The Energy Level Conundrum of Organic Semiconductors in Solar Cells

Abstract

The frontier molecular energy levels of organic semiconductors are decisive for their fundamental function and efficiency in optoelectronics. However, the precise determination of these energy levels and their variation when using different techniques makes it hard to compare and establish design rules. In this work, the energy levels of 33 organic semiconductors via cyclic voltammetry (CV), density functional theory, ultraviolet photoelectron spectroscopy, and low‐energy inverse photoelectron spectroscopy are determined. Solar cells are fabricated to obtain key device parameters and relate them to the significant differences in the energy levels and offsets obtained from different methods. In contrast to CV, the photovoltaic gap measured using photoelectron spectroscopy (PES) correlates well with the experimental device VOC. It is demonstrated that high‐performing systems such as PM6:Y6 and WF3F:Y6, which are previously reported to have negligible ionization energy (IE) offsets (ΔIE), possess sizable ΔIE of ≈0.5 eV, determined by PES. Using various D–A blends, it is demonstrated that ΔIE plays a key role in charge generation. In contrast to earlier reports, it is shown that a vanishing ΔIE is detrimental to device performance. Overall, these findings establish a solid base for reliably evaluating material energetics and interpreting property–performance relationships in organic solar cells.

Reducing non-radiative voltage losses in organic solar cells using molecular encapsulation

“…To understand the improvement in the upon molecular 𝑉𝑜𝑐 encapsulation of the donor polymer, we first studied possible changes in the ionization energy (IE) and electron affinity (EA) of DPP-TT polymer analogs using ultraviolet photoelectron spectroscopy (UPS) and low-energy inverse photoelectron spectroscopy (LE-IPES). 14 For PDPP-TTref, IE and EA were measured to be 5.16 eV and 3.38 eV (Figure 3), respectively, corresponding to a transport gap of 1.78 eV. Though the IE of all-polymer analogs was found to be similar within the experimental uncertainties (~0.05 eV) (PDPP-TT5% -5.12 eV, PDPP-TT10% -5.14 eV, PDPP-TT20% -5.17 eV), the EA of PDPP-TT5%, PDPP-TT10%, and PDPP-TT20% were measured to be 3.48 eV, 3.46 eV, and 3.29 eV, respectively (Figure 3c).…”

“…11,12 So far, the rapid progress in the development of OSCs can be attributed mainly to the design and synthesis of new materials, primarily focusing on minimizing the voltage losses, 10,12 and tuning the optical bandgap and frontier molecular orbital energies. [13][14][15][16] Nevertheless, besides optimizing the energetic offsets, material design strategies are still needed to intrinsically limit the that occur due to the ∆𝑉𝑜𝑐 𝑛𝑟 recombination of the photo-generated charges. Conjugated push-pull polymers based on electron-deficient lactam core of diketopyrrolopyrrole (DPP) have found a wide range of applications for optoelectronic devices, 17 owing to their good electronic properties, photochemical stability, and industrial relevance.…”

Morphology-Dependent Optoelectronic Properties of Pentacene Nanoribbon and Nanosheet Crystallite

“…Furthermore, organic semiconductors can be solution-processed on any substrate inexpensively and even at relatively low temperatures, which is highly advantageous for their scale-up from fundamental studies to industrial-level production [ 5 , 6 , 7 , 8 ]. Current examples of developed organic semiconductors include field-effect transistors (OFETs) [ 9 , 10 , 11 , 12 ], photodetectors (OPDs) [ 13 , 14 , 15 , 16 ], solar cells [ 17 , 18 , 19 ], light-emitting diodes (OLEDs) [ 20 , 21 , 22 ], and so on. To date, a large number of organic optoelectronic devices have been constructed using amorphous or polycrystalline films [ 23 , 24 , 25 ].…”

Halogenated thiophenes serve as solvent additives in mediating morphology and achieving efficient organic solar cells

“…The bulk-heterojunction (BHJ) organic solar cells (OSCs) relying on polymer donors and small-molecule acceptors are gaining considerable attention, mainly because of their unique characteristics such as low-cost device fabrication via the high-throughput printing technology, flexibility, and light weight. [1][2][3][4] In recent years, thanks to the rapid advancement of non-fullerene acceptors featuring narrow bandgaps, strong light absorption in the nearinfrared region (NIR), readily tunable energy levels, and fast electron transportation, OSCs have reached a big breakthrough with power conversion efficiencies (PCEs) nearing 20% [5][6][7][8][9][10] and extrapolated lifetime of over 20 years. 11 Besides the rational design of efficient organic photovoltaic materials, the systematic nanoscale morphology optimization of BHJ photoactive layers is also of critical importance for fabricating high-performance OSCs because light absorption and photoexcitation, exciton diffusion and dissociation, charge transportation, and collection are highly determined by the nanoscale morphology of the BHJ photoactive layers.…”