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CH3NH3PbI3 Perovskite Sensitized Solar Cells Using a D-A Copolymer as Hole Transport Material
Accepted Manuscript Title: CH3 NH3 PbI3 Perovskite Sensitized Solar Cells Using a D-A Copolymer as Hole Transport Material Author: P. Nagarjuna K. Narayanaswamy T. Swetha G. Hanumantha Rao Surya Prakash Singh G.D. Sharma PII: DOI: Reference:
S0013-4686(14)02180-X http://dx.doi.org/doi:10.1016/j.electacta.2014.11.003 EA 23675
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
12-8-2014 30-10-2014 1-11-2014
Please cite this article as: P.Nagarjuna, K.Narayanaswamy, T.Swetha, G.Hanumantha Rao, Surya Prakash Singh, G.D.Sharma, CH3NH3PbI3 Perovskite Sensitized Solar Cells Using a D-A Copolymer as Hole Transport Material, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2014.11.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
CH3NH3PbI3 Perovskite Sensitized Solar Cells Using a D-A Copolymer as Hole Transport Material P. Nagarjuna1, K. Narayanaswamy1, T. Swetha1, G. Hanumantha Rao1, Surya Prakash Singh1* and G. D. Sharma2* 1
Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal road, Tarnaka, Hyderabad-500007, India 2
R & D Center for Engineering and Science, JEC group of Colleges, Jaipur Engineering College, Kukas, Jaipur 303101, India Graphical abstract fx1
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conjugated
D-A
copolymer
(P)
having
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Abstract benzodithiophene
donor
and
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benzo[1,2,5]thiadiazole acceptor was employed as a p-type hole transporting material in solid
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state organic–inorganic hybrid solar cells and compared with the P3HT hole transporting material. In these device we have used organo-lead halide (CH3NH3PbI3) synthesized by us,
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as light harvester. The power conversion efficiency (PCE) of 6.64 % is achieved for the solar
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cell with P which is higher than that for P3HT (4.24 %). The increase in PCE is mainly due to
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the enhancement in FF and Voc and attributed to higher mobility of hole for conjugated
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copolymer than P3HT.
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Key words: Inorganic-organic hybrid solar cells, perovskite, donor--acceptor, hole transport material, power conversion efficiency.
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* Corresponding authors
[email protected] (Surya Prakash Singh)
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Tel: +91-40-27191710; Fax: +91-40-27160921
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[email protected] (G. D. Sharma)
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1. Introduction Dye sensitized solar cells received considerable attention since the first report on 1991 by
Grätzel and his coworkers, due to the excellent power conversion efficiency (PCE) of about 7% at a relatively low cost of fabrication1 and are alternative to the conventional silicon based solar cells. A typical DSSC uses a monolayer of light absorbing sensitizer anchored on nanocrystalline TiO2 to enhance light harvesting in a mesoporous environment in which
charge separation also occurs at this surface. Lot of research has been carried out to enhance the PCE of these devices. As a result, PCE of 12.3 % has been reported in 2011, by Grätzel research group for DSSCs based on an ortho-substituted porphyrin sensitizer, YD2-oC8, cosensitized with organic dye (Y123) with a cobalt electrolyte2 which opened a new research area on porphyrin sensitized DSSCs.3 However, the absorption spectrum of this porphyrin dye covers only the visible spectral region and lack of light harvesting ability beyond 700 nm limits the further enhancement in PCE for the DSSCs based on porphyrin dyes. Moreover, the long term stability of these devices might be an issue for future commercialization due to the use of volatile electrolytes. Various inorganic light absorbers with light harvesting capability
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that extends into the near infrared region were hence sought to replace these organic dyes. Recent studies have reported that organometal halide perovskite mesoscopic solar
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cells with promising PCEs due to their excellent light harvesting and electron transporting properties and are perfectly suitable for use as prospective photovoltaic materials. In 2009,
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Miyasaka et al. reported the first perovskite (CH3NH3PbBr3) sensitized TiO2 solar cell using liquid electrolytes based on iodide and bromide and achieved a PCE of 3.1 % under one sun
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illumination and 3.8 % when CH3NH3PbBr3 was replaced by CH3NH3PbI3.4 Later on Park et
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al.5 in 2011, reported improved PCE of 6.5 % for modified TiO2 solar cells sensitized with
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CH3NH3PbI3 perovskite and an electrolyte based on iodide. They have used Pb(NO3) for
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modify the surface of mesoporous TiO2 film before coating of perovskite film and acts as blocking layer in the solar cells. However, it was observed that the performance of devices
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based on perovskite materials and liquid electrolyte decreased by 80 % in only few hours due
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to the instability of the perovskite material in the presence of liquid electrolyte due to the dissolution of perovskite. Hence it is essential to prepare perovskite based solar cells with a
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solvent free hole transport material (HTM) and called as solid state organic-inorganic hybrid solar cells. Park and coworkers6 reported sub-micrometer thin film solid state solar cells 7
Snaith and co-
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attaining a PCE of 9.7 % with a CH3NH3PbI3 in mesoporous TiO2 film.
workers reported a similar perovskite, CH3NH3PbI3Cl that served as a light absorber for
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mesoscopic thin film solid state solar cells to attain a PCE of 10.9%, for which the mesoporous Al2O3 film served as scaffold to replace the n-type TiO2 electron transporting layer. Recently, same group reported a significant enhanced PCE of 12.3% 8 for perovskite CH3NH3PbI3-xClx solar cells, with the same device structure based on Al2O3. Soak et al. reported almost same PCE for solar cell based on another perovskite CH3NH3Pb(I1-xBrx)3, with the mesoporous TiO2 photoanode.9 Recently, in 2013, Grätzel and co-workers, reported perovskite CH3NH3PbI3 sensitized TiO2 solar cells with a PCE approaching to 15 % which
set a new record for all solid state hybrid mesoscopic solar cells.10 Therefore, this impressive PCE opened new channel for the development of third generation solar cells based on perovskite and HTM with the advantages of very high PCE, low cost, ease of processing and considerable durability11 and efficiency approaching 20% is realistically possible from a solid-state mesoscopic solar cell based on CH3NH3PbX3 (X = Cl, Br, or I).12 In all above perovskite solar cells, a HTM [2,2’,7,7’-tetrakis-(N,N-di-pmethoxyphenylamine) 9,9’-spirobifluorene] (spiro-OMeTAD) has been employed. However, conductivity of spiro-OMeTAD is low (~10-5 S/cm). Moreover, the spirobifluorene core in spiro-OMeTAD molecule is relative expensive due to extensive synthetic processes for preparation. So it would be
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important to develop a cheaper alternative of spiro-OMeTAD for commercial application of hybrid perovskite solar cells.
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Alternative HTM materials with excellent electrical properties may be used in place of spiro-OMeTAD. Soluble donor--acceptor (D--A) can be the alternative choice as HTM.
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Conjugated polymers have been widely used in the field of polymer solar cells because of their easy processability, low cost and mechanical flexibility.13 The solar cells using the
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combination of perovskite with mesoporous TiO2 and P3HT as hole transport material, PCEs
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of 3.8 %, 4.5 %14 and 6.7 %15 have been reported and recently a PCE of 10.8 % have been
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achieved by Coning et al. for CH3NH3PbI2Cl perovskite.16
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To obtain high photovoltaic performance, it is importance that the HTM should show a good hole mobility and compatible highest occupied molecular orbital (HOMO) energy
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level to the semiconductor light absorber. Recently, Qiu, have used a p-type low band gap
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conjugated D-A copolymer PCBTDPP as HTM and achieve an outstanding open circuit voltage of about 1.15 V for CH3NH3PbBr3 sensitized hybrid solar cells and CH3NH3PbI3
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sensitized hybrid solar cell showed a PCE of 5.55% along with good stability.17 Heo et al have reported a ~12 %18 PCE in an organic-inorganic hybrid solar cell using a poly
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(triarylamine) (PTAA) HTM with an average PCE of 9.7% 19 The D-A conjugated polymer P (chemical structure shown in Figure 1) have the deeper HOMO level than P3HT and posses
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good stability than P3HT. Recently, Sharma et al have used this copolymer as donor material for polymer solar cell along with PCBM and achieved a PCE of 3.45-5.30%.20 These demonstrations indicate that the P can be used as HTM for hybrid solar cells. In the present communication, we have used D-A conjugated copolymer P as HTM to investigate the solid state hybrid solar cells consisting of CH3NH3PbI3 as the light harvester and mesoporous TiO2 as photoanode. We have also fabricated the solar cells using P3HT as
HTM for comparison. The inorganic-organic hybrid solar cell with P as hole transport material (PCE=6.64 %), showed higher PCE than that for P3HT (PCE=4.24 %) and attributed to the higher hole mobility of P as compared to P3HT. The enhancement in Voc and FF for cell with P has been related to the deeper HOMO energy level of P and efficient hole transport, respectively. 2.
Experimental details
Perovskite was synthesized following the reported procedure.6 The P3HT was purchased from Aldrich and copolymer was synthesized as reported earlier.20 2.1 Device fabrication and characterization
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A 40 nm thick dense blocking layer of TiO2 was deposited onto the pre-cleaned FTO substrate by spraying pyrolysis deposition carried out using a 20 mM titanium diisopropoxide
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bis(acetylacetone) solution (Aldrich) at 450 C to prevent direct contact between FTO and HTM. The mesoporous TiO2 films were coated from a TiO2 paste (dye Sol 18NT-T paste
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diluted in anhydrous ethanol) by doctor blade technique. The films were dried at 80 C for 1 min and then annealed at 450 C for 30 min. The films were then treated in 40 mM of TiCl4
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aqueous solution at 60 C for 1 hr and heat treatment at 450 C for 30 min to improve the
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interfacial contact with nanocrystalline TiO2. The perovskite CH3NH3PbI3 solution was
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prepared as follow. The synthesized CH3NH3I powder was mixed with PbI2 at a 1:1 mol
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ratio in dimethylformamide (DMF) and stirred over night at 60 C, followed by filtering using a Whattman (0.45 m) filter paper. The prepared solution of perovskite was dripped on
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the top of TiO2 film and then spin cast at 2500 rpm for one minute. After that the perovskite
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sensitized TiO2 film was placed on a top of hot plate at 100 C for 45 min to form crystalline CH3NH3PbI3. The heat treatment is essential for a complete conversion of precursor to final
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organo lead halide. The heated CH3NH3PbI3 film exhibits better optical absorption and higher crystallinity.21 We have used P3HT (commercially available and used without any further
CC
purification)
and
copolymer
P.
The
polymer
HTMs
were
spin
coated
on
CH3NH3PbI3/TiO2/bl-TiO2 /FTO substrate at 2500 rpm for 30s using HTMs solution in
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chlorobenzene and then dried for 1 hr. Finally a gold (Au) cathode electrode was deposited by thermal evaporation using a shadow mask to fix the active area of device around 0.20 cm2. The current-voltage (J-V) characteristics under illumination as well as in dark, of the devices were recorded using a Keithley source meter (model 2400). A xenon lamp coupled with AM 1.5 optical filter (100 mW/cm2) was used as light source for illumination. The incident photon to current conversion efficiency (IPCE) of the devices was measured using a
monochromator and xenon lamp as light source and resulting photocurrent was measured with source meter under short circuit condition. 3.
Results and discussion The optical UV-visible absorption spectra of perovskite CH3NH3PbI3 thin film is
shown in Fig. 2. It can be seen from this figure that the thin film of synthesized perovskite (annealed at 100 C) absorbs a wide range of light from visible to the near infrared with an absorption onset at 790 nm, indicating the formation of CH3NH3PbI3 film on the substrate22. The CH3NH3PbI3 film exhibits an absorption peak around 370 nm and a shoulder band at 484 nm. The optical band gap estimated from the absorption onset wavelength is 1.57 eV is in
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agreement with the value reported in literature. We had also characterized the thin film of CH3NH3PbI3 by X-ray diffraction and shown in Fig. 3. The appearance of strong peaks at
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2=13.98, 28.32 and 31.74 corresponds to the (110), (220) and (310) planes, indicates the formation of the tetragonal perovskite structure.23
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We have used a low bandgap D-A copolymer with 4,8-bis-(5-bromothiophene-2-yl)benzo[1,2,5] thiadiazole as donor unit having HOMO and LUMO energy level of -5.24 eV
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and -3.48 eV, respectively. This copolymer showed a strong absorption band in 590-800 nm
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(Fig.2). The energy levels of copolymer P are well matched with the CH3NH3PbI3
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(conduction band and valance band edge were about -3.92 and -5.96 eV, respectively) for
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efficient charge separation. The valance band of this perovskite is lower than the HOMO of conjugated polymer P. Additionally, conduction band of CH3NH3PbI3 is sufficient higher
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than that of the TiO2 and lower than LUMO of P. Hence, this copolymer P may promote
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charge transport in the hybrid solar cells, when used as HTM. Fig. 4 shows the current-voltage (J-V) characteristics of the devices with and without
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P3HT and P as HTM, under illumination AM1.5 (100 mW/cm2) and the photovoltaic parameters are summarized in table 1. The cell with P as HTM, shows a PCE of 6.64 % with
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Jsc = 11.98 mA/cm2, Voc = 0.84 V and FF = 0.66, where as P3HT as HTM based cell exhibited a PCE of 4.24% with Jsc = 11.83 mA/cm2, Voc= 0.64 V and FF = 0.56. As can be
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seen from figure and table 1, the cell fabricated without HTM also exhibit relatively good photovoltaic response (Jsc =9.22 mA/cm2, Voc=0.62 V and FF= 0.48 and PCE = 2.74 %), though it is inferior to the cells with HTM due to decrease in the Jsc, Voc and FF. This confirms that photogeneration of charge carrier occurs at the TiO2/CH3NH3PbI3 heterojunction24 and the device acts as p-n junction solar cell. In this device, CH3NH3PbI3 acts simultaneously the roles of both light harvester and hoe transporter. Moreover, the
interface between perovskite/ HTM is beneficial to the hole transfer and leading to the higher value of Jsc and FF. As can be seen from the absorption spectra of P3HT and P that these HTM exhibit absorption peak at 520 nm and 595 nm respectively. The IPCE spectra of the devices based with P3HT and P HTM and without HTM are shown in Fig. 5. The spectral overlap between the HTM and perovskite often reduces the light harvesting efficiency of the perovskite and thus loss in the photocurrent of the device. The IPCE spectra of the devices closely matched with the absorption spectra of the perovskite, indicating that in spite of the absorption peak of P3HT and P are about 520 nm and 595 nm, respectively, the polymers does not contribute to
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the photocurrent. Moreover, the similar shape of IPCE spectra for both devices attributed that light absorption by the polymer is negligible and the role of these polymers is limited to the
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charge transporting layer. The cell with P yields relatively higher IPCE values at most of the wavelengths than the cell with P3HT. This finding is consistent with the slightly higher value
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of Jsc for former device, obtained from J-V characteristics, under illumination. The value of Jsc for device based on P is slightly higher than that for P3HT counter
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part, whereas Voc and FF are significantly higher the device based on P. It is reported that the
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Voc of a hybrid solar cell is closely related to the difference between the quasi Fermi level of
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electrons in TiO2 (n-type semiconductor), under illumination and HOMO level of the p-type
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HTM. The conjugated copolymer P has a HOMO energy level of -5.24 eV is deeper than that of P3HT (-5.1 eV), resulting higher value of Voc for the cell based on P as HTM. The
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difference in the HOMO level of P and P3HT is only about 0.14 eV, but the difference in the
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Voc is about 0.20 V. Therefore, the Voc may be attributed to the charge recombination kinetics. The J-V characteristics of the devices in dark (Fig. 6) can give the information of
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charge recombination kinetics in the device. As shown in Figure the device with P has relatively lower dark current and an upper shift in onset, compared to P3HT. This implies that
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P has a superior electron blocking ability than P3HT. Since the dark current is the measure of the recombination current. The small dark current means reduction in the back electron
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recombination. This reduction pushes the quasi Fermi level of TiO2 upwards (more negative), thereby enlarging the difference between the HOMO of P and quasi Fermi level of TiO2, resulting contribution to the enhancement in Voc. Moreover, the high value of Voc for P as compared to P3HT is in agreement with the relative difference in the HOMO levels of P and P3HT because the energy level offset between the HOMO level of donor (HTM) and the LUMO level of acceptor (perovskite) modulates the magnitude of Voc.
It is worth noting that the Jsc of the P based cell is slightly higher than that for P3HT despite of small energetic driving force of regeneration of oxidized perovskite for former device as compared P3HT. Since a driving force of 0.30 mV is sufficient for the regeneration of oxidized perovskite, the off set between the HOMO level of P and valance band edge of perovskite is sufficient for regeneration but lower than that for device based on P3HT. As the back charge recombination for device based on P is suppressed, we explain the slight increase in Jsc by assuming that slow regeneration rate (transfer of electron from P to perovskite) was compensated by the fast charge transport in P HTM medium as compared to P3HT. The difference in the FF can be understand with the assumptions described by Burschka et al., who demonstrated that the conductivity (charge carrier mobility) of an organic semiconductor
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is important for achieving a high photovoltaic performance of the device.25
We have
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measured the charge carrier mobility by fitting J-V characteristics in dark with space charge current limited model. The hole mobility of P is about 6.68 x10-4 cm2/Vs which is higher than
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that for P3HT (2.45 x10-4 cm2/Vs). Therefore, the P exhibited a higher conductivity than P3HT indicates that conjugated polymer P facilities hole transport more efficiently that
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P3HT.26 The FF is related to the series resistance (Rs) and shunt resistance (Rsh). In order to
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obtain a high FF solar cell should exhibits a low Rs and high Rsh. The main difference in the
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devices investigated here is the HTM. This means that the FF value can be partly ascribed to
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the increased hole transporting and electron blocking ability of HTM, which could suppress the possibility for the photogenerated charges to recombine before they reach to the
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collecting electrode. Moreover, the space charge effect in reduced in the TiO2/perovskite/P
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junction as compared to the TiO2/perovskite/P3HT junction due to the small difference in the mobility between electron and hole. It can be also seen from the J-V characteristics under
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illumination, the series resistance (reciprocal of slope of J-V characteristics at Voc) is lower for the device with P as compared to P3HT counterpart also consistent with the fact that the
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hole mobility for P is higher than that for P3HT. The higher value of FF for solar cell with P compared to P3HT counterpart may be ascribed to the reduced series resistance and fast
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charge transport. Moreover, the space charge effect in reduced in the TiO2/perovskite/P junction as compared to the TiO2/perovskite/P3HT junction due to the small difference in the mobility between electron and hole, resulting balanced charge transport6,27 leading to an increase in the FF and PCE. 4.
Conclusion In summary, we have fabricated a solid state organic-inorganic hybrid solar cells
based on a CH3NH3PbI3 perovskite as light harvester and P3HT and D-A conjugated
copolymer P as hole transporting materials. The solar cell based on P showed a considerable higher performance than that for P3HT. The Jsc of the solar cell based on P is slightly higher than that for P3HT, but improvement in PCE was attributed to the mainly due to the enhanced Voc and FF. Acknowledgment We are thankful to Prof. M.L. Keshtov, Institute of Organoelement Compounds of the Russian Academy of Sciences, Vavilova, Moscow, for providing the copolymer. We are also thankful to Dr. S. Biswas, Department of physics, LNMIIT, Jaipur for providing the facilities for device fabrication and their characterization. PN thanks to CSIR for providing senior
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research fellowship. SPS thanks to XII FY CSIR-INTELCOAT (CSC0114) for financial support.
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26 (a) Snaith, H. J.; Grätzel, M. Enhanced Charge Mobility in a Molecular Hole Transporter via Addition of Redox Inactive Ionic Dopant: Implication to DyeSensitized Solar Cells. Appl. Phys. Lett. 2006, 89, 262114, (b) Scholin, R.; Karlsson, M. H.; Eriksson, S. K.; Siegbahn, H.; Johnsson, E. M. J.; Rensmo, H. Energy Level Shifts in Spiro-OMeTAD Molecular Thin Films When Adding Li-TFSI. J. Phys. Chem. C 2012, 116, 26300–26305.
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27 L. J. A. Koster, V. D. Mihailetchi, H. Xie and P. W. M. Blom, Appl. Phys. Lett., 2005, 87, 203502
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Table 1 Photovoltaic performance of devices Jsc (mA/cm2)
Voc (V)
FF
PCE
Without HTM
9.22
0.62
0.48
2.74
P3HT
11.83
0.64
0.56
4.24
P
11.98
0.84
0.66
6.64
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D
M
A
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Fig. 1 Chemical structure of D--A conjugated copolymer P and P3HT
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Fig. 2 Normalized absorption spectra of CH3NH3PbI3, P and P3HT
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Fig. 3 XRD pattern of CH3NH3PbI3 film
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Fig. 4 Current –voltage characteristics of the device using P, P3HT HTM and without HTM, under illumination (100 mW/cm2)
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Fig. 5 IPCE spectra of solar cells with P and P3HT HTMs and without HTM
Fig. 6 Current –voltage characteristics of the device in dark using P and P3HT as HTM
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CH3NH3PbI3 Perovskite Sensitized Solar Cells Using a D-A Copolymer as Hole Transport Material P. Nagarjuna, K. Narayanaswamy, T. Swetha, G. Hanumantha Rao, Surya Prakash Singh and G. D. Sharma
S0013-4686(14)02180-X http://dx.doi.org/doi:10.1016/j.electacta.2014.11.003 EA 23675
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
12-8-2014 30-10-2014 1-11-2014
Please cite this article as: P.Nagarjuna, K.Narayanaswamy, T.Swetha, G.Hanumantha Rao, Surya Prakash Singh, G.D.Sharma, CH3NH3PbI3 Perovskite Sensitized Solar Cells Using a D-A Copolymer as Hole Transport Material, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2014.11.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
CH3NH3PbI3 Perovskite Sensitized Solar Cells Using a D-A Copolymer as Hole Transport Material P. Nagarjuna1, K. Narayanaswamy1, T. Swetha1, G. Hanumantha Rao1, Surya Prakash Singh1* and G. D. Sharma2* 1
Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal road, Tarnaka, Hyderabad-500007, India 2
R & D Center for Engineering and Science, JEC group of Colleges, Jaipur Engineering College, Kukas, Jaipur 303101, India Graphical abstract fx1
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conjugated
D-A
copolymer
(P)
having
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Abstract benzodithiophene
donor
and
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benzo[1,2,5]thiadiazole acceptor was employed as a p-type hole transporting material in solid
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state organic–inorganic hybrid solar cells and compared with the P3HT hole transporting material. In these device we have used organo-lead halide (CH3NH3PbI3) synthesized by us,
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as light harvester. The power conversion efficiency (PCE) of 6.64 % is achieved for the solar
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cell with P which is higher than that for P3HT (4.24 %). The increase in PCE is mainly due to
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the enhancement in FF and Voc and attributed to higher mobility of hole for conjugated
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copolymer than P3HT.
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Key words: Inorganic-organic hybrid solar cells, perovskite, donor--acceptor, hole transport material, power conversion efficiency.
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* Corresponding authors
[email protected] (Surya Prakash Singh)
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Tel: +91-40-27191710; Fax: +91-40-27160921
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[email protected] (G. D. Sharma)
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1. Introduction Dye sensitized solar cells received considerable attention since the first report on 1991 by
Grätzel and his coworkers, due to the excellent power conversion efficiency (PCE) of about 7% at a relatively low cost of fabrication1 and are alternative to the conventional silicon based solar cells. A typical DSSC uses a monolayer of light absorbing sensitizer anchored on nanocrystalline TiO2 to enhance light harvesting in a mesoporous environment in which
charge separation also occurs at this surface. Lot of research has been carried out to enhance the PCE of these devices. As a result, PCE of 12.3 % has been reported in 2011, by Grätzel research group for DSSCs based on an ortho-substituted porphyrin sensitizer, YD2-oC8, cosensitized with organic dye (Y123) with a cobalt electrolyte2 which opened a new research area on porphyrin sensitized DSSCs.3 However, the absorption spectrum of this porphyrin dye covers only the visible spectral region and lack of light harvesting ability beyond 700 nm limits the further enhancement in PCE for the DSSCs based on porphyrin dyes. Moreover, the long term stability of these devices might be an issue for future commercialization due to the use of volatile electrolytes. Various inorganic light absorbers with light harvesting capability
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that extends into the near infrared region were hence sought to replace these organic dyes. Recent studies have reported that organometal halide perovskite mesoscopic solar
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cells with promising PCEs due to their excellent light harvesting and electron transporting properties and are perfectly suitable for use as prospective photovoltaic materials. In 2009,
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Miyasaka et al. reported the first perovskite (CH3NH3PbBr3) sensitized TiO2 solar cell using liquid electrolytes based on iodide and bromide and achieved a PCE of 3.1 % under one sun
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illumination and 3.8 % when CH3NH3PbBr3 was replaced by CH3NH3PbI3.4 Later on Park et
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al.5 in 2011, reported improved PCE of 6.5 % for modified TiO2 solar cells sensitized with
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CH3NH3PbI3 perovskite and an electrolyte based on iodide. They have used Pb(NO3) for
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modify the surface of mesoporous TiO2 film before coating of perovskite film and acts as blocking layer in the solar cells. However, it was observed that the performance of devices
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based on perovskite materials and liquid electrolyte decreased by 80 % in only few hours due
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to the instability of the perovskite material in the presence of liquid electrolyte due to the dissolution of perovskite. Hence it is essential to prepare perovskite based solar cells with a
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solvent free hole transport material (HTM) and called as solid state organic-inorganic hybrid solar cells. Park and coworkers6 reported sub-micrometer thin film solid state solar cells 7
Snaith and co-
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attaining a PCE of 9.7 % with a CH3NH3PbI3 in mesoporous TiO2 film.
workers reported a similar perovskite, CH3NH3PbI3Cl that served as a light absorber for
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mesoscopic thin film solid state solar cells to attain a PCE of 10.9%, for which the mesoporous Al2O3 film served as scaffold to replace the n-type TiO2 electron transporting layer. Recently, same group reported a significant enhanced PCE of 12.3% 8 for perovskite CH3NH3PbI3-xClx solar cells, with the same device structure based on Al2O3. Soak et al. reported almost same PCE for solar cell based on another perovskite CH3NH3Pb(I1-xBrx)3, with the mesoporous TiO2 photoanode.9 Recently, in 2013, Grätzel and co-workers, reported perovskite CH3NH3PbI3 sensitized TiO2 solar cells with a PCE approaching to 15 % which
set a new record for all solid state hybrid mesoscopic solar cells.10 Therefore, this impressive PCE opened new channel for the development of third generation solar cells based on perovskite and HTM with the advantages of very high PCE, low cost, ease of processing and considerable durability11 and efficiency approaching 20% is realistically possible from a solid-state mesoscopic solar cell based on CH3NH3PbX3 (X = Cl, Br, or I).12 In all above perovskite solar cells, a HTM [2,2’,7,7’-tetrakis-(N,N-di-pmethoxyphenylamine) 9,9’-spirobifluorene] (spiro-OMeTAD) has been employed. However, conductivity of spiro-OMeTAD is low (~10-5 S/cm). Moreover, the spirobifluorene core in spiro-OMeTAD molecule is relative expensive due to extensive synthetic processes for preparation. So it would be
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important to develop a cheaper alternative of spiro-OMeTAD for commercial application of hybrid perovskite solar cells.
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Alternative HTM materials with excellent electrical properties may be used in place of spiro-OMeTAD. Soluble donor--acceptor (D--A) can be the alternative choice as HTM.
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Conjugated polymers have been widely used in the field of polymer solar cells because of their easy processability, low cost and mechanical flexibility.13 The solar cells using the
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combination of perovskite with mesoporous TiO2 and P3HT as hole transport material, PCEs
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of 3.8 %, 4.5 %14 and 6.7 %15 have been reported and recently a PCE of 10.8 % have been
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achieved by Coning et al. for CH3NH3PbI2Cl perovskite.16
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To obtain high photovoltaic performance, it is importance that the HTM should show a good hole mobility and compatible highest occupied molecular orbital (HOMO) energy
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level to the semiconductor light absorber. Recently, Qiu, have used a p-type low band gap
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conjugated D-A copolymer PCBTDPP as HTM and achieve an outstanding open circuit voltage of about 1.15 V for CH3NH3PbBr3 sensitized hybrid solar cells and CH3NH3PbI3
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sensitized hybrid solar cell showed a PCE of 5.55% along with good stability.17 Heo et al have reported a ~12 %18 PCE in an organic-inorganic hybrid solar cell using a poly
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(triarylamine) (PTAA) HTM with an average PCE of 9.7% 19 The D-A conjugated polymer P (chemical structure shown in Figure 1) have the deeper HOMO level than P3HT and posses
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good stability than P3HT. Recently, Sharma et al have used this copolymer as donor material for polymer solar cell along with PCBM and achieved a PCE of 3.45-5.30%.20 These demonstrations indicate that the P can be used as HTM for hybrid solar cells. In the present communication, we have used D-A conjugated copolymer P as HTM to investigate the solid state hybrid solar cells consisting of CH3NH3PbI3 as the light harvester and mesoporous TiO2 as photoanode. We have also fabricated the solar cells using P3HT as
HTM for comparison. The inorganic-organic hybrid solar cell with P as hole transport material (PCE=6.64 %), showed higher PCE than that for P3HT (PCE=4.24 %) and attributed to the higher hole mobility of P as compared to P3HT. The enhancement in Voc and FF for cell with P has been related to the deeper HOMO energy level of P and efficient hole transport, respectively. 2.
Experimental details
Perovskite was synthesized following the reported procedure.6 The P3HT was purchased from Aldrich and copolymer was synthesized as reported earlier.20 2.1 Device fabrication and characterization
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A 40 nm thick dense blocking layer of TiO2 was deposited onto the pre-cleaned FTO substrate by spraying pyrolysis deposition carried out using a 20 mM titanium diisopropoxide
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bis(acetylacetone) solution (Aldrich) at 450 C to prevent direct contact between FTO and HTM. The mesoporous TiO2 films were coated from a TiO2 paste (dye Sol 18NT-T paste
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diluted in anhydrous ethanol) by doctor blade technique. The films were dried at 80 C for 1 min and then annealed at 450 C for 30 min. The films were then treated in 40 mM of TiCl4
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aqueous solution at 60 C for 1 hr and heat treatment at 450 C for 30 min to improve the
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interfacial contact with nanocrystalline TiO2. The perovskite CH3NH3PbI3 solution was
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prepared as follow. The synthesized CH3NH3I powder was mixed with PbI2 at a 1:1 mol
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ratio in dimethylformamide (DMF) and stirred over night at 60 C, followed by filtering using a Whattman (0.45 m) filter paper. The prepared solution of perovskite was dripped on
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the top of TiO2 film and then spin cast at 2500 rpm for one minute. After that the perovskite
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sensitized TiO2 film was placed on a top of hot plate at 100 C for 45 min to form crystalline CH3NH3PbI3. The heat treatment is essential for a complete conversion of precursor to final
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organo lead halide. The heated CH3NH3PbI3 film exhibits better optical absorption and higher crystallinity.21 We have used P3HT (commercially available and used without any further
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purification)
and
copolymer
P.
The
polymer
HTMs
were
spin
coated
on
CH3NH3PbI3/TiO2/bl-TiO2 /FTO substrate at 2500 rpm for 30s using HTMs solution in
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chlorobenzene and then dried for 1 hr. Finally a gold (Au) cathode electrode was deposited by thermal evaporation using a shadow mask to fix the active area of device around 0.20 cm2. The current-voltage (J-V) characteristics under illumination as well as in dark, of the devices were recorded using a Keithley source meter (model 2400). A xenon lamp coupled with AM 1.5 optical filter (100 mW/cm2) was used as light source for illumination. The incident photon to current conversion efficiency (IPCE) of the devices was measured using a
monochromator and xenon lamp as light source and resulting photocurrent was measured with source meter under short circuit condition. 3.
Results and discussion The optical UV-visible absorption spectra of perovskite CH3NH3PbI3 thin film is
shown in Fig. 2. It can be seen from this figure that the thin film of synthesized perovskite (annealed at 100 C) absorbs a wide range of light from visible to the near infrared with an absorption onset at 790 nm, indicating the formation of CH3NH3PbI3 film on the substrate22. The CH3NH3PbI3 film exhibits an absorption peak around 370 nm and a shoulder band at 484 nm. The optical band gap estimated from the absorption onset wavelength is 1.57 eV is in
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agreement with the value reported in literature. We had also characterized the thin film of CH3NH3PbI3 by X-ray diffraction and shown in Fig. 3. The appearance of strong peaks at
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2=13.98, 28.32 and 31.74 corresponds to the (110), (220) and (310) planes, indicates the formation of the tetragonal perovskite structure.23
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We have used a low bandgap D-A copolymer with 4,8-bis-(5-bromothiophene-2-yl)benzo[1,2,5] thiadiazole as donor unit having HOMO and LUMO energy level of -5.24 eV
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and -3.48 eV, respectively. This copolymer showed a strong absorption band in 590-800 nm
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(Fig.2). The energy levels of copolymer P are well matched with the CH3NH3PbI3
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(conduction band and valance band edge were about -3.92 and -5.96 eV, respectively) for
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efficient charge separation. The valance band of this perovskite is lower than the HOMO of conjugated polymer P. Additionally, conduction band of CH3NH3PbI3 is sufficient higher
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than that of the TiO2 and lower than LUMO of P. Hence, this copolymer P may promote
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charge transport in the hybrid solar cells, when used as HTM. Fig. 4 shows the current-voltage (J-V) characteristics of the devices with and without
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P3HT and P as HTM, under illumination AM1.5 (100 mW/cm2) and the photovoltaic parameters are summarized in table 1. The cell with P as HTM, shows a PCE of 6.64 % with
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Jsc = 11.98 mA/cm2, Voc = 0.84 V and FF = 0.66, where as P3HT as HTM based cell exhibited a PCE of 4.24% with Jsc = 11.83 mA/cm2, Voc= 0.64 V and FF = 0.56. As can be
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seen from figure and table 1, the cell fabricated without HTM also exhibit relatively good photovoltaic response (Jsc =9.22 mA/cm2, Voc=0.62 V and FF= 0.48 and PCE = 2.74 %), though it is inferior to the cells with HTM due to decrease in the Jsc, Voc and FF. This confirms that photogeneration of charge carrier occurs at the TiO2/CH3NH3PbI3 heterojunction24 and the device acts as p-n junction solar cell. In this device, CH3NH3PbI3 acts simultaneously the roles of both light harvester and hoe transporter. Moreover, the
interface between perovskite/ HTM is beneficial to the hole transfer and leading to the higher value of Jsc and FF. As can be seen from the absorption spectra of P3HT and P that these HTM exhibit absorption peak at 520 nm and 595 nm respectively. The IPCE spectra of the devices based with P3HT and P HTM and without HTM are shown in Fig. 5. The spectral overlap between the HTM and perovskite often reduces the light harvesting efficiency of the perovskite and thus loss in the photocurrent of the device. The IPCE spectra of the devices closely matched with the absorption spectra of the perovskite, indicating that in spite of the absorption peak of P3HT and P are about 520 nm and 595 nm, respectively, the polymers does not contribute to
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the photocurrent. Moreover, the similar shape of IPCE spectra for both devices attributed that light absorption by the polymer is negligible and the role of these polymers is limited to the
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charge transporting layer. The cell with P yields relatively higher IPCE values at most of the wavelengths than the cell with P3HT. This finding is consistent with the slightly higher value
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of Jsc for former device, obtained from J-V characteristics, under illumination. The value of Jsc for device based on P is slightly higher than that for P3HT counter
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part, whereas Voc and FF are significantly higher the device based on P. It is reported that the
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Voc of a hybrid solar cell is closely related to the difference between the quasi Fermi level of
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electrons in TiO2 (n-type semiconductor), under illumination and HOMO level of the p-type
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HTM. The conjugated copolymer P has a HOMO energy level of -5.24 eV is deeper than that of P3HT (-5.1 eV), resulting higher value of Voc for the cell based on P as HTM. The
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difference in the HOMO level of P and P3HT is only about 0.14 eV, but the difference in the
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Voc is about 0.20 V. Therefore, the Voc may be attributed to the charge recombination kinetics. The J-V characteristics of the devices in dark (Fig. 6) can give the information of
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charge recombination kinetics in the device. As shown in Figure the device with P has relatively lower dark current and an upper shift in onset, compared to P3HT. This implies that
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P has a superior electron blocking ability than P3HT. Since the dark current is the measure of the recombination current. The small dark current means reduction in the back electron
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recombination. This reduction pushes the quasi Fermi level of TiO2 upwards (more negative), thereby enlarging the difference between the HOMO of P and quasi Fermi level of TiO2, resulting contribution to the enhancement in Voc. Moreover, the high value of Voc for P as compared to P3HT is in agreement with the relative difference in the HOMO levels of P and P3HT because the energy level offset between the HOMO level of donor (HTM) and the LUMO level of acceptor (perovskite) modulates the magnitude of Voc.
It is worth noting that the Jsc of the P based cell is slightly higher than that for P3HT despite of small energetic driving force of regeneration of oxidized perovskite for former device as compared P3HT. Since a driving force of 0.30 mV is sufficient for the regeneration of oxidized perovskite, the off set between the HOMO level of P and valance band edge of perovskite is sufficient for regeneration but lower than that for device based on P3HT. As the back charge recombination for device based on P is suppressed, we explain the slight increase in Jsc by assuming that slow regeneration rate (transfer of electron from P to perovskite) was compensated by the fast charge transport in P HTM medium as compared to P3HT. The difference in the FF can be understand with the assumptions described by Burschka et al., who demonstrated that the conductivity (charge carrier mobility) of an organic semiconductor
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is important for achieving a high photovoltaic performance of the device.25
We have
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measured the charge carrier mobility by fitting J-V characteristics in dark with space charge current limited model. The hole mobility of P is about 6.68 x10-4 cm2/Vs which is higher than
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that for P3HT (2.45 x10-4 cm2/Vs). Therefore, the P exhibited a higher conductivity than P3HT indicates that conjugated polymer P facilities hole transport more efficiently that
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P3HT.26 The FF is related to the series resistance (Rs) and shunt resistance (Rsh). In order to
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obtain a high FF solar cell should exhibits a low Rs and high Rsh. The main difference in the
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devices investigated here is the HTM. This means that the FF value can be partly ascribed to
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the increased hole transporting and electron blocking ability of HTM, which could suppress the possibility for the photogenerated charges to recombine before they reach to the
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collecting electrode. Moreover, the space charge effect in reduced in the TiO2/perovskite/P
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junction as compared to the TiO2/perovskite/P3HT junction due to the small difference in the mobility between electron and hole. It can be also seen from the J-V characteristics under
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illumination, the series resistance (reciprocal of slope of J-V characteristics at Voc) is lower for the device with P as compared to P3HT counterpart also consistent with the fact that the
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hole mobility for P is higher than that for P3HT. The higher value of FF for solar cell with P compared to P3HT counterpart may be ascribed to the reduced series resistance and fast
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charge transport. Moreover, the space charge effect in reduced in the TiO2/perovskite/P junction as compared to the TiO2/perovskite/P3HT junction due to the small difference in the mobility between electron and hole, resulting balanced charge transport6,27 leading to an increase in the FF and PCE. 4.
Conclusion In summary, we have fabricated a solid state organic-inorganic hybrid solar cells
based on a CH3NH3PbI3 perovskite as light harvester and P3HT and D-A conjugated
copolymer P as hole transporting materials. The solar cell based on P showed a considerable higher performance than that for P3HT. The Jsc of the solar cell based on P is slightly higher than that for P3HT, but improvement in PCE was attributed to the mainly due to the enhanced Voc and FF. Acknowledgment We are thankful to Prof. M.L. Keshtov, Institute of Organoelement Compounds of the Russian Academy of Sciences, Vavilova, Moscow, for providing the copolymer. We are also thankful to Dr. S. Biswas, Department of physics, LNMIIT, Jaipur for providing the facilities for device fabrication and their characterization. PN thanks to CSIR for providing senior
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research fellowship. SPS thanks to XII FY CSIR-INTELCOAT (CSC0114) for financial support.
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18 Heo, J. H.; Im, S. H.; Noh, J. H.; Mandal, T. N.; Lim, C. S., Chang, J. A.; Lee, Y. H.; Kim, H. J.; Sarkar, A.; Nazeeruddin, M. K.; Grätzel, M.; Seok, S. I. Efficient Inorganic–Organic Hybrid Heterojunction Solar Cells Containing Perovskite Compound and Polymeric Hole Conductors. Nat. Photonics 2013, 7, 486–491. 19 Bi, D. Q.; Yang, L.; Boschloo, G.; Hagfeldt, A.; Johansson, E. M. J. Effect of Different Hole Transport Materials on Recombination in CH3NH3PbI3 PerovskiteSensitized Mesoscopic Solar Cells. J. Phys. Chem. Lett. 2013, 4, 1532–1536.
20 Keshtov, M. L.; Marochkin, D. V.; Kochurov, V. S.; Khokhlov, A. R.; Koukaras, E. N.; Sharma, G. D. New Conjugated Alternating Benzodithiophene-Containing Copolymers with Different Acceptor Units: Synthesis and Photovoltaic Application. J. Mater. Chem. A 2014, 2, 155–171. 21 Amalie, D.; Nicolas, T.; Thomas, M.; Peng Gao.; Nazeeruddin, M. K.; Grätzel, M. Effect of Annealing Temperature on Film Morphology of Organic–Inorganic Hybrid Pervoskite Solid-State Solar Cells. Adv. Funct. Mater. doi: 10.1002/adfm.201304022. 22 Jun, Y. J.; Yi, F. C.; Mu, H. L.; Shin, R. P.; Tzung, F. G.; Peter, C.; Ten, C. W. CH 3 NH3 PbI3 Perovskite/Fullerene Planar-Heterojunction Hybrid Solar Cells. Adv. Mater. 2013, 25, 3727–3732.
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23 Chang, J. A.; Rhees, J. H.; Im, S. M.; Lee, Y. H.; Kim, H. J.; Seok, S. I.; Zazeeruddin, M. K.; Grätzel M. High-Performance Nanostructured Inorganic-Organic Heterojunction Solar Cells. Nano. Lett. 2010, 10, 2609–2612.
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24 Etgar L.; Gao, P.; Xue, Z.; Peng, Q.; Chandiran, A. K.; Liu, B.; Nazeeruddin, M. K.; Grätzel, M. Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells. J. Am. Chem. Soc. 2012, 134, 17396–17399.
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25 Burschka, J.; Dualeh, A.; Kessler, F.; Baranoff, E.; Cevey-Ha, N. L.; Yi, C. Y.; Nazeeruddin, M. K.; Grätzel, M. Tris(2-(1H-pyrazol-1-yl)Pyridine)Cobalt(III) as ptype Dopant for Organic Semiconductors and Its Application in Highly Efficient SolidState Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2011, 133, 18042-18045.
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D
M
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26 (a) Snaith, H. J.; Grätzel, M. Enhanced Charge Mobility in a Molecular Hole Transporter via Addition of Redox Inactive Ionic Dopant: Implication to DyeSensitized Solar Cells. Appl. Phys. Lett. 2006, 89, 262114, (b) Scholin, R.; Karlsson, M. H.; Eriksson, S. K.; Siegbahn, H.; Johnsson, E. M. J.; Rensmo, H. Energy Level Shifts in Spiro-OMeTAD Molecular Thin Films When Adding Li-TFSI. J. Phys. Chem. C 2012, 116, 26300–26305.
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27 L. J. A. Koster, V. D. Mihailetchi, H. Xie and P. W. M. Blom, Appl. Phys. Lett., 2005, 87, 203502
A
Table 1 Photovoltaic performance of devices Jsc (mA/cm2)
Voc (V)
FF
PCE
Without HTM
9.22
0.62
0.48
2.74
P3HT
11.83
0.64
0.56
4.24
P
11.98
0.84
0.66
6.64
PT
CC
EP
TE
D
M
A
N
U
SC
RI
Fig. 1 Chemical structure of D--A conjugated copolymer P and P3HT
A
Fig. 2 Normalized absorption spectra of CH3NH3PbI3, P and P3HT
RI
PT CC
EP
TE
D
M
A
N
U
SC
Fig. 3 XRD pattern of CH3NH3PbI3 film
A
Fig. 4 Current –voltage characteristics of the device using P, P3HT HTM and without HTM, under illumination (100 mW/cm2)
PT RI SC
A
CC
EP
TE
D
M
A
N
U
Fig. 5 IPCE spectra of solar cells with P and P3HT HTMs and without HTM
Fig. 6 Current –voltage characteristics of the device in dark using P and P3HT as HTM
TOC
A
CC
EP
TE
D
M
A
N
U
SC
RI
PT
CH3NH3PbI3 Perovskite Sensitized Solar Cells Using a D-A Copolymer as Hole Transport Material P. Nagarjuna, K. Narayanaswamy, T. Swetha, G. Hanumantha Rao, Surya Prakash Singh and G. D. Sharma