Since the first report of solar cells based on organic-inorganic halide perovskites  , various perovskite-type solar cells consisting of ABX3 compounds (A = CH3NH3, HC(NH3)2 or Cs, B = Pb or Sn, X = I, Cl or Br) have been extensively studied  -  . Conversion efficiencies over 20% have recently been achieved for perovskite-type solar cells  -  . However, photovoltaic properties of perovskite-type solar cells strongly depend on fabrication process, microstructure and electronic structure of materials. For perovskite-type solar cells, instabilities against humidity, temperature, and continuous light irradiation are also crucial issue  . Therefore, detailed investigation of perovskite-type solar cells is required to achieve conversion efficiency greater than currently achieved.
2,2’,7,7’-tetrakis[N,N-di(p-methoxyphenyl)amino]-9,9’-spirobifluorene (spiro-OMeTAD) is used in perovskite-type solar cells as a hole transport layer because spiro-OMeTAD can yield high conversion efficiency. However, spiro- OMeTAD is an expensive organic compound. To circumvent the problem, exploitation of alternate hole transport materials has actually been performed     .
Polysilanes are promising candidates for the alternate hole transport materials because they are one-dimensional silicon-based materials   , and can provide good electrical, optical and photovoltaic properties     . Very recently, effects of polysilane-doped spiro-OMeTAD hole transport layers were investigated by our group, and the polysilane-doped spiro-OMeTAD hole transport layers were found to increase conversion efficiency of the perovskite- type photovoltaic devices  . However, photovoltaic properties of perovskite- type solar cells with polysilanes as hole transport layers have not yet been clarified.
The purpose of the present work is to fabricate of perovskite CH3NH3PbI3-based photovoltaic devices with polysilane hole transport layers. In the present study, poly(methyl phenylsilane) (PMPS) and decaphenylcyclopentasilane (DPPS) were used as the hole transport layers, as shown in Figure 1(a) and Figure 1(b), respectively. The photovoltaic devices with the polysilane layers were fabricated by a simple spin-coating method in air atmosphere. Structures, optical and photovoltaic properties of the polysilanes and CH3NH3PbI3 were also investigated. The structures and optical properties of the polysilanes and CH3NH3PbI3 were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and optical spectroscopy. Besides, the photovoltaic properties of the devices were evaluated by measuring current density- voltage (J-V) characteristics and incident photon to current conversion efficiency (IPCE). Carrier transport mechanism based on the obtained results was discussed.
Figure 1. Molecular structures of (a) PMPS and and (b) DPPS.
2. Experimental Procedures
First, polysilane films were prepared on cleaned glass substrates by a spin-coat- ing method  . PMPS (Osaka Gas Chemicals, OGSOL SI-10-10, molecular weight (Mw): 16,300, 12 mg) and DPPS (Osaka Gas Chemicals, OGSOL SI-30- 10, Mw: 945.11, 12 mg) powders were separately dissolved in ο-dichloro-ben- zene (Wako Pure Chemical Industries, 500 μL). Triphenylborate solution (Sigma-Aldrich, 25 μL) was added into the polysilane solutions as a p-type dopant  . Non-doped and boron (B)-doped polysilane solutions were stirred at room temperature. The polysilane solutions were dropped on the glass substrates, and spun by a spin coater (Mikasa, MS-A 100) at 1500 rpm for 30 s. This process was repeated until the desired thickness. Then, CH3NH3PbI3-based photovoltaic devices with polysilane hole transport layers were also fabricated to investigate photovoltaic properties of the polysilanes. The detailed fabrication process was described in our previous reports       , except for those of CH3NH3PbI3 and polysilane layers. In the present work, 0.15 and 0.30 M compact TiOx precursor solutions were prepared from titanium diisopropoxide bis(acetyl-acetonate) (Sigma-Aldrich, 0.055 mL and 0.11 mL) and 1-butanol (Nacalai Tesque, 1 mL). The 0.15 M TiOx precursor layer was spin-coated onto cleaned fluorine (F)-doped tin oxide (FTO) substrate by the spin coater at 3000 rpm for 30 s. After that, the TiOx was dried on a hot plate (As One, ND-1) at 125˚C for 5 min. Likewise, the 0.30 M TiOx precursor layer was spin-coated onto the 0.15 M TiOx layer at 3000 rpm for 30 s, and dried at 125˚C for 5 min. This process was repeated twice. Finally, the TiOx layer was annealed in an electric furnace (as one, SMF-1) at 500˚C for 30 min. After cooling to room temperature, mesoporous TiO2 layer was spin-coated onto the compact TiO2 layer at 5000 rpm for 30 s, and was dried on the hot plate at 125˚C for 5 min. The mesoporous TiO2 layer was sintered in the electric furnace at 500˚C for 30 min. Prior to spin-coating of the mesoporous TiO2, the TiO2 paste was prepared by dispersing TiO2 powder (Aerosil, P-25) in ultrapure water. Poly(ethylene glycol) (Nacalai Tesque, averaged molecular number: 20,000, 10 mg), acetylacetone (Wako Pure Chemical Industries, 10 μL) and a surfactant (Sigma-Aldrich, Triton X-100, 5 μL) were added in the TiO2 paste. The paste was stirred for 30 min, and was left overnight without stirring. For preparation of CH3NH3PbI3 layers, a combination of a one-step solution deposition method        and hot air flow-assisted spin-coating technique   was adopted in the present work. The preparation process of the CH3NH3PbI3 layers were as follows: CH3NH3I (Showa Chemical, 99.8 mg) and PbI2 (Sigma-Aldrich, 289.3 mg) were dissolved in a mixed solvent consisting of γ-butyrolactone (Wako Pure Chemical Industries, 300 μL) and N,N-dimethylformamide (DMF) (Nacalai Tesque, 200 μL)  . The molar ratio of the solutes was 1:1. The CH3NH3PbI3 solution was stirred at 60˚C over 2 h. Prior to spin-coating, the solution was filtered using a 0.20 μm poly(vinylidene difluoride) syringe filter unit (Advantec, Dismic 13 HP). Moreover, the substrate was kept at 50˚C by a heating gun, checked by a thermometer. The CH3NH3PbI3 solution was immersed into the mesoporous TiO2 layer, and was spun at 2000 rpm for 60 s. During the spin- coating, hot air from the heating gun was continuously flown. The spin-coated CH3NH3PbI3 layer was then annealed on the hot plate at 100˚C for 1 min. After that, B-doped PMPS (PMPS:B) and DPPS (DPPS:B) solutions were dropped onto the CH3NH3PbI3 layers, and were spun at 1500 rpm for 30 s without hot air flow. The spin-coating process was repeated twice. Finally, gold (Au) electrodes were deposited onto the polysilane layers by a vacuum evaporation system (Sanyu Electron Co., Ltd., SVC-700TM/SVC-700-2). The present photovoltaic devices are denoted as FTO/TiO2/CH3NH3PbI3/PMPS:B or DPPS:B/Au.
The PMPS, PMPS:B, DPPS and DPPS:B films were characterized by an X- ray diffractometer (Bruker Corporation, D2 PHASER). Optical absorption spectra of the polysilane films were collected by an ultraviolet-visible-near inferred (UV-VIS-NIR) spectrometer (Jasco Corporation, V-770). The CH3NH3PbI3 layers in the FTO/TiO2/CH3NH3PbI3/PMPS:B/Au and FTO/TiO2/CH3NH3PbI3/ DPPS:B/Au devices were also evaluated by XRD and SEM (Jeol Ltd., JSM- 6010PLUS/LA) attached with an EDS detector. The optical absorption spectra of the CH3NH3PbI3 in the photovoltaic devices with the polysilane hole transport layers were collected using the UV-VIS-NIR spectrometer. The J-V characteristics of the FTO/TiO2/CH3NH3PbI3/PMPS:B/Au and FTO/TiO2/CH3NH3PbI3/ DPPS:B/Au were measured using a potentiostat (Hokuto Denko Corporation, HSV-110) under simulated AM 1.5 (100 mW・cm−2) irradiation conditions. The light was irradiated from the bottom side of FTO-coated glass substrate using a solar simulator (San-Ei Electric Co., Ltd., XES-301S). Effective area of the devices was 0.090 cm2. IPCE spectra of the devices were also collected using an IPCE measurement system (Enli Technology Co., Ltd., QE-R). All measurements were carried out at room temperature.
3. Results and Discussion
Figure 2(a) shows XRD patterns of as-prepared PMPS, PMPS:B, DPPS and DPPS:B films. Broad peaks around 24˚ were the signals from the sample holder. All the as-prepared films were amorphous. Figure 2(b) exhibited heat-treated PMPS, PMPS:B, DPPS and DPPS:B films. The heat treatment was carried out at 100˚C for 10 min. The DPPS and DPPS:B films were crystallized, as shown in Figure 2(b), which agreed with our previous report  . Compared with the XRD pattern of heat-treated DPPS film, a peak shift to lower diffraction angle was observed for the heat-treated DPPS:B one, suggesting a structural change of DPPS by B doping and heat trearment. On the other hand, the PMPS and PMPS:B films were not crystallized. In the present work, non-heated PMPS:B and DPPS:B hole transport layers were prepared on the CH3NH3PbI3-based photovoltaic devices.
Figure 3(a) and Figure 3(b) show optical absorption spectra of the as-pre- pared PMPS, PMPS:B, DPPS and DPPS:B films. Absorption intensities of PMPS and PMPS:B films shown in Figure 3(a) were almost same regardless of B doping. Some marked features were observed at 223, 243, 340 and 435 nm for PMPS and PMPS:B films. The features at 223, 243 and 340 nm can be assigned
Figure 2. XRD patterns of (a) as-prepared and (b) heat-treated polysilanes prepared on glass substrates.
Figure 3. Optical absorption spectra of (a) PMPS (dashed green line) and PMPS:B (solid blue line) films, and (b) DPPS (dashed black line) and DPPS:B (solid red line) films. Inset in (a) is enlarged optical absorption spectra of PMPS and PMPS:B films.
to Si-C bond-related transition  , π-π* transition of phenyl group  and σ-σ* transition of polysilane chain  , respectively. The feature at 435 nm shown in inset in Figure 3(a) was derived from charge transfer excitation  . In contrast, absorption intensity of DPPS:B film shown in Figure 3(b) was larger than that of DPPS one, which was presumably due to B doping. A marked feature was observed at 272 nm for the DPPS and DPPS:B films, which would be assigned to σ-σ* transition. From the edges of the σ-σ* transitions, energy gaps of the PMPS, PMPS:B, DPPS and DPPS:B films were estimated to be 3.49, 3.49, and 3.68 and 3.66 eV, respectively.
XRD patterns of FTO/TiO2/CH3NH3PbI3/PMPS:B/Au and FTO/TiO2/CH3NH3PbI3/ DPPS:B/Au photovoltaic devices are shown in Figure 4. The diffraction peaks corresponding to CH3NH3PbI3 with a cubic system () were observed. Some sharp diffraction peaks of PbI2 were simultaneously observed at 12.5˚ and 25.9˚, assumed that phase separation of PbI2 from CH3NH3PbI3 occurred  . From the XRD patterns in Figure 4, lattice constants (a) and unit cell volumes (V) of the CH3NH3PbI3 were estimated, as presented in Table 1. The a and V values of the CH3NH3PbI3 in the two devices were larger than those of our previous reports     because these structural parameters strongly depend on growth conditions. Crystallite sizes (D) of the CH3NH3PbI3 were estimated using Scherrer’s equation: D = 0.9λ/βcosθ. Here, λ, β and θ are the X-ray wavelength (0.154184 nm), full width at half maximum of diffraction peaks and Bragg angle, respectively. The D values presented in Table 1 were almost same regardless of difference in hole transport materials.
A SEM image of the FTO/TiO2/CH3NH3PbI3/PMPS:B/Au device is shown in Figure 5(a). Many diamond-, cross- and cubic-shaped grains with sizes of 15 - 30 nm were observed. Figure 5(b) represents an EDS spectrum for the FTO/TiO2/CH3NH3PbI3/PMPS:B/Au device. It was confirmed that Pb, I, Si and B were contained in the device. Ti and Sn respectively derived from TiO2 and FTO were also detected. Elemental mapping images of Pb, I, Si and B in the device collected by EDS detector are shown in Figures 5(c)-(f), respectively. The contents of Pb, I, Si and B are presented in Table 2. The elemental mapping images clarified that the grains shown in Figure 5(a) were CH3NH3PbI3 because distribution of Pb and I atoms corresponded to the grains. Simultaneously, it was confirmed that Si and B atoms in the PMPS:B were distributed all over the CH3NH3PbI3 layer. However, content of B was below detection limit (BDL).
Likewise, a SEM image of the FTO/TiO2/CH3NH3PbI3/DPPS:B/Au device is shown in Figure 6(a). Many cross-shaped grains with sizes of 20 - 40 nm were observed, considered that PMPS:B and DPPS:B affected surface morphology of
Figure 4. XRD patterns of FTO/TiO2/CH3NH3PbI3/PMPS:B/Au and FTO/TiO2/CH3NH3PbI3/DPPS:B/Au photovoltaic devices.
Table 1. Lattice constants (a), volumes (V) and crystallite sizes (D) of CH3NH3PbI3 in FTO/TiO2/CH3NH3PbI3/DPPS:B/Au and FTO/TiO2/CH3NH3PbI3/PMPS:B/Au photovoltaic devices.
Figure 5. (a) SEM image of CH3NH3PbI3 surface in FTO/TiO2/CH3NH3PbI3/PMPS:B/Au photovoltaic device. (b) EDS spectrum for FTO/TiO2/CH3NH3PbI3/PMPS:B/Au device. (c)-(f) Elemental mapping images of (b) Pb M line, (c) I L line, (d) Si K line and (d) B K line.
CH3NH3PbI3 because of their different molecular structures. An EDS spectrum for the FTO/TiO2/CH3NH3PbI3/DPPS:B/Au device is shown in Figure 6(b). Pb, I, Si and B atoms were confirmed as well as the FTO/TiO2/CH3NH3PbI3/PMPS:B/Au device. Elemental mapping images of Pb, I, Si and B in the device are shown in Figures 6(c)-(f). It is considered that the cross-shaped grains corresponded to CH3NH3PbI3. The contents of Pb, I, Si and B are presented in Table 2. The content of Si in the FTO/TiO2/CH3NH3PbI3/DPPS:B/Au device was smaller than that in the FTO/TiO2/CH3NH3PbI3/PMPS:B/Au one. The difference would be attributed to molecular weight of the polysilanes.
Optical absorption spectra of the FTO/TiO2/CH3NH3PbI3/PMPS:B/Au and FTO/TiO2/CH3NH3PbI3/DPPS:B /Au photovoltaic devices are shown in Figure 7(a). Broad absorption spectra were obtained in the range of 400 - 780 nm. Two peak structures at 415 and 500 nm would be derived from TiO2 and PbI2, respectively   . From the optical absorption spectra, energy gaps of the CH3NH3PbI3 layers in the devices were estimated by Tauc’s formula: (hνα)n = A(hν − Eg), where h, ν, α, A, Eg and n are the Plank constant, light frequency, optical coefficient, proportional constant, energy gap, and power index which depends on the nature of the transition, respectively. In the present study, n = 2 was used for the CH3NH3PbI3 because CH3NH3PbI3 is a direct transition semiconductor  . As shown in Figure 7(b), the Eg of the CH3NH3PbI3 in the two devices were 1.59 eV, comparable with reported Eg of CH3NH3PbI3 prepared with a mixed solvent consisting of γ-butyrolactone and DMF  . However, there was no peak
Figure 6. (a) SEM image of CH3NH3PbI3 surface in FTO/TiO2/CH3NH3PbI3/DPPS:B/Au photovoltaic device. (b) EDS spectrum for FTO/TiO2/CH3NH3PbI3/DPPS:B/Au device. (c)-(f) Elemental image mapping images of (b) Pb M line, (c) I L line, (d) Si K line and (d) B K line.
Table 2. Compsitions of FTO/TiO2/CH3NH3PbI3/DPPS:B/Au photovoltaic devices. BDL means below detection limit.
Figure 7. (a) Optical absorption spectra of FTO/TiO2/CH3NH3PbI3/PMPS:B/Au and FTO/TiO2/CH3NH3PbI3/DPPS:B/Au photovoltaic devices. (b) Tauc plots of FTO/TiO2/ CH3NH3PbI3/PMPS:B/Au and FTO/TiO2/CH3NH3PbI3/DPPS:B/Au photovoltaic devices.
related to DPPS:B and PMPS:B layers, presumably due to their film thickness.
Figure 8(a) shows J-V characteristics of the FTO/TiO2/CH3NH3PbI3/PMPS:B/Au and FTO/TiO2/CH3NH3PbI3/DPPS:B/Au photovoltaic devices under light irradiation. The photovoltaic devices showed clear rectifying behavior with short-circuit current density (Jsc) and open circuit voltage (Voc). A Jsc of 4.56 mA∙cm−2, Voc of 0.610 V, fill factor (FF) of 0.551 and conversion efficiency (η) of 1.53% were obtained for the FTO/TiO2/CH3NH3PbI3/PMPS:B/Au device. Compared with the device with PMPS:B hole transport layer, on the other hand, photovoltaic properties of the FTO/TiO2/CH3NH3PbI3/DPPS:B/Au device were improved. From the J-V characteristics of the FTO/TiO2/CH3NH3PbI3/DPPS:B/Au device shown in Figure 8(a), a Jsc of 6.96 mA cm−2, Voc of 0.578 V, FF of 0.454 and η of 1.83% were obtained. These difference were attributed to molecular structures of the polysilanes and Si content in the hole transport layers. In fact, improvement of conversion efficiency on perovskite-based photovoltaic devices with different types of polysilane-doped spiro-OMeTAD hole transport layers were recently reported, and photovoltaic properties of the perovskite-based photovoltaic devices depended on the contents of Si  . The η, however, were still lower than the perovskite- based solar devices with spiro-OMeTAD, phthalocyanine, and copper thiocyanate hole transport materials     . IPCE spectra of the FTO/TiO2/ CH3NH3PbI3/PMPS:B/Au and FTO/TiO2/CH3NH3PbI3/DPPS:B/Au photovoltaic devices are shown in Figure 8(b). Broad IPCE spectra were obtained in the wavelength range from 300 and 800 nm, indicating that exciton and/or free charge generation occurred in the CH3NH3PbI3 layer. In addition to these results, holes in the CH3NH3PbI3 were effectively transported through the polysilane layers because intensity of the IPCE spectrum is proportional to the η, suggesting that the DPPS:B layer had a good mobility compared with the PMPS:B one. Integrated Jsc shown in Figure 8(b) agreed with the Jsc obtained from the J-V characteristics in Figure 8(a). Effective energy gaps () can also be determined  . By extrapolating the linear part of the graph to meet (hν * IPCE)2 = 0, the of the two devices were estimated.
Figure 8. (a) J-V curves of FTO/TiO2/CH3NH3PbI3/PMPS:B/Au and FTO/TiO2/ CH3NH3PbI3/DPPS:B/Au photovoltaic devices. (b) IPCE spectra and integrated Jsc of FTO/TiO2/CH3NH3PbI3/PMPS:B/Au and FTO/TiO2/CH3NH3PbI3/DPPS:B/Au photovoltaic devices.
to be 1.57 eV (not shown), which agreed with the Eg shown in Figure 7(b). These values indicate that the spectral mismatches between AM 1.5 solar simulator, IPCE measurement system and UV-VIS-NIR spectrophotometer were small.
To explain charge transport, energy band diagrams of the FTO/TiO2/ CH3NH3PbI3/PMPS:B/Au and FTO/TiO2/CH3NH3PbI3/DPPS:B/Au devices are shown in Figure 9(a) and Figure 9(b), respectively. In Figure 9, previously reported values     and energy gaps of PMPS:B and DPPS:B estimated from the absorption spectra shown in Figure 2 were used for the energy levels. In Figure 9(a), although the lowest unoccupied molecular orbital (LUMO) energy level of PMPS was assumed approximately −2.0 eV  , an energy level shift of LUMO by B doping would be conceivable  . In Figure 9(b), it is assumed that the LUMO of DPPS is close to that of poly(di-n-butylsilane) of −1.7 eV  because the LUMO of DPPS has not yet experimentally and theoretically been investigated. By B doping into DPPS, energy levels of DPPS would be shifted. The charge generation occurs in the CH3NH3PbI3 layer by light irradiation from the FTO bottom side. Electrons in the conduction band of CH3NH3PbI3 layer are transferred to FTO anode through TiO2. Simultaneously, holes in the valence band of CH3NH3PbI3 are transported to Au cathode through the polysilanes. Compared to CH3NH3PbI3-based photovoltaic device with PMPS:B layer, it is considered that an effective hole transport occurred from the valence band of CH3NH3PbI3 to Au cathode through the DPPS:B in the CH3NH3PbI3-based one, which was due to proper Si content and molecular structures of the DPPS:B. Furthermore, a heat treatment-like effect was caused for the DPPS:B layer during Au evaporation, which likely to lead to crystallization and increase in mobility of DPPS:B layer.
Finally, composition ratio of Pb and I atoms presented in Table 2 is very sensitive to photovoltaic performance. Compared with photovoltaic devices fabricated without air blow-assisted spin-coating method    , the
Figure 9. Energy band diagrams of (a) FTO/TiO2/CH3NH3PbI3/PMPS:B/Au and (b) FTO/TiO2/CH3NH3PbI3/DPPS:B/Au devices.
present CH3NH3PbI3 layers were Pb-rich and I-poor, suggesting that the CH3NH3PbI3 layers were n-type. Types of conductivity of perovskite compounds strongly depend on composition ratio of themselves. In fact, Wang et al. reported that PbI2-rich CH3NH3PbI3 behaved like an n-type semiconductor  . To improve the photovoltaic performance of perovskite-type solar cells with polysilane hole transport layers, investigation of electrical, optical and photovoltaic properties of polysilanes including p-type dopants should be necessary.
Perovskite-type photovoltaic devices with polysilane hole transport layers were fabricated and were investigated. As-deposited PMPS:B and DPPS:B layers were amorphous, and their optical absorption spectra showed some marked features originated from transitions. J-V characteristics of the CH3NH3PbI3-based photovoltaic devices with PMPS:B and DPPS:B layers exhibited different photovoltaic performance each other. Conversion efficiency of the CH3NH3PbI3-based photovoltaic device with DPPS:B layer was slightly higher than that with PMPS:B one. These results were attributed to molecular structures of polysilanes and Si content in the polysilane hole transport layers of the present devices. In order to realize inexpensive perovskite-type solar cells with polysilane hole transport layers, further investigation of polysilanes, including electrical, optical and photovoltaic properties, and optimization of dopants and their concentration would be required.
This work was partly supported by Super Cluster Program of Japan Science and Technology Agency (JST) and JSPS KAKENHI Grant Number 25420760.