Strong Photocurrent from Two-Dimensional Excitons in Solution-Processed Stacked Perovskite Semiconductor Sheets

Room-temperature photocurrent measurements in two-dimensional (2D) inorganic–organic perovskite devices reveal that excitons strongly contribute to the photocurrents despite possessing binding energies over 10 times larger than the thermal energies. The p-type (C6H9C2H4NH3)2PbI4 liberates photocarriers at metallic Schottky aluminum contacts, but incorporating electron- and hole-transport layers enhances the extracted photocurrents by 100-fold. A further 10-fold gain is found when TiO2 nanoparticles are directly integrated into the perovskite layers, although the 2D exciton semiconducting layers are not significantly disrupted. These results show that strong excitonic materials may be useful as photovoltaic materials despite high exciton binding energies and suggest mechanisms to better understand the photovoltaic properties of the related three-dimensional perovskites.

S-2 METHODS Device Fabrication: The 2D inorganic-organic perovskite (C 6 H 9 C 2 H 4 NH 3 ) 2 PbI 4 is used as the photoresponsive material in all configurations. For configuration 1, pre-cleaned, patterned transparent ITO (indium tin oxide) coated glass slides are used as the anode, onto which CHPI (40 mM solution/N,N-dimethylformamide) is spin-coated at 2000 rpm for 60 sec (film thickness~800 nm) and then annealed in air at 60 °C for 10 mins. The cathode is made by thermally evaporating aluminium (Al) or gold (Au) (thickness ~100 nm) on top of the CHPI/ITO substrate to complete this configuration 1 [Al/CHPI/ITO or Au/CHPI/ITO] ( Figure   1c). The photodetector configurations (2 and 3, Figure 6a,c) using CHPI as the active absorbing layer are fabricated utilizing a TiO 2 compact layer, with a mesoporous TiO 2 scaffold layer (Dyesol, <50 nm particle size) in the case of 3, as the electron transport layer (ETL) and 2,2´,7,7´-tetrakis-(N,Ndi-p-methoxyphenylamine)9,9´-spirobifluorene (spiro-OMeTAD, Sigma Aldrich) as the hole transport layer (HTL). A 50 nm thin compact layer of TiO 2 (c-TiO 2 ) was deposited on the F-doped SnO 2 (FTO) coated glass by spin-coating (1500 rpm/60 sec) from a mildly hydrochloric acidic solution (2 M) of titanium isopropoxide in dry ethanol. Subsequently the films were heat treated at 500 °C for 30 mins in air and left to cool to room temperature. CHPI solution (40 mM/N,N-dimethylformamide) was spin coated (2000 rpm/60 sec) onto the c-TiO 2 /FTO substrate in air and substrates were annealed at 60 °C for 10 mins. The hole transport material, spiro-OMeTAD, was then spin-coated from a chlorobenzene solution (including lithium bis(trifluoromethylsyfonyl) imide salt and tertbutylpyridine as additives) (2000 rpm/45 sec) on the CHPI/mp-TiO 2 /c-TiO 2 /FTO substrate.
For scanning photocurrent experiments, a thin CHPI film (100 nm) is fabricated by intercalating the organic moiety (C 6 H 9 C 2 H 4 NH 3 I) into a thin PbI 2 film. 26  In order to better distinguish the different contributions to the photocurrent we use singular value decomposition (SVD) which again identifies two main components ( Figure   S3a The Singular value decomposition (SVD) function in Matlab programme has been utilised in SVD analysis, which takes the data as a 3D matrix and tries to decompose it into vectors.
SVD describes the concept in linear algebra of factorisation of a matrix, which yields its singular vectors. In our case we took the voltage as one dimension, the wavelength as a second, and photocurrent as third. The photocurrent data was then decomposed in vectors which had the same spectrum and the same voltage dependence.
In our case the input matrix PC ij is a photocurrent (PC) matrix of our photovoltaic devices with the first dimension as the variation of PC with excitation wavelength λ, and the second dimension as the variation of PC with voltage V.
We use the built-in function 'svd' in Matlab to decompose the matrix into its components.
One of the obtained matrices defines the spectral components of the photocurrent, which we label 'SVD component 1,2,…', while the other matrix defines the variation of these components with voltage.