Highly efficient halogen-free solvent processed small-molecule organic solar cells enabled by material design and device engineering

206

220

include the strong electron-electron interactions and strong electron-phonon couplings as well as the complicated donor/ acceptor (D/A) interface morphology, which are fundamentally different from inorganic semiconductors. [23,24] Therefore, the accurate simulating of OPVs requires high-level theoretical methods in quantum chemistry, quantum dynamics and statistical mechanics, and in recent years there have been substantial progress for the theoretical understanding of many microscopic processes such as charge transport, [25] exciton dissociation, [26,27] singlet fission, [28][29][30] etc. These methods are useful for few benchmark systems but cannot be used to explore the chemical space, i.e., screen a large number of candidate materials.The predictive power of a theoretical model by high-throughput virtual screening has been explored in the recent years. [31][32][33][34][35][36][37][38][39][40] For example, in the Harvard Clean Energy Project (CEP), [41] Aspuru-Guzik and co-workers have screened ≈2.3 million compounds by employing the Scharber model [42,43] to find out efficient new donor molecules for OPVs. Here, the computed energies of the highest occupied molecular orbital (HOMO)/the lowest unoccupied molecular orbital (LUMO) of organic molecules are calibrated to experimentally determined values, and then employing an averaging scheme, energies of HOMO/LUMO are estimated to use them as input in the Scharber model. However, the prediction of PCE of an OPV is much more challenging compared to the energies of orbitals. Very recently, the same research team [44] noticed that the Scharber model, widely used in these virtual screening works, is not accurate enough for the prediction of PCE, and calibration of PCE by considering molecular similarity, molecular weight and band gap energy leads to improvement in correlation between experimental (49 OPVs) and calculated PCE (Pearson's coefficient (r) is increased from 0.30 to 0.43). Further, the Scharber model is only optimized for the PC 61 BM acceptor [42] and a more general model is desired to take account of nonfullerene molecules.In an OPV, energy conversion is accomplished by four consecutive steps: i) absorption of photons and exciton formation, ii) exciton diffusion to D/A interface, iii) exciton dissociation, and iv) transport of holes/electrons to the respective electrode. Taking account of all these processes, the efficiency of a device depends on many microscopic properties of the organic material such as optical gap, charge-carrier mobility, ionization To design efficient materials for organic photovoltaics (OPVs), it is essential to identify the largest number of parameters that control their properties and build a model using these parameters (known as descriptors) for the prediction of the power conversion efficiency (PCE). By constructing a dataset for 280 small molecule OPV systems, it is found that for all high-performing devices, frontier molecular orbitals of donor molecules are nearly degenerated and in such cases, orbitals other than just high...

Section: Importance Of Orbitals Energetically Close To Frontier Orbitmentioning

confidence: 99%

Toward Predicting Efficiency of Organic Solar Cells via Machine Learning and Improved Descriptors

Sahu

Rao

Troisi

et al. 2018

206

220

“…As for the photoactive layer materials used in OSCs, considerable efforts have been devoted to the molecular design to high performance donor and acceptor materials. The PCEs of the opaque OSC devices based on fullerenes and nonfullerene acceptors have reached ≈11% [9][10][11][12] (uncertified) and over 13% [4,13] (certified by the National Institute of Metrology, China), respectively.…”

Section: Introductionmentioning

confidence: 99%

Flexible and Semitransparent Organic Solar Cells

Cui

et al. 2017

632

449

Flexible and semitransparent organic solar cells (OSCs) have been regarded as the most promising photovoltaic devices for the application of OSCs in wearable energy resources and building‐integrated photovoltaics. Therefore, the flexible and semitransparent OSCs have developed rapidly in recent years through the synergistic efforts in developing novel flexible bottom or top transparent electrodes, designing and synthesizing high performance photoactive layer and low temperature processed electrode buffer layer materials, and device architecture engineering. To date, the highest power conversion efficiencies have reached over 10% of the flexible OSCs and 7.7% with average visible transmittance of 37% for the semitransparent OSCs. Here, a comprehensive overview of recent research progresses and perspectives on the related materials and devices of the flexible and semitransparent OSCs is provided.

“…While according to the acceptor used, they can be divided into fullerene solar cells and non-fullerene solar cells. [14,15] Although currently both fullerene-and non-fullerene-based allsmall-molecule solar cells present high PCEs of over 11%, [16,17] the defects in each system should be clearly addressed. [9] In contrast, a noticeable gap exists between small-molecule solar cells and polymer solar cells whether in terms of performances or the research scale.…”

mentioning

confidence: 99%

Constructing High‐Performance All‐Small‐Molecule Ternary Solar Cells with the Same Third Component but Different Mechanisms for Fullerene and Non‐fullerene Systems

Chang

Zhu

et al. 2019

donors used, OSCs fall into two categories: polymer solar cells and small-molecule solar cells. While according to the acceptor used, they can be divided into fullerene solar cells and non-fullerene solar cells. Polymer solar cells have always been the focus of attention, and a breakthrough, that is., a power conversion efficiency (PCE) of over 15% has been achieved for this type of device. [9] In contrast, a noticeable gap exists between small-molecule solar cells and polymer solar cells whether in terms of performances or the research scale. However, small molecule donors still have some unique advantages over polymer donors, such as definite chemical structure, extremely high purity and excellent reproducibility, which could be better suited for future large-scale applications of OSCs. [10][11][12][13] Particularly, there has been a new upsurge for developing all-small-molecule solar cells owing to the widespread use of non-fullerene acceptors. [14,15] Although currently both fullerene-and non-fullerene-based allsmall-molecule solar cells present high PCEs of over 11%, [16,17] the defects in each system should be clearly addressed. For instance, due to the weak absorption ability of fullerene acceptors, such as [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM) and [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM), in the short-wavelength and near-infrared regions, it is difficult to obtain much higher photocurrent. [18] On the other hand, when selecting a non-fullerene acceptor, like IDIC, [19] which possesses superior crystallization capability, [20] it is hard to achieve ideal film morphology with nanoscale phase separation. Therefore, overcoming these issues for binary devices is of great importance for continued evolution of all-small-molecule solar cells.Here, utilizing the individual advantages of both fullerene and non-fullerene acceptors has been demonstrated as an attractive strategy to resolve the above-mentioned problems. [21] Recently, several cases of all-small-molecule ternary solar cells with significant increase in the PCE, which employed one small molecule donor material, one fullerene acceptor (PC 71 BM) and one non-fullerene acceptor, have been reported. [21,22] In these cases, PC 71 BM has played an important role in modulating the film morphology of the ternary system, which could significantly facilitate charge generation and transportation and suppress charge recombination. As a result, the short-circuit current Two types of all-small-molecule ternary solar cells consisting of two smallmolecule donors and one acceptor (fullerene/non-fullerene) are developed. Interestingly, both these devices have a common component: a carefully designed medium bandgap small molecule, which possesses appropriate energy levels and displays good compatibility with the host donor. In the fullerene system, the charge-relaying role of the additive donor is confirmed by the improved charge transportation and suppressed charge recombination. While in the non-fullerene system, the mixed face-on and ed...