molecules

Article Preparation and Characterization of Thin-Film Solar Cells with Ag/C60/MAPbI3/CZTSe/Mo/FTO Multilayered Structures

Tsung-Wen Chang 1, Chzu-Chiang Tseng 1, Dave W. Chen 1 , Gwomei Wu 1,*, Chia-Ling Yang 2 and Lung-Chien Chen 3

1 Department of Electronic Engineering, Institute of Electro-Optical Engineering, Chang Gung University, Chang Gung Memorial Hospital, Taoyuan 333, Taiwan; [email protected] (T.-W.C.); [email protected] (C.-C.T.); [email protected] (D.W.C.) 2 Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan; [email protected] 3 Department of Electro-Optical Engineering, National Taipei University of Technology, Taipei 106, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-321-188-00

Abstract: New solar cells with Ag/C60/MAPbI3/Cu2ZnSnSe4 (CZTSe)/Mo/FTO multilayered structures on glass substrates have been prepared and investigated in this study. The electron-

transport layer, active photovoltaic layer, and hole-transport layer were made of C60, CH3NH3PbI3 (MAPbI3) , and CZTSe, respectively. The CZTSe hole-transport layers were deposited  ◦ ◦  by magnetic sputtering, with the various thermal annealing temperatures at 300 C, 400 C, and ◦ 500 C, and the film thickness was also varied at 50~300 nm The active photovoltaic MAPbI3 films Citation: Chang, T.-W.; Tseng, C.-C.; were prepared using a two-step spin-coating method on the CZTSe hole-transport layers. It has Chen, D.W.; Wu, G.; Yang, C.-L.; been revealed that the crystalline structure and domain size of the MAPbI3 perovskite films could be Chen, L.-C. Preparation and substantially improved. Finally, n-type C60 was vacuum-evaporated to be the electronic transport Characterization of Thin-Film Solar layer. The 50 nm C60 thin film, in conjunction with 100 nm Ag electrode layer, provided adequate Cells with Ag/C60/MAPbI3/CZTSe/ electron current transport in the multilayered structures. The current density–voltage Mo/FTO Multilayered Structures. characteristics were evaluated and compared with the thin-film microstructures. The photo-electronic Molecules 2021, 26, 3516. https:// doi.org/10.3390/molecules26123516 power-conversion efficiency could be improved to 14.2% when the annealing temperature was 500 ◦C and the film thickness was 200 nm. The thin-film solar cell characteristics of open-circuit Academic Editors: voltage, short-circuit current density, fill factor, series-resistance, and Pmax were found to be 1.07 V, Braulio García-Cámara and 19.69 mA/cm2, 67.39%, 18.5 Ω and 1.42 mW, respectively. Ricardo Vergaz

Keywords: thin-film solar cell; C60; perovskite; CZTSe Received: 1 May 2021 Accepted: 7 June 2021 Published: 9 June 2021 1. Introduction Publisher’s Note: MDPI stays neutral More than 80% of the world’s current energy demands are satisfied by exhaustible with regard to jurisdictional claims in fossil fuels. It is already having significant impacts on human health and the global published maps and institutional affil- environment. The world has been on the verge of irreversible climate change for years. iations. Moreover, global energy demands have been predicted to be doubled by the year 2050 [1]. The quest for a realistic solution is on the priority list for the research community. On the other hand, organic–inorganic hybrid materials, particularly the perovskite family, have shown great potential for industrial applications in field-effect transistors, solar cells, Copyright: © 2021 by the authors. light-emitting diodes, biochemical sensors, and photon detectors [2–4]. Licensee MDPI, Basel, Switzerland. In the last decade, the power-conversion efficiency (PCE) of halide perovskite This article is an open access article (CH3NH3PbX3, X = Cl, Br or I)-based thin film photovoltaic devices has been improved dra- distributed under the terms and matically [5–8]. Organic metal halide materials have shown excellent properties for photon conditions of the Creative Commons harvesting for solar cells, such as a low and a large excitation diffusion length [9]. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ The optical band gap of organic metal perovskite (CH3NH3PbI3; MAPbI3) is about 1.5 eV. 4.0/). Therefore, this material favors efficient carrier generation and transport to electrodes. It

Molecules 2021, 26, 3516. https://doi.org/10.3390/molecules26123516 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 3516 2 of 14

can harvest sunlight radiation from the (300 nm) to the infrared region (800 nm) by adequately adjusting the composition of the three-dimensional perovskite. Stranks et al. reported a large range length of excitation diffusion greater than 1 mi- crometer in the mixed halide perovskite [10] Miyasaka et al. studied organo-metal halide perovskite solar cells with a mesoporous TiO2 structure. In 2009, they used CH3NH3PbI3 and CH3NH3PbBr3 nano-crystals as the active absorbing materials, and achieved an effi- ciency of 3.8% [11]. In 2012, Kim et al. developed fully solid-state perovskite solar cells 0 0 with a fluorine-doped oxide (FTO)/mesoporous TiO2/perovskite/2,2 ,7,7 -Tetrakis(N,N- diphenylamino)-9,9-spirobifluorene (Spi- roOMeTAD)/gold structure, achieving an effi- ciency greater than 9% [12]. Jeng et al. investigated the first inverted planar structure of a perovskite solar cell, and a similar device structure was used in an [13]. This inverted structure was typically represented as indium tin oxide (ITO)/poly (3,4-ethylenedioxythiophene):poly (styrene sulfonate)(PEDOT:PSS)/perovskite/(6,6)-phenyl-C61-butyric acid methylester (PCBM)/silver. The PEDOT:PSS has been generally used as a hole-transport material (HTM). The conventional and inverted structures could be distinguished by the direc- tions of the transport of carriers. However, the traditional hole-transport material of PEDOT:PSS was somewhat acidic. It could corrode the ITO and reduced the stability of the devices [14,15]. In practice, FTO has been developed to be used to replace ITO as the transparent conducting oxide (TCO) when a post-annealing process in air is required for the thin film structures. FTO could be deposited on top of an optical glass-substrate, such as in dye-sensitized solar cells or perovskite-based solar cells, where a TiO2 layer is needed on top of the TCO and will require thermal annealing treatment. The ITO’s electrical properties can be degraded in the presence of oxygen at relatively high temperatures, such as around 500 ◦C. On the other hand, FTO is much more stable in such high annealing temperature conditions. Metal–selenide materials have received considerable attention owing to their excellent optical characteristics and electronic properties. These materials are very attractive for ap- plications in quantum dots [16–19], photovoltaic devices and gas sensors [20–22]. Todorov et al. investigated CZT(S,Se) as an absorber layer in Kesterite solar cells. This lower-cost and earth-abundant material exhibited a power conversion efficiency above 11% [23]. In addition, Cd-free kesterite-based Cu2ZnSnSe4 (CZTSe)/In2S3 could be fabricated by chem- ical spray pyrolysis [24]. An aqueous solution has also been developed as the HTM layer in halide photovoltaic solar cells [25]. Cu2ZnSn(SxSe1-x)4 has emerged as a promising candidate for scalable photovoltaic applications, and its band gap ranges between 1.0 eV and 1.5 eV, depending on the value of x [26,27]. However, few investigations of metal– selenide as a carrier-transport material in perovskite solar cells have been reported [28,29]. In addition, fullerene C60 is an exciting all-carbon molecule [30]. Wojciechowski et al. studied an efficient perovskite solar cell by using it as an n-type compact layer for normal n-i-p structures. They suggested that the solar cells with C60 carrier charge selective layers could preserve more photocurrent due to increased electronic coupling. It also decreased the non-radiative decline on contact under forward bias [31]. Warner et al. also evaluated the nanostructure in copper phthalocyanine-C60 solar cell blends. Their solar cell’s optimal efficiency could be improved by an alternative film growth procedure [32–34]. In this study, CH3NH3PbI3 perovskite solar cells have been prepared using CZTSe as a hole-transport layer. C60, an n-type semiconductor material, was vacuum evaporated to be the electronic; transport layer (ETL). The solar cells’ photo-electronic characteristics were investigated using a Keithley 2420 programmable source meter system under an irradiation power density of 1000 W/m2. The purpose of this study has been to understand the effects of thin film thicknesses and energy level alignments in multilayered structures. The processing parameters should also play important role in improving the of next generation perovskite solar cells. A successful development in the field could help to pave the way for new thin-film solar cells to contribute to at least part of the energy demand. This would indeed be beneficial to the global environment. Molecules 2021, 26, x FOR PEER REVIEW 3 of 14

in the field could help to pave the way for new thin-film solar cells to contribute to at least

Molecules 2021, 26, 3516 part of the energy demand. This would indeed be beneficial to the global3 of 14 environment.

2. Results and Discussion

2. ResultsFigure and Discussion1 shows the corresponding energy levels of the Ag/C60/CH3NH3PbI3 (MAPbI3)

perovskiteFigure1 shows/Cu2ZnSnSe the corresponding4 (CZTSe energy)/Mo/FTO levels of multilayered the Ag/C 60/CH photovoltaic3NH3PbI3 (MAPbI structures3) [35–37] on perovskite/Cuglass substrates2ZnSnSe that4 (CZTSe)/Mo/FTO have been investigated multilayered in this photovoltaic study. The structures proposed [35–37 cell] structure can onbe glass improved substrates by that varying have been different investigated material in this layer study. properties The proposed, such cell as structure thickness, carrier con- can be improved by varying different material layer properties, such as thickness, carrier concentration,centration, and defectdefect density. density. It has It has been been noted noted that holes that may holes travel may more travel favorably more favorably than thanelectrons electrons.. The The device device performancesperformances are are also also highly highly influenced influenced by the different by the thin different filmstacks, stacks, as as well well as as the electronicelectronic properties properties at the at interfaces. the interfaces. Nevertheless, Nevertheless, the energy the energy level leveldiagram diagramss indeed indeed suggest suggesteded a truea true heterojunction heterojunction planar planar thin-film thin solar-film cell. solar cell.

Figure 1. The energy level diagrams of Ag/C60/MAPbI3/CZTSe/Mo/FTO multilayered thin-film Figure 1. The energy level diagrams of Ag/C /MAPbI /CZTSe/Mo/FTO multilayered thin-film solar cell on glass substrate. 60 3 solar cell on glass substrate.

TheThe top-view top-view SEM SEM micrographs micrographs of theof the CZTSe CZTSe films films on Mo/FTOon Mo/FTO glass glass substrates, substrates, following the followingthermal annealing the thermal process annealing at the process various at thetemperatures various temperatures,, are displayed are displayedin Figure 2 in. It has been clearly Figureshown2. that It has the been grain clearly size increased shown that with the the grain increasing size increased annealing with thetemperature. increasing A crystallized and annealing temperature. A crystallized and dense CZTSe film was observed when the dense CZTSe film was observed when◦ the annealing temperature was exceeding 300 °C . However, annealingin Figure temperature 2a, the CZTSe was thin exceeding film surface, 300 C. However,annealed in at Figure 300 °C2a,, has the CZTSeappeared thin with film relatively smaller surface, annealed at 300 ◦C, has appeared with relatively smaller crystalline grains. Some crystalline grains. Some defect pinholes can be clearly observed. The defect phenomena were likely defect pinholes can be clearly observed. The defect phenomena were likely caused by thecaused incomplete by the sintering incomplete of the sintering film material. of theOn film the material. other hand, On Figurethe other2b shows hand, larger Figure 2b shows larger crystallinecrystalline grains grains with with less less defect defect pinholes pinholes for the for CZTSe the CZTSe thin film thin that film had that been h annealedad been annealed at at a higher atemperature higher temperature of 400 of °C 400. In◦C. addition In addition,, after after the the highest highest 500 500 °C◦C annealing annealing temperaturetemperature process for one processhour (see for oneFigure hour 2c (see), the Figure thin 2filmc), the surface thin film reveal surfaceed the revealed largest the crystalli largestne crystalline grains, with almost undis- grains,cernible with defect almost pinholes undiscernible at the defectsame magnification pinholes at the samescale. magnification scale.

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(a)

(b) (c)

FigureFigure 2. 2. TopTop-view-view SEM SEM micrograph micrographss of of the the sputtered sputtered CZTSe CZTSe films films that that were were thermally thermally anneal annealeded at at (a ()a )300 300 °C◦C,, (b ()b 400) 400 °C◦,C, andand (c ()c 500) 500 °C◦C..

AA low lowerer annealing annealing temperature temperature would would produce produce thin thin films films with withlots of lots defects of defects or pin- or holespinholes on the on surface the surface.. It is known It is known that high that-quality high-quality perovskite perovskite film is filmcritical is criticalfor the forfabri- the cationfabrication of multilayered of multilayered thin-film thin-film photovoltaic photovoltaic device devices.s. The X- Theray X-raydiffraction diffraction (XRD)(XRD) pat- ternspatterns of the of MAPbI the MAPbI3 perovskite3 perovskite films films on onCZTSe CZTSe HTM HTM films films,, following following thermal thermal annealing annealing atat the the various various temperatures temperatures,, are examinedexamined inin Figure Figure3 .3 The. The spectra spectra of of the the MAPbI MAPbI3 perovskite3 perov- skitefilms films that that were were deposited deposited on the on CZTSe the CZTSe HTM HTM films films that were that thermallywere thermally annealed annealed at 300 at◦C , ◦ ◦ 300400 °C ,C, 400 and °C , 500and 500C all°C showedall showed one one dominant dominant CH CH3NH3NH3PbI3PbI33 (110)(110) diffraction peak peak at at 14.3°,14.3◦ indica, indicatingting a strong a strong preferential preferential and and prominent prominent orientation orientation in the in crystal the crystal growth growth di- rection.direction. When When the theannealing annealing temperature temperature was was increased, increased, the the full full-width-width at athalf half-maximum-maximum (FWHM(FWHM)) of of the the CH CH3NH3NH3PbI3PbI3 (3110(110)diffraction) diffraction peak peak was was slightly slightly improved. improved. A A smaller smaller FWHMFWHM sugges suggestedted a a larger larger averaged averaged domain sizesize withwith improvedimproved perovskite perovskite crystallization crystalliza- tionin thein the multilayer multilayer structures structures of theof the thin-film thin-film solar solar cells. cells.

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FigureFigure 3. 3.XRD XRD spectra spectra of of MAPbI MAPbI3/CZTSe/Mo/FTO3/CZTSe/Mo/FTO multilayered multilayered thin thin-film-film structures structures after thermal annealingannealing at at various various temperatures. temperatures.

TheThe function function of the the MAPbI MAPbI3 perovskite3 perovskite film filmis as isan as active an active photovoltaic photovoltaic layer, and layer, the CZTSe and thefilm CZTSe provides film the provides hole-transporting the hole-transporting layer. The CZTSe layer. thin film The layer CZTSe and thin the MAPbI film layer3 thin and film thelayer MAPbIcan be 3annealedthin film and layer sintered can bealtogether. annealed Their and energy sintered band altogether. gaps are similar Their for energy trapping band light. gaps As a areresult, similar this fortrapping trapping of light light. operates As a result,to form this pairs trapping of an excited of light elec operatestron and an to associated form pairs electron of an - excitedhole for electron subsequent and anrecombination. associated electron-holeIt can affect the for photo subsequent-electronic recombination. power conversion It can efficiency affect theand photo-electronic further correlations power between conversion films and efficiency the solar cells’ and multilayer further correlations structures. between films and theFigure solar 4 cells’ shows multilayer the room structures.-temperature photoluminescence (PL) spectra of the MAPbI3 perovskite films that were deposited on CZTSe/Mo/FTO glass substrates following the Figure4 shows the room-temperature photoluminescence (PL) spectra of the MAPbI 3 perovskiteannealing filmstemperature that were at deposited300 °C , 400 on °C CZTSe/Mo/FTO, and 500 °C . The glass corresponding substrates followingemission peak the annealingwas clearly temperature observed at ~ 3001.08◦ C,eV. 400 The◦C, photoluminescence and 500 ◦C. The corresponding was measured emission using fluores- peak wascence clearly spectrophotometer observed at ~1.08 equipment eV. The photoluminescence (Hitachi F-7000). wasThe measuredintensity usingof the fluorescencePL spectrum spectrophotometerpeak is related to equipmentthe lifetime (Hitachi of the F-7000). in The the intensityMAPbI3 perovskite of the PL spectrum film. The peak exciton is relatedappears to when the lifetime a mobile of theconcentration exciton in theof energy MAPbI in3 perovskite a crystal is film.formed The by exciton an excited appears elec- whentron aand mobile an associated concentration electron of energy-hole. As in athe crystal optical is formedstimulation by an was excited intensified electron during and an the associatedexperiments, electron-hole. the As theincrease opticald in stimulation number and was the intensified electron during-hole pairs the experiments, could cancel theeach excitons other increasedout. The current in number density and– thevoltage electron-hole (J–V) characteristics pairs could were cancel evaluated each other using out. a TheKeithley current 2420 density–voltage programmable (J–V) source characteristics meter under were irradiation evaluated from using a 1000 a KeithleyW xenon 2420lamp. programmableThe system forward source scan meter rate under was 0.1 irradiation V/s. The irradiation from a 1000 power W xenon density lamp. on the The surface system of forwardthe solar scan cell ratesample was was 0.1 V/s.nominally The irradiation calibrated power to 100 densitymW/cm on2. As the the surface annealing of the temper- solar 2 cellature sample was wasincreased nominally from calibrated300 °C to 500 to 100 °C, mW/cmthe PL intensity. As the of annealing the MAPbI temperature3 perovskite was film ◦ ◦ increasedalso beco fromme in 300creasedC to. 500ThisC, indicat the PLed intensity that the ofdecomposition the MAPbI3 perovskite rate of excitons film also at the become inter- increased.face between This the indicated MAPbI3 that material the decomposition and the CZTSe rate layer of excitons was quickly at the achieved interface. The between carrier thelifetime MAPbI3 is materialgenerally and limited the CZTSe by surface layer-related was quickly nonradiative achieved. recombination The [38]. The is generallyMAPbI3/CZTSe limited by films surface-related were grown nonradiative by annealing recombination, which resulted [38]. Thein ohmic MAPbI contact3/CZTSe for- filmsmation, were and grown the low by annealing,contact resistance which resultedof the CZTSe in ohmic film contactwith the formation, back electrode and theMo lowfilm. contactThe stability resistance of MAPbI of the3 perovskite CZTSe film solar with cells the is back related electrode to not only Mo film.selective The contact stability mate- of MAPbIrials, but3 perovskite also the MAPbI solar cells3 perovskite is related’s constitutional to not only selective element contacts and morph materials,ology. but In alsoaddi- thetion, MAPbI MAPbI3 perovskite’s3 perovskite constitutionalmaterials are sensitive elements to and moisture morphology. and can In be addition, degraded MAPbI rapidly.3 perovskiteCZTSe thin materials film can are be sensitivean ideal HTM to moisture candidate and to can be beincorporated degraded rapidly.into the CZTSebulk MAPbI thin 3 filmperovskite can be an layer ideal to HTMimprove candidate the device to be’s stability incorporated [39]. into the bulk MAPbI3 perovskite layer to improve the device’s stability [39].

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FigureFigure 4. 4.RoomRoom-temperature-temperature PL PL spectra spectra of of MAPbI MAPbI3 deposited3 deposited on on CZTSe/Mo/FTO CZTSe/Mo/FTO glass glass substrates substrates followingfollowing the the annealing annealing treatment treatmentss at at 300, 300, 400, 400, and and 500 500 °C◦C..

FigureFigure 55 displaysdisplays an an example example of of t thehe cross cross-sectional-sectional scanning scanning electron electron micrograph micrograph (SEM(SEM)) of of the the Ag/C Ag/C60/MAPbI60/MAPbI3/CZTSe/Mo/FTO3/CZTSe/Mo/FTO multilayered multilayered thin-film thin-film structures structures on a glass on a ◦ substrateglass substrate.. This sample This samplewas annealed was annealed at 500 °C at 500duringC duringits preparation its preparation.. These thin These films thin exhibitfilms exhibitmainly mainly smooth smooth interface interfaces,s, free free of pinholes, of pinholes, outgrowth outgrowth orientations orientations and and micro micro cracks.cracks. P Particulatesarticulates originate originate from from a a target target material viavia thethe mechanismmechanism of of splashing splashing [29 [29].]. In Inthe the experiments, experiments it, it was was found found that that the the distribution distribution density density of of particulates particulates could could be be signifi- sig- ◦ nificantcantlyly reduced reduced by by the the thermal thermal annealing annealing process. process. The The 500 500C °C temperature temperature treatment treatme wasnt wassufficiently sufficient highly high to produce to produce a clean, a clean, smooth smooth surface. surface The. SEMThe SEM cross-sectional cross-sectional image image clearly clearlyrevealed revealed a densely a densely packed packed columnar columnar structure. structure. After all,After Figure all, Figure6 illustrates 6 illustrate the plotteds the current density–voltage (J–V) curves for the Ag/C /MAPbI /CZTSe/Mo/FTO multilay- plotted current density–voltage (J–V) curves for the 60Ag/C60/MAPbI3 3/CZTSe/Mo/FTO mul- 2 tilayeredered thin-film thin-film solar solar cells cellswith thewith various the various thermal thermal annealing annealing processes processes under 100 under mW/cm 100 illumination (AM 1.5G). In addition, the summarized photovoltaic characteristics of these mW/cm2 illumination (AM 1.5G). In addition, the summarized photovoltaic characteris- thin-film solar cell devices are listed in Table1. The photo-electronic power-conversion tics of these thin-film solar cell devices are listed in Table 1. The photo-electronic power- efficiency was improved from 12.6% to 14.2% as the annealing temperature was increased conversion efficiency was improved from 12.6% to 14.2% as the annealing temperature from 300 ◦C to 500 ◦C, mainly owing to the increase in the number of photon-generated was increased from 300 °C to 500 °C, mainly owing to the increase in the number of pho- carriers that were extracted and injected into the electrode because of the series resistance ton-generated carriers that were extracted and injected into the electrode because of the (Rs) in the device. The solar cell device exhibited a lower series resistance of 18.5 Ω when series resistance (Rs) in the device. The solar cell device exhibited a lower series resistance its HTM film was annealed at the higher temperature of 500 ◦C. This is depicted by the of 18.5 Ω when its HTM film was annealed at the higher temperature of 500 °C . This is smoothness of the interface between the multilayered films of MAPbI3 and CZTSe, which depicted by the smoothness of the interface between the multilayered films of MAPbI3 resulted in good coverage. As already indicated in Table1, the annealing at 300–500 ◦C and CZTSe, which resulted in good coverage. As already indicated in Table 1, the anneal- reduced the series resistance of the devices. In addition, the open-circuit voltage (V ) ing at 300–500 °C reduced the series resistance of the devices. In addition, the open-circuitOC increased from 0.95 V to 1.07 V. The short-circuit current density (Isc) slightly improved voltagefrom 19.57 (VOC mA/cm) increased2 to19.69 from mA/cm 0.95 V2 . to Furthermore, 1.07 V. The the short fill factor-circuit (FF) current was good density at 67–68%. (Isc) 2 2 slightlyThe fill improved factor is defined from 19.57 as the mA/cm ratio of the to maximum19.69 mA/cm electric. Furthermore, output power the Pmax fill factor of the ( solarFF) wascell good to the at product 67–68%. of The the fill open-circuit factor is defined voltage as and the the ratio short-circuit of the maximum current atelectric the maximum output powerelectric Pmax power of the output. solar cell That to is,the it product is the maximum of the open power-circuit of voltage the rectangle and the in short the- current-circuit currentvoltage at characteristic the maximum plot. electric In fact, power the fill output. factor isThat strongly is, it is affected the maximum by series power resistance of the and rectangleshunt resistance. in the current It is- commonvoltage characteristic to use only fill plot. factor In fact, to simultaneouslythe fill factor is strongly summarize affected these bytwo series effects. resistance Any increaseand shunt in resistance. series resistance It is common or decrease to use in only shunt fill resistance factor to simultane- can reduce ouslythe fill summarize factor, which these in two turn effects. reduces Any the increase conversion in series efficiency. resistance The equivalent or decrease circuit in shunt of an resistanceideal solar can cell reduce can be the mathematically fill factor, which modeled in turn [40 reduce]. Thes photo-generated the conversion efficiency. current density The equivalentis simulated circuit by a of constant an ideal current solar cell source, can whichbe mathematically provides a constant modeled current [40]. The under photo light- generatedillumination. current However, density the is simulated FF cannot reachby a constant 100% due current to the source fact that, which the J–V provides curve can a constant current under light illumination. However, the FF cannot reach 100% due to the

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fact that the J–V curve can never be rectangular. It has been noted that most model equa- neverfact that be rectangular.the J–V curve It can has never been notedbe rectangular. that most I modelt has be equationsen noted that for FFmost are model somewhat equa- tions for FF are somewhat empirical. The underlying factors may sometimes have a com- empirical.tions for FF The are underlying somewhat factorsempirical. may The sometimes underlying have factors a competitive may sometimes relationship have a with com- petitive relationship with each other. Their interactions determine how the current flows eachpetitive other. relation Theirship interactions with each determine other. Their how interaction the currents determine flows in the how device. the current The FF flows also in the device. The FF also decreases with an increased cell area. Therefore, these factors decreasesin the device. with T anhe increased FF also decrease cell area.s with Therefore, an increas theseed factorscell area. often There interactfore, these with factors each often interact with each other in a complicated way. In practice, can otheroften in interact a complicated with each way. other In in practice, a compl solaricated cell way. efficiency In practice can, besolar expressed cell efficiency by three can be expressed by three important parameters: open-circuit voltage, short-circuit current, importantbe expressed parameters: by three open-circuit important voltage,parameter short-circuits: open-circuit current, voltage, and fill short factor.-circuit As a current, result, and fill factor. As a result, to improve the efficiency of solar cells, we must increase the toand improve fill factor. the efficiency As a result of, solarto improve cells, we the must efficiency increase of thesolar open-circuit cells, we must voltage increase and the the open-circuit voltage and the short-circuit current, such as the photocurrent, a fill factor, in short-circuitopen-circuit current, voltage suchand the as theshort photocurrent,-circuit current, a fill such factor, as the inorder photocurrent, to reduce a seriesfill factor, resis- in order to reduce series resistance and leakage current. The Pmax was found to be up to tanceorder and to reduce leakage ser current.ies resistance The Pmax and was leakage found current to be up. The to 1.42Pmax mW was. The found improvement to be up to 1.42 mW. The improvement in photovoltaic performance may be attributed to the increase in1.42 photovoltaic mW. The improvement performance mayin photovoltaic be attributed performance to the increase may inbe theattributed speed of to the the excitonincrease in the speed of the exciton decomposition, such that the electrons are separated and ex- decomposition,in the speed of such the thatexciton the decomposition, electrons are separated such that and the extracted electrons quickly are separated to the Mo/FTO and ex- tracted quickly to the Mo/FTO glass substrates. glasstracted substrates. quickly to the Mo/FTO glass substrates.

Figure 5. Cross-sectional SEM micrograph of Ag/C60/MAPbI3/CZTSe/Mo/FTO multilayered struc- FigureFigure 5. 5. Cross-sectionalCross-sectional SEM micrograph micrograph of of Ag/C Ag/C60/MAPbI/MAPbI3/CZTSe/Mo/FTO/CZTSe/Mo/FTO multilayered multilayered struc- tures on glass substrate. 60 3 structurestures on glass on glass substrate. substrate.

Figure 6. J–V curves for the Ag/C60/MAPbI3/CZTSe/Mo/FTO multilayered thin-film solar cells with FigureFigure 6. 6.J–V J–V curves curves for for the the Ag/C Ag/C60/MAPbI3/CZTSe/Mo/FTO/CZTSe/Mo/FTO multilayered multilayered thin thin-film-film solar solar cells cells with various thermal annealing processes60 under3 100 mW/cm2 illumination (AM 1.5G). withvarious various thermal thermal annealing annealing processes processes under under 100 100mW/cm mW/cm2 illumination2 illumination (AM (AM 1.5G 1.5G).).

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Table 1. Photovoltaic characteristics of the Ag/C60/MAPbI3/CZTSe/Mo/FTO solar cells following

Tablethe CZTSe 1. Photovoltaic thermal annealing characteristics treatment of the at Ag/Cthe various60/MAPbI temperatures.3/CZTSe/Mo/FTO solar cells following the CZTSe thermal annealing treatment at the various temperatures. Annealing Tempera-Annealing Voc Jsc FF Eff Rs Pmax Voc 2 Jsc FF Eff Rs Pmax tureTemperature.. (V) (mA/cm ) 2(%) (%) (Ω) (mW) ◦ (V) (mA/cm ) (%) (%) (Ω) (mW) (°C ) ( C) 300 3000.95 0.9519.57 19.5767.77 67.7712.6 12.620.1 20.1 1.26 1.26 400 4001.01 1.0119.63 19.6367.58 67.5813.4 13.419.3 19.3 1.34 1.34 500 1.07 19.69 67.39 14.2 18.5 1.42 500 1.07 19.69 67.39 14.2 18.5 1.42

TheThe multilayeredmultilayered structural parameters of of the the Ag/C Ag/C6060/MAPbI/MAPbI3/CZTSe/Mo/FTO3/CZTSe/Mo/FTO thin- thin-filmfilm solar solar cells cells were were also alsoinvestigated investigated in the in experiments. the experiments. For Forexample, example, Figure Figure 7 shows7 shows the the current density–voltage curves for the thin-film solar cells with the CZTSe HTM films current density–voltage curves for the thin-film solar cells with the◦ CZTSe HTM films of ofdifferent different thicknesses. thicknesses. All All the the CZTSe CZTSe films films were were annealed annealed at at 500 500 °CC,, and the photovoltaicphotovoltaic 2 cellcell teststests werewere measured measured under under 100 100 mW/cm mW/cm2illumination illumination (AM (AM 1.5G). 1.5G) The. The solar solar cell cell photo- pho- voltaic characteristic parameters of the Ag/C60/MAPbI3/CZTSe/Mo/FTO thin-film solar tovoltaic characteristic parameters of the Ag/C60/MAPbI3/CZTSe/Mo/FTO thin-film solar cells are summarized in Table2. The PCE value of the Ag/C 60/MAPbI3/CZTSe/Mo/FTO cells are summarized in Table 2. The PCE value of the Ag/C60/MAPbI3/CZTSe/Mo/FTO solar cells was found to be improved from 11.7% to 14.2% as the thickness of the CZTSe solar cells was found to be improved from 11.7% to 14.2% as the thickness of the CZTSe HTM film increased from 50 nm to 200 nm. However, further increasing the thickness of HTM film increased from 50 nm to 200 nm. However, further increasing the thickness of the CZTSe HTM film to 300 nm would reduce the PCE to 12.9%, owing to the increased the CZTSe HTM film to 300 nm would reduce the PCE to 12.9%, owing to the increased series resistance effect in the solar cell, as shown in Table2. series resistance effect in the solar cell, as shown in Table 2.

Figure 7. J–V curves for the Ag/C60/MAPbI3/CZTSe/Mo/FTO thin-film solar cells with the various Figure 7. J–V curves for the Ag/C60/MAPbI3/CZTSe/Mo/FTO thin-film solar cells with the various CZTSe film thicknesses, also under 100 mW/cm2 illumination (AM 1.5G). CZTSe film thicknesses, also under 100 mW/cm2 illumination (AM 1.5G).

Table 2. Photovoltaic characteristics of the Ag/C60/MAPbI3/CZTSe/Mo/FTO solar cells with various CZTSe film thicknesses, all thermally annealed at 500 ◦C.

CZTSe (HTM) Voc Jsc FF Eff Rs Pmax Thickness (V) (mA/cm2) (%) (%) (Ω) (mW) (nm) 50 1.02 19.56 58.64 11.7 21.4 1.17 100 1.03 19.59 63.43 12.8 20.2 1.28 150 1.05 19.61 65.56 13.5 19.8 1.35 200 1.07 19.69 67.39 14.2 18.5 1.42 250 1.05 19.62 65.53 13.5 19.6 1.35 300 1.04 19.60 63.22 12.9 20.6 1.29

Molecules 2021, 26, x FOR PEER REVIEW 9 of 14

Table 2. Photovoltaic characteristics of the Ag/C60/MAPbI3/CZTSe/Mo/FTO solar cells with various CZTSe film thicknesses, all thermally annealed at 500 °C .

CZTSe (HTM) Voc Jsc FF Eff Rs Pmax Thickness (V) (mA/cm2) (%) (%) (Ω) (mW) (nm) 50 1.02 19.56 58.64 11.7 21.4 1.17 100 1.03 19.59 63.43 12.8 20.2 1.28 150 1.05 19.61 65.56 13.5 19.8 1.35 200 1.07 19.69 67.39 14.2 18.5 1.42 Molecules 2021, 26, 3516 9 of 14 250 1.05 19.62 65.53 13.5 19.6 1.35 300 1.04 19.60 63.22 12.9 20.6 1.29

OnOn thethe otherother hand,hand, thethe fillfill factorfactor ofof thethe thin-filmthin-film solarsolar cellcell waswas increasedincreased fromfrom 58.64%58.64% toto 67.39%67.39% asas thethe filmfilm thickness thickness was was increased increased from from 50 50 nm nm to to 200 200 nm. nm. However, However, when when the the thicknessthickness furtherfurther increasedincreased toto 300300 nm,nm, thethe fillfill factorfactor degradeddegraded toto aa lowerlower valuevalue atat 63.22%.63.22%. TheThe FF FF is is an an important important parameter parameter that that determines determines the powerthe power conversion conversion efficiency efficiency of a solar of a cell.solar As cell. discussed As discussed earlier, earlier, there arethere several are several factors factors that can that significantly can significantly influence influence FF. They FF. evenThey interact even interact with each with other. each Aother. more A fundamental more fundamental investigation investigation can be further can be carriedfurther outcar- toried resolve out to the resolve complicated the complicated relationship. relationship In our measurement. In our measurement results, the resul seriests, resistance the series valueresistance declined value from declined 21.4 Ω fromto 18.5 21.Ω4 ,Ω while to 18 the.5 Ω, HTM while film the thickness HTM film increased thickness from increased50 nm tofrom 200 50 nm. nm Nevertheless, to 200 nm. Nevertheless, a further increase a further in filmincrease thickness in film to th 300ickness nm increasedto 300 nm the in- seriescreased resistance the series value resistance to 20.6 Ωvalue. These to 2 effects0.6 Ω. onThese the serieseffects resistance on the series of the resistance solar cells of arethe detailedsolar cells in are Table detailed2. In addition, in Table the 2. In open-circuit addition, the voltage open has-circuit been voltage shown tohas be been in the shown range to 2 ofbe 1.02–1.07 in the range V. The of 1.0 short-circuit2–1.07 V. The current short density-circuit current was maintained density was at 19.5–19.7maintained mA/cm at 19.5–. The19.7 Pmax mA/cm was2. The estimated Pmax was 1.17–1.42 estimated mW. The1.17– external1.42 mW. quantum The external efficiency quantum (EQE) efficiency spectra at(EQE different) spectra thicknesses at different are thicknesses presented are in Figure presented8. The in Figure improvement 8. The improvement seems to be more seems profound than the change in the J value. Nevertheless, the maximum EQE value was to be more profound than the changeSC in the JSC value. Nevertheless, the maximum EQE foundvalue inwas the foun 200d nm in the sample. 200 nm sample.

Figure 8. EQE spectra for the Ag/C60/MAPbI3/CZTSe/Mo/FTO thin-film solar cells with various Figure 8. EQE spectra for the Ag/C /MAPbI /CZTSe/Mo/FTO thin-film solar cells with various CZTSe film thicknesses. 60 3 CZTSe film thicknesses. Any intrinsic characteristics of semiconductor materials inevitably involve more or Any intrinsic characteristics of semiconductor materials inevitably involve more or less resistance. Therefore, series resistance will be generated in the solar cell. This is related less resistance. Therefore, series resistance will be generated in the solar cell. This is related to the junction depth, the impurity concentration of p-type, and the n-type in the region. to the junction depth, the impurity concentration of p-type, and the n-type in the region. On the other hand, any p-type or n-type’s electrode in a solar cell has another current that does not pass through the ideal p–n junction, which will cause a leakage of current. This

may be due to the recombination of electron/electron-hole pairs in the depletion zone. The internal recombination current or surface recombination current of the semicon- ductor material is an incomplete isolation of the elemental leakage current of the device. The leakage current then develops shunt-resistance due to the metal contact penetrating the p–n junction. However, the existence of the series resistance makes the short-circuit current relatively small, and the parallel resistance is not enough to reduce the open-circuit voltage. These two factors present the main causes for the decrease in the conversion efficiency of a solar cell. When the series resistance is increased, the slope of the straight line of the current–voltage curve becomes smaller. In addition, the parallel resistance is not large enough, and the slope of the straight line of the reverse direction becomes large. As to the fill factor, open-circuit voltage and short-circuit current, it can be drastically decreased, Molecules 2021, 26, 3516 10 of 14

which in turn causes a reduction in the PCE conversion efficiency. According to the current density–voltage curves and the PCE values, the optimal thickness of the CZTSe HTM thin film was recommended to be 200 nm. It has been noted that the photovoltaic stability is very important for the practical application of solar cells [25,41]. Long-term operation stability tests can be further carried out on solar cells and modules. These include indoor continuous irradiance tests, as well as outdoor irradiance tests, perhaps under simulated conditions. Additionally, an accelerated test can involve high temperatures and moisture. This is important, particularly for the perovskite solar cells. We plan to start by investigating the device’s components in encapsulated test structures. On the other hand, international standards for measuring photovoltaic modules should be followed, such as thermal cycling and humidity–freeze tests. A minimum loss in conversion efficiency is then highly desired.

3. Materials and Methods In the experiments, molybdenum (Mo) metal thin-film was sputtered on FTO-coated glass substrates (Ruilong Glass) to be used as a back electrode contact layer [42]. A relatively high metal work function provides possible ohmic contact to the absorber layer. It decreases the barrier height for the charge carrier at the back contact interface, and in conjunction with a suitable surface recombination speed, can improve solar cell performance. The FTO glass substrates were subsequently cleaned using acetone (Merck), ethanol (Sigma-Aldrich), and deionized water, each for 10 min. Molybdenum is a kind of metal element material with chemical symbol Mo. Its atomic number is 42. It is a gray transition metal, used as a back electrode layer in this solar cell’s multilayer structures. The Mo thin film was prepared by RF magnetron sputtering using a Mo target (Ultimate Materials Technology) on commercially available FTO glass substrates. The Ar flow rate and radio-frequency (RF) power were maintained at 40 sccm and 80 W, respectively. The Mo thin film’s thickness was 200 nm approximately. Moreover, the CZTSe thin film layers were deposited by RF magnetron sputtering using a CZTSe target (Ultimate Materials Technology). This time, the Ar flow rate and RF power were maintained at 30 sccm and 60 W, respectively. The CZTSe film was sputtered on Mo metal thin film as a hole-transport layer in the multilayer structures [43]. The solution-processable MAPbI3 were prepared on CZTSe films to fab- ricate an inverted perovskite solar cell. The optical, structural, and surface properties of CZTSe films could then be examined. The relationship between the performance of solar cells and the properties of CZTSe films is useful in designing a photovoltaic device. The final CZTSe thin film had a thickness of ~200 nm, although several different thicknesses were also deposited for comparison. After the sputtering, the CZTSe thin films were ◦ ◦ ◦ annealed at 300 C, 400 C, or 500 C in a tube furnace for one hour. The PbI2 (Alfa-Aesar) and MAI (Luminescence Technology) were dissolved in a co-solvent containing dimethyl sulfoxide (Emerald Scientific) and γ-butyrolactone (Sigma-Aldrich) (in volume ratio = 1:1) to constitute a perovskite precursor solution. The precursor solution was then coated onto the CZTSe films by a two-step spin-coating method at 1000 rpm and 5000 rpm for 10 and 20 s, respectively, in a glove box that was filled with highly pure nitrogen. The wet spinning thin film was quenched by dropping 50 µL of anhydrous toluene (Sigma Aldrich Corporation, St. Louis, MO, USA) at 17 s. After the process, the thin film of ◦ perovskite precursor solution was annealed at 100 C for 10 min. The MAPbI3 perovskite thin film with a thickness of ~600 nm was thus deposited. Furthermore, the C60 (fullerene) is an n-type semiconductor material. Here, its func- tion was as an electronic transport layer in the solar cell’s multilayer structures. C60 powder (Uni-Onward Co., Pingzhen District, Taoyuan City, Taiwan) was prepared on a molybde- num boat in the vapor chamber of a vacuum evaporation system. It was deposited directly on the MAPbI3 perovskite films. The C60 thin film with a thickness of 50 nm was deposited approximately. C60 can increase the strength of metal materials by alloying, plastic defor- mation and heat treatment. Due to the existence of a three-dimensional highly delocalized Molecules 2021, 26, 3516 11 of 14

electron conjugated structure in the C60 molecule, it has good optical and nonlinear optical properties [44,45]. In addition, C60 is an inexpensive and easily accessible material. The last electrode layer of silver (Ag) was deposited over the C60 n-type semiconductor material. A silver ingot (Gredmann Materials Technology) was prepared on a tungsten boat in the vapor chamber for evaporation. It was deposited above the C60 material. The silver thin film electrode had a thickness of about 100 nm. Additionally, the molybdenum/tungsten boats were connected to an external power supply and provided with a maximum current of 100 A and 90 A, respectively. The evaporation equipment model was PSE-1.5KVA. The power supply equipment mainly provided a direct current to the molybdenum boat or tungsten boat, separately. Due to the resistance effect, the temperature of the molybdenum or tungsten boats would be increased significantly. Subsequently, the C60 and the silver electrode were deposited at thicknesses of 50 and 100 nm, respectively, by the thermal vacuum evaporator equipment. The Ag/C60/MAPbI3/CZTSe/Mo/FTO multilayered thin-film solar cell samples were further covered with a shadow mask to define an active area of 0.5 × 0.2 cm2 during the C60/silver deposition step. Thus, Figure9 depicts a schematic illustration of the com- plete structure. The crystalline microstructures of the solar cell thin films were examined using a PANalytical X’Pert Pro DY2840 X-ray diffractometer (XRD) with Cu Kα radiation (λ = 0.154 nm). A field-emission scanning electron microscope (Zeiss Gemini SEM) was used to observe the surface morphology of the thin-film solar cells. The photoluminescence (PL) was measured using a fluorescence spectrophotometer (Hitachi F-7000, Tokyo, Japan). The current density–voltage (J–V) characteristics were evaluated using a Keithley 2420 programmable source meter under irradiation from a 1000 W xenon lamp. The equipment Molecules 2021, 26, x FOR PEER REVIEW 12 of 14 forward scan rate was set at 0.1 V/s. Finally, the irradiation power density on the surface of the solar cell sample was calibrated to 100 mW/cm2.

Figure 9. Schematic illustration of the multilayered structure of Figure 9. Schematic illustration of the multilayered structure of Ag/C60/MAPbI3/Cu2ZnSnSe4/ Ag/C60/MAPbI3/Cu2ZnSnSe4/Mo/FTO thin-film solar cell. Mo/FTO thin-film solar cell. 4.4. Conclusions Conclusions ThisThis paper paper presents presents the the preparation, preparation, and and investigations investigations into into the the characteristics, characteristics, of of photovoltaicphotovoltaic thin-film thin-film solar solar cells cells with with Ag/C Ag/C60/MAPbI60/MAPbI3/CZTSe/Mo/FTO3/CZTSe/Mo/FTO multilayer struc-struc- tures.tures. The The MAPbI MAPbI3 3perovskite perovskite films films were were deposited deposited on on CZTSe CZTSe HTM HTM films films using using a precursor a precur- solutionsor solution made made of PbI of2 PbIand2 MAI.and MAI. The thermalThe thermal annealing annealing process process improved improved the perovskite the perov- filmskite in film terms in ofterms its crystalline of its crystalline structure structure and domain and size.domain This size process. This ensuredprocess aensured smooth a interfacesmooth interface with good with surface good coverage surface coverage in the multilayered in the multilayered thin-film thin structures.-film structures In combi-. In nationcombination with the with designed the design filmed thicknesses film thicknesses and energy and energy level alignments,level alignments it was, it efficientwas effi- cient in reducing carrier recombination. Thus, it could result in rapid charge transition and low series resistance at the interface. From the experiments, we can conclude that the recommended solar cell device should have an active absorbing MAPbI3 perovskite film of 600 nm in thickness, a CZTSe HTM film of 200 nm in thickness, and a C60 ETL film of 50 nm in thickness, respectively. The thin-film multilayer structures have good omhic con- tact and minimal defects in their films. The thin-film solar cells were evaluated under 100 mW/cm2 illumination (AM 1.5G) for their current density–voltage curves. The photovol- taic characteristics exhibited improved the power-conversion efficiency from 12.6% to 14.2%. The open-circuit voltage was increased from 0.95 V to 1.07 V. The short-circuit cur- rent density was slightly improved from 19.57 mA/cm2 to 19.69 mA/cm2. The solar cell maintained a good fill factor at 67–68%. The device’s series resistance value was also re- duced to 18.5 Ω. Furthermore, the maximum electric output power Pmax of the solar cell was 1.42 mW. We improved the MAPbI3 perovskite nanostructured using CZTSe HTM. The CZTSe films were deposited by one-step magnetron sputtering and an- nealed at different temperatures. It has been confirmed that the CZTSe film can be used as an HTM with C60 ETL to enhance the photovoltaic characteristic parameters.

Author Contributions: Conceptualization, T.-W.C. and C.-C.T.; methodology, T.-W.C. and C.-C.T.; formal analysis, D.W.C. and L.-C.C.; investigation, G.W.; resources, D.W.C.; data curation, C.-L.Y.; writing—original draft preparation, C.-C.T.; writing—review and editing, G.W.; supervision, G.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Ministry of Science and Technology (Taiwan) under re- search contracts MOST105-2221-E-182-059-MY3 and MOST-108-2221-E-182-020-MY3. The APC was funded by CGMH support of CMRP246 and CMRPD3G0063. Institutional Review Board Statement: Not applicable.

Molecules 2021, 26, 3516 12 of 14

in reducing carrier recombination. Thus, it could result in rapid charge transition and low series resistance at the interface. From the experiments, we can conclude that the recommended solar cell device should have an active absorbing MAPbI3 perovskite film of 600 nm in thickness, a CZTSe HTM film of 200 nm in thickness, and a C60 ETL film of 50 nm in thickness, respectively. The thin-film multilayer structures have good omhic contact and minimal defects in their films. The thin-film solar cells were evaluated under 100 mW/cm2 illumination (AM 1.5G) for their current density–voltage curves. The photovoltaic char- acteristics exhibited improved the power-conversion efficiency from 12.6% to 14.2%. The open-circuit voltage was increased from 0.95 V to 1.07 V. The short-circuit current density was slightly improved from 19.57 mA/cm2 to 19.69 mA/cm2. The solar cell maintained a good fill factor at 67–68%. The device’s series resistance value was also reduced to 18.5 Ω. Furthermore, the maximum electric output power Pmax of the solar cell was 1.42 mW. We improved the MAPbI3 perovskite nanostructured photovoltaics using CZTSe HTM. The CZTSe films were deposited by one-step magnetron sputtering and annealed at different temperatures. It has been confirmed that the CZTSe film can be used as an HTM with C60 ETL to enhance the photovoltaic characteristic parameters.

Author Contributions: Conceptualization, T.-W.C. and C.-C.T.; methodology, T.-W.C. and C.-C.T.; formal analysis, D.W.C. and L.-C.C.; investigation, G.W.; resources, D.W.C.; data curation, C.-L.Y.; writing—original draft preparation, C.-C.T.; writing—review and editing, G.W.; supervision, G.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Ministry of Science and Technology (Taiwan) under research contracts MOST105-2221-E-182-059-MY3 and MOST-108-2221-E-182-020-MY3. The APC was funded by CGMH support of CMRP246 and CMRPD3G0063. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. Sample Availability: Samples not available.

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