Enhancing efficiency of perovskite solar cell via surface microstructuring: Superior grain growth and light harvesting effect

Available online at www.sciencedirect.com

ScienceDirect Solar Energy 112 (2015) 12–19 www.elsevier.com/locate/solener

Enhancing efficiency of perovskite solar cell via surface 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 b

Center of Excellence in Solar Energy, Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune 411008, India 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 harvesting properties of perovskite crystallites. Instead of using a mesoporous TiO2 layer, we have modified the surface microstructuring of the TiO2 film by dispensing nano assembled TiO2 submicron structures (nanobeads, NBs) on TiO2 compact layer. With this new approach solar cell efficiency was improved significantly through an increase in both Jsc and Voc. This high efficiency is attributed to crystallite size of the perovskite phase. These also act as light scattering centers giving higher current density and reduced recombination effects giving higher open circuit voltage. Ó 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) in 1991 by Gra¨tzel and co-workers was a crucial breakthrough in the world of photovoltaics (O’Regan and Gra¨tzel, 1991). Significant collective efforts on various sensitizers (Mathew et al., 2014; Gao et al., 2008), co-adsorbers (Allegrucci et al., 2009), co-sensitizers (Kuang et al., 2007), new counter electrodes (Zhang et al., 2012;

⇑ Corresponding authors.

E-mail addresses: [email protected] (S.G. Mhaisalkar), [email protected] (S.B. Ogale). http://dx.doi.org/10.1016/j.solener.2014.11.016 0038-092X/Ó 2014 Elsevier Ltd. All rights reserved.

Tathavadekar et al., 2014), new redox electrolytes (Feldt et al., 2010), etc. over the past 20 years have not only pushed the efficiencies higher but have also brought out several new ways of making robust and durable DSSCs. A significant component of this research has focused on the use of different inorganic oxide morphologies like hierarchical spheres (Liao et al., 2011; Koo et al., 2008), mesoporous 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 taking advantage of the high surface area and enhanced light scattering abilities. High DSSC conversion efficiencies using spherical morphologies are reported by Koo et al.

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(2008) and Sauvage et al. (2010). The work by Koo et al. elaborates the use of nano-embossed spherical hollow TiO2 exhibiting bifunctional properties of efficient generation of photo-excited electrons as well as good light scattering property to yield an impressive efficiency of 10.34% with ruthenium N719 sensitizer (Koo et al., 2008). Sauvage et al. used the photoanode made up of submicron size TiO2 nanobeads which gave an efficiency of 10.6% over a conventional P25 photoanode (8.5%) with C101 sensitizer (Sauvage et al., 2010). Significant increase in performance of the cell in both the cases was mainly attributed to the high surface area and superior light scattering abilities of the particles leading to improved current density values. However, the DSSC architecture has some drawbacks of use of liquid electrolyte, hence research is being pursued on making DSSCs with solid hole transporting materials (HTMs) such as CuI, and CuSCN which have moderate hole mobility. Unfortunately, there is a problem of infiltration of such HTMs into the mesoporous TiO2 matrix (Hagfeldt et al., 2010; O’Regan et al., 2002). Some recent noteworthy research by Chung et al. however showed that the use of CsSnI3 (a p-type direct band gap inorganic perovskite semiconductor with high hole mobility) can yield a fairly high DSSC efficiency of 10.8% (Chung et al., 2012). A major breakthrough was reported recently in the domain of sensitized solar cells with the use of organo-metal halide perovskite (Kazim et al., 2014; Noh et al., 2013; Noel et al., 2014). Indeed, the organo-metallic halide perovskite materials have since gained significant importance as light harvesters in mesoscopic solar cells due to their large absorption coefficient, high charge carrier mobilities (Xing et al., 2013), solution processability, and tunable 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 in solar cells were mainly focused on their use as sensitizer in liquid electrolyte based sensitized solar cells which could give power conversion efficiencies up to 5–6%. Subsequently, a remarkable efficiency of 9.7% was reported by Park and coworkers using CH3NH3PbI3 as a light absorber deposited on a sub-micrometer thick (0.6 lm) mesoporous TiO2 film and spiro-MeOTAD as HTM (Kim et al., 2012). These devices however suffered from poor fill factor due to poor charge transport properties of spiro-MeOTAD. Gra¨tzel and coworkers later introduced p-type dopant in the form of a cobalt complex to improve the charge transport properties of spiro-MeOTAD (Noh et al., 2013). Snaith and coworkers showed a record efficiency of 10.9% with Voc of 1.1 V by replacing the mesoporous TiO2 with an insulating mesoporous Al2O3 (Ball et al., 2013). The mesoporous Al2O3 acted as a scaffold for a few nanometer thin layer of CH3NH3PbI2Cl transporting electronic charges out of the device through FTO anode while the spiro-MeOTAD collected the holes. Here mixed halide perovskite CH3-

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NH3PbI2Cl was used, which acted as both a light absorber as well as an electron transporter. Liu and Kelly have recently shown room temperature processed ZnO based perovskite solar cells with a robust and reproducible efficiency of 15.7% (Liu and Kelly, 2014). The morphology of perovskite formed within the mesoporous metal oxide film or within the planar heterostructure architecture has been shown to dramatically affect the photovoltaic performance (Kim et al., 2012; Liu and Kelly, 2014; Heo et al., 2013; Burschka et al., 2013; Saliba et al., 2014). The earlier method by Gra¨tzel and co-workers for perovskite formation within TiO2/Al2O3 mesoporous film included spin coating of premixed solutions of organic and inorganic counterparts followed by heating (Kim et al., 2012). This approach gave a non-uniform nano-pillar type growth of perovskite over mesoporous TiO2 (Heo et al., 2013). Later the same group showed a uniform growth of perovskite within mesopores of metal oxide via sequential deposition route which gave an outstanding efficiency of 15% in all-solution processed perovskite solar cells (Burschka et al., 2013). Very recently Snaith and coworkers have shown that the perovskite crystal domain size and subsequent thermal annealing play a crucial role in CH3NH3PbI2Cl mixed halide perovskite based mesoporous as well as planar architecture solar cells (Saliba et al., 2014). Various efforts are currently being directed on improving the efficiency of these perovskite solar cells which include use of 1D structures (Kim et al., 2013), gold plasmonic effects (Zhang et al., 2013), making the perovskite cells more cost effective by reducing the processing temperature etc. Dharani et al. made use of TiO2 nanofiber network for the growth of perovskite using sequential deposition process. The open porosity of the TiO2 nanofiber network was found to be responsible for good contacts across the TiO2/perovskite/spiro-MeOTAD interfaces leading to an efficiency of 9.8% (Dharani et al., 2014). Herein we introduce an interesting new variant for the perovskite cell architecture wherein we have dispensed nano-assembled light harvesting TiO2 submicron structures (nanobeads, NBs, 300 ± 100 nm size) on the TiO2 bottom compact layer (instead of the usual 20–30 nm nanoparticulate layer, NPs). As stated earlier, use of such NBs 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 light harvesting centers. In this work we show that the use of such NBs adds another advantage in the context of perovskite cell design, namely a dramatic enhancement of the perovskite grain size, leading to much improved solar cell performance (efficiency increase by about 17%, through an increase in both Jsc and Voc). Higher mean perovskite grain size reduces the interface density and facilitates carrier transport. We discuss the possible reasons for the enhanced quality factors for the cell due to such surface micro-structuring of the TiO2 layer.

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2. Experimental methods 2.1. Synthesis of TiO2 NBs TiO2 nanobeads (NBs, nanoparticle-assembled submicron spheres) were synthesized by a procedure reported by Chen et al. (2010). This synthesis protocol involves sol–gel process followed by solvothermal treatment. In brief, Hexadecylamine (HDA) was used as a surface directing agent which was added to titanium butoxide to yield high surface area porous beads. A paste was made using ethyl cellulose and a-terpineol by the previously reported method (Ito et al., 2007). In order to make a film, this paste was diluted with ethanol (1:3.5 wt/wt) and used for further experiments. 2.2. Solar cell device fabrication First, fluorine doped tin oxide (FTO, <14 X/sq., 2.2 mm thick) on glass, used as a substrate, was chemically etched into the required device pattern using Zn powder and diluted HCl. Then it was cleaned thoroughly with soap solution, deionized water, and finally with ethanol. A blocking layer of TiO2 having thickness of around 100 nm was formed on FTO by spray pyrolysis at 450 °C using a solution of titanium di-isopropoxide bis acetyl acetonate in ethanol (1:9 by volume). This blocking layer was sintered at 450 °C for 30 min followed by 40 mM TiCl4 treatment at 70 °C for 30 min to ensure that no pinholes remain. A layer of TiO2 (NBs) or TiO2 nanoparticles (NPs) (in separate samples) were deposited on the blocking layer by spin coating of the respective nanoparticle dispersions at 4000 rpm. For making the TiO2 NP film, commercially available TiO2 NP paste Dyesol 18NRT was diluted with ethanol (1:3.5 wt/wt) and used. These substrates were dried at 150 °C and sintered at 500 °C for 30 min. The average thickness of TiO2 NB film was 400 ± 50 nm and that of TiO2 NP was around 375 ± 25 nm. A hybrid perovskite, CH3NH3PbI3, was deposited onto the TiO2 films as reported in the literature (Burschka et al., 2013). In this sequential deposition process 1 M PbI2 solution in DMF (N,N-dimethyl formamide) was spin coated at 6000 rpm for 5 s on the films followed by film drying at 70 °C for 30 min. These films were then dipped in a 8 mg/mL solution of CH3NH3I in iso-propanol for 20 min. The films were washed with iso-propanol and dried at 70 °C for 30 min. Hole transporting material (HTM) was prepared by doping 10 wt.% of Co(III) complex (FK-102) in a solution of spiro-MeOTAD (2,20 ,7,70 -tetrakis(N,N-di-p-methoxyphenylamine)-9,90 -spiro bifluorene) in chlorobenzene (120 mg/mL). In the above solution, 24 lL of lithium bis(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 layer at 4000 rpm for 30 s. For the top contact, 100 nm of gold was deposited by thermal evaporation method on the

masked substrates. The active area of the cell was kept at 0.2 cm2 and rest of the area was masked with a black tape. The cells made using TiO2 NB and TiO2 NP were named as NB cells and NP cells, respectively. 2.3. Characterization Morphology of TiO2 nanobeads is observed by transmission electron microscope (TEM) using Technai 300. Field emission scanning electron microscope (FESEM) images were recorded to observe perovskite growth and cross section of the solar cell device using JEOL, JSM7600F, 5 kV and FEI, Nova NanoSEM 450. I–V characteristics of the solar cell were measured under AM 1.5 G using San-EI Electric, XEC-301S. Incident photon to current conversion 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 solvothermal methods (Chen et al., 2010). The XRD pattern of TiO2 NBs shown in supporting information Fig. S1 indicates pure anatase phase. These NBs have diameter 300 ± 100 nm. It is clearly seen from the TEM images (Fig. 1(b) and (c)) that these TiO2 NBs are made from interconnected small granular TiO2 particles (10–15 nm) exhibiting their porous nature. These porous NBs possess fairly high surface area of ca. 82 m2/g. From N2 adsorption data, a mean pore size for these particles is found out to be ca. 12 nm. The surface of such nanoporous submicron particles can provide for the nucleation of high quality CH3NH3PbI3 crystal growth. Using sequential deposition process (Burschka et al., 2013), CH3NH3PbI3 was then grown on the TiO2 (NP or NB) films. The XRD patterns recorded for TiO2 NP and TiO2 NB films loaded with CH3NH3PbI3 are shown in supporting information Fig. S2. The peaks at 2h values of 14.08°, 24.43°, 28.45°, 31.81°, 40.51°, 43.05° correspond to the (1 1 0), (2 0 2), (2 2 0), (3 1 0), (2 2 4), (3 1 4) planes of the desired perovskite structure having tetragonal phase (Qiu et al., 2013; Baikie et al., 2013). The absorbance data recorded for the PbI2 and perovskite loaded TiO2 films are shown in Fig. 2(a) and (b). The absorbance spectra for the case of PbI2 loaded on TiO2 films show the characteristic absorbance of PbI2 in the region from 400 to 450 nm. It shows that the deposition of PbI2 in TiO2 NB film is considerably higher than TiO2 NP film. This increased PbI2 deposition is attributed to the surface microstructuring of TiO2 film as discussed in the next section. The absorption spectra of perovskite loaded TiO2 films cover almost all the visible region shown in Fig. 2(b). The dramatically increased PbI2 deposition for the TiO2 NB case ultimately leads to higher perovskite formation (with enhanced grain size as shown below) as confirmed from the absorbance spectra of perovskite loaded

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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.

film. In the supporting information Fig. S3, we have also shown the transmittance and DRS comparison for PbI2 and CH3NH3PbI3 loaded TiO2 films. These data establish the light harvesting and thereby enhanced light trapping character of the perovskite grown TiO2 NB film. The difference in the formation of PbI2 layer and ultimately perovskite crystal for the two types of TiO2 films (NP and NB) was analyzed using FESEM images. The FESEM images of PbI2 deposited and CH3NH3PbI3 deposited films of TiO2 NP and TiO2 NB are shown in Fig. 3(b), (e), (c), and (f), respectively. Fig. 3(d) shows the FESEM image of the TiO2 NB film where TiO2 NBs

were dispensed on the blocking layer. A single layer of NB particles (nanoparticulate monolayer kind of assembly) having an overall thickness of less than 500 nm was deposited. Making thicker layers was considered undesirable from the standpoint of carrier recombination since it eventually leads to poor performance (Kim et al., 2013). From Fig. 3(d) it is clear the TiO2 NBs do not cover the entire available area, leaving some vacant spaces between any two TiO2 NB particles, where TiO2 blocking layer is exposed. This specific arrangement and also the rough surface topology of the film are beneficial for the formation of large perovskite crystallites. A mesoporous film is obtained

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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 NB film, TiO2 NB film coated with PbI2 and the CH3NH3PbI3 crystals grown on TiO2 NB film, respectively. Figures (g) and (h) show the cross-section images of the entire devices made using TiO2 NPs and TiO2 NBs, respectively.

for the case of TiO2 NPs as shown in Fig. 3(a). Overall smoothness of the film is much more as compared to the TiO2 NB case, as expected. Fig. 3(b) and (e) shows the morphologies for PbI2 coated TiO2 NP and NB films, respectively. Because of comparatively smoother nature of the TiO2 NP film surface the 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 puddles between NB inter-particle spaces. It is clearly seen that the PbI2 deposition occurs on the TiO2 NBs as well as on the exposed TiO2 blocking layer. This leads to increased PbI2 content in this NB case as compared to the standard TiO2 NP case, as confirmed by absorbance studies. After dipping the PbI2 loaded films in the solution of CH3NH3I, the perovskite CH3NH3PbI3 formed on the TiO2 NP and TiO2 NB films appears as shown in Fig. 3(c) and (f) respectively. The size of perovskite crystals grown on the TiO2 NP film is in the range of 100 ± 50 nm, whereas it is in the range of 400 ± 100 nm for perovskite crystals grown on the TiO2 NB film. Also from Fig. 3(f) it is clear that perovskite formation takes place on the TiO2 surface as well as

on the TiO2 blocking layer where PbI2 puddles are formed during spin coating. The enhanced grain growth of the perovskite crystallites in the NB case is attributed to higher amount of PbI2 retained on the TiO2 NB film as well as on the specific morphology of the film. On the other hand the perovskite crystal size is smaller in case of TiO2 NP because of the relatively smoother nano-grained nature of the surface for PbI2 (and hence subsequent perovskite) growth. The advantage of larger perovskite crystallites formed on TiO2 NB film is that the grain boundary density is very less which must contribute to reduced recombination at the TiO2/CH3NH3PbI3/HTM interface (Liu and Kelly, 2014). Also the large crystallite size should lead to scattering of light, increasing the effective mean path of light in the film which in turn can increase the current density (Liu and Kelly, 2014). Further, to fabricate solar cell devices, in each case a layer of HTM was spin coated on perovskite-loaded films under identical experimental conditions and a top gold contact was made. Fig. 3(g) and (h) shows the cross section images of the devices fabricated using TiO2 NPs and TiO2 NBs. In the case of the device made using TiO2 NP, a TiO2perovskite layer of 370 nm was deposited on which a 250 nm thick HTM overlayer was spin-coated. Finally

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100 nm gold layer was deposited as the top contact. In the case of TiO2 NB device, the TiO2 layer consists of single TiO2 NB particles dispensed on the blocking layer, and therefore the overall mean thickness of the film depends on the diameter of the TiO2 particle and the way they are assembled. With the dilution used for spin coating, the total mean thickness of the TiO2 NB-perovskite layer was around 350 ± 50 nm. Therefore a nearly 400 nm thick overlayer of HTM was deposited on the same. Indeed, the NBs are dispensed as isolated submicron particulates on the surface and are not piled up. Also it clear that the TiO2 NB-perovskite layer appears to be rough, thereby increasing contact area of the perovskite–HTM interface. Around 100 nm gold was deposited on the HTM overlayer as a top contact. We made different sets of NP and NB based devices in separate sample making protocols and compared their conversion efficiencies. The NP based devices showed efficiency values in the range 7.7 ± 0.5%, while the NB based devices showed consistently higher efficiencies in the range 9.8 ± 1.0%. The variability was noted to be somewhat higher for the NB case because the spin coating for nanobead dispersed sol has higher variability of micro-particulate distribution. In the following we present and discuss the data for one set, while in the supporting information we present data close to the best case scenario observed in terms of the enhancement of quality factors noted. Fig. 4 shows the I–V characteristics for few solar cell devices made using TiO2 NPs and TiO2 NBs, named as NP cell and NB cell, respectively. These cells were made in separate batches using same materials and processing parameters retaining the peripheral conditions constant, to check reproducibility. Table 1 represents the solar cell parameters for a typical NP cell and NB cell made in one batch. TiO2 NB cell exhibits an efficiency over 9% which outperforms the efficiency of 7.7% achieved for the TiO2 NP cell. In this case, the solar cell device fabricated using TiO2 NB exhibits a remarkable increment of open circuit voltage (Voc = 0.99 V) over that (0.9 V) obtained for the device fabricated using TiO2 NP. Larger Voc in NB cell is attributed to the larger CH3NH3PbI3 crystal size which reduces the grain boundary density and in turn the charge

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

Voc (V)

Jsc (mA/cm2)

FF

%g

NP cell NB cell

0.9 0.99

13.1 17.4

0.65 0.52

7.7 9.0

carrier recombination. The short circuit current density (Jsc) obtained for TiO2 NB cell is 17.4 mA/cm2 which is again significantly higher than 13.1 mA/cm2 realized for the TiO2 NP cell. This dramatic increase in Jsc for TiO2 NB cell is mainly because of the higher amount of CH3NH3PbI3 loading. Moreover, the bigger crystallite size also leads to enhanced scattering of light (harvesting) which contributes positively to the Jsc. The fill factor (FF) for the TiO2 NB cell is however somewhat lower (0.52) as compared to the TiO2 NP cell. In a second batch, a similar trend was observed in Voc and Jsc values for NP cell and NB cell. The NB cell shows a remarkable efficiency of 10.8% exhibiting an excellent Voc of 1 V and Jsc of 18.2 mA/cm2. These data are presented in the supporting information, Fig. S4 and Table S1. The Incident photon to current conversion efficiency (IPCE, DC measurement) plot for devices made from batch 1 (supporting information, Fig. S5) clearly showed an overall increase in the % IPCE for the case of NB cell as compared to the NP cell. Moreover, relatively large CH3NH3PbI3 crystallite size in TiO2 NB case could be expected to lead to higher degree of scattering of light in the longer wavelength region, resulting into a substantial increase in the % IPCE in this wavelength region, as seen. Unfortunately however, due to the known rather rapid degradation of un-sealed perovskite cells the integrated current density from IPCE, which had to be measured elsewhere after several hours duration, was considerably lower than the observed cell current density. We will pursue this aspect later in sealed cells.

4. Conclusion We have introduced an interesting new variant of surface microstructuring in the domain of perovskite solar cells. We have shown that the cells fabricated using nanostructured submicron beads of TiO2 (NBs) exhibit significantly higher conversion efficiency as compared to the cells fabricated using TiO2 nanoparticles (NPs). This high efficiency is attributed to larger quantity as well as crystallite size of the perovskite phase. These also act as light scattering centers giving higher current density and reduced recombination effects giving higher open circuit voltage.

Acknowledgment

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

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

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