Symposium EN02 : Advances in Perovskite Solar Cell Devices and Applications

Due to interesting features of direct band gap, high and balanced carrier mobility, long electron-hole diffusion length, low non-radiative Auger recombination, and high internal quantum efficiency, perovskite solar cells (PVSCs) have been consideration as the great potential candidate for the next-generation high-performance photovoltaics. Various approaches have been proposed to improve device performances, and enhance power conversion efficiency (PCE) up to 22%. However, limited works discuss the loss mechanism and quantify the efficiency loss for perovskite solar cells.
In this work, we will unveil the loss mechanism and quantify the loss factors of perovskite solar cells. Through investigating the device performance of various fabricated perovskite solar cells, the three dominant loss factors of optical loss, non-radiative recombination loss, and ohmic loss are identified quantitatively. The perovskite-interface induced surface recombination, ohmic loss, and current leakage are also analyzed. Our theoretical and experimental results show that for experimentally optimized perovskite solar cells with the power conversion efficiency of 19%, optical loss of 25%, non-radiative recombination loss of 35%, and ohmic loss of 35% are the three dominant loss factors for approaching the 31% efficiency limit of perovskite solar cells. We also find that the optical loss will climb up to 40% for a thin-active-layer design. Moreover, a misconfigured transport layer will introduce above 15% of energy loss. Finally, the perovskite-interface induced surface recombination, ohmic loss, and current leakage should be further reduced to upgrade device efficiency and eliminate hysteresis effect. Consequently, the work offers a guideline to the researchers for optimizing perovskite solar cells and ultimately approaching the Shockley-Queisser limit of photovoltaics [1].


[1] W.E.I. Sha#, H. Zhang#, Z.S. Wang, H.L. Zhu, X. Ren, F. Lin, A.K.-Y. Jen, W.C.H. Choy*, Adv. Energy Mater., in press.

A major industrial application of perovskite photovoltaic cells will be large area fabrication of extremely lightweight flexible power sources in demand of electric vehicles and self-charging small devices in IoT society. Such application is not easy for Si solar cell, in particular for the purpose of power devices useful under weak indoor light. Perovskite solar cell capable of high voltage output can work with high performance even under very low light intensity. We fabricated Cs-doped FAMA perovskite photovoltaic cells on glass substrate [1] and plastic film substrate (125 um); the latter works with efficiency 15-18% exhibiting relatively small ideality factor <1.5 in intensity dependence of Voc [2]. The cell maintains high voltage under indoor illumination, indicating the usefulness of the device as power source to the circuitry of wireless IoT devices. The plastic film perovskite device showed robust stability against mechanical bending over 1000 times. Device was fabricated by low temperature preparation of metal oxide electron transporting layer (ETL). By tuning the composition of ETL at junction structure, hysteresis was successfully removed. Heat-resistant and hydrophobic hole transport materials replaced spiro-OMeTAD to improve the device stability against ambient air and long-term light exposure. We focused on use of low temperature-prepared AM-free FA/Cs perovskite absorber in glass-based and plastic-based cells to enhance thermal resistance. The FA/Cs P3HT perovskite cell was tolerant of large temperature changes (-80 to +100C). The device durability against thermal impacts is a significant issue in R&D but it is improving by tuning the perovskite and hole transporting materials. Perovskite cells were also subjected to stability examination against exposure to high energy particle radiation such as electron and proton for the purpose to know radiation tolerance in comparison to Si, GaAs-based commercial solar cells. Based on these data, versatile applications of lightweight perovskite photovoltaic devices in IoT and smart sustainable system industries will be discussed.

References
[1] T. Singh and T. Miyasaka, Adv. Energy Mat. 2017, DOI: 10.1002/aenm.201700677
[2] T. Singh and T. Miyasaka, submitted.

Since the first report on the solid-state perovskite solar cell with power conversion efficiency (PCE) of 9.7% and 500 h-stability in 2012 by our group, perovskite photovoltaics have received great attention. As a result, the highest PCE of 22.1% was reported in 2017. It is believed that perovskite solar cell is promising next-generation photovoltaics due to superb performance and very low cost. Although high photovoltaic performance was demonstrated, perovskite solar cell is suffering from current-voltage hysteresis. Since hysteric perovskite solar cell cannot guarantee long-term stability, development of methodology toward hysteresis-free perovskite solar cell is important. The hysteresis in perovskite solar cell is especially pronounced in normal mesoporous structure having TiO₂ electron transfer layer. Inverted structure was proved to show non-hysteric behavior, which is however inferior to the normal structure in terms of PCE. Thus, it is important to explore effective ways to remove hysteresis in normal perovskite solar cell employing TiO₂. In this talk, defect, both bulk and surface defects, is emphasized as origin of the hysteresis. Interfacial engineering is found to be one of effective methods to reduce hysteresis and improved improve stability simultaneously. However, interfacial engineering might not be sufficient to remove hysteresis completely. We successfully discovered a universal approach toward hysteresis-free perovskite solar cell, which will be discussed in detail.

Organic-inorganic hybrid perovskite solar cells (PeSCs) are a promising solar technology with rapidly increasing power conversion efficiency (PCE). One of the key advantages of PeSCs is their solution processability. This allows PeSCs to be manufactured by cost-effective industrial roll-to-roll processes. However, rapid progress in the technology has been predominantly made by spin coating, a laboratory process that is not compatible/transferable to the roll-to-roll process. Typically, only a small fraction of reported processes developed by spin coating are applicable to the roll-to-roll process and, therefore, process re-optimization is required for the roll-to-roll process. CSIRO has been developing deposition processes by roll-to-roll compatible deposition methods and actual roll-to-roll processes. In this presentation, various approaches used in slot die coating of perovskite layers in batch and roll-to-roll processes will be presented. To realize a defect-free uniform perovskite layer, various deposition parameters including deposition temperature, coating speed, additives and drying methods are optimized. The sequential deposition process was modified to be suitable for the roll-to-roll process, producing PeSCs on flexible substrate with up to 11% PCE. The more ideal one-step deposition was also developed by additive and blowing-assisted slot die coating, and roll-to-roll produced PeSCs showed over 11% PCE without hysteresis. Hot deposition has also been found to be suitable in the roll-to-roll process. Recent progress on this process will be also presented.

Two-dimensional metal halide perovskites (2D-MHP), first synthesized in the 1990’s ^[1], have recently been the subject of considerable attention. In addition to interesting (opto)electronic properties linked to their reduced dimensionality, they appear to exhibit higher resistance to the environment, e.g., moisture, than their 3D-MHP counterparts, thereby offering great potential for various applications. The importance of these materials warrants in-depth investigations of their electronic properties.^[2–4] Here we present recent electronic structure measurements of a series of solution-processed films of 2D butylammonium methylammonium lead iodide compounds, BA₂MA_n-1Pb_nI_3n+1, n=1 - 4. XRD, AFM, UV-vis absorption, and ultra-violet and inverse photoemission spectroscopies are used to investigate these compounds. We measure valence and conduction band spectra, and determine ionization energy (IE), electron affinity (EA) and single particle gap as a function of n. We find that the single particle gap decreases from 2.77 eV for n=1 to 1.87 eV for n=4 (and 1.6 eV for the 3D-MHP MAPbI₃), with IE decreasing and EA increasing in a nearly symmetric fashion, in contrast to previous results.^[2] We use the single particle gap and the onset of optical absorption at the exciton peak to calculate the exciton binding energy E_B. In agreement with previous results, E_B is found to be large for n=1 and 2 (390 and 110 meV, respectively). However, we find that the exciton binding energy decreases very rapidly thereafter, reaching below 50 meV by n=3. Finally, a simple model is presented to justify the electron and hole levels and the single particle gap in these quantum wells structures.

[1] D. B. Mitzi, C. A. Feild, W. T. A. Harrison, A. M. Guloy, Nature 1994, 369, 467.
[2] D. H. Cao, C. C. Stoumpos, O. K. Farha, J. T. Hupp, M. G. Kanatzidis, J. Am. Chem. Soc. 2015, 137, 7843.
[3] K.-G. Lim, S. Ahn, Y.-H. Kim, Y. Qi, T.-W. Lee, Energy Environ. Sci. 2016, 9, 932.
[4] K. Yao, X. Wang, Y. Xu, F. Li, L. Zhou, Chem. Mater. 2016, 28, 3131.

Recently, organometal halide perovskite solar cells (PSCs) have received great attention. The power conversion efficiency (PCE) of PSCs have shown a dramatic increase and certified PCEs adopting mixed organic cations and halide anions have reached up to 22%. The PCE is considerably affected by photovoltaic property of each component of a PSC. Particularly, because crystal quality of materials is strongly concerned with the electronic properties such as carrier transport, investigation of detailed crystallographic information of the perovskite light absorber is essential. In spite of the significance in the crystallographic information, however, microstructural observation for crystal structure analysis of the perovskite layer has not been actively conducted. In this talk, we will report a microstructural observation about phase coexistence in the perovskite light absorber through transmission electron microscope (TEM) observation.

To obtain the crystallographic information of the perovskite light absorber, a pure methylammonium lead iodide (MAPbI₃) layer was formed through spin-coating method assisted by antisolvent in a planar type PSC (Au/Spiro-MeOTAD/MAPbI₃/TiO₂/FTO/Glass). MAPbI₃ precursor solution used is 1.4 M and the spin-coated MAPbI₃ film was annealed at 100 ^oC for 30 min.

Surprisingly, during the high resolution (HR) TEM observation, we found the coexistence of tetragonal and cubic structures in the perovskite layer. Additionally we found such a mixture condition affetcs on the photovoltaic peroformance of the perovskite solar cells. This new observation is expected to be an important clue of the enhancement of perovskite crystal quality for highly efficient PSCs.

Perovskite solar cells have emerged as a prominent candidate in the field of photovoltaics. With their efficiency comparable to silicon solar cells and easy of fabrication involving low temperature solution processing and self-assembly, Perovskites are an attractive prospect to reduce costs and implement large scale solar energy adoption. However, the main hurdle in the way of success of Perovskites is their sensitivity to moisture. We present passivation schemes to protect organic–inorganic lead halide perovskite solar cells from degradation when exposed to moisture and also encapsulation schemes for vacuum environment like space applications. In specific, hydrophobic BAI (Butyl Ammonium Iodide) that has been used for passivation of 2D layered perovskite solar cells [1] improves perovskite stability and efficiency even in solution processed 3D perovskites [2, 3]. The device architecture in our experiments is ITO Indium tin oxide/ NiO Nickle Oxide/ MAPbI₃ Methyl Ammonium Lead Iodide/ BAI/ PCBM [6, 6]-phenyl-C₆₁-butyric acid methyl ester/Ag Silver. For fabrication of the passivated cells, a layer of BAI solution was span at 4000 rpm for 20 seconds followed by heat at 100°C for 10 minutes, over the perovskite layer. The concentrations of the BAI solutions varied from 2mg/ml to 10mg/ml. 2mg/ml gave the best results so far with efficiency of 12.1% after 18 days for passivated cells as compared to 12.3% for as-fabricated cells without passivation which degraded to about 9% efficiency after the 18 day interval (measured under solar simulator spectra of AM 1.5G). The suggested hypothesis of the mechanism of the BAI passivation, which will be studied in this paper, is that the long chains of the butyl group extend outwards from the perovskite layer blocking other molecules to react with the perovskite layer and in effect passivating the perovskite surface. In x-ray diffraction (XRD) analysis, there is a pronounced peak for PbI₂ seen for 6-day old cells without passivation which is not present in the 6-day old passivated cells, further providing evidence for passivation using BAI. The absorption between 500 nm and 1000 nm wavelength for as-grown perovskite and 6-day old passivated perovskite agree with each other and the curves are almost identical. We further investigate the passivation scheme, structure, and potential improvements using x-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and opto-electronic measurements.

Success of passivation schemes will propel the promising perovskite solar cell technology from research in inert glove boxes to industry-wide adoption.

References:
1) 10.1038/nature18306
2) 10.1021/nn5036476
3) 10.1038/nenergy.2017.135

We elucidate how the surface chemistry of compact TiO₂ electron-selective contacts dramatically affects the nucleation, growth, bulk composition and energetics of device-relevant hybrid perovskite (PVSK) thin films that are processed via solution- and vacuum-based deposition methods. The surface chemistry and energy level alignment of compact TiO₂ thin films, which are grown by either chemical vapor deposition (CVD) or sol-gel methods, is systematically modified by combinations of annealing temperature/environment, plasma activation and end-functional silane modification. The TiO₂ surface chemical composition (e.g., stoichiometry, hydroxyl concentration, monolayer composition, etc.) is quantified by high-resolution, monochromatic X-ray photoelectron spectroscopy (XPS), and the frontier orbital energetics are determined using ultraviolet photoelectron spectroscopy (UPS), which shows that the work function and energy level alignment of the TiO₂ contact can be tuned by approximately 1 V depending on the treatment conditions. In situ XPS and UPS measurements of incrementally co-evaporated methylammonium lead triiodide (MAPbI₃) films reveal that initial nucleation and subsequent growth of MAPbI₃ PVSK films strongly depends on the chemical functionality of the TiO₂ surface, where silane-modified surfaces surpress chemical reactions between the PVSK precursors (e.g., methylammonium iodide) and TiO₂ surface species (e.g., hydroxyls). X-ray diffraction (XRD) studies of solution-processed PVSK films based on methylammonium (MA) and formamidinium (FA) organic cations reveal that the bulk film composition and crystallinity are controlled by the TiO₂ work function and surface energy, where low work function and low surface energy TiO₂ contacts show enhanced conversion of the precursors to the PVSK phase and higher crystallinity. In addition, scanning electron microsope (SEM) images show that the TiO₂ surface free energy, which is tailored by end-functional silane monolayers and plasma activation, and processing conditions have a strong influence on the morphology (e.g., grain size) of both vacuum- and solution-processed PVSK films based on MA and FA organic cations. These combined studies show how the formation mechanism, interfacial/bulk energetics, chemical composition, crystallinity and morphology of device-relevant PVSK active layers is critically dependent on the surface chemistry and energetics of TiO₂ contacts, which have significant consequences related to the processing, stability and operation of next-generation optoelectronic device platforms.

Organic-inorganic perovskites solar cells have emerged in a short span of time as a potential photovoltaic technology. However, the instability of these PSCs under operational conditions has impeded their large-scale deployment. Such instability issues could arise from the degradation of absorber layer itself or by virtue of the charge extraction layers. To mitigate these issues, we have explored the following promising strategies. 1) Crystal cross-linking to passivate the perovskite surfaces and grain boundaries; as the ionic nature of organic-inorganic perovskites renders them inherently sensitive towards reactive species, such as water molecules present in the form of a moisture. 2) Identification of new absorber material which contains less electrophilic organic cations and thermodynamically more stable inorganic frame work, and 3) Using all-inorganic charge extraction layers which are extremely cheap and stable. In my presentation, how one could achieve extraordinary operational stability by employing these highly promising strategies will be discussed.

References:

1. Li, X.; Dar, M. I. et al. Nat. Chem. 2015, 7, 703-711.
2. Arora, N., Dar, M. I.* et al. Science 2017, 358, 768-771
3. Arora, N., Dar, M. I.* et al. Nano Letters 2016, 16, 7155-7162.

Tin fluoride (SnF₂) is widely used as an effective additive for lead-free tin-based perovskite solar cells. However, the function of SnF₂ and the mechanism in improving the film morphology are still not clear. In this work, it is clearly demonstrated that SnF₂ can play a crucial role in the crystal nucleation process. Due to the limited solubility, SnF₂ creates more nucleuses for the crystal growth and therefore enables more uniform thin film with high coverage. It is confirmed that this mechanism can be applied to the growth of both thin film and single crystal. As a result of tin fluoride-assisted heterogeneous nucleation, an MASnIBr₂-based perovskite solar cell with a high and stable power conversion effciency of 3.70% is demonstrated.^[1]
References:
[1] Min Xiao, Jia Zhu et al, Adv. Opt. Mater. 2017, 1700615.

A new type perovskite, a hybrid material with both organic and inorganic components, has appeared to be a wonder for its excellent optical absorption, long range charge-carrier diffusion and apparent tolerance to defects. In the last few years, it has been emerged as a primary candidate material for various photovoltaic, optoelectronic and photoelectronic applications. In just a few years, the power conversion efficiency (PCE) of the perovskite solar cells has been improved from 3.8% to >22%. Moreover, the solar cell fabrication processes based on the planar architecture have been particularly enthusiastic thanks to their low temperature fabrication and compatibility with a range of substrates. Comparing solution deposition and vacuum deposition, the vacuum processes for thermal co-deposition and sequential deposition of PbCl₂ and CH₃NH₃I materials are recognized as efficient means to prepare perovskite film with good uniformity and high surface coverage.
A vacuum deposition process has been developed to fabricate high efficiency perovskite solar cells with high stability using alternating layer-by-layer vacuum deposition. The new deposition process allows us to relax the strict deposition monitoring and control measures, while realizing superior uniformity in film morphology, surface coverage and smoothness, together with crystalline phase purity. The power conversion efficiencies for the planar device is as high as 19.6% on rigid glass substrate, the highest reported at the time. More importantly, we have developed a superior low temperature TiO₂ coating and transferred the cell fabrication process onto lightweight flexible polymeric substrate. The highest cell efficiency achieved was over 16%, it is also the highest efficiency among the flexible perovskite cells reported. Our current status for the rigid thin film cell efficiency is over 21.5% and that for the flexible device over 18.3%, both are the highest for their respective category. Meanwhile, the devices show very good stability over long term exposure in ambient with very low degradation. After a representative cell was exposed in ambient lab condition for a year, its final cell efficiency is as high as over 95% of its initial efficiency with its degradation accounts for only smaller than 5%. Further analysis on the stability of the perovskite solar cells will be discussed.
We have also developed a series of single-crystalline perovskites with superior stability and optoelectronic performance.
References：
[1] D. Yang, R. Yang, X. Ren, X. Zhu, Z. Yang, C. Li, S. Liu*, Advanced Materials, http://dx.doi.org/10.1002/adma.201600446.
[2] D. Yang, R. Yang, J. Zhang, Z. Yang, S. Liu, C. Li, Energy Environ. Sci. 2015, 8, 3208.
[3] D. Yang, Z. Yang, W. Qin, Y. Zhang, S. Liu, C. Li, J. Mater. Chem. A 2015, 3, 9401.

One critical issue related perovskite solar cells is their relatively low stability compared with other thin film solar cell technologies. In htis talk, I will present our progress in understanding the intrinsic stability of perovskite materials related to material structure and operation conditions, including strain, ferroelastic domain boundaries, light, and defects. Many of these properties are unique to halide perovskites in good and bad way in terms the impact to solar cell stability. And our pregress on stability enhancement will also be presented in ehnahcing the overall device stability under real device operation condition. It is shown that efficient perovskite devices (PCE>20%) can be made stable for over six months under one sun illumination at evevrated temperatures.

The rapid progress in metal halide perovskites has generated great interest in the fabrication of tandems on silicon to enable the next generation of solar cells. Optimized light harvesting is essential to achieve the highest current density and efficiency in tandems. Specifically, high infrared transmission through the perovskite top cell is critical. We show here that texturing the top surface of the perovskite considerably reduces coherent reflections in the infrared.
The antisolvent method we use to deposit perovskite films results in a film with a rippled surface that micron-wide ridges with 300-nm-deep trenches. For comparison, we fabricate smooth perovskites using a two-step interdiffusion method. The smooth perovskites appear shiny, indicative of highly specular reflection. In contrast, the textured perovskites are grey on top and angle resolved transmission measurements confirm a significant increase in haze. To understand the mechanism behind the formation of this textured perovskite surface, we performed stress measurements and found that this morphology is caused by wrinkling that occurs because a compressive stress of ~20 MPa arises after the antisolvent drip. However, perovskites fabricated without an antisolvent are shown to remain in tension throughout processing, and no texturing is observed in the films. Experiments are underway to understand the factors that determine whether there is compressive or tensile stress in the films. We will point out examples in the literature in which scanning electron microscope images revealed the same buckling pattern we have observed, but the mechanism of the patterns formation was not identified.
To evaluate the effect of the textured perovskite surface, we fabricated 4-terminal perovskite-silicon tandems. When comparing perovskites with smooth and textured surfaces, we observed a ~1 mA/cm² increase in current density in the silicon bottom cell under the textured surfaces. Reflectance measurements confirm a decrease in infrared reflections by the textured perovskite, leading to higher transmittance and an increased EQE in the silicon. With this higher current density, we achieve >25% efficient perovskite-silicon tandems.

Since the first reports of metal halide perovskite solar cells, intense research focus has centered on CH₃NH₃PbI₃, and related family of materials. Although the initial priority of research was on increasing solar cell efficiencies, focus has now shifted to improve the stability of these solar cells, especially for long-term atmospheric exposures.
The hydrophilicity and volatility of methylammonium cations (MA⁺) turn the archetypical metal halide perovskite vulnerable to degradation through humidity and thermal exposure. The prospects for advancing device stability are contingent upon exploring structural variants, including bilayer 2D/3D perovskites and surface passivation approaches which yield more environmentally stable PSCs. Through molecular design of organic A-site cations, we developed a series of Ruddlesden-Popper perovskites, (CHMA)₂(MA)_n-1Pb_nI_3n+1, in which a thin lower dimensional perovskite was formed over the 3D perovskite layer in one-step deposition. The bilayer 2D/3D hybrid perovskites possess striking moisture resistance and displayed high ambient stability up to 65 days. In order to modulate the formation of bilayer 2D/3D perovskites for high efficiency PSCs, top surface of well-formed 3D perovskite is converted into 2D perovskite in two-steps deposition, resulting a better bilayer perovskite layer in term of perovskite quality, morphology and interfacial contacts. Not only the enhanced moisture tolerance from the hydrophobicity of long chain organic cations, this approach could also suppress surface defects and vacancies of solution-processed perovskite which in turn results in higher power conversion efficiency (PCE) with excellent moisture stability.
Surface passivation approach is another way to improve the stability of PSCs. Different from conventional organic cations, utilization of hydrophobic fluorinated organic salt to passivate highly efficient triple-cations perovskite without triggering the formation of 2D perovskite has been demonstrated. The passivated perovskite thin film displays narrower band gap (halide substitution), longer PL lifetime and a notable improvement in PCE. More importantly, the passivated PSCs show remarkable stability for more than 169 days under ambient conditions at an average relative humidity (RH) of 55% without any significant change in its initial PCE. These findings provide new insights into intrinsic bilayer perovskite formation and passivation through careful design of the organic components in the perovskite structure to achieve a more stable and highly efficient perovskite material for photovoltaics.
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Perovskite solar cells (PSCs) have been received great interest in recent years due to the rapid increase of their solar power conversion efficiency (PCE). However, in order to commercialize PSCs there are still many challenges need to be solved, including fabricating high efficiency and highly stable PSCs using simple, scalable, affordable and eco-friendly process technologies. In this talk, I will introduce several efforts our team have done towards this objective. First, by simply adding 4-tert-butylpyridine (tBP) into the PbI2 precursor solution to enhance its hydrophobicity, we can fabricate highly stable PSCs in ambient air with PCE > 12.5% using spin coating method. Then I will present our work on improving PSCs performance and stability using inorganic perovskite quantum dots (QDs). All-inorganic cesium lead halide perovskite (CsPbX3, X = Cl, Br, I) nanocrystals (NCs) have been prepared, which exhibit near-unity photoluminescence (PL) quantum yields, narrow emission peak widths and anion-tunable absorption/emission wavelengths. By introducing stable α-CsPbI3 QDs as an interface layer between the perovskite film and the hole transport material (HTM) layer to improve the energy band matching, the PCEs of devices have been increased from 15.17% to 18.56%, with substantial improvement on stability as well. Thirdly, we have developed a heat assisted spin-coating (HASP) process to fabricate PSCs with inverted structure and using PbAc2 as the precursor. This new process allows us to avoid using the toxic anti-solvents such as toluene and chlorobenzene. The PCEs can reach 19.12% on glass substrate and 14.87% on flexible PEN substrate. Over 80% of the initial PCEs can be remained for 20 days in air without encapsulation, and for 60 days under simple encapsulation. Finally, I will introduce our work on developing new hole transporting materials to replace Spiro-OMeTAD in order to fabricate high-efficiency and stable PSCs with much lower cost.

Thin-film solution-processed solar cells based on a hybrid organohalide lead perovskite semiconductor have achieved certified power conversion efficiencies (PCEs) exceeding 22%. Early efforts to bring this technology from the lab to the market quickly revealed certain disadvantages of perovskites, including the use of toxic lead, the diffusion of ionic defects causing a hysteresis effect, long-term stability, water sensitivity, the complexity of the ink formulation, as well as the cost efficiency and compatibility of the interface materials. Of these, a critical limitation on commercializing this technology is the absence of suitable hole-transporting materials (HTMs) that offer full performance without sacrificing long-term stability, and with low material costs and printability from green solvents.

In this work¹, we present a generic interface architecture that combines solution-processed, reliable, and cost-efficient hole-transporting materials, without compromising efficiency, stability or scalability of perovskite solar cells. Tantalum doped tungsten oxide (Ta-WOx)/conjugated polymer multilayers offer a surprisingly small interface barrier and form quasi-ohmic contacts universally with various scalable conjugated polymers. Using a simple regular planar architecture device, Ta-WOx doped interface-based perovskite solar cells achieve maximum efficiencies of 21.2% and combined with over 1000 hours of light stability based on a self-assembled monolayer. By eliminating additional ionic dopants, these findings open up the whole class of organics as scalable hole-transporting materials for perovskite solar cells.

References
1. Yi Hou. et al. A generic interface to reduce the efficiency-stability-cost gap of perovskite solar cells. Science, In press.

In recent few years, the efficiency of organic-inorganic metal halide perovskite-based solar cells (PSCs) has been improved rapidly, because of the significant efforts in materials development and device fabrications. Hole transporting materials (HTMs) play an important role to the PSCs in charge extraction and interface modification.^[1] Currently, the most extensively studied and applied HTM for perovskite devices is 2,2’,7,7’-tetrakis(N,N’-di-p-methoxyphenylamine)-9,9’-spirobifluorene (Spiro-OMeTAD),^[2-4] which is expensive because of its relative long synthesis and purification processes. For achieving large area solar cells, devices with satisfactory efficiencies and low-cost materials are urgently required. Thus, the development of high efficient and low-cost HTMs is very necessary for promoting PSCs from lab scale experimentation towards industrial scale application.

We have been working on the development of new organic and polymeric HTMs. For example, two HTMs were synthesized by connecting the arylamine side groups to the both sides of thiophene and/or benzene moieties, named as HTM1 and HTM2, respectively, in which the synthesis procedures are much shorter and simpler than those of Spiro-OMeTAD. When applied in the PSCs devices, the thiophene-based HTM1 and benzene-based HTM2 showed short circuit photocurrent densities (J_scs) of 21.08 mA cm^-2 and 15.83 mA cm^-2 , open circuit voltages (V_ocs) of 1.01 V and 0.79 V and fill factors (FFs) of 0.59 and 0.46 respectively. The devices with the two HTMs showed average power conversion efficiencies (PCEs) of 13.7% and 6.4 % with best PCEs of 14.7% and 7.5%, while the device with Spiro-OMeTAD has a best PCE of 15.8% under the similar device preparation method and measurement conditions. These observations indicated that thiophene-based HTM1 has a comparable performance to the Spiro-OMeTAD. These results showed that selecting a proper π-linker is important for the performance of the HTMs. And the simple material of HTM1 is a promising HTM with the potential to replace the expensive Spiro-OMeTAD due to its comparable performance with much simpler synthesis route and much reduced cost (less than 1/10 folds of that of Spiro-OMeTAD).

Reference
1. Calió, L.; Kazim, S.; Grätzel, M.; Ahmad, S., Angew. Chem. Int. Ed. 2016, 55, 14522-14545.
2. Ding, I. K.; Tétreault, N.; Brillet, J.; Hardin, B. E.; Smith, E. H.; Rosenthal, S. J.; Sauvage, F.; Grätzel, M.; McGehee, M. D., Adv. Funct. Mater. 2009, 19, 2431-2436.
3. Jeon, N. J.; Lee, H. G.; Kim, Y. C.; Seo, J.; Noh, J. H.; Lee, J.; Seok, S. I., J. Am. Chem. Soc. 2014, 136, 7837-40.
4. Liu, D.; Kelly, T. L., Nat. Photon. 2014, 8, 133-138.

All-solid-state organic-inorganic lead perovskites have attracted much attention, due to its rapid and tremendous development within the photovoltaic field. However, toxic lead-containing still a big issue of this field. We developed and characterized methylammonium tri-halide tin perovskites (MASnIBr_2-xCl_x) for carbon-based mesoscopic solar cells free of the hole-transporting layers. There is a question whether iodide and chloride co-crystalize together in a perovskite structure. We found that the iodide and chloride can form the stable single phase, but based on certain amounts of bromide within the structure. Three different halides (Cl, Br and I) co-crystalized inside the crystal of tin perovskites, MASnIBr_2-xCl_x, according the stoichiometric ratios, from x = 0 (SnBr₂/SnCl₂ = 100/0) to x = 0.5 (SnBr₂/SnCl₂ = 75/25). When the SnCl₂ ratio was higher than 25 % (x > 0.5), phase separation occurred to generate MASnI_3-yBr_y and MASnCl_3-zBr_z these two phases. The MASnIBr_1.8Cl_0.2 (SnCl₂ = 10 %) device showed the best device performance with V_OC/mV = 380, J_SC/mA cm^-2 = 14.0, FF = 0.573 and PCE/% = 3.1 with great long-term stability and reproducibility. Transient PL decay measurements were carried out to show the intrinsic problem of tin-based perovskites with the averaged lifetimes less than 100 ps, for which the MASnIBr_1.8Cl_0.2 film featured the longest lifetime over the others to account for its greater device performance than the others. What is more, we also investigated the effect of a bi-functional ammonium cation, EOAI, on structure and energy levels of a tin-based perovskite, FASnI₃. Following the increase of EOAI ratio within the EOA_yFA_(1-y)SnI₃, the valence band maximum of tin perovskite can be modified linearly from deficient -4.91 eV to appropriated -5.50 eV and the crystal structure will transform from 3D to a 2D/3D hybrid structure. Finally, the power conversion efficiency of tin-based carbon electrode perovskite solar cells improved 5 times, which that compared with the FASnI₃ reference device. X-ray diffraction (XRD) analysis, ultraviolet photoelectron spectra (UPS) and electrochemical impedance spectra (EIS) were performed to understand the crystal structures and the photovoltaic performances of the devices for this series of tin perovskite solar cells.

Perovskite solar cells have shown remarkable efficiencies beyond 21%, through organic and inorganic cation alloying. However, the role the inorganic cations plays is not well-understood. By using synchrotron-based micro X-ray fluorescence, we show that alkali metals K, Rb and Cs, mostly segregate into well-defined pockets without fully incorporating. In this presentation I will show how these alkali metals influence the distribution of other elements and how that improves electronic dynamics, including lifetimes above 3 µs and homogenous photobleaching in transient absorption visualized by ultrafast microscopy. Solar cell performance is compromised by large amounts of Rb/K and high efficiency is seen with small amounts of the alkali metal. Remarkably, the high concentration of Rb and K agglomerations do not affect the open-circuit voltage, average lifetimes and photoluminescence distribution, further indication of perovskite’s defect tolerance.

We fabricated ZnO nanorod arrays decorated with Au nanoparticles for use as the electron transport layers in perovskite solar cells. We show with current-voltage measurements, impedance analysis and photoluminescence spectroscopy that the Au nanoparticles reduce recombination losses at the ZnO-perovskite interface. We compare the performance of double cation-mixed halide and triple cation-mixed halide perovskites in this architecture. The use of Au nanoparticles in the solar cells resulted in an increase in the open circuit voltage (V_oc) and fill factor, leading to an increase in the maximum power conversion efficiency from 11.52 % to 12.62 % (double cation) and from 11.9 % to 12.66 % (triple cation). We discuss these results in terms of trap filling and surface passivation by the Au nanoparticles at the ZnO-perovskite interface.

Grain boundaries (GBs) and grain surfaces (GSs) are the most prominent microstructural features that play significant roles in determining the physical properties and photovoltaic functions of the organic-inorganic halide perovskite (OIHP) thin films. While enormous effort has been devoted to modifying/functionalizing the OIHP GBs and GSs and making them benign, the microstructures in these modified/functionalized OIHP thin films have been somehow random and/or uncertain. Herein, we demonstrate several unique chemical approaches to functionalize the OIHP GBs and GSs confocally at the nanoscale. The key to the unprecedent success of the confocal functionalization is the strong molecular interaction between OIHPs grains and functionalizing agents. Microscopic characterization methods including analytical transmission electron microcopy have been employed to confirm the precisely controlled microstructures in our OIHP thin films. Combined experimental and theoretical studies have showed that the confocal functionalization of OIHP GBs and GSs not only leads to electronic passivation of defects, but also protects OIHP grains from moisture/oxygen ingression and unfavorable phase transformation. As a result, highly efficient and stable perovskite solar cells are demonstrated. The concept of confocal functionalization of the OIHP GBs and GSs is paving the way for developing higher-performance perovskite solar cells of the future.

To date, solid-state perovskite solar cells (PSCs) have been developed with power conversion efficiency exceeding 22%<span style="font-size:10.8333px">.</span> PSCs comprise entirely of Earth-abundant elements, can be manufactured in various geometries, can be solution-processed, and possesses ideal solar cell material characteristics. This meteoric rise in PCEs has created a renaissance in studies on perovskites for light-emitting diodes, lasing, photodetectors etc. In addition to its outstanding performance, perovskite solar cell have a simple and low-cost solution-based fabrication process which opens the possibility of commercialization. We have demonstrated upscaling of monolithic perovskites solar device with high efficiency and stability using solution based processing and low cost electrode materials. Monolithic devices of size 10cm x 5cm (active area 30cm²) and 10cm x 10cm (active area 70cm²) with efficiency 10.5% and 10.74% have been realized. Fabrication of large area solar cells were done by fully screen-printed process along with slot die coating of perovskite solution. Perovskite infiltration process and quality of carbon (conductivity and surface area) plays key roles in improving the device performance. Recent progress in the pursuit of 900 cm² monolithic devices and a 3S-3P module comprised of 10x10 substrates encapsulated in a window with junction box will also be covered.

Understanding and controlling charge and energy flow in state-of-the-art semiconductor quantum-wells has enabled high-efficiency optoelectronic devices. Two-dimensional Ruddlesden-Popper layered perovskites (RPPs) have recently emerged as an alternative to the classic bulk organic-inorganic hybrid perovskites, mainly due to significantly improved photo- and chemical-stability in optoelectronic devices [1]. Few recent encouraging developments in optoelectronic applications, notably in energy harvesting and light emitting [1-3], have already been demonstrated in these two-dimensional layered perovskites. RPPs are solution-processed quantum-wells wherein the band gap can be tuned by varying the perovskite layer thickness, which modulates the effective electron-hole confinement. We report that, counterintuitive to classical quantum-confined systems where photo-generated electrons and holes are strongly bound by Coulomb interactions or excitons, the photo-physics of thin films made of Ruddlesden-Popper perovskites with a thickness exceeding two perovskite crystal-units (>1.3 nanometers) is dominated by lower energy states associated with the local intrinsic electronic structure of the edges of the perovskite layers [3]. These states provide a direct pathway for dissociating excitons into longer-lived free-carriers that significantly improve the performance of solar cell devices.

References
[1] Tsai et al., Nature (2016), 536, 312-316.
[2] M. Yuan et al., Nat. Nanotechnol. (2016), 11, 872-877.
[3] Blancon et al., Science (2017), 355, 1288-1292.

Perovskite solar cells (PSCs) technology has attracted a big attention in the solar cell community due to their exceptional performance as the power conversion efficiency (PCE) surged to world record 22%. Though the highest PCE of 22.1% up to date used PTAA as polymeric hole transporting material (HTM) but it has some disadvantages such as super high cost and reproducibility. In contrast to polymers, small molecules possess advantages, e.g. batch-to-batch reproducibility, easy purification, high purity & definite structure. Among small molecular HTMs for PSCs, Spiro-OMeTAD has been employed intensively as standard HTM. Despite of the remarkable performance (20.8%), some main drawbacks of Spiro-OMeTAD, including high cost and multistep synthesis, can hamper the progress of low cost and large area flexible PSCs.^{1, 2}

Herein, six new simple cost efficient solution processable small molecular HTMs, namely TPA-BPV-TPA, TPA-BP-TPA, TPA-TVT-TPA, TPA-NAP-TPA, TPA-ANT-TPA, and ACE-ANT-ACE, using triphenylamine(TPA) and acenaphthylene(ACE) as end-capping groups with different cores are reported. The variation in cores is aimed to change the highest occupied molecular orbital (HOMO) energy level of each HTM to match ot with the HOMO level of perovskite, enhancing the hole extraction and efficient performance.^3-6 Thereafter, TPA-ANT-TPA, ACE-ANT-ACE, TPA-BPV-TPA and TPA-BP-TPA were implemented in mesoporous perovskite devices whereas TPA-TVT-TPA and TPA-NAP-TPA were fabricated in inverted ones. In case of conventional layouts, while doped TPA-BPV-TPA based devices give efficiency around 16.42%, dopant-free TPA-ANT-TPA ones achieves an overall efficiency of 17.5%. Notably, both TPA-BPV-TPA and TPA-ANT-TPA exhibits an impressive stability compared to Spiro-OMeTAD under identical aging condition. Additionally, TPA-ANT-TPA possesses the low synthetic cost of $67/g compared to that of Spiro-OMeTAD ($91/g). For inverted architecture, the devices based on pristine TPA-TVT-TPA and TPA-NAP-TPA as HTMs are found to be of 16.32% and 14.63%, respectively. The cut-price and straightforward synthesis with elegant scale up makes these classes of materials important for the industry to produce high-throughput printed perovskite solar cells for large area applications.

[1]. D. Bi, W. Tress, M. I. Dar, P. Gao, J. Luo, C. Renevier, K. Schenk, A. Abate, F. Giordano, J.-P. C. Baena, J.-D. Decoppet, S. M. Zakeeruddin, M. K. Nazeeruddin, M. Grätzel, A. Hagfeldt, Sci. Adv. 2016, 2, e1501170
[2] N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, Nature 2015, 517, 476
[3] H. D. Pham, Z. Wu, L. K. Ono, S. Manzhos, K. Feron, N. Motta, Y. Qi, P. Sonar, Adv. Electron. Mater. 2017, 3, 1700139
[4] H. D. Pham, H. Hu, K. Feron, S. Manzhos, H. Wang, Y. M. Lam, P. Sonar, Solar RRL. 2017, 1, 1700105
[5] Patent filed
[6] H. D. Pham, T. T, Do, J. Kim, C. Charbonneau, S. Manzhos, K. Feron, W. C. Tsoi, J. Durrant, S. M. Jain, P. Sonar, manuscript submitted to Adv. Ener. Mater.

Perovskite is a promising light harvester for use in photovoltaic solar cells. In recent
years, the power conversion efficiency of perovskite solar cells has been dramatically
increased, making them a competitive source of renewable energy.
This work will discusses new directions related to organic inorganic perovskite and their applications in solar cells.

In low dimensional systems, stability of excitons in quantum wells is greatly enhanced due to the confined effect and the coulomb interaction. The exciton binding energy of the typical 2D organic-inorganic perovskites is up to 300 meV and their self-assembled films exhibit bright photoluminescence at room temperature.

In this work we will show the dimensionality in the perovskite structure. The 2D perovskite structure should provide stable perovskite structure compare to the 3D structure. The additional long organic cation, which is added to the perovskite structure (in the 2D structure), is expected to provide hydrophobicity, which will enhance the resistivity of the perovskite to humidity. Moreover we will demonstrate the use of 2D perovskite in high efficiency solar cells.

Organometal halide perovskite is used mainly in its “bulk” form in the solar cell. Confined perovskite nanostructures could be a promising candidate for efficient optoelectronic devices, taking advantage of the superior bulk properties of organo-metal halide perovskite, as well as the nanoscale properties. In this work, we present facile low temperature synthesis of two-dimensional (2D) lead halide perovskite nanorods (NRs). These NRs show a shift to higher energies in the absorbance and in the photoluminescence compared to the bulk material, which supports their 2D structure. X-ray diffraction (XRD) analysis of the NRs, demonstrates their 2D nature combined with the tetragonal 3D perovskite structure. In addition, by alternating the halide composition, we were able to tune the optical properties of the NRs. Fast Fourier Transform, and electron diffraction show the tetragonal structure of these NRs. By varying the ligands ratio (e.g. octylammonium to oleic acid) in the synthesis, we were able to provide the formation mechanism of these novel 2D perovskite NRs. 2D perovskite NRs are promising candidates for a variety of optoelectronic applications, such as light emitting diodes, lasing, solar cells and sensors.

In this study, we explored the formation of Pb-Sn binary perovskite solar cells using Pb(CH₃COO)₃ and SnI₂ as starting materials. We successfully fabricated CH₃NH₂Pb_0.75Sn_0.25I₃ perovskite solar cells with a power-conversion efficiency (PCE) of nearly 10%, using a fast crystallization process. The effects of annealing time and temperature on the film properties were investigated. We found that a short-duration annealing at 100-105^oC facilitated the crystallization of the CH₃NH₂Pb_0.75Sn_0.25I₃ phase. We also found that the molar ratio of MAI: (Pb+Sn) influenced the film properties. The incorporation of excessive Pb and Sn ions improved the power conversion efficiency of the perovskite solar cells.

Perovskite solar cells (PVSCs) have attracted great attention recently due to their high power conversion efficiency (PCE) with low cost despite their short history. Development of engineering method of perovskite crystallization has boosted the PCE of PVSCs up to 22.7% recently. Among the several fabrication methods, dripping anti-solvents to perovskite precursors in DMF:DMSO mixed solvents is one of the promising way to high quality perovskite films, leading to superior performances. It is known that lewis base and acid interaction by using DMSO forming intermediate states prevents abrupt crystallization. On the other hand, anti-solvents which are casted to perovskite precursors such as chlorobenzene and ethyl ether accelerate crystallization of perovskites by super-saturation. It is considered that kinetics of perovskite precursor during the formation of perovskite crystals directly result in optical and electrical properties of PVSCs. In this sense, we tried to focus on the kinetic control of Cecium (Cs)-mixed intermediate states during the formation of perovskite crystals. By putting the petri-dish on the perovskite substrates right after spin-coating with lower temperature for certain time (stage 1) and further annealing without petri-dish for 1h at 100 celsius (stage 2) for complete conversion to black phase of perovskite, evaporation kinetics of intermediate states can be easily controlled. The remaining DMF and DMSO vapors inside petri-dish decelerate decomposition of intermediate states at stage 1. The optimum temperature at stage 1 depends on types of perovskite compositions. The addition of cation dopants such as Cs, Rubidium (Rb) and Potassium (K) decreases intermediate state binding energy. Thus, for adequate control of intermediate states, lower temperature at stage 1 is required especially for cations-doped perovskite precursors. The mirror-like color, which is indicative of intermediate states, is retained for several seconds or minutes depending on the annealing temperature with petri-dish at stage 1. From the measurement of AFM and SEM, the grain sizes and the surface roughness are largely affected by the time at which intermediate states are retained at stage 1. Photoluminescence (PL) measurement also reveals that adequate control of vaporization kinetics of intermediates enhances the PL intensity, indicating that non-radiative recombination is suppressed. The engineering of kinetic control at stage 1 has resulted in high PCE recording 21.3% with negligible hysteresis for Cs-mixed PVSCs.

The past several years have witnessed a burgeoning of the research on halide perovskite photovoltaic (PV) materials. These materials are structurally distinct from all previous high-efficiency PV materials, such as Si, GaAs, CdTe, and CIGS. Recently, the halide perovskites have also been explored for other optoelectronic devices, such as LEDs. These breakthroughs have motivated us to study other perovskite materials that are potentially semiconductors, instead of dielectrics. Recently, we explored the chalcogenide perovskite materials in quest of suitable ones for PV applications. A number of promising candidates have been identified. In this talk, I will further explore two-dimensional chalcogenide perovskites. In particular, I will discuss the possibility of existence of such materials, including their structures, thermodynamic and kinetic stabilities from a density-functional-theory point of view. I will also discuss their electronic and optical properties in the context of nanoelectronic and optoelectronic applications.

Perovskite solar cells (PSC) exhibit the potential to be the next-generation thin-film photovoltaic technology, having already reached power conversion efficiencies (PCEs) of >22% for lab-scale devices within a few years. State-of-the-art PSCs utilize high temperature (> 450 °C) processed TiO₂ as the electron transport layer. However, upscaling techniques such as roll-to-roll processing on flexible substrates or inkjet printing on large area devices demand low temperature fabrication routes. In this work, we tackle this challenge using TiO₂ nanoparticles (TiO₂-np) instead, which are synthesized at only 72 °C. The TiO₂-np are crystalline in nature and the process allow precise control over particle size, doping, as well as dispersing capability in different solvents. Our results show that TiO₂-np can form compact films upon spin-coating, thus reducing recombination processes at the interfaces. When spin-coated CH₃NH₃PbI₃ is used as the perovskite absorber, devices exhibiting a high initial efficiency (> 19%) and a stabilized PCE of 18.2% (measured at constant voltage) can be realized. Furthermore, the synthesized nanoparticles demonstrate high compatibility (in terms of efficiency) with other perovskite absorber layers, including co-evaporated CH₃NH₃PbI₃ and triple cation perovskite, Cs_0.1(MA_0.17FA_0.83)_0.9Pb(I_0.83Br_0.17)₃. We also show that TiO₂-np can be used to achieve efficient PSCs both in thin (30 nm) and thick (75 nm) layer configurations. In addition, the doping of the TiO₂-np with niobium (Nb) results in a significant improvement for the thicker layers. Moreover, when dispersed in appropriate solvents, we demonstrate both inkjet printing and slot die coating processes for the TiO₂-np. PSCs fabricated with such processes show high initial PCE of >17%, paving the way for high throughput and digitally printed PSCs.

Bacteriorhodopsin (bR) has been deposited on TiO₂ and gold substrates separately. An ultraviolet photoemission experiment (UPS) reveals the energy-level alignment and the Fermi energy of the bR/TiO₂ system while combined x-ray absorption measurements (XANES) and X-ray photoemission spectroscopy (XPS) indicate that in bR the lowest unoccupied molecular orbital (LUMO) is located about 2 eV above the highest occupied molecular orbital (HOMO). A separate Time of Flight Secondary In Mass Spectrometry (TOF-SIMS) study, conducted to characterize the attachment of bR to Au substrate, confirm successful Thiol-Gold bond formation with films thicker than 10 nm. Principal component Analysis (PCA) of TOF-SIMS data further separated the samples based on ratio peak intensities of C₂H₃⁺, C₂H₅⁺ to that of Ca²⁺, Cu²⁺.The HOMO-LUMO gap of retinal was spectroscopically determined to be 2.49 eV. For comparison, we also performed DFT calculations to determine the HOMO-LUMO gap of free retinal and Spiro OMe-TAD. Using the G-311G basis set, the calculated HOMO-LUMO gap was 2.69 eV for Retinal and 1.1 eV for Spiro respectively. The chromophore can be stabilized in the bR protein by the HOMO-LUMO interaction with the protein environment. Based on these spectroscopy results and DFT calculations, new solar cell architectures combining perovskite and bR are proposed and discussed.

Methylammonium lead trihalides (MAPbX₃) have great potential as light harvesters for perovskite solar cells due to their unique optical and electronic properties. In general, conventional growth techniques apply spin-coated precursors on a substrate followed by annealing for the processing of the lead halide perovskites; however, use of toxic solvents and high temperature hinder device stability and performance. To avoid annealing processes, the solution-based methods have been developed, which involve the formation of perovskite colloidal particles in solution. I will introduce a new one-step solution technique to facilitate in situ crystal formation of methylammonium lead bromide and methylammonium lead chloride perovskites at the micron (~1-10 µm) to nano scale (< 500 nm).¹ As a substrate-free approach, the crystal pre-growth allows crystallization in alcohols (methanol, ethanol, 2-propanol, 1-butanol, and 2-butanol) at room temperature followed by a direct precipitation of the perovskite material for a large-area deposition. This room-temperature processable technique, however, differs from the in situ growth method of methylammonium lead iodide in alcohols that eliminates treatment at the boiling point of the alcohols.²
The techniques used to characterize the perovskite crystals involve high-energy synchrotron XRD, wide angle x-ray scattering (WAXS), Fourier transform infrared (FTIR) in reflection (ATR, % R), UV-Vis-NIR (% R), micro Raman spectroscopy, and the solid state ¹H, ¹³C, and ²⁰⁷Pb –MAS NMR. Based on the analysis results, the perovskites show improvement in air/moisture for their chemical stability (<1.5 months, in a fume hood under varying humidity level). Thermogravimetric analysis and in situ techniques of powders also determine their thermal stability (~150°C -MAPbCl₃, ~250°C -MAPbl₃, ~350°C -MAPbBr₃). The poor yield of methylammonium lead iodide in toluene confirms that the alcohols catalyze the growth process through a substitutional reaction mechanism but the mechanism in toluene follows a different path. Indeed, the theoretical calculations reveal that the growth reaction in alcohols is exothermic. Moreover, I will discuss the role of solvent polarity (polar, apolar, non-polar), the type of solvent (protic vs. aprotic), the reactivity order of the alcohols, effect of their different binding affinity, and other reaction parameters on the growth mechanisms. ¹M. Acik, et al. in prep., ²M. Acik, et al. Adv. Energy Mater., 1701726 (2017).

We examined the charge transfer mechanism of Cs₂SnI₆ and clarified the function of its surface state in photovoltaic devices. From a cyclic voltammetry study, we found that the faradaic reactions of the iodine species derived from Cs₂SnI₆ induce charge transfer through a surface state of Cs₂SnI₆, mainly at +0.9 V vs. the normal hydrogen electrode. This potential is located in the mid-gap state of Cs₂SnI₆ and its surface state charging was confirmed by Mott-Schottky measurements. This mid-gap charge transfer was further proved in photovoltaic devices. More specifically, we developed dye-sensitized solar cells with quasi-solid Cs₂SnI₆-based regenerator, or conventional liquid electrolyte. The performances of the Cs₂SnI₆-based regenerator were strongly dependent on the highest occupied molecular orbital (HOMO) of the organic dyes. In particular, BT-HT featuring the lowest HOMO (−5.65 eV) provided a 79% enhancement in the photocurrent density (14.1 mA cm⁻²) in the Cs₂SnI₆-based regenerator compared to that of a conventional liquid electrolyte (7.9 mA cm⁻²). This unprecedented finding can be explained as follows: i) fast charge transfer through Cs₂SnI₆; ii) efficient charge regeneration owing to lower HOMO of BH-HT than mid-gap state (−5.43 eV); and iii) lesser early recombination in the Cs₂SnI₆-based regenerator. To further confirm this mid-gap charge transfer, we demonstrated a correlation between performance of Cs₂SnI₆ as a light absorber and the TiO₂ conduction band by Sn doping in TiO₂. Our findings confirm the importance of surface state engineering in future designs of Cs₂SnI₆ lead-free perovskite devices.

Perovskite solar cells are currently one of the most promising photovoltaic technologies for highly efficient and cost-effective energy production. In only several years, an unprecedented progression of preparation procedures and material compositions delivered a prototype technology that exploits most of the potential for perovskites as photovoltaic materials. However, there remains a vast scope to demonstrate that perovskite solar cells are stable under working conditions
Migration of ions within the perovskite crystal lattice has been widely investigated to explain the “hysteresis” of current-voltage (J-V) characteristic of perovskite solar cells. The results of these studies indicated that, regardless of the particular device architecture and materials composition, halides (and their vacancies) migrate within the perovskite layer and accumulate at the interface with selective charge contacts. Depending on particular voltage and light bias conditioning, accumulation of ions (and their vacancies) reduces the charge collection efficiency. This mechanism has been suggested as the most likely cause of J-V “hysteresis”, but it may also have a significant impact on the long-term stability of devices under working conditions. Understanding the effect of ion migration on device long-term performance is of paramount importance because it will answer the question whether or not there is an intrinsic instability that may ultimately prevent from using perovskites for photovoltaics.
In this talk, I will demonstrate ion migration in perovskite solar cells working under different voltage bias conditions and I will discuss the impact of ion migration on the initial device power conversion efficiency and long-term stability. Thus, I will demonstrate perovskite solar cells stabilised for several hundred hours close to the initial efficiency.

Perovskite solar cells (PSCs) have shown remarkable progress in efficiency as compared to CIGS and CdTe thin-film solar cells, however, instability of PSCs and toxicity of Pb remain issues for commercialization. While the quest to improve efficiency in CdTe, Cu(In,Ga)(S,Se)₂ (CIGS), Cu(Zn,Sn)(S,Se)₂ (CZTSSe) and related thin film solar cells has taken decades to reach the 10-23% level, organic-inorganic perovskites (e.g., CH₃NH₃PbI_3-xCl_x) have emerged with a relatively quick demonstration of efficiencies of 22.1% and simple low-temperature, low-cost processing. The use of a tandem structure could realistically improve efficiency of a CIGS device (22.3%) to beyond 30%. Perovskite tandem solar cells have shown efficiencies above 25% with silicon and 17.8% with CIGS bottom cells respectively. Despite these attractive attributes, there are areas that need to be addressed on the fundamental materials level such as materials stability and replacement of Pb with a non-toxic and sustainable alternative. CsSnI3 is a highly desirable substitute for Pb-based perovskites, but poor stability of the material has prevented fabrication of devices that can withstand sustained operation or even processing under ambient air conditions. The inorganic compound Cs2SnI6 has received recent attention as an alternative to Sn-based halide perovskites for photovoltaic device applications. In comparison to Sn- and Pb-based halide perovskites, Cs2SnI6 has been shown to feature enhanced stability in ambient environments due to the stable oxidation state of Sn (Sn4+ in Cs2SnI6 as compared to Sn2+ in CsSnI3). The structure of Cs2SnI6 has been described as a defect variant of AMX3 perovskites, with half of the Sn atoms removed resulting in shorter Sn-I bond length and hence improved stability.

In this paper, we demonstrate a ~1.6 eV direct bandgap SnF₂ doped Cs₂SnI₆ films processed via solution processing in air. The films are stable when annealed in dark upto 1000 hours at 100 C. Cs2SnI6 compound has been reported to display intrinsic n-type conductivity (with carrier concentrations of 10^14 cm-3 and 5 x 10^16 cm-3), and it has been shown that it can be doped p-type with SnI2 (with carrier concentrations of 10^14 cm-3), demonstrating the ambipolar nature of this material. Here we demonstrate a p-type Cs2SnI6 film with carrier concentration of 5 x 10^14 cm-3 and mobility of about 18 cm2/V-sec, which is comparable to CH3NH3PbI3. Effect of doping at different concentration of SnF2 will be discussed on carrier concentration and hole mobility. SCAPS calculation results for improving device efficiency will also be discussed. We have observed a strong effect of solvent (GBL, DMF or DMSO) on the resulting Cs2SnI6 phase, which will also be discussed here. The serendipitous discovery of this simple solution process sheds new light on possible properties of Cs2SnI6 which have not yet been reported experimentally.

We have already reported that mixed metal perovskite (MA(Pb)x(Sn)yI3:SnPb-PVK) is able to harvest the energy to IR region (up to 1000 nm) (1). The SnPb PVK has a potential to possess ideal band gap (about 1.4eV), which is better than that of MAPbI₃ (around 1.55eV) (1). The short circuit current (Jsc) was high, reaching to 33 mA/cm² (for comparison, 24mA/cm² for MAPbI₃) because of the wide range of light harvesting from visible to IR region. However, the open-circuit voltage (Voc) was lower than 0.3 V. We discuss why the perovskite solar cells consisting of Sn have low efficiency, compared to MAPbI3 from the view point of hetero-interface architecture(2-5). We focused on the Voc loss for the evaluation of these hetero-interfaces. In solar cells with TCO/c-TiO₂/mp-TiO₂/SnPb-PVK/SPIRO/Au (A) structure, the Voc loss was about 0.9eV, which was larger than that of conventional MAPbI₃ (0.44 eV). We found that Ti-O-Sn linkages are present at the hetero-interface between TiO₂ and SnPb PVK and create new traps (charge recombination center). In order to remove the hetero-interface, inversion structure (TCO/PEDOT-PSS/SnPb-PVK/C60/Au)(B) was made. The Voc loss for (B) decreased to 0.5 eV. In addition, the insertion of F1 (trifluorobutylannmonium iodide) thin layer at the interface between the SnPb PVK and C60 layers was also effective for enhancing the efficiency. We have already reported the similar results using MAPbI3 (6). Decreasing crystal defect is another issue for enhancing the efficiency. After coping with these issues causing charge recombination, the Voc loss decreased to 0.43 eV and 17.24% efficiency was obtained. Voc loss for SnPb mixed metal PVK was almost the same as that for Pb-PVK solar cells, leading the conclusion that SnPb PVK has potential to high efficiency solar cell. These directions to enhancing efficiency were applied to Pb-free Sn-perovskite solar cells. Pathways for enhancing the efficiency from 2% to 5.52% efficiency will be discussed.

1. Y. Ogomi, et al., J. Phys. Chem. Lett. 2014, 5, 1004-1011; 2. S. Nakabayashi, et al., J. Photonics for Energy; 2015, 5, 057410; 3. Y. Ogomi, et al., J. Phys. Chem. C, 2014, 118, 16651-16659; 4. Q. Shen, et al., Phys. Chem. Chem. Phys. 2014, 19984-19992; 5. Y. Ogomi, et al., Chem. Phys. Chem. 2014, 15, 1062-1069; 6. H. Moriya, et al., ChemSusChem., 2016, 9, 2634-2639.

Mesoscopic perovskite solar cells (PSCs) have captured intensive attention in the field of energy conversion due to the advantages of low material cost, simple fabrication process and high power conversion efficiency. Benefiting from the optimization of perovskite absorber deposition approaches, the design of new material systems, and the diversity of device concepts, the efficiency of PSCs have increased from 2.19% in 2006 to a certified 22.1% in 2016. Such extremely fast increasing efficiency enables this photovoltaic technology challenge the current commercialized solar cells. However, typical perovskites of methylammonium lead halides (CH₃NH₃PbX₃, X = Cl, Br, I) are usually sensitive to moisture in ambient air, and thus require an inert atmosphere to process. We demonstrate a moisture-induced transformation of perovskite crystals in a triple-layer scaffold of TiO₂/ZrO₂/Carbon to fabricate printable PSCs. An additive of ammonium chloride (NH₄Cl) is employed to assist the crystallization of perovskite, wherein the formation and transition of intermediate CH₃NH₃X-NH₄PbX₃(H₂O)₂ (X = I or Cl) enables high-quality perovskite CH₃NH₃PbI₃ crystals with preferential growth orientation. Correspondingly, the intrinsic perovskite devices based on CH₃NH₃PbI₃ achieve an efficiency of 15.6% and a lifetime of over 130 days in ambient condition with 30% relative humidity. This ambient-processed printable perovskite solar cell provides a promising prospect for mass-production, and will promote the development of perovskite-based photovoltaics.

Unceasing demands for the fabrication of low-cost highly efficient light harnessing devices compelled photovoltaic community to investigate newer materials that has eventually led to the evolution of hybrid perovskite solar cells. Primarily due to the enticing optical, excitonic, and electrical properties, organic-inorganic lead perovskites are considered as suitable light absorbing materials for the development of efficient solar cells. Thus far, much of the focus has been on iodide-based perovskite solar cells, which display record efficiencies mostly because of high current densities. In comparison, bromide-based perovskites bring forth higher open-circuit voltage, which could be beneficial for tandem solar cell applications and in driving electrochemical reactions, including water splitting reactions and CO₂ reduction. However, extrinsic as well as intrinsic instability of perovskite solar cells has proven to be an invincible bottleneck towards their commercialization. In my presentation, I will discuss the fabrication of formamidinium lead bromide (CH(NH₂)₂PbBr₃) perovskite solar cells yielding photovoltage above 1.5 V, a record for CH(NH₂)₂PbBr₃ based devices. The outstanding photovoltage was obtained after improving the quality of the absorber layer and by optimizing the charge extraction layers. The causation of high photovolatge was investigated using a combination of techniques, such as field emission scanning electron microscopy, photothermal deflection absorption spectroscopy, impedance spectroscopy, time-integrated and time-resolved photoluminescence. In addition to insights gained through various structural, morphological and spectroscopic characterization techniques, the operational stability of the devices examined under aggressive conditions will also be discussed.

References:
1. Arora et al. submitted 2018.
2. Arora et al. Adv. Funct. Mater. 2016, 26, 2846-2854.
3. Arora et al. Nano Letters 2016, 16, 7155-7162.
4. Dar, M. I. et al. Chem Phy Lett. 2017, 683, 211-215.

Lead (Pb) based perovskite solar cells(PSCs) are continuously getting improved in terms of stability, reproducibility and efficiency (22.1%)[1]. However, presence of Pb in PSCs has toxicity concern to environment and human health. Therefore, in recent time a lot of research has been done in looking for a Pb free solar cells based on tin (Sn), germanium (Ge) such as FASnI₃, MAGeI₃ etc [2]. Although efforts are being done as described to solve the toxicity issue, but it costs the reduction in solar cell efficiency. So one of the alternative is to mix Pb with Sn making PSC relatively less toxic with the advantage of increased absorption spectra [3] and comparative photoconversion efficiency (PCE) to Pb PSCs. Our group is involved in fabricating the Sn/Pb alloyed solar cells and so far best PCE of 15.93% [4] has been achieved. It is well known that Sn/Pb possess low air stability due to rapid oxidation of Sn²⁺ to Sn⁴⁺, which results in relatively faster degradation of solar cell performance compared to purely Pb based solar cells. To elucidate this problem we tried to explore the role of multiple monovalent cations such as rubidium (Rb⁺), cesium(Cs⁺), formamidinium [(CH₃(NH₂)₂⁺, FA⁺], methylammonium [(CH₃NH₃)⁺, MA⁺] etc. on the A position of ABX₃ perovskite crystal structure [5], which is already an effective way to improve the stability and reproducibility of Pb based PSCs. Therefore, the present research work discusses about the role of multiple monovalent cations in improving the stability and performance of Sn/Pb PSCs as answered by X-ray diffraction (XRD) pattern, thermogravimetric analysis (TGA), scanning electron microscopy etc. We fabricated the solar cells with precursor solutions such as (FASnI₃)_0.6(MAPbI₃)_0.4 as (FAMA), (CsI)x[(FASnI3)0.6(MAPbI3)0.4]1-x as (Cs)x(FAMA)1-x. In result we found that with the addition of Cs(5%) showed increase in PCE from 10.26% (FAMA) to 11.66%. This improvement in efficiency was clearly observed in IPCE as well. Cs(5%) containing solar cell exhibits more than 60% absorption at 900 nm with a high photocurrent of 28 mA/cm². We will finally report an optimized solar cell performance of ~16% with the further discussion on the role of interfacial engineering.

Reference
1. W.S Yang & S.I Seok et al, Science, 2016, 348, 1234-1237.
2. T.M Koh & N. Mathews et al, J.Mater. Chem. A, 2015, 3, 14996-15000.
3. Y. Ogomi & S. Hayase et al, J. Phys. Chem. Lett., 2014, 5, 1004-1011.
4. S. Hayase, Electrochemistry, 2017, 85(5), 222–225.
5. M. Saliba & M. Gratzel et al, Energy & Environ. Sci., 2016, 9, 1989-1997.

Organic-inorganic hybrid halide perovskites have emerged as an interesting class of materials that have excellent photovoltaic properties for application to solar cells. In the last four years, the power conversion efficiency of perovskite solar cells (PSCs) over 20% has recently been realized through systematic optimization of materials and fabrication processes. However, the stability of PSCs is still not satisfied to practical applications, and the actual degradation mechanisms of PSCs are not well understood. Here, we firstly investigate the degradation mechanisms of CH₃NH₃PbI₃-based PSCs using a thermally stimulated current technique. We observed that a large density of hole traps is formed in PSCs degraded by continuous solar illumination and that the formation of hole traps is strongly related to the stability.¹ One source of the traps is metallic lead resulting from photodegradation of CH₃NH₃PbI₃ under continuous light irradiation. We greatly extended the lifetime of PSCs under standard laboratory weathering testing from 150 hours to 4000 hours by suppressing the formation of Frenkel defect-metallic lead.² Furthermore, we revealed influence of phase transition on the device stability, and developed efficient and thermally stable PSCs under standard thermal cycling test.
Second, we focus on perovskite light-emitting diodes that are promising for the next generation of lighting and displays because of their low cost, excellent color purity, and high performance. Morphology and crystallinity engineering in 3D perovskite emitters has led to an external quantum efficiency of 8.5% for green emission. Alternatively, quasi-2D perovskites exhibit an efficiency up to 12% in the near-infrared region, but they have not yet delivered efficiencies higher than this in pure green devices. Here, we discuss quenching mechanism of excited states by the organic cation that is a major loss path and must be avoided to achieve high efficiencies. In our optimized devices, the external quantum and current efficiencies of the champion green-emitting devices reached over 10%. Our findings will provide guidance for the development of future high-performance opto-electronic perovskite devices such as lasing.

References
¹ C. Qin, T. Matsushima, T. Fujihara, W. J. Potscavage, Jr., and C. Adachi, Advanced Materials, 28, 466 (2016).
² C. Qin, T. Matsushima, T. Fujihara, and C. Adachi, Advanced Materials, 29, 1603808 (2017).

Hybrid organic-inorganic perovskites have recently gained tremendous attention as they have enabled bright, widely tunable light-emitting diodes, photodetectors with high detectivity, as well as solar cells that have reached certified power conversion efficiency (PCE) to 22.1% for single junction devices. The detection of ultraviolet (UV) light has a wide range of applications, such as chemical, environmental and biological analysis and monitoring, flame and radiation detection, astronomical studies, and optical communications. UV-A in the range of 320 to 400 nm can penetrate deep into the skin and cause skin damage. Hence, UV-A photodetectors are highly desired to monitor the UV-A intensity. Methylammonium lead chloride (CH₃NH₃PbCl₃) as a wide-bandgap perovskite (Eg = 3.17 eV) shows a high optical absorption coefficient in UV-A range while transparent to visible light, which makes it a promising candidate for visible-blind UV-A photodetectors. In this work, CH₃NH₃PbCl₃ perovskite thin films were fabricated via a two-step spin coating and solvent-vapor-assisted thermal annealing method under low temperature. A PbCl₂ DMSO solution with the concentration of 350 mg/mL was spin coated onto cleaned ITO-coated glass substrates followed by thermal annealing at 70 °C for 10 min. A CH₃NH₃Cl 2-propanol solution with the concentration of 20 mg/mL was then spin coated onto the PbCl₂ layers and annealed at 70 °C for 30 min to facilitate interdiffusion. DMSO-vapor-assisted thermal annealing was introduced at 100 °C after the interdiffusion annealing for 1 h. The films exhibited cubic crystalline structure verified with XRD and pinhole-free morphologies from SEM images. The possible charge traps were investigated via the analysis of photoluminescence spectra of perovskite films prepared with different lead chloride (PbCl₂) precursor concentrations while maintaining the same concentration of methylammonium chloride (CH₃NH₃Cl). The PL results showed that the CH₃NH₃PbCl₃ perovskite thin films fabricated with 350 mg/mL PbCl₂ and 20 mg/mL CH₃NH₃Cl exhibited the PL peak at the shortest wavelength and almost no tail following the PL peak, indicating that the film had less trap states than films made with other PbCl₂ concentrations. Prototypical UV photodetectors with the structure of ITO/CH₃NH₃PbCl₃/Poly (triaryl amine) (PTAA)/Al were fabricated and showed low dark current density 1.60 × 10^-5 mA/cm² under -1 V reverse bias, strong photoresponse in 300-400 nm region, and a high UV-visible rejection ratio (defined by responsivity in UV range divided by that at 500 nm) up to 500 under 0 or -0.5 V bias. All the results demonstrated that low-temperature solution-processed CH₃NH₃PbCl₃ perovskite thin films offer a great potential for making flexible, lightweight visible-blind UV-A photodetectors.

Elucidating the fundamental effects responsible for phase stability and band gap tuning in hybrid perovskites is of critical concern for the further engineering of these promising materials. With recent experimental evidence suggesting increased phase stability in perovskite alloys, the field has labeled entropy stabilization as a likely hypothesis. We clarify the phenomena of entropy stabilization and show, using first-principles density functional theory calculations, that perovskite alloys are not uniquely entropy stabilized by quantifying the site-specific enthalpy and entropy contributions of different cations and anions. We propose that only a small set of these materials are entropy stabilized, and that observed stability effects are primarily enthalpy driven. We also unravel the effect of alloying on three distinct sublattices (A, B and X in perovskite ABX₃) on the band gap in perovksite alloys, and provide insights into the general trends of the band gap evolution with chemical composition. Our results show that alloying on the B-sublattice have the strongest effect on the band gap as it influences both the valence band and conduction band of the alloy band structure, and explains the origins of recent experimentally observed large band gap reduction in Pb-Sn based alloys.

Hybrid organic-inorganic perovskite solar cells (PSCs) based on MAPbI₃ have attracted tremendous interest in the photovoltaic field. Power conversion efficiencies of PSCs have increased from 3%¹ to 22.1%² over the past 7 years due to their unique optoelectronic properties^3-5 and compositional versatility⁶. However, the environmental and thermal instability of MAPbI₃ remains a major challenge for this material’s application in commercial solar energy. Recently, improving the stability of perovskites by tuning the composition has received great attention. Replacing the MA⁺ ion with other cations such as FA⁺ and Cs⁺ while replacing a portion of the iodide anions with Br^- or Cl^- has led to significantly improved stability compared to MAPbI₃.

Our research focuses on the thermal stability in air of the mixed cation organic inorganic lead halide perovskites Cs_0.17FA_0.83Pb(Br_0.17I_0.83)₃ and Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃. For the latter compound, containing both MA⁺ and FA⁺ ions, thermal decomposition of the perovskite phase was observed to occur in two stages. The first stage of decomposition occurs at an increased rate compared to the second stage and is only observed at relatively low temperatures (T<150 °C). For the second stage, we find the decomposition rate and the activation energy both have similar values for Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃ and Cs_0.17FA_0.83Pb(Br_0.17I_0.83)₃, which suggests that the first stage mainly involves reaction of the MA⁺ and the second stage mainly FA⁺. This work is particularly important in light of the high and increasing interest in designing the composition of perovskites to make them more stable.

References
[1] Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T., Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. Journal of the American Chemical Society 2009, 131, (17), 6050-6051.
[2] Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S. I., Iodide management in formamidinium-lead-halide–based perovskite layers for efficient solar cells. Science 2017, 356, (6345), 1376-1379.
[3] Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J., Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, (6156), 341-344.
[4] Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C., Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342, (6156), 344-347.
[5] Gratzel, M., The light and shade of perovskite solar cells. Nat Mater 2014, 13, (9), 838-842.
[6] Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I., Compositional engineering of perovskite materials for high-performance solar cells. Nature 2015, 517, (7535), 476.

The short lifetime of perovskite solar cells (PSCs) has emerged as an urgent issue which needs to be resolved for practical applications. While low reactive energy between lead halide and methyl ammonium halide allows facile formation of perovskite structure, it leads to easy deterioration of perovskite structure under an ambient condition. The structural weakness of perovskite films to humidity or oxygen has been widely studied. It was reported that enhanced lifetime of PSCs can be achieved with compositional variation by introducing cesium or formamidinium to perovskite layer. However, its origin is not well understood. The efficient PSCs are composed of n-type and p-type adjacent buffer layers to enhance charge extraction from perovskite layer. Hence, it is essential to study the interface degradation mechanism between perovskite layer and buffer layer or buffer layer and electrode. Here, we will suggest interface degradation mechanism of inverted PSCs by observing the decay of photovoltaic properties around 50 devices over 1000 hours. At days 10 (240 hours), 20 (480 hours), 30 (720 hours), and 40 (960 hours) after the fabrication of cells, we performed Ag electrode restoration process by peeling-off Ag electrode and re-evaporate Ag electrode. This process results in the power conversion efficiency recovery with interesting variation of photovoltaic parameters. We found direct evidence of perovskite film degradation by time of flight secondary ion mass spectrometry (TOF-SIMS). The iodide ions were diffused from deteriorated perovskite film and accumulated under Ag electrode, resulting in device degradation. The diffusion of iodide ions had thermo-dynamical reaction with fullerenes which induced n-doping effect of [6,6]-phenyl C71 butyric acid methyl ester (PCBM). We analyzed PCBM-halide radical in depth by low temperature measurement (200K ~ 300K). This analysis supports the disorder model for the open circuit voltage increase. Finally, long-term degradation mechanism of inverted PSCs is proposed.

We have developed a new method to dynamically measure the absolute intensity steady-state photoluminescence and the mean carrier diffusion length simultaneously in neat hybrid perovskite films. The measurements reveal four distinct regimes of material changes and show that photoluminescence brightening often coincides with losses in carrier transport, such as in degradation or phase segregation. The evolution of any perovskite film can be plotted parametrically with time on a graph of radiative efficiency versus diffusion length. We show how different environments affect the evolution of and coupling of material properties for MAPbI3 and high bandgap mixed halide perovskites.

We also investigate the origin of phase segregation and its implication for tandems with several mixed halide large-bandgap (∼1.75 eV) compositions. We show explicitly that MAPb(I_0.6Br_0.4)₃ and (MA_0.9,Cs_0.1)Pb(I_0.6,Br_0.4)₃, termed “MA” and “MACs”, respectively, rapidly phase segregate in the dark upon 1 sun equivalent current injection. This is direct experimental evidence that conduction band electrons or valence band holes are the culprit behind phase segregation. In contrast, (FA_0.83,Cs_0.17)Pb(I_0.66,Br_0.34)₃, or “FACs,” prepared at only 75 °C resists phase segregation below 4 sun injection. FACs prepared at 165 °C yields larger grains and withstands higher injected carrier concentrations before phase segregation. Both the phase-stable FACs and phase-segregating MACs devices sustain near constant power output at 1 sun and do not affect the current output of a CIGS bottom cell when used as an incident light filter. Further, optimization of novel surface passivation method applied to the 1.75 eV bandgap FACs films resulted in an enhancement of the photoluminescence quantum yield (PLQY) of over an order of magnitude, an increase of 80 meV in the quasi-Fermi level splitting (to 1.29 eV), an increase in diffusion length by a factor of 3.5, and enhanced open-circuit voltage and short-circuit current in devices.

We compile our findings of quasi-Fermi level splitting and phase segregation to discuss routes toward development of phase-stable, high bandgap HPs with minimal voltage deficit. We will further elaborate on the role of composition in stability and quality of high bandgap HPs by presenting recent combinatorial data on thousands of other cation compositions.

References
Stoddard, R. J.; Eickemeyer, F. T.; Katahara, J. K.; Hillhouse, H. W., Correlation between Photoluminescence and Carrier Transport and a Simple In Situ Passivation Method for High-Bandgap Hybrid Perovskites. The Journal of Physical Chemistry Letters 2017, 8 (14), 3289-3298.

Braly, I. L.; Stoddard, R. J.; Rajagopal, A.; Uhl, A. R.; Katahara, J. K.; Jen, A. K.-Y.; Hillhouse, H. W., Current-Induced Phase Segregation in Mixed Halide Hybrid Perovskites and its Impact on Two-Terminal Tandem Solar Cell Design. ACS Energy Lett., 2017, 2 (8), pp 1841–1847

With the emergence of highly efficient lead halide perovskites there is a need to understand the excited state behavior and charge separation events following photoexcitation. Mixed halide lead perovskites offer a useful strategy for continuous tuning of the semiconductor bandgap. For example, by varying the halide composition of methylammonium lead iodide/bromide (CH₃NH₃PbBr_xI_3-x (x=0 to 3)) it is possible to tune the bandgap between 1.55 eV and 2.43 eV. In addition to photovoltaic applications these mixed halide perovskites offer rich photophysical properties with lasing applications. The excited state characterization using emission and transient absorption spectroscopy has allowed us to probe the photoinduced processes. Of particular interest are mixed halide lead perovskites (e.g.,CH₃NH₃PbI_3-xBr_x) which offer flexibility of tuning bandgap. Interestingly, they also undergo phase segregation to create Iodine-rich and Bromide- rich regions when subjected to visible irradiation. This intriguing aspect of halide ion movement in these mixed halide films can be tracked from the changes in the photoluminescence and absorption spectra. The photovoltaic performance of perovskite solar cells with varying degree of halide treatment will also be discussed

All-perovskite tandem solar cells offer the exciting possibility of surpassing limits on single junction solar cells. Low band gap ABX₃ perovskites with mixtures of tin and lead at the B-site have recently had breakthrough success, but face unique challenges. Our work maps oxidation stability and optoelectronic properties - band gap, carrier lifetime and photoluminescence quantum efficiency - across a wide compositional space, varying tin-lead ratio at the B-site and varying the A-site cation, to identify the optimal composition for the bottom cell in a tandem.

Band gap tuning is critical for subcells in monolithic tandems to absorb complementary spectra. We map band gap across a wide range of A-site and B-site cation compositions and identify mechanisms by which the A-site cation alters the band gap both by lattice contraction and octahedral tilting. The smallest band gaps result from an A-site cation that produces a contracted cubic lattice with no octahedral tilting.

While pure Sn perovskites are unstable to oxidation, we show that mixing Sn and Pb at the B-site drastically improves stability because mixed Sn-Pb perovskites are forced to go through a less favourable mechanism for oxidation. Our results suggest that the low-gap perovskites most likely to succeed will have tin contents of 30 - 50%. While these compounds have slightly larger band gaps than ideal, the small price in band gap will likely be worth large gains in stability. We also study how the A-site cation can improve oxidation stability. We demonstrate tin-lead perovskite solar cells that maintain 80% of their efficiency over 10 hours of maximum power tracking in air with no encapsulation. The drop in performance is reversible in the dark, showing that there is little irreversible oxidation of the perovskite.

Due to lower absorption cross-sections of tin-lead perovskites, thick layers are needed to absorb most above-gap light, which necessitates long carrier diffusion lengths. While early low gap perovskites had short carrier lifetimes (< 1 ns), compositional tuning and improved processing has raised the liftetime to over 250 ns. Further, tin-lead perovskite solar cells have already attained impressive open circuit voltage relative to their band gap despite having much lower radiative efficiency compared with pure lead perovskites. Improving the photoluminescence quantum efficiency (PLQE) can lead to even higher voltages. We map PL lifetime and PLQE across compositions and study the reasons for voltage loss across compositions.

Taken together, our results form a framework for rational selection of compositions and processing methods of low band gap tin lead perovskites for efficient and stable all-perovskite tandem solar cells. With a 1.68 eV perovskite top cell over one such low band gap perovskite, we demonstrate a record 21.4% efficient mechanically stacked all-perovskite tandem.

Hybrid halide perovskites have achieved efficient operation in optoelectronic devices, but their commercialization is hindered by chemical instability under operating conditions. Understanding both the origin of this instability and its connection to performance is key to the development of perovskite optoelectronic devices beyond the laboratory. In particular, ion migration is proposed as an important limiting mechanism in hybrid perovskite devices causing chemical and performance-related instabilities such as hysteresis in scanning current-voltage measurements. Despite the concern regarding ion migration, open questions remain as to what is moving and how any stoichiometric changes relate to device performance.
Here we identify a direct relationship between halide migration and local optoelectronic quality. By using nanoprobe X-ray fluorescence with 200 nm resolution we mapped elemental changes in thin single crystals of methylammonium lead bromide (CH₃NH₃PbBr₃) while systematically applying an electric field across the crystal in a lateral back-contact device. This experiment revealed the migration of Br^- across the perovskite crystal in the opposite direction of the electric field. Photoluminescence (PL) mapping reveals PL intensity increases in halide-rich regions and decreases in halide-poor ones, with quasi-reversible variation observed over multiple voltage biasing cycles. Nudged elastic band calculations indicate that the alignment of the methylammonium cation under bias forms channels that facilitate halide migration along the field direction. The direct link between halide migration and intrinsic optoelectronic response clarifies that halide migration is a challenge that is intrinsic to the absorber and one that plays a determining role in the performance limits of perovskite devices.

Semiconducting metal-halide perovskites present various types of chemical interactions which give them a characteristic fluctuating structure sensitive to the operating conditions of the device, to which they adjust. This makes the control of structure-properties relationship, especially at interfaces where the device realizes its function, the crucial step in order to control devices operation. In particular, given their simple processability at relatively low temperature, one can expect an intrinsic level of structural/chemical disorder of the semiconductor which results in the formation of defects.
Here I will discuss the role of defect physics in determing the open circuit voltage of metal halide perovskite solar cells and present strategies for the optimization of devices which include: 1) the engineering of the charge extracting layer (CEL), which accounts not only for the energy level alignment between the CELs and the perovskite, but also for the quality of the microstructure of the perovskite bulk film that is driven by the substrate surface; and 2) the use of inks based on colloidal suspensions of nanoparticles which lead to a high level of control over the material quality and device reliability, and offer more versatile processing routes by decoupling crystal growth from film formation.

Photovoltaic devices based on hybrid organic-inorganic perovskite absorbers have reached outstanding performance over the past few years, surpassing power conversion efficiency of over 22%. This talk we discuss recent progress at the National Renewable Energy Lab (NREL) on the challenges in hybrid perovskite solar cells (HPSCs) targeting large scale energy generation. This talk will highlight work at NREL to develop understand and enhance stability of HPSCs inline with realizing a scalable HPSC technology. Talk will discussing efforts to controlling interfaces in the devices and key aspects of material formation and processing for high-volume manufacturing. In the case of stability, an examination of different perovskite active layers their formation and resulting interfacial electronic structure with typical and novel contacts with the HPSCs stack will be presented. Our studies at NREL indicate interface formation of the active layer with different carrier transport materials has direct implications for performance and it evolution over time in the resulting devices. Results of interface studies combined with time resolved spectroscopy, structural studies and device level studies to validate impacts on interface electronics and carrier dynamics will be shown and the technological relevance discussed. In particular work on mini-module devices at the 26cm² will be presented along with performance data for these systems.

Two-dimensional Ruddlesden–Popper (RP) halides are under intense investigation for photovoltaic and optoelectronic applications due to advantages in stability, diversity and tunability. Here we present results of a detailed investigation of the crystal structures of a prototypical groups of RP halides PEA₂PbX₄ (PEA= phenylethylammonium) using density functional theory calculations. We find PEA cations energetically prefer (by ~50 meV/f.u.) the same orientation within one PEA layer while the opposite in two adjacent PEA layers, which agrees with a most recent experiential study [1] but differs from other earlier reports, including one of our own [2]. Orientations of PEA cations further mediate the distortion of PbI₆ octahedral layer. Such trends are robust when using both Perdew–Burke–Ernzerhof (PBE) generalized gradient and strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximations with and without revised Vydrov–van Voorhis (rVV10) van der Waals functional. We investigate the origins of the preferred PEA cation orientation and PbI₆ distortion direction based on analyses of the electronic structure. Our study provides insights into the nature of the chemical bonding determining the structure of layered hybrid Ruddlesden–Popper halides.

Acknowledgement: This work was supported by the Singapore Berkeley Research Initiative for Sustainable Energy(SinBeRISE) Program.

[1] K.-Z. Du, Q. Tu, X. Zhang, Q. Han, J. Liu, S. Zauscher, D. B. Mitzi, Inorg. Chem. 56 (2017) 9291–9302.
[2] K. Thirumal, W.K. Chong, W. Xie, R. Ganguly, S.K. Muduli, M. Sherburne, M. Asta, S. Mhaisalkar, T.C. Sum, H. S. Soo, N. Mathews, Chem. Mat. 29 (2017) 3947–3953.

In this work, we demonstrate that the electronic heterogeneity in lead halide perovskite thin films is not intrinsic to the material, but depends on synthesis method, orientation, and grain size. These results have significant implications for the design of thin films with uniform electronic properties for high efficiency solar cells, and to the fundamental understanding of structure property relationships at the perovskite surface.

In the literature, photoluminescence (PL), time resolved photoluminescence (trPL), and charge transfer have been reported to vary for adjacent grains. Also, surface voltage and current have been reported to vary across different facets of the same crystal. This spatial variation implies that all bulk measurements correspond to ensemble averages, rather than intrinsic properties of the material. Furthermore, it is not currently known if the perovskite solar cells are limited by uniform loss or by spatially localized spots of poor efficiency, such as misoriented grains. In-depth examination of the causes of the spatially varying electronic properties, and their impact on macroscopically observable properties is necessary to accurately study the fundamental properties of this unique material.

However, many inconsistencies are currently present in the literature. Some optical studies conclude that perovskite thin films exhibit large spatial variations in trap site density on different grains while others conclude no variations in trap site density, or attributed PL quenching to other phenomena. Synthesizing these studies into one body of knowledge is difficult due to the different thin film morphologies and specific imaging techniques used by each group. Therefore, further work is needed for the field to reach a conclusion regarding the causes of the observed electronic heterogeneity.

Here we developed a methodology to separate the contributions the morphological and electronic contributions to PL and trPL heterogeneities in methylammonium lead iodide thin films, and reconcile the inconsistencies found in the literature. Spatially resolved PL and trPL are combined with light reflectance, transmission measurements and atomic force microscopy in order to quantitatively map the electronic properties and morphologies of perovskite thin films, and statistical techniques were used to separate the PL features caused by morphological variation from those due to true electronic heterogeneity. By comparing samples with different crystallographic orientations and grain sizes the factors causing electronic heterogeneities have been isolated. Our results provide a crucial step toward understanding the factors which control heterogeneity in trap site density at the surface, and toward bridging the gap between thin films and single crystals.

Lead halide hybrid perovskite semiconductors have emerged as attractive candidates for photovoltaic applications owing to their large absorption coefficients, easy synthesis and property tailoring via composition engineering. MAPbX₃ perovskites (where MA = methylammonium and X = Br/Cl) were the subject of recent studies on the partial substitution of Pb to obtain stable mid-gap states with tunable energy level [1,2]. This paves the path towards a new class of multi-junction devices called intermediate band (IB) photovoltaics, which can theoretically surpass the Shockley-Queisser (S-Q) limit of solar conversion efficiency with additional absorption of sub-gap photons [3,4]. Density functional theory (DFT) calculations revealed that 1/8^th substitution of Pb by Co in MAPbBr_yCl_3-y (y ∈ {0-3}) perovskites (ideal parent semiconductors with band gaps between 2 eV and 2.3 eV) creates mid-gap energy states, which was experimentally confirmed via absorption and photoluminescence spectra with sub-gap absorption observed between 1.65 eV and 2 eV [1]. The lack of a second sub-gap absorption feature indicated that the mid-gap states were unfilled at the current level of substitution. This prompted the study of alternative substituents besides Co that can replace Pb in MAPbBr_yCl_3-y with a thermodynamic penalty similar to the thermal energy, and create desirable half-filled mid-gap states. Here, we report the crystal and electronic structure changes upon substitution of Pb by several 3d transition and Group I, II, III, IV and V metals of suitable ionic sizes (based on perovskite stability tolerance and octahedral factors) in MAPbBr_yCl_3-y systems (y ∈ {0-3}), specifically noting trends in parent band gap and substituent energy states within the band gap. Each metal substitution was studied in different charged states and charge transition energy levels were determined. We further compared formation energies of external metal substitution and various intrinsic point defects (vacancy, self-interstitial and anti-site) as a function of chemical potentials of constituent species [5], and determined the thermodynamic equilibrium growth conditions necessary for creating a stable external substitution that compensates for dominant intrinsic defects. Based on this work, promising candidates were identified as Pb-substituents in lead-based hybrid perovskites to create IB photovoltaic materials.

REFERENCES

[1] M.D. Sampson et al., J. Mater. Chem. A. 5, 3578 (2017).
[2] M.T. Klug et al., Energy & Environ. Sci. 10, 236 (2017).
[3] A. Luque et al., Nat. Photonics. 6, 146–152 (2012).
[4] A. Luque et al., Phys. Rev. Lett. 78, 5014-5017 (1997).
[5] Y. Yan et al., Springer International Publishing, Switzerland (2016).

Organohalide lead (hybrid) perovskites have emerged as competitive semiconducting materials for photovoltaic devices due to their high performance and low cost. To further the understanding and optimization of these materials, solution-based methods for interrogating and modifying perovskite thin films are needed. In this work, we report a hydrofluoroether (HFE) solvent-based electrolyte for electrochemical processing and characterization of organic−inorganic trihalide lead perovskite thin films. Organic perovskite films are soluble in most of the polar organic solvents, and thus until now, they were not considered suitable for electrochemical processing. We have enabled electrochemical characterization and demonstrated a processing toolset for these materials utilizing highly fluorinated electrolytes based on a HFE solvent. Our results show that chemically orthogonal electrolytes based on HFE solvents do not dissolve organic perovskite films and thus allow electrochemical characterization of the electronic structure, investigation of charge transport properties, and potential electrochemical doping of the films with in situ diagnostic capabilities.

Hybrid perovskites hold tremendous promise for next-generation solar cells, more than any other recently developed low-cost active PV material. However, stresses are generated in perovskite films during processing and magnified in service by environmental effects such as thermal cycling, resulting in the evolution of defects and propagation of damage for fragile materials. Unfortunately, perovskite layers are exceptionally fragile as measured by their fracture energy—moreso than OPVs by an order of magnitude and c-Si or CIGS solar cells by two orders of magnitude. Surprisingly, despite the significance of film stresses for device stability, the origin and value of stresses in perovskite films has been largely overlooked.

We report on the stress values of perovskite films, which are tensile and can exceed 50 MPa in magnitude, a value which is comparable to the yield stress of copper. These stress values provide a significant driving force for fracture, contributing to perovskite instability and reducing device performance. We show a direct correlation between processing temperature and film stress, a result of the extremely high thermal expansion coefficient of perovskites. No evidence of stress relaxation is observed, and the film stress is purely elastic due to the thermal expansion mismatch between the perovskite and substrate. Methods for reducing stress in perovskite films are presented, which include using lower processing temperatures and substrates with high thermal expansion coefficients. From these guidelines, we show improved environmental stability and demonstrate the feasibility of reliable perovskite devices on flexible substrates resistant to temperatures up to 85°C without encapsulation.

Mesoporous structure perovskite solar cell (MS PSCs) is constructed by front contact, electron selective contact and metal back contact [1,2]. For this configuration the mesoporous structure plays an important role, due to its porosity, pore size and morphology can determine the coverage, morphology of the perovskite layer and carrier lifetime [3,4]. One of the most used mesoporous materials for this application has been the titanium dioxide, for itself shows excellent properties such as chemical and biological stability, optical transparency, low toxicity and inexpensive production. This work presents the fabrication of the mesoporous titanium dioxide (mp-TiO₂) applied as a scaffold layers on perovskite solar cells. We developed two different synthesis of mp-TiO₂ by changing the templates and the acidity in a way to study the dispersion of the particles and the final mesoporosity obtained. The first synthesis was composed by titanium isopropoxide (TTIP), acetic acid (CH₃COOH), polyvinylpyrrolidone (PVP) and ethanol (C₂H₆O). The second synthesis was obtained by adding titanium isopropoxide (TTIP), hydrochloric acid (HCl), Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly (ethylene glycol) (P-123) and ethanol (C₂H₆O). The two different mp-TiO₂ precursor solutions were spin-coated at different speeds onto previous prepared blocking layer of TiO₂ (bl-TiO_2, 30 nm) deposited on FTO substrates. The obtained layers (FTO/bl-TiO₂/mp-TiO₂) were thermally treated at different temperatures to induce the crystallinity. After that a layer of perovskite was deposited by spin coating, followed by spin-coating of Spiro-OMeTAD as hole transport materials (HTL). We studied the volume porosity obtained by using the Volume Average Theory. The porosities obtained were from 40-44% when the spin speed is varied. The best power efficiency (PCE) obtained was about 13% with Spiro-OMeTAD.

Rerences:

1)P. Wang, J. Zhang, R. Chen, Z. Zeng, X. Huang, L. Wang, J. Xu, Z. Hu and Y. Zhu. Electrochimica Acta 227 (2017) 180-184.
2)N.-G. Park. The Journal of Physical Chemistry Letters 4 (2013) 2423-2429.
3)Y. Zhao, K. Zhu. The journal of Physical Chemistry Letter 4 (2013) 2880-2884.
4)S. Li, Y. Wang, C. Tsai, C. Wen, C. Yu, Y. Yang, J. Lin, D. Wang, C. Chen, Y. Yeha, C. Chen. Nanoscale y (2015) 14532-14537.

The organic-inorganic hybrid perovskite materials, especially methyl ammonium lead iodide (MAP), have garnered a huge interest of the photovoltaic community due to their insurmountable charge carrier properties.^1,2 The rapid surge of the photon to current conversion efficiencies (PCE) with MAP as a light absorber, in a span of 9 years³ showed promise to bring a breakthrough in the photovoltaic technologies, due to the ease of manufacture and the low cost of fabrication, until the literature started highlighting the instability of this material. The degradation of this material stems from its limited tolerance to moisture, oxygen and light.^4,5 Other than the inherent propensity of this material to degrade under ambient conditions, we have demonstrated the role of substrate in dictating the rate and product of degradation, and also devised a simple, chemical route to regenerate the degraded perovskite.⁶ Additionally, in an attempt to fabricate stable and robust solar cells, we have synthesized a novel hybrid compound, imidazolium lead iodide (ImP) which has the same stoichiometry ABX₃ as the perovskite, but crystallographically is a hexagonal structure.^7,8 We show, based on the structural analysis and our experiments, that ImP is much more stable to ambient conditions as compared to the conventional perovskite MAP and has the potential to serve as a promising candidate for photovoltaic devices.⁸

References
1. Wehrenfennig, C.; Eperon, G.; Johnston, M.; Snaith, H.; Herz, L. Advanced Materials 2013, 26, 1584-1589.
2. Motta, C.; El-Mellouhi, F.; Sanvito, S. Scientific Reports 2015, 5, 12746.
3. Park, N. The Journal of Physical Chemistry Letters 2013, 4, 2423-2429.
4. Dkhissi, Y.; Weerasinghe, H.; Meyer, S.; Benesperi, I.; Bach, U.; Spiccia, L.; Caruso, R.; Cheng, Y. Nano Energy 2016, 22, 211-222.
5. Tong, C.; Geng, W.; Tang, Z.; Yam, C.; Fan, X.; Liu, J.; Lau, W.; Liu, L. The Journal of Physical Chemistry Letters 2015, 6, 3289-3295.
6. Seth, C.; Khushalani, D. RSC Adv. 2016, 6, 101846-101852.
7. Weber, O.; Marshall, K.; Dyson, L.; Weller, M. Acta Crystallographica Section B Structural Science, Crystal Engineering and Materials 2015, 71, 668-678.
8. Seth, C.; Khushalani, D. (Manuscript under preparation)

Compact TiO₂ is well-known electron transport material in sensitized solar cells and perovskite solar cell. However, because of the unsuitable energy level and slow electron extraction of TiO₂, the planar TiO₂ perovskite solar cell showed low power conversion efficiency with high hysteresis. Here, the planar TiO₂ electron transport layer is modified with p-type CuI for perovskite solar cells. It is found that the polarity of the CuI-modified TiO₂ surface pull electrons to the surface of the TiO₂, which improves electron extraction and makes barrier-free energy level. Furthermore, the radiative recombination is also reduced by high extraction. The final device shows a 19.0% high efficiency with reduced hysteresis by removal of trap s<span style="font-size:10.8333px">tate</span>.

The film quality of perovskite (CH₃MH₃PbI₃) is very crucial to the performance of perovskite solar cells. Here we present two methods to grow high quality perovskite films via a simple and rapid process. Polymer PAN and a series of hydrochlorides with different organic amine cations were introduced into CH₃NH₃PbI₃ solution as a additive to obtain solar cells with high power conversion efficiency (PCE). Based on the device performance, 2D material MoS₂ and GO as interfacial layers make perovskite solar cells with a planar heterojunction have high PCE up to 19.14% with high V_OC of 1.35 V as well as a significant enhancement of stability.

Hybrid organic-inorganic halide perovskites are low-cost solution-processable solar cell materials with photovoltaic properties that rival those of crystalline silicon. The perovskite films are typically sandwiched between thin layers of hole and electron transport materials, which efficiently extract photogenerated charges. This affords high-energy conversion efficiencies but results in significant performance and fabrication challenges. Herein we present a simple charge transport layer-free perovskite solar cell (PSC), comprising only a perovskite layer with two interdigitated gold back-contacts. Charge extraction is achieved via self-assembled molecular monolayers (SAMs) and their associated dipole fields at the metal/perovskite interface. Photovoltages of approximately 600 mV generated by SAM-modified PSCs are equivalent to the built-in potential generated by individual dipole layers. Efficient charge extraction results in photocurrents of up to 12.1 mA cm^-2 under simulated sunlight, despite a large electrode spacing. Full control of the charge extraction process in photovoltaic devices via SAM dipole-fields as shown here renders electronically matched charge extraction layers redundant and has potential application well beyond the field of PSCs.

Perovskite solar cells have had a rapid advance in photovoltaic devices. In 2009, an efficiency of 3.81% was recorded and in 2017 an efficiency of over 22% has been achieved. However, stability problems still arise due to it rapidly decomposing when exposed to the environment thus limiting its use and commercialization. Humidity has been an important factor. In the present work, the interaction between CH₃NH₃PbI₃ and H₂O vapor is studied. The perovskite was synthesized in two steps by the drop casting method. Uniform black-colored films were obtained. They were immediately subjected to an accelerated degradation using QUV Accelerated Weathering Tester Q-Lab. It was characterized by optic microscopy, UV-Vis spectroscopy, XRD and SEM. The effect of the PbI₂/ CH₃NH₃I proportions was demonstrated regarding the stability after the accelerated degradation. The results showed that the perovskite degradation is reversible when it is no longer exposed to humidity.

Project CONACYT Problemas Nacionales 2015-01-1739.

The electronic-ionic mixed conducting nature of organometal halide perovskites (OHPs) necessitates the investigation of ion migration for complete understanding of the operation principle in OHP-based photovoltaic devices.^[1] Herein, the origins affecting the temporal transient of photovoltage/current profiles in (FAPbI₃)_0.83(MAPbBr₃)_0.17-based solar cells and their Cs-doped counterparts will be discussed (with FA standing for (NH₂)₂CH⁺, and MA for CH₃NH₃⁺, respectively). Rapid crystallization of OHPs by conventional antisolvent method has long been one of the most successful approach, but there are still room for the investigation of the impacts of thermal annealing on the OHPs’ characteristics: the microstructural evolution (and microstructure-related optical properties), mobile ionic species altering the energy landscape, and concomitant change of photocarrier kinetics in OHPs-based devices.^[2,3] Frequency and time-dependent analyses, such as impedance spectroscopy and transient voltage/current measurements under controlled light/bias conditions, are used to understand the influence of ion migration on the photocarrier recombination in various timescales. Conductive atomic force microscopy further elaborates the effect of mobile ions on the electronic traps and the related photocurrent behaviours. These findings, all combined, will provide clues on the critical factors enabling the efficient and long-term stable OHP solar cells.

[1] R. A. Belisle, W. H. Nguyen, A. R. Bowring, P. Calado, X. Li, S. J. C. Irvine, M. D. McGehee, P. R. F. Barnes, and B. C. O’Regan, Energy Environ. Sci. 2017, 10, 192.
[2] T. Hwang, B. Lee, J. Kim, S. Lee, B. Gil, A. J. Yun, and B. Park, Adv. Mater. (accepted).
[3] T. Matsui, J.-Y. Seo, M. Saliba, S. M. Zakeeruddin, and M. Grätzel, Adv. Mater. 2017, 29, 1606258.

Perovskite solar cells (PSCs) have demonstrated high power conversion efficiency (PCE) but poor long-term stability and remarkable hysteresis. To date, the most efficient PSCs have the n-i-p device architecture and use Li-TFSI/t-BP as standard dopants for hole-transporting layer (HTL). However, such dopants not only induce deleterious effects on stability but also significantly affect the hysteresis of PSCs. Here, we demonstrate that a novel Lewis acid tris(pentafl uorophenyl)borane (B(C₆F₅)₃) can be employed as an effective dopant for PTAA to realize a new record fill factor 0.81 in TiO₂-based n-i-p PSCs and a record PCE as high as 19.01% among the ever reported PSCs based on single dopant in HTL, versus 17.77% for the control device with Li-TFSI/t-BP doped PTAA. To the best of our knowledge, it is the first case that PSC based on a single dopant for HTL shows higher efficiency than that of the state-of-the-art Li-TFSI/t-BP. Besides, the B(C₆F₅)₃-based PSC displays lower J-V hysteresis and much better long-term stability up to 70 days in high humidity environment without encapsulation. We believe that this work opens up new avenue for high-efficiency, hysteresis-less and stable PSCs exploring novel dopants as alternatives to Li-TFSI/t-BP.

Methylammonium lead halide perovskites (MAPbX₃) as promising light absorbers have great potential in perovskite solar cells (PSCs) due to notable characteristics of optimal band gaps, long-range exciton diffusion lengths, high ab-sorption coefficients and simple solution approaches. To date, the highest power conversion efficiency (PCE) of 22.1% has been achieved in a typically sandwiched device architecture. PSCs are still facing some challenges, such as the TiO₂/MAPbI₃ heterojunction interface loss, hysteresis behavior and reproducibility. Hence, we systematically studied the independent role of 2-, 3- and 4-pyridinesulfonic acid (PA) self-assembled monolayers (SAMs) on perovskite crys-tallinity and illuminated the binding energies of the above-mentioned SAMs on TiO2 and perovskite surfaces for highly efficient PSCs with reduced TiO₂/MAPbI₃ heterojunction interface loss, negligible hysteresis and high reproducibility. Through use of the 4-PA SAMs, the PSC exhibits striking improvements to the reach the highest efficiency of 18.90%, which constitutes an enhancement of ∼20% compared to that of PA SAMs-unmodified PSC (14.65%). Our work high-lights the importance of SAMs dipolar interactions and binding energies of SAMs on TiO₂ and perovskite surfaces at electron transporting layer (ETL)/perovskite interfaces and paves the way for further optimizing the performances of PSCs.

Perovskite solar cells (PSCs) have been received great attention in recent years due to their high power conversion efficiency (PCE) and low temperature solution process, and thus can be fabricated on flexible substrate such as metal-coated PEN or PET plastic. The highest PCEs of PSCs have been increased from initially 3.8% to now more than 20% [1]. However, most high-efficiency PSCs have been developed using non-scalable fabrication technologies such as spin-casting, and in strictly-controlled atmosphere like in N₂ filled glovebox. Hence, it is critical to study on the fabrication of high-quality PSCs with scalable printing techniques such as doctor blade-coating under the ambient condition, in order to accomplish the final commercialization [2].
In this work, we have developed a fully printed process under ambient condition to fabricate flexible PSCs. The structure we employ is a typical inverted “p-i-n” structure, and we adopt one-step method to fabricate the perovskite layer with lead iodide as lead source. PEDOT:PSS and PCBM serve as the hole transport layer and electron transport layer respectively. Except that Ag electrode is deposited by thermal evaporation, all other layers are prepared by blade-coating method under the ambient condition. The test results show that, the PCE of solar cells with 0.1cm2 active area can achieve PCE of 8.95%, with Jsc= 17.4 mA/cm2, Voc=0.83V and FF=62%. The SEM images indicate that the perovskite layer is highly crystalline and has uniform surface coverage. The detailed information about the effect of ink and printing parameters on the properties and microstructures of PCSs will be presented at the Meeting. We believe this preparation technique has great potential to fabricate large area and high quality flexible PSCs, and lay down the foundation for the industrialization of PSCs.
References
1. Yang, W., Park, B., Jung, E., Jeon, N., Kim, Y., Lee, D., Shin, S., Seo, J., Kim, E., Noh, J. and Seok, S. (2017). Iodide management in formamidinium-lead-halide–based perovskite layers for efficient solar cells. Science, 356(6345), pp.1376-1379.
2. Yang, Z., Chueh, C., Zuo, F., Kim, J., Liang, P., & Jen, A. (2015). High-Performance Fully Printable Perovskite Solar Cells via Blade-Coating Technique under the Ambient Condition. Advanced Energy Materials, 5(13), 1500328.

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A novel organic-inorganic hybrid perovskite CH₃NH₃PbX₃ (X = Cl, Br, I) has emerged as a strong competitor in photovoltaic applications due to its superior characteristics including high absorption coefficient, broad absorption spectrum, tunable band gap, long carrier lifetime, high balanced hole and electron mobility, etc.^{[1, 2]} The first embodiment of perovskite solar cells (PSCs) only demonstrated a power conversion efficiency (PCE) of 3.8%, which by now has been boosted to 22.1%.^[3-4] However these PCEs have been obtained by using Spiro-OMeTAD as the hole-transporting material (HTM), which is too expensive for mass production. In this work, we designed an HTM-free PSC with the structure of ITO/Cu(mtsc)I-doped CH₃NH₃PbI₃/PCBM/BCP/Ag. Cu(mtsc)I (mtsc: 2-methyl-3-thiosemicarbide) was first synthesized, and then doped into the precursor solution at an extremely low concentration to form a bulk heterojunction type perovskite layer ,and thus the HTM layer is not needed. After optimization, a PCE of 18.33% has been achieved on glass-substrate. Compared to the traditional p-type copper-based semiconductor such as CuI, Cu(mtsc)I has better energy level matching and is easier to form a chelate with the CH₃NH₃PbX₃ material, resulting in much higher power conversion efficiencies.
References
[1] S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, H. J. Snaith, Science 2013, 342, 341.
[2] G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Gräetzel, S. Mhaisalkar, T. C. Sum, Science 2013, 342, 344.
[3] A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, Journal of the American Chemical Society, vol. 131, no. 17, pp. 6050-6051, 2009.
[4] W. Yang, B. Park, E. Jung, N. Jeon, Y. Kim, D. Lee, S. Shin, J. Seo, E. Kim, J. Noh and S. Seok, Science, vol. 356, no. 6345, pp. 1376-1379, 2017.
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Organic–inorganic CH₃NH₃PbI₃ (MAPbI₃) inverted-structure perovskite films were prepared using Pb(CH₃COO)₂ (Pb(OAc)₂) and CH₃NH₃I (MAI) as source materials. The structural, optical and photoelectronic properties of the resulting MAPbI₃ films varied with the annealing temperature and the Pb(OAc)₂/MAI molar ratio. A CH₃NH₃PbI₃ film annealed at 80^oC for 15min showed the highest power conversion efficiency (PCE) of more than 10%. It was also found that the Pb(OAc)₂/MAI molar ratio greatly influenced the structure and morphology of the MAPbI₃ films. A suitable amount of excess Pb(OAc)₂ (about 5mol% excess Pb) in the solution made the film smoother and improved the film crystallinity. This enhanced the PCE to about 13~15%. However, for films derived from solution with low Pb(OAc)₂/MAI or high Pb(OAc)₂/MAI ratios, defects such as pits or pinholes were easily formed, with low crystallinity. These decreased the lifetime of the carriers and thephotoelectrical properties of the final solar cells.

Inverted perovskite solar cells with a lead acetate precursor and the architecture of glass/ITO/PEDOT:PSS/MAPbI₃/PCBM/Ag have been synthesized. These cells have shown an average power conversion efficiency (PCE) of 10.8%. We have demonstrated that the addition of a zirconium acetyloacetonate (ZrAcac) buffer layer with an optimized concentration of 0.002M to 0.003M in ethanol on top of a PCBM layer which is the conventional electron transport layer has resulted in an improved PCE of 12.5%. We investigated the effect of concentration of ZrAcac on the device performance and attribute the 15% increase in the PCE (over the control device) to improved light harvesting and more efficient electron transport to the respective counter electrode.

Thin films based on two-dimensional metal halide perovskites (2D MHPs) have achieved exceptional performance and stability in numerous optoelectronic device applications. Simple solution processing of the 2D MHPs provides opportunities for manufacturing devices at drastically lower cost. A key to high device performance is to align the 2D MHP layers, during the solution processing, vertical to the electrodes to achieve efficient charge transport. It is, however, yet to be understood how the vertical orientations of 2D MHPs layers on substrates can be obtained. Here we report the formation mechanism of such vertically orientated 2D MHPs in which the nucleation and growth arise from the liquid-air interface, confirmed by our studies on substrate dependence, in-situ grazing incidence X-ray diffraction measurement and optical measurements. The vertical orientation originates from the anisotropic environment at the liquid-air interface. As a consequence, choice of substrates can be liberal from polymers to metal oxides depending on targeted application while still maintaining the vertical orientation for efficient charge transfer. We demonstrate control over the degree of preferential orientation of the 2D MHP layers from almost complete vertical orientation to partial random orientation, and show that the degree of orientation has a drastic impact on solar cell performance.

Organic-inorganic lead halide perovskites have shown impressive power conversion efficiency (PCE) in a range of solar cell architectures. Despite the multiple ionic compositions that have been reported so far, the presence of organic-constituents is a common attribute in all the high efficiency formulations, with the methylammonium (MA) and formamidinium (FA) cations being the sole realistic options available to date. In this study, we demonstrate a novel three-dimensional (3D) perovskite with improved material stability as a result of the incorporation of an alternative organic cation, guanidinium, into the MAPbI₃ crystal structure. The new perovskite material shows enhanced thermal stability and optoelectronic properties, that allows high power conversion efficiencies over 20 %, an important progress within the perovskite field.

Because of the low-temperature process and high device efficiency, organic-inorganic metal halide perovskite absorbers offer an unusual opportunity for fabricating low-cost, high efficiency polycrystalline perovskite thin-film tandem solar cells. Tandem solar cells combining wide-bandgap and low-bandgap subcells can be fabricated in the two-terminal (2-T) or four-terminal (4-T) configuration, each exhibiting unique advantages. In this talk, we will present our recent results on the optimization of the subcells and the resulting 4-T and 2-T tandem cells. For the wide-bandgap subcell, we use FA_0.8Cs_0.2Pb(I_0.7Br_0.3)₃ as the absorber, which exhibits a bandgap of ~1.75 eV. For the low-bandgap subcell, we use FA_0.6MA_0.4Sn_0.6Pb_0.4I₃ as the absorber, which has a bandgap of ~1.25 eV. We further applied various approaches such as using additives and/or post-deposition treatment to improve the quality of the absorbers. We will also discuss key challenges experienced in fabricating high efficiency tandem solar cells.

The high quality phase pure organic-inorganic perovskite such as MA1-xFAxPbI3 or FAPbI3 planar films with extended absorption and enhanced thermal stability were difficult to deposit by regular simple solution chemistry approaches due to the crystallization competition between the easy-to-crystallize but unwanted δ-FAPbI3/MAPbI3 and FAxMA1-xPbI3 or FAPbI3 requiring rigid crystallization condition. A faclile 2D-3D conversion was developed to transform compact 2D mixed composition HMA1-xFAxPbI3Cl perovskite precursor films into 3D MA1-xFAxPbI3 (x=0.1-0.9) perovskites.The high quality phase pure MA1-xFAxPbI3 (x=0.1-0.9) and FAPbI33 perovskite films via 2D-3D conversion achieve a >20% efficiency with higher thermal stability. The all-inorganic α-CsPbI3 perovskite with the most suitable band gap for tandem solar cell application faces an issue of phase instability under ambient conditions. We discovered that a small amount of 2D EDAPbI4 perovskite containing ethylene diamine (EDA) cation stabilizes the α-CsPbI3 to avoid the undesirable formation of the non-perovskite delta phase, achieveing a record efficiency of 11.8%.In all, the 2D-3D conversion and 2D/3D hybridzation would be a promising strategy for fabricating and stabilizing lead halide perovskite solar cells.

Organic–inorganic halide perovskites are receiving remarkable attention in the field of energy materials. The reaction of hybrid lead halide perovskites with Li metal has recently been proposed for a number of potential applications. However, the mechanisms for Li uptake in such materials, such as intercalation and conversion, are still unknown. Using a combination of density functional theory and electrochemical and diffraction techniques, we consider Li intercalation and conversion reactions in CH₃NH₃PbI₃, CH₃NH₃PbBr₃ and CH₃NH₃PbCl₃. Our simulations suggest that conversion reactions with Li are far more energetically preferable in these materials than Li intercalation. The calculations confirm the formation of Pb metal as a result of Li conversion in all three materials, and this is supported by an X-ray diffraction analysis of CH₃NH₃PbBr₃. The results of this study provide fresh insights into lithium and halide perovskite reactions that will hopefully drive further exploration of these materials for a wider variety of energy applications.

On the basis of concerted research efforts worldwide, there is no doubt that outstanding power conversion efficiency (PCE) can be achieved in perovskite solar cells. However, to move forward this technology towards commercialization, developments of up-scaling processes with high PCE and stability is important [1-3]. At OIST, a team of researchers in the Energy Materials and Surface Sciences Unit has been making concerted efforts to develop processes aiming at high PCE, high-throughput, and minimum batch-to-batch variation, and compatible with large-area perovskite solar cells and modules. In this talk, we will present our recent progress using chemical vapor deposition [4-8] and spray coating [9] to fabricate perovskite solar cells and modules. Also, we will introduce a novel methylamine gas-induced crystallization process [10, 11], which provides insights into the formation of perovskite films.

[1] L.K. Ono, N.-G. Park*, K. Zhu*, W. Huang*, Y.B. Qi*, ACS Energy Lett. 2 (2017) 1749.
[2] L. Qiu, L.K. Ono, Y.B. Qi*, Mater. Today Energy (2017) DOI:10.1016/j.mtener.2017.09.008.
[3] M. Remeika, Y.B. Qi*, J. Energy Chem. (2017) in press.
[4] L.K. Ono, M.R. Leyden, S. Wang, and Y.B. Qi*, J. Mater. Chem. A 4 (2016) 6693.
[5] M.R. Leyden, Y. Jiang, and Y.B. Qi*, J. Mater. Chem. A 4 (2016) 13125.
[6] M.R. Leyden, M.V. Lee, S.R. Raga, and Y.B. Qi*, J. Mater. Chem. A 3 (2015) 16097.
[7] M.R. Leyden, L.K. Ono, S.R. Raga, Y. Kato, S.H. Wang, and Y.B. Qi*, J. Mater. Chem. A 2 (2014) 18742.
[8] Y. Jiang, M. R. Leyden, L. Qiu, S. Wang, L. K. Ono, Z. Wu, E. J. Juarez-Perez, Y. B. Qi*, Adv. Funct. Mater. (2017) in press.
[9] M. Remeika, S.R. Raga, S. Zhang, and Y.B. Qi*, J. Mater. Chem. A 5 (2017) 5709.
[10] S.R. Raga, L.K. Ono, and Y.B. Qi*, J. Mater. Chem. A 4 (2016) 2494.
[11] Y. Jiang, E.J. Juarez-Perez, Q. Ge, S. Wang, M.R. Leyden, L.K. Ono, S.R. Raga, J. Hu, and Y.B. Qi*, Mater. Horiz. 3 (2016) 548.

The unprecedented progress in power conversion efficiency (PCE) of halide perovskite solar cells (HPSCs) have attracted a great attention. The PCE has reached to 22.7 % which is above that of conventional thin film solar cells such as CIGS and CdTe cells. It is notable that the high PCE can be achieved by solution process at low temperature under 200 ^oC because it shows great potential for low-cost, flexible, and light weight in photovoltaic devices. The progress in PCE is attributed to developments in terms of device architecture, halide material, and fabrication process based on material engineering. In initial stage, control of morphology of halide thin film was a key factor for improvement of PCE. Strategy to introduce a mediator during formation of halide crystal film successfully fabricated a compact halide thin film with several hundred nanometers, resulting in breakthrough of PCE. In particular, intramolecular exchanging process using PbI₂(DMSO) intermediator thin film was developed for high quality formamidinium lead iodide (FAPbI₃) film. Recently, we also reported key finding for the breakthrough in the PCE that is to reduce deep level trap density of the halide film. The trap is a main non-radiative recombination site which causes to lower open circuit voltage in photovoltaic device. Triiodide ions (I₃^-) are introduced to reduce the deep level traps formed during growing halide perovskite film. We found that the addition of the triiodide ion solution into organic halide solution during formation of halide perovskite film leads to reduction in deep level trap density which is markedly different from the addition of iodine solution prior the conversion to the triiodide ion. Furthermore, the photovoltaic performance of HPSC is not only related to halide materials themselves but also is dependent on n(p)-type oxide and organic semiconducting layers. Therefore, design of the heterojunction between halides and n(p)-type layers will be also discussed for further improvement of PCE in HPSCs.

Hybrid organic-inorganic lead halide perovskite solar cells have made rapid advancements in efficiency, approaching and overtaking those of other thin-film technologies (1). Their cheap and facile deposition, alongside their tunable bandgap, makes them an ideal candidate for tandem devices, with promise to boost the existing efficiency of single-crystal silicon cells at low additional cost (2). Before commercialization can be achieved, however, the stability of perovskite solar cells must be improved. While moisture exposure can be mitigated through careful encapsulation, the thermal stability of the cell, with respect to both intrinsic degradation of the absorber material and extrinsic reactions with other layers, is critical.
We evaluate thermal stability of semitransparent FA_0.83Cs_0.17Pb(I_0.83Br_0.17)₃ perovskite solar cells at 85C in a nitrogen environment for up to 1000 hours, and show that the primary factor in cell degradation is reaction with a metal contact. Using depth profiling in x-ray photoelectron spectroscopy alongside SIMS techniques, we show that silver, copper, and gold not only create a driving force for iodine migration from the perovskite, but also surprisingly have the potential to diffuse through a sputtered tin-doped indium oxide (ITO) window layer, an atomic layer deposited (ALD) tin oxide layer, and an evaporated fullerene electron transport layer into the perovskite, harming the performance of the perovskite solar cell.
The poor barrier quality of the transparent conducting oxide (TCO) is due largely to diffusion channels in domain boundaries created by a proliferation of the existing rough perovskite morphology. We investigate several solutions, including spin-coating the fullerene layer, adding an ALD titania barrier layer, and using amorphous indium zinc oxide (IZO) as an alternative TCO. We discuss the performance and viability of each solution as well as implications for perovskite/silicon and perovskite/perovskite tandem solar cell design. Solving this problem is critically important because metal grid lines are commonly used to reduce series resistance in modules. Moreover our experiments reveal pinholes that could enable water to enter solar cells or volatile cations (e.g. methylammonium or formamidinium) to leave. Having an extremely impermeable contact should prevent multiple types of degradation.

1 .Best Research-Cell Efficiencies (NREL, 2017); https://www.nrel.gov/pv/assets/images/efficiency-chart.png.
2. K. A. Bush, A. F. Palmstrom, et al., Nat. Energy 2, 17009 (2017).

Herein we demonstrate that the surface composition of TiO₂ electron-selective interlayers strongly influence the electrical properties of the interlayer and the nucleation/growth of MA-doped FAPbI₃ perovskite active layers for use in PVs. Simultaneous control of active layer nucleation/growth and interfacial electrical properties with the underlying contact is predicted to significantly influence the energy conversion efficiencies and stabilities of perovskite PVs. Variable incident-angle Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) was used to monitor depth-dependent changes in the perovskite film structure on TiO₂ surfaces that were compositionally modified with O₂-plasma and/or UV-light pretreatment as measured by X-ray photoemission. Conducting tip AFM (cAFM) was used to map how these oxide pretreatments also affect the electrical properties of the oxide and the combined TiO₂/perovskite sample. TiO₂ thin films deposited by chemical vapor deposition are extremely reactive toward ambient small molecules and to perovskite precursor ions as a result of 5-6 different proposed surface defects that can act as adsorption/redox sites. Controlling these native or reacted surface defects by O₂-plasma or UV-light pretreatment impacts TiO₂ charge harvesting ability and interactions with a nucleating film. For the first time, angle-resolved GIWAXS shows that such oxide pretreatments influence the structure of the perovskite thin films first at the TiO₂/perovskite interface which then propagates into the remaining bulk film. By advancing the incident X-ray angle from slightly below to slightly above the critical angle of the perovskite film, thus changing the X-ray probe depth between tens and hundreds of nanometers, GIWAXS reveals a preference for <101> parallel perovskite crystal orientation closer to the buried TiO₂/perovskite interface, versus the top of the film, in all sample types. Furthermore, O₂-plasma treated oxide films, followed by UV-light exposure, show the highest perovskite film preferred orientation and crystallinity at this buried interface amongst all oxide pretreatments. cAFM also demonstrates that UV-O₂-samples contain the most electrically active TiO₂ interlayer when surveyed across the surface, which translates, along with the highest perovskite film orientation/crystallinity, to the most conductive TiO₂/perovskite heterojunction for optimal electron harvesting. We believe these results are explained by UV-O₂ treatment removing chemi- and physi-sorbed carbon-based species at or proximal to Ti³⁺ defects which can then act as nucleation sites for perovskite precursor ions and as electron injection points. The fact that interface chemistry at nanometer length scales at the TiO2/perovskite heterojunction controls perovskite structure tens to hundreds of nanometers away from that interface, as well as overall sample electrical activity, highlights the the the importance of this interface in the optimization of perovskite PVs.

The race to commercialize hybrid perovskite solar cells (HPSCs) is currently limited by device stability, which is influenced by the structure of the perovskite compound, its interaction with the environment (moisture, oxygen, etc) and its intrinsic electrochemical stability.¹ It is only recently that electrochemical ionic processes have been suspected by several groups for triggering reversible efficiency losses.^2,3 However, under operational conditions the direct observation of ionic dynamics is often blurred by other processes associated with free and trapped carriers. To this extent, reverse bias in the dark is an interesting regime that does not involve thermally injected or photogenerated carriers and allows us to isolate the role of mobile ions accumulating at the contacts. In addition, by studying a solar cell in these conditions, one can simulate cell shading in a solar module. When a cell is shaded in a module, the current flowing through the cell drops and a negative voltage builds up within the shaded cell as the illuminated cells try to push current through it.Since it is challenging to incorporate bypass diodes into thin-film solar panels, reverse bias breakdown of shaded cells can be a serious reliability problem.
In this talk, we will discuss the effect of reverse bias on HPSCs with different contacts.⁴ We will present various electrochemical measurements which show that at relatively low values of the applied reverse bias (of the order of -1V to -4V), mobile ions accumulating at the contacts induce the formation of a few-nanometer-thick depletion layer, which triggers electron tunneling. We will then demonstrate that at prolonged reverse bias, an ionic electrochemical reaction takes place and leads to reversible performance losses. Finally, we will discuss the implications of these findings for perovskite solar cells and modules.

¹Leijtens, T et al. J. Mater. Chem. A. (2017) 5, 11483- 11500
²Denf X. et al. J. Mater. Chem. C, 2016, 4, 9060-9068
³Domanski K. et al. Energy Env. Sci., 2017,10, 604-613
⁴Bowring A. et al. Adv. Energy Mater., 2017, just accepted.

I will present and discuss recent data where we observe strongly temperature dependent Stokes shift in the electronic spectra of both hybrid and inorganic lead-halide perovskites. This behavior is distinct from other crystalline semiconductors. Standard solid state semiconductor physics fail to reproduce the temperature dependence. Dielectric solvation theories, originally developed for liquid phase molecular photophysics and chemistry, capture the experimental observation well. The efficacy of dielectric solvation theory indicates the break-down of quantized harmonic phonon picture in lead-halide perovskites. Instead of being stablized by harmonic phonon quanta, electronic excited states are solvated by the dielectric response from an anharmonic vibrational continuum. Similarities between hybrid and inorganic lead-halide perovskites show organic cations are neither a crucial nor a necessary component for dielectric solvation of charge carriers. The dielectric response is provided by the general perovskite framework normal modes.

Studies of organometallic perovskite solar cells have been remarkably grown within several years, but still with the concerns of poor stability and insufficient power conversion efficiency (PCE). To overcome the limit from these concerns, modification of perovskite materials should be addressed [1-3]. Herein, we have demonstrated imidazole (C₃H₄N₂) as a cation dopant for conventional methylammonium lead iodide (MAPbI₃). Being aromatic hydrocarbon species with small ionic radius, imidazole cations were appropriately alloyed with MAPbI₃ [4]. Also, delocalized π-bond in imidazole allowed the formation of unprecedented kind of hybrid perovskite with both high electrical conductivity and stability. Optimal content of imidazole incorporated in MAPbI₃ led to the improved PCE approaching ~18.0% with negligible hysteresis, and both materials and device stability were largely improved when stored in various conditions, such as air, light, and high temperature. Also, devices with large active area of 2 cm² exhibited PCE as high as ~12.2%, further addressing the effect of imidazole on the formation of high quality nanostructures and devices.

[1] J. Kim, T. Hwang, S. Lee, B. Lee, J. Kim, G. S. Jang, S. Nam, and B. Park, Sci. Rep. 6, 25648 (2016).
[2] T. Hwang, B. Lee, J. Kim, S. Lee, B. Gil, A. J. Yun, and B. Park, Adv. Mater. (2017) (accepted).
[3] J. Lu, L. Jiang, W. Li, F. Li, N. K. Pai, A. D. Scully, C.-M. Tsai, U. Bach, A. N. Simonov, Y.-B. Cheng, and L. Spiccia, Adv. Energy Mater. 7, 1700444 (2017).
[4] G. Kieslich, S. Sun, and A. K. Cheetham, Chem. Sci. 5, 4712 (2014).
Corresponding Author: Byungwoo Park: [email protected]

Cs-substituted mixed cation hybrid perovskites are promising materials for solar cell applications, due to their excellent photo-electronic properties and improved stability. Although power conversion efficiencies (PCEs) as high as 21.1% have been reported, devices are mostly processed by spin coating (e.g., the anti-solvent method) with an active area of < 0.3 cm², which is difficult for further scaling-up. Here we report a scable method to fabricate Cs-substituted perovskite i.e., Cs_xFA_1-xPbI₃ by performing Cs cation exchange on hybrid CVD grown FAPbI₃. The perovskite film shows high uniformity over a large area of 10 cm × 10 cm. The champion perovskite module (5 cm × 5 cm) based on Cs_0.07FA_0.93PbI₃ with an active area of 12.0 cm² shows a module PCE of 14.6% and PCE loss/area of 0.17%/cm², demonstrating the significant advantage of this method toward scaling-up. Our in-depth study shows that when the perovskite films contain 6.6% Cs⁺ in bulk, i.e., Cs_0.07FA_0.93PbI₃, solar cell devices show not only significantly increased PCEs but also substantially improved stability, due to favorable energy level alignment with TiO₂ electron transport layer and spiro-MeOTAD hole transport layer, increased grain size and improved perovskite phase stability.

[1] Y. Jiang, M. R. Leyden, L. Qiu, S. Wang, L. K. Ono, Z. Wu, E. J. Juarez-Perez, Y. B. Qi^*, Adv. Funct. Mater. 2017, accepted.

Organic-inorganic halide perovskite solar cells (PSCs) have experienced high-speed developments with the highest power conversion efficiency (PCE) of 22.1%, resulting in huge commercial prospects. However, the commonly used electron selective layer (ESL), TiO₂, always needs high-temperature post-treatment during its fabrication process, which hinders commercial applications of PSCs. Developing low-temperature processed ESLs with high performances can significantly promote the industrialization of PSCs. In this review, current progress in low-temperature processed ESLs, especially non-TiO₂ inorganic, organic materials and quantum dots (QDs), such as SnO₂, WO_x, C60, BaSnO3, etc., are summarized, including several results of our group. Their fabrication methods, properties, and applications in PSCs are highlighted in this review. We also discuss the importance of further developments of the low-temperature ESLs. These low-temperature processed ESLs will provide more possibilities for designing novel structural PSCs and promoting the commercialization of PSCs.

Recently, Pb-based organometal halide perovskite solar cells have passed 20 % power conversion efficiency (PCE). However, the main issue hampering commercialization is the amount of toxic lead used in this type of solar cells. For that reason, research is focused on Pb-free materials, especially Sn- or Bi-based perovskites. In literature, Sn-based perovskites are the most efficient alternative achieving maximum PCE of 6 %, but suffer from instantaneous degradation in ambient air. The more stable but less investigated counterpart is Bi-based perovskites.
Most efficient hybrid organic-inorganic Bi-based perovskite solar cells reported in literature reach PCE up to 0.2 % and consist of methylammonium bismuth iodide (MBI). Due to the air stability of Bi-based perovskites, it becomes possible to analyze effects of ambient conditions on the solar cell characteristics. In this work, we present efficient MBI perovskite solar cells which are stable in ambient air. The cells are processed by spin-coating under inert atmosphere of nitrogen, employing a standard non-inverted stack composed of compact and mesoporous TiO₂, the perovskite layer and Spiro-MeOTAD sandwiched between FTO and evaporated gold contacts. Structural and morphological characterization of processed layers and devices are carried out by scanning electron microscopy. Photovoltaic measurements are performed under simulated AM1.5 sunlight of 100 mW/cm² illumination. We highlight the influence of several concentrations of the perovskite solution (0.15 M - 0.3 M) on the photovoltaic characteristics. The highest PCE was achieved with 0.2 M yielding a maximum photocurrent of 0.56 mA/cm². We observed that exposing the solar cell to ambient air is essential to obtain the largest short-circuit current and open-circuit voltage. The cells exhibit reproducible open-circuit voltages of 0.73 V, which is one of the highest value published for this type of solar cell. The PCE increases over time from 0.004 % directly after processing up to 0.17 % after 48 h. This is most likely caused by the oxidation process of Spiro-MeOTAD which strongly increases its conductivity, overcompensating a potential degradation of the MBI perovskite. Extending the exposure to ambient air leads to a slow decrease of photovoltaic performance. Our experiments show that after 300 h exposure to air, the cells still deliver 56 % of their maximum PCE and 84 % of their maximum open-circuit voltage.
In order to improve Bi-based perovskite solar cells, the use of mixed-cation compositions is beneficial to accomplish higher photocurrents and consequently cell efficiency. It is obvious that Bi-based perovskites, as high-bandgap materials, provide great potential in tandem devices and hybrid organic-inorganic solar cells to be a low-cost and environmentally friendly alternative for expensive inorganic solar panels.

With the advancement in metal halide perovskite (MHP) research, it is now increasingly manifested that mixed cations and halides have many advantages over the single cation/halide MHPs. However, to push the commercialization of mixed cation/halide based MHP, myriad of stability properties, such as ink shelf life, must be understood. Here, significant stoichiometric changes are observed in highly alloyed formamidinium (FA), methylammonium (MA), cesium (Cs), lead (Pb), bromide (Br) and iodide (I) containing perovskite films that were fabricated from perovskite ink with different aging in the dark under N₂. This stoichiometric instability is caused by a change in ink composition over time where the ability of MA⁺, Cs⁺ and I^- to incorporate into the perovskite film is compromised with ink aging. Such change is initiated by the hydrolysis of dimethylformamide and resulted formation of dimethylamine. Dimethylamine then induce Lewis base-acid reaction with Cs⁺ and Pb²⁺. As a result, the quantity of reactive Pb²⁺ present in the ink is reduced over time, which leads to less MA⁺ and I^- incorporation into the perovskite film as they are energetically not favored to coordinate with Pb²⁺ compared to their counterparts (FA⁺, Br^-). To address this ink aging induced film stoichiometry change, solid precursor salts were ball milled into a stable powder, which was proven, via various optoelectronic characterizations, to be a very effective way of storing large quantities of highly alloyed perovskites precursor materials.

The power conversion efficiency of perovskite solar cells is continuously improving, however, the fundamental operational principles and the degradation mechanisms are insufficiently revealed, causing the delay of commercialization. Electrical, physical, and chemical variations need to be carefully investigated to establish causes of the degradation, and some macro-/mesoscopic studies have been made up to date. Nonetheless, for in-depth understanding of the origin, observations in nanoscale where photo-excited carrier generations and collections actually take place should be made. In this presentation, we show nanoscale observations of photocurrent generations of methylammonium lead iodide (CH₃NH₃PbI₃) perovskite solar cells, and uncover their origins of degradation mechanisms [1]. For the first time, we employ a novel near-field scanning photocurrent microscopy (NSPM) technique to image nanoscale photocurrent generations of perovskite solar cells. We investigate how photocurrent generations vary with respect to the sample annealing temperature, and observe increased photocurrent generations at grain boundaries of perovskite solar cells annealed at moderate temperature (100 °C). However, we observe the opposite carrier generation patterns (i.e., reduced photocurrent generations at grain boundaries and increased photocurrent generations at grain interiors) in perovskite solar cells annealed at higher temperature (130 °C). By combining results from our novel NSPM with other characterization techniques including electron microscopy and x-ray diffraction, we reveal that the spatial pattern of photocurrent is caused by material inhomogeneity and dynamics of segregation of lead iodide. Next, we examine the nanoscale signatures of aging caused by continuous light exposure under normal operation to establish the degradation mechanisms of perovskite solar cells. We image multi-stage nanoscale photocurrent generation patterns of a perovskite solar cell as a function of time while the cell is degrading under the extended light exposure. It is found that the extended light exposure drives further structural and compositional changes of materials revealed by the nanoscale photocurrent imaging. The nanoscale observation of perovskite solar cells’ operation depending on the sample preparation temperature and on the degradation conditions would suggest pathways to further improve high-efficient perovskite solar cells with long-term stability.

[1] Dongheon Ha*, Yohan Yoon*, Ik Jae Park, Paul M. Haney, Sangwook Lee, and Nikolai B. Zhitenev, Nanoscale mapping of photocurrent generation in perovskite solar cells, Energy and Environmental Science (under review) (* indicates equal contribution)

As perovskites have moved toward commercialization, the importance of mechanical reliability is receiving a significant increase in attention. Perovskites are susceptible to delamination and fracture from processing, handling, and operational conditions. We describe the mechanical properties of state-of-the-art perovskite solar cells, which have incorporated various compositions and microstructures to improve performance and chemical stability. The aim of this work was to understand how composition affected perovskite mechanical integrity and determine design criteria to increase cohesion and reliability toward the development of module-scale devices.

We report on recent studies characterizing the intrinsic mechanical integrity of perovskite compositions and fully explore the role of various cation combinations, additives, and microstructure on perovskite cohesion. Adding cations to the perovskite decreased mechanical integrity, largely due to smaller grain sizes and increased concentration of PbI₂. Microindentation hardness testing was performed to estimate the fracture toughness of single-crystal perovskite, and the results indicated perovskites are inherently fragile, even in the absence of grain boundaries and defects. Introducing plastically deformable cations led to a modest improvement in cohesion, and the most robust architecture was observed by infusing perovskite into a porous TiO₂/ZrO₂/C layer that provided extrinsic reinforcement to mechanical and environmental stressors. These developments fulfill design for reliability criteria and are the type of device architectures that could transition perovskites from lab-scale to commercialization.

The mechanically fragile nature of perovskites is a property that, if ignored, could inhibit the long-term success of perovskite solar cells as a viable solar technology. Designing robust cells with long operational lifetimes—in addition to high-efficiency—must be a primary focus for perovskites to be commercially realized.

Halide perovskite materials have emerged in the past few years as novel materials for use in the active layer of heterojunction photovoltaic cells and new emissive layers in light emitting diode (LED) displays. In photovoltaic applications they promise to lower costs, and improve efficiency of commercial photovoltaic cells. Their low temperature processability may also lead to interesting new applications in existing solar cell technologies. In LED applications, they exhibit other desirable properties such as color tunability, simple device structures, and facile processability. Although they exhibit many desirable properties, there are still challenges that must be overcome before commercialization can be realized such as observed hysteresis, and short operational lifetimes. In this work it is shown that these issues can be overcome by using a deterministic nucleation process. Using a patterned transition metal nucleation promoter, it is shown that grain size can be controlled on silicon and indium tin oxide substrates. This process can be adopted to many different perovskite systems and produces large and extremely uniform grains. These grains exhibited superior stability compared to nanocrystalline films, and had no observed hysteresis effects. Using these large grain perovskites, photodetectors and single layer LEDs are fabricated.

A range of electron transfer layers (ETL) and hole transfer layers (HTL) have been actively investigated for perovskite solar cells (PSCs). In this work, we demonstrate the ability to utilize < 2 nm of such layers on both sides of perovskites devices. On the n-side, we show that ultrathin (1 nm) layer of vapor-deposited C₆₀ can switch on the perovskite device by primarily enabling efficient electron extraction and eliminating space charge at the interface which are investigated by utilizing time-resolved fluorescence microscopy and impedance spectroscopy. The study is also suggests that the ultrathin C₆₀ layer alone is sufficient to eliminate hysteresis by enabling efficient electron extraction. In contrast, the device shows poor efficiency (PCE < 1%) and large hysteresis in the absence of the 1 nm of C₆₀. On the p-side, we systematically investigate the role of PEDOT:PSS in inverted structures. PEDOT, one of the most popular HTL layers in inverted PSCs, typically suffer from lower device performance, lower photocurrent, and inferior open-circuit voltages. In this work, we report an ultrathin PEDOT layer as the HTL for efficient inverted structure PSCs. The ultrathin layer of PEDOT can significantly change the wetting property of transparent electrode surface, leading to changes in the grain size, crystallization, and injection properties of the upper perovskite film. We will systematically discuss the role of each transport layer and underlying mechanisms for their key function. Surprisingly by utilizing both ultrathin fullerene layer and ultra-thin PEDOT layer, we show that PCEs of over 18% are possible. These demonstrations ultimately aid in the understanding of the true role of these layers in perovskite solar cells, and highlights that very little is needed to make efficient halide perovskites photovoltaics beyond just the perovskite itself and ohmic electrodes. This represents the simplest high efficiency halide perovskite demonstrated to date. This will ultimately enable simpler manufacturing and lower fabrication costs while changing the way researchers think about perovskite semiconductor heterojunctions and perovskite photovoltaic design.

In organic-inorganic metal halide perovskite solar cells (PSCs), reported high performance hole transporting materials (HTMs) are mostly limited to spiro compounds and/or arylamine derivatives, which are derived from 2,2′,7,7′-tetrakis-(N,N-di-p-methoxy-phenyl-amine)-9,9′-spiro-bifluorene (Spiro-OMeTAD). These amorphous HTMs typically require additional dopants to increase their hole mobility. However, such Li-salt dopants often accelerate degradation of the device due to their hygroscopic nature. Recently, our group reported a crystalline HTM molecule (IDID-F) containing indolo[3,2-b]indole (IDID) core unit, which has centro-symmetric π-conjugated backbone. [1] The PSC devices using Li-salt doped IDID-F as HTM showed higher PCE (~19%) and better device stability than those using Spiro-OMeTAD. In addition, even without the use of any dopants, IDID-F shows moderately high PCE over 13% due to its high crystallinity and charge carrier mobility. However, it is still considerably low compared with that of the PSCs using dopants. It was thus speculated that the strong secondary interactions between the IDID-F molecules are positive in developing high crystallinity but are negative in significantly limiting its solubility, which causes poor film morphology and high series resistance to induce low fill factor and thus low PCE. Therefore, the development of IDID-based HTMs possessing high solubility with controlled crystallinity is demanded and actually targeted in this work for highly efficient dopant-free PSCs.
Here, we report new IDID-based crystalline HTMs bearing various alkyl chains for non-doped PSCs. The new IDID derivatives show improved solubility and processability that can ensure uniform film morphologies while maintaining high crystallinity to induce high hole mobility. The PSC devices using these new IDID derivatives as an HTM exhibit excellent PCE over 18% without any additional dopants as well as good device stability. In addition, it should also be noted that the IDID derivatives show one of the highest open circuit voltage (~1.1 V) among the reported methylammonium lead iodide-based PSCs, which is attributed to their deep lying HOMO level (~5.20 eV). The steady-state photoluminescence (PL) and time-resolved PL studies clearly revealed that the IDID derivatives have much improved hole extracting and transporting capabilities compared with those of Spiro-OMeTAD and previously reported IDID-F. To elucidate the origin of the superior hole transport ability, we analyzed the films of IDID derivatives by grazing incidence wide angle X-ray scattering method. The clear diffraction patterns observed from the IDID derivatives indicates high crystallinity of the films.
References:
[1] I.Cho, Chem. Sci., 2017, 8, 734-741

Hybrid organic-inorganic solar cells, utilizing the organometal halide perovskites materials as the light harvesters have attracted much attention due to their low-cost solution process ability and high efficiency. the HTM-free PSCs with affordable and easily prepared electrode, especially the low-temperature paintable carbon-based PSCs,For the simplest low-temperature paintable carbon-based perovskite solar cell (PSC), the commercial carbon paste utilized to prepare carbon electrode damages the well-fabricated perovskite films, resulting in a bad photoelectric property. Herein, we successfully prepared a low temperature perovskite-friendly carbon paste for PSCs with propylene glycol monomethyl ether acetate (PGMEA) as a solvent, which has showing shown a good electrical conductivity. With the exclusive carbon paste, the perovskite film keeps a dense and uniform morphology, leading to the a champion power conversion efficiency is of up to 12.7% and a champion open-circuit voltage is of up to 1.07 V which is the highest value for carbon-based PSCs with MAPbI₃ as light harvester. Furthermore, the long-time stability test shows that the solar cells with the as-prepared CE carbon electrode retain more than 85% of its initial power conversion efficiency after 960 h.

Technologies of today such as smart consumer electronics, electric vehicles and smart grids all depend on the development of high energy density batteries along with efficient and reliable recharging source. Photovoltaic (PV) can become an efficient battery recharging source. Conventional method consists of PV and battery as two separate units connected via long electric wires. These systems tend to be bulky, inflexible, less efficient and expensive. Intrinsic integration of PV with batteries provides a compact yet efficient energy solution in comparison to the traditional discrete systems. Most of the reports on PV integration with energy storage are focused on capacitive storage. Here, we demonstrate an efficient design of integration of PV and battery using a single junction perovskite solar cell monolithically integrated with a Li₄Ti₅O₁₂-LiCoO₂ lithium ion battery enabled by DC-DC converter with maximum power tracking.

Thermal-assisted blade-coating (TABC) technique has been widely applied for scalable production of perovskite solar cells (PSCs) while it is not applicable in ambient atmosphere due to the adverse impact of humility to perovskite films. Here we report a modified TABC method to achieve high quality perovskite films in a harsh ambient condition, which were prepared with lead acetate trihydrate (PbAc₂-3H₂O) as the lead source and followed with a low-temperature, short time annealing treatment. The as-prepared perovskite films were preferably orientated and had large grain domains up to 100 µm. Based on these films, the conversion efficiency of PSCs reaches 15.8±0.6%, nearly 40% boosting compared to that of PbAc₂ sourced devices (11.4±1.0%). We found that small amount of hydrate water in PbAc₂-3H₂O lead source lead to dense and oriented nuclei at the blade-coating stage. The concomitant MAPbI₃-xH₂O on the surface and grain boundaries of perovskite films isolated humility in ambient and upon the annealing process, melted to form a quasi-liquid nutrition pool for the growing up of MAPbI₃ grain domains by Ostwald ripening. As such, the final quality of perovskite films is greatly improved and so is the performance of PSCs. This work provides an original understanding on the role of hydrate water in the formation of perovskite films by the TABC method and the robust strategy present here contributes a significant progress towards scalable production of high efficiency PSCs in ambient condition.

Lead-based halide perovskites have demonstrated superior optoelectronic properties since the emergence of perovskite solar cells (PSCs). However, the toxicity and phase stability of lead halide perovskite brings inevitable concerns with the practical application for the solar panel. Based on the environmental friendly element of Titanium (Ti), we have predicted a series of Ti vacancy-ordered double perovskite compounds, Cs₂TiI₆, Rb₂TiI₆, K₂TiI₆ and In₂TiI₆, which possess optimal bandgap and suitable absorption. Here, we successfully synthesis the Titanium-based materials which indicates plausible photovoltaic applications because of the proper band-gap range. Furthermore, we prepared the pinhole-free titanium-based thin film by the evaporation method. The electrons/holes diffusion length of such thin films with proper crystallographic textures illustrates indicated the possible construction of planer solar cell. Thus, this work provides new direction in the design and development of high performance Ti-based thin-film PSCs of the future,

Perovskite solar cells (PSCs) adopting inverted planar heterojunction architectures have attracted considerable attention because of the low-temperature solution processing of all functional layers, versatility of energy-band engineering, and a simplified device structure^{[1, 2]}. However, their low power conversion efficiency (PCE) (typically <20%) in comparison to over 22% reported for the best cells based on mesoporous-TiO₂ regular structure^{[3, 4]}, becomes a critical challenge for the future large-scale applications. To overcome these scientific challenges, we report that the mixed-cation perovskite films containing formamidinium have enabled to deposit onto the PTAA hole-selective contact layer by dual-source precursor solution, and this results in a rapid rise in PCE that exceeded 20%^[5]. Subsequently, we further obtain more perfect perovskite films via controlling the defects densities both inside grain boundaries and at the surface, in case average Urbach energy of mixed-cation mixed-halide perovskite film is reduced to 14.2 meV. Ultimately, we’ve achieved over 21.5% PCE in inverted structure PSCs that is comparable to the regular perovskite solar cells containing mesoporous TiO₂.

Reference:
[1] Y. Lin, L. Shen, J. Dai, Y. Deng, Y. Wu, Y. Bai, X. Zheng, J. Wang, Y. Fang, H. Wei, W. Ma, X.C. Zeng, X. Zhan, J. Huang, Adv. Mater., 2017, 29, 1604545.
[2] Y. Wu, X. Yang, W. Chen, Y. Yue, M. Cai, F. Xie, E. Bi, A. Islam, L. Han, Nat. Energy, 2016, 1, 16148.
[3] X. Zheng, B. Chen, J. Dai, Y. Fang, Y. Bai, Y. Lin, H. Wei, X. Zeng, J. Huang, Nat. Energy, 2017, 2, 17102.
[4] W. S. Yang, B. W. Park, E. H. Jung, N. J. Jeon, Y. C. Kim, D. U. Lee, S. S. Shin, J. Seo, E. K. Kim, J. H. Noh, S. I. Seok, Science, 2017, 356, 1376-1379.
[5] D. Luo, L. Zhao, J. Wu, Q. Hu, Y. Zhang, Z. Xu, Y. Liu, T. Liu, K. Chen, W. Yang, W. Zhang, R. Zhu, Q. Gong, Adv. Mater., 2017, 29, 1604758.

Recent advances in the use of organic–inorganic hybrid perovskites for optoelectronics have been rapid, with reported power conversion efficiencies of up to 22 per cent for perovskite solar cells. Improvements in stability have also enabled testing over a timescale of thousands of hours. However, large-scale deployment of such cells will also require the ability to produce large-area, uniformly highquality perovskite films. A key challenge is to
overcome the substantial reduction in power conversion efficiency when a small device is scaled up: a reduction from over 20 per cent to about 10 per cent is found when a common aperture area of about 0.1 square centimetres is increased to more than 25 square centimetres. Here we report a new deposition route for methyl ammonium lead halide perovskite films that does not rely on use of a common solvent or vacuum: rather, it relies on the rapid conversion of amine complex precursors to perovskite films, followed by a pressure application step. We disclosed that the molecular interactions between CH₃NH₃I and CH₃NH₂ occurred mainly through the interaction between -NH₃⁺ and -NH₂.
The deposited perovskite films were highly crystallized, free of pin-holes, highly uniform and had a grain size of 0.8-1.0um, which was 3-4 times larger than that of spin-coating processed films. Importantly, the new deposition approach can be performed in air at low temperatures, facilitating fabrication of large-area perovskite devices. We reached a certified power conversion efficiency of 12.1 per cent with an aperture area of 36.1 square centimetres for a mesoporous TiO₂-based perovskite solar module architecture.

Hybrid organic / inorganic perovskite solar cells (PSCs) are among the most competitive emerging photovoltaic technologies and its efficiency has increased considerably in the last few years. In this context, inorganic hole transport materials (HTMs) that replace more expensive spiro-OMeTAD had become an intense research area. NiO has been used as HTM due to its high hole mobility, broad Eg and good stability. Nevertheless, it requires an inverted solar cell architecture due to the nature of the sol-gel deposition method, which is incompatible with deposition onto perovskite. In this work, P3HT:NiO nanocomposites were prepared and optimized to be used as HTM in the conventional architecture: glass/TCO/compact TiO₂/ mesoporous TiO₂/ Perovskite/P3HT: NiO/Au. The nanocomposite P3HT: NiO is formed with functionalized and as-obtained NiO nanoparticles, using chlorobenzene as a common solvent. The concentration of NiO in the composite was optimized in order to improve the efficiency of the PSCs. P3HT: NiO films were morphologically and electrically characterized by SEM, XRD, and conductivity studies. PSCs elaborated with P3HT: NiO as HTM were characterized by surface photovoltage spectroscopy (SPS) and impedance spectroscopy, as well as J-V and external quantum efficiency curves. Improved conductivity was observed in both types of nanocomposites relative to P3HT, and doping of P3HT by functionalized NiO could be inferred from the different film colors.

Interfacial engineering is essential for making highly efficient and stable solar cells and minimizing energetic losses at interfaces. ITO modification via self-assembled monolayers (SAM) is a very useful method to tune work function, improve the cell performance and stability in photovoltaic devices. In this study, for the first time, boronic acid based Fluorine terminated SAM molecules used to modify ITO surface in planar perovskite solar cells. The results showed that SAM treatment reduced the work function of ITO, significantly enhanced cell performance and passivated trap states in ITO/PEDOT:PSS interface. Improvements in long term stability of cells have also been tested for thirty days and found that SAM modified cells conserved %80 of its first day efficiency. Our results represent a treatment route to achieve hysteresis free, stable and highly efficient (% 16) planar perovskite solar cells.

Organic-inorganic lead halide perovskite solar cells (PSCs) have attracted much attention as renewable energy conversion technologies due to their low cost, long exciton diffusion length and superior light absorption. Despite recent breakthroughs in power conversion efficiency (PCE) exceeding 22%, the PSCs still have limitations in terms of their processing temperature, hysteresis, reproducibility and lab-scale processing method. Up to now, most of the high performance PSCs were produced by a one-step anti-solvent dropping method (one-step method) during spin-coating process for uniform perovskite layer.
However, the PSC devices using one-step method indispensably require annealing and also show large deviations of PCE because the film quality depends on anti-solvent dropping time and substrate size. Although the two-step process is potentially more beneficial than one-step method for large area device, it demands long reaction time for conversion of metal halide to perovskite, which is undesirable for mass production. Therefore, developing new processing method for ensuring fast reaction and uniform film is highly imperative.
Here, we report an innovative two-step method controlling the crystallinity and uniformity of metal halide (PbI₂) via the treatment of various solvent on top of PbI₂ layer. Unlike a typical two-step sequential deposition, the solvent-treated PbI₂ is completely converted to perovskite film within 20 seconds after dropping methylammonium iodide (MAI). The PSC devices using our two-step method were not affected by the rate of voltage sweep and showed a deviation of less than 3%. Above all, there is no hysteresis feature.
Furthermore, our universal method is successfully applied to the mixed-cation-halide perovskite (MA_1-xFA_xPbI_3-yBr_y) as well as methylammonium lead iodide (MAPbI₃), achieving above 18% even without any annealing process for each layers. Our novel processing method will be valuable for the commercialization of perovskite electronic applications.

Power conversion efficiency (PCE) of organic-inorganic hybrid perovskite photovoltaics has reached over 22% from research cells fabricated by spin coating. Although the efficiency is already high enough to enter photovoltaic market, there are still a lot of issues to be address for commercialization of the technology. One of the issues is manufacturability of the technology. Champion devices have been fabricated predominantly by spin coating and there has been only few demonstrations of high performance perovskite PV by industry-relevant methods. In this presentation, high performance perovskite solar cells fabricated by slot-die coating, the most common method for production of printed PVs, will be presented. To achieve high performance, we engaged hot deposition method. Improved performance of perovskite PV with increased grain size of perovskite crystals by hot deposition has been reported. We carried out systematic study on temperature effects of substrate and solution on crystal growth and device performance. The results clearly show temperature effects on nucleation of perovskite crystals and improved performance with optimized high temperature condition. Furthermore, this optimized method was evaluated as the high reproducible process with increasing device area up to 10cm × 10cm module which can be the potential for production. Demonstration of hot deposition in roll-to-roll process will be also presented.

Abstract
The role of additives on the performance of CsPbI₃-based perovskite solar cells (PSCs) is investigated. As additives, HCl, HBr, HI, NH₄I, NH₄Cl, and NH₄Br were used. One of the major disadvantages in the widely used methylammonium-based perovskite material is its hygroscopic nature, which is a self-contained property of the methylammonium ion. The NH₃ group of methylammonium is hydrogen bonded to H₂O, which is a water molecule. Since the bonding force is more stable than the CH₃NH₃-PbI₃ hydrogen bond, H₂O is attracted to bond, resulting in collapse of the crystal structure. As a result, the overall stability of the device is decreased. So, inorganic PSC, CsPbI₃, was chosen in this experiment. Because CsPbI₃ needs high annealing temperature(>300°C), we added HCl, HBr, HI, NH₄I, NH₄Cl, and NH₄Br as additives and investigated the effect of them. It is expected that the additives increase the solubility of low materials, reducing the process temperature. The CsPbI₃ layer was synthesized by one-step coating of CsI mixed with PbI₂ and a additives in N,N-dimethylformamide respectively. We investigated the power conversion efficiency of PSCs depend on the annealing temperature and additive content. In this presentation, we will discuss the effect of additives.
Acknowledgment
This research was supported in part by National Research Foundation of Korea (NRF) grants provided by the Korean government (MSIP) (Nos 2015K1A3A1A59073839, 2017H1D8A1030599, 2017K1A3A1A67014432) and in part by Korea Agency for Infrastructure Technology Advancement grant funded by Ministry of Land, Infrastructure and Transport (17IFIP-B133622-01).

Among various all-inorganic halide perovskites exhibiting better thermo stability than organic-inorganic halide perovskites, α-CsPbI₃ with the most suitable band gap for tandem solar cell application suffers an issue of phase instability under ambient conditions. We discovered that a small amount of two-dimensional (2D) EDAPbI₄ perovskite containing the bication ethylenediamine (EDA) could stabilize the α-CsPbI₃ thus avoid the undesirable formation of the nonperovskite δ phase. Moreover, not only the 2D perovskite of EDAPbI₄ facilitate the formation of α-CsPbI₃perovskite films exhibiting high phase stability at room temperature for months and at 100°C for >150 hours but also the α-CsPbI₃ perovskite solar cells (PSCs) display highly reproducible efficiency of 11.8%, a record for all-inorganic lead halide PSCs. Therefore, using the bication EDA presents a novel and promising strategy to design all-inorganic lead halide PSCs with high performance and reliability.

We present a simple method for improving the thermal stability of organometal halide perovskite crystals that also improves the power conversion efficiencies of the associated perovskite solar cells. We demonstrate that the thermal degradation of perovskite crystals occurs predominantly at their grain boundaries due to the migration of components. To improve the thermal stability of the perovskite crystal, phenyl-C[61]-butyric acid methyl ester (PCBM) is added to the perovskite precursor. It is observed that the decay of the power conversion efficiency of the solar cells slows and that the generation of the decomposed product PbI₂ decreases as the amount of PCBM added increases. Moreover, by varying the grain size of perovskite crystals through the use of hot-casting method, we reveal that grain boundary significantly influences on their thermal stability. The improved thermal stability of perovskite crystals upon the addition of PCBM is attributed to the formation of PCBM-nX (n=1, 2, 3, ..., X= Cl, I), which chemically passivates perovskite grain boundaries and prevents halogens at the grain boundaries from exiting their crystal lattice. This study offers a simple method for improving thermal stability of perovskites without performance losses and opens up the possibility of the use of various molecular additives to achieve highly stable perovskite solar cells.

Mixed-halide perovskites, CH₃NH₃PbI_3−xCl_x, enabled fabrication of highly efficient perovskite solar cells (PeSCs). Chloride ions make the morphology, carrier diffusion length, and stability of perovskite films better. However, the reported device performances of mixed-halide PeSCs have showed non-significant difference of efficiencies with those of the triiodide perovskite, CH₃NH₃PbI₃, devices. This makes uncertainty for the benefits from the presence of Cl ions in the mixed-halide perovskite films. To be clear an effect of Cl^- inclusions on optoelectronic properties of perovskite devices, suitable device structure and spectroscopic analysis should be introduced. In this work, we fabricated lateral-structured perovskite device to study electrical properties of perovskite devices, which showed a clear contrast of photoelectric property between mixed-halide and triiodide perovskites and furthermore provided a suitable device for spectroscopic measurements on actual device architecture.

In this study, high efficiency perovskite solar cells (PSCs) are achieved using atomic layer deposited (ALD) TiO₂ as electron transporting layer. Compared to the conventional solution processed TiO₂ , at which high annealing temperature (500 degree) is required, the ALD method not only offers a much lower processing temperature (150 degree), but also provides a thin film with lower roughness and higher purity. The perovskite solar cell with ALD-TiO₂ exhibit a power conversion efficiency of 16.86%, comparable to those with high temperature processed TiO₂. The ALD technique shows a great potential for the wearable, flexible and lager-area planar industrial production.
To investigate how the processing condition affect the TiO₂ layers and the solar cell performance, we compared two different fabrication method: solution processing (SP) and ALD. A conventional PSCs structure of Glass /FTO/TiO₂/ MAPbI₃(Cl)/Spiro-OMeTAD/Au is adopted. To achieve a better quality of perovskite layer, we doped a small amount (1%) of MACl into precursor. The SP-TiO₂ layer was deposited by spin-coating a precursor of TiAcAc and 1-Butanol, followed by an annealing step at 500 degree; while the ALD-TiO₂ was fabricated with Titanium tetrakis (dimethylamide) and water as oxidant by atomic layer deposition kept at 120 degree. The thin film morphology is measured by scanning electron microscope (SEM, ZEISS SUPRA 55) and atomic force microscopy (AFM, Bruker MultiMode 8). The thickness of FTO,TiO₂ and perovskite layer were 280nm, 30nm and 400nm. ALD-TiO₂ exhibited a better smoothness in surface morphology, with a RMS of 7.08nm at 150 degree and 10.2nm at 500 degree, in contrast the SP-TiO₂ shows a RMS of 7.32nm and 13.6nm in order. More pin-holes could be observed in the SP-TiO₂ and the crystals grow bigger along with annealing temperature, leading to a higher RMS as well. In contrast, the XRD pattern of ALD-TiO₂ reveals that the as-prepared ALD-TiO₂ is amorphous, with anatase phase crystal formed when it subject to post annealing treatment, even at a low temperature . Furthermore, we employed Fourier Transform Infrared Spectroscopy (FTIR, Perkinelmer FT-IR Spectrometer) to probe the organic residue and chemical bonds. In the sample by SP method, we discovered the residual Ti-OH bonds weak-peaks, H-OH bonds weak-peaks , and the Ti-O bonds strong-peaks in around 700 cm^-1 area, while a residue-less condition was showed in the samples of ALD method. In performance of PSCs, the ALD-150 method based PSCs exhibited a comparable performance including PCE of 16.86%, as 14.78% by ALD-500 method,16.67% by SP-500 method, which revealed that ALD-TiO₂ annealing at 150 degree had an enough crystallization and electronic transporting properties fitting the morphology analysis above.
In brief, the ALD-TiO₂ layer presented a crystallization which satisfy the photovoltaic applicaiton and had a great potential for the further PSCs production especially in flexible and wearable photovoltaic devices.

In single-junction solar cells, organic-inorganic halide perovskites that are solution processed have proven to be excellent absorbers. Halide based perovskites also offer bandgap tunability which make them good candidates for wide bandgap top-cell in a dual junction solar cell. Organic-inorganic halide perovskites are however prone to degradation due to low formation energies and tolerance factors. We aim to develop a stable perovskite phase with bandgap close to 1.75 eV to develop a high-efficiency tandem device with CIGS thin film solar cells. Recent CsPbI2Br films processed in our laboratory with bandgap of 1.9 eV have demonstrated close to 12% efficiency devices with t₈₀ life of > 600 hours at 1-sun and 65 ^oC and Voc conditions in air.

To improve lifetime of CsPbI2Br devices and tailor bandgap, we study the effect of mixed A and B cations via addition of Rb and Cd to partially replace Cs and Pb respectively. Thin films of (Rb,Cs)(Pb,Cd)I₂Br being processed via spin coating and doctor blade process, with thickness < 500 nm, and effect of composition on bandgap and structure is determined via UV-Vis-NIR spectroscopy and X-ray diffraction. Results of a design of experiments with mixed A and B cations, and annealing temperature will be presented. Preliminary DFT results indicate addition of Cd lowers the tendency of halide segregation, thereby making the films more stable. Also, a stabilization of perovskite phase is observed with mixed Rb and Cs cations due to entropic gains and the small internal energy input required for the formation of their solid solution. Superstrate p-i-n devices for (Rb,Cs)PbI₂Br and CsPbI₂Br with stabilized power conversion efficiency >12% have been demonstrated and stability results will be presented. Development of (Rb,Cs)(Pb,Cd)I₂Br is currently under progress. (Rb,Cs)PbI₂Br and CsPbI₂Br devices do not show any measurable halide segregation under light soaking tests, however, a slow and steady decrease in Voc is observed. The devices are fabricated with F:SnO₂/Zn(O,S) front contact and carbon-based back contact. As previously presented, (Rb,Cs)PbI₂Br and CsPbI₂Br devices show a positive temperature coefficient measured between 25 ^oC to 65 ^oC. The mixed cation and mixed anion perovskites show good stability, but have lower power conversion efficiency as compared to FA/MA mixed organic cation formulations. Further improvement is needed to lower Voc deficit for these wide bandgap devices.

Spiro-OMeTAD is currently the dominant hole-transport material (HTM) in perovskite solar cells. However, commonly employed dopants, such as LiTFSI, severely impact stability and lifetime. For instance, due to its hygroscopic nature, the use of LiTFSI can lead to water absorption that ultimately decomposes the perovskite. Additionally, lithium ions have been shown to diffuse from the HTL through the perovskite to the electron-transport layer. Recently, the use of a cost-effective carbazole-cored hole-transport material, EH44, and its pre-oxidized TFSI salt as a dopant has been shown to maintain up to 94% of its power conversion efficiency unencapsulated over 1,000 hours in ambient conditions. This outperforms Spiro-OMeTAD and analogous pre-oxidized TFSI salt as dopant. To this end, we have prepared a series of cross-linkable carbazole-cored hole-transport materials (HTMs) with varying levels of conjugation for improved hole mobility and conductivity within the HTL. Pre-oxidized salts for each HTM have been synthesized for use as lithium-free dopant in devices. Thermal properties of new HTMs are superior to EH44 while maintaining proper energetic alignment with the perovskite. These improvements will facilitate the commercialization of cost-effective, efficient, and stable perovskite solar cells in the imminent future. This is collaborative work with the National Renewable Energy Lab (NREL).

Organometal halide perovskite (OHP) photovoltaic (PV) cells have shown dramatic increases in power conversion efficiency over the previous 7 years, yet they still face a number of challenges that must be met to enable widespread commercialization. Meeting these challenges involves material and interface development and optimization throughout the whole photovoltaic device stack. OHP photovoltaics usually contain both electron and hole transport layer (HTL), which influence charge extraction, recombination, and thus the overall PV performance. Herein, we introduce a new family of triarylaminoethynyl silanes moieties as HTLs to investigate how the photovoltaic performance depends on the ionization energy (IE) of the HTL and provide a new and versatile HTL material platform. We find that all molecules in this series of triarylaminoethynyl silanes can serve as efficient HTLs for OHP PVs, despite the 0.5 eV variation in IEs. We further studied the influence of the HTL IE on the PV performance of Methylammonium lead iodide (MAPbI₃) based devices, by applying a series of eleven different hole transport materials with IEs varying from 4.74 to 5.84 eV. The ideal HTL IE range for maximum PV efficiency is dependent on the perovskite processing conditions. In the Pb(OAc)₂ processed devices, 4.8 to 5.3 eV is found to yield the maximum PV performance, while in the PbI₂ processed devices the performance is relatively insensitive to HTL IE between 4.8 and 5.8 eV.

In the quest to further improve the efficiency of perovskite solar cells, the absorption layer in these devices is being extensively study. However, the optimization of the interface between the photoelectrode and perovskite layer is also essential for increasing overall efficiency. ZnO is a promising metal-oxide photoelectrode due to its versatility to make a variety of nanostructures (i.e. nanorods) with enhanced electron extraction rate derived from larger surface areas.
In order to increase the power conversion efficiency in the ZnO nanorods-based perovskite solar cells, improving the coverage of the MAPbI₃ layer on ZnO is an essential step. During device fabrication, exposed ZnO nanorods in direct contact with the hole transfer layer (CuSCN) create a direct charge recombination pathway that negatively affects the charge transfer rate and collection efficiency.
To reduce carrier recombination caused by the poor coverage of the perovskite layer (CH₃NH₃PbI₃, MAPbI₃) on the surface of the ZnO photoelectrode, surface modification is performed on the exposed ZnO nanorods. After deposition and annealing of the MAPbI₃ layer, a solid state CuSCN layer is spin-coated on top. The thickness of the MAPbI₃ layer is modulated by controlling the length of ZnO nanorods, leading to different absorbances, charge extraction rate, and photocurrent density in the fabricated devices.
In this work, the effects of surface treatment pre-deposition of the MAPbI₃ layer are observed and compared in the performance of perovskite solar cells. Several treatment methods are studied to maximize surface coverage of the perovskite layer on the ZnO nanorods. The distinct changes in surface morphology are observed by scanning electron microscope image characterization pre- and post-surface treatment. The modified ZnO surface shows improved power conversion efficiency in the treated perovskite solar cells.

The purpose of this project is to design and fabricate a lead-free perovskite solar cell. The solar cell has to be non-toxic as well as being stable in a natural open air environments with a specific resistance to moisture. This design incorporates the use of organic and inorganic materials, as well as the efficient use of the perovskite crystalline structure that is produced by using the properties of Bismuth (III). The advantages of using perovskite for the active layer include but are not limited to broad a light absorption spectrum, tunable band gaps, long charge carrier diffusion, and low fabrication cost. Perovskite solar cells are especially promising when considering the technology lifespan. The procedure done for this solar cell design allowed for investigation using different sets of equipment like hot plate and a sputter coater. The titania was also experimented with by incorporating transparent and reflective pastes. With this different build of this cell the metal contact layer was palladium deposited by sputter coating. This process didn’t produce uniform layers at this level in the research. Further time will allow for more uniformity and consistent values for the layers produced. The TiO2 blocking layer has a difference in thickness of .03 µm, as well as the TiO2 mesoporous layer with a thickness variance range of .03 µm. The result was a tested photoreactive cell. This research shows great promise along with lots of room for improvement. The future and current scope of the project will be on improvement of manufacturing conditions and techniques to further the quality of layer properties of perovskite.

Organic-inorganic halide perovskites are a rising star in the optoelectronic community having excellent properties suitable for photovoltaic, light detection and light emission applications. However, the wide band gap nature of commonly used methyl ammonium lead halide perovskites has no photosensitivity at near-infrared wavelengths beyond 800nm, thus limiting their applications. Recently, partial substitution of Pb by tin (Sn) in organic–inorganic lead halide perovskites has been demonstrated as an effective way to reduce the bandgap of halide perovskites. By utilizing anti-solvent dripping process during perovskite film deposition, a mixed Pb-Sn perovskite (MAPbI₃)_0.4(FaSnI₃)_0.6, has been synthesized with a bandgap of 1.25eV, thus extending its absorption spectrum beyond 1000 nm. In this study, we investigated the influence of various anti-solvents including diethyl ether, chloroform, and toluene on the Sn-contained low-bandgap halide perovskite films and their device performance including IR photodetectors and solar cells.

Hybrid perovskites has shown promising performance for use as solar materials. Their power conversion efficiency has been found to be rising. This notwithstanding the unusual current-voltage hysteresis and low –frequency dielectric response has hampered their widespread applicability. The diffusion of native defect has been suggested as a reason for this. We calculate the halide migration energy in typical hybrid perovskites and identify mechanisms that slow down or reduce the halide migration process. The effect of cation doping was also examined.

Herein, we report perovskite solar cells (PSCs) with the highest reported power conversion efficiency (PCE) using phthalocyanine complexes as a dopant-free hole-transport layer (HTL). As low-cost hole-transport materials, phthalocyanine complexes can provide high thermal and chemical stability with high carrier mobility. Five tetra-alkyl substituted phthalocyanine copper(II) complexes with different alkyl chain lengths were synthesized, characterized, and used as HTLs in PSCs. The effect of the alkyl side chain length on the structural and electronic properties of the HTL in PSC devices was investigated. We found that tetra-propyl-substituted copper(II) phthalocyanine (CuPrPc), which has the strongest π–π interaction and a face-on molecular orientation, deposited on perovskite by spin coating leads to higher hole mobility than the other prepared tetra-alkyl-substituted copper(II) complexes and it forms a highly hydrophobic surface that considerably enhances the stability of the perovskite layer. A highly efficient PSC device was fabricated, and a stable PCE of 17.0% was achieved. The results show that CuPrPc has potential as a hole-transport material for fabricating PSCs.

Organic-inorganic metal halide perovskite materials offer wide range of positives such as lost cost materials, efficient photovoltaic cells, ease of fabrication and appears particularly promising for the next-generation solar cell devices. However, its comprehensive behavior in the reverse-bias is critical to understand the cell performance, stability and the abnormal hysteretic effects. In this study, we investigated on a contrasting study to understand the impact of reverse bias on different contacts such as Silver (Ag), Gold (Au), and Molybdenum Oxide with aluminum (MoO_X/Al). Furthermore, provide insights to the effects of hole transport layers such as spiro and EH44 (2,7-Di(N,N- dimethoxyphenylamino)-N-(2- ethylhexyl)carbazole) with ((FA_0.79MA_0.16Cs_0.05)_0.97Pb(I_0.84Br_0.16)_2.97) perovskite material. These findings further emphasize the versatility and performance potential of these perovskite materials for other optoelectronic applications.

In the past decade, perovskite semiconductors have rapidly emerged as a promising photovoltaic materials with high power conversion efficiency (PCE). State-of-the-art the of the perovskite solar cells (PSCs) achieved the certified power conversion efficiency (PCE) exceeding 22% with the mesoporous device architecture, which demonstrates the potential of the PSCs among the solar cell technologies. However, the planar heterojunction architecture which possesses distinct advantages such as thinner device, short & low temperature device fabrication, and low processing cost, still exhibited inferior device performance as compared to the mesoporous devices. In order to further improve the planar heterojunction PSCs, improved and optimized interface is critical. To optimize the interface, we developed Zn-doped NiOx films as the hole-transporting layer in PSCs. As is well known, NiOx is a promising hole-transporting material for optoelectronics because of its high hole mobility, good chemical stability, and high optical transparency. However, pristine NiOx has unsatisfactory electrical properties, which could deteriorate the device performance. By considering that Zn has a similar atomic size with Ni, we adopted Zn as a dopant of NiOx to employ the Zn-doped NiOx in the planar heterojunction PSCs. As a result, 5% Zn-doped devices exhibited overal PCE up to 12.39% with improvements of a Voc by 4.0%, a Fill factor (FF) by 8.8% and a PCE by 9.7%, as compared to that of pristine NiOx. In the presentation, details of morphology investigations, optoelectronic properties of the Zn-NiOx and its devices will be discussed.

Perovskite photovoltaics have attracted remarkable attention recently due to their exceptional power conversion efficiencies (PCE). The quality of perovskite thin film is an important key for high-performance solar cells. For perovskite absorbers derived from solution-based deposition, thermal annealing is typically required to remove solvents and to achieve high crystallinity of the films. However, the annealing process can reduce device fabrication yield and the energy input increases the device pay-back time. Additionally, the thermal annealing may also hinder application of perovskite technology in tandem photovoltaics and flexible optoelectronics. Therefore, developing room-temperature method to deposit high-quality perovskite films is necessary. Here, we report an additive-based process to obtain high-quality methylammonium lead iodide films with micron-sized grains (>2 mm) and microsecond carrier lifetimes (τ₁= 931.94 ± 89.43 ns; τ₂ = 320.41 ± 43.69 ns) at room temperature. Solar cells employing such films demonstrate 18.22% PCE with significantly improved current-voltage hysteresis and stability without encapsulation. Moreover, we find that the grain size in perovskite film from solution process strongly depends on the precursor aggregate size in the film-deposition solution and tuning the aggregate properties enables enlarging grains to the micron scale. These results offer a new pathway for more versatile, cost-effective perovskite processing.

Today's perovskite solar cells's power conversion efficiency(PCE) has been increased up to 22.1%. Most of perovskite solar cells use TiO2 for electron transport layer (ETL). But in this studies we used ZnO for ETL. ZnO has larger band gap than TiO2, so ZnO films can transmit more sunlight than TiO2 films. Also ZnO films can be obtained at lower temperature (~350°C) than TiO (~550°C). Despite of these benefit of ZnO films the reason why we did not use ZnO is because of unstability of perovskite films deposited on ZnO films. The ZnO films' surface has lots of hydroxyl groups. These hydroxly groups disassemble the perovskite into Methylammoniumiodide (MAI) and Leadiodide (PbI ) because of this problem we don't use ZnO for ETL in perovskite solar cells. But we can solve these problems by pre-annealing. Pre-annealing can reduce the hydroxyl groups on the perovskite films, and it helps to stabilize the perovskite films deposited on ZnO films. Moreover, pre-annealing can help to increase the size of the perovskite grains about 1 , and it contributes to increase mobility of electron. Increasing of mobility carries over to rise the PCE of perovskite solar cells. As a result, pre-annealing on 240°C devices exhibited overall PCE up to 3.49% with improvements of a V by 60.31%, a Fill factor (FF) by 45% ,a J by 165.01% and a PCE by 512.28%, as compared to that of pristine perovskite. The stability of perovskite also increased. Compared to pristine perovskite, pre-annealing perovskite shows 150% better stability at the atmosphere. In the presentation, details of morphology investigations, optoelectronic properties of ZnO, and its devices will be discussed.

Recently, low dimensional bismuth halide perovskites emerged as a promising alternative to lead based 3D hybrid perovskites in the field of optoelectronics, including solution processable thin film solar cells. Lead based metal halide perovskite solar cells have already achieved 22.7% certified solar to electric power conversion efficiency for small area devices (<1 cm²). However, one of the major problems encountered with this new technology, apart from structural and chemical stability, is the toxicity associated with heavy metal lead.
Potentially less toxic bismuth halide perovskites semiconductors possess promising optoelectronic properties including a high absorption coefficient on the order of 10⁴ cm^-1 and can be processed from solution using a variety of wet chemical deposition techniques and additives. Achieving full surface coverage along with precise control over nucleation sites has proven to be difficult by employing techniques, derived from the lead halide perovskite counterpart.
In this study, we investigated the influence of bismuth xanthate precursor and various solvents for the single-step deposition of methylammonium bismuth iodide perovskite (CH₃NH₃)₃Bi₂I₉ thin-films with full surface coverage. The use of acetonitrile solvent in the perovskite synthesis led to pinhole-free absorber layers. Solar cell in n-i-p stack configuration (FTO/c-TiO₂/mp-TiO₂/(CH₃NH₃)₃Bi₂I₉/Spiro-OMeTAD/Au) achieved a remarkably high fill factor of 0.74-0.77 with negligible hysteresis in the current-voltage sweep. Bismuth ethyl xanthate was used as a non-halide precursor additive for the deposition of (CH₃NH₃)₃Bi₂I₉ films. Additionally, a small amount of cesium iodide was incorporated in a multi cation approach to benefit from its known effects such as improvement of stability and phase-pure crystallization. The addition of bismuth ethyl xanthate led to solar cells with improved current density (0.7 mA/cm²), fill factor (0.63) and power conversion efficiency (0.26%) compared to devices fabricated without bismuth ethyl xanthate (J_SC= 0.5 mA/cm², FF= 0.58, PCE= 0.17%).

Perovskite solar cells based on organic-inorganic materials, which have the perovskite crystal structure, present superb power conversion efficiency (PCE) exceeding 22%. Demonstration of the high efficiency devices in a short time is in general attributed to developing of various film formation methods. One of the well-known processes for the film formation is that pouring an anti-solvent during spin-coating of a perovskite material to make an intermediate film which turns to the final perovskite film. This film formation method makes it possible to make high quality methylammonium lead iodide (MAPbI₃) thin films with well-crystallized large grains and uniform film thicknesses. It is known that intermediate phase formers should have lone pair electrons to easily participate in forming the Lewis adduct. In this work, we demonstrate that N-methyl-2-pyrrolidone or trimethyl phosphate with methylammonium and lead iodide make intermediate films, and finally form high quality MAPbI₃ films. Dimethyl sulfuroxide, N-methyl-2-pyrrolidone, and trimethyl phosphate form different intermediate phases, and finally transform to smooth perovskite films with well-crystallized grains. Perovskite solar cells based on the three different films were fabricated, and the performances of the cells as well as the film formation principles were investigated.

Organic-Inorganic perovskite solar cells (PSCs) have been attracted great attention because of their high power conversion efficiency (PCE), low production cost, and high flexibility. The most intensively studied light absorbing material is perovskite-structured CH₃NH₃PbI₃ (MALI). Interestingly, it is reported that a small amount of chlorine incorporation into MALI increases charge carrier diffusion lengths (from 129 nm to 1069 nm), which enables planar structured PSCs with high PCEs. However, whether chlorine exists at the final perovskite film is under debate. Some studies report negligible amount or absence of chlorine in the final film, while others report detection of chlorine from the final film. In this study, we observed microstructure and chlorine content of Cl-incorporated MALI thin films with increasing temperature, using an in-situ nano-Auger spectroscopy and an in-situ scanning electron microscopy system. Precipitates begin to appear at the surface of Cl-incorporated MALI films, from lower temperatures compared to the MALI thin films. Moreover, grains of Cl-incorporated MALI films grow faster than those of MALI films. Local concentrations of chlorine at intragrain and the vicinity of grain boundary were analyzed to understand the microstructural evolution of the perovskite films.

Future high performance PV devices are expected to be tandem cells consisting of a low-bandgap bottom cell and a high-bandgap top cell. In this study, we developed a cradle-to-end of use life cycle inventory to evaluate the environmental impacts, primary energy demand (PED), and energy payback time (EPBT) for four integrated two-terminal tandem solar cells comprised of either Si bottom and lead-based perovskite (PK_Pb) top cell (Si/PK_Pb), copper indium gallium selenide (CIGS) and PK_Pb (CIGS/PK_Pb), copper zinc tin selenide (CZTS) and PK_Pb (CZTS/PK_Pb), or tin-lead based perovskite (PK_Sn,Pb) and PK_Pb (PK_Sn,Pb/PK_Pb). The environmental impacts from single junction Si solar cells were used as a reference point to interpret the results. We found that the environmental impacts for a 1 m² area of a cell was largely determined by the bottom cell impacts and ranged from 50 % (CZTS/PK_Pb) to 120 % of a Si cell. The ITO layer used in Si/PK_Pb, CZTS/PK_Pb, and PK_Sn,Pb/PK_Pb is the most impactful after the Si and CIGS absorbers, and contributed up to 70 % (in PK_Sn,Pb/PK_Pb) of the total impacts for these tandem PVs. Manufacturing a single two-terminal device was found to be a more environmentally friendly option than manufacturing of the two constituent single-junction cells, and can reduce the environmental impacts by 30 % due to the exclusion of extra glass, encapsulation, front contact and back contact layers. PED analysis indicated that PK_Sn,Pb/PK_Pb manufacturing has the least energy-intensive processing, and the EPBTs of Si/PK_Pb, CIGS/PK_Pb, CZTS/PK_Pb, and PK_Sn,Pb/PK_Pb tandems were found to be ~13, ~7, ~2, and ~1 months, respectively. On an impacts/kWh of Si basis the environmental impacts of all the devices were much higher (up to ~10 times). These results can be attributed to the low photoconversion efficiency (PCE) and short lifetime that were assumed. While PK_Sn,Pb/PK_Pb has higher impacts than Si based on today’s low PCE (21 %) and short lifetime (5 yr) assumptions, it can outperform Si if its lifetime and PCE reach 16 yr and PCE of 30%. Of the configuration considered, the PK_Sn,Pb/PK_Pb structure is the most environmentally friendly technology.

Silicon photovoltaic (PV) devices are leading materials for solar cell devices due to its abundances and cost effectiveness for the mass-productions. But, its material-induced limit has shown the critical point of development due to its saturations of performances. The current development of perovskite solar cells is attractive due to high conversion efficiency (up to 20%) and fast development last decade. Furthermore, tunable bandgap of perovsite material such as CH₃NH₃Pb(I_1-XBr_X)₃ where 0<x<1 provide advantages of materials selections due to its broader bandgap ranges (1.5 eV to 2.3 eV).
Theoretical and material induced limit for single junction solar cell address the potential issues of improving efficiencies without assisting other approaches. Thus, the multijunction configuration can be good options due to its efficient spectrum managements and efficiency improvements. Typically, silicon with perovskite multijunction configuration can be good candidate because of using existing materials (silicon) with new materials. Thus, we develop the detailed balance equation (DBE) of silicon/perovskite tandem configurations for two and three junction PV devices to see the potentials.
First, we use Si as a bottom junction material. Then, we consider three perovskite materials on top of silicon materials such as CH₃NH₃PbI₃, CH₃NH₃PbBr₃ and CH₃NH₃Pb(I_1-XBr_X)₃ where x is 0~1. Typically, the tunable bandgap of CH₃NH₃Pb(I_1-XBr_X)₃ can provide wide-options for this tandem configuration (for top junction materials of triple junctions). For this research, we initially conduct ab initio calculations for finding the band structures and its optical properties. Then, these parameters could apply into the conventional DBE with inclusion of non-radiative recombination parameters. Therefore, we could discuss the reasonable expectations for Si/perovskite tandem configuration.

Hybrid perovskites are unique class of semiconductors where high crystalline quality thin films with excellent electronic and optical properties can be produced using solution-processing. This has led to a wide range of high-efficiency optoelectronic devices such as solar cells, light emitting diodes and detectors. However, the demonstration of room temperature operated hybrid perovskites-based field effect transistor has remained elusive. This is largely due to the non-reproducibility induced by polar nature of the perovskite structure coupled with ionic movement, which screens the capacitively coupled gate voltage, has also resulted in a hysteresis in the transconductance of FETs.
In this study, we for the first-time report high-performance, hysteresis-free ambipolar FETs using highly crystalline hybrid perovskites thin films, which operate at room temperature. To achieve this, we systematically improved the film composition, morphology, crystallinity as well as investigated the effect of different high-K dielectrics between the perovskites and gate. As a result, we obtained FETs with high trans-conductance with low subthreshold slopes with an on/off ration >10⁴, which is the highest reported to date. Moreover, we observe ambipolar transport at room temperature with a proper choice of the gate-dielectric, which suggests that the Fermi energy can be tuned continuously to inject both electrons and holes into the channel. We anticipate that these results will create a perfect platform for the systematic investigation of the electronic properties of hybrid perovskites materials and also lead to opportunities for exploring perovskite based novel devices such as ultrasensitive photo-transistors and spin FETs, which have been theoretically predicted but never realized.

We have developed superstrates for perovskite solar cells that feature increased transparency and conductivity due to the incorporation of effectively transparent contacts (ETCs). They increase the short circuit current density by more than 1 mA/cm² compared to standard indium tin oxide (ITO) on glass superstrates. Our superstrates feature effectively transparent grid fingers, which enable a significant reduction in the ITO thickness required for current extraction with a high fill factor. They are composed of soda-lime glass with a thin (~40 µm) layer of polydimethylsiloxane (PDMS) that features triangular cross-section microscale grooves, which are infilled with a conductive silver ink and subsequently coated by a thin (~30 nm) ITO layer such that high lateral conductivity (< 5 Ω/sq) is achieved without altering the surface properties of standard perovskite superstrates. Due to the reduction of the ITO thickness, parasitic absorption is greatly reduced and antireflection properties are optimized leading to up to a 1 mA/cm² increase in short circuit current density. High lateral conductivity is obtained by spacing the triangular silver lines closely (~80 µm distance). Whereas such densely-spaced grid fingers would normally cause excessive shading losses, here, their triangular cross-section and high aspect ratio serve to reflect all incident light to metal-free areas of the superstrate, leading to >99% effective transparency as demonstrated in our previous work for such contacts applied to other solar cells (Adv. Optical Mater. 4 (10), 1470-1474 (2016); Photovoltaic Specialists Conference (PVSC) IEEE 43rd, 3612-3615, (2016); Sustainable Energy and Fuels, 1 (3), 593-598, (2017)). FACsPbI₃ perovskite solar cells were fabricated on these superstrates and showed improved external quantum efficiency, with an average integrated short circuit current increase of 1 mA/cm². Computational modelling of optical and electrical properties guided the device design. Here, we will present computational simulations, fabrication methods and experimental results on increased perovskite solar cell performance.
The above described approach constitutes an effective and scalable way of enhancing the short-circuit current density in perovskite solar cells, and incorporates only materials that are widely used in the photovoltaic industry. The area fraction devoted to macroscopic grid fingers and busbars can be further reduced on large scale solar cells and modules, as compared with conventional designs. Furthermore, our superstrates may find application in thin film tandem solar cell architectures as well as in other optoelectronic devices.

The stability issue is a big challenge in perovskite solar cell. We study the degradation of perovskite and perform coordination enginnering to enhance the stability. The intermediate compounds of degradation and formation are identified and new reaction routes are developed. Large gain and high performance perovskite solar cell with efficiency above 20% are achieved. Through the interface passivation, the optoelectronic and stability are greatly enhanced, aiming to promote the commercialization step.

Perovskite solar cells (PSCs) have been attracting much attention due to their high performance and low fabrication cost. Recently, the power conversion efficiency (PCE) of the PSCs has surpassed other thin film technologies and reached 22.7%.[1] In most of the reports on PSCs, a sandwiched p-i-n structure was adopted to fabricate the PSCs. Owing to the advances in film formation technique and interfacial modification, the internal quantum efficiency of the sandwich PSCs has been approaching 100%. In order to further improve the performance of the PSCs, we need to fundamentally change the structure of the PSCs.
The integrated-back-contacted (IBC) structure has been adopted in the single crystal silicon solar cells and achieved the world record PCE due to the lower light loss compared to the sandwich structure.[2] If we can fabricate PSCs in the IBC structure, it is possible to further improve the PCE. In order to demonstrate whether and to what extent the IBC structure improves the PCE of the PSCs, we used a numerical simulation method to form the IBC-PSCs, and investigated the effects of the structure parameters (contact width, gap), defects (bulk, interface), and electrical parameters (lifetime, mobility) on the performance of the IBC-PSCs. The results clearly indicate that the PCE (22%) of IBC-PSCs is 10% higher than that (20%) of the sandwich PSCs when the dimension of the contacts is relatively small (based on CH₃NH₃PbI₃ material).[3] The results in this work unveil a new approach to further boost the performance of PSCs, and provide valuable guidelines for the design and the fabrication of IBC-PSCs.

[1] Best Research-Cell Efficiencies, www.nrel.gov/pv/assets/images/efficiency-chart.png. Accessed on Oct. 31, 2017.
[2] K. Yoshikawa et al., Nature Energy 2017, 2, 17032.
[3] T. Ma et al., submitted.

The efficiency of perovskite solar cells has skyrocketed from 3.8% to 22.1% in the past few years. For the best perovskites, the open-circuit voltage deficit, defined as the difference between bandgap and open-circuit voltage (V_oc), is only 0.37 V, approaching other best technologies such as GaAs [1]. In contrast to the remarkable achievement in V_oc, the fill factors (FF) of perovskite solar cells are usually around 60%-70%. Even in the champion device, the FF is only 80%, which is considerably lower than that of GaAs (86.5%) [2], despite the fact that it has higher bandgap and comparable V_oc deficit. Therefore, understanding the FF loss, is the key to further boost the efficiency of perovskite solar cells.
Suns-Voc technique is well established for silicon solar cells to acquire pseudo-current-voltage characteristics, from which one could obtain the recombination-limited FF of the device. Furthermore, by comparing the pseudo-current-voltage curve to the one-sun current-voltage curve (IV), the series resistance at maximum power point can be determined. To apply this technique on perovskite solar cells, we equipped our IV tester with neutral-density filters. Distinct from the Sinton flash Suns-Voc tool, our setup utilizes a continuous light source, therefore it can measure accurately on cells even with hysteresis. Moreover, the continuous lamp has class A spectrum, and it maintains the same at reduced illumination by applying neutral-density filters, which prevents errors induced by the change of spectrum.
We first verified our setup by measuring a silicon cell, and it shows excellent agreement with the Sinton tool. Performing IV and Suns-Voc measurements on a methylammonium lead iodide (MAPI) perovskite solar cell, we observed the series resistance decreases when the operating point moves from maximum power point to Voc. In contrast, similar analysis on the silicon cell shows the series resistance stays constant, in other words, no illumination dependency. Such illumination-dependent series resistance was observed on organic solar cells before, and is believed to be caused by the low carrier mobility of the bulk material [3]. However, in perovskite solar cells, the carrier transport is limited by the contact layers instead of bulk [4]. We are further investigating this by applying different contact layers to perovskite, and by the time we present, we would have more insight about the origin of this illumination-dependent series resistance.

Chemical reactivity of metal halide perovskite (formula ABX₃) materials needs to be understood not only in the context of intrinsic stability, but also when interfaced with dissimilar materials that are required to fabricate devices. Both the B-site metal cation and halide anion (X) readily participate in redox reactions. Additionally, the A-site cation is commonly a protonated organic leading to strong acid/base chemistry. We have characterized the chemical interactions of the perovskite material, mainly CH₃NH₃PbI₃, interfaced with the three main categories of commonly available materials: metals, oxides, and organics. We show that B-site cation redox chemistry predominates with reactive metals while underpotential deposition occurs with noble metals catalyzing a pathway to metallic Pb formation. The chemistry with metal oxides can be related to the same Faradaic reactions that give rise to gas sensing behavior meaning voltage and illumination shift the steady state concentrations of products. Weak chemical interactions between perovskite and organic materials include the permeability/solubility of iodine in the organic while strong reactions chemically alter both the perovskite and organic, even in inert atmospheres. This research identifies key reaction and degradation mechanisms to ensure high fidelity characterization of intrinsic material properties and facilitate targeted improvements to perovskite device stability.

Record efficiency of Perovskite/Silicon tandem solar cell 23.6% is approaching to highest silicon solar cell efficiency 26.6%. However, it still has lower efficiency than a single silicon device [1-2]. This is mainly because of the restriction at current matching between monolithically connected top and bottom solar cells [2]. Since most Perovskite solar cells are made by solution process, every reported monolithic Perovskite/Silicon tandem was fabricated without front side texture of bottom silicon. Front side texture is essential to bust up current output and maximize tandem device performance. As a consequence, fabricating Perovskite above the textured silicon solar cells is essential.
We herein will talk about a novel method of fabricating Perovskite on the textured silicon surface and talk about the analysis of Perovskite above silicon texture. We used dry 2-step process which is deposit precursor materials and convert it into Perovskite. Conformal PbO precursor film firstly fabricated on the textured silicon surface with sputtering process. Then, exposed it to solid MAI source. As a result, we could construct conformal and uniform Perovskite film on the textured silicon surface. Films were investigated with XRD, μ-PL 3D mapping and SEM. XRD peak around 14.2 degree and PL emission around 770 nm were obtained. XRD peak shift about 0.1 degree was observed compared to conventional spin-coated Perovskite. This can be the evidence of stress induced by the structural characteristic of texture and volumetric expansion during conversion [3].
Due to arbitrary nature of random texture, it is not suitable for detail analysis for stress. For this reason, we fabricated patterned texture with photolithography technique and wet etching. Perovskite on patterned texture was analyzed by TRPL, XRD, PL 3D mapping, TEM and SEM. Carrier lifetime was measured with TRPL with high spatial resolution to analyze thin film characteristics. The average value of 5.7 ns was obtained with TiO₂ layer, and the lifetime was uniform when measured over 10 points. About 0.1 degree XRD pick shift and 3.0 nm (~769 nm to 766 nm) PL peak position difference at the specific part of the pattern texture was observed. XRD pick shift represents changed d-spacing. Changes in d-spacing can be evidence of stress in the film, and this stress may shift the bandgap. As a way to mitigate XRD peak shift and PL peak difference, we also applied porous precursor and silicon texture rounding respectively and analyze it.
We presented a new method for manufacturing Perovskite on the textured surface of silicon and presented results of the analysis. Fabricating Perovskite on a textured silicon substrate is the substantial step in maximizing the efficiency of Perovskite/Silicon tandem solar cells.

[1] NREL Efficiency Chart. (accessed October 29, 2017).
[2] Bush, et al. Nature Energy 2 (2017): 17009.
[3] Chen, et al. Journal of the American Chemical Society 136.2 (2013): 622-625.

In recent years perovskite photovoltaics have reached impressive research lab scale power conversion efficiencies (PCE) >22%. Although the efficiency of perovskite photovoltaics is competitive with other photovoltaic technologies, commercialization is impeded by their low stability in ambient conditions and elevated application relevant conditions. The hole-transporting layer (HTL) within these devices is crucial for high performance. Typically the HTL in high performing perovskite devices is Spiro-OMeTAD and a lithium-based dopant. Spiro-OMeTAD takes multiple challenging synthetic steps, therefore it is expensive, and the lithium dopant reduces device stability. Initial work using a synthetically simple carbazole-based HTL, EH44, has shown similar PCE to Spiro-OMeTAD and superior stability in ambient conditions over 1000 hrs without encapsulation. We have developed a series of novel carbazole-based hole transporting materials (HTMs) with increased conjugation and preoxidized salts of these HTMs were used as a lithium-free dopant when preparing the HTL to increase hole mobility and conductivity within the HTL. We have also demonstrated a crosslinkable HTM to enhance the device long-term stability. The hole mobility of these HTMs were measured using space charge limited current (SCLC). The device stability was characterized under ambient moisture and oxygen conditions, as well as at elevated temperatures of 80°C. The enhanced stability of these HTMs and simplified synthetic methods should make the commercialization of perovskite photovoltaics easier.

There is great interest in Pb-free alternatives to hybrid lead perovskites. Replacing of Pb2+ with Sn2+, to form MASnI3, not only removes Pb but also reduces the bandgap to 1.3 eV, leading to a higher Shockley–Queisser limit. Unfortunately, the photovoltaic performance of MASnI3 solar cells is limited to just ~7% because a) the diffusion lengths in Sn perovskite films are much lower than Pb perovskite films, and b) MASnI3 films degrade way too quickly in air due to oxidation of Sn2+ to Sn4+. Both diffusion length and stability are the function of film morphology in terms of crystallite grain sizes. Grain-boundary defects lead to carrier recombination and degradation initiates at the grain boundaries due to permeation of oxygen and moisture through it.

In this work we demonstrate a post-deposition vapor anneal treatment that improves the grain size of MASnI3 films from 100-200 nm to 1-2 microns, one of the highest numbers ever reported for this material. The increase in the film crystallinity was confirmed using XRD, where the FWHM of the films reduced from 0.35° in as-deposited film to 0.32° in annealed film. The improvement in the morphology of the MASnI3 layer is the key to the realization of stable and higher efficient lead-free perovskite solar cells.

Perovskite films were spincoated followed by annealing at 100°C. The as-deposited films showed the tetragonal MASnI3 with primary peak position at 24.7° corresponding to (111) crystallographic plane. SEM images show continuous films with grain-size of just 100-200 nm. For improving the morphology, the MASnI3 films were exposed to Methylamine (MA) vapors for 8-10 seconds. The MA vapors react with MASnI3, forming an optically bleached stable Lewis pair complex of SnI2.xMA at room temperature. The left over methylammonium iodide (MAI) also crystalizes out as confirmed by XRD, leading to dusky white film. To recover the perovskite, the SnI2.xMA complex needs to breakdown. This is accomplished by an annealing step in which temperature is progressively increased from 20°C to 150°C, followed by 1 hour annealing at 150°C. During annealing at 150°C, the complex breaks off to SnI2 and volatile MA. The SnI2 reacts with the crystallized MAI already in the film, forming MASnI3 film which is black in color. XRD confirms that the annealed film is phase-pure tetragonal MASnI3, with lower FWHM than as-deposited films with primary peak at 14.1° corresponding to (001) plane. SEM confirms that the annealed films have larger grains of 1-2 um. Annealing at lower temperatures resulted in partial conversion of SnI2.xMA complex, whereas annealing at higher temperatures lead to the degradation of the perovskite film into SnI2. The slow ramping of the temperature is also important because it allows precise control on the nucleation and grain-growth of the MASnI3 films. The work demonstrates that careful grain-growth can enable large grain size even in Sn-perovskites, which hitherto has been a challenge.

Lead halide perovskite semiconductors have recently gained interest for their application in a wide range of different optoelectronic devices, demonstrating impressive performances in solid-state photovoltaic devices [1] with impressive efficiencies exceeding 20% and great potential for lighting and lasing application [2,3]. These materials not only show exceptional primary optoelectronic properties such as a direct gap,[4] small exciton binding energy,[5] low carrier recombination rates,[6] ambipolar transport,[7] and tunability of the bandgap range from the near infrared (NIR)[8] to the ultraviolet,[9] they are also very attractive for their low cost, easy processability for mass production (e.g., printing from solution)[10] and for a large availability of their chemical components.
Usually, the semiconductor films are processed from precursors, such as PbX2 and CH3NH3X dissolved in toxic and high boiling point solvents (e.g., dimethylformamide–DMF, N-methylpirrolidone–NMP, …) to obtain Methylammonium Lead Halide Perovskite (MAPbX3 or CH3NH3PbX3) polycrystalline thin films.[11] Upon deposition, the constituent ions self-assemble during crystallization directly upon the selected substrate, when treated at temperatures around 100 °C. Such process appears simple, however, it presents relevant drawbacks. First of all, it makes very difficult the reproducibility of the thin film morphology and the related optoelectronic properties of the material. In fact, the thin films can exhibit varying morphologies determined by different factors including precursor ratio, solvent, processing additives, substrate roughness and surface energy, atmospheric/environmental conditions, annealing temperature, and treatment time.[12]

In this work we present a totally new and reproducible way to synthetize good quality of MAPbI3 Perovskite submicron – sized Crystals dispersed in an environmentally friendly and low boiling point solvent (isopropanol), avoiding the use of toxic solvents. The procedure is based on adding the PbI2 powder in MAI solution after controlling the size and shape of the PbI2 precursor crystals. The dispersion that we obtain, is an ink and can be easily printed by using few different printing techniques. We decided to bar coat the ink on top of gold interdigitated to produce photodetectors in a simple planar geometry with performances that are comparable to the state of the art perovskite photodetectors, optimized for multilayers vertical structure.

[1] a) Science 2012, 338, 643; b) Sci. Rep. 2012, 2, 591.
[2] Nat. Mater. 2015, 14, 636.
[3] a) Nat. Nanotechnol. 2014, 9, 687; b) ACS Nano 2014, 8, 10947.
[4] Appl. Phys. Lett. 2015, 107, 091904.
[5] Nat. Phys. 2015, 11, 582.
[6] Acc. Chem. Res. 2016, 49, 146.
[7] J. Phys. Chem. Lett. 2013, 4, 4213.
[8] J. Phys. Chem. C 2014, 18,16458.
[9] Nat. Nanotechnol. 2015, 10, 391.
[10] J. Mater. Chem. A 2015, 3,
9092.
[11] J. Mater. Chem. A 2016, 4, 6755.
[12] a) J. Mater. Chem. A 2015, 3, 8970; b) Chem. Rev. 2016, 116, 4558.

Following the unprecedented rise in photovoltaic power conversion efficiency in the past five years, metal halide perovskites (MHPs) have emerged as a new class of photoactive materials that promise to dramatically impact the solar industry. Their extraordinary electrical and optical properties are currently undergoing extensive studies while being explored for numerous other applications that span beyond energy generation.^[1-2] In particular, the realization of MHP photodetectors can enable a wide range of optoelectronic devices, including image sensors, night vision systems, healthcare monitor systems, optical communications and electro-optical circuitry. Among the different types of photodetectors, phototransistors not only combine the functionalities of photodiode and read-out circuits but also offer the ability to deliver high-gain-bandwidth detection with unparalleled photosensitivity due to their intrinsic gain characteristics.

Here, we report on a universal approach for realizing hybrid organic-perovskite photojunction transistors (HOPTs) with unmatched operating characteristics. The combination of perovskites and organic semiconductors allows precise control of the photoresponsivity beyond the fundamental properties of compositional materials for different transistor operation regimes. To shed light on this unusual, but highly versatile device property, advanced materials characterization techniques, such as Raman, photocurrent, and photoluminescence mapping with spatial resolution down to 1 µm, are used to study how the charge transport of photo-generated carriers takes place within the active region of the HOPTs.

By integrating multiple HOPTs, electro-optical circuits that are able to sense and process electrical as well as optical signals, are demonstrated. To the best of our knowledge, this is the first perovskites-based electro-optical circuitry demonstrated to date and paves the way to exciting new developments. Furthermore, by taking advantage of the large library of organic semiconductors in combination with recently developed hybrid perovskite compounds, we are able to demonstrate HOPTs capable of sensing photons with wavelengths beyond the optical spectrum (>800 nm) and in the near-infrared (NIR) region without compromising the device’s superior photosensitivity. The resulting HOPTs exhibit high photoresponsivity (~10⁴) with the ability for low-light detection down to a few nW. Finally, in addition to the highly tuneable opto-electrical properties, these prototypical HOPTs are fully solution-processable at low temperatures (<100 °C), and as such highly compatible with the emerging sector of inexpensive, large-area printed opto/electronics.

References
[1] S. D. Stranks, H. J. Snaith, Nat. Nanotechnol. 2015, 10, 391-402.
[2] Y.-H. Lin, P. Pattanasattayavong, T. D. Anthopoulos, Adv. Mater. 2017, DOI: 10.1002/adma.201702838.

Performance, stability and cost represent key needs for large-scale commercialization of perovskite photovoltaics and relevant optoelectronics. Electron-transport-layer-free (ETL-free) device architectures are promising designs for perovskite photovoltaics, simultaneous providing a simpler configuration, opportunities for ultra-low cost and convenience for building versatile optoelectronics (e.g., all-perovskite tandem photovoltaics and flexible photovoltaics). Unfortunately, all current-generation ETL-free perovskite photovoltaics suffer from compromised performance, due to insufficient understanding/control of carrier dynamics in the ETL-free perovskite devices. Herein, we use external quantum efficiency to reveal that typical ETL-free photovoltaics exhibit insufficient carrier collections due to substantially slow carrier injection process at the ETL-free interface. Moreover, we demonstrate that enhancing carrier lifetimes in perovskite films can efficiently tailor the carrier collection efficiency at the slow-injection-rate interface, making the carrier dynamics in ETL-free devices approach to that in ETL-containing devices. Benefiting from such understanding, ETL-free photovoltaics with power conversion efficiency (PCE) of 19.5%, nearly-eliminated hysteresis and good stability are successfully realized by using perovskite films with microsecond carrier lifetimes. Such PCE is comparable to the PCE (20.7%) from analogous ETL-containing photovoltaics using the same perovskite films and is also the best result among all the reported ETL-free perovskite solar cells to date. This research offers opportunities for versatile perovskite optoelectronics, simultaneously providing high performance, stability and potential for ultra-low cost.

Building on our recent advancements addressing the mechanical instability of perovskite solar cells using scaffold-reinforced compound solar cells (CSCs) in which planar perovskite devices are partitioned into many smaller microcells by a reinforcing scaffold, we have designed a new architecture with integrated light management for highly efficient and mechanically robust perovskite solar cells. The CSCs exhibited a fracture resistance of ~13 ± 3 J m^–2—a 30-fold increase over previously reported planar perovskite devices. However, device efficiency decreased based on the photo-inactivity of the scaffold material, where the short-circuit current scaled inversely with the scaffold dimensions (i.e., larger scaffolds resulted in lower current and efficiency). In this work, we demonstrate how efficiency losses can be mitigated using a robust and low-cost light management system, which directs light away from the scaffolds and into the perovskite microcells. This architecture is material independent and compatible with all perovskite solar cell compositions along with exhibiting some self-tracking ability to reduce system losses in efficiency at lower illumination angles. Device performance is shown to be stable under continuous illumination, and the low concentrated light is thus not deleterious to the perovskite. These developments are significant steps toward demonstrating robust perovskite solar cells with major improvements in reliability and service lifetimes while maintaining high efficiency that can ultimately compete with CIGS, CdTe, and c-Si cells.

Rapid Spray Plasma Processing (RSPP) is a high throughput, scalable route towards perovskite solar module manufacturing. In contrast to conventional spin-coating, RSPP uses clean dry air to produce a combination of plasma reactive species (photons, metastables, and radicals) and thermal energy which rapidly converts the perovskite film after spray-coating. High energy metastables and radical molecules diffuse through the thin plasma-liquid interface, inducing dissociation and volatilization of solvent molecules, and assisting the conversion into the perovskite structure. Deep UV photons emitted by the air plasma are absorbed by the DMF, superheating the solution. Convection with the plasma gas and reactive species quickly and efficiently transfers energy to the perovskite precursor-solvate, curing the film in a top-down manner. The result is a poly-disperse grain distribution, with grains ranging from 10s to 100s of nanometers in diameter, formed in under 250 milliseconds. We implemented in-situ wide angle x-ray scattering during single, double, and triple cation perovskite growth to further characterize the nucleation and growth mechanisms at play. The use of extremely bright synchrotron radiation and ultrafast detector speeds allowed us to resolve both direct crystallization and indirect crystallization reactions, indicated by transient peaks during the scattering measurement. The synergistic energy transfer from convection with the background gas and the interaction with plasma reactive species directly resulted in the mixed nucleation and growth reaction. We previously showed that the plasma annealing process caused significant mechanical toughening, with the current results suggesting that the mixed reaction pathway is responsible for the unique grain structure. Lab scale solar devices deposited with this method achieved excellent power conversion efficiencies (PCE) and large open circuit voltages (V_OC) ; we observed 15.7% PCE for MAPbI₃ and 1.08V V_OC for Cs_0.25FA_0.75Pb(Br_0.15I_0.85)₃ on PEDOT:PSS. The combination of high throughput manufacturing with excellent optoelectronic properties make RSPP very attractive to the single and multiple junction markets.

Meanwhile, the efficiency of Pb-based perovskite solar cells is comparable to that of silicon photovoltaics. However, upscaling devices for production is still a challenge. In recent studies, chemical vapor deposition (CVD) of perovskite solar cells has been recognized as an alternative to the widely used solution processing techniques leading to homogeneous perovskite layers on large areas. Moreover, increasing concern is related to the toxicity of Pb. Therefore, less toxic alternatives like Sn- or Bi-based perovskites are being explored. Compared to Sn perovskite solar cells, Bi-based devices showed a great stability over days of storage in ambient conditions, whereas power conversion efficiency (PCE) is still much lower. Additionally, halide compounds of Bi feature significantly higher vapor pressures and thus are more suitable for CVD compared to their Pb halide counterparts. Without the need for orthogonal solvents, CVD Bi-based perovskites also offer a great potential as high-bandgap material in tandem devices.
In this work, CVD of methylammonium bismuth iodide employing N₂ as carrier gas is studied. In the pressure regime of 10 hPa, methylammonium iodide (MAI) is heated up to 150 °C whereas BiI₃ is heated to 250 °C. The evaporation of both precursors is controlled via the carrier gas flux through each source. To avoid parasitic gas phase prereactions of both precursors, perovskite films are formed by alternating CVD processes. Here BiI₃ and MAI are consecutively deposited over several cycles with time lengths varied between 600 and 3600 s. Single-layer depositions on titanium oxide coated FTO substrates are carried out to investigate film formation and thicknesses. Finally, complete solar cells employing an additional solution-processed Spiro-MeOTAD film with evaporated gold as top electrode were fabricated. SEM measurements are used to investigate the effect of the process parameters. The crystal structure and porosity of the perovskite layer depend strongly on the deposition rate of the perovskite and the substrate temperature. It was found that substrate temperatures in the range of 60°C to 80°C are needed to achieve an in-situ perovskite formation. IV measurements show that CVD results in significantly denser layers with reduced parasitic series resistance and improved fill factors as compared to solution-processed reference layers. Overall, a maximum PCE of 0.03% and an open-circuit voltage of 0.7 V could be achieved with vapor-deposited methylammonium bismuth iodide which is comparable to those of solution-processed solar cells. Cross-section SEM images reveal that the vapor-deposited perovskite poorly penetrates the mesoporous titanium dioxide structure and show adhesion defects at the surface resulting in relatively low currents. Possibilities for future improvements are choosing planar cell geometries, introducing a vapor-deposited hole transport layer and further process optimizations.

Organic–inorganic hybrid perovskite has led to the development of new solar cells with outstanding efficiency reaching 22.1% of certified efficiency. In perovskite solar cells (PSCs), perovskite is sandwiched between a working electrode (fluorine-doped tin oxide, FTO) and a counter electrode (gold, Au). In order to transport charges and block opposite charges, charge transport layers are inserted between perovskite and the electrodes. In the conventional structure, a hole transport layer prevents perovskite from exposure to air. Therefore, it is necessary to investigate dopant-free and hydrophobic polymeric hole transport materials (HTMs). In this study, a novel polymeric HTM (PTEG) is synthesized by controlling the solubility using a tetraethylene glycol (TEG) group. The planar-PSC employing PTEG exhibits an efficiency of 19.8% without any dopants, which corresponds to the highest value reported to date. This study offers a fundamental strategy for designing and synthesizing various polymeric HTMs.

Hybrid perovskites represent a potential paradigm shift for the creation of low-cost solar cells. Current power conversion efficiencies (PCEs) exceed 22%. However, despite this, record PCEs are still far from their theoretical Shockley−Queisser limit of 31%. To increase these PCE values, there is pressing need to understand, quantify and microscopically model charge recombination processes in full working devices. Here, we present a complete microscopic accounting of charge recombination processes in high efficiency (18-19% PCE) hybrid perovskite (mixed cation and methylammonium lead iodide) solar cells. We employ diffraction-limited optical measurements along with relevant kinetic modeling to establish, for the first time, local photoluminescence quantum yields, trap densities, trapping efficiencies, charge extraction efficiencies, quasi-Fermi-level splitting, and effective PCE estimates. Correlations between these spatially-resolved parameters, in turn, reveal factors which limit these state-of-the-art hybrid perovskite solar cells.

Improving the temperature stability for perovskite solar cells is essential for the commercial success of this technology. The issue becomes especially pressing for solar cells operated in a hot and arid climate. In this work we present perovskite solar cell structures with different architectures, and improved temperature stability suitable for operation in the hot/arid climate of Saudi Arabia. Meteorological data is used to establish the relevant operating conditions for solar cells in this region. Hole and electron transport layers with improved temperature stability are introduced and used for the fabrication of upright – for single junction operation – and inverted – for operation in tandem with silicon – solar cell architectures. A series of measurements with varying temperature, injection, and spectrum are used to mimic the operation conditions of these solar cells, and establish energy yield and degradation behavior. These metrics are in turn used to estimate LCOE, and determine under which conditions perovskite single junction and tandem solar cells can economically compete with established silicon and CdTe solar cells.
Initial assessment shows that the most relevant operating conditions for energy yield in Saudi Arabia vary between 400 W/m² and 1000 W/m², with temperatures ranging from 40^○C to 60^○C. In extreme cases, operating temperatures can reach close to 70^○C. We also find that most energy is generated at specific spectral conditions with only small variations, however overall occurring spectra in Saudi Arabia vary over a wide range, with implications especially for two terminal tandem solar cells. Specific to the area is also a comparably low relative humidity and the absence of frost. These conditions mark opportunities for the operation of perovskite solar cells, but also pose challenges for encapsulation and packaging.
We have fabricated first test samples with standard testing condition efficiencies of greater than 20%, which are currently exposed to varying spectrum and temperature conditions, to study their behavior and establish initial energy yield estimations. In a next step we will vary the electron and hole transport layer in these samples, to improve stability and performance under high temperature conditions. We intend to combine these assessments with a bottom-up cost analysis, to establish the most relevant challenges and opportunities for perovskite solar cells in hot and arid climates.

An important component of improving the commercial viability of perovskite solar cells is the development of device architectures that are compatible with low-temperature processing methods. In this work, we investigate the performance of solar cells in which the hole transport layer (HTL), composed of delafossite CuCrO₂, is deposited at room temperature. The HTL films are prepared by spin-coating suspensions of hydrothermally-synthesized CuCrO₂ nanoparticles, whose small size (~10 nm) and nearly spherical shape enable the formation of highly compact films, even for very thin layers. While DFT calculations predict that both rhombohedral and hexagonal polytypes of the delafossite structure may be present due to the similarity in their formation energies, their optoelectronic properties should be essentially identical, and well-suited for the role as HTL in perovskite solar cells. Experimental measurements confirm that the CuCrO₂ films are highly transparent and possess favorable valence band alignment relative to the MAPbI₃ absorber, as well as p-type conductivity. Solar cells fabricated using a glass/ITO/CuCrO₂/MAPbI₃/C₆₀/BCP/Ag device architecture perform with low hysteresis and stabilized power conversion efficiency of over 14%. These results pave the way for the development of cost-effective, low-energy-input device architectures based on CuCrO₂ HTLs.

Organometallic halide perovskite solar cells have gained considerable interest in recent times. Despite the devices’ low cost, reported power conversion efficiencies have risen rapidly and now exceed 22 %. [1] However, most reported efficiencies have been obtained on an active area markedly smaller than 1 cm². Demonstrating similar efficiencies on upscaled devices for industry has been a principal challenge faced by the technology.

While TiO₂ is the most commonly used electron transport layer (ETL) in the field, it has been shown to produce pronounced hysteresis in the current-voltage curve for planar devices. TiO₂ also reduces the long term stability of the cell namely due to its photocatalytic action under UV illumination. Band gap engineering studies in recent years suggest SnO₂ as a viable replacement due to its enhanced conduction band alignment which may allow for improved electron extraction when compared to TiO₂ based devices. [2]


Here we demonstrate the deposition of nominally undoped TiO₂ and SnO₂ ETLs by thermal atomic layer deposition (ALD) on fluorine doped tin oxide (FTO) and indium doped tin oxide (ITO) coated glass substrates. These are then fabricated into planar perovskite devices with an active area up to 1 cm². ALD growth of metal oxides is carried out below 200 °C to accommodate silicon-perovskite heterojunction cells. [3]

The resulting cell performances of 18.3 % power conversion efficiency (PCE) on 0.09 cm² and 15.3 % PCE on 1.04 cm² active areas are discussed in conjunction with the significance of growth parameters and ETL composition.

[1] https://www.nrel.gov/pv/assets/images/efficiency-chart.png
[2] J. P. Correa Baena et al., Energy Environ. Sci., 8, 2928
[3] J. P. Mailoa et al., Appl. Phys. Lett. 106, 121105

The past few years have witnessed a rapid evolution of hybrid organic-inorganic perovskite solar cells (PSCs) with both low cost and boosted high power conversion efficiency (PCE) over 22%. Despite the achievements, MAPbI₃ suffers from inherent instability over ambient operation conditions due to the low formation energy of the material itself and the high hydrophilicity of the organic cations. Efforts such as developing novel device architectures as well as exploring novel materials have been tried to improve the device stabilities. Among them, the two-dimensional (2D) perovskites that are crafted using bulkier alkylammonium cations in place of methylammonium exhibit appealing environmental stability. However, the insulating alkylammonium spacer cations hinder charge transport and limit the efficiencies of the devices based on such perovskites. In this scenario, an exploration of alternative yet effective organic spacer cations that increase the charge transfer is imperative to enhance the efficiency.
Herein, we design such an alternative bi-functional conjugated cation AB. We report the first time the incorporation of AB in 2D/3D perovskites and its implementation on solar cells. The use of bi-functional conjugated cations enhances significantly the performance of the cells, reaching a highest power conversion efficiency of 15.6% with improved stability. The efficiency remained around 90% of the initial value after 100 h continuous illumination, much more stable than MAPbI₃ perovskite. By comparing this cation with a mono-functional cation and a bi-functional non-conjugated cation with similar structure, we found that the bi-functional conjugated cation not only benefits the growth of perovskite crystals in the mesoporous network, but also facilitates the charge transport. Our approach helps explore new rational designs of cations in perovskites.

Organic-inorganic hybrid perovskite (OIHP) solar cells have achieved over 22% powder conversion efficiency, which are promising candidates for next generation of energy converter. However, the stability problems have hindered their commercialization. While encapsulation techniques have been developed to protect OIHP solar cells from external stimuli such as moisture, oxygen and ultraviolet light, understanding the intrinsic instability origin of perovskite films is needed to improve their stability.
In this talk, we will report the OIHP films fabricated by existing methods are strained, caused by mismatched thermal expansion of perovskite films and substrates during the thermal annealing process. The strain was characterized by both in-plane and out-of-plane XRD. The strain accelerates degradation of perovskite films under illumination, which can be explained by increased ion migration in strained OIHP films. This study points out an avenue to enhance the intrinsic stability of perovskite films and solar cells by reducing residual strain in perovskite films.

Transparent photovoltaic (TPV) technologies offer an exciting approach to produce smart windows on buildings, vehicles, mobile electronics, and greenhouses. TPVs can both regulate the transmission of solar heat and provide electricity generation by photoelectron conversion of the invisible part of the solar spectrum. While there is substantially less overall solar photon flux in the ultraviolet, efficiencies up to 7% are theoretically achievable due to high photovoltages > 2V. The approach of selectively harvesting ultraviolet-only photons can enable integration of more traditional semiconductors such as GaN, ZnO, and more enticingly halide perovskites. To date, reported ultraviolet (UV) harvesting TPV devices are still limited by low efficiency (£ 0.1% for inorganic-based devices) and/or low average visible transparency (AVT £ 60%). In this work, we develop a new platform of TPVs based on halide perovskite semiconductors where the bandgap is precisely tuned with a range of compositions to selectively harvest only UV photons with bandgaps between 400-440 nm. We will discuss the methods we developed to enable this tunability and the challenges that exist to control the morphology. Based on this insight, we demonstrate TPV devices with power conversion efficiencies (PCEs) up to 0.52%, average visible transparency (AVT) up to 70.7%, and color rendering index (CRI) over 92. These transparencies are among the highest ever reported for wavelength-selective TPVs. This approach offers theoretical efficiencies up to 7% with 100% visible transparency and CRI > 90. While these first demonstrations are limited by quantum efficiencies of only 20-30%, practical optimization of these perovskite cells could quickly yield TPVs with PCEs in the 3-5% as quantum efficiencies approach 90%. Such devices would rival state of the art TPVs that selectively harvest near-infrared light while also providing a route to higher efficiency multi-junction TPVs when synergistically coupled with near-infrared harvesting TPVs. This work ultimately creates an enticing new direction for halide perovskite research.

It has become clear in the past few years that the commercial success or failure of perovskite solar cells hinges upon thier stability. Thus, over the past few years research efforts focused on improving perovskite solar cell stability have greatly intensified. Recent work has provided insight into moisture instability, thermal instability, and phase instability of the halide perovskites themselves. Building off this important knowledge base, we begin with a standard TiO₂/perovskite/sprio-OMeTAD/Au device architecture and then systematically investigate the degradation mechanisms associated with TiO₂, spiro-OMeTAD, and Au, replacing each of these layers to mitigate identified degradation pathways. This work results in a detailed understanding of the major device-level degradation mechanisms and, importantly, their suppression. Ultimately, this strategy improves device stability by over 3 orders of magnitude. The final SnO₂/perovskite/EH44/MoO_x/Al device stack retains 94% (88% average) of its peak power conversion efficiency despite 1000 hrs of continual, unencapsulated operation in ambient conditions. This dramatic improvement in stability, despite the combined stresses of UV-light, oxygen, and moisture, demonstrates the importance of carefully designed interfaces for realizing true long-term perovskite solar cell stability.