Impact of Ga gradient and alkali treatment on minority charge carrie dynamics in CuInGaSe2 solar cells

Abstract

CuInGaSe2 solar cells, commonly known as CIGS solar cells, have achieved high device e fficiency owing to two major research breakthroughs: the implementation of a Ga gradient into the CIGS absorber, and the alkali atoms treatment. Though many studies have been focused on optimising these two techniques to further enhance device efficiencies, the minority charge carrier dynamics inside such complex system are still under investigation. Hence, in this thesis, transient absorption spectroscopies (TAS) in the time range of picosecond to nanosecond and microsecond to second are utilized primarily to investigate the minority charge carrier dynamics inside such devices. Chapter 3 presents the results of steady state and transient optical studies on CIGS (no Ga gradient and no alkali treatment), CuInSe2 (CIS), and Sb2Se3 thin fi lm, respectively. The study on CIGS/CIS is to set a foundation for the following two chapters. Additionally, the results of Sb2Se3 are included because Sb2Se3 also belongs to the Se-based chalcogenide family. In all three materials, two major transient absorption features are discovered: 1. A photobleach signal appeared in picosecond-nanosecond timescale, which is assigned to the ground state bleach, and 2: a positive absorption signal appeared in microsecond to second scale, which is linked to the presence of trapped charge carriers. Analyzing these two TA features, it is found that CIGS/CIS has a longer minority carrier lifetime and a smaller amount of trapped charges compared to Sb2Se3. In Chapter 4, a Ga-graded CIGS device with RbF+NaF post deposition treatment (PDT) is investigated primarily by TAS, which allows to have a real-time observation of minority carriers dynamics. A drift-di usion kinetic model is established, which successfully deconvolutes the two competitive kinetics, recombination and drift, in a Ga-graded CIGS device. Furthermore, the drift time constant obtained from the kinetic model enables to calculate the minority carrier mobility (16.3 4.0 cm2V-1s-1) in the high Ga region of a Ga-graded CIGS solar cell. As far as I am aware of, this is the fi rst time that the minority carrier mobility in that region is determined. Chapter 5 makes use of the kinetics model established in Chapter 4 to analyze the impact of Na and Rb atoms treatments on minority carrier mobility and recombination throughout the Ga-graded CIGS device. It is found that Na atoms prolong the minority carrier lifetime throughout the CIGS absorber regardless of the [Ga]=([Ga]+[In]) (GGI) ratio. On the other hand, passivation with Rb atoms works best in the domain with low Ga concentration, suggesting an ine ective passivation of Rb atoms in the regions of high Ga concentration.