Dopant-Free Hydrogenated Amorphous Silicon Thin-Film Solar Cells

Oct 21, 2013 - ... and the equation S2 and Figure S3 show the dependence of Jsc, FF, and ...... Hung , L. S.; Tang , C. W.; Mason , M. G.; Raychaudhur...
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Dopant-Free Hydrogenated Amorphous Silicon Thin-Film Solar Cells Using Molybdenum Oxide and Lithium Fluoride Ji-Hwan Yang,‡,○ Hyung-Hwan Jung,†,▽,○ Jihoon Seo,†,○ Kwang-Dae Kim,† Dong-Ho Kim,† Dong-Chan Lim,† Sung-Gyu Park,† Jae-Wook Kang,§ Myungkwan Song,† Min-Seung Choi,† Jung-Dae Kwon,† Kee-Seok Nam,† Yongsoo Jeong,† Se-Hun Kwon,∥ Yun Chang Park,⊥ Yong-Cheol Kang,# Kwun Bum Chung,▽ Chang Su Kim,*,† Koeng Su Lim,*,‡ and Seung Yoon Ryu*,†,◆ †

Advanced Functional Thin Films Department, Korea Institute of Materials Science (KIMS), 797 Changwondaero, Seongsangu, Changwon 641-831, Republic of Korea ‡ Department of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea § Department of Flexible and Printable Electronics, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju 561-756, Republic of Korea ∥ School of Materials Science and Engineering, Pusan National University, 63beon-gil 2, Busandaehak-ro, Geumjeong-gu, Busan 609-735, Republic of Korea ⊥ Measurement & Analysis Team, National Nanofab Center, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea # Department of Chemistry, Pukyong National University, 45, Yongso-ro, Namgu, Busan 609-735, Republic of Korea ▽ Department of Physics, Dankook University, Mt. 29, Anseo-Dong, Chonan 330-714, Republic of Korea S Supporting Information *

ABSTRACT: Toxic doping gases are usually used to produce hydrogenated amorphous silicon (a-Si:H) layers in thin-film solar cells (TFSCs). Hence, an alternative structure that avoids the use of toxic gases is desirable. In this work, we replaced both the p-type-a-Si:H and n-type-a-Si:H layers simultaneously in a normal TFSC to form a structure that is dopant-free. Molybdenum oxide (MoO3) and lithium fluoride were used as the p-type and n-type layers, respectively. The effects of the deposition method and the thickness of the MoO3 layer on the device performance were investigated. The power-conversion efficiency of the optimized hybrid solar cell reached a maximum of 7.08%, which is remarkable considering the novel structure of the dopant-free devices. The light stability of the devices with and without MoO3 was also compared: the light stability of the device with MoO3 was found to be much better than that of the device without MoO3 and with p-i-n Si layers. This was ascribed to the insignificant number of defect sites generated by the nondoping elements, which led to a less contaminated, more compact, and smoother oxide surface, resulting in an increase in the electron lifetime and improved light stability. This work opens up a new direction toward the development of a truly dopant-free device that does not involve the use of toxic gases during fabrication and provides the potential for further enhancement of the efficiency of future dopant-free solar cells.

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INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) thin-film solar cells (TFSCs) have great potential for next-generation photovoltaic devices because Si thin films can be deposited on bendable substrates like plastic and stainless steel, which is an essential requirement for flexible technologies.1−12 In these cells, p-typeintrinsic-n-type (p-i-n) Si layers form orderly stacks that allow the cells to achieve an adequate built-in potential (Vbi). The production process of these devices is simple and inexpensive, especially when compared with that of poly-Si cells.1−12 However, because toxic doping gases (such as diborane (B2H6) and phosphine (PH3) for the fabrication of p- and n© 2013 American Chemical Society

type a-Si:H layers, respectively) are used to produce the layers, much care and expensive facilities are mandatory for achieving adequate safety both in the experimental environment of a university laboratory and in the mass production line.1−12 In addition, the doping process is harmful in terms of the device light stability because of the dangling Si bonds and recombination centers in the Si layer.1−12 Received: March 30, 2013 Revised: October 19, 2013 Published: October 21, 2013 23459

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Figure 1. (a) Schematic device structure of the dopant-free solar cell. (b) Band diagram of the dopant-free solar cells used in this work. (c) BFSTEM cross-sectional image. (d) TEM EDX elemental mapping of the dopant-free solar cell.

layer,22−26,28,29 with its wide optical bandgap of ∼3.2 eV, excellent electrical conductivity, and work function ranging from 5.8 to 6.9 eV.12,19,20 Thus, MoO3 is well-suited for use as a p-type layer22−26,28,29 and can replace p-a-Si:H and PEDOT:PSS in conventional p-i-n Si and organic TFSCs, respectively.22−26 Lim et al. investigated the use of a buffer layer with WO37 and boron-doped amorphous hydrogenated diamond-like carbon (a-DLC:H)11 in p-i-n Si layers and alternative device structures in which both the p-a-Si:H and n-a-Si:H layers are replaced with only one layer of MoO330 or lithium fluoride (LiF) in conventional p-i-n Si TFSCs.31 These studies were aimed only at finding the theoretical relationship between the layers, without focusing on commercial aspects such as the light stability and endurance of the devices against H2O or O2 exposure in the device atmosphere.7,11,30,31 However, the reports suggested that a-Si:H solar cells using MoO3 instead of a p-type Si layer30 with n-a-Si:H or those using LiF instead of an n-type Si layer31 with p-a-SiC:H offer a number of advantages over traditional TFSCs. For instance, these cells deliver good open-circuit voltages (Voc) and can be produced without the use of either B2H6 or PH3. However, from previous studies, information regarding light stability or the mechanism related to device performance upon exposure to H2O and O2 from the environment is scarce. Furthermore, from these studies, it is unclear which parameters are the main contributors to the stability degradation. It is well known that the doping elements induce Si dangling bonds and severe stability degradation.1−12 Therefore, the substitution of both doping layers, that is, p-a-Si:H and n-a-Si:H, is a possible strategy for improving the light stability. With regard to the n-type layer, it is known that LiF has a good dipole moment on the thin-film scale, that is, at a thickness of ≤1 nm. Furthermore, the Fermi level of aluminum (Al) can be adjusted by reacting LiF with Al vapor.31−36 Such LiF n-type layers are mainly used in organic light-emitting

To substitute these overelaborate doping layers, avoid the use of toxic gases such as B2H6 and PH3, and cut the cost of device fabrication, researchers have investigated various types of TFSCs in which the p-type Si or n-type Si layers are replaced in devices with the conventional p-i-n Si stacked structure and organic/inorganic structures. For instance, a C60 derivative, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), with the structure of the p-a-Si:H/intrinsic Si layer (i-a-Si:H), has been introduced as a hole-blocking layer, replacing the n-a-i:H layer.12 In addition, the p-a-Si:H layer has been replaced to construct poly(styrenesulfonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT:PSS)/i-a-Si:H/n-a-Si:H solar cells.13 PEDOT:PSS has also been spin-coated onto n-type Si, silicon nanowire arrays,14 and silicon nanocones15 as a hole-transporting layer. Furthermore, a-Si:H cells that include poly-(3hexylthiophene) (P3HT),16,17 poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinlylene) (MEH-PPV),17 binding alkyl chain molecules,18 or polyaniline (PANI)19 have also been fabricated. However, there has been little focus on the light stability data and on the detailed working mechanism of the devices. This is because normally PEDOT:PSS is infamous for its poor conductivity, chemical degradation with transparent conducting oxides, and photo-oxidization owing to water (H2O) absorption or exposure to oxygen (O2).13−20 The application of various metal oxides such as tungsten oxide (WO3),20 vanadium oxide (V2O5),21 molybdenum oxide (MoO3),22−26 and nickel oxide (NiO)27 as hole-transporting layers has been examined to overcome the obstacles faced with using PEDOT:PSS as the p-type layer. These metal oxide layers show endurance against moisture and O2 present in the device environment, and they can be deposited in a simple and costeffective way through both vacuum evaporation and solution processes. Among the metal oxides, MoO322−26 shows outstanding attributes and hence is an obvious choice. MoO3 has been used widely in various optoelectronic devices and is known for its outstanding properties as a window 23460

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chamber with a base pressure of ∼2 × 10−6 Torr by thermal evaporation using a shadowed mask to define an active cell area of 0.25 cm2. Obtaining an optimized LiF thickness was crucial because an excessively thin layer leads to poor Voc and leakage current owing to inadequate thin-film coverage, while an excessively thick layer leads to high series resistance (Rs) and negatively affects the Jsc and FF.31 The optimized device performance of the dopant-free solar cell was obtained when the thicknesses of the LiF and Al layers were ∼1.4 nm and >100 nm, respectively. The top electrode was obtained by thermally evaporating Al during device fabrication. The J−V characteristics of the solar cell devices were measured using a Keithley 2400 source meter under AM1.5 simulated illumination with an intensity of 100 mW/cm2 from a solar simulator (Peccell Technologies, PEC-L11). An incident photocurrent efficiency measurement system equipped with a 200 W Xe lamp and a grating monochromator was used for the measurement of the external quantum efficiency (EQE), and the light intensity was measured using a calibrated Si solar cell. An ultraviolet−visible spectrophotometer (Varian, Cary5000) was used for obtaining the optical data including the transmittance and absorption spectra. Because of the scattering effect caused by the surface texturing, the total transmittance including the diffused transmittance was measured using an integrating sphere. Transmission electron microscope (TEM) and energy-dispersive X-ray spectroscopy (EDX) were carried out in a scanning transmission electron microscope corrected for spherical aberration (Cs-corrected STEM) to obtain the cross-sectional microstructure of the devices and the elemental composition, respectively. Oxygen diffusion into the intrinsic layer for the p-i silicon and MoO3-i silicon structures were investigated by secondary ion mass spectroscopy (SIMS) after light soaking. This means that the samples were exposed to AM1.5 simulated illumination with an intensity of 100 mW/cm2 for 10 h. MoO3 layer of 7.5 nm in thickness or boron-doped p-a-Si:H layer of 12 nm in thickness was deposited by sputtering or VHF-PECVD, respectively, on a textured FTO glass substrate. Then, a layer of i-a-Si:H of 100 nm in thickness was commonly deposited. The SIMS analysis involved profiling from the i-a-Si:H layer toward the FTO glass substrate.

diodes.31−36 Thus, many studies have been conducted on the use of a LiF/Al double layer as the cathode in organic electronic devices.31−36 However, to our knowledge, there have been no studies that report the replacement of both p-a-Si:H and n-a-Si:H layers simultaneously in a normal device structure to form genuinely dopant-free a-Si:H TFSCs. The cost of producing these dopant-free TFSCs is likely to be lower than that of producing a-Si:H TFSCs using toxic gases. The reasons for the cost reduction would include the better safety and the lower cost of a simple thermal or sputtering process compared with the expensive plasma-enhanced chemical vapor deposition (PECVD) process. We report the development of a dopant-free solar-cell structure using a metal oxide (MoO3) as the p-type layer and LiF as the n-type layer. The effects of the thickness of the MoO3 layer and of the deposition method on the device performance and light stability are investigated in detail. The power conversion efficiency (PCE) of the optimized hybrid solar cell reaches a maximum of 7.08%, which is remarkable considering the novel structure of the dopant-free devices. The PCE of our previous conventional cells using conventional ptype-intrinsic-n-type (p-i-n) Si layers was 7.53%.8 The values of Voc, the short-circuit current density (Jsc), and fill factor (FF) of the hybrid cell are 0.65 V, 16.08 mA/cm2, and 0.67, respectively. Furthermore, the light stability of the device with MoO3/LiF layers is much better than that of the conventional device with the normal p-i-n Si structure.

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EXPERIMENTAL SECTION The schematic structure of the dopant-free solar cell device is shown in Figure 1a. MoO3 thin films were deposited on textured commercially available fluorine-doped tin oxide (FTO) glass substrates (Pilkington) with a sheet resistance of around 7 Ω/□, using the thermal evaporation or sputtering method. The FTO glasses were cleaned in an ultrasonic cleaner for 10 min each with acetone and isopropyl alcohol and then dried with nitrogen gas before being introduced into the thermal evaporation chamber or sputtering chamber. The MoO3 was deposited by thermal evaporation with ∼0.2 Å/s deposition rate on the cleaned FTO glasses in a high vacuum chamber with a base pressure around 2 × 10−6 Torr. Also, the MoO3 was deposited by sputtering with the conditions of a RF power of 50 W, Ar flow of 50 sccm (without O2), the operating pressure of 5 mT, and the distance of 5 cm between target and substrate at room temperature. After MoO3 deposition in either the thermal evaporation or sputtering chamber, vacuum was broken and the samples were exposed to air to be quickly transferred into the PECVD system. The PECVD chamber was initially evacuated to a pressure below 2 × 10−6 Torr. A mixture of SiH4 and H2 (50 sccm/50 sccm) was introduced into the chamber, and the working pressure of 400 mTorr was controlled by adjusting a throttle valve. Amorphous i-Si:H thin films of 450 nm in thickness were deposited on MoO3coated FTO glass by applying a very high frequency (VHF) power (40.68 MHz) of 30 W. The power density of PECVD was 0.075 W/cm2, considering the electrode dimension. The layers were optimized individually by tuning the deposition parameters to obtain dopant-free solar cells with suitable properties. The distance from the upper electrode with showerhead to the substrate was ∼15.7 mm, and the substrate temperature during the a-Si:H PECVD process was 250 °C. To prepare the n-type layer and the reflective cathode, LiF and Al were used. LiF and Al were deposited in a high vacuum

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RESULTS AND DISCUSSION Figure 1a shows the device structure of the metal oxide/i-aSi:H/LiF dopant-free solar cell designed and synthesized in this work. The MoO3 thin films were deposited as a p-type layer on textured FTO glass through thermal evaporation or sputtering with gaseous Ar alone in the absence of O2. The subsequent intrinsic layer and LiF/Al were deposited using VHF-PECVD to obtain a high-quality thin-film Si layer (and not by radio frequency (RF)-PECVD)37 and thermal evaporation, respectively. Figure 1b shows the energy band diagram of the proposed dopant-free solar cell. Even though MoO3 is an n-type material, its Fermi level is well-matched with that of FTO, and hence MoO3 can provide an adequate Vbi with the LiF n-type layer. MoO3 has been considered to improve hole injection and extraction effectively, with an electron affinity (EA) of 6.7 eV, ionization potential (IP) of 9.7 eV, and Fermi level (EF) of 6.9 eV, revealing a strongly n-type material.28 The charge transfer from the valence band (VB) of i-a-Si:H and deep conduction band (CB) of MoO3 to the degenerate n-type FTO substrate originates from the fact that the EF and CB of MoO3 are pinned 23461

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Figure 2. (a) Transmittance characteristics of the dopant-free solar cells with different MoO3 layer thicknesses that have been deposited by different methods. T-MoO3 denotes MoO3 deposited by thermal evaporation and S-MoO3 denotes sputtered MoO3. (b) Absorption spectra of the dopantfree solar cells with different MoO3 thicknesses that were deposited by different methods. (c) XPS profiles for T-MoO3 and S-MoO3 layers. (d) Conductivity of T-MoO3 and S-MoO3 layers as a function of thickness.

with the EF and VB of i-a-Si:H.28 The hole injection and electron extraction between the VB of i-a-Si:H and the CB of MoO3 occur at the same time; this is also known to act as a charge-generation layer at the interface.28 The cross-sectional BF-STEM (Bright Field-Scanning Transmission Electron Microscopy) images of the dopant-free solar cells show welldefined individual layers. A thin 20 nm MoO3 layer on the textured FTO is shown in Figure 1c. Figure 1d shows the elemental mapping of the dopant-free solar cells obtained by TEM EDX, from which uniform distributions of the component elements can be observed clearly. The interfaces appeared sharp, and there was no evidence of layer-by-layer mixing or physical damage that may have occurred during the PECVD and thermal evaporation or sputtering of MoO3. Therefore, from the sharp interfaces, the process proposed in this work has addressed the major challenge of the fabrication of dopant-free solar cells with a sufficiently thick intrinsic layer that connects electrically with the p- and n-type layers.12,20,30,31 First, the optical properties of the MoO3 layers deposited on textured FTO glass were investigated. Figure 2a shows the transmittance data exhibited by MoO3 layers (0, 10, 20, and 30 nm thick) deposited through thermal evaporation (T-MoO3) and by the MoO3 layer (7.5 nm thick) deposited by sputtering (S-MoO3). The transmittances of the 0-, 10-, and 20-nm-thick T-MoO3 layers were nearly identical. However, the transmission exhibited by the 7.5-nm-thick S-MoO3 layer was much worse than that of any other samples. This is because the sputtering was conducted using only gaseous Ar in the absence of O2, which produced highly metal-rich layers. If S-MoO3 was deposited using gaseous O2, the transmittance of the sample might be similar to that shown by the T-MoO3 layers. As shown in Figure 2b, the absorption increased when the thickness of the MoO3 layer increased beyond 20 nm, which can be expected to affect the device performance. Figure S1 in the Supporting Information shows the transmittance and

absorption of 3, 5, 7.5, and 10 nm thick S-MoO3 layers deposited on textured FTO glass, which indicate that both the optical transmittance and absorption data are proportional to the thickness of the MoO3 layers. The compositions of S-MoO3 (metal-rich) and T-MoO3 (O2-rich) were completely different, as confirmed in Figure 2c by X-ray photoelectron spectroscopy (XPS) analysis and explained in detailed in the Supporting Information. The calculated values of x and y in T-MoOx and S-MoOy were 2.95 and 2.43, respectively. A metallic valence band structure appeared when S-MoOy was heavily reduced from Mo6+ to Mo5+ and Mo4+ for Mo 3d, and from O−Mo6+ to O−Mo5+ and O−Mo4+ for O 1s, as shown in Figure 2c.28 The metal-like electrical properties of strongly reduced S-MoOy lead to absorption loss, which is not beneficial for high transparency.28 However, the gap states generated in the bandgap of i-a-Si:H at the interface with metal-rich S-MoOy are able to promote an ohmic hole extraction.28 These results can indicate that SMoOy layers could be better for the collection of photogenerated charge carriers owing to the more favorable band structure and are finally able to increase Jsc. Figure 2d shows the conductivities of T-MoO3 and S-MoO3. The conductivity of S-MoO3 was much better than that of the T-MoO3 layer for all thicknesses because the S-MoO3 thin film was obtained as a metal-rich layer without O2. Also, metallic valence band structure was induced by the sputtered MoOy layer.28 The presence or absence of oxygen is important because the conductivity might be crucially related to Rs and Jsc and ultimately affects the device performance. Interestingly, as the thickness of the S-MoO3 layer was increased, the conductivity increased and the values became saturated at a thickness of ∼30 nm. In comparison, the conductivities of the T-MoO3 layers were similar for the various thicknesses. However, the transmittance of the S-MoO3 thin film decreased with increasing thickness. In contrast, the O2-rich T-MoO3 23462

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Figure 3. (a) Energy band diagram and device physics of the dopant-free solar cells under illumination. (b) J−V characteristics under illumination of the dopant-free solar cells containing MoO3 layers of different thicknesses and those that have been deposited by different methods. (c) Dark current of dopant-free solar cells. (d) Variation of EQE of the dopant-free solar cells used in this work with wavelength.

Table 1. Photovoltaic Parameters of the Dopant-Free Solar Cells with Different MoO3 Layersa device structure FTO FTO/T-MoO3 (10 nm) FTO/T-MoO3 (20 nm) FTO/T-MoO3 (30 nm) FTO/S-MoO3 (7.5 nm) a

Jsc (mA/cm2) 14.97 15.39 14.66 13.99 16.08

V0C (V)

fill factor

0.31 0.61 0.72 0.67 0.65

0.50 0.58 0.65 0.64 0.67

efficiency (%)

Rsh (Ω·cm2)

Rs (Ω·cm2)

2.35 5.53 6.98 6.07 7.08

× × × × ×

15.73 6.25 4.59 6.10 3.95

1.70 6.01 3.12 1.01 3.9

3

10 103 104 104 104

T-MoO3 is thermally evaporated MoO3, and S-MoO3 is sputtered MoO3.

Table 2. Photovoltaic Parameters of the Dopant-Free Solar Cells with Different MoO3 layersa device structure FTO/S-MoOi (3 nm) FTO/S-MoO3 (5 nm) FTO/S-MoO3 (7.5 nm) FTO/S-MoO3 (10 nm) a

Jsc (mA/cm2) 15.88 16.32 16.08 14.27

V0 (V) 0.49 0.62 0.65 0.66

fill factor 0.62 0.66 0.67 0.68

efficiency (%)

Rsh (Ω·cm2)

Rs (Ω·cm2)

4.87 6.59 7.08 6.43

× × × ×

6.57 5.10 3.95 5.36

3.1 3.1 3.9 2.8

3

10 104 104 104

S-MoO3 is sputtered MoO3.

7.5 nm thick S-MoO3 are 0.72 and 0.31 eV, respectively, as shown in Table 1. This means that ΔVbi is 0.41 eV, as given by eq 1.7

layer showed little variation in conductivity and transmittance with thickness. Figure 3a shows the band diagram of dopant-free solar cells compared with the conventional p-i-n a-Si:H structure. The MoO3 band structure was suitably bent and yielded moderate Vbi and Voc values, as seen from the Φ1 and Φ2 levels. Φ1 and Φ2 can be defined as the interface Schottky barriers between FTO and i-a-Si:H without MoO3 and between the CB of MoO3 and the VB of i-a-Si:H, respectively. The energy-level alignment and band bending are determined for the MoO3/i-a-Si:H interface along with the mechanisms of charge injection and extraction occurring at the interface. The very large ionization energy of MoO3 (9.7 eV) prevents any hole transport through the VB.28 However, the energy alignment between the CB of MoO3 and the VB of i-a-Si:H is preferable for charge transfer between the two materials.28 Therefore, Vbi can be enhanced by ΔVbi, because the interface Schottky barrier (Φ1) between FTO and i-a-Si:H is changed by the interface barrier (Φ2) formed between MoO3 and i-a-Si:H. The Voc values with and without

ΔVbi = Φ1 − Φ2

(1)

Even though MoO3 is an n-type layer, it has a high work function and acts as a p-type layer in combination with LiF/Al as the n-type layer. In other words, these layers can create an intrinsic electric field through the intrinsic layer, inducing a voltage drop that is fully dependent on Vbi.30 In fact, the difference between the Fermi levels of MoO3 and LiF/Al determines this Vbi change effectively. The associated carriers generated in the intrinsic layer are divided by the Vbi value created by the MoO3 and LiF/Al layers in the dopant-free structure when the solar cells are exposed to light and the generated carriers are collected by both electrodes.30,31 A new interface state close to the Fermi level could be found, which can improve hole injection through electron extraction between 23463

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diminished Vbi, and thus Voc decreased when the MoO3 layer was thinner.30 However, the PCE values of all of the cells were quite poor compared with those of state-of-the-art devices, with initial efficiencies typically between 9 and 11%. The Voc value is mainly determined by the Vbi imposed by the doped layers in a conventional p-i-n structure or MoO3 and LiF in the dopantfree structure. MoO3 has a deep work function of 6.9 eV,28 and LiF/Al has a lowered effective work function below the EF of Al, 4.1 eV due to the high dipole moment of LiF.31 However, the low efficiencies, particularly Voc, originate from the reduction in the internal electric field due to charging at the interface defect sites31 because the vacuum is broken for a while and the system is exposed to ambient air between each MoO3 evaporation, i-a-Si:H PECVD, and LiF/Al deposition step. The FF seemed to increase from 0.50 without MoO3 and saturated at around 0.65 to 0.67 with both T-MoO3 and SMoO3 depending on the thickness of the MoO3 layer, while Jsc decreased when the thickness of the T-MoO3 layer increased from 10 to 30 nm. Jsc was higher for the device containing the 7.5 nm thick MoO3 layer, even though the transmittance of the 7.5 nm thick MoO3 was the lowest. The LiF/Al bilayer was also chosen for use in the dopant-free devices to obtain an i-layer/ dielectric insulator Schottky junction with a smaller interface barrier.31 An intrinsic Si layer forms a Schottky junction with an electrode having a low work function that has formed an ultrathin dielectric insulator with significant dipole characteristics.31 As shown in Table 2, as the thickness of the S-MoO3 was increased, the Voc values of the devices increased because the moderate thickness of the MoO3 layer allows for an improvement in Vbi generated in combination with the LiF ntype layer. The FF and PCE values of devices are also dependent on the thickness of the S-MoO3. However, the Jsc of the devices containing MoO3 greater than 5 nm in thickness decreased owing to the heavily degraded transmittance and absorption, as shown in Figure S1 in the Supporting Information. Therefore, the lower PCE of 10 nm thick SMoO3 device is originated from the lowest Jsc value, even though the Voc and FF values are higher than those of 7.5 nm thick S-MoO3 device. Consequently, the dopant-free solar cell devices containing MoO3 as the p-type layer and LiF as the ntype layer showed an improvement in Voc in comparison with devices without the MoO3 layer. The optimum thicknesses of T-MoO3 and S-MoO3 were 20 and 7.5 nm, respectively. Figure 3c shows the dark current characteristics of the dopant-free solar cells containing MoO3 layers of different thicknesses. Interestingly, the dark current density of the device with no MoO3 in the reverse bias region was lower than those shown by any other devices, while the current density in the forward bias region was higher than that for any other devices. Following the equivalent circuit model shown in the Supporting Information, the J−V characteristics can be summarized as equation S2 in the Supporting Information, and when J is equal to zero, V is equal to Voc, as described by eq 2.1−12

the VB of i-a-Si:H and the CB of MoO3 as a charge-generation process.28 Figure 3b shows the current density−voltage (J−V) characteristics of the dopant-free solar cells under AM1.5 simulated illumination with an intensity of 100 mW/cm2. Tables 1 and 2 also summarize the device performance characteristics, including the shunt resistance (Rsh) and Rs of the devices containing MoO3 deposited by thermal evaporation and sputtering, respectively. The Rsh and Rs values were calculated by using the inverse of the slope of the J−V curves as determined in the dark at around 0 and 1 V, respectively;12 their physical meaning and the correlation with the equivalent circuit are explained in detail in the Supporting Information. In comparison, Jsc, Voc, FF, and PCE of the reference p-i-n Si device were 13.12 mA/cm2, 0.80 V, 0.72, and 7.53%, respectively.8 It is clear that as T-MoO3 becomes thinner, Jsc increases because of the reduced light absorption in the window layer. It is also better than the Jsc values of the reference p-i-n Si device because p-a-Si:H is known to exhibit higher optical absorption. However, the Jsc of the device with S-MoO3 of 7.5 nm in thickness is better than any other devices with T-MoO3, even though the transmittance of 7.5 nm S-MoO3 is worse than that of any other T-MoO3. This is because as shown by the XPS analysis on that metallic valence band structure induced by the sputtered MoOy layer28 the gap states in the bandgap of i-aSi:H at the interface with S-MoO3 might promote an ohmic hole extraction.28 These results can be explained by the fact that S-MoO3 layers could be better for the collection of photogenerated charge carriers owing to their more favorable band structure and can thus lead to an increase in Jsc. It is known that a high Rsh and low Rs value are beneficial for the device performance. The lower PCEs of solar cells containing MoO3 layers with thicknesses of over 20 nm (Table 1) showed a degradation in performance owing to the decreased Jsc, increased absorption, lower Rsh, and higher Rs than that for devices with 20 nm thick MoO3 layers, even though the transmittances of the layers of both these thicknesses are similar.30 However, the solar cell containing 7.5 nm thick S-MoO3 yielded even better photovoltaic parameters, as shown in Tables 1 and 2. This indicates that even though the optical transmittance of the 7.5 nm thick SMoO3 layer is worse, the Jsc, FF, Rsh, and Rs values of the device were better than those exhibited by any other sample. The increase in Rsh prevents the leakage current paths as well as carrier recombination within the solar cell, which induces the increase in FF.12 In the Supporting Information, the equivalent circuit is investigated in detail, as shown in Figure S2 in the Supporting Information, and the equation S2 and Figure S3 show the dependence of Jsc, FF, and also Voc on Rs and Rsh. In addition, the device containing the 20 nm thick T-MoO3 layer showed a higher Voc than those containing MoO3 layers with thicknesses below 20 nm (Table 1), indicating that the internal barrier at the FTO/MoO3 interface is affected by the ntype characteristic of the MoO3 film. When the thickness of the MoO3 film in the dopant-free solar cells is greater than 20 nm, the Voc also seem slightly decreased or saturated. Hence, in this work, 20 nm is considered as the critical value for the T-MoO3 layer thickness for device performance in the dopant-free structure in combination with a LiF/Al n-type layer.30,31 When the T-MoO3 film was thinner than 20 nm, the band alignment at the MoO3/i-Si interface varied with the thickness of the MoO3 layer. A thin MoO3 layer with a reduced barrier

Voc =

⎡ ⎞⎤ ⎛J ⎞ Jph ⎛ nkT ⎢ ⎜1 − Voc ⎟⎥ ≈ nkT ln⎜ ph ⎟ ln 1 + ⎜ ⎟ ⎢ q J0 ⎜⎝ Jph AR sh ⎟⎠⎥⎦ q ⎝ J0 ⎠ ⎣ (2)

Hence, Voc is independent of Rs and is related to the three variables Rsh, J0, and Jph. However, if the Rsh values are large or 23464

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Figure 4. (a) SIMS data indicating oxygen diffusion to the intrinsic layer for p-i silicon layer and MoO3-i silicon layer. The samples were exposed to AM1.5 simulated illumination with an intensity of 100 mW/cm2 for 10 h. Soaking means that after the samples were illuminated for 10 h SIMS examinations for samples were investigated. On a substrate of textured FTO glass, MoO3 of 7.5 nm thickness or boron-doped p-a-Si:H of 12 nm thickness was deposited by sputtering and VHF-PECVD, respectively. Then, i-a-Si:H of 100 nm thickness was deposited. SIMS profiles were acquired from i-a-Si:H toward FTO glass. (b) Schematic diagram of the cell and direction used to obtain the SIMS profile. (c) Light stability characteristics of the dopant-free solar cells obtained during illumination for 1 h. (d) Light stability characteristics of selected dopant-free and p-i-n conventional solar cells taken over 10 h under illumination.

almost the same for each device, the Voc can be regarded as being also independent of Rsh.1−12 In general, when the value of Rsh is below 1000 Ω•cm2, Rsh should not be ignored for the exact value of Voc.1−12 Figure 3d shows the typical photocurrent action spectra measured separately for the dopant-free solar cells. The EQE is plotted as a function of the wavelength of the light. The EQE of all of the devices containing MoO3 was higher than that of those without MoO3 in the short wavelength range extending from 350 to 550 nm. Furthermore, the conventional devices with p-i-n layers showed a poor EQE from 300 to 550 nm and a better EQE from 550 to 750 nm when compared with the other devices. This is expected, because it is well-known that p-a-Si:H shows poor transmittance in the short-wavelength region38 from 300 to 550 nm, and it is believed that for dopant-free solar cells the poorer red response might be attributed to recombination loss due to the formation of tremendous metal-induced gap states or chemical reactions causing defect species at the i-a-Si:H and LiF/Al interface.31 This means that electron−hole pairs could be collected better from 550 to 750 nm in the conventional devices with p-i-n layers than in the dopant-free solar cells. In contrast, the oxide surface prevents the formation of silicides at the interface, which is more compact, and with fewer induced charged or neutral defects,39 more associated holes and electrons are generated electrically at the interface from 350 to 550 nm for the dopant-free devices than for devices without MoO3, despite the similarity in the optical transmittance

characteristics. In a previous work, Lim et al. reported that the better EQE was related directly to the better transmittance at short wavelengths.30 However, they used Asahi textured FTO glass and deposited an intrinsic 200-nm-thick Si layer through RF-PECVD using a different gas mixture ratio. Here we used Pilkington textured FTO glass and deposited a 450 nm thick intrinsic Si layer through VHF-PECVD. Hence, it can be believed that our experiments induced unexpectedly complicated optical paths and electrical processes. Consequently, the intrinsic a-Si:H layer deposited directly on the FTO electrode resulted in a Schottky contact and low Voc. The use of MoO3 layers replacing the standard p-doped a-Si:H layer enhances the solar-cell response in the short-wavelength range (600 nm) the device with an n-doped a-Si:H layer performs better than that with LiF. As proof of the presence of the oxidized interface between MoO3 and the intrinsic Si layer, the SIMS characteristics of the dopant-free and conventional p-i-n structures were investigated, as shown in Figures 4a,b and Figure S5 in the Supporting Information. On a substrate of textured FTO glass, a MoO3 layer of 7.5 nm in thickness or boron-doped p-a-Si:H of 12 nm in thickness was deposited through sputtering and RF-PECVD, respectively. Then, i-a-Si:H of 100 nm in thickness was deposited through VHF-PECVD. The SIMS profiles were acquired from i-a-Si:H toward FTO glass. We are convinced that the oxidized interface generated a smoother surface, which is more compact and contains fewer induced, charged, or neutral defects,39 and more electron−hole pairs are generated at the interface from 350 to 550 nm,38,39 even though the 23465

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Figure 5. Light stability characteristics of the dopant-free solar cells containing MoO3 of different thicknesses under 1 and 10 h of illumination. (a) Ilayer indicates a device structure of FTO/i-a-Si:H/LiF/Al. (b) PIN denotes the device structure of FTO/p-a-Si:H/i-a-Si:H/n-i-Si:H/Ag. (c−f) TMoO3 denotes thermally evaporated MoO3 and S-MoO3 denotes sputtered MoO3. Each graph indicates the thickness of MoO3 and evaporation method. (g−i) Stability data for selected devices such as PIN, T-MoO3 (20 nm), and S-MoO3 (7.5 nm), respectively, were obtained over 10 h.

Figure 4c,d shows the light stability over 1 h of all of the devices and the stability over 10 h of selected devices, respectively. The light stability is worse for the devices without MoO3 because of the absence of the p-layer and intrinsic potential, which allows the holes and electrons to be captured by the intrinsic layer and the interface. The main reason for the overall performance degradation is the degradation of the FF, as shown in Figure 5. The light stability of Jsc, Voc, FF, and PCE over 1 h of all devices with such as i-layer, p-i-n layers, T-MoO3 (10 nm), T-MoO3 (20 nm), T-MoO3 (30 nm), and S-MoO3 (7.5 nm) are shown in Figure 5a−f, respectively. The stability of Jsc, Voc, FF, and PCE over 10 h of selected devices with such

optical transmittance characteristics are almost identical. Hence, the oxide layer is beneficial for the EQE at short wavelengths, and it may also improve the light stability, which was investigated as described later. The analysis confirmed that there is definite O2 diffusion into the intrinsic layer in the dopant-free solar cells, and we suspect that an oxide interface is present between MoO3 and the intrinsic Si layer. The multilayer samples were exposed to AM1.5 simulated illumination with intensity of 100 mW/cm2 for 10 h. The label soaking in Figure 4a implies that SIMS examinations were carried out after illumination of the samples for 10 h. 23466

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as p-i-n layers, T-MoO3 (20 nm), and S-MoO3 (7.5 nm) are also shown in Figure 5g−i, respectively. This could be ascribed to the higher importance of the interface between p and i layers than that between the i and n layers and the recombination of carriers reducing Rsh and increasing Rs, leading to poor FF values.38−40 In the device with normal p-i-n layers, the doping elements in the doping layers can act as defect sites and thus are easily degraded under illumination.38−40 This effect induces p,nlayer degradation and finally degradation of Voc because the residual boron doping or boron diffusion at the interface reduces the electrical field.38−40 However, an effective barrier can be made by the carrier accumulation at defect sites, which forces the increase in Voc of the p-i-n device, again leading to saturation.38−40 The Voc value of device #2 with normal p-i-n layers showed quick degradation and a subsequent quick recovery, which indicated that the defect sites in the p,n-a-Si:H initial layers induced degradation; however, ultimately, the carriers were captured and accumulated at the defect sites. This accumulation is helpful in the recovery and saturation of Voc.38−40 However, these data and this explanation based on only the initial state of light-induced degradation over approximately 10 h are pure speculation and are inadequate. More detailed study and analysis are needed for at least 1000 h, to give information such as the defect density, light intensity change, and temperature dependence. In comparison, the Voc values obtained for the dopant-free structures containing MoO3 were stable with increasing operation duration owing to the lower number of defect sites because of the absence of dopants, leading to a less contaminated oxide surface that was compacted and smooth, resulting in an increase in the electron lifetime and thus giving improved light stability.38−40 This was confirmed by the SIMS data, as shown in Figures 4a,b, and Figure S5 in the Supporting Information. As the thickness of the MoO3 layer increased, the light stability improved because the thicker MoO3 prevented the penetration of H2O and O2 from the device environment. The device with 10 nm thick MoO3 was more stable than that with 20 nm thick MoO3. However, the stabilities of the two devices switched after 1 h, following the general trend of thickness dependence. Considering the long-term light stability, that is, over a period of 10 h, the devices faithfully followed the trend of thickness dependence. The data indicated that the Vbi between MoO3 and LiF/Al was stable under illumination. However, the interface between MoO3 and i-a-Si:H can also cause FF degradation and ultimately result in a decrease in light stability because FF is also related to the electrical interface. The improved performances indicate that the dopant-free structures proposed in this paper can be considered as alternatives to conventional structures.

with MoO3 show better stability than the conventional p-i-n Si structure as the operation time increases. The work reported here opens a new direction toward the development of truly dopant-free devices that can be fabricated without using toxic gases. Furthermore, the device structure reported in this work also provides the potential for the further enhancement of the efficiency of future dopant-free solar cells.

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ASSOCIATED CONTENT

S Supporting Information *

Transmittance and absorption spectra of the dopant-free solar cells containing S-MoO3 of different thicknesses. XPS analysis for T-MoO3 and S-MoO3. The theory of equivalent circuit analysis. SIMS data for each element’s diffusion to the intrinsic layer for p-i silicon layer and MoO3-i silicon layer. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*S.Y.R.: Tel: +82-41-530-2295. E-mail: [email protected]. kr. *K.S.L.: Tel: +82-42-350-3427. E-mail: [email protected]. *C.-S.K.: Tel: +82-55-280-3696. E-mail: [email protected]. kr. Present Address ◆

S.Y.R.: Department of Information Display, Sunmoon University, Asan, Chungnam, 336-708, Republic of Korea. Author Contributions ○

J.-H.Y., H.-H.J., and J.S. contributed equally to this paper.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present research was supported by the research fund (2013-PNK3290) of the Korea Institute of Materials Science. REFERENCES

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CONCLUSIONS In summary, we have designed and fabricated high-performance dopant-free solar cells and investigated the effects of the deposition method and MoO3 thickness on the device performance. The results of our study indicate that substitution with MoO3 and LiF/Al can provide an efficient intrinsic potential for the electrons and holes generated from the intrinsic Si owing to the formation of a moderate electric field. The PCE of the optimized dopant-free solar cells reaches a maximum value of 7.08%. Furthermore, the values of Voc, FF, and Jsc of the optimized devices are 0.65 V, 0.67, and 16.08 mA/cm2, respectively. In addition, the dopant-free structures 23467

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