CdTe Nanocrystals

Subscriber access provided by UNIVERSITY OF ADELAIDE LIBRARIES

Letter

High-Efficiency Aqueous-Processed Polymer/CdTe Nanocrystals Planar Heterojunction Solar Cells with Optimized Band Alignment and Reduced Interfacial Charge Recombination Qingsen Zeng, Lu Hu, Jian Cui, Tanglue Feng, Xiaohang Du, Gan Jin, Fangyuan Liu, Tianjiao Ji, Fenghong Li, Hao Zhang, and Bai Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09901 • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 7, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

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

ACS Applied Materials & Interfaces

High-Efficiency Aqueous-Processed Polymer/CdTe Nanocrystals Planar Heterojunction Solar Cells with Optimized Band Alignment and Reduced Interfacial Charge Recombination Qingsen Zeng,‡ Lu Hu,‡ Jian Cui, Tanglue Feng, Xiaohang Du, Gan Jin, Fangyuan Liu, Tianjiao Ji, Fenghong Li, Hao Zhang and Bai Yang* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, 130012, P. R. China. *

Address correspondence to [email protected]

ABSTRACT Aqueous-processed nanocrystal solar cells have attracted increasing attentions due to the advantage of environment-friendly nature, which provide a promising approach for large-scale production. The urgent affair is boosting the power conversion efficiency (PCE) for further commercial applications. The low PCE is mainly attributed to the imperfect device structure, which leads to abundant nonradiative recombination at the interfaces. In this work, an environment-friendly and efficient method is developed to improve the performance of aqueous-processed CdTe nanocrystal solar cells. Polymer/CdTe planar heterojunction solar cells (PHSCs) with optimized band alignment are constructed, which result in reduced interfacial charge recombination, enhanced carrier collection efficiency and built-in field. Finally, a champion PCE of 5.9%, which is a record for aqueous-processed solar cells based on CdTe nanocrystals, is achieved after optimizing the photovoltaic device. KEYWORDS: aqueous-processed, planar heterojunction solar cells, band alignment, charge recombination, polymer/CdTe

1

ACS Paragon Plus Environment


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

Colloidal nanocrystals (NCs), prized for their size-tunable bandgap,1 good stability2-4 and solution processability,5-10 have generated great interest for their applications in optoelectronic devices including photodetectors,11 light-emitting diodes,12-14 lasers15 and solar cells.16-26 As a result of rapid progress in both surface passivation and device architecture, a certified PCE exceeding 11% has been achieved based on PbS NCs.27 Recently, aqueous-processed CdTe NC photovoltaic devices have attracted attention increasingly owing to the whole environment-friendly processes, including synthesis of NCs and device fabrication, without using volatile toxic organic solvents,28-33 which provides a promising approach for large scale production. Insulating organic ligands, solubilizing the NCs originally, hamper the charge transport through the films. Our group demonstrated that thermal treatment of CdTe NCs could effectively remove the mercaptoethylamine (MA) ligands.34 In addition, the sulfhydryl groups on the ligands could react with Cd ions during annealing process to form CdS shells around the CdTe NCs. The formation of CdTe-CdS bulk heterojunction is extremely beneficial for charge dissociation and transportation, leading to a PCE near 4%.34 Great advances in CdTe NCs surface passivation and crystal growth have helped increase PCE to be around 5% recently.35-36 In general, aqueous-processed NC photovoltaic devices consist of a photoactive layer between an electron transfer layer (ETL), typically TiO2 or SnOx,34, 37 and a hole transfer layer (HTL), usually MoO3-x.34-36 MoO3-x is widely applied in photoelectronic devices due to its low toxicity and cost effectiveness. Solar cells with MoO3-x as HTL have realized great fill factors and efficiencies,34, 38 which is mainly due to the fast hole extraction rate of MoO3-x.34 However, the nature of n-type and low conduction 2


Page 2 of 24

Page 3 of 24

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


band (CB) result in inferior electron-blocking capability of MoO3-x,38-39 leading to abundant nonradiative recombination at the interface and thus limited charge carrier collection efficiency, which dramatically reduce device photocurrent output as well as the open-circuit voltage (Voc). More recently, p-doping spiro-OMeTAD with high CB was employed to replace MoO3-x as HTL,40 hence improved device performance and reduced interfacial charge recombination were obtained. Nevertheless, volatile toxic organic solvent (chlorobenzene) is used during the spin-coating process of spiro-OMeTAD, which is incompatible with the all-environment-friendly processes of aqueous-processed CdTe solar cells. Therefore, eco-friendly high-performance HTL materials should be explored for the development of aqueous-processed CdTe NC solar cells. In this work, we develop an environment-friendly and efficient method to promote the performance of aqueous-processed CdTe NC photovoltaic devices. The commercially

available

polyelectrolytes,

poly-[3-(potassium-6-hexanoate)

thiophene-2, 5-diyl] (P3KT), are used as HTL in the improved photovoltaic devices. The choice of P3KT polyelectrolytes based on these two facts: first, they possess suitable energy levels matched well with CdTe, which can efficiently collect hole carriers and block electrons at the same time; in addition, the S atoms on the polymer backbone and the carboxy groups on the side chain can coordinate with the CdTe NCs,41,42 which can not only reduce the interfacial dangling bonds of CdTe NCs, but facilitate to hole transfer and collection. We discover that the incorporation of P3KT at the CdTe/MoO3-x interface results in enhanced PCE and carrier collection efficiency. The introduction of P3KT forms optimized band alignment at the CdTe/HTL contact, and thus considerably reduces interface recombination. Finally, a 3



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

Page 4 of 24

champion PCE of 5.9%, which is a record for aqueous-processed photovoltaic devices based on CdTe NCs, is achieved after optimizing the photovoltaic device. The prototypical device architecture of the best-performing aqueous-processed CdTe/P3KT PHSCs is shown in Figure 1a. Low temperature prepared SnO2, which is compatible with sintered CdTe NC solar cells and has faster electron extraction rate than TiO2,37 is selected as ETL. The photoactive CdTe solid is deposited via a layer-by-layer procedure. The CdTe NCs are perfectly passivated by chloride ions, which also act as sintering promoters, during the synthetic process.36 Prior to P3KT deposition, a simple annealing process is performed to remove the ligands and promote crystal growth. A gold electrode with MoO3-x is deposited on top to collect photogenerated holes. Ultraviolet photoelectron spectroscopy (UPS) was conducted to measure the energy levels of P3KT. UPS result of P3KT is presented in Figure S1. The bandgap of P3KT is 1.89 eV determined from the absorption spectrum (Figure S2). Therefore, we derive the valence band (VB) and the CB values for P3KT as -5.15 and -3.26 eV, respectively. The energy levels of P3KT match well with that of CdTe. The

band

alignment

demonstrates

that

P3KT

acts

as

a

hole-extraction/electron-blocking layer between the CdTe film and the anode (Figure 1b). The optimal energy level alignment will provide a driving force for the hole transfer and prevent electron from flowing in the opposite direction, thus leading to enhanced charge collection and reduced interfacial carrier recombination. To understand the effect of P3KT on the surface morphology of photoactive layer, atomic force microscope (AFM) images were captured as shown in Figure1c and d. The surface of layer-by-layer deposited CdTe film shows uniform, tightly packed structures, without any large-scale pinholes that can lead to device shorting. A 4


Page 5 of 24

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


smoother surface is observed after polymer covering due to the reduced root mean square (RMS) roughness from 6.77 to 6.47 nm. The smoother surface can enhance the physical contact between the CdTe/P3KT layer and HTL, leading to decreased interfacial contact resistance, which benefits for the improvement of fill factor (FF) values.43 The current density-voltage (J-V) curves are exhibited in Figure 2a, and the detailed photovoltaic parameters are summarized in Table 1. Solar cells with P3KT exhibit considerably better performance than the control group for all metrics: The Voc increases from 0.51 to 0.55 V with the incorporation of P3KT, the short-circuit current density (Jsc) is improved from 17.3 to 18.3 mA cm-2, and the FF increases from 0.54 to 0.55. Correspondingly, a higher PCE (5.5%) is achieved for the solar cells with P3KT compared with that without P3KT (4.8%). To find the origins of the performance improvement, the external quantum efficiency (EQE) measurements were first conducted and presented in Figure 2b. The EQE of the device with P3KT is enhanced throughout the whole spectrum, especially showing an average increase of ~7% between 390 and 630 nm, which is consistent with the improved Jsc. As we know, the EQE is the combination of the absorption and internal quantum efficiency (IQE) which is closely related with charge collection efficiencies. Because only a thin P3KT layer (about 3 nm) is incorporated at the interface, the contribution of P3KT to light absorption is negligible (Figure S3). We consequently attribute the enhanced EQE to the improved charge extraction efficiencies. To confirm this, the IQE curves were achieved from the specific value between the EQE (Figure 2b) and reflected absorption (Figure S3). As shown in Figure 2c, the IQE spectrum shows clear improvement after introducing P3KT, 5



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

especially a broad band conversion efficiency exceeding 70% at wavelengths between 400 and 820 nm. On the basis of absorption and IQE results, we conclude that the EQE enhancement mainly stems from P3KT-induced charge collection enhancement. To gain more insight, we exhibits the dependence of the photocurrent density (Jph) on the effective voltage (Veff) for the devices with and without P3KT in Figure 2d. Here Jph=JL - JD (JL is the current densities under illumination and JD is the current densities in the dark) and Veff = V0 - V (V0 and V are the voltage at Jph = 0 and applied voltage, respectively). Generally, it is considered that, at high Veff (3.5 V in this case), photocurrent is saturated without recombination (Jsat).44 The Jsat can be read from Figure 2d, and it is only limited by the absorbed incident photon flux and independent of the temperature and bias.44 Thus, it is rational to deduced that the maximum exciton generation rate (Gmax, given by Jsat = eGmaxL, where L is the thickness of the photoactive layer) is similar for the two devices due to the semblable absorption. The Jph/Jsat ratio is the product of exciton dissociation and charge collection probabilities.44 The control devices have a Jph/Jsat ratio of 88.9%, under short-circuit conditions, while a higher Jph/Jsat ratio (90.3%) of the CdTe/P3KT PHSCs is obtained. PHSCs also present a higher Jph/Jsat ratio (71.1%) than that of the control devices (68.4%) under maximum power output conditions. This indicates that PHSCs exhibit enhanced charge extraction and collection under both short-circuit and maximum power output conditions, which is in accord with the improved Jsc and FF for PHSCs as presented in Table 1. The enhanced performance of the devices with P3KT comes from the optimization of charge collection. Electrochemical impedance spectra (EIS) were conducted to further analyze the charge dynamics. From the EIS analysis, we can monitor the 6


Page 6 of 24

Page 7 of 24

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


detailed electrical properties of the interfaces, which cannot be observed by direct current measurements. The EIS were recorded at the bias of 0 V under dark condition. Only one semicircle presents on the Nyquist plot (Figure 3a). According to previous work, the semicircle in each Nyquist plot comprises two highly overlapping arcs, which represents the charge transportation and transfer processes.23,36 In this circumstances, an equivalent circuit including a single charge-transfer recombination resistance (Rre) component is applied to analyze the carrier dynamics of CdTe NC photovoltaic devices (Figure 3a, inset). As exhibited in Figure 3a, the Rre values are determined by the diameter of EIS semicircles, and it increases obviously after incorporating P3KT. The higher Rre value corresponds to the reduced recombination loss. The carrier recombination happens not only at the electron donor–acceptor interface but also during their jump-tunneling process to the electrodes.36 In this study, the main difference is at the hole transfer interfaces. Therefore, the carrier recombination is obviously suppressed at the CdTe/HTL contact for the CdTe/P3KT PHSCs, which agrees well with the foregoing results that PHSCs exhibit improved charge collection. The Voc of aqueous-processed CdTe NC solar cells is determined by the built-in potential (Vbi) generated from the p-n junction between p-type CdTe and n-type TiO2.40 Once n-type MoO3-x contacts with CdTe, a small potential in the opposite direction with Vbi is built, thus leading to decreased Voc. The incorporation of P3KT can block the contact between CdTe and MoO3-x partially, thus, P3KT/CdTe PHSCs presents higher Voc together with higher Vbi. Capacitance-voltage (C−V) analysis was carried out to measure the built-in field in the solar cells. Figure 3b exhibits the plots

7



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

of C-2 with bias voltage (V). The relationship between the capacitance (C), V, and Vbi is expressed by the Mott−Schottky equation:

C -2=

2( Vbi − V ) 2 A eεε 0 NA

where A is the active area, e the elementary charge, ε the relative dielectric constant (10.6 for CdTe), ε0 the permittivity of vacuum, and NA is the net acceptor concentration in active layer. The value of Vbi is read from the intercept of the fitted line with the horizontal axis.40 From the diagram, a higher Vbi of P3KT/CdTe PHSCs is observed than that of the devices without P3KT, which consists with the Voc results of the J-V curves in Figure 2a. Based on the aforesaid results, the incorporation of P3KT at the CdTe/MoO3-x interface exhibits optimal band alignment, resulting in reduced interfacial charge recombination and enhanced built-in field. After further optimizing the thickness of the photoactive layer, a champion solar cell is achieved for the P3KT/CdTe PHSC with active layer of 320 nm, which displays a Voc of 0.54 V, FF of 0.56 and quite high Jsc of 19.5 mA cm-2 (Figure 4a, Table 1). This Jsc is the maximum value for the reported all-aqueous-processed CdTe NC photovoltaic devices and even higher than the devices with spiro-OMeTAD as HTL, demonstrating that the modification of interface with P3KT facilitates the charge extraction. Therefore, a recorded PCE of 5.9% is achieved for the aqueous-processed CdTe NC photovoltaic devices. Figure 4b shows the corresponding EQE spectra of the PHSCs with the different thickness of CdTe layer. After increasing the thickness of CdTe NCs from 240 to 320 nm, the EQE spectrum shows obvious enhancement in long-wavelength regions as a result of higher absorption fraction. However, once the thickness of CdTe reaches 400 nm, the 8


Page 8 of 24

Page 9 of 24

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


EQE values in short-wavelength regions drastically decrease, suggesting that the thickness of photoactive layer has exceeded the carrier diffusion length. As shown in Table 1, all the integrating Jsc from EQE spectra is consistent with the Jsc from the J-V curves. In conclusion, we develop an environment-friendly and efficient method to promote the performance of aqueous-processed CdTe NC photovoltaic devices. The P3KT/CdTe PHSCs with optimized band alignment are constructed, which results in reduced interface recombination and thus enhanced carrier collection efficiency, and built-in field. A champion PCE of 5.9% is achieved after optimizing the photovoltaic device.

This

work

highlights

the

potential

to

achieve

more

efficient

aqueous-processed solar cells through heterojunction modification and careful optimization of the interfaces.

METHODS Preparation of CdTe nanocrystals: CdTe nanocrystals were prepared according to the previous reported recipe.36 0.42 mL of NaHTe (0.67 M) aqueous solution was injected into105 mL of N2-saturated MA (30 mM) and CdCl2 (12.5 mM) solution with the pH range of 5.70-5.74. The obtained solution was refluxed for 55 min to make the CdTe nanocrystals grow. Afterwards, the CdTe nanocrystals was precipitated by adding isopropanol and centrifuged. The precipitates were rinsed with ethanol twice and dried in a vacuum oven. Finally, the nanocrystals were dispersed in deionized water and the concentration was 140 mg mL-1. Preparation of P3KT solution: Typically, 1 mg/mL P3KT (Rieke Metals) solution was obtained by dissolving 3 mg of P3KT in 2.985 mL of deionized water with 15 uL 9



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

of ammonium hydroxide (AR, 25-28%). The function of ammonium hydroxide is facilitating the dissolution of P3KT. The mixed solution was stirred overnight to dissolve the polymer sufficiently. Device fabrication: The pre-patterned ITO-glasses were cleaned using detergent, deionized water, chloroform, acetone, and ethanol step by step. Next, the ITO-glass were coated with the SnO2-x precursors immediately after drying within N2 flow. The SnO2-x layer was prepared and treated according to the previous work.37 The photoactive CdTe NC layer was deposited by spin-coating the colloidal solution at a speed of 700 rpm for 60 s. The CdTe NC layer was annealed at 315 °C for 2 min. This procedure was repeated multiple (3-5) times to build up 240-400 nm thick films. After achieving ideal thickness, the whole photoactive layer was annealed at 375 °C for 10 min to promote CdTe crystal growth. The cooled CdTe NCs were coated with the P3KT solution immediately at a speed of 2000 rpm. The influence of the thickness of P3KT on photovoltaic performance was systematically studied, and the optimal thickness was around 2-3 nm. All the manufacturing processes were performed in air ambient except for the annealing process in the glove box. And then, the P3KT-covered CdTe was annealed at 300 °C for 5min to remove the residual water and ammonium hydroxide. Finally, 10 nm MoO3-x and 60 nm gold were evaporated with a mask at a pressure below 10-5 Torr. The overlap of the anode and cathode were the device areas to be 0.05 cm-2.

Characterization: AFM images were obtained by tapping mode with a Nanoscope IIIa scanning probe microscope from Digital Instruments. Capacitance–voltage (C-V) measurements of the photovoltaic devices were achieved by the CV module of 10


Page 10 of 24

Page 11 of 24

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


Agilent B1500A Semiconductor Device Analyzer. Devices were tested in air ambient. The solar simulators were Newport Oriel Sol3A solar simulators with xenon lamps. A calibrated Si reference solar cell was used to set the intensity of the lamp to 100 mW/cm2 AM1.5 conditions. The EQE spectra were measured by a lock-in amplifier (Enlitech, Taiwan) under short-circuit conditions. UPS measurements were conducted in a UPS system which was equipped with a VG Scienta R3000 analyzer. A monochromatized He Iα irradiation with energy of 21.22 eV is the light source for UPS measurements. The EIS spectra were conducted in a CHI 660E electrochemical workstation under dark conditions.

ASSOCIATED CONTENT Supporting Information Absorption spectrum, bandgap caculation and molecular structure of P3KT; Absorption of the CdTe photovoltaic devices and CdTe/P3KT PHSCs and UPS spectra of P3KT film.

AUTHOR INFORMATION Corresponding Author *Bai Yang, E-mail: [email protected]

Notes

The authors declare no competing financial interest. 11



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

Author Contributions ‡Q. Zeng and L. Hu contributed equally to this work. All authors contributes to write the manuscript and have given approval to the final version.

ACKNOWLEDGMENT This work was financially supported by the National Basic Research Program of China (973 Program) under Grant No. 2014CB643503, JLU Science and Technology Innovative Research Team 2017TD-06, and the National Science Foundation of China (NSFC) under Grant Nos. 51433003.

ABBREVIATIONS PCE, power conversion efficiency; HTL, hole transfer layer; NCs, nanocrystals; PHSCs, planar heterojunction solar cells; ETL, electron transfer layer; CB, conduction band; VB, valence band; FF, fill factor; J-V, current density-voltage; Voc, open-circuit voltage; IQE, internal quantum efficiency; Veff, effective voltage; Jsc, short circuit current density; Vbi, built-in potential; EQE, external quantum efficiency.

REFERENCES 1.

Zhou, J.; Pu, C.; Jiao, T.; Hou, X.; Peng, X. A Two-step Synthetic Strategy

toward Monodisperse Colloidal CdSe and CdSe/CdS Core/Shell Nanocrystals. J. Am. Chem. Soc. 2016, 138, 6475-6483.

12


Page 12 of 24

Page 13 of 24

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


2.

Chuang, C. H.; Brown, P. R.; Bulovic, V.; Bawendi, M. G. Improved

Performance and Stability in Quantum Dot Solar Cells through Band Alignment Engineering. Nat. Mater. 2014, 13, 796-801. 3.

Zhang, X.; Johansson, E. M. J. Reduction of Charge Recombination in PbS

Colloidal Quantum Dot Solar Cells at the Quantum Dot/ZnO Interface by Inserting a MgZnO Buffer Layer. J. Mater. Chem. A 2017, 5, 303-310. 4.

Bernechea, M.; Miller, N. C.; Xercavins, G.; So, D.; Stavrinadis, A.;

Konstantatos, G. Solution-processed Solar Cells Based on Environmentally Friendly AgBiS2 Nanocrystals. Nat. Photon. 2016, 10, 521-525. 5.

Jasieniak, J.; MacDonald, B. I.; Watkins, S. E.; Mulvaney, P. Solution-processed

Sintered Nanocrystal Solar Cells via Layer-by-layer Assembly. Nano Lett. 2011, 11, 2856-2864. 6.

Kurley, J. M.; Panthani, M. G.; Crisp, R. W.; Nanayakkara, S. U.; Pach, G. F.;

Reese, M. O.; Hudson, M. H.; Dolzhnikov, D. S.; Tanygin, V.; Luther, J. M.; Talapin, D. V. Transparent Ohmic Contacts for Solution-processed, Ultrathin CdTe Solar Cells. ACS Energy Lett. 2017, 2, 270-278. 7.

Crisp, R. W.; Panthani, M. G.; Rance, W. L.; Duenow, J. N.; Parilla, P. A.;

Callahan, R.; Dabney, M. S.; Berry, J. J.; Talapin, D. V.; Luther, J. M. Nanocrystal Grain Growth and Device Architectures for High-efficiency CdTe Ink-based Photovoltaics. ACS nano 2014, 8, 9063-9072. 8.

Hu, L.; Li, D.-B.; Gao, L.; Tan, H.; Chen, C.; Li, K.; Li, M.; Han, J.-B.; Song, H.;

Liu, H.; Tang, J. Graphene Doping Improved Device Performance of ZnMgO/PbS Colloidal Quantum Dot Photovoltaics. Adv. Funct. Mater. 2016, 26, 1899-1907.

13



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

9.

Chen, Z.; Du, X.; Zeng, Q.; Yang, B. Recent Development and Understanding of

Polymer–Nanocrystal Hybrid Solar Cells. Mater. Chem. Front. 2017, 1, 1502-1513. 10. Sun, Y.; Liu, Z.; Yuan, J.; Chen, J.; Zhou, Y.; Huang, X.; Ma, W. Polymer Selection toward Efficient Polymer/PbSe Planar Heterojunction Hybrid Solar Cells. Org. Electron. 2015, 24, 263-271. 11. García de Arquer, F. P.; Armin, A.; Meredith, P.; Sargent, E. H. Solution-processed Semiconductors for Next-generation Photodetectors. Nat. Rev. Mater. 2017, 2, 16100. 12. Oh, N.; Kim, B. H.; Cho, S.-Y.; Nam, S.; Rogers, S. P.; Jiang, Y.; Flanagan, J. C.; Zhai, Y.; Kim. J.-H.; Lee. J.; Yu, Y.; Cho, Y. K.; Hur, G.; Zhang, J.; Trefonas, P.; Rogers, J. A.; Shim, M. Double-heterojunction Nanorod Light-responsive LEDs for Display Applications. Science 2017, 355, 616-619. 13. Dai, X.; Zhang, Z.; Jin, Y.; Niu, Y.; Cao, H.; Liang, X.; Chen, L.; Wang, J.; Peng, X. Solution-processed, High-performance Light-emitting Diodes based on Quantum Dots. Nature 2014, 515, 96-99. 14. Colvin, V. L.; Schlamp, M. C.; Allvisatos, A. P. Light-emitting Diodes Made from Cadmium Selenide Nanocrystals and a Semiconducting Polymer. Nature 1994, 370, 354-357. 15. Xu, Y.; Chen, Q.; Zhang, C.; Wang, R.; Wu, H.; Zhang, X.; Xing, G.; Yu, W. W.; Wang, X.; Zhang, Y.; Xiao, M. Two-photon-pumped Perovskite Semiconductor Nanocrystal Lasers. J. Am. Chem. Soc. 2016, 138, 3761-3768. 16. Fu, W.; Wang, L.; Zhang, Y.; Ma, R.; Zuo, L.; Mai, J.; Lau, T. K.; Du, S.; Lu, X.; Shi, M.; Li, H.; Chen, H. Improving Polymer/Nanocrystal Hybrid Solar Cell

14


Page 14 of 24

Page 15 of 24

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


Performance via Tuning Ligand Orientation at CdSe Quantum Dot Surface. ACS Appl. Mater. Interfaces 2014, 6, 19154-60. 17. Martin, T.R.; Katahara, J. K.; Bucherl, C.N.; Krueger, B.W.; Hillhouse, H.W.; Luscombe, C.K. Nanoparticle Ligands and Pyrolized Graphitic Carbon in CZTSSe Photovoltaic Devices. Chem. Mater. 2016, 28, 135−145. 18. Jin, G.; Chen, Z.; Dong, C.; Cheng, Z.; Du, X.; Zeng, Q.; Liu, F.; Sun, H.; Zhang, H.; Yang, B. Aqueous-processed Insulating Polymer/Nanocrystal Hybrid Solar Cells. ACS Appl. Mater. Interfaces 2016, 8, 7101-7110. 19. Crisp, R. W.; Pach, G. F.; Kurley, J. M.; France, R. M.; Reese, M. O.; Nanayakkara, S. U.; MacLeod, B. A.; Talapin, D. V.; Beard, M. C.; Luther, J. M. Tandem Solar Cells from Solution-processed CdTe and PbS Quantum Dots Using a ZnTe-ZnO Tunnel Junction. Nano Lett. 2017, 17, 1020-1027. 20. MacDonald, B. I.; Martucci, A.; Rubanov, S.; Watkins, S. E.; Mulvaney, P.; Jasieniak, J. J. Layer-by-layer Assembly of Sintered CdSexTe1-x Nanocrystal Solar Cells. ACS Nano 2012, 6, 5995-6004. 21. Chen, H. C.; Lai, C. W.; Wu, I. C.; Pan, H. R.; Chen, I. W.; Peng, Y. K.; Liu, C. L.; Chen, C. H.; Chou, P. T. Enhanced Performance and Air Stability of 3.2% Hybrid Solar Cells: How the Functional Polymer and CdTe Nanostructure Boost the Solar Cell Efficiency. Adv. Mater. 2011, 23, 5451-5455. 22. Zhang, H.; Kurley, J. M.; Russell, J. C.; Jang, J.; Talapin, D. V. Solution-processed, Ultrathin Solar Cells from CdCl3--Capped CdTe Nanocrystals: the Multiple Roles of CdCl3- Ligands. J. Am. Chem. Soc. 2016, 138, 7464-7467. 23. Zeng, Q.; Chen, Z.; Zhao, Y.; Du, X.; Liu, F.; Jin, G.; Dong, F.; Zhang, H.; Yang, B. Aqueous-processed Inorganic Thin-film Solar Cells Based on CdSexTe1-x 15



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

Nanocrystals: the Impact of Composition on Photovoltaic Performance. ACS Appl. Mater. Interfaces 2015, 7, 23223-23230. 24. Cao, Y.; Stavrinadis, A.; Lasanta, T.; So, D.; Konstantatos, G. The Role of Surface Passivation for Efficient and Photostable PbS Quantum Dot Solar Cells. Nat. Energy 2016, 1, 16035. 25. Liu, Z.; Sun, Y.; Yuan, J.; Wei, H.; Huang, X.; Han, L.; Wang, W.; Wang, H.; Ma, W. High-efficiency Hybrid Solar Cells Based on Polymer/PbSxSe1-x Nanocrystals Benefiting from Vertical Phase Segregation. Adv. Mater. 2013, 25, 5772-5778. 26. Du, J.; Du, Z.; Hu, J. S.; Pan, Z.; Shen, Q.; Sun, J.; Long, D.; Dong, H.; Sun, L.; Zhong, X.; Wan, L. J., Zn-Cu-In-Se Quantum Dot Solar Cells with a Certified Power Conversion Efficiency of 11.6%. J. Am. Chem. Soc. 2016, 138, 4201-4209. 27. Liu, M.; Voznyy, O.; Sabatini, R.; Garcia de Arquer, F. P.; Munir, R.; Balawi, A. H.; Lan, X.; Fan, F.; Walters, G.; Kirmani, A. R.; Hoogland, S.; Laquai, F.; Amassian, A.; Sargent, E. H. Hybrid Organic-inorganic Inks Flatten the Energy Landscape in Colloidal Quantum Dot Solids. Nat. Mater. 2017, 16, 258-263. 28. Li, J.-H.; Li, Y.; Xu, J.-T.; Luscombe, C.K. Self-assembled Amphiphilic Block Copolymers/CdTe Nanocrystals for Efficient Aqueous-processed Hybrid Solar Cells. ACS Appl. Mater. Interfaces 2017, 9, 17942−17948. 29. Chen, Z.; Du, X.; Jin, G.; Zeng, Q.; Liu, F.; Yang, B. Unravelling the Working Junction of Aqueous-processed Polymer–nanocrystal Solar Cells towards Improved Performance. Phys. Chem. Chem. Phys. 2016,18, 15791-15797. 30. Zhao, Y.; Zeng, Q.; Liu, X.; Jiao, S.; Pang, G.; Du, X.; Zhang, K.; Yang, B. Highly Efficient Aqueous-processed Polymer/Nanocrystal Hybrid Solar Cells with an

16


Page 16 of 24

Page 17 of 24

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


Aqueous-processed TiO2 Electron Extraction Layer. J. Mater. Chem. A 2016, 4, 11738-11746. 31. Liu, F.; Chen, Z.; Du, X.; Zeng, Q.; Ji, T.; Cheng, Z.; Jin, G.; Yang, B. High Efficiency Aqueous-processed MEH-PPV/CdTe Hybrid Solar Cells with a PCE of 4.20%. J. Mater. Chem. A 2016, 4, 1105-1111. 32. Xue, H.; Wu, R.; Xia, Y.; Tan, Q.; Qin, D.; Wu, H.; Huang, W. Recent Progress on Solution-processed CdTe Nanocrystals Solar Cells. Appl. Sci. 2016, 6, 197. 33. Du, X.; Chen, Z.; Li, Z.; Hao, H.; Zeng, Q.; Dong, C.; Yang, B. Dip-coated Gold Nanoparticle Electrodes for Aqueous-solution-processed Large-area Solar Cells. Adv. Energy Mater. 2014, 4, 1400135. 34. Chen, Z.; Zhang, H.; Zeng, Q.; Wang, Y.; Xu, D.; Wang, L.; Wang, H.; Yang, B., In Situ Construction of Nanoscale CdTe-CdS Bulk Heterojunctions for Inorganic Nanocrystal Solar Cells. Adv. Energy Mater. 2014, 4, 1400235. 35. Chen, Z.; Liu, F.; Zeng, Q.; Cheng, Z.; Du, X.; Jin, G.; Zhang, H.; Yang, B. Efficient Aqueous-processed Hybrid Solar Cells from a Polymer with a Wide Bandgap. J. Mater. Chem. A 2015, 3, 10969-10975. 36. Zeng, Q.; Chen, Z.; Liu, F.; Jin, G.; Du, X.; Ji, T.; Zhao, Y.; Yue, Y.; Wang, H.; Meng, D.; Xie, T.; Zhang, H.; Yang, B. Aqueous-processed Polymer/Nanocrystals Hybrid Solar Cells: the Effects of Chlorine on the Synthesis of CdTe Nanocrystals, Crystal Growth, Defect Passivation, Photocarrier Dynamics, and Device Performance. Solar RRL 2017, 1, 1600020. 37. Du, X.; Zeng, Q.; Jin, G.; Liu, F.; Ji, T.; Yue, Y.; Yang, Y.; Zhang, H.; Yang, B. Constructing Post-permeation Method to Fabricate Polymer/Nanocrystals Hybrid Solar Cells with PCE Exceeding 6%. Small 2017, 13, 1603771. 17



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

Page 18 of 24

38. Gao, J.; Perkins, C. L.; Luther, J. M.; Hanna, M. C.; Chen, H. Y.; Semonin, O. E.; Nozik, A. J.; Ellingson, R. J.; Beard, M. C. n-Type Transition Metal Oxide as a Hole Extraction Layer in PbS Quantum Dot Solar Cells. Nano Lett. 2011, 11, 3263-3266. 39. Subbiah,

J.;

Kim,

D.

Y.;

Hartel,

M.;

So,

F.

MoO3/Poly(9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine) Double-Interlayer Effect on Polymer Solar Cells. Appl. Phys. Lett. 2010, 96, 063303. 40. Du, X.; Chen, Z.; Liu, F.; Zeng, Q.; Jin, G.; Li, F.; Yao, D.; Yang, B. Improvement in Open-circuit Voltage of Thin Film Solar Cells from Aqueous Nanocrystals by Interface Engineering. ACS Appl. Mater. Interfaces 2016, 8, 900-907. 41. Wei, H.; Fang, Y.; Yuan, Y.; Shen, L.; Huang, J. Trap Engineering of CdTe Nanoparticle for High Gain, Fast Response, and Low Noise P3HT:CdTe Nanocomposite Photodetectors. Adv. Mater. 2015, 27, 4975–4981. 42. Zhang, H.; Zhou, Z. Yang, B; Gao, M. The Influence of Carboxyl Groups on the Photoluminescence of Mercaptocarboxylic Acid-stabilized CdTe Nanoparticles. J. Phys. Chem. B 2003, 107, 8-13. 43. Lin, X.; Yang, Y.; Nian, L.; Su, H.; Ou, J.; Yuan, Z.; Xie, F.; Hong, W.; Yu, D.; Zhang, M.; Ma, Y.; Chen, X. Interfacial Modification Layers Based on Carbon Dots for Efficient Inverted Polymer Solar Cells Exceeding 10% Power Conversion Efficiency. Nano Energy 2016, 26, 216-223. 44. Chen, Y.; Wang, S.; Xue, L.; Zhang, Z.; Li, H.; Wu, L.; Wang, Y.; Li, F.; Zhang, F.; Li, Y. Insights Into the Working Mechanism of Cathode Interlayers in Polymer Solar Cells via [(C8H17)4N]4[SiW12O40]. J. Mater. Chem. A 2016, 4, 19189-19196.

18


Page 19 of 24

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


FIGURES and TABLES

Figure 1 (a) Device structure of CdTe/P3KT planar heterojunction solar cells. (b) Schematic energy level diagram. The energy levels of P3KT were deduced from UPS results. The energy levels of other materials were achieved from previous work (SnOx,37 CdTe,30 MoOx40). (c)-(d) AFM images (2 µm × 2 µm) of the CdTe films (c) and CdTe/P3KT bilayer (d). The RMS roughness of the two samples are 6.77 (without P3KT) and 6.47 nm (with P3KT), respectively.

19



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

Figure 2 (a) J-V characteristics of the aqueous-processed CdTe NC solar cells and CdTe/P3KT PHSCs. The PCE was measured under 100 mW cm-2 AM 1.5G illumination, which has been corrected by a calibrated Si solar cell. Corresponding external quantum efficiency (b) and internal quantum efficiency (c) curves. (d) Photocurrent density as a function of the effective voltage of the CdTe NC solar cells and CdTe/P3KT PHSCs.

20


Page 20 of 24

Page 21 of 24

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


Figure 3 (a) Electrochemical impedance spectra of the two cells characterized at the bias of 0 V in the dark with a frequency ranging from 1 Hz to 100 kHz. The inset is the equivalent electrical circuit diagram for analysis of charge dynamics. (b) Mott−Schottky capacitance curves of the two devices.

21



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

Figure 4 (a) J-V characteristics of the aqueous-processed CdTe/P3KT PHSCs with different thickness of CdTe NCs. (b) External quantum efficiency curves and integrated current density of the CdTe/P3KT PHSCs.

22


Page 22 of 24

Page 23 of 24

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


TABLE 1 Photovoltaic performance of the devices, under the illumination of AM1.5G, 100 mW cm-2. The parameters in the brackets are the average values calculated from 6 devices. The integrated Jsc is calculated from the EQE spectra. Samples

Voc (V)

Jsc (mA cm-2)

FF

PCE (%)

Integrated Jsc (mA cm-2)

W/o P3KT-240 nm

0.51 (0.50)

17.3 (17.0)

0.54 (0.53)

4.8 (4.5)

16.1

With P3KT-240 nm

0.55 (0.53)

18.3 (17.9)

0.55 (0.55)

5.5 (5.2)

17.1

With P3KT-320 nm

0.54 (0.53)

19.5 (19.0)

0.56 (0.55)

5.9 (5.5)

19.5

With P3KT-400 nm

0.51 (0.49)

19.3 (18.9)

0.55 (0.54)

5.4 (5.0)

18.6

23



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

ToC figure

24


Page 24 of 24

CdTe Nanocrystals

Recommend Documents