Performance Enhancement of Planar Heterojunction Perovskite Solar

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Article http://pubs.acs.org/journal/acsodf

Performance Enhancement of Planar Heterojunction Perovskite Solar Cells through Tuning the Doping Properties of Hole-Transporting Materials He Xi,*,† Shi Tang,‡ Xiaohua Ma,† Jingjing Chang,‡ Dazheng Chen,‡ Zhenhua Lin,‡ Peng Zhong,† Hong Wang,† and Chunfu Zhang*,‡ †

School of Advanced Materials and Nanotechnology, Xidian University, 266 Xinglong Section of Xifeng Road, Xi’an 710126, China Wide Bandgap Semiconductor Technology Disciplines State Key Laboratory, School of Microelectronics, Xidian University, 2 South Taibai Road, Xi’an 710071, China

‡

S Supporting Information *

ABSTRACT: Chemical doping has been widely used to finely tune the electrical properties of organic hole-transporting materials (HTMs) that find widespread applications in perovskite solar cells (PSCs). Here, to shed light on the precise role of chemical p-doping in affecting the chargetransport properties of HTMs and photovoltaic performance of PSCs, two kinds of representative dopants, including lithium bis(trifluoromethane)sulfonimide (LiTFSI) and two Co(III) complexes tris[2-(1H-pyrazol-1-yl)-4-tert-butylpyridine]cobalt(III)tris[bis(trifluoromethylsulfonyl)imide] (FK209) and tris[2-(1H-pyrazol-1-yl)pyridine]cobalt(III)tris[bis(trifluoromethylsulfonyl)imide] (FK102), are employed as the p-type dopant models to dope the 2,2′,7,7′-tetrakis[N,N-di-p-methoxyphenylamine]-9,9′-spirobifluorene (spiro-OMeTAD) HTM. Both dopants can facilitate the generation of oxidized spiro-OMeTAD radical cation and improve hole mobility. Co-doping of FK209 and LiTFSI is necessary to achieve an optimal doping property and best device performance with power conversion efficiency of 17.8% compared to that of the FK209-doped device (13.5%) and the LiTFSI-doped device (15%). UV−vis absorption, spacecharge-limited current measurements, and steady-state and time-resolved photoluminescence measurements have confirmed that with the co-doping of the two kinds of p-dopants in a proper ratio the doped spiro-OMeTAD exhibits a high charge carrier mobility and charge carrier transfer/collection capability.

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INTRODUCTION Organic−inorganic hybrid perovskite solar cells (PSCs) have received considerable attention as one of the most promising candidates for the next generation of high-efficiency, low-cost thin film photovoltaics due to their appealing features of large absorption coefficient, broadband light absorption, tunable band gap, long charge carrier diffusion length, excellent ambipolar charge mobility, low nonradiative recombination rate, and simple fabrication.1−4 Significant efforts have been devoted to the achievement of high performance of PSCs, including optimizing the morphology,5,6 crystallization process,7,8 and composition manipulation of the perovskite active layer9,10 and also the interface engineering between the perovskite and electrodes,11−13 enabling the power conversion efficiency (PCE) of PSCs leaping from 3.8% to over 20% in a relatively short period.14,15 Very recently, a certified world record PCE of 22.1% was obtained, making PSCs comparable to current commercial technologies.4,16 In PSCs, organic hole-transporting materials (HTMs) are usually employed to play a crucial role in charge transport, extraction, and retardation of undesired charge recombination at the perovskite/HTM interface.17,18 Although several © 2017 American Chemical Society

simplified PSC devices without hole-transporting layers (HTLs) have been developed, PCE of only 10−12.8% was achieved.17,19,20 Therefore, the development of devices incorporating HTMs between the perovskite and metal electrode in PSCs remains an attractive goal. In this regard, numerous HTMs have been explored, including organic small molecules, conjugated polymers, and p-type inorganic semiconductors, resulting in reasonable efficiencies of 13− 18%.17,21−26 Only few can reach efficiency over 20%, such as 2,2′,7,7′-tetrakis[N,N-di-p-methoxyphenylamine]-9,9′-spirobifluorene (spiro-OMeTAD), polytriarylamine, and the recently reported 2′,7′-bis(bis(4-methoxyphenyl)amino) spiro[cyclopenta[2,1-b:3,4-b′]dithiophene-4,9′-flurene] (FDT).4,10,15,27 Among these effective HTMs used in PSCs to date, the amorphous organic molecule spiro-OMeTAD, which is first synthesized by Grätzel’s group and traditionally employed as a solid-state replacement for liquid electrolyte in solid-state dyeReceived: December 13, 2016 Accepted: January 17, 2017 Published: January 31, 2017 326

DOI: 10.1021/acsomega.6b00465 ACS Omega 2017, 2, 326−336

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sensitized solar cells (ssDSCs),28 has now shown to be the most efficient. However, most of the organic HTMs suffer from low hole mobility and conductivity in their pristine form, even the state-of-the-art spiro-OMeTAD, which leads to slower charge transport and a decreased efficiency. One of the most useful strategies to increase the charge-transport capacity and alter the conductivity of the organic hole conductor is employing chemical p-type doping. Hence, p-type dopants or ionic additives such as bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) and cobalt complexes, namely, tris[2(1H-pyrazol-1-yl)-4-tert-butylpyridine]cobalt(III)tris[bis(trifluoromethylsulfonyl) imide] (FK209) and tris[2-(1Hpyrazol-1-yl)pyridine]cobalt(III)tris[bis(trifluoromethylsulfonyl) imide] (FK102), are developed and shown to significantly increase the conductivity and hole mobility of spiro-OMeTAD to some extent in ssDSCs and PSCs.25,29,30 In addition, other kinds of p-type dopants or additives have also been widely studied for the improvement of conductivity of spiro-OMeTAD, such as Ag-TFSI,31,32 F4TCNQ,33 protic ionic liquids,34 SnCl4,35 CuI,36 CuSCN,37 copper(II) phthalocyanine (CuPC),38 and iridium complex,39 resulting in improved PCE of ssDSCs and PSCs effectively. The doped HTMs are generally closely dependent on the doping conditions, such as the ambient environment including moisture, oxygen, and light. This ambience dependence may be the cause of the poor stability and reproducibility of the devices. Thus, it is a great challenge to precisely control the oxidation process and improve the device reproducibility, as the oxidation-induced charge dynamics is complicated and susceptible to extrinsic environment. For instance, the oxidation of spiro-OMeTAD can only be achieved upon air exposure when LiTFSI is doped, which is actually dependent on light.40,41 Qi et al. reported that ambient air exposure, especially the humidity, would cause the LiTFSI dopant to redistribute across the whole spiro-OMeTAD films, resulting in an increased conductivity. They also suggested that controlled environments of H2O vapor and dry O2 could individually enhance the conductivity and mobility of LiTFSI-doped spiroOMeTAD films to a certain extent.42 In addition, Qi and coworkers also observed decrease in charge mobility when spiroOMeTAD films were exposed to air, O2, and H2O.43 Meng et al. and Qi et al. reported that tert-butylpyridine (tBP) functioned as a morphology controller for HTL films in PSCs. It could effectively reduce phase separation of LiTFSI and spiro-OMeTAD in the stock solution before spin-coating, leading to a uniform HTL film by decreasing inhomogeneous areas, which further helped realize uniform charge-transport properties across the HTL and long-term device stability.44,45 Furthermore, Snaith’s group reported that the steady-state efficiency of PSCs with a mesoporous structure was closely related to the use of tBP in HTLs. They proposed that tBP could improve the charge selectivity of the perovskite/HTL interface for holes through the chemical interaction between tBP and perovskite, independent of the nature of HTM used, thus resulting in an increment in the steady-state efficiency. Besides, they also found that tBP is likely to induce p-doping of the perovskite layer.46 Seok and co-workers studied the comparative effect of Co(III) complex FK209 and the codoping of FK209 and Li salt on the electrical conductivity of spiro-OMeTAD and the performance of mesoscopic TiO2/ CH3NH3PbI3 heterojunction PSCs. They concluded that the addition of FK209 to spiro-OMeTAD could effectively enhance the open-circuit voltage (Voc) because of the reduced carrier

recombination and lower Fermi level of HTM and that the synergistic effect of FK209 and LiTFSI resulted in a PCE of 10.4%.47 Efforts have been made in previous studies to understand the poorly defined doping mechanism of the pdopants on HTMs and the effects of extrinsic environment on it, as well as the effect of doping on the performance of ssDSCs and PSCs; most of them are about the functionality of the doped spiro-OMeTAD film itself or the resulting photovoltaic characteristics of PSCs with a mesoporous oxide layer. Considering the crucial role of HTMs in hole transport and extraction and the rapid increase in the efficiency of planar PSCs that have a simpler structure and better reproducibility, the studies on the p-doping mechanism of HTMs may provide an effective strategy to further boost the PCEs of planar heterojunction PSCs; however, a deep understanding of the specific effect of p-doping on the charge-transport properties of the HTMs and how it affects the charge carrier dynamics at the perovskite/HTL interface are still lacking. To further improve the device performance, systematic investigations on the effect of p-doping on the interface properties between the perovskite layer and HTL and the overall photovoltaic behavior of PSCs are desirable. In this study, we investigated two kinds of representative dopants including LiTFSI and two Co(III) complexes FK102 and FK209 (as shown in Figure 1) that find important

Figure 1. Molecular structures of spiro-OMeTAD, LiTFSI, FK102, and FK209.

applications in PSCs as the “redox active” p-type dopant model. We introduced small-molecule spiro-OMeTAD doped with the above different dopants as the HTL in PSCs, with CH3NH3PbI3−XClX as the light absorber and TiO2 as the electron transport layer (ETL). The effects of these dopants on the electrical properties of HTM, charge-transport properties at the perovskite/HTM interface, and the resulting device 327


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Figure 2. (a) Device structure of the PSC in this study (FTO/TiO2/PCBM/CH3NH3PbI3−XClX/Spiro-OMeTAD/Ag). (b) Top-view SEM image of the perovskite film grown on PCBM/TiO2/FTO. (c) Energy level diagram of corresponding materials used in the device. (d) Cross-sectional SEM image of the complete device with FK209/Li-spiro as a HTM.

Figure 3. (a) J−V characteristics for the champion PSCs prepared using different dopants measured under AM 1.5G illumination of 100 mW/cm2 and (b) incident photo-to-electron conversion efficiency (IPCE) spectra of the PSCs based on different dopants.

and FF of 0.74, is achieved, which is higher than that of LiTFSIdoped device and Co(III)-doped device by 20−30%.

characteristics are systematically studied. We found that the incorporation of either LiTFSI or Co(III) complexes into spiroOMeTAD could effectively generate charge carriers and improve its hole mobility. When LiTFSI and FK209 (FK102) are codoped in spiro-OMeTAD in a proper ratio, approximately 2 orders of magnitude increment in hole mobility is observed compared to that in the dopant-free spiro-OMeTAD film. It also increases the fill factor (FF), Jsc, and open-circuit voltage (Voc) of PSCs simultaneously, leading to a better device performance, which is primarily due to the improved charge carrier mobility, increased hole-collection efficiency, and reduced charge carrier recombination at the perovskite/HTM interface. As a result, for PSCs based on the mixed dopants, a high efficiency of 17.8%, with a Voc of 1.1 V, Jsc of 21.7 mA/cm2,

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RESULTS AND DISCUSSION PSCs with a planar n−i−p architecture of FTO/TiO2/PCBM/ CH3NH3PbI3−XClX/Spiro-OMeTAD/Ag (Figure 2a) were fabricated in this study to investigate the role of these dopants in affecting the charge-transport properties at the perovskite/ HTM interface and the resulting device characteristics. Here, the compact TiO2/PCBM layer serves as an ETL, wherein the PCBM layer is employed to passivate the defects of the bottom TiO2 layer and diminish the current density−voltage (J−V) hysteresis.48 Spiro-OMeTAD is used as the HTM, where chemical additives, including LiTFSI and two cobalt complexes FK102 and FK209, are introduced to oxidize spiro-OMeTAD. 328


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Table 1. Photovoltaic Parameters of PSCs Prepared Using FK102-Spiro, FK209-Spiro, Li-Spiro, FK102/Li-Spiro, FK209/LiSpiro, and Dopant-Free Spiro-OMeTAD as the HTMsa Jsc [mA/cm2]

HTM FK209/Li-spiro FK102/Li-spiro Li-spiro FK209-spiro FK102-spiro pristine-spiro

b

best averagec bestb averagec bestb averagec bestb averagec bestb averagec bestb averagec

21.70 21.13 21.25 20.78 20.96 20.18 20.50 20.08 20.12 19.94 19.89 19.05

± 0.57 ± 0.47 ± 0.68 ± 0.42 ± 0.46 ± 0.75

Voc [V] 1.11 1.09 1.09 1.09 1.10 1.08 1.08 1.07 1.05 1.04 1.02 1.02

FF [%]

± 0.02 ± 0.02 ± 0.02 ± 0.01 ± 0.01 ± 0.02

74.1 70.3 70.9 68.4 64.7 60.9 61.2 59.6 54.5 52.6 30.2 26.9

± 3.8 ± 2.5 ± 3.8 ± 1.6 ± 1.9 ± 3.3

PCE [%]

Rs (Ω cm2)

Rsh (Ω cm2)

17.8 16.4 ± 1.4 16.5 15.3 ± 1.2 15.0 13.9 ± 1.1 13.5 12.8 ± 0.7 11.5 10.8 ± 0.7 6.1 5.3 ± 0.8

5.24

1758

6.64

1132

8.36

947

9.79

1095

11.19

848

37.15

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a

Performances of the devices were measured with 0.125 cm2 working area. bMaximum values of two batches of 16 devices. cAverage values of two batches counting 16 devices in total.

Figure 4. Statistics parameters of (a) Jsc, (b) Voc, (c) FF, and (d) PCE for the PSCs prepared under different doping conditions: FK209/Li-spiro (black), FK102/Li-spiro (red), Li-spiro (olive), FK209-spiro (blue), FK102-spiro (magenta), and dopant-free spiro (dark yellow).

It is notable that the single tBP-mixed spiro-OMeTAD is not considered in this work. One of the main reasons is that tBP is a polar solvent similar to γ-butyrolactone (GBL), which shows good solubility for perovskite. It is most probable that tBP can erode the perovskite film within the cell, which is a great threat to the long-term stability of PSCs.49,50 Thus, small amount of tBP is commonly used together with LiTFSI as an additive to promote the dissolution of Li salt and to prevent phase separation of LiTFSI and spiro-OMeTAD, leading to a uniform HTL film.44,45 Herein, the corrosion effect of tBP on perovskite films is demonstrated by UV−vis absorption spectroscopy (Figure S1). The CH3NH3PbI3−XClX film was fabricated by the traditional one-step solvent engineering method, and a continuous uniform perovskite film was obtained as shown in

Figure 2b. The energy level diagram of each component in PSCs is depicted in Figure 2c. In the case of PCBM, the valance band and conduction band are around −5.8 and −4.1 eV, respectively, which can reduce the energy mismatch between TiO2 and the perovskite layer and thus facilitate electron transport from perovskite to the TiO2 layer while blocking the holes. The highest occupied molecular orbital (HOMO) energy level of spiro-OMeTAD lies near −5.2 eV and well-matched with that of CH3NH3PbI3−XClX (−5.4 eV) for efficient hole extraction.51 Figure 2d shows the cross-sectional scanning electron microscopy (SEM) image of a representative spiroOMeTAD-based device in which the mixture of FK209 and LiTFSI is added as the dopant, indicating a clear layer-by-layer structure of the device. 329


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observed as shown in Figure 3, especially in terms of Jsc and FF. FF is greatly improved to 60.9 ± 3.8%, combined with an increased Jsc of 20.18 ± 0.68 mA/cm2 and Voc of 1.08 ± 0.02 V, yielding the average PCE of 13.9%. The champion device achieves a PCE of 15% and FF of 64.7%. LiTFSI shows a positive effect on spiro-OMeTAD in generating free carriers, which might further help in decreasing interface-related recombination and improving the charge collection efficiency, resulting in a great increase in Jsc and FF (Figure 4a,c). This is also in agreement with the changes in Rs. Rs decreases from 37.15 (in pristine-spiro PSCs) to 8.36 Ω cm2 with the incorporation of LiTFSI into spiro-OMeTAD; the increased Jsc and decreased Rs for Li-spiro-based PSCs suggest a lower charge-transport resistance and higher hole extraction efficiency from perovskite to the anode.52 Upon further addition of the Co(III) complex into Li-spiro, the FF and Jsc are strikingly increased compared with those for the Li-spiro-based PSCs. The FK209/Li-spiro based devices achieved an average PCE of 16.4%, with a Jsc of 20.13 ± 0.57 mA/cm2, Voc of 1.09 ± 0.02 V, and FF of 70.3 ± 3.8% (the best-performing device shows a remarkable Jsc of 21.7 mA/cm2, a FF of 74.1%, and an overall PCE of 17.8%). As seen from Table 1, the shunt resistance (Rsh) of the FK209/Li-spiro-based device is significantly increased to 1758 Ω cm2 as compared to that of the Li-spirobased device (947 Ω cm2). The Rsh is critically related to the leakage current of the photovoltaic device, which would induce interface carrier’s recombination. The higher Rsh in FK209/Lispiro-based PSCs indicates that less leakage current occurs in the device and subsequently generates higher FF value. Meanwhile, a decreased Rs of 5.24 Ω cm2 was also observed, both of which are well-consistent with the observations of the photovoltaic performance for FK209/Li-spiro. The devices using FK102/Li-spiro have a similar trend to FK209/Li-spiro. These results reveal that the combined use of LiTFSI and Co(III) salt shows better hole doping for spiro-OMeTAD to generate free carriers and improve its hole mobility, thereby achieving a win−win situation for the reduced charge recombination and efficient hole transport and extraction from perovskite to the anode. Apart from the FF and Jsc, Voc increases from 1.03 V for pristine-spiro-based PSCs to 1.08 V for Li-spiro-based PSCs and 1.09 V for FK209/Li-spiro-based PSCs (Figure 4b). The Voc is primarily determined by the difference between the quasiFermi level of TiO2 and the HOMO level of HTMs (spiroOMeTAD with different dopants) for the PSCs, which is related to the potential losses (defined as Eg − qVoc) in the photovoltaic system. The Voc/Eg ratio is generally used to estimate the potential loss,9 and the Voc/Eg ratios for PSCs using pristine-spiro, FK209-spiro, Li-spiro, and FK209/Li-spiro are 0.64, 0.66, 0.67, and 0.68, respectively. The Voc/Eg value becomes higher with the use of p-type dopants, especially for the FK209/Li-spiro, which means that a reduced charge recombination and less potential loss occurred in the device with the co-addition of LiTFSI and FK209, which is responsible for its increased FF and improved PCE. On the other hand, the hole doping of spiro-OMeTAD can facilitate the formation of oxidized spiro-OMeTAD. This would lead to a shift of its intrinsic HOMO level to a lower level that further expands the gap between the quasi-Fermi level of TiO2 and the HOMO level of spiro-OMeTAD, resulting in a higher Voc.30,40 Therefore, the higher Voc of FK209/Li-spiro-based PSCs suggests the superior hole-doping ability of the combination

J−V characteristics for PSCs using different dopants measured under standard AM 1.5G of 100 mW/cm2 are represented in Figure 3a, and the corresponding photovoltaic parameters are summarized in Table 1. The statistical analysis is also shown in Figure 4. All of the devices are fabricated by the same experimental process except for the different doping conditions. Here, dopant-free spiro-OMeTAD is denoted pristine-spiro and FK102- and FK209-incorporated spiroOMeTAD’s are represented as FK102-spiro and FK209-spiro, respectively, and when doped with LiTFSI + tBP, FK102 + LiTFSI + tBP, and FK209 + LiTFSI + tBP, the HTMs are named Li-spiro, FK102/Li-spiro, and FK209/Li-spiro, respectively. The chemical dopants greatly affect the photovoltaic performance of PSCs. The devices based on dopant-free spiroOMeTAD possess an average PCE of 5.3% with a Jsc of 19.05 ± 0.75 mA/cm2, Voc of 1.02 ± 0.02 V, and FF of 26.9 ± 3.3%, and the poor FF is obviously the main reason that limits their performance, which is critically related to the strong hole accumulation and high carrier recombination in the device caused by the low hole mobility and conductivity of spiroOMeTAD in its pristine form.25 Using the Co(III) complexes FK102 and FK209 as dopants, we observed an increased device performance compared to that of the pristine-spiro-based PSCs. The best-performing device based on FK209-spiro shows a Jsc of 20.5 mA/cm2, Voc of 1.08 V, FF of 61.2%, and an overall PCE of 13.51%. Although the FK102-spiro-based PSC exhibits an efficiency of only 11.5%; with a Jsc of 20.12 mA/cm2, Voc of 1.05 V, and a poor FF of 54.5%, which shows a great improvement compared to that in the pristine device; it is much inferior to the FK209-spiro-based PSC. The difference in performance parameters and primarily in FFs for the PSCs using the two Co(III) complexes as p-dopants is quite beyond our expectation as only tert-butyl groups are introduced as endcaps in the pyridine−pyrazole ligand for FK209 as compared with the molecular structure of FK102 (as shown in Figure 1). It is known that the oxidation of spiro-OMeTAD is accompanied by the reduction of Co(III) to Co(II), which has been confirmed by X-ray photoelectron spectroscopy (XPS), as shown in Figure S2. Taking FK209 for example, the Co 2p3/2 peak in the FK209 film centered at 782.9 eV corresponds to that of Co3+ in FK209. For FK209/Li-spiro and FK209-spiro films, the Co 2p3/2 peak becomes broaden with the peak intensity obviously decreased and the Co 2p3/2 peak position shifts to a lower binding energy. This proves the formation of a low valence state of Co. Additionally, the oxidation of spiro-OMeTAD using Co(III) oxidants is an equilibrium reaction, which means that both the reacted and unreacted Co complexes will remain in the HTM solutions. Thus, the enhanced solubility of FK209 induced by the tertbutyl group brings out a better dispersion of the Co(II) salt in the spiro-OMeTAD matrix, obviating crystallization of the Co(II) salt in the solution and increasing charge carrier transport in the HTL. In addition, FK209 is reported to show higher Co(III)-to-spiro-OMeTAD+ conversion yield than that by FK102, and the concentration of the oxidized spiroOMeTAD radical cation is closely correlated with chargetransport resistance at the perovskite/spiro-OMeTAD interfaces and the final performance.29 Meanwhile, the series resistance (Rs) for FK209-spiro obtained from the J−V curves measured in the light is lower (9.79 Ω cm2) than that of FK102-spiro (11.19 Ω cm2), corresponding to the higher Jsc for the FK209-spiro-based device (Table 1). By applying Li-spiro as a HTM, a significantly enhanced performance is also 330


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Figure 5. (a) UV−vis absorption spectra of spiro-OMeTAD in CB solution with different doping conditions. The inset shows the enlarged view of the absorption peak at 520 nm. (b) UV−vis absorption spectra of Li-spiro in CB solution measured after storage in the dark and illumination under standard AM 1.5G of 100 mW/cm2 for 30 min.

intensity at 520 nm to the value at 390 nm for FK209/Li-spiro and FK102/Li-spiro is obviously much higher than that of FK209-spiro to FK102-spiro, indicating the formation of high levels of spiro+. As the doping concentration in this work for each dopant is maintained, we can conclude that the co-doping of Co(III) oxidants and ionic additive LiTFSI exhibits a superior hole-doping level, thus contributing to the performance improvement, which coincides well with the aforementioned photovoltaic behavior. The oxidation reaction between spiro-OMeTAD and oxygen is reported to be a photoinduced reaction, which can be accelerated by LiTFSI and does not occur in the absence of either light or oxygen.53 During the oxidation process, photons participate in the reaction, whereas LiTFSI does not directly involve in the reaction but instead acts as a catalyst to promote the oxidative reaction upon air exposure. Lithium ions can react with oxygen to generate lithium oxide, facilitating the formation of oxidized spiro-OMeTAD; meanwhile, bulk anion TFSI− (shown in Figure 1) acts as a counterion to stabilize the spiroOMeTAD cation radical.31,40,41,54−56 As a result, Li-spiro stored in the dark does not show any absorption signal at 520 nm due to the inhibited photoinduced oxidization. To further confirm the light-driven oxidation of spiro-OMeTAD with LiTFSI, the absorption spectra of Li-spiro in CB solution after 30 min illumination under AM 1.5G of 100 mW/cm2 were also recorded, as shown in Figure 5b. The absorption band of oxidized spiro-OMeTAD at 520 nm can be clearly recognized after illumination, suggesting the occurrence of the oxidation reaction between spiro-OMeTAD and LiTFSI, which results in a significant increase in the concentration of spiro+. The result can be further demonstrated by the absorption spectra of these HTM films as prepared and after illumination, as shown in Figure S3. As it can be observed, the dopant-free spiroOMeTAD film shows a very weak absorption band at ∼550 nm, originated from oxidized spiro-OMeTAD. A similar behavior for the absorption band at 440−800 nm is also observed for Li-spiro. It is probably due to the oxidation reaction between spiro-OMeTAD and oxygen during the spincoating process and absorption test, during which light is unavoidable. When FK209 was added into spiro-OMeTAD, regardless of the presence of LiTFSI, the absorption spectra of FK209-spiro and FK209/Li-spiro rose from 440 to 800 nm with a relatively high intensity, indicating the formation of oxidized spiro-OMeTAD, which is attributed to the charge

of LiTFSI and the Co(III) complex into the HTM, which has been further confirmed later. Figures 3b and S3 show the IPCE spectra of the champion PSCs using these different dopants. The p-dopants greatly affect the IPCE curves. The IPCEs of FK209/Li-spiro- and FK102/Li-spiro-based devices exhibit higher values compared to those of Li-spiro and FK209-spiro within the wavelength range of 400−740 nm, with a maximum value of 90% (FK209/ Li-spiro) at 510 nm, which is in good agreement with the increase in the measured Jsc. To more fully elucidate the above PSC performance, the spiro-OMeTAD films prepared under different doping conditions were studied in detail. The hole-doping level, that is, the concentration of oxidized spiro-OMeTAD radical cations, is found to have great influence on the chargetransport resistance at the HTM/active-layer interface and thereby on the overall performance of PSCs. The oxidized spiro-OMeTAD can be easily monitored by UV−vis absorption spectroscopy in chlorobenzene (CB) solution with a characteristic absorption band at 520 nm.53 Figure 5a shows the UV−vis absorption spectra of spiro-OMeTAD with different dopants in CB solution, where all solutions are stored in the dark before the measurement. To clearly compare the hole-doping effects of different dopants on spiro-OMeTAD to generate additional charge carriers, the doping concentrations for UV−vis absorption measurement are kept consistent with those used for device fabrication. The concentration for the spiroOMeTAD matrix in CB solution is 7.35 × 10−6 M; for other doped HTMs, different amounts of dopants are further added into the matrix solutions, the corresponding dopant concentrations are as follows: Li-spiro, 2.66 × 10−4 M LiTFSI; FK102spiro or FK209-spiro, 5.63 × 10−5 M FK102 or FK209, respectively; and FK102/Li-spiro or FK209/Li-spiro, 2.66 × 10−4 M LiTFSI and 5.63 × 10−5 M FK102 or FK209, respectively. It can be seen that dopant-free spiro-OMeTAD displays an intense absorption band at 390 nm, when doped with Co(III) oxidants, or the mixture of the Co(III) complex and LiTFSI, a new absorption band appears at 520 nm whether the existence of LiTFSI or not. Because FK102 and FK209, as well as their Co(II) analogues, show negligible absorption in the visible region, the peak at 520 nm can be ascribed to the monocation radical of spiro-OMeTAD (spiro+), indicating that charge transfer occurs between spiro-OMeTAD and the Co(III) oxidant.29,30,60 It is notable that the ratio of absorption 331


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Figure 6. (a) Device architecture of the hole-only device used in this work. (b) The hole mobility characteristics of FK209/Li-spiro, FK209-spiro, Lispiro, and dopant-free spiro-OMeTAD obtained from the SCLC J−V curves of hole-only devices under dark conditions.

Figure 7. (a) Steady-state PL spectra excited at 450 nm and (b) time-resolved PL decay transient spectra monitored at 770 nm for MAPbI3−XClX thin films with and without the spiro-OMeTAD layer prepared under different doping conditions after excitation at 445 ± 10 nm.

transfer interaction between spiro-OMeTAD and FK209. It is notable that with the 30 min illumination treatment, those bands of oxidized spiro-OMeTAD for Li-spiro and FK209/Lispiro films ranging from 440 to 800 nm are obviously enhanced due to the light-driven oxidation of spiro-OMeTAD with LiTFSI. Thus, it is reasonable to infer that the use of these two Co(III) oxidants can faciliate the oxidization of spiro-OMeTAD (i.e., the formation of spiro+) by light-independent charge transfer interaction between the Co(III) complex and spiroOMeTAD even in the dark, enabling FK102 and FK209 to act as stable and effective p-dopants for the organic semiconductors. In addition, Figure S4 shows the difference in the photovoltaic performance of the device based on Li-spiro measured directly after preparation and after storage under ambient air with a controlled humidity of 15% (about 16 h). It can be observed that the PCE for Li-spiro is greatly enhanced by 25% after exposure to the air with the relative humidity lower than 30% to prevent the decomposition of the perovskite layer. The performance boost is primarily dominated by the increased FF (Table S1). Actually, a similar trend but with much smaller changes is also observed for the Co(III) oxidants. The behavior can be attributed to two possible factors: one is the improved ohmic contact between the HTL and the metal anode because of the diffusion of oxygen into the spiro-

OMeTAD/Ag interface, which creates a thin silver oxide layer on the surface of Ag and decreases its work function from −4.3 to −5.1 eV, thus improving the charge collection efficiency.58 Another one is that the oxidization mechanism of spiroOMeTAD is primarily dependent on the spectral ranges. In the long-wavelength range (>450 nm), the oxidization of spiroOMeTAD can be enhanced with the assistance of perovskite.41 Thus, a higher amount of oxidized spiro-OMeTAD was formed after a certain time of storage in air, which increased the conductivity of the spiro-OMeTAD layer and enhanced charge transfer at the Ag/Spiro-OMeTAD interface, improving the PSC performance. Moreover, for Li-spiro, the exposure to air can drive the oxidation process to completion as both oxygen and light play a critical role in the oxidization process. The product, spiro-OMeTAD radical cation, can increase the hole concentration and charge carrier mobility of the HTM layer, leading to decreased charge recombination and high FF values. However, for Co(III)-doped HTM, it is not clear yet whether any chemical process with oxygen or moisture occurred during the storage. A deep understanding of the effect of light, oxygen, and moisture on the interface properties of the perovskite layer and the Co(III)-doped HTL and the performance of PSCs will be discussed in detail in our future study. The hole mobilities of the doped spiro-OMeTAD films are also measured using the space-charge-limited-current (SCLC) 332


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HTMs tested here, clearly indicating the better hole-collection ability of co-doping of FK209 and LiTFSI into spiro-OMeTAD. The results show a definite effect of chemical p-doing in improving the charge carrier transport/extraction properties of the HTM, and we believe that the device efficiency can be further increased by precisely controlling the doping ratios between these p-dopants.

method to further understand the chemical p-type doping effect on the hole-transporting properties of the HTM, for which the hole-only devices with the device architecture of ITO/ PEDOT:PSS/HTMs/Au are fabricated in this study. J−V characteristics are measured under dark conditions, as shown in Figures 6 and S5. The mobility of pristine-spiro-OMeTAD is calculated to be 1.47 × 10−5 cm2 V−1 s−1. The extracted hole mobilities of FK209/Li-spiro and FK102/Li-spiro are 1.1 × 10−3 and 5.33 × 10−4 cm2 V−1 s−1, respectively, both of which are considerably higher than those of the Li-spiro (2.27 × 10−4 cm2 V−1 s−1), FK209-spiro (8.9 × 10−5 cm2 V−1 s−1) and FK102-spiro (3.62 × 10−5 cm2 V−1 s−1) films. The higher hole mobility of FK209/Li-spiro (FK102/Li-spiro) can be ascribed to the synthetic effect of the Co(III) oxidant and LiTFSI, which exhibits a higher doping level as mentioned, thus contributing to the improved device efficiency and, in particular, to high FF values. The observed association between the hole mobility and the PCE of PSCs based on different dopants suggests the importance of high charge carrier mobility, which is mainly dominated by p-doping in this study. To investigate the charge transfer processes associated with these p-type dopants in spiro-OMeTAD at the perovskite and doped-HTL interface, steady-state photoluminescence (PL) and time-resolved PL decay transient spectra were recorded for these thin films prepared on glass substrates and the thickness of the perovskite film was maintained at an identical level. As can be seen from Figure 7a, the CH3NH3PbI3−XClX film itself displays a strong emission peak near 770 nm, and PL is significantly quenched when the spiro-OMeTAD layer (with/ without dopants) is covered onto the perovskite films, suggesting efficient charge carrier transfer at the perovskite/ HTM interface. The PL intensity at 770 nm is observed to be quenched to around 1.5 and 1.0% for perovskite covered by FK209-spiro and Li-spiro, respectively, in comparison to that for the perovskite film. The FK209/Li-spiro-based bilayer film exhibits almost entirely quenched PL with ∼0.6% PL intensity, indicating that FK209/Li-spiro can extract holes into the HTL more efficiently. The PL lifetime of these bilayer thin films was measured by monitoring the peak emission at 770 nm. The PL decay spectra are fitted with a biexponential decay model, as shown in Figures 7b and S6. The related fitted decay times are summarized in Table S2, in which the lifetimes for fast decay (τ1) and slow decay (τ2) are included. It is known that the fast and slow decay processes reflect the charge carrier capturing by defects and the radiative decay process, respectively; the latter is mainly associated with the charge carrier transfer process across the interfaces.59 Hence, in this study, slow decay lifetimes are used for comparison. The pristine CH3NH3PbI3−XClX film exhibits the largest PL lifetime with τ2 of 117.18 ns, whereas the PL lifetimes are significantly shortened when spiro-OMeTAD with dopants is coated onto perovskite. The τ2 values are calculated to be 31.46, 25.99, 22.35, 16.67, and 12.38 ns for perovskite films containing the doped HTMs FK102-spiro, FK209-spiro, Li-spiro, FK102/Li-spiro, and FK209/Li-spiro, respectively. All of them reduce the PL lifetime to a greater degree than that by perovskite/pristine-spiro (τ2 = 60.78 ns), with perovskite/FK209/Li-spiro exhibiting the fastest decay. This can be attributed to the efficient charge carrier transfer and high hole mobility induced by the chemical p-doping of spiroOMeTAD. Both the steady-state and time-resolved PL decay results matched well with the above-mentioned characterization results. There is no doubt that the FK209/Li-spiro-based PSCs exhibited the highest Jsc, Voc, and FF values for all of the doped

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CONCLUSIONS Chemical p-doping of organic HTMs has established widespread applications as an effective functional modifier, represented by the most attractive PSCs. In this work, we choose LiTFSI and two Co(III) oxidants FK102 and FK209 as the “redox active” p-type dopant models to study the effect of different types of p-type doping on the charge-transport properties at the perovskite/HTM interface and photovoltaic performance of PSCs. It was observed that both Co(III) oxidants and LiTFSI doping show positive effects on spiroOMeTAD to generate radical cations and improve its hole mobility. More strikingly, the combination of FK209 and the ionic additive LiTFSI exhibits a superior hole-doping ability, resulting in a greatly increased FF and PCE in PSCs. By incorporating CH3NH3PbI3−XClX as the active layer, the PCE of the device increased from 13.5% for FK209-spiro to 15% for Li-spiro and 17.8% for FK209/Li-spiro, with a synchronized enhancement in FF (74.1%), Jsc (21.7 mA/cm2), and Voc (1.11 V). SCLC, steady-state, and time-resolved PL measurements have confirmed that with the coexistence of these p-type dopants in a proper ratio the doped spiro-OMeTAD exhibits a high carrier mobility (1.1 × 10−3 cm2 V−1 s−1) and a more efficient charge carrier transfer/extraction capability. In addition, the optical absorption results also indicated that the introduction of Co(III) oxidants into Li-spiro can further help to avoid significant reliance on light-induced doping of LiTFSI in spiro-OMeTAD via a charge transfer interaction between FK209 (FK102) and spiro-OMeTAD occurring even in the dark. This study sheds light on the key correlation between the chemical doping and the resulting changes in the chargetransport properties of HTMs and the subsequent effects on the photovoltaic performance of PSCs, providing a useful avenue for the optimization of PSCs.

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EXPERIMENTAL SECTION Materials. All chemicals and reagents were used as received without further purification. Methylammonium iodide (MAI, 99.8% purity), tris[2-(1H-pyrazol-1-yl)pyridine]cobalt(III)tris[bis(trifluoromethylsulfonyl) imide] (FK102), and tris[2-(1Hpyrazol-1-yl)-4-tert-butylpyridine]cobalt(III)tris[bis(trifluoromethylsulfonyl)imide] (FK209) were obtained from Dyesol Ltd. Spiro-OMeTAD was supplied by Merck Inc. GBL (anhydrous, >99.9% purity) was bought from Aladdin Ltd. Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS, Clevios P VP Al 4083) was supplied by Heraeus Holding GmbH. Other materials, including lead(II) iodide (PbI2, 99.999% purity), lead(II) chloride (PbCl2, 99.999% purity), [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM, >99.9% purity), dimethyl sulfoxide (DMSO, ≥99.8% purity), toluene (anhydrous, ≥99.8% purity), CB (anhydrous, 99.8% purity), acetonitrile (anhydrous, 99.8% purity), 1-butanol (anhydrous, 99.8% purity), 4-tBP (96% purity), titanium diisopropoxide bis(acetylacetonate) (75 wt % in isopropanol), titanium(IV) chloride (TiCl4, ≥98.0% purity), and bis333


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The UV−visible absorption spectra were measured on a Perkin-Elmer Lambda 950 spectrophotometer. The corresponding HTM solutions were prepared with a similar procedure for the solutions used in device fabrication, followed by dilution treatment. For example, for pristine-spiroOMeTAD, 90 mg of spiro-OMeTAD was dissolved in 1 mL of CB, then the solution was diluted 104 times with CB to obtain the concentration of 7.35 × 10−6 M. However, for Lispiro, 45 μL of the 0.592 M LiTFSI/acetonitrile solution and 10 μL of tBP were added into the spiro-OMeTAD matrix solution (90 mg/mL in CB), which was then diluted 104 times with CB to obtain the concentrations of 2.66 × 10−4 and 7.35 × 10−6 M for LiTFSI and spiro-OMeTAD, respectively. A similar dilution process was also performed for other HTM solutions to obtain the concentration of 5.63 × 10−5 M for FK102 or FK209. It is notable that all solutions were stored in the dark prior to measurement. The absorption spectra in the solid state were measured on the spin-coated films. The thin films were prepared by spin-coating each HTM solution onto the glass substrates at 4000 rpm for 45 s, followed by drying in a drier at room temperature for 2 h. PL spectra were collected on an Edinburgh Instruments FLS920 spectrofluorometer, and the excitation wavelength was 450 nm. Time-resolved PL spectra were also performed on FLS920, which is synchronously equipped with an EPL picosecond pulsed laser (445 ± 10 nm), and the emission at 770 nm was monitored. SEM images were obtained on a JSM-7800F SEM. The chemical states of Co were analyzed by XPS (ESCALAB 250Xi).

(trifluoromethane)sulfonimide lithium salt (LiTFSI, 96% purity), were purchased from Sigma-Aldrich. Solar Cell Fabrication. F-doped SnO2 glass substrates (FTO, Pilkington, TEC8, 8 Ω/sq) were patterned by etching with Zn powder and 2 M HCl diluted in deionized water. The patterned FTO substrates were cleaned sequentially by ultrasonication in detergent, deionized water, acetone, and alcohol for 15 min each. After drying with a N2 stream, UV− ozone was further used to treat the substrates for 15 min. A thin compact TiO2 layer was deposited following the procedures in the literature:57 A 0.15 M titanium diisopropoxide bis(acetylacetonate) solution in 1-butanol was spin-coated onto FTO substrates at 4000 rpm for 40 s and annealed at 125 °C for 5 min; then, the similar process was repeated twice with a 0.3 M titanium diisopropoxide bis(acetylacetonate) solution. The as-prepared film was sintered at 500 °C for 15 min. After cooling down to room temperature, the coated substrates were immersed into a 40 mM TiCl4 aqueous solution at 70 °C for 30 min and heat-treated at 500 °C for 15 min. Then, the substrates were transferred into a nitrogen-filled glovebox and spin-coated with a PCBM solution (10 mg/mL in CB) at 6000 rpm for 40 s, followed by heating at 100 °C for 5 min. To make a uniform perovskite layer, a perovskite precursor solution consisting of 1.26 M PbI2, 0.14 M PbCl2, and 1.35 M MAI in the cosolvent of DMSO/GBL (3:7 vol ratio) was stirred at 60 °C for 2 h and then deposited onto the PCBM layer at 1000 rpm for 20 s and at 5000 rpm for 60 s. It is notable that 320 μL of anhydrous toluene was injected onto the spinning film after 25 s at 5000 rpm. Subsequently, the as-prepared substrates were annealed at 100 °C for 15 min with the thickness of the perovskite layer of about 300 nm. Next, the HTM was spin-coated on the FTO/ TiO2/PCBM/CH3NH3PbI3−XClX substrate at 4000 rpm for 45 s using the prepared HTM solutions. For dopant-free devices, a HTM solution containing 90 mg of spiro-OMeTAD in 1 mL of CB was used. For the doped devices, a Li-spiro solution was prepared by dissolving spiro-OMeTAD in CB (90 mg/mL), into which 45 μL of LiTFSI/acetonitrile solution (170 mg/mL) and 10 μL of tBP were added. The FK102-spiro and FK209spiro solutions were prepared by addition of 75 μL of Co(III) complex FK102/acetonitrile and FK209/acetonitrile solutions (75 mM), respectively, into the spiro-OMeTAD/CB (90 mg/1 mL) solution. For FK102/Li-spiro and FK209/Li-spiro solutions, the Li-spiro solution was employed with a further addition of 75 μL of the FK102/acetonitrile and FK209/ acetonitrile solutions (75 mM), respectively. Before the final thermal evaporation, all of the prepared HTM films were exposed to a controlled environment with a relative humidity of 15% for 2 h to facilitate the oxidation of spiro-OMeTAD. After that, a 100 nm silver electrode was deposited through a shadow mask to create a device area of 0.125 cm2. Measurements and Characterization. All J−V curves were recorded using a Keithley 2400 source meter unit under simulated AM 1.5G illumination at an intensity of 100 mW/ cm2 with a XES-70S1 solar simulator. The system was calibrated using a NREL-certified monocrystal Si photodiode detector before device testing. The incident photo-to-current conversion efficiency (IPCE) spectra were recorded using a solar cell quantum efficiency measurement system (SCS10X150; Zolix instrument. Co. Ltd). All of the measurements were carried out in air at room temperature without encapsulation, and the device performance was measured with a well-defined area of 0.125 cm2.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.6b00465. UV−vis absorption spectra of the perovskite film processed with different levels of tBP; XPS spectra of Co 2p peaks for FK209, FK209/Li-spiro, and FK209spiro films; UV−vis absorption spectra of pristine-spiro, Li-spiro, FK209-spiro, and FK209/Li-spiro films as prepared and after illumination; J−V characteristics measured before and after exposure to ambient air; SCLC measurement of the spiro-OMeTAD films prepared under different doping conditions; timeresolved PL decay transient spectra for MAPbI3−XClX films with/without spiro-OMeTAD layer fabricated under different doping conditions; fitted decay times of perovskite films with/without spiro-OMeTAD layer prepared under different doping conditions (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.X.). *E-mail: [email protected] (C.Z.). ORCID

He Xi: 0000-0003-0684-4979 Jingjing Chang: 0000-0003-3773-182X Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. 334


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ACKNOWLEDGMENTS We thank the Fundamental Research Funds for the Central Universities (grant nos. JB151406 and JB151402), Natural Science Foundation of Shaanxi, China (grant no. 2016JQ6035), Natural Science Foundation of Ningbo, China (grant no. 2016A610033), Young Talent fund of University Association for Science and Technology in Shaanxi, China (grant no. 20160206), the Key Research and Development Plan Project of Shandong Province (grant no. 2015GGX104012), and National Natural Science Foundation of China (grant nos. 61574107, 51503167, and 11604250) for financial support.

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Performance Enhancement of Planar Heterojunction Perovskite Solar

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