Energy levels in dilute-donor organic solar cell photocurrent generation: A thienothiophene donor molecule study
Energy levels in dilute-donor organic solar cell photocurrent generation: A thienothiophene donor molecule study
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DOI：
10.1016/j.orgel.2021.106137
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发表时间：
2021-05
影响因子：
3.2
通讯作者：
L. Murthy;Aaron Kramer;Boya Zhang;Jin Su;Yi‐Sheng Chen;Ken‐Tsung Wong;W. Vandenberghe;J. Hsu
中科院分区：
文献类型：
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作者：
L. Murthy;Aaron Kramer;Boya Zhang;Jin Su;Yi‐Sheng Chen;Ken‐Tsung Wong;W. Vandenberghe;J. Hsu
为了研究这些有机太阳能电池（OSCs）中的光电流产生机制，我们设计并合成了四种基于噻吩（TT）的小分子给体，它们的最高占据分子轨道（HOMO）水平从−6.4 eV到−5.1 eV不等，跨越了[6,6]-苯基- c70 -丁酸甲酯（PC71BM）受体的HOMO值。我们测量了基于tt的给体：PC71BM薄膜的电子和光学特性、OSC电流密度电压特性和外部量子效率，并进行了密度泛函理论（DFT）计算。我们的研究结果表明，光电流的产生强烈依赖于中心TT基团，氰基（-CN）取代己氧基（-OHex）。当供体重量为1 wt%时，TTOHex:PC71BM器件的光电流比纯PC71BM器件高7倍，当供体重量为5 wt%时增加到12倍。相比之下，TTCN:PC71BM器件即使有10 wt%的供体也不会产生额外的光电流。在基于tt的给体：PC71BM器件中，光电流的产生主要取决于给体分子相对于PC71BM的HOMO值，这表明II型能级排列对于促进给体-受体界面激子解离的重要性。所有TT:PC71BM器件的光电压与纯PC71BM器件相当，为0.85-0.90 V，由于非辐射复合而具有低电压损耗。由于低空穴迁移率，TTOHex:PC71BM器件的填充因子较低，为~10−8cm2/V。在激子解离之后，根据三种可能的机制来分析空穴输运：隧道、渗透途径和空穴回输。我们发现空穴回输机制可以解释所有的实验结果，因此是tt基供体：PC71BM稀释供体OSCs中光电流产生的主要空穴输运机制。
To investigate photocurrent generation mechanisms in these organic solar cells (OSCs), we design and synthesize four thienothiophene (TT)-based small-molecule donors with the highest occupied molecular orbital (HOMO) levels varying from −6.4 eV to −5.1 eV, which span across the HOMO value of the [6,6]-phenyl-C70-butyric acid methyl ester (PC71BM) acceptor. We measure TT-based donor:PC71BM films’ electronic and optical properties, OSC current density-voltage characteristic, and external quantum efficiency, and perform density functional theory (DFT) calculations. Our results show that photocurrent generation depends strongly on the substitutions of the center TT groups, cyano (-CN) versus hexyloxy (-OHex). With 1 wt% donor, TTOHex:PC71BM devices produce seven times, increasing to twelve times for 5 wt % donor, higher photocurrent than neat PC71BM devices. In contrast, TTCN:PC71BM devices do not generate additional photocurrent even with 10 wt% donor. The photocurrent generation in TT-based donor:PC71BM devices depends critically on the HOMO value of the donor molecule with respect to that of PC71BM, indicating the importance of type II energy level alignment to facilitate exciton dissociation at the donor-acceptor interface. The photovoltage of all TT:PC71BM devices are comparable to neat PC71BM devices, 0.85–0.90 V, with a low voltage loss due to non-radiative recombination. The fill factor of TTOHex:PC71BM devices are low due to the low hole mobility, ~10−8cm2/V. Following exciton dissociation, hole transport is analyzed according to three possible mechanisms: tunneling, percolation pathways, and hole back transfer. We find that the hole back transfer mechanism can explain all experimental results and therefore is the primary hole transport mechanism for photocurrent generation in TT-based donor:PC71BM dilute-donor OSCs.