Yingzhi Jin

Advanced Energy Materials, Volume 8, Issue 29, October 15, 2018.

Advanced Energy Materials, Volume 8, Issue 30, October 25, 2018.

Advanced Energy Materials, Volume 8, Issue 30, October 25, 2018.

Advanced Energy Materials, Volume 8, Issue 30, October 25, 2018.

Non‐fullerene acceptors with rotatable end‐groups (ITEN and ITPN) are synthesized by introducing ethynyl or phenylethynyl substituents to modify the well‐known acceptor (ITIC). The ITPN‐based device shows enhanced charge transport capacities and demonstrates a PCE of 12.6%, demonstrating the synergistic effect of rotatable end‐groups and extending the π‐conjugated length is effective to enhance the photovoltaic performances of A–D–A acceptors.

Abstract

Unlike varieties of well‐known nonfullerene acceptors with rigid chemical structures, an acceptor–donor–acceptor (A–D–A)‐type acceptor with conjugated rotatable end groups, a phenylethynyl‐substituted acceptor (ITPN) is designed and synthesized. Compared with the ethynyl‐substituted acceptor (ITEN) and a well‐know small molecular electron acceptor (ITIC), ITPN molecules show reduced binding energy and reorganization energy, facilitated by the rotatability and the extended π conjugation of phenylethynyl end groups. ITPN shows stronger molecular aggregation and more ordered arrangement, resulting in enhanced charge‐transport properties. In the polymer solar cells, applying the fluorinated polymer (PBDB‐TF) as the electron donor, the device based on ITPN yields a maximum power conversion efficiency of 12.6%, higher than that of an ITEN‐ or ITIC‐based device. The work demonstrates that chemically modulating the end groups by the synergistic effect of rotatable end groups and extending the π‐conjugation length is an effective and straightforward way to improve the photovoltaic performance of A–D–A‐type small molecular acceptors.

A nonfullerene semitransparent tandem organic solar cell is fabricated by combining a medium‐bandgap photoactive layer based on P3TEA:FTTB‐PDI₄ and a narrow‐bandgap PTB7‐Th:IEICS‐4F blend as front and back subcells, respectively. As a result of matching current generation, a high efficiency of 10.5% is realized with a decent average transmittance of 20%.

Abstract

Semitransparent organic solar cells have great potential for building integrated photovoltaics and power‐generating windows owing to their advantages of light weight, mechanical flexibility, and color tunability. However, the performance of previous semitransparent organic solar cells have been limited by their relatively weak optical absorptions. In this paper, an efficient nonfullerene semitransparent tandem organic solar cell that exhibits a broad absorption from 300 to 1000 nm is reported. The rear subcell is based on a narrow‐bandgap nonfullerene acceptor named IEICS‐4F that exhibits a strong crystallinity and high electron mobility. As a result, the IEICS‐4F‐based single‐junction opaque and semitransparent organic solar cells yield high efficiencies of 10.3% and 7.5%, respectively. To further enhance light harvesting of the single‐junction semitransparent organic solar cells while maintaining a decent transmittance, a semitransparent tandem organic solar cell is fabricated by incorporating a medium‐bandgap P3TEA:FTTB‐PDI₄ blend as the front subcell. A high efficiency of 10.5% is recorded with an average transmittance of 20%.

Unique J‐architecture from a new fused ring–based nonfullerene acceptor is demonstrated. The well correlations between the single crystal data and the graze‐incidence X‐ray diffraction (GIXRD) data give a clear picture of the acceptor molecule packing in the donor:acceptor blend films and the assignments of the well often used GIXRD signals. A power conversion efficiency of 10.5% is obtained.

Abstract

An interesting and important question emerges with the rapid advances of the highly efficient fused‐ring nonfullerene acceptors; that is, how the acceptor molecules form aggregates in its blended film with a donor polymer/small molecule so as to offer highly efficient exciton diffusion and electron transport? To answer this question, a new acceptor molecule, 3,9‐bis(5‐methylene‐4‐one‐6‐(1,1‐dicyanomethylene)‐cyclopenta[c]thiophen‐2,8‐dimethyl)‐5,5,11,11‐tetrakis(4‐n‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (ITCT‐DM), is designed and synthesized herein and its unique interpenetrating J‐architecture is presented in which the acceptor molecules form compacted and displaced ππ‐stacks with the distances of 3.1−4.2 Ǻ. Again the crystal structure data are correlated with the grazing‐incidence X‐ray diffraction (GIXRD) data of the pure acceptor and its polymer:acceptor blended films, which gives a clearer picture about the origins of the acceptor's GIXRD signals in both the pure and its blended films. Again, these results unveil the key roles of the uses of 1,8‐diiodooctane (DIO) and thermal annealing treatment in optimizing the acceptor phase morphologies in the donor:acceptor blended film, and the combination of the thermal annealing and DIO treatment leads to obtain higher crystallinity for both the donor and acceptor phases, more compacted packing, and finer morphologies. A power conversion efficiency of 10.5% is obtained.

The primary factor limiting the efficiency of photocurrent generation is identified in polymer:nonfullerene acceptor (NFA) blends and its implications for materials design in the optimisation of organic solar cell performance is discussed. The magnitude of this geminate recombination loss pathway is found to be the key determinant of the efficiency of photocurrent generation in polymer:NFA blend solar cells.

Abstract

Herein, a meta‐analysis of the device performance and transient spectroscopic results are undertaken for various donor:acceptor blends, employing three different donor polymers and seven different acceptors including nonfullerene acceptors (NFAs). From this analysis, it is found that the primary determinant of device external quantum efficiency (EQE) is the energy offset driving interfacial charge separation, ΔE _CS. For devices employing the donor polymer PffBT4T blended with NFA and fullerene acceptors, an energy offset ΔE _CS = 0.30 eV is required to achieve near unity charge separation, which increases for blends with PBDTTT‐EFT and P3HT to 0.36 and ≈1.2 eV, respectively. For blends with PffBT4T and PBDTTT‐EFT, a 100 meV decrease in the LUMO of the acceptor is observed to result in an approximately twofold increase in EQE. Steady state and transient optical data determine that this energy offset requirement is not associated with the need to overcome the polymer exciton binding energy and thereby drive exciton separation, with all blends studied showing efficient exciton separation. Rather, the increase in EQE with larger energy offset is shown to result from suppression of geminate recombination losses. These results are discussed in terms of their implications for the design of donor/NFA interfaces in organic solar cells, and strategies to achieve further advances in device performance.

All‐small‐molecule organic solar cells employing three nonfullerene acceptors (F‐0Cl, F‐1Cl, and F‐2Cl) are investigated. End group chlorination leads to redshifted absorption, enhanced crystallinity, and high electron mobility. F‐2Cl with highest crystallinity gives the best device performances with power conversion efficiency of 9.89 and 10.76%, respectively, when small molecule DRCN5T and DRTB‐T are used as donors.

Abstract

While a wide variety of nonfullerene acceptors are developed and perform well in combination with polymer donors, only a few nonfullerene acceptors can work well with small molecule donors. Here, all‐small‐molecule solar cells with high performance enabled by a new type of small molecule acceptors (F‐0Cl, F‐1Cl, and F‐2Cl), which contain linear alkyl side chains and end groups substituted with various number of chlorine atoms, are reported. End group chlorination leads to redshifted absorption, enhanced crystallinity, and high electron mobility. These properties make them competitive as electron acceptors for all‐small‐molecule solar devices. When combined with two popular small molecule donors DRTB‐T and DRCN5T, these nonfullerene acceptors offer power conversion efficiencies up to 10.76 and 9.89%, which are among the top efficiencies reported in all‐small‐molecule solar cells and indicate the great potential of all‐small‐molecule solar devices.

Polymer–fullerene bulk‐heterojunction layers made by spontaneous spreading on a water surface and subsequently transferred to glass substrates are used to fabricate unique bilayer–ternary solar cells that possess two complementary absorber layers stacked on top of each other. Surprisingly, these novel devices can exhibit external quantum efficiencies exceeding 100% under bias illumination.

Abstract

A new method is presented to fabricate bilayer organic solar cells via sequential deposition of bulk‐heterojunction layers obtained using spontaneous spreading of polymer–fullerene blends on a water surface. Using two layers of a small bandgap diketopyrrolopyrrole polymer–fullerene blend, a small improvement in power conversion efficiency (PCE) from 4.9% to 5.1% is obtained compared to spin‐coated devices of similar thickness. Next, bilayer–ternary cells are fabricated by first spin coating a wide bandgap thiophene polymer–fullerene blend, followed by depositing a small bandgap diketopyrrolopyrrole polymer–fullerene layer by transfer from a water surface. These novel bilayer–ternary devices feature a PCE of 5.9%, higher than that of the individual layers. Remarkable, external quantum efficiencies (EQEs) over 100% are measured for the wide bandgap layer under near‐infrared bias light illumination. Drift‐diffusion calculations confirm that near‐infrared bias illumination can result in a significant increase in EQE as a result of a change in the internal electric field in the device, but cannot yet account for the magnitude of the effect. The experimental results indicate that the high EQEs over 100% under bias illumination are related to a barrier for electron transport over the interface between the two blends.

Advanced Energy Materials, Volume 8, Issue 28, October 5, 2018.

Introducing polar cyano groups to the end of dithienosilole‐thienopyrrolodione polymer side chains leads to an increase in polymer dielectric constant, without significantly affecting the optical gaps and energy levels. However, this structural modification results in inferior device parameters and ultimately lower power conversion efficiencies, because of the drastic decrease in hole mobilities which is caused by higher energetic disorders.

Abstract

To better understand the correlation of the dielectric properties with the photovoltaic response in conjugated polymer:fullerene bulk heterojunction materials, the concept of introducing minimal structural change is employed to increase the polymer dielectric constant via polar cyano groups added to the end of butyl or octyl side chains in the poly(dithienosilole‐thienopyrrolodione) system. Density functional theory calculations confirm that the polar groups do not affect the polymer electronic structure but can lead to an increase in overall dipole moment depending on the polymer chain conformation. Despite the increased dielectric constant (from 2.7 to 4.3 for cyano‐octyl side chains and from 2.7 to 3.2 for the cyano‐butyl analogues), the device characteristics employing the cyano‐containing polymers are inferior to those of the devices made with unfunctionalized alkyl chains. It is found that the hole mobilities for the cyano‐containing polymers are two orders of magnitude lower compared to those for the parent polymers and suggest this is due to an increase in energetic disorder caused by the strong local permanent dipoles associated with the cyano groups. The study highlights the complexity in the relationship between the dielectric constant of organic materials, the morphologies that are induced, and their photovoltaic performance.

A tandem organic photovoltaic cell combining a nonfullerene‐acceptor‐based ternary cell with a fullerene small‐molecule binary subcell is reported. The cell combines vacuum‐ and solution‐deposited layers, achieving a power conversion efficiency of 15.4%.

Abstract

The paucity of near‐infrared (NIR) organic materials with high absorption at long wavelengths, combined with large diffusion lengths and charge mobilities, is an impediment to progress in achieving high‐efficiency organic tandem solar cells. Here a subcell is employed within a series tandem stack that comprises a solution‐processed ternary blend of two NIR‐absorbing nonfullerene acceptors and a polymer donor combined with a small‐molecular‐weight, short‐wavelength fullerene‐based subcell grown by vacuum thermal evaporation. The ternary cell achieves a power conversion efficiency of 12.6 ± 0.3% with a short‐circuit current of 25.5 ± 0.3 mA cm⁻², an open‐circuit voltage of 0.69 ± 0.01 V, and a fill factor of 0.71 ± 0.01 under 1 sun, AM 1.5G spectral illumination. The success of this device is a result of the nearly identical offset energies between the lowest unoccupied molecular orbitals (LUMOs) of the donors with the highest occupied molecular orbital (HOMO) of the acceptor, resulting in a high open‐circuit voltage. A tandem structure with an antireflection coating combining these subcells demonstrates a power conversion efficiency of 15.4 ± 0.3%.

Efficient nonfullerene organic solar cells (OSCs) are realized by combining a donor polymer, PffBT2T‐TT, and a small‐molecular acceptor, O‐IDTBR, which have identical bandgaps and close energy levels. Despite the small energy offsets for both hole and electron transfer, this system can still achieve efficient charge separation and a high efficiency of 10.4%.

Abstract

State‐of‐the‐art organic solar cells (OSCs) typically suffer from large voltage loss (V _loss) compared to their inorganic and perovskite counterparts. There are some successful attempts to reduce the V _loss by decreasing the energy offsets between the donor and acceptor materials, and the OSC community has demonstrated efficient systems with either small highest occupied molecular orbital (HOMO) offset or negligible lowest unoccupied molecular orbital (LUMO) offset between donors and acceptors. However, efficient OSCs based on a donor/acceptor system with both small HOMO and LUMO offsets have not been demonstrated simultaneously. In this work, an efficient nonfullerene OSC is reported based on a donor polymer named PffBT2T‐TT and a small‐molecular acceptor (O‐IDTBR), which have identical bandgaps and close energy levels. The Fourier‐transform photocurrent spectroscopy external quantum efficiency (FTPS‐EQE) spectrum of the blend overlaps with those of neat PffBT2T‐TT and O‐IDTBR, indicating the small driving forces for both hole and electron transfer. Meanwhile, the OSCs exhibit a high electroluminescence quantum efficiency (EQE_EL) of ≈1 × 10⁻⁴, which leads to a significantly minimized nonradiative V _loss of 0.24 V. Despite the small driving forces and a low V _loss, a maximum EQE of 67% and a high power conversion efficiency of 10.4% can still be achieved.

Advanced Materials, EarlyView.

We investigate the industrial viability of highly efficient organic solar cells (OSCs) based on several representative non-fullerene acceptors (NFAs) by taking into consideration the three essential parameters: power conversion efficiency, photo-stability, and materials cost. End-group and side-chain modifications of NFAs strongly influence long-term photo-stability. Promising extrapolated operational lifetime approaching 10 years has been demonstrated with the most stable system. Industrial figure of merit (i-FoM) analysis highlights the importance of lowering the synthetic complexity of the NFAs for commercialization of this technology.

Advanced Functional Materials, Volume 28, Issue 42, October 17, 2018.

Advanced Materials, Volume 30, Issue 34, August 23, 2018.

Advanced Materials, Volume 30, Issue 40, October 4, 2018.

Advanced Energy Materials, Volume 8, Issue 27, September 25, 2018.

Advanced Functional Materials, Volume 28, Issue 40, October 4, 2018.

Advanced Materials, Volume 30, Issue 38, September 20, 2018.