10001483_P-i-n perovskite solar cells with high open-circuit voltage

P-i-n perovskite solar cells with high open-circuit voltage Beitao Ren,a Gancheong Yuen,a Hoi-Sing Kowk,b and Guijun LIa a College of Electronic Science and Technology, Shenzhen University, Shenzhen, P.R.China. b State Key Lab on Advanced Displays and Optoelectronics Technologies, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. Abstract: With photocurrents in perovskite solar cells close to their practical limit, it is imperative to improve their open-circuit voltage to go beyond a loss-in-potential less than 100 mV. However, state-of-the-art p-i-n perovskite solar cells are reported with a Voc of around 1.1-1.5V, limiting their efficiency improvement. Herein, we demonstrate the Voc of p-i-n perovskite solar cells can be successfully improved via anode contact engineering. First of all, by introducing a partially oxidized nickel layer, we are able to remove the potential barrier for hole transport and enhance the crystallinity of the hole transporting layer, both of which are believed to contribute to the Voc and FF improvement. Furthermore, by separating the inorganic NiMgOx hole transporting layer from the perovskite absorbing layer with a poly(4-vinylpyridine) (PVP) insulating layer, the interfacial recombination could be effectively suppressed, the Voc climbs to an impressive value of 1.15 V along with a power conversion efficiency of 19.3%. Finally, a substrate-type perovskite solar cell is fabricated with an extremely high Voc of 1.18 V, representing a very low voltage deficit in the p-i-n perovskite solar cells. Our works provide an avenue for further reducing the loss-in-potential of perovskite solar cells. Corresponding author: Guijun Li. Dr Guijun Li received his Bachelor degree and Master degree from Nankai University. He then studied with Professor Hoi-Sing Kwok at HKUST, where he received his PhD degree in ECE in 2016. From 2009 to 2012, he worked at BOE and then Astronergy. He joined PSKL of HKUST as a research associate after his graduation and then as an assistant professor of Shenzhen University in 2017. Dr.Guijun Li’s research interest is in the development and applications of optoelectronics with current emphasis on (1) photovoltaic devices, (2) light emitting devices based on quantum-dots and perovskite materials, (3) smart window. Dr.Guijun Li has published/co-authored more than 40 research papers and held more than 10 patents/patent applications. email: guijun@szu.edu.cn Introduction Halide perovskites embrace a unique combination of intriguing attributes, including strong light absorption, long carrier diffusion length, low non-radiative recombination rates, and solution processability. State-of-the-art perovskite solar cells (PSCs) have achieved an impressive power conversion efficiency (PCE) of over 22% within several years of development, primarily thanks to the recent advancements in composition engineering, morphology manipulation, and interface engineering. [1,2] A variety of PSC structures have been designed and implemented, ranging from mesoscopic to planar structures with a n–i–p or a p–i–n architecture. To date, the top- performance PSCs have been fabricated with a mesoporous n-i-p configuration. [3,4] The open-circuit voltage (Voc) of these n-i-p structured devices, which is indicative of the potential for the cell efficiency, has been put forward to an exceptional value of ~1.24 V, only 90 mV below the theoretical limit of ~1.33 V. While the n-i-p PSCs have made significant progress in terms of their loss-in-potential and efficiency, the p-i-n devices are still struggling to make headway in this regard. The highest efficiency achieved for the p-i-n cells is around 20%, along with a Voc of around 1.1-1.15 V, which is still far inferior to their counterparts. [5,6] In the paper, we will address the above issues by incorporating an ultrathin Ni layer at the interface of the TCO/HTL and introducing an insulating layer between the HTL and the perovskite layer. The PSCs with the Ni/NiMgOx/PVP multilayer-structured anode exhibit an outstanding Voc of 1.15 V and a PCE of 19.3%. Furthermore, a substrate-type p-i-n solar cell, which is an important element for the future development of all perovskite-based tandem cells, is also fabricated with an exceptional high Voc of 1.18 V, representing one of the lowest loss-in-potential reported so far for the p-i-n perovskite solar cells. Results and Discussion Figure. 1. (P-i-n perovskite solar cell with a multilayer-structured anode. (a) schematic device structure; (b) energy band diagram; (c) cross-section SEM image. (d) The Ni 2p3/2 and O1s XPS spectra of as-deposited Ni, Ni after 300 oC annealing and NiOx; (e) XRD spectra of as-deposited Ni, Ni after 300 oC annealing and NiOx; (f) SIMS spectra of an ITO/NiMgOx (right)with and (left)without a Ni interfacial layer.) Figure. 1(a) shows the device structure of a p-i-n inverted planar perovskite solar cell. Figure.1 (b) is the corresponding schematic of the energy band diagram. The cross-sectional scanning electron micrograph (SEM) of a typical p-i-n cell with a Ni/NiMgOx/PVP multilayer anode is shown in Figure.1(c). In order to prevent the full oxidization of the Ni during the post-annealing process of the NiNgOx, the annealing temperature is set as low as 300 oC. At this temperature, as evidenced by the Ni 2p3/2 and O1s X-ray photoelectron spectroscopy (XPS) spectra (Figure.1(d)), Ni is partially oxidized. This is further confirmed by the X-ray Diffraction (XRD) measurement (Figure 1(e)). Figure. 1(f) shows the secondary-ion mass spectrometry (SIMS) profiles for Indium (In) and Ni. In the structure without an ultrathin Ni layer, there is a significant overlap between the Ni and In, suggesting that the In diffuses into the NiMgOx. The diffusion of In is retarded and suppressed by using an ultrathin Ni layer. Figure. 2. ((a) Typical J–V curves of perovskite solar cells with different thickness of Ni interfacial layer; (b)Voc distribution of perovskite solar cells with a PVP interfacial layer for different Ni thickness; (c)FF distribution of perovskite solar cells with a PVP interfacial layer for different Ni thickness; (d) the J–V curve of the champion cell. The champion cell is made with a Ni thickness of 4 nm and with a PVP interlayer.) The resultant perovskite solar cells performance with different thicknesses of Ni ultrathin layers under AM1.5G illumination conditions is shown in Figure 2(a) . With the increasing of the Ni thickness, the Voc gradually climbs from 1.065 V to a maximum value of 1.110 V at the Ni thickness of 6 nm. The Voc can be further boosted by introducing an insulating PVP layer at the NiMgOx/perovskite interface. As shown in Figure 2(b and c). The highest FF is achieved with the Ni thickness of 4 nm, and a maximum PCE of 19.3% is obtained with a Ni (4 nm)/NiMgOx/PVP multilayer-structured anode (Figure 2(d)). Figure. 3(A) schematic device structure of a substrate-type perovskite solar cell. Light incident from the electron transporting layer side. Ni is used as the back contact; (b) Mg 2p XPS spectra of films with different Mg concentrations. (B) J–V curves of the substrate-type perovskite solar cell measured by forward scan. The successful improvement of Voc with a multilayer-structured anode inspires us to develop a substrate-type PSCs, in which light incidents from the ETL side of the device (Figure 3(a)). In this type of solar cell structure, the Ni back contact can be made as thick as possible. An extremely high Voc of 1.18 V is obtained for the perovskite solar cells with a Ni/NiMgOx/PVP multilayer back contact (Figure 3(b). However, the photocurrent is not as good as that of the inverted planar structure solar cells, due to the optical loss in the ultrathin Ag layer (~10 nm), which we used as the front transparent contact. Further developing of the front contact material, or using other soft deposition methods will be helpful to address this issues in the future. Even so, the high Voc achieved in the substrate-type device is very important for the development of the high-efficiency all-perovskite tandem solar cells. Conclusion In summary, we have proposed an effective strategy to achieve high Voc for the inverted p-i-n planar structured PSCs by using an Ni/NiMgOx/PVP multilayer-structured anode. Firstly, the ultrathin Ni layer was found to facilitate the hole transport by removing the potential barrier between the NiMgOx and ITO front contact and to increase the crystallinity of the NiMgOx by retarding the In diffusion into the NiMgOx. Secondly, the PVP interfacial layer could suppress the non-radiative recombination rate at the interface of between the perovskite and the HTL. As a result, a Voc of 1.15 V and a PCE of 19.3% are obtained. Furthermore, a substrate-type PSC with an extremely high Voc of 1.18V was also demonstrated using the Ni/NiMgOx/PVP multilayer back contact, which is good news for the future development of high-efficiency all perovskite-based tandem solar cells. These results found here also open feasible routes to reduce the loss-in-potential for a variety of advanced photovoltaic solar cells. Reference [1] M. A. Green, A. Ho-Baillie, H. J. Snaith, Nat. Photonics 2014, 8, 506. [2] G. Li, J. Y. L. Ho, M. Wong, H.-S. Kwok, Phys. Status Solidi RRL – Rapid Res. 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