Electron transport layers (ETLs), e.g., metal oxides, organic small molecules, or conjugated polymers, play a vital role in both performance and photo-thermal stability in organic solar cells (OSCs). Herein, we explored hybrid organic-inorganic electron transport materials by forming complexes between typical electron transport layers and ferrocene (Fc)-based molecules. Experimental and theoretical investigations revealed van der Waals interaction between the ETL and Fc compounds, which allows fine-tuning of the electrode work function to improve charge extraction properties and reduce trap-assisted recombination. As a result, OSCs showed improved fill factor (FF) and power conversion efficiency (PCE) for five donor-acceptor blends and three ETLs, with FF and PCE exceeding 80% and 20.1%, respectively. Finally, we demonstrated improved photostability for the hybrid ETLs with devices that retained 80% of their initial performance for 700 h when degraded under operating conditions (ISOS-L-1I).

Three new donor-acceptor (D-A) conjugated polymers based on benzo[1,2-b:4,5-b ']dithiophene (BDT) and benzo [c][1,2,5]thiadiazole (BT), namely, PB-BT, PB-BTf and PB-BTPf, were synthesized with different substituents (Hydrogen atoms, Fluorine atoms and fluorophenyl groups) on BT unit. Experimental results and theoretical calculations indicate that molecularly tuning of side chains on BT moeity simultaneously influences the energy levels and intermolecular packing of the resulted polymers by modulating their electron affinity and molecular coplanarity. The polymer solar cells (PSCs) based on a blend of PB-BTf/[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) exhibits the best photovoltaic performance among the three copolymers, with a open-circuit voltage (Voc) of 0.79 V and a power conversion efficiency (PCE) of 8.43%. Furthermore, polymer PB-BTPf containing fluorophenyl groups shows a higher Voc of 0.99 V due to its more low-lying highest occupied molecular orbital (HOMO) compared with other two structural polymeric analogues. The results here provide further fundamental insights into the relationship between the fluorination on electron-withdrawing moiety and the photovoltaic performance for the conjugated polymers applied in fullerene organic solar cells.

Currently, high-performance organic solar cells (OSCs) are mainly composed of narrow band gap (NBG) nonfullerene acceptors and medium band gap (MBG) polymer donors, whereas the solar cells based on NBG polymer donors and MBG nonfullerene acceptors were much less successful. Herein, we report a new diketopyrrolopyrrole (DPP) polymer (PffBT-DPP) with a band gap of 1.33 eV for use in nonfullerene OSCs. When blended with a fullerene acceptor [6, 6]-phenyl C71 butyric acid methyl ester ([70]PCBM) and an MBG nonfullerene acceptor MeIC, the binary OSCs offered a power conversion efficiency (PCE) of 6.8 and 2.0%, respectively. Interestingly, a much higher PCE of 9.0% was achieved for the PffBT-DPP/[70]PCBM/MeIC ternary OSC, which is the sum of the two binary OSCs. The phase-separated morphology in this ternary OSC was characterized and correlated to the device performance. Moreover, the PCE of the ternary OSC is the best result for OSCs employing polymer donors with an optical band gap of <1.40 eV and nonfullerene acceptors. This work demonstrates the potential of constructing high-performance OSCs by employing NBG polymer donors and MBG nonfullerene acceptors.

A new polymer donor based on 3,3′-difluoro-2,2′-bithiophene (2F2T) and difluorobenzoxadiazole (ffBX), named 2F2T-ffBX, is designed and synthesized. The organic solar cell (OSC) based on 2F2T-ffBX donor and [6,6]-phenyl-C60-butyl acid methyl ester ([60]PCBM) acceptor exhibits a high efficiency of 7.3% with a high open-circuit voltage (Voc) of 1.03 V. When blended with perylenediimide-based acceptor (PDI6), the corresponding OSC shows a higher Voc of 1.19 V with a low energy loss of 0.50 eV but a much lower efficiency of 2.0%. The detailed analyses including charge generation, transport, recombination properties, and morphology were performed to understand the performance of corresponding devices.

Fine energy level modulation without negatively affecting other properties is realized for random conjugated copolymers, allowing rigorous investigation of the relationship of the chemical structure and device performance in solar cells.

An alkyl thiophene unit was employed for the first time as a side chain substituent on an asymmetric benzodithiophene (BDT) building block in the design of novel light-harvesting polymers.

Organometal halide perovskite (CH3NH3PbI3) solar cells with excellent photovoltaic performance have obtained great attention. In this work, conductive atomic force microscopy is used to investigate the conduction mechanism of perovskite film, and the results clearly show that grain boundaries are beneficial to the charge transport in perovskite solar cells. However, there are large gaps between grains in some as-prepared perovskite films, and the related grain boundaries or grains have poor charge transport capability, which leads to undesirable photovoltaic performance. After iso-propyl alcohol treatment, the charge transport capabilities of both grain boundaries and grains are improved. The power conversion efficiency of related device using PEDOT:PSS as hole transport material is increased from 13.72% to 15.65% with concurrently improved open circuit voltage, short circuit current density and fill factor. This research addresses that grain boundaries are quite important and proper post modification is an effective way to improve the efficiency of planar perovskite solar cells.

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.

Interfacial buffer layer is a primary requirement for highly efficient and stable organic solar cells (OSCs) and has become a significant part of OSC research today. Here, highly dispersed functionalized reduced graphene oxide (FGr) was obtained as a stable electron buffer layer (EBL) for high-performance conventional solar cells. The power conversion efficiency of FGr-based devices (9.47% for PTB7-Th:PC71BM and 4.05% for P3HT:PC71BM) outperforms PFN-based devices (8.94% and 3.52%). Furthermore, the stability of the devices was greatly improved with FGr as EBL compared to PFN, and only dropped 7.4% of its original value after stored for 61 days. The results indicate that solution-processed functionalized reduced graphene oxide is a promising interfacial material and an excellent candidate as electron buffer layer.

One of the most important factors that limits the efficiencies of bulk‐heterojunction organic solar cells (OSCs) is the modest open‐circuit voltage (Voc) due to their large voltage loss (Vloss) caused by significant nonradiative recombination loss. To boost the performance of OSCs toward their theoretical limit, developing high‐performance donor: acceptor systems featuring low Vloss with suppressed nonradiative recombination losses (<0.30 V) is desired. Herein, high performance OSCs based on a polymer donor benzodithiophene‐difluorobenzoxadiazole‐2‐decyltetradecyl (BDT‐ffBX‐DT) and perylenediimide‐based acceptors (PDI dimer with spirofluorene linker (SFPDI), PDI4, and PDI6) are reported which offer a high power conversion efficiency (PCE) of 7.5%, 56% external quantum efficiency associated with very high Voc (>1.10 V) and low Vloss (<0.60 V). A high Voc up to 1.23 V is achieved, which is among the highest values reported for OSCs with a PCE beyond 6%, to date. These attractive results are benefit from the suppressed nonradiative recombination voltage loss, which is as low as 0.20 V. This value is the lowest value for OSCs so far and is comparable to high performance crystalline silicon and perovskite solar cells. These results show that OSCs have the potential to achieve comparable Voc and voltage loss as inorganic photovoltaic technologies.

Two novel copolymer donors based on one-dimensional (1D)-two-dimensional (2D) asymmetrical benzodithiophene (BDT) units (BDTPH-H and BDTPH-OR) and 2,3-diphenyl-5,8-di(thiophen-2-yl) quinoxaline (DTQx) were synthesized and compared with one-dimensional or two-dimensional symmetric BDT unit based photovoltaic polymers. Both asymmetrical polymers exhibited promising photovoltaic performance compared with 1D and 2D symmetric BDT polymers. The power conversion efficiency (PCE) of PBDTPH-DTQx based polymer solar cells is 5.6%, with balanced V-OC = 0.7 V, J(SC) = 11.89 mA cm(-2) and FF = 67.3%, which is almost the highest PCE compared with similar BDT unit and fluorine-free substituted DTQx based polymer. What is more, PBDTPH-DTQx shows better performance than PBDTPHO-DTQx due to the single phenyl making DTQx more planar and with better pi-pi stacking than alkoxy phenyl. These findings demonstrate that the 1D-2D asymmetrical BDT units are also applicable to poor planarity fluorine-free substituted quinoxaline acceptor system.

The micro-structure ordering of the π–conjugated polymer film self-assembling from solution governs its electrical characteristics and performances for organic electronics. The side chains appended on the conjugated backbone could impact the self-assembling characteristics of the polymer, which show us how to design novel substituents to optimize the polymer arrangement and packing in its solid state. Here, cyclic alkyl chains are proven to be excellent ones for designing of narrow bandgap polymers, the design feature of which with preferential conformations can promote the polymer self-assembly and result in higher degree of lamellar order as well as tighter lamellar packing in the active layer, comparing with linear alkyl chains. The linear ones have poorer ordering in the polymer:fullerene blends as well as in the single crystals of the monomers. The well-organized polymer micro-structures facilitate polymer:fullerene phase separation together with balanced hole/electron transporting, and ultimately, improve the power conversion efficiency impressively compared to linear chains.

In this paper, we reported a crystalline p-type medium-bandgap conjugated D-A polymer asy-PBDBTN based on a symmetry-breaking-modified BDT moiety to combine the advantages of both one-dimension (1D) and two-dimension (2D) symmetric BDTs. Polymer asy-PBDBTN is a highly efficient light-harvesting donor material. Single BHJ PSCs exhibit PCE of 8.88% with PC71BM as acceptor. Also, PCE values of 10.50% are achieved with the use of ITIC as an acceptor to couple asy-PBDBTN with V-oc of 0.942 V, J(SC) of 16.81 mA cm(-2), and FF of 0.663. It is worth noting that lower energy loss is obtained in fullerene-free-based PSCs, which is essential to overcome the trade-off between Voc and J(SC) and boost these two parameters simultaneously for high photovoltaic performance. The combination process of additive and thermal annealing is critical to enhance and retain the pi-pi stacking behavior of donor and fullerene-free acceptor; as a result, the trap-assisted recombination was greatly suppressed. This work demonstrates a great prospect for the construction of the symmetry-breaking BDT-based D-A conjugated polymers toward high-performance PSCs, especially with fullerene-free acceptor material.

Terpolymer systems were realized as a good strategy to combine two incompatible polymers as compared to ternary systems.

A thieno[3,4-b]thibphene-based donor polymer PTBTz-2 was employed to construct fullerene-free solar cell with the classical acceptor ITIC. Interestingly, due to the high extinction coefficients and wide absorption for these two materials, the active layer can harvest a larger fraction of the coverage solar spectrum even in ultrathin film. Furthermore, the simultaneous advantages of appropriate cascade energy level, well-balanced hole/electron mobility (mu(h)/mu(e) = 1.16), and low charge accumulation and recombination, make the PTBTz-2/ITIC-based solar cells exhibit an excellent power conversion efficiency of 10.92% with large short circuit current density of 20.34 mA cm(-2). The results indicate that fine-tailored thieno [3,4-b]thiophene-based polymers would be another type of promising donor materials, except for widely reported efficient benzo[d][1,2,3]triazole (BTA)- or benzo[1,2-c:4,5-c']-dithiophene-4,8-dione (BDD)-based polymers, and would enrich the reservoir of high-performance light-harvesting conjugated polymers.

To advance polymer solar cells (PSCs) toward real-world applications, it is crucial to develop materials that are compatible with a low-cost large-scale manufacturing technology. In this context, a practically useful polymer should fulfill several critical requirements: the capability to provide high power conversion efficiencies (PCEs) via low-cost fabrication using environmentally friendly solvents under mild thermal conditions, resulting in an active layer that is thick enough to minimize defects in large-area films. Here, the development of new photovoltaic polymers is reported through rational molecular design to meet these requirements. Benzodithiophene (BDT)-difluorobenzoxa-diazole (ffBX)-2-decyltetradecyl (DT), a wide-bandgap polymer based on ffBX and BDT emerges as the first example that fulfills the qualifications. When blended with a low-cost acceptor (C60-fullerene derivative), BDT-ffBX-DT produces a PCE of 9.4% at active layer thickness over 250 nm. BDT-ffBX-DT devices can be fabricated from nonhalogenated solvents at low processing temperature. The success of BDT-ffBX-DT originates from its appropriate electronic structure and charge transport characteristics, in combination with a favorable face-on orientation of the polymer backbone in blends, and the ability to form proper phase separation morphology with a fibrillar bicontinuous interpenetrating network in bulk-heterojunction films. With these characteristics, BDT-ffBX-DT represents a meaningful step toward future everyday applications of polymer solar cells.

Three perylenediimide (PDI) acceptors (P2O2, P2N2 and P4N4) were synthesized by functionalizing the bay positions of PDI with benzil, 2,3-diphenylquinoxaline and 2,3,7,8-tetraphenylpyrazino[2,3-g]quinoxaline as linkers, respectively. The photovoltaic properties of the three acceptor molecules have been investigated. The different PDI linker units show different physical and chemical properties of the PDIs. The three PDIs display different non-planar geometrical structures because of the different linker units, which affect the corresponding morphology of the blend films and also influence the charge mobility and fill factor (FF) of the organic solar cells (OSCs). Furthermore, the gradient energy levels of the three PDIs provide an efficient research model for the relationship of device open-circuit voltage (Voc) and energy levels. As the result, the P4N4 based non-fullerene devices show the best photovoltaic performance with a power conversion efficiency (PCE) of 5.71%, whereas the P2O2 and P2N2 based non-fullerene devices show relatively lower PCEs of 2.53% and 3.86%, respectively.

The new strategy, employing thiazole as a π bridge into the backbone of quinoid polymers, enhanced the VOC and photovoltaic performance.

It has been proven that the introduction of F atom and the replacement of alkyl side-chain with alkylthio substituent could be the effective side-chain strategies, to obtain the deep highest occupied molecular orbital (HOMO) energy level with little influence on the optical absorption, and in turn, the small photo energy loss (Eloss). In this work, we combine the advantages of D–A(π)–Q–A(π) strategy and the above side chain engineering, to construct a series of high-performance polymers P1-P3 with high open-circuit voltage (VOC) of exceeding 0.90V in the conventional solar cells. Meanwhile, small Elosss of 0.73–0.78eV are achieved accompanying high VOC and almost unchanged optical bandgaps of ~ 1.70eV, which are the smaller values in comparison with that of the other high-performance polymer systems. More encouragingly, P2-based solar cells exhibit high PCE of 10.30% and VOC of 0.97V, which is one of the highest values for polymer/fullerene based solar cells. Our work not only demonstrates a series of high-efficiency new materials, but also strongly confirms that the combination of D–A(π)–Q–A(π) arrangement and rational side chain engineering is a very promising strategy to construct high-performance photovoltaic polymers with reduced energy loss.

Interface engineering is an important aspect for the improvement of perovskite solar cells (PVSCs). The hole transport layer with good interface contact, transport capability, and matched energy level is indispensable and critical for high‐performance photovoltaic devices. Herein, anode interface engineering with an excellent compatible bilayer of poly(3,4‐ethylene dioxythiophene):poly(styrenesulfo‐nate)/poly(3,4‐ethylene dioxythiophene) (PEDOT:PSS/PEDOT) doped with grafted sulfonated‐acetone–formaldehyde lignin (PEDOT:GSL) via a low‐temperature and water‐soluble process is presented. As a water‐processable interface material, PEDOT:GSL exhibits higher conductivity, as well as better structural and electronic homogeneities compared with PEDTO:PSS. Consequently, the PEDOT:PSS/PEDOT:GSL bilayer with tuned energy level, optical properties, and the combination of the trap passivation of GSL at the anode/perovskite interface can greatly improve charge extraction ability and reduce the interface recombination. Simultaneously, the homogeneous perovskite film is fabricated through optimizing the annealing process. The device with the power conversion efficiency up to 17.80% is achieved, with 32.6% improvement compared to PEDOT:PSS‐only device (13.42%). Our success to achieve high‐performance inverted PVSCs provides new understanding of PEDOT:PSS, and also new guidelines for anode interface engineering to further advancement of PVSCs. This promising approach paves the way to realize solution processable highly efficient PVSCs for potential practical applications.