• Energy Research

The impact of moisture and heat treatment on the microstructural, chemical, and electrical properties of Cu(In,Ga)Se2 films and their collective effect on the solar cell device performance was studied. X-ray photoelectron spectroscopy and secondary ion mass spectroscopy measurements show that water exposure causes surface modification and alters the alkali metal distribution, while no composition or structural effect was observed. Deep level transient and optical spectroscopies revealed that the trap densities ( NT ) for both the EV + 0.65 eV and EV + 0.98 eV traps increase after water exposure, while the majority carrier concentration ( NA ) decreases. Time-resolved photoluminescence (PL) and steady-state PL measurements indicated the presence of static, not dynamic, quenching. Reduction of open-circuit voltage ( V OC) and fill factor (FF) was observed for the devices but was not associated with a change of recombination mechanism, which remains in the absorber space charge region. A small increase in series resistance and shunt conductance accounts for most of the FF change, while the modification in both NA and NT yield most of the change in V OC. A gradient of majority carrier concentration, related to the alkali profile, also yields a small voltage-dependent current collection after moisture and heat treatment.

The impact of moisture and heat treatment on the microstructural, chemical, and electrical properties of Cu(In,Ga)Se2 films and their collective effect on the solar cell device performance was studied. X-ray photoelectron spectroscopy and secondary ion mass spectroscopy measurements show that water exposure causes surface modification and alters the alkali metal distribution, while no composition or structural effect was observed. Deep level transient and optical spectroscopies revealed that the trap densities ( NT ) for both the EV + 0.65 eV and EV + 0.98 eV traps increase after water exposure, while the majority carrier concentration ( NA ) decreases. Time-resolved photoluminescence (PL) and steady-state PL measurements indicated the presence of static, not dynamic, quenching. Reduction of open-circuit voltage ( V OC) and fill factor (FF) was observed for the devices but was not associated with a change of recombination mechanism, which remains in the absorber space charge region. A small increase in series resistance and shunt conductance accounts for most of the FF change, while the modification in both NA and NT yield most of the change in V OC. A gradient of majority carrier concentration, related to the alkali profile, also yields a small voltage-dependent current collection after moisture and heat treatment.

Electron channeling contrast imaging (ECCI) is a nondestructive diffraction-based scanning electron microscopy (SEM) technique that can provide microstructural analysis similar to transmission electron microscopy (TEM). However, because ECCI is performed within an SEM and requires little to no sample preparation, such analysis can be accomplished in a fraction of the time. Like TEM, ECCI can be used to image a variety of extended defects and enables the use of standard invisibility criteria to provide further defect characterization (e.g., Burgers vector determination). Here, we use ECCI to characterize various extended defects, including threading dislocations, misfit dislocations, and stacking faults, in heteroepitaxial GaP/Si(1 0 0) samples. We also present applications for which ECCI is particularly well suited compared with conventional methods. First, misfit dislocations are surveyed via ECCI across the radius of a 4-in GaP/Si wafer, yielding a proof-of-concept rapid (∼3 h) approach to large-area defect characterization. Second, by simply wet etching away a portion of a thick epitaxial GaP-on-Si layer, we use ECCI to image specific targeted interfaces within a heterostructure. Both of these applications are prime examples of how ECCI is a compelling alternative to TEM in circumstances where the required sample preparation would be prohibitively time-consuming or difficult.

Electron channeling contrast imaging (ECCI) is a nondestructive diffraction-based scanning electron microscopy (SEM) technique that can provide microstructural analysis similar to transmission electron microscopy (TEM). However, because ECCI is performed within an SEM and requires little to no sample preparation, such analysis can be accomplished in a fraction of the time. Like TEM, ECCI can be used to image a variety of extended defects and enables the use of standard invisibility criteria to provide further defect characterization (e.g., Burgers vector determination). Here, we use ECCI to characterize various extended defects, including threading dislocations, misfit dislocations, and stacking faults, in heteroepitaxial GaP/Si(1 0 0) samples. We also present applications for which ECCI is particularly well suited compared with conventional methods. First, misfit dislocations are surveyed via ECCI across the radius of a 4-in GaP/Si wafer, yielding a proof-of-concept rapid (∼3 h) approach to large-area defect characterization. Second, by simply wet etching away a portion of a thick epitaxial GaP-on-Si layer, we use ECCI to image specific targeted interfaces within a heterostructure. Both of these applications are prime examples of how ECCI is a compelling alternative to TEM in circumstances where the required sample preparation would be prohibitively time-consuming or difficult.

Solar cell degradation can occur through many pathways and at the different stages of the fabrication process, notably because of the water condensation. In the case of Cu(In,Ga)Se2 (CIGS) solar cells, the impact of water ingress on Mo back contact can be substantial for not only the film itself but also the device performance and reliability. Microstructural modification, change in film morphology, loss in reflectance, and increased resistivity because of the moisture ingress were observed via X-ray diffraction, transmission electron microscopy, spectroscopic ellipsometry, and four-point probe measurements, respectively. Secondary ion mass spectrometry measurements revealed drastic changes in the alkali (Na, K) profiles in both the CIGS and Mo layers, likely because of the modification of their diffusion coefficient through Mo. This, in turn, negatively impacts the solar cell efficiency by decreasing both fill factor and open-circuit voltage. Additional experiments, modifying the substrate and utilizing a NaF postdeposition treatment, highlight the mechanisms of degradation as being due to both a modification of the Mo/CIGS interface and the lack of alkali diffusion after water ingress.

Solar cell degradation can occur through many pathways and at the different stages of the fabrication process, notably because of the water condensation. In the case of Cu(In,Ga)Se2 (CIGS) solar cells, the impact of water ingress on Mo back contact can be substantial for not only the film itself but also the device performance and reliability. Microstructural modification, change in film morphology, loss in reflectance, and increased resistivity because of the moisture ingress were observed via X-ray diffraction, transmission electron microscopy, spectroscopic ellipsometry, and four-point probe measurements, respectively. Secondary ion mass spectrometry measurements revealed drastic changes in the alkali (Na, K) profiles in both the CIGS and Mo layers, likely because of the modification of their diffusion coefficient through Mo. This, in turn, negatively impacts the solar cell efficiency by decreasing both fill factor and open-circuit voltage. Additional experiments, modifying the substrate and utilizing a NaF postdeposition treatment, highlight the mechanisms of degradation as being due to both a modification of the Mo/CIGS interface and the lack of alkali diffusion after water ingress.