光電所所訊

Role of Cathode Connecting Layers and Energetic Alignment in Organic Photovoltaic Devices

Professor Chih-I Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所吳志毅教授

Open-circuit voltage is one of the key parameters affecting the performance of organic photovoltaic devices. Several electronic mechanisms originating from energy band alignment within the path of charge transport and carrier extraction are the major factors that limit the V_oc. We demonstrated that the influence of energetic disorder due to introduced cathode buffer layers between active layer and metal cathode can contribute to energetic favorable charge transport and extraction, leading to V_oc enhancement. To explore the dependence of V_oc on the energetic properties of the contacting materials prior to cathode deposition, we proposed a comprehensive model for buffer layers based on molecular materials commonly employed in OLED and OPV cells.

Energy band alignment of various cathode buffer layers in P3HT/PCBM based bulk-heterojunction (BHJ) solar cell are checked via in situ ultra-violet photoemission spectroscopy (UPS) and the results are organized as follows: First, we clarified the correlation between charge transport property and carrier extraction ability at buffer layers with energetic favorable band alignment. This includes a presentation of qualitative models for the dependence of HOMO and LUMO offset between active layer and buffer layer on the shunt resistance and series resistance, and short-circuit current density, or saturated value of V_oc based on Shockley’s theory for p-n junctions. Second, the dependence of V_oc and OPV operating parameters on the cathode contacting materials with inappropriate energy offset or transport ability, such as narrow band gap and hole transport materials, were obtained. We investigated not only the LUMO offset related to electron transport, but also the influence of HOMO offset on carrier extraction at cathode contact. Experimental details and device performance are provided in our recent publication. As a result, the performance of P3HT/PCBM based BHJ solar cells with various buffer materials can be classified into three class according to the energy offset. Class I contained large HOMO and LUMO offset so the better hole blocking and electron transport ability lead to a saturated value, 0.6 V, of V_oc in P3HT/PCBM system. Class II has either large HOMO offset or LUMO offset making a small decrease in V_oc and a poor V_oc for Class III due to inappropriate energy offset of buffer materials. Finally, we summarize the key ﬁndings to propose and verify a new model which gives predictions for the maximum V_oc in BHJ solar cells consisting of buffer materials.

Organic solar cell with 95.6% internal quantum efficiency

Professor Jiun-Haw Lee

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所李君浩教授

With suitable device and fabrication design, an organic solar cell (OSC) with 95.6% internal quantum efficiency (IQE) was demonstrated with boron subphthalocyanine chloride (SubPc) and C₆₀ as the electron donor and acceptor materials, respectively. To improve the open circuit voltage (V_OC), SubPc/C₆₀ heterojunction structure was employed. Suitable deposition rate of SubPc was needed between the tradeoff between V_OC and fill factor. 3-nm N,N-dicarbazolyl-3,5-benzene (mCP) was inserted between indium-tin-oxide (ITO) anode and SubPc which served as the exciton and electron blocking layer. Combined with layer thickness optimization, IQE of such an OSC reached 95.6% at the absorption peak 590 nm.


Fig. 1. (a) J–V characteristics and (b) IQE of OSC with different mCP thickness.