Efficient and cost-effective thin film solar cells processed by direct pulsed laser crystallization and direct pulsed laser recrystallization techniques at room temperature

Martin Yi Zhang, Purdue University

Abstract

Motivated by the depletion of fossil fuel energy and increasing demand for cheaper energy alternatives, improvement of conversion efficiency and reduction of manufacturing cost have been long-time pursuit in the solar cell industry. In this study, novel post thermal processing methods called direct pulsed laser crystallization (DPLC) and direct pulsed laser recrystallization (DPLR) are introduced to rapidly and economically manufacture high performance thin film solar cells. Both DPLC (to process photoactive layer) and DPLR (to process transparent conductive oxide (TCO) layer) are operated at room temperature and atmospheric conditions, required very limited amount of resource, energy, and equipment to startup; they could also be easily integrated for mass productions. Both finite element analysis (FEA) simulation and experiments are carried out to understand the methodology and mechanism of DPLC and DPLR techniques. An FEA multiphysics model that couples electromagnetic module with heat transfer module is built and examined to reveal the correlation among (1) material properties-including electrical and thermal conductivities (σ and κ), material size (d), substrate type; (2) applied processing conditions — including laser wavelength (λ), laser fluence ( F), laser pulse number (N), and (3) resulting temperature (T) within the thin film under investigation. In DPLC, by using different photoactive nanoparticles, multiphysics simulation effectively predicts the correlation between T and F, N, and d. In DPLR, TCO material such as alumina-doped zinc oxide (AZO) is analyzed and a correlation between T and F, N, substrate type is analyzed in details. Comparing T with melting point, an evaluation to see if certain nanoparticles would experience crystallization or recrystallization process would be accessed. Under the guidance of simulation, appropriate laser processing conditions could be chosen. After careful selection of processing parameters (λ, F, N), it is found that short laser pulses induced rapid temperature increase and decrease within the target film are mainly responsible for the abnormal crystal growth and recrystallization in photoactive and TCO nanomaterials, respectively. Utilization of nano-scale target materials are also advantageous considering size effects on melting point and laser-nanoparticle interactions which significantly decreases the laser fluence and temperature during DPLC and DPLR processing. Thermal-treated photoactive and TCO thin films are characterized from various aspects including morphology, structure, optical, and electrical properties. It is found that original nanoparticles of both photoactive and TCO materials gradually merge and impinge with neighboring counterparts as laser pulsed are applied; finally a thermodynamic equilibrium will be arrived where crystal growth would stop. Crystallization/Recrystallization of target materials effectively decreases the film resistivity and increases the charged carrier's Hall mobility which potentially increases the movement strength of electrons/hole pairs in thin solar cell devices. Slight decrease on carrier concentration density and increase of absorptance (transmittance) are observed for photoactive (TCO) materials, respectively. This would increase the amount of solar light irradiation that is being accepted by these layers which would potentially increase the amount of effective electron/hole pairs in the solar cells devices. Upon selection of high repletion rate, high power industry-widely used diode pumped solid state (DPSS) laser system which has power up to 100 W, repletion rate of 100 kHz, very rapid processing at scale of meters per second would be achieved with DPLC and DPLR.

Degree

Ph.D.

Advisors

Cheng, Purdue University.

Subject Area

Industrial engineering|Mechanical engineering|Materials science

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