Perovskite solar cells (PSCs) based on organic-inorganic metal halide light absorbers are regarded as a promising photovoltaic candidate after rapidly rising the energy conversion efficiencies. At the same time, questions on the performance control and its characterizations are being addressed. In this thesis, I will mainly focus on two areas including designing new hole transport materials (HTMs) and interface engineering to enhance the charge carrier transport. The two area study aims to understand their material properties, inform photovoltaic design, improving PCE and device stability

Firstly, in chapter 3, I design and introduce a family of polyfluorene derivative polymers as superior HTMs because polyfluorene (PF) copolymers can be an ideal alternative to spiro-OMeTAD, given thei...[ Read more ]

Perovskite solar cells (PSCs) based on organic-inorganic metal halide light absorbers are regarded as a promising photovoltaic candidate after rapidly rising the energy conversion efficiencies. At the same time, questions on the performance control and its characterizations are being addressed. In this thesis, I will mainly focus on two areas including designing new hole transport materials (HTMs) and interface engineering to enhance the charge carrier transport. The two area study aims to understand their material properties, inform photovoltaic design, improving PCE and device stability

Firstly, in chapter 3, I design and introduce a family of polyfluorene derivative polymers as superior HTMs because polyfluorene (PF) copolymers can be an ideal alternative to spiro-OMeTAD, given their low price, high hole mobility and good processability. In particular, one of the candidates-TFB, has achieved a 10.92% PCE under the one-step perovskite deposition condition, considerably higher than that with spiro-OMeTAD (9.78%). A relatively high efficiency (about 13% efficiency) has been achieved with the two-step perovskite deposition method. Also these polymer based PSCs present better stability than those based on sprio-OMeTAD. Another candidate in Chapter 4 for replacing spiro-OMeTAD is an inorganic material. Normally inorganic materials have advantages of low-cost and better device stability than organic materials. Among various p-type semiconductors, I considered nickel oxide (NiO) might be an ideal p-type hole extraction layer material because of its wide band gaps and electron blocking properties. In this thesis, I develop a new nickel oxide nancystallines (NCs) for hole extraction and transport layer of perovskite solar cells. The well-crystalized NiO NCs film had yielded a PCE reaching up to 9.11% and better stability than organic HTM based PSCs. Currently to compete with spiro-OMeTAD based PSCs, I have improved this type of solar cells to 14.7% efficiency with good device stability.

The second part concerns with interface modification, particularly between perovskite absorber and charge collecting layers. In Chapter 5, a successful approach is to utilize the synthesised GQDs to modify the interface between the perovskite absorber and electron transport layer. Here, I observe that a higher performance conversion efficiency (PCE) perovskite solar cell could be achieved to 10.15%, when inserting an ultrathin layer of GQDs between the perovskite and the mesoporous titanium dioxide. This efficiency is significantly higher than 8.81% without inserting GQDs. I apply ultrafast transient absorption spectroscopy measurement to test the electron extraction between perovskite and TiO₂ mesoporous film. Here, a considerably faster electron extranction time (90~106 ps) with GQDs perovskite/TiO₂ film, than 260~307 ps of pristine perovskite/TiO₂ film. This work has demostrated graphene based carbon materials to facilitate the charge transfer from perovskite absorber to electron acceptor in the energy conversion devices. Chapter 6 is to use a PCBM:PFNOX Blend Electron Transport Layer by adding a small perovskite (1.5wt%) of PFNOX into the PCBM layer. The efficiency has been improved from 10% to 15%, by improving the film quality and further decreased the recombination of electron-hole pairs. This simple and effective approach gives potential for interface engineering by doping with other polymer, small molecular to further improve the efficiency and stability. Finally, in chapter 7 we for the first time synthesizes mesoporous SnO₂ MSCs to prepared high quality and highly efficient electron transport materials, Although the strong recombination in the SnO₂ MSCs based PSCs was found to limit the efficiency to less than 4.0% (highest efficiency~3.76%) due to the low photovoltage and photocurrent. After treating the SnO₂ MSCs with making a thin TiO₂ layer, the cell efficiency was dramatically increased to as high as 8.54%, indeedly much higher than the TiO₂ MSC PSCs record (7.20%). Significantly, the TiO₂-coated SnO₂ MSCs device shows the similar transport properties to the pristine SnO₂ MSCs device and higher than TiO₂ based PSCs.