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.

Solution processed zinc oxide (ZnO) nanoparticles (NPs) with excellent electron transport properties and a low-temperature process is a viable candidate to replace titanium dioxide (TiO2) as electron transport layer to develop high-efficiency perovskite solar cells on flexible substrates. However, the number of reported high-performance perovskite solar cells using ZnO-NPs is still limited. Here we report a detailed investigation on the chemistry and crystal growth of CH3NH3PbI3 perovskite on ZnO-NP thin films. We find that the perovskite films would severely decompose into PbI2 upon thermal annealing on the bare ZnO-NP surface. X-ray photoelectron spectroscopy (XPS) results show that the hydroxide groups on the ZnO-NP surface accelerate the decomposition of the perovskite films. To reduce the decomposition, we introduce a buffer layer in between the ZnO-NPs and perovskite layers. We find that a commonly used buffer layer with small molecule [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) can slow down but cannot completely avoid the decomposition. On the other hand, a polymeric buffer layer using poly(ethylenimine) (PEI) can effectively separate the ZnO-NPs and perovskite, which allows larger crystal formation with thermal annealing. The power conversion efficiencies of perovskite photovoltaic cells are significantly increased from 6.4% to 10.2% by replacing PC61BM with PEI as the buffer layer.

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power conversion efficiency (PCE) of BHJ devices strongly depends on phase separation of the donor and acceptor. However, the formation of BHJ morphology is an extremely complicated process and the formed morphology is also a highly delicate balance involving many parameters such as domain size, purity, miscibility, etc. To achieve high-performance devices, much effort has been devoted to delicately control the morphology of the active layer (such as the blend ratio between the donor and acceptor, [9][10][11] the type of solvent, [12][13][14] the type and amount of processing additive, [15][16][17] and thermal and solvent annealing [18][19][20] ) to obtain optimal interpenetrated nanoscale phase separation. What is worse, the morphology control becomes much more challenging when the device area is scaled up, for example, the morphology of printed film may vary drastically when the processing condition changes. For this reason, there are typically large performance drops when OPV devices are scaled up to large area or processed using printing techniques. [21,22] Moreover, green solvent is a prerequisite for the commercialization of OSCs. However, for the BHJ structure, the solubility and miscibility of both the donor and acceptor must be taken into consideration together, which limits the selection of a processing solvent. Most OSCs were processed with toxic halogen solvents, and only a few high-performances donor/acceptor combinations were reported in the literature using low toxicity/nontoxic solvents.In contrast, sequential deposition of the donor and acceptor materials followed by post thermal annealing can lead to a similar BHJ morphology and reasonably efficient OSC devices. The interpenetration between donor and acceptor during the solution processing ensures the D/A interfaces for separation of the charges. [23][24][25] The bilayer structure should be more favorable for charge transport as the separated charges can easily transport to each electrode through the D or A layer with low possibility of recombination. [26][27][28] In addition, as both donor and acceptor layers can be processed and optimized independently, this bilayer structure should show less dependence on the D/A ratio, solvent, additive, etc., when compared with the BHJ structure. These properties make the bilayer structure very attractive for achieving high-performance OSCs and While the performance of laboratory-scale organic solar cells (OSCs) continues to grow over 13%, the development of high-efficiency large area OSCs still lags. One big challenge is that the formation of bulk heterojunction morphology is an extremely complicated process and the formed morphology is also a highly delicate balance involving many parameters such as domain size, purity, miscibility, etc. The morphology control becomes much more challenging when the device area is scaled up. In this work, a highly efficient (12.9%) nonfullerene organic solar cell processed using a sequential bilayer deposition method from nonhalogenated solvents, is reported. Using this bi...

The contact between the n-type interlayer and the donor provides an extra interface for charge dissociation.