Light-emitting layer suitable for bright luminescence

A light-emitting layer B comprising:

• (a) a first emitter compound (a) having a non-exited state S0(a), a first excited singlet state S1(a) and a first excited triplet state T1(a);
• (b) a second emitter compound (b) having a non-exited state S0(b), a first excited singlet state S1(b) and a first excited triplet state T1(b);
• and
• (c) optionally, one or more host components (c),
• wherein the energy level of S1(a) is higher than that of S1(b), the energy level of S1(b) is higher than that of T1(b) and wherein the light emitting layer B is further characterized by one or more of the following:
• (ia) the rate of reverse intersystem crossing from T1(a) to S1(a) is higher than the rate of excitation energy transfer from S1(a) to S1(b),
• (ib) the rate of reverse intersystem crossing from T1(a) to S1(a) is higher than the rate of excitation energy transfer from T1(a) to T1(b), or
• (ii) the energy level of T1(b) is higher than that of T1(a).