Investigation on cascade photo-excitation via intermediate band (IB) is promising for improving the efficiency of IB-type solar cells (IBSCs). Increasing nitrogen (N) concentration in GaP changes an ensemble of discrete N-N pair levels to form the IB as well as introducing defect levels acting as nonradiative recombination (NRR) centers. In continuation of detecting NRR centers in GaP 1Àx N x (x > 0.5%), a study is made for the lower N concentration region of 0.105% to understand an original formation of defect levels and their properties. Elimination of temperature quenching by immersing the sample into liquid nitrogen reveals a distribution of NRR centers inside the forbidden energy gap and the shift of Fermi energy depending on above-gap excitation (AGE) density. Profound understanding of IB and defects of GaP 1Àx N x leads to a proper optimization of IBSCs.
Presence and influence of nonradiative recombination (NRR) centers in an intermediate band (IB)‐type material, GaP1–xNx (x=0.75%), are studied by two‐wavelength excited photoluminescence (TWEPL) method and time‐resolved photoluminescence (TRPL) measurement at 77 K. With the use of below‐gap excitation (BGE) light in addition to an above‐gap excitation (AGE), the PL peak intensity is found to increase which indicates the presence of NRR centers and a secondary excitation from the IB to conduction band (CB). Depending on the effect of different BGE energies, an energy diagram on the distribution of NRR centers and NRR process is interpreted. The saturation of PL increase is attributed to the trap‐filling effect in NRR centers, which allows us to modify the rate equation. The NRR parameters are evaluated by a qualitative simulation of the modified rate equations of one‐level model together with the lifetime determined by TRPL. In continuation of evaluating NRR parameters by rate equation analysis, the addition of TRPL measurement improves accuracy and approaches the determination of NRR parameters. A successful characterization of NRR centers leads to a proper optimization of IB‐type solar cells (IBSCs).
This study presents two-wavelength excited photocurrent (TWEPC) measurements in GaP1-xNx grown by metalorganic vapor phase epitaxy. TWEPC measurements reveal that photocurrent generation is significantly enhanced when above band gap excitation and below band gap excitation (BGE) sources are applied simultaneously. With increasing BGE photon energy, a large increase in photocurrent is observed. The external quantum efficiency measurements show that the effect of BGE light is higher with a higher density of tail states present. The extended numerical study by rate equations reproduced the results in a good manner. Furthermore, the simulation results showed that the addition of the BGE light affects the electron occupancy as well as the electron lifetime which is found to be 0.1 ns in this study.
Investigation on cascade photo‐excitation via intermediate band (IB) is promising for improving the efficiency of IB‐type solar cells (IBSCs). Increasing nitrogen (N) concentration in GaP changes an ensemble of discrete N–N pair levels to form the IB as well as introducing defect levels acting as nonradiative recombination (NRR) centers. In continuation of detecting NRR centers in GaP1−xNx (x > 0.5%), a study is made for the lower N concentration region of 0.105% to understand an original formation of defect levels and their properties. Elimination of temperature quenching by immersing the sample into liquid nitrogen reveals a distribution of NRR centers inside the forbidden energy gap and the shift of Fermi energy depending on above‐gap excitation (AGE) density. Profound understanding of IB and defects of GaP1−xNx leads to a proper optimization of IBSCs.
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