Flexible micromorph tandem a-Si/μc-Si solar cells

Abstract

The deposition of a stack of amorphous (a-Si:H) and microcrystalline (μc-Si:H) tandem thin film silicon solar cells (micromorph) requires at least twice the time used for a single junction a-Si:H cell. However, micromorph devices have a higher potential efficiency, thanks to the broader absorption spectrum of μc-Si:H material. High efficiencies can only be achieved by mitigating the nanocracks in the μc-Si:H cell and the light-induced degradation of the a-Si:H cell. As a result, μc-Si:H cell has to grow on a smooth substrate with large periodicity (>1 μm) and the a-Si:H cell on sharp pyramids with smaller feature size (∼350 nm) to strongly scatter the light in the weak absorption spectra of a-Si:H material. The asymmetric intermediate reflector introduced in this work uncouples the growth and light scattering issues of the tandem micromorph solar cells. The stabilized efficiency of the tandem n-i-p/n-i-p micromorph is increased by a relative 15% compared to a cell without AIR and 32% in relative compared to an a-Si:H single junction solar cells. The overall process (T<200 °C) is kept compatible with low cost plastic substrates. The best stabilized efficiency of a cell deposited on polyethylene-naphthalate plastic substrate is 9.8% after 1000 h of light soaking at Voc, 1 sun, and 50 °C.

Recombination via tail states in polythiophene:fullerene solar cells

“…The most promising candidates for future generations of low-cost, thin-film solar cells that can be processed on flexible substrates [1][2][3][4][5][6] in roll-to-roll processes are two very different material systems: amorphous and microcrystalline, hydrogenated silicon on the one side and polymer:fullerene 7,8 solar cells on the other side. On first sight, the two technologies seem to be quite different, with the traditional semiconductor Si with decades of research experience on the one side and with the emerging field of polymer optoelectronics on the other side.…”

Sandwiching intermediate reflectors in tandem solar cells for improved photon management

“…Furthermore, light induced degradation 3 limits the feasible thickness of the top cell to values between 200 and 400 nm. 4 Hence, the efficiency of the tandem solar cell is usually limited by the insufficient absorption of light in the top cell. Therefore, the current density of the top cell has to be increased in order to fully exploit the potential of the micromorph approach.…”

Randomly Textured Surfaces

“…Therefore, the total current density is given by the minimal current density of the two cells. Due to the effects of light induced degradation, the thickness of the aSi:H top cell is much stronger limited than the thickness of the bottom cell [45]. The larger optical band gap of a-Si:H as compared to μc-Si:H leads to a higher voltage at the top cell.…”

“…One promising way to achieve this is the implementation of intermediate layers (IL) between the top and the bottom cell [43,45,46]. The basic functionality is the spectrally dependent reflection of light in the spectral range, where the absorption of both cells is comparable.…”