Band Offset Engineering in ZnSnN2

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Letter

Band offset engineering in ZnSnN2-based heterojunction for low-cost solar cells Kashif Javaid, Weihua Wu, Jun Wang, Junfeng Fang, Hongliang Zhang, Junhua Gao, Fei Zhuge, Lingyan Liang, and Hongtao Cao ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00427 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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Band offset engineering in ZnSnN2-based heterojunction for low-cost solar cells

Kashif Javaid1,2,3, Weihua Wu1, Jun Wang4, Junfeng Fang1, Hongliang Zhang1, Junhua Gao1, Fei Zhuge1, Lingyan Liang*,1,5 and Hongtao Cao*,1

1

Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of

Sciences (CAS), Ningbo 315201, People’s Republic of China. 2

International School, University of Chinese Academy of Sciences, Beijing 100049, China.

3

Department of Physics, Govt. College University Faisalabad (GCUF), Allama Iqbal Road,

38000 Faisalabad, Pakistan. 4

Department of Microelectronic Science and Engineering, Faculty of Science, Ningbo University

, Ningbo 315211, China. 5

Key Laboratory of Semiconductor Materials Science, Beijing Key Laboratory of Low

Dimensional Semiconductor Materials and Devices, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, 100083, China.

Abstract A new ternary-alloy, zinc-tin nitride (ZnSnN2), is considered as one of the most promising absorber materials for photovoltaic applications due to its ideal band gap, rich ternary-chemistry, robust optical absorption, and low cost. In the present work, we demonstrate the ZnSnN2-based P-N and P-i-N heterojunctions to study the band offset engineering for the development of highefficiency inorganic solar cell. The P-i-N heterojunction is composed of p-SnO, i-Al2O3 and nZnSnN2 constituents. The inclusion of i-Al2O3 buffer layer has remarkably improved the solar cell efficiency by regulating the conduction band offset and interface energy gap. It is believed

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that our present work will offer a promising approach to manufacture ZnSnN2-based heterojunctions with better band alignment for novel photovoltaic applications.

Keywords ZnSnN2 thin films, Al2O3 buffer layer, interface energy gap, conduction band offset, band alignment engineering, heterojunction solar cell

With the rapid increasing demand for low-cost solar cells, the earth-abundant semiconductor materials possessing high absorption coefficient together with an optimum band gap are considered as a promising class of materials.1 In this framework, a plenty of binary and ternary compounds of III-V (e.g., GaN) and I-III-VI2 (e.g., CIGS) materials have been extensively investigated, in particular for thin film-based photovoltaic applications.2-3 Although these material-analogs demonstrated appealing results, the researchers are still facing some sever constraints regarding the manufacturing yield, toxicity, humidity and environmental issues for their practical application.1, 4 Thus, in view of all these circumstances, a versatile ternary alloy of zinc-tin nitride (ZnSnN2) belonging to II-IV-V2 materials is claimed to be an excellent choice for photovoltaic absorber material due to its direct and tunable band gap,5-6 which is considered as an ideal material for photovoltaic applications.7-8 The exploitation of ZnSnN2 thin film as an absorber layer is quite beneficial due to its ionic nature as compared to other conventional covalent-semiconductors, which makes it more-defect tolerance.9-10 Recently, a mathematical model proposed by Arca et al, to simulate the solar cell performance, exhibited a maximum efficiency up to 23% based on defect-free ZnSnN2-crystal.11 In our previous study, a P-N heterostructure solar cell was fabricated by pairing p-SnO with n-ZnSnN2, showing a power conversion efficiency (PCE) of ~0.37 %.12 It is believed that the low PCE of ZnSnN2-based


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heterojunction devices is mainly deprived due to poor band alignment in type-II heterojunction along with inappropriate interface (effective) energy gap.13-14 The role of conduction band offset (∆Ec) regarding efficient carrier transportation towards respective electrode and/or rapid recombination at the trapping states should be taken into consideration.15 In order to alleviate such non-idealities of P-N heterojunction, different strategies had been put forward, such as insertion of interfacial layer,15 inclusion of hole and/or electron blocking layer,16 fabrication of some regular 3-D patterning of absorbing medium,17 and insertion of ultra-thin dielectric buffer layers.14 Among all these strategies, the route of sandwiching i-buffer layer in between the p- and n-type heteropartners is rather simple, sophisticated and much effective, which can significantly reduce the space charge recombination to enable a high forward current.14, 18 In this letter, we investigated the band offset engineering for ZnSnN2-based P-N and P-i-N heterojunction solar cells via buffer layer insertion and post-device-fabrication annealing. To the best of our knowledge, it is the first time that a tri-layer ZnSnN2/Al2O3/SnO P-i-N heterojunction has been successfully fabricated with a PCE exceeding 1.5%. The present work would definitely bestow a simple and an adept approach for the future maturity of ZnSnN2-based optoelectronic device applications. Figure 1a shows the XRD patterns of the as-deposited and annealed ZnSnN2 thin films. The as-deposited film is amorphous/nanocrystalline in nature without any characteristic peaks. However, four peaks with relatively low intensity were observed in the annealed film at 2-theta of 30.81°, 32.99°, 34.90° and 54.77°, corresponding to (200), (002), (201) and (320) plane, respectively, matching well with the stimulated spectra of ZnSnN2. The annealing-induced enhancement of crystallinity is in agreement with the previous reports.

19-23

The average grain

size of ~10 nm and the inter-planer spacing of ~2.56 Å were estimated based on the (201) peak,


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as summarized in Table S1 in the supporting material. HRTEM image of annealed ZnSnN2 film is shown in Figure 1b. As seen in the inset of Figure 1b, only a few regions are crystallized after annealing with an average d-spacing of 0.253 nm corresponding to (201) plane, which is almost the same as that derived from XRD analysis. Figure 2 (a-b) shows the AFM images along with height and phase profiles of the as-deposited and annealed ZnSnN2 thin films. Generally, after annealing treatment, the augmentation of crystal grains occurs, leading to multi-faceted surfaces with larger root-mean-square (RMS) roughness.23-24 For the as-obtained sample, tiny grains were found all around the surface with a RMS roughness of 2.35 nm over a scanning area of 1µm×1µm. However, larger sized-grains are presented in the annealed sample with a RMS roughness of 7.13 nm, indicating tiny grains coalescence into large ones after annealing.19 Such temperature-dependent structural changes of ZnSnN2 thin films are compatible with the previous reports.20, 22 Figure 2 (c-d) present the SEM images of as-deposited and annealed ZnSnN2 thin films. It is noticed that the average grain size has increased after annealing, making the surface more rough, which is consistent with XRD and AFM results. The average grain size has been estimated by drawing a random straight line on the SEM image. The number of grains underlying the line is counted thereof. The average grain size is obtained by dividing the total number of grains by the actual line length. Moreover, it is double checked by Image-J software following the same procedure. The depiction of P-N and P-i-N heterojunctions is portrayed in Figure 3 (a-b). In this letter, three kinds of devices have been designed with ITO as bottom electrode and Ni/Au as top electrode in all. Device-1 is as-deposited P-N junction without annealing treatment, and device-2 is unannealed P-i-N junction-based device, and then device-3 is with P-i-N structure after


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annealing. The heterostructure device fabrication scheme is described in the supporting information in Figure S1. Figure 4a illustrates the J-V response of all fabricated devices ranging from -2 V to 2 V in the dark. The heterojunction exhibits knee voltage (Vknee) of 1.10 V, 1.06 V, and 1.00 V with a rectification ratio of ~500, ~2×103 and 5×103 corresponding to device-1, 2 and 3, respectively. The inset of Figure 4a shows the variation of the leakage currents at reverse biasing, corresponding to each device. It is reported that the reverse saturation current is primarily originated from the improper band offsets, which may be suppressed efficiently with the insertion of very thin buffer layer.6 All the presented devices demonstrate better rectifying behaviors as compared to our earlier reports.12 It should be noted that the electrical performance of annealed diode based on P-N structure had been investigated ever before

12

, so we did not

repeat to check it anymore. The inset of Figure 4b displays the extracted ideality factor of 4.3, 2.9 and 2.5, corresponding to the device 1, 2 and 3, respectively. The J-V response of device-2, containing an ultra-thin buffer layer has been improved significantly, as compared to device 1, shown in Figure 4b. It can be explained by the reduction of interface defect density and junction resistance for better quality of hetero-interface.11 For the P-i-N devices, post-annealing further assisted to mitigate the non-idealities by improving the quality of hetero-interface,12, 25 as seen that the ideality factor was modified from 2.9 to 2.5. In addition, the series resistance of device-3 is smaller than that of device-2, or even better than that of device-1 although incorporation of ultra-thin Al2O3 layer, as demonstrated in Figure 4c. All the diode parameters, corresponding to each device are summarized in Table S2 in the supporting information. Cross-sectional SEM image in Figure 4d confirms the device architecture along with thickness estimation of each layer.


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The thickness of each thin film was also double-checked via spectroscopic ellipsometry measurement. Figure 5a demonstrated the photovoltaic behavior of heterostructure devices under 1-sun A.M 1.5 (100 mW/cm2). The 3-D bar graph (inset of Figure 5a) shows the solar cell parameters corresponding to each device. The fabricated PV-devices have been frequently retested for more than 3 months with an average time span of 15 days to examine the device stability and all the devices exhibited marginal performance fluctuation during this period. The detailed information related to open circuit voltage (Voc), fill factor (FF), short-circuit current density (Jsc) and power conversion efficiency (PCE) is summarized in Table S3 in the supporting information. Under illumination, the incident light is transmitted through the p-SnO layer and get absorbed in the depletion region of n-ZnSnN2 to create electron-hole pairs (inset of Figure 5a). The transmittance spectra is illustrated in the supporting information as Figure S2. The internal electric field (i.e., built-in potential) drifts the photogenerated carriers towards their respective sides under the influence of band offsets.26 Finally, the Ohmic contact assists a lot to extract these charge carriers for electrical conduction. But, in case of the P-N junction (device-1), the PCE remained low due to undersized ∆Ec, resulting in large dark current via non-radiative recombination.13 However, the presence of buffer layer showed a progressive improvement in device 2 and 3. The incorporation of an ultra-slim buffer layer can passivate the surface of SnO on one hand, and serve as hole-blocking layer by restricting the back-flow of holes from SnO to ZnSnN2 under light illumination, in turn to increase the open circuit voltage by aiding the directional flow of photo-dissociated carriers.14, 16 The efficient light-trapping in the absorbing medium has been achieved via device architecture design by controlling the thickness of each layer to enhance the minority carrier diffusion length as well as through post-fabrication annealing to improve the


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crystallinity of the absorbing layer. The present study shows a PCE efficiency of 1.54% for ZnSnN2-based P-i-N solar cell, which is more or less 4-times higher than that of previously reported P-N junction solar cell.12 The capacitance-voltage (C-V) measurements are performed to determine the built-in voltage (qVbi), as displayed in Figure 5b. The extracted qVbi of ~ 0.58 eV is rather higher than the previously reported ZnSnN2-based P-N junction device.15 As we know that the built-in-voltage could drastically affect the photo-generated carrier dynamics.27 The band offset character along with the effective energy gap (Eg,IF) for P-N heterojunction (device-1) is epitomized in Figure 5b (inset) in the dark. In accordance with known and measured parameters, it is revealed that ‫∆׀‬Ev‫∆׀ > ׀‬Ec‫׀‬, indicating a higher potential barrier for hole injection from pSnO to n-ZnSnN2 in the dark. Therefore, holes could not easily be injected to n-type layer that may possibly lead to tunneling-effect, in which the minority electrons can tunnel a short distance in the vicinity of space charge region to adjust the band offsets by creating tunneling-induced dipoles on either side of depletion region.28 In order to realize the specific type of band offset (i.e., cliff or spike), we use a typical sign convention, i.e., the conduction band offset would be negative (∆Ec ≤ 0), if the conduction band of one semiconductor having larger band gap lies above the conduction band of the other hetero-partner.13 According to this rule, the sum of ∆Ec and ∆Ev should be equal to the difference in band gaps of both hetero-partners, i.e., ∆Ec=Eg(SnO)Eg(ZnSnN2)-∆Ev. In device-1, a cliff-type offset is revealed with ∆Ec = -0.2 eV, indicating that an undersized band offset, that is not capable of restricting the back-flow of photo-generated electrons towards SnO. Additionally, the interface band gap energy (i.e., Eg,IF =0.5 eV) is rather lower than the band gap of each p- and n-type semiconductor, resulting in high probability of recombination at trap states in the forbidden energy gap of SnO,14 as shown in Figure 5b (leftinset). Arca et al.11 claimed that that a moderately-high cliff-type offset is more feasible for solar


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cell applications than spike-type one, which are mainly reliant on the built-in-voltage (qVbi).12 In this framework, P-i-N heterojunction has almost doubled the built-in-voltage (qVbi ~ 0.58 eV) as compared to P-N junction device.12 Although the efficiency of the proposed solar cell is rather improved, it is still far from the real applications. Nevertheless, the rich-ternary chemistry of ZnSnN2 semiconducting alloy (see Figure S3 in the supporting information) together with an ideal band gap engineering is the primary motive to implement this material as a potential photovoltaic material. In summary, we demonstrated ZnSnN2-based P-N and P-i-N heterojunction solar cells. The annealed P-i-N heterostructure exhibited a forward-to-reverse current ratio of 5×103 in the dark. Under 1-sun illumination, P-i-N heterostructure solar cell flaunted the maximum power conversion efficiency (PCE) of 1.54% with Voc of 0.36 V, Jsc of 7.5 mA/cm2 and FF of 0.57. It is believed that the presence of ultra-thin Al2O3-buffer layer in between p-SnO and n-ZnSnN2 contributed a lot to refine the solar cell performance. In fact, the buffer layer provided surface passivation to SnO as well as it served as hole-blocking layer by restricting the flow of photogenerated holes from SnO to ZnSnN2 to boost up the directional-carrier transportation under illumination. A slim buffer layer also regulated the conduction band offset by increasing the built-in-voltage (more or less 2-times) to enhance the open-circuit-voltage. Moreover, the postfabrication annealing was proved to be beneficial regarding tuning of the interface effectiveenergy-gap to realize a cliff-type band offset, resulting in decreased recombination losses. Our present study reveals a promising approach towards low-temperature development of ZnSnN2based heterostructure optoelectronic devices.


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Experimental section Fabrication of P-N and P-i-N heterojunction Tin monoxide (SnO) as the p-type semiconducting window layer was grown on ITO glass at room temperature via electron beam evaporation (EBE) technique. The deposition of ZnSnN2 thin films as n-type absorber material from an alloy Zn:Sn (At.% 3:1) is carried out at a dc-power of 120 W in N2-ambient in magnetron sputtering. The entire deposition was carried out at a base pressure of 6×10-4 Pa with N2-flow rate of 12.5 sccm. For the fabrication of P-i-N heterojunction structure, an ultra-thin film of i-Al2O3 as an intrinsic buffer layer is sandwiched between n- and p-type semiconducting layers by using EBE technique. Top electrodes (Ni/Au) were evaporated on ZnSnN2 surface via EBE using shadow mask. To investigate the annealing effects, the fabricated devices prior to top-electrode deposition are subjected to tube furnace at a temperature of 350 °C for 3 hours under N2-atmosphere. Characterization techniques The phase composition of thin film was probed by an X-ray diffractometer (Bruker AXS D8 Advance X-ray diffractometer, Germany). The optical characteristics of thin films were examined by a spectroscopic ellipsometer (M-2000DI, J.A. Woollam Co., Inc.). The surface morphology has been examined by scanning electron microscope (SEM, Hitachi S4800) coupled with energy dispersive X-ray (EDX, Oxford X-max), atomic force microscopy (AFM Veeco Dimension 3100) and high-resolution transmission electron microscopy (HRTEM Tecnai F20). The current-voltage (J-V) characteristics of fabricated solar cells were scrutinized by Keithley2400 source meter under 1-sun AM1.5 solar spectra (100 mWcm-2) at room temperature.


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Supporting Information

Figure Captions Figure S1:

The schematic illustration of P-N and P-i-N heterostructure photovoltaic devices fabricated on ITO glass.

Figure S2:

The optical band gaps of as-deposited and annealed ZnSnN2 thin films and transmittance spectra of bare ITO (as reference), SnO/ITO (window layer) and Pi-N heterostructure device is shown in the inset.

Figure S3:

Crystal structure of ZnSnN2 (0001).

Table Captions Table S1:

Summary of microstructural and optical properties of as-deposited and annealed ZnSnN2 thin films.

Table S2:

Diode parameters derived from J-V measurements performed in dark for all fabricated devices (1-3).

Table S3:

Solar cell parameters derived from J-V measurements performed under 1-sun spectra for all fabricated devices (1-3).

Corresponding author information: *E-mail: [email protected] (L. Y. L), Phone/Fax: +86 574 8668 8163 *E-mail: [email protected] (H. T. C), Phone/Fax: +86 574 8668 8163


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Acknowledgements This work is supported by the National Natural Science Foundation of China (61474126), Natural Science Foundation of Zhejiang Province (LY16F040002), the program for Ningbo Municipal Science and Technology Innovative Research Team (Grant No. 2016B10005). We also acknowledge the financial support from CAS-TWAS president's fellowship program (201418), administered by University of Chinese Academy of Sciences (UCAS) and The World Academy of Sciences (TWAS).

Notes: The authors have no competing financial or any other conflict of interest.


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FIGURE CAPTIONS

Figure 1. (a) XRD-patterns of as-deposited and annealed ZnSnN2 thin films with stimulated spectra and EDX-analysis (inset), (b) HRTEM image of annealed ZnSnN2 thin film and inset shows a magnified region showing a d-spacing of ~0.253 nm . Figure 2. Annealing induced structural modifications in as-deposited and annealed ZnSnN2 thin films; (a-b) AFM images with phase and height profiles (insets), (c-d) SEM micrographs at a scan size of 500 nm. Figure 3. Schematic illustration of fabricated heterojunction devices (a) with P-N junction, (b) with P-i-N junction. Figure 4. (a) J-V characteristics of all heterojunction devices in the dark and the inset reveals the variation in leakage currents corresponding to each device, (b) The plots of ‫׀‬J‫ ׀‬vs. V in logarithmic scale providing the ideality factors by linear fitting (inset), (c) The plots between dV/d(lnI) and (I) represent the series resistance for each device, (d) Cross-sectional SEM image of the P-i-N heterojunction with top-surface view (inset). Figure 5. (a) J-V response of P-N and P-i-N heterostructure devices under 1-sun AM 1.5 solar spectra along in conjunction with a 3-D bar graph showing solar cell parameters, and the phenomena of photo-generated carriers transportation and recombination (insets), (b) C-V measurements for post-annealed P-i-N junction device, accompanied with band diagrams of P-N junction without any interface state (top-inset) and P-i-N junction under the influence of interface states and tunneling induced dipoles (bottom inset).


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For Table of Contents Use Only Band offset engineering in ZnSnN2-based heterojunction for low-cost solar cells Kashif Javaid1,2,3, Weihua Wu1, Jun Wang4, Junfeng Fang1, Hongliang Zhang1, Junhua Gao1, Fei Zhuge1, Lingyan Liang*,1,5 and Hongtao Cao*,1

Graphical Abstract In the present work, we demonstrate the ZnSnN2-based P-N and P-i-N heterojunctions to study the band offset engineering for the development of high-efficiency inorganic heterostructure solar cell. The P-i-N heterojunction is composed of p-SnO, i-Al2O3 and n-ZnSnN2 constituents. In this work, two experimental strategies have been introduced, firstly: the inclusion of an ultrathin buffer layer of Al2O3 and secondly: the post-fabrication annealing. The role of Al2O3 was proved quite beneficial to overcome the heterojunction non-idealities and the phenomenon of post-fabrication annealing further tuned the band alignment. The annealed P-i-N heterostructure device exhibited a forward-to-reverse current ratio of 5×103 in the dark. Under 1-sun illumination, this optimized heterostructure solar cell flaunted the maximum power conversion efficiency (PCE) of 1.54% with Voc of 0.36 V, Jsc of 7.5 mA/cm2 and FF of 0.57. Our present study reveals a promising approach towards low-temperature development of ZnSnN2-based heterostructure optoelectronic devices.


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Band Offset Engineering in ZnSnN2

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