Efficiency Multi-junction Solar Cells Research Articles

Computational Optimization for CdS/CIGS/GaAs Layered Solar Cell Architecture

Multi-junction solar cells are vital in developing reliable, green, sustainable solar cells. Consequently, the computational optimization of solar cell architecture has the potential to profoundly expedite the process of discovering high-efficiency solar cells. Copper indium gallium selenide (CIGS)-based solar cells exhibit substantial performance compared to those utilizing cadmium sulfide (CdS). Likewise, CIGS-based devices are more efficient according to their device performance, environmentally benign nature, and thus, reduced cost. Therefore, the paper introduces an optimization process of three-layered n-CdS/p-CIGS/p-GaAs (NPP)) solar cell architecture based on thickness and carrier charge density. An in-depth investigation of the numerical analysis for homojunction PPN-junction with the ’GaAs’ layer structure along with n-ZnO front contact was simulated using the Solar Cells Capacitance Simulator (SCAPS-1D) software. Subsequently, various computational optimization techniques for evaluating the effect of the thickness and the carrier density on the performance of the PPN layer on solar cell architecture were examined. The electronic characteristics by adding the GaAs layer on the top of the conventional (PN) junction further led to optimized values of the power conversion efficiency (PCE), open-circuit voltage (VOC), fill factor (FF), and short-circuit current density (JSC) of the solar cell. Lastly, the paper concludes by highlighting the most promising results of our study, showcasing the impact of adding the GaAs layer. Hence, using the optimized values from the analysis, thickness of 5 (μm) and carrier density of 1×1020 (1/cm) resulted in the maximum PCE, VOC, FF, and JSC of 45.7%, 1.16 V, 89.52%, and 43.88 (mA/m2), respectively, for the proposed solar cell architecture. The outcomes of the study aim to pave the path for highly efficient, optimized, and robust multi-junction solar cells.

TCAD simulation of germanium-based heterostructure solar cell employing molybdenum oxide as a hole-selective layer

Molybdenum Oxide (MoO x ) has been used as a hole-extraction film for photovoltaic (PV) applications; however, its interaction with Germanium (Ge)-based solar cells is less understood. For the first time, this paper aims to physically model the Ge solar cell that incorporates MoO x for hole transportation at the front side of the PV device facing the sunlight. However, the charge transportation process within the PV device is influenced by several design parameters that need optimization. A higher work function of MoO x increases the barrier height against minority carriers of electrons which is beneficial for extricating holes at the front interface of MoO x /Ge. A progressive reduction in the recombination of charge carriers has been observed by including a passivation layer of amorphous silicon (i-a-Si:H). Similarly, inserting a passivation and back surface field (BSF) stack of i-a-Si:H strengthens the electric field and likewise reduces the recombination at the rear side of the device. An enhanced doping concentration of BSF assists in the favorable alignment of energy bands for improved charge transportation within the solar cell as the rear passivation maintains the field strength for accelerated movement of charge carriers. However, optimizing the thickness of the front-passivation film is challenging due to the parasitic absorption of light at larger thicknesses. A comparative study with the reference device revealed that the proposed device exhibited a step-increase in the conversion efficiency (η) from 4.23% to 13.10%, with a higher J sc of 46.4 mA cm−2, V oc of 383 mV, and FF of 74%. The proposed study is anticipated to meet the research gap in the physical device modelling of Ge-based solar cells employing high work function MoO x as a carrier-selective layer that could be conducive to the development of highly efficient multijunction solar cells.

Multilayer anti-reflective coating with ultra-low refractive index SiO2 nanopillars for high efficiency multi-junction GaAs solar cells

The anti-reflective coating (ARC) is an essential component in inverted metamorphic triple-junction gallium arsenide (GaAs) solar cells (IMM-SCs) to improve the light absorption and efficiency. The common approach to realize AR function is to construct a multilayer thin-film with a gradient refractive index (n) change from air to the solar cell. In this study, SiO2 nanopillars (SiO2-NPs) are implemented as the top layer in multilayer thin-films to enhance their broadband anti-reflection capability. The transfer matrix method and electron beam (oblique angle) deposition process are applied for the design and fabrication of the multilayer thin-films, respectively. By optimizing the thickness and n of SiO2-NPs thin-films, an IMM-SC with a SiO2-NPs/Al2O3/TiO2 ARC is achieved with an average reflectivity as low as 5.506% in the 300–1300 nm range. The proposed ARCs have significant implications for the window layer designs of high-performance multi-junction solar cells.

Monolithic Two-Terminal Perovskite/Perovskite/Silicon Triple-Junction Solar Cells with Open Circuit Voltage >2.8 V.

The efficiency of perovskite/silicon tandem solar cells has exceeded the previous record for III-V-based dual-junction solar cells. This shows the high potential of perovskite solar cells in multi-junction applications. Perovskite/perovskite/silicon triple-junction solar cells are now the next step to achieve efficient and low-cost multi-junction solar cells with an efficiency potential even higher than that for dual-junction solar cells. Here we present a perovskite/perovskite/silicon triple-junction solar cell with an open circuit voltage of >2.8 V, which is the record value reported for this structure so far. This is achieved through employing a gas quenching method for deposition of the top perovskite layer as well as optimization of interlayers between perovskite subcells. Moreover, for the measurement of our triple-junction solar cells, precise measurement procedures are implemented to ensure the reliability and accuracy of the reported values.

Sterically Suppressed Phase Segregation in 3D Hollow Mixed-Halide Wide Band Gap Perovskites.

Band gap tuning in mixed-halide perovskites enables efficient multijunction solar cells and LEDs. However, these wide band gap perovskites, which contain a mixture of iodide and bromide ions, are known to phase segregate under illumination, introducing voltage losses that limit stability. Previous studies have employed inorganic perovskites, halide alloys, and grain/interface passivation to minimize halide segregation, yet photostability can be further advanced. By focusing on the role of halide vacancies in anion migration, one expects to be able to erect local barriers to ion migration. To achieve this, we employ a 3D "hollow" perovskite structure, wherein a molecule that is otherwise too large for the perovskite lattice is incorporated. The amount of hollowing agent, ethane-1,2-diammonium dihydroiodide (EDA), varies the density of the hollow sites. Photoluminescence measurements reveal that 1% EDA in the perovskite bulk can stabilize a 40% bromine mixed-halide perovskite at 1 sun illumination intensity. These, along with capacitance-frequency measurements, suggest that hollow sites limit the mobility of the halide vacancies.

Modeling Lattice Matched Dilute Nitride Triple and Quadruple Junction Solar Cells on Virtual SiGe Substrate

A lattice matched triple junction solar cell (TJSC) structure with a GaAs0.58 P0.42 top cell and bandgap tunable GaNxAs1-x-zPz middle and bottom cells on virtual SiGe substrate is proposed in this study. SiGe/Si substrate is preferred as it is a low-cost substrate and because it provides a lattice constant at which bandgap tunable dilute nitride materials that are appropriate for highly efficient multijunction solar cells can be obtained. By changing the nitrogen content in GaNxAs1-x-zPz, the bandgap of the middle and bottom subcells is adjusted to the optimum values. The bandgap of the top cell is constant at 1.95 eV. Three models with different values of surface recombination velocities and Shockley–Read–Hall recombination lifetimes are applied to the presented TJSC structure. Peak efficiencies of 48.9%, 40.6% and 33.7% are achieved at EG2 = 1.45 eV and EG3 = 1.04 eV for Model 1, EG2 = 1.45 eV and EG3 = 1.15 eV for Model 2, and EG2 = 1.5 eV and EG3 = 1.17 eV for Model 3, respectively. A fourth bandgap adjustable GaNxAs1-x-zPz junction is inserted into the system and a significant improvement is obtained under high sun concentration for Models 1 and 2. The presented original results are very promising because the variable bandgaps provide very efficient absorption of incoming spectrum.

Overview of High-efficiency Multi-junction Solar Cells and Discussion about Roles of Surface, Interface and Defects

Development of high-efficiency solar cells is very important for clean energy society based on photovoltaics. Multi-junction solar cells are very attractive as high-efficiency solar cells because 2-junction and 3-junction solar cells have high-efficiency potential of more than 36% and 42%, respectively. This paper overviews present status of multi-junction solar cells. As a results of reducing non-radiative recombination and resistance losses, 39.5% efficiency has been demonstrated with 3-junction solar cell. In addition, understanding and controlling defects, surface recombination and interface recombination are shown to be very important in order to improve efficiency of multi-junction solar cells. Brief overview for Si tandem solar cells composing of III-V, II-VI, chalcopyrite, perovskite top cells and Si bottom cell is also presented.

Enhancement of multijunction solar cell efficiency using a cover layer of Eu3+, Tb3+ and Eu3+/Tb3+ doped GeO2-PbO-Al2O3 glasses as spectral converter of solar radiation

The present study reports the multijunction solar cell efficiency enhancement using a cover layer of GeO2-PbO-Al2O3 glasses single doped with Eu3+ and Tb3+ (with and without Ag nanoparticles (NPs)) and codoped with both of them. We highlight that Al2O3 is a network modifier that promotes the occupation of rare-earth ions in more asymmetric sites, and the change in the coordination environment improves the luminescence that benefits the solar cell efficiency. The influence of the downconversion process and the energy transfer mechanisms between Tb3+ and Eu3+ ions are investigated using different concentrations of Eu2O3 and Tb4O7 in single and codoped samples. Moreover, the plasmonic effects of Ag NPs on the solar cell efficiency increase are also studied and compared with those promoted by different doping concentration to reach the best condition for the multijunction solar cell performance. However, in all cases, it is the contribution of the highest luminescence and considerable transmittance that led to the solar cell best performance. The sample single doped with Eu2O3 promoted an increase of 14%; on the other hand, an increase of 13% was observed for the glass doped with Tb4O7 and with 0.5% of AgNO3. For the case of the codoped samples, solar cell efficiency increase of 10% was reached when the largest Eu2O3 concentration was used.

Multiscale Optical Modeling of Perovskite-Si Tandem Solar Cells

With the success of silicon (Si) solar cell technology, research and development on higher efficiency multijunction solar cells is gaining much attention. Tandem cells with a perovskite top cell and a Si bottom cell show particular potential. However, the optical modeling of such devices is complicated by the broad range of length scales involved; the optically thin layers and nanoscale features of a perovskite solar cell require some version of wave optics or even full field electromagnetic (EM) calculations, while the micrometer scale structuring and large dimensions of Si cells are much more manageable using geometrical (ray) optics. In the present work, a method for combining EM and ray optical calculations is developed and described in detail, with examples provided in the software Comsol Multiphysics. For regions with thin films or nanoscale features, EM wave calculations are performed using the finite element method. These calculations provide the phase and amplitude of the waves diffracted into different orders, of which only the regular reflection and transmission are typically of relevance for nanoscale periodicity. In the ray optics simulation, the corresponding regions are implemented as diffracting interfaces, with deterministic transformations of the Stokes vector components according to the EM wave calculations. Meanwhile, the absorbed intensity of intersecting rays is recorded. The method is applied to separate perovskite and Si solar cells and to a few tandem solar cells of relevance for two- versus four-terminal configurations. Corrections for strongly absorbing media in the ray tracing algorithm, which use generalized versions of the Fresnel coefficients, Snell’s law and the Beer-Lambert law, are also evaluated. In a typical Si solar cell with a front surface structure of inverted pyramids, such corrections are found to reduce the absorption by up to 0.5 percentage units compared to a conventional ray tracing calculation. The difference is concluded to originate mainly from reduced absorption rates of inhomogeneous waves, rather than from enhanced escape probabilities for (quasi-) trapped rays at the Si front surface. The method is further applied to evaluate the effects of a plasmonic nanoparticle array, embedded in a perovskite solar cell stack that is located directly on the microstructured Si surface.

Hot-carrier multijunction solar cells: sensitivity and resilience to nonidealities

The resilience against non-idealities of hot-carrier multijunction solar cells (HCMJSCs) is assessed and compared with two references, namely a multijunction solar cell (MJSC) and a hot carrier solar cell (HCSC). We investigate the impact on the efficiency of three deviations from the ideal case: nonoptimal design, internal limitations, and nonstandard operation conditions. We show that the HCMJSC maintains a high efficiency even when materials with nonoptimal bandgaps are considered, broadening the range of candidate materials for its implementation. We also show that the requirement for hot carriers’ thermalization is much less stringent than with the standard HCSC architecture, allowing to surpass the best MJSC efficiency with currently achievable thermalization coefficients. Finally, we estimate the influence of nonstandard illumination by varying the AM spectrum and estimate numerically the yearly averaged efficiency of devices installed in two different locations. Preliminary results on temperature dependence are also presented.

Design optimization of CdTe/Si tandem solar cell using different transparent conducting oxides as interconnecting layers

Due to the efficiency limit placed by Shockley-Queisser for the single junction solar cells, interest has now shifted to the design and fabrication of cost-effective and more efficient multijunction or tandem solar cells. In this study, wxAMPS 3.0 numerical simulation software has been used in designing and optimizing a CdTe/Si based tandem solar cell structure utilizing some selected transparent conducting oxides (TCOs) as electrical and optical interconnectors. Among the three TCOs (SnO2, ITO and AZO) used here, AZO at a thickness of 20 nm illustrated the best efficiency of 26.7156% with Voc = 2.1261 V, Jsc = 13.8071 mA/cm2 and FF = 0.9100 when the thickness of CdTe and Si layers were fixed at 1 µm and 300 µm, respectively. The best possible efficiency for the design was obtained at 38.0417% by reducing the p-CdTe layer thickness from 1 µm to 0.2 µm. This shows that, with proper design and optimized fabrication process, CdTe/Si tandem can push the efficiency beyond the existing single junction limit.

A Brief Review of High Efficiency III-V Solar Cells for Space Application

The demands for space solar cells are continuously increasing with the rapid development of space technologies and complex space missions. The space solar cells are facing more critical challenges than before: higher conversion efficiency and better radiation resistance. Being the main power supply in spacecrafts, III-V multijunction solar cells are the main focus for space application nowadays due to their high efficiency and super radiation resistance. In multijunction solar cell structure, the key to obtaining high crystal quality and increase cell efficiency is satisfying the lattice matching and bandgap matching conditions. New materials and new structures of high efficiency multijunction solar cell structures are continuously coming out with low-cost, lightweight, flexible, and high power-to-mass ratio features in recent years. In addition to the efficiency and other properties, radiation resistance is another sole criterion for space solar cells, therefore the radiation effects of solar cells and the radiation damage mechanism have both been widely studied fields for space solar cells over the last few decades. This review briefly summarized the research progress of III-V multijunction solar cells in recent years. Different types of cell structures, research results and radiation effects of these solar cell structures under different irradiation conditions are presented. Two main solar cell radiation damage evaluation models—the equivalent fluence method and displacement damage dose method—are introduced.

Experimental and Modeling Study on the High-Performance p++-GaAs/n++-GaAs Tunnel Junctions with Silicon and Tellurium Co-Doped InGaAs Quantum Well Inserted

The development of high-performance tunnel junctions is critical for achieving high efficiency in multi-junction solar cells (MJSC) that can operate at high concentrations. We investigate silicon and tellurium co-doping of InGaAs quantum well inserts in p++-GaAs/n++-GaAs tunnel junctions and report a peak current density as high as 5839 A cm−2 with a series resistance of 5.86 × 10−5 Ω cm2. In addition, we discuss how device performance is affected by the growth temperature, thickness, and V/III ratio in the InGaAs layer. A simulation model indicates that the contribution of trap-assisted tunneling enhances carrier tunneling.

Numerical simulation of highly efficient lead-free perovskite layers for the application of all-perovskite multi-junction solar cell

The numerical simulation of lead-free perovskite cesium tin germanium halide (CsSnGeI3) and methyl ammonium germanium halide (CH3NH3GeI3) solar cells have been performed. It has been elucidated from the numerical simulation that as compare to the CH3NH3GeI3 based structure, CsSnGeI3 based structure depicts better photovoltaic performance under constant illumination condition. To enhance the device photovoltaic performance, the effects of different hole transport layer (HTL), defect density of the absorber layer, metal work function and the effect of temperature has been studied. In the present investigation, the effect of hole transport layers are analyzed by correlating the built-in voltage (Vbi) with the open circuit voltage (VOC). It has been obtained from the simulation results, that Vbi has a significant impact on VOC as the higher Vbi corresponds to the highest VOC. Moreover, the effect of defect density and metal work function is also studied. It is obtained from the simulation results, that the optimum defect density is found to be 1 × 1014 cm−3 and metal work function should be greater than or equal to 5 eV for both lead-free CH3NH3GeI3 and CsSnGeI3 based perovskite layer. Furthermore, to suggest possible lead-free alternatives for all-perovskite two-terminal multi-junction solar cell, an attempt is made to match the current of bottom perovskite layer i.e., CsSnGeI3 based perovskite layer to the top perovskite layer i.e., CH3NH3GeI3 based perovskite layer. To attain the current matching for bottom perovskite layer (CsSnGeI3 based perovskite layer) to top perovskite layer (CH3NH3GeI3 based perovskite layer), the bottom perovskite sub cell is fed with the filtered spectrum which is obtained by transmission from top electrode. After, the realization of current matching the short circuit current density of CsSnGeI3 based perovskite layer drops from 25.75 mA/cm2 to 15.32 mA/cm2 which is similar to CH3NH3GeI3 based perovskite absorber layer. In addition, it has been obtained from literature, that both the perovskite layers can be fabricated using solution–processing low-temperature technology. Hence, the numerical study suggests the possible alternatives for wide and narrow bandgap perovskite layers for the application to obtain highly efficient lead-free all-perovskite multi-junction solar cells.

Photovoltaic Concentration: Research and Development

Concentrator Photovoltaic (CPV) technology, by using efficient optical elements, small sizes and high efficiency multi-junction solar cells, can be seen as a bright energy source to produce more cost-effective electricity. The main and basic idea is to replace the use of expensive solar cells with less expensive optical elements made from different materials. This paper aims to give to the readers a rapid and concise overview of CPV and the main characteristics to be considered when designing a CPV system. It reviews the main optical configurations presented in the literature, their advantages and drawbacks, as well as the recent progress in the concentration ratio and the major performances achieved in the field. The paper considers the more recent works, their optical designs, as well as their optical and electrical performances. It also relates the major achievements on the industrial side with the major milestones in CPV developments.

Experimental set-up for testing MJ photovoltaic cells under ultra-high irradiance levels with temperature and spectrum control

• An experimental indoor setup has been developed for test PV cells at +1000 suns. • The set-up is intended to experimental testing, not for cell model extraction. • Irradiance, spectrum and temperature are controlled and measured. • I-V curves at +1000 suns are reported under controlled temperature and spectrum. The interest in Ultra-High Concentrator Photovoltaic (UHCPV) devices has motivated the use of solar cells under ultra-high irradiance levels (+1000 suns). Temperature effects and variations in the spectrum of the incoming light on the solar cells need to be analysed to understand their performance under real operating conditions. In this work an experimental setup, based on an indoor multi-flash solar simulator, is presented. This arrangement allows high efficiency multi-junction solar cells to be characterized up to 1500 suns of irradiance under controlled conditions of temperature and spectrum. Also, different I-V curves obtained by varying each of these parameters separately are attached.

Cascade optical coupling and quantum efficiency measurements of MJ SCs

The paper proposes an experimental approach to determine the external quantum efficiency of multijunction solar cells with pronounced optical coupling. A new technique based on the cascade mechanism of coupling has been elaborated to separate the influence of the induced luminescent flux from the external illumination and to establish the absolute values of photosensitivity without negative impact of optical interaction between subcells. The novelty of this work lies in the method extended to multijunction devices with N-number of subcells.

Ultrathin microcrystalline hydrogenated Si/Ge alloyed tandem solar cells towards full solar spectrum conversion

Thin film solar cells have been proved the next generation photovoltaic devices due to their low cost, less material consumption and easy mass production. Among them, micro-crystalline Si and Ge based thin film solar cells have advantages of high efficiency and ultrathin absorber layers. Yet individual junction devices are limited in photoelectric conversion efficiency because of the restricted solar spectrum range for its specific absorber. In this work, we designed and simulated a multi-junction solar cell with its four sub-cells selectively absorbing the full solar spectrum including the ultraviolet, green, red as well as near infrared range, respectively. By tuning the Ge content, the record efficiency of 24.80% has been realized with the typical quadruple junction structure of a-Si:H/a-Si0.9Ge0.1:H/μc-Si:H/μc-Si0.5Ge0.5:H. To further reduce the material cost, thickness dependent device performances have been conducted. It can be found that the design of total thickness of 4 mm is the optimal device design in balancing the thickness and the PCE. While the design of ultrathin quadruple junction device with total thickness of 2 mm is the optimized device design regarding cost and long-term stability with a little bit more reduction in PCE. These results indicated that our solar cells combine the advantages of low cost and high stability. Our work may provide a general guidance rule of utilizing the full solar spectrum for developing high efficiency and ultrathin multi-junction solar cells.

Impact of the III-V/Ge nucleation routine on the performance of high efficiency multijunction solar cells

This paper addresses the influence of III-V nucleation routines on Ge substrates for the growth of high efficiency multijunction solar cells. Three exemplary nucleation routines with differences in thickness and temperature were evaluated. The resulting open circuit voltage of triple-junction solar cells with these designs is significantly affected (up to 50 mV for the best optimization routine), whereas minimal differences in short circuit current are observed. Electroluminescence measurements show that both the Ge bottom cell and the Ga(In)As middle cell present a VOC gain of 25 mV each. This result indicates that the first stages of the growth not only affect the Ge subcell itself but also to subsequent subcells. This study highlights the impact of the nucleation routine design in the performance of high efficiency multijunction solar cell based on Ge substrates.

The Influence of Solar Spectrum and Concentration Factor on the Material Choice and the Efficiency of Multijunction Solar Cells

In this work, we revisit the theoretical study on the conversion efficiency of series-connected multijunction solar cells. The theoretical method, based on the detailed balance model, is then applied to devices with 2 to 6 junctions under different illumination conditions. As results, (i) we show that the peaks in the efficiency distribution occur for recurrent values of bottom junction bandgap energy corresponding to atmospheric absorption in the solar spectrum, and (ii) we demonstrate that variations in the number of junctions, in the incident solar spectrum, and in the concentration factor lead to changes in the optimum bandgap energy set but that the bottom junction bandgap energy only changes among the recurrent values presented before. Additionally, we highlight that high conversion efficiencies take place for a broad distribution of bandgap energy combination, which make the choice of materials for the device more flexible. Therefore, based on the overall results, we propose more than a hundred III-V, II-VI and IV semiconductor material candidates to compose the bottom junction of highly efficient devices.