text: in order to prepare photovoltaic electrodes , fluorine doped tin oxide ( fto ) glass substrates ( 3 mm thick ) supplied by mansolar . the glass substrates ( 2 × 2 cm ) were ultrasonically cleaned in propanol followed by acetone , each for 5 minutes . other typical substrates which may be used include indium doped tin oxide ( ito ) glass and polymers e . g . pet . a deposition layer medium was made , comprising niox nanoparticles (˜ 50 nm ) suspended in methanol ( 20 mg / ml ) as a dispersion medium . this deposition layer medium was deposited by spraying using a nebulizer ( burgener mira mist atomizer ) which uses an inert gas to break up the suspension into small aerosol droplets . in this case , the inert gas used was nitrogen at a flow rate of about 2 liters / min . the nebulizer was moved over the surface of the substrate in a raster pattern using a computer numeric control ( cnc ) device with a line speed of 20 mm / s and a step interval of 1 mm . the distance from the tube orifice to the substrate was fixed at 10 mm . during deposition , the glass substrates were mounted over a heating block to maintain the substrate temperature at approximately 50 degrees c . the methanol evaporated once deposited to leave a layer of loosely bound niox nanoparticles on the substrate . samples prepared in this way were subjected to microwave plasma processing as will now be described with reference to fig1 . some comparative tests were done against samples prepared in the same way but subjected to conventional furnace sintering in air using a carbolite furnace ( rhf 1200 ). fig1 shows an apparatus used in the manufacture of a photovoltaic electrode , comprising a plasma chamber 10 which is pressure controlled using a gas supply inlet 12 and a vacuum outlet 14 . in the processes described below , the pressure was controlled to form a plasma at a pressure of 20 mbar in an argon and oxygen atmosphere in a ratio 10 : 1 ( argon : oxygen ). a sample stage 16 is located within the chamber 10 to support one or more substrates ( not shown ) upon a sample holder 18 for processing . the sample stage is height - adjustable , rotatable , and is water - cooled . in the set - up used to generate the results described herein , three samples were treated at a time upon the sample holder 18 . a muegge microwave power supply 20 operating at 2 . 4 kw and 2 . 45 ghz provides microwave energy 22 via a tunable waveguide 24 having a tuner 26 , through a quartz window 28 into the chamber 10 , where it excites a plasma ball 30 located above the sample holder 18 . substrate temperatures were measured using a lascon qp003 two - colour pyrometer ( not shown ) from dr merganthaler gmbh & amp ; co . the niox film thickness was measured by step height measurement using a wyko nt1100 optical profilometer in vertical scanning interferometry ( vsi ) mode . for the cross sectional investigations , the coatings were mounted on stubs using double - sided carbon tape , and sputter coated with platinum , using a emitech k575x sputter coating unit , to prevent surface charging by the electron beam . samples were then examined using a fei quanta 3d feg dualbeam ( fei ltd , hillsboro , usa ). x - ray diffraction ( xrd ) measurements were carried out using a siemens d500 diffractometer operating at 40 kv and 30 ma with cu kα radiation in normal diffraction mode at 0 . 2 °/ min scan rate . niox coatings were sensitized with 0 . 3 mm erythrosin b ( ery ) dye , in a 99 . 8 % ethanol solution for 24 h . the dye adsorption was investigated in transmission mode using an analytikjena specord 210 uv - vis spectrophotometer in the wavelength range of 350 - 700 nm . the photovoltaic performance ( i - v characteristic ) of dye sensitized niox coatings were analyzed in two electrode configuration using 870 w m − 2 am 1 . 5 solar simulator and platinum coated fto was used as a counter electrode . the p - type behavior of ery - sensitized niox coatings was observed using a custom made photoelectrochemical cell in three - electrode configuration : working electrode was ery - niox on fto ; counter electrode was platinum , where sce was utilized as a reference electrode . the electrolyte was 0 . 5 m lii and 0 . 05 m i 2 in propylene carbonate ( from sigma - aldrich ). loosely adherent niox particulate layers were prepared from the metal oxide / methanol slurry using the spray technique described above . the layer thickness was maintained between 1 - 2 μm . referring to fig2 , the effect of sintering time on crystallite size was evaluated for samples sintered from 1 to 10 minutes using the microwave plasma sintering technique described above , prior to addition of the dye . for brevity , this microwave plasma sintering technique is referred to as “ rapid discharge sintering ” or “ rds ”. fig2 shows the x - ray diffraction data in the nio ( 200 ) plane for samples sintered for 1 , 3 , 5 , 7 and 10 minutes . using the scherrer equation to examine the xrd data , an increase in crystallite size from 6 . 5 to 19 . 0 nm was observed on increasing the sintering time from 1 to 10 minutes . the scherrer formula gave a crystallite size of 6 . 5 nm for each of the samples sintered for 1 minute , 3 minutes and 5 minutes . for the sample sintered at 7 minutes the crystallite size was calculated at 12 nm , while for 10 minutes the size was 19 nm . referring to fig3 , in order to compare the performance of rds technique with conventional furnace treatments , the niox coatings were also sintered at 450 ° c . for 5 minutes in a box furnace . the properties of the furnace sintered coatings were then compared with those obtained using the rds technique . xrd examination of the sintered niox coatings demonstrated a significantly smaller crystallite size of 6 . 5 nm for the microwave plasma sintered samples , as compared to the 14 nm obtained after the furnace treatment . thus the smaller grain size along with more homogeneous heating / sintering is achieved using the rds technique thus helping to maintain the mesoporous structure of the niox nanoparticles . referring to fig4 , after treatment of the rds samples with the ery - b dye , the uv - vis absorption spectra of the samples prepared under different sintering times showed a gradual decrease of the amount of adsorbed dye for the coatings with the smaller crystallite size to those with the largest crystallites . the line with the highest peak in fig4 is the reference of the ery - b dye in solution . referring to fig5 , comparative data can be seen for the 5 minute rds sample and the 5 minute furnace sintered sample . again the reference is shown for ery - b solution . from this it can be seen that the rds sample has a far greater degree of dye absorption , probably due to rougher surface morphology . fig6 shows the p - type behavior of ery - sensitized niox coatings ( rds5 ). the curves in dark and under uv illumination demonstrated cathodic photocurrents of ery - sensitized niox coatings with an onset of photocurrent at approximately + 120 mv vs . sce reference . next , the open current photovoltage ( v oc ), the short circuit photocurrent density ( i sc ) and overall photocurrent efficiency ( η ), were measured as a function of sintering time . fig7 details the i - v characteristics of the ery sensitized niox coatings sintered at different times ( thickness : 1 - 2 μm ). though dye adsorption levels were higher for the 1 minute sintered coatings , the 5 minutes sintered sample ( rds5 ) exhibited the highest efficiency . these sintering conditions facilitate a high level of dye diffusion , while maintaining interconnectivity between individual oxide grains . thus the mesoporous sintered metal oxide structure facilitates efficient charge injection from the ery dye . a subsequent study with 2 . 5 μm thick niox coatings also demonstrated a similar trend . fig8 shows focused ion beam / scanning electron microscope ( fib / sem ) cross section images of niox coatings obtained after 5 minute sintering using ( a ) the furnace and ( b ) microwave plasma . in each image , one can see the sintered niox layer 34 , the fto layer 36 , and the underlying glass substrate 38 . it is clear from these images that the rds sintered coating exhibits a higher level of bonding at the interface 40 between the niox coating and fto layer , as seen by the elimination of the dark gap seen a this interface in fig8 ( b ). a possible explanation for this is that the rds treatment involves volumetric heating , which provides more effective heating inside the metal oxide coating matrix than obtained with the conductive heating obtained using the furnace . indeed the latter treatment may give rise to selective heating of the outer surface of a coating to produce a heat affected zone [ 37 ]. from fig8 it is also clear that the rds sintered oxide yields a much rougher surface morphology , which would also assist dye adsorption ( fig3 b ). finally , the photovoltaic performances ( open circuit voltage , short circuit current , fill factor , and percent efficiency of both rds5 and cs5 ( i . e . the notation cs5 denotes the 5 minutes furnace sintered sample ) coatings were measured as detailed in table 1 , and comparative values are given for two of the best performing electrodes as reported in the literature , namely he et al . [ j . he , h . lindström , a . hagfeldt , s .- e . lindquist , solar energy materials and solar cells , 62 ( 2000 ) 265 - 273 ] and nattestad et al . [ a . nattestad , m . ferguson , r . kerr , y - b . cheng , u . bach , nanotechnology , 19 ( 2008 ) 295 - 304 ]. the measurements carried out by he and by nattestad were also obtained under the same test methodology . the nattestad results were obtained using the dye erythrosin - j rather than the erythrosin - b the furnace sintered coatings reported here are broadly similar in efficiency to the values reported in the literature while those obtained with the rds treatment exhibit significantly higher performance , i . e . a tenfold increase of conversion efficiency was observed for the 5 minute rds sintered niox coatings as compared to the 5 minute furnace sintered sample . results are given in table 1 for samples sintered in the furnace for both 5 minutes ( cs5 ) and 30 minutes ( cs30 ). an n - type electrode based on titanium dioxide and ery - b was made according to the same techniques as described above . using the same fto glass substrates , a deposition layer slurry was created using titanium dioxide nanoparticles (“ aerosil ® p 25 ” from evonik industries ) having an average particle size of 21 nm , and methanol ( 25 mg / ml ). this mixture was again sprayed on the glass substrate using a nebulizer , in this case in a layer 9 microns thick , and samples were subjected to both conventional and microwave plasma sintering . fig9 shows comparative fib - sem cross - section images for ( a ) conventionally sintered ( at 500 degrees c . for 30 minutes ) and ( b ) microwave plasma sintered samples sintered for 5 minutes . the sem images again show the layers designated with the same reference numerals : sintered tio2 layer 34 , fto layer 36 , glass 38 and the interface 40 between the sintered tio2 and the fto substrate . as with the niox samples described above , one can again see that the rds sample in fig9 ( b ) exhibit far less of a gap at interface 40 , which strongly indicates better electrical connectivity and structural integrity relative to the cs sample in fig9 ( a ). accordingly , the technique of applying to a substrate a deposition layer including semiconductor particles , removing the dispersion medium , and then exposing the weakly bound particle layer to a microwave plasma under conditions leading to sintering of the particles , gives rise to a mesoporous semiconductor layer which is strongly bound with good electrical connectivity to the underlying substrate , and this has been demonstrated both for n - type tio2 and p - type niox . in the case of the niox photocathodes , using the materials and methods described herein it can be seen that a 5 minute microwave plasma treatment cycle provides optimal conversion efficiency , and improved adhesion to fto substrates compared with that obtained using furnace treatments . the 44 % increase in the quantity of adsorbed dye in the case of the rds treated coatings significantly contributed to the tenfold increase in light - to - current conversion efficiency , compared with that obtained with the furnace sintered coatings . this enhanced performance of the microwave plasma sintered coatings is associated with their smaller grain size after sintering , higher surface roughness and enhanced level of interconnectivity between grains in the mesoporous metal oxide structure . fig1 shows the comparative xrd spectra of the box furnace treated and microwave plasma treated tio2 coatings , and it can be seen that both exhibit very similar xrd spectra . degussa p25 tio 2 nanoparticles with an average size between 20 - 25 nm were deposited on ito - pen coated substrate ( where ito stands for indium doped tin oxide and pen for polyethylene naphthalate ). the tio 2 was prepared in a suspension form by grinding the nanoparticles powder in an alumina mortar in order to breakdown the agglomerated particles . the ground paste was then transferred into a recipient using methanol solvent vehicle and diluted to a final concentration of 25 - 30 mg / ml and further sonicated using a sonication horn probe . the tio 2 suspension was applied to the plastic substrate using a roll - to - roll spraying technique . in this technique the suspension is pumped through a nebulizer , shown in fig1 , and with the assistance of a pressurised gas ( nitrogen ) is atomised and projected at the surface of the plastic substrate mounted onto a cnc controlled ( x - y - z ) pneumatic table . in addition to the tio 2 suspension , a second suspension consisting of titanium isopropoxide ( tip ) ( 20 - 25 mmol / l ) precursor in propan - 2 - ol was co - applied using a second nebuliser . the titanium isopropoxide ( tip ) is used to enhance the adhesion of the tio 2 coating to the plastic substrate . the thickness of the tio 2 coating ( varying between 4 to 10 μm ) is controlled by the amount of tio 2 in the suspension and / or the number of passes of the nebuliser over the substrate . in the tests described below and illustrated with reference to fig1 - 19 the tio 2 coatings had a thickness between 4 and 6 microns . this thickness will influence solar to electricity conversion efficiency due to parameters such as the electron transport properties of the coating structure . also , the quantity of dye adsorbed will be influenced by the tio 2 coating thickness . after the coating deposition the samples were allowed to relax for approx . 20 minutes to thoroughly evaporate the carrying vehicle , leaving the powdered semiconductor particles in a deposition layer on the substrate . sintering of the dried tio 2 coatings was then carried out in oxygen plasma generated using a 2 . 45 ghz microwave generator . the plasma gas pressure was maintained between 4 - 5 mbar with a sample treatment time of 5 minutes . the plasma processing apparatus was as shown and as previously described in relation to fig1 , except that oxygen plasma was employed at 4 - 5 mbar instead of a 10 : 1 argon : oxygen mixture at 20 mbar ( although argon : oxygen mixtures or other plasmas could equally be employed ), and ( 2 ) a mask was overlaid on the sample as described below . the samples were held on the cooling stage of the microwave system using a mask ( in this case a 1 mm thick glass slide ) to ensure its flatness and intimate contact with the stage as illustrated in fig1 . the presence of the cooling stage ensures the integrity of the polymeric substrate ; as shown in fig1 the use of an intermediate sample holder ( in this case a glass cover as thin as 0 . 5 mm in thickness ) resulted in melting the polymer . this is an indication that the plasma gas temperature exceeded 270 - 300 ° c . ( as the melting temperature of pen is reported to be 270 ° c .— see e . l . bedia , s . murakami , t . kidate and s . kohjiya ; polymer 42 ( 2001 ) 7299 - 7305 ). fig1 shows results of the tio 2 coatings subjected to rockwell hardness test and bending tests . the coatings were found to easily flake off after indentation was applied to the substrate without the use of tip precursor ( fig1 a ) whereas stability was greatly improved in the same test carried out on a sample prepared with tip precursor ( fig1 b ). as shown in fig1 c , 14 d and 14 e , the use of tip in the deposition process significantly improved the stability of the tio 2 coating on the substrate both in initial condition ( fig1 c ) and when bent through 20 degrees ( fig1 d ) and 40 degrees ( fig1 e ). the coatings were also evaluated using both high resolution scanning electron microscopy ( hrsem ) and focus ion beam ( fib ) cross sectional analysis . these analyses further confirmed the weakened nature of the adhesion of the tio 2 coating deposited without tip and its intimate contact with the substrate when tip is used as a co - depositing layer ( see fig1 ). the sem micrograph of the top surface of the tio 2 coatings indicated micro - crack formation in the coating when deposited on the plastic substrate ( fig1 ). this may be related to the spraying parameters and may be eliminated or significantly reduce by further optimisation of the deposition parameters . further to the morphological analysis of the tio 2 coating , the photovoltaic performance was assessed by assembling dssc &# 39 ; s and recording their current - voltage ( iv ) characteristics . fig1 shows the iv curve of tio 2 coatings on ito - pen substrates sintered in microwave plasma for 5 minutes or a conventional furnace for 60 minutes at 150 ° c . the iv curve of a tio 2 coating on an fto - glass substrate sintered at 500 ° c . for 60 minutes is also shown . table 2 compares the conversion efficiency ( η ) of the same coatings . it is found that the pen samples sintered in the microwave plasma exhibit 30 - 35 % higher conversion efficiencies when compared to the one sintered in the furnace while it reaches 60 % of the conversion efficiencies obtained on the fto - glass substrate . furthermore the overall processing cycle time of the samples in the microwave system is only 10 - 15 minutes including the time taken for loading / unloading of the samples and pumping down of the system . fig1 is a more detailed illustration of the steps involved in the flexing test , showing that a flex involved conforming the substrate and coating to both the interior ( concave ) surface of an annular cylinder and to the outer ( convex ) surface . fig1 shows the iv curves of cells of dyed tio 2 coatings subjected to repeated bending at 20 ° ( as illustrated in fig1 ) before assembling the cell . it is found that the bending does not alter the iv characteristics of the coatings .