nanomaterials Article

Solvent Engineering for High-Performance PbS Quantum Dots Solar Cells Rongfang Wu 1,† , Yuehua Yang 1,† , Miaozi Li 3 , Donghuan Qin 1, * and Lintao Hou 2, * 1

2

3

* †

ID

, Yangdong Zhang 2

Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials & Devices, South China University of Technology, Guangzhou 510640, China; [email protected] (R.W.); [email protected] (Y.Y.) Siyuan Laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials, Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Department of Physics, Jinan University, Guangzhou 510632, China; [email protected] School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China; [email protected] Correspondence: [email protected] (D.Q.); [email protected] (L.H.); Tel.: +86-20-87114346 (D.Q.) These authors contribute equally.

Received: 3 July 2017; Accepted: 25 July 2017; Published: 28 July 2017

Abstract: PbS colloidal quantum dots (CQDs) solar cells have already demonstrated very impressive advances in recent years due to the development of many different techniques to tailor the interface morphology and compactness in PbS CQDs thin film. Here, n-hexane, n-octane, n-heptane, isooctane and toluene or their hybrids are for the first time introduced as solvent for comparison of the dispersion of PbS CQDs. PbS CQDs solar cells with the configuration of PbS/TiO2 heterojunction are then fabricated by using different CQDs solution under ambient conditions. The performances of the PbS CQDs solar cells are found to be tuned by changing solvent and its content in the PbS CQDs solution. The best device could show a power conversion efficiency (PCE) of 7.64% under AM 1.5 G illumination at 100 mW cm−2 in a n-octane/isooctane (95%/5% v/v) hybrid solvent scheme, which shows a ~15% improvement compared to the control devices. These results offer important insight into the solvent engineering of high-performance PbS CQDs solar cells. Keywords: colloidal quantum dots; solar cells; PbS; solvent engineering

1. Introduction Colloidal quantum dots (CQDs) heterojunction solar cells that can be fabricated by simple solution-processing techniques are under intensive research due to their potential for low cost, air stable and large area efficient photovoltaic devices [1–10]. Among all kinds of CQDs, PbS CQDs allow bandgap tuning easily through the quantum size effect by changing the diameter of CQDs, which allows efficient light harvesting over a wide range of the visible/infrared wavelength in multiple junction solar cells formed by single material [11–14]. The most common and efficient PbS CQDs solar cells architecture consists of PbS CQDs active layer and TiO2 or ZnO electron acceptor layer to form depleted heterojunction. Comparing with the PbS CQDs solar cells in configuration of Schottky diode or normal device architecture, PbS/TiO2 -depleted device with the inverted structure has many merits. For example, the placement of the junction on the illuminated side reduces the recombination rate of minority carriers [15]. On the other hand, the band alignment of PbS CQDs and TiO2 will block holes transfer to TiO2 at the interface. Thanks to the development of materials and device processing techniques, the power conversion efficiency (PCE) of CQD solar cells has increased from 0.00004% [16] in 2005 to certified 10.6% [17] in 2016. Due to the large surface to volume ratio in CQDs Nanomaterials 2017, 7, 201; doi:10.3390/nano7080201

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materials, substantial unsaturated dangling bonds will create electronic trap states, which will promote the recombination of charge carriers that is detrimental to the CQDs solar cells performance [13]. To overcome this drawback, a series of ligands strategies including organic, organic–inorganic hybrid or atomic ligands have been developed to passivate trap states of CQDs and a dramatic improvement in device performance is obtained [18–21]. Band alignment between the PbS CQDs and TiO2 is also critical for favoring electron injection to the conduction band of TiO2 . For example, by doping TiO2 with Sb or Zr [22], great improvement of PCEs was obtained in CQD solar cells arising mainly from an increase in short-circuit current (Jsc ). Another way to improve the CQD solar cells performance is decreasing the recombination at the metal oxide-semiconductor interface. In this strategy, introducing passivation ligands [23–26] around CQD can increase the width of the depletion region and optimize the electron collecting efficiency. Improvements in PbS CQDs solar cells can further offer guidance on better understanding as well as eliminating the factors that affect device performance. As a high ratio of surface area to volume existed in PbS CQDs solar cells, reducing the number of recombination centers is a most important way for achieving high device performance [27–32]. In particular, the nature of the morphology and compactness of PbS CQDs thin film are key factors that determine solar cells performance. The evaporating rate, viscosity and dispersability of solvent will affect the aggregations and the compactness of CQDs film. In the previous research, however, few works have been focused on the solvent effects on the device performance. n-octane is widely used as a single solvent for the preparation of PbS CQDs solution. The boiling point of n-octane is as high as 125.7 ◦ C, so postponed annealing at ~50 ◦ C for up to 10 h must be carried out after the deposition of PbS CQDs film in order to remove the existed solvent. Most recently, Lan et al. [17] employed a co-solvent (toluene and dimethylformamide) to incorporate iodide on CQDs before device fabrication. They found that improved passivation was obtained, which resulted in a certified efficiency of 10.6%. Herein, for the sake of adjusting the evaporating rate and viscosity of n-octane, we selected n-hexane, n-heptane, isooctane, toluene, n-octane and their hybrid as solvent for the dispersion of PbS CQDs. As shown in Table 1, the boiling point for n-heptane, isooctane, toluene and n-hexane is 99, 98, 111 and 69 ◦ C, lower than that of n-octane (125.7 ◦ C). On the other hand, toluene has high polarity of 2.4 in comparison to other solvents used in this case. Devices with configuration of FTO (F doped SnO2 )/ZnO/TiO2 /PbS/Au are then fabricated by using these PbS CQDs solutions. We find that the performance of PbS CQDs solar cells is highly dependent on the content of solvent used. In single-solvent strategy, devices show high efficiency up to 6.0% in the case of toluene or n-octane solvent, while only 4~5% efficiency is obtained in the case of isooctane, hexane or heptanes solvent. It should be noted that, in the hybrid solvent scheme, the device performance can be further improved. For example, by adding less than 20% (v/v) isooctane into n-octane, the PCE is higher than that with pure n-octane and as high as 7.64% PCE is obtained in 5% isooctane/95% (v/v) n-octane mixture. Our research gives more insight into the solvent engineering for achieving high-performance PbS CQDs solar cells. Table 1. Summarized properties of different solvent. Solvent Name

Polarity

Viscosity (Pa·s)

Boiling Point (◦ C)

Absorption Wavelength (nm)

n-octane isooctane heptane n-hexane toluene

0.06 0.10 0.20 0.06 2.40

0.53 0.53 0.41 0.33 0.59

125 99 98 69 111

200 210 200 210 285

2. Experiment Procedure 2.1. Materials Zinc acetate hydrate, ethanol amine, 2-methoxyethanol, titanium butoxide, triethanolamine, mercaptopropionic acid, hexamethyldisilathiane (TMS) and octyl-phosphine acid (OPA) were

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purchased from Aladdin (Shanghai, China). Oleic acid (OA), lead oxide, oleic amine (OLA), 1-octadece (ODE) were purchased from Alfa Aesar (Beijing, China). All chemicals were used directly without any further purification. 2.2. Ti-Sols, Zn2+ Precursor and PbS CQDs Fabrication The preparation of Ti-sols was carried out under ambient condition, as described in the literature [24]. Zn2+ precursor was prepared by adding zinc acetate and ethanolamine into the solution of 2-methoxyethanol with a concentration of 0.5 mol L−1 . The resulting solution was refluxed at 60 ◦ C for 2 h under ambient conditions. The final Zn2+ precursor was filtered through 0.45 µm filter before use. PbS CQDs were prepared by a typical synthetic procedure on a literature method [32]. A typical synthetic procedure was given below: in a three-necked flask, 0.45 g PbO, 1.5 mL OA, and 16.5 mL ODE were added. The mixtures were heated to 120 ◦ C and pumped down for 6 h to remove any moisture and impurity. Then high purity (>99.99%) N2 was introduced to the flask; 0.20 mL TMS was mixed with 5 mL 1-octadece in a syringe and quickly injected into the mixture. The hotplate was removed away immediately after the injection and the mixture was cooled down to ~90 ◦ C. At the same time, hybrid ligands were prepared by mixing 0.72 mmol CdCl2 , 2 mL OLA, and 0.048 mmol OPA in a three-necked flask and degassed/refluxed at 90 ◦ C. The hybrid ligands were injected into PbS CQDs mixture at this temperature and the reaction was cooled down to room temperature. After washing and centrifugation, PbS CQDs were dissolved into different solvents or hybrid solvent with a concentration of 25 mg/mL. 2.3. Devices Fabrication PbS/TiO2 CQDs heterojunction solar cells with a triple-layer structure, containing ZnO window layer, transparent TiO2 layer and PbS CQDs light absorber layer, were prepared according to a procedure reported before [32]. Firstly, FTO glass was cleaned in deionized (DI) water and isopropanol solution for 10 min under ultrasonic treatment. Zn2+ precursor was then deposited on cleaned FTO substrate at 2500 rpm for 15 s and put on a hot plate at 300 ◦ C for 30 min to remove any organic solvent and impurity. One layer of Ti-sols was then spin-coated on the FTO/ZnO substrate at 2500 rpm for 15 s, following annealing at 500 ◦ C for 1 h. After cleaning and drying, several drops of PbS CQDs in different solvents were put onto the FTO/ZnO substrate and spin-casted at 3000 rpm for 15 s. 3-Mercaptopropionic acid (MPA) was used for ligands exchange, as described in the literature [31]. It should be noted that the one-step coating PbS CQDs layer thickness for different solvents are different due to the difference in solvent volatilizing rate. In order to obtain PbS CQDs film with similar thickness of ~200 nm, for PbS/n-octane solution, four layers of PbS CQDs should be deposited, while five layers of PbS CQDs for PbS/toluene solution, five layers for PbS/isooctane solution and five layers of PbS CQDs for PbS/heptane solution. 3. Results and Discussion The TEM image (see Figure 1a) suggests that the CQDs are highly crystalline and homogeneous with average diameter 4.5 nm. The absorption spectra of PbS CQDs in different organic solvent are shown in Figure 1b. The PbS CQDs are purified by twice precipitating with acetone and ethanol. Following this, the products are re-dispersed in n-octane, isooctane, n-heptane, n-hexane and toluene with the same concentration of 25 mg/mL. It is mentioned that PbS CQDs can be well dispersed in all the above solvents even with a concentration up to 80 mg/mL, as shown in the inset of Figure 1b. The initial absorption spectrum peak for all PbS CQDs samples is located at ~1080 nm with full width at half maximum of ~150 nm, which indicates that these kinds of solvents do not have any changes on the physical properties of PbS CQDs. It is also found that there are almost no changes in absorption spectrum for the PbS CQDs samples after storage for several weeks, which implies that the PbS CQDs samples fabricated by the above means are very stable. This phenomenon can be explained by the

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successfully hybrid passivation strategy adopted in this case (PbS CQDs is passivated by CdCl2 + OPA + OLA),Nanomaterials which is 2017, also7,confirmed by Sargent et al. [25]. x FOR PEER REVIEW 4 of 12

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Figure 1. (a) TEM image of as prepared PbS CQDs (b) UV-absorption spectra of PbS CQDs in different

Figure 1. (a) TEM image of as quantum prepared PbS CQDs (b) UV-absorption spectra of PbS CQDs in different solvents. CQD = colloidal dots. solvents. CQD = colloidal quantum dots. The device configuration of PbS CQDs solar cells are provided in Figure 2a. A 40 nm layer of Figure 1. (a) TEM image of as prepared PbS CQDs (b) UV-absorption spectra of PbS CQDs in different

ZnO thin film is deposited of onPbS FTO CQDs substrate by thermal decomposition of zinc acetate to make a The device configuration solvents. CQD = colloidal quantum dots. solar cells are provided in Figure 2a. A 40 nm layer of smooth and defects-free surface. Two layers of TiO 2 film (~100 nm) is then deposited on ZnO to create ZnO thin film is deposited on FTO substrate by thermal decomposition of zinc acetate to make a an electron acceptor layer. PbSofCQDs in different solvents are then spin-coated of ofthe The device configuration PbS CQDs solar cells are provided in Figure 2a. A deposited 40on nmtop layer smooth and defects-free surface. Two layers of TiO film (~100 nm) is then on ZnO to 2 FTO/ZnO/TiO 2 substrate. To eliminate the thickness effects on the solar cells performance, all PbS ZnO thin film is deposited on FTO substrate by thermal decomposition of zinc acetate to make a create an electron acceptor layer. PbS CQDs in different solvents are then spin-coated on top of the CQDs film are have similar thickness using thenm) same device processing technique. smooth andcontrolled defects-freetosurface. Two layers of TiOby 2 film (~100 is then deposited on ZnO to create A FTO/ZnO/TiO To eliminate theinthickness effectsisare on the solar cells typical cross-section SEM view of PbS CQDs solar cellssolvents device shown in Figure 2b.performance, The PbS 2 substrate. an electron acceptor layer. PbS CQDs different then spin-coated on top of CQDs the all PbS active layer is very compact and homogeneous with the thickness 200 nm. FTO/ZnO/TiO 2 substrate. To eliminate the thickness effects on the thearound solar performance, all PbStechnique. CQDs film are controlled to have similar thickness by using samecells device processing CQDs film are controlled to have similar thickness using the same device processing A typical cross-section SEM view of PbS CQDs solarby cells device is shown in Figuretechnique. 2b. TheAPbS CQDs typical cross-section SEM view of PbS CQDs solar cells device is shown in Figure 2b. The PbS CQDs active layeractive is very compact and homogeneous with the thickness around 200 nm. layer is very compact and homogeneous with the thickness around 200 nm.

Figure 2. Cont.

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Figure 2. (a) Schematic of FTO/ZnO/TiO2/PbS/Au device architecture (b) The cross-sectional SEM

Figure 2. (a)ofSchematic of FTO/ZnO/TiO 2 /PbS/Au device architecture (b) The cross-sectional SEM images a representative device. images of a representative device. Figure 2. (a) has Schematic FTO/ZnO/TiO 2/PbS/Au device architecture (b) The cross-sectional SEM The solvent a greatofeffect on thin film solar cells performance, which has been demonstrated images of a representative device. in previous work [32–38]. A compact, defects-free and smooth surface is crucial for high-performance The solvent has a great effect on thin film solar cells performance, which has been demonstrated PbS CQDs solar cells. The viscosity and evaporating rate of solvent will affect the aggregation of PbS in previous [32–38]. A compact, and smooth surfacewhich is crucial for high-performance Thework solvent has a great effect ondefects-free thin film solar cells performance, has been demonstrated CQDs on the TiO2, so the device performance of PbS CQDs solar cells can be tailored by using in previous work [32–38]. A compact, defects-free andrate smooth surface will is crucial forthe high-performance PbS CQDs solar cells. The viscosity and evaporating of solvent affect aggregation of PbS different solvents and their hybrid. The current density–voltage (J–V) curves for PbS CQDs solar cells PbS solar cells. The viscosity and evaporating rate of solvent will affect the aggregation of PbS CQDswith onCQDs the TiO , so the device performance of PbS CQDs solar cells can be tailored by using different 2 single solvent measured under AM 1.5 G are presented in Figure 3a, while Figure 3b presents CQDs ontheir the TiO 2, so the device performance of PbS CQDs solar cells can be tailored by using solvents and hybrid. The current (J–V) curves for PbS CQDs determined solar cells with the dark J–V curves for these devices. density–voltage The Jsc, Voc, fill factor (FF), PCE value of devices different solvents and their hybrid. The current density–voltage (J–V) curves for PbS CQDs solar cells singlefrom solvent measured under AMin 1.5Table G are presented Figure 3a, while 3b curve presents the dark J–V curves are summarized 2. The rectifierinratio calculated fromFigure dark J–V (Figure with single solvent measured under AM 1.5 G are presented in Figure 3a, while Figure 3b presents 3b) for for n-octane, n-heptane, and(FF), toluene 442, 1000, 188, 47 and from 251, J–V J–V curves these isooctane, devices. The Jsc , V ocn-hexane , fill factor PCEdevices value are of devices determined the dark J–V curves for these devices. The Jsc, Voc, fill factor (FF), PCE value of devices determined respectively. It is clear that 2. theThe device fabricated using n-hexane solvent lowest curves are summarized in Table rectifier ratio calculated from dark J–Vshows curvethe (Figure 3b) for from J–V curves are summarized in Table 2. The rectifier ratio calculated from dark J–V curve (Figure performance with J sc of 19.33 mA/cm2, Voc of 0.49 V, FF of 43.42 and PCE of 4.12%. On the other hand, n-octane, isooctane, n-heptane, n-hexane and toluene devices are 442, 1000, 188, 47 and 251, respectively. 3b) for n-octane, isooctane, n-heptane, n-hexane and toluene devices are 442, 1000, 188, 47 and 251, n-heptane and isooctane devices show similar device performance with PCE ~5%. The toluene and It is clear that theItdevice fabricated thesolvent lowestshows performance with Jsc respectively. is clear that the using devicen-hexane fabricatedsolvent using shows n-hexane the lowest n-octane devices show the highest performance. The Jsc, Voc, FF and PCE for typical n-octane device of 19.33 mA/cm2with , V2ocJscof FF of2, 43.42 andV,PCE 4.12%. otherOn hand, n-heptane performance of 0.49 19.33 V, mA/cm Voc of 0.49 FF ofof43.42 and On PCEthe of 4.12%. the other hand, and are 23.50 mA/cm , 0.56 V, 54.38 and 6.67%, respectively. It should be noted that the toluene device n-heptane andshow isooctane devices show similar device performance with PCE ~5%.and Then-octane toluene and isooctane devices similar device performance with PCE devices also shows high device performance with a PCE of 6.59% and a~5%. high The Voc oftoluene 0.56 V. To the best of our n-octane devices show the highest performance. The J sc , V oc , FF and PCE for typical n-octane device showknowledge, the highestthere performance. The on Jsc ,PbS Voc ,CQDs FF and PCE forfabricated typical n-octane 23.50 mA/cm2 , are no reports solar cells by usingdevice solventare with relative 2, 0.56 V, 54.38 and 6.67%, respectively. It should be noted that the toluene device are 23.50 mA/cm 0.56 V, 54.38 and 6.67%, It should be noted that the toluene device also shows high device high polarity such asrespectively. toluene. also shows high device performance with a PCE of 6.59% and a high Voc of 0.56 V. To the best of our performance with a PCE of 6.59% and a high Voc of 0.56 V. To the best of our knowledge, there are no knowledge, there are no reports on PbS CQDs solar cells fabricated by using solvent with relative 25 reports on PbS CQDs solar cells fabricated by using solvent with relative(a) high polarity such as toluene. high polarity such as toluene. Current(mA/cm Density2)(mA/cm2) Current Density

20 25 15

15 5 10 0 5 -5 0 -10 -0.2 -5 -10 -0.2

(a)

Octane Isooctane Heptane Hexane Octane Toluene Isooctane Heptane Hexane Toluene

20 10

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.3

0.4

0.5

0.6

Volatage (V) -0.1

0.0

0.1

0.2

Volatage (V)

Figure 3. Cont.

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1000

(b)

-2

Current Density (mA cm )

100 10 1 0.1 0.01

Octane Isooctane Heptane Hexane Toluene

1E-3 1E-4 1E-5 -3

-2

-1

0

1

2

3

Voltage (V)

Figure 3. J–V characteristics of PbS/TiO2 solar cells fabricated by using different single solvent (a)

Figure 3. J–V characteristics of PbS/TiO2 solar cells fabricated by using different single solvent (a) under light (b) under dark. under light (b) under dark. Table 2. Photovoltaic parameters obtained from the J–V curves (Figure 3a) for solar cells with different solvent. Tablesingle 2. Photovoltaic parameters obtained from the J–V curves (Figure 3a) for solar cells with different single solvent. Power Conversion

Solvent

Solvent n-octane isooctane heptane n-hexane toluene

Power Conversion Efficiency Efficiency (PCE) (%)(%) (PCE)

n-octane isooctane heptane n-hexane toluene

6.67 4.62 4.69 4.12 6.59

6.67 4.62 4.69 4.12 6.59

Jsc (mA cm−2)

Jsc (mA

cm−2 )

22.04 22.04 19.00 19.00 21.63 21.63 19.30 19.30 20.60 20.60

Voc (V)

Voc (V) 0.540.54 0.520.52 0.520.52 0.490.49 0.560.56

Fill Factor (FF) (%)

Fill Factor (FF) (%)

56.04 46.76 41.70 43.57 57.13

56.04 46.76 41.70 43.57 57.13

To achieve high device performance of PbS CQDs solar cells, a smooth, compact and

To achieve high device of PbS CQDs solarrecombination cells, a smooth, andand homogeneous homogeneous surface is performance necessary to decrease the carrier in compact the interface collect charges to the electrodes efficiently. To further investigate the properties PbS CQDs activeto the surface is necessary to decrease the carrier recombination ininterface the interface andofcollect charges layer, we probe the surface morphology of PbS CQDs samples by atomic force microscope (AFM). electrodes efficiently. To further investigate the interface properties of PbS CQDs active layer, we probe As described in the experiment section, the PbSby CQDs thinforce filmsmicroscope are prepared(AFM). by spin-coating PbS in the surface morphology of PbS CQDs samples atomic As described CQDs solution onto FTO/ZnO substrate. All samples AFM measurement consists of two layers of the experiment section, the PbS CQDs thin films are prepared by spin-coating PbS CQDs solution PbS CQDs. As shown in Figure 4, the surface morphology of all samples is uniform and smooth, but onto FTO/ZnO substrate. All samples AFM measurement consists of two layers of PbS CQDs. there are some differences in details. The particles size is small in the case of n-octane or isooctane As shown in Figure 4, the surface morphology of all samples is uniform and smooth, but there are sample while large aggregation is found in the case of n-hexane or toluene sample. The root mean somesquare differences in details. The particles is small the case of n-octane or isooctane sample (RMS) value for n-octane, isooctane,size n-hexane and in toluene samples are 7.86, 14.20 nm, 19.57 nm whileand large aggregation is found in the samples case of has n-hexane toluene sample. root mean square 11.70 nm, respectively (n-heptane similar or morphology as that ofThe isooctane sample). (RMS) isooctane, and toluene samples 7.86, and 14.20 nm, 19.57 nm and It value is clearfor thatn-octane, films fabricated fromn-hexane n-octane or toluene solvents showare smooth compact surface, will reduce the number ofsamples grain boundaries andmorphology decrease leakage current. Thus, high PCE is It is 11.70 which nm, respectively (n-heptane has similar as that of isooctane sample). obtained in the case of n-octane or toluene device because of low interface defect density, which clear that films fabricated from n-octane or toluene solvents show smooth and compact surface, is which consistent our J–Vof curves will reduce the to number grainmeasurement. boundaries and decrease leakage current. Thus, high PCE is obtained in the case of n-octane or toluene device because of low interface defect density, which is consistent to our J–V curves measurement.

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Figure 4. AFM (atomic force microscope) images of PbS CQDs thin films prepared by using different Figure 4. AFM (atomic force microscope) images of PbS CQDs thin films prepared by using different solvent (a) n-octane; (b) isooctane; (c) n-hexane; (d) toluene. solvent (a) n-octane; (b) isooctane; (c) n-hexane; (d) toluene.

The solvent physical–chemical property has significant effect on solution-processed thin film The cells, solvent physical–chemical property significant effect For on solution-processed solar solar which had been reported in has previous research. further increasing thin the film device cells, which hadsolvent been reported previous research. For further rate, increasing the device performance, performance, mixturesin are used to affect the evaporating CQDs aggregation and finally solvent mixtures are used affect the evaporating aggregation and finally the quality the quality of CQDs thin to film. We selected n-octane rate, as theCQDs main solvent and isooctane or toluene as the doping the two n-octane doping solvents have different boiling point and thethe addition of CQDs thin solvent. film. WeAsselected as the main solvent and isooctane or polarity, toluene as doping of them n-octane will surely change the evaporating ratepoint and polarity of solvent mixture,of etc. The in solvent. Asinthe two doping solvents have different boiling and polarity, the addition them polarity of n-octane will bethe increased with the addition of toluene. Figuremixture, 5a showsetc. the The J–V curves of of n-octane will surely change evaporating rate and polarity of solvent polarity PbS CQDs solar cells fabricated by using n-octane/isooctane mixture. From the data summarized in n-octane will be increased with the addition of toluene. Figure 5a shows the J–V curves of PbS CQDs Table 3, one can see that the device performance is improved with the increase of isooctane content solar cells fabricated by using n-octane/isooctane mixture. From the data summarized in Table 3, (v/v) to the 20%,device then drops down above 20%. It iswith notedthe that the champion devicecontent with PCE of 0 onefrom can 0see that performance is improved increase of isooctane from 7.64% is obtained in the case of n-octane/isooctane (95%/5%, v/v) device, while this value is 6.67% for (v/v) to 20%, then drops down above 20%. It is noted that the champion device with PCE of 7.64% control device using pure n-octane, showing a ~15% increase mainly arising from a 15% increase in is obtained in the case of n-octane/isooctane (95%/5%, v/v) device, while this value is 6.67% for Jsc. The PCE values of 10 and 20% isooctane doping n-octane devices are up to 7% respectively, which control device using pure n-octane, showing a ~15% increase mainly arising from a 15% increase in Jsc . is higher than that of the control device. However, when isooctane exceeds 20%, device performance The PCE values of 10 and 20% isooctane doping n-octane devices are up to 7% respectively, which is decreases almost linearly with the content increase of isooctane. The enhanced Jsc for low content higher than that of the control device. However, when isooctane exceeds 20%, device performance isooctane devices is confirmed by the external quantum efficiency (EQE) measurement shown in decreases almost linearly with the content increase of isooctane. The enhanced Jsc for low content Figure 5b. Devices fabricated with low isooctane content show higher EQE in the wavelength region isooctane is confirmed bysuggests the external quantum efficiency (EQE)ismeasurement showna in from 400devices to 800 nm. This finding that the carrier transfer efficiency improved by adding Figure 5b. Devices fabricated with low isooctane content show higher EQE in the wavelength region small amount of isooctane. On the contrary, descendant devices performance is found in the case of from 400 to 800 nm. This finding suggests that the carrier transfer efficiency is improved by adding the hybrid solvent schemes based on n-octane and toluene. The J–V curves for n-octane/toluene a small amount of isooctane. On the contrary, descendant performance is found in the case ofinthe hybrid solvent devices are shown in Figure 5c and thedevices corresponding EQE spectrum is described hybrid solvent schemes based on n-octane and toluene. The J–V curves for toluene n-octane/toluene hybrid Figure 5d. It is clear that the device performance decreases linearly with the content increase solvent devices are shown in devices Figure 5c and lower the corresponding spectrum described in Figure 5d. and all the hybrid solvent show PCE than thatEQE of n-octane oristoluene single-solvent It isdevice, clear that theis device decreases with toluene We content increase which mainly performance arising from the decreaselinearly in Voc and FFthe of devices. speculate that and low-all film show may be obtained in this case due to large difference in polaritydevice, for thequality hybrid CQDs solventthin devices lower PCE than that of n-octane or toluene single-solvent n-octane/toluene hybrid solvent. which is mainly arising from the decrease in Voc and FF of devices. We speculate that low-quality CQDs thin film may be obtained in this case due to large difference in polarity for n-octane/toluene hybrid solvent.

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30

Isooctane content

90 (b)

(a)

5% 10 % 20 % 40 % 60 %

Current Density (mA/cm2)

80 20

70 60

10

EQE (%)

5% Isooctane 10% Isooctane 20% Isooctane 40% octane 60% octane

50 40 30

0

20 10

-10 -0.2

0.0

0.2

0.4

0

0.6

400

Voltage (V)

800

1000

1200

Wavelength (nm) 90

(c)

5 % toluene 10 % toluene 20 % toluene 40 % toluene 60 % toluene

20

(d)

Toluene content

80

5% 10 % 20 % 40 % 60 %

70 60 EQE (%)

Current Density (mA/cm2)

30

600

10

50 40 30

0

20 10

-10 -0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0

400

600

800

1000

1200

Wavelength (nm)

Voltage (V)

Figure 5. 5. (a) characteristic of PbS CQDs solarsolar cells cells with with n-octane/isooctane hybridhybrid solventsolvent and (b) Figure (a)J–V J–V characteristic of PbS CQDs n-octane/isooctane the corresponding EQE spectrum; (c) J–V characteristic of PbS CQDs solar cells with n-octane/toluene and (b) the corresponding EQE spectrum; (c) J–V characteristic of PbS CQDs solar cells with hybrid solvent and (d) the corresponding EQE spectrum. EQE spectrum. n-octane/toluene hybrid solvent and (d) the corresponding Table3.3. Photovoltaic parameters obtained obtained from from the the J–V J–V curves Table Photovoltaic parameters curves (Figure (Figure 5a,c) 5a,c) for for solar solar cells cells with with n-octane/isooctane and n-octane/toluene hybrid solvent. n-octane/isooctane and n-octane/toluene hybrid solvent. Isooctane/n-octane (v/v)

Isooctane/n-octane (v/v)

5%

5%

10% 10% 20% 20% 40%

40% 60% 60% Toluene/n-octane (v/v) Toluene/n-octane 5% (v/v)

PCE (%) PCE (%)

7.64

7.64 7.26 7.26 6.99 6.99 6.08 6.08 5.64

5.64 PCE (%) PCE 5.84(%)

Jsc (mA cm−2)

Jsc (mA cm−2 )

26.29

26.29 26.38 26.38 23.10 23.10 22.90 22.90 21.10

(mA cm ) Jsc20.87 −2

5.31 5.84 5.09 5.31 3.85 3.51

21.57 20.87 21.74 21.57 23.20 18.80

40%

3.85

23.20

20%

5.09

21.74

0.54

0.54 0.540.54 0.540.54 0.54 0.530.54

Jsc (mA21.10 cm−2 )

10% 5% 20% 10% 40% 60%

Voc (V)

Voc (V)

0.53 Voc (V) Voc (V) 0.53 0.50 0.53 0.51 0.410.50 0.42 0.51 0.41

FF (%)

FF (%)

Rs (Ω × cm−2)

Rs (Ω × cm−2 )

53.82

53.82 50.93 50.93 56.04 56.04 49.17 50.43 49.17

4.97 5.47 4.83 7.02 7.94

4.97

52.80 FF (%) 49.24 52.80 45.91 40.48 49.24 44.45

45.91 40.48

205.55

205.55 109.21109.21 173.90173.90 104.33 132.37104.33

5.47 4.83 7.02

2) FF (%) 50.43 Rs (Ω × cm−7.94

−2 ) 132.37 Rsh (Ω × cm

Rs (Ω × cm ) 9.29 11.37 9.29 12.19 8.20 11.37 8.52 −2

12.19 8.20

Rsh (Ω × cm−2)

Rsh (Ω × cm−2 )

Rsh (Ω × cm−2) 131.23 109.94 131.23 104.65 55.56109.94 100.00 104.65 55.56

60% 3.51 18.80 with different 0.42 44.45 contents,8.52 100.00 To investigate the morphology changes solvent AFM measurement is carried out for PbS CQDs thin film prepared by using n-octane/isooctane and n-octane/toluene To investigate morphology different solvent is mixture. As shownthe in Figure 6, the changes thin filmwith is homogeneous andcontents, compactAFM for allmeasurement samples with carried out for PbS CQDs thin film bythe using n-octane/isooctane and n-octane/toluene root-mean-square (RMS) value ~7.5 nm.prepared However, domain size is quite different in detail for thin mixture. As shown in Figure 6, the thin film is homogeneous and compact for all samples with large rootfilms fabricated with different n-octane/isooctane content. In the case of low isooctane content, mean-squareof(RMS) valueis~7.5 nm.with However, the content domainincrease size is quite detail 6a,b). for thin aggregation PbS CQDs found isooctane fromdifferent 5 to 10%in (Figure Onfilms the fabricatedwhen withthe different n-octane/isooctane Insamples the casehave of low isooctane content,aslarge contrary, isooctane content is up to content. 20% (60% similar morphology that aggregation of PbS CQDs is found withinisooctane content increase from 5 to pointing). 10% (Figure 6a,b). On of 40% samples), a lot of cracks appear the whole thin film (see the arrow The cracks the contrary, content increase is up to 20% similar morphology as PbS that become biggerwhen with the isooctane content from(60% 20 tosamples 40%. Ashave Au will be deposited on the of 40%thin samples), a lotcracks of cracks the whole thin film (see thewill arrow pointing). The carrier cracks CQDs film, large may appear result ininmore interface defects, which increase interface become biggerand withlow theJsc isooctane content increase from 20 to 40%. AsbeAu will beout deposited the recombination is expected in this case. However, it should pointed that the on width PbS CQDs thin film, large cracks may result in more interface defects, which will increase interface carrier recombination and low Jsc is expected in this case. However, it should be pointed out that the

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of cracks is around 10 nm, much lower than the thickness of the CQDs active layer (around 200 nm), width of cracks is around 10 nm, much lower than the thickness of the CQDs active layer (around so it does not result in short, but a decreased in device performance. Furthermore, based on the above 200 nm), so it does not result in short, but a decreased in device performance. Furthermore, based on discussion, the increase in device performance for low isooctane content mixture is a result of compact the above discussion, the increase in device performance for low isooctane content mixture is a result and large aggregation derived from fast solvent mixture evaporation rate, while excessive isooctane of compact and large aggregation derived from fast solvent mixture evaporation rate, while excessive content can result in cracks and lower device performance. It is interesting that the morphology of isooctane content can result in cracks and lower device performance. It is interesting that the n-octane/toluene samples is quite different from those of n-octane/isooctane samples. As shown in morphology of n-octane/toluene samples is quite different from those of n-octane/isooctane samples. Figure 7, no cracks are found in all CQDs thin film with different toluene content. The morphology As shown in Figure 7, no cracks are found in all CQDs thin film with different toluene content. The is not homogeneous with RMS around 13.5 nm and some holes are found in all samples. The large morphology is not homogeneous with RMS around 13.5 nm and some holes are found in all samples. fluctuation may result in large interface defect, which will lead to low device performance. The large fluctuation may result in large interface defect, which will lead to low device performance.

Figure 6. AFM (atomic force microscope) images for PbS CQDs thin film fabricated by using different Figure 6. AFM (atomic force microscope) images for PbS CQDs thin film fabricated by using different n-octane/isooctane solvent (a) 5% isooctane (b) 10% isooctane (c) 20% isooctane (d) 40% isooctane. n-octane/isooctane solvent (a) 5% isooctane (b) 10% isooctane (c) 20% isooctane (d) 40% isooctane.

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Figure 7. AFM (atomic force microscope) images for PbS CQDs thin film fabricated by using different Figure 7. AFM (atomic force microscope) images for PbS CQDs thin film fabricated by using different n-octane/toluene solvent (a) 5% toluene (b) 10% toluene (c) 20% toluene (d) 40% toluene. n-octane/toluene solvent (a) 5% toluene (b) 10% toluene (c) 20% toluene (d) 40% toluene.

4. Conclusions 4. Conclusions In conclusion, different solvents or their hybrid are developed to fabricate PbS CQD solar cells. conclusion, different or their are different developed to fabricate PbS CQDthe solar cells. The In device performance cansolvents be tailored byhybrid selecting solvents or changing solvent The device performance can be by selecting different solvents or6.0% changing the solvent mixture content. In the case of tailored single-solvent strategy, high PCE up to is obtained in themixture case of content. In the case of single-solvent strategy, high PCE up to 6.0% is obtained in the case of or n-octane or toluene devices, while PCE below 5% is obtained for isooctane, heptane orn-octane n-hexanal toluene devices, while PCE below 5% is obtained for isooctane, heptane or n-hexanal devices. By using devices. By using hybrid solvent, the device performance can be further tailored. PCE as high as hybrid solvent, thein device performance can be further tailored. as is high as 7.64% obtained in the 7.64% is obtained the case of 5% isooctane/n-octane device,PCE which ~15% higheristhan the control case of 5% isooctane/n-octane device, which is ~15% higher than the control devices. The changes devices. The changes in device performance mainly originated from different solvent physicalin device performance mainly originated from different solvent physical-chemical which chemical properties, which affect the morphology of final PbS CQDs thin film. Weproperties, believe that if an affect the morphology of final PbS CQDs thin film. We believe that if an optimized PbS CQDs optimized PbS CQDs film, an optimized device structure or optimized device processing technics is film, an optimized device structure can or optimized processing technics is presented, the way device presented, the device performance be furtherdevice increased. Our results present an effective to performance can be further increased. Our results present an effective way to improve the performance improve the performance of solution-processed PbS CQDs solar cells. of solution-processed PbS CQDs solar cells. Acknowledgments: We thank the financial support of the National Natural Science Foundation of China (Nos. Acknowledgments: We thank the financial support of the National Natural Science Foundation of China 91333206, 61274062 and 11204106), National Science Foundation for Distinguished Young Scholars of China (Nos. 91333206, 61274062 and 11204106), National Science Foundation for Distinguished Young Scholars of China (Grant No. No. 51225301), 51225301), Guangdong Guangdong Province Province Natural Natural Science Science Fund Fund (No. (No. 2014A030313257) 2014A030313257) and and the the Fundamental Fundamental (Grant Research Funds Funds for for the the Central Central Universities. Universities. Research Author Contributions: Donghuan Qin and Rongfang Wu conceived and designed the experiments; Rongfang Wu, Yuehua Yang and Miaozi Li performed the experiments; Lintao Hou and Rongfang Wu analyzed the data; Lintao Hou, Rongfang Wu, Yuehua Yang and Yangdong Zhang contributed reagents/materials/analysis tools; Donghuan Qin and Rongfang Wu wrote the paper.

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Author Contributions: Donghuan Qin and Rongfang Wu conceived and designed the experiments; Rongfang Wu, Yuehua Yang and Miaozi Li performed the experiments; Lintao Hou and Rongfang Wu analyzed the data; Lintao Hou, Rongfang Wu, Yuehua Yang and Yangdong Zhang contributed reagents/materials/analysis tools; Donghuan Qin and Rongfang Wu wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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