RAW DATA EXPERIMENTAL Powder X-ray diffraction (PXRD) Initial compositional screening and solution processing experiments made use of PXRD data measured on a Panalytical X’Pert Pro diffractometer using Co Ka1 radiation (wavelength = 1.7890 Å) in Bragg–Brentano geometry and an X’Celerator detector. Phase identification was carried out using the X’Pert HighScore Plus (Version 2.2a) with the PDF-2-ICDD database. PXRD data of capillaries used for photostability assessment were measured on a Bruker D8 Advance diffractometer using monochromated Mo Ka1 radiation (wavelength = 0.7093 Å). TopasAcademic (Version 5) was used to perform Pawley fittings of the data. VESTA was used for graphical representation of the structures. Single crystal X-ray diffraction (SCXRD) Data were collected at 100K on a Rigaku MicroMax-007 HF diffractometer with a molybdenum rotating anode microfocus source and a Saturn 724+ detector using Rigaku Crystal Clear v2.0. Unit-cell indexation, data integration, and reduction were performed using Rigaku CrysAlisPro v171.38.43. The structure was solved and refined using SHELX-2013, implemented through Olex2. SEM and TEM EDX Compositional Analysis Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) was used to measure the composition as a direct elemental analysis technique. Measurements were carried out using a Hitachi S-4800 SEM with an Oxford Instruments model 7200 EDS X-ray detector. Quantification was carried out using the microanalysis suite of the Inca Suite software (Version 4.15). All powders and crystals were sputtered with 15 nm Au to limit charging effects. Transmission Electron Microscopy (TEM) EDX was carred out using an JEOL JEM 2000FXII TEM microscope operating a W electron source operated at 200 kV, using an EDAX EDX detector, with quantification carried out using EDAX Genesis Spectrum (Version 5.217, 21-Jan-2008). All samples were prepared by spreading a finely ground powder onto carbon coated Au grids. Beam intensity had to be lowered, by increasing the spot size of the beam, as to not decompose the samples. All compositions calculated from EDX measuments were normalised to the nominal iodide content unless otherwise stated Photostability For the experiments, a Solar Light Model 16S-300-002 Solar Simulator was used which has a spectral output that complies with air mass 1.5 (AM1.5) per the ASTM standard definition. The combination of neutral density filters and lamp-to-sample distance allowed for the tuning of the intensity of the incident light to 1000 W m-2 as measured by a Solar Light Pyranometer PMA2144 and datalogging radiometer PMA2100. Sample temperatures were monitored using a T-type thermocouple and were found to stay below 35°C. For measurements in sealed atmospheres, powder was loaded into thin-walled (0.01 mm wall thickness) borosilicate capillaries under ambient air, dry synthetic air, and helium, and these were sealed with a gas-oxygen torch. The capillaries were then placed in the solarsimulator and subjected to the full solar spectrum at an intensity of1000 W m-2. Raman Spectroscopy The measurements were carried out on a Renishaw inVia Reflex with a Leica microscope utilising a 633 nm wavelength red laser with a maximum power of 6.5 mW. The powder samples were measured in borosilicate capillaires and were exposed to 0.5% of the maximum laser power to avoid decomposition of the sample. The spot size was 5 µm. Photoluminescence (PL) Spectroscopy The Cu2AgBiI6 film was mounted in a gas-exchange helium cryostat (Oxford Instruments, OptistatCF2) and photoexcited by a 398 nm picosecond pulsed diode laser (PicoHarp, LDH-D-C-405M). The resultant PL was collected and coupled into a grating spectrometer (Princeton Instruments, SP-2558), which directed the spectrally dispersed PL onto a photon-counting detector (PDM series from MPD), whose timing was controlled with a PicoHarp300 TCSPC event timer. A laser fluence of 200 nJ cm-2 was used for both the spectral and transient measurements, which were both taken at a temperature of 295 K. The PL decay trace in Figure S32 was measured at a wavelength of 720 nm. The PL decay trace was fitted by a stretched exponential function I = I0 exp(-(t/t)ß), where ß is the distribution coefficient and t is the time taken for the PL intensity to drop to I0/e. Such stretched exponential functions have been used to phenomenologically account for the presence of a local distribution of monoexponential decay rates, whose average lifetime is given by tav = (t/ß) G(1/ß), where G is the gamma function. The PL spectrum for MAPbI3 in Figure 10b of the main text was measured using the same experimental setup, except it was mounted in a cold-finger cryostat (Oxford Instruments, MicrostatHe) and detected with an iCCD (PI-MAX4, Princeton Instruments), under a laser excitation fluence of 490 nJ cm-2.The spectrum was previously published in Wright et al. Fourier Transform Infrared (FTIR) UV-Visible absorption measurements using FTIR spectrometer. The UV-Visible absorption spectra in panels a and b of Figure 10 in the main text were measured using a Bruker Vertex 80v Fourier transform infrared (FTIR) spectrometer, configured with a tungsten halogen lamp illumination source, a CaF2 beamsplitter and a silicon detector. The samples were mounted in a gas-exchange helium cryostat (Oxford Instruments, OptistatCF2). The MAPbI3 absorpion spectrum was previously published in Davies et al. THz photoconductivity An amplified laser system (Spectra Physics, MaiTai–Empower–Spitfire) with a central wavelength of 800 nm, 35 fs pulse duration and 5 kHz repetition rate was used to generate THz radiation via the inverse spin Hall effect, using an emitter made of 2 nm of tungsten / 1.8 nm of Co40Fe40B20/ 2 nm of platinum, supported by a quartz substrate. The transmitted THz radiation was detected using free-space electro-optic sampling with a 1 mm thick ZnTe (110) crystal, a Wollaston prism and a pair of balanced photodiodes. The THz pulse was measured in transmission geometry. The pump beam was frequency-doubled to 400 nm by a ß-barium-borate (BBO) crystal. Mobilities were calculated from the initial transmitted signal at time = 0 ps at fluences of 7.8, 15.3 µJcm^(-2), as outlined in Wehrenfennig et al.,10 and measurements were carried out under low vacuum (<10^(-2) mbar). J–V characterisation J–V characterisation was measured using a Keithley 2400 sourcemeter and simulated air-mass 1.5 global tilt (AM1.5G) solar irradiation using a Wavelabs Sinus-220 light-emitting diode array, calibrated with a certified Si reference cell. The areas being measured were defined by using a black anodised aluminium masks placed directly in contact with glass side of the substrate and an enclosed sample holder, to shadow the rest of the device. Photothermal deflection spectroscopy (PDS) PDS is an ultrasensitive absorption measurement technique that detects heating of the sample due to the non­radiative relaxation of absorbed light and is insensitive to reflection and scattering. PDS enables the detection of absorbance signals with 5–6 orders of magnitude weaker than the band edge absorption. For the measurements, a monochromatic Pump light beam is shined on the sample (film on Quartz substrate), which on absorption produces a thermal gradient near the sample surface via non-radiative relaxation induced heating. This results in a refractive index gradient in the area surrounding the sample surface. This refractive index gradient is further enhanced by immersing the sample in an inert liquid FC-72 Fluorinert® (3M Company) which has a high refractive index change per unit change in temperature. A fixed wavelength CW laser probe beam is passed through this refractive index gradient producing a deflection proportional to the absorbed light at that particular wavelength, which is detected by a photo-diode and lock-in amplifier combination. Scanning through different wavelengths gives us the complete absorption spectra. Because this technique makes use of the non-radiative relaxation processes in the sample, it is immune to optical effects like interference and scattering. Furthermore, PDS technique is a powerful technique to measure the sub-bandgap tail states in a semiconductor up to an absorption coefficient of 1 cm-1. This enables us to probe precisely the Urbach tail states, which is a convolution of the absorption contributions from the defects, dopants, and thermal potential fluctuations in a semiconductor. Therefore, a parameter known as Urbach energy (Eu) can be extracted from PDS technique, which gives information about the structural order in the material. The Urbach energy can be calculated from the inverse of the slope of a linear fit to the Urbach tail in the absorption spectra plotted on a natural logarithmic scale. It has a unit of energy (eV) per decade but generally, only energy unit (eV) is explicitly specified. Optical Modelling The generalised transfer matrix method was used to model the optical response of the stack. The python libraries Numpy and Scipy were used to perform the calculations. Transfer matrix calculations take the complex refractive index spectrum and thickness for each layer as input. The calculation provides us with absorptance of each layer, and the transmittance and reflectance of the stack. We assumed perfect internal quantum efficiency and calculated the short circuit current J_sc as the overlap integral of the AM1.5 solar spectrum with the absorptance. The JV curve of each sub cell was modelled as a single diode: J=J_sc-J_0 e^(qV/nkT) Here, n is the ideality factor. The recombination current J_0 is calculated through the principle of detailed balance: J_0=08EQE_PV (E)·?BB,300K(E)·dE Here ?_(BB,300K) (E) is the blackbody photon flux at 300K, and EQE_PV (E) is the absorptance calculated from the transfer matrix calculation. The following diode parameters were assumed for the perovskite sub-cell: R_(Shunt,perov)=5 kO·cm^2, R_(Series,perov)=4.2 O·cm^2,n_perov=1.0, EQE_(EL,perov)=0.01. The calculated JV parameters for the perovskite subcell were: PCE = 18.1%, VOC = 1.16 V, JSC = 19.0 mA/cm2, FF = 0.82. The following were assumed for the Si sub-cell: R_(Shunt,Si)=10 kO·cm^2, R_(Series,Si)=0.4 O·cm^2,n_Si=1.04, EQE_(EL,Si)=0.0056. The calculated JV parameters for the Silicon sub-cell were: PCE = 12.1%, VOC = 0.75 V, JSC = 19.0 mA/cm2, FF = 0.84. The thickness of the LiF (20-200nm) the two ITO layers (20-200nm), and Cu2AgBiI6 (1200-1800nm) were varied with the indicated bounds using a differential evolution algorithm till a PCE maximum was obtained. The stack used as input for the Transfer Matrix Calculations is given in the S.I. The source of optical constants for each layer are also cited. For Cu2AgBiI6, a “synthetic” absorption co-efficient was created by splicing a 79 meV tail (between 1.0 eV-1.88 eV) to the extinction co-efficient obtained from reflectance and transmittance measurements (See S.I.). The extinction co-efficient was set to zero below 1.0 eV. This was then converted into the extinction coefficient which was transformed into the refractive index using the Kramer’s Kronig relation (See S.I.). RAW DATA CONTENTS DEVICES J-V curve FB-SC Cu2AgBiI6 devices Figure 4a J-V curve SC-FB Cu2AgBiI6 devices Figure 4a Stabilised output Cu2AgBiI6 devices Figure 4b OPTICAL Cs2AgBiBr6_PLtransient Figure S16b Cu2AgBiI6_FTIRabsorption Figure 3a Cu2AgBiI6_PLspectrum Figure 3a Cu2AgBiI6_PLtransient Figure S16b Cu2AgBiI6_PLtransientFit Figure S16b FTIR absorption Cs2AgBiBr6 Figure3a MAPbI3_FTIRabsorption Figure 3a MAPbI3_PLspectrum FigureS15 PDS Cu2AgBiI6 film FigureS16a PXRD AgBiI4 stability CONTROL 1week Figure S7 AgBiI4 stability_air_1week Figure S7 Cu2AgBiI6 film Figure S9 Cu2AgBiI6 powder Figure 1i Cu2AgBiI6 stability_air_1week Figure S7 Cu2AgBiI6 stability_CONTROL_1week Figure S7 CuBiI4 powder Figure S4 CuI-BiI3 solid solution PXRD_0p18 Figure S20 CuI-BiI3 solid solution PXRD_0p21 Figure S20 CuI-BiI3 solid solution PXRD_0p23 Figure S20 CuI-BiI3 solid solution PXRD_0p25 Figure S20 CuI-BiI3 solid solution PXRD_0p27 Figure S20 CuI-BiI3 solid solution PXRD_0p29 Figure S20 CuI-BiI3 solid solution PXRD_0p32 Figure S20 RAMAN SPECTROSCOPY AgBiI4 stability exp Figure S8 Cu2AgBiI6 stability exp Figure S8 SCXRD All_twins_hklf5 SEM and TEM EDX SEM EDX Cu2AgBiI6 crystal Figure S2 SEM EDX CuBiI4 powder Figure S5 TEM EDX Cu2AgBiI6 film Figure 2 TEM EDX Cu2AgBiI6 powder cooled from 350C Figure S1 TEM EDX Cu2AgBiI6 powder not corrected Figure 2 TEM EDX Cu2AgBiI6 powder quenched 350C_1 Figure S1 TEM EDX Cu2AgBiI6 powder quenched from 350C Figure S1 TEM EDX Cu2AgBiI6 powder quenched from melt Figure S1 Simulated Tandems Figure S17 Cu2AgBiI6 optimal thickness c-Si_subcell_tandem_simulated_JV_data Cu2AgBiI6_subcell_tandem_simulated_JV_data Tandem_simulated_JV_data Tandem_simulated_EQE_data Figure S18 Cu2AgBiI6 other thicknesses Cu2AgBiI6_Si_EQE_530nm Cu2AgBiI6_Si_EQE_1000nm Cu2AgBiI6_Si_EQE_1400nm Cu2AgBiI6_subcell_JV_530nm Cu2AgBiI6_subcell_JV_1000nm Cu2AgBiI6_subcell_JV_1400nm Si_subcell_JV_530nm Si_subcell_JV_1000nm Si_subcell_JV_1400nm tandem_JV_530nm tandem_JV_1000nm tandem_JV_1400nm Figure S19 Cs2AgBiBr6 Cs2AgBiBr6_Si_tandem_EQE