Direct Evidence of Less Charge Accumulation in Highly Durable Polymer Solar Cells Using Operando Electron Spin Resonance Spectroscopy

Abstract

Inverted-type polymer solar cells with poly[naphtho [1,2-c:5,6-c']bis [1,2,5]thiadiazole-5,10diyl [3,4'-bis(2-butyloctyl)[2,2'-bithiophene]-5,5'-diyl]thiazolo [5,4-d]thiazole-2,5-diyl[3',4-bis (2-butyloctyl)[2,2'-bithiophene]-5,5'-diyl]] (PTzNTz) have attracted attention because of its high power conversion efficiency and thermal stability. However, the internal deterioration mechanism hasn't yet been completely clarified. Here, we report operand electron spin resonance (ESR) spectroscopy of the highly durable PTzNTz polymer solar cells to investigate the mechanism of the device-performance deterioration at a molecular level. We identify the layer with accumulated charges in the PTzNTz cells by the analyses of light-induced components in the ESR spectra of the thin films and cells with PTzNTz. We observe the correlation between the increase in the number of spins and the decrease in the device performance. Notably, compared to the previously reported ESR studies of polymer solar cells, the PTzNTz solar cells have less charge accumulation and higher durability. This correlation holds even if another hole-transport layer of PTzNTz solar cells is utilized. The calculation with density functional theory is performed to analyze the charge-accumulation states in the cells. The present results are useful to develop further highly efficient and durable polymer solar cells.

Deterioration mechanism of perovskite solar cells by operando observation of spin states

“…The measured temperatures were between 4 K and room temperature. The parts of the above-mentioned ESR spectroscopy characterization method have been described in the previous works [22][23][24][25][26]37,38,43 .…”

Analyses of PTzNTz Polymer Solar Cells Using ESR Spectroscopy

“…It is very interesting to investigate the charge accumulation in PTzNTz polymer solar cells. In our previous studies, the signals of PTzNTz solar cells have been identified as a hole signal of PTzNTz and an electron signal of PC 71 BM under simulated solar irradiation [23]. We also have found that the ESR intensity of the PTzNTz cell with a hole transport layer (HTL) of WO x is smaller than that with MoO x HTL.…”

“…We used the continuous-wave ESR method in the experiments which performs a lockin detection with an external magnetic field (H) modulation of 100 kHz. Thus, photogenerated charges with a lifetime (<10 μs) cannot be observed, and it is possible to observe only long-lived (or accumulated) photogenerated charges with an unpaired spin with a long lifetime (>10 μs) in thin films and solar cells [23][24][25][26].…”

“…The present ESR system significantly reduces experimental errors because the fluctuation of experimental conditions such as microwave power etc. can be calibrated by using the signal of the marker sample [23,26].…”

“…In our study, the substrate plane was parallel to the H direction. All measured light-induced spectra were averaged under 1 h irradiation [23,26].…”