Yingzhi Jin

In this work, a new benzo[1,2-b:4,5-b′]dithiophene (BDT) building block containing alkylthio naphthyl as a side chain is designed and synthesized, and the resulting polymer, namely PBDTNS-BDD, shows a lower HOMO energy level than that of its alkoxyl naphthyl counterpart PBDTNO-BDD. An optimized photovoltaic device using PBDTNS-BDD as a donor exhibits power conversion efficiencies (PCE) of 8.70% and 9.28% with the fullerene derivative PC₇₁BM and the fullerene-free small molecule ITIC as acceptors, respectively. Surprisingly, ternary blend devices based on PBDTNS-BDD and two acceptors, namely PC₇₁BM and ITIC, shows a PCE of 11.21%, which is much higher than that of PBDTNO-BDD based ternary devices (7.85%) even under optimized conditions.

Ternary blend devices containing a new building block show much better power conversion efficiencies than their corresponding binary counterparts. These new building blocks contain alkylthio naphthyl as a side chain. Blending with two acceptors, PC₇₁BM and ITIC, results in PBDTNS-BDD devices with a power conversion efficiency of 11.21%, which is much higher than that of the PBDTNO-BDD (7.85%) analogue under optimized conditions.

Multijunction solar cells are designed to improve the overlap with the solar spectrum and to minimize losses due to thermalization. Aside from the optimum choice of photoactive materials for the respective sub-cells, a proper interconnect is essential. This study demonstrates a novel all-oxide interconnect based on the interface of the high-work-function (WF) metal oxide MoO_x and low-WF tin oxide (SnO_x). In contrast to typical p-/n-type tunnel junctions, both the oxides are n-type semiconductors with a WF of 5.2 and 4.2 eV, respectively. It is demonstrated that the electronic line-up at the interface of MoO_x and SnO_x comprises a large intrinsic interface dipole (≈0.8 eV), which is key to afford ideal alignment of the conduction band of MoO_x and SnO_x, without the requirement of an additional metal or organic dipole layer. The presented MoO_x/SnO_x interconnect allows for the ideal (loss-free) addition of the open circuit voltages of the two sub-cells.

A novel all-oxide recombination interconnect for organic tandem solar cells is reported. A large interface dipole between the high-work-function (WF) metal oxide MoO_x and low-WF tin oxide (SnO_x) affords ideal alignment of the conduction band of the two n-type metal oxides. The actual recombination of electrons with holes occurs at the interface of organic/MoO_x of the lower sub-cell.

The tunnel junction (TJ) intermediate connection layer (ICL), which is the most critical component for high-efficient tandem solar cell, generally consists of hole conducting layer and polyethyleneimine (PEI) polyelectrolyte. However, because of the nonconducting feature of pristine PEI, photocurrent is open-restricted in ICL even with a little thick PEI layer. Here, high-efficiency homo-tandem solar cells are demonstrated with enhanced efficiency by introducing carbon quantum dot (CQD)-doped PEI on TJ–ICL. The CQD-doped PEI provides substantial dynamic advantages in the operation of both single-junction solar cells and homo-tandem solar cells. The inclusion of CQDs in the PEI layer leads to improved electron extraction property in single-junction solar cells and better series connection in tandem solar cells. The highest efficient solar cell with CQD-doped PEI layer in between indium tin oxide (ITO) and photoactive layer exhibits a maximum power conversion efficiency (PCE) of 9.49%, which represents a value nearly 10% higher than those of solar cells with pristine PEI layer. In the case of tandem solar cells, the highest performing tandem solar cell fabricated with C-dot-doped PEI layer in ICL yields a PCE of 12.13%; this value represents an ≈15% increase in the efficiency compared with tandem solar cells with a pristine PEI layer.

High-efficiency homo-tandem solar cells with enhanced power conversion efficiency (PCE) are demonstrated by introducing carbon quantum dot (CQD)-doped PEI on a tunnel junction intermediate connection layer. The inclusion of CQDs in the PEI layer leads to improved electron extraction properties and better series connection. The best tandem solar cell fabricated with the CQD-doped PEI layer yields a PCE of 12.13%.

The use of a ternary component is a facile and effective method to further improve the device performances of binary organic solar cells (OSCs) comprising one donor and one acceptor. Recently, the rapid progress of highly efficient nonfullerene acceptor materials has offered a new opportunity for studying ternary OSCs because of the flexibility of ternary components, and the photovoltaic performance of ternary OSCs has been promoted quickly. In this research news article, some strategies for materials selection of the ternary components are concisely summarized and the recent progress in ternary OCSs based on nonfullerene acceptors, and the challenges and perspectives of ternary OSCs are also presented.

Fullerene-free ternary organic solar cells integrating complementary absorptions are greatly inspired by the prosperous development of nonfullerene acceptors with their great performance and abundant varieties. This research news demonstrates the main functions of third components and some design guidelines for ternary composition and summarizes the recent advanced progress for high-performance ternary organic solar cells based on nonfullerene acceptors.

The collection efficiency of photogenerated charges in polymer solar cells (PSCs) is strongly influenced by the built-in field (E_in) that develops across the photoactive materials. Here, by investigating the E_in-development regimes in PSCs by introducing two types of interlayers, electric dipole layers (EDLs) and charge transport layers (CTLs), the device architecture is optimized to result in a larger E_in. By incorporating a pair of EDLs on both sides of the photoactive layer, the E_in is modulated by shifting the vacuum energy at each metal–semiconductor interface, providing a larger E_in than that in conventional PSCs using typical CTLs, such as metal oxides and/or conducting polymers. These devices using paired EDLs exhibit an average PCE of 9.8%, which far surpasses the average PCE of ≈8.5% for paired CTLs.

Polymer solar cells with a new device architecture for reinforcing built-in electric field is demonstrated. A pair of strong electric dipole layers on both sides of the photoactive layer maximizes internal electric field in the operating state under short circuit condition, which permits an efficient “sweep out” of photo generated charges compared to those of typical charge transport layers.

Most nonfullerene acceptors developed so far for high-performance organic solar cells (OSCs) are designed in planar molecular geometry containing a fused-ring core. In this work, a new nonfullerene acceptor of DF-PCIC is synthesized with an unfused-ring core containing two cyclopentadithiophene (CPDT) moieties and one 2,5-difluorobenzene (DFB) group. A nearly planar geometry is realized through the F···H noncovalent interaction between CPDT and DFB for DF-PCIC. After proper optimizations, the OSCs with DF-PCIC as the acceptor and the polymer PBDB-T as the donor yield the best power conversion efficiency (PCE) of 10.14% with a high fill factor of 0.72. To the best of our knowledge, this efficiency is among the highest values for the OSCs with nonfullerene acceptors owning unfused-ring cores. Furthermore, no obvious morphological changes are observed for the thermally treated PBDB-T:DF-PCIC blended films, and the relevant devices can keep ≈70% of the original PCEs upon thermal treatment at 180 °C for 12 h. This tolerance of such a high temperature for so long time is rarely reported for fullerene-free OSCs, which might be due to the unique unfused-ring core of DF-PCIC. Therefore, the work provides new idea for the design of new nonfullerene acceptors applicable in commercial OSCs in the future.

A new nonfullerene acceptor (DF-PCIC) is designed and synthesized by utilizing noncovalent interactions. Organic solar cells (OSCs) with DF-PCIC as the acceptor exhibit the best efficiency of 10.14% with a high fill factor of 0.72. More importantly, excellent morphological stability is achieved for DF-PCIC-based devices, which is meaningful for the future practical applications of OSCs.

Two series of new polymers with medium and wide bandgaps to match fullerene (PC₇₁BM) and fullerene-free 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC) acceptors are designed and synthesized, respectively. For constructing the key donor building blocks, the effective symmetry-breaking strategy is employed. Two common aromatic rings (thiophene and benzene) are chosen as one side substituted groups in the asymmetric benzodithiophene (BDT) monomers. In addition, another rigid benzene ring is inserted between aryl and thioether in the side chains, which results in larger twisting and destroying the aggregation and forming longer lever arms. As a result, highly ordered polymers (PBDTsTh-FBT and PBDTsPh-FBT) with strong aggregation properties can blend well with roughly spherical PC₇₁BM, while amorphous polymers (PBDTsThPh-BDD and PBDTsPhPh-BDD) with long and rigid aryl rings show good miscibility with elongated ITIC, and finally, both devices exhibit excellent power conversion efficiencies over 10%. Thus, it clearly shows that the asymmetric BDT unit is an excellent donor building block to construct highly efficient photovoltaic polymers. Meanwhile, this work demonstrates that it is not necessary that high-performance fullerene-free polymer solar cells (PSCs) require highly ordered microstructures in the blending films, different from the fullerene-based PSCs.

Two series of new asymmetric benzodithiophene building block based polymers with medium and wide bandgaps to match fullerene and fullerene-free acceptors are designed and synthesized, respectively. The significant function of the benzene rings in controlling morphology is revealed, which would be a promising strategy to further design prospective light-harvesting polymers for high-performance polymer solar cells.

A new synthetic route, to prepare an alkylated indacenodithieno[3,2-b]thiophene-based nonfullerene acceptor (C8-ITIC), is reported. Compared to the reported ITIC with phenylalkyl side chains, the new acceptor C8-ITIC exhibits a reduction in the optical band gap, higher absorptivity, and an increased propensity to crystallize. Accordingly, blends with the donor polymer PBDB-T exhibit a power conversion efficiency (PCE) up to 12.4%. Further improvements in efficiency are found upon backbone fluorination of the donor polymer to afford the novel material PFBDB-T. The resulting blend with C8-ITIC shows an impressive PCE up to 13.2% as a result of the higher open-circuit voltage. Electroluminescence studies demonstrate that backbone fluorination reduces the energy loss of the blends, with PFBDB-T/C8-ITIC-based cells exhibiting a small energy loss of 0.6 eV combined with a high J_SC of 19.6 mA cm⁻².

The synthesis of a novel alkylated indacenodithioeno[3,2-b]thiophene (C₈-IDTT) based nonfullerene acceptor (C₈-ITIC), is reported. Compared to ITIC with phenylalkyl side chains, the acceptor exhibits a redshifted absorption with increased absorptivity. Solar cell power conversion efficiencies (PCEs) of up to 13.2 % are achieved, with the high PCE attributed to the broad absorption, high crystallinity of C8-ITIC and low voltage loss.

The commercialization of nonfullerene organic solar cells (OSCs) critically relies on the response under typical operating conditions (for instance, temperature and humidity) and the ability of scale-up. Despite the rapid increase in power conversion efficiency (PCE) of spin-coated devices fabricated in a protective atmosphere, the efficiencies of printed nonfullerene OSC devices by blade coating are still lower than 6%. This slow progress significantly limits the practical printing of high-performance nonfullerene OSCs. Here, a new and relatively stable nonfullerene combination is introduced by pairing the nonfluorinated acceptor IT-M with the polymeric donor FTAZ. Over 12% efficiency can be achieved in spin-coated FTAZ:IT-M devices using a single halogen-free solvent. More importantly, chlorine-free, blade coating of FTAZ:IT-M in air is able to yield a PCE of nearly 11% despite a humidity of ≈50%. X-ray scattering results reveal that large π–π coherence length, high degree of face-on orientation with respect to the substrate, and small domain spacing of ≈20 nm are closely correlated with such high device performance. The material system and approach yield the highest reported performance for nonfullerene OSC devices by a coating technique approximating scalable fabrication methods and hold great promise for the development of low-cost, low-toxicity, and high-efficiency OSCs by high-throughput production.

A new nonfullerene combination composed of a high-performance polymer and a nonfluorinated small molecule is presented. It holds great potential for additive-free and halogen-free processing. Small and pure domains and face-on molecular packing collectively enable the first demonstration of ≈11% efficiency air-processed and stable nonfullerene solar cells by blade-coating techniques. Additionally, complete solvent–morphology–performance relations are established for further improvements.

Recently, a new type of active layer with a ternary system has been developed to further enhance the performance of binary system organic photovoltaics (OPV). In the ternary OPV, almost all active layers are formed by simple ternary blend in solution, which eventually leads to the disordered bulk heterojunction (BHJ) structure after a spin-coating process. There are two main restrictions in this disordered BHJ structure to obtain higher performance OPV. One is the isolated second donor or acceptor domains. The other is the invalid metal–semiconductor contact. Herein, the concept and design of donor/acceptor/acceptor ternary OPV with more controlled structure (C-ternary) is reported. The C-ternary OPV is fabricated by a sequential solution process, in which the second acceptor and donor/acceptor binary blend are sequentially spin-coated. After the device optimization, the power conversion efficiencies (PCEs) of all OPV with C-ternary are enhanced by 14–21% relative to those with the simple ternary blend; the best PCEs are 10.7 and 11.0% for fullerene-based and fullerene-free solar cells, respectively. Moreover, the averaged PCE value of 10.4% for fullerene-free solar cell measured in this study is in great agreement with the certified one of 10.32% obtained from Newport Corporation.

The concept and design of ternary organic photovoltaics with a more controlled structure via sequential solution process is reported. The power conversion efficiencies of all four organic photovoltaics (fullerene-based or fullerene-free) with this structure are enhanced by 14–21% relative to those with simple ternary blend.

Ternary organic solar cells (OSCs) have attracted much research attention, as they can maintain the simplicity of the single-junction device architecture while broadening the absorption range of OSCs. However, one main challenge that limits the development of ternary OSCs is the difficulty in controlling the morphology of ternary OSCs. In this paper, an effective approach to control the morphology is presented that leads to multiple cases of efficient nonfullerene ternary OSCs with efficiencies of up to 11.2%. This approach is based on a donor polymer with strong temperature dependent aggregation properties processed from hot solutions without any solvent additives and a pair of small molecular acceptors (SMAs) that have similar surface tensions and thus low propensity to form discrete phases. Such a ternary blend exhibits a simplified bulk-heterojunction morphology that is similar to the morphology of previously reported binary blends. As a result, an almost linear relationship between V_OC and film composition is observed for all nonfullerene ternary devices. Meanwhile, by carefully designing a control system with a large interfacial tension, a different phase separation and V_OC dependence is demonstrated. This morphology control approach can be applicable to more material systems and accelerates the development of the ternary OSC field.

Multiple cases of efficient nonfullerene ternary organic solar cells with efficiencies of up to 11.2% based on an effective morphology-control approach are presented. This approach is based on a donor polymer with strong temperature dependent aggregation and a pair of small molecular acceptors with similar surface tensions. Increased crystallinity of the acceptor phase is observed and discussed.

Mimicking human skin's functions to develop electronic skins has inspired tremendous efforts in design and synthesis of novel soft materials with simplified fabrication methods. However, it still remains a great challenge to develop electronically conductive materials that are both stretchable and self-healable. Here it is demonstrated that a ternary polymer composite comprised of polyaniline, polyacrylic acid, and phytic acid can exhibit high stretchability (≈500%) and excellent self-healing properties. The polymer composite with optimized composition shows an electrical conductivity of 0.12 S cm⁻¹. On rupture, both electrical and mechanical properties can be restored with ≈99% efficiency in a 24 h period, which is enabled by the dynamic hydrogen bonding and electrostatic interactions. It is further shown that this composite is both strain and pressure sensitive, and therefore can be used for fabricating strain and pressure sensors to detect a variety of mechanical deformations with ultrahigh sensitivity. The sensitivity and sensing range are the highest among all of the reported self-healable piezoresistive pressure sensors and even surpass most flexible mechanical sensors. Notably, this composite is prepared via a solution casting process, which potentially allows for large-area, low-cost fabrication electronic skins.

Artificial skin: mimicking human skin's functions to develop skin-like electronics has inspired tremendous efforts in developing novel soft materials. It is shown that a ternary polymer composite comprised of polyaniline, polyacrylic acid, and phytic acid can exhibit high stretchability (≈500%) and excellent self-healing properties for electronic skin applications with ultrahigh sensitivity.

Achieving efficient bulk-heterojunction (BHJ) solar cells from blends of solution-processable small-molecule (SM) donors and acceptors is proved particularly challenging due to the complexity in obtaining a favorable donor–acceptor morphology. In this report, the BHJ device performance pattern of a set of analogous, well-defined SM donors—DR3TBDTT (DR3), SMPV1, and BTR—used in conjunction with the SM acceptor IDTTBM is examined. Examinations show that the nonfullerene “All-SM” BHJ solar cells made with DR3 and IDTTBM can achieve power conversion efficiencies (PCEs) of up to ≈4.5% (avg. 4.0%) when the solution-processing additive 1,8-diiodooctane (DIO, 0.8% v/v) is used in the blend solutions. The figures of merit of optimized DR3:IDTTBM solar cells contrast with those of “as-cast” BHJ devices from which only modest PCEs <1% can be achieved. Combining electron energy loss spectrum analyses in scanning transmission electron microscopy mode, carrier transport measurements via “metal-insulator-semiconductor carrier extraction” methods, and systematic recombination examinations by light-dependence and transient photocurrent analyses, it is shown that DIO plays a determining role—establishing a favorable lengthscale for the phase-separated SM donor–acceptor network and, in turn, improving the balance in hole/electron mobilities and the carrier collection efficiencies overall.

A set of structurally analogous small-molecule (SM) donors with distinct side-chain manifolds shows significant differences in their performance patterns in bulk-heterojunction (BHJ) devices with the nonfullerene SM acceptor IDTTBM. Reducing the lengthscale of the phase-separated network between donor and acceptor effectively suppresses nongeminate recombination in the BHJ active layers and improves the carrier mobility balance.

In this work, room-temperature-operated ultrasensitive solution-processed perovskite photodetectors (PDs) with near infrared (NIR) photoresponse are reported. In order to enable perovskite PDs possessing extended NIR photoresponse, novel n-type low bandgap conjugated polymer, poly[(N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalene diimide-2,6-diyl) (2,5-dioctyl-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione-5,5′-diyl)] (NDI-DPP), which has strong absorption in the NIR region, is developed and then employed in perovskite PDs. By the formation of type II band alignment between NDI-DPP with single-wall carbon nanotubes (SWCNTs), the NIR absorption of NDI-DPP is exploited, which contributes to the NIR photoresponse for the perovskite PDs, where perovskite is incorporated with NDI-DPP and SWCNTs as well. In addition, SWCNTs incorporated with perovskite active layer can offer the percolation pathways for high charge-carrier mobility, which tremendously boosts the charge transfer in the photoactive layer, and consequently improves the photocurrent in the visible region. As a result, the perovskite PDs exhibit the responsivities of ≈400 and ≈150 mA W⁻¹ and the detectivities of over 6 × 10¹² Jones (1 Jones = 1 cm Hz^1/2 W⁻¹) and over 2 × 10¹² Jones in the visible and NIR regions, respectively. This work reports the development of perovskite PDs with NIR photoresponse, which is terrifically beneficial for the practical applications of perovskite PDs.

Room temperature operated uncooled broadband ultrasensitive photodetectors with the responsivities of 400 and 150 mA W^-1 and the detectivities of over 6 × 10¹² and 2 × 10¹² Jones in the visible and near infrared regions are realized by utilization of perovskite incorporated with novel n-type low-bandgap conjugated polymer and single-wall carbon nanotubes through type II band alignment.