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Characteristics of PTB7‐Th:C 70 bulk heterojunction photocells under low‐light illumination: Critical effect of dark parallel resistance
Author(s) -
Tada Kazuya
Publication year - 2017
Publication title -
physica status solidi (a)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.532
H-Index - 104
eISSN - 1862-6319
pISSN - 1862-6300
DOI - 10.1002/pssa.201700018
Subject(s) - photoresistor , photocurrent , light intensity , optoelectronics , materials science , heterojunction , halogen lamp , chemistry , optics , physics
Solution‐processed organic photocell is one of the most promising candidates for the permanent power source for wireless devices. Since the device is not necessarily to be placed outdoor, and indoor light is generally weaker than outdoor light by 2–3 orders of magnitude, the characterization of the photocell under low‐light illumination is important for this purpose. In this study, bulk heterojunction photocells based on a low‐energy gap polymer poly[[4,8‐bis[5‐(2‐ethylhexyl)thiophene‐2‐yl]benzo[1,2‐b:4,5‐b′ ]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl]] (PTB7‐Th) as well as unmodified C70 prepared with a halogen‐free solvent 1,2,4‐trimethylbenzene, are characterized under low‐light illumination. The combination of a halogen‐free solvent with an unmodified fullerene potentially provides a way to develop environmentally friendly organic photocells. It is found that the photocells show higher power conversion efficiency (PCE) under lower light illumination intensity. That is, a device showing PCE of 4.9% under 1 sun illumination shows PCE over 9% at 3′ 10− 4 sun. A sublinear dependence of the short‐circuit photocurrent to the light intensity, as well as the increased fill‐factor at reduced illumination, is found to be a key to the high PCE at low‐light. The latter originates from the photoconduction in the composite. The critical effect of dark parallel resistance on the low‐light performance of the photocells is also demonstrated.