Inverted perovskite solar cells with air stable diketopyrrolopyrrole-based electron transport layer

Solar Energy 186 (2019) 9–16

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Inverted perovskite solar cells with air stable diketopyrrolopyrrole-based electron transport layer

T

Shikha Sharmaa, Nobuya Sakaib, Suman Raya, Satyaprasad P. Senanayakc, Henning Sirringhausc, ⁎ Henry J. Snaithb, Satish Patila, a

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, United Kingdom c Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, United Kingdom b

A R T I C LE I N FO

A B S T R A C T

Keywords: Diketopyrrolopyrrole Perovskite solar cells

One of the possible causes of degradation of perovskite solar cells is the instability of the electron transporting layer. In this regard, design of air stable electron transport organic semiconductors, compatible with perovskite energy levels presents challenges due to inherent vulnerability to traps, presumably originating due to water and/or oxygen. In this work, we demonstrate air stability of diketopyrrolopyrrole-based molecule (TDPP-CN4) at ambient conditions and its application as electron transporting layer (ETL) in perovskite solar cells. We investigated electron mobility and air stability of TDPP-CN4 by fabricating top-gate bottom-contact (TG-BC) thin film transistors and compared with PCBM at ambient conditions. Both TDPP-CN4 and PCBM exhibit electron transport properties with mobility of 0.13 cm2 V−1 s−1 and 0.03 cm2 V−1 s−1 respectively. However, we found remarkable air stability of the TDPP-CN4 in the OFET measurements under ambient conditions. These excellent properties of TDPP-CN4 render them as potential ETL layer in inverted planar heterojunction perovskite solar cells. Our preliminary device studies show remarkable short-circuit current (Jsc) ∼ 17.4 mA/cm2 with moderate open-circuit voltage (Voc) of 0.50 V. These results suggest that the electron mobility and air-stability of diketopyrrolopyrrole-based molecule hold a promise as ETL in perovskite solar cells at ambient conditions.

1. Introduction

as a barrier to fabricate perovskite solar cells on flexible substrates (Gu et al., 2016). Contrary to this, the metal-oxide-free inverted device architecture (p-i-n) exploiting the planar heterojunction formed by halide perovskite with conventional organic transport layer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) as HTM and fullerene derivative [6,6]phenyl-C6-butyric acid methyl ester (PCBM) as ETL removes the restraint of extreme annealing temperatures (Docampo et al., 2013), which simplifies the cell configuration. At present, PCBM is the most commonly used ETL in inverted perovskite solar cells. The high electron affinity and reasonable charge carrier mobility makes PCBM suitable for promoting charge separation at the interface between perovskite and fullerene (Wu et al., 2015; Azimi et al., 2011). One of the major issues with PCBM is ambient instability, induced by hydrophilicity of ester group in the molecular backbone (Zhao et al., 2016). Furthermore, PCBM is also prone to photochemical transformation from monomer to dimeric/polymeric structures on exposure to light (Zhou et al., 1993). Relatively poor ambient and photostability as well as uninflected tunability (Zhao et al., 2016) of energy levels of PCBM layer are the major limitations towards improved device efficiency as well as further entangles the device fabrication process.

Among recently emerged photovoltaic concepts, organo-metal halide based perovskites have proved successful in terms of cost and efficiency but encountered materials and device instability from ambient humidity and oxygen in the air (Manshor et al., 2016). However, the simplicity and ease of device fabrication from solution with the realization of efficiencies > 21% made serious claims for commercialization as future photovoltaic technology (Yang et al., 2015). In a conventional perovskite solar cell, the device architecture constitutes the light-absorbing perovskite layer in a sandwiched configuration between mesoscopic metal oxide such as TiO2, Al2O3, ZnO as electron transport layer (ETL) and organic semiconductors such as tetrakis[N,N-di(4methoxyphenyl)amino]-9,9-spirobifluorene (spiro-OMeTAD), poly[bis (4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) as hole transporting material (HTM) (Roldán-Carmona et al., 2015). Despite the high performance of such n-i-p structure based perovskite devices, the essential requirement of high temperature sintering process for the preparation of mesoporous TiO2 layer and large photocurrent hysteresis pose an obstacle in determining accurate power conversion efficiencies and act ⁎

Corresponding author.

https://doi.org/10.1016/j.solener.2019.04.071 Received 4 October 2018; Received in revised form 19 March 2019; Accepted 23 April 2019 Available online 06 May 2019 0038-092X/ © 2019 Published by Elsevier Ltd on behalf of International Solar Energy Society.

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Scheme 1. Synthesis of dicyanomethylene-end-capped quinoidal diketopyrrolopyrrole.

Fig. 1. Typical transistor characteristics of TDPP-CN4 with L = 20 μm, W = 1 mm (a) Output and (b) transfer curves of TDPP-CN4, (c) Mobility variation under ambient condition for PCBM and TDPP-CN4 based thin-film transistors. Inset shows the device schematic.

synchronization of energy levels with metal electrodes, morphology including its ambient stability when incorporated in OPVs. Stability of ETL however stands out as the most necessitated variable affecting the overall photovoltaic device performance. However, n-channel organic semiconductors are extremely prone to air and moisture oxidation, which is accredited to the inherent atmospheric reactivity of the anion generated by electron injection in organic semiconductors. The inexorable demand for robust n-type materials can be in a way achieved through judicious design of molecular backbone and substantial shift of LUMO energy levels below −4 eV within electrochemical stability window (Ribierre et al., 2015; Quinn et al., 2017; Yu et al., 2017).

Therefore, attention has increasingly turned to the development of alternative electron transport materials for inverted perovskite solar cells. Compared with the prevalent fullerene acceptors, the development of non-fullerene organic semiconductors (Zhao et al., 2016; Sun et al., 2016; Shao et al., 2016; Gu et al., 2017; Wu et al. 2017) will upsurge the library of alternate ETL and pave the way to address some of the limitations of fullerene derivatives. Besides other interfacial layers, ETL simulates a significant role in high performance photovoltaic devices, facilitating the extraction and transport of electronic charge carriers while blocking holes at the interface. The optimized function of ETL resonates among several factors such as charge carrier mobility,

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Fig. 2. (a) Concentration dependent UV–visible aggregation studies of TDPP-CN4, (b) relative PL intensity of perovskite thin films with and without PCBM and TDPPCN4, (c) relative PL lifetimes of perovskite thin films with and without PCBM and TDPP-CN4.

Strong electron withdrawing groups e.g. nitro, keto, cyano stabilize the LUMO rendering thermodynamic stability from unwanted oxidation of molecular anions. Quinoidal oligomers strategically tuned with electron-deficient cyano and acyl groups have been extensively researched in literature as n-channel semiconductors with excellent mobility and low-lying conduction band energies below −4 eV (Zhang et al., 2014; Suzuki et al., 2011; Ribierre et al., 2011; Mori et al., 2014). Among library of organic semiconductors, diketopyrrolopyrrole (DPP) based oligomers and polymers have emerged as promising candidates as electron transport materials (Kanimozhi et al., 2012; Mukhopadhyay et al., 2016; Kumar and Patil, 2015). In this work, we utilized quinoidal form of DPP, 2,2′-((5E,5′E)-5,5′-(2,5-bis(2-octyldodecyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrole-1,4-diylidene)bis(thiophene-5,2(5H) diylidene))dimalononitrile (TDPP-CN4) with cyano groups at terminal positions to enhance the electron affinity and air stability. Organic field effect transistors fabricated from TDPPCN4 exhibited n-type transport with electron mobility ∼0.13 cm2 V1 −1 s and ambient stability for more than 5 days. We fabricated inverted perovskite solar cells employing FA0.85Cs0.15Pb(I0.8Br0.2)3 as the active layer, PEDOT:PSS as the hole transporting material and BCP as the hole blocking layer. The preliminary devices exhibit high short circuit current (Jsc) ∼ 17.4 mA/cm2 and moderate open circuit voltage, with overall power conversion efficiency of ∼3.5%. These results reveal that DPP-based ETL are promising air stable alternatives to PCBM.

hydride through a Pd-catalyzed Takahashi coupling reaction in anhydrous tetrahydrofuran (Scheme 1). The detailed synthetic procedure for its preparation is described in SI. TDPP-CN4 was obtained in moderate yields through air oxidation of intermediate dihydro compounds as adapted from previous work (Qiao et al., 2012; Ray et al., 2017). The chemical structure of TDPP-CN4 was established by 1H and 13C NMR (SI). The cyclic voltammetry measurements of TDPP-CN4 was carried out in dichloromethane and we observe a reversible two-step reduction process with negligible response upon oxidation at anodic potential (Fig. S1). The unusually deeper LUMO of 4.46 eV lies well below the conduction band edge of perovskite, facilitating efficient charge extraction from the perovskite to the ETL. Furthermore, the low-lying LUMO energy level of TDPP-CN4 stabilizes anion generated after electron injection against the ambient atmosphere, which clearly suggests that this molecule holds a great potential as air stable electron transport material in inverted perovskite solar cells. The UV–visible spectroscopy of TDPP-CN4 in dichloromethane was carried out to investigate the optical properties. The absorption spectrum of TDPP-CN4 shows weak (300–500 nm) and strong (500–780 nm) electronic band along with vibronically broadened peak at 606 nm (Fig. S2). The vibronically broadening of spectral response at the higher wavelength region can be attributed to the planar rigid quinoidal skeleton of the chromophore.

2. Results and discussion

3. Charge transport and stability measurements

TDPP-CN4 was synthesized according to the literature-reported procedure with necessary modifications from the corresponding dibromosubstituted DPP precursor using malononitrile and sodium

Top gate bottom contact organic field effect transistors (OFET’s) were fabricated with cytop dielectric layer, Ag source-drain electrode and TDPP-CN4 or PCBM as the semiconducting layer. Both the 11

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Fig. 3. (a) Device architecture, (b) energy level diagram, device performance of an inverted perovskite solar cell employing TDPP-CN4/PCBM as electron-transport layer, (c) MAPbI3 as absorbing layer (d) FA0.85Cs0.15Pb(I0.8Br0.2)3 as absorbing layer.

stability of DPP based quinoidal oligomer in OFETs to perovskite solar cells. Preliminary spectroscopic studies were performed to investigate the ETL properties of TDPP-CN4 in inverted perovskite devices. As reported in literature (Bai et al., 2015), the propensity of aggregation of ETL layer on the surface of perovskite can have a significant effect on the photovoltaic behavior by affecting the charge carrier mobility or electron injection barrier at the perovskite/acceptor interface. To evaluate the change in spectral features on varying the concentration of sample solution, an UV–visible concentration dependent study was carried out on TDPP-CN4 in chlorobenzene. The concentration of TDPP-CN4 was varied from 2.4 × 10-5 M to 1 × 10-6 M and observed absorbtion spectra exhibit similar vibronic features without any noticeable shift in wavelengths, which clearly rules out any aggregation in solution (Fig. 2a), that could be extrapolated as absence of aggregating behavior of TDPP-CN4 when spin-coated from solution on the perovskite layer. Photoluminescence (PL) measurements of pristine MAPbI3 perovskite thin film and bilayer perovskite/ TDPP-CN4 thin films were systemically analyzed along with the investigation of PL spectrum of a bilayer perovskite/PCBM thin film for comparison (Fig. 2b). Both TDPP-CN4 and PCBM quench the emission peak of perovskite lying around 788 nm, suggesting the effective electron transfer from perovskite to TDPP-CN4/PCBM. However, the magnitude of PL quenching is higher in case of TDPP-CN4 as compared to PCBM. The singlet exciton lifetime measurement of perovskite by timecorrelated single-photon counting (TCSPC) were carried out to confirm the PL quenching of perovskite by TDPP-CN4 as shown in Fig. 2c. A rapid decay in the photoilluminated carrier lifetime of pure perovskite film was observed for bilayer perovskite/TDPP-CN4 film in comparison to perovskite/PCBM, ascertaining the fact that TDPP-CN4 serves as a

semiconducting molecules exhibited clean n-type transport as observed from the linear and saturation regime of the output characteristics and hysteresis free transfer characteristics (shown in Figs. 1 and S3). It should be noted that the output characteristics of both the molecules exhibit an injection barrier indicated by higher onset voltage (Vd ∼ 4 V) of the output characteristics in TDPP-CN4 and a slightly concave shape of the characteristics in PCBM devices. The field effect mobility is estimated from the saturation regime as 0.13 cm2/Vs for TDPP-CN4 and 0.03 cm2/Vs for PCBM based devices which is of similar orders of magnitude as in the reported literature (Von Hauff et al., 2005; Wang et al., 2015). The inclusion of electron-withdrawing DPP skeleton into the quinoidal architecture possessing terminal cyano groups ensures high electron affinity and resistance towards aerial oxidation. In order to validate the air stability of the devices we performed measurements at regular intervals till we observe a significant shift in the threshold voltage while the devices were exposed to ambient condition. TDPPCN4 exhibits retention of charge transport characteristics for > 5 days with slow decrement in drain current after 7 days. In contrast, devices with PCBM demonstrate similar magnitude of drain current for about 3 days, after which device performance begins to deteriorate (shown in Fig. 1). These comparative studies clearly indicate high air stability of quinoidal DPP based electron acceptors compared to commonly used PCBM in FETs. This enhanced air stability of TDPP-CN4 can be ascribed to deeper LUMO energy level of TDPP-CN4 which also promotes efficient injection of electrons into the semiconducting channel by metal electrodes. Detailed insights into the comparative charge transport behavior of OFETs fabricated with TDPP-CN4 and standard n-type molecule PCBM in ambient conditions have established the remarkable air stability of electron transport in nitrile molecule compared to PCBM. These results encouraged us to exploit and extrapolate the satisfactory operating 12

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Fig. 4. Statistical distribution of device parameters for MAPbI3 perovskite with PCBM/TDPP-CN4 blends as ETM.

Fig. 5. Statistical distribution of device parameters for FA0.83Cs0.17Pb(I0.8Br0.2)3 perovskite with PCBM/TDPP-CN4 blends as ETL.

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Table 1 Comparison of device performance of acceptor layers in inverted perovskite solar cells. Perovskite composition

ETL

Jsc [mA cm−2]

Voc [V]

FF

PCE [%]

MAPbI3

TDPP-CN4 PCBM

17.2 14.7

0.50 0.96

38.0 69.0

3.40 9.70

FA0.85Cs0.15Pb(I0.8Br0.2)3

TDPP-CN4 PCBM PCBM:TDPP-CN4 PCBM:TDPP-CN4 PCBM:TDPP-CN4 PCBM:TDPP-CN4

(1:0.5 wt%) (1:5 wt%) (1:15 wt%) (1:25 wt%)

17.4 19.0 14.8 16.4 7.83 13.1

0.53 0.92 0.83 0.68 0.44 0.50

38.7 64.4 74.7 42.8 18.4 24.9

3.53 11.2 9.11 4.70 0.63 1.62

(1:0.5 wt%) (1:5 wt%) (1:15 wt%) (1:25 wt%)

17.7 19.7 16.6 16.5 7.70 9.20

0.50 0.89 0.89 0.77 0.50 0.50

40.5 50.7 48.4 44.0 27.6 26.7

3.57 8.82 7.10 5.56 1.06 1.22

FA0.85MA0.15Pb(I0.8Br0.2)3

TDPP-CN4 PCBM PCBM:TDPP-CN4 PCBM:TDPP-CN4 PCBM:TDPP-CN4 PCBM:TDPP-CN4

of 15 nm via spin-coating process from chlorobenzene solution. Finally, Au (50 nm) electrode was deposited to complete this device (Sakai et al., 2017). The absorption spectra of thin perovskite films fabricated on glass substrate for all three compositions MAPbI3, FA0.85Cs0.15Pb(I0.8Br0.2)3 and FA0.85MA0.15Pb(I0.8Br0.2)3 is shown in SI (Fig. S4). TDPP-CN4 as ETL was tested in perovskite solar cells employing planar ITO/ PEDOT:PSS/FA0.85Cs0.15Pb(I0.8Br0.2)3/ETL (PCBM and TDPP-CN4)/Au device architecture. Fig. 3d shows current-voltage characteristics of the best devices employing TDPP-CN4 and PCBM in planar PSC. The TDPPCN4 ETL based device exhibit device efficiency of 3.53% (Jsc = 17.4 mA/cm2, Voc = 0.53 V, FF = 38.7%). When 0.5 wt% of TDPP-CN4 blended with PCBM, the device shows 9.11% (Jsc = 14.8 mA/cm2, Voc = 0.83 V, FF = 74.7%), which is comparable to the neat PCBM on a like-to-like comparison (power conversion efficacy (PCE) = 11.2%, Jsc = 19.0 mA/cm2, Voc = 0.92 V, FF = 64.4%). Thus, we could anticipate that the reduced performance of TDPP-CN4 based inverted perovskite devices compared to PCBM ones results as an outcome from low open-circuit voltage and decrement in fill factor. ETL morphology has a significant impact on the device parameters through pinhole-formation, high trap densities and non-uniform films. The reduced Jsc and fill factor in the case of TDPP-CN4 can be correlated with poor morphological control and unoptimal surface coverage on the perovskite film. The non-homogeneous surface coverage of electron acceptor layer onto the perovskite crystallites and associated pinholes act as trap defects, impeding electron transport and current leakage through direct contact between perovskite and contact metal electrode. The statistical distribution of the PSC parameters employing TDPP-CN4 and a blend of TDPP-CN4/PCBM as ETLs is shown in Fig. 5. Other than FA0.85Cs0.15Pb(I0.8Br0.2)3, we have carried out the compositional tuning of lead halide based perovskite precursor by mixing formamidinium (FA) and methylammonium (MA) cations in stoichiometric quantities, FA0.85MA0.15Pb(I0.8Br0.2)3 in order to compare device efficiencies with both perovskites. The optimized current density-voltage curves of FAMA-based perovskite using TDPP-CN4 and PCBM as ETL is shown in Fig. S5. Considering the excellent electron mobility of TDPP-CN4, we used mixture of PCBM and TDPP-CN4 as ETL to further improve the device performance. However, we did not observe improved device performance for TDPP-CN4/PCBM blends compared to neat PCBM as depicted by the frequency distribution of corresponding device measures. The photovoltaic parameters of the optimized devices utilizing PCBM and TDPPCN4 as the electron transport layers for a series of hybrid inorganicorganic perovskite cells are summarized in Table 1. The outcome of these studies suggests that, the synthesis and device optimization of such air-stable DPP based acceptor can expedite a new inclination for further study in perovskite solar cells. It is clear that, the

suitable candidate as ETL for potential application in inverted planar heterojunction perovskite solar cells. Inspired by the prelusive spectroscopic measurements with MAPbI3 perovskite and TDPP-CN4/PCBM as ETL, we fabricated planar heterojunction perovskite photovoltaics with the device configuration ITO/ PEDOT:PSS/MAPbI3/ETL (PCBM and TDPP-CN4)/Au (Fig. 3a, 3b) demonstrating PCE of 3.4% (Jsc = 17.2 mA/cm2, Voc = 0.50 V, FF = 38.0%). The current density-voltage characteristics and statistical distribution of photovoltaic parameters for the device are shown in Figs. 3c and 4, respectively. It is clearly evident from the device results that current density is greater for TDPP-CN4 ETL based perovskite cells compared to those of PCBM based cells, indicating better electron extraction and charge transport behavior at the perovskite/TDPP-CN4 interface. However, the Voc value was observed lower than for the usual optimized perovskite solar cell. This could primarily be attributed to deeper LUMO of TDPP-CN4 (4.46 eV; see SI) with respect to conduction band edge of perovskite layer (∼3.9 eV). As per literature reports, MAPbI3 is known to be not suitable as active layer in inverted perovskite solar cells owing to its thermal and iodide segregation instabilities (Conings et al., 2015). Formamidinium (FA) based mixed halide perovskites possess high structural and thermal stability (Eperon et al., 2014; Binek et al., 2015; Pang et al., 2014; Stoumpos et al., 2013; Stranks and Snaith, 2015; Pellet et al., 2014) as well as enhanced charge carrier diffusion lengths (Stranks et al., 2014) compared to pure iodide counterparts. We have used two different compositions of perovskite namely FA0.85Cs0.15Pb(I0.8Br0.2)3 and FA0.85MA0.15Pb(I0.8Br0.2)3 as photoactive layer (McMeekin et al., 2016; Jeon et al., 2015; Yi et al., 2016; Lee et al., 2015). The device fabrication process involved spin casting about 50 nm-thick poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) hole transport layer on the top of indium tin oxide (ITO) glass substrate at 4000 rpm for 40 s under ambient conditions, followed by an annealing treatment at 150 °C for 10 min. To obtain a FA0.85Cs0.15Pb(I0.8Br0.2)3 precursor solutions, FAI (formamidinium iodide), CsI, PbBr2 and PbI2 were dissolved in a mixed solvent of anhydrous N,N-dimethylformamide (DMF; Aldrich) and anhydrous dimethyl sulfoxide (DMSO; Aldrich) in 4:2 vol ratio to obtain a stoichiometric solution with desired composition and a molar concentration of 1 M. Precursors for other perovskite compositions MAPbI3 and FA0.85MA0.15Pb(I0.8Br0.2)3 were prepared using the procedure given in SI. For the deposition, 80 μL of the precursor solution was spin-coated per substrate in a two-step process, spinning for 10 s at 1000 rpm and 35 s at 6000 rpm, and 10 s before the end of the second spin-coating step, the spinning substrate was quenched with 200 μL of chlorobenzene (Sigma-Aldrich). Thereafter, the perovskite films were annealed at 100 °C in an oven. TDPP-CN4 was used as ETL with a thickness 14

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air stability and excellent electron mobility of TDPP-CN4 can be of great benefit in perovskite solar cells to improve the stability and charge extraction at the interface of ETL and perovskite.

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