Perovskite solar cells (PSCs) have rapidly emerged as one of the hottest topics in the photovoltaics community owing to their high power‐conversion efficiencies (PCE), and the promise to be produced ...at low cost. Among various PSCs, typical 3D perovskite‐based solar cells deliver high PCE but they suffer from severe instability, which restricts their practical applications. In contrast to 3D perovskites, 2D perovskites that incorporate larger, less volatile, and generally more hydrophobic organic cations exhibit much improved thermal, chemical, and environmental stability. 2D perovskites can have different roles within a solar cell, either as the primary light absorber (2D PSCs), or as a capping layer atop a 3D perovskite absorbing layer (2D/3D PSCs). Tradeoffs between PCE and stability exist in both types of PSCs—2D PSCs are more stable but exhibit lower efficiency while 2D/3D PSCs deliver exciting efficiency but show relatively poor stability. To address this PCE/stability tradeoff, the challenges both the 2D and 2D/3D PSCs face are identified and select works the community has undertaken to overcome them are highlighted in this review. It is ended with several recommendations on how to further improve PSCs so their performance and stability can be commensurate with application requirements.
Solar cells incorporating 2D perovskites show tradeoffs between efficiency and stability. The challenges these solar cells face are identified and select works the community has undertaken to overcome them are highlighted in this review. Several recommendations are proposed on how to further improve perovskite solar cells so their performance and stability can be commensurate with application requirements.
While typical perovskite solar cells (PSCs) with doped Spiro-OMeTAD as a hole transport material (HTM) have shown rapid increase in their power-conversion efficiencies (PCEs), their poor stability ...remains a big concern as the dopants and additives used with Spiro-OMeTAD have a strong tendency to diffuse into and degrade the perovskite active layer under normal operating conditions. Aiming to push forward the development of PSCs, many dopant-free small-molecular HTMs have been reported based on energetic considerations for charge transfer and criteria for charge transport. However, the PCEs of the state-of-the-art PSCs with dopant-free small-molecular HTMs are still inferior to those using doped Spiro-OMeTAD, and little attention has been paid to the interactions between the HTM and perovskite absorber in PSCs. Here, we report a facile design concept to functionalize HTMs so that they can passivate perovskite surface defects and enable perovskite active layers with lower density of surface trap states and more efficient charge transfer to the hole transport layer. As a consequence, perovskite solar cells with a functionalized HTM exhibit a champion PCE of 22.4%, the highest value for PSCs using dopant-free small molecular HTMs to date, and substantively improved operational stability under continuous illumination. With a
T
80
of (1617 ± 7) h for encapsulated cells tested at 30 °C in air, the PSCs containing the functionalized HTM are among the most stable PSCs using dopant-free small-molecular HTMs. The effectiveness of our strategy is demonstrated in PSCs comprising both a state-of-the-art MA-free perovskite and MAPbI, a system having more surface defects, and implies the potential generality of our strategy for a broad class of perovskite systems, to further advance highly efficient and stable solar cells.
Incorporation of a hole-transport material that also passivates surface defects results in perovskite solar cells with superior efficiency and stability.
The rapid development of organic electrochemical transistor (OECTs)‐based circuits brings new opportunities for next‐generation integrated bioelectronics. The all‐polymer bulk‐heterojunction (BHJ) ...offers an attractive, inexpensive alternative to achieve efficient ambipolar OECTs, and building blocks of logic circuits constructed from them, but have not been investigated to date. Here, the first all‐polymer BHJ‐based OECTs are reported, consisting of a blend of new p‐type ladder conjugated polymer and a state‐of‐the‐art n‐type ladder polymer. The whole ladder‐type polymer BHJ also proves that side chains are not necessary for good ion transport. Instead, the polymer nanostructures play a critical role in the ion penetration and transportation and thus in the device performance. It also provides a facile strategy and simplifies the fabrication process, forgoing the need to pattern multiple active layers. In addition, the development of complementary metal–oxide–semiconductor (CMOS)‐like OECTs allows the pursuit of advanced functional logic circuitry, including inverters and NAND gates, as well as for amplifying electrophysiology signals. This work opens a new approach to the design of new materials for OECTs and will contribute to the development of organic heterojunctions for ambipolar OECTs toward high‐performing logic circuits.
An all‐polymer bulk‐heterojunction (BHJ) organic electrochemical transistor (OECT) with good ambipolar property is achieved through the blending of a new p‐type ladder polymer and a state‐of‐the‐art n‐type ladder polymer. The concept of the BHJ provides a facile strategy to develop ambipolar OECTs with balanced electron and hole transport, and ultimately to achieve high amplification and easy scaling for integrated circuits.
The performance of three-dimensional (3D) organic-inorganic halide perovskite solar cells (PSCs) can be enhanced through surface treatment with 2D layered perovskites that have efficient charge ...transport. We maximized hole transport across the layers of a metastable Dion-Jacobson (DJ) 2D perovskite that tuned the orientational arrangements of asymmetric bulky organic molecules. The reduced energy barrier for hole transport increased out-of-plane transport rates by a factor of 4 to 5, and the power conversion efficiency (PCE) for the 2D PSC was 4.9%. With the metastable DJ 2D surface layer, the PCE of three common 3D PSCs was enhanced by approximately 12 to 16% and could reach approximately 24.7%. For a triple-cation–mixed-halide PSC, 90% of the initial PCE was retained after 1000 hours of 1-sun operation at ~40°C in nitrogen.
To understand degradation routes and improve the stability of perovskite solar cells (PSCs), accelerated aging tests are needed. Here, we use elevated temperatures (up to 110°C) to quantify the ...accelerated degradation of encapsulated CsPbI
3
PSCs under constant illumination. Incorporating a two-dimensional (2D) Cs
2
PbI
2
Cl
2
capping layer between the perovskite active layer and hole-transport layer stabilizes the interface while increasing power conversion efficiency of the all-inorganic PSCs from 14.9 to 17.4%. Devices with this 2D capping layer did not degrade at 35°C and required >2100 hours at 110°C under constant illumination to degrade by 20% of their initial efficiency. Degradation acceleration factors based on the observed Arrhenius temperature dependence predict intrinsic lifetimes of 51,000 ± 7000 hours (>5 years) operating continuously at 35°C.
A cap against aging
Accelerated aging tests for perovskite solar cells must take into account several degradation pathways. Zhao
et al
. found that for all-inorganic cesium lead triiodide (CsPbI
3
) solar cells, a two-dimensional Cs
2
PbI
2
Cl
2
capping layer stabilized the interface between the CsPbI
3
absorber and the copper thiocyanate hole-transporter layer and boosted its power conversion efficiency to 17.4% (see the Perspective by Habisreutinger and Reese). Accelerated testing at various temperatures up to 110°C and approximately 65% relative humidity revealed an Arrhenius temperature dependence on efficiency degradation. The solar cell should maintain 80% of its efficiency for more than 5 years at 35°C. —PDS
A two-dimensional capping layer on a CsPbI
3
perovskite stabilizes a solar cell for high-temperature and high-humidity operation.
Organic–inorganic hybrid perovskite materials are emerging as semiconductors with potential application in optoelectronic devices. In particular, perovskites are very promising for light‐emitting ...devices (LEDs) due to their high color purity, low nonradiative recombination rates, and tunable bandgap. Here, using pure CH3NH3PbI3 perovskite LEDs with an external quantum efficiency (EQE) of 5.9% as a platform, it is shown that electrical stress can influence device performance significantly, increasing the EQE from an initial 5.9% to as high as 7.4%. Consistent with the enhanced device performance, both the steady‐state photoluminescence (PL) intensity and the time‐resolved PL decay lifetime increase after electrical stress, indicating a reduction in nonradiative recombination in the perovskite film. By investigating the temperature‐dependent characteristics of the perovskite LEDs and the cross‐sectional elemental depth profile, it is proposed that trap reduction and resulting device‐performance enhancement is due to local ionic motion of excess ions, likely excess mobile iodide, in the perovskite film that fills vacancies and reduces interstitial defects. On the other hand, it is found that overstressed LEDs show irreversibly degraded device performance, possibly because ions initially on the perovskite lattice are displaced during extended electrical stress and create defects such as vacancies.
High‐efficiency CH3NH3PbI3 perovskite light‐emitting devices are demonstrated. The external quantum efficiency is boosted from 5.9% to 7.4% by subsequent electrical scans, which is related to excess ion motion reducing nonradiative decay channels, while overstressing the device will degrade device performance due to nonexcess ion migration.
Typical lead‐based perovskites solar cells show an onset of photogeneration around 800 nm, leaving plenty of spectral loss in the near‐infrared (NIR). Extending light absorption beyond 800 nm into ...the NIR should increase photocurrent generation and further improve photovoltaic efficiency of perovskite solar cells (PSCs). Here, a simple and facile approach is reported to incorporate a NIR‐chromophore that is also a Lewis‐base into perovskite absorbers to broaden their photoresponse and increase their photovoltaic efficiency. Compared with pristine PSCs without such an organic chromophore, these solar cells generate photocurrent in the NIR beyond the band edge of the perovskite active layer alone. Given the Lewis‐basic nature of the organic semiconductor, its addition to the photoactive layer also effectively passivates perovskite defects. These films thus exhibit significantly reduced trap densities, enhanced hole and electron mobilities, and suppressed illumination‐induced ion migration. As a consequence, perovskite solar cells with organic chromophore exhibit an enhanced efficiency of 21.6%, and substantively improved operational stability under continuous one‐sun illumination. The results demonstrate the potential generalizability of directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates surface traps in perovskite active layers to yield highly efficient and stable NIR‐harvesting PSCs.
A near‐infrared (NIR)‐harvesting perovskite solar cell with a power‐conversion efficiency of 21.6% and an operational half‐life of 1900 h is achieved by directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates defects in the perovskite active layer.
Solution-processed hybrid organic–inorganic perovskites (HOIPs) from organoammonium halide and lead halide precursors form efficacious active layers for photovoltaics, light-emitting diodes, and ...flexible electronics. Though solvent–solute coordination plays a critical role in HOIP crystallization, the influence of solvent choice on such interactions is poorly understood. We demonstrate Gutmann’s donor number, D N, as a parameter that indicates the coordinating ability of the processing solvent with the Pb2+ center of the lead halide precursor. Low D N solvents interact weakly with the Pb2+ center, favoring instead complexation between Pb2+ and iodide and subsequent crystallization of perovskite. High D N solvents coordinate more strongly with the Pb2+ center, which in turn inhibits iodide coordination and stalls perovskite crystallization. Varying the concentration of high-D N additives in precursor solutions tunes the strength of lead–solvent interactions, allowing finer control over the crystallization and the resulting morphology of HOIP active layers.
Organic photovoltaic cells that employ Y‐series non‐fullerene acceptors (NFAs) have recently achieved impressive power‐conversion efficiencies (>18%). To fulfill their commercial promise, it is ...important to quantify their operational lifetimes and understand their degradation mechanisms. In this work, the spectral‐dependent photostability of films and solar cells comprising several Y‐series acceptors and the donor polymer PM6 is investigated systematically. By applying longpass filters during aging, it is shown that UV/near‐UV photons are responsible for the photochemical decomposition of Y‐series acceptors; this degradation is the primary driver of early solar cell performance losses. Using mass spectrometry, the vinylene linkage between the core and electron‐accepting moieties of Y‐series acceptors is identified as the weak point susceptible to cleavage under UV‐illumination. Employing a series of device characterization, along with numerical simulations, the efficiency losses in organic photovoltaic cells are attributed to the formation of traps, which reduces charge extraction efficiency and facilitates non‐radiative recombination as the Y‐series acceptors degrade. This study provides new insights for molecular degradation of organic photovoltaic absorber materials and highlights the importance of future molecular design and strategies for improved solar cell stability.
By applying longpass filters during photo‐aging experiments, the spectral‐dependent photostability of Y‐series acceptors and solar cells that comprise them is investigated. These tests reveal that high‐energy photons are responsible for degradation. Employing device characterization and numerical simulations, solar cell efficiency losses are attributed to trap formation due to photochemical degradation of vinylene groups in Y‐series acceptors.