Direct pulsed laser crystallization of copper indium diselenide nanocrystal thin films for photovoltaics

In current research, novel, rapid, low cost technique called direct pulsed laser crystallization (DPLC) to manufacture economic, high performance thin film solar cells (TFSC) is introduced. DPLC improves the TFSC´s efficiency by reducing internal defects such as grain boundaries and inter-crystal gaps. Fewer defects allow more effective electrons thus produce stronger currents. DPLC has a good potential to be utilized in mass production because of its simplicity and rapidness: it does not require vacuum and high temperature systems, and it completes in the scale of nanoseconds. Optimal DPLC process makes 20-nm-diameter nanocrystals growing into 4-μm-wide big crystals without loss in chemical and structural properties. The number of grain boundary drops significantly and density of inter-crystal gaps decreases from 2.5×10¹, cm^-3 to 4×10¹⁰ cm^-3. Improvement on optical properties was observed, for instance change on band gap (E_g) leads to broader (~100 nm) acceptance of solar spectrum. An increase of 20% in optical absorptance could be obtained in the visible and near-infrared (NIR) regions after DPLC. Another feature is selectivity which means that DPLC processes only the intended layer while let other layers alone. The mechanism of DPLC has been studied and revealed. DPLC delivers laser pulses to nanoscale absorber material (e.g. CuInSe₂). It induces the crystal growth due to abnormal crystal growth and thermal-driven crystal rapid melting/solidification which are driven by the thermodynamic driven forces caused by pulsed laser induced rapid heating and cooling processes. Future experiments will be done to measure the conversion efficiency of TFSC manufactured using proposed method.