'Bio-inspired' Carbon Hole Transporting Layer Derived from Aloe Vera

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‘Bio-inspired’ Carbon Hole Transporting Layer Derived from Aloe Vera Plant for Cost Effective Fully Printable Mesoscopic Carbon Perovskite Solar Cells Sawanta S Mali, Hyungjin Kim, Jyoti V Patil, and Chang Kook Hong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08383 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018

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ACS Applied Materials & Interfaces

‘Bio-inspired’ Carbon Hole Transporting Layer Derived from Aloe Vera Plant for Cost Effective Fully Printable Mesoscopic Carbon Perovskite Solar Cells Sawanta S. Mali, Hyungjin Kim, Jyoti V. Patil and Chang Kook Hong* *Polymer Energy Materials Laboratory, School of Advanced Chemical Engineering, Chonnam National University, Gwangju, S. Korea-500757 *Email: [email protected] (CKH)

Abstract: Herein, we introduce a new ecofriendly naturally extracted cross-linked carbon nanoparticles as a hole transporting layer (C-HTL) prepared by ‘ancient Indian method’ for carbon based printable mesoscopic perovskite solar cells (C-PSCs), which is low-cost so far used

for

fully

printable

PSCs.

The

fabricated

PSCs

having

Glass/FTO/mp-

TiO2/ZrO2/perovskite/AV-C configuration exhibited current density (JSC) of 20.50 ±0.5 mAcm-2, open circuit voltage (VOC) of 0.965 ±0.02 V and fill factor (FF) of 58 ± 2%, resulting in 12.3 ±0.2 % power conversion efficiency (PCE) for MAPbI3 perovskite absorber. The aloe-vera processed carbon C-HTL based PSCs yields up to 12.50 % power conversion efficiency and 15.80 % efficiency for conventional spiro-MeOTAD based HTM. The air and moisture stability >1000 h at >45% relative humidity (RH) for cross-linked AV-C nanoparticles based PSCs. This stability is very high compared to conventional spiroMeOTAD HTM based PSCs. The prepared carbon nanoparticles facilitate an excellent penetration of perovskite absorber in triple-layer-based scaffold which enables a high-quality perovskite crystal results in high PCE. This novel bio-inspired AV-C cross-linked nanoparticle based natural carbon C-HTL layer is low-cost till date. We believe that this technique would be suitable for fully printable and helpful towards air-moisture-stable PSCs. Keywords: Bio-inspired cross-linked carbon nanoparticles, low cost hole extraction layer, air-stable, humidity-stable, fully printable perovskite solar cells.

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1. Introduction The perovskite solar cells (PSCs) could be the most promising game changer alternative material for high-cost and toxic silicon photovoltaic technology. Although the PSCs demonstrated >22 %1-7 power conversion efficiency (PCE) through different compositions of MAPbX/FAPbX (herein MAPbX stands for methylammonium lead halide and FA stands for formamidinium lead halide) and mixing of variety of cations such as RbI, CsI and using different hole transporting layers (HTLs). However, the stability and preparation of HTL is most tedious issue. As per as conventional HTMs concern, they show most promising conversion efficiency compared to other HTMs such as PCPDTBT,9 CuI, 10 NiO, 11 Cu2O,12,13 CuSCN14,15 etc. However, beyond 22% efficiency has only been possible for expensive hole transporting materials (HTMs) such as poly [bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA)7 and (2,2’,7,7’-tetrakis(N,N-pdimethoxyphenylamino)-9,9’-spirobifluorene (spiroMeOTAD).

The

precise

doping

of

4-tertbutylpyridine

(TBP),

bis(trifluoromethane)sulfonimide lithium salt (LiTFSI), and FK 209 Co(III) – TFSI salt (FK209) are still remained. The uniform deposition of these HTMs via spin coating method and cost these HTMs hampers the low cost large scale production technique. Although PTAA based PSCs showed good air-stability but it suffers from high cost. In general, the PTAA and spiro-MeOTAD have respective cost 3000 USD/gm and 500 USD/gm which is higher than any other HTM materials for PSCs.16,17 Now it is well known that, the self-stability of perovskite material has been extended up to several limits via different aspects such as cation doping (CsI, RbI) 18,19 and tin (Sn) substitution by Yang, Z. et al. 20, cross-linking additives21, pin-hole compact perovskite layer by unique composition of Pb(CH3CO2)2·3H2O, PbCl2, and MAI,

22

or use of 2D/3D mixed hybrid perovskite.23,24 Different aspects such as metal oxide

coating, vacuum flash technique, HTM free PSCs have been employed in to extend stability up to several hours.25-29 However, the self-degration due to high sensitivity towards air and moisture, irreversible degration of cations and hygroscopic nature of MAX or FAX (X: halide 2 ACS Paragon Plus Environment

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group such as Cl, Br, I etc.), the stability and cost issue still remained. Therefore, to protect perovskite absorbing layer from air/moisture and avoid self-degration of cations via screen printed carbon materials are the unique idea towards stability of perovskite photovoltaic technology. After the pioneer word by Mei et al.,30 many reports discussed the importance of carbon based HTL towards air and water stability.31-36 Authors demonstrated, the screen printed mp-TiO2 and ZrO2 used as ETL followed by screen printed carbon as a HTL. The fabricated fully printable hole-conductor free, mesoscopic triple-layered-based scaffold devices show 12.8% PCE with >1000 hours’ ambient air stability. Recently, we have also demonstrated the cation degration can be controlled my providing extra cations from carbon HTL while moisture stability can be improved by compact carbon sealing.37 The preparation method of such carbon based HTL is most convenient, eco-friendly and cheapest one. Therefore, carbon HTL based PSCs opens a new era towards low-cost and highly stable PSCs. Similarly, printable PSCs has been demonstrated by Han et al.38 with 6.64% PCE. However, the use of unique composition of 2D/3D perovskite junction perovskite yielded 12.9 % PCE with one-year ultrastability.23,39 Therefore, the HTM free40-46 and carbon-based HTMs electrodes47-54 for PSCs open a new interest towards air-water stability. Hu et al. developed bifunctional conjugated organic molecule 4-(aminomethyl) benzoic acid hydroiodide (AB) as an organic cation with 15.6% PCE. These devices also show high stability.55 Furthermore, more than 10 % stable PCE having 10 x 10 cm2 module via printable technique is also achievable using this carbon based HTMs.56 Zhang et al.57 used SrCl2 based perovskite absorber layer with 16 % efficiency. On the other hand, the candle soot technique yielded 11.02% efficiency.58 Therefore, development in full printable, low cost, eco-friendly and airstable HTL for PSCs is urgent need towards commercialization of PSCs at large scale. As per our best knowledge, so far commercial carbon conducting paste or graphite + carbon colloidal paste has been used for carbon PSCs (C-PSCs). This study focused on a new ‘bio-inspired’ 3 ACS Paragon Plus Environment


methodology using natural carbon extracted from aloe-vera plant (herein AV-C) with or without MAI composite as a HTL. This synthesized AV-C based C-HTL is low-cost till date and showed good air-moisture stability. We believe that, this natural AV-C cross-linked CHTL is eco-friendly and low-cost till date.

In this investigation, cross-linked AV-C nanoparticles with or without MAI composite based HTL used for fully printable mesoscopic triple-layer-based scaffold of TiO2/ZrO2/AVC PSCs. The solar cell performance and stability of these AV-C based PSCs were compared with conventional spiro-MeOTAD HTM. We have used identical device ETL configuration while we have changed different insulating (ZrO2) layer and C-HTLs. Here, we used, for twostep (spin-coated mp-TiO2 and ZrO2) and one step (for fully screen-printed mp-TiO2, ZrO2 and AV-C) method. Here we developed a fully-printable mesoscopic triple-layered PSCs based on AV-C which is cost-effective till date and stable over 1000 h in >45 % relative humidity (RH).

2. Results and discussion The bio-inspired AV-C powder sample was prepared by ‘ancient Indian method’ as shown in Fig. 1. Firstly, the Aloe Vera gel was removed from Aloe Vera leaves and dried under sunlight for 1 day (Step-I) on SS-substrate. The dried gel was subjected to oil lamp (soybean oil) as shown in Fig. 1 (Step-II) for overnight. In this process, the dried gel has been converted in to black colored powder. The black powder scratched from SS-substrate and grinded in ball milling. The black colored powder sample was washed with 2M HCl solution and DI water to remove unreacted dried aloe-vera gel and maintained pH up to 7. The washed AV-C sample was dried at 100 °C overnight and used for further process. The ball-milled fine AV-C powder was washed 1M HCl followed by annealing at 1000 °C in flow argon gas. The thermal properties of as synthesized and annealed AV-C was analyzed by thermal gravimetric analysis (TGA). Fig. S-1 represents the TGA curves of as synthesized and 1000 °C annealed 4 ACS Paragon Plus Environment

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AV-C samples. The magnified graphs of as synthesized AV-C exhibited a different step weight-loss and finally reached up to 16.62 %. In TGA curves, the weight-loss with respect to temperature can be discussed as follows: 1% from 25 °C to 320 °C, 4.18% from 320 to 620 °C and finally 9.6% from 620 to 950 °C. The initial weight loss at around 100-320 °C is observed for as synthesized AV-C. This initial weight loss occurred due to many impurities such as unreacted aloe-vera gel, moisture, hydrocarbons, sulphur compounds and degradation of amorphous carbon. In case of annealed AV-C sample, the total weight loss is negligible (~5.12 %) due to absence of amorphous carbon and proper graphitization. These results clearly revealed that, annealed samples contains more graphitization. Therefore, the fine AVC black product was annealed at 1000 °C in argon flow. The AV-C processed carbon paste was prepared by properly mixing in chlorobenzene (1:2 wt./vol.) solvent by using ball milling.

The morphology of as synthesized and annealed AV-C was evaluated by field emission scanning electron microscopic (FESEM) technique. Fig. 2a revealed the SEM image of as synthesized AV-C sample having uniform carbon nanoparticles which are cross-linked to each other. The AV-C sample annealed at 1000 °C exhibits the agglomeration of carbon nanoparticle (Fig. 2b) to form highly uniform cross-linked carbon nanoparticles. This is may be due to burning of unreacted aloe-vera gel and evaporation of moisture. As synthesized AVC sample having 55 nm average particle size which was reduced up to 45nm after sintering. However, there is no clear difference has been observed between pre and post treated samples. Therefore, these samples were further subjected to XRD measurements. Fig. 2c shows XRD patterns of as synthesized and annealed AV-C samples. The XRD analysis revealed that, the both samples show broad peaks (002) at 2=24.47° and (100) at 2=43.32°. These two peaks suggesting that both samples having C-C (sp2) bonding structure with good crystallinity. However, the intensity of annealed sample is much higher than as prepared indicates the highly crystalline layer structure of cross-linked carbon nanoparticles. It is well known that 5 ACS Paragon Plus Environment


such type of behavior possible only in graphene or reduced graphene oxide (rGO). Therefore, we have recorded Raman spectroscopy of both samples. Raman spectroscopy is powerful tool for analysis of carbon materials. Raman analysis carried out in order to elucidate the crystalline nature of both samples. The disordered peak (D-band) around 1350 cm-1 and graphitic peak (G-band) around 1588 cm-1 are corresponding to E2g mode which confirms the graphite related sp2-bonding. It is well known that, the ID/IG ratio represents the degree of structural disorder. In the present case of as synthesized AV-C and annealed AV-C sample exhibited 1.01 and 1.12 ID/IG ratio respectively which suggest that annealed AV-C sample has a high degree of graphitization. These XRD and Raman analysis indicates that the AV-C synthesized by this method is made up of turbostratic carbon structure.

The transmission electron microscopy (TEM) images of annealed AV-C cross-linked nanoparticles are shown in Fig. 2a-c. Fig. 2a-c shows typical TEM images of annealed AV-C nanoparticles revealed nearly 40-45 nm carbon nanoparticles are formed which are crossed linked to each other. Higher magnified TEM image shows the AV-C nanoparticles are regular spherical in shape. The SAED pattern revealed the annealed AV-C nanoparticles are crystalline in nature with layered stacking structure. The high resolution TEM (HRTEM) image shows d002=0.22 nm which is consistent with XRD analysis. In order to check any mechanical damage after ball milling in cross-linked AV-C carbon nanoparticles, we have recorded SEM and TEM of AV-C after 12 ball milling (Fig. S-2). Interestingly, even after 12 hours ball milling the cross-linked carbon nanoparticle architecture was intact, Fig. S-2. We have checked the resistive of the screen printed film fabricated by AV-C and commercial colloidal carbon paste and our results showed 6.99 cm2 and 78.72 cm2 respectively. For commercial carbon paste we have used PELCO® Conductive Graphite, Product No. 16053, having Sheet resistance @ 1 mil 1200 /sq. The present study is mainly focused on synthesis of carbon from naturally available aloe-vera leaf. We believe that this is promising method to 6 ACS Paragon Plus Environment

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prepare eco-friendly carbon material for solar cells therefore we mentioned the term cheapest materials. The elemental composition of the annealed AV-C has been analysed by XPS measurements. As depicted in survey spectrum of AV-C (Fig. 3d), two obvious C1s peak located at 284.6 eV and O1s peak located at 533.89 eV are observed. The high-resolution spectrum of C1s (Fig. 2e) shows three peaks located at 284.60, 286.84, and 289.21 eV, respectively C-C, C-O and C=O bounding.59-61 Oxygen is promising heteroatom that has been widely studied to modify the surface of carbon materials. Fig. 2f shows the prominent peak at 534.06 eV is identified as ether or phenol O (C-O-C or C-OH), and the other two peaks at 532.44 and 535.94 eV is related to C=O and carboxylic O(C(O)OH), respectively. The peak located at 530.33 eV is corresponds to the highly conjugated quinone or pyridone O.62,63

Initially we have fabricated hetero-structured PSCs based on spin-coated mp-TiO2. The AV-C + MAI hole transporting layer (HTL) for hetero-structured PSCs was prepared by ball-milling process as shown in Fig.4 (a-c). The AV-C + MAI ratio was optimized as per our previous report with few modifications.37 The ball-milled fine AV-C + MAI powder sample was mixed with desired amount chlorobenzene (CB) and prepared viscous paste. The MAPbI3-xClx was synthesized by two step spin coating method. More details can be found elsewhere.37 In order to prepare complete MAPbI3-xClx based hetero-structured PSCs, the paste prepared from AV-C + MAI composition was covered on to spin coated mpTiO2/MAPbI3-xClx electrode bye doctor blade technique and dried for 30 min at 100 °C. Fig. 3(d, e) shows typical cross-sectional SEM images of fabricated PSC at different magnifications. Fig. 4e shows highly magnified cross-sectional SEM micrograph of fabricated device after drying. Nearly 450 nm compact pin-hole free perovskite layer was formed between AV-C and mp-TiO2. Here, the thickness of mp-TiO2 was 200 nm which contain perovskite percolation and its crystallization. Due to this compact capping layer facilitates higher photovoltaic properties. Highly uniform AV-C+MAI HTL with ~10 m thickness 7 ACS Paragon Plus Environment


deposited on to MAPbI3-xClx perovskite layer.37 The smooth surface-interface between MAPbI3-xClx and AV-C confirmed by cross-sectional analysis. Two step perovskite method results in highly uniform compact layer with ~400-500 nm grain size perovskite absorber has been formed between mp-TiO2 and AV-C HTL (Fig. 4f). Fig. 4g represents the energy level diagram of AV-C-PSC. The device operation could be work as discussed previously instead of carbon here AV-C layer acts as HTL.37 The J-V characteristics was recorded under illumination at 100 mAcm-2. The optimized device exhibits VOC of 0.953 V, JSC of 15.22 mAcm-2, and FF of 0.50, with = 7.32% efficiency, Fig. 4h. Although, the power conversion efficiency of this device is lower than reported, but from these results we can confirm that, the developed AV-C could be used as C-HTL for C-PSCs.

Therefore, further have changed our device configuration as shown Fig. 4a. Here, we have deposited mp-TiO2 (~200 nm) ETL and ZrO2 (~200 nm) as an insulating layer by using spin coating technique. Fig. 5a shows typical cross-sectional micrograph of fabricated device using spin coated TiO2/ZrO2 and doctor blade AV-C. This device also shows formation of ~180 nm perovskite capping layer onto ZrO2 layer. Fig. 5b and c respectively show energy level diagram and schematic device configuration. In the present device configuration, when the MAPbI3-xClx absorbs the light, it generates electron in the CB having -3.75 eV energy level and hole on the VB -5.30 eV level. From energy level diagram, it is clear that, the conduction band (CB) of TiO2 (-4.10 eV) and ZrO2 (-3.4 eV) the photoexcited electrons must travel in CB of the TiO2, while hole could transfer in to AV-C layer. The optimized device exhibits VOC of 0.838 V, JSC of 18.05 mAcm-2, FF=0.35 results in =5.29 % efficiency. This low FF due to insufficient thickness of TiO2 and ZrO2, Fig. 5d.64

In order to prepare the fully printable mesoscopic triple-layer-based scaffold, the mpTiO2 and ZrO2 paste was deposited on FTO/Bl-TiO2 by screen-printing techniques and annealed for 30 min at 450 °C for each layer. Further, the AV-C paste in CB solvent was 8 ACS Paragon Plus Environment

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screen printed onto ZrO2 layer and annealed at 400 °C for 30 min. The prepared 1.2 M MAI + 1.2 M PbI2 in GBL solution was dripped on to mesoscopic triple-layer-based scaffold for proper infiltration. After proper infiltration, this perovskite loaded samples were annealed at 50 °C for 1 h in ambient condition. From cross sectional analysis at different magnification it is revealed that the AV-C processed PSCs having mp-TiO2 (~2m)/ZrO2 (~1m) and doctor blade AV-C HTL (Fig. 6a). Both interface between TiO2 and ZrO2 shows smooth (Fig. 6b). Here ZrO2 prevent electrons from photoexcited perovskite in to mp-TiO2 from reaching the back contact.30 Fig. 6c shows schematic representation of fully printable-mesoscopic triplelayered mesoscopic PSCs. Here cross-linked AV-C nanoparticles HTL having thickness >10 m was controlled by screen sprinting method as shown in Fig.6a. The cross-linked AV-C nanoparticles provides much higher pore filling results in highly crystalline MAPbI3 crystal in the scaffold architecture.64 The crystal structure of MAPbI3 has been represented in Fig. 6d. The structural properties of MAPbI3 perovskite deposited on TiO2 and ZrO2 and its optical properties was studied by XRD and photoluminescence technique (Fig. S3, S4). The XRD spectrum of mp-TiO2/perovskite exhibits the strong peak at 2=14.1° which can be assigned (110) of tetragonal MAPbI3 crystal. All samples show strong peak at 14.1° which clearly exhibited formation of highly crystalline MAPbI3 layer. Further, we have also recorded PL of MAPbI3 deposited on either single or double layered TiO2, ZrO2. The PL measurements was carried out from front side excitation. Also note that, for spin-coated (SC) sample we observed capping layer while SP sample all perovskite solution was penetrated in to oxide scaffold. All samples show peak ~775 nm which is evidence of crystalline nature of MAPbI3 phase. Interestingly all screen printed (SP) oxide based MAPbI3 sample shows drastic quenching in PL intensity which suggest that proper extraction of electrons from perovskite to oxide. Fig. 6e shows digital image of fabricated fully printable mesoscopic triple-layeredbased PSCs from top and back side. The optimized AV-C-PSC exhibits VOC of 0.985 V, JSC 9 ACS Paragon Plus Environment


of 21.65 mAcm-2, FF of 0.59 results in 12.48% efficiency. The hysteresis analysis of AV-CPSC was recorded as per standard protocol (Fig. S-5). The screen printed device having FTO/mp-TiO2/ZrO2/MAPbI3/AV-C configuration exhibits PCE values of 11.42 % and 12.75 % for the forward and reverse scans, respectively. The forward scan exhibits V OC of 0.961 V, JSC of 20.85 mAcm-2, and FF of 0.57 resulting in 11.42 % efficiency. While, the reverse scan exhibits a PCE to 12.75 % with VOC of 0.985 V, JSC of 21.22 mAcm-2 and FF of 0.61, results in 12.75 % PCE. In order to confirm the proper infiltration, we have recorded EDS line mapping. Fig. S6, S7 shows EDS line-mapping of mp-TiO2/ZrO2/C (herein C-PSC) and TiO2/ZrO2/AV-C (herein AV-C-PSC) mesoporous triple-layered based printable perovskite cells (AV-C-PSC). We have also compared commercial carbon for carbon based PSCs (C-PSCs). The respective cross-sectional micrograph and J-V plot are shown in Fig. S-8. The C-PSC exhibits VOC of 0.859 V, Jsc of 20.11 mAcm-2, FF of 0.61 results in 11.06 % efficiency.

After completion of AV-C based PSCs we investigated the photovoltaic properties in air and under RH >45%. From these results it is clear that, our developed AV-C HTL shows higher efficiency than commercial carbon ink. The cross-linked AV-C nanoparticles facilitates better pore-filling in the mesoporous ZrO2/TiO2 scaffold film.37,50 Here AV-C-PSCs also shows equal current density while lower VOC may be due to low fermi level of carbon. It is well known that, the stability is a vital factor for the onsite application of photovoltaic technology.65 Therefore, the air-stability and moisture stability of AV-C-PSCs were tested under different condition.

Stability testing under ambient condition Here we used three different types HTL such as conventional spiro-MeOTAD, commercial colloidal carbon ink and AV-C processed cross-linked carbon nanoparticles. The conventional spiro-MeOTAD HTM PSCs was prepared by as per our our previous report with 10 ACS Paragon Plus Environment

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few modification (Fig. S-9).37 After completion of spiro-MeOTAD based device, the J-V performance was recorded under 100 mWcm-2 illumination. For average solar cell parameters, we have studied minimum 20 devices with identical experimental conditions of each type and shown average photovoltaic performance. The device having spiro-MeOTAD HTM yielded a Jsc of 21.25 ± 0.5 mAcm-2, VOC of 1.046 ± 0.05 V and FF of 71 ± 2 % resulting in 15.80 ± 0.5 % PCE (Fig. 7a). Fig. 7b shows the average photovoltaic properties of AV-C processed HTL under 100 mAcm-2 illumination. The optimized AV-C-PSC exhibited Jsc of 21.50 ± 0.5 mAcm-2, VOC of 0.965 ± 0.02 V, FF of 58 ± 2% resulting in 12.30 ± 0.2 % efficiency. Fig. 7c represents the histograms of PCE of PSCs deposited by different techniques. The device performance of spin coated and screen printed devices has been summarized in Table 1. Here, the spin coated TiO2 (SC-TiO2) and doctor blade coated AV-C based PSCs show average 6.5% efficiency. While, the SC-TiO2-ZrO2 and screen printed AV-C based PSCs show average 4.5% efficiency. This low efficiency for SC-TiO2-ZrO2 sample is mainly due to lower thickness of mp-TiO2 (~200 nm). This much thickness is not enough to absorb sufficient light. Therefore, we have deposited TiO2 and ZrO2 by screen printing technique. The triple-layersbased fully printable PSCs provides an optimum surface area for the MAPbI3 perovskite absorber which can facilitates an efficient injection efficiency results in higher power conversion efficiency. Furthermore, the cross-linked AV-C provides a unique architecture for proper penetration and fast hole transportation. The stabilized PCE of conventional spiroMeOTAD and AV-C based PSCs has been represented in Fig. 7d. The stabilized devices show 15.80 % and 12.20 % efficiency respectively for spiro-MeOTAD and AV-C based PSCs. Further, we studied the internal resistivity analysis of our champion device using impedance spectroscopy (IS) and analyzed by Z-view analysis software as shown in Fig. S-10. We have also compared commercial colloidal paste based PSCs. Here, the series resistance of the perovskite device represented by Rs and the recombination resistance represented by R1 in parallel with constant phase element (CPE1). These parameters represent the AV11 ACS Paragon Plus Environment


C/Perovskite interface charge separation. On the other hand, R2 and CPE2 represents the charge interface resistivity between Perovskite/TiO2 interface.66 Here, the additional charge transport resistance (Rtr) parameter represents the electron transport resistance through TiO2 and ZrO2 insulating layer. This Rtr parameter raised due to porous ZrO2 insulating spacer layer. The Nyquist plots of the PSC are analysed by a resistance element parallel with a constant phase element (CPE). The low-frequency semicircle of AV-C based PSCs is smaller in diameter than commercial carbon based PSCs which suggests that there is limited recombination rate with low internal resistance stem from the cross-linked carbon nanoparticles. Furthermore, these cross-linked porous AV-C nanoparticles are filled by perovskite crystals that can further decreases the internal resistance of the device.

The long-term stability of all AV-C based PSCs devices were tested under ambient condition at room temperature with relative humidity (RH) varied from 30-65 % without any further encapsulation. Note that, for stability testing of spiro-MeOTAD based PSCs, the spiroMeOTAD based devices were stored in glove box (dry nitrogen) without any encapsulation and tested in ambient condition. Usually, the spiro-MeOTAD based PSCs are not moisture stable. Therefore, for spiro-MeOTAD based PSCs we have only few hours’ stability in air. For humidity testing, the humidity was maintained by homemade humidity chamber, Fig. S11. The long term stability of spiro-MeOTAD and AV-C fully printable mesoscopic PSCs, Fig. 7e. For comparison, we have also fabricated the C-PSCs based on commercial available ink. Initially the carbon ink was dried and then added desired amount of chlorobenzene. This ink was printed on our champion device instead of AV-C ink. From long term air-stability study, it is clear that, the fully printable triple layered mesoscopic AV-C based PSCs retained >85 % PCE even after 1000 hours. While spiro-MeOTAD based shows very poor efficiency after few hours. Furthermore, the stability testing under >65% RH shows very high stability of AV-C based PSCs and retained > 80% PCE after 1000 hours (Fig. S-10). Interestingly, it is 12 ACS Paragon Plus Environment

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observed that, our AV-C based devices showed very similar stability performance (Fig. 7e, Fig. S-12). These promising results revealed that better stability of the PSCs using crosslinked AV-C nanoparticle based HTL, which is a best-alternative to the traditional, addives doped high-cost HTMs.

3. Conclusions

In summary, we developed a novel eco-friendly and cost-effective HTL for PSCs which is very easy to synthesize, earth-abundant and provides high air-stability compared to conventional HTM. The controlled AV-C based PSCs exhibited 12.48% efficiency due to complete infiltration of perovskite absorber through unique cross-linked morphology of AVC. These printable cross-linked AV-C nanoparticles plays a significant role in the air-stable and moisture-stable triple-layered scaffold. The fabricated AV-C-PSCs shows 12.48% efficiency with >1000 h air-stability with negligible loss during moisture study. Although our developed natural C-HTL shows moderate PCE efficiency but it has high air-stable properties with zero commercial value which makes a cheapest perovskite solar cells towards commercialization. This novel aloe-vera processed carbon hole hole-conductor with unique synthetic method and printable feasibility makes long-life to the perovskite absorbing layer results in slow degradation of the perovskite layer. The developed method provides a key technique towards fully printable highly stable and low-cost perovskite photovoltaic technology. This methodology will also be helpful to different composition of mixed-halide perovskite to cross 20% efficiency. 4. Experimental Section Deposition of the mesoporous TiO2 ETL layer for conventional spiro-MeOTAD based PSCs: Compact TiO2 layer (Bl-TiO2 or c-TiO2), ~200 mp-TiO2 layer were deposited on cleaned UV-plasma patterned FTO-coated glass substrates (TEC15, Pilkington) by spin 13 ACS Paragon Plus Environment


coating.3 The TiO2 precursor of commercial paste was used as per previous literature. Please note, here we have used commercial TiO2 paste (Dyesol DSL-30NR-D) diluted in ethanol (1:6 wt. %)

Two-step method for synthesis of MAPbI3-xClx Methylammonium lead iodide (MAI) was synthesized as per our previous report and MAPbI3xClx.

layer deposited by two-step method discussed in our previous report.3,35, 37 Finally, the

AV-C + MAI ink in chlorobenzene was used for spin-coated electrode of viscous paste was doctor bladed on mp-TiO2+MAPbI3-xClx loaded films, and then heated at 100 °C for 60 min. Screen printed TiO2 ETL layer for printable PSCs: Nearly 1 m mp-TiO2 layer was screen-printed on Bl-TiO2 followed by annealing. Similarly, ZrO2 (Solaronix, ZR/SP) insulating layer (~2 m) was deposited by screen-printing followed by annealing at 450 °C for 30 min.23

Synthesis of Aloe-vera processed carbon based hole extraction material: In typical experiment, the Aloe Vera gel was removed from Aloe Vera leaves and dried under sunlight for 1 day (Step-I). The dried gel was subjected to oil lamp as shown in Fig. 1 (step-II) for overnight. In this process, the dried gel has been converted to in black colored powder. The black powder scratched from SS-substrate and grinded using ball milling. The fine AV-C powder was washed 1M HCl and further annealed in argon atmosphere at 1000 °C. The AV-C processed carbon paste was prepared by were mixed properly mixed using ball milling in chlorobenzene (1:2 wt.%/vol. %) solvent and used for screen printing technique.

Fabrication of fully printable PSCs. For fully printable MAPbI3 based device, AV-C paste was prepared in chlorobenzene and screen printed onto thick ZrO2/mp-TiO2 electrode followed by annealing at 400 °C for 30 min. The perovskite solation was prepared by dissolving 1.2 M MAI and 1.2 M PbI2 in 14 ACS Paragon Plus Environment

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butyrolactone (GBL) solution followed by stirring overnight at 60 °C. The fresh filtered perovskite solution was dripped onto AV-C/ZrO2/TiO2 electrode followed by drying at 50 °C for 1h.

Synthesis of spiro-MeOTAD HTM: The conventional HTM was prepared deposited as per previous reports.3

Characterizations Thermal analysis, top and cross sectional SEM micrographs and transmission electron

microscopy (TEM) micrographs was recorded as per our previous tecnique.3

However, for SEM and TEM analysis few AV-C samples were recorded before and after ball milling. The elemental information regarding the deposited samples were analyzed using an X-ray photoelectron spectrometer (XPS). The sheet resistance of screen printed AV-C and commercial carbon paste was measured by four probe method at room temperature, CMTSERIES, Chang Min Co., LTD. The impedance spectroscopy (IS) was conducted as per our previous experimental conditions in order to get Nyquist plots and analyzed by Z-view3.5e (Scribner Associates) software.3, 67

Solar cell characterization The photocurrent-voltage (J-V) curves were measured using a solar simulator (McScience, K201, LAB50) under AM 1.5G simulated solar illumination (100mWcm-2). Before use simulator was calibrated by a standard silicon solar cell. The active area was defined by using gold contact 0.09 cm2 (0.3 x 0.3 cm2) and black metal mask used in order to avoid overestimation of current density. For fully printable mesoscopic PSCs with active area 0.36 cm2 (0.6 x 0.6 cm) was defined by etched FTO and carbon contact. The device performance was measured by black mask with same active area. Associated Content 15 ACS Paragon Plus Environment


Supporting Information Further materials characterization, SEM/TEM analysis, XRD, EDS mapping, Nyquist plots and device stability testing. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest Acknowledgements This work was supported by Korea Research Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016H1D3A1909289) for an outstanding overseas young researcher. This research is also supported by the National Research Foundation of Korea (NRF) (NRF2017R1A2B4008117). This work was also supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2018R1A6A1A03024334). This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2018R1C1B6008218).


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Figure 1. Schematic representation of preparation of aloe-vera processed cross-linked carbon nanoparticles via ‘ancient Indian method’.



Figure 2. Morphological and structural analysis of AV-C (a) surface morphology of as synthesized AV-C powder sample (b) annealed at 1000 °C (c) XRD patterns of as synthesized and annealed AV-C powder (d) respective Raman spectra.


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Figure 3 Structural and compositional analysis of AV-C (a) TEM microscopic images of AV-C of annealed sample (b) TEM image of single AV-C nanoparticle. Inset shows SAED pattern (c) HRTEM image (d) XPS survey spectrum of AV-C (e) high-resolution C1s core level spectrum (f) core-level spectrum of O1s.



Figure 4. Hybrid hole extraction layer based PSC (a-c) Processed involved in the synthesis of AV-C + MAI based hole extraction layer for mesoporous triple-layer-based scaffold (d,e) cross sectional SEM images of fabricated device at different magnification (f) top view of MAPbI3-xClx capping layer deposited on mp-TiO2 (g) Energy level diagram of each layer used in the AV-C based PSCs. Energy levels has been taken as per previous literature while fermi level of AV-C carbon was considered as graphitic carbon (~5.0 eV) (h) J-V characteristics of heterostructured AV-C/PSCs based on MAPbI3-xClx perovskite.


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Figure 5. Spin coated mp-TiO2 and ZrO2 for MAPbI3-xClx based PSCs (a) an artificially coloured cross-sectional SEM image of the AV-C based PSCs (b) Energy level diagram of each layer used in the AV-C based PSCs (c) respective schematic illustration of the complete perovskite solar cells configuration (d) J-V characteristics of respective device configuration.



Figure 6. Fully printable mesoscopic triple-layer-based scaffold achieved by screen printed of mp-TiO2, ZrO2 and AV-C HTL (a, b) Cross sectional SEM images of the AV-C based PSCs at different magnification (c) schematic illustration of the complete perovskite solar cells configuration (d) MAPbI3 perovskite crystal structure. (e) optical images fabricated AV-C based PSC front view (left) and back (right) view (f) J-V characteristics of champion AV-C based PSCs.


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Figure 7. Average photovoltaic properties of twenty perovskite solar cells and its airstability (a) J-C characteristics of conventional spiro-MeOTAD based PSCs (b) J-V curve of AV-C HTL based PSC. The solar cell parameters and optical images of each device has been represented in respective inset. (c) histograms of PSCs fabricated under different experimental conditions (Notation: SC-TiO2: spin-coated mp-TiO2 and doctor blade coated AV-C; SCTiO2-ZrO2: spin coated mp-TiO2-ZrO2 and screen printed AV-C, SP-TiO2-ZrO2: all mp-TiO2ZrO2 and AV-C layers were deposited by screen printing technique) (d) Stabilized efficiency of the PSCs based spiro-MeOTAD HTM (green) and AV-C HTL (black) tracking up to 150 sec yielding stabilized efficiency of 15.80% and 12.48% respectively. (e) Air-stability of uncapsulated conventional spiro-MeOTAD, AV-C and commercial carbon based PSCs at 35 % RH. Commercial carbon based device stability has been given for comparison.


ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

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Table 1. Photovoltaic properties of PSCs based on different hole extraction layers. Note: SC: spin coating, SP: screen printing, DC: Drop casting Device configuration FTO/Bl-TiO2/mp-TiO2(SC)/MAPbI3(SC)/spiroMeOTAD(SC)/Au FTO/Bl-TiO2/mp-TiO2(SC) /MAPbI3(SC)/AV-C FTO/Bl-TiO2/mp-TiO2(SC)/ZrO2(SC) /MAPbI3(SC)/AV-C FTO/Bl-TiO2/mp-TiO2(SP)/ZrO2(SP) /MAPbI3(DC)/AV-C(SP) FTO/Bl-TiO2/mp-TiO2(SP)/ZrO2(SP) /MAPbI3(DC)/C(SP)

HTL spiro-MeOTAD AV-C AV-C AV-C C

JSC (mAcm-2) 21.25

VOC (V) 1.046

0.71

 (%) 15.80

15.22 18.05 21.65 20.11

0.953 0.838 0.985 0.859

0.50 0.35 0.59 0.61

7.32 5.29 12.48 11.06


FF

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The table of contents: Bio-inspired carbon hole transporting layer: Alove-vera processed cross-linked carbon nanoparticles has been synthesized and used in fully printable C-PSCs. The crossed-linked carbon nanoparticles facilitates effective large surface area which and desired porosity in order to penetrate perovskite solution effectively and crystallize well. The fabricated C-PSCs exhibited >12% efficiency with more than 1000 h air-stability. Keywords: Bio-inspired cross-linked carbon nanoparticles, hole extraction layer, air-stable fully printable perovskite solar cells, Sawanta S. Mali, Hyungjin Kim, Jyoti V. Patil Chang Kook Hong. Title: ‘Bio-inspired’ Carbon hole extraction layer derived from Aloe Vera plant for cost effective fully printable mesoscopic perovskite solar cells ToC figure


'Bio-inspired' Carbon Hole Transporting Layer Derived from Aloe Vera

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