Chen Weijie

An efficient interface passivation strategy for integrated perovskite/organic solar cells (IPOSCs) based on layered RP perovskite is demonstrated. The polymer PM6 is developed as the passivation layer to reduce the interface defects and suppress the nonradiation recombination in IPOSCs, leading to an improved V _OC from 1.06 to 1.12 V. The optimized IPOSC exhibits a champion efficiency of 19.15%, much higher than the control device (PCE = 16.33%).

Abstract

Integrated perovskite/organic solar cells (IPOSCs) have shown great potential in broadening the light absorption range and improving the photovoltaic performance. However, the severe interface charge recombination and unmatched energy levels between perovskite and organic photoactive layers hinder their performance improvement. Here, an efficient interface passivation strategy for IPOSCs based on a layered Ruddlesden–Popper (RP) perovskite and high photovoltaic performance is successfully demonstrated. It is found that an ultrathin conjugated polymer (PM6) layer could passivate the surface defects of perovskite film, tuning the energy level and suppress the nonradiative recombination loss, leading to efficient interface contact between RP perovskite and organic photoactive layers, boosting the open-circuit voltage from 1.06 to 1.12 V and the efficiency from 17.23% to 19.15%. Importantly, the optimized device shows extended photocurrent response to 930 nm with a peak intensity close to 50% from 800 to 931 nm. The results indicate that interface passivation using a functionalized polymer could be an efficient strategy to improve the photovoltaic performance of integrated devices.

Publication date: 11 November 2021

Source: Chem, Volume 7, Issue 11

Author(s): Xinliang Fu, Tingwei He, Shifu Zhang, Xiaojuan Lei, Yuanzhi Jiang, Di Wang, Pingchuan Sun, Dongbing Zhao, Hsien-Yi Hsu, Xiaofang Li, Mei Wang, Mingjian Yuan

All-inorganic halide perovskite films that can adsorb O₂ by halide vacancies at grain boundaries regardless of light condition have demonstrated higher intrinsic oxygen sensitivity than organic–inorganic halide perovskite films. A CsPbBr₃-based oxygen sensor is developed with both ultrahigh sensitivity and long-term stability. The donor–acceptor competition by halide vacancy filling is revealed as the mechanism of oxygen sensing.

Abstract

Oxygen detection by organic–inorganic halide perovskites (OIHPs) has demonstrated advantages in operating temperature, response time, and reversibility over traditional materials. However, OIHPs can only sense O₂ in light and the unavoidable O₂ exposure during detection easily induces the degradation of OIHPs. The trade-off between sensitivity and stability makes the OIHP-based oxygen sensors impractical. By replacing organic groups with Cs, the compact films of all-inorganic halide perovskites (AIHPs) that can adsorb O₂ at grain boundaries in dark are developed. AIHPs show conductance increase of 1875.5% from 1 × 10⁻⁵ to 700 Torr of O₂ pressure, associated with full reversibility and long-term stability. Combining experiments and modeling, this work reveals the donor–acceptor competition via halide vacancy filling leading to the modulation of carrier concentration and mobility. This work offers understandings on oxygen sensing by perovskite materials and paves the way for further optimization of AIHPs as promising oxygen sensors with high sensitivity and stability.

This Minireview summarizes the recent developments on interfaces in inorganic perovskite solar cells, with special focus on the fundamental understanding of how interfaces influence the performance of devices. Directions for developing highly efficient and stable inorganic perovskite solar cells by interface engineering are also provided.

Abstract

Owing to their superior thermal stability, metal halide inorganic perovskite materials continue to attract interest for photovoltaics applications. The highest reported power conversion efficiency (PCE) for solar cells based on inorganic perovskites is over 20 %. As this PCE corresponds to 73 % of the theoretical limit, there remains more room for further improving the device PCEs than for improving organic–inorganic hybrid perovskite solar cells (PSCs). The main loss is in the photovoltage, which is limited by interfaces in terms of non-radiative recombination caused by traps and energy-level mismatch. Furthermore, inefficient charge extraction at interfacial contacts reduces the photocurrent and fill factor. This Minireview summarizes the recent developments in the fundamental understanding of how the interfaces and interfacial layers influence the performance of solar cells based on inorganic perovskite absorbers. An outlook for the development of highly efficient and stable inorganic PSCs from the interface point of view is also given.

The dynamical behaviors of CsPbI₃ perovskite across temperature and structural phase transitions are investigated theoretically. Two iodine-octahedral-tilt modes soften in the Pm3¯m phase; one sub-THz mode maintains very low frequency in the P4/mbm phase arising from the temporal exploration of various structural states, and it has mixed fluctuations of antiphase iodine tiltings and Cs antipolar motions.

Abstract

Lattice dynamics are often regarded as signatures of the underlying crystal structure. Here, a first-principle-based effective Hamiltonian method combined with molecular dynamics simulations is used to study dynamical behaviors of CsPbI₃ perovskite across temperature and structural phase transitions. A single (short-range tilting) parameter in this effective Hamiltonian is varied in order to make the temperature range of the intermediate tetragonal P4/mbm phase, existing in-between the cubic Pm3¯m and orthorhombic Pnma phases, either broader than observed or completely disappearing. Comparing the dynamics of these different cases allows one to conclude that real CsPbI₃ perovskite should have i) two iodine-octahedral-tilt related modes that differ in frequency but both significantly soften as the temperature decreases within the cubic phase toward the Pm3¯m-to-P4/mbm transition; and ii) one mode that maintains a very low frequency (of the order of 1.0 cm⁻¹) in the entire region of P4/mbm stability, as a result of the temporal exploration of various structural states. Such latter sub-THz mode mixes fluctuations of antiphase iodine tiltings and Cs antipolar motions because of a trilinear energetic coupling.

Nature, Published online: 03 September 2021; doi:10.1038/s41586-021-03839-y

Optimizing perovskite vertical crystallization is achieved by the newly developed buried composite layer by introducing graphitic carbon nitride (g-C₃N₄) into tin oxide, which benefits from the pre-nucleation of lead-rich species induced by the rich amino groups on g-C₃N₄, fulfilling the enhanced efficiencies in solar cells. Our findings provide new insight into vertically manipulating the perovskite films from the buried contact.

Planar-heterojunction perovskite solar cells (PSCs) have experienced rapid evolution in recent years because of the low-temperature processing, suitable alignment, and high mobility of the tin oxide buried contact layer. However, improper SnO₂ surface states and poor crystallinity of the top perovskite films are still the main obstacles for the planar PSCs in which performance always lags behind their mesoporous counterparts. Herein, a new buried contact is reported by introducing graphitic carbon nitride (g-C₃N₄) into the commonly used SnO₂ which performs outstanding transmittance, conductivity, and surface states for a high-quality electron-transporting layer. Moreover, the vertical composition and crystallinity of the top perovskite film are manipulated by rich amino groups on the edge of the g-C₃N₄ nanosheets which induce the prenucleation of the lead-rich species at the buried interface. Benefiting from the high-quality buried contacts and the optimized perovskite layers, the resultant PSCs achieve a champion efficiency of 21.5% with all photovoltaic parameters enhanced in comparison with their control counterparts (<20%).

The nanophotonic perovskite solar cell covered with an array of nanodome or nanohole structures can provide enhanced light incoupling compared with the planar device, resulting in improved J _SC by ∼10–15% and strengthening of the omnidirectional capabilities. Such optimized nanophotonic front contacts can benefit from realizing 30% power conversion efficiency in the case of perovskite/perovskite tandem solar cells.

The front contact of solar cells greatly influences the optoelectronic performance of perovskite solar cells (PSCs) by controlling the coherent light propagation as well as charge transport within the device. Herein, the nanophotonic front contact consisting of multilayer nanodomes and nanoholes for high-efficiency perovskite single-junction and perovskite/perovskite tandem solar cells (PVK/PVK TSCs) is investigated. The optical and electrical characteristics of solar cells are investigated by conducting an advanced 3D numerical approach with the combination of finite-difference time-domain (FDTD) and finite-element method (FEM) simulations embedded with the particle swarm optimization (PSO) algorithm. The numerical modeling is validated by fabricating a set of efficient PSCs, optimized to a power conversion efficiency (PCE) of 17.9%, V _OC of 1.07 V, J _SC of 21.8 mA cm⁻² and fill factor (FF) of 77%. The nanophotonic device results in improved J _SC by 10−15%, resulting from 10−15% enhanced light incoupling compared with the planar device, while also strengthening the omnidirectional capabilities at angles of illumination as high as 40°. The optimized nanophotonic front contact results in PCEs of >23% and >30% (matched J _SC ≈18 mA cm⁻²) for single-junction PSCs and PVK/PVK TSCs, respectively. Details of the nanophotonic front contact, device, and fabrication process are provided.

This work reports a novel n-type organic passivator for SnO₂, via regulation of the molecular structures toward efficient charge transport and suppressed recombination at the SnO₂/perovskite interfaces. An impressive power conversion efficiency over 23% is achieved by the resulting perovskite solar cells.

Abstract

SnO₂ has been universally applied as electron transporting layer (ETL) towards the fabrication of highly efficient perovskite solar cells (PSCs), owing to its unique advantages including low-temperature solution-processability, high optical, transmittance and good electrical conductivity. Uncoordinated Sn-dangling bonds on SnO₂ surface exist as deep traps to capture the photogenerated carriers, causing hysteresis and device instability. Fullerene derivatives, though being widely utilized as the passivator for SnO₂, are highly prone to self-aggregate due to their π-cage structures, which hampers passivation. Herein, π-conjugated n-type small molecules with better film formation ability are innovatively designed, to improve passivation effectiveness. By exploring the interplay between molecular stacking of small molecules and charge transporting/recombination dynamics at the SnO₂/perovskite interface, it is unveiled that a more compact molecular packing of the organic passivators yields superior interfacial characteristics, in terms of fewer trap states, lower charge recombination and higher electron transporting efficiency. An impressive PCE over 23% is achieved with the assistance of this new-type SnO₂-passivator, which is among the highest reported value for triple-cation perovskite systems to date. This work offers an original concept for the design and synthesis of ETL passivators towards the development of high performance and stable PSCs.

Genetic modification of M13 bacteriophages amplifies amino acid K, which functions as a perovskite growth template and a stronger passivator than the wild-type virus in PSCs. The modified virus-added PSCs exhibit a higher PCE (23.6%) than wild-type M13 virus-added devices (22.8%). The observed enhancement is attributed to slightly larger perovskite grains, stronger grain boundary passivation, and improved hole conductivity.

Abstract

Perovskite solar cells (PSCs) are considered to be one of the most promising solar energy harvesters owing to their high power conversion efficiency (PCE). To increase their PCE even further, additives are used; however, some of these additives pose certain disadvantages, which limit their applications to PSCs. Therefore, in this study, the nature-inspired ecofriendly M13 bacteriophage is genetically engineered to maximize its performance as a perovskite crystal growth template and as a passivator for PSCs. The genetic manipulation of the M13 bacteriophage enhances the Lewis coordination between the perovskite materials and single-stranded virus by amplifying a designated amino acid group. Among the 20 types of amino acids, lysine (Lys or K), arginine (Arg or R), and methionine (Aug or M) exhibit the strongest interaction with the perovskite materials. Results suggest that the K-amplified genetically engineered M13 bacteriophage is the most effective. The K-type M13 virus-inoculated PSCs yield a PCE of 23.6% in the laboratory. This device, when taken to a national laboratory for verification, exhibits a certified forward and reverse bias-combined efficiency (22.3%), which, to the best of the authors’ knowledge, is one of the highest efficiencies reported among the biomaterial-based PSCs.

The novel structurally defined and covalently linked donor–acceptor dyad 4 is implemented into single-material organic solar cells as the essential ambipolar and photoactive layer. The combination of an oligothiophene donor and PC₇₁BM fullerene as acceptor not only leads to enhanced 5.34% power conversion efficiency, but also to impressive long-term stability after 750 hours (one month) of continuous illumination.

Abstract

A novel donor–acceptor dyad, 4, in which the conjugated oligothiophene donor is covalently connected to fullerene PC₇₁BM by a flexible alkyl ester linker, is synthesized and applied as photoactive layer in solution-processed single-material organic solar cells (SMOSCs). Excellent photovoltaic performance, including a high short-circuit current density (J _SC) of 13.56 mA cm⁻², is achieved, leading to a power conversion efficiency of 5.34% in an inverted cell architecture, which is substantially increased compared to other molecular single materials. Furthermore, dyad 4-based SMOSCs display excellent stability maintaining 96% of the initial performance after 750 h (one month) of continuous illumination and operation under simulated AM 1.5G irradiation. These results will strengthen the rational molecular design to further develop SMOSCs for potential industrial application.

By designing new donor/acceptor materials and combining a ternary blending strategy, a maximum power conversion efficiency (PCE) of 19.0% (certified value of 18.7%) in single-junction organic photovoltaic (OPV) cells is achieved. It is demonstrated that finely tuning the light utilization and photophysical processes of the active layer has great potential for further improving the PCEs of OPV cells.

Abstract

Improving power conversion efficiency (PCE) is important for broadening the applications of organic photovoltaic (OPV) cells. Here, a maximum PCE of 19.0% (certified value of 18.7%) is achieved in single-junction OPV cells by combining material design with a ternary blending strategy. An active layer comprising a new wide-bandgap polymer donor named PBQx-TF and a new low-bandgap non-fullerene acceptor (NFA) named eC9-2Cl is rationally designed. With optimized light utilization, the resulting binary cell exhibits a good PCE of 17.7%. An NFA F-BTA3 is then added to the active layer as a third component to simultaneously improve the photovoltaic parameters. The improved light unitization, cascaded energy level alignment, and enhanced intermolecular packing result in open-circuit voltage of 0.879 V, short-circuit current density of 26.7 mA cm⁻², and fill factor of 0.809. This study demonstrates that further improvement of PCEs of high-performance OPV cells requires fine tuning of the electronic structures and morphologies of the active layers.

Nature Materials, Published online: 30 August 2021; doi:10.1038/s41563-021-01081-5

The control of molecular packing through hydrogen-bond-directed self-assembly bestows robustness to the structure of hole transporting layers (HTL). A comparative study between two analogous pyrene-based small molecules proves that self-assembled HTLs benefit the solution processing of Pb–Sn perovskite solar cells and improve their efficiency and stability, outperforming poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS).

Lead–tin (Pb–Sn) hybrid perovskite materials possess ideal narrow bandgaps (1.2–1.4 eV) for efficient single-junction and tandem solar cells. Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is commonly used as hole transport layer (HTL) for Pb–Sn perovskite solar cells (PSCs), despite its poor stability with these perovskites. Here, two new octacyclic heteroaromatic molecules, pyrenodiindole (PDI) and pyrenodi-(7-azaindole) (PDAI), are presented as the HTL for narrow-bandgap (1.23 eV) p–i–n Pb-Sn PSCs. The self-assembled reciprocal hydrogen-bonded solid-state structure of PDAI bestows robustness compared to PDI, making it less vulnerable in processing the perovskite film on top, and improves the reproducibility of device fabrication. Transient photocurrent measurements and light-intensity-dependent device characteristics indicate that PDI and PDAI possess similar hole extraction properties to PEDOT:PSS. As a result, similar open-circuit voltages and fill factors are obtained in the PSCs. Interestingly, the use of thin PDI and PDAI as HTL in PSCs changes the optical interference and reduces parasitic absorption in the near-infrared region, resulting in an improved short-circuit current density. Consequently, a higher power conversion efficiency of 16.1% is obtained for PDI and PDAI, compared to 15.1% for PEDOT:PSS. In addition, the self-assembled structure of PDAI led to a notable enhancement of device stability.

A cold-aging induced aggregation approach is demonstrated to enhance device efficiency of organic solar cells via tuning of the pre-aggregates of polymer donor PM6 in solution and therefore in its solid photovoltaic blend films with a range of fused-ring and non-fused-ring non-fullerene electron acceptors.

Abstract

The molecular ordering and pre-aggregation of photovoltaic materials in solution can significantly affect the nanoscale morphology in solid photoactive layers, and play a vital role in determining the power conversion efficiency (PCE) of organic solar cells (OSCs). Herein, a cold-aging strategy is reported to mediate the pre-aggregation of PM6 polymer in solution through a disorder-order transition, which leads to dense and fine PM6 aggregates with enhanced π−π stacking in its blend thin films with either fused-ring and non-fused-ring non-fullerene acceptors (NFAs) including Y6-BO, N3, IT-4F, and PTIC. The fine aggregates of PM6 and slightly enlarged NFA domains improve the continuous networks with enhanced and balanced charge mobility. The resulting OSCs all demonstrate enhanced PCEs compared to their counterparts without any cold-aging treatments, with PM6:Y6-BO OSC being most effective from 16.6% to 17.7%, demonstrating the universality of this approach. This can be further optimized upon casting of the cold-aging solution with the presence of solvent vapor, resulting in a champion PCE of 18.0% for PM6:Y6-BO OSC, which is the highest PCE of this OSC reported in the literature. This work provides a rational guide for optimizing non-fullerene OSCs via aggregation control before and during the solution casting process.