Low-molecular weight organic semiconductors for organic and perovskite solar cells

Resumen

The ever-increasing demand for energy in the world coupled with higher awareness of global climate change has cast the research spotlight on renewable energy sources. Since the sun provides more energy hourly than humans consume in a year, photovoltaic devices are considered a promising technology especially since the efficiency of perovskite solar cells (PSCs) exceeds 24 %. Despite this achievement, there are still several challenges limiting industrial fabrication, such as instability due to the doping and exposure to moisture. Additionally, materials used in organic and perovskite solar cells tend not to be low-cost nor easy to prepare. Spiro-OMeTAD is the most widely-employed hole-transporting material (HTM) in PSCs, yet it contributes to upwards of 30 % of the overall price due to complex synthesis. On the one hand, the use of polymers in organic solar cells (OSCs) have many advantages such as high conductivity, solution processing and inexpensive preparation of large area devices. Yet, batch-to-batch reproducibility of molecular weight is an important hurdle, which may impacttheir optical and electrochemical properties. In order to address these problems, this PhD thesis explored the design, synthesis and characterization of two novel families of organic small molecules (SMs) having two different applications in photovoltaics: absorbers for OSCs and HTMs used for perovskite solar cells. The objective of this work was to understand the underlying requirements for efficient absorbers and HTMs in OSCs and PSCs, respectively. To that aim, eight new organic semiconductors were designed for their low-cost synthesis, easy preparation and purification. Additionally, these materials made possible the reproducible fabrication oflow-cost devices with high efficiencies. The following section briefly describes the contents, motivation and conclusions for each chapter of this thesis. Chapter 1 introduces the necessary concepts for the study of two different types of photovoltaic devices. The most efficient small molecules with electron donor properties are described depending on their application in organic or perovskite solar cells. Chapter 3 describes the methods and techniques utilized in the synthesis and characterization of the new organic semiconductors and in the fabrication and characterization of the devices fabricated using these materials. Chapter 4 details the synthesis and characterization of two novel series of small molecules with electron donor properties: the benzothiadiazole (BT) family formed by CS01, CS03, EP02 and LCS01; and the triphenylamine and carbazole-based family that contains CS02, CS04, CS05 and CS06. Inert atmosphere was maintained during the synthesis of these materials. Generally, purification was performed following workup by silica gel column chromatography, or by recrystallization. The synthesized materials were characterized by 1H NMR, 13C NMR and high-resolution mass spectroscopy (HRMS). In order to know the thermal behaviour of the small molecules, thermogravimetric (TGA) and differential scanning calorimetry (DSC) were carried out. In addition, absorption/emission spectroscopy and cyclic voltammetry measurements were collected to study the optical and electrochemical properties. The BT family of molecules was found to have suitable energy levels to be used in both OSCs and PSCs, and the triphenylamine and carbazole-based new HTMs, in PSCs. Chapter 5 studies the fabrication and photovoltaic characterization of binary and ternary bulk heterojunction solar cells using the BT family as absorbers with one or two electron acceptors as components of the active layer. The device performance of the new absorbers was investigated in binary and ternary BHJ OSCs with PC71BM and two different non-fullerene electron acceptors (DPP8 and MPU3). Solvent vapour annealing was applied to control the morphology of the active layer. Efficiencies up to 6.35 % and 5.59 % were achieved for LCS01/EP02:PC71BM active layer respectively. When the active layer contained the SMs blended with one of the non-fullerene electron acceptors, PCEs arrived to 7.81 % and 8.91 %, for CS01/EP02:MPU3. An increase in the voltage was observed for all of them when compared with PC71BM due to the higher LUMO level of MPU3. When ternary BJH OSCs were fabricated, the current density was enhance leading to higher PCEs (9.94 % and 9.62 % % for CS01/EP02:PC71BM:MPU3 devices) because of the addition of a third component into the active layer that has absorption in the visible range of the solar spectrum contributing to the photogenerated electrons. Chapter 6 delves into the optimization process of the device fabrication of perovskite solar cells using HTMs described in Chapter 4, as well as studying their photovoltaic properties: current density, voltage, fill factor, efficiency and incident-photon-tocurrent efficiency. The relationship between molecular structure/optoelectronic properties with the device performance and the HTM layer thicknesses are explored. PSCs was carried out by depositing the different layers that composed the devices by spray pyrolysis, spin coating and thermal evaporation techniques. Afterwards, devices were characterized using a solar simulator recording the J-V curve (currentvoltage), incident photon-to-current efficiency (IPCE) and scanning electron microscopy (SEM) to know the thickness of each layer. The optimization process indicated that the thickness of the hole-transport layer plays an important role in PSCs, even with relative high hole mobilities. CS01 and LCS01 showed the best efficiency (18.09 % and 17.84 %) when compared with the other molecules in the BT family due to their more extended 􀊌-conjugated system and their methoxy substituents. CS05 is the champion molecule with 19.38 %, including in its structure TPA and carbazole moieties. The introduction of the carbazole group, in this case, was an excellent issue. Likewise, CS02 and CS04 efficiencies were 15.40 % and 18.05 %, respectively, showing that the introduction of the substituted TPA also has a positive effect. CS06 showed no hole injection when tested with the triplecation mix perovskite. Nevertheless, devices employing this molecule with MAPBr3 achieved an exceptional PCE of 6.28 % (6.70 % and 4.77 % for the references using spiro-OMeTAD PTAA). Chapter 7 recapitulates the relevant conclusions from each chapter in order and offer an outlook regarding the future of these projects.