Yingzhi Jin

Flexible and semitransparent organic solar cells (OSCs) have been regarded as the most promising photovoltaic devices for the application of OSCs in wearable energy resources and building-integrated photovoltaics. Therefore, the flexible and semitransparent OSCs have developed rapidly in recent years through the synergistic efforts in developing novel flexible bottom or top transparent electrodes, designing and synthesizing high performance photoactive layer and low temperature processed electrode buffer layer materials, and device architecture engineering. To date, the highest power conversion efficiencies have reached over 10% of the flexible OSCs and 7.7% with average visible transmittance of 37% for the semitransparent OSCs. Here, a comprehensive overview of recent research progresses and perspectives on the related materials and devices of the flexible and semitransparent OSCs is provided.

Flexible and semitransparent organic solar cells (OSCs) are regarded as the most promising photovoltaic devices for the application of OSCs in wearable energy resources and building-integrated photovoltaics. Here, a comprehensive overview of recent research progresses and perspectives on the related materials and devices of the flexible and semitransparent OSCs is provided.

For the commercial development of organic photovoltaics (OPVs), laboratory-scale OPV technology must be translated to large area modules. In particular, it is important to develop high-efficiency polymers that can form thick (>100 nm) bulk heterojunction (BHJ) films over large areas with optimal morphologies for charge generation and transport. Here, D₁-A-D₂-A random terpolymers composed of 2,2′-bithiophene with various proportions of 5,6-difluoro-4,7-bis(thiophen-2-yl)-2,1,3-benzothiadiazole and 5,6-difluoro-2,1,3-benzothiadiazole (FBT) are synthesized. It is found that incorporating small proportions of FBT into the polymer not only conserves the high crystallinity and favorable face-on orientation of the D-A copolymer FBT-Th4 but also improves the nanoscale phase separation of the BHJ film. Consequently, the random terpolymer PDT2fBT-BT10 exhibits a much improved solar cell efficiency of 10.31% when compared to that of the copolymer FBT-Th4 (8.62%). Moreover, due to this polymer's excellent processability and suppressed overaggregation, OPVs with 1 cm² active area based on 351 nm thick PDT2fBT-BT10 BHJs exhibit high photovoltaic performance of 9.42%, whereas rapid efficiency decreases arise for FBT-Th4-based OPVs for film thicknesses above 300 nm. It is demonstrated that this random terpolymer can be used in large area and thick BHJ OPVs, and guidelines for developing polymers that are suitable for large-scale printing technologies are presented.

D₁-A-D₂-A-type random terpolymers and organic photovoltaics (OPVs) are developed introducing that the resulting polymer achieves a high efficiency of 10.31%. Furthermore, reproducibility of 1 cm² OPVs shows a high efficiency up to 9.42% using thick active layers in the range of 250–380 nm.

Side-chain engineering is an important strategy for optimizing photovoltaic properties of organic photovoltaic materials. In this work, the effect of alkylsilyl side-chain structure on the photovoltaic properties of medium bandgap conjugated polymer donors is studied by synthesizing four new polymers J70, J72, J73, and J74 on the basis of highly efficient polymer donor J71 by changing alkyl substituents of the alkylsilyl side chains of the polymers. And the photovoltaic properties of the five polymers are studied by fabricating polymer solar cells (PSCs) with the polymers as donor and an n-type organic semiconductor (n-OS) m-ITIC as acceptor. It is found that the shorter and linear alkylsilyl side chain could afford ordered molecular packing, stronger absorption coefficient, higher charge carrier mobility, thus results in higher J_sc and fill factor values in the corresponding PSCs. While the polymers with longer or branched alkyl substituents in the trialkylsilyl group show lower-lying highest occupied molecular orbital energy levels which leads to higher V_oc of the PSCs. The PSCs based on J70:m-ITIC and J71:m-ITIC achieve power conversion efficiency (PCE) of 11.62 and 12.05%, respectively, which are among the top values of the PSCs reported in the literatures so far.

Side-chain engineering is performed to optimize photovoltaic properties of the 2D-conjugated polymer donors. The polymer solar cells with m-ITIC as acceptor and J70 and J71 polymer donors with shorter and linear alkyl substituents in their alkylsilyl side chains achieve power conversion efficiency of 11.62% and 12.05%, respectively.

Based on the most recently significant progress within the last one year in organic photovoltaic research from either alkylthiolation or fluorination on benzo[1,2-b:4,5-b′]dithiophene moiety for high efficiency polymer solar cells (PSCs), two novel simultaneously fluorinated and alkylthiolated benzo[1,2-b:4,5-b′] dithiophene (BDT)-based donor–acceptor (D–A) polymers, poly(4,8-bis(5′-((2″-ethylhexyl)thio)-4′-fluorothiophen-2′-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)-alt-2′-ethylhexyl-3-fluorothieno[3,4-b]thiophene-2-carboxylate (PBDTT-SF-TT) and poly(4,8-bis(5′-((2″-ethylhexyl)thio)-4′-fluorothiophen-2′-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)-alt-1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (PBDTT-SF-BDD), namely, via an advantageous and synthetically economic route for the key monomer are reported herein. Synergistic effects of fluorination and alkylthiolation on BDT moieties are discussed in detail, which is based on the superior balance between high V_oc and large J_sc when PBDTT-SF-TT/PC₇₁BM and PBDTT-SF-BDD/PC₇₁BM solar cells present their high V_oc as 1.00 and 0.97 V (associated with their deep highest occupied molecular orbital level of −5.54 and −5.61 eV), a moderately high J_sc of 14.79 and 14.70 mA cm⁻², and thus result a high power conversion efficiency of 9.07% and 9.72%, respectively. Meanwhile, for PBDTT-SF-TT, a very low energy loss of 0.59 eV is pronounced, leading to the promisingly high voltage, and furthermore performance study and morphological results declare an additive-free PSC from PBDTT-SF-TT, which is beneficial to practical applications.

Superior balance between high V_oc and large J_sc is realized via synergistic effect of fluorination and alkylthiolation on benzo[1,2-b:4,5-b′] dithiophene (BDT) moiety, leading to new efficient conventional BDT-based polymer solar cells are achieved with high power conversion efficiency of 9.07% for poly(4,8-bis(5′-((2″-ethylhexyl)thio)-4′-fluorothiophen-2′-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)-alt-2′-ethylhexyl-3-fluorothieno[3,4-b]thiophene-2-carboxylate and 9.72% for poly(4,8-bis(5′-((2″-ethylhexyl)thio)-4′-fluorothiophen-2′-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)-alt-1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione.

Nanostructures produce unique optical and electronic properties, which have the potential to meet the goals of third-generation photovoltaic devices. However, most nanostructures bring accompanying optical or electrical losses to solar cells. In article number 1602385, Jr-Hau He and Hsin-Ping Wang postulate that the concurrent design of both optical and electrical components will be an imperative route toward breaking the present-day limit of nanostructured solar cells.

Despite the potential of ternary polymer solar cells (PSCs) to improve photocurrents, ternary architecture is not widely utilized for PSCs because its application has been shown to reduce fill factor (FF). In this paper, a novel technique is reported for achieving highly efficient ternary PSCs without this characteristic sharp decrease in FF by matching the highest occupied molecular orbital (HOMO) energy levels of two donor polymers. Our ternary device—made from a blend of wide-bandgap poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-alt-2,5-dioctyl-4,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione) (PBDT-DPPD) polymer, narrow-bandgap poly[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2- 6-diyl)] (PTB7-Th) polymer, and [6,6]-phenyl C₇₀-butyric acid methyl ester (PC₇₀BM)—exhibits a maximum power conversion efficiency of 10.42% with an open-circuit voltage of 0.80 V, a short-circuit current of 17.61 mA cm⁻², and an FF of 0.74. In addition, this concept is extended to quaternary PSCs made by using three different donor polymers with similar HOMO levels. Interestingly, the quaternary PSCs also yield a good FF (≈0.70)—similar to those of corresponding binary PSCs. This study confirms that the HOMO levels of the polymers used on the photoactive layer of PSCs are a crucial determinant of a high FF.

Highly efficient ternary polymer solar cells (PSCs) are successfully demonstrated by matching the highest occupied molecular orbital (HOMO) energy levels of two donor polymers. Ternary or quaternary PSCs made using two or more donor polymers with similar HOMO levels allow efficient charge transport, and consequently offer notably a higher fill factor and power conversion efficiency.

To maximize the short-circuit current density (J_SC) and the open circuit voltage (V_OC) simultaneously is a highly important but challenging issue in organic solar cells (OSCs). In this study, a benzotriazole-based p-type polymer (J61) and three benzotriazole-based nonfullerene small molecule acceptors (BTA1-3) are chosen to investigate the energetic driving force for the efficient charge transfer. The lowest unoccupied molecular orbital (LUMO) energy levels of small molecule acceptors can be fine-tuned by modifying the end-capping units, leading to high V_OC (1.15–1.30 V) of OSCs. Particularly, the LUMO energy level of BTA3 satisfies the criteria for efficient charge generation, which results in a high V_OC of 1.15 V, nearly 65% external quantum efficiency, and a high power conversion efficiency (PCE) of 8.25%. This is one of the highest V_OC in the high-performance OSCs reported to date. The results imply that it is promising to achieve both high J_SC and V_OC to realize high PCE with the carefully designed nonfullerene acceptors.

The existence of the driving force in organic solar cells (OSCs) often creates a problematic trade-off between the open-circuit voltage and short-circuit current. Here, fine-tuning driving force by gradually decreasing the acceptor energy level has afforded high open-circuit voltage (>1.15 V) and efficient charge generation (>60%) at the same time, which is instructive to the development of more efficient OSCs.

High-performance colored aesthetic semitransparent organic photovoltaics (OPVs) featuring a silver/indium tin oxide/silver (Ag/ITO/Ag) microcavity structure are prepared. By precisely controlling the thickness of the ITO layer, OPV devices exhibiting high transparency and a wide and high-purity color gamut are obtained: blue (B), green (G), yellow-green (YG), yellow (Y), orange (O), and red (R). The power conversion efficiencies (PCEs) of the G, YG, and Y color devices are greater than 8% (AM 1.5G irradiation, 100 mW cm⁻²) with maximum transmittances (T_MAX) of greater than 14.5%. An optimized PCE of 8.2% was obtained for the YG OPV [CIE 1931 coordinates: (0.364, 0.542)], with a value of T_MAX of 17.3% (at 561 nm). As far as it is known, this performance is the highest ever reported for a transparent colorful OPV. Such high transparency and desired transmitted colors, which can perspective see the clear images, suggest great potential for use in building-integrated photovoltaic applications.

High-performance colored aesthetic semitransparent organic photovoltaics (OPVs) featuring a silver/indium tin oxide/silver microcavity structure are demonstrated. Colored OPVs of high purity and a wide color gamut are obtained: blue, green, yellow-green, yellow, orange, and red. The highest power conversion efficiency was 8.2% for the yellow-green device, with CIE 1931 coordinates of (0.364, 0.542) and a transmittance of 17.3% at 561 nm.

The considerable improvement on the power conversion efficiency (PCE) for emerging nonfullerene polymer solar cells is still limited by considerable voltage losses that have become one of the most significant obstacles in further boosting desired photovoltaic performance. Here, a comprehensive study is reported to understand the impacts of charge transport, energetic disorder, and charge transfer states (CTS) on the losses in open-circuit voltage (V_oc) based on three high performing bulk heterojunction solar cells with the best PCE exceeding 11%. It is found that the champion poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)-benzo[1,2-c:4,5-c′]dithiophene-4,8-dione))] (PBDB-T):IT-M solar cell (PCE = 11.5%) is associated with the least disorder. The determined energetic disorder in part reconciles the difference in V_oc between the solar cells. A reduction is observed in the nonradiative losses (ΔV_nonrad) coupled with the increase of energy of CTS for the PBDB-T:IT-M device, which may be related to the improved balance in carrier mobilities, and partially can explain the gain in V_oc. The determined radiative limit for V_oc combined with the ΔV_nonrad generates an excellent agreement for the V_oc with the experimental values. The results suggest that minimizing the energetic disorder related to transport and CTS is critical for the mitigation of V_oc losses and improvements on the device performance.

Voltage losses and charge transport in three representative bulk heterojunction solar cells are investigated. By temperature-dependent open-circuit voltage (V_oc) analysis and photovoltaic electroluminescence spectroscopy, we find that the increased V_oc in the champion IT-M cell with an excellent balance in mobility is associated with reduced energetic disorder at the D/A interface and non-redative recombination losses at charge transfer states.

Recently, the influence of molecular weight (M_n) on the performance of polymer solar cells (PSCs) is widely investigated. However, the dependence of optimal thickness of active layer for PSCs on M_n is not reported yet, which is vital to the solution printing technology. In this work, the effect of M_n on the efficiency and especially optimal thickness of the active layer for PBTIBDTT-S-based PSCs is systematically studied. The device efficiency improves significantly as the M_n increases from 12 to 38 kDa, and a remarkable efficiency of 10.1% is achieved, which is among the top efficiencies of wide-bandgap polymer:fullerene PSCs. Furthermore, the optimal thickness of the active layer is also greatly increased from 62 to 210 nm with increased M_n. Therefore, a device employing a thick (>200 nm) active layer with power conversion efficiency exceeding 10% is achieved by manipulating M_n. This exciting result is attributed to both the improved crystallinity, thus hole mobility, and preferable polymer orientation, thus morphology of active layer. These findings, for the first time, highlight the significant impact of M_n on the optimal thickness of active layer for PSCs and provide a facile way to further improve the performance of PSCs employing a thick active layer.

As the molecular weight (M_n) of PBTIBDTT-S increases from 12 to 38 kDa, the efficiency and optimal thickness of the active layer are simultaneously improved from 6.99% to 10.11% and from 62 to 210 nm, respectively. This result demonstrates the importance of M_n in achieving highly efficient devices under a thick active layer.

The development of new flexible and stretchable sensors addresses the demands of upcoming application fields like internet-of-things, soft robotics, and health/structure monitoring. However, finding a reliable and robust power source to operate these devices, particularly in off-the-grid, maintenance-free applications, still poses a great challenge. The exploitation of ubiquitous temperature gradients, as the source of energy, can become a practical solution, since the recent discovery of the outstanding thermoelectric properties of a conductive polymer, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS). Unfortunately the use of PEDOT:PSS is currently constrained by its brittleness and limited processability. Herein, PEDOT:PSS is blended with a commercial elastomeric polyurethane (Lycra), to obtain tough and processable self-standing films. A remarkable strain-at-break of ≈700% is achieved for blends with 90 wt% Lycra, after ethylene glycol treatment, without affecting the Seebeck voltage. For the first time the viability of these novel blends as stretchable self-powered sensors is demonstrated.

Stretchable self-powered sensors are developed via blending elastomeric polyurethane (Lycra) with the best current organic thermoelectric material, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS). In doing so, the main technological constrains of PEDOT:PSS, namely brittleness, processability, and costs, have, at the same time, been overcome. An unprecedented strain at break (≈700%) is reached, while maintaining high electrical conductivity (≈79 S cm⁻¹) and Seebeck coefficient (≈16 µV K⁻¹).

The power conversion efficiencies (PCEs) of state-of-the-art organic solar cells (OSCs) have increased to over 13%. However, the most commonly used solvents for making the solutions of photoactive materials and the coating methods used in laboratories are not adaptable for future practical production. Therefore, taking a solution-coating method with environmentally friendly processing solvents into consideration is critical for the practical utilization of OSC technology. In this study, a highly efficient PBTA-TF:IT-M-based device processed with environmentally friendly solvents, tetrahydrofuran/isopropyl alcohol (THF/IPA) and o-xylene/1-phenylnaphthalene, is fabricated; a high PCE of 13.1% can be achieved by adopting the spin-coating method, which is the top result for OSCs. More importantly, a blade-coated non-fullerene OSC processed with THF/IPA is demonstrated for the first time to obtain a promising PCE of 11.7%; even for the THF/IPA-processed large-area device (1.0 cm²) made by blade-coating, a PCE of 10.6% can still be maintained. These results are critical for the large-scale production of highly efficient OSCs in future studies.

Highly efficient non-fullerene organic solar cells (OSCs) are fabricated, processed with environmentally friendly solvents, tetrahydrofuran/isopropyl alcohol (THF/IPA) and o-xylene/1-phenylnaphthalene, respectively. The highest power conversion efficiency (PCE) of 13.1% can be achieved by adopting the spin-coating method, which is the top result for OSCs. When the blade-coating method is used in an ambient atmosphere, the THF/IPA-processed device maintains a high PCE of 11.7%.

In this work, a nonfullerene polymer solar cell (PSC) based on a wide bandgap polymer donor PM6 containing fluorinated thienyl benzodithiophene (BDT-2F) unit and a narrow bandgap small molecule acceptor 2,2′-((2Z,2′Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDIC) is developed. In addition to matched energy levels and complementary absorption spectrum with IDIC, PM6 possesses high crystallinity and strong π–π stacking alignment, which are favorable to charge carrier transport and hence suppress recombination in devices. As a result, the PM6:IDIC-based PSCs without extra treatments show an outstanding power conversion efficiency (PCE) of 11.9%, which is the record value for the as-cast PSC devices reported in the literature to date. Moreover, the device performances are insensitive to the active layer thickness (≈95–255 nm) and device area (0.20–0.81 cm²) with PCEs of over 11%. Besides, the PM6:IDIC-based flexible PSCs with a large device area of 1.25 cm² exhibit a high PCE of 6.54%. These results indicate that the PM6:IDIC blend is a promising candidate for future roll-to-roll mass manufacturing and practical application of highly efficient PSCs.

An efficient polymer solar cell (PSC) based on a polymer donor PM6 containing BDT-2F unit and an n-type organic semiconductor acceptor, IDIC, is developed. The power conversion efficiencies of PSCs without extra treatments reach up to 11.9% and are insensitive to the active layer thickness (95–225 nm) and device area (0.20–0.81 cm²), with values of over 11%.

Crystallizable, high-mobility conjugated polymers have been employed as secondary donor materials in ternary polymer solar cells in order to improve device efficiency by broadening their spectral response range and enhancing charge dissociation and transport. Here, contrasting effects of two crystallizable polymers, namely, PffBT4T-2OD and PDPP2TBT, in determining the efficiency improvements in PTB7-Th:PC₇₁BM host blends are demonstrated. A notable power conversion efficiency of 11% can be obtained by introducing 10% PffBT4T-2OD (relative to PTB7-Th), while the efficiency of PDPP2TBT-incorporated ternary devices decreases dramatically despite an enhancement in hole mobility and light absorption. Blend morphology studies suggest that both PffBT4T-2OD and PDPP2TBT are well dissolved within the host PTB7-Th phase and facilitate an increased degree of phase separation between polymer and fullerene domains. While negligible charge transfer is determined in binary blends of each polymer mixture, effective energy transfer is identified from PffBT4T-2OD to PTB7-Th that contributes to an improvement in ternary blend device efficiency. In contrast, energy transfer from PTB7-Th to PDPP2TBT worsens the efficiency of the ternary device due to inefficient charge dissociation between PDPP2TBT and PC₇₁BM.

Contrasting effects in determining device efficiencies are observed when incorporating two crystallizable, high-mobility conjugated polymers, namely, PffBT4T-2OD and PDPP2TBT, as the secondary donor materials to PTB7-Th:PC₇₁BM-based solar cells, due to different energy transfer between the electron-donating polymers in the ternary photovoltaic blends.

Organic solar cell optimization requires careful balancing of current–voltage output of the materials system. Here, such optimization using ultrafast spectroscopy as a tool to optimize the material bandgap without altering ultrafast photophysics is reported. A new acceptor–donor–acceptor (A–D–A)-type small-molecule acceptor NCBDT is designed by modification of the D and A units of NFBDT. Compared to NFBDT, NCBDT exhibits upshifted highest occupied molecular orbital (HOMO) energy level mainly due to the additional octyl on the D unit and downshifted lowest unoccupied molecular orbital (LUMO) energy level due to the fluorination of A units. NCBDT has a low optical bandgap of 1.45 eV which extends the absorption range toward near-IR region, down to ≈860 nm. However, the 60 meV lowered LUMO level of NCBDT hardly changes the V_oc level, and the elevation of the NCBDT HOMO does not have a substantial influence on the photophysics of the materials. Thus, for both NCBDT- and NFBDT-based systems, an unusually slow (≈400 ps) but ultimately efficient charge generation mediated by interfacial charge-pair states is observed, followed by effective charge extraction. As a result, the PBDB-T:NCBDT devices demonstrate an impressive power conversion efficiency over 12%—among the best for solution-processed organic solar cells.

An acceptor-donor-acceptor nonfullerene acceptor NCBDT is reported. NCBDT exhibits a low optical bandgap of 1.45 eV and broadened absorption range. The PBDB-T:NCBDT-based device achieves an impressive PCE of 12.12% and J_sc over 20 mA cm^-2—one of the best results for solution-processed OSCs. Further photophysical study reveals slow (≈400 ps) yet efficient free charge generation.

Bulk-heterojunction organic photovoltaic materials containing nonfullerene acceptors (NFAs) have seen remarkable advances in the past year, finally surpassing fullerenes in performance. Indeed, acceptors based on indacenodithiophene (IDT) have become synonymous with high power conversion efficiencies (PCEs). Nevertheless, NFAs have yet to achieve fill factors (FFs) comparable to those of the highest-performing fullerene-based materials. To address this seeming anomaly, this study examines a high efficiency IDT-based acceptor, ITIC, paired with three donor polymers known to achieve high FFs with fullerenes, PTPD3T, PBTI3T, and PBTSA3T. Excellent PCEs up to 8.43% are achieved from PTPD3T:ITIC blends, reflecting good charge transport, optimal morphology, and efficient ITIC to PTPD3T hole-transfer, as observed by femtosecond transient absorption spectroscopy. Hole-transfer is observed from ITIC to PBTI3T and PBTSA3T, but less efficiently, reflecting measurably inferior morphology and nonoptimal energy level alignment, resulting in PCEs of 5.34% and 4.65%, respectively. This work demonstrates the importance of proper morphology and kinetics of ITIC donor polymer hole-transfer in boosting the performance of polymer:ITIC photovoltaic bulk heterojunction blends.

Three high-fill-factor OPV polymers, PTPD3T, PBTI3T, and PBTSA3T are paired with the high performance acceptor, ITIC. A maximum power conversion efficiency of 8.43% is achieved with PTPD3T:ITIC blends due primarily to increased short-circuit current density. Ultrafast hole-transfer from ITIC to PTPD3T is observed by femtosecond transient absorption measurements due to the superior blend morphology and improved charge transport versus PBTI3T:ITIC and PBTSA3T:ITIC blends.

Two novel wide-bandgap copolymers, PBDT-TDZ and PBDTS-TDZ, are developed based on 1,3,4-thiadiazole (TDZ) and benzo[1,2-b:4,5-b′]dithiophene (BDT) building blocks. These copolymers exhibit wide bandgaps over 2.07 eV and low-lying highest occupied molecular orbital (HOMO) levels below −5.35 eV, which match well with the typical low-bandgap acceptor of ITIC, resulting in a good complementary absorption from 300 to 900 nm and a low HOMO level offset (≤0.13 eV). Compared to PBDT-TDZ, PBDTS-TDZ with alkylthio side chains exhibits the stronger optical absorption, lower-lying HOMO level, and higher crystallinity. By using a single green solvent of o-xylene, PBDTS-TDZ:ITIC devices exhibit a large open-circuit voltage (V_oc) up to 1.10 eV and an extremely low energy loss (E_loss) of 0.48 eV. At the same time, the desirable high short-circuit current density (J_sc) of 17.78 mA cm⁻² and fill factor of 65.4% are also obtained, giving rise to a high power conversion efficiency (PCE) of 12.80% without any additive and post-treatment. When adopting a homotandem device architecture, the PCE is further improved to 13.35% (certified as 13.19%) with a much larger V_oc of 2.13 V, which is the best value for any type of homotandem organic solar cells reported so far.

Two novel 1,3,4-thiadiazole-based wide-bandgap copolymers, PBDT-TDZ and PBDTS-TDZ, are developed for efficient nonfullerene organic solar cells. The single-junction devices processed by a green solvent of o-xylene exhibit a high power conversion efficiency (PCE) of 12.80% with a low energy loss of 0.48 eV. The PCE is finally improved to 13.35% when using a homotandem device architecture.

Naphtho[1,2-b:5,6-b′]dithiophene is extended to a fused octacyclic building block, which is end capped by strong electron-withdrawing 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile to yield a fused-ring electron acceptor (IOIC2) for organic solar cells (OSCs). Relative to naphthalene-based IHIC2, naphthodithiophene-based IOIC2 with a larger π-conjugation and a stronger electron-donating core shows a higher lowest unoccupied molecular orbital energy level (IOIC2: −3.78 eV vs IHIC2: −3.86 eV), broader absorption with a smaller optical bandgap (IOIC2: 1.55 eV vs IHIC2: 1.66 eV), and a higher electron mobility (IOIC2: 1.0 × 10⁻³ cm² V⁻¹ s⁻¹ vs IHIC2: 5.0 × 10⁻⁴ cm² V⁻¹ s⁻¹). Thus, IOIC2-based OSCs show higher values in open-circuit voltage, short-circuit current density, fill factor, and thereby much higher power conversion efficiency (PCE) values than those of the IHIC2-based counterpart. In particular, as-cast OSCs based on FTAZ: IOIC2 yield PCEs of up to 11.2%, higher than that of the control devices based on FTAZ: IHIC2 (7.45%). Furthermore, by using 0.2% 1,8-diiodooctane as the processing additive, a PCE of 12.3% is achieved from the FTAZ:IOIC2-based devices, higher than that of the FTAZ:IHIC2-based devices (7.31%). These results indicate that incorporating extended conjugation into the electron-donating fused-ring units in nonfullerene acceptors is a promising strategy for designing high-performance electron acceptors.

A novel fused-ring electron acceptor (IOIC2) based on naphthodithiophene is designed and synthesized, and compared with a naphthalene-based counterpart (IHIC2). The IOIC2-based single-junction binary-blend organic solar cells exhibit efficiencies up to 12.3%, much higher than that of IHIC2 (7.45%).