In situ investigation of light soaking in organolead halide perovskite films

Organolead halide perovskite solar cells (PSCs) have generated extensive attention recently with power conversion efficiency (PCE) exceeding 23%. However, these PSCs exhibit photoinduced instability in the course of their current-voltage measurements. In this work, we study the light-induced behavior in CH3NH3PbI3-xClx films in situ, by employing wide-field photoluminescence (PL) microscopy to obtain both the spatially and temporally resolved PL images simultaneously. Along with the increase in the PL intensity under continuous illumination, some areas render PL inactive. By characterizing the excitation energy dependent long-time PL decay behavior, we suggest that the PL quenching can be ascribed to a localized accumulation of iodide ions driven by the optical field. This ion localization leads to an enhancement of non-radiative recombination. The appearance of the PL inactive areas in the perovskite film impedes its photovoltaic device performance approaching the theoretical maximum PCE. Therefore, the herein presented real-time investigation of the light soaking of perovskite films is a versatile and adaptable method providing more details to improve the performance of PSCs.</p

Formation Criteria of High Efficiency Perovskite Solar Cells in Ambient Conditions

The field of lead halide perovskites for solar cell applications has recently reported impressive power conversion (PCE) above 22 % using complex mixed cation formulations. Very importantly, highest PCE have been obtained using totally dry environmental conditions increasing the processing costs (i.e. use of glovebox). In this work devices processed in air under different ambient conditions are prepared with PCE approaching 19 % for the simplest lead halide perovskite (MAPbI3, MA= Methyl ammonium). It is shown that the PbI2:MAI:additive complex needs to be generated in the correct stoichiometry (1:1:1) where additives are any highly polar molecule able to stabilize the complex (i.e. H2O or Dimethylsulphoxide (DMSO)). At high humidity conditions H2O is incorporated into the complex and only small concentrations of further additives are needed. Precursor formulations not adequately balanced for the humidity conditions lead to films with poor morphology as evidenced by SEM. These films show negative multiiodide plumbate chemical defects as observed by absorbance measurements. These chemical defects act as recombination centers reducing the photocurrent and Fill Factor in photovoltaic devices. In addition, it is shown the undesirable high conductivity of the perovskite hydrates (8x10-1 Scm-1), up to seven orders of magnitude higher than the pure MAPbI3, indicating that the presence of hydrates may act as shunting pathways that can significantly reduce the open circuit potential

What Controls the Rate of Ultrafast Charge Transfer and Charge Separation Efficiency in Organic Photovoltaic Blends.

In solar energy harvesting devices based on molecular semiconductors, such as organic photovoltaics (OPVs) and artificial photosynthetic systems, Frenkel excitons must be dissociated via charge transfer at heterojunctions to yield free charges. What controls the rate and efficiency of charge transfer and charge separation is an important question, as it determines the overall power conversion efficiency (PCE) of these systems. In bulk heterojunctions between polymer donor and fullerene acceptors, which provide a model system to understand the fundamental dynamics of electron transfer in molecular systems, it has been established that the first step of photoinduced electron transfer can be fast, of order 100 fs. But here we report the first study which correlates differences in the electron transfer rate with electronic structure and morphology, achieved with sub-20 fs time resolution pump-probe spectroscopy. We vary both the fullerene substitution and donor/fullerene ratio which allow us to control both aggregate size and the energetic driving force for charge transfer. We observe a range of electron transfer times from polymer to fullerene, from 240 fs to as short as 37 fs. Using ultrafast electro-optical pump-push-photocurrent spectroscopy, we find the yield of free versus bound charges to be weakly dependent on the energetic driving force, but to be very strongly dependent on fullerene aggregate size and packing. Our results point toward the importance of state accessibility and charge delocalization and suggest that energetic offsets between donor and acceptor levels are not an important criterion for efficient charge generation. This provides design rules for next-generation materials to minimize losses related to driving energy and boost PCE.Engineering and Physical Sciences Research Council, Winton Programme for the Physics of Sustainability, University of Cambridge, China Scholarship Council, SoltechThis is the final version of the article. It first appeared from the American Chemical Society via http://dx.doi.org/10.1021/jacs.6b0513

Potassium ions as a kinetic controller in ionic double layers for hysteresis-free perovskite solar cells

Since ion migration and interaction with external contacts has been regarded as one of the origins for photocurrent density (J)–voltage (V) hysteresis and phase segregation in perovskite solar cells (PSCs) under operational condition, control of ionic movement in organic–inorganic halide perovskites presents a big challenge for achieving hysteresis-free and stable PSCs. As a universal method, potassium doping into bulk perovskite films to minimize or eliminate the hysteresis was proposed. Here, we report direct observation of moderately retarded ion migration in K+-doped (FAPbI3)0.875(CsPbBr3)0.125 perovskite by in situ photoluminescence (PL) imaging. However, more impressive is the effect on the kinetics for generation of the ionic double layer in the vicinity of the contacts as it is reduced by two orders of magnitude on the time scale when devices are doped with K+ as detected by impedance spectroscopy. A significantly reduced hysteresis in the K+-doped perovskite is responsible for more prolonged stability exhibiting ∼96% of initial power conversion efficiency (PCE) after 22 days than relatively short-lived perovskites undoped with K+ ions. This work highlights the clear correlation of ion migration and a fast generation of the double layer close to the contacts with severe hysteresis and long-term instability in PSCs and the importance of K+ ions in reducing the kinetics affecting the ionic attachment to the contact surface

Understanding the Improvement in the Stability of a Self-Assembled Multiple-Quantum Well Perovskite Light-Emitting Diode.

We fabricate two-dimensional Ruddlesden-Popper layered perovskite films by introducing 1-naphthylmethylamine iodide into the precursor, which forms a self-assembled multiple-quantum well (MQW) structure. Enabling outstanding electroluminescence properties, light-emitting diodes (LEDs) using the MQW structure also demonstrate significant improvement in stability in comparison with the stability of devices made from formamidinium lead iodide. To understand this, we perform electroabsorption spectroscopy, wide-field photoluminescence imaging microscopy and impedance spectroscopy. Our approach enables us to determine the mobility of iodide ions in MQW perovskites to be (1.5 ± 0.8) × 10-8 cm2 V-1 s-1, ∼2 orders of magnitude lower than that in three-dimensional perovskites. We highlight that activated ion migration is a requirement for a degradation pathway in which a steady supply of ions is needed to modify the perovskite/external contact interfaces. Therefore, the improvement in stability in a MQW perovskite LED is directly attributed to the suppressed ion migration due to the inserted organic layer acting as a barrier for ionic movement

Unravelling the role of vacancies in lead halide perovskite through electrical switching of photoluminescence

Methylammonium lead triiodide perovskite based solar cells have attracted lots of attention but many physical characteristics of this material remain elusive. Here Li et al. reveal the role of defects in the carrier recombination dynamics in photoluminescence experiments and present a model to describe it

Origins and mechanisms of hysteresis in organometal halide perovskites

Inorganic–organic halide organometal perovskites, such as CH3NH3PbI3 and CsPbI3, etc, have been an unprecedented rising star in the field of photovoltaics since 2009, owing to their exceptionally high power conversion efficiency and simple fabrication processability. Despite its relatively short history of development, intensive investigations have been concentrating on this material; these have ranged from crystal structure analysis and photophysical characterization to performance optimization and device integration, etc. Yet, when applied in photovoltaic devices, this material suffers from hysteresis, that is, the difference of the current–voltage (I–V) curve during sweeping in two directions (from short-circuit towards open-circuit and vice versa). This behavior may significantly impede its large-scale commercial application. This Review will focus on the recent theoretical and experimental efforts to reveal the origin and mechanism of hysteresis. The proposed origins include (1) ferroelectric polarization, (2) charge trapping/detrapping, and (3) ion migration. Among them, recent evidence consistently supports the idea that ion migration plays a key role for the hysteretic behavior in perovskite solar cells (PSCs). Hence, this Review will summarize the recent results on ion migration such as the migrating ion species, activation energy measurement, capacitive characterization, and internal electrical field modulation, etc. In addition, this Review will also present the devices with alleviation/elimination of hysteresis by incorporating either large-size grains or phenyl-C61-butyric acid methyl ester molecules. In a different application, the hysteretic property has been utilized in photovoltaic and memristive switching devices. In sum, by examining these three possible mechanisms, it is concluded that the origin of hysteresis in PSCs is associated with a combination of effects, but mainly limited by ion/defect migration. This strong interaction between ion motion and free charge carrier transport can be modulated by the prevalent crystalline structure, chemical passivation, and an external photo/electrical field

What Controls the Rate of Ultrafast Charge Transfer and Charge Separation Efficiency in Organic Photovoltaic Blends.

In solar energy harvesting devices based on molecular semiconductors, such as organic photovoltaics (OPVs) and artificial photosynthetic systems, Frenkel excitons must be dissociated via charge transfer at heterojunctions to yield free charges. What controls the rate and efficiency of charge transfer and charge separation is an important question, as it determines the overall power conversion efficiency (PCE) of these systems. In bulk heterojunctions between polymer donor and fullerene acceptors, which provide a model system to understand the fundamental dynamics of electron transfer in molecular systems, it has been established that the first step of photoinduced electron transfer can be fast, of order 100 fs. But here we report the first study which correlates differences in the electron transfer rate with electronic structure and morphology, achieved with sub-20 fs time resolution pump-probe spectroscopy. We vary both the fullerene substitution and donor/fullerene ratio which allow us to control both aggregate size and the energetic driving force for charge transfer. We observe a range of electron transfer times from polymer to fullerene, from 240 fs to as short as 37 fs. Using ultrafast electro-optical pump-push-photocurrent spectroscopy, we find the yield of free versus bound charges to be weakly dependent on the energetic driving force, but to be very strongly dependent on fullerene aggregate size and packing. Our results point toward the importance of state accessibility and charge delocalization and suggest that energetic offsets between donor and acceptor levels are not an important criterion for efficient charge generation. This provides design rules for next-generation materials to minimize losses related to driving energy and boost PCE.Engineering and Physical Sciences Research Council, Winton Programme for the Physics of Sustainability, University of Cambridge, China Scholarship Council, SoltechThis is the final version of the article. It first appeared from the American Chemical Society via http://dx.doi.org/10.1021/jacs.6b0513