Enhancing the Photovoltaic Performance of Perovskite Solar Cells

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Enhancing the Photovoltaic Performance of Perovskite Solar Cells with a Down-Conversion Eu-Complex Ling Jiang,†,‡ Wangchao Chen,†,‡ Jiawei Zheng,†,‡ Liangzheng Zhu,†,‡ Li’e Mo,† Zhaoqian Li,† Linhua Hu,† Tasawar Hayat,§ Ahmed Alsaedi,§ Changneng Zhang,*,† and Songyuan Dai*,∥ †

Key Laboratory of Photovolatic and Energy Conservation Materials, Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Anhui 230088, P. R. China ‡ University of Science and Technology of China, Hefei, 230026, P. R. China § NAAM Research Group, Department of Mathematics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia ∥ Beijing Key Laboratory of Novel Thin Film Solar Cells, North China Electric Power University, Beijing, 102206, P. R. China ABSTRACT: Organometal halide perovskite solar cells (PSCs) have shown high photovoltaic performance but poor utilization of ultraviolet (UV) irradiation. Lanthanide complexes have a wide absorption range in the UV region and they can down-convert the absorbed UV light into visible light, which provides a possibility for PSCs to utilize UV light for higher photocurrent, efficiency, and stability. In this study, we use a transparent luminescent down-converting layer (LDL) of Eu−4,7-diphenyl-1,10-phenanthroline (Eu-complex) to improve the light utilization efficiency of PSCs. Compared with the uncoated PSC, the PSC coated with Eu-complex LDL on the reverse of the fluorine-doped tin oxide glass displayed an enhancement of 11.8% in short-circuit current density (Jsc) and 15.3% in efficiency due to the Eu-complex LDL re-emitting UV light (300−380 nm) in the visible range. It is indicated that the Eu-complex LDL plays the role of enhancing the power conversion efficiency as well as reducing UV degradation for PSCs. KEYWORDS: down-conversion, Eu-complex, luminescent, UV stability, perovskite solar cells

1. INTRODUCTION

Lanthanide luminescence is an ideal candidate for extending the spectral response range of PSCs to the UV region. Lanthanide ions have the ability to absorb UV light and re-emit visible light due to their specific 4f electronic structure.22−24 Hence, applying lanthanide luminescence down-conversion technology to PSCs is a powerful approach to enhance the efficiency of PSCs while reducing their UV degradation to improve their stability.25 Much research using lanthanide luminescence to improve the photovoltaic performance of PSCs has been proposed.26−28 However, the spectral response of the material-based inorganic host (YVO 4 :Eu 3 + , ZnGa2O4:Eu3+, or CeO2:Eu3+) is narrow or near-far ultraviolet, which could impede the larger availability of UV light. Lanthanide complexes possess a broad band absorption in the UV region and can effectively transfer the absorbed UV energy to lanthanide ions (i.e., antenna effect), which could help PSCs to achieve higher photocurrent, efficiency, and stability. At present, there is little study on the utilization of lanthanide complexes for PSCs to enhance the photovoltaic performance. Europium (Eu3+) is one of the most efficient

Recently, organometal halide perovskites have been considered as one kind of promising photoelectric conversion material for perovskite solar cells (PSCs) due to the advantages of a high light absorption coefficient,1,2 narrow band gap,3 great charge carrier mobilities,4,5 nanoscale structures,6 and simple synthetic method.7,8 On the basis of those advantages, PSCs have exhibited outstanding character for photoelectric conversion and one certified power conversion efficiency (PCE) of 22.1%.9 Generally, the spectral response range of PSCs is mainly concentrated in the visible region (about 44% of the total incident solar irradiation). This brings a major issue for the light-harvesting of PSCs, since 50% of solar irradiation lies in the ultraviolet (UV) and infrared (IR) regions; thus, the active device cannot use it, which limits the light conversion efficiency for PSCs. To improve the light utilization efficiency of PSCs, researchers have utilized different fabrication methods, such as photoanode modification,10−12 halogen ion doping,13−15 and hole-transport material (HTM) design.16−18 Moreover, a UV filter film, fluorescent carbon dots, or zinc phthalocyanine are used to mitigate UV degradation for better performance of perovskite.19−21 However, these methods cannot directly improve the spectral response of PSCs. © 2017 American Chemical Society

Received: July 12, 2017 Accepted: July 28, 2017 Published: July 28, 2017 26958

DOI: 10.1021/acsami.7b10101 ACS Appl. Mater. Interfaces 2017, 9, 26958−26964

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the preparation of Eu-complex luminescent down-converting layer (LDL).

Figure 2. UV−vis absorption spectra (a) and transmission spectra (b) of Eu-complex LDL (Eu-complex content: 0.0, 0.5, 1.0, and 1.5 wt %).

down-converting ions, which has high emission intensity,29 and 4,7-diphenyl-1,10-phenanthroline as a ligand (phenanthroline derivative) with wide UV absorption is matched well with Eu3+ ion. In this work, we propose the use of Eu−4,7-diphenyl-1,10phenanthroline (Eu-complex) to increase the light-harvesting of PSCs. A transparent luminescent down-converting layer (LDL) consisting of Eu-complex is placed on the reverse of the fluorine-doped tin oxide (FTO) glass of the device. The coating converts the UV light (300−380 nm) to visible light that matches the absorption of PSCs and increases the utilization of the incident solar light. In comparison with the devices uncoated with LDL, those with LDL show greatly increased photocurrent density and PCE, and also, the UV stability of PSCs without a package is enhanced.

titanium diisopropoxide and 0.4 mL of bis (acetylacetonate)] was sprayed on patterned FTO substrates at 460 °C to form a dense blocking layer of TiO2. The mesoporous TiO2 layer was prepared by spin-coating of the diluted TiO2 paste at 4000 rpm for 20 s. The substrates were dried at 80 °C and then annealed at 500 °C for 30 min. The CH3NH3PbI3 layer was formed by a two-step spin-coating method. A 50 μL portion of a mixed solution of PbI2 and CH3NH3I was deposited onto the mesoporous TiO2 layer at 1000 rpm for 20 s and 4000 rpm for 30 s, and at the last 10 s before the end of the second spin-coating step, the substrates were treated with chlorobenzene. The substrates were annealed at 105 °C for 1 h. Spiro-MeOTAD was used as the HTM layer and was put on the CH3NH3PbI3 layer by spin-coating of 30 μL of prepared solution [72.3 mg, 28.8 μL of 4-tert-butylpyridine, 17.5 μL of lithium bis(trifluoromethylsulfonyl)imide, and 8 μL of Co(III)-complex dissolved in 1 mL of chlorobenzene] at 3000 rpm for 20 s. Finally, Au backelectrode with a thickness of 60 nm was thermally evaporated on the HTM layer. Characterization. The morphologies of the LDLs (surface and cross-sectional) and the PSCs (cross-sectional) as well as the EDX spectrum of LDL were all investigated by scanning electron microscopy (FEI XL-30 SFEG coupled to a TLD). The UV−vis absorption spectra and transmission spectra were examined using the U-3900H UV−vis spectrophotometer (Hitachi). Photoluminescence (PL) spectra were recorded using a fluorescence spectrophotometer (QM400-TM). The J−V curves were measured using a solar simulator (solar AAA simulator, Oriel) with a source meter (Keithley Instruments, Inc.) at 100 mW/cm2, AM 1.5G illumination. The irradiance was calibrated using a Si reference cell certified by NREL. With a 50 mV/s scan rate, the applied bias voltages for the reverse scan and forward scan were from 1.2 to −0.1 V and from −0.1 to 1.2 V. The photovoltaic performance of our devices is not confirmed from independent certification laboratories. The forward bias for stability characterization of the PSC with Eu-complex was held close to 0.802 V. The active area was attached with a mask of 0.09 cm2. The IPCE values were carried out as a function of wavelength from 300 to 900 nm (PV Measurements, Inc.). The UV-light-soaking test was under a light source (UV−Hg-2000, Beijing Lighting Research Institute). There was no control over the temperature and relative humidity levels during the measurements and the PSCs were not encapsulated. To avoid errors, the validity of solar cell data was checked and measured according to previous reports.30−34

2. MATERIALS AND METHODS Materials. Europium(III) nitrate hexahydrate and 4,7-diphenyl1,10-phenanthroline were purchased from J&K. Polyvinylpyrrolidone (K 30) was obtained from TCI. Lead(II) iodide, dimethyl sulfoxide, lithium bis(trifluoromethylsulfonyl) imide (Li-TFSI), 4-tert-butylpyridine (TBP), and titanium isopropoxide were purchased from Aldrich. Methylammonium Iodide and Spiro-MeOTAD were obtained from Xi’an p-OLED. Acetylacetone, N,N-dimethylformamide, ethanol, and chlorobenzene were from Sinopharm. TiO2 paste (18 NRT) was purchased from Dyesol. All chemicals were commercially available and used without further purification. Synthesis of Eu-Complex LDL. First, 4,7-diphenyl-1,10-phenanthroline (2 mmol) was dissolved well in 20 mL of ethanol. Then 1 mmol of Eu(NO3)3·6H2O in 20 mL of ethanol was slowly added to the above solution with constant stirring of the mixture for a better reaction. After magnetic stirring of the mixed solution for a half-hour, a white precipitate of Eu-complex was formed. Then the complex was purified by centrifugation and washed with ethanol three times and then dried at 40 °C for 12 h. Second, the as-prepared Eu-complex in the range of 0.5−1.5 wt % (a content higher than 1.5 wt % cannot completely dissolve) was added to ethanol containing 2 g of PVP, and the mixed solution was magnetic stirred for 2 h to ensure that it was dissolved completely. Then the mixed solution was used to form a film on the back surface of the FTO glass by the spin-coating method and was dried at 60 °C to remove the ethanol, as shown in Figure 1. Device Fabrication. For the fabrication of PSCs, a TiO2 precursor solution [7 mL of anhydrous 2-propanol containing 0.6 mL of 26959


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Figure 3. (a) PL excitation spectra of Eu-complex LDLs. (b) PL emission spectra of Eu-complex LDLs (Eu-complex content: 0.5, 1.0, and 1.5 wt %).

3. RESULTS AND DISCUSSION Figure 2a shows the UV−vis absorption spectra of LDL with different Eu-complex content from 0.0 to 1.5 wt % in PVP. The LDL without Eu-complex (pure PVP) shows almost no absorption, indicating that the PVP will not affect the passing light much. The LDLs with Eu-complex display a strong and broad absorption band corresponding to the π → π* transition of the ligand in the Eu-complex, and with the increase in the content of Eu-complex, it becomes more intense. But when the content is more than 1.5 wt %, the LDL losses its transparency, impeding the passage of visible light. Furthermore, the absorption peak shows a small red shift of 3 nm from 313 to 316 and of 6 nm from 313 to 319 nm when the Eu-complex content is 1.0 and 1.5 wt %, respectively. This is caused by the increase of attractive polarization forces between Eu-complex and PVP, which lowers the energy level of the excited state in Eu-complex, resulting in a slight reduction of the energy difference between the excited and unexcited states. The absorption band from 300 to 380 nm of Eu-complex LDL makes it possible to absorb the UV light widely. The transmission spectra of LDL with different Eu-complex doping concentrations are shown in Figure 2b. It is found that the transmittance of LDL without Eu-complex (only PVP) is above 60% in the range of 300−380 nm, revealing that most of the light can pass through the PVP, while with the increase of the doping concentration, the transmittance of LDL containing Eu-complex is decreased gradually from 45% to 15%, and these results are consistent with that of the absorption. Furthermore, all the LDLs have a higher transmittance in the visible light range, which means that the spectral response of the LDL is concentrated in the ultraviolet region. The PL excitation spectra of as-synthesized LDL with Eucomplex (0.5, 1.0, and 1.5 wt %) are shown in Figure 3a. The intense and broad excitation band from 300 to 380 nm in the UV region is attributed to the ligand (4,7-diphenyl-1,10phenanthroline) in Eu-complex. With the increase of Eucomplex content in PVP, the excitation band intensity enhances gradually, which is the same as the trend in absorption. It should be noticed that there also appears a slight red shift in the excitation spectra with an increase in the Eu-complex content, further revealing the interaction between Eu-complex and PVP. The excitation band from 300 to 340 nm is matched with the absorption spectra, confirming that the energy is entirely transferred from ligand to Eu3+ ions in Eu-complex, as expected. The highly effective energy transfer from ligand to Eu3+ ions makes it possible to sensitize Eu3+ ions well even in the region of 340−380 nm with weak absorption intensity. Meanwhile, the

wide spectral response performance of 1.5 wt % content of Eucomplex in PVP makes it suitable to use in active PSC devices as down-conversion materials. Eu-complex LDL shows several PL peaks at 578, 519, 614, 652, and 700 nm assigned to the electronic transition of 5D0 → 7 F0, 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3, and 5D0 → 7F4 forbidden transitions of Eu3+, respectively (Figure 3b). A slightly widened peak of Eu-complex LDL is caused by the coordination between PVP and Eu-complex. The emission spectra show the characteristic red luminescence of the trivalent europium ion upon UV excitation, and the luminescence intensity variation is following the changes in excitation spectra. The Eu-complex with a quantum yield of 37% introduced in PVP effectively converts the higher-energy photons in the nearUV region of the solar spectrum into lower-energy photons of long wavelengths, which is well-matching the absorption spectrum of the perovskite layer. It is found that Eu-complex in PVP as effective down-conversion material could convert UV light into visible range and help to broaden the absorption scope of the PSCs. The schematic structure of the PSC-coating Eu-complex LDL (1.5 wt %) is shown in Figure 4. When the radiant light enters into PSCs from the FTO side coated with Eu-complex LDL, most low-energy photons (visible light) pass through the LDL and are absorbed by the perovskite layer, while some highenergy photons (UV light) are absorbed by the LDL and then

Figure 4. Schematic illustration explaining the mechanism of enhanced photoelectric performance of PSC by the transparent Eu-complex LDL (1.5 wt % content of Eu-complex in PVP). 26960


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Figure 5. (a) SEM image of 1.5 wt % Eu-complex LDL. (b) Cross-sectional SEM image of PSC with 1.5 wt % Eu-complex LDL on the back surface of the FTO glass (back side of the device). (c) Cross-sectional SEM image of the PSC with 1.5 wt % Eu-complex LDL (front side of the device). (d) EDX spectrum of 1.5 wt % Eu-complex LDL. (e) Digital photograph of 1.5 wt % Eu-complex LDL when exposed to UV light at 320 nm.

Figure 6. IPCE curves of PSCs with Eu-complex LDL (Eu-complex content: 0.0 and 1.5 wt %).

Figure 7. (a) J−V curves of reverse and forward scans (1 sun, AM 1.5G) for the PSCs with Eu-complex LDL (Eu-complex content: 0.0 and 1.5 wt %). (b) Photocurrent density and PCE as a function of time for the PSC with 1.5 wt % Eu-complex LDL held close to 0.802 V forward bias.

better photovoltaic performance. To investigate the elemental composition of Eu-complex LDL, energy dispersive X-ray (EDX) analysis was carried out. As shown in Figure 5d, the EDX spectrum of the LDL shows obvious diffraction peaks from C, N, and O elements; in addition, some slender peaks of the element Eu also appear in the EDX spectrum, which proves the presence of Eu in the LDL. When exposed to UV light at 320 nm, Eu-complexes in PVP emit red light and exhibit excellent down-conversion fluorescence emission properties (Figure 5e). To confirm the influence of Eu-complex LDL on the spectral photoelectric response of PSCs, the incident photon to current conversion efficiency (IPCE) spectra of both the best PSCs

converted into low-energy photons (red light) for the perovskite layer. This helps to improve the utilization of ultraviolet light, extending the spectral response range to the UV region and reducing the charge loss in PSCs, which is better for the PSC stability.35 Figure 5a shows the SEM image of 1.5 wt % Eu-complex LDL. It is shown that the surface morphology of Eu-complex LDL is uniform and there is no obvious impurity in it. The thickness of the LDL is about 1 μm, as shown in Figure 5b. Figure 5c displays the cross-sectional SEM images of PSC with 1.5 wt % Eu-complex LDL (front side of the device), and from it we can learn that the perovskite layer of the PSC has a uniform thickness and a flat surface, which helps to achieve 26961


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Figure 8. Performance parameter distributions of 20 PSCs without Eu-complex LDL and 20 PSCs with 1.5 wt % Eu-complex LDL: (a) PCE, (b) Jsc, (c) Voc, and (d) FF (reverse scan from 1.2 to −0.1 V).

Figure 9. Normalized stability with error lines of the PSCs with Eu-complex LDL (Eu-complex content: 0.0 and 1.5 wt %; reverse scan from 1.2 to −0.1 V).

for both cells exhibited similar profiles from 400 to 850 nm, where the Eu-complex LDL shows no absorption. The current enhancement by Eu-complex LDL is pronounced in short wavelengths, and the integral current in the range of 300−850 nm increased from 16.28 to 16.76 mA/cm2. The IPCE enhancement is beneficial for the improved photovoltaic performance of PSC owing to the UV light absorption and subsequent red emission.

with and without Eu-complex LDL were measured in the wavelength range from 300 to 850 nm (Figure 6a). As expected from the former discussion, in the range of 300−380 nm, the IPCE curves of both the PSCs share extremely different profiles (Figure 6b). The difference in the UV region is caused by the Eu-complex and the IPCE value of the PSC with the Eucomplex LDL shows clear enhancement compared with that of PSC without Eu-complex LDL, indicating that the spectral response of PSC is improved by Eu-complex LDL. IPCE curves 26962


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voltages (Voc) of the two kinds of cells do not display a significant degradation (Figure 9c). The fill factor (FF) of the PSC with Eu-complex has a better performance than that of the PSC without Eu-complex during continuous UV illumination (Figure 9d), indicating the better crystallinity of perovskite in cells.37 In addition, the PSCs both without and with LDL show a narrow distribution, indicating that the errors are small, and the increase of PCE should come from the light conversion and UV blocking contributed by the LDL. In summary, the Eucomplex luminescent down-converting layer in the PSCs is responsible for the enhancement of photocurrent density as well as the decrease of UV degradation.

The current density−voltage (J−V) plots for the best PSC devices without and with Eu-complex LDL are shown in Figure 7a almost without hysteresis. For the PSC without Eu-complex LDL, a short-circuit current density (Jsc) of 17.27 mA·cm2, an open-circuit photovoltage (Voc) of 1.03 V, and a fill factor of 73.54% were obtained, which correspond to a holistic power conversion efficiency (PCE) of 13.14%. When the content of Eu-complex increased up to 1.5 wt %, a 17.5% increase in Jsc from 13.14 to 15.44 mA·cm2 resulted. The enhanced photocurrent is partly due to the improved absorption in the region of 300−380 nm wavelengths, as seen from the IPCE spectra (Figure 6b). On the other hand, it is because of the better structure of perovskite (uniform thickness and flat surface) in the PSC. As the LDL converts UV light into visible light, it helps to avoid the damage caused by UV light to the perovskite structure. Consistent with the former analysis, the PCE of PSC with 1.5 wt % Eu-complex is higher than that of PSC without Eu-complex because of the increase of UV utilization and the decline of UV degradation by Eu-complex. The down-converting material provides additional red photons, which are rapidly absorbed by the perovskite absorber material, giving rise to the observed photocurrent enhancement shown by the IPCE. Moreover, there is a slight enhancement in the value of photovoltage. The use of down-converting phosphor material has been shown to lead to the improvement of shortwavelength spectral response because parts of the UV spectrum are converted to visible region, which is then absorbed by the active material.36 The photocurrent density and PCE as a function of time for the PSC with 1.5 wt % Eu-complex LDL held close to 0.802 V forward bias are shown in Figure 7b. The photocurrent density and a PCE over 15% of the PSC remained stable within 300 s, suggesting that Eu-complex LDL is conducive to an efficient and stable PSCs under light illumination. The performance parameter distributions of 20 PSCs both without and with Eu-complex LDL are summarized in Figure 8. Obviously, the Jsc is significantly enhanced with the application of Eu-complex LDL by converting UV light. To confirm the effect of Eu-complex LDL on the stability of PSC, we conducted a UV-light-soaking measurement on both PSCs without and with Eu-complex. The variations of the solar cell parameters with time of the two types of PSCs are plotted in Figure 9. The PSC without Eu-complex LDL on the reverse of the FTO glass showed poor stability, and a loss of above 50% in efficiency was observed after 10 h (Figure 9a). Perovskite in PSCs is known to be sensitive to UV light; after prolonged absorption of UV light, the structure of perovskite can be destroyed, which causes the electron−hole recombination, leading to a decrease in charge collection efficiency. This process lowers the short-circuit photocurrent, and the current values drop with time upon illumination, which leads to a decrease in efficiency. The PSC with Eu-complex shows better photocurrent behavior than the PSC without Eu-complex. For the PSC with Eu-complex on the glass side, the Jsc value reduced by 3% after 2 h of UV illumination, confirming that there is almost no change in photocurrent. However, the Jsc value of PSC without Eu-complex drops to 90% of the initial value (Figure 9b). This indicates that the Eu-complex LDL has the ability to arrest the UV degradation effects and improve the stability of the PSC. The Jsc in PSC without Eu-complex degrades to 48% and that of PSC with Eu-complex to 74% of their respective initial values after 10 h. The open circuit

4. CONCLUSION In conclusion, an effective approach to enhance the photovoltaic performance of PSC by the introduction of an Eucomplex transparent luminescent down-converting layer is reported. The Eu-complex LDL has a broad absorption in the UV region, and the present results confirm that an Eu-complex transparent luminescent film on the glass side of the PSCs can improve its photovoltaic performance. By means of downconversion technology, the spectral response range of PSC has been successfully extended to the UV region, leading to a gain of photocurrent for active PSCs. The PSC coated with LDL of Eu-complex (1.5 wt % content) reaches a PCE of 15.44%, which is 17.5% higher than that of PSC without Eu-complex LDL. The short-circuit current density (Jsc) of PSC shows an extreme increase from 17.27 to 19.30 mA·cm2 due to the effective utilization of UV light. Moreover, the introduction of Eu-complex LDL in PSC could decrease the UV degradation by converting the UV light into visible light, so the stability of the PSC is remarkably enhanced. Additionally, the open circuit voltage (Voc) is also increased slightly. Therefore, a downconverting material with an ideal absorption band that absorbs available short-wavelength UV light in the range of 300−380 nm and emits visible light that is well-matched with the spectral response of PSC would be a promising choice for enhancing the photoelectric performance and UV stability of PSCs. The results of our research might provide new avenues to improve the performance of PSCs.

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AUTHOR INFORMATION

Corresponding Authors

*C.Z. e-mail: [email protected]. *S.D. e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Basic Research Program of China (under Grant No. 2015CB932200); the External Cooperation Program of BIC, Chinese Academy of Science (Grant No. GJHZ1607); and the National Natural Science Foundation of China (Grants 21173227, U1205112).

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