Practical Efficiency Limits Research Articles

Evaluating the Practical Efficiency Limit of Silicon Heterojunction–Interdigitated Back Contact Solar Cells by Creating Digital Twins of Silicon Heterojunction Solar Cells with Amorphous Silicon and Nanocrystalline Silicon Hole Contact Layers

Improving power conversion efficiency (PCE) in photovoltaics has driven innovative approaches in solar cell design and technology. Silicon heterojunction (SHJ) solar cells exhibit advantages in PCE due to their effective passivating contact structures. SHJ–interdigitated back contact (SHJ–IBC) solar cells have the potential to surpass traditional SHJ cells, attributed to their advantage in short‐circuit current (JSC). Herein, Silvaco Atlas technology computer‐aided design is used to create digital twins of high‐efficiency SHJ solar cells with amorphous silicon and nanocrystalline silicon hole selective contact (HSC) layers. Using parameters from digital twins of SHJ solar cells, the practical efficiency limit of SHJ–IBC solar cells is assessed. The results show that SHJ–IBC cells can achieve potential efficiencies of 27.01% with amorphous HSC and 27.38% with nanocrystalline HSC. Further efficiency augmentation to 27.51% can be achieved by narrowing the gap from 80 to 20 μm. This study not only advances comprehension of SHJ–IBC solar cells but also provides insights into optimizing geometrical configurations for improved performance. The utilization of digital twins provides a valuable tool for predicting and evaluating the performance of SHJ–IBC solar cells, contributing substantively to the ongoing development of high‐efficiency photovoltaic technology.

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.

Practical efficiency limit of laser power converters based on lead halide perovskite

An ideal laser power converter (LPC) has a power conversion efficiency (PCE) limit of 100%; however, in a real device, additional losses degrade the PCE limit appreciably. We developed an approach to estimate the PCE of perovskite LPCs based on the state-of-the-art perovskite solar cells with a variety of losses. The practical PCE limit is found to be 60.2% under a 785 nm laser with power density of 50 mW/cm2, which result is corroborated by our experimentally measured PCE of 58.6%. Despite a comparable PCE to that of GaAs LPCs, the power output of current perovskite LPC is much lower due to the series resistance, which may be low enough for solar cells but needs to be reduced by one order of magnitude for LPCs. Moreover, the PCE can be further improved by confining emission angle and resorting to wider bandgap perovskite. Finally, a power output over 7 W/cm2 with PCE over 80% is estimated to be within reach for perovskite LPCs.

Limits to the Energy-Conversion Efficiency of Air-Bridge Thermophotovoltaics

As the energy economy becomes increasingly decarbonized, low-cost energy storage grows ever more important. Thermal batteries in combination with thermophotovoltaic (TPV) cells are one major source of storage. The lowest-loss TPV cells utilize an air bridge (AB) with a gold back reflector. In this work, the authors determine a 55.5% thermodynamic efficiency limit for AB-TPVs at an emitter temperature of 1400 K, based on detailed balance. Including losses from nonradiative recombination, finite resistance, and free-carrier absorption, the practical efficiency limit is 48.6%. This work provides a road map for evaluating and limiting losses, leading to even higher AB-TPV efficiencies.

Crystalline silicon solar cells with thin poly‐SiOx carrier‐selective passivating contacts for perovskite/c‐Si tandem applications

AbstractSingle junction crystalline silicon (c‐Si) solar cells are reaching their practical efficiency limit whereas perovskite/c‐Si tandem solar cells have achieved efficiencies above the theoretical limit of single junction c‐Si solar cells. Next to low‐thermal budget silicon heterojunction architecture, high‐thermal budget carrier‐selective passivating contacts (CSPCs) based on polycrystalline‐SiOx (poly‐SiOx) also constitute a promising architecture for high efficiency perovskite/c‐Si tandem solar cells. In this work, we present the development of c‐Si bottom cells based on high temperature poly‐SiOx CSPCs and demonstrate novel high efficiency four‐terminal (4T) and two‐terminal (2T) perovskite/c‐Si tandem solar cells. First, we tuned the ultra‐thin, thermally grown SiOx. Then we optimized the passivation properties of p‐type and n‐type doped poly‐SiOx CSPCs. Here, we have optimized the p‐type doped poly‐SiOx CSPC on textured interfaces via a two‐step annealing process. Finally, we integrated such bottom solar cells in both 4T and 2T tandems, achieving 28.1% and 23.2% conversion efficiency, respectively.

Validated Method for Evaluating the Four‐Terminal Perovskite/Si Tandem Cell Performance and its Efficiency Potential

Recently perovskite/crystalline Si (cSi) tandem cells draw considerable attention because their high efficiency can reduce the levelized cost of electricity and increase the power density of photovoltaics (PVs) to accelerate the energy transition. While the theoretical limits for tandem cells are well known, the practical limits are less clear. Herein, a new method is presented to calculate the efficiency of a four‐terminal (4T) tandem based on the performance of single‐junction perovskite and cSi cells, using their detailed‐balance efficiency fraction. This calculation method is validated with experiments on 4T perovskite/cSi tandem cells that provide a maximum efficiency of 28.0% and with the literature data available for similar configurations. A maximum efficiency of about 36% is estimated for 4T perovskite/cSi tandem cells that would use present record perovskite and cSi PV cells. This can be regarded as the practical efficiency limit for 4T perovskite/cSi tandem devices.

Realization and simulation of interdigitated back contact silicon solar cells with dopant-free asymmetric hetero-contacts

Crystalline silicon (c-Si) solar cells using interdigitated back contact (IBC) configurations are one of the most promising candidates to reach the practical efficiency limits of c-Si solar cells. However, the complexity of the process flow hinders the mass production of the IBC cells with conventional doped regions. One of the simple fabrication methods is to introduce the dopant-free carrier-selective contacts, which utilizes the fabrication processes with low temperature, e.g., the thermal evaporation or the spin coating. In this paper, we investigated efficiency close to 20% silicon IBC solar cells with dopant-free asymmetric hetero-contacts. In this solar cell configuration, the high work function material MoOx was chosen as the hole transporting layer, while the low work function material LiF was chosen as the electron transporting layer, respectively. The simulation results indicate that the perspective efficiency exceeding 22% for this type of cells is achievable with the optimized pitch width and improved passivation quality of the contacts, which has a great potential for the industrialization of IBC solar cells with simple fabrication processes.

Advanced method for electrical characterization of carrier-selective passivating contacts using transfer-length-method measurements under variable illumination

Carrier-selective passivating contacts have been demonstrated to be crucial to reach the practical efficiency limit of single junction, crystalline silicon (c-Si) based solar cells. Yet, the electrical transport losses affecting the collection of photogenerated carriers remain to be addressed. To this aim, different methodologies and characterization techniques are currently used. In this contribution, we propose the concept of shell as a new terminology to describe carrier-selective passivating contacts. Then, we present a novel characterization methodology using transfer length method (TLM) measurement under variable illumination to investigate the charge-carrier transport in amorphous/crystalline silicon heterojunction (SHJ) n-type contact stacks. We use technology computer-aided design simulation to model a TLM structure and to identify the physical phenomena and the key parameters affecting the contact resistivity (ρc) and the charge carrier accumulation of such contact stacks. Then, the simulation results are compared with experimental data by performing variable-illumination TLM measurements of actual SHJ n-type contact stacks. Specifically, we demonstrate that illumination has a strong impact on the measured ρc value, highlighting the importance of measuring ρc under maximum power point conditions for a relevant characterization of solar cell transport losses. In addition, we investigate the dependence of ρc to a change in the injected carrier density within the c-Si bulk to compare the illumination responses of different SHJ n-type contact stacks. In the quest for maximal efficiency, this method may insightfully complete other characterization techniques to further understand and study the electrical transport in solar cells.

Comparing optical performance of a wide range of perovskite/silicon tandem architectures under real-world conditions.

Since single junction c-Si solar cells are reaching their practical efficiency limit. Perovskite/c-Si tandem solar cells hold the promise of achieving greater than 30% efficiencies. In this regard, optical simulations can deliver guidelines for reducing the parasitic absorption losses and increasing the photocurrent density of the tandem solar cells. In this work, an optical study of 2, 3 and 4 terminal perovskite/c-Si tandem solar cells with c-Si solar bottom cells passivated by high thermal-budget poly-Si, poly-SiOx and poly-SiCx is performed to evaluate their optical performance with respect to the conventional tandem solar cells employing silicon heterojunction bottom cells. The parasitic absorption in these carrier selective passivating contacts has been quantified. It is shown that they enable greater than 20mA/cm2 matched implied photocurrent density in un-encapsulated 2T tandem architecture along with being compatible with high temperature production processes. For studying the performance of such tandem devices in real-world irradiance conditions and for different locations of the world, the effect of solar spectrum and angle of incidence on their optical performance is studied. Passing from mono-facial to bi-facial tandem solar cells, the photocurrent density in the bottom cell can be increased, requiring again optical optimization. Here, we analyse the effect of albedo, perovskite thickness and band gap as well as geographical location on the optical performance of these bi-facial perovskite/c-Si tandem solar cells. Our optical study shows that bi-facial 2T tandems, that also convert light incident from the rear, require radically thicker perovskite layers to match the additional current from the c-Si bottom cell. For typical perovskite bandgap and albedo values, even doubling the perovskite thickness is not sufficient. In this respect, lower bandgap perovskites are very interesting for application not only in bi-facial 2T tandems but also in related 3T and 4T tandems.

Intrinsic losses in photovoltaic laser power converters

Using photovoltaic cells as photovoltaic laser power converters (PLPCs) is a potential technology for long-range wireless power transfer. Intrinsic processes that limit the performance of PLPCs have not been fully investigated. Based on a thermodynamic model, we categorize and calculate the intrinsic losses in PLPCs. We use the experimental data of silicon and gallium arsenide to take into account the unavoidable Auger process. We find that the entropic loss generated during the absorption and emission of radiation is the major loss mechanism. Importantly, we show that in the presence of nonideal absorptivity and volumetric entropy production via Auger recombination, using lasers with photon energy equal to the bandgap of the PLPC can be impractical, e.g., comparable efficiencies can be achieved in much thinner silicon PLPCs illuminated by lasers with higher photon energies. We also investigate the methods of diminishing the intrinsic losses with respect to the Auger process: by intensifying the laser irradiance, the proportion of entropic loss in input power can be arbitrarily reduced; by using spectral and angular filters, the intrinsic losses can be diminished via absorption enhancement or emission restriction. Additionally, we discuss the practical efficiency limit of PLPCs accounting for the entropy production due to finite carrier mobilities. The results in this work estimate the potentials for efficiency improvements, which are fundamental to the design of PLPCs.

Semitransparent CdTe solar cell with over 70% near-infrared transmittance

CdTe solar cell is a promising alternative to conventional silicon solar cells and it is quite potential to be utilized as a top sub-cell for tandem solar cells to improve the power conversion efficiency of the existing single-junction devices, whose efficiency has been approaching their practical efficiency limit. Thus, the research and development of semitransparent CdTe solar cells are urgently required. In this work, semitransparent CdTe solar cells were prepared by utilizing 900-nm-thick ultrathin CdTe absorber and CuCl/ITO transparent back contacts. The ultrathin CdTe in the present work was prepared by magnetron sputtering and the effects of substrate temperature and deposition ambient were extensively investigated. CdTe thin films with minimal dislocation density and lowest internal strain were fabricated at a substrate temperature of 235 °C with pure Ar atmosphere. The improved optical–electrical properties of CdTe absorber were then contributing on device performance, and as a result, we successfully fabricated a semitransparent CdTe solar cell with an optimal efficiency of 8.60%. It is worth noting that the transmittance around 1000 nm in the near-infrared region (NIR) is over 70%, which is the highest value among the as-reported semitransparent CdTe solar cells. The present work paves a way to optimize the performance and improve the optical transmittance of semitransparent CdTe solar cells so that it strongly supports their potential applications in bifacial and/or tandem configurations, building integrated photovoltaics, and so on.

Near‐Infrared‐Transparent Perovskite Solar Cells and Perovskite‐Based Tandem Photovoltaics

AbstractMetal halide perovskite solar cells (PSCs) have gained tremendous attention due to their high power conversion efficiencies (PCEs) and potential for low‐cost manufacturing. Their wide and tunable bandgap makes perovskites an ideal candidate for tandem solar cells (TSCs) with well‐established narrow bandgap photovoltaic technologies, such as crystalline silicon and Cu(In,Ga)Se2, to boost the PCEs beyond the Shockley–Queisser limit at affordable additional cost. Although perovskite‐based TSCs have shown rapid progress over the past few years, they are far from reaching their practical efficiency limit. In addition, technology commercialization needs to overcome several challenges such as processing upscalability and long‐term operational stability of solar modules. In this review, a comprehensive overview of the recent progress of perovskite‐based TSCs is provided and the key challenges in the field are discussed. First, the structural and optoelectronic properties of metal halide perovskite materials and preparation methods of metal halide perovskite layers are introduced. Next, the important constituents of near‐infrared (NIR) transparent PSCs for achieving high efficiency along with high NIR transmittance are highlighted. Then the developments in perovskite‐based TSCs are reviewed, the limiting factors are outlined, and strategies to boost the efficiencies well beyond 30% are provided. Finally, the review is concluded by highlighting the bottlenecks for commercialization and providing an outlook for technology advancement.

Numerical Analysis of Pb‐Free Perovskite Absorber Materials: Prospects and Challenges

Optoelectronic properties of organic–inorganic halide perovskites are exceptional with solar cells showing efficiency comparable with conventional photovoltaic technologies. However, with issues of material stability and toxicity of Pb, it is important to understand if Pb can be replaced while maintaining the high power conversion efficiencies of (FA,MA,Cs)Pb(I,Br)3. Herein, practical efficiency limits of Pb and Pb‐free perovskite absorbers are analyzed using a 1D simulator for n‐i‐p or p‐i‐n device structures. SCAPS‐1D baseline models for perovskite absorber materials with and without Pb are developed to numerically reproduce the experimental current density–voltage (JV) and external quantum efficiency (EQE) of champion devices from literature. From these baseline models, the efficiency limits are determined based on optimizing the interface band alignments, reduction in midgap defect density, increased absorption coefficient, and no parasitic losses. SCAPS‐1D simulations suggest that 1) theoretically determined efficiency limit of Cs2PtI6 perovskites is comparable with (FA,MA,Cs)Pb(I,Br)3 perovskites, 2) FA4GeSbCl12 is a promising photoabsorber; and 3) for efficient photoconversion with Sn‐, Ge‐, Ti‐, or Ag‐based compounds, a reduction of defect density and increase in absorption coefficient is needed.

Anomalous carrier dynamics and localization effects in nonpolar m-plane InGaN/GaN quantum wells at high temperatures

Nonpolar InGaN/GaN quantum wells (QWs) have gained significance for efficient light emitting since it eliminates polarization-related effects and allows for higher radiative capability. Although much progress has been made in analyzing carrier localization effects at cryogenic temperatures, carrier dynamics at above room temperatures are still not well understood. In this work, we have observed and explored anomalous carrier dynamics of two nonpolar m-plane InGaN/GaN QWs at high temperatures by combining scanning transmission electron microscopy (STEM) and photophysical characterization. Both experimental and theoretical results suggest that carrier lifetime in both samples increases with temperature in a certain range of temperature. However, the reduced integrity and uniformity of QWs make carriers more prone to the nonradiative Shockley-Reed-Hall recombination as temperature rises. Moreover, both acoustic and optical phonon scatterings dominate from 300 to 600 K through the analysis on the evolution of photoluminescence spectra. Overall, these detailed studies provide insights into approaches to evaluate carrier dynamics at elevated temperatures and improve emitting performance to further push the practical efficiency limit.

Exploring the practical efficiency limit of silicon solar cells using thin solar-grade substrates

For commercially-viable solar-grade silicon, thinner wafers and surface saturation current densities below 1 fA cm−2, are required to significantly increase the practical efficiency limit of solar cells.

Efficiency limits of fans

The purpose of this work is to identify upper efficiency limits of industrial fans such as axial rotor-only fans, axial with guide vanes, centrifugal rotor-only and centrifugal with volute. The efficiency limit is always a function of the class, the design point within the class and the definition of efficiency (total-to-static and total-to-total). The characteristic Reynolds number is another relevant parameter. First, based on analytical and empirical loss models, a theoretical efficiency limit is estimated. A set of idealizing assumptions in the loss models yields efficiencies which are assumed to be an insuperable limit but may be unrealistically high. Second, more realistic efficiency limits are estimated using a computational fluid dynamics-based optimization scheme, seeking for the best designs and hence the maximum achievable efficiencies in all classes. Given the self-imposed constraints in the geometrical parameter space considered, the thus-obtained practical efficiency limits can only be exceeded by admitting more complex geometries of the fans.

Analysis of the performance of InxGa1−xN based solar cells

We have modeled InxGa1−xN single homo-junction solar cells considering realistic carrier transport parameters. It is shown that the maximum efficiency will be less than 19% for an Indium content around 60%. This practical efficiency limit is due to technological issues such as the residual high electron background making it difficult to have p-type doping, causing a low open circuit-voltage and the reduction of the absorber depletion region, and as a result a drop in the photo-current generation. Besides, the difficulty for incorporating In concentrations higher than 40% without phase separation in addition to highly defective material should also be considered. The model does not take in account the carrier lifetime variation as a function of the In content because there are no experimental studies about this yet. To overcome this lack of knowledge, the solar cell with the highest possible In content was modeled by varying the carrier lifetimes from picoseconds to nanoseconds giving calculated efficiencies in the range from 3.9 to 18.9%, respectively. These results explain the poor experimental efficiencies already reported for InxGa1−xN single homo-junction solar cells and suggest that, even in the best case, the expected efficiency will be below that obtained for more conventional Si and GaAs solar cells. Hence, our analysis indicates that alternative ways, such as using nanoparticles or nanowires engineered for making competitive solar cells using this kind of materials, should be looked for in the near future.

Dynamic EEE Coalescing: Techniques and Bounds

Frame coalescing is one of the most efficient techniques to manage the low power idle (LPI) mode supported by Energy Efficient Ethernet (EEE) interfaces. This technique enables EEE interfaces to remain in the LPI mode for a certain amount of time upon the arrival of the first frame (time-based coalescing) or until a predefined amount of traffic accumulates in the transmission buffer (size-based coalescing). This paper provides new insights on the practical efficiency limits of both coalescing techniques. In particular, we derive the fundamental limits on the maximum energy savings considering a target average frame delay. Additionally, we present new open-loop adaptive variants of both time-based and size-based coalescing techniques. These proposals dynamically adjust the length of the sleeping periods in accordance with actual traffic conditions to reduce energy consumption while keeping the average delay near a predefined value simultaneously. Analytical and simulation results show that the energy consumption of both proposals is comparable to the fundamental limits. Consequently, we recommend the usage of the time-based algorithm in most scenarios because of its simplicity as well as its ability to bound the maximum frame delay at the same time.

Ultra-thin atomic layer deposited aluminium oxide tunnel layer passivated hole-selective contacts for silicon solar cells

In this work, we use ultra-thin thickness-controllable spatial atomic layer deposited (ALD) aluminium oxide (AlOx) tunnel layers, which contain high negative fixed charges (Qf), capped by highly boron-doped polysilicon layers to form tunnel layer passivated contacts. The high Qf of the tunnel layers is expected to enhance the carrier selectivity of these passivated hole-extracting contacts. The dependence of the ALD AlOx tunnel layer contact passivation performance on its thickness is investigated. Furthermore, two different thermal charge activation conditions, i.e., fast firing using a belt furnace at 700 and 800 °C are compared. The best measured recombination current density J0 and implied open-circuit voltage iVoc of the developed AlOx/polysilicon passivated contacts with a symmetrical AlOx/SiNx stack passivation are 6.6 fA/cm2 and 723 mV, respectively. Based on the measured J0 and on the measured total contact resistivity of the passivated contact, the practical efficiency limit of a rear-side full area passivated contact solar cell with a conventionally diffused front side is calculated to be as high as 23.2%. Additionally, three rear-side metallization schemes: (1) thermally evaporated full-area silver contacts; (2) screen-printed non-firing-through aluminium contacts and (3) screen-printed firing-through silver-aluminium contacts, are compared. Finally, rear-emitter solar cells, using a rear-side hole-selective AlOx tunnel layer passivated contact, are fabricated, which shows an efficiency of up to 20.5%. While the proposed hole-selective passivated contact scheme appears to be promising based on the simulation prediction, the efficiency of the fabricated cells is largely limited by the non-optimized front-side reflectance and recombination losses as well as the use of non-optimized rear-side metallization schemes.

Performance loss analysis and design space optimization of perovskite solar cells

While the performance enhancement witnessed in the field of perovskite solar cells over the recent years has been impressive, a detailed analysis of various loss mechanisms is required for further performance optimization. Here, we address the same through numerical simulations of optical and electrical characteristics. We quantify various losses like optical losses (5%–6%), recombination losses (3%–4%), and resistive losses against the Auger recombination induced practical efficiency limits. Through this, we identify schemes to reduce these losses and hence lead to an increase in efficiency. In addition, we find that the optimum thickness of the perovskite (with material parameters comparable to MAPbI3-xClx and Eg = 1.55 eV) for solar cell fabrication is around 300 nm (comparable to the well-established value); however, the same could be as large as 900 nm for a trap free perovskite (∼ms as the minority carrier Shockley-Read-Hall recombination lifetime). The analyses also enable us to provide the design charts that could lead to >25% efficient perovskite solar cells on the planar structure.