Transient Photocurrent Elucidating Carrier Dynamics and Potential of Bulk Heterojunction Solar Cells Fabricated by Thermal Precursor Approach

Abstract

Transient photocurrents are measured to clarify carrier dynamics in a series of bulk heterojunction solar cells that are composed of [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) and diketopyrrolopyrrole‐tetrabenzoporphyrin conjugates substituted by straight alkyl groups (Cn‐DPP‐BP), of which the device parameters are largely dependent on the length of the substituted alkyl groups (Cn). The time profile of the observed transients indicates the presence of both unimolecular and bimolecular charge recombination processes in the active layer of the cells. Analysis of the photocurrent decays detected at various bias voltages using a drift model that considers charge extraction by electrodes and charge recombination in the active layer enables elucidation of the charge drift mobility, the initial photocarrier density, and the charge recombination rate constants. The bimolecular recombination rate constant is correlated with Cn and this is the main factor that determines the power conversion efficiency (η) of the Cn‐DPP‐BP:PCBM solar cells. The Cn‐dependent fill factor can be interpreted in terms of the bias voltage dependence of the ratio between the extraction and recombination charges (Qex/Qre). Comparison of η and Qex/Qre at the short‐circuit condition predicts that η for the Cn‐DPP‐BP:PCBM solar cell can be potentially improved up to 6%.

Microscopic Structures, Dynamics, and Spin Configuration of the Charge Carriers in Organic Photovoltaic Solar Cells Studied by Advanced Time-Resolved Spectroscopic Methods

“…Then, we review our recent efforts to understand the behavior of the local charge carrier dynamics in the above BHJ thin films and compare it with those in neat thin films. Finally, we present recent studies on C n -DPP-BP:PC 61 BM devices to study charge recombination and carrier extraction by electrodes by taking submicrosecond time-resolved photocurrent measurements . Here, n stands for the alkyl chain length of the DPP units.…”

“…In this study, we used time-resolved THz spectroscopy to measure the charge carrier dynamics of diketo­pyrrolopyrrole-linked tetrabenzo­porphyrin (DPP-BP) BHJ thin films mixed with PC 61 BM . We also compare the charge carrier dynamics of the same systems on a microsecond time scale by taking transient photocurrent measurements . Here, we focus on the dependence of the alkyl chain length on the charge carrier recombination dynamics in BHJ OPVs based on DPP-BP …”

Realtime Observation of Carrier Trapping and Recombination in a Bulk Heterojunction Film by Simultaneous Optical and Electrical Detection

“…For this reason, conventional analysis of Δi is based on an assumption of nondispersive mobility or time-independent number density. 11,12 To avoid such ambiguous assumption, in this study, we have developed a novel methodology for acquiring transient optical absorption (denoted by ΔA) due to the photocarriers together with Δi at the same time. In the case of donor−acceptor type organic semiconductor films, the photogenerated electrons and holes are respectively localized on the acceptor and donor molecules after completion of charge separation from the exciton.…”

“…There have been reported two major loss mechanisms for photogenerated carriers, which are trapping and recombination. − Both the processes are induced by diffusion and drift motions of the carriers, which are complicated due to structural and energetical disorders in the amorphous organic films. Time-resolved measurement of electric conductance due to photocarriers injected by a pulsed light is a powerful method for studying these dynamic processes. − It is known for disordered organic films that carriers have a broad distribution of mobility (μ). Hence, transient electric current Δ i is expressed by the product of μ and number densities for the photogenerated electrons (denoted by the subscript “e”) and holes (denoted by the subscript “h”) as S , E , and e respectively denote the cross-sectional area of the organic film, the electric field, and the elementary charge.…”

“…Therefore, it is difficult to evaluate the two loss mechanisms separately and quantitatively from the decay kinetics of Δ i . For this reason, conventional analysis of Δ i is based on an assumption of nondispersive mobility or time-independent number density. , …”