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Photoinduced Field-Effect Passivation from Negative Carrier Accumulation for HighEfficiency Silicon/Organic Heterojunction Solar Cells Zhaolang Liu,†,‡,∥ Zhenhai Yang,†,∥ Sudong Wu,† Juye Zhu,† Wei Guo,† Jiang Sheng,*,† Jichun Ye,*,† and Yi Cui*,§ †
Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, People’s Republic of China ‡ School of Materials Science and Engineering, Shanghai University, Shanghai 200072, People’s Republic of China § Department of Material Science and Engineering, Stanford University, Stanford, California 94305, United States S Supporting Information *
ABSTRACT: Carrier recombination and light management of the dopant-free silicon/organic heterojunction solar cells (HSCs) based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) are the critical factors in developing high-efficiency photovoltaic devices. However, the traditional passivation technologies can hardly provide efficient surface passivation on the front surface of Si. In this study, a photoinduced electric field was induced in a bilayer antireflective coating (ARC) of polydimethylsiloxane (PDMS) and titanium oxide (TiO2) films, due to formation of an accumulation layer of negative carriers (O2− species) under UV (sunlight) illumination. This photoinduced field not only suppressed the silicon surface recombination but also enhanced the built-in potential of HSCs with 84 mV increment. In addition, this photoactive ARC also displayed the outstanding light-trapping capability. The front PEDOT:PSS/Si HSC with the saturated O2− received a champion PCE of 15.51% under AM 1.5 simulated sunlight illumination. It was clearly demonstrated that the photoinduced electric field was a simple, efficient, and low-cost method for the surface passivation and contributed to achieve a high efficiency when applied in the Si/PEDOT:PSS HSCs. KEYWORDS: photoinduced negative carrier accumulation, photoactive field-effect passivation, antireflective coating, synergistic effect, heterojunction solar cells
I
issues. Among all, the surface carrier recombination at the Si/ PEDOT:PSS heterointerface is still one of primary issues which prevent the HSCs from reaching a theoretic PCE limit.15 When PEDOT:PSS is deposited on the polished-side of silicon wafer, a large number of micropore defects are left at the Si/ PEDOT:PSS heterointerface, especially when the textured Si substrate is used.16 In order to passivate this naked surface, the dielectric passivation layers (SiOx and AlOx) are usually used to suppress the surface carrier recombination. However, the thickness of these layers needs to be thin enough (usually 350 nm), which is suitable for an antireflection coating (ARC) application.27 Thus, the TiO2 layer has been used to trap the sunlight in the Si/ PEDOT:PSS system.28,29 In this study, a bilayer ARC of polydimethylsiloxane (PDMS) and TiO2 was applied, where PDMS was introduced for the purpose of preventing the carriers from transporting to PEDOT:PSS layer, and TiO2 had a dual functions of light-trapping and producing the O2− species which induce a photoinduced electric field. The
layer. Under this thickness, these ultrathin dielectric layers do not provide the efficient surface passivation. Usually, minority charge carrier lifetime (τn) is only ca. 20 μs, far below the normal microsecond level.17,18 Therefore, a large number of photogenerated charge carriers are lost at the Si/PEDOT:PSS interface, resulting in severe degradation in PCE. Titanium oxide (TiO2) has been widely used in the photovoltaics, due to its outstanding features including large band gap, high transparency, good environment stability, high refractive index, and electron mobility.19 Under ultraviolet (UV) irradiation, the generated electron−hole pairs are separated and move to the different regions of TiO 2 nanoparticles, forming the photocurrent.20 Furthermore, the holes can oxidize OH− and H2O molecules absorbed on TiO2 surfaces into hydroxyl radicals (•OH), while the electrons in the conduction band can reduce the absorbed O2 molecules to form the superoxide (O2−) in both air and water. Subsequently, • OH as a highly reactive species is easily reduced by the ambient oxygen in a few seconds. In addition, O2− as the less reactive species is adsorbed on the TiO2 surfaces and remains several tens of seconds (lifetime >50 s).21−23 Consequently, the 12688
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Figure 3. (a) EPR spectra of TiO2 film after 1 min with and without UV illumination at 2 K. (b) Surface potential images of TiO2 film after the different durations of UV illumination. (c) Typical C−V curves of PEDOT:PSS/PDMS/Ag accompanied by TiO2 film after the different durations of UV illumination; right figure is the equivalent circuit model.
First, as an ARC, the optical performance of TiO2 film is evaluated comprehensively. The compacted TiO2 film made of nanoparticles with an average diameter of 4.01 nm has a relatively high refractive index of ca. 2.1 (Figure S3). PDMS layer has a similar low refractive index with the PEDOT:PSS film. In order to ensure the optimal TiO2 thickness, a detailed theoretical simulation was implemented on a basis of the film thicknesses. Figure 2a shows the reflectance mapping of PEDOT:PSS/Si with the bilayer ARC, illustrating how the TiO2 film influence on the light antireflection. With increasing the thickness of TiO2 film, there is a large red shift of spectrum valley from around 500 to 1000 nm. In addition, the reflectance of short wavelength dramatically increases. For an optimal light absorption of silicon, the reflectance should be required to be low in visible wavelength range (400−900 nm). Therefore, a thin TiO2 film ( 1 × 1012
irradiation duration and intensity were measured systematically, as shown in Figure 4. Under UV soaking, the τn of silicon deposited by PEDOT:PSS, PEDOT:PSS/PDMS, and PEDOT:PSS/TiO2 layers is not changed at all. The reasons for this are that PEDOT:PSS and PDMS layers cannot absorb the light and generate the charge carriers. Furthermore, the TiO2 layer is directly coated on the PEDOT:PSS layer to form the p−n heterojunction, which consumes the photogenerated charge carriers without an accumulation. The insulating PDMS layer fully prohibits the carriers of the TiO2 layer from transporting to PEDOT:PSS layer and ensures the negative carriers to be stored in the TiO2 film. With extending the UV soaking duration (0.1 mW/cm2), the τn of Si/ PEDOT:PSS coated by the bilayer ARC increases significantly up to around 70 μs, which is double of that of Si/PEDOT:PSS (32 μs). Owing to weakening the field force by the thickness of PEDOT:PSS layer, the photogenerated excess electrons of TiO2 layer does not induce the τn to increase under UV irradiation, until the O2− radicals pile up to be enough field force to induce the band offset of silicon. In addition, with increasing the UV intensity, the τn of Si/PEDOT:PSS with the bilayer ARC also increases significantly up to around 126 μs at 3 mW/cm2. The thickness of PDMS has an influence on the field-effect passivation (Figure S7). If PDMS layer is too thin to form the continuous film, some electrons should be consumed, which no doubt will weaken the electric field. On the other hand, if PDMS layer is too thick, the long distance also reduces the electric field intensity. The thickness of TiO2 film also provides little effect on the surface passivation, since thicker film would store more negative carriers. However, the trapped carriers on the rear surface of TiO2 layer primarily impact the band offset at the silicon surface. Therefore, the negative carriers in the TiO2 layer obviously reduce the concentration of 12691
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ACS Nano cm−2. Therefore, with the presence of surface charge carriers, the balance of energy banding under thermal equilibrium is broken and band edge of silicon bends to some extent, leading to a changed barrier height of built-in field and thus the photovoltaic performance (including Voc and PCE). This photoinduced negative carrier accumulation layer will be conducive to the efficiency development of Si/PEDOT:PSS HSC. Equipped with the light-trapping and photoinduced electric field of bilayer ARC, the photovoltaic performance of Si/ PEDOT:PSS HSC will be improved significantly. From the results of Jloss, τn, and ψbi, the thicknesses (39 nm) increases reflective light to be a low photocurrent, resulting in a poor performance. Figure 6 shows the current density−voltage (J−V) character-
should be considered: (1) in the light section, the bilayer ARC manage the light path to improve the light harvesting; (2) in electric section, the photoinduced electric field not only efficiently suppresses the surface carrier recombination at the Si/PEDOT:PSS heterojunction but also enlarges the ψbi for high separation efficiency of photogeneration charge carriers, resulting in a high charge carrier collection efficiency. Additionally, we investigated the difference of performance of HSCs with bilayer ARC with and without the UV filter (Figure S10). After filtering the UV, fewer negative carriers are provided, so the performance of HSC reduces significantly. On the other hand, HSC without the PMDS/TiO2 ARC keeps the performance. Furthermore, prolonging the light duration, the photovoltaic performance of HSC with the PMDS/TiO2 ARC also increases to a large extent in Figure 7. However, the performance of HSC with the TiO2 ARC decays rapidly, especially in Voc, FF, and PCE, because the holes of PEDOT:PSS layer are consumed by the photogenerated electrons of TiO2 layer.37,38 There is more 100 mV drop of Voc because of the low ψbi of Si/PEDOT:PSS from a low work function.37 In addition, the series resistance (Rs) of HSC with the TiO2 ARC increases due to the PEDOT:PSS conductivity decreasing, leading to a low FF.38 The HSC with the PDMS ARC almost maintains the photovoltaic performance under light soaking. The PDMS is an efficient encapsulated layer to prevent from being exposed to the atmosphere. The influence from the moisture absorption of PEDOT:PSS and the formation of oxide layer at the interface of Si/PEDOT:PSS can be prohibited, which develops the air stability of device. The performance of HSC without ARC becomes better first under light irradiation, however, after 150 s the performance starts to decay drastically because of the influence of moisture and oxygen. Therefore, this photoactive ARC of PDMS/TiO2 bilayer is beneficial for improving the photovoltaic performance and stability of Si/PEDOT:PSS HSCs.
Figure 6. J−V curves of Si/PEDOT:PSS HSCs based on the different ARCs.
istics of Si/PEDOT:PSS HSCs based on the different ARCs under AM 1.5G simulated sunlight illumination, and the photovoltaic parameters are summarized in Table 2. The Jsc Table 2. Photovoltaic Properties of Si/PEDOT:PSS HSCs with the Different ARCs under Simulated AM 1.5G Illumination devices
Voc (mV)
Jsc (mA/cm2)
FF (%)
PCE (%)
PEDOT PEDOT + PDMS PEDOT + TiO2 PEDOT + PDMS + TiO2
604 ± 3 605 ± 2
25.65 ± 0.52 26.62 ± 0.46
77.43 ± 0.48 76.47 ± 0.57
12.01 ± 0.54 12.32 ± 0.59
602 ± 5
29.48 ± 0.74
75.66 ± 0.69
13.44 ± 0.77
630 ± 5
30.89 ± 0.61
77.45 ± 0.53
15.01 ± 0.58
CONCLUSIONS In summary, we introduce a photoinduced electric field in the ARC, of which the charge carrier density and potential can be modulated by UV illumination (sunlight). This photoinduced electric field is particularly useful in the photovoltaic application, providing an efficient field-effect passivation at the silicon surface and enlarging the ψbi of Si/PEDOT:PSS HSCs. This photoactive ARC of PDMS/TiO2 bilayer also has a superior capability of light-trapping, similar to that of a traditional random pyramid structure. It is worth noting that the PCE of front-type Si/PEDOT:PSS HSC breaks through 15% only based on the simple photoactive ARC, which also can be applied in other solar cells that require an electric field to suppress the surface recombination or tune the ψbi value. The self-encapsulated, photoactive ARC with tunable electric field under UV irradiation displays a great potential for application in the development of photovoltaics and optoelectronics.
values of HSCs only coated PDMS or TiO2 layer both increase, especially 3.89 mA/cm2 enhancement of TiO2 layer than 25.65 mA/cm2 of HSC without the ARC, corresponding to the simulated Jph results (Figure S4). However, PDMS or TiO2 ARCs alone play a single role in the light-trapping to enhance the Jsc. Only combined with PDMS and TiO2 layers, this ARC has a synergistic effect of light-trapping and electric field to improve the photovoltaic performance. The HSC with the bilayer ARC receives an excellent efficiency of 15.01% with a Voc of 630 mV, a Jsc of 30.89 mA/cm2, and a FF of 77.45%, being UV part of simulated light in origin. Compared to the reference HSC, there is a 25 mV increment in Voc due to higher ψbi induced by this negative carrier accumulation layer. In addition, there is a 5.24 mA/cm2 enhancement of Jsc in the solar cell with the bilayer ARC. Two reasons for this Jsc improvement
EXPERIMENTAL SECTION TiO2 Synthesis. The solution of TiO2 nanoparticle dispersion was prepared as follows:39,40 A 470 μL tetrabutyl titanate was added dropwise into the 2.5 mL ethanol with 123 μL nitric acid under mild stirring of 500 rpm. After 2 h stirring, 114 μL deionized water was added into the mixture for the hydrolysis of tetrabutyl titanate, and this reaction still went on 1 h. Finally, the resulted solution was diluted by isopropanol to form the 0.127 M TiO2 solution. This transparent 12692
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Figure 7. Photovoltaic performance including (a) Voc, (b) Jsc, (c) FF, (d) PCE, and (e) Rs of Si/PEDOT:PSS HSCs with the different ARCs under the various duration of light irradiation. Simulation Method. In this study, we used the AFORS-HET software to solve Maxwell’s, drift-diffusion and Poisson equations for implementing the optical and electric simulation of Si/PEDOT:PSS HSCs. The thicknesses of Si wafer, PEDOT:PSS, and PDMS were fixed at 300 μm, 70 and 10 nm, respectively. Since these HSCs were considered as the stratified and planar structure, two-dimensional model was used in the simulation. In the optical module, the perfectly matched layers were adopted on the front and rear sides to restrict the calculated domain. The wavelength range of 300−1150 nm was considered to implement the light response and absorption bandgap of HSCs. In the electrical module, the dynamic processes of charge carriers, including the photogenerated charge carrier transport, recombination, and collection, were carried out under the AM 1.5G sunlight illumination. Additionally, the photoelectric constants (the refractive index, electron and hole mobility, minority charge carrier lifetime and recombination velocity, work function, and band gap, etc.) could be found in publications elsewhere.1,2,36 Characterization. A particle size analyzer (Zetasizer Nano ZS, Malvern) was used to measure the diameter of TiO2 nanoparticles in solution. The transmittance of TiO2 film was measured by a UV−vis spectrophotometer (Lambda 950, PerkinElmer), and reflectance was measured by a reflectance analyzer (HELIOS LAB-RE, AudioDevGmbH). EPR (Bruker EMX-10/12 plus) was used to examine the paramagnetic species in TiO2 layer at 2 K temperature under the O2-rich vacuum of 10−3 mbar. Kelvin probe studies were performed to analyze the ϕs by using a Digital Instruments Dimension 3100 Nanoscope IV, with the conductive tapes to ensure a good ohmic contact to substrate. C−V curves of PEDOT:PSS/PDMS/Ag
dispersion of TiO2 nanoparticles was ready for the spin-coating without centrifugation, washing, and dispersing. Fabrication of Si/PEDOT:PSS HSCs. The devices of Si/ PEDOT:PSS HSCs were prepared as previously reported.1,11 Oneside polished, n-type (100) silicon wafers (resistivity, 0.05−0.1 Ω·cm) with the thickness of 300 ± 15 μm were used as the substrates of devices. First, the 20 × 20 mm2 square pieces were washed by the RCA cleaning process.1 The cleaned pieces were exposed to ambient atmosphere for 1 h. Then, the commercial PEDOT:PSS solution (Clevios PH1000) with the additives of the dimethyl sulfoxide (DMSO, 5 wt %) and Triton X-100 (0.1 wt %) was spin-coated on the polished side of wafer at a speed of 3500 rpm and immediately thermally treated to form a 70 nm film at 140 °C for 10 min. A silver grid was prepared by the screen-printing with a low-temperature sintered silver paste and then thermal treatment for 10 min to be an active area of 10 × 10 mm2. A PDMS (0.5 wt %) ethyl acetate solution was spin-coated above the PEDOT:PSS layer to form a 10 nm-thick dielectric layer. Then the transparent dispersion of TiO2 was continually spin-coated on the PDMS layer as an ARC, with 5 min thermal treatment at 140 °C. Finally, the rear electrode of Al was prepared by thermal evaporation with the thickness of 150 nm. The UV illumination was conducted by a 5 W UV LED lamp (254 nm), and the UV intensity was controlled by using neutral density filters and measured by using a UV power meter (TOPCON UVR-T1). For comprehensively comparing the optical-electric characteristics, three styles of reference HSCs were used: (a) Si/PEDOT:PSS, (b) PDMS/ Si/PEDOT:PSS, and (c) TiO2/Si/PEDOT:PSS. 12693
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ACS Nano accompanied by TiO2 film (MOS structure) and HSCs were measured by a Parameter Analyzer (Keithley 4200-SCS) at a signal frequency of 1 kHz. The τn values of wafers were measured by a photoconductance lifetime tester (WCT-120, Sinton, USA) without or with UV irradiation. J−V characteristics of HSCs were recorded with a digital source meter (Keithley, USA) under a simulated AM 1.5G sunlight (100 mW/cm2) illumination provided by a xenon lamp (Oriel, USA). A standard multicrystalline silicon solar cell (Oriel, model 91150 V) calibrated the intensity of simulated irradiation. The front of solar cells were shielded by the opaque mask with an open area of 7 × 8 mm2. The temperature was controlled at 25 ± 0.5 °C during the measurements. EQE system used a 300 W xenon light source with a spot size of 1 × 3 mm2, which was calibrated with a silicon photodetector also from Newport.
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ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b07222. SEM image of cross-section; absorption spectrum, photonic band gap and transmittance of TiO2 thin film; diameter distribution and XRD pattern of TiO2 nanoparticles; refractive index; simulated photocurrent density of HSCs with TiO2 ARC; schematic and C−V of MOS structure curves with UV different intensities; minority charge carrier lifetime as a function of thicknesses with the various duration of UV irradiation; energy band structure and built-in potential; J−V curves of HSCs as a function of TiO2 film thicknesses and UV filter (PDF)
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Jiang Sheng: 0000-0002-8617-0945 Yi Cui: 0000-0002-6103-6352 Author Contributions ∥
These authors contributed equally to the work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS We acknowledge the Steady High Magnetic Field Facility in High Magnetic Field Laboratory, Chinese Academy of Sciences for the EPR measurement. This work was supported by the National Natural Science Foundation of China (grant nos. 21403262 and 61574145), Natural Science Foundation of Zhejiang Province (grant nos. LR16F040002 and LY15F040003), Major Project and Key S&T Program of Ningbo (grant no. 2016B10004), and International S&T Cooperation Program of Ningbo (grant nos. 2015D10021 and 2016D10011). REFERENCES (1) Sheng, J.; Fan, K.; Wang, D.; Han, C.; Fang, J.; Gao, P.; Ye, J. Improvement of the SiOx Passivation Layer for High-Efficiency Si/ PEDOT:PSS Heterojunction Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 16027−16034. (2) Sheng, J.; Wang, D.; Wu, S.; Yang, X.; Ding, L.; Zhu, J.; Fang, J.; Gao, P.; Ye, J. Ideal Rear Contact Formed via Employing a Conjugated 12694
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