Efficiently and economically harnessing the solar energy via solar cell devices is one of promising solutions to address the global energy crisis. This thesis mainly focuses on a novel family of photoactive layer materials, namely organic-inorganic lead halide perovskite hybrids, and their corresponding solar cell devices, due to their potential for achieving outstanding power conversion efficiency and low-cost processibility. Specifically, the main research themes of this thesis are to achieve high performance perovskite hybrid solar cells through optimizing device structures, developing novel functional perovskite materials, and elucidating the underlying physics and mechanisms for guiding us to construct high performance solution-processed perovskite hybrid solar cells. This dissertation contains four parts and 10 chapters.
In PART I, a broaden overview on both solar cell device and material is given, which specifically reviews the importance of solar energy and solar cells, comparison between previous-generation solar cells and perovskite hybrid solar cells, history of perovskite hybrid materials for solar cell application in Chapter 1 and describes the theoretical background of solar cell devices and material used for fabrication of solar cells in Chapter 2.
PART II mainly includes the detailed projects on solar cell device engineering. Firstly, in Chapter 3, we employ a highly electrical conductive, polyethylene oxide (PEO)-doped poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as the hole extraction layer (HEL) for the planar heterojunction (PHJ) perovskite hybrid solar cells (pero-HSCs). The dramatically enhanced electrical conductivity of the PEO-doped PEDOT:PSS HEL provides an efficient pathway for the hole extraction, transport, and collection from the perovskite active layer to the anode. As a result, a significantly enhanced short-circuit current (JSC) of 23.42 mA cm-2, a slightly enlarged open-circuit voltage (VOC) of 0.88 V, an enhanced FF of 80.10% and a correspondingly dramatically enhanced power conversion efficiency (PCE) of 16.52%, which is a ~45% enhancement as compared with that from the PHJ pero-HSCs incorporated with the pristine PEDOT:PSS HEL, are observed. In Chapter 4, we utilize a solution-processed ultrathin layer of an ionomer, 4-lithium styrenesulfonic acid/styrene copolymer (LiSPS), to re-engineer the interface of methylammonium lead iodide (CH3NH3PbI3) in PHJ pero-HSCs. The ionomer can sufficiently modify the rough surface of the perovskite and optimize the charge extraction efficiency between perovskite photoactive layer and the charge transport layer. As a result, PHJ pero-HSCs with an increased photocurrent density of 20.90 mA cm-2, an enlarged ¿ll factor of 77.80%, a corresponding enhanced power conversion ef¿ciency of 13.83%, high reproducibility, and low photo-current hysteresis, are achieved. In Chapter 5, because one major limitation to increasing the efficiency of pero-HSCs is the fact that the diffusion length of the electrons is shorter than that of the holes, to facilitate the electron extraction efficiency in pero-HSCs and to make this efficiency comparable with that of the holes, we fabricated BHJ pero-HSCs by mixing perovskite materials with water-/alcohol soluble fullerene derivatives. The observed enhanced JSC and enlarged FFs were a result of the balance in the charge carrier extraction efficiency and the enlarged interfacial area between the perovskite materials and the fullerene derivatives. Significantly improved power conversion efficiencies were obtained for these BHJ pero-HSCs. A greater than 22% increase in power conversion efficiency was observed for the BHJ pero-HSCs compared with planar heterojunction pero-HSCs. A remarkable 86.7% FF, the highest reported value for pero-HSCs, was observed for the BHJ pero-HSCs. Our strategy of using a BHJ structure in pero-HSCs offers an efficient and simple way to further increase the performance of these devices.
PART III mainly discusses the detailed projects on novel perovskite materials development. To fabricate homogeneous and high-quality perovskite thin ¿lms via low-temperature solution processing is always a challenge to realizing high-ef¿ciency pero-HSCs, in Chapter 6, we firstly report a development of an approach to realize smooth surface morphology of CH3NH3PbI3 perovskite thin ¿lms via using strong-polar ethanol solution rather than less-polar isopropanol solution, which was previously used as the solvent for preparing perovskite thin ¿lms. In comparison with the pero-HSCs processed from isopropanol solution, more than 40% enhanced ef¿ciency is observed from pero-HSCs processed from ethanol solution. The enhanced ef¿ciency is attributed to a homogeneous high-quality perovskite thin ¿lm with dramatically low root-mean-square roughness and completely conversion of lead (II) iodide (PbI2) to CH3NH3PbI3. In Chapter 7, we report the development and investigation of novel CH3NH3PbI3: x Nd3+(where x = 0, 0.1, 0.5, 1.0, and 5.0 mol%) perovskite hybrid materials, where Pb2+ is partially substituted by an inequivalent rare-earth metal cation, neodymium (Nd3+), which was never reported in previous studies. By conducting the charge carrier mobility measurements and film morphology studies, it is found that solution-processed CH3NH3PbI3: x Nd3+ thin films exhibit significantly improved and more balanced charge carrier mobilities, and superior film quality with dramatically reduced trap-states and pin-holes, as compared with pristine CH3NH3PbI3 thin film. As a result, a descent power conversion efficiency of 20.56% for solar cells and a superior photodetectivity of ~ 1014 cm Hz1/2 W-1 from 375 nm to 800 nm at room temperature for photodetector, are observed from solution-processed perovskite photovoltaics by novel CH3NH3PbI3: x Nd3+ thin films. All these results demonstrate that our method provides a simple and facial way to boost the device performance of perovskite photovoltaics. In Chapter 8, we report the utilization of polyethylene oxide additives to anchoring the ions in the perovskite lattice to suppress the formation of point defect or the migration of ions/vacancy, for simultaneously enhancing device efficiency, minimizing photocurrent hysteresis and enhancing device stability. Consequently, efficient solar cell devices with power conversion efficiency of 19.01% with extremely low hysteresis index of 0.001 and long-term device shelf half-life time of 504 hrs (without encapsulation, stored in 50% humidity air) have been achieved. Chemical, structural and morphological analysis show that the PEO additive acts as a crosslink between neighboring perovskite crystal domains via the strong hydrogen bonding of `-OH…I-’ and `O…H-NH2CH3+’ to the perovskite.
In PART IV, a brief summerization on our works in terms of both device and material engineering is presented in Chapter 9, that is, for optimizing the device configuration as well as address critical issues in previously wide-applied hybrid perovskite thin films, we mainly developed novel ideas on: (i) modifying anode buffer layer for efficient hole extraction; (ii) modifying the interfacial electrical coherence on the i-n junction; (iii) developing a bulk heterojunction concept for efficient charge extraction; for novel materials part, we also focused on three major parts: (i) optimizing the thin film quality of perovskite; (ii) tuning the crystal lattice structure by inequivalent metal doping; (iii) anchoring the ion within the perovskite lattice for reducing hysteresis and improving device stability. Finally, an outlook is given in the Chapter 10 for guiding our future work.