3-D solar cells by electrochemical-deposited Se layer as extremely-thin absorber and hole conducting layer on nanocrystalline TiO2 electrode
© Nguyen et al.; licensee Springer. 2013
Received: 16 July 2012
Accepted: 13 December 2012
Published: 3 January 2013
A three-dimensional selenium solar cell with the structure of Au/Se/porous TiO2/compact TiO2/fluorine-doped tin oxide-coated glass plates was fabricated by an electrochemical deposition method of selenium, which can work for the extremely thin light absorber and the hole-conducting layer. The effect of experimental conditions, such as HCl and H2SeO3 in an electrochemical solution and TiO2 particle size of porous layers, was optimized. This kind of solar cell did not use any buffer layer between an n-type electrode (porous TiO2) and a p-type absorber layer (selenium). The crystallinity of the selenium after annealing at 200°C for 3 min in the air was significantly improved. The cells with a selenium layer deposited at concentrations of HCl = 11.5 mM and H2SeO3 = 20 mM showed the best performance, resulting in 1- to 2-nm thickness of the Se layer, short-circuit photocurrent density of 8.7 mA/cm2, open-circuit voltage of 0.65 V, fill factor of 0.53, and conversion efficiency of 3.0%.
Keywords3-D solar cells Nanocrystalline TiO2 electrode Se layer
Three-dimensional (3-D) solar cells were developed by Nanu et al. and O'Hayre et al. [1–4]. The structure of these solar cells is similar to dye-sensitized solar cells (DSCs) [5–8]; however, this kind of 3-D solar cell does not use a liquid electrolyte like DSC. Hence, 3-D solar cells can get better stability than DSCs. The other advantage of 3-D solar cells is a short migration distance of the minority carriers and, therefore, reduces the recombination of electrons and holes . In addition, 3-D solar cells are easily fabricated by non-vacuum methods such as spray pyrolysis and chemical bath depositions; consequently, they are well-known as low cost solar cells. The major photoabsorber materials in the 3-D compound solar cells have been CuInS2[1–4, 9], CuInSe2, Se , Sb2S3[12–17], CdSe [18, 19], and CdTe [20, 21]. In the 3-D compound solar cells, the buffer layer between the TiO2 and absorber layer was commonly utilized to block charge recombination between electrons in TiO2 and holes in hole-transport materials [1–4, 9, 10, 12–16].
In this paper, we study 3-D solar cells using selenium for the light absorber layer. Selenium is a p-type semiconductor with a band gap of 1.8 and 2 eV for crystal and amorphous states, respectively. Flat selenium solar cells were researched by Nakada in the mid-1980s [22, 23]. The selenium solar cells with a superstrate structure showed the best efficiency of 5.01% under AM 1.5 G illumination. In our work, the selenium layer was prepared by electrochemical deposition (ECD), a non-vacuum method, resulting in the extremely thin absorber (ETA) [11–21]. The similarly structured solar cells (3-D selenium ETA solar cells deposited on nanocrystalline TiO2 electrodes using electrochemical deposition) were also studied by Tennakone et al. , which were composed with hole-conducting layer of CuSCN. The Se layer worked just to be a photoabsorber.
In this report, on the other hand, the 3-D Se ETA solar cells worked without a CuSCN layer. We did not use any buffer layers between the n-type electrode porous TiO2 and the selenium photoabsorber layer, or any additional hole-conducting layer. Hence, the Se layer worked bi-functionally as photoabsorber and hole conductor. The effect of the TiO2 particle size, HCl and H2SeO3 concentrations, and annealing temperature on the microstructure and photovoltaic performance was investigated thoroughly.
In order to confirm the crystallinity of selenium before and after annealing, X-ray diffraction (XRD) (Mini Flex II, Rigaku Corporation, Tokyo, Japan) was carried out. The cross-section and surface morphology of the samples were measured by scanning electron microscopy (SEM) (JSM-6510, JEOL Ltd., Tokyo, Japan). The coverage on nanocrystalline TiO2 by Se was observed by high resolutiontransmission electron microscopy (JEM 2100 F, JEOL Ltd.). Absorption spectra were measured by an ultraviolet–visible spectroscopy (Lambda 750 UV/VIS spectrometer, PerkinElmer Inc., MA, USA). Photovoltaic measurements employed an AM 1.5 G solar simulator equipped with a xenon lamp (YSS-80, Yamashita Denso Corporation, Tokyo, Japan). The power of the simulated light was calibrated to 100 mW cm−2 using a reference Si photodiode (Bunkoukeiki Co., Ltd., Tokyo, Japan). J-V curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with a DC voltage current source (6240A, ADCMT Corporation, Tokyo, Japan).
Results and discussion
The surface of porous TiO2 is rather rough (see Figure 3b) because the particle size of TiO2 nanoparticles is big, approximately 60 nm. However, the surface became smoother after depositing selenium as shown in Figure 3d. Figure 3f shows the surface morphology of selenium-coated porous TiO2 after annealing at 200°C for 3 min in the air. The surface is rougher than that of before annealing. Big particles were observed in this sample. The appearance of big particles and a rough surface is due to the improvement of the crystallinity of selenium after annealing, as mentioned in the XRD section above.
3-D selenium ETA solar cells using an extremely thin absorber Se layer on nanocrystalline TiO2 electrodes were fabricated by electrochemical deposition method. The crystallinity of the selenium layer after annealing at 200°C for 3 min in the air was significantly improved, and the band gap became narrower in comparison to the sample both with and without annealing at 100°C. The photovoltaic performance features of the best 3-D selenium ETA solar cells are JSC = 8.7 mA/cm2, VOC = 0.65 V, FF = 0.53, and η = 3.0%. These results are interesting for PV researchers because the fabrication method for this kind of solar cells is quite simple. However, in order to get a higher efficiency, the photocurrent density should be more improved.
Part of this work was funded by the Innovative Solar Cells Project (NEDO, Japan).
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