Analysis of Interfacial Layer-Induced Open-Circuit Voltage Burn-In Loss in Polymer Solar Cells on the Basis of Electroluminescence and Impedance Spectroscopy

Stable and robust open-circuit voltage (<i>V</i>_OC) is essential to achieve a long lifetime for polymer solar cells (PSCs). Here, we investigate the <i>V</i>_OC burn-in loss mechanism on the basis of the analysis of electroluminescence quantum efficiency (EQE_EL) and impedance measurements in amorphous PSCs, with an inverted structure having different electron transport layers (ETLs) of ZnO nanoparticles (NPs) and the sol–gel processed ZnO layer. We found that both charge recombination and energetic disorder account for a substantial proportion of the <i>V</i>_OC burn-in loss. Moreover, varying the ETL significantly affected the degree of <i>V</i>_OC burn-in loss, although relative contribution of these two factors remained constant. To accurately extract charge recombination-induced <i>V</i>_OC loss, we applied a novel yet effective method that relates the EQE_EL of PSCs to charge recombination-induced <i>V</i>_OC loss. Additional analyses, including those focused on light intensity (<i>P</i>_light)-dependent <i>V</i>_OC and density of states, will provide an inclusive perspective on the degradation mechanism of <i>V</i>_OC and development of stable PSCs

Modular Fabrication of Hybrid Bulk Heterojunction Solar Cells Based on Breakwater-like CdSe Tetrapod Nanocrystal Network Infused with P3HT

We demonstrate the modular fabrication of nanocrystal/polymer hybrid bulk heterojunction solar cells based on breakwater-like CdSe tetrapod (TP) nanocrystal networks infused with poly­(3-hexylthiophene) (P3HT). This fabrication method consists of sequential steps for forming the hybrid active layers: the assembly of a breakwater-like CdSe TP network followed by nanocrystal surface modification and the infusion of semiconducting polymers. Such a modular approach enables the independent control of the nanoscopic morphology and surface chemistry of the nanocrystals, which are generally known to exhibit complex correlations, in a reproducible manner. Using these devices, the influence of the passivation ligands on solar cell characteristics could be clarified from temperature-dependent solar cell experiments. We found that a 2-fold increase in the short-circuit current with 1-hexylamine ligands, compared with the value based on pyridine ligands, originates from the reduced depth of trap states, minimizing the trap-assisted bimolecular recombination process. Overall, the work presented herein provides a versatile approach to fabricating nanocrystal/polymer hybrid solar cells and systematically analyzing the complex nature of these devices

Influence of External Pressure on the Performance of Quantum Dot Solar Cells

We report the influence of post-treatment via the external pressure on the device performance of quantum dot (QD) solar cells. The structural analysis together with optical and electrical characterization on QD solids reveal that the external pressure compacts QD active layers by removing the mesoscopic voids and enhances the charge carrier transport along QD solids, leading to significant increase in <i>J</i>_SC of QD solar cells. Increasing the external pressure, by contrast, accompanies reduction in FF and <i>V</i>_OC, yielding the trade-off relationship among <i>J</i>_SC and FF and <i>V</i>_OC in PCE of devices. Optimization at the external pressure in the present study at 1.4–1.6 MPa enables us to achieve over 10% increase in PCE of QD solar cells. The approach and results show that the control over the organization of QDs is the key for the charge transport properties in ensemble and also offer simple yet effective mean to enhance the electrical performance of transistors and solar cells using QDs

Nanostructured Electron-Selective Interlayer for Efficient Inverted Organic Solar Cells

We report a unique nanostructured electron-selective interlayer comprising of In-doped ZnO (ZnO:In) and vertically aligned CdSe tetrapods (TPs) for inverted polymer:fullerene bulkheterojunction (BHJ) solar cells. With dimension-controlled CdSe TPs, the direct inorganic electron transport pathway is provided, resulting in the improvement of the short circuit current and fill factor of devices. We demonstrate that the enhancement is attributed to the roles of CdSe TPs that reduce the recombination losses between the active layer and buffer layer, improve the hole-blocking as well as electron-transporting properties, and simultaneously improve charge collection characteristics. As a result, the power conversion efficiency of PTB7:PC₇₀BM based solar cell with nanostructured CdSe TPs increases to 7.55%. We expect this approach can be extended to a general platform for improving charge extraction in organic solar cells

Plasmonic Organic Solar Cells Employing Nanobump Assembly <i>via</i> Aerosol-Derived Nanoparticles

We report the effect of a nanobump assembly (NBA) constructed with molybdenum oxide (MoO₃) covering Ag nanoparticles (NPs) under the active layer on the efficiency of plasmonic polymer solar cells. Here, the NPs with precisely controlled concentration and size have been generated by an atmospheric evaporation/condensation method and a differential mobility classification and then deposited on an indium tin oxide electrode <i>via</i> room temperature aerosol method. NBA structure is made by enclosing NPs with MoO₃ layer <i>via</i> vacuum thermal evaporation to isolate the undulated active layer formed onto the underlying protruded NBA. Simulated scattering cross sections of the NBA structure reveal higher intensities with a strong forward scattering effect than those from the flat buffer cases. Experimental results of the device containing the NBA show 24% enhancement in short-circuit current density and 18% in power conversion efficiency compared to the device with the flat MoO₃ without the NPs. The observed improvements are attributed to the enhanced light scattering and multireflection effects arising from the NBA structure combined with the undulated active layer in the visible and near-infrared regions. Moreover, we demonstrate that the NBA adopted devices show better performance with longer exciton lifetime and higher light absorption in comparison with the devices with Ag NPs incorporated flat poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Thus, the suggested approach provides a reliable and efficient light harvesting in a broad range of wavelength, which consequently enhances the performance of various organic solar cells