

This research introduces a novel approach to synthesizing titanium dioxide (TiO2) nanomaterials using the sol–gel method, specifically aimed at enhancing the performance of Silicon Heterojunction (SHJ) solar cells. The innovation lies in utilizing Iso-Propanol Alcohol and Titanium Tetra Isopropoxides for synthesis, leveraging TiO2’s wide bandgap and low work function to replace the conventional n-doped amorphous silicon layer. This approach is expected to overcome the limitations of traditional materials and significantly improve electron transport efficiency. Advanced characterization techniques were employed to assess the synthesized TiO2 nanomaterials. Atomic Force Microscopy (AFM) measured the roughness and 3D profile, while UV-V spectrophotometry evaluated the absorption properties. The structure and surface morphology were meticulously analyzed using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). These comprehensive analyses ensured a detailed understanding of the nanomaterials’ physical properties. Simulations conducted with the AFORS-HET simulator revealed that TiO2, with a refractive index compatible with a-Si:H(n) and a work function of 4.6 eV, significantly enhances SHJ solar cell performance. The results demonstrated a remarkable 22.57 % improvement in efficiency, achieving an open-circuit voltage (Voc) of 716.8 mV, a short-circuit current density (Jsc) of 37.71 mA/cm2 and a fill factor (FF) of 83.49 %. These findings underscore the potential of TiO2 nanomaterials to revolutionize SHJ solar cell technology by providing a more efficient and effective electron transport layer. Future research will focus on refining the nanostructure of TiO2 further to enhance its photovoltaic applications, aiming to push the boundaries of current solar cell efficiencies even further. This study marks a significant advancement in the field of photovoltaic materials, presenting a novel and promising solution to improving solar energy conversion.

DLTS analysis of interface and near-interface bulk defects induced by TCO-plasma deposition in carrier-selective contact solar cells

We investigate the electrical characteristics of defects at the SiO2/Si interface, within the adjacent Si crystal, and through the depth profile of the bulk defect using three-dimensional deep-level transient spectroscopy (3D-DLTS). These defects are introduced by the reactive plasma deposition technique employed for depositing transparent conductive oxides in the fabrication of carrier-selective contact-type solar cells. To control the surface potential near the Si surface, we apply a varying voltage to obtain DLTS signals as functions of both temperature and Fermi level at the SiO2/Si interface. Using machine learning for 3D-DLTS spectral analysis, we estimate the capture cross sections, energy levels, densities, and depth profiles of these process-induced defects. The experimental results indicate the existence of three types of electron traps within the bulk defects, ranging from the interface to a depth of ∼70 nm. The electrical properties of these bulk defects suggest the presence of oxygen-related defects within Si. On the other hand, regarding the properties of interface defects, the capture cross sections and the defect densities are estimated as a function of their energy levels. They suggest that the defects at the SiO2/Si interface are likely oxygen-related PL centers.

Silicon surface passivation of industrial n-type CZ Si (111) by Al2O3 layers deposited by thermal ALD process for application in carrier selective contact solar cells

Silicon surface passivation of industrial n-type CZ Si (111) by Al2O3 layers deposited by thermal ALD process for application in carrier selective contact solar cells

Back-Contact Interdigitated Carrier-Selective Cell: Numerical Demonstration of 30 mW/cm² Output Power Density in Standard Albedo Condition

This brief proposes a new crystalline-Si (c-Si) solar cell structure, based on bifacial back-contact and carrier-selective contact solar cell technology. The proposed device, known as back-contact interdigitated carrier selective (BICS), offers the advantage of using existing c-Si solar cell structures. The bifaciality provided by the passivated emitter and rear cell (PERC), back-contact provided by interdigitated back contact (IBC), and carrier-selective contact provided by tunnel oxide passivated contact (TOPCon) are featured simultaneously. The local and full TOPCons are, respectively, applied to the emitter and back surface field (BSF) region. The proposed device has an output density of over 30 mW/cm2 under standard conditions—room temperature with a 20% albedo condition. A comparison with our previously proposed bifacial solar cell, heterojunction back contact (HBC)+, reveals the advantages of the proposed device. The proposed device is operable under a relatively low temperature under 45 °C, whereas HBC+ is suitable for higher temperatures. Therefore, the choice between the proposed device and HBC+ should be based on the climate of the installation location.

Analysis of recombination centers near an interface of a metal–SiO2–Si structure by double carrier pulse deep-level transient spectroscopy

This paper proposes a new double carrier pulse deep-level transient spectroscopy (DC-DLTS) method that is applicable for evaluating metal–insulator–semiconductor (MIS) structures and the recombination centers in carrier-selective contact solar cells. Specifically, this study evaluated recombination characteristics of defects induced in bulk Si near SiO2/Si interfaces by reactive plasma deposition (RPD). In this method, a pulse voltage was first applied to inject majority carriers. Subsequently, a second pulse voltage was applied, which allowed minority carriers to be injected into the MIS structure. With these two types of carrier injections, carriers were recombined in recombination-active defects, and the DC-DLTS spectrum changed. During the injection of minority carriers, some majority carriers were thermally emitted from the defects, resulting in a decrease in the signal intensity. The recombination activity was analyzed by considering the effect of thermal emission on the change in signal intensity. The number of induced defect types and defect properties were estimated using Bayesian optimization. According to the results, three types of electron traps were generated using the RPD process. Based on the DC-DLTS results, defects with energy level 0.57 eV below the conduction band and capture cross section of ∼10−15 cm2 act as recombination centers.

Experimental and Numerical Simulation of Molybdenum Oxide Films with Wide Bandgap and High Work Function for Carrier-Selective Contact Solar Cells

In silicon heterojunction (SHJ) solar cells, a wide bandgap material with a high work function is widely used as the hole extraction pathway to attain high efficiency. We introduced a molybdenum oxide (MoOx) film as an effective hole-transfer layer in carrier selective contact (CSC) solar cells by virtue of its wide bandgap along with high work function. The passivation characteristics, optical and electrical properties of MoOx films were investigated by differing thickness and work function. The combination of 6 nm hydrogenated intrinsic amorphous silicon (a-Si:H(i)) and 7 nm thermally evaporated MoOx passivation layers provides excellent passivation properties, reduces carrier recombination, and improves the cell performance. The synthesized CSC solar cells showed promising results, with an open-circuit voltage (Voc) of 708 mV, short-circuit current (Jsc) = 37.38 mA cm−2, fill factor (FF) = 74.59%, and efficiency (η) = 19.75%. To justify the obtained result, an AFORS HET simulation was conducted based on the experimental results. The high work function and wide bandgap MoOx/c-Si(n) interface developed a considerable built-in potential and suppressed the electron–hole pair recombination mechanism. The CSC solar cell’s simulated performance was enhanced from 1.62 to 23.32% by varying the MoOx work function (ΦMoOx) from 4.5 to 5.7 eV.

DLTS with Bayesian Inference for Analyzing RPD Induced Defects in Bulk near SiO2/Si Interface

Bulk defects are induced in the Si crystal near the SiO2/Si interface in a carrier-selective contact solar cell by indium tin oxide (ITO), a reactive plasma deposition (RPD) process. These electrical properties were evaluated by deep level transient spectroscopy (DLTS) with Bayesian inference. The bulk defects were distinguished from the interface by varying the pulse voltage Vp. The capacitance transient decay signals were converted to the DLTS spectra by Fourier transformation. The DLTS spectrum was deconvolved into signals originating from different types of defects using Bayesian optimization. Three types of electron traps, three types of hole traps in the bulk, and one interface defect were generated by RPD. The electron trap E1 and hole trap H1 had almost the same energy levels near the mid-gap of Si, and they had large capture cross-sections. Their concentrations were almost the same. Therefore, it is suggested that E1 and H1 signals were obtained from the same defect and that this defect may act as a recombination center because it efficiently captures both the electrons and holes.

Performance Analysis of Silicon Carrier Selective Contact Solar Cells With ALD MoOx as Hole Selective Layer

Carrier selective contact (CSC) architecture is one of several promising candidates for high-efficiency silicon solar cells operating close to the thermodynamic limit. However, an industrially feasible process is lacking for manufacturable CSC cells. Specifically, although evaporated molybdenum oxide (MoOx) has been actively researched as a hole selective layer, a more industry-compatible deposition process is critically needed. Atomic layer deposition (ALD) has been widely employed in the industry for material stacks requiring precise thickness control and uniformity. In this paper, we present a performance analysis of CSC solar cells fabricated using ALD MoOx as the hole selective layer. As a first step, the standard ALD recipe was modified to obtain sub-stoichiometric MoOx with a higher work function due to increased oxygen vacancy concentration. The modified MoOx recipe resulted in low open-circuit voltage values in fabricated not-passivated CSC solar cells. A 4 nm thick amorphous silicon layer inserted in the hole selective stack showed significant improvement in minority lifetime but worse solar cell performance due to increased series resistance. Besides indicating a strong trade-off between passivation and series resistance, the solar cell data highlights series resistance as a key performance limiter in CSC solar cells.

Effects of tunneling oxide defect density and inter-diffused carrier concentration on carrier selective contact solar cell performance: Illumination and temperature effects

In this study, technology computer-aided design (TCAD) was used to investigate carrier selective contact solar cell performance based on the boundary of tunneling oxide and electron selective contact layer properties. The role of tunneling oxide quality in the passivation and inter-diffusion properties of the plasma-enhanced chemical vapor deposition of phosphorus-doped amorphous silicon as an electron selective contact layer is studied for carrier selective contact solar cells. Tunnel oxide quality varies according to the ratio of Si4+ to Si2+ state. For the Si4+/Si2+ ratio of 4, open-circuit voltage of 730 mV and the lowest interface trap density of 5.3 × 1010 cm−2 eV−1 are achieved. The change in diffusion depth of the doped layer with respect to the annealing temperature is analysed by transmission electron microscopy (TEM)/energy dispersive X-ray spectroscopy (EDS) measurement. Carrier selective contact solar cell parameters are optimized by incorporating the experimental values of interface trap density, inter-diffused impurity concentration, and depth in the Quokka 3 TCAD tool. Solar cell conversion efficiency of 24.31% was obtained. To understand the response of carrier selective solar cell to the environmental changes, the output characteristics of the solar cell were studied by varying the illumination temperature by 10% of the standard test conditions (STC) and temperature from 277 K to 377 K (difference of 100 K).

Numerical analysis of p-type and n-type based carrier-selective contact solar cells with tunneling oxide thickness and bulk properties

Tunnel Oxide Passivated Contact (TOPCon) is a c-Si solar cell structure with tunneling oxide near the rear electrode. A full-area tunneling oxide provides sufficient passivation and low internal resistance with 1D carrier transport. Carrier tunneling mechanism is the key factor of a TOPCon solar cell; the thicker the tunneling oxide, the better is its ability to prevent minority-carrier tunneling. However, excessively thick oxide degrades majority-carrier tunneling. Therefore, the relationship between tunneling oxide thickness and cell performance is important. In this study, TOPCon solar cell structures are evaluated by varying their tunneling oxide thickness and bulk properties for both p-type and n-type bulk using numerical simulation. It is observed that the difference in bulk type is crucial to TOPCon solar cell performance: n-type material shows better performance because of its smaller minority-carrier tunneling current density and wider range of effective tunneling oxide thickness compared to p-type.

Ambient annealing influence on surface passivation and stoichiometric analysis of molybdenum oxide layer for carrier selective contact solar cells

Effective carrier contact with excellent surface passivation is essential for archiving high efficiency silicon solar cells. In this study, optical, passivation and stoichiometric characteristics of MoOx thin films were analyzed by varying thickness, temperature and ambient annealing. The 10 nm thick MoOx films showed high optical transmittance (89.81%) in the visible near infra-red (NIR) wavelength (380–1100) nm region and wide optical bandgap of 3.25 eV. An improved minority carrier lifetime of 2.24 ms was recorded for O2 ambient at 130 °C due to less additional defects in the MoOx films. X-rays photoelectron spectroscopic analysis confirmed that O2 ambient annealing greatly influenced the MoOx stoichiometric and reduce the formation defective state densities at the Fermi energy initiating from the tail of an oxygen vacancy and inducted the defect band within the energy bandgap. Carrier selective contact (CSC) solar cells were fabricated by using 10 nm thick MoOx film as hole extraction layer exhibited the high performance as; open circuit voltage (Voc) = 708 mV, current density (Jsc) = 37.80 mA/cm2, fill factor (FF) = 74.95% and efficiency (η) = 20.06%. These results implies that the MoOx is an important material to use in future CSC solar cell applications with the concept of new advantages such as dopant free, non-toxic and low temperature fabrication process.

Effect of ITO capping and its deposition parameters on electrical properties of MoO3/Si carrier-selective contact solar cell

Silicon solar cells with heterojunction carrier-selective contact are fabricated using molybdenum trioxide for passivation and hole selectivity. No additional thin intrinsic a-Si:H layer passivation is used in any of the experiment. The MoO3/Si heterojunction showed diode behaviour but it degraded in ambient conditions. Deposition of tin-doped indium oxide (ITO) over-layer well protected the device from degradation. However, the deposition parameters of ITO layer greatly influenced electrical properties of MoO3/Si device. The MoO3–ITO interface properties strongly depend upon ITO deposition parameters such as sputter power and substrate temperature. This dependence influences particularly the series resistance and hence, fill factor. With optimization of ITO deposition parameters and metal contact annealing, we could double the fill factor of our cells. Using these optimized conditions, a double heterojunction test cell structure is made using titanium dioxide as electron selective layer. This MoO3/Si/TiO2 solar cell reached JSC of 27.3 mA/cm2, VOC of 530 mV and active area efficiency of 7.86% under 1 Sun simulated radiation. Recently, we have achieved JSC exceeding 30 mA/cm2 and VOC exceeding 600 mV for our reference cells. This a-Si-free approach offers low thermal as well as low overall budget crystalline silicon-based solar cell fabrication route.

Performance Evaluation of Passivated Silicon Carrier-Selective Contact Solar Cell

In this paper, a solar cell with amorphous silicon (a-Si) passivation that uses transition metal oxides (TMOs) for charge carrier collection has been modeled and analyzed for its performance. Charge carrier selection has been incorporated through selective band alignment provided by NiO and TiO2 TMOs for electrons and holes, respectively. The presence of a-Si interfacial layers for defect passivation prevents carrier loss due to high interface state densities at the Si/TMO interfaces. This structure is a potential candidate for high-efficiency solar cells (>25%) with a realistic approach towards the solution of high defect density. Optical analysis of the proposed structure has been done to analyze the amount of light entering the silicon and capable of generating electron-hole pairs. The amount of light lost has also been quantified into reflection loss, parasitic absorption, and back metal absorption. Impact of surface passivation quality and doping of NiO and TiO2 on its performance has been analyzed through the detailed device simulations. This provides a realistic pathway for engineering of passivation quality and doping of metal oxides for improved performance.