CdTe Nanocrystals

Letter www.acsami.org

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 S Supporting Information *

ABSTRACT: Aqueous-processed nanocrystal solar cells have attracted increasing attention due to the advantage of its environmentally friendly nature, which provides 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 environmentally 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 results 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

C

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 ntype and low conduction band (CB) result in inferior electronblocking 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

olloidal nanocrystals (NCs), prized for their size-tunable bandgap,1 good stability,2−4 and solution processabil5−10 have generated great interest for their applications in ity, optoelectronic devices including photodetectors,11 light-emitting diodes,12−14 lasers,15 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 environmentally 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 NC surface passivation and crystal growth have helped increase PCE to be around 5% recently.35,36 © 2017 American Chemical Society

Received: July 8, 2017 Accepted: September 6, 2017 Published: September 6, 2017 31345

DOI: 10.1021/acsami.7b09901 ACS Appl. Mater. Interfaces 2017, 9, 31345−31351

Letter

ACS Applied Materials & Interfaces

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 (c) CdTe films and (d) CdTe/P3KT bilayer. The RMS roughness of the two samples are 6.77 (without P3KT) and 6.47 nm (with P3KT), respectively.

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 (b) external quantum efficiency and (c) internal quantum efficiency curves. (d) Photocurrent density as a function of the effective voltage of the CdTe NC solar cells and CdTe/P3KT PHSCs.

should be explored for the development of aqueous-processed CdTe NC solar cells. In this work, we develop an environmentally 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 31346


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ACS Applied Materials & Interfaces Table 1. Photovoltaic Performance of the Devices, under the Illumination of AM1.5G, 100 mW cm−2a samples w/o P3KT-240 nm with P3KT-240 nm with P3KT-320 nm with P3KT-400 nm a

Voc (V) 0.51 0.55 0.54 0.51

(0.50) (0.53) (0.53) (0.49)

Jsc (mA cm−2) 17.3 18.3 19.5 19.3

(17.0) (17.9) (19.0) (18.9)

0.54 0.55 0.56 0.55

FF

PCE (%)

(0.53) (0.55) (0.55) (0.54)

4.8 5.5 5.9 5.4

(4.5) (5.2) (5.5) (5.0)

integrated Jsc (mA cm−2) 16.1 17.1 19.5 18.6

The parameters in the brackets are the average values calculated from 6 devices. The integrated Jsc is calculated from the EQE spectra.

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 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. The 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 Figure 1c, 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 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, we first conducted the external quantum efficiency (EQE) measurements and presented then 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 to 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, 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 P3KTinduced 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 shortcircuit and maximum power output conditions, which is in accord with the improved Jsc and FF for PHSCs as presented in Table 1. 31347


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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 of C2− with bias voltage (V). The relationship between the capacitance (C), V, and Vbi is expressed by the Mott−Schottky equation

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 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

C −2 =

2(Vbi − V ) A2 eεε0NA

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

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.

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

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. 31348


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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.

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 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 environmentally friendly and efficient method to promote the performance of aqueousprocessed 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.

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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/acsami.7b09901. 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 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

METHODS

Fenghong Li: 0000-0002-5707-7629 Hao Zhang: 0000-0002-2373-1100 Bai Yang: 0000-0002-3873-075X

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 N2saturated 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. Afterward, 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 μL 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 prepatterned 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 glovebox. And then, the P3KT-covered CdTe was annealed at 300 °C for 5 min 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 1 × 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 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

Author Contributions †

Q.Z. and L.H. contributed equally to this work. All authors contributed to write the manuscript and have given approval to the final version. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (973 Program) under Grant 2014CB643503, JLU Science and Technology Innovative Research Team 2017TD-06, and the National Science Foundation of China (NSFC) under Grant 51433003.

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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, opencircuit voltage; IQE, internal quantum efficiency; Vef f, effective voltage; Jsc, short circuit current density; Vbi, built-in potential; EQE, external quantum efficiency

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REFERENCES

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