Towards intermediate band solar cells based on (Cr + N) co-doped TiO2 – PART 1: Nitrogen incorporation

(Cr + N) co-doped TiO 2 has been proposed as a candidate for intermediate band materials, and we calculate that an intermediate band solar cell based on TiO 2 can reach a limiting efficiency of ∼44 % under AM1.5 solar radiation. The ideal smallest bandgap, i.e. the photon energy for the absorption onset, is ∼1.3 eV, aligning with reported optical properties for (Cr + N) co-doped TiO 2 . Since the ideal doping concentration is unknown, we use natural spread combinatorial pulsed laser deposition (nc-PLD) to fabricate co-doped films on 50.8 mm silicon wafers. This method enables a continuous compositional spread (CCS), with nominal Cr and N concentrations ranging from 2 to 10 at-% across the wafer. Unlike other PLD approaches reported, doping is achieved via separate ablation of TiO 2 and CrN targets, offering high flexibility in doping levels. In our initial attempt, both targets were ablated in an oxygen background gas. While Cr was incorporated as intended, nitrogen content was only ∼1/6 of Cr. To improve N incorporation, we deposited test films using nitrogen gas or alternated between nitrogen and oxygen. Based on these results, we propose a scheme where CrN is ablated in nitrogen, then capped with a thin TiO 2 layer also deposited in nitrogen to prevent exposure to oxygen. The remainder of the TiO 2 is then deposited in oxygen. In Part 2 of this work, we present results from this optimized scheme, in which (Cr + N) co-doped TiO 2 films with Cr and N contents comparable within experimental uncertainty were successfully deposited.