Solüsyon yöntemiyle CZTS sentezi ve fotovoltaik devreler için CZTS ince film oluşturma ve film özelliklerini inceleme

Thin film solar cells are second generation solar cell module. Today's avaliable thin film solar cells suffer from low efficiency of a-Si, shortage of tellerium in structure of CdTe and indium in structure of CIS and toxicity of cadmium in structure of CdTe. Quaternary Cu2ZnSn(SxSe1-x)4 (CZTSSe) compound using an absorber layer in structure of thin film solar cell is considered as a promising candidate for low-cost and high efficiency solar cells because of non-toxic, earth abundant and low cost materials. It can be also synthesized low cost solution based method while other quaternary Cu2InGaS4 (CIGS) and ternary CuInS2 (CIS) structure used in thin film solar cell compounds are synthesized expensive vacuum-based technique. This situation make CZT(SSe) more favorable to be used in thin film solar cells as an absorber layer. CZTS was synthesized with solution-based technique by using precursors which are copper, zinc, tin and sulfur were used for synthesize. Oleylamine (OLA) was used as surfactant and solvent during solution based synthesis. In this study, CZT(SSe) hybrid solar cell structure was occurred. First step, CZTS was synthesized by using precursors of copper zinc, tin and sulfur precursors which are copper (II) acetylacetonate, zinc acetylacetonate hydrate tin (IV) bis(acetylacetonate) dibromide and elemental sulfur, respectively. These precursors were added at different amount for each synthesis experiment. OLA was used as both solvent and surfactant during synthesis. These precursors were mixed in a flask at determined temperature around 230 oC for an hour. After reaction of precursors in OLA, toluene and 2-proponol were added to obtained reaction for washing. Copper poor and zinc rich composition was achieved after several reaction experiments. Centrifuge process was applied to final supernatant of solution reaction in tubes of 2 ml. It was centrifuged at 12000 rpm for 10 minutes. Supernatant that was wasted from centrifuge was poured with adding toluene and IPA at equal value for next centrifuge step. After solution was completely mixed with ultrasonic bath, it was centrifuged again at 8000 rpm for 20 minutes. After supernatant was completely poured, tubes fill with toluene to be stable ink. After washing process, final solvent was mixed toluene to make stable CZTS ink. In the second step, CZTS ink in toluene solution was deposited by spin coating method at around 3000 rpm for 1 minute. Quality of films, which were coated at 3000 rpm, was not ideal to be used in absorber layer in thin film solar cell because of non- continuous film formation. Film uniformity was enhanced by keeping dropped CZTS ink on substrate for few minutes before spinning was started. Heat treatment was applied to coated samples at 175 °C to remove solvent like toluene and 2-propanol (IPA) that added in centrifuge step for each coating step. Deposited CZTS and CZTSSe layers were characterized with X-Ray diffraction (XRD) and Raman spectroscopy to determine structure of compounds. Not only crystal structure but also composition of CZTS and CZTSSe is important for copper (Cu) poor and zinc (Zn) rich composition, which mostly employed in literature. Therefore, Energy-dispersive X-Ray spectroscopy (EDX) was used to determine composition of annealed samples. Cu/Zn+Sn and Zn/Sn ratio was estimated by EDX characterization. Scanning electron microscopy (SEM) was used to investigate film formation. Two annealing steps were applied to CZTS coated molybdenum substrate at determined temperature. First annealing process was applied in inert atmosphere on hot plate between 350 °C and 425 °C to define ideal annealing condition. Final compositions of samples that annealed between 350 °C and 425 °C were determined by EDX. They were also characterized by XRD and Raman spectroscopy to determine structure of compounds. UV spectroscopy was used to estimate absorption and transmittance. The UV-vis-NIR absorption spectrum was recorded. Recorded spectrum took the wavelength when the absorption intensity start to take off. Then we can derive the optical band gap using equation: Eg (eV) = 1240/(wavelength in nm). Annealing processes were applied either to remove surfactant or to enhance crystallization of CZTS grains as a third stage. Even without heat treatment, as-synthesized CZTS film shows that kesterite structure was achieved. Temperature range between 350 oC and 425 oC was used to determine ideal heat treatment in nitrogen atmosphere for pure compounds without secondary phases. There was no secondary phases up to 400 oC. XRD result of sample annealed at 400 oC showed secondary phases related oxide compound of cations. Annealed sample at 350 oC and 375 oC showed the best result in terms of Cu poor and Zn rich compound. Increasing annealing temperature caused to increase composition of Cu compound while composition of Zn compound decrease. Amount of sulfur (S) in CZTS structure decrease during heat treatment at any temperature. Composition studies show that increasing annealing temperature increase consantration of Cu while decrease consantration of Zn. More annealing temperature means the more loss of S and Zn during annealing process. Post annealing step was applied to create CZTSSe at determined temperature under selenium (Se) vapor. Selenization was applied to increase formation and crystallization of film. Selenium black 99+ powder was used as selenium source during selenization. Tube furnace was used for selenization. Heat zone of the furnace was determined to locate both selenium powder and samples. Se source was located in tube where temperature was 500 oC while samples were kept around 350 oC because evaporated Se vapor can deposit cooler surfaces. Therefore, deposition rate can be increased by this method. Deposited selenium diffused throughout film and replaced sulfur in CZTS. Therefore, composition of CZTSSe compound was achieved at 500 oC for 40 minutes. Hybrid solar cell structure was fabricated as a structure with [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) used n type organic semiconductor. Device structure was constituted of Mo/CZTS-CZT(SSe)/PCBM/Ag. PCBM was coated on CZTS or CZT(SSe) coated molybdenum to create p-n junction. Silver, which is used forward electrode in solar cell structure, was deposited on PCBM via thermal evaporation method. Incident photon to converted electron (IPCE) and solar simulator were used to examine solar cell performance. IPCE applies monochromatic light to cell to establish amount of current converted incident photon at determined wavelength. This article deals with the term as a measurement of a device's electrical sensitivity to light. Solar simulator apply light at determined standard to simulate sunlight to detect conventional performance of solar cell. Current (I) versus voltage (V) behavior of a solar cell under simulated sunlight. To sum up, quaternary CZT(S,Se) compound was achieved successfully with composition of Cu poor and Zn rich. Reaction condition was adjusted 230 oC for an hour in nitrogen atmosphere. We standardized heat treatment at 350 oC for an hour in nitrogen atmosphere.