82
T. Kaji
and mixed buffer layers for the ultra-thick OPVs. The effect of antireflection film is
also demonstrated in addition.
4.3.1 Buffer Layer Selection for Stable Comparison
In this study, the buffer layer is optimized in order to increase the film thickness
while stabilizing J sc and other characteristics in the device with an active layer
whose thickness exceeds 400 nm. In this section, the selection of this buffer layer is
explained. For this stabilization, LiF was used as an anode buffer layer just below
the Ag electrode to suppress the intercalation of Ag into the organic film, and an
Alq 3 :C 60 mixed buffer was used to uniformly cover the active layer. It has been
reported [30–33] that such a buffer mixture improves the electronic conductivity of
the buffer layer while maintaining the amorphous property such as the combination of
BCP:PBD, BCP:C 60 , and BPhen:C 60 . Although the authors tried many other mixed
buffers, Alq 3 :C 60 was the best for this purpose of the stabilization to the thickness.
Of course, if a single buffer layer of BCP or Alq 3 with a thickness of about
10 nm, which is used for ordinary OPVs, can be used without any problems in many
cases. Usually between the single buffer and the mixed buffer, almost no difference
in performance could be observed except the difference in loss due to the optical
absorption of the buffers. The difference occurred when the optimum thickness of
the single buffer varied due to the surface roughness of the active layer. This is because
if the BCP or Alq 3 single buffer is too thick (in a few nanometers), the J sc sharply
decreases, but in the case of the mixed buffer, the J sc only gradually decreases. Using
the mixed buffer, now, it is possible to make a stable comparison. Because of this
difference, in this study, Alq 3 :C 60 mixed buffer was used for the thickness-dependent
characteristic investigation of the OPV using a crystalline organic mixed active layer
with a rough surface.
The underlying interface buffer used a series of layers of F 4 TCNQ (0.6 nm)/CuI
(3 nm)/ZnPc (2 nm)/C 60 (2.5 nm)/ZnPc (1 nm). The F 4 TCNQ/CuI/ZnPc layer just
above the ITO electrode acted as a hole transport layer to improve the anodic
contact
11,27 . The C 60 /ZnPc layer served as a template layer to reproduce the starting
block of growth of the active ZnPc:C 60 blend layer. This template contributed greatly
to the reproducibility of repeated production of the device. The cell with all these
buffer layers also showed good thermal stability during the two heating/cooling
cycles: heating to 100 °C for 1 h and cooling to 25 °C without bias. They showed no
deterioration in J–V characteristics.
To quantitatively distinguish the effect of these layers from the effect of coevaporant-induced crystallization, we made a set of OPV cells with a 600-nm-thick
ZnPc:C 60 active layer with or without an underlying interfacial layer. The J–V characteristics of these cells and the incident photon-to-current efficiency (IPCE) spectra
are compared in Fig. 4.5a and b. Crystallization with co-evaporant has the greatest
effect on J sc , but requires a lower interfacial layer to significantly improve J sc and
T. Kaji
and mixed buffer layers for the ultra-thick OPVs. The effect of antireflection film is
also demonstrated in addition.
4.3.1 Buffer Layer Selection for Stable Comparison
In this study, the buffer layer is optimized in order to increase the film thickness
while stabilizing J sc and other characteristics in the device with an active layer
whose thickness exceeds 400 nm. In this section, the selection of this buffer layer is
explained. For this stabilization, LiF was used as an anode buffer layer just below
the Ag electrode to suppress the intercalation of Ag into the organic film, and an
Alq 3 :C 60 mixed buffer was used to uniformly cover the active layer. It has been
reported [30–33] that such a buffer mixture improves the electronic conductivity of
the buffer layer while maintaining the amorphous property such as the combination of
BCP:PBD, BCP:C 60 , and BPhen:C 60 . Although the authors tried many other mixed
buffers, Alq 3 :C 60 was the best for this purpose of the stabilization to the thickness.
Of course, if a single buffer layer of BCP or Alq 3 with a thickness of about
10 nm, which is used for ordinary OPVs, can be used without any problems in many
cases. Usually between the single buffer and the mixed buffer, almost no difference
in performance could be observed except the difference in loss due to the optical
absorption of the buffers. The difference occurred when the optimum thickness of
the single buffer varied due to the surface roughness of the active layer. This is because
if the BCP or Alq 3 single buffer is too thick (in a few nanometers), the J sc sharply
decreases, but in the case of the mixed buffer, the J sc only gradually decreases. Using
the mixed buffer, now, it is possible to make a stable comparison. Because of this
difference, in this study, Alq 3 :C 60 mixed buffer was used for the thickness-dependent
characteristic investigation of the OPV using a crystalline organic mixed active layer
with a rough surface.
The underlying interface buffer used a series of layers of F 4 TCNQ (0.6 nm)/CuI
(3 nm)/ZnPc (2 nm)/C 60 (2.5 nm)/ZnPc (1 nm). The F 4 TCNQ/CuI/ZnPc layer just
above the ITO electrode acted as a hole transport layer to improve the anodic
contact
11,27 . The C 60 /ZnPc layer served as a template layer to reproduce the starting
block of growth of the active ZnPc:C 60 blend layer. This template contributed greatly
to the reproducibility of repeated production of the device. The cell with all these
buffer layers also showed good thermal stability during the two heating/cooling
cycles: heating to 100 °C for 1 h and cooling to 25 °C without bias. They showed no
deterioration in J–V characteristics.
To quantitatively distinguish the effect of these layers from the effect of coevaporant-induced crystallization, we made a set of OPV cells with a 600-nm-thick
ZnPc:C 60 active layer with or without an underlying interfacial layer. The J–V characteristics of these cells and the incident photon-to-current efficiency (IPCE) spectra
are compared in Fig. 4.5a and b. Crystallization with co-evaporant has the greatest
effect on J sc , but requires a lower interfacial layer to significantly improve J sc and