Interfacial engineering and molecularly designed additive for stable perovskite solar cells

The past decade has witnessed the rapid development of halide perovskite-based solar cell (PSC) performance. With several spin-off companies had been founded to expedite the commercialization of PSCs, stability remains the bottleneck to market this new technology with high confidence. Despite growing size of literature body investigating stability aspects of PSCs, there are several questions seem scarcely discussed or even visited. Firstly, with strong reliance to metal-oxide based carrier selective contacts or transport layers, metal oxide/perovskite interfaces are inevitable, and their interfacial stability should be examined in-depth. In response, it is hypothesized that even with relatively acidic TiO2, metal oxide/perovskite interface is susceptible to protonation reaction by organic A-site cation at elevated temperatures. To overcome that, metal oxide surface was functionalized with sulfate moieties. This study shows that sulfate functionalization suppresses the amount of hydroxide groups inherently present on metal oxide surface and cuts off the pathway to perovskite’s A-site deprotonation. Secondly, there has been also a wide variety of metal oxide-based transport layer precursors, from ethanolic solutions to aqueous colloidal dispersions, that do not seem converging to one or a few established recipes any time soon. However, in term of colloid-based metal oxide precursors, the relationship between colloidal particle size and resulting transport layer’s morphological and electrical properties has hardly been investigated. Here, size tunable SnO2 nanoparticle precursor synthesis was reported and different nanoparticle sizes were applied to act as interfacial layer between TiO2/perovskite in planar PSCs. The results show that the smallest size gives rise to the best charge carrier transport property and accordingly device performance. Lastly, reports on moisture-resistant PSCs are mainly realized by hydrophobic doping, surface treatment, and encapsulation techniques. At the same time, there emerge few reports that suggest hydrophilic moieties could also improve stability in humid atmosphere. However, studies combining the best of both worlds are yet to be seen. Here, we utilize molecular design principles and come up with a novel amphiphilic molecule called Cor-TEG which is a marriage between hydrophobic aromatic corannulene sulfone and hydrophilic triethylene glycol side chains. Cor-TEG is introduced as dopant to perovskite precursor solution. Our study demonstrates that adding a minute amount of Cor-TEG to perovskite precursor solution results in enlarged perovskite grains, improved charge transport property and enhanced and moisture resistance that can be attributed to Lewis base sites, delocalized electrons in the aromatic core, and the hydrophilic side chains, respectively, present in a single molecule.Doctor of Philosoph

Halide perovskite-based indoor photovoltaics: recent development and challenges

Metal halide perovskite solar cell (PSC) technology is yet to make its way to enter the outdoor solar energy harvesting market as a single junction or a tandem cell; recent studies have already sparked huge interest in PSC for indoor photovoltaic (iPV) applications. The spark is further amplified by the vision of extensive use of low power Internet of Things (IoT) sensors in smart and sustainable buildings. To date, halide perovskite-based solar cells have exceeded 40% efficiency in indoor lighting, which is way above other emerging PV cells such as organic photovoltaic cells and dye-sensitized solar cells. Thanks to tremendous efforts on defect reduction and interfacial engineering in the field of PSCs, the strategies can be directly applied to low light intensity indoor settings where defect and trap states are very detrimental to device performance. Here, we summarize the recent developments of PSCs for iPV applications with emphasis on device engineering approaches and feasibility of PSCs to power smart sensors in indoor setting. We also provide a technical explanation for indoor light profile and respective iPV cell efficiency measurement. Last, we discuss existing challenges in halide perovskite-based iPV cells and an outlook to pave the way for PSC to enter the trillion dollar-sized IoT market by 2025.Ministry of Education (MOE)Submitted/Accepted versionThe authors would like to acknowledge funding support from Ministry of Education (MOE) under AcRF Tier 2 grant (2019-T2-2- 106) and National Robotics Programme (W1925d0106)

Self-healable organic electrochemical transistor with high transconductance, fast response, and long-term stability

The major challenges in developing self-healable conjugated polymers for organic electrochemical transistors (OECTs) lie in maintaining good mixed electronic/ionic transport and the need for fast restoration to the original electronic and structural properties after the self-healing process. Herein, we provide the first report of an all-solid-state OECT that is self-healable and possesses good electrical performance, by utilizing a matrix of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and a nonionic surfactant, Triton X-100, as a channel and an ion-conducting poly(vinyl alcohol) hydrogel as a quasi-solid-state polymer electrolyte. The fabricated OECT exhibits high transconductance (maximum 54 mS), an on/off current ratio of ∼1.5 × 103, a fast response time of 6.8 ms, and good operational stability after 68 days of storage. Simultaneously, the OECT showed remarkable self-healing and ion-sensing behaviors and recovered ∼95% of its ion sensitivity after healing. These findings will contribute to the development of high-performance and robust OECTs for wearable bioelectronic devices.Ministry of Education (MOE)W.L.L. would like to acknowledge funding support from her NTU start-up grant (M4081866), Ministry of Education (MOE), under the AcRF Tier 2 grant (2018-T2-1-075) and the A*STAR AME Young Individual Research Grant (Project no. A1784c019). We would like to acknowledge the Facility for Analysis, Characterization, Testing and Simulation (FACTS), Nanyang Technological University, Singapore, for use of their GIWAXS facility (Xenocs Nano-inXider)

Universal spray-deposition process for scalable, high-performance, and stable organic electrochemical transistors

Organic electrochemical transistors (OECTs) with high transconductance and good operating stability in an aqueous environment are receiving substantial attention as promising ion-to-electron transducers for bioelectronics. However, to date, in most of the reported OECTs, the fabrication procedures have been devoted to spin-coating processes that may nullify the advantages of large-area and scalable manufacturing. In addition, conventional microfabrication and photolithography techniques are complicated or incompatible with various nonplanar flexible and curved substrates. Herein, we demonstrate a facile patterning method via spray deposition to fabricate ionic-liquid-doped poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-based OECTs, with a high peak transconductance of 12.9 mS and high device stability over 4000 switching cycles. More importantly, this facile technique makes it possible to fabricate high-performance OECTs on versatile substrates with different textures and form factors such as thin permeable membranes, flexible plastic sheets, hydrophobic elastomers, and rough textiles. Overall, the results highlight the spray-deposition technique as a convenient route to prepare high-performing OECTs and will contribute to the translation of OECTs into real-world applications.ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Novel amphiphilic corannulene additive for moisture-resistant perovskite solar cells

The addition of amphiphilic triethylene glycol based corannulene molecules provides multiple Lewis basic sites that assist in perovskite grain growth, and improve the charge carrier collection and moisture resistance of perovskite solar cells. This study paves the way for utilization of more molecules from corannulene families in perovskite research.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)National Research Foundation (NRF)Submitted/Accepted versionThis research was supported by NTU start-up grants (M4081866, M4081566), Ministry of Education (MOE) under AcRF Tier 2 grant (2018-T2-1-075), A*STAR AME IAF-ICP Grant (No. I1801E0030), A*STAR AME IRG A1883c0006 and National Research Foundation, Prime Minister’s Office, Singapore, under Energy Innovation Research Program and Intra-CREATE collaborative grant (NRF2015EWT-EIRP003-004, NRF2018-ITC001-001 and Solar CRP: S18-1176-SCRP)

Additives in halide perovskite for blue light-emitting diodes : passivating agents or crystallization modulators?

Successful adoption of defect management and carrier confinement strategies in Ruddlesden-Popper (RP) perovskites has driven the impressive improvements to performance of perovskite-based light-emitting diodes (PeLEDs) seen to date. Although functional additives have been advantageous in mitigating defects, their influence over crystallization behavior of RP (L2Am-1PbmX3m+1) perovskites has yet to be fully studied. This is especially important for blue-emitting mono-halide RP perovskites, where stringent control over m domain distribution is needed for efficient PeLEDs. Herein, we investigate the effect of tri-phenyl-phosphine-oxide (TPPO) on crystallization behaviour of blue RP (PBA2Csm-1PbmBr3m+1) perovskites. Despite TPPO addition, its absence in the resulting film eliminates its role as a passivating agent. Instead, TPPO acts as crystallization and phase distribution modulator – promoting the formation of a narrow distribution of higher m domains with higher Br content. In doing so, an enhancement of ~35% was noted with champion device yielding efficiency of 3.8% at λ of 483 nm.Ministry of Education (MOE)National Research Foundation (NRF)Accepted versionThis research was funded by Ministry of Education, Singapore (MOE2018-T2-2-083) and (MOE2019-T2-2-097). The photophysical measurements are made possible through the support from the Ministry of Education, Singapore under its AcRF Tier 2 grant MOE-T2EP50120-0004, and the National Research Foundation, Singapore under its NRF Investigatorship (NRF-NRFI2018-04). We acknowledge the Facility for Analysis, Characterization, Testing and Simulation, Nanyang Technological University, Singapore, for use of their electron microscopy/X-ray facilities