Single‐Walled Carbon Nanotubes as an Additive in Organic Photovoltaics: Effects on Carrier Generation and Recombination Dynamics

Abstract

Employing single‐walled carbon nanotubes (SWNTs) as an additive in the active layer of organic photovoltaics (OPV) to improve charge extraction is gaining weight in the research community. While SWNTs can transport charge carriers orders of magnitudes faster than conventional polymers, they also quench excitons and trap‐free charges, reducing device performance. Such influences of SWNTs on OPV device performance remain inadequately explored and incompletely understood. Herein, the impact of SWNTs, enriched in (6,5) chirality, on the charge generation and extraction properties of two different OPV devices is studied by femtosecond‐microsecond transient absorption spectroscopy and transient photovoltage/photocurrent techniques. It is shown that depending on the donor material properties (e.g., energetics), SWNTs can reduce carrier generation by quenching the excitons and increase carrier recombination through trapping processes. Then, by performing an analytical calculation of the current–voltage characteristics of the devices using the parameters determined via the transient measurements, it is shown that the increased recombination also relates to morphological changes induced by the SWNTs. These results shed light on the performance limiting factors of the SWNTs when incorporated into the active layer of the solar cells. This work thus can accelerate the blend optimization process for high‐performance organic solar cells.

Comprehensive Study of Carrier Recombination in High‐Efficiency CdTe Solar Cells Using Transient Photovoltage

“…Experimentally, TPC measurements are performed the same as TPV except the device output is connected to the 50 Ω oscilloscope input (approximating a short-circuit condition), as opposed to the 1 MΩ channel used in TPV (approximating an open-circuit condition), to allow current to flow. [20,29,32] The integration of the TPC decay curve (laser perturbation induced short-circuit current density change ΔJ SC (t)) ΔQ, when normalized by the amplitude of the perturbation-induced voltage change (ΔV OC (t = 0)), yields the differential capacitance, C(V OC ) = ΔQ/ΔV OC , as illustrated in Figure 4. This data, when processed across different V OC or bias-light intensities, enables the determination of stationary carrier density via Equation ( 9).…”

“…Under such conditions, the steady-state current density ( J ext ) is proportional to the extraction rate and can be expressed as a function of generation and recombination rates. [20] J…”

“…[28] Both TPV and TPC have been applied to a variety of photovoltaic materials but not yet to CdTe devices. [20,[29][30][31][32][33] Here we use TPV and TPC to investigate a CdTe device. Since these techniques are new to the CdTe community, we start by comparing TRPL to TPV and TPC measurements.…”

Excitation Energy Transfer via Electron Transfer between a Semi‐Conducting Single Walled‐Carbon Nanotube and Encapsulated Zinc Porphyrin or Photovoltaic Application

“…Depending on the characteristics of the donor material, SWNTs can either reduce charge generation by quenching excitons or increase charge recombination through trapping processes. [39][40][41] At present, various organizations worldwide are working to enhance the optical and electrical properties of thin films in preparation for the upcoming generation of OSC. Herein, the literature has documented that various guest substances, including organic molecules, have been enclosed within SWNTs.…”

“…The study explores how SWNTs, particularly those enriched in (6,5) chirality, impact the generation and extraction of charges in various OPV devices. Depending on the characteristics of the donor material, SWNTs can either reduce charge generation by quenching excitons or increase charge recombination through trapping processes [39–41] . At present, various organizations worldwide are working to enhance the optical and electrical properties of thin films in preparation for the upcoming generation of OSC.…”