Chia-Chang Chou

Improvement in the Power Conversion Efficiency of Bulk Heterojunction Photovoltaic Device via Thermal Postannealing of Subphthalocyanine:C70 Active Layer

The authors report an efficient organic photovoltaic device based on subphthalocyanine (SubPc):C70 bulk heterojunction (BHJ) via the postannealing treatment. The power conversion efficiency is improved from 4.5% to 5.5% due to the increase in short-circuit current density () from 8.8 to 12.7 mA/cm2 with the expense of decreased fill factor from 52% to 42%. From external quantum efficiency measurements, the spectral shape-independent enhancement over the entire spectrum suggests that the increased mainly originates from improved charge collection efficiency. To confirm this inference, the hole and electron mobilities in the BHJ are estimated from the space-charge limited current, showing improved transport properties at the optimum temperature. Moreover, the morphologic change is also studied as a function of annealing temperature. A larger grain size is observed with increasing temperature due to the phase separation of SubPc and C70. However, at higher temperatures the strong aggregation of C70 molecules may interrupt the pathway of SubPc, resulting in hindered charge transport and, hence, reduced .

Improving the performance of subphthalocyanine/C60 planar heterojunction organic photovoltaic device through the insertion of molybdenum oxide anodic buffer

Here, an efficient subphthalocyanine (SubPc)/C60 heterojunction organic photovoltaic device is demonstrated by using MoO3 as the anodic buffer. In comparison with the device without any treatments, the insertion of MoO3 leads to a significant increase in open-circuit voltage due to a better energy level alignment of the SubPc, which is similar to the use of oxygen-plasma. In addition, MoO3 serves as an optical spacer to tune the SubPc/C60 interface at the optimum optical field distribution. As a result, the short-circuit current density is considerably improved as predicted using the simulation model based on the transfer matrix. A slightly increased fill factor implies the efficient hole extraction after the insertion of MoO3. Moreover, the device with MoO3 as anodic buffer shows an elongated lifetime as compared with the device with oxygen-plasma treatment.

Origin of the Improved Power Conversion Efficiency of Pentacene/C60 Heterojunction Photovoltaic Devices through the Purification of Donor Material

The authors report the impact of the crystallinity property of the electron donor on the performance of a pentacene/C60 organic photovoltaic device. After subjecting pentacene to sublimation twice, all the photovoltaic parameters showed significant improvements leading to enhancement of the power conversion efficiency from 0.9 to 2.2% under air mass 1.5G solar illumination. This is attributed to the well-packed molecular structure in the pentacene thin film, as observed by X-ray diffraction, which leads to high carrier mobility and hence high photocurrent. Moreover, the elimination of microscopic pinholes or defect sites due to the improvement in the degree of the pentacene thin film reduces the dark current and therefore increases the photovoltage. The external quantum efficiency and space-charge limited current are used to analyze the relationship between the quality of thin film electron donor and device performance.

Efficient planar-mixed heterojunction photovoltaic device based on subphthalocyanine:C70 structure

In this report, we demonstrate an efficient planar-mixed heterojunction organic photovoltaic (OPV) device employing a mixed structure of subphthalocyanine (SubPc) donor and C₇₀ acceptor. Compared to a SubPc:C₆₀ cell, the SubPc:C₇₀ cell exhibits high performance with a fill factor (FF) of 52%, short-circuit current density (J_SC) of 8.8 mA/cm², open-circuit voltage (V_OC) of 1 V, and power conversion efficiency of 4.6% under air-mass 1.5 G illumination (1 sun). The high V_OC is mainly due to the wide interface gap between SubPc and C₇₀ as in the SubPc:C₆₀ device. On comparing the absorption spectra of C₇₀ and C₆₀, C₇₀ shows the higher absorption coefficient and wider absorption band, leading to the significantly improved J_SC. Moreover, the FF of the device using C₇₀ is enhanced to 52%; by comparison it is 45% when using C₆₀. In order to understand this phenomenon, the space-charge limited current is measured for estimating the carrier mobility of SubPc, C₇₀, and C₆₀. It reveals that the electron mobility of C₇₀ is lower than that of C₆₀ by orders of magnitude but is close to the hole mobility of SubPc. As a result, a better charge balance condition is achieved when SubPc is blended with C₇₀ and therefore a higher FF is obtained. Consequently, C₇₀ seems to be a more suitable acceptor for the mixed structure to develop a highly efficient OPV device due to the improvements in both electrical and optical properties.