Chalcogenide As Inorganic Transport Layer in Perovskite Solar Cell

Fill factor (FF) decit and stability is a primary concern with the perovskite solar cell. Resistance values and band alignment at junction interface in perovskite are causing low ll factor. Moisture sensitivity of methylammonium lead halide perovskite is causing a stability issue. We tried to solve these issues by using inorganic hole transport layer (HTL). FF is sensitive to the band offset values. We study the band alignment/band offset effect at the Perovskite /HTL junction. Inorganic material replacing Spiro-MeOTAD can enhance the stability of the device by providing an insulation from ambient. Our simulation study shows that the earth abundant p-type chalcogenide materials of SnS as HTL in perovskite is comparable to SpiroMeOTAD eciency.


Introduction
Terawatt scale Perennial supply of pollution-free energy at a competitive cost is a desirable for sustainable growth. Photovoltaic is potential solution subjected to availability of low cost, nontoxic material, and fabrication technology. Perovskite Methylammonium Lead Halide (CH 3 NH 3 PbI 3 or MAPbI 3 ) is the material with ideal properties of light absorption, defect-free bandgap, high mobility and diffusion length. In a short span of time perovskite has achieved high power conversion e ciency of 22.1% [1]. The organic component in the perovskite solar cell device structure has the challenge of stability due to moisture, thermal and photosensitivity [2,3]. The sensitivity of the organic MA (methylammonium) toward polar solvent like water leads to their dissociation in presence of moisture. A moisture resistant perovskite material or suitable passivation from moisture is probable stability solution. Better insulation from thermal and ambient can be provided by encapsulation using inorganic layers. TiO 2 as electron transport layer (ETL) on one side and another inorganic material as hole transport layer (HTL) can better shield the perovskite layer. Low mobility of holes, unstable (thermal, Photo and moisture stability) issues in polymer HTL like Spiro-MeOTAD, PEDOT: PSS etc. could be overcome by using inorganic layers. Inorganic materials which are tried for HTL in literature are listed in Table. 1. On this line we explored inorganic Chalcogens materials (CIGS, CZTS, SnS, Cu 2 S, PbS, Sb 2 S 3 , FeS 2 ) for HTL, where all the elements are nontoxic, abundant [4] and can be economically processed for solar cell application. The p-type nature of these chalcogens with high acceptor density and hole mobility can be exploited for HTL. Inorganic layer with low thermal conductivity and resilient to the ambient condition can enhance the lifetime of perovskite by encapsulating them from outside environment. In order to discuss and gain some insight into the feasibility of inorganic HTL here, we performed simulation study with inorganic hole transport layer in perovskite.

Chalcogens
Chalcogens we selected here (CIGS, CZTS, SnS, Cu 2 S, PbS, Sb 2 S 3 , FeS 2 ) are prominent p-type absorber material with high absorption coe cient (~10 5 cm -1 ) and bandgap lying invisible region (0.5-1.5eV). Their material properties are listed in Table 2. These are all abundant material available enough for terawatt scale energy generation, nontoxic and cost-effective. Solution-processed fabrication techniques make them strong for the low-cost application. These are Multifunction material with application ranging from (i) prominent absorber layer in inorganic and hybrid solar cells, (ii) counter electrode in DSSC and batteries, (iii) as a photocatalyst, (iv) as a gas sensor (v) Counter electrode in quantum dot solar cell, (vi) Thermoelectric (vii) ultra-fast photo-detector and optoelectronic applications. All the selected chalcogens material binary (SnS, Cu 2 S, PbS, Sb 2 S 3 , FeS 2 ) and quaternary (CIGS, CZTS) have higher hole mobility than Spiro-MeOTAD.

Simulation Detail
Solar cell capacitance simulator SCAPS-1D version 3.3.0.2 is used for the theoretical study. SCAPS model semiconductor and electrostatic equation in 1D environment to simulate a thin lm solar cell. Using Newton Raphson and Gummel iteration it solves 1D Poisson equation and continuity equation for heterostructures.
To begin with, we have modeled a p-i-n structure FTO/TiO 2 /MAPbI 3 /Spiro-MeOTAD and simulated the performance of the solar cell. The reported material parameters of perovskite as thickness 350 nm, bandgap 1.55 eV, the dielectric constant of 10, a value of N A of ~10 13 /cm 3 , the diffusion length of electrons in perovskite, L p~1 000 nm, we used similar values of L n , the electron mobility of ~10 cm 2 /Vs, radiative recombination coe cient (cm³/s) 10 -13 . Neutral defects density of 10 14 at the center of the bandgap with a Gaussian distribution and characteristic energy of 0.1eV; and the capture cross-section of electrons and holes of 2×10 −14 cm −2 . The absorption constant A (1/cm eV^(½)) was assumed to be 10 5 . The device is illuminated from FTO layer side with AM1.5 spectrum using relevant SCAPS option at temperature 300 K. Front contact is from the FTO layer and a back contact is taken from metal contact with work function taken to be 5.1eV. For the series resistance (R s ) and shunt resistance (R sh ), we have used typical values of 5Ω-cm 2 and 5k Ω-cm 2 . The schematic of the device is shown in Fig. 1a.
The relevant material parameters used in the simulation were obtained from the literature and are listed in Table 3 [23][24][25]. Quantum e ciency (QE) curve shows maximum light absorption is within perovskite region.

Results And Discussion
Design of the solar cell does affect the mechanism of the photovoltaic action and e ciency. The structure should be optimized for e cient separation and collection of photogenerated generated charge carrier. (a) Built-in Electric eld, (b) effective force eld (work function difference), (c) discontinuity at an energy level at the interface, are some of the design related options implemented for better performance of the photovoltaic device. The interface at heterojunction, control voltage generation and current ow, potential variation, the electric eld at either side of the junction. Perovskite con guration of FTO/TiO 2 /Perovskite/HTL is p-i-n type.
p-i-n structures is suitable for large built-in potential across absorber layer. Built-in potential depends on the carrier density of p and n layer. The effective force eld can be induced by the appropriate choice of the contacts work function. For design point of view, an HTL should be having a high hole to electron mobility ratio so that it acts as a selective contact. To gain large built-in voltage charge density should be higher in n and p region of a pin. Higher charge density will also shift the space charge layer into the intrinsic layer an advantageous situation for charge drift. Figure 2a shows the Higher builtin voltage resulting from higher density of carrier in p and n layer of pin structure. The slope of band energy level in i layer (absorber) also increases with the carrier density in p and n, as shown in gure 2a. A spike to be present at the valence band offset to lower the interfacing recombination between E V,HTM and E C,Perovskite [23,27]. The reason for the low recombination at slightly positive valence band offset is the low value of ∆. The ∆ is the energy difference E C Perovskites -E V HTL as shown in the Fig. 2. The cliff at the perovskite/HTL junction lowered the ∆ shown in gure 2b, enhancing the chances of recombination of an electron from perovskite to hole from HTL. The presence of spike at the junction increases the ∆ value shown in Fig. 2c and thus lower the chances of recombination of an electron from perovskite and hole from HTL. The high Spike value will impede hole transfer from perovskite to HTL. The slightly positive values of VBO are advantageous and their optimum value can be ascertained by simulation. Device optimization by simulation for an HTL (p layer) in p-i-n the optimizing conditions are i) A positive VBO at the p-i junction.
ii) High hole density in p,n layer ( p + -i structure), a) High built-in voltage b) Lead to the higher eld in absorber (i) layer.

HTM layer parameter
For the simulation of inorganic HTL, we inserted an interface defect layer (IDL) at HTL perovskite junction.
The width and defect density of IDL is taken to be 10nm and 10 17 cm -3 . High defect density in IDL is used to simulate the interfacial recombination caused by lattice mismatching and defect state. CZTS have bandgap tail state of multivalent defect near to conduction band and valence band. We have assumed multivalent defect of charge state {3+/2+, 2+/+, +/0} in CB at E t =1.40eV with characteristic energy of 0.05eV. Acceptor  Table 2.
These inorganic material are p-type with 2-4 order high hole density than perovskite and have a similar bandgap. Simulation is run for these sul de as HTL in Perovskite solar cell. HTL parameter of thickness, hole density are simulated. Fig. 3 shows the dependence of inorganic HTL device performance on hole density, thickness. The hole density in the HTL is varied from 10 14 to 10 18 cm -3 to simulate the effect on e ciency. E ciency rises with hole density for all HTL. As doping concentration in HTL increases, the resistivity of HTL layer reduces. This increases the V OC and FF whereas the current remains constant. Carrier (hole) concentration will affect the bend banding and potential distribution at the junction. Perovskite has a low doping thus higher doped HTL will induce banding in Perovskite absorber layer thus creating a potential distribution across perovskite layer. Higher carrier concentration will cause higher built-in voltage in the device and is favorable until limiting diffusion length at high carrier density, Auger and Radiative HTL junction, an interface defect density is assumed. A 10 nm thick layer is inserted at this junction to simulate any defect density arising from electrical or mechanical dissimilarity.

VBO and contact optimization of HTL layer
VBO is the discontinuity in the energy level at the junction shown by a vertical line in Fig. 3. VBO is given by (E gHTM +X HTL )-(E gPerov .+ X Perov ), E g is the bandgap, X is electron a nity. The simulated band diagram of the device Inorganic HTL/Perovskite shows valence band offset. The High negative offset introduces a cliff, favorable path way for hole transport from perovskite to HTL case shown in Fig. 2b for Perovskite /SpiroMeOTAD. High positive offset (X HTL +E gHTL >X Abs +E gAbs ) introduces a spike forming a barrier for hole transfer from the absorber to HTL as the shown in Fig. 2c. At zero Valence band offset make continuity at the junction. The ∆ (as shown in the gure should be large to avoid interfacial recombination between E V HTL and E C Perovskite . The positive value of VBO will be suitable to increase ∆. However large VBO will inhibit the hole transfer from absorber to HTL. The height of spike should be less than the built-in voltage in absorber. SnS show comparable e ciency to Spiro, therefore, we optimized SnS for further e ciency enhancement. The electron a nity of the HTL layer is varied to vary the VBO and the band alignment. The band alignment is simulated for positive and the negative and zero valence band offset. SnS/perovskite has a E VBO of 0.15eV shown in band diagram Fig. 4. The effect of Spike and Cliff on device performance is studied. The simulation is run for range of VBO from negative to positive value. The defect at the junction of perovskite/HTL is simulated by inserting a IDL (interface defect layer) with a defect density of 10 17 cm -3 .
The dependence of cell e ciency on valence Band offset at interface is shown in Fig. 6. For VBO values from -0.05 to .3eV the e ciency increases and peaks at a value of 0.15eV. Interface defect density at the junction highly affects the negative valence band. Recombination current dependence on valence band offset is shown in the inset of Fig. 4. Recombination current is minimum at the slightly positive band offset region.
The e ciency maximizes at VBO where recombination current is minimum. Interface defect density is assumed to be high (10 17 cm -3 ). Recombination current depends upon the defect density at the interface.