Molecular vibrations reduce the maximum achievable photovoltage in organic solar cells

Abstract

The low-energy edge of optical absorption spectra is critical for the performance of solar cells, but is not well understood in the case of organic solar cells (OSCs). We study the microscopic origin of exciton bands in molecular blends and investigate their role in OSCs. We simulate the temperature dependence of the excitonic density of states and low-energy absorption features, including low-frequency molecular vibrations and multi-exciton hybridisation. For model donor-acceptor blends featuring charge-transfer excitons, our simulations agree very well with temperature-dependent experimental absorption spectra. We unveil that the quantum effect of zero-point vibrations, mediated by electron-phonon interaction, causes a substantial exciton bandwidth and reduces the open-circuit voltage, which is predicted from electronic and vibronic molecular parameters. This effect is surprisingly strong at room temperature and can substantially limit the OSC's efficiency. Strategies to reduce these vibration-induced voltage losses are discussed for a larger set of systems and different heterojunction geometries.

Long-lived and disorder-free charge transfer states enable endothermic charge separation in efficient non-fullerene organic solar cells

“…However, most fullerene systems with reduced offset show lower EQE 15 – 17 , thus implying that thermally activated separation of the CTEs is not efficient in most systems involving fullerene acceptors. A likely reason for this is the high degree of electronic disorder found in many fullerene blends (Urbach energy ≥40 meV) which traps the CTEs in low-energy sites, from which they cannot escape to form free charges or regenerate singlet excitons due to fast non-radiative recombination via vibrational relaxation 4 , 30 , 33 as well as triplet formation 5 , 6 , 49 . Fullerene systems with small offsets that do show decent performance were found to have low electronic disorder (for examples, PIPCP:PCBM 11 and PNOz4T:PCBM 29 ).…”

“…The combination of low E S 1 – E CTE offsets and high EQE EL results in greatly reduced photovoltage losses (≤0.6 V), and state-of-the-art materials with broad spectral overlap with sunlight can now achieve over 17% PCE in single-junction devices 28 . Considerable effort has been directed toward exploring charge separation process within this material class, with recent studies exploring the roles of exciton-CT state hybridization 29 – 32 , molecular vibrations 33 , electrostatic interactions 34 , 35 , electronic delocalization 36 among other properties. However, the precise mechanism and dynamics of electron–hole separation in these efficient OSC systems have remained unclear.…”

A Cost‐Effective, Aqueous‐Solution‐Processed Cathode Interlayer Based on Organosilica Nanodots for Highly Efficient and Stable Organic Solar Cells

“…significantly increased light absorption and reduced energy losses. [1][2][3][4][5][6] Recently, the PCE of single-junction OSCs is approaching the 18% milestone, exceeding the efficiency threshold expected for industrial viability. [7][8][9][10] Besides efficiency, long-term operational stability, such as thermal-and photostability, is a major concern for the practical application of OSCs.…”

Highly Efficient All‐Polymer Solar Cells Enabled by Random Ternary Copolymer Acceptors with High Tolerance on Molar Ratios

“…However, the PYE30‐ and PYE40‐based devices have detected significant decreases in V oc , which might be consistent with the down‐shifted LUMO levels and the absorption tails observed in PVE30 and PYE40 blends, as shown in Figure S4, Supporting Information. [ 12,50,51 ]…”

“…However, the PYE30-and PYE40-based devices have detected significant decreases in V oc , which might be consistent with the down-shifted LUMO levels and the absorption tails observed in PVE30 and PYE40 blends, as shown in Figure S4, Supporting Information. [12,50,51] In contrast to V oc , the J sc values increased gradually. The J sc values are also well matched with the integrated J sc values which were calculated from the external quantum efficiency (EQE) spectra, as shown in Table 1.…”