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Solar Energy 112 (2015) 12–19

Enhancing efficiency of perovskite solar cell viasurface microstructuring: Superior grain growth

and light harvesting effect

Mukta C. Tathavadekar a,b, Shruti A. Agarkar a,b, Onkar S. Game a,b, Umesh P. Bansode a,b,Sneha A. Kulkarni c, Subodh G. Mhaisalkar c,⇑, Satishchandra B. Ogale a,b,⇑

a Center of Excellence in Solar Energy, Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune 411008, Indiab Academy of Scientific and Innovative Research, Anusandhan Bhawan and Network Institute of Solar Energy (CSIR-NISE), New Delhi, India

c Energy Research Institute, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore

Received 25 June 2014; received in revised form 30 October 2014; accepted 16 November 2014

Communicated by: Associate Editor Hari Mohan Upadhyaya

Abstract

We have introduced a novel approach to enhance the perovskite solar cell efficiency by controlling the grain growth and light har-vesting properties of perovskite crystallites. Instead of using a mesoporous TiO2 layer, we have modified the surface microstructuringof the TiO2 film by dispensing nano assembled TiO2 submicron structures (nanobeads, NBs) on TiO2 compact layer. With this newapproach solar cell efficiency was improved significantly through an increase in both Jsc and Voc. This high efficiency is attributed tocrystallite size of the perovskite phase. These also act as light scattering centers giving higher current density and reduced recombinationeffects giving higher open circuit voltage.� 2014 Elsevier Ltd. All rights reserved.

Keywords: Perovskite solar cells; TiO2 nanobeads; Grain growth; Light harvesting

1. Introduction

The discovery of Dye Sensitized Solar Cells (DSSCs) in1991 by Gratzel and co-workers was a crucial break-through in the world of photovoltaics (O’Regan andGratzel, 1991). Significant collective efforts on various sen-sitizers (Mathew et al., 2014; Gao et al., 2008), co-adsorb-ers (Allegrucci et al., 2009), co-sensitizers (Kuang et al.,2007), new counter electrodes (Zhang et al., 2012;

http://dx.doi.org/10.1016/j.solener.2014.11.016

0038-092X/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors.E-mail addresses: subodh@ntu.edu.sg (S.G. Mhaisalkar), sb.oga-

le@ncl.res.in (S.B. Ogale).

Tathavadekar et al., 2014), new redox electrolytes (Feldtet al., 2010), etc. over the past 20 years have not onlypushed the efficiencies higher but have also brought outseveral new ways of making robust and durable DSSCs.A significant component of this research has focused onthe use of different inorganic oxide morphologies like hier-archical spheres (Liao et al., 2011; Koo et al., 2008), meso-porous nanobeads (Sauvage et al., 2010; Archana et al.,2013; Yang et al., 2010; Chen et al., 2009), nanofibers(Chuangchote et al., 2008; Shengyuan et al., 2011;Naphade et al., 2014), etc. to enhance the efficiency by tak-ing advantage of the high surface area and enhanced lightscattering abilities. High DSSC conversion efficienciesusing spherical morphologies are reported by Koo et al.

M.C. Tathavadekar et al. / Solar Energy 112 (2015) 12–19 13

(2008) and Sauvage et al. (2010). The work by Koo et al.elaborates the use of nano-embossed spherical hollowTiO2 exhibiting bifunctional properties of efficient genera-tion of photo-excited electrons as well as good light scatter-ing property to yield an impressive efficiency of 10.34%with ruthenium N719 sensitizer (Koo et al., 2008). Sauvageet al. used the photoanode made up of submicron size TiO2

nanobeads which gave an efficiency of 10.6% over a con-ventional P25 photoanode (8.5%) with C101 sensitizer(Sauvage et al., 2010). Significant increase in performanceof the cell in both the cases was mainly attributed to thehigh surface area and superior light scattering abilities ofthe particles leading to improved current density values.

However, the DSSC architecture has some drawbacks ofuse of liquid electrolyte, hence research is being pursued onmaking DSSCs with solid hole transporting materials(HTMs) such as CuI, and CuSCN which have moderatehole mobility. Unfortunately, there is a problem of infiltra-tion of such HTMs into the mesoporous TiO2 matrix(Hagfeldt et al., 2010; O’Regan et al., 2002). Some recentnoteworthy research by Chung et al. however showed thatthe use of CsSnI3 (a p-type direct band gap inorganicperovskite semiconductor with high hole mobility) canyield a fairly high DSSC efficiency of 10.8% (Chunget al., 2012). A major breakthrough was reported recentlyin the domain of sensitized solar cells with the use of org-ano-metal halide perovskite (Kazim et al., 2014; Nohet al., 2013; Noel et al., 2014). Indeed, the organo-metallichalide perovskite materials have since gained significantimportance as light harvesters in mesoscopic solar cellsdue to their large absorption coefficient, high charge carriermobilities (Xing et al., 2013), solution processability, andtunable optical (Xing et al., 2014; Boix et al., 2014;Kulkarni et al., 2014) and electronic (Kazim et al., 2014)properties.

The early efforts by Kojima et al. (2009) and Im et al.(2011) on the use of organometal halide perovskite insolar cells were mainly focused on their use as sensitizerin liquid electrolyte based sensitized solar cells whichcould give power conversion efficiencies up to 5–6%.Subsequently, a remarkable efficiency of 9.7% wasreported by Park and coworkers using CH3NH3PbI3 asa light absorber deposited on a sub-micrometer thick(0.6 lm) mesoporous TiO2 film and spiro-MeOTAD asHTM (Kim et al., 2012). These devices however sufferedfrom poor fill factor due to poor charge transport prop-erties of spiro-MeOTAD. Gratzel and coworkers laterintroduced p-type dopant in the form of a cobalt com-plex to improve the charge transport properties ofspiro-MeOTAD (Noh et al., 2013). Snaith and coworkersshowed a record efficiency of 10.9% with Voc of 1.1 V byreplacing the mesoporous TiO2 with an insulating meso-porous Al2O3 (Ball et al., 2013). The mesoporous Al2O3

acted as a scaffold for a few nanometer thin layer ofCH3NH3PbI2Cl transporting electronic charges out ofthe device through FTO anode while the spiro-MeOTADcollected the holes. Here mixed halide perovskite CH3-

NH3PbI2Cl was used, which acted as both a light absor-ber as well as an electron transporter. Liu and Kellyhave recently shown room temperature processed ZnObased perovskite solar cells with a robust and reproduc-ible efficiency of 15.7% (Liu and Kelly, 2014).

The morphology of perovskite formed within the meso-porous metal oxide film or within the planar heterostruc-ture architecture has been shown to dramatically affectthe photovoltaic performance (Kim et al., 2012; Liu andKelly, 2014; Heo et al., 2013; Burschka et al., 2013;Saliba et al., 2014). The earlier method by Gratzel andco-workers for perovskite formation within TiO2/Al2O3

mesoporous film included spin coating of premixed solu-tions of organic and inorganic counterparts followed byheating (Kim et al., 2012). This approach gave a non-uni-form nano-pillar type growth of perovskite over mesopor-ous TiO2 (Heo et al., 2013). Later the same group showed auniform growth of perovskite within mesopores of metaloxide via sequential deposition route which gave an out-standing efficiency of 15% in all-solution processed perov-skite solar cells (Burschka et al., 2013). Very recentlySnaith and coworkers have shown that the perovskite crys-tal domain size and subsequent thermal annealing play acrucial role in CH3NH3PbI2Cl mixed halide perovskitebased mesoporous as well as planar architecture solar cells(Saliba et al., 2014).

Various efforts are currently being directed on improv-ing the efficiency of these perovskite solar cells whichinclude use of 1D structures (Kim et al., 2013), goldplasmonic effects (Zhang et al., 2013), making the perov-skite cells more cost effective by reducing the processingtemperature etc. Dharani et al. made use of TiO2 nano-fiber network for the growth of perovskite using sequen-tial deposition process. The open porosity of the TiO2

nanofiber network was found to be responsible for goodcontacts across the TiO2/perovskite/spiro-MeOTADinterfaces leading to an efficiency of 9.8% (Dharaniet al., 2014).

Herein we introduce an interesting new variant for theperovskite cell architecture wherein we have dispensednano-assembled light harvesting TiO2 submicron struc-tures (nanobeads, NBs, 300 ± 100 nm size) on the TiO2

bottom compact layer (instead of the usual 20–30 nmnanoparticulate layer, NPs). As stated earlier, use of suchNBs has been reported earlier (Sauvage et al., 2010;Archana et al., 2013; Yang et al., 2010; Chen et al.,2009) in the case of DSSCs as high surface area lightharvesting centers. In this work we show that the useof such NBs adds another advantage in the context ofperovskite cell design, namely a dramatic enhancementof the perovskite grain size, leading to much improvedsolar cell performance (efficiency increase by about17%, through an increase in both Jsc and Voc). Highermean perovskite grain size reduces the interface densityand facilitates carrier transport. We discuss the possiblereasons for the enhanced quality factors for the celldue to such surface micro-structuring of the TiO2 layer.

14 M.C. Tathavadekar et al. / Solar Energy 112 (2015) 12–19

2. Experimental methods

2.1. Synthesis of TiO2 NBs

TiO2 nanobeads (NBs, nanoparticle-assembled submi-cron spheres) were synthesized by a procedure reportedby Chen et al. (2010). This synthesis protocol involvessol–gel process followed by solvothermal treatment. Inbrief, Hexadecylamine (HDA) was used as a surface direct-ing agent which was added to titanium butoxide to yieldhigh surface area porous beads. A paste was made usingethyl cellulose and a-terpineol by the previously reportedmethod (Ito et al., 2007). In order to make a film, this pastewas diluted with ethanol (1:3.5 wt/wt) and used for furtherexperiments.

2.2. Solar cell device fabrication

First, fluorine doped tin oxide (FTO, <14 X/sq., 2.2 mmthick) on glass, used as a substrate, was chemically etchedinto the required device pattern using Zn powder anddiluted HCl. Then it was cleaned thoroughly with soapsolution, deionized water, and finally with ethanol. Ablocking layer of TiO2 having thickness of around100 nm was formed on FTO by spray pyrolysis at 450 �Cusing a solution of titanium di-isopropoxide bis acetyl acet-onate in ethanol (1:9 by volume). This blocking layer wassintered at 450 �C for 30 min followed by 40 mM TiCl4treatment at 70 �C for 30 min to ensure that no pinholesremain. A layer of TiO2 (NBs) or TiO2 nanoparticles(NPs) (in separate samples) were deposited on the blockinglayer by spin coating of the respective nanoparticle disper-sions at 4000 rpm. For making the TiO2 NP film, commer-cially available TiO2 NP paste Dyesol 18NRT was dilutedwith ethanol (1:3.5 wt/wt) and used. These substrates weredried at 150 �C and sintered at 500 �C for 30 min. The aver-age thickness of TiO2 NB film was 400 ± 50 nm and that ofTiO2 NP was around 375 ± 25 nm.

A hybrid perovskite, CH3NH3PbI3, was deposited ontothe TiO2 films as reported in the literature (Burschka et al.,2013). In this sequential deposition process 1 M PbI2 solu-tion in DMF (N,N-dimethyl formamide) was spin coatedat 6000 rpm for 5 s on the films followed by film dryingat 70 �C for 30 min. These films were then dipped in a8 mg/mL solution of CH3NH3I in iso-propanol for20 min. The films were washed with iso-propanol and driedat 70 �C for 30 min.

Hole transporting material (HTM) was prepared bydoping 10 wt.% of Co(III) complex (FK-102) in a solutionof spiro-MeOTAD (2,20,7,70-tetrakis(N,N-di-p-methoxy-phenylamine)-9,90-spiro bifluorene) in chlorobenzene(120 mg/mL). In the above solution, 24 lL of lithiumbis(trifluoromethanesulfonyl)imide dissolved in acetonitrile(520 mg/mL) and 37 lL of ter-butylpyridine were added.The HTM was then spin coated on the perovskite layerat 4000 rpm for 30 s. For the top contact, �100 nm of goldwas deposited by thermal evaporation method on the

masked substrates. The active area of the cell was kept at0.2 cm2 and rest of the area was masked with a black tape.The cells made using TiO2 NB and TiO2 NP were named asNB cells and NP cells, respectively.

2.3. Characterization

Morphology of TiO2 nanobeads is observed by trans-mission electron microscope (TEM) using Technai 300.Field emission scanning electron microscope (FESEM)images were recorded to observe perovskite growth andcross section of the solar cell device using JEOL, JSM-7600F, 5 kV and FEI, Nova NanoSEM 450. I–V character-istics of the solar cell were measured under AM 1.5 G usingSan-EI Electric, XEC-301S. Incident photon to currentconversion efficiency (IPCE) was recorded using PVE300(Bentham) in DC mode and zero bias light was used.

3. Results and discussions

Fig. 1(a) shows the spherical morphology of the TiO2

NBs prepared by combining sol gel and solvothermalmethods (Chen et al., 2010). The XRD pattern of TiO2

NBs shown in supporting information Fig. S1 indicatespure anatase phase. These NBs have diameter300 ± 100 nm. It is clearly seen from the TEM images(Fig. 1(b) and (c)) that these TiO2 NBs are made frominterconnected small granular TiO2 particles (10–15 nm)exhibiting their porous nature. These porous NBs possessfairly high surface area of ca. 82 m2/g. From N2 adsorptiondata, a mean pore size for these particles is found out to beca. 12 nm. The surface of such nanoporous submicron par-ticles can provide for the nucleation of high quality CH3-

NH3PbI3 crystal growth.Using sequential deposition process (Burschka et al.,

2013), CH3NH3PbI3 was then grown on the TiO2 (NP orNB) films. The XRD patterns recorded for TiO2 NP andTiO2 NB films loaded with CH3NH3PbI3 are shown in sup-porting information Fig. S2. The peaks at 2h values of14.08�, 24.43�, 28.45�, 31.81�, 40.51�, 43.05� correspondto the (110), (202), (220), (310), (22 4), (314) planes ofthe desired perovskite structure having tetragonal phase(Qiu et al., 2013; Baikie et al., 2013).

The absorbance data recorded for the PbI2 and perov-skite loaded TiO2 films are shown in Fig. 2(a) and (b).The absorbance spectra for the case of PbI2 loaded onTiO2 films show the characteristic absorbance of PbI2 inthe region from 400 to 450 nm. It shows that the depositionof PbI2 in TiO2 NB film is considerably higher than TiO2

NP film. This increased PbI2 deposition is attributed tothe surface microstructuring of TiO2 film as discussed inthe next section. The absorption spectra of perovskiteloaded TiO2 films cover almost all the visible region shownin Fig. 2(b). The dramatically increased PbI2 deposition forthe TiO2 NB case ultimately leads to higher perovskite for-mation (with enhanced grain size as shown below) as con-firmed from the absorbance spectra of perovskite loaded

Fig. 1. (a)–(c) Show TEM images of TiO2 NBs for different magnifications.

(a) (b)

Fig. 2. (a) and (b) show the UV–Visible absorbance spectra of PbI2 loaded and CH3NH3PbI3 loaded TiO2 (NP and NB) films on FTO substrate,respectively.

M.C. Tathavadekar et al. / Solar Energy 112 (2015) 12–19 15

film. In the supporting information Fig. S3, we have alsoshown the transmittance and DRS comparison for PbI2

and CH3NH3PbI3 loaded TiO2 films. These data establishthe light harvesting and thereby enhanced light trappingcharacter of the perovskite grown TiO2 NB film.

The difference in the formation of PbI2 layer and ulti-mately perovskite crystal for the two types of TiO2 films(NP and NB) was analyzed using FESEM images. TheFESEM images of PbI2 deposited and CH3NH3PbI3

deposited films of TiO2 NP and TiO2 NB are shown inFig. 3(b), (e), (c), and (f), respectively. Fig. 3(d) showsthe FESEM image of the TiO2 NB film where TiO2 NBs

were dispensed on the blocking layer. A single layer ofNB particles (nanoparticulate monolayer kind of assembly)having an overall thickness of less than 500 nm was depos-ited. Making thicker layers was considered undesirablefrom the standpoint of carrier recombination since it even-tually leads to poor performance (Kim et al., 2013). FromFig. 3(d) it is clear the TiO2 NBs do not cover the entireavailable area, leaving some vacant spaces between anytwo TiO2 NB particles, where TiO2 blocking layer isexposed. This specific arrangement and also the rough sur-face topology of the film are beneficial for the formation oflarge perovskite crystallites. A mesoporous film is obtained

Fig. 3. FESEM images of the sample in different stages of the sequential deposition process. (a)–(c) represent the FESEM images of TiO2 NP film, TiO2

NP film coated with PbI2 and the CH3NH3PbI3 crystals grown on TiO2 NP film, respectively. Images in (d)–(f) represent the FESEM images of TiO2 NBfilm, TiO2 NB film coated with PbI2 and the CH3NH3PbI3 crystals grown on TiO2 NB film, respectively. Figures (g) and (h) show the cross-section imagesof the entire devices made using TiO2 NPs and TiO2 NBs, respectively.

16 M.C. Tathavadekar et al. / Solar Energy 112 (2015) 12–19

for the case of TiO2 NPs as shown in Fig. 3(a). Overallsmoothness of the film is much more as compared to theTiO2 NB case, as expected.

Fig. 3(b) and (e) shows the morphologies for PbI2

coated TiO2 NP and NB films, respectively. Because ofcomparatively smoother nature of the TiO2 NP film surfacethe resultant PbI2 layer is also flat. In the case of TiO2 NB,during spin coating of the PbI2 solution on the TiO2 film,the NBs act as an obstacle for uniform solution spreading.Hence, the arrangement of TiO2 NB creates PbI2 puddlesbetween NB inter-particle spaces. It is clearly seen thatthe PbI2 deposition occurs on the TiO2 NBs as well as onthe exposed TiO2 blocking layer. This leads to increasedPbI2 content in this NB case as compared to the standardTiO2 NP case, as confirmed by absorbance studies. Afterdipping the PbI2 loaded films in the solution of CH3NH3I,the perovskite CH3NH3PbI3 formed on the TiO2 NP andTiO2 NB films appears as shown in Fig. 3(c) and (f)respectively.

The size of perovskite crystals grown on the TiO2 NPfilm is in the range of 100 ± 50 nm, whereas it is in therange of 400 ± 100 nm for perovskite crystals grown onthe TiO2 NB film. Also from Fig. 3(f) it is clear that perov-skite formation takes place on the TiO2 surface as well as

on the TiO2 blocking layer where PbI2 puddles are formedduring spin coating. The enhanced grain growth of theperovskite crystallites in the NB case is attributed to higheramount of PbI2 retained on the TiO2 NB film as well as onthe specific morphology of the film. On the other hand theperovskite crystal size is smaller in case of TiO2 NP becauseof the relatively smoother nano-grained nature of the sur-face for PbI2 (and hence subsequent perovskite) growth.The advantage of larger perovskite crystallites formed onTiO2 NB film is that the grain boundary density is very lesswhich must contribute to reduced recombination at theTiO2/CH3NH3PbI3/HTM interface (Liu and Kelly, 2014).Also the large crystallite size should lead to scattering oflight, increasing the effective mean path of light in the filmwhich in turn can increase the current density (Liu andKelly, 2014).

Further, to fabricate solar cell devices, in each case alayer of HTM was spin coated on perovskite-loaded filmsunder identical experimental conditions and a top goldcontact was made. Fig. 3(g) and (h) shows the cross sectionimages of the devices fabricated using TiO2 NPs and TiO2

NBs. In the case of the device made using TiO2 NP, a TiO2-perovskite layer of �370 nm was deposited on which a�250 nm thick HTM overlayer was spin-coated. Finally

Table 1Solar cell parameters for the NP cell and NB.

Device Voc (V) Jsc (mA/cm2) FF % g

NP cell 0.9 13.1 0.65 7.7NB cell 0.99 17.4 0.52 9.0

M.C. Tathavadekar et al. / Solar Energy 112 (2015) 12–19 17

�100 nm gold layer was deposited as the top contact. Inthe case of TiO2 NB device, the TiO2 layer consists of singleTiO2 NB particles dispensed on the blocking layer, andtherefore the overall mean thickness of the film dependson the diameter of the TiO2 particle and the way they areassembled. With the dilution used for spin coating, thetotal mean thickness of the TiO2 NB-perovskite layer wasaround 350 ± 50 nm. Therefore a nearly 400 nm thickoverlayer of HTM was deposited on the same. Indeed,the NBs are dispensed as isolated submicron particulateson the surface and are not piled up. Also it clear that theTiO2 NB-perovskite layer appears to be rough, therebyincreasing contact area of the perovskite–HTM interface.Around 100 nm gold was deposited on the HTM overlayeras a top contact.

We made different sets of NP and NB based devices inseparate sample making protocols and compared their con-version efficiencies. The NP based devices showed efficiencyvalues in the range 7.7 ± 0.5%, while the NB based devicesshowed consistently higher efficiencies in the range9.8 ± 1.0%. The variability was noted to be somewhathigher for the NB case because the spin coating for nano-bead dispersed sol has higher variability of micro-particu-late distribution. In the following we present and discussthe data for one set, while in the supporting informationwe present data close to the best case scenario observedin terms of the enhancement of quality factors noted.

Fig. 4 shows the I–V characteristics for few solar celldevices made using TiO2 NPs and TiO2 NBs, named asNP cell and NB cell, respectively. These cells were madein separate batches using same materials and processingparameters retaining the peripheral conditions constant,to check reproducibility. Table 1 represents the solar cellparameters for a typical NP cell and NB cell made in onebatch. TiO2 NB cell exhibits an efficiency over 9% whichoutperforms the efficiency of 7.7% achieved for the TiO2

NP cell. In this case, the solar cell device fabricated usingTiO2 NB exhibits a remarkable increment of open circuitvoltage (Voc = 0.99 V) over that (0.9 V) obtained for thedevice fabricated using TiO2 NP. Larger Voc in NB cell isattributed to the larger CH3NH3PbI3 crystal size whichreduces the grain boundary density and in turn the charge

Fig. 4. Solar cell characteristic for NP cell and NB cell.

carrier recombination. The short circuit current density(Jsc) obtained for TiO2 NB cell is 17.4 mA/cm2 which isagain significantly higher than 13.1 mA/cm2 realized forthe TiO2 NP cell. This dramatic increase in Jsc for TiO2

NB cell is mainly because of the higher amount of CH3-

NH3PbI3 loading. Moreover, the bigger crystallite size alsoleads to enhanced scattering of light (harvesting) whichcontributes positively to the Jsc. The fill factor (FF) forthe TiO2 NB cell is however somewhat lower (0.52) as com-pared to the TiO2 NP cell. In a second batch, a similartrend was observed in Voc and Jsc values for NP cell andNB cell. The NB cell shows a remarkable efficiency of10.8% exhibiting an excellent Voc of 1 V and Jsc of18.2 mA/cm2. These data are presented in the supportinginformation, Fig. S4 and Table S1.

The Incident photon to current conversion efficiency(IPCE, DC measurement) plot for devices made frombatch 1 (supporting information, Fig. S5) clearly showedan overall increase in the % IPCE for the case of NB cellas compared to the NP cell. Moreover, relatively largeCH3NH3PbI3 crystallite size in TiO2 NB case could beexpected to lead to higher degree of scattering of light inthe longer wavelength region, resulting into a substantialincrease in the % IPCE in this wavelength region, as seen.Unfortunately however, due to the known rather rapiddegradation of un-sealed perovskite cells the integratedcurrent density from IPCE, which had to be measured else-where after several hours duration, was considerably lowerthan the observed cell current density. We will pursue thisaspect later in sealed cells.

4. Conclusion

We have introduced an interesting new variant of sur-face microstructuring in the domain of perovskite solarcells. We have shown that the cells fabricated using nano-structured submicron beads of TiO2 (NBs) exhibit signifi-cantly higher conversion efficiency as compared to thecells fabricated using TiO2 nanoparticles (NPs). This highefficiency is attributed to larger quantity as well as crystal-lite size of the perovskite phase. These also act as light scat-tering centers giving higher current density and reducedrecombination effects giving higher open circuit voltage.

Acknowledgment

The authors would like to acknowledge DST-APEX,MNRE-TAPSUN and CSIR for funding support.

18 M.C. Tathavadekar et al. / Solar Energy 112 (2015) 12–19

Appendix A. Supplementary material

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.solener.2014.11.016.

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