Gradated Mixed Hole Transport Layer in a Perovskite Solar Cell

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Gradated Mixed Hole Transport Layer in a Perovskite Solar Cell: Improving Moisture Stability and Efficiency Guan-Woo Kim, Gyeongho Kang, Mahdi malekshahi, Gang-Young Lee, and Taiho Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07071 • Publication Date (Web): 01 Aug 2017 Downloaded from http://pubs.acs.org on August 1, 2017

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ACS Applied Materials & Interfaces

Gradated Mixed Hole Transport Layer in a Perovskite Solar Cell: Improving Moisture Stability and Efficiency Guan-Woo Kim, Gyeongho Kang, Mahdi Malekshahi Byranvand, Gang-Young Lee, and Taiho Park* Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 37673, Korea E-mail: [email protected]

KEYWORDS: mixed hole transport layer, perovskite solar cell, stability, vertical separation, surface energy

ABSTRACT: We demonstrate a simple and facile way to improve the efficiency and moisture stability of perovskite solar cells using commercially available hole transport materials (HTMs), 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene

(spiro-OMeTAD)

and

poly(3-hexylthiophene) (P3HT). The hole transport layer (HTL) composed of mixed spiroOMeTAD and P3HT exhibited favorable vertical phase separation. The hydrophobic P3HT was more distributed near the surface (the air atmosphere), whereas the hydrophilic spiro-OMeTAD

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was more distributed near the perovskite layer. This vertical separation resulted in improved moisture stability by effectively blocking moisture in air. In addition, the optimized composition of spiro-OMeTAD and P3HT improved the efficiency of the solar cells by enabling fast intramolecular charge transport. In addition, suitable energy level alignment facilitated charge transfer. A device fabricated using the mixed HTL exhibited enhanced performance, demonstrating 18.9% power conversion efficiency and improved moisture stability.

1. INTRODUCTION Organic–inorganic hybrid perovskites have become promising materials for various optoelectronics because of their superior optical and electronic properties.1-5 In particular, perovskite solar cells (PSCs) have attracted considerable attention with efficiency reaching 22%.6-9 With the mixing of formamidinium (NH=CHNH3+, FA) and methylamine (CH3NH3+, MA), high efficiency has been easily attained.10 Despite the high efficiency and relatively low cost of the materials used in PSCs, the stability of perovskites is still uncertain, which has become the main issue in PSC researches.11 The stability of PSCs has been actively investigated, and the degradation has been attributed to moisture,12-15 light,16 oxygen,17 and temperature.18 Moisture, in particular, can accelerate the transformation of perovskites to methylamine or formamidinium halide salt and lead halide. It can eventually destroy the structure of the perovskite, directly deteriorating its performance as a photoactive material. Therefore, numerous studies have focused on the moisture stability of perovskites.19-22 Conventional PSCs as an n-i-p type comprise fluorine-doped tin oxide (FTO), electron transport layer (ETL), perovskite, hole transport layer (HTL), and metal electrode. In this device structure, hole transport materials


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(HTMs) are in direct contact with moisture in air. Therefore, the moisture stability of PSCs can be considerably improved with the introduction of a hydrophobic HTM.23-25 Several attempts to improve the properties of HTMs to achieve more efficient and stable PSCs have been reported.26-31 Although highly efficient and stable PSCs which employ inorganic HTM have been intermittently reported in an n-i-p structure, a number of studies focus on the organic HTMs due to their superior processability and tunability.32,33 Triarylamine-based HTMs, 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene

(spiro-OMeTAD)

and

poly(triarylamine) (PTAA), are the most common commercialized HTMs and are still widely used; they exhibit high efficiency and reproducibility. However, because of their low hole mobility, such materials require dopants such as bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) and tert-butylpyridine (tBP).34,35 However, these dopants absorb moisture from air and destroy the structure of the perovskite, shortening the lifetime of PSCs.36 Therefore, various dopant-free HTMs have been investigated to improve the stability of PSCs.37-39 We have already reported a newly developed random copolymer (RCP) as a dopant-free HTM for PSCs.36 This RCP imparts the device with not only high efficiency but also high stability. However, the synthesis and processability of such HTMs are usually complicated, and their commercialization has been slow. Consequently, using spiro-OMeTAD and PTAA is still the most practical approach to PSC fabrication although dopants shorten the PSC lifetime. However, they form a hydrophilic layer on the perovskite; this layer cannot completely prevent the penetration of moisture, which accelerates the degradation of perovskite. Solving the aforementioned problem requires the introduction of a hydrophobic layer on the triarylamine-based HTL. This layer should be conductive and have an adaptable energy level. Various conducting polymers can be applied; however, introducing such a layer onto the HTL


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during the solution process is not feasible. The deposition of a conducting polymer layer onto the HTL can be achieved by mixing a polymer and the triarylamine-based HTM.40,41 When two mixed materials are spin-coated, vertical separation occurs. Phase separation of the mixture occurs during the spin-coating process and is governed by various variables such as the solvent, viscosity, and surface properties.42-45 We have already proved that the vertical separation of mixed materials is strongly related to their surface energy in a polymer and a small molecule mixed system.40,41 In general, hydrophobic materials have low surface energies and hydrophilic materials have high surface energies. When two materials are mixed and form a layer, the comparatively hydrophobic material with a low surface energy is preferentially distributed near the surface (air), whereas the other is preferentially distributed near the substrate.


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Figure 1. (a) Schematic of the mixed hole transport layer (HTL) comprising spiro-OMeTAD and P3HT. (b) Energy diagram for perovskite solar cell employing the mixed HTL. Herein, we use a mixed HTL that consists of spiro-OMeTAD and poly(3-hexylthiophene) (P3HT). P3HT was adopted because of its compatible energy levels, hydrophobicity, and costeffectiveness. We expected P3HT distributed near the surface to effectively block moisture in air, resulting in improved device moisture stability (Figure 1a). In terms of efficiency, intramolecular charge transport through P3HT should enable high hole mobility in the mixed HTL,46 which is advantageous for charge collection (Figure 1a). The higher highest occupied molecular orbital (HOMO) energy level of P3HT compared to that of spiro-OMeTAD should also facilitate effective charge transfer (Figure 1b). In addition, better contact between spiroOMeTAD and the perovskite layer can offset the shortcomings of P3HT. To demonstrate the improved efficiency and moisture stability of this system, we systematically investigated the mixed HTM system in planar PSCs by varying the ratio of P3HT to spiro-OMeTAD.

2. EXPERIMENTAL 2.1 Contact angle measurement Contact angle images were obtained using white LED module (Surface and electro-optics 300A). DI-water and diiodomethane (DIM) were employed as dropping liquids to calculate the surface energy. The samples were prepared as follows: glass/spiro-OMeTAD, mixed HTLs, or P3HT. All samples were prepared with dopants (LiTFSI and tBP). The surface energy was calculated using Wu-harmonic mean method, ሺ૚ + ‫ܛܗ܋‬ીሻࢽ࢒ = ૝ሺ

ࢊ ࢽࢊ ࢒ ࢽ࢙

ࢽࢊ ࢏

࢖ ࢖

+ ࢽࢊ࢙ +

ࢽ࢒ ࢽ࢙ ࢖

ࢽ࢒

࢖

+ ࢽ࢙ ሻ . For the

calculation, the polar surface tensions are adopted as 50.7 mJ/m2 for DI-water and 1.8 mJ/m2 for


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DIM, respectively, and the dispersion surface tensions are adopted as 22.1 mJ/m2 for DI-water and 49 mJ/m2 for DIM, respectively. 2.2 Solar cell fabrication Perovskite solar cells were fabricated on the FTO coated glass substrates. For the electron transport layer, tin oxide (SnO2) solution (0.1128g SnCl2·2H2O in 5 mL ethanol) is spin-coated at 2000 rpm for 60 s on the UV-ozone treated substrates, forming ~50 nm layer.47-49 SnO2 layer is annealed at 200 °C for 30 min. Perovskite layer was deposited using anti-solvent method. Precursor for (FAPbI3)0.85(MAPbBr3)0.15 was prepared, mixing 187.94 mg of formamidinium iodide (FAI), 529 mg of lead iodide (PbI2), 22.67 mg of methylamine bromide (MABr), and 74.32 mg of lead bromide (PbBr2) in 1 mL DMF:DMSO = 4:1 (vol %) solvent. Prepared precursor was spin-coated on UV-ozone treated SnO2 layer in the two step: 1) 2000 rpm for 10 s, 2) 6000 rpm for 30 s. In the second step, toluene is quickly dropped after 10 s. Perovskite layer is annealed at 100 °C for 90 min. Hole transport material (HTM) solutions (spiro-OMeTAD: 72.3 mg spiro-OMeTAD with 27.8 µL tBP, 17.8 µL LiTFSI (520 mg/mL in acetonitrile), and 1 mL chlorobenzene, P3HT: 15 mg P3HT with 20.4 µL tBP, 20.4 µL LiTFSI (28.3 mg/mL in acetonitrile) and 1 mL chlorobenzene, mixed HTM: solution mixed with the ratio written) were spin-coated to attain the thickness of ~100 nm layer. Finally, silver or gold electrode (100 nm) was thermally evaporated in high vacuum condition 2.3 Device characterization Using a Keithley 2400 SMU and an Oriel xenon lamp (450 W) with an AM1.5 filter, J-V curves of solar cells were obtained in air under AM 1.5G illumination of 100 mW/cm2 (Oriel 1 kW solar simulator), which was calibrated with a KG5 filter certified by National Renewable Energy Laboratory (NREL). The current density-voltage (J-V) curves of all devices were


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measured by 0.04 V/s of scan rate with 200 ms voltage settling time. The active area of device is 0.09 cm2. 2.4 SCLC measurement Space charge limited current (SCLC) was measured to obtain hole mobility of devices employing mixed HTMs which is controlled by various ratios. PEDOT:PSS (Clevios P, VP AI 4083) was spin-coated on UV-ozone treated indium tin oxide (ITO) substrate at 5000 rpm for 30 sec and then annealed at 140 °C for 10 min. The P3HT solution (15 mg/mL in chlorobenzene), spiro-OMeTAD solution (72.3 mg/mL in chlorobenzene), and mixed HTM solution with various ratios were spin-coated to attain the thickness of ~200 nm. After drying, gold electrode (100 nm) was thermally evaporated in high vacuum condition. Obtained J-V curves were fitted to SCLC model to calculate the hole mobility using Child’s law. 2.5 IPCE measurement Constant 100 W Xenon lamp source with an automated monochromator filters (5-position filter wheel) and 0.76 mm x 1.0 mm rectangular spot size was used for incident photon-tocurrent efficiency (IPCE) spectra. The measurements were conducted in the wavelength range from 300 to 850 nm, chopped at 4 Hz for high signal-to-noise (IQE-200B model). 2.6 Transient photoluminescence measurement Transient photoluminescence measurements were performed using time correlated single photon counting (TCSPC) system (HAMAMATSU/C11367-31). For TCSPC measurements, a pulsed laser source was laser diode with a wavelength of 474 nm, a repetition rate of 100 kHz, fluence of ~ 4 nJ/cm2 and a pulse width of 70 ps. In this study, an excitation wavelength of 474 nm and an emission wavelength of 770 nm were used. The samples were excited from the glass


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side under ambient conditions. Each of samples was prepared in the configuration of glass/(FAPbI3)0.85(MAPbBr3)0.15/mixed HTM. 2.7 Stability test Various sets of devices were kept at relative 30 % humidity. The humidity was controlled in the dark box using precise hygrometer. The devices were periodically measured in the humid air.

3. RESULTS AND DISCUSSION

Figure 2. (a) Contact angle images of the mixed hole transport layer (HTL) about DI-water and diiodomethane (DIM) (b) Contact angle values and surface energies of mixed HTL, which has different mixing ratio. (c) Secondary ion mass spectroscopy (SIMS) depth profile of perovskite solar cell employing the mixed HTL.


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When the mixture of spiro-OMeTAD and P3HT is coated to form the HTL, vertical separation occurs depending on the surface energy of each material. To estimate the vertical separation in the HTL, we measured the contact angles of the mixed HTL by varying the ratio of P3HT to spiro-OMeTAD and subsequently used secondary-ion mass spectroscopy (SIMS) to characterize the PSC with the mixed HTL (Figure 2). Contact angle values were obtained using deionized water (DI-water) and diiodomethane (DIM) (Figure 2a). In Figure 2b, the contact angle values of the mixed HTL synergistically increase with increasing ratio of P3HT. In addition, dynamic contact angle measurement exhibited an identical tendency (Figure S1). The surface energy calculated from contact angle values dramatically decreased when even a small quantity of P3HT was introduced. In addition, the surface energy value of the mixed HTL exponentially decreases with increasing ratio of P3HT, showing the synergistic relationship between spiro-OMeTAD and P3HT. If P3HT and spiro-OMeTAD exhibit a uniform distribution in the mixed HTL, the surface energy values will decrease linearly, not synergistically. The observed synergy indicates that vertical separation occurs in the mixed HTL and that P3HT is mainly located near the surface. The SIMS data (Figure 2c) show the normalized ratio of sulfur to nitrogen ([S]/[N]) as a function of film depth. The [S] and [N] approximately indicate [P3HT] and [spiro-OMeTAD], respectively. Near the perovskite layer, the signal of [N] could be overestimated due to interference of nitrogen in perovskite. However, the overestimation is negligible until the 100-nm depth region because the HTL is coated approximately 100-nm thick (Figure S2). The [S]/[N] ratio decreases as the detection depth becomes deeper, indicating that P3HT is predominantly distributed near the surface, whereas spiro-OMeTAD is predominantly distributed near the perovskite layer. This vertical separation in the mixed HTL can induce two positive effects in terms of efficiency. One effect is better hole


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extraction ability because of good contact between the spiro-OMeTAD layer and the perovskite layer; the other effect is that holes produced in the perovskite will be transported and collected faster than when spiro-OMeTAD is solely used because of intramolecular charge transport through the P3HT. The high HOMO energy level of P3HT will also provide a gradual charge transport pathway. In addition, the dramatically increased hydrophobicity of the mixed HTL should help extend the device lifetime against moisture compared to the lifetime of the spiroOMeTAD device. We measured atomic force microscopy (AFM) of HTLs to investigate their morphologies (Figure S3). When small quantity of P3HT is added, a periodic structure which can improve the perovskite absorption due to the enhanced cavity effects is observed as reported50 and this may affect the device performance.


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Figure 3. Histograms for various ratios of the mixed HTL: (a) short circuit current density (JSC), (b) open circuit voltage (VOC), (c) fill factor (FF), and (d) power conversion efficiency (PCE)


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We fabricated a series of PSCs (0.09 cm2 of active area) with the following planar structure: FTO/tin oxide (SnO2)/(FAPbI3)0.85(MAPbBr3)0.15/HTL/Ag (Figure S2). Figure 3 presents the histograms of the parameters of the devices fabricated using HTLs with various ratios of P3HT to spiro-OMeTAD. (The subscript indicates the weight ratio of each material.) The improved power conversion efficiency (PCE) compared to that of the device with spiro-OMeTAD is mainly attributed to the increased short circuit current density (JSC) and fill factor (FF); these improved parameters can be attributed to the improved hole mobility in the HTL (Figures 3a and 3c). Except for the P3HT device, the devices exhibit similar open circuit voltage (VOC) values (Figure 3b). This similarity is related to the interface between the perovskite layer and the HTL. Spiro-OMeTAD is predominantly located near the perovskite layer; thus, charge transfer from the perovskite layer to the HTL is dominantly determined by spiro-OMeTAD, resulting in a similar VOC distribution.51 When the ratio of P3HT increases, the VOC distribution becomes low and broad because of inevitable contact between the perovskite and P3HT. Finally, the device with P3HT exhibited the lowest efficiency because high HOMO energy level of P3HT induces VOC

loss

and

inefficient

hole

extraction.

These

results

suggest

that

the

spiro-

OMeTAD0.92:P3HT0.08 mixed HTL is the optimum composition for attaining the highest efficiency and reproducibility. Thus, we conducted detailed analysis of the HTL with this ratio to understand its properties and photovoltaic performance as an optimum condition.


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Figure 4. (a) Current density-voltage (J-V) curves obtained from the spiro-OMeTAD, spiroOMeTAD0.92:P3HT0.08, and P3HT devices. (b) Steady-state current and stabilized power output measured at a maximum power point (0.83 V) and incident-photon-to-current efficiency (IPCE) curve for spiro-OMeTAD0.92:P3HT0.08 device. (c) Space charge limited current (SCLC) curves and (d) transient photoluminescence (PL) decay obtained from spiro-OMeTAD, spiroOMeTAD0.92:P3HT0.08, and P3HT samples. The best current density–voltage (J–V) curve for each HTM is presented in Figure 4a (Table 1). Each device exhibited some hysteresis, which is allowable for perovskite planar devices (Figure S4 and Table S1). The device with the spiro-OMeTAD0.92:P3HT0.08 HTL exhibited a


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PCE of 18.9% with a JSC of 24.5 mA/cm2, a VOC of 1.01 V, and an FF of 76.2%. These values are much higher than those of the devices fabricated using P3HT and spiro-OMeTAD. The steadystate current and stabilized power output measured at a maximum power point (0.83 V) are presented in Figure 4c. They quickly reached their maximum values and maintained the values for a long time. In addition, the incident-photon-to-current efficiency (IPCE) curve for the best device with a mixed HTL shows high IPCE values over a broad wavelength range from 400 to 750 nm (inset in Figure 4c). The calculated JSC value from the IPCE curve is 23.2 mA/cm2, which is slightly lower than JSC in J-V curve. The difference between two JSCs is very small and permissible. To determine why the addition of a small quantity of P3HT increases the system efficiency, we investigated the hole mobility and extraction ability of each HTL. To characterize the hole mobility, we conducted measurements of the space charge limited current (SCLC) in the configuration of indium-tin oxide (ITO)/PEDOT:PSS/HTM/Au and the obtained J–V curves were fitted to an SCLC model (Figures 4c and S5).52 Hole mobilities were calculated using Child’s law. The hole mobilities of spiro-OMeTAD, spiro-OMeTAD0.92:P3HT0.08 HTL, and P3HT were 9.63 × 10−4, 1.82 × 10−3, and 3.78 × 10−3 cm2/Vs, respectively (Table 1 and Table S2 for other mixed HTMs). The increased hole mobility of the optimum HTL (spiroOMeTAD0.92:P3HT0.08) can be related to fast intramolecular charge transport in P3HT. The intramolecular charge transport through P3HT conjugated backbone is faster than intermolecular charge transport.46 Therefore, hole mobility in the mixed HTL increases as the ratio of P3HT increases.


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Table 1. Summary of the photovoltaic parameters obtained from the best devices prepared using spiro-OMeTAD, mixed HTL, and P3HT. Measurements except for hole mobility were performed under AM 1.5 solar illumination.a JSC VOC (mA/cm2) (V) Spiro-OMeTAD 23.4 1.00 b Mixed HTL 24.5 1.01 P3HT 22.8 0.972 a Cell size: 0.09 cm2 bSpiro-OMeTAD0.92:P3HT0.08

FF (%) 72.1 76.2 63.5

PCE (%) 16.9 18.9 13.9

µh (cm2/Vs) 9.63 × 10-4 1.82 × 10-3 3.78 × 10-3

To investigate the hole extraction ability of the different HTLs, we measured their transient photoluminescence

(PL)

decay.

glass/(FAPbI3)0.85(MAPbBr3)0.15

and

We

prepared

samples

in

the

configuration

glass/(FAPbI3)0.85(MAPbBr3)0.15/HTLs

with

of

various

compositions. On the basis of the transient PL decays (Figures 4d and S4), the lifetime of charges in the perovskite was calculated to be 176.89 ns. The optimum mixed HTLs spin-coated onto the perovskite layer substantially reduced the lifetimes of holes: 3.09 ns for spiroOMeTAD, 4.32 ns for spiro-OMeTAD0.92:P3HT0.08 HTL, and 12.39 ns for P3HT, indicating that a tiny quantity of P3HT in the mixed HTL decreases the hole extraction ability of the HTL. Fundamentally, spiro-OMeTAD that has high surface energy forms better contact with perovskite layer and it affects the hole extraction ability. Accordingly, the hole extraction ability of the HTL becomes weaker with increasing ratio of P3HT (Figure S6). In addition, the steady PL exhibited the same tendency (Figure S7). This behavior is attributable to the contact between the P3HT and the perovskite with increasing quantity of P3HT, despite dominant spiroOMeTAD distribution near the perovskite layer. These results imply that the optimum ratio of spiro-OMeTAD and P3HT is 92:8 (spiro-OMeTAD0.92:P3HT0.08), which exhibits moderately


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good mobility and extraction ability. These results are well consistent with those obtained from the J–V curves.

Figure 5. Stability test of each HTL device at 30 % relative humidity. Finally, the moisture stability of unencapsulated PSCs was tested at approximately 30 % relative humidity. For stability tests, the electrode of the devices was replaced with a gold electrode because of the reactivity of silver with halides. Each of the devices was stored in a dark box wherein the humidity was strictly maintained. However, the degradation rate was too fast because of the inevitable exposure to extremely high humidity during J–V measurements conducted in the summer. At 30% relative humidity (Figures 5 and S8), the efficiency of the mixed HTM device decreases more slowly than that of the spiro-OMeTAD device. This result supports the aforementioned concept that vertical separation, which results in P3HT being located mainly near the surface, is beneficial for improving the moisture stability of the device. The results of the stability tests indicate that the mixed HTL, which comprises a commercially available material, can improve the moisture stability as well as the efficiency of the device.


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Unfortunately, a dramatic improvement in stability was not observed, likely because of the low glass-transition temperature (Tg) of P3HT. When the Tg of a HTM is lower than its working temperature, molecular motions in the HTL become active and finally form grain boundaries that can function as trap sites.53-55 These trap sites can give rise to a decrease in the efficiency after repeated measurements.55

4. Conclusions In summary, we reported a novel method to improve the moisture stability and efficiency of PSCs by mixing the commercially available HTMs spiro-OMeTAD and P3HT. When these two materials were mixed and spin-coated, vertical separation occurred, particularly depending on their surface energies. The separation led to a layer wherein P3HT was predominantly located near the surface, whereas spiro-OMeTAD was predominantly located near the substrate. This gradated structure facilitated charge extraction and transport and increased the hydrophobicity, resulting in improved performance. However, an excess of P3HT hindered the charge transfer from the perovskite to the HTL. Consequently, the device including the HTL with the optimum mixing ratio (spiro-OMeTAD0.92:P3HT0.08) maximally exhibited 18.9% PCE. In addition, the moisture stability was slightly improved in the mixed HTM devices. These results suggest an easy way to develop a useful HTL using commercially available HTMs. This demonstration of the effectiveness of this approach is expected to accelerate the development of HTL as well as new HTMs.

ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available free of charge on the ACS Publications website: SEM image, hysteresis of solar cells, SCLC results, PL results, stability test

AUTHOR INFORMATION Corresponding Author *T. Park ([email protected])

Notes The authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant (Code No. 2015R1A2A1A10054230) funded by the Korea government (MSIP) and Nano Material Technology Development Program (2012M3A7B4049989).


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TABLE OF CONTENTS


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Gradated Mixed Hole Transport Layer in a Perovskite Solar Cell

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