awn

Publication date: April 2021

Source: Nano Energy, Volume 82

Author(s): Boer Tan, Sonia R. Raga, Kevin James Rietwyk, Jianfeng Lu, Sebastian O. Fürer, James C. Griffith, Yi-Bing Cheng, Udo Bach

Publication date: April 2021

Source: Nano Energy, Volume 82

Author(s): Narges Yaghoobi Nia, Mahmoud Zendehdel, Mojtaba Abdi-Jalebi, Luigi Angelo Castriotta, Felix U. Kosasih, Enrico Lamanna, Mohammad Mahdi Abolhasani, Zhaoxiang Zheng, Zahra Andaji-Garmaroudi, Kamal Asadi, Giorgio Divitini, Caterina Ducati, Richard H. Friend, Aldo Di Carlo

Publication date: April 2021

Source: Nano Energy, Volume 82

Author(s): Claudia Caddeo, Alessio Filippetti, Andrea Bosin, Christine Videlot-Ackermann, Jörg Ackermann, Alessandro Mattoni

The power conversion efficiencies (PCEs) of single‐junction organic solar cells have now reached over 18%. Recent progress that has been made in understanding the morphology and the device photophysics of high performing polymer:non‐fullerene acceptor blends and some of the major challenges that must be overcome to attain PCEs of over 20% are highlighted.

Abstract

The power conversion efficiencies (PCEs) of single‐junction organic solar cells (OSC) have now reached over 18%. This rapid recent progress can be attributed to the development of new nonfullerene electron acceptors (NFAs) that are paired with suitable high performing polymer electron donors. Substantial improvements in the PCEs and long‐term stability enabled by NFA OSCs have allowed the development and integration of these systems into many niche and novel applications. Here, the recent progress that has been made in understanding the device photophysics of high performing polymer:NFA blends is highlighted. As the bulk heterojunction morphology is intrinsically linked to the device photophysics, this review focuses on studies that have provided noteworthy morphological insights using advanced techniques such as solid‐state NMR and resonant soft X‐ray scattering. Through this, some of the major challenges that must be overcome to attain PCEs of over 20% in NFA OSCs are addressed.

A narrow bandgap polymer acceptor PYF‐T with fluorinated end groups on monomer sub‐units is synthesized, showing stronger and red‐shifted absorption, lower‐lying frontier molecular orbitals, higher electron mobility, enhanced intermolecular packing, and without sacrificing photovoltage compared to its non‐fluorinated counterpart (PY‐T). When employed in all‐polymer solar cells, PYF‐T yields an outstanding efficiency of 14.10%.

Abstract

Fluorination of end groups has been a great success in developing efficient small molecule acceptors. However, this strategy has not been applied to the development of polymer acceptors. Here, a dihalogenated end group modified by fluorine and bromine atoms simultaneously, namely IC‐FBr, is first developed, then employed to construct a new polymer acceptor (named PYF‐T) for all‐polymer solar cells (all‐PSCs). In comparison with its non‐fluorinated counterpart (PY‐T), PYF‐T exhibits stronger and red‐shifted absorption spectra, stronger molecular packing and higher electron mobility. Meanwhile, the fluorination on the end groups down‐shifts the energy levels of PYF‐T, which matches better with the donor polymer PM6, leading to efficient charge transfer and small voltage loss. As a result, an all‐PSC based on PM6:PYF‐T yields a higher power conversion efficiency (PCE) of 14.1% than that of PM6:PY‐T (11.1%), which is among the highest values for all‐PSCs reported to date. This work demonstrates the effectiveness of fluorination of end‐groups in designing high‐performance polymer acceptors, which paves the way toward developing more efficient and stable all‐PSCs.

This work is the first to integrate the multimechanism recombination parameters with the density of states (DOS) distribution and effective bandgap in the framework of a semiempirical analytical model of temperature and light intensity dependent V _oc. The proposed approach is expected to be a useful tool for quantifying the full spectrum of recombination‐ and DOS‐related parameters of nonfullerene organic solar cells.

Abstract

The relationship of the temperature–light intensity dependence of open‐circuit voltage V _oc in nonfullerene‐based organic solar cells with their material characteristics and multimechanism recombination parameters is described. The systematic variation of the effective bandgap E _g,eff and the electrode layers allows the observation of different relative contributions of bimolecular, bulk, and surface trap‐assisted recombination mechanisms. The complementary advantages of the analytical model and the established voltage‐impedance spectroscopy technique provide a useful tool to quantify multimechanism recombination parameters, effective density of states N _c, and energetic disorder σ in organic solar cells under operating conditions. The validity of the proposed model to understand the temperature and light intensity dependent of V _oc is shown by applying it to four different donor:nonfullerene acceptor blend systems with conventional or inverted device architectures.

Herein, excitation wavelength‐dependent charge generation dynamics in nonfullerene solar cells is revealed by time‐dependent density functional theory (TDDFT)‐based nonadiabatic dynamics simulations.

Unraveling the charge generation dynamics at the donor–acceptor (D−A) interfaces is crucial for improving the photovoltaic performances of nonfullerene acceptor‐based organic solar cells (OSCs). Herein, time‐dependent density functional theory (TDDFT) based nonadiabatic dynamics simulations are used to explore the ultrafast photoinduced dynamics at a nonfullerene D–A PTB7@PDI interface. Based on the results, it is found that such an interface exhibits distinct charge generation processes upon excitation with different wavelengths. The excitation at ≈591 nm mainly results in the local exciton |PTB7 ^∗ >, whereas the charge transfer exciton |PTB7 ⁺ PDI ⁻ > also has minor contribution. Later on, the electron transfer from PTB7 to PDI, i.e., channel I charge generation process, occurs in 1 ps. The situations are much more complex when the excitation is conducted using ≈487 nm light. The initial populated excitons include local excitons |PDI ^∗ >, |PTB7 ^∗ >, and charge transfer exciton |PTB7 ⁺ PDI ⁻ >, after which both channel I and channel II charge generation take place ultrafast. However, in both situations, the charge generation processes occur within a few picoseconds, which is consistent with previous experimental work. Such ultrafast charge generation processes in a wide range of solar spectra are one of the reasons responsible for the excellent photovoltaic properties of such OSCs.

A thin‐film AlGaInP/AlGaAs/InGaAs/InGaAs inverted metamorphic multijunction (IMM) solar cell with a bandgap of 1.96/1.53/1.16/0.83 eV is fabricated, and the efficiency reaches 34.89%. The analysis of the performance characteristics of four subcells concludes that the key to limiting the overall efficiency improvement is the deep‐level recombination of the AlGaInP top subcell and the bulk recombination of 0.83 eV InGaAs bottom subcell.

A thin‐film AlGaInP/AlGaAs/InGaAs/InGaAs inverted metamorphic multijunction solar cell with a bandgap of 1.96/1.53/1.16/0.83 eV is fabricated. The photoelectric conversion efficiency reaches 34.89% with an open‐circuit voltage of 3.54 V under AM1.5 G spectrum. The analysis of individual subcells is the key to evaluating the performance of multijunction solar cells. The current density versus voltage characteristics of four subcells are calculated using optoelectronic reciprocity relation between the external quantum efficiency and the different injection current densities electroluminescence. The analysis of the performance characteristics of four subcells concludes that the key to limiting the overall efficiency improvement is the deep‐level recombination of the AlGaInP top subcell and the bulk recombination of 0.83 eV InGaAs bottom subcell. Targeted optimization of the top subcell and the bottom subcell is expected to significantly improve efficiency.

Herein, the doping effect on semiconductor oxide‐based electron transport materials, especially for the typical TiO₂, in perovskite solar cells is reviewed and emphasized by classifying the doping ions according to the main family of elements from the critical factors of lattice optimization, carrier transporting improvement, and interface modification.

From the perspective of the device structure of perovskite solar cells (PSCs), the electron transport layer is one of the essential components and plays a significant role in suppressing carrier recombination. Furthermore, its decisiveness is related to the quality of perovskite film, the rapid interface carrier extraction, and the bandgap alignment. However, the deficiency of the semiconductor oxides based electron transport materials, especially for most studied TiO₂, is that their carrier mobility is one to three orders of magnitude lower than the most commonly used hole transport materials, leading to an imbalanced carrier flux and unpredicted hysteresis. Doping new ions are the most effective ways to improve electron mobility and tune the bandgap, while the fundamental mechanism of doping in the majority of cases are still lacking. Herein, the doping effect on semiconductor oxides is reviewed and emphasized by classifying the doping ions according to the critical factors of lattice optimization, a carrier transporting improvement, and interface modification. This review is the first systematic summary of the ion doping characteristics in oxide electron transport layers of PSCs. Finally, the implementation of doping ions in electron transport materials is briefly discussed for further enhancing the photovoltaic performance of PSCs.

High‐quality and all‐inorganic CsPbI₂Br perovskite film with lower defects and improved hydrophobicity is prepared via a facile additive engineering with trace S₈, resulting in inverted solar cells with improved efficiency and stability.

Though prized for excellent thermal stability, inorganic perovskites are still behind organic/inorganic hybrid perovskites due to their high‐density defects and poor hydrophobicity. Herein, trace hydrophobic S₈ is used as additive to optimize the solution‐processed CsPbI₂Br perovskite film. A series of characterizations reveal that S₈ additive not only leads to retarded crystallization of α‐CsPbI₂Br perovskite at low temperature (<150 °C) via self‐formed Pb(S₈) _x ²⁺ intermediate but also induces efficient grain‐boundary passivation via distinctive PbS coordination interaction and reduced wettability on perovskite surface, which all point to the formation of the perovskite film with reduced defects and improved hydrophobicity. As a result, the inverted perovskite solar cells (PSCs) based on the optimized all‐inorganic perovskite of CsPbI₂Br:S₈ deliver an increased power conversion efficiency (PCE) from 12.76% to 14.46% as well as remarkably enhanced device stability under thermal or ambient condition. This work thus provides a simple way as well as new insights for boosting the performance of solution‐processed all‐inorganic perovskite.

In the structure of perovskite solar cells, N‐type semiconductor AgBiS₂ and dimethyl sulfoxide solvent mixed polyethylene glycol are used for perovskite film treatment. Finally, the perovskite solar cells with dual‐interfacial modification exhibite a remarkable improvement of power conversion efficiency from 18.58% to 21.19%, as well as show the excellent long‐term and moisture stability.

Although the research on perovskite solar cells (PSCs) has achieved rapid progress, its efficiency and stability still need to be further improved to meet the industrial requirements. The defects located inside the cells, on the surfaces, interfaces, or grain boundaries, will primarily affect carrier transportation through the formation of nonradiative recombination centers and hinder the further enhancement of the power conversion efficiency (PCE). Herein, a straightforward and simple defect passivation method is developed to increase the PCE and stability of PSCs. In the device, the N‐type semiconductor AgBiS₂ is introduced by thermal evaporation as a modified layer between the perovskite films and electron transport layer, which can improve the charge transport characteristic and bandgap optimization of PSCs. Simultaneously, dimethyl sulfoxide (DMSO) solvent mixed polyethylene glycol (PEG) is used for solvent annealing treatment, which can further improve the quality of perovskite film and reduce the trap density by increasing grain size and enhancing the crystallinity. As a result, the PSCs with dual‐interfacial modification exhibit a remarkable improvement in PCE from 18.58% to 21.19% with exceptional long‐term and moisture stability. This work provides an innovative insight for fabricating the stable and efficient PSCs toward the industrialization.

Nonfullerene acceptors offer new opportunities for high efficiencies in organic solar cells, but the suppression of photodegradation of the materials in the presence of dioxygen is essential. The complex behavior of the reactive oxygen species superoxide and singlet oxygen in the degradation of the donor polymer poly(3‐hexylthiophene), the small molecule acceptor 5,5′‐[(9,9‐dioctyl‐9H‐fluorene‐2,7‐diyl)bis(2,1,3‐benzothiadiazole‐7,4‐diylmethylidyne)]bis[3‐ethyl‐2‐thioxo‐4‐thiazolidinone], and their blends is unraveled in detail.

Abstract

With rapid advances in material synthesis and device performance, the long‐term stability of organic solar cells has become the main remaining challenge toward commercialization. An investigation of photodegradation in blend films of the donor polymer poly(3‐hexylthiophene) (P3HT) and the rhodanine‐flanked small molecule acceptor 5,5′‐[(9,9‐dioctyl‐9H‐fluorene‐2,7‐diyl)bis(2,1,3‐benzothiadiazole‐7,4‐diylmethylidyne)]bis[3‐ethyl‐2‐thioxo‐4‐thiazolidinone] (FBR) is presented in an ambient atmosphere. The photobleaching kinetics of the pure materials and their blends is correlated with the generation of radicals and triplet excitons using optical and magnetic resonance techniques. In addition, spin‐trapping methods are employed to identify reactive oxygen species (ROS). In films of P3HT, FBR, and the P3HT:FBR blend, superoxide is generated by electron transfer to molecular oxygen. However, it is found that the generation of singlet oxygen by energy transfer from the FBR triplet state is responsible for the poor stability of FBR and for the accelerated photodegradation at later times of the P3HT:FBR blend. In the early stage of degradation of the neat blend, it is protected from singlet oxygen by the fast donor–acceptor charge transfer, which competes with triplet exciton formation. These results provide initial input for a rational design of donor and acceptor materials through tuning the molecular singlet and triplet energy levels to prevent ROS‐related photodegradation.

A narrow bandgap polymer acceptor PYF‐T with fluorinated end groups on monomer sub‐units is synthesized, showing stronger and red‐shifted absorption, lower‐lying frontier molecular orbitals, higher electron mobility, enhanced intermolecular packing, and without sacrificing photovoltage compared to its non‐fluorinated counterpart (PY‐T). When employed in all‐polymer solar cells, PYF‐T yields an outstanding efficiency of 14.10%.

Abstract

Fluorination of end groups has been a great success in developing efficient small molecule acceptors. However, this strategy has not been applied to the development of polymer acceptors. Here, a dihalogenated end group modified by fluorine and bromine atoms simultaneously, namely IC‐FBr, is first developed, then employed to construct a new polymer acceptor (named PYF‐T) for all‐polymer solar cells (all‐PSCs). In comparison with its non‐fluorinated counterpart (PY‐T), PYF‐T exhibits stronger and red‐shifted absorption spectra, stronger molecular packing and higher electron mobility. Meanwhile, the fluorination on the end groups down‐shifts the energy levels of PYF‐T, which matches better with the donor polymer PM6, leading to efficient charge transfer and small voltage loss. As a result, an all‐PSC based on PM6:PYF‐T yields a higher power conversion efficiency (PCE) of 14.1% than that of PM6:PY‐T (11.1%), which is among the highest values for all‐PSCs reported to date. This work demonstrates the effectiveness of fluorination of end‐groups in designing high‐performance polymer acceptors, which paves the way toward developing more efficient and stable all‐PSCs.

This work is the first to integrate the multimechanism recombination parameters with the density of states (DOS) distribution and effective bandgap in the framework of a semiempirical analytical model of temperature and light intensity dependent V _oc. The proposed approach is expected to be a useful tool for quantifying the full spectrum of recombination‐ and DOS‐related parameters of nonfullerene organic solar cells.

Abstract

The relationship of the temperature–light intensity dependence of open‐circuit voltage V _oc in nonfullerene‐based organic solar cells with their material characteristics and multimechanism recombination parameters is described. The systematic variation of the effective bandgap E _g,eff and the electrode layers allows the observation of different relative contributions of bimolecular, bulk, and surface trap‐assisted recombination mechanisms. The complementary advantages of the analytical model and the established voltage‐impedance spectroscopy technique provide a useful tool to quantify multimechanism recombination parameters, effective density of states N _c, and energetic disorder σ in organic solar cells under operating conditions. The validity of the proposed model to understand the temperature and light intensity dependent of V _oc is shown by applying it to four different donor:nonfullerene acceptor blend systems with conventional or inverted device architectures.

A PCE of 16.17% is achieved in the doctor‐bladed PM6:Y6‐2Cl device with CF:CB co‐solvent, which is much higher than those of CF‐ and CB‐processed devices. Of note is that the use of this co‐solvent approach in the other two high‐performance photovoltaic systems is also confirmed, demonstrating its good generality of employing in the printing organic solar cells.

Abstract

Studies of the relationship between blend microstructure and photovoltaic performance are becoming more common, which is a prerequisite for rationally improving device performance. Non‐fullerene acceptors usually have planar backbone conformation and strong intermolecular packing, normally resulting in excessive phase separation. Herein, an effective co‐solvent blending strategy to turn the molecular organization of a chlorinated small molecule acceptor Y6‐2Cl and phase separation of the corresponding active layer with PM6 as donor is demonstrated. The in situ photoluminescence measurements and relevant morphological characterizations illustrate that the film‐forming process is fine‐turned when using the mixtures of chloroform (CF) and chlorobenzene (CB) solvents, and the blend showed high domain purity with suitable phase‐separated networks. Thus, better exciton dissociation and charge generation, more balanced charge transport, and less recombination loss are obtained in the co‐solvent blade‐coated devices. As a result, a maximum power conversion efficiency (PCE) of 16.17% is achieved, which is much higher than those of CF‐ and CB‐bladed devices (14.08% and 11.44%, respectively). Of note is that the use of this co‐solvent approach in the other two high‐performance photovoltaic systems is also confirmed, demonstrating its good generality of employing in the printing organic solar cells.

A comprehensive review of the advances in poly(3,4‐ethylenedioxythiophene) (PEDOT) fundamentals (synthesis and doping/dedoping), properties’ tuning (transmittance, conductivity, work function, chemical reactivity), film fabrication methods to versatile photovoltaic applications (single‐junction, tandem, semitransparent, colorful, flexible and ultraflexible, fully printed solar cells) is presented.

Abstract

Poly(3,4‐ethylenedioxythiophene) (PEDOT) is a very unique polymer. It can be very conductive, highly transparent, and environmentally stable. It is highly switchable between its oxidation state and neutral state. It forms a micellar complex with polystyrene sulfonate in aqueous conditions, and then can be solution‐processible and printable. Based on these advantages, PEDOT has been widely used as conductors and transparent electrodes in electronics and optoelectronics. The device performance is highly correlated with the structure and properties of PEDOT. In this review, advances in the synthesis, optoelectronic and chemical properties are comprehensively described and analyzed, as well as the strategies for tuning these properties to fulfill the requirement for device applications. Film processing techniques (printing and transfer printing) for the conducting polymer are also presented. Then, the applications of PEDOT as conductors for versatile organic and perovskite solar cells (single‐junction, tandem, semitransparent, colorful, flexible and ultraflexible, fully printed solar cells) are summarized. Finally future study directions for PEDOT in terms of conductivity enhancement and application‐oriented formulations are discussed.

Three new dithieno[2,3‐d;2ʹ,3ʹ‐dʹ]benzo[1,2‐b;4,5‐bʹ]dithiophene based small‐molecule donors with different branching points for alkyl side chains are designed and synthesized for all small molecular organic solar cells. Modifying the branching points tunes the properties in the aggregation state, and an optimal nanofiber‐based hierarchical morphology for efficient charge separation and transport is successfully demonstrated.

Abstract

The optimization of bulk heterojunction morphology is one of the most challenging topics in all‐small‐molecule organic solar cells. Herein, three small molecular donors based on dithieno[2,3‐d;2′,3′‐d′]benzo[1,2‐b;4,5‐b′]dithiophene (DTBDT) unit by systematically moving the branching point of the alkyl chain have been designed, synthesized, and applied in organic solar cells. Modifying the branching points enables the properties of the aggregation state to be tuned, and an efficient nanofiber‐based hierarchical morphology is successfully demonstrated by combining with different nonfullerene acceptors. The molecules with far branching points can form nanofibers in active layers, and theses nanofibers help the charge separation and charge transport in a large donor‐rich or acceptor‐rich domain of approximately 100 nm. Using nonfullerrene Y6 as an acceptor, the highest power conversion efficiency of 14.78% is obtained, which is one of the highest efficiencies in all‐small‐molecule organic solar cells. The strategy of modification of alkyl side chain branching points can be a practical way to actualize crystallinity control and active layer morphology for improving the performance of all‐small‐molecule organic solar cells.

Three benzo[1,2‐b:4,5‐b']dithiophene‐thienothiophene π‐bridged N‐octylthieno[3,4‐c]pyrrole‐4,6‐dione‐based polymer donors named as PBDT‐X (X=H, F, Cl) are developed. While a planar accepting unit helps improve the crystallinity, all three photovoltaic parameters are simultaneously increased with the introduction of halogen atoms. PBDT‐Cl:Y6‐based devices yield an efficiency of 15.63%, attributed to the enhanced crystallinity, hole mobility, and domain purity.

Abstract

In this work, a new series of polymer donors consisting of thienothiophene π‐bridged N‐octylthieno[3,4‐c]pyrrole‐4,6‐dione (8ttTPD) and benzo[1,2‐b:4,5‐b']dithiophene (BDT) units for producing highly efficient organic solar cells (OSCs) paired with a Y6 acceptor is developed. The incorporation of the highly planar 8ttTPD unit enhances crystalline properties as well as hole mobilities of the BDT‐based polymers that typically have amorphous features. Further, the 2D side chains with halogen atoms (fluorine and chlorine) are designed as another handle to control the crystallinity and energy levels of the BDT‐based polymer donors: PBDT‐X (X = H, F, or Cl). Synergistic effects of incorporated 8ttTPD unit and the halogenated 2D side chain generate significantly enhanced charge transport and recombination properties of the OSCs, which is mainly attributed to optimized crystallinity and hole mobility of the polymer donors. Therefore, the PBDT‐Cl:Y6‐based OSCs exhibit the highest power conversion efficiency (PCE) of 15.63% with simultaneous improvements of open‐circuit voltage, short‐circuit current density, and fill factor, which outperforms the PCEs of PBDT‐H:Y6 (11.84%) and PBDT‐F:Y6 (14.86%).

The T2‐CNORH organic passivation layer (OPL) is used to obtain low energy loss organic photovoltaics. The T2‐CNORH‐deposited PM6:Y6 device exhibits a power conversion efficiency (PCE) of 15.5% with low non‐radiative energy loss (0.203 eV). Furthermore, the OPL improves various photoactive layer systems with a best PCE of 16.4% for the PM6:Y7 system.

Abstract

Despite the tremendous development of various high‐performing photoactive layers in organic photovoltaic (OPVs) cells, improving their performance remains the most important challenge in the field. Here, an effective and compatible strategy (i.e., the concept of vacuum deposition of an organic passivation layer (OPL) on the photoactive layer) is presented to enhance the efficiency of the state‐of‐the‐art photoactive systems, where easy‐deposition processable T2‐ORH and T2‐CNORH OPLs are used. After the deposition process, T2‐ORH forms 2D‐like edge‐on crystalline structure, and the 3D‐like face‐on crystalline growth is induced in T2‐CNORH. Resulting from its relatively higher crystalline features and increased wettability with the cathode interfacial material, the performance of T2‐CNORH‐deposited OPVs with both small and the scaled‐up areas surpass devices without OPL and with T2‐ORH. Experimental studies are conducted linking conductivity, electroluminescence quantum efficiency, carrier transport, and recombination dynamics to find the reasons for the performance difference. Furthermore, by applying the T2‐CNORH to other photoactive platforms, the efficiencies are enhanced by 4.4–9.0% relative to those of the corresponding control devices; an optimal 16.4% efficiency is achieved, which validates its great applicability for photoactive layers that will be developed in the near future.

The piezo‐phototronic effect is introduced to improve the performance of Si/ZnO heterojunction photodiodes with different Fermi‐levels in Si. It is found that when the Fermi‐level of Si moves from the bottom of the conduction band to the top of valence band, the magnitude of the performance improvement in Si/ZnO heterojunction PD is increased.

Abstract

The piezo‐phototronic effect has been extensively investigated to improve the performance of optoelectronic devices. However, the modulations in different energy band structures are quite distinctive, and adverse effects may be produced. Therefore, it is essential to investigate the modulation law in the optoelectronic devices with different energy band structures. Here, five kinds of Si/ZnO heterojunction photodiodes (PDs) with different energy band structures are fabricated and the piezo‐phototronic effect is systematically investigated on their photoresponse performance. For the p‐Si/n‐ZnO PDs, significant performance improvement is achieved by the piezo‐phototronic effect, with the magnitude of improvement increasing with doping concentration of p‐Si. For the n‐Si/n‐ZnO PDs, performance improvement is only achieved when the n‐Si is lightly doped, with a lower magnitude compared to that of the p‐Si/n‐ZnO PDs. The in‐depth working mechanism regarding to the different energy band structures is revealed. It is concluded that when the Fermi‐level of Si moves from the bottom of conduction band to the top of the valence band, the magnitude of performance improvement in Si/ZnO heterojunction PD increases. This study not only presents an in‐depth understanding regarding the piezo‐phototronic effect in Si/ZnO heterojunction PDs, but also provides guidance to optimize the piezo‐phototronic effect in optoelectronic devices.