Crystalline photoactive copper indium diselenide thin films by pulsed laser crystallization of nanoparticle-inks at ambient conditions
Direct pulsed laser crystallization (DPLC) of large area printed nanoparticle-inks is investigated to form a crystalline copper indium diselenide (CIS) thin film. This technique has great potential to be utilized in scalable device production due to its atmospheric processing and high yield rate. It is found that CIS nanoparticles of 20 nm-diameter can grow to micro scale large crystals in a film after DPLC. The internal imperfections including grain boundary, inter crystal gaps and voids were reduced significantly. The effects of laser intensity and pulse number on the deposited thin film are investigated. The localized field enhancement around nanoparticle contacts is believed to be responsible for nanoparticle welding and crystal growth near the CIS film surface area during DPLC. The complete crystallization of CIS nanoparticles along the depth direction is attributed to thermal diffusion driven rapid melting/solidification. Moreover, it was found that the microstructure change in the CIS film after DPLC influences the optical and electrical performance significantly. A typical increase of 7.9% (visible range) and 19.2% (near infrared range) in the optical absorbance is obtained under optimal DPLC conditions and a band-gap shrinkage (ΔEg) of the CIS thin film after DPLC leads to broader acceptance of solar spectrum of 100 nm. The results from the Raman spectrum and photoluminescence (PL) of the thin films under various laser conditions reveal that the improvement of the optoelectronic properties of the CIS thin film is related to the reduction of the crystal defect level in the thin film.