Transient Photocurrent Response of Small‐Molecule Bulk Heterojunction Solar Cells

Seifter, Jason; Sun, Yanming; Heeger, Alan J.

doi:10.1002/adma.201305160

Cited by 64 publications

(72 citation statements)

References 57 publications

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“…[4c,7a] As in classic TPC, it is not possible with this experiment to dissociate the effect of the two carrier species. [11] However, as shown in Table 1, the mobilities calculated using Equation (4) are close to the geometrical average of the electron and hole mobilities. To conclude these simulations show that the simple model proposed previously is valid when there is no dispersion and that the mobility can be calculated using this simple experiment.…”

Section: Resultsmentioning

confidence: 99%

“…The advantage of this technique is that the device is maintained under conditions close to realistic operating conditions with an illumination intensity always close to 1 sun and under constant bias, as opposed to other transient techniques, such as TOF, [5a] time-resolved electric-field-induced second harmonic generation (TREFISH)/TOF, [6b,c] and transient photocurrent (TPC), [11] which are based on pulsed light. Keeping the device under illumination and bias close to the maximum power point ensures that the charge carrier densities [12] and the electrical field, [12b] which can influence the charge carrier motion, will be similar to those observed in a working solar cell leading to a characterization of the transport properties closer to what happens under steady-state conditions.…”

Section: Resultsmentioning

confidence: 99%

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Charge Carrier Extraction in Organic Solar Cells Governed by Steady‐State Mobilities

Corre

Doumon

Chatri

et al. 2017

Advanced Energy Materials

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However, time-of-flight (TOF) experiments and Monte Carlo (MC) simulations have shown that if the degree of both energetic and positional disorder is high enough, the fluctuations of the intersite distances create fast diffusion routes. [5] These routes increase the mobility of carriers located in high energetic states leading to their extraction before they have the time to thermalize, thus creating dispersion in current extraction on a short time scale. [5] Although the relative importance of the dispersive effect on OPV devices performance is conflicting in the literature, [6] new reports have suggested that it is necessary to consider the influence of the dispersion effect in current extraction and that steadystate mobilities are not relevant to describe the transport in OPV devices. [6b,c] In their study, they set up a MC simulation and an optoelectrical measurement based on a laser light pulse from which they measured the charge carrier mobility on a time scale of 100 fs after the exciton generation. [6b,c] In the first nanoseconds, they observed a charge carrier mobility orders of magnitude higher than the one measured by steady-state measurements. This high mobility is due to the carriers being excited on the upper part of the DOS that are extracted before losing their energy through thermalization. [6b,c] The authors conclude that the steadystate mobilities are not pertinent to make relevant statements on OPV performance and that the thermalization loss plays a key role in the extraction.On the other hand, van der Kaap and Koster used a MC simulation to show that in an organic diode the thermalization has a limited impact on the performance. [6a] In addition, other reports have also demonstrated that SCLC analysis can be successfully applied to organic solar cells and gives valuable insight into materials transport properties. [4b,7] In fact, many studies have shown that the power conversion efficiency (PCE) [4b] and fill factor (FF) [7] depend on the steady-state mobilities. Furthermore, it has also been shown that drift-diffusion simulations, which assume a near-equilibrium state and use steady-state mobilities as an input, successfully describe the characteristics of organic transistors, light-emitting diodes, and solar cells. [7a,b,8] Transient signals of OPV devices have also been well reproduced by drift-diffusion simulation. For example, Albrecht et al. have been able to fit the time-delayed collection field (TDCF) signal using drift-diffusion simulations. Even though they also see that during the first 50 ns after the charge generation a small effect of mobility relaxation has to be taken into account in order to reproduce the transient signal, it still shows that those simulations are suitable to describe the transient behavior of organic solar cells. [9] These studies raise Charge transport in organic photovoltaic (OPV) devices is often characterized by steady-state mobilities. However, the suitability of steady-state mobilities to describe charge transport has recently been called...

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Charge Carrier Extraction in Organic Solar Cells Governed by Steady‐State Mobilities

Corre

Doumon

Chatri

et al. 2017

Advanced Energy Materials

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“…This energetic relaxation is often understood to cause a time-dependent mobility and therefore explain dispersive current transients [115,116], yet we will show here that this commonly-used model is inconsistent with our observations in high efficiency organic solar cell materials.…”

Section: Introductioncontrasting

confidence: 49%

“…This decay in photocurrent can occur due to two mechanisms, a reduction in carrier mobility, and/or a reduction in the number (or concentration) of moving carriers. The former, a time-dependent hot carrier mobility, is presently commonly believed to be the cause of dispersion in organic semiconductors [112,115,116] and it is usually understood to originate from a loss of energy as carriers thermalize within their density of states [118,119,120]. Higher energy carriers are expected to have a higher hopping probability, and hence a higher velocity [121,122,123], so the thermalization within the density of states causes the carrier mobility to decline.…”

Section: Introductionmentioning

confidence: 99%

Charge Generation and Transport Phenomena in Disordered Organic Semiconductors and Photovoltaic Diodes

Stolterfoht¹

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Organic semiconductors are of great interest for a broad range of optoelectronic applications due to their solution processability, chemical tunability and mechanical flexibility. In contrast to traditional banded semiconductors, organic semiconductors are intrinsically disordered systems and exhibit therefore much lower charge carrier mobilities. They are also low dielectric constant systems -as such, their photo-excitations are excitonic at room temperature with the electron-hole pair remaining Coulombically-bound. Blending organic semiconductors with differing electron affinities, so-called electron acceptors and donors, creates molecular heterojunctions which deliver the required driving force for exciton separation. These so-called bulk heterojunctions (BHJs) are the dominant architecture for creating organic photovoltaic cells and photodetectors. However, not least because of an incomplete understanding of the underlying physical mechanism that control the conversion of photons to free charges, and the subsequent extraction of these free charges to the electrodes, these organic optoelectronic systems still lag behind their inorganic counterparts.Motivated by these factors, the work described in this thesis advances the fundamental understanding of the loss mechanisms associated with charge photogeneration and charge transport phenomena in BHJ organic solar cells and photodetectors, and presents new experimental methodologies to study these processes. The transport of photogenerated charge carriers in the percolated donor: acceptor pathways towards the extracting electrodes has been studied in-depth via existing and newly developed transient photovoltage and steady-state characterization techniques. A simple but conclusive understanding has been developed which allows for minimization of the detrimental recombination of free charge carriers for given device parameters such as film thickness, applied voltage and the mobility of the slower carrier type (either electrons or holes). The models' predictions have been experimentally validated for many different (> 25) BHJ solar cell systems. Inspired by the need to selectively optimize the processes which control the photocurrent output of the cell, such a charge photogeneration and extraction, a technique to quantify and disentangle both efficiencies has been developed as a next step. Corroborated by transient absorption spectroscopy, the dynamics of the photocarrier generation process was examined. The results suggest that the so-called charge-transfer state (CTS) separation limits the conversion of photons to free charges and the photocurrent output from short-circuit to the maximum power point, strongly depending on the slower carrier ii mobility. These findings were explained by the ability of the slower carriers to leave the donor: acceptor interface via 1) a high enough mobility, 2) a sufficiently large domain size, and 3) enough conduction pathways (entropy). This work also shows that the dynamics of charge extraction and CTS dissociation are simi...

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“…It is known that the optimized phase separation of donor/acceptor (D/A) plays the key role in determining the exciton dissociation and charge carriers transport for obtaining high efficiency OSCs [6,7]. Small molecular photovoltaic materials have been considered as the potential candidates due to their advantages of facile synthesis, uniform batch-to-batch, high charge carrier mobility and great crystallinity [8][9][10]. Recently, the PCE of solution processed small molecule OSCs has been rapidly increased by adapting high extinction coefficient narrow band gap materials.…”

Section: Introductionmentioning

confidence: 99%

Revealing the effect of donor/acceptor intermolecular arrangement on organic solar cells performance based on two-dimensional conjugated small molecule as electron donor

Sun

Zhang

et al. 2015

Organic Electronics

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Transient Photocurrent Response of Small‐Molecule Bulk Heterojunction Solar Cells

Cited by 64 publications

References 57 publications

Charge Carrier Extraction in Organic Solar Cells Governed by Steady‐State Mobilities

Charge Carrier Extraction in Organic Solar Cells Governed by Steady‐State Mobilities

Charge Generation and Transport Phenomena in Disordered Organic Semiconductors and Photovoltaic Diodes

Revealing the effect of donor/acceptor intermolecular arrangement on organic solar cells performance based on two-dimensional conjugated small molecule as electron donor

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