Effect of Metal-Precursor Stacking Order on Volume-Defect Formation in CZTSSe Thin Film: Formation Mechanism of Blisters and Nanopores.

In this study, we investigated the effect of the stacking order of metal precursors on the formation of volume defects, such as blisters and nanopores, in CZTSSe thin-film solar cells. We fabricated CZTSSe thin films using three types of metal-precursor combinations, namely, Zn/Cu/Sn/Mo, Cu/Zn/Sn/Mo, and Sn/Cu/Zn/Mo, and studied the blister formation. The blister-formation mechanism was based on the delamination model, taking into consideration the compressive stress and adhesion properties. A compressive stress could be induced during the preferential formation of a ZnSSe shell. Under this stress, the adhesion between the ZnSSe film and the Mo substrate could be maintained by the surface tension of a metallic liquid phase with good wettability, or by the functioning of ZnSSe pillars as anchors, depending on the type of metal precursor used. Additionally, the nanopore formation near the back-contact side was found to be induced by the columnar microstructure of the metal precursor with the Cu/Zn/Mo stacking order and its dezincification. Based on the two volume-defect-formation mechanisms proposed herein, further development of volume-defect-formation suppression technology is expected to be made.

CZTSSe Formation Mechanism Using a Cu/Zn/SnS Stacked Precursor: Origin of Triple CZTSSe Layer Formation.

In this study, to control the formation of non-uniformly distributed large voids and Cu-Sn alloy agglomeration, which leads to local compositional misfit and secondary phase formation, a SnS compound precursor was applied instead of metal Sn to avoid compositional non-uniformity. Using a Cu/Zn/SnS stacked precursor, a temperature tracking experiment was conducted to confirm the formation controllability of the void and the secondary phase. According to the results of this temperature-profile tracking experiment, it was confirmed that the large void was successfully controlled; however, an additional ZnSSe secondary phase layer was formed in the middle of the CZTSSe upper layer and small voids were distributed relatively uniformly in the bottom CZTSSe layer. An efficiency of approximately 8% was obtained when the Cu/Zn/SnS stacked precursor was used. The origins of the low short-circuit current and fill factor are posited to be caused by the increase of the energy bandgap of the CZTSSe layer due to the SnS precursor, the thin top CZTSSe layer (around 600 nm) of the triple CZTSSe layer, and the diffusion length extension of the minor carriers caused by bypassing the ZnSSe phase.

Effect of Al2O3 Dot Patterning on CZTSSe Solar Cell Characteristics.

In this study, a 5-nm thick Al2O3 layer was patterned onto the Mo electrode in the form of a dot to produce a local rear contact, which looked at the effects of this contact structure on Cu2ZnSn(S1-xSex)4 (CZTSSe) growth and solar cell devices. Mo was partially exposed through open holes having a square dot shape, and the closed-ratios of Al2O3 passivated areas were 56%, 75%, and 84%. The process of synthesizing CZTSSe is the same as that of the previous process showing 12.62% efficiency. When the 5-nm-Al2O3 dot patterning was applied to the Mo surface, we observed that the MoSSe formation was well suppressed under the area coated of 5-nm-Al2O3 film. The self-alignment phenomenon was observed in the back-contact area. CZTSSe was easily formed in the Mo-exposed area, while voids were formed near the Al2O3-coated area. The efficiency of the CZTSSe solar cell decreased when the Al2O3 passivated area increased. The exposure area and pitch of Mo, the collecting path of the hole, and the supplying path of Na seemed to be related to efficiency. Thus, it was suggested that the optimization of the Mo-exposed pattern and the additional Na supply are necessary to develop the optimum self-aligned CZTSSe light absorber.

Self-Alignment of Bottom CZTSSe by Patterning of an Al2O3 Intermediate Layer.

When CZTSSe is synthesized using a metal precursor, large voids of nonuniform size form at Mo back contact side. Herein, we demonstrate that the voids and CZTSSe in the lower part of the CZTSSe double layer can be controlled by using an Al2O3-patterned Mo substrate. The CZTSSe in the lower part self-aligns on the Mo-exposed area, while the voids self-align on the Al2O3-coated area. The origin of the self-alignment is expected to be the difference in bonding characteristics between liquid Sn and the metal or oxide surface, e.g., Al2O3. Good wettability generally forms between nonreactive liquid metals and metal surfaces due to the strong metallic bonding. By contrast, poor wettability generally forms between nonreactive liquid metals and oxide surfaces due to the weak van der Waals bonding between the liquid metal and the oxide layer. When the patterning was added, the device efficiency tended to decrease from 8.6% to 10.5%.

Flexible Cu2ZnSn(S,Se)4 solar cells with over 10% efficiency and methods of enlarging the cell area.

For kesterite copper zinc tin sulfide/selenide (CZTSSe) solar cells to enter the market, in addition to efficiency improvements, the technological capability to produce flexible and large-area modules with homogeneous properties is necessary. Here, we report a greater than 10% efficiency for a cell area of approximately 0.5 cm2 and a greater than 8% efficiency for a cell area larger than 2 cm2 of certified flexible CZTSSe solar cells. By designing a thin and multi-layered precursor structure, the formation of defects and defect clusters, particularly tin-related donor defects, is controlled, and the open circuit voltage value is enhanced. Using statistical analysis, we verify that the cell-to-cell and within-cell uniformity characteristics are improved. This study reports the highest efficiency so far for flexible CZTSSe solar cells with small and large areas. These results also present methods for improving the efficiency and enlarging the cell area.

Secondary Phase Formation Mechanism in the Mo-Back Contact Region during Sulfo-Selenization Using a Metal Precursor: Effect of Wettability between a Liquid Metal and Substrate on Secondary Phase Formation.

Recently, highly efficient CZTS solar cells using pure metal precursors have been reported, and our group created a cell with 12.6% efficiency, which is equivalent to the long-lasting world record of IBM. In this study, we report a new secondary phase formation mechanism in the back contact interface. Previously, CZTSSe decomposition with Mo has been proposed to explain the secondary phase and void formation in the Mo-back contact region. In our sulfo-selenization system, the formation of voids and secondary phases is well explained by the unique wetting properties of Mo and the liquid metal above the peritectic reaction (η-Cu6Sn5 → ε-Cu3Sn + liquid Sn) temperature. Good wetting between the liquid Sn and the Mo substrate was observed because of strong metallic bonding between the liquid metal and Mo layer. Thus, some ε-Cu3Sn and liquid Sn likely remained on the Mo layer during the sulfo-selenization process, and Cu-SSe and Cu-Sn-SSe phases formed on the Mo side. When bare soda lime glass (SLG) was used as a substrate, nonwetting adhesion was observed because of weak van der Walls interactions between the liquid metal and substrate. The Cu-Sn alloy did not remain on the SLG surface, and Cu-SSe and Cu-Sn-SSe phases were not observed after the final sulfo-selenization process. Additionally, Mo/SLG substrates coated with a thin Al2O3 layer (1-5 nm) were used to control secondary phase formation by changing the wetting properties between Mo and the liquid metal. A 1 nm Al2O3 layer was enough to control secondary phase formation at the CZTSSe/Mo and void/Mo interfaces, and a 2 nm Al2O3 layer was enough to perfectly control secondary phase formation at the Mo interface and Mo-SSe formation.

Transanal natural orifice transluminal endoscopic surgery total mesorectal excision in animal models: endoscopic inferior mesenteric artery dissection made easier by a retroperitoneal approach.

PURPOSE: We report the performance of natural orifice transluminal endoscopic surgery (NOTES) low anterior resection in animals using transanal total mesorectal excision (TME) with laparoscopic assistance and endoscopic inferior mesenteric artery (IMA) dissection. METHODS: Four pigs weighing 45 kg each, and one dog weighing 25 kg, underwent surgery via a transanal approach. The rectum was occluded transanally using a purse-string suture, approximately 3-4 cm from the anal verge. The rectal mucosa was incised circumferentially just distal to the purse-string. A SILS or GelPOINT port was inserted transanally. Transanal TME was assisted by laparoscopy and proceeded up to the peritoneal reflection. More proximal dissection, including IMA dissection, was performed along the retroperitoneal avascular plane by endoscopy alone and facilitated by CO2 insufflation. The IMA was clipped and divided endoscopically. The mobilized rectosigmoid were exteriorized transanally and transected. A colorectal anastomosis was performed using a circular stapler with a single stapling technique. RESULTS: Endoscopic dissection of the IMA was successful in all five animals. The mean operation time was 125 minutes (range, 90-170 minutes). There were no intraoperative complications or hemodynamic instability. The mean length of the resected specimen was 14.4 cm (range, 12-16 cm). CONCLUSION: A NOTES retroperitoneal approach to the IMA with CO2 insufflation and intact peritoneal covering overcame the difficulties of retraction and exposure of endoscopic dissection in animals.

Evaluation of plating conditions for the recovery of ²¹°Po on a Ag planchet.

The polonium-210 in the sea and its radiological consequences have been widely studied. Current processes for (210)Po recovery from seawater vary significantly. We compared selected processes to determine optimal conditions for recovery in modestly equipped laboratories. Plating (210)Po onto a Ag planchet with constant stirring for 15 h at room temperature after preconcentration from seawater samples with Mn was preferred, achieving more than 96% recovery with 3% or less precision. Possible contaminants were masked only by ascorbic acid treatment.

Transgastric cecectomy in canine models: natural orifice transluminal endoscopic surgery (NOTES).

BACKGROUND: Natural orifice transluminal endoscopic surgery (NOTES) for bowel resection is a challenging procedure. We studied the feasibility and safety of NOTES by performing cecectomy in dogs, which is equivalent to simple bowel resection in man. METHODS: Three dogs underwent transgastric endoscopic cecectomy with laparoscopic assistance. Antibiotics and irrigation were administered as preoperative preparation. A single-channel endoscope was introduced through the stomach into the peritoneal cavity, and the cecum was dissected and ligated with an L-knife and endoloops, respectively. Laboratory blood testing, peritoneal bacterial culture, and radiologic testing were performed perioperatively. Animals underwent autopsies on postoperative day (POD) 14. RESULTS: All subjects survived surgery and remained alive until POD 14. Mean operative time was 126.6 min. The absence of bacterial infections and anatomic leaks was verified by peritoneal swap culture, blood count, air leak testing, and bowel contrast radiology. However, a temporary mild elevation in white blood cell count was noted. No abscess or sign of infection was observed during autopsy, although adhesion was evident. CONCLUSIONS: Transgastric endoscopic cecectomy with minimal laparoscopic assistance is a feasible and safe procedure. Cecectomy in dogs is equivalent to procedures such as appendectomy, resection of Meckel's diverticulum, and oophorectomy in man. Natural orifice transluminal endoscopic surgery could prove to be a good optional surgical procedure.