Junction Si Solar Cells Research Articles

Thin silicon heterojunction solar cells in perovskite shadow: Bottom cell prospective

Perovskite/Silicon (Pero-Si) tandem with silicon heterojunction (SHJ) bottom cells is a promising highly efficient concept, which in the case of mass production will likely rely on the same wafer feedstock as the single junction Si solar cells. The thickness of these wafers is constantly decreasing for economic and sustainability reasons. We forecast that Si bottom cells for mass produced Pero-Si tandems will be based on wafers thinner than 100 μm. In our work we study challenges and opportunities related to this likely wafer thinning for the performance of the SHJ bottom cells operating in Perovskite shadow. We study SHJ cells prepared on 80 μm thick wafers in comparison to the reference cells based on 135 μm thick wafers addressing two issues: passivation and light management. Effects of passivating layer thickness, back reflector and antireflection coating are studied under AM1.5G standard test conditions, attenuated AM1.5G irradiance, and under Perovskite-filtered spectrum. We show that major wafer thickness reduction of 40% turns to only approx. 0.35%abs loss in the bottom cell efficiency. This minor loss can be reduced even further using highly technological ITO/MgF2/Ag back reflector and MgF2 anti-reflection coating. Our work shows that significant potential for Pero-Si tandems is waiting to be explored in the perovskite shadow from the SHJ bottom cell perspective.

Lithography‐free fabrication of a local contact interlayer for Si‐based tandem solar cells

AbstractCurrently, the Si solar cell market share is dominated by PERC solar cells. Although the efficiency of PERC solar cells has been steadily increasing, it is expected to reach the practical efficiency limit in the near future. The thin film/PERC Si tandem cell technique can be one of the solutions to overcome the single‐cell efficiency limit. In this study, we developed a novel interlayer fabrication technology for the diffused junction Si solar cells of the PERC and Al BSF cell architectures. We combined laser contact opening (LCO) and laser‐induced forward transfer (LIFT) processes to fabricate local contact opening with low contact resistance while maintaining the high passivation performance of the Si bottom cell. The dielectric‐passivated emitter of the Si solar cell was ablated locally by the LCO process, and subsequently, the Ti nanoparticles were transferred selectively by the LIFT process to the opened emitter region followed by transparent conducting oxide deposition. Laser process parameters were carefully optimized to fabricate low‐loss interlayers. We applied the developed interlayer fabrication technology to the Si bottom cells of Al BSF and PERC cells. Finally, we demonstrated successfully the perovskite/PERC Si tandem cell with an interlayer developed in this study. The developed interlayer fabrication technology does not include a photolithography step and vacuum deposition processes for buffer metals; thus, we expect it may be more compatible with the mass production of thin film/diffused junction Si tandem solar cells.

Passivating contact-based tunnel junction Si solar cells using machine learning for tandem cell applications

Tandem solar cells are a key technology for exceeding the theoretical efficiency limit of single-junction cells. One of the most promising combinations is the silicon–perovskite tandem cells, considering their potential for high efficiency, fabrication on a large scale, and low cost. While most research focuses on improving each subcell, another key challenge lies in the tunnel junction that connects these subcells, significantly impacting the overall cell characteristics. Here, we demonstrate the first use of tunnel junctions using a stack of p+/n+ polysilicon passivating contacts deposited directly on the tunnel oxide to overcome the drawbacks of conventional metal oxide-based tunnel junctions, including low tunneling efficiency and sputter damage. Using Random Forest analysis, we achieved high implied open circuit voltages over 700 mV and low contact resistivities of 500 mΩ cm2, suggesting fill factor losses of less than 1% abs for the operating conditions of a tandem cell.

Vertically-aligned p-n junction Si solar cells with CdTe/CdS luminescent solar convertors

In this work, flat solar cells with vertically aligned p-n junctions are designed. Luminescent down-shifting layers are used for enhancement of the solar cells’ efficiency. The solar cells are fabricated with the architecture of double aligned p-n junctions and contain additional coatings with CdTe/CdS luminescent nanocrystals. Current-voltage characteristics of the solar cells are measured using a solar simulator with a pulsed halogen light source. The influence of the average diameter of the CdTe/CdS nanocrystals in the series of 2.8, 3.6 and 4 nm and related to its emission at the wavelengths of 548, 609, and 642 nm, respectively, on the solar cells’ efficiency is studied and discussed. A flat solar cell with vertically aligned p-n junctions containing luminescent down-shifting layers of the nanocrystals that are 4.0 nm in diameter demonstrates a power conversion efficiency of 9% on an area of 1cm2.

Optical simulation of CsPbI3/TOPCon tandem solar cells with advanced light management

Monolithic perovskite/Si tandem solar cells (TSCs) have experienced rapid development in recent years, demonstrating its potential to exceed the Shockley–Queisser limit of single junction Si solar cells. Unlike typical organic-inorganic hybrid perovskite/silicon heterojunction TSCs, here we propose CsPbI3/TOPCon TSC, which is a promising architecture in consideration of its pleasurable thermal stability and good compatibility with current PERC production lines. The optical performance of CsPbI3/TOPCon TSCs is simulated by the combination of ray-tracing method and transfer matrix method. The light management of the CsPbI3/TOPCon TSC begins with the optimization of the surface texture on Si subcell, indicating that a bifacial inverted pyramid with a small bottom angle of rear-side enables a further minimization of the optical losses. Current matching between the subcells, as well as the parasitic absorption loss from the front transparent conductive oxide, is analyzed and discussed in detail. Finally, an optimized configuration of CsPbI3/TOPCon TSC with a 31.78% power conversion efficiency is proposed. This work provides a practical guidance for approaching high-efficiency perovskite/Si TSCs.

Enhancing the performance of p_n junction Si solar cells by integrating silver core&gold shell nano-particles

Enhancing the performance of p_n junction Si solar cells by integrating silver core&gold shell nano-particles

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.

Passivating contacts and tandem concepts: Approaches for the highest silicon-based solar cell efficiencies

The efficiency of photovoltaic energy conversion is a decisive factor for low-cost electricity from renewable energies. In recent years, the efficiency of crystalline silicon solar cells in mass production has increased annually by about 0.5–0.6%abs per year. In order to maintain this development speed, new technologies must be developed and transferred to industrial production. After the transition from full area Al back surface field cells to passivated emitter and rear contact cells, passivating contacts are an important step to get as close as possible to the efficiency limit of single junction Si solar cells. The theoretical background and the two prominent technologies for passivating contacts are presented and discussed. After implementing passivating contacts, the fundamental limit of single junction Si solar cells of 29.4% is in reach. Multi-junction solar cells are the most promising option to achieve efficiencies greater than 30%. Tandem technologies based on crystalline silicon as bottom cells have the advantage that they are based on a mature technology established on a gigawatt scale and can partially use the existing production capacity. In addition, silicon has an ideal bandgap for the lower subcell of a tandem solar cell. The two most promising material candidates for the top cell, i.e., III/V and perovskites, will be discussed. The presented technology routes show that silicon is able to maintain its outstanding position in photovoltaics in the coming years.

On the indentation-assisted phase engineered Si for solar applications

The rhombohedral phase of silicon is fabricated over a large-area within the diamond cubic Si wafer using spherical nanoindentation. The ultraviolet and visible spectrum showed a ~50% lower reflectance for the rhombohedral Si compared to the pristine Si. The estimated refractive index of ~6.7 at 630 nm for the rhombohedral Si is ~35% higher than the diamond cubic Si, which agrees well with the theoretical calculations. The rhombohedral Si with enhanced light absorption ability is utilized as a solar absorber layer for the bare p-n junction Si solar cell and observed to show ~10 times improvement in the photocurrent density.

Toward Low-Cost 4-Terminal GaAs//Si Tandem Solar Cells

Mechanically stacked III–V-on-Si (III–V//Si) tandem solar cells have demonstrated efficiencies beyond what can theoretically be achieved by single junction Si solar cells, but III–V costs are currently at least an order of magnitude higher than Si costs. Recent techno-economic analysis shows that costs could be substantially reduced by replacing traditional metalorganic vapor phase epitaxy (MOVPE) with a lower-cost III–V deposition technique, such as hydride vapor phase epitaxy (HVPE). This study analyzes the performance of an HVPE-grown GaAs top cell incorporated into a 4-terminal (4T) GaAs//Si tandem cell that achieved an efficiency of 29%, which is the highest solar cell efficiency fabricated without expensive deposition techniques such as MOVPE or MBE. We compare these results to an MOVPE-grown GaAs//Si tandem cell that has the same structure. Finally, we model optimizations to the HVPE-grown GaAs top cell and provide a near-term pathway to 31.4% efficiency with a low-cost III–V deposition technique.

Screen printed, large area bifacial N-type back junction silicon solar cells with selective phosphorus front surface field and boron doped poly-Si/SiOx passivated rear emitter

This paper reports on the effect of screen printed metallization on the passivation quality of a boron doped poly-Si/SiOx passivated contact (PC) structure composed of a very thin Si oxide (∼15 Å) capped with boron doped poly-Si. Our boron doped poly-Si/SiOx passivated contact (p-Poly Si/SiOx PC) with a SiNx capping layer gave excellent surface passivation with a very low saturation current density of ∼5 fA/cm2. After screen printed metallization on poly-Si with a metal coverage of ∼10%, this value increased to ∼17 fA/cm2. This paper also demonstrates the fabrication of screen printed, large area (239 cm2), high efficiency (∼21%) n-base bifacial back junction Si solar cells with p-Poly-Si/SiOx PC on the rear and a phosphorus implanted n++-n+ selective front surface field. Detailed analysis is performed to quantify recombination and extract the saturation current density contributions (J0) from each layer of the cell including the metallized front surface field and the tunnel oxide passivated contact. Finally, 2D device modeling of this back junction cell is performed by implementing a simple approach which replaces the p-Poly-Si/SiOx PC by an equivalent p-n junction with the same J0 and gives a good match between the measured and simulated cell parameters using the extracted J0 and recombination velocity values.

Black Silicon With Ultra‐Low Surface Recombination Velocity Fabricated by Inductively Coupled Power Plasma

Black silicon is a naturally antireflective Si surface with great potential for high‐efficiency solar cells. In particular, black silicon surfaces can be obtained using reactive ion etch in a maskless, single‐step process regardless of crystallinity and with minimal material loss. Surface damage from the etching process, however, result in surfaces with high recombination velocity, thus limiting solar cell efficiency. We have developed a method to texture Si surfaces using non‐cryogenic reactive ion etch with a plasma sustained exclusively by inductively coupled power, thereby minimizing surface damage. We achieved a target reflectance of 3% or lower in the wavelength range 300–1000 nm after an etch time of 2 min. Surfaces coated with Al2O3 deposited by atomic layer deposition showed recombination velocity as low as 6.9 cm s−1 on p‐type Czochralski wafers, almost the same values as measured on planar reference surfaces (6.8 cm s−1). This corresponds to an implied open circuit voltage as high as 757 mV for a cell with thickness of 180 μm and base resistivity of 4 Ω cm. These results indicate that our method for texturing of Si surfaces is suitable for fabrication of high‐efficiency single junction Si solar cells.

Influence of surface structure of n-type single-crystalline Si solar cells on potential-induced degradation

Potential-induced degradation (PID) in photovoltaic (PV) modules based on n-type single-crystalline Si solar cells (a bifacial cell, interdigitated back contact cells, and a hetero junction (HJ) cell) was experimentally investigated by applying high voltages to the modules. The power output of a PV module with an n-type bifacial-front junction (FJ) Si solar cell decreased by about 17% by applying −1000V at 85°C for 10min, whereas no degradation was observed by applying +1000V at 85°C for 10min. The spectrum of external quantum efficiency and transient absorption kinetics of the module after PID tests indicated that surface charge recombination between electron and hole on the Si cell was enhanced. PID in n-type Si PV modules can be approximately explained by surface polarization enhancing front surface recombination on Si cells, which corresponds to a loss of passivation effect by a silicon nitride (SiNx) layer. On the other hand, no PID was observed in a PV module with an n-type bifacial-rear junction (RJ) Si solar cell and a commercial PV module based on an HJ cell by applying both positive and negative voltages. High conductive layers of transparent conductive oxide (TCO) on the top of HJ Si solar cells would significantly effective to suppress PID in n-type Si PV modules.

Simulated Study on the Effects of Substrate Thickness and Minority-Carrier Lifetime in Back Contact and Back Junction Si Solar Cells

The BCBJ (Back Contact and Back Junction) or back-lit solar cell design eliminates shading loss by placing the pn junction and metal electrode contacts all on one side that faces away from the sun. However, as the electron-hole generation sites now are located very far from the pn junction, loss by minority-carrier recombination can be a significant issue. Utilizing Medici, a 2-dimensional semiconductor device simulation tool, the interdependency between the substrate thickness and the minority-carrier recombination lifetime was studied in terms of how these factors affect the solar cell power output. Qualitatively speaking, the results indicate that a very high quality substrate with a long recombination lifetime is needed to maintain the maximum power generation. The quantitative value of the recombination lifetime of minority-carriers, i.e., electrons in p-type substrates, required in the BCBJ cell is about one order of magnitude longer than that in the front-lit cell, i.e., 5 × 10−4 sec vs. 5 × 10−5 sec. Regardless of substrate thickness up to 150 μm, the power output in the BCBJ cell stays at nearly the maximum value of about 1.8 × 10−2 W·cm−2, or 18 mW·cm−2, as long as the recombination lifetime is 5 × 10−4 s or longer. The output power, however, declines steeply to as low as 10 mW·cm−2 when the recombination lifetime becomes significantly shorter than 5 × 10−4 sec. Substrate thinning is found to be not as effective as in the front-lit case in stemming the decline in the output power. In view of these results, for BCBJ applications, the substrate needs to be only mono-crystalline Si of very high quality. This bars the use of poly-crystalline Si, which is gaining wider acceptance in standard front-lit 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.

Thermally Stable Silver Nanowires-Embedding Metal Oxide for Schottky Junction Solar Cells.

Thermally stable silver nanowires (AgNWs)-embedding metal oxide was applied for Schottky junction solar cells without an intentional doping process in Si. A large scale (100 mm(2)) Schottky solar cell showed a power conversion efficiency of 6.1% under standard illumination, and 8.3% under diffused illumination conditions which is the highest efficiency for AgNWs-involved Schottky junction Si solar cells. Indium-tin-oxide (ITO)-capped AgNWs showed excellent thermal stability with no deformation at 500 °C. The top ITO layer grew in a cylindrical shape along the AgNWs, forming a teardrop shape. The design of ITO/AgNWs/ITO layers is optically beneficial because the AgNWs generate plasmonic photons, due to the AgNWs. Electrical investigations were performed by Mott-Schottky and impedance spectroscopy to reveal the formation of a single space charge region at the interface between Si and AgNWs-embedding ITO layer. We propose a route to design the thermally stable AgNWs for photoelectric device applications with investigation of the optical and electrical aspects.

Large area tunnel oxide passivated rear contact n‐type Si solar cells with 21.2% efficiency

AbstractThis paper reports on the implementation of carrier‐selective tunnel oxide passivated rear contact for high‐efficiency screen‐printed large area n‐type front junction crystalline Si solar cells. It is shown that the tunnel oxide grown in nitric acid at room temperature (25°C) and capped with n+ polysilicon layer provides excellent rear contact passivation with implied open‐circuit voltage iVoc of 714 mV and saturation current density J0b′ of 10.3 fA/cm2 for the back surface field region. The durability of this passivation scheme is also investigated for a back‐end high temperature process. In combination with an ion‐implanted Al2O3‐passivated boron emitter and screen‐printed front metal grids, this passivated rear contact enabled 21.2% efficient front junction Si solar cells on 239 cm2 commercial grade n‐type Czochralski wafers. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Low-Cost Synthetic Routes for Fabricating Tandem/Multi-Junction Photoelectrochemical Devices

Using sunlight to convert widely available and inexpensive feed stocks such as water, CO2, and industrial wastes (including halogenides) reliably to fuels and value added chemicals, efficiently and cost-effectively, has been, and continues to be a major goal. Most such reactions require a minimum of 1eV or more photon energies. For example, the minimum free energy required to split water reversibly is 1.23 eV. However, kinetic limitations and other sources of inefficiencies (for example, water oxidation on photoanodes is kinetically sluggish, and CO2 reduction at photocathodes needs high overpotentials) make the practical energy needed to carry such reactions even considerably higher. A single-junction photosynthetic device would therefore need to base on a semiconductor with a band gap (Eg) > 2.5 eV for it to carry out water splitting or CO2 reduction, precluding the exploitation of a substantial portion of the solar spectrum. Using tandem/multi-junction photovoltaic architectures is an attractive strategy to overcome these limitations. Such strategies have been estimated to be capable of achieving ~18% solar-to-hydrogen conversion efficiencies. John Turner and his colleagues [1] demonstrated a solar-to-H2 conversion efficiency of 12.4% using multi-junction III-V semiconductors in 1990’s. However, the high cost and complexity associated with device fabrication using Si and III-V semiconductors to produce triple junction devices have impeded commercialization. Here, we report a novel and low-cost wet chemical synthesis route to fabricate a tandem junction PEC device through direct deposition of inexpensive and efficient metal oxide/sulfide based photoanodes on single junction Si solar cells which act as photocathodes. The above structures were able to generate sufficiently high cell voltages to drive valuable, and, at times, challenging photoelectrochemical processes sustainably. Reference: Khaselev, O. & Turner, J. A monolithic photovoltaic-photoelectrochemical device for hydrogen production via water splitting. Science (New York, N.Y.) 280, 425–7 (1998).

Ion-implanted and screen-printed large area 20% efficient N-type front junction Si solar cells

This paper reports on the fabrication of high efficiency (~20%) front junction n-type Si solar cells on 239cm2 Cz using ion implanted boron emitter and phosphorus back surface field (BSF) in combination with screen printed metallization. Cell efficiencies of 19.8% and 20.0% were achieved with SiO2/SiNx and Al2O3/SiNx passivation of boron implanted emitter, respectively, supporting the superiority of Al2O3 passivation. This is consistent with low boron emitter saturation current densities of 76 and 45fA/cm2 achieved for boron emitter passivated with SiO2/SiNx and Al2O3/SiNx stacks, respectively. Saturation current density in metal contact area of boron emitter and phosphorus BSF was measured directly by varying the metal contact coverage. Detailed analysis of saturation current density showed that the performance of our 20% is largely limited by saturation current density associated with recombination on metal contact area of boron emitter and bulk of phosphorus BSF, which accounted for almost ~50% of the total saturation current density.

Designing III-V multijunction solar cells on silicon

Single junction Si solar cells dominate photovoltaics but are close to their efficiency limits. This paper presents ideal limiting efficiencies for tandem and triple junction multijunction solar cells subject only to the constraint of the Si bandgap and therefore recommending optimum cell structures departing from the single junction ideal. The use of III-V materials is considered, using a novel growth method capable of yielding low defect density III-V layers on Si. In order to evaluate the real potential of these proposed multijunction designs, a quantitative model is presented, the strength of which is the joint modelling of external quantum efficiency and current-voltage characteristics using the same parameters. The method yields a single parameter fit in terms of the Shockley-Read-Hall lifetime. This model is validated by fitting experimental data of external quantum efficiency, dark current, and conversion efficiency of world record tandem and triple junction cells under terrestrial solar spectra without concentration. We apply this quantitative model to the design of tandem and triple junction solar cells, yielding cell designs capable of reaching efficiencies without concentration of 32% for the best tandem cell and 36% for the best triple junction cell. This demonstrates that efficiencies within a few percent of world records are realistically achievable without the use of concentrating optics, with growth methods being developed for multijunction cells combining III-V and Si materials.