Tailored phase conversion under conjugated polymer enables

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Tailored phase conversion under conjugated polymer enables thermally stable perovskite solar cells with efficiency exceeding 21% Lei Meng, Chenkai Sun, Rui Wang, Wenchao Huang, Zipeng Zhao, Pengyu Sun, Tianyi Huang, Jingjing Xue, Jin-Wook Lee, Chenhui Zhu, Yu Huang, Yongfang Li, and Yang Yang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10520 • Publication Date (Web): 19 Nov 2018 Downloaded from http://pubs.acs.org on November 19, 2018

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Journal of the American Chemical Society

Tailored phase conversion under conjugated polymer enables thermally stable perovskite solar cells with efficiency exceeding 21% Lei Meng†,‡, Chenkai Sun‡, Rui Wang†, Wenchao Huang†,§, Zipeng Zhao†, Pengyu Sun†, Tianyi Huang†, Jingjing Xue†, Jin-Wook Lee†, Chenhui Zhu∥, Yu Huang†, Yongfang Li‡,*and Yang Yang†,* †Department

of Materials Science and Engineering, University of California, Los Angeles, California 90095, United

States. ‡Beijing

National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. §Department

of Materials Science and Engineering, Monash University, Wellington Road, Clayton, VIC 3800,

Australia. ∥Advanced

Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.

Supporting Information Placeholder

ABSTRACT: The precise control of stoichiometric balance and ionic defects on the surface of solution-processed perovskite is critical to the performance and stability of perovskite solar cells (pero-SCs). Here, we introduce a low-cost and stable conjugated donor polymer (PTQ10) as interfacial layer in the planar n-i-p structured pero-SCs. The polymer was applied to the perovskite intermediate phase before the thermal annealing. This treatment significantly reduced the loss of surface organic cation during thermal annealing. Importantly, the kinetics of phase conversion of perovskite was influenced and perovskite crystal showed a more preferential orientation. Moreover, the polymer proved to be an effective hole extraction layer due to the proper energy alignment with perovskite. Finally, a champion power conversion efficiency (PCE) of the planar pero-SCs was achieved at 21.2% with a high fill factor of 81.6%. The devices also showed great ambient and thermal stability. This work presents a facile way of perovskite surface control to achieve high performance pero-SCs.

INTRODUCTION Perovskite solar cells (pero-SCs) have been developing rapidly with power conversion efficiency (PCE) grew from 3.8% to 23.3% in recent years,1–4 owing to their long carrier diffusion lengths and low carrier recombination loss in the bulk perovskite itself.5–7 Despite of the excellent bulk properties, however, defects at the surface and grain boundaries of perovskite were found to be still detrimental to the photovoltaic performance and stability of devices. In order to restrict the formation of defects, achieving a high quality perovskite film is critical. Normally, the formation of high quality perovskite film has been achieved by retarding the unbalanced crystal growth, through the formation of intermediate phase,8 and improving the nucleation of perovskite, using antisolvent.9,10 However, due to the natural instability of the organic salt compound and weak binding in the organicinorganic hybrid perovskite, organic cations tend to escape from the perovskite crystal during thermal annealing, resulting in non-stoichiometric surface.11,12 This uncontrollable decomposition of perovskite would cause

large amounts of ionic defects on the surface, such as under-coordinated lead ions or clusters, which serve as non-radiative recombination sites.7 Surface recombination was also found to be critical to total carrier lifetime in polycrystalline perovskite films.13 Therefore, controlling the surface stoichiometric ionic balance is important to the performance of pero-SCs. To mitigate or resolve this issue, different approaches were introduced to tackle the problem involving lead defects. Neol et al. reported that ionic defects on the surface can be mitigated by introduction of designed surface interfacial layer between perovskite and a chargeselective material as surface passivation agent to stabilize the device and improve the performance.14 Bi et al. adopted PMMA on top of the surface of perovskite to control the film growth and surface composition.15 Zheng et al. used quaternary ammonium halides to passivate charged defects and reduce charge trap density.16 Several works also introduced small organic molecules as passivation layer to reduce the surface defects and nonradiative

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a

C6H13

PTQ10

C8H17

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d

b

O N

N

F

F

S n

c

w/ PTQ10

w/o PTQ10

Figure 1. Effect of conjugated polymer PTQ10 treatment on two-step made FAPbI3 films. (a) Molecular structure of PTQ10. (b) Tauc plot of FAPbI3 film regarding the treatment of PTQ10 and determination of the optical bandgap. (c) Scanning electron microscope (SEM) images of perovskite films with and without PTQ10 treatment (for the perovskite treated with PTQ10, the polymer was completely washed away by soaking into chloroform before taking SEM images). The scale bar in the figures represents length of 1 μm. (d) X-ray photoelectron spectroscopy (XPS) analysis including the 4f7/2 and 4f5/2 spectra of Pb element on perovskite film surface.

recombination sites.17–20 However, most of the introduced organic interlayers are electrically insulating and limited by their thickness to avoid the exceedingly high series resistance in the final devices. Hence, it is necessary to develop functional interlayers with both the capability of charge transport and the cation-preserving effect. In this work, a low-cost and stable conjugated polymer poly[(thiophene)-alt-(6,7-difluoro-2-(2hexyldecyloxy)quinoxaline)] (PTQ10)21 was applied as a dual-functional interlayer to achieve the aforementioned purposes in (FAPbI3)1−x(MAPbBr3)x pero-SCs. Firstly, PTQ10 serves as a dense polymeric anchor layer to suppress the volatilization of organic cations during the thermal annealing process. This cation-preserving phase conversion (CPC) function prevents the surface cations from escaping and ensures the stoichiometric balance of perovskite crystal. Secondly, PTQ10 serves as a hole selective layer to effectively improve the extraction and transport of holes. Finally, a power conversion efficiency of 21.2% is achieved for the planar pero-SCs. Due to the improved perovskite film quality and, importantly, the exclusion of metal salts in the hole transport layer (HTL),22,23 the devices are able to withstand more than 430 hours under continuous thermal stress at 85°C.

RESULTS AND DISCUSSION Effect of PTQ10 on Perovskite. In this work, we adopted a conventional two-step method to make perovskite active layer as the model system.24,25 PbI2 was first dissolved in mixed solvents of N,Ndimethylformamide (DMF) and Dimethyl sulfoxide (DMSO). This DMSO additive is well-known for its ease of formation of PbI2-DMSO complex film due to its Lewis basicity, which assist intercalation of organic cations.8,26–28 PbI2 film was then coated on the Glass/ITO/SnO2

substrate. In the second step, alkylammonium halide salts including formamidinium iodide (FAI) mixed with small amount of methylammonium bromide (MABr) and methylammonium chloride (MACl) was spin-cast on top of PbI2-DMSO complex film to form an intermediate PbI2/FAI film. Under conventional process conditions, (FAPbI3)1−x(MAPbBr3)x perovskite can be formed after 150 °C thermal annealing in the ambient air (Supporting Information). Addition of MAPbBr3 is responsible for stabilizing the perovskite phase and the ratio is relatively small compared with FAPbI3.29 X-ray photoelectron spectroscopy (XPS) analysis (Figure S1) was conducted to determine the value of x, which was found to be around 0.06. In the later discussion we simply use FAPbI3 for convenience. However, the formation of intermediate PbI2/FAI phase is only one of the critical processes for high quality perovskite film. Due to the thermal instability of perovskites, a certain amount of FA+ cations still tend to escape out of the film surface while undergoing the thermal annealing process: HC(NH2)2PbI3 → PbI2 + HC(NH2)2I

(1)

The largely unbalanced ionic composition at the surface will lead to charge carrier trapping and poor stability of solar cells. To protect the intermediate PbI2/FAI film and preserve the surface cations, conjugated polymer PTQ10 (chemical structure as shown in Figure 1a) dissolved in chloroform is coated on this intermediate-state film (as illustrated in Figure S2). The film thickness of adopted PTQ10 upon the drop-casting is around 60 nm, which functions as a CPC layer to slow down the diminishing of


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Journal of the American Chemical Society organic components. Considering the decomposition of perovskite as a first order kinetics process:30

𝑘𝑛 = ―𝑑𝑛/𝑑𝑡,

( )/(𝑡

𝑘 = 𝑙𝑛

𝑛1 𝑛2

2

(2)

― 𝑡1),

(3)

Where k is the kinetic constant, n1 and n2 are the number of moles of reactant at time t1 and t2, respectively. PTQ10 capping layer on the PbI2/FAI intermediate film will preserve the FA+ cations from escaping and remain n2 value of the final state close to n1. Therefore, kinetic constant of decomposition reaction will be very small, hindering the decay process. Additionally, the polymer chain of PTQ10 may form molecular interactions with the surface FA+, as shown in the Fourier-transform infrared spectroscopy (FTIR) in Figure S3, which might further improve the effectiveness of CPC function of PTQ10 on the perovskite surface. After thermal annealing of the intermediate-state film with PTQ10 layer altogether in the ambience, the resulting films showed a significant difference in surface morphology compared to the conventional control film without PTQ10, as shown in the scanning electron microscope (SEM) images in Figure 1c. The PbI2 and perovskite phases show obvious contrast difference and it can be observed that large amounts of white PbI2 phase evenly distributes on the surface of conventional perovskite film. The large amount of PbI2 phase reveals the decomposition of perovskite crystal, as a result of FA+ cations escape out of the perovskite surface during annealing process. Consequently, too much excess PbI2 phase will be detrimental to device performance. While for the perovskite film treated with PTQ10, no clear PbI2 phase was observed. The PTQ10 treated perovskite film demonstrates larger grain size compared to bare film as shown in atomic force microscopy (AFM) analysis (Figure S4). Importantly, because of the inherent higher hole mobility of PTQ10, its thickness could achieve over 60 nm that will lead to a more effective FA+ cations preserving function compared to the insulating interfacial materials reported in literatures (Figure S5). The Tauc plot in Figure 1b shows the optical bandgap of the PTQ10 treated perovskite to be 1.52 eV, which is 0.2 eV smaller than the control perovskite film due to the coexistence of PbI2 and FAPbI3 phases. The escape of FA+ should cause under-coordinated lead ion (Pb2+) or metallic Pb defects due to the nonstoichiometric at the surface. X-ray photoelectron spectroscopy (XPS) analysis was conducted to investigate the nature of defects for perovskite films. The results are shown in Figure 1d. The two main peaks of Pb 4f7/2 and Pb 4f5/2 possess binding energy of 138.5 eV and 143.4 eV, respectively. However, for the two-step made perovskite film without PTQ10 treatment, two additional peaks can be observed at 136.7 eV and 141.5 eV, which can be

attributed to metallic lead on the surface.31 The appearance of metallic lead may be due to the higher level of iodine and cation vacancies. PTQ10 treatment effectively reduced the peak intensity, indicating the preserved amount of organic cations and reduced undercoordinated lead atoms. Similarly, the PTQ10 thickness dependence was observed as shown in Figure S6 and the trend was consistent with the SEM characterization. Metallic lead is known to be the main non-radiative center and detrimental to the performance of the peroSCs.32 a

b

Perovskite (001) Polymer (100)

PbI2

c

d

Figure 2. Grazing incident wide angle X-ray scattering (GIWAXS) analysis and characterization of perovskite film. (a, b) GIWAXS 2D patterns of control FAPbI3 and the one treated with PTQ10 during thermal annealing process. (c) Pole figure of perovskite (001) peak extracted from 2D patterns (a) and (b). (d) Schematic diagram of preferential orientation of perovskite crystal after PTQ10 treatment.

Crystal Orientation Analysis. Grazing incident wide angle X-ray scattering (GIWAXS) is a powerful technique to investigate the crystallization behavior of perovskite films.33,34 Figure 2a and b compare 2D scattering patterns of perovskite films with and without PTQ10 surface treatment. (1D scattering profile extracted from 2D scattering patterns shown in Figure S7). The peaks located at q=0.90 and 0.99 Å-1 correspond to the PbI2 (001) and perovskite (001) peaks respectively.35–37 It is worth noting that the perovskite film without PTQ10 treatment exhibits a high intensity of PbI2 (001) peak. In contrast, the intensity of PbI2 (001) peak in the perovskite film with PTQ10 treatment is significantly reduced and the intensity of perovskite (001) peak enhanced. Little amount of PbI2 (001) peak can still be observed and it is known a controlled amount of PbI2 is favorable for hole transport in the n-i-p structured devices.38,39 Additionally, a new scattering peak emerges at q=0.28 Å-1, which is identified as the alkyl stacking peak of polymer PTQ10.21 The existence of this polymer layer can effectively reduce the tendency for perovskite decomposition and maintain the high crystallinity of perovskite film.



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Figure 3. Photovoltaic performance and photoluminescent properties. (a) Energy levels of perovskite and interfacial layers and band alignment diagram. (b) Current density-voltage (J-V) curves of the pero-SCs under the illumination of AM1.5G, 100 mW/cm2. (c) External quantum efficiency (EQE) spectra of the pero-SCs with PTQ10. (d) Steady-state power conversion efficiency (PCE) measurement. (e) steady-state and (f) time resolved PL spectra of bare perovskite film and the one incorporating PTQ10 capping layer. Table 1. Photovoltaic performance of the control device and target device from J-V measurements based on reverse (1.2 V ⟶ 0 V, step 0.02 V) and forward scans (0 V ⟶ 1.2 V, step 0.02 V). Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

PTQ10 (treatment) (Reverse)

1.12

23.15

81.57

21.21

PTQ10 (treatment) (forward)

1.12

22.84

80.24

20.58

PTAA (Reverse)

1.06

22.77

78.09

18.91

PTAA (Forward)

1.05

22.69

72.34

17.24

Hysteresis-index (%) 2.97

The crystallite orientation information of perovskite film can also be monitored from 2D scattering patterns. The perovskite film without PTQ10 surface treatment exhibits a uniform scattering intensity of the perovskite (001) peak, indicating that the orientation of perovskite crystallites is randomly distributed. On the other hand, the perovskite crystallites show a preferential orientation in the film treated with PTQ10. As shown in pole figure of perovskite (001) peak (Figure 2c), the strong azimuthal angle dependency of the (001) peak is observed in the CPC made perovskite films. The peak intensity at azimuthal angle between 50° and 60° exhibits the highest value. The highly ordered crystallite orientation strongly affects the electronic properties of perovskite film due to the alignment of polarization and ferroelectric domains (crystal orientation illustrated in Figure 2d).40 The different crystallite orientation could be caused by the different degrees of degradation of the perovskite with or without PTQ10 during heat treatment.33 The growth mechanism of perovskite crystal can be related to (1) stage of formation of PbI2−DMSO complex or intermediate state of PbI2−DMSO−FAI; (2) phase transformation from intermediate states to crystalline perovskite.41 In our case of two-step method (details mentioned in Supporting Information), the second step is the most important stage. The duration of the evaporation of solvents (DMF

8.83

and DMSO) is a critical factor for the formation of perovskite crystallites in this stage. The polymer capping layer serves as the function of slowing down the crystallites formation kinetics. The reduced evaporation rate of solvents molecules will provide a more liquid environment to support the diffusion of ions and allow longer time to form crystal facets with thermodynamically favored orientation. Moreover, the conversion of perovskite phases is a result of rearrangement of ions and reconstruction of the intermediate states. Hereby, the capping polymer layer on top of perovskite guarantees the sufficiency of cations for the full phase transformation and modifies the conditions to form crystal orientations. Taking advantage of higher crystallinity and preferred crystallite orientation, faster charge transport and less non-radiative recombination are expected to occur on the perovskite film, resulting in better device performance. Photovoltaic and Photoluminescence Properties. In this work, planar n-i-p structure was adopted for the device configuration as shown in Figure S8a and crosssectional SEM image was shown in Figure S8b. The PTAA layer doped with TPFB was adopted as a buffer layer between PTQ10 and Ag electrode. The band energy alignments of perovskite and charge transport layers are shown in Figure 3a. Ultraviolet photoelectron spectroscopy (UPS) is conducted to measure and


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Journal of the American Chemical Society calculate the detailed energy levels. The secondary electron cut off edges of perovskites and HTLs were shown in Figure S9, and valence band edges were measured as well. The Fermi level (EF) and energy level of valence band maximum (VBM) of perovskite are determined by UPS as 4.31 eV and 5.49 eV, respectively. VBM is determined via linear extrapolation of the edge of valence band spectra acquired by the UPS measurement. Considering that the optical bandgap of FAPbI3 is around 1.52 eV as measured, the conduction band minimum (CBM) can be calculated as 3.97 eV (ECBM = EVBM - Eg). Similarly, work function of 5.39 eV makes PTQ10 an effective hole extraction layer. When Compared to PTAA alone as HTL, the deeper energy level of PTQ10 provides a more optimal energy alignment with valence band of perovskite and the energy loss of carriers extracted from perovskite would be reduced, resulting in higher opencircuit voltage (Voc) of solar cells. However, the PTAA buffer layer is still necessary to ensures formation of better ohmic contact between PTQ10 and Ag electrode. This cascade energy transfer through HTL can effectively boost the fill factor (FF) of pero-SCs close to 82%. Current density vs. voltage (J-V) curves of the optimized device using PTQ10 CPC and control device (w/o PTQ10) are shown in Figure 3b, in which the highest PCE was achieved at 21.21% (Jsc: 23.15 mA/cm2, Voc: 1.12 V, FF: 81.57%) while the PCE of controlled device was 18.91% (Jsc: 22.77 mA/cm2, Voc: 1.06 V, FF: 78.09%). The detailed parameters are shown in Table 1. The higher Voc of device with PTQ10-treated perovskite is due to the better energy alignment enhancing holes extraction and the decreased non-radiative recombination sites through suppressing surface Pb defects (Figure 1d). External quantum efficiency (EQE) spectra of the device were demonstrated in Figure 3c. The integrated Jsc of the champion device via the PTQ10 CPC method is 22.91 mA/cm2 that matches well with the Jsc measured from J-V scan (discrepancy ~1%). A stabilized PCE of 20.7% was achieved with the target device as shown in Figure 3d. The device with PCE over 20% was highly reproducible with optimized process and a statistic PCE distribution of over 40 devices was demonstrated in Figure S10. The hysteresis behavior was demonstrated in Figure S11. The perovskite treated with PTQ10 shows less pronounced hysteresis behavior and here we use hysteresis index (H=(PCEreverse-PCEforward)/ PCEreverse) to determine the degree of hysteresis. The PTQ10 involved pero-SC has a low H of 2.97% while the control device has much larger value (8.83%). The perovskite film made via the PTQ10 CPC has a limited amount of PbI2 on the perovskite surface compared with the one formed by regular two-step process. Severe organic cation-deficient perovskite system (in the control devices) would contain ionic defects, that can screen the external bias, which is a common cause of enlarged current-voltage hysteresis.42,43 Similarly, it takes longer time for the steady power output to stabilize (as shown in Figure S12). PbI2 is known as a ptype semiconductor and it has been reported that small amount of excess PbI2 on the surface in the n-i-p

structured device is beneficial to the charge transfer from perovskite to HTL.44 However, greatly enhanced holes transfer rate may also surpass the electron transfer rate on the ETL side and cause charge transfer imbalance and accumulation at interfaces, resulting in hysteresis behavior.25 Therefore, precisely controlled perovskite growth and composition is necessary for enhancing device performance. Steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) characterization were conducted to study the charge transfer dynamic between FAPbI3 films and PTQ10. Figure 3e shows the steady-state PL spectra of the FAPbI3 film and the one with PTQ10 polymer as HTL on ITO/glass substrates. The PL intensity of the perovskite with PTQ10 as hole extraction layer was largely reduced by more than half of the magnitude compared to the bare perovskite film. And the PTQ10 HTL displayed more stronger PL quenching effect than that of the PTAA HTL, as shown in Figure S13. This strongly suggests that the PTQ10 layer coated on perovskite can extract charge carriers more efficiently, which is consistent with the higher FF and less hysteresis. In addition, the peak position slightly red-shifted from 779.2 nm to 785.7 nm with the PTQ10 treatment in the perovskite film formation process. The results are consistent with the Tauc plot in Figure 1b, which relates to the purer phase of FAPbI3 and reduced amount ratio of PbI2 in the perovskite film. The TRPL was measured at the wavelength of emission peak as shown in Figure 3f. For the reference perovskite, the decay of photoluminescence was fit with exponential decay model and lifetime τ is 680.3 ns. The decay of photoluminescence of the perovskite film treated with PTQ10 was fit and yielded a decay lifetime of 318.5 ns, suggesting that the faster decay was a dominating factor in the process of depopulation of photogenerated carriers, which results from the charge collection through the PTQ10/FAPbI3 interface. The shortened lifetime indicates an efficient charge carrier extraction and leads to the enhanced photovoltaic performance. a

b

Figure 4. Transient photovoltage (TPV) and transient photocurrent (TPC) measurement. (a) TPV and (b) TPC measurements of devices with PTQ10 CPC treatment, PTQ10 only as HTL but no treatment involved and control cell.

To further study and differentiate the effect of PTQ10 upon perovskite, we prepared and categorized three types of devices: (1) ITO/SnO2/FAPbI3/PTQ10/PTAA/Ag (perovskite was treated with PTQ10 during film formation); (2) ITO/SnO2/FAPbI3/PTQ10/PTAA/Ag



(perovskite with PTQ10 coated after normal film formation without treatment); (3) ITO/SnO2/FAPbI3/PTAA/Ag (Reference device). The scenarios were illustrated in Figure S14 and photovoltaic performance of these devices was shown in Figure S15 and Table S1. Comparing device (2) with (1), the better performance of device (1) supports that the PTQ10 layer could preserve the FA+ cation in perovskite crystal and the CPC process by PTQ10 is critical for the improved performance and, more importantly, reduced hysteresis. Meanwhile, both device (1) and (2) present a higher PCE than device (3). This could be as a result of the better hole extraction with the presence of PTQ10, which will be further confirmed in the following part. The charge carrier lifetime and charge carrier extraction ability was analyzed via transient photovoltage (TPV) and transient photocurrent (TPC) measurements, respectively, as shown in Figure 4. In the measurement, three types of devices were prepared: The TPV measurements were conducted under open-circuit condition and the results were shown in Figure 4a. When the solar cell was illuminated under pulse light, there will be photon-generated electrons and holes, followed by the generation of photovoltage. After the pulsed light was off, the voltage decreased owing to the recombination of electrons and holes at defect sites. The decay time of the photovoltage of device (2) and (3) were almost the same, suggesting similar perovskite film quality, while device (1) shows longer decay time than both device (2) and (3). It can be explained that the PTQ10 CPC positively affects the perovskite film quality in the annealing process and the resulting film has less non-radiative recombination sites due to the reduced metallic lead on the surface. Figure 4b shows the photocurrent decay profile under short circuit condition for the devices. The decay time in both devices with PTQ10 decreased a lot compared to the reference devices with PTAA alone. Benefiting from the cascade energy alignment of FAPbI3/PTQ10/PTAA, the double HTLs showed better charge carrier extraction capability and reduced the extraction time by a factor of four, which is probably the origin of reduced hysteresis in the device (1).

b

a

c

PTAA (Ref.)

d

w/ PTQ10

Figure 5. Devices stability and energy-dispersive X-ray (EDX) spectroscopy analysis. (a) Device ambient stability measured at room temperature with average humidity of 40% (w/o encapsulation). (b) Devices thermal stability upon 85°C continuous annealing in nitrogen box. (c) EDX line scan of aged reference devices for silver, iodine and lead. (d) EDX line scan of aged PTQ10 involved devices for silver, iodine and lead.

Ambient and Thermal Stability. Finally, the ambient and thermal stability of the devices with PTQ10 as HTL and control devices were compared. It is widely known that HTL is one of the critical factors in maintaining the stability of pero-SCs. To test the effectiveness of the protection capability of PTQ10, ambient stability test was carried out at 25°C under 40% relative humidity without any encapsulation. As displayed in Figure 5a, the devices with PTQ10 showed over 86% retention of the initial value during the test for 1480 h and, on the other hand, the control devices show severe degradation for the identical period of test time. A plausible explanation for the improved humidity stability is the hydrophobic nature of the conjugating polymer with the long alky side chains. The wetting angles of water on the surface of PTAA and PTQ10 was measured as shown in Figure S16. PTAA is somewhat hydrophobic with a wetting angle of 80.1°, which has been previously reported. PTQ10, on the other hand, has a wetting angle of 103.6° which is much more hydrophobic compared to PTAA and, thus provides an additional level of barrier against ambient moisture. To further investigate device stability, we conducted thermal stability test of the as-prepared devices at 85°C in nitrogen box. The devices with PTQ10 as HTL shows superior temperature resistance compared with reference devices as shown in Figure 5b and Figure S17 (Some of the device J-V curves at certain time are collected and shown in Figure S18). The target device performance retained 85% of original efficiency after continuous thermal stress for over 430 hours (T85 = 430 h), while the control devices have average T85 of around 200 h. The property of thermal stability is mostly attributed to high quality perovskite film, the intrinsic stability of interfaces and the rate of the ion diffusion of both perovskite and silver electrode. To understand the mechanism of enhanced thermal stability, scanning transmission electron microscopy (STEM) and energy-dispersive X-ray (EDX) spectroscopy analysis were conducted to investigate the degree of ion diffusions at the HTL in devices, and the samples were collected after more than 400 hours of 85°C thermal treatment. The line scans of three elements (Pb, I and Ag) through the whole devices were shown in Figure 5c and d. It can be observed that both metallic electrode and iodide ions diffuse at the same time during aging process. However, the Ag atoms diffusion is much more pronounced and Ag tends to accumulate at the interface between HTL and perovskite surface in the form of AgI,45 as shown in the EDX elemental mapping in Figure S19. Interestingly, the EDX mapping also shows clear Ag clusters in bulk perovskite film (Figure S19a). Accumulated AgI at the interface and in the bulk is accountable to the increased series


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Journal of the American Chemical Society resistance (as shown in Figure S20). At the same time, the devices with PTQ10 as interfacial layer are more resistant with Ag diffusion and less AgI was formed on the surface of the perovskite. The slowed diffusion rate of metal may be due to the thicker double HTLs. In addition, PTQ10 CPC treated perovskite film has balanced stoichiometric and more stable phase, thus further prevents the thermal decomposition of perovskite during the thermal stress test.

CONCLUSION In this work, we introduced conjugated polymer PTQ10 as a dual-functional (cation-preserving and hole extracting) interfacial layer in the planar n-i-p pero-SCs. It offers a deep energy level to form a good energy alignment with the valence band of FA-based perovskite and facilitate the hole extraction. A cation-preserving phase conversion technique was also adopted to prevent the surface cations from escaping during thermal annealing process and ensure the stoichiometric balance of perovskite. An optimized perovskite crystal orientation was also achieved and led to a champion device efficiency of 21.2% with controlled hysteresis. At the same time, the devices show greatly enhanced thermal stability at 85°C beyond 400 hours. We believe our approach will provide more insights into the precise control of stoichiometric balance at the surface of perovskite films and selection of new interfacial materials for the high performance pero-SCs.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental details, Figures S1−S20, and Table S1. (PDF)

AUTHOR INFORMATION Corresponding Author * [email protected]. * [email protected]

Author Contributions The manuscript was written through contributions of all authors.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Air Force Office of Scientific Research (AFOSR, Grant No. FA9550-15-1-0333), Office of Naval Research (ONR, Grant No. N00014-17-1-2484), National Science Foundation (NSF, Grant No. ECCS-EPMD-1509955), and Horizon PV. Y.L. thanks the support from the National Natural Science Foundation of China (NSFC, No. 51820105003). STEM and EDS was conducted using the facilities in the Irvine Materials Research Institute (IMRI) at the University of California−Irvine. GIWAXS analysis was

performed in beamline 7.3.3 in Advanced Light Source Lawrence Berkeley National Laboratory. W.H. has been supported by the Australian Government through the Australian Renewable Energy Agency (ARENA).

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Journal of the American Chemical Society SYNOPSIS TOC

C6H13

PTQ10

C8H17

O N

N

F

F

S n

Ag

PCE: 21.2%

PTAA

PTQ10

FAPbI3 SnO2


9

Tailored phase conversion under conjugated polymer enables

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