Improving Efficiency and Reproducibility of Perovskite Solar Cells

Sep 7, 2017 - Here, we report that the performance of perovskite solar cells (PSCs) can be improved by aggregation control in polyelectrolytes interla...
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Improving Efficiency and Reproducibility of Perovskite Solar Cells through Aggregation Control in Polyelectrolytes Hole Transport Layer Xiaodong Li,† Ying-Chiao Wang,† Liping Zhu,† Wenjun Zhang,†,‡ Hai-Qiao Wang,†,‡ and Junfeng Fang*,†,‡ †

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China University of Chinese Academy of Sciences, Beijing 100049, China

‡

S Supporting Information *

ABSTRACT: Here, we report that the performance of perovskite solar cells (PSCs) can be improved by aggregation control in polyelectrolytes interlayer. Through counterions tailoring and solvent optimization, the strong aggregation of polyelectrolytes P3CT-Na can be broken up by P3CT-CH3NH2. When using P3CT-CH3NH2 to replace P3CT-Na as hole transport layer, the average efficiency is greatly improved from 16.9 to 18.9% (highest 19.6%). Importantly, efficiency over 15% is obtained in 1 cm2 devices with P3CT-CH3NH2, ∼50% higher than that with P3CT-Na (10.3%). Our work demonstrates the important role of aggregation control in polyelectrolytes interlayer, providing new opportunities to promote its application in PSCs. KEYWORDS: perovskite solar cells, polyelectrolytes, hole transport layer, aggregation control, counterions

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CPEs interlayers, the device efficiency is still not high enough (∼16%) compared with conventional TiO2-based devices. Generally, film property of CPEs is not only related to their inherent chemical structure, but also related to the aggregation state.18−20 The aggregation state of CPEs can strongly affect the film morphology, fluorescence quantum yields, charge injection barrier and carrier mobility.21−24 Here, we find that in perovskite solar cells, the device efficiency and reproducibility can be impressively improved by controlling the aggregation of the polyelectrolyte interlayer. Through counterions tailoring (from Na+ to CH3NH3+) combining with solvent selection (from H2O to methanol), the strong aggregation of P3CT-Na in our previous report is effectively broken up by P3CTCH3NH2. When using P3CT-CH3NH2 to replace P3CT-Na, a much smooth interlayer film can be obtained on both rigid and flexible substrate, which is important for the growth of uniform perovskite layer on top. In addition, the hole mobility is also improved in P3CT-CH3NH2, facilitating holes collection in perovskite solar cells. As a result, the average efficiency is greatly increased from 16.9 to 18.9% (highest 19.6%) on rigid substrate. Importantly, devices with P3CT-CH3NH2 exhibit much better reproducibility, and efficiency of 15.4% is realized even in 1 cm2 devices. While for P3CT-Na, the efficiency of 1 cm2 devices is just 10.3%, about 50% lower than that with

nverted perovskite solar cells have attracted considerable attention since the report of perovskite/fullerene heterojunction structure in 2013.1 Compared with conventional TiO2based devices that require high-temperature sintering (∼450 °C), inverted perovskite solar cells are easier to fabricate with a low-temperature solution process, making it compatible with both rigid and flexible substrates.2−4 Typically, inverted perovskite solar cells consist of multiple layers, where electronic communication at each interface is critically important for achieving high efficiency. As such, besides morphology optimization of perovskite layer, interface engineering between perovskite and electrode is also an effective method to enhance device performance.5,6 Among various interlayer materials, conjugated polyelectrolytes (CPEs) containing π-delocalized backbone with charged pendant groups have been widely used in optoelectronic devices because of their good wettability and low temperature solution processability.7−12 In 2015, a polyelectrolyte interlayer based on dithiophene and benzothiadiazole unit (CPE-K) was introduced in perovskite solar cells and efficiency over 12% was obtained.13 Subsequently, we reported another thiophenebased polyelectrolyte interlayer (P3CT-Na), further increasing device efficiency to 16.6%.14 Next, a series of polyelectrolytes interlayers based on arylene-vinylene (PVBT-SO3),15 benzene and thiophene (PhNa-1T),16 fluorine and triphenylamine (HSL1, HSL2)17 were developed, greatly enriching the material system and promoting device efficiency to ∼16%. Although many efforts have been conducted to the synthesis of new © 2017 American Chemical Society

Received: August 10, 2017 Accepted: September 7, 2017 Published: September 7, 2017 31357

DOI: 10.1021/acsami.7b11977 ACS Appl. Mater. Interfaces 2017, 9, 31357−31361

Letter

ACS Applied Materials & Interfaces

conformational defects in polyelectrolytes.25 The hole mobility of P3CT-Na and P3CT-CH3NH2 is measured using space charge limited current (SCLC) method (details in experimental section). As shown in Figure 1c, when using P3CT-CH3NH2 to replace P3CT-Na, the hole mobility can be increased from 0.77 × 10−5 to 1.08 × 10−5 cm2V−1s−1 (fitted line in Figure S3), thus facilitating holes collection in perovskite solar cells, which is beneficial for the improvement of device performance. The aggregation of P3CT-Na can severely affect the film morphology when coated on ITO. In scanning electron microscopy (SEM) images (Figure 2a), dots-like aggregates

P3CT-CH3NH2. Furthermore, P3CT-CH3NH2 also shows promising potential in flexible devices and the efficiency can be up to 18.2%, which is among the highest performance for flexible perovskite solar cells. Our results confirm that apart from new CPEs synthesis, the control of aggregation formation is also an effective method to improve device performance, opening up a new direction for further study of CPEs interlayer in perovskite solar cells. The inverted device configuration and chemical structure of P3CT-Na, P3CT-CH3NH2 are shown in Figure 1a. P3CT-Na

Figure 2. (a) SEM and (b) 3D AFM images of P3CT-Na and P3CTCH3NH2 coated on glass/ITO (the scan size was 20*20 μm); (c) SEM images of 30 nm perovskite coated on ITO/P3CT-Na and ITO/ P3CT-CH3NH2.

Figure 1. (a) Device configuration and chemical structure of P3CT-Na and P3CT-CH3NH2; (b) normalized absorbance and PL spectra of P3CT-Na (in H2O) and P3CT-CH3NH2 (in methanol) solution; (c) J−V curve of hole-only devices (ITO/MoO3/P3CT-Na or P3CTCH3NH2 (40 nm)/MoO3/Al).

randomly distribute in P3CT-Na films, in agreement with its strong aggregation, whereas for P3CT-CH3NH2, the film is uniform without any aggregates. This phenomenon is more obvious under atomic force microscopy (AFM) images as shown in Figure 2b. For the perovskite layer, its film morphology, especially interface morphology can greatly affect the final device performance.26 Conventional SEM measurement can only observe the film morphology at top interface (the side away from ITO). As to the bottom interface (the side near ITO), it is unable to observe. Here, we overcome this problem by preparing two types of perovskite films with different thickness. The thick one (∼500 nm) is used to simulate the top interface and the thin one (∼30 nm) is to simulate the bottom interface. As a result, the film morphology at top interface is compact and uniform whether coated on P3CT-Na or P3CT-CH3NH2 (Figure S4). However, at bottom interface, the perovskite film on P3CT-Na is no longer compact as large particles appear (Figure 2c), which is caused by the aggregates of P3CT-Na beneath. And the perovskite film on P3CT-CH3NH2 is still uniform, indicating the homogeneous film morphology throughout the entire bulk layer. Figure 3a shows the photoluminescence (PL) spectra of perovskite films using excitation light of 530 nm from ITO side. The perovskite coated on P3CT-CH3NH2 exhibits much

and P3CT-CH3NH2 possess the similar chemical structure but different aggregation state. Through counterions tailoring (from Na+ to CH3NH3+) and solvent selection (from H2O to methanol), the solution color is changed from dark red of P3CT-Na to orange of P3CT-CH3NH2 (P3CT-Na cannot be dissolved in methanol as shown in Figure S1). The absorption peak of P3CT-Na solution appears at 540 nm (Figure 1b), which is 45 nm red-shift compared to that of P3CT-CH3NH2 (495 nm), indicating the severe aggregation of P3CT-Na.23 Dynamic light scattering (DLS) is conducted to further investigate the aggregation state (Figure S2). P3CT-Na exhibits broad size distribution from 15 to 100 nm. While for P3CTCH3NH2, a narrow size distribution appears around 56 nm. The large aggregates in P3CT-Na would increase the light scattering, leading to decreased fluorescence yield.19,22 As shown in Figure 1b, the PL intensity of P3CT-Na is about 10 times lower than that of P3CT-CH3NH2, further implying the strong aggregation in P3CT-Na. All these results indicate that compared with P3CT-Na, the aggregation of P3CT-CH3NH2 is indeed suppressed, which may decrease the density of 31358

DOI: 10.1021/acsami.7b11977 ACS Appl. Mater. Interfaces 2017, 9, 31357−31361

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ACS Applied Materials & Interfaces

P3CT-CH3NH2, much longer than that on P3CT-Na (τ2 of 86.3 ns), implying the longer carriers diffusion length and suppressed recombination in bulk perovskite on P3CTCH3NH2, which further confirms the existence of traps in perovskite coated on P3CT-Na. Figure 4a shows the device performance when using P3CTCH3NH2 as hole transport layer. The best efficiency is up to

Figure 3. (a) PL spectra of perovskite films excited at 530 nm; (b) time-solved photoluminescence decay of perovskite coated on P3CTNa and P3CT-CH3NH2, excited at 377 nm, emission at 770 nm.

Figure 4. (a) J−V curves of perovskite solar cells with P3CT-CH3NH2 on rigid glass/ITO; (b) efficiency distribution among 100 separated devices; (c) J−V curves of large area devices with 1 cm2; (d) EQE of devices with P3CT-CH3NH2.

stronger PL intensity and higher quenching efficiency with PCBM compared to that on P3CT-Na, indicating the high quality perovskite film on P3CT-CH3NH2. Generally, PL spectra are mainly induced by the spontaneous radiative recombination from band edge transition and the emission from traps states would lead to red-shift in PL peak.27 As shown in Figure 3a, the PL peak of perovskite film on P3CT-Na appears at 776 nm, about 4 nm red-shift compared to that on P3CT-CH3NH2, indicating the existence of traps in perovskite film on P3CT-Na. To confirm the traps location, a PCBM layer is coated on perovskite surface to passivize the traps around top interface.27 After PCBM coating, the red-shift of PL peak still exists and further red-shifts to 778 nm (enlarged normalized PL spectra in Figure S5). As traps around top interface have been passivized by PCBM, the red-shifted PL peak may be mainly caused by traps distributed around bottom interface (the side near ITO), considering the relative short penetration length (∼80 nm) of 530 nm light.27 These results indicate that there are indeed traps located around bottom interface in perovskite film coated on P3CT-Na, which agrees with the noncompact perovskite morphology as shown in Figure 2c. Time-solved photoluminescence decay is conducted to further study the traps states in perovskite film as shown in Figure 3b. The decay spectra can be fitted to biexponential decay with a fast component τ1 and a slow component τ2 (fitted results in Table S1). The fast decay component can be attributed to the interfacial charge separation property and the slow decay component is assigned to the nonradiative recombination property in bulk.28,29 In the section of fast decay component, perovskite films on P3CT-CH3NH2 exhibit a shorter τ1 (1.8 ns) than that on P3CT-Na (τ1 of 3.0 ns), indicating the more efficient charge separation and transfer at interface between perovskite and P3CT-CH3NH2, in agreement with its high hole mobility as shown in Figure 1c. In the section of slow decay component, the time constant τ2 is 170.4 ns for perovskite on

19.6% (forward scan) with Voc of 1.09 V, Jsc of 22.2 mA/cm2, and FF of 81%. The devices with P3CT-CH3NH2 have negligible hysteresis (reverse scan 19.3%) and the stabilized power output reaches 19.1% (Figure S6). For P3CT-Na, although it exhibits almost the same work function with P3CTCH3NH2 (UPS data in Figure S7), the device performance is relatively inferior (Table S2, J−V hysteresis in Figure S13), especially in device reproducibility. As shown in Figure 4b, the devices with P3CT-CH3NH2 exhibit good reproducibility and the average efficiency is 18.9 ± 0.37% among 100 separated devices. While for P3CT-Na, a broad efficiency distribution is observed ranging from 15% to 18%, leading to an average efficiency of 16.9% ± 0.86% (Average Shifted Histogram in Figure S8). The inferior reproducibility of P3CT-Na based devices strongly limits its practical application in large area devices. As shown in Figure 4c, in 1 cm2 area devices with P3CT-Na, the efficiency is just 10.3% with low Voc of 1.05 V and FF of 45%. For P3CT-CH3NH2, the efficiency of 1 cm2 devices can be up to 15.4% with Voc of 1.11 V, Jsc of 21.2 mA/ cm2 and FF of 65%, which is 50% higher than that with P3CTNa (devices hysteresis and efficiency distribution in Figure S9). The external quantum efficiency (EQE) measurement demonstrates high quantum conversion efficiency throughout the whole wavelength range in P3CT-CH3NH2 based devices (Figure 4d), leading to an integrated Jsc of 22.0 mA/cm2, in agreement with that obtained from J−V curves. Apart from rigid devices, P3CT-CH3NH2 also exhibits promising potential in flexible solar cells. When using P3CTCH3NH2 on PET/ITO substrate, high performance flexible perovskite solar cells can be obtained with efficiency up to 18.2% with Voc of 1.09 V, Jsc of 22.1 mA/cm2, and FF of 75% (Figure 5a), which is among the highest efficiency of flexible 31359

DOI: 10.1021/acsami.7b11977 ACS Appl. Mater. Interfaces 2017, 9, 31357−31361

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ACS Applied Materials & Interfaces Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Project was supported by National Natural Science Foundation of China (51773213, 61474125), National Youth Top-notch Talent Support Program, Key Research Program of Frontier Sciences, CAS (QYZDB-SSW-JSC047), K.C.Wong Education Foundation (rczx0800), China Postdoctor-al Science Foundation funded project (2017M610380), and Zhejiang Provincial Natural Science Foundation of China (LR14E030002).

Figure 5. (a) J−V curves of flexible devices with P3CT-CH3NH2; (b) bending test of flexible devices with a curvature radius of 7 mm.

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perovskite solar cells and much higher than that with P3CT-Na (Figure S14). Furthermore, the flexible devices exhibit little hysteresis (reverse scan 17.6%) and high stabilized power output (17.5%) as shown in Figure S10. Bending tests are conducted to verify the mechanical stability of flexible devices as shown in Figure 5b.30 After 1000 cycles bending at curvature radius of 7 mm, the efficiency is still maintained at 17.1% (J−V curves in Figure S11) without any significant degradation (∼6% efficiency loss), demonstrating the excellent mechanical stability of flexible devices with P3CT-CH3NH2. In conclusion, through counterions tailoring combining with solvent selection, the aggregation of P3CT-Na polyelectrolyte in H2O is effectively suppressed by using P3CT-CH3NH2 in methanol. As a result, the device performance can be greatly improved with efficiency up to 19.6% on rigid substrate and 18.2% on flexible substrate when using P3CT-CH3NH2 to replace P3CT-Na as hole-transport interlayer. Importantly, the devices with P3CT-CH3NH2 exhibit excellent reproducibility and efficiency over 15% is achieved in 1 cm2 devices. Our results indicate that a “poor” or “excellent” polyelectrolyte interlayer is defined not only by its chemical structure but also by the aggregation states. For future mechanistic study of polyelectrolytes interlayer, the effect of electrical, optical and morphology properties induced by aggregation states should also be taken into consideration. This work would open up a broader approach to regulate the photoelectric property of polyelectrolytes interlayer, further promoting its application in perovskite solar cells.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b11977. Experimental section, DLS spectra, fitted line of hole mobility measurement, SEM images, enlarged PL spectra, UPS, XPS data, device maximum power output, efficiency distribution, device hysteresis, table of device parameters, and fitted result of time-solved photoluminescence decay (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ying-Chiao Wang: 0000-0002-6459-0770 Junfeng Fang: 0000-0003-2094-8678 Author Contributions

The manuscript was written through contributions of all authors. 31360

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