Investigation of Interface Properties for ClAlPc/C₆₀ Heterojunction-Based Inverted Organic Solar Cell

Abstract

The interface properties of organic donor–acceptor heterojunction comprising chloroaluminium phthalocyanine (ClAlPc) and C60 in an inverted organic solar cell configuration have been investigated, with particular emphasis on the molecular orientation and interfacial energy level alignment through the combination of in situ near-edge X-ray absorption fine structure (NEXAFS) and ultraviolet photoelectron spectroscopy (UPS) measurements. In situ NEXAFS measurements reveal that ClAlPc molecules on C60 film on indium tin oxide (ITO) electrode adopt a lying configuration with their molecular π-plane nearly parallel to the substrate surface. Such lying configuration of ClAlPc films can facilitate the efficient charge transport in heterojunction based solar cell devices. As confirmed by in situ UPS measurements, the large ionization potential of ClAlPc leads to a large energy offset between the highest-occupied-molecular-orbital (HOMO) of ClAlPc and the lowest-unoccupied-molecular-orbital (LUMO) of C60 at the ClAlPc/C60 heterojunction interface, as compared with that of the copper phthalocyanine (CuPc)/C60 heterojunction, thereby resulting in a relatively large open circuit voltage in an inverted solar cell model device.

Surface strain mediated dipole alignment of ClAlPc on Au(111)

“…Dipolar phthalocyanine molecules, such as chlorogallium phthalocyanine (ClGaPc), 16,17 chloroaluminum phthalocyanine (ClAlPc), 18 vanadyl phthalocyanine (VOPc), 19 titanyl phthalocyanine (TiOPc), 20 and tin phthalocyanine (SnPc), 21 a class of large p-conjugated molecules with intrinsic dipole moment perpendicular to the p-plane, 22 have attracted intensive attention for their prospective applications in molecular nanoelectronics and organic electronics. [23][24][25] For instance, unidirectionally aligned molecular dipoles can effectively modify the substrate work function and stimulate the charge injection or collection at the molecule-electrode interface. 26,27 Reversible molecular switching (i.e., dipole-up versus dipole-down orientation) has been realized in monolayer ClAlPc, VOPc on graphite, 28,29 VOPc in hydrogen-bonded networks on graphite, 30 and SnPc on Cu(111), 31 which might fuel ultrahigh density information storage eventually.…”

Buffer-Free Inverted Photovoltaic Cells with Hybrid C60 /CuI Heterojunction

“…Using cathode buffers, such as LiF or bathocuproine BCP, is widely recommended in order to avoid the “S‐kink” in the illuminated J‐V dependences of the normal devices through reduction of interfacial effects (see, e.g., and references therein). However, with inverted devices, the insertion of a buffer between the cathode and C 60 is not always helpful . Our experiments showed that, in fact, insertion of BCP or LiF between the Al and C 60 layers deteriorates cell performance (Table S1, Supporting Information).…”

“…Besides, the structure of the CuI layer deposited over C 60 is somewhat different from that being the case when the CuI layer is beneath C 60 (Figure S2, Supporting Information). This circumstance may potentially affect the photovoltaic performance of cells …”

“…First, the U oc increases from 0.51 to 0.65 V when PET is swapped for glass, which is the maximal value observed for the devices with the inverted heterojunction − Table . Such an increase cannot be solely attributed to the improvement of R SH , it is likely to involve some changes in the band diagram of a photovoltaic device on glass due to chemical or morphological reasons, for example . Second, the FF value rises from 48 to 64%, being accompanied by a huge increase in the rectification ratio RR (see, also Figure S3, Supporting Information).…”

Perylene bisimide as the cathode modifier in organic photovoltaics: the role of aggregation morphology on the interlayer performance

“…Here, we report modication of ITO surface by the in situ formed nano-aggregates of PBI-1, and their different morphology effects on the electronic properties of ITO electrode by UPS measurements. [36][37][38] The nanobers and nanoparticles of PBI-1 on ITO surface were obtained via spin-coating from chloroform and THF solutions respectively. To minimize the effect of lm thickness on the electronic properties, the thickness of the PBI-1 on ITO was kept 10 nm by control over the solution concentration and revolving speed of the spin-coater, as discussed below.…”