Recent Progress in All-Solution-Processed Organic Solar Cells

All-solution-processed organic solar cells(OSCs)(from the bottom electrode to the top electrode)are highly attractive thanks to their low cost,lightweight and high-throughput production.However,achieving highly efficient all-solution-processed OSCs remains a significant challenge.One of the key issues is the lack of high-quality solution-processed electrode systems that can replace indium tin oxide(ITO)and vacuum-deposited metal electrodes.In this paper,we comprehensively review recent advances in all-solution-processed osCs,and classified the devices as the top electrode materials,including silver nanowires(AgNWs),conducting polymers and composite conducting materials.The correlation between electrode materials,properties of electrodes,and device performance in all-solution-processed OSCs is elucidated.In addition,the critical roles of the active layer and interface layer are also discussed.Finally,the prospects and challenges of all-solution-processed OSCs are presented.

Benzothiadiazole-based materials for organic solar cells

The past few years have witnessed power conversion efficiency(PCE)of organic solar cells(OSCs)skyrocketing to the value of 20%due to the outstanding advantages of organic photoactive materials.The latter,which consist of donor and acceptor materials,indeed play important roles in OSCs,and particularly one building block has attracted considerable research attention,namely benzothiadiazole(BT).The diversity of OSCs based on the BT structure have indeed sprung up,and the progressive increase in PCE values is more than just eye-catching since it heralds a renewal and bright future of OSCs.This review analyzes significant studies that have led to these remarkable progresses and focuses on the most effective BT small-molecules and BT polymers for OSC reported in the last decades.The pivotal structure-property relationships,donor-acceptor matching criteria,and morphology control approaches are gathered and discussed in this paper.Lastly,we summarize the remaining challenges and offer a personal perspective on the future advance and improvement of OSCs.

Control of vertical phase separation in high performance non-fullerene organic solar cell by introducing oscillating stratification preprocessing

Non-fullerene organic solar cell(NFOSC)has attracted tremendous attention due to their great potential for commercial applications.To improve its power conversion efficiency(PCE),generally,sequential solution deposition(SSD)methods have been employed to construct the graded vertical phase separation(VPS)of the bulk-heterojunction(BHJ)active layer for efficient exciton separation and charge transition.However,a variety of orthogonal solvents used in the SSD may lead to the unpredicted change in the BHJ morphology and introduce additional defects inside the BHJ bulk thus complicate the fabrication process.Here,a simple oscillating stratification preprocessing(OSP)is developed to facilitate the formation of graded VPS among the BHJ layer.As a result,a significant improvement is obtained in PCE from 10.96%to 12.03%,which is the highest value reported among PBDB-T:ITIC based NFOSC.

Simple non-fullerene electron acceptors with unfused core for organic solar cells

Two simple electron acceptors based on unfused bithiophene core and 1,1-dicyanomethylene-3-indanone end group were easily prepared via three synthetic steps. These acceptors exhibited broad absorption in the range of 300 nm to 800 nm, aligned energy levels and high crystallinity. When combined with a wide band gap donor polymer in non-fullerene solar cells, an initial power conversion efficiency of 2.4% was achieved. The relatively low efficiencies were due to the large phase separation in blended thin films, which is originated from their high aggregation tendency in thin films. Our results suggest that these electron acceptors with unfused core are promising candidates for commercial application of solar cells due to the low cost starting materials and facile synthesis.

Achieving over 16% efficiency for single-junction organic solar cells

To achieve high photovoltaic performance of bulk hetero-junction organic solar cells(OSCs), a range of critical factors including absorption profiles, energy level alignment, charge carrier mobility and miscibility of donor and acceptor materials should be carefully considered. For electron-donating materials, the deep highest occupied molecular orbital(HOMO) energy level that is beneficial for high open-circuit voltage is much appreciated. However, a new issue in charge transfer emerges when matching such a donor with an acceptor that has a shallower HOMO energy level. More to this point, the chemical strategies used to enhance the absorption coefficient of acceptors may lead to increased molecular crystallinity, and thus result in less controllable phase-separation of photoactive layer. Therefore, to realize balanced photovoltaic parameters, the donor-acceptor combinations should simultaneously address the absorption spectra, energy levels, and film morphologies. Here, we selected two non-fullerene acceptors, namely BTPT-4F and BTPTT-4F, to match with a wide-bandgap polymer donor P2F-EHp consisting of an imidefunctionalized benzotriazole moiety, as these materials presented complementary absorption and well-matched energy levels. By delicately optimizing the blend film morphology, we demonstrated an unprecedented power conversion efficiency of over 16% for the device based on P2F-EHp:BTPTT-4F, suggesting the great promise of materials matching toward high-performance OSCs.

Highly crystalline acceptor materials based on benzodithiophene with different amount of fluorine substitution on alkoxyphenyl conjugated side chains for organic photovoltaics

Organic solar cells based on narrow bandgap small-molecule acceptors(SMAs)with highly crystalline characteristics have attracted great attentions for their superiority in obtaining high photovoltaic efficiency.Employing highly crystalline SMAs to enhance power conversion efficiencies(PCEs)by regulating and controlling morphology and compatibility of donor and acceptor materials has turned out to be an effective approach.In this study,we synthesized three different crystalline SMAs by using fluorine substitution on alkoxyphenyl conjugated side chains to modulate the relationship of crystallinity and morphologies,namely ZY1(zero F atoms),ZY2(two F atoms),and ZY3(four F atoms).The three SMAs show the broad absorption edges and similar frontier orbital energy levels,generating the analogical(over 0.9 V)open circuit voltage(VOC)of the polymer solar cells(PSCs).As a result,the PM6:ZY2-based PSCs yield a PCE of 10.81%with a VOC of 0.95 V,a short-circuit current density(JSC)of 16.154 mA cm^(-2),and a fill factor(FF)of 0.71,which is higher than that of 9.17%(PM6:ZY1)and 6.37%(PM6:ZY3).And the PCE(17.23%)of the PM6:Y6:ZY2 based ternary PSCs is also higher than that of 16.32%PM6:Y6 based binary device.Obviously,the results demonstrate that adding fluorine atoms on the conjugated side chains to construct high crystalline materials is a positive strategy to effectively increase the efficiencies of binary and ternary PSCs.

Selective fluorination on donor and acceptor for management of efficiency and energy loss in non-fullerene organic photovoltaics

Although fluorination has been proved effective to modulate optoelectronic properties and film morphology,knowledge of managing power conversion efficiency(PCE)and energy loss(Eloss)of organic photovoltaics(OPVs)by selective fluorination on the donor and/or acceptor is lacking.Herein we designed and synthesized three 1,2,3-benzotriazole(BTA)-based linear polymers(PE45,PE46 and PE47)with different numbers of fluorine atom substitution on the conjugated phenyl side chain.Two classic non-fullerene acceptors(NFAs)Y5(without fluorination)and Y6(with fluorination)were utilized to manage the device efficiency and energy loss.The results revealed that increasing fluorine substitutions on polymer donor improved the OPV efficiencies when the fluorinated Y6 was used as the acceptor,whereas decreased the PCEs when the non-fluorinated Y5 was used as the acceptor.The energy loss declined with the growing fluorine substitutions on polymer donor,and Y5 systems gave the lower values in comparison with the corresponding Y6 cases.It turns out that PE47:Y6 achieved the highest PCE of 15.58%with an open-circuit voltage(VOC)of 0.84 V(Eloss=0.56 e V)due to the highest and balanced hole/electron mobilities,suppressed bimolecular recombination and fibril network morphology,which is the highest value in the BTA-based polymers.Furthermore,PE47:Y5 attained an ultralow non-radiative energy loss of 0.15 e V,which is one of the lowest values among the reported OPVs.Our work could not only give a direct path on how to manage the efficiency and energy loss by selective fluorination on donor and acceptor,but also show a deep understanding on charge generation,transport and collection induced by selective fluorination.

具有接近15%能量转换效率的有机太阳能电池

对叠层有机太阳能电池中的前、后子电池之间的光谱匹配性进行了优化调制.最终,选用带隙为1.24 eV的PTB7-Th:IEICO-4F作为后电池,带隙为1.59 eV的J52-2F:IT-M作为前电池制备了叠层电池.在这个叠层有机太阳能电池中,这2个子电池不仅具有互补的吸收光谱,还具有高的外量子效率,实现了对300~1000 nm范围内太阳发射光谱的高效利用.此外,前、后电池的能量损失均得到有效的控制,分别为0.64和0.53 eV.因此,从光伏性能具体参数上来看,本文制备的叠层器件兼具了高短路电流密度(JSC)和高开路电压(VOC)的优势,分别达到13.3 mA/cm^2和1.65 V.该电池在本实验室内部测试的光伏效率高达14.9%,封装之后效率略有降低,由中国计量科学研究院(NIM)验证的光伏效率达到14.0%.这是目前有机光伏领域的国际最高结果.

半经验模型指导受体分子结构优化获得效率超过15%的有机太阳能电池

本工作中,在我们发展的半经验模型的指导下,设计合成了两个A-D-A骨架受体分子,6T-OFIC和5T-OFIC.因其较长的共轭骨架,6T-OFIC的截止吸收波长为946 nm.相比于6T-OFIC,因其骨架少一个噻吩单元,5T-OFIC分子的截止吸收波长为927 nm,更接近半经验模型所预测的最佳单结器件的截止吸收波长.最终,基于PM6:5T-OFIC的器件获得了13.43%的能量转换效率,高于PM6:6T-OFIC器件.以F-2Cl作为第三组份,基于PM6:F-2Cl:5TOFIC的三元器件其填充和开压得到了明显的提高,最终获得了15.45%的能量转换效率.