Light-Induced Electroluminescence Patterning: Interface Energetics

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Light Induced Electro-Luminescence Patterning: Interface Energetics Modification at Semiconducting Polymer and Metal-Oxide Heterojunction in a Photodiode Sanyasi R Bobbara, Ehab Salim, Regis Barille, and Jean-Michel Nunzi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07033 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018

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The Journal of Physical Chemistry

Light Indu ed Ele tro-Lumines en e Patterning: Interfa e Energeti s Modi ation at Semi ondu ting Polymer and Metal-oxide Heterojun tion in a Photodiode †

Sanyasi Rao Bobbara,

‡

Ehab Salim,

¶

Régis Barille,

∗,†,§

and Jean-Mi hel Nunzi

†Department

of Physi s, Queen's University, Kingston, ON - K7L 3N6, Canada.

‡Department

of Physi s, Fa ulty of S ien e, Mansoura University, 35516, Egypt

¶Department §Department

of Physi s, Université d' Angers, Angers - 49045, Fran e.

of Chemistry, Queen's University, Kingston, ON - K7L 3N6, Canada.

E-mail: nunzijmqueensu. a

Phone: +1(613)533-6749

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ACS Paragon Plus Environment

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Abstra t Understanding the inje tion barriers and defe t states at the metal-organi or inorgani - organi interfa es is one of the key hallenges to improve the e ien y of hybrid ele troni devi es. In this paper, polymer and metal-oxide based photodiodes are subje ted to light-soaks in order to probe the interfa e and bulk indu ed defe ts and energeti s. Polymers, P3HT and PCDTBT, were used as a tive medium in an ìnverted' sandwi h-type devi e onguration to study the ee t of light-soak on urrent-voltage,

harge trapped and stored, ele trolumines en e, photovoltage and photo urrent hara teristi s. The results olle tively demonstrate a modi ation to the athode onta t and polymer interfa e energeti s. UV-assisted photo-desorption of oxidizing agents at the interfa e of nanostru tured zin oxide derived from a sol-gel pre ursor and the polymer lowers the magnitude of athode work-fun tion. As a result, we have realized an e ient LED sten illed out of the diode after UV exposure. The work fun tion and interfa e barrier modi ation followed by energy band-bending within the devi e is proposed. Our results emphasize the role of unintentional inje tion barriers and a solution to the issue often en ountered in the hybrid organi -inorgani ele troni devi es.

Introdu tion With the in reasing demand for alternative lean energy resour es as well as exible and

heaper ele troni s, organi -inorgani hybrid materials seem to be the potential andidates to pat h up the zone. 14 While exible organi materials oer the opti al band-gap and ele tri al properties' tunability, the inorgani semi ondu tors assist in e ient urrent inje tion/extra tion and in improved me hani al and photo-stability. 58 In hybrid devi es, the physi o- hemi al intera tions at the organi -inorgani heterojun tion play a riti al role in determining the devi e performan es. Depending upon the onditions under whi h the heterojun tions are formed, the urrent inje tion/extra tion, photogenerated ex iton disso iation, re ombination at the these interfa es ould be severely ae ted. 9,10 The unpredi table 2


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behaviour is attributed to unexpe ted surfa e states, adsorbates-indu ed defe t states et . In order to improve the harge extra tion/inje tion/transport in these devi es, it is important to understand the defe ts and any unintentional energeti barriers. In devi es su h as photovoltai s, the ele tri al hara teristi s are ae ted by a prolonged exposure to light itself. 11,12 In this work we have used the light-soak ee ts as a means to probe the energeti s in polymer-based photodiodes. Light-soak tests on organi devi es have been studied previously by other groups. 13,14 The s-shape in the fourth quadrant of light urrent-voltage (IV) hara teristi s has been a wellknown issue that drasti ally ae ts the power onversion e ien y (PCE) of photovoltai devi es. 15 Tress et al. 16 attributed it to the dieren e in the ele tron and hole mobilities whereas Fin k and S hwartz 17 explained it in terms of higher re ombination velo ities at the ele trode interfa es. It was shown that exposing the devi e to ozone, 13 UV light, 18 hydrogen plasma, 19 reverse bias annealing 20 or additional interlayer 2124 one an orre t the s-shape and therefore improve the PCE. In other words, the energeti s at the interfa es had been engineered to meet the requirements. In dark-IV hara teristi s, an in rease in spa e harge limited urrent (SCLC) and a de rease in the ohmi -regime urrent after light-soaking the devi e were reported before, 25,26 in ontrast to the results presented here. The urrentvoltage hara teristi s observed in the dark upon thermal annealing as reported by Chiguvare et al. 27 and Seemann et al. 28 were similar to devi e hara teristi s from the light-soak to be dis ussed in this work. Charge extra tion by linearly in reasing voltage (CELIV) showed a redu tion in harges extra ted upon light-soak . 26,28 Seemann et al. 28 dis ussed the ee t of light-soak on the devi e as de-doping the oxygen ae ted polymer bulk. Sundqvist et al. 26 explained the redu tion in harge extra tion from variation in work-fun tion of the ele tron-transporting metal oxide layer in the devi e. The redu tion in the work-fun tion of the athode should indeed be followed by an in rease in the built-in potential drop a ross the devi e. The builtin potential a ross the a tive medium an be tra ked by monitoring photovoltage. Similar 3



in rease in open- ir uit voltage has been re ently reported by Kusumi et al., 29 although it highlights the role of the ITO itself and not the ele tron transporting layer (ETL). In most re ent works the UV light-soak ee ts have been attributed to either the modi ation of the athode work fun tion 30 or to overall va uum shift due to interfa e dipole layer. 31 These hara teristi s of the metal oxides have been used to advantage as UV sensors 32 and gas sensors. 33 Most of the light-soak studies hitherto were done on bulk heterojun tion devi es. In the present work, we propose to study devi es based on a single polymer sandwi hed between harge transport/blo king layers and onta ts. The opti al tests were arried out under an opti al mi ros ope in order to rule out any spatial defe ts and ele trode-edge ee ts. 34 The studies were done on en apsulated devi es in order to ut out the ee ts of extrinsi fa tors during the devi e testing pro edures. The diodes have been studied for

urrent-voltage hara teristi s, harge extra tion using CELIV, photovoltage, ele trolumines en e (EL) and laser beam indu ed urrent (LBIC) spatial s ans before and after the light-soaks. While kelvin probe mi ros opy (KPM) 35 and ultra-violet photoele tron spe tros opy (UPS) 30 an be performed to determine the work-fun tion hanges due to various layers, it is not possible to perform it on a fully-fun tional devi e. In addition the use of UPS did not seem appropriate to test the devi es here, as we have noti ed UV exposure at about (365 ± 10)nm over 30 se onds had drasti ee t on the devi e hara teristi s although reversible. We show here the impa t of light-soaking on a single polymer-based devi e and in parti ular on the interfa e barriers and predi t the probable energy band-bending that are burried in `sandwi h-type' devi es through simple non-perturbative opto-ele troni te hniques.

Experimental Methods Sample preparation

: ITO- oated glass slides were leaned by soni ation in soap water, dis-

tilled water, a etone and isopropanol for 5 minutes ea h in a sequen e. The air-dried slides

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were treated under oxygen-plasma for 7 minutes before depositing ZnO layer. 0.75 M solution of Zin a etate dihydrate mixed in 2-methoxyethanol and monoethanolamine at volume ratio 19:1 was stirred for about 48 hours in the dark. The ZnO pre ursor solution was spin- oated at 4000 rpm for 40 se onds on the oxygen-plasma treated ITO- oated glass slides. The slides were then annealed at 180 for 30 minutes and ooled down to room temperature before being transferred into the glovebox. Regioregular Poly-(3 hexylthiophene-2,5-diyl), P3HT, of mole ular weight (50-70) kDa and (90-94)% regioregularity was bought from Reike metals In . Poly[N-9'-heptade anyl-2,7 arbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)℄, PCDTBT, of mole ular weight (20-40) kDa was bought from Solariz. P3HT was dissolved in 1,2-di hlorobenzene ( on . 20 mg/mL) inside the glovebox by stirring for 24 hours at room temperature in the dark. PCDTBT mixed in di hlorobenzene( on . 6 mg/mL) was stirred for 48 hours at 50 inside the glove box in the dark. The polymer lms were spin- oated on top of ZnO layer at 600 rpm for 60 se onds. The samples were then annealed for 10 minutes at 120 . Subsequently, the samples were transferred into thermal vapour deposition hamber to deposit a 7 nm thi k Molybdenum Oxide (MoO3 ) followed by a 100 nm thi k silver ele trode. The P3HT, PCDTBT and ZnO lm thi knesses were measured to be (180 ± 10) nm, (70 ± 10) and (40 ± 5) nm respe tively, using a Dektak stylus prolometer. The a tive area of the devi es are (0.070 ± 0.005)cm2 . The devi es were all en apsulated using epoxy and a thi k glass slide before taking them out of glovebox. Devi e hara terization

: The steady-state urrent-voltage hara teristi s were measured by

sweeping the voltage and measuring urrent using Keithley 2400 sour e measurement unit (SMU). Charge stored and extra ted out of the devi es were quatied using the CELIV te hnique. The ele trolumines en e was re orded at various biases using single-photon avalan he photodiodes (APDs,Mi ro Photon Devi es) aligned onfo ally to the obje tive fo al spot on the sample on the opti al mi ros ope (Axio Observer, Carl Zeiss). The voltage bias was

ontrolled using a Keithley SMU2400. An average dark ount rate of (25 ± 10) Hz was 5



re orded with the APDs. The ounts were intergrated over 2 se onds and averaged over 3 data points at ea h bias voltage. The bias voltage, urrent and EL ount measurements were syn hronized using a Labview program. LBIC tests to probe the spatial photo urrents were arried out with a green diode laser (λ = 532 nm) externally triggered at 400 Hz. The laser was oupled into the opti al mi ros ope with lens obje tive (10x , NA = 0.65) to ex ite the devi e over a pixel of diameter

∼ 2 µm at fo us. The photo urrent from ea h pixel was amplied using a lo k-in amplier. The mi ros ope's stage holding the devi e was moved in steps of 5 µm using stepper motors/ ontrollers from Thorlabs. The devi e spatial oordinates and orresponding photo urrent measured and logged at ea h pixel were syn hronized using Labview program. The average opti al power of the laser was measured to be (250 ± 2) nW. The light soaks were done with a Hg ar -lamp and a halogen lamp in ombination with appropriate interferen e lters to sele t narrow range of wavelengths where needed. The halogen lamp of intensity 60 mW/cm2 and Hg ar -lamp of intensity 300 mW/cm2 over the full visible spe trum were set for the tests. The devi es were exposed to the light from the glass substrate side of the devi e. Opti al spe tra were re orded using O eanopti s spe trometer USB2000. Fouriertransform infrared spe trum was re orded using Bruker's Alpha spe trometer in attenuated total ree tion (ATR) mode. A more detailed s hemati of the opto-ele troni set-up to study the devi e hara teristi s in-situ with multiple te hniques is shown in Figure S1.

Results and dis ussion Figure 1(a) and Figure 1(b) show the layer onguration of the devi e and its relevant energy band edges and levels before onta t, respe tively. The energy levels values depi ted in Figure 1(b) were taken from the literature published. For materials P3HT, PCDTBT, ZnO, MoO3 , Ag used in making devi es, the preparation te hniques implemented in this work exhibit amorphous nature and therefore depi ting dis rete energy levels is an oversimpli-

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ation. Nevertheless, it explains the I-V hara teristi s of the harge generation, transport dire tion and energy barriers. The energy levels of the materials were taken from the work of S hlesinger et al. 36 for ZnO; Kröger et al. 37 and Greiner et al. 38 for MoO3 , Zhou et al. 39 for ITO and silver; Beenken et al. 40 and Blouin et al. 41 for P3HT and PCDTBT, respe tively. Within ±100 meV of the mentioned band-edges, the proposed phenomenology in the dis ussion se tion still holds. The ee tive diodi hara teristi of the devi e is possible from the hole-blo king nature of the ZnO layer with the re ti ation ratio of about 104 in the presen e of ZnO layer (Figure S2). Figure 1( ) shows the ee t of the light-soak on urrent-voltage

hara teristi s taken in the dark, before and after 10 minutes of white light illumination with a mer ury ar lamp. After the light-soak it was observed that the urrent in the ohmi regime was higher, and a shifted onset of SCLC and/or redu ed urrent at any given voltage in the SCLC regime.

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(a)

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Figure 1: a) S hemati of layers onstituting the ìnverted' sandwi h-type devi e, b) Energy band edges (in eV) orresponding to ondu tion, valen e bands and fermi levels of ea h layer before onta t, ) Dark J-V hara teristi s of the devi e ompared before and after 10 minutes of light-soak using Hg ar lamp. The inset shows the J-V hara teristi s of the same devi e in linear s ale. FB- Forward Bias, RB- Reverse Bias. The redu ed resistan e in the ohmi regime suggests that either there is an in rease in thermal harge arriers from the photo-a tivated dopants in polymer bulk and/or an in rease in inje tion urrent as a result of redu ed inje tion barrier. While the in rease in the thermal

harge on entration supports the delayed onset of SCLC, a redu ed inje tion barrier should have advan ed the onset of SCLC in voltage. So, it is likely that there is a higher thermal

harge arrier density along with a higher ele tron inje tion barrier between ITO and ZnO that supports the observed IV hara teristi s. A shift of about 300 mV in the onset of

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2

SCLC after light-soak is observed at 1 mA/cm of inje ted urrent density. To distinguish the signi an e of the two auses, the harge density stored in the devi e was studied using CELIV. 42 CELIV was performed on the devi e before and after light-soak. Figure 3(a) and Figure 3(b) show the s hemati of the CELIV ir uit and the typi al input voltage and urrent through the devi e. The input ramp-amplitude was set at 3V over 300 µs in these tests. The oset biases (Vi (s)) were swept from negative(reverse-biased) to positive voltages until the advent of SCLC regime in the forward bias. The Agilent 33220A arbitrary waveform generator was used to design and generate the ramp signals, and the urrent through the devi e is monitored a ross a sensing series resistor of 50Ω using Agilent DSO Inniivision os illos ope. Firstly, in either ase, the total harge extra ted out of the devi e in reased as the initial bias oset varied from the reverse on towards the forward bias until it saturates, as shown in Figure 3( ). The onset of saturation indi ates the at-band ondition at whi h point the SCLC regime dominates. In the SCLC regime, the harge extra tion pro ess ompetes with the inje tion pro ess. Se ondly, the light-soak leads to redu ed number of harges extra ted. This rules out the possibility of the in rease in the thermal harge arrier density or atleast not signi ant enough. The redu ed harge density extra ted is explained on the basis of an in reased built-in voltage (Vbi ) that drains out the harges under short- ir uit onditions. The hange in the built-in voltage a ross the a tive layer was further tested by measuring the photovoltage a ross the devi e during the light-soak. The output photovoltage from the devi e was re orded using an os illos ope with a 100 MΩ input impedan e during the light-soak. With the devi e apa itan e measured to be (3-5) nF, the observed dynami s of the photovoltage is not limited by the time onstant RC, where R is the external load and C is the devi e apa itan e. In order to observe the hanges in the photovoltage over longer periods, the light-soak for this experiment was arried out with a less intense halogen lamp. The hange in the photovoltage, ∼ 70mV , seen in Figure 2 is likely from the modied energy levels at the athode interfa e to be dis ussed in details shortly. The in rease in the 9



photovoltage is indi ative of in rease in the built-in voltage. The higher built-in voltage is then most likely due to redu ed work-fun tion of ZnO, assuming that the ohmi onta t at the anode is inta t.

Figure 2: The photovoltage from the devi e re orded during the light-soak using halogen lamp. In Figure 3(d), the ee t of wavelengths of the light-soak on the harges extra ted from the devi e is shown. The devi e was soaked under light of wavelength bands (525 ± 25) nm, (365 ± 10) nm and (610 ± 35) nm with intensities 12 mW/cm2 , 1.5 mW/cm2 and 40

mW/cm2 , respe tively. The arrows in Figure 3(d) indi ate the time-stamps when the devi e was soaked in ea h band of light for about 1 to 3 minutes. The fra tional de rease in harges extra ted is maximum with (365 ± 10) nm soak i.e. with photons of energy slightly higher than the band-gap of ZnO (∼ 3.2eV ), su ient to reat e-h pairs in the ZnO layer. Figure S4 and Figure S5 show the absorption spe tra of ZnO lm and the ontribution of ZnO layer in the devi e through in ident photon to urrent onversion e ien y (IPCE) measurement, respe tively. Another interesting feature to be noti ed is the signal reversibility. The harges extra ted resumes to the pre-soak values with the green and orange light soak and partially with the UV light soak.

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Figure 3: a)S hemati of ir uit for CELIV test, b) CELIV input and typi al output signal. Vi 's are the initial bias osets swept from reverse bias (RB) towards forward bias (FB), also indi ated in the typi al dark-IV hara teristi s of a diode shown to its right, b) Charges extra ted out of the devi e for an initial bias oset swept from a reverse bias and onto forward bias voltages taken before and after illumination. The inset shows the CELIV signal before and after illumination with initial oset bias set at 300 mV, ) Time-evolution of harges extra ted using CELIV, with inje tion at 300 mV, tested at three dierent wavelength bands used for soaking the devi es. The arrows indi ate the time the devi es were exposed to light of wavelength bands (525 ± 25) nm, (365 ± 10) nm and (610 ± 35) nm. The variation in the workfun tion of the onta ts should indeed ae t the ele trolumines en e from the devi e. 43 For forward bias voltage above built-in potential, the harge inje tion is limited by the energy barriers and the depletion widths at the interfa es. Figure 4(a) shows the ee t of sele tive wavelength light-soak has on the ele trolumines en e from the devi e, where the light-soaks were done in a sequen e starting with long wavelengths (610 ± 35)nm towards shorter wavelengths (365 ± 10) nm at dis rete wavelength

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bands. The exposure to dierent wavelengths of light on EL onrmed that the observed ee ts were from the ultra-violet omponent of the illumination. There is an in rease in the radiative re ombination e ien y from the devi e after a brief exposure to UV light, whereas no hanges were observed for the wavelengths ≥ 420 nm. The abrupt hange in the EL further supports the idea that the observed ee ts are due to the hanges in the athodepolymer interfa e energeti s. The e ient EL is possible from an e ient double arrier inje tion into the devi e. With an ohmi onta t for the hole-inje tion at the anode already in pla e, the UV light-soak helps in improving the ele tron-inje tion from the athode into the polymer ondu tion band. A very e ient EL was seen in PCDTBT-based devi es is shown in Figure 4(b), although similar enhan ements were re orded in P3HT-based devi e. The PCDTBT-based devi e showed immediate response to light-soak and an e ient inje tion of ele trons into the polymer ondu tion band onsidering it's deep ondu tion band (-3.6 eV) ompared to P3HT (-2.9 eV). The athode interfa e modi ation in the devi e is demonstrated by shining UV light through a mask of letter `Q' sten illing the letter in EL out of the devi e at 2.5 V, as shown in Figure 4( ) and Figure 4(d). The PL and EL spe tra of PCDTBT-based devi e along with dark J-V urves an be found Figure S8 and Figure S9. This method presents a way of making submi ron-sized LEDs only limited by the dira tion-limit due to UV light wavelength and opti s used. The onset of EL at about 1.05 V in Figure 4(b) indi ates the photons of energy ≤ 1.05eV (i.e. λ ≥ 1181nm)were being dete ted by the sili on avalan he photodiode (Si-APD). It is not lear yet if the EL seen at the onset is limited by the radiative re ombinations within the devi e or from the Si-APD's responsivity limit. Nevertheless, the dete tion of low-energy photons at 1.05 V proves that the radiative re ombination is happening at the interfa e of ZnO and the polymer promoting hybrid harge transfer ex iton (HCTE) re ombinations. 44 The EL signal reversibility after light-soak was also observed similar to the CELIV reversibility tests.

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Figure 4: a) Dependen e of ele trolumines en e on soak-light wavelength for a P3HT-based devi e, b) PCDTBT-based devi e, ) devi e exposed to UV light through a mask of letter `Q' and fo ussed by an mi ros ope obje tive, d) lo alized ele trolumines en e from a PCDTBTbased devi e at 2.5 V bias after UV exposure. Further the P3HT-based devi e was tested for short- ir uit photo urrent through LBIC s ans before and after UV light exposure. The devi e was soaked under UV-light for 3 minutes over an area, designated by `S' in Figure 5(a), of diameter approximately 600 µm at the opti al mi ros ope's fo us and LBIC s ans were performed over the UV-exposed spot intermittently for upto 45 hours after UV-exposure.

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(b)

Figure 5: a) S hemati showing the a tive zone À' of the devi e in grey olor, ir ular spot `S' exposed to UV light and the dotted line representing the LBIC s an, b) LBIC s ans performed before and after UV exposure tested on a P3HT-based devi e at dierent intervals. The LBIC s an immediately after UV exposure showed a de reased photo urrent in the form of a `valley' shown in Figure 5(b). Although there were no noti eable hanges in the photo urrent for at least upto 160 minutes, the photo urrent re overy was lear after 20 hours in the dark, and in 45 hours after UV exposure there was a very signi ant re overy in the photo urrent. The hanges observed ould either be attributed to the athode or anode interfa es. The role of MoO3 was ruled out when the enhan ement of EL was observed in the devi es without

MoO3 layer upon UV light-soak (see Figure S3). Some of the hara terization methods dis ussed above were also performed on the devi e without ZnO layer on the athode side. Unlike the devi e with ZnO layer, the SCL urrent is higher after light-soak, with no EL and no hange in the already low photovoltge (∼50 mV) in these devi es. There was no observable EL in the voltage range studied is understable onsidering the very large ele tron inje tion barrier from ITO to P3HT; and the very small photovoltage is from the two onta ts with almost similar workfun tions. The ele tron inje tion into the ondu tion band of the polymer without ZnO layer was ine ient. The CELIV tests ould not be performed on these devi es, as the te hnique demands for at least one blo king onta t. Further the signal 14


Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reversibilty tests and undiminished photolumines en e indi ate that the observed ee ts were not from any form of degradation or alteration to the polymer itself. Based on the results dis ussed above, the photophysi s at the interfa e before and after light-soak is summarized in Figure 7. ZnO, in its pristine form, is known for its n-type

hara teristi s although the origin of su h behaviour is still under debate. The dopants in ZnO, believed to be from the ex ess of oxygen va an ies and/or zin interstials, 4548 are the favourable sites for adsorption of mole ules su h as O2 and H2 O , represented by A in Figure 7(a). The nanostru tured ZnO with defe ts traps mole ular oxygen and moisture from the surrounding, whi h in our ase is readily available from the ambien e as the ZnO layer was prepared in open-air onditions. Figure S6 in the supplementary information shows the surfa e topography of the ZnO lm with an average roughness of ∼9 nm. The FTIR spe trum in the ATR mode was used to probe the mole ular level intera tion at the surfa e of the ZnO lm. Figure 6 shows the FTIR spe trum of the ZnO thin lm spin- oated on glass slide and annealed at 180, as well as the ee t of UV exposure. UV-light in the wavelength range (365 ± 10)nm was used to soak the devi es for 5 minutes. The two peaks at 1420 m−1 and 1570 m−1 are attributed to the resonant vibrations of C=O and the broad peak between (2950 - 3750) m−1 is from -OH vibrations of arboxylate group and C-H vibrations, in a

ordan e with other reported work. 49 After UV-light exposure, the peaks P1, P2 and P3 were suppressed indi ating disruption of the C=O and -OH bonds. A similar redu tion in the aforementioned peaks has been observed from thermal annealing. Figure S7 shows the FTIR spe tra of ZnO lm spin- oated on a glass slide and annealed at 150 and 250 , and similar ee t has been reported before. 50 A re ent work of Wu et al. 51 showed the presen e of intermediate omplexes of Zn(OH)2 in the preparation of ZnO lms from sol-gel, espe ially under low annealing temperatures 52 and higher H2 O ontent, as in the present ase. UV exposure is most likely aiding in de omposing the Zin a etate dihydrate into Zn(OH)2 that further de omposes to ZnO and H2 O. This forms one of the routes of photodesorbing water mole ules o the ZnO surfa e. 15



0.1

Absorbance (A)

0.08

Before UV exposure After UV exposure

0.06 0.04

P1

P2

P3

0.02 0 −0.02 1350

1750

2150 2550 2950 −1 Wavenumber (cm )

3350

3750

0.02 ∆A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0 −0.02 −0.04

1500

2000

2500 3000 −1 Wavenumber(cm )

3500

Figure 6: The ee t of UV light exposure on FTIR spe trum(top) of the ZnO lm spin- oated on a glass slide and annealed at 180. The bottom spe trum emphasizes the dieren e in absorban es. ZnO an readily donate ele trons to the surfa e adsorbed a

eptor mole ules, represented by A− in Figure 7(b). The mole ular adsorption at the interfa e of ZnO/P3HT oxidizes ZnO that eventually brings its fermi level down towards its midgap, represented by the level Ef 1 in Figure 7( ) and 7(d). During UV light-soak, photons of energy larger than the bandgap of ZnO generate ele tron-hole pairs in the ZnO, su h that the holes aid in redu ing the oxidizing agents and lead to mole ular desorption from the ZnO surfa e. 33,53,54 The free ele trons ompensate the positive harge within ZnO; thereby raising the fermi-level of ZnO ba k to its pristine ondition represented by Ef 2 in Figure 7( ) and 7(e). Figures 7(d) & 7(e) are the possible band-bending trends at the ITO/ZnO interfa e before and after light-soak, respe tively. The redu ed work fun tion of ZnO in turn leads to in reased built-in voltage drop a ross the a tive medium. At the ITO/ZnO interfa e, the shift in the fermi level of ZnO leads to thinner S hottky depletion region as shown in Figure 7(d) with in reased e ien y of e-h re ombination but impedes the photogenerated ele tron extra tion from the polymer at zero bias. This is in a

ordan e with the observed `valley' of redu ed photo urrent in Figure 5(b).

16


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Figure 7: S hemati of ZnO/Adsorbates and their orresponding energy levels a) before and b) after harge transfer; Band alignment at the interfa e of ITO/ZnO ) before onta t, d) after onta t and e) after UV exposure upon onta t. A (A− ) - Adsorbate(ion), Ec Condu tion-band edge, Ev - Valen e-band edge, Ef i - Fermi levels of ZnO, VL - Va uum level. After light-soak, the higher built-in eld a ross the polymer medium needs to be ompensated with higher applied bias to a hieve the at-band onditions. The shifted onset of SCLC and harges extra ted in CELIV towards higher forward bias is in a

ordan e with this phenomenon. In the ase of e ient EL, the ele tron inje tion into the ondu tion band of ZnO is more e ient due to thinner depletion width at the interfa e of ITO and ZnO, and the HCTE re ombination is favoured at the interfa e of ZnO and polymer.

Con lusion In summary, we showed the signi an e of understanding the energeti s at the organi inorgani interfa es and the role of defe ts on the devi e perfoman es. We have attempted 17



to orrelate the light-soak ee ts on simple opto-ele troni te hniques su h as urrent-voltage, CELIV, photovoltage/ urrent, EL and FTIR spe tra. The layered devi es based on polymers and metal-oxides have been subje ted to light soaks at dis rete wavelength bands, of whi h UV regime had a severe impa t on the devi e hara teristi s. The UV light-soak post devi e preparation has modied the ele tron-inje tion barrier at the athode (ITO/ZnO) leading to an improved ele trolumines en e e ien y of the devi e. The suppressed urrent in the SCLC regime in the urrent-voltage hara teristi s after the light-soak is from the higher built-in eld a ross the polymer. However, with the e ient ele tron inje tion into the polymer ondu tion band, the possibility of trap-assisted re ombination and harge transport may also have ontributed to the redu ed SCLC. The unusual in rease in the photovoltage during light-exposure assures the redu tion in athode work-fun tion, and the heating due to light-soak is negligible whi h would otherwise lead to de rease in photovoltage. UVassisted photodesorption of the moisture ontent within the ZnO layer and surfa e oxidative adsorbates is likely responsible for the observed ee ts. Sin e the light-soak tests were performed on a fully fun tional en apsulated devi es, the readsorption of the photodesorbed mole ules was noti ed in the form of reversible EL, CELIV, photovoltage/ urrent signals. Further tests on signal reversibility ould give useful information on the mole ular diusion and/or the defe t migration within the devi e.

A knowledgement This resear h was supported by Canada's federal funding agen y, National S ien es and Engineering Resear h Coun il (NSERC) Dis overy Grants program (RGPIN-2015-05485).

Supporting Information Available The Supporting Information in ludes:

18


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Opto-ele troni s set-up employed to hara terize the devi es in-situ, the ee t of the buer layers on the devi e hara teristi s of the P3HT-based diode, zin oxide layer AFM s anned topology, its visible and infrared absorption spe tra, PCDTBT-based devi e's urrent-voltage hara teristi s along with its photo-/ele tro-lumines en e spe tra. This material is available free of harge via the Internet at http://pubs.a s.org/.

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Light-Induced Electroluminescence Patterning: Interface Energetics

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