JIALINGBO

Since 2000, researchers at University of Washington have significantly contributed to the study of organic semiconductors, providing new scientific insights and breaking records. The ideas and experiences that have been gained along the way are summarized and reflected upon, and through this reflection insights are offered into future directions.

Abstract

Research at the University of Washington regarding organic semiconductors is reviewed, covering four major topics: electro-optics, organic light emitting diodes, organic field-effect transistors, and organic solar cells. Underlying principles of materials design are demonstrated along with efforts toward unlocking the full potential of organic semiconductors. Finally, opinions on future research directions are presented, with a focus on commercial competency, environmental sustainability, and scalability of organic-semiconductor-based devices.

Narrowband near-infrared organic photodiodes are reported based on a dipolar squaraine dye. J-type coupling in the solid state allows SQ-H thin films to combine favorable NIR absorption at 1040 nm and charge carrier mobility. The bulk-heterojunction with PC₆₁BM yields an organic photodiode with external quantum efficiency of 12.3% at 1050 nm with a full-width half-maximum of 85 nm under short-circuit condition.

Abstract

A highly sensitive short-wave infrared (SWIR, λ > 1000 nm) organic photodiode (OPD) is described based on a well-organized nanocrystalline bulk-heterojunction (BHJ) active layer composed of a dicyanovinyl-functionalized squaraine dye (SQ-H) donor material in combination with PC₆₁BM. Through thermal annealing, dipolar SQ-H chromophores self-assemble in a nanoscale structure with intermolecular charge transfer mediated coupling, resulting in a redshifted and narrow absorption band at 1040 nm as well as enhanced charge carrier mobility. The optimized OPD exhibits an external quantum efficiency (EQE) of 12.3% and a full-width at half-maximum of only 85 nm (815 cm⁻¹) at 1050 nm under 0 V, which is the first efficient SWIR OPD based on J-type aggregates. Photoplethysmography application for heart-rate monitoring is successfully demonstrated on flexible substrates without applying reverse bias, indicating the potential of OPDs based on short-range coupled dye aggregates for low-power operating wearable applications.

A “two-in-one” strategy is applied to form an acceptor alloy for fine-tuning the donor/acceptor energy alignment and blend morphology. Enhanced hole transfer and suppressed charge recombination in the alloy acceptor consisting of AQx-3 and Y6 enable a power conversion efficiency of over 18%, which is the highest documented for ternary organic solar cells utilizing two nonfullerene acceptors.

Abstract

The trade-off between the open-circuit voltage (V _oc) and short-circuit current density (J _sc) has become the core of current organic photovoltaic research, and realizing the minimum energy offsets that can guarantee effective charge generation is strongly desired for high-performance systems. Herein, a high-performance ternary solar cell with a power conversion efficiency of over 18% using a large-bandgap polymer donor, PM6, and a small-bandgap alloy acceptor containing two structurally similar nonfullerene acceptors (Y6 and AQx-3) is reported. This system can take full advantage of solar irradiation and forms a favorable morphology. By varying the ratio of the two acceptors, delicate regulation of the energy levels of the alloy acceptor is achieved, thereby affecting the charge dynamics in the devices. The optimal ternary device exhibits more efficient hole transfer and exciton separation than the PM6:AQx-3-based system and reduced energy loss compared with the PM6:Y6-based system, contributing to better performance. Such a “two-in-one” alloy strategy, which synergizes two highly compatible acceptors, provides a promising path for boosting the photovoltaic performance of devices.

Fabrication of phase-pure films of 2D perovskites using a novel, simple, and scalable method, referred to as the phase-selective method, is demonstrated. Phase-purity is enabled by the presence of sub-micrometer-sized seeds in the precursor-solution that preserves the memory of the dissolved single-crystals. A photovoltaic efficiency of 17.1% with a V _OC of 1.20 V and stability T _97.5 = 800 h at MPP is reported.

Abstract

2D perovskites are a class of halide perovskites offering a pathway for realizing efficient and durable optoelectronic devices. However, the broad chemical phase space and lack of understanding of film formation have led to quasi-2D perovskite films with polydispersity in perovskite layer thicknesses, which have hindered device performance and stability. Here, a simple and scalable approach is reported, termed as the “phase-selective method”, to fabricate 2D perovskite thin films with homogenous layer thickness (phase purity). The phase-selective method involves the dissolution of single-crystalline powders with a homogeneous perovskite layer thickness in desired solvents to fabricate thin films. In situ characterizations reveal the presence of sub-micrometer-sized seeds in solution that preserve the memory of the dissolved single crystals and dictate the nucleation and growth of grains with an identical thickness of the perovskite layers in thin films. Photovoltaic devices with a p–i–n architecture are fabricated with such films, which yield an efficiency of 17.1% enabled by an open-circuit voltage of 1.20 V, while preserving 97.5% of their peak performance after 800 h under illumination without any external thermal management.

A low-cost thiophene-based hole-transporting material, triarylamine-substituted bithiophene (BT-4D), is used as a hole-transporting material in perovskite solar cells with comparable photovoltaic performance to that of 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene. The solar cell using BT-4D demonstrates exceptional long-term stability by retaining 98% of its initial power conversion efficiency after 1186 h under continuous 1-sun illumination in an inert atmosphere.

Abstract

Triarylamine-substituted bithiophene (BT-4D), terthiophene (TT-4D), and quarterthiophene (QT-4D) small molecules are synthesized and used as low-cost hole-transporting materials (HTMs) for perovskite solar cells (PSCs). The optoelectronic, electrochemical, and thermal properties of the compounds are investigated systematically. The BT-4D, TT-4D, and QT-4D compounds exhibit thermal decomposition temperature over 400 °C. The n-i-p configured perovskite solar cells (PSCs) fabricated with BT-4D as HTM show the maximum power conversion efficiency (PCE) of 19.34% owing to its better hole-extracting properties and film formation compared to TT-4D and QT-4D, which exhibit PCE of 17% and 16%, respectively. Importantly, PSCs using BT-4D demonstrate exceptional stability by retaining 98% of its initial PCE after 1186 h of continuous 1 sun illumination. The remarkable long-term stability and facile synthetic procedure of BT-4D show a great promise for efficient, stable, and low-cost HTMs for PSCs for commercial applications.

For a solar cell, the spatial distribution of minority carriers plays a key role in determining the recombination flux. By introducing a front surface gradient to push the minority carriers away from the defect-rich surface, a high open-circuit voltage (93% of the Shockley–Queisser limit) and power conversion efficiency (22.36%) are archieved for perovskite solar cells without defect passivation.

Abstract

The recombination flux in a solar cell is determined by not only recombination centers, but also the spatial distribution of minority carriers. For halide perovskite solar cells (PSCs), although there has been a tremendous amount of work focusing on defect passivation, the issue of carrier distribution is not as well studied as for other types of solar cells. Here in this work, with the incorporation of perovskite quantum dots, the concept of the front surface gradient in PSCs using a solution process is successfully realized. Evidenced by multiple characterization techniques, the minority carriers are pushed away from the defect-rich surface by the gradient of valence band maximum, which effectively reduces surface recombination without compromising photocurrent. As a result, the normal structured hybrid PSCs and MAPbI₃ cells exhibit open-circuit voltages exceeding 93% and 90% of their respective Shockley–Queisser limits, and the power conversion efficiencies reach 22.36% and 20.53%, respectively.

A robust route simultaneously allows effective defect passivation and reduced energy difference between the valence band edge of the perovskite and the highest occupied molecular orbital of the hole transport layer (HTL) via the judicious placement of strongly polar molecules at the perovskite/HTL interface.

Abstract

The ability to passivate defects and modulate the interface energy-level alignment (IEA) is key to boost the performance of perovskite solar cells (PSCs). Herein, we report a robust route that simultaneously allows defect passivation and reduced energy difference between perovskite and hole transport layer (HTL) via the judicious placement of polar chlorine-terminated silane molecules at the interface. Density functional theory (DFT) points to effective passivation of the halide vacancies on perovskite surface by the silane chlorine atoms. An integrated experimental and DFT study demonstrates that the dipole layer formed by the silane molecules decreases the perovskite work function, imparting an Ohmic character to the perovskite/HTL contact. The corresponding PSCs manifest a nearly 20 % increase in power conversion efficiency over pristine devices and a markedly enhanced device stability. As such, the use of polar molecules to passivate defects and tailor the IEA in PSCs presents a promising platform to advance the performance of PSCs.

A universality strain-regulation approach—crosslinking-enabled strain-regulating crystallization (CSRC)—is introduced to eliminate intrinsic tensile strain in perovskite film, which significantly boosts the perovskite solar cells’ (PSCs) stability and performance. The CSRC approach precisely modulates the perovskite film strain through synchronous cooperation of perovskite crystallization manipulation and in situ chemical crosslinking process, as showcased with several types of crosslinking agents.

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Abstract

α-Formamidinium lead triiodide (α-FAPbI₃) represents the state-of-the-art for perovskite solar cells (PSCs) but experiences intrinsic thermally induced tensile strain due to a higher phase-converting temperature, which is a critical instability factor. An in situ crosslinking-enabled strain-regulating crystallization (CSRC) method with trimethylolpropane triacrylate (TMTA) is introduced to precisely regulate the top section of perovskite film where the largest lattice distortion occurs. In CSRC, crosslinking provides in situ perovskite thermal-expansion confinement and strain regulation during the annealing crystallization process, which is proven to be much more effective than the conventional strain-compensation (post-treatment) method. Moreover, CSRC with TMTA successfully achieves multifunctionality simultaneously: the regulation of tensile strain, perovskite defects passivation with an enhanced open-circuit voltage (V _OC = 50 mV), and enlarged perovskite grain size. The CSRC approach gives significantly enhanced power conversion efficiency (PCE) of 22.39% in α-FAPbI₃-based PSC versus 20.29% in the control case. More importantly, the control PSCs’ instability factor—residual tensile strain—is regulated into compression strain in the CSRC perovskite film through TMTA crosslinking, resulting in not only the best PCE but also outstanding device stability in both long-term storage (over 4000 h with 95% of initial PCE) and light soaking (1248 h with 80% of initial PCE) conditions.

An electrical loss management strategy by using SMe-TATPyr molecule manipulating dopant-free Poly(3-hexylthiophene) (P3HT) has been developed and employed to fabricate efficient and thermally stable CsPbI₂Br solar cells. The P3HT/SMe-TATPyr presents optimized molecular orientation, favorable energy level alignment and effective defect passivation. Based on P3HT/SMe-TATPyr HTLs, the fabricated devices yield a record-high efficiency of 16.93 % for CsPbI₂Br solar cells with dopant-free HTLs.

Abstract

Inorganic cesium lead halide perovskites offer a pathway towards thermally stable photovoltaics. However, moisture-induced phase degradation restricts the application of hole transport layers (HTLs) with hygroscopic dopants. Dopant-free HTLs fail to realize efficient photovoltaics due to severe electrical loss. Herein, we developed an electrical loss management strategy by manipulating poly(3-hexylthiophene) with a small molecule, i.e., SMe-TATPyr. The developed P3HT/SMe-TATPyr HTL shows a three-time increase of carrier mobility owing to breaking the long-range ordering of “edge-on” P3HT and inducing the formation of “face-on” clusters, over 50 % decrease of the perovskite surface defect density, and a reduced voltage loss at the perovskite/HTL interface because of favorable energy level alignment. The CsPbI₂Br perovskite solar cell demonstrates a record-high efficiency of 16.93 % for dopant-free HTL, and superior moisture and thermal stability by maintaining 96 % efficiency at low-humidity condition (10–25 % R. H.) for 1500 hours and over 95 % efficiency after annealing at 85 °C for 1000 hours.

Featuring a fused tricyclic core, an organic small molecule was intentionally synthesized to reduce defects density and improve hole transportation in perovskite devices via π-Pb²⁺ interactions, confirmed by multiple characterizations and simulation.

Abstract

Molecular doping is an of significance approach to reduce defects density of perovskite and to improve interfacial charge extraction in perovskite solar cells. Here, we show a new strategy for chemical doping of perovskite via an organic small molecule, which features a fused tricyclic core, showing strong intermolecular π-Pb²⁺ interactions with under-coordinated Pb²⁺ in perovskite. This π-Pb²⁺ interactions could reduce defects density of the perovskite and suppress the nonradiative recombination, which was also confirmed by the density functional theory calculations. In addition, this doping via π-Pb²⁺ interactions could deepen the surface potential and downshift the work function of the doped perovskite film, facilitating the hole extraction to hole transport layer. As a result, the doped device showed high efficiency of 21.41 % with ignorable hysteresis. This strategy of fused tricyclic core-based doping provides a new perspective for the design of new organic materials to improve the device performance.

Publication date: September 2021

Source: Nano Energy, Volume 87

Author(s): Sourabh Pal, Arup Ghorai, Dipak K. Goswami, Samit K. Ray

Nature, Published online: 02 June 2021; doi:10.1038/s41586-021-03518-y

Three cyanoacetate-containing donor-acceptor compounds (CA, CAMA, CAFA) are designed for perovskite modifications, combining surface passivation and secondary grain growth for synergistic effects. With proper selection of cation, the optimal CAMA contributes to lower energy barriers and fewer trap states, thus accounting for CAMA-treated PSCs with higher efficiency and better stability than the reference cells.

Abstract

Interfacial engineering methods have been developed to solve defect issues of perovskite solar cells (PSCs). However, traditional surface passivation has limited effects on eliminating defect-forming residuals, while secondary grain growth (SGG) is restricted by limited choices of additives and intrinsic properties of perovskites. Here, a pincer strategy of taking advantages of surface passivation and SGG is proposed to modify both exterior and interior of CH₃NH₃PbI₃ (MAPbI₃) perovskite, by employing cyanoacetate-containing donor-acceptor compounds (CA-D-A) including 2-cyano-3-(3,4,5-trimethoxyphenyl)acrylic acid (CA), methanaminium 2-cyano-3-(3,4,5-trimethoxyphenyl)acrylate (CAMA), and aminomethaniminium (Z)-2-cyano-3-(3,4,5-trimethoxyphenyl)acrylate (CAFA). In comparison to untreated perovskite, CA-D-A treated perovskites present better crystallinity because of SGG, lower trap densities due to the synergistic effect of surface passivation and SGG, and tuned energy levels induced by CA-D-A. Accordingly, the CA-D-A treated MAPbI₃-based PSCs exhibit higher open-circuit voltage and fill factor than the control PSC without any treatment, leading to improved power conversion efficiency (PCE) and enhanced device stability, especially the CAMA treated PSCs with an average PCE promoted from 17.77 (control PSCs) to 18.71%, and importantly an excellent PCE of 19.71% through further optimization. This work provides an effective strategy for developing highly efficient and stable PSCs with the assistance of both surface passivation and SGG.

A self-sufficient micrometer-level vacuum growth chamber based on encapsulated MAPbBr₃ microcrystals is designed by Rui Chen, Zhipeng Wei, and co-workers, as described in article number 2100466. Perovskite materials with enhanced environmental, thermal, and optical stability through the reduction of deep-level trap states due to self-structural healing are observed. This greatly improves the lasing performance and service cycle.

Ethylenediamine (EDA) coating changes the p-type tin-lead perovskite to n-type, increases the built-in potential, and decreases the open-circuit voltage (V _oc) loss in perovskite solar cells. With Br inclusion into the lattice and passivation by EDA, the highest power conversion efficiency of 21.74% and Voc of 0.86 V is achieved using Cs_0.025FA_0.475MA_0.5Sn_0.5Pb_0.5I_2.975Br_0.025 perovskite film with a bandgap of 1.25 eV.

Abstract

Tin-lead perovskite solar cells (PSCs) show inferior power conversion efficiency (PCE) than their Pb counterparts mainly because of the higher open-circuit voltage (V _oc) loss. Here, it is revealed that the p-type surface of perovskite transforms to n-type, based on post-treatment by a Lewis base, ethylenediamine. This approach forms a graded band structure owing to the rise of the Fermi-energy level at the surface of the perovskite layer, and increases the built-in potential from 0.56 to 0.76 V, which increases the V _oc by more than 100 mV. It is demonstrated that EDA can lower the defect density (Sn⁴⁺ amount) by screening perovskite against oxygen, and by bonding with undercoordinated Sn on the surface. This study further explores the role of Br anion inclusion in the perovskite lattice from the viewpoint of reducing the lattice strain and Urbach energy. Finally, a high V _oc of 0.86 V is obtained, corresponding to a voltage deficit of 0.39 V, using a perovskite absorber with a bandgap of 1.25 eV and the highest PCE (21.74%) reported so far for Sn-Pb PSCs is achieved.

The effect of interfacial properties on charge-initiated triplet sensitization in perovskite-sensitized solid-state upconversion devices is investigated. Trap-assisted triplet sensitization is demonstrated via modification of interfacial trap densities of the devices through surface treatment while monitoring the upconversion performance. Devices with more interfacial traps show brighter upconversion, highlighting the importance of interfacial control in perovskite-sensitized upconversion devices.

Abstract

Photon upconversion via triplet–triplet annihilation (TTA) has promise for overcoming the Shockley–Queisser limit for single-junction solar cells by allowing the utilization of sub-bandgap photons. Recently, bulk perovskites have been employed as sensitizers in solid-state upconversion devices to circumvent poor exciton diffusion in previous nanocrystal (NC)-sensitized devices. However, an in-depth understanding of the underlying photophysics of perovskite-sensitized triplet generation is still lacking due to the difficulty of precisely controlling interfacial properties of fully solution-processed devices. In this study, interfacial properties of upconversion devices are adjusted by a mild surface solvent treatment, specifically altering perovskite surface properties without perturbing the bulk perovskite. Thermal evaporation of the annihilator precludes further solvent contamination. Counterintuitively, devices with more interfacial traps show brighter upconversion. Approximately an order of magnitude difference in upconversion brightness is observed across different interfacial solvent treatments. Sequential charge transfer and interfacial trap-assisted triplet sensitization are demonstrated by comparing upconversion performance, transient photoluminescence dynamics, and magnetic field dependence of the devices. Incomplete triplet conversion from transferred charges and consequent triplet-charge annihilation (TCA) are also observed. The observations highlight the importance of interfacial control and provide guidance for further design and optimization of upconversion devices using perovskites or other semiconductors as sensitizers.

The involvement of guanidinium in perovskite bulk film and CH₃O-PEABr passivation on the perovskite surface synergistically suppresses the trap states. The charge carrier lifetimes of perovskite films increase by tenfold and fivefold to 981 ns and 8.02 µs at the crystal surface and in its bulk, respectively. The decreased nonradiative recombination loss translates to a record efficiency of 40.1%.

Abstract

Perovskite solar cells exhibit not only high efficiency under full AM1.5 sunlight, but also have great potential for applications in low-light environments, such as indoors, cloudy conditions, early morning, late evening, etc. Unfortunately, their performance still suffers from severe trap-induced nonradiative recombination, particularly under low-light conditions. Here, a holistic passivation strategy is developed to reduce traps both on the surface and in the bulk of micrometer-thick perovskite film, leading to a record efficiency of 40.1% under 301.6 µW cm⁻² warm light-emitting diode (LED) light for low-light solar-cell applications. The involvement of guanidinium into the perovskite bulk film and 2-(4-methoxyphenyl)ethylamine hydrobromide (CH₃O-PEABr) passivation on the perovskite surface synergistically suppresses the trap states. The charge carrier lifetimes of the perovskite film increase by tenfold and fivefold to 981 ns and 8.02 µs at the crystal surface and in its bulk, respectively. The decreased nonradiative recombination loss translates to a high open-circuit voltage (V _oc) of 1.00 V, a high short-circuit current (J _sc) of 152.10 µA cm⁻², and a fill factor (FF) of 79.52%. Note that this performance also stands as the highest among all photovoltaics measured under indoor light illumination. This work of trap passivation for micrometer-thick perovskite film paves a way for high-performance, self-powered IoT devices.

The design principle for lead-free perovskites and the progress of typical lead-free perovskite photodetectors are reviewed and discussed. The outlook for future research and applications is then explored.

Abstract

State-of-the-art photodetectors which apply hybrid perovskite materials have emerged as powerful candidates for next-generation light sensing. Among them, lead-based ones are the most popular beyond doubt on account of their unique and superior optoelectronic properties. Nevertheless, trade-off toward commercialization exists between nontoxicity and high performance, with the poor stability of lead-based perovskites, indicating that it is indispensable to substitute lead with nontoxic element meanwhile bringing about a comparable figure of merit of photodetectors and relatively long-term stability. Herein, recent advances in lead-free perovskite photodetectors are reviewed, analyzing the principle while designing new materials and highlighting some remarkable progress, which are comparable, even superior, to lead-based photodetectors. Furthermore, their potential strategy in optical communication, image sensing, narrowband photodetection, etc., is examined and a perspective on developing new materials and photodetectors with superior properties for more practical applications is provided.