Nanocrystal Hybrid Solar Cells

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Aqueous-Processed Insulating Polymer/Nanocrystal Hybrid Solar Cells Gan Jin,† Zhaolai Chen,‡ Chunwei Dong,‡ Zhongkai Cheng,‡ Xiaohang Du,‡ Qingsen Zeng,‡ Fangyuan Liu,‡ Haizhu Sun,*,† Hao Zhang,‡ and Bai Yang*,‡ †

College of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: A novel kind of hybrid solar cell (HSC) was developed by introducing water-soluble insulating polymer poly(vinyl alcohol) (PVA) into nanocrystals (NCs), which revealed that the most frequently used conjugated polymer could be replaced by an insulating one. It was realized by strategically taking advantage of the characteristic of decomposition for the polymer at annealing temperature, and it was interesting to discover that partial decomposition of PVA left behind plenty of pits on the surfaces of CdTe NC films, enlarging surface contact area between CdTe NCs and subsequently evaporated MoO3. Moreover, the residual annealed PVA filled in the voids among spherical CdTe NCs, which led to the decrease of leakage current. An improved shunt resistance (increased by ∼80%) was achieved, indicating the charge-carrier recombination was effectively overcome. As a result, the new HSCs were endowed with increased Voc, fill factor, and power conversion efficiency compared with the pure NC device. This approach can be applied to other insulating polymers (e.g., PVP) with advantages in synthesis, type, economy, stability, and so on, providing a novel universal cost-effective way to achieve higher photovoltaic performance. KEYWORDS: aqueous-processed, polymer, nanocrystal, hybrid, solar cell

1. INTRODUCTION In recent years, nanocrystal solar cells (NCSCs) and conjugated polymer/NC hybrid solar cells (HSCs) fabricated from solution have been developed rapidly because of their advantages such as large area coverage, low-cost fabrication, and broad spectral tunability to match the sun’s spectrum.1−8 Especially, the usage of solvents such as clean water is advocated because of their environmentally friendly ideology without sacrificing the superiority of the third generation solar cells.9−16 For example, CdTe NCSCs with power conversion efficiency (PCE) of 3.98% and methoxy-substituted poly(phenylenevinylene) (MPPV):CdTe HSCs with PCE of 5.18% have been fabricated through aqueous methods.12,17 The results showed that the introduction of conjugated polymers into NCs effectively improved the photovoltaic performance because the charge-carrier lifetime and interfacial carrier injection were largely enhanced. Most studies have concentrated on designing conjugated polymers in HSCs;18−20 however, it is attractive to replace the conjugated polymer with the insulating one due to the advantages in synthesis, types, economy, stability, as well as large-area applications. In addition, a high shunt resistance (Rsh) which is probably related to the device structure and film morphology is required for an efficient solar cell. A small Rsh lowers the current flowing through diode (junction) and, hence, reduces the Voc. © XXXX American Chemical Society

Therefore, the Rsh needs to be maximized to relieve the power loss caused by the current that bypasses the device junction and loads through an alternate current path due to the low Rsh.21−24 To improve the Rsh, one effective approach is to enhance the collection of charges from the hole transport layer (HTL) and/or electron transport layer (ETL), which can be achieved by improving the surface contact area between active layer and charge-carrier transport layer. The surface contact area is normally enlarged through building different structures of charge transport layer such as porous and clubbed TiO2, ZnO or etching active layer such as Si.25−31 For example, Sargent and co-workers utilized ZnO nanowires as template and filtrated with TiO2 to obtain nanowire network electrode, which exhibited superior performance than the planar one.26 These methods have made significant progresses in improving photovoltaic performance in spite of the complex operational process. Another problem in SCs is the existence of leakage current especially in sintered NCSCs because the voids among spherical NCs may accommodate evaporated electrode materials.32,33 Therefore, it is a challenge to develop novel method that can enhance the surface contact area between Received: January 5, 2016 Accepted: March 2, 2016

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DOI: 10.1021/acsami.6b00155 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Device structure of pure CdTe NCs with thickness of (a) 100 and (b) 140 nm, PVA:CdTe with thickness of (c) 140 nm, and (d) corresponding J−V curves. NCs solution was centrifuged at a speed of 8000 rpm with the addition of isopropyl alcohol to remove superfluous salts and MA. Subsequently, the NCs was dried in a vacuum oven and then dissolved in deionized water at concentrations of 70 and 120 mg mL−1. The charaterization of the obtained CdTe NCs is given in Figure S1. 2.4. Preparation of Aqueous PVA:CdTe Solution. The aqueous PVA:CdTe solution was prepared by adding PVA water solution with different concentrations into MA-capped CdTe NCs solution (70 mg mL−1) and then stirred strongly for a uniform mixed solution. The solution volume ratio of PVA to CdTe was 1:40. 2.5. Preparation of the Aqueous-Processed Devices. The photovoltaic devices were constructed by combining the spin-coating and vacuum evaporation methods. ITO-coated glasses first underwent ultrasonic treatment in chloroform, acetone, and isopropyl alcohol and were rinsed with deionized water before drying in N2 flow, followed by oxygen plasma treatment for 5 min. Then spin-coating of the TiO2 layer (∼30 nm) at 2000 rpm for 10 s, followed by annealing for 20 min at 450 °C in ambient condition to convert the TiO2 precursor into anatase-phase TiO2. For the CdTe NCSCs, the first active layer (∼90 nm) was fabricated by spin-coating the aqueous solution of CdTe NCs (120 mg mL−1) at a speed of 700 rpm for 60 s in ambient condition, and subsequently annealed at 300 °C in the glovebox for 2 min. Then, the same process was repeated for the second layer (∼50 nm) except that the concentration of CdTe NCs solution was 70 mg mL−1. For the PVA:CdTe HSCs, the first active layer (∼70 nm) was fabricated by spin-coating the PVA:CdTe solution at a speed of 700 rpm for 60 s in ambient condition, and subsequently annealed at 300 °C in the glovebox for 2 min. The same process was repeated for the second layer (∼70 nm) except that the annealing time was 1 h. Finally, a 10 nm MoO3 film and 60 nm Au electrodes were evaporated on top of the active layer through a mask at a pressure below 10−5 Torr, leading to an active area of 5 mm2. 2.6. Characterization. UV−vis spectra were acquired on a Shimadzu 3600 UV−vis−NIR spectrophotometer. Atomic force

active layer and transport layer, and simultaneously avoid leakage current. In this work, water-soluble poly(vinyl alcohol) (PVA) was incorporated into aqueous-processed CdTe NCs to obtain insulating polymer/NC HSCs in which the surface contact area between CdTe NCs and the evaporated HTL (MoO3) was enlarged and at the same time, the voids among CdTe NCs were successfully filled in. These two factors played significant roles in improving the Rsh of about 80%. As a consequence, the photovoltaic performance was enhanced with increased Voc and fill factor (FF) compared with the pure CdTe NCSCs.

2. EXPERIMENTAL SECTION 2.1. Materials. Tellurium powder (200 mesh, 99.8%) and PVA (average Mw 13 000−23 000, 98% hydrolyzed) were purchased from Aldrich Chemical Corp. 2-Mercaptoethylamine (MA, 98%) was obtained from Acros. Sodium borohydride (NaBH4, 99%) and CdCl2 (99+ %) were commercially obtained. All of the solvents were of analytically pure grade and used as received. 2.2. Preparation of TiO2 Precursor. Typically, 4 mL of tetrabutyl titanate was dissolved in 2 mL of isopropyl alcohol in a conical flask for 5 min. 210 μL water and 17 μL concentrated HCl were mixed with 4 mL isopropyl alcohol for 5 min. Then, this solution was dropped into the conical flask over about 10 min, and the mixture was stirred for 12 h at room temperature. Before use, the resultant TiO2 precursor was diluted with isopropyl alcohol. 2.3. Preparation of Aqueous MA-Capped CdTe NCs. In a typical synthesis, a freshly prepared solution of NaHTe was injected into 12.5 mM N2-saturated CdCl2 solutions in the presence of MA at a pH range of 5.5−6.0. The molar ratio of Cd/MA/Te was set as 1:2.4:0.2. The resultant precursor solution was refluxed at 100 °C for 60 min to maintain the growth of CdTe NCs. After preparation, the B


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ACS Applied Materials & Interfaces microscopy (AFM) images were recorded in tapping mode with a Digital Instruments NanoScope IIIa under ambient conditions. The film thicknesses were measured on an Ambios Tech XP-2 profilometer. The current density−voltage (J−V) characterization of PV devices under dark condition and white-light illumination from an SCIENCETECH 500-W solar simulator (AM 1.5G, 100 mW cm−2) was carried out on computer-controlled Keithley 2400 Source Meter measurement system in air. External quantum efficiency (EQE) was measured under illumination of monochromatic light from the xenon lamp using a monochromator (JobinYvon, TRIAX 320) and detected by a computer-controlled Stanford SR830 lock-in amplifier with a Stanford SR540 chopper. Thermal gravity analysis (TGA) was studied with a Q500 thermalgravimetric analyzer (American TA Company). The electrochemical impedance spectra (EIS) measurements were conducted from a CHI 660E electrochemical workstation in dark condition at zero bias voltage with a frequency ranging from 1 Hz to 100 kHz. The transient absorption (TA) setup consisted of 400 nm pump pulses doubled from 800 nm laser pulses (∼100 fs duration, 250 Hz repetition rate) generated from a mode-locked Ti:sapphire laser/ amplifier system (Solstice, Spectra-Physics) and broadband white-light probe pulses generated from 2 mm thick water. The relative polarization of the pump and the probe beams were set to the magic angle. The TA data were collected using a fiber coupled spectrometer connected to a computer. X-ray diffraction (XRD) patterns were identified by Rigaku D/Max 2500 V/PC X-ray diffractometer with graphite monochromated Cu Kα radiation (λ = 0.15418 nm). X-ray photoelectron (XPS) was investigated by using ESCALAB 250 spectrometer with a mono X-ray source Al Ka excitation (1486.6 eV). FT-IR spectra were taken on a Nicolet AVATAR 360 FT-IR spectrophotometer. Transmission electron microscopy (TEM) images were recorded on a JEOL-2010 electron microscope operating at 200 kV. All the other measurements were performed under ambient atmosphere at room temperature.

heating. Figure 2a presents the TGA curve of the pure PVA. The first 28 min corresponds to the period of increasing the

Figure 2. (a) TGA curve of PVA. (b) FT-IR spectra of PVA and annealed PVA.

temperature from room temperature to 300 °C, while the subsequent 1 h corresponds to the annealing period under 300 °C. This heat treatment process is the same with that of PVA:CdTe device fabrication. It is clearly observed that decomposition occurs on PVA, and less than half of the original weight resides after 1 h annealing. Figure 2b shows FTIR spectra of PVA before and after annealing. Obviously, the hydroxyl peak (∼3400 cm−1) and aldehyde group peak (∼1730 cm−1) decrease abruptly after annealing while new peaks of ether bond (∼1156 cm−1), ketone group (∼1705 cm−1), CC (∼1500 cm−1), and C−H in CC (∼3020 cm−1) form. Therefore, the decomposition of PVA happens probably because the hydroxyl of the side chain possesses potential reactions of elimination and condensation or the backbone has a possibility of breakage. Although it is difficult to explicitly characterize the exact structure of annealed PVA, the possible structure has been inferred and listed in Figure S2. Photographs of PVA and annealed PVA are provided in the Supporting Information (Figure S3) and show the fluorescence of annealed PVA excited at 365 nm, which suggests the existence of conjugated structures (maybe −CC−CC−, −CC−O− CC−, etc.). The photoluminescence (PL) spectrum of annealed PVA excited at 365 nm is provided in Figure S4, further indicating the formation of the conjugated structure. Then, it is guessed that chemical reaction might occur between PVA and CdTe NCs or the changed PVA during annealing might accept charge carriers and hence improve the device performance. Considering that 2-mercaptoethylamine (2-MA), the ligands on the surface of the CdTe NCs, contains characteristic C, N, S elements while PVA contains O element, XPS analysis for Cd 3d, Te 3d, C 1s, N 1s, S 2p and O 1s is used to characterize the valence states of these elements. It is observed from Figure 3 that no changes occur in valence states between identical

3. RESULTS AND DISCUSSION 3.1. Device Structures. Figure 1 shows the structures of the photovoltaic devices, in which TiO2 and MoO3 are used as electron and hole transport layer, respectively. The active layers are processed by spin-coating method. The active layers in Figure 1a,b are the pure CdTe NCs while it is the mixture of PVA and CdTe (PVA:CdTe) in Figure 1c. Figure 1d gives the J−V curves of the three devices. The black line is the J−V curve of CdTe device with the total active-layer thickness of 100 nm, which has a PCE of 2.56% (black ■). When PVA is introduced into CdTe, the PCE increases to 4.32% (blue ★), improved about 70%. Moreover, it is found that the thickness of the active layer increases to 140 nm under the same rotate speed after adding PVA because of the high viscosity of PVA water solution. Undoubtedly, this will save raw materials during the fabrication of the devices. Since a thicker active layer leads to more absorbance within a range of thickness, the higher Jsc and PCE may be resulted from the change of the thickness. Therefore, the device of the pure CdTe NCs with the same active-layer thickness of 140 nm is fabricated. The result shows that the PCE is 3.63% (red ●), which is still lower than that of PVA:CdTe device. Besides, even if four layers of the pure CdTe NCs with total thickness of ∼200 nm are used as active layer, the record PCE (3.98%) is still lower than that of PVA:CdTe device.16 Therefore, we conclude that introducing PVA can enhance the PCE of pure CdTe NCSCs. 3.2. Partial Decomposition of PVA. To investigate the mechanism of PCE improvement, we first characterized PVA by TGA measurement considering that the device undergoes a high temperature annealing process. As known, thermostability is an important problem because macromolecules always decompose into various molecules during high temperature C


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Figure 3. XPS spectra of Cd 3d, Te 3d, C 1s, N 1s, S 2p, and O 1s in CdTe and PVA:CdTe films. There exhibit no changes in valence states between identical elements after the introduction of PVA, indicating that no reaction occur between CdTe and PVA after annealing.

calculated hole and electron mobilities of CdTe are 1.50 × 10−4 and 1.85 × 10 −4 cm2 V −1 s −1 , respectively, and the corresponding parameters of PVA:CdTe are 1.38 × 10−4 and 1.89 × 10−4 cm2 V−1 s−1, respectively. The comparable mobilities between CdTe and PVA:CdTe indicate introducing trace of PVA will not affect the charge-carrier mobilities of CdTe. The results of XRD, XPS and SCLC analysis suggest that no reactions occur between PVA and CdTe even under high temperature of 300 °C heat treatment. Figure 4c exhibits the hole and electron mobility of the annealed PVA. The device structures measuring hole and electron mobility are ITO/PEDOT:PSS/PVA/MoO3/Au and ITO/TiO2/PVA/Al, respectively. The thickness of the PVA is about 130 nm detected by the Ambios Tech. XP-2 profilometer. The calculated hole-only mobility is 2.14 × 10−8 cm2 V−1 s−1, and the electron-only mobility is 6.77 × 10−10 cm2 V−1 s−1. This charge-carrier mobility is too poor to act as a suitable transport layer compared with CdTe itself (10−4 magnitude of hole and electron calculated above). In addition, the J−V curves of designed planar heterojunction (PHJ) ITO/ TiO2/CdTe (∼140 nm)/PVA/MoO3/Au devices with different

elements after introducing PVA. The XRD patterns of CdTe with and without PVA are shown in Figure 4a. The peaks at 23.60, 39.36, and 46.56° are assigned to the (111), (220) and (311) faces of CdTe. The consistent peak positions demonstrate that introduction of PVA will not change the crystalline of CdTe. Figure 4b exhibits the charge-carrier mobilities of CdTe and PVA:CdTe through space-chargelimited-current (SCLC) method. The device structures measuring hole and electron mobility are ITO/PEDOT:PSS/ CdTe (or PVA:CdTe) /MoO3/Au and ITO/TiO2/CdTe (or PVA:CdTe)/Al, respectively. The charge-carrier mobility is calculated according to the Mott−Gurney equation: J = 9ε0εr μ(V − Vbi − Vr)2 /8L3

where ε0 is the permittivity of free space (8.85 × 10−12 F m−1), εr is the dielectric constant of CdTe (assume to 10), μ is the hole/electron mobility, V is the applied voltage, Vr is the voltage drop due to contact resistance and series resistance across the electrodes, Vbi is the built-in voltage and L is the film thickness. The thickness of the CdTe and PVA:CdTe is about 140 nm detected by the Ambios Tech. XP-2 profilometer. The D


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Figure 4. (a) XRD patterns of CdTe with and without PVA. (b) Charge-carrier mobility of CdTe and PVA:CdTe. (c) Charge-carrier mobility of annealed PVA. (d) J−V curves of PHJ ITO/TiO2/CdTe (∼140 nm)/PVA/MoO3/Au devices with different PVA thickness.

Figure 5. AFM (5 × 5 μm) height images of (a) CdTe (b) PVA:CdTe and (c, d) corresponding sections located at diagonals.

PVA thickness are presented in Figure 4d. The details are summarized in Table S1. The Jsc and FF drop sharply compared with CdTe device, indicating that there is hindrance in carrier transport. This result suggests that the annealed PVA acts as insulator even with an ultrathin thickness. Therefore, it is concluded that neither the chemical reaction between PVA and CdTe NC nor the transportation of charge carriers by PVA occurs after annealing. 3.3. Morphology and Dark J−V Measurement. It is reasonable to investigate the change of the active layer

morphology in the following studies. Tapping mode AFM is applied to characterize the microstructure of the active layer. Figure 5a,b exhibit the height graphs of CdTe and PVA:CdTe films. Large CdTe NCs (∼35 nm) form after annealing under 300 °C to ensure the formation of interpenetrated network according to our previous report.12 The CdTe film shows rough surface with a root-mean-square (RMS) roughness of 4.753 nm, which increases into 6.457 nm after doping with PVA, indicating that introduction of PVA can improve the roughness of the film. This enhanced surface roughness is mainly formed E


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Figure 6. (a) TA spectroscopy of CdTe/MoO3 and PVA:CdTe/MoO3. (b) J−V curves of CdTe and PVA:CdTe devices under dark condition. (c) EIS of the CdTe and PVA:CdTe devices. (d) J−V curves of PVA:CdTe devices with different PVA concentrations.

plays an important role in filling in the voids, which is confirmed through the dark current measurement shown in Figure 6b. The CdTe device (140 nm) displays a high dark current in the reverse direction while this region is distinctly suppressed in the PVA:CdTe device (140 nm). The voids among spherical CdTe NCs are filled in with annealed PVA, and therefore, the leakage current is effectively restricted because the penetration of the evaporated electrode materials through the active layer is confined. This result demonstrates that decomposed PVA will not generate pinholes that lead to leakage current because the depth of the pits is too shallow compared to the film thickness. Therefore, introducing PVA can increase surface contact area between CdTe layer and MoO3 and simultaneously avoid leakage current. 3.4. Electrochemical Impedance and Photovoltaic Performance. The measured EIS of the CdTe and PVA:CdTe devices in dark under zero bias are shown in Figure 6c. The series resistance (Rs) and Rsh of the CdTe device are measured to be 3.4 and 3038 Ω cm2, and the corresponding parameters of PVA:CdTe device are 18.6 Ω and 5535 Ω cm2, respectively. Apparently, the PVA:CdTe device has a tremendous improvement in the Rsh, indicating that the extraction of hole by MoO3 is largely improved. The slightly increased Rs is a result of the introduction of insulating polymer PVA, which has an adverse effect on Jsc. However, it is accessible because of the relatively large augment in FF and Voc, which finally leads to a decent enhancement in PCE. The proposed mechanism is consistent with the improvements of FF and Voc for PVA:CdTe devices. In Table 1, the FF (48.2%) and Voc (0.50 V) of CdTe device with a thickness of 140 nm are higher than CdTe device (45.2% and 0.48 V) with a thickness of 100 nm. The enhanced FF and Voc explain that thickening active layer leads to decreased leakage current due to the existence of pinholes in the CdTe layers. Moreover, under the same thickness (140 nm), PVA:CdTe device exhibits more excellent values (53.9% and 0.56 V) compared with CdTe (140 nm) device. Thus, it is suggested that introduction of PVA

by the decomposition of PVA, which leads to plenty of pits on the surface of CdTe NC film. Figure 5c and d are the sections of CdTe and PVA:CdTe films, showing the fluctuation of film surface. The selected sections are located in the diagonals (black lines) taken from Figure 5a,b. These pits are measured to be ∼20 nm in depth and ∼100 nm in width. As a result, the surface contact area is enlarged between CdTe layer and subsequently evaporated MoO3, which is benefit for the hole collection. The AFM graphs of CdTe and PVA:CdTe before annealing are presented in Figure S5, which shows that the film is very smooth (RMS = 0.616, 0.794 nm, respectively). This further indicates that the enhanced surface roughness is mainly formed after the decomposition of PVA. TA spectroscopy is used to characterize the hole transfer from CdTe to MoO3. From Figure 6a, the TA decay spectrum of PVA:CdTe/MoO3 exhibits faster decay rate than that of CdTe/MoO3, that is to say, after introducing PVA, the hole transfer yield is enhanced. This is the consequence of enlarged surface contact area between CdTe layer and MoO3. To visually describe this process, Scheme 1 is imported to demonstrate the hole transfer from CdTe to MoO3 in these two devices in which large contact area leads to better hole collection. On the other hand, the residual annealed PVA still Scheme 1. Illustration of Hole Transfer from CdTe NCs to MoO3 in CdTe and PVA:CdTe Devices

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ACS Applied Materials & Interfaces Table 1. Different Photovoltaic Devices with Champion PCE and Corresponding Jsc, Voc, and FF Valuesa device CdTe (100 nm) CdTe (140 nm) PVA:CdTe (140 nm) a

Jsc (mA cm−2)

Voc (V)

FF (%)

PCE (%)

11.8 [ 11.5 ± 0.4] 15.1 [14.5 ± 0.6] 14.3 [14.2 ± 0.3]

0.48 [0.476 ± 0.012] 0.50 [0.496 ± 0.013] 0.56 [0.554 ± 0.012]

45.2 [43.5 ± 3.2] 48.2 [46.6 ± 3.1] 53.9 [52.5 ± 1.4]

2.56 [2.28 ± 0.28] 3.63 [3.33 ± 0.30] 4.32 [4.17 ± 0.15]

Values in brackets are averages; 10 devices are tested to obtain average values.

leads to improved photovoltaic performance by suppressing the leakage current and simultaneously enhancing surface contact area between CdTe and MoO3. Figure 6d presents photovoltaic performance with different concentrations of PVA solution. It is observed that the PCE of PVA:CdTe devices with PVA concentrations of 2, 3, and 4% are 4.32, 4.18, and 3.85%, respectively, which are higher than that of the pure CdTe NCSCs (3.63%) under the same thickness. When the concentration of PVA solution reaches 8%, the photovoltaic performance is severely restricted due to the vast introduction of insulating polymer PVA and the Jsc has a dramatic decrease. The details are summarized in Table S2, implying that the advisible range of PVA concentration is 2− 4%. In addition, the influence of annealing time and temperature on the morphology and performance of PVA:CdTe devices has been systematically investigated in which the optimized temperature is 300 °C and the time is 1 h (Table S3−S7 and Figure S6−S10). Figure 7a presents the absorption spectra of annealed PVA, CdTe, and PVA:CdTe with the same film thickness of ∼140 nm. The annealed PVA has a weak absorption before 370 nm. For the same thickness of CdTe and PVA:CdTe layers, the PVA:CdTe layer consists of CdTe, annealed PVA and a part of pits, indicating the content of CdTe is a little lower than that of the pure CdTe layer. Therefore, the absorption of the pure CdTe layer is slightly higher than that of PVA:CdTe layer. Figure 7b presents the EQE curves of CdTe (100 and 140 nm) and PVA:CdTe (140 nm) devices. It is easy to understand that the EQE values of the 100 nm CdTe device are lower than those of 140 nm CdTe device because of the thinner film. The EQE values of PVA:CdTe (140 nm) device are almost the same with those of CdTe (140 nm) device between 370 and 800 nm. Although the values of PVA:CdTe are lower than those of CdTe between 300 and 370 nm, which results from the absorbance of light by annealed PVA, the ratio of this waveband (300−370 nm) to the whole absorption (300−800 nm) is too little to affect the current. This result indicates that the added polymer should possess extremely weak or no absorption in order to minimize the waste of the absorbed light. Not much difference between the two currents of CdTe and PVA:CdTe devices demonstrates that increased surface contact area between active layer (CdTe) and HTL (MoO3) do a positive effect on improving Jsc even if PVA:CdTe layer possesses weaker absorption and the residual annealed PVA also hinders charge transport between CdTe NCs. It is strongly believed that some other insulating polymers can be used according to our findings. We tried other insulating polymers such as polyvinylpyrrolidone (PVP) and chitosan. The PVP:CdTe device (film thickness of ∼100 nm) obtains PCE of 2.85% with Jsc of 9.96 mA cm−2, Voc of 0.56 V and FF of 51.1% (Figures 7c and 8 and Table S8), which is better than the pure CdTe device (100 nm). However, because too much

Figure 7. (a) UV−vis−NIR absorption spectra of PVA (black), CdTe (red), PVA:CdTe (blue) after annealing. (b) EQE curves of 100 and 140 nm CdTe and 140 nm PVA:CdTe devices. (c) J−V curves of chitosan:CdTe, PVP:CdTe, PPV:CdTe, and PVA:PPV:CdTe devices with active-layer thickness of ∼100 nm.

decomposition happened on chitosan under 300 °C annealing, the pits are measured to be hundreds of nanometers in width and ∼35 nm in depth. Therefore, chitosan:CdTe device gets an obviously decrease in Voc, FF and PCE because the huge pinholes form after annealing which leads to leakage current (Figures 7c and 8 and Table S8). This result suggests that G


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Figure 8. (a) AFM analysis charts of chitosan:CdTe film. RMS = 15.382 nm. (b) AFM analysis charts of PVP:CdTe film. RMS = 6.475 nm. (c) AFM analysis charts of PPV:CdTe film. RMS = 2.363 nm. (d) AFM analysis charts of PVA:PVP:CdTe film. RMS = 6.729 nm.

current, leading to enhanced photovoltaic performance including Rsh, FF, and Voc. The selected polymers should be limited by the following conditions: (1) no reactions with active layer, (2) absorption as weak as possible, and (3) partial decomposability under required temperature. Insulating polymers such as PVA have advantages in synthesis, types, economy, and stability. This work provides a novel universal avenue toward improved photovoltaic performance, which is suitable for large-area applications.

appropriate decomposition temperature is needed for the selected insulating polymers. We introduced PVA into poly(phenylenevinylene) (PPV):CdTe HSC (film thickness of 100 nm) and achieved higher photovoltaic performance. The PPV:CdTe film is smooth and the pits (∼5 nm depth) are resulted from elimination of tetrahydrathiphane in the branch of PPV precursor under 300 °C annealing (Figure 8c). The PVA:PPV:CdTe film is rougher than PPV:CdTe film and the pits are measured to be ∼20 nm (Figure 8d). The enhanced factors are Voc and FF (Figure 7c and Table S8), indicating that this method is of help for improving the performance of conjugated polymer/NC HSCs. The photovoltaic performance of chitosan:CdTe and PVP:CdTe devices under different annealing temperature are exhibited in Figures S11 and S12 and in Tables S9 and 10 in which chitosan shows the best result at 350 °C and PVP shows the best results at 300 °C.

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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.6b00155. UV spectra, TEM image, and size distribution of assynthesized CdTe NCs; possible structures of annealed PVA; photo images of PVA and annealed PVA; PL spectra of annealed PVA; AFM height graphs of CdTe and PVA:CdTe before annealing; photovoltaic performance about PHJ devices with different PVA thickness; optimization of the PVA:CdTe devices morphology and performance under different annealing temperature and time; and optimization of chitosan:CdTe and PVP:CdTe devices under different annealing temperature. (PDF)

4. CONCLUSIONS In summary, we have fabricated insulating polymer/NC HSCs and investigated the role of insulating polymer in the cells. The pyrolytic property of the polymer resulted in a preferable surface morphology between active layer and HTL. The enlarged surface contact area guaranteed efficient hole transfer and the residual annealed polymer suppressed the leakage H


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel: +86-431-85099667. Fax: +86-431-85099667. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NSFC (51433003, 21574018), the Science Technology Program of Jilin Province (20130204025GX), the National Basic Research Program of China (2014CB643503), Jilin Provincial Education Department (543),and Jilin Provincial Key Laboratory of Advanced Energy Materials (Northeast Normal University).

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ACS Applied Materials & Interfaces (31) Wang, H.; Kubo, T.; Nakazaki, J.; Kinoshita, T.; Segawa, H. PbS-Quantum-Dot-Based Heterojunction Solar Cells Utilizing ZnO Nanowires for High External Quantum Efficiency in the near-Infrared Region. J. Phys. Chem. Lett. 2013, 4, 2455−2460. (32) Jasieniak, J.; MacDonald, B. I.; Watkins, S. E.; Mulvaney, P. Solution-Processed Sintered Nanocrystal Solar Cells Via Layer-byLayer Assembly. Nano Lett. 2011, 11, 2856−2864. (33) Dongaonkar, S.; Servaites, J. D.; Ford, G. M.; Loser, S.; Moore, J.; Gelfand, R. M.; Mohseni, H.; Hillhouse, H. W.; Agrawal, R.; Ratner, M. A.; Marks, T. J.; Lundstrom, M. S.; Alam, M. A. Universality of Non-Ohmic Shunt Leakage in Thin-Film Solar Cells. J. Appl. Phys. 2010, 108, 124509.

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