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Solution-processable metal halide perovskites show immense promise for use in photovoltaics and other optoelectronic applications. The ability to tune their bandgap by alloying various halide anions (for example, in CH3NH3Pb(I1-x Br-x)(3), 0 < x < 1) is however hampered by the reversible photoinduced formation of sub-bandgap emissive states. We find that ion segregation takes place via halide defects, resulting in iodide-rich low-bandgap regions close to the illuminated surface of the film. This segregation may be driven by the strong gradient in carrier generation rate through the thickness of these strongly absorbing materials. Once returned to the dark, entropically driven intermixing of halides returns the system to a homogeneous condition. We present approaches to suppress this process by controlling either the internal light distribution or the defect density within the film. These results are relevant to stability in both single- and mixed-halide perovskites, leading the way toward tunable and stable perovskite thin films for photovoltaic and light-emitting applications.

Transient mobility spectroscopy (TMS) is presented as a new tool to probe the charge carrier mobility of commonly employed organic and inorganic semiconductors over the relevant range of charge densities. The charge density dependence of the mobility of semiconductors used in hybrid and organic photovoltaics gives new insights into charge transport phenomena in solid state dye sensitized solar cells. Copyright © 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

One of the greatest attributes of metal halide perovskite solar cells is their surprisingly low loss in potential between bandgap and open-circuit voltage, despite the fact that they suffer from a non-negligible density of sub gap defect states. Here, we use a combination of transient and steady state photocurrent and absorption spectroscopy to show that CH3NH3PbI3 films exhibit a broad distribution of electron traps. We show that the trapped electrons recombine with free holes unexpectedly slowly, on microsecond time scales, relaxing the limit on obtainable open-circuit voltage (VOC) under trap-mediated recombination conditions. We find that the observed VOCs in such perovskite solar cells can only be rationalized by considering the slow trap mediated recombination mechanism identified in this work. Our results suggest that existing processing routes may be good enough to enable open circuit voltages approaching 1.3 V in ideal devices with perfect contacts.

We present the synthesis and device characterization of new hole transport materials (HTMs) for application in solid-state dye-sensitized solar cells (ssDSSCs). In addition to possessing electrical properties well suited for ssDSSCs, these new HTMs have low glass transition temperatures, low melting points, and high solubility, which make them promising candidates for increased pore filling into mesoporous titania films. Using standard device fabrication methods and Z907 as the sensitizing dye, power conversion efficiencies (PCE) of 2.94% in 2-μm-thick cells were achieved, rivaling the PCE obtained by control devices using the state-of-the-art HTM spiro-OMeTAD. In 6-μm-thick cells, the device performance is shown to be higher than that obtained using spiro-OMeTAD, making these new HTMs promising for preparing high-efficiency ssDSSCs.

Mesoporous single crystals (MSCs) of TiO2 are promising materials for more efficient dye sensitized solar cells and other energy conversion or storage devices, since they combine high surface area with large crystalline domain size. In this work, we investigate the charge transport properties of TiO2 MSCs after annealing them within a confining template at temperatures from 500 to 850 C. We observe that higher anneal temperatures do not change the crystal phase, as in nanocrystalline TiO 2, but do influence the MSC absorption spectrum in a manner consistent with the signature of increased oxygen-vacancy defects. By comparing MSC film conductivity in vacuum and in air, we infer that these anneal-induced defects increase the background charge density in TiO2. Subsequently, we measure higher effective mobility in annealed MSCs using transient mobility spectroscopy (TMS), consistent with higher anneal temperatures filling sub-bandgap trap states by n-doping TiO2. Finally, we measure faster charge transport rates in solid-state dye sensitized solar cells as well as increased open-circuit voltages at low light intensity with increasing MSC anneal temperature. This study leverages the fixed geometry and crystal phase of MSCs under thermal treatment to identify and isolate the doping effect of annealing at high temperature, previously inaccessible for mesoporous anatase TiO2. The results offer insight into the influence of doping on charge transport in TiO2-based solar cells and the tunability of MSCs for use in enhancing device performance. © 2013 American Chemical Society.

This work is concerned with the modeling of perovskite based hybrid solar cells formed by sandwiching a slab of organic lead halide perovskite (CH3NH3PbI3-xCIx) photo-absorber between (n-type) acceptor and (p-type) donor materials -typically titanium dioxide and spiro. A model for the electrical behavior of these cells is formulated based on drift-diffusion equations for the motion of the charge carriers and Poisson's equation for the electric potential. It is closed by (i) internal interface conditions accounting for charge recombination/generation and jumps in charge carrier densities arising from differences in the electron affinity/ionization potential between the materials and (ii) ohmic boundary conditions on the contacts. The model is analyzed by using a combination of asymptotic and numerical techniques. This leads to an approximate - yet highly accurate - expression for the current-voltage relationship as a function of the solar induced photo-current. In addition, we show that this approximate current-voltage relation can be interpreted as an equivalent circuit model consisting of three diodes, a resistor, and a current source. For sufficiently small biases the device's behavior is diodic and the current is limited by the recombination at the internal interfaces, whereas for sufficiently large biases the device acts like a resistor and the current is dictated by the ohmic dissipation in the acceptor and donor. The results of the model are also compared to experimental current-voltage curves, and good agreement is shown.

The solid-state dye-sensitized solar cell (DSSC) was introduced to overcome inherent manufacturing and instability issues of the electrolyte-based DSSC and progress has been made to deliver high photovoltaic efficiencies at low cost. However, despite 15 years research and development, there still remains no clear demonstration of long-term stability. Here, solid-state DSSCs are subjected to the severe aging conditions of continuous illumination at an elevated temperature. A fast deterioration in performance is observed for devices encapsulated in the absence of oxygen. The photovoltaic performance recovers when re-exposed to air. This reversible behavior is attributed to three related processes: i) the creation of light and oxygen sensitive electronic shunting paths between TiO2 and the top metal electrode, ii) increased recombination at the TiO2/organic interface, and iii) the creation of deep electron traps that reduce the photocurrent. The device deterioration is remedied by the formation of an insulating alumino-silicate shell around the TiO2 nanocrystals, which reduces interfacial recombination, and the introduction of an insulating mesoporous SiO2 buffer layer between the top electrode and TiO2, which acts as a permanent insulating barrier between the TiO2 and the metal electrode, preventing shunting. Encapsulated solid-state dye-sensitized solar cells (ssDSSCs) show a reversible and quick deterioration in performance while aging under inert atmosphere. This is attributed to the activation of deep traps and the change in Schottky barrier at the TiO2 surface. This is remedied by forming an insulating alumino-silicate "shell" around the TiO2 nanocrystals, and by introducing an insulating mesoporous SiO2 "buffer layer" between the top electrode and TiO2. © 2014 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

Perovskite solar cells have rapidly risen to the forefront of emerging photovoltaic technologies, exhibiting rapidly rising efficiencies. This is likely to continue to rise, but in the development of these solar cells there are unusual characteristics that have arisen, specifically an anomalous hysteresis in the current-voltage curves. We identify this phenomenon and show some examples of factors that make the hysteresis more or less extreme. We also demonstrate stabilized power output under working conditions and suggest that this is a useful parameter to present, alongside the current-voltage scan derived power conversion efficiency. We hypothesize three possible origins of the effect and discuss its implications on device efficiency and future research directions. Understanding and resolving the hysteresis is essential for further progress and is likely to lead to a further step improvement in performance.

The power conversion efficiency of hybrid solid-state solar cells has more than doubled from 7 to 15% over the past year. This is largely as a result of the incorporation of organometallic trihalide perovskite absorbers into these devices. But, as promising as this development is, long-term operational stability is just as important as initial conversion efficiency when it comes to the development of practical solid-state solar cells. Here we identify a critical instability in mesoporous TiO₂-sensitized solar cells arising from light-induced desorption of surface-adsorbed oxygen. We show that this instability does not arise in mesoporous TiO₂-free mesosuperstructured solar cells. Moreover, our TiO₂-free cells deliver stable photocurrent for over 1,000 h continuous exposure and operation under full spectrum simulated sunlight.

Lithium salts have been shown to dramatically increase the conductivity in a broad range of polymeric and small molecule organic semiconductors (OSs). Here we demonstrate and identify the mechanism by which Li(+) p-dopes OSs in the presence of oxygen. After we established the lithium doping mechanism, we re-evaluate the role of lithium bis(trifluoromethylsulfonyl)-imide (Li-TFSI) in 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9'-Spirobifluorene (Spiro-OMeTAD) based solid-state dye-sensitized solar cells (ss-DSSCs). The doping mechanism consumes Li(+) during the device operation, which poses a problem, since the lithium salt is required at the dye-sensitized heterojunction to enhance charge generation. This compromise highlights that new additives are required to maximize the performance and the long-term stability of ss-DSSCs.

We present a joint experimental and theoretical study with the aim of investigating the individual effects of Li-TFSI and EMIM-TFSI additives on the optical and charge transfer properties of D102 dye in solution as well as at the dye-sensitized TiO2 interface. Experimental results show that while the spectral shifts are negligible for the dye in solution, when moving to the TiO2 film in air the addition of both lithium and EMIM salts clearly gives rise to a slight red-shift with the appearance of a lower-energy shoulder in the absorption spectrum. Computational modelling confirms the weak tendency to have stable dye/additive complexes in solution and predicts appreciable spectral red-shifts as a consequence of the interaction of the dye with Li + and EMIM+ cations. We also predict a strong effect of the additives on the electronic coupling between the dye's LUMO and the TiO 2 conduction band states, reflecting on the calculated injection rates. Further, by modeling the formation and the optical response of selected dye/additive aggregate models, we find a general broadening of the absorption band, accounting for the experimentally observed lower-energy shoulder in the D102 absorption spectrum recorded on TiO2 films where Li and EMIM salts are added. © 2013 The Royal Society of Chemistry.

The recent development of efficient binary tin- and lead-based metal halide perovskite solar cells has enabled the development of all-perovskite tandem solar cells, which offer a unique opportunity to deliver high performance at low cost. Tin halide perovskites, however, are prone to oxidation, where the Sn2+ cations oxidize to Sn4+ upon air exposure. Here, we identify reaction products and elucidate the oxidation mechanism of both ASnI3 and ASn0.5Pb0.5I3 (where A can be made of methylammonium, formamidinium, cesium, or a combination of these) perovskites and find that substituting lead onto the B site fundamentally changes the oxidation mechanism of tin-based metal halide perovskites to make them more stable than would be expected by simply considering the decrease in tin content. This work provides guidelines for developing stable small band gap materials that could be used in all-perovskite tandems.

Metal halide perovskite solar cells are rapidly becoming increasingly competitive with conventional PV technologies. While their efficiencies have been often touted as exceptional, they have received a lot of criticism for an apparent lack of stability. This perspective describes some of the most pressing stability concerns facing perovskite solar cells, and describes some of the recent advances made in this area. We will demonstrate that the solutions to the areas of structural, thermal, and environmental stability are closely linked, and that rational design of the perovskite and careful encapsulation can result in efficient and stable perovskite solar cells. We will conclude with some very promising results, demonstrating perovskite solar cells passing an IEC damp heat stability test.

Organic–inorganic perovskites such as CH3NH3PbI3 are promising materials for a variety of optoelectronic applications, with certified power conversion efficiencies in solar cells already exceeding 21%. Nevertheless, state-of-the-art films still contain performance-limiting non-radiative recombination sites and exhibit a range of complex dynamic phenomena under illumination that remain poorly understood. Here we use a unique combination of confocal photoluminescence (PL) microscopy and chemical imaging to correlate the local changes in photophysics with composition in CH3NH3PbI3 films under illumination. We demonstrate that the photo-induced ‘brightening’ of the perovskite PL can be attributed to an order-of-magnitude reduction in trap state density. By imaging the same regions with time-of-flight secondary-ion-mass spectrometry, we correlate this photobrightening with a net migration of iodine. Our work provides visual evidence for photo-induced halide migration in triiodide perovskites and reveals the complex interplay between charge carrier populations, electronic traps and mobile halides that collectively impact optoelectronic performance., Seventh Framework Programme (European Commission) (FP7/2007–2013) , under grant agreement 604032, MESO project), Seventh Framework Programme (European Commission) (People Programme (Marie Curie Actions), FP7/2007-2013/ under REA grant agreement number PIOF-GA-2013-622630)), United States. Dept. of Energy (DOE (DE-SC0013957)), National Science Foundation (U.S.) (NSF Graduate Research Fellowship (DGE-1256082)), Engineering and Physical Sciences Research Council (EPSRC) (Supergen Supersolar project), Massachusetts Institute of Technology. Libraries (contributions to Open Access article processing fees), United States. Dept. of Energy. Office of Basic Energy Sciences (Center for Excitonics, an Energy Frontier Research Center, Award No. DE-SC0001088)

As the record single-junction efficiencies of perovskite solar cells now rival those of copper indium gallium selenide, cadmium telluride and multicrystalline silicon, they are becoming increasingly attractive for use in tandem solar cells due to their wide, tunable bandgap and solution processability. Previously, perovskite/silicon tandems were limited by significant parasitic absorption and poor environmental stability. Here, we improve the efficiency of monolithic, two-terminal, 1-cm2 perovskite/silicon tandems to 23.6% by combining an infrared-tuned silicon heterojunction bottom cell with the recently developed caesium formamidinium lead halide perovskite. This more-stable perovskite tolerates deposition of a tin oxide buffer layer via atomic layer deposition that prevents shunts, has negligible parasitic absorption, and allows for the sputter deposition of a transparent top electrode. Furthermore, the window layer doubles as a diffusion barrier, increasing the thermal and environmental stability to enable perovskite devices that withstand a 1,000-hour damp heat test at 85 ∘C and 85% relative humidity. Perovskite solar cells can complement silicon photovoltaics in multijunction devices. Here, the authors optimize light harvesting in monolithic perovskite-on-silicon devices and fabricate a certified 23.6% efficient, 1 cm2 tandem solar cell with a perovskite device that withstands damp heat tests.