Nongeminate Recombination Research Articles

Nongeminate Recombination in All-Polymer Solar Cells with Different Crystallinities

One of the most challenging issues facing the organic photovoltaic community is to realize a high fill factor (FF) even with thick active layers. This is because the thick active layer is beneficial for photon absorption but makes charge collection difficult, which is primarily restricted by nongeminate recombination in solar cells. In this work, we have studied nongeminate recombination in four kinds of polymer solar cells based on blends of donor-conjugated polymers with different crystallinities and acceptor-conjugated polymers with a naphthalene diimide unit by using transient photovoltage and photocurrent techniques. As a result, we find that nongeminate recombination is considerably suppressed with an increasing degree of crystallinity of donor polymers, leading to a high FF of more than 0.6 even with an active layer thickness of 300 nm. The origin of such a phenomenon is further discussed in terms of variations in the states of mixed phases with a cascaded energy structure between crystalline domains and amorphous domains evaluated by conductive atomic force microscopy.

Effects of Recombination Order on Open-Circuit Voltage Decay Measurements of Organic and Perovskite Solar Cells

Non-geminate recombination, as one of the most relevant loss mechanisms in organic and perovskite solar cells, deserves special attention in research efforts to further increase device performance. It can be subdivided into first, second, and third order processes, which can be elucidated by the effects that they have on the time-dependent open-circuit voltage decay. In this study, analytical expressions for the open-circuit voltage decay exhibiting one of the aforementioned recombination mechanisms were derived. It was possible to support the analytical models with experimental examples of three different solar cells, each of them dominated either by first (PBDBT:CETIC-4F), second (PM6:Y6), or third (irradiated CH3NH3PbI3) order recombination. Furthermore, a simple approach to estimate the dominant recombination process was also introduced and tested on these examples. Moreover, limitations of the analytical models and the measurement technique itself were discussed.

Increasing Molecular Packing Density and Dielectric Constant in Non-Fullerene Acceptors Enables to Boost the Fill Factor Approaching 0.82 in Organic Solar Cells

SUMMARYThe fill factor (FF) of organic solar cells (OSC), a critically important photovoltaic parameter, is yet sub-optimal, often less than 0.8. To further reduce the FF gaps with regard to the Shockley-Queisser upper limit, we present a study unveiling the impacts of dielectric properties on obtaining high FFs and photovoltaic efficiencies in OSCs. With increasing the dielectric constant (e) of non-fullerene acceptors (NFAs) and bulk heterojunction (BHJ) films, afforded by the increase of molecular packing density (MPD) of NFAs in solid states, we show that the FF penalties related to non-geminate charge recombination in OSCs can be appreciably mitigated. This conceptual modulation on FFs is embodied by a series of NFA-based BHJs where the differentiated MPDs and resultant changes of e in BHJs are reflected by the difference in FFs. In particular, as exemplified by an NFA model system L8-BO tailored with branched side chains, the enlargements of MPD and e allow to achieve an impressive FF of 0.815 with efficiency of 18.23% in OSCs featuring an impressively low bimolecular recombination and reduced sensitivity of FF on the BHJ film thickness.

On Charge Carrier Density in Organic Solar Cells Obtained via Capacitance Spectroscopy

AbstractThe determination of the voltage‐dependent density of free charge carriers via capacitance spectroscopy is considered an important step in the analysis of emerging photovoltaic technologies, such as organic and perovskite solar cells. In particular, an intimate knowledge of the density of free charge carriers is required for the determination of crucial parameters such as the effective mobility, charge carrier lifetime, nongeminate recombination coefficients, average extraction times, and competition factors. Hence, it is paramount to verify the validity of the commonly employed approaches to obtain the density of free charge carriers. The advantages, drawbacks, and limitations of the most common approaches are investigated in detail and strategies to mitigate misleading values are explored. To this end, two types of nonfullerene organic solar cells based on a PTB7‐Th:ITIC‐2F blend and a PM6:Y6 blend, respectively, are used as a case study to assess how subsequent analyses of the nongeminate recombination dynamics depend on the chosen approach to calculate the density of free charge carriers via capacitance spectroscopy.

Electrical performance of PTB7-Th:PC71BM solar cell when in contact with the environment

The goal of this work is to make a contribution to the complex subject of organic solar cells degradation when operating in contact with ambient air. For this we built solar cells of the bulk heterojunction type composed of PTB7-Th (poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo [1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno [3,4-b]thiophene-) -2- carboxylate- 2–6-diyl)]) and PC71BM ([6,6]-Phenyl-C71-butyric acid methyl ester), respectively, as donor and acceptor materials of electrons. We present a series of current–voltage curves (J-V), both in the dark and under illumination, initially with the device immersed in an inert atmosphere and then exposing it to the ambient environment. J-V curves is then analyzed in terms of the Mott-Gurney equation, for measurements in dark, and by an analytical expression for the photocurrent, which allow us to check how the charge carrier mobility and the non-geminate recombination coefficient vary with time of exposure to air. We conclude that the charge carrier mobility (μ) for cells made with the PTB7-Th:PC71BM blend varies very little when in contact with air, and similar influence is observed on the recombination coefficient (k), since the Langevin coefficient reduction factor (ζ) is about 0.2 when the device is in inert atmosphere, reaching a value of almost 0.9 when exposed to air for 43 h.

Revealing the structural effects of non-fullerene acceptors on the performances of ternary organic photovoltaics under indoor light conditions

Abstract Strong absorption and tunable energy levels of non-fullerene small molecules offer enormous potential for organic photovoltaics (OPVs) to harvest energy from various light sources. However, the selection of suitable materials with an effective optical, electrical, and compatible morphological properties remains a big challenge. In this work, we demonstrated the effect of employing different non-fullerene acceptors (NFAs) as a crystalline modulator in efficient ternary-blend OPVs for indoor applications. Two different types of NFAs with different chemical structures and crystallinity, i.e. amorphous (PDI2 and PDI4) and semi-crystalline (IDT and IDDT), were introduced as the third component into a host OD:PC71BM binary-blend system. The optimized ternary-blends exhibited the power conversion efficiencies of 20.54% (OD:PC71BM:PDI2), 18.27% (OD:PC71BM:PDI4), 21.13% (OD:PC71BM:IDT), and 19.05% (OD:PC71BM:IDDT), respectively, under indoor light source (1000 lux, LED lamp) thereby completely outperforming the host binary-blend (14.15%). The variation of the OPV performance was investigated and correlated with the photophysical and morphological properties of the corresponding NFAs. Particularly, the addition of semi-crystalline IDT among other NFAs enabled the formation of compact nanoscale morphology with well-dispersion and smaller π-π stacking distance, thereby resulting in the efficient charge separation, the faster charge carrier transport, the reduced energetic disorder, and the suppression of non-geminate recombination. Overall, this study indicated that the addition of the third component with appropriate morphological properties can effectively improve the OPV performance under indoor light conditions.

Experimentally Calibrated Kinetic Monte Carlo Model Reproduces Organic Solar Cell Current–Voltage Curve

Kinetic Monte Carlo (KMC) simulations are a powerful tool to study the dynamics of charge carriers in organic photovoltaics. However, the key characteristic of any photovoltaic device, its current–voltage (J–V) curve under solar illumination, has proven challenging to simulate using KMC. The main challenges arise from the presence of injecting contacts and the importance of charge recombination when the internal electric field is low, i.e., close to open‐circuit conditions. Herein, an experimentally calibrated KMC model is presented that can fully predict the J–V curve of a disordered organic solar cell. It is shown that it is crucial to make experimentally justified assumptions on the injection barriers, the blend morphology, and the kinetics of the charge transfer state involved in geminate and nongeminate recombination. All of these properties are independently calibrated using charge extraction, electron microscopy, and transient absorption measurements, respectively. Clear evidence is provided that the conclusions drawn from microscopic and transient KMC modeling are indeed relevant for real operating organic solar cell devices.

Quantification of Photophysical Processes in All‐Polymer Bulk Heterojunction Solar Cells

All‐polymer solar cells lag behind the state‐of‐the‐art in small molecule nonfullerene acceptor (NFA) bulk heterojunction (BHJ) organic solar cells (OSCs) for reasons still unclear. Herein, the efficiency‐limiting processes in all‐polymer solar cells are investigated using blends of the common donor polymer PBDT‐TS1 with different acceptor polymers, namely P2TPD[2F]T and P2TPDBT[2F]T. Combining data from steady‐state optical spectroscopy and time‐resolved photoluminescence, transient absorption, and time‐delayed collection field experiments, provides not only a concise but also quantitative assessment of the losses due to limited photon absorption, geminate and nongeminate charge carrier recombination, field‐dependent charge generation, and inefficient carrier extraction. Although both systems exhibit a similar charge separation efficiency in the absence of external bias, charge separation is significantly enhanced in P2TPDBT[2F]T‐based blends when biased. Kinetic parameters obtained via pulsed laser spectroscopy are used to reproduce the experimentally measured device current–voltage (J–V) characteristics and indicate that low fill factors originate either from nongeminate recombination competing with charge extraction, or from a pronounced field dependence of charge generation, depending on the acceptor polymer. The methodology presented here is generic and can be used to quantify the loss processes in BHJ OSCs including both all‐polymer and small molecule NFA systems.

Buildup of Triplet-State Population in Operating TQ1:PC71BM Devices Does Not Limit Their Performance

Triplet generation in organic solar cells has been considered a major loss channel. Determining the density of the triplet-state population in an operating device is challenging. Here, we employ transient absorption (TA) spectroscopy on the quinoxaline-thiophene copolymer TQ1 blended with PC71BM, quantify the transient charge and triplet-state densities, and parametrize their generation and recombination dynamics. The charge recombination parameters reproduce the experimentally measured current-voltage characteristics in charge carrier drift-diffusion simulations, and they yield the steady-state charge densities. We demonstrate that triplets are formed by both geminate and nongeminate recombination of charge carriers and decay primarily by triplet-triplet annihilation. Using the charge densities in the rate equations describing triplet-state dynamics, we find that triplet-state densities in devices are in the range of charge carrier densities. Despite this substantial triplet-state buildup, TQ1:PC71BM devices exhibit only moderate geminate recombination and significantly reduced nongeminate charge recombination, with reduction factors between 10-4 and 10-3 compared to Langevin recombination.

Mediated Non-geminate Recombination in Ternary Organic Solar Cells Through a Liquid Crystal Guest Donor.

The approach via ternary blends prompts the increase of absorbed photon density and resultant photocurrent enhancement in organic solar cells (OSCs). In contrast to actively reported high efficiency ternary OSCs, little is known about charge recombination properties and carrier loss mechanisms in these emerging devices. Here, through introducing a small molecule donor BTR as a guest component to the PCE-10:PC71BM binary system, we show that photocarrier losses via recombination are mitigated with respect the binary OSCs, owing to a reduced bimolecular recombination. The gain of the fill factor in ternary devices are reconciled by the change in equilibrium between charge exaction and recombination in the presence of BTR toward the former process. With these modifications, the power conversion efficiency in ternary solar cells receives a boost from 8.8 (PCE-10:PC71BM) to 10.88%. We further found that the voltage losses in the ternary cell are slightly suppressed, related to the rising charge transfer-state energy. These benefits brought by the third guest donor are important for attaining improvements on key photophysical processes governing the photovoltaic efficiencies in organic ternary solar cells.

Anisotropic Geminate and Non-Geminate Recombination of Triplet Excitons in Singlet Fission of Single Crystalline Hexacene.

Singlet fission is believed to improve the efficiency of solar energy conversion by breaking up the Shockley-Queisser thermodynamic limit. Understanding of triplet excitons generated by singlet fission is essential for solar energy exploitation. Here we employed transient absorption microscopy to examine dynamical behaviors of triplet excitons. We observed anisotropic recombination of triplet excitons in hexacene single crystals. The triplet exciton relaxations from singlet fission proceed in both geminate and non-geminate recombination. For the geminate recombination, the different rates were attributed to the significant difference in their related energy change based on the Redfield quantum dissipation theory. The process is mainly governed by the electron-phonon interaction in hexacene. On the other hand, the non-geminate recombination is of bimolecular origin through energy transfer. In the triplet-triplet bimolecular process, the rates along the two different optical axes in the a-b crystalline plane differ by a factor of 4. This anisotropy in the triplet-triplet recombination rates was attributed to the interference in the coupling probability of dipole-dipole interactions in the different geometric configurations of hexacene single crystals. Our experimental findings provide new insight into future design of singlet fission materials with desirable triplet exciton exploitations.

Nongeminate charge recombination in organic photovoltaics

Suppression of nongeminate recombination due to long charge lifetime and low nonradiative loss is essential to highly efficient organic photovoltaics.

Morphology and carrier non-geminate recombination dynamics regulated by solvent additive in polymer/fullerene solar cells.

In this study, PBDTTT-E (based on benzo [1,2-b:4,5-b′] dithiophene (BDT) and thieno [3,4-b] thiophene (TT)) as a donor and fullerene derivative PC71BM (phenyl-C71-butyric acid methyl ester) as an acceptor with and without 1,8-diiodooctane (DIO)-treated copolymer solar cells were investigated. The device based on PBDTTT-E with treated DIO showed remarkably high current density (Jsc), fill factor (FF) and similar open-circuit voltage (Voc). Charge carrier lifetime (τn), density (n) and non-geminate recombination rate (krec) in the photoactive layers were measured by employing transient photovoltage (TPV) and charge extraction (CE) techniques. Based on krec and n, J–V curves were reconstructed. The DIO optimized the morphology of the active layer and its PBDTTT-E:PC71BM interfaces were increased. Therefore, compared to the device without the treated DIO, the device with the treated DIO showed larger electron mobility, longer carrier lifetime (τn) and lower non-geminate recombination rate (krec), which enhances the carrier transport and restrains the non-geminate recombination, realizing the higher Jsc and FF. In addition, that the DIO-treated devices can weaken the role of other factors (such as field dependent geminate recombination) in limiting device performance. The results provide some hints of improved device performance upon DIO as an additive in the D–A type polymer/fullerene solar cells.

Investigating ultrafast carrier dynamics in perovskite solar cells with an extended π-conjugated polymeric diketopyrrolopyrrole layer for hole transportation.

Here, we show a new diketopyrrole based polymeric hole-transport material (PBDTP-DTDPP, (poly[[2,5-bis(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl]-alt-[[2,2′-(4,8-bis(4-ethylhexyl-1-phenyl)-benzo[1,2-b:4,5-b′]dithiophene)bis-thieno[3,2-b]thiophen]-5,5′-diyl]])) for application in perovskite solar cells. The material performance was tested in a solar cell with an optimized configuration, FTO/SnO2/perovskite/PBDTP-DTDPP/Au, and the device showed a power conversion efficiency of 14.78%. The device charge carrier dynamics were investigated using transient absorption spectroscopy. The charge separation and recombination kinetics were determined in a device with PBDTP-DTDPP and the obtained results were compared to a reference device. We find that PBDTP-DTDPP enables similar charge separation time (<∼4.8 ps) to the spiro-OMeTAD but the amount of nongeminate recombination is different. Specifically, we find that the polymeric PBDTP-DTDPP hole-transport layer (HTL) slows-down the second-order recombination much less than spiro-OMeTAD. This effect is of particular importance in studying the charge transportation in optimized solar cell devices with diketopyrrole based HTL materials.

Effects of Annealing Temperature on the Performance of Organic Solar Cells Based on Polymer: Non-Fullerene Using V2O5 as HTL

The annealing temperature and thickness are two important factors to optimize the morphology of the active layer for a better performance of inverted polymer solar cells (iPSCs). Herein, the effects of the annealing temperature and the thickness of the active layer on the performances of iPSCs based on PBDB-T:IT-M are analyzed. Titanium oxide and vanadium oxide are used as electron and hole transporting layers, respectively. The device made with PBDB-T:IT-M layer (ca. 90 nm thick) thermally annealed at 100 °C exhibits the best performing under simulated AM 1.5G light. The idealities factor of iPSCs with two different annealing temperatures are determined by measuring the open-circuit voltage as a function of light intensity. The study shows that recombination losses in iPSCs annealed at 160 °C are governed by non-geminate recombination mechanisms, while in iPSCs annealed at 100 °C, the recombination losses are mainly due to band-tail trap states. Additional impedance spectroscopy measurements reveal that the device with an annealing temperature of 160 °C exhibits a higher charge-transfer in the bulk layer. However, the device thermally annealed at 100 °C shows lower charge-transfer resistance through all layers involved in the charge extraction. The results of this work show the importance of the annealing temperature on the charge-transfer at the active layer/vanadium oxide interface.

Design of narrow bandgap non-fullerene acceptors for photovoltaic applications and investigation of non-geminate recombination dynamics

A new narrow bandgap non-fullerene electron acceptor was designed, synthesized, and characterized for near-infrared organic photovoltaics.

Afterglow Effects as a Tool to Screen Emissive Nongeminate Charge Recombination Processes in Organic Photovoltaic Composites.

Disentangling temporally overlapping charge carrier recombination events in organic bulk heterojunctions by optical spectroscopy is challenging. Here, a new methodology for employing delayed luminescence spectroscopy is presented. The proposed method is capable of distinguishing between recombination of spatially separated charge carriers and trap-assisted charge recombination simply by monitoring the delayed luminescence (afterglow) of bulk heterojunctions with a quasi time-integrated detection scheme. Applied on the model composite of the donor poly(6,12-dihydro-6,6,12,12-tetraoctyl-indeno[1,2-b]fluorene-alt-benzothiadiazole) (PIF8BT) polymer and the acceptor ethyl-propyl perylene diimide (PDI) derivative, that is, PIF8BT:PDI, the luminescence of charge-transfer (CT) states created by nongeminate charge recombination on the ns to μs timescale is observed. Fluence-dependent, quasi time-integrated detection of the CT luminescence monitors exclusively emissive charge recombination events, while rejecting the contribution of other early-time emissive processes. Trap-assisted and bimolecular charge recombination channels are identified based on their distinct dependence on fluence. The importance of the two recombination channels is correlated with the layer's order and electrical properties of the corresponding devices. Four different microstructures of the PIF8BT:PDI composite obtained by thermal annealing are investigated. Thermal annealing of PIF8BT:PDI shrinks the PDI domains in parallel with the growth of the PIF8BT domains in the blend. Common to all states studied, the delayed CT luminescence signal is dominated by trap-assisted recombination. Yet, the minor fraction of fully separated charge recombination in the overall CT emission increases as the difference in the size of the donor and acceptor domains in the PIF8BT:PDI blend becomes larger. Electric field-induced quenching measurements on complete PIF8BT:PDI devices confirm quantitatively the dominance of emissive trap-limited charge recombination and demonstrates that only 40% of the PIF8BT/PDI CT luminescence comes from the recombination of fully-separated charges, taking place within 200 ns after photoexcitation. The method is applicable to other nonfullerene acceptor blends beyond the system discussed here, if their CT state luminescence can be monitored.

Synthesis, Structure and Photoluminescence Properties of 2D Organic–Inorganic Hybrid Perovskites

Two-dimensional (2D) layered hybrid organic–inorganic perovskites have potential applications in solar cells, electroluminescent devices and radiation detection because of their unique optoelectronic properties. In this paper, four 2D layered hybrid organic–inorganic halide perovskites of (C6H5CH2NH3)2PbCl4, (C6H5CH2NH3)2PbBr4, (C6H5CH2NH3)2PbI4 and (C4H9NH3)2PbBr4 were synthesized by solvent evaporation. Their crystal structure and surface morphology were studied. The effects of different halogens and organic amines on perovskites’ absorption spectra were investigated, and the photoluminescence (PL) properties were studied by femtosecond ultrafast spectroscopy. The experimental results show that the four perovskites are well crystallized and oriented. With the increase of halogen atom number (Cl, Br, I) in turn, the UV-Vis absorption spectra peaks of perovskites redshift due to the increasing of the layer spacing, but organic amines have little effect on the spectra of perovskites. The PL intensity increases with increasing laser power, but the lifetime decreases with increasing laser power, which is mainly due to the non-geminate recombination. This research is of great significance for realizing the spectral regulation of organic–inorganic hybrid perovskites and promoting their application in nano-photonics and optoelectronic devices.

End group tuning in small molecule donors for non-fullerene organic solar cells

Two novel small molecules, which consist of electron donating benzodithiophene core bridged through bithiophene π-spacer with terminal either dicyanovinyl (DCV-Me) or n-butyl cyanoester (CNAB) electron-withdrawing groups, were designed and used as donor materials for all-small molecule OSCs (all-SM-OSCs) with a non-fullerene acceptor (IDIC). The novel donor oligomers were firstly characterized by thermal gravimetric analysis, differential scanning calorimetry, UV–Vis spectroscopy, and cyclic voltammetry as well as studied by density functional theory calculations. The simple change of the DCV-Me to CNAB group leads to more pronounced crystallinity, higher solubility and higher energy levels in the donor BDT-2T-CNAB. The photovoltaic devices based on the BDT-2T-CNAB:IDIC blend exhibit higher short-circuit current (Jsc) and fill factor, and thus much higher power conversion efficiency (PCE) of 6.17% than those of BDT-2T-DCV-Me:IDIC devices (1.56%). Compared to the BDT-2T-DCV-Me system, the BDT-2T-CNAB based device shows smoother film surface morphology, and superior exciton dissociation, charge generation and charge carrier mobilities as well as lower non-geminate recombination losses. The results clearly demonstrate that the design of new small molecule donors for high-performance all-SM-OSCs should aim to choose suitable end acceptor units, among which the alkyl cyanoester groups are one of the most promising.

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.