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.

Clarification of the internal deterioration mechanism in polymer solar cells is an important issue for further improvement of not only device durability but also initial device performance. In particular, the charge accumulation at polymerchain ends has been discussed for such an internal deterioration mechanism. However, the direct correlation between polymerchain-end structures and performance deterioration has not yet been investigated from a microscopic viewpoint. Here, we report direct evidence of the internal deterioration mechanism due to the charge accumulation at polymer-chain ends by operando electron spin resonance (ESR) spectroscopy at the molecular level. Using semiconducting polymers with different molecular chain-end structures, that is, poly[(3,4-ethylenedioxythiophene-2,5-diyl)-(9,9-dioctylfluorene-2,7-diyl)] (PEDOTF) with bromine ends (PEDOTF-Br) and that with hydrogen ends (PEDOTF-H), we demonstrate the correlation between the chain-end structures and solar-cell durability, showing a larger amount of charge accumulation and larger performance deterioration for PEDOTF-Brbased solar cells compared to those for PEDOTF-H-based solar cells, owing to the inhabitation of charge transport and the formation of additional potentials in the cells. The origins of observed charge accumulation are precisely clarified with the fitting analyses of operando light-induced ESR spectra and density functional theory calculation, demonstrating that the main internal deterioration is due to the charge accumulation at polymer-chain ends from a microscopic viewpoint. Because such charge accumulation also affects the initial device performance, the optimization of polymer-chain ends will be important for further improvement of not only device durability but also initial device performance for other polymer solar cells.

Inverted-type polymer solar cells with semiconducting polymers based on PTzNTz have attracted attention because of its high power conversion efficiency (PCE) and thermal stability. In recent years, the PCE of the polymer solar cells has been remarkably improved. However, the effect of these hole transport layers (HTLs) on PTzNTz solar cells have not yet been completely clarified. Here, we report operand electron spin resonance (ESR) spectroscopy of the PTzNTz layered samples to investigate the deterioration mechanism of the PTzNTz solar cells at a molecular level. We have observed monotonic increases in the ESR intensity of the PTzNTz layered samples, where the increases in the ESR intensity of PTzNTz active layers with WOx HTL are smaller than those with MoO x HTL. The present results would be useful to develop further highly efficient and durable polymer solar cells.

The performance of organic solar cells has been remarkably improved recently, where the cell structures are important for achieving high efficiency and stability. The formation and accumulation of long-lived charges in the cells are critical for the efficiency and stability of the cells; however, their relations with the cell structures have not yet been clarified from a microscopic viewpoint. Here, we report the microscopic mechanism of high efficiency and stability of inverted polymer fullerene solar cells compared to conventional cells even though the same photoactive layers are utilized. We directly observe the formation and accumulation of long-lived charges in these cells with electron spin resonance at a molecular level. We find the reduced effects of the formation and accumulation of long-lived charges in the inverted cells on their efficiency and stability compared to the case of conventional cells. These findings provide a striking advance in fundamental understanding, which is useful for further clarifying the operation mechanism of organic solar cells as well as further improving their efficiency and stability.

The performance of organic solar cells has remarkably improved recently, where the cell structures are important for achieving high efficiency and stability. The formation and accumulation of long-lived charges in the cells are critical for the efficiency and stability of the cells; however, their relations with the cell structures have not yet been investigated from a microscopic viewpoint. Here, we show the origin of higher efficiency and stability of inverted organic solar cells compared to those of conventional cells, even though the same photoactive layers with fullerene acceptors are utilized. We directly study the formation and accumulation of long-lived charges in these cells with electron spin resonance (ESR) at a molecular level for as-prepared states and operando states. We find the reduced effects of formation and accumulation of long-lived charges for the efficiency and stability in the inverted cells compared to the case of the conventional cells. These findings are useful for deeply understanding the operation mechanism of organic solar cells as well as further improving their efficiency and stability.