Impact of Triplet Excited States on the Open‐Circuit Voltage of Organic Solar Cells

Abstract

efficiency (PCE). [4,5] The main reason is their low open-circuit voltage (V OC ) as compared to the optical gap (E opt ) of the main absorbing materials. [6] All photovoltaic (PV) technologies suffer from voltage losses, arising from fundamental radiative recombination and parasitic nonradiative recombination. Radiative recombination is inevitable, and is the only recombination process taking place in an ideal solar cell. [7][8][9][10] This process determines the upper limit of the V OC , denoted as the radiative open-circuit voltage V r . In reality, the measured V OC is lower than V r due to the presence of nonradiative decay channels, lowering V r by ΔV nrRau has shown that ΔV nr is proportional to the natural logarithm of the quantum efficiency of emission (EQE EL ). [7] The validity of Equation (1) for OSCs has been shown previously, [11,12] where ΔV nr typically accounts for 0.25-0.40 V of the total voltage losses (ΔV OC = E CT − V OC ). [8,[12][13][14] This is a much higher value than in inorganic and Perovskite solar cells, where ΔV nr ≤0.15 V. [15][16][17] In addition to voltage losses due to radiative and nonradiative recombination, OSCs suffer voltage losses because the photogenerated excitons on the donor (D) or acceptor (A) undergo a charge transfer to form an interfacial charge-transfer (CT) state with energy E CT . However, it has been recently shown that the energy difference between the optical gap of the donor or acceptor and the CT state (E opt − E CT ) can be minimized to less than 0.05 eV [13,18] and even down to 0.01 eV, [6] without sacrificing efficient free charge carrier generation. Therefore, in the OSCs with the currently lowest voltage losses, nonradiative recombination is the main reason for the low V OC as compared to other PV technologies employing absorber with similar optical gaps.In a previous study, we have shown for a whole range of solution and vacuum processed OSCs that ΔV nr correlates with E CT . This led us to the conclusion that nonradiative decay is mediated by CT state decay via electron-phonon coupling. [12] However, in the related OLED technology, the major nonradiative decay channel is mediated by the triplet excited states. [19] In OSCs, triplet states are present on both the D and A materials, and for high voltage OSCs the energy of the lowest energy The best organic solar cells (OSCs) achieve comparable peak external quantum efficiencies and fill factors as conventional photovoltaic devices. However, their voltage losses are much higher, in particular those due to nonradiative recombination. To investigate the possible role of triplet states on the donor or acceptor materials in this process, model systems comprising Zn-and Cu-phthalocyanine (Pc), as well as fluorinated versions of these donors, combined with C 60 as acceptor are studied. Fluorination allows tuning the energy level alignment between the lowest energy triplet state (T 1 ) and the charge-transfer (CT) state, while the replacement of Zn by Cu as the central metal in the Pcs leads to a largely enhanc...

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 ).…”

Reduced Nonradiative Energy Loss Caused by Aggregation of Nonfullerene Acceptor in Organic Solar Cells

“…However, OSCs still face a critical challenge in achieving high power conversion efficiencies (PCEs). It has been recognized that reducing the energy loss ( E loss ), referring to as the energetic offset from the bandgap ( E g ) to the open‐circuit voltage ( V OC ) of a solar cell, is one of the main pathways to promote the PCE . The energy difference between E g and qV OC can be split up in three terms: 1) E g – , which is also identified as Δ E 1 in some reported works, is the difference between the potential energy of the electron–hole pairs and the free energy at zero collected current.…”

“…[29][30][31][32][33][34][35] As is known, for an OSC under open-circuit condition, an electron-hole pair at the CT state transits to the ground state by either radiative recombination or nonradiative recombination, so the electroluminescence (EL) emission can be used as a tool to monitor the nonradiative recombination. [7][8][9][10][11][12][13][14][15] The energy difference between E g and qV OC can be split up in three terms: 1) E g -qV OC SQ , which is also identified as ΔE 1 in some reported works, is the difference between the potential energy of the electron-hole pairs and the free energy at zero collected [28,30,36] For instance, for an OSC based on the polymer donor P3TEA and acceptor SF-PDI2, the electroluminescence quantum efficiency (EQE EL ) was as high as 5 × 10 −5 , corresponding to a ΔE 3 of 0.26 eV, [20] while for OSCs based on polymer donors and fullerene acceptors, ΔE 3 can surpass 0.40 eV.…”

Nonradiative Energy Losses in Bulk-Heterojunction Organic Photovoltaics

“…Thus, research into the fundamental mechanisms and limitations for recombination in organic bulk heterojunction solar cells is urgently required. Recombination, in general, can be rate-limited either by transport of charge carriers towards each other or by dissipation of the energy of the polaron pair, which is believed to occur via a so-called interfacial charge-transfer (CT) state in organic heterojunction solar cells [6,7]. These two processes of transport and energy dissipation have to happen in series, and therefore the slower of the two processes is the rate-limiting process.…”