- Nano Express
- Open Access
Si/PEDOT:PSS Hybrid Solar Cells with Advanced Antireflection and Back Surface Field Designs
© The Author(s). 2016
- Received: 2 June 2016
- Accepted: 21 July 2016
- Published: 8 August 2016
Molybdenum oxide (MoO3) is one of most suitable antireflection (AR) layers for silicon/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (Si/PEDOT:PSS) hybrid solar cells due to its well-matched refractive index (2.1). A simulation model was employed to predict the optical characteristics of Si/PEDOT:PSS hybrid solar cells with the MoO3 layers as antireflection coatings (ARCs), as well as to analyze the loss in current density. By adding an optimum thickness of a 34-nm-thick ARC of MoO3 on the front side and an effective rear back surface field (BSF) of phosphorus-diffused N + layer at the rear side, the hybrid cells displayed higher light response in the visible and near infrared regions, boosting a short-circuit current density (J sc) up to 28.7 mA/cm2. The average power conversion efficiency (PCE) of the Si/PEDOT:PSS hybrid solar cells was thus increased up to 11.90 %, greater than the value of 9.23 % for the reference devices.
- Hybrid solar cells
- Antireflection layer
Organic/inorganic hybrid solar cells that combine the advantages of crystalline silicon (c-Si) and organic solar cells have attracted much attention in recent years owing to their solution-based treatment, simplified processing, and routinely increased power conversion efficiency (PCE) [1–3]. The key role of the organic materials in a hybrid solar cell is to form a heterojunction with Si [4, 5]. Among numerous materials, PEDOT:PSS, with its electron blocking and hole transporting characteristics, seems to be a promising candidate for organic p-type layer [6, 7]. Moreover, PEDOT:PSS is amenable to solution-based processes and shows good conductivities of up to 1000 S/cm, suitable work functions in the range of 4.8–5.2 eV, and high transparency in a visible light range [8–11]. All of these advantages enable silicon/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (Si/PEDOT:PSS) hybrid solar cells to achieve a PCE above 13 % [12–17], up to now. Aiming to improve the performance of the Si/PEDOT:PSS hybrid solar cells to the theoretical boundary , a range of research activities have been carried out, e.g., antireflection (AR) layer coating [19–23], PEDOT:PSS property tuning [24, 25], Si surface texturing [26–28], and back surface field (BSF) layer forming [18, 29].
Adding an AR layer seems to be an effective way to improve the PCE of Si/PEDOT:PSS hybrid cells, with the mechanism of taking advantage of the difference in optical path between the top and bottom sides of the AR layer. As a result, the reflection was reduced and the transmission was enhanced. Ultimately, higher intensity of the light reaching the absorber results in higher short-circuit current density (J sc). However, limited by the quite lower reflective index (1.2–1.6) for the PEDOT:PSS layer, it is critical to precisely figure out appropriate materials and optimum thickness of the AR layer. Meanwhile, special designs are also needed at the rear side of the hybrid solar cells in order to efficiently collect the photo-generated carriers related to the incident light with a longer wavelength.
In this study, we selected a molybdenum oxide (MoO3) film as an antireflection (AR) layer because of its suitable refractive index (~2.1) for the Si/PEDOT:PSS device. By simulating the light absorption and transmission behavior of the MoO3 AR layer of different thicknesses in different wavelength ranges, we obtained a more precise thickness value without the hassle of intensive experimenting. For the purpose of forming a uniform film and without damaging the interface on the PEDOT:PSS film, we deposited the MoO3 layer by thermal evaporation. Besides, a phosphorus-diffused BSF layer was applied onto a medium-doped substrate to suppress recombination and to promote the carrier collection efficiency at the rear side. Furthermore, gendering by the simulated results, we reached an optimum thickness of 34 nm experimentally. The Si/PEDOT:PSS solar cells with BSF and the MoO3 AR layer we fabricated exhibited better light response and external quantum efficiency (EQE) within the wavelength range of 500–1100 nm, resulting in obviously improved performance in comparison with the reference cells, with the V oc from 572.6 to 599.8 mV, the J sc from 23.3 to 28.7 mA/cm2, and the PCE from 9.23 to 11.90 % under the simulated solar illumination (AM 1.5 G, 100 mW/cm2).
In this study, we predicted the optical performance of solar cells utilizing the full-wave finite-element method which solves Maxwell’s equations within a unit cell surrounded by periodic boundary condition and perfectly matched layers . The wavelength-dependent refractive index (n) of a Si material was taken from Palik’s data , and the spectral response in the wavelength range of 300–1200 nm for hybrid solar cells was considered. The overall performance was evaluated with standard AM 1.5 G illumination under normal incidence. The photocurrent density (J ph), total current density (J tot), total loss percentage (P loss), loss current density (J loss), reflection loss (R), and parasitic absorption loss were calculated by integrating the absorption and reflection spectrum of the solar cell [15, 32].
The thickness of the spin-coated PEDOT:PSS film was obtained by a step profiler (Veeco Dektak150). The refractive index and extinction coefficient (k) of thermal-evaporated MoO3 AR layer were tested with spectroscopic ellipsometry (J.A. Woollam M-2000 DI). Using a spectrophotometer (Helios LAB-re, with an integrating sphere), the reflectivity of the PEDOT:PSS and MoO3 layers was measured in the wavelength range of 400–1100 nm. After the irradiation intensity was calibrated using a standard silicon photovoltaic device (Oriel, model 91150V), the current density-voltage (J-V) characteristic of the hybrid solar cells was tested with a Keithley 2400 digital source meter (Keithley) under simulated sunlight (100 mW/cm2) illumination provided by a xenon lamp (Oriel) with an AM 1.5 filter. The open area of the cells was 0.7 cm × 0.8 cm, with 0.11 cm2 shaded by the grid of Ag electrodes. The Newport silicon detector and 300 W Xenon Light Source with a spot size of 1 × 3 mm were used to measure the EQE.
To further investigate the optimum thickness of the MoO3 AR layer, we simulated the J loss-thickness curve quantitatively (Fig. 2b). And we also refined the loss current; it can be divided into three parts, namely the reflection loss current density (R), parasitic absorption loss caused by PEDOT:PSS, and MoO3. As the thickness of PEDOT:PSS is constant, its parasitic absorption is almost fixed. Compared with reflection loss, the materials’ parasitic absorption loss are rather small with the increasing of MoO3 thickness. Such small parasitic absorptions also can be predicted by the k curves in Fig. 1b. In general, the tendency of the total current loss is consistent with reflection loss which results in the optimal thickness of approximately 34 nm. Considering the J tot value of 43.77 mA/cm2, the J ph of the hybrid cell is 37.00 mA/cm2.
The parasitic absorptions of PEDOT:PSS and MoO3 are in the wavelength range of 350–1100 nm, as illustrated in Fig. 2c. It can be found that MoO3 displays higher absorption especially in the ultraviolet band. Both MoO3 and PEDOT:PSS have small parasitic absorptions. It corresponds to the k curve in Fig. 1b and also supports the small current loss caused by parasitic absorptions in Fig. 2b.
To match the simulated and experimented refractive indices, we simulated the reflectivity of PEDOT:PSS heterojunction and with a 34-nm-thick MoO3 layer (Fig. 2d). It can be found that the simulated curves fitted well with the experimental ones in the wavelength range of 400–1100 nm. Both the simulated and experimented data showed much lower refractive indices after the deposition of 34 nm MoO3 compared with the single PEDOT:PSS layer. Combining the small parasitic absorptions of PEDOT:PSS and MoO3, the great reduction of reflectivity in full-wave band will result in the enhancement of short-circuit current obviously.
Performance indicators of Si/PEDOT:PSS heterojunction solar cells
V oc (mV)
J sc (mA/cm2)
R s (Ω cm2)
572.6 ± 1.2
23.2 ± 0.1
69.3 ± 0.5
9.23 ± 0.20
5.5 ± 0.7
598.1 ± 0.4
25.3 ± 0.1
70.2 ± 0.6
10.63 ± 0.18
4.6 ± 0.9
BSF and MoO3
599.8 ± 0.8
28.7 ± 0.2
69.2 ± 0.8
11.90 ± 0.11
4.3 ± 1.0
The EQE of each sample cell was measured, as shown in Fig. 3b. As expected, the cells with the MoO3 AR layer and BSF design displayed a higher EQE value in the visible and near infrared regions in comparison with the reference and BSF cells (which is consistent with the increase of J sc), benefiting from increased intensity of the incident light to the p-n junction. In the ultraviolet band, the unexpected lower EQE value was caused by parasitic absorption of MoO3 material. Unlike PEDOT:PSS, MoO3 displayed a high-extinction coefficient and parasitic absorption in this ultraviolet wavelength region (Figs. 1b and 2c).
Diode ideality factor (n), reverse saturation current density (J s ), and Schottky barrier height (Φ bi) values of Si/PEDOT:PSS heterojunction solar cells
J s (A/cm2)
2.88 × 10−7
BSF and MoO3
1.54 × 10−7
We have applied a simulation model to analyze the photocurrent of Si/PEDOT:PSS heterojunction solar cells. The optical properties we simulated matched well with the experiment results. By coating a MoO3 AR layer with a thickness of 34 nm, the performance of BSF-involved Si/PEDOT:PSS hybrid solar cells can be significantly improved. Solar cells with a higher PCE of 11.90 % was eventually achieved through enhancement of short-circuit current density.
AR, antireflection; ARCs: antireflection coatings; BSF, back surface field; J sc, short-circuit current density; c-Si, crystalline silicon; EQE, external quantum efficiency; FF, fill factor; J loss, loss current density; J ph, photocurrent density; J tot, total current density; J-V, current density-voltage; k, extinction coefficient; MoO3, molybdenum oxide; N, refractive index; PCE, power conversion efficiency; PEDOT:PSS, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate); P loss, loss percentage of total current density percentage; total loss percentage; R, reflection loss; R s, series resistance
This work is supported by Zhejiang Provincial Natural Science Foundation (No. LY14F040005, LR16F040002), National Natural Science Foundation of China (Grant No. 61404144, 51472044), Major Project and Key S&T Program of Ningbo (No. 2016B10004), International S&T Cooperation Program of Ningbo (Grant No. 2015D10021), “Thousand Young Talents Program” of China, One Hundred Person Project of the Chinese Academy of Sciences, and the Instrument Developing Project of the Chinese Academy of Sciences (No. yz201328).
YS, ZY and PG carried out the design and drafted the manuscript. JH, XY, JS and SW commented on the results and revised the manuscript. YX and JY conceived the design and supervised the research. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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- Wendy U, Janke J, Alivisatos A (2002) Hybrid nanorod-polymer solar cells. Science 295:2425–2427View ArticleGoogle Scholar
- Schmidt J, Titova V, Zielke D (2013) Organic-silicon heterojunction solar cells: open-circuit voltage potential and stability. Appl Phys Lett 103:183901View ArticleGoogle Scholar
- Chen J-Y, Yu M-H, Chang S-F, Wen Sun K (2013) Highly efficient poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/Si hybrid solar cells with imprinted nanopyramid structures. Appl Phys Lett 103:133901View ArticleGoogle Scholar
- DuBow JB, Burk DE, Sites JR (1976) Efficient photovoltaic heterojunctions of indium tin oxides on silicon. Appl Phys Lett 29:494View ArticleGoogle Scholar
- Schunck JP, Coche A (1979) Efficient indium tin oxide/polycrystalline silicon solar cells. Appl Phys Lett 35:863View ArticleGoogle Scholar
- Modestino MA, Walczak KA, Berger A, Evans CM, Haussener S, Koval C, Newman JS, Ager JW, Segalman RA (2014) Robust production of purified H2in a stable, self-regulating, and continuously operating solar fuel generator. Energy Environ Sci 7:297–301View ArticleGoogle Scholar
- Xia Y, Ouyang J (2011) PEDOT:PSS films with significantly enhanced conductivities induced by preferential solvation with cosolvents and their application in polymer photovoltaic cells. J Mater Chem 21:4927View ArticleGoogle Scholar
- Lee BR, Kim JW, Kang D, Dong Wook L, Ko SJ, Hyun Jung L, Lee CL, Kim JY, Shin HS, Song MH (2012) Highly efficient polymer light-emitting diodes using graphene oxide as a hole transport layer. ACS Nano 6:2984–2991View ArticleGoogle Scholar
- Wei WR, Tsai ML, Ho ST, Tai SH, Ho CR, Tsai SH, Liu CW, Chung RJ, He JH (2013) Above-11 %-efficiency organic–inorganic hybrid solar cells with omnidirectional harvesting characteristics by employing hierarchical photon-trapping structures. Nano Lett 13:3658–3663View ArticleGoogle Scholar
- Alemu D, Wei H-Y, Ho K-C, Chu C-W (2012) Highly conductive PEDOT:PSS electrode by simple film treatment with methanol for ITO-free polymer solar cells. Energy Environ Sci 5:9662View ArticleGoogle Scholar
- He L, Lai D, Wang H, Jiang C (2012) High-efficiency Si/polymer hybrid solar cells based on synergistic surface texturing of Si nanowires on pyramids. Small 8:1664–1668View ArticleGoogle Scholar
- Liu R, Lee ST, Sun B (2014) 13.8% efficiency hybrid Si/organic heterojunction solar cells with MoO3 film as antireflection and inversion induced layer. Adv Mater 26:6007–6012View ArticleGoogle Scholar
- Tsai ML, Wei WR, Tang L, Chang HC, Tai SH, Yang PK, Lau SP, Chen LJ, He JH (2016) Si hybrid solar cells with 13 % efficiency via concurrent improvement in optical and electrical properties by employing graphene quantum dots. ACS Nano 10:815–821View ArticleGoogle Scholar
- Yang P, Xie D, Zhao Y, Xu J, Li X, Teng C, Sun Y, Li X, Zhu H (2016) NO2-induced performance enhancement of PEDOT:PSS/Si hybrid solar cells with a high efficiency of 13.44. Phys Chem Chem Phys 18:7184–7189View ArticleGoogle Scholar
- He J, Yang Z, Liu P, Wu S, Gao P, Wang M, Zhou S, Li X, Cao H, Ye J (2015) Enhanced electro-optical properties of nanocone/nanopillar dual-structured arrays for ultrathin silicon/organic hybrid solar cell applications. Adv Energy Mater. doi:10.1002/aenm.201501793 Google Scholar
- Jeong S, Garnett EC, Wang S, Yu Z, Fan S, Brongersma ML, McGehee MD, Cui Y (2012) Hybrid silicon nanocone–polymer solar cells. Nano Lett 12:2971–2976View ArticleGoogle Scholar
- Yu P, Tsai CY, Chang JK, Lai CC, Chen PH, Lai YC, Tsai PT, Li MC, Pan HT, Huang YY, Wu CI, Chueh YL, Chen SW, Du CH, Horng SF, Meng HF (2013) 13 % efficiency hybrid organic/silicon nanowire heterojunction solar cell via interface engineering. ACS Nano 7:10780–10787View ArticleGoogle Scholar
- Zielke D, Pazidis A, Werner F, Schmidt J (2014) Organic-silicon heterojunction solar cells on n-type silicon wafers: the BackPEDOT concept. Solar Energy Materials Solar Cells 131:110–116View ArticleGoogle Scholar
- Q. Liu, R. Ishikawa, S. Funada, T. Ohki, K. Ueno, H (2015) Shirai, Highly Efficient Solution-Processed Poly(3,4-ethylenedio-xythiophene):Poly(styrenesulfonate)/Crystalline-Silicon Heterojunction Solar Cells with Improved Light-Induced Stability. Advanced Energy Materials. doi:10.1002/aenm.201500744.
- Wang M, Li S, Zhang P, Wang Y, Li H, Chen Z (2015) A modified sequential method used to prepare high quality perovskite on ZnO nanorods. Chem Phys Lett 639:283–288View ArticleGoogle Scholar
- Zhang P, Li S, Liu C, Wei X, Wu Z, Jiang Y, Chen Z (2014) Near-infrared optical absorption enhanced in black silicon via Ag nanoparticle-induced localized surface plasmon. Nanoscale Res Lett 9:519–523View ArticleGoogle Scholar
- \Zhang T, Zhang P, Li S, Li W, Wu Z, Jiang Y (2013) Black silicon with self-cleaning surface prepared by wetting processes. Nanoscale Res Lett 8:351–355Google Scholar
- Li H, Li S, Wang Y, Sarvari H, Zhang P, Wang M, Chen Z (2016) A modified sequential deposition method for fabrication of perovskite solar cells. Solar Energy 126:243–251View ArticleGoogle Scholar
- Thomas JP, Srivastava S, Zhao L, Abd-Ellah M, McGillivray D, Kang JS, Rahman MA, Moghimi N, Heinig NF, Leung KT (2015) Reversible structural transformation and enhanced performance of PEDOT:PSS-based hybrid solar cells driven by light intensity. ACS Appl Mater Interfaces 7:7466–7470View ArticleGoogle Scholar
- Liu Q, Khatri I, Ishikawa R, Ueno K, Shirai H (2013) Effects of molybdenum oxide molecular doping on the chemical structure of poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) and on carrier collection efficiency of silicon/poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) heterojunction solar cells. Appl Phys Lett 102:183503View ArticleGoogle Scholar
- Wang H, Wang J (2015) Hybrid Si nanocones/PEDOT:PSS solar cell. Nanoscale Res Lett 10:191View ArticleGoogle Scholar
- Ge Z, Xu L, Cao Y, Wu T, Song H, Ma Z, Xu J, Chen K (2015) Substantial improvement of short wavelength response in n-SiNW/PEDOT:PSS solar cell. Nanoscale Res Lett 10:998Google Scholar
- Pudasaini PR, Ruiz-Zepeda F, Sharma M, Elam D, Ponce A, Ayon AA (2013) High efficiency hybrid silicon nanopillar-polymer solar cells. ACS Appl Mater Interfaces 5:9620–9627View ArticleGoogle Scholar
- He L, Jiang C, Wang H, Lai D, Heng Tan Y, Seng Tan C, Rusli (2012) Effects of nanowire texturing on the performance of Si/organic hybrid solar cells fabricated with a 2.2 μm thin-film Si absorber. Appl Phys Lett 100:103104View ArticleGoogle Scholar
- Yang Z, Shang A, Zhan Y, Zhang C, Li X (2013) Ultra-broadband performance enhancement of thin-film amorphous silicon solar cells with conformal zig–zag configuration. Opt Lett 38:5071View ArticleGoogle Scholar
- Palik E (1985) Handbook of optical constants of solids. Academic Press, OrlandoGoogle Scholar
- Yang Z, Li X, Wu S, Gao P, Ye J (2015) High-efficiency photon capturing in ultrathin silicon solar cells with front nanobowl texture and truncated-nanopyramid reflector. Opt Lett 40:1077View ArticleGoogle Scholar
- Kern W, Puotinen DA (1970) Cleaning solutions based on hydrogen peroxide for use in silicon semiconductor technology. Rca Rev 31:187–205Google Scholar
- Peters M, Guo S, Liu Z (2016) Full loss analysis for a multicrystalline silicon wafer solar cell PV module at short-circuit conditions. Prog Photovoltaics Res Appl 24:560–569View ArticleGoogle Scholar