Fullerene Bulk Heterojunction Research Articles

Overcoming the voltage losses caused by the acceptor‐based interlayer in laminated indoor OPVs

AbstractHarvesting indoor light to power electronic devices for the Internet of Things has become an application scenario for emerging photovoltaics, especially utilizing organic photovoltaics (OPVs). Combined liquid‐ and solid‐state processing, such as printing and lamination used in industry for developing indoor OPVs, also provides a new opportunity to investigate the device structure, which is otherwise hardly possible based on the conventional approach due to solvent orthogonality. This study investigates the impact of fullerene‐based acceptor interlayer on the performance of conjugated polymer–fullerene‐based laminated OPVs for indoor applications. We observe open‐circuit voltage (VOC) loss across the interface despite this arrangement being presumed to be ideal for optimal device performance. Incorporating insulating organic components such as polyethyleneimine (PEI) or polystyrene (PS) into fullerene interlayers decreases the work function of the cathode, leading to better energy level alignment with the active layer (AL) and reducing the VOC loss across the interface. Neutron reflectivity studies further uncover two different mechanisms behind the VOC increase upon the incorporation of these insulating organic components. The self‐organized PEI layer could hinder the transfer of holes from the AL to the acceptor interlayer, while the gradient distribution of the PS‐incorporated fullerene interlayer eliminates the thermalization losses. This work highlights the importance of structural dynamics near the extraction interfaces in OPVs and provides experimental demonstrations of interface investigation between solution‐processed cathodic fullerene layer and bulk heterojunction AL.

Doping with Niobium Nanoparticles as an Approach to Increase the Power Conversion Efficiency of P3HT:PCBM Polymer Solar Cells.

Metal additive processing in polymer: fullerene bulk heterojunction systems is recognized as a viable way for improving polymer photovoltage performance. In this study, the effect of niobium (Nb) metal nanoparticles at concentrations of 2, 4, 6, and 8 mg/mL on poly(3-hexylthiophene-2,5-diyl) (P3HT)-6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends was analyzed. The effect of Nb volume concentration on polymer crystallinity, optical properties, and surface structure of P3HT and PCBM, as well as the enhancement of the performance of P3HT:PC61BM solar cells, are investigated. Absorption of the P3HT:PC61BM mix is seen to have a high intensity and a red shift at 500 nm. The reduction in PL intensity with increasing Nb doping concentrations indicates an increase in PL quenching, suggesting that the domain size of P3HT or conjugation length increases. With a high Nb concentration, crystallinity, material composition, surface roughness, and phase separation are enhanced. Nb enhances PCBM's solubility in P3HT and decreases the size of amorphous P3HT domains. Based on the J-V characteristics and the optoelectronic study of the thin films, the improvement results from a decreased recombination current, changes in morphology and crystallinity, and an increase in the effective exciton lifespan. At high doping concentrations of Nb nanoparticles, the development of the short-circuit current (JSC) is associated with alterations in the crystalline structure of P3HT. The highest-performing glass/ITO/PEDOT:PSS/P3HT:PCBM:Nb/MoO3/Au structures have short-circuit current densities (JSC) of 16.86 mA/cm2, open-circuit voltages (VOC) of 466 mV, fill factors (FF) of 65.73%, and power conversion efficiency (µ) of 5.16%.

Photoinduced spin-orbital coupling effect at donor: acceptor interface in non-fullerene organic solar cells

Non-fullerene based organic solar cells show excellent optoelectronic properties. In this study, firstly, we measured the magneto-photocurrent (MPC) effect to monitor the spin orbit coupling (SOC) action of electron-hole pairs at the donor: acceptor (D:A) interfaces, introduced by the dipole-dipole interaction in non-fullerene ITIC molecules. The MPC of non-fullerene based bulk heterojunctions show a broader line-shape, which implies a stronger SOC in electron-hole pairs at D:A interface as compared to the fullerene based bulk heterojunctions under photo-excitation. Secondly, the light-induced electron paramagnetic resonance (LEPR) results further give a direct evidence for stronger SOC in electron-hole pairs at D:A interface in non-fullerene bulk-heterojunction. Theoretically, increasing SOC provides a pathway for singlet electron-hole pairs to convert into triplet electron-hole pairs, leading to more spin states ready for dissociation. Therefore, the increased SOC will increase the magneto-photocurrent, and improve the performance of non-fullerene solar cells, we consider that photoexcitation can present a new approach to strengthen the SOC effect in organic solar cells toward better photovoltaic actions. Non-fullerene based organic solar cells show excellent optoelectronic properties and device performance. We think these excellent properties come from a stronger spin orbit coupling effect in electron-hole pairs at D: A interface under photoexcitation as compared to the fullerene bulk-heterojunctions, leading to increase the magneto-photocurrent. In addition, the capacitance-frequency is also reveal that the stronger SOC for non-fullerene bulk-heterojunctions originates from the larger electron cloud deformation electron-hole pairs caused by photoinduced dipole-dipole interactions. Therefore, we think the photoexcitation present a new approach to strengthen the SOC effect in organic solar cells toward better photovoltaic actions. • Non-fullerene based solar cell of ITO/ZnO/Active layer/MoO 3 /Ag shows a higher PCE of 11.30%. • A strong SOC exists in electron-hole pairs at D:A interface in the non-fullerene under lighting. • The strong SOC comes from the larger electron cloud deformation. • Photo-excitation can present a new approach to strengthen the SOC effect in organic solar cells.

Power losses in conventional and inverted non-polymeric donor:fullerene bulk heterojunction solar cells - The role of vertical phase separation in BQR:PC71BM blends

The performance of bulk heterojunction (BHJ) organic solar cells can be affected by a range of factors including the materials combination, processing solvent, post deposition annealing, and/or whether they are used in a conventional or inverted architecture. In this study we compared conventional and inverted BHJ solar cells composed of a non-polymeric donor (5 Z ,5′ Z )‐5,5'‐[(5‴,5‴''''‐{4,8‐bis[5‐(2‐ethylhexyl)‐4‐ n -hexylthiophen‐2‐yl]benzo[1,2‐b:4,5‐b']dithiophene‐2,6‐diyl}bis{3′,3'',3‴‐trihexyl‐[2,2':5′,2'':5'',2‴‐quaterthiophene]‐5‴,5‐diyl})bis(methanylylidene)]bis[3‐ n -hexyl‐2‐thioxothiazolidin‐4‐one] (BQR) and [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) as the acceptor. It was found that the conventional device structure had power conversion efficiencies nearly two and a half times that of the inverted device, 9.0% versus 4.0%. Through a combination of Shockley equivalent circuit fitting, optical modelling, and light intensity dependent photocurrent measurements we identified that the origin of the power losses for the inverted architecture relative to the conventional device structure arose from a larger component of bimolecular recombination. Neutron reflectometry measurements showed that the origin of the larger bimolecular recombination losses for the inverted device was due to the PC 71 BM phase separating, with a PC 71 BM rich layer located near the anode reducing the hole extraction efficiency. • BQR and PC 71 BM conventional and inverted BHJ solar cells are compared. • The conventional device structure was two and a half times more efficient. • Increased bimolecular recombination identified as the origin of the power losses for the inverted architecture. • A PC 71 BM rich layer near the anode reduced the hole extraction efficiency in inverted device.

P3HT:PCBM polymer solar cells from a didactic perspective

Here, we studied the influence of pre- and post-thermal annealing on the performance of polymer:fullerene bulk heterojunction solar cells using the conventional architecture, comprising a conjugated polymer, namely, poly(3-hexylthiophene-2,5-diyl) (P3HT) and a fullerene derivative [6,6]-phenyl-C60-butyric acid methyl ester (PC60BM) as a photoactive layer. The non-annealed active layer device exhibited a power conversion efficiency of <1 % , which was significantly lower than the pre- and post-annealed devices. To investigate the impact of pre- and post thermal annealing on the natural morphological state of the polymer, regiorandom (P3HT-I) and regioregular (P3HT-II) type P3HT were compared in photoactive layers. In general, P3HT-I is amorphous, whereas P3HT-II is semi-crystalline. Changes in solar cell performance were associated with changes in carrier extraction efficiencies influenced by the annealing conditions. The charge photogeneration processes were investigated using spectroscopic techniques, including electroluminescence, steady-state, and time-resolved photoluminescence spectroscopy. Finally, to explore the morphological changes upon annealing, atomic force microscopy and electroluminescence imaging measurements were performed on films and solar cells, respectively.

Intrachain Delocalization Effect of Charge Carriers on the Charge-Transfer State Dynamics in Organic Solar Cells

We studied the charge-generation mechanism in low-bandgap polymer (P4TNTz-2F)-fullerene bulk heterojunction (BHJ) organic solar cells (OSCs) using transient absorption (TA) spectroscopy. The highly crystalline nanowire structure of P4TNTz-2F in a blend film prepared with chlorobenzene (CB) and 1,8-diiodooctane (DIO) induced more long-lived charge carriers than those in a blend film prepared with CB only. Pump-wavelength-dependent TA data revealed that the increased charge-delocalization by the intrachain ordering of P4TNTz-2F in the blend film prepared with CB/DIO is the key factor to increasing the OSC efficiency. The intrachain charge-delocalization increased the charge-transfer (CT) state lifetime and suppressed geminate recombination losses, resulting in the efficient dissociation of CT states into free carriers. Our findings provide new insights into the excited-state dynamics study of BHJ blends, which can serve as a good guide for the development of novel OSC materials.

Highly Selective Cu Staining of Sulfur-Containing Polymers Facilitates 3D Nanomorphology Reconstruction of Polymer:Fullerene Blends in Organic Solar Cells by FIB-SEM Tomography.

The distinction of different organic materials in phase mixtures is hampered in electron microscopy because electron scattering does not strongly differ in carbon-based materials that mainly consist of light elements. A successful strategy for contrast enhancement is selective staining where one phase of a material mixture is labeled by heavier elements, but suitable staining agents are not available for all organic materials. This is also the case for bulk-heterojunction (BHJ) absorber layers of organic solar cells, which consist of interpenetrating networks of donor and acceptor domains. The domain structure strongly influences the power conversion efficiency, and nanomorphology optimization often requires real-space information on the sizes and interconnectivity of domains with nanometer resolution. In this work, we have developed an efficient approach to selectively stain sulfur-containing polymers by homogeneous Cu infiltration, which generates strong material contrast in scanning (transmission) electron microscopy (S(T)EM) images of polymer:fullerene BHJ layers. Cross-section lamellae of BHJ layers are prepared for STEM by focused-ion-beam milling and are attached to a Cu lift-out grid as a copper source. After thermal treatment at 200 °C for 3 h in air, sulfur-containing polymers are homogeneously infiltrated by Cu, while the fullerenes are not affected. Selective Cu staining is applied to map the phase distribution in PTB7:PC71BM BHJ layers fabricated with different processing additives to tailor the nanomorphology. The strong contrast between polymer and fullerene domains is the prerequisite for the three-dimensional reconstruction of the domain structure by focused-ion-beam/scanning-electron-microscopy tomography.

Molecular Insights into the Mechanical Properties of Polymer–Fullerene Bulk Heterojunctions for Organic Photovoltaic Applications

We investigate the mechanical properties of $\pi$-conjugated polymeric materials composed of regioregular poly(3-hexylthiophene) (P3HT) and fullerene C$_{60}$ using coarse-grained molecular dynamics simulations. Specifically, we perform tensile simulations of P3HT:C$_{60}$ composites with varied degrees of polymerization and C$_{60}$ mass fractions to obtain their stress-strain responses. Decomposition of stress tensor into kinetic energy and virial contributions indicates that the tensile moduli of the pure P3HT samples are greatly dependent on non-bonded interactions and on bonded interactions associated with bond-stretching, while the addition of C$_{60}$ leads to an increase in the tensile modulus originating from enhanced non-bonded interactions associated with C$_{60}$. Additionally, the tensile strength of the P3HT:C$_{60}$ samples correlates well with molecular chain entanglements, which are characterized by the average number of kinks per chain obtained from primitive path analysis. We also find that the upper and lower yield points characterizing strain softening become more pronounced with increasing C$_{60}$ mass fraction. Persistent homology analysis indicates that the emergence of the yield points correlates well with the coalescence of microvoids in the course of tensile deformation, resulting in the generation of larger voids. These results provide a fundamental understanding of the molecular determinants of the mechanical properties of $\pi$-conjugated polymer-fullerene composites, which can also help to interpret and predict the mechanical properties of other polymer composites containing fullerene.

Molecular Dynamics Investigation of the Mechanical Properties of Polymer–Fullerene Bulk Heterojunctions

Molecular Dynamics Investigation of the Mechanical Properties of Polymer–Fullerene Bulk Heterojunctions

Multifunctional Diode Operation of Tetracene Sensitized Polymer:Fullerene Heterojunctions with Simultaneous Electroluminescence in Visible and NIR Bands

AbstractMultifunctional operation of tetracene/polycyclopentadithiophene‐benzothiadiazole (PCPDTBT), and tetracene sensitized PCPDTBT:fullerene bulk heterojunctions (BHJs) that work as an organic photovoltaic (OPV), organic light emitting diode (OLED), and organic photodetector (OPD) is unveiled. The multifunctional operation is underlined by a synergetic interplay between singlet and triplet energy transfer and the judicious alignment of energy levels of donor–acceptor cascade. Introduction of tetracene is shown to enhance electroluminescence (EL) at 800 nm despite the observation of a near‐complete photoluminescence quenching in BHJ systems. Signatures of triple energy transfer mediated via charge transfer states of BHJ to tetracene become apparent as the proportion of fullerene in the BHJ is increased. An optimal balance between exciton population is achieved with BHJ 1:2 that produces EL peaks at 530 and 800 nm while maintaining a respectable OPV and OPD performance. The proposed ternary system unlocks the potential of having multifunctionality within one single diode structure, and it also offers new design rules for multispectral OLEDs.

Effect of fullerene substituent on low-light characteristics of polymer: fullerene bulk heterojunction solar cells

A halogen-free solvent 1,2,4-trimethylbenzene dissolves both unmodified fullerenes and conjugated polymers such as PTB7-Th. This unique feature enables the study on the effect of the substituent of fullerene in polymer:fullerene bulk heterojunction solar cells under low-light illumination. Although the power conversion efficiency (PCE) of the PTB7-Th:C70 solar cell was higher than that of the PTB7-Th:C70-PCBM solar cell under one sun illumination, the latter showed higher PCE than the former under 10−3 sun illumination. This result comes from the fact that C70-PCBM yields higher open-circuit voltage (VOC) at one sun and smaller dependence of VOC on illumination light intensity than C70.

Disentanglement of Degradation Mechanisms by Analyzing Aging Dynamics of Environmentally Friendly Processed Polymer Solar Cells

Lifecycle assessments suggest preventing halogenated solvents or solvent additives for environmentally friendly polymer solar cells. Thus, the active layers of polymer:fullerene bulk heterojunction solar cells based on poly{[4,8‐bis‐(2‐ethyl‐hexyl‐thiophene‐5‐yl)‐benzo[1,2‐b:4,5‐b0]dithiophene‐2,6‐diyl]‐alt‐[2‐(20‐ethyl‐hexanoyl)‐thieno[3,4‐b]thiophen‐4,6‐diyl]} (PBDTTT‐CT) and the fullerene derivative [6,6]‐phenyl‐C70‐butyric acid methyl ester (PC70BM) are cast from m‐xylene solutions. Ortho‐vanilline is used as a nonhazardous and nontoxic solvent additive. Completed photovoltaic devices are subjected to accelerated laboratory weathering tests. Photovoltaic parameters are periodically obtained from current–voltage recordings of the solar cells twice an hour under well‐defined aging conditions following the International Summit on Organic Photovoltaic Stability (ISOS) protocols. An analysis of aging kinetics reveals the superposition of two individual degradation mechanisms, of which one is assigned to continued intermixing and the other one to the formation of a blocking layer by interfacial segregation.

Towards high performance visible-blind narrowband near-infrared photodetectors with integrated perovskite light filter

Narrowband near-infrared (NIR) light detection is essential for optical imaging, machine vision, and spectroscopy. In this work, we demonstrate a strategy towards visible-blind narrowband NIR organic photodetectors by integrating perovskite thin film as the light filter. Lead phthalocyanine (PbPc):Fullerene (C60) bulk heterojunction which demonstrates high efficiency in photoexciton dissociation is used as the photosensitive layer due to excellent absorption of PbPc in NIR light region. Perovskite layer which has a strong absorption in the visible region but a weak absorption in the NIR region serves as a light filtering layer. More importantly, we have further improved the device performance by inserting copper phthalocyanine (CuPc) electron blocking layer between the ITO anode and PbPc:C60 bulk heterojunction. A high photoresponsivity (R) of 1.09 A/W, an external quantum efficiency (EQE) of 167%, and a narrow spectral response with the full width of half maximum of ~60 nm for NIR light of 808 nm wavelength is obtained.

Imaging of polymer:fullerene bulk-heterojunctions in a scanning electron microscope: methodology aspects and nanomorphology by correlative SEM and STEM

Scanning transmission electron microscopy (STEM) at low energies (≤ 30 keV) in a scanning electron microscope is well suited to distinguish weakly scattering materials with similar materials properties and analyze their microstructure. The capabilities of the technique are illustrated in this work to resolve material domains in PTB7:PC71BM bulk-heterojunctions, which are commonly implemented for light-harvesting in organic solar cells. Bright-field (BF-) and high-angle annular dark-field (HAADF-) STEM contrast of pure PTB7 and PC71BM was first systematically analyzed using a wedge-shaped sample with well-known thickness profile. Monte-Carlo simulations are essential for the assignment of material contrast for materials with only slightly different scattering properties. Different scattering cross-sections were tested in Monte-Carlo simulations with screened Rutherford scattering cross-sections yielding best agreement with the experimental data. The STEM intensity also depends on the local specimen thickness, which can be dealt with by correlative STEM and scanning electron microscopy (SEM) imaging of the same specimen region yielding additional topography information. Correlative STEM/SEM was applied to determine the size of donor (PTB7) and acceptor (PC71BM) domains in PTB7:PC71BM absorber layers that were deposited from solution with different contents of the processing additive 1,8-diiodooctane (DIO).

Real‐Time Study on Structure Formation and the Intercalation Process of Polymer: Fullerene Bulk Heterojunction Thin Films

Fullerene Intercalation In article number 1900508, Tobias Unruh and co-workers report on the formation process of a PC60BM:pBTTT-C14 active layer of a bulk heterojunction organic photovoltaic solar cell during evaporation of the solvent:additive mixture. By careful evaluation of the in-situ grazing incidence small-angle X-ray scattering measurements, a conclusive picture of the 5-step mechanism of fullerene intercalation and additive-tuned evolution of the nanoscopic film structure during the drying process could be obtained.

Understanding the Morphology of High-Performance Solar Cells Based on a Low-Cost Polymer Donor.

A low-cost and high-performance bulk heterojunction (BHJ) solar cell comprising an emerging polymer donor, poly[(thiophene)-alt-(6,7-difluoro-2-(2-hexyldecyloxy)quinoxaline)] (PTQ10), shows an efficiency of 12.7%. To improve the performance of the solar cells, a better understanding of the structure-property relationships of the PTQ10-based devices is crucial. Here, we fabricate PTQ10/nonfullerene and fullerene BHJ devices, including PTQ10/IDIC, PTQ10/ITIC, and PTQ10/PC71BM, processed with or without thermal annealing and additive and provide detailed descriptions of the relationships between the morphology and performance. PTQ10 is found to be highly miscible with nonfullerene IDIC and ITIC acceptors and poorly miscible with fullerene PC71BM acceptors. Thermal annealing promotes the crystallization of PTQ10 and phase separation of all PTQ10/IDIC, PTQ10/ITIC, and PTQ10/PC71BM devices, leading to an increased power conversion efficiencies (PCEs) of the PTQ10/IDIC and PTQ10/ITIC devices but a decreased PCE of PTQ10/PC71BM devices with 1,8-di-iodooctane (DIO) additive. Without thermal annealing, DIO greatly improves the morphology of PTQ10/PC71BM, leading to a higher PCE. The results show that the degree of phase separation and ordering in the PTQ10-based devices significantly influences device performance. The morphology-property correlations demonstrated will assist in the rational design of these low-cost polymer donor-based solar cells to achieve even higher performance.

Real‐Time Study on Structure Formation and the Intercalation Process of Polymer: Fullerene Bulk Heterojunction Thin Films

Fullerene intercalation between the side chains of conjugated polymers has a detrimental impact on both charge separation and charge transport processes in bulk heterojunction (BHJ) organic photovoltaic cells (OPVs). In situ grazing incidence X‐ray scattering experiments allow to characterize the structure formation, drying kinetics, and intercalation in blends of phenyl‐c61‐butyric acid methyl ester (PC60BM) and poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) named (pBTTT‐C14) from their 1,2‐orthodichlorobenzene (oDCB) solutions with different volume fractions of dodecanoic acid methyl ester (Me12) as a solvent additive. The structure formation process during evaporation of the solvent:additive mixture can be described by five periods, which are correlated to a multistep contraction of the lamellar stacking of the bimolecular crystals. The onset of crystallization is delayed by increasing the additive volume fraction in the coating solution leading to a promoted crystallinity. A conclusive picture of fullerene intercalation and additive‐tuned structural evolution during the drying of thin films of the polymer:fullerene BHJ blends will be presented.

Thermally induced degradation of PBDTTT-CT:PCBM based polymer solar cells

Thermally induced degradation of photovoltaic performance in bulk-heterojunction (BHJ) polymer solar cells as a result of changes either in the active layer morphology or at interfaces during operation at elevated temperature is a common phenomenon. In this work, we have studied the thermal stability of a high performance polymer:fullerene BHJ PSCcomprising a conjugated polymer poly{[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl]-alt-[2-(20-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl]} (PBDTTT-CT) and a fullerene derivative [6,6]-phenyl-C70-butyric acid methyl ester photoactive layer within a conventional device architecture of glass/ITO/PEDOT:PSS/active layer/Mg/Al. By varying the temperature exposure conditions, the degradation path has been identified as an interfacial change in the device rather than a bulk effect. Furthermore, charge carrier dynamics studied by open circuit corrected charge carrier extraction has shown that post-annealed devices suffer from charge extraction due to the development of interfacial changes as compared to the non-treated devices in both pristine and with 1,8-Diiodooctane added scenarios.

Influence of binary additives into the solvent for preparation of polymer and fullerene bulk heterojunction solar cells by convective deposition method

Abstract Binary additives of 1,8-diiodooctane (DIO) and 1,3,5-trichlorobenzene (135-TCB) were utilized for preparation of 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)-3fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) films for bulk heterojunction (BHJ) solar cells. The addition of binary additives of DIO 3.0 vol% + 135-TCB 2.1 vol% into PTB7-Th:PC71BM films exhibited nano-sized of PTB7-Th networks homogeneous distribution and smaller sizes than the film without additive and with DIO 3.0 vol%. The out-of-plane orientation of PTB7-Th:PC71BM films were improved by addition of binary additives and 135-TCB. A rectification ratio of the BHJ solar cells were improved by using binary additives of DIO 3.0 vol% + 135-TCB 2.1 vol%. As a consequence, fill factor (FF) and power conversion efficiency (PCE) of the device with binary additives increased up to 0.69 and 7.4% as compared with FF (0.66) and PCE (6.9%) of the device with DIO 3.0 vol%. The external quantum efficiency the device with binary additives was enhanced in the region from 550 nm to 750 nm as compared with that of the device with DIO 3.0 vol%. This result was probably caused by the improvement of PTB7-Th orientation, and phase separation between the PTB7-Th and PC71BM.

Impact of Initial Bulk‐Heterojunction Morphology on Operational Stability of Polymer:Fullerene Photovoltaic Cells

AbstractControlling initial bulk‐heterojunction (BHJ) morphology of photoactive layer is critical for device efficiency of organic photovoltaic (OPV) cells. However, its impact on performance, specifically long‐term operational stability is still poorly understood. This is mainly due to limitations in direct measurements enabling in situ monitoring of devices at a molecular level. Here, a thermal annealing preconditioning step is utilized to tune initial morphology of model polymer:fullerene BHJ OPV devices and molecular resonant vibrational spectroscopy to identify in situ degradation pathways. Direct spectroscopic evidence is reported for molecular‐scale phase‐segregation temperature (TPS) which critically determines a boundary in high efficiency and long operational stability. Under operation, initially well‐mixed blend morphology (no annealing) shows interface instability related to the hole‐extracting layer via de‐doping. Likewise, initially phase‐segregated morphology at a molecular level (annealed above TPS) shows instability in the photoactive layer via continuous phase‐segregation between polymer and fullerenes in macroscales, coupled with further fullerene photodegradation. The results confirm that a thermal annealing preconditioning step is essential to stabilize the BHJ morphology; in particular annealing below TPS is critical for improved operational stability while maintaining high efficiency.