Photocarrier dynamics in undoped and Na-doped Cu₂ZnSnS₄single crystals revealed by ultrafast time-resolved terahertz spectroscopy

Abstract

We investigated the effects of sodium doping on the photocarrier dynamics in Cu 2 ZnSnS 4 (CZTS) single crystals using optical pump-THz probe transient reflectivity (THz-TR) and time-resolved photoluminescence (PL) spectroscopy. The THz-TR and PL decay dynamics are influenced by sodium doping, and their sodium-induced changes are consistent with each other. These time-resolved measurements revealed that the lifetime of photocarriers increases with sodium doping. This result indicates that a part of defects is suppressed by doping sodium into CZTS and implies that sodium doping improves the charge transport properties of CZTS, leading to an improvement in the performance of CZTS-based solar cells.

Free-carrier dynamics and band tails in Cu2ZnSn(SxSe1−xPhuong1, Okano2, Yamashita3 et al. 2015Phys. Rev. BSelf Cite201171View full textAdd to dashboardCite

“…Owing to the existence of a considerable number of ionized defects in the multinary compound CZTSSe [16], large electrostatic potential fluctuations occur and modify the near-band-edge structure, leading to the formation of high-density band tails below the band edge [14,15,[17][18][19][20][21]. The exponential tail detected in the low-energy side of the PLE spectrum reflects the light absorption of the below-band-edge tails, this has been calculated theoretically to be only a function of the band-gap energy E g and the average amplitude of the electrostatic potential fluctuations γ [19,22].…”

“…samples shows a significant tail in the low-energy region, a broad peak structure, whose energetic location depends obviously on the anion composition, and a reduction in the high-energy region. Under high-energy photoexcitation, photocarriers are generated near the surface region of the CZTSSe samples, where a larger number of nonradiative recombination centers exist [13][14][15]. The high-energy-side decrease in the PLE spectrum thus is due to an enhancement of nonradiative recombination occurring near the surface region [13][14][15].…”

“…Under high-energy photoexcitation, photocarriers are generated near the surface region of the CZTSSe samples, where a larger number of nonradiative recombination centers exist [13][14][15]. The high-energy-side decrease in the PLE spectrum thus is due to an enhancement of nonradiative recombination occurring near the surface region [13][14][15]. Based on various experimental techniques including PLE, photocurrent, and TR measurements, we have previously shown that the band-gap energy of the pure-sulfide CZTSSe sample (x = 1.0) could be determined approximately around the PLE peak energy [14].…”

Impact of the selenisation temperature on the structural and optical properties of CZTSe absorbers

“…To interpret data gained from PL measurements, it is essential to have reliable values of the bandgap E g . This can be obtained from photoluminescence excitation (PLE) data, which do not require a p-n junction and can be used to analyse films deposited on Mo coated glass [18].…”

Review—Light Emission from Thin Film Solar Cell Materials: An Emerging Infrared and Visible Light Emitter

“…[71][72][73][74][75][76][77][78] Figure 8 shows that the CZTS material properties improve through Na ion doping. 78 The strong drop in the PLE intensity for high-energy excitation of the undoped CZTS single crystal in Figure 8a (blue data) evidences that defects are formed near the surface, which reduces the photocarrier generation efficiency. The red data in Figure 8a indicates that the surface-related recombination centers are reduced by Na doping, which suggests that also the PC increases.…”