Organic solar cells do not have record power conversion efficiencies. Instead, they promise to be cheap, lightweight, flexible, and tunable. A recurring issue is their durability. Organic materials are more sensitive to environmental stress than pure silicon, and small morphology changes can lead to a steep drop in performance. Zicheng Ding, Jun Liu, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Wei Ma, Xi’an Jiaotong University, China, Yongsheng Chen, Nankai University, Tianjin, China, and colleagues have developed an organic solar cell that is extraordinarily heat resistant, and it is efficient, as well. The stability boost comes from an unusual cell composition and a little molecular tweaking.
Finding the Perfect Blend
In the heart of an organic solar cell is the active layer. It absorbs the sunlight and forms the excitons, the excited states of an electron and a hole bound together by the Coulomb force. Research on organic cells aims at finding and developing organic semiconductor materials that split the excitons and can conduct the electrons and holes separately and efficiently into the respective electrodes. The two semiconductor materials are complementary: One is an electron donor, the other one is an electron acceptor.
A critical point is how these materials blend together in the active layer. The active layer morphology should be as flat and lean as possible, with the two semiconductors tightly entangled. The blend morphology defines how large the electronic interface is, in which ways the molecules interact, whether the charges can separate—in short: the performance of the device.
Today, the best organic solar cells, which convert around 10 % of the solar energy into electric power, use a conjugated polymer as the electron donor and an organic molecule as the electron acceptor. Scientists found this combination so successful that it has somewhat ousted other variations in the last years. That might change now. The team demonstrated that the reverse combination of molecular donor and polymeric acceptor works as well and brings its own advantages. Only a little adjusting on the molecular structure is needed.
Electron donor molecules that can be used in organic solar cells usually have a stretched-out conjugated aromatic system, which gives rise to an essentially flat molecular structure. But flat conjugated molecules tend to form stacks. This is detrimental for the active layer because stacks can develop into larger crystalline domains, the material phases separate, the interfaces shrink, and with that the performance of the device is lowered. Therefore, researchers try to avoid crystallization in the blend at all costs.
To suppress crystallization, the team made the structure of the organic donor “bumpier.” They chose an existing organic donor molecule, which was long, flat, and symmetric, with two protruding alkyl thienyl groups in its center. Then they replaced the thienyl groups with bulky alkyl carbazoyl groups. Calculations showed that the protrusions in the middle of the molecule more than doubled in size. With that bump in the middle, stacking should become less favorable.
The morphology of the active layer with the non-stacking donor molecule turned out to be much leaner. The material phases appeared tightly entangled, and the efficiency of the device climbed from 3 % to 8 %. The active layer film was also extraordinarily heat resistant: The team kept the film at 180 °C for seven days and the efficiency remained at nearly 90 %. Existing organic cells fail under these conditions.
The other reason for the good performance of the organic cell was the choice of the polymer acceptor. It was was also a new development and contained a boron–nitrogen coordination bond, which enabled both high electron affinity and charge mobility, according to the researchers.
The team points out that the research on organic cells with “inverse” donor–acceptor combinations is still promising. If more acceptor polymers and donor molecules are developed for this purpose, more surprises can be expected.
- Efficient and thermally stable organic solar cells based on small molecule donor and polymer acceptor,
Zijian Zhang, Junhui Miao, Zicheng Ding, Bin Kan, Baojun Lin, Xiangjian Wan, Wei Ma, Yongsheng Chen, Xiaojing Long, Chuandong Dou, Jidong Zhang, Jun Liu, Lixiang Wang,
Nat. Commun. 2019.