† Corresponding author. E-mail:
Project supported by the National Key Research and Development Program of China (Grant No. 2018YFB1500103), the National Natural Science Foundation of China (Grant No. 61674084), the Overseas Expertise Introduction Project for Discipline Innovation of Higher Education of China (Grant No. B16027), and the Science and Technology Project of Tianjin, China (Grant No. 18ZXJMTG00220).
Coper thiocyanate (CuSCN) is generally considered as a very hopeful inorganic hole transport material (HTM) in semitransparent perovskite solar cells (ST-PSCs) because of its low parasitic absorption, high inherent stability, and low cost. However, the poor electrical conductivity and low work function of CuSCN lead to the insufficient hole extraction and large open-circuit voltage loss. Here, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is employed to improve conductivity of CuSCN and band alignment at the CuSCN/perovskite (PVK) interface. As a result, the average power conversion efficiency (PCE) of PSCs is boosted by ≈ 11%. In addition, benefiting from the superior transparency of p-type CuSCN HTMs, the prepared bifacial semitransparent n–i–p planar PSCs demonstrate a maximum efficiency of 14.8% and 12.5% by the illumination from the front side and back side, respectively. We believe that this developed CuSCN-based ST-PSCs will promote practical applications in building integrated photovoltaics and tandem solar cells.
Expanding application fields and improving power conversion efficiency (PCE) of solar cells have become inescapable issues to further promote the development of photovoltaic (PV) market. Semitransparent solar cells (ST-SCs) provide an attractive choice to address aforementioned problems, and thus receive overwhelming attention. For example, ST-SCs can be utilized not only as color-tunable building integrated PV (BIPV),[1] wearable electronics,[2] and PV vehicles[3] to excite future applications, but also as sub-cell of multi-junction[4] or bifacial PV devices[5] to convert more solar energy into electricity, and therefore improve PCE. Organic–inorganic hybrid perovskite solar cells (PSCs) have emerged as a promising candidate for ST-SCs, because opaque PSCs have achieved PCE of 25.2%[6] with unprecedented speed attributing to its direct band gap, high absorption coefficient, low exciton binding energy, and a long electron–hole diffusion length. Simple device architecture, facile fabrication process, and higher PCE make the planar heterojunction stand out from all PSCs device structures, especially the planar n–i–p device.[7–9] Different from opaque solar cells, both charge collection efficiency and light transmittance of HTMs are vital to avoid influencing light absorption in perovskite active layer and realize high performance of ST-PSCs.[10] Generally, 2,2’,7,7’-tetrakis (N,N-di-p-Methoxyphenylamine)-9, 9’-spirobifluorene (Spiro-OMeTAD)[11] or poly (triarylamine) (PTAA)[12] are used as HTMs in most efficient PSCs. Unfortunately, both of them are unstable and expensive, which will greatly affect their wide application in the future. Besides, the serious light absorption in short-wavelength range is also harmful to light harvesting of perovskite absorption layer. Therefore, it is urgent and essential to explore alternative HTMs.[13]
Recently, copper thiocyanate (CuSCN) is paid much attention due to its excellent material stability, extremely low price, and compatible HOMO (highest occupied molecular orbited) level with perovskite. And it is noteworthy that the hole mobility of CuSCN of 0.01 cm2 · V−1 · s−1–0.1 cm2 · V−1 · s−1 is remarkably higher than that of common organic HTMs, such as 10−3 cm2 · V−1 · s−1 of PTAA and 4 × 10−5 cm2 · V−1 · s−1 of Spiro-OMeTAD.[14] A report of the PSCs based opaque CuSCN with PCE over 20%[15] confirmed that CuSCN can be employed as excellent HTM. In addition, the low parasitic absorption also allows it as potential candidate of HTMs for ST-PSCs. For instance, Jen and co-authors[16] adopted CuSCN as HTM instead of PEDOT:PSS to prepare ST-PSCs to obtain PCE of 10% and average visible transmittance (AVT) of 25%. Fan and co-authors[17] fabricated bifacial semitransparent n–i–p planar PSCs, which achieved a maximum efficiency of 12.47% and 8.74% by illumination from the front side and back side, respectively. Wang and co-authors[18] used the CuSCN and ITO as the HTMs and back electrode in the PSCs, respectively. Compared with the Spiro-OMeTAD based ST-PSCs, the CuSCN-based ST-PSCs exhibited higher PCE of 14.2% and 13.3% by illumination from the front side and back side, respectively. Despite many efforts have been made in employing CuSCN as HTM, few attentions were focused on the poor electrical conductivity and small work function of CuSCN. Those problems will cause weak built-in field within limited film thickness, and at the same time cause energy level offset at the CuSCN/perovskite interface.[19] Consequently, it can be deduced that there will be a large amount of hole accumulation and electron–hole recombination at the interface of CuSCN/perovskite due to the low holes transfer efficiency of intrinsic CuSCN.
In this work, F4TCNQ was doped into CuSCN to be used as the HTM in CsFAMA based n–i–p planar perovskite solar cells, which optimize the energy level alignment at the I/P interface in order to achieve better hole transport. Ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS) indicated that, after F4TCNQ doping, the energy level positions of CuSCN HTMs changed, in which Fermi level (EF) decreased from −4.82 eV to −4.90 eV and valence band maximum (VBM) −EF decreased from 0.71 eV to 0.68 eV. Therefore, the average conversion efficiency of CsFAMA mixed cation PSCs was improved by about 11%, and a PCE of 16.6% for a champion cell. In addition, for bifacial semitransparent n–i–p planar PSC using CuSCN as HTM, maximum PCE of 14.8% and 12.5% were achieved by illuminated from the front side and the rear side, respectively.
Lead bromide (PbBr2, 99.99%), copper thiocyanate (CuSCN, 99%), cesium iodide (CsI, 99.999%), chlorobenzene (CB, 99.8%), dimethylformamide (DMF, 99.9%), and dimethyl sulfoxide (DMSO, 99.9%) were purchased from Sigma-Aldrich. Tin(IV) oxide colloid precursor (15% in H2O colloidal) was purchased from Alfa Aesar. F4TCNQ (99%) were purchased from materwin. Lead iodide (PbI2, 99.99%), formamidinium iodide (FAI), and methylammonium bromide (MABr) wear purchased from Xi’an Polymer Light Technology Corp. Diethyl sulfide (DES, 98%) was purchased from Shanghai TCI.
ITO glass substrates were cleaned with detergent, deionized water, acetone, and isopropanol for 30 min each time. The previously cleaned ITO glass substrates were exposed to ultraviolet light at room temperature for 10 min, spin coated SnO2 layer at 4000 rpm for 30 s then annealed at 150 °C for 30 min in ambient air. The CsFAMA mixed precursor solution was prepared in a glove box. Precursor solution of perovskite was prepared by mixing PbI2 (548.62 mg), PbBr2 (57.06 mg), FAI (178.94 mg), MABr (17.41 mg), CsI (27.04 mg) in DMF/DMSO (V/V 780 μl/220 μl). Subsequently, perovskite films were prepared by spin coating in a two-step programme at 1000 rpm and 5000 rpm for 10 s and 30 s. During the second step, 100 μl of chlorobenzene was dropped on the spinning substrate 10 s prior to the end of the programme and finally annealed on a hot plate at 130 °C in a glove box for 20 min.
The F4TCNQ was dissolved in chlorobenzene at the concentration of 1 mg/ml and added it in CuSCN(30 mg/ml, DES) solution at a volume ratio of 1%, 3%, 5%, and 7%, respectively. Doped-CuSCN and undoped-CuSCN layers were spin-coated on the ITO/SnO2/CsFAMA substrates at the speed of 4000 rpm for 30 s. Drying was performed at room temperature for 10 min, and then it was annealed at 80 °C for 5 min to form the CuSCN films. MoOX with thickness of 9 nm was thermally evaporated in high vacuum of 1 × 10−4 Pa, and grew at a slow rate of 0.1 Å/s, while the substrate temperature was kept at room temperature. Then ITO with thickness of 100 nm was deposited at room temperature by DC magnetron sputter deposition. Finally, gold grid electrode with thickness of 100 nm was deposited by thermal evaporation on the top.
X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo VG ESCALAB 250X with Al Ka radiation as excitation source, and used the C 1s neutral carbon peak at 284.8 eV to calibrate the binding energy. Ultraviolet photoelectron spectroscopy (UPS) with a VG Scienta R4000 analyzer and using a monochromatic He I light source (21.22 eV) was used to characterize the molecular energy levels. Secondary electron cutoff (SEC) was observed with a sample bias of −5 V. The method to determine the work function (ϕ) was based on the difference between the photon energy and the binding energy of the secondary cutoff edge. Scanning electron microscope (SEM, FEI NanoSEM650) was used to measure the surface morphology of undoped and doped CuSCN. In addition, surface roughness of undoped and doped CuSCN was characterized by atomic force microscope (AFM, NanoNavi-SPA400). The transmittance and absorbance of undoped and doped CuSCN were characterized with a UV–vis-NIR spectrophotometer (Cary 5000, VARIAN). A fluorescence spectrophotometer (Edinburgh Instruments, FLS 980) was used to record room temperature steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL). The photocurrent density–voltage (J–V) curves of devices were measured under one sun illumination by using a Keithley 2400 digital source meter. The dark current density (ITO/SnO2/PVK/HTM/Au) and trap state density (ITO/HTM/PVK/HTM/Au) were both examined by Keithley 2400 digital source meter under dark condition. External quantum efficiency (EQE) spectrum was obtained without any voltage bias in the case where the EQE system (QEX10, PV Measurement Co, Ltd.) was used in the DC mode.
The device schematic diagram of the undoped and doped CuSCN are illustrated in Fig.
Based on the aforementioned problem, the band diagram of the CuSCN/perovskite in the PSCs is shown in Fig.
Steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) decay measurements are used to understand the hole extraction capability and recombination characteristics of the devices with different HTMs. As shown in Fig.
The dark current–voltage (J–V) characteristics of hole-only devices based on ITO/NiOx/perovskite/undoped or doped CuSCN/Au were used to evaluate carrier recombination at the interface of CuSCN/Perovskite. The space-charge-limited currents (SCLC) curves is used to measure the trap state density state (ntrap). As shown in Figs.
The photovoltaic performance of the CsFAMA perovskite PSCs with undoped and doped CuSCN HTMs is analyzed and is shown in Fig.
The long-term stability of these PSCs can also be evaluated by the J–V characteristics monitoring of the devices stored in glove box (RH < 10%) at room temperature. As shown in Fig.
As depicted in Fig.
The vertical conductivity of the undoped and doped film are measured by J–V test system. Figure
As shown in Fig.
In summary, a molecular doping strategy is used to tailor the electronic properties of inorganic CuSCN hole transporting layer. The doped CuSCN has a superior conductivity and well-matched energy level with perovskite, which is beneficial for hole transferring from perovskite to doped CuSCN and reducing the recombination at perovskite/CuSCN interface. As a result, the champion opaque cells with doped CuSCN exhibits PCE of 16.6%, Jsc of 21.5 mA/cm2, Voc of 1.10 V, FF of 70.5%. When employing as HTM in bifacial semitransparent n–i–p planar PSCs, the maximum efficiency of 14.8% and 12.5% are achieved by illuminating from the front side and rear side, respectively. This work can be used for reference to improve the efficiency of ST-PSCs based on CuSCN and will promote the development of BIPV.
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