Evolution of Morphological and Chemical Properties at p-n Junction of Cu(In,Ga)Se2 Solar Cells with Zn(O,S) Buffer Layer as a Function of KF Postdeposition Treatment Time.

We carried out KF postdeposition treatment (PDT) on a Cu(In,Ga)Se2 (CIGS) layer with a process time varying from 50 to 200 s. The highest CIGS solar-cell efficiency was achieved at a KF PDT process time of 50 s; in this condition, we observed the highest level of K element at the near-surface of the CIGS layer and the perfectly passivated pinholes on the CIGS surface. At process times above 150 s, the oversupplied KF agglomerated into large islands and was subsequently eliminated during the deposition of the chemical bath deposition (CBD)-Zn(O,S) buffer layer owing to the islands' water-soluble characteristics. As a result, the growth mechanism of the CBD-Zn(O,S) layer varied as a function of KF PDT process time. X-ray photoemission spectroscopy (XPS) measurements were used to examine the dependency of the chemical state on the KF PDT process time, and from the results, we formulated a chemical reaction model based on the shift in the elemental binding energy following deposition of the CBD-Zn(O,S) buffer layer. The chemical states of the K-In-Se phase, which have a beneficial effect on the solar-cell performance owing to the formation of durable and improved p-n junctions, are formed only at a KF PDT process time of 50 s. We derived band alignments from the XPS depth profiles by extracting the conduction- and valence-band offsets, and we used optical-pump-THz-probe spectroscopy to measure the ultrafast photocarrier lifetimes related to the defect states following KF PDT. Our key findings can be summarized as follows: (i) photocarrier transport is beneficial at a low barrier height, and (ii) the photocarrier lifetime increases when the K-In-Se phases are formed on the CIGS surface, which allows K+ ions to be effectively substituted into Cu vacancies.

Sodium-assisted passivation of grain boundaries and defects in Cu2ZnSnSe4 thin films.

The long stagnation of the photo-conversion efficiency of kesterites below 13% is a source of frustration in the scientific community. In this study, we investigated the effects of sodium on the passivation of grain boundaries and defects in Cu2ZnSnSe4 (CZTSe) grown on a soda-lime glass (SLG) and borosilicate (BS) glass. Because BS glass does not inherently contain sodium, we placed a thin layer of NaF between CZTSe and Mo. The composition of the samples is Cu-poor and Zn-rich. The distribution of sodium and its contributions to phase formation and defects were examined by cross-sectional energy-dispersive X-ray profiling, Raman scattering spectroscopy and imaging, surface potential and photoluminescence. From the experimental results, it can be strongly claimed that sodium ions segregate predominantly near the grain boundaries and reduce CuZn-related defects. These local surface imaging analyses provided the exact locations of the secondary phases. In particular, the photo-assisted scanning probe method enabled us to observe the changes in the optoelectrical properties of the thin films and the carrier behavior within the materials. Further studies with distinct alkali ions and optimal processing conditions will pave a way to improve the performance of kesterite solar cells.

Metal-agglomeration-suppressed growth of MoS2 and MoSe2 films with small sulfur and selenium molecules for high mobility field effect transistor applications.

This work reports a breakthrough technique for achieving high quality and uniform molybdenum dichalcogenide (MoX2 where X = S, Se) films on large-area wafers via metal-agglomeration-suppressed growth (MASG) with small chalcogen (X-) molecules at growth temperatures (TG) of 600 °C or lower. In order to grow MoS2 films suitable for field effect transistors (FETs), S-molecules should be pre-deposited on Mo films at 60 °C prior to heating the substrate up to TG. The pre-deposited S-molecules successfully suppressed the agglomeration of Mo during sulfurization and prevented the formation of protruding islands in the resultant sulfide films. The small X-molecules supplied from a thermal cracker reacted with Mo-precursor film to form MoX2. The film quality strongly depends on the temperatures of cracking and reservoir zones, as well as TG. The MoS2 film grown at 570 °C showed a thickness variation of less than 3.3% on a 6 inch-wafer. The mobility and on/off current ratio of 6.1 nm-MoS2 FET at TG = 570 °C were 59.8 cm2 V-1 s-1 and 105, respectively. The most significant advantages of the MASG method proposed in this work are its expandability to various metal dichalcogenides on larger substrates as well as a lower TG enabled by using reactive small molecules supplied from a cracker, for which temperature is independently controlled.

Enhanced sulfurization reaction of molybdenum using a thermal cracker for forming two-dimensional MoS2 layers.

We propose a method to fabricate two-dimensional (2D) molybdenum disulfide (MoS2) layers to overcome issues in typical fabrication processes by promoting the sulfurization reaction of molybdenum (Mo). A thin sputtered-Mo layer was sulfurized using a sulfur (S) thermal cracker to form 2D MoS2 layers. The effects of key process parameters such as cracking-zone temperature (TC-zone), thickness of the sputtered-Mo layer, and Ar pressure during deposition of the Mo layer were systematically investigated. The degree of thermal treatment of evaporated S vapor is controlled by varying TC-zone. The higher TC-zone enabled easy formation of thin MoS2 layers at a low substrate temperature of 250 °C due to the greatly enhanced sulfurization reaction. The thickness of the final MoS2 layers was controlled by changing the initial thickness of the sputtered-Mo film. Ultra-thin MoS2 film about 2-layers-thick was obtained by sulfurizing a 2 Å-thick Mo film. The chemical state of the MoS2 layers largely depended on the Ar pressure during the sputtering process of the initial Mo. Lower Ar pressure enhanced MoS2 formation due to more efficient substitution of the MoS2 phase for the MoO3 phase. By using the S thermal cracker, we demonstrate a method to easily fabricate 2D MoS2 layers, excluding some problematic issues such as toxic and expensive reactants, non-vacuum conditions susceptible to contamination, and high substrate temperature.

Light-soaking effects and capacitance profiling in Cu(In,Ga)Se2 thin-film solar cells with chemical-bath-deposited ZnS buffer layers.

We fabricated Cu(In,Ga)Se2 (CIGS) solar cells with chemical-bath deposited (CBD) ZnS buffer layers with different deposition times. The conversion efficiency and the fill factor of the CIGS solar cells reveal a strong dependence on the deposition time of CBD-ZnS films. In order to understand the detailed relationship between the heterojunction structure and the electronic properties of CIGS solar cells with different deposition times of CBD-ZnS films, capacitance-voltage (C-V) profiling measurements with additional laser illumination were performed. The light-soaking effects on CIGS solar cells with a CBD-ZnS buffer layer were investigated in detail using current density-voltage (J-V) and C-V measurements with several different lasers with different emission wavelengths. After light-soaking, the conversion efficiency changed significantly and the double diode feature in J-V curves disappeared. We explain that the major reason for the improvement of efficiency by light-soaking is due to the fact that negatively charged and highly defective vacancies in the CIGS absorber near the interface of CBD-ZnS/CIGS were formed and became neutral due to carriers generated by ultra-violet absorption in the buffer layer.

Behavior of Photocarriers in the Light-Induced Metastable State in the p-n Heterojunction of a Cu(In,Ga)Se2 Solar Cell with CBD-ZnS Buffer Layer.

We fabricated Cu(In,Ga)Se2 (CIGS) solar cells with a chemical bath deposition (CBD)-ZnS buffer layer grown with varying ammonia concentrations in aqueous solution. The solar cell performance was degraded with increasing ammonia concentration, due to actively dissolved Zn atoms during CBD-ZnS precipitation. These formed interfacial defect states, such as hydroxide species in the CBD-ZnS film, and interstitial and antisite Zn defects at the p-n heterojunction. After light/UV soaking, the CIGS solar cell performance drastically improved, with a rise in fill factor. With the Zn-based buffer layer, the light soaking treatment containing blue photons induced a metastable state and enhanced the CIGS solar cell performance. To interpret this effect, we suggest a band structure model of the p-n heterojunction to explain the flow of photocarriers under white light at the initial state, and then after light/UV soaking. The determining factor is a p+ defect layer, containing an amount of deep acceptor traps, located near the CIGS surface. The p+ defect layer easily captures photoexcited electrons, and then when it becomes quasi-neutral, attracts photoexcited holes. This alters the barrier height and controls the photocurrent at the p-n junction, and fill factor values, determining the solar cell performance.

Photovoltaic Performance and Interface Behaviors of Cu(In,Ga)Se2 Solar Cells with a Sputtered-Zn(O,S) Buffer Layer by High-Temperature Annealing.

We selected a sputtered-Zn(O,S) film as a buffer material and fabricated a Cu(In,Ga)Se2 (CIGS) solar cell for use in monolithic tandem solar cells. A thermally stable buffer layer was required because it should withstand heat treatment during processing of top cell. Postannealing treatment was performed on a CIGS solar cell in vacuum at temperatures from 300-500 °C to examine its thermal stability. Serious device degradation particularly in VOC was observed, which was due to the diffusion of thermally activated constituent elements. The elements In and Ga tend to out-diffuse to the top surface of the CIGS, while Zn diffuses into the interface of Zn(O,S)/CIGS. Such rearrangement of atomic fractions modifies the local energy band gap and band alignment at the interface. The notch-shape induced at the interface after postannealing could function as an electrical trap during electron transport, which would result in the reduction of solar cell efficiency.