Fabrication, characterization and modelling of high efficiency inverted polymer solar cells
- Autores: Jose Guadalupe Sanchez Lopez
- Directores de la Tesis: Josep Pallarès Marzal (dir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2018
- Idioma: español
- Tribunal Calificador de la Tesis: Lluís F. Marsal Garví (presid.), Emilio J. Palomares Gil (secret.), Elizabeth Van Itallie (voc.)
- Programa de doctorado: Programa de Doctorado en Tecnologías para Nanosistemas, Bioingeniería y Energía por la Universidad Rovira i Virgili
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX hdl.handle.net
- Resumen
During the last decade, many progress on the renewable energy field research has been achieved. The advances on the renewable energy technologies still allow obtaining good performances with lower cost than fossil fuel prices. For this reason, the interest of public and private organizations in clean, renewable and sustainable energy sources such as hydroelectric, wind power, bioenergy, geothermal, ocean and solar has increased. Solar energy is one of the most common renewable energy technology because of the possibility to use the energy of sun directly. Some technologies such as solar thermal heating and cooling, concentrating solar power (CSP) and solar photovoltaics (PV) capture the solar energy (in form of radiant light or heat) to transform it in a useful energy for human benefit. Among them, the PV is the most developed solar energy technology. The Solar PV (also called solar cells) convert the sunlight energy (photons) into electricity (voltage) by the photovoltaic effect.
Nowadays, important advances in solar cells technology field have been achieved. There have been emerged new technologies alternative energy source based on inorganic and organic materials to the traditional solar cells systems.
Motivation Since the first evidence of photovoltaic effect in polymer:fullerene blend, the organic solar cells based on these materials were considered as a promising low-cost energy source. Recently, the research on organic solar cells based on polymer:fullerene materials have progressively growing due to the lastly efficiencies over 10%. Several research groups have focused on the development of more efficient polymer materials for OSCs applications. Moreover the studies on the manufacturing process and device' engineering allowed to develop different OSC's architecture with more efficient interpenetrating network morphology. The development of OSCs with inverted architecture, as well as the synthesis of polymers based on thieno[3,4-b]-thiophene and benzodithiophene marked a turning point in the research of high efficient OSCs. These two progress allowed to fabricate single junction OSCs based on polymers materials (PSCs) with both high photon conversion and charge carrier extraction efficiencies.
Moreover, the development of PTB-based polymers and inverted architecture prompted to researchers to develop PSCs with a tandem architecture.
For these reason, this thesis is focused on the fabrication, characterization and modelling of single junction high efficient PSCs with inverted architecture (iPSCs). The iPSCs concerned were fabricated with two of the most efficient PTB-based polymer blended with a fullerene used as active layer.
The major aim of this thesis is:
Fabrication, characterization and modelling of high efficient polymer:fullerene-based solar cells with inverted architecture.
Additionally, there are several specific aims:
1. To analyze the effect of an ultra-thin layer of titanium oxide as an electron transport layer in the performance of inverted polymer solar cells based on PTB7: PC70BM.
2. To analyze the stability and lifetime of the PTB7PC70BM-based iPSCs with TiOx (TiOx-iPSCs) in comparison to similar PSCs which use different electron transport layers.
3. To analyze the stability of TiOx-iPSCs under different degradation conditions.
4. To determine the loss mechanisms in TiOx-iPSCs based on PTB7-Th:PC70BM by electrical and optical characterization.
5. To analyze the effect of of zinc oxide film deposited by inkjet printing (ZnO-IJP) as an electron transport layer on the performance of PTB7-Th:PC70BM-based PSCs, for use in the large-scale manufacture of PSCs.
6. To analize the performance of iPSCs based on PTB7-Th:PC70BM with ZnO-IJP in comparison to similar iPSCs using ZnO deposited by different thin layer deposition techniques.
7. To determine the loss mechanisms in iPSCs based on PTB7-Th:PC70BM with ZnO deposited by different techniques by electrical, photophysical and optical characterization.
The thesis is organized as follows:
Chapter 2 presents a literature review as introduction to the organic semiconducting materials. The basics, operation principles and performance parameters of organic solar cells are described. The different architectures of organic solar cells are introduced. A brief introduction to organic solar cell based on polymer:fullerene is also presented. Finally, the state-of-the-art of the most important semiconducting polymer used for organic solar cell applications are described.
Chapter 3 introduces the polymer donor and fullerene acceptor materials used for the devices fabrication. Moreover, the materials used as buffer layers and electrodes are presented. The operation principle of the different thin-films coating and printing technologies used for the devices fabrication are described. The full step-by-step process for the fabrication of devices is also described. Finally, the methodology for the devices characterization used in this thesis is discussed.
Chapter 4 describes the fabrication and characterization of iPSCs based on PTB7:PC70BM using a titanium oxide (TiOx) film as electron transport layer (ETL). The effects of TiOx on the performance parameters and stability of iPSCs compared to the effects on conventional PSCs and iPSCs using PFN as ETL is presented. The results of impedance spectroscopy analysis on encapsulated and non-encapsulated TiOx-iPSCs under ambient conditions are discussed. Finally, the effects of the palladium-doped TiOx layer on the performance of iPSCs are shown.
Chapter 5 presents the effects of ZnO layers deposited by inkjet printing as electron transport layer ETL on the performance of i-PSCs based on PTB7-Th:PC70BM active layers. The performance of iPSCs with ZnO-IJP are compared to that of iPSCs with ZnO deposited by spin coating (ZnO-SC) and thermal evaporation techniques (ZnO-TE). Finally, results obtained from electrical, optical and photophysical characterization measurements of iPSCs with ZnO-IJP, ZnO-SC and ZnO-TE are discussed.
Chapter 6 presents the general conclusions of the thesis and resume the overall results achieved during this research. The further works related to this research are also presented.
Main conclusions Herein the fabrication, modelling and characterization of polymer-based organic solar cells with inverted architecture (iPSCs) were described. The thesis was focused on the study of the simultaneously improvement of efficiency and long-term stability of iPSCs based on polymer:fullerenes. The polymers PTB7 and PTB7-Th were used as the electron donor materials, whereas the fullerene PC70BM was used as the electron acceptor. The zinc oxide (ZnO) and titanium oxide (TiOx) were used as electron transport layers (ETL), moreover PFN was used for comparison purpose. All iPSCs were characterized by optical, electrical and photophysical methods in order to understand the loss mechanisms involved in the degradation process of these devices. Equivalent circuit models were used to analyze the dark J-V characteristics and the impedance spectroscopy data to identify the origin of the loss mechanisms.
Chapter 3 describes the polymers and fullerene materials that conform the active layer. Subsequently, the materials used as buffer layer, ETL or hole transport layer (HTL), for the fabrication of PSCs with inverted architecture were presented. The materials used as buffer layers for conventional PSCs fabrication were also presented. Afterward, the thin-film deposition techniques, as well as the step-by-step process for the polymer solar cells fabrication with conventional and inverted architecture were described. Finally, the methodology for the current vs voltage, external quantum efficiency, charge carrier extraction, transient photovoltage, and impedance spectroscopy measurements used for the characterization of iPSCs were discussed.
In chapter 4, the stability study of high efficiency iPSCs was described. The devices were fabricated based on PTB7:PC70BM using an ultra-thin film of TiOx as ETL. The performing and stability of TiOx-iPSCs were compared to that of conventional PSCs and iPSCs using PFN as ETL. TiOx-iPSCs exhibited an efficiency 5% and 11% higher than that of PFN-iPSCs and conventional PSCs. Under dry nitrogen (N2) atmosphere, TiOx-iPSCs shown a lifetime (PCE = 80 %) of 6048 h, that is five times longer than that of conventional PSCs (1200 h) and PFN-iPSC (1300 h). Moreover, TiOx-iPSCs stability was analyzed by exposing encapsulated and non-encapsulated TiOx-iPSCs to ambient conditions (23 °C and 45% humidity). Under these conditions, the non-encapsulated devices exhibited a lifetime of several hours and an electron mobility 75% lower than that of devices under N2 atmosphere. This low stability is attributed to the degradation of the contact interfaces driven by ambient oxygen and water. On the other hand, the lifetime of encapsulated TiOx-iPSCs under ambient conditions was extended by up to 120 h with no observed loss in electron mobility. Moreover, it was demonstrated that the shunt losses of iPSCs can be reduced by using TiOx doped with a small amount of palladium [0.125 mM]. As a consequence, the efficiency of devices with Pd-doped TiOx is up to 4% higher than that of iPSCs with pristine TiOx.
The effects of ZnO layers deposited by inkjet printing (ZnO-IJP) as ETL on the performance of iPSCs based on PTB7-Th:PC70BM were explored in Chapter 5. The motivation was to demonstrate that ZnO-IJP layers can be successfully applied to the fabrication of efficient iPSCs. The morphology of the ZnO-IJP layers was analyzed by AFM, and compared to that of ZnO layers deposited by spin coating (ZnO-SC) and thermal evaporation (ZnO-TE). The study shown that the band structure and non-geminate recombination kinetics of PTB7-Th:PC70BM layer is affected by the morphology of the ZnO underlayer. Charge carrier and transient photovoltage measurements reveal that non-geminate recombination is governed by deep trap states in iPSCs with ZnO-IJP, whereas trapping is less significant for devices with ZnO-SC and ZnO-TE. It was found that the efficiency of iPSCs with ZnO-IJP is mainly limited by their slightly lower JSC since the low photon conversion efficiency in the visible part of the solar spectrum. In spite of this current limitations, the performance of iPSCs with ZnO-IJP compares very favorably with that of devices with ZnO-SC and ZnO-TE.
In conclusion, high efficient and stable organic solar cells based on PTB7:PC70BM and PTB7-Th:PC70BM with inverted architecture were demonstrated in this thesis. Moreover, TiOx used as an electron transport layer is crucial for improving the efficiency and the stability of iPSCs. Finally, it was also demonstrated that ZnO layers deposited by inkjet printing can be successfully applied to the fabrication of high efficiency iPSCs at laboratory scale.
Future works In spite of the good results obtained in this thesis, several experiments could not been done due to lack of time, e g. the iPSCs fabrication and their characterization are time consuming, moreover the stability experiments are carried out as longer as possible. For these reasons, future works concern deeper analysis of loss mechanism, stability under different conditions, new materials for active and buffer layers. The further work that could be tested is as follows:
- Optimization the synthesis of Titanium-doped to improve the stability and charge extraction in iPSCs.
- Optimization of encapsulation process to improve the long-term stability of iPSCs under real operational conditions.
- Optimization of the depositing parameters of the ZnO-ink by the inkjet printing to obtain a high uniform layer.
- Fully inkjet printed polymer solar cell fabrication in a flexible substrate.
- Optical, electrical and photophysical characterization of fully printed iPSCs, as well as the study of their stability over time.
- Fabrication, characterization and modelling of iPSCs based on new polymers and non-fullerenes materials.
- Fabrication, characterization and modelling of iPSCs based on ternary blends (polymer:fullerene:non-fullerenes).
