High-efficiency Si Solar Cells Research Articles

One-step Cu-assisted chemical etching at room temperature of inverted pyramid array for high-performance crystalline silicon solar cells

Previous studies on the one-step Cu-assisted texturization of silicon wafers have predominantly focused on etching conditions at relatively high temperatures, which are unsuitable for the mass production of solar cells in industrial settings. In this study, we introduced the one-step Cu-assisted chemical etching (Cu-ACE) technique at room temperature to obtain large-area silicon inverted pyramid (IP) array structures. Based on this structure, diamond wire saw (DWS) mc-Si solar cells can achieve a balance between enhanced light trapping and lower carrier recombination by optimizing the concentration of the component Cu(NO3)2/HF/H2O2 in the etchant solution and the etching time during the Cu-ACE process. As a result, these solar cells deliver a power conversion efficiency (PCE) of 18.20 %, which is 1.99 % absolute higher than solar cells using traditional etching techniques. This work paves a new pathway toward the mass production of high-efficiency crystalline Si solar cells, offering a cost-effective and simple texturization approach.

(Invited, Digital Presentation) Advanced Passivating Contacts for Silicon Photovoltaics

Carrier selective passivating contacts, which provide dual functions of surface defect passivation and extracting one type of charge carriers, is a key enabler for high efficiency Si solar cells. Here we introduce two passivating contact materials developed in our lab: one is the hydrogenated nanocrystalline silicon (nc-Si:H) and the other is the metal-oxide (non-silicon) material. For the nc-Si:H, it is demonstrated that the use of the (p)nc-Si:H/(i)a-Si:H stack instead of the conventional (p)a-Si:H/(i)a-Si:H in silicon heterojunction solar cells provides 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 our reference cell, and a best cell efficiency of 23.5%. For metal-oxide material, we have developed an atomic-layer-deposited titanium oxide (TiOx) that functions as an efficient and transparent hole-selective passivating contact without using buffer layer, leading to the proof-of-concept solar cells with efficiencies above 20%. An example of applying the developed contact material as a recombination junction layer of the perovskite/Si tandem solar cell will be presented.

Local Enhancement of Dopant Diffusion from Polycrystalline Silicon Passivating Contacts.

Passivating contacts consisting of heavily doped polycrystalline silicon (poly-Si) and ultrathin interfacial silicon oxide (SiOx) films enable the fabrication of high-efficiency Si solar cells. The electrical properties and working mechanism of such poly-Si passivating contacts depend on the distribution of dopants at their interface with the underlying Si substrate of solar cells. Therefore, this distribution, particularly in the vicinity of pinholes in the SiOx film, is investigated in this work. Technology computer-aided design (TCAD) simulations were performed to study the diffusion of dopants, both phosphorus (P) and boron (B), from the poly-Si film into the Si substrate during the annealing process typically applied to poly-Si passivating contacts. The simulated 2D doping profiles indicate enhanced diffusion under pinholes, yielding deeper semicircular regions of increased doping compared to regions far removed from the pinholes. Such regions with locally enhanced doping were also experimentally demonstrated using high-resolution (5-10 nm/pixel) scanning spreading resistance microscopy (SSRM) for the first time. The SSRM measurements were performed on a variety of poly-Si passivating contacts, fabricated using different approaches by multiple research institutes, and the regions of doping enhancement were detected on samples where the presence of pinholes had been reported in the related literature. These findings can contribute to a better understanding, more accurate modeling, and optimization of poly-Si passivating contacts, which are increasingly being introduced in the mass production of Si solar cells.

A comparative study on the band diagrams and efficiencies of silicon and perovskite solar cells using wxAMPS and AMPS-1D

wxAMPS is a popular software widely used in simulating state-of-art solar cells; however, the official documentation for it does not exist. Herein, we compiled information from literature and compared the wxAMPS simulation results with the software it was developed from, AMPS-1D. We first delineated three new tunneling models introduced in wxAMPS: trap-assisted, intra-band, and band-to-band, and highlighted its salient features of the GUI, file I/O, data visualization, and numerical methods. Conventional c-Si solar cells were then used to elucidate the origin of the photovoltaic response. It was evident that devices operating on the built-in electrostatic field achieved a higher efficiency (3%) than those on the effective field (0.05%) and Dember effect (almost zero). High-efficiency Si solar cells in p-n and p-i-n junctions were further studied. It was found both hole and electron transport layers play critical roles in optimizing the power conversion efficiencies. An occasional convergence issue was observed in calculating charge carrier densities in p-i-n device, in which case the slightly deviated JV characteristics can be mainly attributed to the tunneling models. When running the batch simulation on homojunction perovskite solar cells, the voltage bias step size was tuned smaller than 0.01 V in wxAMPS to produce data of higher accuracy than AMPS-1D. Among all the performance parameters, the fill factor was found to be most sensitive to the voltage bias step.

Efficient passivation of n-type and p-type silicon surface defects by hydrogen sulfide gas reaction

An efficient surface defect passivation is observed by reacting clean Si in a dilute hydrogen sulfide–argon gas mixture (<5% H2S in Ar) for both n-type and p-type Si wafers with planar and textured surfaces. Surface recombination velocities of 1.5 and 8 cm s−1 are achieved on n-type and p-type Si wafers, respectively, at an optimum reaction temperature of 550 °C that are comparable to the best surface passivation quality used in high efficiency Si solar cells. Surface chemical analysis using x-ray photoelectron spectroscopy shows that sulfur is primarily bonded in a sulfide environment, and synchrotron-based soft x-ray emission spectroscopy of the adsorbed sulfur atoms suggests the formation of S–Si bonds. The sulfur surface passivation layer is unstable in air, attributed to surface oxide formation and a simultaneous decrease of sulfide bonds. However, the passivation can be stabilized by a low-temperature (300 °C) deposited amorphous silicon nitride (a-Si:N X :H) capping layer.

Omnidirectional and Broadband Antireflection Effect with Tapered Silicon Nanostructures Fabricated with Low-Cost and Large-Area Capable Nanosphere Lithography.

In this report, we present a process for the fabrication and tapering of a silicon (Si) nanopillar (NP) array on a large Si surface area wafer (2-inch diameter) to provide enhanced light harvesting for Si solar cell application. From our N,N-dimethyl-formamide (DMF) solvent-controlled spin-coating method, silica nanosphere (SNS in 310 nm diameter) coating on the Si surface was demonstrated successfully with improved monolayer coverage (>95%) and uniformity. After combining this method with a reactive ion etching (RIE) technique, a high-density Si NP array was produced, and we revealed that controlled tapering of Si NPs could be achieved after introducing a two-step RIE process using (1) CHF3/Ar gases for SNS selective etching over Si and (2) Cl2 gas for Si vertical etching. From our experimental and computational study, we show that an effectively tapered Si NP (i.e., an Si nanotip (NT)) structure could offer a highly effective omnidirectional and broadband antireflection effect for high-efficiency Si solar cell application.

Enhanced Stability of Exposed PECVD Grown Thin n + Poly-Si/SiO x Passivating Contacts With Al2O3 Capping Layer During High Temperature Firing

Carrier selective poly-Si/SiOx contacts have become a very strong contender for next generation high-efficiency Si solar cells as well as Si-based tandem cells. A thin unmetallized poly-Si/SiOx passivated contact on the top surface of the bottom Si cell provides an excellent opportunity for higher efficiency two-terminal Si-based tandem cells. However, this article shows that when the manufacturable low-cost screen-printed contacts are applied on the rear side of the Si cell, the exposed poly-Si/SiOx contact on the front undergoes appreciable degradation in J0 during high temperature contact firing without a capping layer on the front poly-Si. In this article, unmetallized n-TOPCon structures with ~10-nm poly-Si showed significant degradation after a typical simulated firing process (~750 °C), resulting in decreased implied Voc from 728 to 708 mV and increased J0 from 5 to 15 fA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . We found that a ~7-nm Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> capping layer on poly-Si provides excellent protection with no degradation and can also be easily removed after firing for subsequent processing. For comparison, PECVD grown ~20 nm SiNx and SiOx layers were also investigated for firing stability. The SiNx capping layer was also found to be effective in preserving stability, but the SiOx failed due to lack of hydrogen supply. Large area n-type front junction Si solar cells were fabricated with a boron diffused screen-printed emitter on the front and Ag evaporated thin n-TOPCon on the rear to demonstrate ~0.3% higher absolute efficiency when the rear n-TOPCon was capped with Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> during the firing of the front screen-printed contacts.

(Keynote) Development History of High Efficiency Silicon Heterojunction Solar Cell- from Discover to Practical Use

Silicon heterojunction (SHJ) solar cells are attracting attention as high-efficiency Si solar cells. The basicSHJ structure is p-type amorphous Si (a-Si)/i-type (non-doped) a-Si/n-type mono-crystalline Si (c-Si). Wehave developed and actively evolved this SHJ solar cells from early 1990s, and introduced the moduleequipped with SHJ solar cells named as well-known “HIT”. The features of HIT are: (1) high efficiency,(2) good temperature characteristics, that is, a small output decrease even in the temperature environmentactually used, (3) easy application to double-sided power generation (bifacial module) using symmetricstructure. Although bifacial module has been receiving much attention in recent years, we launched it asearly as 2000.How was this excellent solar cell born? Originally, it was born in the process of developing thin-filmpolycrystalline Si (poly-Si) solar cells as the bottom cells of tandem-type solar cells with a-Si top cells.As an initial stage of the development, we used p-type a-Si as a window layer and n-type c-Si wafer as aphotovoltaic layer because the characteristics of c-Si was clearer than thin film poly-Si. The insertion of ilayerbetween p-type a-Si and n-type c-Si came from two ideas to improve the interface properties. Onewas a structural approach (control the Si bonding network between c-Si and a-Si with microcrystal bufferlayer), and the other was an attempt to suppress the mutual diffusion of impurities. For the former, it wasconfirmed that it was better to use a-Si instead of microcrystals to passivate the c-Si surface effectively,and for the latter, it was found that better characteristics could be obtained by surface passivation of ilayerwith few defects, instead of the effect of impurity separation. The surface passivation technologyusing amorphous semiconductor film was a new invention different from conventional passivation usinginsulating films such as SiOx and SiNx. With this new structure, a large improvement of about 30 mV inopen circuit voltage (Voc) was confirmed. Next we applied this approach to the back side, and again wefound that i-layer could greatly reduce interface defects. Thus, the basic structure of the symmetrical SHJsolar cell was determined. With this structure (Ag electrode/TCO/(p/i)-a-Si/n-c-Si/(i/n)-a-Si/TCO/Agelectrode), we achieved a conversion efficiency of 20% (cell size of 1 cm x 1 cm) in 1994, and after thatthe efforts for mass production started. To expand to a practical size, we newly developed the electrodeforming technology using low-temperature curing type silver paste, and successfully started the massproduction in 1997.In parallel, our R&D team continued to make various conversion efficiency improvements whichincluded optimizing a-Si deposition conditions, developing new TCO materials and deposition methods,and lowering the resistivity of silver paste. In addition, we succeeded in demonstrating in experiments forthe first time that the open-circuit voltage increased associated with the thinning of wafers that had beenpredicted in simulations. The excellent surface passivation capability of i-layer minimized losses whenthinning wafers and maintained the conversion efficiency, which was a very advantageous feature fromthe viewpoint of cost reduction. At the end of 2012 (announced in 2013), we achieved a cell conversionefficiency of 24.7% with a wafer thickness of 98 μm, demonstrating that both low cost and highefficiency can be obtained. Some of these technologies were introduced into mass production.In 2014, we applied SHJ technology the back-contact type solar cell, and achieved a conversionefficiency of 25.6% with both the high Isc due to elimination of front electrode and the high Voc due toexcellent surface passivation. This was a new world record at that time for non-concentrating siliconbasedsolar cells at the research level, the previous one having stood for 15 years.

Investigation of Pd/MoOx/n-Si diodes for bipolar transistor and light-emitting device applications

Sub-stoichiometric molybdenum oxide (MoOx) has recently been investigated for application in high efficiency Si solar cells as a “hole selective” contact. In this paper, we investigate the electrical and light-emitting properties of MoOx-based contacts on Si from the viewpoint of realizing functional bipolar devices such as light-emitting diodes (LEDs) and transistors without any impurity doping of the Si surface. We realized diodes on n-type Si substrates using e-beam physical vapor deposition of Pd/MoOx contacts and compared their behavior to implanted p+n-Si diodes as a reference. In contrast to majority-carrier dominated conduction that occurs in conventional Schottky diodes, Pd/MoOx/n-Si diodes show minority-carrier dominated charge transport with I–V, C–V, and light-emitting characteristics comparable to implanted counterparts. Utilizing such MoOx-based contacts, we also demonstrate a lateral bipolar transistor concept without employing any doped junctions. A detailed C–V analysis confirmed the excessive band-bending in Si corresponding to a high potential barrier (&amp;gt;0.90 V) at the MoOx/n-Si interface which, along with the observed amorphous SiOx(Mo) interlayer, plays a role in suppressing the majority-carrier current. An inversion layer at the n-Si surface was also identified comprising a sheet carrier density greater than 8.6×1011 cm−2, and the MoOx layer was found to be conductive though with a very high resistivity in the 104 Ω-cm range. We refer to these diodes as metal/non-insulator/semiconductor diodes and show with our device simulations that they can be mimicked as high-barrier Schottky diodes with an induced inversion layer at the interface.

Fully screen-printed bifacial large area 22.6% N-type Si solar cell with lightly doped ion-implanted boron emitter and tunnel oxide passivated rear contact

We report on the fabrication of fully screen-printed bifacial large area (239 cm2) high-efficiency n-type Si solar cells with ion-implanted homogeneous boron emitter on the front side and carrier-selective tunnel oxide passivated contact (TOPCon) on the rear. Our phosphorus-doped poly-Si/SiOx passivated contact with SiNx capping layer gave excellent surface passivation with a very low recombination current density (J0_TOPCon) of ~1 fA/cm2 after a simulated firing treatment at ~770 °C without metallization. After screen-printed and fire-through metallization on n-TOPCon with ~13% metal coverage, metallized J0_TOPCon value increased to only ~5 fA/cm2. In addition, homogenous boron implanted emitter (~180 Ω/) passivated with ALD Al2O3 layer capped with PECVD SiNx/SiOx double-layer antireflection (DLAR) coating gave excellent passivation with emitter recombination current density of ~12 fA/cm2 prior to metallization. The industrial screen-printed, fire-through contacts with floating busbars on this boron emitter gave metallized emitter recombination current density of ~31 fA/cm2. This resulted in a low-cost industrial screen-printed n-type bifacial Si solar cell with 22.6% efficiency and 702 mV open-circuit voltage (Voc).

High-Efficiency p-Type Si Solar Cell Fabricated by Using Firing-Through Aluminum Paste on the Cell Back Side.

Firing-through paste used for rear-side metallization of p-type monocrystalline silicon passivated emitter and rear contact (PERC) solar cells was developed. The rear-side passivation Al2O3 layer and the SiNx layer can be effectively etched by the firing-through paste. Ohmic contact with a contact resistivity between 1 to 10 mΩ·cm2 was successfully fabricated. Aggressive reactive firing-through paste would introduce non-uniform etching and high-density recombination centers at the Si/paste interface. Good balance between low resistive contact formation and relatively high open-circuit voltage can be achieved by adjusting glass frit and metal powder content in the paste. Patterned dot back contacts formed by firing-through paste can further decrease recombination density at the Si/paste interface. A P-type solar cell with an area of 7.8 × 7.8 cm2 with a Voc of 653.4 mV and an efficiency of 19.61% was fabricated.

Large-area MACE Si nano-inverted-pyramids for PERC solar cell application

Silicon (Si) nanostructures are regarded as a competitive candidate for the front texture of future high-efficiency Si solar cells owing to their excellent light-trapping properties. In this paper, we present nano-inverted-pyramids (NIPs) textures on the surface of Cz Si wafer with an area of 156 × 156 mm2 fabricated by employing the metal-assisted chemical etching (MACE) technique. The optical properties of the surface are investigated in detail before and after the application of a SiOx/SiNx antireflection coating. A passivated emitter and rear contact (PERC) solar cell with NIPs (NIPs-PERC) is analyzed at the cell as well module level using simulation. The simulation results indicate that the NIPs-PERC solar cell possesses a higher efficiency (η) of 1.4% relatively than that of the traditional PERC solar cell, benefiting from a significant better reflection performance in both the short as well as long wavelength range. Furthermore, simulations show that a 60-cell module with NIPs-PERC solar cells can yield a peak power of 310 W which is 8 W higher than a traditional PERC module. The novel NIPs-PERC solar cell shows high potential for mass production and opens a broad way for the application of NIPs textures to other high-performance solar cells.

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.

Structural Properties of Al-O Monolayers in SiO2 on Silicon and the Maximization of Their Negative Fixed Charge Density.

Al2O3 on Si is known to form an ultrathin interfacial SiO2 during deposition and subsequent annealing, which creates a negative fixed charge ( Qfix) that enables field-effect passivation and low surface recombination velocities in Si solar cells. Various concepts were suggested to explain the origin of this negative Qfix. In this study, we investigate Al-O monolayers (MLs) from atomic layer deposition (ALD) sandwiched between deliberately grown/deposited SiO2 films. We show that the Al atoms have an ultralow diffusion coefficient (∼4 × 10-18 cm2/s at 1000 °C), are deposited at a constant rate of ∼5 × 1014 Al atoms/(cm2 cycle) from the first ALD cycle, and are tetrahedral O-coordinated because the adjacent SiO2 imprints its tetrahedral near-order and bond length into the Al-O MLs. By variation in the tunnel-SiO2 thickness and the number of Al-O MLs, we demonstrate that the tetrahedral coordination alone is not sufficient for the formation of Qfix but that a SiO2/Al2O3 interface within a tunneling distance from the substrate must be present. The Al-induced acceptor states at these interfaces have energy levels slightly below the Si valence band edge and require charging by electrons from either the Si substrate or from Si/SiO2 dangling bonds to create a negative Qfix. Hence, tunneling imposes limitations for the SiO2 and Al2O3 layer thicknesses. In addition, Coulomb repulsion between the charged acceptor states results in an optimum number of Al-O MLs, i.e., separation of both interfaces. We achieve maximum negative Qfix of ∼5 × 1012 cm-2 (comparable to thick ALD-Al2O3 on Si) with ∼1.7 nm tunnel-SiO2 and just seven ALD-Al2O3 cycles (∼8 Å) after optimized annealing at 850 °C for 30 s. The findings are discussed in the context of a passivating, hole-selective tunnel contact for high-efficiency Si solar cells.

High-efficiency, single-crystalline, p- and n-type Si solar cells: Microstructure and chemical analysis of the glass layer

Recent investigations of the front side metallization of high-efficiency mono-crystalline Si solar cells proved that the glass layer formed at the Si/metallization interface during the screen printing plays an essential role for the charge transport, both in n- and p-type cells. High-efficiency cells (~ 18.0% for p-type and ~ 20% for n-type cells) show similar microstructure of the glass layer and similar temperature dependence of the series resistance. From this it is concluded that the microstructure of the glass layer determines the series and contact resistance of the front side metallization.The glass layers of high-efficiency cells contain a high density of nano-Ag colloids and other precipitates which reduce the contact resistance. A percolation model was proposed and is best suited to describe the charge transport in a dirty semiconductor containing metallic precipitates.Quantitative chemical composition of the glass layer of p- and n-type cells was investigated by Energy Dispersive X-ray (EDX) microanalysis in SEM and TEM. The chemical composition of the glass layer showed (SiOx)Pb, as main constitutes and Zn, Ti, Al, Ag, P and B as minor constituents with mole fractions above the detection limit of EDX. The glass layer is therefore considered to be a dirty semiconductor rather than a perfect insulator. The mole fractions of Zn (~ 1at%) and Al (~ 1at%) were quantitatively analyzed. Such analyses are important to correlate microstructural features with electrical properties of the front side metallization.We could prove that in p-type cells the efficiency of the cells correlated with the chemical composition of the glass layer. In n-type cells, with Al-containing pastes EDX spectroscopy yielded Al beyond the detection limit in the glass layer, whereas for Al free pastes the Al mole fraction was below the detection limit of EDX and yielded reduced efficiencies. In p-type cells pastes with enhanced Zn mole fraction yielding zinc oxide phases in the bulk Ag finger and Zn mole fractions up to 5at% in the glass layer.

Improved optical absorption in visible wavelength range for silicon solar cells via texturing with nanopyramid arrays.

Surface-texture with silicon (Si) nanopyramid arrays has been considered as a promising choice for extremely high performance solar cells due to their excellent anti-reflective effects and inherent low parasitic surface areas. However, the current techniques of fabricating Si nanopyramid arrays are always complicated and cost-ineffective. Here, a high throughput nanosphere patterning method is developed to form periodic upright nanopyramid (UNP) arrays in wafer-scale. A direct comparison with the state-of-the-art texture of random pyramids is demonstrated in optical and electronic properties. In combination with the antireflection effect of a SiNx coating layer, the periodic UNP arrays help to provide a remarkable improvement in short-wavelength response over the random pyramids, attributing to a short-current density gain of 1.35 mA/cm2. The advanced texture of periodic UNP arrays provided in this work shows a huge potential to be integrated into the mass production of high-efficiency Si solar cells.

Surface Passivation Properties of ${\textrm{HfO}}_{2}$ Thin Film on n-Type Crystalline Si

Atomic layer deposited hafnium oxide is shown to provide good surface passivation of low resistivity, n-type crystalline Si wafers after a low temperature anneal. The surface passivation is related to a fixed negative charge, as well as an excellent interface with the crystalline Si wafer. In this paper, the influence of four deposition parameters on the HfO2 passivation properties, namely precleaning, precursors, deposition temperature, and postannealing temperature, is discussed. Minority carrier lifetimes of 1.9 ms (surface recombination velocity (SRV) 7.7 cm/s) on float zone n-type wafers and 1.7 ms (SRV 11 cm/s) on Czochralski n-type wafers, under optimized deposition conditions and a postannealing process, have been measured. A significant improvement of the surface passivation is observed after 100 h light soaking, resulting in a carrier lifetime of 2.5 ms. Fitting of the results by a two-defect charge trapping/detrapping model indicates that additional light-induced negative charges enhance the field effect passivation, which is also consistent with the experimental results. Due to its high refractive index and the obtained good surface passivation of Si wafers, HfO2 has a great potential as a surface passivation material, e.g., in the fabrication of high-efficiency Si solar cells.

High-efficiency selective boron emitter formed by wet chemical etch-back for n-type screen-printed Si solar cells

Front metal contact induced recombination and resistance are major efficiency limiting factors of large-area screen-printed n-type front junction Si solar cells with homogeneous emitter and tunnel oxide passivated back contact (TOPCON). This paper shows the development of a selective boron emitter (p+/p++) formed by a screen-printed resist masking and wet chemical etch-back process, which first grows a porous Si layer and subsequently removes it. Various wet-chemical solutions for forming porous Si layer are investigated. An industrial compatible process with sodium nitrite (NaNO2) catalyst is developed to uniformly etch-back the ∼47 Ω/◻ atmospheric pressure chemical vapor deposited heavily doped boron emitter to ∼135 Ω/◻ by growing a 320 nm porous Si layer within 3 min and subsequently removing it. After etching back, the boron emitter was subjected to a thermal oxidation to lower the surface concentration and the emitter saturation current density J0e. Various etched-back emitters were evaluated by measuring J0e on symmetric test structures with atomic layer deposited aluminum oxide (Al2O3) passivation. Very low J0e of 21, 14, and 9 fA/cm2 were obtained for the 120, 150, and 180 Ω/◻ etched-back emitters, respectively. A solar cell with a selective emitter (65/180 Ω/◻) formed by this etch-back technology and with an Al/Ag contact on the front and TOPCON on the back gave an open-circuit voltage (Voc) of 682.8 mV and efficiency of 21.04% on n-type Czochralski Si wafer. This demonstrates the potential of this technology for next generation high-efficiency industrial n-type Si solar cells.

A 14.7% Organic/Silicon Nanoholes Hybrid Solar Cell via Interfacial Engineering by Solution-Processed Inorganic Conformal Layer.

We demonstrated a high-performance Si-organic hybrid heterojunction solar cell utilizing low-temperature and liquid-phase-processed TiO2 as an interlayer between poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and Si nanoholes to produce a conformal contact on the surface of the Si nanostructure. The hydrophilic TiO2/Si-nanohole surface enabled the PEDOT:PSS to flow into the spacing of the close-packed nanoholes. Scanning electron microscopy images were used to confirm the PEDOT:PSS nanohole filling induced by the TiO2. With forming gas annealing of the double-sided TiO2, high Voc (0.63 V) and Jsc (35.7 mA/cm2) values were obtained, yielding a high power conversion efficiency of 14.7%. The high Voc was attributed to the surface passivation of Si by annealed TiO2. The X-ray photoelectron spectroscopy investigation at the TiO2/Si interface indicates the TiOx signal decreased and the TiO2 and SiOx signals increased after annealing. The Si-O bonding found in the O 1s study appeared in the form of Si-O-Si bonding to serve surface passivation. The band alignment of the PEDOT:PSS/TiO2/n-Si hetero-interfaces was postulated and plotted. The Vbi in the system after annealing was assumed to be higher because of the reduction of bulk and surface states that yield high Voc. After annealing, the Vbi increased from 0.805 to 0.905 V. The reduction of surface recombination velocity proved the passivation ability of TiO2 after annealing. With proven surface passivation and conformal PEDOT:PSS/Si nanohole interfaces for enhanced contact, this Si-organic hybrid heterojunction solar cell with solution-processed TiO2 interlayers has excellent potential for application as a high-efficiency and low-cost Si solar cell.

Enhanced Absorption and &lt;1% Spectrum-and-Angle-Averaged Reflection in Tapered Microwire Arrays

We report ordered, high aspect ratio, tapered Si microwire arrays that exhibit an extremely low angular (0° to 50°) and spectrally averaged reflectivity of <1% of the incident 400–1100 nm illumination. After isolating the microwires from the substrate with a polymer infill and peel off process, the arrays were found to absorb 89.1% of angular averaged incident illumination (0° to 50°) in the equivalent volume of a 20 μm thick Si planar slab, reaching 99.5% of the classical light trapping limit between 400 and 1100 nm. We explain the broadband absorption by enhancement in coupling to waveguide modes due to the tapered microstructure of the arrays. Time-resolved microwave photoconductivity decay measurements yielded charge-carrier lifetimes of 0.75 μs (more than an order of magnitude higher than vapor–liquid–solid-grown Si microwires) in the tapered microwires, resulting in an implied Voc of 0.655 V. The high absorption and high aspect ratio in these ordered microwire arrays make them an attractive platform for high-efficiency thin-film crystalline Si solar cells and as well as for the photoelectrochemical production of fuels from sunlight.