Minority Carrier Diffusion Lengths in Silicon Slices and Shallow Junction Devices

Measurements of surface photovoltage as a function of the wavelength of incident light provide a convenient method for the determination of minority carrier diffusion length in semiconductors. The diffusion length in silicon slices with a thickness greater than twice the diffusion length has been measured by the steady-state surface photovoltage method with a single laboratory reproducibility of ± 10% over a long period of time. Radial variations in diffusion length in silicon slices as well as the effects of heat treatment have been studied. The diffusion length in the base region of shallow junction devices has been measured by the collection of short-circuit current as a function of the wavelength of incident light. The single laboratory reproducibility of this method for the determination of diffusion length in single crystalline and polycrystalline silicon solar cells is also about ± 10%. Prepared for the Division of Solar Energy of the U. S. Energy Research and Development Administration under Con-tract No. E(04-3)1285.     

Quantum efficiency analysis of thin-layer silicon solar cells withback surface fields and optical confinement

Thin-layer silicon solar cells utilize surface textures to increase light absorption and back surface fields to prevent recombination at the silicon-substrate interface. We present an analytical model for the internal quantum efficiency that accounts for light trapping and also considers carrier generation and recombination in back surface fields or substrates. We introduce a graphical representation of experimental data, the so-called Parameter-Confidence-Plot, which allows one to draw maximum information on diffusion lengths and surface recombination velocities from quantum efficiency measurements. The analysis is exemplified for state of the art thin-layer silicon solar cells with and without back surface fields     

Modeling extremely thin absorber solar cells for optimized design

Extremely thin absorber (ETA) solar cells based on inorganic semiconductors are theoretically analyzed by a model that considers the absorber as being a pin junction, with tunneling‐assisted defect recombination. Tunneling recombination turns out to be very important in ETA solar cells, owing to the high electrical fields in the absorber, which establishes a minimum thickness for the absorber layer, which is calculated to be around 15 and 20 nm for CdTe and CuInS₂, respectively. Nevertheless, 15% efficient CdTe and CuInS₂ ETA solar cells are possible, even at low diffusion lengths down to 10 nm. Additionally, the modeling provides optimum values for the thickness and number of absorber layers for CdTe and CuInS₂ ETA solar cells, as a function of the minority‐carrier diffusion length and diffusion constant, for cells with and without light trapping. The calculations predict that light trapping serves two purposes: to enhance the cell efficiency by up to 5% absolute, and to use a simpler structure compared with the situation without light‐trapping. Copyright © 2004 John Wiley & Sons, Ltd.     

Photocurrent enhancement in thin film amorphous silicon solar cells with silver nanoparticles

Silver nanoparticles embedded in a dielectric material have strong scattering properties under light illumination, due to localized surface plasmons. This effect is a potential way to achieve light trapping in thin‐film solar cells. In this paper we study light scattering properties of nanoparticles on glass and ZnO, and on silver coated with ZnO, which represent the back reflector of a solar cell. We find that large nanoparticles embedded in the dielectric at the back contact of amorphous silicon solar cells lead to a remarkable increase in short circuit current of 20% compared to co‐deposited cells without nanoparticles. This increase is strongly correlated with the enhanced cell absorption in the long wavelengths and is attributed to localized surface plasmons. We also discuss the electrical properties of the cells. Copyright © 2010 John Wiley & Sons, Ltd.     

Photocurrent enhancement in thin‐film silicon solar cells by combination of anti‐reflective sub‐wavelength structures and light‐trapping textures

Dielectric films with anti‐reflective sub‐wavelength structures are applied to thin‐film silicon solar cells to improve the light incoupling at the front surface. It is verified that modification of the refractive index of the incident medium using dielectric films with sub‐wavelength structures is beneficial to reduce the average reflectivity of Si solar cells with an anti‐reflective coating based on optical interference. It is also shown that the sub‐wavelength structure must be combined with a proper light‐trapping texture to enhance the absorption within thin‐film silicon solar cells. The effectiveness of dielectric films with sub‐wavelength structures is demonstrated by an increase of the short‐circuit current density of a microcrystalline silicon cell from 29.1 to 30.4 mA/cm² in a designated area of 1 cm². The optical interplay between the dielectric films and the light‐trapping textures is also discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

Separation of local bulk and surface recombination in crystalline silicon from luminescence reabsorption

Spontaneous photoemission of crystalline silicon provides information on excess charge carrier density and thereby on electronic properties such as charge carrier recombination lifetime and series resistance. This paper is dedicated to separating bulk recombination from surface recombination in silicon solar cells and wafers by exploiting reabsorption of spontaneously emitted photons. The approach is based on a comparison between luminescence images acquired with different optical short pass filters and a comprehensive mathematical model. An algorithm to separate both front and back surface recombination velocities and minority carrier diffusion length from photoluminescence (PL) images on silicon wafers is introduced. This algorithm can likewise be used to simultaneously determine back surface recombination velocity and minority carrier diffusion length in the base of a standard crystalline silicon solar cell from electroluminescence (EL) images. The proposed method is successfully tested experimentally. Copyright © 2009 John Wiley & Sons, Ltd.     

双面抛光单晶硅片少子扩散长度的测量

本文改进了常规表面光电压测试少子扩散长度法,采用环形下电极消除了薄样品背面光电压信号对测量结果的影响;应用阻尼最小二乘法数据处理原理对实验数据进行“曲线拟合”,求出少子扩散长度和背面表面复合速度。本文讨论了该方法的测量范围。     

偏压量子效率测试在薄膜太阳能电池特性分析中的应用

薄膜太阳能电池在不同偏压下的量子效率(QE)会呈现非常不一样的结果.对不同波长范围内偏压量子效率的分析可以研究薄膜太阳能电池窗口层区域杂质补偿情况、主结势垒高低、背势垒高度等,还可以得出耗尽区宽度以及少子扩散长度等重要参数.通过实验测量与理论分析,给出了薄膜太阳能电池耗尽区宽度(W)和少子扩散长度(Ln)与偏压量子效率的关系,提出了一种新的拟合耗尽区宽度(W)和少子扩散长度(Ln)的方法,探讨了偏压量子效率测试在薄膜太阳能电池特性分析中的应用.     

Influence of wafer thickness on the performance of multicrystalline Si solar cells: an experimental study

The influence of the thickness of silicon solar cells has been investigated using neighbouring multicrystalline silicon wafers with thickness ranging from 150 to 325 μm. For silicon solar cell structures with a high minority‐carrier diffusion length one expects that J_sc would decrease as the wafer becomes thinner due to a shorter optical path length. It was found experimentally that J_sc is nearly independent of the thickness of the solar cell, even when the minority‐carrier diffusion length is about 300 μm. This indicates that the Al rear metallisation acts as a good back surface reflector. A decrease in J_sc is observed only if the wafer thickness becomes less than about 200 μm. The observed trend in V_oc as a function of the wafer thickness has been explained with PC1D modelling by a minority‐carrier diffusion length in the Al‐oped BSF which is small in relation to the thickness of the BSF. This effectively increases the recombination velocity at the rear of the cell. We have shown that the efficiency of solar cells made with standard industrial processing is hardly reduced by reducing the wafer thickness. Solar cell efficiencies might be increased by better rear surface passivation. Copyright © 2002 John Wiley & Sons, Ltd.     

Back‐contacted back‐junction n‐type silicon solar cells featuring an insulating thin film for decoupling charge carrier collection and metallization geometry

In this study, back‐contacted back‐junction n‐type silicon solar cells featuring a large emitter coverage (point‐like base contacts), a small emitter coverage (point‐like base and emitter contacts), and interdigitated metal fingers have been fabricated and analyzed. For both solar cell designs, a significant reduction of electrical shading losses caused by an increased recombination in the non‐collecting base area on the rear side was obtained. Because the solar cell designs are characterized by an overlap of the B‐doped emitter and the P‐doped base with metal fingers of the other polarity, insulating thin films with excellent electrical insulation properties are required to prevent shunting in these overlapping regions. Thus, with insulating thin films, the geometry of the minority charge carrier collecting emitter diffusion and the geometry of the interdigitated metal fingers can be decoupled. In this regard, plasma‐enhanced chemical vapor deposited SiO₂ insulating thin films with various thicknesses and deposited at different temperatures have been investigated in more detail by metal‐insulator‐semiconductor structures. Furthermore, the influence of different metal layers on the insulation properties of the films has been analyzed. It has been found that by applying a SiO₂ insulating thin film with a thickness of more than 1000 nm and deposited at 350 °C to solar cells fabricated on 1 Ω cm and 10 Ω cm n‐type float‐zone grown silicon substrates, electrical shading losses could be reduced considerably, resulting in excellent short‐circuit current densities of more than 41 mA/cm² and conversion efficiencies of up to 23.0%. Copyright © 2012 John Wiley & Sons, Ltd.     

Nanoparticle‐enhanced light trapping in thin‐film silicon solar cells

A systematic investigation of the nanoparticle‐enhanced light trapping in thin‐film silicon solar cells is reported. The nanoparticles are fabricated by annealing a thin Ag film on the cell surface. An optimisation roadmap for the plasmon‐enhanced light‐trapping scheme for self‐assembled Ag metal nanoparticles is presented, including a comparison of rear‐located and front‐located nanoparticles, an optimisation of the precursor Ag film thickness, an investigation on different conditions of the nanoparticle dielectric environment and a combination of nanoparticles with other supplementary back‐surface reflectors. Significant photocurrent enhancements have been achieved because of high scattering and coupling efficiency of the Ag nanoparticles into the silicon device. For the optimum light‐trapping scheme, a short‐circuit current enhancement of 27% due to Ag nanoparticles is achieved, increasing to 44% for a “nanoparticle/magnesium fluoride/diffuse paint” back‐surface reflector structure. This is 6% higher compared with our previously reported plasmonic short‐circuit current enhancement of 38%. Copyright © 2011 John Wiley & Sons, Ltd.     

Nanowire and nanohole silicon solar cells: a thorough optoelectronic evaluation

Photovoltaic devices with nanostructured active layers have attracted considerable attention for their outstanding light‐trapping capability. Although with the promise of an efficient light‐conversion, the realistic performance is still far from expectation. This is because the detailed electrical mechanisms have seldom been included into the design, leading to a substantial discrepancy between prediction and reality. This paper reports a complete optoelectronic simulation for nanowire and nanohole solar cells by addressing electromagnetic and carrier‐transport response in a coupled finite‐element method. The effects of surface/bulk recombination are quantified and compared for nanowire and nanohole solar cells with radial and axial doping profiles. Our results reveal that the axially doped silicon cells are extremely sensitive to surface recombination because of the large surface‐to‐volume ratio and lateral recombination loss, eventually reducing the photocurrent and light‐conversion efficiency. Relatively, radially doped silicon cells with a moderate nanowire length show some improvement relative to axially doped cells, but nevertheless remain very sensitive to recombination losses. Comparison of the light‐trapping and electrical performance between nanowire and nanohole solar cells is also given. The methodology is applicable for nanostructured solar cells based on various semiconductor materials and system configurations, and is expected to play a promising role in accurately predicting the performance of the new‐generation light‐conversion devices. Copyright © 2015 John Wiley & Sons, Ltd.     

Highly transparent modulated surface textured front electrodes for high‐efficiency multijunction thin‐film silicon solar cells

To further increase the efficiency of multijunction thin‐film silicon (TF‐Si) solar cells, it is crucial for the front electrode to have a good transparency and conduction, to provide efficient light trapping for each subcell, and to ensure a suitable morphology for the growth of high‐quality silicon layers. Here, we present the implementation of highly transparent modulated surface textured (MST) front electrodes as light‐trapping structures in multijunction TF‐Si solar cells. The MST substrates comprise a micro‐textured glass, a thin layer of hydrogenated indium oxide (IOH), and a sub‐micron nano‐textured ZnO layer grown by low‐pressure chemical vapor deposition (LPCVD ZnO). The bilayer IOH/LPCVD ZnO stack guarantees efficient light in‐coupling and light trapping for the top amorphous silicon (a‐Si:H) solar cell while minimizing the parasitic absorption losses. The crater‐shaped micro‐textured glass provides both efficient light trapping in the red and infrared wavelength range and a suitable morphology for the growth of high‐quality nanocrystalline silicon (nc‐Si:H) layers. Thanks to the efficient light trapping for the individual subcells and suitable morphology for the growth of high‐quality silicon layers, multijunction solar cells deposited on MST substrates have a higher efficiency than those on single‐textured state‐of‐the‐art LPCVD ZnO substrates. Efficiencies of 14.8% (initial) and 12.5% (stable) have been achieved for a‐Si:H/nc‐Si:H tandem solar cells with the MST front electrode, surpassing efficiencies obtained on state‐of‐the‐art LPCVD ZnO, thereby highlighting the high potential of MST front electrodes for high‐efficiency multijunction solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

Design of Ag nanograting for broadband absorption enhancement in amorphous silicon thin film solar cells

Light trapping is one of the key issues to improve the light absorption and increase the efficiency of thin film solar cells. The effects of the triangular Ag nanograting on the absorption of amorphous silicon solar cells were investigated by a numerical simulation based on the finite element method. The light absorption under different angle and area of the grating has been calculated. Furthermore, the light absorption with different incident angle has been calculated. The optimization results show that the absorption of the solar cell with triangular Ag nanograting structure and anti-reflection film is enhanced up to 96% under AM1.5 illumination in the 300–800 nm wavelength range compared with the reference cell. The physical mechanisms of absorption enhancement in different wavelength range have been discussed. Furthermore, the solar cell with the Ag nanograting is much less sensitive to the angle of incident light. These results are promising for the design of amorphous silicon thin film solar cells with enhanced performance.     

Optical properties of thin‐film silicon solar cells with grating couplers

The effect of grating couplers on the optical properties of silicon thin‐film solar cells was studied by a comparison of experimental results with numerical simulations. The thin‐film solar cells studied are based on microcrystalline silicon (μc‐Si:H) absorber layers of thickness in the micrometer range. To investigate the light propagation in these cells, especially in the red wavelength region, three‐dimensional power loss profiles are simulated. The influence of different grating parametres—such as period size, groove height, and shape of the grating—was studied to gain more insight into the light propagation within thin‐film silicon solar cells and to determine an optimized light trapping scheme. The effect of the TCO front and TCO back side layer thickness was investigated. The calculated quantum efficiencies and short‐circuit current densities are in good agreement with the experimental data. The simulations predict further optimization criteria. Copyright © 2006 John Wiley & Sons, Ltd.     

Design principles for light trapping in thin silicon films with embedded dielectric nanoparticles

We investigate the light‐trapping effects of dielectric nanoparticles embedded within the active semiconductor layer of a thin‐film solar cell. The baseline model consists of a 1·0 µm slab of crystalline silicon on an aluminum back contact topped with a 75 nm Si₃N₄ anti‐reflective coating. Using finite‐difference time‐domain simulations, we calculate the absorption gain due to a periodic array of SiO₂ nanospheres with characteristic depth, diameter, and pitch. Under optimal conditions, absorption gain due to embedded spheres can reach as high as 23·4% relative to the baseline geometry. Using Au‐core/SiO₂‐shell nanoparticles, it is even possible to reach 30%. We then infer a series of design principles from our data that include trade‐offs between broadband scattering efficiency, poor absorption at long wavelengths, and semiconductor displacement. We also find that the optimal spacing between particles is approximately 400 nm. Above this distance, each scatterer acts in near isolation from any neighboring particles, and absorption gain is approximately linear with area coverage. Such gains are also expected for disordered as well as ordered arrays. These results demonstrate the potential of embedded dielectric nanoparticles as a tool for enhancing carrier generation in thin silicon solar cells. Copyright © 2011 John Wiley & Sons, Ltd.     

SHORT COMMUNICATION: ACCELERATED PUBLICATION: Multicrystalline silicon solar cells exceeding 20% efficiency

This paper presents the first conversion efficiency above 20% for a multicrystalline silicon solar cell. The application of wet oxidation for rear surface passivation significantly reduces the process temperature and therefore prevents the degradation of minority‐carrier lifetime. The excellent optical properties of the dielectrically passivated rear surface in combination with a plasma textured front surface result in a superior light trapping and allow the use of substrates below 100 μm thickness. A simplified process scheme with laser‐fired rear contacts leads to conversion efficiencies of 20·3% for multicrystalline and 21·2% for monocrystalline silicon solar cells on small device areas (1 cm²). Copyright © 2004 John Wiley & Sons, Ltd.     

Large-scale,adhesive-free and omnidirectional 3D nanocone anti-reflection films for high performance photovoltaics

An effective and low-cost front-side anti-reflection(AR) technique has long been sought to enhance the performance of highly efficient photovoltaic devices due to its capability of maximizing the light absorption in photovoltaic devices. In order to achieve high throughput fabrication of nanostructured flexible and anti-reflection films, large-scale, nano-engineered wafer molds were fabricated in this work. Additionally, to gain in-depth understanding of the optical and electrical performance enhancement with AR films on polycrystalline Si solar cells, both theoretical and experimental studies were performed. Intriguingly,the nanocone structures demonstrated an efficient light trapping effect which reduced the surface reflection of a solar cell by17.7% and therefore enhanced the overall electric output power of photovoltaic devices by 6% at normal light incidence. Notably, the output power improvement is even more significant at a larger light incident angle which is practically meaningful for daily operation of solar panels. The application of the developed AR films is not only limited to crystalline Si solar cells explored here, but also compatible with any types of photovoltaic technology for performance enhancement.     

Computer analysis of induced-inversion layer MOS solar cells

A computer analysis of induced inversion layer MOS solar cells is described. The analysis simultaneously solves Poisson's equation and the continuity equation in one dimension and provides a very effective method for solar cell evaluation. Numerical solutions of the carrier continuity equation in the inversion layer illustrate how cell designs may be improved in order to obtain higher short wavelength spectral response. Very shallow junctions (on the order of 0.07-0.1 μm) are shown to be optimum with higher electric fields in a direction to aid the collection of carriers generated by very high energy photons. The results also indicate that induceed inversion layer cells are less sensitive to surface recombination velocity variations than diffused p-n junction cells and have higher minority carrier lifetime. Furthermore, the effect of a p-p⁺ low-high junction on the back surface is examined and the results indicate that it is insignificant when the substrate doping concentration is optimized. High inversion layer sheet resistance values are evaluated and minimized with the contact diffusion used in the analysis designed to reduce the high inversion layer sheet resistance. Design improvements in cell performance are evaluated and identified with further improvement possible here. Conversion efficiency for silicon of 17.3% at AMO in the inversion layer solar cell is predicted assuming 95% transmission through the transparent conductor.     

Two‐dimensional simulation of thin‐film silicon solar cells with innovative device structures

We investigate the optical and electrical properties of thin‐film silicon solar cells by means of numerical simulations. The optical design under investigation is the encapsulated‐V texture, which is capable of absorbing sunlight corresponding to a maximum short‐circuit current density of 35 mA cm⁻². Because the layer thickness can be restricted to only 4 μm, the encapsulated‐V structure also provides a good collection efficiency for photogenerated charge carriers. The results for our simulations suggest that practical efficiencies above 12% can be expected for Si material with a minority carrier lifetime as low as 10 ns. Increased lifetimes of 100 ns allow for about 14% efficiency. The benefit of multiplejunctions within the device structure strongly depends on surface recombination. The efficiency of a single‐junction cell can be improved by more the 3% absolute with a multi‐junction device if the surface combination velocity is as high as 10⁵ cm s⁻¹. For moderate surface recombination, the gain is only 1%. Copyright © 1999 John Wiley & Sons, Ltd.