Electronic Structure of Nonionic Surfactant ... - ACS Publications

Subscriber access provided by CAL STATE UNIV BAKERSFIELD

Surfaces, Interfaces, and Applications

Electronic Structure of Nonionic Surfactant-Modified PEDOT:PSS and its Application in Perovskite Solar Cells with Reduced Interface Recombination Dongguen Shin, Donghee Kang, Jae-Bok Lee, Jong-Hyun Ahn, Il-Wook Cho, Mee-Yi Ryu, Sang Wan Cho, Na Eun Jung, Hyunbok Lee, and Yeonjin Yi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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.

Page 1 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Electronic Structure of Nonionic Surfactant-Modified PEDOT:PSS and its Application in Perovskite Solar Cells with Reduced Interface Recombination Dongguen Shin,†,‡ Donghee Kang,†,‡ Jae-Bok Lee,§ Jong-Hyun Ahn,§ Il-Wook Cho,║ Mee-Yi Ryu,║ Sang Wan Cho,┴ Na Eun Jung,†,‡ Hyunbok Lee*,║ and Yeonjin Yi*,†,‡ †Institute

of Physics and Applied Physics and ‡van der Waals Materials Research Center,

Yonsei University, 50 Yonsei-ro, Seodaemun-Gu, Seoul 03722, Republic of Korea §School

of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro,

Seodaemun-Gu, Seoul 03722, Republic of Korea ║Department

of Physics, Kangwon National University, 1 Gangwondaehak-gil, Chuncheon-si,

Gangwon-do 24341, Republic of Korea ┴Department

of Physics, Yonsei University, 1 Yonseidae-gil, Wonju-si, Gangwon-do 26493,

Republic of Korea

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 24

ABSTRACT The interfacial properties of organolead halide perovskite solar cells (PSCs) affect the exciton and charge transport dynamics significantly. Thus, proper modification of the interfaces between perovskite and charge transport layers is an efficient method to increase the power conversion efficiency (PCE) of PSCs. In this work, we explore the effect of a nonionic surfactant, i.e., Triton X-100 (TX) additive, in the poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS) hole transport layer. The electronic structure of TXmodified PEDOT:PSS is investigated with ultraviolet/X-ray photoelectron spectroscopy and X-ray absorption spectroscopy with various TX concentrations. The surface of the TXmodified PEDOT:PSS layer showed high TX content, and thus the semi-metallic properties of PEDOT:PSS were suppressed conspicuously by its insulating nature. With the TX-modified PEDOT:PSS, the PCE of methylammonium lead iodide (MAPbI3) PSCs increased significantly. To elucidate the origin of the improved device performance, the electrical properties and photoluminescence were investigated comprehensively. Consequently, it was found that the TX additive inhibits interface recombination between PEDOT:PSS and MAPbI3, which is caused by the suppression of semi-metallic properties of the PEDOT:PSS surface. Hence, we fabricated flexible PSCs successfully using a graphene electrode and TX-modified PEDOT:PSS.

KEYWORDS PEDOT:PSS, organolead halide perovskite solar cells, energy-level alignment, interface recombination, flexible perovskite solar cells


2

Page 3 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction The device performance of organolead halide perovskite solar cells (PSCs) has improved rapidly. Currently, the reported highest power conversion efficiency (PCE) exceeds 23% owing to the recent intensive studies.1 It is well known that the interfacial properties including the energy-level alignment and morphology affect charge transport and exciton quenching in PSCs significantly.2–5 Thus, the proper modification of interfaces is critical in increasing the PCE of PSCs. From the standpoint of energy-level alignment at the anode interface, the energy offset between the Fermi level (EF) of an anode and the valence band maximum (VBM) of a perovskite layer should be minimized. Hence, various hole transport layers (HTLs), such as poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)

(PEDOT:PSS),

poly(3-

hexylthiophene-2,5-diyl (P3HT), poly(thieno[3,4-b]-thiophene-co-benzodi-thiophene (PTB7), poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]

(PTAA),

poly[N,N'-bis(4-butylphenyl)-

N,N'-bis(phenyl)-benzidine] (poly-TPD), NiO, and CuSCN, have been inserted between the anode and the perovskite layer.6–9 Among them, PEDOT:PSS is a popular HTL owing to its high work function and conductivity, as demonstrated in organic solar cells (OSCs).10–13 However, PEDOT:PSS also exhibits intrinsic vulnerabilities that render PSCs difficult to attain the best PCE. Representatively, the semi-metallic characteristics of PEDOT:PSS induce interface recombination, thus causing a detrimental effect to the device performance of PSCs, OSCs, and organic light-emitting diodes.14–18 In PSCs, this interface recombination is the primary culprit of the deterioration in short-circuit current density (JSC) and open-circuit voltage (VOC).19–21 Hence, the molecular modification of PEDOT:PSS, which tailors its electronic properties, is necessary to achieve highly efficient PSCs. An efficient method to engineer the electronic properties of PEDOT:PSS is by mixing a nonionic surfactant into PEDOT:PSS. This nonionic surfactant is speculated to form the long nanofibril structure of hydrophobic PEDOT in PEDOT:PSS.22,23 Particularly, Triton X-100


3


Page 4 of 24

(TX) is a well-known nonionic surfactant used for this purpose. In addition, the TX additive renders a uniform coating of PEDOT:PSS on a hydrophobic graphene surface. This enables the PEDOT:PSS HTL to be used with a graphene electrode in flexible devices.22,23 In the literature, the electrical properties and stability of OSCs with the graphene electrode upon stretching was highly improved using the TX-modified PEDOT:PSS.22,23 Thus, the TX addition is a promising strategy to expand the use of PEDOT:PSS for various device applications. However, a fundamental understanding of the electronic structure of TX-modified PEDOT:PSS and its application in PSCs have not been reported yet. In the present study, we investigate the electronic structure of TX-modified PEDOT:PSS with ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) and X-ray absorption spectroscopy (XAS) measurements. By varying the TX concentration, the electronic structure of PEDOT:PSS is changed significantly by the surface TX content. To investigate the effect of the TX additive on the device performance of PSCs, PSCs with TX-modified PEDOT:PSS were fabricated and their photovoltaic performance was characterized. Consequently, a noteworthy enhancement in the PCE of PSCs was attained using the TX-modified PEDOT:PSS. The reduced interface recombination between MAPbI3 perovskite and PEDOT:PSS by the TX additive was demonstrated by dark current density–voltage (Jd–V), capacitance–voltage (C–V), and photoluminescence (PL) measurements. Combined with the measured electronic structure, we established the operation mechanism of selective hole transport in PSCs. Hence, we fabricated flexible PSCs successfully on a polyethylene terephthalate (PET) substrate with a graphene electrode and TX-modified PEDOT:PSS.

2. Experimental details Experimental details about sample and device fabrication, UPS, XPS, and XAS measurements, electrical characterization, and PL measurement are provided in Supporting Information.


4

Page 5 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3. Results and discussion To investigate the effect of the TX additive on the valence electronic structure of the PEDOT:PSS film, UPS spectra were obtained. The measured UPS spectra of the SEC region and the valence band (VB) region are shown in Figures 1a and b, respectively. For comparison, the UPS spectrum of the neat TX film was also measured. The UPS spectra of the SEC region were displayed with the kinetic energy in abscissa such that the SEC indicates the work function (Ψ) of the sample directly. To view the energy shift clearly, the SEC region spectra were normalized. In the SEC region, the Ψ of PEDOT:PSS without TX was measured to be 5.30 eV. However, as the TX concentration in PEDOT:PSS increased, the Ψ decreased gradually: 4.96 eV for 0.5 wt% TX, 4.88 eV for 1.0 wt% TX, and 4.60 eV for 3.0 wt% TX. In the VB region, as the TX concentration increased, the highest occupied molecular orbital (HOMO) features of PEDOT:PSS attenuated. Simultaneously, the spectral intensity near the EF from PEDOT:PSS also decreased gradually, as shown in the Figure 1b inset. These indicate that the semi-metallic properties of PEDOT:PSS are reduced by the TX additive. Meanwhile, the HOMO features of TX were shown around 2 eV, and its spectral intensity increased gradually with the energetic shift toward a higher binding energy. The HOMO onset of TX was observed at 1.37 eV for 0.5 wt% TX, 1.60 eV for 1.0 wt% TX, and 1.86 eV for 3.0 wt% TX. This energetic shift would be attributed to the formation of an interface dipole by TX. Notably, with only 3.0 wt% of the TX additive, the HOMO features were in good agreement with those of the neat TX film (dashed red line). Because the photoelectron spectroscopy measurement is highly surface-sensitive (topmost several nanometers), this indicates the high TX content on the PEDOT:PSS surface. In the literature, the polyethylene glycol additive showed a high content on the PEDOT:PSS surface, and a similar phenomenon occurred in the current case.24 This would be attributed to the lower surface energy of TX than that of PEDOT:PSS.


5


Page 6 of 24

Figure 1. UPS spectra of (a) the SEC region and (b) the VB region of TX-modified PEDOT:PSS films with different TX concentrations (red dashed line shows the neat TX spectrum shifting rigidly and the ★ mark indicates the TX HOMO peak) (inset: magnified EF region).

To investigate the electronic structures of core levels of TX-modified PEDOT:PSS films, we obtained the XPS O 1s, C 1s, and S 2p core level spectra of PEDOT:PSS with different concentrations of the TX additive, as presented in Figures 2a-c. In all core levels, the peak position and spectral shape of the neat PEDOT:PSS spectrum matched well those in the literature.25–27 The O 1s peaks of the neat PEDOT:PSS were located at 532.9 eV and 531.5 eV. These were assigned to the C-O bonding on PEDOT and the S-O bonding on PSS.25 The C 1s peaks of the neat PEDOT:PSS were observed at 286.0 eV and 284.3 eV, originating mainly from the C-O and C-C bonding, respectively. The S 2p1/2 and 2p3/2 peaks of PEDOT:PSS were deconvoluted into three components: PSS-H+ at 169.5 eV and 168.3 eV, PSS-Na+ at 168.9 and 167.7 eV, and PEDOT at 165.1 eV and 163.9 eV. As the TX concentration increased, the


6

Page 7 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


spectral intensity for the S-O bonding on PEDOT:PSS decreased gradually in the O 1s and S 2p spectra, while that for the C-O bonding on TX increased in the O 1s and C 1s spectra. These changes indicate that the TX is abundant on the surface, considering the surface sensitivity of XPS measurements. Meanwhile, all core levels of PEDOT:PSS shifted gradually toward a higher binding energy as the concentration of TX increased. Thus, the XPS results coincide well with the UPS results.

Figure 2. (a) O 1s, (b) C 1s, and (c) S 2p XPS core level spectra of TX-modified PEDOT:PSS films with the different concentrations of the TX additive.

To understand the molecular orientation of the TX-modified PEDOT:PSS, XAS spectra were obtained, as presented in Figure 3. As the XAS measurements were conducted with the TEY mode, the measured spectra reflect bulk properties of the PEDOT:PSS film. Figure 3a shows the C K-edge XAS spectra of PEDOT:PSS films with different TX concentrations obtained with an incident beam angle of 90° relative to the surface plane. The absorption peak of the lowest energy at 283.4 eV is the C 1s → π*C=C transitions of aromatic hydrocarbons and


7


Page 8 of 24

thiophene,28–30 and its intensity decreased as the TX concentration increased. The small absorption peaks at 285.2 eV and 285.8 eV correspond to C 1s → σ*C-H transitions.28–30 The peaks at 287.0 eV and 291.8 eV originate from C 1s → σ*C-O transitions and C 1s → σ*C-C transitions, respectively.28–30 However, their intensities increased gradually as the TX concentration increased. Meanwhile, the peak at the 300.0 eV originates from C 1s → σ*C=C transitions29,30 and it followed the tendency of changes in π*C=C transitions. These spectral changes indicate the increase in the TX content in the PEDOT:PSS films as the TX concentration increased. To study the molecular orientation of each film, angle-dependent XAS spectra were obtained (90° and 20°), as shown in Figures 3b-e. The changes in spectral intensity of the C 1s → π*C=C transitions with different incident angles were significantly lower at the TX-modified PEDOT:PSS than the neat PEDOT:PSS for all TX concentrations. Thus, the TX additive causes PEDOT:PSS to be oriented randomly in the bulk. This random orientation could be linked with a previously reported model where TX induces the phase separation of PEDOT:PSS.31 With the combined measured electronic structure using UPS, XPS, and XAS, we conclude that the bulk TX-modified PEDOT:PSS shows a disordered structure of PEDOT, PSS, and TX, while the high TX content is present on the surface and shifts the energy levels toward a higher binding energy.


8

Page 9 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a) C K-edge XAS spectra of PEDOT:PSS films with different TX concentrations, which were obtained with an incident beam angle of 90° relative to the surface plane. Angledependent C K-edge XAS spectra with incident angles of 90° and 20° relative to the surface plane of PEDOT:PSS (b) without TX, (c) with 0.5 wt% TX, (d) 1.0 wt% TX, and (e) 3.0 wt% TX.

To explore the role of the TX additive in PEDOT:PSS on the device performance of PSCs, we fabricated inverted PSCs with various TX concentrations. The device structure and measured J–V curves are shown in Figure 4, and photovoltaic parameters are summarized in Table 1. The significant change in the J–V characteristics was observed with the change in TX concentration. The control PSCs without TX showed the worst performance among the devices with the JSC of 17.36 mA cm−2, VOC of 0.90 V, and fill factor (FF) of 71.10%, yielding a PCE of 11.12%. With 0.5 wt% TX, the device performance was improved: JSC of 21.15 mA cm−2, VOC of 0.92 V, and FF of 73.10%, yielding a PCE of 14.34%. As the TX concentration increased to 1.0 wt%, the photovoltaic parameters increased further: JSC of 23.10 mA cm−2, VOC of 0.94 V, and FF of 74.98%, yielding a PCE of 16.23%. This is the best PCE among the devices,


9


Page 10 of 24

which is a 46% increase from that of the control PSC. The enhancement in the PCE is mainly attributed to the increases in JSC and VOC. This originates from the reduced interface recombination between PEDOT:PSS and perovskite by the insulating TX. From the UV-vis absorption spectra, scanning electron microscopy (SEM) images, and X-ray diffraction (XRD) spectra, we observed that the absorbance, morphology, and crystallinity of MAPbI3 perovskite films were very similar on PEDOT:PSS and TX-modified PEDOT:PSS (see Figures S1 and S2). This indicates that the TX additive does not significantly affect the formation of the upperlying MAPbI3 layer. Thus, the change in photovoltaic performance would be attributed to the modified interfacial properties between PEDOT:PSS and MAPbI3, which is crucial in the charge extraction efficiency of devices. When the TX concentration increased to 3.0 wt%, however, JSC, VOC, FF, and PCE decreased to 19.20 mA cm−2, 0.92 V, 73.10%, and 12.93%, respectively. Thus, the optimum concentration of TX should be incorporated into PEDOT:PSS. The PSCs with 1.0 wt% TX also exhibited the better long-time stability than that without TX (see Figure S3).

Figure 4. (a) Schematic device structure and (b) J–V curves of MAPbI3 PSCs with different TX concentrations under 1 sun illumination.


10

Page 11 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Photovoltaic parameters of PSCs fabricated with different TX concentrations in PEDOT:PSS.a TX concentrations (wt%)

VOC (V)

JSC (mA cm−2)

FF (%)

PCE (%)

0.90 (0.91 ± 17.36 (16.54 ± 71.10 (70.59 ± 11.12 (10.55 ± 0.02) 0.63) 2.75) 0.43) 0.92 (0.91 ± 21.15 (21.00 ± 73.10 (71.58 ± 14.34 (13.81 ± 0.5 0.01) 0.44) 1.41) 0.44) 0.94 (0.93 ± 23.10 (22.83 ± 74.98 (72.71 ± 16.23 (15.55 ± 1.0 0.01) 0.88) 1.12) 0.61) 0.92 (0.91 ± 19.20 (16.96 ± 73.10 (71.27 ± 12.93 (10.97 ± 3.0 0.02) 2.64) 1.65) 1.88) a (average value and standard deviation from 8 devices are shown in parentheses) 0

To investigate the role of the TX additive in PEDOT:PSS on the interface recombination, we performed the Jd–V (dark condition) and C–V (dark and light illumination condition) measurements for PSCs with different TX concentrations. In Figure 5a, the Jd–V characteristics of PSCs exhibited the different behaviors as the TX concentration was varied, especially in the low-bias regime (< 0.80 V). The PSCs with the higher TX concentration exhibited a lower J value in the low-bias regime under the dark condition. This bias regime is lower than the turnon voltage, demonstrating a steep increase in J by charge injection from the electrodes. Therefore, this J value would be highly contributed by leakage current, which is attributed to the semi-metallic properties of PEDOT:PSS. Therefore, using TX, the leakage current was reduced effectively.15 To substantiate this assumption, C–V characteristics were also measured under 1 sun illumination (Clight) or the dark condition (Cdark). Because the absorbance of MAPbI3 film was not significantly changed by the TX concentrations in PEDOT:PSS (Figure S1), the difference between Clight and Cdark (ΔC) is primarily related to the number of photogenerated carriers without the interface recombination between PEDOT:PSS and MAPbI3. Remarkably, the PSCs with the higher TX concentration showed a much higher ΔC value. Therefore, the carriers were much less recombined at the interface of MAPbI3/(PEDOT:PSS with TX), resulting in the enhanced JSC and VOC of PSCs. These Jd–V


11


Page 12 of 24

and ΔC–V results clearly show that the improved performance of PSCs with TX-modified PEDOT:PSS originates from the suppression of interface recombination.

Figure 5. (a) Jd–V curves and (b) (Clight–Cdark)–voltage curves of MAPbI3 PSCs with different TX concentrations.

To verify our hypothesis, we conducted the steady-state and time-resolved PL measurements of the MAPbI3 perovskite film on glass and on PEDOT:PSS/ITO with different TX concentrations at room temperature. In Figure 6a, the PL peak of MAPbI3 film on glass was observed at 765 nm corresponding to the emission energy of 1.62 eV, which is in good agreement with the literature.32–34 The PL intensity of MAPbI3 on PEDOT:PSS was decreased significantly by non-radiative recombination. However, a considerable increase in the PL intensity was observed with TX-modified PEDOT:PSS. Moreover, the higher TX concentration showed the higher PL intensity. This increase in the radiative recombination of photogenerated carriers is attributed to the reduced exciton quenching at the interface between PEDOT:PSS and MAPbI3 by TX. To study the carrier dynamics in detail, time-resolved PL spectra were obtained, as shown in Figure 6b. The measured time-resolved PL spectra were well fitted with a single exponential decay function, I(t) = A𝑒 ―

𝑡𝜏


12

Page 13 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


where I(t), A, and  are the PL decay function, pre-exponential constant, and recombination lifetime, respectively.35,36 The evaluated τ of perovskite on glass is 84.0 ns, and on PEDOT:PSS without TX, with 0.5 wt% TX, 1.0 wt% TX, and 3.0 wt% TX is 21.2 ns, 33.9 ns, 35.3 ns, and 39.3 ns, respectively. The increased τ by the TX additive, indicating the decrease in nonradiative recombination, is a direct evidence of the reduced interface recombination.19,20,37 The reduced recombination at the interface between MAPbI3 and TX-modified PEDOT:PSS accords well with its increased steady-state PL intensity. Therefore, we conclude that the semimetallic properties of PEDOT:PSS that induce the PL quenching are suppressed efficiently by the TX additive.

Figure 6. (a) Steady-state and (b) time-resolved PL spectra of the MAPbI3 perovskite films on glass and on PEDOT:PSS/ITO with different TX concentrations.

The energy-level diagrams of MAPbI3/PEDOT:PSS and MAPbI3/(PEDOT:PSS with 1.0 wt% TX) derived from the measured UPS spectra were illustrated in Figure 7. The measured UPS spectra of MAPbI3 on PEDOT:PSS without TX and with 1.0 wt% TX are presented in Figure S4. All energy levels except the MAPbI3 conduction band minimum (CBM) were measured with UPS, whereas the MAPbI3 CBM was estimated from the reported transport gap


13


Page 14 of 24

(1.70 eV).4 Although the Ψ of underlying PEDOT:PSS is decreased from 5.30 eV to 4.88 eV by the 1.0 wt% TX additive, the energy levels of MAPbI3 are identical. In both cases, the VBM and CBM of MAPbI3 are located at 0.66 eV below the EF and 1.04 eV above the EF. Thus, the hole transport barrier (Φh) and electron transport barrier (Φe) are 0.66 eV and 1.04 eV, respectively. However, in the neat PEDOT:PSS, the electron blocking ability cannot be expected owing to their density of states crossing the EF. This semi-metallic property results in significant interface recombination during device operation. Thus, an additional electron blocking layer should be used for high device performance.38–40 In this regard, TX is a wide band gap material (4.03 eV, from UV-vis spectra shown in Figure S1) that can block electron transport into the semi-metallic PEDOT:PSS. Thus, TX reduces the interface recombination when located between PEDOT:PSS and MAPbI3.19–21 This would be the origin of PSC performance enhancement. The selective charge transport at the interface between MAPbI3 and the electrode is critical in achieving the high PCE of the PSCs. However, the insulating TX might also hinder hole transport. If the hole current dominates at the anode interface, the photovoltaic performance is improved by electron blocking. Meanwhile, if the electron current dominates at the anode interface, the photovoltaic performance is deteriorated by recombination. Thus, we calculated the hole and electron currents to the anode via tunneling through TX. To compare the hole and electron tunneling current density (Jh and Je), we employed a metal-insulator-semiconductor model as described below38,41,42: 𝐽 = 𝐴𝑇2𝑒

―

4𝜋𝛿 ℎ

where A is the Richardson’s constant

1/2

[

(2𝑚 ∗ 𝜒)

𝑞𝑉

]

∙ 𝑒 ―𝛷/𝐾𝐵𝑇 ∙ 𝑒𝑛𝑘𝐵𝑇 ― 1

4πqm ∗ 𝑘2 ħ

, χ is the energy barrier for tunneling, δ is the

tunneling width, Φ is the charge transport barrier, kB is Boltzmann’s constant, q is the elementary charge, V is the bias voltage, and n is the ideality factor. If we approximate TX on


14

Page 15 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the surface as an effective layer, the thickness of the TX layer (i.e., δ) is estimated to be 20 Å for 1.0 wt% by the electron effective attenuation length of the XPS S 2p core level spectra (Figure 2c). Consequently, the Jh/Je ratio is higher than 5 × 106 owing to the significantly higher χ for electron than the χ for hole. This indicates that TX can block electrons at the interface efficiently, while holes can be transported well to the anode via tunneling. However, if the TX concentration is extremely high, its effective thickness is increased further, δ is increased, and the tunneling probability is decreased. Thus, the TX concentration over 1.0 wt% deteriorates the Jh, thus resulting in the decreased PCE of the PSCs, as shown in Figure 4b.

Figure 7. Energy-level diagrams of (a) MAPbI3 on PEDOT:PSS without TX and (b) with 1.0 wt% TX (unit: eV).

Based on the understanding of the electronic structure and reduced interface recombination of TX-modified PEDOT:PSS, we investigated its applicability into flexible PSCs. Owing to the high conductivity upon mechanical bending, graphene has been employed in flexible devices as an electrode.43 Because graphene is hydrophobic, PEDOT:PSS cannot be deposited with a spin-coating method on graphene. However, as aforementioned, TX-modified


15


Page 16 of 24

PEDOT:PSS can be coated uniformly on the graphene electrode (see Figures S5 and S6). Thus, we fabricated flexible PSCs with the structure of Ag/BCP/C60/MAPbI3/(PEDOT:PSS with 1.0 wt% TX)/graphene on PET. Graphene was synthesized with chemical vapor deposition on the Cu substrate, and subsequently transferred sequentially to a PET substrate four times.22 Figure 8 shows the measured J–V curves of the flexible PSC under 1 sun illumination and the inset shows the photograph of the device. The JSC, VOC, and FF values of flexible PSCs are 20.47 mA cm−2, 0.88 V, and 67.00%, respectively, yielding a PCE of 12.01%. The device performance might be improved further by optimizing the fabrication condition. However, this result clearly shows that TX-modified PEDOT:PSS can be applied to not only ITO but also hydrophobic graphene, which is promising for future flexible device applications.

Figure 8. J–V curve of the flexible MAPbI3 PSC with the structure of Ag/BCP/C60/MAPbI3/(PEDOT:PSS with 1.0 wt% TX)/graphene on PET under 1 sun illumination (inset: photograph of flexible PSC).

4. Conclusion We investigated the electronic structure of TX-modified PEDOT:PSS with UPS, XPS,


16

Page 17 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and XAS measurements. The electronic structure of PEDOT:PSS was altered significantly as the TX concentration changed. The TX content was high on the surface; thus, the semi-metallic properties of PEDOT:PSS were reduced. In addition, we observed that the TX-modified PEDOT:PSS was composed of disordered PEDOT, PSS, and TX. The PCE of PSCs increased significantly with the TX-modified PEDOT:PSS. Although the Ψ of PEDOT:PSS was decreased by 1.0 wt% TX, the energy offset for the hole transport was not varied. However, the C–V and PL measurements revealed that the interface recombination was highly suppressed by the insulating TX. This was the origin of the improved device performance of PSCs with the TX additive. Hence, the TX-modified PEDOT:PSS could also be applied successfully to flexible PSCs with graphene electrodes on a PET substrate. Therefore, TX-modified PEDOT:PSS can be used as an efficient HTL for optoelectronic devices in a versatile manner.

▪ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS publications website at DOI: xxx/xxxxxxx. Experimental details, UV-vis spectra, SEM images, and XRD spectra of MAPbI3 films with various TX concentrations, PCE degradation of MAPbI3 PSCs without TX or with 1.0 wt% TX, UPS spectra of MAPbI3 films on the PEDOT:PSS without TX and with 1.0 wt% TX, contact angle images of TX-modified PEDOT:PSS, and SEM image of TX-modified PEDOT:PSS on the graphene electrode (PDF).

▪ AUTHOR INFORMATION Corresponding Authors *E-mail:

[email protected] (H. L.)

*E-mail:

[email protected] (Y. Y.)


17


Page 18 of 24

Notes The authors declare no competing financial interests.

▪ ACKNOWLEDGMENTS This study was supported by National Research Foundation of Korea [NRF2018R1D1A1B07051050,

2018R1A6A1A03025582,

2017R1A2B4002442,

2018K1A3A7A09057410, and 2017R1A5A1014862 (SRC program: vdWMRC center)], and MOTIE [Ministry of Trade, Industry & Energy (10079558)], and Samsung Display Company and an Industry-Academy joint research program between Samsung Electronics and Yonsei University, and the Graduate School of Yonsei University Research Scholarship Grants in 2018. PL measurements were performed with instrument at the Central Laboratory of Kangwon National University. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.

▪ REFERENCE (1)

NREL Efficiency Chart. https://www.nrel.gov/pv/assets/images/efficiency-chart.png (accessed Apr 11, 2017).

(2)

Hawash, Z.; Raga, S. R.; Son, D.-Y.; Ono, L. K.; Park, N.-G.; Qi, Y. Interfacial Modification of Perovskite Solar Cells Using an Ultrathin MAI Layer Leads to Enhanced Energy Level Alignment, Efficiencies, and Reproducibility. J. Phys. Chem. Lett. 2017, 8, 3947–3953.

(3)

Westbrook, R. J. E.; Sanchez-Molina, D. I.; Manuel Marin-Beloqui, D. J.; Bronstein, D. H.; Haque, D. S. A. Effect of Interfacial Energetics on Charge Transfer from Lead Halide Perovskite to Organic Hole Conductors. J. Phys. Chem. C 2018, 122, 1326–1332.


18

Page 19 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(4)

Shin, D.; Kang, D.; Jeong, J.; Park, S.; Kim, M.; Lee, H.; Yi, Y. Unraveling the Charge Extraction Mechanism of Perovskite Solar Cells Fabricated with Two-Step Spin Coating: Interfacial Energetics between Methylammonium Lead Iodide and C60. J. Phys. Chem. Lett. 2017, 8, 5423–5429.

(5)

Schulz, P.; Edri, E.; Kirmayer, S.; Hodes, G.; Cahen, D.; Kahn, A. Interface Energetics in Organo-Metal Halide Perovskite-Based Photovoltaic Cells. Energy Environ. Sci. 2014, 7, 1377–1381.

(6)

Liu, T.; Jiang, F.; Qin, F.; Meng, W.; Jiang, Y.; Xiong, S.; Tong, J.; Li, Z.; Liu, Y.; Zhou, Y. Nonreduction-Active Hole-Transporting Layers Enhancing Open-Circuit Voltage and Efficiency of Planar Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2016, 8, 33899–33906.

(7)

Wang, Q.; Bi, C.; Huang, J. Doped Hole Transport Layer for Efficiency Enhancement in Planar Heterojunction Organolead Trihalide Perovskite Solar Cells. Nano Energy 2015, 15, 275–280.

(8)

Lee, J.; Kang, H.; Kim, G.; Back, H.; Kim, J.; Hong, S.; Park, B.; Lee, E.; Lee, K. Achieving Large-Area Planar Perovskite Solar Cells by Introducing an Interfacial Compatibilizer. Adv. Mater. 2017, 29, 1606363.

(9)

You, J.; Meng, L.; Song, T.-B.; Guo, T.-F.; Yang, Y.; Chang, W.-H.; Hong, Z.; Chen, H.; Zhou, H.; Chen, Q.; et al. Improved Air Stability of Perovskite Solar Cells via Solution-Processed Metal Oxide Transport Layers. Nat. Nanotechnol. 2016, 11, 75–81.

(10)

Huang, D.; Goh, T.; Kong, J.; Zheng, Y.; Zhao, S.; Xu, Z.; Taylor, A. D. Perovskite Solar Cells with a DMSO-Treated PEDOT:PSS Hole Transport Layer Exhibit Higher Photovoltaic Performance and Enhanced Durability. Nanoscale 2017, 9, 4236–4243.


19


(11)

Page 20 of 24

Liu, D.; Li, Y.; Yuan, J.; Hong, Q.; Shi, G.; Yuan, D.; Wei, J.; Huang, C.; Tang, J.; Fung, M.-K. Improved Performance of Inverted Planar Perovskite Solar Cells with F4-TCNQ Doped PEDOT:PSS Hole Transport Layers. J. Mater. Chem. A 2017, 5, 5701–5708.

(12)

Wang, Q.; Chueh, C.-C.; Eslamian, M.; Jen, A. K. Y. Modulation of PEDOT:PSS PH for Efficient Inverted Perovskite Solar Cells with Reduced Potential Loss and Enhanced Stability. ACS Appl. Mater. Interfaces 2016, 8, 32068–32076.

(13)

Kim, W.; Kim, S.; Chai, S. U.; Jung, M. S.; Nam, J. K.; Kim, J.-H.; Park, J. H. Thermodynamically Self-Organized Hole Transport Layers for High-Efficiency Inverted-Planar Perovskite Solar Cells. Nanoscale 2017, 9, 12677–12683.

(14)

Bubnova, O.; Khan, Z. U.; Wang, H.; Braun, S.; Evans, D. R.; Fabretto, M.; HojatiTalemi, P.; Dagnelund, D.; Arlin, J.-B.; Geerts, Y. H.; et al. Semi-Metallic Polymers. Nat. Mater. 2014, 13, 190–194.

(15)

Tvingstedt, K.; Gil-Escrig, L.; Momblona, C.; Rieder, P.; Kiermasch, D.; Sessolo, M.; Baumann, A.; Bolink, H. J.; Dyakonov, V. Removing Leakage and Surface Recombination in Planar Perovskite Solar Cells. ACS Energy Lett. 2017, 2, 424–430.

(16)

Hains, A. W.; Marks, T. J. High-Efficiency Hole Extraction/Electron-Blocking Layer to Replace

Poly(3,4-Ethylenedioxythiophene):Poly(Styrene

Sulfonate)

in

Bulk-

Heterojunction Polymer Solar Cells. Appl. Phys. Lett. 2008, 92, 023504. (17)

Yan, H.; Lee, P.; Armstrong, N. R.; Graham, A.; Evmenenko, G. A.; Dutta, P.; Marks, T. J. High-Performance Hole-Transport Layers for Polymer Light-Emitting Diodes. Implementation

of

Organosiloxane

Cross-Linking

Chemistry

in

Polymeric

Electroluminescent Devices. J. Am. Chem. Soc. 2005, 127, 3172–3183. (18)

Luo, H.; Lin, X.; Hou, X.; Pan, L.; Huang, S.; Chen, X. Efficient and Air-Stable Planar Perovskite Solar Cells Formed on Graphene-Oxide-Modified PEDOT:PSS Hole Transport Layer. Nano-Micro Lett. 2017, 9, 39.


20

Page 21 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19)

Zheng, Y.; Shi, W.; Kong, J.; Huang, D.; Katz, H. E.; Yu, J.; Taylor, A. D. A Cytop Insulating Tunneling Layer for Efficient Perovskite Solar Cells. Small Methods 2017, 1, 1700244.

(20)

Wolff, C. M.; Zu, F.; Paulke, A.; Toro, L. P.; Koch, N.; Neher, D. Reduced InterfaceMediated Recombination for High Open-Circuit Voltages in CH3NH3PbI3 Solar Cells. Adv. Mater. 2017, 29, 1700159.

(21)

Wang, Q.; Dong, Q.; Li, T.; Gruverman, A.; Huang, J. Thin Insulating Tunneling Contacts for Efficient and Water-Resistant Perovskite Solar Cells. Adv. Mater. 2016, 28, 6734–6739.

(22)

Lee, J.-B.; Rana, K.; Seo, B. H.; Oh, J. Y.; Jeong, U.; Ahn, J.-H. Influence of Nonionic Surfactant-Modified PEDOT:PSS on Graphene. Carbon 2015, 85, 261–268.

(23)

Oh, J. Y.; Shin, M.; Lee, J. B.; Ahn, J.-H.; Baik, H. K.; Jeong, U. Effect of PEDOT Nanofibril Networks on the Conductivity, Flexibility, and Coatability of PEDOT:PSS Films. ACS Appl. Mater. Interfaces 2014, 6, 6954–6961.

(24)

Alemu Mengistie, D.; Wang, P.-C.; Chu, C.-W. Effect of Molecular Weight of Additives on the Conductivity of PEDOT:PSS and Efficiency for ITO-Free Organic Solar Cells. J. Mater. Chem. A 2013, 1, 9907–9915.

(25)

Xing, K. Z.; Fahlman, M.; Chen, X. W.; Inganäs, O.; Salaneck, W. R. The Electronic Structure of Poly(3,4-Ethylene-Dioxythiophene): Studied by XPS and UPS. Synth. Met. 1997, 89, 161–165.

(26)

Hwang, J.; Amy, F.; Kahn, A. Spectroscopic Study on Sputtered PEDOT·PSS: Role of Surface PSS Layer. Org. Electron. 2006, 7, 387–396.

(27)

Park, S.; Jeong, J.; Hyun, G.; Kim, M.; Lee, H.; Yi, Y. The Origin of High PCE in PTB7 Based Photovoltaics: Proper Charge Neutrality Level and Free Energy of Charge Separation at PTB7/PC71BM Interface. Sci. Rep. 2016, 6, 35262.


21


(28)

Page 22 of 24

Lee, H.; Puodziukynaite, E.; Zhang, Y.; Stephenson, J. C.; Richter, L. J.; Fischer, D. a; DeLongchamp, D. M.; Emrick, T.; Briseno, A. L. Poly(Sulfobetaine Methacrylate)s as Electrode Modifiers for Inverted Organic Electronics. J. Am. Chem. Soc. 2015, 137, 540–549.

(29)

Mauger, S. A.; Li, J.; Özmen, Ö. T.; Yang, A. Y.; Friedrich, S.; Rail, M. D.; Berben, L. A.; Moulé, A. J. High Work-Function Hole Transport Layers by Self-Assembly Using a Fluorinated Additive. J. Mater. Chem. C 2014, 2, 115–123.

(30)

Stöhr, J. NEXAFS Spectroscopy; Springer Berlin Heidelberg, 1992.

(31)

Yoon, S.-S.; Khang, D.-Y. Roles of Nonionic Surfactant Additives in PEDOT:PSS Thin Films. J. Phys. Chem. C 2016, 120, 29525–29532.

(32)

Chiang, C.-H.; Tseng, Z.-L.; Wu, C.-G. Planar Heterojunction Perovskite/PC71BM Solar Cells with Enhanced Open-Circuit Voltage via a (2/1)-Step Spin-Coating Process. J. Mater. Chem. A 2014, 2, 15897–15903.

(33)

Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Gratzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electron- and Hole-Transport Lengths in OrganicInorganic CH3NH3PbI3. Science 2013, 342, 344–347.

(34)

Hu, Z.; Fu, W.; Yan, L.; Miao, J.; Yu, H.; He, Y.; Goto, O.; Meng, H.; Chen, H.; Huang, W. Effects of Heteroatom Substitution in Spiro-Bifluorene Hole Transport Materials. Chem. Sci. 2016, 7, 5007–5012.

(35)

Bi, D.; Yi, C.; Luo, J.; Décoppet, J.-D.; Zhang, F.; Zakeeruddin, S. M.; Li, X.; Hagfeldt, A.; Grätzel, M. Polymer-Templated Nucleation and Crystal Growth of Perovskite Films for Solar Cells with Efficiency Greater than 21%. Nat. Energy 2016, 1, 16142.

(36)

Gonzalez-Carrero, S.; Galian, R. E.; Pérez-Prieto, J. Maximizing the Emissive Properties of CH3NH3PbBr3 Perovskite Nanoparticles. J. Mater. Chem. A 2015, 3, 9187–9193.


22

Page 23 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(37)

Zuo, L.; Guo, H.; deQuilettes, D. W.; Jariwala, S.; De Marco, N.; Dong, S.; DeBlock, R.; Ginger, D. S.; Dunn, B.; Wang, M.; et al. Polymer-Modified Halide Perovskite Films for Efficient and Stable Planar Heterojunction Solar Cells. Sci. Adv. 2017, 3, 1700106.

(38)

Wei, J.; Li, H.; Zhao, Y.; Zhou, W.; Fu, R.; Pan, H.; Zhao, Q. Flexible Perovskite Solar Cells Based on the Metal–insulator–semiconductor Structure. Chem. Commun. 2016, 52, 10791–10794.

(39)

Ratcliff, E. L.; Meyer, J.; Steirer, K. X.; Armstrong, N. R.; Olson, D.; Kahn, A. Energy Level Alignment in PCDTBT:PC70BM Solar Cells: Solution Processed NiOx for Improved Hole Collection and Efficiency. Org. Electron. 2012, 13, 744–749.

(40)

Yan, H.; Scott, B. J.; Huang, Q.; Marks, T. J. Enhanced Polymer Light-Emitting Diode Performance Using a Crosslinked-Network Electron-Blocking Interlayer. Adv. Mater. 2004, 16, 1948–1953.

(41)

Depas, M.; Vermeire, B.; Mertens, P. W.; Van Meirhaeghe, R. L.; Heyns, M. . Determination of Tunnelling Parameters in Ultra-Thin Oxide Layer Poly-Si/SiO2/Si Structures. Solid-State Electron. 1995, 38, 1465–1471.

(42)

Kao, K. C.; Hwang, W. Electrical Transport in Solids, With Particular Reference to Organic Semiconductors, Pergamon P.; Oxford, 1981.

(43)

Tan, R. K. L.; Reeves, S. P.; Hashemi, N.; Thomas, D. G.; Kavak, E.; Montazami, R.; Hashemi, N. N. Graphene as a Flexible Electrode: Review of Fabrication Approaches. J. Mater. Chem. A 2017, 5, 17777–17803.


23


Page 24 of 24

TOC GRAPHICS


24

Electronic Structure of Nonionic Surfactant ... - ACS Publications

Recommend Documents