Highly Air Stable Tin Halide Perovskite Photovoltaics using a Bismuth Capped Copper Top Electrode

Abstract An effective approach is reported to enhance the stability of inverted organo‐tin halide perovskite photovoltaics based on capping the cathode with a thin layer of bismuth. Using this simple approach, unencapsulated devices retain up to 70% of their peak power conversion efficiency after up to 100 h testing under continuous one sun solar illumination in ambient air and under electrical load, which is exceptional stability for an unencapsulated organo‐tin halide perovskite photovoltaic device tested in ambient air. The bismuth capping layer is shown to have two functions: First, it blocks corrosion of the metal cathode by iodine gas formed when those parts of the perovskite layer not protected by the cathode degrade. Second, it sequesters iodine gas by seeding its deposition on top of the bismuth capping layer, thereby keeping it away from the electro‐active parts of the device. The high affinity of iodine for bismuth is shown to correlate with the high polarizability of bismuth and the prevalence of the (012) crystal face at its surface. Bismuth is ideal for this purpose, because it is environmentally benign, non‐toxic, stable, cheap, and can be deposited by simple thermal evaporation at low temperature immediately after deposition of the cathode.


Experimental Procedures
Grazing incidence small-angle X-ray scattering (GISAXS) measurements were made using a Xenocs Xeuss 2.0 equipped with a micro-focus Cu Kα source collimated with Scatterless slits.The scattering was measured using a Pilatus 300k detector with a pixel size of 0.172 mm × 0.172 mm.The distance between the detector and the sample was calibrated using silver behenate (AgC 22 H 43 O 2 ), giving a value of 2.480(5) m.The magnitude of the scattering vector (q) is given by q=4π sin(θ)⁄λ, where 2θ is the angle between the incident and scattered X-rays and λ is the wavelength of the incident X-rays.This gave a q range for the detector of 0.003 Å-1 and 0.13 Å-1 in the horizontal plane.This q range allows crystallite sizes between 1 and 200 nm to be determined.Samples were aligned such that the surface was parallel to the beam and in the centre of the beam.To maximize the scattering signal from the Ag or Cu layers the sample was positioned at an incidence angle (αi) of 0.3° which is close to the critical angle of 0.4° for Ag or Cu with Cu Kα radiation.Integrating the in-plane scattering as a function of q allows the horizontal radius of the crystallites to be determined.The horizontal scattering was fitted using spheres with a lognormal distribution of the radius using the Irena analysis package. [10]

Figure S2|
Figure S2|Evolution of the J SC , V OC, FF and  for unencapsulated perovskite PV devices

Figure S3|
Figure S3|Evolution of the J SC , V OC, FF and  for unencapsulated perovskite PV devices

Figure S4|
Figure S4|Plot of the time taken to achieve peak power conversion efficiency (PCE) vs time

Figure S5|
Figure S5| Representative photographs of PPV devices supported on 1.44 cm 2 glass substrates before (left) and after (middle and right) degradation in air.The colour change from deep red to transparent yellow in those areas not covered by the cathode is indicative of perovskite degradation.

Figure S6|
Figure S6| SEM images of the perovskite film underneath the electrode after removal of the electrode and the organic semiconductor layers: (a) aged device (145 hours testing from peak power conversion efficiency) and (b) fresh device (i.e.not tested).

Figure S7|
Figure S7| EDX analysis elemental maps for the cross-sectional TEM image shown in Figure 2 of the main manuscript.The cross-section was taken from near to the edge of the Cu / Bi cathode of an aged PPV device with the structure: ITO|PEDOT:PSS (Al 4083)|perovskite|C 60 |BCP|Cu|Bi.

Figure S8|
Figure S8| TEM cross-section and EDX line profile along the yellow line indicated in the TEM image.Please note that the sample cross-section is mounted on a Cu grid for TEM analysis and so the Cu signal is not a true representation of the Cu distribution across the sample.

Figure S9| Upper:
Figure S9| Upper: Large area SEM image showing part of a PPV device cathode after stability testing with middle and edge areas indicated by A and B respectively.Lower: Representative higher resolution SEM images taken from the middle (A) and the edge (B).

Figure S10|
Figure S10| Elemental compositions (atomic percentages) determined using EDX at the locations indicated in TEM image above.The composition of the oxide layer is estimated from spectrum 4.

Figure S11| (
Figure S11| (Left) Photograph and the schematic (Right) of the experimental set-up used for monitoring the resistance change of bismuth capped copper electrodes exposed to I 2 gas emanating from an adjacent decomposing perovskite film.

Figure S12| Figure S13|
Figure S12| XRD pattern of a 100 nm Cu film supported on a Si wafer coated with a C 60 (32.5nm) layer and BCP (5 nm) layer, before and after exposing to constant illumination for 20 hours in ambient air.

Figure S15|
Figure S15| Powder X-ray diffraction patterns of Bi films (35 nm) supported on silicon wafer (blue) and Cu (100 nm) coated Si wafer (red).The very intense peak at  28.5 o and the peak at  43 o result from the underlying single-crystal silicon substrate and polycrystalline Cu film respectively.

Figure S16|
Figure S16| EDX spectra of a 35 nm Bi film on Cu (Blue) and 35 nm Bi film on a silicon wafer after exposure to the same I 2 gas environment for the same duration.

Table S2|
Stability of unencapsulated organo-tin perovskite PV devices tested in ambient air under 1 sun continuous simulated solar illumination reported in the literature to date.
Table S3|The Sn:I ratio at the edge and centre of the cathode of a degraded PPV device determined using spatially resolved EDX analysis.The edge and middle regions are indicated in FigureS8.

Table S4|
Two point electrical resistance of strips (2 mm wide and 10 mm long) of Cu (100 nm) | Bi (35 nm) supported on glass substrates pre-coated with C 60 |BCP layer.Electrical contact was made at either end of each strip with contact to the Bi and Cu layers.