Passivating Contacts Research Articles

Potential high efficiency of GaAs solar cell with heterojunction carrier selective contact layers

Advanced configurations of gallium arsenide (GaAs) solar cells are proposed, using hydrogenated amorphous silicon (a-Si: H) and zinc oxide (ZnO) layers as passivation and carrier selective contacts (CSC), as alternatives to the conventional epitaxial GaInP CSC layers. The cell operation is simulated based on the AFORS-HET simulation program. The results show that with a wide-gap ZnO window layer, single-junction GaAs solar cells can reach a higher short-circuit current density (Jsc) and open-circuit voltage (Voc) than that obtained with conventional GaInP CSC layers without affecting the fill factor (FF). Notably, wide-gap and n-type doping ZnO layers show great potential as electron selective contacts in GaAs solar cells with high barriers at the valence band for blocking hole-transport. An efficiency of 30% is reached for a single-junction GaAs cell with a front ZnO electron selective contact. A 4-terminal GaAs/c-Si tandem solar cell configuration with an estimated conversion performance of 35.16% is proposed. These results indicate enormous potential for the development of low-cost and high-efficiency GaAs-based solar cells in the future.

P‐Type Dopants As Dual Function Interfacial Layer for Efficient and Stable Tin Perovskite Solar Cells

Interfacial engineering plays a key role for the stability and efficiency of perovskite photovoltaics, especially for the tin perovskite solar cells (TPSCs). Herein, a simple and effective interfacial layer approach to modify the heterointerface between hole transport layer (HTL) and perovskite active layer is reported. By the deposition of a p‐type dopant, tetrafluoro‐tetracyanoquinodimethane (F4TCNQ) onto the commonly used poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS) HTL, a favorable energy level alignment with the mixed‐cation tin perovskite is obtained. Moreover, the F4TCNQ interfacial layer introduces a passivated contact and suppressed trap density at the HTL/perovskite interface through halogen bonding. As a result, the optimized TPSC yields an improved efficiency of 8.11% as compared with 6.41% of the reference device. Meanwhile, the stability of the TPSC is also improved due to the hydrophobicity of the F4TCNQ interfacial layer. This work demonstrates that the incorporation of an interfacial layer at the HTL/perovskite interface is a feasible approach to boost the efficiency and stability of inverted TPSC.

Dopant activity for highly in-situ doped polycrystalline silicon: hall, XRD, scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM)

Progressing miniaturization and the development of semiconductor integrated devices ask for advanced characterizations of the different device components with ever-increasing accuracy. Particularly in highly doped layers, a fine control of local conduction is essential to minimize access resistances and optimize integrated devices. For this, electrical Atomic Force Microscopy (AFM) are useful tools to examine the local properties at nanometric scale, for the fundamental understanding of the layer conductivity, process optimization during the device fabrication and reliability issues. By using Scanning Capacitance Microscopy (SCM) and Scanning Spreading Resistance Microscopy (SSRM), we investigate a highly in situ doped polycrystalline silicon layer, a material where the electrical transport properties are well known. This film is deposited on a oxide layer as a passivating contact. The study of the nano-MIS (SCM) and nano-Schottky (SSRM) contacts allows to determine the distribution and homogeneity of the carrier concentration (active dopants), especially by investigating the redistribution of the dopants after an annealing step used for their activation. While the chemical analysis by Secondary Ions Mass Spectroscopy (SIMS) quantifies only the dopant concentration in the polycrystalline layer, the comparison with macroscopic characterization techniques as Hall effect measurements, supported with XRD characterization, shows that careful SCM and SSRM measurements can be used to highlight the dopant activation. This analysis gives a complete investigation of the local electrical properties of the passivating contact when the parameters (applied voltages and applied forces) of the AFM nano-contacts are correctly controlled.

Quantum Transport across Amorphous-Crystalline Interfaces in Tunnel Oxide Passivated Contact Solar Cells: Direct versus Defect-Assisted TunnelingSupported by the National Natural Science Foundation of China (Grant Nos. 61704083 and 61874060), the Natural Science Foundation of Jiangsu Province (Grant No. BK20181388), and NUPTSF (Grant No. NY219030).

Tunnel oxide passivated contact solar cells have evolved into one of the most promising silicon solar cell concepts of the past decade, achieving a record efficiency of 25%. We study the transport mechanisms of realistic tunnel oxide structures, as encountered in tunnel oxide passivating contact (TOPCon) solar cells. Tunneling transport is affected by various factors, including oxide layer thickness, hydrogen passivation, and oxygen vacancies. When the thickness of the tunnel oxide layer increases, a faster decline of conductivity is obtained computationally than that observed experimentally. Direct tunneling seems not to explain the transport characteristics of tunnel oxide contacts. Indeed, it can be shown that recombination of multiple oxygen defects in a-SiO x can generate atomic silicon nanowires in the tunnel layer. Accordingly, new and energetically favorable transmission channels are generated, which dramatically increase the total current, and could provide an explanation for our experimental results. Our work proves that hydrogenated silicon oxide (SiO x :H) facilitates high-quality passivation, and features good electrical conductivity, making it a promising hydrogenation material for TOPCon solar cells. By carefully selecting the experimental conditions for tuning the SiO x :H layer, we anticipate the simultaneous achievement of high open-circuit voltage and low contact resistance.

Passivation Properties and Formation Mechanism of Amorphous Halide Perovskite Thin Films

AbstractLead halide perovskites are among the most exciting classes of optoelectronic materials due to their unique ability to form high‐quality crystals with tunable bandgaps in the visible and near‐infrared using simple solution precipitation reactions. This facile crystallization is driven by their ionic nature; just as with other salts, it is challenging to form amorphous halide perovskites, particularly in thin‐film form where they can most easily be studied. Here, rapid desolvation promoted by the addition of acetate precursors is shown as a general method for making amorphous lead halide perovskite films with a wide variety of compositions, including those using common organic cations (methylammonium and formamidinium) and anions (bromide and iodide). By controlling the amount of acetate, it is possible to tune from fully crystalline to fully amorphous films, with an interesting intermediate state consisting of crystalline islands embedded in an amorphous matrix. The amorphous lead halide perovskite has a large and tunable optical bandgap. It improves the photoluminescence quantum yield and lifetime of incorporated crystalline perovskite, opening up the intriguing possibility of using amorphous perovskite as a passivating contact, as is currently done in record efficiency silicon solar cells.

Solution-Doped Polysilicon Passivating Contacts for Silicon Solar Cells.

In this work, we present a simple and efficient solution-doping process for preparing high-quality polycrystalline silicon (poly-Si)-based passivating contacts. Commercial phosphorus or boron-doping solutions are spin-coated on crystalline silicon (c-Si) wafers that feature SiO2/poly-Si layers; the doping process is then activated by thermal annealing at high temperatures in a nitrogen atmosphere. With optimized n- and p-type solution doping and thermal annealing, n- and p-type poly-Si passivating contacts featuring simultaneously a low contact recombination parameter (J0c) of 2.4 and 12 fA/cm2 and a low contact resistivity (ρc) of 29 and 20 mΩ·cm2 are achieved, respectively. Taking advantage of the single-sided nature of these solution-doping processes, c-Si solar cells with poly-Si passivating contacts of opposite polarity on the respective wafer surfaces are fabricated using a simple coannealing process, achieving the best power conversion efficiency (PCE) of 18.5% on a planar substrate. Overall, the solution-doping method is demonstrated to be a simple and promising alternative to gas/ion implantation doping for poly-Si passivating-contact manufacturing.

‘SolarEYE’ loss analysis of screen-printed, n-type silicon solar cells with ‘monoPoly’ PECVD rear passivated contacts

We present cell results and loss analysis summaries for screen printed, five-busbar (5BB), M2-size, n-type silicon solar cells made with rear-passivated contacts deposited with PECVD (referred to as ‘n-monoPoly’ cells). The PECVD process avoids the need to mask the front during the formation of the rear passivated contact. A median open circuit voltage (VOC) of 702 mV and a peak cell VOC of 705 mV was demonstrated. Improvements to the passivated contact stack led to a passivated rear surface recombination current density (J0-poly) of 5 fA/cm2 and an area-normalised rear metal recombination current density (J0-met) of 3 fA/cm2 for fire-through contacts. The cell models extracted from SolarEYE loss analysis were used to identify the largest VOC and FF losses. The best-fit cell model for 5BB n-monoPoly cells was used to: (i) project realistic 12 busbar (12BB) cell efficiencies; and (ii) simulate the output of the devices under bifacial operation. This 12BB n-monoPoly cell structure with a uniformly-doped p+ emitter has a median efficiency potential of 23.6%, and a potential bifacial output of 7 W with a rear irradiance factor of 0.3 – this would translate to an output of 12.5 W for M12-size wafers with the same rear irradiance. If a selectively-deposited passivated contact is added on the front, the median batch efficiency and VOC of screen-printed n-monoPoly cells can reach 24.7% and 720 mV for these 12BB devices.

Light-Promoted Electrostatic Adsorption of High-Density Lewis Base Monolayers as Passivating Electron-Selective Contacts.

Achieving efficient passivating carrier‐selective contacts (PCSCs) plays a critical role in high‐performance photovoltaic devices. However, it is still challenging to achieve both an efficient carrier selectivity and high‐level passivation in a sole interlayer due to the thickness dependence of contact resistivity and passivation quality. Herein, a light‐promoted adsorption method is demonstrated to establish high‐density Lewis base polyethylenimine (PEI) monolayers as promising PCSCs. The promoted adsorption is attributed to the enhanced electrostatic interaction between PEI and semiconductor induced by the photo‐generated carriers. The derived angstrom‐scale PEI monolayer is demonstrated to simultaneously provide a low‐resistance electrical contact for electrons, a high‐level field‐effect passivation to semiconductor surface and an enhanced interfacial dipole formation at contact interface. By implementing this light‐promoted adsorbed PEI as a single‐layered PCSC for n‐type silicon solar cell, an efficiency of 19.5% with an open‐circuit voltage of 0.641 V and a high fill factor of 80.7% is achieved, which is one of the best results for devices with solution‐processed electron‐selective contacts. This work not only demonstrates a generic method to develop efficient PCSCs for solar cells but also provides a convenient strategy for the deposition of highly uniform, dense, and ultra‐thin coatings for diverse applications.

Structural and optical studies of molybdenum oxides thin films obtained by thermal evaporation and atomic layer deposition methods for photovoltaic application

MoOX (X < 3) has shown its promising potential as an efficient hole-selective passivating contact in crystalline Si solar cells. The device performance highly depends on the film properties of MoOX film, which is significantly affected by different synthesis methods. In this work, Si solar cells with c-Si(p)/MoOX rear contacts were demonstrated, where the MoOX films were realized by thermal evaporation (TE), atomic layer deposition (ALD), and UV-assisted ALD (UV-ALD) methods. A pronounced efficiency drop was disclosed with the order of TE, ALD, and UV-ALD MoOX. Subsequently, the contact propertieis, crystallinity, chemical states, roughness, density, and refractive indices of MoOX films were systematically characterized by a series of microscopic and spectroscopic analyses. It is found that the TE film is composed of nanocrystals, while ALD methods yield amorphous feature with a smaller density and refractive indices. A mild UV illumination (3.5 mW/cm2) slightly reduces the film roughness, while a stronger (35 mW/cm2) one increases the film density, roughness, and growth rate significantly.

A round Robin-Highliting on the passivating contact technology

The aim of this work is to demonstrate the maturity of the TOPCon technology by conducting a round-robin on symmetrically processed lifetime samples in the leading European PV institutes EPFL, ISC, CEA-INES, ISFH, IMEC and Fraunhofer ISE within the H2020 funded project called HighLite. For all layers, dark saturation current-densities ranging between 2 and 10 fA/cm2 can be reported. Simultaneously, no metal induced recombination for the two lower sintering temperatures have been observed pointing towards a true passivated contact. Furthermore, contact resistivities below 10 mΩcm2 have been achieved. It seems that the industrial passivating contact matured to a fully passivated and conducting contact enabling full efficiency potential. The fact that this can be realized using either PECVD or LPCVD from various manufacturer is expected to drive costs down and contribute to the increased adoption of the TOPCon technology.

Development of novel carrier selective passivating contacts for crystalline silicon solar cells

Development of novel carrier selective passivating contacts for crystalline silicon solar cells

Status and prospective of high-efficiency c-Si solar cells based on tunneling oxide passivation contacts

Current photovoltaic market is dominated by crystalline silicon (c-Si) solar modules and this status will last for next decades. Among all high-efficiency c-Si solar cells, the tunnel oxide passivated contact (TOPCon) solar cell has attracted much attention due to its excellent passivation and compatibility with the traditional c-Si solar cells. The so-called tunnel oxide passivated contact (TOPCon) consists of an ultra-thin silicon oxide layer less than 2 nm in thickness and a heavily doped poly-Si layer, which is used for implementing effective passivation and selective collection of carriers. This TOPCon solar cell has some advantages including no laser contact opening, no light-induced degradation and no elevated temperature-induced degradation because of N-type c-Si wafer, compatibility with high temperature sintering and technical scalability. This paper first introduces the basic structure and principles of TOPCon solar cells, then compares the existing methods of preparing ultra-thin silicon oxide layer and heavily doped poly-Si layer, and finally points out the future research direction of this cell based on the analysis of the current research status.

Inkjet printing of phosphorus dopant sources for doping poly-silicon in solar cells with passivating contacts

In this work application of inkjet printing of phosphorus inks as dopant source for fabrication of silicon solar cells with tunnel oxide passivating contacts (TOPCon) was demonstrated. Inkjet printing is of great importance for the formation of selectively doped TOPCon layers of industrial TOPCon cells to provide a low contact resistance, while creating the requirements for screen printing metallization. Rheological characterizations were carried out in order to determine the complex viscosity and elastic effects of the inks. Printing processes were developed and optimized accordingly, given the significant relevance of rheology in inkjet printing. It was realized that inkjet printing processes have a direct influence on the electrical properties of passivating contacts. Droplets with a diameter of below 35 μm and lines with sharp edges and width of down to 70 μm were obtained. Excellent passivation quality was achieved for inkjet-printed n-type poly-Si surfaces with an implied open-circuit voltage of iVoc = 733 mV and an implied fill factor of iFF = 86.4%. For n-type passivating contacts, doped by inks containing phosphorus, the junction characteristics were investigated using double-sided contacted silicon solar cells. With these cells, Voc = 703 mV, FF = 72 %, and conversion efficiency of η = 17.1 % were realized. For ion-implanted poly-Si surfaces, herein used as reference, η = 18.2 % was obtained, underlining the high potential of investigated inkjet printing processes.

Clean-Room Lithographical Processes for the Fabrication of Graphene Biosensors.

This work is on developing clean-room processes for the fabrication of electrolyte-gate graphene field-effect transistors at the wafer scale for biosensing applications. Our fabrication process overcomes two main issues: removing surface residues after graphene patterning and the dielectric passivation of metallic contacts. A graphene residue-free transfer process is achieved by using a pre-transfer, sacrificial metallic mask that protects the entire wafer except the areas around the channel, source, and drain, onto which the graphene film is transferred and later patterned. After the dissolution of the mask, clean gate electrodes are obtained. The multilayer SiO2/SiNx dielectric passivation takes advantage of the excellent adhesion of SiO2 to graphene and the substrate materials and the superior impermeability of SiNx. It hinders native nucleation centers and breaks the propagation of defects through the layers, protecting from prolonged exposition to all common solvents found in biochemistry work, contrary to commonly used polymeric passivation. Since wet etch does not allow the required level of control over the lithographic process, a reactive ion etching process using a sacrificial metallic stopping layer is developed and used for patterning the passivation layer. The process achieves devices with high reproducibility at the wafer scale.

Nanocrystalline‐silicon hole contact layers enabling efficiency improvement of silicon heterojunction solar cells: Impact of nanostructure evolution on solar cell performance

AbstractHydrogenated amorphous silicon (a‐Si:H) is a key enabler in high‐efficiency crystalline silicon solar cells known as the silicon heterojunction technology. Although efforts have been devoted to replacing doped a‐Si:H contact layer by hydrogenated nanocrystalline silicon (nc‐Si:H) to take advantage of its superior optoelectrical properties, it is still unclear whether the nc‐Si:H outperforms the a‐Si:H at the high efficiency level. Here, we show that boron‐doped (p)nc‐Si:H prepared by plasma‐enhanced chemical vapor deposition (PECVD) acts as an efficient hole contact layer, providing not only a mitigation of the parasitic absorption loss but also improvements in passivation and electrical contact properties. This results in an efficiency increase by 0.3%–0.6% absolute compared to the reference cell with the (p)a‐Si:H, and a best cell efficiency of 23.54%. We find that the critical thickness of the (p)nc‐Si:H layers required for gaining high efficiency (tc ~ 15–30 nm) is a factor of 3–6 greater than that of the (p)a‐Si:H. UV Raman spectroscopy and electrical conductivity measurements reveal that the tc of the (p)nc‐Si:H is associated with the layer growth needing for the surface coalescence of nanocrystals, determining the hole selectivity and the contact resistivity at the electrode/(p)nc‐Si:H interface. Our results suggest that such nanostructure evolution can be hastened by using a very‐high‐frequency PECVD process.

Application of Phosphorus‐Doped Polysilicon‐Based Full‐Area Passivating Contact on the Front Textured Surface of p‐Type Silicon

A p‐type crystalline silicon (c‐Si) passivated emitter and rear contact (PERC) nowadays have become mainstream in the highly competitive photovoltaic market. Herein, the recently popular passivating contact concept on the front textured surface of p‐type c‐Si PERC solar cells is applied. The full‐area textured passivating contact consists of an ultrathin SiO2 film of ≈1.5 nm thickness grown with thermal oxidation and phosphorus‐doped polysilicon (poly‐Si) contact layer by low‐pressure chemical vapor deposition. A detailed investigation of poly‐Si with different crystalline structures, doping conditions, and thicknesses on the passivation effect and parasitic absorption loss is carried out. Preliminary achievement of 21.3% efficiency is realized in large‐area (244.3 cm2) p‐PERC c‐Si solar cells without the need for additional laser selective redoping. Theoretical calculation expects that the cell efficiency can be enhanced to 23.4% by decreasing the recombination current to a reasonable level. It is demonstrated that further improvement of low‐cost p‐PERC c‐Si solar cells is feasible using the full‐area textured passivating contact processes which are fully compatible with existing production lines.

Can interface charge enhance selectivity in tunnel layer passivated contacts? Using negatively charged aluminium oxide capped with dopant free PEDOT or boron doped polysilicon

Tunnel layer passivated contacts are widely used for selective extraction of electrons or holes in silicon solar cells. We investigate if a tunnel layer with high negative interface charge can further enhance the hole extraction ability (selectivity) of a passivated contact and thus the efficiency of silicon solar cells. High negative-charged, atomic layer deposited AlOx dielectric tunnel layers, capped with either conventional boron doped polysilicon (p+-poly-Si) or low-temperature, dopant-free poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), are investigated in order to form hole selective passivated contacts. Ultra-thin AlOx films in the tunnel regime (0.13–2 nm) are studied, measuring their passivation quality (τeff), fixed charge density (Qtot) and defect density (Dit) prior to capping, and contact recombination density (J0,contact) and effective contact resistivity (ρcontact) after capping with either PEDOT:PSS or p+-poly-Si. The measured J0,contact and ρcontact values are then used to calculate carrier selectivity and efficiency potential if rear-side integrated in a solar cell.Low temperature deposited PEDOT:PSS capped passivated contacts behave differently compared to high temperature deposited p+-poly-Si capped passivated contacts. Yes, negatively charged AlOx tunnel layers can enhance hole extraction capability/selectivity of a passivated contact. For PEDOT:PSS capped passivated contacts, the efficiency potential deploying optimized negatively charged AlOx tunnel layers (1.5 nm, annealed at 425 °C) was measured to be 1.1% higher than conventional reference-moderately charged SiOx tunnel layer (26% as compared to 24.9%). However, this is not observed for p+-poly-Si capped passivated contacts due to boron in-diffusion and associated charge compensation mechanism, a consequence of the high-temperature diffusion process.

Interface analysis of ultrathin SiO2 layers between c‐Si substrates and phosphorus‐doped poly‐Si by theoretical surface potential analysis using the injection‐dependent lifetime

AbstractPassivated contact structures are often representative of tunnel oxide passivated contact (TOPCon) and polycrystalline silicon on oxide (POLO) solar cells. These passivated contact technologies in silicon solar cells have experienced great strides in efficiency. However, characteristics analysis of poly‐Si/SiO2 applied to TOPCon and POLO solar cells as a carrier‐selective and passivated contact is still challenging because the silicon oxide film is very thin (&lt;1.5 nm), poly‐Si and silicon oxide properties change during thermal treatment for passivation effects, and dopant diffusion from poly‐Si layer to the silicon wafer occurs. In this study, the interfacial analysis was performed by applying an algorithm based on the extended Shockley–Read–Hall (SRH) theory to the P‐doped poly‐Si/SiO2/c‐Si structure. Quantitative parameters of the P‐doped poly‐Si/SiO2/c‐Si interface were extracted by fitting the measured and simulated lifetime curves with algorithms, such as Dit (interface trap density) and Qf (fixed charge), from which we were able to elucidate the passivation effect of the interface. The interface analysis method using this algorithm is meaningful in that it can quantify the passivation characteristics of TOPCon with very thin silicon oxide film. The interface characteristics were also analyzed using the injection‐dependent lifetime after thermal treatment of P‐doped poly‐Si/SiO2/c‐Si samples for passivation effect. After the 850°C thermal treatment, the following best passivation effects were verified, namely, ψs = 0.248 eV, Dit = 1.0 × 1011 cm−2·eV−1, Qf = 2.4 × 1012 cm−2, and J02 = 370 pA·cm−2. Through the analysis model using carrier lifetime theory, we investigated quantitatively the passivation properties of P‐doped poly‐Si/SiO2/c‐Si.

Degradation Mechanism and Stability Improvement of Dopant-Free ZnO/LiFx/Al Electron Nanocontacts in Silicon Heterojunction Solar Cells

Dopant-free passivating contacts for photovoltaics have the potential to be deposited at low costs while providing excellent surface passivation and low contact resistance. However, one pressing is...

Optoelectronic properties of ultrathin ALD silicon nitride and its potential as a hole-selective nanolayer for high efficiency solar cells

Fully exploiting the power conversion efficiency limit of silicon solar cells requires the use of passivating contacts that minimize electrical losses at metal/silicon interfaces. An efficient hole-selective passivating contact remains one of the key challenges for this technology to be deployed industrially and to pave the way for adoption in tandem configurations. Here, we report the first account of silicon nitride (SiNx) nanolayers with electronic properties suitable for effective hole-selective contacts. We use x-ray photoemission methods to investigate ultra-thin SiNx grown via atomic layer deposition, and we find that the band alignment determined at the SiNx/Si interface favors hole transport. A band offset ratio, ΔEC/ΔEV, of 1.62 ± 0.24 is found at the SiNx/Si interface for the as-grown films. This equates to a 500-fold increase in tunneling selectivity for holes over electrons, for a film thickness of 3 nm. However, the thickness of such films increases by 2 Å–5 Å within 48 h in cleanroom conditions, which leads to a reduction in hole-selectivity. X-ray photoelectron spectroscopy depth profiling has shown this film growth to be linked to oxidation, and furthermore, it alters the ΔEC/ΔEV ratio to 1.22 ± 0.18. The SiNx/Si interface band alignment makes SiNx nanolayers a promising architecture to achieve widely sought hole-selective passivating contacts for high efficiency silicon solar cells.