Operando dynamics of trapped carriers in perovskite solar cells observed via infrared optical activation spectroscopy.

Conventional spectroscopies are not sufficiently selective to comprehensively understand the behaviour of trapped carriers in perovskite solar cells, particularly under their working conditions. Here we use infrared optical activation spectroscopy (i.e., pump-push-photocurrent), to observe the properties and real-time dynamics of trapped carriers within operando perovskite solar cells. We compare behaviour differences of trapped holes in pristine and surface-passivated FA0.99Cs0.01PbI3 devices using a combination of quasi-steady-state and nanosecond time-resolved pump-push-photocurrent, as well as kinetic and drift-diffusion models. We find a two-step trap-filling process: the rapid filling (~10 ns) of low-density traps in the bulk of perovskite, followed by the slower filling (~100 ns) of high-density traps at the perovskite/hole transport material interface. Surface passivation by n-octylammonium iodide dramatically reduces the number of trap states (~50 times), improving the device performance substantially. Moreover, the activation energy (~280 meV) of the dominant hole traps remains similar with and without surface passivation.

Solution-Processable Cu3BiS3 Thin Films: Growth Process Insights and Increased Charge Generation Properties by Interface Modification.

Cu3BiS3 thin films are fabricated via spin coating of precursor solutions containing copper and bismuth xanthates onto planar glass substrates or mesoporous metal oxide scaffolds followed by annealing at 300 °C to convert the metal xanthates into copper bismuth sulfide. Detailed insights into the film formation are gained from time-resolved simultaneous small and wide angle X-ray scattering measurements. The Cu3BiS3 films show a high absorption coefficient and a band gap of 1.55 eV, which makes them attractive for application in photovoltaic devices. Transient absorption spectroscopic measurements reveal that charge generation yields in mesoporous TiO2/Cu3BiS3 heterojunctions can be significantly improved by the introduction of an In2S3 interlayer, and long-lived charge carriers (t50% of 10 µs) are found.

Engineering Stable Lead-Free Tin Halide Perovskite Solar Cells: Lessons from Materials Chemistry.

Substituting toxic lead with tin (Sn) in perovskite solar cells (PSCs) is the most promising route toward the development of high-efficiency lead-free devices. Despite the encouraging efficiencies of Sn-PSCs, they are still yet to surpass 15% and suffer detrimental oxidation of Sn(II) to Sn(IV). Since their first application in 2014, investigations into the properties of Sn-PSCs have contributed to a growing understanding of the mechanisms, both detrimental and complementary to their stability. This review summarizes the evolution of Sn-PSCs, including early developments to the latest state-of-the-art approaches benefitting the stability of devices. The degradation pathways associated with Sn-PSCs are first outlined, followed by describing how composition engineering (A, B site modifications), additive engineering (oxidation prevention), and interface engineering (passivation strategies) can be employed as different avenues to improve the stability of devices. The knowledge about these properties is also not limited to PSCs and also applicable to other types of devices now employing Sn-based perovskite absorber layers. A detailed analysis of the properties and materials chemistry reveals a clear set of design rules for the development of stable Sn-PSCs. Applying the design strategies highlighted in this review will be essential to further improve both the efficiency and stability of Sn-PSCs.

Halide Chemistry in Tin Perovskite Optoelectronics: Bottlenecks and Opportunities.

Tin halide perovskites (Sn HaPs) are the top lead-free choice for perovskite optoelectronics, but the oxidation of perovskite Sn2+ to Sn4+ remains a key challenge. However, the role of inconspicuous chemical processes remains underexplored. Specifically, the halide component in Sn HaPs (typically iodide) has been shown to play a key role in dictating device performance and stability due to its high reactivity. Here we describe the impact of native halide chemistry on Sn HaPs. Specifically, molecular halogen formation in Sn HaPs and its influence on degradation is reviewed, emphasising the benefits of iodide substitution for improving stability. Next, the ecological impact of halide products of Sn HaP degradation and its mitigation are considered. The development of visible Sn HaP emitters via halide tuning is also summarised. Lastly, halide defect management and interfacial engineering for Sn HaP devices are discussed. These insights will inspire efficient and robust Sn HaP optoelectronics.

High Power Irradiance Dependence of Charge Species Dynamics in Hybrid Perovskites and Kinetic Evidence for Transient Vibrational Stark Effect in Formamidinium.

Hybrid halide perovskites materials have the potential for both photovoltaic and light-emitting devices. Relatively little has been reported on the kinetics of charge relaxation upon intense excitation. In order to evaluate the illumination power density dependence on the charge recombination mechanism, we have applied a femtosecond transient mid-IR absorption spectroscopy with strong excitation to directly measure the charge kinetics via electron absorption. The irradiance-dependent relaxation processes of the excited, photo-generated charge pairs were quantified in polycrystalline MAPbI3, MAPbBr3, and (FAPbI3)0.97(MAPbBr3)0.03 thin films that contain either methylamonium (MA) or formamidinium (FA). This report identifies the laser-generated charge species and provides the kinetics of Auger, bimolecular and excitonic decay components. The inter-band electron-hole (bimolecular) recombination was found to dominate over Auger recombination at very high pump irradiances, up to the damage threshold. The kinetic analysis further provides direct evidence for the carrier field origin of the vibrational Stark effect in a formamidinium containing perovskite material. The results suggest that radiative excitonic and bimolecular recombination in MAPbI3 at high excitation densities could support light-emitting applications.

Additive-Free, Low-Temperature Crystallization of Stable α-FAPbI3 Perovskite.

Formamidinium lead triiodide (FAPbI3 ) is attractive for photovoltaic devices due to its optimal bandgap at around 1.45 eV and improved thermal stability compared with methylammonium-based perovskites. Crystallization of phase-pure α-FAPbI3 conventionally requires high-temperature thermal annealing at 150 °C whilst the obtained α-FAPbI3 is metastable at room temperature. Here, aerosol-assisted crystallization (AAC) is reported, which converts yellow δ-FAPbI3 into black α-FAPbI3 at only 100 °C using precursor solutions containing only lead iodide and formamidinium iodide with no chemical additives. The obtained α-FAPbI3 exhibits remarkably enhanced stability compared to the 150 °C annealed counterparts, in combination with improvements in film crystallinity and photoluminescence yield. Using X-ray diffraction, X-ray scattering, and density functional theory simulation, it is identified that relaxation of residual tensile strains, achieved through the lower annealing temperature and post-crystallization crystal growth during AAC, is the key factor that facilitates the formation of phase-stable α-FAPbI3 . This overcomes the strain-induced lattice expansion that is known to cause the metastability of α-FAPbI3 . Accordingly, pure FAPbI3 p-i-n solar cells are reported, facilitated by the low-temperature (≤100 °C) AAC processing, which demonstrates increases of both power conversion efficiency and operational stability compared to devices fabricated using 150 °C annealed films.

Phosphorene Nanoribbon-Augmented Optoelectronics for Enhanced Hole Extraction.

Phosphorene nanoribbons (PNRs) have been widely predicted to exhibit a range of superlative functional properties; however, because they have only recently been isolated, these properties are yet to be shown to translate to improved performance in any application. PNRs show particular promise for optoelectronics, given their predicted high exciton binding energies, tunable bandgaps, and ultrahigh hole mobilities. Here, we verify the theorized enhanced hole mobility in both solar cells and space-charge-limited-current devices, demonstrating the potential for PNRs improving hole extraction in universal optoelectronic applications. Specifically, PNRs are demonstrated to act as an effective charge-selective interlayer by enhancing hole extraction from polycrystalline methylammonium lead iodide (MAPbI3) perovskite to the poly(triarylamine) semiconductor. Introducing PNRs at the hole-transport/MAPbI3 interface achieves fill factors above 0.83 and efficiencies exceeding 21% for planar p-i-n (inverted) perovskite solar cells (PSCs). Such efficiencies are typically only reported for single-crystalline MAPbI3-based inverted PSCs. Methylammonium-free PSCs also benefit from a PNR interlayer, verifying applicability to architectures incorporating mixed perovskite absorber layers. Device photoluminescence and transient absorption spectroscopy are used to demonstrate that the presence of the PNRs drives more effective carrier extraction. Isolation of the PNRs in space-charge-limited-current hole-only devices improves both hole mobility and conductivity, demonstrating applicability beyond PSCs. This work provides primary experimental evidence that the predicted superlative functional properties of PNRs indeed translate to improved optoelectronic performance.

Lewis Base Passivation Mediates Charge Transfer at Perovskite Heterojunctions.

Understanding interfacial charge transfer processes such as trap-mediated recombination and injection into charge transport layers (CTLs) is crucial for the improvement of perovskite solar cells. Herein, we reveal that the chemical binding of charge transport layers to CH3NH3PbI3 defect sites is an integral part of the interfacial charge injection mechanism in both n-i-p and p-i-n architectures. Specifically, we use a mixture of optical and X-ray photoelectron spectroscopy to show that binding interactions occur via Lewis base interactions between electron-donating moieties on hole transport layers and the CH3NH3PbI3 surface. We then correlate the extent of binding with an improvement in the yield and longer lifetime of injected holes with transient absorption spectroscopy. Our results show that passivation-mediated charge transfer has been occurring undetected in some of the most common perovskite configurations and elucidate a key design rule for the chemical structure of next-generation CTLs.

Degradation mechanism of hybrid tin-based perovskite solar cells and the critical role of tin (IV) iodide.

Tin perovskites have emerged as promising alternatives to toxic lead perovskites in next-generation photovoltaics, but their poor environmental stability remains an obstacle towards more competitive performances. Therefore, a full understanding of their decomposition processes is needed to address these stability issues. Herein, we elucidate the degradation mechanism of 2D/3D tin perovskite films based on (PEA)0.2(FA)0.8SnI3 (where PEA is phenylethylammonium and FA is formamidinium). We show that SnI4, a product of the oxygen-induced degradation of tin perovskite, quickly evolves into iodine via the combined action of moisture and oxygen. We identify iodine as a highly aggressive species that can further oxidise the perovskite to more SnI4, establishing a cyclic degradation mechanism. Perovskite stability is then observed to strongly depend on the hole transport layer chosen as the substrate, which is exploited to tackle film degradation. These key insights will enable the future design and optimisation of stable tin-based perovskite optoelectronics.

Ultrathin polymethylmethacrylate interlayers boost performance of hybrid tin halide perovskite solar cells.

Introducing a polymethylmethacrylate (PMMA) layer at the (PEA)0.2(FA)0.8SnI3 perovskite/hole transport layer interface leads to a remarkable improvement in the photogenerated current density and fill factor, resulting in an increase in the power conversion efficiency from 6.5% to 10%. PMMA is proposed to mitigate interfacial charge losses and to induce a more favourable distribution of 2D perovskite phases, elucidating a pathway towards the development of high-performance tin-based perovskite solar cells.

2D Phase Purity Determines Charge-Transfer Yield at 3D/2D Lead Halide Perovskite Heterojunctions.

Targeted functionalization of 3D perovskite with a 2D passivation layer via R-NH3I treatment has emerged as an effective strategy for enhancing both the efficiency and chemical stability of ABX3 perovskite solar cells, but the underlying mechanisms behind these improvements remain unclear. Here, we assign a passivation mechanism where R-NH3I reacts with excess PbI2 in the MAPbI3 film and unsaturated PbI6 octahedra to form (R-NH3)2(MA)n-1PbnI3n+1. Crucially, we show that precise control of the 2D (R-NH3)2(MA)n-1PbnI3n+1 layer underpins performance improvements: n = 1 yields over a 2-fold improvement in hole injection to the HTL; n > 1 deteriorates hole injection. Ultrafast transient absorption spectroscopy suggests this n-dependence is rooted in the fact that fast (<6 ns) hole injection does not occur between the 3D and 2D layers. These results help explain contemporary empirical findings in the field and set out an important design rule for the further optimization of multidimensional perovskite optoelectronics.

Bioinspired scaffolds that sequester lead ions in physically damaged high efficiency perovskite solar cells.

Hydroxyapatite nanoparticles (HAP NPs) are blended with TiO2 NPs to prepare mixed mesoporous scaffolds which are used to prepare high efficiency perovskite solar cells (PSCs) with a best power conversion efficiency (PCE) of 20.98%. HAP not only increases the PCE but also limits the concentration of Pb released in water from intentionally broken PSCs by ion sequestration thereby potentially offering a promising in-device fail-safe system.

Stability of Lead and Tin Halide Perovskites: The Link between Defects and Degradation.

The field of photovoltaic research has been lately dominated by the rapid evolution of low-cost and high-efficiency hybrid organic lead halide perovskite solar cells. Despite the considerable progress made in the efficiency of such devices, the achievement of long-term material and device stability remains a challenge. In this Perspective, insights into the role structural defects play in the stability of these perovskite absorbers are examined, highlighting the critical importance of vacancy type defects as the initiation sites for moisture-, oxygen-, and light-induced degradation and the approaches that are emerging to help overcome these issues. In the second part of the Perspective we consider the stability of tin-based perovskites. Here, the Sn4+ defects that arise upon material degradation are described along with the strategies being developed to enhance stability and decrease their formation. Finally, the discussion is extended to innately more stable layered tin-based perovskites, identifying them as a route to the development of efficient lead-free perovskite solar cells.

Ligand-free preparation of polymer/CuInS2 nanocrystal films and the influence of 1,3-benzenedithiol on their photovoltaic performance and charge recombination properties.

Bulk heterojunction solar cells based on conjugated polymer donors and fullerene-derivative acceptors have received much attention in the last decade. Alternative acceptors like organic non-fullerene acceptors or inorganic nanocrystals have been investigated to a lesser extent; however, they also show great potential. In this study, one focus is set on the investigation of the in situ growth of copper indium sulfide nanocrystals in a conjugated polymer matrix. This preparation method allows the fabrication of a hybrid active layer without long-chain ligands, which could hinder charge separation and transport. In contrast, surfactants for the passivation of the nanocrystal surface are missing. To tackle this problem, we modified the absorber layer with 1,3-benzenedithiol and investigated the influence on charge transfer and solar cell performance. Using ToF-SIMS measurements, we could show that 1,3-benzenedithiol is successfully incorporated and homogeneously distributed in the absorber layer, which significantly increases the power conversion efficiency of the corresponding solar cells. This can be correlated to an improved charge transfer between the nanocrystals and the conjugated polymer as revealed by transient absorption spectroscopy as well as prolonged carrier lifetimes as disclosed by transient photovoltage measurements.

Toward Improved Environmental Stability of Polymer:Fullerene and Polymer:Nonfullerene Organic Solar Cells: A Common Energetic Origin of Light- and Oxygen-Induced Degradation.

With the emergence of nonfullerene electron acceptors resulting in further breakthroughs in the performance of organic solar cells, there is now an urgent need to understand their degradation mechanisms in order to improve their intrinsic stability through better material design. In this study, we present quantitative evidence for a common root cause of light-induced degradation of polymer:nonfullerene and polymer:fullerene organic solar cells in air, namely, a fast photo-oxidation process of the photoactive materials mediated by the formation of superoxide radical ions, whose yield is found to be strongly controlled by the lowest unoccupied molecular orbital (LUMO) levels of the electron acceptors used. Our results elucidate the general relevance of this degradation mechanism to both polymer:fullerene and polymer:nonfullerene blends and highlight the necessity of designing electron acceptor materials with sufficient electron affinities to overcome this challenge, thereby paving the way toward achieving long-term solar cell stability with minimal device encapsulation.

Correction: Polymeric hole-transport materials with side-chain redox-active groups for perovskite solar cells with good reproducibility.

Correction for 'Polymeric hole-transport materials with side-chain redox-active groups for perovskite solar cells with good reproducibility' by Rosinda Fuentes Pineda et al., Phys. Chem. Chem. Phys., 2018, 20, 25738-25745.

Polymeric hole-transport materials with side-chain redox-active groups for perovskite solar cells with good reproducibility.

Two monomers, M:OO and M:ON, and their corresponding polymers, P:OO and P:ON, were prepared from styrene derivatives N,N-diphenyl-4-vinyl-aniline with different substituents (-OCH3 and -N(CH3)2) in the N-phenyl para positions. The polymers were synthesised and fully characterised to study their function as hole transport materials (HTMs) in perovskite solar cells (PSCs). The thermal, optical and electrochemical properties and performance of these monomers and polymers as HTMs in PSCs were compared in terms of their structure. The polymers form more stable amorphous glassy states and showed higher thermal stability than the monomers. The different substituent in the para position influenced the highest occupied molecular orbital (HOMO) level, altering the oxidation potential. Both monomers and polymers were employed as HTMs in perovskite solar cells with a device configuration FTO/bl-TiO2/mp-TiO2/CH3NH3PbI3/HTM/Au resulting in power conversion efficiencies of 7.48% for M:OO, 5.14% for P:OO, 5.28% for P:ON and 3.52% for M:ON. Although showing comparatively low efficiencies, the polymers showed much superior reproducibility in comparison with Spiro-OMeTAD or the monomers, suggesting further optimisation of polymeric HTMs with redox side groups is warranted.

Exploring the validity and limitations of the Mott-Gurney law for charge-carrier mobility determination of semiconducting thin-films.

Using drift-diffusion simulations, we investigate the voltage dependence of the dark current in single carrier devices typically used to determine charge-carrier mobilities. For both low and high voltages, the current increases linearly with the applied voltage. Whereas the linear current at low voltages is mainly due to space charge in the middle of the device, the linear current at high voltage is caused by charge-carrier saturation due to a high degree of injection. As a consequence, the current density at these voltages does not follow the classical square law derived by Mott and Gurney, and we show that for trap-free devices, only for intermediate voltages, a space-charge-limited drift current can be observed with a slope that approaches a value of two. We show that, depending on the thickness of the semiconductor layer and the size of the injection barriers, the two linear current-voltage regimes can dominate the whole voltage range, and the intermediate Mott-Gurney regime can shrink or disappear. In this case, which will especially occur for thicknesses and injection barriers typical of single-carrier devices used to probe organic semiconductors, a meaningful analysis using the Mott-Gurney law will become unachievable, because a square-law fit can no longer be achieved, resulting in the mobility being substantially underestimated. General criteria for when to expect deviations from the Mott-Gurney law when used for analysis of intrinsic semiconductors are discussed.

Reducing hole transporter use and increasing perovskite solar cell stability with dual-role polystyrene microgel particles.

Perovskite solar cells (PSCs) are a disruptive technology that continues to attract considerable attention due to their remarkable and sustained power conversion efficiency increase. Improving PSC stability and reducing expensive hole transport material (HTM) usage are two aspects that are gaining increased attention. In a new approach, we investigate the ability of insulating polystyrene microgel particles (MGs) to increase PSC stability and replace the majority of the HTM phase. MGs are sub-micrometre crosslinked polymer particles that swell in a good solvent. The MGs were prepared using a scalable emulsion polymerisation method. Mixed HTM/MG dispersions were subsequently spin-coated onto PSCs and formed composite HTM-MG layers. The HTMs employed were poly(triaryl amine) (PTAA), poly(3-hexylthiophene) (P3HT) and Spiro-MeOTAD (Spiro). The MGs formed mechanically robust composite HTMs with PTAA and P3HT. In contrast, Spiro-MG composites contained micro-cracks due the inability of the relatively small Spiro molecules to interdigitate. The efficiencies for the PSCs containing PTAA-MG and P3HT-MG decreased by only ∼20% compared to control PSCs despite PTAA and P3HT being the minority phases. They occupied only ∼35 vol% of the composite HTMs. An unexpected finding from the study was that the MGs dispersed well within the PTAA matrix. This morphology aided strong quenching of the CH3NH3PbI3-xClx fluorescence. In addition, the open circuit voltages for the PSCs prepared using P3HT-MG increased by ∼170 mV compared to control PSCs. To demonstrate their versatility the MGs were also used to encapsulate P3HT-based PSCs. Solar cell stability data for the latter as well as those for PSCs containing composite HTM-MG were both far superior compared to data measured for a control PSC. Since MGs can reduce conjugated polymer use and increase stability they have good potential as dual-role PSC additives.

Fast oxygen diffusion and iodide defects mediate oxygen-induced degradation of perovskite solar cells.

Methylammonium lead halide perovskites are attracting intense interest as promising materials for next-generation solar cells, but serious issues related to long-term stability need to be addressed. Perovskite films based on CH3NH3PbI3 undergo rapid degradation when exposed to oxygen and light. Here, we report mechanistic insights into this oxygen-induced photodegradation from a range of experimental and computational techniques. We find fast oxygen diffusion into CH3NH3PbI3 films is accompanied by photo-induced formation of highly reactive superoxide species. Perovskite films composed of small crystallites show higher yields of superoxide and lower stability. Ab initio simulations indicate that iodide vacancies are the preferred sites in mediating the photo-induced formation of superoxide species from oxygen. Thin-film passivation with iodide salts is shown to enhance film and device stability. The understanding of degradation phenomena gained from this study is important for the future design and optimization of stable perovskite solar cells.