Polymer solar cells (PSCs) possess the unique features of semitransparency and coloration, which make them potential candidates for applications in aesthetic windows. Here, the authors fabricate inverted semitransparent PSCs with high-quality hybrid Au/Ag transparent top electrodes and fine-tuned dielectric mirrors (DMs). It is demonstrated that the device color can be tailored and the light harvesting in the PSCs can be enhanced by matching the bandgap of the polymer donors in the active layer with the specifically designed maximum-reflection-center-wavelengths of the DMs. A detailed chromaticity analysis of the semitransparent PSCs from both bottom and top (mirror) views is also carried out. Furthermore, the inverted semitransparent PSCs based on PTB7-Th:PC71BM with six pairs of DMs demonstrate a maximum power conversion efficiency (PCE) of 7.0% with an average visible transmittance (AVT) of 12.2%. This efficiency is one of the highest reported for semitransparent PSCs, corresponding to 81.4% of the PCE from opaque counterpart devices. The device design and processing method are also successfully adapted to a flexible substrate, resulting in a device with a competitive PCE of 6.4% with an AVT of 11.5%. To the best of our knowledge, this PCE value is the highest value reported for a flexible semitransparent PSC.

In perovskite based planar heterojunction solar cells, the interface between the TiO2 compact layer and the perovskite film is critical for high photovoltaic performance. The deep trap states on the TiO2 surface induce several challenging issues, such as charge recombination loss and poor stability etc. To solve the problems, we synthesized a triblock fullerene derivative (PCBB-2CN-2C8) via rational molecular design for interface engineering in the perovskite solar cells. Modifying the TiO2 surface with the compound significantly improves charge extraction from the perovskite layer. Together with its uplifted surface work function, open circuit voltage and fill factor are dramatically increased from 0.99 to 1.06 V, and from 72.2% to 79.1%, respectively, resulting in 20.7% improvement in power conversion efficiency for the best performing devices. Scrutinizing the electrical properties of this modified interfacial layer strongly suggests that PCBB-2CN-2C8 passivates the TiO2 surface and thus reduces charge recombination loss caused by the deep trap states of TiO2. The passivation effect is further proven by stability testing of the perovskite solar cells with shelf lifetime under ambient conditions improved by a factor of more than 4, from ∼40 h to ∼200 h, using PCBB-2CN-2C8 as the TiO2 modification layer. This work offers not only a promising material for cathode interface engineering, but also provides a viable approach to address the challenges of deep trap states on TiO2 surface in planar perovskite solar cells.

We report the cooperative assembly of the fullerene-styrene cyano-(octyloxy)benzene triad (PCBB-CN-C8) and poly(3-hexylthiophene) (P3HT) to form an active layer of polymer solar cells (PSCs) with a well-defined microstructure and an enhanced stability of morphology. A favorable synergistic effect of the three functional moieties (C60, styrene cyano and tri(octyloxy) chains) in PCBB-CN-C8 can not only induce P3HT to assemble into long-range ordered periodic fibrils giving an interpenetrating network but also can form PCBB-CN-C8 crystallized domains without the need for external treatment. The characterization of the microstructure and morphology of P3HT:PCBB-CN-C8 blend films by two-dimensional grazing incidence X-ray diffraction, transmission electron microscopy and atomic force microscopy reveals that the P3HT fibrils possess a highly crystallized lamellar phase, and the spacing of the periodic P3HT fibrils is approximately 10 nm depending on the PCBB-CN-C8 crystallites, which fill in the P3HT interpenetrating network. Bulk heterojunction PSCs based on P3HT:PCBB-CN-C8 exhibit an improved open-circuit voltage and an excellent power conversion efficiency of 4.20%, which is greater than that of control PSCs based on P3HT:PCBB-C8 and the devices based on P3HT:PCBM with thermal annealing. We believe that the cooperative assembly of the active layer using the synergistic effect of the fullerene triad is a general approach that can be used to develop external treatment-free technology and improve the morphological stability of the active layer and photovoltaic performance.

•Well defined graphene oxide (pr-GOs) were successfully synthesized and applied in OSCs.•The pr-GOs as hole extraction layer (HEL) possessing controllable multiple functionalities.•Dependence between device performance and multiple functionalities of pr-GOs as HEL.•pr-GO-3 exhibited superior device efficiency, stability, and repeatability as compared with PEDOT:PSS.A simple method for synthesizing a series of graphene oxide with precise oxidation (pr-GO) (mild oxidation, moderate oxidation and severe oxidation) by strictly controlling pre-oxidation steps, oxidant content and oxidation time has been successfully developed. The well defined pr-GO as hole extraction layer (HEL) presented multiple functionalities, like modulation of work function, enhanced interfacial dipole, and excellent film-forming properties, which had significantly improved the efficiency and stability of organic solar cells. The P3HT:PC61BM system device based on pr-GO-3 HEL, which possessing well defined electronic structure and moderate oxidation, exhibited an improved 3.74% in power conversion efficiency and better air-stability compared to that of other pr-GOs and conventional PEDOT:PSS based devices. The well defined electronic structure pr-GO (i.e., suitable work function, larger interfacial dipole, and high repeatability) will provide better understanding in utilizing pr-GO film as HEL in future solar cell applications.A simple method for synthesizing a series of graphene oxide with precise oxidation (pr-GO) (mild oxidation, moderate oxidation and severe oxidation) has been successfully developed. pr-GOs present multiple functionalities which is an excellent hole extraction material. Moderate oxidated pr-GO-3 as hole extraction layer (HEL) exhibited an improved 3.74% in power conversion efficiency and better air-stability compared to devices with other pr-GOs and conventional PEDOT:PSS HEL.

A series of acceptor–donor–acceptor-based small molecules (SMs) with varied crystallinity were successfully synthesized. The processing additive can induce the SMs to self-organize as nanofibrils with higher crystallinity and controlled scales of nanofibrils, which have significant influence on the photovoltaic performance.

A bulk tri(octyloxy)benzene moiety grafted onto fullerene (PCBB-C8) was successfully synthesized, which could effectively induce poly(3-hexylthiophene) (P3HT) to form highly ordered bulk heterojunction structure without any external treatment. This ordered active layer exhibits good photovoltaic performance.

•Narrow band gap molecule can be obtained by tuning central units in A–D–A structure.•Small molecule with a mediated oligothiophene conjugated length exhibits higher crystallinity and more ordered structure.•Higher crystallization molecule showed better power conversion efficiency.•The optimized TT(HTTzHT)2:PC71BM based device exhibit a high PCE of 3.24% in OSCs.A series of high coplanar alternative linear small molecules with acceptor–donor–acceptor (A–D–A) structure containing electron-accepting tetrazine (Tz) moiety and electron-donating oligothiophenes (OTs) moiety, alkylated thiophene attached to both sides of the Tz moiety were designed and synthesized. The influences of varied oligothiophene length on small molecules’ optical and electrochemical properties, crystallization, self assembling morphology in blend film with (6,6)-phenyl-C61-butyric acid methyl ester (PC61BM), and photovoltaic properties for the application as donor materials in organic solar cells (OSCs) were studied. The optical and electrochemical properties of small molecules showed that the HOMO and LUMO energy levels were determined by the number of OTs moiety and electron-accepting ability of Tz in the alternative small molecules, respectively. Meanwhile, the varied OT moieties can significantly affect the hierarchical structures when mixed with PC61BM. The molecule with intermediate conjugate moity length showed the highest ordering in its crystalline state, as revealed by differential scanning calorimetry (DSC) and X-ray diffraction experiments, and best photovoltaic properties when blended together with PC61BM or (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) as active layer in photovoltaic devices. The results indicate that hierarchical structures controlled by adjusting the conjugate moity length of small molecules is an effective way to improve the performance of OSCs. The photovoltaic device based on TT(HTTzHT)2:PC71BM with 1% DIO additives showed the best performance, with a Jsc of 7.87 mA/cm2 and a PCE of 3.24%.A series of high coplanar alternative linear small molecules with acceptor–donor–acceptor (A–D–A) structure containing electron-accepting tetrazine moiety and electron-donating olighothiophenes (OTs) moiety, alkylated thiophene attached to both sides of the Tz moiety were designed and synthesized. The influences of varied oligothiophene length on small molecules’ optical and electrochemical properties, crystallization, hierarchical structure in blend film with (6,6)-phenyl-C61-butyric acid methyl ester, and photovoltaic properties for the application as donor materials in organic solar cells were studied. The hierarchical structure can be tuned by changing the OTs based on the A–D–A structure, which exhibit great effect on the photovoltaic properties.

A bulk tri(octyloxy)benzene moiety grafted onto fullerene (PCBB-C8) was successfully synthesized, which could effectively induce poly(3-hexylthiophene) (P3HT) to form highly ordered bulk heterojunction structure without any external treatment. This ordered active layer exhibits good photovoltaic performance.

We report the cooperative assembly of the fullerene-styrene cyano-(octyloxy)benzene triad (PCBB-CN-C8) and poly(3-hexylthiophene) (P3HT) to form an active layer of polymer solar cells (PSCs) with a well-defined microstructure and an enhanced stability of morphology. A favorable synergistic effect of the three functional moieties (C60, styrene cyano and tri(octyloxy) chains) in PCBB-CN-C8 can not only induce P3HT to assemble into long-range ordered periodic fibrils giving an interpenetrating network but also can form PCBB-CN-C8 crystallized domains without the need for external treatment. The characterization of the microstructure and morphology of P3HT:PCBB-CN-C8 blend films by two-dimensional grazing incidence X-ray diffraction, transmission electron microscopy and atomic force microscopy reveals that the P3HT fibrils possess a highly crystallized lamellar phase, and the spacing of the periodic P3HT fibrils is approximately 10 nm depending on the PCBB-CN-C8 crystallites, which fill in the P3HT interpenetrating network. Bulk heterojunction PSCs based on P3HT:PCBB-CN-C8 exhibit an improved open-circuit voltage and an excellent power conversion efficiency of 4.20%, which is greater than that of control PSCs based on P3HT:PCBB-C8 and the devices based on P3HT:PCBM with thermal annealing. We believe that the cooperative assembly of the active layer using the synergistic effect of the fullerene triad is a general approach that can be used to develop external treatment-free technology and improve the morphological stability of the active layer and photovoltaic performance.