TiOx Electron Transport Layer Boosts Efficiency

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Carbon quantum dots/TiOx electron transport layer boosts efficiency of planar heterojunction perovskite solar cells to 19% Hao Li, Weina Shi, Wenchao Huang, En-Ping Yao, Junbo Han, Zhifan Chen, Shuangshuang Liu, Yan Shen, Mingkui Wang, and Yang Yang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b05177 • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 2, 2017

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Carbon quantum dots/TiOx electron transport layer boosts efficiency of planar heterojunction perovskite solar cells to 19% Hao Lia, Weina Shia, Wenchao Huangb, En-Ping Yaob,d, Junbo Hanc, Zhifan Chena, Shuangshuang Liua, Yan Shena, Mingkui Wanga,*, Yang Yang b, * a Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, Hubei, China b Department of Materials Science and Engineering, University of California Los Angeles, 405, Hilgard Ave, Los Angeles, CA 90095-9000 c Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, Hubei, China d Advanced Optoelectronic Technology Center, National Cheng Kung University, No. 1, University Rd., East District, Tainan City 70101, Taiwan.

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Abstract: In planar n-i-p heterojunction perovskite solar cells, the electron transport layer (ETL) plays important roles in charge extraction and determine the morphology of the perovskite film. Here, we report a solution-processed carbon quantum dots (CQDs)/TiO2 composite that has negligible absorption in the visible spectral range, a very attractive feature for perovskite solar cells. Using this novel CQDs/TiO2 ETL in conjunction with a planar n-i-p heterojunction, we achieved an unprecedented efficiency of ~19% under standard illumination test conditions. It was found that a CQDs/TiO2 combination increases both the open circuit voltage and shortcircuits current density as compared to using TiO2 alone. Various advanced spectroscopic characterizations including ultrafast spectroscopy, ultra-violet photoelectron spectroscopy and electronic impedance spectroscopy elucidate that the CQDs increases the electronic coupling between the CH3NH3PbI3-xClx and TiO2 ETL interface as well as energy levers that contribute to electron extraction.

Keywords: perovskite, heterojunction, solar cell, quantum dots, electron transport


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Photovoltaic (PV) cells convert solar energy into electricity which are important in providing renewable energy. Among the various PV technologies, organic-inorganic hybrid lead halide perovskites such as CH3NH3PbI3 and CH3(NH2)2PbI3 have emerged as promising materials since their utilization as pigments in sensitized solar cells.1 Organic-inorganic hybrid lead halide perovskites have shown excellent carrier transport (carrier mobility, µh ≈ 0.6 cm2 V-1 s-1), high absorption coefficients (~ 5.7×104 cm-1 at 600 nm), long electron and hole diffusion length (> 1000 nm).2 Perovskite solar cells can be easily fabricated with feasible methods such as spin coating3, dip coating4, two-step interdiffusion5, chemical vapor deposition6, atomic layer deposition7. Planar perovskite solar cells using solution-processed photoactive layers have reached the PCEs close to 20% using compositional or interfacial engineering.8,9 To date, the certified maximum value of PCE for perovskite solar cells has reached 22.1%.10 For a conventional planar perovskite solar cells, the mostly used ETL and hole transport layer (HTL) materials reported in literatures are TiO2 and 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenyl amine)-9,9'-spiro bifluorene (spiro-MeOTAD), respectively. The TiO2 has been used as an excellent ETL with suitable energy levels of conduction band (~ -3.9 eV) and valence band (~ 7.2 eV) but poor conductivity (~ 1.1×10-5 S cm-1). In planar perovskite solar cells devices, the ETL extracts and transports photo-generated electrons from perovskite absorber to the indium-tin oxide (ITO) contact, meanwhile serving as a hole-blocking layer. However, an electron transfer potential barrier between CH3NH3PbI3 and the compact TiO2 formed by surface states induces the charge acumination within CH3NH3PbI3 and thus undesirable large current-voltage hystersis observed for CH3NH3PbI3/TiO2 planar heterojunctions.11,12 The latter renders it very difficult to determine the true PCE for the planar perovskite solar cells.13 Careful surface engineering is highly necessary in pursuit of better performance by selection of ETLs with smooth and compact


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surface, appropriate conduction band, superior electron mobility and conductivity to attenuate excessive charge accumulation and guarantee effective electron injection. Carbon material (C60, graphene oxide, etc)14, 15 and metal ions (Mg2+, Nb5+, Li+, etc)16-19 have been successfully used to modify TiO2 ETL, showing synergistically improved PCEs in the range of 9.4-12.8%. For instance, Y- or Li-doped TiO2 has been utilized as efficient ETL for planar perovskite solar cells devices with efficiency up to over ~19% due to more efficient electron transport properties (charge extraction or injection).9,18 Most of the enhanced electronic properties were evidenced in these high-efficiency perovskite solar cells with a mesoporous architecture. Therefore, it is highly desirable to develop an efficient ETL with excellent electron transport property for further improving the photovoltaic performance of planar perovskite solar cells. Herein, we introduced, for the first time, a homogeneous CQDs/TiO2 nanocolloid to form high quality ETL for efficient perovskite solar cell using a planar ITO/TiO2/MAPbI3Cl3-x/spiroOMeTAD/Au structure as framework. CQDs are a new class of carbon nanomaterials composed of discrete, quasi-spherical nanoparticles with sizes below 10 nm that have emerged recently and have garnered much interest as potential competitors to conventional semiconductor quantum dots.20,21 Notably, the CQDs exhibit light harvesting ability and electron reservoir property.22 Due to the excellent optical and electric properties, CQDs/semiconductor composites have been widely used for photo-catalytic water splitting and photo-degradation of organic pollutants.23-28 In this article, we demonstrate that CQDs adding leads to a significant improvement of efficient charge carrier extraction and injection in perovskite solar cells between the TiO2 and perovskite layers. The increasing electron transport property at the interface of TiO2/perovskite successfully enhances the short-circuit current density (JSC) and open-circuit voltages (VOC) with a PCE of ~19% and reduces the photocurrent hysteresis as well.


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Figure 1a compares the transmission spectra of ITO glass covered with the pristine TiO2 and CQDs/TiO2 composited films. The ITO substrate serves as a reference. A highly transparent substrate is necessary for a high output photocurrent. The CQDs/TiO2 nano-crystalline complex ETL was prepared by adding certain amount of CQDs ethanol dispersion liquid with various weight percentage (0.1, 1, 10, 25, 50 wt.%) into a separately prepared TiO2 nano-crystalline solution, which were denoted as C0.1/T, C1.0/T, C10/T, C25/T, C50/T in this study, respectively. After coating with TiO2 or CQDs/TiO2, the transmission of the substance remains above 80% in a longer wavelength range. The optical transmission peak of the CQDs/TiO2 films shifts to longer wavelength compared to the TiO2 only. In order to verify whether the CQDs work on TiO2 film or not, ultraviolet photoelectron spectroscopy (UPS) was employed to characterize the surface energy (Figure 1b). The Fermi level (EF) of TiO2 and C10/T were calculated to be -3.97 eV and -4.01 eV, respectively. The valance band maximum (EVB) of TiO2 and C10/T were found to be -7.25 eV and -7.35 eV, respectively. The conduction band minimum (ECB) of TiO2 and C10/T were evaluated to be -3.95 eV and -4.05 eV, respectively. A deeper ECB of C10/T would increase the driving force for electron injection from the CB (conduction band) of perovskite to the TiO2. Figure 1c describes the energy diagram of TiO2 and CQDs/TiO2 based ETLs, from which we can see the visually changes. Figure S2 further compares the UPS characteristic for the TiO2, C10/T, C50/T samples. As the CQDs weight percentage increases to 50%, the sample CB energy level negatively shifts to -4.33 eV and the VB energy level to -7.63 eV. This would lead to deterioration of devices performance due to an incompatible energy level between the TiO2 ETL with high CQDs content and MAPbI3-xClx perovskite layer. High-resolution transmission electron microscopy (HRTEM) image (Figure 1d) shows the lattice spacing of 0.36 nm, corresponding to the (101) facets of TiO2. The lattice spacing of 0.23 nm was observed


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corresponding to (100) facet of CQDs marked with red arrows, demonstrating successful formation of CQDs/TiO2 composite. A homogenous distribution of C, Ti, and O in SEM-EDS (scanning electron microscope-energy dispersive spectrometer) mapping images (Figure S3) indicates that CQDs are evenly dispersed in TiO2 matrix. The ETL/perovskite interface plays an important role in charge collection in perovskite solar cells, where a pinhole-free and uniform TiO2 layer is highly desired for effective charge extraction. Figure S4 presents tapping mode atomic force microscopy (AFM) images to identify surface roughness of CQDs/TiO2 substances. The root-mean-square roughness (Ra) value increases slightly with the increasing contents of CQDs in the TiO2, from 3.44 nm for the TiO2 to 5.48 nm for the C50/T, which attributes to the small dimension of CQDs (about 1-3 nm as shown in the TEM image in Figure 1d) compared to TiO2 nanoparticle. This confirms us that the film remains uniform and smooth after the adding of CQDs. In addition, the surface roughness of the MAPbI3Cl3-x perovskite films deposited onto the CQDs/TiO2 substances shows slightly but ignorable difference as presented in Figure S5. The UV-visible absorption spectra of the perovskite layers on CQDs/TiO2 and TiO2 shows nearly identical absorbance (Figure S6a). Moreover, the XRD and SEM measurements (Figure S6b-d) were carried out to explore the crystalline morphologies as well, showing almost the same crystalline characteristic and similar morphologies for these samples. Therefore, we conclude the device performance improvements may come from the changes of optical and electronic properties between the interfaces which will be discussed below. X-ray photoelectron spectroscopy (XPS) was used to confirm the existence and chemical state of elements in the CQDs/TiO2 and TiO2 ETL (Figure S7). As shown in Figures 2a and 2d, the O1s signal can be assigned to Ti-O bonds and O-H bonds with two peaks centered at 530.3 eV


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and 531.6 eV, respectively. The pronounced shoulder at the higher energy (~ 531.6 eV) in the CQDs/TiO2 (Figure 2a) compared with the pure TiO2 (Figure 2d) indicates the presence of residual O-H bonds groups from CQDs. The de-convolution of C1s spectra (Figures 2b and 2e) show three peaks centered at 284.9.0, 286.4 and 288.8 eV that are attributed to C-C, C-O and C=O, respectively. The sample with the incorporation of CQDs shows the stronger C 1s peak at 286.4 eV and 288.8 eV compared to the pure TiO2, indicating the existence of CQDs. The weak C 1s peak located at the same position in pure TiO2 sample originates from the carbon contamination, which usually presents at the surface of materials. The enhancement of the peaks corresponding to the C-O and C=O bonds may be credited to the presence of hydroxyl, carbonyl and carboxylic acid groups on CQDs surfaces.29 Figures 2c and 2f shows the de-convolution Ti 2p spectra. The intensity of the Ti 2p peaks, namely, the 2p3/2 peak at 458.8 eV and the Ti 2p1/2 peak at 464.5 eV, decreased after the adding of CQDs. However, it is worth noting that neither 281.5eV peak in the C 1s spectrum nor 454.9eV peak in the Ti 2p spectrum corresponding to the C 1s and Ti 2p3/2 spin orbital peaks can be observed in the XPS results of the CQDs/TiO2 composite.30 This suggests no chemical bond is formed between the TiO2 and the CQDs. It also could be reasonable to infer that the additive of CQDs has negligible effect on the structural properties of the nano-crystal TiO2, since the incorporation process is conducted after synthesis of the TiO2 nano-crystals. Thus, the additives may act as intermediate material between nanocrystals particles. To study the charge transporting properties of the ETLs, electron-only devices based on the architectures of Au/ETL/Au on glass substrates were prepared and the electron mobility were extracted by fitting the data using the space charge-limited current (SCLC) model. As shown in Figure S8, the CQDs/TiO2 exhibited higher current with extracted electron mobility of 8.28×10-4


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cm2 V-1 s-1 than that of TiO2 at 5.27×10-4 cm2 V-1 s-1 by fitting data using the SCLC model with the Mott-Gurney law J D =

9εε 0 µV b2 , where ε0 is the permittivity of free space, ε is the 8L3

dielectric constant of TiO2 (= 55)31, µ is the charge carrier mobility and L is the film thickness (40 nm). The increasing of the electron mobility suggested that the adding of CQDs is good for the electron transporting. Perovskite solar cells were fabricated with a configuration of ITO/ETL/MAPbI3-xClx/spiroMeOTAD/Au. The measured statistical representation photovoltaic parameters of perovskite solar cells employing ETL incorporated with different contents of CQDs via reverse bias scanning at a scan rate of 20 mV s-1 under the simulated irradiation of AM 1.5G (100 mW cm-2) were shown in Figure 3. Each box presents the parameter distribution of 20 devices under similar working conditions. When compared with the reference without CQDs, it was found that the JSC and VOC increased greatly when the adding content of CQDs is below 10%. The average JSC was increased from 16.6 mA cm-2 to around 20.2 mA cm-2 for the adding of CQDs (10 wt.%), and VOC was increased to about 1.12 V. With continuous increasing CQDs contents, the FF decreased drastically, causing a decrease of PCE. With the optimized adding amount of CQDs (10 wt.%), the average PCEs of perovskite solar cells is enhanced from 12.7% to 17.6%. Figure 4a presents the current density-voltage (J-V) characteristics of solar cells with and without the adding of CQDs (10 wt.%). The champion CQDs-device shows a VOC of 1.14 V, a JSC of 21.36 mA cm-2, a fill factor of 78%, yielding an impressive PCE up to 18.9%. To confirm the enhancement of the photocurrent, we record the corresponding incident photon conversion efficiency (IPCE) curve of the optimized perovskite solar cells (Figure 4b). The IPCE curves of all devices show a wide photo-response from 350 to 800 nm, which are consistent with the


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absorption spectrum of MAPbI3Cl3-x (Figure S6a). The device based on C10/T ETL has a higher IPCE response than that of TiO2 substrate device in the wavelength from the 650 nm to 750 nm. The integrated JSC obtained from the IPCE spectra are 19.4 and 21.2 mA cm-2 for the TiO2 substrate and C10/T substrate devices, respectively. Both are in good agreement with the measured values of JSC. It can be deduced that the increase of JSC after adding of CQDs can be ascribed to an increased electron extraction ability which will be discussed below. Furthermore Figure S9 presents the IPCE spectra for devices using ETL with different ratios of CQDs/TiOx to clarify the changes brought by the optical effect of CQDs on the device performance. As increasing the content of CQDs in the ETL, the IPCE response shows clear increase in the wavelength from the 650 nm to 750 nm. Figure S10 shows the down-converted optical photoluminescence spectra of CQDs which was excited in the range of 410-650 nm. When the excitation wavelength (the sharp peak comes from the laser) changes from 410 to 650 nm, the photoluminescence peak correspondingly shifts from 450 to 700 nm. The CQDs film shows absorbance mainly below 350 nm (Figure S11). The down-converted photoluminescence spectra of CQDs was reported to red shift by about 50 nm compared to exciting light.32 Though the CQDs could contribute to the entire light harvesting of CH3NH3PbI3-xClx films by downconversion, it is difficult to quantitatively determine this function for its relatively week absorption as compared to the perovskite film. It is well-known that an anomalous hysteresis is frequently observed in perovskite solar cells, which has been attributed to the charge selective layers, ionic movement, or ferroelectric effects.33-35The photocurrent hysteresis behavior is significantly suppressed in the devices using CQDs/TiO2 ETL as shown in Table 1. We speculate this originating from the efficient charge transfer at the perovskite/TiO2 interface and the reduced electron transfer potential barrier between CH3NH3PbI3-xClx and ETL interface.11


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Ultra-fast spectroscopy characterization was an effective way to investigate the charge transfer process at the CQDs/TiO2/perovskite interface. Transient photoluminescence (PL) decays were used to elucidate the dynamics of charge transfer process at the MAPbI3-xClx/TiO2 interfaces. The time-resolved PL characteristics of the pristine TiO2 or C10/T based perovskite films are shown in Figure 5a. The sample were pumped with picosecond laser pulses with a wavelength at 450 nm and a repetition rate of 76 MHz and probed at 770 nm. The PL lifetimes (τPL) were determined to be 4.64 ns and 2.64 ns, respectively, by a single-exponential fitting to the PL decay. The shorter PL lifetime for the C10/T substance suggests efficient carrier extraction from MAPbI3-xClx film. As shown in Figure S12a, a faster PL decay is observed as increasing the contents of CQDs in the ETL, indicating that an excess CQDs benefits to the electron transport. However, this would not benefit to devices’ performance as the CQDs content increases from 10% to 50% (weight percentage) for an energy losses could be brought by an inappropriate energy levers as evidenced by UPS characteristics (Figure S2). Nanosecond transient absorption spectroscopy (ns-TAS) characterization was further performed to understand the role of the CQDs in the ETL. As shown in Figure 5b and 5c, the photo-bleaching (PB) negative and photo-absorption (PA) positive peaks can be easily observed at around 760 nm and 500-600 nm, respectively. The PB negative peak appears at 760 nm is related to the band gap or exciton transition of MAPbI3-xClx film which is consistent with the steady-state PL quenching experiment (Figure S12b), while the PA positive peak at about 500600 nm can be attributed to the absorption of transient species.36, 37 After adding CQDs, the nsTAS spectra exhibit several distinct features from those without CQDs. An obvious decrease of the peak intensity was observed by comparing the negative PB peak at about 760 nm between MAPbI3-xClx coated TiO2 and C10/T substrates. This phenomenon well supports the result of


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steady-state PL quenching experiment, that is, the existence of CQDs contributes to the exciton transition or charge extraction process from the MAPbI3-xClx film. The positive PA peak at 520 nm increases with the amount of CQDs, suggesting there are strong excited states between MAPbI3-xClx and the ETL interface. Thus, we conclude that the electron extraction from perovskite to TiO2 is significantly enhanced by adding CQDs. Figure 5d shows the kinetic decay traces of photo-bleaching features, which are fitted to a single exponential function. The time constant τTAS is related to the excited-stated decay and free carrier recombination dynamics in the perovskite layers. The τTAS of perovskite/(C10/T) sample (~ τTAS1=324 ns) is much higher than that of perovskite on TiO2 (~ τTAS2=262 ns), indicating that the photo-excited carriers in the perovskite solar cells with C10/T ETL have a longer lifetime and higher opportunity to be collected by the ETL.38-40 This is caused by adding of CQDs for the promoted carrier extraction process. Consequently, the recombination opportunity is reduced at the same time, resulting in a longer free carrier recombination process. There is an electron transfer potential barrier between CH3NH3PbI3-xClx and TiO2 interface which has been discussed in the introduction section.11 Adding CQDs into the TiO2 ETL enhances the electronic coupling for efficiently mediating the electron transfer from the perovskite to ETL, which may presumably provide a super-fast electron funnel as illustrated in Figure S13. Therefore, we attribute the augmented VOC for the devices applying CQDs/TiOx ETL to the fast electron extraction ability verified by the ultra-fast spectroscopy characterization, which substantially reduces the interfacial charge accumulation and thus allows for less recombination opportunity between ETL and perovskite interface. Electronic impedance spectroscopy (IS) characterization was further carried out to examine the electronic processes at the perovskite/ETL interface under illumination. Figure 6a shows the Nyquist plot resulting from solar cells based on the TiO2 substrate and C10/T substrate at the


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same forward bias (1 V) serving as an example. The resulting frequency analysis shows three separated semicircles in the Nyquist diagram. The first arc in high-frequency region is possibly assigned to the charge transport in the ETL layer, HTM layer, and/or at ETL/ITO, HTM/Au interface. The second arc in the intermediate-frequency region is associated with the charge recombination process in the active film, and/or at the interface of the selective contacts with the perovskite layer, from which the recombination resistance (Rct) can be obtained.41 As presented in Figure 6a, the two arcs are merged into a compressed semi-arc due to the very close RC time constant. The feature at low-frequency represents a process with long relaxation time can be attributed to ion migration during the perovskite layer.42 We have fitted the IS data based on an equivalent circuit model of three series resistance capacitance circuits as shown in the inset of Figure 6a. Figure 6b presents the series resistance (RS) as a function of bias, showing an Ohmic contact behavior. A lower RS is obtained for the C10/T substrate based device, suggesting that the modification of CQDs is efficient in charge transportation. The lower RS might be attributed to the augmented short current and fill factor of the devices using C10/T ETL. Figure 6c presents the recombination resistance (Rct) for the charge recombination process at the TiO2/CH3NH3PbI3 interface. It is clear that C10/T substrate based device displays a larger Rct than the pure TiO2 based device, indicating that the interfacial charge recombination process is slower in the former. In summary, a simple solution-processed effective ETL for efficient perovskite solar devices has been developed. With optimized CQDs modification contents, the planar heterojunction perovskite solar cell devices could achieve power conversion efficiency as high as ~19%. The high performance of such perovskite solar cells can be attributed to the improvement of electron mobility, electron extraction ability, and well-matched energy levers by the adding CQDs, which lead to remarkable increases on JSC and VOC. The CQDs offer significant possibility to search for


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compact layer alternatives possessing enhanced conductivity with prospects for high performance perovskite solar cells. Methods Synthesis of Nano-crystalline TiO2 and CQDs solution precursors: In brief, 0.5 mL of TiCl4 (Aldrich, 99.8%) was added slowly to 2 mL of ethanol and then mixed in 10 mL of anhydrous benzyl (Aldrich, 99.8%), leading to a yellow solution. The vial was loosely sealed and sol was heated at 80 oC for 9 h. A 1 mL volume of the resulting suspension was precipitated in 12 mL of diethyl ether and centrifuged at 4500 rpm to isolate the nanoparticles from the solvent and the un-reacted precursor. The final precipitate was dispersed in ethanol, leading to a transparent solution of nanocrystals with a concentration of 5.3 mg mL-1. CQDs were synthesized through a one-step alkali-assisted ultrasonic chemical method.32 In a typical synthesis, 9.0 g of glucose was dissolved in 50 mL of deionized water in a glass beaker. Then an equal volume of NaOH solution (1 M) was added into the solution of glucose, followed by ultrasonic treatment (300 W, 40 KHz) for 4 h at room temperature. The raw solution obtained from glucose/NaOH was adjusted to pH=7 with HCl, before a certain amount of ethanol was added drop by drop into the solution under continuous stirring. After that, the solution was treated by adding a suitable amount of MgSO4 (10-12 wt.%), stirred for 20 min and stored for 24 h to remove the salts and water. Other experimental details including device fabrication, characterization are shown in the supporting information.


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a)

b)

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C10/T

50

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3.34 eV

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3.28 eV

0 400

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0.35nm (101)

-5.43 eV -7.25 eV -7.35 eV

Figure 1. a) Transmittance spectra of ITO glass, ITO/TiO2 and ITO/C10/T Samples, b) UPS spectra describing the cut-off energy (Ecut-off) and Fermi edge (EF, edge) for TiO2 and C10/T, respectively. c) Schematic illustration of energy levels of the ITO, ETLs (before and after the adding of CQDs), CH3NH3PbI3 perovskite. d) High-resolution transmission electron microscopy (HRTEM) image of the C10/T substance.


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Counts /s

3x10

O-H

4

c)

CQD/TiO2-C 1s Peak

3

8x10

3x104

CQD/TiO2-Ti 2p Peak

C-C 284.9 eV

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2x10

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531.6 eV

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6x10

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4x10

C=O 288.8 eV

458.8 eV Ti 2p3/2

2x104

464.5 eV Ti 2p1/2

1x104

4

1x10

3

2x10

0 530

290

d)

e)

8.0x103

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3x104

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2x104 531.6 eV O-H

286.4 eV C-O

2x104

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1x104

288.8 eV C=O

2.0x103

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455

458.8 eV Ti 2p3/2

6.0x103 4.0x103

460

Pure TiO2-Ti 2p Peak

284.9 eV C-C

Ti-O

465

Binding energy (eV)

f)

Pure TiO2-C 1s Peak

530.3 eV

3x104

470

285

Binding energy (eV)

Counts /s

535

Binding energy (eV)

Counts /s

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

Nano Letters

0 535

0.0 295

530

Binding energy (eV)

290

285

280

Binding energy (eV)

470

465

460

455

Binding energy (eV)

Figure 2. X-ray photoelectron spectroscopy of CQDs/TiO2 composite ETL layers for the O 1s peaks, C 1s peaks and Ti 2p peaks (a, b, c) and pure TiO2 O 1s peaks, C 1s peaks and Ti 2p peaks (d, e, f).


15

Nano Letters

b)

a)

20

0.5

0.0

c)

B

TiO2

C D E C0.1/T C1.0/T C10/T

F C25/T

10

0

G C50/T

100

B

C

D

E

F

G

TiO2 C0.1/T C1.0/T C10/T C25/T C50/T

d) 20

PCE (%)

Voc (V)

-2

Jsc (mA cm )

1.0

Fill Factor (%)

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

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50

0

B

TiO2

C D E C0.1/T C1.0/T C10/T

F C25/T

G C50/T

10

0

B

TiO2

C D E C0.1/T C1.0/T C10/T

F C25/T

G C50/T

Figure 3. Device statistics of a) Voc, b) Jsc, c) Fill factor, and d) PCE of perovskite solar cell employing ETL incorporated with different contents of CQD-based additives. Each box presents the parameter distribution of 20 devices under similar working conditions. The small square symbol (□) inside the boxes represents the mean, whereas the line across the boxes represents the median.


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a)

b)

25 100 20

20

80

IPCE (%)

-2

J (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

Nano Letters

10

15 60

TiO2

40

C10/T TiO2

20

TiO2

C10/T

C10/T

0 0.0

0 0.5

1.0

U (V)

400

500

600

700

10 5 0 800

Integrated Current (mA cm-2)

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Wavelength (nm)

Figure 4. a) J-V curves of the perovskite solar cells based on ITO/C10/T and the control device based on the ITO/TiO2 substrate, using a metallic mask with an aperture area of 0.108 cm2 under AM 1.5G one sun (100 mW cm-2) illumination with a rate of 20 mV s-1 in reverse scan direction, b) The IPCE spectra and integrated current date of corresponding devices.


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b)

TiO2/perovskite (C10/T)/perovskite

(C10/T)/perovskite

0.1

∆ O.D.

a)

0.0 30 ns 130 ns 330 ns 430 ns

-0.1

0

2

4

6

8

500

10

600

700

800

Wavelength (nm)

t (ns)

c)

d) 0.1

0.0

1

Delta OD (norm.)

TiO2/perovskite

∆ O.D.

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

Normalized PL intensity

Nano Letters

τTAS1=323.9 ns

τTAS2=261.5 ns

0.5

30 ns 130 ns 330 ns 430 ns

-0.1

500

600

TiO2/Perovskite (C10/T)/Perovskite

700

800

0

Wavelength (nm)

200

400

Delay Time(ns)

Figure 5. Photoelectronic properties of perovskite (MAPbI3Cl3-x) films with different ETLs. PL spectra of perovskite/ETL films excited by a 500 nm light source from the air side. a) Timeresolved PL decay transients measured at 770 nm for pristine TiO2/perovskite (black), CQDdoped TiO2/perovskite (red) films after excitation at 500 nm. The gray solid lines are the singleexponential fits of the PL decay transients. Transient absorption spectra of b) ITO/C10/T/perovskite (MAPbI3Cl3-x) films and c) the control sample based on the ITO/TiO2/perovskite (MAPbI3Cl3-x) films substrate, excitation at 500 nm for 30 ns (black), 130 ns (red), 330 ns (green), and 430 ns (blue). d) Normalized kinetic traces for photobleaching probed at 760 nm for perovskite layers on C10/T and pristine TiO2 substrates.


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4

10

b)

TiO2

RCT

CCE

CCT

Rion

TiO2 C10/T

Cion

10

0

0

10

20

30

2

Z' ( Ω cm )

2

RCE

2

Rs

RS (Ω cm )

20

TiO2 C10/T

3

10 2

C10/T

c) RCT (Ω cm )

a) -Z'' ( Ω 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

Nano Letters

5

2

10

1

10

0

10

40 -1

0

10

0.8

1.0

1.2

U (V)

0.8

0.9

1.0

1.1

U (V)

Figure 6. Impedance spectroscopy of perovskite solar cell with ITO/C10/T substance and the control device based on the ITO/TiO2 substrate at around VOC under illumination with an LED array emitting white light. a) Nyquist plot obtained under illumination at a bias of 1.0 V. b) the series resistance (Rs) as a function of the bias, c) recombination resistance (Rct) derived from midfrequency region as a function of bias. The inset shows the equivalent circuit for fitting the impedance spectroscopy data obtained from the perovskite solar cells in this study.


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Table 1. Photovoltaic parameters of the perovskite solar cells based on ITO/C10/T substrate and the control device based on the ITO/TiO2 substrate.

Device

Scan direction

VOC [V]

JSC [mA cm−2]

FF [% ]

PCE [%]

A

RS

1.036

19.1

77

15.15

(champion)

FS

1.036

14.53

63

9.52

C10/T

RS

1.136

21.36

78

18.89

(champion)

FS

1.09

20.4

78

17.5

Device architecture: ITO/TiO2(CQD)/Perovskite/spiro-OMeTAD/Au. RS(FS), scan from reverse (forward) direction.


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Nano Letters

Additional information Supporting Information Available: Experimental details, characterization with UPS, EDS, AFM, SEM, XPS, and photoelectronic properties. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Contributions H. L. Y.S. and M. W. designed the experiments. E. Y., W. H. and Y.Y. gave suggestions for data analysis. H.L. and W.S. performed the devices fabrication and characterization. Z.C. and S.L. carried out the SEM and UPS measurements. J.H. conducted the PL measurement. All authors discussed the results and commented on the manuscript. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Corresponding Author E-mail: [email protected] (M.W.), [email protected] (Y.Y.)

Acknowledgements Financial support from the 973 Program of China (No.: 2014CB643506 and 2013CB922104), the China Scholarship Council (No.: 201506165038), the Natural Science Foundation of China (No.: 21673091), the Natural Science Foundation of Hubei Province (No.: ZRZ2015000203), Technology Creative Project of Excellent Middle & Young Team of Hubei Province (No.: T201511), the Wuhan National High Magnetic Field Center (2015KF18). YY acknowledges the


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Nano Letters

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

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grants from Air Force Ofﬁce of Scientiﬁc Research (Grant No. FA9550-15-1-0333, Program Manager Dr. Kenneth Caster) and UC-Solar Institute (grant number: MR-15-328386) to support the research at UCLA. The authors thank the Analytical and Testing Centre of Huazhong University of Science & Technology for the measurements of the samples. The authors declare no competing financial interests.


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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

Nano Letters

Abbreviations CQDs, carbon quantum dots; ETL, electron transport layer; PCE, power conversion efficiency; MAPbI3-xClx, CH3NH3PbI3-xClx; HTL, hole transport layer; spiro-MeOTAD, 2,2',7,7'-tetrakis(N,N-di- pmethoxyphenylamine)-9,9'-spiro bifluorene; ITO, indium-tin oxide; C60, graphene oxide; rGO, Reduced graphene oxide; UPS, Ultraviolet photoelectron spectroscopy; PV, Photovoltaic; JSC, short-circuit current density; VOC, open-circuit voltages; CB, conduction band; HRTEM, High-resolution transmission electron microscopy; XPS, X-ray photoelectron spectroscopy; AFM, Tapping mode atomic force microscopy; EDS, Energy dispersive spectrometer; PL, Steady-state photoluminescence; J-V, photocurrent-voltage; IPCE, incident photon conversion efficiency; AC, alternating current; IS, impedance spectra; SEM-EDS, scanning electron microscope- Energy dispersive spectrometer; SCLC, space charge-limited current; ns-TAS, Nanosecond transient absorption spectroscopy; PB, photo-bleaching; PA, photo-absorption.


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42. Zhang, H.; Qiao, X.; Shen, Y.; Moehl, T.; Zakeeruddin, S.; Grätzel, M.; Wang, M. J. Mater. Chem. A 2015, 3, 11762-11767.


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PCE=19% 20 -2

J (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

Nano Letters

10

Perovskite

G lass/ITO CQDs

0 0.0

TiO2

0.5

1.0

U (V)


TiOx Electron Transport Layer Boosts Efficiency

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