Dr Imalka Jayawardena

Marcus Lee Lecturer
PhD (Surrey), BSc (Hons) in Engineering (Moratuwa, Sri Lanka)


Areas of specialism

Photovoltaics; Perovskites; Organic Semicondutors; Light Emitting Devices; X-Ray detection and Imaging; Nanoelectronics


Research interests

Research collaborations


Postgraduate research supervision

My teaching

My publications


Tan YY, Jayawardena KDGI, Adikaari AADT, Tan LW, Anguita JV, Henley SJ, Stolojan V, Carey JD, Silva SRP (2012) Photo-thermal chemical vapor deposition growth of graphene,CARBON 50 (2) pp. 668-673 PERGAMON-ELSEVIER SCIENCE LTD
Jayawardena KDGI, Fryar J, Ravi S, Silva P, Henley SJ (2010) Morphology Control of Zinc Oxide Nanocrystals via Hybrid Laser/Hydrothermal Synthesis, JOURNAL OF PHYSICAL CHEMISTRY C 114 (30) pp. 12931-12937 AMER CHEMICAL SOC
Jayawardena KDGI, Opoku C, Fryar J, Silva SRP, Henley SJ (2011) Excimer laser accelerated hydrothermal synthesis of ZnO nanocrystals & their electrical properties,APPLIED SURFACE SCIENCE 257 (12) pp. 5274-5277 ELSEVIER SCIENCE BV
Jayawardena KDGI, Rozanski LJ, Mills CA, Silva SRP (2015) The true status of solar cell technology, Nature Photonics 9 (4) pp. 207-208
Alenezi MR, Alshammari AS, Jayawardena KD, Beliatis MJ, Henley SJ, Silva SR (2013) Role of the Exposed Polar Facets in the Performance of Thermally and UV Activated ZnO Nanostructured Gas Sensors.,J Phys Chem C Nanomater Interfaces 117 (34) pp. 17850-17858
ZnO nanostructures with different morphologies (nanowires, nanodisks, and nanostars) were synthesized hydrothermally. Gas sensing properties of the as-grown nanostructures were investigated under thermal and UV activation. The performance of the ZnO nanodisk gas sensor was found to be superior to that of other nanostructures (S g
Han S, Adikaari AADT, Jayawardena KDGI, Nismy NA, Silva SRP (2012) Control of nanocrystal surface defects for efficient charge extraction in polymer-ZnO photovoltaic systems,Journal of Applied Physics 112 (6)
Factors determining the photovoltaic device performance of blends of regioregular poly(3-hexylthiophene) (rr-P3HT) and ZnO nanostructures are reported. A decrease in the crystallinity of rr-P3HT upon the formation of ZnO (through hydrolysis) is observed through optical absorption spectroscopy. Increasing the humidity level for the ZnO formation leads to a decrease in the photoluminescence of the rr-P3HT:ZnO blend together with improved photovoltaic device performance. This is attributed to more efficient charge extraction due to a decrease in the radiative trap sites on the ZnO surface as a result of decreasing ZnO surface area with increasing humidity level. © 2012 American Institute of Physics.
Rozanski LJ, Smith CTG, Gandhi KK, Beliatis MJ, Dabera GDMR, Jayawardena KDGI, Adikaari AADT, Kearney MJ, Silva SRP (2014) A critical look at organic photovoltaic fabrication methodology: Defining performance enhancement parameters relative to active area,Solar Energy Materials and Solar Cells 130 pp. 513-520
With the ever-increasing focus on obtaining higher device power conversion efficiencies (PCEs) for organic photovoltaics (OPV), there is a need to ensure samples are measured accurately. Reproducible results are required to compare data across different research institutions and countries and translate these improvements to real-world production. In order to report accurate results, and additionally find the best-practice methodology for obtaining and reporting these, we show that careful analysis of large data sets can identify the best fabrication methodology. We demonstrate which OPV outputs are most affected by different fabrication or measurement methods, and identify that masking effects can result in artificially-boosted PCEs by increasing fill factor and current densities, requiring care when selecting which mask to use. For example, our best performing devices (>6% efficiency) show that the smallest mask areas have not produced a surfeit of the highest performers, with only 11% of the top performing devices measured using a 0.032 cm2 mask area, while 44% used the largest mask (0.64 cm2). This trend holds true for efficiencies going down to 5%, showing that effective fabrication conditions are reproducible with increasing mask areas, and can be translated to even larger device areas. Finally, we emphasise the necessity for reporting the best PCE along with the average value in order to implement changes in real-world production. © 2014 Elsevier B.V.
Beliatis MJ, Rozanski LJ, Jayawardena KDGI, Rhodes RW, Anguita JV, Mills CA, Silva SRP (2014) Hybrid and Nano-composite Carbon
Sensing Platforms,
In: Demarchi D, Tagliaferro A (eds.), Carbon for Sensing Devices 5 pp. 105-132 Springer International Publishing Switzerland
Carbon nanomaterials offer a number of possibilities for sensing
platforms. The ability to chemically functionalize the surfaces of the
nano-carbon, using hybrid or nano-composite structures, can further
enhance the material properties. Complementary to the addition of
any requisite chemical or biochemical functionality, such
enhancements can take the form of improved electrical, optical or
morphological properties which improve the transduction capabilities
of the carbon nano-material, or facilitate detection of the transduced
signal, for example by improving charge transfer to detection
electronics. Here we review the methods of producing hybrid and
nano-composite carbon structures for sensing systems, highlighting
the advantages of the functionalization in each case and benchmark
their performance against existing carbon-only devices. Finally, we
detail some of the recent applications of hybrid and nano-composite
carbon technologies in a wide variety of sensor technologies.
Henley SJ, Fryar J, Jayawardena KDGI, Silva SRP (2010) Laser-assisted hydrothermal growth of size-controlled ZnO nanorods for sensing applications,NANOTECHNOLOGY 21 (36) 365502 pp. --- IOP PUBLISHING LTD
Pulsed laser irradiation is used to seed the low-temperature hydrothermal growth of ZnO nanorods. UV laser irradiation produces ZnO nanoparticles in solution that act as nucleation seeds for the subsequent hydrothermal growth of the nanorods. By systematically varying the seed density and/or the concentration of the reactants, the diameter of the nanorods can be controlled over a wide range with a narrow size distribution. The nanorods are linked into multi-pod structures, due to nucleation at a central seed, but ultrasonic processing of the solutions is shown to yield isolated nanorods. Three-dimensional networks of these multi-pod structures are fabricated by drop-casting the solutions onto inter-digitated electrodes. These devices are used to detect ethanol, water vapour and UV light exposure.
Jayawardena KDGI, Amarasinghe KMP, Nismy NA, Mills CA, Mills CA, Silva SRP (2015) Effect of solution processed and thermally evaporated interlayers on the performance of backgrated polymer solar cells, Thin Solid Films
© 2015 Elsevier B.V. Polymer solar cells are fast gaining momentum as a potential solution towards low cost sustainable energy generation. However, the performance of architectures is known to be limited by the thin film nature of the active layer which, although required due to low charge carrier mobilities, limits the optical coupling to the active layer. The formation of periodic backgratings has been proposed as a solution to this problem. Here, we investigate the effect of solution processed and thermally evaporated interlayers on the performance of backgrated polymer solar cells. Analysis of device performance under standard conditions indicates higher power conversion efficiencies with the incorporation of the evaporated interlayer (5.7%) over a sol-gel processed interlayer (4.9%). This is driven by a more conformal coating as evidenced through two orders of magnitude higher electron mobilities (10-5 versus 10-7 cm2 V-1 s-1) as well as the balanced electron and hole transport observed for the former architecture. It is believed that these results will catalyse further development of such device engineering concepts for improved optical coupling in thin film photovoltaics.
Han SJ, Adikaari AADT, Jayawardena KDGI, Nismy NA, Hak Kim Y, Won Kim J, Hahn YB, Silva SRP (2013) Band alignment effects at the metal electrode interface of poly(3-hexylthiophene):zinc oxide hybrid photovoltaics,Applied Physics Letters 102 (8)
Photoelectron spectroscopy is used to investigate the role of titanium oxide as an interfacial layer between a hybrid regioregular poly(3- hexylthiophene):zinc oxide photoactive layer and the Al back contact. The inspection of chemical bonds through X-ray photoemission spectroscopy core peaks indicates that the inner structure of the rr-P3HT:ZnO photo-active layer is preserved, subsequent to the deposition of the TiOx layer. Furthermore, the band alignment of rr-P3HT:ZnO/TiOx and TiO x/Al interfaces gives rise to the enhancement in device efficiency from 1.08% to 1.22% upon incorporating the TiOx layer, which is associated with the additional open circuit voltage obtained in the interface of P3HT:ZnO/TiOx. © 2013 American Institute of Physics.
Sam FLM, Razali MA, Jayawardena KDGI, Mills CA, Rozanski LJ, Beliatis MJ, Silva SRP (2014) Silver Grid Transparent Conducting Electrodes for Organic Light Emitting Diodes,Organic Electronics 15 (12) pp. 3492-3500 Elsevier
Polymer organic light emitting diodes (OLEDs) were fabricated using thin silver hexagonal grids replacing indium tin oxide (ITO) as the transparent conducting electrodes (TCE). Previous literature has assumed that thick metal grids (several hundred nanometres thick) with a lower sheet resistance ( 80 %) compared to thinner grids would lead to OLEDs with better performance than when thinner metal grid lines are used. This assumption is critically examined using OLEDs on various metal grids with different thicknesses and studying their performances. The experimental results show that a 20 nm thick silver grid TCE resulted in more efficient OLEDs with higher luminance (10 cd/A and 1460 cd/m2 at 6.5 V) than a 111 nm thick silver grid TCE (5 cd/A and 159 cd/m2 at 6.5 V). Furthermore, the 20 nm thick silver grid OLED has a higher luminous efficiency than the ITO OLED (6 cd/A and 1540 cd/m2 at 6.5 V) at low voltages. The data shows that thinner metal grid TCEs (about 20 nm) make the most efficient OLEDs, contrary to previous expectations.
Nismy NA, Jayawardena KD, Adikaari AA, Silva SR (2011) Photoluminescence Quenching in Carbon Nanotube-Polymer/Fullerene Films: Carbon Nanotubes as Exciton Dissociation Centres in Organic Photovoltaics., Advanced Materials 23 (33) pp. 3796-3800 Wiley
Fryar J, Jayawardena KDGI, Silva SRP, Henley SJ (2012) The origin of the metal enrichment of carbon nanostructures produced by laser ablation of a carbon-nickel target, Carbon
Compositional analysis of metal-containing carbon thin films and nanostructures produced by pulsed laser ablation of a carbon-nickel target revealed significantly higher fractions of nickel in the materials than in the target used to produce them. Ablation of mixed targets is used routinely in the synthesis of carbon nanotubes and to enhance the conductivity of amorphous carbon films by metal incorporation. In this extensive study we investigate the physical mechanisms underlying this metal-enrichment and relate changes in the dynamics of the ablation plumes with increasing background gas pressure to the composition of deposited materials. The failure to preserve the target atom ratios cannot, in this case, be attributed to conventional mechanisms for non-stoichiometric transfer. Instead, nickel-enrichment of the target surface by back-deposition, combined with significantly different propagation dynamics for C atoms, Ni atoms and alloy clusters through the background gas, appears to be the main cause of the high nickel fractions. © 2012 Elsevier Ltd. All rights reserved.
Beliatis MJ, Gandhi KK, Rozanski LJ, Rhodes R, McCafferty L, Alenezi MR, Alshammari AS, Mills CA, Jayawardena KDGI, Henley SJ, Silva SRP (2014) Hybrid graphene-metal oxide solution processed electron transport layers for large area high-performance organic photovoltaics, Advanced Materials 26 (13) pp. 2078-2083
Solution processed core-shell nano-structures of metal oxide-reduced graphene oxide (RGO) are used as improved electron transport layers (ETL), leading to an enhancement in photocurrent charge transport in PCDTBT:PC 70BM for both single cell and module photovoltaic devices. As a result, the power conversion efficiency for the devices with RGO-metal oxides for ETL increases 8% in single cells and 20% in module devices. © 2014 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Tan YY, Tan LW, Jayawardena KDGI, Anguita JV, Carey JD, Silva SRP (2011) Field effect in chemical vapour deposited graphene incorporating a polymeric gate dielectric, Synthetic Metals 161 (21-22) pp. 2249-2252
We have investigated the room temperature long channel field effect characteristics of a single graphene layer transistor incorporating a poly-4-vinyl-phenol (PVP) organic insulating layer, as an alternative to conventional oxide gate dielectric materials. High purity copper foils were used in the chemical vapour growth of the graphene layer and visible Raman analysis confirmed the presence of a high quality mono-layer carbon film. Using a channel length of 50 ¼m, a field effect hole mobility of 37 cm2/Vs was calculated, which demonstrates the possibility of an all carbon graphene based large area transistor with carrier mobilities above those found in conventional long channel all organic electronic transistors. © 2011 Elsevier B.V. All rights reserved.
Dabera GDMR, Jayawardena KDGI, Prabhath MRR, Yahya I, Tan YY, Nismy NA, Shiozawa H, Sauer M, Ruiz-Soria G, Ayala P, Stolojan V, Adikaari AADT, Jarowski PD, Pichler T, Silva SRP (2013) Hybrid carbon nanotube networks as efficient hole extraction layers for organic photovoltaics,ACS Nano 7 (1) pp. 556-565
Transparent, highly percolated networks of regioregular poly(3-hexylthiophene) (rr-P3HT)-wrapped semiconducting single-walled carbon nanotubes (s-SWNTs) are deposited, and the charge transfer processes of these nanohybrids are studied using spectroscopic and electrical measurements. The data disclose hole doping of s-SWNTs by the polymer, challenging the prevalent electron-doping hypothesis. Through controlled fabrication, high- to low-density nanohybrid networks are achieved, with low-density hybrid carbon nanotube networks tested as hole transport layers (HTLs) for bulk heterojunction (BHJ) organic photovoltaics (OPV). OPVs incorporating these rr-P3HT/s-SWNT networks as the HTL demonstrate the best large area (70 mm2) carbon nanotube incorporated organic solar cells to date with a power conversion efficiency of 7.6%. This signifies the strong capability of nanohybrids as an efficient hole extraction layer, and we believe that dense nanohybrid networks have the potential to replace expensive and material scarce inorganic transparent electrodes in large area electronics toward the realization of low-cost flexible electronics. © 2012 American Chemical Society.
Tan YY, Jayawardena KDGI, Adikaari AADT, Tan LW, Anguita JV, Henley SJ, Stolojan V, Carey JD, Silva SRP (2012) Photo-thermal chemical vapor deposition growth of graphene, Carbon 50 (2) pp. 668-673
The growth of graphene on Ni using a photo-thermal chemical vapor deposition (PT-CVD) technique is reported. The non-thermal equilibrium nature of PT-CVD process resulted in a much shorter duration in both heating up and cooling down stages, thus allowing for a reduction in the overall growth time. Despite the reduced time for synthesis compared to standard thermal chemical vapor deposition (T-CVD), there was no decrease in the quality of the graphene film produced. Furthermore, the graphene formation under PT-CVD is much less sensitive to cooling rate than that observed for T-CVD process. Growth on Ni also allows for the alleviation of hydrogen blister damage that is commonly encountered during growth on Cu substrates and a lower processing temperature. To characterize the film's electrical and optical properties, we further report the use of pristine PT-CVD grown graphene as the transparent electrode material in an organic photovoltaic device (OPV) with poly(3-hexyl)thiophene (P3HT)/phenyl-C61-butyric acid methyl ester (PCBM) as the active layer where the power conversion efficiency of the OPV cell is found to be comparable to that reported using pristine graphene prepared by conventional CVD. © 2011 Elsevier Ltd. All rights reserved.
Silva SRP, Beliatis MJ, Jayawardena KDGI, Mills CA, Rhodes RW, Rozanski LJ (2014) Hybrid and nano-composite materials for flexible organic electronics applications, In: Logothetidis S (eds.), Handbook of flexible organic electronics: Materials, Manufacturing and Applications 3 Woodhead Publishing
Flexible organic electronics have recently progressed from ?organic-only? semiconductor devices, based on thin films of organic materials (small molecules and polymers), to hybrid and nano-composite materials - a family of truly advanced materials designed at the nanoscale which offer enhancements in device performance and a reduction in production costs over their traditional inorganic predecessors. These hybrid and nano-composite materials are attractive given the potentially wide range of available organic semiconductors (both small molecule and polymeric) and nanoparticle types (carbon allotropes, metal oxides, metal nanostructures etc.). Here, we emphasise the variety and potential of these materials and introduce some of the production methods, properties and limitations for their use in flexible electronics applications.
Smith CTG, Rhodes RW, Beliatis MJ, Jayawardena KDGI, Rozanski LJ, Mills CA, Silva SRP (2014) Graphene oxide hole transport layers for large area, high efficiency organic solar cells,Applied Physics Letters 105 (7) 073304 American Institute of Physics
Graphene oxide (GO) is becoming increasingly popular for organic electronic applications. We present large active area (0.64 cm^2), solution processable, poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]:[6,6]-Phenyl C71 butyric acid methyl ester (PCDTBT:PC70BM) organic photovoltaic (OPV) solar cells, incorporating GO hole transport layers (HTL). The power conversion efficiency (PCE) of ~5% is the highest reported for OPV using this architecture. A comparative study of solution-processable devices has been undertaken to benchmark GO OPV performance with poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) HTL devices, confirming the viability of GO devices, with comparable PCEs, suitable as high chemical and thermal stability replacements for PEDOT:PSS in OPV.
Jayawardena KDGI, Tan YY, Fryar J, Shiozawa H, Silva SRP, Henley SJ, Fuge GM, Truscott BS, Ashfold MNR (2011) Highly conductive nanoclustered carbon:nickel films grown by pulsed laser deposition,Carbon 49 (12) pp. 3781-3788
An enhancement by 5 orders of magnitude of the electrical conductivity of nanoclustered carbon films is reported by incorporation of metallic atoms, but without significant morphological changes. Films were deposited by 248 nm pulsed laser ablation of both a pyrolytic graphite target and a mixed carbon-nickel (C:Ni) target, and structural analysis revealed that similar film morphologies were obtained when deposition was carried out using either target. Compositional analysis demonstrated a preferential incorporation of nickel over carbon in the resulting films (cf. the composition of the target). This non-stoichiometric transfer was also observed for films grown by 193 nm laser ablation of the C:Ni target, for which the enhancement was more pronounced, indicating that the ablation mechanism and the subsequent transfer are important in determining the eventual film composition. © 2011 Elsevier Ltd. All rights reserved.
Abdel-Rahman MAE, Lohstroh A, Jayawardena I, Henley SJ (2012) The X-ray detection performance of polycrystalline CVD diamond with pulsed laser deposited carbon electrodes,Diamond and Related Materials 22 pp. 70-76
The deposition of amorphous carbon electrical contacts on a diamond radiation detector by Pulsed Laser Deposition (PLD) is introduced as a novel technique for producing tissue equivalent X-ray dosimeters. Three devices were fabricated with the following electrical contacts: pure amorphous carbon (labelled Poly-C), amorphous carbon mixed with Nickel (PLD) (labelled Poly-C/Ni) and conventional sputtered Pt (labelled Poly-Pt). To examine the performance of PLD carbon as a contact, a set of X-ray detection characteristics was studied and compared to those of Poly-Pt. This investigation includes current-voltage characteristics, linearity and dose rate dependence, sensitivity and specific sensitivity, photoconductive gain, stability, reproducibility and time response (rise and fall-off times). The experimental results suggest that Poly-C/Ni is suitable for an X-ray dosimeter. It shows a high signal to noise ratio (SNR) of ~ 3300, approximately linear relationship between the photocurrent and the dose rate and a sensitivity of 65 nC/Gy. In addition the current signal is stable and reproducible (within 0.26%) and the rise and fall-off times are less than 1.1 and 0.4 s, respectively. © 2011 Elsevier B.V. All rights reserved.
Schirru F, Lohstroh A, Jayawardena KDGI, Henley SJ, Sellin PJ (2013) X-ray induced photocurrent characteristics of CVD diamond detectors with different carbon electrodes,JOURNAL OF INSTRUMENTATION 8 IOP PUBLISHING LTD
Beliatis MJ, Gandhi KK, Rozanski LJ, Rhodes R, McCafferty L, Alenezi MR, Alshammari AS, Mills CA, Jayawardena KD, Henley SJ, Silva SR (2014) Hybrid graphene-metal oxide solution processed electron transport layers for large area high-performance organic photovoltaics., Adv Mater 26 (13) pp. 2078-2083
Solution processed core-shell nano-structures of metal oxide-reduced graphene oxide (RGO) are used as improved electron transport layers (ETL), leading to an enhancement in photocurrent charge transport in PCDTBT:PC70 BM for both single cell and module photovoltaic devices. As a result, the power conversion efficiency for the devices with RGO-metal oxides for ETL increases 8% in single cells and 20% in module devices.
Jayawardena KDGI, Rozanski LJ, Mills CA, Beliatis MJ, Nismy NA, Silva SRP (2013) ?Inorganics - in - Organics?: Recent Developments and Outlook for 4G Polymer Solar Cells, Nanoscale 5 (18) pp. 8411-8427 Royal Society of Chemistry
Recent developments in solution processable single junction polymer solar cells have led to a significant improvement in power conversion efficiencies from ~5% to beyond 9%. While much of the initial efficiency improvements were driven through judicious design of donor polymers, it is the engineering of device architectures through the incorporation of inorganic nanostructures and better processing that has continued the efficiency gains. Inorganic nano-components such as carbon nanotubes, graphene and its derivatives, metal nanoparticles and metal oxides that have been central role in improving device performance and longevity beyond those achieved by conventional 3G polymer solar cells. The present work aims to summarise the diverse roles played by the nanosystems and features in state of the art next generation (4G) polymer solar cells. The challenges associated with the engineering of such devices for future deployment are also discussed.
Han S, Fei Z, Jayawardena KDGI, Beliatis MJ, Hahn YB, Adikaari AADT, Heeney MJ, Silva SRP (2015) ZnO hybrid photovoltaics with variable side-chain lengths of thienothiophene polymer, Thin Solid Films 576 pp. 38-41
© 2014 Elsevier B.V. All rights reserved.The effect of the side-chain length of poly(3,6-dialkylthieno[3,2-b]thiophene-co-bithiophene) (pATBT) on the performance of hybrid polymer-metal oxide photovoltaics (PVs) utilizing zinc oxide (ZnO) acceptor is investigated. The pATBT attached with a dodecyl side chain (pATBT-C12) in hybrid photovoltaics with ZnO was compared to pATBT with a hexadecyl side chain (pATBT-C16). Atomic force microscopic analysis reveals a smoother surface for the pATBT-C16 photoactive layer compared to the pATBT-C12. For hybrid PVs using pATBT-C16, the relative intensity of the external quantum efficiency (EQE) increased particularly in wavelength region associated with the ZnO. Furthermore, the EQE spectrum shows a red shift for pATBT-C16 indicating better structural ordering compared to hybrid PVs with pATBT-C12. As a result, the hybrid PV utilizing pATBT-C16:ZnO blend layer is observed to display a better performance with a power conversion efficiency of 1.02% compared to 0.672% of pATBT-C12:ZnO PV.
Jayawardena KDGI, Rhodes R, Gandhi KK, Prabhath MRR, Dabera GDMR, Beliatis MJ, Rozanski LJ, Henley SJ, Silva SRP (2013) Solution processed reduced graphene oxide/metal oxide hybrid electron transport layers for highly efficient polymer solar cells,Journal of Materials Chemistry A 1 (34) pp. 9922-9927
We report new solution processable electron transport layers for organic photovoltaic devices based on composites of metal oxides and reduced graphene oxides. Low bandgap polymer cells fabricated using these nanohybrid transport layers display power conversion efficiencies in the range of 7.4-7.5% which is observed to be an improvement over conventional metal oxide or thermally evaporated electron transport layers. This efficiency enhancement is driven mainly by improvements in the short circuit current (from
Nismy NA, Jayawardena KDGI, Adikaari AADT, Silva SRP (2015) Nano-engineering of hybrid organic heterojunctions with carbon nanotubes to improve photovoltaic performance, Organic Electronics: physics, materials, applications 22 pp. 35-39
© 2015 Elsevier B.V. All rights reserved.Organic-inorganic hybrid photovoltaics are beginning to show significant promise as a low cost highly efficient route towards renewable energy generation. Of the hybrid architectures available, carbon nanotube incorporated organic photovoltaics is considered to be among the most promising. Herein, the optical and electronic effects of localizing multiwalled carbon nanotubes in the donor polymer is investigated in comparison to its incorporation into the bulk heterojunction architecture (triple heterojunction scheme) through photoluminescence quenching and dark diode characteristics analysis. A significant improvement in photoluminescence quenching is observed when the nanotubes are localized in the donor polymer where the active layer is formed through a sequential deposition route in comparison to the triple heterojunction scheme. However, the former architecture also leads to a higher recombination of carriers due to the introduction of trap states as observed through space charge limited conduction analysis. In comparison, the triple heterojunction scheme shows a lower dark current and hence a significantly improved photovoltaic device performance (3.8% in comparison to 2.6% for the sequentially deposited architecture). This indicates that the formation of the triple heterojunction is the more ideal scheme for improving device performances in organic-inorganic hybrid architectures.
Dharmasena Randunu Devage Ishara Gihan, Jayawardena Imalka, Mills Christopher, Deane Jonathan, Anguita Jose, Dorey Robert, Silva S (2017) Triboelectric Nanogenerators: Providing a Fundamental Framework,Energy & Environmental Science 10 (8) pp. 1801-1811 Royal Society of Chemistry
A new model which comprehensively explains the working principles of contact-mode Triboelectric Nanogenerators (TENGs) based on Maxwell?s equations is presented. Unlike previous models which are restricted to known simple geometries and derived using the parallel plate capacitor model, this model is generic and can be modified to a wide range of geometries and surface topographies. We introduce the concept of a distance-dependent electric field, a factor not taken in to account in previous models, to calculate the current, voltage, charge, and power output under different experimental conditions. The versatality of the model is demonstrated for non-planar geometry consisting of a covex-conave surface. The theoretical results show excellent agreement with experimental TENGs. Our model provides a complete understanding of the working principles of TENGs, and accurately predicts the output trends, which enables the design of more efficient TENG structures.
Jayawardena K. D. G. Imalka, Thirimanne Hashini M., Tedde Sandro Francesco, Huerdler Judith E., Parnell Andrew J., Bandara R. M. Indrachapa, Mills Christopher A., Silva S. Ravi P. (2019) Millimeter-Scale Unipolar Transport in High Sensitivity Organic?Inorganic Semiconductor X-ray Detectors,ACS Nano American Chemical Society
Hybrid inorganic-in-organic semiconductors are an attractive class of materials for optoelectronic applications. Traditionally, the thicknesses of organic semiconductors are kept below 1 ¼m due to poor charge transport in such systems. However, recent work suggests that charge carriers in such organic semiconductors can be transported over centimeter length scales opposing this view. In this work, a unipolar X-ray photoconductor based on a bulk heterojunction architecture, consisting of poly(3-hexylthiophene), a C70 derivative, and high atomic number bismuth oxide nanoparticles operating in the 0.1?1 mm thickness regime is demonstrated, having a high sensitivity of ?1 cm?3. The high performance enabled by hole drift lengths approaching a millimeter facilitates a device architecture allowing a high fraction of the incident X-rays to be attenuated. An X-ray imager is demonstrated with sufficient resolution for security applications such as portable baggage screening at border crossings and public events and scalable medical applications.
Bandara R. M. I., Jayawardena K. D. G. I., Adeyemo S. O., Hinder S. J., Smith J. A., Thirimanne H. M., Wong N. C., Amin F. M., Freestone B. G., Parnell A. J., Lidzey D. G., Joyce H. J., Sporea R. A., Silva S. R. P. (2019) Tin(iv) dopant removal through anti-solvent engineering enabling tin based perovskite solar cells with high charge carrier mobilities,Journal of Materials Chemistry C Royal Society of Chemistry
We report the need for careful selection of anti-solvents for Sn-based perovskite solar cells fabricated through the commonly used anti-solvent method, compared to their Pb-based counterparts. This, in combination with the film processing conditions used, enables the complete removal of unwanted Sn4+ dopants, through engineering the anti-solvent method for Sn-based perovskites. Using a Cs0.05(FA0.83MA0.17)0.95Pb0.5Sn0.5I3 perovskite, charge carrier mobilities of 32 ± 3 cm2 V?1 s?1 (the highest reported for such systems through the optical-pump terahertz probe technique) together with ?2 short circuit current densities are achieved. A champion efficiency of 11.6% was obtained for solvent extraction using toluene (an 80% enhancement in efficiency compared to the other anti-solvents) which is further improved to 12.04% following optimised anti-solvent wash and thermal treatment. Our work highlights the importance of anti-solvents in managing defects for high efficiency low bandgap perovskite materials and develops the potential for all-perovskite tandem solar cells.
Jayawardena K.D.G.I., Bandara R. M. I., Monti M., Butler-Caddle E., Pichler T., Shiozawa H., Wang Z., Jenatsch S., Hinder S. J., Masteghin M. G., Patel M., Thirimanne H.M., Zhang W., Sporea R.A., Lloyd-Hughes J., Silva S.R.P. (2019) Approaching the Shockley?Queisser limit for fill factors in lead?tin mixed perovskite photovoltaics,Journal of Materials Chemistry A 8 (2) pp. 693-705 The Royal Society of Chemistry
The performance of all solar cells is dictated by charge recombination. A closer to ideal recombination dynamics results in improved performances, with fill factors approaching the limits based on Shockley?Queisser analysis. It is well known that for emerging solar materials such as perovskites, there are several challenges that need to be overcome to achieve high fill factors, particularly for large area lead?tin mixed perovskite solar cells. Here we demonstrate a strategy towards achieving fill factors above 80% through post-treatment of a lead?tin mixed perovskite absorber with guanidinium bromide for devices with an active area of 0.43 cm2. This bromide post-treatment results in a more favorable band alignment at the anode and cathode interfaces, enabling better bipolar extraction. The resulting devices demonstrate an exceptional fill factor of 83%, approaching the Shockley?Queisser limit, resulting in a power conversion efficiency of 14.4% for large area devices.
Li Bowei, Xiang Yuren, Jayawardena Imalka, Luo Deying, Watts John, Hinder Steven, Li Hui, Ferguson Victoria, Luo Haitian, Zhu Rui, Silva Ravi, Zhang Wei (2020) Tailoring Perovskite Adjacent Interfaces by Conjugated Polyelectrolyte for Stable and Efficient Solar Cells,Solar RRL Wiley-Blackwell
Interface engineering is an effective means to enhance the performance of thin?film devices, such as perovskite solar cells (PSCs). Herein, a conjugated polyelectrolyte, poly[(9,9?bis(32?((N,N?dimethyl)?N?ethyl?ammonium)?propyl)?2,7?fluorene)?alt?2,7?(9,9?dioctylfluorene)]di?iodide (PFN?I), is used at the interfaces between the hole transport layer (HTL)/perovskite and perovskite/electron transport layer simultaneously, to enhance the device power conversion efficiency (PCE) and stability. The fabricated PSCs with an inverted planar heterojunction structure show improved open?circuit voltage (Voc), short?circuit current density (Jsc), and fill factor, resulting in PCEs up to 20.56%. The devices maintain over 80% of their initial PCEs after 800 h of exposure to a relative humidity 35?55% at room temperature. All of these improvements are attributed to the functional PFN?I layers as they provide favorable interface contact and defect reduction.
Jayawardena K. D. G. I., Silva S. M., Misra R. K. (2020) Solution processed perovskite incorporated tandem photovoltaics: developments, manufacturing, and challenges,Journal of Materials Chemistry C Royal Society of Chemistry
Photovoltaics based on organic?inorganic as well as all inorganic semiconducting perovskites have emerged as a high performing technology at a lower cost. Intense research carried out in this regard over the last decade has resulted in single junction power conversion efficiencies that now exceed 25%. Furthermore, combining wide-bandgap perovskites with other narrow-bandgap absorbers such as silicon has enabled efficiencies exceeding 24%. Such tandem architectures provide the possibility of increasing the power conversion efficiency with either a little increase in cost or even at a lower cost. In this review, we discuss the emerging tandem concepts that incorporate perovskites in at least one sub-cell and can also be printed on roll-to-roll manufacturing lines. Initially, we discuss the progress in the field of perovskite/silicon tandem architectures followed by a discussion on perovskite/copper indium gallium selenide tandem devices. Then, recent progress in all-perovskite tandem devices is discussed. This is then followed by developments combining perovskites and organic bulk heterojunction absorbers. Subsequently, we discuss roll-to-roll and sheet-to-sheet printing techniques that can be used for scaled-up manufacturing of the above tandem architectures, which is then followed by reported work on perovskites that have utilised these printing techniques. Finally, we discuss prospects and future directions focusing on material stability and elimination of toxic solvents that are typically used in lab-scale perovskite solar cell fabrication processes and on less well investigated applications such as energy harvesting in space.