A major challenge for organic solar cell (OSC) research is how to minimize the tradeoff between voltage loss and charge generation. In early 2019, we reported a non-fullerene acceptor (named Y6) that can simultaneously achieve high external quantum efficiency and low voltage loss for OSC. Here, we use a combination of experimental and theoretical modeling to reveal the structure-property-performance relationships of this state-of-the-art OSC system. We find that the distinctive π–π molecular packing of Y6 not only exists in molecular single crystals but also in thin films. Importantly, such molecular packing leads to (i) the formation of delocalized and emissive excitons that enable small non-radiative voltage loss, and (ii) delocalization of electron wavefunctions at donor/acceptor interfaces that significantly reduces the Coulomb attraction between interfacial electron-hole pairs. These properties are critical in enabling highly efficient charge generation in OSC systems with negligible donor-acceptor energy offset.

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counterparts and is very attractive for developing semitransparent organic solar cells (ST-OSCs), which have great potential application in building integrated photovoltaic, agricultural greenhouse, car window, and so on. [5,6] Different from opaque device that is concerned mostly with power conversion efficiency (PCE), ST-OSCs also value the transparency in the visible region and adopt average visible transmittance (AVT) to assess their see-through function. [7] However, there is a trade-off relationship between the see-through function and photovoltaic performances since enhancing the light transmission generally leads to lower amount of photons to be utilized. Therefore, efforts including material designing and device engineering are widely employed to reconcile the intrinsic contradiction between AVT and PCE. [8][9][10][11][12][13][14][15][16] Ideally, the photon-absorbing materials in ST-OSCs should have the main absorption in the NIR region and weak absorption in the visible region. [17] Previously, various narrow bandgap polymer donors were synthesized and matched with fullerene acceptors in ST-OSCs. [18,19] Recently, with the aid of advances in materials designing of NIR-absorbing non-fullerene electron acceptors, the PCEs of ST-OSCs have already surpassed 14%. [20,21] Meanwhile, optical engineering technologies, including photonic crystals, [22,23] antireflective device structures, [24,25] or optical microcavity structures, [26,27] have been applied to improve the PCEs and optimize the transmission spectra with deliberately designed layout but energy-consuming process. Other strategies, including multi-component, [28,29] sequentially deposited active layer [30,31] and thickness optimization [20] are also implemented to regulate photon harvesting range and improve PCEs in these fields.More directly, lowering the large-bandgap donor content in bulk-heterojunction (BHJ) has been demonstrated as a facile strategy to tune the transmittance spectrum of active layer. [32][33][34] The AVTs of D18-Cl:Y6-1O films have been continuously changed from 30.3% to 68.0% through ranging the donor and acceptor weight ratio (D:A ratio) from 1.1:1.6 to 0.1:1.6. [4] These previous works imply the feasibility of achieving high AVT from low-donor content ST-OSCs. Nevertheless, the performance optimization of OSCs with dilute donors still remains a challenge, mainly originating from the imbalanced charge Semitransparent organic solar cells (ST-OSCs) have promising prospects for building or vehicle integrated solar energy harvesting with energy generation and see-through function. How to achieve both an adequate average visible transmittance (AVT) and high-power conversion efficiency (PCE) is always the key issue. Herein, a simple but effective strategy for constructing high performance ST-OSCs by introducing a small molecule [2-(9-H-Carbazol-9-yl) ethyl] phosphonic acid (2PACz) into a low-donor content active layer is reported. The fill factor is improved from 70.5% to 75.5% and correlated to the mitigated charge recombination a...

Nonfullerene organic solar cells (OSCs) have achieved an impressive power conversion efficiency (PCE) over the past few years, showing a great potential for real applications. However, the study on the photostability and degradation mechanism of nonfullerene OSCs is far behind than that of fullerene‐based solar cells, which is crucial for the commercial applications of the technology. Herein, an efficient and stable nonfullerene OSC based on PCE10:rhodanine‐benzothiadiazole‐coupled indacenodithiophene with branched 2‐ethylhexyl side chains (EH‐IDT) is fabricated from environmentally benign solvent. The PCE10:EH‐IDT solar cell shows a high PCE of 9.17% and a long operational lifetime (T80) of 2132 h, compared with other two OSCs based on 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone)‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2’,3’‐d’]‐s‐indaceno[1,2‐b:5,6‐b’]‐dithiophene (ITIC) and another fuse ring acceptor with withdrawing units of 1,1‐dicyanomethylene‐3‐indanone and hexyl side chains (IDIC) nonfullerene acceptors, with tested lifetimes of only 221 and 558 h, respectively. As indicated by the Flory–Huggins interaction parameters, ITIC and IDIC have poor miscibility with PCE10, which leads to morphology degradation, suppressed charge generation, increased trap states, and charge recombination in the photoaging test, which accounts for the significant loss of short‐circuit current density and fill factor during operation. The improved miscibility of the donor and the acceptor results in a more stable morphology, and the PCE10:EH‐IDT solar cells thus achieve an outstanding overall performance that combines high efficiency and superior photostability and paves the way for the potential practical applications of OSCs.

Ternary blending of light-harvesting materials has been proven to be a potential strategy to improve the efficiency of solution-processed organic solar cells (OSCs). However, the optimization of a ternary system is usually more complicated than that of a binary one as the morphology of conventional ternary blend films is very difficult to control, thus undermining the potential of ternary OSCs. Herein, we report a general strategy for better control of the morphology of ternary blend films composed of a polymer donor and two nonfullerene small-molecule acceptors for high-performance OSCs using the sequential layer-by-layer (LbL) deposition method. The resulting LbL films form a bicontinuous interpenetrating network structure with high crystallinity of both the donor and acceptor materials, showing efficient charge generation, transport, and collection properties. In addition, the power conversion efficiencies (PCEs) of the ternary LbL OSCs are less sensitive to the blending ratio of the third component acceptor, providing more room to optimize the device performance. As a result, optimal PCEs of over 11, 13, and 16% were achieved for the LbL OSCs composed of PffBT4T-2OD/IEICO-4F:FBR, PBDB-T-SF/IT-4F:FBR, and PM6/Y6:FBR, respectively. Our work provides useful and general guidelines for the development of more efficient ternary OSCs with better controlled morphology.