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The emphasis of this project is the study of ZnTe:O as an intermediate band solar cell (IBSC) material. Oxygen states in ZnTe provide radiative optical transitions within the bandgap, and may be used as a solar cell material with three absorption bands: (1) valence band to conduction band, (2) valence band to intermediate band, and (3) intermediate band to conduction band. The incorporation of the extra optical absorption bands corresponding to intermediate band states, transitions (2) and( 3), provide a path for lower energy photons to be absorbed in the structure, and extraction of electrons with energy corresponding to standard optical transitions from valence band to conduction band. In this project, we have studied the growth of ZnTe:O by molecular beam epitaxy, the optical absorption properties, the insertion of this material in a diode device, and the development of a solar cell model for intermediate band materials.  

ZnTe:O samples were grown by molecular beam epitaxy using solid source effusion cells for Zn and Te, and an RF plasma source for oxygen and nitrogen incorporation. Nitrogen is used to achieve p-type ZnTe for diode structures.  A typical oxygen flow rate of 1 sccm is used, where a concentration of 1x10¹⁹ cm^-3 is estimated based on the assumption of a similar incorporation rate for oxygen and nitrogen. Room-temperature photoluminescence spectra for ZnTe:O grown on GaAs indicate both a bandedge response for ZnTe at 2.3 eV and a strong sub-bandgap response related to oxygen doping in the range of 1.6-2.0 eV. In a majority of the ZnTe:O samples, the spectrum is dominated by emission from oxygen states. In comparison, the photoluminescence spectrum for an undoped ZnTe sample shows a sharper bandedge transition, and a much weaker defect emission ( residual oxygen present in the growth chamber). The strong emission for the oxygen defect in ZnTe denotes a highly radiative transition, and is therefore a highly desirable characteristic for the IB solar cell. Complementary optical absorption spectra inferred from transmission measurements were investigated on sapphire substrates under varying oxygen partial pressure. A sharp bandedge response is observed for ZnTe without oxygen, while increasing sub-bandgap optical absorption is observed with increasing oxygen. The sub-bandgap optical absorption is consistent with the photoluminescence characteristics, and is similarly attributed to oxygen defects in ZnTe.

The spectral response for diodes fabricated with ZnTe and ZnTe:O absorber layers show a sharp bandedge response near 2.25 eV for ZnTe,  while the ZnTe:O diode exhibits enhanced spectral response below the bandedge, down to  1.5 eV. It should be noted that the spectral response was obtained using monochromatic light, and does not necessarily represent the true spectral response of a solar cell where a full spectrum is incident on the device. The monochromatic incidence does not provide a means of “pumping” carriers from the valence band to intermediate band in order to enable absorption from the intermediate to the conduction band. The current-voltage characteristics for ZnTe and ZnTe:O solar cells under AM 1.5 illumination show a clear photovoltaic response for both sets of devices, where an increased short circuit current density (Jsc) and reduced open circuit voltage (Voc) are observed for the ZnTe:O, with  an overall improvement of 50% in the power conversion efficiency. The increase in Jsc is approximately double and is consistent with the extended spectral response below the bandedge and corresponding larger number of photo-generated carriers. The source for the approximate 15% reduction in Voc may be due to a number of factors, including increased non-radiative recombination and carrier occupation in the IB lowering the Fermi level position.

The multi-photon process associated with intermediate-band transitions was further investigated using illumination by 650 nm (1.91 eV) and 1550 nm (0.8 eV) laser sources corresponding to energies below the ZnTe bandedge. The 650 nm laser provides energy necessary to excite an optical transition between the valence band and IB, while the 1550 nm laser provides the energy necessary to excite a transition from IB to conduction band. The ZnTe diodes did not exhibit any detectable response at these wavelengths. The ZnTe:O diodes do not show any detectable response for sole illumination at 1550nm, but do demonstrate significant response for illumination at 650nm, consistent with the response observed at 1.91 eV. The observed response for monochromatic 650 nm illumination suggests that carriers are excited into the IB and are subsequently promoted to the conduction band by further 650 nm photons, thermionic emission, or tunneling. The addition of 1550 nm illumination to 650 nm illumination (simultaneous excitation) results in an increase in photocurrent and open circuit voltage. The photovoltaic response increases monotonically with increasing 1550nm power density, providing supporting evidence that the intended two-photon process for IB solar cells may be occurring in the ZnTe:O device.      

To understand the unique behavior of these devices, a one-dimensional IBSC drift-diffusion model has been developed. The results are consistent with prior 0-D models verifying the ability to achieve high efficiency in the case of low non-radiative recombination and good electronic transport properties. The drift-diffusion model has identified that space-charge effects are significant for IBSC with lightly-doped regions, where devices would have low occupation of IB states and corresponding low conversion efficiencies. A doping compensation scheme is proposed to clamp the IB quasi-Fermi level at the IB position to reduce space-charge effects and to maximize optical generation. The compensated base doping scheme also eliminates the intrinsic dependence of efficiency on base doping due to space-charge effects and increases the maximum achievable efficiency to >40%, near the values predicted for 0-D IBSC devices. The 1-D drift diffusion model facilitates the future design of IBSC and related devices using the established framework of solving carrier continuity and electrostatic equations.

Figure 1: Spectral response of diodes comparing ZnTe and ZnTe:O absorber regions, where clear sub-bandgap response is observed in the ZnTe:O device corresponding to oxygen states.  

Figure 2: Current-voltage characteristics of diodes under simulated AM1.5 illumination comparing ZnTe and ZnTe:O  absorber regions, where a 50% enhancement in efficiency is observed for the ZnTe:O device corresponding to oxygen state transitions.