Bandgap gradients in (Ag,Cu)(In,Ga)Se2 thin film solar cells deposited by three-stage co-evaporation

Abstract

Ag,Cu)(In,Ga)Se 2 (ACIGS) solar cells are optimized at bandgaps greater than 1.2 eV by varying composition profile of the absorber layer using a three-stage evaporation process. Numerical modeling and cumulative process data provides insight into the process. Silver alloying CIGS changes the optimized bandgap profile by reducing carrier concentration, and reducing bandgap gradients. The minimum bandgap position is controlled by the point when the film reaches I/III stoichiometry during the second stage of the three-stage process. We achieved a 19.9% efficient solar cell with V OC = 732 mV at a bandgap of 1.2 eV based on quantum efficiency.

Wide‐gap (Ag,Cu)(In,Ga)Se2 solar cells with different buffer materials—A path to a better heterojunction

“…While Ga accumulation towards the back contact is evident for Ag/I = 0.2 and 0.5, a more linear grading is detected for the highest Ag concentration. A flattening of the GGI profile was seen previously for high Ag contents . However, the GGI value at the interface to the buffer layer (GGI IF ) is the same for these three films (GGI IF = 0.68).…”

“…A flattening of the GGI profile was seen previously for high Ag contents. 35 However, the GGI value at the interface to the buffer layer (GGI IF ) is the same for these three films (GGI IF = 0.68). In the case of samples with GGI = 0.85, a much weaker GGI grading is observed, ranging from about GGI IF = 0.84 to GGI BC = 0.87 at the back contact.…”

“…33,34 The exchange of Cu by Ag also affects the charge distribution and free carrier density. A reduced p-type doping is frequently reported when adding Ag, 30,35,36 and even n-type doping is suggested for AgInSe 2 . 23 It is found that Na supply via in-diffusion from a soda-lime glass (SLG) substrate is significantly enhanced by Ag addition, 28 which could result in "too much" Na incorporation and may be the reason for the reduced doping density.…”

Experimental Investigations of Phase Equilibria in Ternary Ag-Cu-Ga System

“…All ternary alloys were prepared from pure elements: Ag, Cu and Ga. 30 samples (total mass about 0.50 g) were sealed and melted in evacuated quartz tubes. After alloying, the samples were homogenized in two separately operating furnaces at first at 1200°C for 10 h. Then, they were annealed in another furnace (Linn High Therm VMK 10) at 200°C (samples 1-15), 450°C (samples [16][17][18][19][20][21][22][23][24] and 650°C (samples 24-30) for 8 weeks to form equilibrium phases. After this equilibrium treatment, the tubes were quenched in cold water.…”

“…[17,18] It is claimed that after Ag addition these thin film solar cells (ACIGS) reached the efficiency of almost 20%. [19] Thus, there is a mounting evidence that gallium alloys can be interesting future materials.…”