Highly mismatched alloys are a class of materials whose fundamental properties are dramatically modified through the substitution of a relatively small fraction of host atoms with an element of very much different electronegativity. In ZnTe, the incorporation of a small amount of isoelectronic O leads to the formation of a narrow, O-derived intermediate band (IB, E-) located well below the conduction band (CB, E+) edge of the ZnTe through an anticrossing interaction between localized states of O and the CB of the ZnTe matrix. Therefore, ZnTe1-xOx (ZnTeO) alloy is one of the potential candidates for an absorber material in a bulk intermediate band solar cell (IBSC). We have previously demonstrated the generation of photocurrent induced by two-step photon absorption (TSPA) in ZnTeO IBSCs using n-ZnO window layer. Here, we review our recent progress on the development of ZnTeO based IBSCs using n-ZnS window layer and Cldoped ZnTeO. With n-ZnS window having a small conduction band offset with ZnTe, the open circuit voltage of ZnTeO IBSC was improved. Cl-doping was performed to introduce electrons into the IB of ZnTeO that is required to be halffilled with electrons for the efficient operation of an IBSC. Low temperature photoluminescence spectra indicated that the doped Cl atoms act as donors in ZnTeO. The improved photovoltaic properties were demonstrated in the IBSC using Cl-doped ZnTeO.
Highly mismatched ZnTe1-xOx (ZnTeO) alloy is one of the potential candidates for an absorber material in a bulk intermediate band solar cell (IBSC) because a narrow, O-derived intermediate band IB (E-) is formed well below the conduction band CB (E+) edge of the ZnTe. We have previously demonstrated the generation of photocurrent induced by two-step photon absorption (TSPA) in ZnTeO IBSCs using n-ZnO window layer. However, because of the large conduction band offset (CBO) between ZnTe and ZnO, only a small open circuit voltage (Voc) was observed in this structure. Here, we report our recent progress on the development of ZnTeO IBSCs with n-ZnS window layer. ZnS has a large direct band gap of 3.7 eV with an electron affinity of 3.9 eV that can realize a smaller CBO with ZnTe. We have grown n-type ZnS thin films on ZnTe substrates by molecular beam epitaxy (MBE), and demonstrated ZnTe solar cells and ZnTeO IBSCs using n-ZnS window layer with an improved VOC. Especially, a n-ZnS/i-ZnTe/p-ZnTe solar cell showed an improved Voc of 0.77 V, a large short-circuit current density of 6.7 mA/cm2 with a fill factor of 0.60, yielding the power conversion efficiency of 3.1 %, under 1 sun illumination.
The subband features E‒ and E+ for the conduction band of III-V dilute nitride alloys make them promising for intermediate band solar cell application. However, presence of bandgap states can limit the two-step photon absorption activity, a necessary requirement for IBSC functionality. A model analysis is performed to characterize the density of states. The sub-band tails states are characterized using a temperature-dependent map of photo-modulated reflectance spectroscopy for GaNAs thin films grown on GaAs substrates using molecular beam epitaxy. The effect of indium and antimony incorporation on the subband features were investigated. Marked improvements in the thin films were observed both for the lower (E‒) and the upper (E+) conduction bands (CB) when In was introduced with marginal enhancement by Sb. These improvements are associated with suppression of tail states below both the E‒ and E+ bands. Sb rather mainly plays a surfactant role improving the abruptness of the GaNAs/GaAs hetero-interface.
ZnO1-xSex films have been prepared through pulsed laser deposition as a step toward stable films with a band gap
appropriate for water splitting. The films show a clear red shift in absorption with increasing Se content and a shift
in the flat band voltage toward spontaneity. Due to the films' electron affinities, there exists a natural tunnel
junction between these n- ZnO1-xSex films when grown on the p-side of a Si diode. The overall performance,
emphasized by flat band potential measurements, can be improved by growing films on Si p-n diodes.
The direct gap of the In1-xGaxN alloy system extends continuously from InN (0.7 eV, in the near IR) to GaN (3.4 eV, in the mid-ultraviolet). This opens the intriguing possibility of using this single ternary alloy system in single or multi-junction (MJ) solar cells. A number of measurements of the intrinsic properties of InN and In-rich In1-xGaxN alloys (0 < x < 0.63) are presented and discussed here. To evaluate the suitability of In1-xGaxN as a material for space applications, extensive radiation damage testing with electron, proton, and alpha particle radiation has been performed. Using the room temperature photoluminescence intensity as a indirect measure of minority carrier lifetime, it is shown that In1-xGaxN retains its optoelectronic properties at radiation damage doses at least 2 orders of magnitude higher than the damage thresholds of the materials (GaAs and GaInP) currently used in high efficiency MJ cells. Results are evaluated in terms of the positions of the valence and conduction band edges with respect to the average energy level of broken-bond defects (Fermi level stabilization energy EFS). Measurements of the surface electron concentration as a function of x are also discussed in terms of the relative position of EFS. The main outstanding challenges in the photovoltaic applications of In1-xGaxN alloys, which include developing methods to achieve p-type doping and improving the structural quality of heteroepitaxial films, are also discussed.
We have studied the effects of composition and hydrostatic pressure on the direct optical transitions at the Γ point of the Brillouin zone in MBE-grown ZnOxSe1-x and ion-implantation-synthesized Zn1-yMnyOxTe1-x alloys. We observe a large O-induced band-gap reduction and a change in the pressure dependence of the fundamental band gap of the II-O-VI alloys. The effects are similar to those previously observed and extensively studied in highly mismatched III-N-V alloys. Our results are well explained in terms of the band anticrossing model that considers an anticrossing interaction between the highly localized oxygen states and the extended states of the conduction band of II-VI compounds. The O-induced modification of the conduction band structure offers an interesting possibility of using small amounts of O to engineer the optoelectronics properties of group II-O-VI alloys.
We present recent results of calculations of charge transfer and electron mobilities in nominally undoped AlGaN/GaN heterostructures. It has previously been proposed that the two-dimensional electron gas (2-DEG) originates from donor- like defects on the surface of the AlGaN barrier. We have made detailed calculations of a model in which these defects are created under thermodynamic equilibrium at the growth temperature and show that the spontaneous and strain-induced piezoelectric fields in the AlGaN barrier enhance the formation of these defects. In calculating the low temperature electron mobility in these structures, we consider all the major scattering mechanisms including acoustic phonons, Coulomb scattering from charged centers, and alloy disorder scattering. The relative importance of the different scattering mechanisms depends strongly on the 2-DEG density. At densities smaller than about 2x1012cm-2, the mobility is limited by Coulomb scattering. At higher densities, alloy disorder scattering becomes the dominant electron scattering process. Finally, we have calculated the ratio of the transport to quantum lifetimes ((tau) t/(tau) q for various AlGaN/GaN heterostructures and find that the value of the ratio cannot be used to infer the nature of the dominant scattering mechanism, as is traditionally assumed.
Incorporation of a few percent of nitrogen into conventional III-V compounds to form III-N-V alloys such as GaNAs and GaNP leads to a large reduction of the fundamental band gap. We show experimentally and theoretically that the effect originates from an anti-crossing interaction between the extended conduction-band states and a narrow resonant band formed by localized N states. The interaction significantly alters the electronic band structure by splitting the conduction band into two nonparabolic subbands. The downward shift of the lower conduction subband edge is responsible for the N-induced reduction of the fundamental band-gap energy.