Time-delayed Collection Field Research Articles

On the Impact of Bimolecular Recombination on Time‐Delayed Collection Field Measurements and How to Minimize Its Effect

The time‐delayed collection field (TDCF) technique is a popular method to quantify the field and temperature dependences of free charge generation in organic solar cells. Because the method relies on the extraction of photogenerated charge carriers, bimolecular recombination not only between the photogenerated carriers but also between the photogenerated and dark‐injected carriers affects its accuracy, particularly at forward bias. In this work, drift–diffusion simulations are employed to quantify the recombination losses in conventional and modified TDCF measurements, where the latter technique intends to reduce the impact of dark injection. It is shown that parameters such as the generation profile, carrier mobilities, and effective density of states affect the recombination losses in both measurements. Importantly, modified TDCF enables to reduce the recombination losses at forward bias, especially beyond the open‐circuit voltage. However, conventional TDCF is preferable for studies at reverse bias due to a better depletion of the active layer prior to the emergence of the photogenerated carriers. Measurements on a ZR1:Y6 blend with fast recombination are in good agreement with the simulation results. This work shows that artifacts in TDCF measurements related to non‐geminate recombination can be accounted for and minimized through an informed choice of the experimental conditions.

The physical meaning of time-delayed collection field transients from disordered devices

Charge carrier mobility and recombination determine the performance of many opto-electronic devices such as solar cells, sensors, and light-emitting diodes. Understanding how these parameters change as a function of material choice, charge carrier density, and device geometry is essential for developing the next generation of devices. The time-delayed collection field (TDCF) technique is becoming a widely used method to measure both recombination and carrier transport with values derived from this method being widely reported for many material systems. However, most novel materials are highly disordered with a high density of trap states and standard TDCF theory neglects the influence of these states. In this work, we examine how reliable TDCF can be as a measurement technique when the device contains significant energetic disorder. We identify regimes where the results can be relied upon and where the results should be taken with more caution. Finally, we provide simple and easy to use experimental tests to help the experimentalist decide whether the physical processes are dominated by trap states.

Anticorrelated photoluminescence and free charge generation proves field-assisted exciton dissociation in low-offset PM6:Y5 organic solar cells

Understanding the origin of inefficient photocurrent generation in organic solar cells with low energy offset remains key to realizing high-performance donor-acceptor systems. Here, we probe the origin of field-dependent free-charge generation and photoluminescence in non-fullereneacceptor (NFA)-based organic solar cells using the polymer PM6 and the NFA Y5—a non-halogenated sibling to Y6, with a smaller energetic offset to PM6. By performing time-delayed collection field (TDCF) measurements on a variety of samples with different electron transport layers and active layer thickness, we show that the fill factor and photocurrent are limited by field-dependent free charge generation in the bulk of the blend. We also introduce a new method of TDCF called m-TDCF to prove the absence of artifacts from non-geminate recombination of photogenerated and dark charge carriers near the electrodes. We then correlate free charge generation with steady-state photoluminescence intensity and find perfect anticorrelation between these two properties. Through this, we conclude that photocurrent generation in this low-offset system is entirely controlled by the field-dependent dissociation of local excitons into charge-transfer states.

Spray-Deposited Aluminum-Doped Zinc Oxide as an Efficient Electron Transport Layer for Inverted Organic Solar Cells

Spray-deposited thin films of zinc oxide (ZnO) and aluminum-doped zinc oxide (Al-ZnO) are characterized in detail to get insight into the role of a dopant in the matrix. ZnO and Al-ZnO are implemented as electron transport layers (ETLs) in inverted organic solar cells (IOSCs) with PTB7-Th as a donor and IEICO-4F as a nonfullerene acceptor, forming the bulk heterojunction (BHJ) photoactive layer. Organic solar cells (OSCs) based on the ZnO ETL exhibit a short-circuit current density (JSC) of 24.46 mA/cm2 and an open-circuit voltage (VOC) of 0.68 V, yielding a power conversion efficiency (PCE) of 9.3%. A solar cell based on the Al-ZnO ETL yields a higher JSC of 25.16 mA/cm2 and a VOC of 0.71 V, resulting in a PCE of 10.5%, which indicates that Al doping improves the device performance. Time-delayed collection field (TDCF) measurements yielded field-independent charge generation for both devices. Furthermore, steady-state photoluminescence (PL), time-resolved PL, and transient absorption measurements confirm reduction in the number of defect states in Al-ZnO thin films compared to ZnO thin films and efficient charge transfer, yielding an overall improved IOSC device performance.

Probing Charge Generation Efficiency in Thin-Film Solar Cells by Integral-Mode Transient Charge Extraction.

The photogeneration of free charges in light-harvesting devices is a multistep process, which can be challenging to probe due to the complexity of contributing energetic states and the competitive character of different driving mechanisms. In this contribution, we advance a technique, integral-mode transient charge extraction (ITCE), to probe these processes in thin-film solar cells. ITCE combines capacitance measurements with the integral-mode time-of-flight method in the low intensity regime of sandwich-type thin-film devices and allows for the sensitive determination of photogenerated charge-carrier densities. We verify the theoretical framework of our method by drift-diffusion simulations and demonstrate the applicability of ITCE to organic and perovskite semiconductor-based thin-film solar cells. Furthermore, we examine the field dependence of charge generation efficiency and find our ITCE results to be in excellent agreement with those obtained via time-delayed collection field measurements conducted on the same devices.

Efficiency Limits in Wide‐Bandgap Ge‐Containing Donor Polymer:Nonfullerene Acceptor Bulk Heterojunction Solar Cells

A precise picture of the photophysics that determine the efficiency of nonfullerene acceptor (NFA) bulk heterojunction (BHJ) organic solar cells is still missing, yet needed to mitigate the remaining loss channels. Herein, charge carrier generation and recombination dynamics in blends of a novel wide‐bandgap germanium‐containing donor polymer, namely, PEHGeNDT‐BT, paired with either O‐IDTBR or O‐IDTBCN as NFA in BHJ solar cells are investigated by (ultrafast) transient spectroscopy and time‐delayed collection field (TDCF) experiments. Photovoltaic devices yield moderate power conversion efficiencies (PCEs) of 5.3% when using O‐IDTBCN as the acceptor, and only about 2% when using O‐IDTBR as the acceptor, the latter severely limited by its low photocurrent and moderate fill factor (FF) of ≈43%. Time‐resolved photoluminescence experiments reveal limited exciton quenching as one loss channel in O‐IDTBR‐based blends, accompanied by significant geminate recombination further reducing the photocurrent, while field‐dependent charge generation is identified as the origin of the low FF. Geminate recombination is less in the O‐IDTBCN blend and charge generation is field‐independent, leading to improved photocurrent and FF. Carrier drift‐diffusion simulations of the devices’ current–voltage (J–V) characteristics confirm that the experimentally determined kinetic parameters and process yields can reproduce the measured J–V curves under steady‐state solar illumination.

Charge Photogeneration in Non‐Fullerene Organic Solar Cells: Influence of Excess Energy and Electrostatic Interactions

AbstractIn organic solar cells, photogenerated singlet excitons form charge transfer (CT) complexes, which subsequently split into free charge carriers. Here, the contributions of excess energy and molecular quadrupole moments to the charge separation process are considered. The charge photogeneration in two separate bulk heterojunction systems consisting of the polymer donor PTB7‐Th and two non‐fullerene acceptors, ITIC and h‐ITIC, is investigated. CT state dissociation in these donor–acceptor systems is monitored by charge density decay dynamics obtained from transient absorption experiments. The electric field dependence of charge carrier generation is studied at different excitation energies by time delayed collection field (TDCF) and sensitive steady‐state photocurrent measurements. Upon excitation below the optical gap, free charge carrier generation becomes less field dependent with increasing photon energy, which challenges the view of charge photogeneration proceeding through energetically lowest CT states. The average distance between electron–hole pairs at the donor–acceptor interface is determined from empirical fits to the TDCF data. The delocalization of CT states is larger in PTB7‐Th:ITIC, the system with larger molecular quadrupole moment, indicating the sizeable effect of the electrostatic potential at the donor–acceptor interface on the dissociation of CT complexes.

Reliability of charge carrier recombination data determined with charge extraction methods

Charge extraction methods are popular for measuring the charge carrier density in thin film organic solar cells and to draw conclusions about the order and coefficient of nongeminate charge recombination. However, results from such studies may be falsified by inhomogeneous steady state carrier profiles or surface recombination. Here, we present a detailed drift-diffusion study of two charge extraction methods, bias-assisted charge extraction (BACE) and time-delayed collection field (TDCF). Simulations are performed over a wide range of the relevant parameters. Our simulations reveal that both charge extraction methods provide reliable information about the recombination order and coefficient if the measurements are performed under appropriate conditions. However, results from BACE measurements may be easily affected by surface recombination, in particular for small active layer thicknesses and low illumination densities. TDCF, on the other hand, is more robust against surface recombination due to its transient nature but also because it allows for a homogeneous high carrier density to be inserted into the active layer. Therefore, TDCF is capable to provide meaningful information on the order and coefficient of recombination even if the model conditions are not exactly fulfilled. We demonstrate this for an only 100 nm thick layer of a highly efficient nonfullerene acceptor (NFA) blend, comprising the donor polymer PM6 and the NFA Y6. TDCF measurements were performed as a function of delay time for different laser fluences and bias conditions. The full set of data could be consistently fitted by a strict second order recombination process, with a bias- and fluence-independent bimolecular recombination coefficient k2 = 1.7 × 10−17 m3 s−1. BACE measurements performed on the very same layer yielded the identical result, despite the very different excitation conditions. This proves that recombination in this blend is mostly through processes in the bulk and that surface recombination is of minor importance despite the small active layer thickness.

Energy-Gap Law for Photocurrent Generation in Fullerene-Based Organic Solar Cells: The Case of Low-Donor-Content Blends.

The involvement of charge-transfer (CT) states in the photogeneration and recombination of charge carriers has been an important focus of study within the organic photovoltaic community. In this work, we investigate the molecular factors determining the mechanism of photocurrent generation in low-donor-content organic solar cells, where the active layer is composed of vacuum-deposited C60 and small amounts of organic donor molecules. We find a pronounced decline of all photovoltaic parameters with decreasing CT state energy. Using a combination of steady-state photocurrent measurements and time-delayed collection field experiments, we demonstrate that the power conversion efficiency, and more specifically, the fill factor of these devices, is mainly determined by the bias dependence of photocurrent generation. By combining these findings with the results from ultrafast transient absorption spectroscopy, we show that blends with small CT energies perform poorly because of an increased nonradiative CT state decay rate and that this decay obeys an energy-gap law. Our work challenges the common view that a large energy offset at the heterojunction and/or the presence of fullerene clusters guarantee efficient CT dissociation and rather indicates that charge generation benefits from high CT state energies through a slower decay to the ground state.

Impact of Structural Polymorphs on Charge Collection and Nongeminate Recombination in Organic Photovoltaic Devices

The formation of different types of structural polymorphs of poly(3-hexyl-thiophene) (P3HT) affects the performance of organic photovoltaic (OPV) devices that use thermally annealed P3HT:PCBM[60] blend films as a photoactive layer. Here it is demonstrated that when densely packed and nondensely packed P3HT polymorphs coexist in the P3HT:PCBM[60] layer, nongeminate charge recombination is fast; however, in a device nongeminate recombination is effectively overruled by efficient and fast charge carrier extraction. In stark contrast, when only a less densely packed P3HT polymorph is present in the blend, nongeminate charge recombination losses are less pronounced, and the charge carrier extraction efficiency is lower. The antagonistic nongeminate charge recombination and charge carrier extraction processes in these systems are monitored by time-delayed collection field (TDCF) and ultrafast transient absorption (TA) experiments. Furthermore, resonance Raman spectroscopy reveals that in the absence of the densely packed P3HT polymorph the energetic disorder present in the P3HT:PCBM[60] blend is higher. High-resolution atomic force microscopy imaging further identifies pronounced differences in the layer morphology when the polymorph distribution varies between unimodal and bimodal. These results indicate that less densely packed P3HT polymorphs increase disorder and impede charge collection, leading to a reduction of the device fill factor.

Apparent Field‐Dependence of the Charge Carrier Generation in Organic Solar Cells as a Result of (Bimolecular) Recombination

Charge carrier generation in organic solar cells is often reported to depend on the electric field. This is, however, not measured directly but derived from transient charge carrier extraction experiments based on the time delayed collection field (TDCF) method. In this work, numerical simulations of TDCF experiments are presented which – when analyzed in the same way as reported in literature – result in a field‐dependence of charge carrier generation despite the fact that a field‐independent generation is used. This discrepancy is shown to be caused by recombination of photogenerated charge carriers occurring in the time range prior to and during extraction. This apparent field‐dependence becomes more pronounced for larger recombination coefficients and decreasing charge carrier mobilities, very much in accordance with experimental TDCF data from literature. Even an apparent voltage‐ and time‐dependence of the bimolecular recombination coefficient is reproduced in the simulations although a constant, voltage‐independent one is used. These findings strongly question whether TDCF is an appropriate method to detect a potential field‐dependence of the photocurrent generation and the recombination coefficient. Our study shows that all experimental results can consistently be explained without the assumption of a field‐dependence of the charge carrier generation and the bimolecular recombination coefficient.

Wide‐Bandgap Small Molecular Acceptors Based on a Weak Electron‐Withdrawing Moiety for Efficient Polymer Solar Cells

Narrow‐bandgap small molecular acceptors (SMAs) with absorption extending into the near‐infrared spectral region such as ITIC derivatives are widely investigated, while the development of their wide‐bandgap counterparts remains largely unexplored. Wide‐bandgap non‐fullerene acceptors (NFAs) are highly desirable and beneficial for constructing efficient device layouts such as ternary blend and tandem solar cells that require multiple light‐harvesting materials with different regions of absorption. In this contribution, the design and synthesis of two wide‐bandgap SMAs (IDT‐TBA and IDDT‐TBA), consisting of a weak electron‐withdrawing moiety (1,3‐diethyl‐2‐thiobarbituric acid, TBA) is presented. Compared to ITIC, this molecular design strategy results in energetically down‐shifted HOMO levels and hence much enlarged bandgaps of 1.91 eV for IDT‐TBA and 1.78 eV for IDDT‐TBA, respectively. Further photovoltaic performance evaluation demonstrates power conversion efficiencies (PCEs) of 6.5% for IDT‐TBA and 7.5% for IDDT‐TBA, respectively, when using PBDB‐T as the electron donor polymer. In addition, time‐delayed collection field (TDCF) experiments suggest that both IDT‐TBA and IDDT‐TBA based cells exhibit field‐independent charge generation with external charge generation efficiencies exceeding 90%, implying negligible geminate recombination losses. The results demonstrate that TBA units are promising and attractive building blocks as weak electron‐withdrawing acceptors to construct wide‐bandgap high‐efficiency SMAs for efficient organic photovoltaic devices.

(Invited) Photocarrier Generation in Single-Absorber Organic Solar Cells

Power Conversion efficiency of organic photovoltaic devices has exceeded 10% by taking advantage of bulk heterojunction (BHJ) structure, where donor and acceptor molecules are microscopically mixed. A remained problem is photovoltage loss from absorbed photon energy. In the BHJ system, LUMO offset is required to promote photodissociation at the donor-acceptor interface, causing serious photovoltage loss. To solve this problem, we are approaching it from single absorber cells, where there is no donor-acceptor interface. Generally, photogeneration efficiency in organic film bulk is quite low because of large exciton binding energy of Frenkel exciton. On the other hand, photogeneration from CT excitons occurs almost spontaneously in the BHJ system. If we could generate charge-transfer (CT) excitons in the single-component film directly, photovoltage loss would be reduced. Recently, we found intramolecular charge-transfer molecules including donor and acceptor units within one molecule show considerable high performance in the single-absorber device. In particular, DTDCPB having triphenyl amine unit (donor) and benzothiadiazole unit (acceptor) was a promising material for the single-absorber system. In this presentation, photogeneration process in the single-component film of CT molecules was investigated from several viewpoints. Molecular design for intramolecular charge-transfer absorption is discussed from the viewpoint of quantum chemical calculation and organic synthesis. The photocarrier dynamics in the single-absorber solar cells is investigated by several techniques such as intensity-modulated photovoltage/photocurrent (IMVS/IMPS) and time delayed collection field (TDCF). Exciton binding energy, that is a key factor for photodissociation in the film bulk, was estimated by Onsager-Braun analysis by using a planar device. Figure 1

Probing the pathways of free charge generation in organic bulk heterojunction solar cells

The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges.

Organic solar cells based on anthracene-containing PPE–PPVs and non-fullerene acceptors

Lately, non-fullerene acceptors (NFAs) have received increasing attention for use in polymer-based bulk-heterojunction (BHJ) organic solar cells (OSCs), as improved photovoltaic performance compared to classical polymer–fullerene blends could be demonstrated. In this study, polymer solar cells based on a statistically substituted anthracene-containing poly(p-phenylene ethynylene)-alt-poly(p-phenylene vinylene)s (PPE–PPVs) copolymer (AnE-PVstat) as donor in combination with a number of different electron accepting materials were investigated. Strong photoluminescence quenching of the polymer donor indicates intimate intermixing of both materials. However, the photovoltaic performances were found to be poor compared to blends that use fullerene as acceptor. Time-delayed collection field (TDCF) measurements demonstrate: charge generation is field-independent, but bimolecular recombination processes limit the fill factor and thus the efficiency of devices.

Intercalated vs Nonintercalated Morphologies in Donor-Acceptor Bulk Heterojunction Solar Cells: PBTTT:Fullerene Charge Generation and Recombination Revisited.

In this Letter, we study the role of the donor:acceptor interface nanostructure upon charge separation and recombination in organic photovoltaic devices and blend films, using mixtures of PBTTT and two different fullerene derivatives (PC70BM and ICTA) as models for intercalated and nonintercalated morphologies, respectively. Thermodynamic simulations show that while the completely intercalated system exhibits a large free-energy barrier for charge separation, this barrier is significantly lower in the nonintercalated system and almost vanishes when energetic disorder is included in the model. Despite these differences, both femtosecond-resolved transient absorption spectroscopy (TAS) and time-delayed collection field (TDCF) exhibit extensive first-order losses in both systems, suggesting that geminate pairs are the primary product of photoexcitation. In contrast, the system that comprises a combination of fully intercalated polymer:fullerene areas and fullerene-aggregated domains (1:4 PBTTT:PC70BM) is the only one that shows slow, second-order recombination of free charges, resulting in devices with an overall higher short-circuit current and fill factor. This study therefore provides a novel consideration of the role of the interfacial nanostructure and the nature of bound charges and their impact upon charge generation and recombination.

Charge Generation and Mobility-Limited Performance of Bulk Heterojunction Solar Cells with a Higher Adduct Fullerene

Alternative electron acceptors are being actively explored in order to advance the development of bulk-heterojunction (BHJ) organic solar cells (OSCs). The indene–C60 bisadduct (ICBA) has been regarded as a promising candidate, as it provides high open-circuit voltage in BHJ solar cells; however, the photovoltaic performance of such ICBA-based devices is often inferior when compared to cells with the omnipresent PCBM electron acceptor. Here, by pairing the high performance polymer (FTAZ) as the donor with either PCBM or ICBA as the acceptor, we explore the physical mechanism behind the reduced performance of the ICBA-based device. Time delayed collection field (TDCF) experiments reveal reduced, yet field-independent free charge generation in the FTAZ:ICBA system, explaining the overall lower photocurrent in its cells. Through the analysis of the photoluminescence, photogeneration, and electroluminescence, we find that the lower generation efficiency is neither caused by inefficient exciton splitting, nor ...

Dispersive Non-Geminate Recombination in an Amorphous Polymer:Fullerene Blend.

Recombination of free charge is a key process limiting the performance of solar cells. For low mobility materials, such as organic semiconductors, the kinetics of non-geminate recombination (NGR) is strongly linked to the motion of charges. As these materials possess significant disorder, thermalization of photogenerated carriers in the inhomogeneously broadened density of state distribution is an unavoidable process. Despite its general importance, knowledge about the kinetics of NGR in complete organic solar cells is rather limited. We employ time delayed collection field (TDCF) experiments to study the recombination of photogenerated charge in the high-performance polymer:fullerene blend PCDTBT:PCBM. NGR in the bulk of this amorphous blend is shown to be highly dispersive, with a continuous reduction of the recombination coefficient throughout the entire time scale, until all charge carriers have either been extracted or recombined. Rapid, contact-mediated recombination is identified as an additional loss channel, which, if not properly taken into account, would erroneously suggest a pronounced field dependence of charge generation. These findings are in stark contrast to the results of TDCF experiments on photovoltaic devices made from ordered blends, such as P3HT:PCBM, where non-dispersive recombination was proven to dominate the charge carrier dynamics under application relevant conditions.

Charge carrier recombination dynamics in perovskite and polymer solar cells

Time-delayed collection field experiments are applied to planar organometal halide perovskite (CH3NH3PbI3) based solar cells to investigate charge carrier recombination in a fully working solar cell at the nanosecond to microsecond time scale. Recombination of mobile (extractable) charges is shown to follow second-order recombination dynamics for all fluences and time scales tested. Most importantly, the bimolecular recombination coefficient is found to be time-dependent, with an initial value of ca. 10−9 cm3/s and a progressive reduction within the first tens of nanoseconds. Comparison to the prototypical organic bulk heterojunction device PTB7:PC71BM yields important differences with regard to the mechanism and time scale of free carrier recombination.

Free carrier generation and recombination in PbS quantum dot solar cells

Time Delayed Collection Field and Bias Assisted Charge Extraction (BACE) experiments are used to investigate the charge carrier dynamics in PbS colloidal quantum dot solar cells. We find that the free charge carrier creation is slightly field dependent, thus providing an upper limit to the fill factor. The BACE measurements reveal a rather high effective mobility of 2×10−3 cm2/Vs, meaning that charge extraction is efficient. On the other hand, a rather high steady state non-geminate recombination coefficient of 3×10−10 cm3/s is measured. We, therefore, propose a rapid free charge recombination to constitute the main origin for the limited efficiency of the PbS colloidal quantum dots cells.