Highly Efficient Thermally Co-evaporated Perovskite Solar Cells and Mini-modules

The rapid improvement in the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has prompted interest in bringing the technology toward commercialization. Capitalizing on existing industrial processes facilitates the transition from laboratory to production lines. In this work, we prove the scalability of thermally co-evaporated MAPbI3 layers in PSCs and mini-modules. With a combined strategy of active layer engineering, interfacial optimization, surface treatments, and light management, we demonstrate PSCs (0.16 cm2 active area) and mini-modules (21 cm2 active area) achieving record PCEs of 20.28% and 18.13%, respectively. Un-encapsulated PSCs retained ∼90% of their initial PCE under continuous illumination at 1 sun, without sample cooling, for more than 100 h. Looking toward tandem and building integrated photovoltaic applications, we have demonstrated semi-transparent mini-modules and colored PSCs with consistent PCEs of ∼16% for a set of visible colors. Our work demonstrates the compatibility of perovskite technology with industrial processes and its potential for next-generation photovoltaics

From lab to fab : investigating facile and scalable methods of perovskite solar cell fabrication

Perovskite solar cells have seen dramatic improvements in terms of device efficiency. In a short span of a decade, the device efficiency improved from 3.8% to beyond 24%. Low cost of precursor materials coupled with highly facile spin coating techniques used to fabricate perovskite solar cells have contributed to the impressive improvement of efficiencies in the short time frame. However, most of perovskite solar cells that reported efficiencies beyond 23% employs a spin-coating technique for cell fabrication, followed by an evaporation step to deposit the contacts. Also, these high efficiency numbers are only recorded on small areas smaller than 1cm2, which has little real-world applications. Furthermore, this spin coating method has high material wastage, and usage of the thermal evaporator limits the throughput required by industrial needs. These scaling issues, coupled with the low stability of the perovskite material in the presence of moisture, remain as one of the main tumbling blocks that hinders the commercialization of this promising technology. An alternative architecture that could pave the pathway towards the commercialization of perovskite solar cells would be the fully printable mesoscopic design employing carbon material as a counter electrode. The mesoscopic layers are printed and sintered, and the device is completed by infiltrating the pores with perovskite precursor solution. This highly scalable and facile method of perovskite solar cell fabrication, coupled with high throughput and little material wastage, seem to offer the best compromise of what it takes to commercialize this promising technology. This architecture offers reasonable efficiencies of up to 15%, offers high scalability to 70cm2 in size. Furthermore, the carbon electrode serves a dual-purpose role of charge collector and extractor, as well as a protective layer against moisture ingression. Like the earlier section, the high efficiencies of fully printable mesoscopic solar cells are reported based on small surface area with large amount of masking. This could lead to erroneous and exaggerated PCE values. In this thesis, systematic experiments have been outlined with the aims to properly evaluate and further improve the performance of the fully printable mesoscopic perovskite solar cell. Firstly, the active area effect on the efficiency, as well as the effect of masking on the device efficiency is being investigated. Secondly, systematic scale up of the printable solar cell is being discussed. A 70cm2 module with 10% PCE values is fabricated. Next, experiments are planned to enhance the device performance. Optimization of additive concentration improved pore filling and device performance. A thinly printed Cu:Niox is introduced as an p-type oxide interlayer between carbon and ZrO2. A well aligned energy level of the p-type oxide with the valence band of the perovskite material should enhance charge extraction, improving device performance. The last part of the chapter also includes preliminary investigation of encapsulation. Various means of encapsulation are discussed. The second part of this chapter discusses about the optimization the patterning process using laser ablation. A combination of encapsulation and laser processes are essential to ensure the translation of perovskite technology from the lab to commercial scale. Finally, the final chapter draws a conclusion and summarizes the thesis. Further works are also being discussed, which will help to facilitate the push towards commercialization of printable perovskite solar cells.Master of Engineerin

Design of Perovskite Thermally Co‐Evaporated Highly Efficient Mini‐Modules with High Geometrical Fill Factors

Perovskite solar cells (PSCs) have emerged as a promising technology for next-generation photovoltaics thanks to their high power-conversion-efficiency (PCE). Scaling up PSCs using industrially compatible processes is a key requirement to make them suitable for a variety of applications. Herein, large-area PSCs and perovskite solar modules (PSMs) are developed based on co-evaporated MAPbI3 using optimized structures and active area designs to enhance PCEs and geometrical fill factors (GFFs). Small-area co-evaporated PSCs (0.16 cm2) achieve PCE over 19%. When the PSCs are scaled-up, the thin films high quality allows them to maintain consistent Voc and Jsc, while their fill factors (FF), which depend on the substrate sheet resistance, are substantially compromised. However, PSCs with active areas from 1.4 to 7 cm2 show a substantially improved FF when rectangular designs with optimized length to width ratios are used. Reasoning these results in the PSM design with optimal subcell size and for specific dead areas, a 6.4 cm2 PSM is demonstrated with a record 18.4% PCE and a GFF of ≈91%. Combining the high uniformity of the co-evaporation deposition with active areas design, it is possible to scale up 40 times the PSCs with PCE losses smaller than 0.7% (absolute value).National Research Foundation (NRF)Accepted versionThis research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under Energy Innovation Research Program (Grant numbers: NRF2015EWT-EIRP003-004, NRF-CRP14-2014-03, Solar CRP: S18-1176-SCRP) and Intra-CREATE Collaborative Grant (NRF2018- ITC001-001)

Self-powered organic electrochemical transistors with stable, light-intensity independent operation enabled by carbon-based perovskite solar cells

Wearable sensors and electronics for health and environment monitoring are mostly powered by batteries or external power supply, which requires frequent charging or bulky connecting wires. Self-powered wearable electronic devices realized by integrating with solar cells are becoming increasingly popular due to their ability to supply continuous and long-term energy to power wearable devices. However, most solar cells are vulnerable to significant power losses with decreasing light intensity in the indoor environment, leading to an errant device operation. Therefore, stable autonomous energy in a reliable and repeatable way without affecting their operation regime is critical to attaining accurate detection behaviors of electronic devices. Herein, we demonstrate, for the first time, a self-powered ion-sensing organic electrochemical transistor (OECT) using carbon electrode-based perovskite solar cells (CPSCs), which exhibits a highly stable device operation and independent of the incident light intensity. The OECTs powered by CPSCs maintained a constant transconductance (gm) of ~60.50±1.44 μS at light intensities ranging from 100 mW cm-2 to 0.13 mW cm-2. Moreover, this self-powered integrated system showed good sodium ion sensitivity of -69.77 mV decade-1, thereby highlighting its potential for use in portable, wearable, and self-powered sensing devices.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)W.L.L. would like to acknowledge funding support from Ministry of Education (MOE) under AcRF Tier 2 grant Nos. (2018-T2-1-075 and 2019- T2-2-106), A*STAR AME IAF-ICP Grant (No. I1801E0030), and National Robotics Programme (W1925d0106)

Ultrafast THz photophysics of solvent engineered triple-cation halide perovskites

Solution processed thin film organic-inorganic perovskites are key to the large scale manufacturing of next generation wafer scale solar cell devices. The high efficiency of the hybrid perovskite solar cells is derived mainly from the large carrier mobility and the charge dynamics of films, which heavily depend on the type of solvent used for the material preparation. Here, we investigate the nature of conduction and charge carrier dynamics of mixed organic-inorganic cations [methylammonium (MA), formamidinium (FA), and cesium (Cs)] along with the mixed halides [iodine (I) and bromine (Br)] perovskite material [Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3] synthesized in different solvents using optical pump terahertz probe (OPTP) spectroscopy. Our findings reveal that carrier mobilities and diffusion lengths strongly depend on the type of solvent used for the preparation of the mixed cation perovskite film. The mixed cation perovskite film prepared using dimethylformamide/dimethylsulfoxide solvent shows greater mobility and diffusion length compared to γ-butyrolactone solvent. Our findings provide valuable insights to improve the charge carrier transport in mixed cation perovskites through solvent engineering.MOE (Min. of Education, S’pore)Published versio

Suppressing the δ-phase and photoinstability through a hypophosphorous acid additive in carbon-based mixed-cation perovskite solar cells

Despite a meteoric rise in the efficiency and promising scalability aspects, the operational stability of halide perovskites poses a serious concern for the commercialization of this technology. A paradigm shift from thermally unstable MA+ (methylammonium)-based perovskites to stable FA+ (formamidinium) and Cs+ (cesium)-based mixed halide perovskite variants is a step in this direction. However, phase stabilization of mixed-cation halide perovskites within a triple-layer scaffold remains a major challenge. In this work, we demonstrate two-step sequential fabrication of FA+- and Cs+-based halide perovskites with formulation Cs0.05FA0.95Pb(IBr)3 in a triple-mesoscopic scaffold with a carbon layer as the back electrode. A strong but reversible performance degradation is observed under light illumination. Addition of hypophosphorous acid (HPA) into the perovskite precursor solution improves the operational stability of the cells. A striking correlation between phase- and operational stability was observed. From structural analysis, it was found that HPA tends to suppress the formation of a hexagonal yellow phase and promotes trigonal black phase formation. Further optical analysis of the cells showed the improvement in the optoelectronic properties in terms of defects and carrier recombination in the perovskite formed by HPA addition supported by external quantum efficiency and photoluminescence measurements. A stable 12% power conversion efficiency was achieved by tuning the composition and optimizing the process conditions for Cs0.05FA0.95Pb(IBr)3-based triple-mesoscopic perovskite solar cells.National Research Foundation (NRF)N.M. and S.G.M. would like to acknowledge funding from the Singapore National Research Foundation through the IntraCREATE Collaborative Grant (NRF2018-ITC001-001) and the Competitive Research Program: NRF-CRP14-2014-03

Improving the performance of carbon-based perovskite solar modules (70 cm2) by incorporating cesium halide in mesoporous TiO 2

We present the fabrication of highly efficient large-area carbon-based perovskite solar cells (C-PSCs) using CsX (X = Cl, Br, and I)-modified mesoporous (mp) TiO2 beads of 40 nm size as an electron transport material. Here, triple-layered scaffolds made of cesium halide-modified TiO2 exhibit efficient charge extraction as confirmed by enhanced photoluminescence quenching and inhibit the UV-activated degradation processes of perovskite, leading to an enhanced operational stability. Among the three cesium halide modifications, devices containing CsBr-modified TiO2 showed the highest short-circuit current density, yielding a photoconversion efficiency (PCE) of 12.59% of the device, with 0.7 cm(2) active area and 11.55% for a large-area module (70 cm(2)). These devices are stable in an ambient atmosphere (25 degrees C, 65-70% RH) over 2700 h as well as at a high temperature (85 degrees C) over 750 h with virtually no hysteresis

Effect of formamidinium/cesium substitution and PbI2 on the long‐term stability of triple‐cation perovskites

Altering cation and anion ratios in perovskites has been an excellent avenue in tuning the perovskite properties and enhancing the performance. Recently, MA/FA/Cs triple cation mixed halide perovskites have demonstrated efficiencies reaching up to 22 %. Similar to the widely explored MAPbI3, excess PbI2 is added in these perovskite films to enhance the performance. Previous reports demonstrate that the excess PbI2 is beneficial for the performance. However, not much work has been conducted about its impact on stability. Triple cation perovskites (TCP) deploy excess PbI2 up to 8 %. Thus, it is imperative to analyze the role of excess PbI2 in the degradation kinetics. In this paper, we have varied the amount of PbI2 in the triple cation perovskite films and monitored the degradation kinetics by X-ray diffraction (XRD) and optical absorption spectroscopy. We found that the inclusion of excess PbI2 adversely affects the stability of the material. Faster degradation kinetics is observed for higher PbI2 samples. However, excess PbI2 samples showed superior properties such as enhanced grain sizes and better optical absorption. Thus, careful management of the PbI2 quantity is required to obtain better stability and alternative pathways should be explored to achieve better device performance rather than adding excess PbI2.NRF (Natl Research Foundation, S’pore)Accepted versio

A large area (70 cm2) monolithic perovskite solar module with a high efficiency and stability

Monolithic perovskite modules with active areas of 31 cm2 and 70 cm2 and with power conversion efficiencies (PCE) of 10.46% and 10.74%, respectively, were fabricated using scalable printing processes. An ambient stability of more than 2000 h with less than a 5% reduction in efficiency is demonstrated. The electrical quality of the mesoscopic hole transporter and its facilitation of efficient infiltration is paramount.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Cu-doped nickel oxide interface layer with nanoscale thickness for efficient and highly stable printable carbon-based perovskite solar cell

The power conversion efficiency (PCE) of hole conductor free carbon-based perovskite solar cells (PSCs) is restricted by the poor charge extraction and recombination losses at the carbon-perovskite interface. For the first time we successfully demonstrated incorporation of thin layer of copper doped nickel oxide (Cu:NiOx) nanoparticles in carbon-based PSCs, which helps in improving the performance of these solar devices. Cu:NiOx nanoparticles have been synthesized by a facile chemical method, and processed into a paste for screen printing. Extensive X-ray Absorption Spectroscopy (XAS) analysis elucidates the co-ordination of Cu in a NiOx matrix and indicates the presence of around 5.4% Cu in the sample. We fabricated a monolithic perovskite module on a 100 cm2 glass substrate (active area of 70 cm2) with a thin Cu:NiOx layer (80 nm), where the champion device shows an appreciated power conversion efficiency of 12.1% under an AM 1.5G illumination. To the best of our knowledge, this is the highest reported efficiency for such a large area perovskite solar device. I-V scans show that the introduction of Cu:NiOx mesoporous scaffold increases the photocurrent, and yields fill factor (FF) values exceeding 57% due to the better interface and increased hole extraction efficiency. Electrochemical Impedance Spectroscopy (EIS) results reinforce the above results by showing the reduction in recombination resistance (Rrec) of the PSCs that incorporates Cu:NiOx interlayer. The perovskite solar modules with a Cu:NiOx layer are stable for more than 4500 h in an ambient environment (25 °C and 65% RH), with PCE degradation of less than 5% of the initial value.NRF (Natl Research Foundation, S’pore)Accepted versio