- Nano Express
- Open Access
Efficiency Enhancement Mechanism for Poly(3, 4-ethylenedioxythiophene):Poly(styrenesulfonate)/Silicon Nanowires Hybrid Solar Cells Using Alkali Treatment
© Jiang et al. 2016
- Received: 16 March 2016
- Accepted: 22 April 2016
- Published: 25 May 2016
The efficiency enhancement mechanism of the alkali-treated Si nanowire (SiNW) solar cells is discussed and analyzed in detail, which is important to control the useful photovoltaic process. All the results demonstrate that the photovoltaic performance enhancement of alkali-treated SiNW device steps from the formation of the good core-shell heterojunction, which consequently enhances the junction area, promotes fast separating and transporting of electron and hole pairs, and reduces the carrier surface combination. It also indicates that alkali treatment for SiNWs is a promising processing as an economical method for the formation of good core-shell SiNW/polymer heterojunction.
- Silicon nanowire array
- Alkali treatment
- Hybrid solar cells
Organic conjugated polymer/Si hybrid solar cells have received intensive attention due to their promising features, such as low-cost processing, light weight, low-temperature process, and large junction area [1–3]. Among the conjugated molecules, poly(3,4-ethylene dioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) is the commonly used conjugated polymer for the conjugated molecules/Si hybrid device [4–9]. PEDOT:PSS plays a major role as a hole transporting path as well as for the formation of heterojunction with Si .
Si nanowires (SiNWs) in the hybrid solar cells have been used for enhancing the light harvest in devices and could overcome high reflection issue effectively [11, 12]. Moreover, the large PEDOT:PSS/Si junction area enhances charge collection by shortening the paths traveled by minority carriers. To date, the PCEs of hybrid solar cells composed of SiNWs and PEDOT:PSS have been developed recently up to 14 % [13, 14].
However, the difficulty lies in the preparation of highly efficient core/shell nanowire photovoltaic junction in a simple way. Since the gaps between SiNWs are normally too small to be filled with the conductive polymer, If PEDOT:PSS do not infiltrate into gaps, then the photoexcited carriers are easily trapped by high-density surface defects owing to the large non-passivated surface area of nanostructures before being collected by electrodes . PEDOT:PSS and the Si nanostructures are needed to allow polymers to infiltrate and form thin films on the SiNW surface. To this regard, the tapered Si nanostructures allowed for conformal polymer surface coverage via spin coating have been exploited [15, 16].
To obtain the tapered SiNWs in a simple way, the alkali treatment process was proposed , which easily tapered and separated the SiNWs from the bundled nanowires in a simple, cost-effective way, and the tapered SiNWs also demonstrated the improved photovoltaic characteristics. In our previous work, we found the phenomena that the alkali treatment process also could tapper the SiNWs and improve efficiency of the SiNW/PEDOT:PSS hybrid solar cells , then we attributed photovoltaic improvement to the good filling of PEDOT:PSS. However, our previous work just found the phenomena and deduced the conclusion from the morphology of scanning electronic microscopy (SEM) images; it still demands further in-depth investigations to provide more evidences and understand how alkali treatment processing could improve the photovoltaic performance in the organic/inorganic hybrid solar cells, which plays an important role in controlling the useful photovoltaic process.
In this work, we find the photovoltaic improvement of the alkali-treated SiNW/PEDOT:PSS device stems from the formation of good core-shell PEDOT:PSS/SiNW heterojunctions; the alkali-treated process provides superior coverage of PEDOT:PSS down to the bottom of the NWs, then enlarges PEDOT:PSS/Si junction area and enhances charge collection by shortening the paths traveled by the electron and hole pairs (EHPs). Our major investigation is to understand how the alkali treatment processing can dramatically improve the photovoltaic performance. The facile processing of alkali treatment is comparable to the chemical polish etching (CPE) and dry etching processing, which could promote the filling of PEDOT:PSS and formation of good core-shell heterojunction [15, 19].
Fabrication of SiNWs Combining Metal-assisted Etching and Alkali Treatment
Nanostructures were fabricated on n-type, polished (100) oriented Si wafers with 500-μm thickness and 1 to 3-Ω/cm resistivity. Firstly, silicon wafers were ultrasonically vibrated in acetone and ethanol at room temperature for 10 min to remove organic contaminations, respectively. The cleaned silicon wafers were immediately immersed in a mixed solution of deionized water, HF (4.6 mol/L) and AgNO3 (0.02 mol/L) for 3 min to grow nanostructures. The samples after etching were found to be wrapped with thick dendritic Ag structure [20, 21]. Silver nanoparticles were easily removed using nitric acid (68 wt%) dipping for 20 min after completion of the electroless etching. The samples after Ag removal were then rinsed with de-ionized water followed by dipping in 10 wt% HF solutions to remove the native oxide, rinsed with de-ionized water again, and dipped in solution of 2.5 M NaOH at 25 °C for 30 s.
Hybrid Solar Cell Fabricating
Prior to device fabrication, the produced textured samples went through a rigorous cleaning procedure. At first, the samples were cleaned by immersing them in a solution consisting of H2O2 (30 %), H2SO4 (68 %), and DI water in the volume ratio of 1:1:5 at 120 °C for 10 min to remove any other organic residues. The samples were transferred to a DI water bath for 10 min. Again, the samples were immersed in a solution comprised of H2O2 (30 %), HCl (37 %), and DI water in the volume ratio of 1:1:5 at 80 °C for 10 min to remove any metallic contamination. The samples were then transferred to a DI water bath for 10 min. Finally, the samples were cleaned in a diluted HF (2 %) solution for 60 s to remove the native oxide and subsequently treated in UV-ozone cleaner for 15 min to improve the hydrophilic nature; this processing obviously improves the photovoltaic performance from 6.8 % in our previous work  to 9.1 %, the reason would be further investigated in the future. Immediately, the highly conductive PEDOT:PSS (Clevios PH500) mixed with 5 wt% dimethyl sulphoxide and 1 wt% Triton X-100 (surfactant) solution was spin-cast at 500 rpm for 15 s and 6000 rpm for 45 s to form a core-shell radial junction. The samples were then annealed on a hot plate at 140 °C for 10 min to remove the solvent. Finally, a 10-nm Cu film was deposited through a shadow mask (5 mm × 5 mm) as the top anode, a 200-nm-thick Al film was deposited on the backside for cathode contact.
Nanoscale surface and cross-sectional morphology of the SiNWs were characterized using a SEM (SUPRA™ 40). The morphology of single nanowire was characterized using the transmission electron microscope (TEM). The reflectivity of the substrates was measured by a UV-3600 spectrophotometer with an integrating sphere in the wavelength range of 300–1200 nm. The internal quantum efficiency (IQE) measurements were carried out by using a calibrated Newport 818-UV sensor and a monochromator (Acton Spectra Pro 2300i) with a lock-in amplifier (SR-830) and a SR540 optical chopper. The photovoltaic current density-voltage (J-V) characteristics of the solar cells were measured at air mass (AM) 1.5 G illumination.
Photovoltaic properties of the hybrid Si planar, SiNWs, and alkali-treated SiNWs solar cells
V oc (V)
J sc (mA/cm2)
To further investigate the reasons for the improved efficiency, it is important to gain insight into the correlation between J sc enhancement and carrier collection enhancement in the active layer. The IQE is measured as shown in Fig. 1d. One can see that IQE of the alkali-treated SiNW device is above 50 % over a wide wavelength range between 400 and 1000 nm, but SiNWs and planar devices have a maximum IQE of 40 % at 400 and 550 nm, severely decreases from 400 to 1100 nm. Compared with PEDOT:PSS/SiNW solar cells and PEDOT:PSS/Si planar solar cells, IQE of alkali-treated SiNWs device is the highest and reaches a peak value of 60 % at the wavelength of 600 nm. The IQE of planar solar cells is the lowest and reaches its peak value of 37 % at the wavelength of 500 nm. In short, the photoresponse for the alkali-treated SiNWs device is significantly better than that of SiNWs and planar one.
To further understand the limiting factors of the device performance, J-V characteristics of the devices in the dark are measured as shown in Fig. 1b. By comparing the dark-current measurement results, one can find that the SiNW/PEDOT:PSS with alkali treatment possesses small reversed saturated current compared to that of one without alkali treatment, which suggests that the recombination of carriers is reduced . In our work, the devices possess the same materials and the analogue device structure; the difference should be caused by the alkali treatment. The results indicate that introduction of alkali treatment processing could suppress the recombination of photogeneration carriers,
Figure 1c shows the reflectance spectra of planar, SiNWs, and alkali-treated SiNWs, respectively. The reflectance of alkali-treated SiNWs is slightly higher than the area densities of the NWs that are accordingly decreased. Therefore, alkali treatment could not contribute to the light absorption, so the additional enhanced J sc of alkali-treated device must step from the enlarged junction area.
For the planar and SiNWs device, the distance of photogenerated carriers in the silicon substrate transporting to the top contact is about several micrometers, which is the length of SiNWs. The poor carrier collection efficiency in the deep region of the silicon substrate causes low photocurrent and low IQE .
To reveal how the EHP can be fast separated and collected in the core-shell heterojunctions, and to further understand the related mechanism for the enhanced performance of alkali treated devices, we investigated the charge dissociation probabilities in the alkali treatment devices, which indicates the charge collection efficiency and the charge recombination reducing under working condition . Figure 4e shows the J-V characteristics in a wide reverse bias range under AM 1.5G illumination. The results are plotted as the net photocurrent (J ph = J L−J D) dependence on the effective applied voltage V eff, where J L and J D are the current density under illumination and in the dark, respectively, V is the applied voltage, and V o is the compensation voltage at which J ph = 0.
At a large reverse voltage (V eff = 1.2 V), J ph reaches saturation for both devices. Noticeably, with increasing effective voltage, the saturation photocurrent (J ph) in the device with alkali treatment is reached earlier than that in the device without alkali treatment, suggesting that the cell with alkali treatment has the better charge transportability. These results suggest that the charge collection efficiency of the alkali-treated device was markedly enhanced, compared with the device without alkali treatment.
Parameters of SiNWs and alkali-treated SiNWs solar cells determined by IS measurements under dark conditions
In summary, the SEM and TEM showed that alkali treatment is well-suited for the hybrid Si/polymer solar cell since anisotropic etching can taper and spare the SiNWs which allows for conformal polymer coating to form the good core-shell heterojunctions, which results in fast collecting and separating EHPs. This hybrid structure can reduce carrier recombination ratio due to the short transportation distance of carriers in NWs and passivation of PEDOT:PSS, leading to enhanced carrier collection efficiency at the electrodes. This report emphasizes the importance of the alkali treatment process application in Si hybrid solar cells.
This work is supported by Chinese Nature Science Foundation Committee (No. 61540016), Henan Provincial Basic and Frontier Project (No. 152300410088), and Henan normal university youth backbone teachers (No. 5101029470611).
JY carried out the fabircation of SiNWs and analysis of photo response properties, and drafted themanuscript. XG participated in experimental design and the sequence alignment of manuscript. RQ participated the SEM characterization. HL carried out the modifying SiNWs using alkali solution. CX carried out the TEM charaterizaton. HM participated the analysis of photoelectric peroperties. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
- He L, Jiang CY, Wang H, Lai D (2012) High efficiency planar Si/organic heterojunction hybrid solar cells. Appl Phys Lett 100:073503View ArticleGoogle Scholar
- Masahiro O, Tang ZG, Ryo I, Gotou T, Keiji U, Shirai H (2012) Efficient crystalline Si/poly(ethylene dioxythiophene):poly(styrene sulfonate): graphene oxide composite heterojunction solar cells. Appl Phys Express 5:032301View ArticleGoogle Scholar
- Liu QM, Masahiro O, Tang ZG, Ryo I, Keiji U, Shirai H (2012) Highly efficient crystalline silicon/Zonyl fluorosurfactant-treated organic heterojunction solar cells. Appl Phys Lett 100:183901View ArticleGoogle Scholar
- Abdul MS, Muhammad NA, Han-don U, Sang-won J, Koun CH, Sang-woo K, Jung-ho L (2012) A stamped PEDOT:PSS–silicon nanowire hybrid solar cell. Nanotechnology 23:145401View ArticleGoogle Scholar
- Shiu SC, Chao JJ, Hung SC, Yeh CL, Lin CF (2010) Morphology dependence of silicon nanowire/poly(3, 4-ethylenedioxythiophene):poly(styrenesulfonate) heterojunction solar cells. Chem Mater 22:3108–3113View ArticleGoogle Scholar
- Zhang FT, Song T, Sun BQ (2012) Conjugated polymer-silicon nanowire array hybrid Schottky diode for solar cell application. Nanotechnology 23:194006View ArticleGoogle Scholar
- Lu WH, Chen Q, Wang B, Chen LW (2012) Structure dependence in hybrid Si nanowire/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) solar cells: understanding photovoltaic conversion in nanowire radial junctions. Appl Phys Lett 100:023112View ArticleGoogle Scholar
- Thiyagu S, Hsueh CC, Liu CT, Syu HJ, Lin TC, Lin CF (2014) Hybrid organic-inorganic heterojunction solar cells with 12 % efficiency by utilizing flexible film-silicon with a hierarchical surface. Nanoscale 6:3361–3366View 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 7:10780–11078View ArticleGoogle Scholar
- Pietsch M, Bashouti M, Christiansen S (2013) The role of hole transport in hybrid inorganic/organic silicon/poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) heterojunction solar cells. J Phys Chem C 11:79049–79055Google Scholar
- Zhang J, Song T, Shen X, Yu X, Lee ST, Sun B (2014) A 12 %-efficient upgraded metallurgical grade silicon–organic heterojunction solar cell achieved by a self-purifying process. ACS Nano 8:11369–11376View ArticleGoogle Scholar
- He L, Jiang CY, Wang H, Lai D (2011) Simple approach of fabricating high efficiency Si nanowire/conductive polymer hybrid solar cells. IEEE Electron Device Lett 32:1406–1408View ArticleGoogle Scholar
- Liu Y, Zhang Z, Xia Z, Zhang J, Liang F, Li Y, Song T, Yu X, Lee ST, Sun B (2016) High performance nanostructured silicon-organic quasi p-n junction solar cells via low-temperature deposited hole and electron selective layer. ACS Nano 10:704–712View ArticleGoogle Scholar
- Zhang Y, Cui W, Zhu Y, Zu F, Liao L, Lee ST, Sun B (2015) High efficiency hybrid PEDOT:PSS/nanostructured silicon Schottky junction solar cells by doping-free rear contact. Energy Environ Sci 8:297–302View ArticleGoogle Scholar
- Jeong S, Garnett E, Wang S, Yu Z, Fan S, Brongersma M, Mcgehee M, Cui Y (2012) Hybrid silicon nanocone–polymer solar cells. Nano Lett 12:2971–2976View ArticleGoogle Scholar
- Bai F, Li M, Huang R, Li Y, Trevor M, Musselman K (2014) A one-step template-free approach to achieve tapered silicon nanowire arrays with controllable filling ratios for solar cell applications. RSC Adv 4:1794–1798View ArticleGoogle Scholar
- Jung Y, Guo Z, Jee SW, Um HD, Lin KT, Lee JH (2010) A strong antireflective solar cell prepared by tapering silicon nanowires. Opt Express 18:A186–A292Google Scholar
- Gong X, Jiang Y, Li M, Liu H, Ma H (2015) Hybrid tapered silicon nanowire/PEDOT:PSS solar cells. RSC Adv 5:10310–10317View ArticleGoogle Scholar
- Subramani T, Syu H, Liu C, Hsueh C, Yang S, Lin C (2016) Low-pressure-assisted coating method to improve interface between PEDOT:PSS and silicon nanotips for high-efficiency organic/inorganic hybrid solar cells via solution process. ACS Appl Mater Interfaces 8:2406–2415View ArticleGoogle Scholar
- Peng K, Wang X, Wu XL, Lee ST (2009) Fabrication and photovoltaic property of ordered macroporous silicon. Appl Phys Lett 95:143119View ArticleGoogle Scholar
- Peng K, Xu Y, Wu Y, Yan Y, Lee ST, Zhu J (2005) Aligned single-crystalline Si nanowire arrays for photovoltaic applications. Small 1:1062–1067View ArticleGoogle Scholar
- Wang X, Peng K, Pan X, Chen X, Yang Y, Li L, Meng XM, Zhang WJ, Lee ST (2011) High-performance silicon nanowire array photoelectrochemical solar cells through surface passivation and modification. Angew Chem Int Ed 50:9861–9865View ArticleGoogle Scholar
- Tsai SH, Chang HC, Wang HH, Chen SY, Lin CA, Chen SA, Chueh YL, He JH (2011) Significant efficiency enhancement of hybrid solar cells using core-shell nanowire geometry for energy harvesting. ACS Nano 5:9501–9510View ArticleGoogle Scholar
- Liang ZC, Zeng F, Song H, Shen H (2013) Effect of porous Si and an etch-back process on the performance of a selective emitter solar cell. Sol Energy Mater Sol Cells 109:26–32View ArticleGoogle Scholar
- He Z, Zhong C, Huan X, Wong WY, Wu H, Chen L, Su S, Cao Y (2011) Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in polymer solar cells. Adv Mater 23:4636–4643View ArticleGoogle Scholar
- Braña AF, Forniés E, López N, García BJ (2015) High efficiency Si solar cells characterization using impedance spectroscopy analysis. J Phys Conf Ser 647:012069View ArticleGoogle Scholar
- Yadav P, Tripathi B, Pandey K, Kumar M (2015) Investigating the charge transport kinetics in poly-crystalline silicon solar cells for low-concentration illumination by impedance spectroscopy. Sol Energy Mater Sol Cells 133:105–112View ArticleGoogle Scholar
- Garcia-Belmonte G, Boix PP, Bisquert J, Sessolo M, Bolink HJ (2010) Simultaneous determination of carrier lifetime and electron density-of-states in P3HT:PCBM organic solar cells under illumination by impedance spectroscopy. Sol Energy Mater Sol Cells 9:366–375View ArticleGoogle Scholar
- Chal P, Shit A, Nandi AK (2016) Dye-sensitized solar cell from a new organic n-type semiconductor/polyaniline composite: insight from impedance spectroscopy. J Mater Chem C 4:272–285View ArticleGoogle Scholar
- Bashouti YM, Pietsch M, Brönstrup G, Sivakov V, Ristein J, Christiansen S (2014) Heterojunction based hybrid silicon nanowire solar cell: surface termination, photoelectron and photoemission spectroscopy study. Prog Photovolt Res Appl 2:1050–1061View ArticleGoogle Scholar
- Sinton RA, Cuevas A (1996) Contactless determination of current-voltage characteristics and minority carrier lifetimes in semiconductors from quasi-steady-state photoconductance data. Appl Phys Lett 69:2510–2512View ArticleGoogle Scholar