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Developing a bridge capping layer between perovskite and hole transport layer materials (HTMs) in the n-i-pperovskite solar cells (pero-SCs) is an effective approach to modify the morphology of HTMs and passivate the perovskite simultaneously. Herein, we select the quinoxaline-based bifunctional passivation agent, quinoxalin-6-yl-methylamine hydrochloride (QxMACl), as the bridging layer, and a D-A copolymer PBQ12 containing the same quinoxaline unit as an HTM for the n-i-ppero-SCs. Due to the π-πstacking among the common quinoxaline units in the bridge layer and HTM, QxMACl induces the π-πstacking of the PBQ12 film and improves the film morphology of HTMs with better conductivity. Additionally, QxMACl can effectively passivate the perovskite surface, and PBQ12 possesses appropriate energy levels and high hole mobility. The pero-SCs based on FAPbI3with PBQ12/QxMACl treatment showed a higher power conversion efficiency (PCE) of 24.05% and outstanding stability, maintaining 95.4% and 92.1% of its initial PCE after 750 h of storage and after over 800 h of thermal annealing at 85 °C, respectively. To further enhance the PCE of the PBQ12/QxMACl-based devices, we developed a non-metal ion dopant for the PBQ12 HTM. Through trace doping of PBQ12 HTM by the non-metal ion dopant, the PCE of the PBQ12/QxMACl-based devices reached 25.24% (the calibrated PCE of 24.55% by the National Institute of Metrology, China).

The medium-bandgap polymerized small molecule acceptors (PSMAs) have broad application scenarios. However, the effort in the molecular design of the high-performance medium-bandgap PSMAs is limited. In this article, we introduce alkoxy groups as outer side chains and as substituents of the thiophene π-bridges of the high-performance PSMA PY-IT to synthesize a medium-bandgap PSMA PO-TO. Due to that the non-covalent interaction between the alkoxy groups and the terminal groups of the small molecule acceptor (SMA) unit can weaken the intramolecular charge transfer (ICT) effect, the bandgap of PO-TO is enlarged and its absorption is blue-shifted compared with PY-IT, while the absorbance of PO-TO solution and film is enhanced significantly compare with that of PY-IT. When blended PO-TO with the polymer donor PBQx-TF, the corresponding all-polymer solar cells (all-PSCs) exhibit an open-circuit voltage (Voc) exceeding 1.04 V with a power conversion efficiency (PCE) of 13.75%. Furthermore, PO-TO was used as the third component to fabricate ternary all-PSCs with PBQx-TF as the polymer donor and PY-IT as the main polymer acceptor, and the ternary all-PSCs based on PBQx-TF:PY-IT:PO-TO (1:1:0.2, w/w/w) demonstrated a high PCE of 17.71% with simultaneously improved Vocof 0.940 V, short-circuit current density (Jsc) of 24.60 mA cm−2and fill factor (FF) of 76.81%. In comparison, the binary all-PSCs based on PBQx-TF:PY-IT showed a PCE of 16.77%. This result indicates that introducing alkoxy groups is a promising strategy for synthesizing high-performance medium-bandgap PSMAs.

Isomerization of functional groups in photovoltaic materials plays a vital role in molecular packing and their photovoltaic performance. In this work, two narrow-band gap polymer acceptors, PY5-TTz-FT and PY5-TTz-CT, are synthesized by copolymerization of the small-molecule acceptor (SMA) Y5 as the main backbone and thiophene-thiazolothiazole (TTz)-thiophene with isomerized ethylhexyl substitution on the thiophene π-bridges as the linking unit. The polymerized SMA (PSMA) PY5-TTz-CT with the ethylhexyl substituents close to the TTz unit exhibits red-shifted absorption spectra and a relatively higher lying lowest unoccupied molecular orbital (LUMO) energy level, in comparison with PY5-TTz-FT with the ethylhexyl substituents far from the TTz unit. However, PY5-TTz-CT exhibits poorer photovoltaic performance than that of PY5-TTz-FT in the all-polymer solar cells (all-PSCs) with PBDB-T as the polymer donor, due to the poor morphology of the PBDB-T:PY5-TTz-CT active layer, while the optimized all-PSC based on PBDB-T:PY5-TTz-FT shows a higher power conversion efficiency of 12.73%. The results imply that isomerization of polymer acceptors is an effective strategy to tune the molecular packing of the photovoltaic materials and photovoltaic performance of the all-PSCs.

To reduce synthetic cost of the classic fluorinated bithienyl benzodithiophene (BDTT-F) unit, here, an alpha-fluorinated bithienyl benzodithiophene unit, namely, α-BDTT-F (F atom in the α position of the lateral thiophene unit), is developed by the isomerization strategy of exchanging the positions of the F atom and flexible alkyl chain on the lateral thiophene unit of the BDTT-F unit. The α-BDTT-F unit was synthesized with less synthetic steps, higher synthetic yield, and less purification times from the same raw materials as those of the BDTT-F unit, thus with low synthetic cost. Theoretical calculation indicates that the α-BDTT-F unit possesses a similar twisted conformation and electronic structures as those of the BDTT-F unit. The α-BDTT-F-based polymer α-PBQ10 exhibits similar light absorption and energy levels as those of the corresponding BDTT-F-based polymer PBQ10 but marginally increased molecular aggregation and stronger hole transport than PBQ10. In consequence, the α-PBQ10:Y6-based polymer solar cell demonstrates a slightly enhanced power conversion efficiency (PCE) of 16.26% compared with that of the PBQ10:Y6-based device (PCE = 16.23%). Also, the PCE is further improved to 16.77% through subtle microscopic morphology regulation of the photoactive layer with the fullerene derivative indene-C60 bisadduct as the third component. This work provides new ideas for the design of low-cost and high-efficiency photovoltaic molecules.

Poor stability of spiro-OMeTAD hole transport materials (HTM) with dopant is a major obstacle for the commercialization of perovskite solar cell (pero-SC). Herein, we demonstrate a series of quinoxaline-based D-A copolymers PBQ5, PBQ6 and PBQ10 as the dopant-free polymer HTMs for high performance pero-SCs. The D-A copolymers are composed of fluorothienyl benzodithiophene (BDTT) as D-unit, difluoroquinoxaline (DFQ) with different side chains as A-unit, and thiophene as π-bridge, where the side chains on the DFQ unit are bi-alkyl for PBQ5, bi-alkyl-fluorothienyl for PBQ6, and alkoxyl for PBQ10. All the three copolymers are adopted as the dopant-free HTM in the pero-SCs. The planar n-i-pstructured pero-SCs based on (FAP-bI3)0.98(MAPbBr3)0.02with PBQ6 HTM demonstrated the high power conversion efficiency (PCE) of 22.6% with Vocof 1.13 V and FF of 80.8%, which is benefitted from the suitable energy level and high hole mobility of PBQ6. The PCE of 22.6% is the highest efficiency reported in the n-i-pstructured pero-SCs based on dopant-free D-A copolymer HTM. In addition, the pero-SCs show significantly enhanced ambient, thermal and light-soaking stability compared with the devices with traditional spiro-OMeTAD HTM.

Molecular frontier orbital energy level and aggregation behavior regulation of polymer donors are feasible ways to improve the photovoltaic performance of polymer solar cells (PSCs). Here, we design and synthesize a new D–A copolymer donor PBQ10 based on bithienyl benzodithiophene D-unit and monoalkoxy-substituted bifluoroquinoxaline A-unit, which shows an obviously downshifted highest occupied molecular orbital energy level in comparison with the control polymer PBQ7 with a dialkoxyphenyl substituent on the bifluoroquinoxaline A-unit. Moreover, PBQ10 exhibits more preferential face-on molecular orientation and tighter π–π stacking in the vertical direction of the substrate than PBQ7,which significantly improves the hole mobility of PBQ10 to 5.22 × 10–4cm2V–1s–1in comparison with that (1.71 × 10–4cm2V–1s–1) of PBQ7. As a result, the PBQ10-based PSC with Y6 as the acceptor demonstrates an impressive power conversion efficiency (PCE) of 16.34% with simultaneously increased open-circuit voltage and fill factor, which is significantly increased compared with the PBQ7-based PSC with a PCE of 13.45% and is one of the highest PCEs in binary PSCs. The result suggests that rational side-chain optimization of the polymer donor is an efficient way to regulate the molecular energy level and self-assembly feature and thus to improve the PCE of PSCs.

Achieving efficient charge transfer at small frontier molecular orbital offsets between donor and acceptor is crucial for high performance polymer solar cells (PSCs). Here we synthesize a new wide band gap polymer donor, PTQ11, and a new low band gap acceptor, TPT10, and report a high power conversion efficiency (PCE) PSC (PCE = 16.32%) based on PTQ11–TPT10 with zero HOMO (the highest occupied molecular orbital) offset (ΔEHOMO(D–A)). TPT10 is a derivative of Y6 with monobromine instead of bifluorine substitution, and possesses upshifted lowest unoccupied molecular orbital energy level (ELUMO) of −3.99 eV and EHOMOof −5.52 eV than Y6. PTQ11 is a derivative of low cost polymer donor PTQ10 with methyl substituent on its quinoxaline unit and shows upshifted EHOMOof −5.52 eV, stronger molecular crystallization, and better hole transport capability in comparison with PTQ10. The PSC based on PTQ11–TPT10 shows highly efficient exciton dissociation and hole transfer, so that it demonstrates a high PCE of 16.32% with a higher Vocof 0.88 V, a large Jscof 24.79 mA cm–2, and a high FF of 74.8%, despite the zero ΔEHOMO(D–A)value between donor PTQ11 and acceptor TPT10. The PCE of 16.32% is one of the highest efficiencies in the PSCs. The results prove the feasibility of efficient hole transfer and high efficiency for the PSCs with zero ΔEHOMO(D–A), which is highly valuable for understanding the charge transfer process and achieving high PCE of PSCs.