High-efficiency Silicon Solar Cells Research Articles

High-efficiency multi-crystalline black silicon solar cells achieved by additive assisted Ag-MACE

It is important for uniform texture on crystallites with different orientation of diamond wire sawn multi-crystalline silicon solar cells by Ag metal-assisted chemical etching method (Ag-MACE). In this paper, an additive was adopted to make the nanostructures on crystallites with different orientation of multi-crystalline silicon wafers with roughly the same depth and diameter. This additive contains biological enzymes, which are macromolecular proteins with specific catalytic functions. The uniform texture produced by the additive is demonstrated to be helpful in improving the cell performance in terms of surface morphology, reflectivity, effective minority carrier lifetime, auger recombination rate, internal quantum efficiency and external quantum efficiency measurements as well as electroluminescence spectra characterizations. The chemical mechanism of additive assisted etching reaction during the texturing process was discussed. Finally, mass-produced multi-crystalline silicon solar cells made by Ag-MACE with the additive achieve a maximum efficiency and average efficiency of 19.56%, and 19.24%, respectively, which were 0.65% and 0.64% absolutely higher than that without the additive. Moreover, the effective minority carrier lifetime and auger recombination rate of the former was nearly twice and a third of that of the latter respectively.

21.3%-efficient n-type silicon solar cell with a full area rear TiOx/LiF/Al electron-selective contact

In this work, we investigate an efficient electron-selective passivating contact with TiOx/LiF/Al contact structure, which offers both low surface recombination and specific contact resistance. Optimized TiOx layer thickness of 4 nm provides high quality surface passivation, achieving minority carrier lifetime of 3.03 ms on 5 Ω cm n-type wafers, with a saturated current density J0 of 23 fA/cm2. In addition, inserting a 1 nm LiF between the 4 nm TiOx and Al reduces the contact resistivity to 18 mΩ cm2. The low contact resistivity of TiOx/LiF/Al contact is attributed to barrier reduction from the low work function of LiF/Al stack. A champion solar cell efficiency of 21.3% has been achieved for an n-type crystalline silicon device with a full-area rear TiOx/LiF/Al contact, demonstrating the excellent potential of this passivating contact for fabricating high-efficiency silicon solar cells.

Analysis of edge losses on silicon heterojunction half solar cells

Halved and shingled solar cells are a powerful technology to reduce cell-to-module losses and ultimately increase the output power of a photovoltaic module. The combination of this approach with high-efficiency silicon solar cells architectures, like heterojunction or passivated contacts solar cells, is a promising option, but comes also with new challenges. In particular, the low level of carrier recombination in these cells makes them highly sensitive to any additional defects, as it is the case for the unpassivated edge created when splitting in half a solar cell. In order to optimize the cutting processes, an accurate method to characterize edge losses, adapted to high-efficiency solar cells, is required. In this work, two approaches are proposed and compared. In the first one, edge losses are modelled with a parallel diode, and the associated recombination current (J02edge) is fitted with current-voltage measurements, as usually done in previous studies. The conditions of application of this method are widely discussed and precisely redefined for an accurate extraction of parameters in the case of high-efficiency solar cells. The second method proposed consists in simply calculating relative losses out of the main current-voltage parameters. We demonstrate that simple estimation of open-circuit voltage (Voc) losses is perfectly adapted to our characterization needs, provided that adapted measurement methodology is applied. Results obtained with both methods are then compared on heterojunction cells and discussed. If these two characterization techniques both provide accurate results for the laser scribing processes tested, the method based on open-circuit voltage loss comparison shows slightly less dispersion. Moreover, this last method does not rely on curve fitting, making it faster and insensitive to modeling issues. The Voc approach appears thus more adapted for a fine optimization of different cutting processes and eventual characterization step implemented in an industrial environment. However, the alternative J02edge method based on curve fitting remains of high interest, for example to compare cell losses obtained with usual values reported in the literature, provided that extraction of this parameter is done with the conditions assessed in this work.

Anodic oxidations: Excellent process durability and surface passivation for high efficiency silicon solar cells

We investigate the versatility of anodically grown silicon dioxide (SiO2) films in the context of process durability and exceptional surface passivation for high efficiency (>23%) silicon solar cell architectures. We show that a room temperature anodic oxidation can achieve a thickness of ~70 nm within ~30 min, comparable to the growth rate of a thermal oxide at 1000 °C. We demonstrate that anodic SiO2 films can mask against wet chemical silicon etching and high temperature phosphorus diffusions, thereby permitting a low thermal budget method to form patterned structures. We investigate the saturation current density J0 of anodic SiO2/silicon nitride stacks on phosphorus diffused and undiffused silicon and show that a J0 of <10 fA cm−2 can be achieved in both cases. Finally, to showcase the anodic SiO2 films on a device level, we employed the anodic SiO2/silicon nitride stack to passivate the rear surface of an interdigitated back contact solar cell, achieving an efficiency of 23.8%.

High Efficiency Silicon Solar Cells

High Efficiency Silicon Solar Cells

High efficiency silicon solar cells with back ZnTe layer hosting IPV effect: a numerical case study

To get enhanced long-wavelength absorption, an impurity photovoltaic (IPV) mechanism was implemented within a transparent conducting oxide (TCO) at the rear of a solar cell. The numerical simulation of the N+/P (silicon)/IPV-TCO device was carried out by using SCAPS-1D program which allows the inclusion of optically active defects. In the proposed heterostructure configuration, ZnTe is a suitable material as back surface reflector. In analyzing the Si/ZnTe interface, lattice mismatch, energy band alignment and defects density were considered with appropriate treatment. In particular, to cure the detrimental 12% lattice mismatch at the interface, a thin silicon amorphous layer was inserted in-between, allowing 22.98% conversion efficiency. With adapted ZnTe Lucovsky's model for the optical capture cross sections and introduction of an oxygen radiative IPV defect (O2 level at 0.4 eV below the conduction band), a conversion efficiency of 27.15% was ultimately achieved. The experimental feasibility of the high-efficiency heterostructure device is evaluated.

Optical properties of a random inverted pyramid textured silicon surface studied by the ray tracing method

The random inverted pyramid texture has been extensively studied experimentally in high-efficiency monocrystalline silicon solar cells due to its superior optical properties. In this paper, a random inverted pyramid structure model was established, and the optical performance was studied by the ray tracing method. It has five common light paths and can achieve lower reflectance than random upright pyramids. To further analyze the optical properties of random inverted pyramids, a simplified random inverted pyramid model, which consists of two overlapping inverted pyramids, was studied and analyzed in detail. The proportions of the light path related to different overlapping regions, and the reflectance ranging from 10.11% to 10.64%, can be obtained, which is lower than the reflectivity of random upright pyramids. We believe that the random inverted pyramid texture can have a wide range of applications in high-efficiency monocrystalline silicon solar cells.

Activating and optimizing evaporation-processed magnesium oxide passivating contact for silicon solar cells

Irrespective of the success on reduction of contact resistivity, lack of chemical passivation of evaporated metal oxides heavily hinders their applications as passivating contacts, such contacts can be an alternative route for high efficiency and cost effective silicon solar cells. Here, we demonstrate that electron beam evaporated magnesium oxide (MgOx) thin film can work as a promising electron-selective passivating contact for n-Si solar cells after a post-annealing treatment and an alumina-initiated atomic hydrogenation. 10 nm MgOx on n-Si provided a surface recombination velocity down to 14.9 cm/s while 1 nm MgOx showed a low contact resistivity of 14 mΩ cm2. Comprehensive characterizations revealed the formation of Si–O–Mg bonds and the activation of atomic hydrogens were the main reasons for such high-level passivation. A PERC-like dopant-free rear contact was formed by using the 1 nm-MgOx as electron-collector and the 10 nm-MgOx as passivating layer, the resultant solar cells achieved 27% increment in efficiency and 51 mV increase in open-circuit voltage in comparison with reference devices. The ways of improving passivation quality of MgOx and novel design of contact structure open up the possibility of using evaporation-processed metal oxides as effective and low-cost carrier-selective passivating contacts for n-Si photovoltaic devices.

Dual-Function Electron-Conductive, Hole-Blocking Titanium Nitride Contacts for Efficient Silicon Solar Cells

Summary High-performance passivating contact is a prerequisite for high-efficiency crystalline silicon (c-Si) solar cells. In this work, an electron-conductive, hole-blocking contact based on titanium nitride (TiN) deposited by reactive magnetron sputtering is presented. Quasi-metallic TiN combined with an ultrathin SiO2 passivation layer (SiO2/TiN) is demonstrated to be an effective electron-selective contact on c-Si, featuring a low-contact resistivity of 16.4 mΩ.cm2 and a tolerable recombination current parameter of ∼500 fA/cm2. By implementing the dual-function SiO2/TiN contact, which acts simultaneously as a surface passivating layer and metal electrode, an efficiency of 20% is achieved by an n-type c-Si solar cell with a simple structure. This work not only demonstrates a way to develop efficient n-type c-Si solar cells with dual-function metal nitride contacts at a low cost but also expands the pool of available carrier transport materials, from metal oxides to metal nitrides, for photovoltaic devices.

Computational Modeling of Polycrystalline Silicon on Oxide Passivating Contact for Silicon Solar Cells

Polycrystalline silicon on oxide (POLO) junction passivating contacts have recently been demonstrated as carrier selective contacts for high-efficiency silicon solar cells. The carrier transport through these contacts has been attributed to two competing mechanisms: 1) carrier tunneling through ultrathin oxide and 2) transport through weak spots (pinholes) – the nanoscale regions where oxide thickness has been completely or partially compromised during the processing. In this paper, we use two dimensional device simulations to compare the relative effects of these mechanisms on solar cell characteristics with n-type POLO contact. We show that variation in pinhole areal density ( $ {D}_{\text {ph}}$ ) or the tunnel oxide thickness ( $ {t}_{\text {ox}}$ ) both result in qualitatively similar trends in the cell characteristics under dark and light. For a given $ {t}_{\text {ox}}$ , an exponential variation in $ {D}_{\text {ph}}$ results in trends that are similar to those for a linear variation in $ {t}_{\text {ox}}$ . The effect of pinholes on contact resistance ( $\rho _{\text {c}}$ ) and saturation current density ( ${J} _{\text {o}}$ ) is most significant for relatively thicker oxides (≥ 2 nm). For $ {t}_{\text {ox}}~\leq1$ nm, $\rho _{\text {c}}$ and ${J} _{\text {o}}$ become essentially insensitive to pinholes for $ {D}_{\text {ph}}\,\, . The modeling results are compared with a set of published experimental data to predict the possible roles of various mechanisms.

Silicon surface passivation by transparent conductive zinc oxide

Surface passivation is essential for high-efficiency crystalline silicon (c-Si) solar cells. Despite the common use of transparent conductive oxides (TCOs) in the field of solar cells, obtaining surface passivation by TCOs has thus far proven to be particularly challenging. In this work, we demonstrate outstanding passivation of c-Si surfaces by highly transparent conductive ZnO films prepared by atomic layer deposition. Effective surface recombination velocities as low as 4.8 cm/s and 11 cm/s are obtained on 3 Ω cm n- and p-type (100) c-Si, respectively. The high levels of surface passivation are achieved by a novel approach by using (i) an ultrathin SiO2 interface layer between ZnO and c-Si, (ii) a sacrificial Al2O3 capping layer on top of the ZnO film during forming gas annealing, and (iii) the extrinsic doping of the ZnO film by Al, B, or H. A combination of isotope labeling, secondary-ion mass spectrometry, and thermal effusion measurements showed that the sacrificial Al2O3 capping layer prevents the effusion of hydrogen from the crystalline ZnO and the underlying Si/SiO2 interface during annealing, which is critical in achieving surface passivation. After annealing, the Al2O3 capping layer can be removed from the ZnO film without impairing the high levels of surface passivation. The surface passivation levels increase with increased doping levels in ZnO, which can be attributed to field-effect passivation by a reduction in the surface hole concentration. The ZnO films of this work are suitable as a transparent conductor, an anti-reflection coating, and a surface passivation layer, which makes them particularly promising for simplifications in future solar cell manufacturing.

Understanding the charge transport mechanisms through ultrathin SiOx layers in passivated contacts for high-efficiency silicon solar cells

We report on the microscopic structure of the SiOx layer and the transport mechanism in polycrystalline Si (poly-Si) passivated contacts, which enable high-efficiency crystalline Si (c-Si) solar cells. Using electron beam induced current (EBIC) measurements, we accurately map nanoscale conduction-enabling pinholes in 2.2 nm thick SiOx layers in a poly-Si/SiOx/c-Si stack. These conduction enabling pinholes appear as bright spots in EBIC maps due to carrier transport and collection limitations introduced by the insulating 2.2 nm SiOx layer. Performing high-resolution transmission electron microscopy at a bright spot identified with EBIC reveals that conduction pinholes in SiOx can be regions of thin tunneling SiOx rather than a geometric pinhole. Additionally, selectively etching the underlying poly-Si layer in contacts with 1.5 and 2.2 nm thick SiOx layers using tetramethylammonium hydroxide results in pinhole-like etch features in both contacts. However, EBIC measurements for a contact with a thinner, 1.5 nm SiOx layer do not reveal pinholes, which is consistent with uniform tunneling transport through the 1.5 nm SiOx layer. Finally, we theoretically show that reducing the metal to the c-Si contact size from microns, like in the p-type passivated emitter rear contact, to tens of nanometers, like in poly-Si contacts, allows lowering of the unpassivated contact area by several orders of magnitude, thus resulting in excellent passivation, as has been demonstrated for these contacts.

Photovoltaics literature survey (no. 149)

Photovoltaics literature survey (no. 149)

Application of SiO2 passivation technique in mass production of silicon solar cells

Effective surface passivation is one of the primary prerequisites for high-efficiency silicon solar cells. In this paper, high-quality silicon dioxide (SiO2) films with excellent surface passivation abilities have been realized by thermal oxidation and plasma-enhanced chemical vapor deposition, respectively. By employing SiO2 capped with hydrogenated silicon nitride (SiNx:H) (SiO2/SiNx:H stacks) as the front passivation layers, the emitter saturation current density has been reduced from 90.5 fA/cm2 to 62.3 fA/cm2. We have successfully mass-produced thermal-oxidized p-type Czochralski-Si (CZ-Si) solar cells with high conversion efficiency (η) of 20.1%, which is 0.2% absolutely higher than that of the conventional Al back surface field (Al-BSF) solar cells. With a rational design of process integration, we have further presented a cost-effective way to fabricate high-efficiency SiO2 passivated emitter and rear cells (PERCs) at an existing production line. The introduction of SiO2/SiNx:H stacks on the rear surface can effectively increase the long wavelength response and the rear surface recombination is also suppressed with a rather low surface recombination velocity of 26 cm/s achieved after a post-annealing process. The industrial SiO2 passivated p-type CZ-Si PERCs possess outstanding performances with the average η of 21.3% and the highest η of over 21.9%, absolute 1.3% increment in average η compared with the conventional Al-BSF solar cells. Moreover, we have demonstrated that the relative low illumination response (below 50 W/m2) loss of our SiO2 PERCs over a day is extremely limited to be less than 0.2%.

Scalable Techniques for Producing Field-Effect Passivation in High-Efficiency Silicon Solar Cells

This paper presents techniques to tailor and optimize the field-effect passivation of silicon surfaces using the deposition and field-assisted migration of potassium ions. While field-effect passivation of this nature has previously been demonstrated using laboratory scale techniques, in this paper, it is shown how it can be realized using fast and scalable ion deposition and migration methods. Together, the deposition and migration processes are seen to produce excellent improvements to the surface passivation of oxidized 1-Ω·cm n-type float-zone silicon. Effective lifetimes as high as 2.1 ms are observed, in the best case yielding a surface recombination velocity as low as 3.3 cm/s with a corresponding surface dark saturation current density of 8.4 fA/cm2.

Role of polysilicon in poly-Si/SiO x passivating contacts for high-efficiency silicon solar cells.

In this study, we focused on understanding the roles of a polysilicon (poly-Si) layer in poly-Si/SiOx/c-Si passivating contacts. Passivating contact formation conditions were varied by changing the doping method, annealing temperature and time, polysilicon layer thickness, and polysilicon doping concentration. Our observations indicated that the roles of polysilicon are contact, in-diffusion barrier action, field effect, gettering, and light absorption. Based on the observations, a iVOC of 741 mV was obtained. Finally, to increase JSC with high VOC, the polysilicon was etched after hydrogenation to reduce light absorption with high passivation quality. iVOC was not affected by etching; moreover, by etching the polysilicon from 300 nm to 60 nm, the cell efficiency increased from 20.48% to 20.59% with increasing JSC, constant VOC, and fill factor.

Numerical exploration for structure design and free-energy loss analysis of the high-efficiency polysilicon passivated-contact p-type silicon solar cell

In this work, the application of the p-type and n-type polysilicon passivated contact on industrial-level p-type silicon solar cell is studied using numerical simulation. The effects of (i) the structure design, (ii) the bulk lifetime and resistivity of the p-type wafer, and (iii) the carrier selectivity of polysilicon passivated contact on cell performances are investigated. Furthermore, the corresponding energy-loss pathways are classified by using free energy loss analysis (FELA). In essence, the rear-junction solar cell with the n-type polysilicon passivated-contact generates more internal power because of the better surface passivation and less front metallization shading, but the efficiency potential is limited by the low lifetime of the state-of-the-art p-type Czochralski (Cz) wafer. Thus, the p-type polysilicon passivated contact serving as the back-surface field would be more favorable if the lifetime of the p-type Cz silicon were less than 350 μs. Over the long term, the lifetime of the p-type wafer possibly becomes the bottleneck of the high-efficiency polysilicon passivated-contact solar cells. Finally, we present the roadmap toward the 23% industrial p-type silicon solar cell with the p-type or n-type polysilicon passivated contact.

Versatile Hole Carrier Selective MoOx Contact for High Efficiency Silicon Heterojunction Solar Cells: A Review

Excellent surface passivation and carrier selective contact formed by the metal oxide induced junctions is required for future high efficiency silicon solar cells. Due to wide optical bandgap and high work function of molybdenum oxide (MoOx, x < 3) films envisioned as a superior hole selective layer in organic light emitting diodes and photovoltaics applications. We have studied the influence of ultrathin MoOx layer, as a hole-selective contact for high efficiency of silicon heterojunction (SHJ) solar cell. MoOx films can be deposited by atomic layer deposition, magnetron sputtering and thermal evaporation. Due to higher work function of MoOx films, a potential barrier can develop against electrons while it supports the hole carriers flow hence current density of SHJ solar cells can be enhanced. A summary of single layer and solar cell characteristics of MoOx layer for the application of carrier selective contact and dopant-free asymmetric heterocontact (DASH) solar cells is reported.

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.

Thermal stability of Al2O3/TiO2 stacks for boron emitter passivation on n-type silicon solar cells

Boron-diffused samples passivated by Al2O3/TiO2 stacks with different types of TiO2 were investigated. An emitter saturation current density of 11 × 10−15 A/cm2 was obtained for Al2O3/TiO2 passivated samples with symmetrical 100 Ω/sq. boron diffusion. The surface passivation of Al2O3/TiO2 stacks was strongly impacted by the temperature, gas, and sequence during two-step annealing. In addition, the saturation current density of the Al2O3/TiO2 stacks is influenced by the TiO2 layers with an O3 treatment. Enhanced passivation performance of the Al2O3/TiO2 stacks was achieved by a combination of appropriate TiO2 capping layer and post-annealing. The results of this study provide reliable criteria for the design of thermally-stable Al2O3/TiO2 stacks on high-efficiency silicon solar cells.