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Overcoming the Challenges of Large Area High Efficiency Perovskite Solar Cells Jincheol Kim, Jae Sung Yun, Yongyoon Cho, Da Seul Lee, Benjamin Wilkinson, Arman Mahboubi Soufiani, Xiaofan Deng, Jianghui Zheng, Adrian Shi, Sean Lim, Sheng Chen, Ziv Hameiri, Meng Zhang, Cho-Fai Jonathan Lau, Shujuan Huang, Martin A. Green, and Anita W. Y. Ho-Baillie ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00573 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 8, 2017

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ACS Energy Letters

Overcoming the Challenges of Large Area High Efficiency Perovskite Solar Cells Jincheol Kim,† Jae Sung Yun,*,† Yongyoon Cho,† Da Seul Lee,† Benjamin Wilkinson,† Arman Mahboubi Soufiani,† Xiaofan Deng,† Jianghui Zheng,† Adrian Shi,† Sean Lim,‡ Sheng Chen,† Ziv Hameiri,† Meng Zhang,† Cho Fai Jonathan Lau,† Shujuan Huang,† Martin A. Green,† and Anita W. Y. Ho-Baillie*,†

† Australian Centre for Advanced Photovoltaics (ACAP), School of Photovoltaic and Renewable and Engineering, University of New South Wales, Sydney 2052, Australia ‡ Electron Microscope Unit, University of New South Wales, Sydney, NSW 2052, Australia

Corresponding Author * Jae Sung Yun * Anita W. Y. Ho-Baillie School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney 2052, Australia E-mail: [email protected] and [email protected] Phone: +61 2 9385 4257

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Abstract For the first time, we report large area (16cm2) independently certified efficient single perovskite solar cells (PSCs) by overcoming two challenges associated with large area perovskite solar cells. The first challenge of achieving a homogeneous and densely packed perovskite film over a large area is achieved using an anti-solvent spraying process. The second challenge of removing the series resistance limitation of transparent conductor is achieved by incorporating a using metal grid designed using a semi-distributed diode model. A 16 cm2 perovskite solar device at the cell level rather than at the module level is demonstrated using the modified solution process in conjunction with the use of metal grid. The cell is independently certified to be 12.1 % efficient. This work paves the way towards highly efficient and large perovskite cells suitable without single junction perovskite solar cells and silicon-perovskite tandems.

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The past few years have seen the rapid progress in a new class of solar cells based on mixed organic-inorganic halide perovskites.1 The highest certified PCE of a perovskite solar cell of 22.1% has been achieved on an aperture area of 0.046 cm2.2 For more realistic PV performance evaluation, PCE measured for cell area ≥1 cm2 is encouraged and some promising results for ≥1 cm2 devices have been reported recently. Certified PCE at 19.6 % for a 1.0 cm2 device have been demonstrated using an vacuum-flash assisted solution process.3 To date, the highest certified efficiency for a close to 1 cm2 perovskite solar cell is 19.7%.4 The highest certified PCE for a Si-Perovskite tandem is 23.6% on an area of 1 cm2.5 Two main challenges prohibit the creation of larger efficient perovskite solar cell. Both effects limit maximum device area to a few cm2. The first problem is the dramatic reduction in perovskite film quality and uniformity for areas much larger than 1 cm2. The second problem is the approximately linear increase in series resistance with cell area. Perovskite layer deposition via spin coating has been widely used to fabricate a dense and uniform absorber film, and represents the state of the art.3, 6-12 However, as cell size grows beyond, film uniformity declines beyond acceptable levels, degrading performance and limiting the development of large area PSCs. Other deposition techniques have been attempted, such as blade coating,13-15 roll to roll printing,16-17 and spraying.

18-21

However,

device performance in these works still lag behind those fabricated by spin coating.3, 10-11, 22-23 To date, a promising technique to control the nucleation and crystal growth kinetics for high quality perovskite film is the use of anti-solvent crystallization.10, 24-28 Although such process can produce a high-quality film, the dropping of anti-solvent during spinning is only effective over a small area. While parallel efforts were made on developing anti-solvent spraying for perovskite film deposition29 on small area (0.09cm2), here, we report a spray anti-solvent (SAS) method that is suitable for large area to facilitate rapid nucleation and uniform crystallization of a large area perovskite film with densely packed grains.


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To circumvent the quadratic scaling of series resistance (RS) with cell width, one method is to physically isolate the large area device into thin strips followed by series connection30 which can be a combination of laser ablation, physical scribing or chemical etching and metal deposition requiring high precision alignment to minimize “dead-area” losses.31 This architecture is acceptable for a single junction perovskite module, but it is unsuitable if the PSC is to be incorporated into a tandem such as Si-Perovskite tandem cell which also prevents current matching and optimal optical design. Another solution to the series resistance problem is the use of metal grid such as that reported in a literature32 which demonstrated an “inverted” perovskite cell on 25 cm2 at 6.8 % according to in-house measurement. In this work, we present a semi-distributed diode model for the optimization of metal grid design and implement the metal grid to reduce series resistance of a 16cm2 cell without any isolation and alignment steps. By combining the advantages of the modified perovskite deposition method using SAS and the implementation of metal grid, we have demonstrated a large area 16 cm2 cell with a certified efficiency at 12.1 %. This is the highest certified efficiency for a perovskite device larger than 10 cm2 at the cell level rather than at the module level. The fabrication details of (HC(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15 perovskite film (with 5 mol% of excess PbI2) by conventional anti-solvent process that involves the dropping of anti-solvent (DAS) and the modified anti-solvent processes can be found in the Supplementary Material. The modified anti-solvent process involves the spraying of anti-solvent during the last stage of the perovskite precursor spinning process. As part of the optimization, we investigated the effects of different spraying pressure and time on the microstructure of the film. Results are shown in Figure S1. It was found that spraying pressure is the key parameter as it determines droplet size, number of droplets and nozzle flow rate33. When the spraying pressure is too low, the anti-solvent droplet size is too large and the number of droplets are 4 ACS Paragon Plus Environment

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not sufficient to induce sufficient nucleation sites for uniform film growth resulting in holes in the film (Figure S1a). When the spray pressure is sufficiently high, the droplet size becomes smaller and the number of droplets increases, thereby increasing the number of perovskite nucleation sites resulting in more compact and dense films as observed (Figure S1b and S1c). Another advantage of increasing the spraying pressure is the ability to deliver the anti-solvent at a high speed to drive the super-saturation at a faster rate than the conventional drop and spin anti-solvent process. In the case when the perovskite film is fabricated by the DAS method, the speed of dispersion and the area of coverage are limited by the speed limit of the spin coater, which relies on the centrifugal force from the spinning to deliver the anti-solvent across the surface. For the SAS method, the dimethyl sulfoxide (DMSO) to dimethylformamide (DMF) ratio in the perovskite precursor is also optimised as this is related to the amount and properties of DMSO-PbI2 complexes 24 which affects the film crystallisation process 34-35. It is found that a DMSO content of 20% in DMF (which is used for all device fabrication in this work) results in the best cell performance for DAS and SAS method (Figure S2). To compare the uniformity of the films prepared by the DAS and SAS methods, perovskite films are deposited on 5 cm by 5 cm fluorine doped tin oxide (FTO) coated glass substrates. Same spin coating parameters such as spinning speed and time are used for both processes. The difference is in the method of delivery the ant-solvents. Figure 1a shows an optical image and scanning electron microscopy (SEM) images of the perovskite film fabricated by the DAS immediately after the spinning process and before annealing. The upper SEM image was taken near the corner of the substrate while the lower SEM image was taken near the center of the substrate. Figure 1b shows the optical images of the DAS processed perovskite film after annealing, which has been transformed from a yellow transparent film to a dark opaque film. UV-visible absorption measurements (spot size of 0.45


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cm2) were also carried out on the four corners and the center of the film. Figure 1c and 1d show similar images and absorption spectra but for the film fabricated by SAS.

Figure 1. Perovskite film on 5cm by 5cm FTO substrate. Schematic illustration of the processes, optical images of the entire films and SEM images from the corner and from the centre of the pre-annealed perovskite films fabricated using the (a) DAS process and (c) SAS process. Optical images of the entire films, SEM images from the corner and from the centre of the films and UV-visible absorptions (spot size of 0.45 cm2) at the various positions of the post-annealed (100 °C for 20 mins) perovskite films by (b) DAS (d) SAS. The scale bar in SEM images is 150 µm.

Before annealing, the film deposited by the conventional DAS has a sparse network of dendrites with voids (the brighter regions are the underlying FTO) as seen in the SEM images in Figure 1a. In addition, the morphology of these features changes with position on the substrate. The absorption data for the annealed film at various positions also reveal non6 ACS Paragon Plus Environment

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uniformity (Figure 1b). A radial pattern can be observed in the optical image (inset in the absorption graph in Figure 1b) of the annealed film revealing the limitation of the spinning in delivering the anti-solvent uniformly across a large surface. The film made by the new SAS process shows superior surface coverage and denser features (Figure 1c). The denser feature is a result of enhanced interaction between the perovskite precursor and the sprayed anti-solvent droplets, which are fine and abundant. No radial pattern can be observed in the optical image for the film in Figure 1d. The absorption curves are identical at different positions and the SEM images in Figure 1d confirm excellent film uniformity. It is important to note that another advantage of using the spraying process to deliver the anti-solvent is material saving and less wastage. The amount of anti-solvent used by SAS method is only 30% of that required for the DAS method. Figure S3 shows the result when an equivalent amount of chlorobenzene (CBZ) required for spraying is used for the dropping process. Apart from the radial pattern observed showing non-uniformity, the regions near the corners of the sample show inadequate delivery of the anti-solvent over a large area that could otherwise be achieved if the spraying method is used. Figure S4 shows the X-ray diffraction (XRD) patterns of the films made by the DAS and SAS processes before and after annealing. Before annealing, the samples exhibit XRD peaks at around 6.6 ° which is related to the presence of the intermediate phase of DMSOPb2+. This is confirmed (1017 cm-2 signature) by Fourier transform infrared spectroscopy (FTIR) measurements10, 36-37 (Figure S5). Although the annealed DAS and SAS films have similar grain size (Figure S4c and S4f), XRD patterns reveal that there are some differences in crystal structure. We calculated integral peak area for the (110), (200), and (220) perovskite phases, and the (110)/(200) and (220)/(200) ratios (Table S1). For the SAS processed sample, the integral peak areas are slightly larger than those for the DAS processed sample indicating a better crystalline phase. The slightly higher (110)/(200) and (220)/(200) ratios also indicate 7 ACS Paragon Plus Environment

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stronger (110) orientation in the SAS processed perovskite film. This is favourable as it has been shown to result in improved cell performance.38 To obtain insights into the film quality in terms of carrier dynamics, we also conducted the time correlated single photon counting (TCSPC) measurements on five different regions (centre and four corners) of the test structures perovskite/m-TiO2/blTiO2/FTO/glass fabricated by the DAS and SAS methods (Figure S6a). Figures S6b and S6c show the measured PL decay traces and the fitted curves for each region of the two samples. Table S2 summarizes the results of the bi-exponential fitting (see Supporting Information under Materials and Methods for details). The SAS sample has a shorter τ1 than the DAS sample due to higher carrier extraction8, 12, 39. The resultant better device performance will be reported later in this paper. τ2 which corresponds to charge carrier recombination is also lower in the SAS sample compared to the DAS sample. τeff summarized in Table S2 has a large spread in the DAS sample compared to the SAS sample (also see Figures S6 b-d) due to larger variation in film quality across the DAS sample. PL imaging on full devices fabricated by DAS (Figure S7a) and SAS (Figure S7b) methods was performed at open-circuit condition. Figure S7e shows the corresponding image intensity distributions. The area-averaged luminescence intensity counts represent the accumulated photo-generated charge carrier density at open-circuit condition and thus effective carrier lifetime, which are very similar for the two cells. This finding is similar to results from the TCSPC measurements showing comparable lifetimes between the samples. However, a greater non-uniformity is observed across the device fabricated by the DAS technique (cf. Figures 2a and 2b). We also performed EL imaging40 on these cells to obtain further insights into the resistive and carrier recombination losses. In particular, to quantity the non-uniformity which is more prominent in the cell fabricated by the DAS method, the current transport efficiency (fT) maps were calculated41 (see Supporting Information under Materials and Methods for 8 ACS Paragon Plus Environment

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details). The corresponding fT maps are shown in Figures 2c & 2d while fT distributions are shown in Figures 2e & 2f.

Figure 2. Electroluminescence images of full device fabricated by (a) DAS and (b) SAS methods. The corresponding (c, d) current transport efficiency (fT) maps and (e, f) fT distributions of the same devices. The scale bars are 3 mm.

It is found that the peak value of fT for the cell fabricated by the DAS approach (Figure 2c) is lower than that of the SAS approach (Figure 2d). The relationship between fT(x,y) (as a function of x and y) and the spatially varying series resistance, Rs(x,y), terminal voltage, V(x,y), and dark current density, j(x,y), is expressed in Equation 140-41 as follows: 1 ∂j = 1 + R s ( x, y ) ( x, y ) f T ( x, y ) ∂V

(1)

We therefore can conclude that the accumulative series resistance and effective dark saturation current in the DAS device are higher than those fabricated in SAS device.40 This


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explains the higher fill factor (FF) found in the SAS devices which will be later in this paper. The narrower distribution of fT for the SAS device indicates better uniformity in the electrical properties when compared to the DAS device (see standard deviation values provided in the inset Tables of Figures 2e and 2f). We note that the tail at low fT for the SAS cell (Figure 2f) is associated with the damage caused by scratches (see Figure S7 and caption for explanation). To confirm the benefits of the SAS method over the DAS method, full devices of structure gold/spiro-OMeTAD/perovskite/m-TiO2/bl-TiO2/FTO/glass (Figure S7) were first fabricated on 2 cm by 2 cm substrates. We measured the current density-voltage (J-V) characteristics of the different regions (center and four corners) of the cell using a round aperture of 0.159 cm2 as shown in Figure 3. The cell fabricated by SAS process has a higher efficiency with smaller spread of performance across the device. The higher voltage also confirms better film quality consistent with what were observed under the SEM, XRD, time resolved-PL. The superior FF in the SAS device agrees with the improved current transport efficiency as observed from EL imaging. Using the SAS method, we have achieved an independently certified (by Newport) PCE at 18.1 % for 1.2cm2 cell (Figure S9). A certified JSC of 21.4mA/cm2 and a FF of 74.6% is achieved for this cell. In particular, a VOC of 1.13V is amongst the highest for the state of the art cells.

Figure 3. Local (center and four corners) solar cell performance for a perovskite solar cell on a 2cm by 2cm substrate fabricated by the (a) DAS and (b) SAS methods. The local solar cell efficiencies were measured under standard AM 1.5G illumination using a 0.159 cm2 aperture while the total cell area of 1.6 x 1.0 cm2. 10 ACS Paragon Plus Environment

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Figure 4. The current density-voltage (J-V) curves for (a) 1.2 cm2 devices using DAS and SAS methods, (b) 5.8 cm2 device by SAS, (c) 16.0 cm2 device by SAS. Inset images in (c) is an optical image of the device. The solid lines are fitted light J-V curve through the “semidistributed diode” model. Table summarises the electrical parameters as well as the RS, RSH and m(V) values from the fittings.

To illustrate the effect of increased series resistance on large area cells, 5.8 and 16 cm2 cells without metal grid are fabricated (using the SAS method) and the results are shown in Figure 4. To accurately account for the distribution of RS over the area of these large cells, we utilise a semi-distributed diode model (see Supporting Information for detail methods and Figure S10c for the equivalent circuit). Although it has been a common practice to determine RS and RSH from the gradients of the light J-V curve at open-circuit voltage (VOC) and shortcircuit current (JSC), respectively, there can be multiple causes to the gradients. For very large area cells, RS can be large enough to cause a non-zero gradient near JSC, which can appear to indicate a low RSH value. Similarly, when RS is large, a change in RSH has a similar effect on the J-V curve to a change in photocurrent. The ideality factor as a function of voltage (m-V) curve derived from the dark J-V curve contains useful information about the cell parameters. By fitting the light J-V curves (Figure 4) and m-V curves (Figure S10) simultaneously using the “semi-distributed diode” model with parameters listed in Table S3, the J-V parameters 11 ACS Paragon Plus Environment

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become more tightly constrained, allowing the lower limit of RSH to be found, a more accurate account of RS, and the reporting of local ideality factor which is a good indicator of cell quality. The reason for using a diode model that is semi-distributed is to avoid excessive computational complexity while still allowing accurate reproduction of experimental J-V curves to quantify the parasitic losses in the large area cells. The parasitic resistances and minimum local ideality factors of the large area cells without metal grid from the fitting are listed in Table in Figure 4. The effect of cell size on resistive loss is apparent with RS increasing with area. The minimum local ideality factor (m-V) of the 1.2 cm2 cell approaches 2, indicating that recombination is trap limited. The minimum m-V of the larger cells is significantly higher, being closer to 3. RSH is reasonably larger (> 1,000 Ω-cm2) for all cells displaying no trend with cell size. This indicates that surface coverage is consistently high, with no shunting paths through the perovskite. Using the lumped-resistance method as described in the Supporting Information (under Materials and Methods), three grid designs were modelled (see Figure S11 for modelled J-V curves and Table S4 for grid width and spacing). The trade-off between series resistance (reduces with metal coverage) and shading (increase with metal coverage) is apparent. Figure 5 shows the J-V and m-V characteristics of a 16cm2 cell with a metal grid using the V3 design as well as the parasitic resistances and minimum local ideality factors from the fitting. Figure S12 shows the photo of the cell. Comparing results from Figures 3 and 4, it is apparent that the introduction of a metal grid to the 16 cm2 cell is effective in reducing the RS by more than 50% and the minimum local ideality factor to a value below that of the 5.8 cm2 grid-less cell and the 16 cm2 grid-less cell.


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Figure 5. (a) Measured and modelled J-V curves (inset is photo of the cell), (b) measured and (c) modelled m-V for a 16 cm2 cell fabricated by SAS method with a metal grid. The solid line is the fitted light J-V curve through the “semi- distributed diode” model. Table summarises the electrical parameters as well as the RS, RSH and m(V) values from the fitting.

The encapsulated 16 cm2 is certified by Newport confirming an impressive PCE at 12.1 % (Figure S13). A certified JSC of 17.3mA/cm2 and a FF of 61.9% is achieved for this cell. In particular, a very high VOC of 1.13V is maintained for this 16 cm2 cell. This is the highest certified efficiency for a perovskite device larger than 10 cm2 at the cell level rather than at the module level. Table S5-8 summarizes the electrical characteristics of cells fabricated in this work measured at the University of New South Wales showing relative reliability of our in-house measurement. It is expected that improvements in metal grid design will further reduce RS and therefore improve performance for these large cells in future work (Figure S14 and Table S8). The large cells fabricated by the SAS in this work when encapsulated have sufficient stability for independent certifications. Results in Figure S15 show that the 16 cm2 cell is stable after 2 months of storage in ambient conditions. A short movie in the Supplementary Material (Movie S1) shows the ability of the 2-month old certified 16 cm2 cell in powering a fan. The good stability of the large device is likely to be due to the smaller perimeter to area ratio (1 cm-1 for 16 cm2 device as opposed to 3.8 cm-1 for 1.2 cm2 device) with less exposure 13 ACS Paragon Plus Environment

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to the ambient including possible moisture ingress at the edges of the device. We re-measured the 16 cm2 cell 5 months after fabrication (Figure S16). The cell’s maximum power point (~ 0.76 V) current density stabilizes within seconds at ~16 mA cm-2 resulting in a PCE of 12%. To conclude, we have developed an anti-solvent spraying process (SAS), which allows high quality perovskite film to be fabricated over a large area. The advantage of SAS method compared to the conventional anti-solvent dropping (DAS) method includes less anti-solvent usage, rapid delivery of the anti-solvent, faster rate of super-saturation and denser nucleation and the production of more uniform over large area. Time resolved-PL measurement, and EL/PL imaging also confirms better film quality for better electrical properties such as interfacial series resistance and lower effective dark saturation current in the device fabricated by the SAS method. A semi-distributed diode model is used to optimise grid design without computational complexity and good accuracy with a trade-off between the series resistance from the FTO/metal grid and the shading loss from the metal grid. Through the inclusion of a metal grid, we have successfully demonstrated a 16 cm2 perovskite device at the cell level with a certified highest efficiency at 12.1 %.

ASSOCIATED CONTENT

Supporting Information Materials preparation, sample fabrication, characterization method, and additional data.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] and [email protected]. Phone: +61 2 9385 4257

Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT The Australian Centre for Advanced Photovoltaics (ACAP) encompasses the Australianbased activities of the Australia U.S. Institute for Advanced Photovoltaics (AUSIAPV) and is supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). We thank the Analytical Centre at UNSW for their technical support.

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