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.
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.
Results and Discussion
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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