Impact of CH3NH3PbI3-PCBM bulk heterojunction active layer on the photovoltaic performance of perovskite solar cells

Accepted Manuscript Research paper Impact of CH3NH3PbI3-PCBM bulk heterojunction active layer on the photovoltaic performance of perovskite solar cells Dhirendra K. Chaudhary, Pankaj Kumar, Lokendra Kumar PII: DOI: Reference:

S0009-2614(17)30745-5 http://dx.doi.org/10.1016/j.cplett.2017.07.069 CPLETT 34995

To appear in:

Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

17 April 2017 25 July 2017 26 July 2017

Please cite this article as: D.K. Chaudhary, P. Kumar, L. Kumar, Impact of CH3NH3PbI3-PCBM bulk heterojunction active layer on the photovoltaic performance of perovskite solar cells, Chemical Physics Letters (2017), doi: http:// dx.doi.org/10.1016/j.cplett.2017.07.069

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Impact of CH3NH3PbI3-PCBM bulk heterojunction active layer on the photovoltaic performance of perovskite solar cells Dhirendra K Chaudhary1, Pankaj Kumar2, Lokendra Kumar†1 1

Molecular Electronics Research Laboratory, Physics Department, Science Faculty, University of Allahabad, Allahabad-211 002, U. P., India 2

CSIR-National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi-110012, India

Abstract We report here the impact of CH3NH3PbI3-PCBM bulk heterojunction (BHJ) active layer on the photovoltaic performance of perovskite solar cells. The solar cells were prepared in normal architecture on FTO coated glass substrates with compact TiO 2 (c-TiO2) layer on FTO as electron transport layer (ETL) and poly(3-hexylthiophene) (P3HT) as hole transport layer (HTL). For comparison, a few solar cells were also prepared in planar heterojunction structure using CH3NH3PbI3 only as the active layer. The bulk heterojunction CH 3NH3PbI3PCBM active layer exhibited very large crystalline grains of 2-3 m compared to 150 nm only in CH3NH3PbI3 active layer. Larger grains in bulk-heterojunction solar cells resulted in enhanced power conversion efficiency (PCE) through enhancement in all the photovoltaic parameters compared to planar heterojunction solar cells. The bulk-heterojunction solar cells exhibited  9.25% PCE with short circuit current density (Jsc) of 18.649 mA/cm2, open circuit voltage (Voc) of 0.894 V and Fill Factor (FF) of 0.554.

There was 36.9%

enhancement in the PCE of bulk-heterojunction solar cells compared to that of planar heterojunction solar cells. The larger grains are formed as a result of incorporation on PCBM in the active layer.

Email for correspondence: [email protected]

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Keywords: Bulk-heterojunction, Perovskite solar cells, Hysteresis, c-TiO2, P3HT, grain size.

Introduction Photovoltaic (PV) technology has become one of the prominent technologies towards the production of long lasting carbon neutral, and environment friendly energy. The PV devices are semiconductors based and in recent years, a new contender has emerged that promises to give highly cost effective and efficient, solution processed PV devices [1-3]. These are the organic-inorganic metal halide perovskite semiconductors and have stimulated resurgence of interest due to their excellent optical and electrical properties towards photovoltaic applications [4-6]. These semiconductors exhibit high absorption coefficient, long electron-hole diffusion length, ambipolar charge transport, and tunable optical band gaps [7-9]. PV devices fabricated using these materials have shown over 20% PCE approaching to the commercially available Si based solar cells [10-12]. Demonstrations of efficient devices via solution processing in short time domain affirm the meteoric rise in this technology. Quality of films plays a very important role in controlling the photovoltaic performance of solar cells [13-15]. In perovskite solar cells, grain boundaries, surface coverage, grain size distribution, nature of crystallinity and surface morphology of perovskite film have been observed to be the key factors in affecting their performance [16-19]. Therefore, in order to realize efficient solution processed perovskite solar cells, control over the surface morphology has become one of the major challenges [20]. We report here the formation of 23 m sized perovskite grains and more than 99% surface coverage on the c-TiO2 coated FTO substrates. Formation of larger perovskite grains is achieved by the presence of PCBM in active layer that not only helps in growth of perovskite crystals but also formulate bulkheterojunctions with perovskite and contribute to enhanced photovoltaic performance

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compared to those without PCBM. The solar cells were prepared in Glass/FTO/cTiO2/Perovskite Film/P3HT/Ag configuration, where c-TiO2 worked as an ETL and P3HT as HTL. CH3NH3PbI3 was choosen as perovskite semiconductor and to improve the film quality and to formulate bulk-heterojunctions, we introduced a little amount of PCBM in perovskite during its growth. Though introduction of PCBM had no effect of the crystal structure but it had a great impact on the morphology and size of the crystalline grains. Introduction of PCBM resulted in formation of larger grains of 2-3 m with an excellent 99.96 % surface coverage. The solar cells with perovskite: PCBM bulk-heterojunctions exhibited improved PCE of 9.25 % with negligible hysteresis in J-V characteristics, however those without PCBM exhibited 5.38% PCE with significant hysteresis. The PCBM content in CH3NH3PbI3PCBM bulk heterojunction bound the grains of PbI 2 layer which forms a dense layer of perovskite. The dense layer of PbI2 is supposed to be responsible for the larger grain size of the perovskite films and improvement in device performances. Formation of larger crystalline grains by addition of PCBM has also been observed by Chiang et al. where they prepared solar cells on poly(ethylene dioxythiophene):poly(styrinsulfonate) (PEDOT:PSS) coated ITO substrates using CH3NH3PbI3-PCBM heterojunctions as photoactive material [21]. The solar cells exhibited hysteresis free J-V characteristics with power conversion efficiency of 16.0 % [21]. Some groups have also used polymeric materials (e.g. PTB7-CH3NH3PbI3 composite) to enhance the quality of perovskite film [22]. The growth of perovskite films is observed to be highly dependent on the substrate’s surface energy. Formation of larger crystalline grains even on the TiO2 coated FTO substrate, validates the role of PCBM in formation of larger grain size with enhanced substrate coverage irrespective of substrate type. We report here the growth and photovoltaic performance of CH3NH3PbI3-PCBM-BHJ hybrid perovskite films on TiO2 coated substrate, which has not been explored yet. Thought the solar cell efficiency was lower than that reported by Chiang et al. on PEDOT:PSS coated glass substrates but it

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validates the role of PCBM mixture in perovskite films. Rapid development of perovskite solar cells requires detailed investigations towards reproduction in cells performance with different structures and configurations and present study as a contribution in that direction.

2. Experimental details 2.1 Synthesis of CH3NH3I For the synthesis of CH3NH3I310 ml of hydroiodic acid (57 wt% in water) was added drop wise into 24 ml of methylamine (33 wt % in absolute ethanol) in 250 ml round bottom flask under constant stirring at 0oC for 2 h. Thereafter, the solution was subjected to rotary evaporator and solvents were evaporated at 50˚C resulting a yellowish white precipitate of CH3NH3I (MAI). The MAI precipitate was again dissolved in 80 ml of absolute ethanol and precipitated using 300 ml of diethyl ether. The precipitate was filtered out and dissolved in 80 ml of absolute ethanol again and precipitated out by 300 ml of diethyl ether. Finally a white power of MAI was achieved which was used in device fabrication after drying at 70˚C for overnight.

2.2 Preparation of TiO2 precursor solution Initially, a solution was prepared by drop wise adding 370 μL of titanium isopropoxide in 2.53 ml of ethanol with constant stirring. This solution was kept for stirring and another solution containing 350 μL diluted HCl (3:1) in 2.53 mL ethanol was prepared separately. The acidic solution of ethanol was drop wise added in ethanol containing titanium isopropoxide at constant stirring. The final solution was stirred for 30 min to obtain a homogenous, clear and transparent precursor solution.

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2.3 Preparation of perovskite films The perovskite films were grown by two step spin coating technique using separate PbI2 and MAI solutions. For planer and bulk-heterojunction solar cells two separate solution of PbI2 e.g.0.75 M PbI2 and 0.75 M PbI2 with 0.1 wt% PCBM were prepared in anhydrous N,N-Dimethylformamide (DMF). To obtain a homogeneous solution of PCBM in DMF, this solution was sonicated for 30 min and then overnight stirred at 80˚C. For MAI solution it was dissolved in isopropanol at 50 mg/ml and stirred at room temperature. For perovskite films, the PbI2 and PbI2-PCBM solutions were spin coated on to the substrates and dried for some time, which was followed by spin coating of MAI solution and annealing at 80 oC. After completion of annealing process dark brown films of CH3NH3PbI3 perovskite were obtained.

2.4 Preparation of solar cells FTO coated glass substrates of sheet resistivity 7-10 Ω/sq were purchased from Sigma Aldrich and patterned by wet chemical etching method using HCl and Zn metal powder. Afterwards the substrates were cleaned with the deionized water, acetone and isopropanol for 10 min in ultrasonic cleaner and dried in vacuum oven at 100oC for 15 min. Thereafter TiO2 precursor solution was spin coated on top of pre-cleaned substrates at 2000 rpm for 30 sec. The TiO2 coated substrate was first dried at 100˚C and then sintered at 450˚C for 40 min to get compact TiO2 (c- TiO2) layer. For the growth of planar CH3NH3PbI3 and bulk heterojunction CH3NH3PbI3-PCBM perovskite films the prepared PbI2 and PbI2: PCBM solutions were spin coated on top of cTiO2 coated FTO substrates at 2000 RPM for 60 sec. These films were annealed at 70˚C for 5 minutes. Thereafter, 40 l of MAI solution was spin coated on the top of as grown PbI 2 and PbI2: PCBM films. Deposition of MAI on PbI 2 films changed the colour of films from yellow to dark brown. The films were annealed at 80˚C for 60 min. Afterwards, P3HT (16 mg/ml in 5

chlorobenzene) was spin coated on top of the pure and BHJ perovskite layers at 2000 RPM for 60 sec. Spin coating of all the films was carried out in ambient atmosphere. Finally, a 100 nm silver (Ag) top electrode was deposited via thermal evaporation at 0.3-0.5 Å/sec in an evaporator chamber at the base pressure of 1x10 -6 Torr using shadow masks. Thickness of the deposited silver films was measured in-situ using quartz crystal thickness monitor. The active area of devices was estimated to be 0.04 cm2.

2.5 Material characterization To characterize the materials different tools and techniques were used. Surface morphology of the perovskite films was investigated using Zeiss-Field Emission Scanning Electron Microscope (FE-SEM) and Bruker Dimension Icon Atomic Force Microscopy (AFM). ImageJ image processing software was used for the estimation of perovskite grain size and surface coverage area from SEM micrographs by applying a suitable threshold function as reported in our previous report17. Crystal structural properties of the perovskite films were studied using Proto A-XRD diffractometer equipped with CuKα (λ=1.54 Å) radiation. UV-Vis absorption (EA) spectra of the films were obtained using Unicam 5625 UV-Vis spectrometer. The Fourier Transform Infra Red (FTIR) spectra of the perovskite films were recorded using Bomem MB-3000 ATR-FTIR spectrometer. Photoluminescence (PL) spectra were recorded for various samples in steady-state using Horiba Fluorolog with a 450 W Xenon lamp excitation source. All the spectra were corrected for instrumental response using a calibration lamp of known emissivity.

2.6 Photovoltaic characterization Current density–voltage (J–V) measurements of the solar cells were performed under simulated AM 1.5 sunlight, from Photo Emission Technology, USA (PET-SS50AAA-EM), a class AAA solar simulator calibrated for 100 mW cm-2 light intensity (in constant intensity 6

mode) using National Renewable Energy Laboratory (NREL) calibrated silicon reference cell. The J–V curves were recorded with a Keithley 2400 source meter with 0.286 V/Sec without and sweep delay. Hysteresis in the J-V characteristics of the devices was estimated by changing the scan direction.

3. Result and discussions Fig. 1 shows the schematic structure and energy level diagrams of the devices under study. For photon harvesting the light is made incident on the cell from FTO side. Light absorption in perovskite film causes exciton generation, which have to dissociate into free electrons and holes. The excitons diffuse to the perovskite-P3HT interface and dissociate there via transferring hole to P3HT and leaving behind the electrons in perovskite film. The electrons are transported to TiO2 and collected at FTO electrode whereas holes are collected at Ag electrode. Presence of PCBM in CH3NH3PbI3-PCBM bulk-heterojunction layer assists the excitons to dissociate within the perovskite film itself that results in reduced recombination losses and enhanced photo-current. Not only enhanced exciton dissociation, PCBM results in formation of larger crystal grains with enhanced substrate coverage that reduces recombination losses and provides high FF to the solar cells. The device fabrication steps for CH3NH3PbI3 and CH3NH3PbI3-PCBM bulk heterojunction based solar cells are shown in supplementary information. Fig. 2 shows the SEM images of CH 3NH3PbI3 and CH3NH3PbI3-PCBM thin films on c-TiO2 coated FTO substrates.

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Figure 1: Schematic diagrams for (A) device architecture and (B) energy level of the device.

Figure 2: Scanning electron micrograph of (A) CH3NH3PbI3 and (B) bulk heterojunction CH3NH3PbI3-PCBM thin film. Fig. 3(A) corresponds to the SEM micrograph of CH 3NH3PbI3 film grown using pure PbI2 solution, whereas Fig. 3 (B) corresponds to that of CH 3NH3PbI3-PCBM film grown using PbI2-PCBM solution. CH3NH3PbI3 film exhibited 100 – 150 nm nano-crystals with many pin holes in the film. On the other hand CH3NH3PbI3-PCBM film exhibited quite larger crystal grains with size 2-3 m without any pin holes in the film. This observed grain size in CH3NH3PbI3-PCBM BHJ hybrid perovskite film was 10-15 times larger than in CH3NH3PbI3 films. Here, we have used the two steps processing root for the growth of perovskite films, where perovskite film is the result of intercalation of the PbI2 and MAI and the morphology of the PbI2 film plays an important role in controlling the final perovskite film morphology. It 8

is observed that pure PbI2 films consist of small PbI2 crystals with rough and porous kind of morphology (see supplementary information). This porous morphology causes high surface area of the films. Therefore, when the MAI is deposited onto the PbI 2 film it reacts rapidly with PbI2 and due to larger surface area and porous surface morphology if forms smaller grain size (100-150 nm). Whereas the PbI2-PCBM film was quite smooth and dense, which formed very dense perovskite layer when intercalation reaction occurs on deposition of MAI. PCBM helps in formation of dense PbI2 film due reduced compressive stress introduced by volume expansion and fill the lest empty spaces in PbI 2 film. This yields continuous dense bulk heterojunction CH3NH3PbI3-PCBM films containing larger grain size ( 2-3 m) with excellent surface coverage area. A schematic illustration of both interpretations is shown in Fig. 3. Fig. 4 shows the AFM images of CH3NH3PbI3 and CH3NH3PbI3-PCBM perovskite films. Smaller grain size with many pin holes in CH3NH3PbI3 film can be clearly seen from 2D AFM plots shown in supplementary information.

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Figure 3: Mechanism of perovskite grain formation with (A) pure PbI2 and (B) PCBMPbI2 bulk heterojunction with 0.1 wt% of PCBM.

Figure 4: AFM micrograph of (A) CH3NH3PbI3 and (B) CH3NH3PbI3-PCBM bulk heterojunction perovskite thin films.

The line profile of AFM also shows a wide variation in the surface of perovskite film. Whereas, the CH3NH3PbI3-PCBM bulk heterojunction hybrid film is characterized by the compact and dense morphology with larger grain size. The AFM micrographs of both the films were consistent with the morphology acquired by SEM. The root mean square surface (RMS) roughness of CH3NH3PbI3 and CH3NH3PbI3-PCBM films are found to be the 54.4 nm and 50.9 nm respectively. These values suggest that within the measurement errors the CH3NH3PbI3 only films has a higher disordered surface.

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Figure 5: X-ray differactogram of PbI2, CH3NH3I, pure CH3NH3PbI3 and CH3NH3PbI3PCBM. (# stands for the peak corresponding to PbI2) Fig. 5 shows the XRD differactogram of PbI 2, CH3NH3I, CH3NH3PbI3 and CH3NH3PbI3-PCBM films. The diffraction pattern of CH3NH3PbI3 consisted peaks at positions 14.1˚, 20.0˚, 24.6˚, 28.4˚, 31.8˚ and 43.2˚ of 2θ degree which correspond to the planes (110), (112),

(202), (220), (310), and (330) of tetragonal crystal structure of

CH3NH3PbI3. The lattice parameter has been calculated using XRDA 3.1 software and found to be a = 8.82 and c = 12.73 Å. In diffraction profile of CH 3NH3PbI3 no additional peaks corresponding to PbI2 or CH3NH3I were observed which suggests complete conversion of PbI2 into CH3NH3PbI3. The mean coherent scattering domain size of crystallites was calculated using Debye–Scherrer formula and found to be 48.6 nm for CH3NH3PbI3. In CH3NH3PbI3-PCBM BHJ hybrid perovskite films few peaks are suppressed in the XRD profile and only the prominent peaks at (110), (202) and (220) are observed in XRD profile. This may be due to preferential growth of perovskite films along c-axis and the growth morphology of the films can be observed in SEM micrograph of the respective samples. The mean coherent scattering domain size of crystallites is found to be 49.0 nm in CH3NH3PbI3-

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PCBM BHJ hybrid films. The observed characteristics of crystallite is in good agreement with our predictions that the larger grain size in the perovskite films are formed by the addition of larger crystallites in presence of PCBM. The peak at 12.12˚ of 2θ position is due to presence of PbI2 in the perovskite layer (in XRD profile the peak is represented by #). Whereas the effect of PbI2 in device performance is unclear some report shows the enhancement in device performance due to presence of PbI 2 via reducing the energy level difference within the devices and some report shows that the presence of PbI 2 in perovskite layer hampers the device performances by hampering the crystallinity of the perovskite films [23,24].

Figure 6: UV-Vis absorbance spectra pure and PCBM-bulk heterojunction perovskite films. The electronic absorption spectra of both perovskite films are shown in Fig. 6, The absorption spectra show the BHJ thin film of CH3NH3PbI3-PCBM exhibits enhance absorbance spectra as compared to the CH3NH3PbI3 though the films were deposited in identical conditions. The enhancement in absorbance of the BHJ CH3NH3PbI3-PCBM films could be due to the formation of larger grain size in CH3NH3PBI3-PCBM. The sharper absorption edge and 12

enhanced absorbance could be the due to combined effect of well-defined crystal morphology, high degree of crystallinity and good surface coverage of the CH3NH3PbI3 [17]. The larger grain size leads to enhancement in the absorbance that may be due to the large grain size increases the distance of light propagation due to back scattering of incident light from the comparatively rough surface [17]. The band gap of both the films has been estimated using Tauc plot of UV-Vis (see supplementary information) absorbance spectra and found to be 1.56 eV and 1.54 eV for the CH3NH3PbI3 and CH3NH3PbI3-PCBM respectively. The difference in band gap is also confirmed by the normalized PL spectra of both samples (see supplementary information).

Figure 7: ATR-FTIR spectra of CH3NH3PbI3 and CH3NH3PbI3-PCBM BHJ films. The ATR-FTIR spectra of photoactive CH3NH3PbI3 and CH3NH3PbI3-PCBM layers are shown in Fig. 7. After preparation, CH3NH3PbI3 perovskite and CH3NH3PbI3-PCBM composite thin films illustrate main characteristic peaks of lead perovskite crystals; i.e. wide strong peak at 910cm−1 for CH3-NH3 rock, peak at 989 cm−1 for C-N stretch, peaks at

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1403 cm−1 and 1439 cm−1 for C-H vibration bands, and peaks at 1482 cm−1 and 1570 cm−1 for H-N vibration bands.

Figure 8: Photoluminescence spectra of CH3 NH3PbI3 and BHJ CH3NH3PbI3-PCBM with a P3HT quenching layer.

The steady-state photoluminescence emission spectra of CH3NH3PbI3, CH3NH3PbI3-PCBM photoactive thin films were recorded using excitation wavelength 557 nm as shown in Fig. 8. The CH3NH3PbI3 film shows a PL emission peak at the onset of absorption edge of which shows the direct band gap characteristics of the CH 3NH3PbI3. The PL emission spectra of CH3NH3PbI3-PCBM film exhibited reduced PL intensity, which can be attributed to the presence of PCBM that quenches the PL. This quenching clearly shows the effective charge separation within the CH3NH3PbI3-PCBM layer. The PL emission peak almost disappeared when a thin film of P3HT was deposited on perovskite film. This suggests to believe that in CH3NH3PbI3-PCBM layer the separated electrons and holes were in the same layer and some of them were recombined again to give some PL intensity. However in CH 3NH3PbI3-

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PCBM/P3HT the holes are transferred to P3HT and electrons are left in perovskite film resulting in great PL quenching.

Figure 9: J-V characteristics of the devices based on (A) CH3NH3PbI3 and (B) CH3NH3PbI3-PCBM active layers, under illumination condition in reverse and forward sweep directions.

The J-V characteristics of the solar cells based on CH3NH3PbI3 and CH3NH3PbI3-PCBM photoactive layers in forward and reverse scans are shown in Fig. 9. The photovoltaic parameters of both the devices are shown in Table 1. The best performing devices with CH3NH3PbI3-PCBM BHJ show the PCE up to 9.25 % with very small hysteresis in J-V characteristics. This obtained power conversion efficiency of the devices was  36.8% larger than the devices with CH3NH3PbI3 only active layer. Also, the devices with CH3NH3PbI3 exhibit anomalous hysteresis in J-V characteristics. The anomalous hysteresis behaviour in the J-V characteristics of perovskite solar cells can be due to several factors viz. perovskite film quality, ion migration, ferroelectricity in perovskite films, grain size, optoelectronic properties of materials used as hole and electron transport layer etc.

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Device

Scan direction

Jsc (mA/cm2)

Voc (V)

FF

Efficiency (%)

Forward Scan

15.135

0.753

0.434

4.959

Reverse scan

15.990

0.820

0.445

5.836

Forward Scan

17.642

0.885

0.552

8.6369

Reverse scan

18.649

0.894

0.554

9.249

FTO/TiO2/CH3NH3PbI3/P3HT/Ag

FTO/TiO2/CH3NH3PbI3PCBM/P3HT/Ag

Table 1: Photovoltaic performance of solar cells based on CH 3NH3PbI3 and CH3NH3PbI3-PCBM BHJ active layers in forward and reverse scan directions. The CH3NH3PbI3 devices exhibit the anomalous hysteresis that could be due to smaller grain size [17,18]. The small grain size of the perovskite films increases the grain boundaries. The few micron order grain size in the CH3NH3PbI3-PCBM perovskite films significantly reduces the grain boundaries and also the larger grain size of the perovskite films reduces the ferroelectric behaviour of the films in used scan range.

Conclusions We have studied the effect of PCBM on the growth of perovskite crystals and as a result the effect on photovoltaic performance of perovskite solar cells. CH 3NH3PbI3 films were grown in the presence of PCBM via two step solution processing method. Incorporation of PCBM resulted in bulk-heterojunction network in perovskite film with larger crystal grains. The structural and optical properties of the films were investigated using SEM, AFM, XRD, UV-Vis, Photoluminescence and ATR-FTIR spectroscopy. SEM and AFM micrographs showed compact perovskite films with the grain size of 2 to 3 micron in CH3NH3PbI3-PCBM bulk-heterojunction film whereas CH3NH3PbI3 film without PCBM 16

exhibited smaller

grains of size

around 100 to 150 nm. The solar cells based on

CH3NH3PbI3-PCBM bulk-heterojunction layer showed the PCE 9.25% which was 36.9% enhancement in the PCE compared to that of based on CH 3NH3PbI3 only. The enhanced PCE with CH3NH3PbI3-PCBM active layer can be attributed to the improvement in film quality and reduced recombination losses.

Acknowledgement The financial support to this work was provided by the Department of Science & Technology under FTP grant number SERB/F/5521 and DST-FIST and UGC-CAS grants to Physics Department, University of Allahabad.

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Graphical abstract:

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Research highlights 

CH3NH3PbI3-PCBM Bulk heterojunction hybrid films were grown on c-TiO2 thin films.



The hybrid films exhibited very large crystalline grains of 2-3 m compared to 150 nm only in CH3NH3PbI3 active layer.



There was 36.9% enhancement in the PCE of bulk-heterojunction solar cells compared to that of planar heterojunction solar cells.



Passivation in hysteresis has been also observed in devices with CH3NH3PbI3-PCBM Bulk heterojunction hybrid films.

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