Toward High-Efficiency Organic Photovoltaics: Perspectives on the Origin and Role of Energetic Disorder

Abstract

The power conversion efficiency of organic photovoltaics is strongly limited by relatively large energy loss, which is partially due to the disordered nature of organic semiconductors. This disordered nature not only hinders the rational design of molecules with excellent photophysical properties but also prevents a more thorough understanding of the inherent link between microscopic parameters and physical phenomena. In this Perspective, we demonstrate that the injection-dependent emission line-shape in organic semiconductors is primarily associated with a state-filling effect, where the extent of spectral blue-shift can be a strong indicator for energetic disorder. Molecular geometry with rigidity and coplanarity not only promotes preferential faceon stacking that narrows the energetic distribution of subgap states but also impedes torsional deformations of the conjugated backbone away from planarity, thereby facilitating larger π-electron delocalization. These structural characteristics explain the seemingly contradictory high radiative efficiency of low-bandgap nonfullerene molecules, providing promising molecular design strategies to realize high-efficiency organic photovoltaics.

Effects of energetic disorder in bulk heterojunction organic solar cells

“…Such broadening of the CT state manifold has been identified as a major source of voltage losses because the occupation of low lying CT states fosters recombination and increases the voltage loss in the blends, thereby limiting the efficiency of fullerene-based solar cells. 16,24,28,29…”

“…Such broadening of the CT state manifold has been identified as a major source of voltage losses because the occupation of low lying CT states fosters recombination and increases the voltage loss in the blends, thereby limiting the efficiency of fullerene-based solar cells. 16,24,28,29 With the development of new photovoltaic materials, especially non-fullerene acceptors (NFAs), the PCEs of OSCs have significantly improved. One potential reason accounting for the surge in PCE is that the energetic offset for generating free charge carriers in the OSCs is no longer a constraint in NFAbased devices.…”

“N‐π‐N” Type Oligomeric Acceptor Achieves an OPV Efficiency of 18.19% with Low Energy Loss and Excellent Stability

“…The low degree of energetic disorder at interfaces in the active layers enables free charge separation to outcompete trap‐assisted non‐radiative recombination of the CT state, gaining excellent device performance with small Δ E . [ 28 , 29 , 31 ] Hence, Urbach energy ( E U ) is obtained through an exponential fit to the FTPS‐EQE spectra to evaluate the degree of energetic disorder (Figure 5b and Figure S9 , Supporting Information). [ 32 ] The PM6/ 2BTP‐2F‐T obtains the smallest E U with a value of 22.74 meV, compared to that of PM6/monomer ( E U = 23.92 meV) and PM6/ PYF‐T‐ o ( E U = 24.79 meV), indicating the smallest width of the tail of the electronic density of disorder (DOS).…”

Improved Predictions of Organic Photovoltaic Performance through Machine Learning Models Empowered by Artificially Generated Failure Data

“…Research in the field of organic photovoltaics (OPV) has surged over the last decade, triggered by the discovery of the efficient, narrow bandgap ( E g ) non-fullerene small molecular acceptor (NFA). − A p-type donor (D)–acceptor (A) polymer possessing a median E g is blended with an n-type, narrow E g NFA to form a p/n bulk heterojunction (BHJ) network applicable to a wide range of the sunlight spectrum. , The resultant photoactive layer enables the maximization of the short-circuit current density ( J SC ), open-circuit voltage ( V OC ), and fill factor (FF) owing to the fine-tuning of energy levels and improvements in charge carrier mobilities and balance. , As a consequence, high power conversion efficiencies (PCE) are achieved by single junction p/n cells (∼18%) − and ternary blend (p/n/n or p/p/n) cells (∼19%). − The structural diversity of NFAs (generally A–D–A) and D–A polymers is of significant interest to synthetic chemists; however, it is not possible to completely survey the vast molecular space. Quantum chemical calculations and molecular dynamics simulations can furnish an approximate estimation of the optical and electrochemical properties of new materials; − however, complex structure–property relationships characteristic of OPV materials has hampered the efficient exploration of materials.…”