Optical modeling of finely textured amorphous silicon solar cells

It has long been known that the use of finely textured transparent conducting oxide layers substantially improves the performance of thin film amorphous silicon (a-Si:H) solar cells. Major efforts to understand the nature of this effect and to fully capture its potential have been made by researchers using advanced modeling techniques. In this work, modeling the oblique angle optical performance and use of an effective medium approximation to simulate microrough interfaces suggests that effective interface grading makes a significant contribution to optical enhancement.     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells

We have investigated the photovoltaic (PV) characteristics of both glow discharge deposited hydrogenated amorphous silicon (a-Si:H) on crystalline silicon (c-Si) in a n⁺ a-Si:H/undoped a-Si:H/p c-Si type structure, and DC magnetron sputtered a-Si:H in a n-type a-Si:H/p c-Si type solar cell structure. It was found that the PV properties of the solar cells were influenced very strongly by the a-Si/c-Si interface. Properties of strongly interface limited devices were found to be independent of a-Si thickness and c-Si resistivity. A hydrofluoric acid passivation prior to RF glow discharge deposition of a-Si:H increases the short circuit current density from 2.57 to 25.00 mA/cm² under 1 sun conditions.DC magnetron sputtering of a-Si:H in a Ar/H₂ ambient was found to be a controlled way of depositing n type a-Si:H layers on c-Si for solar cells and also a tool to study the PV response with a-Si/c-Si interface variations. 300 Å a-Si sputtered onto 1–10 ω cm p-type c-Si resulted in 10.6% efficient solar cells, without an A/R coating, with an open circuit voltage of 0.55 V and a short circuit current density of 30 mA/cm² over a 0.3 cm² area. High frequency capacitance-voltage measurements indicate good junction characteristics with zero bias depletion width in c-Si of 0.65 μm. The properties of the devices have been investigated over a wide range of variables like substrate resistivity, a-Si thickness, and sputtering power. The processing has focused on identifying and studying the conditions that result in an improved a-Si/c-Si interface that leads to better PV properties.     

N型单晶硅衬底上非晶硅/单晶硅异质结太阳电池计算机模拟

采用德国HMI研发的AFORS-HET软件模拟了N型衬底非晶硅,单晶硅异质结太阳电池的特性,结果表明随着发射层厚度的增加,短路电流下降,电池的短波响应变差.在非晶硅,单晶硅异质结界面处加入不同的界面态密度(Dit).发现当Dit1012cm-2·eV-1时,电池的开路电压和填充因子均大幅减小,导致电池效率降低.当在非晶硅,单晶硅异质结界面处加入本征非晶缓冲层后,电池性能明显改善,但是缓冲层厚度应控制在30nm以内.模拟的a-Si/i-a-Si:H/c-Si/i-a-Si:H/n a-Si双面异质结太阳电池的最高转换效率达到28.47%.     

a-Si:H(p)/c-Si(n)异质结太阳电池的模拟优化

通过AFORS-HET软件模拟了TCO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n)/Ag结构的硅异质结电池中硅衬底电阻率、本征非晶硅薄膜厚度、发射极材料特性以及TCO功函数对电池性能的影响。结果表明:在其它参数不变的条件下,硅衬底电阻率越低,转换效率越高;发射极非晶硅薄膜厚度对短路电流有较大影响,发射极掺杂浓度低于7.0×10¹⁹cm^-3时,电池各项性能参数都极差;TCO薄膜功函数应大于5.2 eV,以保证载流子的输运收集。     

Amorphous silicon solar cell computer model incorporating the effects of TCO/a-Si:C:H junction

A simulation model of amorphous silicon solar cells ASPIN has been extended to incorporate the material properties of the TCO/a-Si:C:H interface region, which plays an important role in p-i-n a-Si:H solar cells and can strongly influence their photoelectrical characteristics. The analysis includes the impact of band bending at the front a-Si:C:H surface due to the difference between the work functions of TCO and a-Si:C:H and the influence of an increased surface density of states at the TCO/a-Si:C:H interface on internal and external characteristics.     

A-Si:H buffer in a-SiGe:H solar cells

Profiled a-SiGe:H-buffer layers between the doped and the absorption layers of amorphous silicon germanium (a-SiGe:H) solar cells are routinely used to avoid bandgap discontinuities and high-defect densities at the p/i- and i/n interface. Here, we present a much simpler approach replacing the profiled a-SiGe:H-buffer layers at both interfaces by a-Si:H-buffer layers. It is demonstrated that for a-SiGe:H solar cells (thickness of the E_G=1.5 eV part is 54 nm) these structures yield similar open circuit voltage V_OC and fill factor (FF) compared to the bandgap profiled layer at the same short circuit current density j_SC. The influence of thickness, optical bandgap and position of the buffer layers on the solar cell performance is investigated.     

Enhancement of optical absorption for below 5 μm thin-film poly-Si solar cell on glass substrate

Optical confinement effect of thin-film polycrystalline-Si (poly-Si) solar cell on glass substrate fabricated at low-temperature has been investigated as a function of cell thickness of less than 5 μm. We found that it is possible to fabricate the textured Si thin film in situ on a glass substrate and that the reflectance at long-wavelength light is reduced by surface texturing. Thin-film poly-Si solar cell and a-Si:H/(0.45 μm)/poly-Si (5 μm) tandem solar cell exhibit the efficiency of 8.6% and 12.8%, respectively. The numerical study in terms of the light trapping explains the excellent high short-circuit current density (_sc above 27 mA/cm² at the 4.7 μm thin-film poly-Si solar cell.     

Plasma-induced TCO texture of ZnO:Ga back contacts on silicon thin film solar cells

This paper considers texturing of ZnO:Ga (GZO) films used as back contacts in amorphous silicon (a-Si) thin film solar cells. GZO thin films are first prepared by conventional methods. The as-deposited GZO surface properties are modified so that their use as back contacts on a-Si solar cells is enhanced. Texturing is performed by simple dry plasma etching in a CVD process chamber,at power=100 W, substrate temperature=190 °C (temperature is held at 190 °C because thin film solar cells are damaged above 200 °C), pressure=400 Pa and process gas H₂ flow=700 sccm. Conventional a-Si solar cells are fabricated with and without GZO back contact surface treatment. Comparison of the with/without texturing GZO films shows that plasma etching increases optical scattering reflectance and reflection haze. SEM and TEM are used to evaluate the morphological treatment-induced changes in the films. Comparison of the a-Si solar cells with/without texturing shows that the plasma treatment increases both the short-circuit current density and fill factor. Consequently, a-Si solar cell efficiency is relatively improved by 4.6%.     

Performance of spray-deposited ZnO:In layers as front electrodes in thin-film silicon solar cells

The surface morphology, optical and electrical properties of spray-deposited ZnO:In layers are characterized and compared to ASAHI U-type SnO₂:F. Both TCO layers were implemented as front electrodes in a-Si:H single-junction solar cells. The similar V_oc of the solar cells indicates a good electrical contact between the ZnO:In layer and the a-Si:H material. The difference in solar cell performance between cells on ASAHI U-type TCO (10%) and the spray-deposited ZnO:In (8.4%) is mainly due to a Jsc-loss, caused by the lower ZnO:In bandgap and insufficient surface texturing. Surface roughening experiments of ZnO:In layers have been carried out.     

Preparation of (n) a-Si: H/(p) c-Si heterojunction solar cells

Heterojunction solar cells have been manufactured by depositing n-type a-Si: H on p-type 1–2Ω cm CZ single crystalline silicon substrates. Although our cell structure is very simple - neither a BSF nor a surface texturing is used - a conversion efficiency of 13.1% has been achieved on an area of 1 cm². In this paper the technology is described and the dependence of the solar cell parameters on the properties of the n-type a-Si: H layer is discussed. It is shown that this cell type exhibits no degradation under light exposure.     

A new texturing geometry for producing high efficiency solar cells with no antireflection coatings

Various schemes to trap weakly absorbed light into solar cells have been proposed. These schemes include texturing the cell, texturing the cover glass and geometric arrangements of the individual cells. The perpendicular slats geometry is considered to be the best cell texturing design for light trapping. In this paper a new cell surface texturing design is proposed which, without the use of anti-reflection coatings, can outperform the perpendicular slats geometry with a double layer anti-reflection coating by virtue of efficient internal light trapping and a decrease in the front surface reflectance. The single sided texture uses three perpeendicular planes on the front surface and a planar back surface. The three perpendicular planes provide a triple bounce for the incoming light and efficient confinement for light which has entered the cell. TEXTURE, a raytracing program for textured cells, was used to predict the performance of this new design. A quantitative comparison with other texturing schemes is also provided. It is shown that for a cell without an anti-reflection coating on the front and a 98% effective back surface reflector, the new design produces a maximum short circuit current density of 40.99 mA/cm² as compared to 41.46 mA/cm² and 35.16 mA/cm² for the perpendicular slats geometry and flat surfaces, respectively, with a conventional single layer AR coating on the front. Effects of different front surface reflection coefficients are examined to show that as the front reflectance is decreased by improved antireflection coatings, the importance of the triple bounce is reduced and most promising surface texturing schemes approach the same value of maximum current.     

Large-area multicrystalline silicon solar cell fabrication using reactive ion etching (RIE)

Surface texturing of crystalline silicon wafer improves the conversion efficiency of solar cells by the enhancement in antireflection property and light trapping. Compared to antireflection coating, it is a more permanent and effective scheme. Wet texturing with the chemicals such as alkali (NaOH, KOH) or acid (HF, HNO₃, CH₃COOH) is too difficult for thinner wafer to apply due to a large amount of silicon loss. However, Plasma surface texturing using Reactive Ion Etching (RIE) can be effective in reducing the surface reflectance with low silicon loss. In this study, we have fabricated a large-area (156×156 mm) multicrystalline silicon (mc-Si) solar cell by mask less surface texturing using a SF₆/O₂ reactive ion etching. We have accomplished texturing with RIE by reducing silicon loss by almost half of that in wet texturing process. By optimizing the processing steps, we achieved conversion efficiency, open circuit voltage, short circuit current density, and fill factor as high as 16.1%, 619 mV, 33.5 mA/cm², and 77.7%, respectively. This study establishes that it is possible to fabricate the thin multicrystalline silicon solar cells of low cost and high efficiency using surface texturing by RIE.     

p-i interface engineering and i-layer control of hot-wire a-Si:H based p-i-n solar cells using in-situ ellipsometry

In this paper we report on the effect of monitoring the i-layer region near the p-i interface with the help of in-situ kinetic and spectroscopic ellipsometry on the performance of hot-wire deposited hydrogenated amorphous silicon p-i-n solar cells. It is very clearly observed that the microstructure at the p-i interface region in terms of the Si---Si bond packing density and surface roughness significantly affects the cell performance. The filament temperature, T_Fil, was the main parameter varied to control the above mentioned two properties near the p-i interface as well as in the bulk i-layer. In order to achieve significant enhancement in the cell performance we extended the idea of the “soft start”, earlier employed for the glow discharge deposited solar cells, to the hot-wire deposited i-layer. We were able to control the i-layer properties at the p-i interface and in the bulk independently and correlate these to the cell performance. It is shown that a major increase in cell performance can be achieved by improving the microstructure of the growing film directly at the p-i interface. Most interestingly, no significant deterioration in cell efficiency has been observed if only the p-i interface was properly controlled but the i-layer was of lower quality. These results are also shown to be consistent with model calculations of a numerical simulation. Our results therefore provide a clue to prepare hot-wire a-Si:H based solar cells with high efficiency and in the whole at high growth rates, which is needed for a more economic a-Si:H solar cell production.     

Efficiency limits for single-junction and tandem solar cells

Basic limitations of single-junction and tandem p–n and p–i–n diodes are established from thermodynamical considerations on radiative recombination and semi-empirical considerations on the classical diode equations. These limits are compared to actual values of short-circuit current, open-circuit voltage, fill factor and efficiency for amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon solar cells. For single-junction cells, major efficiency gains should be achievable by increasing the short-circuit current density by better light trapping. The limitations of p–i–n junctions are estimated from recombination effects in the intrinsic layer. The efficiency of double-junction cells is presented as a function of the energy gap of top and bottom cells, confirming the ‘micromorph’ tandem (a-Si:H/μc-Si:H) as an optimum combination of tandem solar cells.     

Fabricating high performance a-Si solar cells by alternately repeating deposition and hydrogen plasma treatment method

The performance of a p-i buffer layer in pin amorphous silicon solar cell was improved by the “alternately repeating deposition and hydrogen plasma treatment method (ADHT)”. The optical bandgap of the a-Si film was increased by hydrogen plasma treatement. The wide optical bandgap and the high photoconductive a-Si:H films without carbon could be fabricated by the ADHT method. The conversion efficiency of the solar cell with a-Si:H buffer layer was almost the same as that using an a-SiC:H buffer layer. Second, the a-Si (ADHT) films were applied to the n-i buffer layer. The insertion of a-Si (ADHT) films between the i-layer and the n-layer was effective to improve the cell performance, especially the fill factor. With the use of high performance a-Si p-i and n-i buffer layer deposited by ADHT method, a cell conversion efficiency of 12.9% was obtained.     

Analysis of light scattering in a-Si:H-based solar cells with rough interfaces

The main features of a recently developed semi-coherent optical model for a-Si:H thin film solar cells with rough interfaces are presented. In contrast to the previous optical models, the model takes into account also the interference fringes observed in measured wavelength-dependent characteristics of a-Si:H solar cells. The simulations of the quantum efficiencies of the cells with different intrinsic a-Si:H layer thicknesses and interface root mean square (rms) roughness of 40 nm are shown and compared with the measured data.     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells with a microcrystalline silicon buffer layer

Undoped hydrogenated amorphous silicon (a-Si:H)/p-type crystalline silicon (c-Si) structures with and without a microcrystalline silicon (μc-Si) buffer layer have been investigated as a potential low-cost heterojunction (HJ) solar cell. Unlike the conventional HJ silicon solar cell with a highly doped window layer, the undoped a-Si:H emitter was photovoltaically active, and a thicker emitter layer was proven to be advantageous for more light absorption, as long as the carriers generated in the layer are effectively collected at the junction. In addition, without using heavy doping and transparent front contacts, the solar cell exhibited a fill factor comparable to the conventional HJ silicon solar cell. The optimized configuration consisted of an undoped a-Si:H emitter layer (700 Å), providing an excellent light absorption and defect passivation, and a thin μc-Si buffer layer (200 Å), providing an improved carrier collection by lowering barrier height at the interface, resulting in a maximum conversion efficiency of 10% without an anti-reflective coating.     

Nanoimprint patterning for tunable light trapping in large-area silicon solar cells

We demonstrate the flexibility of UV nanoimprint lithography for effective light trapping in p-i-n a-Si:H/μc-Si:H tandem solar cells. A textured polymeric layer covered with pyramidal transparent conductive oxide structures is shown as an ideal system to promote front light scattering and thus enhanced photocurrent. The double structure incorporated into micromorph tandem thin film silicon solar cells is systematically investigated in order to find a relationship between interface morphology, optical properties and photovoltaic characteristics. To prevent the formation of defects during cell growth, a controllable smoothing of the imprinted texture is developed. Modules grown on polymer structures smoothed via multi-replication show excellent performance reaching a photocurrent of 12.6 mA/cm² and an efficiency of 12.8%.     

Optimum design and its experimental approach of a-Si/ /poly-Si tandem solar cell

A series of technical data on four-terminal a-Si/ /poly-Si stacked solar cells has been reported. The developed device has some unique significances such as high achievable efficiency, and low cost with almost no light induced degradation. It has been shown on a poly-Si bottom cell that an efficiency of 17.2% has been obtained by employing high conductivity with wide optical band-gap p-type μc-SiC as a window material and n-type μc-SiC as a back ohmic contact with BSF effects. On the optically transparent a-Si top cell, an optimum design has been experimentally made with the device structure of p μc-SiC/p a-SiC/i a-Si/n μc-Si/ITO, and an efficiency of 7.25% has been obtained with a 100 nm thick i-layer, while the best efficiency is 12.3% for p-i-n single-junction solar cell with 500 nm i-layer thickness deviced by Ag back-electrode. With the 100 nm thick ultrathin top cell, a total conversion efficiency as high as 21.0% has been achieved on a-Si/ /poly-Si four-terminal tandem solar cells.     

Design and Optimization of Amorphous Based on Highly Efficient HIT Solar Cell

The objective of this paper is to improve the power conversion efficiency of HIT solar cell using amorphous materials. A high efficiency amorphous material based on two dimensional heterojunction solar cell with thin intrinsic layer is designed and simulated at the research level using Synopsys/RSOFT-Solar Cell utility. The HIT structure composed of TCO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n⁺)/Ag is created by using of RSOFT CAD. Optical characterization of the cell is performed by Diffract MOD model based on RCWA (rigorous coupled wave analysis) algorithm. Electrical characterization of the cell is done by Solar cell utility using based on Ideal diode method. In addition, optimization of the different layer thickness in the HIT structure is executed to improve the absorption and thereby the photocurrent density. The proposed HIT solar cell structure resulted in an open circuit voltage of 0.751 V, a short circuit current density of 36.37 mA/cm² and fill factor of 85.37% contributing to the total power conversion efficiency of 25.91% under AM1.5G. Simulation results showed that the power conversion efficiency is improved by 1.21% as compared to the reference HIT solar cell. This improvement in high efficiency is due to reduction of resistive losses, recombination losses at the hetero junction interface between intrinsic a-Si and c-Si, and optimization of the thicknesses in a-Si and c-Si layers.