Effective Hole Transport Layer Research Articles

Low-cost kesterite (CZTS) bilayers as an effective hole-transport layer for perovskite solar cells

This study investigates the utility of Copper Zinc Tin Sulfide (CZTS) thin films as inorganic hole-transporting layers (HTLs) in low-temperature perovskite solar cells (PSCs). The primary goal is to improve power conversion efficiencies (PCE) in PSCs using CZTS thin films fabricated through solution-based methods, with CH3NH3PbI3 perovskite and ZnO serving as the light-absorbing and electron-transporting components, respectively. Precise control over CZTS thin film growth significantly influences the formation of uniform perovskite layers. The study underscores the importance of achieving dense ZnO films with suitable grain sizes to confirm consistent coverage of perovskite films, enabling efficient charge transport while reducing recombination at the CZTS/perovskite interface. Results reveal that CZTS/CZTS bilayers exhibit superior photoelectrochemical performance compared to conventional CZTS(A)/FTO and CZTS(B)/CZTS(A) configurations. Evaluation of water splitting performance demonstrates that the CZTS/CZTS bilayer photoelectrode achieves the maximum photocurrent density, reaching 12 mA/cm2 vs. Ag/AgCl, with an Incident Photon-to-Current Efficiency of 9 % at 400 nm and an applied potential of 0.4V vs. Ag/AgCl. The study comprehensively examines CZTS impact on PSC photovoltaic properties and stability, culminating in a high-performance device with a PCE potential of up to 7 %. This underscores the pivotal role of CZTS− perovskite contacts in determining PSC photovoltaic performance and highlights CZTS potential as a versatile HTM for various PSCs designs.

Carbazole Derivative Doped Polyoxometalate Inorganic Clusters as Hole Transport Layer to Enhance the Photovoltaic Performance of Organic Solar Cells

Polyoxometalate inorganic clusters (PICs) as a kind of environmentally friendly material can potentially act as an effective hole transport layer (HTL) in organic solar cells (OSCs) due to their thermally stable, low-cost, easily soluble, and excellent redox properties. However, their low conductivity often limits the applications in high-efficiency OSCs. Here, we use a carbazole derivative (2PACz) to modify the bivalent tin (Sn2+) doped phosphomolybdic acid (H3PMo12O40) (termed HPMO-Sn) to further increase the conductivity. The resulted HPMO-Sn:2PACz HTL displays an improved hole transport ability, enhanced conductivity, increased exciton dissociation efficiency, and low nonradiative energy loss in OSCs. As a result, OSCs employing HPMO-Sn:2PACz as the HTL and PM6:Y6 as the active layer exhibit higher power conversion efficiency (17.62%) than those using HPMO-Sn (efficiency: 16.40%) or PEDOT:PSS (efficiency: 16.45%) as the HTL with enhanced open-circuit voltage and fill factor. Our results provide an effective method to elevate the conductivity of PICs and demonstrate the possibility of using the modified PICs as an effective HTL to achieve high-efficiency OSCs.

Interfacial fluorinated polymer layer enabling highly stable, fast response, high-performance self-powered PEA2PbBr4 ultraviolet photodetector

A fast-response, self-powered UV photodetector is highly desirable in health and safety applications to monitor UV exposure, especially to the human body. PEDOT: PSS is known as an effective hole transport layer often used in the fabrication of organic electronics. However, the moisture-sensitive PEDOT: PSS resulted in poor surface coverage and degradation of perovskite material, leading to lower performance and instability of the device. In this work, adding hydrophobic Nafion layer on top of PEDOT: PSS and perovskite interface dramatically increased the photocurrent and stability. The best device based on PEA2PbBr4 films with a Nafion layer exhibited a high photocurrent of 0.34 mA/cm2 and a fast response speed of 15.6 μs at 0 V bias. The responsivity and detectivity of the optimized device were calculated to be 25 mA/W and 2.54 × 1011 Jones, respectively. The improved device performance is a direct result of the enhanced surface coverage as shown by field-emission scanning electron microscope and energy dispersive x-ray spectroscopy. Furthermore, the PEA2PbBr4 films and devices based on Nafion coated PEDOT: PSS showed a better UV light stability than those without the Nafion layer due tothe hydrophobic nature of the Nafion layer. As a result of this simple technique, high-performance PEA2PbBr4 photodetector can be fabricated.

Graphene oxide modified PEDOT:PSS as an efficient hole transport layer for enhanced performance of hybrid silicon solar cells

In recent years, PEDOT:PSS/n-Silicon (n-Si) based hybrid solar cells (HSCs) have drawn a significant attention as promising alternative to the conventional Si solar cell technology. However, poor electrical performance of the PEDOT:PSS limits the power conversion efficiency (PCE) of the device. Herein, tailoring the properties of the PEDOT:PSS via graphene oxide (GO) is investigated to use PEDOT:PSS-GO as an efficient hole transport layer (HTL) for the enhanced performance of the PEDOT:PSS/n-Si HSCs. A small amount of GO incorporation resulted in significantly improved structural, electrical, Si surface passivation properties of the PEDOT:PSS and hence the photovoltaic performance of the PEDOT:PSS-GO/n-Si HSCs. The study reveals that the oxygen functional groups on 2D-GO sheets interact with PEDOT:PSS and facilitates the improved electrical conductivity by >2 folds without affecting its optical properties. Further, the effective passivation of n-Si surface by the composite film has been found with enhanced minority carrier lifetime (∼1.7 folds). The PEDOT:PSS-GO/n-Si HSCs could achieve the highest PCE of 11.22% on device dimension of 1 cm2, which is absolute ∼4.14% higher with respect to that with the pristine PEDOT:PSS. The improved performance is attributed to the effective charge transport properties of the PEDOT:PSS-GO, interface passivation and thus efficient separation and collection of the photo-carriers for an optimum GO addition. The study establishes that PEDOT:PSS-GO composite could be used as an effective HTL for the efficient PEDOT:PSS/n-Si HSCs.

A transparent self-healable multistimuli-responsive novel supramolecular Co(II)-metallogel derived from adipic acid: Effective hole transport layer for polymer solar cells

A fascinating approach for the instantaneous synthesis of adipic acid as a low molecular weight gelator and cobalt(II)-ion based metallogel through one-shot mixing in N,N-dimethyl formamide medium has been attained under ambient condition and at room temperature. The rheological study was applied to investigate the mechanical property of the synthesized Co(II)-metallogel. Through thixotropic experiments, the self-healing ability of the synthesised cobalt metallogel is established. The layered structure of the metallogel was envisaged through FESEM and the present elements were characterized through EDX mapping investigations. The potential metallogel formation approach has been examined using FT-IR, and the Powder X-ray diffraction pattern verifies the crystallinity of the Co(II)-metallogel's xerogel form. The absorption characteristics of this gel showed transparent in the visible to near infrared region of solar spectrum. The semiconducting property of the metallogel has been further confirmed and verified and with electronic device characterization and charge career mobility evaluation and found that this gel exhibits quite high hole mobility. We have used this as hole transport layer for polymer solar cells using PM6:Y6 as active layer and compared with the MoO3 as hole transport layer. The power conversion efficiency for Co(II)-metallogel (Co-AD) as hole transport layer 13.15 % which is higher than that for MoO3 counterpart (12.17 %). The higher power conversion efficiency for Co(II)-metallogel is associated with enhanced value of fill factor owing to the more efficient charge transport and suppressed charge recombination.

10.3% Efficient Green Cd-Free Cu2 ZnSnS4 Solar Cells Enabled by Liquid-Phase Promoted Grain Growth.

Small grain size and near-horizontal grain boundaries are known to be detrimental to the carrier collection efficiency and device performance of pure-sulfide Cu2 ZnSnS4 (CZTS) solar cells. However, forming large grains spanning the absorber layer while maintaining high electronic quality is challenging particularly for pure sulfide CZTS. Herein, a liquid-phase-assisted grain growth (LGG) model that enables the formation of large grains spanning across the CZTS absorber without compromising the electronic quality is demonstrated. By introducing a Ge-alloyed CZTS nanoparticle layer at the bottom of the sputtered precursor, a Cu-rich and Sn-rich liquid phase forms at the high temperature sulfurization stage, which can effectively remove the detrimental near-horizontal grain boundaries and promote grain growth, thus greatly improving the carrier collection efficiency and reducing nonradiative recombination. The remaining liquid phase layer at the rear interface shows a high work function, acting as an effective hole transport layer. The modified morphology greatly increases the short-circuit current density and fill factor, enabling 10.3% efficient green Cd-free CZTS devices. This work unlocks a grain growth mechanism, advancing the morphology control of sulfide-based kesterite solar cells.

Na2S decorated NiOx as effective hole transport layer for inverted planar perovskite solar cells

NiOx have been considered as an excellent material for hole transport layer (HTL) of perovskite solar cells because of its wide band gap, excellent optical transmittance, proper chemical stability and energy level matching with perovskite materials. However, the power conversion efficiency (PCE) of inverted perovskite solar cells (PSCs) is hindered by the low conductivity of unmodified NiOx. Here, we reported an operative Na2S treated NiOx HTL, which could optimize the conductivity of the NiOx layers and the crystallinity of perovskite. The synergistic effect of Na cations and S anions could strengthen the conductivity of the NiOx HTL, improve the interface contact between NiOx and perovskite, and enhance the quality of the perovskite film, so as to boost photoelectricity performance of the device. The carrier dynamics have shown that doping of Na2S could not only diminish the interface defects of the HTL/perovskite layer, but also improve the hole extraction at the NiOx/perovskite interface. All these factors led to the improvements in short-circuit density (Jsc), fill factor (FF) and PCE of inverted planar perovskite solar cells. Consequently, Na2S-decorated NiOx HTL contributes to a stable device which attains a champion efficiency of 16.9%.

High‐Performance Inkjet‐Printed Blue QLED Enabled by Crosslinked and Intertwined Hole Transport Layer

AbstractA novel crosslinkable 4,4′‐(9,9‐dimethyl‐9H‐fluorene‐2,7‐diyl)bis(N‐phenyl‐N‐(4‐vinylphenyl)aniline) (FLTA‐V) based on TFB repeat unit is designed and synthesized. A blended film with optimized 2:1 weight ratio of TFB and FLTA‐V is employed to act as the effective hole transport layer (HTL) for quantum dot light‐emitting diode (QLED). The resulted HTL is crosslinked and exhibits excellent solvent‐resistant property without any initiators. It also forms a robust network structure and the TFB molecules are intertwined and imprisoned in the network. The HTL then becomes denser, which enhances both intermolecular interactions and π‐π stacking to further promote efficient charge transportation. Moreover, the closeness of highest occupied molecular orbital levels between TFB (−5.4 eV) and FLTA‐V (−5.6 eV) is beneficial for hole injection from HTL to QD layer. The increased surface energy of blended HTL ensures better electrical contact between HTL and QDs, which reduces the leakage current of the device. The combination of these favorable factors has led to the fabricated blue QLED showing a remarkable enhancement of external quantum efficiency (EQE) from 7.23% for the control TFB‐based device to 10.20% for blended HTL‐based device. In addition, a maximum of 6.82% of EQE for inkjet‐printed blue QLED has been achieved, which is the best‐reported high‐efficiency inkjet‐printed blue QLED to date.

A novel strategy for enhancing photoelectrochemical performance of Ca2Fe2O5 photocathodes: An integrated experimental and DFT-based approach

Using a wet chemical approach, high-density Ca2Fe2O5 (CFO) nanoflakes were successfully synthesized on a NiO/FTO layer that is an effective hole transport layer (HTL). We found that a two-step annealing strategy could further increase the photoelectrochemical (PEC) efficiency of CFO photocathodes. It was elucidated that two-step annealing was beneficial in various aspects such as morphology, chemical state, optical properties, and impedance. In addition, the NiO HTL further improved the charge transport properties. The CFO on NiO HTL fabricated via two-step annealing demonstrates a photocurrent density of up to −313.7 µA/cm2, which is 76 % higher than that of the HTL-free CFO fabricated via single-step annealing. Through DFT calculations, it is revealed that the increase in the concentration of oxygen vacancies at the CFO surface, which was obtained by the two-step annealing process, is beneficial for lowering the relative energy throughout the hydrogen evolution reaction.

Interfacial engineering with NiOx nanofibers as hole transport layer for carbon-based perovskite solar cells

Although perovskite solar cells (PSCs) have made revolutionary progress in terms of power conversion efficiency (PCE), to achieve long-term stability and low-cost device manufacturing for commercialization of the devices, selection of proper hole transport layer (HTL) and affordable back contact are still crucial to realize the upscale manufacturing. However, the carbon-based perovskite still faces great challenges to further improve the device performance due to the low quality of the carbon/perovskite interface. Inorganic NiOx is a superior HTL candidate due to its favorable energy band alignment, superior chemical stability, high hole mobility, and low-cost manufacturing. To address the poor interface quality of the carbon-based PSCs, we report electrospun NiOx fibers as an effective HTL, resulting in highly efficient device performance of PCEs up to 13.73% when deployed in our carbon-based PSCs. NiOx has been introduced as an interfacial layer between the perovskite and the carbon counter electrode to study its impact on interfacial modification and device performance.

The effect of MoO3 interlayer on electro-physical characteristics of the perovskite solar cells

This report shows that sequentially deposited CuPc and MoOx films are an effective hole-transport layer (HTL) for perovskite solar cells (PSCs). PSCs with only CuPc hole-transport layer have a low power conversion efficiency (PCE) of 3.51%, which mostly due to a poor fill factor (FF) of 29.4%. The incorporation of the MoOx interlayer between the CuPc layer and the anode (Ag) significantly boosted device FF and PCE up to 63.2% and 8.12%, respectively. The analysis of the impedance spectra of PSCs indicates that the MoOx interlayer reduces HTL resistance and increases the recombination resistance at the perovskite/HTL interface, which increases an effective charge carrier lifetime. Moreover, MoOx electric transport properties depends on the stoichiometry of the crystal structure, which can be varied by tuning vacuum level during the thermal sputtering of MoOx layers. PSCs based on the MoOx layer deposited at low vacuum (10−2 Pa) have reached PCE of 8.12% owing to the optimization of the interlayer energy levels and electric transport properties of MoOx films. In this work, we have developed the double hole-transport layer (HTL) for PSCs, which can prevent perovskite degradation on perovskite/HTL interface and hinder a severe charge recombination.

CZTS nanoparticles as an effective hole-transport layer for Sb2Se3 thin-film solar cells

Sb2Se3 is an emerging and promising thin-film photovoltaic absorber material. However, for commonly CdS/Sb2Se3 heterojunction device structure, low carrier concentration and back contact barrier will prevent the further improvement of device efficiency. Here we have successfully applied Cu2ZnSnS4 (CZTS) nanoparticles (NPs) as a hole-transport layer (HTL) to construct a low-cost n-i-p Sb2Se3 solar cell with FTO/CdS/Sb2Se3/CZTS/Carbon configuration. We found that CZTS HTL can effectively suppress the back interface carrier recombination and improve carrier collection efficiency in the Sb2Se3 solar cells. The average Jsc, FF, and efficiency of Sb2Se3 solar cells with an optimum CZTS HTL are increased by 10.64%, 7.91%, and 17.14%, respectively, compared with Sb2Se3 solar cells without a HTL. Consequently, with CZTS NPs as HTL and carbon as electrode, the n-i-p Sb2Se3 solar cells have achieved a champion efficiency of 6.06%.

Development of a new family of a hole transport layer based on Pm1−xZnxTe QDs for flexible fluorine derivative solar cell

Fluorine derivative solar cells (FDSCs) are considered a promising candidate for next-generation photovoltaics because they hold promise for the realization of lightweight and flexible devices that can be fabricated by room-temperature solution approach. Herein, a new family of hole transport materials with high LUMO and HOMO energy levels was prepared. A series of Pm1−xZnxTe QDs were synthesized via N-naphthyl-N-phenylaminobiphenyl assisted colloidal approach and used as an effective hole transport layer of fluorine derivative solar cells. The use of Pm1−xZnxTe QDs brings unprecedentedly high voltage of 0.78 V, which is over 25% higher than that of 0.6 V for devices based on NiO and PEDOT:PSS. The device shows a remarkable power conversion efficiency of 7.8%. The enhanced photovoltaic performance by using the Pm1−xZnxTe QDs as HTL into FDSCs should be ascribed to the fast dissociation of the photoinduced excitons, which significantly reduces interface carriers recombination. This novel HTL may pave the way for the realization of lightweight and mechanically flexible solar cell devices with high power conversion efficiency.

Solvent mixture formulation for orthogonal inkjet processing and uniform pixel patterning of quantum dot light-emitting diode

Quantum dot light-emitting diode (QLED) can be directly patterned for pixel array by solution processing, while the limitation of resolution and device stability exists unless a proper solvent system and multilayer design are established. With a small-sized nozzle inkjet printing process, solvent formulation effects on the morphology and precision of printing for the quantum dot layer were investigated. Drop formation as well as ink spreading and wetting behavior on the non-crosslinked organic hole-transporting polymer were optimized with a non-erosive solvent formulation composed of octane and cyclohexane. While the ink composition formed low-contact angle droplet on the surface and was not completely ideal for the defectless pixelating process of high-resolution inkjet, its controllability of wetting at a small-scale nozzle, uniform drying, and orthogonal characteristics with an effective hole transport layer resulted in a high-performance green inkjet QLED with 6.13 cd/A at 19,550 nits highly bright region. This amounted to about 50% of efficiency performance compared to an identical device prepared with spin-coating, but one of the best green inkjet QLED properties reported with less than 120-μm sub-pixel pitch.

Solution-Processed All-inorganic Planar Heterojunction Solar Cells by Employing In Situ Grown Interfacial Layer with Dual Functions: Complementary Absorption and Selective Extraction of Photogenerated Holes.

Photovoltaic conversion of renewable solar energy into electricity for sustainable energy production requires efficient, stable, and low-cost solar cells. Developing solution-processed all-inorganic solar cells is a practical scenario in virtue of the high charge mobility and good stability of inorganic semiconductors. Here, for the first time, we present a solution-processed all-inorganic planar heterojunction solar cell based on the nanoparticle film of copper indium sulfide (CuInS2) by using an antimony trisulfide (Sb2S3) nanoparticle film as an interfacial layer between the CuInS2 photon-harvesting layer and cathode. All of the component layers in the solar cell are in a superstrate architecture and sequentially in situ grown on a transparent conducting glass acting as anode by solution-processing methods. The dependences of device performance on the thickness of Sb2S3 film and the reduction of hole-trapping centers in the Sb2S3 film by thioacetamide treatment are investigated. The optimized all-inorganic device exhibits the best power conversion efficiency of 4.85% under AM 1.5G illumination and an excellent thermal stability. It is found that the Sb2S3 interfacial layer sandwiched between the CuInS2 photon-harvesting layer and counter-electrode has dual functions, that is, to provide complementary absorption after CuInS2 attenuation and to act as an effective hole-transporting layer to selectively extract photogenerated holes for effective charge collection efficiency.

Comparative study on the effect of annealing temperature on sol–gel-derived nickel oxide thin film as hole transport layers for inverted perovskite solar cells

Sol–gel-processed nickel oxide (NiOx) is a promising hole transport layer (HTL) for inverted perovskite solar cells (PSCs) because of its excellent carrier transport properties, low costs, and high stability. Annealing temperature plays a vital role in the sol–gel process as different temperatures can alter the physicochemical properties of the materials. In the present study, we investigated the effect of annealing temperature on NiOx thin films prepared through sol–gel processing from predominantly used precursors: (1) nickel acetate tetrahydrate (Ni-ac) and (2) nickel acetylacetonate (Ni-acac). X-ray diffraction and X-ray photoelectron spectroscopy results revealed that the crystallinity and stoichiometry of both NiOx thin films improve with an increase in annealing temperature, indicating the formation of a standard Ni–O octahedral in a cubic rock NiO structure. Higher annealing temperatures have been shown to improve film optical transparency and electrical conductivity due to the complete decomposition of precursors and improved crystallinity. The device characterization has shown both films can work as an effective HTL for PSCs and showed the best device performance for films annealed at 400 °C. It is believed that this study can provide a basic understanding and strategy for developing highly efficient NiOx HTL in PSCs.

Black phosphorus doped Poly(triarylamine) as hole transport layer for highly efficient perovskite solar cells

Charge transport layer plays a critical role in high-performance perovskite solar cells (PSCs). Herein, few-layered 2D black phosphorus (BP) nanosheet doped poly(triarylamine) (PTAA) is employed as hole transport layer for PSCs. The BP:PTAA significantly improves charge extraction at perovskite/BP:PTAA interface together with the smaller energy barrier, the increased conductivity of the PTAA film, and the formation of the high-quality perovskite film with enlarged crystal gain size, which suppress the interfacial charge recombination and trap-assisted recombination. As a result, the champion device using BP:PTAA produces the higher power conversion efficiency of 20.49% than the control device of 18.26%. Moreover, the remarkable improvement in device stability has been demonstrated attributed to the more hydrophobicity of BP:PTAA and the perovskite layer with less defect states. This work provides an effective hole transport layer for PSCs, which is comparable with the commonly used 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ):PTAA. • The BP:PTAA improves charge extraction at perovskite interface together with the smaller energy barrier in photovoltaics. • Both the interfacial recombination and trap-assisted recombination have been effectively suppressed. • The improved device efficiency of 20.49% has been achieved compared to the control device of 18.26%.

Substrate-dependent Growth of CH3NH3PbI3 Films Deposited by Vacuum Evaporation

CH3NH3PbI3 (MAPbI3) films were prepared by dual-source vacuum evaporation on Au, poly(3,4-ethylenedioxythiophene) (PEDOT), and indium-tin-oxide(ITO) with the similar deposition parameters. The films were characterized with X-ray diffraction (XRD), steady-state photoluminescence (PL) and Raman spectroscopy. The interface electronic structures of co-evaporated MAPbI3 films on different substrates were studied with ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS). The research indicates that Au is more suitable for the film growth than the other substrates, especially when the film is very thin. The poor adsorption of the precursors may make it difficult to form MAPbI3 thin film on ITO. Furthermore, it is found that the charge transfer efficiency at the interface between PEDOT and MAPbI3 is relatively high, which indicates that PEDOT can act as an effective hole transport layer for MAPbI3-based devices.

Single Step Blending of PEDOT:PSS/SPGO Nanocomposite via Low Temperature Solid Phase Addition of Graphene Oxide for Effective Hole Transport Layer in Organic Solar Cells.

Herein, we report the modification of PEDOT:PSS by in-situ direct addition of graphene oxide powder processed by spray dryer (SPGO) for the enhancement in the performance of organic solar cell. The preparation of PEDOT:PSS/SPGO composite was done by direct incorporation of graphene oxide powder at lower temperature i.e., below 5 °C. Raman spectroscopy of the prepared PEDOT:PSS/SPGO nanocomposites at low temperature suggested that low temperature plays a vital role to improve the ability of these composite as hole transport layer by improving adhesive properties of the composite. Atomic force microscopy (AFM) analysis suggested that the adhesive ability of these composite decreased surface roughness and thus providing smoother path for the hole transportation. After the successful synthesis of PEDOT:PSS/SPGO nanocomposites, ITO/PEDOT:PSS/SPGO/PTB7:PC71BM/Al based organic solar cell was fabricated. The J-V curves under AM 1.5G illumination (100 mW/cm²) of the PTB7:PC71BM based OSCs using PEDOT:PSS/SPGO as a HTL exhibit Voc = 0.67 V, Jsc = 17.3 mA, FF = 41.5%, PCE = 4.82%, and device with PEDOT:PSS as HTL exhibit Voc = 0.68 V, Jsc = 16.0 mA/cm², FF = 38.7% and PCE = 4.04%. The enhance PCE in case of PEDOT:PSS/SPGO based devices depicted that the direct inclusion of graphene oxide in PEDOT:PSS increased the PCE almost 16%, which arises due the high conductivity and stable pi-pi stacking of the spray dryer processed graphene sheets with PEDOT:PSS which ease the charge carrier mobility, thus providing feasible path for charge transportation.

High-performing self-driven ultraviolet photodetector by TiO2/Co3O4 photovoltaics

Ultraviolet (UV) may provide the essential to maintain the human body and metabolism, however, excessive exposure to UV light is absolutely harmful. We may take advantage of the strong UV energy for energy conversion devices. A stable, simple and scalable heterojunction (TiO2/Co3O4) thin film was developed for UV-absorbing and visible-blind photovoltaic. Effective hole transport layer (HTL, NiO) insertion established photovoltaics, having a power conversion efficiency of 15.8%. The heterojunction exhibits the improved carrier lifetime from 1.94 ms to 9 ms, resulting in the enhancement of both photovoltage and photocurrent from 0.17 V to 0.78 V and from 0.54 mA cm−2 to 13.5 mA cm−2, respectively. The TiO2/Co3O4 device exhibits a photoresponse to a UV light intensity of 60 μW cm−2 in the presence of sunlight and works stable for over a year. The results indicate metal oxide heterojunction (TiO2/Co3O4) photovoltaic device could be useful for monitoring UV radiation exposure and cost-effective self-powered UV optoelectronics. The functional utilization of metal oxide layers would provide strong benefits for photoelectric devices, such as photodetectors and transparent solar cells.