† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant Nos. 61377065 and 61574064), the Science and Technology Planning Project of Guangdong Province, China (Grant Nos. 2013CB040402009 and 2015B010132009), and the Science and Technology Project of Guangzhou City, China (Grant No. 2014J4100056).
The morphology of the copper iodide (CuI) film as an inorganic p-type material has an important influence on enhancing the performance of polymer solar cells (PSCs). A self-assembled monolayer of 3-aminopropanoic acid (C3-SAM) was used on the surface of indium tin oxide (ITO) before depositing the CuI films. Consequently, a well-distributed and smooth CuI film was formed with pinhole free and complete surface coverage. The root mean square of the corresponding CuI film was reduced from 3.63 nm for ITO/CuI to 0.77 nm. As a result, the average power conversion efficiency (PCE) of PSCs with the device structure of ITO/C3-SAM/CuI/P3HT:PC61BM/ZnO/Al increased significantly from 2.55% (best 2.66%) to 3.04% (best 3.20%) after C3-SAM treatment. This work provides an effective strategy to control the morphology of CuI films through interfacial modification and promotes its application in efficient PSCs.
Polymer solar cells (PSCs) have many unique advantages of easy manufacturing process, low-cost, light weight, mechanical flexibility, and so on. It is a very promising alternative in photovoltaic field.[1–5] In recent years, numerous strategies have been utilized to enhance the performance of PSCs, including the synthesis of efficient photoactive materials,[6,7] the modification of film morphology,[8,9] and the reasonable selection of transport layers.[10–13] Consequently, the power conversion efficiency (PCE) of PSCs has been significantly improved, and it is worth noting that the values were up to over 12% for nonfullerene polymer solar cells that fabricated by ternary bulk-heterojunction[14] or the molecular regulation of acceptors,[15,16] which will further accelerate the commercial application of PSCs. The widely used structure of PSCs is a sandwich structure of a hole transport layer (HTL), photoactive layer, and electron transfer layer (ETL). The most frequently used material of HTLs is poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) for conventional PSCs. Although PEDOT:PSS has a high work function (5.0 eV), high conductivity, and smooth film, its acidic and hygroscopic natures can corrode the indium tin oxide (ITO) anode and deteriorate the device performance. Furthermore, PEDOT:PSS is not very effective to block electrons.[17–19]
In order to solve the above problems of PEDOT:PSS, various inorganic materials have been reported as efficient HTLs in PSCs, such as nickel oxide (NiOx),[20] vanadium oxide (V2O5),[21] copper oxide (CuO),[22] and copper(I) thiocyanate (CuSCN).[23] Recently, due to the characteristics of suitable energy level, high transparency and solution-processed, γ-phase copper(I) iodide (CuI) as HTLs has also been employed in PSCs.[24–30] CuI is a p-type semiconductor with zinc blende structure and has a wide band gap of 3.1 eV. Shao et al.[24] first used p-type CuI as HTLs for high-efficiency poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PC61BM) bulk-heterojunction solar cells, but the PCE of 2.6% achieved by a solution-processed CuI interfacial layer is lower than 3.1% with the vacuum deposited CuI layer. Subsequently, Sun et al.[25] reported that the PSCs based on a CuI layer spin-coated with an appropriate speed reached a very high PCE of 4.15% by increasing the thickness of organic active layer. However, the CuI interfacial layer prepared by spin-coating CuI dispersion on the surface of ITO easily forms large aggregates of CuI particles and island-style polycrystalline films,[24,29,31] leading to large leakage current and unintentionally direct short for the CuI-based PSCs.
In this work, we adopted solution-processed CuI films as HTLs for PSCs (device structure shown in Fig.
PEDOT:PSS (AI 4083) was purchased from Heraeus. The 3-aminopropanoic, copper iodide (99.999%), anhydrous acetonitrile (99.8%), ZnO, and anhydrous 1,2-dichlorobenzene (99.8%) were purchased from Sigma-Aldrich. The poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) were purchased from Luminescence Technology Corp. These materials obtained from commercial way were used without further purification.
The structure of the fabricated PSCs was ITO/HTLs/P3HT:PC61BM/ZnO/Al (as shown in Fig.
The UV-visible absorbance spectra and the x-ray diffraction (XRD) patterns of CuI films were recorded by an ultraviolet-visible (UV-vis) spectrometer (Agilent Technologies 8453 A) and x-ray diffractometer (Bruker D8 Advance), respectively. The surface morphologies of CuI films were analyzed with the atomic force microscopy (AFM, NT-MDT) and scanning electron microscopy (SEM, ZEISS Ultra 55). The distribution of copper (Cu) and iodine (I) elements in the CuI films was investigated utilizing a distribution mapping technique of energy-dispersive x-ray spectroscope (EDS). The current density–voltage (J–V) characteristics of the devices were measured through a Keithley 2400 source measurement system under the illumination of 100 mW/cm2 of AM1.5 G (Oriel model 91160) and in the dark. An optical contact angle meter (OCA20, Data physics) with tilting table was used to carry out contact angle measurements of ITO substrates under an ambient atmosphere. The external quantum efficiency (EQE) measurements were performed by solar cell spectral response measurement system QE-R3011 (Enlitechnology Co. Ltd, Taiwan, China).
The water contact angle measurement was carried out to explore the surface characteristics of ITO treated by C3-SAM. Figure
Figure
To reveal the effect of C3-SAM on the morphology of CuI films, the surface morphology of CuI films has been investigated by the atomic force microscopy (AFM) and scanning electron microscope (SEM). The AFM image of CuI film spin-coated on the surface of raw ITO is exhibited in Fig.
Figure
This pinhole free and uniform CuI film should be important to be used as HTLs of highly efficient PSCs. We fabricated the PSCs using PEDOT:PSS, CuI, and C3-SAM/CuI as HTLs. The device structure is shown in Fig.
In conclusion, we have demonstrated a simple method of modifying the CuI HTLs with C3-SAM on the surface of ITO to enhance the photovoltaic performance of PSCs. Due to the C3-SAM incorporated between the ITO electrode and CuI layer, the CuI nanoparticles were much well-distributed on the surface of ITO and formed an uniform morphology with pinhole free and complete coverage. The best PCE of the devices based on C3-SAM treatment reached 3.20%, which was similar to that of PSCs with PEDOT:PSS as HTLs. This work provides an effective strategy to control the morphology of CuI films and promotes the application of CuI as an attractive inorganic p-type material in efficient PSCs.
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