ZnO Nanowire Array Solar Cells by Pyro-phototronic

Enhanced P3HT/ZnO Nanowire Array Solar Cells by Pyro-phototronic Effect Kewei Zhang,† Zhong Lin Wang,†,‡ and Ya Yang*,† †

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences; National Center for Nanoscience and Technology (NCNST), Beijing 100083, People’s Republic of China ‡ School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States S Supporting Information *

ABSTRACT: The pyro-phototronic effect is based on the coupling among photoexcitation, pyroelectricity, and semiconductor charge transport in pyroelectric materials, which can be utilized to modulate photoexcited carriers to enhance the output performance of solar cells. Herein, we have demonstrated the largely enhanced output performance of a P3HT/ZnO nanowire array photovoltaic cell (PVC) by using the pyrophototronic effect under weak light illuminations. By applying an external cooling temperature variation, the output current and voltage of the PVC can be dramatically enhanced by 18% and 152% under indoor light illumination, respectively. This study realizes the performance enhancement of pyroelectric semiconductor materials-based solar cells via a temperature-variation-induced pyro-phototronic effect, which may have potential applications in solar energy scavenging and self-powered sensor systems. KEYWORDS: ZnO, pyroelectricity, optoelectronic process, pyro-phototronics, heterojunction

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In this work, we utilize the temperature-variation-induced pyro-phototronic effect in ZnO to modulate the optoelectronic process in the P3HT/ZnO nanowire array heterojunction PVC for realizing performance enhancement, which decreases with increasing the illumination intensity. Under indoor light illumination, the output current and voltage of the PVC can be enhanced by 18% and 152% by applying an external cooling temperature variation, respectively. The pyroelectric-enhanced photovoltaic performance mainly arises from two aspects: the inner pyroelectric nanogenerator (PENG) and the PENGinduced band bending at the heterojunction. The demonstrated pyro-phototronic effect has been utilized to tune the performance of the P3HT/ZnO nanowire array PVC when the devices are subjected to temperature variations, which offers an opportunity for enhancing the output performance of optoelectronic devices, such as solar cells, light-emitting diodes, and photon detectors.

nder the growing global need for energy, scavenging energy from various forms of solar energy is attracting substantial interest due to its reliable, abundant, and sustainable nature, including solar photovoltaic cells (PVCs), solar heat, and solar thermal electricity.1−5 To achieve highly efficient energy conversion, great efforts and desirable achievements have been made on the optimization of solar cell configurations as well as the design of advanced materials toward interfacial charge transfer.6−11 However, each research field as stated above is focused on only one kind of solar energy, and very limited research has been conducted on the combination of solar PVC and solar heat.12,13 If the PVC effect and heat effect induced by one source are coupled together, it may largely increase the overall power output. The piezo-phototronic effect is well known as a three-way coupling among piezoelectricity, photoexcitation, and semiconductor characteristics (as depicted in Figure 1a),14 which promotes the ability to modulate the energy band levels and optoelectronic process at the local heterojunction by a strain-induced piezopotential.15−18 The pyro-potential created upon temperature variation can also be theoretically utilized to control the generation, transport, separation, and/or recombination of photoinduced charge carriers. Although the pyroelectric effect has been found in some semiconductor materials,19−21 there has been no report about the pyro-phototronic effect by coupling of pyroelectricity, photoexcitation, and semiconductor characteristics (as depicted in Figure 1b). © 2016 American Chemical Society

RESULTS AND DISCUSSION The solar cell consists of an indium tin oxide (ITO) electrode, a ZnO nanowire array, poly(3-hexylthiophene) (P3HT) film, poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) film, and a Ag nanowire electrode, as illustrated in Figure 1c. Figure 2a presents an optical image of a Received: September 7, 2016 Accepted: October 29, 2016 Published: October 30, 2016 10331

DOI: 10.1021/acsnano.6b06049 ACS Nano 2016, 10, 10331−10338

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Figure 1. Design of a heterojunction solar cell based on pyro-phototronics. (a) Schematic diagram of the piezo-phototronics. (b) Schematic diagram of the pyro-phototronics. (c) Illustration of a P3HT/ZnO nanowire array solar cell demonstrating the pyro-phototronic effect.

After diffusing to the donor/acceptor interface, the excitons dissociate by charge transfer, and then the holes and electrons are transported toward their respective electrode, resulting in a photocurrent. The current density−voltage (J−V) characteristics of the solar cell device were measured in air under simulated AM1.5 sunlight of 100 mW·cm−2 with the Ag side facing the light source and the ITO glass side touching a cooling module. The device shows an open-circuit voltage (Voc) of ∼380 mV, a short-circuit current density (Jsc) of ∼0.86 mA· cm−2, a fill factor (FF) of 33%, and a power conversion efficiency (PCE) of 0.11%, as depicted in Figure 2f. The demonstrated values are comparable to previously published results of P3HT/ZnO solar cells.25−27 In need of special note is that the output of a PENG is much lower than that of a solar cell if full sunlight is applied; thus the solar cell output is dominated to a level comparable to that of the PENG by using very weak indoor light. When measured under LED illumination of 388 Lux, the solar cell device shows a Voc of ∼40 mV and a Jsc of ∼0.1 μA·cm−2 (inset of Figure 2f). As is well known, pyroelectricity is defined as the temperature dependence of the spontaneous polarization in certain anisotropic solids.21 ZnO in a hexagonal wurtzite lattice lacking a center of symmetry shows piezoelectricity as well as

representative solar cell device that has an active area of approximately 1 cm × 1 cm. The cross-sectional scanning electron microscopy (SEM) image in Figure 2b confirms the device architecture, showing the ITO glass densely covered by a layer of ZnO seeds with a thickness of ∼100 nm, followed by a layer of ZnO nanowire array with a thickness of nearly 3 μm, then thin films of P3HT and PEDOT:PSS, and finally a layer of Ag nanowire electrode with a thickness of ∼100 nm. Note that the boundary between compact layers of P3HT and PEDOT:PSS is not resolved in the SEM image. From a topview SEM image of the device (Figure 2c), it can be seen that dense and cross-linked Ag nanowire networks are observed on the surface, indicating the high conductivity of the top electrode. The normalized UV−vis absorbance of the device with light illumination from the Ag nanowires’ side is slightly smaller than that from the ITO side, indicating good transmittance and uniformity of the Ag nanowire film (Supporting Figure S1). Figure 2d illustrates a vertical SEM view of the as-grown ZnO nanowires, revealing the diameters of ZnO nanowires are in the range of 100−150 nm. Figure 2e presents the energy band diagram of the device based on previously reported energy levels.22−24 Following illumination, the P3HT (donor) absorbs photons and generates excitons. 10332


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Figure 2. Device architecture and J−V characteristic for the P3HT/ZnO solar cell. (a) Optical image of a fabricated device. (b) Crosssectional SEM view of a planar solar cell exhibiting well-separated layers. (c) Top-view SEM image of a solar cell showing cross-linked Ag nanowire networks. Inset shows a high-magnification SEM image of the Ag nanowires. (d) Vertical SEM view of the as-grown ZnO nanowire array on ITO glass. (e) Energy band diagram of the vertical heterostructure of ITO/ZnO/P3HT/PEDOT:PSS/Ag. (f) Representative J−V curve for the solar cell under 1-sun illumination. Inset shows the J−V curves on a semilogarithmic scale under LED illumination and in the dark.

pyroelectric effects.19 To demonstrate the pyroelectric ability of the P3HT/ZnO device, the temperature of the device was periodically adjusted by a thermoelectric-based cooling module. Figure 3a−d display the periodic changes of temperature and corresponding differential curves with time in different operating conditions. The temperature is varied from 21 °C (room temperature) to 6 or 52 °C with a peak value of the temperature changing rate (dT/dt) of about 1.2 °C/s (−1.2 °C/s) or 1.5 °C/s (−1.8 °C/s). The measured open-circuit voltage and short-circuit current under the different changes in temperature are correspondingly shown in Figure 3e−h. When the temperature is decreased, positive current/voltage signals are observed, while negative signals are received on increasing

the temperature. Under a variation period of 120 s, the obtained current and voltage peaks are about 15 (−22) nA and 2.1 (−3.4) mV (Figure 3e,f), respectively, for the cooling condition, while smaller values of 11 (−9) nA and 0.8 (−1.4) mV (Figure 3g,h) were obtained for the heating condition; even the temperature variation (both the amount and the rate) is more obvious. After reversely connecting the device to the measurement system, an opposite signal is observed (Supporting Figure S2), confirming that the measured signal is generated by the PENG. Besides, it can be clearly seen that the variation of voltage/current output for the PENG can be accelerated by shortening the period of temperature variation, e.g., from 120 s (Figure 3e,g) to 10 s (Figure 3f,h). By using the 10333


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Figure 3. Pyroelectric characteristics of the P3HT/ZnO solar cell. (a−d) The periodic changes of temperature and corresponding differential curves in cooling and heating conditions. (e−h) Measured currents and voltages of the PENG under the conditions in (a)−(d).

active area (A) of the device in Figure 2a, the pyroelectric coefficient (p) of ZnO in our P3HT/ZnO system can be calculated from the equation I = pA(dT/dt). The estimated value is about 6−18 nC/cm2·K, which is much larger than that of reported ZnO material.20,21,28 This is possibly because the electron transport in the vertical-aligned structure is faster than that through a random polycrystalline network. Herein, the anisotropic polarization of ZnO upon light illumination is applied to drive the electron flow at the interface of the P3HT/ZnO nanowire array. Figure 4a shows that the photocurrent of the device under LED illumination exhibits a linear relationship with increasing light intensity. The corresponding light spectra are displayed in Supporting Figure S3. The temperature was also cooled periodically from room temperature to 6 °C with a peak changing rate of about ±1.2 °C/s, as clarified in Figure 4b. It can be seen from Figure 4c,d that no current/voltage signals from the device are observed under dark conditions and in the absence of temperature variation. Upon LED illumination with 30 lx (77 lx), the current/voltage of the device rapidly increases to a stable value

of 30 nA/29 mV (100 nA/44 mV) owing to the photovoltaic property of the solar cell. When a periodic change of temperature is applied to the device at intervals of 120 s, the current/voltage increases up to a maximum value of 41 nA/42 mV (110 nA/58 mV) with periodic signals at the same intervals. As the temperature variation is stopped and the LED illumination is turned off, the current/voltage signals rapidly disappear. This result indicates that the enhancement of current/voltage signals is attributed to the pyroelectric effect. For the LED illumination with 77 lx, the output current signals are enhanced by about 10% with a sharp rising edge, where about 60% of the total enhancement is due to the inner pyroelectric nanogenerator (Supporting Figure S4). To further investigate the light intensity dependence of the pyrophototronic effect, the output current and voltage of the solar cell device were measured under a series of LED illumination with different light intensity. These results are summarized in Figure 4e,f, which demonstrates the inconsistent characteristics of the current/voltage change for the PVC part and the PENG part. On increasing the light intensity from 77 lx to 2340 lx, the 10334


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Figure 4. Pyroelectric-enhanced photovoltaic properties of the P3HT/ZnO solar cell under LED illumination. (a) Photocurrent of LED illumination with different light intensity. (b) Periodic changes of cooling temperatures and corresponding differential curves. Short-circuit current curves (c) and open-circuit voltage curves (d) of the pyroelectric-enhanced solar cell when it was subjected to periodic temperature variations in (b). Output current values (e) and voltage values (f) of the PVC part and the PENG part in the device and the corresponding enhanced ratios of output currents (g) and voltages (h) by the pyroelectric effect.

output current increases from 100 nA to 2388 nA for the PVC part and from 11 nA to 57 nA for the PENG part (Figure 4e). Differently, the output voltage increases from 42 mV to 152 mV for the PVC part, while slightly decreases from 23 mV to 20 mV for the PENG part (Figure 4f). Although the enhanced ratio decreases on increasing the light intensity, both the current and voltage are enhanced under each light intensity after applying periodic temperature variation (Figure 4g,h, Supporting Figure S5). These results clearly indicate that pyroelectricity has a positive effect on the photovoltaic properties of the P3HT/ZnO nanowire array solar cell. Moreover, it is noted that the enhanced ratio of the voltage (from 13% to 55%) is larger than that of the current (from 2%

to 10%) under the measured light intensities, suggesting this phenomenon has a bright future in practical applications. We have also demonstrated the pyroelectric-enhanced phenomenon under indoor light illumination, which is a normal indoor light source. The corresponding light spectra are displayed in Supporting Figure S3b. To achieve a cooling or heating equilibrium, the applied change of temperature is maintained at an extended period of 10 min, as depicted in Figure 5a. The nature of the output in Figure 5b,c illustrates a similar trend to that diagrammed in Figure 3 and Figure 4. The output current signals of a pyroelectric-enhanced solar cell can be considered as a hybrid of a PENG and a PVC, where the PENG exhibits ac characteristics and the PVC exhibits dc characteristics. Upon indoor light illumination, the output 10335


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Figure 5. Working mechanism of the pyroelectric-enhanced solar cell under indoor light illumination. (a) Periodic changes of temperature and corresponding differential curves in cooling and heating conditions. Short-circuit current curves of the pyroelectric-enhanced solar cell under cooling (b) and heating (c) conditions when it was subjected to the periodic temperature variation in (a). (d−f) Schematic illustrations of the working mechanism of the pyroelectric-enhanced solar cell: (d) energy band diagram of the device in the absence of temperature variation, (e) band bending of the P3HT/ZnO heterojunction during the cooling process, and (f) band bending of the P3HT/ZnO heterojunction during the heating process. Here, both the PENG part and the PVC part are in one circuit. To distinguish the different contributions from PENG and PVC, they are marked in the different circuits.

current rapidly increases to about 105 nA, which further increases to a sharp rising value of 124 nA in the cooling process and then decreases to a sharp falling value of 83 nA in the heating process (Figure 5b). A similar trend is observed for the output voltage as shown in Supporting Figure S6. Under indoor light illumination, the output current and voltage of the device are enhanced by 18% and 152%, respectively. Figure 5c presents the heating process of the device and the corresponding output currents induced by the pyroelectric effect. All these results indicate that the pyroelectric effect has an obvious effect on the photovoltaic behavior of P3HT/ZnO at the heterojunction interface, especially under weak light intensity. Herein, both the PENG part and the PVC part are in one circuit. To distinguish the different contributions from PENG and PVC, they are marked in the different circuits, as diagrammed in Figure 5d−f. The pyroelectric-enhanced photovoltaic performance of the P3HT/ZnO nanowire array solar cell can be explained from the band bending of the P3HT/ZnO heterojunction under the influence of the inner PENG. Initially, the light illumination produces free carriers as photocurrent to transport through the external circuit from Ag to the ITO electrode owing to the photovoltaic effect (step i, Figure 5b). At this stage, no current flow occurs in the PENG circuit, since the polarization is a constant at fixed temperature (Figure 5d). Once the device is cooling, the temperature inside ZnO decreases, resulting in a distribution of polarization

charges across the ZnO nanowires with localized positive charges at the P3HT/ZnO interface (Figure 5e). On one side, the positive charges at the interface of the P3HT/ZnO heterojunction lower the level of the conduction band in ZnO, which can decrease the barrier height at the interface, resulting in an enhancement of the photovoltaic performance. On the other side, the inner pyroelectric field will drive charges in the ZnO nanowires to be redistributed, accompanied with electron flow from the ITO electrode to the Ag nanowires electrode, which is the same direction in the PVC circuit. Consequently, the output signals are enhanced by 18% with a sharp rising edge (step ii, Figure 5b). When the illumination is retained and the temperature stays constant at a new value, the output current reaches a stable plateau that is still larger than the current of PVC (step iii, Figure 5b). The slight decreasing trend is due to the current leakage. Herein, the barrier height at the interface of P3HT/ZnO is still different from the initial state due to the fact that the temperature is lower. This part of the enhancement is due to the band bending of the P3HT/ ZnO heterojunction, which promotes the photovoltaic performance (about 37% of the total enhancement). When the cooled temperature recovers to room temperature, an opposite inner pyroelectric field results in electron flow in the opposite direction, which is the opposite direction of the PVC circuit (Figure 5f). Thus, a falling peak of current is observed (step iv, Figure 5b). After the temperature variation is stopped 10336


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Electrical Measurements. A thermoelectric-based cooling module was used to change the temperature of the device, and a temperature sensor was used to record the temperature variation. The light was illuminated on the top side of the device, and the temperature variation was applied on the bottom side of the device. The output voltage and current signals of the device were measured by a Stanford Research SR560 and a Stanford Research SR570, respectively.

and the indoor light illumination is turned off, the output returns to dark current.

CONCLUSIONS In a P3HT/ZnO nanowire array heterojunction solar cell, the pyroelectricity of ZnO shows promise in controlling the generation, transport, separation, and/or recombination of photoinduced charge carriers. We have presented the largely enhanced performance of P3HT/ZnO nanowire array solar cells by using the temperature-variation-induced pyro-phototronic effect under weak light illuminations. The voltage/ current outputs of the device under heating and cooling conditions show an opposite change and can be enhanced by increasing the rate of temperature variation. Both current and voltage of the device are enhanced under measured light intensity after applying periodic temperature variation, and the enhancement decreases with increasing the light intensity and maintains a stable value. Under indoor light illumination, the output current and voltage of the PVC can be enhanced by 18% and 152% by applying an external cooling temperature variation, respectively. The pyroelectric-enhanced photovoltaic performance mainly arises from two aspects: the inner PENG and the PENG-induced band bending of the heterojunction. Results presented here indicate the presence of pyro-phototronics, which are likely to have important applications in improving the performance of optoelectronic devices, such as light-emitting diodes, solar cells, and photon detectors. Besides, by using pyro-phototronics, energy harvesting from a solar PVC and solar heat induced by one source can be coupled together, which is highly desirable and represents a trend of an all-in-one multiple energy harvesting technology.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06049. Additional characterization and electrical test results (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Author Contributions

Prof. Ya Yang conceived the idea and guided the project. Dr. Kewei Zhang and Prof. Ya Yang designed, fabricated and measured devices. Dr. Kewei Zhang, Prof. Ya Yang, and Prof. Zhong Lin Wang prepared/revised the manuscript by using the obtained experimental data. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by Beijing Natural Science Foundation (2154059), the China Postdoctoral Science Foundation (Grant No. 2015M570988), the National Natural Science Foundation of China (Grant Nos. 51472055, 61404034, 61604012), External Cooperation Program of BIC, Chinese Academy of Sciences (Grant No. 121411KYS820150028), the 2015 Annual Beijing Talents Fund (Grant No. 2015000021223ZK32), the National Key R & D Project from Minister of Science and Technology in China (Grant No. 2016YFA0202701), and the “Thousands Talents” Program for the Pioneer Researcher and His Innovation Team, China.

METHODS Synthesis of ZnO Nanowire Arrays. ZnO nanowire arrays were synthesized on ITO-coated glass (5 Ω/sq, Guluo Glass Co.) that was first cleaned thoroughly by acetone/ethanol sonication and then sputtered with a thin layer of ZnO seeds by a PVD 75 sputtering deposition system (Kurt J. Lesker Co.). Vertical nanowire arrays were grown by immersing the seeded substrates downward into the growth solutions containing 20 mM zinc chloride (ZnCl2), 20 mM hexamethylenetetramine (HMTA), and 5 vol % ammonium hydroxide (NH4OH) at 95 °C for 8 h. The arrays were then rinsed with deionized water and annealed in air at 400 °C for 30 min to remove any residual organics. A portion of each ITO glass was protected with heat-resistant adhesive tape to produce a clean area for electrical contact to ITO before sputtering and growth of ZnO. Device Fabrication and Characterization. The solar cell device was fabricated by using the heterojunction of ZnO nanowire arrays and poly(3-hexylthiophene) film, where ITO and Ag nanowire films were used as electrodes. A 15 mg/mL solution of P3HT (Hange Inc.) in chloroform was preheated to 50 °C, dripped onto the ZnO film, and spun at 2000 rpm for 60 s. The film was annealed in a vacuum oven at 120 °C for 30 min to improve the crystallinity of P3HT. A formulation of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) was prepared in a 1:2 mixture of Clevios PH1000 and 2-propanol, which was spun onto the P3HT film at 1000 rpm for 40 s, followed by 2000 rpm for 10 s, and subsequently annealed at 120 °C for 30 min. The Ag nanowires obtained by using a simple chloride-assisted solvothermal method were spun twice onto the PEDOT:PSS film at 2000 rpm for 10 s. Finally, two Ag-coated Cu wires were adhered to the ITO and Ag nanowire films with silver paste as the electrodes of the device. The current density−voltage characteristics of the device were recorded in the dark and under room light illumination using a VersaSTAT 4. Layer thicknesses were determined by cross-sectional SEM imaging using a Hitachi SU8020 at 10 kV.

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