Materials For High-efficiency Solar Cells Research Articles

Electric field tunable electronic structures and ultrahigh power conversion efficiency of BC6N/MoSe2 van der Waals heterostructure: A promising material for high-efficiency solar cell applications

A novel BC6N/MoSe2 van der Waals heterostructure (vdWH) was fabricated by stacking monolayer BC6N onto monolayer MoSe2. The structural stability, electronic structure, optical properties, and power conversion efficiency (PCE) of the BC6N/MoSe2 vdWH were systematically investigated using first-principles calculations and by considering the effects of electric fields. The results indicated that the stable BC6N/MoSe2 vdWH has an indirect band gap of 1.84 eV, and exhibits a type II band alignment which is not affected by external electric fields. Additionally, the band gap of BC6N/MoSe2 decreased linearly with increasing intensity of the electric field, and the maximum band gap was 1.88 eV under an electric field of −0.05 V/Å. The optical absorptivity of the BC6N/MoSe2 vdWH increased in the visible region, with the highest optical absorptivity (23.1 %) observed in the violet region. The BC6N/MoSe2 vdWH displayed an ultrahigh PCE (22.9 %), which reached 23.6 % under an electric field of −0.05 V/Å. The PCE decreased with increasing intensity of the electric field, but was still high (19.1 %) under an electric field of 0.2 V/Å. The optimal optical absorptivity and ultrahigh and tunable PCE of the BC6N/MoSe2 vdWH indicate its great potential for application as high-efficiency solar cell materials.

Thermally driven phase transition of halide perovskites revealed by big data-powered in situ electron microscopy.

Halide perovskites are promising light-absorbing materials for high-efficiency solar cells, while the crystalline phase of halide perovskites may influence the device's efficiency and stability. In this work, we investigated the thermally driven phase transition of perovskite (CsPbIxBr3-x), which was confirmed by electron diffraction and high-resolution transmission electron microscopy results. CsPbIxBr3-x transitioned from δ phase to α phase when heated, and the γ phase was obtained when the sample was cooled down. The γ phase was stable as long as it was isolated from humidity and air. A template matching-based data analysis method enabled visualization of the thermally driven phase evolution of perovskite during heating. We also proposed a possible atomic movement in the process of phase transition based on our in situ heating experimental data. The results presented here may improve our understanding of the thermally driven phase transition of perovskite as well as provide a protocol for big-data analysis of in situ experiments.

Theoretical Study on the Carrier Mobility and Optical Properties of CsPbI3 by DFT.

The advantages of organic–inorganic hybrid halide perovskites and related materials, such as high absorption coefficient, appropriate band gap, excellent carrier mobility, and long carrier life, provide the possibility for the preparation of low-cost and high-efficiency solar cell materials. Among the inorganic materials, CsPbI3 is paid more attention to by researchers as CsPbI3 has incomparable advantages. In this paper, based on density functional theory (DFT), we first analyze the crystal structure, electronic properties, and work function of two common bulk structures of CsPbI3 and their slices, and then, we study the carrier mobility, exciton binding energy, and light absorption coefficient. Considering that CsPbI3 contains heavy elements, the spin–orbit coupling (SOC) effect was also considered in the calculation. The highest mobility is that electrons of the cubic structure reach 1399 cm2 V–1 S–1 after considering the SOC effect, which is equal to that of traditional solar cells (such as Si-based, PbSe, and PbTe). The lowest exciton binding energy is 101 meV in the cubic bulk structure, which is beneficial to the separation of photogenerated carriers. In the visible region, the absorption coefficient of the cubic structure is the best among all structures, reaching 105 cm–1. Through the study of mobility, exciton binding energy, and light absorption coefficient, it is found that the cubic bulk structure in all structures of CsPbI3 has the best photoelectric performance. This paper can provide some guidance for the experimental preparation of CsPbI3 as a potential high-efficiency solar cell material.

Simulation of photovoltaic material (donor blends PTB7:PC70BM) polymer for solar cell application

Organic solar cells (OSCs) have good tendency to convert photonic energy into electrical energy. We are using PTB7: PC70BM donor polymer with the device structure: FTO/TiO2/ blends / PTB7:PC70BM /Spiro-MeOTAD/Au. The device simulation was done by GPVDM (General-purpose Photovoltaic Device Model) software. The temperature and thickness dependent power conversion efficiency of the donor blends PTB7:PC70BM polymer had observed by the GPVDM simulation software. We found that PTB7:PC70BM shows temperature varying stability in the device. The energy level of the device is explained by the optical simulation. Blends / PTB7:PC70BM layers and metallic interlayer along with suitable front and back metal contacts. This study will be helpful in carefully designing/fabricating the structure of the organic material for high-efficiency solar cells.

Defect-mediated metastability and carrier lifetimes in polycrystalline (Ag,Cu)(In,Ga)Se2 absorber materials

Using a combination of optical and electrical measurements, we develop a model for metastable defects in Ag-alloyed Cu(In,Ga)Se2, one of the leading thin film photovoltaic materials. By controlling the pre-selenization conditions of the back contact prior to the growth of polycrystalline (Ag,Cu)(In,Ga)Se2 absorbers and subsequently exposing them to various stresses (light soaking and dark-heat), we explore the nature and role of metastable defects on the electro-optical and photovoltaic performance of high-efficiency solar cell materials and devices. Positron annihilation spectroscopy indicates that dark-heat exposure results in an increase in the concentration of the selenium–copper divacancy complex (VSe–VCu), attributed to depassivation of donor defects. Deep-level optical spectroscopy finds a corresponding increase of a defect at Ev + 0.98 eV, and deep-level transient spectroscopy suggests that this increase is accompanied by a decrease in the concentration of mid-bandgap recombination centers. Time-resolved photoluminescence excitation spectroscopy data are consistent with the presence of the VSe–VCu divacancy complex, which may act as a shallow trap for the minority carriers. Light-soaking experiments are consistent with the VSe–VCu optical cycle proposed by Lany and Zunger, resulting in the conversion of shallow traps into recombination states that limit the effective minority carrier recombination time (and the associated carrier diffusion length) and an increase in the doping density that limits carrier extraction in photovoltaic devices.

Structural and optical characteristics of Sn-doped CuGaSe2 thin films as a new intermediate band material for high-efficiency solar cells

In this research, a new intermediate band (IB) material Sn-doped CuGaSe2 was synthesized for light absorbing layers of high-efficiency solar cells via ball milling. The experimental investigation indicated that element Sn can be successfully doped in the chalcopyrite CuGaSe2 sample, which enhanced the absorption spectrum significantly in the range of visible and near-infrared light wavelength (500 nm–900 nm). With the increase in the content of Sn, the optical bandgap of CuGa1−xSnxSe2 thin films was tuned from 1.65 eV to 1.41 eV for the doping content x from 0.00 to 0.06. The above results proved that the IB was introduced into the CuGa1−xSnxSe2 thin films, and due to the IB existence, this material leads to lower-energy photo absorption (with energy hν ≤ 1.68 eV). Moreover, the presence of Sn4+ in the host material was testified by x-ray photoelectron spectroscopy. Element composition and mapping analysis further confirmed that the fabricated film is composed of Cu, Ga, Sn, and S, and all elements have a homogeneous distribution without partial aggregation. Photoelectric investigations of the Sn–CuGaSe2 indicated that it is a desirable and promising IB material, which could be another candidate for light absorption layers of high-efficiency solar cells.

Exploring electronic and optical properties of Ge-based perovskites under strain: Insights from the first-principles calculations.

Organic-inorganic hybrid perovskites have attracted extensive attention as promising photovoltiac materials for high-efficiency solar cells. In this study, strain effects on the material properties of Ge-based perovskites are fully investigated by the first-principles calculations. The results indicate that the structural, mechanical, electronic and optical properties of CH3NH3GeX3 (X=Cl, Br, I) are sensitive to external modulations. The band gaps of three Ge-based halide perovskites are well predicted by using the HSE06 functional. By increasing the compressive strain, the band gaps of three compounds decrease. A suitable band gap (1.36eV) of CH3NH3GeI3 can be obtained under a strain of -3%. Moreover, the calculated elastic constants further imply that this compound is stable under this condition. The relationship between the band gap variation and geometry change under the compressive strain is revealed. These results are useful for understanding the effects of strain on the material properties of semiconductors and guiding the experiments to improve photovoltaic performance of Ge-based perovskites.

Gradient band gap perovskite films with multiple photoluminescence peaks

Inorganic-organic hybrid materials have been used as a promising material for high-efficiency solar cells and light-emitting diodes. Here, we fabricated a multilayer perovskite film CH3NH3PbBr3@CH3NH3PbI3 (MAPbBr3@MAPbI3) with a sandwich structure by means of thermal evaporation of MAPbBr3 and MAPbI3 single crystals on glass substrates. The MAPbBr3@MAPbI3 film with the sandwich structure, which consists of a MAPbBr3 bottom layer, a MAPbI3-xBrx interlayer, and a MAPbI3 top layer, exhibits gradient band gap and multiple photoluminescence peaks. The gradient band gap can be controllably turned to cover almost the entire visible spectrum. Adjusting the mass ratio of MAPbI3/MAPbBr3 can transform the MAPbBr3@MAPbI3 film from a Br rich phase to an I rich phase, and in this way, the PL emission peaks gradually shift from the blue region (543 nm) to the red region (822 nm), respectively. The property of tunable multiple photoluminescence peaks has application potential in white light-emitting diodes. The gradient band gap perovskites may be used in colorful or tandem structure solar cells. We believe that the sandwich structure of the multilayer perovskite film will facilitate the design and development of more perovskite-based devices.

Accelerated Discovery of Efficient Solar-cell Materials using Quantum and Machine-learning Methods.

Solar-energy plays an important role in solving serious environmental problems and meeting high-energy demand. However, the lack of suitable materials hinders further progress of this technology. Here, we present the largest inorganic solar-cell material search to date using density functional theory (DFT) and machine-learning approaches. We calculated the spectroscopic limited maximum efficiency (SLME) using Tran-Blaha modified Becke-Johnson potential for 5097 non-metallic materials and identified 1997 candidates with an SLME higher than 10%, including 934 candidates with suitable convex-hull stability and effective carrier mass. Screening for 2D-layered cases, we found 58 potential materials and performed G0W0 calculations on a subset to estimate the prediction-uncertainty. As the above DFT methods are still computationally expensive, we developed a high accuracy machine learning model to pre-screen efficient materials and applied it to over a million materials. Our results provide a general framework and universal strategy for the design of high-efficiency solar cell materials. The data and tools are publicly distributed at: https://www.ctcms.nist.gov/~knc6/JVASP.html, https://www.ctcms.nist.gov/jarvisml/, https://jarvis.nist.gov/ and https://github.com/usnistgov/jarvis.

Two-dimensional Blue-AsP monolayers with tunable direct band gap and ultrahigh carrier mobility show promising high-performance photovoltaic properties.

The successful fabrication of black phosphorene (Black-P) in 2014 and subsequent synthesis of layered black As1-xPx alloys have inspired research into two-dimensional (2D) binary As-P compounds. The very recent success in growing blue phosphorene (Blue-P) further motivated exploration of 2D Blue-AsP materials. Here, using ab initio swarm-intelligence global minimum structure-searching methods, we have obtained a series of novel and energetically favored 2D Blue-AsP (denoted x-AsP, x = I, II, III, IV, V) compounds with As : P = 1 : 1 stoichiometry. They display similar honeycomb structures to Blue-P. Remarkably, the lowest-energy AsP monolayer, namely I-AsP, not only possesses a quasi-direct band gap (2.41 eV), which can be tuned to a direct and optimal gap for photovoltaic applications by in-plane strain, but also has an ultrahigh electronic mobility up to ∼7.4 × 104 cm2 V-1 s-1, far surpassing that of Blue-P, and also exhibits high absorption coefficients (×105 cm-1). Our simulations also show that 30 nm-thick I-AsP sheet-based cells have photovoltaic efficiency as high as ∼12%, and the I-AsP/CdSe heterostructure solar cells possess a power conversion efficiency as high as ∼13%. All these outstanding characteristics suggest the I-AsP sheet as a promising material for high-efficiency solar cells.

Experimental Demonstration of Correlated Flux Scaling in Photoconductivity and Photoluminescence of Lead-Halide Perovskites

Lead-halide perovskites attracted attention as materials for high-efficiency solar cells and light emitting applications. Among their attributes are solution processability, high absorbance in the visible spectral range and defect tolerance, as manifested in long photocarrier lifetimes and diffusion lengths. The microscopic origin of photophysical properties of perovskites is, however, still unclear and under debate. Here, we have observed an interesting universal scaling behavior in a series of (hybrid and all-inorganic) perovskite single crystals investigated via simultaneous measurements of the Hall effect, photoconductivity and photoluminescence. A clear correlation between photoconductivity and photoluminescence as functions of the incident photon flux is observed. While photoconductivity exhibits a crossover in the power-law dependence between power exponents 1 and 1/2, photoluminescence exhibits a crossover between power exponents 2 and 3/2. This correlation is found in all the studied compounds irrespective of the cation type (organic or inorganic) or crystallographic phases. We propose phenomenological microscopic mechanisms that explain these interesting non-trivial power exponents and crossovers between them in this broad class of lead-halide perovskites.

Laser-Induced Flash-Evaporation Printing CH3NH3PbI3 Thin Films for High-Performance Planar Solar Cells.

Organic-inorganic hybrid perovskites have been emerging as promising light-harvesting materials for high-efficiency solar cells recently. Compared to solution-based methods, vapor-based deposition technologies are more suitable in preparing compact, uniform, and large-scale perovskite thin films. Here, we utilized flash-evaporation printing (FEP), a laser-induced ultrafast single source evaporation method employing a carbon nanotube evaporator, to fabricate high-quality methylammonium lead iodide perovskite thin films. Stoichiometric films with pure tetragonal perovskite phase can be achieved using a controlled methylammonium iodide to lead iodide ratio in evaporation precursors. The film crystallinity and crystal grain growth could further be promoted after postannealing. Planar solar cells (0.06 cm2) employing these perovskite films exhibit a champion power conversion efficiency (PCE) of 16.8% with insignificant hysteresis, which is among the highest reported PCEs using vapor-based deposition methods. Large-area (1 cm2) devices based on such perovskite films also achieved a stabilized PCE of 11.2%, indicating the feasibility and scalability of our FEP method in fabricating large-area perovskite films for other optoelectronic applications.

Insights into the increased degradation rate of CH3NH3PbI3 solar cells in combined water and O2 environments

Halide perovskites offer low cost and high efficiency solar cell materials but serious issues related to air and moisture stability remain.

Chalcogenide perovskites for photovoltaics.

Chalcogenide perovskites are proposed for photovoltaic applications. The predicted band gaps of CaTiS3, BaZrS3, CaZrSe3, and CaHfSe3 with the distorted perovskite structure are within the optimal range for making single-junction solar cells. The predicted optical absorption properties of these materials are superior compared with other high-efficiency solar-cell materials. Possible replacement of the alkaline-earth cations by molecular cations, e.g., (NH3NH3)(2+), as in the organic-inorganic halide perovskites (e.g., CH3NH3PbI3), are also proposed and found to be stable. The chalcogenide perovskites provide promising candidates for addressing the challenging issues regarding halide perovskites such as instability in the presence of moisture and containing the toxic element Pb.

Conversion Efficiency of Intermediate Band Solar Cells with GaAs:N δ-Doped Superlattices

The performance of intermediate band solar cells using a GaAs:N δ-doped superlattice (SL) as the optical absorber is analyzed. In GaAs:N δ-doped SLs, both of the E+ and E- bands formed around the N δ-doped layers form SL potentials with the conduction band of the spacer GaAs layers, resulting in the formation of multiple minibands. The conversion efficiency limits of the solar cells are calculated using the detailed balance model based on intermediate band structures composed of the valence band and two minibands formed respectively in the E-- and E+-related SL potentials. A high efficiency of 62.6% for full concentration is expected by properly adjusting the structural parameters of the SL. Alloying other elements such as In to a GaAs:N δ-doped SL is a possible way to further facilitate the development of intermediate band materials for high-efficiency solar cells.

Oxide heterostructures for efficient solar cells.

We propose an unexplored class of absorbing materials for high-efficiency solar cells: heterostructures of transition-metal oxides. In particular, LaVO(3) grown on SrTiO(3) has a direct band gap ∼1.1 eV in the optimal range as well as an internal potential gradient, which can greatly help to separate the photogenerated electron-hole pairs. Furthermore, oxide heterostructures afford the flexibility to combine LaVO(3) with other materials such as LaFeO(3) in order to achieve even higher efficiencies with band-gap graded solar cells. We use density-functional theory to demonstrate these features.

The optical and structural properties of polycrystalline Cu(In,Ga)(Se,S)2 absorber thin films

The pentenary compound semiconductor Cu(In,Ga)(Se,S)2 is one of the most attractive materials for high-efficiency solar cells due to its tunable band gap to match well the solar spectrum. In this study, semiconducting Cu(In,Ga)(Se,S)2 thin films were prepared by a classical two-step growth process, which involves the selenization and/or sulfurization of In/Cu–Ga precursor. During the precursor formation step metallic In/Cu–Ga alloys were deposited onto the Mo-coated soda-lime glass substrates by DC magnetron sputter process. The respective precursors were subsequently reacted with H2Se and/or H2S gasses, at elevated temperatures. By optimizing the selenization parameters, such as the gas concentrations, reaction time, reaction temperature, and the flow of H2Se and H2S, high quality, single phase pentenary films were obtained. The gallium and sulfur diffusion behaviors were found to depend strongly on the selenization/sulfurization profile. The surface morphology, phase structure, and composition of the layers were analyzed by scanning electron microscope, atomic force microscopy, X-ray diffraction, and electron diffraction spectroscopy. Photoluminescence measurements were performed to examine the optical properties of the films.

SYNTHESIS OF PbS NANOCRYSTAL/FUNCTIONALIZED CONDUCTING POLYMERS FOR PLASTIC SOLAR CELLS

Lead-based nanocrystals (NCs) are promising materials for high-efficiency solar cells since they are able to generate multiexcitons with high efficiency. One complication of utilizing these NCs is the insulating ligands capping their surfaces. In this paper, we have successfully developed and characterized a phosphonate-functionalized poly-3-hexylthiophene (POP3HT-50) and used it in the direct synthesis of PbS NCs within the polymeric host matrix without extraneous ligands. Devices made of POP3HT-50/PbS nanocomposites show an order of magnitude improvement in η when compared to that reported for a P3HT/ PbS device (η = 0.011% versus 0.001%). The improved performance is consistent with better electronic contact between PbS NCs and POP3HT-50.

Influence of improved anti-Stokes energy transfer between rare earth ions in ErP5O14 noncrystal on dynamic study

Since the visible quantum cutting in Eu3+–Gd3+ material was reported, the importance, the application, and the significance of the quantum cutting phenomenon have been widely recognized.The rate equations which describe the luminescence dynamic processes for different concentrations of (ErxLa1-xP5O14) noncrystal are established in this paper. The coefficients exp{hc k/kT} is introduced in to the calculation of anti-Stokes energy transfer rate to distinguish Stokes energy transfer.All dynamic processes have been simulated separately with and without considering this coefficient for the energy transfer rate of Er0.01La0.99P5O14,Er0.1La0.9P5O14 and ErP5O14 noncrystals when their 2H11/2 ,4I9/2 and4I11/2 levels are excited.The results show that it is essential to take the coefficient into calculation particularly for ErP5O14 noncrystal where energy transfer plays a key role.There is no influence though the distance between rare earth ions is larger.And it is found that infrared quantum cutting exsists in ErP5O14 noncrystal excited by visible light. The relative nonradiative relaxation rate, the spontaneous emission rate and the energy transfer rate are calculated particularly , The relative energy transfer dynamics is analyzed.It is found that the {4H11/2→4I9/2,4I15/2→4I13/2} energy transfer with a rate of 239500s-1, is the main reason for 2H11/2 energy level to have the infrared quantum cutting, Which is meaningful for finding high-efficiency solar cell materials.

Carrier Multiplication in Graphene

Graphene as a zero-bandgap semiconductor is an ideal model structure to study the carrier relaxation channels, which are inefficient in conventional semiconductors. In particular, it is of fundamental interest to address the question whether Auger-type processes significantly influence the carrier dynamics in graphene. These scattering channels bridge the valence and conduction band allowing carrier multiplication, a process that generates multiple charge carriers from the absorption of a single photon. This has been suggested in literature for improving the efficiency of solar cells. Here we show, based on microscopic calculations within the density matrix formalism, that Auger processes do play an unusually strong role for the relaxation dynamics of photoexcited charge carriers in graphene. We predict that a considerable carrier multiplication takes place, confirming the potential of graphene as a new material for high-efficiency photodevices.