Interfacial and compositional engineering of perovskite solar cells for enhanced device performance and stability

This project was a comprehensive study of interfacial and compositional engineering on perovskite solar cells (PSCs). Significantly enhanced power conversion efficiency and device lifetime of PSC photovoltaic technology were achieved by ion doping based on solution-processed and vapour-assisted treatments. The effects of various amine-contained ligands on enhancing performance of PSC were systematically investigated to reveal controlling defects and interfacial properties in the materials. This work provides several new insights into perovskite solar cells. The research outcomes benefit the development of PSCs based photovoltaic technology

Perovskite-Si Tandem Solar Cells

Since 2009, organic-inorganic - hybrid metal halide perovskite solar cells (PSCs) have achieved great progress on power conversion efficiency (PCE) and stability. Combining PSCs with other photovoltaic (PV) technologies as a tandem architecture is expected to make a breakthrough in theoretical PCE limit of single-junction solar cells and promote commercialization of the emerging PVs. This chapter aims at tracing the evolution of PSCs, interpreting optoelectrical properties of perovskite materials, and demonstrating material engineering on tackling current research problems of tandem solar cells based on atomic scale and nanoscale strategies. Thus, the goal of this study is to gain research interest in perovskite tandem solar cells and push them forward from laboratory to industrial applications.</p

Low-Dimensional-Networked Perovskites with A-Site-Cation Engineering for Optoelectronic Devices

Low‐dimensional‐networked (LDN) perovskites denote materials in which the molecular structure adopts 2D, 1D, or 0D arrangement. Compared to conventional 3D structured lead halide perovskite (chemical formula: ABX3 where A: monovalent cations, B: divalent cations, X: halides) that have been studied widely as light absorber and used in current state‐or‐the‐art solar cells, LDN perovskite have unique properties such as more flexible crystal structure, lower ion transport mobility, robust stability against environmental stress such as moisture, thermal, etc., making them attractive for applications in optoelectronic devices. Since 2014, reports on LDN perovskite materials used in perovskite solar cells, light emitting diodes (LEDs), luminescent solar concentrators (LSC), and photodetectors have been reported, aiming to overcome the obstacles of conventional 3DN perovskite materials in these optoelectronic devices. In this review, the variable ligands used to make LDN perovskite materials are summarized, their distinct properties compared to conventional 3D perovskite materials. The research progress of optoelectronic devices including solar cells, LEDs, LSCs, and photodetectors that used different LDNs perovskite, the roles and working mechanisms of the LDN perovskites in the devices are also demonstrated. Finally, key research challenges and outlook of LDN materials for various optoelectronic applications are discussed

Band alignment tuning at Mo/CZTS back contact interface through surface oxidation states control of Mo substrate

The molybdenum (Mo) back contact is one of the crucial factor that affects the performance of kesterite Cu2ZnSnS4 (CZTS) thin film solar cells. Thick MoS2 which is formed during sulfurization process of CZTS is known to be harmful for the device performance. MoOx on surface of Mo substrate with proper thickness can enhance the efficiency of CZTS solar cells through reducing the thickness of MoS2 layer and tailoring the work function of back contact. However, the role of oxidation states of Mo in MoOx on the performance of solar cells is unclear. Herein the effect of MoOx with different oxidation states on CZTS absorber and intermediate MoS2 layer were investigated. Our results shown that the efficiency of CZTS solar cells on Mo substrate with proper surface oxidation status (MoO2.03) can be as high as 2.86 times of the one fabricated on higher oxidized Mo (MoO2.21), although the thinnest MoS2 layer was present in the latter case. This work discloses the origin of the oxidation states of Mo for the efficiency enhancement of kesterite structured solar cells. The results will be also helpful to improve the performance of compound semiconductor by oxidation states control strategy

Inorganic p-type semiconductors and carbon materials based hole transport materials for perovskite solar cells

Organic-inorganic lead halide based perovskite solar cells (PSCs) have presented a promising prospective in photovoltaic field with current record power conversion efficiency of 22.7%, which is comparable to commercial crystalline silicon cells and even higher than traditional thin film solar cells of CIGS. However, the pressure to enhance device stability under operational condition has driven researches towards development of stable hole transport materials (HTMs) for PSCs. Compared to traditional expensive organic HTMs such as spiro-OMeTAD, there is no doubt that inorganic p-type semiconductors and carbon materials are attractive alternatives that not only possess better stability but also are much cheaper. This review summarized the most recent progress of inorganic hole-transporting materials and carbon materials that have been developed for PSCs. The most recent advancement of device performance using these HTMs was demonstrated. In addition, the research of using various types of carbon materials as additives in HTMs to enhance device performance and stability or as electrical contact in HTM-free PSC was also demonstrated. The effectiveness of each type of materials on mitigating ion migration and degradation of PSC induced by humidity, illumination light intensity and high temperature is discussed. This timely review sheds light on the approaches to tackle the stability issue of PSCs to push the technology towards commercialization through material engineering of HTM

Self-charging flexible solar capacitors based on integrated perovskite solar cells and quasi-solid-state supercapacitors fabricated at low temperature

Self-charging perovskite solar capacitors (SPSCs) that harvest and store solar energy simultaneously can offer sustainable, off-grid power supply for electrical devices. In particular, flexible and lightweight SPSCs are highly desirable in practical applications but are currently restricted by the high annealing temperature needed to make the electron transport layer (ETL) in the devices. Herein, we demonstrate a novel SPSC by integrating a perovskite solar cell (PSC) using amorphous WOx film deposited at room temperature as ETL and a quasi-solid-state supercapacitor (SC). The WOx film with 26 nm thickness yielded a champion power conversion efficiency of 14.14% and 10.80% with the FTO/glass rigid and the ITO/PEN flexible PSCs, respectively. Investigation of the performance of the SPSCs based on the rigid substrate (r-SPSC) and the flexible substrate (f- SPSC) exhibited that the r-SPSC could be photo-charged to 0.68 V within 20 s while the f-SPSC required 25 s to be charged to 0.65 V. Consequently, an overall conversion efficiencies of 2.13% and 1.27% were obtained with the r-SPSCs and the f-SPSC, respectively. This work paves a new way towards making SPSCs that may serve as off-grid electrical power supply in the future

Spiro-OMeTAD or CuSCN as a preferable hole transport material for carbon-based planar perovskite solar cells

A hole transport layer (HTL) plays the role of extracting hole carriers while improving interfacial contacts of perovskite/carbon in planar heterojunction perovskite solar cells using carbon based electrodes (C-PSCs). For the future application of C-PSCs, the HTL also needs to have good stability and easy processability while maintaining high device efficiency. Herein, we compare the behaviour of the most widely used HTL based on spiro-OMeTAD to the much less studied HTL based on CuSCN in planar C-PSCs. The results show that 14.7% power conversion efficiency (PCE) is obtained for CuSCN based C-PSCs with good reproducibility and negligible hysteresis behaviour. In contrast, the C-PSCs using spiro-OMeTAD show a much lower PCE (12.4%) and significant hysteresis phenomenon. We conduct systematic characterisation of the electronic and energetic properties of the C-PSCs to understand this phenomenon. We find that a more favourable energy level alignment of CuSCN with the perovskite than spiro-OMeTAD leads to a reduced charge injection barrier, which favours a more efficient hole extraction capability, inhibited carrier recombination and reduced ionic capacitance. This in turn leads to better PCE and lower hysteresis for the C-PSCs. Moreover, the CuSCN based C-PSCs also demonstrate better stability against moisture with a high PCE retention ratio of 93% under a humid environment (55-70%) for 80 days without encapsulation.</p

2D-3D Mixed Organic-Inorganic Perovskite Layers for Solar Cells with Enhanced Efficiency and Stability Induced by n-Propylammonium Iodide Additives

Device instability has become an obstacle for the industrial application of organic–inorganic metal halide perovskite solar cells that has already demonstrated over 23% laboratory power conversion efficiency (PCE). It has been discovered that the sliding of A-site cations in the perovskite compound through and out of the three-dimensional [PbI6]4– crystal frame is one of the main reasons that are responsible for decomposition of the perovskite compound. Herein, we report an effective method to enhance the stability of the FA0.79MA0.16Cs0.05PbI2.5Br0.5 perovskite film through the incorporation of n-propylammonium iodide (PAI). Both density functional theory calculation and the X-ray diffraction patterns have confirmed the formation of two-dimensional (PA)2PbI4 with the Ruddlesden–Popper perovskite as a result of the reaction between PAI and PbI2 in the perovskite film. X-ray photoelectron spectroscopy reveals less −COOH (carboxyl) groups on the surface of the perovskite film containing (PA)2PbI4, which indicates the suppressed penetration of oxygen and moisture into the perovskite material. This is further confirmed by the surface water wettability test of the (PA)2PbI4 film that exhibits excellent hydrophobic property with over 110° contact angle. Ultraviolet photoelectron spectroscopy demonstrates the introduction of PAI additives that resulted in the upshift of the conduction band minimum of the perovskite by 160 meV, leading to a more favorable energy alignment with an adjacent electron transporting material. As a consequence, enhanced 17.23% PCE with suppressed hysteresis was obtained with the 5% PAI additive (molar ratio) in perovskite solar cells that retained nearly 50% of the initial efficiency after 2000 h in air without encapsulation under 45% average relative humidity

Guanidinium thiocyanate selective Ostwald ripening induced large grain for high performance perovskite solar cells

Organic-inorganic lead halide perovskite has become one of the most attractive materials for future low-cost high-efficiency solar technology. However, the polycrystalline nature of perovskite thin-film often possesses an exceptional density of defects, especially at grain boundaries (GBs) and film surface, limiting further improvement in the power conversion efficiency (PCE) of the perovskite device. Here, we report a simple method to reduce GBs and to passivate the surface of a methylammonium lead tri-iodide (MAPbI3) film by guanidinium thiocyanate (GUTS)-assisted Ostwald ripening post treatment. High-optoelectronic quality MAPbI3 film consisting of micron-sized grains were synthesized by post-treating a MAPbI3 film with GUTS/isopropanol solution (4 mg/mL, GUTS-4). Analysis of the electrochemical impedance spectra (EIS) of the solar cells showed that interfacial charge recombination resistance of the device based on a GUTS-4 post-treated MAPbI3 absorber film was increased by a factor of 1.15 to 2.6, depending on light illumination intensity, compared to the control MAPbI3 cell. This is consistent with results of the open-circuit voltage (Voc) decay and the light intensity dependent photovoltage evolution which shows device with GUTS treatment had one order longer charge carrier lifetime and was more ideal (ideality factor = = 1.25). Further characterization by Kelvin probe force microscope indicated that GUTS-4 treatment shifted the energetics of the MAPbI3 film by ~100 meV towards better energy level alignment with adjacent SnO2 electron transport layer, leading to a more favorable charge extraction process at the MAPbI3/SnO2 interface. As a result, the PCE of PSCs was enhanced from 14.59% to 16.37% and the hysteresis effect was mitigated