Highly Efficient Inverted Perovskite Solar Cells with CdSe QDs/LiF Electron Transporting Layer
© The Author(s). 2017
Received: 26 October 2017
Accepted: 22 November 2017
Published: 6 December 2017
Organic/inorganic hybrid perovskite solar cell has emerged as a very promising candidate for the next generation of near-commercial photovoltaic devices. Here in this work, we focus on the inverted perovskite solar cells and have found that remarkable photovoltaic performance could be obtained when using cadmium selenide (CdSe) quantum dots (QDs) as electron transporting layer (ETL) and lithium fluoride (LiF) as the buffer, with respect to the traditionally applied and high-cost [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The easily processed and low-cost CdSe QDs/LiF double layer could facilitate convenient electron-transfer and collection at the perovskite/cathode interface, promoting an optoelectric conversion efficiency of as high as 15.1%, very close to that with the traditional PCBM ETL. Our work provides another promising choice on the ETL materials for the highly efficient and low-cost perovskite solar cells.
Hybrid organic-inorganic perovskite solar cell has been recognized as a very promising new-generation thin film solar cells based on remarkable improvement in its photovoltaic performance with a present efficiency of as high as 22.1% . Long-term environmental stability could also be obtained with a time scale of several hundred to a thousand hours [2, 3]. In the large family of perovskite solar cells, planar heterojunction with an inverted device skeleton has been highly emphasized and intensively researched because of its appealing potential in mild fabrication process and easily accessible flexibility [4–7]. Typically for this device structure, the perovskite layer is sandwiched between the anode and cathode buffer layers to form a p-i-n-layered energy level alignment. In this structure, the n type layer plays a critical role in accepting electrons and inhibiting holes from the perovskite layer.
Up to now, a variety of semiconducting materials were adopted as electron transporting layer (ETL); the traditional choice is the extensively used C60 and its derivative, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) [7–10]. Through uniform and excellent electrical contact with the underlied perovskite film, the small molecule ETLs can provide remarkable efficiency of as high as 19.9% . Although high efficiency was obtained for organic ETLs, gradual attention arises to the high cost of such ETL materials, the complicated device fabrication process, and the unsatisfied device stability. In comparison, ETL materials based on inorganic nanoparticles appeal great attention because of their potential advantage in low material cost, charge mobility, mild fabrication integration, and promising device stability [11–15]. However, up to now, exploring on inorganic ETLs in inverted structure was relatively rare. M. Grätzel and L. Han et al. developed highly conductive Nb-doped TiO2 film on PCBM to obtain an efficiency of 16.2% with > 90% retained PCE after 1000 h of light soaking . Similarly, Alex K et al. introduced Zn2SnO4 nanocrystalline thin film on PCBM buffer layer to facilitate electron extraction and thus increased the device performance to 17.76% . You et al. and Yang et al. firstly fabricated all-metal-oxide layer-based inverted perovskite solar cell that show 16.1% efficiency and significantly improved stability . Generally, either the quantity of reported works or the photovoltaic performance of this inverted devices lagged behind the traditional structure. More investigation on the inorganic ETL-based inverted perovskite solar cells are needed to accelerate the fast growth of this field.
Here in this work, we developed a novel all-inorganic ETL for the inverted perovskite solar cells, a cadmium selenide (CdSe) quantum dots (QDs)/lithium fluoride (LiF) double layer obtained from spin-coating and thereafter evaporation process. Up to now, the synthesis and optoelectric application of CdSe QDs have been extensively reported as electron acceptor [16–18]. Ultrathin and island-shaped LiF were also widely used in the cathode buffer layers in organic solar cells [19, 20]. All these well-developed references prompt us to consider them as inorganic ETL and cathode buffer layer in the inverted perovskite solar cells. We have found that the CdSe/LiF layer plays an excellent role in extracting and transferring electrons from the underlying perovskite to the above cathode, enabling a photovoltaic conversion efficiency of as high as 15.1% that is very close to the PCBM reference. Our work provides another promising choice on the low cost and all-inorganic electron extraction layer for inverted perovskite solar cells.
Synthesis of CdSe QDs
Cadmium oxide (CdO, 1 mmol), oleic acid (OA, 10 mmol), and 3 g trioctylphosphine oxide (TOPO) were dissolved in a four-neck round bottom flask and pumped at 140 °C under N2 flow for 30 min. After that, the temperature was raised to about 280 °C during which the solution turned clear. A TOP-Se solution (containing 1 mmol Se in 3 ml tri-n-octylphosphine (TOP) was injected into the flask quickly. The reaction was allowed at 260 °C for 4 min, and then, the heating mantle was removed. After the solution was cooled to room temperature, 10 ml acetone was injected to collect the red precipitation by centrifugation at 4500 rpm. The obtained CdSe QDs were cleaned with chlorobenzene (CB)/acetone solvent/antisolvent for at least four times and then dissolved in 30 ml pyridine and stirred at 50 °C overnight to exchange the surface OA ligands. Then, the pyridine-capped CdSe QDs were collected by adding n-hexane to the solution and thereafter centrifuging at 4000 rpm. About 8 ml CB was used to disperse the collected CdSe QDs. The concentration of the final solution was adjusted to 15 mg/ml that was used for solar cell fabrication.
The pre-patterned indium tin oxide (ITO) glass was firstly untrasonicated with deionized water, acetone, and isopropanol separately for 30 min and then dried by N2 blowing. One hundred microliter poly(3,4-ethylenedioxythiophene) poly(styrene-sulfonate) (PEDOT:PSS, VPAI 4083) was spin-coated onto the ITO at 6000 rpm and then dried at 120 °C in air. The organic-inorganic perovskite solution was prepared by mixing 2 mmol MAI and 2 mmol PbI2 in 1.6 ml DMF. The solution was stirred at 70 °C overnight in N2-filled glovebox. The perovskite film was deposited on the substrate through a two-step spin-coating procedure (1000 rpm for 10 s and 6000 rpm for 30 s). One hundred eighty microliter chlorobenzene was deposited quickly at 5 s since the beginning of the second stage of spin-coating. All the perovskite films were annealed at 100 °C for 10 min. After cooling down, the as-prepared CdSe QD chlorobenzene solution was dripped on the perovskite surface, stayed for 5 s, and then spin-coated at different speed to obtain different film thickness. The substrate was transferred into thermal evaporator where a 0.8–1.0-nm LiF ultrathin film or particle islands was deposited (0.2 Å/s, 6 × 10−4 Pa) followed by 20 nm Au and 80 nm Ag. A mask was used to define six separate pixels each with an effective area of 0.04 cm2.
The film topology with and without CdSe/LiF covering were researched by field emission scanning electron microscope (FESEM, JEOL 7006F) and scanning probe microscope (SPA400). X-ray diffraction (XRD) was performed on a Rigaku D/max-gA X-ray diffractometer with Cu Kα radiation. Light absorption properties were measured with ultraviolet-visible-inferred spectrophotometer (Varian Cary-5000). Photoluminescence (PL) spectra were collected on HORIBA Jobin Yvon Fluorlog-3 system. Time-resolved photoluminescence (TRPL) spectroscopy measurements were conducted using a pulse laser (512 nm) for excitation (F980 lifetime spectrometers, Edinburgh Instruments, EI). The TRPL decays at 790 nm were recorded by a time-correlated single-photon counting (TCSPC) spectrometer. The photovoltaic I-V properties were recorded on Keithley 2440 source meter combined with Newport 94043A solar simulator (AM 1.5 illumination). The unencapsulated solar cells were tested at room temperature in air. Typically, light soaking was needed to get a stabilize power conversion efficiency. External quantum efficiency (EQE) was measured on a solar cell IPCE measurement system (Crowntech Qtest Station 500ADX) with a CM110 monochromator, a Keithley 2000 source meter, and a CT-TH-150 Br-W lamp. The surface photovoltage (SPV) spectrum were obtained from a measurement system containing the source of monochromatic light, a lock-in amplifier (SR830-DSP) with a light chopper (SR540). Electrochemical impedance spectra (EIS) were measured from a CHI 660E electrochemical workstation (Chenhua Inc., Shanghai), applying a 10-mV AC signal and scanning in a frequency range between 1 MHz and 1000 Hz at different forward applied bias.
Results and Discussion
To evaluate the charge transfer and collection ability of this novel perovskite/CdSe interface, we characterized the photoluminescence (PL) properties of different samples. The bare MAPbI3 film on ITO glass shows a strong PL peak at about 790 nm (Fig. 2b) while this peak intensity is up to 80% quenched for the sample covered with CdSe/LiF layer. This result reflects that the photon-generated charges could be effectively separated at the perovskite/CdSe interface. Incorporation of the PEDOT:PSS anode buffer layer beneath the perovskite layer further quenches the PL intensity. For further evidence, time-resolved photoluminescence (TRPL) decay spectrum were characterized to probe the effect of inorganic buffer layer on carrier dynamics in the solar cells. For pure perovskite film, it was reported that a longer PL lifetime could be obtained through suppressing charge recombination with mixed antisolvent or surface passivation [21, 22]. Here in this work, we focused on chlorobenzene for easy comparison, although other antisolvent may also play a positive role in fabrication of uniform perovskite films . The results in Fig. 2c show that the TRPL signal of perovskite film covered with CdSe/LiF has a faster decay as compared to the film without cathode buffer, indicating a rapid charge injection from MAPbI3 to CdSe. As shown in Fig. 2d, the perovskite/CdSe contact could form a typical type-II heterojunction that facilitates exciton dissociation and charge transfer. Thus, the results demonstrate that the adopted CdSe QDs/LiF layer is electronically beneficial to charge extraction as a cathode buffer layer. Therefore, it is highly expectable to gain a reasonable photovoltaic performance by applying the PEDOT:PSS/MAPbI3/CdSe/LiF heterostructure. The planar solar cell was thus fabricated with CdSe QDs and PEDOT:PSS as the cathode and anode buffer layer respectively, as is shown in Fig. 2d.
Photovoltaic performance of inverted MAPbI3 solar cells with different thickness (T.) of CdSe QD layer
Jsc (mA cm−2)
In conclusion, we have fabricated planar perovskite solar cells with CdSe quantum dots/LiF electron transporting layer that is compatible to the solution process of the device. The uniform and full coverage of perovskite film through a 25-nm CdSe QDs and 1 nm LiF would provide spacial and electronic convenience for electrons’ transfer and extraction, as indicated from the TRPL, EIS, and SPV characterization and so on. The adoption of this ETL brings a significant increase in photovoltaic efficiency, from 4.8% for that without buffer layer to 14.2% in the optimized target and a maximum of 15.1%. The performance stability is also improved. Our work provides a promising candidate on ETLs for the development of highly efficient and low-cost inverted perovskite solar cells.
We thank Dr. Ling Wei and Dr. Huiping Gao for their kind help on the AFM and PL measurements and discussion.
This work is supported by the National Natural Science Foundation of China (Grant no. 61306019), the Henan Provincial Science Found (Grant no. 162300410026), the Key Member of Young Teachers (Grant no. 2016GGJS-019), and the Henan University Fund.
FRT and WZX carried out the experiments and drafted the manuscript. XDH participated in the sequence alignment. PY and LW conceived the study and participated in its design. WFZ participated in the design of the study and performed the analysis. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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- Yang W, Park B, Jung E, Jeon N, Kim Y, Lee D et al (2017) Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells. Science 356:1376–1379View ArticleGoogle Scholar
- Tan H, Jain A, Voznyy O, Lan X, Garcíade Arquer F, Fan J et al (2017) Efficient and stable solution-processed planar perovskite solar cells via contact passivation. Science 355:722–726View ArticleGoogle Scholar
- Zhang P, Wu J, Wang Y, Sarvari H, Liu D, Chen Z et al (2017) Enhanced efficiency and environmental stability of planar perovskite solar cells by suppressing photocatalytic decomposition. J Mater Chem A 5:17368–17378View ArticleGoogle Scholar
- Jeng J, Chen K, Chiang T, Lin P, Tsai T, Chang Y et al (2014) Nickel oxide electrode interlayer in CH3NH3PbI3 perovskite/PCBM planar-heterojunction hybrid solar cells. Adv Mater 26:4107–4113View ArticleGoogle Scholar
- Brinkmann K, Zhao J, Pourdavoud N, Becker T, Hu T, Olthof S et al (2017) Suppressed decomposition of organometal halide perovskites by impermeable electron-extraction layers in inverted solar cells. Nat Commun 8:13938View ArticleGoogle Scholar
- Ye S, Sun W, Li Y, Yan W, Peng H, Bian Z et al (2015) CuSCN-based inverted planar perovskite solar cell with an average PCE of 15.6%. Nano Lett 15:3723–3728View ArticleGoogle Scholar
- Wolff C, Zu F, Paulke A, Toro L, Koch N, Neher D (2017) Reduced interface-mediated recombination for high open-circuit voltages in CH3NH3PbI3 solar cells. Adv Mater 29:1700159View ArticleGoogle Scholar
- Wang B, Zhang Z, Ye S, Gao L, Yan T, Bian Z et al (2016) Solution-processable cathode buffer layer for high-performance ITO/CuSCN-based planar heterojunction perovskite solar cell. Electrochim Acta 218:263–270View ArticleGoogle Scholar
- Chiang C, Wu C (2016) Bulk heterojunction perovskite–PCBM solar cells with high fill factor. Nat Photonics 10:196–200View ArticleGoogle Scholar
- Ye S, Rao H, Zhao Z, Zhang L, Bao H, Sun W et al (2017) A breakthrough efficiency of 19.9% obtained in inverted perovskite solar cells by using an efficient trap state passivator Cu(thiourea)I. J Am Chem Soc 139:7504–7512View ArticleGoogle Scholar
- Zhang L, Zhang X, Yin Z, Jiang Q, Liu X, Meng J et al (2015) Highly efficient and stable planar heterojunction perovskite solar cells via a low temperature solution process. J Mater Chem A 3:12133–12138View ArticleGoogle Scholar
- Chen W, Wu Y, Yue Y, Liu J, Zhang W, Yang X et al (2015) Efficient and stable large-area perovskite solar cells with inorganic charge extraction layers. Science 350:944–948View ArticleGoogle Scholar
- Docampo P, Ball J, Darwich M, Eperon G, Snaith H (2013) Efficient organometal trihalide perovskite planar-heterojunction solar cells on flexible polymer substrates. Nat Commun 4:2761View ArticleGoogle Scholar
- Liu X, Chueh C, Zhu Z, Jo S, Sun Y, Jen A (2016) Highly crystalline Zn2SnO4 nanoparticles as efficient electron-transporting layers toward stable inverted and flexible conventional perovskite solar cells. J Mater Chem A 4:15294–15301View ArticleGoogle Scholar
- You J, Meng L, Song T, Guo T, Yang Y, Chang W et al (2016) Improved air stability of perovskite solar cells via solution-processed metal oxide transport layers. Nat Nanotechnol 11:75–81View ArticleGoogle Scholar
- Peng X, Manna L, Yang W, Wickham J, Scher E, Kadavanich A et al (2000) Shape control of CdSe nanocrystals. Nature 404:59–61View ArticleGoogle Scholar
- Tan F, Wang Z, Qu S, Cao D, Liu K, Jiang Q et al (2016) A CdSe thin film: a versatile buffer layer for improving the performance of TiO2 nanorod array:PbS quantum dot solar cells. Nano 8:10198–10204Google Scholar
- Zhao T, Goodwin E, Guo J, Wang H, Diroll B, Murray C et al (2016) Advanced architecture for colloidal PbS quantum dot solar cells exploiting a CdSe quantum dot buffer layer. ACS Nano 10:9267–9273View ArticleGoogle Scholar
- Gao D, Helander M, Wang Z, Puzzo D, Greiner M, Lu Z (2010) C60:LiF blocking layer for environmentally stable bulk heterojunction solar cells. Adv Mater 22:5404–5408View ArticleGoogle Scholar
- Tang Z, George Z, Ma Z, Bergqvist J, Tvingstedt K, Vandewal K et al (2012) Semi-transparent tandem organic solar cells with 90% internal quantum efficiency. Adv Energy Mater 2:1467–1476View ArticleGoogle Scholar
- Wang Y, Wu J, Zhang P, Liu D, Zhang T, Ji L et al (2017) Stitching triple cation perovskite by a mixed anti-solvent process for high performance perovskite solar cells. Nano Energy 39:616–625View ArticleGoogle Scholar
- Li S, Zhang P, Wang Y, Sarvari H, Liu D, Wu J et al (2017) Interface engineering of high efficiency perovskite solar cells based on ZnO nanorods using atomic layer deposition. Nano Res 10:1092–1103View ArticleGoogle Scholar
- Ji L, Zhang T, Wang Y, Zhang P, Liu D, Chen Z et al (2017) Realizing full coverage of stable perovskite film by modified anti-solvent process. Nanoscale Res Lett 12:367View ArticleGoogle Scholar
- Zhao C, Chen B, Qiao X, Luan L, Lu K, Hu B (2015) Revealing underlying processes involved in light soaking effects and hysteresis phenomena in perovskite solar cells. Adv Energy Mater 5:1500279View ArticleGoogle Scholar
- Nie W, Blancon J, Neukirch A, Appavoo K, Tsai H, Chhowalla M et al (2016) Light-activated photocurrent degradation and self-healing in perovskite solar cells. Nat Commun 7:11574View ArticleGoogle Scholar
- Christians J, Fung R, Kamat R (2014) An inorganic hole conductor for organo-lead halide perovskite solar cells. Improved hole conductivity with copper iodide. J Am Chem Soc 136:758–764View ArticleGoogle Scholar
- Jung J, Chueh C, Jen A (2015) A low-temperature, solution-processable, Cu-doped nickel oxide hole-transporting layer via the combustion method for high-performance thin-film perovskite solar cells. Adv Mater 27:7874–7880View ArticleGoogle Scholar
- Barnea-Nehoshtan L, Kirmayer S, Edri E, Hodes G, Cahen D (2014) Surface photovoltage spectroscopy study of organo-lead perovskite solar cells. J Phys Chem Lett 5:2408–2413View ArticleGoogle Scholar