Boundary Recombination Research Articles

The roles of carrier concentration and interface, bulk, and grain-boundary recombination for 25% efficient CdTe solar cells

CdTe devices have reached efficiencies of 22% due to continuing improvements in bulk material properties, including minority carrier lifetime. Device modeling has helped to guide these device improvements by quantifying the impacts of material properties and different device designs on device performance. One of the barriers to truly predictive device modeling is the interdependence of these material properties. For example, interfaces become more critical as bulk properties, particularly, hole density and carrier lifetime, increase. We present device-modeling analyses that describe the effects of recombination at the interfaces and grain boundaries as lifetime and doping of the CdTe layer change. The doping and lifetime should be priorities for maximizing open-circuit voltage (Voc) and efficiency improvements. However, interface and grain boundary recombination become bottlenecks for device performance at increased lifetime and doping levels. This work quantifies and discusses these emerging challenges for next-generation CdTe device efficiency.

Effect of Annealing Process on CH3NH3PbI3-XClX Film Morphology of Planar Heterojunction Perovskite Solar Cells with Optimal Compact TiO2 Layer

The morphology of compact TiO2 film used as an electron-selective layer and perovskite film used as a light absorption layer in planar perovskite solar cells has a significant influence on the photovoltaic performance of the devices. In this paper, the spin coating speed of the compact TiO2 is investigated in order to get a high-quality film and the compact TiO2 film exhibits pinhole- and crack-free films treated by 2000 rpm for 60 s. Furthermore, the effect of annealing process, including annealing temperature and annealing program, on CH3NH3PbI3-XClX film morphology is studied. At the optimal annealing temperature of 100°C, the CH3NH3PbI3-XClX morphology fabricated by multistep slow annealing method has smaller grain boundaries and holes than that prepared by one-step direct annealing method, which results in the reduction of grain boundary recombination and the increase of Voc. With all optimal procedures, a planar fluorine-doped tin oxide (FTO) substrate/compact TiO2/CH3NH3PbI3-XClX/Spiro-MeOTAD/Au cell is prepared for an active area of 0.1 cm2. It has achieved a power conversion efficiency (PCE) of 14.64%, which is 80.3% higher than the reference cell (8.12% PCE) without optimal perovskite layer. We anticipate that the annealing process with optimal compact TiO2 layer would possibly become a promising method for future industrialization of planar perovskite solar cells.

A theoretical study of radio wave attenuation through a polycrystalline silicon solar cell

One-dimensional study of both electronic and electrical parameters of a silicon solar cell in the presence or not of an electric field, a magnetic field, or an electromagnetic field does not take into account the grain size and the grain boundary recombination velocity. A three-dimensional study, on the contrary, takes those factors into account. However, the three-dimensional study poses the problem of the attenuation of the wave in the grain of the polycrystalline solar cell as well as the issue of finding the expressions of its components. This study aimed to solve these issues by considering radio waves, which are becoming more and more present in our environment via telecommunication masts. We first obtained the expressions of both the electric field and magnetic field in a grain of a polycrystalline silicon solar cell by solving the dispersion equation. Then we investigated the evolution of the radio wave into the grain by analyzing the behavior of the exponential coefficient that appeared in the expressions of both the electric field and the magnetic field. The study has shown that the attenuation of the radio wave can be neglected through the polycrystalline silicon solar grain and by extension through the polycrystalline silicon solar cell.

Diffusion length and grain boundary recombination activity determination by means of induced current methods

The application of induced current methods for a quantitative description of multicrystalline silicon solar cell properties is demonstrated. For the minority carriers' diffusion length (L) and grain boundary recombination velocity (Vs) determination three types of measurements were used. They included the measurement of EBIC signal dependence on electron beam energy and of EBIC and XBIC grain boundary contrast profiles. The L and Vs values obtained by means of minimization the residual function between measured and model induced current curves are presented. The inaccuracy of obtained parameters is discussed for each of three types of measurements.

Two-Dimensional Finite Element Method Analysis Effect of the Recombination Velocity at the Grain Boundaries on the Characteristics of a Polycrystalline Silicon Solar Cell

To take into account the variation of the recombination velocity at the grain boundaries, we present in this paper a new approach of characterization of the solar cells, based on the two dimensional finite element method. The results of this study on a bifacial polycrystalline silicon solar cell, modelled in the rectangular form, highlighting the effects of the boundary recombination velocity (Sgb) on the solar cell electrical parameters. The photogenerated excess carrier’s density, the photocurrent density; the phototovoltage and the current-voltage characteristics are analyzed, namely. A good agreement with the results given in the literature is observed.

Grain boundary recombination in semiconductors: the grain boundary interaction case

AbstractThe recombination velocity and the barrier height at a grain boundary are investigated with respect to the interaction between two parallel grain boundaries. A self‐consistent calculation has been performed within the Read‐Hall‐Shockley framework. The procedure takes into account of the space‐charge region and the quasi‐neutral region assuming a rigid displacement of the bands. The grain‐boundary energy states are considered in the half‐filled‐level model. The dependence of the recombination parameters are reported versus the excitation and the semiconductor and grain‐boundary characteristics. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High Efficiency Millimeter-Scale Crystalline Perovskite Solar Cells

Current state-of-the-art photovoltaics utilize high purity single-crystal semiconductors grown by sophisticated, high temperature crystal-growth processes. Alternative emerging solar cell technologies based on solution-processed materials promise to deliver power at lower cost, but so far have had limited success due to the presence of defects in the bulk and at the grain boundaries. Improving the crystal size and quality of solution-processed films could lead to unprecedented improvements in photovoltaic performance. However, large-scale crystal growth is challenging in solution-based deposition where molecular kinetics are limited by the low processing temperature. Here, for the first time we demonstrate organometallic perovskites thin-films composed of extra-ordinarily large crystalline grains (1-2 mm), fabricated using a new processing technique. The increase in the grain-size directly correlates to a dramatic increase in the efficiency, as a direct consequence of increased charge carrier mobility and reduction in defect densities. Our measured efficiency values approach ~18%, which is among the highest reported. These devices are extremely robust, exhibiting no degradation with voltage sweep direction or the rate at which the voltage was scanned. We anticipate that this technique for growing high quality large-area crystals at low temperature with low defect concentration will be universally applicable to a wide variety of other material systems thus providing a solution to a long-standing scientific challenge of overcoming polydispersity, defects and grain boundary recombination in solution-processed thin-films. From the perspective of the global photovoltaics community, these results are expected to lead the field towards the synthesis of wafer-scale crystalline perovskites necessary for the fabrication of high-efficiency single-junction and hybrid (semiconductor and perovskite) tandem planar cells. In addition, I will also discuss design principles that govern the photostability in these systems.

Solar cells. High-efficiency solution-processed perovskite solar cells with millimeter-scale grains.

State-of-the-art photovoltaics use high-purity, large-area, wafer-scale single-crystalline semiconductors grown by sophisticated, high-temperature crystal growth processes. We demonstrate a solution-based hot-casting technique to grow continuous, pinhole-free thin films of organometallic perovskites with millimeter-scale crystalline grains. We fabricated planar solar cells with efficiencies approaching 18%, with little cell-to-cell variability. The devices show hysteresis-free photovoltaic response, which had been a fundamental bottleneck for the stable operation of perovskite devices. Characterization and modeling attribute the improved performance to reduced bulk defects and improved charge carrier mobility in large-grain devices. We anticipate that this technique will lead the field toward synthesis of wafer-scale crystalline perovskites, necessary for the fabrication of high-efficiency solar cells, and will be applicable to several other material systems plagued by polydispersity, defects, and grain boundary recombination in solution-processed thin films.

Significantly Enhancing Grain Growth in Cu2ZnSn(S,Se)4 Absorber Layers by Insetting Sb2S3, CuSbS2, and NaSb5S8 Thin Films

The Cu2ZnSn(S,Se)4 (CZTSSe) has gained extensive attention in thin film solar cells due to their potential as a nontoxic, low-cost, and earth-abundant absorber material, and a rapid increase in power conversion efficiencies has been demonstrated in laboratory. Compared with the most successful hydrazine-based solution process, the nanocrystal-based ink method and non-hydrazine molecular precursor solution approach are more eco-friendly for fabricating high-efficiency CZTSSe solar cells. However, it is hard to obtain a complete large-grain CZTSSe absorber thin film which can facilitate the transport of photogenerated carriers while minimize grain boundary recombination. Here, we present a simple and effective strategy to significantly enhance grain growth of CZTSSe absorber layers by insetting Sb2S3, CuSbS2, and NaSb5S8 thin films. The incorporation of Sb-based thin films can induce grain growth in the selenization process, and did not produce the impurity phase confirmed by XRD patterns and Raman spectra. It was found that the order of the crystal growth promotion ability is Sb2S3 > CuSbS2 > NaSb5S8 under the same experimental conditions. The presented approach can be extended to other solution processes of fabricating CZTSSe solar cells to enhance their microstructural properties, which are critical for applications in CZTSSe absorbers with fine-grain layers.

Template-Free Fabrication of Highly-Oriented Single-Crystalline 1D-Rutile TiO2-MWCNT Composite for Enhanced Photoelectrochemical Activity

Template-free synthesis of phase pure one-dimensional (1D), single crystalline rutile titania nanorods or wires at low temperature still remains a challenging task due to its complex nature of surface chemistry. In these 1D structures, charge transport is highly favored. To further modify the electrical conductivity and optoelectronic properties of these 1D nanostructures, various methods such as doping of TiO2 with metal and nonmetal and synthesis of branched and hybrid structures are developed. If these hybrid structures can directly synthesize on the substrate, the transport of the electron will improve due to reduced grain boundary and exciton recombination. In this contribution, for the first time, we have simultaneously synthesized 1D-rutile TiO2-multiwalled carbon nanotube (MWCNT) composite film directly grown on fluorine dope conducting oxide (FTO) substrate along with 1D-rutile TiO2-MWCNT composite powder. The as-grown nanorods films were single-crystalline and oriented vertically with respect to the substrate, having an average height of ∼2 μm. The well connected network of TiO2 with MWCNTs was observed through electron microscopy. The composite film shows positive movement of the flat-band edge and increase in charge carrier density. The TiO2- MWCNT composite was successfully used as photoanode in a dye sensitized solar cell (DSSC) and exhibits a 60% increase in energy-conversion efficiency compared with only TiO2 nanorods.

Grain growth behavior in a columnar-grain-structured silicon wafer for photovoltaic application

With direct growth technologies for obtaining ribbon-type silicon (Si) wafers from molten Si, the horizontal growth process is much faster than the vertical one in terms of the growth rate of the wafer. However, such a fast growth rate is likely to create grains of a relatively small size in the Si wafer, resulting in limited efficiency caused by grain boundary recombination. In this regard, controlling the grain growth behavior of the Si wafer with processing parameters such as the temperature of the substrate and its moving speed is very important in the horizontal growth process. The present investigation produced the experimental findings concerning the processing parameters for controlling the grain size. The size of the grains on the surface of the Si wafer increased to around 180 μm at a lower substrate temperature and a slower the moving speed of the substrate due to the increasing thickness of the Si wafer. However, the growth rate of the grains was observed to have little relation to the variations in the wafer thickness resulting from the processing parameters unless the nucleation behavior had been controlled.

Comparison of thin epitaxial film silicon photovoltaics fabricated on monocrystalline and polycrystalline seed layers on glass

AbstractWe fabricate thin epitaxial crystal silicon solar cells on display glass and fused silica substrates overcoated with a silicon seed layer. To confirm the quality of hot‐wire chemical vapor deposition epitaxy, we grow a 2‐µm‐thick absorber on a (100) monocrystalline Si layer transfer seed on display glass and achieve 6.5% efficiency with an open circuit voltage (VOC) of 586 mV without light‐trapping features. This device enables the evaluation of seed layers on display glass. Using polycrystalline seeds formed from amorphous silicon by laser‐induced mixed phase solidification (MPS) and electron beam crystallization, we demonstrate 2.9%, 476 mV (MPS) and 4.1%, 551 mV (electron beam crystallization) solar cells. Grain boundaries likely limit the solar cell grown on the MPS seed layer, and we establish an upper bound for the grain boundary recombination velocity (SGB) of 1.6x104 cm/s. Copyright © 2014 John Wiley & Sons, Ltd.

Diode laser crystallization processes of Si thin-film solar cells on glass

The crystallization of Si thin-film on glass using continuous-wave diode laser is performed. The effect of various processing parameters including laser power density and scanning speed is investigated in respect to microstructure and crystallographic orientation. Optimal laser power as per scanning speed is required in order to completely melt the entire Si film. When scan speed of 15–100 cm/min is used, large linear grains are formed along the laser scan direction. Laser scan speed over 100 cm/min forms relatively smaller grains that are titled away from the scan direction. Two diode model fitting of Suns -V oc results have shown that solar cells crystallized with scan speed over 100 cm/min are limited by grain boundary recombination (n = 2). EBSD micrograph shows that the most dominant misorientation angle is 60°. Also, there were regions containing high density of twin boundaries up to ~1.2 × 10-8 /cm2 . SiOx capping layer is found to be effective for reducing the required laser power density, as well as changing preferred orientation of the film from ⟨ 110 ⟩ to ⟨ 100 ⟩ in surface normal direction. Cracks are always formed during the crystallization process and found to be reducing solar cell performance significantly.

In situ synthesis of bimetallic Ag/Pt loaded single-crystalline anatase TiO2 hollow nano-hemispheres and their improved photocatalytic properties

Considering that the reduction of particle size can increase the number of active sites, particle size is accepted to be the key factor in the design of photocatalysts. However, serious agglomeration of nanosized particles may cause not only a decreased surface area but also increase grain boundary recombination. Forming a hollow nano-hemisphere in single crystals with reactive sites could be a good method to overcome this drawback. In this work, we report the synthesis of novel anatase TiO2 hollow nano-hemispheres by using a mixture of CH3COO− and F− as size and morphology controlling co-agents that prevent aggregation and increase the number of active sites. Furthermore, bimetallic Ag/Pt nanoparticles (NPs) are uniformly loaded on both interior and exterior of the nano-hemispheres in situ. The Ag/Pt@TiO2 NPs with improved visible-light-harvesting ability, high charge–hole mobility, and low electron–hole recombination exhibited improved photocatalytic performance in the degradation of rhodamine B/ciprofloxacin (RhB/CIP) and hydrogen generation. The results presented here give a promising way toward the development of delicate bimetal@semiconductor composites for board applications in photocatalysis.

Explanation of red spectral shifts at CdTe grain boundaries

We use cathodoluminescence spectrum imaging to investigate the nanoscale properties of CdTe thin-films for solar cells deposited by close-spaced sublimation. Luminescence emission is detected (bands) at ∼1.32 eV and ∼1.50 eV, which are consistent with Z- and Y-bands. For the grains in the as-deposited films, there is a significant redshift in the transition energies near the grain boundaries. The high grain boundary recombination velocity and the donor-acceptor pair (DAP) mechanism of the Z-band transition account for the contrast between grain boundaries and the grain interior. By applying DAP theory, we estimate the concentration of the shallow donor species participating in the Z-band transition to be ∼1017 cm−3.

On the diffusion length and grain size homogeneity requirements for efficient thin-film polycrystalline silicon solar cells

We examine the influence of intragrain defects and grain boundaries on the macroscopic performance of a thin film polycrystalline silicon solar cell. In addition, we evaluate the effect of grain size inhomogeneity on the cell performance via circuit simulations. From an analytical study of charge transport in individual grains and homogeneous grain systems, we obtain the grain size and intragrain diffusion length requirements for a desired efficiency. We identify the conditions under which the grain size and the intragrain diffusion length dominate the cell characteristics. In devices with intragrain effective diffusion length Lmono ⩽ 100 µm and grain boundary recombination velocity SGB ⩽ 104 cm s−1, achieving a larger grain size beyond several µm is not crucial. The inhomogeneous distribution circuit simulations show that grain size inhomogeneity is not the main limiting factor in polycrystalline silicon solar cells. This is so even in thin polycrystalline silicon films with a broad grain size distribution such as those made with aluminum-induced crystallization at low annealing temperature. The main reason is that the optimum bias point for grains of different sizes only differ by about ∼50 mV over a fairly wide grain diameter range 0.5–50 µm even when Lmono = 100 µm and SGB = 105 cm s−1.

Shrinking and Growing: Grain Boundary Density Reduction for Efficient Polysilicon Thin-Film Solar Cells

Polycrystalline Si (poly-Si) thin-film, due to its low Si consumption, low substrate cost, and good stability, is an attractive candidate for cost-effective solar cells, but the as-deposited poly-Si typically has a columnar structure with grain boundaries in between, severely limiting the efficiency of the poly-Si. Here, we report a micropillar poly-Si solar cell that utilizes the columnar structure of the as-deposited poly-Si grains. We first formed submicrometer diameter poly-Si pillars, smaller than the initial grain sizes, and used these pillars as the seeds for the subsequent epitaxial growth of Si, which effectively reduces grain boundary density in the final poly-Si crystal. In addition, the vertically aligned micropillar arrays form radial p-n junctions that further mitigate the grain boundary recombination losses by improving the light absorption and charge-carrier collection efficiencies. Consequently, the maximum efficiency of micropillar poly-Si thin-film solar cells is 6.4%, that is, ∼1.5 times higher than that of the planar cells.

Probing grain boundaries in ceramic scintillators using x-ray radioluminescence microscopy

X-ray radioluminescence microscopy (XRLM), a novel fluorescence microscopy technique under focused x-ray excitation, was used to characterize micro-scale luminescence of Eu:Y2O3 and Ce:YAG transparent ceramics and bicrystals. The diffusion length of a known semiconductor measured by XRLM was found to be in agreement with previously measured values, illustrating its use for characterizing charge carrier transport. Emission intensity was found to drop at the boundaries in both Eu:Y2O3 and Ce:YAG ceramics and bicrystals. The depletion in emission at grain boundaries was ultimately found to be related to charge carrier depletion (through either deep trapping or non-radiative recombination). A charge carrier diffusion model was used to understand the effect of grain boundaries on charge carrier transport in these scintillators. The diffusion model was found to accurately predict the spatial distribution of emission in a Ce:YAG single-crystal as a function of x-ray excitation energy. Structural and chemical characterization of grain boundaries in an Eu:Y2O3 ceramic using transmission electron microscopy and secondary ion mass spectrometry mapping showed an ordered boundary region and no detectable segregation of impurities or Eu, justifying the use of an abrupt boundary condition to determine boundary recombination velocities in these materials. The boundary recombination velocities were then used to show that, for ceramics with grain sizes > ∼20 μm, there would be a minimal effect from the detected charge carrier depletion at grain boundaries on their bulk x-ray radioluminescence intensity. Ultimately, this study illustrates how this new XRLM technique can be used to measure charge carrier diffusion properties and how it may be coupled with microstructural and micro-scale chemical analyses to fully investigate the effect of grain boundaries on scintillator properties.

Cathodoluminescence measurement of grain boundary recombination velocity in vapour grown p-CdTe

A cathodoluminescence (CL) based method for measuring the minority carrier diffusion length (L) and reduced recombination velocity(Sred) of an individual grain boundary is presented. The technique is based on the van Roosbroeck model for the steady-state carrier distribution near a free surface in a semi-infinite solid. Values of L = 0.55±0.03 μm and Sred = 0.23±0.02 are obtained for vapour grown p-type CdTe. The effect of the electron beam generation volume on the accuracy of measurement is also discussed. Intermediate beam voltages (e.g. 15 kV for CdTe) are likely to produce the most reliable results. The method can be used to characterise the electrical activity of grain boundaries in thin-film solar cells.

A contactless method for measuring the recombination velocity of an individual grain boundary in thin-film photovoltaics

A cathodoluminescence-based, contactless method for extracting the bulk minority carrier diffusion length and reduced recombination velocity of an individual grain boundary is applied to vapor grown CdTe epitaxial films. The measured diffusion length was within the range of 0.4–0.6 μm and the grain boundary recombination velocity varied from 500 to 750 cm/s. The technique can be used to investigate the effect of grain boundaries on photovoltaic performance.