Thermally Stable All‐Polymer Solar Cells with High Tolerance on Blend Ratios

Abstract

comprised of a conjugated polymer as the electron donor and a fullerene derivative as the electron acceptor. [2,3] Conventional fullerene acceptors like [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) may be unfavorable for practical application, due to the inefficient absorption in visible and near-infrared (IR) regions, fixed molecular structure, complex purification process, and other complications well-documented in the literature. [4][5][6][7][8] In addition, the morphology of polymer-fullerene blends is very sensitive to thermal annealing, solvent additive, film thickness, and especially D:A ratio, resulting in significantly different device performance. [9][10][11] Recently, people have shown that the integration of either polymeric or molecular acceptor into photovoltaic devices would be advantageous. The chemical structures of nonfullerene acceptor can be easily adjusted to tune their energy levels, and the conjugated molecule or polymer acceptor demonstrates enhanced absorption at long wavelengths relative to fullerenes. [12] Thus nonfullerene photovoltaics can have improved harvest of solar radiation, enhanced thermal and mechanical stability, and reduced open-circuit voltage loss. [13] To date, solution-processed nonfullerene solar cells based on polymer-polymer (all-polymer) and polymer-small molecule blend have achieved power conversion efficiencies (PCEs) of 10% and 13%, respectively. [14][15][16][17][18][19] Exciton dissociation and charge transport are at the core of organic photovoltaics, which is strongly affected by the BHJ blend morphology. [20][21][22][23] In fullerene-based systems, the morphology of thin films is critical to the device performance and has been extensively studied. [24] How the blend morphology of nonfullerene blend affects device efficiency and stability is of growing interest as the PCEs of many nonfullerene solar cells now exceed the best fullerene devices. It is well known that the morphology has been largely influenced by the D:A blend ratio. Early studies on fullerene-based devices observed a drastic change in film morphology when fabricating the devices with increased acceptor content. [25][26][27][28] However, for nonfullerene solar cells, recent efforts mainly focus on materials synthesis and device optimization. To the best of our knowledge, systematical study on the effect of blend ratio on the device morphology, performance, and stability has not been reported, while it is of great importance to help achieve in-depth Tuning the blend composition is an essential step to optimize the power conversion efficiency (PCE) of organic bulk heterojunction (BHJ) solar cells. PCEs from devices of unoptimized donor:acceptor (D:A) weight ratio are generally significantly lower than optimized devices. Here, two high-performance organic nonfullerene BHJ blends PBDB-T:ITIC and PBDB-T:N2200 are adopted to investigate the effect of blend ratio on device performance. It is found that the PCEs of polymer-polymer (PBDB-T:N2200) blend are more tolerant to composition changes, relati...

All‐Polymer Solar Cells: Recent Progress, Challenges, and Prospects

“…Furthermore, most fullerene or small molecular based acceptors suffer from an initial PCE loss of up to 30–40 % (“burn‐in loss”) in times as short as several hundred hours with (Figures a,b). Thus, PBDB‐T:ITIC suffers significant aging and thermal degradation in a short time while those fabricated with 10:1 PBDB‐T:N2200 exhibit remarkable aging and thermal stability . The devices were stored on a hot plate at 80 °C in a glovebox without encapsulation between measurements to investigate the thermal stability.…”

“…Thus, Ma et al. showed that the PCEs of PBDB‐T:P(NDI2OD‐T2) cells vary far less than those based on PBDB‐T:ITIC blends (Figure c) . They fabricated and compared the performance of non‐fullerene PSCs based on PBDB‐T :acceptor blends at various weight blend ratios, from 1:10 to 20:1.…”

Understanding the Interplay of Transport‐Morphology‐Performance in PBDB‐T‐Based Polymer Solar Cells

“…Nonfullerene molecular or polymeric electron acceptors offer unique opportunities for photovoltaic applications due to their flexible structural tunability and new device features that emerge at the donor/acceptor heterojunction. In addition, the adoption of nonfullerene materials can offer exceptional stability which has not been achieved in previous polymer–fullerene systems . This progression demonstrates the potential for nonfullerene PSCs and its advantages for further design opportunities for efficient and stable solar cells for large‐scale applications.…”

“…Conjugated molecules and polymers tend to exhibit better thermal stability than conventional fullerene derivatives. This property presents unique potential as these materials are less prone to thermal stress and exhibit better phase stability . However, the morphology of bulk heterojunction (BHJ) blend film is detrimental to charge transport and recombination.…”

Solution‐Processed Molybdenum Oxide with Hydroxyl Radical‐Induced Oxygen Vacancy as an Efficient and Stable Interfacial Layer for Organic Solar Cells

“…As shown in the XPS results of Figure S9, Supporting Information, the surface chemical compositions of both the PEDOT:PSS and SM5 films exhibit almost negligible changes after thermal annealing at 85 °C for 50 h in ambient air conditions. Therefore, the similarly observed ambient‐air thermal instabilities from PEDOT:PSS‐ and SM5‐based devices are likely to originate from the thermal degradation of the PBDB‐T:ITIC photoactive layer during the initial test period before the HTL films begin to have any noticeable effect on device stability . Ambient air stability of PBDB‐T:ITIC‐based devices without encapsulation at room temperature was also investigated, as shown in Figure c.…”