Organic/inorganic perovskite (CH
3NH
3PbI
3, CH
3NH
3PbI
xCl
3-
x) solar cells have emerged as a highly promising alternative renewable energy source because of their high efficiency and low-cost solution processable manufacturing processes. However, current perovskite solar cells typically require an expensive and air sensitive hole transporter material (HTM, e.g., spiro-MeOTAD) and a noble metal electrode (Au or Ag) deposited by complicated vacuum technology. These drawbacks, if not adequately addressed, will hinder the industrial development and market potential of perovskite solar cells. Therefore, it is highly desirable to study and develop alternative materials and processes that show high performance but are inexpensive, earth-abundant, stable, environmentally friendly, easily processabl...[
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Organic/inorganic perovskite (CH
3NH
3PbI
3, CH
3NH
3PbI
xCl
3-
x) solar cells have emerged as a highly promising alternative renewable energy source because of their high efficiency and low-cost solution processable manufacturing processes. However, current perovskite solar cells typically require an expensive and air sensitive hole transporter material (HTM, e.g., spiro-MeOTAD) and a noble metal electrode (Au or Ag) deposited by complicated vacuum technology. These drawbacks, if not adequately addressed, will hinder the industrial development and market potential of perovskite solar cells. Therefore, it is highly desirable to study and develop alternative materials and processes that show high performance but are inexpensive, earth-abundant, stable, environmentally friendly, easily processable and energy non-intensive. My thesis is focused on the understanding and the development of nanocarbon-based HTM-free perovskite solar cells.
The thesis is divided into 7 chapters. Chapter 1 surveys the current literature and outlines the motivation and objectives of my thesis. Chapter 2 introduces experimental techniques used in my experiments. Major findings of the study are presented and discussed from Chapters 3 to 6, with conclusions and outlooks summarized in Chapter 7.
In chapter 3, I report a new modality of perovskite solar cells that do away with the use of conventional HTMs by directly clamping a selective hole extraction electrode made of candle soot and a deliberately engineered perovskite photoanode. Three generations of clamping solar cells were evolved from direct clamping to rolling-transfer clamping and to chemically promote clamping accelerated by the mechanistic understanding of inner workings. Up to this stage, the third generation clamping solar cells have already achieved a remarkable efficiency of 11.02% and good long term stability. The key soot/perovskite interface, which promotes hole extraction and electron blocking by forming a Schottky junction, was established seamlessly by pre-wetting and reaction embedding the carbon particles into the as-formed perovskite thin film. We named the reaction embedding process as “chemical embedment” interface engineering strategy.
In chapter 4, I extend the “chemical embedment” strategy to the “in-situ and instant transformation” strategy. More specifically, I have demonstrated a successful fabrication of the first planar carbon-based perovskite solar cells for which an inkjet printing technique was developed to deposit the nanocarbon electrode. The inkjet printing technique could not only precisely and controllably pattern the carbon electrode, but also improve the interface between the CH
3NH
3PbI
3 and C electrodes by the instant chemical transformation. By exploiting the C+CH
3NH
3I ink formulation to transform PbI
2 in situ to CH
3NH
3PbI
3, a reinforced interpenetrating interface between the CH
3NH
3PbI
3 and C electrodes was formed in comparison with that using bare C ink, which significantly suppressed charge recombination at the interface. As a result, a considerably high PCE up to 11.60% was achieved.
In chapter 5, I explore different nanocarbon materials for the nanocarbon-based perovskite solar cells using the “chemical embedment” strategy. Through systematic performance comparison between multi-wall carbon nanotube (MWCNT) and the other two representative carbon materials (carbon black and graphite), I discovered two advantages for MWCNT-based devices: i.e., high fill factor (FF) and hysteresis-free performance. I came up with two reasons for the good performance of the MWCNT-based devices. The first reason is the proper size (~ 30 nm in diameter) and 1D chain structure. It is the small size that ensures the appropriate embedment of the MWCNTs into the perovskite film, while the 1D chain structure allows interpenetration to form a continuous, highly conductive and crack-free network film. The second reason is the interpenetrating charge transfer bi-continuous highway at the interface between the CH
3NH
3PbI
3 and the MWCNT network, which is mainly responsible for the high FF and hysteresis-free performance. After preliminary optimization, we are able to achieve a hysteresis-free PCE of 12.67 % with a very impressive FF of 0.80.
In chapter 6, I develop a new interface engineering strategy called “adhesives”. Specifically, I have designed a carbon-based composite cathode (C+epoxy) for HTM-free perovskite solar cells, where epoxy binder can fix perovskite and C electrode tightly but will not hinder hole extraction process from perovskite to carbon. In this fashion, the C+epoxy electrode can serve as both a hole-selective extractor and a water-rejecting barrier. After boosting with a silver paint coating, the whole device is not only efficient at ~11 % but also waterproof. When immersed in water, no performance degradation occurred at the first 80 min. What’s more, the cell stability has be investigated in some harsh environment, like high humidity environment and 50 ℃ thermal stressing, nearly no performance degradation is observed. In other words, the much desired waterproof capability was realized by a two-step conductive paint encapsulation strategy: (1) C+epoxy paint not only serve as an hole extraction electrode but also partly encapsulate the cell; (2) a modified Ag paint further seals the whole device and improve PCE by decreasing contact resistance.
Finally, Chapter 7 summarizes the thesis with conclusions of my work and outlook of this exciting research area.
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