The migration and accumulation of iodide ions create a modulation of the respective interfacial barriers causing the hysteresis in solar cells based on methylammonium lead iodide perovskites. Iodide ions are identified as the migrating species by measuring temperature dependent current-transients and photoelectron spectroscopy. The involved changes in the built-in potential due to ion migration are directly measured by electroabsorption spectroscopy.

photovoltaic technology, where lifetimes greater than 25 years are required. [2] Although the issue of device stability has attracted increased attention of the photovoltaic research community in the last two years, reports that systematically study the fundamental causes (e.g., heat, electrical stress, humidity, oxygen, (UV) light, chemical precursors, processing conditions, influence of film quality and morphology) and mechanisms limiting the material and device stability remain scarce. [2][3][4][5] While the degradation of methylammonium lead iodide (MAPbI 3 ) in humid air has been studied experimentally as well as theoretically and was long thought to be the main factor for material degradation in ambient environment, [6][7][8][9][10][11][12][13][14][15] studies exploring the influence of oxygen and light on the solar cell performance have only recently been reported. [16][17][18][19][20] It has been shown that photoexcited electrons in the perovskite layer can form superoxide (O 2 − ) via electron transfer to molecular oxygen, which through deprotonation of the methylammonium cation in turn results in irreversible material degradation. The severity of the degradation has been linked to the efficiency of electron extraction via the electron extracting layer (EEL): devices employing a compact-TiO 2 /mesoporous Al 2 O 3 or compact-TiO 2 as EELs degraded in dry air on a timescale of less than 1 h, while with the use of a mesoporous TiO 2 layer, an EEL which results in faster electron extraction, the lifetimes were significantly increased. However, in these reports only the degradation of complete photovoltaic device was reported, with limited information on the degradation of the perovskite active layer itself and the impact of its microstructure was not identified.In this work, we systematically study the degradation of MAPbI 3 films under precisely controlled exposure to various oxygen levels (0-20%) under simulated sunlight in order to shed light on the progression of perovskite degradation under these conditions. We investigate two types of perovskite layers that are formed using different fabrication methods. The two recipes allow us to include the effect of layer microstructure on the dynamics of oxygen-induced degradation. We characterize the electronic, optical, compositional, and structural properties of the degraded perovskite films and correlate these results This paper investigates the impact of microstructure on the degradation rate of methylammonium lead triiodide (MAPbI 3 ) perovskite films upon exposure to light and oxygen. By comparing the oxygen induced degradation of perovskite films of different microstructure-fabricated using either a lead acetate trihydrate precursor or a solvent engineering technique-it is demonstrated that films with larger and more uniform grains and better electronic quality show a significantly reduced degradation compared to films with smaller, more irregular grains. The effect of degradation on the optical, compositional, and microstructural properties of the perovsk...

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This paper describes the construction of a phase diagram for the as-cast state in the organic photovoltaic system P3HT:PCBM. Evidence for a transition to a phase-separated state at PCBM concentrations greater than 70 wt % is seen both by DMTA and GIWAXS, and the glass transition temperatures of blends in the single phase state below 70 wt % PCBM are observed to be raised compared to the pure polymer. Pure PCBM is observed to exhibit a thermal transition at 155 °C, an observation unreported to dateoffering insight into crystallites commonly seen in device films. The liquid-crystal phase of P3HT is shown to persist in the presence of up to 41 wt % PCBM. In addition, pure PCBM is shown to be significantly hygroscopic, with important implications for the processing of high-performance devices.

unexpectedly clean electronic properties of these semiconductors have led to the demonstration of proof-of-concept PV devices with an initial effi ciency that outperforms competing 'next-generation' technologies such as dye-sensitized, organic, and quantum dot solar cells, and even matches that of commercially deployed PV. [ 6 ] Other attractive characteristics of hybrid perovskites include the low embedded costs of materials processing and the ability to tune the properties of the semiconductor through changes in chemical composition. [ 1,[7][8][9][10] All of these factors have led to hybrid perovskites being considered as disruptive materials in a wide range of technology applications; [11][12][13][14][15][16] interest from the materials science research community is both substantial and rapidly maturing. Realizing the technology potential of hybrid perovskites necessitates a thorough understanding of the degradation mechanisms that limit the lifetime of devices, and identifying solutions to mitigate these. This exercise is nontrivial because of the various factors that can affect device performance: light, heat, moisture, oxygen, mechanical and electrical stresses, and combinations thereof. [ 17,18 ] Currently, perovskite solar cells (PSCs) based on the "triple layer" architecture with a thick carbon back electrode have demonstrated perhaps the most stable performance characteristics of all PSCs. [ 19 ] These devices, based on the mixed cation perovskite (5-AVA) X (MA) 1− X PbI 3 (where 5-AVA corresponds to 5-ammoniumvaleric acid), have been shown to maintain their initial power conversion effi ciency (PCE) of approximately 10% over 1000 h continuous simulated solar illumination, [ 20 ] dark storage for three months at elevated temperatures and humidity (85 °C/85%) and a minimum of 7 d operation in real-world conditions (Jeddah, Saudi Arabia). [ 21 ] Although clearly commendable, it is apparent that signifi cant work is required to realize PSCs with high stability and high performance, where promising stability metrics are combined with the certifi ed PCE values for champion devices. [ 22 ] To elucidate the underlying mechanisms for degradation in hybrid perovskites and guide the development of intrinsically stable devices, considerable work has been undertaken on the archetypal semiconductor of the research fi eld: CH 3 NH 3 PbI 3 . It is known that this material (and other single or mixed-halide derivatives) are particularly sensitive to moisture, [ 1,7,17,[23][24][25][26] where the proposed chemical reactions involving water result in decomposition of the perovskite into its precursor components via The rapid pace of development for hybrid perovskite photovoltaics has recently resulted in promising fi gures of merit being obtained with regard to device stability. Rather than relying upon expensive barrier materials, realizing market-competitive lifetimes is likely to require the development of intrinsically stable devices, and to this end accelerated aging tests can help identify degradation mechanisms tha...

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Organic photovoltaic devices (OPVs) based on conjugated polymers and fullerene blends offer an attractive method to produce renewable energy. It has been demonstrated that they can be manufactured on fl exible substrates and at low cost over large areas using the high volume technique of roll-to-roll printing. [1][2][3] Whilst much work has been devoted to the donoracceptor system P3HT:PCBM (poly(3-hexylthiophene):[6,6]-phenyl-C 61 -butyric acid methyl ester), attention is now turning to the use of donor materials which have a reduced energy gap (permitting more of the sun's spectral emission to be harvested) and an increased ionization potential (leading to an increased open-circuit voltage and thus greater power conversion effi ciency [ 4 , 5 ] ).One promising new donor polymer for OPV applications is PCDTBT (poly [N-9 ′ -heptadecanyl-2,7-carbazole-alt-5,5-(4 ′ ,7 ′ -di-2-thienyl-2 ′ ,1 ′ ,3 ′ -benzothiadiazole)] whose chemical structure is shown in Figure 1 . [ 4 ] When PCDTBT is blended with the fullerene acceptor PC 70 BM, OPV devices have been created having a power conversion effi ciency of ∼ 6%, [ 6 ] one of the highest OPV effi ciencies to date. A marked difference in the fabrication process of these devices compared to the benchmark P3HT:PCBM system is the requirement for thermal annealing. It has been established that thermal annealing at ∼ 150 ° C [ 7 ] (between the glass transition and melting temperatures of P3HT) is required to drive the crystallization of the two components to form nanoscale, phase-separated domains. [8][9][10] Without such a thermal anneal process, the power conversion effi ciency (PCE) of P3HT:PCBM is limited to around 1%-2% [ 11 ] -a value that is improved to between 4 and 5% in an annealed device. [ 7 , 11 ] Recently we have established that this thermal anneal also modifi es the vertical structure of P3HT:PCBM thin fi lms. [ 12 ] In as-cast P3HT:PCBM fi lm, the surface is relatively depleted in PCBM. However by annealing the fi lm, PCBM is driven to the fi lm surface, an effect that we believe improves the electronic functionality of the device. In PCDTBT:PCBM OPVs however, a high temperature annealing process has been reported to be detrimental to device effi ciency. [ 6 ] Rather a low-temperature anneal/ drying stage at 70 ° C appears suffi cient to fully optimize device effi ciency although the benefi t of this stage has not been explicitly quantifi ed. Devices incorporating a TiO x optical spacing layer are also subsequently annealed at 80 ° C. [ 6 ] In this paper, we use neutron refl ectivity (NR) and grazing incidence wide-angle X-ray scattering (GI-WAXS) to explore the nanoscale structures formed in freshly cast PCDTBT:PCBM thin fi lms and those that have been annealed at a relatively low temperature. We make two main fi ndings. Firstly, we fi nd that the surface of freshly cast PCDTBT:PCBM fi lms is relatively enriched in PCBM with a negative concentration gradient existing away from the fi lm surface. Secondly, thermal annealing at 70 ° C does not signifi cantly m...

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