Quantifying and Understanding Voltage Losses Due to Nonradiative Recombination in Bulk Heterojunction Organic Solar Cells with Low Energetic Offsets

Abstract

Open‐circuit voltage (VOC) losses in organic photovoltaics (OPVs) inhibit devices from reaching VOC values comparable to the bandgap of the donor–acceptor blend. Specifically, nonradiative recombination losses (∆Vnr) are much greater in OPVs than in silicon or perovskite solar cells, yet the origins of this are not fully understood. To understand what makes a system have high or low loss, an investigation of the nonradiative recombination losses in a total of nine blend systems is carried out. An apparent relationship is observed between the relative domain purity of six blends and the degree of nonradiative recombination loss, where films exhibiting relatively less pure domains show lower ∆Vnr than films with higher domain purity. Additionally, it is shown that when paired with a fullerene acceptor, polymer donors which have bulky backbone units to inhibit close π–π stacking exhibit lower nonradiative recombination losses than in blends where the polymer can pack more closely. This work reports a strategy that ensures ∆Vnr can be measured accurately and reports key observations on the relationship between ∆Vnr and properties of the donor/acceptor interface.

Role of Energy Offset in Nonradiative Voltage Loss in Organic Solar Cells

“…The EQE EL for group I is on the order of 10 À6 -10 À8 , which is typical for conventional OSCs. [3,[5][6][7]19,20,22] On the contrary, the EQE EL In panel a, previously published data are also shown by open circles. [3,7,13,29] SM, small molecule donor; P, polymeric donor; F, fullerene acceptor; and NF, nonfullerene acceptor.…”

“…[17,18] However, the voltage loss incurred by charge recombination in OSCs is significantly larger than the SQ limit, mainly because of an extra voltage loss caused by nonradiative charge recombination. [3][4][5][6][7][17][18][19][20] It has been experimentally demonstrated that CT states are intermediates of charge recombination in OSCs. [9,21] The oscillator strengths of CT states are fairly small because of poor spatial overlap between the highest occupied molecular orbital (HOMO) of the donor and lowest unoccupied molecular orbital (LUMO) of the acceptor, resulting in the radiative decay rate k r , in general, being fairly smaller than the nonradiative decay rate k nr .…”

“…Therefore, the CT states predominantly decay nonradiatively to the ground state. The external quantum efficiency of electroluminescence (EQE EL ) of OSCs is typically on the order of 10 À6 -10 À8 , which is equivalent to the nonradiative voltage loss ΔV nr of 0.35-0.46 V. [3,[5][6][7]19,20,22] Therefore, OSCs with smaller ΔV nr are highly required.…”

Understanding the High Performance of over 15% Efficiency in Single‐Junction Bulk Heterojunction Organic Solar Cells

“…[7,15] However, the CT state is not always pronounced in the low-energy tail of the EQE PV spectra, especially in blends where the energetic offsets between the donor and acceptor are low (i.e., low HOMO-HOMO or LUMO-LUMO offsets) [2,5,8,16] , and it can therefore be difficult to distinguish the energy of the CT state from the donor or acceptor singlet state. It is possible, however, to significantly reduce the degrees of freedom in the fitting by 1) performing a simultaneous fit to both the EQE PV (Equation S1) and the EL (Equation S2) spectra [3,9,[17][18][19] using equations derived from Marcus theory, as was first demonstrated by Vandewal et. al.…”

A relatively wide-bandgap and air-stable donor polymer for fabrication of efficient semitransparent and tandem organic photovoltaics

“…Over the past decade, the researchers in the field of organic photovoltaics (OPVs) have made significant progress in terms of synthesizing new donor polymers, nonfullerene devices, stability, and efficiency, resulting in a certified efficiency of 15.6% (5)(6)(7). Some research groups conducted their experiments to synthesize new donor and acceptor polymers with more suitable structures, which allowed them to improve the optical absorption and reduce the voltage losses in OPVs (8)(9)(10)(11). Other than the above points, since the bandgap of the blend in an OPV can be tuned by changing the polymer structures, these organic semiconductors provide a wide range of material choices for the fabrication of semitransparent OPVs applicable in window and tandem solar cells (12)(13)(14)(15).…”