In the third part of this series, we study the influence of trap states in the band gap of semiconductors on the time-resolved luminescence decay (TRL) after a pulsed excitation. The results based on simulations with Synopsys TCAD® and analytical approximations are given for p-doped Cu(In,Ga)Se2 as a working example. We show that a single trap can be mostly described by two parameters which are assigned to minority carrier capture and emission. We analyze their influence on the luminescence decay and study the difference between a single trap and an energetic Gaussian trap distribution. It is found that trap states artificially increase the TRL decay and obscure the recombination dynamics. Thus, there is a demand for experimental methods which can reveal the recombination of minority carriers in a TRL experiment without trapping effect. In this regard, a variation of the device temperature, the excitation frequency, the injection level, as well as a bias illumination may be promising approaches. We study these methods, discuss advantages and disadvantages, and show experimental TRL for prove of concept. At the end, we validate our approach of simulating only band-to-band radiative recombination although photoluminescence spectra often exhibit free-to-bound radiative recombination of charge carriers.

No abstract

The main objective of time-resolved photoluminescence (TRPL) is to characterize minority carrier recombination in semiconductors. However, trap states in the band gap can lead to artificially long decay times thus distorting the measured minority carrier lifetime. In this work, we propose to measure TRPL under elevated temperature and excitation in order to reduce minority carrier trapping. Taking three Cu(In,Ga)Se2 layers as examples, we show that the decay time decreases with increasing temperature—in accordance with simulations. Under increasing excitation, the decay time can become smaller due to trap saturation but also can become larger due to asymmetric hole and electron lifetimes. By comparison of simulation and experiment, we can find the energy, the density, and the electron capture cross-section of the trap which in the present example of Cu(In,Ga)Se2 films gives values of ∼200 meV, ∼1015 cm−3, and ∼10−13 cm2, respectively.

Cu (In,Ga)Se 2 solar cells exhibit higher efficiencies if an appropriate Ga gradient is introduced and if the absorber is doped with sodium (Na) plus a heavier alkali atom such as potassium. However, a Gallium gradient in the presence of Na is challenging because sodium impedes the interdiffusion of elements and influences the gradient. In this contribution, we show that the presence of sodium during growth with the combination of high Ga concentration creates a pronounced gradient that is detrimental for carrier collection. One solution is to avoid Na abundance during absorber growth but to add it to the grown absorber layer via a postdeposition treatment. We investigate the effect of different Na incorporation methods simultaneously with KF-PDT on wide band gap CIGSe absorbers. By preparation on alkali-free substrates and application of alkalis (NaF and KF) onto a grown CIGSe layer, we show by smoothening of the Gallium gradient a large improvement in the solar cell performance from 4% to 8%. Another way to optimize the gradient in the presence of Na is the modification of the three-stage method. This yields the best efficiency of 10% in our laboratory at an integral GGI of 0.8. By means of temperature-dependent JV measurements, we show that the additional postdeposition of KF induces a barrier for the diode current. We conclude that KF-PDT induces a new thin layer at the CIGSe surface that has a lower valence band edge relative to the CIGSe bulk and is responsible for the double-diode behavior. This barrier can also explain the V oc (T) saturation at low temperature.

Solar cells with Cu(In, Ga)Se 2 absorbers rely on the three-stage co-evaporation process with Cu-poor/Cu-rich/Cu-poor absorber deposition conditions for highest efficiency devices. During the three-stage process, the formation and evolution of different selenide phases with changing compositions throughout the process crucially determine the final absorber quality. In this contribution, we monitor the evolution of crystalline phases in real-time with an X-ray diffraction (XRD) line detector setup implemented into an evaporation setup. Using the common three-stage process, we prepare and compare samples covering the full alloying range from CuInSe 2 to CuGaSe 2 . The in situ XRD allows the detection of the crystalline phases present at all times of the process as well as an advanced analysis of the phase evolution through a closer look at peak shifts and the full width at half maximum. For samples with a Ga/(Ga þ In) ratio (GGI) < 0.5, distinct phase transitions associated with the transition to the reported vacancy compounds Cu(In,Ga) 5 Se 8 and Cu(In, Ga) 3 Se 5 are observed. No such indication was found for samples with a GGI > 0.5. For Ga-rich Cu(In, Ga)Se 2 phases with a GGI of 0.55, the XRD analysis evidenced a Ga-rich phase segregation before the stoichiometric point was reached. The above findings are discussed in view of their implication on wide gap solar cell performances.