Perovskite semiconductors have shown great promise for making high-efficiency solar cells at low cost^[1]. Since their first use in solar cells^[2]. The power conversion efficiency (PCEs) of perovskite solar cells (PSCs) have speedily increased. The chemical structure can be tailored by combining various halides or utilizing different aliphatic ammonium ions, leading to tunable band gaps and charge transport properties. ^[3] The certified power conversion efficiency has reached 22.3%^[4]. Despite this rapid increased in power conversion efficiency of perovskite solar cells the maximum current is till upto 25.0 mA/cm². The efficiency limit of perovskite cells without the angular restriction is about 31%, which approaches to Shockley-Queisser limit (33%) achieved by gallium arsenide (GaAs) cells. ^[5] Moreover, theoretically the Shockley-Queisser limit could be possible to reached with only a 200 nm thick perovskite solar cell, through integrating a wavelength dependent angular restriction design with a textured light-trapping structure^[5] Alkylammonium lead (II) trihalide perovskite act as ambipolar charge transport characteristics and function well in solar cells with an inverted structure. For this device architecture, the perovskite forms a heterojunction with an organic charge transport material, such as fullerene (C60), {6,6}-phenyl C61-butyric acid methyl ester (PCBM) or conjugated polymer. The inverted structure (ITO/EEL/Perovskite/HEL/electrode) can be fabricated with reversed electrode polarity. In general, the inverted structure has better ambient stability and compatibility to all solution roll-to-roll application. Perovskite shows high intrinsic carrier mobility and high photon absorption^[6] due to thickness of ~1000 nm there are severe charge recombination and losses^[7a,b,c] Therefore, hampering the resultant current⁸.

       Delicate optical interlayer management is utilized to enhance the current. Inorganic interlayers due to their superior electronic properties and environmental stability compared to their organic counter parts and are considered very good candidates for interface engineering. The transition metal oxides with decent transparency across visible and infrared spectra have been widely proven to form good ohmic contact with absorbers due to their high conductivity and appropriate WF ^{[9 (a),(b),(c) & (d)]} For an ideal Hole extraction layers (HELs), its HOMO level should align with the E_f,h of CH₃NH₃PbI₃ layer while its LUMO level sits above the E_f,e of CH₃NH₃PbI₃. In this direction we show that introduction of an additional buffer HTL of high work function metal oxide layer on NiO_x decreases the concentration of deep-level defects. The interface engineering of this additional buffer layer helps collecting holes effectively hindering electrons this enable the fabrication of PSCs with power conversion efficiency of  21% under standard AM1.5 G conditions. with the highest ever recorded current of 27.8 mA/cm² which is well matched with the current of 27.7 mA/cm² obtained from EQE.

References 

(1) (a) Nature 2013, 501,395. (b) Nature 2013, 499,316 (c) Sci. Rep. 2012,2,591 (d) Science 2013, 342,344. 

(2) J. AM. Chem. Soc. 2009, 131, 6050

(3) (a) J. H. Noh, S.H. Im, J. H. Heo, T.N. Mandal, S.I. Seok, Nano Lett., 2013, (b) M. I. Dar, N. Arora, P. Gao, S. Ahmad, M. Gratzel, M. K. Nazurridin, Nano Lett., 2014 

(4) (a) Science 2014, 345,542 (b) Nature 2015, 517,476 (c) Science 2015, 348,1234 (d) Science 2015, 350, 944 (e)

(5) Applied Physics Letters 2015, 106 (22): 221104

(6) Phys. Chem. Chem. Phys., 2015,17,11516-11520

(7) (a) J. AM. Chem. Soc. 2013, 135, 4656-4659 ; (b) Adv. Mater., 2013,25,6642-6671 ; (c) Nat. Photonics, 2013,7,825-833

(8) J. Phys. Chem. Lett., 2013,4,1821-1828

(9) (a) Nanoscale, vol. 8, no. 22, pp. 11403-11412, 2016 ; (b) Chemical Communications, vol. 52, no. 52, pp. 8099-8102, 2016 ;