Energy-Down-Shift CsPbCl3:Mn Quantum Dots for Boosting the

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Energy-Down-Shift CsPbCl:Mn Quantum Dots for Boosting the Efficiency and Stability of Perovskite Solar Cells Qian Wang, Xisheng Zhang, Zhiwen Jin, Jingru Zhang, Zhenfei Gao, Yongfang Li, and Shengzhong (Frank) Liu ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on May 26, 2017

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Energy-Down-Shift CsPbCl3:Mn Quantum Dots for Boosting the Efficiency and Stability of Perovskite Solar Cells Qian Wang1,#, Xisheng Zhang1,#, Zhiwen Jin1,*,Jingru Zhang1, Zhenfei Gao1, Yongfang Li3 and Shengzhong (Frank) Liu1,2,* #

These authors contributed equally to this work Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education; Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China. E-mail: [email protected], [email protected]

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Dalian National Laboratory for Clean Energy; iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China.

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Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China Abstract: Parasitic absorption by window layer, electrode layer and interface layer in the near UV region is no longer negligible for high efficiency perovskite solar cells. On the other hand, the UV-induced degradation is also a big component for its instability. Herein, CsPbCl3:Mn based quantum dots (QDs) are synthesized and applied onto the

front side of the perovskite solar cells as the energy-down-shift (EDS) layer. It is found that with very high quantum yield (~60%) and larger Stokes shift (> 200 nm), the CsPbCl3:Mn QDs effectively convert the normally wasted energy in the UV region (300-400 nm) into usable visible light at ~590 nm for enhanced PCE. Meanwhile, by converting the UV rays, it eliminated a significant loss mechanism that deteriorates perovskite stability. As a result, EQE in the UV region is significantly increased, leading to an increased JSC (3.77 %) and PCE (3.34 %). Furthermore, the stability of perovskite solar cells has also been improved from 85% 1

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to 97% of their initial efficiency after exposure in UV region with 5 mW/cm2 intensity by 100 hours. In parallel, the organic and silicon solar cells coated by EDS QDs also confirm the above conclusion with both PCE enhancements by 3.21 % and 2.98 %, respectively. These results suggest that the CsPbCl3:Mn QDs play a significant role in improving the efficiency and stability for photovoltaic devices. To our knowledge, this is the first report about CsPbCl3:Mn QDs assisted perovskite solar cells.

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The organic-inorganic hybrid perovskite has become a superstar material for photovoltaic and optoelectronic applications.1-5 Since the pioneering work on solid state perovskite solar cells firstly reported in 2012 which triggered perovskite photovoltaics,6 the power conversion efficiency (PCE) of the perovskite solar cells has been rapidly improved to 22.1% in just a few years.7-11 However, the PCE is still significantly lower than its theoretical prediction and its poor stability still prevent it from large scale commercialization.12-17 Hence, there have been increasing efforts focused on further improving PCE of stabilized solar cells.18-22 As visible light accounts for only half of the solar radiation, the other half is in UV and IR wavelength, it is critical to design an effective strategy to harvest them for high efficiency solar cells.23,24 In addition, the commonly used transparent conductive oxide layer (FTO or ITO) has high absorption in the UV region.25,26 Even though there have been efforts using more transparent electrodes such as graphene27,28 and reduced graphene oxide29, to decrease the parasitic absorption, their cell efficiencies are still low. High-energy UV photons accelerated cell degradation is another major problem30-34 It is therefore desired to convert the harmful UV photons to usable visible light that warrants not only higher PCE but also better stability, as been recently proved.23 Crystalline solar cells35-37 using energy-down-shift (EDS) coating in which the destructive UV light is absorbed and transformed into visible light for improved PCE and long term performance. For a good EDS layer, it needs to display high light absorption in the UV wavelength, good EDS quantum yield, extraordinary 3


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transparency in visible region and large stokes shift to reduce the self-re-absorption losses.35-37 Moreover, structural simplicity, low-cost fabrication, ambient operation and long term stability are also crucial figures of merit for real applications. The wide-bandgap (larger than 3 eV) CsPbCl3 quantum dots (QDs) appear to be ideal candidates based on the above discussion.38-41 Their superior quality in high photoluminescence (PL) quantum yield, size-tunable band gap, high transparency in visible spectrum but sensitive to UV radiation and excellent photostability make the CsPbCl3 QDs attractive for EDS application.42-44 Unfortunately, the virgin CsPbCl3 QDs can only emit at 402 nm due to its narrow band-edge emission.45,46 This relatively small Stokes shift brings about significant self re-absorption loss in the QDs. To overcome this problem, transition metal Mn2+ doped QDs are developed, due to its excellent PL properties, similar ionic radius and valence state with Pb2+.47-49 After excited by UV light, the CsPbCl3:Mn QDs with large Stokes shift (higher than 200 nm) exhibit yellow emission centered at 590 nm through the energy transfer from CsPbCl3 host to Mn2+ ions.50 In this work, we prepared CsPbCl3:Mn QDs solution with quantum yield up to 60% using a hot-injection method, they were then successfully applied onto the perovskite solar cells. It is found that both JSC and PCE are significantly increased by as much as 3.77 % and 3.34 %, respectively. More importantly, the cell stability is improved from 85 % to 97 % of their initial efficiency after exposure in UV region with 5 mW/cm2 intensity by 100 hours. To our knowledge, this is the first report about improving the efficiency and stability of perovskite solar cells using the light 4


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conversion ability of CsPbCl3:Mn QDs. Finally, the effect of QDs on other high performance solar cells (organic and silicon) are also investigated. These results reported here reveal that making full use of the UV light by CsPbCl3:Mn QDs is beneficial to enhance the photovoltaic performance of solar cells in the future. The CsPbCl3:Mn QDs layer was deposited on the front side (transparent side) of the solar cells as shown in the schematic diagram (Figure 1a). Figure 1b schemes the emission model of CsPbCl3:Mn QDs. When excited by the UV light of 365 nm, the CsPbCl3 host absorbs the incident energy and exhibits a narrow band-edge emission peak at 402 nm (the blue line in Figure 1f).51 However, the Mn2+ substitution induces intermediate quantum state in CsPbCl3 QDs, which generates anew recombination pathway.52 Therefore, besides the band-edge emission, energy transfer from the CsPbCl3 host to the Mn2+ ions occurs via a non-radiative pathway.50 The energy transfer process can be certified by the lifetimes of CsPbCl3 QDs. The decay curves of un-doped and Mn2+ doped CsPbCl3 QDs excited by 375 nm and monitored by 402 nm are shown in Figure 1c. The lifetimes are calculated to be 4.55 ns and 3.85 ns, respectively. The reduction of the lifetime indicates the energy transfer from CsPbCl3 host to the Mn2+ ions. The part of photo-induced excitonsin conduction band (CB) of CsPbCl3 can be transferred to the excited state of Mn2+, and then emit the 590 nm emission attributing to the 4T1−6A1 transition of Mn2+ ions.53 The PL properties of CsPbCl3 and CsPbCl3:Mn can be clearly seen from fluorescence mapping spectra in

Figure 1d and 1e. The CsPbCl3 shows almost no emission and CsPbCl3:Mn exhibits strong yellow emission ranging from 450 nm to 750 nm excited by the UV light in the 5


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regions of 260-440 nm. As is well known, the solar spectrum can range from ultraviolet to infrared light, as shown in Figure 1f. However, the perovskite solar cells can only effectively utilize the visible light in the region of 400-800 nm to implement the photoelectric conversion. Therefore, the ultraviolet light in the solar spectrum (the purple area in

Figure 1f) cannot be effectively converted to electric signal by solar cells for the parasitic absorption caused by transparent substrate and electrode,25 and it is harmful to the organic-inorganic hybrid layer.30,31 Coincidentally, the CsPbCl3:Mn QDs exhibit a strong absorption spectrum in the UV region and can convert the UV light to visible light with the quantum efficiency of 60 %, which matches well with the absorption spectrum of perovskite solar cells. When sunlight illuminates the device coated with CsPbCl3:Mn QDs, the UV light is converted to visible light before absorbed by glass and FTO electrode due to the down-conversion effect, and then the converted visible light and native visible light can be absorbed by perovskite solar cells. Thus the CsPbCl3:Mn QDs is deemed to be helpful for enhancing the PCE and stability of perovskite solar cells.

The transmission electron microscopy (TEM) images (Figure2a and 2b) indicate the cubic morphology of as-prepared QDs with an average size of about 8 nm. The inset in Figure 2a is the photograph of the synthesized CsPbCl3:0.1Mn QDs.

Figure2c and 2d show the high-resolution TEM (HRTEM) images of CsPbCl3:0.1Mn QDs with legible crystal lattices, respectively. The interplanar distance of 0.56 nm for

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CsPbCl3:0.1Mn QDs is well consistent with the (001) planes of cubic-phase CsPbCl3QDs.47 The X-ray diffraction (XRD) pattern further confirms that the CsPbCl3:0.1Mn QDs crystallizes in the cubic structure (Pm3m space group) with no MnCl2 phases observable (Figure 2e). Obviously, the XRD pattern of CsPbCl3:0.1Mn exhibits a lattice shrink with a 0.2° shift to the higher angle compared with that of CsPbCl3QDs, which is caused by the substitution of larger Pb2+ (133 pm) by smaller Mn2+ ions (97 pm).52 The XRD results prove the existence of Mn2+ in the CsPbCl3 host. We have also investigated the doping ratio of Mn2+ ions in CsPbCl3 using X-ray photoelectron spectroscopy (XPS). Figure 2f plots the comparison of XPS results for the CsPbCl3 and CsPbCl3:0.1Mn. As expected, the small binding energy peaks at 655 eV corresponding to Mn 2p core-levels which are detected only in the CsPbCl3:0.1Mn QDs.54 It hints that Mn ions enter into the CsPbCl3 lattices and preserves the Mn2+ state. Therefore, both the XRD and XPS results suggest that Mn ions are incorporated into the CsPbCl3 lattice via replacing cation doping rather than surface adsorption or decoration.

In order to investigate the PL properties of CsPbCl3:xMn, a series of samples with different Mn2+molar percentage are prepared. Figure 3a shows the photograph of CsPbCl3:xMn QDs octane solution and films on FTO glass. The scanning electron microscope (SEM) images of CsPbCl3:Mn QDs film morphology are also given in

Figure S1. The left FTO glass is blank, the middle one is coated by CsPbCl3 QDs and the right one is coated by CsPbCl3: 0.1Mn QDs. One can see that all the solution and films are transparent. After excited by 365 nm, CsPbCl3:0.1Mn QDs based solution 7


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and film exhibit the strongest yellow emission (Figure 3b). The XPS analysis of the as-prepared QDs reveal that x is 1%, 2%, 5% and 10%, respectively. Figure 3c and d illustrate the absorption and emission spectra of CsPbCl3:xMn (x= 0, 1%, 2%, 5% and 10%). Obviously, as the increasing of Mn2+ concentration, the absorption and host emission band (408 nm) show a blue shift despite no apparent change in QDs size, which are caused by the lattice contraction corresponding to the results of XRD.47 The emission intensities of Mn2+ (590 nm) increase gradually with increasing the Mn2+ concentration due to the energy transfer from band-edge to the d-d transition of Mn2+ ions. CsPbCl3:0.1Mn shows the strongest emission intensity in the yellow region. Unfortunately, there is no more Mn2+ substitution in CsPbCl3 QDs with the further increase of Mn2+ concentration.

The PL properties of CsPbCl3 and CsPbCl3:0.1Mn films coated on the FTO glass are also studied. The absorption and emission spectra of films are exhibited in

Figure3e and 3f. Clearly, the CsPbCl3 and CsPbCl3:0.1Mn QDs films perform a stronger absorption in the UV region than the blank FTO glass with highly transparent for visible light. In Figure3f, it can be clearly seen that the conversion capability of CsPbCl3:0.1Mn film from UV light to the visible light is significantly enhanced compared to the CsPbCl3 film. Theoretically speaking, the enhancement of ultraviolet absorption and the emission of visible light are helpful to the performance and stability of the perovskite solar cells.

In order to detect whether the down-shifting coating is beneficial for the 8


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perovskite solar cells, the photoelectric performance are measured. Different concentrations (1, 2, 5 and 20 mg/ml) of CsPbCl3:0.1Mn QDs are coated on the front side of perovskite solar cells as shown in Figure 1a. The current density (J)-voltage (V) curves and the external quantum efficiency (EQE) are recorded in Figure4. From the J-V measurements of the perovskite solar cells coated by CsPbCl3:0.1Mn QDs with different concentration in Figure4a, the open-circuit voltage (VOC) almost fixes at 1.105 V indicating that the effect of CsPbCl3:0.1Mn is negligible on the VOC of the perovskite solar cells. The short circuit current (JSC) shows an increase in the range of 1-5 mg/ml and then a significantly decrease in JSC for 20 mg/ml which may resulted from the parasitic absorption by the thicker CsPbCl3:0.1Mn layer.36 The PCE is also calculated. The value of PCE shows the same tendency with the JSC and the increase in PCE from 17.97% to 18.57% origins from the enhancement in JSC. The PCE of the perovskite solar cells coated with QDs layer at 5 mg/ml concentration present the highest value of 18.57% with the amplification of 3.77 % comparing to the uncoated reference cell. The key parameters of the perovskite solar cells coated with different concentration of CsPbCl3:0.1Mn QDs layer are summarized in Table 1.

In Figure4b, the EQE increases in the UV region of 300-420 nm for all samples with different concentration of CsPbCl3:0.1Mn QDs. Compared with the reference solar cells, the optimized performance with highest enhancement of EQE is found for the cells coated with a 5 mg/ml concentration solution. The improvement of EQE in the UV region leads directly to the increase in JSC by coating an EDS layer. In the visible wavelength ranges from 450 nm to 800 nm, the EQE nearly unchanged 9


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comparing to the reference cell except the 20 mg/ml for the parasitic absorption by the thicker CsPbCl3:0.1Mn QDs layer told above.36 From these results of J-V curves and EQEs, it is demonstrated that lower concentration EDS layer (1-5mg/ml) lead to an increase in EQE, JSC and PCE in the UV region, confirming the active effect of the EDS coating on the perovskite solar cells.

We also tested whether the CsPbCl3:0.1Mn QDs layer can enhance device stability in a long term aging test kept in an N2-filled dry glove box and continuously irradiated (100 h) with a UV LED diode (5 mW/cm2 intensity, Figure 4c). This value simulates well the 5% contribution given by UV light (280 to 400 nm) to the overall solar spectral irradiance on Earth (1000 W/m2, AM1.5G) based on the Standard Refer-ence Solar Spectra (ASTM G-173-03).23 The uncoated device in this UV-induced aging test lost 15% of their initial efficiency after 100 hours of exposure. Conversely, CsPbCl3:0.1Mn QDs layer coated device demonstrated excellent stability under the same conditions, retaining 97% of their initial PCE after 100 hours.

The EDS layer is also investigated to detect the universal application possibility of CsPbCl3:0.1Mn QDs on other high performance solar cells, such as organic (J71:ITIC)55 and silicon (HIT)56 solar cells. Figure 5 plots the J-V characteristics and EQE curves of the reference (black dots) and optimized solar cells (red dots). And the corresponding performance parameters are summarized in Table 2. It should be noted that the integrated JSC calculated from the EQE curves are consistent with the J-V measurements, indicating that the latter is well calibrated. Distinctly, all the optimized 10


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cells show the increase of EQE in the UV region (300-420 nm) which lead to the increase of JSC and the significant enhancement of PCE with the amplification of 3.21% and 2.98%, respectively. These results also indicate that EDS layer has a large and significant effect on JSC and PCE for other photovoltaic devices. To further confirm the reproducibility of the devices, 25 individual devices were fabricated, and the PCE distribution histograms are shown in Figure S2. It appears that the PCE has fairly narrow distribution, meaning that the optimized devices present very good reproducibility.

In summary, we have successfully synthesized CsPbCl3:Mn QDs. The structural and optical properties of CsPbCl3:Mn QDs were investigated in detail, revealing that the best PL performance was at 10% Mn substitution with quantum yield up to ~ 60%. Then, the as-prepared CsPbCl3:0.1Mn QDs are employed on the perovskite solar cells as the light conversion materials by converting UV light to visible light. The effects of different concentration of CsPbCl3:0.1Mn QDs on the performance were also studied and the optimized concentration was 5 mg/ml. The results show that CsPbCl3:0.1Mn QDs would increase the JSC and PCE of the device. In particular, the best-recorded enhancements in JSC and PCE are 3.77 % and 3.34%, respectively. Meanwhile, the stability of perovskite solar cells under UV irradiation has also been improved. Furthermore, it can be easily extended to other high performance solar cells, such as organic and silicon solar cells with enhancement of PCE by 3.21 % and 2.98 %, respectively. We deem that the effects of CsPbCl3:0.1Mn QDs on the performance of solar cells are significant in the photovoltaic field. 11


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Supporting Information Experimental section, SEM image of film morphology, and PCE distribution histograms are listed as Supporting Information.

Acknowledgements Q.W. and X.Z. contributed equally to this work. This work was supported by the National Nature Science Foundation of China (61674098), the China Postdoctoral Science Foundation funded project (2016M602759), the National Key Research Program of China (2016YFA0202403), the National University Research Fund (GK261001009, GK20170302 ， GK201603107), the Innovative Research Team (IRT_14R33) and the Chinese National 1000-talent-plan program (Grant NO. 111001034).

Author Contributions statement Q.W., X.Z., and J.Z. performed the experiments, the data analysis and experimental planning. The project was conceived, planned and supervised by Z.J., and S.L. The manuscript was written by Q.W., Z.J., and S.L. All authors reviewed the manuscript.

Competing financial interests The authors declare no competing financial interests.

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Figure 1. (a) Schematic structure of solar cells coated with CsPbCl3:0.1Mn layer. (b) Energy band gap alignment diagram of CsPbCl3:0.1Mn QDs. (c) The decay curves of CsPbCl3 and CsPbCl3:0.1Mn QDs. (d) and (e) the fluorescence mapping spectra of CsPbCl3 and CsPbCl3:0.1Mn QDs, respectively. (f) The solar spectrum (black curve), the emission spectrum of CsPbCl3:0.1Mn (blue curve) after 365 nm excitation, the excitation spectrum (green curve) monitored by 590 nm and the absorption spectrum (red curve) of CsPbCl3:0.1Mn QDs.

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Figure 2. (a) and (b) TEM images of CsPbCl3:0.1Mn QDs, (c) and (d) HRTEM image of CsPbCl3:0.1Mn QDs, (e) XRD and (f) XPS patterns of CsPbCl3 and CsPbCl3:0.1Mn QDs. The inset in (a) is the synthesized CsPbCl3:0.1Mn QDs under 365 nm UV light.

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Figure 3. The photographs of fabricated CsPbCl3:xMn QDs solutions and films: (a) under the visible light and (b) under the 365 nm UV light; (c) the absorption spectra and (d) emission spectra (excited by 365 nm) of CsPbCl3:xMn QDs solution with different Mn2+ concentration; (e) the absorption and (f) emission spectra (excited by 365 nm) of Glass/FTO, CsPbCl3/ Glass/FTO and CsPbCl3:0.1Mn/ Glass/FTO.

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Figure 4. (a) I-V curves and (b) EQE spectra for CsPbCl3:0.1Mn with different coated layer thickness assisted perovskite solar cells. (c) Results of the aging test on the reference and optimized perovskite solar cells with CsPbCl3:0.1Mn QDs layer under continuous UV irradiation (5 mW/cm2) at N2 atmosphere for 100 hours.

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Figure 5. I-V curves and EQE spectra for the reference and optimized solar cells with CsPbCl3:0.1Mn QDs layers: (a) and (b) for the perovskite solar cells; (c) and (d) for the organic solar cells (J71:ITIC); (e) and (f) for the silicon solar cells (HIT).

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Table 1. The key J-V parameters of perovskite solar cells without and with different coated layer thickness of CsPbCl3: 0.1Mn QDs.

Table 2. The key J-V parameters of the reference and optimized three types of solar cells by coated with CsPbCl3:0.1Mn QDs layer.

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Energy-Down-Shift CsPbCl3:Mn Quantum Dots for Boosting the

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