Sn-Doped Zinc Oxide as an Electron Transporting Layer for Enhanced Performance in PbS Quantum Dot Solar Cells.

Colloidal PbS quantum dot solar cells (QDSCs) have been primarily demonstrated in n-i-p structures by incorporating a solution-processed ZnO electron transporting layer (ETL). Nevertheless, the inherent energy barrier for the electron extraction at the ZnO/PbS junction along with the defective nature significantly diminishes the performance of the PbS QDSCs. In this study, by employing Sn-doped ZnO (ZTO) ETL, we have tuned the conduction band offset at the junction from spike-type to cliff-type so that the electron extraction barrier can be eliminated and the overall photovoltaic parameters can be enhanced (open-circuit voltage of 0.7 V, fill factor over 70%, and efficiency of 11.3%) as compared with the counterpart with the undoped ZnO ETL. The X-ray photoelectron spectroscopy (XPS) analysis revealed a mitigation of oxygen vacancies in the ZTO ETL of our PbS QDSCs. Our work signifies the importance of Sn doping into the conventional ZnO ETL for the superior electron extraction in PbS QDSCs.

Spectrally Resolved Exciton Polarizability for Understanding Charge Generation in Organic Bulk Hetero-Junction Diodes.

Despite decades of research, the dominant charge generation mechanism in organic bulk heterojunction (BHJ) devices is not completely understood. While the local dielectric environments of the photoexcited molecules are important for exciton dissociation, conventional characterizations cannot separately measure the polarizability of electron-donor and electron-acceptor, respectively, in their blends, making it difficult to decipher the spectrally different charge generation efficiencies in organic BHJ devices. Here, by spectrally resolved electroabsorption spectroscopy, we report extraction of the excited state polarizability for individual donors and acceptors in a series of organic blend films. Regardless of the donor and acceptor, we discovered that larger exciton polarizability is linked to larger π-π coherence length and faster charge transfer across the heterojunction, which fundamentally explains the origin of the higher charge generation efficiency near 100% in the BHJ photodiodes. We also show that the molecular packing of the donor and acceptor influence each other, resulting in a synergetic enhancement in the exciton polarizability.

Pressurized Back-Junction Doping via Spray-Coating Silver Nanowires Top Electrodes for Efficient Charge Collection in Bifacial Colloidal PbS Quantum Dot Solar Cells.

Colloidal PbS quantum-dot solar cells (QDSCs) have long suffered from inefficient charge collection near the back-junction due to the lack of p-doping strategy, rendering their bifacial photovoltaic applications unsuccessful. Here, we report highly efficient photocarrier collection in bifacial colloidal PbS QDSCs by exploiting spray-coated silver nanowires (AgNWs) top electrodes. During our spray-coating process, pressurized Ag diffusion occurred toward the active layer, which induced effective p-doping and deep-level passivation. By manipulating the spray pressure, optimum AgNWs' stacking morphology enabling an appropriate level of Ag diffusion could be achieved, leading to Jsc over 30 mA/cm2 from the conventional n-i-p structure upon light illumination to the film side. The morphological and electrical behaviors of AgNWs according to the spray pressure are comprehensively explained in relation to the device performance. Finally, 50 bifacial cells were fabricated over 49 cm2 sized glass substrate, demonstrating the large-area processability and functionality of the spray-coated AgNWs with the effective back-junction engineering.