## 2 Materials and Methods  
Thin CuSe/Se films with a thickness of 130 nm and dimensions of 15 \(\times\) 35 mm were synthesized on a glass substrate by successive vacuum thermal deposition of Se and Cu, according to the procedure described in one of our previous publications [14]. To stabilize the residual Se [53; 54], the synthesized film structure was annealed at a temperature of 140 \({}^{\circ}\)C for 30 min. As a result, the selenium from an unstable amorphous phase was transformed into a stable polycrystalline trigonal selenium (\(t\)-Se). It should be noted that currently there are some other methods of film copper selenides structure synthesis that have also been developed [55; 56; 57; 58; 59; 60]. The phase composition of the films was studied at room temperature on a Bruker D2 PHASER diffractometer. We used CuK\(\alpha\) radiation (\(\lambda\) = 0.154 nm). Using the DIFFRAC.EVA universal program, the diffraction curves were smoothed and the background due to X-ray scattering on a glass substrate was subtracted. To study the phase composition of the films, we additionally used a HORIBA HR800 Raman spectrometer with laser excitation at 632.8 nm at the radiation intensity of 14 kW/cm\({}^{2}\). Sample surface images were taken using a scanning electron microscope (SEM) (Thermo Fisher Scientific Quattro S, Brno-Cernovice, Czech Republic). The elemental composition of the synthesized films was determined using an energy-dispersive microanalyzer based on an EDAX \"Octane Elect Plus EDS System\" spectrometer built into the SEM. The optical transmittance spectra of the films were recorded using a two-beam spectrophotometer (PerkinElmer Lambda 650, Shelton, WA, USA). The film thickness was measured using a stylus profilometer. The refractive index \(n\) and the extinction \(\kappa\) of the film were determined using an ellipsometer. The X-ray diffraction pattern (see Figure S1, Supporting Information) and the Raman spectrum (see Figure S2, Supporting Information) demonstrate that the film has two phases and consists of CuSe and \(t\)-Se. The mass percentages of CuSe and \(t\)-Se in the synthesized film are 33.3 and 66.7 wt.%, respectively. The film surface consists of a number of flat petal like structures, which are CuSe crystallites predominantly oriented in radial directions from their centers (see Figure S3, Supporting Information). The centers of these structures are located at distances of about 15-30 \(\upmu\)m from each other. The film is semitransparent in the visible wavelength range (see Figure S4, Supporting Information). The optical transmittance, refractive index \(n\), and extinction coefficient \(\kappa\) at 795 nm are 27.6%, 1.64, and 2.14, respectively. The synthesized film is electrically conductive with a sheet resistance of 39 \(\Omega/\Box\), which is orders of magnitude lower than the corresponding value of many known thin-film semiconductor structures (see, for example, [61; 62]). It should be added that the sheet resistance of CuSe thin films with thicknesses from 50 to 500 nm synthesized by the chemical bath method varies between 23-50 \(\Omega/\Box\)[63]. Meanwhile, the sheet resistance of 280 nm thick bismuth selenide (Bi\({}_{2}\)Se\({}_{3}\)) film obtained by the same method immediately after synthesis is 10\({}^{12}\)\(\Omega/\Box\), and after its annealing in air or in the nitrogen medium, the sheet resistance decreases to 3.6 \(\times\) 10\({}^{3}\)\(\Omega/\Box\)[64]. To measure the laser-induced photocurrent (photovoltage), two film gold electrodes A and B were deposited on opposite short sides of the synthesized film by vacuum thermal deposition. The film under study was placed on a special goniometric device, which made it possible to smoothly change its spatial orientation relative to the incident laser beam. The surface of the film remaining after depositing the electrodes was sufficient to study the generation of photocurrent under the action of a narrow laser beam 1.5 mm in diameter in a wide range of incidence angles.