Investigation of heterojunction interface of CIGSe/doped-ZnO for solar cell applications

Abstract

In first part of this work the optimization of growth temperature (Tgrowth) of CIGSe thin films deposited on Mo coated sodalime glass substrate has been performed in a single-step route from a single quaternary sputtering target by dual ion beam sputtering (DIBS) system. The influence of Tgrowth on the structural, optical, morphological and electrical properties has been investigated and the correlation of these properties has been established. The presence of all elements Cu, In, Ga and Se has been confirmed by X-ray Photoelectron Spectroscopy (XPS). Moreover the Cu-rich film with favourable ratio Ga/(In+Ga) of 0.25 for photovoltaic device is obtained. XRD measurements have shown a strong preferred (112) orientation located at 2θ=26.70o, which indicated the chalcopyrite structure of films. Interestingly no signature of any sort of secondary phase is observed in all the films. Grain size is observed to increase from 14.98 to 43.05 nm with the corresponding rise in Tgrowth from 100 to 400 oC. However, morphological properties demonstrated a smooth and uniform surface at lower Tgrowth and the surface roughness is observed to increase with increasing Tgrowth. The optical bandgap of CIGSe films is seen to lie in the range of 1.48–1.54 eV at room temperature. The absorption coefficient for CIGSe film grown at 400 oC exhibited a marginal increment (~2.3 times) than that for CIGSe film grown at 100 oC. Hall measurements have exhibited the minimum film resistivity of 0.09 Ω cm with a hole concentration of 2.42 × 1018 cm-3 and mobility of 28.60 cm2 V-1 s-1 for CIGSe film grown at 100 oC. Film absorption coefficient is found to enhance nominally from 1 × 105 to 2.3 × 105 cm-1 with increasing Tgrowth from 100 to 400 oC. These results suggest that CIGSe film grown at 100 oC by DIBS system had the best morphological and electrical properties with good structural and optical properties; indicating that lower Tgrowth is better to grow high-quality CIGSe material for photovoltaic application. These results indicate that the high quality CIGSe thin film can be achieved at even low Tgrowth by DIBS system.second part, the effects of Tgrowth (200–600 oC) on GMZO thin films grown on Si (111) substrate by DIBS system is described. The optical, elemental, electrical, crystalline and morphological properties and their correlations have been thoroughly investigated. Transmittance value of more than ~94% in 400–1000 nm region is observed for all GMZO films. The plasmonic features can be detected in the absorption coefficient spectra of GMZO grown at 500 and 600 oC in the form of a peak at ~4.37 eV. The presence of such plasmonic features in the film is confirmed by UPS measurements in form of two broad plasmon peaks. The plasmon peak at higher Tgrowth i.e. 500 oC is more prominent in comparison to that of at lower Tgrowth 200 oC. These plasmon peaks are due to valance bulk plasmons (VBPs) and valance surface plasmons (VSPs) of the nanoclusters embedded within GMZO film. The valence bulk plasmon resonance energy (VPRE) and particle plasmon resonance energy, (PPRE) are calculated. The calculated values of PPRE of different nanoclusters are observed to be in the range of 1.59-5.20 eV in both ZnO and the air media. This indicates the solar light from near UV to Visible and partial near IR (238-780 nm) wavelength is covered. These nanoclusters can easily be tuned and excited at the desirable wavelengths while optimizingthe efficiency of solar cells (SCs) by simple alteration of DIBS Tgrowth. These nanoclusters are extremely promising to enhance the optical scattering and trapping of the incident light, which increases the optical path length in the absorber layer of cost-effective SCs and eventually increases its efficiency. The presence of the nanoclusters in the film is expected to be beneficial for realizing economical and high-performance photovoltaic.In third part, the band alignment at GMZO/CIGSe heterojunction interface is thoroughly investigated and demonstrated in GMZO thin film. A considerable difference is observed in valence band onset (VBOn) for CIGSe and GMZO films before and after short duration sputtering. Hence a few minutes sputtering of both CIGSe and GMZO thin films is essential before another layer is grown. The values of valence band offset (VBOff or ΔE) and conduction band offset (CBOff or ΔE) of the GMZO/CIGSe heterojunction are determined to be 2.69 eV and −0.63 eV, respectively. The negative value of ΔE is a non-ideal alignment at the interface, as it increases the possibilities of interface recombination and limits open circuit Voltage (VOC) [5]. The obtained value of ΔE can be further improved by suitable band-gap engineering of the buffer GMZO layer to realize a flat or better CBOff.In fourth part, the band alignment at GZO/GMZO heterojunction interface and the generation of plasmons in GZO thin film is thoroughly described by spectroscopy ellipsometry and UPS study. The crystalline, electrical, and optical properties and plasmonic features are studied for the individual films, while the band offset is calculated at the GZO/GMZO interface. XRD measurements have demonstrated that both GZO and GMZO films have good crystalline quality with large grain size of 37.28 and 24.30 nm, respectively and high c-axis orientation in (002) crystal plane at 2θ = 34.41o. Hall measurement has demonstrated that GZO and GMZO films are n-type with high carrier concentration of 1.06 × 1021 and 4.36 × 1019 cm-3 and lowresistivity of 4.11 × 10-3 and 7.43 × 10-2 Ω-cm, respectively. The transmittance of GZO film is better than 96% while transmittance of GMZO film is better than 94% in 400 to 800 nm wavelength range and calculated band gap of GZO and GMZO are 3.55 and 3.63 eV, respectively at room temperature. The average experimentally calculated values of VBOff and CBOff at GZO/GMZO interface are -0.016 and -0.064 eV, respectively. The low value of indicates an almost flat band alignment at the interface, which allows the charge transport efficiently at interface and enhances the performance of the SCs based on such heterojunction. Moreover, the band alignment can be further tuned by suitable band gap engineering by varying the Mg and Ga concentration or altering the DIBS deposition condition according to manufacturing requirements and design of experiments. The broad plasmon peaks observed in UPS spectra. The values of PPRE are observed to be in the range of ~1.87-10.04 eV for different nanoclusters in both ZnO and air media. These nanoclusters are extremely promising to enhance the optical scattering and trapping of the incident light, which increases the optical path length in the absorber layer of cost-effective SCs and eventually increases its efficiency.