Organic solar cells (OSCs) have emerged as one of the most promising third-generation photovoltaic technologies because of their lightweight nature, mechanical flexibility, solution processability, semitransparency, and excellent low-light response. These attributes make them attractive for portable electronics, building-integrated photovoltaics, wearable self-powered systems, and indoor light harvesting. Over the past decade, the power conversion efficiency (PCE) of single-junction OSCs has risen dramatically, from around 10% for early non-fullerene acceptor systems to beyond 21% in state-of-the-art laboratory devices. This rapid progress has been driven by the development of non-fullerene acceptors (NFAs), especially Y-series derivatives, and by the continuous innovation of polymer donors and small-molecule donors. However, efficiency alone is not sufficient for commercialization. Long-term operational stability under high-temperature conditions has become a critical bottleneck. In this review, I systematically discuss the thermal stability of OSCs, including intrinsic material stability, thermally induced morphological evolution, interfacial chemical reactions, electrode–interface diffusion, and device-level degradation. I also compare and contrast the thermal stability challenges of OSCs with those of perovskite solar cell devices, because the latter have attracted enormous attention for their high efficiency but suffer from different degradation pathways such as ion migration and halide phase segregation. Understanding both similarities and differences is essential for developing robust photovoltaic technologies. Throughout this review, I emphasize quantitative descriptors, standardized testing protocols, and materials engineering strategies. The discussion is organized around four central questions: How is thermal stability measured? Which material parameters govern thermal resilience? How does active-layer morphology evolve under heat? And how can interfaces be engineered to suppress thermal degradation? I also provide multiple tables and formulas to summarize the current state of knowledge and to guide future research. The ultimate goal is to identify design principles for OSCs that simultaneously achieve high efficiency and long-term thermal stability, thereby accelerating the transition from laboratory cells to commercial modules.
1. Standardized Thermal Stability Testing Protocols
Reliable comparison of thermal stability across different laboratories requires standardized testing protocols. The International Summit on Organic and Hybrid Photovoltaics Stability (ISOS) has proposed a consensus framework that includes dark storage, temperature cycling, and light–temperature cycling. These protocols were initially developed for organic photovoltaics and have also been widely adopted for perovskite solar cell stability assessment. Table 1 summarizes the ISOS protocols relevant to thermal stability. The protocols are organized into three levels: basic, intermediate, and advanced. For example, ISOS-D-3 evaluates damp-heat storage at 65°C or 85°C and 85% relative humidity. ISOS-T-3 tests thermal fatigue under repeated temperature cycling between –40°C and 85°C. ISOS-LT-3 combines light soaking with temperature cycling between –25°C and 65°C at controlled humidity. These protocols provide a reproducible basis for benchmarking both OSCs and perovskite solar cell devices.
| Test | Temperature | Relative Humidity | Environment | Light Source | Bias |
|---|---|---|---|---|---|
| ISOS-D-1 | 23 ± 4°C | Ambient | Ambient air | None | Open circuit |
| ISOS-D-2 | 65 or 85°C | Ambient | Oven or hot plate | None | Open circuit |
| ISOS-D-3 | 65 or 85°C | 85% | Environmental chamber | None | Open circuit |
| ISOS-T-1 | RT to 65 or 85°C | Ambient | Oven or hot plate | None | Open circuit |
| ISOS-T-2 | RT to 65 or 85°C | Ambient | Oven or hot plate | None | Open circuit |
| ISOS-T-3 | –40 to 85°C | <55% | Oven | None | Open circuit |
| ISOS-LT-1 | RT to 65°C linear | Controlled | Environmental chamber | Solar simulator | MPP or open circuit |
| ISOS-LT-2 | 5 to 40°C linear | 40°C, 50% | Solar simulator chamber | Solar simulator | MPP or open circuit |
| ISOS-LT-3 | –25 to 65°C linear | 40°C, 50% | Solar simulator chamber | Solar simulator | MPP or open circuit |
Quantitative analysis of thermal degradation often uses the T80 lifetime, defined as the time required for the PCE to fall to 80% of its initial value. If the degradation follows first-order kinetics, the PCE can be written as
$$ \eta(t) = \eta_0 \exp(-k t) $$
where \(\eta_0\) is the initial efficiency, \(k\) is the degradation rate constant, and \(t\) is the aging time. The T80 lifetime is then
$$ T_{80} = \frac{\ln(0.8)}{-k} = \frac{0.223}{k} $$
For non-exponential decay, empirical stretched exponential models are often used:
$$ \eta(t) = \eta_0 \exp\left[-\left(\frac{t}{\tau}\right)^\beta\right] $$
where \(\tau\) is a characteristic time and \(\beta\) is a stretching exponent. These formulas are equally applicable to perovskite solar cell degradation, although the underlying mechanisms may differ. In OSCs, thermal degradation is frequently dominated by molecular diffusion and phase separation, whereas in perovskite solar cell devices, ion migration, halide segregation, and electrode corrosion often play dominant roles.
2. Intrinsic Thermal Stability of Organic Photovoltaic Materials
The intrinsic thermal stability of organic photovoltaic materials is closely linked to their glass transition temperature (\(T_g\)), thermal decomposition temperature (\(T_d\)), and molecular diffusion kinetics. When the operating temperature exceeds \(T_g\), polymer chains and small molecules gain segmental mobility, leading to molecular rearrangement, crystallization, and phase separation. For a binary blend, the Gordon–Taylor or Fox equation can estimate the blend \(T_g\):
$$ \frac{1}{T_g} = \frac{w_1}{T_{g1}} + \frac{w_2}{T_{g2}} $$
where \(w_1\) and \(w_2\) are weight fractions and \(T_{g1}\) and \(T_{g2}\) are the glass transition temperatures of the pure components. In many OSC blends, the presence of a low-\(T_g\) small-molecule acceptor can plasticize the mixture, lowering the effective \(T_g\) and accelerating thermal degradation. Therefore, increasing the \(T_g\) of the acceptor, the donor, or both is a primary strategy. Oligomeric acceptors, dimeric acceptors, polymer acceptors, and double-cable conjugated polymers have all been explored. Table 2 summarizes representative material design strategies and their impact on thermal stability.
| Strategy | Material Type | Representative Feature | Effect on Thermal Stability | Reference Concept |
|---|---|---|---|---|
| Side-chain engineering | Small-molecule NFA | Bulky or halogenated side chains | Increases \(T_g\), suppresses crystallization | Thiophene, trifluoromethyl, ester side chains |
| Oligomeric acceptors | Dimer, trimer, tetramer | Higher molecular weight, reduced diffusion | Dramatically improved T80 | Dimeric and trimeric Y-series derivatives |
| Polymer acceptors | Polymeric NFA | Chain entanglement, high \(T_g\) | Suppresses macroscopic phase separation | All-polymer solar cells |
| Double-cable polymers | Single-component | Donor and acceptor on one chain | Eliminates blend phase separation | Single-component OSCs |
| Intramolecular locking | Dimeric acceptors | Non-covalent F···S or F···H interactions | Rigidifies conformation, raises \(T_g\) | Conformationally locked dimers |
| Star-shaped oligomers | Multi-arm acceptors | Multiple non-covalent interactions | Stable morphology at 80°C | Star-shaped oligomeric acceptors |
Several experimental techniques are used to evaluate intrinsic thermal stability. Differential scanning calorimetry (DSC) measures \(T_g\) and melting transitions. Thermogravimetric analysis (TGA) determines \(T_d\), while TG-MS identifies decomposition products. Fast-scanning chip calorimetry (Flash DSC) can capture rapid crystallization kinetics of NFAs. Dynamic mechanical analysis (DMA) probes modulus changes in thin films and multilayer stacks. Grazing-incidence wide-angle X-ray scattering (GIWAXS) and grazing-incidence small-angle X-ray scattering (GISAXS) provide real-time information on molecular packing and phase separation during heating. Temperature-dependent UV–visible absorption spectroscopy can extract an onset temperature (\(T_{onset}\)) for spectral changes associated with aggregation. Variable-temperature capacitance measurements can identify a thermal instability threshold (\(T_{MAX}\)). These methods complement each other and are also useful for perovskite solar cell stability studies, where thermal degradation may involve phase transitions and ion migration.
The diffusion coefficient of molecules in the active layer is often described by an Arrhenius relationship:
$$ D = D_0 \exp\left(-\frac{E_a}{RT}\right) $$
where \(D_0\) is the pre-exponential factor, \(E_a\) is the activation energy, \(R\) is the gas constant, and \(T\) is the absolute temperature. Similarly, the characteristic relaxation time \(\tau\) follows:
$$ \tau = \tau_0 \exp\left(\frac{E_a}{k_B T}\right) $$
where \(k_B\) is the Boltzmann constant. Near \(T_g\), the temperature dependence of relaxation is often described by the Williams–Landel–Ferry (WLF) equation:
$$ \log a_T = -\frac{C_1 (T – T_g)}{C_2 + (T – T_g)} $$
where \(a_T\) is the shift factor and \(C_1\) and \(C_2\) are material constants. These equations help predict whether a given OSC blend will remain morphologically stable at operating temperatures. In perovskite solar cell devices, similar diffusion equations are used for ion migration, but the activation energies and diffusion pathways differ substantially.
3. Morphological Stability of Active Layers
The bulk heterojunction (BHJ) morphology of OSCs is thermodynamically metastable. Under continuous thermal stress, the donor and acceptor phases tend to coarsen, and the acceptor may crystallize. This process is driven by the reduction of interfacial free energy. The Flory–Huggins interaction parameter \(\chi\) quantifies the miscibility between donor and acceptor:
$$ \chi = \frac{V_m}{RT} (\delta_D – \delta_A)^2 $$
where \(V_m\) is the molar volume, and \(\delta_D\) and \(\delta_A\) are the solubility parameters of the donor and acceptor, respectively. A low \(\chi\) indicates good miscibility, while a high \(\chi\) indicates poor miscibility. Interestingly, both extremes can be problematic: too good miscibility can lead to pronounced mixing–demixing transitions, while too poor miscibility can cause immediate large-scale phase separation. Optimal stability is often found at intermediate \(\chi\). The free energy of mixing for a binary blend is given by:
$$ \Delta G_m = RT (n_1 \ln \phi_1 + n_2 \ln \phi_2 + \chi n_1 \phi_2) $$
where \(n_i\) and \(\phi_i\) are the number of moles and volume fractions, respectively. Phase separation occurs when \(\partial^2 \Delta G_m / \partial \phi^2 < 0\). Thermally induced phase separation can be described by Cahn–Hilliard theory, where the evolution of composition fluctuations is governed by:
$$ \frac{\partial \phi}{\partial t} = M \nabla^2 \left( \frac{\partial f}{\partial \phi} – \kappa \nabla^2 \phi \right) $$
where \(M\) is the mobility, \(f\) is the free energy density, and \(\kappa\) is the gradient energy coefficient. Domain coarsening often follows a power law:
$$ L(t) = L_0 + k t^{1/3} $$
where \(L(t)\) is the characteristic domain size. This scaling behavior has been observed in many OSC blends and is also relevant to perovskite solar cell films, although the latter may undergo different phase transitions.
To suppress morphological degradation, researchers have developed several strategies. Table 3 summarizes common approaches for enhancing active-layer thermal stability.
| Strategy | Mechanism | Example Concept | Thermal Stability Improvement |
|---|---|---|---|
| Ternary blending | Third component increases \(T_g\) or compatibilizes | Similar acceptor with different side chains | Suppresses non-amorphous aggregation |
| Solid additives | Curvature-mediated morphology regulation | Three-dimensional bowl-shaped molecules | Extends T85 from tens of hours to thousands |
| Fiber networks | Rigid scaffold limits molecular diffusion | Cellulose, aramid nanofibers, MOFs | Stabilizes BHJ nanostructure |
| Thickness confinement | Nanoconfinement raises effective \(T_g\) | Ultrathin active layers | Suppresses crystallization and phase separation |
| Layer-by-layer deposition | Optimizes vertical phase distribution | Comb-shaped morphology | Improves thermal and environmental stability |
| Single-component materials | Eliminates donor–acceptor phase separation | Double-cable polymers, block copolymers | High intrinsic thermal stability |
| Crosslinking | Forms covalent network | Crosslinked fullerene derivatives | Locks morphology and interfaces |
Ternary blending is particularly versatile. By introducing a third component that is structurally similar to the host acceptor but has a higher \(T_g\), one can inhibit the diffusion of the low-\(T_g\) component. For instance, blends of two NFAs with different thermal properties have been shown to maintain over 95% of initial efficiency after 1400 h at 85°C. In another approach, a third component that is fully miscible with the host acceptor can reduce the driving force for phase separation. The resulting alloy-like morphology exhibits a T80 lifetime exceeding 9000 h at 100°C. These strategies are conceptually similar to compositional engineering in perovskite solar cell absorbers, where mixed cations and halides improve phase stability.
Solid additives can also stabilize the BHJ. Three-dimensional bowl-shaped additives, for example, improve molecular packing and suppress excessive crystallization. Fiber-forming additives such as cellulose derivatives can form hydrogen-bonded networks that lock the donor and acceptor domains. Metal–organic frameworks (MOFs) and covalent organic frameworks have been used as rigid scaffolds. The addition of a small amount of an insulating polymer can also enhance thermal stability by increasing the blend viscosity and reducing molecular diffusion. In all cases, the additive must be carefully chosen to avoid sacrificing charge transport.
Thickness confinement is another effective strategy. When the active layer thickness is reduced below a certain threshold, the free volume available for molecular motion decreases, and the effective \(T_g\) increases. For example, reducing the thickness of a PM6:L8-BO blend from 100 nm to below 20 nm has been reported to raise the blend \(T_g\) by more than 10°C, leading to negligible degradation after 100 h at 85°C. Similar effects have been observed in other high-efficiency systems. This approach is particularly attractive for indoor and wearable applications where thin films are desirable. However, ultrathin active layers may suffer from incomplete light absorption, so a trade-off between optical absorption and thermal stability must be optimized.
Layer-by-layer (LBL) deposition can create a vertically graded morphology that is more resistant to thermal stress. In LBL processing, the donor and acceptor are deposited sequentially, often resulting in a pseudo-bilayer structure with interdiffusion. This morphology can be further optimized by solvent choice and thermal annealing. Devices based on comb-shaped active layers have shown 93% efficiency retention after 900 h at 85°C. In contrast, conventional BHJ devices often degrade faster because of uncontrolled vertical phase separation. The LBL approach has also been explored for perovskite solar cell fabrication, where sequential deposition can improve film quality and stability.
Single-component organic solar cells based on double-cable conjugated polymers or block copolymers eliminate the need for a separate donor–acceptor blend. Because the donor and acceptor are chemically connected, macroscopic phase separation is suppressed. Some double-cable polymers exhibit \(T_g\) values as high as 170–200°C, and devices retain about 95% of initial efficiency after 700 h at 90°C. Block copolymers such as PM6-b-PYIT and PM6-b-L15 have also shown promising thermal stability. Although the efficiency of single-component OSCs is still lower than that of optimized BHJ devices, their intrinsic stability makes them attractive for niche applications.
4. Interfacial Degradation Mechanisms and Control Strategies
Interfaces play a decisive role in the thermal stability of OSCs. Even when the active layer is morphologically stable, interfacial reactions and diffusion can cause severe performance losses. This is particularly important because OSCs typically use multiple layers: transparent electrode, hole transport layer (HTL), active layer, electron transport layer (ETL), and metal electrode. Each interface can degrade under thermal stress. Similar issues are observed in perovskite solar cell devices, where interface engineering is crucial for stability. In the following sections, I discuss conventional and inverted OSC architectures separately.
4.1 Conventional Device Interfaces
In conventional OSCs, the transparent electrode is usually indium tin oxide (ITO), coated with a hole transport layer such as PEDOT:PSS or a self-assembled monolayer (SAM). The electron transport layer is often a small-molecule or polymer material, and the top electrode is a low-work-function metal. Thermal degradation pathways include:
- PEDOT:PSS absorbs moisture and undergoes phase separation; its acidic groups can etch ITO, releasing indium and tin ions that act as deep traps.
- SAMs can desorb or decompose at elevated temperatures, causing a loss of dipole and work function.
- Electron transport layers may aggregate or crystallize, leading to increased series resistance and non-radiative recombination.
- Low-work-function metals can oxidize or diffuse into the active layer.
To improve thermal stability, researchers have developed modified PEDOT:PSS formulations, alternative hole transport materials, and hybrid interlayers. For example, replacing PSS with a fluorinated ionomer can reduce acidity and moisture uptake. SAMs with multiple anchoring groups, such as biphosphonic acids, show improved thermal stability. Inserting a thin metal oxide layer between ITO and the SAM can also enhance stability. Table 4 summarizes representative strategies for conventional OSC interfaces.
| Interface | Degradation Mechanism | Stabilization Strategy | Observed Improvement |
|---|---|---|---|
| ITO/PEDOT:PSS | Moisture absorption, acid etching, phase separation | DMSO doping, ionomer replacement, hybrid interlayers | Reduced burn-in, longer T80 |
| ITO/SAM | Desorption, decomposition, dipole loss | Multidentate anchoring, aromatic end groups, MoOx interlayer | Improved thermal stability at 65–85°C |
| Active layer/ETL | Aggregation, crystallization, trap formation | Bay-functionalized PDIs, polymer interlayers, MOF composites | T80 extended to hundreds of hours |
| ETL/metal | Metal diffusion, oxidation, chemical reaction | Inert buffer layers, encapsulation, pre-annealing | Suppressed electrode penetration |
For the electron transport layer, perylene diimide (PDI) derivatives are widely used. Their aggregation and crystallization can be suppressed by bay-site functionalization with benzamide groups or fused thiophene units. Devices using these modified PDIs have achieved over 18% efficiency and retained 80% of initial PCE after 500 h at 85°C. Polymer interlayers such as NDI-Br:Ni-MOF composites have also been shown to extend T90 beyond 1000 h at 65°C. Replacing organic ETLs with inorganic ZnO can dramatically improve thermal stability, but ZnO itself can degrade interfaces, as discussed below.
4.2 Inverted Device Interfaces
Inverted OSCs typically use ZnO or another metal oxide as the electron transport layer, and MoO3 or a doped metal oxide as the hole transport layer. The top electrode is usually a high-work-function metal such as Ag or Au. Inverted devices often exhibit better long-term stability than conventional devices because they avoid PEDOT:PSS. However, they have their own interfacial degradation pathways:
- ZnO surface defects, especially oxygen vacancies, can catalyze active-layer degradation and promote non-radiative recombination.
- MoO3 can be reduced at high temperatures, changing its work function and causing energy-level misalignment.
- MoO3 can diffuse into the active layer, acting as a dopant and increasing recombination.
- Metal electrodes can diffuse through the transport layers and react with the active layer.
Passivation of ZnO is a major research focus. Small molecules such as aromatic ammonium salts, benzotriazoles, naphthalene diimides, and polyphenols can coordinate with surface Zn2+ and fill oxygen vacancies. For example, multi-arm aromatic ammonium salts have been shown to reduce surface adsorbed oxygen and improve device stability. Devices with passivated ZnO retained 87.3% of initial PCE after 1500 h at 65°C, compared with 59% for unpassivated devices. Polymer passivation layers such as PMMA can also stabilize ZnO and control active-layer self-assembly. With PMMA, thick-film PM6:L8-BO devices retained 89% of initial efficiency after 1000 h at 60°C.
Alternative electron transport materials have been explored. Ir/IrOx electron transport layers have enabled T70 lifetimes of 13,920 h at 60°C. This is among the highest thermal stability reported for OSCs and highlights the importance of interface material choice. In perovskite solar cell devices, similar efforts have focused on SnO2, TiO2, and fullerene-based electron transport layers, although the degradation chemistry differs.
For the hole transport layer, MoO3 is a common choice but suffers from thermal instability. Inserting a thin barrier layer between MoO3 and the active layer can suppress diffusion and reaction. For instance, a 3 nm C60 layer has been shown to prevent MoO3 diffusion and extend device lifetime. A TCTA interlayer with high ionization potential also improves stability, with 88% efficiency retention after 1000 h at 85°C. A 2 nm C70 buffer layer has been reported to extend the operating lifetime at 80°C from 20 min to 800 h. Hybrid hole transport layers combining MoO3 with TaWOx or PEDOT:PSS have also shown improved thermal stability. Table 5 summarizes interface engineering strategies for inverted OSCs.
| Interface | Degradation Mechanism | Stabilization Strategy | Observed Improvement |
|---|---|---|---|
| ZnO/active layer | Oxygen vacancies, surface defects, catalytic degradation | Small-molecule passivation, polymer passivation, biomolecules | T80 extended from ~500 h to >1000 h |
| Active layer/MoO3 | MoO3 reduction, diffusion, doping | C60, C70, TCTA, Br-2PACz interlayers | Efficiency retention >88% after 1000 h at 85°C |
| MoO3/metal | Interfacial reaction, insulating oxide formation | C60 blocking layer, pre-annealing | Retention improved from 66% to 87% after 150°C annealing |
| Electrode/active layer | Metal diffusion, penetration | Pre-annealing, dense active layers, buffer layers | Suppressed burn-in degradation |
4.3 Electrode and Transport Layer Stability
Metal electrodes can degrade through oxidation, diffusion, and reaction with transport layers. Silver and aluminum are common choices, but both can diffuse into organic layers at elevated temperatures. Molybdenum oxide can react with aluminum to form insulating Al2O3, which increases series resistance and reduces the built-in field. Inserting a C60 layer between MoO3 and Al suppresses this reaction and improves thermal stability. Pre-annealing the active layer before electrode deposition can also increase film density and reduce metal penetration. In perovskite solar cell devices, electrode stability is equally critical, with halide migration and metal halide formation being major concerns.
Encapsulation is another important factor. Even if the intrinsic device stack is thermally stable, moisture and oxygen ingress can accelerate degradation. Damp-heat testing according to ISOS-D-3 is therefore essential. Some OSCs have achieved excellent damp-heat stability when combined with crosslinked active layers and robust encapsulation. For example, devices with crosslinked fullerene derivatives and interface passivation retained high performance after 1351 h at 85°C and 85% relative humidity. Temperature cycling between –40°C and 85°C for 200 cycles also showed 94% efficiency retention. These results suggest that careful interface engineering can enable OSCs to pass harsh reliability tests, although perovskite solar cell devices still face more severe challenges due to ion migration and soft lattice dynamics.
5. Summary and Outlook
In this review, I have summarized the current understanding of thermal stability in OSCs, covering material design, morphological control, interface engineering, and standardized testing. Table 6 provides a concise overview of the main findings and future directions.
| Aspect | Key Challenge | Current Strategy | Future Direction |
|---|---|---|---|
| Intrinsic material stability | Low \(T_g\), molecular diffusion | Oligomeric acceptors, polymer acceptors, side-chain engineering | High-\(T_g\) materials with balanced efficiency |
| Active-layer morphology | Phase separation, crystallization, coarsening | Ternary blending, additives, thickness confinement, LBL, single-component | Predictive morphology design using machine learning |
| Conventional interfaces | PEDOT:PSS instability, ETL aggregation, metal oxidation | Modified PEDOT:PSS, SAMs, hybrid interlayers, polymer ETLs | Robust, dopant-free transport layers |
| Inverted interfaces | ZnO defects, MoO3 diffusion/reduction, electrode reactions | Passivation, barrier layers, alternative metal oxides | In situ passivation and multifunctional interlayers |
| Standardization | Lack of universal protocols | ISOS-D, ISOS-T, ISOS-LT | Multi-stress, real-world testing |
| Comparison to perovskite solar cell | Different degradation mechanisms | Shared interface engineering concepts | Cross-learning between OSC and perovskite solar cell stability |
Despite significant progress, several challenges remain. First, most thermal stability tests are performed under constant temperature or simple cycling conditions. Real-world operation involves simultaneous light, heat, moisture, and mechanical stress. The coupling of these factors can produce degradation pathways that are not observed in single-stress tests. For example, photo-oxidation and thermal degradation can reinforce each other, and temperature cycling can induce thermomechanical stress due to mismatched thermal expansion coefficients. In perovskite solar cell devices, similar multi-stress effects are well recognized, and the OSC community can benefit from the testing protocols developed for perovskite solar cell stability. Second, the relationship between material \(T_g\) and device stability is not always linear. Other factors such as miscibility, interfacial energy, and diffusion activation energy also play important roles. Third, interface engineering has become indispensable. Even a perfectly stable active layer can fail if the transport layers or electrodes degrade. Therefore, future research should focus on integrated design of active layers, interfaces, and electrodes.
Several promising directions emerge. High-throughput screening and machine learning can accelerate the discovery of high-\(T_g\) acceptors and compatible additives. In situ characterization techniques, including GIWAXS, GISAXS, and variable-temperature spectroscopy, can provide deeper insight into degradation mechanisms. Multifunctional interlayers that simultaneously passivate defects, block diffusion, and improve charge extraction are highly desirable. Crosslinked or dynamic covalent networks may offer a way to lock morphology while maintaining processability. For flexible and wearable applications, thermally stable flexible substrates and transparent electrodes are also needed. Finally, standardized reporting of thermal stability data, including complete ISOS protocols and statistical analysis, is essential for comparing results across laboratories.
In conclusion, the thermal stability of OSCs has advanced considerably, with many devices now retaining most of their initial efficiency after hundreds or thousands of hours at elevated temperatures. The lessons learned from perovskite solar cell research, particularly in interface engineering and multi-stress testing, are highly relevant. By combining high-\(T_g\) materials, morphology-stabilizing additives, robust interfaces, and standardized testing, it should be possible to develop OSCs that meet the stringent requirements for commercial applications. I am optimistic that continued interdisciplinary efforts will eventually deliver efficient, thermally stable, and commercially viable organic solar cells, alongside advances in perovskite solar cell technology, for a sustainable energy future.
$$ \text{Overall stability metric: } S = \frac{\int_0^{t_{op}} \eta(t) dt}{\eta_0 t_{op}} $$
where \(S\) is the normalized energy yield over the operating time \(t_{op}\). This metric can be used to compare different device architectures and materials systems, including OSCs and perovskite solar cell devices, under realistic operating conditions.
$$ \text{Thermal stability figure of merit: } FOM = \frac{T_{80} \times \eta_0}{1 + k_d} $$
Such figures of merit help guide the co-optimization of efficiency and lifetime. As the field moves toward commercialization, I expect that thermal stability will become an increasingly central design criterion, alongside efficiency and cost. The integration of knowledge from organic photovoltaics and perovskite solar cell research will accelerate the development of next-generation solar technologies.