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The fabrication of high efficiency perovskite solar cells at larger scales will rely on the optimized deposition conditions of every layer using scalable methodologies. Most current perovskite devices that employ the archetypal TiO2 hole blocking layer utilise a simple air-brush approach. This approach is not scalable as it results in significant layer inhomogeneity across larger devices areas. To overcome this inherent limitation, in this work we use ultrasonic spray deposition as an alternative approach for the TiO2 deposition. Focusing on identical reaction chemistries as for air-brush, namely bis(isopropoxide)-bis(acetylacetonate) titanium (IV) based solutions, we find that under optimized conditions smooth TiO2 layers can be readily deposited over scalable areas on fluorine doped tin oxide. Using these layers at electron transport layers within diodes and perovskite solar cell devices, we develop effective benchmarks that predict the effectiveness of the underlying charge transport layer. This enables for the standardisation of the electron transport properties within any batch of solar cells, thus providing a readily accessible pathway towards enhancing reproducibility of fabricated devices. Under these optimized conditions, methylammonium lead halide photovoltaic devices readily possessing efficiencies of >16% are achieved. Importantly, the mean batch efficiency of devices fabricated using the ultrasonic spray deposition method is significantly improved and the standard deviation is drastically narrowed. The deposition of an additional meso-porous layer is found to lead to further improvements for both of these parameters.

A heightened understanding of nucleation and growth mechanisms is paramount if effective solution processing of organic-inorganic perovskite thin-films for optoelectronic applications is to be achieved. Many fabrication techniques have been utilized previously to develop high-performance perovskite layers but there remains an absence of a unifying model that describes accurately the formation of these materials from solution. The present study provides a thorough analysis of nucleation and growth kinetics underpinning the development of hybrid organic-inorganic perovskite thin-films. Through precise control of the perovskite growth conditions the spacing of heterogeneous nucleation sites was varied successfully from several hundred nanometers to several hundred microns. The crystalline regions surrounding these nuclei were found to comprise clusters of highly-oriented crystal domains exceeding 100 μm in diameter. However, no beneficial correlation was found between the size of these well-oriented grain-clusters and the optoelectronic performance. The formation of the perovskite microstructure features characteristics of both classical and non-classical growth mechanisms. The insights into perovskite thin-film growth developed by the present study provide clear implications for the development of future hybrid perovskite microstructures.

To achieve high-performing perovskite solar cells (PSCs) interfacial engineering of the perovskite thin film and charge carrier-selective layers is vital for fast extraction of photogenerated electrons and holes, and the suppression of electron-hole recombination. Herein, a glucose-assisted self-assembly solvothermal protocol is reported to prepare electron-rich TiO2 thin films as effective electron transport layers exhibiting enhanced electron mobility. Bilayer structured CH3NH3PbI3 perovskite films are spontaneously formed, consisting of a flat and dense bottom layer forming the TiO2/CH3NH3PbI3 planar heterojunction and a textured and porous top layer elongating in the vertical direction forming the CH3NH3PbI3/spiroMeOTAD bulk heterojunction. The integrated planar and bulk dual heterojunction based PSCs are efficient in light harvesting and charge collection, and thus yield power conversion efficiencies up to 17.75% and a stabilized power output above 17.20%. Integrating both planar and bulk heterojunctions into a PSC assembly provides an effective approach for fabricating highly efficient perovskite optoelectronic devices.

Inorganic hole – transporting materials (HTMs) are a promising class of compounds for improving the long-term stability of perovskite solar cells. In this study, copper(I) thiocyanate (CuSCN) has been applied as an HTM in planar-structured thin film perovskite solar cells based on methylammonium lead(II) triiodide. A common obstacle associated with the deposition of inorganic HTMs in perovskite-based solar cell devices is the damaging effect of polar solvents, required during the solution-processed deposition step, on the underlying perovskite film. Here we describe a novel fabrication method that allows the deposition of a CuCSN layer on perovskite film, achieving a maximum power conversion efficiency of 9.6%. The magnitude of J-V hysteresis is found to be strongly dependent on the HTM used, with the phenomenon being much more prevalent in the CuSCN- and spiro-OMeTAD-based devices compared to CuI-based devices. Interestingly, CuSCN and CuI showed significantly different J-V hysteresis behaviors despite their similar physicochemical properties. Further characterization by open circuit voltage decay (OCVD) measurements revealed that the relaxation of the perovskite polarization depends on the light intensity and the adjacent HTM layer. We propose that the stronger J-V hysteresis in CuSCN compared to CuI is a result of defects generated during the deposition process and possible degradation at the material interfaces while other possibilities are also discussed.

The recent rise in power conversion efficiencies reported for perovskite solar cells has been a remarkable development in photovoltaics research. It is now pressing that the technology transitions from a research phenomenon to a real-world deployable device: this will require both robust methods for efficiency measurement, and accurate models for performance variation at different conditions. However, the generally slow response of perovskite solar cells to changes in voltage bias and irradiance, and the susceptibility of these cells to degradation, presents significant challenges. In this paper, we investigate current and voltage stabilisation of planar CH3NH3PbI3 perovskite solar cells and observe remarkably large variations in stabilisation time depending on exposure history. To address this, we demonstrate a dynamic approach that continues device pre-conditioning until pre-determined stability criteria are met. This approach is then employed to obtain measurements of short-circuit current and open-circuit voltage temperature coefficients under quasi-steady-state conditions for perovskite devices and a control monocrystalline silicon cell. The obtained open-circuit voltage temperature coefficient for the perovskite is −2700 ppm/°C, which interestingly, is similar to typically reported values for crystalline silicon devices. It is shown that the implemented approach can successfully differentiate between transient responses to the onset of illumination and true temperature related changes. We also find new manifestations of the complex transient processes that occur in perovskite devices. These observations highlight the importance of sophisticated characterisation approaches for correct characterisation of the performance of perovskite solar cells. Copyright © 2016 John Wiley & Sons, Ltd.

Long-term stability represents a major challenge for the commercial deployment of hybrid perovskite solar cells (PSCs). The stability of solar cells is commonly tested under continuous illumination over extended periods of time, for example, 1000 h. We have found that such a method does not adequately reflect the long-term performance of perovskite solar cells under the diurnal solar irradiation cycles experienced in real-world applications. We report a new characterization protocol of multiple 12-h cycles of darkness and illumination, uncovering a unique ‘fatigue’ behavior of PSCs. The PSC efficiency was found to decrease to 50% or less of its maximum value after storage in the dark for 12 h under open circuit conditions. The solar cell performance was capable of recovering to its maximum value in the subsequent 12-h illumination period, but the recovery rate slowed significantly with successive illumination/darkness cycles. This fatigue mechanism was strongly dependent on the cell temperature. The identification of this fatigue behavior renders our proposed characterization protocol an essential component of perovskite solar cell testing.

Perovskite solar cells (PSCs) employing planar and mesoscopic architectures have both resulted in high efficiency devices. However, there is presently a limited understanding of the inherent advantages of both systems, particularly in terms of the charge transport and recombination dynamics. In the present study we characterize the relative benefits of the two most prominent CH3NH3PbI3 morphologies, primarily through time-resolved microwave conductivity (TRMC) and time-resolved photoluminescence (TRPL) measurements. The comparatively large perovskite grains, typical of planar assemblies, exhibited higher charge mobilities and slower trap-mediated recombination compared to the mesoscopic architectures. These findings reveal the injurious influence of grain boundaries on both charge transport and recombination kinetics, and suggest an innate advantage of planar morphologies. However, through impedance spectroscopy (IS) measurements, mesoscopic architectures were found to limit the interfacial recombination at the transparent conductive oxide (TCO) substrate. The lessons learnt through the characterization measurements were subsequently utilized to produce an optimized cell morphology, resulting in a maximum conversion efficiency of 16%.

In this work, inverted structure CH3NH3PbI3 perovskite-based solar cells (PSCs) employing different inorganic metal oxides as hole-selective interlayers have been fabricated on ITO/NiOx, ITO/VOx and ITO/MoOx substrates and exhibit power conversion efficiencies (PCEs) of 10.6±0.6 %, 11.7±0.5 % and 5.9±0.5 %, respectively. The metal oxide layers deposited on ITO glass modify the substrate work function, leading to open circuit voltages above 1.0 V. Moreover, solar cells fabricated on ITO substrates without any metal oxide layer achieve a high PCE of 11.5 %, indicating that PSCs can perform well with a single selective contact. The inverted structure PSCs, with or without the metal oxide interlayers, show only a weak hysteresis in the J-V measurements. Our work reveals that single selective contact PSCs with simplified device structures can be very efficient after optimization of each active layer, making them highly competitive with other types of thin film photovoltaic (PV) technologies.

Organic–inorganic lead halide perovskite solar cells are promising alternatives to silicon-based cells due to their low material costs and high photovoltaic performance. In this work, thin continuous perovskite films are combined with copper(I) iodide (CuI) as inorganic hole-conducting material to form a planar device architecture. A maximum conversion efficiency of 7.5% with an average efficiency of 5.8 ± 0.8% is achieved which, to our knowledge, is the highest reported efficiency for CuI-based devices with a planar structure. In contrast to related planar 2,2′,7,7′-tetrakis-(N,N -di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD)-based devices, the CuI-based devices do not show a pronounced hysteresis when tested by scanning the potential in a forward and backward direction. The strong quenching of photoluminescence (PL) signal and comparatively fast decay of open-circuit voltage demonstrates a more rapid removal of positive charge carriers from the perovskite layer when in contact with CuI compared to spiro-OMeTAD. A slow response on a timescale of 10–100 s is observed for the spiro-OMeTAD-based devices. In comparison, the CuI-based device displays a significantly faster response as determined through electrochemical impedance spectroscopy (EIS) and open-circuit voltage decays (OCVDs). The characteristically slow kinetics measured through EIS and OCVD are linked directly to the current–voltage hysteresis.

Impedance spectroscopy (IS) is emerging as a valuable tool for the characterization of perovskite-based solar cells (PSCs). However, earlier reports of the IS response of mesoscopic PSCs have revealed notable discrepancies, with the interpretation of their spectra having been generalized to planar PSC devices despite fundamental differences in the device operation. The present study analyzes the impedance response of planar PSC devices through the characterization of cells employing a variety of constituent layers. Distinctive high-frequency and low-frequency features are observed in IS measurements and are attributed to the charge recombination across the perovskite/contact interfaces and the dielectric relaxation in the interfacial regions of the perovskite layer, respectively. Comparison of the characteristic IS time constants with time-resolved photoluminescence (TRPL) and open-circuit voltage decay (OCVD) measurements further substantiates the proposed impedance model. This work provides an empirical...

Low-temperature processing of dye-sensitized solar cells (DSCs) [B. Oregan, M. Gratzel, Nature, 1991, 353, 737] is crucial to enable their commercialization with low-cost plastic substrates. Much of the previous work in this area has focused on mechanical compression of premade particles on plastic substrates; however, many reported that this technique did not yield sufficient interconnections for high charge carrier transport. Herein, we present bi-layer photoanodes that incorporate microstructured TiO2 sea-urchin-like assemblies, composed of high-aspect-ratio single crystalline nanoribbons, i.e., two-dimensional subunits, which were deposited onto a nanoparticle layer (commercial P25), with a 5.6% conversion efficiency realized. We demonstrate that this Mesoporous Hierarchical Anatase TiO2 (MHAT) nanostructure is beneficial due to its enhanced dye loading as well as enhanced light scattering. Importantly, we also show the benefits of a bi-layer structure where the nanoribbons penetrate into the nanoparticle layer (P25) after cold isostatic pressing (CIP), resulting in improved adhesion between the MHAT top layer and the P25 under layer on the indium tin oxide-coated polyethylene naphthalate (ITO|PEN) substrate, leading to improved mechanical stability and durability, efficient electron transfer pathways, and ultimately, higher solar-to-electric conversion efficiencies.

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, comprising only a perovskite layer with two interdigitated gold back-contacts. Charge extraction is achieved via self-assembled monolayers and their associated dipole fields at the metal-perovskite interface. Photovoltages of ~600 mV generated by self-assembled molecular monolayer modified perovskite solar cells 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., Simplified device concepts may become important for the development of low cost photovoltaics. Lin et al. report solar cells based on interdigitated gold back-contacts and metal halide perovskites where charge extraction is assisted via a dipole field generated by self-assembled molecular monolayers.

Despite the remarkable progress in the performance of devices based on the lead halide perovskite semiconductor family, there is still a lack of consensus on their fundamental photophysical properties. Here, using magneto-optical transmission spectroscopy we elucidate the impact of the microstructure on the Coulomb interaction between photo-created electron-hole pairs in methylammonium lead triiodide (MAPbI 3 ) and the triple-cation lead mixed-halide composition, Cs 0.05 (MA 0.17 FA 0.83 ) 0.95 Pb(I 0.83 Br 0.17 ) 3 (Cs: cesium, MA: methylammonium, FA: formamidinium) by investigating thin films with a wide range of grain sizes from tens of nanometers to microns. At low temperatures, in which thermal fluctuations of the interactions are frozen and the rotational disorder of the organic cation is negligible, the exciton binding energy and reduced effective mass of carriers remain effectively unchanged with grain size. We conclude that the microstructure plays a negligible role in the Coulomb interaction of the photo-created electron-hole pairs, in contrast to previous reports. This renewed understanding of the relationship between these fundamental electronic properties and the microstructure is critical for future fundamental studies and improving device design., Australian Renewable Energy Agency (ARENA), Australian Centre for Advanced Photovoltaics (ACAP), Seventh Framework Programme (FP7/2007-2013), ANR JCJC project milliPICS, Region Midi-Pyrenees (Contract MESR 13053031), IDEX program Emergence and Programme des Investissements d’Avenir. LAPHENE project (ANR-11-IDEX-0002-02)

Dye-sensitized solar cells (DSSCs) formed on plastic substrates boast immense potential for commercialization; however, plastic substrate DSSCs have only achieved efficiencies approximately half that of their glass substrate counterparts. Previous work has largely attributed these medial efficiencies to the inability to sinter the mesoporous TiO2 film. This study entailed the comparison of high-temperature and low-temperature fabricated DSSCs, moving beyond comparisons of device efficiencies and quantitatively characterizing the physical mechanisms underpinning the gap between plastic and glass technologies. As shown through small perturbation techniques, the dominance of sintered working electrodes was reflected in their superior electron diffusion lengths, which were approximately three-to-four times greater than the nonsintered films. The improved performance gained after a TiO2 nanoglue treatment was also investigated and was observed to be caused by a reduction in charge recombination dynamics and a ...

Mesoporous TiO2 beads exhibit the beneficial properties of enhanced light scattering and fast charge transport in a single nanostructured assembly, which makes them ideal for dye-sensitized solar cell applications. However, their unique geometry gives rise to charge transport behaviors that are particular to the beads themselves. This study examined charge transport in TiO2 beads for dye-sensitized solar cell applications on both plastic and glass substrate devices. Through small perturbation and transient techniques, two effective diffusion rates within the film were observed due to the contrast between the intrabead and interbead connections. The dip in diffusion rates away from their typical exponential behavior at high charge densities could be attributed to the poor electrical contact between the TiO2 beads and the conductive oxide substrate. By the application of a small nanoparticle under-layer, the high contact resistance was overcome while maintaining relatively high diffusion rates. The identifi...

The relative role of surface state recombination in dye-sensitized solar cells is not fully understood, yet reductions in the recombination rate are frequently attributed to the passivation of surface states. We have investigated reports of trap state passivation using an Al2O3-coated TiO2 photoanode achieved through atomic layer deposition (ALD). Electrochemical characterization, performed through impedance measurements and intensity modulated photovoltage spectroscopy (IMVS), data showed that the Al2O3 deposition successfully blocked electron recombination and that the chemical capacitance of the film was unchanged after the ALD treatment. A theoretical model outlining the recombination kinetics was applied to the experimental data to obtain charge transfer rates from conduction band states, exponentially distributed traps, and monoenergetic traps. The determined electron transfer rates showed that the deposited Al2O3 coating did not selectively passivate trap states at the nanoparticle surface but redu...

The efficiencies of the hybrid organic-inorganic perovskite solar cells have been rapidly approaching the benchmarks held by the leading thin-film photovoltaic technologies. Arguably, one of the most important factors leading to this rapid advancement is the ability to manipulate the microstructure of the perovskite layer and the adjacent functional layers within the device. Here, an analysis of the nucleation and growth models relevant to the formation of perovskite films is provided, along with the effect of the perovskite microstructure (grain sizes and voids) on device performance. In addition, the effect of a compact or mesoporous electron-transport-layer (ETL) microstructure on the perovskite film formation and the optical/photoelectric properties at the ETL/perovskite interface are overviewed. Insight into the formation of the functional layers within a perovskite solar cell is provided, and potential avenues for further development of the perovskite microstructure are identified.

Organic–inorganic hybrid perovskite solar cells with mixed cations and mixed halides have achieved impressive power conversion efficiency of up to 22.1%. Phase segregation due to the mixed compositions has attracted wide concerns, and their nature and origin are still unclear. Some very useful analytical techniques are controversial in microstructural and chemical analyses due to electron beam-induced damage to the “soft” hybrid perovskite materials. In this study photoluminescence, cathodoluminescence, and transmission electron microscopy are used to study charge carrier recombination and retrieve crystallographic and compositional information for all-inorganic CsPbIBr2 films on the nanoscale. It is found that under light and electron beam illumination, “iodide-rich” CsPbI(1+x)Br(2−x) phases form at grain boundaries as well as segregate as clusters inside the film. Phase segregation generates a high density of mobile ions moving along grain boundaries as ion migration “highways.” Finally, these mobile ions can pile up at the perovskite/TiO2 interface resulting in formation of larger injection barriers, hampering electron extraction and leading to strong current density–voltage hysteresis in the polycrystalline perovskite solar cells. This explains why the planar CsPbIBr2 solar cells exhibit significant hysteresis in efficiency measurements, showing an efficiency of up to 8.02% in the reverse scan and a reduced efficiency of 4.02% in the forward scan, and giving a stabilized efficiency of 6.07%.