## 2 Experimental Section  
Solar cells were fabricated in a superstrate configuration (Figure 1a), with ITO and ZnO as the front contact. CdS and Cd\({}_{0.85}\)Zn\({}_{0.15}\)S ((Cd,Zn)S) serve as the ETLs and, right after, antimony selenide (Sb\({}_{2}\)Se\({}_{3}\)) was deposited by Close-Spaced Sublimation (CSS) (details on growth parameters and technical specifications can be found in previous publications [9; 20]). The back contact was an amorphous thin film composed of iron, sulfur, and oxygen; we refer to this layer as Fe-S-O, and it was coated by an Au layer. All layers, except Sb\({}_{2}\)Se\({}_{3}\), were deposited using either r.f. magnetron sputtering (ZnO, CdS, (Cd,Zn)S) or d.c. magnetron sputtering (ITO, Fe-S-O, Au). The current-voltage (J-V) characteristics of the solar cells were assessed using a continuous LOT-Oriel solar simulator (Oriel, Irvine, CA, USA) equipped with an air mass AM1.5G filter. The simulator was equipped with a 600 W Xenon lamp (Oriel, Irvine, CA, USA) that gives a 1 kW/m\({}^{2}\) light power density. To ensure accuracy, a calibrated pyranometer served as a reference, and the measurements were conducted at the standard temperature of 298 K. For the intensity-dependent current-voltage measurements, neutral density optical filters with optical densities (OD) of 3, 2, 1.3, 1, and 0.3 were employed.  
J-V curves were taken under illumination, using a KEITHLEY 2400 (Keithley, Solon, OH, USA) source meter, between 233 K and 353 K in steps of 20 K. The antimony selenide-based solar cell was placed inside a thermostatic chamber and illuminated through an optical fiber coupled with a Linos photonics LQX 1000 lamp (Qioptiq Photonics, Gottingen, De). To achieve illumination in different spectral ranges, a combination of band-pass optical filters in the range from 1000 nm to 500 nm was employed.  
Capacitance-voltage (CV) measurements in the dark and at room temperature (RT) were carried out with a Keithley 4200-SCS semiconductor characterization system, selecting the highest available frequency (10 MHz) to limit the contribution of deep levels to the capacitance while varying the applied reverse bias within the range of from 0.4 V to 2 V. For admittance spectroscopy, experiments were carried out in the dark and under light using a Hewlett Packard 4284A (Hewlett Packard Enterprise, Palo Alto, CA, USA) PRECISION LCR METER by varying the frequency between 100 Hz and 1 MHz, and the temperature in the range 273 K-353 K.  
To determine the energy gap of CdS and (Cd,Zn)S, UV-vis absorption measurements were acquired with a Varian 2390 UV-VIS-NIR (Varian Inc., Palo Alto, CA, USA) in the range between 300 nm and 1500 nm using a transmission configuration.  
The (scanning) transmission electron microscopy ((S)TEM) investigations were carried out in an aberration-corrected Titan Themis G2 200-type microscope ((Thermo Fisher Scientific, Waltham, MA, USA)) operating at 200 keV equipped with an X-FEG gun, a 4 k \(\times\) 4 k CETA 16 CMOS camera (ThermoFisher Scientific), and a HAADF detector (E.A. Fischione Instruments (Fischione Instruments, Inc., PA, USA)). For the energy dispersive X-ray (EDX) mapping, a Super-X detector was used. The cross-sectional TEM samples were prepared by focused ion beam (FIB) technique using a ThermoFisher Scios 2 Dual Beam microscope with EasyLift\({}^{\rm{TM}}\) nanomanipulator. The TEM lamella was immediately movedto the TEM after preparation in FIB in order to reduce the contamination with air. After the TEM experiments, the lamella was stored in a vacuum.