Elsevier

Nano Energy

Volume 34, April 2017, Pages 257-263
Nano Energy

Full paper
Low-temperature synthesis TiOx passivation layer for organic-silicon heterojunction solar cell with a high open-circuit voltage

https://doi.org/10.1016/j.nanoen.2017.02.024Get rights and content

Highlights

  • An open circuit voltage of 643 mV for textured Si/organic solar cell is achieved by inserting TiOx layer between Si and Al.

  • Si-O-Ti chemical bond can be formed by a low-temperature solution-processed TiOx on Si.

  • The TiOx layer effectively suppresses the recombination losses of Si.

  • The TiOx layer reduces the rear side contact barrier.

Abstract

Organic/silicon (Si) heterojunction solar cells may promise for high performance and low cost solar cell manufacture because they can combine advantages of Si and organic semiconductors. However, the performances of organic-Si solar cells were still mainly jeopardized by Si rear contact recombination losses. Here, titanium oxide (TiOx) layer via low-temperature solution synthesis way was deposited on Si rear side to reduce electrical losses. Minority carrier lifetime, transient photovoltage decay and external quantum efficiency measurements indicated that TiOx could effectively suppress the recombination losses of Si rear side, which reflected in the improvement of open circuit voltage (Voc) and long-wavelength photoresponse. Contact resistance and capacitance-voltage measurements further showed that resistance and rear adverse barrier height (Φ-) between Si and aluminum could be dramatically reduced. A power conversion efficiency (PCE) of 14.3% was achieved for textured Si/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) solar cell by incorporating TiOx layer between Si and aluminum. A Voc of 643 mV was achieved based on this low-temperature processed organic-Si heterojunction solar cell. The outstanding performance was ascribed to Si-O-Ti bonding at TiOx/Si interface, which was identified by X-ray photoelectron spectroscopy. This simple low-temperature solution processed metal oxide layer provided a facile way for Si surface passivation to achieve high performance solar cells.

Introduction

Traditional p-n homojunction silicon (Si) solar cells have achieved power conversion efficiencies (PCE) of 25% and possess the largest market [1], [2]. However, complicated processes and expensive facilities were generally indispensable to achieve a high PCE for a tradition Si solar cell. Alternatively, organic-Si heterojunction based on p-type conjugated polymer, such as poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS), and n-type Si offered an alternative choice toward fabricating simple processes, low cost and high efficiency solar cells [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. PEDOT: PSS could be deposited on Si by a simple solution process followed by moderately temperature (~120 °C) annealing[17], [18], [19], [20]. Besides, organic-Si solar cells were not sensitive to the thickness of PEDOT: PSS film, from 20 nm to 100 nm [5], [11], [21], [22], [23], [24], [25], which might promise a convenient roll-to-roll method for large-area fabrication.

Up to now, the PCE of organic-Si heterojunction solar cell made by a low-temperature (<150 °C) has reached up to ~15% [26], but there were still some noticeable weaknesses compared with all inorganic Si solar cells such as amorphous Si/Si heterojunction or p-n homojunction. One of crucial unfavorable factors was the surface recombination losses that restricted the open circuit voltage (Voc) and the short circuit current density (Jsc) [27], [28]. Previous studies demonstrated that a notable strong inversion layer could be formed by optimizing the organic-Si interface, which offered an efficient quasi p-n type junction for charge separation [29], [30], [31], [32]. Therefore, Si surface passivation played a key role on device performance. For an organic-Si heterojunction device, PEDOT: PSS film has served as a hole transportation layer and anti-reflection layer [22], [33]. And most importantly, PEDOT: PSS film has been demonstrated a passivation layer for Si surface [28], [34]. For further reducing the recombination losses between PEDOT: PSS and Si front surface, several advanced schemes have been developed. Sturm's group utilized conjugate polymer poly(3-hexylthiophene) as electron blocking layer that decreased the reverse saturation current density (J0), reduced the recombination of electrons and holes, which generated a Voc of 590 mV with a PCE of more than 10% [27]. Our group have used 2,2,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene as an interface layer that reduced the carriers recombination with a PCE of 9.3% [3]. Yu's group incorporated organic molecule 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane as passivation layer that inserted into the interface between PEDOT: PSS and Si, where a Voc of 540 mV and a fill factor (FF) of 69.5% and achieved a PCE of 13.01% [28]. Nagamatsu KA et al. deposited Al2O3 layer between Si and PEDOT: PSS by chemical vapor deposition (CVD), which boosted the Voc and improved short-wavelength photons utilization due to the suppression of interface recombination [12]. Compared with front surface modification, the rear side of Si heterojunction solar cells was mainly dependent on junction quality between Si and cathode. However, rear side passivation has not well explored [35], [36]. Titanium oxide (TiOx) deposited by CVD or atomic layer deposition (ALD), can act as a selective contact passivation for rear Si modification [37], [38]. However, since CVD or ALD required special facilities, these methods might not be desired ones for low cost and large area manufacture, such as roll-to-roll process. Solution process of TiOx as passivation layer for Si heterojunction would be an alternative strategy for low cost solar cells, but it has not been systematically explored.

Here, we developed TiOx passivation layer for Si heterojunction solar cells through depositing TiOx precursor solution followed by mild thermal annealing. TiOx layer was synthesized between Si and cathode, which achieved both passivation of Si and reduction of contact barrier between Si and cathode. A Voc of 643 mV with a PCE of 14.3% was achieved for textured Si-PEDOT: PSS heterojunction solar cells.

Section snippets

Materials and devices fabrication

Cleaned (100)-oriented Si substrates (300 µm, n-type, 0.05–0.1 Ω/sq) were cut into 1.5×1.5 cm2 and immersed into an aqueous solution (hydrofluoric acid (4.8 M) and silver nitride (0.02 M)) for 5 min at 25 °C to acquire nanotextured Si structure following our previous report [34]. Then the substrates were immersed into nitric acid for 5 min with additional with deionized (DI) water rinsing to get rid of residual silver. The substrates were dipped into HF solution to remove Si oxide. Finally, the samples

Results and discussion

Fig. S1 showed the structure scheme of Si heterojunction solar cell. TiOx layer was fabricated by spin-coating titanium isopropoxide/IPA solution onto the rear side of Si. PEDOT: PSS solution was deposited on the front side. In order to improve the light harvesting properties, Si surface was textured into microstructure by metal ion-assisted etching and smoothed by tetramethylammonium hydroxide (Fig. 1(a)). These processes could control the surface/volume ratio to balance surface recombination

Conclusion

In conclusion, we had demonstrated a low-temperature solution-processed TiOx interface layer for efficient Si heterojunction solar cells. The formation of Si-O-Ti chemical bond led to reduction in dangling bond on Si surface, which dramatically suppressed the charge recombination of Si rear side. The TiOx layer achieved well passivation effect of Si, which enhanced the Voc and the Jsc of the devices. Besides, TiOx layer decreased contact resistance and formed negative Vbi between Si rear side

Acknowledgments

This work was supported by the National Key Research and Development Program of China (2016YFA0202402), the National Natural Science Foundation of China (91333208, 61176057, 61504089), the Natural Science Foundation of Jiangsu Province of China (BK20130310), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Collaborative Innovation Center of Suzhou Nano Science and Technology.

Yuqiang Liu is a Ph.D. candidate of Materials Science and Engineering in Functional Nano & Soft Materials Laboratory of Soochow University, China. He received M.S. degree in Materials Science and Engineering at Soochow University in 2016. His current research interests focus on passivation and interface engineering of Si heterojunction solar cells and flexible wearable devices.

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      To achieve an additional improvement in the efficiency of Si-based solar cells, a technique is required to reduce the contact barrier on the Si/metal interface [18–20]. Recently, many groups have demonstrated improvements in Si-based solar cell device performance by blocking recombination through using TiOx back passivation [18–21]. The improvement brought by this technique are due to the appropriate energy band of the TiOx layer.

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    Yuqiang Liu is a Ph.D. candidate of Materials Science and Engineering in Functional Nano & Soft Materials Laboratory of Soochow University, China. He received M.S. degree in Materials Science and Engineering at Soochow University in 2016. His current research interests focus on passivation and interface engineering of Si heterojunction solar cells and flexible wearable devices.

    Jie Zhang received his Ph.D. degree in Functional Nano & Soft Materials Laboratory from Soochow University, China, in 2014. After being a research assistant in The Chinese University of Hong Kong, China, in 2015, he joined Faculty of Science, Engineering and Technology, as a postdoc, Swinburne University of Technology, Australia, in 2016. His current research interests include graphene oxide optoelectronic devices and laser nanofabrication design of nanostructures and nanomaterials for energy related photonics research.

    Haihua Wu is a Ph.D. candidate in Institute of Functional Nano & Soft Materials, Soochow University, China. He received B.S. degree in Physics at Capital Normal University in 2009. His main research interest is focused on the optical engineering of organic-inorganic solar cells.

    Wei Cui received his Ph.D. degree in Material Science and Engineering from Soochow University, China, in 2017. He received his B.S. degree in Qufu Normal University in 2010. His current research focuses on fabrication and interface modification of Si-based photoelectrochemical electrodes.

    Rongbin Wang is a joint Ph.D. candidate between institute of Functional Nano and Soft Materials, Soochow University, China and Institute of Physics, Humboldt-University zu Berlin, Germany. He received M.S. degree in Physics at Soochow University in 2015. His main research interest is focused on the electronic structure of organic/organic interface and organic/inorganic interface using photoemission spectroscopy.

    Ke Ding received his B.S. degree from Hefei University of Technology in 2013. He is currently a Ph.D student in Institute of Functional Nano & Soft Materials, Soochow University. His research interesting includes nano-electronic and nano-optoelectronic devices, graphene and relevant devices, and solar cells based on Si nano-arrays.

    Shuit-Tong Lee is a member of Chinese Academy of Sciences and a fellow of Academy of Sciences for the Developing World. He is a distinguished scientist in material science and engineering. Prof. Lee is a professor and the Founding Director of Institute of Functional Nano & Soft Materials and Founding Dean of College of Nano Science & Technology at Soochow University, and Founding Director of Collaborative Innovation Center of Suzhou Nano Science & Technology.

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