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.     

From Recombination Dynamics to Device Performance: Quantifying the Efficiency of Exciton Dissociation,Charge Separation,and Extraction in Bulk Heterojunction Solar Cells with Fluorine‐Substituted Polymer Donors

An original set of experimental and modeling tools is used to quantify the yield of each of the physical processes leading to photocurrent generation in organic bulk heterojunction solar cells, enabling evaluation of materials and processing condition beyond the trivial comparison of device performances. Transient absorption spectroscopy, “the” technique to monitor all intermediate states over the entire relevant timescale, is combined with time‐delayed collection field experiments, transfer matrix simulations, spectral deconvolution, and parametrization of the charge carrier recombination by a two‐pool model, allowing quantification of densities of excitons and charges and extrapolation of their kinetics to device‐relevant conditions. Photon absorption, charge transfer, charge separation, and charge extraction are all quantified for two recently developed wide‐bandgap donor polymers: poly(4,8‐bis((2‐ethylhexyl)oxy)benzo[1,2‐b:4,5‐b′]dithiophene‐3,4‐difluorothiophene) (PBDT[2F]T) and its nonfluorinated counterpart poly(4,8‐bis((2‐ethylhexyl)oxy)benzo[1,2‐b:4,5‐b′]dithiophene‐3,4‐thiophene) (PBDT[2H]T) combined with PC₇₁BM in bulk heterojunctions. The product of these yields is shown to agree well with the devices' external quantum efficiency. This methodology elucidates in the specific case studied here the origin of improved photocurrents obtained when using PBDT[2F]T instead of PBDT[2H]T as well as upon using solvent additives. Furthermore, a higher charge transfer (CT)‐state energy is shown to lead to significantly lower energy losses (resulting in higher V_OC) during charge generation compared to P3HT:PCBM.     

Reduced Recombination in High Efficiency Molecular Nematic Liquid Crystalline: Fullerene Solar Cells

Bimolecular recombination in bulk heterojunction organic solar cells is the process by which nongeminate photogenerated free carriers encounter each other, and combine to form a charge transfer (CT) state which subsequently relaxes to the ground state. It is governed by the diffusion of the slower and faster carriers toward the electron donor–acceptor interface. In an increasing number of systems, the recombination rate constant is measured to be lower than that predicted by Langevin's model for relative Brownian motion and the capture of opposite charges. This study investigates the dynamics of charge generation, transport, and recombination in a nematic liquid crystalline donor:fullerene acceptor system that gives solar cells with initial power conversion efficiencies of >9.5%. Unusually, and advantageously from a manufacturing perspective, these efficiencies are maintained in junctions thicker than 300 nm. Despite finding imbalanced and moderate carrier mobilities in this blend, strongly suppressed bimolecular recombination is observed, which is ≈150 times less than predicted by Langevin theory, or indeed, more recent and advanced models that take into account the domain size and the spatial separation of electrons and holes. The suppressed bimolecular recombination arises from the fact that ground‐state decay of the CT state is significantly slower than dissociation.     

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.     

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.     

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%.     

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.     

Interplay Between Side Chain Pattern,Polymer Aggregation,and Charge Carrier Dynamics in PBDTTPD:PCBM Bulk‐Heterojunction Solar Cells

Poly(benzo[1,2‐b:4,5‐b′]dithiophene–alt–thieno[3,4‐c]pyrrole‐4,6‐dione) (PBDTTPD) polymer donors with linear side‐chains yield bulk‐heterojunction (BHJ) solar cell power conversion efficiencies (PCEs) of about 4% with phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) as the acceptor, while a PBDTTPD polymer with a combination of branched and linear substituents yields a doubling of the PCE to 8%. Using transient optical spectroscopy it is shown that while the exciton dissociation and ultrafast charge generation steps are not strongly affected by the side chain modifications, the polymer with branched side chains exhibits a decreased rate of nongeminate recombination and a lower fraction of sub‐nanosecond geminate recombination. In turn the yield of long‐lived charge carriers increases, resulting in a 33% increase in short circuit current (J _sc). In parallel, the two polymers show distinct grazing incidence X‐ray scattering spectra indicative of the presence of stacks with different orientation patterns in optimized thin‐film BHJ devices. Independent of the packing pattern the spectroscopic data also reveals the existence of polymer aggregates in the pristine polymer films as well as in both blends which trap excitons and hinder their dissociation.     

Polymer Blend Solar Cells Based on a High‐Mobility Naphthalenediimide‐Based Polymer Acceptor: Device Physics,Photophysics and Morphology

A high electron mobility polymer, poly{[N,N’‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5’‐(2,2’‐bithiophene) (P(NDI2OD‐T2)) is investigated for use as an electron acceptor in all‐polymer blends. Despite the high bulk electron mobility, near‐infrared absorption band and compatible energy levels, bulk heterojunction devices fabricated with poly(3‐hexylthiophene) (P3HT) as the electron donor exhibit power conversion efficiencies of only 0.2%. In order to understand this disappointing photovoltaic performance, systematic investigations of the photophysics, device physics and morphology of this system are performed. Ultra‐fast transient absorption spectroscopy reveals a two‐stage decay process with an initial rapid loss of photoinduced polarons, followed by a second slower decay. This second slower decay is similar to what is observed for efficient P3HT:PCBM ([6,6]‐phenyl C₆₁‐butyric acid methyl ester) blends, however the initial fast decay that is absent in P3HT:PCBM blends suggests rapid, geminate recombination of charge pairs shortly after charge transfer. X‐ray microscopy reveals coarse phase separation of P3HT:P(NDI2OD‐T2) blends with domains of size 0.2 to 1 micrometer. P3HT photoluminescence, however, is still found to be efficiently quenched indicating intermixing within these mesoscale domains. This hierarchy of phase separation is consistent with the transient absorption, whereby localized confinement of charges on isolated chains in the matrix of the other polymer hinders the separation of interfacial electron‐hole pairs. These results indicate that local, interfacial processes are the key factor determining the overall efficiency of this system and highlight the need for improved morphological control in order for the potential benefit of high‐mobility electron accepting polymers to be realized.     

Guiding the Selection of Processing Additives for Increasing the Efficiency of Bulk Heterojunction Polymeric Solar Cells

In bulk heterojunction (BHJ) polymeric organic solar cells (OSCs), the use of processing additives in the material formulation has emerged as a promising, cost‐effective, and widely applicable method for optimizing the phase separation between the donor (D) and acceptor (A) materials, thus increasing their efficiency. So far, however, there has been no systematic approach for identifying suitable processing additives for a given D:A system. A method based on the Hansen solubility parameters (HSPs) is proposed for guiding the selection of processing additives for a given D:A combination. The method is applied to the archetypical poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) system. The HSPs of these materials are determined and used to define a set of numerical criteria that need to be satisfied by a processing additive in order for it to be effective in realizing a higher efficiency OSC. Applying the selection criteria results in the identification of three novel processing additives. OSCs made of these formulations demonstrate an increase in their short‐circuit current density (J_SC) and power conversion efficiency (PCE). These results demonstrate the efficiency of these novel processing additives and show that the HSPs represent a useful tool to determine and explore new types of processing additives for BHJ‐OSCs.     

Shelf Life Degradation of Bulk Heterojunction Solar Cells: Intrinsic Evolution of Charge Transfer Complex

Achievement of long‐term stability of organic photovoltaics is currently one of the major topics for this technology to reach maturity. Most of the techniques used to reveal degradation pathways are destructive and/or do not allow for real‐time measurements in operating devices. Here, three different, nondestructive techniques able to provide real‐time information, namely, film absorbance, capacitance–voltage (C–V), and impedance spectroscopy (IS), are combined over a period of 1 year using non‐accelerated intrinsic degradation conditions. It is discerned between chemical modifications in the active layer, physical processes taking place in the bulk of the blend from those at the active layer/contact interfaces. In particular, it is observed that during the ageing experiment, the main source for device performance degradation is the formation of donor–acceptor charge‐transfer complex (–) that acts as an exciton quencher. Generation of these radical species diminishes photocurrent and reduces open‐circuit voltage by the creation of electronic defect states. Conclusions extracted from absorption, C–V, and IS measurements will be further supported by a range of other techniques such as atomic force microscopy, X‐ray diffraction, and dark‐field imaging of scanning transmission electron microscopy on ultrathin cross‐sections.