Se2 Thin Films Research Articles

Effects of Alkali Elements on Copper Indium Gallium Aluminum Selenide Flexible Solar Cells Fabricated on Polyimide Substrates.

Polycrystalline Cu(InGaAl)Se2 (CIGAS) thin films were prepared on polyimide (PI) foils by depositing aluminum (Al) and CIGS precursor layers. Three ceramic CIGS quaternary targets with different sodium (Na) contents were used for investigating the influences of alkali doping at an annealing temperature of 500 °C. The Al concentration was enriched at the front interfaces of absorber films with different Na doping amounts after annealing. Na in the precursor layer diffused to both interfaces during the annealing process, most Na diffused into the Mo layer, and Na existed in the annealed film as compounds Na2Sex and Na2SeO3. An appropriate amount Na element could be beneficial for grain growth in the region beneath the surface. Low Na doping had no significant effect on the crystallization property. High Na doping effected the diffusion of the Cu2-xSe liquid phase and reduced the grain size. On the basis of good crystallization, the passivation effect of Na can effectively improve the performance of cells. A certified power conversion efficiency of 16.19% of a CIGAS flexible solar cell with a 0.54 cm2 active area has been achieved.

Wide absorption spectrum and rapid response time of PEC photodetectors based on MoS2–Se nanocomposites

Photoelectrochemical (PEC) photodetectors convert light into electrical signals via PEC processes, providing high sensitivity, positioning them as promising candidates for next-gen optoelectronic devices. Over the past few years, there has been a significant interest in the field of photodetection regarding 2D materials, owing to their advantageous properties. In this study, the PEC photodetection of hydrothermally synthesized MoS2–Se nanocomposites have been systematically tested. Structural analysis confirms the coexistence of MoS2 and Se phases in the synthesized nanocomposites with crystallite sizes in the nanoscale dimension. Optical studies indicate that MoS2–Se nanocomposites exhibit broadened absorption spectrum extending to UV region, unlike pure MoS2 which absorbs solely in the visible region. This enhances their sensitivity for wide-band photodetectors with a small bandgap (1.01–1.34 eV). Optoelectronic study revealed superior performance of MoS2–Se thin film PEC device with efficient photoresponse with a responsivity of 38.3 μA/W and fast response time of 0.12 s. The novelty of this research resides in the integration of these unique features in MoS2–Se nanocomposites for PEC photodetectors, explored for the first time. The innovative aspect of this study involves employing spin coating to deposit 2D MoS2–Se thin films, which enhances control over film thickness and uniformity, thereby resulting in improved responsivity and photocurrent.

Development of Sb phase change thin films with high thermal stability and low resistance drift by alloying with Se

A single Sb phase demonstrates potential for use in phase change memory devices. However, the rapid crystallization of Sb at room temperature imposes limitations on its practical application. To overcome this issue, Sb is alloyed with Se using a reactive co-sputtering deposition technique, employing both Sb and Sb2Se3 sputter targets. This process results in the formation of Sb-rich Se thin films with varying compositions. Compared to pure Sb, the Sb-rich Se thin films exhibit enhanced thermal stability due to the formation of Sb–Se bonds and reduced resistance drift. In particular, the Sb86.6Se13.4 thin film demonstrates an exceptionally low resistance drift coefficient (0.004), a high crystallization temperature (Tc = 195 °C), a high 10-year data retention temperature (116.3 °C), and a large crystallization activation energy (3.29 eV). Microstructural analysis of the Sb86.6Se13.4 reveals the formation of a trigonal Sb phase with (012) texture at 250 °C, while Sb18Se and Sb2Se3 phases form at 300 °C. Conversely, the Sb98.3Se1.7 thin film shows the formation of the single Sb phase with (001) texture, a Tc of 145 °C, and a low resistance drift coefficient (0.011). Overall, this study demonstrates that the alloying strategy is a viable approach for enhancing thermal stability and reducing resistance drift in Sb-based phase-change materials.

Bandgap-graded Cu2Sn1-xGexS3 thin film solar cells prepared by sputtering SnGe/Cu targets

Cu2Sn1-xGexS3 (CTGS) thin films were created through sulfurizing SnGe/Cu precursors deposited by the sputtering method. The effects of different sulfurization conditions on the crystallinity, morphology, composition, optical-electrical properties and chemical information of the CTGS thin films have been investigated in detail. The results demonstrate that the CTGS thin films obtained by using sulfurization temperature at 550 °C for 20min have the best crystal quality and smooth surface morphology with largest grains and the best optical-electrical properties. Meanwhile, CTGS thin films solar cell with the best performance is found that the CTGS film possesses a graded band gap structure, in which the band gap gradually become larger towards the Mo electrode, and the CdS/CTGS hetero-junction interface shows the “type I″ energy band alignment with a “spike-like” structure. As a result, the obtained bandgap-graded CTGS thin film solar cells with the best power conversion efficiency (PCE) of 3.35 (±0.04) % manifest an open-circuit voltage of 389 (±7.78) mV and a short-circuit current density of 26.98 (±0.54) mA cm−2. This work demonstrates a promising route to develop high-efficiency Cu(In,Ga)Se2 or Cu2SnS3-based thin film solar cells.

Investigation of Physical Properties of Nanostructured Selenium-Based Compound Semiconductors

The study of physical properties of compound semiconductors plays a vital role in the device fabrication point of view. Different investigations suggest that one can tune these properties by impurity dopants, irradiation techniques, adopting different synthesis techniques, etc. In this study, the physical properties of nanostructured thin films of Se–S compound were tuned by different Hg dopant concentrations. X-ray diffraction spectra clarify the preferred crystallite growth occurs in the cubic phase of the corresponding Bravi’s crystal system. The particle size calculated from Bragg’s reflection spectra by using Scherrer’s analytical formula illustrates an enhancement of crystallite size in highly concentrated Hg-doped sample. Similar consequences are found in surface morphological micrographs. The optical band gap reduced from 1.65[Formula: see text]eV to 1.54[Formula: see text]eV on increasing the doping concentration of Hg, according to the computed optical parameters using optical transmission spectra in the wavelength range 200–1100[Formula: see text]nm. The thin-film refractive index dispersion data fits well in accordance with the single oscillator model. Investigation of electrical properties tells that there is significant improvement in electrical conductivity in highly concentrated Hg-doped samples. Therefore, the apparent tuning as observed in the physical properties of the investigated material may have practical uses in industry for a variety of optoelectronic devices.

Interfacial Engineering of Selenium‐Based Photodiodes for Extremely Low‐Noise Photodetection and Single‐Pixel Imaging

AbstractSelenium as one of the most historical semiconductors has been used for photovoltaics and photodetection, thanks to its excellent optoelectronic properties, superior stability, facile and low‐cost fabrication. However, commercialized Se‐based devices are mainly based on its amorphous state, which significantly limits the charge transport and requires relatively large bias voltage. The charge carrier dynamics of high‐performance crystalline Se thin films are barely reported, and related photodiodes have not been realized mainly limited by the large dark current and slow response speed. Here, poly‐crystalline Se thin films via vacuum evaporation, in situ thermal annealing, and engineered the interfaces with various metal electrodes are fabricated, which resulted in enhanced charge transport properties and reduced device leakage. The optimized Se photodiodes exhibited excellent performance metrics, including extremely low dark current density, decent specific detectivity, and superior stability. Furthermore, the optimized Se photodetectors are also introduced for single‐pixel imaging, which demonstrates excellent sensitivity and great potential for real applications.

Optical and chemical properties of As–Se and As–S–Se solution processed thin films prepared via As50Se50 source solution modification

This work presents the prospect of As–S–Se and As–Se thin film preparation from a modified solution of As50Se50 chalcogenide glass in ethylenediamine.

Unlocking the effect of film thickness on the thermoelectric properties of thermally evaporated Cu2-x Se thin films.

Miniaturization is crucial to realize thermoelectric (TE) devices as an energy source for smart utilities. The present work reports the bulk-like ZT value realized in Cu2-x Se thin films in the mid-temperature range. The effect of varying the film thickness on the structural and TE properties were systematically studied, and the obtained results were compared with that of their bulk counterpart. The detailed structural characterizations revealed the nonstoichiometric polycrystalline nature of Cu2-x Se with the unusual presence of monoclinic and cubic phases. Upon increasing the film thickness, the TE measurements showed a simultaneous increase in the electrical conductivity (σ) and Seebeck coefficient (S) due to increased grain size and uniformity. Due to the simultaneous increase in σ and S, an ultrahigh power factor (PF) value of 5185 μW m-1 K-2 and a bulk-like figure of merit ZT ≈ 1.2 were achieved.

Exploring the deposition pathway in the notch region of double-graded bandgap ACIGS solar cells

The bandgap widening of Ag-alloyed Cu(In,Ga)Se2 (ACIGS) thin films poses a significant challenge in enhancing their photocurrent. This study demonstrates correlations between element diffusion behavior and notch-point formation in ACIGS films, which ultimately influence the resulting current circuit voltage and fill factor. Our findings delineate a novel approach for fabricating a suitably tailored band gap grading in ACIGS, which serves as a bottom subcell in tandem configuration. This was achieved by modulating a wide range of process temperatures from 340 to 470 °C, which were measured by pyrothermal, to assess the GGI profile during deposition. Experiments were undertaken to variate the second-stage conditions, striving to sustain the photocurrent, mitigate defects, and enhance crystallinity, achieving an impressive performance of 17.7 % without applying post-deposition treatments or anti-reflection coatings. The efficiencies surpassing 8 % were achieved as we measured these cells under a 715 nm long-pass filter, which corresponds to the bandgap of a typical top cell in tandem applications (1.73 eV photon energy). These results emphasize the significance of understanding the deposition temperature and its impact on the properties of ACIGS films, paving the way for further advancements in solar-cell technology.

Growth and characterization of CT(S,Se) thin films and Al/n-Si/p-CT(S,Se)/Mo heterojunction diode application employing a two-stage process

Growth and characterization of CT(S,Se) thin films and Al/n-Si/p-CT(S,Se)/Mo heterojunction diode application employing a two-stage process

Unveiling the optimal selenization temperature for enhancing efficiency in flexible Cu(In,Ga)Se2 thin film solar cells

This work is centered on the growth of metallic precursors (Cu/In/Ga) through sequential evaporation, followed by a controlled ramping step selenization process aimed at synthesizing thin films based on copper indium gallium diselenide (CIGSe) on stainless steel substrates. Selenization temperatures ranging from 475 to 525 °C were employed to optimize the most suitable recrystallization temperature. A comprehensive characterization was conducted, covering structural, morphological, topographical, optical, and electrical aspects, revealing the distinct role of selenization temperature. Subsequently, solar cells were fabricated using the synthesized absorbers, resulting in an impressive efficiency of 15.89 % for the absorber selenized at 525 °C. This observation underscores the pivotal role of selenization parameters in shaping the quality of CIGSe, directly influencing the performance of solar cells. Consequently, this research not only focuses on refining the growth process of CIGSe through a modified approach but also provides valuable insights for the advancement of flexible solar cell technology.

A Comprehensive Review on the Recent Strategy of Cation Substitution in CZTS(Se) Thin Films to Achieve Highly Efficient Kesterite Solar Cells

Kesterite Cu2ZnSnS4 (CZTS) and selenized CZTS (CZTSSe) are the most potential replacements of Cu(InxGa1−x)Se2 as absorber layers in thin‐film solar cells, aligning with modern green energy policies. In spite of a reported power conversion efficiency (η) of nearly 14.9%, their efficiency still lags much behind their predecessors like cadmium telluride and CIGS. Major obstacles hindering the performance of CZTS‐based thin‐film solar cells pertain to the formation of CuZn and ZnCu antisite defects, along with high density of 2CuZn + SnZn donor defects, which adversely affects open‐circuit voltage and η. To combat these challenges, researchers have explored cation substitution by incorporating alternative isovalent atoms, such as substituting Ag/Li for Cu, Mn/Cd for Zn, and Ge/Ti for Sn, as a potent strategy to inhibit severe potential fluctuations and undesirable tail states caused by Cu–Zn disorder and Sn‐related defects. Recently single‐cation substitution in CZTS (or CZTSSe) has garnered considerable attention, while dual‐cation incorporation has emerged as an intriguing avenue to enhance device performance, although in its nascent stage of research and development. This review discusses recent progress on cation substitution to suppress antisite defects in CZTS (or CZTSSe), leading to high‐efficiency thin‐film solar cells.

Unravelling the intricacies of selenization in sequentially evaporated Cu(In,Ga)Se2 Thin film solar cells on flexible substrates

This study aimed to fabricate copper indium gallium diselenide (CIGSe) thin films using a novel two-step approach. Firstly, we deposited metallic precursors (Cu/In/Ga) onto a Mo-coated stainless steel substrate using thermal evaporation at unintentional substrate temperature. Subsequently, selenization was carried out in a furnace under the presence of an inert gas. The quality of the CIGSe thin films was analyzed to explore the influence of selenization temperature (450 °C–550 °C) and duration (30 and 60 min), while maintaining an inert atmosphere inside the selenization furnace. The structural analysis revealed the progressive development of additional phases over time, resulting in the formation of a complete chalcopyrite CIGSe structure with the preferred reflection on the (112) plane. The absorber layer exhibited a thickness of 2 μm, with atomic ratios of 0.83 for Cu/(In+Ga) and 0.24 for Ga/(In+Ga) in the film selenized at 550 °C. P-type conductivity was observed in the CIGSe thin film, with a carrier concentration of up to 1017 cm−3, and it displayed a well-defined and uniform morphology characterized by a large grain size of approximately 0.9 μm. Utilizing the optimized conditions, we successfully fabricated solar cells on a flexible substrate, achieving a photoconversion efficiency of up to 9.91%. This research delves into the impact of selenization parameters on the growth of CIGSe absorber layers and introduces a new approach that could significantly influence the feasibility and industrialization of flexible solar cells.

Effect of In(O,S) buffer layer on the band alignment and the performance of CZT(S,Se) thin film solar cells

The purpose of this study is to assess how the presence of an indium oxysulfide buffer layer affects the efficiency of copper zinc tin (sulfur,selenium) photovoltaic cells. The effectiveness of the junction in In(O,S)/CZT(S,Se) solar cells was evaluated by examining the influence of conduction band offset using the TCAD simulator. To achieve this objective, a comparison was conducted between a CZT(S,Se) cell having a CdS buffer layer and one having an In(O,S) buffer layer. Ratios of oxygen to sulfur and sulfur to selenium control the band gap alignment at the interface of the In(O,S)-CZT(S,Se) layer. High levels of oxygen in the In(O,S) layer result in a significant conduction band offset, as demonstrated by the simulation results. By incorporating this buffer layer, the efficiency of the cell increased by 3.88%, resulting in a total efficiency of 16.48% for the CZT(S,Se) photovoltaic cell. The study presented valuable perspectives on the function of an In(O,S) buffer layer in boosting the efficacy of CZT(S,Se) photovoltaic cells, highlighting the importance of comprehending the properties of materials used in such apparatus.

Inhibiting aluminum oxidation in CuInAl precursor films: Significance for achieving high-performance Cu(InAl)Se2 solar cells

In this work, we demonstrate the effects of aluminum and aluminum oxide within CuInAl precursor films on the formation process of Cu(InAl)Se2 (CIASe) thin films. We carried out a large number of experiments to optimize the conditions for obtaining CIASe thin films with excellent adhesion and electrical properties. Our findings suggest that the fabrication conditions of the precursor films, such as the vacuum level of sputtering chamber and the structure of the precursor films, significantly affect to the growth of the CIASe layer during selenization annealing. Through a series of experiments, we successfully obtained a 2.4 μm-thick CIASe thin film with strong adhesion to the Mo-coated substrate and fabricated a CIASe solar cell with an efficiency of 7.7%. Our study demonstrates the great potential of the two-step fabrication approach for CIASe thin film production, and further improvements are expected by optimizing the fabrication conditions.

A computational study for understanding the effect of various parameters on the performance of three different CI(G)Se-based thin film solar cells

The copper indium selenium/copper indium gallium selenium (CI(G)Se)-based thin film solar cells (TFSCs) have been fascinating in the photovoltaic market due to their potential to attain high efficiency of solar cells and modules at relatively little cost. This research work introduces the simulation of the CI(G)Se TFSCs through SCAPS software. Some parameters like thickness, bandgap, and carrier concentration of semiconducting materials used in the CI(G)Se TFSCs were optimized to get the solar cell efficiency as high as possible. The optimized efficiencies for CISe, CIGSe, and CIGSe bilayer TFSCs were 22.81, 27.32, and 27.99%, respectively. It is seen from the results that the device’s performance using two absorber layers in CI(G)Se TFSCs was comparatively higher than using a single absorber layer. This outcome is mainly due to the better absorption of photons in CI(G)Se TFSCs containing two absorber layers. The SCAPS software also analyzed the experimentally obtained parameters of CI(G)Se and CdS thin films and noticed more than 20% efficiency, showing potential parameters to use in commercial applications. The effect of defects found in the CI(G)Se absorber layer, CdS buffer layer, and CdS/CI(G)Se interface on the solar cell parameters are primarily studied here. It is observed that the increasing defects in the CI(G)Se TFSCs could impact negatively the device’s performance by recombining the generated charge carriers. Better results were found for CI(G)Se TFSCs at a lower work temperature (by reducing the saturation current, and at a smaller series resistance value as well as a larger shunt resistance value (by reducing the alternative paths for generated charge carriers). Therefore, understanding these results could provide complete information on the optimization process that can serve as a guide to achieve highly efficient electronic devices experimentally.

Low-temperature processed (Ag,Cu)(In,Ga)Se2 thin film solar cells using Se@Ag2Se core-shell hybrid inks and performance optimization

To create (Ag,Cu)(In,Ga)Se2 (ACIGS) thin films using a hybrid ink method, large-scale synthesis of Se@Ag2Se core-shell and Cu-Se nanoparticles was performed, surpassing the scale of previous methods. In particular, although the Se@Ag2Se core-shell was also a precursor for Ag in ACIGS, the amorphous Se (a-Se) in the Se@Ag2Se core-shell acted as a flux during the reaction due to its low melting temperature (221 °C), resulting in the formation of ACIGS thin films at a lower reaction temperature (∼340 °C) and shorter reaction time (30 min) than conventional ones (∼530 °C, 60 min). Furthermore, by varying the amount of Se@Ag2Se core-shell in the hybrid ink, the Ag ratio (Ag/(Ag + Cu)) was controlled, and pure ACIGS without secondary phases was obtained. However, the ACIGS solar cells fabricated by this method had a conversion efficiency of only approximately 4% and relatively low shunt resistance (Rsh) and carrier concentration (∼1015/cm3). To improve the Rsh and carrier concentration, ACIGS was doped with Sb by mixing the Sb precursor into the hybrid ink. The Sb-doped ACIGS exhibited grain growth and grain boundary reduction without secondary phase formation. In addition, capacitance–voltage (C–V) measurements showed an increase in the carrier concentration (∼1016/cm3). Therefore, the conversion efficiency of Sb-doped ACIGS increased with increasing Rsh and open-circuit voltage (VOC).

Modeling of the persistent photoconductivity in Cu(In,Ga)Se2: On the origin of the temperature and light intensity dependences

The temperature and light intensity dependences of persistent photoconductivity (PPC) in Cu(In,Ga)Se2 thin films have not been fully understood so far. We show by means of the numerical simulations that experimental characteristics of PPC can be explained by the combination of two factors: a higher than previously assumed energy barrier for the electron capture process (at least 0.26 eV), and stretched-exponential character of transients. Our findings are compatible with the hypothesis linking PPC in Cu(In,Ga)Se2 with a defect exhibiting the large lattice relaxation, like the Se-Cu divacancy complex (VSe-VCu).

Impact of tellurium as an anion dopant on the photovoltaic performance of wide-bandgap Cu(In,Ga)Se2 thin-film solar cells with rubidium fluoride post-deposition treatment

The development of wide-bandgap Cu(In,Ga)Se2 thin films is crucial in order to reach the theoretical Shockley–Queisser limit values in single-crystal solar cells. However, the performance of solar cells based on wide-bandgap thin film absorbers has lagged significantly compared to that of their narrow-bandgap counterparts. Herein, we develop a feasible strategy to improve the photovoltaic performance of wide-bandgap Cu(In,Ga)Se2 chalcopyrite thin-film solar cells by simultaneously doping with both RbF PDT and Te2− anions as dopants in the absorber layer during the three-stage co-evaporation process. Besides inducing significant change in the GGI gradient, the synergistic effect of the Te2− anion dopant is rather beneficial in terms of controlling grain size, defects in grain boundaries, and charge carrier lifetime for encouraging charge separation and extraction, which contributes to simultaneously boosting short-circuit current density and fill factor. Te-poor devices afford an impressive efficiency of 9.58%, compared to 6.43% for control devices. More importantly, the efficiency and Voc values obtained for wide-bandgap-based thin-film solar cells containing Te anions were the highest compared to their counterparts as reported in the literature. These results demonstrate the role of Te2− anions in wide-bandgap absorber thin films on the photovoltaic performance of thin-film solar cells and the potential of this approach for use in reasonable and effective design of highly efficient wide-bandgap thin-film solar cells.

Remarkable average thermoelectric performance of the highly oriented Bi(Te, Se)-based thin films and devices

Bi(Te, Se)-based compounds have attracted lots of attention for nearly two centuries as one of the most successful commercial thermoelectric (TE) materials due to their high performance at near room temperature. Compared with 3D bulks, 2D thin films are more compatible with modern semiconductor technology and have unique advantages in the construction of micro- and nano-devices. For device applications, high average TE performance over the entire operating temperature range is critical. Herein, highly c-axis-oriented N-type Bi(Te, Se) epitaxial thin films have been successfully prepared using the pulsed laser deposition technology by adjusting the deposition temperature. The film deposited at ∼260 °C demonstrate a remarkable average power factor (PFave) of ∼24.4 μW·cm−1·K−2 over the temperature range of 305–470 K, higher than most of the state-of-the-art Bi(Te, Se)-based films. Moreover, the estimated average zT value of the film is as high as ∼0.81. We then constructed thin-film TE devices by using the above oriented Bi(Te, Se) films, and the maximum output power density of the device can reach up to ∼30.1 W/m2 under the temperature difference of 40 K. Predictably, the outstanding average TE performance of the highly oriented Bi(Te, Se) thin films will have an excellent panorama of applications in semiconductor cooling and power generation.