ICAPS 2020
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Item Effects of ZnO on inverted P3HT:PCBM bulk heterojunction solar cells(Faculty of Science, University of Kelaniya, Sri Lanka, 2020) Wanigasekara, G.; Namawardana, D.G.K.K.; Wanninayake, W.T.M.A.P.K.,; Jayathilaka, K.M.D.C.,; Wijesundera, R.P.; Siripala, W.Low cost, low environmental impact, ease of mass production and many more promising attributes of Organic Solar Cells (OSCs) have inspired researchers to investigate OCSs for increasing their performance and stability in a constant phase. Moreover, in the recent years, OSCs with inverted structures have gained more attention compared to the conventional configuration of the device. In this study, Indium Tin Oxide (ITO) -free inverted OSC devices were fabricated on polished Stainless Steel (SS) substrates with top illumination in order to have the device structure of SS/ZnO/P3HT:PCBM/PEDOT:PSS/Au. A thin film of Zinc Oxide (ZnO) layer was deposited on SS substrates from a solution of Zinc Acetate Dihydrate (ZnC₄H₆O₄·2H2O) using spin coating technique. The active layer was spin-coated from a bulk heterojunction polymer blend of regioregular Poly(3-hexylthiophene) (P3HT) and Phenyl-C61-butyric acid methyl ester (PCBM) on the prepared ZnO layer. On the top of the active layer, Ethylene glycol doped poly(3,4- ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was blade coated as the hole transport layer. Then the stack was annealed before Gold (Au) was sputter coated to make the top contact. The device performance was optimized by varying a number of parameters including the concentration of ZnC₄H₆O₄·2H2O solution, thickness of the ZnO layer, annealing temperature, annealing time, composition of the polymer blend and dopant material of PEDOT:PSS dispersion. Open circuit voltage (Voc) and short circuit current (Jsc) of the devices increased after applying ZnO layer. The thermal annealing improved the fill factor (FF) of the devices. Spectral response measurements reveal that photon energies higher than 1.77 eV are absorbed by the device and photogenerated electron-hole pairs are produced. The best OSC device exhibited Voc of 440 mV with the Jsc of 6.2 mA/cm2 , fill factor (FF) of 30% and maximum power conversion efficiency of 0.05%.Item Fabrication of inverted polymer based organic solar cells on stainless steel substrate(Faculty of Science, University of Kelaniya, Sri Lanka, 2020) Namawardana, D.G.K.K.; Wanigasekara, G.; Wanninayake, W.T.M.A.P.K.; Jayathilaka, K.M.D.C.; Wijesundera, R.P.; Siripala, W.In the past years, polymer based organic solar cells (OSCs) have become a widely researched topic as a potential candidate for producing clean and renewable energy due to their lightweight, high mechanical flexibility, and large-area processability. As an alternative for the conventional device structure, in this study, OSC devices with an inverted structure were fabricated and characterized under the top illumination. Regioregular poly (3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) were used as the electron donor and electron acceptor material respectively for the device fabrication with structure of SS/P3HT:PCBM/PEDOT:PSS/Au. On pre-cleaned stainless steel (SS) substrates, bulk heterojunction polymer blend was spin coated from chlorobenzene solution (20 mg/mL) with a 1:1 weight ratio of P3HT: PCBM and then it was thermally annealed. As a hole-transport-layer (HTL), a thin film of poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) doped with ethylene glycol (10 wt.%) was blade coated on the active layer and the stack was annealed at 120ºC for 10 minutes. As the top contact of the device, gold (Au) was sputter coated. Performances of the fabricated OSC devices were optimized by varying several discrete parameters including the spin rate of the active layer formation, annealing temperature and the annealing time of the active layer. The optimum conditions for the device fabrication with the best performance were at the spin rate of 3000 rev./min for the active layer formation whereas optimum annealing temperature and annealing time were 160ºC and 60 minutes, respectively. The best device produced had an open-circuit voltage (Voc) of 238 mV and a short-circuit current density (Jsc) of 4.36 mAcm-2 . A maximum power conversion efficiency (PCE) of 0.02% with a fill factor (FF) of 23.16% was obtained under 1 sun illumination (AM 1.5G, 1000 Wm-2 ). The spectral response measurements of the fabricated cell indicate that it absorbs photons with energy higher than 1.77 eV to generate electron-hole pairs. It is planned to fabricate a thin film of Zinc Oxide (ZnO) as a potential electron transport layer (ETL) on SS substrate to improve the FF and PCE of the device.Item Comparison of the properties of CZTS semiconductor films grown by sequential and single step electrodeposition techniques(Faculty of Science, University of Kelaniya, Sri Lanka, 2020) Fernando, W.T.R.S.; Jayathilaka, K.M.D.C.; Wijesundera, R.P.; Siripala, W.Cu2ZnSnS4 (CZTS) is a promising semiconductor material suitable for application in low-cost and environmentally friendly thin film solar cells due to its superior optoelectronics properties. It is a perfect absorber material due to its high absorption coefficient (>10-4 cm-1 ) and direct optical bandgap (1.4-1.5 eV). Among the CZTS preparation techniques, electrodeposition is an attractive technique because of its simplicity, low cost and easy process controlling capability. In this investigation, a comparative study on CZTS films grown by two different techniques, namely, sequential electrodeposition and single step electrodeposition, has been carried out. Electrodeposition of Cu, Sn and Zn stack layers followed by sulphurisation with H2S is one of CZTS growth techniques. In this study, growth parameters of sequentially electrodeposited CZTS were optimized to obtain best photoactive CZTS thin films. Electrodeposition parameters of Cu, Sn and Zn have been obtained using voltammograms. Cu thin film was electrodeposited on Mo substrate at –0.89 V vs Ag/AgCl in an electrochemical cell containing 0.4 M CuSO4, 3 M lactic acid and NaOH at pH 11. Deposition of Sn thin film on Mo/Cu electrodes was carried out at -1.2 V vs Ag/AgCl in an electrochemical cell containing 0.055 M, 2.25 M NaOH and 8 ml of sorbitol. Zn thin film was electrodeposited on Mo/Cu/Sn at -1.2 V vs Ag/AgCl in an electrochemical cell containing 0.2 M ZnSO4. In order to grow CZTS, Mo/Cu/Sn/Zn thin films were annealed at 550 oC for 60 min in H2S. In the single step electrodeposition, CZTS thin films on Mo substrate were potentiostatically electrodeposited at -1.05 V vs Ag/AgCl for 40 min in a three electrode electrochemical cell containing 0.02 M copper (II) sulfate pentahydrate (CuSO4·5H2O), 0.01 M zinc sulfate heptahydrate (ZnSO4·7H2O), 0.02 M tin sulfate (SnSO4) and 0.02 M sodium thiosulfate (Na2S2O3) at room temperature. 0.2 M tri-sodium citrate (C6H5Na3O7) was used as the complexing agent and tartaric acid (C4H6O6) was used as the pH control solution. The pH of the bath was maintained at 5. The Ag/AgCl and platinum electrodes were used as the reference and the counter electrodes respectively. Then samples prepared were annealed at 550 oC for 30 min in H2S. CZTS films grown by two techniques were characterized using X-ray diffraction, reflectance, dark and light I-V, spectral response and C-V measurements in a PEC containing 0.1 M sodium acetate. Reflectance measurements reveal that the band gap energy of the films is 1.45 eV and I-V and spectral response measurements reveal that CZTS thin films were photoactive and p-type. The results obtained revealed that high quality photoactive CZTS can be prepared using both techniques. However, I-V and spectral response characteristics revealed that photoactive properties of CZTS thin films prepared by single step electrodeposition technique are superior in comparison to sequentially electrodeposited thin films.Item Enhancement of photovoltaic performance of Cu2O homojunction by introducing a ZnO buffer layer(Faculty of Science, University of Kelaniya, Sri Lanka, 2020) Thejasiri, S.A.A.B.; Kafi, F.S.B.; Wijesundera, R.P.; Siripala, W.Cuprous oxide (Cu2O) is a semiconductor material having the capability of producing a theoretical conversion efficiency of 20% which is acceptable for solar energy applications. In this investigation, we have explored the possibility of improving open circuit voltage (Voc) of Cu2O homojunctions by introducing a ZnO buffer layer in between n- and p-Cu2O layers. The thin buffer layer may be able to develop an additional potential drop across the interface improving Voc without hindering short-circuit current density (Jsc). In this investigation, n-Cu2O thin films were electrodeposited on Ti substrates at -200 mV vs Ag/AgCl for 30 minutes in an acetate bath. Samples were then annealed at 175 oC for 30 min in air. ZnO thin film was deposited on Ti/nCu2O film by employing Successive Ionic Layer Adsorption Reaction (SILAR) technique using 0.1 M Zn(NH3)4 2+ aqueous solution. Resulted samples were annealed at 175 oC for 10 minutes. pCu2O thin film was electrodeposited on Ti/n-Cu2O/ZnO electrode at -450 mV vs. Ag/AgCl for 45 minutes in a lactate bath. Surface of p-Cu2O was exposed to ammonium sulphide vapor in order to prepare an ultra-thin Cu2S layer. Finally, 2x2 mm2 Au spots were sputtered on the coper sulphide layer. A set of Ti/n-Cu2O/ZnO/p-Cu2O/Au devices having different thicknesses of ZnO layers was prepared by changing the number of successive adsorption cycles and characterized them by using dark and light current voltage measurements. Dark and light current voltage characteristics revealed that the device fabricated using 3 cycled ZnO layer produces the best photoactive performance. Without the buffer layer, the device produced Voc of 384 mV and Jsc of 8.1 mAcm-2 , under AM 1.5 illumination. With the ZnO buffer layer the device Voc improved up to 416 mV and Jsc up to 9.1 mAcm-2 . Our results revealed the possibility of improving both Voc and Jsc of the Cu2O homojunction by introducing a ZnO buffer layer.