We demonstrate open-circuit voltages exceeding 1.26 V for CH3NH3PbI3 solar cells by careful process optimization of the perovskite and its interfaces to the electron and hole transport layers. This open-circuit voltage is the highest reported so far in a full MAPI cell stack and only 64 mV below the maximum open circuit voltage that is possible for this material. We confirm these values for the open circuit voltage by independent measurements of the external photoluminescence quantum efficiency reaching values of 5 % for the fully processed solar cell. We further find exceptionally long photoluminescence lifetimes in full cells and in layer stacks involving one or two contact layers. Numerical simulations reveal that these long photoluminescence lifetimes are only possible with extremely low interface recombination velocities between absorber and contact materials.

Time‐resolved photoluminescence (TRPL) is a powerful characterization technique to study carrier dynamics and quantify absorber quality in semiconductors. The minority carrier lifetime, which is critically important for high‐performance solar cells, is often derived from TRPL analysis. However, here it is shown that various nonideal absorber properties can dominate the TRPL signal making reliable extraction of the minority carrier lifetime not possible. Through high‐resolution intensity‐, temperature‐, voltage‐dependent, and spectrally resolved TRPL measurements on absorbers and devices it is shown that photoluminescence (PL) decay times for kesterite materials are dominated by minority carrier detrapping. Therefore, PL decay times do not correspond to the minority carrier lifetime for these materials. The lifetimes measured here are on the order of hundreds of picoseconds in contrast to the nanosecond lifetimes suggested by the decay curves. These results are supported with additional measurements, device simulation, and comparison with recombination limited PL decays measured on Cu(In,Ga)Se2. The kesterite material system is used as a case study to demonstrate the general analysis of TRPL data in the limit of various measurement conditions and nonideal absorber properties. The data indicate that the current bottleneck for kesterite solar cells is the minority carrier lifetime.

postdeposition alkali treatment to improve heterojunction diode quality in Cu(In,Ga) Se 2 (CIGS) solar cells and chloride treatment to passivate grain boundaries in CdTe solar cells. [1] These solar-cell technologies are already commercialized, with lab-scale photovoltaic efficiencies exceeding 22%. [2] However, kesterite-based solar cells, such as Cu 2 ZnSn(S,Se) 4 , which share many of the same characteristics of CIGS and CdTe, significantly lag behind, with a record power conversion efficiency (PCE) of 12.6%. [3] Although the dominant limiting factors for this low performance are a matter of considerable discussion, [4] the following observations are consistent among kesterite absorbers: i) a low photoluminescence quantum yield (PLQY) and a short chargecarrier lifetime, [5] ii) a high value of Urbach band tail energy (larger than 30 meV for S-rich kesterites) and lack of a steep absorption onset, [6,7] and iii) the presence of secondary phases. [4,6,8,9] The extent to which these factors individually affect the photovoltaic performance is debated, but their ubiquity among kesterite absorbers indicates the presence of a large density of point defects. [10][11][12] Specifically, i) the low PLQY arises from the presence of nonradiative mid-gap The identification of performance-limiting factors is a crucial step in the development of solar cell technologies. Cu 2 ZnSn(S,Se) 4 -based solar cells have shown promising power conversion efficiencies in recent years, but their performance remains inferior compared to other thin-film solar cells. Moreover, the fundamental material characteristics that contribute to this inferior performance are unclear. In this paper, the performance-limiting role of deep-trap-level-inducing 2Cu Zn +Sn Zn defect clusters is revealed by comparing the defect formation energies and optoelectronic characteristics of Cu 2 ZnSnS 4 and Cu 2 CdSnS 4 . It is shown that these deleterious defect clusters can be suppressed by substituting Zn withCd in a Cu-poor compositional region. The substitution of Zn with Cd also significantly reduces the bandgap fluctuations, despite the similarity in the formation energy of the Cu Zn +Zn Cu and Cu Cd +Cd Cu antisites. Detailed investigation of the Cu 2 CdSnS 4 series with varying Cu/[Cd+Sn] ratios highlights the importance of Cu-poor composition, presumably via the presence of V Cu , in improving the optoelectronic properties of the cation-substituted absorber. Finally, a 7.96% efficient Cu 2 CdSnS 4 solar cell is demonstrated, which shows the highest efficiency among fully cation-substituted absorbers based on Cu 2 ZnSnS 4 .

The performance of many emerging compound semiconductors for thin‐film solar cells is considerably lower than the Shockley–Queisser limit, and one of the main reasons for this is the presence of various deleterious defects. A partial or complete substitution of the cations presents a viable strategy to alter the characteristics of the detrimental defects and defect clusters. Particularly, it is hypothesized that double cation substitution could be a feasible strategy to mitigate the negative effects of different types of defects. In this study, the effects of double cation substitution on pure‐sulfide Cu2ZnSnS4 (CZTS) by partially substituting Cu with Ag, and Zn with Cd are explored. A 10.1% total‐area power conversion efficiency (10.8% active‐area efficiency) is achieved. The role of Cd, Ag, and Cd + Ag substitution is probed using temperature‐dependent photoluminescence, time‐resolved photoluminescence, current–voltage (IV), and external quantum efficiency (EQE) measurements. It is found that Cd improves the photovoltaic performance by altering the defect characteristics of acceptor states near the valence band, and Ag reduces nonradiative bulk recombination. It is believed that the double cation substitution approach can also be extended to other emerging photovoltaic materials, where defects are the main culprits for low performance.

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