Solar Cell Applications of Solution-Processed Agingase2 Thin Films and Improved Properties by Sodium Doping

nanomaterials

Article Solar Cell Applications of Solution-Processed AgInGaSe2 Thin Films and Improved Properties by Sodium Doping

Xianfeng Zhang 1, Qingxuan Sun 2,*, Maoxi Zheng 1, Zhuohua Duan 1 and Yuehui Wang 1,3,* 1 Zhongshan Institute, University of Electronic Science and Technology of China, Zhongshan 528402, China; [email protected] (X.Z.); [email protected] (M.Z.); [email protected] (Z.D.) 2 Shenzhen Polytechnic, No. 7098, Liuxian Avenue, Nanshan District, Shenzhen 518000, China 3 Guangdong Engineering-Technology Research Center of Nano-Photoelectric Functional Films and Devices, Zhongshan 528402, China * Correspondence: [email protected] (Q.S.); [email protected] (Y.W.); Tel.: +86-755-2673-1975 (Q.S.); +86-760-8831-3456 (Y.W.) Received: 20 November 2019; Accepted: 14 January 2020; Published: 18 March 2020

Abstract: Binary nanoparticle inks comprising Ag2Se, In2Se3, and Ga2Se3 were fabricated via a wet ball-milling method and were further used to fabricate AgInGaSe2 (AIGS) precursors by sequentially spraying the inks onto a Mo-coated substrate. AIGS precursors were annealed under a Se atmosphere for 1 h at 570 ◦C. Na2Se thin layers of varying thicknesses (0, 5, 10, and 20 nm) were vacuum-evaporated onto the Mo layer prior to the AIGS precursors being fabricated to investigate the influence on AIGS solar cells. Sodium plays a critical role in improving the material properties and performance of AIGS thin-film solar cells. The grain size of the AIGS films was significantly improved by sodium doping. Secondary ion mass spectroscopy illustrated slight surficial sodium segregation and heavy sodium segregation at the AIGS/Mo interface. Double-graded band profiles were observed in the AIGS films. With the increase in Na2Se thickness, the basic photovoltaic characteristics of the AIGS solar cells were significantly improved. The highest solar cell conversion efficiency of 6.6% (open-circuit voltage: 775.6 mV, short-circuit current: 15.5 mA/cm2, fill factor: 54.9%, area: 0.2 cm2) was obtained when the Na2Se thickness was 20 nm.

Keywords: AgInGaSe2 solar cell; non-vacuum method; sodium doping; conversion eﬃciency

1. Introduction

The CuInGaSe2 (CIGS) thin-film solar cell is a very promising candidate for future large-area photovoltaic applications. The highest efficiency reported is 23.35% [1], which is comparable to that of monocrystal Si solar cells [2]. Further research of tandem solar cells based on CIGS solar cells is necessary to further increase solar cell efficiency. A wide bandgap of >1.70 eV has been calculated for the top cell as necessary to achieve a high efficiency of >30% when CIGS solar cells are used as the bottom layer [3]. AgInGaSe2 (AIGS) has been considered a suitable candidate as the tandem solar cell top layer. The highest efficiency of AIGS has been reported as 10.7% in our previous work [4]. Furthermore, the fabrication of AIGS absorber layers via a modified three-stage method using a molecular beam epitaxy (MBE) system has received a wealth of interdependent research [5,6]. However, the equipment of the MBE system requires a significantly elevated vacuum level and demands a high maintenance cost, which hinders large-scale applications in the photovoltaic industry. The non-vacuum method was widely used in fabricating CIGS [7,8] and Cu2ZnSnS4 thin films (CZTS) [9,10] because only simple equipment and fabrication process were required, which led to a low product cost. The conversion efficiency of non-vacuum fabricated CIGS and CZTS solar cells has respectively reached 17.1% [11]

Nanomaterials 2020, 10, 547; doi:10.3390/nano10030547 www.mdpi.com/journal/nanomaterials NanomaterialsNanomaterials 20202020, ,1010, ,x 547 FOR PEER REVIEW 2 2 of of 13 13 has respectively reached 17.1% [11] and 12.6% [12]. Herein, AIGS thin films were fabricated via a non- vacuumand 12.6% method [12]. to Herein, reduce AIGS the associated thin films costs. were fabricated via a non-vacuum method to reduce the associatedSodium costs. has been widely adopted to improve device performance and the material properties of CIGS-relatedSodium materials has been widely[13–15]. adopted Numerous to improve reports devicehave demonstrated performanceand the thepositive material influence properties of sodiumof CIGS-related on the chalcopyrite materials [absorber13–15]. Numerous layer properti reportses and have solar demonstrated cell performance. the positive There are influence research of effortssodium that on have the chalcopyrite assigned the absorber improvement layer propertiesassociated andwith solar sodium cell performance.on the chalcopyrite-related There are research film grainefforts size that [16], have while assigned other reports the improvement have focused associated on the mechanism with sodium of how on the sodium chalcopyrite-related doping influences film thegrain electronic size [16 ],and while material other proper reportsties have [17]. focused Sodium on theis observed mechanism to passivate of how sodium defects doping in CIGS-related influences films,the electronic resulting and in improved material propertiessolar cell performance [17]. Sodium [18–20]. is observed to passivate defects in CIGS-related films,In resultingthis work, in improveda new non-vacuum solar cell performance method was [18 ad–20opted]. to fabricate low-cost AIGS absorber In this work, a new non-vacuum method was adopted to fabricate low-cost AIGS absorber layers. layers. Ag2Se, Ga2Se3, and In2Se3 nanoparticle inks fabricated via a wet ball-milling method were used toAg further2Se, Ga fabricate2Se3, and AIGS In2Se substrate3 nanoparticle precursors. inks fabricatedThe AIGS precursor via a wet structure ball-milling is shown method inwere Figure used 1. Suchto further structure fabricate types AIGSensure substrate double-graded precursors. bandgap The AIGSstructures precursor in the structure AIGS precursors, is shown inwhich Figure are1 . importantSuch structure for improving types ensure solar double-graded cell performance bandgap [21]. To structures study the ininfluence the AIGS of precursors,Na on the properties which are important for improving solar cell performance [21]. To study the influence of Na on the properties of of AIGS films and solar cell performance, Na2Se layers of various thicknesses were vacuum- evaporatedAIGS films andonto solar Mo-coated cell performance, soda-lime Na glass2Se layers (SLG) of varioussubstrates thicknesses prior to werethe vacuum-evaporateddeposition of the onto Mo-coated soda-lime glass (SLG) substrates prior to the deposition of the nanoparticle layers. nanoparticle layers. The Na2Se thicknesses were selected as 0, 5, 10, and 20 nm. Thereafter, the AIGS precursorThe Na2Se was thicknesses annealed werein a selectedcontinuously as 0, pumped 5, 10, and two-zone 20 nm. Thereafter,furnace under the AIGSa Se atmosphere precursor was to improveannealed the in agrain continuously size, crystallization, pumped two-zone and electronic furnace underproperties. a Se atmosphere Se flux was to continuously improve the supplied grain size, tocrystallization, prevent decomposition and electronic of the properties. AIGS film Se becaus flux wase of continuously the low-Se suppliedatmosphere. to prevent The as-grown decomposition AIGS filmof the was AIGS used film as becausean absorber of the of low-Se the AIGS atmosphere. solar cell Theto complete as-grown the AIGS full filmsolar was cell used structure. as an absorber The solar of cellthe performance AIGS solar cell was to completedetermined the fullto evaluate solar cell the structure. important The solarrole of cell sodium. performance The research was determined herein providesto evaluate a way the important to improve role the of sodium.properties The of research AIGS films herein and provides enhance a way the to performance improve the propertiesof AIGS photovoltaicof AIGS films devices. and enhance the performance of AIGS photovoltaic devices.

Figure 1. Structure of the AgInGaSe2 (AIGS) precursor. Figure 1. Structure of the AgInGaSe2 (AIGS) precursor. 2. Experimental Approach 2. Experimental Approach 2.1. Fabrication of Nanoparticle Inks 2.1. Fabrication of Nanoparticle Inks In this process, binary Ag2Se, In2Se3, and Ga2Se3 raw materials were provided by Kojundo ChemicalIn this Laboratory process, binary Co., Ltd Ag (Sakado,2Se, In2Se Saitama3, and Ga Prefecture,2Se3 raw Japan).materials And were correspondent provided by nanoparticle Kojundo Chemicalinks were Laboratory fabricated Co., via aLtd wet (Sakado, ball-milling Saitama method, Prefecture, the procedures Japan). And of which correspondent are explained nanoparticle in detail inksin our were previous fabricated work via [22 a ].wet During ball-milling the milling method process,, the procedures 1 mm balls, of 50 whichµm ceramic are explained balls, one in binarydetail incompound our previous powder, work and [22]. 5 During mL ethanol the milling were mixed process, together 1 mm in balls, a mill 50 pot μm and ceramic milled balls for , 40 one h. binary Herein, compoundthree milling powder, machines and of5 mL the ethanol same model were (P-4,mixed Fritsch together Japan in Co.,a mill Ltd., pot Kanagawaand milled Prefecture,for 40 h. Herein, Japan) threewere milling used to machines grind three of ditheﬀerent same powders. model (P-4, Fritsch Japan Co., Ltd., Kanagawa Prefecture, Japan) were usedAfter to ball grind milling, three the different mixtures powders. were ﬁltrated to obtain particles <32 µm. Through this procedure, all particlesAfter ball> milling,32 µm werethe mixtures removed were and filtrated a solution to obtain with particles relatively <32 small μm. Through particles this (<32 procedure,µm) was allobtained. particles Thereafter, >32 μm were 2-(2-ethoxyethoxy)ethanol removed and a solution and with ethanol relatively were usedsmall as particles a dispersion (<32 agentμm) was and obtained.solvent, respectively. Thereafter, The2-(2-ethoxye obtainedthoxy)ethanol solutions were and then ethanol sonicated were for used 1 h to as disperse a dispersion the particles agent inand the solvent,solvent. respectively. Next, the solutions The obtained were centrifuged solutions were twice: then First, sonicated the centrifugation for 1 h to disperse process the was particles conducted in theat asolvent. low speed Next, (1500 the rpm) solutions to remove were particlescentrifuged over twice: micrometers First, the in sizecentrifugation with the upper process solution was conducteddecanted; second,at a low thespeed centrifugation (1500 rpm) processto remove was part repeatedicles over three micrometers more times in at size a speed with of the 6000 upper rpm solution decanted; second, the centrifugation process was repeated three more times at a speed of 6000 rpm to gradually remove larger particles, to finally obtain the desired nanoparticles. Ag2Se,

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Into2Se gradually3, and Ga remove2Se3 nanoparticle larger particles, inks having to finally a concentration obtain the desiredof 200 mg/mL nanoparticles. were obtained Ag2Se, by In adjusting2Se3, and theGa2 quantitySe3 nanoparticle of ethanol. inks The having obtained a concentration nanoparticle of 200inks mg were/mL then were used obtained to fabricate by adjusting AIGSthe precursors quantity usingof ethanol. an inkjet The obtainedprinter. The nanoparticle Ag2Se layer inks was were first then deposited used to fabricateonto the AIGSsubstrate precursors at a thickness using an of inkjet 0.9 μprinter.m, followed The Ag by2Se sequential layer was firstGa2Se deposited3 and In onto2Se3 thelayers substrate having at thicknesses a thickness ofof 0.90.7µ m,and followed 0.25 μm, by respectively.sequential Ga Finally,2Se3 and Ga In2SeSe33 waslayers again having deposited thicknesses onto of the 0.7 top and layer 0.25 µ tom, obtain respectively. a double-graded Finally, Ga2 GaSe3 distribution,was again deposited which tailors onto the band top layer profile to obtainof the AIGS a double-graded films. The AIGS Ga distribution, precursor structure which tailors is shown the inband Figure profile 1. of the AIGS films. The AIGS precursor structure is shown in Figure1.

2.2.2.2. Sodium Sodium Doping Doping Process Process TheThe backback contactcontact of of the the Mo layer,Mo layer, having having a thickness a thickness of 800 nm, of was800deposited nm, was onto deposited an ultrasonically onto an ultrasonicallycleaned SLG substrate. cleaned SLG Na2 substrate.Se layers of Na varying2Se layers thicknesses of varying (50, thicknesses 100, and 20 (50, nm) 100, were and fabricated 20 nm) were onto fabricatedthe SLG/Mo onto substrates the SLG/Mo via a substrates vacuum evaporation via a vacuum method, evaporation while a method, non-coated while Na 2aSe non-coated layer-equivalent Na2Se layer-equivalentsubstrate was fabricated substrate as was a comparison. fabricated as a comparison.

2.3. Fabrication of AIGS Absorbers and Solar Cells 2.3. Fabrication of AIGS Absorbers and Solar Cells Figure2 schematically illustrates the AIGS film fabrication process. The AIGS precursor was Figure 2 schematically illustrates the AIGS film fabrication process. The AIGS precursor was fabricated using an inkjet printer. The Ag2Se, Ga2Se3, and In2Se3 nanoparticle inks were deposited fabricated using an inkjet printer. The Ag2Se, Ga2Se3, and In2Se3 nanoparticle inks were deposited sequentially based on the stoichiometric composition of the AIGS film. The design of the AIGS film sequentially based on the stoichiometric composition of the AIGS film. The design of the AIGS film structure, as shown in Figure1, was to obtain AIGS films having a Ga /(In+Ga) ratio of ~0.75 to achieve structure, as shown in Figure 1, was to obtain AIGS films having a Ga/(In+Ga) ratio of ~0.75 to achieve a bandgap of ~1.70 eV. The precursors were thereafter annealed in a two-zone furnace, where the Se a bandgap of ~1.70 eV. The precursors were thereafter annealed in a two-zone furnace, where the Se powder (99.999%) was placed in one zone and the AIGS precursor placed in the other zone. Figure3 powder (99.999%) was placed in one zone and the AIGS precursor placed in the other zone. Figure 3 shows a schematic of the two-zone annealing furnace. A shutter between the two zones was used to shows a schematic of the two-zone annealing furnace. A shutter between the two zones was used to control the Se flux in the AIGS zone. When the shutter was closed, Se is prevented from flowing to control the Se flux in the AIGS zone. When the shutter was closed, Se is prevented from flowing to the AIGS zone, and only when the shutter is open does the Se flux flow to the AIGS zone. Prior to the AIGS zone, and only when the shutter is open does the Se flux flow to the AIGS zone. Prior to annealing, the shutter was left in the open position and both zones of the furnace were evacuated to annealing, the shutter was left in the open position and both zones of the furnace were evacuated to ~2.0 10 5 Pa, using a molecular pump to remove oxygen. Thereafter, the shutter between the two ~2.0 × 10−−5 Pa, using a molecular pump to remove oxygen. Thereafter, the shutter between the two zones was set to the closed position, and the substrate and Se sources were heated. The Se source zones was set to the closed position, and the substrate and Se sources were heated. The Se source was was first heated to 150 C in 40 min, to obtain a high Se flux prior to heating the AIGS substrate. first heated to 150 °C in◦ 40 min, to obtain a high Se flux prior to heating the AIGS substrate. The The temperatures of the AIGS substrate and Se flux profile are shown in Figure4. The substrate was temperatures of the AIGS substrate and Se flux profile are shown in Figure 4. The substrate was heated to 570 C in 30 min, with the thermocouple connected to the substrate to ensure a consistent heated to 570 °C◦ in 30 min, with the thermocouple connected to the substrate to ensure a consistent and accurate temperature. When the substrate temperature was increased to 250 C, the shutter and accurate temperature. When the substrate temperature was increased to 250 °C,◦ the shutter between the two zones was opened in the presence of a N2 carrier gas to carry Se to the AIGS zone. between the two zones was opened in the presence of a N2 carrier gas to carry Se to the AIGS zone. TheThe temperature temperature was was isothermally isothermally maintained maintained at at 570 570 °C◦C for for 1 1 h h throughout throughout the the annealing annealing process. process. Next,Next, the the substrate substrate temperature temperature was decreased to room temperature at a rate of ~7.5 °C/min,◦C/min, and and only only whenwhen the the substrate temperature temperature dec decreasedreased to to 350 350 °C◦C was was the the shutter shutter closed closed to to stop stop the the supply supply of of Se Se flux.flux. Finally, Finally, the the as-grown as-grown AIGS AIGS films films were were re removedmoved from from the annealing furnace and and immediately characterized.characterized. During During the the annealing annealing process, process, the the molecular molecular pump pump was was shut shut down down and and the the annealing annealing furnacefurnace was was continuously continuously pumped pumped using using a a mechanical mechanical pump pump to to maintain maintain an an annealing annealing pressure pressure of of 10 Pa, aiming at providing sufficient sufficient Se fluxflux to promote selenization ofof thethe AIGSAIGS films.films.

(a) (b) (c)

FigureFigure 2. SchematicSchematic of of the the AIGS AIGS film ﬁlm fabrication process: ( (aa)) Na Na22SeSe fabrication fabrication process; process; ( (bb)) AIGS AIGS precursorprecursor annealing process; and ( c) schematic of the AIGS ﬁlm.film.

A typical AIGS solar cell structure can be described as Mo/AIGS/CdS/i-Zno/ZnO:Al/Al. In this work, a 50 nm thick CdS buffer layer was deposited onto the as-grown AIGS films by a chemical bath

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Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 13 NanomaterialsA typical 2020 AIGS, 10, x FOR solar PEER cell REVIEW structure can be described as Mo/AIGS/CdS/i-Zno/ZnO:Al/Al. In 4 ofthis 13 work,deposition a 50 nmmethod, thick CdSfollowed buﬀer by layer a sputter-deposited was deposited onto i-ZnO the (80 as-grown nm) layer AIGS and ﬁlms an Al-doped by a chemical ZnO bath(600 depositionnm)deposition layer. Finally, method, method, a followed front-contactfollowed by by a sputter-depositeda Al sputter-deposited grid was depos i-ZnOited i-ZnO onto (80 (80 nm)the nm) top layer layerlayer and andby an an Al-doped an evaporation Al-doped ZnO ZnO (600method. nm)(600 layer.nm) layer. Finally, Finally, a front-contact a front-contact Al grid Al grid was depositedwas depos ontoited onto the topthe layertop layer by an by evaporation an evaporation method. method.

FigureFigure 3.3. Schematic of the two-zone annealing furnace. Figure 3. Schematic of the two-zone annealing furnace.

Figure 4. AIGS substrate temperature and Se ﬂuxflux proﬁleprofile during thethe annealingannealing process.process. Figure 4. AIGS substrate temperature and Se flux profile during the annealing process. 2.4.2.4. CharacterizationCharacterization 2.4. Characterization TheThe morphologymorphology of of the the annealed annealed AIGS AIGS films wasfilms characterized was characterized using a scanningusing a electronscanning microscope electron (JSM-7001F,microscopeThe morphology JEOL,(JSM-7001F, Tokyo, of Japan)JEOL,the annealed equippedTokyo, AIGSJapan) with films aequipped JED-2300T was characterized with energy-dispersive a JED-2300T using a spectroscopy energy-dispersivescanning electron (EDS) systemspectroscopymicroscope operating (JSM-7001F,(EDS) at ansystem acceleration JEOL, operating Tokyo, voltage at anJapan) of acceleration 10 kV. equipped EDS for voltage compositionalwith ofa 10JED-2300T kV. analysis EDS forenergy-dispersive was compositional measured at ananalysisspectroscopy acceleration was measured (EDS) voltage system ofat an 15 kV.accelerationoperating The particle at voltage an sizeacceleration of of 15 the kV. inks Thevoltage was particle measured of 10 size kV. of by EDSthe transmission inks for wascompositional measured electron microscopybyanalysis transmission was (TEM, measured electron JEM-2100F, at microscopyan acceleration JEOL, Tokyo, (TEM, voltage Japan). JEM- of2100F, Crystallization 15 kV. JEOL, The particleTokyo, of the sizeJapan). AIGS of filmsthe Crystallization inks was was characterized measured of the byAIGSby X-raytransmission films diffraction was characterized electron (XRD, microscopy SmartLab, by X-ray Rigaku,(TEM, diffraction JEM- Tokyo,2100F, (XRD, Japan) JEOL,SmartLab, with Tokyo, a 40 Rigaku, kV Japan). voltage Tokyo, Crystallization and Japan) 20 mA with current. of a the 40 ThekVAIGS voltage elemental films wasand depth characterized20 mA profile current. of the by AIGSX-rayThe elemental filmdiffraction was measured dep (XRD,th profile SmartLab, by secondary of the Rigaku, AIGS ion mass Tokyo,film spectroscopy was Japan) measured with (SIMS, a by40 + TOF.SIMS5,secondarykV voltage ion Hitachi,and mass 20 Tokyo, mAspectroscopy current. Japan), usingThe(SIMS, elemental a 3TOF.SIMS5, keV primary depth Hitachi, Csprofileion Tokyo, beam.of the The Japan),AIGS strong film using signal was a 3 of measuredkeV the Moprimary back by electrodeCssecondary+ ion beam. was ion used Themass asstrong spectroscopy a characterization signal of (SIMS, themarker Mo TOF.SIMS5, back of theelectrode AIGS Hitachi, film was rear Tokyo,used surface. as Japan), a characterization The using solar cella 3 keV performance marker primary of + wastheCs AIGS measuredion beam. film withTherear strong asurface. 913 CV signal The type solarof current–voltage the cell Mo performance back el (J–V)ectrode tester was was measured (AM1.5), used as provided witha characterization a 913 by anCV solar type marker simulator current– of (LP-50B,voltagethe AIGS EKO,(J–V) film Tokyo, testerrear surface. (AM1.5), Japan). The The provided solar simulator cell by performance was an calibratedsolar simulator was with measured a standard(LP-50B, with GaAsEKO, a 913 solar Tokyo, CV cell type toJapan). obtain current– The the 2 standardsimulatorvoltage (J–V) illumination was testercalibrated (AM1.5), density with (100 aprovided standard mW/cm byGaAs). Thean solasolar externalr cellsimulator to quantum obtain (LP-50B, the efficiency standard EKO, (EQE) illuminationTokyo, of the Japan). AIGS density solar The cells(100simulator wasmW/cm characterized was2). calibratedThe external by with a QE quantum a tester standard (QE-2000, effici GaAsency Otsuka sola(EQE)r cell Electronicsof theto obtain AIGS Co., thesolar Ltd.,standard cells Oosaka, was illumination characterized Japan). density by a 2 QE(100 tester mW/cm (QE-2000,). The externalOtsuka Electronicsquantum effici Co.,ency Ltd., (EQE) Oosaka, of theJapan). AIGS solar cells was characterized by a 3.QE Results tester (QE-2000, and Discussion Otsuka Electronics Co., Ltd., Oosaka, Japan).

3.1.3. Results Nanoparticle and Discussion Ink Characterization 3. Results and Discussion 3.1. NanoparticleTo perform Ink the Characterization measurements, the nanoparticle inks were diluted 20 times to reduce the concentration3.1. Nanoparticle of Ink the Characterization particles in the ink. Additionally, the inks were sonicated for 1 h prior to To perform the measurements, the nanoparticle inks were diluted 20 times to reduce the being dipped onto the TEM sample holder micro-grid to reduce particle agglomeration. Figure5a–c concentrationTo perform of thethe particles measurements, in the ink. the Additiona nanoparticlelly, the inks inks were were diluted sonicated 20 fortimes 1 h toprior reduce to being the shows the TEM micrographs of the nanoparticle distributions in the inks of Ag Se, In Se , and Ga Se , dippedconcentration onto the of TEMthe particles sample holderin the ink.micro-grid Additiona to rlly,educe the particle inks were agglomeration. sonicated2 for Figure 12 h 3prior 5a–c to shows being2 3 respectively.dipped onto Asthe shown TEM sample in Figure holder5a, the micro-grid Ag 2Se particle to reduce size varies particle from agglomeration. several nm to ~10Figure nm. 5a–c Although shows the TEM micrographs of the nanoparticle distributions in the inks of Ag2Se, In2Se3, and Ga2Se3, the particle size distribution is broad, there are almost no observed particles >10 nm. The yellow the TEM micrographs of the nanoparticle distributions in the inks of Ag2Se, In2Se3, and Ga2Se3, respectively. As shown in Figure 5a, the Ag2Se particle size varies from several nm to ~10 nm. arrows in the ﬁgure illustrate particle clusters. The cluster size is ~10 nm; unfortunately, speciﬁc Althoughrespectively. the Asparticle shown size in distribution Figure 5a, isthe broad, Ag2Se there particle are almostsize varies no observed from several particles nm >10 to nm.~10 Thenm. yellowAlthough arrows the particle in the sizefigure distribution illustrate particleis broad, clusters. there are The almost cluster no observedsize is ~10 particles nm; unfortunately, >10 nm. The specificyellow arrowsparticle insizes the are figure difficult illustrate to identify particle because clusters. of the The vague cluster boundary. size is Judging~10 nm; from unfortunately, Figure 5b, specific particle sizes are difficult to identify because of the vague boundary. Judging from Figure 5b,

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2 3 particlethethe GaGa2Se sizesSe3 particleparticle are di ffi sizesizecult distributiondistribution to identify becauseisis relativelyrelatively of the uniformuniform vague boundary.andand nono significantsignificant Judging fromagglomerationsagglomerations Figure5b,the areare observed. The particles exhibit a regular size of several tens of nanometers, as illustrated by the Gaobserved.2Se3 particle The sizeparticles distribution exhibit isa relativelyregular size uniform of several and no tens significant of nanometers, agglomerations as illustrated are observed. by the Theyellowyellow particles arrows.arrows. exhibit LargeLarge a particlesparticles regular size ofof ~10~10 of nmnm several cancan alsoalso tens bebe of observed,observed, nanometers, asas shownshown as illustrated inin thethe circle,circle, by the atat yellow aa significantlysignificantly arrows. 2 2 Largelesserlesser particlesdegree,degree, however,however, of ~10 nm whenwhen can also comparedcompared be observed, toto thethe as AgAg shown2SeSe ink.ink. in TheThe the circle,averageaverage at particle aparticle significantly sizesize ofof lesser InIn2Se,Se, degree, FigureFigure 5c, is <10 nm, with the smallest particles having a size observed to be ~10 nm, as illustrated by the however,5c, is <10 whennm, with compared the smallest to the particles Ag2Se ink. having The averagea size observed particle sizeto be of ~10 In2 nm,Se, Figureas illustrated5c, is < 10by nm,the withcircle,circle, the whilewhile smallest thethe largestlargest particles particleparticle having isis a observedobserved size observed toto bebe to ~50~50 be nm, ~10nm, nm,asas shownshown as illustrated byby thethe arrows. byarrows. the circle, Thus,Thus, while thethe wet-wet- the largestmillingmilling particle processprocess is isis observed concludedconcluded to betoto ~50bebe efficient nm,efficient as shown andand thethe by nanoparticle thenanoparticle arrows. Thus, inksinks theareare wet-millingobtainedobtained forfor process allall threethree is concludedmaterials.materials. to be efficient and the nanoparticle inks are obtained for all three materials.

((aa)) ( (bb)) ( (cc))

Figure 5. Transmission electron microscopy images of the (a) Ag2Se, (b) Ga2Se3, and (c) In2Se3 FigureFigure 5.5. Transmission electronelectron microscopymicroscopy imagesimages ofof thethe (a(a)) Ag Ag2Se,2Se, (b(b)) GaGa2Se2Se33,, andand (c(c)) InIn2Se2Se33 nanoparticle inks. nanoparticlenanoparticle inks.inks.

3.2.3.2.3.2. CharacterizationCharacterization ofof AIGSAIGS FilmsFilmsFilms TheTheThe as-milledas-milledas-milled nanoparticlenanoparticlenanoparticle inksinks werewerewere usedusedused tototo fabricatefafabricatebricate thethethe AIGSAIGSAIGS precursorsprecursorsprecursors havinghavinghaving aaa structurestructurestructure shownshownshown in inin Figure FigureFigure1 1..1. The TheThe morphology morphologymorphology of ofof the thethe AIGS AIGSAIGS precursor, precprecursor,ursor, as asas shown shownshown in inin Figure FigureFigure6 a, 6a,6a, exhibits exhibitsexhibits a a compacta compactcompact andandand flat flatflat surface surfacesurface with withwith nonono observableobservable defects,defects, suchsuch asas pinpin holesholes oror large large surficial surficialsurficial fluctuations.fluctuations.fluctuations. However, However, thethethe specificspecificspecific graingraingrain sizesizesize couldcouldcould not notnot be bebe measured, measured,measured, because bebecausecause of ofof the thethe unobvious unobviousunobvious grain graingrain boundaries. boundaries.boundaries. JudgingJudgingJudging fromfromfrom thethethe cross-sectionalcross-sectionalcross-sectional imageimageimage ofofof thethethe AIGSAIGSAIGS precursor,precursoprecursor,r, asasas shownshownshown ininin FigureFigureFigure6 6b,b,6b, the thethe AIGS AIGSAIGS film filmfilm shows showsshows a aa compactcompactcompact structure structurestructure along alongalong the thethe thickness thicknesthicknesss direction. direction.direction. The TheThe particle particleparticle size sizesize can cancan be bebe clearly clearlyclearly measured. measured.measured. AlthoughAlthoughAlthough thethethe precursorprecursor isis composedcomposed composed ofof of threethree three layers,layers, layers, onlyonly only aa singlesingle a single layerlayer layer ofof filmfilm of film cancan be canbe observedobserved be observed asas aa resultresult as a result ofof thethe ofsimilaritysimilarity the similarity ofof particleparticle of particle sizesize forfor size alalll thethe for nanoparticlenanoparticle all the nanoparticle inks.inks. ToToinks. improveimprove To improvethethe crystallinity,crystallinity, the crystallinity, graingrain size,size, grain andand size,elementalelemental and elemental distributiondistribution distribution ofof thethe AIGSAIGS of thefilms,films, AIGS high-temperaturehigh-temperature films, high-temperature annealingannealing annealing (570(570 °C)°C) ofof(570 thethe AIGSAIGS◦C) of precursorsprecursors the AIGS precursorsunderunder aa SeSe under atmosphereatmosphere a Se atmosphere waswas conducted.conducted. was conducted. DetailsDetails Details ofof thethe of annealing theannealing annealing processprocess process areare are describeddescribed described inin thethethe experimentalexperimentalexperimental section. section.section. Furthermore,FurthermorFurthermore,e, the thethe samplessamples werewere immediatelyimmediately characterized characterizedcharacterized after afterafter beingbeingbeing removedremoved fromfromfrom the thethe annealing annealingannealing chamber. chamber.chamber.

((aa)) ((bb))

FigureFigureFigure 6.6.6. ((aa)) ScanningScanningScanning electronelectron microscopymicroscopy surfacesurface morphologymorphology of of the the ( (aa)) AIGSAIGS precursor,precursor, andand (((bb))) cross-sectionalcross-sectionalcross-sectional micrograph micrographmicrograph of ofof the thethe AIGS AIGSAIGS precursor. precursor.precursor.

2 ToToTo study studystudy the thethe role rolerole of ofof Na NaNa2Se2SeSe on onon the thethe AIGS AIGSAIGS film filmfilm and andand solar solarsolar cell cell properties,cell properties,properties, a series aa seriesseries of experiments ofof experimentsexperiments were 2 conductedwerewere conductedconducted by post-depositing byby post-depositingpost-depositing various variousvarious Na2Se layerNaNa2SeSe thicknesses layerlayer thicknessesthicknesses of 5, 10, ofof and 5,5, 10, 2010, nm. andand A2020 film nm.nm. without AA filmfilm 2 Nawithoutwithout2Se post-deposition NaNa2SeSe post-depositionpost-deposition was used waswas as used aused reference. asas aa reference.reference. The improvement TheThe improvementimprovement in the AIGS inin thethe film AIGSAIGS grain filmfilm size graingrain in size thesize presenceinin thethe presencepresence of Na isofof clear NaNa isisbased clearclear basedbased on the onon observations thethe observobservationsations from from Figurefrom FigureFigure7a–d. 7a–d.7a–d. For the ForFor AIGS thethe AIGSAIGS precursor precursorprecursor in the inin 2 absencethethe absenceabsence of any ofof post-depositedanyany post-depositedpost-deposited Na2Se NaNa (see2SeSe Figure (see(see FigureFigure7a), the 7a),7a), AIGS thethe film AIGSAIGS exhibits fifilmlm exhibitsexhibits a lesser a degreea lesserlesser of degreedegree quality ofof qualityquality havinghaving aa broadbroad graingrain sizesize distribution,distribution, wherewhere thethe graingrain sizesize ofof thethe smallersmaller particlesparticles isis <20<20 nm,nm, 2 withwith thethe largerlarger graingrain sizessizes beingbeing >1>1μμm.m. ByBy applyingapplying thethe NaNa2SeSe post-depositionpost-deposition priorprior toto fabricatingfabricating

Nanomaterials 2020, 10, 547 6 of 13 having a broad grain size distribution, where the grain size of the smaller particles is <20 nm, with the Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 13 larger grain sizes being >1µm. By applying the Na2Se post-deposition prior to fabricating the AIGS theprecursors, AIGS precursors, the grain sizethe grain was significantly size was significantl increasedy increased with a reduction with a reduction in surface in roughness. surface roughness. The grain size gradually increased as a function of Na2Se thickness. Tailoring the thickness of Na2Se to 5 nm The grain size gradually increased as a function of Na2Se thickness. Tailoring the thickness of Na2Se initiates growth of the grain size, where a maximum value is observed with a Na2Se thickness of 10 nm. to 5 nm initiates growth of the grain size, where a maximum value is observed with a Na2Se thickness ofAdditionally, 10 nm. Additionally, some of the some grains of extendedthe grains throughout extended throughout the film, with the some film, of with the smallersome of grains the smaller being observed at the bottom of the AIGS precursor. When the thickness of Na2Se increased to 20 nm, the grains being observed at the bottom of the AIGS precursor. When the thickness of Na2Se increased to grain size uniformity decreased significantly. Liquid Na2Se has been reported to play a critical role in 20Nanomaterials nm, the grain 2020, 10 size, x FOR uniformity PEER REVIEW decreased significantly. Liquid Na2Se has been reported to play 6 of a 13 criticaltriggering role the in mechanismtriggering the during mechanism grain growth during [23 grain]. Furthermore, growth [23]. other Furtherm researchore, has other demonstrated research has Se to be chemically adsorbed onto Na to form polyselenide Na2Sex during the annealing process, which demonstratedthe AIGS precursors, Se to bethe chemicallygrain size was adso significantlrbed ontoy increasedNa to form with polyselenide a reduction inNa surface2Sex during roughness. the promotes the crystallization of the AIGS film because of the reaction between the metal precursor and annealingThe grain process,size gradually which incr promoteseased as the a function crystallizat of Naion2Se of thickness. the AIGS Tailoring film because the thickness of the reactionof Na2Se reactive Se derived from Na2Sex [24]. betweento 5 nm initiatesthe metal growth precursor of the and grain reactive size, whereSe derived a maximum from Na value2Sex [24]. is observed with a Na2Se thickness of 10 nm. Additionally, some of the grains extended throughout the film, with some of the smaller grains being observed at the bottom of the AIGS precursor. When the thickness of Na2Se increased to 20 nm, the grain size uniformity decreased significantly. Liquid Na2Se has been reported to play a critical role in triggering the mechanism during grain growth [23]. Furthermore, other research has demonstrated Se to be chemically adsorbed onto Na to form polyselenide Na2Sex during the annealing process, which promotes the crystallization of the AIGS film because of the reaction between the metal precursor and reactive Se derived from Na2Sex [24].

(a) 0 nm (b) 5 nm (c) 10 nm (d) 20 nm

Figure 7. 7. SurfaceSurface and and morphology morphology of ofthe the AIGS AIGS films ﬁlms showing showing the important the important role of role the of Na the2Se Na post-Se 2 depositionpost-deposition process. process. (a) No (a )Na No2Se Na post-deposition.2Se post-deposition. Na2Se Na thicknesses2Se thicknesses of (b of) 5, (b ()c 5,) 10, (c) and 10, and (d) (20d) nm. 20 nm.

Figure8 shows the cross-sectional morphology of a typical AIGS solar cell. Pinholes and a severe rough surface can be observed. Figure9 shows XRD patterns of AIGS films with di fferent thicknesses of Na2Se layers. Main peaks of the chalcopyrite AIGS films are illustrated by blue lines. It is concluded that(a) all0 nm the AIGS films ( showb) 5 nm a typical chalcopyrite (c) 10 nm structure. It can (d) also20 nm be observed that the peakFigure intensity 7. Surface of AIGSand morphology films gradually of the increaseAIGS films with showing the increase the important in Na2 Serole thickness, of the Na2 indicatingSe post- a betterdeposition crystallinity. process. (a) No Na2Se post-deposition. Na2Se thicknesses of (b) 5, (c) 10, and (d) 20 nm.

Figure 8. Cross-sectional SEM image of a typical AIGS solar cell.

Figure 8 shows the cross-sectional morphology of a typical AIGS solar cell. Pinholes and a severe rough surface can be observed. Figure 9 shows XRD patterns of AIGS films with different thicknesses of Na2Se layers. Main peaks of the chalcopyrite AIGS films are illustrated by blue lines. It is concluded that all the AIGS films show a typical chalcopyrite structure. It can also be observed that the peak intensity of AIGS films gradually increase with the increase in Na2Se thickness, indicating a better crystallinity. FigureFigure 8. 8.Cross-sectional Cross-sectional SEMSEM imageimage ofof a a typical typical AIGS AIGS solar solar cell. cell.

Nanomaterials 2020, 10, 547 7 of 13 NanomaterialsNanomaterials 20202020,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 7 7 of of 13 13

FigureFigure 9.9. XRDXRD patterns patterns of of AIGS AIGS films ﬁlmsfilms with with differentdidifferentﬀerent thicknessesthicknesses ofof NaNa22Se.Se.

FigureFigure 1010 a–d a–da–d show showshow the thethe depth depthdepth profile profprofileile results resultsresults of ofof the thethe element elementelement distribution distridistributionbution of the ofof the AIGSthe AIGSAIGS solar solarsolar cells cellscells as a function of Na Se thickness (0, 5, 10, and 20 nm) by the SIMS method. All the samples were fabricated asas aa functionfunction of2of NaNa22SeSe thicknessthickness (0,(0, 5,5, 10,10, andand 2020 nm)nm) byby thethe SIMSSIMS method.method. AllAll thethe samplessamples werewere on SLG substrates. The sample in the absence of any post-deposition of Na Se was used as a reference. fabricatedfabricated onon SLGSLG substrates.substrates. TheThe samplesample inin thethe absenceabsence ofof anyany post-depositionpost-deposition2 ofof NaNa22SeSe waswas usedused asas aSodiuma reference.reference. was SodiumSodium not added waswas to notnot the addedadded film intentionally,toto thethe filmfilm inteinte tontionally,ntionally, have all to theto havehave Na derivedallall thethe NaNa from derivedderived the SLG fromfrom substrate. thethe SLGSLG substrate.Thesubstrate. free surface TheThe freefree of thesurfacesurface AIGS ofof film thethe wasAIGSAIGS set filmfilm as thewaswas starting sesett asas thethe point startingstarting of the pointpoint SIMS ofof measurement. thethe SIMSSIMS measurement.measurement. The dotted ThelineThe showsdotteddotted the lineline elemental showsshows thethe profile elementalelemental of Mo, andprofileprofile the of rapidof MoMo onset,, andand ofthethe the rapidrapid curve onsetonset was used ofof thethe to curve identifycurve waswas the usedused bottom toto identifyofidentify the AIGS thethe absorber.bottombottom ofof The thethe Ag, AIGSAIGS In, Ga,absorber.absorber. and Se TheThe elements Ag,Ag, In,In, in Ga, allGa, the andand four SeSe sampleselementselements show inin allall similar thethe fourfour distribution samplessamples showprofilesshow similarsimilar having distributiondistribution small fluctuations profileprofiles throughouts havinghaving smallsmall the absorberfluctuationsfluctuations layer. throthro Anughoutughout obvious thethe ‘hump’ absorberabsorber structure layer.layer. near AnAn obvioustheobvious surface ‘hump’‘hump’ of the structurestructure Mo layer nearnear in thethe surfacesurface Se profile ofof thethe was MoMo observed, layerlayer inin the indicatingthe SeSe profileprofile segregation waswas observed,observed, of Se, indicatingindicating which is attributed to the formation of a MoSe layer that is commonly observed in chalcopyrite solar cells [25]. segregationsegregation ofof Se,Se, whichwhich isis attributedattributed2 toto thethe formationformation ofof aa MoSeMoSe22 layerlayer thatthat isis commonlycommonly observedobserved inin chalcopyriteFurthermore,chalcopyrite solarsolar the Ga cellscells distribution [25].[25]. Furthermore,Furthermore, profile shows thethe a GaGa ‘valley’ distributiondistribution structure profileprofile having showsshows a minimum aa ‘valley’‘valley’ concentration structurestructure havingathaving ~500 aa nm minimumminimum beneath concentrationconcentration the surface of atat the ~500~500 AIGS nmnm film. beneathbeneath As the thethe AIGS surfacesurface film ofof bandthethe AIGSAIGS profile film.film. is influencedAsAs thethe AIGSAIGS by filmfilm Ga bandcontentband profileprofile [26], isis a influenced double-gradedinfluenced byby GaGa bandgap contentcontent [26],is[26], inferred, aa double-gradeddouble-graded which is beneficial bandgapbandgap forisis inferred,inferred, photon whichabsorptionwhich isis beneficialbeneficial [27,28], forandfor photonphoton thus, promotes absorptionabsorption solar [27,28],[27,28], cell performance. andand ththus,us, promotespromotes solarsolar cellcell performance.performance.

((aa)) 00 nmnm ( (bb)) 55 nmnm

Figure 10. Cont.

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Figure 10. Elemental distribution profiles of AIGS films as a function of Na2Se thickness by secondary FigureFigure 10.10. Elemental distribution proﬁlesprofiles ofof AIGSAIGS ﬁlmsfilms asas aa functionfunction ofof NaNa22Se thickness by secondary ionionion massmassmass spectroscopyspectroscopyspectroscopy (SIMS): (SIMS):(SIMS): ( ((aa)) 0,0, ((bb)) 5,5, ((cc)) 10,10, andand ((dd)) 2020 nm. nm.

TheThe AIGSAIGS filmsfilms werewere alsoalso characterizedcharacterized byby SIMSSIMS toto investigateinvestigate NaNa distribution.distribution. Figure 1111 showsshows thethe AIGS film film sodium sodium profile profile as as a afunction function of of Na Na2Se2 Sethickness. thickness. For Forthe AIGS the AIGS film without film without any post- any post-depositeddeposited Na2Se, Na 2theSe, sodium the sodium element element was was observed observed to todi diffuseffuse from from the the SLG SLG substrate. substrate. Thus, thethe sodiumsodium concentrationconcentration gradually increasedincreased fromfrom thethe frontfront surfacesurface toto rearrear surface.surface. However,However, aa slightslight intensityintensity decreasedecrease inin thethe profileprofile waswas observedobserved byby focusingfocusing onon thethe toptop surfacesurface ofof thethe film,film, indicatingindicating slightslight sodiumsodium segregationsegregation atat thethe toptop surface.surface. AnAn obviousobvious sodiumsodium segregationsegregation nearnear thethe AIGSAIGS/Mo/Mo interfaceinterface waswas observed,observed, whichwhich isis widelywidely reportedreported in in CIGSCIGS solarsolar cellscells [[29,30].29,30]. ForFor thethe AIGSAIGS filmfilm havinghaving thethe 55 nm nm Na Na2Se2Se post-deposition post-deposition layer, layer, sodium sodium di ffdiffusionusion to theto the surface surface was was significantly significantly promoted, promoted, with thewith sodium the sodium concentration concentration increased increased throughout throughout the film. the With film. increased With increased Na2Se thickness, Na2Se thickness, the sodium the concentrationsodium concentration level in the level AIGS in filmthe graduallyAIGS film increased gradually with increased a concomitant with a increased concomitant segregation increased of sodiumsegregation at the of AIGS sodium/Mo at interface. the AIGS/Mo interface.

FigureFigure 11.11. SIMSSIMS sodiumsodium proﬁleprofile asas aa functionfunction ofof NaNa22SeSe thickness.thickness.

FigureFigure 1212 showsshows thethe X-rayX-ray photoelectronphotoelectron spectroscopyspectroscopy spectraspectra ofof sodiumsodium andand oxygenoxygen onon thethe surfacesurface ofof thethe AIGSAIGS ﬁlmsfilms asas aa functionfunction ofof NaNa22Se post-deposition thickness.thickness. AsAs thethe thickness ofof sodiumsodium increased,increased, anan obviousobvious NaNa ((1s1s)) peakpeak waswas observed,observed, FigureFigure 1211a,a, whichwhich isis consistentconsistent withwith thethe previousprevious SIMSSIMS observationsobservations ofof surfacesurface sodiumsodium segregation.segregation. Additionally, thethe NaNa ((1s1s)) peakpeak waswas accompaniedaccompanied byby anan O ( 1s)) peak, peak, Figure Figure 11b,12b, with with a asynchronous synchronous change change in in the the intensity intensity of ofthe the spectra. spectra. During During the theexperiment, experiment, the thesamples samples were were exposed exposed to toair air for for <5< 5min. min. These These results results indicate indicate that that oxygen waswas absorbedabsorbed ontoonto thethe AIGSAIGS surfacesurface withinwithin minutesminutes forfor samplessamples comprisingcomprising highhigh surfacesurface sodium levels, whilewhile thethe absorption absorption of oxygenof oxygen was lowwas for low samples for sa withmples lower with surface lower sodium surface levels. sodium The relationshiplevels. The betweenrelationship sodium between and sodium oxygen and has oxygen previously has beenpreviously reported beenbeen in reportedreported the case inin of thethe CZTS casecase [ 31 ofof], CZTSCZTS and a [31],[31], similar andand conclusiona similar conclusion was obtained was inobtained this work. in this work.

Nanomaterials 2020, 10, 547 9 of 13 NanomaterialsNanomaterials 2020 2020, 10, 10, x, xFOR FOR PEER PEER REVIEW REVIEW 9 9 of of 13 13

(a()a ) (b(b) )

FigureFigureFigure 12.12. 12. X-ray X-ray photoelectronphotoelectron photoelectron spectroscopyspectroscopy spectroscopy measurementsmeasurements measurements of of samplessamples samples containingcontaining containing variousvarious various sodiumsodium sodium contents:contents:contents: (a ()a Na) Na spectra spectra and and ((b ()b oxygenoxygen) oxygen spectra.spectra. spectra.

3.3.3.3.3.3. Properties Properties of of AIGS AIGS Solar Solar Cells Cells TheTheThe as-grownas-grown as-grown AIGSAIGS AIGS filmsfilms films werewere were usedused used toto to completecomplete complete thethe the fullfull full structurestructure structure ofof of thethe the solarsolar solar cells.cells. cells. TheThe The AIGSAIGS AIGS solarsolarsolar cellcell cell performanceperformance performance waswas was characterizedcharacterized characterized underunder under aa a standardstandard standard conditioncondition condition ofof of 100100 100 mWmW/cm mW/cm/cm22 irradiation2irradiation irradiation forfor for illumination.illumination.illumination. A A high-precision high-precision monocrystalline monocrystalline Si Si solarsola solar r cell cell was was used used to to calibratecalibrate calibrate thethe the intensityintensity intensity ofof of thethe the solarsolarsolar simulator simulatorsimulator to obtain toto obtainobtain a standard a a standardstandard light intensity lightlight forintensityintensity characterization. forfor characterization.characterization. Basic photovoltaic BasicBasic characteristics photovoltaicphotovoltaic ofcharacteristicscharacteristics the AIGS solar of of the cells the AIGS asAIGS a functionsolar solar cells cells of as Na as a2 Seafunction function amount of of are Na Na illustrated2Se2Se amount amount in are Figureare illustrated illustrated 13. Considering in in Figure Figure the13. 13. systematicConsideringConsidering error, the the systematic measurements systematic error, error, were measurements measurements conducted onwe we fourrere conducted conducted different samples.on on four four different Figuredifferent 13 samples. a–dsamples. shows Figure Figure the open-circuit13a–d13a–d shows shows voltage the the open-circuit open-circuit (Voc), short-circuit voltage voltage ( V( currentVoc),oc), short-circuit short-circuit (Jsc), fill current factor current ( FF( J(sc),J),sc), andfill fill factor conversionfactor ( FF(FF),), and eandffi ciencyconversion conversion (Eff), 2 respectively.efficiencyefficiency (E (Eff), Theff), respectively. respectively. area of the The solar The area area cell of wasof the the 0.2solar solar cm cell cell. As was was the 0.2 0.2 thickness cm cm2. 2As. As the of the Na thickness thickness2Se increased, of of Na Na2Se2 allSe increased, theincreased, basic parametersallall the the basic basic of parameters parametersVoc, Jsc, FF of, andof V Voc,ocE J,ff scJ,scof FF, FF the, and, and AIGS E Eff ffofsolar of the the cellAIGS AIGS increased. solar solar cell cell The increased. increased. highest Thesolar The highest highest cell conversion solar solar cell cell 2 econversionfficonversionciency of efficiency 6.6%efficiency (Voc: of 775.6of 6.6% 6.6% mV, ( V(JVocsc:oc ::775.6 15.5775.6 mAmV, mV,/cm J scJ:sc, :15.5FF 15.5: 54.9%)mA/cm mA/cm was2, 2,FF obtainedFF: :54.9%) 54.9%) when was was obtained the obtained thickness when when of thethe the Nathicknessthickness2Se layer of of wasthe the Na 20 Na nm.2Se2Se layer Thelayer basicwas was 20 characteristics20 nm. nm. The The basic basic of thecharacteri characteri other solarsticsstics cellsof of the the were other other as follows:solar solar cells cellsE ffwere =were2.6% as as 2 withfollows:follows:Voc =E Eff620.4 ff= =2.6% 2.6% mV, with withJsc = V 9.2Voc oc= mA=620.4 620.4/cm mV, mV,, and J scJ scFF= =9.2 =9.245.7% mA/cm mA/cm for2, 20and, nmand FF Na FF =2 Se; =45.7% 45.7%Eff =for for3.6% 0 0nm withnm Na NaV2Se;oc2Se;= Eff 662.1Eff = =3.6% mV,3.6% 2 2 Jwithscwith= 11.8V Voc oc= mA =662.1 662.1/cm mV, mV,, and J scJ scFF= =11.8 =11.846.1% mA/cm mA/cm for2, 5 2and, nmand FF Na FF =2 Se;=46.1% 46.1%Eff = for for4.4% 5 5nm nm with Na NaV2Se;oc2Se;= E E685.4ff =ff =4.4% 4.4% mV, with withJsc =V Voc13.3 oc= =685.4 mA685.4/ cmmV, mV,, andJscJ sc= =13.3FF 13.3= mA/cm48.3% mA/cm for2, 2and, 10and nmFF FF = Na =48.3% 48.3%2Se. for for 10 10 nm nm Na Na2Se.2Se.

(a()a ) (b(b) )

Figure 13. Cont.

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((cc)) ((dd))

Figure 13. Photovoltaic parameters of solar cells as a function of Na2Se thickness: (a) Open-circuit FigureFigure 13.13. Photovoltaic parameters of solarsolar cellscells asas aa functionfunction ofof NaNa22SeSe thickness:thickness: ((aa)) Open-circuitOpen-circuit voltage,voltage, ((bb)) short-circuitshort-circuit current,current, (((ccc))) ﬁllfillfill factor,factor,factor, and andand ( (d(dd))) conversion conversionconversion e efficiency.efficiency.ﬃciency.

J–VJ–V andand EQEEQE curvescurves of of the the champion champion efficiency eefficiencyfficiency for for the the AIGSAIGS solarsolar cellscells asas aaa functionfunctionfunction ofofof NaNaNa222SeSe thicknessthickness are are shownshown inin FigureFigureFigure 1414a–c.14a–c.a–c. In In Figure Figure 1414a,14a,a, whenwhen thethe thicknessthicknessthickness ofofof NaNaNa222SeSe waswas <<20<2020 nm, nm, an an obviousobvious roll-overroll-overroll-over in inin the thethe high-voltage high-voltagehigh-voltage region regionregion was waswas observed. observed.observed. This ThisThis phenomenon phenomenonphenomenon can cancan be attributed bebe attributedattributed to two toto twofactors:two factors:factors: (1) MoSe (1)(1) MoSeMoSe2 observed22 observedobserved at the atat thethe AIGS AIGS/MoAIGS/Mo/Mo interface interfaceinterface and andand (2) (2) a(2) high aa highhigh recombination recombinationrecombination rate raterate because becausebecause of oflow-qualityof low-qualitylow-quality solar solarsolar cells. cells.cells. In InIn this thisthis work, work,work, both bothboth factors factorsfactors are areare suggested suggestedsuggested to toto be bebe influential, influential,influential, leading leadingleading toto thethe final finalfinal result.result. Figure Figure 14b1414bb shows shows thethe J–VJ–V curvecurve of of the the AIGS AIGS solarsolar cellcell withwith 2020 NaNa222SeSe layers. layers. No No crossover crossover was was observedobserved between between the the two two curves. curves. The The EQEEQE curvecurve of of the the AIGS AIGS solar solar cell cell significantly significantlysignificantly improved improved with with NaNa22SeSe thickness,thickness, asas shownshown inin Figure Figure 13c.1313c.c. TheThe QEQE curvescurves showshow aa rapid rapid drop drop in in the the infrared infrared region region at at ~750~750 nm, nm, correspondingcorresponding to to the the absorption absorption edge edge of of the the AIGSAIGS film.film.film. Accordingly, Accordingly, the the bandgapsbandgaps of of the the AIGSAIGS filmsfilmsfilms werewere calculatedcalculated toto bebe ~1.65~1.65 eV. eV. Furthermore,Furthermore, the thethe drops dropsdrops at atat ~510 ~510~510 andand ~380~380 nm nm are are attributed attributed toto the the absorption absorption edgesedges ofof thethe CdSCdS andand ZnOZnO layerslayelayersrs [[32],[32],32], respectively.respectively. TheTheThe correlationcorrelationcorrelation betweenbetweenbetween J JJscscsc andand thethe EQEEQE curvecurve is is givengiven asas [[33][33]33] Z ∞ J = q QE(E)b (E, T )dE, (1) 𝐽 =𝑞 𝑄𝐸sc𝐸𝑏𝐸, 𝑇 d𝐸,s s (1) 𝐽 =𝑞 𝑄𝐸 𝐸 𝑏 0𝐸, 𝑇 d𝐸, (1)

wherewhere qq isis the the elementary elementary chargecharge andand bbss isis thethe irradiationirradiation of of thethe solarsolar spectra.spectra. ForFor For standardstandard standard conditions,conditions, conditions, airair mass mass 1.5 1.5 (AM(AM 1.5)1.5) isis used,used, andand bbsss isis available available from from [34]. [[34].34]. Based Based on on Equation 1, Figure Figure 13c,1313c,c, and and the the 2 2 2 solarsolar irradiationirradiationirradiation spectrum, spectrum,spectrum,Jsc JJscofsc ofof the thethe AIGS AIGSAIGS solar solarsolar cell wascellcell calculatedwaswas calculatedcalculated as 8.6 as mAas 8.68.6/cm mA/cmmA/cmfor 0 nm,2 forfor 10.6 00 nm,nm, mA /10.6cm10.6 2 2 2 2 2 mA/cmformA/cm 5 nm,2 forfor 12.8 55 mAnm,nm,/cm 12.812.8for mA/cmmA/cm 10 nm,2 forfor and 1010 14.2 nm,nm, mA andand/cm 14.214.2for mA/cmmA/cm 20 nm.2 The forfor deviation2020 nm.nm. TheThe of Jdeviationscdeviationwas calculated ofof JJscsc waswas by calculatedthecalculatedQE curve byby derivedthethe QEQE curvecurve from derived thederivedJsc obtained fromfrom thethe by JJscscthe obtainedobtainedJ–V curve, byby thethe and J–VJ–V is curve,curve, related andand to a isis lower relatedrelated illumination toto aa lowerlower illuminationintensityillumination of the intensityintensity QE measurement ofof thethe QEQE measurementmeasurement when compared whenwhen to co thatcomparedmpared of a sun-irradiated toto thatthat ofof aa sun-irradiatedsun-irradiated equivalent. equivalent.equivalent.

((aa)) ((bb))

Figure 14. Cont.

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(c)

Figure 14. Performance ofFigure AIGS 14. champion Performance solar of cells: AIGS (a) championJ–V curves solar as a functioncells: (a) J–V of Na curves2Se thickness; as a function of Na2Se thickness; (b) light and dark J–V curve(b) light of AIGS and solardark cellJ–V withcurve 20 of nm AIGS Na 2solarSe; ( ccell) quantum with 20 enmﬃciency Na2Se; as (c a) functionquantum efficiency as a function of Na2Se thickness. of Na2Se thickness.

4. Conclusions 4. Conclusions

Ag2Se, In2Se3, and Ga2AgSe32Se,nanoparticle In2Se3, and inks Ga2Se were3 nanoparticle fabricatedvia inks a wetwere ball-milling fabricated via method, a wet which,ball-milling method, which, thereafter, were used tothereafter, fabricate were AIGS used precursors to fabricate via a AIGS spray precursors method. The via AIGS a spray films method. were obtained The AIGS films were obtained by annealing the AIGSby precursors annealing underthe AIGS a Se precursors atmosphere under for 1 a h Se at atmosphere 570 ◦C. The influencefor 1 h at 570 of sodium °C. The influence of sodium doping on the AIGS solardoping cells on was the evaluated AIGS solar by cells vacuum-evaporating was evaluated by variousvacuum-evaporating thicknesses of various Na2Se thicknesses of Na2Se layers (0, 5, 10, and 20layers nm) onto (0, 5, the 10, Mo and layer 20 nm) prior onto to fabricatingthe Mo layer the prior AIGS to precursors. fabricating Thethe AIGS grain precursors. size The grain size and crystallization of theand AIGS crystallization films were of significantly the AIGS films improved were significantly by sodium doping.improved Slight by sodium surficial doping. Slight surficial sodium segregation andsodium heavy segregation sodium segregation and heavy at sodium the AIGS segregation/Mo interface at the were AIGS/Mo observed interface by SIMS. were observed by SIMS. Basic photovoltaic characteristicsBasic photovoltaic of the characteristics AIGS solar cells of werethe AIGS significantly solar cells improved were significantly as a function improved as a function of of increasing sodium content.increasing The sodium highest content. solar cell The conversion highest sola effir cellciency conversion of 6.6% (Vefficiencyoc: 775.6 of mV, 6.6% Jsc :(Voc: 775.6 mV, Jsc: 15.5 2 15.5 mA/cm2, FF: 54.9%,mA/cm2, with an FF: area 54.9%, of 0.2 with cm an) was area obtained of 0.2 cm when2) was the obtained Na2Se thicknesswhen the wasNa2Se 20 thickness nm. was 20 nm. The The bandgaps of the AIGSbandgaps thin films of the were AIGS calculated thin films as 1.65were eV, calculated according as to 1.65 the QEeV, curveaccording of the to AIGS the QE curve of the AIGS solar cell. solar cell.

Author Contributions: Conceptualization: X.Z. Characterization: M.Z. Funding acquisition: Q.S. and Y.W. Draft Author Contributions: Conceptualization: X.Z. Characterization: M.Z. Funding acquisition: Q.S. and Y.W. Draft review and editing: Z.D. review and editing: Z.D. Funding: Part of the work was financially supported by a Grant for Special Research Projects of Zhongshan Institute (Funding No. 417YKQ10)Funding: Part and Keyof the Project work of was Science financially and Technology supported Plan by ofa ZhongshanGrant for Special City (Grant Research No. Projects of Zhongshan 2018B1018). This work wasInstitute also financially (Funding No. supported 417YKQ10) by the and National Key Project Science of FoundationScience and of Technology China (Grant Plan Nos. of Zhongshan City (Grant 61302044 and 61671140).No. 2018B1018). This work was also financially supported by the National Science Foundation of China (Grant Acknowledgments: WeNos. thank 61302044 Edanz Group and 61671140). (www.edanzediting.com /ac) for editing a draft of this manuscript. Conflicts of Interest: TheAcknowledgments: authors declare no We conflict thank of Edanz interest. Group (www.edanzediting.com/ac) for editing a draft of this manuscript.

Conflicts of Interest: The authors declare no conflict of interest.. References

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