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Triplet–triplet annihilation-based photon upconversion (TTA-UC) is a promising mechanism for harvesting lower energy photons by converting them to a higher energy. Photons generated from this process can be used for numerous applications, including photovoltaics, infrared sensing and imaging, biomedicine and photochemical reactions. Recently, bulk metal halide perovskite semiconductors have been introduced as triplet sensitizers for the TTA-UC process. While relatively efficient upconversion has been achieved at low fluences, the full potential of these materials as triplet sensitizers has not been unlocked. Here, we examine four pathways for device optimization and improvements, while discussing relevant works and potential further improvements. Finally, we discuss the outlook and bright future of such perovskite materials as triplet sensitizers and their important role in solid-state upconversion applications.

Solid-state bulk lead halide perovskite thin films have recently shown progress as triplet sensitizers in infrared-to-visible photon upconversion (UC) schemes. Common systems pair lead halide perov...

Summary Studying nanoscale photophysical processes is mandatory to fully understand the complex optoelectronic properties in semiconductor materials used in photovoltaics and light emitting diodes. In this perspective, we target specific scanning probe techniques, which combine scanning tunneling microscopy (STM) with optical methods to unravel the localized optoelectronic properties of semiconductors under realistic electric and optical fields, down to the nanoscale. Combining optical spectroscopy with STM yields a powerful platform that allows for simultaneous imaging of the surface morphology and the electronic structure down to the atomic level, a resolution that is otherwise not accessible due to the optical diffraction limit. Incident wavelengths spanning the electromagnetic spectrum from the terahertz region to X-rays have been coupled into the STM tip-sample junction to investigate the nanoscale properties of semiconductor materials, whereas the reverse process of luminescence can give insight on local recombination processes. Imagine the potential of a tool capable of detecting both localized absorption and spontaneous and stimulated emission processes of semiconductor materials at the nanoscale. The role of every atom, defect, or electronic interaction could be disentangled, tailored, or harnessed to its maximum capacity.

The processes of singlet fission and triplet fusion could allow state-of-the-art photovoltaic devices to surpass the Shockley–Queisser limit by optimizing the utilized solar spectrum by reducing thermal relaxation and inaccessible sub-bandgap photons, respectively. Triplet fusion demands precise control of the spin-triplet state population, and requires a sensitizer to efficiently populate the triplet state of an acceptor molecule. In this perspective, we highlight the established field of sensitized upconversion and further examine alternative triplet sensitization routes, including the possibility of bulk solid-state semiconductors as triplet sensitizers, which provide a new avenue for charge transfer-based triplet sensitization rather than excitonic triplet energy transfer.

High capital expenditures associated with manufacturing thin silicon wafers make it difficult for the industry to scale up and prevent novel technologies from entering the market. Thin wafers fail largely due to breakage during solar cell processing and handling. One of the root causes for breakage is sub-mm edge cracks in the silicon wafer, and these cracks cannot be reliably detected by most commercially-available crack detection tools. In this work, we first investigate the correlation between wafer thickness and critical crack length, and explain the importance of detecting sub-500-μm edge cracks as the wafer thickness is reduced. Secondly, we extend our previous work of micro-crack detection to demonstrate an edge illumination technique using a near-infrared laser to image edge cracks less than 500 μm in length in multicrystalline silicon. Thirdly, we investigate two fundamental edge illumination mechanisms based on dark-field imaging; namely, direct and vicinal illumination. We will then compare these methods to a state-of-the-art rear illumination method. The advantages and disadvantages of both illumination methods are presented and provide an in-depth analysis of light-crack interaction. In particular, we find that the robustness of vicinal illumination is due to diffuse reflectance. The diffuse reflectance has less dependence on crack configurations, while direct illumination has more dependence on the crack configurations because it utilizes specular reflectance. Our results show that our proposed prototype can detect sub-mm edge cracks in multicrystalline silicon wafers, which is an important step in enabling thin silicon wafer manufacturing.

The role electron-transport layers (ETLs) play in perovskite solar cells (PSCs) is still widely debated. Conduction band alignment at the perovskite/ETL interface has been suggested to be an import...

Green-to-blue photon upconversion bears great potential in photocatalytic applications. However, current hybrid inorganic-organic upconversion schemes utilizing spherical CdSe nanocrystals are often limited by energetic polydispersity, low quantum yields and an additional tunneling barrier resulting from the necessity of surface-passivating inorganic shells. In this contribution, we introduce anisotropic CdSe nanoplatelets as triplet sensitizers. Here, quantum confinement occurs in only one direction, erasing effects stemming from energetic polydispersity. We investigate the triplet energy transfer from the CdSe nanoplatelets to the surface-bound triplet acceptor 9-anthracene carboxylic acid. We further focus on the influence of nanoplatelet stacking and singlet back transfer on the observed upconversion efficiency. We obtain an upconversion quantum yield of 5.4% at a power density of 11 W/cm2­ using the annihilator 9,10-diphenylanthracene, and a low efficiency threshold Ith of 237 mW/cm2.

In this review, we highlight the current advancements in the field of triplet sensitization by three-dimensional (3D) perovskite nanocrystals and bulk films. First introduced in 2017, 3D perovskite sensitized upconversion (UC) is a young fast-evolving field due to the tunability of the underlying perovskite material. By tuning the perovskite bandgap, visible-to-ultraviolet, near-infrared-to-visible or green-to-blue UC has been realized, which depicts the broad applicability of this material. As this research field is still in its infancy, we hope to stimulate the field by highlighting the advantages of these perovskite nanocrystals and bulk films, as well as shedding light onto the current drawbacks. In particular, the keywords toxicity, reproducibility and stability must be addressed prior to commercialization of the technology. If successful, photon interconversion is a means to increase the achievable efficiency of photovoltaic cells beyond its current limits by increasing the window of useable wavelengths.

Microcracks in silicon solar cells reduce the mechanical strength of the wafer and cause breakage during manufacturing, transportation, and field operation. Therefore, there is a need to trace where microcracks initiate in the manufacturing line. As wafers become thinner, the critical crack length required for fracture significantly decreases for the same loading conditions. Currently, very few industry-standard tools can reliably detect submillimeter cracks, which will become more critical for thinner wafers. In this work, we demonstrate a technique to detect submillimeter cracks located at the edges of various multicrystalline silicon wafers and solar cells. The proposed technique, which is based on near-infrared dark-field imaging with vicinal laser illumination from the wafer edge, has several advantages over state-of-the-art optical transmission imaging and dark-field scattering techniques. Moreover, we adapt this technique to achieve the high-throughput requirement of inline metrology; hence, it can be used to detect submillimeter cracks in a manufacturing line. With a high-frame-rate line-scan camera, this proposed crack technique is designed to theoretically achieve a scanning throughput of less than 1 s per wafer.

Photon upconversion, particularly via triplet–triplet annihilation (TTA), could prove beneficial in expanding the efficiencies and overall impacts of optoelectronic devices across a multitude of technologies. The recent development of bulk metal halide perovskites as triplet sensitizers is one potential step toward the industrialization of upconversion-enabled devices. Here, we investigate the impact of varying additions of bromide into a lead iodide perovskite thin film on the TTA upconversion process in the annihilator molecule rubrene. We find an interplay between the bromide content and the overall device efficiency. In particular, a higher bromide content results in higher internal upconversion efficiencies enabled by more efficient charge extraction at the interface likely due to a more favorable band alignment. However, the external upconversion efficiency decreases as the absorption cross section in the near infrared is reduced. The highest upconversion performance is found in our study for a bromide content of 5%. This result can be traced back to a high absorption cross section in the near infrared and higher photoluminescence quantum yield in comparison to the iodide-only perovskite and an increased driving force for charge transfer.

Bismuth-based materials have been studied as alternatives to lead-based perovskite materials for photovoltaic applications. However, poor film quality has limited device performance. In this work, we developed a solvent-engineering method and show that it is applicable to several bismuth-based compounds. Through this method, we obtained compact films of methylammonium bismuth iodide (MBI), cesium bismuth iodide (CBI), and formamidinium bismuth iodide (FBI). On the basis of film growth theory and experimental analyses, we propose a possible mechanism of film formation. Additionally, we demonstrate that the resultant compact MBI film is more suitable to fabricate efficient and stable photovoltaic devices compared to baseline MBI films with pinholes. We further employed a new hole-transporting material to reduce the valence-band offset with the MBI. The best-performing photovoltaic device exhibits an open-circuit voltage of 0.85 V, fill factor of 73%, and a power conversion efficiency of 0.71%, the highest r...

Impurity contamination in thin-film solar cells remains an uncertain risk due to the little-known impact of impurities on recombination. Building upon previous work, in which we intentionally contaminated lead halide perovskite (LHP) solar cells with iron, we further examine the distribution and charge state of iron-induced defects in LHP films using synchrotron-based X-ray techniques. X-ray absorption measurements suggest that iron-rich regions, which form among iron feedstock concentrations that exceed 100 ppm, most closely resemble the chemistry of Fe2O3. Iron distributed within the bulk may form a mix of Fe2+and Fe3+, the latter of which is not expected to be recombination active, potentially allowing LHPs to incorporate more iron than traditional semiconductors. X-ray beam induced current measurements show little correlation between the presence of iron-rich regions and charge collection, which further suggests low recombination activity at these sites. These results further elucidate the recombination behavior caused by iron incorporation in LHP films, revealing insight into how inhomogeneous incorporation of impurities may mitigate photovoltaic performance degradation., National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (award number DMR-1419807), United States. Department of Energy. Office of Science User Facility (Contract No. DE-AC02-06CH11357), National Science Foundation (U.S.) (NSF EECS Award No. 1541959), Martin Family Society of Fellows for Sustainability, National Science Foundation (U.S.). Graduate Research Fellowship (Grant No. 1122374), National Science Foundation (U.S.). (CA No. EEC-1041895)

Recent advances in perovskite-sensitized photon upconversion via triplet-triplet annihilation (TTA) in rubrene have yielded several unanswered questions about the underlying mechanism and processes occurring at the interface. In particular, the near-infrared perovskite emission is not significantly quenched and a rapid reversible photobleach of the upconverted emission can be observed under fairly low excitation densities of 3.2 mW/cm2. In this contribution, we investigate the perovskite/organic interface in more detail and conclude that non-covalent interactions between the organic layer and perovskite result in surface trap passivation. In addition, band bending results in a charge space region at the perovskite/rubrene interface, which precharges the rubrene interface with holes. Upon initial illumination, electrons can rapidly transfer to the excited triplet state of rubrene, followed by efficient TTA upconversion. As the device is continuously illuminated, the precharged holes are consumed and a new equilibrium is reached, resulting in the previously investigated steady-state upconversion efficiency.

Crystalline silicon comprises 90% of the global photovoltaics (PV) market and has sustained a nearly 30% cumulative annual growth rate, yet comprises less than 2% of electricity capacity. To sustain this growth trajectory, continued cost and capital expenditure (capex) reductions are needed. Thinning the silicon wafer well below the industry-standard 160 μm, in principle reduces both manufacturing cost and capex, and accelerates economically-sustainable expansion of PV manufacturing. In this analysis piece, we explore two questions surrounding adoption of thin silicon wafers: (a) What are the market benefits of thin wafers? (b) What are the technological challenges to adopt thin wafers? In this analysis, we re-evaluate the benefits and challenges of thin Si for current and future PV modules using a comprehensive technoeconomic framework that couples device simulation, bottom-up cost modeling, and a sustainable cash-flow growth model. When adopting an advanced technology concept that features sufficiently good surface passivation, the comparable efficiencies are achievable for both 50 μm wafers and 160 μm ones. We then quantify the economic benefits for thin Si wafers in terms of poly-Si-to-module manufacturing capex, module cost, and levelized cost of electricity (LCOE) for utility PV systems. Particularly, LCOE favors thinner wafers for all investigated device architectures, and can potentially be reduced by more than 5% from the value of 160 μm wafers. With further improvements in module efficiency, an advanced device concept with 50 μm wafers could potentially reduce manufacturing capex by 48%, module cost by 28%, and LCOE by 24%. Furthermore, we apply a sustainable growth model to investigate PV deployment scenarios in 2030. It is found that the state-of-the-art industry concept could not achieve the climate targets even with very aggressive financial scenarios, therefore the capex reduction benefit of thin wafers is advantageous to facilitate faster PV adoption. Lastly, we discuss the remaining technological challenges and areas for innovation to enable high-yield manufacturing of high-efficiency PV modules with thin Si wafers., U.S. Department of Energy (DOE) (Award DE-EE0007535)

Reducing silicon usage by adopting thinner wafers can significantly reduce capital expenditure (capex) and cost, and thus accelerate the growth of manufacturing capacity and deployment. In this work, we evaluated potential benefits of thin Si wafers for current and future PV modules. We apply a technoeconomic framework that couples bottom-up cost model, and a cash-flow growth model to analyze PV modules with thin wafers. First, we show that, comparing the current PERC with 160 µm thick wafers to the high-efficiency concept with 50 µm thin wafers, the capex is reduced from 0.39 to 0.2 $/(W/year) while the cost is from 0.32 to 0.2 $/W. Second, the potential of accelerated deployment is analyzed for the high-efficiency thin wafer concept. We found the significant advantages of higher growth rate (~20% relative) and higher deployment plateau (~3 times) than the current PERC modules.

Photon upconversion via triplet–triplet annihilation could allow for the existing efficiency limit of single junction solar cells to be surpassed. Indeed, efficient upconversion at subsolar fluences has been realized in bulk perovskite-sensitized systems. Many questions have remained unanswered, in particular, regarding their behavior under photovoltaic operating conditions. Here, we investigate the impact of repeated and continuous illumination on bilayer perovskite/rubrene upconversion devices. We find that variations of the underlying perovskite carrier recombination dynamics greatly impact the upconversion process. Trap filling and triplet sensitization are in direct competition: more saturated trap states in the perovskite and, thus, longer underlying perovskite photoluminescence lifetimes allow for an increased number of carriers to diffuse to the perovskite/rubrene interface and undergo charge extraction to the triplet state of rubrene. As a result, the upconversion efficiency is greatly influenced by the underlying trap density: the upconverted photoluminescence intensity increases by two orders of magnitude under continuous illumination for 4 h. This shows that the upconversion efficiency is difficult to define for this system. Importantly, these results indicate that perovskite-sensitized upconversion devices exhibit peak performance under continuous illumination, which is a requirement for their successful integration into photovoltaics to help overcome the Shockley–Queisser limit in single junction solar cells.

Nanometer-sized metal clusters are prime candidates for photoactivated catalysis, based on their unique tunable optical and electronic properties, combined with a large surface-to-volume ratio. Due to the very small optical cross sections of such nanoclusters, support-mediated plasmonic activation could potentially make activation more efficient. Our support is a semi-transparent gold film, optimized to work in a back-illumination geometry. It has a surface plasmon resonance excitable in the 510–540 nm wavelength range. Ptn clusters (size distribution peaked at n = 46 atoms) have been deposited onto this support and investigated for photoactivated catalytic performance in the oxidative decomposition of methylene blue. The Pt cluster catalytic activity under illumination exceeds that of the gold support by more than an order of magnitude per active surface area. To further investigate the underlying mechanism of plasmon-induced catalysis, the clusters have been imaged with optically-assisted scanning tunneling microscopy under illumination. The photoactivation of the Pt clusters via plasmonic excitation of the support and subsequential electronic excitation of the clusters can be imaged with nanometer resolution. The light-induced tunneling current on the clusters is enhanced relative to the gold film support.

Correction for ‘Revisiting thin silicon for photovoltaics: a technoeconomic perspective’ by Zhe Liu et al., Energy Environ. Sci., 2020, 13, 12–23, DOI: 10.1039/C9EE02452B.

Lead halide perovskites have emerged as promising absorber materials over the last decade to increase the efficiency of photovoltaics beyond its current limits. However, to further optimize the performance of perovskites more detailed studies need to be performed, which allow for the correlation of film morphology and local electronic properties at the nanoscale. Here, we present a scanning tunneling microscopy (STM) approach to probe the effect of an applied electric field of a methylammonium formamidinium lead triiodide perovskite thin film on the film response by current–voltage spectroscopy, current imaging tunneling spectroscopy, differential conductance mapping, and x-ray absorption spectroscopy by means of synchrotron x-ray STM. We find a strong correlation between the measurement conditions and the obtained current–voltage characteristics when imaging under opposite bias polarities. In particular, we find similarities to already observed poling effects for lead halide perovskites, which result in either a positively or negatively charged interface due to ion and vacancy migration. Our results provide insight into the influence of measurement conditions such as bias polarity on the performance assessment of perovskite thin films by STM.

Lead halide-based perovskite thin films have attracted great attention due to the explosive increase in perovskite solar cell efficiencies. The same optoelectronic properties that make perovskites ideal absorber materials in solar cells are also beneficial in other light-harvesting applications and make them prime candidates as triplet sensitizers in upconversion via triplet-triplet annihilation in rubrene. In this contribution, we take advantage of long carrier lifetimes and carrier diffusion lengths in perovskite thin films, their high absorption cross sections throughout the visible spectrum, as well as the strong spin-orbit coupling owing to the abundance of heavy atoms to sensitize the upconverter rubrene. Employing bulk perovskite thin films as the absorber layer and spin-mixer in inorganic/organic heterojunction upconversion devices allows us to forego the additional tunneling barrier owing from the passivating ligands required for colloidal sensitizers. Our bilayer device exhibits an upconversion efficiency in excess of 3% under 785 nm illumination.

The same optical and electronic properties that make perovskite thin films ideal absorber materials in photovoltaic applications are also beneficial in photon upconversion devices. In this contribution, we investigate the rubrene-triplet sensitization by perovskite thin films based on methylammonium formamidinium lead triiodide (MAFA). To elucidate the role of trap states which affect the free carrier lifetimes, we fabricate MAFA perovskite thin films with three different thicknesses. By measuring the change in the photoluminescence properties under different excitation fluences, we find that the prevalent recombination mechanism shifts from monomolecular for thinner films to bimolecular recombination for thicker MAFA films, indicating a reduction in shallow trap-assisted recombination. The addition of rubrene shows a passivating effect on the MAFA surface, but adds an additional quenching pathway due to charge transfer to the triplet state of rubrene. We observe that the threshold for efficient triplet-triplet annihilation shifts to lower incident powers with increasing MAFA thickness, which suggests that the charge transfer to the triplet state competes with non-radiative trap filling. Hence, injection of free electrons and holes into the upconverting organic semiconductor can provide a new avenue for sensitization of rubrene, and may allow us to move away from the necessity of efficient excitonic singlet-to-triplet converters.

By harvesting sub-bandgap photons, we have a path to overcome the Shockley-Queisser limit in photovoltaics (PVs). We investigate semiconductor nanocrystal (NC) sensitized upconversion via triplet-triplet annihilation (TTA) in organic semiconductors (OSCs). Since this process relies on optically inactive triplet states in the OSCs, we utilize PbS NCs to directly sensitize the triplet state via energy transfer. This is possible due to the strong spin-orbit coupling in PbS NCs, resulting in rapid spin-dephasing of the exciton. Current technology allows for upconversion of light with a photon energy above $\sim 1.1$ eV. However, while internal efficiencies are rapidly improving, the low external device efficiencies render them impractical for applications, as devices are based on a single monolayer of NCs. Our results show simply increasing the PbS NC film thickness does not show improvement in the efficiency due to poor exciton transport between PbS NCs. Here, we present a new strategy to increase the external upconversion efficiency by utilizing thin tinbased halide perovskites as the absorbing layer. Resonant energy transfer from the perovskite to the PbS NCs allows for subsequent sensitization of the triplet state in rubrene.

Chiral junctions of carbon nanotubes have the potential of serving as optically or electrically controllable switches. To investigate optoelectronic tuning of a chiral junction, we stamp carbon nanotubes onto a transparent gold surface and locate a tube with a semiconducting-metallic junction. We image topography, laser absorption at 532 nm, and measure I-V curves of the junction with nanometer spatial resolution. The bandgaps on both sides of the junction depend on the applied tip field (Stark effect), so the semiconducting-metallic nature of the junction can be tuned by varying the electric field from the STM tip. Although absolute field values can only be estimated because of the unknown tip geometry, the bandgap shifts are larger than expected from the tip field alone, so optical rectification of the laser and carrier generation by the laser must also affect the bandgap switching of the chiral junction.

The high capital expenditure (capex) necessary to manufacture crystalline silicon PV modules negatively affects the levelized cost of electricity (¢/kWh) and critically impacts the rate at which the PV industry can scale up. Wafer, cell, and module fabrication with thin free-standing silicon wafers is one key to reduce capex. Thin wafers reduce capex associated with silicon refining and wafer fabrication, which together sum to 58% of the total capex of silicon module manufacturing. In addition, thin wafers directly and significantly reduce variable costs. However, introducing 50 μm thin free-standing wafers into today's manufacturing lines result in cracking, creating a yield-based disincentive. Due to the brittle nature of silicon, wafer breakage is the major concern due to the high stress that is induced during processes in manufacturing lines. In this paper, we describe an improved method for edge micro-crack detection that can help enable low-capex, thin free-standing Si wafers. We present a method of detecting and measuring cracks along wafer edges by using a dark-field IR scattering imaging technique which enables detection of edge cracks at the micron scale. Keywords: Capex; polysilicon; thin free-standing wafer; edge crack detection; IR scattering; dark-field imaging, United States. Department of Energy (Grant DE-EE0007535)

Nature employs self-assembly to fabricate the most complex molecularly precise machinery known to man. Heteromolecular, two-dimensional self-assembled networks provide a route to spatially organize different building blocks relative to each other, enabling synthetic molecularly precise fabrication. Here we demonstrate optoelectronic function in a near-to-monolayer molecular architecture approaching atomically defined spatial disposition of all components. The active layer consists of a self-assembled terrylene-based dye, forming a bicomponent supramolecular network with melamine. The assembly at the graphene-diamond interface shows an absorption maximum at 740 nm whereby the photoresponse can be measured with a gallium counter electrode. We find photocurrents of 0.5 nA and open-circuit voltages of 270 mV employing 19 mW cm−2 irradiation intensities at 710 nm. With an ex situ calculated contact area of 9.9 × 102 μm2, an incident photon to current efficiency of 0.6% at 710 nm is estimated, opening up intriguing possibilities in bottom-up optoelectronic device fabrication with molecular resolution., Two-dimensional, self-assembled heteromolecular networks often lack functionality. Here the authors study the photoresponse of self-assembled heteromolecular networks, while controlling their positions and interfaces at an atomic level, suggesting bottom-up assembly of optoelectronics devices.