para-azaquinodimethane (p-AQM) based quinoidal-donor (Q-D) alternating copolymers offer a unique opportunity to investigate the properties of quinoidal polymers. It was however reported that the bandgap increases with increasing number of...

In solar-cell applications, metal chalcogenides have been widely used as light-harvesting materials, especially CIGS and CdTe solar cells, demonstrating a power conversion efficiency (PCE) of 23.35% [1] and over 22%. [2,3] Achieving net-zero carbon emission in the near future, and also due to the scarcity and toxicity of Ga and Cd, restricts further development. In recent years, the antimony chalcogenides family have been considered as a promising contender for absorber material, especially antimony selenide, Sb 2 Se 3 , antimony sulfur, Sb 2 S 3 , and antimony sulfoselenide, Sb 2 (S,Se) 3 . These materials show excellent photoelectric properties, such as tunable bandgap (1.1-1.7 eV), high-absorption coefficient (>10 5 cm À1 ), stability against moisture, as well as low toxicity, environmental friendliness, earth abundance, and finally, simple preparation process. [4][5][6][7][8][9][10] So far, the Sb 2 (S,Se) 3 solar-cell PCE has been quickly enhanced (i.e., over 10%) by material preparation, device configuration, defect passivation, and interface modification. [5,[11][12][13] And these devices are made up of an expensive organic Spiro-OMeTAD hole-transport layer (HTL), demonstrating poor device stability due to the bis(trifluoromethane) sulfonimide lithium salt (Li-TFSI) doping into Spiro-OMeTAD. Moreover, the toxicity of the dopant 4-tert-butylpyridine, additive acetonitrile, and the solvent chlorobenzene are hazardous to the human central nervous system. [6,14,15] It is well-known that the HTL plays a crucial role in determining the device's photovoltaic (PV) performance by extracting photogenerated holes from the absorber to the electrode and it's generating a built-in electric field at the rear interface. Therefore, choosing the HTL material with high hole mobility, high stability, low toxicity, and low cost is essential to enhance PV performance with improved device stability. According to the published reports, notably, the Tao Chen group employed different HTLs for Sb 2 (S,Se) 3 solar cells than conventional Spiro-OMeTAD, such as CsPbBr 3 perovskite quantum dots, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) doped copper phthalocyanine, and DTPThMe-ThTPA, demonstrating PCE 7.82%, [16] 8.57%, [17] and 9.7%, [18] respectively. However, the aforementioned HTLs are prepared by the solution process, which is unsuitable for large-scale productions. In that consideration, a thermally evaporated low-cost inorganic manganese sulfide (MnS) semiconductor is an excellent choice for the HTL, and recently, Qian et al. and Wang et al. used this MnS-HTL in Sb 2 (S,Se) 3 solar cells, achieving 9.6% [6] and 9.24% [19] of PCE. Also, their results evidently explain that the post-annealing treatment for MnS-HTL significantly influences the PV performance.

Four new small molecules – CTDP, BCTDP, CFDP, and BCFDP having D‐π‐A‐π‐D molecular architecture, possessing carbazole and benzocarbazole as electron donors, diketopyrrolopyrrole core as acceptor and thiophene/furan acting as spacer/bridge between donor (carbazole and benzocarbazole) and acceptor (diketopyrrolopyrrole) units are synthesized. All the four compounds exhibited absorption in the range of 300 to 700 nm, and, in particular, more intense absorption found in the 500 to 660 nm region. The estimated band gaps are found to be 1.92 eV for CTDP, 1.92 eV for BCTDP, 1.94 eV for CFDP, and 1.92 eV for BCFDP from their intersection point of absorption and emission spectra. The electrochemical studies revealed that the highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels of all the four compounds, CTDP (−5.03/−3.65 eV), BCTDP (−5.03/−3.65 eV), CFDP (−4.94/−3.65 eV), and BCFDP (−4.90/−3.62 eV) are well matched with PCBM and expected to be act as donor materials in small molecule bulk hetero junction organic solar cells. All the compounds are thermally stable up to 382–416°C.

Understanding the structure‐property relationship is one of the critical factors in the practical design of organic small molecules for bulk heterojunction organic solar cells (BHJ‐OSCs). In this study, we designed and synthesized two low‐band‐gap organic small molecule donors, TRh and PhRh. To study the effect of the π‐conjugated spacer, the A‐π‐D‐π‐A architecture was constructed with an efficient dithienopyrrole donor (D) and rhodanine acceptor (A) units. Two types of π‐conjugated spacers, thiophene in TRh and phenyl in PhRh, are used to fine‐tune the various properties. A detailed investigation of the effect of π‐conjugated spacers on optoelectronic, thermal, and photovoltaic properties have carried out. TRh is planar, more conjugated, has a more prolonged absorption wavelength and leads to a low band gap of 1.48 eV. DFT studies demonstrate that TRh has lower BLA (Bond length alternation) and has more quinoid structure than PhRh. PhRh is thermally more stable than TRh. PhRh displayed a high glass transition temperature (Tg) at 193 °C and a 5% decomposition temperature at 402 °C. The inverted device architecture of BHJ‐OSCs blended with PC71BM displayed the PCE of 2.78% for TRh and 1.10% for PhRh. TRh : PC71BM blend exhibits higher hole and electron mobilities than PhRh : PC71BM blend.