What's Next for Organic Solar Cells? The Frontiers and Challenges

Abstract

The power conversion efficiency (PCE) of organic solar cells (OSCs) is improved dramatically in recent years and now approaches >19% for single‐junction cells and >20% for tandem cells. Therefore, the practical use of OSCs is becoming a reality. This perspective summarizes the state of the art of OSC characteristics and discusses the challenges that remain in further improving PCE. The short‐circuit current density (J SC) of the state‐of‐the‐art OSCs almost approaches 30 mA cm−2. As the internal quantum efficiencies of these devices exceed 90%, for further improvement in J SC, it is necessary to suppress reflection at interfaces and increase the active layer thickness to absorb as many photons as possible. Further suppression of the nonradiative voltage loss is imperative, as there is still large room for improvement compared to inorganic and perovskite counterparts. Hence, increasing the exciton lifetime and photoluminescence quantum yield of nonfullerene acceptors are pivotal to improving the open‐circuit voltage of OSCs. The fill factors of the latest OSCs approach 80%; however, because the optimized active layer thickness remains ≈100 nm, further suppression of bimolecular charge recombination is needed. Finally, this perspective discusses to what extent PCE can be improved and what can be done to achieve this.

Role of the energy offset on the charge photogeneration and voltage loss of nonfullerene acceptor-based organic solar cells

“…19 Therefore, as shown in Figure 3a, the maximum EQEPV values were also plotted against ΔVnr. EQEPV again showed a clear threshold at approximately 0.2 V; EQEPV dropped sharply in the region where ΔVnr was less than 0.2 V. This result is consistent with the fact that little OSCs reported to date exhibited high EQEPV with ΔVnr less than 0.2 V. 9 Briefly, by plotting EQEPV against the three different criteria (ΔV, ΔIEPYSA, and ΔVnr), we observed a clear threshold above which efficient charge photogeneration could be ensured in any of the three criteria, indicating that a D:A interface with a certain amount of energy offset is required for efficient charge photogeneration, contrary to what some reports have claimed. 28,29 Ideally, it is the most straightforward to discuss the relationship between EQEPV and the energy offset between Eg and ECT; however, accurately determining ECT of some blends is challenging.…”

“…In general, a large VOC is expected to provide a high FF. 9,52,53 Nevertheless, as shown in Figure 6a, the FFs of our devices decreased at high VOC region. To deepen our understanding, FF was plotted against ΔVnr (Figure 6b, see also Figure S17, Supplementary Information), where FF dropped in the ΔVnr < 0.2 V region, similar to the case of EQEPV.…”

“…[1][2][3][4][5][6][7] Nevertheless, a large voltage loss ΔV, which is defined as the difference between the optical bandgap Eg and open-circuit voltage VOC (ΔV = Eg/q − VOC, where q is the elementary charge), is a significant disadvantage of OSCs, restricting further improvement in the PCEs of OSCs. 8,9 Although the ΔV has been continuously decreased in the past decade, state-ofthe-art OSCs still exhibit ΔVs of ~0.5 V or more, which remains considerably larger than those of their inorganic and perovskite counterparts, where ΔV of less than 0.4 V can be achieved. 10 Large voltage losses in OSCs primarily originate from the following two sources.…”

“…In the Shockley-Queisser (SQ) framework, 20 nonradiative charge recombination leads to a non-ideal extra voltage loss ΔVnr. 9,16,19,[21][22][23][24][25] Conventional fullerene-based OSCs typically exhibit ΔVnrs of approximately 0.4 V, 24 which are substantially larger than those of their inorganic and perovskite counterparts. The origin of the large ΔVnr can be rationalized by the extremely low photoluminescence quantum yields (PLQYs) of the CT states.…”

Machine learning of atomic force microscopy images of organic solar cells

“…[7][8][9][10][11] High performance is achieved through efficient photoenergy conversion processes comprising photoabsorption, exciton diffusion, charge separation and charge transport with minimal loss. [12][13][14][15] One of the essential factors in facilitating these multi-physical phenomena is optimising the bulk heterojunction (BHJ) structure of a p/n-blended lm, which is fabricated through a solution process, including a p/n blend ratio, solvent, additive and annealing. [16][17][18][19][20][21] The resulting morphological and crystalline features of the BHJs are characterized by atomic force microscopy (AFM), [22][23][24] transmission electron microscopy (TEM), [25][26][27] X-ray diffractometry (XRD) [28][29][30] and neutron scattering.…”