Xiaoling Ma

Nematic liquid crystal materials as a morphology regulator for ternary small molecule solar cells with power conversion efficiency exceeding 10

Solution-processed small molecule solar cells (SMSCs) are fabricated based on DRCN5T:PC71BM as active layers, the power conversion efficiency (PCE) is markedly increased from 3.63% to 9.11% for the active layers undergoing up-side-down thermal annealing and solvent vapor annealing post-treatments. The PCE improvement should be attributed to the appropriate phase separation consisting of enhanced crystallinity of donor and purified acceptor domain at nanoscale. The nematic liquid crystal small molecule BTR is selected as the second donor and morphology regulator to prepare ternary SMSCs. The champion PCE of ternary SMSCs was improved to 10.05% by mixing 1.5 wt% BTR, which corresponds to a 10.3% PCE improvement compared with the optimized binary SMSCs. The performance improvement is mainly attributed to the further optimized phase separation and complementary photon harvesting between DRCN5T and BTR, which could be well demonstrated from absorption spectra, two dimensional grazing incidence X-ray diffraction (2D-GIXD) and transmission electron microscopy (TEM).

Efficient ternary non-fullerene polymer solar cells with PCE of 11.92% and FF of 76.5%

Non-fullerene polymer solar cells (PSCs) attract more attention due to the constantly refreshed power conversion efficiency (PCE) based on the versatile non-fullerene acceptors. In this work, PCEs of 10.51% and 11.02% were obtained for two kinds of non-fullerene PSC with IDT6CN-M or ITCPTC as the acceptor and PBDB-T as the donor. ITCPTC has a relatively narrow bandgap and a high absorption coefficient compared with IDT6CN-M, which explains well the relatively large short-circuit current density of 17.44 mA cm−2 for the ITCPTC based binary PSCs. Meanwhile, the IDT6CN-M based binary PSCs exhibit a relatively high fill factor (FF) of 75.3% and an open-circuit voltage of 0.915 V. A PCE of 11.92% and a FF of 76.5% were achieved for the ternary PSCs with 60 wt% ITCPTC content in the acceptors, which should be attributed to the enhanced photon harvesting and their good compatibility for a synergistic improvement of exciton utilization and charge transport in the ternary active layers. The FF of 76.5% is among the top values of ternary non-fullerene PSCs.

A liquid crystal material as the third component for ternary polymer solar cells with an efficiency of 10.83% and enhanced stability

Herein, a liquid crystal material, BTR, was elaborately selected as the third component to complement PTB7-Th for the fabrication of highly efficient ternary polymer solar cells (PSCs). Via incorporating 10 wt% BTR in their donors, the champion power conversion efficiency (PCE) of the PSCs increased from 10.08% to 10.83%, resulting from the enhanced short circuit current (JSC) of 19.23 mA cm−2 and fill factor (FF) of 72.21%. The small amount of incorporated BTR may prefer to distribute in the PTB7-Th networks and has good miscibility with PC71BM. The excitons on BTR may dissociate into free charge carriers at the BTR/PC71BM interfaces and also transfer their energy to PTB7-Th through Forster resonance energy transfer, resulting in improved exciton utilization. Moreover, the molecular arrangement and morphology of the active layers could be optimized by incorporating appropriate amount of BTR as a nucleating agent, also leading to enhanced stability of the ternary PSCs. The positive effects of BTR on the performance improvement of PSCs were confirmed from the inverted and conventional structures of the cells.

Energy level modulation of non-fullerene acceptors enables efficient organic solar cells with small energy loss

Two new non-fullerene (NF) acceptors, namely BDTIT-M and BDTThIT-M, were rationally designed to optimize the energy levels and optical bandgap. BDTIT-M is derived by changing the end-group of NFBDT into slightly weak DCI-M, and BDTThIT-M is obtained by adding two conjugated thiophene side-chains into a ladder-type core of BDTIT-M. By incorporating with the polymer donor PBDB-T, BDTIT-M based organic solar cells (OSCs) deliver a higher PCE of 11.31% compared to that of NFBDT based cells, which is mainly attributed to the increased VOC and FF. A higher PCE of 12.12% with a small energy loss of ∼0.588 eV is achieved compared with BDTThIT-M based OSCs, benefiting from the elevated LUMO level, narrowed bandgap, and enhanced absorption coefficient and electron mobility of BDTThIT-M compared with BDTIT-M. The combination of a methyl-modified end-group and conjugated side-chain should be an efficient strategy to elevate the LUMO and HOMO levels with different amplitudes for realizing simultaneous improvement in VOC and JSC.

Simultaneously Enhanced Efficiency and Stability of Polymer Solar Cells by Employing Solvent Additive and Upside-down Drying Method

The morphology of active layer plays an important role in determining the power conversion efficiency (PCE) and stability of polymer solar cells (PSCs), which strongly depend on the dynamic drying process of active layer. In this work, an efficient and universal method was developed to let active layer undergo upside-down drying process in a covered glass Petri dish. For the PSCs based on PTB7-Th:PC71BM, the champion PCEs were improved from 8.58% to 9.64% by mixing 3 vol % 1,8-di-iodooctane and further to 10.30% by employing upside-down drying method. The enhanced PCEs of PSCs with active layers undergoing upside-down drying process are mainly attributed to the optimized vertical phase separation, the more ordered and tightly packed π-π stacking of polymer molecules. Meanwhile, PC71BM molecules can be frozen in more ordered and tightly packed π-π stacking polymer network, which lead to the enhanced stability of PSCs. The universality of upside-down drying method can be solidly confirmed from PSCs with PTB7:PC71BM, PffBT4T-2OD:PC71BM, or PBDT-TS1:PC71BM as active layers, respectively. The molecular packing and phase separation of blend films with different solvent additives and drying methods were investigated by grazing incidence X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy.

Ternary nonfullerene polymer solar cells with efficiency >13.7% by integrating the advantages of the materials and two binary cells

Highly efficient ternary polymer solar cells (PSCs) are fabricated from two well-compatible small molecular nonfullerene acceptors (INPIC-4F and MeIC1) and one polymer donor, PBDB-T. The power conversion efficiency (PCE) of the INPIC-4F or MeIC1 based binary PSCs reaches 12.55% and 11.53%. Based on these efficient binary PSCs, a high PCE of 13.73% is achieved in the ternary PSCs with 50 wt% MeIC1 in the acceptors, resulting from the simultaneously improved short circuit current (JSC) of 21.86 mA cm−2, open circuit voltage (VOC) of 0.88 V and fill factor (FF) of 71.39%. The PCE improvement of the ternary PSCs should be mainly attributed to the simultaneously optimized photon harvesting and film morphology of the ternary active layers. This result may provide more in-depth insight into the material selection criteria for fabricating highly efficient ternary PSCs: (i) the complementary absorption spectra and good compatibility of the used materials; (ii) the complementary photovoltaic parameters of the corresponding two binary PSCs.

Ternary non-fullerene polymer solar cells with an efficiency of 11.6% by simultaneously optimizing photon harvesting and phase separation

Ternary non-fullerene polymer solar cells (PSCs) were fabricated with J71 as a donor and mixed ITIC:MeIC2 as acceptors. The power conversion efficiency (PCE) of the ternary PSCs reaches 11.6% for 20 wt% MeIC2 in the acceptors, which is larger than the 10.7% or 10.2% for binary PSCs with ITIC or MeIC2 as an acceptor. The PCE improvement of ternary PSCs is mainly attributed to the simultaneously enhanced short circuit current density (JSC) of 18.1 mA cm−2 and fill factor (FF) of 70.5%. The photon harvesting of ternary active layers can be optimized by adjusting MeIC2 content due to the complementary asborption of ITIC and MeIC2, leading to the enhanced JSC. Meanwhile, phase separation of ternary active layers can be further optimized for efficient exciton dissociation and more balanced charge transport. The improved FF can be well explained from the more balanced charge transport evaluated by the ratios of hole mobility to electron mobility in the active layers.

Efficient Ternary Organic Solar Cells with Two Compatible Non-Fullerene Materials as One Alloyed Acceptor

Efficient ternary organic solar cells (OSCs) are fabricated by employing a polymer PBT1-C as the donor and two non-fullerene materials, MeIC and MeIC2, as one alloyed acceptor. The optimized ternary OSCs with 30 wt% MeIC2 in acceptors achieve a power conversion efficiency (PCE) of 12.55%, which is much higher than that of 11.47% for MeIC-based binary OSCs and 11.41% for MeIC2-based binary OSCs. The >9.4% improvement in PCE is mainly attributed to the optimized photon harvesting and morphology of ternary active layers, resulting in the simultaneously improved short-circuit current and fill factor. Furthermore, good compatibility and similar lowest unoccupied molecular orbital energy levels of MeIC and MeIC2 are beneficial to form one alloyed acceptor for efficient electron transport in the ternary active layers. This work may provide new insight when selecting the third component for preparing efficient ternary OSCs.

Simultaneously improved efficiency and average visible transmittance of semitransparent polymer solar cells with two ultra-narrow bandgap nonfullerene acceptors

One narrow bandgap donor (PTB7-Th) and two ultra-narrow bandgap acceptors (COi8DFIC and IEICO-4F) were elaborately selected to fabricate semitransparent ternary polymer solar cells (PSCs). To find out the optimal content of IEICO-4F in the acceptors, a series of opaque PSCs were first fabricated with 100 nm of Ag as the electrode. The power conversion efficiency (PCE) of the optimized opaque ternary PSCs arrived at 11.94% by incorporating 15 wt% IEICO-4F in the acceptors. Then, the thickness of Ag was adjusted to balance its electrical conductivity and transmittance, and the 100 nm of Ag was replaced by 15 nm of Ag as a semitransparent electrode. The semitransparent electrode possesses high transmittance in the visible light range and low transmittance in the long wavelength range, which is propitious for photon harvesting of ternary active layers with two ultra-narrow bandgap acceptors. The optimized semitransparent ternary PSCs show a synchronously improved PCE of 8.23% and an average visible transmittance (from 370 nm to 740 nm) of 20.78% compared with COi8DFIC based semitransparent binary PSCs. This work suggests the potential of a ternary strategy for fabricating highly efficient semitransparent PSCs with narrow or ultra-narrow bandgap materials as active layers.

Efficient ternary organic solar cells with high absorption coefficient DIB-SQ as the third component

A series of organic solar cells (OSCs) are prepared with PTB7:PC71BM as the host materials and DIB-SQ as the third component. The power conversion efficienty (PCE) of OSCs can be improved from 6.79% to 7.92% by incorporating 6 wt% DIB-SQ into donors, resulting from the enhanced short circuit current density (J SC) and fill factor (FF). The increased J SC of the optimized ternary OSCs should be attributed to the enhanced photon harvesting of teranry active layer by incorporating DIB-SQ. Meanwhile, FF of the optimized ternary OSCs should be due to the optimied phase separation. The open circuit voltage (V OC) of tenray OSCs can be maintained at a constant of 0.75 V, indicating that all photogenerated holes willl be transported along the channels formed by PTB7.