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Black Phosphorus Quantum Dots for Holes Extraction of Typical Planar Hybrid Perovskite Solar Cells Wei Chen, Kaiwen Li, Yao Wang, Xiyuan Feng, Zhenwu Liao, Qicong Su, Xinnan Lin, and Zhubing He J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02843 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 14, 2017

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The Journal of Physical Chemistry Letters

Black Phosphorus Quantum Dots for Holes Extraction of Typical Planar Hybrid Perovskite Solar Cells Wei Chena†, Kaiwen Lib†, Yao Wangc, Xiyuan Fenga, Zhenwu Liaoc, Qicong Sua, Xinnan Linb*, Zhubing Hea* a

Department of Materials Science and Engineering, Shenzhen Key Laboratory of Full Spectral Solar Electricity Generation (FSSEG), Southern University of Science and Technology, No. 1088, Xueyuan Rd., 518055, Shenzhen, Guangdong, P.R. China

b

Shenzhen Key Laboratory of Integrated Microsystems, School of Electronic and Computer Engineering, Peking University Shenzhen Graduate School, No. 2199,Lishui Rd., 518055, Shenzhen, Guangdong, P.R. China c

Materials Characterization and Preparation Center (MCPC), South University of Science and Technology of China, No. 1088,Xueyuan Rd., 518055, Shenzhen, Guangdong, P.R. China † Equal

contributions to this work. *Correspondence and requests for materials should be addressed to Z.B. He (email: [email protected]) and X.N. Lin ([email protected]).

1

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Abstract Black phosphorus, famous as two-dimensional (2D) materials, shows such excellent properties for opto-electronic devices as tunable direct band-gap, extremely high hole mobility (300-1000cm2/(Vs)) and etc. In this paper, facile processed black phosphorus quantum dots (BPQDs) were successfully applied to enhance holes extraction at the anode side of the typical p-i-n planar hybrid perovskite solar cells, which remarkably improved the performance of devices with photon conversion efficiency ramping up from 14.10% to 16.69%. Moreover, more detailed investigations by c-AFM, SKPM, SEM, hole-only devices and photon physics measurements, discover further the hole extraction effect and work mechanism of the BPQDs, such as nucleation assistance for the growth of large grain size perovskite crystals, fast hole-extraction, more efficient hole transfer and suppression of energyloss recombination at the anode interface. This work definitely paves a way for discovering more and more 2D materials with high electronic properties to be used in photovoltaics and opto-electronics.

TOC Graphic

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For the past few years, MAPbI3 as a paradigm of organic-inorganic hybrid perovskite photovoltaic materials have triggered a large scale research campaign for high conversion efficiency solar cells1-7,even breaking through the Schockley-Queisser (S-Q) limit of single junction ones8, owing to their unique properties such as long carrier diffusion length and carrier lifetime, low electron-hole recombination rate constant, and inverse temperature carrier mobility, which prevent the charge carriers from scattering by charge defects, optical phonons and so on9,10.So far, the certified champion cell conversion efficiency of hybrid perovskite solar cells (HPSCs) has surpassed 22%11, but it is now turning a platform as the time goes always, which still have a large distance from so called S-Q limit12. In fact, such peripheral materials in HPSC devices with low electronic properties as hole transport materials (HTM), electron transport materials (ETM) et al, for example 10-4 cm2/(Vs) in hole mobility for popular Spiro-OMeTAD13,14, hinder the whole progress of HPSCs, which is deemed as an challenge. Inorganic materials with high carrier mobility are good candidates to take place of organic molecules with much lower carrier mobility and remarkably upgrade the performance of HPSCs. On those demands, in place of organic ones, various kinds of higher mobility inorganic ETMs, dense and thin layers, have been used to improve both the performance and stability of HPSCs. However, high mobility inorganic HTMs are especially rare although such inorganic materials as CuI, CuSCN, CuInS and etc.15-20, have been adopt in the structure of HPSCs, which shows either

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disappointed hole mobility, mismatched valence band with perovskite absorb layers, unstable in process and utilization, complicated chemical composition or unreliable phase control. Hence, simple structure, facile processed, well matched with perovskite and chemically stable inorganic HTMs with high mobility and stable phase are on-demand currently. As a famous two dimensional (2D) material, black phosphorus (BP), elemental phosphorus, has a layered structure composed of wrinkle hexagonal

honeycombs

in

orthorhombic

crystal

phase

in

ambient

environment.21 Few layered BP shows excellent electronic and optoelectronic properties. It has been identified direct physical band-gap of 0.3 eV for bulk and near 2 eV for single layer by high resolution scanning tunneling spectroscopy

while

the anisotropic

quasi-particle band-gap

and the

anisotropic exciton binding energy of monolayer BP were revealed to be 2.2 and 0.9 eV, respectively.21-23 Besides the tunability of layer-dependent bandgap, BP shows very high hole mobility. The Hall hole mobility of BP reported by Xia et al. ranged from 400-1000 cm2/(Vs) for 8 nm and 15nm in thickness. The field effect hole mobility of 300-1000 cm2/(Vs) was also confirmed by various groups24-28 and the mobility value might be enhanced to 2000-4000 cm2/(Vs) when few layers BP encapsulated in hBN layers at low temperature.29 So, thin layer BP, as a high hole mobility p-type semiconductor, meets well most of the requirements of desirable HTMs for high efficiency HPSCs. In this work, BP quantum dots (BPQDs) were studied on the platform of HPSCs to investigate the holes extraction effect of BPQDs by advantage of those superior electronic and optoelectronic properties of thin layers BP

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mentioned above. In an inverted p-i-n planar HPSC device configured as “ITO/PEDOT:PSS/with

or

without

BPQD/Perovskite/PCBM/ZrAcAc/Ag”,

BPQDs were discovered to remarkably enhance the device performance owing to their strong holes extraction effect yielded from high mobility and well matched band alignment with both MAPbI3 and PEDOT:PSS. The optimal enhancement was achieved by tuning the thickness of BPQDs thin layer. The effect was explored in-detail through a series of powerful protocols, including conductive atom force microscopy (c-AFM), scanning kelvin probe microscopy (SKPM), SEM and photon-physical characteristics. A systematic series of convincing data were collected to prove the mechanism illustrated in the following context. The bulk crystalline BPs were synthesized from red phosphorus under high pressure and high temperature (see detailed synthesis procedures in the Experimental Section)30.It’s known that liquid exfoliation method is frequently employed to prepare QDs.31,32 Thus, BPQDs in isopropanol (IPA) were obtained by using sonication and centrifugation processes with high-yield. The obtained BPQDs can be easily dispersed in IPA in color of clear yellow, and stable without any noticeable aggregation at room temperature for more than one month, as the inset in Figure 1a shows. The absorption curve of this solution is similar to most of reported results mentioned above,31,32 and the optical band-gap is deduced to be 2.25 eV (Figure 1b), which is corresponding to the quasi-particle (hole) band-gap reported by Xia et al23 and size confinement. Furthermore, the BPQDs were characterized by Raman spectroscopy (Figure 1c). There are three main peaks in the Raman spectra. The peak at ca. 361.4 cm-1 can be ascribed to one out-of-plane

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phonon modes (A1g), while the other two peaks are in-plane modes, which can be defined as B2g (438.1 cm-1) and A2g (465.5 cm-1), respectively.33,34 Although BP shows strong anisotropic behavior, the polarization dependent Raman peaks’ intensity contrast is not obvious, which also show the uniformity in both the shape and size of BPQDs. Transmission electron microscopy (TEM) was used to investigate the morphology of BPQDs. As we can see from TEM images in Figure 1d, the BPQDs show narrow size distribution and the average size of BPQDs is ca. 5.2 nm. High-resolution TEM (HRTEM) images of BPQDs show the lattice fringe of 0.34 nm, which can be ascribed to the (021) plane of crystalline BP (Figure 1e and 1f).31,32,35

Figure 1a) absorption spectrum for the black phosphorus quantum dots (BPQDs) in isopropanol (IPA) solution, and the solution shown in the inset is clear yellow color; b) Tauc plot for the corresponding absorption spectrum presented in a); c) Raman spectrum for the BPQDs; d) Typical TEM image of BPQDs; e) HRTEM image of the BPQDs; f) Zoom-in HRTEM image of the BPQDs as indicated in area marked in (e) with a red dash box, the lattice distance is 0.34nm for (021) plane of BP crystals.

Next we investigated the chemical composition of BPQDs through X-ray photoelectron spectroscopy (XPS). As shown in Figure S1a, the XPS survey

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scan spectra show a predominant P2p peak (ca. 129.88eV) and O1s peak (ca. 532.07eV). The O signal is likely to come from the oxidation of BP sample when exposed in the atmosphere during XPS sample preparation, which have been observed in previous reports

31,32,36

. The high-resolution XPS spectrum

for P2p shows doublets at around 130.24eV and 129.46eV, which are corresponding

to

2p1/2

and

characteristic of crystalline BP

2p3/2 36,37

binding

energy,

respectively

and

.Additionally, the strong sub-band peak at

high-energy region (ca. 133.86 eV) is a sign of the oxidation of BPQDs (Figure S1b).36 As we declared in the introduction, BP crystals show high holes mobility of ptype behavior, which motivates us to use them in solar cells, especially currently hot HPSCs as an excellent HTM candidate. The few layers BP flakes synthesized in our experiments were applied to construct BP based FETs, shown in the inset of Figure S2. From the Ids-Vg curve in Figure S2, we can deduce the BP-FET on-state transconductance gm=1.37uS and then calculate its field-effect hole mobility (µeff) to be 118.8 cm2/(Vs) and the corresponding hole density (n) arrives to 4.7×1014 cm-3 depending on the following two equations, ௚೘ ௅೐೑೑

ߤ௘௙௙ = ௐ஼

(1)

৶ = σ/(eߤ௘௙௙ )

(2)

೚ೣ ௏೏ೞ

Where Leff is channel length, Cox is capacitance of 300nm thick SiO2 layer, W is the width of the channel, σ is conductivity of the channel, respectively. Although the measured hole mobility is a little bit far from the values reported,24-26 it, combining with considerable hole concentration, could match 7



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that requirement of MAPBI3 and is also much larger than most of other HTMs reported in the HPSCs. Basing on the understandings mentioned above, BPQDs were brought into HPSCs as a holes extraction layer. For the first, it’s worth noting that BPQDs are easily dissolved in a highly stable IPA solution (Figure 1a), which is compatible with our solution process for the fabrication of HPSCs. Thus, BPQDs were spun onto PEDOT:PSS layers to investigate the impact of BPQDs on the hole extraction behavior at the anode interface of planar p-i-n HPSCs. The typical HPSCs device architecture is presented in Figure 2a, where the notable organic-inorganic hybrid perovskite, CH3NH3PbI3, acted as photon-absorber, PEDOT:PSS and ZrAcAc modified PC61BM

38,39

were

employed as hole transport layer (HTL) and electron transport layer (ETL), respectively. Different from that BP flakes were used as electron-transfer layer,40 the BPQDs inserted between the layers of perovskite and PEDOT:PSS were applied to explore the enhancement of hole extraction from the perovskite absorb layer. The band alignment of each layer in HPSCs is sketched in Figure 2b. All the corresponding data were cited from literatures

39

, except

the BPQDs thin films. The energy levels for BPQDs thin films were determined by its absorption spectrum and valance band spectrum from UPS measurements. According to the absorption characteristic (Figure 1a), the band gap for BPQDs is calculated to be ca. 2.25 eV while the onset of the valence band (VB) of BPQDs film coated on silicon substrate for three times is determined to be ca. 5.2 eV basing on the UPS data shown in Figure S3. The valence band level of the BPQDs film matches well with both that of 8


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CH3NH3PbI3 perovskite film (5.4 eV) and the HOMO level of PEDOT:PSS (5.1 eV), facilitating holes transfer from perovskite to HTL.41 The HPSCs architecture was further characterized by cross-section scanning electron microscopy (CS-SEM). Because the BPQDs and ZrAcac layers are very thin, Figure 2c clearly shows the perovskite film was sandwiched by the modified PEDOT:PSS and PCBM layers, while both of the two organic layers show dark color due to the poor electron diffraction ability of organic materials. The thickness of perovskite film was determined to be ca. 310 nm. To investigate the influence of the BPQDs layer thickness on the device performance, the BPQD thin films were deposited on the surface of PEDOT:PSS layer in process by repeating coating of as-prepared BPQDs solution (See in Experimental Section) from 0 to 5 times. The resulted current density–voltage (J–V) characteristics of those HPSCs with and without BPQDs interlayer were plotted in Figure 2d. As the coating count of BPQDs solution increases from 0 to 3, the best device conversion efficiency ramped up from 14.10% to 16.69% associating with enhancements of all the key solar cells parameters, which can be attributed to the formation of a continuous BPQDs film in the course. The continuous BPQDs film contributes to more effectively collect the holes diffused from the perovskite absorber at the interface and transport them quickly to HTL. When the coating times increase further, the device conversion efficiency declined from 16.69% to 14.78%, which can be ascribed to that the arising thickness of partial-oxidized BPQDs film would cause the increasing of series resistance (Rs) in devices, which is identical to the variation trend of both series and shunt resistance (Rsh) listed in Table 140. Besides Rs and Rsh, all the other parameters of those HPSCs

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were summarized at Table 1. In a brief summary, for reference devices without BPQDs interlayer, the averaged PCE of 13.13 % (+0.97/-1.4) was achieved in average values of short current density (Jsc) of 19.35 mA/cm2 (+0.39/-0.31), open circuit voltage (Voc) of 0.903 V (+0.012/-0.014) and fill factor (FF) of 75.1% (+3/-5.8), respectively. Apparently, inserting BPQDs between PEDOT:PSS and perovskite film leads to the remarkable improvement of the device performance. The averaged PCE of 15.26 % (+1.43 /-0.96) was obtained after three times coating of BPQDs (named as PEDOT/BPQD3). As we can check, for the optimal cell of each kind, the main improvement comes from the augment of open-circuit voltage, increasing by almost 11% from 0.903 V for the optimal reference device to 0.989 V for the optimal PEDOT/BPQD3 devices. On the other hand, the short-circuit current rised by ca. 4% from 19.35 to 20.24 mA/cm2 correspondingly. To confirm the above improvements, external quantum efficiency (EQE) measurements were utilized to analyze the holes extraction effect of BPQDs in devices. As shown in Figure 2e, the EQE for PEDOT/BPQD3 devices in the entire visible region obviously demonstrated better photon-responsibility than that of the reference device. The Jsc of PEDOT/BPQD3 device integrated from the EQE spectrum was determined to be 20.13 mA/cm2, which is higher than the value of PEDOT reference device (19.17 mA/cm2) (Table 1), which is consistent with the Jsc enhancement observed in J-V characteristics. According to the statistics of the devices performance summarized in Figure 2F and Figure S4, including all the reference (38 cells) and PEDOT/BPQD3 based (39 cells)

devices, the comparison result show clearly that the averaged efficiency value shifts from 13.13% (+0.97/-1.4) for the reference devices to 15.26 % (+1.43 /-

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0.96) for PEDOT/BPQD3 based devices, and the distribution also move unambiguously correspondingly. The statistics data clearly demonstrates the good reproducibility of HPSCs in our process and also the advantage of BPQDs as holes extraction interlayer. The hole extraction effect and plausible mechanism of BPQDs will be discussed in-detail in the following text. To ensure the J-V measurement is reliable, we tested the hysteresis effect of our best reference and BPQDs based cells by changing the scanning direction42-44. It is found that there were negligible photocurrent hysteresis effect for our solar cells, which have further been demonstrated by measuring the J-V curves under different scanning rates (Figure S5). That confirms further the excellent charge extraction at both anode and cathode sides in our devices45.

Figure 2 a) Device configuration of the p-i-n planar HPSCs studied here; b) Energy level diagram of each layer in the device; c) Cross-section scanning electron microscopy (SEM) of an optimal device; d) J-V characteristics in 1 sun illumination (100 mW/cm2) in forward scan; e) external quantum efficiency (EQE) spectra of the optimal HPSCs without and with different times of BPQDs coating as interlayers varying from 0 to 5 times. For example, “BPQD1” means one time coating of BPQDs as an interlayer on the surface of as-prepared PEDOT:PSS in process; f) Device performances statistics for the HPSCs with BPQD3 (the optimal kind) and without BPQD as interlayers.

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Table 1 Summary of the photovoltaic parameters in 1 sun illumination (100 mW/cm2) for the optimal HPSCs without and with different times of BPQDs coating as interlayers varying from 0 to 5 times. For example, “BPQD3”denotes three times coating of BPQDs as an interlayer on the surface of as-prepared PEDOT:PSS in process. c Jsc Rs Rsh Jsc Voc(V) FF(%) PCE(%) 2 (mA/cm ) (Ω/cm2) (KΩ/cm2) (mA/cm2) a 19.74 0.915 78.1 14.10 PEDOT 19.17 12.7 4.5 b (19.35) (0.903) (75.1) (13.13) 20.31 0.970 79.1 15.58 PEDOT/BPQD1 19.89 11.2 4.8 (19.87) (0.959) (75.7) (14.43) 20.45 0.988 78.8 15.92 PEDOT/BPQD2 20.07 10.3 5.2 (19.98) (0.972) (75.3) (14.62) 20.56 1.014 80.0 16.69 PEDOT/BPQD3 20.13 8.7 5.9 (20.24) (0.989) (76.2) (15.26) 20.30 1.007 77.5 15.84 PEDOT/BPQD4 19.96 11.9 5.2 (19.79) (0.979) (73.7) (14.28) 20.17 0.963 76.1 14.78 PEDOT/BPQD5 19.75 14.5 4.3 (19.61) (0.932) (72.4) (13.23) a b Photovoltaic parameters of the best cell; Averaged values based on total cells from c different batches; The short circuit current density calculated from EQE spectra. Please note that the average value of conversion efficiency of each kinds of cells maybe not equal to the product of the corresponding three parameters, including Jsc, Voc and FF.

Devices

In order to figure out in-detail the hole extraction mechanism of BPQDs integrated in the inverted planar HPSCs, several powerful protocols, including c-AFM, SKPM, SEM and photon-physical characteristics, were employed to investigate the characteristics and analysis of the BPQDs involved interfaces. Since the BPQD3 device sample is the best performance cell, BPQD3 represent the BPQDs based devices in contrast to the reference ones without BPQDs in the following analysis. As presented at Figure3a and 3b, the morphology before and after coating of BPQDs shows almost no change, owing to a very thin BPQDs film yielded from low concentration precursor solution in IPA. While the conductivity for the 12


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PEDOT:PSS films was enhanced slightly after coating with BPQD3 (Figure3c and 3d), it is consistent with the enhancement of the photocurrent of the

HPSCs after integration of BPQD3. Furthermore, the surface potential variation of the PEDOT:PSS films with and without BPQD3 were recorded by SKPM measurements. According to Figure 3e and 3f the averaged surface potential of the PEDOT:PSS coated with BPQD3 decreased ca. 0.1V compared to the PEDOT:PSS films. This indicated the work function of PEDOT:PSS films was increased after coating with BPQDs, which demonstrated that holes could be extracted at higher energy level. That is consistent with the enhancement of the Voc for the PEDOT/BPQD3 compared to the PEDOT reference device45.

a

c

e

PEDOT/BP4 PEDOT b

PEDOT/BPQD3

PEDOT d

PEDOT/BPQD3

PEDOT f

PEDOT/BPQD3

Figure3 Atom force microscopy (AFM) topographical images for (a) ITO/PEDOT film and (b) ITO/PEDOT/BPQD3 film; Conductive AFM images for (c) ITO/PEDOT film and (d) ITO/PEDOT/BPQD3 film; Scanning kelvin probe microscopy (SKPM) images for (e) ITO/PEDOT film and (f) ITO/PEDOT/BPQD3 film.

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a

c

b

d

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Figure4 Scanning electron microscopy (SEM) images for the perovskite film on (a) ITO/PEDOT and (b) ITO/PEDOT/BPQD3; AFM images for the perovskite film on (c) ITO/PEDOT and (d) ITO/PEDOT/BPQD3. Size of each AFM images is 5 X 5 µm.

It has been well recognized that the morphology of the perovskite films are strongly correlated to the supporting interface46. Highly crystalline perovskite film with large grain size and uniform morphology are likely to achieve high performance47. In our case, the morphologies of perovskite films on different supporting interfaces have been explored by SEM and AFM measurements, as shown in Figure 4. It’s interesting that the crystalline grain size of the MAPbI3 films formed on PEDOT/BPQD3turns is larger than that only on bare PEDOT (Figure 4a and 4b). BPQDs are believed to offer nucleation sites for perovskite, hence to reduce the nuclei density and finally to form larger grain size in the course. On the other hand, the surface root-mean-square (Rms) roughness was also decreased from 13.2 nm for perovskite film on PEDOT to 11.5 nm for that on PEDOT/BPQD3 (Figure 4c and 4d), indicating higher

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quality perovskite films can be obtained through the nucleation assistance of BPQDs at the interface of PEDOT/Perovskite.

b

0.1

50

100

150

2

ITO/Perovskite ITO/PEDOT/Perovskite ITO/PEDOT/BPQD3/Perovskite

Current density (mA/cm )

Normalized Intensity

a1

1E-3 PEDOT/BP-QD3 PEDOT

1E-4

VTFL VTFL

1E-5

200

0.1

1

Time (ns)

Voltage (V)

c

d PEDOT/BPQD Linear Fit PEDOT Linear Fit

10

PEDOT/BPQD3 Linear Fit PEDOT Linear Fit

1.0

Voc(V)

2

Jsc (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

0.1

0.8

0.6

0.1

1

10

100 2

0.1

1

10

100 2

Light density (mW/cm )

Light density (mW/cm )

Figure 5 a) Time resolved photoluminescence (TR-PL) spectra of perovskite films with different interfaces; b) J-V curves under dark condition for the hole-only devices with structure:

ITO/PEDOT/with

or

without

BPQD3/perovskite/Spiro-MeOTAD/Ag.

The

corresponding VTFL kink points are presented as well; (c) Jsc and (d) Voc as function of illuminated light intensity for the p-i-n perovskite solar cells with or without BPQD interfaces.

To reveal the impacts of the BPQDs on the interface charge transfer and the electron properties, we measured the photoluminescence spectroscopy (PL) of perovskite films on various interfaces, as plotted at Figure S6. Highest PL intensity was observed in the perovskite films on ITO substrates. For the perovskite films deposited on PEDOT/ITO interface, the PL intensity greatly quenched due to the charge transfer from perovskite to PEDOT HTL. Those quenching effect was further enhanced after BPQD3 coated onto the perovskite/PEDOT interface, indicating more efficient charge transfer in the

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perovskite/BPQD/PEDOT case, which can be further confirmed by timeresolved PL measurements about the charge-transfer kinetics between perovskite layer and HTL. Figure 5a displayed the PL decay curves of perovskite films on different interfaces. Since the PL decays show nonlinear features, the PL lifetime ߬ was calculated using a bi-exponential fitting model48. The average lifetime ߬ for perovskite directly on ITO was 51.3 ns. Considerably, the lifetime was significantly reduced to 16.5 ns when the perovskite films were coated on PEDOT/ITO. When BPQD3 was involved in, the smallest PL lifetime turned up with a value of 10.7 ns in the case of perovskite/BPQD3/PEDOT/ITO, which clearly double check the powerful hole transfer ability of BPQDs and hence strengthen hole injection rate from the perovskite to the PEDOT/BPQDs HTM in contrast to the PEDOT-only HTM. It has been also recognized that the fast PL decay is beneficial to suppress the charge recombination at the perovskite/HTM interface49-51. Therefore, the Jsc and FF are significantly enhanced, in agreement with the J-V and the EQE measurement (Figure 2). Furthermore, hole-only devices with structure: ITO/PEDOT/with or without BPQD3/CH3NH3PbI3/Spiro-OMeTAD/Ag were fabricated to reveal the charge trap-state density in the perovskite active layer under dark condition. Figure 5b shows the J-V curves of the devices with and without BPQD3 interfaces.

As we can see, both curves showed linear response, which indicates the ohmic response of the devices at low applied bias and kink points, as pointed out at each curve. When the applied bias goes higher than the kink point, the current density dramatically increases, which means that the trap-states are completely filled. The kink point is so called as trap-filled limit voltage (VTFL)52. 16


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It has been reported that the trap-state density is positively correlated to the VTFL.53 Therefore, the smaller VTFL for the hole-only device with BPQDs than the reference device indicates the trap-state in the perovskite film are effectively passivated by BPQDs at the PEDOT/perovskite interface54,55. The lower trap-state density is likely to be one of intrinsic factors leading to the enhancement of performance. We further investigated the charge recombination behavior of the HPSCs with and without BPQDs interfaces by measuring the Jsc and Voc as a function of illumination intensity. The corresponding J-V curves under various illumination conditions were presented in Figure S 7. Jsc and Voc of both types of all HPSCs demonstrated linear correlation, as shown in Figure 5c and d. In the case of Jsc, PEDOT/PBQD3 device showed a slightly large slop of 0.989 than that for PEDOT reference device (0.966). But those two slopes essentially are extremely close to 1, indicating negligible bimolecular recombination for both types of devices55. In term of Voc vs. light intensity (Figure 5d), It has been pointed out the slope deviated from (kT/q) closely related to trap-assisted recombination behavior51. The PEDOT/BPQD3 device gave a much smaller kT/q slope of 1.05 than the value from PEDOT reference device (1.57), which indicates a small energy loss from trap-assisted recombination in the former device compared with the latter52. That also declares directly the antirecombination effect of BPQDs interlayer in the devices owing to quick charge transfer properties of the BPQDs involved. In summary, basing on reliable synthesis of them, facile processed BPQDs have been successfully integrated into the typical p-i-n planar HPSCs as electron-extraction layers. Depending on their high hole mobility and suitable 17



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Page 18 of 25

energy band level, the devices performance were remarkably improved only by the BPQDs with the PCE ramping up from 14.10% to 16.69%. Besides the clear demonstrations of J-V characteristics in comparison of devices of interest, more detailed investigations by c-AFM, SKPM, SEM, hole-only devices and photon physics measurements discovered further the hole extraction effect and work mechanism of the BPQDs, such as nucleation assistance for the growth of large grain size perovskite crystals, fast holeextraction rate, more efficient hole transfer and suppression of energy-loss recombination at the anode interface. This work clarifies clearly the understanding of black phosphorus’ electronic properties and its reasonable role in the structure of solar cells, which definitely pave a way for discovering more and more 2D materials with high electronic properties to be used in photovoltaics and opto-electronics.

Acknowledgements

The authors thank Dr. R. Gu, Dr. Y. Qiu, Dr. T.B. Yang (Materials Characterization and Preparation Center of SUSTC) for the characterizations involved in this work. This work was funded by National Natural Science Foundation of China (No. 11304147), Natural Science Foundation of Shenzhen Innovation Committee (No. JCYJ20150529152146471), and the Shenzhen Key Laboratory Project (No.ZDSYS201602261933302).We also acknowledge

the

start-up

funding

and

internal

funding

(Nos.FRG-

SUSTC1501A-61, FRG-SUSTC1501A-67) of the Southern University of Science and Technology (China). Dr. X.N. Lin also thanks the support from Natural

Science

Foundation

of

Shenzhen

Innovation 18


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Committee(No.SGJL20150217094454668).

Supporting information:

More information supplementary to this paper could be found in the Supporting Information materials.

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TOC

Facile processed black phosphorus quantum dots with high hole mobility over 100 cm2/(Vs), integrated at the anode side of the typical p-i-n planar hybrid perovskite solar cells, remarkably improve the performance of the devices through enhancing hole-extraction and suppression of energy-loss recombination at the interface.


Black Phosphorus Quantum Dots for Hole ... - ACS Publications

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