Origin of open circuit voltage in planar and bulk heterojunction organic thin-film photovoltaics depending on doped transport layers

Abstract

The aim of this article is to investigate the origin of the open circuit voltage ͑V oc ͒ in organic heterojunction solar cells. The studied devices consist of buckminsterfullerene C 60 as acceptor material and an oligophenyl-derivative 4 , 4Ј-bis-͑N , N-diphenylamino͒quaterphenyl ͑4P-TPD͒ as donor material. These photoactive materials are sandwiched between indium tin oxide and p-doped hole transport layers. Using two different p-doped hole transport layers, the built-in voltage of the solar cells is independently changed from the metal contacts. The influence of the built-in voltage on the V oc is investigated in bulk and planar heterojunctions. In bulk heterojunctions, in which doped transport layers border directly on the photoactive blend layer, V oc cannot exceed the built-in voltage significantly. Though, in planar heterojunctions, V oc is identical with the splitting of quasi-Fermi levels at the donor-acceptor interface and is thus primarily determined by the difference of the lowest unoccupied molecular orbital of C 60 and the highest occupied molecular orbital of 4P-TPD. In planar heterojunctions, the open circuit voltage can exceed the built-in voltage. Furthermore, the investigations show that the efficiency of organic solar cells can be improved by using p-doped charge transport layers with optimized energy level alignment to the active materials. The optimized planar heterojunction shows a fill factor of up to 65.5% and a V oc of 0.95 V. For solar cells with insufficient energy level alignment between the photoactive layer system and the hole transport layer, a reduced V oc in bulk heterojunction cells and a characteristic S shape of the I-V characteristics in planar heterojunction cells are observed.

Built-in voltage of organic bulk heterojuction p-i-n solar cells measured by electroabsorption spectroscopy

“…The work function of the ETL material C 60 doped by W 2 (hpp) 4 is shown for different doping concentrations in Table I. It decreases for higher doping concentration, i.e., as expected a higher doping concentration shifts the Fermi level towards the electron transport level (LUMO).…”

“…6 In contrast, strong changes of the open-circuit voltage with the built-in field were observed for bulk heterojuction solar cells. 4,5,7,8 In addition, a strong correlation of the photocurrent to the internal electric field was previously detected in BHJ solar cells, 9 indicating V BI plays a crucial role for the charge carrier extraction.…”

“…2 Here, open-circuit voltages (V OC ) larger than V BI were observed. 3,4 Nevertheless, a strong influence on the shape of the j-V characteristic is observed, leading to signifcant s-kinks for small built-in voltages. 4,5 An enhancement of both the short-circuit current and the fill factor of more than 10 % was previously observed in PHJ solar cells for an increased V BI .…”

“…3,4 Nevertheless, a strong influence on the shape of the j-V characteristic is observed, leading to signifcant s-kinks for small built-in voltages. 4,5 An enhancement of both the short-circuit current and the fill factor of more than 10 % was previously observed in PHJ solar cells for an increased V BI . 6 In contrast, strong changes of the open-circuit voltage with the built-in field were observed for bulk heterojuction solar cells.…”

“…3,4,8,10,11 Still, interface effects and barriers often result in remarkable a ellen.siebert@iapp. variations of the energy level alignment of these materials.…”