Energy Losses in Small‐Molecule Organic Photovoltaics

Abstract

processible PV technology has recently appeared as well. [5] Comparing the different technologies in terms of PCE, which is specified as the product of the short-circuit current density j SC , the open-circuit voltage V OC and the fill factor FF divided by the incoming light intensity under standard AM 1.5G illumination conditions, OPVs can well compete with their inorganic counterparts in terms of j SC or, more precisely, the external quantum efficiency and also with minor trade-off in FF, but clearly suffer from lower V OC at a given energy gap E g of the light absorbing material. While this socalled bandgap-voltage offset can be as low as 0.3-0.4 eV in Si and GaAs [6] and only a little larger in perovskite cells, [7] OPV cells exhibit energy losses of at least 0.6 eV -in many cases, however, this offset can approach and even exceed 1 eV. [8] This is currently one of the main bottlenecks toward making OPVs competitive with inorganic PV cells.In this research news, we provide the required background information on the appearance of energy losses in OPV cells, by which we mean the difference between the equivalent of the optical gap and the measured open-circuit voltage that is frequently also denoted as voltage loss, and discuss recent progress toward better understanding their origin and strategies to reduce them. To keep focused, we will mainly address small molecules as active organic semiconductors, which are being processed into thin films by vacuum deposition techniques. Compared to frequently studied π-conjugated polymers, the synthesis of small molecules is more reproducible. Moreover, a rigorous purification of small molecules is easier, which gives the opportunity to reproducibly investigate well-defined systems. The application of vacuum deposition techniques prevents the use of solvents, which as a third component in wet chemical processing can strongly influence the morphology. [9] Thus, active layers of small molecules prepared by vacuum deposition methods mark a well-controlled model system for fundamental studies such as the origin of energy losses in OPV devices. However, we expect that most of the findings can be transferred to solution-processed OPVs as well, which have considerably higher complexity in terms of local morphology and phase behavior. Excitonic Organic Solar CellsIn order to properly address energy losses in OPVs it is useful to look at their working principles in more detail (see also [10] ).

The Impact of Local Morphology on Organic Donor/Acceptor Charge Transfer States

“…As many of the works discussed in this Progress Report have shown, there is a general trend for CT state energy to decrease as the structural order at the interface increases, which, in turn, is linked to a reduction in V OC . Numerous works have reported an empirical relationship between V OC and E CT in OPVs: q V OC = E CT − Δ E loss , where Δ E loss = 0.5 − 0.7 eV (see Figure 7 ). The origin of this V OC loss has been attributed to a combination of factors, including the CT state binding energy, CT state energetic broadening due to interfacial disorder, the degree of interfacial mixing, and CT state lifetime (i.e., recombination) .…”

Energy Loss in Organic Photovoltaics: Nonfullerene Versus Fullerene Acceptors

“…[6][7][8][9][10] As a result, HJs using NFAs show as low as 0.1 to 0.2 eV, Δ of 0.2 to 0.3 eV and a total approaching 0.6 eV, [11][12][13][14][15] compared to fullerene-based HJs with > 0.3 eV, Δ of 0.3 to 0.4 eV. [16][17][18] While energy loss mechanisms have been discussed for years, [5,[19][20][21] to our knowledge there has been no quantitative analysis as to why NFAs have both and comparatively low. As a result, unambiguous guidelines for molecular designs have been lacking.…”

An Effective Method for Recovering Nonradiative Recombination Loss in Scalable Organic Solar Cells

“…[ 25 ] Absorber material (with an optical bandgap, E gap ) utilizes the energy of photons in sunlight, and the primarily formed species is a strongly bound electron–hole pair. To break through the constraint of the Coulomb force, it consumes a part of energy to separate into freely moving charge carriers, [ 26 ] and this part is defined as Δ E CT . The separated electrons and holes are extracted by the interface layer and finally collected by the electrodes on both sides.…”

“…By analyzing the three parts of the energy loss in detail, reduced energy loss can be understood more clearly. According to the balanced theory, the V oc can be defined as Equation (1) [ 26 ] …”

“…The first part, Δ E 1 = E gap − E CT , depends on the formation of CT state in charge transfer process. Interface energetics and the morphology of active layers, [ 26 ] including introduction of the third component, will be outmost importance for the E CT . The next part, Δ E 2 = q ·Δ, can be seen from Equation (2) that E CT has a decisive influence on the magnitude of the radiative recombination …”