Overcoming the Challenges of Large Area High Efficiency Perovskite Solar Cells

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1 Supporting Information 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 Materials and Methods Materials All chemicals were purchased from Sigma-Aldrich or Alfa Aesar or Lumtec (Luminescence Technology Corp.) and used without further purification. HC(NH 2 ) 2 I were synthesized by reacting 15 g formamidine acetate and 30 ml HI (57 wt% in water) in 100 ml ethanol at 0 C for 2 hours with stirring. The precipitate was recovered by rotary evaporator at 50 C for 1 hour. The white crystals were obtained by washing with diethyl ether followed by recrystallization from Ethanol. CH 3 NH 3 Br were synthesized by reacting 11 ml methylamine solution (33 wt% in water) and 10 ml HBr (48 wt% in water) in 100 ml ethanol at 0 C for 2 hours with stirring. The precipitate was recovered by rotary evaporator at 50 C for 1 hour. The white crystals were obtained by washing with diethyl ether followed by recrystallization from Ethanol.

2 To prepare 1.2M HC(NH 2 ) 2 PbI 3 (or CH 3 NH 3 Br 3 ) solution, the prepared HC(NH 2 ) 2 I (or CH 3 NH 3 Br) is mixed with PbI 2 (or PbBr 2 ) in Dimethylformamide (DMF) : Dimethyl sulfoxide (DMSO) mixed solvent (1:0, 1:0.25, or 1:0.17 volume ratio) at room temperature. For the solution of (HC(NH 2 ) 2 PbI 3 ) 0.85 (CH 3 NH 3 PbBr 3 ) 0.15, prepared HC(NH 2 ) 2 PbI 3 and CH 3 NH 3 Br 3 solution are mixed with the corresponding volume ratio. In case of perovskite solution which we used for this study, extra PbI 2 (5 mol% to HC(NH 2 ) 2 PbI 3 ) were mixed with the prepared (HC(NH 2 ) 2 PbI 3 ) 0.85 (CH 3 NH 3 PbBr 3 ) 0.15 solution and heated at 60 C for 30 mins. Solar Cell Fabrication A dense blocking layer of TiO 2 (bl-tio 2 ) was deposited by spray pyrolysis using 20 mm titanium diisopropoxide bis(acetylacetonate) solution at 450 C on the clean FTO glass (TEC8) with metal grid for large area device. On top of the bl-tio 2 layer, 150 mg ml -1 of mesoporous TiO 2 paste (m-tio 2, Dyesol 30 NR-D) in ethanol was spin-coated at 5000rpm (acceleration of 2000 rpm s -1 ) for 10 sec. Then, the substrates were annealed at 100 C for 10 mins followed by sintering at 500 C for 30 mins. The prepared perovskite solution was spun at 2000 rpm (acceleration of 200 rpm s -1 ) and 6000 rpm (acceleration of 2000 rpm s -1 ) for 10 sec and 30 sec, respectively. During the last 20 sec of the second spin-coating step, the anti-solvent chlorobenzene was drop-casted (110 µl or 688 µl) or sprayed (RG-3L, ANEST IWATA) for 3 sec at the pressure from 20 psi to 60 psi. The perovskite film was dried on a hot plate at 100 C for 20 min. A solution of Spiro-OMeTAD containing 72.3 mg Spiro-OMeTAD, 17.5 µl of a 520 mg ml -1 lithium bis (trifluoro-methylsulphonyl)imide in acetonitrile and 31.2 µl of 4-tertbutylpyridine in 1 ml chlorobenzene was spin-coated on the perovskite/m-tio 2 /bl-tio 2 /FTO substrate at 2000 rpm (acceleration of 1200 rpm s -1 ) for 20 s. All films on m-tio 2 were prepared in nitrogen filled glovebox. A gold electrode was deposited by thermal evaporation. Characterization method Optical image were taken using digital single-lens reflex camera (Nikon D800) and UV-VIS absorption (the optical reflection and transmission spectra) was measured using Perkin Elmer Lambda1050 UV/Vis/NIR spectrophotometer. X-ray diffraction (XRD) patterns were measured

3 using a PANalytical Xpert Materials Research diffractometer system with a Cu Kα radiation source (λ = nm) at 45 kv and 40 ma. Top-view SEM images were obtained using a field emission SEM (NanoSEM 230). For cross-section SEM image, the sample was milled using the ZEISS Auriga FIB/SEM to produce the cross section and the cross section was then imaged using the FEI SEM450 SEM. Fourier transform infrared spectroscopy (attenuated total reflectance (ATR) accessories, cm -1 ) is carried out using a Perkin Elmer Spotlight 400 FT-IR imaging system on thin perovskite films on FTO glass. The time-resolved PL were performed by a microtime200 microscope (Picoquant) using the TCSPC technique with an excitation of 405 nm laser detected with a 750/40 nm band-pass filter. PL imaging of complete solar device was conducted using LED with excitation wavelength of 635 nm and illumination intensity of about 4.8 mw cm -2 ( cm -3 ). A nm bandpass filter was used before the camera lens to exclusively detect the emission from the perovskite active layer. For electroluminescence (EL) measurements, a commercially available 1-mega-pixel Si-CCD camera (Princeton Instruments, Trenton, NJ, USA) was used to detect the luminescence signal. A nm bandpass filter was used before the camera lens to exclusively detect the emission from the perovskite active layer. The voltage bias control and current reading was performed by a source measurement unit (Agilent Technologies). The J V measurements were performed using a solar cell I V testing system from Abet Technologies, Inc. (using class AAA solar simulator and Keithley 2400 source meter) under an illumination power of 100 mw cm -2 with an 1.2 cm 2, 5.8 cm 2 or 16 cm 2 aperture and a scan rate of 0.1 V s -1. All J-V measurements were undertaken at room temperature in ambient condition. To remove noise in the dark J-V data (J d (V)), in particular, at low voltages, a Gaussian filter was applied to smooth the data out so that the derivative of Log[J d (V)] was continuous without modifying the behaviour above the maximum power point. Fitting of the PL decay traces The PL decay traces can be fitted using the bi-exponential function in equation S1 to deduce two decay components for each sample where I is PL decay, α 1 is weighting of τ 1, α 2 is weighting of τ 2, τ 1 is the fast component and τ 2 is the slow component.

4 t t I = α1 exp + α2 exp τ1 τ 2 (S1) By taking into account of the weighting α i for each lifetime component, τ i, the effective lifetime τ eff, 10 can be found using equation S2. τ eff = i i α τ 2 i i α τ i i (S2) Calculating current transport efficiency maps from EL images Current transport efficiency is defined as the ratio of the differential change in the light-induced current generated at a specific point in the device with respect to the differential change in the current collected at the terminals. Deploying the reciprocity theorem in a linearized circuit form, for incrementally different external voltage biases (Va), f T is calculated using Equation S r r r V j( ) ln( φel ( )) ft ( ) = (S3) V a ( V a ) V th where V j is the internal junction voltage of the device which is a function of position r. V th is the thermal voltage and Ф EL ( r ) is the spatial distribution of the EL signal captured by the camera. In order to arrive at equation S3 for calculating the current transport efficiency f T ( r ) for incrementally different external voltage biases (V a ), it is assumed that the internal junction voltage, V j, of the device which is a function of position, r, is much greater than the thermal voltage, V th. Presuming the device temperature, T, is remained at 300 K during the 30 seconds measurement in this study V th = k b T/q 25.9 mev, where k b and q are the Boltzmann constant and unit charge, respectively. All parameters in the right-hand side term are measurable r including φel(r), which is the spatial distribution of the EL signal captured by the camera (see the related Experimental Section). The spatial distribution of this parameter is influenced by both the series resistance and the diode saturation current non-uniformity 1. In order to collect sharp images with clearly resolved features, the EL imaging was conducted at voltage biases slightly higher than (i.e. 20 mv) the measured V oc (1100 mv and 1124 mv for dropping and spaying

5 method, respectively). We note that the reported peak value for the photocurrent transport efficiency is underestimated with respect to the collection efficiency at short-circuit condition since the differential biasing is slightly larger than V oc 3-4. We have assumed that within the range of the incremental voltage change of 10 mv the photovoltaic external quantum efficiency, EQE PV (E), remains unchanged. Methods for J-V and m-v fitting In order to determine how the series resistance (R S ) and shunt resistance (R SH ) changed with cell size, the J-V curves were fitted using a semi-empirical model based on the work by Nair et al, given in equation S4. The terms RR, RS and RA represent the radiative, Shockley- Reed-Hall and Auger recombination currents respectively, and are presented in equations S5, S6, and S7. (S4) (S5) (S6) (S7) For cells with a front grid, the shading needs to be accounted for, and is represented by S. In this way, shading acts to increase recombination, reducing V OC. The current collection efficiency, η collect, is based on the work 7 for n-i-p diodes, given the built in voltage (V bi ), carrier diffusion length (L) and active layer thickness (W). The V bi was approximated as, where N b is the carrier density of states in the ETM and HTM, and n i is [ ] sh s collect ph R R J V V J RA V J RS V J RR S J J = ), ( ), ( ), ( 1 1 η + = 1 ) ( ), ( 0 T K R J V q Exp J V J RR b s R + = 1 2 ) ( 2 ), ( T K R J V q Exp n W q V J RS b s i τ SRH + = 1 2 ) ( 3 2 ), ( 3 T K R J V q Exp W n A q V J RA b s i = i b B bi n N Log q T K V 2

6 the intrinsic carrier density in the perovskite layer. J R0 is the radiative recombination coefficient, determined using the Transfer Matrix Method described by C. Katsidis and D. Siapkas, 8 and the radiative recombination approximation described by Green. 9 Surface recombination was ignored, and incorporated into the bulk lifetime value (τ SRH ) for fitting. This is adequate given that the modelled perovskite thickness is the same value in every simulation. These parameter values used for this fitting are given in Table S3. For a single value of R S, the simulated JV curve has a sharp knee. The measured perovskite cells have a fairly rounded knee, as a result of the value of R S varying over the cell area. This prevents the simulated JV curve from being accurately matched to the measured JV curve over the entire voltage range, especially near the maximum power point. By introducing a two-cell model, with each cell having a different value of R S, the distribution of R S can be more accurately approximated, and the J-V curve matched almost exactly. In equation S8, the current of two diodes are added in parallel, the first diode covering a fraction of the total area f 1 with Rs = Rs 1, and the second covering a fraction of the total area (1-f 1 ), with Rs = Rs 2. Both diodes share the same value of R SH. The equivalent circuit diagram is depicted in Figure S10c. J( V) = f1 J1( RS1, V) + (1 f1) J2( RS 2, V) (S8) The local ideality factor m(v) of the cells were also computed according to equation S9: [ V ] q d( Log J d ( ) m( V ) = KBT dv 1 (S9) Where J d is the dark current of the cell. Methods for calculating R S from front grid design The resistance of the front grid was modelled using the lumped-resistance method described by Sokolic et al., 5 and A. Burgers. 6 Specifically the resistances (Ω-m 2 ) of busbars, fingers and the sheet resistance (R b, R f, R TCE ) are given by equations S10-12, where ρ is the material resistivity;

7 d is the material height; and w the material width. L b, L f and L TCE are the lengths of the busbar, fingers and transparent conductive electrode (TCE) spacing respectively. R b 1 ρm L = 3 d w m 2 b m (S10) R f 1 ρ m L = 12 d w m 2 f m (S11) R TCE 1 ρ L = 12 d 2 TCE (S12) Crucially resistance increases for all elements with the square of element length, and decreases with increasing element height. Hence decreasing length and maximizing height are key criteria for reducing R S. Increasing the number of busbars decreases the finger length, reducing finger resistance while increasing shading. Similarly increasing the number of fingers decreases the length of the TCE regions, reducing TCE resistance while increasing shading. By optimizing this trade-off, the efficiency can be maximized for a given grid design.

8 Figure S1. Effect of different spray pressures on film morphology. Schematic illustration of sprayed chlorobenzene on perovskite film and SEM images (scale bar: 1 mm) of the perovskite film fabricated by anti-solvent spraying at different pressures: (a) psi, (b) 40 psi, and (c) psi. Figure S2. Box plots of the (a) V OC, (b) J SC, (c) FF, and (d) PCE of devices fabricated by SAS method as a function of DMSO content in the perovskite precursor.

9 Figure S3. Optical images showing perovskite film on glass fabricated by DAS method but uses equivalent amount of chlorobenzene required for SAS (a) before and (b) after annealing at 100 C for 20 mins. Scale bars are 1 cm. Figure S4. XRD patterns for perovskite films fabricated by DAS method (a) before and (b) after annealing for 20 minutes. XRD patterns for perovskite films fabricated by SAS method (d) before and (e) after annealing for 20 minutes. SEM images of annealed perovskite films fabricated by (c) DAS and (f) SAS methods. The scale bar in SEM images: 1 µm. FTO peaks are marked with hash marks (#). For the annealed films, both exhibit perovskite peaks at 13.92, 19.76, 24.35, 28.09, and which are indexed to (110), (200), (202), (220), and (222). Also, XRD peak at 12.5 arises from the (001) lattice planes of hexagonal (2H polytype) PbI 2 which is due to 5 mol% of excess PbI 2 in the (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 solution for improving photovoltaic performance

10 Figure S5. Fourier transform infrared spectroscopy (FTIR) results for perovskite films on FTO/glass by (a) DAS and (b) SAS methods before annealing. Typical stretching vibrations for DMSO S=O (ν(s=o)) appears at 1045 cm 1. The shift to 1,017 cm 1 is due to the intermediate phase of DMSO-Pb 2+.

11 Figure S6. (a) Positions of the perovskite/m-tio 2 /bl-tio 2 /FTO/glass samples on which time resolved PL was carried out. The samples were illuminated from the glass side. PL decay traces and fitted curves for each region of the samples fabricated by the (b) DAS and (c) SAS methods. (d) The effective lifetimes from the samples fabricated by the two methods.

12 Figure S7. Photoluminescence images at open-circuit condition on full devices gold/spiro- OMeTAD/perovskite/m-TiO 2 /bl-tio 2 /FTO/glass fabricated by (a) DAS and (b) SAS methods. Filtered PL images of the same (c) DAS (d) SAS devices only showing features that have intensity < 15,500 counts. (e) The intensity distribution of the (a) and (b). We note that the features in (c) and (d) and the tail (on the left hand side) in the intensity distribution in (e) are associated with the damaged (scratched) regions in the cell, which is more prominent in the SAS device in this case. The scale bars are 3 mm.

13 Figure S8. Cross-sectional SEM image of our solar cell devices. Scale bar is 500 nm.

14 Figure S9. Independent certification (19 th Sep 2016) from Newport confirming a power conversion efficiency of 18.04% for a 1.2 cm 2 cell.

15 Figure S10. (a) Measured and (b) modelled local ideality factor as a function of voltage (m-v) of the 1.2, 5.8 and 16 cm 2 cells without front metal grid. (c) The equivalent circuit diagram for the model. The rough features in measured m-v at low voltage are a result of signal noise in the dark I-V curve at low current values. As the cell size increases, the error in current becomes proportionally smaller, resulting in smoother curve. Decreasing R SHUNT increases the height of the peak at 0.6V, and increases the height of the local ideality factor near 1.0V. The measured data hence gives effective minimum R SH value that aids with J-V parameter fitting.

16 Figure S11. Modelled J-V curves for a 16 cm 2 cell with different grid designs. The drop in J SC is predominantly due the shading incurred through the use of a grid. The increase in FF due to reduced R S results in improved efficiencies. Figure S12. 16cm 2 cell fabricated using grid design 3. The widths of busbar and fingers are 2mm and 100 µm, respectively. The spacing between the fingers is 6.5 mm.

17 Figure S13. Independent certification (30 th Aug 2016) from Newport confirming a power conversion efficiency of 12.1 % for a 16 cm 2 perovskite single-cell.

18 Figure S14. Modelled J-V curves for a 16 cm 2 cell with improved grid designs for future work.

19 Figure S15. Performance of encapsulated large (16 cm 2 ) perovskite solar cell when stored in the ambient. (a) Temperature and relative humidity recorded over the 2-month period. Normalized (b) J SC, (c) V OC, (d), FF, and (e) PCE during storage. Error bars are standard deviations of each parameter.

20 Figure S16. Stabilized current density and power conversion efficiency at maximum power point (0.76 V) of the encapsulated large (16 cm 2 ) perovskite solar cell measured under continuous illumination for 45 seconds. The cell is 5 months old after fabrication and has undergone outdoor demonstrations and has been transported internationally.

21 Table S1. Calculated integral peak area underneath (110), (200), and (220) perovskite phases for film fabricated by DAS and SAS processes Integral peak area (a.u.) Perovskite phase DAS SAS (110) (200) (220) Ratio (110)/(200) (220)/(200) Tables S2. Bi-exponential fitting results of PL decay traces of perovskite/m-tio 2 /bl- TiO 2 /FTO/glass samples fabricated by the DAS and SAS methods. τ 1 and τ 2 are the decay time, and α 1 and α 2 are the weighting of fast and slow component, respectively. DAS SAS Region α 1 α 2 τ 1 [ns] τ 2 [ns] τ eff [ns] α 1 α 2 τ 1 [ns] τ 2 [ns] τ eff [ns] a 39% 61% % 50% b 53% 47% % 48% c (center) 39% 61% % 56% d 36% 64% % 54% e 35% 65% % 50%

22 Table S3. Parameters used for J-V and m-v fitting. parameter Symbol Value Units Source Photo generated current J ph ma/cm2 Experimental Shading fraction S Experimental Radiative dark current JR0 3x10-16 ma/cm2 Simulation Perovskite thickness W 400 nm Experimental Shockley Reed-Hall bulk lifetime τ SRH us - Perovskite intrinsic carrier density ETM and HTM carrier density of states n i 10 6 cm -3 Calculated N b 3x10 20 cm -3 ref 12 Auger recombination coefficient A Cm- 6 ref 13 Carrier Diffusion length L 800 nm ref 14 Band gap E g 1.57 ev Experimental Perovskite intrinsic carrier density was calculated using the method described in reference 15. The value of n i strongly determines the V OC. Further model assumptions include: n=p, N b =N C =N V, L=L h =L e, where n and p are the electron and hole carrier densities, N C and N V are the conduction and valence band density of states, and L h and L e are the diffusion lengths of holes and electrons respectively.

23 Table S4. Cell efficiencies calculated for different grid designs including bus bar width, finger width, and finger spacing. Cell w/o grid Grid V1 Grid V2 Grid V3 (implemented) Busbar width [µm] Finger width [µm] Finger & busbar height [nm] Average Finger spacing [mm] Metal resistivity [Ω-cm] N/A N/A N/A N/A N/A 9.3x x x10-6 Shading [%] J SC [ma/cm 2 ] Calculated Efficiency [%] 5.5% 8.5% 10.7% 12.0%

24 Tables S5. Electrical characteristics of 1.2 cm 2 (aperture area) perovskite solar cell fabricated by conventional DAS method measured at UNSW in V OC ->J SC direction at a sweep rate of 0.1 V/s. Cell No. Jsc (ma/cm2 Voc (mv) FF (%) Efficiency (%) Average Standard deviation

25 Tables S6. Electrical characteristics of 1.2 cm 2 (aperture area) perovskite solar cell fabricated by new SAS method measured at UNSW in V OC ->J SC direction at a sweep rate of 0.1 V/s. Cell No. Jsc (ma/cm2) Voc (mv) FF (%) Efficiency (%) Average Standard deviation

26 Tables S7. Electrical characteristics of 16 cm 2 perovskite single-solar-cell with metal grid fabricated by SAS method measured at UNSW in V OC ->J SC direction at a sweep rate of 0.1 V/s. Cell No. Jsc (ma/cm2) Voc (mv) FF (%) Efficiency (%) Average Standard deviation

27 Table S8. Cell efficiencies calculated for improved grid designs including bus bar width, finger width, and finger spacing. Cell w/o grid Grid V3 Grid V4 Grid V5 Busbar width [µm] Finger width [µm] Finger & busbar height [nm] Average busbar spacing [mm] Average Finger spacing [mm] Metal resistivity [Ω-cm] N/A N/A N/A N/A N/A N/A N/A 9.3x x x10-6 Shading [%] J SC [ma/cm 2 ] Calculated Efficiency 5.5% 12.0% 14.2% 15.2%

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