Naphthalene Diimide-Based n-Type Polymers: Efficient Rear

Jul 5, 2017 - Naphthalene Diimide-Based n-Type Polymers: Efficient Rear Interlayers for High-Performance Silicon–Organic Heterojunction Solar Cells ...
30 downloads 13 Views 6MB Size
Download PDF
Naphthalene Diimide-Based n‑Type Polymers: Efficient Rear Interlayers for High-Performance Silicon−Organic Heterojunction Solar Cells Yujie Han, Yuqiang Liu, Jianyu Yuan,* Huilong Dong, Youyong Li, Wanli Ma, Shuit-Tong Lee, and Baoquan Sun* Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou 215123, China S Supporting Information *

ABSTRACT: Silicon−organic heterojunction solar cells suffer from a noticeable weakness of inefficient rear contact. To improve this rear contact quality, here, two solution-processed organic n-type donor− acceptor naphthalene diimide (NDI)-based conjugated polymers of N2200 and fluorinated analogue F-N2200 are explored to reduce the contact resistance as well as to passivate the Si surface. Both N2200 and F-N2200 exhibit high electron mobility due to their planar structure and strong intermolecular stacking, thus allowing them to act as excellent transporting layers. Preferential orientation of the polymers leads to reduce contact resistance between Si and cathode aluminum, which can enhance electron extraction. More importantly, the substitution of fluorine atoms for hydrogen atoms within the conjugated polymer can strengthen the intermolecular stacking and improve the polymer−Si electronic contact due to the existence of F···H interactions. The power conversion efficiencies of Si-PEDOT:PSS solar cells increased from 12.6 to 14.5% as a consequence of incorporating the F-N2200 polymer interlayers. Subsequently, in-depth density functional theory simulations confirm that the polymer orientation plays a critical role on the polymer−Si contact quality. The success of NDI-based polymers indicates that planar conjugated polymer with a preferred orientation could be useful in developing high-performance solution-processed Si−organic heterojunction photovoltaic devices. KEYWORDS: Si−organic hybrid solar cell, conjugated polymer, fluorination, density functional theory, interfaces

S

surface carrier recombination. A traditional method of Ohmic contact between Al and Si via high doping Si side is generally used to improve the electrical contact properties.23 However, this process requires a high temperature (∼800 °C) as well as toxic gases, such as diborane or phosphine. Consequently, several alternative low-temperature schemes have been proposed to replace the high-temperature doping method, such as incorporating 8-hydroxyquinolinolato lithium,24 poly(ethylene oxide),25 organic electrolytes,2 cesium carbonate,19 and titanium oxide26,27 layer between Si and Al. These methods effectively improve the junction quality between Si and Al without the use of high temperatures or toxic gases. However, the inherent coupling interaction between the modified layer and Si is still unclear. Recently, n-type conjugated polymers have proven to be crucial in organic non-fullerene solar

ilicon−organic heterojunction solar cells combine the advantages of high-performance crystalline Si with lowcost organic materials, which has the potential to be an alternative Si solar cell with a high power conversion efficiency (PCE).1−10 After a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer is deposited on ntype Si, a strong inversion layer is formed on the Si side; this promotes the separation and extraction of photogenerated electrons−holes.2,11,12 During the past decade, several schemes, such as light-harvest texture,13−15 solvent treatment,16−18 and surface modification,19−21 have been proposed to improve the Si−organic performance. These schemes lead to the PCE of SiPEDOT:PSS solar cells up to ∼16%;22 however, the PCE still failed to match that of a traditional homojunction Si solar cell. Therefore, the research on the development of Si−organic solar cells needs to be further explored to boost device performance. Currently, aluminum cathodes are directly deposited on the Si rear side for electron collection, which jeopardizes the performance of the solar cell due to the Schottky barrier and © 2017 American Chemical Society

Received: May 4, 2017 Accepted: July 5, 2017 Published: July 5, 2017 7215

DOI: 10.1021/acsnano.7b03090 ACS Nano 2017, 11, 7215−7222

Article

www.acsnano.org

Article

ACS Nano

Figure 1. (a) Synthesis routes of polymers N2200 and F-N2200. 2D-GIWAXS patterns of (b) N2200 and (c) F-N2200 films on a Si(100) substrate. Schematic of intermolecular planarity stacking of (d) N2200 and (e) F-N2200 on a Si substrate.

cells.28,29 In comparison to semiconducting metal oxide materials, various conjugated organic molecules exhibit excellent film-forming properties. More importantly, the electrical properties can be modulated by tailoring the molecular structure. Naphthalene diimide (NDI)-based conjugated polymers have proven to be the most successful n-type polymer, owing to their planar structure, sufficient intermolecular contacts, and high electron mobility.30,31 However, nonfullerene n-type conjugated polymers have never been as well explored as the interlayers in Si−organic hybrid solar cells. Herein, the widely used NDI-based polymer poly{[N,N0bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6diyl]-alt-5,50-(2,20-bithiophene)} [P(NDI2OD-T2)); Polyera ActivInk N2200] and its fluorinated analogue F-N2200 are adopted as the interfacial layers between Si and Al for Si− organic heterojunction solar cells.32,33 Besides the high mobility and planarity, the lowest unoccupied molecular orbital (LUMO) of both polymers are closed to the conduction band (CB) of Si (−4.05 eV), allowing electrons to easily extract to the polymer layer. The offset between the polymer highest occupied molecular orbital (HOMO) and Si valence band (VB) is large (∼0.32 eV) enough to block holes.33 The peak PCE of a solar cell with N2200 and F-N2200 increases from 12.6% (reference device) to 13.5 and 14.5%, respectively. Density functional theory (DFT) simulation and experimental results indicate that both N2200 and F-N2200 enhance the interfacial

contact quality, which is beneficial for solar cell performance. Moreover, substitution of the fluorine atom for hydrogen within the conjugated polymer can both enhance the polymer intermolecular stacking and decrease the contact distance between Si and the polymer backbone from 5.4 Å (Si/N2200) to 4.2 Å (Si/F-N2200), which strengthens their electronic coupling. According to the measurements including minority carrier lifetimes, surface potential, contact resistance, and transient photovoltage decay, it indicates that this simple efficient strategy can effectively modify the rear interface between Si and Al to further enhance charge transfer and builtin potential (Vbi). These results suggest that the conjugated polymer may act as general and facile interlayers that enhance the performance of Si−organic photovoltaic devices.

RESULTS AND DISCUSSION Chemical structures of both polymers are shown in Figure 1a. N2200 is synthesized following our previous report.34 F-N2200 is synthesized by a similar procedure using a high-yield Stille cross-coupling polymerization between fluorinated distannide bithiophene and a dibromide dialkyl NDI unit with palladium catalysts. Both of them exhibit similar solubility in common organic solvents such as chlorobenzene and chloroform. The synthetic details are listed in the Supporting Information. Average molecular weights (M n) and dispersities (Đ) determined by gel permeation chromatography confirm the 7216

DOI: 10.1021/acsnano.7b03090 ACS Nano 2017, 11, 7215−7222

Article

ACS Nano

Figure 2. (a) Device structure of Si/organic solar cell; (b) J−V characteristics under simulated AM 1.5G illumination at 100 mW cm−2; (c) J− V characteristics in the dark; (d) corresponding EQE curves of the optimized Si/organic solar cell devices with/without a polymer interlayer.

microscopy (AFM) are performed to investigate relevant structural features in solid polymer thin films. We examined the polymer films casted from chlorobenzene solutions on a Si(100) substrate. The 2D-GIWAXS scattering images for polymer films are shown in Figure 1b,c, and the inplane and out-of-plane line-cuts are presented in Figure S4. Both polymers favor a “face-on” orientation, with stronger π−π stacking reflection peaks in the out-of-plane direction. F-N2200 displays improved coplanarity orientation, and the calculated molecule face-to-face distances for the “π−π stacking” are around 4.0 Å for both polymers. It is worth noting that fluorinated polymer shows improved ordering and crystallinity along in-plane directions (Figure 1d,e), which may be attributed to the synergistic effect of F···S and F···H intramolecular interaction.35,36 AFM height images of N2200 and F-N2200 on a Si surface are shown in Figure S5. The root mean square values for F-N2200 and N2200 are ∼0.83 and ∼1.10 nm, respectively. According to GIWAX measurement, the F-N2200 layer is smoother and more uniform. It is known that charge transport properties in conjugated polymers are strongly affected by the molecular ordering and crystallinity in solid films.38−40 Therefore, F-N2200 is expected to exhibit better charge transport in the devices, which is verified by the following experiments. Si−organic heterojunction solar cells are based on the structure of silver grids/PEDOT:PSS/textured Si/polymer/Al, as shown in Figure 2a. Si substrates are etched by metal-ionassisted etching and further smoothed from tetramethylammonium hydroxide solution,41 which balances the textured Si surface for light-trapping and surface recombination. SEM image of the textured Si in Figure S6 exhibits irregular distributional inverted pyramids. PEDOT:PSS is spin-coated onto the Si front side followed by mild annealing. An inversion layer at the Si/PEDOT:PSS interface is expected to increase the built-in potential;12 meanwhile, PEDOT:PSS can also suppress

similar properties of these two polymers. The DFT simulations for the two polymers are first performed at the B3LYP/631G(d) level, with the alkyl chains of all the molecules truncated as methyl groups for more resource-effective calculations. Figure S1 shows the corresponding results from the DFT calculations and detailed dihedral angles between each unit on the backbone. It is considered that the atomic radius of F is similar to that of H (van der Waals radius, r = 1.35 Å, comparable to H, r = 1.2 Å), which does not cause too much deleterious steric hindrance.35 However, the dihedral angles between two thiophene rings increase from 165.7° (N2200) to 179.8° (F-N2200), implying the fluorination can improve planarity of the polymer backbone, which may be attributed to the noncovalent interaction between the F atom and sulfur atom.36 To explore the effect of fluorination on molecular properties, temperature-dependent ultraviolet−visible (UV−vis) absorption spectrum measurements are performed in dilute solutions of chlorobenzene (0.01 mg mL−1). According to the solution absorption spectra (Figure S2), it is worth noting that the vibronic peak of F-N2200 (∼700 nm) is more pronounced than that in N2200 solution, suggesting improved intermolecular contacts in F-N2200. As shown in Figure S2a,b, at an elevated temperature (100 °C), both polymers are well dissolved and disaggregated. With gradually decreasing temperatures, the polymers begin to aggregate and their absorption spectra show a red shift to a different extent. The drastic absorption shifts in both N2200 and F-N2200 indicate strong aggregation of the polymer chains even in solution.37 As shown in Figure S3, the thin film absorption curve of F-N2200 also exhibits a remarkable vibronic peak compared to that in N2200, indicating a better intermolecular contact in the solid state. Meanwhile, two-dimensional (2D) grazing incidence wideangle X-ray scattering (2D-GIWAXS) and atomic force 7217

DOI: 10.1021/acsnano.7b03090 ACS Nano 2017, 11, 7215−7222

Article

ACS Nano the carrier recombination at the interface.41 N2200 or F-N2200 are spin-coated on the Si rear side without any further treatment. Current density−voltage (J−V) characteristics of the optimized devices under simulated air mass (AM) 1.5G illumination at 100 mW/cm2 are shown in Figure 2b. All the electrical output characteristics are summarized in Table 1. A

standard AM 1.5G spectrum for the different devices, as shown in Figure S8. All the calculated Jsc values are within 5% error compared to the corresponding measured Jsc ones. However, all the devices still show a limited Jsc of ∼30 mA/cm2, which may result in parasitic absorption of PEDOT:PSS film as well as light reflection from the whole device.42 The recombination of ∼10 mA/cm2 to light reflection and ∼6 mA/cm 2 to PEDOT:PSS parasitic absorption results in low Jsc. As discussed above, although N2200 and F-N2200 share the same backbone, side-chain structure, and similar molecular weight, their related electrical output characteristics for the corresponding Si heterojunction device are different. The only difference between two polymers lies in replacement of H with a F atom in the thiophene rings. According to previous GIWAXS measurements, slightly improved crystallinity and morphology are observed in the F-N2200 film. We believed that the enhanced performance of the F-N2200-based device is attributed to improved electrical properties at the interfaces between Al and Si. In order to further clarify this effect, DFT simulation is used to gain insight into the mutual effects between Si and N2200/F-N2200 at the molecular level. Here, the Si(100) surface is chosen, which is consistent with experimental conditions. As shown in Figure 3, the simulated

Table 1. Electrical Output Characteristics of Si/Organic Solar Cells with and without a Polymer Interlayer, under Illumination of AM 1.5G at 100 mW cm−2 device structurea

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

reference N2200 F-N2200

0.613 0.631 0.635

29.2 29.9 31.1

0.702 0.718 0.733

12.6 13.5 14.5

a

Reference: Ag grids/PEDOT:PSS/Si/Al; N2200: Ag grids/PEDOT:PSS/Si/N2200/Al; F-N2200: Ag grids/PEDOT:PSS/Si/FN2200/Al.

reference device based on Ag grids/PEDOT:PSS/Si/Al structure exhibits a short-circuit current density (Jsc) of 29.2 mA/cm2, an open-circuit voltage (Voc) of 0.613 V, a fill factor (FF) of 0.702, and a PCE of 12.6%. When N2200 is deposited on the rear side of Si, the device exhibits an increased Jsc of 29.9 mA/cm2, a Voc of 0.631 V, and a FF of 0.718 and yields a slightly improved PCE of 13.5%. When we replace N2200 with F-N2200, the device achieves a higher PCE of 14.5%, with a Jsc of 31.1 mA/cm2, a Voc of 0.635 V, and a FF of 0.733. Obviously, all the parameters are enhanced when incorporating either N2200 or F-N2200 as the interlayer. In comparison with the reference device, the PCE exhibits ∼10 and ∼15% PCE enhancement for the device based on N2200 and F-N2200, respectively. Obviously, a solar cell device adopting fluorinated polymer F-N2200 achieves performance better than that of N2200. Figure 2c illustrates the J−V curves in the dark. The devices incorporating either N2200 or F-N2200 exhibit a lower reverse saturation current density (J0) that is directly correlated with Voc. The following equation interprets the relationship between J0 and Voc:

Voc =

⎞ KT ⎛ Jsc ln⎜⎜ + 1⎟⎟ q ⎝ J0 ⎠

(1)

Figure 3. DFT simulation of (a) N2200 and (b) F-N2200 on the Si(100) surface in three-dimensional view. The simulation is based on the micromolecular structure of the conjugated polymer instead of their monomer.

where K is Boltzmann’s constant, q is electron charge, and T is temperature. According to eq 1, the lower J0 of the devices based on N2200 or F-N2200 is beneficial to achieve higher Voc. In addition, the F-N2200-based device exhibits further improved diode properties. To evaluate the photoresponse of Si−organic solar cells with and without polymer interlayer, external quantum efficiency (EQE) of the optimized devices are measured, as shown in Figure 2d. Upon incorporating either N2200 or F-N2200, the device EQE values are enhanced in the long-wavelength spectra region compared to the reference one, and the device with FN2200 as an interlayer achieves the highest EQE values. In Figure S7, there is an obvious EQE enhancement with a mean value of ∼10% in the long-wavelength region for the F-N2200based device. It is known that rear side recombination could deteriorate the utilization of long-wavelength photons,27 thus the EQE enhancement in this region reveals the enhancement of rear side contact quality with the polymer interlayer. The calculated Jsc is obtained by integrating the EQE curve with a

distance between the Si(100) surface and polymer backbone of N2200 is 5.4 Å (Figure 3a). In the case of fluorinated polymer F-N2200, due to the initial F···H interaction, the simulated distance between the Si(100) surface and polymer backbone is reduced to 4.2 Å (Figure 3b). The simulated distance between F-N2200 and Si is 20% shorter than that between N2200 and Si. In addition, as mentioned above, F-N2200 exhibits more planar backbone structure on Si. A shorter distance between H (Si) and F (F-N2200) atoms and planarity of F-N2200 would result in its strong electrical coupling with Si. As shown Table S1, absorption energy between Si and F-N2200 is 0.53 eV, which is slightly larger than that of Si/N2200 (0.50 eV), which indicates efficient charge transfer between F-N2200 and Si. An exponential function is used to describe the relationship of 7218

DOI: 10.1021/acsnano.7b03090 ACS Nano 2017, 11, 7215−7222

Article

ACS Nano

Figure 4. Sketches of the device structure and I−V curves of different Al pad areas on (a) Si, (b) Si/N2200, and (c) Si/F-N2200. Al pad diameters are 1, 0.8, 0.5, 0.3, 0.2, and 0.1 cm.

Figure 5. (a) Minority carrier lifetime mapping of Si/N2200 (35 μs), Si (24 μs), and Si/F-N2200 (46 μs) on one Si substrate; the mapping area is 1 cm × 1 cm; the injection level is ∼2 × 1017 cm−3. (b) SKPM mapping of surface potential difference for Si/F-N2200 on one Si substrate; the mapping area is 30 μm × 30 μm.

Al. However, Si/N2200/Al and Si/F-N2200/Al devices exhibit linear curves, which indicates their Ohmic contact. The resistances with either N2200 or F-N2200 decrease dramatically compared to that in crude Si/Al contact with the same pad size. The higher resistance of Si/Al is due to the Schottky contact.27 The resistances of Si/F-N2200/Al are 50.72, 50.97, 51.54, 52.02, 52.92, and 54.10 Ω for the pad with diameters of 1, 0.8, 0.5, 0.3, 0.2, and 0.1 cm, respectively. Under the same experimental conditions, Si/N2200/Al exhibits slightly increased resistances of 51.89, 52.53, 53.44, 54.80, 56.20, and 58.23 Ω. Si/F-N2200/Al displays lower resistance than that of Si/N2200/Al, which may further verify the better intermolecular stacking and transportation quality between F-N2200 and the Si substrate. Minority carrier lifetime (τeff) mapping of Si substrates with and without polymer interlayers is measured to further analyze the effect of conjugated polymer on the Si substrate. As shown in Figure 5a, the bare Si substrate displays a τeff of 24 μs, whereas Si/N2200 and Si/F-N2200 exhibit τeff of 46 and 35 μs, respectively. In order to avoid τeff measurement error in different substrates, all the measurements are conducted on one Si substrate, according to the following eq 3:

charge transfer rate (ket) and distance (D), as expressed in eq 2:43 ket = k 0 exp( −βD)

(2)

where β is attenuation factor and k0 is a constant value. The ket value is inversely exponential to the D value, which means a tiny change of distance between Si and N2200 (F-N2200) would generate dramatically different ket. The distance between F-N2000 and Si is smaller than that of the N2200 system. Therefore, charge transfer of Si/F-N2200 should be more efficient than that of Si/N2200. In order to evaluate the contact quality in the presence of the polymer, the contact resistances of Si/Al, Si/N2200/Al, and Si/ F-N2200/Al are extracted according to the previous reported method.44 As shown in Figure 4, a device structure of Al/ polymer/Si/LiF/Al is used to investigate the contact resistance. Circular Al pads with different diameters display various current−voltage (I−V) curves. When Al directly deposits on Si, the contacts change from Ohmic to non-Ohmic contact with decreasing pad areas, which is in line with the previous report.,44 The non-Ohmic contact means a higher contact resistance and adverse barrier for electron transfer from Si to 7219

DOI: 10.1021/acsnano.7b03090 ACS Nano 2017, 11, 7215−7222

Article

ACS Nano S + Seff2 1 1 = + eff1 τeff τbulk d

reference device (708 mV). The higher Vbi can enhance the charge separation in the photovoltaic device, which is consistent with the above results.

(3)

where τbulk is bulk Si lifetime, d is Si substrate thickness, Seff1 is the Si front surface recombination velocity, and Seff2 is the rear Si. Because τbulk, d, and Seff2 are identical, the τeff enhancement of samples should be correlated with the contact quality of the Si rear side. Namely, N2200 or F-N2200 deposited on Si are beneficial for enhanced τeff. A longer τeff promises a higher Voc and Jsc. The longer τeff in the Si/F-N2200 system is consistent with its stronger electrical coupling interaction. Scanning Kelvin probe microscopy (SKPM) measurement is performed to probe the change of surface potential of the FN2200 layer upon contact with Si. As shown in Figure 5b, onehalf of the Si substrate is covered with an insulating polymer of poly(methyl methacrylate) (PMMA) thin film. Then F-N2200 is deposited on the whole Si substrate including the PMMAcoated region. The insulating PMMA layer blocks charge transfer between Si and F-N2200, and it is expected that no charge transfer could occur in the presence of the PMMA layer. As shown in Figure 5b, there is ∼40 mV of surface potential between F-N2200/Si and F-N2200/PMMA/Si. The F-N2200/ Si side shows a work function lower than that of F-N2200/ PMMA/Si, indicating an electron carrier transfer from conjugated polymer to Si. As F-N2200 interface layer could achieve Ohmic contact between Si and Al, the device series resistance (Rs) should be reduced. Herein, Rs (Figure S9) is extracted from J−V curves according to the following equation:19 d (V ) nKT = R sAeff J + d(ln J ) q

CONCLUSION In conclusion, we have demonstrated that both n-type conjugated polymers, N2200 and F-N2200, can be used as an efficient interlayer between Si and Al for high-performance Si/ PEDOT:PSS heterojunction solar cells. Either N2200 or FN2200 can be used between Al and Si to create their Ohmic contact, which, in turn, decreases the contact resistance and suppresses carrier recombination near the rear side. Furthermore, substitution of F with H within the conjugated polymer can further enhance the intermolecular stacking and improve the polymer−Si contact quality; this is due to the F···H strong interactions. Consequently, the PCE of Si-PEDOT:PSS hybrid solar cells increases from 12.6 to 14.5% (F-N2200) with polymer interlayer. Incorporating the conjugated polymer interlayer into the solar cells results in more effective electron transport and collection in the cathode, thus suggesting that the planar conjugated polymer with preferred orientation can be used in the development of high-performance solutionprocessed organic−inorganic hybrid photovoltaic devices.

EXPERIMENTAL SECTION Device Fabrication. All devices are fabricated on n-type Si (100oriented, 0.05−0.1 Ω/sq, 300 μm) substrates. The cleaned polished Si substrates were immersed into hydrofluoric acid (HF) (4.8 M) and silver nitride (AgNO3) (0.02 M) at room temperature for 5 min. Then, the Si substrate is dipped into nitric acid solution and HF in sequence to dissolve the residual Ag and Si oxide, respectively. Finally, the Si sample is immersed in tetramethylammonium hydroxide solution (1% w/w) for 1 min and cleaned by deionized water. PEDOT:PSS (Clevios PH 1000) solution mixed with 5 wt % dimethylsulfoxide (Sigma-Aldrich) and 1 wt % Triton (Sigma-Aldrich) is spin-coated onto the front textured silicon surface and then annealed at 125 °C for 20 min. N2200 and F-N2200 are synthesized according to a previous report.33 They are dissolved in chlorobenzene with a concentration of 1 mg/mL. The polymer solutions are spin-coated onto the rear side of Si at 3000 rpm (thickness of ∼3.0 nm). After that, 200 nm thick Ag grid electrodes are deposited onto the PEDOT:PSS layer through a shadow mask, and 200 nm thick Al film is deposited onto the back side by vacuum thermal evaporation (Kurt J. Lesker, Mini-Spectros glovebox organic/metal evaporator). The device active area is 0.5 cm2. Characterization. 2D-GIWAXS measurements were performed at the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory on Beamline 7.3.3. DFT calculations for the two monomers and polymer/Si are performed by the DMol3 program.47,48 Absorption spectra are obtained with a UV−vis spectrophotometer. J−V characteristics are tested by a solar simulator (Newport 91160, AM 1.5 G, 100 mW/cm2). EQE spectra are measured by Newport monochromator and Keithley source meter. I−V curves of circleshaped Al pads with and without N2200 or F-N2200 layers are measured by a probe station (Cascade M150). The minority carrier lifetimes are mapped by microwave-detected photoconductivity MDPmap (Freiberg Instrument GmbH). SKPM and AFM are conducted by a Veeco instrument (Multimode V). C−V data are measured by a 6500B impedance analyzer (Wayne Kerr). The transient photovoltage decay data are tested by a pulsed 532 nm laser light source under white light bias, and signals are recorded on a Tektronix oscilloscope.

(4)

where Aeff is the effective area of the solar cell. Rs of the FN2200-based device (2.28 Ω·cm2) is smaller than the reference one (3.41 Ω·cm2). A reduced Rs promises a higher FF, which can further interpret the higher FF of 0.733 in F-N2200-based devices. Transient photovoltage decay measurement is a method to explore the charge carrier recombination of solar cells.45,46 The devices are connected to an oscilloscope and illuminated with “light-bias” white light to generate a Voc. A small optical perturbation is generated by a pulsed laser (532 nm), which generates a transient voltage with an amplitude. Transient voltage decay can therefore be used to monitor the charge loss kinetics. The decay reveals the probability of carrier recombination in the device. As shown in Figure S10a, the device with F-N2200 exhibits a slower decay tendency compared with the reference one, indicating that the FN2200 could decrease the carrier recombination rate in the Si− organic solar cells, which is consistent with the longer τeff of the Si/F-N2200 system. In addition, built-in potential (Vbi) is correlated with the Voc of the solar cell. The Vbi values are calculated from capacitance−voltage (C−V) curves, according the following equation:46 2(Vbi − V ) 1 = 2 qεND C

(5)

where ε and ND are permittivity and doping level of silicon, respectively. 1/C2 is linear with bias of V. Vbi could be acquired by fitting the intercept value in Figure S10b. The device with FN2200 exhibits a Vbi of 787 mV in comparison to that of the 7220

DOI: 10.1021/acsnano.7b03090 ACS Nano 2017, 11, 7215−7222

Article

ACS Nano

(3) Liu, Q.; Ishikawa, R.; Funada, S.; Ohki, T.; Ueno, K.; Shirai, H. Highly Efficient Solution-Processed Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate)/ Crystalline-Silicon Heterojunction Solar Cells with Improved Light-Induced Stability. Adv. Energy Mater. 2015, 5, 1500744. (4) He, L.; Jiang, C.; Wang, H.; Lai, D.; Rusli. High Efficiency Planar Si/Organic Heterojunction Hybrid Solar Cells. Appl. Phys. Lett. 2012, 100, 073503. (5) Shiu, S.-C.; Chao, J.-J.; Hung, S.-C.; Yeh, C.-L.; Lin, C.-F. Morphology Dependence of Silicon Nanowire/Poly(3,4-ethylenedioxythiophene): Poly(styrenesulfonate) Heterojunction Solar Cells. Chem. Mater. 2010, 22, 3108−3113. (6) Shen, X.; Sun, B.; Liu, D.; Lee, S. T. Hybrid Heterojunction Solar Cell Based on Organic−Inorganic Silicon Nanowire Array Architecture. J. Am. Chem. Soc. 2011, 133, 19408−19415. (7) He, J.; Gao, P.; Liao, M.; Yang, X.; Ying, Z.; Zhou, S.; Ye, J.; Cui, Y. Realization of 13.6% Efficiency on 20 μm Thick Si/Organic Hybrid Heterojunction Solar Cells via Advanced Nanotexturing and Surface Recombination Suppression. ACS Nano 2015, 9, 6522−6531. (8) He, J.; Gao, P.; Ling, Z.; Ding, L.; Yang, Z.; Ye, J.; Cui, Y. HighEfficiency Silicon/Organic Heterojunction Solar Cells with Improved Junction Quality and Interface Passivation. ACS Nano 2016, 10, 11525−11531. (9) He, J.; Yang, Z.; Liu, P.; Wu, S.; Gao, P.; Wang, M.; Zhou, S.; Li, X.; Cao, H.; Ye, J. Enhanced Electro-Optical Properties of Nanocone/ Nanopillar Dual-Structured Arrays for Ultrathin Silicon/Organic Hybrid Solar Cell Applications. Adv. Energy Mater. 2016, 6, 1501793. (10) Gao, P.; He, J.; Zhou, S.; Yang, X.; Li, S.; Sheng, J.; Wang, D.; Yu, T.; Ye, J.; Cui, Y. Large-Area Nanosphere Self-Assembly by a Micro-Propulsive Injection Method for High Throughput Periodic Surface Nanotexturing. Nano Lett. 2015, 15, 4591−4598. (11) Yu, X.; Shen, X.; Mu, X.; Zhang, J.; Sun, B.; Zeng, L.; Yang, L.; Wu, Y.; He, H.; Yang, D. High Efficiency Organic/Silicon-Nanowire Hybrid Solar Cells: Significance of Strong Inversion Layer. Sci. Rep. 2015, 5, 17371. (12) Erickson, A. S.; Zohar, A.; Cahen, D. n-Si−Organic Inversion Layer Interfaces: A Low Temperature Deposition Method for Forming a p−n Homojunction in n-Si. Adv. Energy Mater. 2014, 4, 1301724. (13) He, L.; Jiang, C.; Rusli; Lai, D.; Wang, H. Highly Efficient SiNanorods/Organic Hybrid Core-Sheath Heterojunction Solar Cells. Appl. Phys. Lett. 2011, 99, 021104. (14) Jeong, H.; Song, H.; Pak, Y.; Kwon, I. K.; Jo, K.; Lee, H.; Jung, G. Y. Enhanced Light Absorption of Silicon Nanotube Arrays for Organic/Inorganic Hybrid Solar Cells. Adv. Mater. 2014, 26, 3445− 3450. (15) Jeong, S.; Garnett, E. C.; Wang, S.; Yu, Z.; Fan, S.; Brongersma, M. L.; McGehee, M. D.; Cui, Y. Hybrid Silicon Nanocone−Polymer Solar Cells. Nano Lett. 2012, 12, 2971−2976. (16) Thomas, J. P.; Leung, K. T. Defect-Minimized PEDOT:PSS/ Planar-Si Solar Cell with Very High Efficiency. Adv. Funct. Mater. 2014, 24, 4978−4985. (17) Liu, Q.; Ono, M.; Tang, Z.; Ishikawa, R.; Ueno, K.; Shirai, H. Highly Efficient Crystalline Silicon/Zonyl Fluorosurfactant-Treated Organic Heterojunction Solar Cells. Appl. Phys. Lett. 2012, 100, 183901. (18) Thomas, J. P.; Zhao, L.; McGillivray, D.; Leung, K. T. HighEfficiency Hybrid Solar Cells by Nanostructural Modification in PEDOT:PSS with Co-Solvent Addition. J. Mater. Chem. A 2014, 2, 2383−2389. (19) Zhang, Y.; Cui, W.; Zhu, Y.; Zu, F.; Liao, L.; Lee, S.-T.; Sun, B. High Efficiency Hybrid PEDOT: PSS/Nanostructured Silicon Schottky Junction Solar Cells by Doping-Free Rear Contact. Energy Environ. Sci. 2015, 8, 297−302. (20) Avasthi, S.; Lee, S.; Loo, Y.-L.; Sturm, J. C. Role of Majority and Minority Carrier Barriers Silicon/Organic Hybrid Heterojunction Solar Cells. Adv. Mater. 2011, 23, 5762−5766. (21) Liu, Y.; Zhang, J.; Wu, H.; Cui, W.; Wang, R.; Ding, K.; Lee, S.T.; Sun, B. Low-Temperature Synthesis TiOx Passivation Layer for

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03090. Synthesis of polymers N2200 and F-N2200; chemical structure from DFT calculations of N2200 and F-N2200; temperature-dependent UV−vis absorption spectra of N2200 and F-N2200 in chlorobenzene solution at different temperatures; UV−vis absorption spectra of N2200 and F-N2200 films on quartz substrates; line-cut profile curves of 2D scattering images for neat polymer films in the in-plane and out-of-plane directions; AFM height images of F-N2200 and N2200 films on polished silicon substrates; SEM images of the textured silicon; EQE improvement of F-N2200 in the region of 600− 1100 nm; calculated current density derived by integrating EQE with photon flux density of standard AM 1.5G solar spectrum for the devices; resistance calculated from current−voltage curves of the devices with and without F-N2200 layer; transient photovoltage decay and C−V curves of the devices with and without FN2200; DFT calculation of adsorption energy of polymer on the Si surface (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yuqiang Liu: 0000-0003-3494-6390 Jianyu Yuan: 0000-0002-5131-1285 Youyong Li: 0000-0002-5248-2756 Shuit-Tong Lee: 0000-0003-1238-9802 Baoquan Sun: 0000-0002-4507-4578 Author Contributions

Y.H. and Y.L. contributed equally to this work. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (2016YFA0202402), the National Natural Science Foundation of China (91333208, 61674108, 61504089, 61674111), the Natural Science Foundation of Jiangsu Province of China (BK20170310), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the 111 Project and Collaborative Innovation Center of Suzhou Nano Science and Technology. REFERENCES (1) Yu, P.; Tsai, C.-Y.; Chang, J.-K.; Lai, C.-C.; Chen, P.-H.; Lai, Y.C.; Tsai, P.-T.; Li, M.-C.; Pan, H.-T.; Huang, Y.-Y.; Wu, C.-I.; Chueh, Y.-L.; Chen, S.-W.; Du, C.-H.; Horng, S.-F.; Meng, H.-F. 13% Efficiency Hybrid Organic/Silicon-Nanowire Heterojunction Solar Cell via Interface Engineering. ACS Nano 2013, 7, 10780−10787. (2) Liu, Y.; Zhang, Z.-G.; Xia, Z.; Zhang, J.; Liu, Y.; Liang, F.; Li, Y.; Song, T.; Yu, X.; Lee, S.-T; Sun, B. High Performance Nanostructured Silicon−Organic Quasi p−n Junction Solar Cells via Low-Temperature Deposited Hole and Electron Selective Layer. ACS Nano 2016, 10, 704−712. 7221

DOI: 10.1021/acsnano.7b03090 ACS Nano 2017, 11, 7215−7222

Article

ACS Nano Organic-Silicon Heterojunction Solar Cell with a High Open-Circuit Voltage. Nano Energy 2017, 34, 257−263. (22) He, J.; Gao, P.; Yang, Z.; Yu, J.; Yu, W.; Zhang, Y.; Sheng, J.; Ye, J.; Amine, J. C.; Cui, Y. Silicon/Organic Hybrid Solar Cells with 16.2% Efficiency and Improved Stability by Formation of Conformal Heterojunction Coating and Moisture-Resistant Capping Layer. Adv. Mater. 2017, 29, 1606321. (23) Zhao, J.; Wang, A.; Green, M. A.; Ferrazza, F. 19.8% Efficient “Honeycomb” Textured Multicrystalline and 24.4% Monocrystalline Silicon Solar Cells. Appl. Phys. Lett. 1998, 73, 1991−1993. (24) Zhang, Y.; Zu, F.; Lee, S. T.; Liao, L.; Zhao, N.; Sun, B. Heterojunction with Organic Thin Layers on Silicon for Record Efficiency Hybrid Solar Cells. Adv. Energy Mater. 2014, 4, 1300923. (25) Wang, D.; Sheng, J.; Wu, S.; Zhu, J.; Chen, S.; Gao, P.; Ye, J. Tuning Back Contact Property via Artificial Interface Dipoles in Si/ Organic Hybrid Solar Cells. Appl. Phys. Lett. 2016, 109, 043901. (26) Lee, Y.-T.; Lin, F.-R.; Chen, C.-H.; Pei, Z. A 14.7% Organic/ Silicon Nanoholes Hybrid Solar Cell via Interfacial Engineering by Solution-Processed Inorganic Conformal Layer. ACS Appl. Mater. Interfaces 2016, 8, 34537−34545. (27) Yang, X.; Bi, Q.; Ali, H.; Davis, K.; Schoenfeld, W. V.; Weber, K. High-Performance TiO2-Based Electron-Selective Contacts for Crystalline Silicon Solar Cells. Adv. Mater. 2016, 28, 5891−5897. (28) Shi, S.; Yuan, J.; Ding, G.; Ford, M.; Lu, K.; Shi, G.; Sun, J.; Ling, X.; Li, Y.; Ma, W. Improved All-Polymer Solar Cell Performance by Using Matched Polymer Acceptor. Adv. Funct. Mater. 2016, 26, 5669−5678. (29) Yuan, J.; Guo, W.; Xia, Y.; Ford, M. J.; Jin, F.; Liu, D.; Zhao, H.; Inganäs, O.; Bazan, G. C.; Ma, W. Comparing the Device Physics, Dynamics and Morphology of Polymer Solar Cells Employing Conventional PCBM and Non-Fullerene Polymer Acceptor N2200. Nano Energy 2017, 35, 251−262. (30) Hwang, Y.-J.; Courtright, B. A. E.; Ferreira, A. S.; Tolbert, S. H.; Jenekhe, S. A. 7.7% Efficient All-Polymer Solar Cells. Adv. Mater. 2015, 27, 4578−4584. (31) Kim, T.; Kim, J.-H.; Kang, T. E.; Lee, C.; Kang, H.; Shin, M.; Wang, C.; Ma, B.; Jeong, U.; Kim, T.-S.; Kim, B. J. Flexible, Highly Efficient All-Polymer Solar Cells. Nat. Commun. 2015, 6, 8547. (32) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dotz, F.; Kastler, M.; Facchetti, A. A High-Mobility Electron-Transporting Polymer for Printed Transistors. Nature 2009, 457, 679−686. (33) Jung, J. W.; Jo, J. W.; Chueh, C. C.; Liu, F.; Jo, W. H.; Russell, T. P.; Jen, A. K. Y. Fluoro-Substituted n-Type Conjugated Polymers for Additive-Free All-Polymer Bulk Heterojunction Solar Cells with High Power Conversion Efficiency of 6.71%. Adv. Mater. 2015, 27, 3310− 3317. (34) Yuan, J.; Ma, W. High Efficiency All-Polymer Solar Cells Realized by the Synergistic Effect Between the Polymer Side-Chain Structure and Solvent Additive. J. Mater. Chem. A 2015, 3, 7077−7085. (35) Yuan, J.; Ford, M. J.; Zhang, Y.; Dong, H.; Li, Z.; Li, Y.; Nguyen, T.-Q.; Bazan, G. C.; Ma, W. Toward Thermal Stable and High Photovoltaic Efficiency Ternary Conjugated Copolymers: Influence of Backbone Fluorination and Regioselectivity. Chem. Mater. 2017, 29, 1758−1768. (36) Nguyen, T. L.; Choi, H.; Ko, S. J.; Uddin, M. A.; Walker, B.; Yum, S.; Jeong, J. E.; Yun, M. H.; Shin, T. J.; Hwang, S.; Kim, J. Y.; Woo, H. Y. Semi-Crystalline Photovoltaic Polymers with Efficiency Exceeding 9% in a ∼ 300 nm Thick Conventional Single-Cell Device. Energy Environ. Sci. 2014, 7, 3040−3051. (37) Steyrleuthner, R.; Schubert, M.; Howard, I.; Klaumünzer, B.; Schilling, K.; Chen, Z.; Saalfrank, P.; Laquai, F.; Facchetti, A.; Neher, D. Aggregation in a High-Mobility n-Type Low-Bandgap Copolymer with Implications on Semicrystalline Morphology. J. Am. Chem. Soc. 2012, 134, 18303−18317. (38) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.-S.; Ree, M. A Strong Regioregularity Effect in Self-Organizing Conjugated Polymer Films and High-Efficiency Polythiophene:Fullerene Solar Cells. Nat. Mater. 2006, 5, 197−203.

(39) Ying, L.; Hsu, B. B. Y.; Zhan, H.; Welch, G. C.; Zalar, P.; Perez, L. A.; Kramer, E. J.; Nguyen, T.-Q.; Heeger, A. J.; Wong, W.-Y.; Bazan, G. C. Regioregular Pyridal[2,1,3]thiadiazole π-Conjugated Copolymers. J. Am. Chem. Soc. 2011, 133, 18538−18541. (40) Yuan, J.; Dong, H.; Li, M.; Huang, X.; Zhong, J.; Li, Y.; Ma, W. High Polymer/Fullerene Ratio Realized in Efficient Polymer Solar Cells by Tailoring of the Polymer Side-Chains. Adv. Mater. 2014, 26, 3624−3630. (41) Zhang, J.; Song, T.; Shen, X.; Yu, X.; Lee, S.-T.; Sun, B. A 12%Efficient Upgraded Metallurgical Grade Silicon−Organic Heterojunction Solar Cell Achieved by a Self-Purifying Process. ACS Nano 2014, 8, 11369−11376. (42) Jäckle, S.; Liebhaber, M.; Gersmann, C.; Mews, M.; Jäger, K.; Christiansen, S.; Lips, K. Potential of PEDOT: PSS as a Hole Selective Front Contact for Silicon Heterojunction Solar Cells. Sci. Rep. 2017, 7, 2170. (43) Adams, D. M.; Brus, L.; Chidsey, C. E.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; et al. Charge Transfer on the Nanoscale: Current Status. J. Phys. Chem. B 2003, 107, 6668−6697. (44) Wan, Y.; Samundsett, C.; Bullock, J.; Hettick, M.; Allen, T.; Yan, D.; Peng, J.; Wu, Y.; Cui, J.; Javey, A.; et al. Conductive and Stable Magnesium Oxide Electron-Selective Contacts for Efficient Silicon Solar Cells. Adv. Energy Mater. 2017, 7, 1601863. (45) Ip, A. H.; Thon, S. M.; Hoogland, S.; Voznyy, O.; Zhitomirsky, D.; Debnath, R.; Levina, L.; Rollny, L. R.; Carey, G. H.; Fischer, A.; Kemp, K. W.; Kramer, I. J.; Ning, Z.; Labelle, A. J.; Chou, K. W.; Amassian, A.; Sargent, E. H. Hybrid Passivated Colloidal Quantum Dot Solids. Nat. Nanotechnol. 2012, 7, 577−582. (46) Shuttle, C. G.; O’Regan, B.; Ballantyne, A. M.; Nelson, J.; Bradley, D. D. C.; de Mello, J.; Durrant, J. R. Experimental Determination of the Rate Law for Charge Carrier Decay in a Polythiophene: Fullerene Solar Cell. Appl. Phys. Lett. 2008, 92, 093311. (47) Delley, B. An All-Electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules. J. Chem. Phys. 1990, 92, 508−517. (48) Delley, B. From Molecules to Solids with the DMol 3 Approach. J. Chem. Phys. 2000, 113, 7756−7764.

7222

DOI: 10.1021/acsnano.7b03090 ACS Nano 2017, 11, 7215−7222