Beyond Fullerenes: Indacenodithiophene-Based Organic Charge

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Beyond fullerenes: Indacenodithiophene-based organic charge transport layer towards upscaling of low-cost perovskite solar cells Dechan Angmo, Xiaojin Peng, Jinshu Cheng, Mei Gao, Nicholas Rolston, Kallista Sears, Chuantian Zuo, Jegadesan Subbiah, Seok-Soon Kim, Hasitha Chandana Weerasinghe, Reinhold H. Daustkardt, and Doojin Vak ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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ACS Applied Materials & Interfaces

Beyond fullerenes: Indacenodithiophene-based organic charge transport layer towards upscaling of low-cost perovskite solar cells Dechan Angmo,a Xiaojin Peng,a,b Jinshu Cheng,b Mei Gao,a Nicholas Rolston,c Kallista Sears,a Chuantian Zuo,a Jegadesan Subbiah,d Seok-Soon Kim,a.e Hasitha Weerasinghe,a Reinhold H. Dauskardt,c and Doojin Vaka* a

Flexible Electronics Laboratory, Manufacturing Flagship, CSIRO, Clayton, VIC 3168, Australia.

b

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, P. R. China.

c

Department of Materials Science and Engineering, Stanford University, CA 94305-4034, USA.

d

School of Chemistry, Bio 21 Institute, University of Melbourne, Parkville, VIC 3010, Australia.

e

Department of Nano and Chemical Engineering, Kunsan National University, Kunsan, Jeollabuk-do 54150, Korea

KEYWORDS: perovskite solar cell, non-fullerene, Indacenodithiophene, ITIC, charge transport layer

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ABSTRACT

Phenyl-C61-butyric acid methyl ester (PCBM) is universally used as the electron-transport layer (ETL) in the low-cost inverted planar structure of perovskite solar cells (PeSCs). PCBM brings tremendous challenges in upscaling of PeSCs using industry-relevant methods due to its aggregation behavior which undermines power conversion efficiency and stability. Herein, we highlight these, seldom reported, challenges with PCBM. Furthermore, we investigate the potential of non-fullerene Indacenodithiophene (IDT)-based molecules by employing a commercially available variant, ITIC, as a PCBM replacement in ambient-processed PeSCs. Films fabrication by lab-based spin-coating and industry-relevant slot-die coating method are compared. While similar power conversion efficiencies are achieved with both types of ETL in a simple device structure fabricated by spin-coating, the nano fibriller morphology of ITIC compared to the aggregated morphology of PCBM films enables improved mechanical integrity and stability of ITIC devices. With slot-die coating, the aggregation of PCBM is exacerbated leading to significantly lower power conversion efficiency of devices than spin-coated PCBM as well as slot-die coated ITIC devices. Our results clearly indicate IDT-based molecules have great potential as an ETL in PeSCs offering superior properties and upscaling compatibility than PCBM. Thus, we present a short summary of recently emerged non-fullerene IDT-based molecules from the field of organic solar cells and discuss their scope in PeSCs as electron or hole-transport layer.

INTRODUCTION Inorganic-organic hybrid perovskite solar cells (PeSCs) offer high power conversion efficiency (PCE) with solution-based processing – a combination that appeared elusive in other thin-film solar cells. Thus, PeSCs received an overwhelming interest in the scientific community leading to a rapid progress in the technology as evident in the certified efficiency having reached 22.1% in a short span of time.1 In early years of development, PeSCs faced challenges relating to uniform film formation of the perovskite film, poor reproducibility, and poor stability. As significant


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advances have been made with respect to PCE, stability, and reproducibility, 2–6 the focus must turn to assessing the compatibility of the materials and processes for commercial translation. Drastically lowering cost of materials, manufacturing and installation while maintaining high efficiency and stability is imperative for PeSCs to become commercially feasible. The inherent solution compatibility of perovskite materials in combination with lower temperature sintering requirement provides an unprecedented golden opportunity at commercial feasibility. These merits would allow for fast and continuous manufacturing using vacuum-free scalable printing and coating methods in a roll-to-roll (R2R) or a sheet-to-sheet line, similar to those applied in graphics and textile printing. To harness the full potential of perovskites, however, all functional materials comprised in the PeSCs, must be co-developed. Ideally, an all-solution based vacuumfree process incurring low-temperature drying/sintering steps and low-cost abundant and nontoxic raw materials while delivering high efficiency and high stability is needed. Currently, three main architectures of PeSCs are pursued: the mesoporous structure, the n-i-p structure (alternatively, known as the traditionnal planar), and the p-i-n structure (alternatively, known as the inverted planar structure). All architectures have resulted in efficiency above 20%,1,5–9 however each structure possesses distinct advantages and disadvantages. The mesoporous structure employs an inorganic metal oxide scaffold such as TiO2, ZrO2, Al2O3 on a TiO2 compact layer-both layers require prolonged sintering conditions at elevated temperatures around 400-500 °C.1,5,10 This imply higher embedded energy in the processing and also rules out their compatibility with low-cost flexible substrates and fast R2R production. Similarly, the planar n-i-p structure uses similar metal oxides as electron-transport layer (ETL), thus imposing similar limitations as the mesoporous structure. Low temperature alternatives, most notably ZnO, reacts with the perovskite and therefore hinders stability.11 Additionally, both mesoporous and planar ni-p structure demonstrate pronounced current-voltage (IV) hysteresis owing to imbalanced charge transport through these metal oxides.12,13 Several low-temperature materials are reported lately such as SnO2 and TiO2:Cl and may prove effective at addressing the current challenges.8,10,14


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Meanwhile, p-i-n planar structure is a low-cost scalable structure embodying low temperature steps, less expensive raw materials, processing simplicity while enabling hysteresis-free cells.7 In its simple form, this structure comprises substrate/ITO/HTL/perovskite/ETL/BE (HTL: hole transport layer; BE: back electrode). In this structure, fullerene acceptors, particularly PCBM, are almost exclusively used as the ETL.7 However, PCBM is hydrophobic in nature and has a tendency to aggregate. The challenges with PCBM segregation and aggregation are widely known in the field of organic solar cells where PCBM or other fullerene compounds intermixed with a polymer material forms the charge-generating active layer.15–17 However, any such issues have not been reported thus far in PeSCs where PCBM itself form a single layer unsupported by a polymer matrix unlike in organic solar cells. Thus, the challenge with aggregation is likely to be more severe in PeSCs. The aggregation of PCBM may not be reflected in laboratory PeSCs as spin-coating is the commonly employed fabrication method and generally no thermal annealing is pursued for the PCBM layer. In spin-coating, layer spreading and drying are concurrent dynamic processes and it may result in seemingly uniform films which might explain the rather lack of literature on film formation of PCBM only films in general and in the context of PeSCs in particular. Additionally, annealing processes accelerate the aggregation of PCBM as shown in organic solar cells.15–17 In PeSCs, the absence of the annealing processes for PCBM due to fear of negatively influencing the underlying perovskite layer has further prohibited the identification of the issues with PCBM. The mechanical analysis of p-i-n PeSCs structure have revealed PCBM layer to be the most brittle layer and susceptible to fracture, thus warranting replacement.18 Herein, we will demonstrate these challenges with PCBM using spin-coating which is the common lab-scale processing method as well as slot-die coating which is a scalable industry-compatible process. Recently, non-fullerene acceptor materials based on indacenodithiophene (IDT) chemical backbone have been reported in organic solar cells to highly efficient compared to fullerene-based acceptor materials. Although IDT was used in engineering of donor polymers in organic solar


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cells for several years,19 its use as an acceptor molecule in organic solar cell was first demonstrated just three years ago by Zhan et al. who synthesized ITIC (3,9-bis(2-methylene-(3(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-sindaceno[1,2-b:5,6-b’] dithiophene).20 ITIC was found to be excellent acceptor material in bulk heterojunction having similar electronic properties as fullerene but also enabling light-harvesting in the visible and near-infrared (NIR) regions. This has inspired subsequent engineering of the material by various groups,20–34 consequently enabling record efficiency in organic solar cells reaching 14% (ref. 31). ITIC comprise of indacenodithienol [3,2-b]thiophene (IT) backbone with four 4-hexylphenyl groups substituted on it and end-capped with 2-(3-oxo,2,3-dihydroinden-1-lidene) malononitrile (INCN) groups (Figure 1). According to Zhan et al., the push-pull structure of ITIC induces intramolecular charge transfer leading to high electron mobility, the three electron withdrawing groups – one carbonyl and two cyano groups – placed on each INCN downshifts LUMO levels and the presence of four rigid 4-exylphenyl substituents out of the IT main plane restricts molecular planarity, aggregation, and large phase-separation in bulk heterojunction organic solar cells.20 Thus, ITIC has a bandgap of 1.59 eV with HOMO and LUMO levels of -5.48 eV and 3.89 eV with respect to the vacuum level, respectively. Additionally, electron mobility is estimated to be ~2.6 ×10-4 cm2V-1s-1.20 The energy levels of ITIC compares well with PCBM which has a HOMO and LUMO level of

5.89 and 3.91 eV with respect to vacuum level,

respectively. While ITIC’s electron mobility is an order of magnitude lower than PCBM’s (2×10-3 cm2V-1s-1),7,25,35,36 it has superior mobility as TiO2 indicating that the mobility should be sufficient to be used as an electron transport layer.37 All these factors should make ITIC an obvious replacement to PCBM in a MAPbI3-based PeSCs which has conduction band level (CBL) and valence band level (VBL) of -3.9 and -5.4 eV, respectively.38 We use this perovskite material and the ITIC (only commercially available at the time of this study) as a representative of IDT-based molecules to investigate their potential as charge transport layer in PeSCs. At the end of this


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paper, we present a synopsis on the state-of-art of IDT-based materials and discuss their scope and opportunities as either ETL or HTL for PeSCs.

RESULTS AND DISCUSSION Device structure MAPbI3 (CH3NH3PbI3) was employed as the perovskite photoactive material in a simple p-i-n device structure (Figure 1a). The structure comprised Glass|ITO|PEDOT:PSS|MAPbI3|ETL|Ag with ITIC and PCBM as the ETL. The simplest device structure was initially used so that the role of the ETLs could be more clearly investigated while minimizing the effects of other structural complexities which gets introduced by using additional layers such as buffer interlayers. Energy level alignment in the device structure is illustrated in Figure 1c. All devices were fabricated in uncontrolled ambient condition with relative humidity ranging between 30-55% and temperature between 20-30 °C. All drying steps were kept below 140 °C to keep the process compatible on low-cost flexible substrates such as polyethylene terephthalate which can withstand up to 140 °C temperature. Additionally, only non-halogenated solvents were employed. Devices were fabricated on pre-patterned ITO substrates where six devices could be fabricated on each substrate (Figure 1b). This layout enables probing large-area reproducibility as six cells are fabricated across 2.5 cm wide substrates.

Photovoltaic properties Specular, highly homogenous, pinhole-free, perovskite films were processed using the perovskite solution formulation reported by Ahn et al.6 (Figure 2a). The perovskite precursor ink comprised of MAI:PbI2:DMSO in 1:1:1 molar ratio. A non-toxic anti-solvent system, specifically tailored for uncontrolled environment was dropped on the rotating substrate 20 seconds after the precursor ink. The ambient conditions for all experiments were monitored. Temperature and humidity varied in the range of 20-30 °C and 30-55 %, respectively. The root-mean-square roughness of the


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perovskite films was ~12 nm measured over 20 by 20 µm2 area. First, the perovskite film thickness was optimized by varying the spin-coating conditions and using standard PCBM as the ETL. The optimized thickness of the perovskite layer processed in ambient condition was ca. 400 nm (Figure 2) similar to that reported by Ahn et al.6 (Figure 2c-d). This thickness is higher than the generally reported for MAPbI3 fabricated under inert conditions (~300 nm), nevertheless MAPbI3 film thicknesses above 200 nm led to complete absorption and PCE should not be impeded by using this thicker perovskite layer.39,40 Further reducing the thickness led to large drop in efficiency while increasing the thickness led to issues with incomplete solution spreading on the substrate, so that thickness above 400-450 nm was not possible to yield complete film coverage over the substrate (Figure 3-a). To evaluate ITIC as an ETL layer and compare it with PCBM, both types of devices were processing conditions were kept same and both devices types were processed identically on days with similar ambient conditions. The ITIC and PCBM solution concentrations and deposition volume were varied to control the final film thickness. For ITIC, the highest average efficiency was achieved using a solution concentration of 5 mg/ml and a deposition volume of 50 µl compared with 20 mg/ml ink concentration and 50 µl quantity for PCBM (Figure 3-b). These conditions resulted in an optimum thickness of 20 nm and 40 nm for ITIC and PCBM, respectively (Figure 2 c-d). Although chlorobenzene is most common solvent for dissolving PCBM in lab-scale devices, it cannot be adopted by industry due to regulations on halogenated solvents. Thus, toluene was chosen as the ETL solvent because of its eco-friendliness. The optimum concentrations of ITIC (5 mg/ml) and PCBM (20 mg/ml) were readily soluble in toluene.41 The optimized ITIC and PCBM devices resulted in similar best PCE values of 11.1% (Table 1, Figure 3-c). This PCE value compares well with reported values for MAPbI3 devices processed in ambient conditions as well as in inert environment using the same device configuration.42-44 Table 1 also gives the average and standard deviation of all six cells produced on the same substrate as


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the best device, and gives us an indication of the uniformity and reproducibility over this larger area. The spread is found to be representative cells on any randomly selected substrate. The optimized ITIC devices consistently displayed lower variation in PCE among all six cells on each substrate with the PCE varying less than 10%. In contrast, a considerable variation was observed amongst the PCBM devices with the highest efficiency measured for the cells positioned at the center of any substrate and then dropping towards the edges (Figure 3-d). The biggest difference in photovoltaic parameters is observed for the fill factor (FF) which is higher for the ITIC devices than the PCBM devices. This is related to poor contact between the electrode and the PCBM layer which in-turn is related to morphology as discussed in section 2.3. Reproducibility over different cells is also given in Figure 3 e-h. As evident, ITIC devices leads to less spread in performance attributed particularly to open circuit voltage (Voc) and FF. While Voc compares well with other studies which used similar structure with PCBM as ETL,7 its value is far from the thermodynamic limit in MAPbI3 (1.33 eV) and much lower than the highest reported Voc for MAPbI3 (>1.2 V).40 This is due to the low built-in field in the device because the contacts have similar work functions. It should be possible to improve the Voc through further device optimization and device design, for example, by tuning the work function of HTL layer,40,45-46 introducing a buffer interlayer between the ETL and the Ag electrode42,47 and further optimizing the perovskite morphology and composition.40,48 In particular, the HTL work function has a significant impact on the Voc in p-i-n structures due to its effect on monomolecular recombination.45 In contrast to Voc, the Jsc of the device is reasonably close to the theoretical limit of 23 mA·cm-2 for MAPbI3 system considering parasitic absorption by the preceding layers and interfaces to the perovskite (Glass/ITO/PEDOT:PSS) and losses due to transmission and reflection. FF is however the efficiency limiting parameter. The poor FF results from trapassisted recombination at the interfaces due to poor energy band alignment between the HTL and the perovskite as well as between the ETL and the Ag electrode. Additional losses, albeit less


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dominant, are attributed to the presence of traps at the grain boundaries which act as recombination centre and display faster nonradioactive decay, increasing series resistance.49 The grain size of perovskite films in this study were 100-200 nm which is significantly lower than >500 nm reported in films processed under dry conditions50,51 (Figure 2-b). While the perovskite films were already optimized for the ambient processing conditions, the band alignment can be tackled by lowering the HTL work function and using a thin cathode buffer interlayer to improve band alignment.47 For example, Lu et al. noticed an improvement in FF from 56 % to 77 % by employing a 10 nm interlayer/buffer layers based on bathocuproine (BCP).42 To further improve device performance including Voc and FF, we started to fine-tune the device structure while maintaining the merits of this device structure: low-temperature drying requirements and solution-process ability. The work-function of PEDOT:PSS was modified simply by adding sodium polystyrene sulfonate (PSS-Na) which is referred hereafter as mPEDOT:PSS (modified PEDOT:PSS). This lowers the work function of PEDOT:PSS from -5.2 to -5.4 eV towards that of the MAPbI3 conduction band.46 This modification improved in Voc (Table 1) and hysteresis (Figure 4) for both ITIC and PCBM devices. However, Jsc dropped slightly, thus the cumulative effect was only a marginal improvement in PCE of PCBM devices and a small decrease in PCE of ITIC devices (Table 1, Figure 4). Inserting a solution-processed buffer interfacial layer based on ethoxylated polyethylene amine (PEIE) between the ETL and the Ag electrode resulted in marked improvement in PCE of the PCBM devices from 11.1 to 14.3% due to an increased Jsc and FF. Additionally, hysteresis was significantly reduced to negligible (Figure 4 and Table S1). On the other hand, the interfacial layer negatively influenced ITIC devices by drastically suppressing FF while Voc and Jsc remained unchanged. The JV curve suggests a complete barrier to charge transport. At first, we speculated that a higher thickness of the PEIE layer was responsible for the charge transport barrier. However, optimizing the thickness of the PEIE layer did not impact the nature of the IV curve. We suspect the complex chemical moieties of ITIC interfere in the organization and orientation of dipole moment of the interfacial


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buffer layer. To circumvent this, we mixed PCBM and ITIC to see if the problem could be avoided. A maximum PCE of 12.94 % (Voc:1V, Jsc: 18.28 mA/cm2 and FF: 71%) with negligible hysteresis was achieved, however the reproducibility of the mixed ITIC-PCBM devices became very poor. Further work is necessary to elucidate the mechanism behind this and further explore buffer interlayer compatible with ITIC or similar organic molecules. Whilst we were reviewing this manuscript, reaction between ITIC and amine-based interlayer was reported, thus attesting to our conclusion that ITIC is not the efficiency limiting material but the incompatibility of the buffer interlayer is.52

Morphology and film formation The morphology of PCBM and ITIC were compared by atomic force microscopy (AFM) (Figure 5). Samples were prepared as for the solar cells but without the Ag electrode. ITIC films on perovskites comprise of an interconnected network of nano-fibriller structure with interspersed clusters of fibers, resulting in fiber widths ranging from 20 nm to 120 nm. The fibers are characteristics of low molecular weight polymers. They have been characterized to be crystallites which display long-range, three dimensional periodicity and are responsible for high carrier mobilities.53 Low molecular weight polymers allow non-entangled conformations leading to formation of isolated and extended chain crystal structures resulting in polycrystalline one-phase morphology.54In contrast, PCBM (molecular weight ~0.91 Kg/mol) has similar molecular weight as ITIC (1.43 Kg/mol) but forms a completely different morphology owing to the presence of the bulky C60 moiety. The AFM images indicate PCBM organizes into much larger PCBM-rich islands. Further zooming into these PCBM rich domains reveal a granular structure, analogous to the observations reported by Tang et al.55 These granular structure compose of PCBM nanocrystals of various sizes.55 As there was no annealing process after the deposition of ETL, it is likely that these particles are PCBM crystals as PCBM self-organizes in micro- or nanocrystalline structure upon slow-solvent evaporation.56 PCBM only films are seldom reported but


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PCBM in blends with conjugated polymers are widely studied as PCBM is the most used acceptor in bulk heterojunction organic solar cells.57 Several studies have revealed that the size of PCBM aggregates/agglomerates increases over time in blends where PCBM is in mixture with a high molecular weight conjugated polymer.15–17 In PeSCs in which PCBM exists as a pure film, the aggregation tendency is likely to be higher which may prove a hindrance to stability. The impact of this morphological difference can be seen in the degradation of the silver electrode over time. We observed that the silver electrodes in PCBM-based devices lose their specularity even when stored in a glove-box, whereas they remain reflective and unchanged in ITIC devices even after months of storage in glovebox. This suggests ITIC provides a barrier between the perovskite layer and the electrodes preventing halide ions to migrate to the silver electrode. In contrast, the degradation of silver electrode in PCBM devices attributed to the above mentioned organization of PCBM into large domains which concurrently leads to many voids in the film, providing easy diffusion of the ions to the Ag electrode through the ETL or vice versa (Figure 5).58-59 We qualitatively explored this by removing old samples from the glovebox and leaving them without any encapsulation in ambient conditions for two weeks. Images (a) and (b) in Figure 6 are images of ITIC and PCBM devices taken immediately after their removal from the glove-box where they had been stored for 37 and 25 days, respectively. Images (c) and (d) are of the same devices after two weeks of storage in air. It is evident that the silver electrode on PCBM appears hazy and has developed stains indicating degradation due to the silver electrode reaction with the halide in the MAPbI3, forming silver iodide.60 On the other hand, the silver electrode on the ITIC device has retained its shine despite being an older device. From the front-electrode side, PCBM devices have turned yellow indicating escape of methylammonium ions whereas no change in colour was observed in ITIC based devices (Insets in Figure 6 c-d, respectively), thus confirming ITIC’s additional advantage as barrier material in addition to the ETL properties. Such a result is in conjunction with a previous report which also found poly (3-hexylthiophene-2,5-


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diyl) or P3HT – a conjugated polymers HTL materials used in n-i-p device structure – imparted barrier properties.11 Film formation and scaling consideration The process of film formation is more passive in industry-compatible coating processes than in spin-coating. One promising process for industrial production of perovskites is slot-die coating. In slot-die coating, a solution in directed through a slot-die head and a meniscus of the ink is formed between the head and the substrate with the use of a flow-mask and meniscus guide. The meniscus is continually fed with ink as coating proceeds by the movement of the substrate or the slot-die head. In this manner, one dimensional facile coating in the form of stripes of various sizes could be accomplished , which are interconnected with the deposition of back electrode layer to form modules.61

Slot-die is one of the most desirable coating techniques for scale-up as it

requires little or no maintenance and is a very cost-friendly R2R/sheet-to-sheet compatible technique and it is one of the most suitable techniques for low viscosity solutions.62 PeSCs can ideally be processed with just slot-die method except for the electrode which makes PeSCs an incredibly accessible and easily scalable technology. With these in mind, we explored the process of film formation of ITIC and PCBM on perovskite films and on glass using an in-house developed laboratory slot-die coater mounted on a 3D printer63 (Figure S-1). To focus only on the ETL, all other conditions were kept same as in the spin-coated devices. To achieve similar thickness as the spin-coated optimized conditions, the slot-die coating conditions of PCBM and ITIC were each optimized until the absorption coefficient of the films matched to that of the respective optimized spin-coated films. Once processing parameters on slot-die coating were established which includes flow rate, coating speed and meniscus height, ITIC and PCBM solutions were each slot-die coated on glass and on devices fabricated with spin-coating up to the perovskite layer (Glass/ITO/PEDOT:PSS/MAPbI3). Spin-coated ITIC and PCBM were also fabricated on the same day. Optical microscopy was used to compare ITIC and PCBM films formed using spin-coating and slot-die coating (Figure 7).


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With Spin-coating, ITIC film on perovskite forms uniform film with full coverage. In contrast, spin-coated PCBM coalesces into segregated domains. The aggregation in PCBM film is more severe on films fabricated directly on glasses. With slot-die coating, ITIC forms highly uniform film glass and on the perovskite films. In contrast, highly non-uniform films are formed with slotdie coating of PCBM on both glass and perovskite films. Aggregation at the coating edges and sparse deposition towards the center of the coating width, analogous to the coffee-ring effect observed in coating of colloidal solutions, is observed in PCBM films. The coffee-ring effect explains the tendency of outward flow of ink from the center of the print to the pinned contact lines during the drying process due to higher evaporation rate at the contact line.64 This could be related to poor adhesion of PCBM to the perovskites as the same solvent were used for both ITIC and PCBM and no annealing steps were involved. Further zooming in to the morphology reveal aggregate formation with PCBM, similar to morphology observed in the spin-coated films albeit on a larger scale. This large-scale aggregation has arisen because of the passive drying process in slot-die coating compared to the dynamic film formation in spin-coating. As a result, the photovoltaic properties of slot-die coated PCBM devices were significantly lower than slot-die coated ITIC largely contributed by reduced FF (Table 2, Figure 7-i).

Fracture Energy of ITIC and PCBM Films The mechanical integrity of ITIC was characterized using a double cantilever beam (DCB) test, a standard technique for measuring the fracture energy (Gc) of thin films. DCB samples were fabricated by depositing ITIC on ITO-glass and bonding an identical superstrate. The loaddisplacement curve was then measured to determine Gc using a simple mechanical testing system consisting of a load cell and a servo actuator. The measured Gc of ITIC films on glass was 2.7 ± 0.9 J·m-2 (Figure 7-j). In comparison, PCBM is one of the most fragile materials used in PeSCs, with a Gc below 0.2 J·m-2.18,33 Thus, ITIC is a more mechanically robust alternative to PCBM and other fullerene derivatives.


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Further mechanical tests were performed to determine the Gc of ITIC and PCBM deposited on top of perovskite films to be more representative of a solar cell structure. In this case, the Gc of ITIC on perovskite was measured to be 0.56 ± 0.12 J·m-2 (Figure 7-k). This value is markedly higher than the Gc of 0.14 ± 0.06 J·m-2 for PCBM on perovskite and lower than the reported fracture energy of ITIC because the perovskite has been shown to also be mechanically fragile.65 The fracture path for the ITIC-perovskite samples occurred in the perovskite, indicating that the ITIC is not the weakest layer in the solar cell, which is the case for PCBM.

Perspective: Indacenodithiophene-based non-fullerene molecules for PeSCs Since Zhan’s report on ITIC as an acceptor molecule in organic solar cells, ITIC has gathered significant interest in the organic solar cells community. By chemically tailoring various units on indacenodithienol [3,2-b] thiophene or the IT backbone, numerous variants have been reported19 (Table 3). This has led to a rapid increase in record PCE of organic solar cells from 6 % to 14 %.31 These IDT-based molecules have electron mobility on par or even superior to existing ETL materials and a bandgap fitting many perovskites formulations. Most of these high mobility nonfullerene acceptors along with their properties of interest are listed in Table 3 and their band-gap alignment with many common perovskites compositions is illustrated in Figure 8.

20–32

Several

variants are superior to ITIC and are more suitable for PCBM replacement, however they were not commercially available at the time of this study. As can be deducted from Figure 8, ITIC, IDTBR, ITIC-Th, IT-4F, and COi8DFIC can all fulfill the role of ETL in MAPbI3-based PeSCs. IT-4F can be used with a range of compositions of mixed organic cation compositions (FAxMA1XPbI3),

21

MAPbBr3 and lead-free MASnI3.21,66 None of the existing variants would be compatible

for perovskite compositions with deeper LUMO level such as pure FAPbI3 (NH2CHNH2PbI3; HOMO:4.00 eV, LUMO: 5.67 eV) or deeper HOMO level such as mixed Pb/Sn formulations.21,6669

Similarly, none are suitable as ETL for the stable perovskite formulations such as

Cs0.05FA0.81MA0.14PbI2.55Br0.45.5,14,70 Instead, all except IT-4F are suitable to be employed as HTL


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with this mixed perovskite formulation with IT-CC being the best. Hole mobility values for these molecules are not generally reported (except for COi8DFIC which has even higher hole than electron mobility) as only electron and HOMO and LUMO level are of interest in the field of organic solar cells, but provided these molecules display ambipolar mobilities or hole-mobility relatively close to their respective electron mobility as listed in Table 3 and it is very well likely,71–73 this may mean that these non-fullerene IDT-based molecules can potentially replace the current state-of-the-art HTL, Spiro-MeOTAD in the n-i-p structure. Spiro-MeOTAD is ten times more expensive than gold, possesses a modest hole mobility (1.67x10-5 cmV-1s-1) without doping, inhibits stability especially with the use of dopants and obstructs scalability as it is extremely difficult to print based on our experience with scaling-up.74,75 Thus, the non-fullerene alternatives could potentially signify a breakthrough in HTL in PeSCs but their hole mobilities need to be evaluated. Unlike in organic solar cells, PCBM and equivalent materials do not actively take part in exciton generation and dissociation but only functions as a selective charge transport layer. Yet, the design framework applied in chemically tailoring these molecules as acceptors in organic solar cells is still applicable for designing charge transport layers for PeSCs. That is, the HOMO and the LUMO of the ETL molecules should be deeper than the conduction and valence band of the perovskites, respectively. For HTL, however, the opposite design strategy is required – the HOMO and LUMO of the HTL must be shallower than the conduction and valence band of the perovskite, respectively. It is unclear at this point however how the difference in conduction band level (CBL) and the HOMO for ETL and valence band level (VBL) and the LUMO in case of HTL may impact device photovoltaic properties in PeSCs. It must be noted that we encountered large batch-to-batch variation of the IDT-based molecules which also needs to be addressed by the organic chemists, which can be addressed by scaling-up the organic synthesis.

CONCLUSION


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In summary, we have explored the applicability of recently emerged IDT-based non-fullerene molecules in the field of organic solar cells as ETL in PeSCs. Although several high performance IDT-based molecules have been reported, ITIC was chosen due to its commercial availability and explored here a representative of this class of materials as listed in Table 3. ITIC was integrated in ambient-processed low-cost and solution-processable p-i-n planar structure with industrialcompatible choice of solvents and compared with the state-of-the-art ubiquitous counterpart, PCBM. Differences in film formation and morphology with spin-coating and slot-die coating were investigated which clearly demonstrated ITIC to be a better alternative to PCBM owing to its nano-fibrillar structure and non-aggregation tendencies leading to highly uniform film formation unlike PCBM which form non-uniform films and large aggregates. These factors influences the mechanical robustness of the interface as mechanical testing indicated that the fracture energy of ITIC films exhibited a 16-fold increase compared to PCBM. Furthermore, PCE of ITIC-based devices is similar to PCBM-based devices in spin-coated devices but significantly higher with slot-die coating because of the poor film formation in PCBM which is exacerbated in slot-die coating. Further tuning of the devices structure was carried out by modulating the work function of the HTL and by introducing a solution-processed interfacial layer.

The organic

interfacial materials proved incompatible with the ITIC while it increased the efficiency of PCBM devices to 14.2 % which is the highest for a low-temperature ambient processed PeSCs. Further work needs to be carried out to find a compatible interfacial material with the organic ETL. The results and discussions presented in this study highlights significant development potential with non-fullerene small molecules for application in PeSCs. The perspective section explored this in some depth. In all, further pursuit of these IDT-based molecules can be specially tailored for PeSCs as either HTL or ETL with the goal of achieving five key attributes: 1) high mobility; 2) energy level alignment with respect to the perovskites for either blocking holes or blocking electrodes; 3) stability; 4) printability; and 5) scalability in material supply and low-cost of manufacturing owing to low material consumption and no annealing requirement.


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EXPERIMENTALS Materials and Device fabrication Solar cell devices were processed on pre-patterned indium tin oxide (ITO) coated glass substrates (Shenzhen Display, 5 Ω·sq−1). Prior to device fabrication, the substrates were successively cleaned by sonicating in detergent solution (5 vol.% Deconex 12 PA), deionized water, acetone and propan-2-ol for 5 minutes each followed by drying using a N2 gun. The cleaning step was completed by exposing the substrates to a UV-ozone light (Novascan PDS-UVT) at room temperature for 15 minutes. PEDOT:PSS (HC Starck, Baytron P AI 4083) was coated at 3000 rpm at an acceleration speed of 10,000 rpm/s for 50 seconds followed by drying on a hot plate at 140 °C for 20 minutes to ensure complete removal of moisture. Modified PEDOT:PSS (m-PEDOT:PSS) was prepared by adding 1 mL PEDOT:PSS (HC Starck, Baytron P AI 4083), 60 mg sodium polystyrene sulfonate (molecular weight ~70,000, Sigma-Aldrich), and 5 mL deionized water and stirring at room temperature for 10 min. The m-PEDOT:PSS was spin coated at 5000 rpm for 30s and annealed at similar condition as regular PEDOT:PSS. All PEDOT:PSS solutions were filtered (0.2 µm RC filter) prior to use. The perovskite solution was prepared under nitrogen environment following ref. 6. Briefly, PbI2 (99.9985%, Alfa Aeser), MAI (Dyesol, now GreatCell Solar), and DMSO (anhydrous, SigmaAldrich) were mixed in 1:1:1 molar ratio in 600 mg of DMF (anhydrous, Sigma-Aldrich). The solution sealed in a vial was taken out of the glovebox and stirred on a hotplate at 70 °C for 2-3 hours. The solution was cooled down to room temperature before spin coating at ambient conditions in a fume hood cabinet. Temperature and relative humidity inside of the fume hood cabinet were 25-30 °C and 30-55 %, respectively. 100 µl solution was spin-coated using a twostep program: 1000rpm-5000rpm/s-10s and 3500rpm-10000rpm/s-30s. At 10 seconds into the


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second step, 4 ml of a non-halogenated anti-solvent was dropped. The film is observed to convert to complete transparent. The films were then annealed at 100 °C for 2 minutes in air on a hot-plate upon which the film readily turns to greyish-brown and becomes highly specular. PCBM (Nano-C) or ITIC (1-material) in different concentrations were prepared in toluene and stirred on a hot-plate at 70 °C for 2-3 hours. 50 µl of the PCBM or ITIC were spin-coated on the perovskite films at a spin-speed of 3000 rpm for 45 seconds. Concentration and spin-coating volume were optimized. No annealing steps were carried out. The substrates were then transferred to an evaporator (Angstrom) located in a glovebox and 100 nm Ag (99.9% pure, Kurt J. Lesker Company) was deposited through a shadow mask under 10-7 Torr vacuum condition to give an active area of 10 mm2.

Device characterization The current density-voltage (IV) scan of the devices were carried out at 1000 Wm-2 AM 1.5G illumination conditions using a 1 kW Oriel solar simulator as the light source in conjunction with a Keithley 2400 source measurement unit. For accurate measurement, the light intensity was calibrated using a reference silicon solar cell (PV Measurements Inc.) certified by the National Renewable Energy Laboratory. Solar simulator was located in a glovebox. Mechanical Testing: DCB samples were prepared by spin coating the films of interest on 30 mm x 30 mm ITO-glass slides and adhering an identical glass superstrate with a thin, brittle epoxy (E-20NS, Hysol) cured for 24 hours in a N2 drybox. Before bonding, a 5 nm Cr/200 nm Al barrier film was evaporated onto the samples as a barrier to the epoxy. Any residual epoxy was carefully wiped away from the edges of the DCB sample before curing and scraped away with a razor blade after curing. The mechanical test was performed by loading under displacement control in a thinfilm cohesion testing system (Delaminator DTS, Menlo Park, CA) from which a load, P, versus displacement, ∆, curve was recorded. The fracture energy, Gc (Jm–2), was measured in terms of the critical value of the strain energy release rate, G. Gc can be expressed in terms of the critical


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load, Pc, at which crack growth occurs, the crack length, a, the plane-strain elastic modulus, E′, of the substrates and the sample dimensions: width, B and half-thickness, h. Gc was calculated from equation (1):76 G =

12P a h 1 + 0.64 (1) a B E h

An estimate of the crack length was experimentally determined from a measurement of the elastic compliance, d∆/dP, using the compliance relationship in equation (2): a=

/

d∆ BE h ∗ 8 dP

− 0.64 ∗ h (2)

All Gc testing was carried out in laboratory air environment at ~25 °C and ~40 % R.H. The sample was pulled apart with a displacement rate of 1 µm·s–1 until reaching Pc before unloading slightly to calculate d∆/dP. The samples were subsequently loaded to Pc and the process repeated until the crack traveled through the entire length of the sample. Material Characterization Scanning Electron Microscope (SEM) imaging was carried out using Zeiss Merlin field emission microscope using an electron gun voltage of 5 KV and a working distance of ~5 mm. Images were acquired with secondary electron and in-lens detectors. Cross-sections were milled using a FEI Helios NanoLab 600 Dual Beam FIB-SEM. A protective Pt layer was first deposited followed by milling at beam currents between 0.92 nA to 9.7 pA. Optical measurements were carried out using Olympus BX60 microscope in the bright or dark field mode. Atomic force microscopy (AFM) was performed on an Asylum Research MFP-3D instrument in tapping mode. Mechanical testing for adhesion of ITIC and PCBM on perovskite films was conducted on according to procedure reported

earlier.18

Ag

films

were

excluded

from

the

samples

(Glass/ITO/PEDOT:PSS/MAPI3/ITIC or PCBM).


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxxx. Picture of slot-die coating process, JV decay curve of perovskite solar cells with ITIC and PCBM, photovoltaic parameters associated with Figure 4. (Pdf) AUTHOR INFORMAITON Corresponding Author *E-mail: [email protected] Orchid ID Dechan Angmo: 0000-0003-4029-0017 Doojin Vak: 0000-0001-7704-5563

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT D. Angmo and X. Peng contributed equally. The authors would like to acknowledge Mark Greaves, Melisja de Vries and Gediminas Gervinskas for their expert advice and knowledge on SEM and FIB. CSIRO’s OCE post-doctoral grant is acknowledged for supporting D. Angmo, the Chinese Government’s Fundamental Research Funds for the Central Universities (2016-JL-002) for X. Peng, and CSIRO Julius Career Award funding for D. Vak. N. Rolston and R. Dauskardt were supported by the Bay Area Photovoltaics Consortium (BAPVC) (Grant number: DE-


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EE004946). Additional support was provided by the National Science Foundation Graduate Research Fellowship, awarded to N. Rolston under award no. DGE-1656518.This work was partially supported by the Australian Centre for Advanced Photovoltaics (ACAP) program funded by the Australian Government through the Australian Renewable Energy Agency (ARENA). This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF).

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FIGURES AND TABLES FIGURE CAPTIONS Figure 1 [a] Device structure of the perovskite planar heterojunction solar cells employed in this study: Glass/ ITO/PEDOT:PSS/MAPbI3/PCBM/Ag. [b] Schematic illustration of device layout. Six cells were fabricated across 25 mm glass substrate. [c] Schematic diagram of energy level alignment among various functional layers in the device along with chemical structure of ITIC and PCBM. Figure 2 [a] A image of ambient-processed specular perovskite films reflecting CSIRO logo. [b] Scanning Electron Microscope image showing morphology of the perovskite film. [c-d] SEM crosssection image of optimized cells with ITIC and PCBM as electron transport layer respectively. Figure 3 [a] Effect of spin-coating conditions (rpm) of the perovskite films on the PCE of the cells (optimized with PCBM ETL). [b]Effect on concentration of the ETL (ITIC vs PCBM) on the final PCE of the cells. [c] Current-voltage curve of the best cells with PCBM or ITIC as ETL when characterized under standard 1 sun illumination condition (1000 W m-2, AM 1.5G) in reverse scan mode from 1.0 to 0 V with 20 mV voltage steps. [d] Variation of the PCE of six cells fabricated across one 25 mm2 substrate. [e] Reproducibility of PCBM and ITIC based spin-coated devices. Sample size for PCBM and ITIC are 12 and 10 cells, respectively. Figure 4 Current-voltage curves of: [a] ITIC-based devices with regular PEDOT:PSS; modified PEDOT:PSS (m-PEDOT:PSS); and m-PEDOT:PSS with the use of an interfacial cathode buffer layer based on PEIE. [b] PCBM-based devices with similar device modifications as in [a]. Hysteresis in JV curves of PCBM and ITIC-based cells incorporating mPEDOT as HTL: [c] without interfacial buffer layer and [d] with interfacial buffer layer. [e] Hysteresis in mixed ITIC+PCBM layer with and without the interfacial buffer layer. Devices were characterized under standard 1 sun illumination condition (1000 W m-2, AM 1.5G) in 20 mV steps in reverse and forward scan conditions from 1 or 1.1 to 0 V. Figure 5 Morphology images of ITIC [a-b] and PCBM [c-d] characterized under Atomic Force Microscope (AFM). Scale bar in [a] and [c] correspond to 2 µm and in [b] and [d] correspond to 400 nm. Figure 6 Pictures of: [a] ITIC-based and [b] PCBM-based cells taken right after removing from 37 days and 25 days of storage in glovebox, respectively. [c] and [d] corresponds to pictures taken after subsequent storage of [a] and [b] respectively over two weeks under uncontrolled ambient conditions. Inset in [c] and [d] shows the view from the glass/ITO front side. Figure 7 Optical images: spin-coated [a] PCBM on glass and [b] ITIC on glass; slot-die coated [c] PCBM on glass and [d] ITIC on glass; spin-coated [e] PCBM on perovskite film and [f] ITIC on perovskite film; slot-die coated [g] PCBM on perovskite film and [h] ITIC on perovskite film. Insets show higher magnification optical images taken from the center of the film except in [e] and [f] which show SEM images owing to reaching resolution limit of the optical microscope. Scale bars in [a-b, e-f] = 20 µm; insets in [a –b, e-f] = 1 µm; [c-d, g-h] = 500 µm, inset in [c-d, g-h] = 10 µm. [i] Current-voltage curve of the best cells comprising slot-die coated PCBM or ITIC as electron transport layer. Measurements conducted under 1000 W m-2, AM 1.5G conditions in reverse scan mode from 1.0 to 0 V in 20 mV voltage steps. [j-k] The measured Gc of PCBM or ITIC films deposited on [j] ITO-glass and [k] perovskite, showing a marked increase in mechanical integrity for ITIC. Figure 8 Energy level diagram depicting some common perovskite formulations and several IDTbased or related high efficiency molecules from various sources (See Table 3).


Page 28 of 34

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Figure 1

Figure 2



Figure 3

Figure 4


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Figure 5

Figure 6



Figure 7

Figure 8


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Table 1. Key photovoltaic parameters of PeSCs with spin-coated ETL materials measured under standard conditions (1000 W m-2 AM1.5G). Values in parenthesis shows average of 6 cells measured across with the width of 2.5 cm substrate. ETL layer

HTL

Cathode interlayer

Voc [V]

Jsc [mA cm-2]

FF [%]

PCE [%]

ITIC

PEDOT:PSS

No

0.88 (0.85±0.03)

20.67 (20.61±0.44)

61.12 57.97±1.67)

11.11 (10.61±0.68)

ITIC

m-PEDOT:PSS

No

0.94 (0.94±0.00)

19.48 (18.65±1.1)

59.29 (58.44±1.43)

10.85 (10.23±0.54)

ITIC

m-PEDOT:PSS

Yes

0.95 (0.94±0.02)

18.40 (18.46±1.12)

30.05 (29.83±1.89)

5.31 (5.20±0.15)

PCBM

PEDOT:PSS

No

0.90 (0.84±0.10)

21.77 (21.50±0.40)

56.64 (53.78±2.39)

11.10 (9.71±1.53)

PCBM

m-PEDOT:PSS

No

0.96 (0.92±0.07)

19.94 (19.32±0.59)

61.84 (55.17±8.95)

11.84 (9.97±2.52)

PCBM

m-PEDOT:PSS

YES

0.96 (0.96±0.01)

21.38 (20.90±0.35

69.47 (70.17±0.68)

14.26 (14.03±0.27)

ITIC+PCBM

m-PEDOT:PSS

YES

1.00 (1.00±0.01)

18.28 (18.38±0.15)

70.79 (69.59±0.58)

12.94 (12.80±0.12)

Table 2. Key photovoltaic parameters of PeSCs with slot-die coated ETL materials measured under standard conditions (1000 W m-2, AM 1.5) with the structure ITO/PEDOT/MAPbI3/PCBM or ITIC/Ag. Corresponding IV curve in Figure 7. Slot-die Coated ETL layer ITIC PCBM

Voc [V] 0.84 0.80

Jsc [mA cm-2] -20.19 -20.15

FF [%] 54.06 48.51

PCE [%] 9.17 7.82

Table 3 Compilation of ITIC and related IDT-based non-fullerene materials of interest to PeSCs. Electron Mobilitya [cm2 V-1 s-1]

Optical Bandgapb [eV]

HOMOc [eV]

LUMOc [eV]

ref

PCBM

10-3

~3.0

-5.89

-3.91

[7][20}[35-36]

ITIC-CC

9.26 × 10−4

1.67

-5.47

-3.76

[27]

m-ITIC

2.45×10-4

1.58

-5.54

-3.82

[26]

1.57

-5.4

-3.82

[34]

1.60

-5.50

- 3.83e

[24][28][29]]

Materials

IEIC

IT-M

2.1 x 10-4

d1.10

× 10−4

COi8DFIC

1.55 ×10-5

1.26

-5.50

-3.88

[31-32]

ITICc,f

1.60 — 2.6×10-4

1.59

-5.50 – -5.48

-3.89 -3.83

[20], [23], [26]



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IDTBR

n/a

1.6

-5.5

-3.9

[22]

ITIC-Th

6.1×10-4

1.60

-5.66

-3.93

[30]

IT-4F

5.05 ×10-4

1.80

-5.66

-4.14

[23]

aSpace-charge-limited

Current (SCLC) method; bUV-Vis spectrophotometer; cCyclic Voltammetry (CV); d Values only reported in blend with a conjugated polymer PBDB-T. eHOMO level was measured by adding the optical bandgap value to the CV measured LUMO resulting in overestimation (3.35 eV). The value is corrected to account for the difference in measurement method by comparing LUMO level of ITIC reported in this publication against ref. [20] where both HOMO and LUMO where measured with CV. fUltraviolet photoelectron spectroscopy.

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Beyond Fullerenes: Indacenodithiophene-Based Organic Charge

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