Manis-Levy Hadar, Abutbul Ran Eitan, Grosman Arieh, Peled Hadar, Golan Yuval, Ashkenasy Nurit, Sa'Ar Amir, Shikler Rafi, Sarusi Gabby
Electro-optics and Photonics Engineering Dept., School of Electrical and Computer Engineering, Ben-Gurion University of the Negev, Be'er-Sheva, Israel. Electrical and Computer Engineering Dept., School of Electrical and Computer Engineering, Ben-Gurion University of the Negev, Be'er-Sheva, Israel. Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Be'er-Sheva, Israel.
Nanotechnology. 2020 Apr 3;31(25):255502. doi: 10.1088/1361-6528/ab7ef7. Epub 2020 Mar 11.
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.
低成本短波长红外（SWIR）光伏（PV）探测器和太阳能电池备受关注，然而当今的主要生产技术基于成本高昂的InGaAs层外延生长。在本研究中，采用化学浴沉积（CBD）技术从溶液中制备了p型量子受限（QC）PbS纳米域（NDs）结构的层，该结构经设计可吸收1550 nm（Eg = 0.8 eV）的SWIR光。这些层使用两种不同的CBD方案生长在两种不同的n型CdS中间层（Eg = 2.4 eV）之上，衬底为氟氧化锡（FTO）。获得了两种类型的CdS/PbS异质结用作SWIR PV探测器。这两种所得器件表现出相似的光致发光行为，但对SWIR光照的电响应却截然不同。一种类型的CdS/PbS异质结对SWIR光表现出光伏响应，而另一种仅在施加偏压下才对SWIR光表现出光响应。为了阐明这一有趣现象，并且由于这两种异质结之间唯一的差异可能是CdS层的掺杂水平，我们通过表面光电压（SPV）测量了该层的掺杂水平。这为两种器件产生了不同的极化，表明两种不同制备方案的CdS掺杂水平不同，霍尔效应测量也证实了这一点。我们在超带隙光照下针对CdS进行了电流 - 电压测量，得到的电响应表明存在一个对空穴从CdS转移到PbS无障碍的情况。结果表明，不同的响应确实源于CdS/PbS异质结界面处能带结构的变化，这是由于CdS的不同掺杂水平所致。我们发现，与具有相似结构的太阳能电池或可见光探测器不同，在SWIR光电探测器中，形成了一种I型异质结，其界面处存在一个势垒，限制了光激发电子从QC - PbS注入到CdS一侧。较高n掺杂的CdS在CdS一侧产生一个窄的耗尽区，具有一个尖峰状势垒，其宽度窄到足以使隧道电流通过，从而产生光伏电流。我们的结果表明，对于CBD生长的对SWIR敏感的CdS/PbS - NDs异质结，在零偏压下可获得约2%的外部量子效率（EQE）和约20%的内部量子效率（IQE）。
