Hock Beng Lee

Controlled Defects of Fluorine-incorporated ZnO Nanorods for Photovoltaic Enhancement

Anion passivation effect on metal-oxide nano-architecture offers a highly controllable platform for improving charge selectivity and extraction, with direct relevance to their implementation in hybrid solar cells. In current work, we demonstrated the incorporation of fluorine (F) as an anion dopant to address the defect-rich nature of ZnO nanorods (ZNR) and improve the feasibility of its role as electron acceptor. The detailed morphology evolution and defect engineering on ZNR were studied as a function of F-doping concentration (x). Specifically, the rod-shaped arrays of ZnO were transformed into taper-shaped arrays at high x. A hypsochromic shift was observed in optical energy band gap due to the Burstein-Moss effect. A substantial suppression on intrinsic defects in ZnO lattice directly epitomized the novel role of fluorine as an oxygen defect quencher. The results show that 10-FZNR/P3HT device exhibited two-fold higher power conversion efficiency than the pristine ZNR/P3HT device, primarily due to the reduced Schottky defects and charge transfer barrier. Essentially, the reported findings yielded insights on the functions of fluorine on (i) surface –OH passivation, (ii) oxygen vacancies (Vo) occupation and (iii) lattice oxygen substitution, thereby enhancing the photo-physical processes, carrier mobility and concentration of FZNR based device.

Two-dimensional CdS intercalated ZnO nanorods: a concise study on interfacial band structure modification

The controllable growth of metal sulfide–metal oxide based nanomaterials with a tunable band gap structure is vital in the fabrication of new generation optoelectronic devices. In this paper, two-dimensional hierarchical CdS/ZnO nanorod arrays were successfully grown via a low temperature hydrothermal-SILAR method. A concise mechanism related to the surface and band gap modification on the CdS/ZnO nanorods was investigated under various CdS deposition cycles (N). The diameter and surface roughness properties of the sample were found to be linearly dependent on the value of N. A bathochromic shift in the optical energy band gap revealed the quantum size effects of the CdS/ZnO nanorods, as well as the induced interface band state and energy band split in the ZnO band state. An impressive improvement in the crystallinity of the sample was also observed under the CdS treatment. The correlation between the optical band gap and photovoltaic efficiency was evaluated. The results proved that the ZnO nanorod/CdS devices exhibited a threefold higher power conversion efficiency in comparison to a pristine ZnO nanorod device.

Improving light extraction of flexible OLEDs using a mechanically robust Ag mesh/ITO composite electrode and microlens array

In this study, we designed a highly flexible, mechanically robust Ag mesh/ITO composite transparent conducting electrode (TCE) integrated with a microlens array (MLA) to improve the light extraction of organic light-emitting diodes (OLEDs). The mechanical flexibility and durability of a flexible OLED based on a Ag mesh/ITO composite TCE were superior to those of a device with a conventional ITO electrode, retaining ∼100% initial luminance for up to 1000 cycles at a bending radius of ∼2 mm. Additionally, both experimental and theoretical characterizations suggest that the performance of a flexible OLED, such as its external quantum efficiency and angular-dependent emission, vastly improves after the incorporation of MLA due to the additional light scattering induced by the MLA. The combined use of Ag mesh and MLA is a practical way to overcome the brittleness of ITO and improve the out-coupling efficiency of flexible OLEDs (enhancement ratio of ∼1.51).

Enhanced photovoltaic performance of CdS-sensitized inverted organic solar cells prepared via a successive ionic layer adsorption and reaction method

One-dimensional ZnO nanorods (ZNRs) synthesized on fluorine-doped tin oxide (FTO) glass by hydrothermal method were modified with cadmium sulfide quantum dots (CdS QDs) as an electron transport layer (ETL) in order to enhance the photovoltaic performance of inverted organic solar cell (IOSC). In present study, CdS QDs were deposited on ZNRs using a Successive Ionic Layer Adsorption and Reaction method (SILAR) method. In typical procedures, IOSCs were fabricated by spin-coating the P3HT:PC61BM photoactive layer onto the as-prepared ZNRs/CdS QDs. The results of current-voltage (I-V) measurement under illumination shows that the FTO/ZNRs/CdS QDs/ P3HT:PC61BM/ PEDOT: PSS/Ag IOSC achieved a higher power conversion efficiency (4.06 %) in comparison to FTO/ZNRs/P3HT:PC61BM/PEDOT: PSS/Ag (3.6 %). Our findings suggest that the improved open circuit voltage (Voc) and short circuit current density (Jsc) of ZNRs/CdS QDs devices could be attributed to enhanced electron selectivity and reduced interfacial charge carrier recombination between ZNRs and P3HT:PC61BM after the deposition of CdS QDs. The CdS QDs sensitized ZNRs reported herein exhibit great potential for advanced optoelectronic application.One-dimensional ZnO nanorods (ZNRs) synthesized on fluorine-doped tin oxide (FTO) glass by hydrothermal method were modified with cadmium sulfide quantum dots (CdS QDs) as an electron transport layer (ETL) in order to enhance the photovoltaic performance of inverted organic solar cell (IOSC). In present study, CdS QDs were deposited on ZNRs using a Successive Ionic Layer Adsorption and Reaction method (SILAR) method. In typical procedures, IOSCs were fabricated by spin-coating the P3HT:PC61BM photoactive layer onto the as-prepared ZNRs/CdS QDs. The results of current-voltage (I-V) measurement under illumination shows that the FTO/ZNRs/CdS QDs/ P3HT:PC61BM/ PEDOT: PSS/Ag IOSC achieved a higher power conversion efficiency (4.06 %) in comparison to FTO/ZNRs/P3HT:PC61BM/PEDOT: PSS/Ag (3.6 %). Our findings suggest that the improved open circuit voltage (Voc) and short circuit current density (Jsc) of ZNRs/CdS QDs devices could be attributed to enhanced electron selectivity and reduced interfacial charge carrie...

Effect of growth durations on the formation of ZnO nanorods prepared using continuous microwave heating technique

The effect of growth durations on the formation of ZnO nanorods thin film under continuous microwave irradiation was investigated. ZnO nanorods were synthesized using a microwave-assisted seed mediated growth method. ZnO nanoseed layer was prepared on Si substrate using a sol-gel technique. The microwave-assisted seed mediated growth of ZnO nanorods was conducted at different growth durations, namely 15, 20 and 30 min by fixing the irradiation power at 110 W. XRD diffractogram shows that the hexagonal wurtzite crystal structure for all the samples exhibited a preferred growth orientation along the c-axis. FESEM micrographs reveal with increasing heating duration, the average diameter of ZnO nanorods increased considerably and contrarily, the surface density of nanorods reduced. PL analysis suggests that an extended growth duration has resulted in the creation of defects in nanorods films owing to the insufficient supply of precursor ions during growth process.

Tailoring the photovoltaic performance of inverted hybrid solar cells by replacing PEDOT:PSS with V2Ox as hole-extraction layer

For nanoscale fabrication of organic photovoltaic device, the utilization of a hole extraction layer (HEL) is essential to prevent the wrong flow of charge carriers and overcome the Schottky barrier at electrode-polymer interface. In recent years, the degradation issue of PEDOT:PSS based device which arises from its acidic nature and extremely hygroscopic properties has prompted researchers to find an appropriate transition metal oxide as replacement. In current work, we introduced an entirely solution-processed substoichiometric vanadium oxide (V2Ox) as HEL in inverted polymer:fullerene based device. We demonstrated the efficiency of substoichiometric V2Ox layer in enhancing the device performance of inverted organic solar cells, with the PCE of the device increased from 1.91 to 2.89%. The V2Ox prepared herein was found to exhibit broad optical absorption, highly selective charge transporting properties and excellent film transparency. A robust correlation between the hole extraction nature and electroni...

Growth concentration effect on oxygen vacancy induced band gap narrowing and optical CO gas sensing properties of ZnO nanorods

Band gap energy and surface defect on the nanostructure play an important role especially in determining the performance and properties of the optical based gas sensor. In this report, ZnO nanorods (ZNRs) with various growth concentrations were successfully synthesized using a facile wet chemical approach. The gas sensing performance of the ZNRs samples with different concentrations were tested toward the highly hazardous carbon monoxide (CO) gas at a concentration of 10 ppm operated at room temperature. It was found that the 40 mM ZNRs sample exhibited the highest response coupled with the shortest response time (123.3 ± 1.3 s) and recovery time (7.7 ± 0.3 s). The high response and accelerated sensing reaction were attributed to the band gap narrowing of the 40 mM ZNRs induced by the increase in oxygen vacancy related defect states, and it is directly proportional to the CO gas sensing activity. These defects acted as the oxygen trap sites which will promote the oxygen adsorption on the surface of ZNRs a...