Ilke Celik^{1 3} Zhaoning Song^{1 2} Adam Phillips² Randy Ellingson² Yanfa Yan² Michael Heben² Defne Apul³

1, University of Toledo, Toledo, Ohio, United States
3, University of Toledo, Toledo, Ohio, United States
2, University of Toledo, Toledo, Ohio, United States

Future high performance PV devices are expected to be tandem cells consisting of a low-bandgap bottom cell and a high-bandgap top cell. In this study, we developed a cradle-to-end of use life cycle inventory to evaluate the environmental impacts, primary energy demand (PED), and energy payback time (EPBT) for four integrated two-terminal tandem solar cells comprised of either Si bottom and lead-based perovskite (PK_Pb) top cell (Si/PK_Pb), copper indium gallium selenide (CIGS) and PK_Pb (CIGS/PK_Pb), copper zinc tin selenide (CZTS) and PK_Pb (CZTS/PK_Pb), or tin-lead based perovskite (PK_Sn,Pb) and PK_Pb (PK_Sn,Pb/PK_Pb). The environmental impacts from single junction Si solar cells were used as a reference point to interpret the results. We found that the environmental impacts for a 1 m² area of a cell was largely determined by the bottom cell impacts and ranged from 50 % (CZTS/PK_Pb) to 120 % of a Si cell. The ITO layer used in Si/PK_Pb, CZTS/PK_Pb, and PK_Sn,Pb/PK_Pb is the most impactful after the Si and CIGS absorbers, and contributed up to 70 % (in PK_Sn,Pb/PK_Pb) of the total impacts for these tandem PVs. Manufacturing a single two-terminal device was found to be a more environmentally friendly option than manufacturing of the two constituent single-junction cells, and can reduce the environmental impacts by 30 % due to the exclusion of extra glass, encapsulation, front contact and back contact layers. PED analysis indicated that PK_Sn,Pb/PK_Pb manufacturing has the least energy-intensive processing, and the EPBTs of Si/PK_Pb, CIGS/PK_Pb, CZTS/PK_Pb, and PK_Sn,Pb/PK_Pb tandems were found to be ~13, ~7, ~2, and ~1 months, respectively. On an impacts/kWh of Si basis the environmental impacts of all the devices were much higher (up to ~10 times). These results can be attributed to the low photoconversion efficiency (PCE) and short lifetime that were assumed. While PK_Sn,Pb/PK_Pb has higher impacts than Si based on today’s low PCE (21 %) and short lifetime (5 yr) assumptions, it can outperform Si if its lifetime and PCE reach 16 yr and PCE of 30%. Of the configuration considered, the PK_Sn,Pb/PK_Pb structure is the most environmentally friendly technology.