Collection-limited theory interprets the extraordinary response of single semiconductor organic solar cells

Abstract

The bulk heterojunction (BHJ) organic photovoltaic (OPV) architecture has dominated the literature due to its ability to be implemented in devices with relatively high efficiency values. However, a simpler device architecture based on a single organic semiconductor (SS-OPV) offers several advantages: it obviates the need to control the highly system-dependent nanoscale BHJ morphology, and therefore, would allow the use of broader range of organic semiconductors. Unfortunately, the photocurrent in standard SS-OPV devices is typically very low, which generally is attributed to inefficient charge separation of the photogenerated excitons. Here we show that the short-circuit current density from SS-OPV devices can be enhanced significantly (∼100-fold) through the use of inverted device configurations, relative to a standard OPV device architecture. This result suggests that charge generation may not be the performance bottleneck in OPV device operation. Instead, poor charge collection, caused by defect-induced electric field screening, is most likely the primary performance bottleneck in regular-geometry SS-OPV cells. We justify this hypothesis by: (i) detailed numerical simulations, (ii) electrical characterization experiments of functional SS-OPV devices using multiple polymers as active layer materials, and (iii) impedance spectroscopy measurements. Furthermore, we show that the collection-limited photocurrent theory consistently interprets typical characteristics of regular SS-OPV devices. These insights should encourage the design and OPV implementation of high-purity, high-mobility polymers, and other soft materials that have shown promise in organic field-effect transistor applications, but have not performed well in BHJ OPV devices, wherein they adopt less-than-ideal nanostructures when blended with electron-accepting materials.T he photovoltaic properties of semiconducting polymers arranged in a simple metal-polymer-metal sandwich structure were first demonstrated in 1993 by several groups (1-3). The efficiency values of these early photovoltaic (PV) cells were minuscule mainly because of the poor short-circuit current densities (J sc ∼10 μA cm −2 ) under standard 1 sun illumination. By definition,where J ph ðV Þ is the voltage-dependent photocurrent, η ex is the exciton diffusion-dissociation (or free-charge generation) efficiency, η C is the charge (free-carrier) collection efficiency, and J max is the maximum current density obtained by integrating absorption spectra (J max ∼ 10 − 20 mA cm −2 for typical semi-conducting polymers). The difference between J sc and J max was attributed to an inefficient η ex (<0.1%) arising from ultrashort exciton diffusion lengths (∼10-15 nm) in semiconducting polymers and was eventually interpreted by Onsager's theory of geminate pair recombination (4-6). This classical interpretation assumes, on the other hand, that η C ∼ 100%. To improve η ex , Heeger and co-workers (7) and Holmes and coworkers (8) introduced the concept of bulk heterojunction (BHJ) devices in 1995. Her...

Current Status of Outdoor Lifetime Testing of Organic Photovoltaics

“…A collection‐limited theory also confirmed the dependence of device efficiency on irradiance level, as shown in Figure . Here, it was found that the space‐charge density increased with increasing irradiance level.…”

“…Thus, the recombination mechanisms can largely determine V oc of OPVs. As shown in Figure c, V oc varies logarithmically with illumination intensity, with its slope being equal to kT / e . From Equation , it can be seen that V oc is particularly susceptible to the density of states (DOS) of the acceptor LUMO and donor HOMO.…”

“…Dotted lines correspond to results from the self‐consistent numerical simulations for typical inorganic solar cells. Reproduced with permission . Copyright 2015, National Academy of Sciences of the United States of America.…”

Integrated Front–Rear-Grid Optimization of Free-Form Solar Cells

“…14 is reduced by a certain factor. For a OPV, photocurrent is observed to be voltage-dependent [31][32][33] and this behavior is especially prominent in the reverse regime. Since our model uses the data only for a small range of positive voltage values, we assume that photocurrent is independent of voltage.…”

“…However, for organic solar cells, Eq. 1 does not fit very well [31][32][33]. While modeling, this needs to be taken into account and the behavior of the device needs to be accurately modeled.…”