Grain Boundary Engineering of Halide Perovskite CH3NH3PbI3 Solar

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Grain Boundary Engineering of Halide Perovskite CHNHPbI Solar Cells with Photochemically-Active Additives

Nastaran Faraji, Chuanjiang Qin, Toshinori Matsushima, Chihaya Adachi, and Jan Seidel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00804 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 13, 2018

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The Journal of Physical Chemistry

Grain Boundary Engineering of Halide Perovskite CH3NH3PbI3 Solar Cells with Photochemically-Active Additives

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2,3

Nastaran Faraji , Chuanjiang Qin , Toshinori Matsushima

2–4

, Chihaya Adachi

2–4

, and Jan

1*

Seidel

1

School of Materials Science and Engineering, UNSW Australia, Sydney NSW 2052,

Australia 2

Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744

Motooka, Nishi, Fukuoka 819-0395, Japan 3

Japan Science and Technology Agency (JST), ERATO, Adachi Molecular Exciton

Engineering Project, 744 Motooka, Nishi, Fukuoka 819-0395, Japan 4

International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu

University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan *

correspondence: [email protected], phone +61 2 9385 4442

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ACS Paragon Plus Environment

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Abstract In this study, we investigate the nanoscale effects of photochemically-active additives of benzoquinone (BQ), hydroquinone (HQ), and tetracyanoquinodimethane (TCNQ) on grain boundaries in CH3NH3PbI3 solar cells. We employ scanning probe microscopy under light illumination, in particular Kelvin probe force microscopy, to study surface potential changes under laser light illumination. The recently found improvement in efficiency of BQ added solar cells can be clearly seen in vanishing contact potential differences at grain boundaries under illumination, rendering the material more uniform in solar cell operating conditions. These effects are observed for BQ, but not for HQ and TCNQ. Our findings shed light onto halide perovskite materials and functional additive design for improved solar cell performance.

Introduction Exploring new pathways for making inexpensive photovoltaic devices is a main goal in photovoltaic technology development

1, 2

. Organic-inorganic hybrid materials have been

shown to be promising candidates with facile processing, stable 3, tuneable band gap 4, extremely high optical absorption 5, and long electron-hole diffusion length 6. Although these materials have numerous properties which makes them ideal for applications in photovoltaics cells, detrimental effects such as water sensitivity and material degradation remain a challenge 7. Specifically, CH3NH3PbI3 perovskite has seen tremendous attention as light harvesting material

8, 9

. Chemical modifications such as replacing halide ions, e.g.

CH3NH3PbI3-xBrx have led to increasing photovoltaic efficiencies in this material

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together

with attempts to attain a better morphology of the perovskite layer for improved efficiency. Similarly, mixed halides, e.g. CH3NH3PbI3−xClx, were investigated and an efficiency of 15.4% was achieved

11

. End of 2013 efficiencies of 16.2% using a mixed halide

CH3NH3PbI3−xBrx (10-15% Br) and a poly-triarylamine hole transport material reported. Later, a confirmed efficiency of 17.9%

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were

was achieved by mixing the lower

bandgap CH(NH2)2PbI3 material with CH3NH3PbBr3 as the photovoltaic active layer. Zhou et. al. fabricated CH3NH3PbI3 perovskite on doped TiO2 with an yttrium and modified indium tin oxide cathode with polyethylenimine ethoxylated to reduce the contact barrier

14

. An

efficiency of 20.1% was independently confirmed in late 2014, as demonstrated by Seok and co-workers

[15]

. In this work, high-quality CH(NH2)2PbI3 films were fabricated by direct 2


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intramolecular exchange of dimethylsulfoxide (DMSO) molecules intercalated in PbI2 with formamidinium iodide. To date, efficiencies for a perovskite solar cells as high as 22.1% have been reported

16

. More recently, the influence of grain boundaries (GBs) in fabricated

solar cell devices has gained attention as a key factor for device performance

3, 17-20

and

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chemical engineering of such boundaries has become a focus .

Besides excelling in efficiency, producing stable perovskite solar cells is also challenging. Qin et al. previously reported eliminating water inclusion which is a source of hole trap formation and degradation that can significantly improve the stability of perovskite solar cells

22

. Moreover, in

23

it was reported that introducing BQ as redox active-organic

molecule into a precursor solution of methylammonium iodide and PbI2 improves both morphology and stability. In this work, GBs of organic-inorganic halide perovskite films were characterized by Kelvin probe force microscopy (KPFM) to investigate their role in CH3NH3PbI3 solar cell devices with photochemically-active additives. Experimental KPFM has been used as a tool to measure the contact potential difference (CPD). These measurements were performed with a modified AIST-NT Smart SPM scanning probe microscope and a tunable laser source for measurements under light illumination. KPFM allows for high lateral resolution measurements of spatial variations of the electrical properties of the devices on the nanometre scale. KPFM is a surface potential detection method that determines the CPD during scanning by compensating the electrostatic forces between the probe and the sample, which has been used for photovoltaic SPM measurements 24

. KPFM measurements were performed using Pt-coated AFM cantilevers (Mikromasch

HQ:NSC35/Pt) as the probe. The wavelength of laser light used for the AFM beam deflection was 1300 nm, which is outside the absorption range of the investigated sample. Films of the perovskite CH3NH3PbI3 were prepared on a film of poly (3, 4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) coated on indium-tin-oxide (ITO) substrates by spin casting from a precursor solution of CH3NH3I (MAI), PbI2, and BQ (or HQ, TCNQ). During spin casting, intermolecular interactions between MAI and BQ (or HQ, TCNQ) compete with the reaction between MAI and PbI2 to form the three dimensional perovskite structures, thus reducing the speed of crystallization. After annealing at 90 °C in a nitrogen-filled glove box, CH3NH3PbI3 films containing BQ (or HQ, TCNQ) additives were 3



obtained. Experimental details are provided in our previous literature 23.

Results and discussion The topography of CH3NH3PbI3 films with and without additives was investigated by AFM measurements and is shown in Figure 1(a). The effect of additives on CH3NH3PbI3 thin films can be seen in changes in the grain sizes of the films. To rule out surface morphology effects on the device performance, thin films without additives were fabricated and then additives induced into the perovskite system. All films have similar rms surface roughnesses of a few nanometers, which were extracted from AFM images (BQ: 10 nm, HQ: 19 nm, TCNQ: 8 nm, without additive: 12 nm). Figure 1(b) shows X-ray diffraction (XRD) 2θ/θ scans of the investigated samples. The CH3NH3PbI3 thin films keep their tetragonal crystallinity for different additives.

Figure 1. Morphological and structural characteristic of perovskite films with different additives. a) AFM topography image of films without and with BQ, HQ and TCNQ additive. Scan area is 4×4 µm². b) XRD patterns for films without and with BQ, HQ and TCNQ additive.

Figure 2(b) indicates the external quantum efficiency measured over the broad range of 300 to 850 nm wavelength for the given device geometry schematically shown in Figure 2(a). The CH3NH3PbI3 thin film with BQ as an additive shows overall improved device efficiencies, as seen in Figure 2(b).

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Figure 2. External quantum efficiency (EQE) spectra of the CH3NH3PbI3 thin films containing 0.5% of additives. a) Schematic drawing of device. b) EQE of CH3NH3PbI3 thin films without any additive (black line), with BQ (red line), HQ (green line) and TCNQ (purple line).

To investigate correlations of device efficiency on the macroscale and potential nanoscale origins, we performed KPFM measurements on all samples. Figure 3 shows the topography

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Figure 3. (a-d) AFM topography image of the CH3NH3PbI3 thin films without and with additives of BQ, HQ and TCNQ respectively. (e-h) Corresponding CPD images recorded for CH3NH3PbI3 thin films in dark and (i-l) under laser illumination. (m-p) Histogram distributions in dark and in light condition are shown for each sample.

and surface potential of halide thin films without and with BQ, HQ and TCNQ as additives. At grain boundaries surface potential changes in general are not expected unless the band structure bends upward or downward 25. In that case, changes in CPD images reflect the change of the work function in the measured area. Brighter areas in CPD images indicate 6


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a higher work function. It needs to be mentioned that, since KPFM measures the contact potential difference between the probe and the sample surface, the same tip is used for all the measurements. CPD measurements under light illumination with 600 nm wavelength and intensity of 3.02 kW/m2 were carried out. Acquired KPFM data in dark condition is shown in Figure 3(e-h). Grain boundaries show a visibly lower contact potential difference than the grain interiors. Interestingly, under illumination (Figure 3(i-l)), this local difference or barrier at the GBs is reduced, the effect is most prominent in the BQ added samples, where CPD appears almost flat across GBs. This finding indicates that band bending at GBs is reduced under illumination, due to photovoltaic carrier generation and potential screening at the GBs. For this case the material thus looks more uniform in the surface potential landscape and we believe that it has a positive effect on the photovoltaic performance, as proven by the external quantum efficiency measurements discussed earlier. In addition, the overall average CPD change upon illumination is the largest in the BQ added samples at 300mV compared to 245…265mV for all other samples, which can be seen in the CPD histogram plots in Fig. 3(m-p).

Figure 4. Cross-section data of topography and CPD for CH3NH3PbI3 thin films a) without additives, b) with BQ, c) HQ and d) TCNQ as additives.

Cross-section profiles of Figure 3(a-l) were extracted for the same area of the thin films in dark and under laser illumination and are shown in Figure 4 to see the difference in CPD better and for simplicity of comparison. For each image the cross-section data for 7



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topography and CPD are shown together, so the behaviour of grain boundaries can be seen clearly. In dark condition, nearly in all extracted profiles the contact potential drops at grain boundaries. The reduced CPD change at GBs is most visible for the BQ added sample. A schematic representation of associated qualitative changes in band structure is shown in Fig. 5. The schematic only shows a general concept, bending could be up or down. For the described effect the important thing is the amount of bending or lack thereof. a)

b) GB

GB

600 nm

c) GB

600 nm

∆ CPDGB EC

EC ∆ CPDGB

EF EV

Eg = 1.5 eV

EC

∆ CPDGB

EF

EF

EV

EV

HQ, TCNQ

BQ

Figure 5. Schematic band diagrams qualitatively illustrating the electronic structure around grain boundaries (GB) for a) sample in dark condition, b) for samples with added HQ and TCNQ under illumination, c) for samples with added BQ under illumination. ∆ CPDGB indicates the change of contact potential difference at the grain boundaries.

Conclusions We have demonstrated that the GBs in CH3NH3PbI3 films with additives of BQ, HQ, and TCNQ play a crucial role in halide perovskite solar cell performance. KPFM measurements showed that a potential barrier is formed along the GBs and higher CPD changes at GBs are found for HQ and TCNQ, while the opposite is true for BQ. These measurements confirm that photogenerated carriers are interacting differently with GBs in these samples leading to differences in overall device efficiencies. Further studies on the composition or different types of dopings at GBs are required to fully understand how the GB engineering can benefit halide solar cell device development.

Acknowledgements We acknowledge support by the Australian Research Council through Discovery Grants. JS further acknowledges travel support by I2CNER and UNSW strategic seed funding.

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Grain Boundary Engineering of Halide Perovskite CH3NH3PbI3 Solar

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