Band Alignment Optimization Research Articles

Efficient Sb2(S,Se)3/Zn(O,S) solar cells with high open-circuit voltage by controlling sulfur content in the absorber-buffer layers

The purpose of this study is the efficiency improvement of the Sb2Se3 solar cells. In this study, an experimental Sb2Se3 solar cell (glass/Mo/MoSe2/Sb2Se3/CdS/ZnO/ZnO:Al/Ag) with an efficiency record of 9.2% has been simulated. Absorber/buffer interface engineering plays a significant role in enhancing the efficiency of Sb2Se3 solar cells. To achieve this purpose, two approaches have been considered: first, adding sulfur to Sb2Se3 absorber, and second, using an alternative buffer with a wider bandgap instead of conventional CdS buffer. The effects of various x = Se/(S + Se) ratios of Sb2(S1−xSex)3 absorber layer on the photovoltaic performance were investigated. For Sb2(S,Se)3/CdS solar cell, optimum Se/(S + Se) mole fraction of 0.6 < x < 0.8 leads to improved efficiency. Also, ZnO1−ySy buffer layer was applied to replace the conventional CdS buffer layer of Sb2Se3 solar cells to reduce parasitic absorption and improve the short-circuit current density (Jsc). Bandgap for ZnO1−ySy semiconductor is higher than CdS. So, this leads to improved external quantum efficiency at short wavelengths. Optimization of band alignment through ZnO1−ySy buffer layer reduces interface carrier recombination and improves open-circuit voltage (Voc). The band alignment at the Sb2(S1−xSex)3/ZnO1−ySy interface is optimized by adjusting the selenium-to-sulfur ratio in the absorber layer and sulfur-to-oxygen ratio in the buffer layer. This work reveals that the most suitable interface for Sb2(S1−xSex)3/ZnO1−ySy heterojunction is formed when sulfur mole fractions ranging 0.5–0.6 in the Zn(O,S) buffer layer and 0.1–0.2 in the Sb2(S,Se)3 absorber layer. The results show that the efficiency improves from 9.2% to 15.65%, which represents a 70% improvement compared with the conventional Sb2Se3/CdS solar cell.

Interface Engineering of Cu(In,Ga)Se2 Solar Cells by Optimizing Cd- and Zn-Chalcogenide Alloys as the Buffer Layer.

The optimization of band alignment at the buffer/absorber interface is realized by tuning compositions of Cd and Zn chalcogenides as the buffer layer toward high-efficiency Cu(In,Ga)Se2 (CIGS) solar cells. Using the special quasi-random structure (SQS) approach, we construct randomly disordered ZnxCd1-xSySe1-y alloys and ZnSxO1-x alloys as alternatives to the traditional CdS buffer layer. The compositional dependence of formation energies, lattice parameters, band-gap energies, and band alignments of ZnxCd1-xSySe1-y and ZnSxO1-x alloys is investigated by first-principles density functional theory calculations. For quaternary ZnxCd1-xSySe1-y alloys, we find that the miscibility temperatures and the bandgap bowing coefficients are proportional to the lattice mismatch between the mixing elements. The linear dependence of lattice parameters, trinomial dependence of band-gap energies and band-edge positions on the alloy-composition of ZnxCd1-xSySe1-y alloys are established. For ZnSxO1-x alloys, we find the lattice parameters also exhibit a linear dependence on its composition. Because of the large lattice mismatch and the chemical disparity between ZnO and ZnS, the bowing coefficient for the bandgap energies of ZnSxO1-x alloys is composition dependent, and is larger for dilute ZnSxO1-x alloys. With the optimization criteria of moderate spike-like conduction band offset, large valance band offset, sufficiently wide bandgap, and lattice match with respect to the CIGS absorber, we illustrate the optimal composition range of both ZnxCd1-xSySe1-y alloys and ZnSxO1-x alloys as the buffer layer of the CIGS solar cells. Our work demonstrates that ZnxCd1-xSySe1-y alloys and ZnSxO1-x alloys are promising buffer layers for high-efficiency CIGS solar cells.

Minimizing Voltage Losses in Perovskite Solar Cells

Organic–inorganic metal halide perovskite solar cells (PSCs) have achieved an unprecedented increase in power conversion efficiency (PCE) from 3.8% to 25.2% in the past decade. Given that the open‐circuit voltage (Voc) of PSCs is well below its theoretical limit due to the trap‐mediated charge recombination and energy band alignment mismatch, there is still considerable scope to further increase the PCE by minimizing Voc losses. Herein, a fundamental understanding on the origins of voltage losses in PSCs is summarized. Then, the strategies for mitigating the voltage losses of PSCs are critically reviewed, including crystal growth control, grain‐boundary defect passivation, interfacial defect passivation, and band alignment optimization. Finally, the current challenges are discussed and the future directions are outlined to further boost the Voc of PSCs toward their radiative limit.

Towards high efficiency Cd-Free Sb2Se3 solar cells by the band alignment optimization

In this paper, we investigated the effect of Zn1-yMgyO/ZnO1-xSx double buffer layers on the antimony selenide (Sb2Se3) solar cells performance. Our results show that the magnesium and sulfur concentrations in proposed double buffer layers have a significant effect on the band alignment optimization at the junctions. The flexibility with Zn1-yMgyO/ZnO1-xSx double buffer layers ranging from 0 to 25% magnesium and 10%–70% sulfur allows an optimal conduction band offset (CBO) for Sb2Se3 solar cells. An improved open-circuit voltage (Voc) was observed in the Sb2Se3 solar cell with Zn0.93Mg0.07O/ZnO0.4S0.6 proposed double buffer layers due to appropriate CBO values at the junction interfaces. The short-circuit current density (Jsc) values increased as the sulfur concentration in the ZnO1-xSx buffer layer increased from 0 to 0.7 (x value), which could be related to the increase in the optical band gap of the ZnO1-xSx layer. The results demonstrated that under small positive offset conditions (spike-like), the carrier recombination rate at the interface is reduced, and consequently, the Sb2Se3 solar cell performance is improved. A Sb2Se3 solar cell with the optimum CBO amount (+0.3 eV) represented a conversion efficiency of 15.46%, which exhibited a 68% enhancement in comparison to the traditional CdS/Sb2Se3 solar cell.

Synergistic Interface Energy Band Alignment Optimization and Defect Passivation toward Efficient and Simple‐Structured Perovskite Solar Cell

Efficient electron transport layer–free perovskite solar cells (ETL‐free PSCs) with cost‐effective and simplified design can greatly promote the large area flexible application of PSCs. However, the absence of ETL usually leads to the mismatched indium tin oxide (ITO)/perovskite interface energy levels, which limits charge transfer and collection, and results in severe energy loss and poor device performance. To address this, a polar nonconjugated small‐molecule modifier is introduced to lower the work function of ITO and optimize interface energy level alignment by virtue of an inherent dipole, as verified by photoemission spectroscopy and Kelvin probe force microscopy measurements. The resultant barrier‐free ITO/perovskite contact favors efficient charge transfer and suppresses nonradiative recombination, endowing the device with enhanced open circuit voltage, short circuit current density, and fill factor, simultaneously. Accordingly, power conversion efficiency increases greatly from 12.81% to a record breaking 20.55%, comparable to state‐of‐the‐art PSCs with a sophisticated ETL. Also, the stability is enhanced with decreased hysteresis effect due to interface defect passivation and inhibited interface charge accumulation. This work facilitates the further development of highly efficient, flexible, and recyclable ETL‐free PSCs with simplified design and low cost by interface electronic structure engineering through facile electrode modification.

Impact of annealing temperature on band-alignment of PLD grown Ga2O3/Si (100) heterointerface

Cost-effective integration with existing silicon CMOS electronics has been one of the primary motivations for most of the emerging non-silicon devices. In this context, β-gallium oxide (Ga2O3) films were deposited on Si (100) substrate using pulsed laser deposition (PLD) technique in this research. After deposition, samples were further annealed at 600 °C and 800 °C under vacuum. X-ray diffractometer (XRD) was employed to observe the crystallinity variation due to the annealing. The crystallinity of samples degrades with annealing at 600 °C and an incremental improvement in crystallinity was again exhibited at 800 °C due to the possible rearrangement of the Ga and O atoms to their optimal sites. Further, x-ray photoelectron spectroscopy (XPS) was used for identification of elements and chemical composition. XPS results were also analyzed to locate the position of Ga 2p and Si 2p core levels and calculate the valance band offset (VBO) at Ga2O3/Si interface. Reflected electron energy loss spectroscopy (REELS) and ultraviolet photoelectron spectroscopy (UPS) were utilized to calculate the bandgap and work function, respectively. Moreover, valance band maxima and Fermi level position of the as-deposited and annealed samples were calculated using UPS. The bandgaps for the as-deposited, annealed samples at 600 °C and 800 °C were estimated to be 4.72 ± 0.05, 4.52 ± 0.05, and 4.48 ± 0.05 eV, respectively. Consequently, the work function of the sample increases with annealing due to increase in the oxygen vacancies and resulting bandgap narrowing. The band offsets (valance band, conduction band) were determined to be (3.35 ± 0.05 eV, 0.26 ± 0.02 eV), (3.55 ± 0.05 eV, 0.15 ± 0.02 eV) and (3.54 ± 0.05 eV, 0.17 ± 0.02 eV) for the as-deposited, annealed samples at 600 °C and 800 °C, respectively. This optimization of band alignment with annealing temperature shall be very useful for controlling the carrier transport at the interface for Ga2O3/Si based devices. Thus, a trade-off is to be performed between VBO and crystallinity of the Ga2O3/Si to operate the devices efficiently and reliably respectively.

Achieving 14.4% Alcohol-Based Solution-Processed Cu(In,Ga)(S,Se)2 Thin Film Solar Cell through Interface Engineering.

An optimization of band alignment at the p-n junction interface is realized on alcohol-based solution-processed Cu(In,Ga)(S,Se)2 (CIGS) thin film solar cells, achieving a power-conversion-efficiency (PCE) of 14.4%. To obtain a CIGS thin film suitable for interface engineering, we designed a novel "3-step chalcogenization process" for Cu2- xSe-derived grain growth and a double band gap grading structure. Considering S-rich surface of the CIGS thin film, an alternative ternary (Cd,Zn)S buffer layer is adopted to build favorable "spike" type conduction band alignment instead of "cliff" type. Suppression of interface recombination is elucidated by comparing recombination activation energies using a dark J- V- T analysis.

Superlattice Barrier HgCdTe nBn Infrared Photodetectors: Validation of the Effective Mass Approximation

Implementation of the unipolar barrier detector concept in HgCdTe-based compound semiconductor alloys is a challenging problem, primarily because practical lattice-matched materials that can be employed as the wide bandgap barrier layer in HgCdTe nBn structures present a significant valence band offset at the n-type/barrier interface, thus impeding the free flow of photogenerated minority carriers. However, it is possible to minimize the valence band offset by replacing the bulk HgCdTe alloy-based barrier with a CdTe–HgTe superlattice barrier structure. In this paper, an $8 \times 8$ k.p Hamiltonian combined with the nonequilibrium Green’s function formalism has been employed to numerically demonstrate that the single-band effective mass approximation is an adequate numerical approach, which is valid for the modeling, design, and optimization of band alignment and carrier transport in HgCdTe-based nBn detectors incorporating a wide bandgap superlattice barrier.

Humidity controlled sol-gel Zr/TiO2 with optimized band alignment for efficient planar perovskite solar cells

This work draws attention to environmental influence to function layers interfaces and the band alignment optimization of organic-inorganic metal halide planar perovskite solar cell (PSC), demonstrating an effective route to enhance the PSC photovoltaic conversion efficiency (PCE) and stability. The PCE of the PSC presents a sensitive correlation with the relative humidity (RH) during the electron transport layer (ETL) preparation. Through optimizing the RH condition, the PSCs show outstanding performances using sol-gel TiO2 ETL prepared under 55 RH%. Moreover, we systematically investigate the Zr doping effect in the TiO2 ink with a variation of Zr molar percentages from 0% to 7.5%. A high PCE of 15.06% with a good fill factor of 0.76 is obtained at 2mol% Zr doping, in contrast to 12.25% of the undoped device. The Mott-Schottky analysis reveals that the elevated open-circuit voltage (Voc) is attributed to the upshifting of flat band potential of TiO2 from −4.15eV to −4.02eV (vs. vacuum) after doping. Notably, the as-made cell shows low hysteresis and elevated stability with only 15% degradation of PCE (35% for PSC with pyrolyzed TiO2) after 550h/14 RH% ageing without encapsulation.

Band alignments of different buffer layers (CdS, Zn(O,S), and In2S3) on Cu2ZnSnS4

The heterojunctions of different n-type buffers, i.e., CdS, Zn(O,S), and In2S3 on p-type Cu2ZnSnS4 (CZTS) were investigated using X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) Measurements. The band alignment of the heterojunctions formed between CZTS and the buffer materials was carefully measured. The XPS data were used to determine the Valence Band Offsets (VBO) of different buffer/CZTS heterojunctions. The Conduction Band Offset (CBO) was calculated indirectly by XPS data and directly measured by NEXAFS characterization. The CBO of the CdS/CZTS heterojunction was found to be cliff-like with CBOXPS = −0.24 ± 0.10 eV and CBONEXAFS = −0.18 ± 0.10 eV, whereas those of Zn(O,S) and In2S3 were found to be spike-like with CBOXPS = 0.92 ± 0.10 eV and CBONEXAFS = 0.87 ± 0.10 eV for Zn(O,S)/CZTS and CBOXPS = 0.41 ± 0.10 eV for In2S3/CZTS, respectively. The CZTS photovoltaic device using the spike-like In2S3 buffer was found to yield a higher open circuit voltage (Voc) than that using the cliff-like CdS buffer. However, the CBO of In2S3/CZTS is slightly higher than the optimum level and thus acts to block the flow of light-generated electrons, significantly reducing the short circuit current (Jsc) and Fill Factor (FF) and thereby limiting the efficiency. Instead, the use of a hybrid buffer for optimization of band alignment is proposed.

Optimized electronic structure of a Cu(In,Ga)Se2 solar cell with atomic layer deposited Zn(O,S) buffer layer for high power conversion efficiency

The band alignment optimization of a Cu(In,Ga)Se2 solar cell with a Zn(O,S) buffer layer yielded a power conversion efficiency of 16.7%.

Recent development in colloidal quantum dots photovoltaics

The increasing demand for sustainable and green energy supply spurred the surging research on high-efficiency, low-cost photovoltaics. Colloidal quantum dot solar cell (CQDSC) is a new type of photovoltaic device using lead chalcogenide quantum dot film as absorber materials. It not only has a potential to break the 33% Shockley-Queisser efficiency limit for single junction solar cell, but also possesses low-temperature, high-throughput solution processing. Since its first report in 2005, CQDSCs experienced rapid progress achieving a certified 7% efficiency in 2012, an averaged 1% efficiency gain per year. In this paper, we reviewed the research progress reported in the last two years. We started with background introduction and motivation for CQDSC research. We then briefly introduced the evolution history of CQDSC development as well as multiple exciton generation effect. We further focused on the latest efforts in improving the light absorption and carrier collection efficiency, including the bulk-heterojunction structure, quantum funnel concept, band alignment optimization and quantum dot passivation. Afterwards, we discussed the tandem solar cell and device stability, and concluded this article with a perspective. Hopefully, this review paper covers the major achievement in this field in year 2011–2012 and provides readers with a concise and clear understanding of recent CQDSC development.