Phase segregation in Cu2ZnSnSe4 thin films for photovoltaic applications. The effects of precursor microstructure and selenium activity during selenization of electrodeposited metallic precursors.

This thesis develops the synthesis of thin film copper-zinc-tin-selenide kesterite absorber layers, Cu2ZnSnSe4 (CZTSe), suitable for solar cell applications. The kesterites are highly sensitive to stoichiometry deviations and most of the kesterite growth technologies suffer from the segregation of secondary phases, which can be detrimental to the performance of the solar cell. This thesis examines the phase segregation occurring during CZTSe absorber growth during a two-step electrodeposition – annealing process. A method towards reducing phase segregation in the CZTSe absorber is developed by manipulating the microstructure of Cu-Sn-Zn precursors and the annealing conditions. A power conversion efficiency improvement from 0 % to 5.94 % was possible by changing the microstructure of the metallic precursor from a bi-layered with Zn-rich alloy on the top to a matrix-type. This is explained by the higher reactivity of Zn with selenium vapour than Cu or Sn. The synthesis route used in this study consisted of three stages: sequential electrodeposition of Cu, Sn and Zn layers, soft pre-alloying of the resulting metallic stacks, and thermal annealing in the presence of selenium vapour. The pre-alloying step is introduced in order to develop uniform microstructure to the precursors by formation of Cu-Sn and Cu-Zn alloys. Depending on the stacking order of the metals in the electrodeposited precursors, two types of microstructure are developed during the pre-alloying process: a bi-layered structure with the Cu-Zn alloy on the top and the Cu-Sn alloy on the bottom and a matrix-type microstructure with the Zn-rich phase in the Sn-rich matrix. It is shown that the precursor microstructure determines the further segregation of phases during CZTSe growth. Using a rapid thermal annealing system (RTP) it is possible to resolve the mechanism of CZTSe formation at short time intervals. Identification and location of secondary phases is studied by four characterization techniques: XRD, Raman spectroscopy, SEM/EDX and SIMS. The influence of the partial pressure of selenium above the precursor on the CZTSe growth is investigated by theoretical computation and compared to experimental results. It is shown that the rate-limiting step of kesterite formation is the arrival of selenium species from the source to the precursor sample. This is correlated to the thermodynamic feasibility of the metal- vapour phase selenium reactions, since the selenium partial pressure above the sample will determine which phases are prone to form. The studies of the CZTSe formation mechanism show that Zn is the first metal to react with selenium vapour. This explains why the presence of Zn on the precursor surface enhances the ZnSe phase segregation during the CZTSe growth

The Effect of Soft Pre-Annealing of Differently Stacked Cu-Sn-Zn Precursors on the Quality of Cu2ZnSnSe4 Absorbers

ABSTRACTCu2ZnSnSe4 p-type semiconductors currently investigated for use in thin film solar cells can be synthesized by firstly depositing a metallic precursor and secondly annealing the precursor in selenium vapor. Differently stacked Cu-Sn-Zn metallic precursors were characterized after a soft annealing at 350°C under nitrogen atmosphere. For the stack where the Sn and Zn were in direct contact with sufficient Cu to form a stable alloy, a bi-layered structure consisting of Cu-Sn on the bottom and Cu-Zn on the top was formed. Contrarily, when Zn was not in direct contact with Cu, the metals diffused to form a stable alloy and the system segregates horizontally, forming a mixed columnar structure. These two types of precursors were selenized under exactly the same conditions to form kesterite absorbers for solar cell devices. Using this approach the improvement from 0.44% power conversion efficiency for the bi-layered precursor to 4.5% for the mixed precursor was achieved.</jats:p

CONTENT OF REDUCED GLUTATHIONE FORM AS A BIOMARKER OF OXIDATIVE STRESS IN SPINACH PLANTS GROWING IN SOIL CONTAMINATED WITH ZINC

The aim of the paper is the assessment of the efficiency of anti-oxidative system in spinach plants growing in the substratum polluted with zinc. The assessment was conducted on the basis of changes of reduced glutathione (GSH) form concentration in the plant aboveground organs. Spinach plants, ‘Matador’ c.v., were cultivated in soils contaminated with zinc in two pot experiments conducted in 2010 and 2011. The experimental substratum was light, slightly soil with granulometric composition of sandy silt loam. Zinc in the acetate form, was supplied to the soil in four doses corresponded to this metal critical concentrations in soil with 0, I, II and III degrees of pollution with this element according to IUNG classification. Simultaneously, the control with natural Zn content in soil was maintained. Zn concentrations in spinach ranged from 412.8 to 1722 mg ∙ kg-1 d.m. and increased with growing degree of substratum pollution with this element. Over the course of the vegetation period the content of Zn in plants was generally greater. GSH content in spinach grown in both years of experiments fluctuated from 31.70 to 238 µg ∙ g-1 f.m. The biggest content of this compound in spinach was stated in the initial phase of plants growth. The plants tolerated only the first two Zn doses supplied to the substratum. Spinach growing in the objects where zinc additions to the soil equalled II and III degree of substratum pollution died shortly after germination. The plants from these objects in the initial growth phase contained significantly less GSH than spinach from the objects with two first degrees (0 and I) of substratum pollution with zinc or from the control. The content of reduced glutathione form in spinach is a good biomarker of oxidative stress caused by zinc presence in plants. Synthesis of a bigger amount of GSH conditions spinach plant resistance to over the norm zinc content in soil. The efficiency of anti-oxidative system in spinach is bigger in the initial phase of this plant growth

ROADSIDE RESERVOIRS OF RAIN WASTEWATER AS AN ELEMENT OF WATERS QUALITY PROTECTION

This paper presents an assessment of the quality of rain wastewater from the drainage of the national road No. 4 between Sułków - Łysokanie and the impact of changes in the intensity of traffic on their content of Zn and Pb. Material consisted of samples of sewage flowing out directly of the road and collected in 16 reservoirs, located on both its sides, taken in July 2011 during the precipitation. Physicochemical parameters (pH, electrolytic conductivity, COD and total nitrogen and chloride) as well as zinc and lead were analysed. The results were compared with those obtained in 2007 and 2008 for wastewater of the same objects. There were found significant differences of physico-chemical parameters and of both metals content in the wastewater flowing into reservoirs and stagnant in them. The average concentration of metals in the wastewater flowing out of the road and accumulated in the reservoirs were similar, but showed less variability and lower maximum levels, so build reservoirs reached the intended target of retardation of water resources degradation. Comparing the waste collected in the same reservoirs in 2007-2011 showed that the average Zn concentration in rain wastewater draining from the road No. 4 has decreased since 2007, from 1.2 to 4 times, which should be regarded as a positive effect of reducing the amount of traffic vehicles, mostly trucks, after being put into use in 2009, the A4 motorway between Kraków-Szarów. The concentration of Pb since 2008 has not changed significantly, which may result from the fact that has not changed significantly traffic intensity, especially cars, which are moving residents of the area. The Zn concentration in the studied rain wastewater flowing out of the road was correlated with the value of COD, and in the case of reservoirs located on the south side of the road with the values of pH and COD. Pb concentration in the effluent flowing out of the road into all reservoirs and located on the north side was correlated with the volume of reservoirs and sealed surface and total their catchment area. Pb content in the wastewater flowing into reservoirs located on the south side of the road and gathered them was correlated with the value of COD, and stored in reservoirs on the northern side depend on the pH value

ASSESSMENT OF THE BLACK SEA ECOSYSTEM POLLUTION WITH COPPER AND CADMIUM IN SELECTED BAYS OF SEVASTOPOL REGION

A high level of anthropopressure has been registered in Sevastopol region, connected with its strategic role as the main city in the region, but also due to Russian Black Sea Fleet stationing there for many years. A significant source of the Black Sea contamination in Sevastopol area is the industry located in this city, municipal waste and agriculture. Implementing measures aimed at protection of the Black Sea and the evolution of their results requires monitoring conducted in the regions with various levels of anthropopressure. The work was aimed at the assessment of copper and cadmium content in water and algae in selected bays of the Black Sea in the vicinity of Sevastopol. Samples of water and algae were collected in August 2012 from eight Sevastopol bays (Galubaja, Kozacha, Kamyshova, Kruhla, Strieletska, Pishchana, Pivdenna and Sevastopolska) and from the open sea in the vicinity of Fiolent. Algae (Cystoseira barbata and Ulva rigida) were collected from the same places. Collected water was preserved on the sampling place and brought to the laboratory where its copper and cadmium concentrations were assessed. Collected algae were rinsed in distilled water, dried, then homogenised and mineralised. Copper and cadmium content were determined in the mineralizates using ASA method with electrothermal atomisation. Cadmium concentration in water ranged from 0.13 to 1.74 µg Cd∙dm-3, and copper from 7.07 to 22.56 µg Cd∙dm-3. Considerable differences in the content of the analysed elements were registered in individual bays. The highest content was assessed in Galubaja and Sevastopolska bays, whereas the lowest one in the water collected in the open sea and in Pivdenna bay. Copper concentrations in the analysed algae fluctuated from 3.375 to 14.96 mg Cu∙kg-1 d.m. No differences were noted in this element content between the algae species. Cadmium content in the algae ranged from 0.133 to 1.133 mg Cd∙kg-1 d.m. Higher accumulation of cadmium was observed in Cystoseira barbata than in Ulva rigida. The value of copper bioaccumulation coefficient (BC) ranged from 181 to 1201, whereas cadmium from 181 to 5256. The contents of the analysed metals, both in biotic and abiotic elements of the studied ecosystems point to anthropogenic enrichment and the results obtained for Sevastopolska, Galubaja and Kozacha bays indicate a hazardous, excessive bioaccumulation of copper and cadmium and to potential threat to the life of aquatic organisms and seafood consumers

Electrodeposition of Kesterite thin films for photovoltaic applications: Quo vadis?

This paper aims at providing an updated overview of the main achievements in the development of solar cells based on Cu2ZnSn(S,Se)4 (CZTS(Se)) kesterite absorbers obtained by electrodeposition. Although undoubtedly challenging, the ultimate goal is to learn from the past works and build a solid framework for future advances in this field. What is the reason for the lower efficiency of electrodeposited CZTS(Se)-based devices (8%) compared to the world record efficiency achieved with a hydrazine-based solution approach (12.6%)? Can this gap be filled, or there are intrinsic limitations for this achievement? The review is divided into the three main electrodeposition approaches: sequential elemental layer, alloy co-deposition, and chalcogenide co-deposition. It is argued that considerable technical challenges must be overcome for the latter approach to be successfully applied

Is it Possible to Grow Thin Films of Phase Pure Kesterite Semiconductor? A ZnSe case study

The kesterite semiconductor Cu2ZnSnS(e)4is seen as a suitable absorber layer to replace Cu(In,Ga)Se2in thin film solar cells, if thin film photovoltaics are to be deployed on the terawatt scale. Currently the best devices, and hence the best kesterite absorber layers are grown away from stoichiometry and are zinc rich and copper poor, presumably leading to the formation of ZnS(e). However, it has been shown that secondary phases present in an absorber layer reduce device performance. If growth in Zn rich conditions seems to be mandatory, then any secondary phases formed should be grown on the surface of the absorber layer so that they may be easily removed by etching. Therefore, it is important to know how and why secondary phases form, and if possible, how to segregate them to the surface of the absorber layer. Here we show that ZnSe is formed at the initial stages of absorber formation from annealing metal stacks in selenium vapor. Further we demonstrate that the way the precursor metals are distributed on the substrate leads to different absorber layer performances in full devices. The importance of selenium vapor pressure is highlighted in respect to the order of selenisation of the metals, Zn before Cu. Additionally, the importance of selenium and tin selenide vapor pressure during annealing is reviewed with regard to avoiding a decomposition of the Cu2ZnSnSe4to ZnSe and Cu2Se phases. Regardless of the atmosphere above the absorber, the reaction of the absorber with molybdenum appears unavoidable without the use of a passivation strategy. Counter-intuitively, it is demonstrated that for our absorber layers grown under Zn-rich conditions, removal of the ZnSe is harmful for device performance. \ua9 2013 Materials Research Society

Quantification of surface ZnSe in Cu2ZnSnSe4-based solar cells by analysis of the spectral response

Absorber layers consisting of Cu2ZnSnSe4 (CZTSe) and surface ZnSe in variable ratios were prepared by selenization of electroplated Cu/Sn/Zn precursors and completed into full devices with up to 5.6 % power conversion efficiency. The loss of short circuit current density for samples with increasing ZnSe content is consistent with an overall reduction of spectral response, pointing to a ZnSe current blocking behavior. A feature in the spectral response centered around 3 eV was identified and attributed to light absorption by ZnSe. A model is proposed to account for additional collection of the carriers generated underneath ZnSe capable of diffusing across to the space charge region. The model satisfactorily reproduces the shape of the spectral response and the estimated ZnSe surface coverage is in good qualitative agreement with analysis of the Raman spectral mapping. The model emphasizes the importance of the ZnSe morphology on the spectral response, and its consequences on the solar cell device performance