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.
<div id="UMS_TOOLTIP" style="position: absolute; cursor: pointer; z-index: 2147483647; background-color: transparent; top: -100000px; left: -100000px; background-position: initial initial; background-repeat: initial initial;"> </div>

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.