Yingzhi Jin

A new class of non-fullerene acceptor, Y6, by employing a ladder-type electron-deficient-core-based central fused ring with a benzothiadiazole core is reported. Organic photovoltaics made from Y6 in conventional and inverted architectures each exhibited a high efficiency of 15.7%, measured in two separate labs. Inverted device structures certified at Enli Tech Laboratory demonstrated an efficiency of 14.9%. Y6-based devices maintained the efficiency of 13.6% with an active layer thickness of 300 nm.

Ultrafast hole transfer mediated by polaron pairs in all-polymer photovoltaic blends

Ultrafast hole transfer mediated by polaron pairs in all-polymer photovoltaic blends, Published online: 23 January 2019; doi:10.1038/s41467-019-08361-4

All-polymer solar cells have shown high efficiencies but the ultrafast charge transfer processes are less known. Here Wang et al. show that polaron pairs play vital role facilitating the hole transfer, which is quite different from the exciton dominated pathway in polymer-fullerene blends.

To predict the open‐circuit voltage (V _oc) of polymer–fullerene solar cells, three independent methods, square‐wave voltammetry (SWV), ultraviolet photoelectron spectroscopy, and density functional theory, are compared. For 19 diketopyrrolopyrrole polymers, SWV gives the best correlation. Remarkably, the slope of V _oc with the blend's electrochemical gap is less than unity and possible reasons for this result are discussed.

Abstract

For 19 diketopyrrolopyrrole polymers, the highest occupied molecular orbital (HOMO) energies are determined from i) the oxidation potential with square‐wave voltammetry (SWV), ii) the ionization potential using ultraviolet photoelectron spectroscopy (UPS), and iii) density functional theory (DFT) calculations. The SWV HOMO energies show an excellent linear correlation with the open‐circuit voltage (V _oc) of optimized solar cells in which the polymers form blends with a fullerene acceptor ([6,6]‐phenyl‐C₆₁‐butyl acid methyl ester or [6,6]‐phenyl‐C₇₁‐butyl acid methyl ester). Remarkably, the slope of the best linear fit is 0.75 ± 0.04, i.e., significantly less than unity. A weaker correlation with V _oc is found for the HOMO energies obtained from UPS and DFT. Within the experimental error, the SWV and UPS data are correlated with a slope close to unity. The results show that electrochemically determined oxidation potentials provide an excellent method for predicting the V _oc of bulk heterojunction solar cells, with absolute deviations less than 0.1 V.

A novel wide‐bandgap nonfullerene acceptor TfIF‐4FIC is synthesized. PBDB‐T‐2F:TfIF‐4FIC‐based organic solar cell acquires a power conversion efficiency (PCE) of 13.1%, a high open‐circuit voltage of 0.98 V, which is the best performed device with bandgap larger than 1.60 eV. When using PBDB‐T‐2F:TfIF‐4FIC as front cell and PTB7‐Th:PCDTBT:IEICO‐4F as back cell to construct tandem device, PCE of 15% is achieved.

Abstract

A tandem organic solar cell (OSC) is a valid structure to widen the photon response range and suppress the transmission loss and thermalization loss. In the past few years, the development of low‐bandgap materials with broad absorption in long‐wavelength region for back subcells has attracted considerable attention. However, wide‐bandgap materials for front cells that have both high short‐circuit current density (J _SC) and open‐circuit voltage (V _OC) are scarce. In this work, a new fluorine‐substituted wide‐bandgap small molecule nonfullerene acceptor TfIF‐4FIC is reported, which has an optical bandgap of 1.61 eV. When PBDB‐T‐2F is selected as the donor, the device offers an extremely high V _OC of 0.98 V, a high J _SC of 17.6 mA cm⁻², and a power conversion efficiency of 13.1%. This is the best performing acceptor with such a wide bandgap. More importantly, the energy loss in this combination is 0.63 eV. These properties ensure that PBDB‐T‐2F:TfIF‐4FIC is an ideal candidate for the fabrication of tandem OSCs. When PBDB‐T‐2F:TfIF‐4FIC and PTB7‐Th:PCDTBT:IEICO‐4F are used as the front cell and the back cell to construct tandem solar cells, a PCE of 15% is obtained, which is one of best results reported to date in the field of organic solar cells.

Three polymers L24, L68, and L810 are developed as donor materials for organic solar cells. As the alkyl side chain of the fluorobenzotriazole (FTAZ) unit increases, the L810‐based device exhibits lower energy loss, better molecular face‐on orientation, and a higher absorption coefficient. Consequently, the power conversion efficiency is improved to 12.1%, which is one of the highest values for FTAZ‐based devices.

Abstract

The fluorobenzotriazole (FTAZ)‐based copolymer donors are promising candidates for nonfullerene polymer solar cells (PSCs), but suffer from relatively low photovoltaic performance due to their unsuitable energy levels and unfavorable morphology. Herein, three polymer donors, L24, L68, and L810, based on a chlorinated‐thienyl benzodithiophene (BDT‐2Cl) unit and FTAZ with different branched alkyl side chain, are synthesized. Incorporation of a chlorine (Cl) atom into the BDT unit is found to distinctly optimize the molecular planarity, energy levels, and improve the polymerization activity. Impressively, subtle side chain length of FTAZ realizes a dramatic improvement in all the device parameters, as revealed by the short‐current density (J _sc) improved from 7.41 to 20.76 mA cm⁻², fill‐factor from 36.3 to 73.5%, and even the open‐circuit voltage (V _oc) from 0.495 to 0.790 V. The best power conversion efficiency (PCE) of 12.1% is obtained from the L810‐based device, which is one of the highest values reported for FTAZ‐based PSCs so far. Notably, the corresponding external quantum efficiency curve keeps a very prominent value up to 80% from 500 to 800 nm. The notable performance is discovered from the reduced energy loss, improved molecular face‐on orientation, the down‐shifted energy levels, and optimized absorption coefficient regulated by side‐chain engineering.

The morphology control of nonfullerene blends is very important to improve the device performance. Here, the high boiling point cosolvent is added to the poly(3‐hexylthiophene):(5Z,5′Z)‐5,5′‐((7,7′‐(4,4,9,9‐tetraoctyl‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis (benzo [c][1,2,5]thiadiazole7,4diyl)) bis(methanylylidene)) bis (3‐ethyl‐2‐thioxothiazolidin‐4‐one) (P3HT:O‐IDTBR) blend, which separates the crystallization process of P3HT and O‐IDTBR, and prolongs film forming time. As a result, the crystallinity, lateral and phase separation structure are optimized simultaneously, leading to high performance of 7.18%.

Abstract

The morphology with the interpenetrating network and optimized vertical phase separation plays a key role in determining the charge transport and collection in polymer:nonfullerene small molecular acceptors (SMAs) solar cells. However, the crystallization of polymer and SMAs usually occurs simultaneously during film‐forming, thus interfering with the crystallization process of each other, leading to amorphous film with undesirable lateral and vertical phase separation. The poly(3‐hexylthiophene) (P3HT):O‐IDTBR blend is selected as a model system, and controlling film‐forming kinetics to solve these problems is proposed. Herein, a cosolvent 1,2,4‐triclorobenzene (TCB) with selective solubility and a high boiling point is added to the solution, leading to prior crystallization of P3HT and extended film‐forming duration. As a result, the crystallinity of both components is enhanced significantly. Meanwhile, the prior crystallization of P3HT induces solid–liquid phase separation, hence rationalizing the formation of the nano‐interpenetrating network. Moreover, the surface energy drives O‐IDTBR to enrich near the cathode and P3HT to migrate to the anode. Consequently, a highly crystalline nano‐interpenetrating network with proper vertical phase separation is obtained. The optimal morphology improves charge transport and suppresses bimolecular recombination, boosting the power conversion efficiency from 4.45% to 7.18%, which is the highest performance in P3HT‐based binary nonfullerene solar cells.

A high‐efficiency parallel‐like ternary organic photovoltaic device is developed through synergetic effects among a wide‐bandgap donor polymer, a narrow‐bandgap nonfullerene acceptor, and fullerene acceptors. Morphological optimization of the ternary devices via the incorporation of fullerenes yields simultaneous enhancement of the charge generation and extraction. An efficiency of 12.1% at an energy loss of 0.61 eV is realized.

Abstract

Ternary organic photovoltaic (OPV) devices with multiple light‐absorbing active materials have emerged as an efficient strategy for realizing further improvements in the power conversion efficiency (PCE) without building complex multijunction structures. However, the third component often acts as recombination centers and, hence, the optimization of ternary blend morphology poses a major challenge to improving the PCE of these devices. In this work, the performance of OPVs is enhanced through the morphological modification of nonfullerene acceptor (NFA)‐containing binary active layers. This modification is achieved by incorporating fullerenes into the layers. The uniformly dispersed fullerenes are sufficiently continuous and successfully mediate the ordering of NFA without charge or energy transfer. Owing to the simultaneous improvement in the charge generation and extraction, the PCE (12.1%) of these parallel‐linked ternary devices is considerably higher than those of the corresponding binary devices (9.95% and 7.78%). Moreover, the additional energy loss of the ternary device is minimized, compared with that of the NFA‐based binary device, due to the judicious control of the effective donor:acceptor composition of the ternary blends.

Protonation of polyethylenimine ethoxylated (PEIE) can effectively passivate the chemical reaction between the PEIE and a nonfullerene (NF) active layer. As a result, the PEIE can work very efficiently as a low‐work‐function interface for NF solar cells. These flexible solar cells exhibit power conversion efficiency up to 12.5% with a room‐temperature‐processed PEIE interface.

Abstract

Nonfullerene (NF) organic solar cells (OSCs) have been attracting significant attention in the past several years. It is still challenging to achieve high‐performance flexible NF OSCs. NF acceptors are chemically reactive and tend to react with the low‐temperature‐processed low‐work‐function (low‐WF) interfacial layers, such as polyethylenimine ethoxylated (PEIE), which leads to the “S” shape in the current‐density characteristics of the cells. In this work, the chemical interaction between the NF active layer and the polymer interfacial layer of PEIE is deactivated by increasing its protonation. The PEIE processed from aqueous solution shows more protonated N⁺ than that processed from isopropyl alcohol solution, observed from X‐ray photoelectron spectroscopy. NF solar cells (active layer: PCE‐10:IEICO‐4F) with the protonated PEIE interfacial layer show an efficiency of 13.2%, which is higher than the reference cells with a ZnO interlayer (12.6%). More importantly, the protonated PEIE interfacial layer processed from aqueous solution does not require a further thermal annealing treatment (only processing at room temperature). The room‐temperature processing and effective WF reduction enable the demonstration of high‐performance (12.5%) flexible NF OSCs.

A synergistic effect is proposed by employing a dielectric mirror and a ternary strategy to precisely tune the color perception as well as semitransparent organic solar cell (ST‐OSC) performance. It results in the highest efficiency reported for neutral‐color ST‐OSCs to date.

Abstract

Neutral‐colored semitransparent organic solar cells (ST‐OSCs) have attracted considerable attention owing to their unique application in no‐visual‐obstacle building‐integrated photovoltaics. Toward this promising potential application, a synergistic effect is first proposed by employing a dielectric mirror and ternary photoactive layer with near‐infrared absorption to tune the color perception as well as ST‐OSC performance precisely. As a result, a neutral‐color ST‐OSC with high average transmittance of over 21% is successfully constructed, and a remarkable color‐rendering index approaching 100 and high power conversion efficiency (PCE) of 9.37% are simultaneously achieved. To the best of our knowledge, this is the highest PCE reported for neutral‐color ST‐OSCs to date. Importantly, this synergistic effect is demonstrated to be a universal strategy that is not only suitable for various photoactive layer systems, but can also be implanted in flexible substrate. The resulting neutral‐color flexible ST‐OSCs also show a promising PCE of 8.76%.

Herein, significantly improved ambient operational stability, including air processability and long‐term stability in polymer‐polymer solar cells relative to polymer‐PCBM devices is demonstrated. It is shown that all‐polymer blends exhibit excellent stability, with an efficiency approaching 9% despite being processed under high‐humidity conditions. Additionally, the all‐polymer cell shows improved stability under thermal stress and ambient conditions without encapsulation.

Abstract

In this work, the way in which ambient moisture impacts the photovoltaic performance of conventional PCBM and emerging polymer acceptor–based organic solar cells is examined. The device performance of two representative p‐type polymers, PBDB‐T and PTzBI, blended with either PCBM or polymeric acceptor N2200, is systemically investigated. In both cases, all‐polymer photovoltaic devices processed from high‐humidity ambient conditions exhibit significantly enhanced moisture‐tolerance compared to their polymer–PCBM counterparts. The impact of moisture on the blend film morphology and electronic properties of the electron acceptor (N2200 vs PCBM), which results in different recombination kinetics and electron transporting properties, are further compared. The impact of more comprehensive ambient conditions (moisture, oxygen, and thermal stress) on the long‐term stability of the unencapsulated devices is also investigated. All‐polymer solar cells show stable performance for long periods of storage time under ambient conditions. The authors believe that these findings demonstrate that all‐polymer solar cells can achieve high device performance with ambient processing and show excellent long‐term stability against oxygen and moisture, which situate them in an advantageous position for practical large‐scale production of organic solar cells.