Dr Jae Sung Yun

Ambient energy harvesting has great potential to contribute to sustainable development and address growing environmental challenges. Converting waste energy from energy-intensive processes and systems (e.g., combustion engines and furnaces) is crucial to reducing their environmental impact and achieving net-zero emissions. Compact energy harvesters will also be key to powering the exponentially growing smart devices ecosystem that is part of the Internet of Things, thus enabling futuristic applications that can improve our quality of life (e.g., smart homes, smart cities, smart manufacturing, and smart healthcare). To achieve these goals, innovative materials are needed to efficiently convert ambient energy into electricity through various physical mechanisms, such as the photovoltaic effect, thermoelectricity, piezoelectricity, triboelectricity, and electromagnetic power transfer. By bringing together the perspectives of experts in various types of energy harvesting materials, this Roadmap provides extensive insights into recent advances and present challenges in the field. Additionally, the Roadmap analyzes the key performance metrics of these technologies in relation to their ultimate energy conversion limits. Building on these insights, the Roadmap outlines promising directions for future research to fully harness the potential of energy harvesting materials for green energy anytime, anywhere.

The past decade has seen the unprecedentedly rapid emergence of a new class of solar cells based on mixed organic-inorganic halide perovskites. The power conversion efficiency (PCE) of halide perovskite solar cells since then has quickly risen above 25% in single-junction devices and 30% in tandem devices. Twin domains within polycrystalline grains have been recently reported in this material, nevertheless, their roles associated with both ionic and charge carrier transport properties are still to be fully understood. Here, combining molecular dynamic (MD) simulations with nanoscale scanning probe microscopy investigations, we reveal unique properties of the twin domains that exhibit vital channels for ion migration and influence charge separation and collection. Our nanoscale elemental analysis using photo-induced force microscopy reveals that the domain structure possesses an alternating chemical compositional variation, rich and poor in cations for low topography domains (LTDs) and high topography domains (HTDs) respectively. Also, Kelvin probe force microscopy (KPFM) measurements confirm that LTDs provide a confined tunnel for cation vacancy migration. This phenomenon is supported by the MD simulation which suggests the presence of the twin domain wall causes a reduction in the crystal symmetry and appearance of a strain field. Lastly, KPFM and conductive AFM (c-AFM) under illumination show that both photovoltage and photocurrent are higher at LTDs due to the effective charge collection by ion accumulation. This work highlights important elements of the nanoscale intragrain feature that may pave the way to high-efficiency perovskite solar cells.

Mixed-halide perovskites (MHPs) have attracted attention as suitable wide-band-gap candidate materials for tandem applications owing to their facile band-gap tuning. However, when smaller bromide ions are incorporated into iodides to tune the band gap, photoinduced halide segregation occurs, which leads to voltage deficit and photoinstability. Here, we propose an original post-hot pressing (PHP) treatment that suppresses halide segregation in MHPs with a band gap of 2.0 eV. The PHP treatment reconstructs open-structured grain boundaries (GBs) as compact GBs through constrained grain growth in the in-plane direction, resulting in the inhibition of defect-mediated ion migration in GBs. The PHP-treated wide-band-gap (2.0 eV) MHP solar cells showed a high efficiency of over 11%, achieving an open-circuit voltage (V oc) of 1.35 V and improving the maintenance of the initial efficiency under the working condition at AM 1.5G. The results reveal that the management of GBs is necessary to secure the stability of wide-band-gap MHP devices in terms of halide segregation.

To expedite the commercialization of perovskite solar cells (PSCs), researchers are exploring the feasibility of employing nickel phthalocyanine (NiPc) as a hole transport material (HTM) due to its cost-effectiveness, excellent thermal stability, and suitability for solution coating. However, the low LUMO energy level of the NiPc may limit its ability to block photoelectrons generated in the perovskite layer from recombining with holes, which can reduce the overall efficiency of the solar cell. One solution is to use cascaded bilayers with HTMs that have relatively higher LUMO levels. In this study, a bilayer consisting of NiPc and poly(3-hexylthiophene) (P3HT) is employed as the HTM, where the P3HT exhibits vertical phase separation during the coating process. By optimizing the mixing amount of P3HT into the NiPc, a record power conversion efficiency of 23.11%, the highest reported for NiPc-based PSCs is achieved. Moreover, an excellent long-term stability is demonstrated by encapsulating the PSC in polyisobutylene, with the device retaining 90% of its initial efficiency after exposure to 85 & DEG;C and 85% relative humidity for 1000 h.

In this study, we propose a strategic interface engineering method for optimizing the power density and power conversion efficiency (PCE) of perovskite solar cells (PVSCs) under low-intensity indoor light conditions. The insertion of a polar bathocuproine (BCP) layer at the electron transport interface significantly improved the photovoltaic properties, in particular, the fill factor and open circuit voltage, in a low-intensity light environment. Based on the systemic characterizations of surface trap states and carrier dynamics using Kelvin probe force microscopy, we revealed that BCP facilitated efficient charge carrier separation and electron extraction under low-intensity light illumination due to surface passivation and dipole-induced suppressed charge recombination. The beneficial role of BCP enabled excellent indoor PCEs of 27.04 and 35.45% under low-intensity light-emitting diode and halogen lights, respectively. Modification of the electron transport layer interface using polar molecules is a simple but highly effective method for optimizing the indoor performance of PVSCs.

The use of inexpensive, highly efficient, and long-term stable hole-transporting layers (HTLs) while facilitating the fabrication process has become a critical issue for PSC commercialization. Among organic HTLs, copper phthalocyanine (CuPc) has been increasingly studied owing to its low cost and excellent thermal stability. Nevertheless, CuPc has a low energy level in the conduction band, resulting in low efficiency due to a poor electron barrier. In this study, an efficient and stable PSC is fabricated by combining CuPc with an ultrathin poly(methyl methacrylate) (PMMA) interlayer, which is deposited on a [(FAPbI3)0.95(MAPbBr3)0.05] absorption layer (here, FAPbI3 and MAPbBr3 denote formamidinium lead triiodide and methylammonium lead tribromide, respectively). PMMA in perovskite has been found to reduce perovskite surface defects and series resistance as well as the electronic barrier to HTL. The optimum concentration of PMMA allows for the fabrication of the PSC with a PCE of 21.3%, which is the highest PCE for PSCs featuring metal phthalocyanines as the HTL reported to date. The stability of the encapsulated PSC exceeds 80% after 760 h at 85 °C under 85% RH conditions.

Recently, kesterite-based absorbers and related compounds have been considered as promising eco-friendly light absorber materials for thin-film solar cells (TFSCs). However, the device performances of kesterite-based TFSCs are limited because of the formation of defects and poor interfacial properties. In this study, we developed a strategic approach to improve the device performances of Cu2ZnSn­(S,Se)4 (CZTSSe) solar cells using back-interface passivation of the absorber layer and further reduced the formation of defects through Ge doping. The application of CuAlO2 (CAO) as an intermediate layer near the back interface efficiently improves the grain growth and minimizes the detrimental Mo­(S,Se)2 thickness. In addition, the Ge nanolayer deposited over the CAO layer improves the absorber bulk quality, effectively suppresses the defect density, and reduces the nonradiative carrier recombination losses. As a result, the short-circuit current density, fill factor, and power conversion efficiency of the champion device with the CAO and Ge nanolayer improved from 31.91 to 36.26 mA/cm2, 0.55 to 0.61, and 8.58 to 11.01%, respectively. This study demonstrates a potential approach to improve the performances of CZTSSe TFSCs using a combination of back-interface passivation and doping.

[Display omitted] •We investigated interface dipole effects on indoor perovskite solar cells.•QDPPO enabled homogeneous charge distribution and suppressed recombination.•Perovskite solar cells based on QDPPO exhibited iPCE of 36.90% at 800 lux light.•Mini-modules based on QDPPO generated power output of 2.4 mW at 1,000 lux light. Indoor-light harvesting-technology based on perovskite solar cells have attracted significant attention owing to their promising photovoltaic properties as indoor power generators. We investigated the effect of interfacial dipoles on the performance of perovskite solar cells in low-intensity indoor light environments. Interfacial dipoles were controlled by inserting different polar layers with different molecular dipole moments (BCP, QPPO and QDPPO) on top of electron transport layers (ETLs). Significantly improved uniformity of interfacial dipoles, in the QDPPO layer, effectively reduced charge recombination and enabled persistent fill factors (FF’s) under low-intensity light environments. Perovskite solar cells based on QDPPO exhibited indoor power density (iPD) and indoor power conversion efficiency (iPCE) of 65.63 μW/cm2 and 27.49 % under 800 lux LED, which were further enhanced up to 88.09 μW/cm2 and 36.90 % by employing additional passivation layer under 800 lux LED. Finally, using QDPPO, we successfully demonstrated perovskite photovoltaic mini-modules with a high power output of 2.4 mW under a 1,000 lux halogen, which can be applied in Internet-of-Things sensors under indoor light conditions.

Interest in photovoltaics (PVs) based on Earth-abundant halide perovskites has increased markedly in recent years owing to the remarkable properties of these materials and their suitability for energy-efficient and scalable solution processing. Formamidinium lead triiodide (FAPbI )-rich perovskite absorbers have emerged as the frontrunners for commercialization, but commercial success is reliant on the stability meeting the highest industrial standards and the photoactive FAPbI phase suffers from instabilities that lead to degradation - an effect that is accelerated under working conditions. Here, we critically assess the current understanding of these phase instabilities and summarize the approaches for stabilizing the desired phases, covering aspects from fundamental research to device engineering. We subsequently analyse the remaining challenges for state-of-the-art perovskite PVs and demonstrate the opportunities to enhance phase stability with ongoing materials discovery and in operando analysis. Finally, we propose future directions towards upscaling perovskite modules, multijunction PVs and other potential applications.

Spiro-OMeTAD, one of the most widely used hole-transport materials (HTMs) in optoelectronic devices, typically requires chemical doping with a lithium compound (LiTFSI) to attain sufficient conductivity and efficient hole extraction. However, the doping step requires an activation process that comprises exposure of the blend films to an ambient atmosphere. Additionally, the lithium dopant induces crystallization, and its hygroscopic nature negatively impacts device performance and lifetime. Here we report a facile approach based on the incorporation of a low-cost alkylthiol additive (1-dodecanethiol, DDT) in the spiro-OMeTAD HTM. We discover that DDT provides a more efficient and controllable doping process with significantly reduced doping duration, enabling the HTM to achieve comparable performance before air activation. The coordination between DDT and LiTFSI increases the concentration of dopants in the HTM bulk, reduces their accumulation at interfaces, and enhances the structural integrity of the HTM under wetting, heat and light stress. We fabricate perovskite solar cells using DDT-treated spiro-OMeTAD as the HTM. Our best devices exhibit a certified power conversion efficiency of 23.1%. Furthermore, the devices can retain 90% of peak performance under continuous illumination for 1,000 h. Our findings represent an important step forward in the production of doped spiro-OMeTAD, as well as its reliable application and future device commercialization.

Copper (Cu) is present not only in the electrode for inverted-structure halide perovskite solar cells (PSCs) but also in transport layers such as copper iodide (CuI), copper thiocyanate (CuSCN), and copper phthalocyanine (CuPc) alternatives to spiro-OMeTAD due to their improved thermal stability. While Cu or Cu-incorporated materials have been effectively utilized in halide perovskites, there is a lack of thorough investigation on the direct reaction between Cu and a perovskite under thermal stress. In this study, we investigated the thermal reaction between Cu and a perovskite as well as the degradation mechanism by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Kelvin probe force microscopy (KPFM). The results show that high temperatures of 100 °C induce Cu to be incorporated into the perovskite lattice by forming “Cu-rich yet organic A-site-poor” perovskites, (Cu x A1–x )­PbX3, near the grain boundaries, which result in device performance degradation.

With the growing need for cost-effective and sustainable Internet of things (IoT) technologies, kesterite-based solar cells are gaining popularity. We report the fabrication of an efficient CZTSSe absorber layer with improved V oc loss and its possible use in indoor photovoltaic applications. The double cation incorporation (co-doping) approach is employed with Ag and Ge to achieve this. The devices fabricated and tested under standard illumination (1 sun) and low light intensity conditions showed enhanced device performances and lower V oc losses after co-doping. Under indoor light conditions, V oc of 290 mV with white LED (WLED) and 310 mV with fluorescent lamp (FL-4000K) was achieved at the lowest intensity of 400 lux, while a value exceeding 350 mV was obtained at 1200 lux with FL-4000K for the CZTSSe:Ag–Ge device. V oc recoveries of >60% under all intensity conditions and >70% at 1200 lux with both WLED and FL-4000K were achieved. Moreover, the CZTSSe:Ag–Ge device showed efficiencies of 4.95% and 5.85% under WLED and FL-4000K at 1200 lux, respectively. The prototype device also demonstrated successful test results under indoor conditions. These achievements are attributed to the enhanced carrier density, reduced density of defects, and low carrier recombinations.

Solar cells made from Cu 2 ZnSn(S,Se) 4 (CZTS)-derived materials have been widely studied for their favourable material properties utilized in photovoltaic energy conversion. Drawbacks of the materials are associated with low open circuit voltage (V oc) resulting from non-radiative recombination at grain boundaries and interfaces. Considerable work has focused on the incorporation of sodium (Na), which is found to passivate trap states and reduce electronic losses. Here we present evidence that Na + as well as several ionic species (Se 2À and Zn 2+), do not remain stationary after device fabrication, but in fact migrate under electrical biasing. Furthermore, this ionic migration can be manipulated at room temperature by exposing the device to an external electric forming field. We outline a novel procedure that can effectively control and adjust ionic movement and associated local distribution in fully fabricated devices. Our results show that this simple treatment leads to favourable improved device performance and provides insight into light-induced reduction in performance which may be partially reversible.

The optoelectronic performance of organic−inorganic halide perovskite (OIHP)-based devices has been improved in recent years. Particularly, solar cells fabricated using mixed-cations and mixed-halides have outperformed their single-cation and single-halide counterparts. Yet, a systematic evaluation of the microstructural behavior of mixed perovskites is missing despite their known composition-dependent photoinstability. Here, we explore microstructural inhomogeneity in (FAPbI3)x(MAPbBr3)1−x using advanced scanning probe microscopy techniques. Contact potential difference (CPD) maps measured by Kelvin probe force microscopy show an increased fraction of grains exhibiting a low CPD with flat topography as MAPbBr3 concentration is increased. The higher portion of low CPD contributes to asymmetric CPD distribution curves. Chemical analysis reveals these grains being rich in MA, Pb, and I. The composition-dependent phase segregation upon illumination, reflected on the emergence of a low-energy peak emission in the original photoluminescence spectra, arises from the formation of such grains with flat topology. Bias-dependent piezo-response force microscopy measurements, in these grains, further confirm vigorous ion migration and cause a hysteretic piezo-response. Our results, therefore, provide insights into the microstructural evaluation of phase segregation and ion migration in OIHPs pointing toward process optimization as a mean to further enhance their optoelectronic performance.

The development of perovskite solar cells (PSCs) with low recombination losses, at low processing temperatures is an area of growing research interest as it enables compatibility with roll-to-roll processing on flexible substrates as well as with tandem solar cells. The inverted or p-i-n device architecture has emerged as the most promising PSC configuration due to the possibility of using low temperature processable organic hole transport layers and more recently, self-assembled monolayers such as, [4-(3,6-Dimethyl-9H-carbazol-9-yl)butyl]phosphonic Acid (Me-4PACz). However, devices incorporating these interlayers suffer from poor wettability of the precursor leading to pin hole formation and poor device yield. Here, we demonstrate the use of alumina nanoparticles (Al2O3 NPs) for pinning the perovskite precursor on Me-4PACz, thereby improving the device yield. While similar wettability enhancements can also be achieved by using poly[(9,9-bis(3’-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]dibromide (PFN-Br), a widely employed surface modifier, the incorporation of Al2O3 NPs results in significantly enhanced Shockley-Read-Hall recombination lifetimes exceeding 3 μs, which is higher than those on films coated directly on Me-4PACz and on PFN-Br modified Me-4PACz. This translates to a champion power conversion efficiency of 19.9% for PSCs fabricated on Me-4PACz modified with Al2O3, which is a ∽20% improvement compared to the champion device fabricated on PFN-Br modified Me-4PACz.

In order to shield perovskite solar cells (PSCs) fromextrinsicdegradation factors and ensure long-term stability, effective encapsulationtechnology is indispensable. Here, a facile process is developed tocreate a glass-glass encapsulated semitransparent PSC usingthermocompression bonding. From quantifying the interfacial adhesionenergy and considering the power conversion efficiency of devices,it is confirmed that bonding between perovskite layers formed on ahole transport layer (HTL)/indium-doped tin oxide (ITO) glass andan electron transport layer (ETL)/ITO glass can offer an excellentlamination method. The PSCs fabricated through this process have onlyburied interfaces between the perovskite layer and both charge transportlayers as the perovskite surface is transformed into bulk. The thermocompressionprocess leads the perovskite to have larger grains and smoother, denserinterfaces, thereby not only reducing defect and trap density butalso suppressing ion migration and phase segregation under illumination.In addition, the laminated perovskite demonstrates enhanced stabilityagainst water. The self-encapsulated semitransparent PSCs with a wide-band-gapperovskite (E (g) & SIM; 1.67 eV) demonstratea power conversion efficiency of 17.24% and maintain long-term stabilitywith PCE > & SIM;90% in the 85 & DEG;C shelf test for over 3000h and with PCE > & SIM;95% under AM 1.5 G, 1-sun illuminationinan ambient atmosphere for over 600 h.

Grain boundaries (GBs) in polycrystalline halide perovskite solar cells play a significant role in not only device performance but also stability. The first section of this chapter (Section ) provides fundamental aspects of grain boundaries. In the following section (Section ), both positive and negative roles of GBs in the device performance of organic‐inorganic halide perovskite (OIHP) solar cells are discussed along with various engineering approaches that tune the GB properties. In Section , enhanced ion transport through GBs is depicted that causes the I–V hysteresis in OIHP solar cells. Finally, in Section , moisture penetration along the GBs is illustrated that ultimately degrade the entire structure and deteriorate the perovskite solar cell stability.

In this work, we report on the design principles of high-power perovskite solar cells (PSCs) for low-intensity indoor light applications, with a particular focus on the electron transport layers (ETLs). It was found that the mechanism of power generation of PSCs under low-intensity LED and halogen lights is surprisingly different compared to the 1 Sun standard test condition (STC). Although a higher power conversion efficiency (PCE) was obtained from the PSC based on mesoporous-TiO2 (m-TiO2) under STC, compared to the compact-TiO2 (c-TiO2) PSC, c-TiO2 PSCs generated higher power than m-TiO2 PSCs under low-intensity (200-1600 Lux) conditions. This result indicates that high PCE at STC cannot guarantee a reliable high-power output of PSCs under low-intensity conditions. Based on the systemic characterization of the ideality factor, charge recombination, trap density, and charge-separation, it was revealed that interfacial charge traps or defects at the electron transport layer/perovskite have a critical impact on the resulting power density of PSC under weak light conditions. Based on Suns-VOC measurements with local ideality factor analyses, it was proved that the trap states cause non-ideal behavior of PSCs under low-intensity light conditions. This is due to the additional trap states that are present at the m-TiO2/perovskite interface, as confirmed by trap-density measurements. Based on Kelvin probe force microscopy (KPFM) measurements, it was confirmed that these traps prohibit efficient charge separation at the perovskite grain boundaries when the light intensity is weak. According to these observations, it is suggested that for the fabrication of high-power PSCs under low-intensity indoor light, the interface trap density should be lower than the excess carrier density to fill the traps at the perovskite's grain boundaries. Finally, using the suggested principle, we succeeded in demonstrating high-performance PSCs by employing an organic ETL, yielding maximum power densities up to 12.36 (56.43), 28.03 (100.97), 63.79 (187.67), and 147.74 (376.85) mu W/cm(2) under 200, 400, 800, and 1600 Lux LED (halogen) illumination which are among the highest values for indoor low-intensity-light solar cells.

Development of efficient solar cells under indoor light has attracted tremendous attention because of the Internet of Things revolution. Here we investigate the effect of chlorine in perovskite precursors for indoor light applications. Use of chlorine has an effect on the photovoltaic performance of perovskite solar cells, especially under low-intensity indoor light. Based on the characterization of leakage current, crystalline structure, and Urbach tail, we reveal that chlorine doping of the perovskite layer influences the movement of photo-generated carriers and ions because of the smaller bulk defects in perovskite. In particular, we suggest that chlorine doping in perovskite facilitates hole extraction on its top surface and contributes to suppression of ion migration and non-radiative recombination, as confirmed by Kelvin probe force microscopy measurements. We demonstrate high performance of perovskite solar cells with a maximum power density of 35.25 (231.78) μW/cm2 under 400 lux light-emitting diode (halogen) illumination. [Display omitted] The effect of chlorine in perovskite precursors for indoor light applicationsDefect density characterization with varied chlorine contentEfficient charge separation within grain boundaryHighly efficient perovskite solar cells under low light intensity Kim et al. investigate the effect of chlorine in perovskite precursors for indoor light applications. Use of chlorine has a significant effect on the photovoltaic performance of perovskite solar cells, especially under low-intensity indoor light. They demonstrate 35.25 and 231.78 μW/cm2 under 400-lux LED and halogen illumination.

Despite over a decade of research on metal halide perovskites (MHPs) in the context of photovoltaic applications, understanding the nature of electronic and ionic processes associated with current-voltage (I-V) hysteretic behavior has been limited. Here, we explore the hysteretic behavior in (FAPbI(3))(0.85)(MAPbBr(3))(0.15) perovskite devices with lateral Cr electrodes by applying first order reversal curve (FORC) bias waveform in I-V, Kelvin probe force microscopy (KPFM) measurements, and in-situ chemical imaging by time-resolved time-of-flight secondary ion mass spectrometry (tr-ToF-SIMS). In dark, we reveal pronounced hysteretic behaviors of charge dynamics in the off-field by probing time-dependent current and contact potential difference (CPD). Under illumination, transient and hysteretic behaviors are significantly reduced. The tr-ToF-SIMS results reveal that the hysteretic behaviors are strongly associated with accumulation of Br- ions at the interfaces. In addition, the low mobility MA(+) ions result in transient behavior and contribute to the hysteretic phenomena. It was shown that Pb2+ ions can be reduced at the interfaces due to electrochemical reactions with the electrode in the presence of charge injection and photogenerated charges. These hysteretic behaviors associated with charge dynamics, ion migration, and interfacial electrochemical reaction are critical to further improve the performance and stability of MHPs photovoltaics and optoelectronics.

Perovskite solar cells (PSCs) have now achieved power conversion efficiencies (PCEs) over 25%, but their long-term stability under illumination and thermal stress is still a major barrier to commercialisation. Herein, we demonstrate the evaluation of light-induced degradation activation energy (Ea) of encapsulated semi-transparent PSCs by using the commonly employed method in crystalline silicon solar cells. Different parameters showed different activation energies where primary degradation is due to increase in series resistance, which also led to reduction in short-circuit current. Open-circuit voltage and shunt resistance also change with different Ea, suggesting the mechanism of the reduction is likely to be due to different reasons. Despite each parameter exhibiting slight variation over time for each temperature, the overall trend converges, indicating that each parameter is likely to be primarily reduced by a single dominant reaction. We also report the main cause of irreversible device degradation is not due to the decomposition of the perovskite layer as confirmed by X-ray diffraction characterisation. Instead, our pole figure map and absorption spectra analysis indicate that a loss of crystal symmetry occurs due to ion migration within the device that induce oxidation of 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (Spiro-OMeTAD). Our work provides a better understanding through quantification of the degradation processes of encapsulated semi-transparent PSCs over time, which is essential for further progress and development of stable perovskite-Si tandem solar cells. •The light-induced degradation in semi-transparent solar cell is investigated.•The primary loss of PCEs in PSCs was due to an increase in series resistance.•XRD results indicate stability of perovskite layer, with degradation occurring in other layers.•Diffusion of ions back into the perovskite layer results in a recovery from LID.

For semitransparent devices with n-i-p structures, a metal oxide buffer material is commonly used to protect the organic hole transporting layer from damage due to sputtering of the transparent conducting oxide. Here, a surface treatment approach is addressed for tungsten oxide-based transparent electrodes through slight modification of the tungsten oxide surface with niobium oxide. Incorporation of this transparent electrode technique to the protective buffer layer significantly recovers the fill factor from 70.4% to 80.3%, approaching fill factor values of conventional opaque devices, which results in power conversion efficiencies over 18% for the semitransparent perovskite solar cells. Application of this approach to a four-terminal tandem configuration with a silicon bottom cell is demonstrated.

Effectively incorporating alkali metals or alternative isovalent cations into Cu2ZnSnS4 (CZTS) is considered one of the most promising strategies for realizing a step-change improvement in the photovoltaic device performance. Herein, the local distribution of Na and Cd by a moisture-assisted postdeposition annealing (MAPDA) treatment combined with a subsequent heterojunction heat treatment is manipulated. The MAPDA treatment facilitates the controllable reduction of the Na concentration, thus promoting the spontaneous diffusion of Cd into the heterojunction region. A subsequent 150 degrees C low-temperature heterojunction heat treatment after MAPDA treatment enables further modification of Cd and Na distributions, leading to significantly enhanced optoelectronic properties at the CZTS/CdS heterojunction and greatly improved device performance with a peak conversion efficiency of 9.40%. The modified heterojunction significantly improves quasi-Fermi-level splitting under low-photon injection, making CZTS solar cells more feasible in low-light applications. This work provides an effective approach to simultaneously manipulate the distribution of Na and Cd, enabling pronounced modification of the heterojunction quality of CZTS solar cells and boost of conversion efficiencies. Insights gleaned herein may also be applicable to manipulating other critical trace elements in chalcogenide materials in general.

Recent efficiency advancements in kesterites have reinforced the use of Cu2ZnSn(S,Se)(4) (CZTSSe) in indoor photovoltaic applications. However, the performance of kesterites under low light intensity conditions is mainly hindered by deep-level defects. In this study, a strategic approach of silver (Ag) and germanium (Ge) cation substitution to cure these defects are employed. The Ag-doped CZTSSe (CZTSSe:Ag) and Ge-doped (CZTSSe:Ge) samples experimentally demonstrated a significant improvement in kesterite device performance under all intensities of LED and white fluorescent lamp conditions are prepared. Interestingly, the CZTSSe:Ag device exhibited the highest performance levels, i.e., 1.2-1.5 and 2.5-3 times better than those of Ge-doped CZTSSe:Ge and undoped CZTSSe, respectively. This improved device performance is mainly attributed to the reduced energy level of deep-level defects in CZTSSe:Ag. Moreover, these defects assisted in the generation of a larger potential difference between the grain boundary and grain interior in the CZTSSe:Ag sample, attracting minority carriers near the grain boundary. Consequently, the improved carrier separation process reduced the carrier recombination losses and enhanced the power output under low light intensity conditions. This Ag and Ge cation substitution in kesterite is found to be an effective approach to improve the device performance under low light intensity conditions.

This report addresses indium oxide doped with titanium and tantulum with high near-infrared transparency to potentially replace the conventional indium tin oxide transparent electrode used in semitransparent perovskite devices and top cells of tandem devices. The high near-infrared transparency of this electrode is possibly explained by the lower carrier concentration, suggesting less defect sites that may sacrifice its optical transparency. Incorporating this transparent electrode into semitransparent perovskite solar cells for both the top and bottom electrodes improved the device performance through possible reduction of interfacial defect sites and modification in energy alignment. With this indium oxide-based semitransparent perovskite top cell, we also demonstrated four-terminal perovskite–silicon tandem configurations with improved photocurrent response in the bottom silicon cell.

For the fabrication of low-dimensional perovskite solar cells, understanding the effect of precursor preparation on film formation is critical to achieve high-quality perovskite film and, therefore, high efficiency in related solar devices. Herein, the two methods to prepare phenethylammonium-based mixed perovskite precursors with the same chemical composition are reported. These methods are called 1) different phase (DP) and 2) same phase (SP) methods as the former involves the mixing of a 3D perovskite precursor with a 2D perovskite precursor, whereas the latter involves the mixing of quasi-2D perovskite precursors. The films prepared by these methods are characterized by X-ray diffraction, Kelvin probe force microscopy, and scanning electron microscopy, revealing different perovskite structures. The power conversion efficiency (PCE) of the champion cells by DP and SP methods reaches 19.1% and 18.9%, respectively. Results of the aging test show a dramatic improvement in the stability of SP perovskite devices maintaining 86% of its initial performance after exposure to a relative humidity (RH) 8 +/- 5% for 1000 hr and over 80% of its initial PCE after continuous 1 sun illumination (including UV) at RH 70%. The new insights provided by this work are important to design perovskite precursor preparation methods for the best device performance and stability.

Recently, perovskite solar cells have shown excellent performance under indoor light conditions. In a new approach using directional illumination combined with nanoscale scanning probe microscopy (SPM) characterization, morphology dependent‐charge transport measurements are performed to provide a comprehensive understanding of the optoelectronic behavior of (FAPbI3)0.85(MAPbBr3)0.15 containing 5 vol% cesium (Cs5vol%) with various electron transport layers (ETLs), i.e., SnO2, c‐TiO2, and [6,6]‐phenyl‐C61‐butyric acid methyl ester/SnO2 under indoor light. This approach allows the identification of the charge transport properties of the perovskite film and the perovskite/ETL interface separately. The light is applied from the top of the perovskite film to study the electronic properties of the surface. Lower photocurrent and lower surface photovoltage (SPV) are observed under top‐illumination conditions. The electronic interface behavior is investigated using bottom‐illumination and short excitation wavelengths, such as blue LED light. Higher photocurrent and higher SPV are observed under blue light illumination from the bottom. These results suggest that the charge transport capability is enhanced near the p–n junction. Conductive atomic force microscopy results show that SnO2 enhances the charge collection properties of the perovskite's grain boundaries (GBs). Kelvin probe force microscopy results confirm that SnO2 exhibits homogeneous and high surface potential because of the lowest trap states at GBs. A nanoscale morphology dependent photovoltaic characterization is performed in order to better understand perovskite solar cells with different types of electron transport layers under indoor light conditions for IoT applications. It is indicated in the results that the layer type does not only impact the interface properties but strongly influences the charge transport properties at the grain boundary, and thereby, indoor solar cell performance.

The present work applies a focal point of materials-related issues to review the major case studies of electron transport layers (ETLs) of metal halide perovskite solar cells (PSCs) that contain graphene-based materials (GBMs), including graphene (GR), graphene oxide (GO), reduced graphene oxide (RGO), and graphene quantum dots (GQDs). The coverage includes the principal components of ETLs, which are compact and mesoporous TiO2, SnO2, ZnO and the fullerene derivative PCBM. Basic considerations of solar cell design are provided and the effects of the different ETL materials on the power conversion efficiency (PCE) have been surveyed. The strategy of adding GBMs is based on a range of phenomenological outcomes, including enhanced electron transport, enhanced current density-voltage (J-V) characteristics and parameters, potential for band gap (E-g) tuning, and enhanced device stability (chemical and environmental). These characteristics are made complicated by the variable effects of GBM size, amount, morphology, and distribution on the nanostructure, the resultant performance, and the associated effects on the potential for charge recombination. A further complication is the uncertain nature of the interfaces between the ETL and perovskite as well as between phases within the ETL.

Halide perovskite-based photovoltaic (PV) devices have recently emerged for low energy consumption electronic devices such as Internet of Things (IoT). In this work, an effective strategy to form a hole-selective layer using phenethylammonium iodide (PEAI) salt is presented that demonstrates unprecedently high open-circuit voltage of 0.9 V with 18 mu W cm(-2) under 200 lux (cool white light-emitting diodes). An appropriate post-deposited amount of PEAI (2 mg) strongly interacts with the perovskite surface forming a conformal coating of PEAI on the perovskite film surface, which improves the crystallinity and absorption of the film. Here, Kelvin probe force microscopy results indicate the diminished potential difference across the grain boundaries and grain interiors after the PEAI deposition, constructing an electrically and chemically homogeneous surface. Also, the surface becomes more p-type with a downshift of a valence band maximum, confirmed by ultraviolet photoelectron spectroscopy measurement, facilitating the transport of holes to the hole transport layer (HTL). The hole-selective layer-deposited devices exhibit reduced hysteresis in light current density-voltage curves and maintain steadily high fill factor across the different light intensities (200-1000 lux). This work highlights the importance of the HTL/perovskite interface that prepares the indoor halide perovskite PV devices for powering IoT device.

Membrane-based photothermal crystallization -a pioneering technology for mining valuable minerals from seawater and brines -exploits self-heating nanostructured interfaces to boost water evaporation, so achieving a controlled supersaturation environment that promotes the nucleation and growth of salts. This work explores, for the first time, the use of two-dimensional graphene thin films (2D-G) and three dimensional vertically orientated graphene sheet arrays (3D-G) as potential photothermal membranes applied to the dehydration of sodium chloride, potassium chloride and magnesium sulfate hypersaline solutions, followed by salt crystallization. A systematic study sheds light on the role of vertical alignment of graphene sheets on the interfacial, light absorption and photothermal characteristics of the membrane, impacting on the water evaporation rate and on the crystal size distribution of the investigated salts. Overall, 3D-G facilitates the crystallization of the salts because of superior light-to-heat conversion leading to a 3-fold improvement of the evaporation rate with respect to 2D-G. The exploitation of sunlight graphene-based interfaces is demonstrated as a potential sustainable solution to aqueous wastes valorization via recovery in solid phase of dissolved salts using renewable solar energy.

For the fabrication of perovskite solar cells (PSCs) using a solution process, it is essential to understand the characteristics of the perovskite precursor solution to achieve high performance and reproducibility. The colloids (iodoplumbates) in the perovskite precursors under various conditions were investigated by UV–visible absorption, dynamic light scattering, photoluminescence, and total internal reflection fluorescence microscopy techniques. Their local structure was examined by in situ X-ray absorption fine structure studies. Perovskite thin films on a substrate with precursor solutions were characterized by transmission electron microscopy, X-ray diffraction analysis, space-charge-limited current, and Kelvin probe force microscopy. The colloidal properties of the perovskite precursor solutions were found to be directly correlated with the defect concentration and crystallinity of the perovskite film. This work provides guidelines for controlling perovskite films by varying the precursor solution, making it possible to use colloid-engineered lead halide perovskite layers to fabricate efficient PSCs.

Kesterite-based thin-film solar cells (TFSCs) have recently gained significant attention in the photovoltaic (PV) sector for their elemental earth abundance and low toxicity. An inclusive study from the past reveals basic knowledge about the grain boundary (GB) and grain interior (GI) interface. However, the compositional dependency of the surface potential within GBs and GIs remains unclear. The present work provides insights into the surface potential of the bulk and GB interfaces. The tin (Sn) composition is sensitive to the absorber morphology, and therefore, it significantly impacts absorber and device properties. The absorber morphology improves with the formation of larger grains as the Sn content increases. Additionally, the presence of Sn(S,Se)(2) and increased [Zn-Cu + V-Cu] A-type defect cluster density are observed, validated through Raman analysis. The secondary ion mass spectroscopy analysis reveals the altered distribution of sulfur (S) and sodium (Na) with higher near-surface accumulation. The synergistic outcome of the increased density of defects and the accumulation of S near the interface provides a larger GB and GI difference and expedites carrier separation improvement. Consequently, at an optimum compositional ratio of Cu/(Zn+Sn) = similar to 0.6, the power conversion efficiency (PCE) is significantly improved from 6.42 to 11.04% with a record open-circuit voltage (V-OC) deficit of 537 mV.

In this study, we prepared three kesterite thin-film solar cells, Cu2ZnSnSe4(CZTSe), Cu2ZnSn(S,Se)(4)(CZTSSe), and Cu2ZnSnS4(CZTS), and based on low light intensity measurements, examined the possibility of using kesterite devices for indoor applications. Interestingly, all the prepared cells exhibited nearly the same device efficiency under standard test conditions of 1 sun; however, under illumination with low-intensity halogen and LED lamps (200-400 lux), the power output of CZTSSe was twice that of CZTSe and CZTS. CZTSe (58%) and CZTS (37%) showed relatively larger open-circuit voltage drops than CZTSSe (29%). Suns-V(oc)measurements revealed that the ideality factor of CZTS and CZTSe increased as the light intensity decreased, which indicates severe recombination caused by deep-level defects at low light intensities. Furthermore, admittance spectroscopy measurements revealed that CZTSe and CZTS have deep trap energy levels, whereas CZTSSe has comparatively shallower trap energy levels; this validates the rapid open-circuit voltage drop under low light intensity conditions. Kelvin probe force microscopy measurements showed that CZTSSe exhibited a higher photovoltage (86 mV) under illumination at 400 lux compared with that under dark conditions. In addition, our results indicated that the CZTSSe sample showed relatively much higher charge separation at GBs (grain boundaries) owing to the downward band bending at the GBs. The findings revealed that for deeper energy levels, the open-circuit voltage reduction was faster; in addition, an absorber layer with shallower defects and efficient charge separation at the GBs can induce high power conversion efficiency under low-light conditions.

Organometal perovskite single crystals have been recognized as a promising platform for high-performance optoelectronic devices, featuring high crystallinity and stability. However, a high trap density and structural nonuniformity at the surface have been major barriers to the progress of single crystal-based optoelectronic devices. Here, the formation of a unique nanoisland structure is reported at the surface of the facet-controlled cuboid MAPbI(3) (MA = CH3NH3+) single crystals through a cation interdiffusion process enabled by energetically vaporized CsI. The interdiffusion of mobile ions between the bulk and the surface is triggered by thermally activated CsI vapor, which reconstructs the surface that is rich in MA and CsI with reduced dangling bonds. Simultaneously, an array of Cs-Pb-rich nanoislands is constructed on the surface of the MAPbI(3) single crystals. This newly reconstructed nanoisland surface enhances the light absorbance over 50% and increases the charge carrier mobility from 56 to 93 cm(2) V-1 s(-1). As confirmed by Kelvin probe force microscopy, the nanoislands form a gradient band bending that prevents recombination of excess carriers, and thus, enhances lateral carrier transport properties. This unique engineering of the single crystal surface provides a pathway towards developing high-quality perovskite single-crystal surface for optoelectronic applications.

A contactless effective series resistance imaging method for large-area perovskite solar cells that is based on photoluminescence imaging with nonuniform illumination is introduced and demonstrated experimentally. The proposed technique is applicable to partially and fully processed perovskite solar cells if laterally conductive layers are present. The capability of the proposed contactless method to detect features with high effective series resistance is validated by comparison with various contacted mode luminescence imaging techniques. The method can reliably provide information regarding the severeness of the detected series resistance through photoexcitation pattern manipulation. Application of the method to subcells in monolithic tandem devices, without the need for electrical contacting the terminals, appears feasible.

Greater stability of low-dimensional halide perovskites as opposed to their three-dimensional counterparts, alongside their high extinction coefficient and thus excellent emission properties, have made them popular candidates for optoelectronic applications. Topological edges are found in two-dimensional perovskites that show distinct electronic properties. In this work, using Kelvin Probe Force Microscopy, performed on butylammonium lead bromide (BA2PbBr4) single crystals with optical bandgap of ~413 nm, we elucidate the electronic response of the edges and their potential impact on photodetector devices. We show that the charge-carriers are accumulated at the edges, increasing with the edge height. Wavelength-dependent surface photovoltage (SPV) measurements reveal that multiple sub-bandgap states exist in BA2PbBr4. As the edge height increases, the SPV amplitude at the edges reduces slightly more as compared to the adjacent regions, known as terraces, indicating relatively less reduction in band-bending at the surface due possibly to increased de-population of electrons from sub-bandgap states in the upper bandgap half. The existence of sub-bandgap states is further confirmed by the observation of below-bandgap emission (absorption) peaks characterised by spectral photoluminescence and photothermal deflection spectroscopy measurements. Finally, we fabricated a photodetector using a millimetre size BA2PbBr4 single crystal. Noticeable broadband photodetection response was observed in the sub-bandgap regions under green and red illumination, which is attributed to the existence of sub-bandgap states. Our observations suggest edge-height dependence of charge-carrier behaviour in BA2PbBr4 single crystals, a potential pathway that can be exploited for efficient broadband photodetector fabrication.

Incorporating homogeneously dispersed metal single atoms or nanoclusters into bulk matrix can produce functional materials for electrochemical catalysis, energy storage, and electronic devices. However, the instability of single metal atoms (or clusters) against agglomeration and thus loss of active surfaces during high-temperature treatment or reactions remains a major challenge. Here, we report the effect of spatial confinement on suppressing migration and coalescence of metal atoms/clusters in solid films made of stacked and/or overlapping (‘reduced’) graphene oxide, resulting in increased stability of dispersed metal (i.e., Cu, Co, Ni) atoms and nanoclusters at high temperature (1000 °C). We find that pressing has a significant impact on the degree of ‘reduction’ of graphene oxide and the morphology and distribution of metals in the films; the presence of metals influences the thermal ‘reduction’ and graphitization of graphene oxide. This work demonstrates the efficacy of externally applied pressure in controlling the reactivity and mobility of metal atoms/clusters in bulk solids, which can be a useful means for preparing a variety of atomic/nano-metal-based hybrid materials.