Effects of Solvent Additives on Morphology,Charge Generation,Transport, and Recombination in Solution‐Processed Small‐Molecule Solar Cells

The effects of solvent additive (1,8‐diiodooctane (DIO)) on the morphology, charge generation, transport, and recombination in solution‐processed small‐molecule solar cells are studied and these parameters are correlated with device performance. In the optimum nanoscale morphology, which is processed with 0.4% DIO, the phase separation is large enough to create a percolating pathway for carrier transport, yet still small enough to form large interfacial area for efficient charge separation. Complete phase separation in this film reduces the interfacial defects, which occurs without DIO, and hence suppresses the monomolecular recombination. Moreover, balanced charge transport and weak bimolecular recombination lead to a high fill factor (72%). On the other hand, an excess amount of DIO (0.8%) in the solvent results in the over‐aggregation of the donor phase, which disturbs the percolating pathway of the acceptor phase and reduces the electron mobility. The over‐aggregation of the donor phase also shrinks the interfacial area for charge separation and consequently reduces the photocurrent generation.     

Charge Transport and Recombination in Low‐Bandgap Bulk Heterojunction Solar Cell using Bis‐adduct Fullerene

Charge transport and recombination are studied for organic solar cells fabricated using blends of polymer poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐(4,7‐bis(2‐thienyl)‐2,1,3‐benzothiadiazole)‐5,5′‐diyl] (Si‐PCPDTBT) with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (mono‐PCBM) and the bis‐adduct analogue of mono‐PCBM (bis‐PCBM). The photocurrent of Si‐PCPDTBT:bis‐PCBM devices shows a strong square root dependence on the effective applied voltage. From the relationship between the photocurrent and the light intensity, we found that the square‐root dependence of the photocurrent is governed by the mobility‐lifetime (μτ) product of charge carriers while space‐charge field effects are insignificant. The fill factor (FF) and short circuit current density (J_sc) of bis‐PCBM solar cells show a considerable increase with temperature as compared to mono‐PCBM solar cells. SCLC analysis of single carrier devices proofs that the mobility of both electrons and holes is significantly lowered when replacing mono‐PCBM with bis‐PCBM. The increased recombination in Si‐PCPDTBT:bis‐PCBM solar cells is therefore attributed to the low carrier mobilities, as the transient photovoltage measurements show that the carrier lifetime of devices are not significantly altered by using bis‐PCBM instead of mono‐PCBM.     

Effect of Charge Recombination on the Fill Factor of Small Molecule Bulk Heterojunction Solar Cells

Solution‐processed organic BHJ solar cells based on 3,6‐bis[5‐(benzofuran‐2‐yl)thiophen‐2‐yl]‐2,5‐bis(2‐ethylhexyl)pyrrolo[3,4‐c]pyrrole‐1,4‐dione (DPP(TBFu)₂) or poly(3‐hexylthiophene) blended with [6,6]‐phenyl‐C₆₀₍₇₀₎ ‐butyric acid methyl ester (PC₆₀₍₇₀₎ BM) behave differently under various irradiation intensities. Small molecule‐based DPP(TBFu)₂:PC₆₀ BM solar cells show up to 5.2% power conversion efficiency and a high fill factor at low light intensity. At 100 mW cm^?2 illumination, the efficiency and fill factor decrease, resulting in stronger power losses. Impedance spectroscopy at various light intensities reveals that high charge recombination is the cause of the low fill factor in DPP(TBFu)₂:PC₆₀ BM.     

Recombination Losses Above and Below the Transport Percolation Threshold in Bulk Heterojunction Organic Solar Cells

Achieving the highest power conversion efficiencies in bulk heterojunction organic solar cells requires a morphology that delivers electron and hole percolation pathways for optimized transport, plus sufficient donor:acceptor contact area for near unity charge transfer state formation. This is a significant structural challenge, particularly in semiconducting polymer:fullerene systems. This balancing act in the model high efficiency PTB7:PC70BM blend is studied by tuning the donor:acceptor ratio, with a view to understanding the recombination loss mechanisms above and below the fullerene transport percolation threshold. The internal quantum efficiency is found to be strongly correlated to the slower carrier mobility in agreement with other recent studies. Furthermore, second‐order recombination losses dominate the shape of the current density–voltage curve in efficient blend combinations, where the fullerene phase is percolated. However, below the charge transport percolation threshold, there is an electric‐field dependence of first‐order losses, which includes electric‐field‐dependent photogeneration. In the intermediate regime, the fill factor appears to be limited by both first‐ and second‐order losses. These findings provide additional basic understanding of the interplay between the bulk heterojunction morphology and the order of recombination in organic solar cells. They also shed light on the limitations of widely used transport models below the percolation threshold.     

Interplay between Bimolecular Recombination and Carrier Transport Distances in Bulk Heterojunction Organic Solar Cells

In this work, it is demonstrated that bimolecular recombination depends on the distance that free carriers are required to travel in transit to the electrodes in bulk heterojunction organic solar cells. By employing semi‐transparent devices, the carrier transport distance can be controlled via the local light absorption profile with an appropriate choice of the illumination side and incident wavelength. Using a series of light intensity‐dependent measurements, bimolecular recombination is shown to depend on the distance electrons or holes are required to transit the active layer. This effect is demonstrated for three different bulk heterojunction blends, where the restrictive carrier that causes the onset of recombination is identified. The mobility‐lifetime products of the limiting carriers are also estimated using a simple model for carrier extraction, where similar values are obtained regardless of the absorption profile. Implications for 1‐sun performance are also discussed, which provide guidelines for fabricating devices with thicker active layers capable of maximizing light absorption without succumbing to recombination losses.     

Efficient Solution‐Processed Bulk Heterojunction Perovskite Hybrid Solar Cells

Efficient conventional bulk heterojunction (BHJ) perovskite hybrid solar cells (pero‐HSCs) solution‐processed from a composite of CH₃NH₃PbI₃ mixed with PC₆₁BM ([6,6]‐phenyl‐C61‐butyric acid methyl ester), where CH₃NH₃PbI₃ acts as the electron donor and PC₆₁BM acts as the electron acceptor, are reported for the first time. The efficiency of 12.78% is twofold enhancement in comparison with the conventional planar heterojunction pero‐HSCs (6.90%) fabricated by pristine CH₃NH₃PbI₃. The BHJ pero‐HSCs are further optimized by using PC₆₁BM/TiO₂ bi‐electron‐extraction‐layer (EEL), which are both solution‐processed and then followed with low‐temperature thermal annealing. Due to higher electrical conductivity of PC₆₁BM over that of TiO₂, an efficiency of 14.98%, the highest reported efficiency for the pero‐HSCs without incorporating high‐temperature‐processed mesoporous TiO₂ and Al₂O₃ as the EEL and insulating scaffold, is observed from PC₆₁BM modified BHJ pero‐HSCs. Thus, the findings provide a simple way to approach high efficiency low‐cost pero‐HSCs.     

Time‐Dependent Morphology Evolution of Solution‐Processed Small Molecule Solar Cells during Solvent Vapor Annealing

Morphological modification using solvent vapor annealing (SVA) provides a simple and widely used fabrication option for improving the power conversion efficiencies of solution‐processed bulk heterojunction (BHJ) small molecule solar cells. Previous reports on SVA have shown that this strategy influences the degree of donor/acceptor phase separation and also improves molecular donor ordering. A blend composed of a dithienopyrrole containing oligothiophene as donor (named UU07) and [6,6]‐phenyl‐C61‐butyric acid methyl ester as acceptor is investigated with respect to SVA treatment to explore the dynamics of the BHJ evolution as a function of annealing time. A systematic study of the time dependence of morphology evolution clarifies the fundamental mechanisms behind SVA and builds the structure–property relation to the related device performance. The following two‐stage mechanism is identified: Initially, as SVA time increases, donor crystallinity is improved, along with enhanced domain purity resulting in improved charge transport properties and reduced recombination losses. However, further extending SVA time results in domains that are too large and a few large donor crystallites, depleting donor component in the mixed domain. Moreover, the larger domain microstructure suffers from enhanced recombination and overall lower bulk mobility. This not only reveals the importance of precisely controlling SVA time on gaining morphological control, but also provides a path toward rational optimization of device performance.     

Capacitance Spectroscopy for Quantifying Recombination Losses in Nonfullerene Small‐Molecule Bulk Heterojunction Solar Cells

The light intensity dependence of the main photoelectrical parameters of the nonfullerene small‐molecule bulk heterojunction (BHJ) solar cells p‐DTS(FBTTh₂)₂:perylene diimide (T1:PDI) shows that the nongeminate recombination losses play an important role in this system. A simple approach for the quantitative analysis of capacitance spectroscopy data of the organic BHJ solar cells, which allows to determine the density of free charge carriers as a function of applied bias under standard working conditions, is demonstrated. Using the proposed capacitance spectroscopic technique, the nongeminate recombination losses in the T1:PDI solar cells are quantitatively characterized in the scope of the bimolecular‐ and trap‐assisted recombination mechanisms. Their contributions are separately analyzed within a wide range of the applied bias.     

Nongeminate Recombination in Planar and Bulk Heterojunction Organic Solar Cells

Nongeminate recombination in organic solar cells based on copper phthalocyanine (CuPc) and C₆₀ is investigated. Two device architectures, the planar heterojunction (PHJ) and the bulk heterojunction (BHJ), are directly compared in view of differences in charge carrier decay dynamics. A combination of transient photovoltage (TPV) experiments, yielding the small perturbation charge carrier lifetime, and charge extraction measurements, providing the charge carrier density is applied. In organic solar cells, charge photogeneration and recombination primarily occur at the donor–acceptor heterointerface. Whereas the BHJ can often be approximated by an effective medium due to rather small scale phase separation, the PHJ has a well defined two‐dimensional heterointerface. In order to study nongeminate recombination dynamics in PHJ devices the charge accumulation at this interface is most relavent. As only the spatially averaged carrier concentration can be determined from extraction techniques, the charge carrier density at the interface n_int is derived from the open circuit voltage. Comparing the experimental results with macroscopic device simulation, the differences of recombination and charge carrier densities in CuPc:C₆₀ PHJ and BHJ devices are discussed with respect to the device performance. The open circuit voltage of BHJ is larger than for PHJ at low light intensities, but at 0.3 sun the situation is reversed: here, the PHJ can finally take advantage of its generally longer charge carrier lifetimes, as the active recombination region is smaller.     

The Physics of Small Molecule Acceptors for Efficient and Stable Bulk Heterojunction Solar Cells

Organic bulk heterojunction solar cells based on small molecule acceptors have recently seen a rapid rise in the power conversion efficiency with values exceeding 13%. This impressive achievement has been obtained by simultaneous reduction of voltage and charge recombination losses within this class of materials as compared to fullerene‐based solar cells. In this contribution, the authors review the current understanding of the relevant photophysical processes in highly efficient nonfullerene acceptor (NFA) small molecules. Charge generation, recombination, and charge transport is discussed in comparison to fullerene‐based composites. Finally, the authors review the superior light and thermal stability of nonfullerene small molecule acceptor based solar cells, and highlight the importance of NFA‐based composites that enable devices without early performance loss, thus resembling so‐called burn‐in free devices.     

Improving Interfacial Charge Recombination in Planar Heterojunction Perovskite Photovoltaics with Small Molecule as Electron Transport Layer

Although perovskite solar cells (PSCs) have emerged as a promising alternative to widely used fossil fuels, the involved high‐temperature preparation of metal oxides as a charge transport layer in most state‐of‐the‐art PSCs has been becoming a big stumbling block for future low‐temperature and large‐scale R2R manufacturing process. Such an issue strongly encourages scientists to find new type of materials to replace metal oxides. Except for expensive PC₆₁BM with unmanageable morphology and electrical properties, the past investigation on the development of low‐temperature‐processed and highly efficient electron transport layers (ETLs) has met some mixed success. In order to further enhance the performance of all‐solution‐processed PSCs, we propose a novel n‐type sulfur‐containing small molecule hexaazatrinaphtho[2,3‐c][1,2,5]thiadiazole (HATNT) with high electron mobility up to 1.73 × 10^?2 cm² V^?1 s^?1 as an ETL in planar heterojunction PSCs. A high power conversion efficiency of 18.1% is achieved, which is fully comparable with the efficiency from the control device fabricated with PC₆₁BM as ETL. This superior performance mainly attributes from more effective suppression of charge recombination at the perovskite/HATNT interface than that between the perovskite and PC₆₁ BM. Moreover, high electron mobility and strong interfacial interaction via S? I or S? Pb bonding should be also positive factors. Significantly, our results undoubtedly enable new guidelines in exploring n‐type organic small molecules for high‐performance PSCs.     

Doping‐Induced Screening of the Built‐in‐Field in Organic Solar Cells: Effect on Charge Transport and Recombination

We report on the effects of screening of the electric field by doping‐induced mobile charges on photocurrent collection in operational organic solar cells. Charge transport and recombination were studied using double injection (DI) and charge extraction by linearly increasing voltage (CELIV) transient techniques in bulk‐heterojunction solar cells made from acceptor‐donor blends of poly(3‐n‐hexylthiophene):phenyl‐C61‐butyric acid methyl ester (P3HT:PC60BM). It is shown that the screening of the built‐in field in operational solar cells can be controlled by an external voltage while the influence on charge transport and recombination is measured. An analytical theory to extract the bimolecular recombination coefficient as a function of electric field from the injection current is also reported. The results demonstrate that the suppressed (non‐Langevin) bimolecular recombination rate and charge collection are not strongly affected by native doping levels in this materials combination. Hence, it is not necessary to reduce the level of doping further to improve the device performance of P3HT‐based solar cells.     

Quantifying the Nongeminate Recombination Dynamics in Nonfullerene Bulk Heterojunction Organic Solar Cells

In this study, a comprehensive analytical model to quantify the total nongeminate recombination losses, originating from bimolecular as well as bulk and surface trap‐assisted recombination mechanisms in nonfullerene‐based bulk heterojunction organic solar cells is developed. This proposed model is successfully employed to obtain the different contributions to the recombination current of the investigated solar cells under different illumination intensities. Additionally, the model quantitatively describes the experimentally measured open‐circuit voltage versus light intensity dependence. Most importantly, it is possible to calculate the experimental results with the same fitting parameter values from the presented model. The validity of this model is also proven by a combination of other independent, steady‐state, and transient experimental techniques. This new powerful analytical tool will enable researchers in the photovoltaic community to take into account the synergetic contribution from all relevant types of nongeminate recombination losses in different optoelectronic systems and target their analysis of recombination dynamics at any operating voltage.     

Understanding the Role of Thermal Processing in High Performance Solution Processed Small Molecule Bulk Heterojunction Solar Cells

Two similar structural versions of a molecular donor, in which two terminal hexyl‐substituted bithiophene units are connected to a central dithienosilole (DTS) through electron deficient thiadiazolopyridine (PT) units, and which differ only in the position of pyridyl N‐atoms, were explored to study the interplay of crystallization and vertical phase segregation as a result of annealing. The donor materials exhibit greatly contrasting photovoltaic performance despite similarity in molecule structure. The difference in position of the pyridal N‐atom which points away (distal configuration; compound 1) or towards (proximal configuration; compound 2) from the DTS core, modifies the aggregation/molecular packing in the solid state, resulting in differences in the phase segregation and formation of crystalline domains. A systematic study of the temperature dependence of photovoltaic performance reveals a parameter trade‐off: as annealing temperature increases, higher donor crystallinity and ordering results, but increased donor segregation near the surface or decrease in electrode selectivity also occurs, resulting in increased interfacial recombination and hence reduction in open‐circuit voltage (V_oc). The higher crystalline nature of 2 is found to have a higher impact on cell performance despite a competing undesired effect at the donor/aluminum cathode interface, contributing to its superior performance to 1 when blended with [6,6]phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM). Molecule 2 exhibits a performance increase of a factor of two after thermal annealing at 100 °C, achieving a power conversion efficiency of 5.7%.     

Oligothiophene Additive-Assisted Morphology Control and Recombination Suppression Enable High-Performance Organic Solar Cells

Tuning the morphology through processing additives represents one of the most promising strategies to boost the performance of organic solar cells (OSCs). However, it remains unclear how oligothiophene-based solid additives influence the molecular packing and performance of OSCs. Here, two additives namely 2T and 4T, are introduced into state-of-the-art PM6:Y6-based OSCs to understand how they influence the film formation process, nanoscale morphology, and the photovoltaic performance. It is found that the 2T additive can improve the molecular packing of both donor polymer and non-fullerene acceptor, resulting in lower Urbach energy and reduced energy loss. Furthermore, the blend film with 2T treatment displays enhanced domain purity and a more favorable distribution of the acceptor and donor materials in the vertical direction, which can enhance charge extraction efficiency while simultaneously suppressing charge recombination. Consequently, OSCs processed with 2T additive realize a promising efficiency of 18.1% for PM6:Y6-based devices. Furthermore, the general applicability of the additive is demonstrated, and an impressive efficiency of 18.6% for PM6:L8-BO-based OSCs is achieved. These findings highlight that the uncomplicated oligothiophenes have excellent potential in fine-adjustment of the active layer morphology, which is crucial for the future development of OSCs.