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Selenium as a photoabsorber for inorganic–organic hybrid solar cells Kai Wang,a Yantao Shi,*a Hong Zhang,a Yujin Xing,a Qingshun Donga and Tingli Ma*b

Received 27th June 2014, Accepted 11th September 2014

As an inorganic photoabsorber, selenium was used in a mesoscopic solar cell with a hybrid organic–inorganic structure of TiO2/Se/P3HT/PEDOT:PSS/Ag, in which the Se layer was prepared by vacuum thermal deposition

DOI: 10.1039/c4cp02821j

and post thermal treatment. The microstructure, photoelectrical properties, as well as the rationality in structural design of the solar cell were illustrated in detail. Finally, the hybrid solar cell demonstrated a

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photoelectric conversion efficiency of 2.63%.

1. Introduction Hybrid solar cells (HSCs) that incorporate inorganic and organic functional materials have received increasing attention because of their attractive properties such as cost-effectiveness, easy processing, and high efficiency.1–5 Among HSCs, dye-sensitized solar cells (DSCs), which achieved a breakthrough in 1991,6 have obtained a photoelectric conversion efficiency (PCE) of 13% in 2014.7 However, several critical issues should be resolved for the practical application of DSCs. These issues are especially related to the unsatisfactory stability of the cells mainly originating from the leakage of the electrolyte based on organic solvents. Perovskite solar cells (PSCs), which employ organometal halide as a light absorber, have recently been extensively explored, and these cells exhibit surprisingly high PCEs of over 15%.8,9 At present, exploring nonPb-based candidates as photoabsorbers is extremely urgent for PSCs because of the toxicity of the heavy metal Pb. Additionally, problem of the stability that the organometal halide perovskite used in PSCs confronts is also severe. For example, this kind of perovskite is exceedingly sensitive to water. It in turn requires the fabrication of PSCs in an inert atmosphere. Hence, developing novel and high-efficiency HSCs based on stable, earth abundant, and nontoxic materials is necessary. Selenium (Se) is an attractive material and often applied in thinfilm devices such as xerographic plates,10 memory switches,11 and for energy storage, and photoelectric conversion.12–14 Furthermore, Se is nontoxic in nature. Hence, related devices are generally

environmentally friendly and publicly acceptable. For instance, Se can be used in rechargeable lithium batteries and DSCs as a cathode12 and a counter electrode,13 respectively. In addition, Se is stable and possesses a suitable band gap of 1.8 eV.15 Therefore, Se can also be used as a photoabsorber to generate photoelectrons in solar cells. Nakada et al. examined Se-based solar cells in the mid-1980s.16–18 Two structures of ITO/Te/Se/Au and ITO/TiO2/Se/Au have been proposed. Moreover, a higher efficiency of 5.01% was obtained, exhibiting promising features. High PCEs could be achieved because the maximum theoretical efficiency of Se-based solar cell has been estimated to be approximately 20%.19 Ito et al. recently prepared an extremely thin Se absorber layer on mesoporous TiO2 (mp-TiO2) and successfully constructed one solar cell with ITO/compact-TiO2/mp-TiO2/Se/Au, which achieved over 3% PCE without any expensive organic parts.20,21 However, to date, utilization of Se as a photoabsorber in HSCs has not been reported. In this study, we used Se as a photoabsorber to design a novel organic–inorganic HSC with a FTO/compact-TiO2/mp-TiO2/Se/ P3HT/PEDOT:PSS/Ag structure (Fig. 1). In this HSC, the mesoporous TiO2 collected the photo-induced electrons from Se and supported the Se layer. Then, Se as a photoabsorber generated electrons and holes. The organic semiconductor P3HT was used to collect and transport holes from Se to an external circuit.

a

State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology (DUT), 2 Linggong Rd., 116024, Dalian, P. R. China. E-mail: [email protected]; Fax: +86-411-84986237; Tel: +86-411-84986237 b Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu, Kitakyushu, Fukuoka, 808-0196, Japan. E-mail: [email protected]

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Fig. 1 (a) Schematic structure and (b) energy level diagram of Se-based HSCs.

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In addition, PEDOT:PSS in this structure facilitated the transport of holes to the Ag cathode. The unoptimized solar cell achieved a PCE of 2.63%. Higher efficiency can be realized in the future by optimizing structures and materials of these HSCs.

2. Experimental All chemicals and solvents used were of analytical grade. TiO2 compact and mesoporous films were prepared on cleaned fluorine-doped tin oxide glass (FTO) by spin-coating titanium organic sol and TiO2 nanoparticles. Then, an extremely dense and uniform Se layer (ca. 350 nm) was deposited on the TiO2 films using vacuum evaporation technology. At this stage, Se was amorphous, red in colour, and with a band gap of approximately 2 eV.22 After sintering at 200 1C, a black Se layer was obtained because Se was converted into crystalline.18 Subsequently, the P3HT and PEDOT:PSS as hole transport materials (HTMs) were spin-coated on the Se layer individually. In addition, spiro-OMeTAD, a typical HTM, was introduced into the Se-based HSCs for comparison. Finally, as a cathode, an 80 nm Ag layer was prepared by thermal vacuum evaporation. The active area of the HSC was fixed at 0.06 cm2. The films were investigated through field-emission scanning electron microscopy (S-4800, Hitachi, Japan). The phase composition and crystal structure of the Se powders were studied via X-ray diffraction analysis (XRD, D/MAX-2400, Japan) with Cu Ka radiation (g = 0.1541 nm). The band gap of Se film was studied through UV-visible (UV-vis) absorption spectra, which were measured using a spectrophotometer equipped with an integrating sphere setup (JASCO V-570). The current density– voltage ( J–V) curves of DSCs were measured under simulated AM 1.5 illumination (100 mW cm 2, PEC-L15, Peccell, Japan) using a Keithley digital source meter (Keithley 2601, USA). The incident photon-to-electron conversion efficiency (IPCE) spectra was measured in incident light whose wavelength was from 300 nm to 800 nm. Each of the samples was tested 3 to 5 times, and the average data were obtained.

3. Results and discussion Crystallinity of the inorganic photoabsorber considerably affects photoabsorption and electron transfer. In our study, the crystal structure of the Se sample after sintering at 200 1C was characterized by XRD (Fig. 2). The result was obtained from the Se/TiO2/FTO structure because of the poor adhesion of Se on bare FTO. In addition to those of FTO and anatase TiO2, all diffraction peaks of Se can be well designated based on the standards cards (No. 65-1876). Se was identified to have a hexagonal crystal structure. The diffraction peaks at 23.521, 29.691, 41.341, 43.621, 45.371, 48.111, and 51.731 were assigned to the (100), (011), (110), (012), (111), (200), and (021) planes, respectively. A top-view SEM image of Se film deposited on mp-TiO2 after sintering is shown in Fig. 3a. Two regions can be observed from the image, in which one is relatively rough, and the other is relatively smooth. A cross-sectional SEM image of the complete Se-based HSC is shown in Fig. 3b, in which each functional

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Fig. 2

X-ray diffractogram of FTO/TiO2/Se.

Fig. 3 Thin-film topology characterization. (a) SEM top view of FTO/TiO2/Se. (b) Cross-sectional SEM images of a complete device.

layer could be evidently distinguished. To reveal the components of the different presentations, elemental mapping was performed through energy dispersive X-ray (EDX) analysis (Fig. 4). Prior to sintering, Se was uniformly distributed over the TiO2 layer, as identified by the elemental mapping of O and Se (Fig. 4a). After sintering, Fig. 4b shows the cracks in the Se layer, indicating that Se shrank after crystallization. With these cracks in a complete solar cell device, the mp-TiO2 layer (as the underlayer of Se) is exposed and directly connected with HTM. Finally, the undesired shunt pathway and resultant energy loss will occur. Therefore, fabricating high quality, crack-free, and crystalline Se film is critical for the photovoltaic performance of such HSCs. In addition, cross-sectional SEM with EDX analysis showed that Se uniformly penetrated into the mp-TiO2 films (Fig. 4c). Generally, mp-TiO2 film in this kind of HSC has dual functions. On the one hand, this film is an inert scaffold,

Fig. 4 EDX analysis of FTO/TiO2/Se films with elemental mapping. (a) Top views before sintering; (b) top views after sintering; and (c) cross-sectional images after sintering.

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which can ‘‘grasp’’ the absorber tightly because of the poor adhesion of the Se layer to the smooth compact TiO2 layer. On the other hand, the mp-TiO2 is responsible for accepting and transporting photo-induced electrons from Se. Therefore, the mp-TiO2 layer in Se-based HSCs is necessary. The HTM layer is composed of two components, namely, P3HT and PEDOT:PSS. This combination can facilitate the extraction and collection of photo-induced holes. Moreover, direct evaporation of Ag onto the Se layer will result in an extremely large cathode resistance. This resistance is caused by the fact that Ag itself cannot connect well on the rough Se surface. The band gap of Se was measured by UV-visible spectrometry (Fig. 5). The absorption threshold of Se is approximately 700 nm, suggesting that the absorption wavelength of Se can almost extend the entire region of visible light. The transformation of Fig. 5a according to the conventional Tauc equation (ahn)n = A (hn Eg), where A is an empirical constant and n = 2 for the direct band gap semiconductor, is shown in Fig. 5b. The band gap of Se was calculated to be 1.79 eV, which is consistent with the results from other studies.15 Therefore, Se is suitable to be a photoabsorber in solar cells. Moreover, the absorption intensity is slightly enhanced after the Se layer is coated with a thin layer of P3HT. However, almost no change is found for the absorption threshold, suggesting perfect overlap of the absorption ranges of P3HT and Se. Hence, P3HT in the HSCs could not only transfer the holes from Se, but also absorb the incident light and contribute to the photocurrent. Se-based solar cells were measured under AM 1.5, 100 mW cm 2 light illumination. J–V curves of the solar cells are illustrated in Fig. 6, and the detailed parameters are summarized in Table 1. In addition to P3HT, we used spiro-OMeTAD for parallel comparison. P3HT-based solar cells exhibit a PCE of 2.63%. The open-circuit voltage (Voc) and short-circuit current density ( Jsc) of Se-based solar cells are ca. 0.71 V and not more than 10.0 mA cm 2, respectively. The theoretical Voc is estimated to be ca. 1.0 V to 1.2 V based on the difference between the conduction band of TiO2 and the

Fig. 5 Optical property of Se (a) UV-vis absorption spectra; (b) (ahn)2 versus photoenergy.

Fig. 6

(a) J–V and (b) IPCE curves of Se-based solar cells.

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Table 1 HTMs

Photovoltaic parameters of Se-based solar cells with different

HTM

Voc (V)

Jsc (mA cm 2)

FF

Eff (%)

P3HT Spiro-OMeTAD

0.71 0.69

9.71 8.10

0.38 0.33

2.63 1.83

Fig. 7 Effect on PCE of different HTMs.

highest occupied molecular orbital level of HTM. Therefore, the Se-based HSCs possess great potential for performance improvement. As shown in Fig. 7, using spiro-OMeTAD as HTM, a lower PCE of only 1.83% was obtained because of its lower Jsc and fill factor (FF) compared with the P3HT-based one. In Fig. 5a, the absorption intensity is slightly enhanced after the Se layer is coated with a thin layer of P3HT. P3HT could help Se absorb photoenergy and spiro-OMeTAD has no absorption in the visible region,23 which was one of the reasons that solar cells based on P3HT had better performance. The other reasons such as HTM pore infiltration and HTM interactions with ionic additives and the TiO2 surface should be studied deeply in future.24 Hence, P3HT can be said to be better than spiro-OMeTAD as HTM in Se-based HSCs. For the Se-based HSCs with either P3HT or spiroOMeTAD, the FF reflecting the inherent resistance and the degree of charge recombination in solar cells are found to be lower than 0.40. Large inherent resistance is known to always result in photo-induced electrons, which cannot be freely transported and efficiently collected. Therefore, increasing FF is relatively critical for the improvement of the PCE of Se-based HSCs. The IPCE of Se-based HSCs was also measured (Fig. 6b). The maximum efficiency in IPCE is ca. 70%, which appears at the short-wavelength region, i.e., at approximately 440 nm. However, beyond 400 nm, IPCE drastically decreases and gradually approaches zero with increasing wavelength from 700 nm to 800 nm. The IPCE result suggests that, for Se-based HSCs, the spectral coverage within which photons could be converted into electrons is relatively narrow. This condition may be responsible for the lower photocurrent shown in Table 1. To improve the photovoltaic performance of Se-based HSCs, we propose several critical issues and reasonable strategies. The first strategy involves the compatibility of Se with the mp-TiO2, especially in terms of the nanostructures. At present, the mp-TiO2 used in Se-based HSCs is composed of nanoparticles and ca. 300 nm in thickness. Hence, thorough infiltration of Se into this TiO2 matrix

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is extremely difficult. However, massive inter-particle boundaries will slow down electron transport and then hinder charge collection. Tailoring the pore structure to coupling Se and mesoporous film, constructing ‘‘highways’’, such as one-dimensional nanoarrays, and using other oxide semiconductors with superior conductivity (such as ZnO25 and SnO226) are effective strategies to match the mesoporous layer and Se. Second, preparing high quality and crack-free Se film is also crucial to minimize the shunt pathway. In situ preparation of crystalline Se on a mesoporous layer in the solution system may be a feasible and effective method. Third, lowering the band gap of Se to broaden the available light spectrum for Se-based HSCs is also critical. According to previous reports, doping with other elements, such as In, Ge, and Bi, alters the band-gap from 1.8 eV to 1.4 eV.27,28 We believe that these strategies are effective to further improve the performance of Se-based HSCs.

4. Conclusions In summary, we report an organic–inorganic HSC based on Se. Solar cell microstructures and the working principle, as well as the optical and photovoltaic properties, were analyzed in detail. Se-based HSCs were observed to exhibit a PCE of 2.63%. We propose several feasible and effective strategies to solve the critical issues that seriously affect the photovoltaic performance of Se-based HSCs. This study provided a basis for the future study and further improvement of Se-based solar cells.

Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 51273032, 91333104), International Science & Technology Cooperation Program of China (Grant No. 2013DFA51000).

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Selenium as a photoabsorber for inorganic-organic hybrid solar cells.

As an inorganic photoabsorber, selenium was used in a mesoscopic solar cell with a hybrid organic-inorganic structure of TiO2/Se/P3HT/PEDOT:PSS/Ag, in...
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