From a potential absorber material (Cu2ZnSnS4) toward a low-cost advanced thin film solar cell device

Abstract

The focus of this thesis is to use low-cost fabrication and deposition methods to synthesize Cu2ZnSnS4 (CZTS) nanocrystal inks as thin films to study the impacts of several variables to determine the most desirable conditions to improve the efficiency of CZTS solar cell devices. These variables are the hot injection synthesis reaction temperatures and times, tin contents of the composition synthesis, the spin coating speed and acceleration index for the deposition of the inks and the post deposition annealing temperatures and times. From the results of these conditions, CZTS PV devices were built and tested. The films were characterized using X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy and Ultraviolet-visible spectroscopy (UV-vis). Devices were tested by measuring I-V characteristics under illumination.
The low-cost, non-vacuum hot injection technique was used for synthesising the CZTS nanoparticle thin films with spin coating for deposition on a soda lime glass substrate. For the reaction temperatures, the optical data confirms that the energy bandgap decreased with raised reaction temperatures from 1.55 ± 0.02 to 1.34 ± 0.02 eV for the range of temperatures 225-300 °C. The samples fabricated at 300 °C showed absence of secondary phases in Raman spectra. In the reaction times, 15-60 minutes was examined for 225 °C, 250 °C and 275 °C. It was found that the energy bandgap increased with the increase of reaction times for the three reaction temperatures tested from 1.40 ± 0.02 to 1.54 ± 0.02 eV for 225 °C, 1.41 ± 0.01 to 1.56 ± 0.03 eV for 250 °C and 1.35 ± 0.03 to 1.46 ± 0.01 eV for 275 °C. The sample produced at 250 °C for 30 minutes demonstrates well matching peaks and locations with the theoretically expected values for the a, c lattice constants, d-spacing and Raman shifts. For the tin contents, the optical properties showed a trend where the bandgap increases from 1.34 ± 0.04 to 1.71 ± 0.01 eV as the tin contents increases from 0.60-0.90 mmol. Two secondary phases of Cu4SnS4 and CuS were detected by Raman at locations of 317 and 472 cm-1 in the sample of 0.70 mmol Sn. Different spin coating speeds from 1300-9700 rpm were studied where the XRD data revealed that the size of scattering domain decreases with the increase of the spin coating speed from 17 ± 2 to 7 ± 3 nm. However, the optical properties of CZTS seem to be less affected by the spin coating speed as a slight change were demonstrated in the bandgap fluctuating between 1.40 ± 0.03 to 1.52 ± 0.03 eV. Different spin coating acceleration rates from 80-2400 rpm s-2 were investigated. The bandgaps obtained were in the range of the theoretical expected values of between 1.40-1.60 eV. There was a possible secondary phase of SnS present at peak location of 288 cm-1 in the sample of 2400 rpm s-2 observed by Raman. It is found that acceleration index has a more significant effect on thin film properties than spin coating speed. Increasing acceleration index produced films with a smaller scattering domain, more Zn, less S, and a larger energy bandgap.
A range of different annealing temperatures 560 °C, 585 °C, 610 °C, 635 °C with N2:H2S 80:20, and 585 °C with N2 only atmosphere were investigated. From the optical properties, the films demonstrate improvement in uniformity and crystallinity of CZTS with annealing especially the sample of 585 °C with N2 and H2S environment for 60 minutes as the gaps and cracks visible in microscopy images almost disappeared in comparison with the sample before annealing and formed a continuous thin film in the sample. This annealing temperature was chosen to further the investigation, but on the annealing times 30, 60 and 120 minutes. The 60 minutes sample showed a better performance in respect of energy bandgap, uniformity and crystallinity. This sample has an energy bandgap of 1.50 ± 0.01 eV which is near the optimum value to achieve the highest PV device efficiency. In addition, it showed less gaps and cracks in comparison to the sample examined before annealing and 30 minutes. Moreover, no secondary phases were present by Raman measurement in the sample of 60 minutes.
Batches of solar cell devices with structure of Mo/CZTS/CdS/ZnO/ITO/Au grid were built with different CZTS ink conditions for the reason of studying the I-V device performance and efficiency and the composition and structure of the device layers. Of particular relevance are CZTS nanoparticle reaction temperatures and reaction times. The cross-section images of the devices were done by SEM which showed a formation of MoS2 thin layer on top of the Mo back contact. A thick CZTS layer was deposited on top of Mo with thickness between 1630 to 4680 nm in all devices. The illuminated-dark I-V curve of these devices showed PV conversion. Among the devices tested, 2.6 ± 0.05% was the highest efficiency performance reported. However, this is a low efficiency value which indicates that the fabrication of devices has not been fully successful. An EDS mapping image was presented for the highest efficiency performance. This mapping showed a good agreement with SEM images of the formation of a thin layer of MoS2 on top of Mo. The distribution of all measured elements throughout the device is confirmed by EDS mapping. A higher concentration of S in MoS2 compared with CZTS was confirmed by S element map. Cu, Zn, and Sn were distributed approximately evenly and uniformly among the CZTS nanocrystals. It was not possible to correlate overall device efficiency with CZTS fabrication conditions because of the low efficiency due to large recombination losses, meaning that to determine the impact of different absorber layers on device efficiency is not possible at this stage.