The role of CdS doping in improving SWIR photovoltaic and photoconductive responses in solution grown CdS/PbS heterojunctions.

Low cost short wavelength infrared (SWIR) photovoltaic (PV) detectors and solar cells are of very great interest, yet the main production technology today is based on costly epitaxial growth of InGaAs layers. In this study, layers of p-type, quantum confined (QC) PbS nano-domains (NDs) structure that were engineered to absorb SWIR light at 1550 nm (Eg = 0.8 eV) were fabricated from solution using the chemical bath deposition (CBD) technique. The layers were grown on top of two different n-type CdS intermediate layers (Eg = 2.4 eV) using two different CBD protocols on fluoride tin oxide (FTO) substrates. Two types of CdS/PbS heterojunction were obtained to serve as SWIR PV detectors. The two resulting devices showed similar photoluminescence behavior, but a profoundly different electrical response to SWIR illumination. One type of CdS/PbS heterojunction exhibited a PV response to SWIR light, while the other demonstrated a photo-response to SWIR light only under an applied bias. To clarify this intriguing phenomenon, and since the only difference between the two heterojunctions could be the doping level of the CdS layer, we measured the doping level of this layer by means of the surface photo voltage (SPV). This yielded different polarizations for the two devices, indicating different doping levels of the CdS for the two different fabrication protocols, which was also confirmed by Hall Effect measurements. We performed current voltage measurements under super bandgap illumination, with respect to CdS, and got an electrical response indicating a barrier free for holes transfer from the CdS to the PbS. The results indicate that the different response does, indeed, originate from variations in the band structures at the interface of the CdS/PbS heterojunction due to the different doping levels of the CdS. We found that, unlike solar cells or visible light detectors having similar structure, in SWIR photodetectors, a type I heterojunction is formed having a barrier at the interface that limits the injection of the photo-exited electrons from the QC-PbS to the CdS side. Higher n-doped CdS generates a narrow depletion region on the CdS side, with a spike like barrier that is narrow enough to enable tunneling current, leading to a PV current. Our results show that an external quantum efficiency (EQE) of ∼2% and an internal quantum efficiency (IQE) of ∼20% can be obtained, at zero bias, for CBD grown SWIR sensitive CdS/PbS-NDs heterojunctions.

The relative advantages of criminal versus administrative environmental enforcement actions in Israel.

Most environmental enforcement programs have separate criminal and administrative procedures for addressing violations. Pitting "criminal"versus"administrative" enforcement as mutually exclusive alternatives constitutes a "false dilemma". Clearly, the government response should be influenced by the severity of pollution and the specific circumstances of the polluter. Yet, policy makers seeking to improve environmental compliance need to have a clearer picture about the merits of each approach and their relative effectiveness. This research empirically assesses the effectiveness of criminal versus administrative enforcement activities in Israel. After considering the philosophical implications associated with each regulatory approach, the results of a year-long study are presented. A series of performance indicators are utilized, with particular weight on compliance status in the field, to assess the condition of environmental violations several years subsequent to completion of enforcement activities. The state of 100 violations of air, water and hazardous materials laws that had been addressed through the criminal process were matched with results of comparable violations, against which administrative actions were taken. Results suggest that while criminal enforcement is a longer process, compliance following these actions was significantly greater than in administrative enforcement cases. The findings also underline the significance of a systematic follow-up system for tracking violations after enforcement actions are complete. Finally, the study confirms the benefits of targeted assistance to improve compliance among violators of environmental standards.