CdSe Quantum Dot Sensitized Solar Cells with

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High efficiency CdS/CdSe quantum dotsensitized solar cells with two ZnSe layers Fei Huang, Lisha Zhang, Qifeng Zhang, Juan Hou, Hongen Wang, Huanli Wang, Shanglong Peng, Jianshe Liu, and Guozhong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12842 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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ACS Applied Materials & Interfaces

High efficiency CdS/CdSe quantum dot-sensitized solar cells with two ZnSe layers Fei Huang,ab Lisha Zhang,b Qifeng Zhang,a Juan Hou,a Hongen Wang,a Huanli Wang,b Shanglong Peng,a Jianshe Liu,*b Guozhong Cao*a

a

Department of Materials Science and Engineering, University of Washington, Seattle,

Washington 98195-2120, United States E-mail: [email protected]; Tel: +1-206-616-9084

b

College of Environmental Science and Engineering, Donghua University, Shanghai 201620, P.R.

China E-mail: [email protected]

Abstract CdS/CdSe quantum dot sensitized solar cells (QDSCs) have been intensively investigated; however, most of the reported power conversion efficiency (PCE) is still lower than 7% due to serious charge recombination and a low loading amount of QDs. Therefore, suppressing charge recombination and enhancing light absorption are required to improve the performance of QDSCs. The present study demonstrated successful design and fabrication of QDSCs with a high efficiency of 7.24% based on CdS/CdSe QDs with two ZnSe layers inserted at the interfaces between QDs and TiO2 and electrolyte. The effects of two ZnSe layers on the performance of the QDSCs were systematically investigated. The results indicated that the inner ZnSe buffer layer located between QDs and TiO2 serve as a seed layer to enhance the subsequent deposition of CdS/CdSe QDs, leading to higher loading amount and covering ratio of QDs on the TiO2 photoanode. The outer ZnSe layer located between QDs and electrolyte behave as an effective passivation layer which not only reduces the surface charge recombination, but also enhances the light harvesting.

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Keywords: CdS/CdSe quantum dots, ZnSe buffer layer, ZnSe passivation layer, enhanced quantum dots loading, reduced charge recombination, Quantum dot-sensitized solar cell Introduction In order to solve the increasing serious energy crisis and environmental pollution problems, the development of photovoltaic devices for solar power conversion is of great significance. Dye sensitized solar cells (DSCs) have attracted plenty of research during the past 25 years because of their inexpensive, simple fabrication process and high efficiency; however, it is not easy to obtain a low cost commercial dye that can absorb the whole light spectrum region.1-6 Perovskite absorbers in the form of thinner layers can enable complete light absorption.7 In the past few years, perovskite solar cells have been a hot spot in solar cell research fields, however, they are sensitive to the environment (oxygen, moisture, UV light and temperature) because of their low chemical stability.8-10 Due to its good stability, size-dependent tunable band gap, large extinction coefficient and multiple exciton generation (MEG) with single-photon absorption, narrow band gap semiconductor quantum dots have been widely explored as sensitizers for quantum dots sensitized solar cells (QDSCs).11-14 In view of the MEG effect the theoretical photovoltaic conversion efficiency of QDSCs can reach up to 44%.15 However, the highest power conversion efficiency of QDSCs was still lower than 12%.16-19 Considerable efforts has been put forward to improve the performance of QDSCs to meet the demand of effective solar power conversion devices. Mainly including: (1) introducing passivation layer (i.e. ZnS, or ZnSe) on the QDs’ surfaces to suppress recombination processes in the photoelectrode/QDs/electrolyte interfaces,20-21 or introducing wide band gap semiconductors (i.e. SiO2, TiO2) on the surface of photoelectrode to reduce electron losses,22-23 (2) adopting ex-situ synthesized high quality QDs with reduced surface trap states,24-25 (3) choosing narrow band gap semiconductors to extend the absorption range to longer wavelength or near infrared region,26-27 (4) enhancing the deposition amount of homogeneous distributed QDs,28 and (5) introduction of transition metal ion dopants such as Mn2+ to modify the optoelectronic properties of QD sensitizers.29-31 Significant improvement in the performance of QDSCs has been demonstrated with these methods. Taking the advantages of a wide light absorption range, superior electron transfer properties, good stability, easy synthesis, CdS/CdSe co-sensitized QDSCs have attracted lots of attention in 2


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the research community.32-36 However, CdS and CdSe QDs were always synthesized via a low temperature synthesis method, such as chemical bath deposition (CBD) and successive ionic layer adsorption and reaction (SILAR) or electrodeposition method.6, 34, 37-38 The QDs obtained by the low-temperature chemical processes contain a large number of surface trap states, which can act as charge recombination centers. And it’s very difficult to get a sufficient and evenly deposition of QDs on the whole surface of photoanode (TiO2 and ZnO).39 The low surface coverage of QDs on metal oxide film electrode will, on the one hand, lead to insufficient absorption of light; on the other hand, result in large portion area of bare TiO2 surface contact with the electrolyte directly, leading to severe charge recombination at the TiO2 oxide/electrolyte interface. Previous work has demonstrated that when ZnSe was introduced as a passivation layer in CdS/CdSe quantum dot-sensitized solar cells (QDSCs), compared with the solar cells without passivation layer and the most widely used ZnS passivation layer, it presented ~90% and ~30% improvement in power conversion efficiency.40-41 The present investigation included the design and fabrication of CdS/CdSe quantum dot-sensitized TiO2 solar cells by inserting two ZnSe layers: one between TiO2 and QDs as a buffer layer and another between QDs and electrolyte as a passivation layer. Both inner and outer ZnSe layers lead to the enhancement of current density, open-circuit voltage, and the consequently PCE of the resultant cell devices. The champion QDSC based on the CdS/CdSe QDs sensitizer with two ZnSe layers exhibited a PCE of 7.24% under AM 1.5 G one sun illumination, which is among the best results of CdS/CdSe co-sensitized solar cells so far reported in literature.

Experimental section

2.1 Preparation of TiO2/CdS/CdSe photoanodes with two ZnSe layers All the chemicals used in this work were analytical grade reagents without further purification. The preparation of mesoporous TiO2 films and deposition of CdS/CdSe QDs all referred to previous work.39-40, 42 A transparent TiO2 mesoporous film was prepared via doctor blading the TiO2 paste on a clean F: SnO2-coated (FTO, 6-8Ω/square) glass substrate following by sintering at 500 oC for 30 min to remove the organic impurities and improve the crystallinity. CdS QDs were deposited by successive ionic layer adsorption and reaction (SILAR) method on the surface of 3



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TiO2 mesoporous films with five cycles. CdSe QDs were deposited on the TiO2/CdS film via a chemical bath deposition (CBD) procedure for 3 h at room temperature. The ZnSe layers were also deposited by SILAR method according to our previous publication.40 Briefly, the films were first immersed in 0.1 M Zn2+ solution for 2 min and then immersed in 0.1 M Se2- for another 2 min. Following each immersion, the films were thoroughly rinsed with deionized water to remove the unbound ions and dried. In this process, the Se2- solution was always purged with N2. According to previous work,40 three SILAR cycles of ZnSe outer layer are the optimized thickness. The optimization of the SILAR cycles of inner ZnSe layer is shown in Figure S1 (ESI†). So this immersion was repeated five times to get the optimum thickness. 2.2 Electrolyte, counter electrode and device assembly The electrolyte used in this work was freshly prepared polysulfide solution, which was made by dissolving 1 M sulfur and 1M sodium sulfide in 10 ml deionized water before each test. The counter electrode is a nanostructured Cu2S film on brass foil, in brief, immersing a brass foil into 37% HCl at 80 oC for 20 min, taking it out to rinse with deionized water and ethanol, drying in air, then immersing it into the freshly prepared electrolyte for 5min, this process was repeated two times, resulting in the formation of nanostructured Cu2S on the brass foil. The device was assembled into sandwich-type with a scotch tape placed between the QDs sensitized photoanode and the counter electrode. 2.3 Characterization We used the scanning electron microscope (SEM, JSM-7000) equipped with an EDX spectrometer to characterize the surface morphology and element composition of the photoanodes. The photocurrent density-voltage characteristics (J-V curves) of the solar cells were recorded by an HP 4155A programmable semiconductor parameter analyzer under AM 1.5 simulated sunlight with a power density of 100 mW cm-2. To measure the light absorption properties of the photoanodes, Perkin Elmer Lambda 900 UV/VIS/IR Spectrometer was used. Electrochemical impedance spectroscopy (EIS) was carried out to analyze the electronic and ionic processes in the QDSCs using a Solartron 1287A coupled with a Solartron 1260 FRA/impedance analyzer. The photoluminescence (PL) spectra and PL decay curves were recorded with a luminescence spectrometer FLsp920. The incident photon-to-current conversion efficiency (IPCE) spectra were obtained using 7-SCSpec response measurement system. 4


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3. Results and discussion

Figure 1 Top view SEM micrographs of CdS/CdSe QDs based (a, a1) TiO2/QDs, (b, b1) TiO2/ZnSe/QDs, (c, c1) TiO2/QDs/ZnSe, and (d, d1)TiO2/ZnSe/QDs/ZnSe films. Figure 1 is the SEM images showing the surface morphologies of respective CdS/CdSe QDs based TiO2/QDs, TiO2/ZnSe/QDs, TiO2/QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe films. No appreciable difference was observed in all the four samples; the mesoporous structure did not exhibit any apparent change. However, careful observation did reveal a small increase in particle size and a slightly higher filling density of the pores with the deposition of inner or outer ZnSe layer. The change is vague and it is not possible for us to calculate the specific enhancement of the particle size and coverage ratio of the QDs

. Figure 2 UV-visible absorption spectra of TiO2/CdS, TiO2/ZnSe/CdS films.

To evaluate the influence of inner ZnSe layer on the subsequent deposition of CdS QDs, we have measured the UV-vis absorption spectra of the TiO2/CdS and TiO2/ZnSe/CdS films, as shown in Figure 2. Compared with the TiO2/CdS film, the light absorption of the TiO2/ZnSe/CdS film show 5



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a great enhancement not only in absorbance but also a red-shift in absorption edge. The light absorption of ZnSe is not longer than 460nm because it has a wider bandgap (2.7 eV), so the noticeable enhancement was not because of the ZnSe layer.28 This result demonstrates that for the bare TiO2 film, the deposition amount of CdS QDs was low, while, with the presence of inner ZnSe layer, the deposition amount of CdS QDs was significantly enhanced, which was demonstrated by the higher light absorbance and the larger red shift of the absorption range. Thus the presence of the ZnSe layer can enhance the deposition of CdS QDs; the ZnSe layer might have served as a seeding layer to enhance the nucleation and growth of CdS QDs with high quality, leading to a high coverage ratio on the surface of photoanodes.

Figure 3 (a) UV-visible absorption spectra and (b) (Ahv)2 vs. hv curves of the CdS/CdSe QDs based TiO2/QDs, TiO2/ZnSe/QDs, TiO2/QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe films.

Figure 3 (a) shows the light absorption spectra of CdS/CdSe QDs based TiO2/QDs, TiO2/ZnSe/QDs, TiO2/QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe films over the wavelength ranging from 300 to 800 nm under the same measurement condition. Obviously, both the deposition of inner ZnSe layer and outer ZnSe layer have a great and different influence on the optical absorption of the CdS/CdSe QDs based TiO2/QDs films. With the deposition of inner ZnSe layer, the photoelectrode shows an increase in absorbance, while with the deposition of outer ZnSe layer, the photoelectrode shows not only an enhancement in absorbance but also a noticeable red-shift in absorption range. When two ZnSe layers were deposited, the photoelectrode shows a further increase in absorbance compared with the photoanode with single ZnSe layer, and a same red shift in absorption range. The higher absorbance indicates the amount of QDs deposited on the photoelectrode with inner 6


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ZnSe layer is higher than that without ZnSe layer. The inner ZnSe layer appears to favor the deposition of CdS QDs, and in turn CdS layer promotes the growth of CdSe QDs.33, 43 Thus the deposition amount of CdSe QDs was increased, which leads to a higher light absorbance. The introduction of ZnSe outer layer results in an increased absorbance and a significant red-shift of the absorption edge, in accordance with the results reported earlier.40-41 ZnSe has a bandgap of 2.7 eV, which is wider than 1.74 eV of CdSe QDs, so the change in absorption was not caused by ZnSe itself.28 According to litteratures, the high absorption of the photoelectrode might be attributed to: (1) the partial overlap of the exciton wave functions of CdS/CdSe QDs and the outer ZnSe layer resulting from an interaction between the two parts;21,

44

(2) increased deposition

amount and particle size of CdSe QDs, The light absorption of TiO2/CdS/CdSe photoanodes with different SILAR cycles of ZnSe outer layer, and the atom ratios of the elements in TiO2/CdS/CdSe photoanodes were examined and the results were shown in Figure S2 (ESI†)and Figure S3 (ESI†). The absorption shows a strong dependence on the number of SILAR cycles for deposition of ZnSe passivation layer, and the red shift of the absorption edge mainly occurs in the first cycle. The amount of Cd is more than the total amount of S and Se combined in TiO2/CdS/CdSe photoanode. So the excessive cadmium existing on the surface of TiO2/CdS/CdSe film would react with Se2during the first ZnSe SILAR deposition process, leading to a further growth and formation of the CdSe QDs. Although these explainations sounds reasonable, the oxidization of Se ions appeared taking place on the surface of ZnSe layer which could have contribution to the higher absorption. More detailed experiments are still going on to illustrate this issue, seperately. As demonstrated in Figure 3 (b), the optical band gap of the QDs can be estimated by extrapolating the linear portion of the (Ahv)2 versus hv plots, according to equation (1):45-47

(ℎ) = ℎ −

(1)

where A is the absorbance, c is a constant, v is the photon frequency and h is plank constant. Obviously, both the inner and outer ZnSe layer can contribute to enhanced light absorption, and the TiO2/CdS/CdSe photoanode with two ZnSe layers shows a higher light absorption compared to the photoanode without or with single ZnSe layer.

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Figure 4 (a) Nyquist plot and (b) Bode plot curves of the CdS/CdSe QDs based TiO2/QDs, TiO2/ZnSe/QDs, TiO2/QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe solar cells measured under forward bias (-0.6 V) in dark condition

Table 1 Electrochemical impedance results of QDSCs Samples

R1 (Ω)

Rct (Ω)

τn (ms)

TiO2/QDs

1.9

68.0

11.5

TiO2/ZnSe/QDs

1.8

95.3

26.5

TiO2/QDs/ZnSe

1.5

156.2

40.0

TiO2/ZnSe/QDs/ZnSe

1.7

196.3

61.2

To estimate the resistance distribution and charge recombination processes, Figure 4 (a) gives the comparison of electrochemical impedance spectra (EIS) of the TiO2/QDs, TiO2/ZnSe/QDs, TiO2/QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe QDSCs, which were measured under a forward bias (-0.6 V) and dark condition. The spectra composed of two semicircles fitted by an equivalent circuit (inset in Figure 4 (a)).48 The fitting results are listed in Table 1. The first small semicircle, almost invisible, indicates the recombination resistance at the counter electrode/electrolyte interface (R1). Because the same counter electrode and electrolyte were used during the measurement, almost no difference was observed in R1 among these four QDSCs. The second large semicircle indicates the charge transfer resistance at the TiO2/QDs/electrolyte interface and in the TiO2 films (Rct).23, 49 We can see there is a noticeable difference in Rct, the Rct value for the QDSCs with two ZnSe layers is 196.3 Ω, while the Rct for the QDSCs without passivation layer and with single ZnSe passivation layers is only 68.0 Ω, 95.3 Ω and 156.2 Ω. The charge recombination resistance in a QDSC is mainly determined by Rct. A high Rct value means a 8


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reduced recombination of the electrons and holes.23 Figure 4 (b) shows the Bode plots of the TiO2/QDs, TiO2/ZnSe/QDs, TiO2/QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe QDSCs. The curve peak of the Bode plots can be used to extimate the electron lifetime (τn) according to eqn (2):50

=

1

=

1 2

(2)

Where fmin is the peak frequency at the minimum phase angle in the bode plots. The corresponding electron lifetime of the QDSCs is listed in Table 1. The τn of the QDSCs with a two ZnSe layer is 61.2 ms, which is much longer than that of the QDSCs without ZnSe layer or just with single ZnSe layer. Increase in both Rct and τn suggests a reduced interface charge recombination in QDSCs. According to the above discussion, two ZnSe layers have superior ability to suppress interfacial charge recombination processes campared to the single ZnSe layer.

Figure 5 (a) Photoluminescence (PL) curves (b) normalized PL decay curves of the CdS/CdSe QDs based TiO2/QDs, TiO2/ZnSe/QDs, TiO2/QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe films.

Figure 5 (a) shows the photoluminescence (PL) spectra of TiO2/QDs, TiO2/ZnSe/QDs, TiO2/QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe films. In the PL test, the films are illuminated under light source with photon energy above the band-gap energy of QDs. Under light illumination, the QDs are excited by photons and generate electron-hole pairs which will finite momenta existence in the conduction and valence bands, respectively. The PL emission derives from the recombination of the photoinduced electrons and holes.51 Therefore a high emission is an indication of increased recombination of photoinudced electrons and holes. As shown in Figure 5 9



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(a) there are two peaks in each curve around 500nm and 650nm, which correspond to the emission of CdS and CdSe QDs, repectively. The PL emission of CdSe QDs is markedly quenched despite the increase in absorbance after loading with ZnSe outer layer, which indicates the recombination of photo-generated electron-hole pairs is significantly reduced. In addition, the PL emission peak of CdSe QDs also shifts to a longer wavelenth, in a good accordance with the UV-vis results. But with ZnSe inner layer deposition, there’s a slightly increase in PL emission intensity of CdSe QDs. This was due to the fact that with ZnSe inner layer deposited, more CdSe QDs are deposited and more excitons are produced, in according with the enhanced absorbance. For the PL emission of CdS QDs, it is quenched with the deposition of two ZnSe layers, although the exsitense of ZnSe inner layer can lead to high absorbance of CdS QDs as shown in Figure 2. This phenomenon can be attributed to less defects exsiting in the CdS QDs. So the exsistence of ZnSe inner layer helps CdS QDs formation with enhanced crystallinity and better surface quailty. Figure 5 (b) depicts the excited state electron radiative decay curves of the photoanodes, which are employed to analyze the influence of inner and outer ZnSe layer coating on the photo-generated electrons and holes recombination and electrons injection rate. The PL lifetimes of the QDs are estimated to be 10.685ns, 7.909ns, 1.285ns and 0.752ns for the TiO2/QDs, TiO2/ZnSe/QDs, TiO2/QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe films, respectively. This result shows that the introduction of inner and outer ZnSe layer can shorten the PL lifetime of the QDs. The short-lived excited state of the QDs means the enhanced injection kinetics of the photo-generated elctrons in the QDs.52-53 So the introduction of inner and outer ZnSe layer will benefit the electrons from QDs inject to TiO2.

Figure 6 (a) Schematic diagram and (b) band edges structure for efficient transport of excited 10


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electrons and holes of the CdS/CdSe QDs based TiO2/ZnSe/QDs/ZnSe solar cells

Figure 6 (a) present the schematic diagram of TiO2/ZnSe/QDs/ZnSe solar cells consist of a TiO2 photoanode with an inner ZnSe buffer layer, an outer ZnSe passivation layer, CdS/CdSe QDs, a sulfide/polysulfide (S2-/Sx2-) electrolyte and a Cu2S counter electrode. According to the literatures, 21, 28

the band edge structure of all the materials in the photoanode is shown in Figure 6 (b). Under

light illumination, CdS/CdSe quantum dots will be excited by photons and generate electron-hole pairs, then the electrons will transfer to the conduction band of TiO2 and holes will transfer to the electrolyte and released by the redox couples in the electrolyte. Meanwhile, severely charge recombination processes also occurring in the QDSCs and severely decrease the performance.54-55 In this structure, the outer ZnSe passivation layer will not only prevent recombination of electrons in the conduction band of the QDs and TiO2 with the oxidized form of the redox couple in electrolyte, but also facilitates the holes transfer to the electrolyte, and the deposition of ZnSe outer layer also helped to enhance light absorption. Inner ZnSe buffer layer serves as a seed layer benefit the CdS and CdSe QDs deposition, leading to a sufficient and evenly deposition of QDs with high quality on the TiO2 photoelectrode. This will not only help the CdS QDs formation with less defects and decrease the charge recombination between TiO2/electrolyte interfaces, but also enhance light absorption. Herein, we should point out that although ZnSe has a higher conduction band edge than that of CdS/CdSe QDs and TiO2, however, the PL lifetime of the QDs is shortened by the introduction of inner ZnSe layer. This result shows that the inner ZnSe layer does not seem to hinder the electron injection from the conduction band of QDs to the TiO2, instead, more efficient charge injection occurs at the TiO2/ZnSe/QDs interfaces.

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Figure 7 (a) Incident photon-to-current conversion efficiency (IPCE) spectra and (b) light harvesting efficiency (LHE) spectra of the CdS/CdSe QDs based TiO2/QDs, TiO2/ZnSe/QDs, TiO2/QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe solar cells.

Figure 7 (a) presents the incident photon-to-current conversion efficiency (IPCE) spectra to evaluate the light absorption and electron collection characteristics. It was reported in literature,56 the IPCE values are lower than 80% restricted by the reflection of the glass substract. From Figure 7 (a) we can find the IPCE values for the solar cell with two ZnSe layers are very close to 80%, which indicates it’s good performance. The integration of IPCE over the solar spectrum is the Jsc of the solar cells, as expected, the IPCE values of the solar cells with two ZnSe layers is higher than that of the solar cells without ZnSe layer or with single ZnSe layer, which is in a same trend with the variation of Jsc of the corresponding cells. The IPCE was estimated by the equation (eqn (3)):57

IPCE = LHE × ƞ × ƞ

(3)

Where LHE is light-harvesting efficiency, ƞct is charge-transfer efficiency, and ƞcc is charge-collection efficiency. The light harvesting efficiency "LHE" is given by equation (4):58-59

!" = 1 − 10$%&'()&%*

(4)

Figure 7 (b) shows the "LHE" of the QDSCs, which is in a same trend with the light absorption of the photoelectrodes. The LHE of the photoelectrodes with two ZnSe is higher than that without ZnSe layer or with single ZnSe layer. "ƞct" is determined by the energy level difference between the conduction band edge of QDs and the photoanode, which was the driving force for the photoexcited electrons transfer. Theoretically, the energy level difference between the conduction band edge of CdS/CdSe QDs and TiO2 is over 200 mV, which will be sufficient to drive charge transfer process.33 To discuss the effect of different ZnSe layers on the "ƞct", we can refer to the PL decay as shown in Figure 5(b). Figure 5(b) indicates that the injection of eletrons from QDs to TiO2 was accelerated by the introduction of ZnSe layer, so it’s helpful for improving "ƞct". "ƞcc" is 12


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related to the charge recombination in solar cells, the EIS results as well as the J-V curves in dark condition indicates the deposition of ZnSe layers can reduce charge recombination processes. And the deposition of two ZnSe passivation layer is better, corresponding with a much higher charge collection efficiency. Consequently, the TiO2/CdS/CdSe QDSCs with two ZnSe layer show the highest incident photon-to-current conversion efficiency (IPCE).

Figure 8 Comparison of Photocurrent density–voltage (J–V) curves of the CdS/CdSe QDs based TiO2/QDs, TiO2/ZnSe/QDs, TiO2/QDs/ZnSe, and TiO2/ZnSe/QDs/ZnSe solar cells.

Figure 8 compares the J-V curves of the solar cells measured under one sun (AM 1.5, 100mA cm-2) illumination. Table 2 shows the average values of the open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF) and power conversion efficiency (PCE) which are obtained from the J-V curves. The PCE of the CdS/CdSe co-sensitized TiO2 solar cells without ZnSe layer or just with outer ZnSe layer are 3.49%, 5.05% and 6.16%, respectively. When two ZnSe layers were deposited, Jsc and Voc increased to 20.95 mA cm−2 and 0.62 V, the power conversion efficiency reaches 7.03%. Such an enhancement in PCE can be attributed to the enhanced light absorption and reduced charge recombination. The champion QDSC based on the CdS/CdSe QDs sensitizer with two ZnSe layers is exhibiting a PCE of 7.24%. So far the highest PCE of CdS/CdSe co-sensitized QDSCs was 7.11% with nanostructured counter electrode based on non-stoichiometric Cu2-xSe electrocatalysts.60 However, with Au or Cu2S counter electrode, the best PCE values of CdS/CdSe co-sensitized QDSCs was ~6%, still not easy to get up to 7%.61-62 To the best of our knowledge, the PCE of 7.24% is one of the highest values for CdS/CdSe co-sensitized solar cells. 13



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Table 2. Photovoltaic properties of average values obtained from the J–V curves with different ZnSe layers (the average value of each data was obtained by testing at least 6 cells). Samples

Voc (V)

Jsc (mA cm-2)

FF

ƞ(%) (average)

TiO2/QDs

0.51

13.26

0.51

3.49

TiO2/ZnSe/QDs

0.54

18.28

0.51

5.05

TiO2/QDs/ZnSe

0.59

19.30

0.54

6.16

TiO2/ZnSe/QDs/ZnSe

0.62

20.95

0.54

7.03

TiO2/ZnSe/QDs/ZnSe

0.61

21.49

0.55

7.24 (Champion)

Conclusions Inserting two ZnSe layers at the interfaces between QDs and TiO2 and electrolyte has demonstrated to promote the deposition of QDs and to decrease charge recombination, collectively leading to high efficiency QDSCs. The inner ZnSe buffer layer served as a seed layer to increase the subsequent deposition of CdS and CdSe QDs, which led to a sufficient and evenly deposition of high quality QDs, benefitting the efficient photon capturing and suppressing interfacial charge recombination; the outer ZnSe layer acted as a passivation layer not only preventing charge recombination but also enhancing light absorption. As a result, both Voc and Jsc were increased, and an overall power conversion efficiency of 7.24% has been achieved, which is considerably higher than the QDSCs without ZnSe layer or with single ZnSe layer and definitely among the highest reported data for CdS/CdSe co-sensitized QDSCs.

Supporting Information: Optimization of the SILAR cycles of inner ZnSe layer (J-V curves), UV-vis of TiO2/QDs photoanodes with various cycles of outer ZnSe layer and EDS of TiO2/QDs photoanodes. Acknowledgments This work was financially supported by the National Science Foundation (NSF, DMR 1505902) and Fei Huang would also like to acknowledge the scholarship by China Scholarship Council for the scholarship. This work was also supported by National Natural Science Foundations of China 14


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(Nos. 21377023 and 21477019).

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Graphical abstract:

19


CdSe Quantum Dot Sensitized Solar Cells with

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