High-efficiency Silicon Solar Cells Research Articles

High-mobility Hydrogenated Indium Oxide without Introducing Water During Sputtering

The key role of water to obtain high-mobility IO:H (hydrogenated indium oxide) layers has been well documented, but introducing the required tiny amount of water is technologically challenging. We first use simulations to evidence the key role of high mobility for the transparent conductive oxide for high-efficiency crystalline silicon solar cells. Then, we investigate an approach to fabricate high-mobility IO:H that circumvent the introduction of water vapor, relying on water vapor from ambient air. A sputtering tool equipped with a residual gas analyzer allows partial pressure monitoring of hydrogen and water in the system, and to link the gas composition to the properties of the deposited films. To vary the residual water pressure, we varied the pumping time after opening the chamber and before starting the deposition to reach different base pressures (1. 10-5 mbar to 3. 10-7 mbar), which are mostly composed of residual water. An optimum base pressure around 3. 10-6 mbar—and not lower pressures—was found to yield the highest mobility values after annealing. An alternative approach by introducing a small flow of hydrogen together with argon and oxygen is also shown to provide promising results.

Development of n-type Selective Emitter Silicon Solar Cells by Laser Doping Using Boron-doped Silicon Paste

N-type solar cells are being developed as next generation photovoltaic solar cells, and have attracted a significant amount of attention. In addition, the demand for high-efficiency silicon solar cells has increased in order to reduce production costs and save space. In this study, we demonstrate boron laser doping (LD) using a boron-doped NanoGram® Si paste in n-type passivated emitter, rear totally diffused solar cells. The sheet resistance was 54 Ω/sq at the local boron emitter after boron LD. The boron diffusion depth was ∼2.0 μm, and the boron surface concentration was 7 × 1019 atoms/cm3. The n-type solar cells were fabricated using boron LD to have front side boron selective emitters. The characteristics of the newly developed solar cells improved in comparison with those of the reference cells (i.e., those without boron selective emitters), and a maximum efficiency of 19.70% was achieved. The improved efficiency was mainly due to the increase in the short circuit current density and fill factor.

Three-Dimensional TCAD Modeling of Grain Boundaries in High-Efficiency Silicon Solar Cells

Grain boundaries degrade the performance of solar cells based on polycrystalline materials; yet, device models rarely accurately account for all their effects or their spatially localized nature. Here, we use 3-D technology computer-aided design (TCAD) simulations to determine the effect of grain boundaries on the performance of silicon solar cells. We find that grain boundary-limited performance of multicrystalline silicon can approach that of monocrystalline silicon in high-efficiency devices. We identify higher carrier injection as a design feature that improves defect tolerance by reducing grain boundary charging, shrinking the depletion region surrounding the grain boundary.

Simulation of the real efficiencies of high-efficiency silicon solar cells

The temperature dependences of the efficiency η of high-efficiency solar cells based on silicon are calculated. It is shown that the temperature coefficient of decreasing η with increasing temperature decreases as the surface recombination rate decreases. The photoconversion efficiency of high-efficiency silicon-based solar cells operating under natural (field) conditions is simulated. Their operating temperature is determined self-consistently by simultaneously solving the photocurrent, photovoltage, and energy-balance equations. Radiative and convective cooling mechanisms are taken into account. It is shown that the operating temperature of solar cells is higher than the ambient temperature even at very high convection coefficients (~300 W/m2 K). Accordingly, the photoconversion efficiency in this case is lower than when the temperature of the solar cells is equal to the ambient temperature. The calculated dependences for the open-circuit voltage and the photoconversion efficiency of high-quality silicon solar cells under concentrated illumination are discussed taking into account the actual temperature of the solar cells.

Polycrystalline silicon passivated tunneling contacts for high efficiency silicon solar cells

Abstract

Silicon heterojunction solar cells with electron selective TiOx contact

Silicon solar cells featuring carrier selective contacts have been demonstrated to reach ultra-high conversion efficiency. In this work, the electron-selective contact characteristics of ultrathin TiOx films deposited by atomic layer deposition on silicon are investigated via simultaneous consideration of the surface passivation quality and the contact resistivity. Thin TiOx films are demonstrated to provide not only good passivation to silicon surfaces, but also allow a relative low contact resistivity at the TiOx/Si heterojunction. A maximum implied open-circuit voltage (iVoc) of ~703mV is achieved with the passivation of a 4.5nm TiOx film, and a relatively low contact resistivity of (~0.25Ωcm2 is obtained at the TiOx/n-Si heterojunction simultaneously. N-type silicon solar cell with the champion efficiency of 20.5% is achieved by the implementation of a full-area electron-selective TiOx contacts. A simulated efficiency of up to 23.7% is achieved on the n-type solar cell with a full-area TiOx contact. The efficient, low cost electron-transporting/hole-blocking TiOx layer enables the fabrication of high efficiency silicon solar cells with a simplified process flow.

A large-volume manufacturing of multi-crystalline silicon solar cells with 18.8% efficiency incorporating practical advanced technologies

The optimization processes for the mass-production of high-efficiency multi-crystalline silicon solar cells have been observed in this paper.

Transparent Electrodes in Silicon Heterojunction Solar Cells: Influence on Contact Passivation

Charge carrier collection in silicon heterojunction solar cells occurs via intrinsic/doped hydrogenated amorphous silicon layer stacks deposited on the crystalline silicon wafer surfaces. Usually, both the electron and hole collecting stacks are externally capped by an n-type transparent conductive oxide, which is primarily needed for carrier extraction. Earlier, it has been demonstrated that the mere presence of such oxides can affect the carrier recombination in the crystalline silicon absorber. Here, we present a detailed investigation of the impact of this phenomenon on both the electron and hole collecting sides, including its consequences for the operating voltages of silicon heterojunction solar cells. Based on our findings, we define guiding principles for improved passivating contact design for high-efficiency silicon solar cells.

Tunnel oxide passivated contacts formed by ion implantation for applications in silicon solar cells

Passivated contacts (poly-Si/SiOx/c-Si) doped by shallow ion implantation are an appealing technology for high efficiency silicon solar cells, especially for interdigitated back contact (IBC) solar cells where a masked ion implantation facilitates their fabrication. This paper presents a study on tunnel oxide passivated contacts formed by low-energy ion implantation into amorphous silicon (a-Si) layers and examines the influence of the ion species (P, B, or BF2), the ion implantation dose (5 × 1014 cm−2 to 1 × 1016 cm−2), and the subsequent high-temperature anneal (800 °C or 900 °C) on the passivation quality and junction characteristics using double-sided contacted silicon solar cells. Excellent passivation quality is achieved for n-type passivated contacts by P implantations into either intrinsic (undoped) or in-situ B-doped a-Si layers with implied open-circuit voltages (iVoc) of 725 and 720 mV, respectively. For p-type passivated contacts, BF2 implantations into intrinsic a-Si yield well passivated contacts and allow for iVoc of 690 mV, whereas implanted B gives poor passivation with iVoc of only 640 mV. While solar cells featuring in-situ B-doped selective hole contacts and selective electron contacts with P implanted into intrinsic a-Si layers achieved Voc of 690 mV and fill factor (FF) of 79.1%, selective hole contacts realized by BF2 implantation into intrinsic a-Si suffer from drastically reduced FF which is caused by a non-Ohmic Schottky contact. Finally, implanting P into in-situ B-doped a-Si layers for the purpose of overcompensation (counterdoping) allowed for solar cells with Voc of 680 mV and FF of 80.4%, providing a simplified and promising fabrication process for IBC solar cells featuring passivated contacts.

Investigation of different contact geometries for partial rear metal contact of high-efficiency silicon solar cells

In this paper, we have investigated the dependence of the parameters of partial rear metal contact (PRC) solar cells on two commonly used geometrical configurations, viz. grid contact and square contact. It is demonstrated that while the geometry of rear metallic contact having the same fractional coverage changes the base spreading resistance significantly, rear surface passivation and back reflectance depend only on the fractional coverage of the real metal contact and are independent of the geometry of the contact. The performed analysis indicates that the base spreading resistance is much higher for square contact cells compared to that of grid contact cells having the same fractional coverage. While open-circuit voltage and short-circuit current are found to be essentially independent of the contact geometry, the fill factor is significantly affected by the geometry, indicating that the design of high-efficiency cells with partial rear grid contact is less critical in comparison to that of square contact cells for optimized performance. Results indicate that for a 180 μm cell, an efficiency enhancement of 14% is possible for <10% fractional rear metallization over the baseline efficiency of about 19% for solar cells having full-area rear metallization. For 50 μm thin cells, the corresponding improvement in efficiency is 17%, leading to 22% efficiency solar cells.

Silicon nitride/silicon oxide interlayers for solar cell passivating contacts based on PECVD amorphous silicon

This Letter demonstrates improved passivating contacts for silicon solar cells consisting of doped silicon films together with tunnelling dielectric layers. An improvement is demonstrated by replacing the commonly used silicon oxide interfacial layer with a silicon nitride/silicon oxide double interfacial layer. The paper describes the optimization of such contacts, including doping of a PECVD intrinsic a‐Si:H film by means of a thermal POCl3 diffusion process and an exploration of the effect of the refractive index of the SiNx. The n+ silicon passivating contact with SiNx /SiOx double layer achieves a better result than a single SiNx or SiOx layer, giving a recombination current parameter of ∼7 fA/cm2 and a contact resistivity of ∼0.005 Ω cm2, respectively. These self‐passivating electron‐selective contacts open the way to high efficiency silicon solar cells. (© 2015 WILEY‐VCH Verlag GmbH &amp;Co. KGaA, Weinheim)

Cover Picture: Progress in Low Cost and High Efficiency Silicon and Inorganic Semiconductor Solar Cells (Isr. J. Chem. 10/2015)

Cover Picture: Progress in Low Cost and High Efficiency Silicon and Inorganic Semiconductor Solar Cells (Isr. J. Chem. 10/2015)

Progress in Low Cost and High Efficiency Silicon and Inorganic Semiconductor Solar Cells

Progress in Low Cost and High Efficiency Silicon and Inorganic Semiconductor Solar Cells

Improving the Short-Wavelength Spectral Response of Silicon Solar Cells by Spray Deposition of YVO4 :Eu3+ Downshifting Phosphor Nanoparticles

Europium-doped yttrium vanadate downshifting phosphor nanoparticles (NPs) have been coated on top of monocrystalline silicon solar cells, having efficiency more than 15%, by a spray deposition technique. The effects of phosphor NPs on solar cells with antireflection coating (ARC) have been studied. The optimized quantity of phosphor NPs provides a photocurrent enhancement of ~2.1% for monosilicon solar cells. External quantum efficiency data of high-efficiency ARC layer-coated silicon solar cells conclusively show that the enhancement in short-wavelength spectral response is mainly due to downshifting effects of the phosphor NPs. A small increment in a long-wavelength spectral response is also observed due to the scattering effects of phosphor NPs, which also results in a small enhancement of effective diffusion length of minority carriers in the base region.

Combining randomly textured surfaces and one-dimensional photonic crystals as efficient light-trapping structures in hydrogenated amorphous silicon solar cells

One of the foremost challenges in achieving high-efficiency thin-film silicon solar cells is in devising an efficient light trapping system because of the short optical path length imposed by the inherent thin absorption layers. In this paper, an efficient light trapping system is proposed using a combination of randomly textured surfaces and a one-dimensional photonic crystal (randomly textured photonic crystal; RTPC). The influence of the texture on the optical performance of RTPCs is discussed using the results of an experiment and a finite-difference time-domain simulation. This RTPC back reflector (BR) can provide high reflectivity and strong light scattering, resulting in an increased photocurrent density of the hydrogenated amorphous silicon (a-Si:H) solar cell. As a result, the highly textured RTPC BR yielded an efficiency of 9.6% for a-Si:H solar cell, which is much higher than the efficiency of 7.6% on flat AZO/Ag BR and 9.0% on textured AZO/Ag BR. This RTPC BR provides a new approach for creating high-efficiency, low-cost thin-film silicon solar cells.

Multifunctional ICP-PECVD Silicon Nitride Layers for High-efficiency Silicon Solar Cell Applications

Silicon nitride layers can be deposited by a variety of methods leading to different physical characteristics, such as optical, electrical and mechanical properties. In photovoltaic applications highly hydrogenated (>20% H concentration) and transparent layers are necessary to passivate the surface (Seff°<20°cm/s) while allowing for optimal internal quantum efficiency. Secondary properties, such as a low pinhole density (<100/mm2) and various barrier-functions for even very thin films (<20°nm) are useful in more complex solar cell processes. This work investigates high deposition rate (2-7°nm/s) PECVD SiNx:H layers from inductively-coupled plasma (ICP) technology. The ICP-technology is capable of creating a variety of such layers, or even only one layer, with all these properties for cost reduction of high-efficiency silicon solar cell processes.

Passivating boron silicate glasses for co-diffused high-efficiency n-type silicon solar cell application

Doping layers commonly have but one function: supplying the dopants to form a doped region within a substrate. This work presents B doping layers/stacks, which at the same time supply dopant atoms, passivate the B-doped crystalline Si surface sufficiently well (j0E &amp;lt; 50 fA/cm2), and show optical properties suitable for anti-reflective coating. Furthermore, these boron silicate glasses can act as a barrier against parasitic P in-diffusion during a co-diffusion step. The boron emitters diffused from the inductively coupled plasma plasma-enhanced chemical vapor-deposited B containing SiOx layers are investigated and optimized concerning passivation quality and contact properties for high-efficiency n-type solar Si cell designs. It is shown that even 10 nm thin SiOx:B films already allow for suitable emitter sheet resistance for screen-printed contacts. Furthermore, SiOx:B layers presented here allow for iVOC values of 675 mV and contact resistivity of 1 mΩcm2 for commercial Ag instead of Ag/Al pastes on the diffused boron emitter passivated with the SiOx:B layer supporting the contact formation. All of these properties can be achieved within one single B doping layer/stack.

Towards the efficiency limits of silicon solar cells: How thin is too thin?

It is currently possible to fabricate crystalline silicon solar cells with the absorber thickness ranging from a few hundreds of micrometres (conventional wafer-based cells) to devices as thin as 1μm. In this work, we use a model single-junction solar cell to calculate the limits of energy conversion efficiency and estimate the optimal absorber thickness. We have found that the limiting efficiency for cells in the thickness range between 40 and 500μm is very similar and close to 29%. In this regard, we argue that decreasing the thickness below around 40μm is counter-productive, as it significantly reduces the maximum achievable efficiency, even when an optimal light trapping is implemented. We analyse the roles of incomplete light trapping and extrinsic (bulk and surface) recombination mechanisms. For a reasonably high material quality, consistent with present-day fabrication techniques, the optimal thickness is always higher than a few tens of micrometres. We identify incomplete light trapping and parasitic losses as a major roadblock for improving the efficiency upon the current record of 25.6% for silicon solar cells. Finally, considering the main parameters that impact the solar cell performance, we quantify the requirements for achieving a given efficiency, which helps us to establish a proper design strategy for high efficiency silicon solar cells.

High-efficiency thin-film silicon solar cells realized by integrating stable a-Si:H absorbers into improved device design

We report that thin-film silicon solar cells exhibiting high stabilized efficiencies can be obtained by depositing hydrogenated amorphous silicon (a-Si:H) absorbers using triode-type plasma-enhanced chemical vapor deposition. The improved light-soaking stability and performance of solar cells are also realized by optimizing the device design, such as p and p–i buffer layers. As a result, we attain independently confirmed stabilized efficiencies of 10.1–10.2% for a-Si:H single-junction solar cells (absorber thickness: ti = 220–310 nm) and 12.69% for an a-Si:H (ti = 350 nm)/hydrogenated microcrystalline silicon (µc-Si:H) tandem solar cell fabricated using textured SnO2 and ZnO substrates, respectively. The relative efficiency degradations of these solar cells are ∼10 and 3%, respectively, under 1 sun illumination at 50 °C for 1000 h.

Surface passivation of crystalline silicon by sputtered AlOx/AlNx stacks toward low-cost high-efficiency silicon solar cells

Recently, excellent surface passivation has been achieved for both p- and n-type silicon solar cells using AlOx/SiNx:H stacks deposited by atomic layer deposition and plasma-enhanced chemical vapor deposition. However, alternative materials and deposition methods could provide practical options for large-scale manufacturing of commercial solar cells. In this study we demonstrate that AlOx/AlNx stacks fabricated by reactive radio-frequency magnetron sputtering can provide fairly good surface passivation (Smax of ∼30 cm/s) regardless of AlOx thickness, which is found to be due to the high negative fixed charge density (Qeff of −2.8 × 1012 cm−2) and moderately low interface trap density (Dit of 2.0 × 1011 eV−1·cm−2). The stacks also show fairly good antireflection performance in the visible and near-infrared spectral region. The demonstrated surface passivation and antireflection performance of in situ reactively sputtered AlOx/AlNx stacks make them a promising candidate for a surface-passivating antireflection coating on silicon solar cells.