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Organic Monomolecular Layers Enable Energy Level Matching for Efficient Hole Transporting Layer Free Inverted Perovskite Solar Cells Weiguang Kong, Wang Li, Changwen Liu, Hui Liu, Jun Miao, Weijun Wang, Shi Chen, Manman Hu, Dedi Li, Abbas Amini, Shaopeng Yang, Jianbo Wang, Baomin Xu, and Chun Cheng ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07627 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Organic Monomolecular Layers Enable Energy Level Matching for Efficient Hole Transporting Layer Free Inverted Perovskite Solar Cells Weiguang Kong,a,b,c# Wang Li,a# Changwen Liu,a# Hui Liu,a Jun Miao,a Weijun Wang,a Shi Chen, a Manman Hu,a Dedi Li,a Abbas Amini,d,e Shaopeng Yang,b Jianbo Wang,c Baomin Xu, a* Chun Chenga* a

Department of Materials Science and Engineering, Southern University of Science and

Technology, Shenzhen, Guangdong Province 518055, China b

Hebei Key Laboratory of Optic-electronic Information Materials, National-Local Joint

Engineering Laboratory of New Energy Photoelectric Devices, College of Physics Science and Technology, Hebei University, Baoding 071002, P. R. China c

Department of Physics, Wuhan University, Wuhan, Hubei Province 430072, China

d

Center for Infrastructure Engineering, Western Sydney University, Kingswood, NSW 2751,

Australia e Department

of Mechanical Engineering, Australian College of Kuwait, Mishref, Kuwait

ABSTRACT: High efficiency hole transport layer-free perovskite solar cells (HTL-free PSCs) with economical and simplified device structure can greatly facilitate the commercialization of PSCs. However, eliminating the key HTL in PSCs results usually in a severe efficiency loss and poor carrier transfer due to the energy level mismatching at ITO/perovskite interface. In this study, we solve this issue by introducing an organic monomolecular layer (ML) to raise the effective work function of ITO with the assistance of interface dipole created by Sn-N bonds. The energy level alignment at the ITO/perovskite interface is optimized with a barrier-free contact which favors efficient charge transfer, and suppressed non-radiative carrier recombination. The HTL-free PSCs based on the ML modified ITO yields an efficiency of 19.4%, much higher than those of HTL-free PSCs on bare ITO (10.26%) comparable to the state of the art PSCs with HTL. This study provides an in-depth understanding of the mechanism of interfacial energy level alignment, and facilitates the design of advanced interfacial materials for simplified and efficient PSCs devices. KEYWORDS: solar cells, monomolecular layers, interface dipole, energy level alignment, interface recombination

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In the past few years, perovskite photovoltaics (PVs) have received great attentions owing to their low-temperature processing, solution processability and high efficiencies. However, for perovskite solar cells (PSCs), hole-transporting layer (HTL) materials (such as Sipro-OMeTAD, PTAA) are expensive for scale-up applications. To overcome this issue, plenty of pioneering works have been reported on HTL-free PSCs.1-3 HTL-free PSCs not only reduce the production cost, but also simplify processing steps. The absence of HTL in PSC structure usually causes a large energy barrier for holes (∆Eh) to transport across the interface of perovskite/electrode; this fundamentally decreases the performance of PSCs. ∆Eh is defined as the energy difference between the Fermi level (EF) of electrode and the valance band maximum (VBM) of perovskites.4 For high efficiency HTL-free PSCs, it is highly desired to reduce as low as possible or wholly eliminate ∆Eh. To minimize ∆Eh, one strategy is to approach the VBM of perovskites towards EF of the electrode. Li and co-authors introduced graded CsPbBrxI3-x to PSCs to raise up the VBM of the perovskite towards EF of the electrode. The power conversion efficiency (PCE) of the modified HTL-free PSCs reaches to 11.33% at maximum, but is still better than the control HTL-free PSC (~8.00%).5

Recently,

Huang

utilized

2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodime-thane

(F4TCNQ) doped MAPbI3 to fabricate HTL-free PSCs.6 F4TCNQ doping induced an upward bending of the valence band at the ITO/perovskite interface with eliminated ∆Eh as the author claimed. The final HTL-free PSCs gave an average efficiency of 18.85%. However, engineering the VBM of perovskites to minimize ∆Eh at ITO/perovskite interface is quite challenging, since the electronic structure of perovskites depends highly on the specifics of these compositions which vary with the preparation and ambient conditions.7, 8 In fact, the existence of electron states that tail into the band gap of perovskite materials, results in a Fermi-level pinning at these states, and consequently, ∆Eh can hardly approach to zero.9-13 Another strategy to minimize ∆Eh is to shift ITO EF towards VBM of perovskites. To this end, organic monomolecular layer (ML) materials, which can regulate EF of electrodes, have been widely used in organic electronics.14-16 Despite of this property, organic ML has rarely been utilized to fabricate high-efficiency HTL-free PSCs. On the one hand, in PSC area, the energy-level alignment at the electrode/HTL interface has been often oversimplified in previous reports.17 On the other hand, different from organic materials, strong ionic nature of perovskites may eliminate the impact of organic ML on EF of the electrode by strong chemical interaction with it.18 Up to now, the feasibility of using organic ML in constructing efficient HTL-free PSCs is yet unclear and rarely reported. In this work, we report the application of 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl) benzenamine] (TAPC) as an organic ML to effectively modify EF of ITO. It is found that TAPC ML can chemically bind to ITO surface by forming strong N-Sn bonds as identified by XPS analysis. The polar N-Sn bond acts as a dipole layer and results in a significant increase of effective work function of ITO from 4.32 to 5.35 eV; this is attributed to strong charge transfer at the interface of ITO/TAPC ML. The increased work function of ITO converts the band bending direction of the perovskite from downward to upward. Upon the transport of the holes from


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perovskite to ITO, ∆Eh is dramatically decreased from 1.67 to 0.05 eV, while the electron transport barrier (∆Ee) is increased from 0.3 to 1.55 eV. Thereby, the charge transport and extraction efficiency are greatly improved, and the possibility of charge recombination at the interface of electrode/perovskite is effectively decreased. The efficiency of proposed system (HTL-free PSCs with TAPC ML on ITO) exceeds from 19%, while PSC on bare ITO can only yield a low efficiency of 10.26%. This interesting improvement in the performance of PSCs originates mainly from the greatly enhanced open circuit voltage (VOC) and FF. In fact, the ultralow dark saturated current density J0 (1.9×10-7 mA/cm2) and small ideal factor n (1.9) confirm lower resistance series (RS) and less carrier recombination loss. RESULTS/DISCUSSION

Figure 1 TAPC with mono-molecule thickness on ITO and quartz. (a) N ls XPS curves of ITO/TAPC ML and ITO; (b) absorption spectra of TAPC film, and TAPC ML amplified 20 times. (c) AFM image of ITO covered with TAPC ML and (d) AFM image of pristine ITO as the reference. TAPC ML is fabricated either by spin-coating or thermal evaporation followed by a 110ºC thermal annealing treatment. To ensure the formation of TAPC ML, chlorobenzene (CB) is used to remove the redundant TAPC layers. It is noted that TAPC ML can hardly be removed from ITO surface even after washing with CB for several times. This is because of the strong corresponding


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interaction of Sn-N bonds in TAPC ML and ITO as identified by X-ray photoelectron spectroscopy (XPS) analysis (see Supporting Information, Figure S1). From XPS results in Figure 1a, the weak peak at ~398 eV is assigned to N 1s, indicating that TAPC remains on the surface of ITO. Figure 1b shows the absorption spectra of TAPC ML and TAPC film with the thickness of 30 nm on quartz. The bandgap of TAPC film is determined to be 3.5 eV, while it shifts to 3.23 eV for TAPC ML as shown in Figure S2a. The energy difference could be derived from the chemical interactions between the substrate and organic ligands (Figure S2b and c) which changes the conjugated structures of the chromophore of the organic molecules as being reported.19 As shown in Figure 2b, the largest absorbance of TAPC ML at 4.0 eV is 1/20 of that of TAPC film. The thickness of TAPC ML is therefore estimated to be ~1.5 nm, according to A=αd where A is the absorbance, α is the absorption coefficient and d (=30 nm) is the thickness of TAPC film. In Figure 1b, an absorption shoulder at 3.1 eV is observed in both TAPC ML and the TAPC film, which results from the shallow electronic states inside the band gap. These shallow electronic states close to the HOMO (or LUMO) level of TAPC working as the p- or n-type dopant can shift the Fermi level of TAPC from mid-gap toward HOMO (or LUMO) level as shown in Figure S4. As reported by Oehzelt, a pronounced charge transfer across the interface of ITO/TAPC can lead to a steeper potential energy change for ITO/TAPC ML which is further studied in the following.20 As TAPC ML is rather thin, naked eyes cannot distinguish the existence of TAPC ML on ITO surface as shown in Figure S3. The surface topography of bare ITO and ITO covered with a TAPC ML (ML-ITO) is characterized by atomic force microscopy (AFM) analysis as shown in Figures 1c and 1d. It is obvious that TAPC ML on ITO does not change the surface roughness. The root-mean-square (RMS) roughness (rRMS) of bare ITO varies from 2.2 to 2.6 nm, while it ranges from 2.3 to 2.5 nm for ML-ITO. This can be explained by the short length of TAPC molecules (1.5 nm) compared to the roughness of ITO (RRMS≈2.4 nm).


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Figure 2 Electronic structure change induced by the interface dipole. (a) UPS spectra of ITO, TAPC ML and TAPC film in HOMO region (left) and in secondary-cutoff region (right). (b) The Fermi level of ITO and the electronic structures of TAPC ML and TAPC film, which is directly derived from UPS results. (c) The formation of dipole layer and the energy level alignment at the interface of ITO and TAPC; Blue line represents the vacuum level (VL). (d) Vacuum level shift at the surface of ITO caused by the interface dipole, the green dashed lines indicate the interactions between Sn and N, rather than the real Sn-N bonds. The Fermi level of ITO, and HOMO level of TAPC ML and TAPC film are investigated by ultraviolet photoelectron spectroscopy (UPS) measurement (Figure 2a). This method has been widely used in surface science.21 As shown in Figure 2a, EF is determined by hν-E0, where hν (=21.22 eV) is the photon energy, E0 is the binding energy value where the secondary electron cutoff. VL is the vacuum level which is sensitive to the surface characteristics of the sample. The work function of bare ITO is VL-EF=4.32 eV, which is similar to previous reports, 14, 22, 23 while the work function of TAPC ML and TAPC film are determined as 3.68 and 5.16 eV, respectively. Figure 2b shows the Fermi level and electronic structures of TAPC ML and TAPC film. The lowest unoccupied molecular orbitals (LUMO) levels of TAPC ML and TAPC film are determined by the absorption spectra shown in Figure 1b. In Figure 2b, the Fermi level of TAPC film is close to the HOMO level (∆E=0.44eV), indicating TAPC film is self-doped p-type organic


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semiconductor. This result is consistent with the absorption spectra which shows an obvious sub-band absorption peak at 3.1eV (Figure 1b). Similar to p-type doped spiro-OMeTAD, the sub-band absorption peak comes from the electronic states close to the HOMO level which is in favor of efficient charge transfer. 24 By contrast, the Fermi level of TAPC ML lies approximately at the middle of the bandgap, i.e., 1.67eV, above the HOMO level, and 1.56 eV below the LUMO level. This fact suggests that TAPC ML is intrinsic. Due to the large bandgap (3.23 eV) of TAPC, TAPC ML can be regarded as an insulator and does not act as the traditional HTL materials to construct the p-i bulk heterojunction with the perovskite in PSCs. The role of TAPC ML is much similar to that of the polytetrafluoroethylene (PTFE, an organic insulator material) to increase the work function of ITO via an interface dipole in organic PVs.25 Figure 2c shows the energy level alignment at the interface of ITO/TAPC where the Fermi level throughout TAPC is aligned with ITO. Here, Sn-N bonds construct a dipole layer at the interface of ITO and TAPC ML with mainly one molecule thickness, along with a band bending toward the interior region of TAPC film.26 The dipoles at ITO/TAPC interfaces introduce an electrical field pointing from TAPC towards ITO surface which finally results in a vacuum level shift at the surface of ITO. Figure 2d shows the effective work function change at the surface of ITO caused by the interface dipole. The effective work function (Φeff) of ITO is determined by Equation (1):20 (1)

Φ𝑒𝑓𝑓 = Φ + 𝐸ID

where EID (=1.03 eV) is the energy of interface dipole and Φ (=4.32 eV) is the work function of ITO. The large EID can be caused by the strong van der Waals force between ITO and TAPC ML via Sn-N bonds.27 The effective work function (Φeff) of ML-ITO is calculated as 5.35 eV according to Equation (1) which is close to the valence band maximum (VBM) of the perovskite.


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Figure 3. Schematic band diagrams of anode/perovskite interface in HTL-free PSCs: (a) ITO/perovskite; (b) ML-ITO/perovskite interface. The blue solid line represents the vacuum level, and black dashed line is the Fermi level of ITO. (c) Photon-generated charge carrier transfer and extraction in HTL-free PSCs with (c) downward and (d) upward band-bending direction. Figure 3 shows schematic band diagrams of the interface of ITO/perovskite and ML-ITO/perovskite in PSCs according to Schottky-Mott limit on the basis of the relative alignment of energy levels, where VBM and EF of the perovskites are, respectively, set at -5.4 eV and in the middle of band gap. The bandgap of perovskite is 1.6 eV determined by the near-band-edge absorption spectra shown in Figure S5. In this case, the dipole at the interface of ITO/perovskite is negligible due to the limited charge transfer at the interface between the intrinsic perovskite and ITO.20 As illustrated in Figures 3a and 3c, a downward energy band configuration is formed in the perovskite film. ∆Eh at the interface of ITO/perovskite is determined to be 1.08 eV, too large for efficient hole extraction, while the ∆Ee is 0.52 eV, insufficient to block electrons. Both large ∆Eh and small ∆Ee can induce a severe carrier interface-mediated recombination loss at ITO/perovskite interface which will be studied in-depth in the following. On the other hand, the downward energy band configuration partially offsets the built-in potential and is expected to decrease the VOC for PSCs.28 As shown in Figures 3b and 3d, an approximate Ohmic contact and an upward band-bending configuration are formed at the interface of ML-ITO/perovskite. ∆Eh at ML-ITO/perovskite interface is reduced from initial 1.08 eV to very low magnitude of 0.05 eV, meanwhile, the ∆Ee increases from initial 0.52 eV to 1.55 eV. The upward band-bending as the driving force in the perovskite enables an efficient carrier transfer and extraction by the corresponding electrodes and is in favor of reducing the carrier interface-mediated recombination. Finally, large VOC and an enhancement in the performance of HTL-free PSCs are expected, which is investigated as follows.13 It has been reported that organic ML changes the wettability of substrates by replacing the terminating -OH groups of ITO which may influence the nucleation and further the surface morphology of the perovskite (Figure S6).14 Here, a modified solvent annealing process along with a low annealing temperature is applied to optimize the morphology of perovskite; the detail procedure can be seen in the Experimental Section. Figure S5 shows the scanning electron microscopy (SEM) images and the XRD patterns of perovskite films on ITO and ML-ITO. As shown in Figure S7, the surface morphology of perovskite films on different substrates exhibits similar crystal grain sizes and pinhole-free textures. In addition, the XRD peak positions and intensities of perovskite films are similar. These results indicate that the quality of all the perovskite films in this study is almost the same. The changes in device performances are mainly from the variations at the junctions of the perovskite and HTL (or ETL) in PSCs. The oxygen-plasma etching/UV ozone treatment has been widely used to modify the wettability and the work function of ITO. However, it is concerned that the oxygen-plasma etching/UV ozone treatment could result in a degradation of the organic layer and/or introduce deep trap centers into the perovskite, which would sharply deteriorates the performance of the perovskite solar cells. 29-31


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Comparably, TAPC monolayer can strongly anchor on ITO surface and increase the work function of ITO. In addition, the perovskite precursor solution on it shows a very small contact angle (16.8º) which is in favor of the uniform perovskite film formation with a full coverage to the substrate. Therefore, TAPC monolayer can replace the oxygen-plasma etching/UV ozone treatment and eliminate the potential negative effects of the oxygen-plasma etching/UV ozone treatment on PSCs.

Figure 4 Optical characterization of the perovskite on different substrates. (a) Steady PL and (b) TRPL spectra of the perovskite on different substrates. The photon-generated carrier transfer and recombination dynamic at the interface of ITO/perovskite and ML-ITO/perovskite are studied by the steady and time-resolved photoluminescence (PL and TRPL) spectra. At the interface of ITO/perovskite, two dynamic processes coexist and compete with each other, i.e., carrier charge transfer and interface-mediated recombination. In photoelectric devices, the interface-mediated recombination is detrimental to the device performances and should be eliminated.13,

23, 32, 33

To minimize the carrier loss by

interface-mediated recombination, an optimized energy level alignment, as well as a deep traps free interface is highly desired. Figure 4a shows the steady PL spectra of the perovskite on different substrates. The PL quenching at the interface is quantified by the relative PL integrated intensity change (∆I), where PL intensity from the perovskite on quartz is used as the reference. As shown in Figure 5a, it is found that the PL intensity change (∆I=15) at the ITO/perovskite interface is much larger than that at the ML-ITO/perovskite interface (∆I=6). This result manifests the carrier interface-mediated recombination is effectively suppressed at the interface of ML-ITO/perovskite. Figure 4b shows the TRPL spectra of perovskite on different substrates. For better understanding of the carrier recombination dynamic at the interface of ITO/perovskite, we introduce surface recombination velocity (S), which is independent of the quality of perovskite bulk, by Eq. (2): 1 𝜏𝑚𝑒𝑎𝑠

1

𝑆

(2)

= 𝜏𝑏𝑢𝑙𝑘 + 𝑑

where τmeas is the average lifetime PL lifetime (the method and the values are shown in Table S1),


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τbulk is the PL lifetime obtained from perovskite on quartz which is calculated as 176 ns, and d (=500 nm) is the perovskite film thickness. For the perovskite on ITO, S is calculated as 2.5 m/s, while S for the perovskite on ML-ITO, decreases sharply to 1.16 m/s. The decreased S indicates that the possibility for carrier interface-mediated recombination at ML-ITO/perovskite interface is effectively reduced; this is consistent with the results from the steady PL measurement. It is worth noting that the perovskite on quartz shows a rigid single exponential PL decay curve (Figure 5b). This fact indicates that the surface states are shallow and delocalized for the perovskite films. This can be further evidenced by the near-band-edge absorption spectra of the perovskite films on either ITO or ML-ITO. No obvious electronic states within the band gap are observed as shown in Figure S4. In this situation, the interfaces of ITO/perovskite and ML-ITO/perovskite can be regarded as deep traps free.34 As a result, we can conclude that TAPC ML can effectively suppress the carrier interface-mediated recombination through optimization the energy level alignment at the interface of ITO/perovskite. This is beneficial for the charge carrier transfer and extraction, and further study of HTL-free PSCs.

Figure 5. Dark current-voltage curves of trap density and carrier mobility of the perovskite films on (a) ITO and (b) ML-ITO. n=1 is the Ohmic region, n=2 is the SCLC region, and in between is the trap-filled limited region (TFL). The onset of TFL region is defined as VTFL. ITO is positive biased during the measurement. The dark current-voltage curves of the perovskite films on ITO and ML-ITO are measured based on the hole only devices: ITO/Perovskite/MoO3/Ag and ML-ITO/Pervskite/MoO3/Ag and the result is shown in Figure 5. The current is injected from ITO (or ML-ITO) to the perovskite and is very sensitive to the changes in the contact barrier ∆Eh.35,

36

As shown in Figure S8 in

Supporting Information, the current injected from ML-ITO is comparable to that from PTAA indicating an Ohmic contact (∆Eh=0) is formed at ML-ITO/perovskite interface. By contrast, there is a considerable decrease in the current injected from ITO due to the large contact barrier ∆Eh at ITO/perovskite interface.37,

38

This fact suggests our physical model in Figure 3 is reasonable.

When ∆Eh is approaching zero, the current is due to space-charge-limited current (SCLC) which is the maximum electrostatic current allowed to pass through the perovskite layer. In this case, the hole mobility of the perovskite films can be obtained with a Mott-Gurney law:


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𝑉2

(3)

J = 8𝜀0𝜀𝜇𝑑3

Here, ε (=25) is the relative dielectric constant of the perovskite, and ε0 is the vacuum permittivity. 2

−1 −1

The hole mobility of the perovskite on ML-ITO is calculated to be 1.13 ×10-2 cm V s

similar

to the values for the perovskite on TAPC film with the thickness of 30 nm.39 This fact further demonstrates the ∆Eh is eliminated at the interface of ML-ITO/perovskite. By contrast, the hole 2

−1

mobility of the perovskite on ITO is calculated to be 2.4×10-3 cm V

s

−1

near one order of

magnitude smaller than the reported values. In this case, the current in the hole only device is injection limited (rather than space charge limited) due to the large contact barrier (∆Eh>0.3eV) at the interface of ITO/perovskite that cannot be neglected.38 Here we also calculate the trap density Ntrap in perovskite films which is determined by Equation (4), since the carrier mobility is also sensitive to the trap density of the semiconductor. 𝑉𝑇𝐹𝐿 =

q𝑁𝑡𝑟𝑎𝑝𝑑2

(4)

2𝜀𝜀0

where, VTFL is the onset voltage of the trap-filled limit (TFL) region which is linearly proportional to Ntrap, and q is the elemental charge. As shown in Figure 5, the VTFL is set at 0.59 V and 0.46 V for the perovskite on ITO and ML-ITO, respectively. The perovskite films on ITO and ML-ITO exhibit comparable trap state densities, Ntrap, in the order of 1×1015 cm−3 (Table S2), two orders of magnitude lower than that in MAPbI3 films.40 This fact confirms that the quality of all the perovskite films on different substrates is the same and the injected current from ITO (or ML-ITO) to the perovskite is only related to the contact barrier ∆Eh.

Figure 6 Solar cell architecture and characterization. (a) Configuration of PSCs. Notice: the green dashed lines indicate the interactions between Sn and N, rather than the real Sn-N bonds. (b) Cross-sectional SEM image of HTL-free PSCs. (c) J-V curves of PSCs based on ITO and ML-ITO with different scanning directions. (d) EQE spectrum and integrated current density of the champion PSC. (e) Distribution of PCEs based on 30 individual devices for PSCs based on ML-ITO (f) Stability measurement of JSC and efficiency of PSCs under continuous one simulated sun soaking. To further investigate the role of TAPC ML, HTL-free PSCs are fabricated by using ML-ITO


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and bare ITO (reference) as substrates, respectively. Figure 6a shows the energy level diagram of PSCs with the presence of TAPC ML between ITO and perovskite. Figure 6b illustrates the cross-sectional SEM image of HTL-free PSCs based on ML-ITO. The bright dots in Figure 7b are the platinum particles which are out of the intention to increase the conductivity of the sample during the SEM measurement. Figure 6c presents J-V curves for the champion PSC based on ML-ITO, as well as the reference PSC based on bare ITO. The PSC based on bare ITO possesses 10.26% PCE. By contrast, the champion PSC based on ML-ITO without any current leakage (Figure S9) gives the PCE as high as 19.4% which is higher than the highest (18.8%) PCE reported to-date for the devices based on TAPC film (30 nm).39 The improved performance for HTL-free PSC based on ML-ITO is mainly from the enhanced FF and VOC as shown in Table 1.41 The VOC of the HTL-free PSC based on ML-ITO reaches 1.12 V, which is comparable to the PSCs with eliminated interfacial recombination by the passivation layer.13, 23 By contrast, PSCs based on ITO give a maximum VOC (0.83 V) which is similar to the previous reported values for HTL-free PSCs.41,

42

Figure 6d shows the external quantum efficiency (EQE) of the champion

device. The integrated JSC (=21.02 mA∙cm-2) from the EQE measurement agrees well with the measured JSC under AM 1.5 G one sun illumination. We also analyzed the distribution of PCEs based on 30 individual devices for PSCs based on ML-ITO as shown in Figure 6e. The majority of PCEs lies between 17.0 and 19.0%, exhibiting high uniformity and reproducibility. The average PCE of the PSCs based on ML-ITO is 18.5%, still better than that (17.55%) based on TAPC film (30 nm) as reported.39 To explain the evolution in FF and VOC, series resistance RS, dark saturated current density J0 and ideal factor n are investigated. The influence of RS, J0 and n on device has been well established in diode based solar cells, according to the classical diode equation:

[

J = 𝐽𝑆𝐶 ― 𝐽0𝑒𝑥𝑝

𝑞(𝑉 + 𝐽𝑅𝑠) 𝑛𝑘𝐵𝑇

]

(7)

where J is the device output current density, KB is Boltzmann constant and T is temperature. The detailed method and the results are shown in Figure S6.43 The resulting parameters are all tabulated in Table 1. The RS for PSCs based on ML-ITO is calculated 1.8 Ω comparable to the state of the art PSCs with HTL. For PSCs based on bare ITO, RS is sharply increased to 5.2 Ω by almost 3 times. The increased RS has been reported to be the main reason resulting in the significant decrease in FF.6 J0 is identified as the carrier recombination current density in thermal equilibrium and is directly related to the recombination velocity. The smaller J0 in PSC indicates slower carrier recombination velocity and accordingly a promoted VOC consistent with a high FF (the relationship between VOC and J0 can be seen in the Supporting Information).43 J0 for the PSCs on ML-ITO is calculated as 1.9×10-7 mA/cm2 which is 4 orders of magnitude smaller than that on bare ITO which is 2.5×10-3 mA/cm2. The ideal factor n is accounted for the carrier recombination loss in the space charge region of a diode.44 It has been reported that the ideality factor n for the perovskite solar cells should be close to 2, since the Shockley- Read-Hall (SRH) is expected to be


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the dominant carrier recombination mechanism in intrinsic perovskite.45 However, some factors, such as series resistance effect and parasitic shunt effects, etc., could result in ideality factor even larger than 3.45, 46 In our case, n for the HTL-free PSCs based on TAPC monolayer modified ITO is calculated to be 1.9, indicating the carrier transport through ETL and HTL is sufficiently high and current is limited by recombination rate in perovskite layer. By contrast, n for the HTL-free PSCs based on bare ITO is much larger than 2, which is determined to be 2.7 in forward scanning mode and 3 in backward scanning mode as shown in Figure S10 (the interpretation of the hysteresis can be found in Ref.(46). In our case, n (>2) for the HTL-free PSCs based on bare ITO could be due to the increased RS and J0 as discussed above which dominate the carrier recombination mechanism. Hysteresis is another concern for the PSC community. Although the origin of hysteresis is still controversial, it is widely accepted that a balanced carrier extraction via interface engineering can lead to diminishing hysteresis in PSCs.47 In our case, the obvious hysteresis in the PSC on ITO can be interpreted by the lower hole-extraction efficiency with respect to that on ML-ITO, where the electron extraction is the same for the two prototype PV devices. Figure 6f shows the stability of JSC and photocurrent density (20.5 mA/cm2) of the PSC based on ML-ITO, measured at 0.94 V giving a stabilized PCE exceeding 19% after a 600s continuous one simulated sun soaking under ambient condition with the relative humidity ~40%. Table 1. Photovoltaic parameters of PSCs with different substrates. Scanning

ML-ITO/Perovskite/C60/BC

VOC

JSC

FF 2

mode

(V)

(mA/cm )

(%)

Forward

1.12

22.07

78.89

η (%)

Backward

1.11

21.57

78.76

18.86

Forward

0.75

21.5

58.5

9.5

Backward

0.83

21.35

57.8

Ideal

(mA/cm2)

factor

1.8

1.9×10-7

1.9

5.2

2.5×10-3

19.42

P/Ag

ITO/Perovskite/C60/BCP/Ag

J0

RS (Ω)

10.26

2.7 3

CONCLUSIONS High-efficiency HTL-free PSC based on ML-ITO and the underlying mechanism have been introduced in this study. TAPC ML anchors on ITO via Sn-N bonds forms an interfacial dipole at the ITO surface. It is found that the interface dipole at the ITO/ML interface can increase the effective work function of ITO. By utilizing TAPC ML modified ITO as the substrate, the perovskite grown on that possesses an upward energy band-bending at the ITO/perovskite interface. Correspondingly, ∆Eh of the holes to transfer from the perovskite to ITO is decreased from 1.08 eV to as small as 0.05 eV, while ∆Ee is increased from 0.52 to 1.55eV. As a result, the surface recombination velocity is sharply decreased from 2.5 to 1.16 m/s. SCLC result verifies the


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ACS Nano

efficient hole transfer and extraction mainly based on the optimized energy level alignment at ITO/perovskite interface. Finally, the champion HTL-free PSCs based on ML-ITO produce an efficiency of over 19%, which is almost two times larger than that PSCs/ITO (10.26%). The efficiency enhancement is mainly originated from the improved FF and VOC, which is directly related to the decreased series resistance and the saturated dark current density. In particular, the HTL-free PSCs/ML-ITO shows a negligible hysteresis and a robust stability under illumination by simulated sunlight. Our work highlights the significance of the energy level alignment at the interface of ITO/HTL. It provides an in-depth understanding for the mechanisms of HTL-free PSCs and sheds light on the design of advanced interfacial materials for simplified PSCs devices. METHODS/EXPERIMENTAL Materials. Indium tin oxide (ITO)-coated glasses were purchased from Ying Kou You Xuan Trade Co. Ltd., China, and the sheet resistance was measured to be 15 Ω sq-1. PbI2 (99.9985%) was purchased from Alfa Aesar, United States, TAPC (97%). MAI, PbCl2, C60 and BCP were purchased from Xi’an Polymer Light Technology Corp, China. All solvents, such as N, N-dimethylformamide (DMF), isopropanol, chlorobenzene, and dimethyl sulfoxide (DMSO), were purchased from Sigma-Aldrich, United States. These commercially available materials were used directly without further purification. Methods. ITO glass was firstly cleaned by detergent, acetone and isopropanol in sequence interval of 20 min in an ultrasonic bath. After drying with nitrogen flow, ITO glass was treated by UV-ozone treatment for 15 min before usage. TAPC ML was fabricated by thermal evaporation of TAPC powder onto the precleaned ITO glass inside a vacuum chamber with a pressure of 2×10-4 Pa, followed by 20 min-annealing at 110°C. TAPC ML on ITO is washed by chlorobenzene (CB) before deposition of perovskite films. The MAPbI3-xClx precursor solution was prepared by dissolving 497.88 mg PbI2, 33.37mg PbCl2, 190.8 g MAI in 0.9 ml DMF and 0.1 ml DMSO mixed solvent. After stilled for 12 h, the MAPbI3-xClx precursor solution was dripped on the ITO substrates and spin-coated at 700 rpm for 3s and 5000 rpm for 25s. 100 μL of CB was dripped within 1s onto the substrates at 17s before spin-coating was finished. Then, the as-prepared perovskie films were transferred onto a hotplate at 40oC and covered with a petri dish until the perovskite films turned to dark brown. Afterwards, the petri dish was removed and the perovskite films were annealed at the same temperature for another 2h. Then, a higher temperature annealing at 95oC was applied for 10 min to obtain the high quality perovskite films. Finally, C60 (35 nm), BCP (8 nm), and Ag (100 nm) were sequentially deposited on the perovskite film by thermal evaporation through a shadow mask with a defined active area of 0.10 cm2. Characterizations. The morphology of perovskite and HTLs films were characterized by scanning electron microscopy (SEM, TESCAN MIRA3) at a 5 kV accelerating voltage. Atomic force microscope (AFM) images of HTLs films were collected on a multimode SPM (Bruker). UPS analysis was conducted on the X-ray photoelectron spectrometer (Thermo Fischer, ESCALAB 250Xi) with an unfiltered He I (21.22 eV) gas discharge lamp and a hemispherical


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analyzer. Steady-state and time-resolved PL spectra were measured using an Edinburgh FLS5 spectroscopy system, and pulsed laser excitation at 405 nm. The excitation density of 5 nJ·cm-2 was used to avoid nonlinear effects. Photocurrent density-voltage (J-V) curves were measured under AM 1.5 G one sun illumination (100 mW/cm2) with a solar simulator (Enlitech SS-F7-3A) equipped with a 300 W xenon lamp and a Keithley 2400 source meter. The light intensity was adjusted by an NREL-calibrated Si solar cell. During measurement, the cell was covered by a mask with 0.1 cm2 aperture. The external quantum efficiency (EQE) values were measured using an EQE system (Enlitech QE-R3011) containing a Xenon lamp, a monochromator, a Si detector for calibration, and a dual-channel power. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. Figures showing the XPS spectra of TAPC on ITO, bandgap determination of TAPC monolayer, SEM and absorption spectra of the perovskite, contact angle measurements, SCLC measurements, dark J-V curves and the methods for calculation n, J0 and PL lifetime. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], [email protected]. Author Contributions W. K., W. L. and C. L. contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Project funding from the Ministry of Science and Technology of China (Grants Nos. 2016YFA0202400 and 2016YFA0202404), the Peacock Team Project funding from Shenzhen Science and Technology Innovation Committee (Grant No. KQTD2015033110182370), the National Natural Science Foundation of China (Grant No. 51776094), the Guangdong Natural Science Funds for Distinguished Young Scholars (Grant No. 2015A030306044), and the Guangdong-Hong Kong joint innovation project (Grant No. 2016A050503012). REFERENCES (1) Etgar, L., Hole-transport Material-free Perovskite-Based Solar Cells. MRS Bulletin. 2015, 40, 674-680. (2) Zuo, C.; Bolink, H. J.; Han, H.; Huang, J.; Cahen, D.; Ding, L., Advances in Perovskite Solar Cells. Adv. Sci. 2016, 3, 1500324. (3) Maniarasu, S.; Korukonda, T. B.; Manjunath, V.; Ramasamy, E.; Ramesh, M.; Veerappan, G., Recent Advancement in Metal Cathode and Hole-Conductor-Free Perovskite Solar Cells for Low-Cost and High Stability: A route Towards Commercialization. Renewable Sustainable Energy Rev. 2018, 82, 845-857.


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ACS Nano

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