Improved Performance of Silicon Nanowire-Based Solar Cells with

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C: Energy Conversion and Storage; Energy and Charge Transport

Improved Performance of Silicon Nanowire Based Solar Cells with Diallyl Disulfide Passivation Yunjun Rui, Tianmu Zhang, Dewei Zhu, Yongji Feng, Alexander N. Cartwright, Mark T. Swihart, Ying Yang, Tianyou Zhang, Cheng-Ping Huang, Hengyu Wang, and Dawei Gu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10542 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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The Journal of Physical Chemistry

Improved Performance of Silicon Nanowire Based Solar Cells with Diallyl Disulfide Passivation

Yunjun Rui,*,†,‡ Tianmu Zhang,§ Dewei Zhu,‡ Yongji Feng,† Alexander N. Cartwright,§ Mark T. Swihart,*,‡ Ying Yang,† Tianyou Zhang,† Chengping Huang,† Hengyu Wang,† and Dawei Gu,†

Department of Physics, Nanjing Tech University, Nanjing 210009, China

†

Department of Chemical and Biological Engineering, University at Buffalo (SUNY),

‡

Buffalo, New York 14260, United States Department of Electrical Engineering, University at Buffalo (SUNY), Buffalo, New

§

York 14260, United States

ABSTRACT Silicon nanowires (SiNWs) have attracted increasing attention for their enhanced light harvesting and large junction area of the photovoltaic devices compared to planar silicon wafers. However, high surface recombination velocity deteriorates the photovoltaic performance of the SiNW based solar cells. Therefore, a passivation step is necessary in order to avoid this effect. Here, a small organic molecular, diallyl disulfide (DADS), has been employed to passivate the surface of SiNWs. This passivation process could be carried out just under the condition of UV illumination and room temperature. It was found that covalent Si-C bonds were formed between DADS and the Si surface, which was experimentally proved to reduce the surface recombination of photo-generated carriers. Compared with cells employing oxide or

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hydrogen-passivated SiNWs, the power conversion efficiency (PCE) of devices employing DADS-passivated SiNWs was 7.2%, which was improved by a factor of 3.8 and 1.6, respectively. Moreover, the solar cell using DADS-passivated SiNWs exhibited good stability in air. The S-shaped current-voltage curves were not observed due to the high oxidation resistance for the DADS modified surface. This simple and effective UV-initiated passivation procedure with DADS can lower the cost and improve photovoltaic performance of SiNW-based solar cells.

1. INTRODUCTION The application of silicon nanowires (SiNWs) in photovoltaics, along with other applications like biosensing, has attracted much recent attention. SiNW-based devices have key advantages of CMOS compatibility and potential low cost.1-3 A widely used method to growing SiNWs is a bottom-up vapor-liquid-solid growth in the presence of a metal catalyst.3 However, Peng et al. reported a simpler method to produce the SiNWs by a metal ion assisted chemical etching (MACE) process, during which metal-induced local oxidation and dissolution occur on the surface of a Si substrate in aqueous hydrogen fluoride solution.4 The etching process creates a SiNW array with controllable density and position determined by the initial states of the metal catalyst.5 This etching technique is also amenable to large scale SiNW array fabrication. However, nitric acid (HNO3), which is used to remove metal nanoparticles after SiNW fromation, oxidizes the freshly etched SiNWs to form a thick oxide layer. Typically, these oxidized SiNWs are dipped into a dilute hydrofluoric acid (HF) solution to remove the oxide layer and achieve a hydrogen-terminated surface.6-9 Aouida et al. reported that hydrogen passivation increased the minority carrier lifetime and reduced the surface recombination velocity of the SiNWs.9 Likewise,

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Jevasuwan et al. obtained the hydrogen-passivated SiNW solar cells with a relatively high power conversion efficiency (PCE) of 6.3% through an H2 annealing technique, resulting from effective reduction of surface charge recombination.10 Although hydrogen termination of SiNWs improves performance, the Si-H bond is unstable and easily oxidized into silicon oxide (SiOx) in ambient air. The thickness of native SiOx is a critical parameter determining the performance of SiNW solar cells due to Si surface passivation and charge recombination.11-17 The Rusli group studied the effect of Si surface native oxide on the performance of planar Si/organic heterojunction solar cells and concluded that the optimal thickness of SiOx was 1.5 nm for the best PCE (10.6%) after 2h exposure in air of the fresh Si-H surface.12 As reported there, when the SiOx thickness was increased to 2.1 nm, only 5.2% PCE was obtained in their experiment, showing that a thicker than optimal SiOx layer will impede carrier transport and collection. In addition, due to the large surface-to-volume ratio of the SiNWs, forming a 1.5 nm native SiOx layer on the surface takes a relatively short time (1.5h).13,14 Moreover, if the oxidization process of SiNW surface was carried out in aqueous solution, controlling the thickness of SiOx passivation layer was quite difficult. Wei et al. indicated that the oxide passivation of freshly-etched Si NWs should be completed within only 1 second in boiling water.15 Rusli et al. firstly employed the ozone treatment to form a sacrificial oxide layer, and then partially thinned the oxide layer to a suitable thickness (1~2 nm) by HF etching process.16 Therefore, optimal surface oxidation thickness is a key factor to fabricate SiNW devices with consistent high performance, but is not easily achieved. In many studies, the surface of SiNWs are passivated with more robust materials. Silicon nitride (SiNx), hydrogenated amorphous silicon (a-Si:H) and amorphous aluminum oxide (a-Al2O3) thin films have all been used respectively in SiNW-based

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solar cells.18-20 However, adding an extra passivation layer makes the fabrication of the solar cells more complex and expensive. Compared with these inorganic materials, surface passivation with an organic molecular can be simple and effective.21-29 In a two-step chlorination/alkylation process (converting the surface Si-H bonds firstly into Si-Cl bonds, and then into Si-C bonds), a monolayer of organic compound can be bonded to Si on the surface forming covalent Si-C bonds.23,24 The Si surface that was chemically passivated by Si-alkyl bonds was shown to have a lower surface recombination velocity and was more resistant to oxidation in air.24-26 A more direct wet-chemical approach can be used to form Si-C bonds with the use of an olefin (such as a 1-alkene, CnH2n).27-29 These chemically grafting processes often require either heating (e.g. to 200℃) or high vacuum (10-5 Torr).27,28 The surface passivation of SiNWs with small organic molecules like a 1-alkene or their derivatives at room temperature has rarely been reported.29 In our previous work, we presented a facile and effective functionalization method to produce a stable hydrogen-terminated Si nanocrystals (NC) ink with short ligands, which allows charge transport through films of passivated NCs.30 Under UV illumination at room temperature, the Si NC surface was passivated by diallyl disulfide (DADS). These NCs were employed as absorber layer in a photodiode and showed pretty good stability in air. Building on that study with NCs, here we report, for the first time, DADS can also passivate SiNW surface, resulting in a much higher PCE of 7.2% for the DADS-passivated soalr cells compared with identical devices using hydrogen or SiOx passivated SiNWs. A schematic representation of the UV initiated reaction in which the organic monolayer is covalently bonded to the SiNW surface is shown in Figure 1. The effect of surface passivation was characterized by Fourier Transport infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy 4


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(XPS), current-voltage (J-V) curves, capacitance-voltage (C-V) curves and external quantum efficiency (EQE) spectra. The stability of these DADS-passivated SiNW solar cells in ambient air is also discussed.

Figure 1. (a) Chemical structure of DADS; (b) Three types of passivation procedures for preparing the Si-H, Si-SiOx and Si-DADS surface of SiNWs.

2. EXPERIMENTAL SECTION SiNWs/PEDOT:PSS hybrid solar cells were fabricated on single-crystal Si wafers (n-type, 5 Ωcm, 500  m thickness) by a metal ion assisted chemical etching (MACE) process and spin coating of a PEDOT:PSS organic layer. A Si wafer was first cleaned using standard RCA procedures. Typically, a Si wafer was placed in a solution of NH3H2O (26%), H2O2 (30%) and deionized water (1:2:5 by volume) for about 4~5 min at 100 °C to remove any organic and inorganic contaminations on the surface. It was then immersed in a solution that consisted of HCl (37%), H2O2 (30%), and deionized water (1:2:6 by volume) at 100 °C for 4~5 min to remove metallic residues.11 Then the cleaned wafer was transferred to an aqueous solution of 5M HF and 0.02M AgNO3 for 5 min to grow the vertically aligned Si NWs, followed by dipping into nitric acid (68%) for 30 min to remove the Ag dendritic structures and Ag

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nanoparticles.4,31 Before applying the PEDOT:PSS layer, the SiNW coated wafer was transferred into a nitrogen-filled glove box and immersed in a solution of HF containing isopropanol (IPA) to remove the thick oxide and produce a hydrogen-terminated Si surface, as described in detail in supporting information (Figure S1 and S2). After this HF treatment, the SiNW-coated wafer was sealed in a cuvette containing diallyl disulfide (DADS). UV illumination (254 nm in a Rayonet UV reactor) was applied to the cuvettes to facilitate the reaction of SiNW and DADS. This UV illumination lasted for 2 or 4h. For comparison, some HF treated SiNW samples were exposed to air for 10 days to obtain thin SiOx (~2.3 nm) passivated SiNW samples as shown by TEM images in Figure S4. Subsequently, PEDOT:PSS was spin coated (3000 rpm, 60 s) on top of the SiNWs. The samples were annealed at 120°C under nitrogen to remove the solvents.15 An Ag grid top electrode (100 nm) was deposited through a shadow mask by e-beam evaporation, followed by a back Al contact with a thickness of 100 nm. The active area of the hybrid solar cell was 0.8 cm2. The surface topography of SiNW arrays was observed by scanning electron microscopy (SEM, JSM-5900) and transmission electron microscopy (TEM, JEM 2010 UHR). To obtain the chemical configuration and contents of the surface passivated SiNWs, FTIR ((Nexus 870) and XPS (Thermofisher, Al K  1486 eV) measurement was employed. A series of J-V characteristics were measured using a computerized Keithley 2400 source meter in the dark and under AM1.5G illumination at 100 mW/cm2, supplied by a solar simulator (Oriel, 1000W). The EQE spectra were collected by a spectral response measurement system over a wavelength range of 300-1100 nm. The minority carrier lifetime was obtained from transient photovoltage 6


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decay measurement. A laser light source (532 nm, 9ns, 10Hz) was applied to trigger the open circuit voltage transient, which was recorded on a digital oscilloscope (Rigol DS4054, 500MHz 4GSa/s). The C-V measurement was carried out using an Agilent B1500A semiconductor device analyzer under a high frequency of 100 kHz to evaluate the surface defect density.

3. RESULTS AND DISCUSSION 3.1 SiNW surface passivation with DADS A schematic of the process for preparing the DADS passivated SiNWs under the UV illumination is shown in Figure. 1. As a reference, hydrogen-terminated Si surface (labeled as Si-H) was obtained by removing the thick silicon oxide layer in dilute HF/IPA solution. However, these Si-H bonds on the surface would be naturally re-oxidized in air, forming a thin SiOx passivated layer (labeled as Si-SiOx). Under UV illumination in DADS and in a nitrogen atmosphere, these freshly prepared Si-H bonds were converted into Si-C bonds, forming a covalent bond with DADS (labeled as Si-DADS). Figure 2 shows FTIR spectra of the three types of passivated SiNW surfaces. For the freshly hydrogen-terminated SiNWs, the stretching vibration of Si-H bonds peaked at 2090 cm-1, was clearly observed. After 10 days exposure in air, the surface layer of hydrogen-terminated SiNWs was re-oxidized. As shown in Figure 2, the Si-H peak was decreased and a Si-O peak at 1060cm-1 was dominant, indicating the formation of a Si-SiOx sample32 For the Si-DADS surface formed by dipping the freshly prepared Si-H bonded SiNWs in the DADS solution under UV illumination, the Si-CH2-peaks in the FTIR spectrum centered at 1230 cm-1 indicate the formation of covalent bond between SiNWs and DADS.30,33 The UV-induced hydrosilylation 7



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reaction is shown as: Si-H + H2C=CH-CH2-S-S-CH2-CH=CH2

Si-CH2-CH2-CH2-S-S-CH2-CH=CH2

(1)

The possibility of passivating the SiNW surface by DADS was reported in our previous work.30 However, in this case, based on the FTIR spectra of the DADS-treated samples, we can infer that one of the C=C bonds was eliminated by the reaction with the Si-H, while the other double bond remained, as illustrated by the schematic in Figure 1. The absorbance peaks at vibration frequencies of 910 and 990 cm-1 arise from this remaining double bond (C=C) of the Si-DADS samples as shown in Figure 2. In addition, the absorbance peak corresponding to Si-H bonds (2090 cm-1) was decreased in comparison to the HF treated sample because of the replacement of H-terminated Si sites by DADS functionalized sites. This peak decreased further with increasing UV illumination time.

Figure 2. FTIR spectra of samples with different surface passivation. Si-CH2 bonds formed by covalent attachment of DADS to the Si-H surface under UV illumination are indicated by the peak near 1230 cm-1. Very little evidence of Si-H or Si-O bonds was seen for the DADS passivated samples.

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Covalent Si-C bonds can also be characterized by XPS measurement.33-36 Before analysis, the Si 2p3/2 line at 99.4eV as a reference to correct the charging shift of the binding energies in all cases.34 Figure 3(a) shows the C 1s XPS spectrum for the DADS passivated surface of SiNWs (UV 2h), which was fitted to three peaks.34 The major peak was centered at 284.8 eV (75.3% in total area) and two others, 284.0 (12.3% in total area) and 285.7 eV (12.4% in total area). The binding energy of 284.8 eV is known to be assigned to the signal of C-C bonds. Compared with the Pauling electronegativity of Si (1.90) and C(2.55), the Si-C bond has a smaller binding energy than that of C-C bond. This chemical shift changing from 284.8 to 284.0 eV indicated the fact that Si-C bond was formed on the SiNW surface,as expressed in the UV-induced hydrosilylation reaction (1). The peak of 285.7 eV arises from the C=C bond remained in DADS ligands bonded on the SiNW surface. When the DADS passivation process extended to 4 h, more Si-C bonds were formed and C-C bonds were decreased accordingly, as indicated in Figure 3(b). Therefore, XPS and FTIR data both confirm covalent Si-C bonds between DADS and SiNWs.

Figure 3. The XPS spectra in the C1s region from the DADS passivated SiNW surface with UV illumination for (a) 2h, and (b) 4h. Peaks were fitted with a Gaussian-Lorentzian sum function after a 9



Shirley background subtraction and the corresponding organic groups, and the percentage of total area are labelled.

Other researchers have prepared the organic alkyl monolayer by covalently bonding olefins, such as methyl 10-undecenoate or 1-alkenes (C2H2n, n>4), with the hydrogen-terminated planar Si surface with heating or high vacuum.27,28 One study reported that by means of illumination with a mercury arc lamp (~185 and 253.7 nm), the reaction between Si-H and 1-pentene occured.29 This process was similar to ours. Thus, UV illumination was shown to be an effective method to passivate the Si-H surface of Si nanocrystals as well as SiNWs. Such a passivated SiNW surface with Si-C bonds will greatly suppress surface charge recombination, which is beneficial to the separation and transport of photo-excited carriers in the photovoltaic (PV) devices.

Figure 4. (a) Cross-sectional and (b) top-view SEM images of DADS-passivated SiNWs covered with PEDOT:PSS layer.

3.2 DADS passivated SiNW solar cells with high efficiency Based on these surface passivated SiNWs, SiNW/PEDOT:PSS heterojunction solar cells (HSCs) were fabricated. The cross-sectional and top-view SEM images of the device are presented in Figure 4. The SiNW arrays were covered with a PEDOT:PSS layer as shown in Figure 4(a). The length of the SiNW layer was about 1.2 µm after 5 min MACE etching. Although the base of the SiNWs remained uncoated, the 10


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PEDOT:PSS layer penetrated about halfway through the layer of SiNW. The penetration depth is illustrated by the white arrows in Figure 4(a). Meanwhile, the top-view SEM image in Figure 4(b) shows that the top of the SiNW arrays were covered with PEDOT:PSS. The SEM imaging clearly displays the core (n-type SiNWs)-sheath (PEDOT:PSS acting as a hole conductive polymer) structure. The well-designed structure could increase the light absorption area of the heterojunctions and shorten the carrier diffusion distance.37 Therefore, it greatly enhances the carrier collection efficiency and power conversion efficiency (PCE) of the SiNW based HSCs.7,38

Figure 5. (a) Device structure of SiNWs/PEDOT:PSS HSCs with surface passivation and the corresponding (b) light J-V curves and (c) EQE. (d) The EQE enhancement ratio of Si-DADS and Si-H sample was obtained by dividing them by the EQE of the Si-SiOx sample. The inset of (b) shows a picture of a HSC with Ag grid electrode. The area where there was no grid electrode was covered with a stainless steel mask.

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Figure 5(a) shows the device structures of SiNWs/PEDOT:PSS HSCs. Due to the core-sheath structure, electrons and holes would transport through n-Si and PEDOT:PSS with short distance, respectively. This schematic also illustrates the wavelength dependence of the collection efficiency. Light penetration into the cell is known to be characterized by the optical thickness ( Lopt ) of the material (  1  , noting that the absorption coefficient 

is wavelength dependent).37 For example,

for the light of 450 and 960 nm, Lopt is 0.4 and 75 µm , respectively.39 In our case ,the length of SiNWs was 1.2 µm, as shown in Figure 4. Therefore, the photo-generated carriers induced by light of short wavelengths will be collected near the top of the HSCs compared with those excited by long wavelength photons.7,40,41 The performance of HSCs with three types of passivation surface was characterized by light J-V curves, as shown in Figure 5(b). The inset shows a picture of a Si-DADS solar cell. The devices with DADS-passivated SiNWs clearly provide better performance than the devices with hydrogen or SiOx passivated SiNWs. For the sample treated with DADS under UV illumination for 2 h, the open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and PCE were 488 mV, 28.8 mA/cm2, 49.0%, and 7.02%, respectively. When the DADS passivation process was extended to 4 h of UV illumination, the performance of the solar cell was slightly degraded. The photovoltaic parameters of SiNW HSCs with different surface passivation are listed in Table 1. Each value was obtained by 5 identical cells including an average value and their standard deviation. It is worth noting that the reflectivity of these different passivated SiNW substrates are almost the same, as illustrated in Figure S5 in Supporting Information. However, over a broad wavelength range, the EQE of the Si-DADS devices was much higher than the hydrogen-terminated or SiOx passivated samples as shown in Figure 12


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5(c). In addition, the enhanced EQE was shifted towards shorter wavelength. To identify the wavelength contribution to the PCE, The EQE enhancement ratio of Si-DADS and Si-H sample was obtained by dividing each by the EQE of Si-SiOx sample, respectively, as presented in Figure 5(d). This EQE ratio reaches 2.2 and 1.6 at 450 nm for the Si-DADS and Si-H samples, respectively. For the DADS passivated SiNW solar cell, the light at 960 nm also contributes more to the increased PCE in contrast to the Si-H surface sample. Therefore, the enhanced EQE may be mainly ascribed to the superior surface passivation for the Si-DADS samples. Table 1. Photovoltaic parameters of SiNW hybrid solar cells with different surface passivation.

Samples Si-H Si-SiOx Si-DADS UV 2h Si-DADS UV 4h

Voc (mV) 472±14 432±10 488±6

Jsc (mA/cm2) 23.12±0.47 18.30±0.62 28.80±1.03

FF (%) 43.2±2.1 30.1+1.6 49.0±1.2

PCE (%) 4.41±0.35 1.84±0.24 7.02±0.17

483±11

27.48±1.30

48.5±1.8

6.7±0.22

Surface passivation with chemical groups is well known to play a very important role in the charge separation and transport in SiNW PV devices, and thus in the overall device performance. Lee et al. prepared CH3-terminated SiNW solar cells using the two-step, chlorination/methylation procedure. As a reference, the solar cells with Si-H and Si-SiOx surfaces were also studied. Due to the lowest surface recombination velocity, 3.4% PCE was obtained for the CH3-terminated SiNW sample, which was higher than the other two.6 The Si-H surface has been reported to display a high surface recombination velocity of 500 cm/s, which increases to 2040 cm/s for the native SiOx terminated Si surface.42,43 However, when the Si surface was passivated with alkyl by Si-C bonds, the velocity of surface recombination was

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dramatically suppressed to as low as 25 cm/s.24 In our case, verified by FTIR and XPS spectra in Figure 2 and 3, respectively, Si-C bonds were formed at the DADS passivated SiNW surface, which may be also helpful to reduce the surface recombination velocity and improve the charge transport in PV devices.

Figure 6. Dark J-V curves for the samples with different surface passivation. The DADS-passivated (UV 2h) sample has the best diode rectification characteristics.

To investigate the transport mechanism of solar cells, we measured the dark J-V curves as shown in Figure 6. The dark J-V relation of the samples exhibits obvious rectification due to the Schottky junction between the SiNWs and PEDOT:PSS layers, which can be simulated by a one diode model from a simple equivalent circuit including series resistance (Rs) and shunt resistance (Rsh) with the following equation:44   q (V  JARs )   V  JARs J  J 0  exp    1  nk B T Rsh    

(2)

In Eq. (2), J0 is the reverse saturation current density, n is the diode ideality factor, kB is the Boltzmann constant, T is the absolute temperature and q is electron charge.

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Table 2. Diode parameters, including series resistance (Rs), shunt resistance (Rsh), reverse saturation current densities ( J0) and diode ideality Factors ( n ) obtained from dark J-V curves, as well as the effective carrier lifetime (  eff ) measured using transient photovoltage decay method.

Rs (Ωcm2)

Rsh (Ωcm2)

Si-H

6.7±0.5

570.2±6.0

Si-SiOx

19.0±1.0 2857±15.2

Si-DADS UV 2h Si-DADS UV 4h

5.5±0.4

1805.5±10.8

7.0±0.6

1247.2±7.2

Samples

J0 (mA/cm2) (1.0±0.1) ×10-1 (2.1±0.1) ×10-2 (1.9+0.1) ×10-2 (3.1±0.2) ×10-2

n

 eff (

s )

4.3±0.2

6.5±0.3

4.1±0.2

7.0±0.4

2.9±0.1

12.1±0.5

3.4±0.2

11.0±0.5

From this model, diode parameters for each surface passivated SiNW sample can be calculated, and these parameters are listed in Table 2. Firstly, the shunt resistance was estimated from the linear reverse J-V range of Figure 6, where the current density is given by J  V /( ARsh ) .44 The obtained Rsh has a value of several thousand Ohm (shown in Table 2). Then the other parameters of Rs, J0 and n in Eq. (2) were determined as shown in Figure S6. The DADS passivated sample shows the smallest value of Rs, 5.5 Ωcm2. The small Rs and large Rsh for the DADS-passivated samples are helpful for reducing power loss and leakage current, respectively. The values of n were greater than 1 for all samples under forward bias conditions, reflecting the electron-hole recombination mechanism in our hybrid solar cells.11,33,45 The larger the value of n, the higher the contribution of surface recombination. Note that when the Si-H surface was replaced by Si-C bonds, the value of n decreased from 4.3 to 2.9. So the DADS passivated surface of SiNW reduces the recombination of photo-generated carriers. Moreover, the saturation current density was decreased to 1.9×10-2 mA/cm2, one fifth that of the device with hydrogen-terminated SiNW sample. This carrier transport mechanism was consistent with the fact that alkyl-terminated Si surface 15



decreases the velocity of surface recombination, as discussed before. However, as shown in Table 2, J0, n and Rs all increased when the UV illumination time was increased from 2 to 4 hours. This is attributed to attachment of more DADS on the surface of SiNW with the corresponding reduction of Si-H bond absorbance (2090 cm-1 in FTIR spectra) and increase of Si-C bond (284.1 eV in XPS spectra) during the UV illumination process as indicated in Figure 2 and 3, respectively. Like the oxide passivation, if the passivating layer is too thick, carrier transport and collection will be impeded.11,12 A thicker or denser organic passivation layer has a lower conductivity and therefore degrades the efficiency of carrier collection. Surface recombination can also be evaluated by measuring the transient photovolatage decay in solar cells.24,33 This photovoltage decay ( V ) tendency has an exponential relationship with the effective carrier life (  eff ) through the following: V  A( 

t

 eff

(3)

)

Where A is a constant that fits to the peak height, and  eff is depended on bulk lifetime (  b ), surface recombination ( S ) and the wafer thickness ( d ): 1  eff  1  b  2 S d .11,21,24 For a given Si wafer substrate,  b and d are constant.

Therefore, surface recombination velocity is inversely proportional to the effective carrier lifetime,

 eff

. Figure 7 presents the time-resolved photovoltage decay of PVs

with different passivated SiNW surfaces. According the Eqn. (3), single-exponential fits to these decays in Figure 7 were operated with solid lines. Thus, four  eff were obtained and also listed in Table 2. The devices with DADS passivated SiNW surface have longer carrier lifetimes (>11  s ) compared with other two passivated surface of Si-H (6.5  s ) and Si-SiOx (7.0  s ), which indicates the decrease of surface 16


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recombination due to the presence of Si-C bonds. Therefore, the performance of solar cells through DADS passivatiion was improved, which is consistent with the J-V analysis in the previous part.

Figure 7. The transient voltage decay of devices fabricated with different surface passivated SiNWs. The color lines represent the measured data and the dark lines stand for the fitting ones.

In addition, the different surface recombination velocity was related to the variable defect density at the SiNW/PEDOT:PSS interface. Figure 8 shows the 1/C2-V plot, from which the interfacial defect density (Dit) can be deduced, as described in detail in Ref. 7. The steeper slope implies a larger Dit, leading a higher velocity of surface recombination. In our case, the deduced Dit values for SiNWs with Si-H and Si-DADS surfaces were

6.11012 and

4.2 1012 cm 2eV 1 , respectively. This

indicates DADS could effectively promote the SiNW surface passivation to reduce surface recombination velocity. Since Dit is reduced, the Fermi-level pinning effect will be alleviated, which results in an increase of Voc as well as built-in potential (Vbi).46,47 A 0.016 V increment 17



of Voc was obtained for DADS-passivated PV devices compared with hydrogen terminated ones, as presents in Table 1. While for Vbi, which can be achieved from the x-intercept in the 1/C2-V plot as shown in Figure 8. It is 0.60V for the DADS passivated SiNW solar cells, larger than the value of 0.56V for the hydrogen terminated sample. The increase of Vbi (0.04 V) means that a large internal electric field generated and the charge separation enhanced. Therefore, the improved PV performance discussed above is mainly attributed to the excellent SiNW surface passivation by the small molecular of DADS.

Figure 8.

Room-temperature inverse square of the capacitance (1/C2) versus reverse bias (VR ) plots

for the devices, as labelled.

3.3 Stability of DADS passivated HSCs Surface passivation by Si-C bonds has been reported to improve stability of PV devices. Figure 9 shows the evolution of light J-V curves with the increasing air exposure time for devices with Si-DADS or Si-H surface passivation. The decline 18


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tendency of corresponding values of Jsc and PCE for these two samples is shown in Figure 9(c). From the dark J-V curves (Figure S7), the detailed PV parameters were obtained and listed in Table S1 in Supporting Information. Over time, the J-V curve becomes S-shaped for the Si-H sample, while it maintains the initial shape for the Si-DADS sample.

Figure 9. Light J-V curves for the SiNWs/PEDOT:PSS HSCs with (a) hydrogen and (b) DADS passivation surface, and (c) their corresponding PCE and Jsc as a function of air exposure time.

The different performance for the hydrogen and DADS passivated PV devices worked in ambient atmosphere was ascribed to oxidation of the SiNW surface.6,36,48 Figure 10 gives the XPS spectra of Si 2p and O 1s regions for SiNW surfaces with hydrogen and DADS passivation as as function of air exposure time. In Figure 10 (a), 19



it is observed that both for the freshly prepared Si-H and Si-DADS samples, no obvious signal of oxide was detected. After 5 days storage in air, an intense SiOx peak at 103.4 eV appears in the hydrogen terminated SiNW surface. However, a weak SiOx peak at about 102.8 eV occurs for the DADS passivated surface even it was exposed for a longer time of 10 days. Figure 10 (b) also shows this oxidation tendency of these two types of passivated surfaces. Therefore, the lower binding energy and the weak intensity of SiOx peak verified that more stability in air was achieved for the DADS passivated samples than that for hydrogen terminated ones, which is very critical for the device application.

Figure 10. XPS spectra of (a) Si 2p and (b) O 1s region for SiNW surfaces with hydrogen and DADS passivation as as function of air exposure time. The XPS spectra were corrected by using the Si-Si photoelelctons (99.4 eV) as a reference. The signals of SiOx in 2p and O 1s region were labelled with vertical arrows in the Figure.

Combined with the J-V characteristics and XPS spectra in Figure 9 and 10, respectively, this S-shape is ascribed to the oxidation for the Si-H PV devices. It was reported that such S-shaped J-V curve is generally proposed to be due to the presence of interfacial dipoles or carrier injection barriers.49 A synchrotron photoemission study

20


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has demonstrated that the H-Si surface would pose a negative surface dipole of 0.12eV. While for the Si-SiOx and Si-CH3 surface, a positive surface dipole of 0.15 and 0.35eV would be formed, respectively.6 The negative and positive surface dipoles have the opposite effect on bending down and up the Si energy band at the Si/PEDOT:PSS interface to suppress and promote the carrier separation, respectively.12 So the S-shaped J-V curve would be expected for the Si-H passivated sample (free of SiOx). However, in our case, a normal J-V curves was observed for the Si-H surface solar cells, as shown in Figure 9(a). Because residual SiOx was detected even after 30 min oxide removal in dilute HF solution as shown by the FTIR spectra in Figure 2, the thin SiOx layer will eliminate the effect produced by the Si-H negative surface dipole. Some researchers reported that a higher performance was indeed obtained once the freshly prepared Si-H surface was exposed in air for less than 30 min or for a day.6,16 Therefore, the normal J-V curve for the Si-H surface (0 days) solar cells in our case was attributed to the rapid formation of a small amount of native oxide.6 Although this thin SiOx layer has the advantages of positive dipole as well as the surface passivation, the formation of native oxide is not a well-controlled process and is hence not feasible for practical applications.16 The SiOx layer cannot be thick; otherwise it would become a carrier injection barrier and impede the drift of carriers in the devices, leading to a high series resistance(Rs) as well as a low PCE. As shown in Figure S3, the as-prepared SiNW solar cells (after nitric acid treatment, without oxide removal) have a thick SiOx layer (2.75 nm) on the Si surface, leading to a low PCE (0.21%) and high Rs (54.6 Ωcm2). After dilute HF immersion, the performance of Si-H surface SiNW PV devices was enhanced greatly due to hydrogen passivation as well as the oxide removal. However, as shown in Figure 9(a), S-shaped J-V curves

21



were again observed for these hydrogen-terminated SiNW solar cells after a 5 or 10 day exposure in air. The PCE, Jsc, and FF were all decreased to 2.04%, 20.1 mA/cm2 and 28.4%, respectively, as shown in Table S1. Therefore, this S-shape was attributed to the charge barrier produced by an oxidation layer at the interface of Si-H/PEDOT:PSS, for which the thickness increases with the air exposure time. In contrast, the minimal variation of the J-V curve and the FF for the DADS passivated devices provides evidence that the surface passivated with DADS was oxidation resistant. This DADS passivated SiNW surface not only provides good passivation to suppress photo-generated carrier combination (discussed before) but also excellent air stability for the better sustained PV performance. It was reported that the velocity of surface recombination for such surfaces can be as low as 45 cm/s, even when exposed to ambient air for more than 48h.42 Therefore, in contrast with the Si-H device, higher PCE (4.66%), Jsc (25.45mA/cm2) and FF (43.2%), for the DADS passivated PV device after air exposure for 10 days, were obtained. However, as shown by the FTIR spectra in Figure 2 , though most Si-H bonds have been replaced by Si-C bonds, the DADS passivated sample still has some Si-H bonds remaining on the surface.23-26,30 So, due to the oxidization, the Jsc was decreased from 28.8 mA/cm2 to 25.45 mA/cm2 after the DADS passivated sample was stored in air for 10 days. The forward current density was decreased, especially in the high bias range (>0.6V), as shown in Figure S7. This deterioration is related to increasing of series resistance of 13.3 Ωcm2 because of the oxidization of the surface of SiNWs. Therefore, additional means of preventing oxidation are still needed, such as device encapsulation as well as the methods proposed below.50,51

22


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3.4 Proposal to improve the PV performance. As shown in Figure 5(a), the schematic of the SiNW/PEDOT:PSS structure shows carrier collection near the top of PV device for carriers exited by short wavelength photons, while the carriers generated by longer wavelength photons are more likely to be collected inside the SiNW layer. The collection efficiency will be suppressed if the SiNW surface is poorly passivated. Carriers, especially holes, could not be separated and extracted effectively. This charge transport process is illustrated by short arrows and a cross mark in Figure 5(a). Therefore, the increase of passivated SiNW surfaces was critical to increase the resistance to oxidation, and boost PV performance of the SiNW based solar cells in our case. The first step is decreasing density of SiNWs, which will benefit a complete removal of oxide and more attachment of Si-C bonds. Although the as-prepared SiNWs were treated with HF solution, the removal of oxide on the surface was incomplete, as illustrated by FTIR spectra in Figure S2. As a result, the subsequent surface passivation with organic monolayers was not complete, especially in the bottom region of SiNWs. Some methods have been reported to obtain sparse SiNWs, such as using phosphorus pentachloride (PCl5) or potassium hydroxide (KOH).7,38,52 In addition, because these SiNWs become tapered during the process of reducing density, the PEDOT:PSS can penetrate deeply along the nanowires, as shown in Figure 11. So the junction quality of core-shell electrode-contact is improved.48,53 As a result, the carriers induced by short and long wavelength light could both be collected and therefore contribute to PCE.

23



Figure 11. Schematic cross-section of a SiNWs/PEDOT:PSS HSC with a tapered SiNW and its hybrid passivated surface.

On the other hand, the electrical resistance of organic molecule passivated surface is important.54 In our case, more DADS attachment on SiNW surface hinders photo-generated carrier transport and collection, as discussed above. Therefore, organic molecules with high charge mobility or organic mixtures may be preferable to passivate the surface of optoelectronic devices. Figure 11 also shows a hybrid passivation schematic diagram. The symbol -X on the SiNW surface presents a organic molecule with a shorter chain length than DADS. This hybrid passivation could be expected to exhibit high molecular coverage without overly diminishing the conductivity of the passivated surface. Carrier transport is known to depend on the chain length of organic molecules.55-57 The shorter the chain length, the higher the charge mobility across the interface. In addition, molecules having short length could infiltrate and passivate sites that are difficult for longer molecules to access, as has been demonstrated experimentally.58,59 The Sargent group developed a robust hybrid passivation scheme for colloidal quantum dots, in which halide anions were introduced to passivate trap 24


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sites that were inaccessible to much larger organic ligands.58 Sun et al. demonstrated an effective way to increase the Si-C bonds and stability using a mixture of methyl and allyl groups on Si for the SiNW/PEDOT:PSS hybrid solar cells.33 Compared with the individual methyl or allyl terminations, the coverage ratio of methyl/alkly terminated silicon surface was nearly 100%, and the reported PCE was as high as 10.2%. Therefore, a dense monolayer of mixed small organic molecules may thus provide the best combination of complete passivation with minimal inhibition of carrier transport. Based on our UV-initiated passivation procedure, exploring hybrid passivation with mixed small organic molecules, such as 1-pentene (C5H10) and DADS (C6H10S2) mixture, to improve the coverage of Si-C bonds is very promising, and is underway in our lab.29,60

4. CONCLUSION In conclusion, we have investigated the passivation effect of DADS on the photovoltaic performance of SiNW based solar cells. The SiNW surface could be covalently passivated with DADS through the formation of Si-C bonds under the condition of UV illumination and room temperature. The DADS passivated sample had a high PCE of 7.2%, which was improved by a factor of 1.6 and 3.8 compared with the Si-H surface and Si-SiOx surface samples, respectively. Due to the smaller diode ideality factor (n) large effective carrier lifetime (

 eff

), and the reduction of

interfacial defect density (Dit), the DADS-passivated SiNW surface helps to suppress surface charge recombination. Therefore, excellent performance of SiNW based solar cells with Si-DADS passivation was obtained with small Rs, large Jsc and FF. Moreover, the solar cell using DADS-passivated NWs exhibited good stability in air. The covalent Si-C bonds formed on the SiNW surface were more stable in ambient air 25



than Si-H bonds. Because of the better resistance to oxidation, the electrical stability of the DADS-passivated SiNW solar cells is superior to that of Si-H passivated samples. We observed an S-shaped J-V curve for the hygrogen-terminated SiNW devices, which is mainly ascribed to the charge injection barrier of SiOx layer originated from the oxidation of Si-H bonds in ambient air. Further improved performance of SiNW based solar cells may be achieved by increasing covalent Si-C bonds coverage as well as preserving conductivity of the surface passivated with a mixed organic molecules. Base on our simple and effective UV-initated procedure, this proposal will involve complete removal of oxide, dilute and tapering SiNW , and hybrid passivation with 1-pentene (C5H10) and DADS (C6H10S2).

■ ASSOCIATED CONTENT Supporting Information 1. Oxide removal by HF solution immersion, Oxidation of freshly hydrogen passivated SiNWs in air, reflectivity of SiNW and planar Si substrates, method for determination of Rs, J0 and n, and stability test in air for the hydrogen and DADS passivated SiNW solar cells. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Authors *(Y.R.) E-Mail: [email protected]. Tel.: 86-25-5813-5934. *(M.T.S.) E-Mail: [email protected]. Tel.:1-716-645-1181. Notes The authors declare no competing financial interest. 26


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■ ACKNOWLEDGMENTS We would like to acknowledge the support from Korea Institute of Energy Research (Grant No. GP2012–0024–01), NSFC (61704079), National Undergraduate Training Program for Innovation and Entrepreneurship (201810291080X) and PAPD. We also acknowledge Prof. Jun Xu for his helpful discussion.

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