Junction Silicon Solar Cells Research Articles

Analytical model for simulating thin-film/wafer-based tandem junction solar cells

Replacing the present-day commercial single junction silicon (Si) solar cells with low cost, high efficiency solar cells is imperative, in order to compete with other existing energy technologies. Many research groups have looked into using III–V materials, tandem junction solar cells and thin-film technologies to reach higher efficiencies. However, many of these techniques involve expensive materials or costly manufacturing processes. In this research, we focus on a tandem junction solar cell design that is based on a thin-film perovskite top cell and wafer-based Si bottom cell. In order to analyze the performance of the tandem cell, an analytical model is needed to compute the quantum efficiency and characteristic solar cell data. The highly versatile Matlab-based analytical model presented in this work is capable of modeling different kinds of tandem cells based on a variety of solar absorber combinations. The model allows user to adjust input parameters, such as reflectivity, material thickness, donor and acceptor densities, and carrier lifetimes in order to optimize the quantum efficiency, maximum power output, open circuit voltage, and short circuit current quantities of the cell. Using this analytical model, we were able to design a perovskite and black Silicon (bSi) tandem cell, which reached an efficiency of greater than 30%.

Base Doping Effects on the Efficiency of Vertical Parallel Junction Solar Cells

In this work, we present a theoretical study of a vertical parallel junction silicon solar cell under monochromatic illumination. The internal quantum efficiency (IQE) and the photovoltaic conversion efficiency are calculated, taking into account the base doping density and illumination wave-length. The main purpose of this work is to show how conversion efficiency depends on internal quantum efficiency and the dependence of the later on the base doping density.

23.2% laser processed back contact solar cell: fabrication, characterization and modeling

We describe the manufacturing process for interdigitated back contact back junction silicon solar cells based on laser processes, and present detailed results and analysis to our best cell efficiency of 23.24%. The manufacturing process features two laser doping steps, one for the boron doped emitter and one for the phosphorus doped back surface field. The saturation current densities of thermal oxide passivated laser doped regions are on par with furnace diffused silicon for high efficiency solar cells. Laser ablation locally defines the contact areas through the rear dielectric layer stack, and structures the rear aluminum metallization. The precision of the laser systems in conjunction with the optical setup yields line shaped doping traces with a width of 150 µm and a pitch below 500 µm. The measured optical and electrical properties of our solar cell agree well with 3D simulation results. The measured reflection, transmission, quantum efficiency and current voltage curves in dark and illuminated condition simultaneously agree well with simulation, based on the same data set, giving confidence in the result of a detailed free energy loss analysis. The bulk resistive and recombination losses are identified as the main loss contributors. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.

Effect of incidence angle on the electrical parameters of vertical parallel junction silicon solar cell under frequency domain

The effect of incidence angle on the electrical parameters of vertical parallel silicon solar cell under frequency domain was theoretically analyzed. Based on the diffusion-recombination equation, the expression of excess minority carrier density in the base was established according to the modulation frequency and the illumination incidence angle. The excess minority carrier density, the photocurrent density, the photo voltage, series resistance, shunt resistance, electric power and the space charge region capacitance were calculated and plotted. The objective of this work was to show the effects of solar cell modulation frequency and the illumination incidence angle on these electrical parameters, electric power and space charge region capacitance. Plots of solar cell’s electric power with the junction recombination velocity gave the maximum solar cell’s electric power; Pm. Influence of various parameters of incidence angles on the solar cell’s electric power was also studied.

Anti-radiation of space triple-junction solar cell based on distributed Bragg reflector structure

At present, solar cells are the main sources for spacecrafts. For a long time the bulk of the space power installations has been the solar arrays based on single junction silicon and gallium arsenide solar cells. In recent years a trend has been the active use of triple-junction GaAs solar cell with higher efficiency instead of single junction solar cells. One of the most important characteristics of solar cells used in spacecrafts is the resistance to radiation damages caused by high energy particles of the near-Earth space. According to the spectral response of triple-junction GaAs solar cell and the damage characteristics of the current under the condition of electron irradiation, the physical mechanism of cell attenuation can be determined: the current degradation originates mainly from the GaInAs subcells. These damages form additional centers of nonradiative recombination, which results in the reduction of the minority charge carrier diffusion lengths and in degradation of the solar cells photocurrent.The radiation damage caused by the electron irradiation will shorten the diffusion length of the base region and affect the collection of photo generated carriers. The ways of improving absorption of long wavelength light in GaInAs subcells with a thin base in using the distributed Bragg reflector can be investigated by the mathematical simulation method based on calculating the light propagation in a multilayer structure by means of the TFCalc software which can design optical structure. To estimate the validity of these methods for solar cells structures with distributed Bragg reflector, the spectral dependences of the photoresponse and the reflection coefficient with different base thickness values are calculated and compared with experimental results. Based on the physical mechanism of the degradation, the thickness of middle subcell base layer is reduced, and an appropriate structure of the distributed Bragg reflector is simulated by the TFCalc software. As a result, the new structure solar cells are that the thickness of the base layer is 1.5 m compared with the different middle subcell thickness values, and the distributed Bragg reflector structure with 15 paris of the Al0.1Ga0.9As/Al0.9Ga0.1As with 850 nm central wavelength is embedded in the middle subcell of the base layer, the distributed Bragg reflector has a highest reflectivity of more than 97% in the actual test, and a bandwidth of 94 nm, which can satisfy design requirement. After irradiating the new structure of solar cells, the decay of its short-circuited current is reduced by 50% compared with that of the original structure, and the remaining efficiency factor is increased by 2.3%.

Tunnel oxide passivated rear contact for large area <em>n</em>-type front junction silicon solar cells providing excellent carrier selectivity

Carrier-selective contact with low minority carrier recombination and efficient majority carrier transport is mandatory to eliminate metal-induced recombination for higher energy conversion efficiency for silicon (Si) solar cells. In the present study, the carrier-selective contact consists of an ultra-thin tunnel oxide and a phosphorus-doped polycrystalline Si (<em>poly</em>-Si) thin film formed by plasma enhanced chemical vapor deposition (PECVD) and subsequent thermal crystallization. It is shown that the <em>poly</em>-Si film properties (doping level, crystallization and dopant activation anneal temperature) are crucial for achieving excellent contact passivation quality. It is also demonstrated quantitatively that the tunnel oxide plays a critical role in this tunnel oxide passivated contact (TOPCON) scheme to realize desired carrier selectivity. Presence of tunnel oxide increases the implied <em>V_oc</em> (<em>iV_oc</em>) by ~ 125 mV. The <em>iV_oc</em> value as high as 728 mV is achieved on symmetric structure with TOPCON on both sides. Large area (239 cm²) <em>n</em>-type Czochralski (Cz) Si solar cells are fabricated with homogeneous implanted boron emitter and screen-printed contact on the front and TOPCON on the back, achieving 21.2% cell efficiency. Detailed analysis shows that the performance of these cells is mainly limited by boron emitter recombination on the front side.

Impact of Parasitic Resistances on the Output Power of a Parallel Vertical Junction Silicon Solar Cell

This paper describes the theoretical model for calculating IV-curve of parallel vertical silicon solar cells (SCs) based on solving diffusion-recombination equation for such SC, which was suggested that two IV curve zones (those which are close to the short current and open circuit points) can be linearized. This linearalization allows obtaining the values of shunt (Rsh) and series (Rs) resistances. The evolution of the electric power based on these resistances was illustrated to show the values that shunt and series resistances must have to obtain a good efficiency.

Effect of wavelength on the electrical parameters of a vertical parallel junction silicon solar cell illuminated by its rear side in frequency domain

The influence of the illumination wavelength on the electrical parameters of a vertical parallel junction silicon solar cell by its rear side is theoretically analyzed. Based on the excess minority carrier’s density, the photocurrent density and photovoltage across the junction were determined. From both photocurrent and the photovoltage, the series and shunt resistance expressions are deduced and the solar cell associated capacitance and conversion efficiency are calculated.The aim of this study is to show the influence of the illumination wavelength on the electrical parameters of the cell and the behavior of both parasitic resistances and capacitance versus operating point.

Dependence of the Photocurrent of a Schottky-Barrier Solar Cell on the Back Surface Recombination Velocity and Suggestion for a Structure with Improved Performance

Though Schottky-barrier solar cells have been studied extensively previously, not much work has been done recently on these cells, because of the fact that conventional p-n junction silicon solar cells have much higher efficiency and have attracted the attention of most of the researchers. However, the Schottky-barrier solar cells have the advantage of simple and economical fabrication process. In this paper, the effect of back surface recombination velocity on the minority carrier distribution and the spectral response of a Schottky-barrier silicon solar cell have been investigated and, based on this study, a new design of the cell with a back surface field has been suggested, which is expected to give much improved performance.

Modeling of Increased Open Circuit Voltage through Localized Emitter Area on Silicon Solar Cells

A limited area p-n junction approach can increase the open circuit voltage (Voc) of silicon solar cells when the emitter recombination dominates. A limited area p-n junction silicon solar cell is designed and the effect of reducing emitter areas on the Voc, short circuit current (Jsc) and fill factor (FF) is studied using the 3-D simulator Quokka. This paper compares the modelling result of this approach on structures with different values of emitter recombination. It is concluded that voltage improvement can be achieved when the emitter dominates the total recombination. The result also shows that the current collection and FF can be maintained with an optimized unit cell design. The paper also extends the approach to thin (20μm) solar cells, demonstrating the potential for Voc values up to 770mV when excellent surface passivation is employed.

High efficiency silicon solar cell based on asymmetric nanowire

Improving the efficiency of solar cells through novel materials and devices is critical to realize the full potential of solar energy to meet the growing worldwide energy demands. We present here a highly efficient radial p-n junction silicon solar cell using an asymmetric nanowire structure with a shorter bottom core diameter than at the top. A maximum short circuit current density of 27.5 mA/cm2 and an efficiency of 7.53% were realized without anti-reflection coating. Changing the silicon nanowire (SiNW) structure from conventional symmetric to asymmetric nature improves the efficiency due to increased short circuit current density. From numerical simulation and measurement of the optical characteristics, the total reflection on the sidewalls is seen to increase the light trapping path and charge carrier generation in the radial junction of the asymmetric SiNW, yielding high external quantum efficiency and short circuit current density. The proposed asymmetric structure has great potential to effectively improve the efficiency of the SiNW solar cells.

Capacitance of Vertical Parallel Junction Silicon Solar Cell under Monochromatic Modulated Illumination

The aim of this study is to show the influence of the wavelength on the electrical parameters of vertical parallel junction silicon solar cell by use of impedance spectroscopy technique. The Bode diagrams of the diffusion capacitance are shown for different wavelengths while the solar cell operates under both open circuit and short circuit conditions. The Nyquist diagram of the diffusion capacitance has shown the extension of the space charge region for various wavelengths.

Performance enhancement of ITO/oxide/semiconductor MOS-structure silicon solar cells with voltage biasing.

In this study, we demonstrate the photovoltaic performance enhancement of a p-n junction silicon solar cell using a transparent-antireflective ITO/oxide film deposited on the spacing of the front-side finger electrodes and with a DC voltage applied on the ITO-electrode. The depletion width of the p-n junction under the ITO-electrode was induced and extended while the absorbed volume and built-in electric field were also increased when the biasing voltage was increased. The photocurrent and conversion efficiency were increased because more photo-carriers are generated in a larger absorbed volume and because the carriers transported and collected more effectively due to higher biasing voltage effects. Compared to a reference solar cell (which was biased at 0 V), a conversion efficiency enhancement of 26.57% (from 12.42% to 15.72%) and short-circuit current density enhancement of 42.43% (from 29.51 to 42.03 mA/cm2) were obtained as the proposed MOS-structure solar cell biased at 2.5 V. In addition, the capacitance-volt (C-V) measurement was also used to examine the mechanism of photovoltaic performance enhancement due to the depletion width being enlarged by applying a DC voltage on an ITO-electrode.

Minority Carrier Distribution in the Base Region of a pn Junction Silicon Solar Cell and its Contribution to the Spectral Response

Analytical and experimental research work has been previously by various researchers on silicon solar cells. In this paper, an analytical study has been carried out on the light generated excess minority carrier distribution and photocurrent in the base region of a p + n junction solar cell. The effect of back surface recombination velocity, doping and absorption coefficient on the minority carrier distribution in the n type base layer have been observed and the spectral response component due to this region has been obtained. The minority carrier profile helps in understanding the physics of the solar cells.

20.7% efficient ion‐implanted large area n‐type front junction silicon solar cells with rear point contacts formed by laser opening and physical vapor deposition

ABSTRACTIn this work, we report on ion‐implanted, high‐efficiency n‐type silicon solar cells fabricated on large area pseudosquare Czochralski wafers. The sputtering of aluminum (Al) via physical vapor deposition (PVD) in combination with a laser‐patterned dielectric stack was used on the rear side to produce front junction cells with an implanted boron emitter and a phosphorus back surface field. Front and back surface passivation was achieved by thin thermally grown oxide during the implant anneal. Both front and back oxides were capped with SiNx, followed by screen‐printed metal grid formation on the front side. An ultraviolet laser was used to selectively ablate the SiO2/SiNx passivation stack on the back to form the pattern for metal–Si contact. The laser pulse energy had to be optimized to fully open the SiO2/SiNx passivation layers, without inducing appreciable damage or defects on the surface of the n+ back surface field layer. It was also found that a low temperature annealing for less than 3 min after PVD Al provided an excellent charge collecting contact on the back. In order to obtain high fill factor of ~80%, an in situ plasma etching in an inert ambient prior to PVD was found to be essential for etching the native oxide formed in the rear vias during the front contact firing. Finally, through optimization of the size and pitch of the rear point contacts, an efficiency of 20.7% was achieved for the large area n‐type passivated emitter, rear totally diffused cell. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Characterization of solar cells by thermal transient testing

The current paper deals with the application of thermal transient testing as a characterization tool for solar modules. Based on the measurement of different samples (concentrator solar cell, single junction silicon solar cell) we prove the applicability of this measurement technique and address some specific issues of the characterization of solar cells by the thermal transient method.From the measurement metrics such as junction-to-base plate thermal resistance and thermal capacitance(s) can be derived and can serve as a basis of a multi domain solar cell model. The used technique also enables us to verify the quality of attachment layers in a solar module allowing fair quality control and reliability analysis of these devices. Finally a method is proposed to regain the data that is covered by the initial electric transient following the power step. This initial electric transient can be high in large surface devices like solar cells, and covers valuable data describing the structure near to the p–n junction. To eliminate this, simulated transients were fitted to the part of the actual measured thermal transient where the electric transient already decayed. This way the part of the thermal transient that was covered by the electric transient can be reconstructed.

New Approach to Determine Vertical Parallel Junction Silicon Solar Cell Parameters

The aim of this work is to study the electrical behavior of a vertical parallel junction silicon solar cell by mean of impedance spectroscopy. By solving diffusion-recombination equation, the excess minority carrier density expression was established and the photocurrent and the photo voltage were deduced. We plotted Nyquist diagram of the solar cell impedance and gave corresponding electrical circuits. We then introduced impedance spectroscopy method to access for the first time to some electrical parameters of a vertical junction solar cell (series resistance, parallel resistance, capacitance and inductance).

Amorphous silicon passivated contacts for diffused junction silicon solar cells

Carrier recombination at the metal contacts is a major obstacle in the development of high-performance crystalline silicon homojunction solar cells. To address this issue, we insert thin intrinsic hydrogenated amorphous silicon [a-Si:H(i)] passivating films between the dopant-diffused silicon surface and aluminum contacts. We find that with increasing a-Si:H(i) interlayer thickness (from 0 to 16 nm) the recombination loss at metal-contacted phosphorus (n+) and boron (p+) diffused surfaces decreases by factors of ∼25 and ∼10, respectively. Conversely, the contact resistivity increases in both cases before saturating to still acceptable values of ∼ 50 mΩ cm2 for n+ and ∼100 mΩ cm2 for p+ surfaces. Carrier transport towards the contacts likely occurs by a combination of carrier tunneling and aluminum spiking through the a-Si:H(i) layer, as supported by scanning transmission electron microscopy–energy dispersive x-ray maps. We explain the superior contact selectivity obtained on n+ surfaces by more favorable band offsets and capture cross section ratios of recombination centers at the c-Si/a-Si:H(i) interface.

High efficiency large area n -type front junction silicon solar cells with boron emitter formed by screen printing technology

In this paper, we report on commercially viable screen printing (SP) technology to form boron emitters. A screen-printed boron emitter and ion-implanted phosphorus back surface field were formed simultaneously by a co-annealing process. Front and back surfaces were passivated by chemically grown oxide capped with plasma-enhanced chemical vapor deposition silicon nitride stack. Front and back contacts were formed by traditional SP and firing processes with silver/aluminum grid on front and local silver back contacts on the rear. This resulted in 19.6% efficient large area (239 cm2) n-type solar cells with an open-circuit voltage Voc of 645 mV, short-circuit current density Jsc of 38.6 mA/cm2, and fill factor of 78.6%. This demonstrates the potential of this novel technology for production of low-cost high-efficiency n-type silicon solar cells. Copyright © 2014 John Wiley & Sons, Ltd.

INVESTIGATION ON SILICON SOLAR CELL CAPACITANCE AND ITS DEPENDENCE ON BOTH TEMPERATURE AND INCIDENCE ANGLE

The aim of this work is to investigate a theoretical study of a vertical junction silicon solar cell capacitance under monochromatic illumination. By solving the continuity equation and using a one dimensional model in frequency modulation, we derive the analytical expressions of both excess minority carrier density and photovoltage. Based on these expressions, the solar cell capacitance was calculated; we then exhibited the effects of both temperature and incidence angle on the solar cell capacitance.