Faculty Publications
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Item Room-temperature ultraviolet-ozone annealing of ZnO and ZnMgO nanorods to attain enhanced optical properties(Springer, 2020) Alam, M.J.; Murkute, P.; Sushama, S.; Ghadi, H.; Mondal, S.; Paul, S.; Das, D.; Pandey, S.K.; Chakrabarti, S.ZnO and ZnMgO nanorods have proven to be promising materials for sensing, UV and deep UV based optoelectronic applications. A major drawback of ZnO and ZnMgO based thin films and nanorods is the presence of native point defects which deteriorates their optical efficiency and becomes an impediment to their efficient device applications. The furnace and rapid thermal annealing processes have overcome this up to a great extent but being high temperature processes, they put many fabrication and technological limits in device fabrication. Especially keeping an eye on the future flexible devices, herein we report ultraviolet-ozone (UVO) annealing as a room-temperature, simple and cost-effective annealing method to improve the optical efficiency of ZnO and ZnMgO nanorods along with control of defect states. The ZnO and ZnMgO nanorods were grown by hydrothermal method and annealed in UVO irradiation. UVO annealing substantially improved near band emission and suppressed defect band emissions. It is found that zinc interstitial atoms migrate from the top portion of ZnO nanorods towards the bottom of nanorods after UVO annealing, resulting in reduced zinc interstitial defects in the top portion of nanorods. X-ray diffraction results showed improvement in structural properties. XPS results confirmed suppression of oxygen vacancies and zinc interstitials and improvement in lattice oxygen in the ZnO nanorods after UVO annealing. Optimum times of UVO annealing for ZnO and ZnMgO nanorods were 30 and 50 min respectively. These findings will be helpful for the further development of ZnO and ZnMgO nanorods based high performance optoelectronic devices and sensors. © 2020, Springer Science+Business Media, LLC, part of Springer Nature.Item Experimental investigation and comparative analysis of electron beam evaporated ZnO/MgxZn1-xO/CdxZn1-xO thin films for photodiode applications(Academic Press, 2021) Kumar, R.R.; Shukla, R.; Pandey, S.K.; Pandey, S.K.— This work reports the growth optimization and analysis of ZnO, MgxZn1-xO, and CdxZn1-xO thin films on silicon substrate using an electron beam evaporation system. The crystal phase purity, surface morphology, optical and electrical properties of deposited ZnO, MgxZn1-xO, and CdxZn1-xO thin films were studied. X-ray diffraction (XRD) spectra revealed that the deposited films were polycrystalline in nature with preferred (002) crystal orientation. Field emission scanning electron microscope study showed a dense-packed grained structure with an exact symmetrical distribution. The root-mean-square roughness of 3.03 nm was perceived by atomic force microscopy measurement for MgxZn1-xO thin-film, indicating good morphology of the deposited film. Photoluminescence measurement demonstrated a near-band-edge emission peak around 363 nm for ZnO thin film. The energy band gap obtained for ZnO, MgxZn1-xO, and CdxZn1-xO were 3.36 eV, 3.86 eV, and 2.89 eV, respectively, as measured by Ultraviolet–Visible spectroscopy. The higher amount of photocurrent was detected in illumination condition compared to dark condition with responsivity 0.54 AW-1 for ZnO films, making it suitable for photodiodes applications. © 2020 Elsevier LtdItem Phosphorus doping of ZnO using spin-on dopant process: A better choice than costly and destructive ion-implantation technique(Elsevier B.V., 2021) Mishra, M.; Sushama, S.; Pandey, S.K.; Chakrabarti, S.Radio frequency sputtered ZnO thin films doped with phosphorus (ZnO:P) have been prepared employing spin-on dopant process. In the SOD process, the dopant film has been spin-coated on a silicon substrate and positioned close to the as-deposited undoped ZnO film at high temperature to perform the phosphorus doping. The high-resolution X-ray diffraction measurement reveals that the prepared ZnO:P films are good in crystalline quality which improves further by annealing. It is found that the full-width half-maximum corresponding to (002) peak of SOD processed thin films is much narrower than previously reported ion-implanted thin films, indicating the better crystalline quality of SOD processed phosphorus-doped ZnO thin films. The X-ray photoelectron spectroscopy measurement signifies that the P2O5 decomposes into two phosphorus atoms behaving like an acceptor dopant and five oxygen atoms which may fill in oxygen vacancies at high-temperature annealing. The photoluminescence spectra discover the acceptor bound exciton peak at 3.35 eV and free electron to acceptor level transitions at 3.31 eV. The calculated acceptor binding energy is 127 meV for the phosphorus dopant which works as a shallow acceptor level. It is found that the phosphorus-doped ZnO thin films prepared using the SOD process have much superior structural and optical properties in comparison to previously reported ion-implanted film. This study demonstrates that the SOD process is much superior than the ion-implantation process to produce high-quality ZnO:P thin films for very stable p-type conduction. © 2021 Elsevier B.V.Item Improving the Cu2ZnSn(S,Se)4-Based Photovoltaic Conversion Efficiency by Back-Contact Modification(Institute of Electrical and Electronics Engineers Inc., 2021) Sengar, B.S.; Garg, V.; Siddharth, G.; Kumar, A.; Pandey, S.K.; Dubey, M.; Atuchin, V.V.; Kumar, S.; Mukherjee, S.Back-contact modification using a 10-nm ZnS layer in CZTSSe-based solar cell can play a crucial role in improving photovoltaic conversion efficiency. An ultrathin layer of ZnS is deposited over Mo-coated soda lime glass substrate before depositing CZTSSe using sputtering. The crystal structure of deposited CZTSSe thin films over ZnS is recognized as (112)-oriented, polycrystalline in nature, and free from the presence of any secondary phases such as Cu2(S,Se) or Zn(S,Se). The bandgap of CZTSSe thin films deposited over ultrathin ZnS is observed to increase from 1.49 (deposited over Mo directly) to 1.58 eV at room temperature, as determined by spectroscopic ellipsometry. In addition, numerical simulation has been performed using SCAPS software. The impact of ZnS layer has been simulated by using the defects in the absorber and at the interface of ZnS/CZTSSe. The simulated results have been validated with experimentally fabricated CZTSSe device. Simulated device with ZnS intermediate layer is observed to give rise to a photovoltaic conversion efficiency of 15.2%. © 1963-2012 IEEE.Item Theoretical investigations of band alignments and SnSe BSF layer for low-cost, non-toxic, high-efficiency CZTSSe solar cell(Elsevier Ltd, 2021) Prabhu, S.; Pandey, S.K.; Chakrabarti, S.In this work, a numerical simulation approach is utilized using SCAPS-1D software to model, modify, optimize, and evaluate the CZTSSe solar cell structure. For the CZTSSe solar cell, one possible reason hindering the performance is improper band alignment between the absorber and the buffer layers. With conventional CdS as a buffer layer, having a fixed bandgap, tuning the band alignment is impossible. To overcome this issue, Cd-free zinc oxide-based compounds Zn(O1-xSx), Zn1-xSnxO, and Zn1-xMgxO are explored as buffer layers, and their performance is evaluated. Using their composition-dependent tunable bandgap as an advantage, suitable band alignment with the absorber layer is evaluated for equal or higher performance when compared to CdS. Further performance improvement is attempted by using SnSe as the back surface field (BSF) layer. Band alignment evaluation is also extended to the back contact (Mo)/SnSe interface, whereby an attempt is made to replace Mo with a suitable metal. The Ni is found as a good candidate to replace Mo to achieve high-efficiency solar cell. The same approach is repeated with the transparent conducting oxide layer, and aluminum doped zinc oxide (AZO) is found as a suitable material in place of ITO for optimized solar cell structure. A maximum power conversion efficiency of 17.55% is achieved with an optimized structure. It is also observed that the external quantum efficiency (EQE) of the solar cell is improved significantly in the blue photons region in comparison to the EQE of the champion solar cell. The optimized structure Ni/SnSe/CZT(S0.4Se0.6)/Zn(O0.3S0.7)/i-ZnO/AZO in this work will be very useful to fabricate low-cost and Cd-free high-efficiency kesterite solar cells. © 2021Item Enhancement in structural, elemental and optical properties of boron–phosphorus Co-doped ZnO thin films by high-temperature annealing(Elsevier B.V., 2021) Sushama, S.; Murkute, P.; Ghadi, H.; Pandey, S.K.; Chakrabarti, S.The inherent n-type nature of zinc oxide (ZnO) and its unstable p-type behavior with single dopant species have encouraged researchers to explore the effect of multiple dopants as a viable solution for long-term stability and repeatability. Herein, we report boron (B) and phosphorus (P) co-doped ZnO thin films engineered through an optimized ion implantation technique followed by annealing at 1000 °C in oxygen ambiance. We investigated their structural, chemical, and optical properties to capture the effect of both boron implantation duration and annealing temperature. Co-doping with boron was observed to boost phosphorus incorporation in the film. Compared with P-doping, P–B co-doping increased the dominance of acceptor-bound exciton peak and also, suppressed non-radiative/visible emission which is due to reduced Madelung energy. After high-temperature annealing at 1000 °C, further narrowing of optical emission peaks generated due to acceptor incorporation was observed. Also, the co-doped samples showed stability in the acceptor behavior for more than one year. © 2021 Elsevier B.V.Item Innovative structural engineering of sustainable and environment-friendly Cu2ZnSnS4 solar cell for over 20% conversion efficiency(John Wiley and Sons Ltd, 2022) Prabhu, S.; Pandey, S.K.; Chakrabarti, S.Kesterite Cu2ZnSnS4(CZTS) thin-film technology has been comprehensively investigated over the last decade as a promising candidate in the field of photovoltaic technologies. However, despite numerous strategies to improve the performance, the efficiencies remain stagnant at around 11%. Poorly optimized absorber/buffer interface, non-absorption of higher wavelength photons, and non-ohmic back contact are the primary reasons for the poor performance of the CZTS solar cell. The authors of this paper propose a cadmium-free buffer layer, multiple quantum wells (MQWs) structure, and a back surface field (BSF) layer to overcome these issues, respectively. In this study, the buffer layer, zinc oxysulfide (Zn[O1−xSx]) is considered as an alternative to toxic Cadmium Sulfide (CdS) for better band alignment with the CZTS absorber layer. Cu2ZnSn(SxSe1−x)4 (CZTSSe) is used as a quantum well material in MQWs to increase photon absorption in CZTS solar cells. Tin selenide (SnSe) is used as the BSF layer to reduce the effect of non-ohmic back contact and to improve the open-circuit voltage (Voc) of MQW incorporated CZTS solar cells. Detailed analysis and optimization of the modified structure with higher performance are presented. The simulation results obtained provide imperative guidelines for the fabrication of high-efficiency CZTS solar cells using non-toxic and earth-abundant materials. © 2022 John Wiley & Sons Ltd.Item Unfolding the conductivity reversal n- to p-type in phosphorus-doped ZnO thin films by spin-on dopant (SOD) process(Institute of Physics, 2022) Mishra, M.; Saha, R.; Bhowmick, S.; Pandey, S.K.; Chakrabarti, S.Phosphorus doping induced p-type doping in ZnO thin films based on spin-on dopant (SOD) process is reported in this article. Owing to the reduced dependence on the conventional amenities for diffusion/ion-implantation doping, the SOD process provides a simple and cheap doping method. The effect of SOD process temperature on conductivity ZnO thin films is investigated by altering the temperature from 700°C to 1000°C. Systematic field emission scanning electron microscopy analysis demonstrates the impact of doping temperature on the morphological properties of SOD. The x-ray diffraction measurements reveal that the p-type ZnO thin films had (002) preferred crystal orientation. At the same time, x-ray photoelectron spectroscopy validated the formation of the PZn-2VZn complex, which was responsible for the acceptor behaviour of films. Moreover, the photoluminescence spectra tracked down that the origin of 3.35 and 3.31 eV emission peaks is due to the acceptor bound exciton and free-electron to acceptor level transitions, respectively. Finally, an elevated hole concentration of 2.09 × 1016 cm-3 is achieved with a resistivity of 1.14 ω-cm at 800°C doping temperature. However, the film doped at 900°C and 1000°C showed n-type behaviour due to the generation of high concentration donor defects. Here, we successfully demonstrate that the SOD process has great potential to produce high-quality p-type ZnO thin films suitable for optoelectronic devices applications. © 2022 IOP Publishing Ltd.