ZiQi Sun

The morphology, photophysics, and device performance of solar cells based on the low bandgap polymer poly[[2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl (PBDTTT-EFT) (also known as PTB7-Th) blended with different fullerene acceptors: Phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), phenyl-C₇₁ -butyric acid methyl ester (PC₇₁BM), or indene-C₆₀ bisadduct (ICBA) are correlated. Compared to PC₇₁ BM-based cells – which achieve a power conversion efficiency (PCE) of 9.4% – cells using ICBA achieve a higher open-circuit voltage (V_OC) of 1.0 V albeit with a lower PCE of 7.1%. To understand the origin of this lower PCE, the morphology and photophysics have been thoroughly characterized. Hard and soft X-ray scattering measurements reveal that the PBDTTT-EFT:ICBA blend has a lower crystallinity, lower domain purity, and smaller domain size compared to the PBDTTT-EFT:PC₇₁BM blend. Incomplete photoluminescence quenching is also found in the ICBA blend with transient absorption measurements showing faster recombination dynamics at short timescales. Transient photovoltage measurements highlight further differences in recombination at longer timeframes due to the more intermixed morphology of the ICBA blend. Interestingly, a mild thermal treatment improves the performance of PBDTTT-EFT:ICBA cells which is exploited in the fabrication of a homo PBDTTT-EFT:ICBA tandem solar cell with PCE of 9.0% and V_OC of 1.93 V.

The mixing behavior of a high-efficiency polymer with various fullerene derivatives is investigated. Compared to solar cells using either PC₆₁BM or PC₇₁ BM as acceptor, cells using ICBA have a lower efficiency due to a more intermixed morphology. ICBA blends, however, show a higher thermal stability which is exploited in the fabrication of homo-tandem cells with 9.0% power conversion efficiency.

Nonfullerene polymer solar cells (PSCs) are fabricated with a perylene monoimide-based n-type wide-bandgap organic semiconductor PMI-F-PMI as an acceptor and a bithienyl-benzodithiophene-based wide-bandgap copolymer PTZ1 as a donor. The PSCs based on PTZ1:PMI-F-PMI (2:1, w/w) with the treatment of a mixed solvent additive of 0.5% N-methyl pyrrolidone and 0.5% diphenyl ether demonstrate a very high open-circuit voltage (V_oc) of 1.3 V with a higher power conversion efficiency (PCE) of 6%. The high V_oc of the PSCs is a result of the high-lying lowest unoccupied molecular orbital (LUMO) of −3.42 eV of the PMI-F-PMI acceptor and the low-lying highest occupied molecular orbital (HOMO) of −5.31 eV of the polymer donor. Very interestingly, the exciton dissociation efficiency in the active layer is quite high, even though the LUMO and HOMO energy differences between the donor and acceptor materials are as small as ≈0.08 and 0.19 eV, respectively. The PCE of 6% is the highest for the PSCs with a V_oc as high as 1.3 V. The results indicate that the active layer based on PTZ1/PMI-F-PMI can be used as the front layer in tandem PSCs for achieving high V_oc over 2 V.

Nonfullerene polymer solar cells with PTZ1 as a donor and PMI-F-PMI as an acceptor demonstrate a very high open-circuit voltage (V_oc) of 1.3 V with a higher power conversion efficiency of 6%. The high V_oc is a result of the high-lying lowest unoccupied molecular orbital (LUMO) of −3.42 eV of the PMI-F-PMI acceptor and the low-lying highest occupied molecular orbital (HOMO) of −5.31 eV of the polymer donor.

Photonic nanostructures are created in organo-metal halide perovskites by thermal nanoimprint lithography at a temperature of 100 °C. The imprinted layers are significantly smoothened compared to the initially rough, polycrystalline layers and the impact of surface defects is substantially mitigated upon imprint. As a case study, 2D photonic crystals are shown to afford lasing with ultralow lasing thresholds at room temperature.

A new, easy, and efficient approach is reported to enhance the driving force for charge transfer, break tradeoff between open-circuit voltage and short-circuit current, and simultaneously achieve very small energy loss (0.55 eV), very high open-circuit voltage (>1 V), and very high efficiency (>10%) in fullerene-free organic solar cells via an energy driver.

While SnPb alloyed perovskites have been considered as an effective approach to broaden the absorption spectrum, it is still challenging to modify the crystallization (and thus morphology, crystallinity, and orientation) in a controllable manner and thus boost the efficiency of SnPb alloyed perovskite solar cells. Here, it is unveiled that controlling the crystallization of CH₃NH₃Sn_0.25Pb_0.75I₃ films can be simply realized by adjusting the amount of dimethyl sulfoxide in precursors, which has not been reported in SnPb alloyed perovskite systems. The remarkable perovskite crystallinity enhancement by the 20-fold enhanced (110) plane intensity in the X-ray diffraction spectrum of CH₃NH₃Sn_0.25Pb_0.75I₃ and the preferred (110) orientation with the texture coefficient enhanced by 2.6 times to reach 0.88 are demonstrated. Importantly, it is discovered that the introduction of dimethyl sulfoxide avoids the formation of the colloidal coagulation observed in prolonged-storage precursors and ameliorates inhomogeneous Sn/Pb distributions in resultant perovskite films. Through optimizing perovskite films and device structures, hysteresis-free planar-heterojunction CH₃NH₃Sn_0.25Pb_0.75I₃ solar cells with the efficiency reaching 15.2%, which are the most efficient SnPb alloy-based perovskite solar cells, are achieved.

CH₃NH₃Sn_0.25Pb_0.75I₃ films with enhanced crystallinity, preferred orientation, and ameliorated inhomogeneous Sn/Pb distributions are realized by the controllable crystallization via the introduction of dimethyl sulfoxide. Optimized planar-heterojunction CH₃NH₃Sn_0.25Pb_0.75I₃-based solar cells achieve the power conversion efficiency of 15.2% and no hysteresis, which is the highest value among SnPb alloy-based perovskite solar cells.