Rear-Sided Passivation by SiNx:H Dielectric Layer for Improved Si/PEDOT:PSS Hybrid Heterojunction Solar Cells
© The Author(s). 2016
Received: 24 February 2016
Accepted: 30 May 2016
Published: 28 June 2016
Silicon/organic hybrid solar cells have recently attracted great attention because they combine the advantages of silicon (Si) and the organic cells. In this study, we added a patterned passivation layer of silicon nitride (SiNx:H) onto the rear surface of the Si substrate in a Si/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hybrid solar cell, enabling an improvement of 0.6 % in the power conversion efficiency (PCE). The addition of the SiNx:H layer boosted the open circuit voltage (V oc) from 0.523 to 0.557 V, suggesting the well-passivation property of the patterned SiNx:H thin layer that was created by plasma-enhanced chemical vapor deposition and lithography processes. The passivation properties that stemmed from front PEDOT:PSS, rear-SiNx:H, front PEDOT:PSS/rear-SiNx:H, etc. are thoroughly investigated, in consideration of the process-related variations.
Over the past several decades, crystalline silicon (c-Si) solar cells have dominated the commercial solar cell market due to multiple factors, such as high power conversion efficiency (PCE) , abundance of raw materials, free of toxicological issues, and well-established processing techniques. However, this type of solar cells suffers from drawbacks such expensive processing and large material consumption due to high-temperature treatment and thick substrate required. In recent years, organic photovoltaics emerge as a promising technology in the solar energy field, thanks to simple processing and low material consumption [2–4]. The development of organic solar cells is faced by a grand challenge: the PCE is relatively low due to the low electron–hole separation efficiency. The emergence of c-Si/organic hybrid photovoltaics offers a possible route to low-cost and high-efficiency solar cells by combining the advantages of c-Si and organic materials [5–7]. Recently, poly(3,4ethylenedioxythiophene)/poly (styrenesulfonate) (PEDOT:PSS) has stimulated intense interest in the research community because of its advantageous properties with respect to light transmission and hole conductivity. Up to now, the PCE of PEDOT:PSS hybrid solar cells has been improved to above 13 % [8–10] as a result of efforts in several areas including interface modification [11, 12], surface texturing on Si [13–17], and property tuning of PEDOT:PSS . Typical improvements related to the rear side is to add an ultra-thin interfacial layer of LiF , LiQ , or CsCO3  between the c-Si layer and the back electrode, with the aims to reduce contact resistance and enhance the rear electric field. With this design, the short circuit current density (J sc) and open circuit voltage (V oc) are both enhanced. However, it is critical to precisely control the thickness of these kinds of layers at a certain value, in order to achieve a satisfied contact resistance while not hindering charge carrier collection.
For the purpose of passivating the n-type c-Si, hydrogenated silicon nitride (SiNx:H) is an ideal candidate material. SiNx:H , conventionally deposited by plasma-enhanced chemical vapor deposition (PECVD), is known to be widely used in the Si-based solar cell processing. This dielectric layer contains considerable amount of hydrogen bonds and positive charges (typically several 1012 cm−2) , offering good chemical and field-effect passivation on H-terminated n-type emitter . To date, surface recombination velocities (S eff) below 10 cm/s have been achieved though PECVD method [22, 24].
In this study, we fabricated a hybrid c-Si/organic solar cell with an added passivation layer of PECVD-SiNx:H at the rear side and investigated its characteristics. First, the PECVD-SiNx:H layer was thoroughly characterized by surface recombination velocity, focusing on its relations to some aspects including thickness and chemical bond. Second, photoresist was served to protect the SiNx layer, and chemical etching with diluted hydrofluoric acid (HF) was used to obtain a partial passivation layer with a SiNx-to-substrate ratio of 60 %. After that, a PEDOT:PSS film was formed on the front side of the substrate by spin-coating, followed by the formation of grid-Ag/full-Al contact layers on the front and rear sides by thermal evaporation. A comparison of the SiNx:H-passivated device and control sample showed perceivable increases in both J sc and V oc, improving the PCE by 0.6 to 9.0 % under the simulated solar illumination (AM 1.5, 100 mW/cm2).
A Si wafer (n-type, single-side polished, float zone, 20 × 20 mm, 300 ± 15 μm in thickness, resistance 3-5 Ωcm) underwent standard RCA (Radio Corporation of American)  cleaning and 8 % (volume ratio) HF cleaning. Then, a SiNx:H passivation layer was deposited upon the double sides of the Si substrate from the gas mixture of SiH4 (5 sccm), NH3 (40 sccm), and Ar (40 sccm) for 10 min at a temperature of 350 °C with a pressure of 70 Pa. The prepared films have a thickness of about 100 nm that was measured with a scanning electron microscope (SEM).
A layer of negative photoresist was coated on the SiNx:H layer by spin-coating at a speed of 3000 rpm for 30 s. Then, the masked Si/SiNx:H layer was exposed in the UV for 90 s. The exposed portion of the negative photoresist was then washed away using a developer, and the surface underwent an etching process in 0.25 % HF solution for 30 s, removing the portion of the SiNx:H layer without the protection of photoresist. After washing away the remained photoresist by acetone, a Si substrate partially covered by a SiNx film was obtained.
The surface topography and thickness of the patterned SiNx:H passivation layer were observed by SEM (Hitachi S-4800 SEM). The chemical bonding characteristics of the SiNx:H layers were obtained by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Harrick) and X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD). Using a microwave photoconductance decay (μ-PCD) technique (WT2000PVN, Semilab), the minority carrier lifetimes of the SiNx:H layers were characterized. After calibrating the irradiation intensity of the standard silicon photovoltaic device (Oriel, model 91150 V), the current density–voltage (J–V) characteristics of the hybrid solar cells were 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 area shaded by the grid of Ag electrodes. Newport silicon detector and 300-W xenon light source with a spot size of 1 × 3 mm was used to measure the external quantum efficiency (EQE).
Results and Discussion
SiNx:H Surface Topography
Passivation of the SiNx:H Layer
Chemical Bond Structure of the SiNx:H Layer
Photovoltaic Characteristics of the Solar Cells
Photovoltaic characteristics of Si/PEDOT:PSS heterojunction device with or without a SiNx:H layer
V oc (V)
J sc (mA/cm2)
R s (Ωcm2)
0.523 ± 0.011
24.0 ± 0.18
67.78 ± 0.27
8.40 ± 0.21
7.95 ± 0.42
2570.80 ± 5.78
0.557 ± 0.014
24.8 ± 0.22
65.24 ± 0.22
9.02 ± 0.15
10.39 ± 0.40
5544.18 ± 4.69
Diode ideality factors (n), reverse saturation current densities (J s), and Schottky barrier heights (Φ bi) values of Si/PEDOT:PSS heterojunction solar cells with or without a SiNx:H layer
J s (A/cm2)
Diode ideality factors (n)
Φ bi (eV)
1.08 × 10-6
5.55 × 10-7
In summary, we have demonstrated that the performance of Si/PEDOT:PSS hybrid solar cells can be improved by adding a patterned passivation layer of SiNx:H onto the rear surface of the Si substrate. A PCE of 9 % was achieved for the SiNx:H-coated solar cells. Compared to the cells without rear passivation, a 0.6 % improvement in PCE was obtained. As the shrink of contact areas would increase the R s value, further optimizations on the pattern configurations and the contact between Si and Al are needed to achieve more higher PCE for Si/PEDOT:PSS hybrid cells.
AFM, atomic force microscopy; ATR-FTIR, attenuated total reflectance Fourier transform infrared spectroscopy; J sc, photocurrent density; PCE, power conversion efficiency; PEDOT:PSS, poly(3, 4-ethylenedioxythiophene):poly(styrenesulfonate); V oc, open circuit voltage; XPS, X-ray photoelectron spectroscopy
This work is supported by the Zhejiang Provincial Natural Science Foundation (No. LY14F040005, LR16F040002), the National Natural Science Foundation of China (Grant No. 61404144, 51472044), the International S&T Cooperation Program of Ningbo (Grant No. 2015D10021), and the “Thousand Young Talents Program” of China, One Hundred Person Project of the Chinese Academy of Sciences, the Instrument Developing Project of the Chinese Academy of Sciences (No. yz201328).
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