Essay: Enhancing the performance of perovskite sensitized solar cells

Enhancing the performance of perovskite sensitized solar cells based on core/shell photoanode structure with spiro-MeOTAD HTM electrolyte
ABSTRACT
An attempt has been made to fabricate methyl ammonium tin chloride (CH3NH3SnCl3) perovskite sensitized TiO2 nanostructure photoanode solar cell with hole transport material (HTM) spiro-MeOTAD and graphite coated counter electrode. The TiO2 nanoparticles (TNPs), TiO2 nanoleaves (TNLs), TNLs with MgO core/shell photoanodes were prepared to fabricate perovskite sensitized solar cells (PSSCs). The prepared samples were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The photovoltaic characteristics of the PSSCs, photocurrent density (Jsc), open-circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE) were determined under illumination of AM 1.5 G. Electrochemical impedance spectroscopy (EIS) analysis was carried out to study the charge transport and life time of charge carriers at photoanode/sensitizer/electrolyte interface of the PSSCs. The PSSC made with CH3NH3SnCl3 perovskite sensitized TNLs-MgO core/shell photoanode and
spiro-MeOTAD HTM shows an impressive photovoltaic performance, Jsc = 17.24 mA/cm2,
Voc = 800 mV, FF= 73% and PCE of 9.98% under 100 mW/cm2 light intensity. The advent of such simple solution processed mesoscopic heterojunction solar cells paves the way to realize low-cost and high-efficiency solar cells. By the aid of electrochemical impedance spectroscopy, it is revealed that the core/shell structure with can increase interfacial resistance of photoanode/CH3NH3SnCl3 interface and retard electron recombination process in the photoanode/sensitizer/HTM interface.
Keywords:
Perovskite, solar cell, core/shell, TiO2 nanoleaves, HTM, PCE, EIS
1. Introduction
Perovskite solar cells based on organo-metal halides represent an emerging photovoltaic technology. A highly efficient solar cell material should absorb light over a wide spectral range, generate charges with high efficiency and transport these photogenerated charges to the electrodes with minimum losses. Several groups have reported power conversion efficiencies from 10% to 15% for solution processed organo-metal halide perovskite -based solar cells in the past two years [1−3]. Perovskite showed an absorption coefficient that was 10 times greater than that of the conventional ruthenium-based molecular dye. Since organo-lead halide perovskite is an ionic crystal, it easily dissolves in a polar solvent. Thus, organolead halide perovskite is not suitable for liquid electrolyte-based sensitized solar cells because of stability concerns. This instability problem was solved by substituting a solid hole conductor for the liquid electrolyte. More recently, it was shown that these classes of materials have electron−hole diffusion lengths of at least 100 nm for triiodide absorber [4] and longer than
1 μm for trihalide perovskites [5] However, the fundamental photophysical processes underlying solar cell function need to be understood in order to fully utilize the properties of these materials. For instance, it is not known whether the exceptionally long diffusion lengths are related to molecular excitons, i.e., tightly bound electron−hole pairs, or to highly mobile charge carriers. Hence, the nature of the initial photophysical product is not known. The exact role of the metal oxide TiO2 as electron acceptor and charge transport layer in an perovskite based photovoltaic cell is also not understood.
Organo-metal perovskite with balanced long-range carrier diffusion lengths [4-6] and low-cost have attracted great attention as a new class of light harvesters for solid-state hybrid solar cells in the past few years [7]. Perovskites are unique as active layer in PV modules owing to their ability to deliver high open circuit voltages, under full sun illumination, leading to light harvesting from a broad spectrum of incident solar radiation. In particular, the recent emergence of organic−inorganic halide perovskite-based solar cells promises to deliver one of the lowest cost technologies that is capable of converting sunlight to electricity at the highest efficiencies [8-11]. Good progress has been made in revealing the hybrid metal halide perovskite to deliver impressive power conversion efficiency in solar cells. Synthetic perovskite have been identified as possible inexpensive base materials for high-efficiency commercial photovoltaic [12] they showed a conversion efficiency of up to 15% [3]. A group of methyl ammonium tin and lead halides is of interest for use in solar cells [13]. Among organic-inorganic perovskite-structured semiconductors, the most common one is the triiodide (CH3NH3PbI3). It exhibits high charge carrier mobility and carrier lifetime that allow light-generated electrons and holes to move far enough to be extracted as current, instead of losing their energy as heat within the cell. As we know, lead poisoning is a type of metal poisoning [14] hence it should be replaced by Sn. In thin photovoltaic film, optical management is an important key for harvesting light while ensuring higher efficiency. Organic sensitizers often limit light-harvesting ability because of their low absorption coefficients and narrow absorption bands. Recently, a new class of hybrid organic halide perovskite was introduced as light sensitizing material, showing strong absorption in a broad region of the visible spectrum (direct energy gap down to ~ 1.55 eV), good electron and hole conductivity, and high open circuit voltages in photovoltaic devices [15]. Meanwhile, solid-state DSSC was introduced in 1998 [16], the liquid electrolyte was replaced by organic/inorganic hole-transport materials. The drawback of the perovskite-sensitized liquid-type DSSC was the instability of the deposited CH3NH3PbI3 in liquid electrolyte. Surface protection of the deposited CH3NH3PbI3 is thus to be developed to increase its stability in liquid-based DSSCs. The instability issue was actually solved by replacing liquid electrolyte with solid hole conductor spiro-MeOTAD, where an efficiency as high as 9.7% was achieved from a very thin TiO2 film (~ 0.5 μm) together with excellent long-term stability [17]. Solid-state mesoscopic solar cell employing (CH3NH3)PbI3 perovskite nanocrystals as a light absorber and spiro-MeOTAD as a hole-transporting layer, a strikingly high PCE of 9.7% was achieved with submicron thick films of mesoporous anatase TiO2 under AM 1.5G illumination [9]. Perovskites are hybrid layered materials typically with an AMX3 structure, with A being a large cation, M a smaller metal cation and X an anion from the halide series. They form an octahedral structure of MX6, which forms a three dimensional structure connected at the corners [18-20] as shown in Fig. 1.
Metal oxide nanostructures have also been integrated into DSSCs to maximize the number of adsorbed dyes and there by maximize the photocurrent. Titanium dioxide (TiO2) has several advantages such as chemical stability, non-toxicity, good electrical property and inexpensive material; therefore it is extensively used in many applications such as photocatalyst, hydrogen production and solar cells [21-24]. Anatase, rutile and brookite are three main crystalline structure of titanium dioxide [23]. Anatase TiO2 has band gap energy of 3.2 eV of which the absorption thresholds correspond to 380 nm, suggesting that easy for photoelectron transfer under solar light irradiation. To avoid recombination of charge carriers, the use of core/shell structure, which acts as the energy barrier for physical separation of injected electrons from HTM. It reduced the dark current and increased the open circuit voltage. Accordingly, the enhanced photocurrent and open-circuit voltage have led to a prominent increase in photovoltaic efficiency. MgO has distinctive properties: high refractive index, wide optical band gap, and low absorption and dispersion in visible and near-infrared spectral regions [25]. In the present investigation, an attempt has been made to study the performance of PSSCs fabricated based on various photoanode structures applied with CH3NH3SnCl3 perovskite sensitizer and spiro-MeOTAD electrolyte. The PSSCs are subjected to J-V characteristics for their PCE and other solar cell parameters. The electron impedance parameters at the photoanode/sensitizer/electrolyte interface were investigated by electrochemical impedance spectroscopy.
2. Experimental
All chemicals used in this study were of high purity, which were purchased from Sigma Aldrich, India, and used without further purification unless otherwise stated.
2.1. Preparation of TNPs and TNLs through sol-gel process
TNPs were synthesized using titanium (IV) isopropoxide [TTIP], nitric acid, ethyl alcohol and distilled water through sol-gel process. 70 ml of TTIP was mixed with 100 ml of ethanol, and 50 ml of distilled water was added drop by drop under vigorous stirring for 1 h. This solution was then peptized using 0.1M of nitric acid (430 ml) and heated under reflux at 80° C for 8 h. After this period, a TiO2 sol was prepared. The prepared sol was dried to yield a TiO2 powder. The TiO2 was calcined at 450°C for 1 h in a furnace to get TNPs. TNLs were prepared through alkali hydrothermal process. 2g of TNPs prepared from the sol-gel method was mixed with 100 ml of a 10M NaOH aqueous solution, followed by hydrothermal treatment at 150°C in a Teflon-lined autoclave for 12 h. After the hydrothermal reaction, the treated sample was washed thoroughly with distilled water and 0.1 M HCl and subsequently filtered and dried at 80°C for 1 day. To achieve the desired TNLs size and crystallinity, the sample was calcined at 600°C for 1h [26].
2.2. Preparation of the MgO shell coating
In a typical process, the MgO shell coating was prepared by dipping the TNLs electrode into saturated magnesium methoxide solution and then washing thoroughly with distilled water. After drying, the electrode was calcined at 450°C for 0.5 h. The thickness of the shell layer (~5 nm) was controlled by the dipping time [27].
2.3. Synthesis of metal halide CH3NH3SnCl3 perovskite sensitizer
CH3NH3Cl was synthesized by reacting 30 mL of methylamine (40% in methanol) and
32.3 mL of hydrochloric acid (57 wt% in water) in a 250 mL round-bottom flask at
0 °C for 2 h with stirring. The precipitate was recovered by putting the solution on a rotary evaporator and carefully removing the solvents at 50 °C. The yellowish raw product methyl ammonium chloride (CH3NH3Cl) was washed with diethyl ether by stirring the solution for 30 min, a step which was repeated three times, and then finally recrystallized from a mixed solvent of diethyl ether and ethanol. To prepare CH3NH3SnCl3, readily synthesized CH3NH3Cl (0.395g) and SnCl2 (1.157g), were mixed in 10ml of DMF at 60 °C for overnight with stirring. The small beaker is immediately sealed and kept in dark at room temperature. Aluminium foil is used over the lid of the sample beaker to prevent reaction with sun light [28].
2.4. Fabrication of perovskite sensitized solar cells
The photoanodes using bulk TiO2, TNPs, TNLs, TNLs-MgO core/shell were prepared by ultrasonically mixing for 2 h. The prepared pastes were deposited by doctor-blade technique on FTO (8–10 Ώ/ square) by preparing an active area of 1 cm2. Four types of (Bulk TiO2, TNPs, TNLs, and TNLs-MgO core/shell) photoanodes, which were respectively labeled as PSSC-1, PSSC-2, PSSC-3 and PSSC-4. The film was then heated to 450 ºC at a rate of 15 ºC/min and kept at 450 ºC for 30 min. After cooling to 80ºC, the working electrodes were immersed overnight in a solution of CH3NH3SnCl3 perovskite sensitizer for overnight. One drop of the hole transport material, 0.170 M 2,2′,7,7′-tetrakis(N,N-dipmethoxyphenylamine)9,9′-spirobifluorene (spiro-MeOTAD, Merck), 0.064 M bis(trifluoromethane)sulfonimide lithium salt (LiTFSI, 99.95%, Aldrich) and 0.198 M 4-tert-butylpyridine (TBP, 96%, Aldrich) in the mixed solvent of chlorobenzene (99.8%, Aldrich) and acetonitrile (99.8%, Aldrich) (chlorobenzene : acetonitrile51 : 0.1 v/v) deposited onto the surface of the photoanode. A graphite coated counter electrode was then clipped on top of the photoelectrode to form a photovoltaic device.
2.5. Characterization of PSSCs
The photovoltaic properties of the PSSCs were characterized by recording the photocurrent voltage (J–V) under illumination of AM 1.5G (100mW/cm2). The prepared photoanode using bulk TiO2, TNPs, TNLs, and TNLs- MgO core/shell structures were characterized by X-ray diffraction (XRD) (X’Pert PRO-PANalytical X-ray powder diffractometer). Transmission electron microscopy (TEM) characterization was done by Philips Tecnai10 electron microscope operated at 200 kV. Scanning electron microscopy (SEM) images were recorded by VEGA3 SB electron microscope. Specific surface areas of the TiO2 samples are determined by the use of a nitrogen adsorption apparatus (Micromeritics ASAP 2020). Electrochemical characteristics of the PSSCs were recorded with a potentiostat /galvanostat (Gamry 300). The applied bias voltage and ac amplitude were set at open-circuit voltage of the PSSCs and 10 mV between the FTO/counter electrode and the FTO/photoanode, respectively, and the frequency range explored was 1 mHz to 105 Hz. The impedance spectra were analyzed by an equivalent circuit model interpreting the characteristics of the PSSCs.
3. Results and discussion
3.1. XRD, TEM and SEM analysis
The crystalline structure of the bulk TiO2, TNPs, TNLs and TNLs-MgO core/shell were characterized by XRD and given in Fig. 2. The XRD pattern of TNLs-MgO consists of anatase TiO2 and MgO peaks. The peaks observed at 25.3°, 37.9°, 48.0°, 53.9° and 62.6° are corresponding to the planes (101), (004), (200), (105) and (215) of anatase phase
(JCPDS 21-1272) [29]. The observed peaks are very sharp; implying that the TNLs were well crystallized and additional peaks observed in this diffractogram might be due to the calcinations of the sample during hydrothermal process. The peaks appeared at 43.2° and 62.3° corresponding to the (200) and (220) planes of MgO phase respectively [27]. Figures 3a and 3b show the TEM image of TNPs and TNLs/MgO core/shell respectively. In Fig. 3a depicts that the average diameters of the TNPs are
20–60 nm. From the Fig. 3b, it could be seen that the average diameter of the TNLs-MgO core/shell are 5–10 nm. Fig. 3c show the SEM image of the TNLs has a leaf-like structure. TNLs are very uniform, quite clean, and smooth-surface. It could be seen that the starting material exhibited the nanoparticles, after hydrothermal synthesis the nanoparticles were completely converted to TNLs. The textures of the TNLs are uniform and reasonably dense through there are ample voids between the leaves. The Brunauer-Emmett-Teller (BET) surface area measurement revealed that the TNLs-MgO core/shell had a specific area of
16 m2g−1. This value is much higher than that of TNPs (12 m2g−1) and TNLs (14 m2g−1).
3.2. Photovoltaic characterization
The photocurrent-voltage (J-V) characteristics of the PSSCs with various types of
photoanodes are shown in Fig. 4. The detailed photovoltaic parameters open-circuit voltage (Voc), short-circuit photocurrent density (Jsc), fill factor (FF) and PCE (η) of the PSSCs are presented in Table 1. The overall PCE of PSSCs is varied in the four types of TiO2 photoanodes. The PSSC-4 showed the highest efficiency as compared to the other PSSCs. Photovoltaic parameters for the PSSC-4 are Voc = 800 mV, Jsc = 17.24 mA,
FF = 73%, and η = 9.98%. The PSSC-1 is made with only bulk particle of TiO2 showed lowest photovoltaic parameters, Voc = 750 mV, Jsc = 6.47 mA, FF = 66%, and η = 3.20%. Photovoltaic parameters for the PSSC-2 prepared with the TNPs photoanode are
Voc = 780mV, Jsc = 9.65 mA, FF = 68%, and η = 4.92%. The PSSC-3 is made using TNLs photoanode exhibited the photovoltaic parameters, Voc = 780 mV, Jsc = 14.16 mA,
FF = 70%, and η = 7.73%. The efficiencies of the PSSCs fabricated with different types of photoanode in the order of PSSC-4 > PSSC-3 > PSSC-2 > PSSC-1. These results suggested that PSSC-4 fabricated through TNLs-MgO core/shell structure hold more efficiency as compared to the other PSSCs. When comparing PSSC-4 and PSSC-3, the PSSC-4 showed enhanced efficiency due to core/shell structure of TNLs-MgO photoanode. By enclosure of MgO into the TNLs film, the photovoltaic characteristics have been changed dramatically. As a result of the good photovoltaic parameters for the cell consisting of TNLs-MgO core/shell photoanode, the highest η of 9.98% is obtained at an optimized condition. The overall conversion efficiency was increased by 211% from 3.20 to 9.98 % when comparing PSSC-1 and PSSC-4. The improvement of the efficiency was ascribed to the increased in the
short-circuit current density Jsc, Voc, and FF. These results suggested that the electron injection is efficient through the MgO shell coating compared with the uncoated PSSC-3. The possible mechanisms for the increased Voc is the thin MgO layer apparently decreased the recombination rate of the photo injected electrons with the oxidized sensitizer or ions in the electrolyte by forming an energy barrier thus the core-shell structure enhanced the performance of the PSSCs significantly. The PSSC-1 has lowest photocurrent efficiency because it was made up with the bulk TiO2 photoanode without core-shell structure.
Remarkably, the mesoporous n-type TiO2 with insulating MgO, CH3NH3SnCl3 sensitizer and spiro-MeOTAD HTM electrolyte improved the power conversion efficiency to 9.98%. Perovskites are unique as active layer in PV modules owing to their ability to deliver high open circuit voltages, under full sun illumination, leading to light harvesting from a broad spectrum of incident solar radiation [30]. In the present investigation PSSC-4 has exhibited higher Voc of 800 mV. Hole transporting materials (HTMs) are solid-state mediators which are absolutely free from leakage. In DSCs with HTMs, hole transfer takes place directly from the sensitizer to the HTM and the holes are transported via electron hopping to a metal counter electrode. Here we used HTM, spiro-MeOTAD (2, 2\’, 7, 7\’-tet-rakis-(N, N-di-p-methoxyphenylamine)9,9\’-spirobifluorene). Due to its small molecular size, high solubility, and amorphous nature, solid state PSSC-4 attained the PCE of 9.98%.
3.3. EIS analysis
Electron transport properties were investigated using electrochemical impedance spectroscopy. Figures.5 and 6 show the Nyquist and Frequency-phase plots of PSSCs respectively. Fig. 5 illustrates the Nyquist plots corresponding to the PSSCs based on the metal-halide perovskite sensitized photoanode film under the light intensity of 100mW/cm2. Generally, all the Nyquist plots of PSSCs exhibit three semicircles, which are assigned to electrochemical reaction at the graphite counter electrode, charge transfer at the photoanode/perovskite/HTMs and Warburg diffusion process. The equivalent circuit consist of series resistance (Rs, starting point of the first semicircle in Nyquist plot), electron transport resistance at the counter electrode/electrolyte (RCE, first semicircle in Nyquist plot), recombination resistance (Rrec) at the photoanode/perovskite/HTM (second semicircle in Nyquist plot), the constant phase elements of capacitance Crec (Rrec), CCE (RCE) and Warburg diffusion impedance Zd are shown in Fig.7. The other useful electrochemical parameters of PSSCs are τn (electron life time – equation (1)), Ln (electron diffusion length – equation (2)), Deff (effective diffusion coefficient – equation (3)), µ (electron mobility – equation (4)),
σ (electron conductivity – equation (5)), ηcc (electron collection efficiency – equation (6)) and Conc. (concentration of electrons in the TiO2/sensitizer/HTM interface- equation (7)) were calculated from the following equations [27] and presented in Table 2.
(1)
(2)
(3)
(4)
(5)
(6)
(7)
where, LF – the film thickness of the photoanode (10 µm); S – active area of the PSSCs (1 cm2); P– porosity of the photoanode film (0.6); τd – (Electron transport time) (Rtr× Crec); e – Charge of an electron; KB – Boltzmann constant; T – Room temperature; keff – rate of recombination of the electrons [ ]
From the Nyquist plots, it could be seen that the first and third semicircles are weak when compared with the second semicircle. From the Fig. 5 comparing the middle semicircles of the Nyquist plot of the PSSCs, the Rrec value increases in the order of PSSC-1 < PSSC-2 < PSSC-3 < PSSC-4 which indicates that the larger the value of Rrec, the lower the recombination rate at the photoanode/sensitizer/HTM interface. The PSSC-4 fabricated with TNLs-MgO core/shell photoanode illustrated higher values of Crec, Ln, τn, Deff, μ, σ, and ηcc at the photoanode/sensitizer/HTM interface as compared with other PSSCs. In addition, Rtr and concentration of electrons at the photoanode/sensitizer/HTM interface have lower values as compared with the other PSSCs. These results enabled not only the reduction of the recombination rate but also enhancement of the ηcc in PSSC-4. Control of interfacial electron injection and recombination in photoanode/sensitizer/HTM interface is pivotal for the best performance of PSSCs. This implied that the recombination rate was diminished because the doped MgO could shield the electron back flow through the TNLs to the electrolyte. Thus, the surface modification of photoanode with very thin layer as well as larger band gap shell material improved the photocurrent density. Hence, core-shell material provided faster electron injection, and suppressed recombination at photoanode/sensitizer/HTM interface. In our examination on the performance of core-shell structure based PSSCs, hints that the essential improvement in electron injection efficiency and thereby overall performance can be obtained by proper designing of the second metal oxide with suitable band gap, optimal thickness of shell layer and IEP. In addition, TNLs structure, the grain boundaries effect could be restricted [31, 32]. Moreover, for the same given film thickness, the loading of dye can be much higher in TNLs than the TNPs, for instance, the TNLs has allowed for larger adsorption of the sensitizer. This necessitates the development of new nanostructures for electron separation and conduction such as TiO2 nanorods [33, 34], nanosheets [28] with the objective of improving electron transport and absorber infiltration, as well as exploration of novel HTMs with suitable band alignment and improved hole mobilities [2].The enhanced efficiencies of the TNLs-MgO core-shell PSSCs is achieved by optimizing device architectures, which enhanced light absorption and facilitated electron transport by determining and designing appropriate dimensions of TNLs-MgO, by optimizing the cation concentration in the HTM for promoting electron injection yield from sensitizing CH3NH3SnCl3 molecules to core-shell TNLs-MgO electrodes, or by synthesizing thermally stable TNLs with a stable high surface area. In the core-shell TNLs-MgO photoanode, the higher Rrec suggested that each TNL made it more difficult for an electron to jump outside the nanostructure than to stay within the structure during diffusion; this explained the high collection efficiency and thus high short-circuits photocurrent.
Fig.6 shows the frequency-phase plots of EIS spectra for the PSSCs, made with different photoanodes. Two characteristic peaks associated with the transfer of the photo generated electrons at the surface of TNLs-MgO core/shell and the conducting electrodes, are clearly observed. The frequency peak at the high frequency region can be ascribed to the charge transfer at the interfaces of the electrolyte/counter electrode and the other low frequency region to the accumulation/transport of the injected electrons with TNLs-MgO core/shell porous film and the charge transfer at the interfaces of electrolyte, respectively. From the Fig. 6 Frequency phase plots in the lower frequency regime, it can be seen that the frequency peak of PSSC-1 is shifted to lower frequency when compared with the PSSC-3 and PSSC-4. The characteristic frequency peak of PSSC-4 in higher frequency regime shifted to higher frequency region when comparing other PSSCs. This result could be attributed to the higher electron transport rate and electron diffusion coefficient of PSSC-4. These results indicate that TNLs-MgO core/shell photoanode PSSCs has higher electrons transport and lower recombination rate at photoanode/sensitizer/HTM interface and the same property leading to higher solar cell efficiency. In the present investigation, 9.98% efficiency is a remarkable considering the HTM electrolyte and graphite coated counter electrode employed.
The recent advent of CH3NH3PbX3 (X = Cl, Br, I) as a hole conductor or as a sensitizer in solid-state cells, has yielded a PCE of 15% [3, 9, 30, 36]. However, most of these cells employed mesoporous TiO2 nanoparticles for the loading of perovskite thereby offering scope for the cell performance to be further improvised by employing photoanode materials with better porosity and better charge transport characteristics. In these devices the electrons are transported through the perovskite layer and the holes are transported through the spiro-MeOTAD layer. It is interesting to note that the perovskite layer can function as a light absorber as well as n-type semiconductor for transporting electronic charges out of the device. In the perovskite sensitizer, the inorganic component is a ionic crystal, then native dipoles would occur at the organic−inorganic interface, between the hole transporter and the perovskite. Indeed, the presence of under coordinated halide at the crystal surface gives rise to local excess of negative electrostatic charge, which may trap the holes injected into the
spiro-MeOTAD, generating interfacial dipoles. Recombination of electrons in TiO2 with holes in the perovskite itself is moreover unlikely to be very fast, since the hole density in the perovskite should be relatively low due to their high diffusivity and effective transfer to
spiro-MeOTAD at the planar heterojunction [5]. The same applies for the recombination of holes in spiro-MeOTAD with electrons in the perovskite: the electron density in the perovskite will be very low due to the fast electron transfer to the TiO2 nanoparticles, meaning this recombination mechanism will also be very slow. The understanding gained here allows us to propose design rules for preparing high efficiency TiO2-based perovskite solar cells. The most important factor to control is the completeness of perovskite coverage on the TiO2 nanoparticles, as this reduces recombination rates. This means that higher charge densities can be maintained in the TiO2, improving the charge transport rates and collection efficiencies of the solar cells. This, together with the fact that thinner TiO2 scaffolds serve to concentrate the electron density at a given generation rate, also leads to a raising of the electron quasi Fermi level in the TiO2 to improve the photovoltage of the solar cells.
Our findings show that the perovskite sensitized solar cell PSSC-4 had highly mobile electron and holes are formed within a few picoseconds, and mobilities of both are almost balanced and remain high. Low recombination and almost equal electron and hole mobilities guarantees very efficient charge collection and thus high solar cell efficiency. The results also show that, as a consequence of electron injection from the perovskite to the core/shell photoanode with very high electron mobility, overall mobility is higher. A possible improvement of solar cell performance would be the active materials such that both electron and hole mobilities are on the level of the electron mobility in the perovskite.
Despite the disadvantage of platinum CE in DSSC, higher cost, poor stability in corrosive electrolyte and high temperature processing drives the need for the development of alternative CE materials. In the recent past, a few groups reported on different CE materials like carbon nanotubes (CNT) [3, 28], activated carbon, and graphite [37]. In this study graphite has used as a counter electrode. It has also been reported that humidity degrades the CH3NH3PbI3 performance. Halide alteration has been promising in this regard with increased stability in humid environments at minor performance penalty [38] occurring when a low proportion of Cl is introduced. Lead content is another draw-back for the viability of these cells. Hence, in this investigation PSSCs fabricated by incorporating the core-shell photoanode structure, CH3NH3SnCl3 perovskite sensitizer, the HTM electrolyte and graphite counter electrode offered better PCE performance. In this study Pb was replaced by Sn thus stability is significantly increased. The device PSSC-4 illustrated the PCE of 9.82% after 180 days.
4. Conclusions
TNPs and TNLs were synthesized via sol–gel and hydrothermal transformation respectively. PSSCs were constructed with made of different types of TiO2 working photoanode. The electron transport resistance, electron life time and diffusion length were evaluated in terms of electrochemical impedance spectra and photovoltaic characteristics of the cells. The PSSC-4 based on core-shell structure of TNLs-MgO photoanode hybrids showed a better photovoltaic performance (higher FF, open circuit-voltage and photocurrent density) than the cell purely made of TNPs. The PSSC-4 showed the PCE conversion efficiency of 9.98% of which higher than the other PSSCs. From these results it could be concluded that TNLs-MgO core-shell novel photoanode structure possibly could restrain the recombination process and increase the PCE effectively. The new core/shell with perovskite sensitizer architecture presented a large open spacing and mediator/electrolyte access. As well, the PSSC made up of spiro-MeOTAD electrolyte showed better performance has relatively high ambient and ionic conductivity intimate interfacial contact with TNLs-MgO core/shell and remarkable stability. The present work establishes the cell with the CH3NH3SnCl3 sensitized TNLs-MgO core/shell photoanode heterojunction with spiro-MeOTAD electrolyte had better performance. We conclude that the perovskite is stable in dry ambient air and can be deposited low cost solution processing opens up new avenues for future development of high-efficiency low-cost photovoltaic cells.
Acknowledgments
The authors are thankful to the authorities of Annamalai University, for providing all necessary facilities to carry out the present work successfully. We also thank the anonymous referees who significantly contributed to improving the contents of the manuscript.
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