Lead Selenide Colloidal Quantum Dot Solar Cells Achieving High

Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 3598−3603

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Lead Selenide Colloidal Quantum Dot Solar Cells Achieving High Open-Circuit Voltage with One-Step Deposition Strategy Yaohong Zhang,† Guohua Wu,*,‡ Chao Ding,† Feng Liu,† Yingfang Yao,∥ Yong Zhou,∥ Congping Wu,# Naoki Nakazawa,† Qingxun Huang,† Taro Toyoda,† Ruixiang Wang,§ Shuzi Hayase,⊥ Zhigang Zou,*,∥,# and Qing Shen*,† †

Faculty of Informatics and Engineering, The University of Electro-Communications, Tokyo 182-8585, Japan School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China ∥ Ecomaterials and Renewable Energy Research Center, Jiangsu Key Laboratory for Nano Technology, National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China # Kunshan Sunlaite New Energy Technology Co. Ltd., Suzhou 215347, China § Beijing Engineering Research Centre of Sustainable Energy and Buildings, Beijing University of Civil Engineering and Architecture, Beijing 102616, China ⊥ Faculty of Life Science and Systems Engineering, Kyushu Institute of Technology, Fukuoka 808-0196, Japan

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‡

S Supporting Information *

ABSTRACT: Lead selenide (PbSe) colloidal quantum dots (CQDs) are considered to be a strong candidate for high-efficiency colloidal quantum dot solar cells (CQDSCs) due to its efficient multiple exciton generation. However, currently, even the best PbSe CQDSCs can only display open-circuit voltage (Voc) about 0.530 V. Here, we introduce a solution-phase ligand exchange method to prepare PbI2-capped PbSe (PbSe-PbI2) CQD inks, and for the first time, the absorber layer of PbSe CQDSCs was deposited in one step by using this PbSe-PbI2 CQD inks. One-step-deposited PbSe CQDs absorber layer exhibits fast charge transfer rate, reduced energy funneling, and low trap assisted recombination. The champion large-area (active area is 0.35 cm2) PbSe CQDSCs fabricated with one-step PbSe CQDs achieve a power conversion efficiency (PCE) of 6.0% and a Voc of 0.616 V, which is the highest Voc among PbSe CQDSCs reported to date.

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layer. As a result, the charges transfer between PbSe CQDs is blocked and the new recombination centers of electrons and holes are generated, which significantly limits open-circuit voltage (Voc) of PbSe CQDSCs. Recently, a record power conversion efficiency (PCE) of PbSe CQDSCs has reached 8.2% (cell area 0.071 cm2) by Huang’s group.26 Unfortunately, in this report, the Voc loss of devices are over 0.8 V, the Voc of the devices are lower than 0.530 V. In addition, for high-efficiency PbSe CQDSCs (with PCE over 6%),23,26−28 an ultracomplex method was typically required, i.e., CdSe (or ZnSe) CQDs should be first synthesized as the Se source of PbSe, and then using a Pb2+ cation exchange procedure to synthesize PbSe CQDs, and the PbSe CQDs absorber layer was deposited by using a lengthy multistep LBL dip-coating deposition method. With the solar cell area increasing, an extremely inhomogeneous PbSe film will be predictably resulting from this dip-coating method, which is not applicable to obtain large-area high efficiency

olloidal quantum dots (CQDs) as light harvesting materials show bright prospect in light of their quantum size effect and low-cost solution processability.1−7 Colloidal quantum dot solar cells (CQDSCs), which can be facilely fabricated by a low-cost spin-coating procedure, have attracted more and more interest as an immense promising candidate for the new generation photovoltaic devices.8−16 Lead selenide (PbSe) CQDs are considered to be a strong candidate for the absorber of high-efficiency CQDSCs because of its more remarkable multiple exciton generation (MEG) effect than other materials.17−21 In typical fabrication process of PbSe CQDSCs, PbSe CQDs are deposited on TiO2 or ZnO substrate by a multitimes layer-by-layer (LBL) (such as dipcoating or spin-coating) deposition procedure to obtain a sufficient thickness of CQDs absorber layer.22,23 In the LBL deposition process, PbSe CQDs are subsequently exposed in solvents and surrounded by chemicals, potentially causing new defects and compositional mixing between layers in junction structures, which leads to an inhomogeneous energy landscape in PbSe CQD films24,25 and organic residues in the PbSe CQD films. This uncontrollability of LBL deposition procedure has disadvantages in obtaining a high-quality PbSe CQDs absorber © 2018 American Chemical Society

Received: May 13, 2018 Accepted: June 15, 2018 Published: June 15, 2018 3598

DOI: 10.1021/acs.jpclett.8b01514 J. Phys. Chem. Lett. 2018, 9, 3598−3603

Letter

The Journal of Physical Chemistry Letters

Figure 1. (a) Absorption spectra of OA-capped PbSe (named PbSe-OA) CQDs in octane and PbSe-PbI2 CQDs in DMF. (b) TEM image of PbSeOA CQDs. (c) TEM image of PbSe-PbI2 CQDs. (d) XPS survey of PbSe-PbI2 CQDs. (e) FT-IR absorption spectra of PbSe CQDs films before and after ligand exchange. (f) Absorption spectra of PbSe-OA, PbSe-TBAI, and PbSe-PbI2 CQD solid films.

Figure 2. Comparison of the TA decays of PbSe-TBAI and PbSe-PbI2 CQD solid films: (a) normalized absorption changes (ΔA) of the films which are probed at 940 nm (1.32 eV). (b and c) Spectro-temporal TA maps for PbSe-TBAI and PbSe-PbI2 CQD solid films, respectively. All samples are pumped by 470 nm (2.64 eV) laser pulse with the pump fluence of 6 μJ/cm2 which is used to avoid the presence of Auger recombination. The negative and positive signals in the changes of absorption (ΔA) indicate the photobleaching and photoinduced absorption of exciton state, respectively.

thorough ligand exchanges. PbI 2 also provides better passivation with less recombination and lower trap density in PbSe CQDs absorber layer. Then for the first time, we fabricated PbSe CQDSCs with one-step deposited PbSe-PbI2 CQDs absorber layer and achieved a champion device with an improved Voc of 0.616 V and a PCE of 6.0% (active area of this device is 0.35 cm2, which is about 5 times than most of reported PbSe CQDSCs (usually 0.01−0.07 cm2)20,23,26,27). So far as we know this is the first reported PbSe CQDSCs with Voc

CQDSCs. Therefore, the above-mentioned process for PbSe CQDSCs limits its further development. In our previous research,21 we employed a one-step hot injection method to synthesize PbSe CQDs and used tetrabutylammonium iodide (TBAI) as a ligand source to fabricate PbSe CQDSCs by the LBL spin-coating deposition method. A best PCE of 3.53% was obtained for the large-area PbSe-TBAI CQDSCs (active area about 0.25 cm2), but the Voc of the device is only 0.42 V. Thus, reducing the Voc loss and improving the Voc of PbSe CQDSCs will be an effective way to enhance the PCE of PbSe CQDSCs. In order to simplify the fabrication procedure of PbSe CQDSCs and improve the Voc of PbSe CQDSCs, here, we employ a modified one-step hot injection method to synthesize PbSe CQDs and replace the original ligand oleic acid (OA) of PbSe CQDs by PbI2 in a solution-phase ligand exchange process, which produces PbI2-capped PbSe (PbSe-PbI2) CQD butylamine inks. By using these preprepared PbSe-PbI2 CQD inks, the absorber layer of PbSe CQDSCs can be deposited on TiO2 substrate in one step. Compared with PbSe-TBAI CQD films obtained by the LBL spin-coating deposition method, the PbSe CQD films deposited by this one-step method has less organic residues and faster charge transfer rate due to more

Table 1. Performance Details of PbSe-TBAI and PbSe-PbI2 CQDs-Based CQDSCsa devices PbSeTBAI PbSePbI2

Jsc (mA/cm2) 18.5 ± 1.1 (19.4) 20.6 ± 0.9 (21.2)

Voc (V) 0.578 ± 0.008 (0.581) 0.610 ± 0.007 (0.616)

FF (%) 37.0 ± 1.2 (38.0) 45.3 ± 1.4 (46.1)

PCE (%) 4.0 ± 0.3 (4.3) 5.8 ± 0.2 (6.0)

a

Light intensity for the measurement is AM1.5 G 100 mW/cm2. The values for champion devices were shown in parentheses. All of the devices were measured at 20 °C with the indoor relative humidity of 40% in air.

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DOI: 10.1021/acs.jpclett.8b01514 J. Phys. Chem. Lett. 2018, 9, 3598−3603

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The Journal of Physical Chemistry Letters larger than 0.6 V, and a large-area (0.35 cm2) PbSe CQDSCs with a PCE of 6% up until now. In addition, PbSe-PbI2 CQDbased CQDSCs demonstrated excellent air and light soaking stability, retaining at least 93% of its maximum value even after storage in air for 139 days, and with >99% of the initial PCE maintained after being continuously illuminated under AM 1.5G for 240 min. Figure 1a shows the absorption spectra of OA-capped PbSe (PbSe-OA) and PbSe-PbI2 CQDs in solution. The excitonic peak of PbSe remains well-preserved after PbI2 treatment, and the small blue shift of the excitonic peak may be attributed to the change in dielectric environment of PbSe-PbI2 CQDs in N,N-dimethylformamide (DMF).29,30 Transmission electron microscopy (TEM) images of PbSe CQDs capped with OA (Figure 1b) and PbI2 (Figure 1c) confirmed the integrity of the CQDs and the narrowed CQD−CQD distance after ligand exchange which shall result in enhanced packing densities and conductivity of PbSe-PbI2 CQD films.31,32 In Figure 1d, strong characteristic peaks of iodide were detected in PbSe-PbI2 CQDs film by X-ray photoelectron spectroscopy (XPS) measurement, which suggests good incorporation of iodide into the PbSe-PbI2 CQDs film.33 As we introduced above, compared with the solid-state ligand exchange (or LBL deposition) method, the PbSe CQDs film deposited by one-step deposition method would have perfect ligand exchanging and less organic residues, and this point of view can be verified by Fourier transform infrared absorption spectra (FT-IR) measurement. FT-IR absorption spectra of PbSe-OA, PbSe-TBAI and PbSe-PbI2 CQD solid films reveals quenching of carboxylate stretches of OA (two sets of peaks in the range of 1415−1463 cm−1 and 1515−1551 cm−1) for both of TBAI and PbI2 exchanged PbSe CQDs relative to OA-capped PbSe CQDs (Figure 1e). The result suggests that OA ligands were successfully removed from PbSe CQDs during solid-state and solution-phase ligand exchange process. However, weak peaks at 2853 and 2923 cm−1 which belong to the C−H bending vibration of the hydrocarbon chains (residual organics) have been detected in PbSe-TBAI CQD solid film, which confirms there are organic residues in PbSe-TBAI CQD solid film. In contrast, no obvious peaks in this region can be found for PbSe-PbI2 CQD solid film. In addition, Figure S1 shows X-ray diffraction (XRD) patterns of PbSe-PbI2 CQD solid film, peaks which belong to PbI2 cannot

has less organic residues, meanwhile no excess PbI2 was introduced into the film during the solution-phase ligand exchange process. Compared with the one-step deposition method, repetitive solution treatment in the LBL deposition process will induce a new trap in the CQDs absorber layer.34,35 To verify the degree of trap density in PbSe-TBAI and PbSe-PbI2 CQD solid films, we investigate the Urbach energies in these two CQD solid films (as shown in the inset of Figure 1f).36−38 Fitting the linear exponential part of absorption edge (Urbach tail) according to the Urbach’s rule,39,40 the Urbach energy value can be estimated, which represents the localized tail states of PbSe CQD solid films and is closely correlated to the degree of trap density in PbSe-TBAI and PbSe-PbI2 CQD solid films.37,38 The sharper band tailing is detected for PbSe-PbI2 CQD solid film, which exhibit lower Urach energy value of 45 meV. While for PbSe-TBAI CQD solid film, Urach energy value is around 58 meV. This result indicates PbSe-PbI2 CQD solid film has lower trap density compared to PbSe-TBAI CQD solid film. For the light absorber layer of solar cells, a lower trap density is critical to achieve a high open-circuit voltage (Voc).24,34,36,37,41 Less organic residues and low trap density in PbSe-PbI2 CQD solid film which deposited by the one-step deposition method may have effects on the charge dissociation and energy funneling in the film.24,42 We substantiate this consideration by measuring the charge transfer rate between CQDs (i.e., exciton dissociation rate) and spectral distribution of charge carriers for PbSe-TBAI and PbSe-PbI2 CQD solid films using an ultrafast transient absorption (TA) spectroscopy. In order to avoid the presence of Auger recombination process, a pump fluence of 6 μJ/cm2 is applied here (as shown in Figure S2). In Figure 2a, for both of PbSe-TBAI and PbSe-PbI2 CQD solid films, the TA signals can be well fitted by single exponential decay with a faster decay process which reflects the charge transfer behavior between CQDs−CQDs (as shown in Table S1).43 The values of decay time constant τct in PbSe-TBAI and PbSe-PbI2 CQD solid films are about 184 ± 3 ps and 161 ± 4 ps, respectively. Also, the corresponding charge transfer rates kct = 1/τct of PbSe-TBAI and PbSe-PbI2 CQD solid films are 5.4 × 109 and 6.2 × 109 s−1, respectively. It means that charge transfer of the PbSe-PbI2 CQD solid film is slightly faster than that of the PbSe-TBAI CQD solid film. The faster charge transfer rate of the PbSe-PbI2 CQD solid film is induced by the unobstructed charge transfer channel (no organic and PbI2 residues) and enhanced electronic tunneling effect of the PbSePbI2 CQD solid film, which is beneficial for achieving higher short-circuit photocurrent of CQDSCs. The weights of charge transfer process (A1) in both PbSe-TBAI and PbSe-PbI2 CQD solid films are about 85.5%, respectively. It means that charge transfer efficiency for two type films are similar, and about 85.5% photoexcited charges are transferred before recombination. In addition, we also investigate the effect of trap density on energy funneling in PbSe CQDs solid films. Figure 2b,c shows the spectro-temporal TA maps for PbSe-TBAI and PbSe-PbI2 CQD solid films. The bleach peak of both PbSeTBAI and PbSe-PbI2 CQD solid films are red shifted with time, which demonstrates that charge carriers are funneling to the lowest energy sites,24 and a smaller red shift of the bleach peak (about 20 meV) for the PbSe-PbI2 CQD solid film is observed comparing with PbSe-TBAI CQD solid film (about 38 meV). Larger redshift of bleach peak relates to a higher trap density with energy funneling toward undesired band tailing

Figure 3. (a) Current density−voltage (J−V) curves of PbSe CQDSCs based on PbSe-TBAI and PbSe-PbI2 films as absorber layers. (b) IPCE spectrum and integrated current density of PbSe CQDSCs.

be detected, indicating no PbI2 residues in PbSe-PbI2 CQD solid film. The FT-IR and XRD results jointly indicate that compared with PbSe-TBAI CQD solid film which was fabricated by the LBL deposition method, PbSe-PbI2 CQD solid film which was deposited by one-step deposition method 3600

DOI: 10.1021/acs.jpclett.8b01514 J. Phys. Chem. Lett. 2018, 9, 3598−3603

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Figure 4. (a) Recombination resistance (Rrec) of CQDSCs with PbSe-TBAI and PbSe-PbI2 CQDs as absorber layer extracted from impedance spectroscopy under various bias voltage. (b) Air stability evaluation of PbSe-TBAI and PbSe-PbI2 CQDs-based CQDSCs. The devices were stored and measured at 20 °C with the indoor relative humidity of 25−60%. (c) Continuous illumination of the PbSe-PbI2 CQDs-based CQDSCs device which was measured at 20 °C with the indoor relative humidity of 48% under AM 1.5 G for 240 min.

states.24 The reduced energy funneling in PbSe-PbI2 CQD solid film corroborates a reduced undesired band tailing states and flatter energy landscape, which benefits for CQDSCs reducing Voc loss. The above discussion of charge transport and trap density in PbSe CQDs absorber layer are very helpful for thorough understanding of the performance of PbSe CQDSCs. A device structure of FTO/TiO2/PbSe/Au was employed, and we have measured 12 devices for each type of the CQDSCs based on PbSe-TBAI and PbSe-PbI2 CQDs. The average values of the photovoltaic performance parameters such as short-circuit current density (Jsc), Voc, fill factor (FF), and PCE are shown in Table 1. Figure 2a shows current density−voltage (J−V) curves of PbSe CQDSCs with PbSe-TBAI and PbSe-PbI2 films as the absorber layer (thickness of the PbSe CQDs layer in a typical device was measured by scanning electron microscopy (SEM) is about 230 nm, as shown in Figure S3a,b) under AM1.5G 100 mW/cm2. The champion device was fabricated by PbSe-PbI2 CQDs, and it achieved a PCE of 6.0% (active area is 0.35 cm2, the photograph of the device is shown in Figure S3c), with a Voc of 0.616 V (the highest Voc of PbSe CQDSCs reported to date), a Jsc of 21.2 mA cm−2, and a FF of 46%. In addition, no hysteresis was observed in the devices (see Figure S4 and Table S2). Compared to devices fabricated with PbSe-TBAI CQDs (PCE of 4.3%), all performance parameters of CQDSCs were improved. As we discussed above, the larger Voc of PbSe-PbI2 CQDSCs maybe ascribed to the flatter energy landscape and reduced tailing states in the PbSe-PbI2 CQDs absorber layer, which leads to reduced Voc loss. Moreover, we also evaluate the diode ideality factor (n) of two types CQDSCs from a slope in Voc plotted against the logarithm of Jsc (Figure S5). n can reveal the photocarrier recombination process in a solar cell. If the carrier recombination process is determined by band-to-band recombination, the value of n should be close to 1. In contrast, if the recombination process is dominated by trap-assisted recombination, the value of n is theoretically greater than unity (1 < n < 2).44 The estimated values of n are 1.87 and 1.35 for PbSe-TBAI and PbSe-PbI2-based CQDSCs, respectively. The smaller ideality factor for PbSe-PbI2-based CQDSCs implies that trap-assisted recombination in the PbSe-PbI2 CQDs layer is lower than that in PbSe-TBAI CQDs layer (agreement with Urbach energy and TA results), which mainly benefit from the perfect surface ligand exchange and passivation by PbI2 in the solution-phase ligand exchange process. Traps can capture the photoexcited carriers and work as recombination centers, which will greatly decrease the charge collection efficiency as

well as Jsc of solar cells.21 Just as we discussed above, the charge transfer rate for the PbSe-PbI2 CQD film is faster than that of the PbSe-TBAI CQD film, and the optical absorption spectra are almost the same for two kinds of devices (Figure S6). Thus, the larger Jsc of PbSe-PbI2 CQDs-based CQDSCs (21.2 mA/ cm2) compared to that of PbSe-TBAI CQDs-based CQDSCs (19.4 mA/cm2) should result from the faster charge transfer rate, lower trap density, and higher charge collection efficiency. As shown in Figure 3b, the calculated Jsc from the incident photon to current conversion efficiency (IPCE) spectra were in a good agreement with those from the J−V measurements. Impedance spectroscopy was used to reveal charge carrier recombination in two kinds of PbSe CQDSCs.13,45,46 Recombination resistance (Rrec) of two types of devices was extracted by fitting their impedance spectra using an equivalent circuit plotted in Figure S7c. The low frequency resistance (see Figure S7) is connected with the Rrec of the system, permitting a qualitative analysis of the charge carrier recombination process in the device.13 Figure 4a shows the Rrec extracted for CQDSCs with PbSe-TBAI and PbSe-PbI2 CQDs. Larger Rrec confirms the lower trap assisted recombination in the PbSePbI2-based CQDSCs. In addition to PCE, the stability is also an important criterion for the comprehensive evaluation of CQDSCs. The PbSe CQDSCs were stored in air while their performances were measured over time. In Figure 4b, both PbSe-TBAI and PbSe-PbI2 CQDs-based CQDSCs exhibit excellent long-term air stability for over 145 days (about 3500 h), and all of the photovoltaic parameters increased at the first 6 days. The PCE of PbSe-PbI2-based device retained about 93% of its maximum value even after 139 days, from 6.0 to 5.6%. In addition, PbSe-PbI2-based CQDSCs also exhibit excellent light soaking stability, with >99% of the initial PCE remained after being continuously illuminated under AM 1.5 G for 240 min (see Figure 4c, Figure S8, and Table S3). In summary, we have employed a modified one-step hot injection method to synthesize PbSe CQDs and deposited an absorber layer of PbSe CQDSCs using PbSe-PbI2 CQD inks in one step. Compared with the LBL method, the one-step deposition method allows efficient surface passivation of CQD solids. Less organic residues and lower trap-assisted recombination in PbSe-PbI2 CQD solid film improve charge transport and reduce energy funneling. With these benefits, we fabricated solar cells based on PbSe-PbI2 CQDs and achieved a champion device with a PCE of 6.0% and a Voc of 0.616 V, which is the highest Voc among PbSe CQDSCs and the highest PCE for large-area PbSe CQDSCs reported to date. The air and light soaking stability for the solar cells were also evaluated, which 3601

DOI: 10.1021/acs.jpclett.8b01514 J. Phys. Chem. Lett. 2018, 9, 3598−3603

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The Journal of Physical Chemistry Letters showed excellent air and light soaking stability. This first largearea application for PbSe CQDSCs with high performance paves the way to their future marketization. Especially, the path for improving the Voc would shed light on the development of CQDSCs and other PbSe CQD-based devices, such as the field-effect transistor (FET).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b01514. Experimental details, XRD patterns, transient absorption response, SEM images and photograph of CQDSCs, J− V curve and data measured by forward and reverse scan, light intensity dependence of Voc, impedance spectra, and light soaking stability measurement (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yaohong Zhang: 0000-0001-7789-4784 Yong Zhou: 0000-0002-9480-2586 Taro Toyoda: 0000-0002-2067-3689 Qing Shen: 0000-0001-8359-3275 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Japan Science and Technology Agency (JST) CREST Program, the Beijing Advanced Innovation Center for Future City Design Program, and the MEXT KAKENHI Grant (Grants 26286013, 17H02736).

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REFERENCES

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DOI: 10.1021/acs.jpclett.8b01514 J. Phys. Chem. Lett. 2018, 9, 3598−3603

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DOI: 10.1021/acs.jpclett.8b01514 J. Phys. Chem. Lett. 2018, 9, 3598−3603