AVS2017 Session TF-WeM: Thin Film for Photovoltaics

Metal halide perovskite solar cells (PSCs) have received global attention because of their excellent photovoltaic performance and ease of fabrication. However, much of the focus in perovskite solar cell advance has been on improved device efficiency through trial and error recipe modification. Despite the impressive photovoltaic performances, perovskite solar cells are poorly stable under operation, failing by far the requirements for a widespread commercial uptake.^1-3 Various technological approaches have been proposed to overcome the instability problem, which, while delivering appreciable improvements, are still far from a market-proof solution.^4-5 In this talk we demonstrate stable perovskite devices by engineering an ultra-stable 2D/3D HOOC(CH₂)₂NH₃)₂PbI₄/CH₃NH₃PbI₃ perovskite junction. The 2D/3D composite delivers an exceptional gradually organized multidimensional structure that yields 13% photovoltaic efficiency in a low cost, hole-conductor free architecture and 20% in standard mesoporous solar cells. To demonstrate the up-scale potential of this technology we fabricate 10x10 cm² solar modules by a fully printable, industrial-scale process delivering 11% efficient devices which are stable for >10,000 hours with zero efficiency loss measured under controlled standard conditions. This innovative architecture will likely enable the timely commercialization of perovskite solar cells.

References

1. National Renewable Energy Laboratory, N.R.E.L. http://www.nrel.gov/ncpv/images/efficiency_chart.jpg.

2. M. Saliba, T. Matsui, J. Seo, K. Domanski, J.-P. Correa-Baena, Md.K. Nazeeruddin, S.M. Zakeeruddin, W. Tress, A. Abate, A. Hagfeldt, M. Grätzel Energy Environ. Sci. 9, 1989-1997 (2016).

3. X. Li et al., Improved performance and stability of perovskite solar cells by crystal crosslinking with alkylphosphonic acid ω-ammonium chlorides. Nat. Chem. 7, 703–711 (2015).

4. I. C. Smith, E. T. Hoke, D. Solis-Ibarra, M. D. McGehee, H. I. Karunadasa, A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability. Angew. Chem. 126, 11414–11417 (2014).

5. K. Domanski et al., Not All That Glitters Is Gold: Metal-Migration-Induced Degradation in Perovskite Solar Cells. ACS Nano. 10, 6306–6314 (2016).

Synthesis of methylammonium bismuth iodide (CH₃NH₃)₃Bi₂I₉) is achieved through a single step, atmospheric pressure, chemical vapor deposition (CVD) process. The precursors used are powders of bismuth iodide (BiI₃) and methylammonium iodide (CH₃NH₃I). These precursors are sublimated inside a tube furnace reactor with a well-controlled and predetermined temperature profile. The sublimated vapors are transported via Ar carrier gas to the cooler parts of the tube furnace, where simultaneous condensation and reaction between the two precursors leads to the formation of high quality (CH₃NH₃)₃Bi₂I₉ films on the wafer surface.

Structural and compositional information is obtained via grazing incidence X-ray diffraction (GI-XRD), SEM, TEM and XPS which confirm the (CH₃NH₃)₃Bi₂I₉ phase. Band-edge information is obtained via a combination of cyclic voltammetry and UV-vis spectroscopy. The band gap is revealed to be 1.9 eV. Temperature-dependent Hall measurements detect the recently observed ^[1] ferrielectric phase transition at ~150K via changes to the slope in the concentration vs. temperature and mobility vs. temperature curves. A room temperature electron concentration of 1.48 x 10¹⁹ cm^-3 and mobility of 7.13 cm²/V.sec is obtained for (CH₃NH₃)₃Bi₂I₉ films with good crystallinity.

Reference:

[1] Kamminga, M. E.; Stroppa, A.; Picozzi, S.; Chislov, M.; Zvereva, I. A.; Baas, J.; Meetsma, A.; Blake, G. R.; Palstra, T. T. M., Polar Nature of (CH₃NH₃)₃Bi₂I₉ Perovskite-Like Hybrids. Inorganic Chemistry 2017, 56, 33-41.

Within the class of emerging photovoltaic technologies, perovskite solar cells have exhibited a sky-rocketing conversion efficiency above 20% in just a few years. In this contribution we will address the opportunities which atomic layer deposition (ALD) offers to perovskite solar cells [1] by highlighting the following merits: control on charge carrier transport and recombination processes at complex interfaces [2,3] and compatibility with low-temperature processing [3]. Specifically, two case studies will be presented:

· Plasma-assisted ALD amorphous TiO₂ (cycles consisting of Ti(Cp^Me)(NMe₂)₃ and O₂ plasma exposure steps) has been adopted in MeNH₃PbI₃ perovskite solar cells in a mesoscopic configuration [2], with the purpose of suppressing charge recombination processes at the ITO/mesoscopic scaffold/perovskite interface. The superior performance of 10 nm thick ALD TiO₂ layers (i.e. up to 16% cell efficiency under 1000/m² illumination and 24% under indoor illumination) with respect to conventionally adopted spray pyrolysis TiO₂, is explained by a lower reverse dark current measured for ALD TiO₂. This result points out the superior blocking character of the ALD TiO₂ layer toward electron-hole recombination, which is also confirmed by the evaluation of the electron lifetime from open circuit voltage decay analysis. Since ALD TiO₂ is carried out at temperatures below 150°C, flexible perovskite solar cells built on PET/iTO substrates are also tested, exhibiting a conversion efficiency of 10.8% under indoor illumination, comparable to the one of flexible dye-sensitized solar cells and exceeding the one of flexible a-Si:H solar cells.

· Ultra-thin (15 nm) plasma-assisted ALD amorphous SnO₂ (cycles consisting of Sn(NMe₂)₄ and O₂ plasma exposure steps) is adopted as electron transport layer in a Cs_x(MA_yFA_1-y)_1-xPb(I_z,Br_1-z)₃ perovskite solar cell in an n-i-p thin film configuration. The solar cell efficiency reaches the value of 15.9±0.5%, while the same solar cell configuration with an electron-beam deposited TiO₂ electron transport layer reaches an efficiency of just 10 ±0.5%. UPS analysis shows that the higher efficiency achieved by SnO₂ (with respect to TiO₂) is due to a better energy level alignment between the SnO₂ and perovskite conduction bands, promoting electron transport. Hole transport, instead, is efficiently blocked because of the misalignment between the SnO₂ and perovskite valence bands.

References

[1] V. Zardetto et al., Sustainable Energy and Fuels 1,30 (2017)

[2] F. Di Giacomo et al., Nano Energy 30, 460 (2016)

[3] V. Zardetto et al., Solar Energy, accepted (2017)

Abstract

The Methyl amine lead iodide perovskite (CH₃NH₃PbI₃) shows great potential in solar cells, light-emitting diodes, lasers, and chemical sensors. Base on the surface chemical properties of CH₃NH₃PbI₃, many chemicals with Lewis base group, for instance: pyridine, tetra-ethyl ammonium, were used to enhance the photovoltaic, optoelectronics performance and the stability in ambient environment. However, people found contradict results that Pyridine molecules not only passivate the surface Pb²⁺ sites of CH₃NH₃PbI₃, but also to bleach and recrystallize CH₃NH₃PbI₃. Surface passivation demands the confinement of the reaction at the surface region but recrystallizing and bleaching require the transformation of bulk CH₃NH₃PbI₃. The underlying mechanism for these seemly contradicting results are not well-understood. Our results show, at 25 ºC, partial pressure of pyridine vapor is a determining factor for its reaction behaviors with CH₃NH₃PbI₃: one can modify just the surface of CH₃NH₃PbI₃ by using pyridine vapor of the pressure less than 1.15 torr, but can transform the whole bulk CH₃NH₃PbI₃ film with a pyridine vapor of 1.3 torr or higher. The results indicate, for the first time, that a small change of free energy of pyridine vapor (~ 0.3 kJ/mol) can cause the transition from surface-confined reaction to bulk transformation. It is interesting that in all pressure ranges, pyridinium ions is the main product from the reaction between pyridine and CH₃NH₃PbI₃. The bulk transformation is probably due to the formation of a liquid-like film, which increases the mobility of species to catalyze the reaction between pyridine and CH₃NH₃PbI₃. These findings provide a guidance for designing experiments in applying pyridine and probably other amines to functionalize and transform CH₃NH₃PbI₃ and other hybrid halide perovskites.

Tritantalum pentanitride (Ta₃N₅), a visible-light absorbing material (<600 nm), was formed into a thin layer and tested as a photoelectrode in electrolytic solution under simulated solar light. By the aid of cobalt phosphate catalyst (CoPi), the electrode surface evolved O₂ at a substantial efficiency for photon energy conversion. By inserting a GaN layer between the catalyst and Ta₃N₅, the electrode exhibited an improved stability and durability for O₂ evolution in aqueous electrolytic solution.

The Ta₃N₅ film was prepared by high-temperature (1000-1300 K) nitridation of sputtered Ta film on Ta metallic substrate in NH₃ flow at the atmospheric pressure. Ta sputtering was performed in a RF magnetron setup with a Ta metal target at 4 Pa of purified Ar introduction pressure. The gas impurities were minimized and a trace amount of O₂ was introduced for sputtering. The formed Ta film is amorphous with a thickness approximately 500nm. After nitridation, the formed Ta₃N₅ is a transparent, orange-colored film with a visible light absorption cutoff at xxx nm. The CoPi catalyst was deposited by simulated solar irradiation in Co(NO₃) ₃ solution in neutral phosphate buffer.

The O₂ evolution performance was examined in pH 13 K₃PO₄ buffer under potentiostatic control with a Pt counter electrode and a Ag/AgCl reference electrode. At 1.23 V vs RHE of the anodic potential, The photocurrent by a solar simulator at AM1.5G (1 kWm^-2) reached 7 mAcm^-2 or higher, however rapidly decay in 1 hour. This was due to Ta₃N₅ oxidation, and we introduced a GaN overlayer (thickness ~ 50 nm) by NH₃-nitridation of evaporated Ga₂O₃ layer (electron-beam heating, largely metallized). The same photoelectrochemical test exhibited a photocurrent higher than 8 mAcm^-2, lasting for more than 10 hours, with slow deactivation afterwards. Microscopic observation of the GaN layer showed fine grains (<50 nm in diameter) of GaN, blocking Ta₃N₅ oxidation. The faradaic O₂ evolution was confirmed by anther setup.

The semiconductive properties of Ta₃N₅ and the GaN/Ta₃N₅ junction are currently investigated. Both Ta₃N₅ and GaN in the present case are polycrystalline and contain influential amounts of impurities, reflecting in the resistivity and other transportation properties.

The exceptional uniformity of deposition provides magnetron sputtering with potentially important advantages for the production of thin film CdTe solar cells. For example, the thickness of the absorber layer could be reduced to ~1um saving on material cost. The uniformity is also essential for glazing applications such as power producing semi-transparent windows. Efforts to develop a viable process for the deposition of thin film CdTe solar cells by magnetron sputtering have been hampered for many years by the formation of surface blistering following the cadmium chloride activation process. Delamination of the films at the cadmium sulphide junction is often observed and catastrophic void formation within the CdTe layer and at the junction has also been reported. As a result, the conversion efficiency of sputtered devices has not matched that obtained using lower energy deposition techniques such as Close Space Sublimation or Vapour Transport Deposition. We have discovered that blistering is caused by argon trapped in the CdTe layer during the sputtering process. High Resolution Transmission Electron Microscopy reveals that argon gas bubbles are formed by diffusion during the high temperature device activation process. The bubbles then coalesce to cause surface blistering. Here we report on the development of a viable sputtering process by replacing argon with xenon as the magnetron working gas. Using xenon avoids gas bubble and blister formation. The microstructural integrity of the devices is retained resulting in increased performance of sputtered thin film CdTe photovoltaic devices.

Organic solar cell technology is very low cost, reliable and efficient technology. In this work, we have studied the performance of single layer hybrid organic solar cell, which is based on polyvinyl alcohol (PVA) doped with Camellia sinensis (Black Tea) solution and Zinc Oxide (ZnO) as plasmonics nanoparticles to scatter more light into active layer of solar cell. The broad absorption spectra of the composite material (PVA and tea solution) with various concentrations ranged from UV to visible region. Its energy gap was measured which allowed trapping a large portion of the incident solar light. The band gap is reduced upon the addition of Camellia sinensis (Black Tea) because the addition of Camellia sinensis (Black Tea) caused PVA to get more ordered structure. Characterization of ZnO nanoparticles using XRD and SEM confirmed the hexagonal wurtzite structure and spherical surface morphology of nanoparticles. Six solar cells were fabricated based on various concentrations of PVA doped with Camellia sinensis (Black Tea) solution using silver paste/graphite as counter electrodes, with and without ZnO nanoparticles thin film. The current-voltage (IV) characteristics curves of the fabricated organic solar cells were measured. The IV curves were studied of all soar cells that gave the following efficiencies 0.0216%, 0.0364%, 0.34%, 0.42%, 0.564 and 0.729%. The maximum energy conversion efficiency was calculated to be 0.7%, for an organic solar cell having high concentration of camellia sinensis, with ZnO nanoparticles thin film and silver paste as a counter electrode.

Key Words: Hybrid organic cell, polyvinyl alcohol, Camellia Sinensis, ZnO nanoparticles.

Cu₂ZnSnS₄ (CZTS), a potential low-cost, earth-abundant absorber material, has been under intense investigation for prospective use in thin film photovoltaic (PV) devices. Its high optical absorption coefficient and variable band gap also have potential applications in other areas including photocatalysis, thermoelectrics, and energy storage. Theoretically predicted physical properties, however, have not been realized to date owing to the lack of control over chemical and microstructural heterogeneity and grain size and grain boundary chemistry. Using a modified aerosol spray pyrolysis technique, we have grown phase-pure 25 nm CZTS nanoparticles having no organic ligand contaminant. This is desirable in that grain boundary chemistry is known to degrade both optical and electrical response. Many applications demand that these nanoparticles be consolidated into relatively thick, large grain size films by means of a high temperature annealing treatment that drives impurities to interfaces thereby degrading properties. For PV applications, large grains are required to significantly reduce exciton recombination at grain boundaries. While the particles produced here do not sinter uniformly into polycrystalline thin films, we have shown that introducing a thin amorphous oxide to the surface of the particles does indeed promote uniform grain growth to create thin films with the optimal morphology for absorber layers in PV devices. The improvement is attributed to an increased retention of volatile SnS, a byproduct of CZTS decomposition, which occurs at high temperature. We have also controlled grain growth by incorporating Na₂S into the film prior to annealing by means of a facile dipping technique – sodium is known to enhance grain growth in this material, though its incorporation is generally uncontrolled. Additionally, we present data from in situ Raman spectroscopy measurements designed to simulate the standard CZTS thin film annealing process. Using this approach, we have characterized the time/temperature effects on the vibrational spectrum of annealed CZTS films at high temperature in static variable atmospheres of sulfur vapor with and without a thin surface oxide interfacial coating. This technique is also used to monitor the cation site exchange process (copper and zinc) in the lattice that appears to be dependent on the rate of cooling following high temperature annealing.