- Nano Express
- Open Access
Highly Conductive PEDOT:PSS Transparent Hole Transporting Layer with Solvent Treatment for High Performance Silicon/Organic Hybrid Solar Cells
© The Author(s). 2017
- Received: 22 June 2017
- Accepted: 16 August 2017
- Published: 23 August 2017
Efficient Si/organic hybrid solar cells were fabricated with dimethyl sulfoxide (DMSO) and surfactant-doped poly(3,4-ethylenedioxythiophene): polystyrene (PEDOT:PSS). A post-treatment on PEDOT:PSS films with polar solvent was performed to increase the device performance. We found that the performance of hybrid solar cells increase with the polarity of solvent. A high conductivity of 1105 S cm− 1 of PEDOT:PSS was achieved by adopting methanol treatment, and the best efficiency of corresponding hybrid solar cells reaches 12.22%. X-ray photoelectron spectroscopy (XPS) and RAMAN spectroscopy were utilized to conform to component changes of PEDOT:PSS films after solvent treatment. It was found that the removal of the insulator PSS from the film and the conformational changes are the determinants for the device performance enhancement. Electrochemical impedance spectroscopy (EIS) was used to investigate the recombination resistance and capacitance of methanol-treated and untreated hybrid solar cells, indicating that methanol-treated devices had a larger recombination resistance and capacitance. Our findings bring a simple and efficient way for improving the performance of hybrid solar cell.
- Hybrid solar cells
In recent years, silicon-organic hybrid solar cells are attracting great attention benefit from their advantages such as low-temperature spin-coating process, simple device structure, and low-cost potential [1–7]. Several kinds of organic materials, including conjugated polymers [1–4, 8], conjugated small molecules [9, 10], and fullerene derivatives , are used as hole or electron transporting layer in hybrid solar cells. Among them, poly(3,4-ethylenedioxythiophene): polystyrene (PEDOT:PSS), a conducting polymer widely used as a hole transporting layer or metal-free electrode in organic electronic devices, has been proven to be commendable to act as a hole transporting layer in hybrid solar cells [12–15]. Owing to the rapid development of theory and techniques on high-performance materials [16, 17], hybrid solar cells have gained great progress. Generally, in a Si/PEDOT:PSS heterojunction-based solar device, the incoming light is mostly absorbed by Si. Light-induced charge carriers are then separated under the built-in electric field. In order to get high-power conversion efficiency hybrid solar cells, many efforts have been made to reduce the light reflection of the Si substrate. Therefore, nanostructured Si including nanowires , nanoholes , pyramids , and some other hierarchical structures  are applied to increase the light harvesting of the hybrid solar cells. Although an enhanced short-circuit current intensity (J SC) may be obtained due to the improved light harvesting, the associated large surface/volume ratio of nanostructured Si may cause poor contact between Si and PEDOT:PSS and then serious surface recombination in the hybrid solar cells. What is more, the cost will be increased with complex nanostructure Si fabrication. On the other hand, it has been reported that the conductivity and the contact between PEDOT:PSS and Si could be improved by adding organic co-solvents and non-ionic surfactant, respectively. It has been reported that improvement of surface conductivity of PEDOT:PSS films could been received by acid treatments like formic acid treatment and nitric acid treatment [21, 22]. But acid treatment is too violent for the PEDOT:PSS films and may take adverse effects to the device stability. It is well known that PEDOT:PSS aqueous dispersion is made up of a certain concentration of PSS added to PEDOT. But the insulating PSS that contains sulfonic acid SO3H groups may bring detrimental effects such as low conductivity and lifetime issues. Dimethyl sulfoxide (DMSO) and ethylene glycol (EG) are commonly used as co-solvents to modify the morphology and nanostructure of PEDOT:PSS, and the conductivity could be significantly improved compared to that with other co-solvents [23, 24]. However, it is worth noting that although the morphological structure across the PEDOT:PSS thin film may be modified by the addition of co-solvents, the negative effects brought by PSS still remain, which means the performance of the hybrid solar cells could be further improved.
In this work, we demonstrate planar Si-based hybrid solar cells with an enhanced PCE by a simple post-treatment with methanol. DMSO is used as a co-solvent to improve the conductivity of the PEDOT:PSS thin film; in addition, a further methanol treatment by spin-coating could further improve the conductivity and change the PSS concentration on the surface. A high PCE of 12.22% has been achieved by the methanol-treated hybrid Si/PEDOT:PSS solar cell, which is 28% higher than that of the untreated one. The effects of surface treatment with different alcohols on the hybrid solar cells properties are evaluated. Our work offers a better understanding of using of solvent treatment for further enhancing the device performances of the hybrid Si/organic solar cells. Our experimental results demonstrate that an effective modification of electrical properties occurs in Si/PEDOT:PSS solar cells when implementing methanol treatment on PEDOT:PSS films.
Double-side-polished n-type CZ crystal Si(100) wafers(2.6 ~ 3.5 Ω cm, 450-μm thickness) were first cleaned using acetone, ethanol, and deionized water by ultrasonically soaking for 20 min, respectively. Then, the substrates were treated in a 80 °C piranha solution (3:1 H2SO4/H2O2) for 30 min and washed with deionized water several times. Finally, the samples were immersed in a diluted HF (5%) solution for 5 min to remove the native oxide to obtain H-Si surfaces. The cleaned Si were then transferred into a diluted HNO3 (10%) solution to form a SiO x film to act as a passivation layer [25, 26]. Highly conductive PEDOT:PSS (Clevios PH1000) uniformly mixed with 5 wt% DMSO and 1 wt% Triton X-100 was spin-coated onto the surface of the SiO x -terminated Si substrate at a spin speed of 1500 rpm in air for 60 s. Following that, the samples were annealed at 140 °C for 10 min under nitrogen atmosphere. Solvent treatment with methanol or other alcohols on PEDOT:PSS films was done by dropping 60 μL methanol or other alcohols on the dried PEDOT:PSS films and then spin-coated at 2000 rpm for 60 s. The obtained films were annealed at 120 °C for 10 min under nitrogen atmosphere. Silver grids of 200-nm thickness were deposited by thermal evaporation as the top electrode through a shadow mask and aluminum of 200-nm thickness was deposited on the back side. The deposition process is performed under high vacuum circumstance about ~ 10− 7 Pa. The deposition rate of Ag is controlled at 0.2 Ȧ S− 1 for the first 10 nm and at 0.5 Ȧ S− 1 for the rest of the Ag electrode. And for Al deposition, the deposition rate is controlled at 0.3 Ȧ S− 1 for the first 10 nm, 1 Ȧ S− 1 for the thickness range from 10 to 200 nm, and 5 Ȧ S− 1 for the rest part. The device area is 0.3 cm2.
The current density-voltage (J-V) characteristics of the solar cells were determined by a Keithley 2400 digital source meter under simulated sunlight (100 mW cm− 2) illumination provided by a xenon lamp (Oriel) with an AM 1.5 filter. The radiation intensity was calibrated by a standard silicon photovoltaic device. The external quantum efficiency (EQE) system used a 300 W xenon light source with a spot size of 1 mm × 3 mm which was calibrated with a silicon photodetector. For PEDOT:PSS conductivity measurements, the PEDOT:PSS films are spin-coating on a glass. The conductivity of the PEDOT:PSS films was measured by RST-9 4-point probe instrument. The X-ray photoelectron spectroscopy (XPS) spectra were collected on Thermo ESCALAB 250 equipped with a monochromatized Al Kα source (hν = 1486.8 eV). Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical workstation (CHI660E). EIS spectra are recorded in the frequency range of 10− 1–106 Hz at room temperature. The results of EIS spectra are analyzed and fitted using the Z-view software. Transmittance spectra of the films were measured using a UV-2450 spectrophotometer with PEDOT:PSS films spin-coating on a quartz glass. The surface topography and roughness of PEDOT:PSS films were observed by atomic force microscopy (AFM) in a Digital Instruments Dimension 3100 Nanoscope IV.
PEDOT:PSS/Planar-Si Hybrid Solar Cell Properties
Photovoltaic performance of the hybrid solar cells with PEDOT:PSS treated with different chemicals
V OC (V)
J SC(mA cm− 2)
J SC (cal)b(mA cm−2)
Physical properties of solvents used for film treatments
Polarity (water = 100)
Compared to untreated devices, a slightly higher PCE of 9.98% is achieved for IPA-treated devices, with a J SC of 27.71 mA cm− 1 and a FF of 64.66%. The ethanol-treated devices have a V OC of 0.556 V, a J SC of 28.16 mA cm− 1, and a FF of 68.27%, resulting in a higher PCE of 10.69%. When methanol treatment was used, a highest PCE of 12.22% is achieved with a J SC of 30.58 mA cm− 1 and a FF of 72.01%, which is 28% higher than that of control devices. Obviously, the performance of hybrid solar cells is increased with increasing polarities of chemicals used.
Conductivity and Optoelectronic Properties of Treated PEDOT:PSS Films
In summary, a post-treatment on PEDOT:PSS films with polar solvent has been proposed to enhance the performance of PEDOT:PSS/Si heterojunction solar cells. A high conductivity of 1105 S cm− 1 of PEDOT:PSS was achieved by using methanol treatment as the corresponding hybrid solar cells having a best efficiency of 12.22%, which is 28% higher compared to those with untreated PEDOT:PSS films. RAMAN and XPS results provide strong evidence for the reorganization of PEDOT nanocrystals and reduction of PSS chain along the surface, which jointly enhance the conductivity and therefore the device performance. The enhanced conductivity can be ascribed to the rearrangement of PEDOT moieties on the surface since the PSS matrix can be removed by methanol spin-coating. EIS measurements stated clearly results that the charge recombination loss in hybrid solar cells with methanol treated PEDOT:PSS films is reduced compared to untreated devices. We believe that such low cost approaches of modifying the surface of PEDOT:PSS buffer layer would be promising candidates for photovoltaic application.
This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 11574119, 51202150, and 51272161), Science and Technology R&D Program of Shenzhen (Grant No. JCYJ20160520175916066, JCYJ20150324141711593), China Postdoctoral Science Foundation (Grant No. 2016M590809), State Key Laboratory of Solidification Processing in NWPU (SKLSP201110), and Program of Introducing Innovative Research Team in Dongguan (Grant No. 2014607109).
QDL and JZZ conceived the idea and drafted the manuscript. QDL and JWY carried out the experiments. WGX and XRZ commented on the results. QDL and SSC revised the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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