ZiQi Sun

In p-i-n planar perovskite solar cells (pero-SCs) based on methylammonium lead iodide (MAPbI₃) perovskite, high-quality MAPbI₃ film, perfect interfacial band alignment and efficient charge extracting ability are critical for high photovoltaic performance. In this work, a hydrophilic fullerene derivative [6,6]-phenyl-C₆₁-butyric acid-(3,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)phenyl)methanol ester (PCBB-OEG) is introduced as additive in the methylammonium iodide precursor solution in the preparation of MAPbI₃ perovskite film by two-step sequential deposition method, and obtained a top-down gradient distribution with an ultrathin top layer of PCBB-OEG. Meanwhile, a high-quality perovskite film with high crystallinity, less trap-states, and dense-grained uniform morphology can well grow on both hydrophilic (poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid)) and hydrophobic (polytriarylamine, PTAA) hole transport layers. When the PCBB-OEG-containing perovskite film (pero-0.1) is prepared in a p-i-n planar pero-SC with the configuration of ITO/PTAA/pero-0.1/[6,6]-phenyl-C₆₁-butyric acid methyl ester/Al, the device delivers a promising power conversion efficiency (PCE) of 20.2% without hysteresis, which is one of the few PCE over 20% for the p-i-n planar pero-SCs. Importantly, the pero-0.1-based device shows an excellent stability that can retain 98.4% of its initial PCE after being exposed for 300 h under ambient atmosphere with a high humidity, and the flexible pero-SCs based on pero-0.1 also demonstrate a promising PCE of 18.1%.

It is demonstrated that a new strategy of two-step method provides a simple way to develop high-quality perovskite film. The perovskite solar cells (pero-SCs) show a high power conversion efficiency (PCE) of 20.2% with an excellent device stability. This strategy can also be suitable for fabricating flexible pero-SC giving a promising PCE of 18.1%.

An efficient perovskite photovoltaic-thermoelectric hybrid device is demonstrated by integrating the hole-conductor-free perovskite solar cell based on TiO₂/ZrO₂/carbon structure and the thermoelectric generator. The whole solar spectrum of AM 1.5 G is fully utilized with the ≈1.55 eV band gap perovskite (5-AVA)_x(MA)_1−xPbI₃ absorbing the visible light and the carbon back contact absorbing the infrared light. The added thermoelectric generator improves the device performance by converting the thermal energy into electricity via the Seebeck effect. An optimized hybrid device is obtained with a maximum point power output of 20.3% and open-circuit voltage of 1.29 V under the irradiation of 100 mW cm⁻².

By utilizing the whole AM 1.5 G solar spectrum energy, an efficient perovskite photovoltaic-thermoelectric hybrid device is demonstrated by integrating the perovskite solar cell based on carbon electrode and the thermoelectric generator. An optimized hybrid device is obtained with a maximum point power output of 20.3% and open-circuit voltage of 1.29 V under the irradiation of 100 mW cm⁻².

The application of conjugated polymer and fullerene water-based nanoparticles (NP) as ecofriendly inks for organic photovoltaics (OPVs) is reported. A low bandgap polymer diketopyrrolopyrrole–quinquethiophene (PDPP5T-2) and the methanofullerene PC₇₁BM are processed into three types of nanoparticles: pristine fullerene NPs, pristine polymer NPs, and mixed polymer:fullerene NPs, allowing the formation of bulk heterojunction (BHJ) composites with different domain sizes. Mild thermal annealing is required to melt the nanospheres and enable the formation of interconnected pathways within mixed phases. This BHJ is accompanied by a shrinkage of film, whereas the more compact layers show enhanced mobility. Consistently reduced recombination and better performance are found for mixed NP, containing both, the polymer and the fullerene within a single NP. The optimized solar cell processed by ultrasmall NPs delivers a power conversion efficiency of about 3.4%. This is among the highest values reported for aqueous processed OPVs but still lacks performance compared to those being processed from halogenated solvents. Incomplete crystallization is identified as the main root for reduced efficiency. It is nevertheless believed that postprocessing does not cut attraction from printing aqueous organic NP inks as a trendsetting strategy for the reliable and ecofriendly production of organic solar cells.

The correlation between microstructure and device physics in water processed nanoparticulate organic photovoltaics is investigated. Bulk heterojunction composites with largely different domain sizes are determined by pristine nanoparticle formation, which significantly influence the mobility-lifetime product and nongeminate recombination in the nanoparticle-based solar cells.

The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has now exceeded 20%; thus, research focus has shifted to establishing the foundations for commercialization. One of the pivotal themes is to curtail the overall fabrication time, to reduce unit cost, and mass-produce PSCs. Additionally, energy dissipation during the thermal annealing (TA) stage must be minimized by realizing a genuine low-temperature (LT) process. Here, tin oxide (SnO₂) thin films (TFs) are formulated at extremely high speed, within 5 min, under an almost room-temperature environment (<50 °C), using atmospheric Ar/O₂ plasma energy (P-SnO₂) and are applied as an electron transport layer of a “n–i–p”-type planar PSC. Compared with a thermally annealed SnO₂ TF (T-SnO₂), the P-SnO₂ TF yields a more even surface but also outstanding electrical conductivity with higher electron mobility and a lower number of charge trap sites, consequently achieving a superior PCE of 19.56% in P-SnO₂-based PSCs. These findings motivate the use of a plasma strategy to fabricate various metal oxide TFs using the sol–gel route.

A tin oxide (SnO₂) electron transport layer for a perovskite solar cell is successfully fabricated at extremely high speeds at a genuinely low temperature using atmospheric Ar/O₂ plasma annealing. This plasma-annealed SnO₂ (P-SnO₂) exhibits outstanding electrical conductivity and charge-extraction ability compared to thermally-annealed SnO₂, consequently achieving a superior PCE of 19.56% in P-SnO₂-based PSCs.

The power conversion efficiency of perovskite solar cells (PSCs) has ascended from 3.8% to 22.1% in recent years. ZnO has been well-documented as an excellent electron-transport material. However, the poor chemical compatibility between ZnO and organo-metal halide perovskite makes it highly challenging to obtain highly efficient and stable PSCs using ZnO as the electron-transport layer. It is demonstrated in this work that the surface passivation of ZnO by a thin layer of MgO and protonated ethanolamine (EA) readily makes ZnO as a very promising electron-transporting material for creating hysteresis-free, efficient, and stable PSCs. Systematic studies in this work reveal several important roles of the modification: (i) MgO inhibits the interfacial charge recombination, and thus enhances cell performance and stability; (ii) the protonated EA promotes the effective electron transport from perovskite to ZnO, further fully eliminating PSCs hysteresis; (iii) the modification makes ZnO compatible with perovskite, nicely resolving the instability of ZnO/perovskite interface. With all these findings, PSCs with the best efficiency up to 21.1% and no hysteresis are successfully fabricated. PSCs stable in air for more than 300 h are achieved when graphene is used to further encapsulate the cells.

Surface passivation of ZnO by a thin layer of MgO and protonated ethanolamine readily makes ZnO a very promising electron-transporting material for creating efficient, hysteresis-free and stable perovskite solar cells (PSCs). PSCs, stable in air for more than 300 h, are achieved when graphene is used to encapsulate the cells.

A terthieno[3,2-b]thiophene (6T) based fused-ring low bandgap electron acceptor, 6TIC, is designed and synthesized for highly efficient nonfullerene solar cells. The chemical, optical, and physical properties, device characteristics, and film morphology of 6TIC are intensively studied. 6TIC shows a narrow bandgap with band edge reaching 905 nm due to the electron-rich π-conjugated 6T core and reduced resonance stabilization energy. The rigid, π-conjugated 6T also offers lower reorganization energy to facilitate very low V_OC loss in the 6TIC system. The analysis of film morphology shows that PTB7-Th and 6TIC can form crystalline domains and a bicontinuous network. These domains are enlarged when thermal annealing is applied. Consequently, the device based on PTB7-Th:6TIC exhibits a high power conversion efficiency (PCE) of 11.07% with a high J_SC > 20 mA cm⁻² and a high V_OC of 0.83 V with a relatively low V_OC loss (≈0.55 V). Moreover, a semitransparent solar cell based on PTB7-Th:6TIC exhibits a relatively high PCE (7.62%). The device can have combined high PCE and high J_SC is quite rare for organic solar cells.

Terthieno[3,2-b]thiophene (6T) based low bandgap fused-ring electron acceptor, 6TIC, is developed for highly efficient solar cells, which exhibits a high power conversion efficiency (PCE) of 11.07% with a high J_SC over 20 mA cm⁻² and a high V_OC of 0.83 V with a relatively low V_OC loss (≈0.55 V). Moreover, the semitransparent solar cell based on PTB7-Th:6TIC exhibits a very promising PCE of 7.62%.

Nonfullerene polymer solar cells (PSCs) are fabricated by using one wide bandgap donor PBDB-T and one ultranarrow bandgap acceptor IEICO-4F as the active layers. One medium bandgap donor PTB7-Th is selected as the third component due to the similar highest occupied molecular orbital level compared to that of PBDB-T and their complementary absorption spectra. The champion power conversion efficiency (PCE) of PSCs is increased from 10.25% to 11.62% via incorporating 20 wt% PTB7-Th in donors, with enhanced short-circuit current (J_SC) of 24.14 mA cm⁻² and fill factor (FF) of 65.03%. The 11.62% PCE should be the highest value for ternary nonfullerene PSCs. The main contribution of PTB7-Th can be summarized as the improved photon harvesting and enhanced exciton utilization of PBDB-T due to the efficient energy transfer from PBDB-T to PTB7-Th. Meanwhile, PTB7-Th can also act as a regulator to adjust PBDB-T molecular arrangement for optimizing charge transport, resulting in the enhanced FF of ternary PSCs. This experimental result may provide new insight for developing high-performance ternary nonfullerene PSCs by selecting two well-compatible donors with different bandgap and one ultranarrow bandgap acceptor.

Highly efficient ternary nonfullerene polymer solar cells (PSCs) are fabricated by employing two well-compatible donors with complementary absorption spectra and one ultranarrow bandgap acceptor. The power conversion efficiency and short-circuit current density of ternary PSCs are simultaneously increased to 11.62% and 24.14 mA cm⁻² by incorporating 20 wt% PTB7-Th due to the enhanced photon harvesting and optimized film morphology.

Organic–inorganic perovskites have demonstrated an impressive potential for the design of the next generation of solar cells. Perovskite solar cells (PSCs) are currently considered for scaling up and commercialization. Many of the lab-scale preparation methods are however difficult to scale up or are environmentally unfriendly. The highest efficient PSCs are currently prepared using the antisolvent method, which utilizes a significant amount of an organic solvent to induce perovskite crystallization in a thin film. An antisolvent-free method is developed in this work using flash infrared annealing (FIRA) to prepare methylammonium lead iodide (MAPbI₃) PSCs with a record stabilized power conversion efficiency of 18.3%. With an irradiation time of fewer than 2 s, FIRA enables the coating of glass and plastic substrates with pinhole-free perovskite films that exhibit micrometer-size crystalline domains. This work discusses the FIRA-induced crystallization mechanism and unveils the main parameters controlling the film morphology. The replacement of the antisolvent method and the larger crystalline domains resulting from flash annealing make FIRA a highly promising method for the scale-up of PSC manufacture.

Flash infrared annealing (FIRA) is demonstrated as an antisolvent-free method to prepare methylammonium lead iodide perovskite solar cells with over 18% efficiency. FIRA enables the preparation of pinhole-free perovskite films with micrometer-size crystalline domains over a large area of both glass and plastic substrates. FIRA, as a rapid and environmentally friendly method to scale up perovskite solar cells is proposed.

In the past few years, organic–inorganic metal halide ABX₃ perovskites (A = Rb, Cs, methylammonium, formamidinium (FA); B = Pb, Sn; X = Cl, Br, I) have rapidly emerged as promising materials for photovoltaic applications. Tuning the film morphology by various deposition techniques and additives is crucial to achieve solar cells with high performance and long-term stability. In this work, carbon nanoparticles (CNPs) containing functional groups are added to the perovskite precursor solution for fabrication of fluorine-doped tin oxide/TiO₂/perovskite/spiro-OMeTAD/gold devices. With the addition of CNPs, the perovskite films are thermally more stable, contain larger grains, and become more hydrophobic. NMR experiments provide strong evidence that the functional groups of the CNPs interact with FA cations already in the precursor solution. The fabricated solar cells show a power-conversion efficiency of 18% and negligible hysteresis.

Carbon nanoparticles are incorporated in perovskite solar cells as a degradation inhibitor and perovskite crystal size magnifier. Here, carbon nanoparticles are used with functional groups in the perovskite solution, which interact with formamidinium cations resulting in morphology tuning, increased hydrophobicity, and thermal stability.

Organolead trihalide perovskite MAPbI₃ shows a distinctive combination of properties such as being ferroelectric and semiconducting, with ion migration effects under poling by electric fields. The combination of its ferroelectric and semiconducting nature is used to make a light harvesting, self-powered tactile sensor. This sensor interfaces ZnO nanosheets as a pressure-sensitive drain on the MAPbI₃ film and once poled is operational for at least 72 h with just light illumination. The sensor is monolithic in structure, has linear response till 76 kPa, and is able to operate continuously as the energy harvesting mechanism is decoupled from its pressure sensing mechanism. It has a sensitivity of 0.57 kPa⁻¹, which can be modulated by the strength of the poling field. The understanding of these effects in perovskite materials and their application in power source free devices are of significance to a wide array of fields where these materials are being researched and applied.

The ferroelectric properties of MAPbI₃ films are coupled with a dynamic drain of ZnO nanosheets to make a self-powered tactile sensor that is operational with just light illumination for at least 72 h. The device is developed by decoupling the ion migration and ferroelectric effects in these films based on poling conditions.

Organic solar cells (OSCs) based on bulk heterojunction structures are promising candidates for next-generation solar cells. However, the narrow absorption bandwidth of organic semiconductors is a critical issue resulting in insufficient usage of the energy from the solar spectrum, and as a result, it hinders performance. Devices based on multiple-donor or multiple-acceptor components with complementary absorption spectra provide a solution to address this issue. OSCs based on multiple-donor or multiple-acceptor systems have achieved power conversion efficiencies over 12%. Moreover, the introduction of an additional component can further facilitate charge transfer and reduce charge recombination through cascade energy structure and optimized morphology. This progress report provides an overview of the recent progress in OSCs based on multiple-donor (polymer/polymer, polymer/dye, and polymer/small molecule) or multiple-acceptor (fullerene/fullerene, fullerene/nonfullerene, and nonfullerene/nonfullerene) components.

This progress report provides an overview of the most impactful recent progress in high-performance organic solar cells based on multiple-donor (polymer/polymer, polymer/dye, and polymer/small molecule) or multiple-acceptor (fullerene/fullerene, fullerene/nonfullerene, and nonfullerene/nonfullerene) components, focusing particularly on the interactions between different components from the perspective of morphology and photophysics.

Cesium-based trihalide perovskites have been demonstrated as promising light absorbers for photovoltaic applications due to their superb composition stability. However, the large energy losses (E_loss) observed in inorganic perovskite solar cells has become a major hindrance impairing the ultimate efficiency. Here, an effective and reproducible method of modifying the interface between a CsPbI₂Br absorber and polythiophene hole-acceptor to minimize the E_loss is reported. It is demonstrated that polythiophene, deposited on the top of CsPbI₂Br, can significantly reduce electron-hole recombination within the perovskite, which is due to the electronic passivation of surface defect states. In addition, the interfacial properties are improved by a simple annealing process, leading to significantly reduced energy disorder in polythiophene and enhanced hole-injection into the hole-acceptor. Consequently, one of the highest power conversion efficiency (PCE) of 12.02% from a reverse scan in inorganic mixed-halide perovskite solar cells is obtained. Modifying the perovskite films with annealing polythiophene enables an open-circuit voltage (V_OC) of up to 1.32 V and E_loss of down to 0.5 eV, which both are the optimal values reported among cesium-lead mixed-halide perovskite solar cells to date. This method provides a new route to further improve the efficiency of perovskite solar cells by minimizing the E_loss.

The interfacial properties between CsPbI₂Br absorber and poly(3-hexylthiophene) (P3HT) hole-acceptor are improved by passivating the surface defects of CsPbI₂Br and reducing the energy disorder of P3HT. Consequently, a stable inorganic perovskite solar cell with high power conversion efficiency of 12.02% and minimal energy loss of 0.50 eV is obtained.

A novel small molecule acceptor MeIC with a methylated end-capping group is developed. Compared to unmethylated counterparts (ITCPTC), MeIC exhibits a higher lowest unoccupied molecular orbital (LUMO) level value, tighter molecular packing, better crystallites quality, and stronger absorption in the range of 520–740 nm. The MeIC-based polymer solar cells (PSCs) with J71 as donor, achieve high power conversion efficiency (PCE), up to 12.54% with a short-circuit current (J_SC) of 18.41 mA cm⁻², significantly higher than that of the device based on J71:ITCPTC (11.63% with a J_SC of 17.52 mA cm⁻²). The higher J_SC of the PSC based on J71:MeIC can be attributed to more balanced μ_h/μ_e, higher charge dissociation and charge collection efficiency, better molecular packing, and more proper phase separation features as indicated by grazing incident X-ray diffraction and resonant soft X-ray scattering results. It is worth mentioning that the as-cast PSCs based on MeIC also yield a high PCE of 11.26%, which is among the highest value for the as-cast nonfullerene PSCs so far. Such a small modification that leads to so significant an improvement of the photovoltaic performance is a quite exciting finding, shining a light on the molecular design of the nonfullerene acceptors.

A novel small-molecule acceptor MeIC with a methylated end-capping group is developed. Compared to unmethylated counterparts (ITCPTC), MeIC exhibits higher lowest unoccupied molecular orbital (LUMO) level, tighter molecular packing, and better crystallite quality. MeIC-based polymer solar cells with J71 as donor achieve high power conversion efficiency up to 12.54%, significantly higher than that of the device of ITCPTC.

A fused tris(thienothiophene) (3TT) building block is designed and synthesized with strong electron-donating and molecular packing properties, where three thienothiophene units are condensed with two cyclopentadienyl rings. Based on 3TT, a fused octacylic electron acceptor (FOIC) is designed and synthesized, using strong electron-withdrawing 2-(5/6-fluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)-malononitrile as end groups. FOIC exhibits absorption in 600–950 nm region peaked at 836 nm with extinction coefficient of up to 2 × 10⁵m^–1 cm^–1, low bandgap of 1.32 eV, and high electron mobility of 1.2 × 10^–3 cm² V^–1 s^–1. Compared with its counterpart ITIC3 based on indacenothienothiophene core, FOIC exhibits significantly upshifted highest occupied molecular orbital level, slightly downshifted lowest unoccupied molecular orbital level, significantly redshifted absorption, and higher mobility. The as-cast organic solar cells (OSCs) based on blends of PTB7-Th donor and FOIC acceptor without additional treatments exhibit power conversion efficiencies (PCEs) as high as 12.0%, which is much higher than that of PTB7-Th: ITIC3 (8.09%). The as-cast semitransparent OSCs based on the same blends show PCEs of up to 10.3% with an average visible transmittance of 37.4%.

A fused tris(thienothiophene)-based electron acceptor with strong near-infrared absorption and high electron mobility is designed, synthesized, and applied in as-cast organic solar cells and as-cast semitransparent organic solar cells, which exhibit efficiencies of 12.0% and 10.3%, respectively.

Significant development has been achieved in nonfullerene organic solar cells. However, most of the high-efficiency nonfullerene systems are composed of polymer donors and fused-ring acceptors, and only a few small molecule donors can work well. Herein, a new A–D–A small molecule donor named NDTSR with naphtho[1,2-b:5,6-b′]dithiophene (NDT) as building blocks is synthesized. Two energy levels well-matched fused-ring acceptors ITIC and IDIC are chosen to construct all-small-molecule solar cells with NDTSR, respectively. When mixed with IDIC, a high power conversion efficiency (PCE) of 8.05% is achieved, which is the highest efficiency for NDT-based small molecule donor. However, the NDTSR:ITIC system only exhibits a low PCE of 1.77%. The big difference in the performance of these two systems should be attributed to the different morphology and phase separation resulting from the crystallinity and aggregation ability of the acceptors. The results demonstrate that NDT-based small molecule is a promising candidate donor for all-small-molecule systems, while the crystallinity of fused-ring acceptors is a critical factor for optimizing the phase separation in the active layer.

An all-small-molecule nonfullerene solar cell is constructed with a novel small molecule NDTSR as donor, and ITIC and IDIC as acceptor, respectively. Through enhancing the crystallinity of acceptors, a high power conversion efficiency of 8.05% is obtained, which indicates that the crystallinity of the acceptor is a key factor for the performance of all-small-molecule solar cells.

Polymer aggregation plays a critical role in the miscibility of materials and the performance of all-polymer solar cells (APSCs). However, many aspects of how polymer texturing and aggregation affect photoactive blend film microstructure and photovoltaic performance are poorly understood. Here the effects of aggregation in donor–acceptor blends are studied, in which the number-average molecular weights (M_ns) of both an amorphous donor polymer, poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PBDTT-FTTE) and a semicrystalline acceptor polymer, poly{[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)) are systematically varied. The photovoltaic performance is correlated with active layer microstructural and optoelectronic data acquired by in-depth transmission electron microscopy, grazing incidence wide-angle X-ray scattering, thermal analysis, and optical spectroscopic measurements. Coarse-grained modeling provides insight into the effects of polymer aggregation on the blend morphology. Notably, the computed average distance between the donor and the acceptor polymers correlates well with solar cell photovoltaic metrics such as short-circuit current density (J_sc) and represents a useful index for understanding/predicting active layer blend material intermixing trends. Importantly, these results demonstrate that for polymers with different texturing tendencies (amorphous/semicrystalline), the key for optimal APSC performance, photovoltaic blend morphology can be controlled via both donor and acceptor polymer aggregation.

The templating effects in morphology engineering by regulating aggregation are clearly demonstrated for the first time in all-polymer solar cells, where the morphology may be templated by the amorphous phase in one blend and by the semicrystalline phase in another, all dictated by the degree of polymer aggregation.

In article number 1701659, Jinsong Huang, Jiarong Lian, and co-workers propose a simple hot-substrate deposition method to prepare a thin film with higher coverage and improved uniformity. The hot substrate improves the adhesion of the solvent on the substrate and speeds its drying process to avoid the aggregation of the upmost molecules, so that both reduced current leakage and series resistance are simultaneously realized in perovskite solar cells.

Flexible and self-powered photodetectors (PDs) are highly desirable for applications in image sensing, smart building, and optical communications. In this paper, a self-powered and flexible PD based on the methylammonium lead iodide (CH₃NH₃PBI₃) perovskite is demonstrated. Such a self-powered PD can operate even with irregular motion such as human finger tapping, which enables it to work without a bulky external power source. In addition, with high-quality CH₃NH₃PBI₃ perovskite thin film fabricated with solvent engineering, the PD exhibits an impressive detectivity of 1.22 × 10¹³ Jones. In the self-powered voltage detection mode, it achieves a large responsivity of up to 79.4 V mW⁻¹ cm⁻² and a voltage response of up to ≈90%. Moreover, as the PD is made of flexible and transparent polymer films, it can operate under bending and functions at 360 ° of illumination. As a result, the self-powered, flexible, 360 ° omnidirectional perovskite PD, featuring high detectivity and responsivity along with real-world sensing capability, suggests a new direction for next-generation optical communications, sensing, and imaging applications.

A flexible and self-powered organometallic halide perovskite photodetector is demonstrated that features an impressive detectivity of 1.22 × 10¹³ Jones and a large responsivity of up to 79.4 V mW⁻¹ cm². These results demonstrate a promising approach for developing a flexible and self-powered photodetector featuring high detectivity, responsivity, and excellent compatibility in various situations, particularly for outdoor applications.

The effects of alkyl chain regiochemistry on the properties of donor polymers and performances of non-fullerene organic solar cells are investigated. Two donor polymers (PfBTAZ and PfBTAZS) are compared that have nearly identical chemical structures except for the regiochemistry of alkyl chains. The optical properties and crystallinity of two polymers are nearly identical yet the PfBTAZ:O-IDTBR blend exhibits nearly double domain size compared to the blend based on PfBTAZS:O-IDTBR. To reveal the origins of the very different domain size of two blends, the morphology of neat polymer films is characterized, and it is found that PfBTAZ tends to aggregate into much larger polymer fibers without the presence of O-IDTBR. This indicates that it is not the polymer:O-IDTBR interactions but the intrinsic aggregation properties of two polymers that determine the morphology features of neat and blend films. The stronger aggregation tendency of PfBTAZ could be explained by its more co-planar geometry of the polymer backbone arising from the different alkyl chain regiochemistry. Combined with the similar trend observed in another set of donor polymers (PTFB-P and PTFB-PS), the results provide an important understanding of the structure–property relationships that could guide the development of donor polymers for non-fullerene organic solar cells.

The effects of alkyl chain regiochemistry on the properties of donor polymers and the performance of non-fullerene organic solar cells are investigated. It is found that the alkyl chain regiochemistry has great impacts on the morphology features of the neat and blend films, and the PfBTAZS:O-IDTBR-based cells with small domain size can achieve a high efficiency of 10.4%.

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.