Efficient Vacuum-Deposited Organic Solar Cells by Novel Designs of Device Structures with Analysis of Working Mechanisms

Abstract

Organic photovoltaics (OPVs) has shown large progress in the past several years. Unfortunately, however, the power conversion efficiency (PCE) is still lower than inorganic counterparts. One of the main reasons is the narrow absorption window of organic materials, which can only partially use solar photon flux. Therefore, extending the absorption into the near-infrared region is a critical factor to enhance device performance in OPVs. In this thesis, the novel designs of devices structures with analysis of their working mechanisms are reported to broaden spectral response to the solar photon flux. The templating effect on the device performance is introduced using copper iodide (CuI) as a templating layer to control the crystal structure of lead phthalocyanine (PbPc). The templating layer inserted between ITO and the PbPc layer induces the increase of crystallinity with the preferred orientation and enhanced exciton diffusion length of the PbPc molecules, resulting in the increased short circuit current density (JSC). Moreover, Fill factor (FF) is also enhanced due to the better contact between ITO and the PbPc layer by insertion the templating layer. As the results, the absorption range of the device is extended into the wavelength of 1000 nm with almost twice enhancement of the PCE. Tandem organic photovoltaics (TOPVs) having two identical sub-cells connected by the interconnection unit (ICU) are reported. The TOPVs can cover broad spectrum if the two sub-cells have complementary absorption. To realize efficient TOPVs, the ICU is the most important part because it affect device performance optically as well as electrically. The requirements of the ICU are transparency in the active region of the TOPVs to reduce optical losses, energy level alignment between two sub-cells to eliminate voltage losses and tunable thickness with electrical properties maintained for the current matching. New design of the ICU (electron-transporting layer/metal/p-doped hole-transporting layer) for the TOPVs is introduced, which fulfills all the requirements. Moreover, its working mechanisms are discussed with the analysis of the ICU by systematically varying the doping concentration of the p-HTL in the ICU. The difference between the Fermi level and the highest ocuupied molecular orbital (HOMO) level of the p-HTL is a key factor to influence the open circuit voltage (VOC) in the TOPVs. The FF is related with the conductivity of the p-HTL and the depletion width at the interface between Ag and the p-HTL. The requirement of the p-HTL is also investigated. Dopants with higher charge generation efficiency results in higher VOC due to the reduction of the difference between the Fermi level and the HOMO level of the p-HTL, and higher FF due to the efficient hole transport at the interface between Ag and p-HTL through the tunneling process. Based on the understanding of the new ICU, efficient vacuum-deposited TOPVs, composed of pristine fullerenes as the acceptors and two complementary absorbing donors, 2-((2-(5-(4-(diphenylamino)phenyl)thieno[3,2-b] thiophen-2-yl)thiazol-5-yl)methylene)malononitrile (DTTz) for the visible absorption and 2-((7-(5-(dip-tolylamino)thiophen-2-yl)benzo[c]-[1,2,5]thiadiazol-4-yl)methylene)malononitrile (DTDCTB) for the near-infrared absorption, are realized. The PCE of 9.2% is achieved with higher FF of 0.62 than that of single-junction sub-cells (0.54, 0.57), JSC of 8.7 mA cm−2 and VOC of 1.71 V using 80 nm thick active layers in both sub-cells. Another device concept of ternary blend OPVs to extend absorption of the active layer using two donors, DTTz for the visible absorption and DTDCTB for the near-infrared absorption, co-deposited with C70 in the ternary layer is reported. The PCE of 8.02% for the ternary device is achieved, which is 23% higher than that of the binary OPVs. This enhancement originates from incorporating two donors with the complementary absorption covering from the wavelength of 350 nm to 900 nm with higher hole mobility in the ternary layer than one of the binary layers consisting of one donor and C70, combined with the energy transfer from the donor (DTTz) with lower hole mobility to the one with higher mobility (DTDCTB). This structure fulfills all the requirements for efficient ternary OPVs.