The open-circuit voltage of organic solar cells is usually lower than the values achieved in inorganic or perovskite photovoltaic devices with comparable bandgaps. Energy losses during charge separation at the donor-acceptor interface and non-radiative recombination are among the main causes of such voltage losses. Here we combine spectroscopic and quantum-chemistry approaches to identify key rules for minimizing voltage losses: (1) a low energy offset between donor and acceptor molecular states and (2) high photoluminescence yield of the low-gap material in the blend. Following these rules, we present a range of existing and new donor-acceptor systems that combine efficient photocurrent generation with electroluminescence yield up to 0.03%, leading to non-radiative voltage losses as small as 0.21 V. This study provides a rationale to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.

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Low energy loss and efficient charge separation under small driving forces are the prerequisites for realizing high power conversion efficiency (PCE) in organic photovoltaics (OPVs). Here, a new molecular design of nonfullerene acceptors (NFAs) is proposed to address above two issues simultaneously by introducing asymmetric terminals. Two NFAs, BTP‐S1 and BTP‐S2, are constructed by introducing halogenated indandione (A1) and 3‐dicyanomethylene‐1‐indanone (A2) as two different conjugated terminals on the central fused core (D), wherein they share the same backbone as well‐known NFA Y6, but at different terminals. Such asymmetric NFAs with A1‐D‐A2 structure exhibit superior photovoltaic properties when blended with polymer donor PM6. Energy loss analysis reveals that asymmetric molecule BTP‐S2 with six chlorine atoms attached at the terminals enables the corresponding devices to give an outstanding electroluminescence quantum efficiency of 2.3 × 10−2%, one order of magnitude higher than devices based on symmetric Y6 (4.4 × 10−3%), thus significantly lowering the nonradiative loss and energy loss of the corresponding devices. Besides, asymmetric BTP‐S1 and BTP‐S2 with multiple halogen atoms at the terminals exhibit fast hole transfer to the donor PM6. As a result, OPVs based on the PM6:BTP‐S2 blend realize a PCE of 16.37%, higher than that (15.79%) of PM6:Y6‐based OPVs. A further optimization of the ternary blend (PM6:Y6:BTP‐S2) results in a best PCE of 17.43%, which is among the highest efficiencies for single‐junction OPVs. This work provides an effective approach to simultaneously lower the energy loss and promote the charge separation of OPVs by molecular design strategy.

A series of "double-cable" conjugated polymers were developed for application in efficient single-component polymer solar cells, in which high quantum efficiencies could be achieved due to the optimized nanophase separation between donor and acceptor parts. The new double-cable polymers contain electron-donating poly(benzodithiophene) (BDT) as linear conjugated backbone for hole transport and pendant electron-deficient perylene bisimide (PBI) units for electron transport, connected via a dodecyl linker. Sulfur and fluorine substituents were introduced to tune the energy levels and crystallinity of the conjugated polymers. The double-cable polymers adopt a "face-on" orientation in which the conjugated BDT backbone and the pendant PBI units have a preferential π-π stacking direction perpendicular to the substrate, favorable for interchain charge transport normal to the plane. The linear conjugated backbone acts as a scaffold for the crystallization of the PBI groups, to provide a double-cable nanophase separation of donor and acceptor phases. The optimized nanophase separation enables efficient exciton dissociation as well as charge transport as evidenced from the high-up to 80%-internal quantum efficiency for photon-to-electron conversion. In single-component organic solar cells, the double-cable polymers provide power conversion efficiency up to 4.18%. This is one of the highest performances in single-component organic solar cells. The nanophase-separated design can likely be used to achieve high-performance single-component organic solar cells.

The current work reports a high power conversion efficiency (PCE) of 9.54% achieved with nonfullerene organic solar cells (OSCs) based on PTB7‐Th donor and 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) acceptor fabricated by doctor‐blade printing, which has the highest efficiency ever reported in printed nonfullerene OSCs. Furthermore, a high PCE of 7.6% is realized in flexible large‐area (2.03 cm2) indium tin oxide (ITO)‐free doctor‐bladed nonfullerene OSCs, which is higher than that (5.86%) of the spin‐coated counterpart. To understand the mechanism of the performance enhancement with doctor‐blade printing, the morphology, crystallinity, charge recombination, and transport of the active layers are investigated. These results suggest that the good performance of the doctor‐blade OSCs is attributed to a favorable nanoscale phase separation by incorporating 0.6 vol% of 1,8‐diiodooctane that prolongs the dynamic drying time of the doctor‐bladed active layer and contributes to the migration of ITIC molecules in the drying process. High PCE obtained in the flexible large‐area ITO‐free doctor‐bladed nonfullerene OSCs indicates the feasibility of doctor‐blade printing in large‐scale fullerene‐free OSC manufacturing. For the first time, the open‐circuit voltage is increased by 0.1 V when 1 vol% solvent additive is added, due to the vertical segregation of ITIC molecules during solvent evaporation.

A free‐standing high‐output power density polymeric thermoelectric (TE) device is realized based on a highly conductive (≈2500 S cm−1) structure‐ordered poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate film (denoted as FS‐PEDOT:PSS) with a Seebeck coefficient of 20.6 µV K−1, an in‐plane thermal conductivity of 0.64 W m−1 K−1, and a peak power factor of 107 µW K−2 m−1 at room temperature. Under a small temperature gradient of 29 K, the TE device demonstrates a maximum output power density of 99 ± 18.7 µW cm−2, which is the highest value achieved in pristine PEDOT:PSS based TE devices. In addition, a fivefold output power is demonstrated by series connecting five devices into a flexible thermoelectric module. The simplicity of assembling the films into flexible thermoelectric modules, the low out‐of‐plane thermal conductivity of 0.27 W m−1 K−1, and free‐standing feature indicates the potential to integrate the FS‐PEDOT:PSS TE modules with textiles to power wearable electronics by harvesting human body's heat. In addition to the high power factor, the high thermal stability of the FS‐PEDOT:PSS films up to 250 °C is confirmed by in situ temperature‐dependent X‐ray diffraction and grazing incident wide angle X‐ray scattering, which makes the FS‐PEDOT:PSS films promising candidates for thermoelectric applications.

Organic photovoltaic cells (OPVs) have attracted broad attention and become a very energetic field after the emergence of nonfullerene acceptors. Long-lifetime triplet excitons are expected to be good candidates for efficiently harvesting a photocurrent. Parallel with the development of OPVs based on singlet materials (S-OPVs), the potential of triplet materials as photoactive layers is explored. However, so far, OPVs employing triplet materials in a bulk-heterojunction have not exhibited better performance than S-OPVs. Here, the recent progress of representative OPVs based on triplet materials (T-OPVs) is briefly summarized. Based on that, the performance limitations of T-OPVs are analyzed. The shortage of desired triplet materials with favourable optoelectronic properties for OPVs, the tradeoff between long lifetime and high binding energy of triplet excitons, as well as the low charge mobility in most triplet materials are crucial issues restraining the efficiencies of T-OPVs. To overcome these limitations, first, novel materials with desired optoelectronic properties are urgently demanded; second, systematic investigation on contribution and dynamics of triplet excitons in T-OPVs is necessary; third, close multidisciplinary collaboration is required, as proved by the development of S-OPVs.

The first diketopyrrolopyrrole based donor–acceptor macrocyclic conjugated molecule was developed and its application in organic electronics was systematically studied.