Characteristics of PTB7‐Th:C bulk heterojunction photocells under low‐light illumination: Critical effect of dark parallel resistance

Abstract

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 lowenergy gap polymer poly [[4,8-bis[5-(2-ethylhexyl) [3,4-b]thiophenediyl]] (PTB7-Th) as well as unmodified C 70 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.1 Introduction Photocells based on bulk heterojunction composites consisting of conjugated polymers and fullerenes are attracting much interest of the researchers, partly because the compatibility with low-temperature solution-based process makes them promising candidates for large-area low-cost renewable energy source in the near future [1][2][3][4]. The development of conjugated polymers such as poly [[4,8-bis[(2ethylhexyl)oxy]benzo [1,2-b:4,5-b ]dithiophene-2,6-diyl][3fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) and poly[[4,8-bis[5-(2-ethylhexyl) thiophene-2yl]benzo[1,2-b:4,5-b'f]dithiophene-2,6-diyl][3-fluoro-2-[(2ethylhexyl)carbonyl]thieno [3,4-b]thiophenediyl]] (PTB7-Th) is pushing up the performance of this class of photocells [5,6].The use of highly soluble fullerene derivatives such as [6,6]-phenyl-C 71 -butyric acid methyl ester (C 70 -PCBM), as

Lighting flicker: a blind spot in indoor photovoltaic cell characterization

“…However, the loss becomes high enough to skew the current-voltage curve to lower the FF of the device at a high illumination level and eventually limits the short-circuit photocurrent. The increased FF at low illumination intensity is also observed in photovoltaic devices based on organic semiconductors [15][16][17][18][19] and is considered as a key for the increased PCE at a low illumination level of the class of photovoltaic cells. The PCEs at 10L 0 and 20L 0 are found to be approximately 39% and 23% of the PCE at L 0 , respectively.…”

“…Therefore, photovoltaic devices based on other materials with wider bandgap energy than c-Si such as amorphous silicon (a-Si), III-V semiconductors, organic semiconductors, dye-sensitized TiO 2 , and lead-halide perovskite are attracting much attention of the researchers for indoor light harvesting. [11][12][13][14][15][16][17][18][19][20][21][22][23][24] A semiconductor with wider bandgap energy has smaller intrinsic carrier concentration and thus is expected to yield smaller dark saturation current density, which is another key for efficient indoor photovoltaic cells because the photocurrent density under indoor illumination condition approaches the dark saturation current density.…”

What Do Apparent Series and Shunt Resistances in Solar Cell Estimated by I–V Slope Mean? Study with Exact Analytical Expressions

“…Although the full description of solar cell characteristics requires the numerical solution of a set of differential equations, such model is so complicated to be implemented into the electronic circuit design software. Suitable equivalent‐circuit model of a solar cell is not only necessary for designing efficient electronic circuit powered by the solar cell, but also useful for understanding the loss mechanism taking place in the solar cell …”

“…Such discrimination requires a systematic study including the collection of suitable set of data such as the dependence of estimated circuit parameters on experimental condition like temperature or illumination light intensity. For example, it has been reported that R p in organic solar cell is inversely proportional to the illumination light intensity while that in Si solar cell slightly depends on the light intensity, reflecting the difference in the carrier mobilities in the active material. It is unquestionable that accurate estimation of circuit parameters is critically important for such purpose.…”

“…Since a single solar cell generally outputs relatively large current with low voltage, the evaluation of the series resistance is considered as especially important issue. While parameter extraction through the fitting of current–voltage curve with the model gives adequate value, a variety of methods for the series resistance estimation without fitting had been proposed so far . On the other hand, in the field of solar cell research using emerging materials such as organic and perovskite compounds, it is customary to mention the reciprocal slopes of a current–voltage curve at open‐circuit‐ and short‐circuit‐conditions as apparent series and shunt resistances, respectively .…”