Bulk Heterojunction Quasi-Two-Dimensional Perovskite Solar Cell

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Bulk Heterojunction Quasi-Two-Dimensional Perovskite Solar Cell with 1.18 V High Photovoltage Han Wang, Guanghui Cheng, Jiangsheng Xie, Shenghe Zhao, Minchao Qin, Christopher Chang Sing Chan, Yongcai Qiu, Guangxu Chen, Chunhui Duan, Kam Sing Wong, Jiannong Wang, Xinhui Lu, Jianbin Xu, and Keyou Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17030 • Publication Date (Web): 26 Dec 2018 Downloaded from http://pubs.acs.org on December 27, 2018

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ACS Applied Materials & Interfaces

Bulk Heterojunction Quasi-Two-Dimensional Perovskite Solar Cell with 1.18 V High Photovoltage Han Wang‡, a, Guanghui Cheng‡, b, Jiangsheng Xie a, Shenghe Zhao a, Minchao Qin c, Christopher C. S. Chan b, Yongcai Qiud, Guangxu Chend, Chunhui Duand, Kam Sing Wong b, Jiannong Wang b, Xinhui Lu c, Jianbin Xu a,*, Keyou Yan a, d,* a Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong b Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong c Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong d School of Environment and Energy, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, China ‡H. W and G. C contributed equally to this work. *To whom correspondence should be addressed. Emails: [email protected]; [email protected] 1

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ABSTRACT

Multicomponent quasi-two-dimensional perovskites (Q-2DPs) have efficient luminescence and improved stability, which are highly desirable for light emitting diode (LED) and perovskite solar cell (PSC). However, the lack of radiative recombination at room temperature is still not well understood and the performance of PSC is not good enough as well. The open-circuit voltage (VOC) is even lower than that of three dimensional (3D) PSC with narrower band gap. In this work, we study the energy transfer of excitons between their multiple components by time-resolved photoluminescence (TRPL) and find that charge transfer from high energy states to low energy state is gradually suppressed during elevating temperature owing to trap-mediated recombination. This may reveal the bottleneck of luminescence at room temperature in Q-2DPs, leading to large photovoltage loss in 2D PSC. Therefore, we develop a p-i-n bulk heterojunction (BHJ) structure to reduce the nonradiative recombination and obtain high VOC of 1.18 V for (PMA)2MA4Pb5I15Cl (33.3%PMA) BHJ device, much higher than the planar counterparts. The enhanced efficiency is attributed to the improved exciton dissociation via BHJ interface. Our results provide an important step towards high VOC and stable 2D PSCs, which could be used for tandem solar cell and colorful photovoltaic windows.

KEYWORDS: Two-dimensional perovskite, Trap-mediated recombination, Energy transfer bottleneck, Bulk heterojunction, Large photovoltage

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INTRODUCTION

The efficiency of organic-inorganic halide perovskite solar cells (PSCs) has been pushed from 3.81% 1 to 23.3% 2 within nine years due to excellent electronic and optical properties 3-6,

including tunable bandgap 7, low trap density8, 9, high absorption efficiency

exciton binding energy the solution process

13,

11,

long-range charge-carrier transportation

26,

small

In combination with

these properties make perovskite solar cells a perfect candidate for

future photovoltaic applications. However, stability issues degradation

12.

10,

temperature degradation

26

14-25,

including moisture

and UV light-induced degradation

27,

become

major obstacles towards its future commercialization. Therefore, much more efforts were done for stability engineering. In recent reports, low-dimensional PSCs have shown better moisture stability than three-dimensional (3D) counterpart and these thin films can be deposited without anti-solvent method

28-33.

A typical composition is formulated as

(L)2(S)n-1PbnI3n+1, where L is typically large capping organic cations, such as phenylethylammonium (PEA) cations,37,

38

33, 34,

butylamine (BA)

35, 36,

phenylmethylamine (PMA)

S is typically methylammonium (MA+), formamidinium (FA+) small cations.

The enhanced stability is attributed to hydrophobic capping organic molecules that are resistant to water penetration via the long alkyl chains

39.

Moreover, low-dimensional

perovskite is an ideal candidate for light emitting diode (LED) due to efficient radiative recombination though multiple quantum wells like energy funnels.40-42 In these reports, they found that in the thin films, the stoichiometric incorporation of starting material (L: S: PbI2 = 2: (n-1): n) for a targeting (L)2(S)n-1PbnI3n+1 did not really yield single phase, but produced a series of poly-dispersive mixed phase compounds with n=1, 2, 3…∞. In this manuscript, we use %PMA ratio (molar percentage of PMA molecules as a fraction of total organic



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composition) to represent the concentration of insulating PMA molecules in quasi-2D perovskite (Q-2DP) mixed compounds. For solar cells, initially the low-dimensional PSCs could only yield 4.73 % efficiency

29.

There are two reasons for this low efficiency. First, the out-of-plane transport is restricted by the organic insulating long chain. Second, the size confinement will induce large binding energy, which requires large driving force for charge separation. Mohite, et al. used a novel film deposition method called “hot casting”

43

to obtain a preferred crystalline orientation

film. In this film, 2D slices were standing with respect to the substrate and the charge could be conducted along the 2D slice, thus delivering an efficiency of 12.52 %. In the following work, they demonstrated that the exciton dissociation occurred efficiently at the edge states, but the limited edges still had charge separation problem. Liu et al. found incorporating a small amount of cesium ions was beneficial to reduce trap density and increase grain size, which resulted in 13.7 % high efficiency for solar cell. 39 After that, Zhou et al. reported FA cation incorporation could tune perovskite crystallization orientation and thus delivered 12.81 % efficiency in the PV device.

36

In these reports, the VOC obtained was still much

lower than 3D PSC and probably the charge collection is still highly related to the n = ∞ perovskite. There is still a charge transfer bottleneck from small n perovskites to large n perovskites, resulting energy. In principle, for n=2, 3, 4, 5, the exciton binding energy reaches a value of ~ 200 meV, which means the 2D PSC is actually excitonic solar cell 44. During the energy transfer, there is ~200-500 meV energy loss similar to organic solar cell. In the organic solar cell, bulk heterojunction (BHJ) interface can overcome the Coulomb interactions caused by dielectric confinement and improve the charge separation, leading to the increased VOC accordingly. Previously, researchers employed BHJ in the 3D PSC and achieved fill factor (FF) as high as 0.82 and fair good photovoltage without hysteresis


45.

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However, up to date, there is no report on the implement of BHJ to dissociate the exciton for high-performance 2D PSC. In this work, we investigated the energy transfer behaviors in Q-2DPs using time-resolved photoluminescence spectroscopy (TRPL) and found that the energy transfer from their high energy state to low energy state was gradually suppressed when elevating temperature, particularly at room temperature. Thus, trap-mediated recombination outcompeted the radiative recombination and lowered the photovoltage as well. Then we employed BHJ structure in (PMA)2MAn-1PbnI3nCl Q-2D PSCs to directly facilitate exciton dissociation at the interface which mitigates large energy loss caused by complex exciton transfer inside energy funnels. BHJ structure is fabricated by a two-step process. First, phenyl-C61-butyric acid methyl ester (PCBM)-blended perovskite precursor solution was spin-coated on the TiO2 substrate. Second, during the spin-coating, anti-solvent with poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine](PTAA) incorporation was dripped on the top of PCBM-perovskite blend. Film morphology and crystallographic orientation are controlled by BHJ deposition. The obtained VOC is 1.18 V, much higher than that of the planar counterparts. The device undergoes moisture stability test under 65 %RH for two weeks with no sign of degradation. The improved VOC and the stable performance offer possibility the application in tandem solar cell and colorful photovoltaic windows as well.

RESULTS AND DISCUSSION

Photocarrier behaviors of single phase 2DP. We firstly investigate the optoelectronic property of single phase of (PMA)2PbI3Cl (100%PMA) perovskite. In the absorption spectra, (PMA)2PbI3Cl has only one excitonic peak at 530 nm, suggesting it is the well-defined single phase with 2D layered structure and uniform thickness of the inorganic framework (Figure



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S1). Hence, it provides us a platform to understand the intrinsic excitonic behavior for low-dimensional perovskites. In the power dependent TRPL spectrum (Figure 1a), we find it is more monoexponential decay like under the low excitation and then deviates towards biexponential decay under higher excitation intensity, and similar decay trends are also observed in 33.3%PMA and 40%PMA Q-2DP film (Figure S2).40. The decay traces following bi-exponential function with two decay components can be attributed to coexisting of edge states and 2D states, in which the charges are free carriers and excitons, respectively. The decay traces can be fitted by equation (1) below and average recombination rate is then given as in equation (2) below. (1)

𝐼(𝑡) = 𝐴1exp ( ― 𝑡/𝜏1) + 𝐴2exp ( ― 𝑡/𝜏2) 𝑘𝑎𝑣𝑔 = 1/𝜏𝑎𝑣𝑔 = (𝐴1𝜏1 +𝐴2𝜏2)/(𝐴1𝜏21 + 𝐴2𝜏22)

(2)

where I(t) is the PL intensity at decay time t, τ1(2) the fitted time constant, τavg the average recombination time constant and the reciprocal represents the average recombination rate (kavg).47 Values of kavg allow the determination of the total recombination events. 48 The PL feature with varied temperature is also shown in Figure 1b. We observe increasing PL intensity when reducing temperatures. We infer this behavior is ascribed to the increase of exciton recombination, as the binding energy is larger in the lower temperature region. Besides, the additional redshifted PL peak at 544nm at 17K in Figure 1b is probably indication of trion or other higher order excitonic states, which can be seen in many 2D transition-metal dichalcogenide semiconductors.52 At moderate temperatures (>120K), the excitonic recombination is suppressed due to the thermal dissociation, while trap states are gradually activated, resulting in the slow recombination and decreased PL intensity (see Figure 1c). There is a trade-off between the trap-mediated recombination (PL reduction) and exciton recombination (PL increase) on the decay traces. After increasing the temperature to 6 ACS Paragon Plus Environment

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200 K, the trap-mediated recombination begins to dominate, with the recombination rate speeding up again. Therefore, the PL lifetime first increases and then decreases (see Figure 1d).

41

In 2DPs, traps for either electrons or holes may originate from a wide variety of

sources, including halide vacancies, substitutions, interstitials and/or ion migration.

48

The

lack of efficient PL at room temperature is due to the traps and starting point at 200 K suggests the traps may arise from the activated ion motion in halide perovskite (Figure 1d).50

Figure 1. Photocarrier recombination of single phase 2DP (PMA=100%). (a) Power dependent TRPL spectrum taken at 17 K with excitation intensity from 0.72 to 15.68 nJ/cm2. (b) PL spectra at different temperatures with excitation density of 2.57 nJ/cm2. (c) TRPL profiles at different temperatures with fitting curves by bi-exponential function with excitation density of 2.57 nJ/cm2. (d) The temperature-dependent average recombination rate



from the fitted parameters and peak PL intensity under varied temperatures. Note: 2DP Films are made on glass by spin-coating 2DP precursor dissolved in DMF.

Figure 2. Photocarrier transfer of multicomponent 33.3% PMA Q-2DP. (a) PL spectrum and (b-d) TRPL decay traces for peaks at 570 nm, 613 nm, 640 nm, 760 nm corresponding to n = 2, 3, 4, ∞ 2D slices in 33.3% PMA Q-2DPs at 180 K, 210K and 240K with fitting curves 8 ACS Paragon Plus Environment

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by bi-exponential function with excitation density of 2.57nJ /cm2. (e) Average lifetimes as a function of temperature for different PL peaks. (f) Schematic of cascade excitonic charge transfer processes between 2D slices which are suppressed at elevated temperature. ket represents exciton transfer rate.

Photocarrier transfer behaviors between multicomponents of Q-2DP. For 33.3%PMA Q-2DP sample, as shown in the typical PL spectra in Figure 2a, a series of PL peaks at 570 nm, 613 nm, 640 nm, …760 nm are observed representing multiple components of 2D slices (labeled n=2, 3, 4, …∞), which are also observed in the absorption spectra (Figure S1), consistent with previous work 50. Although the Q-2DP thin films are not in a pure phase, they are especially interesting materials for LED due to the formation of self-organized multiple quantum wells. 41, 42 The PL peaks from high energy state (small n) to low energy state (large n) gradually increase due to the energy transfer. The photocarrier transfer are revealed by TRPL traces (Figure 2b-2d). Since there are large amount of free carriers in n=∞ perovskite and edge state, the decay curves are well fitted by bi-exponential function (Figure 2b-2d) and the average lifetimes are shown in Figure 2e. The lifetime of high-energy PL peaks is always smaller than that of low energy PL peaks (Figure 2a), indicating the exciton transfer from high energy states to low energy states, as schematically shown in Figure 2f. This charge transfer acts as an energy funnel and thus increases PL emission at low energy state. We also further verify the proposed energy transfer behavior by transient absorption spectroscopy (TAS) as shown in Figure S3. Transient absorption bleaching peaks first appear at high bandgap 2D components at around 570 nm and 613 nm and decay quickly followed by rapid and intense increase of absorption signals at much lower bandgap 2D slices around 750nm, this is apparent evidence for charge transfer from Q-2DP film. Based on the temperature 9 ACS Paragon Plus Environment


dependence of TRPL, we can see that, at the low temperature, energy transfer is quite efficient verified by the sharp lifetime difference between energy states. At increasing temperature, the lifetime decreases, suggesting that charge transfer is suppressed by trap-mediated processes. We again infer that ion migration may partially account for the trap (Figure 2e and Figure S4b). We also verify it by measuring temperature dependent steady state PL spectrum, the PL emission peak is reduced and blue shifted with increasing temperatures, probably indicating the presence of trap states (Figure S4). From temperature-dependent effect on the PL decay traces, we can conclude there are ion migration both in 3DP (the 760 nm peak) and low-dimensional perovskites. A little weaker dependence of lifetime on the temperature at emission of n=2, 3, 4 components suggests that low-dimensionality only mitigates the ion migration (Figure 2e), but cannot eliminate it. At 300 K, the ion migration induced traps will seriously retard the charge transfer and degrade the performance and stability as shown in Figure 2f. For 40% PMA Q-2DP, we observe similar photocarrier behaviors (Figure S5). Because the energy transfer is greatly influenced by the trap, the charge extraction is not so efficient. It is better to design an interface to improve the charge collection.

BHJ structure for Q-2D PSC. We propose to employ the BHJ structure which forms mesoscopic interpenetrating network to extract the excitonic charges directly from low-dimensional slices. The fabrication procedure of the Q-2D PSC is shown as Figure 3a. First, we blend perovskite precursor colloid with PCBM (20mg/ml PCBM powder dissolved in chlorobenzene) to form a mixed perovskite precursor. The role of PCBM can form electron extracting channel in PV device after coating on n-type substrate (TiO2/FTO glass). Besides, due to passivation effect of PCBM, the ion migration could be suppressed to reduce hysteresis of Q-2DP as intrinsic absorber (i). Second, during spin-coating of the mixed 10 ACS Paragon Plus Environment

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precursor, we simply drip the precursor film using p-type PTAA chlorobenzene solution (10mg/ml PTAA in cholrobenzene) to extract the holes more effectively. Therefore, the resulting film is actually p-i-n BHJ structure which could facilitate the dissociation of photo-induced excitons (Figure 3b). Besides, we also verify that 2DP layers actually form a graded composition by PL enhancement from back side to front side, with small n value 2DP slices near the TiO2 layer and big n value 2DP slices near spiro-OMeTAD side (Figure S6), which agrees well with the other groups.51

Figure 3. (a) Schematic illustration for fabricating BHJ Q-2D PSC using dripping method and the incorporation of p/n -type polymer to form p-i-n BHJ with Q-2DP.

(b) The

schematic illustration of band alignment for charge separation. The thin film quality is shown in Figure 4. We first apply SEM to compare the difference of quality between BHJ thin film and planar thin film as shown in Figure 4a, 4b. We find planar Q-2DP has poor film coverage, whereas BHJ Q-2D film has uniform film morphology.

Specially, there are grain boundaries with some pinholes even with

anti-solvent technique for planar film. The PCBM/PTAA polymer retarded the volatilization of organic species to generate vacancies and thus delivered high-quality thin film. Next, we further carry out GIWAXS measurement to evaluate crystal phase and crystallinity of 11 ACS Paragon Plus Environment


perovskites in planar architecture and BHJ architecture. In Figure 4c, we find diffraction rings appear for the planar thin film, suggesting the polycrystal thin film. In Figure 4d, we find separated diffraction spots aligned with dashed diffraction rings in BHJ film, which indicated high-quality well-oriented Q-2DP in a textured structure. Besides, polar intensity profile along the ring in the qr range of 0.96 to 1.02 A-1 (Figure S7) clearly shows high crystallization peak at 0 ° , middle peak at 46 ° and low peak at 70 ° indicating vertical crystalline behavior inside the BHJ film in contrast to weaker vertical growth of perovskite film inside planar film. For azimuthally integrated intensity plots as shown in Figure S8, BHJ also shows a higher crystallization peak than planar counterparts in the range of 0.96 to 1.02 A-1. More diffracted peak was also seen in planar film below q 175 μm in Solution-Grown CH3NH3PbI3 Single Crystals. Science 2015, 347 (6225), 967-970. 22 ACS Paragon Plus Environment

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Bulk Heterojunction Quasi-Two-Dimensional Perovskite Solar Cell

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