Two-Step Annealing CZTSSe/CdS Heterojunction to Improve Interface Properties of Kesterite Solar Cells

Solvent Engineering-Enabled Surface Defect Passivation in Cu2ZnSn(S,Se)4 Solar Cells with Low Open-Circuit Voltage Losses and Improved Carrier Lifetime

The efficiency of earth-abundant kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells has been lagging behind the Shockley-Queisser limit primarily due to the presence of deep-level defects. These deep-level defects cause critical issues such as short carrier diffusion length, significant band tailing, and a large open-circuit voltage (Voc) deficit, ultimately leading to low device efficiency. To address these issues, we propose a post-fabrication defect healing strategy by dip-coating the CZTSSe film in dimethylformamide (DMF) solvent. Immersing the absorber layer in DMF (a polar solvent), neutralizes CuSn antisite defects through chemical bonding and facilitates the formation of a dense, smooth CZTSSe film with larger grain size. Deep-level transient spectroscopy revealed a remarkable increase in carrier diffusion length from 93 nm (control device) to 142 nm (champion device), confirming the beneficial effect of solvent-assisted post-treatment on mitigating CuSn antisite defects. The reduction in defect densities led to a decrease in Voc deficit by up to 289 mV, accompanied by an increased champion device efficiency of 11.4 %. This work highlights the huge potential of the DMF post-treatment strategy for defect healing in CZTSSe solar cells.

Endurable IGZO/SnSx/IGZO Heterojunction Phototransistor Arrays for Image Sensors

Optoelectronic devices require stable operation to detect repetitive visual information. In this study, endurable arrays based on heterojunction phototransistors composed of indium-gallium-zinc oxide (IGZO) with a low dark current and tin sulfide (SnSx) capable of absorbing visible light are developed for image sensors. The tandem structure of IGZO/SnSx/IGZO (ISI) enables stable operation under repetitive exposure to visible light by improving the transport ability of the photoexcited carriers through mitigated trap sites and their separation into each IGZO layer. Additionally, the structure promotes recombination by confining the holes. Therefore, the optimal ISI phototransistors exhibit a photoresponsivity of 514.50 A/W and a detectivity of 1.31 × 109 Jones under red light (635 nm) of 1 mW/mm2 and endurable time-dependent photoresponse characteristics, including a slope value of 1.66 × 10-11, without the persistent photoconductivity phenomenon under green light (532 nm) at a frequency of 50 mHz for over 4,000 s. Furthermore, image sensing characteristics of the 6 × 6 arrays based on ISI phototransistors for image sensors are demonstrated by sequentially applying "4" and "2" digit numbers. These technologies contribute to the development of endurable oxide-based optoelectronic devices and provide valuable perspectives on the utility of next-generation image sensors.

Tailoring Li assisted CZTSe film growth under controllable selenium partial pressure and solar cells

It is still critical to prepare a high-quality absorber layer for high-performance Cu2ZnSnSe4 (CZTSe) multi-component thin film solar cell. The gas pressure during the selenization process is commonly referred to as the pressure of inert gas in the tube furnace, while the exact selenium partial pressure is difficult to be controlled. Therefore, the grain growth under different selenium partial pressures cannot be made clear, and the film quality cannot be controlled as well. In this work, we use a sealed quartz tube as the selenization vessel, which can provide a relatively high and controllable selenium partial pressure during the selenization process. To further tailor the grain growth, lithium doping is also utilized. We find that lithium can greatly promote the growth of CZTSe films as the selenium partial pressure is controlled near the selenium saturation vapor pressure. Combined with ALD-Al2O3, the crystallization quality of CZTSe absorber films is significantly enhanced and the efficiency of CZTSe solar cells achieved a significant improvement. This work clarifies the effect of controllable Se pressure on CZTSe film growth and can lead to better results in CZTSe and other multi-compound thin film solar cells.

Element Diffusion Induced Carrier Transport Enhancement in High-Performance CZTSSe Self-Powered Photodetector

Cu2ZnSn(S,Se)4 (CZTSSe) has attracted great interest in thin-film solar cells due to its excellent photoelectric performance in past decades, and recently is gradually expanding to the field of photodetectors. Here, the CZTSSe self-powered photodetector is prepared by using traditional photovoltaic device structure. Under zero bias, it exhibits the excellent performance with a maximum responsivity of 0.77 A W-1, a high detectivity of 8.78 × 1012 Jones, and a wide linear dynamic range of 103 dB. Very fast response speed with the rise/decay times of 0.576/1.792 µs, and ultra-high switching ratio of 3.54 × 105 are obtained. Comprehensive electrical and microstructure characterizations confirm that element diffusion among ITO, CdS, and CZTSSe layers not only optimizes band alignment of CdS/CZTSSe, but also suppresses the formation of interface defects. Such a suppression of interface defects and spike-like band alignment significantly inhibit carrier nonradiative recombination at interface and promote carrier transport capability. The low trap density in CZTSSe and low back contact barrier of CZTSSe/Mo could be responsible for the very fast response time of photodetector. This work definitely provides guidance for designing a high performance self-powered photodetector with high photoresponse, high switching ratio, fast response speed, and broad linear dynamic range.

Toward High Efficient Cu2 ZnSn(Sx ,Se1- x )4 Solar Cells: Break the Limitations of VOC and FF

Increasing the fill factor (FF) and the open-circuit voltage (V_OC ) simultaneously together with non-decreased short-circuit current density (J_SC ) are a challenge for highly efficient Cu₂ ZnSn(S,Se)₄ (CZTSSe) solar cells. Aimed at such target in CZTSSe solar cells, a synergistic strategy to tailor the recombination in the bulk and at the heterojunction interface has been developed, consisting of atomic-layer deposited aluminum oxide (ALD-Al₂ O₃ ) and (NH₄ )₂ S treatment. With this strategy, deep-level Cu_Zn defects are converted into shallower V_Cu defects and improved crystallinity, while the surface of the absorber is optimized by removing Zn- and Sn-related impurities and incorporating S. Consequently, the defects responsible for recombination in the bulk and at the heterojunction interface are effectively passivated, thereby prolonging the minority carrier lifetime and increasing the depletion region width, which promote carrier collection and reduce charge loss. As a consequence, the V_OC deficit decreases from 0.607 to 0.547 V, and the average FF increases from 64.2% to 69.7%, especially, J_SC does not decrease. Thus, the CZTSSe solar cell with the remarkable efficiency of 13.0% is fabricated. This study highlights the increased FF together with V_OC simultaneously to promote the efficiency of CZTSSe solar cells, which could also be applied to other photoelectronic devices.

Growth-control of hexagonal CdS-decorated ZnO nanorod arrays with low-temperature preheating treatment for improved properties and efficient photoelectrochemical applications

The limitations of oxide semiconductor-based solar cells in achieving high energy conversion efficiencies have prompted incessant research efforts towards the creation of efficient heterostructures. Despite its toxicity, no other semiconducting material can fully replace CdS as a versatile visible light-absorbing sensitizer. Herein, we explore the aptness of preheating treatment in the successive ionic layer adsorption and reaction (SILAR) deposition technique and improve the understanding of the principle and the effects of a controlled growth environment on thus-formed CdS thin films. Single hexagonal phases of nanostructured cadmium sulfide (CdS)-sensitized zinc oxide nanorods arrays (ZnO NRs) have been developed without the support of any complexing agent. The influences of film thickness, cationic solution pH and post-thermal treatment temperature on the characteristics of binary photoelectrodes have been investigated experimentally. Interestingly, the preheating-assisted deposition of CdS, which is rarely applied for the SILAR technique, resulted in improved photoelectrochemical performance similar to the post-annealing effect. The X-ray diffraction pattern revealed that optimized ZnO/CdS thin films were polycrystalline with high crystallinity. Examination of the morphology of the fabricated films via field emission scanning electron microscopy showed that film thickness and medium pH altered the growth mechanism of nanoparticles, thereby changing their particle sizes, which had a significant influence on the film's optical behavior. The effectiveness of CdS as a photosensitizer and the band edge alignment for ZnO/CdS heterostructures were evaluated using ultra-violet visible spectroscopy. Facile electron transfer in the binary system as evidenced in electrochemical impedance spectroscopy Nyquist plots, therefore, promotes higher photoelectrochemical efficiencies from 0.40% to 4.30% under visible light illumination as compared with the pristine ZnO NRs photoanode.

GeSe-evoked synchronous strategy for electrodeposited CZGSe solar cells

Sn-free Cu₂ZnGeSe₄ (CZGSe) is emerging as a promising non-toxic and earth-abundant photovoltaic absorber material due to its attractive electrical and optical properties as well as its high theoretical conversion efficiency. Nevertheless, no photovoltaic device fabricated through the green electrodeposition process has yet been reported, likely due to the poor solubility of Ge-based salts and harsh electrodeposition conditions. Herein, we propose a GeSe-evoked synchronous strategy involving a Ge incorporation and selenization-regulated co-heating process of GeSe and Se, following electrodeposition of a Cu-Zn preformed layer. We experimentally found that the low-melting-point GeSe could promote the crystal growth and induce a high-quality bulk absorber layer and good back interface. In the GeSe-promoted sample, it was found that MoSe₂ could ensure a good back quasi-Ohmic contact, and the band bending at the grain boundaries (GBs) was favorably inverted. Moreover, the depletion region width was also prolonged, and the deleterious Cu_Zn near E_F was passivated, leading to an increased carrier separation. In turn, a surprising progress in device performance was found, achieving a ground-breaking efficiency of 3.69%, and it could fill the bank of green electrodeposited CZGSe-based solar cells.

Energy Band Alignment by Solution-Processed Aluminum Doping Strategy toward Record Efficiency in Pulsed Laser-Deposited Kesterite Thin-Film Solar Cell

Kesterite-based Cu2ZnSnS4 (CZTS) thin-film photovoltaics involve a serious interfacial dilemma, leading to severe recombination of carriers and insufficient band alignment at the CZTS/CdS heterojunction. Herein, an interface modification scheme by aluminum doping is introduced for CZTS/CdS via a spin coating method combined with heat treatment. The thermal annealing of the kesterite/CdS junction drives the migration of doped Al from CdS to the absorber, achieving an effective ion substitution and interface passivation. This condition greatly reduces interface recombination and improves device fill factor and current density. The JSC and FF of the champion device increased from 18.01 to 22.33 mA cm-2 and 60.24 to 64.06%, respectively, owing to the optimized band alignment and remarkably enhanced charge carrier generation, separation, and transport. Consequently, a photoelectric conversion efficiency (PCE) of 8.65% was achieved, representing the highest efficiency in CZTS thin-film solar cells fabricated by pulsed laser deposition (PLD) to date. This work proposed a facile strategy for interfacial engineering treatment, opening a valuable avenue to overcome the efficiency bottleneck of CZTS thin-film solar cells.

Plasmonic Local Electric Field-Enhanced Interface toward High-Efficiency Cu2ZnSn(S,Se)4 Thin-Film Solar Cells

The kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have shown a continuous rise in power conversion efficiencies in the past years. However, the encountered interfacial problems with respect to charge recombination and extraction losses at the CdS/CZTSSe heterojunction still hinder their further development. In this work, an additional plasmonic local electric field is imposed into the CdS/CZTSSe interface through the electrostatic assembly of a two-dimensional (2D) ordered Au@SiO2 NP array onto an aminosilane-modified surface absorber. The interfacial electric properties are tuned by controlling the coverage particle distance, and the finite-difference time domain (FDTD) simulation demonstrates that the strong near-field enhancement mainly occurs near the p-n junction interface. It is shown that the imposed local electric field leads to interfacial electrostatic potential (Velec) augmentation and improves the charge extraction and recombination processes. These electric benefits enable remarkable improvements in open-circuit voltage (Voc) and short-circuit current (Jsc), leading to the cell efficiency being increased from 10.19 to 11.50%. This work highlights the dramatic role of the plasmonic local electric field and the use of the 2D Au@SiO2 NP array to modify a surface absorber instead of the extensively used ion passivation, providing a new strategy for p-n junction engineering in kesterite photovoltaics.