Charge Generation and Recombination in an Organic Solar Cell with Low Energetic Offsets

Abstract

Organic bulk heterojunction (BHJ) solar cells require energetic offsets between the donor and acceptor to obtain high short-circuit currents (J SC ) and fill factors (FF). However, it is necessary to reduce the energetic offsets to achieve high open-circuit voltages (V OC ). Recently, reports have highlighted BHJ blends that are pushing at the accepted limits of energetic offsets necessary for high efficiency. Unfortunately, most of these BHJs have modest FF values. How the energetic offset impacts the solar cell characteristics thus remains poorly understood. Here, a comprehensive characterization of the losses in a polymer:fullerene BHJ blend, PIPCP:phenyl-C61-butyric acid methyl ester (PC 61 BM), that achieves a high V OC (0.9 V) with very low energy losses (E loss = 0.52 eV) from the energy of absorbed photons, a respectable J SC (13 mA cm −2 ), but a limited FF (54%) is reported. Despite the low energetic offset, the system does not suffer from fielddependent generation and instead it is characterized by very fast nongeminate recombination and the presence of shallow traps. The charge-carrier losses are attributed to suboptimal morphology due to high miscibility between PIPCP and PC 61 BM. These results hold promise that given the appropriate morphology, the J SC , V OC , and FF can all be improved, even with very low energetic offsets.

Efficient DPP Donor and Nonfullerene Acceptor Organic Solar Cells with High Photon‐to‐Current Ratio and Low Energetic Loss

“…Differently, IEICO‐4F‐based devices exhibit a lifetime twice as short (7.7 µs) than that of ITIC. This result is in agreement with the higher trap‐assisted recombination observed in the plot of V oc versus light intensity, which further confirm the relatively lower FF values in ITIC devices . Besides charge recombination, charge transport is another crucial process for efficient organic photovoltaics.…”

A Highly Crystalline Fused‐Ring n‐Type Small Molecule for Non‐Fullerene Acceptor Based Organic Solar Cells and Field‐Effect Transistors

“…As observed in Figure c, the slopes of V oc versus light intensity plots for PBDB‐T:IDTIC and PBDB‐T:IDTTIC devices are 1.32 and 1.05 kT / q respectively, indicating that trap states in IDTIC‐based devices have a detrimental effect on the transport properties of the devices, in agreement with the lower FF value obtained compared to IDTTIC‐based solar cells. Consequently, we conclude that charge extraction in PBDB‐T:IDTIC is limited by trap‐assisted recombination, whereas IDTTIC‐based devices do not suffer from this limitation …”

Difluorinated Oligothiophenes for High‐Efficiency All‐Small‐Molecule Organic Solar Cells: Positional Isomeric Effect of Fluorine Substitution on Performance Variations

“…Mobility changes usually result from active layer morphology changes. Accordingly, we decided to check the thin‐film microstructures …”